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9 7 I H I G H L I G H T S 2 0 2 1

Multiphase, multiscale chemomechanics of Li-ion batteries at low temperatures

Multiphase, multiscale chemomechanics of composite lithium-ion battery cathodes under low- temperature conditions have been systematically elucidated by advanced synchrotron techniques. The results suggest that, in order to design batteries for use in a wide temperature range, it is critical to develop electrodes that are structurally and morphologically robust when the temperature is switched.

Understanding the behaviour of lithium-ion batteries (LIB) under extreme conditions, e.g. low temperature, is key to a broad adoption of LIBs in various application scenarios. A significant irrevocable capacity loss is important but not well understood. It is thus crucial to gain more insights into the mechanisms causing the LIB performance degradation upon exposure to low-temperature conditions, which could aid engineering of active materials, electrode structure and charge/discharge protocols.

By using a suite of advanced synchrotron techniques including hard X-ray phase-contrast nanoholotomography at beamline ID16A, X-ray powder diffraction (XRD), X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS) and full-field transmission X-ray microscopy (TXM), this study systematically elucidates the multiphase multiscale chemomechanics in nickel-manganese-cobalt oxide (NMC) composite electrodes at low temperatures.

It is observed that, upon exposure to low temperature and then recovering to room temperature, the reversible and anisotropic lattice deformation could lead to irreversible cracking of active cathode particles. A closer look at the spatial arrangement of the cracking patterns over this particle reveals interesting behaviour (Figure 77). While the cycling-induced damage is more severe over the core region of the particle, in good agreement with a number of reported experimental observations [1], the low- temperature-induced cracks exhibit a rather flat depth- dependence profile (Figure 77c). The low-temperature exposure predominantly damages the sub-particle regions that are more intact after electrochemical cycling

Fig. 77: TXM characterisation of single NMC particle damages. a) A 3D rendering of single NMC particle with its central slice and segmented different phases. b) Two selected regions of interest, as indicated in (a), are compared before and after the low- temperature exposure. c) Depth-dependent profiles of the damages caused by electrochemical cycling and by exposure to -40oC. d) The spatial association of the cycling-induced cracks and the low-temperature-induced damages, showing a mild negative correlation. e) The relative frequency histogram of cycling-induced damage and low-temperature-induced damage as a function of the distance from the nearest pore phase.