Cycle-life characterization of automotive lithium-ion batteries with LiNiO2 cathode

A set of lithium-ion cells containing a LiNi0.8 Co0.15 Al0.05 O2 -based positive electrode and a graphite negative electrode were cycled nonintrusively at high power (5C rate) and elevated temperature (40°C). The aged cells were characterized at prescribed cycle numbers (up to 5250 cycles) by a three-electrode cell, capacity measurement, and electrochemical impedance spectroscopy (EIS). Excellent cyclability of these cells under typical hybrid-electric vehicle conditions is demonstrated by 18% capacity fade after 5250 cycles, and the discharge capacity shows a mainly parabolic behavior with the cycle number (N) (dependent on N1/2) in the initial stage and a linear behavior (dependent on N) for subsequent cycles. Using a lithium reference electrode further reveals that the capacity fade during cycling is primarily caused by the positive electrode, where discharge capacity may be limited by a decrease in active lithium intercalation sites in the oxide particles. The increase in full-cell impedance with cycling is evident from the increase in midfrequency arc width (Rw), composed of charge-transfer kinetic resistance (Rct) and Li+ transport resistance through the solid electrolyte interphase (SEI), RSEI. More specifically, the cell-impedance rise comes mainly from the rising Rct and RSEI of the positive electrode. Based on individual electrode EIS spectra and equivalent-circuit analysis, it is found that the RSEI rise in the positive electrode is far more influential than the change in Rct. Therefore, property modification and thickening of the SEI layer of the positive electrode during cycling appear to be dominant factors in cell-impedance rise and power fade.