Cycle-Life Characterization of Automotive Lithium-Ion Batteries with LiNiO2 Cathode

A set of lithium-ion cells containing a LiNi0.8Co0.15Al0.05O2-based positive electrode and a graphite negative electrode were cycled nonintrusively at high power (5C rate) and elevated temperature (40 degrees 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 N-1/2) 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 (R-w) , composed of charge-transfer kinetic resistance (R-ct) and Li+ transport resistance through the solid electrolyte interphase (SEI), R-SEI. More specifically, the cell-impedance rise comes mainly from the rising R-ct and R-SEI of the positive electrode. Based on individual electrode EIS spectra and equivalent-circuit analysis, it is found that the R-SEI rise in the positive electrode is far more influential than the change in R-ct. 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. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3126385] All rights reserved.