Two-Steps Halogen Intercalation Mechanism in Dual-Ion Batteries 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 dual-ion aqueous batteries based on Li-ion (de)intercalation at graphite negative electrode and halides (de)intercalation at graphite positive electrode. However, the exact role of ABS in promoting the intercalation of halogens while preventing halogen gas evolution remains unclear. We find that using ABS do not favor halogen intercalation but rather trihalides formation. Electrochemically-generated trihalides are confined in the halide-rich phase of ABS, preventing their diffusion to negative electrode and selfdischarge. We also revealed that confined trihalides spontaneously intercalate in the graphite positive electrode. ABS thus enable a two-steps electrochemical-chemical intercalation mechanism of halogens. This work paves the way toward the design of novel ABS for developing dual-ion batteries or membrane-less redox-flow batteries with independent anodic and cathodic chemistries.