| Due to the highest theoretical specific capacity(3860 mAh g-1)and the lowest oxidation-reduction potential(-3.04 V vs.standard hydrogen electrode),lithium metal is considered as the most ideal anode material for lithium secondary batteries,which can significantly improve the energy density of lithium batteries.However,the practical application of metal lithium anode is seriously restricted by the instability of Li/electrolyte interface.Due to the serious side reaction between lithium metal and electrolyte,unstable solid electrolyte interface(SEI)will be produced in the plating/stripping process resulting in large capacity loss.The formation of lithium dendrite leads to the safety issues and reduces the cycling stability of lithium batteries.The deposition process of lithium includes two stages:nucleation and growth.The nucleation morphology of lithium is related to the components of electrolyte.The distribution state of lithium nuclei is determined by the surface properties of the substrate.The lithium growth behavior on the substrate surface is closely related to the SEI film.When SEI film has low ionic conductivity and poor mechanical properties,lithium is more likely to grow into dendritic shape.The strategies to improve the cycling performance of cells by changing lithium growth behavior mainly include electrolyte regulation,three-dimensional current collector design and SEI film design.Lithium nucleation is the primary stage of lithium electrochemical deposition.Therefore,precise control of lithium nucleation based on the current collector can effectively regulate the growth behavior of lithium.In this thesis,we mainly investigate the growth behavior of lithium on the substrate,comprehensively considering the solvation chemistry and the in-situ SEI,and design the structure and function of the current collector,which can effectively control the lithium deposition mode and build a stable SEI film to reduce the accumulation of dead lithium in the cycling stage.This study can provide a scientific basis for the design of high safety and stability lithium metal batteries.The main results are as follows:(1)Uniform deposition of lithium with Li2S-enriched SEI.Lithium nuclei formed on Cu have high probability growing into dendritic shape during the growth stage.In order to study the effect of SEI components on the growth behavior of lithium nuclei,Cu7S4 nano-flake arrays(Cu7S4 NFAs)was prepared on the surface of Cu foil by electrochemical oxidation method to form Li2S-enriched SEI films on the surfaces of lithium nuclei.Li2S can improve the ionic conductivity of SEI contributing to uniform and fast transfer of Li+,and keep the uniform distribution of Li+flux.The spherical lithium grown on the surface of Cu7S4 NFAS shows more uniform size distribution than that on Cu,indicating that the Li2S-enriched SEI can effectively inhibit the local preferential growth of lithium particles.The crosssectional analysis of 5 mAh cm-2 lithium deposits shows that the lithium structure on Cu7S4 NFAs is denser due to more uniform deposition.After 157 cycles,compared with Cu foil,the amount of isolated metal dead lithium on the surface of Cu7S4 NFAs is reduced by 11%at 1 mA cm-2,indicating that the growth of lithium dendrite is inhibited.Low dead lithium accumulation can reduce the impedance of the cells during cycling.Li2S-enriched SEI with high ionic conductivity can accelerate the electrode reaction kinetics and significantly reduce the polarization of electrode.Li‖Cu7S4 NFAs cell can maintain nearly 400 cycles at 1 mA cm-2 and 1 mAh cm-2.The symmetrical cell of Cu7S4 NFAs with 5 mAh cm-2 lithium deposition capacity can maintain stable cycling with smaller polarization for more than 1200 h under the conditions of 1 mA cm-2 and 1mAh cm-2.With N/P=1.5 and 1 C,for S‖Li@Cu,after 100 cycles,its specific capacity rapidly decays to 200 mAh g-1,meanwhile the specific capacity of S‖ Li@Cu7S4 NFAs maintains at 500 mAh g-1.(2)Ordered lithium deposition with lithiophilic micro-patterns on Cu.Cu is an ideal lithium deposition substrate for lithium-free anode.However,Coulombic efficiency of Cu at high current density is low,and the study about the effect of initial distribution state of nuclei on lithium growth behavior is insufficient.We propose strategy to regulate the distribution of lithium nuclear barrier on Cu surface.In order to direct the deposition of lithium,femtosecond laser is used to process ordered lithiophilic micro-grooves on Cu surface.It is found that lithium growth behavior shows obvious change when lithium nuclei are concentrated in grooves with limited space.The strong interaction between lithium nuclei can significantly inhibit dendrite growth.In ester-based electrolytes,the typical whisker-like lithium on Cu is transformed into large granular lithium particles,therefore,the possibility of micro short circuit in the cell is significantly reduced.In addition to the controllable lithium growth behavior,the dense lithium deposition structure significantly reduces the specific surface area of lithium and mitigates the side reaction.By quantitative analysis of dead lithium,it is confirmed that the accumulation of SEI formed by the serious side reaction during cycling can accelerate the depletion of electrolyte and the explosive growth of lithium dendrites.At 3 mA cm-2,with ordered and dense lithium deposition on the substrate,the amount of SEI and isolated metallic lithium are reduced by 43.2%and 61.4%,respectively.By changing the random distribution of lithium nuclei on the substrate to the ordered one,less side reaction and controllable lithium growth behavior are achieved at the same time.This design idea of regulating lithium nucleation sites on the current collector can provide guidance for the future work of high-energy-density anode-free cell.(3)Li+solvation structure regulation with functional micro-arrays.Nano hydroxyapatite(nHA)is a particle with negative functional groups(PO4)which has higher Li+adsorption energy than the components of the electrolyte.Li+can be adsorbed on the surface of nHA in electrolyte to form a region with high concentration of Li+.In this region,the interactions between the cations and anions are significantly improved,therefore,more aggregates(AGGs)can be formed.TFSIand NO3-coordinated by more Li+can enter the electrical double layer of the anode more easily,and they are adsorbed on the inner Helmholtz layer reacting with lithium,which improves the conversion efficiency of the beneficial inorganic components of SEI such as LiF and Li3N.The nHA film was processed as ordered micro-arrays by femtosecond laser(LP-nHA),which simultaneously realizes the ordered lithium deposition and high-quality LiF/Li3N SEI.The interactions between Li+and solvent molecules become weak,and the number of solvent molecules involved in the solvated sheath of Li+is reduced.As a result,the de-solvation energy of Li+is reduced by 12 kJ mol-1,and the electrode reaction kinetics is effectively improved.At 3 mA cm-2,Li‖LP-nHA can maintain over 350 cycles,which is nearly 7 times longer than the life of Li‖Cu.The voltage hysteresis of Li‖LP-nHA during cycling is significantly lower than that of Li‖Cu.At 2 mA cm-2 and 2 mAh cm-2,when the cells are in the capacity decay stage,the amount of isolated metallic lithium on LP-nHA is 10 times lower than that on Cu.Due to the generation of high-stability and protective SEI,the capacity decay rate of full cells also be significantly reduced. |
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