Construction Of Lithium Halide-Rich Interface Film For Lithium Metal Anode And Study On Ion Transport Mechanism

In response to global climate change,efficient energy storage and utilization by converting chemical energy into electrical energy has received extensive attention from all walks of life.Among them,graphite-based lithium-ion batteries（LIBs）have dominated the electronics market since 1991.However,with the development of the electric vehicle industry,LIBs cannot meet people’s requirements for long endurance mileage because the energy density is close to its theoretical limit.Lithium metal has the lowest reduction potential（-3.04 V,vs.standard hydrogen electrode）and high specific capacity(3860 m Ah g^-1),which is recognized by the battery community as the ultimate negative electrode choice to promote the development of the next generation of rechargeable high energy density batteries.However,the spontaneous reaction between lithium metal and electrolyte will form a solid electrolyte interface film（SEI）on the surface of lithium electrode.During the battery cycle,with the uneven deposition-dissolution of lithium metal,SEI will be damaged-repaired under the action of surface stress of lithium electrode,and even lithium dendrites will be generated due to uneven electrochemical deposition.The growth of dendrites not only leads to inevitable capacity loss,but also poses a security risk,which hinders its process in large-scale applications.Studies have found that stable SEI can effectively solve many problems of lithium metal anodes,especially lithium halide-rich SEI has attracted much attention.However,the role of lithium halide components in SEI remains controversial.In order to solve many problems of lithium metal electrodes and explore the ion transport mechanism of lithium halide in SEI film,lithium fluoride（Li F）and lithium bromide（Li Br）rich SEI films were constructed and discussed in this paper.The specific research contents are summarized as follows:（1）Polyethylene glycol methylether acrylate（PEGMEA）based gel electrolyte was prepared by in-situ polymerization.At the same time,the in-situ construction of Li F-rich SEI film was realized by introducing fluoroethylene carbonate（FEC）additive into the precursor solution.The gel polymer electrolyte is in-situ formed inside the battery,which improves the interface compatibility between the electrode and the electrolyte,and improves the safety performance of the battery while ensuring the electrochemical performance.The addition of FEC makes the electrolyte obtain a high ionic conductivity of 1.71×10^-4 S cm^-1 at room temperature and a lithium ion migration number of 0.47.The solid-phase network after polymerization of PEGMEA monomer uniformly encapsulates the liquid component and increases the oxidation resistance of the electrolyte to 4.5 V.The assembled Li||Li symmetrical battery exhibits a stable cycle of more than 1400 h at 0.5 m A cm^-2.SEM images show uniform and dense lithium deposition.In addition,the Li||LFP battery with a commercial lithium iron phosphate（LFP）cathode achieves a stable cycle at 0.5 C with a capacity retention of 92.7%.（2）Using the rapid spontaneous reaction between C₈H₁₇Br and lithium metal,a Li Br-rich SEI film was constructed on the surface of lithium metal by simple immersion treatment.XRD,FTIR and XPS characterization confirmed that the main component of the SEI film was Li Br.It was observed by SEM,EDS and AFM that the surface of the SEI film was very uniform and had nano-sized Li Br grains,and the thickness of the SEI film was 2μm.The adsorption energy and diffusion barrier of Li⁺on three surfaces of Li Br were calculated by density functional theory（DFT）.The adsorption energy of Li⁺on the hollow sites of Li Br（111）/（110）surface is-1.82 e V and-1.53 e V,respectively,which is beneficial to the adsorption of Li⁺.The diffusion barrier of Li⁺on the（111）/（100）surface is low,indicating that Li⁺can achieve rapid transport in the Li Br-rich SEI film,which theoretically verifies the feasibility of constructing Li Br-rich SEI film for lithium metal batteries.（3）The electrochemical performance of lithium metal electrode modified by Li Br-rich SEI was characterized by electrochemical test.For symmetrical batteries at room temperature,when the current density is 0.5 m A cm^-2,it exhibits long-term cycle stability of more than 2400 h,indicating the fast transport kinetics of Li⁺.Tafel test shows a large exchange current density of 1.30 m A cm^-2.EIS impedance test shows that the symmetrical battery has a small charge transfer resistance of only3.04Ωafter cycling,which fully proves that the Li Br-rich SEI film has faster charge transfer efficiency.In addition,both in-situ and ex-situ morphology observations show smooth and flat surfaces,which proves that the lithium bromide-rich SEI film can effectively inhibit dendrite growth.When the battery was tested in simulated extreme environment,under the protection of Li Br-rich SEI film,both high temperature and low temperature symmetrical batteries showed longer cycle performance and lower polarization voltage.When matched with the LFP electrode,the capacity retention rate is 88.1%after 500 cycles at 0.5 C.