Tracking Thermodynamic Changes Due to Cycling and Calendar Ageing of Commercial High-Energy Li-Ion Cells: Effects of Relaxation Periods

Abstract

There is a great need for non-invasive tools that enable improved insight into the underlying causes of battery ageing. In this project we investigate cycling and calendar ageing effects on high energy commercial cylindrical cells (NCA|Si-Gr), by tracking thermodynamic and kinetic aspects by the employment of entropy profiling, differential voltage and incremental capacity analysis (DVA/ICA), and differential thermal voltammetry (DTV). Two groups of three cells from the same batch are being subjected to different ageing scenarios, both at 45°C: group A is under calendar aging at 75 % state of charge - SoC at which we observed the most accelerated calendric aging for these cells - and group B, which is being cycled continuously at the maximum current allowed by the manufacturer. To ensure minimal temperature fluctuations during tests and allow proper comparative DTV, cells were submerged in a high conductivity dielectric fluid bath. We revised Osswald’s approach for entropy profiling [1], and also increased the number of titration steps in entropy profiling to 63, for both charge and discharge. Fig. 1 shows the results for the initial 40-day aging period, comparing ICA and entropy profiles, before and after ageing. In addition to the capacity fade of 2.2 % and 6.2 % for groups A and B respectively (Figure 1a), the loss of sharpness present in the ICA peaks is also present in the entropy curves (features A1-A4 in Fig. 1b). This can be attributed to a more heterogeneous lithium distribution and a change in lithium inventory. To track the reversibility of ageing and the time scales of passive electrode effects [2], both groups of cells will be submitted to rest periods after which entropy, DTV and ICA experiments will be periodically repeated.