Structural Studies of Capacity Activation and Reduced Voltage Fading in Li-Rich, Mn-Ni-Fe Composite Oxide Cathode

Li-rich Mn-Ni-Fe (MNF) oxide cathodes are emerging as a low-cost alternative to commercial Ni-Mn-Co (NMC) oxides with the cost of raw iron being three orders of magnitude lower than cobalt. MNF cathodes have demonstrated potential for high capacity and high discharge voltage cathodes, however, the capacity decay and instability of discharge voltage upon cycling still need to be resolved for their successful commercialization. Both phenomena are related to the changes in structure and in this study, we utilize SXRD and XAFS to investigate the structural changes correlated to electrochemical performance of Li 1.2 Mn 0.5 Ni 0.2 Fe 0.1 O 2 cathode. This material, prepared by sol gel synthesis, showed initial discharge capacity of 226 mAhg −1 and 93% capacity retention after 100 cycles. Fitting of the XAFS results provided information on the changes in local environment of each transition metal atom. Such detailed information on atomic environment is reported for the first time and compared to prior studies of NMC cathodes. In the discharged state Mn atoms show a shift to lower oxidation state and an irreversible loss of oxygen near neighbors after cycling, while Ni and Fe show only minor changes in their environment. An observed 5.87% lattice expansion after cycling is suggested to contribute to voltage fade.