Design Of Low Temperature Aqueous Electrolytes And Performance Of The Zinc/Copper Ion Batteries

In recent years,aqueous rechargeable batteries have attracted much attention due to their intrinsic safety and environmental friendliness.Nevertheless,compared with reported organic electrolytes with low melting points such as ethyl acetate,1,3-dioxolane and methyl propionate,the inherent hydrogen bond network structure of aqueous electrolytes at low temperatures will cause them to freeze,which severely limits ion migration and causes the battery to fail to operate.In the northern region of China,the temperature remains below 0°C for nearly five months in a year.This environmental factor fully reflects the importance of low temperature adaptability of aqueous rechargeable batteries.In addition,among many conductive metal anodes,the safe,non-toxic and rich metal zinc and metal copper have relatively high standard electrode potentials,which renders them thermodynamically stable to aqueous electrolytes.Therefore,the main objective of this paper is to realize ultra-low temperature aqueous zinc ion batteries and copper ion batteries with high cycle stability and fast charging/discharging.Among them,bacterial cellulose hydrogel is used as the electrolyte matrix,and the key design principle is to effectively destroy the hydrogen bond structure among water molecules.The aim is to design a highly concentrated and ion-specific aqueous zinc ion and copper ion electrolyte systems.The evolution of the hydrogen bond structure in electrolytes is studied in depth by rationally regulating the ion hydration,and the low-temperature electrochemical behavior of different electrolyte systems is systematically analyzed.The main contents and results of the paper are as follows:First,the bacterial cellulose hydrogel electrolyte matrix with a thickness of138μm and a tensile strength of 18.16 MPa was prepared by a simple freeze-thaw method.Its excellent mechanical properties are nearly three orders of magnitude（21.5 k Pa）higher than that of traditional polyacrylamide hydrogel matrix.Based on this matrix,a spontaneously hygroscopic double-layer gel electrolyte system was designed opposite to the“water-in-salt”process.Studies have shown that the impedance value of the gel electrolyte with a water absorption rate of 9.05 wt.%can decrease from 1.2×10⁵Ωto 1.3×10³Ωat-20℃,and the corresponding ionic conductivity increases from 0.0001 m S cm^-1 to 0.012 m S cm^-1.In obvious contrast,the impedance value of 1.5 mol kg^-1 Zn（OTf）₂+3 mol kg^-1 Li TFSI pure aqueous electrolyte at-20℃ is only 1.66Ω,and the ionic conductivity is 1.20 m S cm^-1.The above results together with XRD,SEM and other tests,reveal that high-concentration pure aqueous electrolytes can achieve higher ionic conductivity and improve the stability of positive/negative electrodes at low temperatures,which is ascribed to the destruction of the hydrogen bond structure among water molecules by high-concentration ion hydrates with the ion specificity.Based on the role of the fast ion transport played by the above-mentioned high-concentration pure aqueous electrolytes,in order to further achieve ultra-low temperature zinc ion batteries with long cycle life,the electrochemical window of chloride and non-chloride cellulose hydrogel electrolytes was systematically investigated using the Hofmeister effect,and the low-temperature electrochemical behavior of high-concentration,synergistic Zn Cl₂-Li Cl,Zn Cl₂-Na Cl and Zn Cl₂-KCl hydrogel electrolytes was studied.Low-temperature Raman spectroscopy and linear sweep voltammetry results show that the electrochemical window of the chloride hydrogel electrolyte was broadened by>1 V at low temperatures due to the significant increase in the proportion of hydrogen bonds.Compared with other synergistic systems,the 3 mol kg^-1 Zn Cl₂+6 mol kg^-1 Li Cl hydrogel electrolyte exhibited a stronger disruption of hydrogen bonds among water molecules and cellulose chains,exposing more amorphous regions and significantly reducing viscosity of the electrolyte,thereby achieving a high ionic conductivity of 1.14m S cm^-1 and a low activation energy of 0.21 e V at-50℃.Benefiting from the widened electrochemical window and excellent ionic conductivity,the assembled Zn//PANI cell can provide a specific discharge capacity of 96.5 m Ah g^-1 and～100%capacity retention after 2000 cycles at-50℃.In order to further obtain a low-temperature fast-charging battery with an ultra-long cycle life,combined with the proton-accelerated ionic conductivity and the good acid resistance of metallic copper in this paper,the Cu（BF₄）₂ electrolyte with solubility advantages and spontaneous H⁺generation due to the hydrolysis of BF₄^-was used to construct a low-temperature copper ion battery.Spectroscopy,electrochemical analysis and molecular dynamics indicated that the Cu（BF₄）₂electrolyte is one of the most effective aqueous electrolytes capable of breaking the hydrogen bond network of water molecules.This hydrogel electrolyte endows the Cu//PANI battery with a super short charging time of 21 s at-30°C and a charge/discharge capability up to 10 A g^-1,a discharge specific capacity of 70m Ah g^-1,and a supercapacitor-comparable power density of 3000 W kg^-1.In addition,it exhibits an ultra-long lifetime of more than 10000 cycles at-50°C,and the constructed Cu//Cu symmetrical cell can maintain a highly reversible copper ions deposition/stripping cycle without dendrites and by-products for up to 40 days.In addition,the full battery can also operate normally at-70°C.Based on the above research on ultra-low temperature copper ion batteries and the fact that metallic copper has a high ductility to form independent copper wires,this paper constructs a weavable ultra-low temperature fiber battery by proton-enhanced hydrogel electrolyte（Cu（BF₄）₂+H₃PO₄）.It is found that the low temperature environment can activate the abundant reactive active sites in the PANI cathode,resulting in enhanced specific capacity and cycling stability of the Cu//PANI battery.Among them,H⁺with the high ionic conductivity instead of copper ions participates in the redox reaction.The corresponding fiber battery can achieve a high discharge specific capacity of 120.1 m Ah g^-1 and a capacity retention of～96.8%after 2000 cycles at-50°C.In addition,the fiber battery with good flexibility shows no obvious capacity decay under severe deformations at-30°C.Integrating fiber batteries into textiles can power 46 parallel-connected red LEDs,an alarm clock,and a sport wristband,showing its potential application in the field of low-temperature wearable electronics.