The Modification Study Of High Voltage LiCoO₂ And Corresponding Gel Electrolyte

At present,the energy density of lithium-ion batteries is mainly restricted by the cathode materials.Among the cathode materials,Li Co O₂ has good cycling performance,rate performance and high real density.However,at current application voltage,it is far from the theoretical specific capacity.Nevertheless,with the voltage increasing,the electrolyte stability becomes poor due to the increased oxidizability of delithiated Li Co O₂ cathodes after charging to high voltages,and the interfacial side reactions between electrode and electrolyte turns to be more serious.The overlap of energy levels for Co and O leads to the generation of oxygen,resulting in the dissolution of Co ions,the destruction of cathode electrolyte interphase（CEI）film on the electrode surface,and the deterioration of performance.In this paper,the modification of Li Co O₂ and electrolyte is studied to improve the stability of Li Co O₂ cathode under high voltage,so as to further explore its application potential.Firstly,the functionalization of nano Si O₂ surface was realized by using the self-polymerization coating characteristics of dopamine.Thereby,a high-voltage tolerant functional nano coating（PDA@Si O₂）was obtained,which was used to complex and modify the Li Co O₂ cathode materials.The close interaction between nano Si O₂ coating and Li Co O₂ particles is emerged,which avoids the agglomeration of nano Si O₂ and enhances the solid superacid function caused by fumed Si O₂,effectively inhibits the interface side reaction and irreversible phase transformation of crystal structure,improves the interface stability and the ionic conductivity of Li Co O₂ cathode under high voltage,and significantly improves the cyclic stability of charge and discharge of Li Co O₂ at 4.6 V high voltage.At room temperature,the functionalized nano-Si O₂ modified Li Co O₂ cathode achieved a reversible capacity of around 190 m Ah g^-1 with the capacity retention of 78%after 200 cycles at 1 C.Also,the modified Li Co O₂ cathode also exhibits high coulomb efficiency and enhanced cycling performance at the high temperature of 50℃.Secondly,the capacity decay of Li Co O₂（LCO）at 4.6 V high voltage is mainly related to the escape of lattice oxygen and its participation in side reactions.Therefore,in this paper,we use Se with low melting point and moderate reducibility to further investigate the modification of LCO by Se coating.According the results,by optimizing the dispersion methods,sintering methods and the amount of Se coated.The results show that,compared with the traditional mechanical ball milling method,Se is dissolved by a volatile solvent（CS₂）for the dispersion of liquid phase method which is used for the first time to realize the effective uniform coating of LCO by Se.The results of different sintering methods show that the chemical properties of Se can be maintained as much as possible while Se is directly coated on the surface of LCO without sintering（L-Se@LCO）,so that the composite electrode Se coated LCO could obtain good cycling performance and high specific capacity.The specific capacity of the prepared L-Se@LCO cathode can reach about 182 m Ah g^-1 at 1 C and high surface load(about 7 mg cm^-2),which is close to that of the original LCO electrode.After 120 cycles,compared with the original LCO electrode,the capacity retention of the modified LCO can reach 82.8%.The cycle performance was significantly improved.According to the mechanistic studies,Se is preferentially oxidized while the cells cycling at 4.6 V,which could protect lattice oxygen.The formation of thin and robust CEI on the surface of L-Se@LCO during charge-discharge cycling.The issues such as phase transitions at surface,lattice distortion and accumulation of microscopic stress are improved,effectively and cycling stability is improved while the specific capacity does not reduce.At last,an in-situ PEGDA-based gel electrolyte is prepared by in-situ thermal polymerization,which is modified with 10 wt%1-Ethyl-3-methylimidazolium bis（trifluoromethylsulfonyl）imide（IL）with low viscosity and high ionic conductivity.The addition of IL broadens the electrochemical window of PEGDA GPE（～5 V）,and improves the conductivity(4.36 m S cm^-1)and thermal stability of PEGDA GPE.Li Co O₂ half cells with PEGDA GPE modified with IL（IL/PEGDA GPE）shows an improved cycle retention at 4.6 V,with capacity retention of 74.3%after 100 cycles at0.5 C.According to the researches,this alkyl imidazoles ionic liquid additives had little effect on the ionic conductivity of PEGDA-based gel electrolyte,and it could still maintain high ionic conductivity while the addition of IL increased to 10 wt%.Therefore,by improving the amount of this alkyl imidazoles ionic liquid further enhance the antioxidant capacity and thermal stability of PEGDA-based gel electrolyte,form a protective layer of gel electrolyte in the cathode particles,effectively inhibits the interface side reaction and retain the structural stability.PEGDA-based gel electrolyte modified by the alkyl imidazoles is a promising electrolyte candidate for high-voltage polymer.