The Study Of Li-ion Organic Electrolytes In Wide Temperature Range

With the development of society,various problems,including energy,environment,and resources,are becoming more and more serious.For social goals of sustainability,researchers are turning their attention to lithium-ion batteries.LIBS is the energy storage system of choice for portable electronics and electric vehicles.As the deployment of energy storage solutions continues to grow,there is an increasing need for LIBs to operate safely and stably in a wide range of operating conditions.However,conventional organic electrolytes used in LIBS cannot function properly at temperatures below 0℃ or above 60℃.Besides,because conventional organic electrolytes are toxic and flammable,they pose serious safety risks,especially in the case of accidents or overheating.Therefore,continuous research and development of electrolytes with high ionic conductivity,excellent electrochemical stability,and normal operation over a wide temperature range are required.In this paper,HOMO and LUMO energy levels of different solvents and lithium salts are calculated based on first principles.Based on the theoretical calculation results,suitable solvents and lithium salts were selected as components of the electrolyte with a wide temperature range.Secondly,molecular dynamics simulation was used to predict the solvation structures of two kinds of electrolyte systems with a wide temperature range,and the results were confirmed by Raman spectroscopy.In the first work,the lithium metal battery using adiponitrile（ADN）/ethylene carbonate（EC）electrolyte has excellent performance,and a new concept was proposed for the formation of stable inorganic-rich SEI film on lithium secondary batteries under a wide operating temperature window by manipulating the competitive decomposition between solvent and lithium salts during the cycle:Li/Cu battery at 100℃,the coulomb efficiency can reach 97.89%;Li/Li symmetric battery can cycle stably for 100 hours at150℃;The LFP/Li battery exhibits excellent cycle stability in the temperature range from-20 to120℃.NCM/Li batteries have 83.4%capacity retention at 100℃.Also,the LTO/Li full battery can work at-40℃ to 150℃.It exhibited high-temperature cycle stability at a high rate of 5C that after 1000 cycles,the capacity retention was87.94%at 120℃,99.93%at 100℃,and 90.38%at 30℃,respectively.Therefore,a simple universal electrolyte system with ADN co-solvent was established to expand the operating temperature window to control the decomposition of EC and lithium salts.In particular,it exhibits excellent electrochemical performance at high temperatures with a variety of lithium secondary batteries,including LFP/Li,LTO/Li,and NCM/Li cells.In addition,molecular dynamics simulation and in situ Raman simulation were used to explore the changes of solvation structure at different temperatures.ADN solvent was introduced to promote the solvation of Li ions.Using ADN-fluoroethylene carbonate（FEC）-dimethoxyethane（DME）-Li NO₃ electrolyte called ADFN electrolyte.Dinitriles have excellent chemical stability,low vapor pressure,and high flash point,which are considered to be good solvents for expanding electrochemical windows and improving the safety of lithium-ion batteries.In addition,it has a unique molecular structure,with two nitrile groups that can participate in the solvation process,so it has a very strong influence on the solvation sheath,soluble and complex with lithium ions.In this work,ADN molecules were introduced to occupy the solvation sites around Li⁺,reduce the solvation of ethers and esters,and have a synergistic effect with NO₃^-anions to promote the solvation of anions and make the whole solvation structure more inorganic.In the process of charge and discharge,ADN is extremely stable and will not decompose.Therefore,the reaction of various anions in the solvation sheath promotes the formation of a more compact and inorganic SEI film on the surface of the Li metal negative electrode.With the ADFN electrolytes,the Li/Cu battery enabled cycling 100 cycles stably,with an average coulombic efficiency of 98.61%,and the cycling time of the Li/Li symmetric battery exceeded 1900 hours.In addition,the capacity retention of Li/NCM523 full battery is 93.9%and 96.6%at rate of 1.0 C and 0.2 C,respectively,and it has good high-voltage performance.Besides,due to the good thermal and electrochemical stability of the ADFN electrolyte,its solvation structure is very strong at high temperatures,thus forming a stable inorganic rich SEI layer.Li/Li Fe PO₄ cells can cycle more than 1000 times stably at 30℃,and the capacity retention rate is 95.20%.When the temperature rises to 80℃,the battery has no capacity loss in 200 cycles,and the battery can cycle more than 100 times at90℃.