Superdense phase between graphite and Li metal, \(\hbox {C}_{2}\) Li, has been significant in the research on both lithium-ion batteries (LIBs) and graphite intercalated compounds (GICs). However, a detailed method for synthesizing \(\hbox {C}_{2}\) Li remains unknown owing to the limited information regarding \(\hbox {C}_{2}\) Li and difficulties in distinguishing \(\hbox {C}_{2}\) Li from \(\hbox {C}_{6}\) Li. Thus, we performed in situ X-ray diffraction measurements on samples with the nominal composition of \(\hbox {C}_{2}\) Li under high pressures and temperatures of up to 10 GPa and 400 \(^{\circ }\hbox {C}\) , respectively. We employed two types of \(\hbox {C}_{2}\) Li samples; one was a mixture of \(\hbox {C}_{6}\) graphite powder and Li metal (C \(_{6}+3\) Li), and the other was a mixture of \(\hbox {C}_{6}\) Li and Li metal in which the \(\hbox {C}_{6}\) Li was prepared by the electrochemical discharge (reduction) reaction that occurs in LIBs. Considering changes in the d-value based on the 001 diffraction peak from \(\hbox {C}_{6}\) Li or \(\hbox {C}_{2}\) Li, C \(_{6}\) Li + 2Li is suitable for synthesizing \(\hbox {C}_{2}\) Li, although the nonaqueous electrolyte used for the electrochemical reaction should be removed to avoid structural transformations to lower-stage compounds such as \(\hbox {C}_{12}\) Li and \(\hbox {C}_{18}\) Li during the heating. These findings pave the way toward a method for synthesizing C \(_{2}\) Li, which could increase the energy density of LIBs and establish GICs with novel physical and electronic properties.
