%0 Journal Article %A Michael C Tucker %A Lenz Kroeck %A Jeffrey A Reimer %A Elton J Cairns %B Journal of The Electrochemical Society %D 2002 %G eng %P A1409-A1413 %R 10.1149/1.1510766 %T The Influence of Covalence on Capacity Retention in Metal-Substituted Spinels ⁷Li NMR, SQUID, and Electrochemical Studies %V 149 %8 09/2002 %X

Variable-temperature $^{7} Li$ 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, ${LiM}_{x} {Mn}_{2 - x} O_{4}$ $(M = Li,$ 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 ${LiCr}_{0.175} {Mn}_{1.825} O_{4}$ remains unchanged upon electrochemical cycling, whereas that of ${LiMn}_{2} O_{4}$ increases with cycling.