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 mu m2 min-1 for LiC12 and 1,619 +/- 51 mu m2 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 mu m2 min-1) and dissolution rate (-676 +/- 30 mu m2 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.

Automated summary

Li metal deposition and internal short circuits in full cells are observed using in situ optical microscopy. Different conditions and effects, such as lithiation gradients, dendrite growth, dissolution rate, self-discharge, and the importance of separator pores, are discussed.