Study On Artificial SEI Films Interfacial Contral For Lithium Metal Anode And Electrochemical Properties

With the application of high energy density in electric vehicles and large-scale energy storage,it is urgent to develop new generation storage system of high energy density and long cycle life.Lithium metal batteries have become the focus of research for their high theoretical specific capacity.However,due to the strong activity of lithium,it is easy to form lithium dendrites on the surface,which causes safety risks and limits its practical application seriously.To cope with the current problem that Solid Electrolyte Interphase?SEI?film produced by lithium metal itself cannot effectively inhibit lithium dendrites in the circulation process,this research proposes to use Zn-MOF,a metal organic framework,as an artificial SEI film to inhibit the formation and growth of lithium dendrites,and to further modify it by using polymer to improve the comprehensive performance of artificial SEI film.In this research,the physical and chemical properties of the prepared artificial SEI film were characterized comprehensively,the electrochemical properties were studied,and the mechanism of protecting lithium anode electrode was explored.In the experiment,a simple method was adopted to grow Zn-MOF film in situ on the surface of copper foil as artificial SEI film at room temperature,and the thickness of Zn-MOF film was regulated by controlling the reaction time.Zn-MOF film has a large of channels that allow lithium ions to diffuse rapidly and the special channels structure can be effective for Li⁺screening.A large number of polar bonds such as Zn-N and C-N exist on the surface,which are conducive to the adsorption of lithium ions.MOF metal center on the anionic complexation hindered the migration of anion,thus increasing the migration number of lithium ions,reducing the formation of space charge area,and inhibiting the growth of lithium dendrites.After 250 cycles under the current density of1 mA cm^-2,the coulombic efficiency of the Zn-MOF film protected lithium anode still reached 97.79%,and the lag voltage remained below 40 mV within 300 hours,showing a good cyclic stability.The discharge capacity of the battery assembled with lithium iron phosphate?LiFePO₄?can reach 134.6 mAh g^-1 after 160 constant current charge and discharge cycles at the discharge rate of 1 C.PVA?polyvinyl alcohol?gel with lithium salt LiTFSI was added to the surface of Zn-MOF film by rotary coating.PVA was used to bond Zn-MOF film to relieve the internal stress and to increase its flexibility.The rigidity of Zn-MOF film and the flexibility of PVA make it easy for the composite artificial SEI film to adapt to the change of anode volume,so as to inhibiting the dendrites of lithium and to improve the utilization ratio of lithium anode.The screening function of Zn-MOF film has a synergistic effect with local high Li⁺concentration in the interface,further improve the transference number of lithium ions,and prolong the time of dendrite growth.The coulombic efficiency of lithium anode protected by Zn-MOF/PVA composite film still reaches 97%after 300 cycles at the current density of 1 mA cm^-2.The lithium sulfur battery assembled with prepared lithium negative electrode has a discharge capacity of881 mAh g^-1 after 50 constant current charge and discharge cycles at the discharge rate of 1 C.Under the current density of 2 C,LiFePO₄ battery has a capacity of 136.9 mAh g^-1 after 50 cycles,showing excellent multiplier performance.