High-performance water-in-salt electrolyte-enabled zinc-graphite batteries with bromine dual electrochemical processes

As an alternative to lithium-ion batteries, aqueous zinc-graphite batteries (ZnGBs) are being explored as safer and low-cost options with the expectation of scalability to large energy storage systems. However, the currently adopted polyatomic and metal complex anion intercalation process at the graphite electrode in ZnGB exhibits poor electrochemical performances. Alternatively, incorporating halogen anions offers exceptional electrochemical performance to graphite electrodes due to their redox process. In this work, ZnGBs are assembled using a LiCl/ZnCl₂/KBr-based water-in-salt electrolyte, which efficiently supplies bromide (Br⁻) ions for conversion into Br_x⁻ and facilitates Br₂ intercalation at the graphite electrode. The conversion and intercalation of bromine together enable the ZnGB to achieve a discharge capacity of 2.73 mAh/cm² with 91.0% of coulombic efficiency (CE) while supporting high current density operations of up to 150 mA/cm². With high energy density (4.56 Wh/cm²), high power density (199.5 mW/cm²), and excellent rate capability (∼93.0% CE at 150 mA/cm²), the ZnGB is shown to operate efficiently for as much as 800 cycles. Beguilingly, an anode-free ZnGB offers enhanced stability for up to 1100 cycles without performance decay, matching the electrochemical performance of Zn metal electrodes. This work provides insights into the bromine reaction mechanism at graphite electrodes and the role of surface exfoliation in enabling efficient Br_x⁻ formation, along with Br₂ intercalation, for achieving high-performance ZnGBs.