Study On Optimization Of High Voltage Electrolyte System And Structure-activity Relationship Of Functional Additives

As one of the main energy storage devices,lithium-ion batteries are widely used in power and aerospace fields.As the demand for its applications continues to expand,the development of systems with high energy density,high power density,and good safety is critical to the development of power and energy storage lithium-ion batteries.Li Ni_0.5Mn_1.5O₄(LNMO)cathode material is characterized by its high energy density(700 Wh kg^-1)and high operating voltage(4.7 V,vs.Li⁺/Li),stable structure,abundant raw materials for preparation,and high safety performance,which is a strong competitor for lithium ion battery system with high energy density and power density.But it has severe interfacial reactions with the electrolyte at high voltage,which is the"bottleneck"that limits the market application of this battery system.In addition to the modification of the bulk LNMO material,optimization of the electrolyte formulation is the main measure and mean to break the above"bottleneck".The most economical and effective way to optimize the electrolyte formulation is to combine multiple functional additives in a commercial electrolyte system.The synthesis of functional additives and the choice of compounding methods are the research nodus.In this paper,through the study of the effects of special functional groups of additives on the performance of lithium-ion batteries,it correlates the structure-activity relationship between additives and battery performance,solves the technical difficulties in the synthesis of functional additives and the selection of compounding methods,and achieves the optimization of electrolyte formulations.The main research content and conclusion are as follows:(1)Synthesis of chelated phosphate lithium difluorobisoxalate phosphate(Li DFBOP)through the reaction of bistrimethylsiloxalate(DTMSO)and lithium hexafluorophosphate(Li PF₆),and high purity product was obtained by the.optimization of reaction conditions(proportion,temperature,time and concentration).Further,with the help of the characteristics of the trimethylsilyl ester protective agent,a general preparation method for a series of lithium salts is proposed.That is,with the help of trimethylsilyl ester,cyclic functional groups(oxalate and sulfate)are introduced into the structure of Li PF₆ and Li BF₄ to obtain a series of new chelated lithium salts.(2)In order to suppress the self-discharge of the high-voltage battery system and improve the battery performance,the above-mentioned synthesized chelated phosphate Li DFBOP was used as an additive at an optimal ratio of 0.25%to optimize the commercial electrolyte.The performance of the optimized electrolyte system matched with the LNMO/Mesophase Carbon Microsphere(MCMB)battery system and self-discharge suppression function analysis showed that the 0.25%Li DFBOP electrolyte significantly increased the specific discharge capacity of the positive electrode LNMO,with an average of 138.5 m Ah g^-1,the capacity retention rate is increased by 15.8%compared to the blank sample.In addition,the negative electrode MCMB also showed a good capacity retention rate(88.1%).The mechanism of Li DFBOP on the positive and negative electrodes was analyzed using secondary time-of-flight ion mass spectrometry(TOF-SIMS)combined with density functional theory calculations.The electron-deficient P element was the key to the preferential decomposition of Li DFBOP.In addition,the structure of cyclic oxalic acid ester can increase the proportion of organic molecules in the interface film to suppress the self-discharge of the battery and improve the overall performance of the battery.(3)In addition to battery self-discharge,overcharging in battery also has an important impact on battery safety applications and performance.Methyl phenylsulfite(MBS),an additive composed of benzene rings and sulfate functional groups,plays a key role in suppressing overcharge of high-voltage LNMO batteries.The 100%overcharge analysis method was used to evaluate the electrolyte system with 0.4%MBS.It shows excellent protection when the voltage exceeded 5.0 V(vs.Li⁺/Li).Combined with theoretical calculation and analysis of the difference in charge density,it can be concluded that the decomposition products of MBS are contained on the surface of LNMO,which includes a benzene ring with a special conjugated structure that can disperse excess electrons during overcharge to protect the battery from overcharge hazards.In addition,MBS can improve the rate performance of the battery,which is mainly due to the sulfate functional group,and its decomposition products Li₂SO₄ and organic sulfur oxide can improve the lithium ion transmission rate of the interface film.(4)Improving electrolyte oxidation stability is another major method for developing high-voltage electrolytes.The nitrile reagent 1,3,6-hexane trinitrile(HTN)with addition of 1%compared with the same proportion of adiponitrile(ADN)due to the rich nitrile functional groups can improve the electrolyte oxidation potential close to 6.0 V(vs.Li⁺/Li).The electrolyte system resistant to oxidative decomposition improves the capacity retention rate in high-voltage LNMO batteries by 30.1%.In addition to increasing the oxidation potential,the mechanism of its effect also reduces the corrosion of electrodes by eliminating acid impurities in the electrolyte.(5)The above-mentioned functional additives are combined to form four types of optimized electrolyte system A-D,which are compared with the blank electrolyte Baseline,and a new scoring method is used to comprehensively analyze the five types of scoring factors(CCS,CEDS,CRS,AMS,and SPS),which are corresponding to battery performance,including cycle performance,energy density,rate performance,adaptability to negative electrode,and safety performance.The electrolyte system A containing Li DFBOP and MBS additives has the best comprehensive performance for LNMO/MCMB battery system.Here,in-situ analysis is used to analyze the gas production of all electrolyte systems during the charge and discharge process by means of in-situ analysis-differential electrochemical mass spectrometry(DEMS).Finally,combined with traditional interface analysis methods and in-situ impedance methods,the mechanism and interface properties of the optimized electrolyte system were analyzed in detail.Due to the excellent synergy between Li DFBOP and MBS in the electrolyte system,the overall performance of the battery is improved from the perspective of suppressing the self-discharge and overcharge of the high-voltage battery system by optimizing the properties of the interface film.The above research is helpful to reveal the general law of the structure-activity relationship between the functional groups of the additives and the battery performance.By clarifying the mechanism of action of the different central ions,ligands and other functional groups of the additives,and clarifying the beneficial interface reactions,it is helpful to guide the application from a theoretical level.For the construction of electrolyte system for high energy density batteries.