Insights into Lithium Surface: Stable Cycling by Controlled 10 Î¼m

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Insights into Lithium surface: Stable Cycling by Controlled 10-µm-deep Surface Relief, Reinterpreting the Natural Surface Defect on Lithium Metal Anode Jinhyeok Ahn, Joonam Park, Ju Young Kim, Sukeun Yoon, Yong Min Lee, Seungbum Hong, Young-Gi Lee, Charudatta Phatak, and Kuk Young Cho ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00805 • Publication Date (Web): 11 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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ACS Applied Energy Materials

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Insights into Lithium Surface: Stable Cycling by

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Controlled 10-µm-deep Surface Relief,

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Reinterpreting the Natural Surface Defect on

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Lithium Metal Anode

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Jinhyeok Ahn,† Joonam Park,‡ Ju Young Kim,§ Sukeun Yoon,∥ Yong Min Lee,‡

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Seungbum Hong,■ Young-Gi Lee,§ Charudatta Phatak,*,⊥ and Kuk Young Cho*,†

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†Department

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Gyeonggi 15588, Republic of Korea ‡

Department of Energy Science and Engineering, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, Republic of Korea

§Research

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of Materials Science and Chemical Engineering, Hanyang University,

Group of Multidisciplinary Sensors, Electronics and Telecommunications Research Institute (ETRI), Daejeon 34129, Republic of Korea

∥Division

of Advanced Materials Engineering & Institute for Rare Metals, Kongju National University, Chungnam 31080, Republic of Korea

ACS Paragon Plus Environment

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ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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■Department

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of Materials Science and Engineering, Korea Advanced Institute of

Science and Technology (KAIST), Daejeon 34141, Republic of Korea ⊥Materials

Science Division, Argonne National Laboratory, Argonne, Illinois 60439,

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United States

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AUTHOR INFORMATION

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Corresponding Author

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*E-mail: [email protected] (C. Phatak); [email protected] (K. Y. Cho)

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ABSTRACT

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The future of next-generation rechargeable batteries, such as lithium-metal, lithium-

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sulfur and lithium-oxygen batteries, hinges on the utilization of metallic lithium as the

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anode. However, the practical application of lithium anodes has been challenging thus far

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due to the uncontrolled growth of lithium dendrites and extremely unstable interfaces

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between lithium and the electrolyte. Extensive investigations have been conducted to


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mitigate these limitations; nevertheless, there is a lack of fundamental insight into physical

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and chemical characteristics, the effect of thickness, and surface morphology of lithium

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anodes. Herein, an overview of the fundamental understanding of the effect of the shape

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of surface reliefs on lithium anode under the different cycling conditions is reported. Two

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different types of 10-μm deep surface reliefs, viz., continuous and discontinuous, were

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fabricated via soft lithography. It was newly found that the lithium-stripping behavior was

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significantly affected by the shape of surface reliefs. Furthermore, it is demonstrated that

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a 10-μm deep surface relief can not only be utilized on a thin lithium anode (20 μm) but

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also enhances the cell cycling stability. It is anticipated that the results will shed light on

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the practical utilization of lithium anodes by using the combination of other physical or

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chemical modification techniques.

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KEYWORDS: lithium metal battery, surface pattern, thin lithium anode, lithium plating and

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stripping, shape of pattern


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TOC GRAPHICS

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1. INTRODUCTION

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Lithium-ion batteries (LIBs) with higher energy and power density are urgently needed

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to address persistent global demands for the electrification of transport that requires an

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alternative battery chemistry beyond the up-to-date LIBs.1, 2 Li metal anode is a common

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core component of the most promising next-generation rechargeable batteries, viz.,

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lithium metal battery (LMB), lithium-sulfur (Li-S), and lithium-oxygen (Li-O2), owing to its

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overwhelming advantages of highest theoretical capacity (3860 mAh g−1) and lowest

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electrochemical potential (−3.04 V versus standard hydrogen electrode) over all other


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candidates.3, 4 The utilization of lithium metal anode facilitates an increase in the energy

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density from that of state-of-the-art LIBs to 500 Wh kg−1 that could replace the combustion

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engine.5 Thus, investigations on lithium metal anodes are receiving intense interest

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presently.

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Despite its attractive advantages, the use of Li anode is challenging in broader practical

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applications owing to its drawbacks such as low coulombic efficiency, poor cycle life, and

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safety concerns. These obstacles result from the high reactivity of Li with electrolytes and

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the inhomogeneous Li plating/stripping (the formation of Li dendrites and pinholes).6, 7

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The former triggers the continuous formation of the solid electrolyte interphase (SEI),

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which eventually results in decreasing coulombic efficiency and drying up of the cells. The

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latter might cause thermal runaway and explosion hazards by internal short circuits and

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capacity loss by the dead Li resulting from the isolation of active Li from bulk Li.5 Thus,

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most efforts have been devoted to suppressing the growth of Li dendrites because it is

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an issue directly connected to the utilization of Li anodes.

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To prevent the growth of Li dendrites, various strategies have been implemented, including the formation of artificial SEI layers on the Li surface,8,

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using novel liquid


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electrolyte systems,10,

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coatings on the Li surface,15, 16 adopting solid polymer or inorganic electrolyte systems,17,

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18

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can be classified as protective coatings or replacing components/searching new materials

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by adopting chemical approaches to overcome dendrite formation. Besides these

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strategies, current studies on a host framework for Li electrodes, such as porous

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conducting carbon foams,21, 22 3D framework,23, 24 3D current collectors,25, 26 and surface-

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patterned Li anode,27-30 are attracting considerable attention because these methods offer

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uniform and reduced current distribution with large specific surface area and controlled Li

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plating/stripping into the hollow space of the host material to suppress dendrite growth on

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the electrode surface. Compared to the chemical aspect of previous studies, the

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investigations on a host framework for Li can be classified as a physical approach that

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involves the modification of the Li anode structure. Among the physical paths to overcome

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problems encountered in LMB, surface patterning on Li metal by our group27-29 and other

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groups30, although in its early stage, provides promising candidates owing to

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reproducibility and feasibility of mass production.28 However, the basics of the patterns

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introducing functional additives,12-14 fabricating protective

and employing a functional separator19, 20 for the practical application of LMB. These


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are considerably unexplored. The fundamental understanding of phenomena at the

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interface of the Li anode is inevitable not only for the improvement of LMB but also for

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providing valuable information for the development of other next-generation rechargeable

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batteries (Li-S, Li-O2). We recently reported a new method to measure the Li metal

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surface modulus, which may give a clue for understanding the structural difference

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between fresh and pressurized Li.31

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The prerequisites for using surface-patterned Li anodes are:

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(1) Considering the size of the pattern, we created depths of 80 and 50 μm. There was

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difficulty in lowering the depth of the pattern owing to the absence of a proper fabrication

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method. At this time, it is unclear whether the small patterns (below 50 μm) affect the

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performance of LIB as bigger patterns do. Furthermore, the effect of the shape of patterns

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is yet unexplored. Soft lithography is suggested to overcome the problems of

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photolithography in semiconductor fabrication, and it also enables the manufacture of

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various small micron-scale patterns on the substrate.32 Here, we adopted this method

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and used a metallic master instead of rubbery stamps for Li patterning.


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(2) Mainly small patterns and even large patterns are not always able to store all the Li

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from the cathode in practical LMB. Thus, overflow, which means the total volume of the

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Li source from the cathode exceeds the volumetric capacity of the surface pattern, is

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common in an application using surface-patterned Li anodes (Figure S1a). This issue was

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only raised recently in our report.29 Unfortunately, few works address overflow in surface-

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patterned Li anode systems in spite of its practical importance.

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(3) It was recently reported that the use of Li metal should be limited to thicknesses

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under 30 μm, or the capacity of 6 mAh cm−2, for viable commercial designs with high-

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energy-density LMB (Figure S1b).33 Minimizing the use of lithium in a unit cell also

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improves the cost-effectiveness. Thus, we extended small patterns on thick (450 μm) as

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well as thinner (20 μm) anodes, which will provide an idea of the effectiveness of small

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patterns on the thin metal anodes as well as thicker ones for promising practical

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applications.

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(4) Most of the present studies used lower current density (< 1 mA cm−2) and showed

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stable operation with the cycling. However, the current density of charging and

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discharging are related to power density and application devices that require high-power


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performance.14 There is considerable debate on the appropriate current conditions of

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LMB, and a few works have suggested that the current density should be raised for

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practical use.34, 35 Thus, the investigation of a wider range of current density is vital for

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new approaches on LMB. We investigated cycling the behaviors of patterned Li under

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three different current densities, viz., 0.1 (low), 1 (moderate to high), and 10 mA cm−2

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(extremely high), to investigate the impact of patterns (continuous and discontinuous) on

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the plating and stripping. An understanding of the impact of current density will add to the

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information on proper operating conditions for LMB (Figure S1c).

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In this work, we prepared two different-shaped continuous and discontinuous surface

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reliefs (CSR and DSR) via soft lithography to compare the structural effect of surface

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reliefs on the Li anode. The CSR and DSR were fabricated with a diameter of 10 μm, both

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in width and depth. Both Li/Li symmetric cells and Li/LiCoO2 (LCO) full cells were

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assembled using the new Li anode, and they were tested under the overflow condition to

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estimate the effect of surface reliefs in practical use. Furthermore, the cycling current

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density was varied from low to extremely high in order to study its impact on Li/Li

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symmetric cells by investigating the morphological transition and cycling stability of the


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cell. A few works36 on thin Li anodes (< 100 μm) have been reported and the applicability

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of surface pattern on the thin Li anodes remains unclear. In this work, from the preliminary

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cell operation study, we found the potential to fabricate small surface patterns even on

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the thin Li anode.

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2. EXPERIMENTAL SECTION

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2.1. Fabrication of Ni masters and PDMS replicas. The Ni master was fabricated by the

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electroplating of Ni on the patterned HAR SU-8 mold. The Ti/Cu/Ti metal layers were

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served as seed layers and adhesion. The SU-8 mold was removed after Ni electroplating,

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and the seed-layers were etched from the Ni master. PDMS replica with surface reliefs

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was prepared from a commercially available liquid prepolymer mixture of a silicone

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elastomer base and curing agent (Sylgard 184, Dow Corning). A mixture of the elastomer

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base and curing agent (91/9 w/w) was vigorously stirred at 300 rpm for 30 min. Any

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bubbles generated during mixing were removed by repeatedly evacuating and purging

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the mixtures in a vacuum oven. PDMS replica with the surface reliefs was obtained by


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thermal curing of the prepolymer mixture on a respective photoresist master at 80 ℃ for

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5 h. The cured PDMS was then peeled off from the master.

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2.2. Electrode preparation. Anode material Li metal foil (both 450 and 20 μm) was

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purchase from Honjo Metal. Micropatterned Li metals were fabricated by pressing Li with

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Ni mold with 20 and 10 kgf for 450 and 20 μm think Li anode, respectively. After imprinting

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surface reliefs on Li metal, Li metal was punched into disks with an area of 1.13 cm−2 (12

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mm in diameter). LiCoO2 (LCO, KD10, Umicore) was used as a cathode material. The

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LCO electrodes were prepared by casting a slurry containing 90:5:5 and 80:10:10 (active

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material, super P, poly(vinylidene fluoride) binder (KF 1300)) onto an Al current collector

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foil, which correspond to the areal capacities of ≈ 1 mAh cm−2 and ≈ 0.3 mAh cm−2,

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respectively. The typical loadings of the NMC electrodes were about 7.3 mg cm−2 and

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2.3 mg cm−2 of active material, respectively. After drying, the electrodes were punched

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into disks with an area of 0.95 cm−2 (11 mm in diameter).

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2.3. Electrochemical measurements. Electrochemical performance measurements were

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carried out using CR2032 coin-type cells. Li metal batteries were assembled with the LCO


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cathode, metallic Li anode, polyethylene (PE) separator (12 μm, W-Scope, Korea), and

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1.0 M LiPF6 in ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (3/7, v/v)

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(electrolyte, Panax Etec Co. Ltd.) as electrolyte in a glove box filled with argon gas

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(99.999 % purity). For the Li/Li symmetric cells, Li metal was used both as a cathode and

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anode. The galvanostatic cycling of Li/Li symmetric cells was conducted using a battery

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tester (WBCS 3000, WonA tech) at current densities of 0.1, 1, and 10 mA cm−2 with

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equivalent capacity of 0.5 mAh cm−2. The rest time was 10 min to alleviate the influence

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of concentration gradients. The galvanostatic charge-discharge tests of Li/LCO full cells

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were conducted using a battery tester (PNE solution, Korea) at 0.2, 1 and 10C (1C=1 mA

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cm−2) in a constant current (CC) in the voltage range of 2.7–4.2 V (vs. Li/Li+). The cells

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were measured with gradually increasing applied current densities from 0.5 to 3 or 10C

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to stabilize the cells under high current densities (3 or 10 C). For the electrochemical test

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of 20 μm Li metal, a moderated condition was adapted 0.3 mA cm−2 was applied with the

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capacity of 0.3 mAh cm−2 for the Li/Li symmetric cell test and cycling of Li/LCO cells was

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evaluated at 0.3 mA cm−2 (1C) in a CC mode. Electrochemical impedance spectroscopy


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(EIS) measurement was performed with an impedance analyzer (VSP, Bio-Logic SAS) at

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an amplitude of 10 mV with a frequency range of 10 mHz to 1 MHz.

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2.4. Scanning Electron Microscopy (SEM) analysis: After electrochemical investigations,

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the cells were carefully disassembled in a dry glove box filled with argon. Samples were

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gently washed with DMC for several times and thoroughly dried under vacuum for 24 h.

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To measure the cross-section images of Li metal, we cut the Li metal samples with Teflon

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coated scissors. The images of surface morphology of PDMS replicas and Li anodes were

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obtained using field-emission scanning electron microscopy (FE-SEM, S-4800, Hitachi).

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2.5. Li metal modeling and simulation: A 3D Li metal battery model, which simulates

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current density distribution, was constructed using Newman's porous electrode model in

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COMSOL Multiphysics 5.4 based on the finite element method.28,

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parameters applied are given in Table S1. The simulation was performed with the

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following conditions: 1) Li metal foil with a thickness of 20μm was taken as an anode in

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the battery model; 2) a constant current density of 1 mA cm−2 at 298.15 K was used in

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the battery model; 3) Any side reaction such as SEI formation was neglected.

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The new model

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3. RESULTS AND DISCUSSION

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Figure 1. (a) Schematic illustration of continuous and discontinuous surface reliefs (CSR

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and DSR) on the Li surface. (b) Bar chart of depth of CSR on the Li anode with variable

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applied force. (c) Surface and cross-sectional SEM images of CSR and DSR on the Li

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surface (CSR Li and DSR Li).

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Figure 1a shows the schematic illustrations of CSR and DSR on the Li surface, which

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are designed for studying the impact of surface patterns on the Li anode performance. In

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brief, CSR refers to the furrow-like structure where the surface reliefs extend end to end,


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whereas DSR is the isolated surface relief with a round appearance at the surface. A

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nickel master, which has the inverse shape of the pattern, is used to fabricate surface

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patterns on Li metal foil (Figure S2). First, we prepared a polydimethylsiloxane (PDMS)

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replica by using the Ni master to confirm the appropriate implementation of the designed

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CSR and DSR with furrow-like and round-shaped well-like surface reliefs, respectively

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(Figure S3).

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As Li is ductile, patterns can be easily made.27 However, a well-defined and repeatable

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pattern on the surface is hindered by the difficulty of the detachment of the mold due to

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the ductility, which is even worse for small patterns. From the experience of pattern

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fabrication, it is understood that the ease of detachment is related to the force applied for

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pattern fabrication. Thus, we designed and constructed an in-house mold-pressing

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machine that could measure the applied force and determine the adequate force value

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for micropatterning (Figure S2). We investigated the resulting pattern depth on the Li

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anode by varying the force applied on a 450-μm thick Li metal foil, as depicted in Figure

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1b. The optimum condition was obtained at an applied force of 20 kgf at room

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temperature. When the force is low, the exact depth as that of the master pattern could


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not be achieved, where a higher applied force results in inhomogeneity of the depths, and

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overflowed residue leading to the distortion of the pattern is observed (Figure 1b and S4).

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Thus, we chose 20 kgf for the micropatterning of the 450-μm Li anode. The distinct

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surface structures of the fabricated surface reliefs for the CSR and DSR on the Li anode

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with an applied force value of 20 kgf are shown in Figure 1c. The surface exhibited no

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additional surface cracks or impurities other than intrinsic natural defects and grain

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boundaries of Li foil.37


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Figure 2. Galvanostatic cycling voltage profiles for Li/Li symmetric cells with the capacity

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of 0.5 mAh cm−2: (a) at 1.0 mA cm−2 and detailed voltage profiles (b) 0–15 h, (c) 200–215

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h, and (d) 500–515 h. (e) at 0.1 mA cm−2. (f) at 10 mA cm−2.

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Successful small-patterning of DSR and CSR formation on the Li metal foil in this work

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is of great importance in managing and understanding the uncontrollable and unavoidable

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“hot spot” of natural Li metal surface. Bare Li necessarily has an irregular morphology

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consisting of cracks and surface defects, which act as uncontrollable “hot spots” for the

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Li plating and stripping, at the surface.38 The characteristic shapes of the surface defects

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are pinholes and irregular valleys, where the inhomogeneous electron density is formed,

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and Li plating and stripping preferably occur (Figure S5). Here, the furrows in the CSR

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and dispersed well-like surface reliefs in the DSR are controllable valleys and pinholes,

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respectively. This can provide a profound understanding of the Li plating and stripping

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and potential of reducing unwanted dendrite formation.

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We evaluated the galvanostatic cycling of Li/Li symmetric cells to investigate the cycling

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stability of Li anodes with the surface reliefs under different current densities, as

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demonstrated in Figure 2. The cells were cycled at current densities of 0.1, 1, and 10 mA

247

cm−2 with an equivalent capacity of 0.5 mAh cm−2 for all the cells. Lower polarization in

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the order of DSR on Li surface (DSR Li), CSR on Li surface (CSR Li), and reference bare

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Li was observed for the Li/Li symmetric cell cycled at the current density of 1 mA cm−2


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(Figure 2a). A smaller polarization indicates reduced current density at the Li surface and

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influence on the reduction in dendrite formation. There is an increase in the voltage gap

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corresponding to the cycle numbers, and eventually, failure in the cell takes place. The

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earlier failure of the cell corresponds to the poorer stability of the cell. The results clearly

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indicate the positive effect of a surface-patterned Li compared to a bare Li anode by the

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surface reliefs’ diminishing specific current density (current/area).27 When the patterned

256

Li anodes are compared, DSR Li is found to be more stable than CSR Li, and this could

257

be clearly observed from the amplified Figure 2a profile at the initial, steady state, and

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cell failure region of Li/Li symmetric cell cycling, which are shown in Figures 2b, 2c, and

259

2d, respectively.

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The flat voltage plateaus of the three cells in the stable region are shown in Figure 2c;

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the CSR Li and DSR Li (0.055 and 0.050 V at the 150th cycle, respectively) remain

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smaller with flat voltage hysteresis than bare Li (0.118 V at 150th cycle), which indicates

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more uniform Li plating/stripping of surface-patterned Li. At the cell failure region shown

264

in Figure 2d, the increasing unstable voltage profile due to electrolyte depletion and

265

formation of dendritic short-circuiting39 is first observed in bare Li. The reduced


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polarization with the stable potential profiles of the Li metal with surface reliefs was

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consistent with the smaller overall impedance of the cells subjected to 400 cycles, as

268

shown in Figure S6.

269

The performance of Li/Li symmetric cells cycled at low (0.1 mA cm−2) and extremely

270

high (10 mA cm−2) current, except 1 mA cm−2, is shown in Figures 2e and 2f, respectively.

271

All Li/Li symmetric cells cycled at a low current density of 0.1 mA cm−2 demonstrated

272

stable operation for at least 1200 h. Interestingly, a significant difference in overpotentials

273

is observed at the initial stage of symmetric cell cycling. Stable Li plating and stripping at

274

the initial stage was observed for the patterned (DSR and CSR) Li anodes (Figure 2e).

275

The current density of 10 mA cm−2 corresponds to the 5C operation of LIB with a

276

cathode having an electrode density of 2 mAh cm−2, which is a harsh condition. Although

277

a lower overpotential of DSR, same as at 1 and 0.1 mA cm−2, was observed, all the cells

278

showed failure after 43 h under 10 mA cm−2 (Figure 2f). The very-early-stage failure

279

indicates that the current density of cycling can overwhelm the effect of pattern existence

280

and the shape at an extremely high current density.


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Figure 3. Surface SEM images of (a–l) CSR Li and (a’–l’) DSR Li after Li/Li symmetric cell

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cycling at 0.1, 1.0, and 10 mA cm−2 with the charge/discharge capacity of 0.5 mA cm−2:

284

(a, c, e) after 1st plating, (b, d, f) after 1st stripping, (g, i, k) after stripping followed by 1st

285

plating, and (h, j, l) after plating followed by 1st stripping of CSR Li. (a’, c’, e’) after 1st

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plating, (b’, d’, f’) after 1st stripping, (g’, i’, k’) after stripping followed by 1st plating, and


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(h’, j’, l’) after plating followed by 1st stripping of DSR Li. All the scale bars correspond to

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20 μm.

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The influence of current density on the Li/Li symmetric cell cycling performance is

290

noteworthy. It implies that a detailed investigation of Li plating and stripping should be

291

conducted at a broader range as well as at the low value where the stability is already

292

secured. We also confirmed that small surface reliefs reduce the polarization of the Li

293

surface despite the overflow condition.

294

Because the difference in the stability was confirmed from the Li/Li symmetric cell

295

performance, the effect of shape on the plating and stripping should be investigated in

296

detail. We investigated both Li anodes used in stripping and plating via the scanning

297

electron microscopy (SEM) after the galvanostatic cycling of the Li/Li symmetric cells. The

298

observation was conducted with a different current density in symmetric cell operation, as

299

shown in Figure 3. There are two stages ((1) and (2)) of Li movement in left Li and right

300

Li. In the first stage, which is denoted as (1), plating of Li at the left Li and stripping of Li

301

at the right Li occur simultaneously. In the second stage, which is (2), stripping of the


22

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302

plated Li at the left Li and plating of Li at the stripped surface of the right Li occur

303

simultaneously. The effect of surface relief shapes, which are continuous furrows and

304

dispersed wells, is shown at the upper and lower side in the figure, respectively. It is the

305

first systematic view of the effect of shape, current density, plating and stripping

306

simultaneously with the Li overflowing cell.

307

Judging from the SEM images of the left Li of both CSR and DSR (Figures 3a, 3c, 3e,

308

3a’, 3c’, and 3e’), Li prefers to be deposited in the surface reliefs. Li-ions tend to be plated

309

on the top edge of the surface reliefs initially and then the plated Li will grow into the inner

310

space of the surface reliefs under low current density (0.1 mA cm−2). At moderate-to-high

311

current density (1.0 mA cm−2), the granular form of Li grows in the surface reliefs. As the

312

plating current density increases up to 10 mA cm−2, the size of the granules increases

313

and needle-like dendrites grow between the granular-shaped Li. Furthermore, Li is

314

deposited not only in the inner space of the surface reliefs but also on the fresh Li surface,

315

which demonstrates the overpowering impact of the extremely high current density on Li

316

plating despite the existence of surface reliefs.


23


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317

The noticeable difference between CSR and DSR is found after the first stripping on

318

the right Li in Figures 3b, 3d, 3f, 3b’, 3d’, and 3f’. Under the low current density of Li

319

stripping, apparently, Li ions were smoothly stripped from both CSR and DSR, so the

320

morphology did not change considerably before stripping. However, upon increasing the

321

current density to 1.0 mA cm−2 or 10 mA cm−2, the significant difference between CSR

322

and DSR was clearly observed. After Li was stripped at 1.0 mA cm−2, enlarged holes

323

along the rims were observed in DSR Li, whereas the Li stripping occurred mainly at the

324

fresh surface in CSR Li. The tendency was coincident in Li stripping at 10 mA cm−2.

325

Relatively controlled morphology after Li stripping in DSR is observed in Figure 3f’,

326

although the extremely high current density overwhelms the effect of surface reliefs in the

327

plating process. This indicates that the difference of the stripping process led to the more

328

stable cycling behavior of Li/Li symmetric cells with DSR Li than CSR Li, as shown in

329

Figure 2. Furthermore, the results suggested that the understanding of not only Li plating

330

but also Li stripping is essential.

331

Generally, after the surface reaction (Li plating or stripping), newly generated “hot

332

spots,” such as deposited Li or pinholes, induce the enlargement of dendritic Li growth or


24

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333

pinholes. The accelerated surface reaction was also observed in the second Li plating

334

and stripping on both left and right Li. The behavior of Li stripping in the stripping of the

335

plated Li at the left Li surface was similar to that of the right Li surface after initial stripping

336

(Figures 3g, 3i, 3k, 3g’, 3i’, and 3k’). The Li stripping behavior on CSR Li showed that Li

337

would detach from the planar surface leaving the previously deposited Li in the CSR. On

338

the other hand, relatively controlled Li stripping was observed in DSR Li in spite of the

339

effect of new “hot spots.”

340

From the SEM images of Li plating at the stripped surface of the right Li (Figure 3h, 3j,

341

3l, 3h’, 3j’, and 3l’), it can be confirmed that newly formed pinholes or enlarged DSR from

342

the previous stripping process acted as a new surface relief. Therefore, the effect of the

343

original continuous patterns was significantly reduced due to the previously generated

344

pinholes. However, because the discontinuous patterns expanded around its rim, Li

345

plating preferentially occurred at the DSR.

346

Furthermore, to investigate the surface morphology of the CSR and DSR Li, we

347

obtained surface SEM images of CSR and DSR Li after 3rd and 50th cycles of Li/Li

348

symmetric cells under the low current density (0.1 mA cm−2) (Figure S7 and S8). There


25


Page 26 of 53

349

was not much difference in surface morphology between the sample after the 1st and 3rd

350

cycles. The accelerated surface reaction was observed, which is determined by the

351

dendritic lithium growth, and deformed the surface reliefs. In addition, although the exact

352

number of cycles, after which the surface reliefs persist by the deposited Li and SEI

353

formation, cannot be estimated, we could find traces of surface reliefs even after the 50th

354

cycle of the cell.

355

Overall, the understanding of the Li stripping process is significantly necessary in

356

addition to Li plating for the improvement of the stability of the Li anode. Both surface

357

reliefs act as arrayed “hot spots” for the initial Li plating process. However, in this work,

358

the Li stripping behavior can be distinguished from the shape of surface reliefs. Above all,

359

we demonstrated that the discontinuous surface reliefs facilitate more controlled Li

360

stripping. This well-controlled stripping affects additional plating or stripping by reducing

361

the generation of additional uncontrolled “hot spots.” Besides, we also confirmed that

362

small surface reliefs could enhance the homogeneity of Li plating/stripping despite the

363

overflow condition.


26

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364

365

Figure 4. Cycling performance of Li/LCO full cells with bare Li, CSR Li, and DSR Li at

366

various C-rates in the voltage range of 2.7–4.2V: (a) 0.2C (0.2 mA cm−2), (b) 1.0C (1.0

367

mA cm−2), and (c) 10C (10 mA cm−2).

368


27


Page 28 of 53

369

Li/LCO cells were assembled, and their cycling performances were evaluated under

370

varying current densities to investigate the effect of the surface reliefs on the

371

electrochemical performances of LMBs.

372

An LCO cathode with a capacity loading of 1.0 mAh cm−2 was fabricated and used for

373

Li/LCO cell cycling tests. The cyclability at 0.2C (0.2 mA cm−2) of bare Li, CSR Li, and

374

DSR Li was compared after up to 200 cycles between 2.7 and 4.2 V (Figure 4a). Slightly

375

higher initial specific capacities of patterned Li were obtained thorough the overall cycling

376

test, which might be attributed to the homogeneous Li deposition at the initial cycle. A

377

drastic decrease in capacity was observed in bare Li after 150 cycles. Among the

378

samples, DSR Li showed the most excellent capacity retention, delivering 81.4 % (114.5

379

mAh g−1) of its initial capacity (140.6 mAh g−1) after 200 cycles. Bare Li and CSR Li

380

showed lower capacity retention of 50.8 % (69.0 mAh g−1) and 76.4 % (107.5 mAh g−1) of

381

their initial capacities (134.6 and 140.6 mAh g−1, respectively), respectively. A remarkable

382

difference in cycling stability was observed at 1C (1 mA cm−2) (Figure 4b). The DSR Li-

383

based cell delivered 88.6 % of its initial capacity after 100 cycles, which showed a large

384

gap between DSR Li (46.8 %) and bare Li (42.7% of initial capacity). This tendency was


28

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385

consistent with the result of the cycling stability of Li/Li symmetric cell at 1 mA cm−2.

386

Primarily, it was important that though both CSR and DSR Li showed longer cycle life

387

under low-current-density cycling, only DSR Li patterns maintained the cycling stability

388

under high current density. It might be mainly caused by the difference in the Li stripping

389

behavior of the surface reliefs shown in Figure 2. The most stable and high value of

390

coulombic efficiency of DSR Li is also shown in Figure S9, which demonstrated stable

391

plating and stripping on the Li anode. Nevertheless, the cycle life of all cells at 10 mA

392

cm−2 was very short (Figure 4c); DSR Li showed relatively high capacity at the initial stage

393

of the cycling, which may be attributed to the homogeneous Li stripping on the Li anode

394

by the DSR. The reduced polarization and improved cycle performances of the surface-

395

patterned Li anodes at low and moderate-to-high current density were consistent with the

396

smaller overall impedance than bare Li, as shown in Figure S10 and Table S2.


29


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397

398

Figure 5. (a) Cross-sectional SEM image of 20-μm Li anode and surface SEM images of

399

(b) bare Li, (c) CSR Li, and (d) DSR Li of 20-μm Li anode. (e) Cycling performance of

400

Li/LCO full cells with bare Li, CSR Li, and DSR Li at 1.0C (0.3 mA cm−2) in the voltage

401

range of 2.7–4.2 V. (f) Galvanostatic cycling voltage profiles for Li/Li symmetric cells at

402

0.3 mA cm−2 with a total capacity of 0.3 mAh cm−2.

403


30

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404

To estimate the performances of Li anodes with surface reliefs further in order to confirm

405

the possibility of the practical application of LMBs with thin Li anodes, the surface reliefs

406

were fabricated on thin Li anodes (20 μm), and their electrochemical performances were

407

investigated. A lower areal capacity loading of LCO with 0.3 mAh cm−2 was used for thin

408

Li/LCO cells to acquire data in milder conditions. Figures 5a and 5b show the cross-

409

sectional and surface SEM images of bare Li and surface-patterned Li anodes. The thin

410

Li anode was laminated with a 10-μm-thick Cu foil. By pressing the Li anode with 10 kgf,

411

the surface reliefs were also well fabricated, similar to the reliefs on the thick Li anode

412

(Figures 5c and 5d). A comparison of cycling performances of thin Li/LCO cells is shown

413

in Figure 5e. The DSR Li showed the most stable cycling performance, delivering 88.9 %

414

(112.0 mAh g−1) of its initial capacity (126.0 mAh g−1) after 100 cycles. However,

415

compared to thick-Li-anode-based cells, a more drastic decrease in capacity was

416

observed in all the cells due to the high reactivity of the thin Li anode. Figure S11 shows

417

the Nyquist plots of thin Li/LCO cells after the 100th cycle. Thin Li anodes with reliefs

418

showed smaller impedances than bare Li (Table S3). The results of the electrochemical

419

performance of thin Li were consistent with those of thick Li. The galvanostatic cycling of


31


Page 32 of 53

420

Li/Li symmetric cell with the 20-μm-thick Li anode with surface reliefs was conducted

421

(Figure 5f). The most stable voltage hysteresis was observed in DSR Li. This implies that

422

the surface reliefs affect the stability of the thin Li anode, though it is more unstable than

423

the thick Li anode.

424


32

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425

Figure 6. Simulation results of current densities distribution on CSR and DSR Li during

426

the charging and discharging process.

427

To verify the effect of the shape of the surface reliefs further, we performed a simulation

428

where Li plating and stripping occur preferentially with current density differences.28, 29

429

We constructed a 3D lithium metal full-cell model that can simulate the current densities

430

on lithium metals with CSR and DSR. Figure 6 indicates the 3D structure and 2D

431

tomography images, which are covered by current density values during the charging or

432

discharging process. DSR has higher maximum current density in the reliefs and lower

433

minimum current density on the ridges of patterns than CSR. Moreover, the difference in

434

average current density between the inner part and on the ridges of DSR is higher than

435

that of CSR. The results indicate that DSR has a higher potential for lithium

436

plating/stripping in the inner space than CSR, which was well matched with the more

437

stable Li plating/stripping behavior in Figure 3.

438

In addition, we compared the other discontinuous-type surface relief—the reverse cubic

439

relief (RCR)—via the simulation method (Figure S12). The simulation results of RCR


33


Page 34 of 53

440

showed that the cubes show better morphology in view of the current density distribution

441

than CSR. Furthermore, although RCR is discontinuous and the dimensions are similar

442

to those of DSR, DSR showed better current distribution in both charging and discharging.

443

This result provides a clue that a change in the structure of surface reliefs could affect the

444

electrochemical performance of LMBs.

445 446

4. CONCLUSIONS

447

In conclusion, we have revealed the influence of the significance of the surface relief

448

shape and current density on the Li plating and stripping from the initial stages of the

449

cycling of LMB. Furthermore, successful fabrication of 10-μm-deep small continuous and

450

discontinuous surface reliefs realized on very thin Li metal (20 μm) as well as a thicker

451

one (450 μm) provides an early step for the practical application of LMB by minimizing

452

the lithium use.

453

We newly found that there is a clear difference in Li stripping phenomena depending on

454

the shape of the relief, whereas similar behavior of Li plating either on the continuous


34

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455

(furrow) or discontinuous (well-like) surface relief was observed from the analysis of SEM

456

images of the cycled Li surface of Li/Li symmetric cells. The Li anode with discontinuous

457

surface reliefs demonstrated the most stable cycling performance in both Li/Li symmetric

458

cell and Li/LCO full cell by the reversible plating/stripping around the surface reliefs. The

459

same trends of cycling performance from thick as well as thin anodes indicate the surface

460

pattern independence of Li metal anode thickness. Nevertheless, under overflow

461

conditions, there was a prominent improvement in cycling performance because of the

462

surface reliefs. This work not only reveals the promising prospects in the fabrication

463

approach for various patterned thin-Li-metal anodes but also reports on the importance

464

of investigation of the Li stripping behavior at different charge-discharge rates in LMB.

465

This work provides an insightful understanding of Li plating and stripping by the small

466

patterns of Li metal anode for practical utilization of LMB.

467

468

ASSOCIATED CONTENT

469

Supporting Information


35


Page 36 of 53

470

The Supporting Information is available free of charge on the ACS Publications website

471

at

472

DOI: xxx.

473

Experimental methods; schematic illustrations of the issues related to utilization of

474

lithium anode; digital camera images of the Ni master and the press machine used for

475

the fabrication of surface reliefs; OM and SEM images of surface patterned Li metal;

476

additional electrochemical analytical data

477

AUTHOR INFORMATION

478

Corresponding Author

479

*E-mail: [email protected] (C. Phatak); [email protected] (K. Y. Cho)

480

ORCID

481

Jinhyeok Ahn: 0000-0002-6022-1548

482

Joonam Park: 0000-0002-3308-7427

483

Yong Min Lee: 0000-0003-2002-2218


36

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484

Seungbum Hong: 0000-0002-2667-1983

485

Charudatta Phatak: 0000-0002-8931-0296

486

Kuk Young Cho: 0000-0002-8577-9767

487

488

Notes

489

The authors declare no competing financial interest.

490

ACKNOWLEDGMENT

491

This research was supported by the National Research Foundation of Korea (NRF)

492

grant funded by Korea government (MSIP) (No. 2018R1A2B6003422).

493 494 495 496

497 498

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TOC Figure 75x50mm (300 x 300 DPI)



(a) Schematic illustration of continuous and discontinuous surface reliefs (CSR and DSR) on the Li surface. (b) Bar chart of depth of CSR on the Li anode with variable applied force. (c) Surface and cross-sectional SEM images of CSR and DSR on the Li surface (CSR Li and DSR Li).


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Galvanostatic cycling voltage profiles for Li/Li symmetric cells with the capacity of 0.5 mAh cm−2: (a) at 1.0 mA cm−2 and detailed voltage profiles (b) 0–15 h, (c) 200–215 h, and (d) 500–515 h. (e) at 0.1 mA cm−2. (f) at 10 mA cm−2.



Surface SEM images of (a–l) CSR Li and (a’–l’) DSR Li after Li/Li symmetric cell cycling at 0.1, 1.0, and 10 mA cm−2 with the charge/discharge capacity of 0.5 mA cm−2: (a, c, e) after 1st plating, (b, d, f) after 1st stripping, (g, i, k) after stripping followed by 1st plating, and (h, j, l) after plating followed by 1st stripping of CSR Li. (a’, c’, e’) after 1st plating, (b’, d’, f’) after 1st stripping, (g’, i’, k’) after stripping followed by 1st plating, and (h’, j’, l’) after plating followed by 1st stripping of DSR Li. All the scale bars correspond to 20 μm.


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Cycling performance of Li/LCO full cells with bare Li, CSR Li, and DSR Li at various C-rates in the voltage range of 2.7–4.2V: (a) 0.2C (0.2 mA cm−2), (b) 1.0C (1.0 mA cm−2), and (c) 10C (10 mA cm−2).



(a) Cross-sectional SEM image of 20-μm Li anode and surface SEM images of (b) bare Li, (c) CSR Li, and (d) DSR Li of 20-μm Li anode. (e) Cycling performance of Li/LCO full cells with bare Li, CSR Li, and DSR Li at 1.0C (0.3 mA cm−2) in the voltage range of 2.7–4.2 V. (f) Galvanostatic cycling voltage profiles for Li/Li symmetric cells at 0.3 mA cm−2 with a total capacity of 0.3 mAh cm−2.


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Simulation results of current densities distribution on CSR and DSR Li during the charging and discharging process.


Insights into Lithium Surface: Stable Cycling by Controlled 10 Î¼m

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