Hierarchical intercalation of tetrafluoroborate anion into graphite electrode from sulfolane

The anion storage behavior of graphite positive electrode in a dual-ion battery is closely related to the solvation of anion in the corresponding electrolyte solution. The classical electrolyte solutions of LiBF 4 -sulfolane (SL) have long been recognized in the community of lithium batteries and still appear promising in dual-ion batteries. Nevertheless, the solvation of BF 4 − by SL has seldom been addressed before. In this study, the solvation states of SL-BF 4 − are adjusted by varying LiBF 4 concentration or introducing auxiliary salts of LiPF 6 or SBPBF 4 (SBP: spiro-(1,1′)-bipyrrolidinium) in the electrolyte solutions of Li/graphite dual-ion cells. The electrochemical storage processes of SL-BF 4 − anions in graphite electrodes are investigated through in situ X-ray diffraction measurements. Two kinds of graphite intercalation compounds (GICs) with contrastive intercalation gallery heights (IGHs) have been discovered, which are ascribed to the storage of different kinds of SL-BF 4 − anions in graphite electrode. The interactions between ions and SL in the electrolyte solutions are characterized by Fourier transform infrared spectroscopy and then correlated with the performance of Li/graphite cells.