Insights into the Degradation Mechanism of Aqueous LiMn₂O₄-Cathode Interfaces by Computational Vibrational...

${\mathrm{LiMn}}_2{O}_4$ (LMO) is a versatile and earth-abundant Li intercalation compound of high interest for sustainable energy storage and conversion. Yet, its broader adoption as a cathode in Li-ion batteries or as a water-splitting electrocatalyst is hindered by severe degradation reactions at the solid-electrolyte interface. This instability is rooted in its mixed-valence ${\mathrm{Mn}}^{3+}$ / ${\mathrm{Mn}}^{4+}$ electronic structure, which is governed by the Li content. Still, the atomistic link between the lithiation state and interfacial degradation remains elusive. Here, we investigate aqueous LMO interfaces across varying lithiation states by atomistic simulations using an ab initio-quality machine learning potential. We demonstrate that the local Li content determines surface Mn oxidation states and reveal their decisive role in interfacial acid-base chemistry. By analyzing the vibrational features of interfacial species, we identify the O-H stretching band as a sensitive spectroscopic probe of the surface electronic structure. Distinct, oxidation-state-dependent shifts reveal a fundamental vulnerability of the mixed-valence spinel surface: the coexistence of Lewis-acidic ${\mathrm{Mn}}^{4+}$ centers and oxygen sites neighboring ${\mathrm{Mn}}^{3+}$ , which are susceptible to electrophilic attack. These findings provide a compelling atomistic rationale extending the conventional ${\mathrm{Mn}}^{3+}$ -centric view of acid-promoted Mn dissolution toward a dual-site mechanism driven by LMO's mixed-valent nature.