Stage-number dependence of intercalated species for fluorosilicate graphite intercalation compounds: pentafluorosilicate vs. hexafluorosilicate

Abstract

Most intercalated ions ever reported for graphite intercalation compounds (GICs) are singly charged, and the number of reports on GICs with doubly charged ions is limited. For fluorosilicate complex anions, only the intercalation of SiF5− was reported as stage-2 GICs, although SiF62− is more commonly known in inorganic compounds. In the present study, chemical states of fluorosilicate GICs (SiFy-GICs) are investigated along with the change in stage numbers. Syntheses of SiFy-GICs at various stage numbers (the mixtures of stage-5 and -4, stage-4 and -3, stage-3 and -2, and stage-3 and -2) clarify that SiFy-GICs at a low stage number have a larger gallery height than those at higher stage numbers. In addition, reactions of SiFy-GICs with PF5 formed PF6-GICs with large weight increase and stage-number decrease, which cannot be explained by the substitution of SiF4 with PF5 to intercalate PF6−. The model that SiF62− and SiF5− are present in GICs at high and low stage numbers, respectively (SiF62− for stage-n (n ≥ 3) and SiF5− for stage-2), can explain this phenomenon, suggesting intercalation of SiF62− into graphite for the first time and stage-number dependency of intercalated species for fluorosilicate GICs.