Protonation Stimulates the Layered to Rock Salt Phase Transition of Ni-Rich Sodium Cathodes

Protonation of oxide cathodes triggers surface transition metal dissolution and accelerates the performance degradation of Li-ion batteries. While strategies are developed to improve cathode material surface stability, little is known about the effects of protonation on bulk phase transitions in these cathode materials or their sodium-ion battery counterparts. Here, using NaNiO2 in electrolytes with different proton-generating levels as model systems, a holistic picture of the effect of incorporated protons is presented. Protonation of lattice oxygens stimulate transition metal migration to the alkaline layer and accelerates layered-rock-salt phase transition, which leads to bulk structure disintegration and anisotropic surface reconstruction layers formation. A cathode that undergoes severe protonation reactions attains a porous architecture corresponding to its multifold performance fade. This work reveals that interactions between electrolyte and cathode that result in protonation can dominate the structural reversibility/stability of bulk cathodes, and the insight sheds light for the development of future batteries. The de-protonation of electrolyte solvents and subsequent protonation of layered NaNiO2 triggers multiple degradation mechanisms. Intercalated protons are primarily responsible for the accelerated layered to rock salt phase formation process, which ends up in porous bulk structures as well as anisotropic surface degradations. image