Electrodeposition of atmosphere-sensitive ternary sodium transition metal oxide films for sodium-based electrochemical energy storage

Significance Layered sodium transition metal oxides constitute an important class of materials with applications including electrochemical energy storage, high-temperature superconductivity, and electrocatalysis. However, electrodeposition of these compounds, an approach commonly used to grow oxides, has been elusive due to their atmosphere instability and intrinsic incompatibility with aqueous electrolytes. Using a molten sodium hydroxide electrolyte, we demonstrate electrodeposition of O3 (O′3)- and P2-type layered sodium transition metal oxides and apply these electrodeposits as high areal capacity cathodes in sodium-ion batteries. The electrodeposits are micrometers thick, polycrystalline, and structurally similar to materials synthesized classically at high temperature. This work enables fabrication of previously inaccessible alkali and alkaline earth ion intercalated, higher valent transition group oxides in important thick film form factors. We introduce an intermediate-temperature (350 °C) dry molten sodium hydroxide-mediated binder-free electrodeposition process to grow the previously electrochemically inaccessible air- and moisture-sensitive layered sodium transition metal oxides, NaxMO2 (M = Co, Mn, Ni, Fe), in both thin and thick film form, compounds which are conventionally synthesized in powder form by solid-state reactions at temperatures ≥700 °C. As a key motivation for this work, several of these oxides are of interest as cathode materials for emerging sodium-ion–based electrochemical energy storage systems. Despite the low synthesis temperature and short reaction times, our electrodeposited oxides retain the key structural and electrochemical performance observed in high-temperature bulk synthesized materials. We demonstrate that tens of micrometers thick >75% dense NaxCoO2 and NaxMnO2 can be deposited in under 1 h. When used as cathodes for sodium-ion batteries, these materials exhibit near theoretical gravimetric capacities, chemical diffusion coefficients of Na+ ions (∼10−12 cm2⋅s−1), and high reversible areal capacities in the range ∼0.25 to 0.76 mA⋅h⋅cm−2, values significantly higher than those reported for binder-free sodium cathodes deposited by other techniques. The method described here 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 structures with extended frameworks.