Research On Wide-temperature Electrolytes For Lithium Batteries

Lithium-ion batteries have been widely used in consumer electronics products and electric vehicles due to their advantages of high energy density,high power density and long cycle life.However,current lithium-ion batteries still display inferior temperature tolerance,as extreme temperatures（>45°C or<0°C）would adversely affect their per-formance（or even pose safety hazards）,which limits their application to scenarios that involves wider temperature range.In this regard,the paper first systematically reviews the diverse challenges faced by lithium batteries in low and high temperatures,and then focuses on the technological innovations in electrolytes to address these challenges and to improve the wide-temperature performance of lithium batteries.This paper primarily contains research on the following topics:（1）Developing low-temperature electrolytes with uniform design method.For the development of low-temperature electrolytes,co-solvents with low viscosities and low melting points,as well as lithium salts and film-forming additives with low impedance are usually necessary,but how to maximize the synergistic effects of these components remains a major challenge.This work developed a multi-component electrolyte system using dual-lithium salts（Li BF₄ and Li FSI）and three solvents（PC,DME and i BA）and an ingenious“uniform design”method was implemented to gain the optima electrolyte formula with the least number of experiments.Lithium/graphite fluoride（Li/CF_x）cells using the optimized electrolyte（X）exhibit huge enhancements in their low temperature performance,retaining more than 52%of the room temperature capacity and an energy density of 679.4 Wh kg^-1（based on cathode active material）at-60°C,holding a leading position in the same type of research.Comprehensive characterizations and theoretical calculations suggest that the lithium-ion desolvation process is the rate-limiting step in the low-temperature kinetics of the cells.In addition,by optimizing the relative content of PC and DME involved in lithium-ion coordination,the difficulty of lithium-ion des-olvation can be effectively reduced as well as the low-temperature performance can be greatly enhanced.Furthermore,Li FSI can introduce more inorganic components（such as Li₃N）in the solid-electrolyte interphase（SEI）and reduce the SEI thickness,thereby promoting the lithium-ion transport in SEI,and further improving the low-temperature performance.（2）Enhancing battery kinetics by modulating the interface with additives.In order to address the issue raised by the unique electrochemical reaction mechanism of fluori-nated graphite（CF_x）cathodes（i.e.,the discharge product Li F can deposit on the surface of CF_x particles and hinder the insertion of subsequent lithium ions into the bulk of the cathode,significantly increasing the cell impedance）,a novel electrolyte additive,BF₃,has been developed in this study.The CF_x cathode with the 0.01 M additive electrolyte can deliver a maximum power density of 23040 W kg^-1 with a high gravimetric energy density of 722.8 Wh kg^-1（15C,based on cathode active material）.By contrast,the CF_xcathode with the non-additive electrolyte can hardly provide capacity at discharge rates above 5C due to severe cell polarizations.The significant improvement in cell kinetics can be attributed to the ability of additive BF₃ to dissolve the Li F deposited on the CF_xsurface,facilitating the subsequent lithium-ion diffusion into the bulk of cathodes.（3）Aluminum foil corrosion prevention of wide-temperature lithium salt Li TFSI.The lithium salt Li TFSI shows excellent thermal/chemical stability,superior ionic con-ductivity and low-temperature performance,making it a wide-temperature lithium salt capable of handling both high and low temperatures.Nevertheless,it can cause serious corrosion to aluminum current collector at higher potentials（>3.8 V vs.Li⁺/Li）,which severely limits its application.This work proposes an ionic liquid additive[EMIM]NO₃based on competitive adsorption mechanism to inhibit the corrosion behavior of alumi-num current collector in Li TFSI-based electrolytes.The Li Co O₂||Li cell with the non-additive electrolyte（1 M Li TFSI dissolved in EC/DEC 1:1 vol.）exhibits rapid capacity reduction during cycling（voltage range:3 V～4.2 V）,with a capacity retention of only33.6%after 10 cycles.By contrast,the cell with the 0.1 M additive electrolyte exhibits significantly improved cycle stability,retaining 90.2%of initial capacity even after 120cycles.Electrochemical characterizations and theoretical calculations indicate that both[EMIM]⁺and NO₃^-can preferentially adsorb on the native Al₂O₃ passivation,excluding TFSI^-from encountering the surface of aluminium current collector,thereby preventing aluminum corrosion.（4）Development of all-temperature-range electrolyte systems.Based on the wide-temperature lithium salt Li TFSl,together with two functional lithium salts Li PO₂F₂ and Li ODFB and the mixed PC/MPN solvent with wide liquid range,high oxidation resist-ance and low lithium-ion binding energy,this work developed an all-temperature-range electrolyte capable of operating between-50°C and 80°C.The Li Co O₂||Li cell using this electrolyte can retain 65.4%of the room temperature capacity at-50℃（0.1C）and exhibit a stable cycling performance at 80℃,maintaining 87.3%of the initial capacity after 150 cycles.By contrast,the Li Co O₂||Li cell using the commercial electrolyte can hardly provide any capacity at-50℃,and exhibits rapid capacity decay during cycling at 80°C,eventually failing to operate after 92 cycles.The superior kinetic properties at low temperatures and bulk/interface stabilities at high temperatures can be attributed to the coordinated optimization of multiple lithium salts and solvents.This work has dev-eloped a practical all-temperature-range electrolyte,while emphasizing the importance of mechanism-level understandings and utilizing the synergistic advantages of multiple components in handling the dual challenges of low and high temperatures.