Energy Storage Materials 24 (2020) 602–609

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Energy Storage Materials

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Vertically aligned carbon nanotubular structure for guiding uniform lithiumdeposition via capillary pressure as stable metallic lithium anodes

Brindha Moorthy a, Joo-Hyung Kim a, Hyun-Wook Lee b,**, Do Kyung Kim a,*

a Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 305-701, Republic of Koreab School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 44919, Republic of Korea

A R T I C L E I N F O

Keywords:Metallic lithium anodesDendriteVertically aligned porous carbon structureCapillary forceTemplate synthesis

* Corresponding author.;** Corresponding author.

E-mail addresses: hyunwooklee@unist.ac.kr (H.-

https://doi.org/10.1016/j.ensm.2019.06.016Received 22 January 2019; Received in revised forAvailable online 21 June 20192405-8297/© 2019 Elsevier B.V. All rights reserved

A B S T R A C T

Metallic lithium anodes hold the key for next-generation high-energy batteries. However, the safety issues andpoor Coulombic efficiency induced by dendritic growth of lithium hinder their commercialization. We addressthese limitations with vertically aligned nanoporous carbon tubes (VNPCT) as a high performing, stable host forlithium. The designed carbon structure can efficiently direct Li deposition inside the tubular structure throughuniform lithium flux and capillary force. Furthermore, the tubular host can greatly reduce the effective currentdensity to eliminate the dendrite formation even at 5 mA cm�2. A high areal lithium deposition of 2mAh cm�2

can be plated efficiently on the designed structure without any dendritic morphology. Additionally, the engi-neered lithium metal host exhibited a stable cycle life and higher Coulombic efficiency of 96% over 200 cycles at acurrent density of 0.5 mA cm�2, which is superior to conventional 2D planar hosts. A full cell performance oflithiated VNPCT (VNPCT@Li) and LiFePO4 cathode was demonstrated.

1. Introduction

The demand for batteries in electric vehicles, and grid storage ap-plications has encouraged a research focus towards the development ofhigh-energy density rechargeable batteries [1,2]. Among various batte-ries, lithium metal batteries (LMBs) have received much significantattention due to their high energy densities. Metallic lithium (Li) has longbeen considered as a promising anode candidate due to its high specificcapacity (3860mAh g�1), low redox potential (�3.04 V vs Li/Liþ), andlightweight (0.53 g cm�3) [3,4]. However, the practical realization of ametallic lithium anodes in LMBs is severely hindered by several safetyand performance issues. The uncontrollable growth of lithium dendritesupon cycling can initiate a thermal runaway and catastrophic failure inbatteries [5]. The performance issues in lithiummetallic anode arise fromtheir poor Coulombic efficiency. The highly reactive Li can easily reactwith organic electrolytes, leading to formation of a passivating layer, theso-called solid electrolyte interface (SEI). However, the formed SEI layeris not stable during repeated cycling and can easily be destroyed [2,6–8].This exposes the fresh Li to the electrolyte, which again consumes boththe Li and electrolyte to reform the SEI layer, thereby lowering theCoulombic efficiency [3–5,9]. In addition, deposited lithium below the

W. Lee), dkkim@kaist.ac.kr (D.K.

m 14 June 2019; Accepted 18 Ju

.

cracked SEI layer may grow faster than other parts, leading to anisotropicgrowth of dendrites. Finally, the formed dendrites can easily penetratethe separator and thereby cause a short circuit and battery failure [3,4,10].

Several research attempts have been made to solve the problemsassociated with metallic lithium anodes. Stabilizing lithium metal anodesurface is crucial for improving the plating/stripping behavior and cyclelife. Thus, can be achieved by tuning the interface engineering throughhigh concentrated electrolytes and using additives in the electrolytes.Various electrolyte additives for a liquid electrolyte, such as vinylenecarbonate (VC), fluroethylene carbonate (FEC), halogenated salts, Csþ

ions, Li2S8, and LiNO3, and concentrated electrolytes (4M LiFSI-DME)have been studied for reinforcing the stable SEI layer formation [2,11–16]. On the other hand, an artificial protective SEI layer, such ashexagonal boron nitride, interconnected hollow carbon nanospheres,polymer coating, and LixSi coating on the surface of Li metal or on the ofcurrent collectors have been investigated to stabilize Li metal depositionand prevent the penetration of Li dendrites [17–20]. Although theseapproaches have controlled the dendrite growth and improved theelectrochemical performance to some extent, the huge volume changeand uneven deposition of Li below the SEI layer remains unchanged. The

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ne 2019

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significant volume change in a lithiummetal electrode during repeated Lideposition/stripping is mainly due to its “hostless” nature. Various hoststructures, including layered graphene oxide, carbon cloth, carbonnanofiber film, and carbonized wood, have been demonstrated formetallic lithium anodes [5,21–23]. Among various approaches, the Lihost structures have attracted considerable interest because they areeffective in guiding the Li deposition, suppressing the growth of Li den-drites by reducing the local current density, and homogenizing the Liþ

flux. Recently, Zhang et al. [23] designed lithiophilic ZnO coatedcarbonized wood channels as a host material for a stable metallic lithiumanodes. The well-aligned channel structure provides excellent pathwaysin which the Li plating/stripping process can take place and effectivelyconfine volume change. Liang et al. [24] demonstrated stable cycling of ametallic lithium anodes using a chemically inert and electronicallyinsulating 3D-oxidized PAN nanofiber mat as host to guide and homog-enize the deposition of Li. Cui and co-workers have pioneered anapproach using nanomaterial that have zero overpotential (Au seeds) toselectively deposit and encapsulate Li in hollow carbon spheres throughheterogeneous seeded growth. Also, In-situ TEM analysis demonstratedthat encapsulated lithium metal anode inside Au particles decoratedhollow carbon spheres could be in semi-fluid state [25]. Liu et al. [26]demonstrated a polyimide coating layer with vertically aligned nano-channels that stabilized the metallic lithium anodes by minimizing thelocal current density and regulating the homogenous Liþ flux distributionthrough nanochannel confinement. Lu et al. [27] reported a rationaldesign of a freestanding Cu nanowire network current collector to sup-press the growth of dendritic lithium. Wang et al. [28] also developed anovel porous current collector with vertical microchannels fordendrite-free Li metal anodes. These kinds of stable host structures with alithium metal guiding root can make it possible to achieve high-energydensity Li-metal, Li-air and Li–S batteries for next-generation energystorage devices. However, it is still challenging to attain a stable andefficient lithium metal anode host at high current densities with highplating capacities.

Here, we report a novel vertically aligned nanoporous carbon tubular(VNPCT) structure engineered from porous anodized aluminum oxidetemplate as a host for stable metallic lithium anodes. The rationallydesigned nanoporous carbon tubular structure with polar functionalgroups facilitate uniform Liþ flux and produce stable SEI layer, thusconfine the lithium metal inside the porous structure and suppress thedendrite formation. By the tailored engineering, we have succeeded inplating a high plating capacity of 2 mAh cm�2 within the space availablein our designed structure through the capillary pressure. The designednano architecture provides a stable cycle life (200 cycles at 0.5mA cm�2)along with a superior plating/stripping performance, even at high cur-rent rates (5mA cm�2). Further, a full cell with LFP cathode and LithiatedVNPCT (VNPCT@Li) shows a stable electrochemical performance.

2. Materials and methods

2.1. Fabrication of the VNPCT-modified electrode

Nanoporous anodized aluminum oxide (AAO) templates were usedfor making a vertically aligned nanoporous carbon tubular (VNPCT)structure. We prepared the nanoporous alumina templates by a two-stepanodization process in 0.3M oxalic acid (99.0%, Sigma-Aldrich) at 25 �C[29]. High purity aluminum foil (thickness 0.25mm, 99.999%,Sigma-Aldrich) was used as the substrate (1� 1 cm�2). The aluminumsubstrate was subjected to electro-polishing at 20 V for 2min in a solu-tion containing mixture of ethanol (99.5%, Samchun Pure Chemicals)and perchloric acid (60%, Sigma-Aldrich) with the ratio of 4:1. The firstanodization process was conducted at a cell potential of 40 V for 5 h.Then, the porous alumina layer was removed by a wet chemical etchingprocess with a solution containing a mixture of 0.4M phosphoric acid(�85wt%, Sigma-Aldrich) and 0.2M chromic acid (�98.0%,Sigma-Aldrich) at 60 �C for 2 h. The second step, anodization, was

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conducted at a constant cell potential of 40 V for 1 h. The nanoporousalumina template was then etched to widen the diameter of the pores bydipping it in 6wt% phosphoric acid for 80min. Then, carbon layers weredeposited on the nanoporous alumina template using a CVD technique at640 �C with a flowmixture of C2H2 and N2 (6% C2H2 and 90%N2) for 2 hand then the furnace was cooled to room temperature under a N2 flow.The carbon coated alumina templates were attached on the stainless steel(SS) electrode using conductive epoxy (CW2400, Chemtronics). The re-sidual aluminum and alumina templates were removed using saturatedHgCl2 (�99.5%, Sigma-Aldrich) and a 0.05M NaOH (97%, SigmaAldrich) solution, respectively, to get the VNPCT-modified electrode,which was directly used for electrochemical characterization.

2.2. Materials and electrochemical characterization

The morphology of the electrodes was analyzed using field emissionscanning electron microscopy (FE-SEM) (Hitachi S-4800). X-ray photo-electron spectroscopy (XPS) analysis was conducted using a ThermoScientific MultiLab 2000 UK spectrometer with Al-Kα as the X-ray source(hν¼ 1486.71 eV). CR2032-type coin cells were assembled in an argonfilled glovebox with the VNPCT structure and pristine stainless steel(SS,0.02mm thickness, FoShan Gangsun Metal Products) as the workingelectrode and metallic lithium foil (99.9%, Sigma-Aldrich) as counterelectrode with a Celgard separator and filled with 1M lithium bis(tri-fluromethanesulfonyl)imide (LITFSI, 99.5%, Sigma Aldrich) in a 1,3-dioxolane (DOL, 99.8%, Sigma Aldrich) and 1,2-dimethoxyethane(DME, 99.5%, Sigma Aldrich) (1:1 in volume) electrolyte with a 0.2Mlithium nitrated (LiNO3, 99.99%, Sigma Aldrich) additive. Electro-chemical impedance spectroscopy was recorded by a Bio-Logic electro-chemical workstation (VSP). The galvanostatic measurements werecarried out using a battery cycler (Won-A-Tech WBS3000). Cycling testswere carried out by first plating lithium onto the working electrode atdesired current rates and capacities, followed by Li stripping up to500mV. For full cell testing, the VNPCT@Li and SS@Li anode wereprepared by electrodepositing Li in advance on the VNPCT modifiedelectrode with the areal capacity of 3mAh cm�2. An excess amount of Liwas used in order to compensate the Li capacity loss during initial cyclesdue to SEI layer formation. The cathode was prepared by mixing com-mercial LFP powder (MTI, Korea), super P and PVDF in the weight ratioof 75/15/10 and dissolved in NMP to form the slurry. Then, slurry wascasted on Al foil current collector and dried under vacuum. The activematerial loading of the LFP cathode was 5mg cm�2. The electrolyte forfull cell tests was 1.0M lithium hexafluorophosphate (LiPF6, 99.99%,Sigma Aldrich) in ethylene carbonate (EC) and dimethylcarbonate(DMC) (V/V¼ 1:1). The galvanostatic charge-discharge tests were per-formed in the voltage range of 4.2–2.5 V vs. VNPCT@Li/SS@Li anode.

3. Results and discussion

A two-step anodization process was done to prepare a nanoporousAAO template, and an additional pore widening step was employed toincrease the diameter of the nanoporous structure so that an easy lithiumdeposition inside the pores can be made. Then, a carbon layer was coatedon the nanoporous AAO template by chemical vapor deposition (CVD)technique after which the template was removed using a wet chemicaletching process and scheme for experimental procedure is presented inFig. 1a. The FE-SEM images in Fig. 1b show the fabricated carbon coatedAAO templates composed of a vertically aligned nanoporous tubularstructure. The cross-sectional FE-SEM image in Fig. 1c clearly confirmsthe vertically aligned tubular structure and its porous nature. FE-SEMimages in Fig. 1d and e shows the top and side views of the finalVNPCT-modified electrode. The top view FE-SEM image shown in Fig. 1dindicates that the VNPCT structure has uniform channels with an averagelength of about 10 μm having an average pore diameter of about 100 nm;and the vertically aligned tubular structure of the carbon is well main-tained even after the removal of AAO template (Fig. 1e). Further

Fig. 1. (a) Fabrication process of vertically aligned carbon tube arrays on SS electrode.(b) SEM image of the carbon coated AAO templates. (c) Cross sectional SEMimage of the vertically aligned carbon tube arrays after AAO template removal. (d–e) Top and side view of the vertically aligned carbon tube arrays. (f) TEM image of abundle of VNPCTs. (g) A HRTEM image of single carbon tube.

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confirmation of the porous and tubular nature of VNPCT is made by TEMand HRTEM analysis as shown in Fig. 1f and g. To examine the polarfunctional groups on the vertically aligned carbon tubular structure,Fourier transform infrared spectroscopy (FTIR) was conducted. The FTIRspectra of the VNPCT structure in Fig. S1 shows the absorption bands ataround ~3535, 2954, 1347, and 1058 cm�1, which correspond to thestretching vibration of O–H, C–H, C–O, and C–OH, respectively [18,30].The etching solution (NaOH) used for the removal of residual Al2O3 layercould be the reason for the oxidative polar functional groups on theVNPCT structure. This oxidative functional groups on the VNPCT struc-ture can offer uniform Liþ flux and stable SEI layer formation duringlithium deposition/stripping. Because these oxidative functional groupscan easily attracts Liþ ions available in the electrolyte due to electrostaticforce which is essential for uniform nucleation and also they can promotethe decomposition LiTFSI which is important to form major components

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of stable SEI layer [31]. The contact angle measurement was conductedby dropping the electrolyte (1M LiTFSI in DOL/DME) onto a pristine SSand VNPCT-modified electrode, to demonstrate the affinity between theLi ions and deposition host (Fig. S2). The VNPCT-modified electrodeshows a better electrolyte wettability than pristine SS electrode, indi-cating the enhanced electrolyte affinity and reduced interface resistance.The improved wettability allows the facile electrolyte penetration intodeeper pores of VNPCT and affords the large uptake of electrolyte in theVNPCT electrode, which is beneficial to homogenize Li ions distributionover the Li deposition host.

A constant amount of Li was galvanostatically deposited onto thepristine SS electrode and VNPCT-modified electrode to systematicallyexplore the benefits of the VNPCT structure in suppressing lithiumdendrite formation. Fig. 2a and b show SEM images of deposited Li on thepristine SS electrode with various areal deposition capacities at a current

Fig. 2. Morphology study of the Li metal deposition: (a–b) SEM image of lithium metal deposited on a bare SS electrode at current density and deposition capacities of1 mA cm�2 & 1mAh cm�2, and 1mA cm�2 & 2mAh cm�2, respectively. (c–d) Top view of lithium metal deposited on the VNPCT-modified electrode at 1 mA cm�2 &1mAh cm�2, and 1mA cm�2 & 2mAh cm�2, respectively.

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density of 1 mA cm�2. For 1mAh cm�2 Li plating, inhomogeneous wire-like lithium deposits can be seen on the pristine SS electrode (Fig. 2a).The average diameters of the lithium dendrites are around 2 μm. Byincreasing the plating capacity to 2mAh cm�2, the surface roughnessincreased and the deposited Li grew into long Li dendrites (Fig. 2b). It iswell known that an uneven lithium deposition creates more side re-actions between Li metal and the electrolyte and decreases theCoulombic efficiency [26,32]. On the other hand, deposition of lithiumonto the VNPCT-modified electrode shows a smooth, roundeddendrite-free morphology. The FE-SEM image in Fig. 2c shows that thesurface of the VNPCT structure was covered with smoother and roundeduniform lithiummetal deposition when the Li was plated at 1mAh cm�2.Even with an increased plating capacity of 2 mAh cm�2, there was uni-form lithium metal deposition with a flat rounded morphology ratherthan dendritic growth (Fig. 2d). This clearly confirms the advantage ofthe vertically aligned nanoporous architecture of the modified electrodethus enables a high areal lithium deposition with dendrite-freemorphology. However, it is challenging to confirm the lithium deposi-tion inside the porous structure because of its low atomic number. But webelieve that the electrodeposited metallic lithium inside the porous car-bon host could be in a semi-fluid state [25]. The modern characterizationtechniques are further required to study the nature of lithium inside thepores and will be pursued in future.

To investigate the efficiency of the nano-engineered VNPCT-modifiedelectrode, lithium metal was galvanostatically deposited/stripped intothe VNPCT structure at the different current densities with the same arealplating capacity of 0.5mAh cm�2 and 1mAh cm�2. A pristine SS elec-trode used as the control electrode and tested under the same conditionsto understand the benefits of our proposed system in stabilizing theelectrochemical performance of metallic lithium anodes. To depositlithium, a VNPCT electrode or a pristine SS electrode used as a workingelectrode and lithium foil served as a counter electrode. The Coulombicefficiency of the cycling process was defined as the ratio of the amount ofLi stripping from the working electrode and the amount of Li plating onthe working electrode at each cycle, which is a generally adopted

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parameter to evaluate the efficiency of metallic lithium anodes. Fig. 3acompares the Coulombic efficiency and cyclic stability of both theVNPCT-modified electrode and the pristine SS electrode at current den-sities of 0.5, 1, 2, 3, and 5mA cm�2 obtained with an areal plating ca-pacity of 0.5mAh cm�2. The low Coulombic efficiencies of the pristine SSand VNPCT modified electrodes in the first few cycles are attributed tothe SEI layer formation [22,32]. The VNCPT electrode delivers lowerinitial Coulombic efficiency than SS electrode (Fig. S3) and this is due to3D structure of VNPCT electrode with larger electrochemical surfacearea. A higher amount of Li-metal can react with ether-based electrolyte,resulting in the large irreversible capacity during first cycle and this is acommon phenomenon observed for carbon based materials. However, itsCoulombic efficiency recovered quickly to higher values in consecutivecycles. The formation of a stable SEI layer during initial cycling is crucialfor attaining a long-term performance; such a phenomenon was notobserved in the planar type pristine SS, thus resulted in a short cycle life.At current densities of 0.5, 1, 2, 3 and 5mA cm�2, the VNPCT-modifiedelectrodes delivered a stable Coulombic efficiency of 97% after 200 cy-cles, 95% after 150 cycles, 94% after 120 cycles, 93% after 80 cycles and91% for 60 cycles, respectively. In contrast, the Coulombic efficiency ofthe cells with pristine SS electrodes displayed an inferior electrochemicalcycling performance with poor Coulombic efficiencies and a short cyclelife. Additionally, the pristine SS electrode did not maintain a stableCoulombic efficiency for long cycles and there were severe fluctuations.In particular, at higher current densities, a significant fluctuation anddecay on the Coulombic efficiency dominated the cycling performancewithin a very few cycles. It is well-known that a high current densityinduces unfavorable dendrite growth in metal anodes [33,34]. The poorelectrochemical performance of the pristine electrode system during Liplating/stripping could have been due to the formation of an unstable SEIlayer, which led to consumption of both the electrolyte and the Li metal.However, the modified VNPCT electrode demonstrated a very stable andextended cycle life, even at high current rates.

In addition to the Coulombic efficiency, voltage polarization (hys-teresis) is also an important parameter in evaluating stability of the

Fig. 3. Electrochemical characterization: (a) Coulombic efficiency of Li plating/stripping at various current densities with the same areal capacities of 0.5, 1, 2, 3, and5mA cm�2 with a deposition capacity of 0.5 mAh cm�2. Voltage profile of Li plating/stripping at different cycles (1, 2, 10, 50, 125, and 150th) at 1mA cm�2. (b)VNPCT-modified electrode. (c) Pristine SS electrode. The areal capacity was 0.5 mAh cm�2 in all cases.

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lithium metal anodes. The voltage polarization (hysteresis) is defined asthe sum of the overpotential for Li plating and Li stripping [35]. The Liplating/stripping profiles of the VNPCT electrode and pristine SS elec-trode at 1mA cm�2 and a plating capacity of 0.5mAh cm�2 with differentcycles (1st, 2nd, 10th, 50th, 125th, and 150th) are shown in Fig. 3b and c.The difference in the voltage hysteresis with additional cycles are clearlyobserved, indicating the advantage of the modified VNPCT over theplanar pristine SS electrode. The voltage hysteresis of the pristine elec-trode varied significantly with cycling, and the percentage of Li strippingdecreased with the consequent cycles. The irreversible Li deposits on theSS electrode resulted in the dead Li and may form a thick insulating SEIlayer on the SS electrode, thus lead to poor cycle life [36]. However, theVNPCT-modified electrode exhibited a highly stable lithium platting/-stripping efficiency with a low voltage hysteresis of 75mV, even after150 cycles (Fig. 3c). The stable lithium plating/stripping in the VNPCTstructure with a low voltage hysteresis was due to the formation of astable SEI layer that induced the uniform Li deposition in the VNPCTstructure. Furthermore, the voltage polarization of 30th Li plating/strip-ping cycle at different current densities with same areal capacity of0.5 mAh cm�2 for VNPCT and pristine SS electrode compared in Fig. S4.The cells with the VNPCT-modified electrode exhibited voltage hyster-esis of 52, 71, 128, 209, and 264mV at current densities of 0.5, 1, 2, 3,and 5mA cm�2, respectively. Although the voltage hysteresis of theelectrode increased with an increasing current, the VNPCT-modifiedelectrode cycled stably at high rates of 3 and 5mA cm�2 and main-tained Coulombic efficiencies higher than 90%. On the other hand, thecells with pristine SS electrode delivered a larger polarization of 55, 80,135, 220, and 280mV under the same test conditions. It can be evidencedthat the modified VNPCT electrodes showed lower polarization thanpristine SS electrode.

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The stable performance of the VNPCT and pristine electrodes werefurther evaluated with a high areal lithium deposition of 1mAh cm�2.The VNPCT-modified electrodes remarkably delivered a superior stabil-ity with a high Coulombic efficiency of ~95% for 50 cycles, 94% for 50cycles, 93% for 47 cycles, and 90% for 39 cycles at current densities of 1,2, 3, and 5mA cm�2, respectively. The stability and Coulombic efficiencyachieved by the VNPCT electrodes with higher lithium deposition werefar superior to pristine SS. The poor performance of the pristine SSelectrodes was due to the formation of dendrites on their planar surfacedue to irregular lithium deposition at high lithium plating capacity(Fig. S5). Further, the VNPCT-modified electrode exhibits decent cyclingperformance at a current density of 1 mA cm�2 and plating capacity of2 mAh cm�2 (Fig. S6). This again confirms the significant role of avertically aligned structure as stable host for a metallic lithium anodes.The superior high rate performance is associated the vertical nanoporousstructure that provides uniform Liþ flux and enables uniform depositionof metallic lithium. In addition, vertical channels of the VNPCT can offera capillary pressure to infuse the metallic lithium during electrochemicalcycling, which may increase the density of the deposited lithium metal.The VNPCT electrode with vertical channels provide a large electroactivesurface area and long range of conduction path for e� and Liþ and reducethe longitude resistance, thus improves the kinetics and stability of theelectrode [37].

To elucidate the origin of the superior performance, ex-situ SEManalysis were performed on VNPCT and pristine SS electrode aftercycling. The morphology of the deposited lithium metal was analyzedafter 200 continuous Li plating/stripping cycles at 0.5 mA cm�2 with aplating capacity of 0.5mAh cm�2. The surface of the cycled pristine SSelectrode had a rough texture with sharp lithium dendrite formation overit, which mainly originated from the repeated inhomogeneous and

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hostless lithium plating/stripping cycles (Fig. 4a and b). Meanwhile, theVNPCT-modified electrode remained a relatively flat lithium metal-deposited surface without any sharp lithium dendritic growth (Fig. 4cand d). Moreover, we analyzed the cross-sectional SEM (Fig. 4e and f) togain more insight into the lithium metal deposition on the VNPCTstructure. The image clearly reveals that the lithium metal uniformlyconfined within the vertically aligned tube space available in the VNPCT-modified electrode. The superior performance of the modified VNPCTelectrode is especially attributed to the highly porous structure alongwith the vertically aligned nanochannels, which promotes capillarypressure for lithium metal deposition. Benefiting from the porousmorphology, the lithium metal was easily confined inside the nano-tubular structure without any volume change, even after repeatedplating/stripping. Furthermore, the vertical tubular architecture alongwith polar functional groups significantly reduced the local currentdensity thus provided the uniform Liþ flux and produced the stable SEIlayer. On the other hand, homogenous Liþ flux and stable SEI layer werenot obtained in the pristine SS electrode due to their planar morphologywhere local current density was relatively high, which favored mossy-like dendrite formation.

We additionally verified the surface composition SEI layer on theVNPCT modified electrode after cycling by X-ray photoelectron spec-troscopy (XPS) Fig. S7 shows the XPS survey spectrum of the VNPCTelectrode before and after cycling (50 cycles). The XPS survey shows theorigin of new peaks due to Li, S, N, and F after cycling peaks, confirmingthe formation of the SEI layer during cycling. As shown in Fig. 5a, the

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deconvoluted C 1s spectrum has three peaks centered around ~284.8,286.3, and 290 eV, which corresponds to the C–C, C–O, and C¼O,functional groups, respectively [16,21]. The two peaks centered at~531.9 and 533.4 eV in the deconvoluted O 1s (Fig. 5b) spectrum areascribed to adsorbed oxygen species and carbonate species, respectively.The C 1s, and O 1s spectra confirm that the SEI layer constitutes a largeamount of Li2CO3, and lithium alkyl carbonates (ROCO2Li). The threepeaks centered at 54.9, 55.7 and 56.4 eV in the Li 1s spectrum (Fig. 5c)could be assigned to the RCO2Li, lithium carbonates and lithium fluoride(LiF), respectively, confirming the existence of lithium carbonate(Li2CO3) in the VNPCT structure. The high-resolution F 1s spectrum(Fig. 5d) has two peaks located at ~688.9 and 685 eV ascribed to –CF3and F�, respectively. This XPS study clearly indicates that the SEI layerover the VNPCT structure also contains LiF, which is a typical product ofLiTFSI decomposition on the surface of Li metal [30]. Additionally, thethree peaks located at ~399.7, 403.6, and 407.7 eV in N1s spectrum(Fig. 5e) correspond to N–S, NO2�, and NO3� functional groups. Li2NxOyis an important component in stabilizing the SEI layer, which resultingfrom the reduction of LiNO3 [34]. For the S 2p spectrum (Fig. 5f), peakslocated around 168 and 161 eV are assigned to Li2SO4

2�/Li2SO32�and

Li2S/Li2S, respectively [16]. Li2S/Li2S2 component forms by thedecomposition of LiTFSI through N–S bond breakage which is promotedonly by oxygen functionalities over carbon. The sulfurized (Li2S/Li2S2)component is essential in stabilizing SEI layer over continuous Li plati-ng/stripping process [31]. Therefore, ROCO2Li, LiF, Li3N, Li2NxOy, andLi2S are considered as the major components of the SEI layer on the

Fig. 4. SEM images of morphologies of the pristineSS electrode and the VNPCT-modified electrode.(a–b) The morphology of Li metal deposited on thepristine SS electrode at a current density of1mA cm�2 and a deposition capacity of 0.5 mAhcm�2, after 200 cycles. (c–d) top view of Li deposi-tion on the VNPCT-modified electrode. (e–f) Cross-sectional SEM image of the VNPCT-modified elec-trode after 200 cycling at a current density of1mA cm�2 with a discharge capacity of 0.5 mAhcm�2, indicating Li deposition on the tubularstructure.

Fig. 5. XPS spectra of (a) C 1s, (b) O 1s, (c) Li 1s, (d) F 1s, (e) N 1s and (f) S 1s for VNPCT-modified electrode after 50 cycles under 1mA cm�2 with areal capacity of0.5 mAh cm�2 in ether-based electrolyte (1 M LiTFSI þ0.2 M LiNO3).

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VNPCT modified electrode, thus provide a stable and strong protectionduring continuous Li plating/stripping cycles [38].Such a stable andstrong SEI layer over the aligned carbon tubes are helpful to maintain theinterface stability, even under harsh high current lithium deposi-tion/stripping [18,25]. The strong, stable SEI layer over the VNPCTstructure inhibits the growth of lithium dendrites over the tubular ar-chitecture. Additionally, the SEI layer prevents further electrolytedecomposition and thereby helps to maintain a stable Coulombic effi-ciency for longer cycles. On the other hand, XPS spectra of the pristine SSelectrode tested under similar condition with VNPCTmodified electrode,is shown in the supporting information (Fig. S8). The composition of SEIlayer over the pristine electrode contains typical by-products due todecomposition of ether-based electrolyte expect Li2S/Li2S2. N–S bondbreaking and formation Li2S/Li2S2 component do not occur on SEI layerover pristine electrode. This unstable SEI layer formation on the pristineelectrode leads to dendritic Li growth and poor cycle life [39].

To demonstrate the efficiency of pre-lithiated VNPCT (VNPCT@Li)and SS (SS@Li) anodes, full cells were assembled utilizing the LiFePO4(LFP) as the cathode. Fig. S9 (a & c) shows the voltage profiles ofVNPCT@Li/SS@Li-LFP at different current rates from 0.2 to 2 C, exhib-iting typical flat discharge plateau of LFP. In comparison to SS@Li-LFPcells, the VNPCT@Li-LFP full cell delivered a stable reversible capacityof 162, 159, 147, and 131mAh g�1 with smaller voltage polarization at0.2, 0.5, 1, and 2 C, respectively. The cycling performance of full cellswere tested at 1 C (Fig. S9 (b and d)). The full cell delivered a stablereversible capacity of 115 and 55mAh g�1 with the capacity retention of83% and 55% after 100 cycles, respectively. The stable electrochemicalperformance of VNPCT@Li full cell demonstrating their benefits over

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planar 2D SS electrodes.

4. Conclusion

In summary, we designed and fabricated a vertically aligned nano-porous carbon tube-modified electrode via a route with anodizedaluminum oxide template to overcome the problems associated withlithium metal anodes. The highly aligned vertical channels with polarfunctional groups enabled the stable SEI layer formation. Moreover, therationally designed nanoporous carbon tubular structure effectivelyreduced the local current density and offered the capillary pressure toencapsulate the metallic lithium within the VNPCT structure. We foundthat a high areal lithium deposition of 2mAh cm�2 lithium could beplated efficiently on the designed structure without any dendriticmorphology due to the formation of a stable SEI layer on top of thedesigned structure. Compared with a pristine SS electrode, the designedlithium metal host exhibited a stable cycle life and higher Coulombicefficiency of 96% over 200 cycles at a current density of 0.5mA cm�2.Further, VNPCT@Li and LFP full cell demonstrates the stable reversiblecapacity of 131mAh g�1 at 2 C. Our results demonstrate that the use ofcarbon structures with vertical porous channels for stabilizing metalliclithium anodes. We hope that stable cycling performance even at highercurrent densities with higher areal capacities can be achieved by tuningthe pore size of the tubular structures.

Conflicts of interest

The authors declare no competing financial interest.

B. Moorthy et al. Energy Storage Materials 24 (2020) 602–609

Data availability statement

The raw/processed data required to reproduce these findings cannotbe shared at this time as the data also forms Part of an ongoing study.

Acknowledgements

This project was supported by the National Research Foundation ofKorea (MSIP) (No. 2017R1A2B2010148) and the Climate ChangeResearch Hub of EEWS from KAIST (Grant No. N11170059). H.-W.L.acknowledges support from International Cooperation in S&T Program(NRF-2016K2A9A1A06934767) and Basic Research Lab Program (NRF-2017R1A4A1015533) through the National Research Foundation ofKorea funded by the Ministry of Science and ICT.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.ensm.2019.06.016.

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