In Situ NMR Investigations of Impacts of Electrolyte Additives on Lithium Metal Cycling in Anode-Free Lithium Metal Batteries

Introducing functional electrolyte additives which could significantly improve solid electrolyte interphase (SEI) composition and lithium deposition morphology is the most economical and effective way to mitigate active lithium loss and improve the cycling performance of anode free lithium metal batteries (AFLMBs). However, due to the lack of real-time detection methods, the precise impact of these additives on lithium deposition/stripping processes and dead lithium evolution still remains unclear. In this work, the effects of fluoroethylene carbonate (FEC) additive in ester-based electrolytes along with LiNO₃ additive in ether-based electrolytes on lithium deposition and dead Li formation in Cu||LiFePO₄ batteries are investigated by <i>in situ</i> nuclear magnetic resonance (NMR) in combinations with post-mortem scanning electron microscopy (SEM). Although the FEC additive shows a negligible impact on initial lithium deposition, it effectively mitigates dead lithium accumulation by altering its growth kinetics from exponential to linear with the cycle repeated. Comparative studies in ether-based electrolytes reveal that LiNO₃ additive significantly affects deposition morphology by promoting dense lithium deposition. However, it fails to suppress dead lithium accumulation as the growth kinetics still follow an exponential trend with the cycle repeated. The failure mode of Cu||LiFePO₄ batteries shifts from dead lithium dominated to SEI dominated after introducing FEC additive, while SEI dominated failure mode persists after introducing LiNO₃ additive. Our findings provide new insights into the role of FEC and LiNO₃ additives on the lithium metal deposition and dead lithium evolution, which could assist in improving the cycling performance of AFLMBs.