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Measurements of the open circuit voltage of Li-ion cells have been extensively used as a non-destructive characterisation tool. Another technique based on entropy change measurements has also been applied for this purpose. More recently, both techniques have been used to make qualitative statements about aging in Li-ion cells. One proposed cause of cell failure is point defect formation in the electrode materials. The steps in voltage profiles, and the peaks in entropy profiles are sensitive to order/disorder transitions arising from Li/vacancy configurations, which are affected by the host lattice structures. We compare the entropy change results, voltage profiles and incremental capacity (dQ/dV) obtained from coin cells with spinel lithium manganese oxide (LMO) cathodes, Li1+yMn2-yO4, where excess Li y was added in the range 0 ≤ y ≤ 0.2. A clear trend of entropy and dQ/dV peak amplitude decrease with excess Li amount was determined. The effect arises, in part, from the presence of pinned Li sites, which disturb the formation of the ordered phase. We modelled the voltage, dQ/dV and entropy results as a function of the interaction parameters and the excess Li amount, using a mean field approach. For a given pinning population, we demonstrated that the asymmetries observed in the dQ/dV peaks can be modelled by a single linear correction term. To replicate the observed peak separations, widths and magnitudes, we had to account for variation in the energy interaction parameters as a function of the excess Li amount, y. All Li-Li repulsion parameters in the model increased in value as the defect fraction, y, increased. Our paper shows how far a computational mean field approximation can replicate experimentally observed voltage, incremental capacity and entropy profiles in the presence of phase transitions.

The propagation of strain within a commercial LiCoO2 (LCO) electrode for lithium-ion batteries is investigated during cycling. An experimental multiscale approach is combined with microstructural, mechanical simulations. The crystal structure exhibits a volume change of 2.32% measured by in operando X-ray diffraction (XRD) measurements. The resulting change in the electrode thickness is about 1.8% and is measured by electrochemical dilatometry. The width of the electrode, volume fraction of active material, and binder geometry all affect the electrode deformation; this is investigated using a representative spherical particle model (RSPM). Thereby, the anisotropic swelling behavior of the electrode is verified, as the in-plane expansion of the electrode is restricted by interactions between the particles, binder, and the current collector. SEM images of the electrode are used to model the electrode expansion in a realistic microstructure. The simulation reveals that load paths form inside the electrode and cause stress peaks inside the binder material. To compare the 2D simulations with experimental data, a 3D RSPM is constructed. Based on these findings, we propose an equation that predicts the expansion of electrodes based on characteristics of the crystal structure.

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