Voltage-Dependent Li Kinetics Leads to Charge-Discharge Asymmetry in Co-Free Li-Rich Li_1.12Ni_0.44Mn_0.44O₂ under Conditions without Transition Metal Migration

Abstract

Understanding factors that influence Li-ion kinetics in positive electrode materials is important for development of Li-ion batteries capable of operating at high rates and low temperatures. Herein, the impact of rate and operating temperature on Li diffusion was studied to explain charge-discharge asymmetry where lithiation is kinetically hindered compared to delithiation. Co-free, Li-rich Li1.12Ni0.44Mn0.44O2 materials with different secondary particle sizes of 4 and 16 μm, but nearly identical primary particle sizes, lattice parameters, and percentage of Ni atoms in the Li layers, were chosen to demonstrate how worsened Li kinetics during lithiation leads to charge-discharge asymmetry. Cells were cycled between 4.4–1.5 V to eliminate transition metal migration and to understand Li kinetics as a function of state of charge. Measurements were done on an Ultra-high precision charger to measure apparent capacity loss due to kinetic limitations. At low and moderate rates, the cell capacity was fully recovered by allowing the cell voltage to pass below 2 V where a voltage plateau corresponding to a Li2MO2-like phase was observed. Li ion diffusion measurements confirmed that Li diffusion is slower at low voltage. Taken together with the rate and temperature dependence, the charge-discharge asymmetry is fully accounted for solely from Li-ion kinetic limitations.