Internal Redox Mediation Governs Charge-Discharge Kinetic Asymmetry in Microscale Li- and Mn-Rich Layered Oxides for...

Integrating Li- and Mn-rich layered oxides (LMRs) into all-solid-state batteries (ASSBs) offers a promising route toward high-energy-density systems beyond conventional lithium-ion batteries with Li[Ni_1‒x‒yCo_xMn_y]O₂ (NCM) cathodes and liquid electrolytes. Although microscale LMRs are attractive for practically relevant electrode-level energy density in sulfide-based ASSBs, their performance remains limited. Here, we elucidate the asymmetric charge-discharge kinetics of LMRs, originating from kinetically sluggish Mn/O redox in Li₂MnO₃-derived domains, as a critical bottleneck limiting operation of microscale LMRs (D₅₀ = 2-12 µm) in ASSBs. During charge, NCM domains facilitate Li₂MnO₃ delithiation via internal redox mediation, whereas the reverse process is ineffective during discharge, leading to pronounced kinetic asymmetry. This asymmetry results in underutilization of Mn/O redox, which is exacerbated by extended solid-state diffusion lengths, thereby causing progressive capacity decline with increasing secondary-particle size. Guided by this mechanistic insight, we introduce Li₂MoO₄ intergranular domains to enhance Li⁺ transport within secondary particles. This strategy enables microscale LMRs (D₅₀ = 2-4 µm) to deliver a high reversible capacity of 200 mA h g^-1 at 0.5 C with stable cycling over 600 cycles. This finding establishes microscale grain-boundary engineering as an effective approach to unlock the full potential of LMR cathodes for sulfide-based ASSBs.