Study On Preparation And Electrochemical Performance Of Li_2xLa_2/3-xTiO₃ Composite Electrolyte

With the development of human society,the next-generation lithium-ion batteries are developing toward high energy density,safety and long cycle life.However,the safety hazards of organic electrolytes such as volatilization,leakage and spontaneous combustion used in traditional lithium-ion batteries,as well as the low actual specific capacity of electrode materials,make it difficult to meet the development requirements of high energy density of lithium-ion battery.The novel solid-state batteries are seen as an important way to solve these problems,the core of which lies in the study of high-performance solid-state electrolyte materials.In this study,Li_3xLa_2/3-xTiO₃（1/24<x≤1/6,LLTO）,a perovskite-type solid-state electrolyte material with high ionic conductivity(～10^-3 S cm^-1),low conductivity activation energy（0.3～0.4 e V）and excellent mechanical properties,was selected as the object,and its preparation by solvothermal method was investigated.Furthermore,in veiw of the poor mechanical performance and unstable electrochemical performance of the LLTO composite electrolyte,the two composite electrolyte materials were prepared respectively by solution casting and tape casting,which use the optimized Li_0.33La_0.557TiO₃（x=0.11）as inorganic solid-state electrolyte filler,and the related electrochemical properties and the optimization mechanism were systematically studied.The study contents are as follows:（1）Study on synthetic process and performance of Li_3xLa_2/3-xTiO₃powder.The Li_3xLa_2/3-xTiO₃ ceramic powders with different lithium contents（x=0.08,0.11,0.12,0.14,0.167）were prepared by the solvothermal method,and the effects of lithium content and sintering temperature on the phase structure were investigated.When x=0.11 and the sintering temperature was not lower than 1000℃,the phase of ceramic powder was purest,which was consistent with the tetragonal phase crystal structure of the perovskite-type solid electrolyte Li_0.33La_0.557TiO₃,and its particle size was around 100～200 nm.The performance tests show that Li_0.33La_0.557TiO₃ ceramic nanoparticles had high thermal stability.（2）Study on preparation and electrochemical performance of LLTO/PEO-Li TFSI composite polymer electrolyte coatings.The LLTO/PEO-Li TFSI composite polymer electrolyte coatings with different additions were prepared on stainless steel sheets and Li Fe PO₄cathode plates by solution casting method,which used LLTO ceramic powder as inorganic filler,PEO and Li TFSI as polymer matrix,respectively.The results showed that the addition of LLTO could reduced the crystallinity of polymer matrix,and LLTO particles were uniformly distributed in the polymer matrix.The electrochemical performance tests showed that compared with the polymer matrix,the 5%LLTO/PEO-Li TFSI composite polymer electrolyte coating with 5 wt.%inorganic filler had the highest performance enhancement,and the first discharge capacity of the assembled solid-state battery was up to 173 m Ah g^-1 at 0.1 C with a Coulomb efficiency of 95.62%.In addition,the addition of LLTO can also improve the interfacial properties between the polymer matrix and the lithium metal anode.（3）Study on preparation and performance of LLTO/PEO composite solid-state electrolyte films.The LLTO/PEO composite solid-state electrolyte films with different additions were prepared on PET substrate by tape casting method through adding LLTO inorganic filler to PEO and Li TFSI polymer matrix and introducing PVP as binder,and the thickness could be as low as 24μm.The results showed that the addition of LLTO can significantly reduce the crystallinity of the polymer matrix,and LLTO particles were uniformly distributed in the polymer matrix and bonded closely with each other.The performance test results showed that the 20-LLTO/PEO composite solid electrolyte film with 20 wt.%inorganic filler had the highest performance enhancement compared with the polymer matrix,which had excellent thermal stability,stability to lithium,electrochemical and mechanical properties,including the tensile strain up to 950%.Meanwhile,the assembled solid-state battery showed good cycling stability and rate performance,and the discharge capacity was still about 76.4 m Ah g^-1 after 200 cycles at 1.0 C and 60℃.