The Synthesis And Performances Of LLZO-PEO Based Composite Solid Electrolytes

The demand for efficient energy storage systems of Li-ion batteries(LIBs)is gaining immense attention due to a great potential to convert into electricity from intermittent renewable energy sources such as solar energy and wind energy.Moreover,LIBs based on ultrahigh energy density have shown attractive application in electronic equipment and electric vehicles.The application of Li metals as anode materials can further improve the energy density of LIBs because metallic Li offers the lowest redox potential(-3.040 V vs.the standard H+/H2)and the highest theoretical specific capacity(≈3.8Ah g-1).However,the actual application of Li metals in conventional batteries meets one critical challenge,that is,the growth of Li dendrites in organic liquid electrolyte systems during Li cycling.Such growth easily poses threats of short circuits.Furthermore,the solid electrolyte interphase layers that are formed at the interface between electrolytes and electrodes cannot bear mechanical deformation.These layers undergo continuous formation and dissolution,leading to low Coulombic efficiency(90%~95% for using ether solvents and 80%~90% for using carbonate solvents)and poor cycle life.Solid-state lithium batteries that use solid-state electrolytes(SSEs)instead of liquid electrolytes have been recognized as a method to resolve these issues.However,in order to ensure good performance for solid-state lithium batteries,practical SSEs should be sought to meet the following: high ionic conductivity(more than 10-4 S cm-2),good thermal,good chemical and electrochemical stabilities,and excellent mechanical properties,flexible surface,and ease of handling.However,SSEs have met no practical application in solid-state batteries up till the present moment.solid polymer electrolytes have better flexibility.However,its low ionic conductivity or mechanical properties seriously restrict its application.Inorganic solid electrolytes including oxide solid electrolytes and sulfide solid electrolytes generally have high ionic conductivity(>10-4 S cm-2).However,oxide solid electrolytes have high interfacial resistance between electrolyte and electrodes due to the high rigidity.sulfide solid electrolytes are extremely sensitive to the external environment,and the mechanical strength is not high enough to effectively inhibit the lithium dendrite.Adding Inorganic solid electrolytes into solid polymer electrolytes to prepare composite solid electrolytes seems to be an effective way to solve the above problems,which combines the advantages of solid polymer electrolytes and inorganic solid electrolytes.Poly(ethylene oxide)(PEO)has been extensively studied as a polymer matrix for solid polymer electrolytes due to its good electrochemical stability and excellent capability of dissolving a wide variety of lithium salts.Garnet-structured Li7La3Zr2O12(LLZO)as inorganic solid electrolyte has received increasing scientific attention due to its high ionic conductivity,good thermal performance,superior electrochemical and chemical stability.In this paper,we focused on the composite solid electrolytes prepared by PEO and LLZO to obtain good comprehensive performance to realize good performance for solid-state lithium batteries.The detailed research content includes:(1)Ga-doped Li6.5La3Zr1.5Bi0.5O12(Li6.5-3x Gax La3Zr1.5Bi0.5O12,0 ≤ x ≤ 0.3)ceramics were prepared via the Sol-Gel method.Then,the influences of Ga contents on the crystal structure,micromorphology,and ionic conductivity of the above ceramics were systematically studied.Our results show that all samples appear to be in cubic phase.Suitably doping Ga contents(x = 0.1)promoted the densification of ceramics with relative density of 93.5% and improved ionic conductivity of 1.7×10-4 S cm-1.This finding on improved ionic conductivity can be attributed to the enhanced densification and occupation of the octahedral 96 h site.(2)PEO/polyvinylidene fluoride(PVDF)blending solid polymer electrolytes with different lithium bis(trifluoromethane sulfonyl)imide(Li TFSI)contents were synthesized by casting.First,the PEO matrix was optimized by adding PVDF.The addition of 30% of PVDF into the PEO yielded a low melting point and crystallinity PEO/PVDF matrix,which is beneficial for the improvement of ionic conductivity.Meanwhile,the PEO/PVDF matrix showed superior mechanical strength and thermostability in comparison with the pure PEO matrix.Second,the effects of lithium Li TFSI concentrations on the micro-structure and ionic conductivity of the blend PEO/PVDF-based solid polymer electrolytes were studied.The weight ratio of Li TFSI and PEO/PVDF is 1:5(Li TFSI:PEO/PVDF = 1:5),maximum conductivities are 2.98×10-5 S cm-1 at 30 °C and 5.56×10-4 S cm-1 at 60 °C.Furthermore,the all solid-state battery using this solid electrolyte film,a Li metal anode,and a Li Fe PO4 cathode delivers initial discharge capacities of 149.6 m Ah g-1(0.1 C,60 °C)and 130.2m Ah g-1(0.5 C,60 °C).Meanwhile,the all solid-state lithium battery also presents good cycling performance and excellent rate stability at 60 °C.(3)PEO/PVDF-Li TFSI-LLZO and PEO/PVDF-Li TFSI-LLZO-EC composite solid electrolytes were synthesized by casting.The PEO/PVDF-Li TFSI-LLZO composite solid electrolyte with 10wt%LLZO shows the maximum conductivity(4.2×10-5 S cm-1 at 30 °C and 7.02×10-4 S cm-1 at 60 °C).In addition,all-solid-state Li Fe PO4| |Li battery assembled this solid electrolyte shows good cycling stability,which retained 96.5% of the maximum capacity after 100 cycles,and columbic efficiency(close to 100%)at 60 °C.The Li| |Li symmetric battery assembled with the solid electrolyte can be steadily cycled for more than 300 h at a current density of 0.2 m A cm-2 at 60 °C.The PEO/PVDFLi TFSI-LLZO-EC composite solid electrolyte with 40wt%EC shows the maximum conductivity(2.64×10-4 S cm-1 at room temperature).Solid-state Li Fe PO4| |Li battery assembled this solid electrolyte can work for more than 80 cycles at 0.2 C(room temperature).(4)A soft-rigid sandwich-structured hybrid electrolyte,where polymer-in-ceramic(PIC)electrolytes(rigid inner structure)are sandwiched in between Celgard separator membranes(soft outer structure),was designed and prepared.Results show that the hybrid electrolyte exhibits high ionic conductivity(4.76×10-4 S cm-1 at room temperature),low interfacial resistance(below 300 Ω for two sides),high thermal stability,and effective inhibition of Li dendrite growth.Moreover,the solid-state Li Fe PO4| |Li battery that used this sandwich-structured hybrid electrolyte delivered an initial discharge capacity of 125.8 m Ah g-1,a high capacity retention of 94.9% after 200 cycles,good rate capability,and an excellent coulombic efficiency(close to 100% except for the value of the initial cycle,which is84%)of 0.2 C at room temperature.