Eutectic Solvent Regulating Solid Electrode Interface For High Performance Lithium Metal Anode

Lithium metal rechargeable battery is regarded as one of the high specific energy secondary batteries candidates,which can meet the demand for industrial power storage,household daily energy storage and communication energy storage.However,there are some issues caused by Lidendrite growth which should be overcome before its practical applications.More attention has been focused on deep eutectic solvents（DESs）as a new green solvent due to its low cost,biodegradability and low toxicity.Although DES have many advantages（high ionic strength,high polarity,supramolecular structure,high dielectric constant,stability in air and environmental friendliness）,little interest in DESs as electrolytes for lithium metal rechargeable batteries.This is possible because the disadvantages of DES with the high viscosity,low electronic conductivity and low voltage decomposition,which limit the further development in lithium metal rechargeable battery.Based on the above-mentioned,this paper mainly focuses on study the eutectic solvent used in lithium metal rechargeable battery.Such as,investigating the performance of lithium metal rechargeable battery with the electrolyte of DES as main component,understanding the mechanism of DES as additive in carbonate-based electrolyte to regulated the SEI composition of lithium metal anode,designing an in-situ polymerization gel electrolyte and evaluating the effect of DES as additive in gel electrolyte for lithium metal rechargeable batteries.The main contents and results are described as follows:1)Effect of eutectic solvent as a main component in electrolyte on the performance of lithium metal rechargeable batteriesEutectic solvent cannot be directly used as electrolyte in lithium metal rechargeable batteries,because of its high viscosity and poor stability with lithium metal.To overcome these issues,FEC is introduced into（LiTFSI）·（Urea）_x electrolyte as SEI film-forming solvent and viscosity diluent.FEC can preferentially decomposed and participated in the SEI formation process on the surface of lithium metal due to high reduction potential.Then,the continues side reaction between lithium metal and（LiTFSI）·（Urea）_x electrolyte can be prevented.Raman spectroscopy,DSC,SEM,CV,self-extinguish time and electrochemical test are used for investigating the properties of DES with different Urea/LiTFSI ratio,as well as,the effect of FEC in（LiTFSI）·（Urea）_x on the interfacial stability of lithium metal anode.Firstly,the electrochemical performance of the cells assembled by solid electrolyte[（LiTFSI）·（Urea）_1.5]with or without FEC are tested and evaluated,which is improved after LiTFSI/FEC saturated solution modified lithium metal anode.Secondly,the non-aqueous electrolyte based on Li⁺[（Urea）₃（TFSI^-）₁]with different FEC concentration are investigated.A series test results suggested that the FEC-50%electrolyte is the optimal electrolyte.And the Li|FEC-50%|LiFePO₄ cells exhibits a long cycling stability with the capacity retention of 94%after 200 cycles at 0.3C.2)Deep eutectic solvent additive regulating SEI of lithium metal anode for high performance lithium metal batteriesTo improve the interfacial stability between lithium metal anode and electrolyte,deep eutectic solvent（Li⁺[（Urea）₃（TFSI^-）₁]）as an additive agent is introduced into carbonate-based electrolyte.Experimental（Raman,XPS,DSC,SEM）and theoretical calculation（MS,DFT）results show that carbonate-based electrolyte with 1 wt.%Li⁺[（Urea）₃（TFSI^-）₁]additive is the optimal electrolyte.Consequently,the lithium symmetrical cell with BE-1%electrolyte exhibits a stable cycling life over 1200 hours at a current density of 0.5 mA cm^-2,and the capacity retention of the Li|BE-1%|LiFePO₄ cells is still maintained 92.1%after 1000 cycles at 1C.This is attributed to that the urea-conjugated ionic is prior absorbed on the surface of metallic lithium,then the interfacial chemistry of SEI formation on the lithium metal anode can be adjusted.Therefore,the robust and dense SEI layer enriched with Li₃N compound is formed to suppress Lidendrite growth and mitigate Limetal corrosion.Furthermore,this SEI layer can decrease the de-solvent energy of Li⁺ions to reduce the Linucleation and deposition overpotential.Thus,the high performance of lithium metal rechargeable batteries is obtained.3)In situ fabricated gel electrolyte with deep eutectic solvent additive for lithium metal batteriesIn this work,LiPF₆ as lithium salt electrolyte,initiator and fluoride source to in situ prepared gel polymer electrolyte.A LiF-rich SEI layer is formed on the surface of lithium metal anode,which is attribute for the process of THF ring-opening polymerization.This process resulting in a great enhancement of LiPF₆ decomposition to form LiF in the SEI layer.Therefore,the growth of lithium dendrite can be preventing by the stable SEI.Results suggested that the as-prepared gel polymer electrolyte presents an ionic conductivity of 1.33m S·cm^-1 at room temperature and shows a high electrochemical stability up to 4.5 V.The lithium symmetrical cell with Li|GPE|Liexhibits stable cycling life over 600 hours at a current density of 0.5 mA cm^-2,and the capacity retention of the Li|GPE|LiFePO₄ cells is maintained88.5%after 400 cycles at 1C.Furthermore,gel electrolyte with deep eutectic solvent additive is also evaluated for lithium metal rechargeable batteries.It is found that Li⁺[（Urea）₃（TFSI^-）₁]as additive can prevent the ring-open polymerization of THF,while THF/335C with（Na TFSI）·（SN）₃ additive as precursor solution can formed a gel state electrolyte.However,the cell assembled by the gel electrolyte with（Na TFSI）·（SN）₃ show a poor cyclic stability.Thus,it is further to development a new DES for PTHF,and used for high performance lithium metal rechargeable batteries.