Three-dimensional Structural Modification And Electrochemical Performance Of Lithium Metal Anode

With the progress of society and the development of science and technology,traditional fossil fuels are becoming more and more difficult to meet the energy needs of human beings,so we have to exploit efficient energy storage system.In addition,with the development of mobile phone devices,electric cars and other fields,the commercial battery systems,including lithium ion battery,gradually fail to meet people's requirements for the energy storage devices.Unlike lithium-ion batteries,lithium metal can provide the highest theoretical specific capacity(3860 mAh/g),the lowest density(0.59 g/cm3),and the most negative electrochemical potential(-3.04 V vs.SHE),and it can significantly improve energy density of batteries.Therefore,lithium metal batteries are expected to replace lithium ion battery as the commercial energy storage system of the next generation.However,due to its active chemical properties,low Lewis acidity,lithium metal batteries are faced with a series of serious problems,which will do harm to their practical application:(1)The primary SEI film on the electrode surface is unstable and brittle,causing side reaction and low Coulombic efficiency.(2)The uneven deposition and dendrite growth on the surface of lithium anode will lead to the deterioration of battery performance and safety hazard.(3)Volume expansion appear during long time cycling.(4)Some lithium metal will fall off to form "dead lithium".To solve these problems,researchers have developed a series of solutions from the aspects of structuring lithium anode,improving electrolyte,adding artificial SEI layer to replace the original SEI layer,etc.In the attempt of modifying unsupported lithium anode,researchers studied many three-dimensional skeleton materials with different functions.Among them,the skeleton with lithiophilicity can attract lithium ions and induce their uniform deposition;the conductive skeleton can establish complete and uniform conductive path inside lithium metal,and reduce local current density;the skeleton with dual functions can play the synergistic effect of the two parts,showing stronger ability to suppress dendrite and stabilize the electrode.In addition,the three-dimensional skeleton with certain strength can inhibit the dendrite growth mechanically.The main research content of this thesis is concluded as follows:(1)the modified 3D copper collector was prepared by kirkendall effect and template etching to modify lithium metal anode.The first method utilizes kirkendall effect between copper and other elements:After long-term diffusion,the two metals will diffuse into each other's lattice,and a large number of holes will form inside copper matrix.Template etching used the natural plant surface with porous or protruded morphology as the original template.With the help of polydimethylsiloxane(PDMS),the morphology can be printed to the surface of 2D copper current collector,equipping it with porous structure.The 3D porous current collectors prepared by the two methods have good conductivity and 3D structures to increase the electrode area,and therefore reduce the local current density and delay the dendrite generation.The internal pores can be used as "cages" to store lithium metal and inhibit dendrite growth.The protrusion structure of the connection between the internal holes can enhance the local electric field and attract lithium ions,thus providing preferential sites for the nucleation of lithium metal.These factors enable the modified 3D copper current collector to maintain a high Coulombic efficiency after long time cycling.During long time symmetric battery test,the structured lithium anode modified by 3D porous copper current collector showed long time cycling lifespan.In full cells,they showed good cycling stability.(2)For the first time,a kind of nitrogen and sulfur codoped Nb2C MXene(NS-Nb2C)skeleton material was synthesized and used for structural modification of lithium metal.Element doping can enlarge the interlamellar spacing and lattice parameter of Nb2C MXene,and therefore suppress lamella restacking and collapse.The heterogeneous atoms brought by doping can act as extra defects to increase active sites for electrochemical reactions.The nitrogen and sulfur sites show synergistic effect between each other,so it can increase its lithiophilicity and conductivity,guide uniform deposition of lithium,suppress dendrite growth,stabilize SEI layer,and maintain the stability of the structured lithium anode.Hence,lithium metal can realize dendrite-free and uniform deposition/stripping in NS-Nb2C skeleton.After 800 cycles,the Coulombic efficiency of NS-Nb2C modified lithium anode was as high as about 1 00%.In the test of symmetric cell,it showed a long lifespan of 1000 h.In full cell,it exhibited good cycling stability.(3)A kind of 3D and conductive skeleton material N-CNT/Sb2MoO6 has been designed and prepared to modify lithium anode.It is composed of 1D conductive nitrogen doped carbon nanotube(N-CNT)and 2D lithiophilic bimetallic oxide Sb2MoO6 nanosheet.The addition of N-CNT will assemble original dispersed nanosheets together to form a ball-like 3D structure.The lithiophilic part and conductive part of the composite material have a synergistic effect on guiding uniform lithium deposition and inhibiting dendrite growth.In addition,the lithiophilicity and conductivity of carbon nanotube are greatly enhanced after the nitrogen doping.By confining lithium dendrites to spherical structure,it can impose an effective physical barrier to their growth.The 3D lithiophilic/conductive N-CNT/Sb2MoO6 ball-like skeleton can maintain a Coulombic efficiency of 94%after 850 cycles.and showed a long lifespan of 1400 h in the symmetric cell test.In full cell,it can exhibit good cycling and rate performance.(4)For the first time,the 3D lithiophilic/conductive N-CNT@Cu2O@Cu network has been synthesized and used to modify lithium anode.The Cu2O spheres wrapped by Cu nanospheres and the nitrogen doping sites on carbon nanotubes can attract lithium ions as lithiophilic sites,so that lithium ions can be more easily distributed in the entire network,and preferentially deposited on the surface of the lithiophilic sites.Core-shell Cu2O@Cu acts as concentrators to promote the combination of the lithiophilic part and the conductive part of the network to play a synergistic effect,and the encapsulation of nano-copper spheres can also improve the conductivity of Cu2O.This network can integrate the whole network by using lithiophilic sites as concentrators to link current transmission channels of multiple carbon nanotube fibers.Its internal conductive pathways are interlaced in all directions,which can give better play to the conductivity of carbon nanotubes,distributing electric field uniformly and lowering the current density,and therefore delaying the generation of dendrite.N-CNT@Cu2O@Cu showed excellent electrochemical performance:it can maintain a Coulombic efficiency of as high as 97%after 1100 cycles,as well as a long lifespan of 1000 h.In full cell,it exhibited superb practical potential.