PNNL

Abstract

Protonation of transition metal oxide cathodes aggravates surface transition metal dissolution and accelerates the performance degradation of Li-ion batteries. While various strategies are developed based the theory to improve cathodes surface stability, little is known about the protonation effect on the bulk phase transition of these cathodes nor their sodium-ion battery counterparts. Here, using NaNiO2 in electrolytes with different proton-generating levels as model systems, we unveil a previously overlooked impact of incorporated protons. The protonation of cathodes lattice oxygens stimulates transition metal migration to the alkaline layer with accelerated layered-rock salt phase transition, leading to bulk structure disintegration and anisotropic surface reconstruction layers. The cathode experiencing severe protonation reactions attains a porous architecture corresponding to its multifold performance fading. The work reveals that interactions of electrolyte and cathode like protonation can dominate the structure reversibility/stability of bulk cathodes and the insight sheds light on the development of future batteries.