In situ X-ray diffraction studies on nominal composition of C₂Li under high pressure and temperature

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