Effect Of Functional Electrolyte On The Improvement Of The Electrochemical Performance Of LiMn₂O₄and LiNi_0.5Mn_1.5O₄and The Action Mechanism

Currently, high-low-temperature, high-security, and high-voltage electrolytes applied to the Manganese-based cathode materials such as LiMn2O4(LMO, hereafter) or LiNio.5Mn1.5O4(LNMO, hereafter) are the research hotspots in the electrolyte field of the electric vehicles and hybrid vehicles of lithium-ion batteries (LIBs). In this paper, the current research situation of LMO and LNMO cathode materials and the research progress of commercial electrolyte are summarized. Various electrolyte additives and novel electrolyte systems are used for improving the cycling performance and storage performance of LMO and LNMO.The component optimization of the functional electrolyte is generalized systematically for the first time. The frontier orbital energy of common solvents is obtained with the Gaussian software calculation. The principle based on the selection of functional electrolyte component is elaborated. Meanwhile, the commercial electrolyte system is evaluated comprehensively. It is indicated that the worse thermal stability and the lower decomposition voltage restrict the application of commercial electrolyte in LMO system at high-temperature and LNMO system at high-voltage.The tests of conductivity and decomposition voltage of different electrolyte systems show that the temperature, dielectric constant and viscosity of solvent, the concentration of lithium salts and additives are the main factors affecting the conductivity and electrochemical windows of electrolytes.The conductivity, decomposition voltage and thermodynamic stability of all the electrolytes are studied, and the electrochemical performance of all the electrolytes are discussed when heptamethyldisilazane (HEMDS), ethanolamine (MEA), LiDFOB, LiBOB,1,3-propane sultone (1,3-PS) and succinonitrile (SN) are added into the commercial electrolyte at different content. The results show that2%HEMDS can inhibit the capacity fading of LMO at moderate-and-low rate after cycling and storing at different temperatures. Appropriate anount of LiBOB, LiDFOB,1,3-PS and SN added in the commercial electrolyte can improve the cycling and storing performance of LMO.The addition of2%HEMDS or MEA into the electrolyte can inhibit the capacity fading of LMO after cycling. However, the former can also increase the initial discharge capacity at low rate.The mixed electrolyte stabilizer added into the commercial electrolyte can apparently improve the storage performance of LMO at high temperature. The possible mechanism may be the reaction of HEMDS with HF and the hydrogen bonds formed between MEA and H2O and HF.Temperature and the charge state of LMO affect the storage performance obviously. After LMO/Li half cell is stored for a week at room and high temperature, the cycling performance of the cell at discharged state is superior to that at half and charged state. When LMO/Li half cell at discharged state is stored for a month at room temperature, the initial coulombic efficiency is up to95.22%, while it still retains96.74%of its initial capacity after cycling for200th at2C.The addition of a small anount of commercial electrolyte, DEC and the mixed solvent of EC:PC:EMC (1:1:3, in volume) in1mol L-1LiBOB/γ-GBL can decrease the viscosity of the electrolyte system and increase the comprehensive electrochemical performance. Both1mol L-1LiBOB and lmol L-1LiDFOB dissolved in EC:PC:EMC (1:1:3, in volume) can significantly improve the cycling performance of LMO at high temperature. When cycled for50th at1C,97%of its initial capacities can be retained. The electrolyte system is compatible well with graphite, and has excellent performance at high-low temperatures.High-purity LiDFOB can be synthesised by liquid phase method with BF3·(C2H5)2O and LiF as the raw materials, and can be applied in the high-temperature electrolyte of LMO. TG-FTIR tests show that the decomposition of the salt at high temperature occurs at three stages.The initial coulombic efficiency is related with the instability of the aluminum current collector and separator, and the irreversible oxidation of electrolyte. The difference of the cycling performance of LNMO at different rates is mainly caused by polarization, the decomposition of electrolyte and separator. The capacity fading is very serious when cycling at60℃for LNMO/Li half cell, and it is even up to39.43%after130th cycles at1C. The comparison between cycling and storing at high temperature shows that the main reason of fast capacity fading at high temperature is the worse electrochemical stability and the lower decomposition voltage of the commercial electrolyte. The electrolyte based on the salt of LiPF6decomposes easily and its thermal stability is worse. Meanwhile, the decomposition product interacts with the cathode materials, causing the dissolution of nickel and manganese. The dissolved nickel and manganese deposit easily on the surface of cathode materials as lithium salts.The addition of LiDFOB and trimethyl borate into the commercial electrolyte can not improve the cycling performance of LNMO obviously.50%TMS added in1mol L-1LiDFOB/EC:PC:EMC (1:1:3, in volume) or5%FEC in1mol L-1LiTFSI/EC:PC:EMC(1:1:3) can improve the decomposition voltage of electrolyte and the cycling performance of LNMO.