The Influence of Covalence on Capacity Retention in Metal-Substituted Spinels ⁷Li NMR, SQUID, and Electrochemical Studies

Variable-temperature ⁷Li nuclear magnetic resonance and superconducting quantum interference device (NMR and SQUID) magnetometry have been used to determine the supertransferred hyperfine coupling constant, an indicator of Li-O-Mn bond covalence, for metal-substituted spinels, LiM_xMn_2−xO₄ (M=Li, Zn, Al, Cr). The Curie and Weiss constants are reported for all compositions studied. The covalence of the Li-O-Mn bond is observed to increase upon substitution for all substituting metals studied. The greatest enhancement in covalence was achieved by substitution of ions that are similar in size to the Mn^3.5+ ion. The covalence values are compared to electrochemical capacity retention of the same compositions when cycled on the 4 V plateau. A positive correlation between the effectiveness of a particular substituent at improving capacity retention and the effectiveness of that substituent at enhancing covalency is observed. The bond covalence for LiCr_0.175Mn_1.825O₄ remains unchanged upon electrochemical cycling, whereas that of LiMn₂O₄ increases with cycling.