Research On Performance Optimization And Interface Modification Of Solid-State Composite Electrolyte Lithium Metal Batteries

Lithium ion batteries（LIBs）are one of the power sources of choice for the twenty-first century energy economy.However,although lithium-ion batteries are relatively satisfactory for portable electronics,their performance is far from satisfactory.Batteries with high energy and power densities,long cyclability,and excellent safety performance are urgently needed in our lives.Most batteries on the market today still use liquid electrolytes.The ideal electrolyte should not only provide suitable ionic conductivity over a wide range of ambient temperatures,but also maintain good electrochemical stability and electrode surface wettability.In most commercial batteries,the performance of liquid electrolytes based on organic solvents and lithium salts has been optimized for high performance in terms of electrical conductivity.However,organic electrolytes often suffer from insufficient electrochemical（thermal）stability,low ion selectivity,and poor safety.Research into safer and more reliable electrolytes is a priority.Compared with traditional liquid-based Li-ion energy storage systems,the application of solid-state electrolytes（SSEs）in Li-ion batteries with expected advantages in terms of safety,operating temperature range,and improved weight and volumetric energy densities has led to of great interest.In addition,the use of SSE can increase the packing density of Li-ion batteries,alleviate self-discharge problems,and improve cycle life.Polyvinylidene fluoride-co-hexafluoropropylene（PVDF-HFP）is a copolymer,which has a strong affinity for the electrolyte,and has a strong electron-absorbing functional group（-C-F-）on the molecular structure.The C-F bond and high dielectric constant facilitate the dissolution and dissociation of lithium salts,supporting high concentrations of charge carriers.Furthermore,compared with PEO,PVDF-HFP exhibits better electrochemical stability,thermal stability and mechanical properties as well as higher anode stability,which makes PVDF-HFP a good candidate for solid-state lithium metal battery system applications.In this paper,PVDF-HFP was used as the polymer matrix,and the modification research is carried out on its basis.Interfacial modification of composite polymer electrolytes and the effect of MOF-based composite solid-state electrolytes on the shuttling of redox mediators（RMs）in lithium-oxygen（Li-O₂）batteries were investigated,respectively.The specific work content is as follows:We electrochemically deposit an aluminum layer on the composite polymer electrolyte interface（CPE）surface by electrochemical methods.The aluminum-containing layer CPE（ACPE）is in close contact with the lithium anode,forming a Li-Al-O fast Li-ion conducting layer between the solid-solid interface.ACPE acts as a lithium ion buffer layer,which promotes the uniform deposition of Li⁺and improves the ionic conductivity.Therefore,the ACPE was assembled into a lithium symmetric battery with good cycling stability.The ionic conductivity of ACPE at room temperature was as high as 2.163×10^-4 S cm^-1.ACPE also has a high Li-ion migration number of 0.72.The lithium symmetric cell had been stably cycled for more than 1600 h in Li deposition/stripping experiments.The composite polymer film has a certain high voltage resistance performance,and the electrochemical window reaches 4.93V.In the cycling test,the ACPE separator performs better in the Li/ACPE/Li Fe PO₄ lithium-ion battery,and the Li/ACPE/Li Fe PO₄ full cell had long-term stable cycling performance at 1C at room temperature.The initial discharge capacity of the Li/ACPE/Li Fe PO4 battery was129.1 m A hg^-1,which was still as high as 110.4 m Ah g^-1 after 300 cycles,with a capacity retention rate of 85.6%.To sum up,it is shown that the interfacial modification in the composite polymer electrolyte can improve the cycle life and interfacial stability of the modified lithium-ion battery to a certain extent.The insulating properties of Li₂O₂,the main discharge product in Li-O₂ batteries,lead to its high decomposition voltage.Benefiting from the tuning of the equilibrium potential of soluble redox mediator（RM）species,the RM becomes an electron carrier,which can effectively reduce the charging voltage for Li₂O₂ decomposition.However,as a strong oxidant,RM⁺is prone to side reactions with Li anode,resulting in redox shuttle and severe Li degradation.In order to alleviate the corrosion of RM on the lithium anode,we introduce MOF-based solid electrolyte,which can inhibit the shuttle of RM molecules and guide the migration of lithium ions uniformly due to its three-dimensional pore structure and highly ordered micropores.The RM we used is TEMPO,which has high oxidation potential and good reversibility.The MOF-based composite solid electrolyte exhibited a wide electrochemical window and a high Li-ion migration number.The MOF-based composite solid electrolyte was assembled into a lithium symmetric battery,which can be stably cycled for 1300 hours at a current density of 0.2 m A cm^2-.The MOF-based composite polymer electrolyte matrix was introduced into the Li-O₂ battery system,which effectively suppressed the shuttle of RM and uniformly deposited and stripped lithium ions.By maximizing the advantages of RM,the Li-O₂ battery can stably perform100 cycles at a high current density of 500 m A g^-1,and the charging voltage cut-off voltage is still below 4.3 V.