Highly Reversible Li Plating Confined in Three-Dimensional

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Highly Reversible Li Plating Confined in 3D Interconnected MicroChannels towards High-Rate and Stable Metallic Lithium Anodes Wei Deng, Wenhua Zhu, Xufeng Zhou, Xiaoqiang Peng, and Zhaoping Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02619 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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ACS Applied Materials & Interfaces

Highly Reversible Li Plating Confined in 3D Interconnected Micro-Channels towards High-Rate and Stable Metallic Lithium Anodes Wei Deng,†,‡ Wenhua Zhu†, Xufeng Zhou†,*, Xiaoqiang Peng† and Zhaoping Liu†,*

†Key Laboratory of Graphene Technologies and Applications of Zhejiang Province and Advanced Li-ion Battery Engineering Laboratory, Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Zhejiang 315201, P. R. China.

‡College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences (UCAS), Beijing 100049, P. R. China *E-mail address: [email protected], [email protected].

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ABSTRACT: Practical application of metallic Li anode in Li ion batteries has been restricted because of dendrite growth of Li which induces poor stability and safety issues. Despite various hosts for Li have been developed to address these issues, it is still a challenge to achieve highly reversible and stable striping/plating behavior of Li, especially at high rate conditions. Herein, we propose a simple method of incorporating Li in commercial carbon fiber cloth (CFC) to realize high-rate and stable metallic Li anodes by confining stripping/plating of Li in micro-channels of ZnO decorated CFC (CFC/ZnO) and dissipating high current densities through conductive carbon fiber networks. The symmetrical cell using this novel anode can run stably for over 1800 h (900 cycles) under 1 mA cm-2 and even 320 h (800 cycles) at 5 mA cm-2, which has rarely been achieved previously through structural evolution of Li metal anode. When it is paired with commercial activated carbon, the as-made Li ion capacitor coin cell can deliver high rate capability (up to 30 A g-1) and long-term cycling stability for over 5000 cycles at 10 A g-1, and large pouch cell can operate as ultrafast charge (~1 min) battery with high energy density of ~50 Wh kg-1.

KEY WORDS: lithium metal anode, carbon fiber cloth, interconnected channels, high rate, long-term stability

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Introduction Tremendous requirements of high-energy density rechargeable Li-ion batteries draw worldwide research interests in advanced anode materials.1-4 For many perspectives, the most attractive anode for a rechargeable battery is metallic lithium (Li) owing to its light-weight, highest theoretical specific capacity and lowest negative electrochemical potential.5-8 However, its high chemical reactivity and characteristic dendrite growth phenomenon have paused practical application.9-10 Especially, high local current density will significantly accelerate the formation and growth velocity of Li dendrites, which causes steep drop of cycle life and severe safety hazards, thus restricts utilization of Li metal anode at high rate conditions.11-12 Consequently, worldwide researchers are focusing on how to solve safety issues and prolong life span of Li metal anode at high current densities for realizing ultrafast charging/discharging and high power density without sacrificing energy density. Previous researches mainly concentrated on the design of tough SEI films by using functional electrolytes or forming artificial layers on the surface of Li metal to slow down the growth of Li dendrites.6, 13-16 Despite inspiring breakthroughs have been made for the past decades, nearly all efforts have been applied on bulk Li metal anodes with 2D planar morphology, which still inevitably induced battery failure, because only surface protection is not effective in accommodating volume change of bulk Li and dissipating high current densities, and poor mass transfer of bulk Li is also not solved. In fact, according to Chazalviel's model, high current density applied on electrodes results in a near zero ion concentration at negative electrode, which 3



easily triggers the formation of Li dendrites.17 Given the factor of ion concentration gradient, creating more contact interfaces between Li and electrolytes is extremely important for alleviate ion concentration issues by producing more uniform ion flux. Thus, recently a new strategy has been developed to realize this, which employs porous hosts to accommodate volume expansion of Li metal. Hosts such as graphene-based materials18-19, porous Cu20, nickel foam21-22 and nano fiber matrix23-24 also can dissipate high current densities to inhibit nucleation of Li dendrites, and facilitate diffusion of electrolytes. However, hosts simply possessing porous morphology and conductivity is incompetent to comply with requirements at high rate conditions. More elaborate structural design of the hosts is needed. For instance, metal (Cu, Ni, Ni-Fe, etc.) foams or meshes recently are used to host Li metal as anode materials,20-21, 25 but metallic Li inside has relatively large size of hundreds of micrometers due to the large pore sizes of such metal hosts. Thus, the restriction effect of the host for guiding the plating behavior of Li metal is limited, and the irreversible deposition of Li metal inside such metal hosts inevitably generates formation of “dead Li” on the surface of the electrode. Thus, it is better that the hosts can not only distribute Li in small scales but also restrict Li plating in local area. On the other side, abundant pathways for electrolyte infiltration are also required to create more electrolyte/Li interfaces, which can prolongs Sand’s time, and is beneficial for stable and homogeneous Li plating.17 Despite that some of reported scaffolds could achieve fine distribution of Li metal, they usually lack highly interconnected porous microstructure for continuous electrolyte infiltration. In most cases, the infiltration of 4


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electrolytes inside the electrode is suppressed to a large extent during charge/discharge processes, as the Li metal fully blocks the original pore structure of the hosts. Thus, it is important to choose appropriate hosts to realize fine distribution of Li for spatial restriction of Li stripping/plating and maximum exposure of electrolyte/Li interfaces. By comparing various possible host materials for Li metal, we found that commercial carbon fiber cloth (CFC), which has been widely used in energy conversion or storage devices, is one of the most potential candidates due to its unique structure.23, 26-27 The characteristic 3D interconnected channels with spatial confinement of ~5 µm resulted from two-step weave manner in CFC, plus its high conductivity, making CFC suitable for hosting Li metal according to aforementioned requirements for appropriate hosts. More specifically, dozens of carbon fibers twinning together will help to minimize size of Li and restrict the plating area of Li deeply inside CFC, and can also provide a steady stream of electrons for highly reversible and high rate Li striping/plating behavior. Meanwhile, the abundant channels among carbon fibers can provide 3D interconnected pathways for electrolyte infiltration. In addition, the macro-voids among vertically weaved carbon fiber bundles are able to facilitate electrolyte infiltration into inner network to create more electrolyte/Li interfaces. Based on the advantages of CFC, we prepared high-performance CFC/ZnO-hosted Li metal anode (CFC/Li) by simply infusing molten Li (@300°C, inert atmosphere) into CFC/ZnO without any further structure modification. The combination of CFC/ZnO and Li metal effectively lowers voltage hysteresis of Li symmetrical cells and 5



enhances the rate (0.5-30.0 A g-1) capability of lithium-ion capacitors (LICs) cells (paired with commercial activated carbon, AC). Since Li plating behavior was guided and restricted by 3D interconnected channels, the Li/SEI interphase was firmly confined without extrusion from the CFC/ZnO host, and ample supply of electrons by carbon fibers together gave rise to highly reversible striping/plating of Li, rather than growth of “dead Li”. As a result, Li symmetrical cells could cycle for over 1800 h at 1.0 mA cm-2, and were extremely stable at a high current density of 5.0 mA cm-2 for 320 h (800 cycles) in unmodified ether-based electrolyte. Moreover, LIC pouch cells assembled by large-area CFC/Li anodes and AC cathodes could deliver energy densities of 65 Wh kg-1 and 50 Wh kg-1 at the ultrafast charge/discharge time of 3 min and 1min, respectively.

Results and Discussions CFC (Figure S1a, SI) was modified with ZnO by dipping CFC in zinc acetate solution and annealing at inert atmosphere to improve its lithiophilicity.28-29 Figure 1a illustrates the typical inner structure of CFC/ZnO, the top-view shows the macroporous structure of CFC/ZnO and the magnified image demonstrate the nano ZnO particles decorated on CFC skeleton. It is consisted of vertically weaved carbon fiber bundles, and the corresponding SEM image is shown in Figure 1c. Each bundle is composed by numerous carbon fibers twinning together with abundant channels as shown in Figure 1d. Surface modification of carbon fiber with ZnO is confirmed in Figure 1e. The XRD pattern of CFC/ZnO is shown in Figure S2 (SI). The main 6


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diffraction peaks confirm the transformation of zinc acetate into ZnO after annealing. Meanwhile, the high magnification SEM images (Figure S3, SI) demonstrate that the ZnO nanoparticles with uniform sizes of 100-200 nm are distributed evenly on each carbon fiber to enhance the lithiophilicity of CFC. The digital photographs in Figure S4 (SI) show that pure CFC without ZnO decoration has no ability for the infusion of molten Li metal. In contrast, the CFC/ZnO scaffold can enable fast incorporation of Li metal since ZnO NPs can react with molten Li to form LiZn and Li2O, which enhances the lithiophilicity of CFC. After Li (~12.0 mg for a CFC disc with diameter of 11 mm) was incorporated with CFC/ZnO (Figure S1b, SI), channels in each carbon fiber bundle were filled with liquid metallic Li which then solidified to be Li metal immobilized by carbon fibers, as is illustrated in Figure 1b and f. It should be noted that CFC/ZnO has not been fully filled with Li. Macro-voids between weaved bundles is still persevered after incorporation of Li (Figure 1g), which facilitate infiltration of electrolytes into CFC/ZnO and lay the foundation for generating more electrolyte infiltration pathways during initial stripping of Li. High magnification SEM image (Figure 1h) confirms that Li metal covers on single carbon fibers and embeds in micro-channels between carbon fibers. Comparing with conventional porous hosts for Li metal, such as metal foams, micro-channels with much smaller width in CFC/ZnO not only confine the size and uniformity of Li metal, but also serve as restriction area to guide plating behavior of Li. To demonstrate the functions of CFC/ZnO scaffold, nonporous CFC/Li electrode (~21.0 mg of Li in a CFC disc with diameter of 11 mm) in which all the pores are fully filled with Li by elongating infusion time has been 7



fabricated as the control sample (named n-CFC/Li). Macro-voids between bundles have been thoroughly filled by Li metal in n-CFC/ Li, and the cross-sectional SEM image shows that extra Li metal layer has covered the entire surface of CFC/ZnO (Figure S5, SI). Stripping/plating behaviors of CFC/Li were characterized to demonstrate the functions of highly interconnected channels in CFC/ZnO. As illustrated in Figure 2a, the macro-voids in CFC/Li enable initial infusion of electrolyte to reach the surface of carbon fiber bundles embedded with Li at the beginning of the stripping process. Then the continuous exposure of fresh micro-voids and surface in the bundles, which are generated by continuous stripping of Li embedded in the micro-channels, facilitates deeper infusion of electrolyte into the interior of carbon fiber bundles as illustrated in Figure 2b. This self-promoting process continuously pushes forward the reaction interface between Li and electrolyte deep inside, resulting in smooth and controllable stripping of Li. To confirm this, CFC/Li electrode was discharged at 2.0 mA cm-2 for 6 h at the first cycle to dissolve Li metal. SEM image in Figure 2c is an indication that top Li around CFC/ZnO was dissolved preferentially and surface of carbon fiber bundles was exposed with interconnected channels after discharge. The inset digital photograph indicates that the color of CFC/Li also changed to black after Li stripping, suggesting the removal of Li and exposure of original CFC/ZnO substrate. Micro-scale channels after striping are clearly observed in Figure 2d, whose morphology is similar with original CFC/ZnO. More importantly, the discharge process did not remove all Li metal, but still left a thin layer of Li covering on 8


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individual carbon fibers (Figure 2e), which is helpful to reduce interfacial energy for plating of Li to eliminate nucleation overpotential.30 During subsequent Li plating process, lithium ions nucleated on the surface of unstripped Li metal layer. The same crystal structure and the reduction of current density by CFC synergistically lowered the nucleation overpotential and promised the uniform nucleation of Li. The ion concentration gradient was effectively alleviated by uniform distributed and interconnected micro-channels in CFC/ZnO,31-32 resulting in highly reversible stripping/plating behavior, which can be confirmed by SEM observation of the structure of CFC/Li at fully charge state after 20 cycles at 2.0 mA cm-2. The cross-sectional SEM image (Figure 2f) of the electrode material demonstrates that the CFC/ZnO scaffold was re-filled with Li metal, but there was no extra Li layer on the outer surface of CFC/Li electrode, though it showed metallic luster (inset in Figure 2f). SEM images of higher magnification (Figure 2g and h) more clearly displays that the micro-channels between carbon fibers were fully filled with Li, and no dendrite-like lithium metal was visualized. The similar morphology of the CFC/Li electrode after 20 cycles comparing with that of the fresh one suggests the prominent effect of CFC/ZnO in guiding highly reversible stripping/plating behavior of Li, due to infinity between Li wrapped carbon fibers after initial stripping process and Li metal, and spatial confinement provided by micro-channels between carbon fibers. Symmetrical cells were assembled using two identical CFC/Li electrodes and ether-based electrolyte for evaluation of long-term cycling stability at high current 9



densities of CFC/ZnO hosted Li. Various current densities were employed to compare the interfacial resistance and nucleation overpotential of bulk Li and CFC/Li cells (Figure 3a). The dash lines represent the charge curves of bulk Li foil from 0.5 mA cm-2 to 10.0 mA cm-2. The nucleation overpotential occurs at a relatively low current density of 2.0 mA cm-2 with voltage hysteresis of 60 mV. The voltage profiles show apparent fluctuation and inclining trends when the current density is above 5.0 mA cm-2, which is also accompanied by more obvious nucleation overpotential. The voltage hysteresis (sum of charge and discharge potential) reaches 150 mV with nucleation overpotential of 15 mV at a current density of 10.0 mA cm-2. The voltage hysteresis of CFC/Li (solid lines at discharge area) is much smaller comparing with that of Li foil at all current densities. Specifically, its voltage hysteresis at low current densities below 2.0 mA cm-2 (11 mV at 0.5 mA cm-2, 18 mV at 1.0 mA cm-2 and 29 mV at 2.0 mA cm-2) are less than half of those for Li foil at the same current density. Even at a high current density of 15.0 mA cm-2, its voltage hysteresis of 120 mV is still 20% lower than that of the Li foil at 10.0 mA cm-2. Moreover, the nucleation overpotential is almost invisible for CFC/Li cells. Cells were also cycled at various current densities (1, 2, 5 and 10 mA cm-2) at a fixed areal capacity of 1.0 mAh cm-2 for monitoring their stability, and the corresponding voltage hysteresis of galvanostatic charge/discharge were measured and presented in Figure 3b, c, e and g. At a current density of 1.0 mA cm-2, voltage hysteresis of CFC/Li cell (Figure 3b) stabilizes at 20 mV (-10 mV to 10 mV) for over 1800 h (900 cycles). This ultralong life-time has rarely been achieved by structural modification of Li metal anode. 10


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Similar cycle life has only been realized by modification of gel electrolyte with boron nitride nanoflakes.33 In contrast, cells of bulk Li has a much higher voltage hysteresis of 50 mV, and can only stabilize for 300 h even with the addition of LiNO3 in the electrolyte to enhance the formation of stable SEI.34-36 The rapid increase of voltage hysteresis after 150 cycles indicates the aggregation of “dead Li” on the surface of bulk Li, which causes high resistances and failure of cells.37 The control sample n-CFC/Li displays slightly better stability for over 420 h than that of Li foil, but its voltage hysteresis still increases during sequent cycles (Figure S6, SI). The voltage profiles of CFC/Li and bulk Li foil at 1.0 mA cm-2 are compared in Figure S7 (SI). The nucleation of Li metal on CFC/Li electrode shows zero overpotential, and no evident increasing of voltage hysteresis is detected even after 500 cycles (1000 h), however, bulk Li foil still displays obvious nucleation overpotential and dramatic increase of voltage along with cycling, and its voltage-capacity curve also slightly fluctuates at such a low current density. When an elevated areal current density of 2.0 mA cm-2 was applied on symmetrical cells (Figure 3c), the voltage of bulk Li foil keeps increasing since the beginning of the cycling without stable period, and the voltage hysteresis soon reaches 100 mV only after 200 h. In contrast, CFC/Li anode exhibits stable voltage for over 750 h. Figure 3d demonstrates zero overpotential of nucleation and stable voltage for the CFC/Li cell even after long-term cycling. However, fluctuation of voltage and obvious overpotential are observed from bulk Li foil cells. The n-CFC/Li cell displays lower voltage hysteresis than that of bulk Li, but can also be stable for only 200 h 11



with abrupt voltage fluctuation afterwards (Figure S8, SI). For further proving the effect of CFC/ZnO in stabilization of Li stripping/plating, areal capacity of 5.0 mAh cm-2 at 2.0 mA cm-2 current density has also been applied. The voltage profile (Figure S9, SI) shows low voltage hysteresis of ~38 mV, and keeps stable for over 300 h at such high areal capacity. More aggressive areal current densities (5.0 mA cm-2 and 10.0 mA cm-2) were then used to further confirm high-rate capability and stability of CFC/Li anode. As is shown in Figure 3e, CFC/Li cells cycled with extremely high stability for over 800 cycles (320 h) at 5 mA cm-2, which has rarely been achieved for Li symmetrical cells by using unmodified electrolyte. Figure 3f exhibits that the voltage of CFC/Li is stable for over 500 cycles at 5.0 mA cm-2 without polarization and fluctuation. As expected, step-by-step polarization, huge nucleation overpotential and significantly expanded voltage hysteresis are observed in bulk Li foil and n-CFC/Li (Figure S10, SI). At the same current density of 5.0 mA cm-2, a much higher areal capacity (3.0 mAh cm-2) was employed to further test the stability of CFC/Li (Figure S11, SI), which could still work properly for over 100 h. The electrochemical impedance spectroscopy (EIS) was employed to further indicate the excellence of CFC/Li on guiding dissolution/deposition behaviors of metallic Li (Figure S12, SI). Fresh symmetrical cells of CFC/Li anode and Li foil were both observed with high resistance level, due to the inert layer on the surface of the electrode. After cycling at 5.0 mA cm-2 for 50 cycles, the oxide layer was broken and the resistance of both cell reduced (19.8 Ω for the CFC/Li, and 30.3 Ω for the Li foil cell). Along with the 12


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cycling, the resistance of the CFC/Li cell slowly increased to 36.7 Ω after 300 cycles, whereas that of the Li foil cell dramatically increased to 58.6 Ω. The much steadier resistance of the cell using CFC/Li anode implies that the CFC/ZnO scaffold is beneficial for enhancing the reversibility of Li stripping/plating. As an ultrahigh areal current density, 10.0 mA cm-2 was almost destructive to Li metal anodes reported in previous literatures.24,

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Few researches have realized stable

voltage longer than 20 h (100 cycles) for Li symmetrical cells based on unmodified ether-based electrolyte at such a high current density. Thanks to the 3D interconnected micro-channels in CFC/ZnO that confine stripping/plating of Li in restricted areas and conductive CFC network that dissipates high current density, in this work, the CFC/Li symmetrical cell was stable at voltage profiles for over 60 h with only slightly increased polarization (Figure 3g). In contrast, the fluctuation of voltage (Figure 3h) of Li foil emerged after less than 5 h because of severe Li dendrite formation and it shows large voltage hysteresis of over 600 mV. The voltage hysteresis of the n-CFC/Li cell also rapidly increases from 160 mV to 400 mV after less than 50 h (Figure S13, SI). Figure 4 compares the life at different areal current densities of Li symmetrical cells from previous literatures and this work with fixed areal capacity of 1 mAh cm-2 (specific data was compared in Table S1).18,

20-21, 23-24, 38-42

Most of

previous literatures showed their stability in low areal current density condition (< 4.0 mA cm-2), however, due to enhancement of CFC/ZnO scaffold, the CFC/Li anodes work stably for over 1800 h, which is much longer than those of with time < 1000 h. Moreover, at high current density of 5.0 mA cm-2 and 10.0 mA cm-2, the high-rate 13



stability is obviously improved, and the cycling time was enlarged for several folds from 40 h to 360 h at 5.0 mA cm-2and 20 h to 60 h at 10.0 mA cm-2, respectively. These comparisons clearly show superiority of CFC/Li anode from our work in high-rate capability and stability than other modified Li anodes. Figure S14 (SI) shows repeated measuring data of the electrochemical performance of CFC/Li anode at the current densities from 1.0 mA cm-2 to 10.0 mA cm-2, which manifests the repeatability of the excellent stability of CFC/Li at such high current densities. To further manifest the function of CFC on guiding stripping/plating behavior of Li metal, the Coulombic efficiency (CE) of pure CFC current collector at the current densities of 1.0, 2.0 and 5.0 mA cm-2 under the areal capacity of 3.0 mAh cm-2 was measured and shown in Figure S15 (SI). The initial CE of all cells is lower than 90%, and even approaching 80% at 5.0 mA cm-2. The relatively low initial CE is mainly ascribed to the large specific surface area of CFC (BET specific surface area of 1093.8 m2 g-,1 Figure S16 in SI), which induces the formation of relatively large amounts of SEI on the surface of CFC after initial charge/discharge processes. However, the CE keeps increasing in the subsequent cycles. It soon approaches 100% only after a few cycles and keeps steady afterwards. More specifically, the CE of the cell tested at 1.0 mA cm-2 quickly rises to 99.0% after several cycles, and lasts for over 140 cycles. The average CE at 2.0 mA cm-2 is also as high as ~98.0% (over 100 cycles). Even at a relatively high current density of 5.0 mA cm-2, the CE also quickly climbs up from ~ 80% in the first cycle to over 95% within 5 cycles, and keeps stable for over 70 cycles. In great contrast, the CE of bare Cu foil fluctuates severely during 14


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the initial 15 cycles, and remains at ~ 70% in the following cycles, indicating the poor reversibility of Li stripping/plating on planar Cu foil. The relatively high and stable CE of cells using CFC current collector suggests that the impact of exposed surface area of CFC on the reversibility of the cells is almost negligible. It can be understood as that most micropores on CFC is filled by metallic Li after initial Li plating, thus the surface area of CFC is enormously reduced and its effect on stripping/plating of Li is minimized. For fully revealing functions of CFC/ZnO in improving long-term stripping/plating behavior of Li, morphology of fully charged CFC/Li and bulk Li electrode after 150 h of cycling at 5.0 mA cm-2 was characterized. The digital photographs in Figure 5a, b show the disassembled CFC/Li cell and bulk Li cell. The separator in CFC/Li cell is still wetted by electrolyte, while the electrolyte is almost completely consumed in bulk Li cell. Bulk “dead Li” stripped from the Li foil is also observed on the separator due to the continuous formation and aggregation of dead Li during cycling of Li foil. SEM images of CFC/Li electrode after cycling are shown in Figure 5c-e. CFC/Li (Figure 5c) exhibits no extra Li layer covering on the surfaces even after 150 h of cycling, comparing with the obvious “dead Li” layer on bulk Li (Figure 5f) and n-CFC/Li (marked by red dashed lines in Figure S17c-d, SI). Such thick “dead Li” layers with huge resistance are the main cause for the polarization and voltage increase of the Li symmetrical cells. Instead of forming “dead Li”, the stripping/plating reaction of Li metal in the CFC/Li electrode is strictly confined in the micro-channels in CFC/ZnO as demonstrated in Figure 5d and e. Low 15



magnification top-view SEM image (Figure S18, SI) shows that most Li/SEI interphase is embedded in the channels or voids of CFC/ZnO with visualized CFC/ZnO network on top. No Li dendrites protruding on the surface are observed, suggesting that the deposition of Li is confined inside CFC. The top-view image (Figure 5d) of CFC/Li electrode demonstrates that intertwining carbon fibers are evidently seen, accompanied by Li/SEI interphase firmly embedded inside. The cell using CFC/Li anode can keep stable voltage for over hundreds even thousands of hours. Though the Li metal inside micro-channels of CFC/ZnO is pulverized after cycling (Figure 5e), the pulverized substance (Li/SEI interphase) confined in micro-channels can still be utilized as they are in intimate contact with conductive carbon fibers which supply abundant electrons for electrochemical reactions. Thus, the pulverized substance has little chance to aggregate into “dead Li”, which ensures highly reversible Li stripping/plating behaviors. In contrast, cross-sectional SEM image (Figure 5f) of bulk Li electrode displays two separated layers in the electrode after cycling. A “dead Li” layer (~200 µm in thickness) is clearly seen on the unreacted Li foil,43 which is in accordance with the digital photograph of disassembled cell. Also, dense “dead Li” around carbon fiber bundles is visualized in the n-CFC/Li electrode (Figure S17c-d, SI), suggesting striping/plating of control sample is similar with planar Li foil. For better reflecting the advantage of CFC/Li as Li metal anode with outstanding rate capability and long-term stability, LIC cells using CFC/Li as the anode and commercial AC as the cathode were assembled and tested. Various rates (from 0.5 A 16


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g-1 to 30 A g-1) were employed to characterize the rate capability of the cells, as is shown in Figure 6a. The coin cell using CFC/Li anode can deliver specific capacity of 47.0 mAh g-1 at 0.5 A g-1, which is the normal capacity for AC. Its capacity slowly reduces to 42.5 mAh g-1 at 2.0 A g-1, 35.4 mAh g-1 at 10.0 A g-1 and 28.1 mAh g-1 at 20.0 A g-1, and a relatively high capacity of 20.7 mAh g-1 can still be achieved at an ultra-high rate of 30 .0A g-1. In sharp contrast, the capacity of the coin cell with Li foil anode rapidly decays with increasing of the current density, and it drops down to less than 5.0 mAh g-1 at 30.0 A g-1. It also shows severe capacity fluctuation when current density is higher than 5.0 A g-1. The discharge profiles of CFC/Li (Figure 6b) and Li foil (Figure 6c) are compared to further display the exceptional rate capability of CFC/Li from the IR drop. At current densities lower than 10.0 A g-1, the starting voltage of discharge profiles (from 4.0 V to 2.0 V) of CFC/Li is higher than ~3.75 V, corresponding to small IR drop of less than 0.25 V. Even at an ultra-high rate of 30.0 A g-1, it can still maintain a high starting voltage of 3.3 V. However, the IR drop increases quickly with rise of the current density for Li foil, which exceeds 1.0 V at 10.0 A g-1, and reaches almost 2.0 V at 30.0 A g-1, resulting in enormous capacity loss at high current densities. Long-term cycling performance of LIC cells is shown in Figure 6d. The CFC/Li cell shows slow decay of the capacity at a high current density of 5.0 A g-1. With an initial capacity of 49.7 mAh g-1, it can still deliver a high capacity of 38.9 mAh g-1 after 5000 cycles, corresponding to total capacity retention of 78.3% and average capacity loss of 0.004% per cycle. Whereas, the capacity of the Li foil cell quickly drops from 30 mAh g-1 to 11.2 mAh g-1 only after 500 cycles, 17



indicating the great improvement of cycling stability from CFC/Li anodes. The CFC/Li cell was also cycled at a more aggressive current density of 10 A g-1, which also exhibits outstanding stability of 74.4% capacity retention after 5000 cycles. As is shown in Figure 6e, the CFC/Li anode was also paired with lithium iron phosphate (LFP, areal loading of ~4.6 mg cm-2) cathode to demonstrate its excellent rate capability and cycling stability. When the rate varies from 0.5C to 4C (1C equals to 170 mA g-1), the specific capacity of CFC/Li-LFP cell are 146.6 mAh g- 1 (0.5C), 139.8 mAh g-1 (1C), 130.5 mAh g-1 (2C) and 116.7 mAh g-1 (4C), respectively. However, the LFP cell using Li foil as the anode only delivers the capacity of 120.1 mAh g-1 at 2C and 98.7 mAh g-1 at 4C, which is much lower than that of the CFC/Li-LFP cell, indicating the enhancement of the rate ability by storing Li in CFC scaffold. Moreover, long-term cycling at 1C shows that the capacity retention of CFC/Li-LFP cell is 77.9% after 200 cycles. In contrast, the Li foil-LFP cell suffers from continuous capacity decay and shows much lower capacity retention of 60.3%, further confirming the excellence of the CFC/Li anode. Thanks to easy fabrication of large area CFC/Li anodes, LIC pouch cells (36 mm in length and 24 mm in width) were successfully assembled and utilized to demonstrate the potential of CFC/Li anode for practical application. As is shown in Figure 6f, three LIC pouch cells are able to light two lamps (5 V, 20 W). When the pouch cells were charged to 4 V in 3 min, they could deliver energy density of ~65 Wh kg-1 based on electrodes at power density of ~1450 W kg-1 (Figure 6g). When further speed up the charge/discharge time to 1 min, these pouch cells could still deliver ~50 Wh kg-1 energy density at power 18


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density of 2580 W kg-1 based on electrodes. These results demonstrate great rate capability and cycling stability of CFC/Li anode in LIC cells, and potential for ultrafast charge batteries.

Conclusion In summary, we reported facile production of large-area high-function Li metal anodes for guiding stripping/plating behavior by confining Li striping/plating in interconnected channels. The macro voids enable edges of Li self-promoting inner Li into electrolyte to exposed channels. With that, the deposited behavior of Li could be restricted in narrow gaps among carbon fibers, providing sufficient electron and dissipating high current density into low local one for dendrite-free plating. Moreover, the incompletely stripped Li covered on carbon fibers could eliminate nucleation overpotential by homogenous crystal structure. These interconnected channels guide the highly reversible striping/plating behaviors at high areal current density of 1.0 mA cm-2 for 1800 h and 5.0 mA cm-2 for 320 h, which is higher than ever reported one by using common electrolyte. By integrating CFC/Li anodes into LIC cells, excellent rate capability and highly improved cycling stability at high current density condition can be acquired. What's more, by using large-area CFC/Li anodes, soft packs of LIC are able to be assembled and serve as ultrafast charge (about 1 min) batteries with high energy density (~50 Wh kg-1). This work opens up the mind for high-performance Li metal anodes by appealing more concerns about design of electrolyte/Li interfaces under the help of 3D and conductive hosts, which may revive safe and stable Li metal

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anodes at high rate condition in the near future.

Experimental Section Preparation of CFC/Li anodes Commercial activated carbon fiber cloth (CFC) was purchased from Nantong Belgae Co., LTD. The CFC was rinsed several times in deionized water, and then soaked in zinc acetate solution (4.0 g zinc acetate dissolved in 100 mL deionized water) for 24 h. The wet CFC was placed in an oven at 60 °C for 6 h and annealed at 800 °C for 2 h at inert atmosphere (pure Ar), and ZnO decorated CFC (CFC/ZnO) was obtained. Polished Li foil (thickness of 0.6 mm, purchased from MTI Co., Ltd.) was placed on nickel foil, and then heated over 300 °C to become molten Li in in a glove box filled with inert atmosphere (oxygen and water content was kept below 0.1 ppm). Molten Li was adsorbed quickly by CFC in a few minutes when ZnO decorated CFC was placed on the edge of molten Li, and CFC/Li was finally obtained after cooling to room temperature. Characterization Methods Electron microscopic images were recorded by a Hitachi S-4800 field emission scanning electron microscope (SEM). The cycled symmetrical coin cells were disassembled in a glove box, and then rinsed by ethylene carbonate (EC) and placed 3 days to be completely dry before characterization. The nitrogen adsorption-desorption isotherms were recorded by Micromeritics ASAP-2020M nitrogen adsorption apparatus. Powder X-ray diffraction (XRD) were performed using an AXS D8

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Advance diffractometer (Cu Kα radiation; receiving slit, 0.2mm; scintillation counter; 40mA, 40 kV) from Bruker Inc. Electrochemical Measurement CR2032-type coin cells and pouch cells were employed to characterize the electrochemical performances. All the cells were assembled in glove box filled with inert atmosphere. A LAND-CT2001A battery test system (Jinnuo Wuhan Corp., China) Arbin BT2000 appliances (purchased from Arbin Instruments) were used to test the symmetrical cells. The impedance spectra recorded by Solartron electrochemical workstation were tested with the amplitude of 5.0 mV and frequency range of 100 kHz-0.01 Hz. For the electrochemical performances of Li symmetrical cells, two CFC/Li electrodes pouched into discs with diameter of 11 mm were paired and used as working electrode and counter electrode. Polypropylene film (Celgard) was used as the separator and commercial ether-based electrolyte (1% lithium nitrate and 1.0 M lithium bistrifluoromethanesulfonylimide (LiTFSI) in 1, 3-dioxolane (DOL) and 1, 2-dimethoxyethane (DME) with volume ratio 1:1, purchased from Jiangsu Duoduo Chemical Material Co., Ltd.) was used as the electrolyte for symmetrical cells with the amount of 50 µL. Li symmetrical cells was tested at 1.0 mA cm-2, 2.0 mA cm-2, 5.0 mA cm-2 and 10 mA cm-2 at the fixed areal capacity of 1.0 mAh cm-2 for long-term cycling performances by recording the variation of voltage hysteresis. And at the same areal capacity, areal current density varies from 0.5 mA cm-2 to 20 mA cm-2 was employed to characterize rate performances. 21



CFC/Li or bulk Li foil anodes was paired with commercial activated carbon (areal loading of 1.6 mg cm-2, purchased from Power Carbon Technology. Co., Ltd.) and assembled into Li ion capacitors (coin cells and pouch cells). For the pouch cells, five large-area CFC/Li anodes was paired with commercial activated carbon coated on both sides of Al foil (four sheets, with the areal mass loading of 8 mg cm-2 on single side). The energy density and power density of LIC punch cells was calculated by the mass of whole CFC/Li anodes and AC/Al foil cathodes. Both coin cells and pouch cells used polypropylene film (Celgard) as the separator and commercial carbonated-based electrolyte (1 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) with volume of 1:1, purchased from Guotai-Huarong New Chemical Material Co., Ltd) (30 uL in coin cells and ~4 g in pouch cells). The voltage range for LIC measurement is between 2.0 V and 4.0 V from 0.5 A g-1 to 30 A g-1 for rate capability, and 5 A g-1 and 10 A g-1 was employed for long-term cycling tests. CFC/Li and Li foil was paired with lithium iron phosphate cathode in coin cells. The charge-discharge voltage range is 2.0-4.0 V, and the rate varies from 0.5 C to 4.0 C (1 C equals to 170.0 mA g-1). The CE was tested by using pure CFC or Cu foil as the working electrode, and Li foil as the counter electrode. The cells are discharged at the current density of 1.0 mA cm-2, 2.0 mA cm-2 and 5.0 mA cm-2 under the areal capacity of 3.0 mAh cm-2, and charged to 1.0V.

Supporting Information

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Additional materials and characterizations of (Word), Supporting information is available free of charge via the Internet at http://pubs.acs.org. Coulombic efficiency of CFC, BET surface area, XRD pattern, Table for comparison, EIS profiles, digital photographs, SEM images and electrochemical performances.

ACKNOWLEDGEMENT This work was supported by National Natural Science Foundation of China (Grant No. 21371176) and Key Research Program of the Chinese Academy of Sciences (Grant No. KGZD-EW-T08).

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Figure 1. (a, b) Schematic illustration of the structure of CFC/ZnO (upper right in panel a is the top view of the CFC and lower right is the magnified illustration showing ZnO nanoparticles on individual carbon fibers), and the preparation and morphology of CFC/Li. (c, d and e) The SEM images with different magnifications showing the morphology of CFC decorated with ZnO nanoparticles. (f, g and h) SEM images with different magnifications of CFC/Li electrode.

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Figure 2. (a, b) Illustration of exposure of highly interconnected micro-channels in CFC/Li during stripping of Li. The thin layers on the carbon fibers with purple color represent remaining metallic Li layer after stripping. (c) Cross-sectional SEM image of CFC/Li, (d) top-view SEM image of CFC/Li and (e) high magnification SEM image of single carbon fiber after 1st striping of Li for 6 h at 2 mA cm-2. Inset in (c) shows the digital photograph of CFC/Li electrode after stripping. (f) Cross-sectional SEM image of CFC/Li, (g) top-view SEM image of CFC/Li and (h) high magnification SEM image of single carbon fiber after 20th plating of Li 2 mA cm-2. Inset in (f) shows the digital photograph of CFC/Li electrode after plating.

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Figure 3. (a) The voltage profiles of Li foil (dashed line, charge profiles) and CFC/Li anode (solid line, discharge profiles) at varying areal current densities. Galvanostatic curves of symmetrical cells at areal current densities of 1 mA cm-2 (b), 2 mA cm-2 (c), 5 mA cm-2 (e) and 10 mA cm-2 (g) using Li foil and CFC/Li electrodes at the areal capacity of 1 mAh cm-2. The voltage profiles of Li foil and CFC/Li cells from different cycles during long-term testing at the current density of 2 mA cm-2 (d), 5 mA cm-2 (f) and 10 mA cm-2 (h).

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Figure 4. The comparison of this work with previous literatures about cycling time of Li symmetrical cells at different working areal current densities.

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Figure 5. Digital photographs of disassembled CFC/Li cell (a) and Li foil cell (b) after cycling at an areal current density of 5 mA cm-2 and areal capacity of 1 mAh cm-2. (c) Low magnification cross-sectional SEM image, and (d) low and (e) high magnification top-view SEM images of CFC/Li electrode after 150 h cycling at the current density of 5 mA cm-2. (f) Cross-sectional SEM images of Li foil after 150 h cycling at the current density of 5.0 mA cm-2.

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Figure 6. (a) Reversible capacity at varying charge/discharge rates of LIC cells using CFC/Li or Li foil as the anode. Galvanostatic discharge/charge profiles of CFC/Li (b) and Li foil (c) at different rates. (d) Cycling profiles of CFC/Li and Li foil at current densities of 5 A g-1 and 10 A g-1. (e) Rate and cycling performance of CFC/Li-LFP cell and Li foil-LFP cell. (f) The demonstration of LIC pouch cells assembled by large area CFC/Li anode and commercialized activated carbon cathode. (g) The plots showing energy density of LIC pouch cells using CFC/Li anodes during cycling at charge/discharge time of 3 min and 1 min.

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Highly Reversible Li Plating Confined in Three-Dimensional

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