Construction And Study On Aqueous Zinc–Based Halogen Battery With High Energy Density

The aqueous zinc–based battery stands out among various energy storage technologies due to its prominent advantages of suitable working potential,high theoretical capacity,low cost and high safety of zinc cathode.However,the energy density of current rocking–chair aqueous zinc–based batteries based on the zinc ion insertion/deinsertion mechanism of oxide cathode materials is low,which is difficult to meet the demand for large-scale energy storage system applications.To solve the above problems,this study designed a halogenated salt deep eutectic solvent（DES）electrolyte system and constructed a new aqueous zinc halogen battery based on Kochen black carbon（KB）cathode material.The effects of electrolyte components and concentrations on the electrolyte ion structure and carbon cathode charging/discharging behavior was systematically investigated.The wors revealed the charge storage mechanism based on halogen anions in carbon cathode,and achieved a significant increase in energy density of aqueous zinc-based battery.An aqueous zinc-bromine battery was constructed by DES electrolyte composed of zinc bromide（Zn Br₂）,choline chloride（Ch Cl）and H₂O,KB cathode and zinc cathode where the effect of electrolyte composition on the electrochemical performance of the battery was studied.It was found that in the Zn Br₂+Ch Cl electrolyte system,Zn²⁺could coordinate with Br^–and Cl^–to form the[Zn Br_4–xCl_x]^2–structure,which greatly improved halogenated salts solubility in water and thus observably reduced the water activity.The above electrolyte leads to effective inhibition of polyhalide formation and irreversible dissolution and even shuttling,which significantly enhanced reversibility of Br^–/Br⁰ redox electric pair reactions.When the electrolyte concentration increased from 70 m Zn Br₂+35m Ch Cl to 80 m Zn Br₂+40 m Ch Cl,the discharge capacity of the KB cathode based on Br^–/Br⁰ redox pair increased from 368 m Ah/g to 507 m Ah/g at 100 m A/g current density with a discharge potential of 1.47 V.The energy density of the battery based on the carbon cathode mass was up to 712 Wh/kg.At the same time,the battery system exhibits good rate performance at different discharge current density and excellent cycling stability.HRTEM as well as XPS reveal that the Br^–deposited on KB forming Br₂ and partially embedded into the interlayer along with the formation of C_n[Br]during the charge process,and Zn²⁺deposition at the negative terminal.The highly reversible reaction occurs for the discharge process.The discharge platform of above mentioned zinc–bromine battery based on the Br^–/Br⁰redox pair is still low.We further constructs an aqueous zinc–dual halogen battery by optimizing the electrolyte components and introducing the Cl^–/Cl⁰ redox pair,thus enhancing the battery discharge potential（average discharge potential 1.6 V）and further improving the battery energy density.It was found that in the 80 m Zn Cl₂+40 m Ch Cl+5 m Zn Br₂ electrolyte,the two redox pairs Br^–/Br⁰ and Cl^–/Cl⁰ can be highly reversible at the KB cathode due to the suppressive the water molecule activity,the reduction of the non–free halogen anion and the interaction of the Br Cl halogen intercompounds.This aqueous zinc–dual halogen battery can delivers a maximum discharge capacity of 550 m Ah/g and achieved higher energy density of 788 Wh/kg.The results of ex–situ SEM,HRTEM and XPS tests demonstrate that the charging mechanism of the DES electrolyte based battery works is as following:The Br^–/Br⁰ redox pair first takes place a reversible reaction where the[Zn Cl_4–xBr_x]^2–carrier strips Br^–on the KB surface and undergoes a deposition transformation reaction on the KB surface to produce Br₂,while part of the Br^–is embedded into the KB interlayer forming C_n[Br].Subsequently,the Cl^–/Cl⁰ redox pair reacts and the[Zn Cl_4–xBr_x]^2–carrier is stripped of Cl^–on the KB surface and a deposition transformation reaction on the KB surface to produce Br Cl.Besides,part of the Cl^–insertion into KB layers accompanied with the production of C_n[Br Cl].For the negative side,the reaction is related to the deposition of zinc ions.In the discharge process,opposite reactions are presented with high reversibility.