Effect Of Indentation Array On The Surface Of Copper Current Collector On The Deposition Behavior Of Lithium Metal

Lithium metal batteries（LMBs）have ultra-high energy density and are expected to become the first choice for next-generation energy storage systems.However,as a key component,the Lithium metal anode（LMA）is prone to grow into dendrite during the electrochemical deposition,resulting in low coulombic efficiency and potential safety hazards,which restricts the commercialization of LMBs.In order to suppress dendrite growth,scholars have proposed methods such as regulating electrolyte composition,constructing three-dimensional anode structure and preparing artificial solid electrolyte interface（SEI）,but it is still difficult to meet the actual needs of high charge-discharge rate and long cycle life,most of the methods have complex preparation processes and are difficult to achieve large-scale applications.The literature survey shows that the applied stress has a significant effect on the electrochemical potential and lithium deposition behavior.Scholars have proposed some theoretical models of the stress effect on lithium deposition.However,the understanding of the effect of prefabrication internal stress is still relatively weak and there is a lack of effective design methods.In this regard,a simple and easy-to-control indentation method was used to prepare micron-scale indentations on the surface of the copper current collector.The morphology characteristics and evolution process of lithium deposition inside and outside the indentations were studied and the parameters such as the depth of indentation and array spacing were analyzed.Using the finite element method（FEM）and theoretical calculations the mechanism of residual stress on the electrochemical deposition behavior of lithium was clarified,furthermore a new and efficient design method for the surface indentation array of metal current collectors of LMAs was proposed and applied to LMBs.The specific research contents are as follows:（1）The nucleation and growth laws of lithium inside and outside the indentations on the surface of copper current collectors during electrodeposition were studied.Indentation with the depths of 15μm,23μm,and 30μm were prepared using Vickers indenter.When the deposition capacity was low(0.1 m Ah cm^-2),the number of nuclei in the indentation was large and a mass of slender nuclei appeared.Significant lithium dendrite growth was observed with the increase of discharge depth(2.0 m Ah cm^-2).In contrast,the lithium cores electrodeposited outside the indentation showed a uniform ellipsoid shape and exhibited uniform growth in all directions with the increase of the deposition capacity.With the increase of current density,the number of lithium nucleation changed,but the above-mentioned lithium deposition characteristics remained unchanged.The morphology of the deposited lithium outside the indentation was significantly affected by the distance from the indentation.With the increase of the distance,the dendrite phenomenon similar to the original copper surface gradually appeared.Using the double indentation experiment,the relationship between the indentation depth and the size of the dendrite-free area was quantitatively studied.The size of the area and the distortion degree of the copper foil were comprehensively optimized,finally the optimal depth and spacing of indentation were established as 23μm and 0.6 mm,respectively.（2）Combining with the classical dendritic growth model the deposition mechanism of lithium metal was analyzed.First,through the calculation of the indentation structure size and the analysis of the surface layer by Fourier transform infrared spectroscopy（FTIR）,The tip effect can promote the preferential deposition of lithium metal inside the indentation,but the oxide film rupture mechanism is difficult to explain the huge difference in the lithium deposition behavior inside and outside the indentation.Then the residual stress distribution within 300μm near the indentation was calculated by FEM,the results showed that compressive stress was dominant inside the indentation and tensile stress was dominant in the outside.Based on the Monroe-Newman model of tip-growth,it was found that the ratio of i_eff/i₀ inside the indentation was greater than that outside the indentation.It was shown that the stress distribution promoted Li deposition to preferentially occur inside the indentation.In addition,since the presence of SEI on the Li metal surface would constrain the volume change caused by the deposition of Li,based on the base-growth compressive stress model,the residual stress inside the indentation would aggravate the extrusion of Li from defects in the SEI and the external tensile stress could alleviate this phenomenon.（3）Design of copper current collector surface indentation microarrays for electrochemical testing.A 10*10 microarray was constructed using Vickers indenter to study the effects of different loads/depths（300 gf,500 gf,1000 gf）and spacing（0.6 mm,1.0 mm）of indentation on the electrochemical performance.Benefiting from reasonable depth and spacing,half-cell tests showd that the 500gf-0.6mm sample could achieve stable cycling for 200 cycles at the current density of 1.0 m A cm^-2;the 500gf-0.6mm@Li symmetric cell exhibited a low-voltage polarization of 30 m V under the current density of 1.0 m A cm^-2 and charge-discharge depth of0.5 m Ah cm^-2,which was attributed to the reversible deposition/dissolution process.The full cell matched with the lithium iron phosphate（LFP）cathode was cycled at 1C and the capacity retention rate was still 89%after 200 cycles.At the same time,the three-dimensional indentation model also showed that the stress distribution was the most reasonable when the depth and spacing of indentation were about 23μm and 0.6 mm,respectively,which provided a new design method for the surface modification of metal current collectors.