Electrochemical intercalation of rubidium into graphite, hard carbon, and soft carbon

Abstract

The electrochemical insertion of Rb into carbonaceous materials, including graphite, was achieved herein. Rubidium ions were reversibly inserted into and extracted from graphite via electrochemical processes using different non-aqueous electrolytes containing rubidium bis(trifluoromethanesulfonyl)amide (RbTFSA) salts in carbonate esters, glymes, and ionic liquids, similar to the process used for other lighter alkali metal ions such as Li⁺ and K⁺. The chemical compositions of the rubidiated graphite were determined to be RbC₈, RbC₂₄, and RbC₃₆ at each step of the electrochemical reduction process. Graphite underwent a phase transition to RbC₈ exhibiting a stage-1 structure, with stage-3 RbC₃₆ and stage-2 RbC₂₄ as intermediates, as confirmed by ex situ and in situ X-ray diffraction and ex situ Raman spectroscopy, similar to the electrochemical phase evolution of staged potassium graphite intercalation compounds (K-GICs). Furthermore, Rb was reversibly inserted into and extracted from graphitizable and non-graphitizable carbons such as pitch-derived soft carbon and commercial hard carbon, along with other alkali metals such as Li, Na, and K.