Surface transformation by a "cocktail" solvent enables stable cathode materials for sodium ion batteries

Coating the surfaces of active materials has become an effective and indispensable path towards the stable operation of practical rechargeable batteries. Improving the affordability of coating processes can bring enormous manufacturing advantages to battery applications. Herein, we report a cheap, simple and efficient method to create conformal coating layers on the primary particles of sodium layered oxide materials for improving battery performance. Mimicking the cathode-electrolyte interfacial reaction in practical cells, we create conformal coating layers via the spontaneous reaction between the oxidative cathode surfaces and a cocktail of reductive organic solvents. The conformal coating layers consist of metal-organic compounds with reduced transition metal cations, i.e., artificial cathode-electrolyte interphases (CEIs). The cells containing these coated cathode materials deliver much improved cycle life while maintaining reasonably high reversible capacity and rate capability. Furthermore, the structural stability and water resistance are enhanced, which can practically help simplify the storage protocol of cathode powders prior to battery manufacturing. The surfaces of most oxide cathode materials (e.g., lithium cathodes and sodium cathodes) are highly oxidative, and thus we expect that the present method, with tailored experimental parameters, can be readily applied to most battery systems.