Quantifying Capacity Loss Mechanisms of Li Metal Anodes beyond Inactive Li0

Capacity losses in Li anodes derive from the formation of electronically isolated Li0 and parasitic reactions that form the solid electrolyte interphase (SEI). Loss quantification has focused on Li0, but SEI losses increasingly dominate at high Coulombic efficiency (CE), becoming the loss mode to minimize. Here, we apply quantitative titration to track an array of key SEI phases: ROCO2Li, Li2C2, RLi, LiF, P-containing phases, and total Li loss, which teach new insights beyond inactive Li0. In 1 M LiPF6 EC/DEC, we demonstrate chemical resolution up to 71% of Li loss and 33% of SEI loss inventory. Expanding to additional carbonate electrolytes, ROCO2Li was consistently the major quantifiable SEI phase, but proportions were invariant with CE. Instead, Li2C2, a minor phase, exhibited clear inverse correlation with CE. These results demonstrate that, while minor phases often receive less focus and are harder to characterize, they can play governing roles in SEI function.