Surface-Modified Na(Ni0.3Fe0.4Mn0.3)O2 Cathodes with Enhanced Cycle Life and Air Stability for Sodium-Ion Batteries

Sodium layered-oxide cathodes show promise as an alternative to lithium layered oxides for grid-scale energy storage applications. Among the many sodium layered-oxide chemistries, O3-type structures with a mixture of nickel, manganese, and iron are of interest due to their high theoretical energy density. The main challenges of these materials include poor cycle life and air stability due to surface reactivity in both electrolytes and ambient moisture. Surface modifications can mitigate both challenges by protecting the underlying cathode material. Phosphate coatings are of particular interest due to their low cost and high conductivity, but they have not yet been explored for O3 sodium layered-oxide materials. Furthermore, the relative benefits of different coating methods and phosphate groups are not well-understood. Herein, the electrochemical performance and air stability of two coating methods are compared, and their unique challenges with an O3 sodium layered oxide are explored. Unlike lithium layered oxides or P2-type sodium layered oxides, O3 sodium layered oxides suffer from irreversible extraction of sodium during the coating process resulting in metal oxide formation. This sodium extraction causes reduced capacity and conductivity at high coating contents and highlights the unique challenges of O3-type sodium layered oxides. Despite these challenges, a 1% (NaPO3)n coating achieves significant improvement over the uncoated material, corresponding to a 14% gain in capacity retention after 100 cycles and only a loss of 30 mA h g–1 after 9 days in humid air, compared to 80 mA h g–1 lost in the uncoated sample.