Charging processes in the cathode LiNi0.6Mn0.2Co0.2O2 as revealed by operando magnetometry

Operando magnetic susceptibility measurements of the cathode material LiNi0.6Mn0.2Co0.2O2 during repetitive electrochemical cycling enable a continuous and volume sensitive monitoring of the transition metal oxidation states. Upon charging a simultaneous oxidation of Co2+- and Ni2+-ions down to a Li-content of x=0.53 occurs, followed by an exclusive oxidation of Ni3+-ions for 0.53>x>0.21. In the low Li concentrations regime x<0.21 Co3+-ions were found to be involved in the charge compensation process although a sufficient amount of Ni3+-ions is still present in the cathode material. While the oxidation process of Ni2+- and Co2+-ions is reversible, the Ni3+- and Co3+-ion oxidation process is found to be at least partially irreversible.