Re-entrant lithium local environments and defect driven electrochemistry of Li- and Mn-Rich Li-Ion battery cathodes

Direct observations of structure-electrochemical activity relationships continue to be a key challenge in secondary battery research. ⁶Li magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy is the only structural probe currently available that can quantitatively characterize local lithium environments on the subnanometer scale that dominates the free energy for site occupation in lithium-ion (Li-ion) intercalation materials. In the present study, we use this local probe to gain new insights into the complex electrochemical behavior of activated 0.5⁶Li₂MnO₃·0.5⁶LiMn_0.5Ni_0.5O₂, lithium- and manganese-rich transition-metal (TM) oxide intercalation electrodes. We show direct evidence of path-dependent lithium site occupation, correlated to structural reorganization of the metal oxide and the electrochemical hysteresis, during lithium insertion and extraction. We report new ⁶Li resonances centered at ∼1600 ppm that are assigned to LiMn₆-TM_tet sites, specifically, a hyperfine shift related to a small fraction of re-entrant tetrahedral TMs (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.