Electrolyte optimization, such as using fluoride-bearing electrolytes, is regarded as an effective way to improve the cycle performance of lithium metal batteries (LMBs), but the promotion mechanisms of the electrolytes are in controversy due to the lack of quantitative understanding of the reaction products during cycling. Here, taking several fluorinated electrolytes as models, we use mass spectrometry titration (MST) and solid state nuclear magnetic resonance (NMR) techniques to quantify the evolution of dead Li metal, solid electrolyte interphases (SEI) and lithium hydride (LiH) during cycling. Our quantitative results clearly disclose that lithium difluoro(oxalato)borate (LiODFB) is able to inhibit the formation of SEI and LiH while fluoroethylene carbonate (FEC) mainly inhibits the formation of dead Li metal. Furthermore, we surprisingly observe a linear correlation between LiH and SEI formation, whereas the commonly mentioned lithium fluoride (LiF) shows a weak correlation with either dead Li metal or SEI. Guided by the clear failure mechanism, we can provide a reasonable explanation for the synergistic effect with the combination of LiODFB and FEC from a quantitative perspective. We believe that a quantitative insight of electrolytes on the failure mechanism of LMBs will guide us to explore the functional electrolytes to achieve the practical application of LMBs.
Keywords: Anode failure mechanism; electrochemistry; lithium fluoride; lithium hydride; lithium metal batteries.