Visualizing electrolyte dynamics and monitoring salt concentration to improve commercial Si-based Li-ion batteries

The race for Li-ion batteries with higher energy density has led researchers toward Si-rich/Carbon composites, though their >200 % volume change induces strong electrolyte dynamics. The challenge is therefore to understand how this electrolyte dynamics contributes to cell formation and degradation under real operating conditions. Herein, we answer this question by combining optical calorimetry, and use of multiplexed tilted fiber Bragg grating sensors (TFBGs), to monitor electrolyte motion and Li-ion concentration gradient within a cell. For proof of concept, we used 21700 cylindrical prototype cells based on either graphite or SiC composite as negative electrode. We found a continuous and irreversible heat generation associated with the solid electrolyte interphase (SEI) formation throughout the entire charging process for SiC, unlike graphite-based cells. In addition, we provided evidence of reversible changes in hydrostatic pressure in SiC cells during cycling, related to the real-time movement of the electrolyte. Interestingly, the concomitant expansion-contraction and electrolyte movement caused depletion and inhomogeneous LiPF6 concentration, with nearly 35 % in the bottom area and 10 % in the middle area of the cell mandrel after 100 cycles. These insights, obtained through operando optical detection of cylindrical cells, should be of great help to battery manufacturers in streamlining formation protocols and reducing manufacturing costs.