Monodispersed Carbon-Coated Cubic NiP2 Nanoparticles Anchored

Mar 21, 2017 - Facile synthesis of N-doped carbon-coated nickel oxide nanoparticles embedded in N-doped carbon sheets for reversible lithium storage...
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Monodispersed Carbon-Coated Cubic NiP2 Nanoparticles Anchored on Carbon Nanotubes as Ultra-Long-Life Anodes for Reversible Lithium Storage Peili Lou,†,‡ Zhonghui Cui,*,† Zhiqing Jia,†,‡ Jiyang Sun,†,‡ Yingbin Tan,† and Xiangxin Guo*,† †

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050 China ‡ University of Chinese Academy of Sciences, Beijing 100039 China S Supporting Information *

ABSTRACT: In search of new electrode materials for lithium-ion batteries, metal phosphides that exhibit desirable properties such as high theoretical capacity, moderate discharge plateau, and relatively low polarization recently have attracted a great deal of attention as anode materials. However, the large volume changes and thus resulting collapse of electrode structure during long-term cycling are still challenges for metal-phosphide-based anodes. Here we report an electrode design strategy to solve these problems. The key to this strategy is to confine the electroactive nanoparticles into flexible conductive hosts (like carbon materials) and meanwhile maintain a monodispersed nature of the electroactive particles within the hosts. Monodispersed carbon-coated cubic NiP2 nanoparticles anchored on carbon nanotubes (NiP2@C-CNTs) as a proof-of-concept were designed and synthesized. Excellent cyclability (more than 1000 cycles) and capacity retention (high capacities of 816 mAh g−1 after 1200 cycles at 1300 mA g−1 and 654.5 mAh g−1 after 1500 cycles at 5000 mA g−1) are characterized, which is among the best performance of the NiP2 anodes and even most of the phosphide-based anodes reported so far. The impressive performance is attributed to the superior structure stability and the enhanced reaction kinetics incurred by our design. Furthermore, a full cell consisting of a NiP2@C-CNTs anode and a LiFePO4 cathode is investigated. It delivers an average discharge capacity of 827 mAh g−1 based on the mass of the NiP2 anode and exhibits a capacity retention of 80.7% over 200 cycles, with an average output of ∼2.32 V. As a proof-of-concept, these results demonstrate the effectiveness of our strategy on improving the electrode performance. We believe that this strategy for construction of high-performance anodes can be extended to other phase-transformation-type materials, which suffer a large volume change upon lithium insertion/extraction. KEYWORDS: nickel phosphide, monodispersed, volume change, structure stability, reaction kinetics, lithium storage ince first commercialized by SONY, the rechargeable lithium-ion battery (LIB) has gained great success in portable electronics and now in electric vehicles (EV) such as those from Tesla Motors. Pursuing new, safe, and lowcost electrode materials with higher capacity than currently commercialized materials has been an important mission of battery research.1,2 As an alternative to graphite, conversionreaction-based materials, such as oxides, fluorides, sulfides, and phosphides, have been extensively investigated as promising anode materials, due to their ability to give high capacity. Among them, metal phosphides distinguish themselves with their moderate discharge plateau and relatively low polarization, without sacrificing their advantages of capacity.3−21 For

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example, the average polarization of phosphides (∼0.4 V) is lower than that of sulfides (∼0.7 V), oxides (∼0.9 V), and fluorides (∼1.1 V).4 For these conversion anodes, low initial Coulombic efficiency and poor cycle stability are two important challenges for their real application.21,22 Recent progress on compensation lithium loss on initial discharge could shed light on solving the initial Coulombic efficiency problem.23−25 Like other phase-transformation-type electrodes (e.g., Si,26−28 MnO,29,30 FeS31,32), the limited cycle life and poor capacity Received: December 8, 2016 Accepted: March 21, 2017 Published: March 21, 2017 3705

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ACS Nano retention still are big problems for the metal-phosphide-based anodes, which results from their large volume change and thus induced pulverization and loss of electrical contact of active materials. Nanostructuring appears as an effective approach to overcome this problem, as evidenced in many well-designed Si, oxides, and sulfides,26,27,32−35 which is beneficial from the increased electrode−electrolyte contact areas and shortened ion/electron diffusion paths.36−38 Several kinds of nanostructured met al phosphides including nanoparticles,4,5,7,10,14−16 nanosheets,12 nanowires,19 and hollow spheres17 have been synthesized. Although some of them indeed show improved lithium storage capability, achieving ultralong cycle life (>1000 cycles) with stable capacity is still a challenge, due to the gradual collapse of electrode structures and the aggregation of nanograins refined in the course of repeated conversion reactions. Nickel phosphides (NiPx), a typical kind of phosphide, have attracted extensive interest in recent years, mainly due to their excellent catalytic capacity for oxygen evolution reaction (OER),39,40 hydrogen evolution reaction (HER),41,42 and hydrotreating reaction,43−45 as well as their impressive ability for lithium storage.4−8,10−12,14−16,18−21 According to the ratio of Ni/P, the NiPx compounds can be divided into metal-rich phases (x ≤ 1, e.g., Ni2P, Ni12P5, and Ni5P4) and phosphorusrich phases (x > 1, e.g., NiP2 and NiP3). The metal-rich phase especially Ni2P deemed as catalysts has been profoundly investigated from theoretical to experimental by several groups,39−50 while the phosphorus-rich phases have been studied as anodes for LIBs, as they can take up more lithium to give much higher theoretical capacities (e.g., 1333 mAh g−1 for NiP2 and 1591 mAh g−1 for NiP3).4−8,18,21 Gillot et al. reported that the ball-milled monoclinic NiP2 anode can give an initial capacity of about 1250 mAh g−1 at 0.1 C and remains at ∼800 mAh g−1 after 11 cycles,4 and Boyanov et al. showed that downsizing the particles of monoclinic NiP2 to the nanoscale improves its capability to take up more than 5 lithium ions per molar of NiP2, leading to a high discharge capacity.8 It is well known that the high conductivity is favorable for fast electron transfer and thus renders a high rate capability. Compared with commonly reported monoclinic NiP2, cubic NiP2 is found to be more electronically conductive, even comparable with pyrolytic carbon.51 For example, sole cubic NiP2 as an anode for solidstate LIBs exhibits stable cycle performance with a reversible capacity of 600 mAh g−1.6 Although with an advantage in conductivity, cubic NiP2 has not yet received much attention, and its electrochemical performance in previous reports is unsatisfactory in terms of reversible capacity, cycle stability, and rate capability. Moreover, construction of desired phosphorusrich-phases-based nanostructures remains a challenge from the perspective of materials synthesis, because the high nucleation energy of phosphorus-rich phases will cause them to decompose into metal-rich structures,52−54 leading to only the nickel-rich phases being widely considered.11,12,14−16,19−21 Herein, we demonstrate an electrode design strategy to solve the above-mentioned problems. The key to the strategy is to confine the electroactive nanoparticles into flexible conductive hosts such as carbon materials and meanwhile maintain a monodispersed nature of the electroactive particles within the hosts. Compared with the microparticles- and the agglomerates of nanoparticles-based electrodes (Scheme 1), such designed electrodes will have several benefits:36,37,55 (1) confining the nanoparticles into flexible conductive hosts with a monodispersed nature can increase the contact areas between them

Scheme 1. (a, b) Schematic illustration of the design and behavior of a conventional microparticle- and agglomeratebased electrode that shows failure of the electrode because of cracking in particles and surface coating layer, which results in loss of electrical contact. (c) Schematic illustration of the design and behavior of our nanostructured electrode that shows a strong maintaining of electrical contact between the broken particles and no cracks in the surface coating layer, because the monodispersed carbon-coated electroactive nanoparticles greatly increase the contact area between the electroactive nanoparticles and coating layer, thus decreasing the average stress suffered by the coating layer during lithium insertion and guaranteeing structure integrity.

and thus decrease the average stress suffered by the hosts upon lithium insertion, ensuring excellent structure integrity during cycling; (2) the interconnected conductive paths formed between monodispersed particles can favor fast electron transfer, guaranteeing superior rate performance; (3) downsizing the electroactive particles to the nanoscale not only shortens the Li+/e− diffusion path but also increases the contact areas between the electroactive materials and the electrolyte, thus facilitating the reaction kinetics determined by Li+ and e− transfer. It is expected that such designed electrodes can effectively restrict/buffer the large volume change and maintain structure integrity during long-term cycling, thus leading to excellent cyclability and capacity retention. Monodispersed carbon-coated cubic NiP2 nanoparticles anchored on carbon nanotubes (NiP2@C-CNTs) as a proof-of-concept were designed and synthesized. As the anodes for LIBs, excellent cyclability and capacity retention are demonstrated, and the relevant mechanism is also discussed. Furthermore, the full cells consisting of NiP2@C-CNTs anodes and LiFePO4 cathodes with impressive cycle stability demonstrate remarkable application potential of NiP2 as anodes for LIBs.

RESULTS AND DISCUSSION The synthesis of the NiP2@C-CNTs nanocomposites is illustrated in Figure 1a. The structure and morphology of the CNTs and the obtained Ni-based nanocrystals were studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). First, the NiO nanoparticle decorated CNT composites (NiO-CNTs) were prepared simply via the reflux method followed by annealing in an Ar atmosphere. As shown in Figure 1c1, crystallized NiO nanoparticles with an average size around 5.1 nm (inset of Figure 1c1) are uniformly coated on the surface of the CNTs. The crystalline nature of NiO is evidenced by the XRD results (Figure 1c2), which is well consistent with a previous report.56 Note that the CNTs used 3706

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Figure 1. (a) Schematic diagram for the synthesis of the NiP2@C-CNTs nanocomposites. The representative TEM images of (b1) acid-treated CNTs, (c1) NiO-CNTs, (d1) Ni@C-CNTs, and (e1) NiP2@C-CNTs and the XRD patterns of (b2) acid-treated CNTs, (c2) NiO-CNTs, (d2) Ni@C-CNTs, and (e2) NiP2@C-CNTs. The insets of c1, d1, and e1 show the particle size distribution of the NiO, Ni, and NiP2, respectively.

Figure 2. Morphology and structure of the as-synthesized NiP2@C-CNTs nanocomposites: (a) TEM, (b) the corresponding HAADF-STEM, (c) HRTEM, (d) SEM, and (e) Raman spectrum. (f) TGA results of the NiP2@C-CNTs nanocomposites tested under oxygen flow; the inset shows the XRD pattern of the residue of the TGA tests.

here were pretreated by refluxing in concentrated nitric acid (HNO3, 65%) to introduce surface functional groups (e.g., carboxylic and hydroxyl) and remove residual metal impurities. The diameter of the acid-treated CNTs is around 50 nm (Figure 1b1). The acid treatment does not destroy their crystalline nature, as confirmed by XRD results (Figure 1b2), in which all the peaks can be indexed to the hexagonal graphite (JCPDS No. 01-003-0401).57 Second, in order to avoid aggregation during phosphorization and get monodispersed NiP2 nanoparticles, the NiO@CNTs were coated with a thin layer of carbon by reacting with polyvinylidene fluoride (PVDF). It can be seen clearly from Figure S1b that all the nanoparticles are evenly wrapped by a thin layer of carbon and their nanosized-granular morphology is well maintained during

the carbon-coating process (Figure 1d1). Surprisingly, the rhombohedral NiO nanoparticles are simultaneously reduced to face-centered cubic Ni nanoparticles by PVDF (Figure 1d2). Besides as a reduction agent reported here, PVDF is previously reported to be an excellent fluorinating agent for the synthesis of inorganic fluorides.58 Finally, the NiP2@C-CNTs were obtained by directly phosphorizing the Ni@C-CNTs using red phosphorus at 700 °C. As shown in the TEM image, carboncoated NiP2 nanoparticles with a monodispersed nature are anchored on the surface of the CNTs (Figure 1e1). The XRD pattern (Figure 1e2) indicates that the final product is the pure NiP2 with primary cubic structure.4,5,7,59 The cubic phase of NiP2 is unexpected, because this phase is reported to be a hightemperature, high-pressure phase.52 According to the statistical 3707

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Figure 3. Electrochemical performance of the NiP2@C-CNTs nanocomposites: (a) typical galvanostatic charge−discharge profiles with a current density of 1300 mA g−1 (∼1C, 1C = full discharge/charge in 1 h); (b) rate performance and capacity retention and cycle performance with a current density of (c) 1300 mA g−1 (∼1 C) and (d) 5000 mA g−1 (∼3.75 C), indicating excellent cyclability and capacity retention.

clear boundary between NiP2 particles for the NiP2-CNT composites. This means that the NiP2 particles without carbon coating layer are prone to merge together, leading the average particle size to increase to 25 nm (Figure S3c). During the synthesis process, the morphology of most CNTs is well retained (Figure 2d). Such well-preserved CNTs can provide solid paths for fast electron transfer, and their hollow structure can relieve some of the volume change during cycling. The Raman spectrum of the NiP2@C-CNTs shown in Figure 2e displays two broad peaks centered at ∼1345 and 1595 cm−1, which correspond to D and G bands, respectively.60 The intensity ratio of the D to G band (ID/IG) is about 0.929, implying the existence of highly crystallized carbon, which can favor fast electron transfer during reaction. Furthermore, the content of NiP2 in the nanocomposites was determined by thermogravimetric analysis (TGA). The TGA results collected under oxygen flow show that the NiP2@CCNTs increase in total weight by about 28.03% in the studied temperature range (Figure 2f), suggesting a large amount of oxygen element being incorporated into the final products. To identify the final products, the TGA residue was subjected to an XRD investigation. As shown in the inset of Figure 2f and Figure S4a, the XRD patterns indicate that the NiP2@C-CNTs transferred into the pure nickel phosphate (Ni(PO3)2) during the TGA tests. Thus, the content of NiP2 in the nanocomposites is calculated to be 71.3% (a detailed calculation is given in the Supporting Information). In the course of the TGA tests, two distinct weight increase zones after a slight weight loss (∼1.6%) ascribed to the dehydration of the NiP2@C-

results based on 100 particles, the average size of Ni-based nanocrystals increases from 5.1 nm for the NiO to about 9.3 nm for the Ni@C in the course of reduction and then increases only slightly to 10.7 nm for the NiP2 during phosphorization (insets of Figure 1c1, d1, and e1). These changes in particle size and phase structure of these Ni-based nanocrystals during synthesis are further evidenced by high-resolution transmission electron microscopy (HRTEM) images, as shown in Figure S1. To further investigate the structure and morphology of the obtained NiP2@C-CNTs, TEM, scanning electron microscopy (SEM), and Raman studies were conducted. The monodispersed granular morphology of the NiP2 nanoparticles with interconnected carbon-coating layer can be seen clearly from Figure 2a,b, especially from the STEM-HAADF (scanning transmission electron microscopy high-angle annular dark-field) image (Figure 2b), where the brighter color represents the element Ni (and P) and the surrounding light region represents the element C. The HRTEM image indicates that the NiP2 nanoparticles are evenly wrapped by a thin layer of carbon (∼3 nm in thickness), as marked with a white dashed line in Figure 2c. Meanwhile, clear lattice fringes with a spacing of 2.44 Å corresponding to the (210) plane of the cubic NiP2 can be identified, which is in good agreement with the XRD results (Figure 1e2) and previous results.7 Such few-layer nanographene-like carbon (∼3 nm) encapsulation nature is further confirmed by more TEM images (Figure S2). This thin carbon layer plays a significant role in synthesizing the monodispersed NiP2@C-CNTs nanocomposites, which is evidenced by its counterpart of NiP2-CNTs. As shown in Figure S3b, there is no 3708

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cycles shows that almost all the NiP2 microparticles are peeled off the Cu current collector due to the large volume change (Figure S7a), thus resulting in a poor cycle stability (Scheme 1a). Contrary to this situation, the NiP2-CNTs anode and the NiP2@C-CNTs anode both with nanosized electroactive particles remain stable with no obvious detachment even after 500 and 1200 cycles, respectively (Figure S7b,c). Accordingly, the cycle performances of the NiP2-CNTs and NiP2@C-CNTs are improved. However, a constant capacity decay during longterm cycling is observed for the NiP2-CNTs (Figure S6c,d and Scheme 1b) but disappears for the NiP2@C-CNTs (Figure 3c). Furthermore, serious capacity decay resulting from the particle pulverization is also observed in the NiO-CNTs nanocomposites (Figure S8),56 although the particle size of the NiO is around 5.1 nm (inset of Figure 1c1). This sharp contrast between the NiP2-CNTs and the NiP2@C-CNTs clearly shows the advantage of our strategy and, more importantly, indicates that protective layers (e.g., carbon layers in this work) that can prevent the detachment of pulverized nanoparticles during long-term cycling are essential for good structure stability and thus enhanced cyclability (Scheme 1c). The rate performance of the NiP2@C-CNTs anode is shown in Figure 3b. According to the detailed charge−discharge profiles and the corresponding dQ/dV plots (Figure S9), only a small increase in the reaction overpotential occurs when the current density increases from 200 to 5000 mA g−1, illustrating the rapid reaction kinetics of the NiP2@C-CNTs anode. The NiP2@C-CNTs anode is able to deliver a stable discharge capacity of 1205, 1049, 956, 891, 831, and 755 mAh g−1 at current densities of 100, 200, 500, 1000, 2000, and 5000 mA g−1, respectively (Figure 3b). After the high-rate cycling, the reversible capacity of the NiP2@C-CNTs anode recovers to 1047 mAh g−1 as the current density decreases to 100 mA g−1. This impressive rate performance indicates that the as-designed NiP2@C-CNTs nanocomposites could tolerate high rate cycling (large volume changes) without damaging its structure integrity, thus guaranteeing the exceptional Li+ and e− mobility and excellent cyclability. The superiority of the NiP2@C-CNTs nanocomposites is further evidenced by the cycle performance at a high current density of 5000 mA g−1. This property is usually deemed as a critical indicator for practical applications. It can be seen clearly that a high reversible capacity of 654.5 mAh g−1 remained after 1500 cycles even at such a high current density of 5000 mA g−1 (Figure 3d), and all the discharge−charge curves are highly overlapped (Figure S10). These results clearly evidence the excellent electrochemical reversibility and super structure stability of such designed NiP2@C-CNTs anodes. It is worth noting that the measurement of the high-rate cyclability was carried out after 10 cycles of low rate cycling (100 mA g−1), which was employed to fully activate the electrodes. Furthermore, the comparison of the performance of the NiP2@C-CNTs with that of previously reported nickel oxideand sulfide-based anodes, nickel phosphides, and non-nickel metal phosphides (Tables S1−S3) indicates that the nickel phosphide (NiP2) as the anode has an advantage of exhibiting a lower discharge plateau than its oxide and sulfide counterparts and that such designed NiP2@C-CNTs nanocomposites exhibit better electrochemical performance, especially the cyclability and capacity retention, than previously reported metalphosphide-based anodes and even some extensively studied nickel-oxide and sulfide-based anodes. More importantly, the above results clearly demonstrate that our strategy is effective

CNTs nanocomposites are observed, which is attributed to the combined effects of the oxidation of NiP2 into Ni(PO3)2 and the partial decomposition of carbon materials. Gillan et al. reported that the NiP2 suffers poor thermal stability, which decomposes into the metal-rich phase of Ni2P over 575 °C.52 This can explain the formation of the mixtures of a metal-rich phase (Ni2P) and nickel phosphate (Ni(PO3)2) when the TGA was done under air flow with a heating rate of 10 °C min−1 (Figure S4b), in which the respective amounts of Ni2P and Ni(PO3)2 are unknown. Beyond the two distinct weight increase zones, one steep weight loss zone at 800−925 °C attributed to the complete oxidation of the carbon components occurs, leading to a ∼22.88% weight loss. Thereafter, no further weight loss can be observed until 1000 °C, as shown in Figure 2f. The effectiveness of our strategy was evaluated based on the CR2025 coin-type cells. Figure 3a shows the typical discharge− charge profiles of the NiP2@C-CNTs anodes in the voltage range of 0.01−3 V at 1300 mA g−1 (∼1 C, 1 C = full discharge/ charge in 1 h). The NiP2@C-CNTs anode delivers a discharge capacity of 1455 mAh g−1 and a charge capacity of 1034 mAh g−1 in the initial cycle, leading to a Coulombic efficiency (CE) of 71%. After a few cycles, the CE value is as high as 99%. The relatively low initial Coulombic efficiency (99.5%. Figure 7a shows the typical charge−discharge profiles of the NiP2@C-CNTs/LiFePO4 full cell cycled between 1 and 3.25 V. The full cell delivers a discharge capacity of 827 mAh g−1 based on the mass of the NiP2@C-CNTs anode with an average output of ∼2.32 V at 2000 mA g−1. After 200 cycles, the discharge capacity remains 668 mAh g−1, resulting in a high capacity retention of 80.7% (Figure 7b). Although here the average output of the NiP2@C-CNTs/LiFePO4 full cell is lower than that of the carbon anode based configuration, the NiP2 anode based full cell holds potential to deliver high energy density due to its ability to give 2 to 3 times larger reversible capacity than the carbon anode (Figure 3). Furthermore, the average output of such a configuration can be increased by resorting to a high-voltage cathode (e.g., LiNi0.5Mn1.5O4). These results indicate that the NiP2@C-CNTs have remarkable application potential for LIBs.

CONCLUSION In this work, we demonstrate an electrode design that can effectively restrict/buffer the large volume changes and guarantee electrode integrity upon repeated lithiation/delithiation. The key of the design is to confine nanosized electroactive 3712

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ization was performed on a field emission scanning electron microscope (FEI, Magellan 400). The transmission electron microscopy, high-resolution TEM, and elemental mappings were collected on a FEI Tecnai G2 F20. Thermogravimetric analysis was performed from room temperature to 1000 °C with a heating rate of 10 °C min−1 by using a NETZSCH STA 449C thermogravimetry analyzer. Raman spectra was recorded in a Thermo DXR with an excitation wavelength of 532 nm. Electrochemical Measurements. The NiP2@C-CNTs films were prepared by coating the slurry of the NiP2@C-CNTs, Super P, and PVDF with a mass ratio of 80:10:10 using NMP as the solvent on Cu foil. The electrode films were dried at 100 °C for 6 h under vacuum and then punched into disks with a diameter of 8 mm. The active material loading was about 1 mg cm−2. For the full cells, the LiFePO4 cathodes were prepared by pressing its gum-like slurry onto Al mesh, which consisted of LiFePO4 (Shenzhen Dynanonic Co., Ltd.), Super P, and PTFE binder with a mass ratio of 70:20:10. After drying at 100 °C for 10 h, the LiFePO4 films were punched into disks with a diameter of 8 mm. The CR2025 coin-type cells were assembled in an argon-filled glovebox (MBraun) using 1 M LiPF6 dissolved in a 1:1 (wt) mixture of ethylene carbonate and diethyl carbonate as the electrolytes and glass fiber (Whatman, GF/B) as the separators. The cells were galvanostatically cycled at room temperature using a program-controlled test system (LAND CT2001A). The specific capacity was calculated based on the mass of the NiP2@C-CNTs. The cyclic voltammetry measurements were carried out on an Autolab electrochemical workstation (PGSTAT302N). For ex situ SEM and TEM analysis, the cycled electrodes were disassembled from the batteries and washed with diethyl carbonate in an argon-filled glovebox.

materials into flexible conductive hosts (e.g., carbon materials) and meanwhile maintain a monodispersed nature of the electroactive particles within the hosts. Monodispersed carbon-coated cubic NiP2 nanoparticles anchored on CNTs (NiP2@C-CNTs) as a proof-of-concept were designed and synthesized. The as-designed NiP2@C-CNTs exhibit impressive electrochemical performance in terms of reversible capacity, cyclability, and capacity retention. Specifically, high reversible capacities of 816 and 654.5 mAh g−1 can be achieved after 1200 cycles at 1300 mA g−1 (∼1C) and after 1500 cycles at 5000 mA g−1 (∼3.75 C), respectively. EIS, CV, and postmortem morphology analyses reveal that the excellent performance of the NiP2@C-CNTs is ascribed to the excellent structure stability and enhanced reaction kinetics. The full cells consisting of NiP2@C-CNTs anodes and LiFePO4 cathodes with an average output of ∼2.32 V exhibit good cycle performance (200 cycles with a high capacity retention of 80.7% at 2000 mA g−1). These results demonstrate the effectiveness of our strategy on improving the electrode performance. We believe that our study may provide inspiration to construct high-performance anodes based on other phase-transformation-type materials, which suffer large volume changes during cycling.

EXPERIMENTAL METHODS Purification of the Carbon Nanotubes (Acid-Treated CNTs). The CNTs used here (Shenzhen Nanotech. Port. Co., Ltd.) were first purified by refluxing in concentrated nitric acid (HNO3, 65%) at 150 °C for 6 h. Then the resulting CNTs were filtered, washed with deionized water until pH ∼7, and dried under vacuum at 100 °C for 12 h. Synthesis of the NiO-CNTs Nanocomposites. The synthetic procedure was performed based on a previous report.41 In a typical synthesis, 500 mg of purified CNTs and 2.9 g (10 mmol) of Ni(NO3)2·6H2O (TCI) were added into 350 mL of 1-Methyl-2pyrrolidincone (NMP) (Sinopharm Chemical Reagent Co., Ltd.) and sonicated for 1.5 h. Then the suspension was transferred into a threenecked flask (500 mL) and refluxed at 180 °C for 1 h with magnetic stirring. The resulting products were collected by centrifugation, washed with deionized water and ethanol several times, and dried at 90 °C under vacuum for 12 h. Finally, the black precipitate was annealed at 350 °C under Ar flow for 1 h, resulting in the NiO-CNT nanocomposites. Synthesis of the Ni@C-CNTs Nanocomposites. Afterward, 100 mg of the above obtained NiO-CNTs composites was mixed with 50 mg of PVDF (Alfa). Then the mixture was pressed into a pellet with a diameter of 10 mm and sintered at 400 °C under Ar flow for 12 h. After cooling, the sample was collected and denoted as Ni@C-CNTs. Synthesis of the NiP2@C-CNTs Nanocomposites. Then, 50 mg of Ni@C-CNTs and 100 mg of red phosphorus (TCI) were ground in an argon-filled glovebox, then sealed into a quartz tube (⦶ 8 mm × 150 mm) under vacuum. The sealed tube was kept at 700 °C for 6 h with a ramp of 5 °C min−1. After cooling, the final products were collected in an argon-filled glovebox and rinsed with carbon disulfide (CS2) to remove the white phosphorus formed from the unreacted red phosphorus. Synthesis of the NiP2-CNTs Composites and NiP2 Microparticles. Both of the samples were synthesized in a sealed quartz tube as described above. The NiP2-CNTs were synthesized by phosphorizing the collected reflux products during synthesizing the NiO-CNTs at 700 °C for 6 h. The NiP2 microparticles were obtained by heating the mixture of Ni powders (100 nm, Aladdin) and red phosphorus at 700 °C for 10 h. These two samples were collected in an argon-filled glovebox and rinsed with carbon disulfide (CS2) to remove the white phosphorus formed from the unreacted red phosphorus. Physicochemical Characterizations. The phase structure was identified by a Bruker D2 diffractometer. The morphology character-

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b08223. Additional experimental data (Figure S1−S15 and Table S1−S3) (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail (Z. Cui): [email protected]. *E-mail (X. Guo): [email protected]. ORCID

Zhonghui Cui: 0000-0002-5195-0263 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Key Basic Research Program of China (2014CB921004), the National Natural Science Foundation of China (51402339), and the China P o s t d o ct o r a l Sc i e n c e Fo un d at i o n ( 20 1 5L H 0 02 6 , 2015M581667). REFERENCES (1) Armand, M.; Tarascon, J. M. Building Better Batteries. Nature 2008, 451, 652−657. (2) Tarascon, J. M.; Armand, M. Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2001, 414, 359−367. (3) Souza, D. C.; Pralong, V.; Jacobson, A. J.; Nazar, L. F. A Reversible Solid-State Crystalline Transformation in a Metal Phosphide Induced by Redox Chemistry. Science 2002, 296, 2012− 2017. (4) Gillot, F.; Boyanov, S.; Dupont, L.; Doublet, M. L.; Morcrette, M.; Monconduit, L.; Tarascon, J. M. Electrochemical Reactivity and 3713

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