Two-Step Mechanism for Halogen Intercalation in Graphite Enabled by Aqueous Biphasic Systems

The electrification of our society requires safer and longer-lasting batteries, for which modular electrolytes, such as aqueous biphasic systems (ABS), are being developed. ABS, i.e., two-phase systems, were shown to enable high-potential aqueous batteries based on Li-ion (de)intercalation at a graphite negative electrode and halides (de)intercalation at a graphite positive electrode. However, the exact role of ABS in promoting the intercalation of halogens while preventing halogen gas evolution has remained unclear. We find that using ABS does not favor halogen intercalation but rather trihalides formation. Electrochemically generated trihalides are confined in the halide-rich phase of ABS, preventing their diffusion to the negative electrode and self-discharge. We also reveal that confined trihalides spontaneously intercalate in the graphite positive electrode. ABS thus enables a two-step electrochemical-chemical intercalation mechanism of halogens. This work paves the way toward the design of innovative ABS for developing intercalation-type batteries or membrane-less redox-flow batteries with independent anodic and cathodic chemistries.