Direct observation of internal short circuits by lithium dendrites in cross-sectional lithium-ion in situ full cells

Li metal deposition and internal short circuits (ISC) are directly observed in full cells using a cross-sectional in situ optical microscopy set-up. The cell chemistries under investigation are graphite/LFP and graphite/NMC 622. Direct observations are made on (i) lithiation gradients without Li metal deposition, (ii) Li metal deposition without ISC, (iii) Li metal deposition with ISC by a dendrite growing above the separator, and (iv) Li metal deposition with ISC induced by mechanical closing of separator pores. The speed of the lithiation fronts in graphite/LPF cells was determined to be 2,736 ± 47 μm2 min−1 for LiC12 and 1,619 ± 51 μm2 min−1 for LiC6. By image processing (binarization) of in situ microscopy image sequences, the accumulated observable Li area as well as the projected two-dimensional areal growth (+1,215 ± 41 μm2 min−1) and dissolution rate (−676 ± 30 μm2 min−1), along with the turning point between deposition and dissolution were estimated. Li dendrite dissolution, re-intercalation, and reorganization under different conditions (CC vs. CV-charging), as well as self-discharge, the fuse-effect, self-healing of dendrites, the importance of overhang areas, and the effect of closed separator pores are discussed in this publication.