Gel Polymer Electrolyte With High Li~+ Ion Transference Number For Lithium Metal Batteries

With the development of consumer electronics and electric vehicles,the demand for high-energy density and high safety storage batteries has increased sharply.Lithium（Li）is regarded as one of potential anode materials of high-energy density batteries owing to its high theoretical specific capacity of 3860 m Ah g^-1and its low reduction potential of-3.04 V versus the standard hydrogen electrode.However,Li metal anode has poor compatibility with liquid electrolyte,which results in unstable interphase,uncontrolled growth of Li dendrites,and safety issues.Solid state electrolytes are considered to be more secure than liquid electrolytes.Inorganic solid electrolytes have superior thermal stability but poor interfacial compatibility.Solid polymer electrolytes have high flexibility and safety,but its low ionic conductivity limits their practical application.Gel polymer electrolytes（GPEs）as an intermediate state between liquid electrolytes and solid polymer electrolytes have been identified as the promising electrolyte to enhance the stability of Li metal batteries due to its excellent flexibility,good electrode/electrolyte interface compatibility and high ionic conductivity.However,GPEs usually have low Li⁺ion transference number.We designed dual-polymer organic-inorganic composite gel electrolytes and multiscale confinement GPE of micron,nano,sub-nano scales.The Lewis acid site of nano inorganic additives,polar groups of polymers and size effect can limit the movement of anions,improve the Li⁺ion transference number and inhibit the growth of Li dendrites.And it provides insight into the design of innovative GPEs for Li metal batteries with high performance.（1）The polymer skeleton was prepared by electrospinning with propylene carbonate（PPC）and PVDF-HFP.PPC has good electrode-electrolyte compatibility and low crystallinity,which can increase the amorphous region of electrolyte by blending with PVDF-HFP.And the polar groups of polymer are conducive to accelerate Li⁺ion transport.The monodispersed Si O₂nanoparticles with particle size only 14 nm was synthesized by using lysine as catalyst,which could reduce the Li⁺ion migration energy barrier,promote the Li salt dissociation and increase the Li⁺ion transference number.The gel polymer electrolyte prepared by the composite of monodisperse Si O₂nanoparticles and polymer skeleton（Si O₂-PPGPE）can effectively accelerate Li⁺ion migration and improve the stability of the interface.In electrochemical performance tests,the Si O₂-PPGPE exhibit a high Li⁺ion transference number of 0.73 and wide electrochemical window of 4.8 V.And it enables a stable life for 300 cycles with a remarkable capacity retention of 90%in NCM811 full cell.（2）We design a novel multiscale gel polymer electrolyte,where nanosized Si O₂and ZSM-5 nanoparticles as inorganic fillers are incorporated into poly（vinylidene fluoride-co-hexafluoropropylene）（PVDF-HFP）-based GPE（denoted as SZ-GPE）with high porosity and mechanical properties,to promote uniform deposition of Li⁺ion,improve interfacial stability and cycling stability.Due to the Lewis acid sites,Si O₂nanoparticles can absorb amounts of PF₆^-and promote the dissociation of Li PF₆.The sub-nanometer confinement effect of ZSM-5 greatly enhance the Li⁺ion transference number.The combination of SZ-GPE has a synergistic effect that can effectively create a reliable solid electrolyte interface（SEI）on the Li metal anode and reduce side reactions.Benefited from the multiscale structure design,SZ-GPE shows a high Li⁺ion transference number of 0.67 and excellent cycling performance.SZ-GPE is prolonged to 1200 h at 1 m A cm^-2.And it enables a stable life for300 cycles with a remarkable capacity retention of 92%in Li||Ni_0.8Co_0.1Mn_0.1O₂（NCM811）full cell.