Structural Design,preparation And Electrochemical Performance Of PVDF-based Composite Electrolytes For Solid-state Lithium-metal Batteries

The rise in popularity of electric vehicles and the growth of the energy storage market have led to an increased demand for electrochemical energy storage systems,which demand comes with higher requirements for safety,energy density,cyclic life.Solid-state lithium-metal batteries with solid-state electrolyte as the ion transport medium can,in principle,satisfy the above requirements,and thus have become a research hotspot in recent years.Meanwhile,the solid-state electrolyte is a crucial component in solid-state lithium metal batteries,facilitating the transport of lithium ions and ensuring interfacial compatibility with the cathode and anode electrodes,as well as tunable mechanical properties.These properties are essential for handling stability of batteries during charging and discharging.However,literatures report on solid-state electrolytes reveal certain limitations,include the low conductivity of lithium ions,interfacial compatibility issues between components,and poor interfacial compatibility between solid-state electrolytes and cathode/anode electrodes.Consequently,there is no solid-state electrolyte that fully meets the aforementioned requirements.To this end,we conducted research in four main areas:1.Preparation of PVDF-based composite solid electrolytes with high ionic conductivity.2.Study on the stability of the interface between PVDF based polymer and fillers.3.Trace filler doping enables high-flux ion transport and stable anode interface.4.The preparation of a high-ionic conductivity composite solid electrolyte with a stable interface of cathode/anode electrodes.The following main conclusions have been drawn:（1）In order to address the issues of low ionic conductivity and slow lithium migration associated with polyvinylidene fluoride-chlorotrifluoroethylene polymer（P（VDF-ctfe））,an innovative modification strategy was adoped by doping lithium-ion ceramic conductors.Herein,based on the“binary electrolytes”of poly（vinylidene fluoride-chlorotrifluoroethylene）（P（VDF-ctfe））and lithium salt（Li TFSI）,a kind of eutectogel hybrid electrolytes（EHEs）with high Li⁺transference number was developed via tuning the spontaneous coupling of charge and vacated space generated by Li-cation diffusion utilizing the Li_6.4La₃Zr_1.4Ta_0.6O₁₂（LLZTO）dopant.Therefore,relevant analytical tests such as Scanning Electron Microscopy,Raman,X-ray photoelectron spectroscopy,and ion migration number have shown that the optimized EHEs-10%possess a high Li-transference number of 0.86 and a high Li⁺conductivity of 0.32 mS cm^-1 at room temperature.Moreover,the prepared EHEs-10%presents excellent lithiumphilic and compatibility,and can be tested stably for 1200 h at 0.3 mA cm^-2with assembled lithium symmetric batteries.Likewise,the EHEs-10%match well with high-mass loading of Li Fe PO₄ and Li Co O₂ cathodes(>10 mg cm^-2)and exhibit remarkable interface stability.This study has deepened the understanding of lithium ion diffusion and migration mechanism and lithium ion coordination/decoupling reaction mechanism of composite solid electrolyte in theory,and significantly improved the lithium ion migration number and ionic conductivity of P（VDF-ctfe）polymers in technology,which solves the problems of low ionic conductivity and slow lithium migration of polymers.（2）The present study is aimed at the dehydrofluorination reaction caused by the incompatible interface between the space charge layers of LLZTO and P（VDF-ctfe）polymers,which leads to the severe gel solidification during the preparation of the composite slurry.An innovative design was carried out based on the interface of P（VDF-ctfe）/LLZTO,proposed an interface passivation strategy with the sp²-sp²bis（catechol）diboron molecule（B₂cat₂）,and designed a chemically neutral B₂cat₂ molecular layer between the interface of LLZTO and P（VDF-ctfe）,made a stable interface between P（VDF-ctfe）and LLZTO filler.UV-visible absorption spectroscopy,X-ray photoelectron spectroscopy and NMR analyses showed that B₂cat₂ was effectively reacted with LLZTO,reducing its surface activity.This strategy effectively passivates the LLZTO surface,overcomes the dehydrofluorination of P（VDF-ctfe）polymer,and achieves efficient interfacial ion transport.In addition,B₂cat₂ can effectively anchor DMF,achieve the riveting of residual DMF molecular,and reduce the influence of free DMF on lithium ion transport and interface stability.Theoretically,the scientific issue of the key influence of B₂cat₂ on the stability of the composite solid electrolyte interface was clarified,and the incompatibility between LLZTO and P（VDF-ctfe）interface was technically solved to achieve controllable preparation of composite solid electrolyte.（3）To solve the problems of difficult preparation of lightweight composite solid electrolyte,sluggish ions conduction at room temperature and poor compatibility of lithium metal anode,an optimization strategy for lithium ion environment based on residual solvent and anion cluster with negative charge was proposed by doping a small amount of La F₃.Thus,high-flux lithium transportation channels were designed,and a lightweight composite solid electrolyte with high ionic transportation and good compatibility with lithium anode can be prepared.Solid-state NMR spectroscopy and molecular dynamics simulation results demonstrate that the microscopic lithium chemical environment of the solid electrolyte is regulated.Time-of-flight secondary ion mass spectrometry,X-ray photoelectron spectroscopy,and in-situ electrochemical impedance spectroscopy tests indicate that the improved compatibility of the lithium anode is due to the organic-inorganic dielectric layer by the in-situ construction during the electrochemical process.As a result,a high ionic conductivity（σ）of 0.7 mS cm^-1and a lithium transfer number of 0.79 are achieved at 25℃.In addition,the optimized Li//ATCSE-3%//Li can deliver favorable compatibility at 0.1 and 0.3 mA cm^-2 with stable Li⁺plating/stripping testing for 2000 h and 1200 h,respectively,demonstrating good lithium compatibility.This study deepens the understanding of the microscopic mechanism of lithium ion environment in theory,and technically solves the problems of slow ion conduction at room temperature,lightweight preparation of composite solid electrolyte and compatibility of lithium metal anode electrode.（4）In order to solve the issues of poor interfacial affinity between the cathode and lithium anode and low ionic conductivity at room temperature,a strategy to improve interfacial ion transport and build the compatibility of heterogeneous interfaces was designed by doping functionalized covalent organic frameworks（COF-SH）.COF-SH modified composite solid electrolytes（CSE-COF-SH）has enhanced affinity for the cathode and anode,as well as high conduction of lithium ions,as which is attributed to the rich thiol/amine polar groups and the numerous 1-dimensional high ion transport channels.First-principles calculations show that the thiol/amine groups can be effective in adsorbing residual solvents,inhibiting the interaction with the lithium ions and increasing the freedom of the lithium ions.As a result,the CSE-COF-SH has a high ionic conductivity(0.24 mS cm^-1)and lithium transfer number（0.54）.AC impedance,X-ray photoelectron spectroscopy and electrochemical testing of lithium metal batteries show that the rich heterogeneous interface significantly improves the interfacial ion transport and the compatibility of the cathode and anode electrodes.The symmetrical lithium battery can achieve a 1200 h of stable lithium plating/stripping at 0.3 mA cm^-2.The batteries assembled with lithium iron phosphate and lithium cobaltate demonstrated excellent capacity retention of 77.4%and 79.3%after 200 cycles at 0.5 C.This study theoretically verified that the thiol/amine polar groups can promote the adsorption of residual solvents and enhance the fast conduction,and technically solved the problem of considering the compatibility of the cathode/lithium anode interface while meeting the requirements of high lithium ion conduction.In summary,the problem that P（VDF-ctfe）based composite solid electrolytes cannot be used in solid-state batteries has been overcome to a large extent through the above research,especially 1.Low ionic conductivity at room temperature;2.Instability of the interface between P（VDF-ctfe）polymer and lithium active ceramic filler;3.Incompatibility between P（VDF-ctfe）composite solid electrolyte and lithium anode electrode;4.Unstable interface between P（VDF-ctfe）polymer and the cathode/anode.This provides a reference for ion transport and interface design for the development of high-performance solid-state batteries.