Bulk-Doped Heavy Element Mitigating the Ligand-to-Metal Charge Transfer of LiCoO2 toward its Robust High-Voltage Charging Stability

Abstract

The anionic redox reaction (ARR), which includes ligand-to-metal charge transfer (LMCT), is a key challenge in the stabilization of the rhombohedral symmetry of LiCoO2 (LCO) cathodes above 4.6 V. One of the promising strategies for suppressing LMCT is to employ heavy-element dopants, which can increase the oxygen dissociation energy of LCO cathodes. However, a correlation between heavy-element doping and the modulated LMCT has not yet been clarified. Herein, the effects of two different heavy-element dopants, W and Sb, on the LMCT of LCO cathodes have been investigated by varying the charging cutoff voltage from 4.6 to 4.8 V. Although W, preferentially segregated onto the crystal surface of LCO, helps to stabilize its surface structure, the inherent surface W occupancy could not modulate Co-O covalency and LMCT, failing to prevent oxygen loss and microcrack evolution from the LCO lattice under high-voltage charging (above 4.6 V). By contrast, Sb, occupying the 3a site of Co, properly modulates Co-O bond length and covalency during charging, thereby suppressing the LMCT and extending the stability of the oxygen framework toward superior cyclic retention upon charging up to 4.8 V. This finding indicates that sustaining the oxygen framework of LCO through a regulated LMCT is one of the keys to improve the energy density of LCO, allowing an increase in charging voltage above 4.6 V. This provides a valuable insight into the importance of the oxygen framework and the interplay between transition metals and oxygen as a determinant for the stability of rhombohedral layered cathode materials.