Improved Electrochemical Performance of Self-Assembled

ARTICLE pubs.acs.org/JPCC

Improved Electrochemical Performance of Self-Assembled Hierarchical Nanostructured Nickel Phosphide as a Negative Electrode for Lithium Ion Batteries Y. Lu,† J. P. Tu,*,† J. Y. Xiang,‡ X. L. Wang,† J. Zhang,† Y. J. Mai,† and S. X. Mao§ †

State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡ Narada Power Source Co., Ltd. Lin'an Economic Development Zone, Hangzhou 311105, China § Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States

bS Supporting Information ABSTRACT: Hierarchical, nanostructured nickel phosphide (h-Ni2P) spheres are synthesized by a one-pot reaction from an organic-phase mixture of nickel acetylacetonate, trioctylphosphine, tri-n-octylamine, and oleylamine (OAm). OAm is used as a surfactant to modify the surface morphology of Ni2P spheres. The h-Ni2P spheres are composed of ordered nanoparticles with 5
10 nm sizes and filled by amorphous carbon. The hierarchical structure can greatly increase the contact area between Ni2P and electrolyte, which provides more sites for Li+ accommodation, shortens the diffusion length of Li+, and enhances the reactivity of the electrode reaction. Also, the amorphous carbon and the hierarchical Ni2P nanostructures can buffer volume expansion and thus increase the electrode stability during cycling. In the context of storage behavior, the h-Ni2P electrode exhibits high capacity as well as Coulombic efficiency. After 50 cycles, the reversible capacity of h-Ni2P spheres is 365.3 mA h g
1 at 0.5 C and 257.8 mA h g
1 at 1 C, much higher than that of Ni2P spheres (97.2 mA h g
1 at 0.5 C). At a high rate of 3 C, the specific capacity of h-Ni2P is still as high as 167.1 mA h g
1.

’ INTRODUCTION In the field of lithium ion batteries, many scientific and practical investigations are currently focused on improving the efficiency of the anode conversion reaction and enhancing the electrode reaction kinetics.1
8 Transition-metal phosphides (TMPs, where M = Fe, Co, Ni, Cu, etc.) have gained much attention due to their low polarization and high gravimetric capacity.2,9
13 The mechanism of Li+ reactivity in TMPs differs from the classical Li+ intercalation/deintercalation or Li-alloying process but involves the formation and decomposition of Li3P. The electrochemical conversion mechanism of TMPs toward Li+ is shown as follows:14
16 MPx þ 3xLiþ þ 3xe
T xLi3 P þ M0

ð1Þ

These so-called conversion reactions17 offer a new type of energy storage involving a larger exchange of Li.18 In recent years, many efforts have been devoted to electrochemically exploring Ni
P systems as negative electrodes.2,13,14,18
20 Although TMPs show capacities greater than that of graphite, as demonstrated for NiP2,2 their quite low capacity retention still falls short to be suitable for practical applications.10,19,21
23 Therefore, the key for exploiting these phosphide-based materials in future applications lies in controlling the kinetics of Li+ insertion, thereby improving the efficiency of the conversion reaction. r 2011 American Chemical Society

Construction of nanoscaled materials usually aims at enhancing their specific area so that the kinetics would be improved since the nanomaterials facilitate the transportation of Li+ ions by offering a shorter solid-state diffusion length, along with a better buffering of the active material volume expansion. This new architecture would lead to a better cyclability, even though, in the normal lithium ion batteries, a critical problem for utilizing nanoscaled materials as negative electrodes is the aggregation of nanoparticles, which would result in poor cycling performances. To obviate the problems mentioned above, the strategy of carbon or metal coated on pristine materials as a shell or composite network has been developed.3,24
27 To advance the engineering of TMPs, we propose in this work to improve the kinetics of reactions of Ni2P versus Li+ together with enhancement of its capacity retention by introducing an amorphous carbon network. The carbon network can act as a barrier to suppress the aggregation and pulverization of active particles and thus increase their structure stability during cycling.27
29 On the other hand, the carbon network has a high electronic conductivity and can improve the conductance of the active materials.30 Using such electrodes, a great improvement in power density and capacity retention can be achieved. Received: August 25, 2011 Revised: September 25, 2011 Published: October 27, 2011 23760

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Herein, we develop a very simple method to synthesize hierarchical, nanostructured Ni2P (h-Ni2P) spheres ultilizing a one-pot reaction from an organic-phase mixture of nickel(II) acetylacetonate (Ni(acac)2), trioctylphosphine (TOP), tri-n-octylamine (TOA), and oleylamine (OAm). OAm is used as a surfactant to modify the surface morphology of Ni2P spheres, as well as the carbon source. The morphology modification mechanism under the effect of OAm is discussed, and the improved electrochemical performance of h-Ni2P spheres obtained in the presence of OAm is investigated.

’ EXPRIMENTAL SECTION Materials. Ni(acac)2 (ca. 95%) and oleic acid (OA; d = 0.85 g mL
1) were purchased from Sinopharm Chemical Reagent Co., Shanghai, and TOP (ca. 97%) and OAm (ca. 80%
90%, d = 0.81 g mL
1) were purchased from J&K Chemical Ltd., Shanghai. TOA (ca. 90%, Aladdin Chemistry Co., Shanghai), hexane, and ethanol were used throughout the course of the investigation. All chemicals were of analytical grade and were used without further purification. Synthesis. The h-Ni2P spheres were synthesized by a one-pot strategy. In a typical synthesis, 1 mmol of Ni(acac)2, 7
8 mmol of TOP, and 4 mmol of OAm were mixed under magnetic stirring. Then the mixture was directly added to 7 mL of TOA and the resulting mixture treated at 320 °C for 2 h under protection of high-purity argon gas. The Ni2P spheres were prepared as mentioned above but using OA as the surfactant. After the mixture was cooled to room temperature, the black product was precipitated out by adding a mixture of hexane and ethanol and separated by centrifugation. The precipitation was washed several times and dried in vacuum. Characterization. The morphologies and microstructures of the products were characterized using scanning electron microscopy (SEM; Hitachi S-4700), field emission SEM (FESEM; FEI Sirion-100, equipped with energy-dispersive X-ray, EDX), X-ray diffractometry (XRD; Rigaku D/max 2550 PC, Cu Kα), and transmission electron microscopy (TEM; JEM 200CX at 160 kV, Tecnai G2 F30 at 300 kV). Electrochemical Investigation. The electrochemical tests were performed using a coin-type half-cell (CR 2025). The working electrodes were prepared by a slurry coating procedure. The slurry consisted of 80 wt % Ni2P spheres, 10 wt % acetylene black, and 10 wt % poly(vinylidene fluoride) (PVDF) dissolved in N-methylpyrrolidinone (NMP) and was incorporated on nickel foam with a 12 mm diameter. After being dried at 90 °C for 24 h in vacuum, the foam was pressed under a pressure of 20 MPa. Test cells were assembled in an argon-filled glovebox with the metallic lithium foil as the counter electrode, 1 M LiPF6 in ethylene carbonate (EC)
dimethyl carbonate (DME) (1:1 in volume) as the electrolyte, and a polypropylene (PP) microporous film (Cellgard 2300) as the separator. The galvanostatic charge
discharge tests were conducted on a LAND battery program-control test system at rates of 0.5
3 C (1 C = 542 mA h g
1) between 0.02 and 3.0 V at room temperature (25 ( 1 °C). Cyclic voltammetry (CV) was performed on a CHI660C electrochemical workstation in the potential range of 0
3.0 V (vs Li+/Li) at a scanning rate of 0.1 mV s
1.

’ RESULTS AND DISCUSSION Ni2P nanoparticles with different structures or morphologies have been prepared by many groups.31
34 Here, entirely simple,

Figure 1. XRD patterns of as-synthesized (a) h-Ni2P and (b) Ni2P nanoparticles.

cost-effective, completely organic phase one-pot strategy based “self-assembly” chemistry has accounted for the isotropic growth of hierarchical Ni2P nanostructures. Controllable synthesis of the hierarchical nanomaterial was achieved with the assistance of OAm as the stabilizer, surfactant agent, and also carbon source. Figure 1 shows the XRD patterns of the as-synthesized powders. All the peaks exclusively match well with hexagonal Ni2P (JCPDS 03-0953). For the pattern of h-Ni2P, only peaks of hexagonal Ni2P can be observed, indicating that the carbon introduced in the composite is amorphous. Broadening in the peaks of h-Ni2P, compared with those of Ni2P, indicates the formation of small-sized Ni2P particles in the composite. Figure 2a shows the SEM image of clewlike h-Ni2P spheres exhibiting a hierarchical structure, in which the skeleton is made up of a large number of nanoparticles as shown in the inset (top). In addition, the existence of amorphous carbon is confirmed according to the EDX result, and the content of carbon in the composite is 8.78 wt % as shown in the inset (bottom). In addition, the Si element comes from the Si substrate. Figure 2b shows the TEM image of h-Ni2P spheres. It clearly shows the uniform size of the spheres and the ordered nanoparticles on the surface of the hierarchical Ni2P sphere. The selected area electron diffraction (SAED), as shown in the inset of Figure 2b, reveals a well-crystallized sample, and all the diffraction points can be ascribed to the Ni2P phase. Highresolution TEM (HRTEM) images of the h-Ni2P spheres are shown in Figure 2c,d, from which the regularity of the lattice image can be observed, indicating a single crystal of nanoparticles with sizes of 5
10 nm. The edge position of h-Ni2P shows that the Ni2P nanoparticles are connected through the amorphous carbon network. Furthermore, the image of the middle position shows that the Ni2P nanoparticles exhibit a continuous net structure. It is noteworthy that the Ni2P network is homogenously filled by amorphous carbon; thus, there are no interspaces left. For comparison as shown in Figure 2e, the Ni2P nanoparticles are 50
100 nm in size using OA as the surfactant and wellcrystallized (SAED, inset), but with no carbon network (EDX result, inset). To understand the formation mechanism of an h-Ni2P sphere with this special structure, the synthetic procedure is illustrated in Figure 3. Compared with the procedure of fabricating Ni2P nanoparticles, the h-Ni2P spheres were grown simply by employing OAm as the surfactant agent. In a previous work, Park et al.35 reported the formation of a Ni(acac)2
OAm complex and then 23761

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Figure 2. (a) SEM image of h-Ni2P with an enlarged image (top inset) and energy-dispersive spectrometry analysis of h-Ni2P spheres (bottom inset). (b) TEM image of h-Ni2P with the corresponding SAED pattern (inset). (c, d) HRTEM images of the fringe and middle positions of h-Ni2P, respectively, showing the hierarchical structure of Ni2P nanoparticles and the amorphous carbon network. (e) TEM image of Ni2P nanoparticles with the corresponding SAED pattern (inset).

Ni nanoclusters at an elevated temperature. Since TOP is an active coordination agent with metallic Ni, the P
C bonds located at the cluster surface may break at elevated temperature, resulting in P atoms diffusing into Ni nanoclusters to form Ni2P nanostructures. In addition, the removal of the CN bridging ligand could be induced at this elevated temperature.36 Thus, it is believed that the conversion of Ni
TOP complexes is carried out at 320 °C under an argon gas atmosphere, and the formation of a carbon network wrapping the Ni2P spheres is induced. The surfaces of Ni
TOP complex precursors are coordinately stabilized by an organic layer of thermally decomposed OAm, which will reduce the size of the nanoparticles and serve as a carbon source for the h-Ni2P spheres during the decomposition of these complexes. Meanwhile, these Ni2P nanoparticles will get together under the influence of OAm to form a hierarchical structure of spheres. In addition, the amount of OAm also has

an effect on the morphology of h-Ni2P (see the Supporting Information). TEM images show that when the addition of OAm is reduced to 2 mmol, the Ni2P nanoparticles can get together to form small hierarchical clusters through amorphous carbon as shells (Figure S1a, Supporting Information). With the addition of OAm increased to 8 mmol, the Ni2P nanoparticles aggregate to form big shapeless clusters through a carbon network (Figure S1b). On the other hand, the microstructure is highly sensitive to the reaction temperature. At 300 °C, with a constant addition of 4 mmol of OAm, h-Ni2P spheres are synthesized with a uniform size of particles (Figure S1c, inset), but these nanoparticles are not closely connected. However, at 350 °C, the Ni2P nanoparticles are quite compact through the carbon network, growing into spheres. Figure 4a shows the cyclability of the h-Ni2 P and Ni 2 P electrodes at different charge
discharge rates. The specific 23762

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Figure 3. Schematice illustration of the formation of h-Ni2P.

reversible capacity of h-Ni2P after 50 cycles is 365.3 mA h g
1 at 0.5 C and 257.8 mA h g
1 at 1 C, much higher than that of Ni2P (97.2 mA h g
1 at 0.5 C). Due to the amorphous carbon network, the h-Ni2P electrode exhibits better capacity retention. After the initial cycles, the specific reversible capacity of the h-Ni 2 P electrode decreases slightly, and the capacity decrease is more minor than that of Ni2P. As we can see, 57.8% capacity of the second cycle could be obtained for the h-Ni2P electrode at 0.5 C. However, the Ni2P electrode can only sustain 34.6% capacity of the second cycle at 0.5 C after 50 cycles. In addition, there 53% capacity could still be obtained for h-Ni2P at 1 C. Figure 4b gives the galvanostatic cycling of the cells at charge
discharge rates from 0.5 to 3 C. With increasing current rate, the discharge capacities of the two materials decrease gradually, indicating a diffusion-controlled kinetic process for the electrode reaction.37,38 It is observed that the h-Ni2P electrode shows a lower fading rate than the Ni2P electrode. At a high rate of 3 C, the specific charge capacity of h-Ni2P is still as high as 167.1 mA h g
1; however, there is nearly no discharge capacity for the Ni2P counterpart. When the rate is lowered to 0.5 C, a stable capacity during cycling is regained for h-Ni2P, but for Ni2P, the capacity just shows a high initial value and then decreases markedly. The cyclability of h-Ni2P is significantly improved, especially at the high rate, suggesting that the hierarchical structure and the carbon network in the composite facilitate charge transportation at high rates. Figure 4b also gives the Coulombic efficiency of h-Ni2P and Ni2P electrodes at different rates. It obviously demonstrates that the h-Ni2P electrode has a higher initial Coulombic efficiency than the Ni2P electrode (64
43%) at 0.5 C. In the subsequent cycles, the Coulombic efficiency of h-Ni2P is steady and almost 100%, while the Coulombic efficiency of Ni2P is unstable and much lower than that of h-Ni2P. At a rate of 1 C, though the Coulombic efficiency of h-Ni2P fluctuates, it still remaines stable and near 100%, which accounts for the effect of the carbon network and hierarchical structure on the improvement of the conductivity and electrode stability. In addition, the addition of OAm and the reaction temperature will also influence the cycling performance (see Figure S2, Supporting Information). However, the optimized synthetic strategy for better electrochemical performance still employs 4 mmol of OAm as the surfactant agent at 320 °C as mentioned in the Experimental Section. Figure 5 shows the CV curves of h-Ni2P and Ni2P electrodes performed over the potential range of 0
3.0 V (vs Li+/Li) at a

Figure 4. (a) Cycling performance of h-Ni2P and Ni2P electrodes at different charge
discharge rates from the 2nd cycle to the 50th cycle. (b) Rate performance and Coulombic efficiency of h-Ni2P and Ni2P electrodes.

scanning rate of 0.1 mV s
1. The reduction peak of h-Ni2P with a maximum at 0.6 V corresponds to the decomposition of Ni2P into metallic Ni and the formation of amorphous Li3P and a solid electrolyte interface (SEI). For the Ni2P electrode, this peak shifts to 0.4 V. The two oxidation peaks of h-Ni2P located at about 1.36 and 2.32 V can be attributed to the decomposition of SEI and Li3P, respectively.19,39,40 Compared with Ni2P, the redox peaks of h-Ni2P are more intensive, suggesting that a larger 23763

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Figure 5. CV curves of h-Ni2P and Ni2P electrodes for (a) the first and (b) the second and third cycles at a scan rate of 0.1 mV s
1 from 0 to 3.0 V. The inset of (a) is the enlarged image corresponding to the dashed square region. +

amount of Li reacts with Ni2P due to the improved conductivity. In addition, the anodic peaks, which can be indexed to the reversible reaction of metallic Ni to Ni2P, are quite obvious and very intensive, indicating a sufficient oxidation reaction process during the anodic cycling. The enlarged curve of the Ni2P electrode, as shown in the inset of Figure 5a, shows that the anodic peak is located at about 2.28 V. Therefore, the separation between the reduction and oxidation peaks (ΔU) of h-Ni2P decreases as compared to that of Ni2P, indicating weaker polarization and better reversibility. This is because the high electronic conductive carbon in h-Ni2P is beneficial for the diffusion of Li ions. In the second and third cycles, as shown in Figure 5b, the area of the anodic peaks of Ni2P is already quite small. Contrarily, the areas of the cathodic and anodic peaks of h-Ni2P are closer than those of Ni2P. The smaller the difference in areas of cathodic and anodic peaks, the higher the Coulombic efficiency of the electrode. Briefly, the carbon network in h-Ni2P facilitates the reversible reaction Ni2P + 3Li+ + 3e
T Li3P + 2Ni, confirming again weaker polarization and better reversibility. Figure 6 displays the typical charge
discharge curves of h-Ni2P and Ni2P electrodes. The initial discharge capacity of h-Ni2P is as high as 994.5 mA h g
1, which is much higher than that of Ni 2 P (653.9 mA h g
1 ). The extra capacity of the electrodes compared with the theoretic capacity of Ni 2 P (526 mA h g
1 for h-Ni2P spheres containing 8.78 wt % carbon) resulted from the formation of an SEI during the first discharging process.40 After the first cycle, the discharge capacity of h-Ni2P

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Figure 6. Discharge
charge curves for (a) h-Ni2P and (b) Ni2P electrodes.

decreases slightly and remains at about 580 mA h g
1. For the Ni2P electrode, the initial discharge capacity is 653.9 mA h g
1, which decreases markedly. Overall, the main reason for the improvement of cycling performance of h-Ni2P is introducing the amorphous carbon network. As is known to all, a poor conductive SEI film will form during the cycling. The SEI film is a gel-like polymer which contains LiF, Li2CO3, and lithium alkyl carbonate (ROCO2Li),39 so more SEI will lead to poorer conductivity. However, for the h-Ni2P electrode, the amorphous carbon is able to keep the Ni2P nanoparticles electrically connected as a network and thus facilitates charge transportation.3,24,41 In addition, during the initial discharge process of both the electrodes, a broad plateau between 1.5 and 1.0 V is ascribed to the formation of an intermediate compound, besides a broad plateau corresponding to the decomposition of Ni2P into metallic Ni as mentioned above. For the Ni2P electrode, the plateau between 1.5 and 1.0 V is quite flat, indicating that the formation of an intermediate compound is dominant and controls the whole reduction process. Also, no obvious plateau between 1.0 and 0.5 V is detected. Therefore, there exists insufficiency of the reduction reaction of the further transformation into Li3P and Ni. Compared with Ni2P, the h-Ni2P electrode shows a slope in the voltage range of 1.0
1.5 V in the initial charge curve, related to the oxidation of Ni0 to Ni1+ or Ni2+. Also, an obvious plateau between 1.0 and 0.5 V is observed, which corresponds to the further reaction into Li3P and metallic Ni. This process has been depicted as a two-step reaction process.42 Furthermore, as shown in Figure 6, there exists a well-pronounced voltage step near 1.0 V, 23764

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Figure 7. Ex situ XRD patterns of h-Ni2P at different potentials of the first discharge cycled between 0.02 and 3 V at a C/60 rate: (a) region of 2θ = 20
80°, (b) enlarged pattern of the region between 2θ = 40° and 2θ = 50° in (a).

which has been described traditionally as the dividing line between the insertion process and the conversion process.2 To determine whether the reaction occurred during the initial discharge process, ex situ XRD patterns of the h-Ni2P electrode at different discharge potentials at C/60 are investigated as shown in Figure 7 by labeling some significant voltages during the discharge process (Figure 5a). With decreasing potential, a decrease in the intensity of the main Ni2P phase is observed. A new set of Bragg peaks is illustrated by black dots, tilted squares, and inverted triangles. To accurately observe the growth of the intermediate phase, selected powder diagrams have been magnified in the angular region (2θ = 40
50°; see the inset of Figure 7b). When the potential reaches 2.0 V, there is no obvious structural change. At the cutoff potential of 1.4 V, besides the peaks of the Ni substrate, there is a broad convex peak around 2θ = 43
44°, where the main hexagonal structured Ni5P2 is located. After the potential reaches 1.0 V, a new set of Bragg peaks appear (illustrated by plus signs in Figure 7b). These peaks can be indexed to Ni12P5, which has been reported as the intermediate compound with a tetragonal structure and lattice constants of a = b = 8.646 Å and c = 5.070 Å.14 Meanwhile, some peaks of Ni5P2 disappear. After the electrode is fully discharged, pursuing the lithiation results in the disappearance of Ni2P at the expense of the formation of hexagonal Li3P (Figure 7a). What is more, it is considered that Li3P mainly exhibits an amorphous structure;43,44 thus, a relatively wide diffraction peak appears at 2θ = 25
35°, where the main diffraction peaks of Li3P

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Figure 8. (a) HRTEM image realized at the end of the fourth discharge and the corresponding FFT in the inset. TEM images of (b) Ni2P and (c) h-Ni2P electrodes after five cycles.

are all located. Interestingly, the Ni:P ratio of Ni5P2 and Ni12P5 is 2.5:1 and 2.4:1, respectively. That is to say, in the initial stage of the first discharge, lithiation will consume a large amount of P3
to form Li3P, thus resulting in the formation of Ni5P2, which deviates from the stoichiometric ratio of 2:1 more obviously. After the lithiation process kinetically maintains stability, metallic Ni0 begins to form through diffusion, thus resulting in the Ni:P ratio becoming small again. To obtain a more exact and straightforward investigation of electrochemical process, TEM/HRTEM measurements were performed on the cycled electrodes. When the h-Ni2P electrode is fully discharged, the initial particles transform into an agglomerate of crystallized 5 nm particles embedded in a crystallized lithiated matrix, as shown in the HRTEM image of Figure 8a. After the careful calculation of the fast Fourier transform (FFT; inset of Figure 8a) and indexing of the so-obtained spots, the nanoparticles and matrix are attributed to metallic Ni and the Li3P phase, respectively. The Ni2P + 3Li+ + 3e
f Li3P + 2Ni conversion reaction expected in the first discharge from the plateau in Figure 6 is thus confirmed by ex situ XRD and HRTEM analysis. Parts b and c of Figure 8 give the TEM images of the fully discharged Ni2P and h-Ni2P electrodes at the fifth cycle. The Ni2P network is disintegrated after discharge and the particles are aggregated severely for the Ni 2 P electrode (Figure 8b). As shown in Figure 8c, however, the network structure of h-Ni2P is undamaged, indicating that the Ni2P nanoparticles are stabilized by the carbon filling. In addition, 23765

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The Journal of Physical Chemistry C the nanocrystalline Ni particles still exhibit good dispersity, which can be attributed to the hierarchical structure to buffer the particle aggregation and volume expansion. The SEM images (Supporting Information, Figure S3) of the fully charged h-Ni2P and Ni2P electrodes are analyzed after 50 cycles. For the Ni2P electrode, the nanoparticles are pulverized and aggregated severely after cycling. However, graininess of h-Ni2P is still visible, though there is slight aggregation, indicating that the h-Ni2P electrode is stabilized by the carbon network. This accounts for again the better reversibility of the h-Ni2P electrode, which is consistent with the cycling performance and CV results. All these results indicate that the electrochemical reversibility of h-Ni2P is built after introduction of the amorphous carbon network and formation of the hierarchical structure.

’ CONCLUSIONS Hierarchical nanostructured Ni2P (h-Ni2P) spheres were successfully synthesized via a one-pot reaction of an organicphase strategy in a stirred solution of Ni(acac)2 as the metal precursor and TOP as the phosphorus source by using OAm as the surfactant. The h-Ni2P spheres composed of ordered nanoparticles show better cycling performance than the Ni2P spheres prepared by using OA as the surfactant, especially at high charge
discharge rate. This is attributed to the larger specific surface area of h-Ni2P, leading to a sufficient contact area for Ni2P/electrolyte, a shortened diffusion length of Li ions, and an enhanced reactivity of the electrode reaction during cycling. In addition, the amorphous carbon and the hierarchical structure of the nanoparticle network can buffer the volume expansion and thus increase the electrode stability during cycling. Therefore, the h-Ni2P spheres synthesized through this facile strategy are a potential anode material for lithium ion batteries. ’ ASSOCIATED CONTENT

bS

Supporting Information. TEM images and cycling performance of Ni2P synthesized under different conditions and SEM images of Ni2P electrodes after cycling. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: +86 571 87952856. Fax: +86 571 8795 2573. E-mail: [email protected].

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