Interlayer Confined Capacitive Response via Solvated Cointercalation in Graphite Layers

Nanofluids confined within two-dimensional materials promote ionic flux, which is essential for achieving ultrahigh-rate capacitor-like responses and high charge storage capacity. Here, we offer quantitative and microscopic insights into the interlayer-confined electric double-layer (EDL) capacitive behavior arising from the cointercalation of Na⁺-xdiglyme ([Na-xG2]⁺) into graphite layers. By leveraging in situ nuclear magnetic resonance, electrochemical quartz crystal microbalance, embedded optical fiber sensors, and other techniques, it demonstrates that a nonconstant Na⁺:G2 ratio during cointercalation into graphite with the evolution of the stages. This aligns with the formation of graphite intercalation compounds (GICs) from stage >3 to 1, and a subsequent transition from battery-like intercalation to interlayer-confined EDL adsorption. The stage 1 GIC with an expanded spacing of 1.168 nm shows confined solvated Na⁺ ions with strong interactions with carbon, which features the formation of highly mobile Na⁺ ions and G2 solvents, leading to the high-rate and stable performance. Our findings offer a deep understanding of the preconditions and microstructure necessary for confined solvated ions in layered materials with capacitor-like electrochemical behavior.