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

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, Na_xMO₂ (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 Na_xCoO₂ and Na_xMnO₂ 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.

Electrochemical synthesis of materials has contributed to significant breakthroughs in materials processing by replacing high-temperature, cost- and energy-intensive pyrometallurgical processes (1). Noteworthy examples include aluminum extraction by Bayer’s process (2), electrowinning of copper (3), titanium extraction through the Kroll process (4), electrolytic production of steel (5), and electrochemical synthesis of cement (6). Additional advantages of electrochemical synthesis include controllable access to metastable polymorphs (7), prevention of crystallite coarsening, and generation of asymmetric, highly oriented structures (8). Specific to the work discussed here on transition metal oxides for electrochemical energy storage, electrodeposition allows direct thermodynamic and kinetic control of the phase formation along with the ability to conformally deposit on topologically complex structures, resulting in improved electron and ion transport kinetics (9). Even though electrodeposition as a technology is well developed for reduction of elements and alloys, studies on multicomponent systems grown by electrolytic oxidation are sparse. Finally, synthesis of materials directly using electrons as the energy packet provides the potential of significant reductions in greenhouse gas emissions and enhanced thermodynamic efficiencies (10).We focus on materials for sodium-ion (Na-ion) batteries due to their potential cost advantages over lithium-ion systems and the equitable distribution of raw materials required for the synthesis of the cathodes relative to lithium-based systems (11). Of particular interest for Na-ion battery cathodes is O3-type layered NaMO₂ (M = Co, Mn, Ni, Fe) with an alpha NaFeO₂-type crystal structure and octahedrally coordinated transition metal ion. Along with electrochemical energy storage applications (12), this class of oxides has also been studied for its catalytic (13) and superconducting (14) properties. Fabrication of NaMO₂-based battery cathodes involves first high-temperature (>700 °C) synthesis of NaMO₂ for over 24 h, followed by grinding, mixing with binders and additives, and slurry casting. Throughout the process, water must be rigorously excluded. While vapor-phase deposition of Na-ion cathodes by methods including pulsed laser deposition and radio frequency magnetron sputtering has been attempted, deposition of polytypes of Na_xCoO₂ (15–18), alpha Na_xMnO₂ (19), and polyanion compounds (20) has faced processing challenges, only the first of which is maintaining Na stoichiometry under vacuum during growth on a substrate heated to around 700 °C. Additionally, the vapor-phase methods have been limited to cathodes with thicknesses of ≤750 nm and growth rates of only tens of nanometers per hour. As a result, the areal capacities are only a few microamps per hour per square centimeter even at modest discharge currents of order 10 µA⋅cm⁻². The direct electrochemical synthesis of thick layered sodium transition metal oxides (STMOs) at reasonable growth rates and temperatures would overcome the abovementioned challenges and serve as a potential starting point for the scalable manufacturing for Na-ion cathodes. As we will show, such electrochemically grown cathodes also have higher volumetric and gravimetric capacities than slurry-based electrodes due to the absence of binder and conductive additives.In early work, most ceramic oxides, including binary (21–23) and ternary oxides (24–27), were formed via aqueous electrodeposition. These systems were generally limited to thinner layers (≤5 µm), exhibited slow growth rates (micrometers per day), and exhibited poor crystallinity. An annealing step is often needed to oxygenate and crystallize the deposit. The general mechanism for anodic electrolytic deposition of ceramic oxides (28) involves the complexation of a transition metal ion in an alkaline solution as stable hydroxo-aquo complexes of the type M^(m+)(OH)_x(H₂O)_y (m = 2–3) (details depend on the transition metal ion concentration, charge on the transition metal ion center, pH, and temperature) (29–32). During the anodic oxidation step, M^m+ is electrochemically oxidized and precipitates as a higher valent oxide and/or proton intercalated oxide. The poor kinetics of the reaction and quality of electrodeposits can be traced back to the choice of water as the solvent, the low temperature of synthesis (room temperature to 80 °C), and the low hydrolysis ratio (i.e., OH⁻/M^m+ = 3 to 5 at pH 14 for low transition metal concentrations). Synthesis temperatures of prior electrodeposited oxide/oxyhydroxides by anodic electrolytic deposition and equivalent solid-state synthesis are summarized in SI Appendix, Fig. S1A. A high hydrolysis ratio (>75) (SI Appendix, Fig. S1B) and high temperature of deposition (>250 °C) (SI Appendix, Fig. S1C) synergistically kinetically benefit the growth of thicker (>10 µm) electrodeposits. Moreover, since the O3 and O′3 polytypes of the STMOs most interesting for energy storage applications are air and moisture sensitive (33), aqueous electrodeposition cannot be employed to grow this class of materials.Here we report a dry molten sodium hydroxide-based solvent-mediated electrodeposition of high-quality thick STMOs with controlled crystallography and phase. By using dry hydroxide salts, we overcome the intrinsic limitations imposed by the inclusion of water in the solvent and broaden the temperature range of the growth conditions. Oxides of transition metals (Co₃O₄, Mn₃O₄, Fe₃O₄, Ni(OH)₂) are first solvated as stable hydroxo complexes in molten NaOH at 350 °C. In comparison to water-based anodic electrodeposition, here, electrodeposition is conducted at an extremely high hydrolysis ratio ([OH⁻]/[M²⁺] = 300:1) and high pH (pH₂O = 10 in Lux Flood bascisity scale) and at an elevated temperature of 350 °C. The combination of these factors improves the kinetics of the deposition rate limiting base catalyzed oxolation (inorganic S_N2) process, due to the extremely high nucleophile concentration [OH⁻], catalyzed by the “base” [OH⁻], and removal of the leaving group H₂O from the bath (as vapor at 350 °C) (31). We specifically demonstrate the direct electrochemical deposition of various thick film (>10 µm) thermodynamically and kinetically controlled highly crystalline STMO polymorphs. Fig. 1 outlines the general electrodeposition scheme. We explore the effects of the applied electrode potential, basicity of the hydroxide solvent medium, transition metal precursor concentration, and temperature of synthesis on the resulting phase assemblage, phase distribution, and electrochemical performance of the electrodeposited Na-M-O phases across various transition metal chemistries.Open in a separate windowFig. 1.Reaction conditions for molten hydroxide-mediated anodic electrodeposition of layered STMOs of general form Na-M-O (M = Co, Mn, Ni, Fe) at 350 °C. SEM image below the reaction shows ∼60-µm-thick Na_xCoO₂ on a nickel foil (nickel foil is the brighter band on left side of image).