Preparation And Performance Of Ether-Based Electrolyte For Low-Temperature Sodium-Ion Batteries

Sodium-ion batteries（SIBs）are expected to be a potential candidate for next-generation electrochemical energy storage because of their evident advantages of abundant resources and low cost.However,SIBs suffer from low discharge capacity,poor rate performance,and dendrite growth when operating at temperatures below 0°C,which seriously damage cell life.Tailoring the composition and solvation structure of electrolyte has been proved to be an effective strategy to solve the above issues.Ether-based electrolytes have low melting point and viscosity,as well as weaker intercalation with alkali-metal ions.Therefore,ether-based electrolytes are promising for operating at low temperature.However,ether-based electrolytes,such as 1,3-dioxolane（DOL）,are easily polymerized induced by Lewis acid,which weaken its ionic conductivity.Ether-based electrolytes also have poor oxidative stability and incompatibility with cathode materials.The current researches mainly focus on the linear ether-based electrolyte,such as diglyme（DIG）-based electrolyte.Compared with DIG,cyclic ethers,such as DOL and tetrahydrofuran（THF）,have lower melting points and viscosities as well as less steric hindrance,which is beneficial to improve the kinetics.Herein,DOL and THF were selected as the main solvents,and DIG was selected as the co-solvent.By optimizing the solvent ratio,a series of ether-based electrolytes were prepared and assembled into batteries for electrochemical performance testing.The main contents and conclusions are as follows:（1）For DIG:DOL electrolytes,when the DOL volume ratio is 60%,the cell can maintain 98.7%of its room temperature（RT）capacity at-40°C.However,the intrinsic ring-opening polymerization of DOL induced by PF₆^-anion leads to poor RT cycling performance.In this paper,tris（2,2,2-trifluoroethyl）borate（TTFEB）,an anion acceptor additive,was introduced into DOL-based electrolyte.By tethering the PF₆^-anion,TTFEB effectively inhibits the polymerization of DOL.In addition,TTFEB promoted the formation of a dense,uniform and inorganics-rich SEI,thereby improving the RT cycling stability and rate performance of the cell,and the capacity retention after 250 cycles was 89.7%.（2）For DIG:THF electrolytes,when the THF volume ratio is 80%,the freezing point of the electrolyte can be significantly decreased,and the ionic conductivity at-40°C is 3.2 m S cm^-1,which is conducive to the rapid Na⁺transmission.By characterizing the electrode after cycling,it can be found that SEI film formed in THF-based system is thin and uniform,which ensuring fast Na⁺transport kinetics.Besides,the co-intercalation of solvated Na⁺facilitate the de-solvation kinetics.As a result,the cycle stability and rate performance of cell at low temperature are significantly improved.Hard carbon（HC）anode can deliver 253.8 m Ah g^-1and 218.7m Ah g^-1capacity at-40°C and-50°C,respectively,corresponding to 84.9%and72.9%of its RT capacity.Na_2/3Mn_2/3Ni_1/4Cu_1/12O₂（MNC）cathode can maintain 88.8%and 80.9%RT capacity at-40°C and-50°C,respectively.The assembled HC||MNC full cell maintains 70.2%of its RT capacity at-40°C,and the capacity retention is90.6%after 400 cycles.