X - R A Y N A N O P R O B E

S C I E N T I F I C H I G H L I G H T S

9 8 H I G H L I G H T S 2 0 2 1 I

(Figure 77d). The relative concentration distributions of the cycling damage and low-temperature damage show a higher degree of association between the low-temperature- induced cracks and the particle s intrinsic porosity (Figure 77e).

At the electrode scale, hard X-ray nanoholotomography offers excellent sensitivity to both the dense NMC particles and the porous matrix of low-Z carbon/binder domains (CBD). With the machine-learning model [2], hundreds of active NMC particles are identified automatically and the morphological deformation of different electrode components is quantified (Figure 78). The results suggest that the low-temperature environment deforms the NMC, CBD and the pore structure in a rather incoherent fashion,

which leads to the particle detachment from the CBD matrix. The deformation mismatch of different cathode components irreversibly provokes the development of local impedance and particle deactivation that could significantly degrade the LIB s fast-charging performance.

This work presents a fundamental understanding of the mechanisms behind the irreversible performance degradation of LIBs upon exposure to low temperature. Besides developing electrolytes that have stable performance, designing batteries for use in a wide temperature range also calls for developing electrode components that are structurally and morphologically robust when the cell is switched between different temperatures.

Fig. 78: Nanoholotomography characterisation of the composite cathode. a-c) 3D visualisation of the composite cathode with the identified NMC particles and segmented inactive phases. d) A representative lateral virtual slice with an enlarged region of interest

compared before and after the low-temperature exposure. e) The detachment caused by low temperature exposure is calculated particle by particle and the corresponding value is used to colour code the map in panel (e). f) The visualisation of the low-temperature-induced

structural deformation field over the region of interest shown in panel (d). g) The statistical distribution of the deformation vectors orientations (in degrees) of all NMC particles. h) The relative amplitudes of different phase deformations normalised to that of the NMC

particles. i) The relative angle of different phase deformations normalised to that of the void phase.