Silver Nanoparticle-doped 3D Porous Carbon Nanofibers as

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Applications of Polymer, Composite, and Coating Materials

Silver Nanoparticle-doped 3D Porous Carbon Nanofibers as Separator Coating for Stable Lithium Metal Anodes Min Liu, Nanping Deng, Jingge Ju, Liyuan Wang, Gang Wang, Yali Ma, Weimin Kang, and Jing Yan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04122 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

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ACS Applied Materials & Interfaces

Silver Nanoparticle-doped 3D Porous Carbon Nanofibers as Separator Coating for Stable Lithium Metal Anodes Min Liua, Nanping Dengb, Jingge Jua, Liyuan Wanga, Gang Wangb, Yali Maa, Weimin Kang*,a, Jing Yan*,a a. State Key Laboratory of Separation Membranes and Membrane Processes/National Center for International Joint Research on Separation Membranes, School of Textile Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China. b. School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China.

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Keywords: Porous carbon nanofiber; Silver nanoparticle; Three-dimensional structure; Lithium dendrite; Electro-blown spinning.

Abstract The ever-increasing demand for electric devices and vehicles prompts the fast development of energy storage systems. Lithium metal is thought to be the most promising electrode for high performance batteries. However, the growth of lithium dendrites impedes the industrial production of lithium metal batteries. Herein, an effective approach is proposed by coating commercial separator with three-dimensional porous carbon fibers loaded with silver nanoparticles (Ag-PCNFs) which can be regarded as a subsidiary of electrode to improve the cycling performance of lithium metal batteries. The porous structure with high specific surface area endows the electrode with a high lithium loading capacity. The silver nanoparticles provide the electrode pro-Li property and excellent electrical conductivity, which are beneficial for the electrochemical reaction and reduce the local current density to attain a dendrite-free electrode. Electrochemical cycling performance of symmetric Li-Li batteries show that Ag-PCNFs coating can hinder dendrite growth and enhance the cycling stability, indicating that Ag-PCNFs acting as host materials can effectively guide the deposition of Li and solve the dendrite problem.


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1. Introduction Excellent energy storage systems are strongly needed for electric vehicles and portable electronics.1 Lithium (Li) metal batteries with high theoretical specific capacity are pointed to be the most promising battery system in the future. However, the formation of dendritic lithium during the cycling process leads to serious irreversibility, limited service life and short circuit problem, greatly hampering the commercial production of lithium metal batteries.2-3 To date, an enormous amount of research effort have been made to improve the cycling stability of Li metal anodes,4-5 including changing the electrolyte and additives,6-7 coating functional interface layer8-9 and constructing three-dimensional (3D) structural current collectors10. Among them, the 3D structural current collectors allow for relatively ordered deposition and stripping of Li at the intended location during charge/discharge process, rather than forming Li dendrites,10-14 can improve the deposition behavior and interface stability of the Li anode.15-16 Porous carbon nanofibers (PCNFs), regarded as 3D structural materials, having excellent mechanical property, structural integrity, excellent electrical conductivity and high specific surface area, can facilitate the conduction of Li ions and electrons.17 Xiang et al.18 demonstrated a hollow carbon nanofiber with appropriate internal and external radius ratios, which could control the deposition of Li by the drift effect of structural stress and allow Li to grow preferentially into internal pores. Electrospinning is one of the simple and effective techniques in producing nano/micro-fibers with 3D structures.19-20 Comparatively, electro-blown spinning (EBS) assisted by high-speed airflow is a novel rapid preparation method with high productivity.


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Common 3D structural host materials, like carbon materials, having bad compatibility with Li metal, exhibit nucleation overpotential during Li nucleation. Figure 1(a, c) demonstrate the working principle of the solid carbon nanofibers (SCNFs) and PCNFs before and after Li deposition. In the SCNFs, Li is only plated on the outer surface; while in PCNFs, Li is plated not only on the outer surface but also in the inner channels, both of which have deposited Li with granular structure. Recently, Cui and his colleagues developed selective Li deposition growing heterogeneously from metal seed.2 Using pro-lithium nanomaterials as seeds, Li preferentially forms nuclei on these seeds,21 followed by controlled deposition on the 3D structure and thus suppress the growth of Li dendrites.2, 16, 27 Yang et al.14 invented CNFs for electroplating silver nanoparticles (AgNPs), which served as seeds to guide the smooth growth of Li. As depicted in Figure 1(b), under the guidance of uniform AgNPs seeds, Li plates on the outer surface of the Ag-CNFs smoothly. Zhang et al.16 prepared a composite Li electrode based on a carbon fiber electroplated by silver layer. However, relative researches on AgNPs and PCNFs composites are rarely reported. To handle the dendrite growth in lithium metal batteries, coating Ag modified CNF on the lithium anode is the first thought. However, the surface coating for lithium anode requires critical conditions, like severe environment and exquisite operative technique. Inversely, coating Ag modified PCNFs on separator is a simple and effective way to inhibit the dendrite growth, which can play the similar effect to that coated on the lithium because both of them are steadily located between the separator and lithium electrode. In this work, we fabricate silver nanoparticle-doped porous carbon nanofibers (Ag-PCNFs) to be coated on the separators. And under the guidance of uniform pro-lithium AgNPs seeds, Li is smoothly plated on the outer surface and in the inner channels of the Ag-PCNFs as shown in Figure 1(d). But if there is no AgNPs seed (Figure


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1(c)), the deposition of Li on the PCNFs is uncontrolable.14,

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The AgNPs and PCNFs

composites are prepared through the electro-blown spinning and carbonization methods. In the case of 1 mA cm-2 and 1 mAh cm-2, the Li-Cu cells with Ag-PCNFs-coated polypropylene separator (Ag-PCNFs-PP) can maintain more than 100 cycles and the Coulombic efficiency is up to 90%. In 0.5 mA cm-2 and 0.5 mAh cm-2, the lithium metal anode with Ag-PCNFs exhibits excellent cycle stability, showing a low overpotential for 550 h in Li-Li cells. The capacity retention rate of the Li-S battery with Ag-PCNFs-PP after 600 cycles at 0.5 C is 77.73%. The controllability of the deposition of Li metal on the AgNPs-doped PCNFs finally form a Li electrode without Li dendrites.

Figure 1 The schematic diagrams of a) SCNFs, b) Ag-CNFs, c) PCNFs, and d) Ag-PCNFs before and after Li deposition. 2. Experimental Section 2.1 Fabrication of the silver nanoparticle-doped porous carbon nanofibers


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The silver-doped nanofibers are prepared using electro-blown spinning (EBS) technique and carbonization process. The EBS is operated at a solution extrusion rate of 20 mL h-1, a voltage of 42 kV, and a receiving distance of 1 m as shown in Figure 2(a). The as-spun solution is mixed with Polytetrafluoroethylene (60% PTFE emulsion, 25 g, porogen), Polyvinyl alcohol (PVA, 10 g, carbon source), Boric acid (BA, 30 uL, thickening agent), and AgNO3 (1.4 g) by mass fraction. The prepared fiber membrane is pre-oxidized in a muffle furnace at 240 °C for 1 h and then carbonized at 800 °C for 1 h under nitrogen, in which AgNO3 is pyrolyzed to silver nanoparticles and the silver nanoparticle-doped porous carbon nanofibers (Ag-PCNFs) are obtained. Figure 2(b) demonstrates the material transformation process for composite nanofiber membrane. The silver nanoparticle-doped carbon nanofibers (Ag-CNFs) without PTFE addition and porous carbon nanofibers (PCNFs) without AgNO3 addition are also prepared as contrast samples.

Figure 2 a) Electro-blown spinning process for fabrication of the as-spun composite nanofiber membrane; b) Schematic illustration for preparing Ag-PCNFs.


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Figure 3 a) The schematic illustration for preparing Ag-PCNFs-PP; b) The front digital images of Ag-PCNFs-PP, PP; c) The back digital images of Ag-PCNFs-PP, PP; d) The cross-sectional SEM image of Ag-PCNFs-PP; e) Configuration of a Li-Li (Li-S) cell with functional AgPCNFs-coated separator. 2.2 Electrochemical measurements For all cells, the electrochemical characterizations are performed using the assembled CR2340-type coin cells. As shown in Figure 3(a), 0.06 g Ag-PCNFs, 0.02 g PVDF and 1.6 g NMP are loaded onto the PP (Ag-PCNFs-PP) using a simple coating method. The coating is controlled to have an average thickness of 3 μm and an average loading of 0.643 g cm-2 shown in Figure 3(d). The important thing is that the Ag-PCNFs coating directly covers the Li foil well after assembling the Li-Li (Li-S) battery, which is beneficial for Ag-PCNFs to guide Li deposition. Ag-CNFs-PP and PCNFs-PP are prepared via the same route as Ag-PCNFs-PP. 3. Results and discussion 3.1 Morphological structures


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The morphological structures of the obtained porous PCNFs with 2 wt%, 4 wt% and 6 wt% AgNO3 are shown in Figure 4(a-c). It can be seen in Figure 4(a) that the porous carbon nanofibers have a 3D porous structure, which is beneficial to regulate the volumetric expansion of Li metal and transmission of electrons inside the cells.22-23 As presented in Figure 4(b), quite fine Ag particles are observed and measured in the PCNFs, and the particle diameter is below 100 nm. The morphological structures of the PCNFs of adding 6 wt% AgNO3 is shown in Figure 4(c). The diameter of AgNPs with 100 nm is larger and more heterogeneous when compared with Figure 4(b). With the increase of silver nitrate content, the Ag particles grow larger due to aggregation with little binding with the PCNFs.14 4 wt% AgNO3 is chosen as the optimal proportion because the size of the obtained AgNPs is moderate and a certain scale is maintained with the carbon fiber, so that the adhesion between the carbon fiber and the AgNPs is strong enough.

Figure 4 SEM images of PCNFs added with a) 2 wt%, b) 4 wt% and c) 6 wt% AgNO3; SEM images of the as-spun composite nanofiber membrane of d1) PCNFs, e1) Ag-PCNFs (with 4 wt% AgNO3) and f1) Ag-CNFs (with 4 wt% AgNO3); SEM images of d2) PCNFs, e2) Ag-PCNFs and f2) Ag-CNFs with higher magnification.


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Figure 4(d1-f1) display SEM images of the primary fiber of PCNFs, Ag-PCNFs and AgCNFs. Among them, the primary fiber of Ag-PCNFs has the smallest fiber diameter under the same experimental conditions. This may be because the addition of silver nitrate in the AgPCNFs solution increases the conductivity of the solution, resulting in the spinning solution subjected to greater draft force in the electric field, thus forming finer fibers. The primary fiber of Ag-CNFs is uneven in diameter because there is no PTFE to improve the solution viscosity. A small fiber diameter of Ag-PCNFs indicates a larger surface area of the fiber, which may increase the degree of graphitization in the carbonization, thereby obtaining higher conductivity, which is advantageous for suppressing the formation of Li dendrites. As shown in Figure 4(d2), the PCNFs have many holes after carbonization of as-spun fibers. The Ag-PCNFs have not only a large number of pore structures which is more capable of supporting Li, but also a good deal of nanoparticles which may be the elemental silver particles in Figure 4(e2). But Ag-CNFs have no obvious particles on the surface in Figure 4(f2).

Figure 5 The SEM a) and TEM b) images of Ag-PCNFs (with 4 wt% AgNO3); c1) SEM image of Ag-PCNFs and c2) The corresponding EDX elemental mapping image of Ag. Figure 5(a-b) show the morphological structures of the PCNFs with 4 wt% AgNO3. The SEM and TEM image reveal that the PCNFs are porous and the AgNPs grow on and in the


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PCNFs. The AgNPs with uniform size are in distribution on porous carbon nanofibers. This makes it easier for Li to find growth sites during deposition and deposits them on the porous carbon skeleton in an orderly manner.14,

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In this process, the AgNPs can be uniformly

synthesized on and in PCNFs by the in situ synthesis method. During the carbonization process, the high temperature enables a strong bond between AgNPs and the PCNFs. The AgNPs in PCNFs are uniformly distributed on the surface, and also distributed inside the PCNFs. In order to investigate the distribution of AgNPs in the PCNFs, the chemical element mapping of AgPCNFs are characterized by energy dispersive x-ray (EDX) spectroscopy and the results are shown in Figure 5(c2). The Ag elements are uniform distributed in the Ag-PCNFs, which demonstrates the homogeneous distribution of silver particles. 3.2 Spectroscopic Analysis The Fourier transform infrared (FTIR) spectrum is used to prove the existence of AgNPs in the PCNFs. The characteristic peaks of Ag-PCNFs in Figure 6(a) are at 3432 (-OH=NH), 28402815 (-CH), 1600 (-C=O) and 1400-1420 (-CN) cm-1.24 The strong peak at 1400-1420 cm-1 corresponds to a dipole moment change in -CN due to the combination of AgNPs with electronegative nitrogen.25 However, the spectrum of PCNFs does not have these characteristic peaks because the absence of AgNPs. The X-ray photoelectron spectroscopy (XPS) is carried out to evaluate the chemical composition and valence state analysis for Ag-PCNFs. As presented in Figure 6(b), two individual peaks at about 368.19 eV and 364.2 eV could be assigned to Ag3d5/2 and Ag3d3/2, which are the characteristic of metal Ag0 and Ag+, respectively.26 The existence of silver ion should be due to the oxidation of AgNPs exposed to the air.27 The XPS analysis further confirme that PCNFs have AgNPs growing on them. The crystal phase of Ag-PCNFs is characterized by XRD testing. As presented in Figure 6(c), the XRD shows a peak at 38.1°,


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which cloud be assigned to the (111), (200) diffraction plane of AgNPs. The intensity of AgPCNFs at (111), (200), (220) and (311) can be clearly seen.26 According to the JCPS cards No. 04-0683, the reflections corresponding to the peak of (111), (200), and (311) are face-centered cubic silver crystals. The XRD of PCNFs obviously does not have these characteristic peaks. The large silver elements in Ag-PCNFs composites are mainly composed of metallic silver. Combined with the previous electron microscope images, silver nanoparticle-doped 3D porous carbon nanofibers are prepared successfully.

Figure 6 a) FTIR spectra of PCNFs and Ag-PCNFs with 4 wt% AgNO3; b) XPS spectra of AgPCNFs with 4 wt% AgNO3; c) XRD patterns of PCNFs and Ag-PCNFs with 4 wt% AgNO3. 3.3 Physical Properties Excellent electrical conductivity and large specific surface area can lower the local current density, leading to low interfacial resistance. Table 1 shows electrical conductivity and specific surface area of PCNFs, Ag-PCNFs and Ag-CNFs. The Ag-CNFs have the lowest conductivity of 1250 S m-1 because their nonporous structure affects the degree of graphitization during the carbonization process. The PCNFs has a good conductivity, which can reach to 2242 S m-1. The


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conductivity of Ag-PCNFs nearly doubles to 4115 S m-1, which can greatly accelerate the electron transport of the entire composite lithium metal electrode. Large specific surface area of Ag-PCNFs provides a spatial location for the deposition of Li, and the growth of Li dendrites will be suppressed under the guidance of AgNPs seeds.18 In Table 1 and Figure S1, the surface area of Ag-CNFs, PCNFs and Ag-PCNFs can reach 13.52, 642.02 and 596.43 m2 g-1, respectively.28 Large surface area of Ag-PCNFs offer a lot of space for lithium depositing, and excellent electrical conductivity can improve the electron transport performance of lithium electrodes, which is one of the necessary factors for excellent electrochemical performance. Table 1 Electrical conductivity and BET surface area of PCNFs, Ag-PCNFs and Ag-CNFs

Sample

Electrical conductivity (S m-1) BET surface area (m2 g-1)

PCNFs

2242

642.02

Ag-PCNFs

4115

596.43

Ag-CNFs

1250

13.52

3.4 Electrochemical Cycling Performance To investigate the electrochemical performance, PP, Ag-CNFs-PP, PCNFs-PP and AgPCNFs-PP are assembled into a Li-Cu cells, and the Cu foil and the Li foil are working electrode and reference electrode, respectively. Figure 7(a) shows Coulombic efficiencies of the Li-Cu cells at 1 mA cm-2 and 1 mAh cm-2. The results provide the perspective on the generation of Li dendrites or "dead Li" during the cycle. After 37 cycles of charge and discharge, the Li-Cu cells with PP exhibit an unstable process, while the Li-Cu cells with Ag-PCNFs-PP remain stable. The internal structural damage of the battery with the PP separator during the charge and discharge cycles result in a decrease in Coulombic efficiency to 0% after 37 cycles. The cell with Ag-


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PCNFs-PP has maintained a Coulombic efficiency of about 90% after 100 cycles. The Li-Cu cell with Ag-CNFs-PP and PCNFs-PP is similar to Li-Cu cell with PP. In Figure S5, the Li-Cu cells with Ag-PCNFs-PP at 2 mA cm-2 and 2 mAh cm-2 are very stable and the Coulomb efficiency is maintained at around 90% after 100 cycles. In summary, the electrochemical cycle performance of Li-Cu batteries with Ag-PCNFs-PP has been greatly improved. The 3D pores make AgPCNFs have ample internal space and good Li loading capacity.18 All of these indicate that AgPCNFs can act as host materials to guide the deposition of Li, which largely solves the dendrite problem.

Figure 7 a) Coulombic efficiencies of the Li-Cu cells at 1mA cm-2 and 1 mAh cm-2; EIS of the Li-Li cells b) before cycling c) after 1 cycle d) after 10 cycles at 0.5 mA cm-2 and 0.5 mAh cm-2; Voltage profile of the Li-Li cells with separator of e) PP, f) Ag-CNFs-PP, g) PCNFs-PP and AgPCNFs-PP at 0.5 mA cm-2 and 0.5 mAh cm-2.


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The electrochemical impedance spectroscopy (EIS) tests are carried out to explore the charge-transfer kinetics of the four modified batteries. As presented in Figure 7(b-d), the interface impedance and stability of Li-Li batteries are evaluated by the EIS under the condition of current density 0.5 mA cm-2 with a total capacity of 0.5 mAh cm-2. The internal resistance of the four batteries after cycling process is obtained by testing the Nyquist plot of the symmetric battery (Fig. 7(b-d)). The semicircle in the high frequency region corresponds to the interface resistance of the electrolyte and the lithium electrode. Before the start of the cycle, the cells with the PP, Ag-CNFs-PP and PCNFs-PP have relatively large interface resistance, which may be because a passivation film is grown on the lithium metal electrode. After the first cycle and 10 cycles, the battery interface resistances decrease.29 This phenomenon is due to the cracking of the passivation film during the formation of lithium dendrites and the increase in the contact surface area of the lithium electrode reaction.30-31 In contrast, the cells with Ag-PCNFs-PP show lower interfacial impedance before cycling, after 1 cycle, and after 10 cycles. The stable interface is derived from the uniform disseminated nucleation and reversible dissolution of Li on Ag-PCNFs, which also makes it have excellent cycle performance.32 Ag-PCNFs with excellent electrical conductivity and high specific surface area are favorable for electrochemical reaction kinetics and current density near a uniform anode, resulting in low interface resistance. A constant current charge and discharge cycles of Li-Li cells is utilized to characterize the stability of deposition and strip metal lithium of the cells with four coated-separators. As presented in Figure 7(e-g), it is a voltage-time diagram of a Li-Li battery at 0.5 mA cm-2 and 0.5 mAh cm-2. The deposition behavior of Li has an important influence on its electrochemical performance. When cells assembled with PP, Li will form Li columns and dendrites, resulting in “dead Li”. The voltage profile is verified by using a Li-Li battery with PP at 0.5 mA cm-2 and 0.5


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mAh cm-2. After 300 h, an overpotential behavior clearly occurs, indicating a large amount of Li dendrites on the surface of the lithium electrode. In the Li-Li cells using bare lithium metal as the electrode, it can be seen that the voltage gradually increases with the increase of the cycle time, which means that the voltage deviation during the lithium deposition/stripping process becomes severe. A similar phenomenon is found in the cells with Ag-CNFs (400 h) and PCNFs (450 h). Compared with PP, the Li-Li batteries with Ag-PCNFs have a tiny voltage polarization, which prove that Ag-PCNFs have the function of alleviating the growth of Li dendrites.14, 16 Li metal can be nucleated and controlled to be deposited on Ag-PCNFs by increasing the seed sites of AgNPs on PCNFs. The lithium metal anode with Ag-PCNFs-PP has a lower potential polarization at 0.5 mA cm-2 for 550 h, showing excellent stability. In Figure S2, the Li-Li cells with the Ag-PCNFs-PP can be operated at a high current density of 2 mA cm-2 for 1500 h, and the voltage polarization is not significantly increased.16, 31, 33 The 3D porous structure allows the anode with Ag-PCNFs to have a rich inner hollow space and a high Li loading capacity.18 Under the guidance of uniform AgNPs seeds, Li is uniformly deposited in the outer and inner channels of PCNFs.14 The above results show that the separator coating improves the cycle performance of the battery. At the same time, Ag-PCNFs make the battery performance excellent, and the performance of all aspects has been improved.


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Figure 8 The EIS a) before cycle b) after 5 cycles of the Li-S cells; c) Discharge capacity and Coulombic efficiency of the Li-S cells at 1 C; d) The rate performance of the Li-S cells; e) Discharge capacity and Coulombic efficiency at 0.5 C of the Li-S cells with Ag-PCNFs. The performance of lithium-sulfur (Li-S) batteries is tested to investigate the function of separator coating on the full-cell battery performance. The interface impedances of Li-S batteries are studied using the EIS. Figure 8(a-b) show the Nyquist plots of the cells with PP, Ag-CNFsPP, PCNFs-PP, Ag-PCNFs-PP before cycle and after 5 cycles. The Nyquist plot is semi-circular at high frequencies and oblique at low frequencies, corresponding to oblique lines of ion diffusion.34 The cells with PP, Ag-CNFs-PP, and PCNFs-PP present larger semicircular diameters than that of the cells with Ag-PCNFs-PP, which illustrates that the cells with PP, AgCNFs-PP, and PCNFs-PP show a larger charge transfer resistance than the cells with AgPCNFs.35 The PCNFs form a highly conductive skeleton on the surface of the lithium electrode,


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which is beneficial for improving the electrochemical performance of the Li-S battery. The diagonal line in the low frequency region indicates that it is easy for lithium ions to diffuse in the battery. From Figure 8(a), the Warburg impedance of the cells with Ag-PCNFs-PP is smaller when comparing with the cells with PP, Ag-CNFs-PP, and PCNFs-PP, which will reduce the difficulty of transporting lithium ions. The improvement in ion transport is due to the highly conductive network structure of Ag-PCNFs. After 5 cycles, the EIS of these cells change, and two similar semicircles appeare as shown in Figure 8(b). The new semicircle in the high frequency region appears because Li2S and Li2S2 are formed during the electrochemical cycle. It is obvious that after charging and discharging, the interface impedance of these cells is correspondingly improved compared with the corresponding fresh batteries.16,

36-39

Lithium

spontaneously aligns to a more suitable position during the cycling process, which can improve the interface impedance of the cell. This phenomenon indicates that Li and Ag-PCNFs are more closely bonded together after a period of cycling.40 This indicates that it has excellent reversible electrochemical properties and excellent electrical conductivity for Ag-PCNFs in the Li-S cells.41 Figure 8 (c) shows the charge and discharge cycle performance of four batteries at 1 C rate. The specific discharge capacity of the Li-S battery with Ag-PCNFs-PP is more stable and higher than those with PP, Ag-CNFs-PP and PCNFs-PP. And it shows that the cells with PP, Ag-CNFsPP, PCNFs-PP and Ag-PCNFs-PP deliver an initial specific discharge capacity of 551.3, 650.2, 682.2 and 685.8 mAh g-1 and the specific capacity can still retain at 370.7, 463.5, 474.3 and 748.2 mAh g-1 after 200 cycles, respectively. These results present that the discharge capacity of Ag-PCNFs is extremely higher and more stable. However, it must be pointed out that the discharge specific capacity of the battery with Ag-PCNFs-PP first decreases and then increases. This is because a part of the electrode material is not activated at the beginning due to


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insufficient electrolyte wetting, and the active material is fully utilized after the activation is completed, particularly at a high C rate of the initial cycle in the Li-S battery. Figure S3(a-d) shows the constant current charge and discharge curves of four batteries at 1 C rate. The Li metal anode with Ag-PCNFs has the smallest displacement change at the 1th, 100th and 200th, which indicates a good capacity retention ratio compared with PP, Ag-CNFs-PP and PCNFs-PP. In Figure 8(d), the discharge capacity of the cells with Ag-PCNFs can reach 806.6, 1072, 807.8, 860.4, 821.1, 1022 and 1086.4 mAh g-1 with the increasing current density of 0.1, 0.2, 0.5, 1, 0.5, 0.2 and 0.1 C, while those with PCNFs are only 1341, 1085, 862.2, 735.4, 775.6, 874.4, and 854 mAh g-1, respectively. Compared with the PCNFs, the batteries with PP and Ag-CNFs-PP have lower discharge capacity. The Li-S cell with Ag-PCNFs at 0.5 C is presented in Figure 8(e). It can be obtained that the initial discharge specific capacity of the battery with Ag-PCNFs-PP can reach 963.8 mAh g-1. After 600 cycles, the discharge specific capacity can reach 749.2 mAh g-1 with a Coulombic efficiency of about 96.82 wt%, achieving a capacity retention rate of 77.73%. Figure S3(e) is the constant current charge and discharge curves of the Li-S battery with AgPCNFs-PP at 0.5 C. In the positions of 1th 100th, 200th, 400th and 600th, the charge and discharge curves of the Li-S battery with Ag-PCNFs-PP have the smallest change, indicating that the lithium metal anode using Ag-PCNFs has good cycle retention. Figure S4 shows the discharge capacity and Coulombic efficiency of the Li-S battery (sulfur loading = 1.62 mg cm-2) with AgPCNFs-PP at 0.5 C. The initial discharge capacity is 953 mAh g-1, and the discharge specific capacity is maintained at 788 mAh g-1 after 300 cycles, achieving a capacity retention rate of 82.68%. This electrochemical cycling performance difference is caused by the uneven peeling of Li in the ultra-thin Li foil with limited capacity. As the cycle period increases, a high proportion of


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"dead Li" increases, and the activated Li decreases.22, 24, 42-43 The 3D porous structure of PCNFs has a large amount of internal space, allowing a large amount of lithium to deposit and grow inside.18 Under the guidance of AgNPs seeds, Li firstly forms lithium seeds, which are finally deposited smoothly inside and outside of the PCNFs,14 thus avoiding the phenomenon of battery polarization.44-45 3.5 Morphological Evolution

Figure 9 Surface SEM images of Li-metal electrode with a) PP b) Ag-CNFs-PP c) PCNFs-PP and d) Ag-PCNFs-PP; The electrodes are obtained from the Li-Li cells cycled for 400 h at 0.5 mA cm-2 and 0.5 mAh cm-2; e-g) Homogeneous Li on and in Ag-PCNFs from the Li-Li cells for 5 cycles at 0.5 mA cm−2 and 0.5 mAh cm-2; h) Schematic illustration of the strategy to deposit Li metal uniformly on and in Ag-PCNFs. In order to observe the deposition of lithium metal after the cycle, the deposited electrode after cycling is observed by SEM. Observation of the lithium anode after cycling process can directly verify the protection of the lithium anode by surface coating. Figure 9(a-d) show the


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surface morphological structure comparisons of Li-metal electrode with PP, Ag-CNFs-PP, PCNFs-PP and Ag-PCNFs-PP. The Li-Li cells are cycled at 0.5 mA cm-2 and 0.5 mAh cm-2 for 400 h. Figure 9(a) shows that the surface of lithium metal with PP becomes rough and there are a large number of random loose pores, in which fine Li rods are visible. The Li ingot eventually turns into Li dendrite and penetrates the membrane, causing a short circuit. The surface of Li anode with Ag-CNFs-PP becomes rough and the Li turns to be lumps and granules as shown in Figure 9(b). Compared with the bare Li anode with PP, the Li electrode has a dense surface and a small Li rod. The surface of Li anode with PCNFs-PP becomes rough and the Li turns to be granules and local flakes in Figure 9(c). The surface of the Li electrode is dense and flat compared to Li-metal electrode with Ag-CNFs-PP. The overall morphology is that the particles interphase with smooth Li sheets. By contrast, in Figure 9(d) Li-metal electrode with AgPCNFs-PP is more uniform and smooth. The electrode surface is dense and smooth in general with few hole and dendrite structures, and Li-metal presents a rugged, smooth fold surface, possibly due to Ag-PCNFs induced Li growth in space which is still better compared to others (Figure 9(a-c)).18, 43 All results indicate that Ag-PCNFs inhibit Li dendrites as shown in Figure 9(d). In order to visually observe the deposition of lithium on Ag-PCNFs, the morphology of metallic lithium grown on Ag-PCNFs is observed by SEM images. Figure 9(e-g) show the SEM images of Li-metal electrode with Ag-PCNFs-PP after 5 cycles at 0.5 mA cm-2 for 0.5 mAh cm-2. It is directly observed that Li metals are uniformly and smoothly plated uniformly on and in AgPCNFs. The diameter of Li-filled Ag-PCNFs is larger than that of Ag-PCNFs before Li deposition. As the cycle increase, the deposited lithium metal occupy various sites of Ag-PCNFs, and finally becomes a Li metal anode without lithium dendrites.16, 46-48 As depicted in Figure


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9(h), under the guidance of uniform AgNPs seeds, Li is smoothly plated on the outer surface and in the inner channels.49 4. Conclusions In this work, the AgNPs and PCNFs composites are prepared through the electro-blown spinning and carbonization methods. A functional Ag-PCNFs layer-coated separator is prepared by a cheap and simple slurry-coating method for Li metal batteries. The high specific surface area of the 3D porous structure endowed Ag-PCNFs with many locations for storing lithium. AgNPs are synthesized in situ as a lithium metal growth site, and the Li with free dendrites are deposited on the 3D substrate. The Coulombic efficiency is maintained at 90% after 100 cycles of Li-Cu cells with Ag-PCNFs-PP at 1 mA cm-2 and 1 mA cm-2. With a current density of 2 mA cm-2 and a capacity of 2 mAh cm-2, the Li metal anode with Ag-PCNFs-PP exhibits excellent stability with no overpotential over 1500 h. The lithium metal electrode with Ag-PCNFs-PP shows excellent performance in a Li-S battery. The initial discharge capacity of the Li-S battery with Ag-PCNFs-PP is 963.9 mAh cm-2 at 0.5 C, and the capacity is 749.2 mAh cm-2 after 600 cycles, achieving the capacity retention rate of 77.73%. The Li-S battery with Ag-PCNFs-PP at 1 C exhibits excellent specific capacity and stability. The lithium electrodes with Ag-PCNFs-PP have excellent electrochemical stability without lithium dendrite and short circuit problems. It is worth mentioned that the electrode with Ag-PCNFs provides a new idea for dendritic Li metal anodes in high performance rechargeable batteries.


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Associated Content Supporting

Information:

Experimental

preparation

methods,

characterization

and

electrochemical measurements; The pristine adsorption-desorption curves for a) PCNFs, b) AgPCNFs and c) Ag-CNFs; Voltage profile of the Li-Li cells with separator of Ag-PCNFs-PP at 2 mA cm−2 and 2 mAh cm-2; The charge-discharge curves of a) PP, b) Ag-CNFs-PP, c) PCNFs-PP and d) Ag-PCNFs-PP at 1 C. e) The charge-discharge curves of Ag-PCNFs-PP at 0.5 C; Discharge capacity and Coulombic efficiency at 0.5 C of the Li-S cells with Ag-PCNFs with 1.62 mg cm-2 sulfur loading; Coulombic efficiencies of the Li-Cu cells with Ag-PCNFs-PP at 2 mA cm-2 and 2 mAh cm-2. Corresponding Author *Corresponding author. E-mail addresses: [email protected] (W. Kang), [email protected] (J. Yan). Notes The authors declare no competing financial interest. Acknowledgements This study was supported by the National Natural Science Foundation of China (51673148, 51678411), the Science, Technology Plans of Tianjin (17PTSYJC00040, 18PTSYJC00180) and the Natural Science Foundation of Tianjin City (17JCYBJC41700), Scientific Research Project of Tianjin Municipal Education Commission (2017KJ067) and Tianjin Scientific Research Foundation for the Returned Overseas Chinese Scholars (2018003).


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Table Of Contents (TOC) graphic


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Silver Nanoparticle-doped 3D Porous Carbon Nanofibers as

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