NiS0.66 Heterostructures and Their

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Assembly of Multifunctional Ni2P/NiS0.66 Heterostructures and Their Superstructure for High Lithium and Sodium Anodic Performance Tian Wu,†,‡ Sanpei Zhang,†,‡ Qiming He,†,‡ Xiaoheng Hong,†,‡ Fan Wang,†,‡ Xiangwei Wu,† Jianhua Yang,† and Zhaoyin Wen*,† †

CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100039, P. R. China S Supporting Information *

ABSTRACT: The combination of structure designs at the microscopic and macroscopic level can efficiently enable electrode materials with greatly enhanced lithium and sodium storage. In this paper, the construction of Ni2P/NiS0.66 heterostructures and their assembly into a superstructure at the nanoscale were successfully achieved by a facile and effective strategy. In the obtained superstructure, the Ni2P/ NiS0.66 heterostructures are homogeneously coated with ultrathin carbon layers (HT-NPS@C) and, at the same time, assembled into a yolk−shell nanosphere. Upon evaluation as the anode materials for Li-ion batteries, the HT-NPS@C delivers a high reversible capacity of 430 mA h g−1 after 200 cycles at 200 mA g−1 and ultrastable cyclability with negligible capacity loss over 500 cycles. Furthermore, the coin-type full cell with the LiNi1/3Co1/3Mn1/3O2 (LNCMO) cathode and HT-NPS@C anode deliver a high specific capacity of 323.5 mA h g−1 after 50 cycles at 0.3 A g−1. Apart from an excellent performance as promising anode materials for LIBs (Li-ion batteries), the Na-ion batteries with HT-NPS@C sphere electrodes also manifest a remarkable electrochemical performance. KEYWORDS: Li-ion battery, Na-ion battery, sulfur dopant, nanocrystals, heterostructures, superstructure

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INTRODUCTION With rechargeable battery technology poised to move into larger-scale applications, such as electric vehicles and portable devices, intensive research has targeted the optimization of Liion batteries (LIBs) and Na-ion batteries (NIBs).1−3 Graphite, the most commonly used commercial anode material for LIBs, suffers from limited theoretical specific capacity (372 mA h g−1) and rate capability.4,5 Meanwhile, it is still challenging for the graphite anode to achieve the ideal practical capacity for NIBs (only 31 mA h g−1).6−9 For the development of highperformance LIBs and NIBs that satisfy the requirements for future energy storage systems, various electrode materials (Si,10 Sn,11 and Ge12) based on an alloy/dealloy mechanism have been exploited and studied.13−15 However, these high-capacity anode materials still face many problems, such as an extremely large volume variation (∼400%) during cycling and a large irreversible capacity. Apart from the Li-alloying materials, transformation-type materials, such as NiO,16 MnF2,17 MoS2,18 and so forth, with a higher theoretical capacity have been recently investigated as alternative anode materials for LIBs. Recently, transition-metal phosphides with high conductivity and activity have attracted great attention as high-performance anode materials for LIBs and NIBs. For instance, Feng et al.19 synthesized sandwiched Ni2P nanoparticles between graphene sheets using a template-assisted strategy. The electrode with © 2017 American Chemical Society

sandwiched Ni2P nanoparticles delivered a high reversible capacity of 625 mA h g−1 at 0.2 C and remarkable high-rate cyclability. Qian and his co-workers20 fabricated core−shell Sn4P3/C nanocomposites by a high-energy mechanical milling method. The electrode based on Sn4P3/C nanocomposites exhibited a high reversible capacity of around 500 mA h g−1 at 0.1 A g−1 for 150 cycles. In addition, various structure designs for metal phosphides including nanotubes,21 nanosheets,22 hollow spheres,23 and nanorods23,24 have been developed to acquire encouraging lithium and sodium storage performance. The design and synthesis of transition-metal phosphide nanostructures have endowed them with a large quantity of active sites and high surface-to-volume ratios for Li and Na storage. Nonetheless, because of their high surface energy, it is inevitable that the designed nanocrystals with large numbers of active surfaces aggregate during long cycles. Moreover, the transport kinetics of charge carriers is greatly hindered across the overaccumulated nanocrystals. Precisely constructing the heterostructures and their assembly into a three-dimensional (3D) superstructure can not only quicken the transport kinetics in the crystal structures but also preserve the nanocrystal Received: June 4, 2017 Accepted: August 1, 2017 Published: August 1, 2017 28549

DOI: 10.1021/acsami.7b07939 ACS Appl. Mater. Interfaces 2017, 9, 28549−28557

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ACS Applied Materials & Interfaces

Figure 1. Schematic representation of the formation process via low-temperature heating processes: step 1, formation of the Ni-precursor solid spheres at 150 °C for 10 h; step 2, preparation of the carbon-coated nanocrystal-assembly nickel phosphide spheres with yolk−shell structure (NP@ C) after phosphidation in N2 at 450 °C for 2 h; step 3, fabrication of Ni2P/carbon yolk−shell spheres with abundant Ni2P/NiS0.66 heterointerfaces and S/C defects (HT-NPS@C) after subsequent sulfidation in N2 at 450 °C for 2 h.

hydrothermal method.25,26 Then, the obtained nanospheres are thermally treated at 450 °C in PH3 vapor (step 2), during which time inner Ni precursors of the nanospheres are phosphorized to Ni2P. Meanwhile, the outer organic layers convert into ultrathin carbon layers (denoted as NP@C). Subsequently, the fast sulfidation process (step 3) is carried out to construct the Ni2P/NiS0.66 heterostructures and the bifunctional sulfur-doped coating layers (HT-NPS@C). The scanning electron microscopy (SEM) image (Figure S1a) and transmission electron microscopy (TEM) image (Figure S1b) show that the as-synthesized Ni precursor consists of microspheres with an average diameter of about 1.3 μm. The broadness and weakness of all the peaks for Ni precursors reveal the poor crystallinity and small size of the crystallite (Figure S2). After being annealed in PH3 vapor, the Niprecursor nanospheres are transformed into Ni2P@C nanospheres, as confirmed by energy dispersive X-ray spectroscopy (EDS) images (Figure S3a−d) and powder X-ray diffraction (XRD) measurements (Figure S4). Compared with the Ni precursors, the NP@C nanospheres show no obvious changes in structure and size. After the subsequent sulfidation with thiourea at the same temperature, the final particles (HTNPS@C) are obtained. The SEM images (Figure 2a,b) of the HT-NPS@C show that these particles exhibit a morphology of well-maintained spheres. XRD analysis further confirms the phase-pure properties of the final particles. As demonstrated in Figure 2c, the HT-NPS@C sample exhibits a clear hexagonal structure of Ni2P (JCPDS 74-1385). Furthermore, a small and weak peak of NiS0.66 (JCPDS 44-1418) at 2θ = 33.15° can also

structures during the discharge and charge process. However, the construction of Ni2P-based heterostructures and their assembly into a superstructure for LIBs and NIBs have rarely been reported. Herein, we demonstrate a simple and effective method to construct Ni2P/NiS0.66 heterostructures and control their assembly into a superstructure (denoted as HT-NPS@C) at the nanoscale. In the obtained superstructure, the Ni2P/NiS0.66 heterostructures are homogeneously coated with ultrathin carbon layers and, at the same time, assembled into a yolk− shell nanosphere. Upon evaluation as the anode materials for Li-ion batteries, the HT-NPS@C delivers a high reversible capacity of 430 mA h g−1 after 200 cycles at 200 mA g−1 and ultrastable cyclability with negligible capacity loss over 500 cycles. Furthermore, the coin-type full cell with a LiNi1/3Co1/3Mn1/3O2 (LNCMO) cathode and an HT-NPS@C anode delivers a high specific capacity of 323.5 mA h g−1 after 50 cycles at 0.3 A g−1. Apart from an excellent performance as promising anode materials for LIBs, the Na-ion batteries with HT-NPS@C sphere electrodes also manifest a remarkable electrochemical performance. We believe the present strategy for HT-NPS@C can be further applied for other nanomaterials to obtain an enhanced electrochemical response.

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RESULTS AND DISCUSSION The overall strategy for the preparation of HT-NPS@C is schematically depicted in Figure 1. The detailed preparation procedures are described in the Supporting Information. In the first step, Ni precursors are prepared from a modified 28550


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Figure 2. (a, b) SEM images of HT-NPS@C. (c) XRD pattern of HT-NPS@C. (d, e) TEM images of HT-NPS@C. (f) Electron diffraction of HTNPS@C. (g−i) HRTEM images of HT-NPS@C, (j) the selected TEM image, and the corresponding elemental EDS mapping images of (k) nickel, (l) phosphorus, (m) carbon, and (n) sulfur.

be found in the pattern. It might be the result of the structural change from Ni2P into NiS0.66 during the sulfidation process. The detailed structure and morphology of the HT-NPS@C samples are studied by TEM and high-resolution TEM (HRTEM) analysis. As shown in Figure 2d, Ni2P spheres possess a yolk−shell architecture with average diameters of the yolk and shell of about 600 and 1000 nm, respectively. The higher magnification TEM image in Figure 2e and the corresponding selected-area electron diffraction (SAED) patterns (Figure 2f) show that the shell and yolk are dotted with ultrafine crystallized Ni2P nanoparticles. Each nanocrystal has a diameter of approximately 25 nm. HRTEM images (Figure 2f,g) further reveal that onionlike carbon shells with a thickness of 2−5 nm cover the surface of the Ni2P nanoparticles. The carbon content in the HT-NPS@C sample estimated from the TGA/DSC analysis (Figure S5) is 3.3 wt %. Notably, the additional lattice spacing of 0.41 nm (Figure 2i) can also be observed, which is assigned to the (011) facet of

hexagonal NiS0.66. Interestingly, the heterostructure constructed by the (011) facet of NiS0.66 and the neighboring (111) facets of Ni2P can be clearly observed from the HRTEM image. More importantly, the Ni2P/NiS0.66 heterostructure is also homogeneously coated by the ultrathin carbon layers at the nanoscale. Although the transition-metal phosphides with well-defined sphere nanostructures have been previously reported,23,27,28 the construction of Ni2P/NiS0.66 heterostructures and their assembly into a superstructure are successfully achieved for the first time. X-ray elemental mappings recorded from an individual yolk−shell sphere (Figure 2j−n) reveal that different elements of Ni (green), P (purple), C (red), and S (yellow) are homogeneously distributed throughout the whole yolk−shell sphere. The TEM-EDS analysis was used to determine the S content in the HT-NPS@C sample. As shown in Figure S6, the S content of HT-NPS@C is about 2.8 atom %, which is consistent with the X-ray photoelectron spectroscopy (XPS) result (ca. 2.5 atom %) (Table S1). 28551


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Figure 3. High-resolution XPS spectra of HT-NPS@C and NP@C in the (a) Ni 2p and (b) P 2p regions. (c) High-resolution S 2p XPS spectra of HT-NPS@C at the surface and an etched domain of 3 nm depth. (d) Nitrogen adsorption−desorption isotherm of the HT-NPS@C. The inset shows the pore size distribution.

respectively.31−33 These results further confirm the successful introduction of the S dopant in the Ni2P/carbon nanocrystals. Full nitrogen sorption isotherms (Figure 3d) of HT-NPS@C were carried out to obtain information on the specific surface area and corresponding pore size distribution. The nitrogen adsorption−desorption isotherms of the as-prepared HTNPS@C indicate type-IV curves of mesoporous materials. The Barrett−Joyner−Halenda (BJH) pore size distribution curve (inset of Figure 3d) shows a broad peak ranging from 2 to 15 nm and a narrow peak at 5.0 nm, confirming the nanoporous nature of HT-NPS@C. The Brunauer−Emmett− Teller (BET) specific surface area of HT-NPS@C is calculated to be 88.4 m2 g−1. Such a high specific area of a yolk−shell structure with mesoporous nanocrystals is desirable for Li-ion and Na-ion batteries, which can provide a sufficient interface between the electroactive materials and the electrolyte.34 Hoping that these interesting HT-NPS@C spheres with abundant Ni2P/NiS0.66 heterointerfaces, S/C defects, and high surface areas are promising anode materials, we have prepared Li-ion batteries with the HT-NPS@C-based cathode and lithium metal foil as the counter electrodes. Figure 4a shows the CV curves of the HT-NPS@C electrode for the initial five scans at a scan rate of 1 mV s−1 from 0.01 to 3 V versus Li/Li+. By the assignment of the cathodic and anodic peaks to the redox reactions, the electrochemical mechanism for Ni2P is considered to be a conversion-type process.35,36 Figure 4b

The detailed surface chemistry of the HT-NPS@C and NP@ C samples was characterized by XPS measurements. Figure 3a− c presents the Ni 2p, P 2p, and S 2p spectra of NP@C and HTNPS@C samples. The survey scan spectra from XPS analysis of the HT-NPS@C and NP@C are shown in Figure S7. The S 2p peak of the HT-NPS@C sample located at ∼160 eV provides evidence of the introduction of S species. As can be seen in Figure 3a, the high-resolution XPS spectrum of the Ni element of the NP@C indicates two main peaks located at 854.2 and 871.6 eV, corresponding to the Ni 2p3/2 and Ni 2p1/2 of the Ni−P bond,29,30 respectively. After sulfidation, the Ni 2p3/2 and Ni 2p1/2 peaks of the HT-NPS@C display an obvious shift to 853.7 and 871.1 eV, respectively. The shifts of the Ni 2p peak and P 2p peak (Figure 3b) suggest the substitution of P by highly electronegative S.31 For an understanding of the chemical bonding of S atoms with carbon, the high-resolution C 1s spectra of HT-NPS@C and NP@C were fitted and are presented in Figure S8a,b. It is noted that the peak located at 283.9 eV, which is assigned to C−S−C, demonstrates the effective atomic hybridization of sulfur into the carbon lattice. Furthermore, in-depth XPS tests were conducted to verify the sulfur dopant. Figure 3c presents the S 2p XPS at the surface and an etched domain of 3 nm depth, where S species both are clearly identified. The S 2p peak can be deconvoluted into four peaks located at 164.9, 163.7, 162.4, and 161.3 eV, which are assigned to S 2p3/2, S 2p1/2 of C−Sx−C, sulfide, and thiolate, 28552


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Figure 4. Electrochemical performance for lithium storage. (a) Cyclic voltammogram curves of HT-NPS@C electrodes scanned between 0.01 and 3.0 V at a rate of 0.1 mV s−1. (b) Galvanostatic curves during the initial five cycles. (c) Rate cycling behavior of the HT-NPS@C and NP@C electrodes. (d, e) Cycling performance of the HT-NPS@C and NP@C electrodes at 0.2 A g−1 for 200 cycles and 0.4 A g−1 for 500 continuous cycles.

can only deliver a reversible capacity of 400 mA h g−1, lower than that of HT-NPS@C (Figure S9). The enhanced reversible capacity suggests that the Ni2P/NiS0.66 and S/C heterointerfaces in HT-NPS@C nanocrystals can produce more Liion insertion sites and faster reaction kinetics for reoxidation of Ni to Ni2P. Electrochemical impedance spectroscopy (EIS) measurements were carried out on the HT-NPS@C and NP@ C electrodes. Figure S10 shows that the HT-NPS@C electrode exhibits the reduced resistance, further confirming its fast Li reaction kinetics. The superiority of the HT-NPS@C electrode is also demonstrated via its prevailing rate capability in Figure 4c. Moreover, the HT-NPS@C electrode shows a high capacity

displays the galvanostatic charge−discharge profiles of the HTNPS@C electrode for the initial five cycles at 0.1 A g−1. The obscure plateau centered at ∼1.9 V is ascribed to the oxidation of Li2S to Li ions and to NiS0.66,37,38 which is in accordance with the CV results. In addition, the HT-NPS@C electrode exhibits a high initial discharge and charge capacity of 992.8 and 538.5 mA h g−1, respectively. The irreversible capacity is attributed to the formation of an SEI layer in the first cycle.39 Notably, the contribution of NiS0.66 to the capacity should be negligible (less than 30 mA h g−1) as the S content is considerably low (2.58 atom % in average). NP@C contains the same content of Ni2P crystals, but the NP@C-based battery 28553


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Figure 5. (a) SEM and (b) TEM images of the HT-NPS@C electrode after 500 cycles at 0.2 A g−1. (c) Structural evolution of the carbon-coated Ni2P/NiS0.66 nanoparticle electrode during cycling.

Figure 6. Full cell lithium-ion battery performance. (a) Charge−discharge profile of the HT-NPS@C//LNCMO cells. Inset shows the cyclic performance profile of the HT-NPS@C//LNCMO cells. (b) Rate performance of the HT-NPS@C//LNCMO cells.

in Figure 5c. During the initial lithiation and delithiation processes, the protective layer is elastic and capable of tolerating a large volume expansion of Ni2P/NiS0.66 nanoparticles without particle pulverization and structural rupture. Significantly, with the effects of the elastic coating, the electrode is expected to retain the original structure, and a stable SEI film is formed on the nanocrystal surface during cycling. Therefore, the carbon-coated Ni2P/NiS0.66 yolk−shell spheres are likely to be preserved over prolonged cycling, thereby leading to high capacity retention and excellent cycling stability. To our knowledge, metal phosphides have rarely been reported in a full coin cell. For a demonstration of the newly fabricated HT-NPS@C as a potential anode in the coin-type full cells, the LiNi1/3Co1/3Mn1/3O2 (LNCMO) cathode was analyzed within the voltage range 1.5−4.0 V. As the capacity delivered by the commercial LNCMO cathodes is around 150 mA h g−1 (Figure S11), the mass of the HT-NPS@C electrode

retention of 78% over 200 cycles (Figure 4d). In contrast, the capacity of the NP@C electrode is 174.2 mA h g−1 after 200 cycles with a much lower capacity retention of 45%. Long-term cycling stability is also a vital parameter for the practicability of high-power batteries. As shown in Figure 4e, the HT-NPS@C electrode was measured at a higher current density of 0.4 A g−1 for 500 cycles. High reversible capacity and sustainable capacity retention could be achieved for the HT-NPS@C electrode, which retains a stable capacity of 423.2 mA h g−1 without significant capacity decay. The enhancement of cycling performance of HT-NPS@C is remarkable compared to those in the previous reports on Ni2P nanomaterials (Table S2). Meanwhile, as shown in Figure 5a,b, the HT-NPS@C superstructure is still detectable even after 500 cycles under the current density of 400 mA g−1. The structural evolution of the carbon-coated Ni2P/NiS0.66 nanoparticles during the initial lithiation and delithiation processes are schematically depicted 28554


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Figure 7. Electrochemical performance of the HT-NPS@C electrodes for NIBs: (a) Cyclic voltammogram curves of the HT-NPS@C electrode collected at a scan rate of 0.2 mV s−1 within the voltage range 0.01−3 V. (b) Galvanostatic curves during initial cycles. (c) Rate cycling behavior. (d) Cycle performance of the HT-NPS@C electrodes and NP@C electrodes at 0.2 A g−1.

was limited to about 1.3 mg cm−2 to match the electrodes. The specific capacity of the HT-NPS//LNCMO is calculated based on the mass of the anode. Figure 6a shows that the HT-NPS@ C anode has an initial specific charge and discharge capacity of 513.3 and 476.7 mA h g−1, respectively. In the inset of Figure 6a, the HT-NPS@C//LNCMO full cell shows a reversible capacity of 349.3 mA h g−1 at the current density of 0.3 A g−1 over 50 cycles, which confirms that the as-prepared anode material is a suitable anode material for lithium-ion batteries. Furthermore, the rate capability of the HT-NPS@C//LNCMO full cell was tested at incremental current densities from 0.2 to 4 A g−1 for every five cycles. The HT-NPS@C electrode exhibits a reversible capacity of 443.5, 326.1, 231.7, 203.6, 140.3, and 83.4 mA h g−1 at 0.2, 0.4, 0.8, 1, 2, and 4 A g−1, respectively (Figure 6b). The rate cycling behavior is similar to the result of half cells in Figure 4c. When the current density returns to the initial rate after high-rate cycling, the capacity can return to 349.8 mA h g−1. These results confirm that the HT-NPS@C holds promise in anode applications for energy storage devices. Apart from the excellent lithium storage performance, the sodium storage behavior of the HT-NPS@C electrodes was also investigated and shown in Figure 7. Figure 7a shows the CV curves of the HT-NPS@C electrode. In the first cycle, the peak at 0.6 V could be attributed to the formation of SEI films and the reduction of Ni2P to Ni and Na3P. The charge− discharge profiles (Figure 7b) display that the initial discharge and charge capacities of the HT-NPS@C electrodes are 519.1

and 320.8 mA h g−1, respectively. Because of the larger size of the Na ion compared to the Li ion, the reversible capacity of the HT-NPS@C electrode for sodium storage is lower than that for lithium storage. Rate performance of the HT-NPS@C and NP@C electrodes in NIBs was also studied. As shown in Figure 7c, the reversible capacities of the HT-NPS@C electrode are 320.8, 222.7, 180.6, 147.1, 133.2, and 111.5 mA h g−1 at current densities of 0.1, 0.2, 0.4, 0.8, 1, 2, and 2 A g−1, respectively. More importantly, when the current density returns to the initial rate after high-rate cycling, the capacity can recover to 189.7 mA h g−1. As expected, the rate performance is superior to those of the NP@C electrode. Figure 7d shows that the HT-NPS@C electrodes exhibit superior cycle stability with a decaying rate of 0.94% capacity per cycle. It is identified that the HT-NPS@C electrode also has high stability upon Na+ intercalation/deintercalation. The above evidence clearly demonstrates that our HTNPS@C anode possesses high reversible Li+/Na+ capacities, an excellent Coulombic efficiency, and a super long-life cycling capability at high rates, which should be tightly related to its unique superstructure that offers the following advantages. First, the prepared HT-NPS@C with large surface area can offer enough interface area between the HT-NPS@C electrode and electrolyte. Moreover, the nanocrystal-assembled spheres provide large amounts of active sites, effectively giving rise to the rate performance of the batteries. Second, the Ni2P/NiS0.66 heterostructures of the nanocrystal in the spheres facilitate the 28555


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ACS Applied Materials & Interfaces fast transport of Li+ and electrons. More importantly, the highly flexible ultrathin carbon nanosheets that homogeneously anchor on the surface of the Ni2P/NiS0.66 heterostructures can effectively alleviate the volume change of HT-NPS@C during cycling, which is beneficial to the ultralong cycling performance.

(4) Kaskhedikar, N. A.; Maier, J. Lithium Storage in Carbon Nanostructures. Adv. Mater. 2009, 21, 2664−2680. (5) Fan, H.-H.; Li, H.-H.; Huang, K.-C.; Fan, C.-Y.; Zhang, X.-Y.; Wu, X.-L.; Zhang, J.-P. Metastable Marcasite-FeS2 as a New Anode Material for Lithium Ion Batteries: Cnfs-Improved Lithiation/ Delithiation Reversibility and Li-Storage Properties. ACS Appl. Mater. Interfaces 2017, 9, 10708−10716. (6) Stevens, D.; Dahn, J. The Mechanisms of Lithium and Sodium Insertion in Carbon Materials. J. Electrochem. Soc. 2001, 148, A803− A811. (7) Doeff, M. M.; Ma, Y.; Visco, S. J.; De Jonghe, L. C. Electrochemical Insertion of Sodium into Carbon. J. Electrochem. Soc. 1993, 140, L169−L170. (8) Wan, F.; Wu, X.-L.; Guo, J.-Z.; Li, J.-Y.; Zhang, J.-P.; Niu, L.; Wang, R.-S. Nanoeffects Promote the Electrochemical Properties of Organic Na2C8H4O4 as Anode Material for Sodium-Ion Batteries. Nano Energy 2015, 13, 450−457. (9) Xing, Y. M.; Zhang, X. H.; Liu, D. H.; Li, W. H.; Sun, L. N.; Geng, H. B.; Zhang, J. P.; Guan, H. Y.; Wu, X. L. Porous Amorphous Co2P/N, B-Co-Doped Carbon Composite as an Improved Anode Material for Sodium-Ion Batteries. ChemElectroChem 2017, 4, 1395. (10) Wu, H.; Yu, G.; Pan, L.; Liu, N.; McDowell, M. T.; Bao, Z.; Cui, Y. Stable Li-Ion Battery Anodes by in-Situ Polymerization of Conducting Hydrogel to Conformally Coat. Silicon Nanoparticles. Nat. Commun. 2013, 4, 1943. (11) Hassoun, J.; Derrien, G.; Panero, S.; Scrosati, B. A Nanostructured Sn−C Composite Lithium Battery Electrode with Unique Stability and High Electrochemical Performance. Adv. Mater. 2008, 20, 3169−3175. (12) Xue, D.-J.; Xin, S.; Yan, Y.; Jiang, K.-C.; Yin, Y.-X.; Guo, Y.-G.; Wan, L.-J. Improving the Electrode Performance of Ge through Ge@C Core−Shell Nanoparticles and Graphene Networks. J. Am. Chem. Soc. 2012, 134, 2512−2515. (13) Dunn, B.; Kamath, H.; Tarascon, J.-M. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928−935. (14) Linic, S.; Christopher, P.; Ingram, D. B. Plasmonic-Metal Nanostructures for Efficient Conversion of Solar to Chemical Energy. Nat. Mater. 2011, 10, 911−921. (15) Ong, S. P.; Chevrier, V. L.; Hautier, G.; Jain, A.; Moore, C.; Kim, S.; Ma, X.; Ceder, G. Voltage, Stability and Diffusion Barrier Differences between Sodium-Ion and Lithium-Ion Intercalation Materials. Energy Environ. Sci. 2011, 4, 3680−3688. (16) Zou, F.; Chen, Y. M.; Liu, K.; Yu, Z.; Liang, W.; Bhaway, S. M.; Gao, M.; Zhu, Y. Metal Organic Frameworks Derived Hierarchical Hollow NiO/Ni/Graphene Composites for Lithium and Sodium Storage. ACS Nano 2016, 10, 377−386. (17) Rui, K.; Wen, Z.; Lu, Y.; Jin, J.; Shen, C. One-Step Solvothermal Synthesis of Nanostructured Manganese Fluoride as an Anode for Rechargeable Lithium-Ion Batteries and Insights into the Conversion Mechanism. Adv. Energy Mater. 2015, 5, 1401716. (18) Zhang, S.; Chowdari, B.; Wen, Z.; Jin, J.; Yang, J. Constructing Highly Oriented Configuration by Few-Layer MoS2: Toward HighPerformance Lithium-Ion Batteries and Hydrogen Evolution Reactions. ACS Nano 2015, 9, 12464−12472. (19) Feng, Y.; Zhang, H.; Mu, Y.; Li, W.; Sun, J.; Wu, K.; Wang, Y. Monodisperse Sandwich-Like Coupled Quasi-Graphene Sheets Encapsulating Ni2P Nanoparticles for Enhanced Lithium-Ion Batteries. Chem. - Eur. J. 2015, 21, 9229−9235. (20) Qian, J.; Xiong, Y.; Cao, Y.; Ai, X.; Yang, H. Synergistic NaStorage Reactions in Sn4P3 as a High-Capacity, Cycle-Stable Anode of Na-Ion Batteries. Nano Lett. 2014, 14, 1865−1869. (21) Lu, Y.; Tu, J.; Xiong, Q.; Qiao, Y.; Zhang, J.; Gu, C.; Wang, X.; Mao, S. X. Carbon-Decorated Single-Crystalline Ni2P Nanotubes Derived from Ni Nanowire Templates: A High-Performance Material for Li-Ion Batteries. Chem. - Eur. J. 2012, 18, 6031−6038. (22) Lu, Y.; Tu, J.-p.; Xiong, Q.-q.; Zhang, H.; Gu, C.-d.; Wang, X.-l.; Mao, S. X. Large-Scale Synthesis of Porous Ni2P Nanosheets for Lithium Secondary Batteries. CrystEngComm 2012, 14, 8633−8641.

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CONCLUSIONS In summary, uniform-nanocrystal-assembled HT-NPS@C was successfully synthesized by a facile hydrothermal method and subsequent heating processes. The Ni2P/NiS0.66 heterointerfaces and S/C defects constructed in HT-NPS@C improve the reaction kinetics and ion-transport speeds, which can accommodate more Li+/Na+ and also facilitate the insertion and extraction of Li+/Na+. In addition, the nanocrystalassembled yolk−shell structure with a high surface area, appropriate pore size, and large pore volume can serve as high-capacity reservoirs for Li+/Na+, and also provide a large electrode/electrolyte contact area and short diffusion distance. Because of these unique advantages, the synthesized HTNPS@C exhibited a high and stable performance when used as anode materials in LIBs and NIBs. We believe that our work could provide a promising strategy for designing other advanced nanocomposite materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07939. Experimental details, SEM, XRD, TEM, TG/DSC, XPS, EDS, EIS, and electrochemical performance of HTNPS@C and NP@C samples (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-21-52411704. Fax: +86-21-52413903. E-mail: [email protected]. ORCID

Zhaoyin Wen: 0000-0003-1698-7420 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for funding from the National Science Foundation of China (NFSC) Projects 51672300, 51402330, and 51372262: opening projects of CAS Key Laboratory of Materials for Energy Conversion. We thank Prof. B. V. R. Chowdari (School of Materials Science and Engineering, Nanyang Technological University) for helpful discussion.

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REFERENCES

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