Continuous monitoring of the bulk oxidation states in $LixNi1/3Mn1/3Co1/3O2$ 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.