Xin Shen, Rui Zhang, Shuhao Wang, Xiang Chen, Chuan Zhao, Elena Kuzmina, Elena Karaseva,Vladimir Kolosnitsyn, Qiang Zhang,*

1 Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China

2 Advanced Research Institute of Multidisciplinary Science, Beijing Institute of Technology, Beijing 100081, China

3 School of Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia

4 Ufa Institute of Chemistry UFRC RAS, Ufa, 450054, Russia

Keywords:Electrochemistry Li dendrites Rechargeable Li batteries In-situ optical microscopy Phase field model Electrochemical engineering

ABSTRACT Lithium(Li)metal anodes promise an ultrahigh theoretical energy density and low redox potential,thus being the critical energy material for next-generation batteries.Unfortunately, the formation of Li dendrites in Li metal anodes remarkably hinders the practical applications of Li metal anodes.Herein, the dynamic evolution of discrete Li dendrites and aggregated Li dendrites with increasing current densities is visualized by in-situ optical microscopy in conjunction with ex-situ scanning electron microscopy.As revealed by the phase field simulations,the formation of aggregated Li dendrites under high current density is attributed to the locally concentrated electric field rather than the depletion of Li ions.More specifically,the locally concentrated electric field stems from the spatial inhomogeneity on the Li metal surface and will be further enhanced with increasing current densities.Adjusting the above two factors with the help of the constructed phase field model is able to regulate the electrodeposited morphology from aggregated Li dendrites to discrete Li dendrites, and ultimately columnar Li morphology.The methodology and mechanistic understanding established herein give a significant step toward the practical applications of Li metal anodes.

1.Introduction

Lithium(Li)metal has attracted widespread interests as an ideal anode for next-generation batteries because of its high theoretical capacity (3860 mA·h·g-1) and low redox potential (-3.04 V versus the stand hydrogen electrode) [1-3].The batteries based on Li metal anodes can deliver a striking improvement in practical energy densities (＞400 W·h·kg-1), especially Li-S and Li-O2systems [4,5].Unfortunately, the practical application of Li metal anodes is rather troublesome due to the uncontrolled formation of Li dendrites,which induces a poor cycle life,low Coulombic efficiency, and even safety concern [6,7].Therefore, a deep understanding of the dynamic evolution of Li dendrites and valid regulation methods are strongly requested [8,9].

Generally, Li dendrites refer to the unordered non-planar electrodeposits[10].In 1979,Chianelli et al.described the observation of Li dendrites for the first time[11].Since then,tremendous studies have been devoted to investigating the morphology of Li dendrites through in/ex-situ microscopy [12-17].In spite of ubiquitous metal dendrites in the electrodeposition,the high reactivity of Li metal, which brings about solid electrolyte interphase(SEI) [18-20], endows more specificity and diversity to Li dendrites.Up to now, there is no consensus of the description on Li dendrites.Li dendrites can be classified into two types:An individual Li dendrite, which can be identified as needle-like [21,22],whisker-like[23],and filament-like structures[24];An aggregated Li dendrite,which is likely to be described as moss-like[25],bushlike[26],and tree-like Li dendrite structures[27].Both the thermodynamic and kinetic factors,such as surface diffusion[28,29],ion/-electron transfer [30-33], stress extrusion [34], and temperature[35,36], have been explored to interpret various final morphologies.In view of dynamic evolution, various growth modes of Li dendrites, including tip growth, root growth, and ‘‘bread expansion”[26],are proposed.However,a comprehensive understanding of the relationship between the individual and aggregated Li dendrites is lacking:1)why individual dendrites cluster into aggregated Li dendrites;2)if individual dendrites can be arranged in an orderly manner with consistent size, direction, and growth rate,which likes the columnar deposited morphology.

In this contribution, multiscale imaging techniques (e.g.in-situ optical microscopy, ex-situ scanning electron microscopy (SEM))were employed to capture the evolution of Li electrodeposition.Specifically, the growth of aggregated Li dendrites inhibits the growth of surrounding discrete Li dendrites.Further mechanistic understanding through correlation with a phase field model is provided.It is revealed that the locally concentrated electric field stemming from the spatial heterogeneity along the Li metal surface is exacerbated by increasing the applied current density, which results the formation of aggregated Li dendrites.

2.Materials and Methods

2.1.Materials

Li foils of 50-μm thickness were purchased from China Energy Lithium Co., Ltd.Copper (Cu) foils were purchased from Shenzhen Kejing Star Technology Co., Ltd.The electrolyte purchased from Beijing Institute of Chemical Reagents is composed of lithium hexafluorophosphate(LiPF6,1.0 mol·L-1)dissolved in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 by volume).Celgard 2400 was employed as the separator.All materials were reserved in an Ar-filled glove box with oxygen and water contents below 0.00001%.

2.2.In-situ optical microscopy

The in-situ optical Li | Li symmetric cell with a quartz window was fabricated and assembled as Fig.1.Specially,lithium foils were rolled on the Cu foils in advance and were employed as the electrodes.The distance between the anode and cathode is about 1.0 mm.All operations were carried out in the glove box.Solartron 1470E electrochemical workstation was employed to provide the constant current and record the voltage-time profile.The optical microscope (Nikon SMZ25) with a camera (Nikon DS-Fi3 Microscope Camera) was used to catch the morphological evolution of Li surface during electroplating.The videos were recorded at 30 frame per second with NIS-Elements D software.

2.3.Ex-situ scanning electron microscopy

Scanning electron microscopy (SEM) images were acquired from JSM 7401F, JEOL Ltd., Japan.The Li metal anodes obtained from in-situ optical Li|Li symmetric cell were rinsed with the pure solvent (DEC) and then dried thoroughly in the glove box prior to characterization.Samples were kept under argon conditions utilizing a home-made transfer vessel.

2.4.Phase field simulation

The phase field simulation is based upon our previous reported model [37].An order parameter ξ is employed to differentiate the phases, defined as ξ = 1 and 0 for the Li metal and electrolyte,respectively.

The morphology evolution under electroplating(Li++e-→Li)is expressed by a modified Butler-Volmer equation,

Fig.1.The in-situ optical cell.(a) Schematic diagram.(b) The optical image of the cell section.

where DLi+is the diffusion coefficient of Li+,cLithe initial concentration of Li atoms.

The electrostatic potential can be expressed by

where σeff＝h（ξ）σLi+（1-h（ξ））σeis the effective electric conductivity.σLiand σerepresent the electric conductivity of electrode and electrolyte, respectively.

Above equations were solved by COMSOL Multiphysics 5.5.A 2D domain(100 μm×100 μm)is adopted.The initial morphology of Li nuclei is representing by semi-spheres with a radius of 0.5 μm and an interval of 4.0 μm.Dirichlet boundary conditions areapplied on the top and bottom boundary for the Li ion concentration and electrostatic potential.The Li ion concentration on the top boundary is fixed to 1.0 mol·L-1, representing the bulk solution;The electrostatic potential on the bottom boundary is fixed to 0 V as grounding.Global equations are employed to satisfy that the internal current density equals to the applied current density.All model parameters are listed in Table 1.

Table 1 Model parameters employed in the phase field simulations

3.Results and Discussion

To track the dynamic evolution of Li dendrites, in-situ optical observation was conducted in a special Li | Li symmetric cell(Fig.1).Compared to postmortem analyses, in-situ optical microscopy can offer an in-situ and true-color observation.Herein a routine electrolyte of 1.0 mol·L-1LiPF6in EC/DEC was used.The electrodeposition was propelled by a constant current density of 1.0 mA·cm-2and the total plating time was set as 3.0 h.

Synchronized morphology and voltage profile of electrodeposition are shown in Fig.2 and Video S1 (Supplementary Material).The open circuit voltage of the Li|Li symmetric cell is around 0 V.Once the constant current is applied, the voltage starts to drop until a minimum, whereafter asymptotically reaches a relatively stable value(Fig.2a).Although a thick electrolyte layer in the optical cell brings a large ohmic resistance,the voltage trace is consistent with that of a coin cell.Such voltage profile is a typical response signal during the Li electrodeposition,which corresponds to the formation of SEI, Li nucleation, and the growth of Li dendrites (Fig.2b-g).Dasgupta and co-workers have carried out immense study to untangling the relationship between voltage variations and morphology evolution [42,43].Herein we focus on the electrodeposited morphology.Theoretically, the thickness of the planar electrodeposited Li (h) is defined by

Fig.2.Observation of Li electrodeposition in the in-situ Li| Li symmetric cell cycled at 1.0 mA·cm-2.(a) Voltage responses with respect to plating time.(b-g)In-situ optical snapshots of Li dendrites growth during electroplating.

Fig.3.Li dendrites growth at 3.0 mA·cm-2 (a-d) and 0.5 mA·cm-2 (e-f).In-situ optical snapshots with a plating capacity of 0.08 mA·h·cm-2 (a,e) and 3.0 mA·h·cm-2 (b, f),respectively.(c, d, g, h) ex-situ SEM images after optical observations.

where iappliedis the applied current density, t the plating time, MLithe atomic weight of Li,ρLithe mass density of Li metal.In this case,the thickness of the planar electrodeposited Li is about 14.5 μm,which is almost invisible under the optical microscope.However,an aggregated Li dendrite with a height of ca.300 μm appeared at a plating capacity of 0.5 mA·h·cm-2(Fig.2d).As the electrodeposition advances,it continues to expand without a preferred grow site,just like a bread fermentation.It is consistent with the description of Li bush growth that reported by Steiger et al.[26].The ultimate thickness of this aggregated dendrite reaches 800 μm, much larger than the theoretical deposition thickness.It can be supposed that the aggregated dendrite becomes a hot spot and attracts all deposited Li ions.Apparently, the aggregated dendrite is undesirable,which not only brings huge volume expansion, but also increases the formation risk of dead Li in the stripping process.

Fig.4.The simulated results of Li dendrites.The simulated(a,c)dendritic morphology evolution,Li ion concentration,and(b,d)corresponding electric field intensity during Li electroplating at 3.0 mA·cm-2 (a, b) and 0.5 mA·cm-2 (c, d), respectively.Inset: A schematic of the initial Li nuclei.

Further observations at various current densities were carried out(Fig.3 and Video S2,Supporting Information).When increasing the current density to 3.0 mA·cm-2,an aggregated Li dendrite was observed(Fig.3a-d).Under a same plating capacity,the Li dendrite formed at a higher current density of 3.0 mA·cm-2(Fig.3b) exhibits a similar height (about 800 μm) but more fractal shape compared to that formed at 1.0 mA·cm-2(Fig.2g).Limited by the resolution of optical microscopy, the fine structure inside the aggregated Li dendrite is invisible.In this regard,postmortem characterization of SEM was carried out after optical observation,achieving a resolution complement [44].Fig.3c and d demonstrates that the aggregated Li dendrites in optical images is not integrated and solid, but consists of Li whiskers.These whiskers are inconsistent in size and are entangled with each other,making it impossible to distinguish the onsets and ends.In contrast, no obvious aggregated Li dendrite formation at 0.5 mA·cm-2before a plating capacity of 2.7 mA·h·cm-2(Fig.3e-g, Video S2, Supporting Information).Only a tiny bump emerged after a plating capacity of 3.0 mA·h·cm-2(marked in Fig.3f).Moreover,it can be found that the surface roughness of Li metal has changed,resulting in the weakness of reflected light on the Li metal surface.There are massive needle-like Li dendrites (a diameter of about 2.0 μm and a height of about 10.0 μm) on the Li metal surface (Fig.3g, h).It is a relatively uniform and compact electrodeposition.

In summary, there are two different phenomena in the above experiments: 1) all Li ions are deposited in one site under a large current density, resulting in a huge aggregated dendrite; 2) the nucleation sites grow into whisker-like Li together under a small current density.Previous work has indicated that the number density of nuclei raises with the increasing of current density [45].Obviously, the surface density distribution of aggregated Li dendrites in the growth process is opposite to the nucleation process.Therefore, the formation of aggregated Li dendrites is determined by the latter growth process rather than the nucleation process.While the current density exceeding the limiting current density, the occurrence of dendritic morphology has been well understood by Sand’s time model and Chazalviel model [46,47].Therefore we estimated the limiting current density by

where F denotes Faraday’s constant,D=3×10-6cm2·s-1the diffusion coefficient,c0the bulk concentration,t+=0.38 the cation transference number, L = 1.0 mm the distance between the two electrodes [40].The calculated value of limiting current density is 9.4 mA·cm-2.It means that the electrodeposition observed in this work is under mixed diffusion and kinetic control,where the depletion of interfacial Li ion is unable.Therefore, it is difficult to interpret the phenomena with the classic models.

An important disturbance that the Sand’s time model transplanted from Cu electrodeposition does not consider is the heterogeneity caused by the SEI and other impurities of the Li surface.In this regard,phase field simulations were performed for explaining above phenomena.Comparing with our previous model [37], spatial heterogeneities stemming from SEI or other factors are involved in this work, represented by a noise function (see Section 2.4).Moreover, the variation of Li nuclei under different current densities, including space density and size, are ignored.We assume that the initial nuclei exist in the form of semi-spheres with a fixed radius of 0.5 μm and a fixed interval of 4.0 μm.

The simulated results and quantitative data are presented in Figs.4 and 5.Fig.4a and c display the simulated morphology of electrodeposited Li as well as the distribution of Li ion concentration at a plating capacity of 0.1,1.0,2.0,and 3.0 mA·h·cm-2,respectively, in which ξ = 1 and 0 represent the Li metal and electrolyte,respectively.The dynamic evolution of morphology is illustrated in Video S3 (Supporting Information).The simulated morphology is consistent with the experiment results.There are aggregated Li dendrites on the Li metal anode surface at a current density of 3.0 mA·cm-2.This inhibits the growth of surrounding discrete Li dendrites.In the case of 0.5 mA·cm-2, discrete Li nuclei grow together at first, presenting a columnar-like morphology (before 1.0 mA·h·cm-2).Then their growth rate diverges, resulting in a variation of diameter and height (2.0 and 3.0 mA·h·cm-2).These discrete dendrites are able to evolve into columnar Li morphology with an efficient regulation.The ratio of current deposit surface area Scurrentto the initial deposit surface area Sinitialaffords an objective indicator for the morphology evolution.It is noted that Scurrent/Sinitialunder ideal planar electrodeposition keeps in a value of 1.0.As shown in Fig.5a, Scurrent/Sinitialunder 3.0 mA·cm-2increases almost linearly to 7.01, 13.10, and 18.47 at 1.0, 2.0, and 3.0 mA·h·cm-2, respectively.In contrast, Scurrent/Sinitialunder 0.5 mA·cm-2sluggishly increases to 5.13, 7.45, and 8.89 at 1.0,2.0,and 3.0 mA·h·cm-2, respectively.This indicates that the larger current density, the higher degree of dendrite fractal.The rapidly rise of exposed surface area renders a large amount of SEI,thereby affecting the performance of the batteries [48].

Fig.5.Quantitative data from phase field simulations.(a) The growth rate of surface area with respect to plating capacity.(b) The electric field intensity around dendrite tips with respect to the position of initial nucleation.

Furthermore, the minimum value of Li ion concentration near the Li surface at 0.5 mA·cm-2and 3.0 mA·h·cm-2is 0.54 mol·L-1(the last column of Fig.4c).Although this value decreases to 0.36 mol·L-1with the increase of current density (3.0 mA·cm-2),it still does not reach 0(the last column of Fig.4a).It demonstrates that the Li ion near the Li surface is never depleted, which agrees with our previous discussion about the limiting current density.Then what caused the transition of aggregated Li dendrites from discrete Li dendrites?

Corresponding electric field distribution gives an intuitive explanation,which is exhibited in Fig.4b and d.At first,the electric field on the surface of Li metal is relatively uniform(0.1 mA·h·cm-2), where small fluctuation is induced by the space heterogeneity.With the electroplating advances, locally electric field around some tips of Li dendrites is quickly enhanced.Compared to the case of a small current density, the tip with locally max electric field forms a strong shielding effect, which intensely attracts the Li ions and finally forms aggregated Li dendrites.The quantitative analyses of the electric field intensity around the tips of Li dendrites with the variation in applied current density and plating capacity are shown in Fig.5b.With the increasing of the applied current density from 0.5 to 3.0 mA·cm-2, about six fold the average electric field intensity at a plating capacity of 0.1 mA·h·cm-2is enhanced from about 1.0 to 6.0 kV·m-1.After further electrodeposition (1.0 mA·h·cm-2), the fluctuation of electric field intensity is amplified.For 0.5 mA·cm-2, the fluctuation is within 2 kV·m-1, whereas it reaches 20 kV·m-1at 3.0 mA·cm-2.

According to the above results and discussion, the aggregation of individual Li dendrites stems from the spatial inhomogeneity,which is further amplified by the current density, resulting in a locally concentrated electric field.Even if the limiting current density is unreached,such locally concentrated electric field is enough to form a hot spot, attracting subsequent deposited Li ions, suppressing the growth of surrounding discrete Li dendrites, and finally developing into aggregated Li dendrites.Therefore, it is insufficient to regulate Li dendrites according to the limiting current density and Sand’s time.Reducing the actual current density of Li metal anodes helps to delay the formation of aggregated Li dendrites, but the most fundamental thing should be to regular the spatial heterogeneity of the Li surface.This is especially important as Li metal batteries evolve into harsh conditions, e.g., electrode with large area (from coin cells to pouch cells), high current density, lean electrolyte, and low negative/positive capacity ratio[49].In this respect,a Li host or an ion re-distribution layer might help flatten the spatial heterogeneity.

4.Conclusions

Various electrodeposited morphologies in the Li metal anodes were visualized by means of in-situ optical microscopy and ex-situ SEM, especially the transition from discrete Li dendrites to aggregated Li dendrites with increasing current densities.Combining with the phase field simulations, it is quantitatively revealed that the dynamic evolution of aggregated Li dendrites is induced by the locally concentrated electric field rather than the depletion of Li ions.Specifically, the locally concentrated electric field originates from the spatial inhomogeneity on the Li metal surface and can be further enhanced by the high operating current density.Adjusting these two factors—with the assistance of the constructed phase field model—is able to regulate the electrodeposited morphology from aggregated Li dendrites to discrete Li dendrites,and finally realize ideal columnar Li morphology.Besides, it is appealed that more accessible and paralleled parameters, especially the kinetic parameters of electrolytes and SEI should be afforded in the future experiment research.The deep and fundamental understanding of various Li electrodeposition morphology established herein sheds light to the regulation of Li dendrites.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22061132002, U1801257, and 21825501),Russell Sage Foundation project (21-43-00006), the National Key Research and Development Program (2016YFA0200102 and 2016YFA0202500), Beijing Municipal Natural Science Foundation(Z20J00043), and the Tsinghua University Initiative Scientific Research Program.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.05.008.