Study On Modification Of Lithium Metal Anode Based On Deposition Process

In 2019,the Nobel Prize in Chemistry was jointly awarded to John B.Goodenough,M.Stanley Whittingham and Akira Yoshino for their outstanding contributions to lithium-ion batteries under the theme "Development of lithium-ion batteries".With the continuous exploration and progress of energy storage technology,Li metal surpasses traditional anode material(graphite)with its ultra-high mass specific capacity(3860 mAh g-1),lowest reduction potential(-3.04 V vs.SHE)and light density,has become research hotspot.Unfortunately,the low Coulombic efficiency and limited cycle life severely hinder the practical commercialization of Li metal anode.Among them,the unstable electrode/electrolyte interface and the formation of lithium dendrite are the main sources of performance degradation problems.Based on the process of Li metal deposition,this thesis aims to explore the key scientific issues at each stage,and rationally design solutions to achieve stable cycling of Li metal batteries in practical applications.The main research contents and conclusions are as follows:(1)Ag nanoparticles modified titanate nanowire arrays for Li metal batterieFrom the point of view of the process of Li metal deposition,the initial problem to be faced is the nucleation,and the overpotential required is generally much higher than the deposition overpotential.And some studies also show that the morphology of the initially deposited Li metal nuclei will directly affect the subsequent Li metal deposition.In this work,Ag nanoparticles(Ag@HTO)were modified on titanate nanowire array structures by silver mirror reaction as bifunctional current collector to achieve highly reversible deposition/stripping of Li metal in carbonate electrolyte.On the one hand,the titanate nanowire array structure homogenizes Li ions to reduce concentration polarization.On the other hand,the modified Ag nanoparticles can effectively enhance the electronic conductivity and provide stable nucleation sites for controllable Li deposition/stripping.Benefiting from these advantages,Ag@HTO‖NCM retains 81.2%capacity retention after 200 cycles at current density of 1 C,while Ti‖NCM fails after only 70 cycles.(2)Research on nitrogen-doped carbonized cellulose for Li metal batteriesConductive current collectors with 3D structures are often used to alleviate the formation of Li dendrites because of their large specific surface area,which can effectively reduce the local current density.However,it cannot change the interfacial problems encountered during Li deposition,so the improvement of Coulombic efficiency is always limited.In this work,we designed a nitrogen-doped carbonized cellulose(N-CF)current collector via polyacene bridging reaction to achieve long cycling and high deposition/stripping efficiency of Li metal in carbonate electrolyte.After further nitrogen doping,the conductivity of carbonized cellulose is further enhanced.Combined with its own 3D network structure,thereby promoting the uniformity of the electric field.And the surface fibrosis caused by nitrogen doping can effectively homogenize the distribution of Li ions on the surface,greatly reducing the concentration polarization.Not only that,the nitrogen-doped sites exhibit strong lithiophilicity with Li metal,which can effectively induce uniform Li deposition.Benefiting from the bridging reaction of nitrogen element,N-CF also exhibits excellent mechanical strength and flexibility,which can greatly improve the stability of the structure during long cycling.(3)In-situ construction of polypyrrole as Li metal protective layer with nickel foamThe artificial SEI is synthesized by pretreatment of the Li metal,so the structure and composition of the artificial SEI film can be effectively controlled.However,strict experimental conditions and equipment are required.Therefore,many studies choose to modify the current collector.In this work,we developed a new method to protect the Li metal by in situ construction of polypyrrole(PPy)on nickel foam(PPy@Ni foam)to achieve the construction of protective layer on the Li metal surface.The PPy protective layer constructed in-situ in an alkaline environment has certain flexibility,can alleviate the volume change of Li metal,and can also isolate the direct contact between the electrolyte and Li metal,so it can effectively suppress the formation of Li dendrites.Therefore,PPy@Ni foam exhibits excellent electrochemical performance.When depositing 2 mAh cm-2 at current density of 1 mA cm-2,the high Coulombic efficiency of 99%can still be maintained after 250 cycles.And when using limited Li/LFP as the cathode material to assemble the full cell,the PPy@Ni foam can guarantee capacity retention rate of 85.5%after 500 cycles.(4)Construction of Li metal surface functionalized SEI by spraying fluorinated carbon nanotubesThrough previous experimental work,we found that the electrochemical reaction at the interface of Li metal is the key to regulating the uniform deposition of Li metal.In this work,we construct an organic-inorganic composite artificial SEI layer with fluorinated carbon nanotubes and PAMPS-Li on Li metal surface by a simple spray technique.Carbon fluoride will form hard carbon and LiF when undergoing electrochemical reaction.Due to insulating property and higher surface energy,LiF can effectively participate in the construction of lithium metal SEI and can alleviate the formation of Li dendrite.As an organic cross-linked structure,PAMPS-Li ensures its flexibility to cope with the volume expansion of Li metal.In the full cell test,the FCNTs@Li‖NCM811 full cell can still maintain capacity retention rate of 80.8%after 300 cycles at current density of 3 C.And under the N/P ratio of 3.1,FCNTs@Li‖NCM811 full cell still has capacity retention rate of 80.26%after 200 cycles.(5)Construction of bi-electrodes protective layer for practical Li metal batteriesIt can be found that the stability of the surface interface of electrode materials in Li metal batteries can determine its cycle life.The overall design of a battery is a systematic project,and it is far from enough to design the interface layer single electrode.It can be reasonably predicted that the simultaneous modification of the interfacial layer on the bi-electrodes will synergistically improve the practical electrochemical performance.Here,we report an electrolyte additive that can realize the modification of the bi-electrodes interfacial layer in Li metal battery.The DFT calculation results show that the C6F3LiN4 additive can take precedence over the electrolyte and lithium salt,and simultaneously form SEI on the anode and cathode.In addition,the FT-IR,TOF-SIMS and XPS depth profile results also showed that the C6F3LiN4 films formed on the electrode surface were rich in long-chain organic molecules and LiF.LiF can act as a component of SEI to suppress the formation of Li dendrite.The formation of long-chain organic molecules can enhance the flexibility of the film and alleviate the volume expansion of Li metal.Benefiting from these advantages,the NCM523 full cell with C6F3LiN4 can still maintain capacity retention rate of 82.6%after 400 cycles.