Diagnostic evaluation of detrimental phenomena in high-power lithium-ion batteries

A pouch-type lithium-ion cell, with graphite anode and LiNi0.8Co0.15Al0.05O2 cathode, was cycled at C/2 over 100% depth of discharge at ambient temperature. The LiNi0.8Co0.15Al0.05O2 composite cathode was primarily responsible for the significant impedance rise and capacity fade observed in that cell. The processes that led to this impedance rise were assessed by investigating the cathode surface electronic conductance, surface structure, composition, and state of charge at the microscopic level with the use of local probe techniques. Raman microscopy mapping of the cathode surface provided evidence that the state of charge of individual LiNi0.8Co0.15Al0.05O2 particles was nonuniform despite the deep discharge at the end of cell testing. Current-sensing atomic force microscopy imaging revealed that the cathode surface electronic conductance diminished significantly in the tested cells. Loss of contact of active material particles with the carbon matrix and thin film formation via electrolyte decomposition not only led to LiNi0.8Co0.15Al0.05O2 particle isolation and contributed to cathode interfacial charge-transfer impedance but also accounted for the observed cell power and capacity loss.