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.