Changing Established Belief on Capacity Fade Mechanisms: Thorough Investigation of LiNi1/3Co1/3Mn1/3O2 (NCM111) under High Voltage Conditions

The further development of lithium ion batteries: operating at high voltages requires basic understanding of the occurring capacity fade mechanisms. In this work, the overall specific capacity loss with regard to reversible and irreversible processes for LiNi1/3Co1/3Mn1/3O2 (NCM111)/Li half cells, cycled at a charge cutoff potential of 4.6 V vs Li/Li+, has been investigated in detail. By means of total X-ray fluorescence (TXRF) technique it was shown that specific capacity losses associated with the amount of dissolved transition metals are negligible, implying a still intact NCM111 active material after 53 cycles. It was demonstrated that the specific capacity fade during cycling at constant specific currents can be mainly attributed to the increase of the delithiation (charge) hindrance, whereas lithiation (discharge) hindrance is only present after a specific current increase, leading to, apparent specific capacity losses, and to decreased Coulombic efficiencies. This could be proven by the determination of the NCM lithiation degree in the discharged state with inductively coupled plasma optical emission spectroscopy (ICP-OES). Moreover, by, decreasing the kinetic hindrance in the NCM material, it was shown that most of the observed specific capacity losses after 53 cycles are reversible: The influence of the active material and the cathode electrolyte interphase (CEI) on the specific capacity fade has been discussed. The results of the X-ray photoelectron spectroscopy (XPS) studies revealed that the CEI thickness is predominately dependent on the applied temperature (thermal-chemical origin) rather than the applied electrode potential (electrochemical origin). Finally, the absence of a fade in specific capacity for LiNi0.5Mn1.5O4 (LNMO) at an even higher charge cutoff potential of 4.95 V vs Li/Li+ points to a strong active material dependence than solely to the impact of electrolyte decomposition and CEI formation.