Origins of Divergent Cycling Trajectories of Lithium Metal Anodes Revealed by Titration Methods

Abstract

Despite their near-ubiquitous use as additives in carbonate-based electrolytes for Li-ion batteries, the divergent effects of vinylene carbonate (VC) and fluoroethylene carbonate (FEC) on the cycling performance of Li metal anodes is not fully understood. Here, we employed cycle-by-cycle titration-based analysis to interrogate the evolving cycling performance of a baseline carbonate electrolyte (1 M LiPF6 in ethylene carbonate/diethyl carbonate, 1:1 v/v, “LP40”) without and with the addition of VC or FEC. FEC led to both higher and more consistent Coulombic efficiencies (CE) compared to the baseline, while VC generated no significant improvement in CE and more erratic cycling. These differences originated in different relative rates of inactive Li0 accumulation, as all three electrolytes formed SEI at the same rate. Further SEI analysis revealed that although all three electrolytes formed Li2O initially, only LP40 + FEC continued generating it throughout cycling; LP40 + FEC also had the densest deposit morphology, least resistive SEI, and most LiF formation. Together, these observations combine to reveal how subtle differences in SEI dramatically alter CE trajectories: the Li2O- and LiF-rich, ionically conductive SEI generated by FEC promotes dense deposits and stable, high-efficiency Li0 removal, while VC’s more resistive SEI culminates in mossy deposits, progressively deteriorating surface stability, and inefficient stripping.