Direct Experimental Probe of the Ni(II)/Ni(III)/Ni(IV) Redox Evolution in LiNi_0.5Mn_1.5O₄ Electrodes

The LiNi_0.5Mn_1.5O₄ spinel is an appealing cathode material for next generation rechargeable Li-ion batteries due to its high operating voltage of ∼4.7 V (vs Li/Li⁺). Although it is widely believed that the full range of electrochemical cycling involves the redox of Ni(II)/(IV), it has not been experimentally clarified whether Ni(III) exists as the intermediate state or a double-electron transfer takes place. Here, combined with theoretical calculations, we show unambiguous spectroscopic evidence of the Ni(III) state when the LiNi_0.5Mn_1.5O₄ electrode is half charged. This provides a direct verification of single-electron-transfer reactions in LiNi_0.5Mn_1.5O₄ upon cycling, namely, from Ni(II) to Ni(III), then to Ni(IV). Additionally, by virtue of its surface sensitivity, soft X-ray absorption spectroscopy also reveals the electrochemically inactive Ni²⁺ and Mn²⁺ phases on the electrode surface. Our work provides the long-awaited clarification of the single-electron transfer mechanism in LiNi_0.5Mn_1.5O₄ electrodes. Furthermore, the experimental results serve as a benchmark for further spectroscopic characterizations of Ni-based battery electrodes.