Roles of Mn and Co in the Air Synthesizability of Layered Oxide Cathodes for Lithium-Based Batteries

High-nickel layered oxides (LiNi_1-x-yMn_xCo_yO₂) are the prevailing cathode materials for high-energy-density lithium-based batteries, but they are plagued with deleterious surface air instabilities stemming from residual lithium formation. These issues severely hinder mass production as cathode calcination is limited to a flowing oxygen atmosphere, which entails high manufacturing costs as opposed to simpler and more economical air calcination. Here, while higher Ni contents are known to worsen air instabilities, the influence of Mn and Co contents on impacting these phenomena are less elucidated. We herein present the synthesis in ambient air and flowing oxygen atmospheres of three cathode variants with the same Ni contents, but varying Mn and Co contents: LiNi_0.7Mn_0.3O₂, LiNi_0.7Mn_0.15Co_0.15O₂, and LiNi_0.7Co_0.3O₂. It is found that the critical parameter influencing the air stability of the cathodes is the average Ni oxidation state, which is greatly dependent on the Mn and Co contents. Substitution of Mn for Ni drives down the Ni oxidation state as Mn exists as Mn4⁺ and reduces surface residual lithium formation, which vastly improves the overall air stability and, therefore, the synthesizability in air, but with a penalty of lowered capacity. In contrast, substitution of Co for Ni maintains Ni³⁺ as Co exists as Co³⁺, offering increased initial capacity, but worsens the air stability and cyclability as the driving force for residual lithium formation and surface reactivity is increased.