The Modification Study Of High Voltage Cathodes And Corresponding Gel Electrolyte

High-voltage cathode with high specific discharge capacity and discharge platform is an important method to improve the energy density of lithium-ion batteries.However,the structure and interface stability of the cathode and the security of electrolyte are threatened at high voltage.In order to solve the problems of high-voltage cathode,this paper focuses on the coating modification of cathode materials and the research of suitable electrolyte to solve the problems of high-voltage cathode materials,which can provide reference for the development of high-energy density and high safety lithium-ion batteries.The main research results are as follows:（1）Direct surface coating of high voltage Li Co O₂cathode with P（VDF-HFP）based gel polymer electrolyteFor high-voltage cycling of lithium-ion batteries,a gel polymer Li-ion conductor layer,P（VDF-HFP）/Li TFSI（PHL）with high electrochemical stability has been coated on the surfaces of as-formed Li Co O₂（LCO）cathodes by a solution-casting technique at low temperature.An LCO cathode coated with around 3μm thickness of the PHL ultrathin membrane,retains 88.4%of its original capacity(184.3 m Ah g^-1)after 200 cycles in the 3.0-4.6 V range with a standard carbonate electrolyte,while the non-coated one retains only 80.4%of its original capacity(171.5 m Ah g^-1).The reason for the better electrochemical behaviors and high-voltage cycling is related to the distinctive characteristics of the PHL coating layer that is compact,has highly-continuous surface coverage and penetrates the bulk of LCO,forming an integrated electrode.The PHL coating layer plays the role of an ion-conductive protection barrier to inhibit side reactions between the charged LCO surface and electrolyte,reduces the dissolution of cobalt ions and maintains the structural stability of LCO.Further,the PHL coated LCO cathode is well preserved,compared to the uncoated one which is severely cracked after 200 cycles at a charging cutoff voltage of4.6 V.（2）High voltage Li Ni_0.5Mn_1.5O₄modified with sulfonated graphene for long-life and high energy density lithium-ion batteriesThe sulfonated graphene（SA-GE）with the function of both conducting ion and electron is utilized to modify high voltage Li Ni_0.5Mn_1.5O₄.It shows that the Li Ni_0.5Mn_1.5O₄modified with 2 wt.%SA-GE exhibits extremely long cycling life,achieving great capacity retention of 98.0%after 1000 cycles at the loading of 2.6 mg cm^-2and 99.0%after 350 cycles at the loading of 6.0 mg cm^-2under 1C.Besides,rate capability is also greatly improved(discharge capacity remains 78.5 m Ah g^-1at 10C).It is revealed that the SA-GE involved layer could significantly inhibit the rapid growth of the cathode electrolyte interphase layer at 5.0 V,which mainly contributes to an enhanced Li⁺diffusion kinetics without sacrificing the electronic conductivity of graphene coating.Thus,the polarization degree and charge transfer resistance are significantly mitigated.Furthermore,structure degradation of the cathode is avoided by reducing the side reactions and the dissolution of transition-metal ions.It is well demonstrated that chemical decorations of graphene modifiers with an optimized amount of functionized sulfonic acid groups can be a promising strategy to construct lithium-ion batteries with long life and high energy density.（3）Functional imidazole ionic liquid modified in-situ PEGDA-based gel electrolyte for 4.6 V Li Co O₂An in-situ PEGDA-based gel electrolyte is prepared by in-situ thermal polymerization,introducing 1-cyanopropyl-3-vinylimidazole bistrifluoromethane-sulfonimide salt containing vinyl and nitrile groups with excellent electrochemical stability.The prepared 1CN-3MZ-TFSI/PEGDA GPE enables a high ionic conductivity of 3.0×10^-3S cm^-1,a low activation energy of 14.68 k Jmol^-1and a wide electrochemical window.Li Co O₂half cells with 1CN-3MZ-TFSI/PEGDA gel electrolyte present a significantly improved cycle retention（98.0%,0.5C,100 cycles）and rate performance(109.9 m Ah g^-1,5C).According to the comparison of control experiments,the results show that the enhancement of electrochemical performance of1CN-3MZ-TFSI/PEGDA gel electrolyte might be due to the strong oxidation resistance of 1CN-3MZ-TFSI containing nitrile group.On the other hand,the vinyl of1CN-3MZ-TFSI can be polymerized with PEGDA to form a protective layer on the surface of the cathode particles,which improves the structure and interface stability of the cathode.The EIS and XRD characterizations of the cycled Li Co O₂cathode further demonstrate that 1CN-3MZ-TFSI/PEGDA GPE can maintain the interface and structural stability of the lithium cobalt oxide cathode at a high voltage of 4.6 V.