High-Resolution 6Li Solid-State Nuclear Magnetic Resonance Reveals Hidden Lithium Dynamics in All-Solid-State Cathodes

Abstract The decisive chemistry of iron-based halide cathodes─where lithium resides, how it evolves, and how disorder governs multielectron redox─has remained largely inaccessible, limiting the rational design of catholyte-free composite cathodes in all-solid-state lithium batteries. Here we make the lithium sublattice directly visible in paramagnetic LiFeCl3 using a 6Li solid-state NMR strategy combining high isotopic enrichment, optimized acquisition, and variable-temperature control. This method resolves and quantifies three Li+ environments, revealing that local Li/Fe disorder drives in-place Fe2+ reduction as the root cause of highly reversible intercalation-conversion chemistry. It further uncovers trace irreversible Fe0 species invisible to conventional spectroscopic and imaging techniques. Guided by this hidden degradation signature, optimization of the voltage window enables 85.9% capacity retention after 500 cycles with a 16.3% energy-density penalty, whereas full-depth cycling retains 68.6% after 100 cycles. Our work establishes lithium-resolved NMR as a platform linking disorder, hidden redox, and reversibility in paramagnetic cathodes.