Electrochemically Grown Sodium Transition Metal Oxides NaxMO2 (M=Co, Mn, Fe, Ni)

Abstract

Electrochemically grown ceramic oxide films have exhibited improved electrochemical, photovoltaic, and catalytic performance. However, product library accessible through conventional aqueous anodic electrolytic deposition of oxides is limited primarily to binary systems and rare examples exist for air and moisture sensitive ternary oxides. Moreover, growth rate, thickness, and functionality of the electrodeposited oxide films are hindered by the solvent of choice, poor reaction kinetics and low temperature of synthesis. Here, we introduce, an intermediate temperature (350oC), dry molten sodium hydroxide mediated binder-free electrodeposition process to grow air and moisture sensitive layered sodium transition metal oxides, NaxMO2 (M=Co, Mn, Ni, Fe), which are conventionally synthesized by solid state reactions at temperatures ≥ 700oC. Albeit the lowest reported synthesis temperature and reaction times, our electrodeposited oxides retain the key structural and electrochemical performance observed in the high temperature bulk synthesized analogues. We demonstrate that tens of microns thick, >75% dense O3 NaxCoO2 and O'3 NaxMnO2 can be deposited at a growth rate of ~5-20 mg/cm2/hr, with near theoretical gravimetric capacity, chemical diffusion coefficient of Na+ ions, and reversible areal capacity in the range ~0.25-0.76 mAh/cm2, which are significantly higher than those reported for binder free sodium cathodes deposited by other techniques. Our method resolves longstanding intrinsic challenges associated with traditional aqueous solution-based electrodeposition of ceramic oxides and opens a general solution chemistry approach for electrochemical processing of hitherto unexplored air and moisture sensitive high valent multinary oxides.