Construction And Properties Study Of Wide Temperature Electrolyte System For Lithium Ion Batteries

The operating temperature range is one of the important performance indicators of the power system.In order to adapt to use in different environments,the power system should have a wide operating temperature range.In civilian fields,based on GJB,batteries could work stably at-20⁶0°C,but current commercial lithium-ion batteries are difficult to operate stably with high performance over a wide temperature range.In this paper,a wide temperature electrolyte system was constructed to broaden the temperature range for stable operation of lithium-ion batteries,and to achieve high discharge capacity and platform voltage at-20⁶0°C,while maintaining an excellent high-low temperature cycling stability.At present,the most effective way to improve the wide temperature performance of lithium-ion batteries is to optimize the electrolyte system.In this paper,the construction of wide temperature electrolyte systems,compatibilities of electrolyte/MCMB anode and electrolyte/LiCoO2,LiFePO4 and LiNi0.5Mn1.5O4 are studied.The application of mixed lithium salts,the performance of additives,the conductivity of electrolyte,the surface chemical reaction between electrolyte and positive and negative electrodes were discussed.Through the screening of suitable lithium salts and additives,the relationship between the thermal stability of lithium salt,the amount of additives added and the battery performance was studied,and the mechanism of action of lithium salt and additives with positive and negative electrode materials was discussed.Finally,the conductivity of the preliminarily constructed electrolyte system was measured in the temperature range of-20⁶0°C,and the electrochemical performance of the battery assembled with the wide temperature electrolyte system was comprehensively analyzed.In the research of the preliminary construction of wide temperature electrolyte,lithium hexafluorophosphate（LiPF6）was replaced by lithium lithium tetrafluoroborate（LiBF4）and lithium difluoro oxalate borate（Li ODFB）with higher thermal stability.Electrolyte application of high temperature range,and mixed lithium salt for the collector in a wide temperature range of applications play a protective role,from the lithium salt broadened the scope of application of electrolyte temperature range.In the aspect of additives,the influence of fluorinated ethylene carbonate（FEC）on the electrochemical performance of the electrolyte was studied.The experimental and theoretical calculations show that when FEC is added to the surface of the electrode prior to the solvent system,when FEC is added in an amount of 5%reduce battery impedance and improve battery cycle stability.The constructed 1mol·L^-1 xLiBF4-（1-x）LiODFB(（x is LiBF4 and LiODFB-based electrolyte in the ratio of the amount of material,x can be taken 1,0.9,0.8,0.7,0.6,0.5,0.4,0.3,0.2,0.1,0）/PC/EC/EMC（1:1:3,v/v/v）+5%FEC Electrolytes have higher conductance in the range of-40 to 60 ^oC,And increases with the increase of LiODFB content.The conductivity of the system is higher than 1 mS·cm^-1 at-40 ^oC.The conductivity of this wide temperature electrolyte system is in accordance with the VTF equation at-40¹0 ^oC,Arrhenius equation is more in the range of 10⁶0 ^oC.In terms of the compatibility between the wide temperature electrolyte and the negative electrode of MCMB,the electrolyte system has good compatibility with the negative electrode material when the LiODFB content x>0.2,and has a high specific discharge capacity And cycle stability,the electrochemical performance of the negative electrode is relatively poor at-20°C,which is the main limiting factor of the electrolyte used in the low temperature of the whole battery.The mixed salt electrolyte system has good compatibility with MCMB due to the fact that LiODFB and FEC are easily reduced and the product covers the surface of MCMB to form a stable and dense solid electrolyte interface（SEI）film,which inhibits the formation of carbonate reduction reaction,the formation of SEI film has high lithium ion conductivity and stability.In the research of the compatibility between LiCoO2 and electrolyte,the mixed lithium salt electrolyte system has better positive compatibility than single lithium salt and LiPF6 system.The positive electrode has good compatibility with mixed salt electrolyte system at room temperature.The electrochemical oxidation rate of single lithium salt carbonate solvent increases at high temperature,and the compatibility of positive electrode and electrolyte decreases obviously.Mixed lithium salt electrolyte system several times into the cycle before the positive electrode surface can generate stable and dense passivation film,then the positive electrode in the electrolyte at high temperature cycle stability increased significantly.At the same time LiCoO2 positive rate performance has improved significantly,mixed salt electrolyte system can improve positive cycle stability at the same time take better ratio rate performance.When the ratio of LiBF4 to LiODFB is 8:2 and 7:3,the cycling performance of LiCoO2 cathode is improved obviously.After 60 cycles of 100 cycles,the capacity retention of 93.6%and 92.5%.The specific capacity of the magnification is 128mAh·g^-1 and 130 mAh·g^-1,can reach more than 95%of 0.2 C magnification.The initial discharge capacities of 0.1C at-20°C were 124.6 mAh·g^-1 and 126.4 mAh·g^-1,respectively,and the capacity retention of 94.3%and 93.8%remained after 300cycles.The study on the compatibility between the electrolyte and the positive electrode of LiFePO4 shows that the mixed electrolyte system has the same good compatibility.When the ratio of LiBF4 to LiODFB is 7:3,LiFePO4/Li cells have excellent electrochemical performance at wide temperature.In the end of this paper,the wide temperature electrolyte was applied to high voltage battery system to study the compatibility with high voltage LNMO cathode.The electrolyte has good compatibility with LNMO under normal temperature and low temperature condition.The addition of LiODFB can effectively inhibit the dissolution of Mn³⁺and improve the circulation stability of LNMO.When the ratio of LiBF4 to LiODFB was 7:3 and 5:5,the capacity retention of LNMO/Li batteries was92.3%and 91.4%after 300 cycles at 25°C.The initial discharge specific capacities of0.1C at-20°C were 105.3 mAh·g^-1 and 98.9 mAh·g^-1,with 92.3%and 91.4%capacity retention after 200 cycles.The mixed salt electrolyte system constructed in this paper is superior to the commercial LiPF6-based electrolyte in wide temperature range,and basically meets the requirements of wide operating temperature range and has good low temperature performance in a 5 V high voltage battery system.The results show that the properties of the electrolyte（such as melting point,boiling point,electrochemical window and ionic conductivity）are the necessary conditions for the wide temperature operation of the electrolyte.The compatibility of the electrolyte with the positive and negative electrodes are the main factors affecting the wide temperature electrolyte.