The Preparation And Research Of PVDF-HFP-based Composite Solid Electrolyte

In recent years,global energy and environmental problems have become increasingly serious.Countries around the world have begun to focus on the research of new energy sources.Among them,the development of lithium batteries has taken the lead.Countries have formulated timetables for eliminating fuel vehicles and comprehensively replacing them with new energy vehicles.It is booming and technology is advancing by leaps and bounds.However,currently facing the traditional liquid electrolyte,the development of lithium batteries has encountered bottlenecks.On the one hand,the battery capacity and endurance requirements are constantly increasing.On the other hand,it challenges the battery safety.In safety accidents,people have gradually realized that the liquid electrolyte is difficult to completely solve the problem of unavailability of both capacity and safety,and the development of solid-state batteries has ushered in a turning point.As the name implies,solid-state batteries use stable solid-state electrolytes instead of flammable,explosive,and leaky liquid electrolytes to fundamentally solve battery safety issues.Not only that,solid-state electrolytes have higher electrochemical windows and increase battery operating voltage.Inhibiting the growth of lithium dendrites,the ideal material with the highest theoretical specific capacity,lithium metal,is used as the negative electrode,which can greatly increase the capacity;at the same time,it has many advantages such as flexibility,foldability,and easy packaging.Solid electrolytes have become the most likely development direction for next-generation lithium batteries.PVDF-HFP has a strong affinity for electrolytes.The presence of a strong electron-withdrawing functional group（-C-F-）facilitates the dissolution of the lithium salt,and strong dielectric properties support a high concentration of carriers.It also has better thermal stability,electrochemical stability and mechanical properties than PEO,which makes PVDF-HFP an excellent candidate for lithium battery system applications.In this paper,PVDF-HFP is used as the polymer matrix,on which the modification research is carried out.The new carbon ball fillers are studied,the in-situ coating of the composite cathode is studied,the difference between the front and back of the cast membrane,and the lithium-air battery Study on the design of layered asymmetric structure electrolyte.The specific research content is as follows:（1）The oxidized nano-carbon spheres are used as a new polymer electrolyte modified filler.Its size is controllable and it is rich in oxygen-containing groups.It disperses well in the polymer matrix and produces surface interactions,which can reduce the polymer’s The crystallization tendency effectively reduces the membrane resistance,and the rich interface produced by the doping of carbon spheres provides various transport routes for Li⁺,thereby improving the ionic conductivity.The ionic conductivity at room temperature reached 1.98×10^-4 S cm^-1 at room temperature,and the lithium ion transfer number t⁺_Lireached 0.82.The lithium plating/stripping cycling at a current density of 0.1 m A cm^-2 for more than 1400h.The composite polymer membrane has a certain high voltage resistance,and the electrochemical window reaches 4.9V.In the cycle test,the 2%Cs-QSPE separator performs better in LFP|QSPE|Li lithium-ion batteries,and the cycle performance of 5%Cs-QSPE is better in Li-O₂ batteries.In the QSPE|Mn₂O₃battery,the battery cycle reaches 204 cycles and the stable cycle exceeds 1000h.In summary,carbon spheres are a promising solid electrolyte filler that can be applied to lithium-ion batteries and Li-O₂batteries,which significantly improves the performance of polymer electrolytes and improves the cycle capacity of batteries,especially in Li-O₂ batteries The effect is more obvious.（2）The traditional solution-casting single-layer membrane cannot solve the solid-solid resistance bottleneck.The composite solid electrolyte is directly cast on the cathode layer to prepare a composite membrane coated with the cathode in situ to enhance the adhesion between the cathode layer and the solid electrolyte membrane The resultant force,and the ability of the solid electrolyte to wet the cathode is enhanced because the pores inside the cathode are filled with the solid electrolyte.There is no need to separately prepare a separate cathode layer and a separate solid electrolyte membrane,achieving excellent cycle performance of the battery and an ultra-high capacity retention rate.PVDF-HFP,Li BOB,and LLZTO are used to prepare a composite polymer electrolyte membrane by mechanical blending.Compared with pure PVDF-HFP,the electrochemical window is significantly improved,reaching5V;at a current density of 0.1 m A cm^-2,the Li|FLB-CPE|Li battery can stably cycle for more than 2000 hours without any significant polarization.The in-situ Li|In-situ coating-CPE|LFP battery has a specific discharge capacity of 155.1 m Ah g^-1in the first circle,and a capacity of 138.1 m Ah g^-1 remaining after 350.The capacity retention rate is as high as 89%.Reflected excellent cycling performance of in-situ coated cathode composite membrane.（3）The traditional solution casting method,as the most commonly used method for preparing composite polymer electrolytes,has its inherent shortcomings.First,during the drying of the membrane,the upper surface due to the large amount of solution volatilization,will cause pores on the upper surface,showing loose and porous Topography;the lower surface is densely deposited.Second,during the drying of the solution,it is inevitable that the additive sinks and the distribution of the filler on both sides is uneven.Such structural differences will have a certain impact on battery performance.When the amount of electrolyte added is small,the difference between the front and back has a greater impact on the performance of Li Fe PO₄ batteries,the capacity of the front-mounted battery decreases significantly,and the Coulomb efficiency is unstable,while the reverse-mounted battery has better cycle performance.When the amount of electrolyte is increased,the difference of Li Fe PO₄ battery membrane is not obvious.For NCM811 batteries,reverse installation is more conducive to the performance of the battery.（4）A double-layer asymmetric composite solid electrolyte（DLCE）designed for different requirements of cathode and anode.The cathode side is composed of polymer and lithium salt,and an interactive flexible layer is constructed,which is stable and in good contact with the cathode.Isolation prevents air from crossing and protects lithium metal from side reactions with air.The anode side is mainly composed of LLZTO,which forms a rigid layer.While improving the ionic conductivity,it induces lithium ion deposition,inhibits lithium dendrite growth,and prevents dendrites from piercing the diaphragm to cause short circuits.Wetting with an appropriate amount of electrolyte,the ionic conductivity of the DLCE membrane reached 5.23×10-4S cm^-1 at room temperature,and the obvious electrochemical decomposition voltage reached 5.2V.The performance of the lithium cycle test at 0.1m A cm^-2,the DLCE membrane was stable for more than 2000h without significant polarization.Li|DLCE|Super P Lithium Oxygen battery achieves excellent performance of more than 360 cycles and stable cycle of more than 1800h.Under this system,Li BOB-DLCE is beneficial for the battery to achieve stable and long cycle.Thickness investigation shows that the thicker DLCE membrane is more conducive to the performance of lithium-oxygen batteries.The DLCE membrane not only performs well in Li-O₂ batteries,but also achieves excellent cycle performance in Li-CO₂batteries and lithium-air batteries.