Solid-state synthesis and electrochemical characterization of LiMyCr0.5−yMn1.5O4 (M = Fe or Al; 0.0 < y < 0.4) spinels

A series of LiMyCr0.5−yMn1.5O4 compositions, where M is either Fe or Al, was synthesized by a conventional solid-state fusion method. Phase-pure materials were obtained for all the Fe-substituted compositions, while at Al stoichiometries of 0.3 and 0.4 the Al-substituted compositions had γ–LiAlO2 as an impurity phase. An increase in the amount of Fe or Al increased the lattice parameters as well as the propensity of these co-dopants to occupy the 8a lithium sites. Electrochemical activity was noted in the 4 and 5-V regions. Fe as a co-substituent increased the currents associated with the high-voltage peaks, while Al as a co-substituent enhanced the high-voltage capability of the spinel. Irrespective of whether the co-substituent was a transition metal or non-transition metal, it altered the electrochemical characteristics of both the Mn4+/Mn3+ and the Cr4+/Cr3+ couples, the effect being more pronounced in the 5-V region. Although increased amounts of Fe or Al rendered the spinels high-voltage active, both the deliverable capacity and capacity retention were lowered. The cumulative capacity obtained with Al-substituted materials was less than with Fe-substituted materials. At a 0.1 C rate between 3.3 and 5.1 V, the Fe co-substituted spinel (y=0.1) gave a first-cycle capacity of 117 mAh g−1, while that with Al was 112 mAh g−1. The corresponding values in the 40th cycle were 88 and 82 mAh g−1.