Abstract: Lithium-ion batteries (LIBs) have become an integral part of electrical energy storage systems; however, the high price leads to a search for alternatives to LIBs. Sodium-ion batteries stand out because sodium is unlimited in the Earth’s crust and sea. Similar to LIBs, layered transition metal oxides are the optimal choice for practical application by virtue of good ion conductivity, high energy density, and facile synthesis. However, the air sensitivity will lead to the degradation of layered oxides, corresponding with poor electrochemical performance. For instance, capacity losses of 20%-50% after exposure to air with ~50% relative humidity for 1 day to 30 days, accompanied by surface carbonate coverage exceeding 20% in many cases. It is also regrettable that the air sensitivity has plagued the commercialization of layered oxides for a long time, resulting in huge cost needs to be spent on storage and transportation. Hence, in this work, recent strategies on air stability are reviewed, and the comprehensive chemical evolution mechanisms of air-exposure are introduced that H₂O and CO₂ have the greatest impact on the structural degradation of layered oxides. In addition, the design principles of layered oxides that choose elements with higher electrochemical potential, increasing the cation competition coefficient, increasing the particle size, surface coating layer modification, and controlling exposed active facets for better air stability are given out. It is hoped that the attention of researchers to air stability can be aroused, and more studies about air stability will be done to address the problem of air sensitivity.

Key words: Sodium-ion batteries, Cathode, Layered oxides, Air sensitivity