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Self-Supporting Hybrid Fiber Mats of Cu3P-Co2P/N-C Endowed with Enhanced Lithium/Sodium Ions Storage Performances Jing Li, Xuefeng Li, Ping Liu, Xingqun Zhu, Rai Nauman Ali, Hina Naz, Yan Yu, and Bin Xiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22367 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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ACS Applied Materials & Interfaces

Self-Supporting Hybrid Fiber Mats of Cu3P-Co2P/N-C Endowed with Enhanced Lithium/Sodium Ions Storage Performances

Jing Li, Xuefeng Li, Ping Liu, Xingqun Zhu, Rai Nauman Ali, Hina Naz, Yan Yu, Bin Xiang*

Hefei National Research Center for Physical Sciences at the Microscale, Department of Materials Science & Engineering, CAS Key Lab of Materials for Energy Conversion, University of Science and Technology of China, Hefei, Anhui 230026, China

*Corresponding author: [email protected]

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ABSTRACT Cu3P has been targeted as alternative anode material for alkali-metal ions batteries recently because of their safety potential and high volumetric capacity. However, designing high rate Cu3P electrode with long durability is still facing huge challenges. Here, we report a self-supporting three-dimensional composite of Cu3P and Co2P interconnected by Ndoped C fibers (Cu3P-Co2P/N-C). The advanced 3D structure not only provides a fast reaction kinetics but also improves the structure stability, leading to excellent rate capability and long-term cycling stability, and pseudo-capacitance behavior is also beneficial to the high rate performance as well. Additionally, the synergistic effects between Cu3P, Co2P and N-doped carbon can increase electrical conductivity and active sites, ensuring more ions storage. The Cu3P-Co2P/N-C anode for lithium ion batteries delivers high discharge capacity, superior rate performance, as well as ultra-long lifespan over 2000 cycles accompanied by a stable capacity around 316.9 mAh/g at 5 A/g. When the 3D structured-material works in sodium ion batteries, it also displays improved electrochemical performance. Our method opens a new sight to design advanced metal phosphides anodes for energy storage devices.

KEYWORDS: self-supporting, Cu3P, N-doped C, energy storage devices, high rate performance


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1. INTRODUCTION Considering the shortage of energy and environmental issues, the clean and costeffective energy storage devices are widely studied. Alkali-metal ions batteries, especially lithium-ion batteries (LIBs) and sodium-ion batteries (NIBs) have aroused broad interest for their high energy density, longevity and environment friendly characteristics.1-5 However, the booming electric vehicles and large-scale storage systems propose more urgent demand to develop high efficient batteries with high power and energy density.6-9 It becomes important to develop alternative anode materials for LIBs/NIBs to fulfill above demands. Recently, metal phosphides have been regarded as one class of potential anodes because of their low lithiation potential, high specific capacity and safety.10 Moreover, the energy storage properties of metal phosphides including Sn4P3, MoP, FeP, Ni2P and so on have been extensively investigated.11-14 Among these conversion-type metal phosphides, Cu3P exhibits an excellent volumetric capacity of 3020 mAh/cm-3 (far in excess of that in commercial graphite), and arouses much concern as anode for LIBs.15,

16

Additionally, the

superior sodium storage performance of Cu3P has been reported, revealing its great potential for alkali-metal ion batteries.17 However, the large volume expansion induced from lithiation/sodiation process and the inferior electrical conductivity are still the major impediments to the application of Cu3P anode. To tackle these problems, the hybrid materials composed of metal phosphides and carbonaceous materials have been fabricated. Owing to their high conductivity, stability and mechanical flexibility merits, the applications of carbonaceous materials not only increase the conductivity but also provide kinetic favorability for fast ions diffusion, as well as buffer the aggregation and volume expansion, leading to enhanced cycling capability and rate performance.18 Thus, the reported Cu3P@C and Cu3P/rGO have shown great improvement in electrochemical performance.15, 19 However, as far as we know, the



reported hybrid materials usually exhibit 2D nanostructure and less attention has been given on the self-supporting three-dimensional (3D) nanocomposites containing N-doped carbon and Cu3P. In fact, the N-doped carbon matrix plays a vital role in increasing the conductivity and increasing ions storage sites.20, 21 Moreover, the 3D structured materials can act as a binder free electrode in LIBs/NIBs and provide more ions diffusion pathways and shorter diffusion lengths, enhanced structure stability during cycling.17 Apart from the application of carbon, designing integrated materials containing different metal phosphides has been confirmed to be an effective way to improve the electrochemical performance owing to their synergistic effect.13, 22 Herein, the flexible 3D hybrid fiber mats composed of Cu3P, Co2P and N-doped C (Cu3P-Co2P/N-C) are creatively fabricated via electrospinning and phosphorization process. Owing to its superior flexibility, our as-synthesized sample can work as the binderfree electrode without additives and collector, reducing the mass of battery and the loss of active materials. Additionally, the synergistic effects of different components and particular structure provide fast reaction kinetics and enhance the structure integrity during cycling. Consequently, the Cu3P-Co2P/N-C shows excellent electrochemical performance in LIBs, containing larger stable capacity at 0.1 A/g (780 mAh/g after 200 cycles), ultralong life span and superior cycling stability (316.9 mAh/g over 2000 cycles retaining at 5 A/g). As well, the enhanced properties of sodium ions storage are obtained in the Cu3PCo2P/N-C anode, which also highlights the great potential applications for advanced alkalimetal ions batteries.


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2. EXPERIMENTAL SECTION Synthesize of Cu3P-Co2P/N-C composites: The Cu3P-Co2P/N-C composites were synthesized via simple electrospinning method coupled with a phosphorization reaction process. Initially, dissolving 0.5 g polyacrylonitrile (PAN) into N, N- Dimethylformamide (DMF, 10 mL) solution via 12 h stirring. Then, 0.8 g Cu(oAc)2·H2O and 0.2 g Co(oAc)2·6H2O were poured into the solution, and finally the blue solution was obtained after 12 h constantly stirring. Next, a syringe equipped with a 21G stainless steel needle was used to draw into the precursor solution, then it was driven by a pump. The Al foil collector was placed in front of the needle (15 cm) and the applied voltage is 15 kV. After electrospinning, the Cu-Co/PAN fibers were dried in the oven overnight at 80 ℃. In order to stabilize and carbonize the fibers, the precursor was annealed at 600 ℃ for 1 h under Ar atmosphere. Subsequently, two crucibles loaded with the pretreatment fibers and NaH2PO2 with a weight ratio of 1:10 were placed in the furnace separately, while the NaH2PO2 was placed at the upstream. Then a phosphorization reaction took place when the samples were heated at 300 ℃ for 1 h in flowing Ar and finally the flexible Cu3P-Co2P/N-C hybrid nanofibers were obtained. Synthesis of Cu3P/N-C, hollow Cu3P-Co2P and Cu3P-Co2P/C nanofibers: The preparation procedures of Cu3P/N-C nanofibers were similar like above except that there is no Co(oAc)2·6H2O in the precursor solution. For comparison, the above Cu-Co/PAN fibers were annealed under air atmosphere at 600 ℃ for 1 h and then phosphorization via the same steps like above, which resulted in the hollow Cu3P-Co2P nanofibers. Furthermore, another carbon source as poly(vinyl pyrrolidone) (PVP) was also used to synthesis the Cu3P-Co2P/C nanofibers without N doped and the synthesis process was similar like Cu3PCo2P/N-C. Characterization: To investigate the chemical compositions and morphologies, the data was collected from MXPAHF X-ray diffractometer (XRD), JSM-6700F scanning



electron microscopy (SEM), JEM-2010 transition electron microscopy (TEM) and ESCALab 250 X-ray photoelectron spectrometer (XPS), respectively. Electrochemical measurements: The flexible Cu3P-Co2P/N-C and Cu3P/N-C composites were used as binder-free electrode materials and assembled into the coin cells (CR2032) in an argon-filled glove box (oxygen and moisture < 0.1 ppm), while the Celgard 2400 polypropylene membrane and Li plate served as the separator and counter electrode respectively. Besides, 1 M LiPF6 dissolved in ethylene carbonate (EC) + diethyl carbonate (DEC) (v/v: 1/1) mixed solvent acted as the electrolyte. On the other hand, the hollow Cu3P-Co2P was mixed with the acetylene black and PVDF with a mass ratio of 7:2:1 to form uniform slurry. Then, the mixed slurry was spread onto copper foil and worked as anode. The sodium ion batteries were assembled with similar procedures, except the counter electrode and separator were Na metal and glass fiber (Whatman GF/D), respectively. And 1 M NaPF6 dissolved in EC/DEC hybrid solvent (v: v = 1: 1) was used as electrolyte. The packed cells were tested on the CT2001A Land system and the CHI660E electrochemical working station to characterize their electrochemical performance.


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3. RESULTS AND DISCUSSION The flexible Cu3P-Co2P/N-C (denoted as CCP/C) nanofibers were fabricated with two steps as demonstrated in experimental sections. Firstly, the Cu-Co/PAN fibers with an uniform diameter of 200 nm were shown in Figure 1a. Subsequently, the CCP/C was generated after calcination and phosphorization in Ar atmosphere. During this process, the PAN transformed into the flexible N-doped carbon mats and the metal precursor migrated outward resulting from the Kirkendall effect, yielding numerous metal phosphide particles on the surface of the fibers (Figure 1b).21, 23 The resulted CCP/C exhibits the flexible threedimensional cross-linked networks which could provide efficient pathways for lithium ions and stable structure during cycling. For comparison, the precursor fibers were annealed in air firstly, then followed with phosphorization in Ar environment. The obtained Cu3P-Co2P (CCP) fibers exhibited typical hollow tubular structure as illustrated in Figure S1b. The removing of the PAN, and the outward migration of metal precursors caused by the Kirkendall effect could be responsible for the hollow structure. In addition, the SEM image of Cu3P/N-C (CP/C) fibers is shown in Figure S1a, which portrays the similar fiber-like structure like CCP/C. The TEM image of the CCP/C single fiber was captured to investigate its microstructure. As illustrated in Figure 1c, the sample displays a typical nanowire structure with a uniform diameter around 200 nm with numerous particles observed on the surface. The particle size of metal phosphides can be obtained by measuring these nanoparticles from the SEM and TEM images. The measured particle sizes are around 30-60 nm, indicating that the particle sizes of these metal phosphides are 30-60 nm. However, the precursor solution containing Cu and Co source for electrospinning must be mixed uniformly owing to the characterization of electrospinning method. Thus, the particles of Cu3P and Co2P on the surface of the fibers are mixed uniformly, which is difficult to distinguish these two different metal phosphides. But considering the uniform mixing of the two metal phosphides, we think that the particle size of Cu3P and Co2P is



similar, which is around 30-60 nm. The high resolution TEM (HRTEM) image shows lattice fringes of the (112) plane for Cu3P and (103) plane for Co2P with corresponding lattice distances of 0.25 nm and 0.20 nm, respectively (Figure 1d). Most importantly, the overlap of the two lattice fringes were observed, confirming the existence of the synergistic effect between the Cu3P and Co2P, which can improve the lithium storage property of CCP/C. The elemental mapping illustrated in Figure 2 confirms the well-dispersed of Cu, Co, C, N and P elements in the CCP/C. The weight ratio of Cu: Co obtained from the energy dispersive X-ray (EDS) spectrum is around 4:1 (Figure S2). To clarify the content of Cu3P, Co2P and N-doped carbon, the thermogravimetric analysis (TGA) of the CCP/C and CP/C after heating to 800 ℃ under air atmosphere were tested. As shown in Figure S3a, 49.2wt.% residues in CP/C is obtained after oxidizing Cu3P and the final product is CuO. During the heating process, the Cu3P and Co2P in CCP/C could be oxidized to form CuO and Co3O4, respectively. Thus, the remaining products is 61.4wt.% of the initial CCP/C as shown in Figure S3b. According to the weight content of residues, it can be calculated that the CuO and Co3O4 contents after oxidizing CCP/C is 41.6% and 19.8%. Then, the mass ratio of the two metal phosphides can be obtained from the following formula: ω (CuO) 𝑀 (𝐶𝑢𝑂)

=

𝜔 (𝐶𝑢3𝑃) 𝑀 (𝐶𝑢3𝑃)

𝜔 (𝐶𝑜3𝑂4)

𝑀 (𝐶𝑜3𝑂4) =

𝜔 (𝐶𝑜2𝑃) 𝑀 (𝐶𝑜2𝑃)

Finally, the calculated mass ratios of Cu3P, Co2P, N-doped C are 38.69%, 19.67% and 41.64%, respectively. The elemental analysis verifies that the carbon and nitrogen contents are 33.23 wt.% and 2.35 wt.%, respectively (Table S1). The chemical compositions were further elucidated by XRD and XPS characterizations. As shown in Figure 3a, the XRD pattern of the CCP/C displays the representative peaks of Cu3P (PDF# 71-2261) and Co2P (PDF# 89-3030) without any second phase. The XPS full spectrum of CCP/C (Figure S4) demonstrates the existence of the Cu, Co, P, C and N, which is consistent with the EDS result. The detailed peaks of Cu


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2p (Figure 3b) exhibits two peaks at 932.93 and 952.79 eV, which are indexed to the Cu 2p3/2 and Cu 2p1/2 of Cu3P. The other two peaks at 935.20 and 955.28 eV are attributed to be the oxidized copper derived from the surface oxidation in the air.17, 24 Besides, the broad peaks centered at 944.19 and 963.25eV are considered as satellite peaks. Similarly, the high resolution spectrum of Co 2p also shows satellite peaks centered at 785.99 and 803.70 eV, respectively (Figure 3c). The Co 2p peaks centered at 779 (2p3/2) and 793.70 eV (2p1/2) are assigned to the Co2P and that Co 2p peaks derived from the oxidized cobalt locate at 781.59 and 798.4 eV, respectively.25 The P 2p spectrum (Figure 3d) can be fitted by four peaks of Cu-P (130.05 eV), C-P (133.23 eV), P-O (133.90 eV), P=O (134.56 eV).15, 24 Also, the four peaks centered at 284.67, 285.47, 286.64 and 288.31 eV in C 1s spectrum (Figure 3e) are assigned to the bonds of C-C, C-P, C-N, O=C-O.24 It further confirms that the nitrogen is doped into the carbon. As for the N 1s spectrum (Figure 3f), the doped N can be divided into three types including pyridinic, pyrrolic and graphitic, which located at 398.59, 400.07 and 401.89 eV, respectively. It has been demonstrated that the nitrogen atoms can not only create numbers of active sites and defects to facilitate lithium/ sodium ions diffusion and transportation, but also increase the conductivity and finally optimize the electrochemical performance as well.20, 22, 24 Subsequently, the electrochemical performance of our flexible CCP/C acted as selfsupporting anode electrode was elucidated. The CV curves of CCP/C at first to third cycles at a scan rate of 0.1 mV/s were measured to investigate the reaction mechanisms (Figure 4a). In the first cathodic scan, two broad peaks centered at 0.84 and 1. 34 V are observed. The cathodic peak at 0.84 V could be related to the lithium insertion and the formation of Li inefficient phase of LixCu3-xP (x=1), while the latter peak at 1.34 V represents the formation of LiP derived from the reaction:15, 19, 26, 27 Li + Co2P + 𝑒 ― →LiP + 2Co

(1)

In the following cathodic scan, the peak located at 1.34 V shifts to the high potential



of 1.5 V, derived from the generation of SEI film.19 And the reduction peak at 0.84 V changes to 0.61 V in the second cycle, corresponding to the formation of Li rich phase LixCu3-xP (Li2CuP).28 The anodic peak at 1.2 V is relevant to the delithiation process as well as the formation of the Li3-xCuxP, which keeps stable potential in the three cycles. Notably, the overlapped CV profiles in the 2nd and 3rd cycles highlight the stability and reversibility of the CCP/C during lithium ions insertion/ extraction process. The similar redox peaks correspond to the insertion and extraction of Li+ into Cu3P are observed in the CV profiles of CP/C and CCP (Figure S5). The CV curve of CCP in 2nd scan (Figure S5b) displays huge shrinkage accompanied with weak redox peaks, indicating the instability and irreversibility of CCP during cycling. Figure 4b shows the galvanostatic charge/discharge curves of CCP/C at 0.1 A/g at different cycles. Obviously, a slope plateau around 1 V is observed in the first discharge, resulted from the insertion of lithium ions. The CCP/C delivers high discharge/charge capacities of 1017.7 mAh/g and 725.2 mAh/g in the first cycle, as well as a large coulombic efficiency of ~ 71.2%. The SEI formation triggers out the irreversible capacity attenuation.29 The initial reversible capacity and coulombic efficiency of CCP/C are larger than those of CP/C (567.6 mAh/g, 64.4%) and CCP (314.6 mAh/g, 35.7%) as illustrated in Figure S6, indicating high reversibility and remarkable capacity retention of CCP/C. The cycling performance of the three samples cycled at 0.1 A/g with a potential range from 0.005 V to 3 V were demonstrated (Figure 4c, Figure S7). Apparently, the pure CCP electrode without carbon modification displays the poorest cycling performance with a huge capacity drop from 880.2 to 314.2 mAh/g in the first cycle and maintaining a low capacity around 174.9 mAh/g after 200 cycling, indicating the inferior capacity stability of CCP. In contrast, both of the CCP/C and CP/C electrodes exhibit higher reversible capacities with improved capacity retention due to the assistance of carbon. It is well known that, the introduction of carbon can alleviate the volume change, suppress the


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pulverization of the electrode materials and increase the conductivity during cycling, leading to improved structure integrity and cycling capability.21,

26

Besides, the self-

standing CCP/C and CP/C can act as electrode directly without binder and conductive agent additives, which can avoid the active materials shedding and prove the mechanical stability.30 Undoubtedly, the designed CCP/C anode obtains the best cycling capability, maintaining a high capacity of 780.9 mAh/g over 200 cycling with ~100% coulombic efficiency, much higher than the reported data. In addition, the capacity retention in CP/C is as good as CCP/C but the capacity is lower in 200th cycle (531.6 mAh/g). Therefore, the application of Co2P generates additional capacity, and the synergistic effect between different components can benefit to the enhancement of the lithium storage property as well. Interesting, the capacity boost phenomenon is observed in Cu3P-Co2P/N-C after fading in initial 30 cycles, which is common in LIBs. This phenomenon may be related to a “pseudo-capacitive type behavior”, derived from the electrochemically active polymer/gel-like film.31 The polymer/gel like film generated at low potential on the surface of materials can not only provide extra capacity through pseudocapacitive process but also protect the conducting path of the active materials, finally leading to the recovery of capacity. Meanwhile, the reactivation effect of the active materials and the structure during lithiation process results in capacity increasing and ultra-long cyclability as well.32, 33 To investigate the effect of the N-doped in Cu3P-Co2P/N-C (CCP/C) on the electrochemical performance, we have prepared the sample of Cu3P-Co2P/C (CCP-C) without N-doped by using the poly(vinyl pyrrolidone) (PVP) as alternative carbon source as well. PVP can be pyrolized to carbon without generating nitrogen element. Figure S8 illustrates the cycling performance of the CCP/C and CCP-C in LIBs. Obviously, the CCP/C shows better cycling stability and larger specific capacity of 651 mAh/g after 20 cycles at 0.1 A/g, while that of CCP-C is 526.3 mAh/g. It is testified that the N-doped could enhance the electrochemical performance of CCP/C, due to that N atoms could create more active sites and defects into



carbon and increase the conductivity to facilitate more ions insertion and transporting, leading to improved cycling capability. The rate performance of the CCP/C, CP/C and CCP were investigated at different current densities (Figure 4d and Figure S9). The great enhanced rate performance is obtained in CCP/C electrode, delivering large discharge capacities of 887.7, 814.6, 736.1, 667.3 and 567.7 mAh/g after every 5 cycles at 0.1, 0.25, 0.5, 1 and 2.5 A/g, respectively. Even retaining at 5 A/g, a large capacity around 488.5 mAh/g is still retained, 3.5 times and 35 times higher than that of CP/C (138.2 mAh/g) and CCP (13.9 mAh/g), respectively. Notably, the capacities of CCP/C are always higher than that of CP/C and CCP at each rate. After returning to 0.1 A/g, the CCP/C gets a restored capacity of 874.3 mAh/g, indicating superior rate capability and reversibility of CCP/C. Impressively, the CCP/C anode has also shown a great cycling performance at high rates of 1 A/g and 5 A/g (Figure 4e, S10). Specifically, a long lifespan over 2000 cycles with a remarkable stable capacity of 316.9 mAh/g is obtained when the current density is as high as 5 A/g, illustrating the great rate performance and cycling stability. To our knowledge, such great rate performance with longevity has not been obtained in the reported Cu-P based materials. We also compares the lithium storage performance of our materials with other reported metal phosphides in Table S2. Our-synthesized materials exhibits superior energy storage properties and great potential in high-rate LIBs. We also tested the structure stability of CCP/C after cycling at 0.1 A/g. The SEM images of the three samples after 10 cycles (Figure 5) reveal that the morphologies of CCP/C and CP/C are well maintained during lithium insertion/extraction process, while the structure of CCP has collapsed, consistent with above analysis. To get rid of the influence of different electrode preparation methods on structure stability, we furtherly prepared the Cu3P-Co2P/N-C and Cu3P/N-C electrodes via the same method like CCP electrode and denoted as CCP/C-P and CP/C-P, respectively. The SEM images of the CCP/C-P and CP/C-P electrodes after cycling are shown in Figure


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S11. It can be seen that the nanowires structures of the two electrodes, prepared by spreading the slurry of the mixed materials of active materials with the acetylene black and PVDF on the copper foil, are still maintained compared to the CCP electrode. It confirms that the structure stability of Cu3P-Co2P/N-C and Cu3P/N-C are improved greatly with the assistance of N-doped carbon. These results illustrate the great enhancement in electrochemical performance of CCP/C as compared to the other two samples, which may be ascribed to the following aspects. Firstly, the three-dimensional cross-linked networks derived from the PAN precursor exhibit many characteristic advantages, such as more diffusion pathways, shorter diffusion length, high specific surface area, fast diffusion rate and stable structure. Secondly, the application of N-doped carbon is beneficial to alleviate the volume change and enhance the conductivity, ensuring fast diffusion kinetics and the structure integrity during cycling. Doping N atoms could provide defects into carbon to allow more lithium ions inserting and diffusing through the materials, resulting in increased specific capacity. Thirdly, the Co2P also contributes extra capacity. The synergistic effect between the two metal phosphides could provide more active sites for ion storage, which further optimizes the electrochemical performance of our designed material.13, 20-22 To explore the reaction kinetics in detail, the CV curves of CCP/C with different scan rates were illustrated in Figure 6a. The redox peaks shift with the increasing of rates and follow the function: log(i) = blog(v) + loga

(2)

where v and i means the scan rate and current; a and b are constants, respectively.34 As we know, the b value obtained from the plots of log (i) vs log (v) corresponds to the ratelimiting process. The diffusion-controlled process works at b= 0.5, while the pseudocapacitive behavior dominants at b=1.35 As shown in Figure 6b, the slopes for the cathodic and anodic peaks are 80.6 and 71.8, respectively, indicating that both of diffusion-



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controlled process and pseudo-capacitive behavior contribute to the lithium storage process. Moreover, the contributions of the two different mechanisms can be quantitatively analyzed in virtue of the following function: I(v) = k1v + k2𝑣1/2

(3)

where the k1v and k2v1/2 represent the current contributed by the pseudo-capacitive behavior and diffusion-controlled process, respectively.36, 37The calculated contribution of pseudo-capacitive (blue region) is ~ 73.4% at 2 mV/s (Figure 6c). Meanwhile, the capacitive contributions of CCP/C at different scan rates are summarized in Figure 6d. As scan rate increasing, the corresponding capacitive effect increases obviously and dominates in the high rate, providing fast Li+ ions diffusion kinetics, and finally leading to high rate performance of CCP/C anode.34, 36, 38, 39 The electrochemical impedance spectroscopy (EIS) of our samples before and after cycling were measured to investigate the reaction kinetics. The semicircles in high frequency correspond to the charge transfer resistance shown in Figure S12. Apparently, the initial CCP/C has the smallest semicircle, and it is still smaller than both of CP/C and CCP after cycling, indicating the excellent conductivity and stability of the CCP/C. Since the metal phosphides have been regarded as the attractive anode materials for NIBs, the sodium- storage properties of CCP/C composite were investigated as well. The CV curves in initial three cycles of CCP/C in a potential range from 0.005 to 3 V were illustrated in Figure 7a. A strong and broad peak positioned at 0.47 V and a weak peak at 0.91 V were observed in the first discharge, which is related to the sodium insertion and the formation of Na3P, accompanied by the SEI film formation. In the following cathodic scan, there are four peaks, corresponding to the multi-step sodiation process to from the Na3P and NaxP. The two anodic peaks located at 1.82 and 2.31 V resulted from the deintercalation process of Na+ and decomposition of Na3P were observed in the initial anodic scan.17, 40-42 Then, the subsequent CV curves display the similar shape to the first


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cycle. Next, Figure 7b displays the galvanostatic discharge/ charge curves of CCP/C examined at 0.1 A/g in 0.005~ 3 V. In the first discharge, a sloping platform around 0.781.1 V appeared due to the formation of SEI film and sodiation process, which is in accordance with the profile of the CV curves. Our as-synthesized sample provides high initial discharge/charge capacities of 429.5/321.2 mAh/g accompanied with high coulombic efficiency of 74.8 %. The irreversible attenuation of initial capacity is due to the electrolyte decomposition and SEI film formation. We compared the cycling capability of CCP/C with that of CP/C at 0.1 A/g in Figure 7c. The CCP/C attains a larger specific capacity of 321.2 mAh/g accompanied with a coulombic efficiency around 74.8% in initial cycle, which is considerably larger than that of CP/C (106.2 mAh/g with 31.5% coulombic efficiency). With the introduction of Co2P, we have improved the cycling capability and reversibility of CCP/C. Impressively, a stable capacity around 166.4 mAh/g is sustained in CCP/C after 50 cycles, 2.1 times higher than that of CP/C (78 mAh/g). Besides, the coulombic efficiency increases rapidly from the initial 74.8% to ~ 99% and keeps stable during

cycling,

reveals

good

cycling

performance

and

stability

during

sodiation/desodiation process. Subsequently, the rate performance of the CCP/C was carried out as displayed in Figure 7d. A capacity of 264.2 mAh/g at 0.05 A/g in 5th cycle is obtained, then the capacity exhibits a decrease as the rate increasing from 0.05 A/g to 0.1, 0.25, 0.5, 1 A/g. When the current density returns to 0.05 A/g, the capacity close to 220 mAh/g would be restored, indicating a good rate capability of CCP/C. Based on the better sodium storage properties of CCP/C than that of CP/C, it is reasonable to deduce that the synergistic effect between Cu3P/C and Co2P, which can provide extra capacity and more active sites for sodium insertion, as well as increase the conductivity to facilitate sodium diffusion. Surely, the Na+ storage performance is weaker than lithium storage performance towing to the larger radius of Na ions than Li ions, which hampers ions diffusion and triggers out larger volume expansion.43, 44



4. CONCLUSION In summary, the rational 3D self-supporting nanocomposites composed of Cu3P, Co2P and N-doped C (CCP/C) were synthesized by a combination method of electrospinning and phosphorization process. Benefit from the 3D flexible structure and various ingredients, the CCP/C nanocomposite are endowed with more and short lithium/ sodium ions diffusion pathway and more active sites for ions insertion, as well as high conductivity. Besides, the application of N-doped carbon can alleviate the volume change during cycling, leading to great structure integrity and cycling stability during lithiation/sodiation process. As a result, the CCP/C delivers excellent energy storage performance in both LIBs and NIBs. Specifically, a stable capacity of 780.9 mAh/g with ~ 100% coulombic efficiency is obtained after 200 cycles at 0.1 A/g in LIBs, which is much higher than that of CP/C and CCP. Most importantly, it shows excellent rate performance and ultra-long lifespan over 2000 cycles with superior capacity stability (316.9 mAh/g retained at 5 A/g). In the NIBs, CCP/C anode shows larger capacity (166.4 mAh/g after 50 cycling) and better rate performance than those of CP/C. Inspired by the enhancement energy storage performance of CCP/C, our designed sample could be an alternative anode for alkali metal ion batteries. And this facile method is suitable to fabricate other metal phosphides and their composites for energy storage devices.


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Supporting information The SEM images of CP/C and CCP. The EDS spectrum, elemental analysis table and XPS full spectrum of CCP/C. the discharge/charge curves, The TG curves of CCP/C and CP/C. CV curves and coulombic efficiency of CP/C and CCP, as well as the rate performance of CCP in LIBs. The coulombic efficiency of CCP/C anode at high rate of 5 A/g. The SEM images of these samples after cycling.



ACKNOWLEDGMENTS This work was supported by the joint fund of the National Key Research and Development Program of China (2017YFA0402902) and the National Natural Science Foundation Committee of China Academy of Engineering Physics (NSAF) (U1630108). We also acknowledge the support of the USTC Center for Micro and Nanoscale Research and Fabrication.


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Figure 1 SEM images of (a) as-electrospun Cu-Co/PAN precursor (b) CCP/C nanofibers. The inset images are the low magnification images. (c) The low magnification TEM image and (d) HRTEM image of CCP/C composites.



Figure 2 (a) The TEM image of CCP/C hybrid materials and the corresponding EDS mapping for (b) Cu, (c) Co, (d) C, (e) N and (f) P elements.


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Figure 3 (a) The XRD patterns of our prepared samples. The XPS spectra of CCP/C: (b) Cu; (c) Co; (d) P; (e) C and (f) N.



Figure 4 The electrochemical performances of CCP/C in LIBs. (a) CV curves of CCP/C in initial three cycles with a scan rate of 0.1 mV/s between 0.005 and 3 V. (b) The discharge/charge curves

in 0.005-3 V at 0.1 A/g. (c) The cycling performances of CCP/C,

CP/C and CCP at 0.1 A/g. (d) Comparison of the rate performances of CCP/C and CP/C at various rates range from 0.1 to 5 A/g. (e) Cycling performance of CCP/C at 1 A/g and 5 A/g.


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Figure 5 The morphologies of (a) CCP/C (b) CP/C and (c) CCP after 10 cycles at 0.1 A/g.



Figure 6 (a) CV curves of CCP/C at different scan rates. (b) Plots of peak current vs sweep rate. (c) Capacitive contribution of CCP/C composites (blue region) at 2 mV/s. (d) Comparison of the ratio of capacitive contributions at various scan rate.


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Figure 7 Sodium storage performances of CCP/C. (a) CV curves of CCP/C in NIBs in initial three scans at 0.1 mV/s. (b) The discharge-charge curves in initial three cycles. (c) Cycling stabilities of CCP/C and CP/C at 0.1 A/g. (d) Rate performance of CCP/C.



254x102mm (150 x 150 DPI)


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