The solid–electrolyte interphase (SEI) is probably the least understood component in Li-ion batteries. Considerable effort has been put into understanding its formation and electrochemistry under realistic battery conditions, but mechanistic insights have mostly been inferred indirectly. Here we show the formation of the SEI between a graphite anode and a carbonate electrolyte through combined atomic-scale microscopy and in situ and operando techniques. In particular, we weigh the graphitic anode during its initial lithiation process with an electrochemical quartz crystal microbalance, which unequivocally identifies lithium fluoride and lithium alkylcarbonates as the main chemical components at different potentials. In situ gas analysis confirms the preferential reduction of cyclic over acyclic carbonate molecules, making its reduction product the major component in the SEI. We find that SEI formation starts at graphite edge sites with dimerization of solvated Li+ intercalation between graphite layers. We also show that this lithium salt, at least in its nascent form, can be re-oxidized, despite the general belief that an SEI is electrochemically inert and its formation irreversible.
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The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.
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This research was supported financially by the National Materials Genome Project (2016YFB0700600), the Guangdong Innovation Team Project (no. 2013N080) and Shenzhen Science and Technology Research Grants (nos. JCYJ20151015162256516, JCYJ20150729111733470 and JCYJ20160226105838578). J.Lu and K.A. acknowledge support from the US Department of Energy under contract no. DE-AC0206CH11357 with the main support provided by the Vehicle Technologies Office, Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy. Support provided by the China Scholarship Council (CSC) during a visit of T.L. to Argonne National Laboratory is acknowledged.
The authors declare no competing interests.
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