Highly Conductive and Robust 3D Host with Excellent Alkali Metal

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Energy, Environmental, and Catalysis Applications

Highly Conductive and Robust 3D Host with Excellent Alkali Metal Infiltration Boosts Ultra-stable Lithium and Sodium Metal Anodes Wan-Sheng Xiong, Yu Xia, Yun Jiang, Yuyang Qi, Weiwei Sun, Dan He, Yumin Liu, and Xingzhong Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03572 • Publication Date (Web): 24 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

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ACS Applied Materials & Interfaces

Highly Conductive and Robust 3D Host with Excellent Alkali Metal Infiltration Boosts Ultrastable Lithium and Sodium Metal Anodes Wan-Sheng Xiong,† Yu Xia,† Yun Jiang,† Yuyang Qi,† Weiwei Sun,‡ Dan He,† Yumin Liu,*,† and Xing-Zhong Zhao†,§

†

Institute for Interdisciplinary Research (IIR), Key Laboratory of Optoelectronic Chemical

Materials and Devices, Ministry of Education, Jianghan University, Wuhan 430056, China. ‡

College of Aerospace Science and Engineering, National University of Defence Technology,

Changsha 410073, China. §

School of Physics and Technology, Key Laboratory of Artificial Micro/Nano Structures,

Ministry of Education, Wuhan University, Wuhan 430072, China.

KEYWORDS: carbon textiles, alkali metal infiltration, stable host, lithium metal anodes, sodium metal anodes.

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ABSTRACT: The direct utilization of metallic lithium and sodium as the anodes for rechargeable batteries would be highly advantageous, which has been considered as one of the most promising choices for next-generation high-energy-density storage devices. Although the induced safety concerns, inferior rate and cycling performance severely hinder the commercialization of lithium-metal batteries (LMB) and sodium-metal batteries (SMB), the recent development of nanotechnology-based solutions really revives the lithium/sodium metal anodes for high-energy batteries. In this work, an ultra-stable carbon textiles (CTs) based host with excellent infiltration for both metallic Li and Na has been designed and exhibits more flat voltage profiles, lower stripping/plating overpotential and better cycling stability both in symmetric cell and full cell configurations even in additive-free carbonate-based electrolyte compared with pure Li/Na electrodes. The highly conductive and mechanically robust 3D CTs not only offer a stable scaffold against hyperactive lithium and sodium, but also enable uniformly nucleation and growth during stripping/plating process, which effectively suppress the dendrite growth and stabilize the electrode dimension. This facile strategy provides new insights into the design of stable hosts with pre-stored alkali metal to address the multifaceted issues in LMBs and SMBs simultaneously.

1. INTRODUCTION To meet the ever-increasing demand of modern life to high-energy-density storage systems for electric vehicles (EVs), portable electronic devices, grid-scale energy storage and other


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applications, the rechargeable batteries beyond the conventional “rocking-chair-based” lithiumion or sodium-ion chemistry have attracted intensive research,1-6 which are of great scientific and technological importance. Li metal has been considered as one of the most promising anode materials attributed to its high theoretical capacity (3,860 mAh g-1), lowest density (0.59 g cm3), and most negative electrochemical potential (-3.040 V vs. SHE).7 Furthermore, the utilization of metallic Li as the anode remarkably promotes the energy density of lithium metal batteries (LMBs) with the employ of Li-free cathodes such as sulfur and oxygen, achieving high specific energy density of 2567 Wh kg-1 and 3505 Wh kg-1, respectively.8 However, the Li metal anodes have been plagued for decades with the problems of dendritic Li formation, severe side reactions, and infinite relative volume change since the pioneering work by Stanley Whittingham in the 1970s.9-12 In order to overcome these formidable challenges of LMBs, increasing efforts have been devoted recently, such as electrolyte optimization for stabilizing Li anodes,13-18 artificial engineering of the anode-electrolyte interfaces,19-27 and employing the stable hosts for high reactive metallic Li.28-32 On the other hand, concerning over potentially rising costs and the availability of global lithium resources, metallic sodium could be another ideal choice for anode material due to its low cost and high natural abundance. Owing to the high reactivity of alkali metal, sodium metal batteries (SMBs) and lithium metal batteries (LMBs) suffered from almost the same obstacles mentioned above. Nevertheless, in contrast to recent rapid development of LMBs, there are a handful of studies on room temperature SMBs.


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Among all the viable strategies, the employment of stable host materials pre-stored metallic Li is an effective approach to address these multifaceted issues. The introduction of stable hosts can minimize the infinite relative volume change during stripping/plating process, which is also benefit to the solid electrolyte interphase (SEI) stability. And the stable SEI avoids the danger of collapse during continuous cycling and the locally formation of Li dendrite. More importantly, the designed and optimized stable hosts afford the opportunity to reduce the large polarization and strong electric field at the vicinity of the Li/Na metal anodes, which further suppress the dendrite growth.33 Recently, the layered composite of Li/reduced graphene oxide was fabricated, achieving a stable cycling performance, low polarization, and small electrode dimensional change.28 Li coated lithiophilic conductive scaffold or polymer matrix also exhibited good cyclability and minimal volume variation.29,31 Notably, excellent lithiophilicity is essential to the hosts for the infiltration of molten Li. In previous work, atomic layer deposition and chemical vapor deposition technology have been employed to obtain a thin and conformal lithiophilic layer.29,31 This process is complex and makes cost up. Thus, it is of great importance to develop a scalable facile approach to afford lithiophilicity of the hosts. Furthermore, despite of the differences in electrolyte composites and electrochemical kinetics for LMBs and SMBs, the development of stable hosts with pre-stored metallic Li/Na could be a universal strategy to facilitate the practical application of metallic Li/Na based high-energy-density storage systems. Herein, we demonstrate a general strategy to construct metallic Li or Na infiltrated 3D scaffold via a scalable oxide deposition that offers the primary infiltration for both Li and Na (Figure 1a).


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Figure 1. (a) Schematic illustration of the fabrication of Li/Na infiltrated 3D SCTs. (b) Surface pictures of pure CTs and 3D SCTs before and after Li/Na infiltration. FE-SEM images of (c-e) Li-3D SCTs, (f-h) 3D SCTs, and (i-k) Na-3D SCTs.


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Carbon textiles (CTs) were employed as the host material for metallic Li or Na on account of the considerations including excellent stability, highly conductive bicontinuous networks, and robust flexibility. Firstly, carbon-based hosts exhibit outstanding mechanical and chemical stabilization under the redox environment in rechargeable batteries, which is essential for the current collect towards electrochemical stripping and plating cycles. Secondly, the highly conductive carbon fiber based 3D networks provide fast electron/ion transport pathway and achieve relatively large surface area for alkali metal deposition, reducing the effective current density and homogenizing the Li/Na ion flux. Furthermore, the CTs woven by carbon fibers with high flexibility (Figure S1a-c) are the ideal choice as unique supporting backbones for the infiltration of alkali metal. This robust scaffold affords the volume stability at the electrode level to stabilize the electrode dimension which effectively avoids the danger of SEI collapse and the locally formation of Li/Na dendrites during continuous cycling. Notably, our highly conductive and robust 3D scaffold with the infiltration for both Li and Na achieved long-term reversibility of Li/Na metal anodes in additive-free carbonate-based liquid electrolytes, which provides new insights into the design of stable hosts with alkali metal infiltration for high-performance LMBs and SMBs.

2. EXPERIMENTAL SECTION 2.1 Scalable oxide deposition on three-dimensional carbon textiles Carbon textiles (CTs, CeTech WOS1002, through-plane resistance < 5 mΩ cm-2) was refluxed in 3 M HNO3 solution for 5 h at 80 ˚C and washed by DI water, followed by drying under vacuum


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at 60 ˚C overnight. Ultra-thin SnO2 nanocrystallines were grown on CTs via a facile chemical bath deposition method. After the sonication for 15 min, the CTs were immersed in the 0.23 M Na2SnO3 aqueous solution at 80 ˚C for 1 h, the obtained CTs with the assembly of SnO2 nanocrystals were rinsed with DI water for several times, followed by annealing in ambient air condition at 450 ˚C for 8 h to achieve desirable alkali metal wetting property. The obtained three-dimensional SnO2 coated carbon textiles (3D SCTs) were cut into small rounds with a diameter of 1.4 cm before the alkali metal infiltration. 2.2 Infiltration of lithium and sodium into the 3D SCTs The infiltration of Li and Na into the obtained 3D SCTs was carried out in an argon-filled glove box with O2 and H2O level below 0.1 ppm. Firstly, metallic Li and Na were heated to melt at 300 °C in a nickel boat on a hotplate. Subsequently, the edge of the 3D SCTs piece was dipped into the molten Li or Na, which infuses into the 3D scaffold steadily and wet the whole matrix. The final obtained Li or Na infiltrated 3D SCTs were labelled as Li-3D SCTs and Na-3DSCTs, respectively. In the final Li-3D SCTs, the mass percent of Li, SnO2, and CTs is approximately 55.8%, 2.5%, and 41.7%, respectively. In the case of Na-3D SCTs, the corresponding mass percent of Li, SnO2, and CTs is approximately 71.8%, 1.6%, and 26.6%, respectively. 2.3 Characterizations SEM images of all samples were observed by a field-emission scanning electron microscopy (FE-SEM, HITACHI SU8010) at 3 kV. The energy-dispersive X-ray elemental mapping was obtained on a IXRF system 550i SDD detector. X-ray diffraction (XRD) measurements were


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performed on a PANalytical XRD system (X’Pert Powder) with Cu Kα radiation (λ = 1.54056 Å). TEM and high-resolution TEM (HR-TEM) were carried out using a FEI Tecnai G20. Symmetric type-2032 coin cells were assembled with two identical electrodes in an argon-filled glove box to evaluate the stripping/plating process of Li and Na. The common electrolyte consists of 1.0 M LiPF6 in ethylene carbonate (EC), ethylene methyl carbonate (EMC) and dimethyl carbonate (DMC) was employed for Li-based symmetric cells and half-cells. And the electrolyte consists of 1.0 M NaClO4 in ethylene carbonate (EC) and dimethyl carbonate (DMC) was purchased for Nabased symmetric cells. The separator used for all the cells was a polypropylene membrane (Celgard). Galvanostatic cycling was conducted on a LANHE test system (CT2001A) at different current density with the stripping/plating capacity of 1 mAh cm-2 in all cases. Electrochemical impedance spectroscopy (EIS) was carried out using a electrochemical workstation (Zahner, Zennium) at room temperature between 100 kHz and 0.01 Hz with an amplitude of 5 mV before initial cycle and after 10 cycles. In order to prepare the working electrodes for the half-cell investigation, LiFePO4 (Shenzhen Kejing Instrument Co. Ltd.), carbon black (conducting agent), and a polyvinylidene fluoride (binder) in a weight ratio of 70:20:10 were mixed in N-methyl-pyrrolidone (NMP) to form a homogeneous slurry, and then pasted on a Cu foil, followed by drying under vacuum at 120 ˚C overnight. The mass of LiFePO4 loaded on each electrode was ca. 0.7 mg. The obtained LiFePO4 electrodes were assembled into 2032 coin-type half cells by employing pure Li foil or Li-3D SCTs as the counter electrode for galvanostatic cycling test. The separator used was the same with that in the symmetric cells. The


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LiFePO4/Li and LiFePO4/Li-3D SCTs cells were cycled between 2.5 and 4.1 V under different current rate (x C = fully discharge within 1/x hours) at room temperature.

3. RESULTS AND DISCUSSION Owing to the large surface tension of molten Li and Na on the carbon fibers, the pristine 3D CTs cannot be well wetted by molten Li and Na as shown in Figure 1b. Thus, an ultra-thin SnO2 layer was firstly coated on the carbon fiber surface via a scalable chemical bath deposition (CBD) approach, considering eco-efficiency, synthetic process and scalability, to afford the infiltration for both Li and Na (Figure 1b). Field-emission scanning electron microscopy (FE-SEM) was employed to characterize the morphology of the obtained SnO2 nanocrystallines on the surfaces of the CTs. Figure 1f shows a well-established texture structure of the obtained 3D SCTs. The high-magnification SEM image (Figure 1g) reveals that the ultra-thin SnO2 nanocrystallines were homogenously deposited on the surface of the carbon fibers. The cross-sectional SEM image further indicated the evident core-shell structure of 3D SCTs (Figure 1h). The crystalline phase of as-synthesized 3D SCTs before and after annealing was determined by X-ray diffraction (XRD) as shown in Figure S1e. All the diffraction peaks observed at the XRD pattern can be indexed to the tetragonal SnO2 phase (JCPDS card No. 21-1250), excluding the peak at ca. 25.7° and 43.6° associated to the (002) and the (101) planes of the CTs (JCPDS card No. 01-0640). High-resolution transmission electron microscopy (HR-TEM) was carried out to further investigate the morphology and structure of the SnO2 nanocrystallines. Figure S2a indicates that


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the crystal size of the obtained ultra-thin SnO2 nanocrystallines was less than 10 nm, and the neighboring lattice fringes with distance of 3.36 Å can be corresponded to (110) planes of the tetragonal SnO2 phase (Figure S2b), which confirmed the XRD results. Figure S1d demonstrates that the ultra-thin SnO2 nanocrystallines can be homogenously deposited on the surface of CTs on a large scale via this facile CBD approach to afford the wetting property for molten Li and Na. After the contact of the 3D SCTs pieces with molten Li, the molten Li could react with this ultra-thin SnO2 layer and provide high driving force to infiltrate Li into the scaffold. The final obtained Li-3D SCTs with metallic luster still maintained the texture structure (Figure 1b). Figure 1c and 1d show SEM top-view of the Li-3D SCTs. The molten Li was fully infused into the highly ordered 3D carbon fibers based conductive scaffold, and the size of the SnO2 nanocrystallines turns larger owing to the spontaneous reaction between SnO2 and molten Li (Figure 1d, inset). Figure 1e shows the cross-sectional SEM images of Li-3D SCTs, which further indicates the full infiltration of molten Li into the carbon fibers based matrix. Most impressively, this 3D SCTs also exhibited good wetting property for molten Na, which has not been reported in previous work. The full infiltration of molten Na into 3D SCTs needed more time than Li, and the Na-3D SCTs was more twinkling than Li-3D SCTs as shown Figure 1b. The surface of the obtained Na-3D SCTs was more uneven than that of Li-3D SCTs as shown in Figure 1i and 1j, which could be attributed to the more reactivity of metallic Na than Li. Evident infiltration of molten Na into 3D SCTs was also observed from the cross-sectional SEM image (Figure 1k).


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In order to further indicate the existence of metallic Li in the 3D SCTs, XRD was employed to characterise the obtained Li-3D SCTs. The main diffraction peaks of the XRD spectra can be indexed to the lithium (JCPDS 01-1131), as shown in Figure S3a. The other impurity peaks could be attributed to the formation of LixSn alloy. Owing to the hyperactivity of metallic Na in air, we employed the energy-dispersive X-ray elemental mapping to characterise the element distribution of the obtained Na-3D SCTs, which confirmed the infiltration of molten Na into this carbon fibers based scaffold (Figure S3b-d). Benefiting from the robust mechanical flexibility of the CTs, the obtained Li/Na-3D SCTs exhibited mechanically robust flexibility as shown in Figure S4, which is fantastic and critical in battery manufacturing. The galvanostatic cycling performance of the symmetric cells based on Li-3D SCTs was evaluated in additive-free carbonate-based liquid electrolytes and compared with pure Li electrode. The cells were cycled at a current density of 1 mA cm-2 with a total stripping/plating capacity of 1 mAh cm-2, and the potential was recorded over time. Figure 2a compares the voltage profiles of symmetric Li-3D SCTs cell and pure Li counterpart through over 100 cycles. For pure Li symmetric cell, it shows a large Li stripping/plating overpotential, which is more than 100 mV at the first few cycles and stabilise at ca. 60 mV versus Li/Li+ subsequently. In contrast, the Li-3D SCTs exhibited a much lower overpotential of ca. 40 mV in the initial cycle and delivered a stable value less than 20 mV over 100 cycles (achieving 15 mV overpotential in the 100 cycle). The overpotential became increasingly pronounced at higher current density as


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Figure 2. Galvanostatic cycling performance of the symmetric cells based on (a) pure Li foil and Li-3D SCTs, (c) pure Na foil and Na-3D SCTs at a current density of 1 mA cm-2. Rate performance of (b) the pure Li and Li-3D SCTs, (d) the pure Na and Na-3D SCTs at various current densities. Voltage profiles of (e) the pure Li and Li-3D SCTs, (f) the pure Na and Na-3D SCTs based symmetric cells at 1 mA cm-2 in different cycles and at different current density. Nyquist plot of the impedance spectra of the symmetric cells before initial cycle and after 10 cycles based on (g, h) pure Li foil and Li-3D SCTs, (i, j) pure Na foil and Na-3D SCTs. The stripping/plating capacity was 1 mAh cm-2 in all cases. shown in Figure 2b. The stripping/plating overpotential of symmetric pure Li cell was ca. 100 and 180 mV at a current density of 3 and 5 mA cm-2, respectively. As expected, the 3D SCTs exhibited much lower overpotential at such high current density, which was ca. 43 and 65 mV at


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3 and 5 mA cm-2, respectively (Figure 2e). It is worth noting that the overpotential of pure Li electrode decreased obviously after cycling at high current rate, which could be attributed to the gradually increased surface area from the formation of Li dendrites during the dramatically stripping/plating process. The necking behavior of the symmetric pure Li cell during stripping/plating can be associated to the dendrite formation and SEI accumulation.34 The remarkably reduced overpotential and improved cycling stability of the cells based on the Li-3D SCTs can be attributed to the highly conductive scaffold, which reduced the effective current density and enabled the uniformly deposition of metallic Li. To further investigate the underling mechanism, EIS measurements were carried out to study the interfacial resistance and the charge transfer resistance at the anode surface, which can be associated with the semi-circle in high frequency region.35 As shown in Figure 2g, pure Li electrode shows a large interfacial resistance of ca. 60 Ω before initial cycle. After 10 cycles at a current density of 1 mA cm-2, the interfacial resistance of pure Li electrode decreased to ca. 40 Ω (Figure 2h), which could be related to the increased surface area from the formation of Li dendrites. In comparison, the symmetric cell based on Li-3D SCTs delivered much lower interfacial resistances of ca. 4.3 Ω before initial cycle and ca. 3.0 Ω after 10 cycles. These results confirmed that the highly conductive carbon fiber based 3D scaffold remarkably lowered the interfacial resistance, enabling fast electron/ion transport and achieving homogenous Li ion flux. In the field of room-temperature Na metal batteries, most efforts have been devoted on the electrolyte optimization.36-39 Recently, more attentions have been attracted on the direct


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construction of the Na metal anodes.40-44 The obtained Na infiltrated 3D SCTs was expected to address poor reversibility of Na metal anodes during repeated stripping/plating. The electrochemical performance of the symmetric cells based on pure Na and Na-3D SCTs was investigated using 2032-type coin cells for repeating stripping/plating experiment. The electrolyte consists of 1.0 M NaClO4 in ethylene carbonate (EC) and dimethyl carbonate (DMC) was employed without any other additives. As shown in Figure 2c, it can be observed that the Na-3D SCTs exhibited a much more stable voltage profile and overpotential (less than 50 mV) over 110 cycles compared with that of pure Na electrode. The voltage fluctuation was frequently shown for the symmetric pure Na cell (Figure 2f), revealing the hyperactive nature of metallic Na. The significantly improved cycling stability and more flat voltage profiles of the Na-3D SCTs could be attributed to the following factors. Firstly, the 3D SCTs offers a stable scaffold against hyperactive molten Na. Secondly, the highly conductive CTs remarkably reduced the current density and achieved more homogenous Na ion flux compared with pure Na electrode. Furthermore, the mechanically robust flexibility of the CTs affords the volume stability at the electrode level to stabilize the electrode dimension of the Na-3D SCTs. The voltage profiles at higher current density further confirmed these fantastic advantages of the Na-3D SCT electrode. The stripping/plating overpotential of symmetric Na-3D SCTs cell was ca. 90 and 180 mV at a current density of 3 and 5 mA cm-2, respectively, which was much lower than that of pure Na electrode (Figure 2d). EIS was also carried out to evaluate the interfacial resistance of pure Na and Na-3D SCTs electrodes in a symmetric cell configuration as shown in Figure 2i and 2j. The


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measurements were conducted on both pristine cells and those after 10 cycles at a current density of 1 mA cm-2. The interfacial resistance for pristine Na and Na-3D SCTs, as determined by the semicircle at high frequency, was ca. 2850 Ω and ca. 375 Ω, respectively, which decreased to ca. 92 Ω and ca. 4.6 Ω after 10 cycles. The huge difference of the interfacial resistance between the symmetric cell based on pure Na and Na-3D SCTs before and after cycling can be associated to the high conductive CTs that provides fast electron/ion transport pathway and achieve relatively large surface area for Na deposition. To further characterize the long-term cycling stability, the symmetric cell based on the Li-3D SCTs was cycled at a current density of 3 mA cm-2 with a stripping/plating capacity of 1 mAh cm-2 over 500 cycles. As shown in Figure 3a, the Li-3D SCTs exhibited an ultra-stable cycling performance with a low overpotential of ca. 40 mV and flat voltage profiles. In comparison, pure Li electrode showed a more fluctuant voltage profiles and gradually increased voltage hysteresis over cycles (Figure S5). After 200 cycles, the overpotential of the symmetric cell based on pure Li was ca. 600 mV, whereas the Li-3D SCTs delivered a low overpotential of ca. 38 mV even in the 500 cycle. This ultra-stable cycling performance offers the exciting possibility to fabricating high-performance Li anodes. In full cells assembled with LiFePO4 cathode, the Li-3D SCTs exhibited a much better battery performance compared with pure Li electrode. As shown in Figure 3b, the battery based on Li-3D SCTs anode achieved much higher LiFePO4 capacity compared with that based on pure Li, especially at higher current rate. The LiFePO4||Li-3D SCTs cell retained a discharge capacity of ca. 162, 145, 124, and 99 mAh g-1 at a rate of 0.5 C, 2 C, 6 C,


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Figure 3. (a) Long-term cycling performance of the Li-3D SCTs symmetric cell at a current density of 3 mA cm-2 over 500 cycles with a stripping/plating capacity of 1 mAh cm-2. (b) Rate performance of the LiFePO4||Li and LiFePO4||Li-3D SCTs cells cycled at various current rates from 0.5 C to 15 C between 2.5 and 4.1 V. (c-f) Comparison of the rate capability for the LiFePO4/Li and LiFePO4/Li-3D SCTs cells with increasing rates from 0.5 C to 15 C. and 15 C, respectively. The current rate was determined via the fully discharge time (x C = fully discharged within 1/x hours). In contrast, the battery based on pure Li anode only delivered a


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discharge capacity of ca. 160, 125, 95, and 60 mAh g-1 at corresponding current rate, respectively. Furthermore, the voltage profiles of the LiFePO4||Li-3D SCTs cell at different current rates exhibited much lower overpotential and more flat voltage plateau compared with the counterpart based on LiFePO4||Li (Figure 3c-f). The characterization of battery performances is consistent with the results of the symmetric cells, which indicates a significantly improved stability of the Li-3D SCTs anode compared with pure Li electrode. To obtain a better understanding of the nucleation and growth of metallic Li during the stripping/plating process, SEM study was conducted on the Li-3D SCTs taken out from the separated symmetric cell after cycling. Figure 4a and 4e show the optical photos of the Li-3D SCTs with Li stripping and plating after 10 cycles. The texture structure with metallic luster can be still observed clearly. The top-view SEM images of the cycled Li-3D SCTs show that infiltrated Li maintained a stable surface around the well-established texture structure of carbon fibers (Figure 4b and 4f). Notably, the stripping and plating process left obvious trace on the surface of corresponding Li-3D SCTs as shown in Figure 4c and 4g. After Li stripping from the electrode, several irregular shapes were formed close to the SnO2 coated carbon fibers (Figure 4c). Whereas, the accumulation of metallic Li with similar irregular patterns were observed on the surface of the plated Li-3D SCTs (Figure 4g). The SEM with high magnification further indicates that there are Li nuclei left on the surface of the Li-3D SCTs after Li stripping with uniformly distribution as shown in Figure 4d. After Li plating, the size of Li nuclei became larger, and maintained the uniform distribution and smooth surface (Figure 4h). The dendrite


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Figure 4. Optical photos of the Li-3D SCTs (a) with Li stripping and (e) with Li plating after 10 cycles at a current density of 1 mA cm-2 with a stripping/plating capacity of 1 mAh cm-2. SEM images of (b-d) Li-3D SCTs with Li stripping and (f-h) Li-3D SCTs with Li plating after 10 cycles at a current density of 1 mA cm-2 with a stripping/plating capacity of 1 mAh cm-2. (i) Schematic illustration of the uniformly nucleation and growth of alkali metal during the stripping/plating process and the fantastic advantages of this Li/Na infiltrated 3D SCTs electrodes. nucleation and growth can be explained via a simple ambipolar diffusion equation at the electrode as follows:45

where is the effective current density of the electrode, and are the mobility of the anion


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and Li ion, is the electronic charge, and is the ambipolar diffusion coefficient. When the value of ⁄ is less than 2 ⁄, where and refer to the initial concentration and the inner electrode distance, the ionic concentration at the negative electrode keeps a steady state with constant concentration gradient and achieves electrostatic potential value, resulting in a smooth deposition of Li ion.23 Clearly, the value of ⁄ is in proportion to the electrode effective current density. The highly conductive carbon fiber based 3D scaffold significantly improved electron/ion transport and relatively large surface area for Li deposition, remarkably reducing the effective current density and homogenizing the Li ion flux. Thus the effectively weakened polarization and electric field at the vicinity of the anode surface enabled this uniform nucleation and growth of metallic Li. In order to further identify the electrode stability after cycling, the surface morphology of pure Li, Li-3D SCTs and Na-3D SCTs after 100 cycles was investigated. As shown in Figure S6a, the repeated stripping/plating cycles causes uneven porous Li electrode with severe crack and exfoliation on the surface of hostless Li foil. Benefiting from the robust 3D scaffold, Li-3D SCTs maintained a stabile and smooth electrode surface after continuous cycling, and the texture structures were faintly visible from the top-view SEM image (Figure S6b). Although pure Na electrode was too soft to be separated from the symmetric cell, the construction of Na infiltrated 3D SCTs was preserved, indicating the stabilization of the electrode dimension (Figure S6c). The cross-sectional SEM images (Figure S6d and S6e) confirmed that the 3D SCTs was still fully infiltrated into the metallic Li and Na after 100 cycles just as the pristine composite electrodes as


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shown in Figure 1e and 1k. By contrast, pure Li electrode shows an uneven cross-section with numerous random holes and porous structures after 100 cycles (Figure S7c), leading to the formation of “dead” Li and Li exfoliation. The cross-section of the cycled Li-3D SCTs and Na3D SCTs exhibited an intact electrode shape with stable interface and undamaged 3D matrix as shown in Figure S7d-i. This ultra-stable electrode stability can be ascribed to the robust carbon fiber based supporting backbones, which minimized the volume change and stabilized the electrode dimension during continuous stripping and plating process.

4. CONCLUSIONS In summary, we demonstrate a general strategy to construct metallic Li or Na infiltrated 3D CTs via a scalable SnO2 deposition that offers excellent alkali metal infiltration. The Li/Na infiltrated 3D SCTs achieve more flat voltage profiles, lower stripping/plating overpotential and better cycling stability both in symmetric cell and full cell configurations even in additive-free carbonate-based electrolyte compared with pure Li/Na electrodes. The highly conductive carbon fiber based 3D scaffold exhibits a low interfacial resistance, reduced the effective current density and homogenized Li/Na ion flux, which enable uniformly nucleation and growth of metallic Li or Na during continuous stripping/plating process. Benefiting from the mechanically robust supporting backbones, the 3D SCTs provide a stable matrix against hyperactive molten alkali metal and afford the volume stability at the electrode level to stabilize the electrode dimension. Our design of ultra-stable hosts with excellent infiltration for both metallic Li and Na could be a


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universal strategy to address the multifaceted issues in LMBs and SMBs, which would further facilitate the practical applications of high-energy-density Li/Na battery systems.

ASSOCIATED CONTENT Supporting Information Supporting Information Available: FE-SEM images for the pure CTs; Photographs of the CTs, 3D SCTs, and Li/Na-3D SCTs; XRD patterns of the CTs, 3D SCTs before/after annealing, and Li-3D SCTs; HR-TEM images of the SnO2 nanocrystallines; EDX mapping of carbon and sodium elements in the Na-3D SCTs; long-term cycling performance of the pure Li symmetric cell; SEM images of different electrodes before and after cycling. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This project was supported by the National Natural Science Foundation of China (Grant No. 61404060, No. 51703080, and No. 21705057) and the Wuhan Youth Science and Technology Program (2015071704011602). We would also like to acknowledge the financial support from the National Postdoctoral Program for Innovation Talents (BX201700103).

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Highly Conductive and Robust 3D Host with Excellent Alkali Metal

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