Solid-electrolyte interphase composition and homogeneity in industrial-scale Li-ion batteries – quantitative 3D tomography using AI-assisted ion beam analysis

Si-based anodes for Li-ion batteries improve battery performance, but introduce new fatigue mechanisms, reducing their cycle-life, compared to graphite anodes. Four 110 × 80 mm 2 electrodes of a 5.4 Ah NCA + Si/G cell after 340 cycles are analysed by ion-beam analysis in 345,000 points, providing a unique insight into the degradation of the Si-anode. Our results show an inhomogeneous state-of-charge (SoC) across the electrodes and Li immobilization at the Si-anode. After the initial formation cycling, the anode contains 10.2 atom% Li (SoC = 0), after 340 cycles this amount increases to 17.5 at.%. The F and P concentrations show most Li is immobilized as LiF. Li-silicates are only a minor fraction of the solid-electrolyte interphase, ruling them out as main driver of the accelerated degradation. The immobilized anode Li results in a cathode Li concentration drop from initially 21 at.% (SoC = 0) to 16 at.% Li after 340 cycles. This Li-loss correlates well with the aging related electrode-potential shift of 130 mV known from literature. A ±6% cathode layer thicknesses inhomogeneity is revealed through the areal density of Ni. The Li loss requires 480 mAh/g Si pre-lithiation for compensating the observed Li-immobilization, which would increase cycle-life and the initial discharge capacity.