Functional Electrolytes To Stable High Voltage Lithium-rich Manganese-based Cathode

In the development of high energy density lithium-ion batteries,the development and application of high-performance cathode materials and electrolytes are very important.High voltage Lithium-rich manganese-based cathode is considered as one of the next generation cathode materials due to its high energy density,low cost and safety.However,the continuous side reaction between electrolyte and the Lithium-rich manganese-based cathode leads to the deterioration of cathode interface and continuous loss of capacity.In this study,the solvation structure of the electrolyte is regulated by solvent and lithium salt to optimize the protective layer formed by the electrolyte in the cathode and stabilize the cathode interface:The"strong coordination"electrolyte with high coordination degree of lithium anion and lithium ion is prepared by introducing the coordination-solvent 1,1,2,2-tetrafluorethyl-2,2,3,3-tetrafluorpropyl ether into the basic electrolyte 1 M LiPF₆FEC/DMC 1wt%LiDFOB.The electrolyte electrochemical window is 5.5 V,used in the cathode surface loads of 1.5 m Ah/cm² Li_1.2Ni_0.13Co_0.13Mn_0.54O₂||Li battery,the capacity retention rate of battery after 400 cycles at 25℃and 100 cycles at 45℃can reach 83.9%and 85.5%,while the capacity retention rates of the battery with conventional high voltage electrolyte for 200 cycles at 25℃and 100 cycles at 45℃are 82.2%and 74.3%respectively.XPS,TEM and XRD tests show that the lithium salt anion（PF₆^-）in the"strong coordination"electrolyte is more involved in the interphase formation,forming a protective layer rich in fluoride on the surface of the cathode,stabilizing the cathode interface and structure.Applied to the pouch cell,it can achieve 370 Wh/kg energy density and provide the cycle life of 36 cycles,but its lithium ion conductivity is low,the pouch cell shows a large polarization at high current,which is not conducive to the cathode capacity.Further based on the above mixed solvents with coordination-solvent participation,lithium salt Li TFSI led with Li PF₆ to develop a"dual salts"electrolyte,so as to promote the application of Li TFSI-based electrolyte in Lithium-rich manganese-based cathode battery.Compared with lithium salt Li PF₆,Li TFSI has stronger binding energy between anions and lithium ions,and can present stronger coordination in the solvated structure to promote the formation of protective layer rich in inorganic components.Moreover,Li TFSI can show advantages of high lithium ion migration number and lithium ion conductivity when applied in the electrolyte.Li PF₆ can reduce the corrosion current of Li TFSI-based electrolyte on aluminum foil and promote its application at high voltage."Dual salts"electrolyte used in the cathode surface loads of 4 m Ah/cm²Li_1.2Ni_0.13Co_0.13Mn_0.54O₂||Li battery,the capacity retention rate of battery after 150cycles can reach 83.8%,and the corresponding"strong coordination"electrolyte capacity at a rate of 75.6%.At the same time,the"dual salts"electrolyte has better rate performance in application,showing a smaller battery polarization under the large current of the pouch cell,and the cathode can provide higher capacity.