Electrochemical impedance spectra of full cells: Relation to capacity and capacity-rate of rechargeable Li cells using LiCoO2, LiMn2O4, and LiNiO2 cathodes

Electrochemical impedance spectra (EIS) are reported for rechargeable lithium cells using cathodes LiMyOx (M=Co, Ni, and Mn) prepared from casting and high-pressure compacting. The composite cathodes were cast from a slurry mixture consisting of 30% NMP solvent and 70% solid (91% LiMyOx, 3% PVDF and 6% KS44 graphite) onto a 25-μm thick Al current collector. The compacted cathodes were made from the cast cathode using a laboratory press. The most compacted cathodes have the smallest impedance and the highest specific capacity and capacity-rate. Three equivalent circuits are proposed according to the effect of compaction pressure on our EIS results and work reported in the literature. The EIS results and relationship to the capacity and capacity-rate are discussed using these circuits. It appears that the ohmic resistance of the composite cathode is an important factor in the overall resistance of the cell and unfavorably affects the performance (capacity and capacity-rate) of the cathode.