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Modification of MnO₂-based cathode materials for rechargeable alkaline batteries Nesvaderani, Farhang

Abstract

Aqueous batteries like the alkaline battery, which utilizes the MnO₂/Zn chemistry, are recently receiving renewed attention due to an urgent desire to develop advanced batteries for storage of energy. MnO₂/Zn batteries offer high energy density, lower cost, and excellent shelf life. The cycleability of such batteries is, however, challenging due to the poor performance of the MnO₂ cathode. Therefore, various phases of MnO₂ materials were synthesized to investigate their cycling performance. A series of electrolytic MnO₂ (EMD) samples were synthesized using different concentrations of sulfuric acid-based electrolysis baths. EMD samples synthesized at a relatively high acidic concentration (2M H₂SO₄), had a 30% higher energy efficiency over a cycling period of 100 cycles and 35% higher capacity at the end of the cycling period. The better cycling performance is attributed to higher surface area, higher structural water content (essential for proton diffusion), and a larger fraction of ramsdellite phase in the 2M EMD structure. Pure ramsdellite MnO₂ was also synthesized and tested. It displayed an improved energy delivery and efficiency over all the EMD samples and its final specific capacity was very comparable to the 2M EMD sample. An alternative electrolyte solution (zinc sulfate) was examined for the cycling performance MnO₂ versus a zinc electrode. Addition of manganese sulfate to the electrolyte, which is reported to inhibit manganese dissolution during cycling, was also studied. This led to a discovery that the manganese sulfate additive leads to deposition of additional MnO₂ on the cathode substrate during the charge step of the cycling regime. Based on this observation, a novel method of producing EMD was designed in the zinc sulfate electrolyte that provides a milder environment for producing the material. This form of EMD, named “neutral” EMD or NEMD, exhibits a specific capacity 3x higher than that of commercial EMD when cycled in the zinc sulfate electrolyte. Furthermore, it was possible to retain at least 67-80% of its capacity after 100 cycles. Although MnO₂ cycled in zinc sulfate can only be utilized with low gravimetric loading of the material, this thesis exhibits a possible method of improving this factor.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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