Expanded interlayer spacing of graphene oxide achieved by electrostatic cation intercalation towards superior sodium ion storage

Graphite has been employed as anode material of lithium ion batteries due to its low cost, unique layered structure, and high conductivity; however, the small interlayer spacing and poor rate capability limit its application in sodium ion batteries. To address these issues, the interlayer spacing of graphene oxide (GO) was controllably enlarged through K + and Ca 2+ pillaring by the electrostatic interaction between the negatively charged GO sheets and the positively charged K + /Ca 2+ . The K + /Ca 2+ pillared in the interlayers of GO can controllably expand the interlayer spacing from 0.78 to 1.01 nm by regulating the K + /Ca 2+ concentrations. The K + /Ca 2+ -pillared GO (K + /Ca 2+ -GO) exhibits high Na + ion storage performance because of expanded interlayer spacing, showing large Na + ion diffusion coefficient (the largest D Na + is 43.8 × 10 −15 cm 2 s −1 ) and high reversible specific capacity (199.3 mAh g −1 at 0.1 A g −1 ). Meanwhile, the 2D layered structure of GO is stabilized by the pillar effects of K + /Ca 2+ to realize a superior cycle stability of Na + insertion/extraction. The relations between the interlayer spacing of K + /Ca 2+ -GO and rate capability are studied and an optimum interlayer spacing of K + /Ca 2+ -GO for high rate Na + storage (0.84 nm and 1.01 nm for K + -GO and Ca 2+ -GO) is obtained. The results provide an essential reference for design of high rate 2D energy storage materials.