Decrease in Capacity in Mn-Based/Graphite Commercial Lithium-Ion Batteries

Abstract

Commercially available lithium-ion batteries (LiMn2O4/LiNi0.8Co0.15Al0.05O2 mixed cathode and graphite anode) are disassembled to determine the reversible capacity of each electrode, the state of charge (SOC) in the operation range, and the lithium content in the graphite anode by inductively coupled plasma optical emission spectroscopy (ICP-OES) after a cycle or storage operation. The origin of the decrease in capacity of the battery is attributed to (i) the decrease in capacity of the cathode active material and (ii) the limited cathode operation range. This leads to a shift to a high SOC of the cathode owing to the irreversible loss of lithium at the anode. We quantitatively explain all degraded battery capacities using the above two factors. The shift in capacity is obtained by discharging from the constant disassembly conditions at a constant open-circuit voltage (OCV), which cannot be obtained by electrochemical analysis of the anode. The determined shift in capacity has a strong correlation with the amount of irreversibly accumulated lithium at the anode determined by ICP-OES.