Symmetric cell approach towards simplified study of cathode and anode behavior in lithium ion batteries

It was observed that the impedance of 18 650 high-power cells using LiNi0.8Co0.2O2 chemistry increased with time during the beginning period of storage. A symmetric cell approach was adopted to isolate the effects of the positive and negative electrodes on the impedance of a laboratory-scale lithium ion cell using the same chemistry. The AC impedance spectra of a normal cell and symmetric cells were measured and compared. The cathode processes were found to be most responsible for the cell impedance and the impedance rise. In addition, the impedance spectra provide indirect evidence that surface layers might be formed on both the positive and negative electrodes.