Li-ion kinetics and polarization effect on the electrochemical performance of Li(Ni_½Mn_½)O₂

Abstract

Galvanostatic intermittent titration technique (GITT) at room temperature (25 °C) and at 50 °C and electrochemical impedance spectroscopy (EIS) at 25 °C as a function of cell voltage and cycle number were performed on the layered cathode, Li(Ni_½Mn_½)O₂ and the results were correlated with its known electrochemical performance. Comparison of the voltage versus capacity profiles and the cycling performance reveals that a higher extraction and reversible capacity can be obtained by means of increasing the upper cut-off voltage or by changing the cycling mode (from constant-current (CC) to constant-current-constant-voltage (CCCV)) at 25 °C or by increasing the temperature to 50 °C for V > 4.2 V versus Li metal, but this leads to capacity-fading over 50 cycles. A minimum in the Li-ion diffusion coefficient (D_Li) versus voltage curve was observed at ~ 3.8 V coinciding with the plateau region in the charge-discharge profiles. The D_Li is almost unchanged with a value of 3.0(±0.5)×10⁻¹⁰ cm²/s in the voltage range, 3.8-4.3 V versus Li. EIS studies as a function of cycle number after charging the cells to ~ 3.8 and 4.4 V, after they were cycled in the CC and CCCV modes show that while the surface film and the bulk contribution of the cell resistances remain small and unchanged, a significant increase in the charge transfer resistance (R_ct) occurs, revealing a pronounced electrode polarization. This polarization may be the reason for the observed capacity-fading in Li(Ni_½Mn_½)O₂ when charged to voltages > 4.2 V.