Preparation,Modification And Performance Of Solid Electrolyte For Solid-state Fluoride Ion Batteries

Solid-state fluoride ion batteries(SSFIBs)based on fluoride ion shuttle are expected to become one of the efficient energy storage systems due to its high theoretical volume energy density(above 5000 Wh L^-1),high safety and other advantages.At present,the solid-solid interface impedance between electrode and electrolyte is large,and the solid electrolyte has a high ionic conductivity only at high temperature,which limits the further development of SSFIBs.Aiming at the above problems,this paper takes fluoride ion solid electrolyte and SSFIBs as research objects,novel high-performance fluoride ion solid electrolytes are prepared and successfully applied to the construction and research of SSFIBs through organic-inorganic composite,high-energy ball-milling,chemical co-precipitation and other methods.The main research contents and results are as follows:1.A flexible La_0.95Ba_0.05F_2.95@PEO solid composite electrolyte(LBF05@PEO)is prepared by organic-inorganic composite method to reduce the impedance of solid-solid interface between electrode and electrolyte in SSFIBs.The ionic conductivity of LBF05@PEO solid composite electrolyte reaches 5.75×10^-6 S cm^-1 at 90?,and the voltage stability window reaches 4.7 V(vs.Li⁺/Li)at 70?.With LBF05@PEO as solid electrolyte,metal Li as anode electrode and Fe F₃ composite material as cathode electrode,Fe F₃/LBF05@PEO/Li cell are prepared.Charge-discharge tests are carried out on the SSFIBs at 70?.The Fe F₃/LBF05@PEO/Li cell has an initial discharge capacity of 113.5 m Ah g^-1 at the current density of 0.1 C(1 C=273 m A g^-1)and the voltage range of 1.0-4.5 V.After reducing the charging cut-off voltage to 3.5 V,the cycling stability is further improved.The successful application of LBF05@PEO solid composite electrolyte in SSFIBs shows that the flexible electrolyte can effectively improve the interface contact between electrolyte and electrode,which provides a good idea for the research and development of SSFIBs.2.Aiming at the problem that the SSFIBs can only work at 150?or above,Pb Sn F₄ solid electrolyte is prepared by high-energy ball-milling and firstly applied to the construction and research of room-temperature SSFIBs(RT-SSFIBs).XRD results show that the Pb Sn F₄ solid solution has a monoclinic structure(?-Pb Sn F₄).After sintering 2 h at 300?,the phase of?-Pb Sn F₄ solid solution is transformed into the orthorhombic Pb Sn F₄(?-Pb Sn F₄)with a higher ionic conductivity.The RT ionic conductivity of?-Pb Sn F₄ solid electrolyte is as high as 5.44×10^-4 S cm^-1,and motion activation energy is 0.26 e V.Sn/Pb Sn F₄/Bi F₃ cell is built by using Sn as anode,Bi F₃ as cathode,and?-Pb Sn F₄ solid solution as solid electrolyte,respectively.The charging-discharging test shows that the initial discharge capacity of Sn/Pb Sn F₄/Bi F₃ battery is175 m Ah g^-1 at RT,and the capacity of 80 m Ah g^-1 is still maintained after 10 cycles.Further,XRD is used to characterize the chemical properties of the composite electrode material before and after discharge.After discharge,the Bi characteristic peak is detected in the Bi F₃ cathode electrode,and the Sn F₂ characteristic peak is detected in the Sn anode electrode,which confirms the charging-discharging behavior of Sn/Pb Sn F₄/Bi F₃ cell.3.Although Pb Sn F₄ solid electrolyte enables the SSFIBs to work at RT,in order to reduce the use of lead and environmental pollution caused by lead,this chapter applies high-energy ball-milling to prepare environmentally friendly Ba Sn F₄ solid electrolyte with high ionic conductivity.XRD results show that Ba F₂ and Sn F₂ produce cubic Ba Sn F₄ solid solution(?-Ba Sn F₄)with fluorite structure after high-energy ball-milling.After 2 h sintering at 300?,the?-Ba Sn F₄ solid solution changes into tetragonal phase(?-Ba Sn F₄)with higher ionic conductivity.EIS results show that the ionic conductivity of the?-Ba Sn F₄ solid electrolyte reaches 2.02×10^-4 S cm^-1 at RT,and the motion activation energy is 0.23 e V.The Sn/Ba Sn F₄/Bi F₃ cell is constructed with?-Ba Sn F₄ solid solution as solid electrolyte,Sn and Bi F₃ as anode and cathode,respectively.The Sn/Ba Sn F₄/Bi F₃ cell has an initial discharge capacity of 125 m Ah g^-1at RT,and has a capacity of 53 m Ah g^-1 after 10 cycles.The successful preparation of environmental-friendly and pollution-free Ba Sn F₄ solid electrolyte has provided a broader space for the research and development of RT-SSFIBs.4.In order to further improve the RT ionic conductivity of Ba Sn F₄ solid electrolyte and the electrochemical performance of SSFIBs,Ba Sn F₄ solid solutions doped with different chemical calculation ratio Nd³⁺are prepared by chemical co-precipitation method and used as solid electrolyte for the first time in the preparation and research of RT-SSFIBs.Ba_1-xNd_xSn F_4+x(0?x?0.08)solid solutions prepared by chemical coprecipitation has flake structure and tetragonal-type crystal structure.Contrast study shows that the crystal structure of sintered Ba_1-xNd_xSn F_4+x solid solutions does not change,and the density is improved.The Ba_0.98Nd_0.02Sn F_4.02 solid electrolyte has optimal electrochemical properties.The ionic conductivity of Ba_0.98Nd_0.02Sn F_4.02 solid electrolyte is as high as 5.8×10^-4 S cm^-1,which is nearly 3 times larger than un-doped Ba Sn F₄ sample.The motion activation energy is 0.15 e V,and electronic conductivity is about 7.8×10^-8 S cm^-1.The RT-SSFIB,in which Ba_0.98Nd_0.02Sn F_4.02 is used as solid electrolyte,Sn as anode and Bi F₃ as the cathode,exhibits the specific discharge capacities of 135 m Ah g^-1 and 95 m Ah g^-1 at the 1st and 20th cycles,respectively.Besides,the rate performance of Sn/Ba_0.98Nd_0.02Sn F_4.02/Bi F₃ cell at the current density range of 12.7-127?A cm^-2 is also obtained for the first time.The successful application of Nd³⁺doped Ba Sn F₄ solid electrolyte in RT-SSFIBs provides a new route for the research of solid electrolyte and RT-SSFIBs.5.To improve the performance of Ba Sn F₄ solid electrolyte for RT-SSFIB,Eu³⁺doped Ba Sn F₄ solid solutions[Ba_1-xEu_xSn F_4+x(0?x?0.06)]are designed and prepared.XRD results show that Ba_1-xEu_xSn F_4+x solid solutions are a tetragonal-type crystal system with P4/nmm space group.Rietveld refinement of the XRD data shows that Eu³⁺can be successfully doped into Ba²⁺in Ba Sn F₄ crystal.After Ba_1-xEu_xSn F_4+x solid solution is sintered at 300?for 2 h,the grains grow further,the surface poriness of the particles is smaller,and the density is improved.A higher ionic conductivity of the Ba_0.98Eu_0.02Sn F_4.02 solid electrolyte achieves 3.8×10^-4 S cm^-1 at RT.The motion activation energy is about 0.15 e V.Besides,the performances of RT-SSFIBs based on Ba1-x Eux Sn F4+x(0?x?0.04)solid electrolytes have been studied,in which Sn and Bi F₃is used as the anode and cathode,respectively.Sn/Ba0.98Eu0.02Sn F4.02/Bi F3 cell has a discharge capacity of 106 m Ah g^-1 for the 1st cycle and 72 m Ah g^-1 for the 20th cycle,respectively.Moreover,the cycling stability and rate capability of Sn/Ba1-xEux Sn F4+x/Bi F3(0?x?0.04)batteries are measured and compared.The results demonstrate that the incorporation of Eu³⁺can significantly improve the ionic conductivity of Ba Sn F₄ solid electrolyte and thus boost the electrochemical performance of RT-SSFIBs.This paper takes the fluoride ion solid electrolyte material as research object and successfully prepares the new solid electrolyte materials,which are applied to the construction and research of SSFIBs.From flexible solid electrolyte to improve electrode and electrolyte interface,to Sn F₂-base solid electrolyte achieves RT-SSFIBs,and then to rare earth metal adulteration promotes Ba Sn F₄ solid electrolyte for RT-SSFIBs,the research of this paper provides theoretical basis and technical support for solid electrolyte and SSFIBs and laid a good foundation for the research and development of SSFIBs.