Precursor-driven Jahn–Teller distortion as a hidden origin of surface instability in Mn-stabilized Ni-rich cathodes

Abstract

Mn-enriched surfaces are widely employed to enhance the structural and thermal stability of high-Ni layered cathodes. However, this study identifies a critical, previously unrecognized vulnerability in Co-free Ni-rich core/Mn-rich shell architectures stemming from precursor processing. We demonstrate that exposing hydroxide precursors to an oxygen-rich atmosphere induces spontaneous surface Mn oxidation, driving the formation of a defective spinel-like phase during solid-state synthesis. This anomalous phase, enriched with Jahn–Teller distorted Mn^4−x species and oxygen vacancies, disrupts the intended protective shell. We elucidate that this altered electronic structure creates a highly reactive interface. At this interface, electron-rich oxygens act as strong Lewis bases. Consequently, they catalyze aggressive electrolyte decomposition via both direct nucleophilic attack and indirect hydrolysis pathways. To clearly isolate this fundamental degradation mechanism, we utilized a model system with an exceptionally high Ni content (>95%). In this highly sensitive system, the reactive interface triggers detrimental crosstalk with the graphite anode, accelerating capacity fading two-fold over 1000 cycles. Crucially, we show that this deleterious phase transition can be mitigated by modulating the synthesis stoichiometry. Specifically, a threefold increase in excess lithium effectively suppresses the defective spinel formation and restores Mn–O covalency. This fundamental structural restoration significantly enhances electrochemical stability, achieving a capacity retention of over 90%. These findings highlight the extreme sensitivity of surface Mn chemistry and offer a vital strategy for engineering robust high-energy cathodes.