Preparation And Properties Of Polymer Electrolytes For All-Solid-State Lithium Metal Batteries

The increasing depletion of traditional fossil energy sources（including coal,oil,etc.）due to the massive energy consumption for human production and living,and the resulting climate and environmental problems have posed a serious challenge to the sustainable development of society.The development of new green and renewable energy sources and their energy storage devices has become one of the hot areas of economic and technological competition in the world in recent years.Lithium-ion batteries are a highly efficient secondary energy storage device with the advantages of high energy density,high output power,low self-discharge,wide operating temperature,no memory effect and environmental friendliness.But the traditional lithium battery on the one hand due to electrolyte leakage,the electrolyte electrode reaction between the intensification of serious safety hazards at high temperatures or high current density,such as spontaneous combustion,self-destruction;on the other hand,the traditional lithium-ion battery are also close to the theoretical limit in energy density and power density and other performance indicators after decades of development,and cannot fully meet the actual needs for producing the batteries with high-performance high-power.Therefore,manufacturing all-solid-state lithium metal batteries with safe,high energy-density and power-density has important economic and strategic significance.Published studies indicate that for an all-solid-state lithium battery the solid-state electrolyte is the bottleneck to limit its widely application and progress.Therefore,it has a role value in application and science to undergo the researches on new solid-state electrolytes with high-performance.In this paper,two new solid-state polymer electrolytes,PEO polyurethane-CA blends capped by siloxane（PU-CA）and polyester polyurethane-b-polysiloxane（PUSR）block copolymers,were designed and prepared.The structures of the polymers and solid-state polymer electrolytes（SPEs）were characterized by FT-IR,XPS and XRD,respectively;their morphology,physical properties and electrochemical properties for the solid polymer electrolytes were investigated as well by scanning electron microscopy,electron tension machine,differential scanning calorimetry and electrochemical workstation,respectively.The effects of different factors on their electrochemical performance of SPEs,such as temperature,lithium salt type and content in SPE,etc.,were investigated by designing orthogonal tests and grey correlation analysis using the controlled variates method.Assembling the Li Fe PO₄ all-solid lithium batteries using these made SPEs in a glove box to verify the SPEs’by testing different performance indexes of these cell,and the main results based on above researches were listed as following:1.Successfully prepared two solid polymeric electrolyte matrices,i.e.siloxane-capped PEO polyurethane-CA blend（PU-CA）and polyester polyurethane-b-polysiloxane（PUSR）block copolymer,and two types of new polymer-based solid electrolytes from the synthesized polymers.2.The results from the orthogonal experiments,their grey correlation analysis and analysis of variance showed that the three variables,PEO molecular weight,CA addition ratio and EO and Li molar ratio in the blend,had almost equal effects on ionic conductivity of the PU-CA electrolyte with correlation parameter magnitudes of 0.7838,0.7963 and 0.811,respectively.3.The ionic conductivity,Li⁺transference number and electrochemical window were2.5×10^-5 S·cm^-1,0.34 and 4.7 V,respectively,at room temperature for the PU-CA-Li BOB polymeric solid electrolyte prepared using a PEO to CA mass ratio of 10:1 and an EO to Li⁺molar ratio of 16:1.Its thermal stability exceeded 200°C,and the maximum fracture strength and maximum elongation reached 2.2 MPa and 13%,respectively.4.The PUSR-Li BOB40 block copolymer solid electrolyte was prepared with ionic conductivity（σ）,Li⁺transference number(t_Li⁺)and electrochemical window up to1.5 x 10^-4 S·cm^-1,0.82 and 5.0 V at room temperature,respectively.Its maximum fracture strength and maximum elongation reached to 5 MPa and170%,respectively.The thermal decomposition temperature is above 320°C.5.For PUSR-based electrolyte systems prepared with three lithium salts（Li BF₄,Li NO₃,Li BOB）respectively.Li BOB salt had the best performance for matching with the PUSR polymer.The Li⁺ionic conductivity and Li+transference number also increased significantly as increase of the addition of Li BOB from 0 to 50%,and in the same time the crystallinity of the electrolyte generally decreased.The electrolyte with 40%Li BOB（PUSR-Li BOB40）shown the lowest crystallinity.The electrochemical window reached maximum（5.5 V）for the electrolyte with 30% Li BOB addition（PUSR-Li BOB30）.6.Two all-solid-state lithium metal batteries（LMBs）,Li|PU-CA-Li BOB16|Li Fe PO₄ and Li|PUSR-Li BOB40|Li Fe PO₄,were assembled,respectively using the prepared polymer solid electrolytes.The combined performance of the two polymeric solid electrolytes was verified and evaluated by testing the overall performance of the batteries.The experimental results show that the Li|PU-CA-Li BOB16|Li Fe PO4 battery has a first discharge specific capacity of 121.8 m Ah·g^-1 at 25°C and 0.5 C,and a capacity retention rate of 96.7%after 50 cycles.The discharge specific capacities at 0.1,0.2,0.5 and 1.0 C were 141.7,128.9,114.7 and 97.5 m Ah·g^-1,respectively,and remained cyclically stable during charging and discharging.For Li|PUSR-Li BOB40|Li Fe PO₄ cells reversible discharge specific capacities of 143.0,124.2,108.8 and 82.3 m Ah·g^-1 were obtained at 0.1,0.2,0.5 and 1.0C,with charge/discharge efficiencies in the region of 95%.The first discharge specific capacity of the Li|PUSR-Li BOB40|Li Fe PO₄ battery was 111.6 m Ah·g^-1 at 25°C,0.5C,with a capacity retention rate of 94.82%after 50 cycles.7.The PUSR block copolymer-based electrolyte with good thermal and mechanical properties has outstanding electrochemical properties,The all solid state polymer metal lithium cell made from PUSR has high energy-and power-density and good cycling stability.Therefore,it is an effective candidate electrolyte material for the preparation of high-performance all-solid polymer metal lithium batteries.