UBC Theses and Dissertations
Lignin-derived hard carbon anode for potassium-ion batteries : interplay among lignin molecular weight, material structures, electrolyte chemistry, and storage mechanisms Wu, Zhenrui
Potassium-ion battery (PIB) is a rising star in the rechargeable battery field due to its potential low cost and high energy for large-scale applications. Hard carbon (HC) is one of the most popular anodes for practical PIBs due to its high K-ion storage and relatively low material cost. Primarily, biomass (such as lignin) represents an abundant source for synthesizing HC. This thesis developed a series of HCs from lignin with different molecular weights (MWs) at serial pyrolysis temperatures (PTs) of 500–1000 °C, exhibiting varying electrochemical performance. The best lignin-derived HC (LHC) was pyrolyzed from medium-MW lignin at 700 °C (M700) and delivered a high reversible specific capacity of ~300 mAh g⁻¹ at 50 mA g⁻¹. M700 exhibited an optimal mixture of graphite-like nanocrystals with the most significant interlayer distance and amorphous structure to maximize K-ion storage from bulk insertion and surface adsorption. The role of electrolytes was then investigated to determine the LHC performance and K-ion storage mechanism. We systematically studied the influence of four electrolyte systems, i.e., two K salts (KPF⁶ and KFSI) in carbonate ester and ether solvents, on ionic mobility, cycling stability, and charge transfer kinetics of M700 in PIBs. It is found that the M700 anode achieved the best cycling stability and kinetics performance in the KFSI EC/DEC electrolyte. Mechanimsic study disclosed that the improved performance could be ascribed to the formation of robust KF-rich SEI resulting from FSI⁻ decomposition, which effectively prevented irreversible side reactions and severe structural decay (e.g., exfoliation and pulverization). The degradation mechanisms of other electrolyte systems are also explained by SEI formation and solvation/desolvation effect. It is expected that this work will provide guidance on the anode and electrolyte selection and design for PIBs in the near future.
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