Influence of Li-ion kinetics in the cathodic performance of layered Li(Ni_1/3Co_1/3Mn_1/3)O₂

Abstract

Galvanostatic intermittent titration technique (GITT) at 25 and 50°C and electrochemical impedance spectroscopy (EIS) at 25°C as a function of cell voltage and cycle number were performed on the layered oxide cathode, Li(Ni_1/3Co_1/3Mn_1/3)O₂. The Li-ion diffusion coefficient D_Li(GITT) obtained for GITT was stable with a value of ~3 ×10⁻¹⁰ cm²/s in the voltage range, 3.8-4.4 V vs. Li. The D_Li(EIS) was higher by an order of magnitude than the D_Li(GITT) in the above voltage range. A minimum in the D_Li vs. voltage curve was observed at ~3.7 V coinciding with the voltage-plateau region in the charge-discharge cycling curves, indicating a possible reversible structural phase transition or order-disorder transition in the compound. EIS studies as a function of cycle number show that the surface film and the bulk contribution to the cell resistances remain stable with cycling. The proportional increase in charge-transfer resistance (R_ct) with cycling observed when charged to 50 mAh/g (~3.7 V) and charged to 4.4 V of the cell indicate the possible influence of phase transition on the charge-transfer kinetics. The variation of D_Li derived from GITT and EIS as a function of cell voltage and D_Li(EIS) with the cycle number and the kinetic parameters obtained from the impedance plots were correlated with the electrochemical performance.