Continuous monitoring of the bulk oxidation states in Li_xNi_1/3Mn_1/3Co_1/3O₂ during charging and discharging

Operando magnetic susceptibility measurements on the Li𝑥Ni1/3Mn1/3Co1/3O2 cathode material during repetitive electrochemical cycling were performed, enabling a continuous and bulk sensitive monitoring of the charge compensation process. Upon charging and Li extraction down to the Li contents of x = 1/3, exclusively Ni undergoes oxidation in two consecutive steps, namely, Ni2+→Ni3+ for x > 2/3 and Ni3+→Ni4+ for 2/3 > x > 1/3 with a continuous transition in between. In the regime of low Li concentrations x < 1∕3, both Co- and O-ions contribute to the charge compensation. While the oxidation of Ni and O during charging is reversible, the Co oxidation is found to be irreversible. The authors thank C. Baumann for performing the dip-coating of the NMC cathodes. This work was performed in the framework of the inter-university cooperation of TU Graz and Uni Graz on natural science (NAWI Graz