The effects of compression on lithium-ion batteries

Abstract

This thesis experimentally investigates the effects of compression on pouch cells. Cells made from stacked electrode layers, were compressed between 0 MPa – 0.43 MPa (@ 40°C,50% SOC) and cycled 2000 times. It was found that pressure had no effect on the rate of degradation. Compression improved the volumetric energy density by 10% at 0.43 MPa down to a 5% improvement at 0.02 MPa. The response of lithium-ion batteries to compression throughout its lifetime can be categorised into four stages: I. Initial compression to the battery: - Pressure is applied from 0 MPa to a target pressure. II. Viscoelastic relaxation, with no change in SOC or temperature: - Passive relaxation of the cell, i.e. pressure/strain relaxes. III. Further relaxation of the battery as a result from electrochemical cycling: - During electrochemical cycling, the pressure/strain of the cell reduces for any given SOC. IV. The operational stage of the battery: - upon completion of Stage III, increases in strain/pressure occur as the battery ages. Stage IV is the most important when the battery is in operation. Stages I – III take place during set-up. The assumption that the pressures/strains at the beginning of Stage IV are similar to those applied in Stage I are incorrect. To define the mechanical state of a compressed cell, the conditions at the end of Stage III should be used and the SOC and temperature declared (30% SOC and 20°C are proposed as a standard). Compression at different rates showed that the response of a battery was highly variable, with no correlation between the measured strains at the end of Stage II and the rate at which the compression was applied in Stage I. Thickness changes due to thermal expansion and during charge/discharge were found to be of the same magnitude. Hence, future compression experiments require accurate thermal management.