In Situ Fabrication of Ni2P Nanoparticles Embedded in Nitrogen and

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In Situ Fabrication of Ni2P Nanoparticles Embedded in Nitrogen and Phosphorus Codoped Carbon Nanofibers as a Superior Anode for Li-ion Batteries Zhuzhu Du, Wei Ai, Jun Yang, Yujiao Gong, Chenyang Yu, Jianfeng Zhao, Xiaochen Dong, Gengzhi Sun, and Wei Huang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03327 • Publication Date (Web): 13 Oct 2018 Downloaded from http://pubs.acs.org on October 13, 2018

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ACS Sustainable Chemistry & Engineering

In Situ Fabrication of Ni2P Nanoparticles Embedded in Nitrogen and Phosphorus Codoped Carbon Nanofibers as a Superior Anode for Li-ion Batteries Zhuzhu Du,1 Wei Ai,2 Jun Yang,1 Yujiao Gong,1 Chenyang Yu,1 Jianfeng Zhao,1 Xiaochen Dong,1 Gengzhi Sun*1,2 and Wei Huang*1,2,3 1

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), National

Jiangsu Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China 2

Institute of Flexible Electronics (IFE), Northwestern Polytechnical University (NPU), 127 West Youyi

Road, Xi’an 710072, China 3

Key Laboratory for Organic Electronics & Information Displays (KLOEID) and Institute of Advanced

Materials (IAM), SICAM, Nanjing University of Posts & Telecommunications, Nanjing 210023, Jiangsu, China

*To whom correspondence should be addressed: [email protected]; [email protected] RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

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Abstract: A facile and cost-effective method has been developed for the fabrication of Ni2P nanoparticles

encapsulated in

N,P-codoped carbon nanofibers (Ni2P@NPCNFs) using the

commercialized products of melamine polyphosphate (MPP) and Ni foam. During the synthetic process, MPP serves as the solid phosphorous source for the phosphidation of Ni foam, and simultaneously performs as carbon source for the in situ growth of functionalized carbon nanofibers, resulting in a favorable architecture of Ni2P well-integrated carbon matrix. With this unique structure, the resultant Ni2P@NPCNFs electrode shows remarkable Li ion storage performance, in terms of high reversible capacity and long-cyclic lifespan (850 mAh g-1 at 200 mA g-1 after 450 cycles), as well as good rate behavior (300 mAh g-1 at 3200 mA g-1). This work not only presents a new avenue to construct advanced Ni2P-based functional nanocomposites for Li-ion batteries but also paves the way to explore unique transition-metal phosphides towards diverse applications.

Keywords: Transition metal phosphides, carbon nanofibers, Li-ion batteries, general strategy.

Introduction Although lithium-ion battery (LIB) is currently the dominating power source for portable electronics,1,2 its energy density (i.e., 210 Wh kg-1 or 650 Wh L-1) is an inherent limitation as a result of intercalation/deintercalation-based charge-discharge mechanism using graphite as anode, which cannot satisfy the fast development of electronic devices and electric vehicles. In order to upgrade the performance of LIBs, significant researches have been devoting to developing superior electrode materials.3–5 Among all kinds of alternative anodes, transition-metal compounds deserve special attention in view of their diverse morphologies and variable valence states.6–8 Different from the traditional graphitic anode which has a theoretical capacity of ~370 mAh g-1, transition-metal compounds store Li through conversion reaction giving higher specific capacities.9,10 Compared with


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their counterparts of oxides, sulfides and fluorides, transition metal phosphides exhibiting weak polarization and low discharge plateau are more suitable for next-generation LIBs.11 As a typical example, NiPx have attracted considerable attentions in LIBs, owing to their changeable components forming phosphorus-rich phases (x > 1, e.g., NiP2 and NiP3) and metal-rich phases (x ≤ 1, e.g., Ni2P and Ni12P5).12 Although phosphorus-rich phases exhibit higher theoretical Li storage capacities, their stability is an issue and the high nucleation energy will finally induce their conversion into metal-rich phases nanostructures.11,13 As a consequence, NiPx anodes with metal-rich-phases, e.g., Ni2P with a theoretical capacity of 542 mAh g-1, have been profoundly studied.14–16 However, the poor electronic conductivity and large volume variations of Ni2P anode during charge-discharge processes normally lead to unsatisfactory performance in terms of low specific capacity, poor coulombic efficiency and short cyclic lifespan.17 Various well-designed Ni2P nanostructures so far have been constructed, including hollow spheres,18 nanoparticles,19 nanowires,20 nanotubes,21 and nanosheets,22 in order to improve their electrochemical performance. Nevertheless, the synthetic strategy usually experiences multisteps with low yield and requires specifically-trained individuals with skills in nanotechnology, which are not appropriate for practical application. Moreover, the insurmountable aggregation of Ni2P nanoparticles during electrochemical reactions naturally leads to unsatisfied cycling performance, especially at high current densities. Alternatively, embedding Ni2P in a conductive matrix could be a feasible solution to solve the above-mentioned problems, for example Ni2P@carbon nanocomposites. In the hybrid, the carbon matrix is expected to not only buffer the volume changes of Ni2P but also provide high conductivity for efficient electron transport. However, it remains a challenge to develop facile and scalable routes for fabricating advanced Ni2P-based anodes towards high-performance Li storage. In this work, we report a scalable approach to prepare Ni2P nanoparticles which are encapsulated in as-grown N,P-codoped carbon nanofibers (Ni2P@NPCNFs). Commercial melamine polyphosphate (MPP) is used as the solid sources of C, N and P, and Ni foam is adopted as the Ni source which is believed to initiate catalytic growth of NPCNFs. The resultant 3D interconnected architecture featuring good conductivity, large accessible surface area and high dispersity of Ni2P nanoparticles displays ACS Paragon Plus Environment

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outstanding electrochemical performance, including high initial discharge capacity (1170 mAh g-1 at 200 mA g-1), long cycle-life (73% retention after 450 cycles) and high-rate capability (300 mAh g-1 at 3200 mA g-1).

Experimental Section Synthesis of Ni2P@NPCNFs: In a general protocol, 160 mg Ni foam was placed in a porcelain boat, and then transferred into the tubular furnace. Next, another porcelain boat loaded with 14 g MPP was put in the left side of the Ni foam, with an approximate separation distance of 15 cm (See Figure 1a). The furnace was heated to 900 °C at 5 °C min-1 under Ar atmosphere, and kept at 900 °C for 2 h. After cooling down to room temperature, Ni2P@NPCNFs was obtained in the second boat without any further post-treatment. Characterization: X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer using Cu Kα source. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were taken on a JEOL JSM-6700F microscope and a JEOL JEM 2100 microscope, respectively. X-ray photoelectron spectroscopy (XPS) was carried out on an ESCALAB MK II spectrometer using Al Kα source. Brunauer-Emmett-Teller (BET) was analyzed on a Micromeritics ASAP 2020 using N2 sorption measurements. Raman spectra were collected on a WITec Alpha300 Series with 532 nm laser. Thermogravimetric analysis (TGA) was conducted on a DTG 60H analyzer in oxygen atmosphere. Inductively coupled plasma optical emission spectrometry (ICP-OES) was performed on an Agilent 730 Series. Electrochemical Characterizations: The electrochemical performance of the materials were tested using coin-type cells. The working electrodes were prepared by coating slurries onto 10 μm thick copper foils, where the slurries were prepared by mixing 80 wt% of active materials, 10 wt% of acetylene black and 10 wt% of polyvinylidene fluoride in N-methyl-2-pyrrolidinone. After drying in a vacuum oven at 100 oC overnight, 1.2 cm diameter circular electrodes were obtained using a punching machine. The weight of active materials loaded on each electrode was controlled to be 1.0 - 1.5 mg. The cells were ACS Paragon Plus Environment

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assembled in an Ar-filled MBRAUN glovebox using pure Li foil as both the reference and counter electrodes. 1 M LiPF6 in 1:1 v/v ethylene carbonate and dimethyl carbonate was used as electrolyte. Electrochemical behaviors of the cells were inquired on a NEWARE multichannel battery testing system and a CHI760D electrochemical workstation.

Results and discussion Figure 1a presents the schematic illustration of preparing Ni2P@NPCNFs via chemical vapor deposition (CVD). MPP was used as the solid source while Ni foam was used as catalyst. 900 oC was chosen as the ideal temperature for the growth according to our previous experience.23,24 During the annealing process, MPP decomposes and releases a large amount of small molecules,25 which trigger the catalytic growth of N,P-codoped carbon nanofibers (NPCNFs).26,27 Simultaneously, Ni was converted to Ni2P following the contact-conversion reaction processes,28-30 resulting in the formation of Ni2P nanoparticles-embedded NPCNFs. SEM images of Ni2P@NPCNFs show an interconnected, 3D porous network of nanofiber scaffolds with dispersed Ni2P nanoparticles (Figure 1b and 1c). This is in sharp contrast to the pristine Ni foam substrate shown in Figure S1. TEM images suggest that the NPCNFs are composed of bamboo-like and cup-stacked fibrous structures with diameters in range of dozens of nanometers to hundreds of nanometers (Figure 1d). Such interesting structures stem from the generation of pentagonal carbon rings during carbon atoms deposit on the Ni surface, which induces changes in the curvature of graphitic layers to accommodate the geometry of the inside ones.31 In NPCNF scaffold, Ni2P nanoparticles at the end tips of the nanofibers show analogous size to the fiber diameter (Figure 1e), whereas the other ones embedded in the nanofibers possess dozens of nanometers in sizes (Figure 1f). This is further evidenced by elemental mapping images (Figure 1g-l), where large spots appear at the end of CNF tips while small ones exist in the nanofibers. The inset of Figure 1d shows the highresolution TEM (HRTEM) image of a typical Ni2P nanoparticle, and a lattice spacing of 0.51 nm is observed, which agrees well with the (010) planes of hexagonal Ni2P.32 The lattice with a spacing of 0.34 nm corresponds to the (002) plane of carbon shell coated on the core nanocrystal.10,33 ACS Paragon Plus Environment

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The crystalline structure of Ni2P@NPCNFs was characterized by XRD (Figure 2a). The peaks located at ~40.7°, ~44.5°, ~47.2°, ~54.2°, ~54.9°, ~66.4°, ~72.6°and ~74.8°correspond to the (111), (201), (210), (300), (211), (310), (311) and (400) planes of hexagonal Ni2P (JCPDS No. 74-1385), respectively.17,34 Additionally, the abroad diffraction peak centered at 2θ = 26.3°is indexed to the (002) plane of carbonaceous materials.10,23 Raman spectrum of the composite displays two remarkable peaks at 1361 and 1569 cm-1 (Figure 2b), which stem from the D (A1g breathing mode) and G (E2g vibrational mode) bands of carbon, respectively.35 This result states the successful growth of carbon nanofibers catalyzed by Ni-based nanoparticles. The detailed elemental composition of Ni2P@NPCNFs was examined by XPS analysis (Figure 2c-f). As shown in Figure 2c, the signals of C, N, O, P and Ni can be distinctly identified from the XPS survey spectrum, in line with the elemental mapping and Raman data. High-resolution Ni 2p spectrum shows three peaks (Figure 2d). The two predominant peaks located at 854.5 and 872.0 eV can be successively assigned to Ni 2p3/2 and Ni 2p1/2 states of Ni2P, while the peak at 862.4 eV is the satellite peak of Ni 2p3/2.5 Correspondingly, two peaks at 129.8 and 133 eV are observed in the high-resolution P 2p region (Figure 2e), attributable to the P 2p3/2 and P 2p1/2, respectively. This reflects P in the form of metal phosphides and oxidized phosphorus species.5,23 Meanwhile, the deconvolution of high-resolution N 1s spectrum suggests the presence of pyridine-like (398.8 eV) and pyrrole-like (401.1 eV) N atoms (Figure 2f).10,36,37 Besides, the signal of O is also detected in Ni2P@NPCNFs, indicating the existence of oxygen-containing functional groups in the carbon matrix, as revealed by the high-resolution C 1s region (Figure S2). Figure S3 depicts the TGA curve of Ni2P@NPCNFs under oxygen flow. It can be seen that the composite shows good thermal stability at temperature below 400 oC, and a rapid mass loss at 450-500 oC can be noted, which is ascribed to the combustion of carbon. The slight increase of mass after 600 oC is associated with the oxidation of Ni2P to yield nickel oxides. Based on the TGA curve, the content of Ni2P in the composite is calculated to be 45.8 wt%. The accurate content of Ni2P in the composite is further determined to be 46.9 wt% according to the ICP-OES analysis (Table 1), in accordance with the TGA results.


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To gain deeper insights into the formation of Ni2P@NPCNFs, we further studied the amount of MPP precursor on the influence of the final product. As shown in the SEM images (Figure S4), when half of the amount of MPP (7 g) was used, no fibrous structure was detected. Alternatively, an interesting nanostructure like "mushroom" was obtained. XRD pattern (Figure S5) suggests that the composite consists of Ni2P nanoparticles encapsulated in N,P-codoped carbon (Ni2P@NPC). It is worth noting that the sizes of the nanoparticles are quite similar to that observed in the SEM images of Ni2P@NPCNFs, indicating an efficient phosphidation of the Ni foam by the constitutional phosphorus of MPP. Thus, we can conclude that the synthesis of NPCNFs is based on a catalytic CVD process, during which small molecular decomposed from enough MPP are catalytically grown into CNFs on Ni-based nanoparticles following a tip-growth mechanism.38,39 On the contrary, Ni2P@NPC was obtained when limited MPP solid source is employed. N2 adsorption/desorption measurements (Figure S6) illustrate that Ni2P@NPCNFs and Ni2P@NPC represent the same feature isothermal curves; however, the BET surface area of Ni2P@NPCNFs (63 m2 g-1) is up to 4.5 times larger than Ni2P@NPC (14 m2 g-1). The significantly improved BET surface area together with the fascinating nanostructure of Ni2P@NPCNFs are expected to express excellent Li storage behavior. Figure 3a depicts the first three cyclic voltammogram (CV) curves of the Ni2P@NPCNFs electrode at the scan rate of 0.5 mV s-1 in the voltage window of 0.005-3 V (vs. Li+/Li). It is notable that the initial CV curve differs from that of the subsequent ones. The cathodic peak in the potential range of 0.2-1.0 V stems from the overlapped regions of solid electrolyte interface (SEI) layer formation and the conversion reaction between Ni2P and Li ions.10,17 This peak was then substituted by a broad peak centered at ~0.54 V, which can be indexed to the reversible conversion reaction, resulting in the generation of Li3P and metallic Ni.12,40 Accordingly, the anodic peak located at ~0.47 V corresponds to their conversion back to Ni2P. The shape of the CV curves has almost no change after the first cycle, indicative of an effective SEI film formation that is capable of affording steady and reversible Li storage kinetics. The cyclic behavior of Ni2P@NPCNFs was evaluated by galvanostatic charge-discharge measurements at the current rate of 200 mA g-1. For comparison, the electrochemical performance of Ni2P@NPC was also 7 ACS Paragon Plus Environment


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studied. Figure 3b shows the corresponding first-cycle charge-discharge profiles of the Ni2P@NPCNFs and Ni2P@NPC electrodes. The initial discharge and charge capacities for Ni2P@NPCNFs are ~1180 and ~660 mAh g-1, respectively, which is significantly larger than that of Ni2P@NPC (~430 and ~260 mAh g-1). The large irreversible capacity loss of the electrodes during the first cycle may be associated with the electrolyte decomposition reactions involving SEI process and the other detrimental side reactions. The relatively low coulombic efficiency of Ni2P@NPCNFs is likely due to the high amount of NPCNFs with large BET surface area.41 However, the electrochemical reactions gradually become reversible upon cycling, as verified by the dramatically increased coulombic efficiency showing > 90% in the second cycle and > 95% in the third cycle (Figure S7). Besides, the Ni2P@NPCNFs electrode also displays impressive cycling performance. As shown in Figure 3c, even after 450 cycles, the reversible capacity of the electrode could still reach as high as 850 mAh g-1. It is interesting to note that the capacity of Ni2P@NPCNFs electrode shows a gradually increasing trend with cycling, which is probably due to the improved accessibility of Li ions to the inner area of the material.10,42 In sharp contrast, the Ni2P@NPC electrode shows a much smaller specific capacity, with only 170 mAh g-1 was sustained. Meanwhile, the Ni2P@NPCNFs electrode exhibits superior rate capability compared with Ni2P@NPC. Figure 3d and Figure S8 present their rate performance under consecutive increased current density. The specific capacities of Ni2P@NPCNFs electrode are 810, 688, 578, 480, 411 and 300 mAh g-1 with the current density stepwisely increased from 100 to 200, 400, 800, 1600 and 3200 mA g-1, respectively. When the current density was switched back to 100 mA g-1, the specific capacity of the Ni2P@NPCNFs electrode can instantly reach 815 mAh g-1, fully recovering to the former value, which suggests its good reversibility. While the performances of Ni2P@NPC electrode are inferior to that of Ni2P@NPCNFs when tested at the same current densities (Figure 3e). To the best of our knowledge, the overall performance of Ni2P@NPCNFs is among the top level of the reported Ni2P-based electrodes. The improved Li storage behavior of Ni2P@NPCNFs can be attributed to the excellent dispersity of Ni2P nanoparticles in NPCNFs, which not only guarantees full utilization of the active material for electrochemical reactions but also enables fast charge-transfer processes (see below). Additionally, the ACS Paragon Plus Environment

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distinctly larger BET surface area of Ni2P@NPCNFs is beneficial for easy electrolyte accessibility and effective ion diffusion. In order to highlight the advantages of Ni2P@NPCNFs nanostructure for Li storage, we also fabricated a controlled sample using red phosphorus as the phosphorus source. The morphology and structure of the resulting sample are shown in Figure S9 and S10. As expected, the controlled sample shows poor electrochemical behaviors (Figure 3c and 3d) compared with those of the Ni2P@NPCNFs and Ni2P@NPC electrodes, because of its poor conductivity and insufficient contact area with the electrolyte. The kinetic properties of Ni2P@NPCNFs and Ni2P@NPC electrodes were evaluated by electrochemical impedance spectroscopy (EIS) measurements (Figure 4a). It is evident that the Nyquist plots of these two samples contain two parts, that is, a semicircle in the high frequency range corresponds to charge-transfer resistance, and a tail (sloped line) at low frequencies is associated with the diffusion resistance. An equivalent circuit (Figure 4b) was applied to analyze the measured impedance data, where Re is the electrolyte and contact resistances, Rsf is the surface film impedance related to SEI, Rct is the charge-transfer resistance, W is the Warburg impedance associated with the diffusion of Li ions into the bulk electrode, and CPE is the double layer capacitance.43 The simulated results of the electrodes using the equivalent circuit model are presented in Figure 4c. Notably, the Ni2P@NPCNFs and Ni2P@NPC electrodes have comparable Re; however, the Rsf (13 Ω) and Rct (95 Ω) of Ni2P@NPCNFs are substantially lower than that of Ni2P@NPC (53 and 119 Ω, respectively), which proves the enhanced kinetic behaviors in Ni2P@NPCNFs electrode in terms of easier migration of Li ions through the SEI film and faster charge-transfer process. To further understand the structural features of Ni2P@NPCNFs, ex situ SEM was carried out on the electrodes before and after cycling. As observed in Figure S11, the pristine and 450-cycled electrodes show no apparent morphology difference except for a fuzzy layer wrapped on the surface of the cycled nanofibers, attributable to the initially formed SEI film on the electrode. In the meantime, Ni2P nanoparticles can also be readily identified from the SEM images of the cycled electrode, indicating the efficient mitigation of volume variations of the active


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materials, which accounts for its good reversibility. With these merits, Ni2P@NPCNFs is able to afford robust stability for electrochemical reactions, thus leading to exceptional Li storage performance.

Conclusions In summary, we have demonstrated that MPP and Ni foam can act as effective synthetic platforms for the preparation of advanced anode with Ni2P nanoparticles embedded in functionalized carbon nanofibers. MPP plays a vital role in determining the morphology of the final product. It not only functions as phosphorous source for converting Ni to Ni2P but also as carbon source for catalytically growing carbon nanofibers. Electrochemical tests suggest that Ni2P@NPCNFs with Ni2P nanoparticles firmly anchored on the NPCNFs shows high Li storage capacity and good cyclic stability. The reversible capacity after 450 cycles retains 850 mAh g-1 at 200 mA g-1, and the structure of the electrode could be preserved after long-term cycling. We believe that our method provides a simple yet cost-effective route for mass production of Ni2P@carbon utilizing commercial products towards sustainable energy systems.

Supporting Information

SEM images of bare Ni foam, High-resolution C 1s XPS spectrum and TGA curve of Ni2P@NPCNFs, SEM images and XRD pattern of Ni2P@NPC, Nitrogen adsorption and desorption isotherms of Ni2P@NPCNFs

and

Ni2P@NPC,

Coulombic

efficiency

and

charge-discharge

profiles

of

Ni2P@NPCNFs and Ni2P@NPC electrodes, SEM images and XRD pattern of the controlled sample, and SEM images of Ni2P@NPCNFs before and after cycling.

Acknowledgments This work was financially supported by National Key Basic Research Program of China (973 Program) (Grant No. 2015CB932200), National Natural Science Foundation of China (Grant No. 61704076), Natural Science Foundation of Jiangsu Province (Grant No. BK20171018), Jiangsu Specially-Appointed


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Professor program (Grant No. 54935012). W.A. thank the support from the Fundamental Research Funds for the Central Universities (31020180QD094).

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Figure captions

Figure 1. (a) Schematic diagram for the synthesis of Ni2P@NPCNFs. (b, c) SEM, (d-f) TEM, and (g-l) elemental mapping images of Ni2P@NPCNFs. The inset of Figure 1d shows a representative HRTEM micrograph of the Ni2P nanoparticle.


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Figure 2. (a) XRD pattern of Ni2P@NPCNFs in comparison with the standard diffraction peaks for hexagonal Ni2P nanostructure (JCPDS no. 74-1385). (b) Raman spectrum, (c) XPS survey spectrum, (d) high resolution Ni 2p spectrum, (e) high resolution P 2p spectrum, and (f) high resolution N 1s spectrum of Ni2P@NPCNFs.


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Figure 3. (a) CV curves of the Ni2P@NPCNFs electrode at a scan rate of 0.5 mV s-1. (b) The initial charge-discharge voltage curves of Ni2P@NPCNFs compared with Ni2P@NPC. (c) Cyclic performances of the electrodes measured at 200 mA g-1. (d) Rate capabilities of the electrodes continuous tested at different current densities. (e) Specific capacity versus current density plots of the Ni2P@NPCNFs and Ni2P@NPC electrodes.


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Figure 4. (a) Nyquist plots and the corresponding simulation results of the electrodes. All the EIS data were recorded in a frequency of 100 kHz to 0.1 Hz employing a 5 mV amplitude. (b) The equivalent circuit model used to fit the experimental data. (c) The fitting data for different electrodes.

Table 1. ICP-OES data of Ni2P@NPCNFs.

Ni content

Ni2P content*

(wt%)

(wt%)

37.10

46.9

Sample

Ni2P@NPCNFs

* Ni2P (wt%) = Ni (wt%)/M(Ni)/2*[M(Ni)*2 + M(P)]


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SYNOPSIS TOC

Synopsis A scalable approach for preparing Ni2P nanoparticles encapsulated in N,P-codoped carbon nanofibers has been developed for high-performance Li storage.


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In Situ Fabrication of Ni2P Nanoparticles Embedded in Nitrogen and

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