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Lithium intercalation in titanium based oxides and sulfides Colbow, Kevin Michael 1988

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LITHIUM INTERCALATION IN TITANIUM BASED OXIDES AND SULFIDES by KEVIN MICHAEL COLBOW B. Sc. (Hon.) Simon F r a s e r U n i v e r s i t y , 1986 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES Department of P h y s i c s We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA APRIL 1988 © KEVIN MICHAEL COLBOW, 1988 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of /^A^yS/C& The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 DE-6G/81) ABSTRACT The L i - T i - S t e r n a r y system was i n v e s t i g a t e d . The elements and/or compounds, such as l i t h i u m s u l f i d e , t i t a n i u m d i s u l f i d e and t i t a n i u m t r i s u l f i d e , were combined at h i g h temperature ( t y p i c a l l y 500-900°C). The s y n t h e s i z e d compounds c o n t a i n i n g one or more phases were s t r u c t u r a l l y c h a r a c t e r i z e d using x-ray powder d i f f r a c t i o n . When L i T i S , (0 < x < 1) was s y n t h e s i z e d at e l e v a t e d temperature, a new p o l y t y p e , 3 R - L i x T i S 2 , was found f o r some valu e s of x. The r e g i o n s of s t a b i l i t y of the 3R p olytype and the w e l l known 1T p o l y t y p e are presented. L i t h i u m can be i n t e r c a l a t e d or d e - i n t e r c a l a t e d from both p o l y t y p e s at room temperature. Ambient temperature L i / 3 R - L i x T i S 2 c e l l s have higher average v o l t a g e s than L i / 1 T - L i T i S , c e l l s . The l i t h i u m s p i n e l oxides are another c l a s s of m a t e r i a l s r e c e i v i n g a t t e n t i o n as cathode m a t e r i a l s i n l i t h i u m secondary b a t t e r i e s . L i T i 2 0 4 i s m e t a l l i c , has the c u b i c s p i n e l s t r u c t u r e and r e a c t s with one f u r t h e r l i t h i u m atom to form L i 2 T i 2 0 4 . The r e l a t e d s p i n e l L i ^ ^ T i 5 / 3 ° ^ > which i s e l e c t r i c a l l y i n s u l a t i n g , a l s o r e a c t s r e v e r s i b l y with one l i t h i u m atom. Both L i / L i T i 2 0 4 and L i / L i 4 / 3 T i 5 / 3 0 4 c e l l s c y c l e r e v e r s i b l y , but have s u b t l e d i f f e r e n c e s i n t h e i r v o l t a g e p r o f i l e s . The d i f f e r e n c e in c e l l behaviour was i n t e r p r e t e d based on the band s t r u c t u r e of L i 1 + x T i 2 _ x 0 4 . The mixed s p i n e l s LiMn Ti„ 0. (0 ^ y ^ 2 ) were a l s o y i y 4 i i i n v e s t i g a t e d . These compounds were s y n t h e s i z e d at high temperature but t h e i r performance as cathodes i n l i t h i u m b a t t e r i e s was not encouraging. i i i TABLE OF CONTENTS ABSTRACT i i LIST OF TABLES v i LIST OF FIGURES v i i ACKNOWLEDGMENTS x 1 INTRODUCTION 1 1.1 THE ELECTROCHEMICAL CELL 3 1.2 BRIEF REVIEW OF L i TiS, WORK 5 1.2.1 Structure of 1T-TiS 2 5 1.2.2 Room Temperature 1 T-Li TiS- 5 A £, 1.2.3 High Temperature Synthesis of. L i MX, 9 1.3 REVIEW OF SPINEL OXIDES 11 1.3.1 Transition Metal Lithium Oxides 11 1.3.2 The Spinel Structure AB 20 4 13 1.3.3 Lithium Spinel Oxides LiM 20 4 16 1.3.4 The So l i d Solution Series L i i + x T i 2 - x ° 4 •**• 1 7 2 EXPERIMENTAL METHODS 2 0 2.1 PROCEDURES 20 2.1.1 Synthesis of Materials 20 2.1.1.1 Sulfides ' 20 2.1.1.2 Oxides 22 2.1.2 Electrochemical C e l l Fabrication 23 2.1.2.1 C e l l Components 23 2.1.2.2 C e l l Assembly 25 2.2 TECHNIQUES 29 2.2.1 Powder X-ray D i f f r a c t i o n 29 2.2.1.1 In-situ X-ray D i f f r a c t i o n 29 2.2.1.2 Preparation of X-ray Samples 31 iv 2.2.2 Electrochemical Measurements of V(x) 2.2.3 R e s i s t i v i t y Measurements 32 33 3 RESULTS AND DISCUSSION 36 3.1 SULFIDES 36 3.1.1 Materials Synthesized 36 3.1.1.1 Li - T i - S Ternary Phase Diagram .... 36 3.1.1.2 3 R - L i x T i S 2 (0 < x < 1) 43 3.1.2 X-ray D i f f r a c t i o n 49 3.1.2.1 a & c vs x in 1T & 3 R - L i x T i S 2 . 50 3.1.3 Electrochemical Measurements 53 3.1.3.1 C e l l s for 1T 6c 3 R - L i x T i S 2 vs L i .. 53 3.1.4 In-situ X-ray D i f f r a c t i o n on 3 R - L i x T i S 2 ... 58 3.2 OXIDES 61 3.2.1 Materials Synthesized 61 3.2.1.1 L i T i 2 0 4 and L i 4 / 3 T i 5 / , 3 0 4 61 3.2.1.2 Mixed Spinel Oxides LiMn T i ~ 0. . 66 y ^ y * 3.2.2 X-Ray D i f f r a c t i o n 70 3.2.2.1 a vs y in LiMn T i ~ 0. 70 y ^ y * 3.2.3 Electrochemical Measurements 70 3.2.3.1 L i T i 2 0 4 and L i 4 / 3 T i 5 / 3 0 4 C e l l s 70 3.2.3.2 Band Structure Arguments 75 3.2.3.3 Li/LiMn 20 4 C e l l s 78 4 POSSIBLE APPLICATIONS 82 5 SUMMARY AND SUGGESTIONS FOR FUTURE WORK 84 BIBLIOGRAPHY 87 Appendix I : T c Measurements on L i T i 2 0 4 89 Appendix II : L i / L i 1 + x T i 2 0 4 c e l l with 25% graphite 94 LIST OF TABLES Table Page I. Some lithium metal oxide structures 15 II. Summary of syntheses of L i - T i - S compounds 37 II I . L a t t i c e parameters of Ti-S compounds 41 synthesized in thi s study IV. Comparison of the x-ray patterns of 1T-LiTiS2 45 and 3R-LiTiS 2 V. Summary of syntheses of L i T i 2 0 4 and 64 L i 4 / 3 T i 5 / 3 ° 4 VI. Summary of L i M n y T i 2 _ ^ 0 4 (0 < y < 2) syntheses 67 performed vi LIST OF FIGURES Figure Page 1. Schematic diagram of an i n t e r c a l a t i o n c e l l 4 2. The "sandwich" structure of T i S 2 6 3. The atomic structure of T i S 2 6 4. V(x) for L i T i S , c e l l s 7 5. Variation of the l a t t i c e parameters, c and a, 8 of L i x T i S 2 with x 6. Schematic diagram of lithium metal oxide 12 structures 7. Half a unit c e l l of the spinel structure 14 8. L a t t i c e parameter a versus composition x for 19 L i 1 + x T i 2 - x ° 4 9. Exploded view of the 27 "cathode-separator-anode" sandwich 10. An experimental test c e l l 28 11. An i n - s i t u x-ray d i f f r a c t i o n c e l l 30 12. Schematic diagram of apparatus used for 34 resistance measurements on powdered samples 13. L i - T i - S ternary phase diagram.at T = 700°C 40 14. 110 projections of the structures of 1T-TiS 2 44 and 3R-Li. TiS~ V I 1 15. Diagram showing 3R:1T r a t i o for various 47 st a r t i n g materials 16. Variation of l a t t i c e parameters of room 51 temperature and high temperature 1 T - L i x T i S 2 with x 17. Variation of l a t t i c e parameters of 3 R - L i x T i S 2 52 with x 18. V(x) for L i / 1 T - L i x T i S 2 c e l l s with an i n i t i a l 54 cathode material of T i S 2 19. V(x) for L i / l T - L i x T i S 2 c e l l s with an i n i t i a l 55 cathode material of 1 T - L i T i S 2 20, V(x) for L i / 3 R - L i x T i S 2 c e l l s with an i n i t i a l 57 cathode material of 3R-LiTiS„ 21. Portions of x-ray d i f f r a c t i o n p r o f i l e s taken 59 during the f i r s t charge and subsequent discharge of a L i / 3 R - L i x T i S 2 c e l l 22. Ternary phase diagram for the L i - T i - 0 system 63 at T = 700-900°C '23. Variation of the a l a t t i c e parameter of 71 LiMn T i 9 _ 0. versus y y £ y 24. V(x) for L i / L i 1 + x T i 2 0 4 c e l l s 72 25. V(x) for L i / L i 4 y 3 + x T i 5 y 3 0 4 c e l l s 73 26. Percent c e l l capacity versus cycle number for 76 L i / L i 1 + x T i 2 0 4 c e l l s v i i i 27. Percent c e l l c a p a c i t y versus c y c l e number f o r 77 L i / L i 4 / 3 + x T i 5 / 3 ° 4 c e l l s 28. Schematic diagram showing the band s t r u c t u r e 79 Of L i T i 2 0 4 and L i 4 / 3 T i 5 / 3 0 4 29. V(x) f o r a L i / L i 1 + x M n 2 0 4 c e l l 80 30. H t o t / g versus temperature f o r L i T i 2 0 4 at 91 H = 60 gauss 31. ^ t o t ^ versus temperature f o r L i T i 2 0 4 and 92 L i 1 1 T i 2 0 4 at H = 5 gauss 32. V(x) f o r a L i / L i 1 + x T i 2 0 4 c e l l with 25% 95 g r a p h i t e by weight i n the cathode 33. Percent c e l l c a p a c i t y versus c y c l e number f o r 96 a L i / L i 1 + x T i 2 0 4 c e l l with 25% graphite' by weight i n the cathode ix ACKNOWLEDGMENT S I would l i k e to express sincere gratitude to my supervisor, Dr. Rudi Haering for his guidance throughout the course of t h i s project. I am also indebted to Dr. Jeff Dahn for his advice, encouragement and enthusiasm. Discussions with both Rudi and Jeff were most h e l p f u l . I wish to extend my gratitude to the entire research and development group at Moli Energy Ltd. for the i r willingness to tackle problems and provide prompt attention in the maintenance of equipment. A special thanks also goes to Dr. Jim Carolan for his assistance in obtaining the superconductivity measurements. F i n a l l y , the author i s grateful for f i n a n c i a l support from the Natural Sciences and Engineering Council in the form of a 1967 postgraduate scholarship. x 1 INTRODUCTION Intercalation is the reversible insertion of guest atoms or molecules into a host structure without d r a s t i c a l l y a l t e r i n g the atomic structure of the host. Suitable hosts must have s i t e s in their l a t t i c e s which are accessible from the surface. Consequently, these materials generally have open passages through which intercalants can d i f f u s e . The c l a s s i c example of an i n t e r c a l a t i o n host i s graphite, where i t is found that many di f f e r e n t types of atoms and molecules can be intercalated between the graphite planes. Dresselhaus and Dresselhaus (1981) review much of the work performed on graphite i n t e r c a l a t i o n compounds. Intercalation occurs -in other layered hosts such as the t r a n s i t i o n metal dichalcogenides (MX2) (Whittingham 1978). Materials with non-layered structures such as the t r a n s i t i o n metal oxides (e.g. V 20g (Murphy et a l . 1979a)) and the spinel oxides (e.g. M 30 4 (M = Fe, Co, Mn) and LiM 20 4 (M = T i , V, Mn) (Thackeray et a l . 1987)) have also been found to be hosts for reversible i n t e r c a l a t i o n . Intercalation systems are of current interest for two reasons. F i r s t , the large number of interesting physical phenomena exhibited by these systems makes these materials a t t r a c t i v e from a basic science viewpoint. Secondly, the technological importance of many intercalation systems renders them interesting for p r a c t i c a l and economic reasons. 1 2 In f a c t , the development of these systems r e s u l t e d in the f i r s t mass produced b a t t e r y based on Li/MoS2 (Haering et a l . 1980). Of the t r a n s i t i o n metal d i c h a l c o g e n i d e s (MX 2), T i S 2 has r e c e i v e d the most a t t e n t i o n i n the l i t e r a t u r e (Dahn and Haering 1979, Murphy et a l . 1979a, Thompson 1978, C h i a n e l l i 1976 and Whittingham 1976). However, apart from L i T i S ~ (0 ^ x < 1) the L i - T i - S t e r n a r y phase diagram has not been e x t e n s i v e l y i n v e s t i g a t e d . Here a p r e l i m i n a r y phase diagram fo r the L i - T i - S system i s determined and the e x i s t e n c e and s t r u c t u r e of a new T i S 2 p o l y t y p e , 3 R - L i x T i S 2 , i s r e p o r t e d . The l i t h i u m s p i n e l oxides are one c l a s s of m a t e r i a l s that are p a r t of the l i t h i u m t r a n s i t i o n metal ox i d e s . We made l i t h i u m secondary b a t t e r i e s using L i T i 2 0 4 and the r e l a t e d compound L i 4 / , 3 T i 5 / , 3 0 4 . Compounds of the form LiMn T i 9 _ O. (0 < y < 2), known y £. y 4 as mixed s p i n e l oxides, were a l s o i n v e s t i g a t e d , and many of these m a t e r i a l s were s y n t h e s i z e d f o r the f i r s t time. The two end members of t h i s s e r i e s , L i T i 2 0 4 and L i M n 2 0 4 , although i d e n t i c a l i n s t r u c t u r e , showed markedly d i f f e r e n t v o l t a g e curves i n L i / L i T i 2 0 4 and L i / L i M n 2 0 4 c e l l s . The behaviour of L i / L i M n T i ~ 0. (0 ^ y < 2) c e l l s , as a r e s u l t , becomes a y ^ y 4 very i n t e r e s t i n g q u e s t i o n . The idea of " t u n i n g " the v o l t a g e of a l i t h i u m secondary b a t t e r y by c o n t r o l l i n g the r a t i o of the two t r a n s i t i o n metals, Mn and T i motivated a study of 3 t h i s s y s t e m . 1.1 THE ELECTROCHEMICAL CELL A n e l e c t r o c h e m i c a l c e l l b a s e d o n i n t e r c a l a t i o n i s r e p r e s e n t e d : L i / o r g a n i c e l e c t r o l y t e c o n t a i n i n g L i + i o n s / L i M X , X ^ u s i n g M X 2 a s t h e c a t h o d e a n d l i t h i u m a s t h e a n o d e . S u c h a c e l l i s s h o w n s c h e m a t i c a l l y i n f i g u r e 1. T h e h a l f c e l l r e a c t i o n s a r e x L i > x L i + + e a t t h e a n o d e a n d x L i + + x e~ + M X 2 > L i x M X 2 a t t h e c a t h o d e . U p o n d i s c h a r g e o f t h e c e l l , e l e c t r o n s m o v e f r o m t h e a n o d e t o t h e c a t h o d e v i a a n e x t e r n a l e l e c t r i c c i r c u i t a n d t h e L i + i o n s m i g r a t e t h r o u g h t h e e l e c t r o l y t e t o t h e c a t h o d e . T h e o v e r a l l c e l l r e a c t i o n i s t h e t r a n s f e r o f l i t h i u m a t o m s f r o m t h e a n o d e t o t h e c a t h o d e x L i + M X 2 > L i x M X 2 U p o n r e c h a r g e , a c u r r e n t i s d r i v e n t h r o u g h t h e e x t e r n a l c i r c u i t s o t h a t t h e e l e c t r o n s f l o w f r o m t h e c a t h o d e t o t h e a n o d e w h e r e t h e y r e c o m b i n e w i t h e l e c t r o n s a n d e l e c t r o p l a t e o n t o t h e l i t h i u m m e t a l a n o d e . T h e v o l t a g e , V ( x ) , o f a L i / L i MX„ c e l l i s g i v e n b y X £ V ( x ) = [u - y ( x ) ] / e w h e r e u a n d M „ ( x ) a r e t h e c h e m i c a l p o t e n t i a l s o f t h e cL C 4 F i g u r e 1. Schematic diagram of an i n t e r c a l a t i o n c e l l . (PC and EC denote propylene carbonate and e t h y l e n e carbonate r e s p e c t i v e l y . ) 5 lithium atoms in the anode and cathode respectively and e i s the electronic charge. V(x) and M c(x) are functions of lithium concentration because the chemical composition of the Li xMX 2 cathode changes as current flows. 1.2 BRIEF REVIEW OF L I X T I S 2 WORK 1.2.1 S t r u c t u r e of l T - T i S 2 Titanium d i s u l f i d e i s a material which i s not found in nature and must be produced s y n t h e t i c a l l y . T i S 2 has the well known cadmium iodide (Cdl 2) structure (Wyckoff 1963). This is a layered structure which consists of S-Ti-S "sandwiches" stacked upon each other and held together by r e l a t i v e l y weak van der Waals forces as depicted in figure 2. Each "sandwich" consists of a plane of titanium atoms between two planes of sulfur atoms, where the atoms are hexagonally close packed. The atomic structure of the "sandwiches" i s shown in figure 3. The prefix 1T in 1T-TiS 2 refers to the 1 layer unit c e l l with trigonal (T) symmetry. 1.2.2 Room Temperature l T - L i x T i S 2 The variation of the voltage with state of charge, V(x), of L i / L i x T i S 2 c e l l s has been the subject of much interest (Thompson 1979 and Berlinsky et a l . 1979). Figure 4 shows the V(x) behaviour of L i / L i TiS~ c e l l s and figure 5, s Ti S Van der Wads gap Figure 2. The "sandwich" s tructure of T i S 2 . Hor izonta l l ines in the f igure represent planes of atoms. lT -T iS 2 Figure 3 . The atomic s tructure of T i S 2 . Titanium atoms are depicted by s o l i d c i r c l e s and su l fur atoms by open c i r c l e s . The pref ix IT re fers to the 1 layer unit c e l l with t r i g o n a l (T) symmetry. 7 CO o > 2.4 2.2 2.0 1.8 T « r T r-F i g u r e 4. Thompson 1978) 0 0.2 0.4 0.6 0.8 1.0 x in L i x T i S 2 The V(x) behaviour of L i T i S - c e l l s . x ^ ~ 2 ( A f t e r 8 -J3.46 0.2 0.4 0.6 x in L i x T i S 2 0.8 1.0 F i g u r e 5. V a r i a t i o n of the l a t t i c e parameters, c and a, of L i x T i S 2 with x. ( A f t e r Dahn et a l . 1982) 9 the v a r i a t i o n of the c and a l a t t i c e parameters (Dahn et a l . 1982). It is evident that the c axis increases by about 10% between x = 0 and x = 1 with most of the increase coming before x = 0.4. In contrast, the a axis increases most rapidly near x = 1 and increases by about 1.5% over the range 0 < x < 1 . Previous studies (Whittingham et a l . 1975 and Whittingham 1978) show that, of the group IV t r a n s i t i o n metal dichalcogenides, only ZrS 2 and HfS 2 undergo a str u c t u r a l t r a n s i t i o n from the 1T to the 3R polytype when intercalated with lithium. McKinnon et a l . (1984) observed that for ZrS 2, this t r a n s i t i o n was completely reversible. The 1T and 3R phases coexist in the range 0.01 ± 0.01 < x < 0.23 ± 0.01 as this system undergoes a. f i r s t order phase t r a n s i t i o n . The l a t t i c e parameters in the two phase region do not change since the system i s made up of varying amounts of two int e r c a l a t i o n phases of fixed lithium content. No such t r a n s i t i o n has been reported to occur for T i S 2 . 1.2.3 High Temperature Synthesis of L i x M X 2 In addition to i n t e r c a l a t i n g lithium atoms into t r a n s i t i o n metal dichalcogenides using electrochemical c e l l s , one can prepare L i MX, compounds by high temperature (H.T.), s o l i d state reactions. Issler (1986) prepared 10 L i x H f S 2 and L i x H f S e 2 by the s o l i d state reaction of the appropriate elements along with L i 2 X (X = S or Se) in sealed evacuated quartz tubes. The required stoichiometries were determined from the following equation: x/2 L i 2 X + M + (2-x/2) X > Li xMX 2 One of the major problems associated with t h i s type of synthesis i s attack of the quartz tube by the lithium metal. This lowers the lithium content or goes so far as to prevent the formation of the Li xMX 2 material. The production of multi-phased compounds i s also a problem. The exact value of the lithium composition, x in the d i f f e r e n t phases cannot be determined. Differences between the behaviour of these high temperature compounds and their room temperature counterparts were discovered. For L i HfS 9 (H.T.) (0.05 < x < 1 .3) Issler (1986) found that the l a t t i c e parameters did not change as a function of lithium composition in the same fashion as the lithium intercalated t r a n s i t i o n metal dichalcogenides prepared by other methods. The electrochemical experiments showed very l i t t l e of the lithium inserted at high temperatures could be deintercalated. Although at high temperature the lithium atoms insetted cannot be removed, the single phased 3R-Li xHfS 2 (H.T.) materials demonstrated very promising in t e r c a l a t i o n battery behaviour (Issler 1986). Syntheses of 11 t h i s type may be of technological significance in the development of high energy density, reversible lithium i n t e r c a l a t i o n batteries based on the t r a n s i t i o n metal dichalcogenides. 1.3 REVIEW OF SPINEL OXIDES 1.3.1 T r a n s i t i o n Metal L i t h i u m Oxides Air-stable lithium t r a n s i t i o n metal oxides often are found in three common structural types. These are: (i) the rocksalt structure; ( i i ) the spinel structure; and ( i i i ) the layered oxide structure. These structure types are described in the l i t e r a t u r e (Murphy et a l . 1983 and Mizushima et a l . 1980), and here only their s i m i l a r i t i e s w i l l be discussed. Although the rocksalt and spinel structures have cubic symmetry, they are b u i l t up of hexagonal close packed oxygen layers having ABCABC stacking just l i k e the layered oxide structure. Figure 6 shows that these structures d i f f e r in the arrangement of M and L i atoms between the hexagonal close packed 0 atom layers. In these structures a l l the M and L i atoms are located in octahedral s i t e s . The spinel diagram of figure 6(b) applies to compounds l i k e L i 2 T i 2 0 4 , where a l l the octahedral s i t e s are f i l l e d . The related spinel compound L i T i 2 0 4 , where some octahedral s i t e s would be l e f t vacant, has i t s M and L i atoms d i s t r i b u t e d between 12 (111) 1/2 Li, 1/2 M O 1/2 Li, 1/2 M O 1/2 Li, 1/2 M O (a) Rocksalt (111) (001) _ — — _ — 3/4 L i > -j/4 M O _ — — _ — 1/4 Li, 3/4 M O — — — — — 3/4 Li, 1/4 M O (b) Spinel O — — — — — Li O — — — — _ M O — — — — — Li O (c) Layered F i g u r e 6 . S c h e m a t i c d i a g r a m o f r o c k s a l t ( a ) , s p i n e l l a y e r e d ( c ) l i t h i u m m e t a l o x i d e s t r u c t u r e s . ( b ) a n d 13 both tetrahedral and octahedral s i t e s . Therefore, the schematic diagram of figure 6(b) does not s t r i c t l y apply to L i T i 2 0 4 . Table I shows representative compounds having each of the structures. 1.3.2 The S p i n e l S t r u c t u r e A B 2 0 4 Compounds with the spinel structure have a generalized formula, AB 2X 4 which i s derived from the mineral, MgAl 20 4 known as s p i n e l . A large variety of cations of d i f f e r e n t size and valence state may enter the spinel structure. Well over a hundred compounds have been found to belong to the spinel family, most of them are oxides, some are chalcogenides and halides. A unit c e l l of the ideal face-centered-cubic (fee) spinel structure of an oxide spinel of composition A B 2 0 4 consists of a cubic-close-packed array of 32 oxygen atoms occupying s i t e s 32e of space group Fd3m. One-eighth of the 64 tetrahedral holes per unit c e l l (sites 8a) and one-half of the 32 octahedral holes per unit c e l l ( s i t e s I6d) are f i l l e d by the cations A and B. Figure 7 shows half a unit c e l l of the spinel structure. The resultant unit c e l l contains eight formula units of AB 20 4. The d i s t r i b u t i o n s of cations in the octahedral and tetrahedral s i t e s of a spinel is usually indicated by putting the cations occupying the octahedral s i t e s in brackets: A[B 0]6., is denoted a "normal" 14 F i g u r e 7. Half a u n i t c e l l of the s p i n e l s t r u c t u r e ( a f t e r Goodenough et a l . 1984) showing the p o s i t i o n o f : 8a (©) and I6d (•) c a t i o n s ; some of the 32e oxygens ( 0 ) ; 8b (•) and 16c (*) i n t e r s t i t i a l s i t e s of one q u a r t e r (two o c t a n t s ) of the u n i t c e l l . TABLE I Some l i t h i u m metal oxide s t r u c t u r e s that are a i r s t a b l e R o c k salt S p i n e l Layered NiO L i N i 0 2 L i x N i 1 - x ° (0 < x < 0.2) L i T i 0 2 L i T i 2 ° 4 L i 2 T i 0 3 TiO L i 2 T i 2 0 4  L i 4 / 3 T i 5 / 3 ° 4 CoO L i C o 0 2 L i A l 0 2 Li 2Mn0 3, L i M n 2 0 4  L i 4 / 3 M n 5 / 3 ° 4 16 s p i n e l , and B[AB]0 4 an " i n v e r s e " s p i n e l . 1.3.3 L i t h i u m S p i n e l Oxides L i M 2 0 4 S p i n e l compounds having the general formula A [ B 2 ] X 4 are c u r r e n t l y r e c e i v i n g a t t e n t i o n as p o s s i b l e s o l i d s o l u t i o n e l e c t r o d e s f o r room temperature l i t h i u m b a t t e r i e s . P a r t i c u l a r a t t e n t i o n has been given to the normal l i t h i u m s p i n e l s L i M n 2 0 4 (Thackeray et a l . 1983, Mosbah et a l . 1983, and Thackeray et a l . 1984), L i T i 2 0 4 (Murphy et a l . 1982, Murphy et a l . 1983, and Cava et a l . 1984) and L i V 2 0 4 (Murphy et a l . 1983). In these s p i n e l s the t e t r a h e d r a l l y c o o r d i n a t e d L i + ion r e s i d e s i n the i n t e r s t i t i a l space of the [ B 2 ] 0 4 framework. The t r a n s i t i o n metals ( T i , Mn and V) are able to form these compounds because of the a v a i l a b i l i t y of both the 3 + 4 + M and M valence s t a t e s . T h i s i s i n f a c t a c r i t e r i a f o r the formation of the l i t h i u m s p i n e l oxides that c o n t a i n one type of t r a n s i t i o n metal. The l a t t i c e c o n s t a n t s f o r these compounds are given below. Compound a, A L i T i 2 0 4 8.41 L i M n 2 0 4 8.25 L i V 2 0 4 8.22 The a l a t t i c e parameter i s r e l a t i v e l y s i m i l a r f o r v a r i o u s l i t h i u m s p i n e l oxides implying that the oxygen s u b l a t t i c e 17 contributes most to the size of the unit c e l l . Lithium insertion/extraction reactions have been demonstrated with L i T i 2 0 4 and i t has been shown that l i t h i a t e d anatase, Lig 5 T i 0 2 , transforms to spinel on heating to 500°C (Murphy et a l . 1982). Of p a r t i c u l a r interest has been the system L i 1 + x T i 2 0 4 in which the [M 2]0 4 framework remains intact for both lithium insertion and extraction over the range -1 < x < 1 (Thackeray et a l . 1983, Mosbah et a l . 1983, and Thackeray et a l . 1984). Only limited data i s available for l i t h i a t e d L i V 2 0 4 ; the unit c e l l is reported to contract on + 3 + l i t h i a t i o n , and in L i 2 V 2 0 4 the L i and V ions occupy the 16c and 16d octahedral s i t e s of the spinel structure respectively (Murphy et a l . 1983). 1.3.4 The S o l i d S o l u t i o n S e r i e s L i i + x T i 2 - x ° 4 The spinel oxides L i T i 2 0 4 and L i 4 y 3 T i 5 y 3 0 4 a r e t n e e n c* members of the s o l i d solution series L i , T i 9 _ 0. for 0 < x < 1/3. Lithium substitutes for titanium in octahedral titanium s i t e s in the L i 4 y 3 T i g ^ 3 0 4 phase (Murphy et a l . 1983). Both materials are stable in a i r , however these two compounds exhibit completely di f f e r e n t properties. L i T i 2 0 4 i s a dark blue, metallic compound whereas L i 4 ^ 3 T i g ^ 3 0 4 ^ s white and insulating. In fact L i T i 2 0 4 i s superconducting below T c = 12 K and t h i s study found that T c = 12.6 K using measurements of the magnetic moment. (See Appendix I for 18 d e t a i l s of these r e s u l t s ) . F i g u r e 8 shows the v a r i a t i o n of the l a t t i c e parameter a with the nominal composition x f o r L i 1 + x T i 2 _ x samples prepared i n dynamic vacuum and s e a l e d tubes and then exposed to a i r at room temperature ( H a r r i s o n et a l . 1985). Both L i T i 2 0 4 and L i 4 y 3 T i g y 3 0 4 are known to r e a c t with one f u r t h e r l i t h i u m atom to form L i 2 T i 2 0 4 and L i y ^ T i g ^ C ^ r e s p e c t i v e l y (Murphy et a l . 1983). In L i 2 T i 2 C > 4 the Ti-2°4 r r a m e w o r k i s v i r t u a l l y unchanged from that i n the normal s p i n e l , whereas a l l the L i ions occupy o c t a h e d r a l s i t e s . S i m i l a r l y i n L i y ^ 3 T i g ^ 3 0 4 a l l the L i i s l i k e l y i n o c t a h e d r a l s i t e s (Murphy et a l . 1983). 19 0 0.1 0.2 0.3 x in L i 1 + X T i 2 _ x 0 4 F i g u r e 8. L a t t i c e parameter a versus composition x f o r L i 1 + x T i 2 _ x 0 4 prepared i n dynamic vacuum (•) and s e a l e d tubes (•) a f t e r exposure to a i r at room temperature. ( A f t e r H a r r i s o n et a l . 1985) 2 EXPERIMENTAL METHODS 2.1 PROCEDURES 2.1.1 S y n t h e s i s of M a t e r i a l s A variety of methods were used to synthesize the materials discussed in t h i s thesis. The variables that proved to be relevant were temperature, time of reaction, surrounding gaseous atmosphere and to some extent the reaction vessel. The d i f f e r e n t methods of preparation w i l l be described in the next sections. 2.1.1.1 Sulfides Ti-S compounds The compounds T i S 2 , T i s 3 f T i 3 S 4 a n d T i s were a l l prepared in the same manner for further use as s t a r t i n g materials for other reactions. This method of preparation involved placing stoichiometric ratios of highly pure (99.9%) Ti and S powders in a glass ampoule, which was then evacuated to less than 30 mT and f i n a l l y sealed. The sample was then heated to 250°C and the temperature raised at approximately 50°C/hr to 550°C, where i t was held for approximately four days. The samples were quenched in a i r and were allowed to cool to room temperature before the ampoules were opened. 20 21 L i - T i - S compounds A l l the high temperature L i - T i - S compounds were prepared by the s o l i d - s t a t e reaction of a variety of sta r t i n g materials, including L i 2 S (99%), L i 2 C 0 3 (99.5%), Ti (99.9%) and the Ti-S compounds synthesized as described above. In a t y p i c a l experiment stoichiometric quantities of the s t a r t i n g materials were mixed, ground and placed in a graphite ampoule. This procedure was carried out inside an inert atmosphere glove box because of the moisture s e n s i t i v i t y of some of the s t a r t i n g materials. The graphite containers are useful as containment vessels as they are apparently inert to 'the lithium containing compounds. However, they must not be allowed to come in contact with -oxygen at elevated temperatures as the carbon reacts with oxygen to form carbon dioxide thus destroying the vessel. The mixture was allowed to react in the graphite ampoule under a flowing argon atmosphere at temperatures ranging between 500-900°C for 20-60 hours depending on the s p e c i f i c experiment. The samples were cooled under argon. Then the graphite ampoules were quickly transferred back to the glove box. 22 2.1.1.2 Oxides The synthesis of spinel oxides i s inherently easier than that of the su l f i d e s because these compounds are stable in a i r . However, t h i s by no means makes i t easy to synthesize pure oxide compounds of the desired stoichiometry. In fact a t o t a l of 125 oxide samples were synthesized in thi s study in an e f f o r t to obtain pure samples. The synthesis of the various spinel oxides was again based on the high temperature so l i d - s t a t e reactions of various st a r t i n g materials. Generally the sta r t i n g materials consisted of a lithium containing compound such as LiOH, L i 2 C 0 3 or L i 2 0 and one or more of a t r a n s i t i o n metal oxide or carbonate such as T i 0 2 , T ^ O j , Mn02 or MnCO^ and/or the tr a n s i t i o n metal elements Ti and Mn. In some cases, depending on the s p e c i f i c experiment, L^TiO-j was synthesized f i r s t and then used as a sta r t i n g material to better contain the highly reactive lithium. Stoichiometric quantities of the st a r t i n g materials were ground together to a fine uniform powder which was pressed into p e l l e t s at 15,000-17,000 p s i . In some cases i t was unnecessary to press the p e l l e t s . The p e l l e t s (or powder) were f i r e d at temperatures ranging between 500-1000°C for durations as short as 1 hour and as long as 4 days. The reaction atmospheres used include a i r , oxygen, argon, dynamic vacuum, sealed vacuum and a variety of 0 9/C0 o 23 gas mixtures. Alumina boats were the r e a c t i o n v e s s e l s of choice normally l i n e d with Pt, Au or Ni f o i l . However s e a l e d s t a i n l e s s s t e e l cans and s e a l e d quartz ampoules were a l s o used. 2 . 1 . 2 E l e c t r o c h e m i c a l C e l l F a b r i c a t i o n 2.1.2.1 C e l l Components An e l e c t r o c h e m i c a l c e l l based on l i t h i u m i n t e r c a l a t i o n i s made up of four components: (1) the host m a t e r i a l f o r i n t e r c a l a t i o n known as the cathode, (2) the l i t h i u m metal anode, (3) the e l e c t r o l y t e , and (4) the sep a r a t o r , which allows L i + ions to pass but prevents d i r e c t s h o r t i n g of the c l o s e l y spaced e l e c t r o d e s . Cathodes were made by evenly spreading a s l u r r y onto a s t r i p of aluminum f o i l . I t i s necessary to i n c l u d e a binder a d d i t i v e i n the s l u r r y mixture so that good adhesion r e s u l t s . The s l u r r i e s were made i n one of two ways. The f i r s t method c o n s i s t e d of mixing p o l y e t h y l e n e oxide (PEO; molecular weight 2,000,000), ethylene carbonate (EC), and the powdered m a t e r i a l of i n t e r e s t i n the r a t i o 2:4:94 r e s p e c t i v e l y by weight. Dichloromethane ( C H 2 C I 2 ) was added to the mixture and s t i r r e d u n t i l a uniform s l u r r y was obtained which normally took approximately 15 minutes. The second method c o n s i s t e d of adding a known q u a n t i t y of a 4% 24 ethylene propylene diene monomer (EPDM) in cyclohexane stock solution to the powdered material to give a f i n a l concentration of 1.0-2.0% EPDM by weight in the cathode depending on the s p e c i f i c material. Cyclohexane was added to make the slurry s u f f i c i e n t l y thin to make spreading possible. Prior to making the s l u r r i e s the powders were s i f t e d through an 80-mesh sieve (180 um) to prevent lumping of the resu l t i n g cathode. The cathode s l u r r i e s were evenly spread onto the surface of an 18 um thick aluminum substrate, and the solvent was allowed to evaporate to cathode dryness. The cathodes were then pressed between a pair of r o l l e r s to increase cathode density and obtain higher cathode u t i l i z a t i o n . The cathodes normally consisted of 30 to 40 mg of powdered material covering 1.2 cm x 1.2 cm square aluminum substrate. When a i r sensitive cathodes were prepared, the above processes were carried out inside an inert atmosphere glove box, otherwise the process was done in a 1% r e l a t i v e humidity dry room. Lithium f o i l (127 um thick supplied by Foote) was used as received for the anode. In order to prevent the formation of lithium oxide or lithium hydroxide surface films, the f o i l was stored in a sealed chamber in the glove box. Anodes were cut to a size which covered the entire cathode surface. In a l l c e l l s approximately 20 times the required amount of 25 m e t a l l i c l i t h i u m was p r e s e n t . The c e l l e l e c t r o l y t e c o n s i s t e d of a 1M s o l u t i o n of l i t h i u m h e x a f l u o r o a r s e n a t e (LiAsF^) i n a 50:50 s o l v e n t mixture of propylene carbonate (PC) and ethylene carbonate (EC) by volume. These s o l v e n t s were s e p a r a t e l y vacuum d i s t i l l e d to reduce moisture content and major i m p u r i t i e s such as propylene g l y c o l i n PC and ethylene g l y c o l i n EC. Analyses of the p u r i f i e d s o l v e n t s showed that d i s t i l l a t i o n reduced the i m p u r i t i e s to l e s s than 20 ppm i n propylene and ethylene g l y c o l and 30 ppm i n water. The L i A s F g s a l t (U.S. A g r i c . Chemical) was used as r e c e i v e d . T h i s s a l t i s used because of i t s high s o l u b i l t y and c o n d u c t i v i t y i n the PC/EC so l v e n t mixture. The s e p a r a t o r s used were Celgard #2500 microporous polypropylene f i l m s , cut i n t o sheets that were l a r g e r than both the cathode and the anode. They were immersed i n the prepared e l e c t r o l y t e and wetted under 180 p s i p r e s s u r e . 2.1.2.2 C e l l Assembly The experimental t e s t c e l l employed i n t h i s work was designed to meet s e v e r a l requirements. Most importantly the cathode and anode must be e l e c t r i c a l l y i s o l a t e d . In a d d i t i o n there must be s u f f i c i e n t p r essure on the e l e c t r o d e s u r f a c e s to p rovide good e l e c t r i c a l c o n t a c t , and at the same time, minimize the i n t e r n a l r e s i s t a n c e of the c e l l . F i n a l l y , the 26 c e l l s must be s e a l e d so as to prevent any a i r from e n t e r i n g the c e l l . C e l l c o n s t r u c t i o n i n v o l v e d the assembly of a cathode-separator-anode sandwich between two s t a i n l e s s s t e e l s u r f a c e s . Two to four drops of e l e c t r o l y t e were added to the cathode and a separator p l a c e d on top. To complete the sandwich, a p i e c e of anode was p l a c e d over the top of the s e p a r a t o r . F i g u r e 9 g i v e s an exploded view of the cathode-separator-anode sandwich. The two metal contact s u r f a c e s which hol d the sandwich together are the p o s i t i v e and the negative t e r m i n a l s of the c e l l . F i g u r e 10 shows an experimental t e s t c e l l . The s p r i n g assembly which i s i n s e r t e d i n t o a N i c k e l - p l a t e d m i l d s t e e l can a p p l i e s a 225 p s i pressure on the cathode-separator-anode sandwich. E l e c t r i c a l i s o l a t i o n i s achieved by the g l a s s - t o - m e t a l s e a l between the anodic c e n t e r p i n and the r e s t of the c e l l . Welding around the rim g i v e s a hermetic s e a l . The EPDM cathodes were pressure wetted by e x e r t i n g 180 p s i on the c e l l p r i o r to welding. 27 Figure 9. sandwich. Exploded view of the "cathode-separator-anode" C E N T E R P«N M E T A L C O N T A C T S U R F A C E S gure 10. An experimental t e s t c e l l . 29 2.2 TECHNIQUES 2.2.1 Powder X-ray D i f f r a c t i o n 2.2.1.1 I n - s i t u X-ray D i f f r a c t i o n Powder d i f f r a c t i o n i s a standard technique used to identify unknown substances. Its theory i s well known ( C u l l i t y 1959) and, therfore, w i l l not be discussed here. However, the limi t a t i o n s associated with t h i s technique w i l l be considered in the following section. The design of the i n - s i t u x-ray d i f f r a c t i o n c e l l used in this thesis was developed by Dahn et a l . (Dahn et a l . 1982). The c e l l , shown in figure 11, was used to monitor changes in the cathode l a t t i c e while i t s composition was altered electrochemically. The top cover, made of brass, incorporates a 0.25 mm thick beryllium window in the recess of the bottom surface. The center beam in the c e l l top prevents the beryllium from flexi n g excessively. A 0.43 mm thick greased polypropylene gasket provides the gas tight seal between the top cover and the stainl e s s steel c e l l base. X-ray measurements were taken using a P h i l i p s powder diffTactometer. The system consisted of a P h i l i p s 1730/10 x-ray generator equipped with a copper tube (PW 2253/20) and a v e r t i c a l goniometer (PW 1050/70). X-rays of undesirable F i g u r e 1 1 . An i n - s i t u x-ray d i f f r a c t i o n c e l l . 31 wavelength are e l i m i n a t e d by a g r a p h i t e monochromator c r y s t a l mounted between the sample and the d e t e c t o r . A P h i l i p s 1386/50 automatic divergence s l i t i s used to i l l u m i n a t e a constant area of the sample as the s c a t t e r i n g angle i s changed (Dahn et a l . 1982). The system can l o c a t e s c a t t e r i n g angles of Bragg peaks to the nearest 0.01°. 2.2.1.2 P r e p a r a t i o n of X-ray Samples X-ray p a t t e r n s were determined f o r a l l the m a t e r i a l s that were s y n t h e s i z e d i n t h i s study. Samples were prepared i n f l a t p l a t e form using the same a i r - t i g h t h o l d e r s used i n the i n - s i t u x-ray d i f f r a c t i o n experiments. Powders were f i n e l y ground and pressed onto a t h i n l a y e r of s i l i c o n grease. A i r s t a b l e samples were prepared on g l a s s s l i d e s i n the same f a s h i o n , however sample t h i c k n e s s e s were of the order of 100 um as compared to 300 um when the h o l d e r s were used. The s u r f a c e of the sample, when mounted on the goniometer, must make equal angles with the i n c i d e n t and d i f f r a c t e d beams. In t h i s geometry, the a b s o r p t i o n f a c t o r becomes independent of t h e t a provided that the sample exceeds a minimum t h i c k n e s s ( C u l l i t y 1959). For a t y p i c a l a b s o r p t i o n c o e f f i c i e n t of 550 cm 1 i n Cu Ka and a maximum theta value of 35°, the minimum r e q u i r e d t h i c k n e s s of the sample i s 36 um. The v a r i a t i o n s i n t h i c k n e s s from sample to sample can 32 l e a d to a systematic s h i f t i n 20. T h i s e f f e c t r e s u l t s from a displacement of the sample from the goniometer a x i s of r o t a t i o n and u l t i m a t e l y a f f e c t s the de t e r m i n a t i o n of the dimensions of the u n i t c e l l (Dahn et a l . 1982). T h e r e f o r e , a c o r r e c t i o n to the peak p o s i t i o n s was i n c l u d e d i n the l e a s t square refinements f o r determining the l a t t i c e c o nstants (Dahn et a l . 1982). 2.2.2 E l e c t r o c h e m i c a l Measurements of V(x) To determine the coarse f e a t u r e s of the v o l t a g e curve, V ( x ) , and to q u a l i t a t i v e l y determine the c y c l e l i f e , e l e c t r o c h e m i c a l t e s t c e l l s were charged and d i s c h a r g e d at constant c u r r e n t s between f i x e d upper and lower v o l t a g e l i m i t s . The c y c l e r s used i n these experiments were capable of m a i n t a i n i n g s t a b l e c u r r e n t s to w i t h i n ± 2%. From the mass of the cathode i n the c e l l , x i s x = I•t•M/m•F where I i s the c u r r e n t , t i s the d u r a t i o n of the c u r r e n t flow ( s e c ) , M i s the molecular weight of the cathode m a t e r i a l , m i s the cathode mass, and F i s Faraday's c o n s t a n t . T h i s equation assumes that the net charge t r a n s f e r i s due t o t a l l y to the i n t e r c a l a t i o n of l i t h i u m i n t o the cathode. In 33 practice, t h i s does not s t r i c t l y hold because of several factors which affect the value of x. F i r s t l y , one must consider the problem of side reactions. Lithium can interact with the various components in the c e l l , reactions of the lithium with the e l e c t r o l y t e is one such example. However, currents drawn by side reactions are t y p i c a l l y very much smaller than those drawn by the cathode material and to a f i r s t approximation, the equation for x can be applied. Another problem i s that of cathode u t i l i z a t i o n , which arises when the entire cathode mass i s not e l e c t r i c a l l y connected to the aluminum substrate or when the cathode material i t s e l f i s not a good conductor. 2.2.3 R e s i s t i v i t y Measurements » In order to obtain good cathode u t i l i z a t i o n , the cathode material should be a f a i r l y good conductor. To determine the approximate r e s i s t i v i t y of the powdered samples, resistance measurements were made using a four-probe technique employing the apparatus schematically shown in figure 12. Small insulating disks with a diameter of 0.325 cm were placed in the bottom contact plate and f i l l e d with the finely-ground samples. Sample thicknesses were determined by recording the d i a l gauge reading before and after the sample was inserted between the upper and lower contacts. Sample thicknesses were in the range of ' ' 34 APPLIED PRESSURE DIAL GAUGE INSULATING DISK LOAD CELL INSULATOR F i g u r e 12. Schematic diagram of apparatus used f o r r e s i s t a n c e measurements on powdered samples. 35 0.15-0.25 cm. The pressure a p p l i e d was determined using a loa d c e l l . The r e s i s t a n c e of the samples was measured at 5,000 p s i using a Fluke 8840A multimeter with four probes. The r e s i s t i v i t y of the powder i s obtained using the equation p = R-A/L where p i s the r e s i s t i v i t y , R i s the r e s i s t a n c e of the sample, L i s the t h i c k n e s s of the sample, and A i s the c r o s s - s e c t i o n a l area of the sample. 3 RESULTS AND DISCUSSION 3.1 SULFIDES 3.1.1 M a t e r i a l s Synthesized 3.1.1.1 L i - T i - S Ternary Phase Diagram The study of any solution or a l l o y system ultimately leads to the construction of a phase diagram, as i t i s the most e f f e c t i v e means of conveying s t r u c t u r a l and phase information. For a system with three components, written in the general form A u B v C i - u - v ' w e n a v e t w o independent compositional variables u,v and the thermodynamic state of the system i s completely determined by two state variables, usually P and T. For condensed matter, the pressure variation i s usually n e g l i g i b l e , leaving only three independent variables u,v and T. The usual convention for representation of a ternary system i s to use an e q u i l a t e r a l triangle as the compositional space. To include more than one temperature, one needs only to add a temperature axis perpendicular to the t r i a n g l e . Many syntheses were attempted with the objective of iden t i f y i n g some of the Li-Ti-S ternary compounds at high temperature. Table II summarizes the reactions attempted and the observed products. From these results i t i s possible to 36 37 TABLE II Summary of syntheses of L i - T i - S compounds # L i - T i - S Compound React ion/ Temperature Products 1 L i T i S l / 4 T i S 2 + l / 2 L i 2 S +3/4Ti 7 0 0 ° C , 800°C L i T i S 2 + T i + L i 2 S 2 L i 2 T i S 2 l / 2 T i S 2 + L i 2 S + 1/2T1 6 0 0 ° C , 700°C L i T i S 2 + T i + L i 2 S 3 L i 1 . 1 T i S 2 . 4 8 T i S 3 + .55Li 2 S +.52Ti 600°C L i T i S 2 + 6Ti + 5 L i 2 S 4 L i x T i S 2 ( 1 - x / 4 ) T i S 2 + x / 2 L i 2 S L i x T i S 2 (0 < x < 1 ) + x /4Ti 5 0 0 - 9 0 0 ° C ( 2 / 3 - x / 6 ) T i S 3 + x / 2 L i 2 S + ( l /3+x/6)Ti 5 0 0 - 9 0 0 ° C L i x T i S 2 5 L i 0 . 9 T i 1 . 1 S 2 . 7 8 T i S 2 + .45Li 2 S +.32Ti 700°C L i 0 . 9 T i 1 . 1 S 2 6 L i 0 . 7 5 T i 1 .25 S2 . 8 l T i S 2 +.375Li 2 S +.44Ti 600°C L l 0 . 7 5 T l 1 , 2 5 S 2 + 8Ti 7 L i 0 . 6 T i 1 . 4 S2 . 8 5 T i S 2 + .3Li 2 S +.55Ti 600°C L i 0 . 6 T i 1 . 4 S 2 + 5Ti 8 L i 0 . 5 T i 1 . 5S2 7 / 8 T i S 2 + l / 4 L i 2 S +5/8Ti 700°C L i 0 . 6 T i 1 . 4 S 2 + TiS 38 9 L i 0 . 4 T i1 .6 S2 . 9 T i S 2 + 700°C . 2 L i 2 S + .7Ti L i 0 . 6 T i 1 . 4 S 2 + TiS 10 L i 0 . 2 T i1 .8 S2 .95TiS 2 + 700°C . 1 L i 2 S + . 85Ti L i 0 . 6 T i 1 . 4 S 2 + T i S 1 1 L i o . 6 T i 2 .4 S2 .85TiS 2 + 700°C . 3 L i 2 S + 1 .55Ti L i 0 . 6 T i 1 . 4 S 2 + T i S + T i 12 L i 0 . 4 T i1 .2 S2 . 9 T i S 2 + 700°C . 2 L i 2 S + .3Ti L i 0 . 4 T i 1 . 2 S 2 1 3 L i 0 . 6 T i 1 . 2 S 2 -85TiS 2 +.3Li 2S +.35Ti L i ^ T i ^ ^ 700°C 39 construct a preliminary L i - T i - S ternary phase diagram at elevated temperature (T = 700°C) as shown in figure 13. The four Ti-S compounds can be synthesized at temperatures between 550-850°C, and thus are included in the phase diagram. Table III gives further d e t a i l on their c r y s t a l structures and the results of the l a t t i c e constant refinements done on the x-ray patterns. Below approximately 700°C, TiS forms the NiAs structure, which i s denoted the low temperature form, ( i . e . T i S ( D ) . Its high temperature form has a structure that can be considered as a r e l a t i v e l y simple "superlattice" on NiAs. The probable arrangement i s described as one in which three molecules are in a unit rhombohedron of the dimensions, a = 9.04 A and 0 = .21.80°. The corresponding hexagonal c e l l containing nine molecules has the edges, a' = 3.417 A and c' = 26.4 A (Wyckoff 1963). A large three phase region i s thought to exist as represented by the triangular region A. The co-existing phases are given by the apices of the tr i a n g l e , namely L i T i S 2 , L i - 2 S a n <^ T * " T n ^ s conclusion i s based on the results of the attempted syntheses of LiTi S and L i 2 T i S 2 whose x-ray patterns consisted of the three phases formentioned. L i 1 i T^- S2 a ^ s o formed these three phases although L i 2 S was barely detectable. The existence of this three phase region i s somewhat surprising because at room temperature L i 2 T i S 2 does exist and can be prepared electrochemically (Dahn F i g u r e 13. L i - T i - S t e r n a r y phase diagram at T Samples are numbered i n accordance with t a b l e I I . 700°C. 41 TABLE I I I L a t t i c e parameters of T i - S compounds s y n t h e s i z e d i n t h i s study Compound L a t t i c e Type L a t t i c e Parameters (A) t h i s work Wyckoff (1963) T i S ( l ) hexagonal a = 3.294 ± 0.002 3.30 c = 6.433 ± 0.002 6.44 T i 3 S 4 hexagonal a = 3.409 ± 0.002 3.431 c = 11.428 ± 0.002 1 1 . 44 T i S 2 hexagonal a = 3.412 ± 0.002 3.412 c = 5.709 ± 0.002 5.695 T i S 3 m o n o c l i n i c a =. 4.971 ± 0.002 4.97 b = 3.405 ± 0.002 3.42 c = 8.815 ± 0.002 8.78 P = 97.93 ± 0.04° 97.17° 42 et a l . 1982). A l i n e of single phase compounds, namely those of the series L i x T i S 2 (0 ^ x < 1) connecting the end members, T i S 2 and L i T i S 2 , e x i s t s . This agrees with the results at room temperature when these compounds are formed electrochemically. The l i n e connecting L i T i S 2 and TiS in figure 13 represents the series L i x T i 2 _ x S 2 (0 < x < 1). The sample numbers of the attempted syntheses are shown in the figure in accordance with table II. For values of x > 0.5, the samples form a single L i x T i 2 _ x S 2 phase that can be indexed based on a hexagonal unit c e l l l i k e L i T i S 2 . For values of x < 0.5, the samples seem to form two phases, a L i T i - S, X £ X ^ phase with x near 0.6. and a TiS phase. The peaks in the x-ray patterns of the samples with x < 0.5 were broader than those with x > 0.5 indicating a decrease in c r y s t a l l i n i t y . It would appear that the l i n e connecting Lip 4^2 * s a t i e l i n e connecting these two phases. Sample 11 was prepared to determine the nature of region C. The result was the formation of three phases namely a L i T i , _ S, phase near x = 0.6, TiS and T i . It would appear that region C i s a three phase region and region B a two phase region consisting of Ti and of a L i T i , S, (0 ^ x < 0.5) compound. X (L. X c. F i n a l l y two samples (12 and 13) were prepared to determine the nature of region D. Both these samples turned 43 out to be single phased and could be indexed based on a hexagonal unit c e l l . Regions E and F were not investigated. 3.1.1.2 3R-Li„TiS„ (0 < x < 1) In the studies of the L i - T i - S ternary system and in p a r t i c u l a r the L i x T i S 2 (0 < x ^ 1) series, two d i s t i n c t phases of the same composition were discovered. The f i r s t of these phases was the well known 1T polytype of L i TiS,, the X £ other was found to be the previously unreported 3R polytype of L i x T i S 2 - A comparison of the x-ray patterns for the two polytypes of L i T i S 2 i s presented in table IV. The patterns are e a s i l y distinguishable from each other. The i n t e n s i t i e s of the 1T (101) peak and the 3R (104) peak provide the best measure of the r e l a t i v e quantities of each polytype in a p a r t i c u l a r sample. Figure 14 shows projections of the structures of 1T-TiS 2 and 3 R - L i x T i S 2 in the (110) plane. In the 1T polytype, the metal atoms are positioned above each other in each T i S 2 repeat layer. The two structures are related by simply displacing adjacent pairs of T i S 2 layers in opposite dire c t i o n s , leaving every t h i r d T i S 2 layer fixed, thus obtaining the 3R phase. The unit c e l l for 3R-Li TiS, now becomes 3 layers thick as compared to just a single layer unit c e l l for the 1T structure. The dashed l i n e s in the figure indicate the unit c e l l s of the structures. The 3R 44 1T-TiS 2 3 R - L i x T i S 2 O s ® Ti O Li F i g u r e 14. 110 p r o j e c t i o n s of the s t r u c t u r e s of 1 T - T i S 2 and 3 R - L i x T i S 2 . The dashed l i n e s i n d i c a t e the u n i t c e l l s of the s t r u c t u r e s , and the l e t t e r s ABCA i n d i c a t e the e q u i v a l e n t x and y c o o r d i n a t e p o s i t i o n s w i t h i n the hexagonal u n i t c e l l , [eg. A<x=0.67,y=0.33) B(x=0,y=0) C(x=0.33,y=0.67)] 45 TABLE IV Comparison of the x-ray patterns of 1T-LiTiS 2 and 3R-LiTiS Intensity abbreviations are as follows, v=very, s=strong, m=medium and w=weak. 1T Polytype 3R Polytype a = 3.461 A , c = 6.182 A a = 3.521 A , c = 18.152 A ( h k l ) 2*obs 2 * c a l c Rel. ( h k l ) 2*obs 2 * c a l c R e l « Int. Int. 001 14. 339 14. 327 vs 003 14. 618 14. 639 m 002 28. 893 28. 883 w 101 29. 706 29. 706 m 100 29. 814 29. 808 w 1 02 30. 927 30. 934 w 101 33. 225 33. 217 vs 1 04 35. 491 35. 465 vs 102 41 . 960 41 . 986 vs 105 38. 574 38. 555 w 003 - 43. 936 vw 009 - 44. 942 vw 1 1 0 52. 887 52. 908 s 1 07 45. 931 45. 938 w 103 53. 938 53. 998 m 108 50. 1 06 50. 1 09 s 1 1 1 55. 1 19 55. 1 10 m 1 10 51 . 908 51 . 934 s 004 59. 873 59. 840 m 1 1 3 54. 301 54. 260 m 201 63. 937 63. 915 m 1 1 6 60. 875 vw 202 69. 742 69. 733 m 201 - 60. 978 vw 204 64. 444 64. 466 m 46 polytype only begins to appear once l i t h i u m atoms have been i n s e r t e d i n t o the van der Waals gap. P o s s i b l y i n the 3R p o l y t y p e , coulomb r e p u l s i o n between the metal atoms and the i n s e r t e d l i t h i u m s i s reduced by s t a g g e r i n g t h e i r p o s i t i o n s in the subsequent l a y e r s . With the goal of making pure 3R-Li T i S - (0 < x < 1) s t o i c h i o m e t r i c q u a n t i t i e s of e i t h e r L i 2 S , T i and T i S 2 , ^^25' T i and T i S g , or L i 2 C 0 3 and T i S 2 were allowed to r e a c t . For r e a c t i o n s of L i 2 C 0 3 and T i S 2 , the s t o i c h i o m e t r y was such that the L i - T i - S r a t i o s were c o n t r o l l e d but the carbon and oxygen from the carbonate group i s thought to be e x p e l l e d as C0 2. However, as we d i s c u s s l a t e r there i s evidence f o r the presence, of some oxygen c o n t a i n i n g compounds. The l i t h i a t e d compounds are handled in an i n e r t atmosphere glove box as they r e a c t r a p i d l y i n a i r . F i g u r e 15 shows s c h e m a t i c a l l y the r e s u l t s of these p r e p a r a t i o n s . I t i s evident from the f i g u r e that when L i 2 S was used as a s t a r t i n g m a t e r i a l the product was predominantly the 3R phase f o r values of x > 0.7 and a temperature range of 500-700°C. As x decreases, the 3R p o l y t y p e no longer forms and the 1T p o l y t y p e i s p r e v a l e n t . At very high temperature, (800-900°C), the 1T polytype i s the more s t a b l e phase. T h i s would suggest that the 1T p o l y t y p e i s the thermodynamically s t a b l e phase. In an e f f o r t to determine the s t a b i l i t y of the 3R polytype at lower temperatures, a sample of L i T i S - was s y n t h e s i z e d at 900°C 47 900 • • S H A " HGA -o o 800 - O • G -URE 1PERAT 700 G o OA -LU f— 600 500 i i G i I A %A -QA -I 0 0.2, 0.4 0.6 0.8 1.0 x in L i x T iS 2 1T 3R O Li 2S, Ti & TiS 2 • A Li 2 S, Ti & TiS 3 A • L i 2 C Q 3 & TiS 2 • F i g u r e 15. Diagram showing 3R:1T r a t i o f o r v a r i o u s s t a r t i n g m a t e r i a l s . Shaded area r e p r e s e n t s the p o r t i o n of the 3R phase and unshaded that of the 1T phase.-48 and i n s t e a d of quenching i t to room temperature q u i c k l y , the sample was slowly c o o l e d over a p e r i o d of two days. The r e s u l t was a pure 1T phase, implying that the 3R poly t y p e may be a metastable phase. The most apparent exception was the formation of the 3R phase at 900°C f o r x > 0.6 when L i 2 C 0 3 was used as a s t a r t i n g m a t e r i a l . Samples prepared with L^CO^ showed the presence of two impurity compounds, namely T i 0 2 and a L i 1 + x T i 2 _ x 0 4 (0 < x < 1/3) compound, that were not evident i n the other p r e p a r a t i o n s . L i 2 C 0 3 i s the obvious source of oxygen a l l o w i n g these oxides to form. The presence of oxygen may s t a b i l i z e the 3R phase. The d i f f i c u l t y in determining the regions of s t a b i l i t y and dependence on r e a c t i o n c o n d i t i o n s of the t r a n s i t i o n metal d i c h a l c o g e n i d e s i s best i l l u s t r a t e d i n tanatalum d i s u l f i d e . T a S 2 i s a substance which has aroused much i n t e r e s t on account of i t s v a r y i n g p r o p e r t i e s i n i t s d i f f e r e n t forms. J e l l i n e k (1962) i n v e s t i g a t e d the Ta-S system by x-ray powder d i f f r a c t i o n and observed the f o l l o w i n g p o l y t y p e s : 1T-, 2H-, 3R- and 6R-TaS 2. D i r e c t s y n t h e s i s at 600°C, 800°C or 1000°C r e s u l t s i n the formation of a l l four p o l y t y p e s . The 6R poly t y p e i s always mixed with the 1T p o l y t y p e , but prolonged h e a t i n g at 800°C y i e l d s an i n c r e a s i n g amount of the 6R p o l y t y p e . Gamble et a l . (1970) prepared 2H-TaS 7, the phase s t a b l e at room temperature, by 49 h e a t i n g s t o i c h i o m e t r i c r a t i o s of the elements i n evacuated quartz ampoules at 900°C f o r s e v e r a l days, f o l l o w e d by slowly c o o l i n g the r e a c t i o n ampoule and i t s c o n t e n t s . I f , however, one i n c l u d e s a small excess of S and quenches the sample i n a i r , the pure 1T form r e s u l t s with the excess S condensing on the tube w a l l s . I f there i s no S excess and one quenches the ampoule, the r e s u l t i n g product i s a mixture of 4H(b)-, 1T- and 6R-TaS 2. The 3R pol y t y p e i s l e s s f r e q u e n t l y obtained than the other TaS 2 forms. J e l l i n e k (1962) r e p o r t e d a 3R phase together with the other TaS 2 forms, with TaSg and with three lower s u l f i d e s of composition T a 1 + x S 2 as a product of d i r e c t r e a c t i o n between elements at 600-800°C and 1000°C. I t i s q u i t e c l e a r that great a t t e n t i o n i s r e q u i r e d f o r p r e p a r a t i v e d e t a i l s to ob t a i n s i n g l e phase s t o i c h i o m e t r i c TaS 2, and f o r that matter other t r a n s i t i o n metal d i c h a l c o g e n i d e s i n c l u d i n g L i T i S , . 3.1.2 X-ray D i f f r a c t i o n Least square refinements of the Bragg peak p o s i t i o n s were performed i n order to determine the l a t t i c e parameters f o r the v a r i o u s IT- and 3R-Li T i S , m a t e r i a l s that were X prepared. Refinements were made to at l e a s t 6 Bragg peak p o s i t i o n s and t y p i c a l l y as many as 8-10 Bragg peaks were used. For both the 1T and 3R s t r u c t u r e s , the Bragg peaks can be indexed to a hexagonal u n i t c e l l , thus the l a t t i c e 50 parameters a and c can be determined. 3.1.2.1 a & c vs x in 1T & 3R~Li„TiS^ A £ Figure 16 shows the variation of the l a t t i c e parameters a and c versus x in 1T-Li TiS- for both the high temperature A and the room temperature material. The room temperature data were c o l l e c t e d by Dahn et a l . (1982) using the i n - s i t u x-ray d i f f r a c t i o n technique. It appears that the l a t t i c e parameters of the high temperature material change in the same fashion as the electrochemically intercalated material. Figure 17 shows the variation of the l a t t i c e parameters a and c/3 versus x in 3R-Li TiS- as determined by i n - s i t u x-ray d i f f r a c t i o n . Most' of the t r a n s i t i o n metal dichalcogenides undergo an increase in the c l a t t i c e parameter when intercalated with lithium. However c/3 is r e l a t i v e l y unchanged through the f u l l range of x for 3 R - L i x T i S 2 . In contrast, the a l a t t i c e parameter of the 3R polytype shows a similar variation to that of the 1T polytype, namely a 1.5% decrease with decreasing x. 51 -£3.46 0.2 0.4 0.6 x in 1T -M TiS, 0.8 1.0 F i g u r e 1 6 . V a r i a t i o n o f l a t t i c e p a r a m e t e r s o f ( • ) r o o m t e m p e r a t u r e a n d ( 0 , A ) h i g h t e m p e r a t u r e I T - L i T i S ~ w i t h x . (Room t e m p e r a t u r e d a t a a f t e r D a h n e t a l . 1 9 8 2 ) . * 52 3.51 3.50 o < 3 . 49 -« 3 . 48 -3.47h 3.46 6.20 6.15 <>< 6 .10^ CO o 6.05 6.00 5.95 <!> J i i J L <> 0 I 0.2 0.4 0.6 x in 3R-Li ' TiS 0.8 0 <> 1.0 F i g u r e 17. V a r i a t i o n of l a t t i c e parameters, a and c/3, of 3 R - L i x T i S 2 with x. 53 3.1.3 E l e c t r o c h e m i c a l Measurements 3.1.3.1 Ce l l s for 1T & 3R-Li..TiS„ vs L i Both L i / l T - L i x T i S 2 and L i / 3 R - L i x T i S 2 c e l l s were cycled at ambient temperature at rates of 30 hours for Ax = 1 . Figure 18 shows the voltage curve, V(x) for a Li/1T-Li TiS-, X c. c e l l where the cathode material was i n i t i a l l y T i S 2 . The second charge and discharge are shown in t h i s figure. These curves agree well with previous results (eg. Thompson 1978 or Dahn et a l . 1982). In addition, Li/1T-Li TiS, c e l l s X i n i t i a l l y employing 1T-LiTiS 2 (synthesized at high temperature) as the cathode material were made to check the r e v e r s i b i l i t y of the lithium that had been inserted at high temperature and thus to see i f these compounds are true i n t e r c a l a t i o n compounds. Figure 19 shows V(x) for the i n i t i a l charge and subsequent discharge for such a c e l l . Lithium could be removed on the f i r s t charge, however long term cycling of these c e l l s was not observed. There is good agreement over the f i r s t few cycles between these L i / 1 T - L i x T i S 2 c e l l s in that the average voltages are the same, however the high temperature material has approximately a 0.1 V lower discharge voltage and a 0.1 V higher charge voltage. This can be attributed to a greater internal resistance of the c e l l due to the higher r e s i s t i v i t y of the cathode material. The cathode u t i l i z a t i o n 54 F i g u r e 18. V(x) f o r L i / l T - L i x T i S 2 c e l l s at 21°C and a discharge r a t e of 30 hours f o r Ax = 1. The cathode m a t e r i a l was i n i t i a l l y T i S 9 . 55 F i g u r e 19. V(x) f o r L i / 1 T - L i x T i S 2 c e l l s at 21°C and a discharge r a t e of 30 hours f o r Ax = 1. The cathode m a t e r i a l was i n i t i a l l y 1 T - L i T i S - s y n t h e s i z e d at high temperature. 56 of the high temperature m a t e r i a l c e l l s was on average lower than the L i / T i S 2 c e l l s . F i g u r e 20 shows V(x) f o r a L i / 3 R - L i T i S - c e l l where the cathode m a t e r i a l was i n i t i a l l y 3 R - L i T i S 2 prepared at T = 900°C u s i n g L i 2 C 0 3 and T i S 2 as the s t a r t i n g m a t e r i a l s . The 3R c e l l s have higher average v o l t a g e s than 1 T c e l l s . The d i f f e r e n c e i n v o l t a g e between the charge and d i s c h a r g e curves can be a t t r i b u t e d to such e f f e c t s as s e r i e s r e s i s t a n c e s due to the anode and the 1M LiAsF^/PC e l e c t r o l y t e . On charge, 3R c e l l s e x h i b i t a step i n the v o l t a g e curve. T h i s step i s only r e s o l v e d at r e l a t i v e l y slow d i s c h a r g e r a t e s , t y p i c a l l y 30-^50 hours f o r Ax = 1 . Even at these slow r a t e s t h i s f e a t u r e becomes l e s s pronounced as one c y c l e s these c e l l s . The reason why the discharge curve i s s h o r t e r than the charge curve w i l l be d i s c u s s e d i n s e c t i o n 3.1.4. The V(x) behaviour of these c e l l s i s s i m i l a r to that of other MX2 i n t e r c a l a t i o n compounds. The v o l t a g e v a r i e s with l i t h i u m composition, and on d i s c h a r g e as the a v a i l a b l e s i t e s f o r l i t h i u m become f i l l e d , a uniform drop i n v o l t a g e i s observed. However upon charg i n g L i / 3 R - L i T i S - c e l l s , l i t h i u m X £* i s e x t r a c t e d from s i t e s of lower energy versus l i t h i u m metal for l a r g e r values of x and a f t e r the step s i t e s of r e l a t i v e l y high energy f o r smaller values of x (x < 0.4). I t i s not c l e a r why we observe such a step only on charge and 5 7 3.6 • 1 1 1 1— 1 1 i i i l 0 0.2 0.4 0.6 0.8 1.0 x in 3 R - L i x T i S F i g u r e 20. v(x) f o r L i / 3 R - L i x T i S 2 c e l l s at 21°C and a d i s c h a r g e r a t e of 30 hours f o r Ax = 1. The'cathode m a t e r i a l was i n i t i a l l y 3 R - L i T i S 2 s y n t h e s i z e d a t high temperature. 58 see no evidence of i t on discharge. The fact that i t disappears on subsequent cycles may indicate that after the i n i t i a l charge these high energy s i t e s are not reversible to lithium insertion and extraction. 3.1.4 I n - s i t u X-ray D i f f r a c t i o n on 3 R - L i x T i S 2 Figure 21 shows portions of x-ray d i f f r a c t i o n p r o f i l e s taken during the f i r s t charge and subsequent discharge of a L i / 3 R - L i x T i S 2 c e l l (an i n - s i t u d i f f r a c t i o n c e l l ) at room temperature. The M i l l e r indices of peaks of the 1T and 3R phases are indicated. The high angle peak is due to the L i anode. The values of x corresponding to these scans are also shown. The scans were taken in chronological sequence from front to back. The 1T phase f i r s t appears in the fourth scan. Apparently, 3 R - L i x T i S 2 is unstable to the formation of 1 T - L i x T i S 2 at room temperature when x i s decreased on the i n i t i a l charge of the c e l l . The t r a n s i t i o n from the 3R phase to the 1T phase begins at x < 0.4. Upon the subsequent discharge, when lithium i s reinserted, the 1T phase does not reform the 3R phase. Instead, co-intercalation of the PC solvent into the T i S 2 cathode material i s thought to occur. This causes the disappearance of the 1T phase from the x-ray pattern shown in figure 21. PC co-intercalation in T i S 2 i s known to occur in 1M LiAsFg/PC e l e c t r o l y t e (West et a l . 1983). Since co-intercalation of PC into T i S 0 renders that 59 Figure 21. In-situ x-ray d i f f r a c t i o n p r o f i l e s taken during the f i r s t charge of a L i / 3 R - L i x T i S 2 c e l l . The values of x are indicated. The f i n a l scan i s a f t e r discharge at x = 0.909. 60 portion of the cathode inactive for further c y c l i n g , one observes a decreased value of Ax for the discharge of the c e l l (figure 20). Further evidence of PC co-intercalation i s the decrease in the 3R peak intensity after one cycle. The s h i f t of the 3R peak to s l i g h t l y higher angle as x decreases, leaves behind the inactive 3R component of the cathode. From the peak i n t e n s i t i e s , t h i s u n u t i l i z e d portion of the cathode i s estimated to be approximately 35%. 61 3.2 OXIDES 3.2.1 M a t e r i a l s Synthesized 3.2.1.1 L i T i 2 Q 4 and L i 4 ^ , T i 5 ^ 3 Q 4 Our purest samples of L i 4 ^ 3 T i g /, 30 4 were obtained by one of two methods. In the f i r s t , the f o l l o w i n g one-step r e a c t i o n was used 2/3 L i 2 C 0 3 + 5/3 T i > L i 4 / 3 T i 5 / 3 0 4 P e l l e t s made up of s t o i c h i o m e t r i c q u a n t i t i e s of the s t a r t i n g m a t e r i a l s were f i r e d a t 900°C i n a i r f o r 20 hours i n an alumina boat, a f t e r which they were quenched i n a i r at room temperature. The l a t t i c e constant of our L i 4 / 3 T i 5 / 3 ° 4 was a = 8.367 ± 0.002 A. H a r r i s o n et a l . (1985) report a value of a 8.358 A which i s smaller than the value obtained i n t h i s study. The r u t i l e form of T i 0 2 was present as a 10% impurity i n the product. In the second, the r e a c t i o n 4/3 LiOH + 5/3 T i 0 2 > L i ' 4 / 3 T i 5 ^ 3 0 4 + 2/3 H 20 was used. T h i s r e a c t i o n was c a r r i e d out under a s t a t i c helium atmosphere at 800°C, in alumina c r u c i b l e s l i n e d with n i c k e l - f o i l . The helium was c l o s e d to the outer atmosphere but the escape of any excess gas pressure was allowed through an o i l bubbler. U n l i k e many c r u c i b l e s , n i c k e l - f o i l was found to be i n e r t to r e a c t i o n with the m a t e r i a l s 62 involved in these reactions and thus made an excellent l i n i n g for the reaction vessels. P e l l e t s of the LiOH/Ti0 2 mixture were pressed p r i o r to f i r i n g . The product of th i s reaction i s substantially pure and gives l a t t i c e constants equal to those of L i ^ ^ T i 5 / 3 O 4 prepared from Li-^CO^. The synthesis of L i T i 2 0 4 i s considerably more dependent on the reaction conditions. Table V shows the results of several of the syntheses attempted. The pe r s i s t i n g problem was that of the reaction forming Li^/-^^5/3°4 a n ^ T*°2 rather than L i T i 2 0 4 . Figure 22 shows a portion of the L i - T i - 0 ternary phase diagram for the temperature range of 700-900°C. The l i n e connecting L i 2 0 and T i 0 2 in the figure represents the series of compositions with the chemical formula L i 2/3-2u T iu 0u+1/3 (0 ^ u < 1/3). The la b e l l e d compositions represent single phase compounds. As shown, L i 4 / 3 T i 5/3°4 and Ti°2 a r e t w o e n <^ members of a two phase region in the L i - T i - 0 ternary phase diagram. L i T i 2 0 4 i s adjacent to this l i n e towards the side of higher Ti content. In many of the reactions, the reaction vessel showed evidence of reaction with one or more of the st a r t i n g materials. If the Ti content was reduced through these "side" reactions, the e f f e c t i v e composition of the product could l i e on or near the l i n e connecting L i ^ ^ T ^ / ^ C ^ a n c * T i 0 2 in the phase diagram. As a result one would observe the co-existence of TiC^ a n < ^ L*4/3 T*5/3°4 a s fc^e reaction 63 F i g u r e 22. Ternary phase diagram f o r the L i - T i - 0 system at T = 700-900°C showing the s e r i e s of compositions along the l i n e connecting L i , 0 and T i 0 o . 64 TABLE V Summary of syntheses of L i T i 9 0 . and L i . / - T i r - z o O Proposed Reaction Temp (°C)/ Observed Atmosphere Products L i T i 2 0 4 1/2 L i 2 C 0 3 + 2 T i 900 A i r L i 4 / 3 T i 5 / 3 ° 4 1/2 L i 2 C 0 3 + 2 T i 0 2 850 A i r H 850 Argon flow II 1/2 L i 2 T i 0 3 + 3/2 T i 0 2 850 A i r II l / 2 L i 2 T i 0 3 + 5 / 4 T i 0 2 + l / 4 T i 850 Sealed tube L i T i 2 0 4 ii 850 Sealed Tube L i T i 2 0 4 + ? n 850 Dynamic vacuum L i T i 2 0 4 + T i 0 2 it 850 Dynamic vacuum L i x T i 2 - x ° 4 + L i 2 T i 0 3 it 850 Sealed tube, Ni II ii 850 Sealed Ni can II l / 2 L i 2 0 + 7 / 4 T i 0 2 + l / 4 T i 850 Sealed Ni can n L i 2 T i 0 3 + T i 0 2 + T i 2 0 3 800 Sealed Ni can L i T i 2 0 4 + ? 1/2 L i 2 C 0 3 + T i 2 0 3 950 80O 2:20CO 2 flow L i T i 2 ° 4 + T i 0 2 L i 2 T i 0 3 + l / 2 T i 0 2 + 1 / 2 T i 2 0 3 850 Sealed tube L i T i 2 0 4 + ? LiOH + T i 0 2 + 1/2 T i 2 0 3 900 Standing He L i 4 / 3 T i 5 / 3 ° 4 + T i 0 2 65 Proposed Reaction Temp (°C)/ Atmosphere Observed Products L i 4 / 3 T i 5 / 3 ° 4 2/3 L i 2 C 0 3 + 5/3 T i 800 A i r L i 4 / 3 T i 5 / 3 ° 4 + 6 T i 0 2 2/3 L i 2 T i 0 3 + T i 800 A i r L i 4 / 3 T i 5 / 3 ° 4 + T i 0 2 2/3 L i 2 T i 0 3 + T i 0 2 850 Sealed Ni can 800 Standing °2 II L i 2 C 0 3 + 5/3 T i 900 A i r 4/3 LiOH + 5/3 T i 0 2 800 Standing He L i 4 / 3 T i 5 / 3 ° 4 + T i 0 2 II 900 Standing He II II 900 Standing He L i 4 / 3 T i 5 / 3 ° 4 H 800 Standing He II 66 products. Our most s u c c e s s f u l method of s y n t h e s i z i n g L i T i 2 0 4 i n v o l v e s a two-step r e a c t i o n L i 2 C 0 3 + T i 0 2 > L i 2 T i 0 3 1/2 L i 2 T i 0 3 + 5/4 T i 0 2 + 1/4 T i > L i T i 2 0 4 Powdered mixtures of l i t h i u m carbonate and t i t a n i u m d i o x i d e (anatase) were f i r e d at 750°C i n a i r f o r approximately 16 hours i n an alumina boat. The s t a r t i n g m a t e r i a l s f o r the p r e p a r a t i o n of L i T i 2 0 4 were s i f t e d together (180 Mm) and s e a l e d under vacuum (50 mT) i n a quartz tube. The tube was then heated to a temperature of 850°C at a r a t e of 150°C/hr and h e l d at t h i s temperature f o r 16 hours. X-ray d i f f r a c t i o n showed no apparent i m p u r i t i e s i n .the L i T i 2 0 4 product, however the w a l l s of the quartz tube were clouded by t h e i r r e a c t i o n with l i t h i u m . The l a t t i c e constant was a = 8.416 ± 0.002 A. H a r r i s o n et a l . (1985) r e p o r t a value of a = 8.405 A which i s again smaller than the value obtained i n t h i s study. 3.2.1.2 Mixed S p i n e l Oxides L i M n ^ T i 2 _ y 0 _ 4 Many samples i n the s e r i e s LiMn T i 9 _ 6. (0 ^ y ^ 2) y ^ y 4 were attempted. The only report of these compounds has been that of B l a s s e (1963) c l a i m i n g the e x i s t e n c e of L i M n T i 0 4 with a l a t t i c e constant of a = 8.30 A. Table VI shows a summary of the most s u c c e s f u l p r e p a r a t i o n s that were 67 TABLE VI Summary of L i M n y T i 2 _ y 0 4 (0 < y < 2) syntheses performed S t o i c h i o m e t r y / Reaction Temp (°C)/ Atmosphere Impurity Product(s) L i M n 2 0 4 1/2 L i 2 C 0 3 + 2 Mn0 2 l / 2 L i 2 0 + 7/4Mn02 + 1/4Mn 1/2 L i 2 C 0 3 + 2 Mn 850 A i r 850 Sealed Can 750 A i r 900 A i r 6Mn 20 3 5? Mn 20 3 6? L i M n 1 . 9 T i 0 . 1 ° 4 l / 2 L i 2 C 0 3 + 1.9Mn02 + . 0 5 T i 2 O 3 950 A i r 6? L i M n l . 7 5 T i 0 . 2 5 ° 4 l / 2 L i 2 C 0 3 + 7/4Mn02 + l / 8 T i 2 0 3 950 A i r 950 80O 2:20CO 2 none L i T i 2 0 4 L i M n 1 . 5 T i 0 . 5 ° 4 l / 2 L i 2 C 0 3 + 3/2MnC03 + l / 4 T i 2 0 3 ! / 2 L i 2 C 0 3 + 3/2Mn02 + l / 4 T i 2 0 3 950 750 2:25C0 2 950 A i r none 5? (on reheat) L i M n l . 2 5 T i 0 . 7 5 ° 4 l / 2 L i 2 C 0 3 + 5/4MnC03 + 3 / 8 T i 2 0 3 950 80O 2:20CO 2 950 A i r none 5LiMn 20 4 68 L i M n T i 0 4 l / 2 L i 2 C 0 3 + MnC0 3 + 1/2T1 20 3 950 80O 2:20CO 2 none 1 / 2 L i 2 C 0 3 + Mn0 2 + l / 2 T i 2 0 3 950 A i r 8? L i M n 0 . 7 5 T i 1 . 2 5 ° 4 l / 2 L i 2 C 0 3 + 3/4MnC03 + 5 / 8 T i 2 0 3 950 80O 2:20CO 2 T i 0 2 + 6? 950 A i r » L i M n 0 . 5 T i 1 . 5 ° 4 1 / 2 L i 2 C 0 3 + l/2MnC0 3 + 3 / 4 T i 2 0 3 950 750 2:25C0 2 T i 0 2 950 A i r »? L i T i 2 0 4 l / 2 L i 2 T i 0 3 + 5/ 4 T i 0 2 + l / 4 T i 850 Sealed tube none 69 performed. The general equation which most of these syntheses was based on i s L i 2 C 0 3 + y MnC03 + (1-y/2) T i 2 0 3 > L i M n y T i 2 _ y 0 4 In some cases Mn02 was used rather than MnC03. In addition, the two end members of th i s series, LiMn 20 4 and L i T i 2 0 4 , were prepared by methods previously discussed and are included in table VI for completeness. Reaction temperatures were found to be necessarily high ( t y p i c a l l y 950°C). At lower temperatures samples would form two d i s t i n c t phases, usually a Mn and a Ti spinel compound. Samples were synthesized under two d i f f e r e n t atmospheres. I n i t i a l l y , the 0 2/C0 2 flowing gas mixtures were used as described by Blasse (1963) and found to be successful, but samples were then prepared in a i r as well. In general, samples with y > 1 were prepared with higher purity with only trace amounts (<10%) of impurity products being detected. However, for y < 1 a larger percentage (10-20%) of impurity products were detected. Q u a l i t a t i v e l y , i t seems that as one moves towards the pure titanium spinel end of the series, the more d i f f i c u l t i t i s to obtain pure samples in an unsealed reaction vessel. Conversely, the samples near the pure Mn spinel end are r e l a t i v e l y easy to prepare. 70 3.2.2 X-Ray D i f f r a c t i o n 3.2.2.1 a vs y in LiMn^Ti 2_ yQ 4 Least square refinements of the x-ray patterns were performed to determine the a l a t t i c e parameter for the series L i M n y T i 2 _ y 0 4 (0 < y < 2). Figure 23 shows the variation of a versus y for both methods of preparation. The c i r c l e s represent the series of samples prepared in the flowing 0 2/C0 2 gas mixture, whereas the squares represent the samples prepared in a i r . It appears that the l a t t i c e parameters for the samples made in the flowing 0 2/C0 2 gas mixture are consistently lower than the corresponding samples made in a i r . The curve included in the figure was v i s u a l l y f i t t e d showing the probable variation of a with y. Very l i t t l e emphasis has been put on the points for y < 1, for reasons explained above. 3.2.3 E l e c t r o c h e m i c a l Measurements 3.2.3.1 L i T i 2 Q 4 and L i 4 ^ 3 T i 5 ^ 3 0 4 C e l l s Figures 24 and 25 show the voltage, V(x), for a L i / L i 1 + x T i 2 0 4 c e l l and a L i / L i 4 ^ 3 + x T i 5 y 3 0 4 c e l l at 21°C for 0 ^ x < 1 respectively. Both curves were obtained at discharge rates of 30 hours for Ax = 1. For an ideal c e l l during a phase t r a n s i t i o n , the surface concentration of 71 y in LiMn y T i 2 _ y 0 4 F i g u r e 23. V a r i a t i o n of the l a t t i c e , parameter a of LiMn T i 2 0 4 versus y f o r the syntheses i n a i r (•) and i n an gas mixture (•). it 72 Figure 24. V(x) for L i / L i + x T i 2 0 . c e l l s at 21°C and a discharge rate of 30 hours for Ax = 1. 73 3.4-3.0-CO 2.6-o -> 2.2-LU -CD < 1.8-o 1.4-> 1.0-0.6--0.2 0 Discharge j 1 «_ Charge J !_ 0.2 0.4 0.6 0.8 1.0 X i n L i 4 / 3 + x T i 5 / 3 ° 4 F i g u r e 25. V(x) f o r L i / L i 4 / 3 + x T i 5 / 3 0 4 c e l l s at 21°C and a di s c h a r g e r a t e of 30 hours f o r Ax = 1. 74 lithium remains fixed while the phase boundary moves through the c r y s t a l . This means that uc i s constant, and from "the equation V(x) = U a - M c(x)] / e, the voltage in a two phase region i s also constant. Thus, the plateaus in the V(x) curves indicate f i r s t order phase tran s i t i o n s and the two phase co-existence of L i T i 2 0 4 / L i 2 T i 2 0 4 and of L i 4 / 3 T i 5 / 3 0 4 / L i ? / 3 T i respectively. In-situ x-ray d i f f r a c t i o n confirmed the two phase co-existence for L i T i 2 0 4 and revealed that there i s a sl i g h t decrease in the size of the unit c e l l on addition of lithium from 8.416 A to 8.380 A. Murphy et a l . (1982) suggest that this decrease results from additional L i minimizing repulsion from other L i and Ti ions by occupying only the remaining octahedral s i t e s , leaving a l l tetrahedral s i t e s vacant. In the case of L i 4 y 3 T i 5 y 3 C > 4 , the a l a t t i c e parameter s l i g h t l y increases from 8.36 A to 8.37 A upon the addition of lithium (Murphy et a l . 1983). The average plateau voltages at 21°C are 1 .338 V for the Li/LiTi 2C> 4 c e l l and 1.562 V for the L i / L i 4 / 3 T i 5 / / 3 0 4 c e l l . L i / L i T i 2 0 4 c e l l s In addition, L i / L i T i 2 0 4 c e l l s show some reversible c e l l capacity near 2.8 V, however upon subsequent cycles t h i s capacity disappears. L i / L i T i 2 0 4 c e l l s cycle reversibly through the f l a t portion of the voltage curve with l i t t l e capacity loss for 75 100 c y c l e s as shown i n f i g u r e 26. F i g u r e 27 shows a s i m i l a r curve f o r L i / L i 4 y 3 T i 5 y 3 0 4 . I t i s s u r p r i s i n g that L*4/3 T*5/3^4 c e ^ ^ - s a r e a ° l e t 0 c y c l e at a l l as t h i s m a t e r i a l i s e l e c t r i c a l l y i n s u l a t i n g and no g r a p h i t e or other a d d i t i v e has been added to these c e l l s to improve the c o n d u c t i v i t y . L i ^ ^ T i ^ ^ C ^ i s a dark blue m a t e r i a l and, on d i s c h a r g e of a L i / L i 4 y 2 T i 5 / 3 1 ^ c e l l , forms a conducting phase at the s u r f a c e of the cathode. However, i t i s not c l e a r how the recharge of the c e l l works, s i n c e the s u r f a c e of the cathode would then be an i n s u l a t i n g L i 4 ^ 3 T i r ^ 3 0 4 phase. In an e f f o r t to i n c r e a s e the c o n d u c t i v i t y and thus the r a t e c a p a b i l i t y of the L i / L i T i 2 0 4 c e l l s , a c e l l with 25% g r a p h i t e by weight in the cathode was assembled. The v o l t a g e curve of t h i s c e l l c o u l d not be e x p l a i n e d i n terms of a simple decrease i n the i n t e r n a l r e s i s t a n c e of the c e l l which would be expected f o r an i n c r e a s e i n c o n d u c t i v i t y . Appendix II shows the v o l t a g e V(x) and the c y c l i n g data f o r t h i s c e l l . 3.2.3.2 Band S t r u c t u r e Arguments It has been r e p o r t e d (Murphy et a l . 1983) that L i can be removed from L i T i 2 0 4 . F i g u r e 24 shows that there i s some r e v e r s i b l e c e l l c a p a c i t y near 2.8 V. Since new L i / L i T i 2 0 4 c e l l s have open c i r c u i t v o l t a g e s near 2.0 V, t h i s high v o l t a g e c a p a c i t y corresponds to removing L i from the s p i n e l 76 100 H t o < CL < o 75 H 5 0 -25 H ' „ •••«•••«« 0 25 50 75 100 CYCLE NUMBER F i g u r e 26. Percent c e l l c a p a c i t y versus c y c l e number f o r L i / L i 1 + x T i 2 0 4 c e l l s . 10CH 10< CYCLE NUMBER F i g u r e 27. Percent c e l l c a p a c i t y versus c y c l e number f L i / L i 4 / 3 + x T i 5 / 3 ° 4 c e l l s ' 78 host. By contrast, figure 25 shows that L i / L i 4 / 3 T ^ 5 / 3 ° 4 c e l l s do not show any capacity at high voltage that would correspond to a similar removal of L i from the L i 4 y 3 T i 5 / r 3 0 4 host material. Figure 28 shows schematically the band structure for (a) L i T i 2 0 4 and (b) L i 4 / , 3 T i 5 / , 3 0 4 (Harrison et a l . 1985). In L i T i 2 0 4 , the T i : t 2 band contains one electron per formula unit, corresponding to the mixed-valence states T i 4 + + T i 3 + . Removal of L i + ions from L i T i 2 0 4 must be accompanied by the loss of an electron to maintain charge n e u t r a l i t y . The existence of such electrons in the T i : t 2 band of L i T i 2 0 4 allows such a removal to take place (figure 25). For L * 4/3T* 5/3<*>4 t* i e T* : t2 ^and * s unoccupied so removal of L i + ions i s not possible due to the lack of the necessary electrons to maintain charge n e u t r a l i t y . The energy band gap of approximately 3.0 eV between the T i : t 2 band and the 0:2p6 band i s apparently too large for the electrochemical removal of L i + ions from the host structure. 3.2.3.3 Li/LiMn 2Q 4 C e l l s Figure 29 shows the voltage curve, V(x), for a Li/LiMn 20 4 c e l l . Since the conductivity of the cathode material i s poor, i t i s necessary to add graphite to the cathode material slurry to improve the conductivity. The c e l l whose voltage curve i s shown in figure 29 had 20% 79 F i g u r e 28. Semi-empirical energy versus d e n s i t y of s t a t e s diagram f o r (a) L i T i 2 0 4 and (b) L i 4 / 3 T i 0 4. ( A f t e r H a r r i s o n et a l . 1985). 80 F i g u r e 29. V(x) f o r a L i / L i 1 + x M n 2 0 4 c e l l at 21°C a t a d i s c h a r g e r a t e of 30 hours f o r Ax = 1. 81 g r a p h i t e by weight added. The d i s c h a r g e curve agrees w e l l with the r e s u l t s obtained by Thackeray et a l . (1983). Acco r d i n g to Thackeray et a l . (1983) e l e c t r o c h e m i c a l i n s e r t i o n of l i t h i u m i n t o LiMn 20^ produces a p l a t e a u i n the V(x) curve f o r the c o m p o s i t i o n a l range 0.1 < x < 0.8 s i g n a l i n g a two phase r e g i o n . A c u b i c phase p e r s i s t s i n the range 0 ^ x < 0.1 and a t e t r a g o n a l (c/a >> 1) phase i n the range 0.8 < x < 1.0. A v a r i e t y of c e l l s of the type Li/ L i M n T i - _ 0. were Y ^ y 4 assembled and s e v e r a l attempts were made to c y c l e them. These c e l l s showed l i t t l e c y c l i n g a b i l i t y . I n i t i a l l y , i t was thought that the high r e s i s t i v i t y of the cathode m a t e r i a l s was the cause, but even the subsequent a d d i t i o n of 20-30% g r a p h i t e d i d not s o l v e the problem. L i / L i M n 1 gTifj 1 ° 4 c e l l s and L i / L i M n 1 7 5 T i Q 25^4 c e l l s were s u c c e s s f u l l y c o n s t r u c t e d and t h e i r v o l t a g e curves were i d e n t i c a l to the curve of f i g u r e 29. However, the cathode u t i l i z a t i o n of these mixed s p i n e l c e l l s was f a r worse than the pure Mn s p i n e l c e l l s . 4 POSSIBLE APPLICATIONS The s p i n e l phases L i T i 2 0 4 and L i ^ ^ T i ^ - j O ^ can be prepared from cheap s t a r t i n g m a t e r i a l s . C e l l s employing these m a t e r i a l s as cathodes show good r e v e r s i b i l i t y and r a t e c a p a b i l i t y . As a r e s u l t , i t would be p o s s i b l e to use these cathode m a t e r i a l s i n p r a c t i c a l c e l l s f o r the 1.5 V rechargeable b a t t e r y market. The t h e o r e t i c a l values of a "AA" c e l l employing L i T i 2 0 4 are compared with the Ni/Cd c e l l below. Ni/Cd L i / L i T i 2 0 4 C a p a c i t y 0.8 A-hrs 0.99 A-hrs Average v o l t a g e 1.2 V 1.34 V C y c l e s 500 ? (>100) A l t e r n a t i v e l y , the s p i n e l phase m a t e r i a l can be employed as an anode in c e l l s that have come to be known as " r o c k i n g c h a i r " c e l l s . These c e l l s c o n t a i n no m e t a l l i c l i t h i u m but r a t h e r employ two host m a t e r i a l s that t r a n s f e r L i atoms i n and out of t h e i r l a t t i c e s d u r i n g charge and d i s c h a r g e . These "r o c k i n g c h a i r " c e l l s , although having lower t h e o r e t i c a l energy d e n s i t i e s , would be expected to ( i ) have longer c y c l e l i f e ( i i ) be more c o s t e f f i c i e n t and ( i i i ) be s a f e r . The p o t e n t i a l d i f f e r e n c e between the two host m a t e r i a l s i s the main f a c t o r determining which m a t e r i a l s should be combined to c o n s t r u c t a " r o c k i n g c h a i r " c e l l . The p o t e n t i a l 82 83 difference should be comparable to "AA" size voltages. Metal oxide electrodes in these "rocking chair" c e l l s can be chosen by considering the main parameters used to characterize host materials: (i) the number of L i atoms that can be intercalated per metal atom; ( i i ) the chemical d i f f u s i o n c o e f f i c i e n t of L i and ( i i i ) the e l e c t r i c a l conductivity. An example would be a L i N i 0 2 / L i T i 2 0 4 c e l l . Since V(x) for L i / L i x N i 0 2 c e l l s i s approximately 4 V and V(x) for L i / L i 1 + x T i 2 0 4 c e l l s i s approximately 1.5 V, cycl i n g L i between the Ni0 2 host and the L i T i 2 0 4 host would result in a c e l l with an average voltage of approximately 2.5 V. It is these "rocking chair" c e l l s that are thought to be the c e l l s that w i l l have the longest cycling capacity as they don't employ metallic lithium. It i s the lithium anode which accounts for a large part of the fade c h a r a c t e r i s t i c s of lithium rechargeable batteries. 5 SUMMARY AND SUGGESTIONS FOR FUTURE WORK T h e i n t r o d u c t i o n o f t h e t h e s i s e s t a b l i s h e d t h e n e e d f o r w o r k o n t h e L i - T i - S t e r n a r y s y s t e m a n d t h e i n t e r e s t f o r a n i n v e s t i g a t i o n o f t h e l i t h i u m s p i n e l o x i d e s a s c a t h o d e m a t e r i a l s i n l i t h i u m s e c o n d a r y b a t t e r i e s . I n p a r t 2 o f t h e t h e s i s we d e s c r i b e d e x p e r i m e n t a l m e t h o d s f o r s y n t h e s i z i n g a n d s t u d y i n g l i t h i u m i n t e r c a l a t i o n c o m p o u n d s a n d a p p l i e d t h e m t o t h e L i - T i - S t e r n a r y s y s t e m a n d t h e s p i n e l o x i d e s . P a r t 3 p r e s e n t e d t h e r e s u l t s o f t h e s y n t h e s e s p e r f o r m e d o n b o t h t h e l i t h i u m t i t a n i u m s u l f i d e s a n d o x i d e s , a s w e l l a s s t r u c t u r a l a n d e l e c t r o c h e m i c a l d a t a . I n s e c t i o n 3.1 we d i s c u s s e d ' t h e r e s u l t s o f t h e w o r k o n s u l f i d e s . A p r e l i m i n a r y p h a s e d i a g r a m f o r t h e L i - T i - S s y s t e m a t h i g h t e m p e r a t u r e w a s d e t e r m i n e d . A l a r g e t h r e e p h a s e r e g i o n w i t h L i T i S 2 , L i 2 S a n d T ^ a s t n e a p i c e s a n d t h e s i n g l e p h a s e L i x T i S 2 (0 < x < 1) w e r e i d e n t i f i e d , h o w e v e r m o r e w o r k i s r e q u i r e d t o v e r i f y t h e e x i s t e n c e o f t h e o t h e r r e g i o n s . A n e w T i S 2 p o l y t y p e , 3 R - L i x T i S 2 (0 < x < 1), w a s r e p o r t e d a n d i t s s t r u c t u r e d e s c r i b e d a n d c o m p a r e d t o t h e m o r e c o m m o n 1 T - L i x T i S 2 (0 < x < 1). X - r a y d i f f r a c t i o n r e v e a l e d t h a t b o t h p o l y t y p e s h a v e a s i m i l a r v a r i a t i o n o f t h e a l a t t i c e p a r a m e t e r v e r s u s x b u t t h a t c r e m a i n s r e l a t i v e l y c o n s t a n t f o r a l l x i n 3 R - L i x T i S 2 (0 < x < 1 ) . We a l s o f o u n d t h a t 3R c e l l s h a v e h i g h e r a v e r a g e v o l t a g e s t h a n 1T c e l l s a n d t h a t 8 4 85 they exhibit a voltage step on recharge. In-situ x-ray d i f f r a c t i o n on 3 R - L i x T i S 2 revealed that the 3R phase i s metastable and converts to the more stable 1T phase when the lithium content i s lowered by de-intercalation. Phase conversion begins for x < 0.4 in L i TiS,. The disappearance of the 1T phase upon subsequent discharge i s thought to be due to co-intercalation of PC in T i S 2 -In section 3.2 work on the oxides was presented and the results of numerous syntheses were summarized. Lithium secondary batteries using L i T i 2 0 4 and the related compound hi ^^Ti^^O ^ were made. The plateaus in the V(x) indicate the co-existence of L i T i 2 0 4 / L i 2 T i 2 0 4 and of L i 4 y 3 T i g ^ 2 0 4 / L i 7 y 3 T i g y 2 0 4 respectively. The average plateau voltages at 21°C are 1.338 V for the L i / L i T i 2 0 4 c e l l and 1.562 V for the L i / L i 4 ^ 3 T i g ^ 3 0 4 c e l l . Arguments based on the band structure of L i 1 + x T i 2 _ x 0 4 were used to interpret the differences in the voltage curves. Compounds of the form LiMn T i , O. (0 < y < 2), known y z y 4 as mixed spinel oxides, were also investigated. The variation of the a l a t t i c e parameter versus y was determined but only c e l l s of the two end members, L i T i 2 0 4 and LiMn 20 4 were successfully made. The mixed spinel compounds of t h i s series have very high r e s i s t i v i t i e s , which impedes c e l l performance. Future work is needed in identifying for certain some 86 of the L i - T i - S (H.T. ) ternary compounds that we have come across in th i s study. The fade mechanisms of L i 4 y 2 T ^ 5 / 3 ° 4 would be useful to know for th i s materials app l i ca t ion in "rocking chair" c e l l s . Work in th i s area could be most b e n e f i c i a l for the development of a new l i th ium rechargeable battery system. Working c e l l s of the LiMn T i ~ 0. (0 ^ y ^ 2) compounds y ^ y * would give useful ins ight into the band structure of these mixed sp ine l oxides. BIBLIOGRAPHY Ber l insky , A . J . ; Unruh, W . G . ; McKinnon, W.R. and Haering, R.R. ( 1 979) S o l i d State Commun. 3J_, 135. Blasse, G. (1963) J . Inorg. Nuc l . Chem. 2_5, 743. Cava, R . J . ; Murphy, D.W. and Zahurak, S. (1984) J . S o l i d State Chem. 5_3, 64. C h i a n e l l i , R.R. (1976) J . C r y s t a l Growth 34, 239. C u l l i t y , B .D. (1959) Elements. of X-ray D i f f r a c t i o n , Addi son-Wesley. Dahn, J . R . and Haering, R.R. (1979) Mat. Res. B u l l . 14, 1259. Dahn, J . R . ; Py, M.A. and Haering, R.R. (1982) Can. J . Phys. 60, 307. Dresselhaus, M.S. and Dresselhaus, G. (1981) Adv. in Physics 30, 139. Gamble, F . R . ; DiSalvo, F . J . ; Klemm, R .A . and Gebal le , T . H . (1970) Science 168, 568. Goodenough, J . B . ; Thackeray, M . M . ; David, W . I . F . and Bruce, P . G . ( 1 984) Rev. Chim. Min. 2_1_, 435. Haering, R . R . ; S t i l e s , J . A . R . and Brandt, K. (1980) U .S . Patent 4224390. Harr i son , M . R . ; Edwards, P . P . and Goodenough, J . B . (1985) P h i l . Mag. B 52, 679. I s s l e r , S . L . (1986) Ph.D. Thes is , Corne l l U n i v e r s i t y . J e l l i n e k , F . (1962) J . Less Comm. Metals 4, 9. L i e t h , R.M.A. (ed.) (1977) Preparation and C r y s t a l Growth of  Mater ia l s with Layered Structures , D. Riedel Publ ishing Company. McKinnon, W.R.; Dahn, J . R . and Levy-Clement, C. (1984) S o l i d State Commun. 50, 101-104. Mizushima, K . ; Jones, P . C . ; Wiseman, P . J . and Goodenough, J . B . (1980) Mat. Res. B u l l . JJ5, 783-789. 87 88 Mosbah, A.; Verbaere, A. and Tournbux, M. (1983) Mat. Res. B u l l . J_8, 1375. Murphy, D.W.; DiSalvo, F.J.; Carides, J . N. and Waszczak, J.V. ( 1978) Mat. Res. B u l l . J_3, 1 395. Murphy, D.W.; Chris t i a n , P.A.; DiSalvo, F.J. and Carides, J.N. (1979a) J. Electrochem. Soc. 126, 349. Murphy, D.W. and Christian, P.A. (1979b) Science 205 , 651. Murphy, D.W.; Greenblatt, M.; Zahurak, S.M.; Cava, R.J.; Waszczak J.V.; Hu l l , G.W. and Hutton, R.S. (1982) Rev. Chim. Min. J_9, 441 . Murphy, D.W.; Cava, R.J.; Zahurak, S.M. and Santoro, A. (1983) S o l i d State Ionics 9&10, 413-418. Thackeray, M.M.; David, W.I.F.; Bruce, P.G. and Goodenough, J.B. ( 1983) Mat. Res. B u l l . J j 5 , 461. Thackeray, M.M.; Johnson, P.J.; de P i c c i o t t o , L.A.; Bruce, v P.G. and Goodenough, J.B. (1984) Mat. Res. B u l l . 19, 179. Thackeray, M.M.; de P i c c i o t t o , L.A.; de Kock, A.; Johnson, P.J.; Nicholas, V.A. and Adendorff, K.T. (1987) J . Power Sources 2_1_, 1-8. Thompson, A.H. (1978) Phys. Rev. Lett. 40, 1511. Thompson, A.H. (1979) J. Electrochem. Soc. 126, 608. West, K.; Jacobsen, T.; Zachau-Christiansen, B. and Atlung, S. ( 1983) Electrochimica Acta 2_8, 97. Whittingham, M.S. and Gamble, F.R. (1975) Mat. Res. B u l l . J_0, 363. Whittingham, M.S. (1976) Science J_92, 1126. Whittingham, M.S. (1978) Progress in Solid State Chem. J_2, 41 . Wyckoff, R.G. (1963) Crystal Structures, Second Edition Vol. 1-4, Wiley and Sons. APPENDIX I : T_ MEASUREMENTS ON L I T I o 0 . C 2 4 Superconductivity in oxide compounds has been known for about two decades. However, the presently known number of superconducting oxide compounds i s s t i l l very small in comparison to the approximately 1000 superconducting intermetallic compounds. Apart from TiO and NbO a l l of these superconducting oxide compounds are ternary compounds. One such ternary compound, L i T i 2 0 4 , becomes superconducting at temperatures up to about 12 K (Harrison et a l . 1985). This compound i s one end member of the homogeneity range of the spinel phase in the L i - T i - 0 ternary system: L*i+ x T*2-x^4 (0 ^ x < 1/3). L i T i 2 0 ^ has a high T c compared with most other oxide superconductors and i s the only oxide superconductor having the spinel structure. When a sample of magnetic material ( i . e . , one having non-zero magnetic s u s c e p t i b i l i t y ) i s placed in a uniform magnetic f i e l d , the f i e l d in the neighborhood of the sample consists of the o r i g i n a l uniform f i e l d plus a non-uniform f i e l d due to the sample. The strength of thi s f i e l d i s a measure of the sample magnetization and hence of i t s s u s c e p t i b i l i t y . The PAR Model 155 Vibrating Sample Magnetometer detects and measures the part of the f i e l d due to the sample. The superconducting t r a n s i t i o n temperature of two 89 90 spinel-phase samples, L i T i 2 0 4 and L i 1 ^ T ^ C^, were determined by the magnetization measurements just described. L i 1 1 T i 2 0 4 was prepared from L i T i 2 0 4 by standard chemical l i t h i a t i o n techniques (Murphy and Christian 1979b) using a benzophenone in tetrahydrofuran (THF) solution. L i T i 2 0 4 powder and an excess of L i metal are slowly added to a s t i r r i n g solution of benzophenone in THF. The solution was l e f t to s t i r overnight and then f i l t e r e d , washed with THF and dried under vacuum. The whole procedure was performed in an inert atmosphere glovebox. Powdered samples weighing 30-40 mg were loaded into sealed ampoules and then placed in the magnetic f i e l d . The magnetic f i e l d R was held constant at 5 gauss and 60 gauss as the temperature was varied and the t o t a l magnetic moment, ^ t o t of the sample, was measured. Figure 30 shows a plot of ^ t o t / 9 versus temperature for the L i T i 2 0 4 sample at 60 ± 1 gauss. The onset of superconductivity, namely T c, was determined to be 12.6 K by the sharp t r a n s i t i o n of the t o t a l magnetic moment per gram of sample. Figure 31 shows a similar plot of ^ t o t / 9 versus temperature but both spinel samples, L i T i 2 0 4 and L i 1 1 T i 2 0 4 are shown. The magnetic f i e l d used here was only 5 gauss. From the figure, T c was found to be 12.6 K and 11.9 K for L i T i 2 0 4 and L i 1 1 T i 2 0 4 respectively. F i r s t l y , T c for L i T i 2 0 4 does not seem to be dependent on the applied magnetic f i e l d . Secondly, i t appears that L i 1 1 T i 2 0 4 has a s l i g h t l y lower 91 — £ CP 0 -.04 -.08 D) -.12 o 15* -.16 -.20 I I I I I I • • I • • I — • — — • — • — • — • — • — I* • I i I I I I I 4 8 12 16 TEMPERATURE (Kelvin) F i g u r e 30. H* = 60 gauss . R t Q t / g ver sus temperature f o r L i T i 2 0 4 at 92 CO E CD CO I o 12 - 1 0 - 2 0 - - 3 0 - 4 0 h I 1 1 1 1 1 • • • • • • • • 1 • 1 • — • • • • • —— • • • • • • • — — • • • • — 1 1 1 1 1 1 1 1 8 12 TEMPERATURE (Kelvin) 16 F i g u r e 31. ^ t o t ^ 9 versus temperature f o r L i T i ^ O ^ (•) and L i K 1 T i 2 0 4 (•) at H = 5 gauss. 2~4 93 t r a n s i t i o n temperature. This result i s somewhat surprising in that figure 28, schematically shows that L i ^ 1 T i 2 0 4 has a larger density of states as compared to L i T i 2 0 4 and thus might be expected to have a higher T . The p o s s i b i l i t y of an impure L i , ^ i - O . sample may explain the lower value of T . APPENDIX II : L I / L I 1 + x T I 2 0 4 CELL WITH 25% GRAPHITE In an e f f o r t to increase the cathode conductivity and thus the rate c a p a b i l i t y of the L i / L i 1 + x T i 2 0 4 c e l l s , a c e l l with 25% graphite in the cathode material was assembled. The batch of L i T i 2 0 4 used for this c e l l had approximately 10% of an unknown impurity. The graphite was mixed and ground into the L i T i 2 0 4 powder prior to making the cathode sl u r r y . The voltage curve, V(x) for this c e l l at 21°C at a discharge rate fo 30 hours for Ax = 1 is shown in figure 32. The curve i s not f l a t for the whole range of x as i t i s for the c e l l s with no graphite (figure 24). It i s clear that the V(x) behaviour can not be explained in terms of a simple decrease in the internal resistance of the c e l l which would result from an increase in conductivity. Further work is required to explain the shape of the voltage curve. Figure 33 shows the percent c e l l capacity versus cycle number for the L i / L i 1 + x T i 2 Q 4 c e l l with 25% graphite. This c e l l retains over 85% of i t s i n i t i a l capacity after 150 cycles with a l l the capacity loss occuring in the f i r s t 10 cycles. 94 95 F i g u r e 32. V(x) f o r a L i / L i 1 + x T i 2 0 4 c e l l with 25% g r a p h i t e by weight i n the cathode. C y c l e d at 21°C at a discharge r a t e of 30 hours f o r Ax = 1. 96 160 C Y C L E NUMBER F i g u r e 33. Percent c e l l c a p a c i t y versus c y c l e number f o r a L i / L i 1 + x T i 2 ° 4 c e l 1 with 25% g r a p h i t e by weight i n the cathode. 

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