Unraveling the LiMn₂O₄/LiPON Interface Degradation in All-Solid-State Thin-Film Lithium Batteries

Spinel LiMn₂O₄ (LMO) is a low-cost cathode with limited lifespan in liquid electrolytes due to Mn dissolution but is expected to be improved in solid-state batteries by suppressing metal dissolution. Using LMO/LiPON/Li solid-state thin film battery as a model system, we demonstrate that LMO shows considerably worse cycling stability with a solid electrolyte than a liquid electrolyte, and the capacity fading mechanism is entirely different from its liquid equivalent. It is disclosed that the capacity fading originates from the LMO/LiPON interface degradation with cycling. An interfacial disordered LMO layer is formed after LiPON deposition, which not only impedes Li⁺ diffusion but also becomes a stress-concentrated region. During cycling, the interfacial layer near the LiPON side evolves into inactive Mn₃O₄/Li₂O together with Mn diffusion into LiPON, while the interfacial layer near the LMO side induces lattice rotation toward the interior LMO film, resulting in fast structural degradation and rapid capacity decay.