Preparation And Electrochemical Performance Of PEO/LLZNO-based Flexible Composite Solid Electrolyte

Lithium-ion batteries have become an important energy carrier in the 21st century because of their good energy storage performance.However,traditional liquid lithium-ion batteries not only have potential safety hazards such as flammability,explosion,and leakage due to the use of liquid electrolytes,but also make it difficult to further increase energy density.The replacement of liquid electrolytes by solid electrolytes can solve this situation and promote the development of lithium-ion batteries.Among the many electrolytes,the garnet-type solid electrolyte LLZO has attracted much attention because of its high ionic conductivity and electrochemical stability.However,the solid garnet type solid electrolyte LLZO has a complicated sintering process;it is difficult to obtain a stable cubic phase structure and a large interface resistance.Polyethylene oxide(PEO)is also popular because of its good flexibility,plasticity and good solubility in lithium salts.PEO and LLZO are combined to construct an organic/inorganic composite electrolyte membrane,and the chemical properties are studied.The main results are as follows:(1)The garnet type solid electrolyte LLZO was prepared by the solid-phase reaction method.In order to stabilize the cubic phase structure of LLZO by introducing Nb element,excessive lithium source was added to reduce the volatilization of lithium under high temperature calcination.The influence of Nb impurity amount,lithium source addition amount,calcination temperature,calcination time on the preparation of LLZO was investigated,and the structure and morphology of LLZO were analyzed by XRD and SEM.The experimental analysis results show that at optimal conditions i.e.the Nb doping amount of 0.5,the excess Li of 10%,and sintering at 950? for 6 h,garnet type solid electrolyte Li6.5La3Zr1.5Nb0.5O12(LLZNO)powder with a stable cubic phase structure was obtained.By ball milling for 12 h,the LLZNO particles can be reduced from 2-3 ?m to 300-400 nm,and then pressed at 20 MPa for 10 min to prepare LLZNO ceramic pellets.Through the design of the buried powder sintering process,the Li element can be effectively reduced at high temperatures.Li6.5La3Zr1.5Nb0.5O12 ceramic sheets are sintered at 1250? for 2 h to obtain complete densification,and the ion conductivity at 25? can reach 3.82×10-4 S cm-1.(2)The solution casting method was used to prepare flexible PEO20-LiTFSI-x wt%LLZNO composite solid electrolyte membrane,which were characterized by XRD and SEM.The commercial LiFePO4 was used as the positive electrode and the metallic lithium was assembled into an all-solid-state lithium ion battery,and its electrochemical performance was tested at 60?.The experimental results show that when the amount of lithium salt added n(EO:Li)is 20:1,LLZNO is 10 wt%,PEO20+LiTFSI+10wt%LLZNO composite electrolyte membrane has the highest ionic conductivity,it can reach 6.21×10-4 S cm-1 at 60?.In addition,the battery electrochemical window can reach 5.2 V,and the ion migration number(tLi+)is 0.32.The constant current charge and discharge test of Li/PEO20-LiTFSI-10 wt%LLZNO/Li symmetric battery shows up to 800 h of stability and reversibility,indicating that the composite electrolyte membrane has good compatibility with Li metal electrodes.The uniform deposition of Li+has sufficient mechanical strength to resist the growth of lithium dendrites.All solid-state battery LiFePO4/PEO20-LiTFSI-10 wt%LLZNO/Li has long cycle stability.The cell delivers an initial discharge capacity of 153.4 mAh g-1 with Coulombic efficiency 96.54%at 0.5 C and 60?.After 200 cycles of stable cycling,the discharge capacity is still 143.3 mAh g-1,and the discharge capacity retention rate is as high as 93.4%.