Developing The Compatible Compatible Interphase Between Garnet-type Solid Electrolyte And Electrode For Solid State Rechargeable Li Batteries

All-solid-state batteries are considered to be one of the most promising next-generation battery technologies due to their outstanding safety,high energy density,and long cycle lifespan.However,their full potential realization is hampered by complex structure,design and electrochemical instability.The main limitation lies in the interface compatibility in terms of stability and safety that the Li should diffuse through.The electrochemical impedances caused by physical contact,volume expansion,elemental diffusion,chemical and electrochemical stability at the solid-solid interfaces remain unresolved.In this context,using Garnet-type solid state electrolyte for its high ionic conductivity,wide electrochemical window,and high chemical and electrochemical stability,solid-state lithium-ion batteries are fabricated with the novel design of cathode/solid electrode composites and separator,which demonstrate superior electrochemical performance.These important findings shed new light on addressing the solid-solid interface issues for Li ion batteries.The main progresses are displayed as follows:(1)The Al-doped LLZO(LLAxZO)was prepared by the sol-gel method with the following high temperature annealing treatments.The effects of Al content and sintering temperature were investigated on the preparation of LLZO.It is found that the Al-doped cubic LLZO(cubic-LLAxZO)was synthesized with the Al content of 0.2 and the sintering temperature at 800℃.The structure,morphology and phase stability of the as-prepared cubic-LLAxZO sample were then studied.The electrochemical window of ca.7.0 V for LLAxZO was measured using cyclic voltammetry.Afterwards,the LLAxZO powder was pressed and sintered into ceramic pellets with the Au as block electrodes to check its ionic conductivity,which is 2×10-5 S/cm by EIS characterization.Then a composite positive electrode with the mixture of LiFePO4 and LLAxZO slurry was prepared to assemble solid-state battery.At a rate of 0.1 C,the battery had a low Coulombic efficiency(first cycle)and a rapid capacity decay thereafter.After electrochemical test,SEM observation demonstrated that the physical contacts between positive electrode layer and LLZO layer were poor enough with obviously particle separation and also plenty of voids in LLAxZO electrolyte itself,which had negative affect on the interface stability and ionic diffusion kinetics.Moreover,an elemental interdiffusion phenomenon between LiFePO4 and LLAxZO was caught by the observation of Zr distribution in the composite electrodes.(2)Using then porous PVDF-HFP membrane as matrix,an organicinorganic composite gel polymer electrolyte(PLxL-GPE,x corresponds the mass ratio of LLZTO vs.PVDF-HFP)was prepared with the LLZTO(Li6.4La3Zr1.4Ta0.6O12)and LiFSI as fillers.The influence of LLZTO on the electrochemical performance of PLxL-GPE membrane was substantially documented.PVDF-HFP With 3D network,the PVDF-HFP has high crystallinity at room temperature,while its bulk modulus is only 1.99 MPa to show the poor mechanical property.The ionic conductivity and electrochemical window of PVDF-HFP membrane were tested to be 2.98×10-5 S/cm,and 3.7V,respectively,at room temperature is.However,the crystallinity of PVDF-HFP is decreased in PLxL-GPE,which facilitates the ionic transport.Moreover,the mechanical strength was significantly enhanced in PLxL-GPE membranes due to the LLZTO fillers,where the PL30L-GPE showed a dense structure with an elastic modulus of 9.42 MPa.Compared with the PVDF-HFP membrane,the PLxL-GPE showed better thermal stability,and there was no significant weight loss before 130℃ by TGA tests.Further electrochemical characterizations indicated that the ionic conductivity of PL30L-GPE was 3.57×10-4 S/cm with the activation energy of 0.133 eV and the electrochemical window was 4.27 V at room temperature.Using the symmetric Li/PL30L-GPE/Li cell,the polarization voltage of the cell maintained at 0.06 V after 500 h electrochemical plating/stripping to show the stability of PL30L-GPE against the Li anode.FTIR test also implied that the structure of PL30L-GPE was not significantly changed before and after cycle.With a Li transference number of 0.62,the PL30L-GPE membrane demonstrated the excellent electrochemical performance in a Li/PL30L-GPE/LiFePO4 full cell.which delivered a room-temperature specific capacity of 150 mAh g-1(after 110 cycles,0.4 C,Coulombic efficiency:98%)with a good capacity retention of 92%,as well as the excellent rate performance at 2 C(about 120 mAh g-1).These important findings provide a rewarding avenue to the design of separator,the protection of Li anode,the improvement of interface and the development of high-performance solid battery.The knowledge on interface reconstruction deepens the understanding of designing stable solid-solid contacts.