Re-Entrant Lithium Local Environments and Defect Driven Electrochemistry of Li- and Mn-Rich Li-Ion Battery Cathodes

⁶Li NMR spectroscopy is used to quantitatively characterize local lithium environments that dominate the free energy for site occupation and to monitor the evolution of local order and low concentration defect formation with the goal of correlating local structural changes with hysteresis and voltage fade phenomena observed in layered lithium and manganese rich TM oxide cathode structures. We have undertaken an isotopic enrichment strategy coupled with very long acquisition times to obtain unprecedented, and quantitative, high-resolution data for cycled electrodes using fully enriched cell components. This strategy has allowed the determination of structure-activity relationships and monitoring the evolution of local order and low concentration defect formation with the goal of correlating local structural changes with hysteresis and voltage fade phenomena. We report new ⁶Li resonances centered at ~1600ppm that are assigned to LiMn₆-TM_tet sites, specifically, a hyperfine shift related to a small fraction of reentrant tetrahedral transition metals (Mn_tet), located above or below lithium layers, coordinated to LiMn₆ units. The intensity of the TM layer lithium sites correlated with tetrahedral TMs loses intensity after cycling, indicating limited reversibility of TM migrations upon cycling. These findings reveal that defect sites, even in dilute concentrations, can have a profound effect on the overall electrochemical behavior.