In Situ Raman and Nuclear Magnetic Resonance Study of Trapped Lithium in the Solid Electrolyte Interface of Reduced Graphene Oxide

Motivated by its high surface area and electrical conductivity, reduced graphene oxide (rGO) flakes have been intensively studied as potential anode materials for lithium ion battery (LIB). The high capacity in rGO (600-1000 mA h g(-1)) compared to graphite (372 mA h g(-1)) suggest that a different lithiation mechanism may be operational in the former. The high capacity of rGO should be attributed to its high surface area and associated defective sites, however, these may act as trapping sites and undergo side reactions with the solvent and lithium ions to form the solid electrolyte interphase (SEI) upon lithiation, resulting in irreversible capacity loss (ICL) during the initial cycles. Elucidating the temporal evolution of SEI layer on rGO and quantifying the amount of trapped lithium will be useful in developing strategies to mitigate the ICL process. Herein, the Li intercalation mechanism in rGO and graphite was investigated using in situ Raman spectroscopy and in situ time-resolved nuclear magnetic resonance (NMR) to provide insights into the origins of the high capacity and ICL loss in rGO. Finally, the dynamic and static SEI passive layer formed on rGO flakes was monitored, and a method was developed to quantify the amount of Li+ trapped in the SEI layer.