| In today's energy storage field,lithium-ion batteries occupy a dominant position.However,the development of commercial lithium-ion batteries is currently facing bottlenecks,limited by safety risks,such as leakage,combustion or even explosion caused by low-boiling organic liquid electrolytes.Using solid electrolyte is the most effective strategy to solve the security problem.Among them,polymer solid electrolyte has become one of the important technical directions because of its low flammability,good flexibility,excellent thermal stability and high safety.However,the low room temperature ionic conductivity and poor interface contact of polymer solid electrolytes are still the main challenges for its commercial development.Aiming at the above problems,this paper proposes to use polyvinylidene fluoride(PVDF)-based polymer electrolyte as a research system to optimize the preparation of PVDF-based composite solid electrolyte and the electrode/electrolyte interface modification to improve the electrolyte's stability against lithium,battery rate and cycle performance.The main research contents and results are as follows:(1)A PVDF-based gel electrolyte membrane(GPE)containing a polyethylene(PE)skeleton was prepared by the process of scraping film-electrolyte immersion.The effects of different lithium salt content on the physical and electrochemical properties of the electrolyte membranes were studied.The results show that when the lithium content is 50%,the ionic conductivity of PVDF GPE is 1.03×10-4 S·cm-1,the electrochemical window is 4.35 V,and the lithium ion migration number is 0.438.Assemble the NCM622/GPE/Li battery and at a current density of 0.5C,the initial specific capacity reaches 154.1 m Ah·g-1,after 100 charge-discharge cycles,the discharge specific capacity only maintains 55.6 m Ah·g-1.The research results indicate that the PVDF electrolyte of gel state is not ideal for improving the cycle stability of solid-state lithium battery.(2)The PVDF-LLZO composite solid electrolyte membrane(CSE)was prepared by scraping coating method by combining lithium lanthanum zirconium oxide(LLZO)and PVDF.The effects of different filler content on the physical and electrochemical properties of the composite solid electrolyte membranes were studied.The study found that when the LLZO filler content is 10%,the tensile strength of PVDF-LLZO CSE is 94.69 MPa,the heat resistance temperature exceeds150?,and it has good mechanical and thermal stability;the room temperature ionic conductivity can reach 1.75×10-4 S·cm-1,the electrochemical stability window is 4.6 V,the lithium ion migration number is 0.657,and it has good lithium dendrite inhibition ability.Under the wetting condition of the micro liquid electrolyte interface,the assembled NCM622/CSE/Li solid state battery has an initial specific capacity of 151 m Ah·g-1 at 0.5 C,and after 100 cycles,the specific capacity remains 108 m Ah·g-1.Compared with the gel state PVDF electrolyte,the cycle stability of the battery is obviously improved.(3)In order to avoid the direct introduction of unsafe liquid electrolyte,a flexible polymer buffer layer of polypropylene carbonate(PPC)-LLZO was constructed at the electrode/electrolyte interface,and the PVDF-LLZO/PPC-LLZO layered structure composite solid electrolyte(LCE)was prepared by scraping coating method.The effects of preparation process on the physical and electrochemical properties of layered composite solid electrolyte membrane were studied.The study found that the layered structure composite solid electrolyte membrane prepared under optimized conditions has an electrochemical stability window of 4.55 V,and the ion conductivity is 1.25×10-4 S·cm-1.Due to the formation of a good interface contact between the electrolyte and the electrode,the assembled NCM622/LCE/Li solid state battery has an initial specific capacity of152.7 m Ah·g-1 at a current density of 0.5 C,and after 200 cycles,the discharge specific capacity remains 106.9 m Ah·g-1.The composite electrolyte membrane also showed good stability against lithium and inhibition of lithium dendrite.At a current density of 0.1 m A·cm-2,the Li/LCE/Li battery can be cycled stably for more than 1600 h. |
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