Important Role of Functional Groups for Sodium Ion Intercalation in Expanded Graphite

Expanded graphite oxide (GO) has recently received a great deal of attention as a sodium ion battery anode due to its superior characteristics for sodium ion storage. Here, we report that the sodium ion intercalation behavior of expanded GO strongly depends on the amounts and ratios of different functional groups. The epoxide-rich GO shows significantly higher specific capacities than those of the hydroxyl-rich counterpart utilizing strong sodium-epoxide attractions and appropriately enlarged interlayer spacing during sodiation. The epoxide-rich GO also enables fast sodium ion transport on account of the diminishment of interlayer hydrogen bonds that could reduce the free volume. Our calculations suggest that the theoretical capacity of epoxide-only GO with a stoichiometry of Na_2.5C₆O₃ can reach 930 mAh g^-1, which is far higher than recent experimental results as well as even those of conventional graphite materials in lithium ion batteries.