Construction Of Lithium Metal Anodes With High Stability And Electrochemical Performances Of Lithium Metal Secondary Batteries

Lithium metal anodes,owing to their extremely high theoretical capacity(3860m Ah g^-1)and low electrode potential（-3.04 V vs.（H⁺/H₂））,have been considered as the“Holy Grail”anodes for the next-generation lithium metal secondary batteries with high energy density.However,unstable electrode surfaces,uncontrollable growth of Li dendrites and huge volume changes of Li metal anodes would result in short cycling-life of Li metal batteries,which plague their commercial applications.In order to solve the above problems,this dissertation focused on the construction of highly stable Li metal anodes and their secondary batteries through the approaches of internal structure design,surface protection of Li metal anodes and separator regulation of Li metal batteries.A series of multifunctional three-dimensional（3D）frameworks with lithiophilicity,artificial surface coating layers for Li metal anodes and modified separators for Li metal batteries have been successfully prepared,and the effects of their structural characteristics on metallic Li deposition/stripping behaviors were systematically studied.On the basis of revealing the“structure-performance”relationships of the modified Li metal anodes and separators,the high-performance Li metal anodes and their secondary batteries with improved cycling stability were obtained.The details are as follows:（1）3D frameworks of lithiophobic nickel foams（NF）were in-situ coated by the metal-organic framework（MOF）-derived porous carbons,and then the Ag-C@NF composite frameworks with high conductivity,super lithiophilicity and stable lithiophilic sites were successfully constructed through embedding super-lithiophilic Agnanoparticles into the pores of carbon coating layers by the in-situ replacement reaction of AgNO₃ with Ni nanoparticles in MOF-derived carbons.The super lithiophilicity of Ag-C@NF composite frameworks not only promotes the rapid impregnation of molten stated Li metal within their pores to achieve Ag-C@NF/Li composite anodes,but also significantly reduces the overpotential of metallic Li deposition during charging/discharging process of the anodes.Moreover,different from the loosely loaded and exposed lithiophilic sites which easily fail in the previously-reported literatures,the nano-Aglithiophilic sites which are tightly immobilized in the pores of carbon coating layer have high structural stability and thus effectively withstand the volume strain caused by Li insertion/extraction during the molten Li impregnation and electrochemically charging/discharging process.Therefore,such stable lithiophilic sites of nano Agcould greatly play their effects on regulating the uniform Li deposition,and thus effectively inhibit the uncontrollable growth of Li dendrites.The coin-type symmetric cells of the Ag-C@NF/Li anodes exhibit low overpotentials of Li deposition and high cycling stability.（2）To reduce the weight of 3D frameworks and ensure the high specific capacity based on the total weight of Li metal anodes,lightweight porous aluminum foams were used as the 3D frameworks for Li metal anodes.The Al foams are extruded onto the Li foils by a simple mechanical extrusion method,and Al@Li composite anodes were successfully constructed.The lithiophilicity of Al foams could provide abundant active sites for metallic Li nucleation,and induce uniform Li deposition within their frameworks.At the same time,the 3D porous structures of Al foams could accommodate the huge volume strain of Li metal anodes during charging/discharging cycles,and reduce the local current density,thus effectively inhibiting the growth of Li dendrites.Therefore,the symmetric cells of Al@Li anodes also exhibit high cycling stability with low overpotentials of Li deposition.After stable charging/discharging for 250 cycles at a current density of 1 mA cm^-2,the Li deposition overpotential of Al@Li symmetric cell is only 5 mV.Moreover,compared with the commonly-used methods of molten Li metal impregnation and electrochemical Li deposition,the mechanical extrusion process for preparing Li composite anodes is much simpler and effective,which is easy to be applied for large scale production.（3）The simple mechanical pressing-assisted transfer printing method was developed to successfully printing-transfer pine needle-liked Agfrom the surface of Cu to bare Li foil.These pine needle-liked Agfractal aggregates are interconnected to form lawn-liked porous L-Ag array layer（L-Ag ）,which are uniformly coated on the surfaces of Li metal anodes.Due to the super lithiophilicity of L-Ag coating layers,L-Ag@Li anodes have excellent electrolyte wettability,thus promoting the rapid transfer of Li⁺flux and uniform deposition of Li metal on the anode surfaces.Moreover,the porous structures of L-Ag coating layers could reduce the local current density for Li metal deposition,which significantly suppress the growth of Li dendrites and improve the cycling stability of Li metal anodes.Therefore,the symmetric cells of L-Ag @Li anodes could be stably charged and discharged for 800cycles at a current density of 2 mA cm^-2,and the overpotentials of Li deposition are only 10 mV.The NMC811||L-Ag @Li coin-typed full cells also have high discharge capacity of 159 mAh g^-1 after 100 cycles at a current rate of 1 C.（4）Based on the structural advantages of super-lithiophilic Agand polymer,a multifunctional Ag/polymer composite protective layer（Ag/PH）was successfully constructed by drop casting a mixed solution of AgNO₃ and PVDF-HFP onto the surface of Li metal anode.Ag,Li₃N,Li N_xO_y and Li F particles formed through the chemical reaction of AgNO₃ and PVDF-HFP with Li metal are uniformly dispersed in the PVDF-HFP matrixes.Firstly,due to the excellent mechanical rigidity of metallic Ag,inorganic Li₃N,Li N_xO_y and Li F particles,as well as the excellent mechanical flexibility of polymeric PVDF-HFP,the Ag/PH composite films have remarkable mechanical properties,which is capable of adapting to the volume strain of Li metal anodes.Besides,due to the super affinity of Ag towards Li,the Ag/PH composite films could effectively reduce the nucleation barrier and deposition overpotential of Li metal.Finally,due to the high Li⁺conductivity of Li₃N and Li F,the Ag/PH composite films have high Li⁺conductivity and migration number.Owing to the multiple synergistic effects,the Ag/PH protective films effectively inhibit the Li dendrite growth,maintain the interface stability and promote the uniform Li deposition,thus improving the cycling performances of Li metal anodes.Consequently,the Ag/PH@Li composite anodes exhibit excellent electrochemical performances in both symmetrical cells and NCM811 coin-typed full cells.After charging/discharging for 100 and 400 cycles at a current rate of 1 C,the reversible capacity of NCM811||Ag/PH@Li coin-typed full cells could be maintained at 173and 133 mAh g^-1,respectively.（5）Based on the separator regulation strategy,a multifunctional MoS₂/graphene@PP separator（MoS₂/G@PP）was successfully constructed by in-situ growth of MoS₂ nanosheets in the porous graphene framework using a one-step hydrothermal method and then coating them on the surface of the commercial PP separator.The MoS₂/G coating layers could improve the electrolyte wettability and Li⁺conductivity of PP separators,and regulate the Li⁺transfer on the surfaces of Li metal anodes as well as the deposition/stripping behavior of Li metal.When the temperatures of hydrothermal reaction are low,the obtained O-doped O-MoS₂/G-LH@PP separators could further suppress the uncontrollable formation of lithium dendrites and improve the cycling stability of lithium metal anodes.Furthermore,DFT theoretical simulation calculations and Li₂S₆ adsorption experiment indicates that compared with graphene and MoS₂,O-MoS₂ have the highest adsorption energy and capability towards Li₂S₆.O-MoS₂/G-LH@PP separators could effectively suppress the shuttle effect of lithium polysulfides and their corrosion on Li metal anodes.Due to the efficient dual-effects of regulating Li deposition/stripping behaviors and lithium polysulfide adsorption capabilities,the coin-type Li-S full cells with O-MoS₂/G-LH@PP separators and high sulfur loading cathodes(～3.0 mg cm^-2)deliver excellent electrochemical performances with high discharge capacity of 1141 mAh g^-1 at the initial cycle at 0.1 C and 772 mAh g^-1after 50 cycles at 0.2 C.