| With the rapid development of electric vehicles and portable electronic devices,people are increasingly pursuing rechargeable batteries with high energy density.Lithium metal has attracted much attention due to due to its ultra-high theoretical specific capacity of 3860 m Ah g-1 and electrode potential as low as-3.04 V compared to standard hydrogen electrodes.Nevertheless,the commercialization of lithium metal batteries still faces numerous obstacles,such as the spreads of dendrite and large volume changes during cyclic charging and discharge,which will cause the attenuation of capacity,greatly weaken the battery life and cycle stability,and even lead to a series of serious safety problems.The spontaneous formation of an unstable natural solid electrolyte interphase（SEI）on metallic lithium further exacerbates these problems.In this thesis,stable artificial SEI was constructed instead of natural SEI to regulate the lithium deposition behavior during cycling,prevent the formation of dendrites inside the battery,and enhance the performance of lithium metal battery.Firstly,fluorinated boron nitride（F-BN）was synthesized by introducing fluorine atom from hexagonal boron nitride（h-BN）with high mechanical strength,and F-BN artificial SEI was constructed on the surface of Cu collector by solution casting.The low electron conductivity of artificial SEI is conducive to inhibiting the reduction and decomposition of electrolyte,while the high ion conductivity and ion transfer number can regulate the uniform arrival of lithium-ion flux to the electrode surface.During the electrochemical cycling,F-BN artificial SEI promotes the generation of lithium fluoride with high interfacial energy to lithium and lithium nitride with rapid ion transfer.The nucleation barrier of lithium is only 35 m V under the cooperative regulation of various beneficial components,and the deposition morphology of lithium remains uniform and dendrite-free.Under the influence of F-BN artificial SEI with optimal thickness,the coulombic efficiency of lithium anode remains stable at 98%and the cycle life exceeded800 hours when the current density is 1 m A cm-2.The F-BN artificial SEI enabled an anode-free full cell assembled with lithium iron phosphate as a positive electrode to realize a capacity of 144 m Ah g-1 and a life of 500 cycles at the rate of 0.2 C.Secondly,the organic polymer polyvinyl alcohol（PVA）was introduced to address the shortcomings of inorganic F-BN,which is brittle and may crack during the cycling.A composite artificial SEI with"rigid and flexible"was prepared with F-BN as the inner layer and PVA as the outer layer,which further improved the effect of artificial SEI.The F-BN inner layer regulates the uniform deposition of lithium and physically inhibits dendrite growth through its inherent high mechanical strength.The flexibility and viscosity of the PVA outer layer enable the artificial SEI to withstand greater volume changes of lithium,ensure the integrity of the artificial SEI during the process of lithium plating/stripping,and extend the protection period of the inner layer of F-BN.Under the dual protection of the inner and outer layer of artificial SEI,the overpotential of the lithium on anode decreases to 15 m V when the current density is 1 m A cm-2,and the cycle life exceeds 1200 hours.When the current density is increased to 3 m A cm-2 or the lithium plating capacity is increased to 3 m Ah cm-2,the assembled symmetrical cells exhibit a stable cycle life of more than 700 hours.The anode-free full cells with composite artificial SEI retains 62.3%of its initial capacity after 100 cycles at a rate of 0.2 C. |
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