Interface Construction And Performance Optimization Of The Positive Electrode Side Of Polymer Electrolyte-based Solid-state Lithium Batterie

Rechargeable solid-state batteries with lithium metal anodes are attracting a lot of attention due to their high energy density and safety.As a key material in solid-state lithium batteries（SSLBs）,solid-state electrolytes have a significant impact on the energy density,cycling performance and safety of solid-state batteries.Polymer electrolytes are widely used because of their light weight,low cost,high flexibility,relative stability to lithium metal.Yet,the commercialization of polymer electrolytes has been hampered by the narrow electrochemical window of the polymer or the polymer-based electrolytes,which are difficult to match with high-voltage cathodes.Previous research has shown that reducing the contact between the polymer electrolytes and the high voltage cathode by adopting surface coating of the cathode material is an effective strategy to increase the energy density of solid-state lithium batteries.In this thesis,the cyclized polyacrylonitrile as a surface coating material for cathode materials was systematically investigated.By further cooperating with ionic liquid interlayers,the electrochemical performance of polymer electrolyte based SSLBs can be greatly improved.The obtained research results are listed below.（1）The cyclized polyacrylonitrile（c PAN）nanoparticles served as an electron-conducting shell was uniformly coated on lithium cobaltate（LCO）cathode particles（LCO@c PAN）.The solid-state LCO@c PAN/PVDF-HFP/Li cell was assembled using LCO@c PAN cathode,poly vinylidenefluoride-hexafluoro propylene（PVDF-HFP）electrolyte,and Li-metal anode.Specially,trace amount of N-methyl-N-propylpiperidine bis（trifluoromethylsulfonyl）imide（PP13-TFSI）-based ionic liquid（IL）was introduced at LCO/PVDF-HFP interface to form an intimate cathodic contact.The electronic-conducting c PAN coating along with the ionic-conducting IL enables the construction of ion-electron conducting network inside the cathode and the intimate LCO/PVDF-HFP contact.The results show that the solid-state lithium battery exhibited 125.2 m Ah g^-1at 30°C,0.1 C and a cut-off voltage of 4.2 V the capacity retention of 87.7%after 100 cycles.（2）The c PAN nanoparticles were encapsulated on Li Ni_0.6Co_0.2Mn_0.2O₂（NCM）cathode particles.The solid-state NCM@c PAN/PEO/Li cell was assembled using NCM@c PAN cathode,polyethylene oxide（PEO）electrolyte,and Li-metal anode with trace amount of PP13-TFSI IL at cathodic interface.The synergistic effect of c PAN coating and IL suppresses the direct contact of PEO with high-voltage NCM.Besides,during cycling,the ionic-conducting cathode electrolyte interphase（CEI）layer consisting of Li F and Li₃N is formed at cathode side,further enhancing the stability of PEO against high-voltage cathode.The results show that the solid-state NCM@c PAN/PEO/Li cell delivers good cycling stability in terms of the cutoff voltages of 4.2,4.25 and 4.3V,along with high capacity retentions of>80%after 100cycles.Moreover,a high capacity retention of 86.2%can be achieved after 50 cycles when the cathode loading is increased up to 6.2 mg cm^-2.The above results show that the polymer electrolyte can be effectively separated from the high voltage cathode by means of coating,avoiding oxidative decomposition of the polymer electrolyte and improving the stability of the polymer electrolyte to the high voltage cathode.In addition,the ionic liquids with high ionic conductivity can be used to improve the solid-solid contact between electrodes and solid electrolytes, achieving the fast ionic transfer at interfaces.Therefore,the cycling performance of SSLBs can be improved through the combination of cathode coating and ionic liquid modification.