Published: October 27, 2011

r 2011 American Chemical Society 23760 dx.doi.org/10.1021/jp208204u | J. Phys. Chem. C 2011, 115, 23760–23767

ARTICLE

pubs.acs.org/JPCC

Improved Electrochemical Performance of Self-Assembled HierarchicalNanostructured Nickel Phosphide as a Negative Electrode for LithiumIon BatteriesY. 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

’ INTRODUCTION

In the field of lithium ion batteries,many scientific and practicalinvestigations are currently focused on improving the efficiency ofthe anode conversion reaction and enhancing the electrodereaction kinetics.1�8 Transition-metal phosphides (TMPs, whereM = Fe, Co, Ni, Cu, etc.) have gained much attention due to theirlow polarization and high gravimetric capacity.2,9�13 The mechan-ism of Li+ reactivity in TMPs differs from the classical Li+ intercala-tion/deintercalation or Li-alloying process but involves theformation and decomposition of Li3P. The electrochemical conver-sion mechanism of TMPs toward Li+ is shown as follows:14�16

MPx þ 3xLiþ þ 3xe� T xLi3P þ M0 ð1Þ

These so-called conversion reactions17 offer a new type of energystorage involving a larger exchange of Li.18

In recent years, many efforts have been devoted to electro-chemically exploringNi�P systems as negative electrodes.2,13,14,18�20

Although TMPs show capacities greater than that of graphite, asdemonstrated for NiP2,

2 their quite low capacity retention stillfalls short to be suitable for practical applications.10,19,21�23

Therefore, the key for exploiting these phosphide-based materi-als in future applications lies in controlling the kinetics of Li+

insertion, thereby improving the efficiency of the conversionreaction.

Construction of nanoscaled materials usually aims at enhanc-ing their specific area so that the kinetics would be improved sincethe nanomaterials facilitate the transportation of Li+ ions by offeringa shorter solid-state diffusion length, along with a better buffering ofthe active material volume expansion. This new architecture wouldlead to a better cyclability, even though, in the normal lithium ionbatteries, a critical problem for utilizing nanoscaled materials asnegative electrodes is the aggregation of nanoparticles, which wouldresult in poor cycling performances. To obviate the problemsmentioned above, the strategy of carbon or metal coated on pristinematerials as a shell or composite network has been developed.3,24�27

To advance the engineering of TMPs, we propose in this workto improve the kinetics of reactions of Ni2P versus Li+ togetherwith enhancement of its capacity retention by introducing anamorphous carbon network. The carbon network can act as abarrier to suppress the aggregation and pulverization of activeparticles and thus increase their structure stability duringcycling.27�29 On the other hand, the carbon network has a highelectronic conductivity and can improve the conductance of theactive materials.30 Using such electrodes, a great improvement inpower density and capacity retention can be achieved.

Received: August 25, 2011Revised: September 25, 2011

ABSTRACT:Hierarchical, nanostructured nickel phosphide (h-Ni2P) spheres are synthe-sized 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 tomodify the surface morphology of Ni2P spheres. The h-Ni2P spheres are composed ofordered nanoparticles with 5�10 nm sizes and filled by amorphous carbon. Thehierarchical structure can greatly increase the contact area between Ni2P and electrolyte,which provides more sites for Li+ accommodation, shortens the diffusion length of Li+, andenhances the reactivity of the electrode reaction. Also, the amorphous carbon andthe hierarchical Ni2P nanostructures can buffer volume expansion and thus increase theelectrode stability during cycling. In the context of storage behavior, the h-Ni2P electrodeexhibits high capacity as well as Coulombic efficiency. After 50 cycles, the reversiblecapacity 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, muchhigher than that of Ni2P spheres (97.2 mA h g�1 at 0.5 C). At a high rate of 3 C, the specificcapacity of h-Ni2P is still as high as 167.1 mA h g�1.

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Herein, we develop a very simple method to synthesizehierarchical, nanostructured Ni2P (h-Ni2P) spheres ultilizing aone-pot reaction from an organic-phasemixture of nickel(II) acetyl-acetonate (Ni(acac)2), trioctylphosphine (TOP), tri-n-octylamine(TOA), and oleylamine (OAm). OAm is used as a surfactant tomodify the surface morphology of Ni2P spheres, as well as thecarbon source. The morphology modification mechanism underthe effect of OAm is discussed, and the improved electrochemicalperformance of h-Ni2P spheres obtained in the presence of OAmis investigated.

’EXPRIMENTAL SECTION

Materials. Ni(acac)2 (ca. 95%) and oleic acid (OA; d = 0.85g mL�1) were purchased from Sinopharm Chemical Reagent Co.,Shanghai, and TOP (ca. 97%) and OAm (ca. 80%�90%, d = 0.81gmL�1) were purchased from J&KChemical Ltd., Shanghai. TOA(ca. 90%, Aladdin Chemistry Co., Shanghai), hexane, and ethanolwere used throughout the course of the investigation. All chemicalswere 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 mmolof TOP, and 4 mmol of OAm were mixed under magneticstirring. Then the mixture was directly added to 7 mL of TOAand the resulting mixture treated at 320 �C for 2 h underprotection of high-purity argon gas. The Ni2P spheres wereprepared as mentioned above but using OA as the surfactant.After the mixture was cooled to room temperature, the blackproduct was precipitated out by adding a mixture of hexane andethanol and separated by centrifugation. The precipitation waswashed several times and dried in vacuum.Characterization. The morphologies and microstructures of

the products were characterized using scanning electron micro-scopy (SEM; Hitachi S-4700), field emission SEM (FESEM; FEISirion-100, equipped with energy-dispersive X-ray, EDX), X-raydiffractometry (XRD; Rigaku D/max 2550 PC, Cu Kα), andtransmission 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). Theworking electrodes were prepared by a slurry coating procedure.The slurry consisted of 80 wt % Ni2P spheres, 10 wt % acetyleneblack, and 10 wt % poly(vinylidene fluoride) (PVDF) dissolvedin N-methylpyrrolidinone (NMP) and was incorporated onnickel foam with a 12 mm diameter. After being dried at 90 �Cfor 24 h in vacuum, the foam was pressed under a pressure of 20MPa. Test cells were assembled in an argon-filled glovebox withthe metallic lithium foil as the counter electrode, 1 M LiPF6 inethylene carbonate (EC)�dimethyl carbonate (DME) (1:1 involume) as the electrolyte, and a polypropylene (PP) micropor-ous 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 roomtemperature (25 ( 1 �C). Cyclic voltammetry (CV) was per-formed on a CHI660C electrochemical workstation in the poten-tial 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 morphologieshave been prepared by many groups.31�34 Here, entirely simple,

cost-effective, completely organic phase one-pot strategy based“self-assembly” chemistry has accounted for the isotropic growthof hierarchical Ni2P nanostructures. Controllable synthesis of thehierarchical nanomaterial was achieved with the assistance ofOAm 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 ofhexagonal Ni2P can be observed, indicating that the carbonintroduced in the composite is amorphous. Broadening in thepeaks of h-Ni2P, compared with those of Ni2P, indicates theformation of small-sized Ni2P particles in the composite.Figure 2a shows the SEM image of clewlike h-Ni2P spheresexhibiting a hierarchical structure, in which the skeleton is madeup of a large number of nanoparticles as shown in the inset (top).In addition, the existence of amorphous carbon is confirmedaccording to the EDX result, and the content of carbon in thecomposite is 8.78 wt % as shown in the inset (bottom). Inaddition, the Si element comes from the Si substrate.

Figure 2b shows the TEM image of h-Ni2P spheres. It clearlyshows the uniform size of the spheres and the ordered nano-particles on the surface of the hierarchical Ni2P sphere. Theselected area electron diffraction (SAED), as shown in the insetof Figure 2b, reveals a well-crystallized sample, and all thediffraction points can be ascribed to the Ni2P phase. High-resolution TEM (HRTEM) images of the h-Ni2P spheres areshown in Figure 2c,d, from which the regularity of the latticeimage can be observed, indicating a single crystal of nanoparticleswith sizes of 5�10 nm. The edge position of h-Ni2P shows thatthe Ni2P nanoparticles are connected through the amorphouscarbon network. Furthermore, the image of the middle positionshows that the Ni2P nanoparticles exhibit a continuous netstructure. It is noteworthy that the Ni2P network is homoge-nously filled by amorphous carbon; thus, there are no interspacesleft. For comparison as shown in Figure 2e, the Ni2P nanopar-ticles are 50�100 nm in size using OA as the surfactant and well-crystallized (SAED, inset), but with no carbon network (EDXresult, inset).

To understand the formation mechanism of an h-Ni2P spherewith this special structure, the synthetic procedure is illustrated inFigure 3. Compared with the procedure of fabricating Ni2Pnanoparticles, the h-Ni2P spheres were grown simply by employ-ing 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

Figure 1. XRD patterns of as-synthesized (a) h-Ni2P and (b) Ni2Pnanoparticles.

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Ni nanoclusters at an elevated temperature. Since TOP is anactive coordination agent with metallic Ni, the P�C bondslocated at the cluster surface may break at elevated temperature,resulting in P atoms diffusing into Ni nanoclusters to form Ni2Pnanostructures. In addition, the removal of the CN bridgingligand could be induced at this elevated temperature.36 Thus, it isbelieved that the conversion of Ni�TOP complexes is carriedout at 320 �C under an argon gas atmosphere, and the formationof a carbon network wrapping the Ni2P spheres is induced. Thesurfaces of Ni�TOP complex precursors are coordinately stabi-lized by an organic layer of thermally decomposed OAm, whichwill reduce the size of the nanoparticles and serve as a carbonsource for the h-Ni2P spheres during the decomposition of thesecomplexes. Meanwhile, these Ni2P nanoparticles will get to-gether under the influence of OAm to form a hierarchicalstructure of spheres. In addition, the amount of OAm also has

an effect on the morphology of h-Ni2P (see the SupportingInformation). TEM images show that when the addition of OAmis reduced to 2 mmol, the Ni2P nanoparticles can get together toform small hierarchical clusters through amorphous carbon asshells (Figure S1a, Supporting Information). With the additionof OAm increased to 8 mmol, the Ni2P nanoparticles aggregateto form big shapeless clusters through a carbon network (FigureS1b). On the other hand, the microstructure is highly sensitive tothe reaction temperature. At 300 �C, with a constant addition of4 mmol of OAm, h-Ni2P spheres are synthesized with a uniformsize of particles (Figure S1c, inset), but these nanoparticles arenot closely connected. However, at 350 �C, the Ni2P nanopar-ticles are quite compact through the carbon network, growinginto spheres.

Figure 4a shows the cyclability of the h-Ni2P and Ni2Pelectrodes at different charge�discharge rates. The specific

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 ofNi2P nanoparticles and the amorphous carbon network. (e) TEM image ofNi2P nanoparticles with thecorresponding SAED pattern (inset).

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reversible capacity of h-Ni2P after 50 cycles is 365.3 mA h g�1 at0.5 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 theinitial cycles, the specific reversible capacity of the h-Ni2Pelectrode decreases slightly, and the capacity decrease is moreminor than that of Ni2P. As we can see, 57.8% capacity of thesecond cycle could be obtained for the h-Ni2P electrode at 0.5 C.However, the Ni2P electrode can only sustain 34.6% capacity ofthe 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 givesthe galvanostatic cycling of the cells at charge�discharge ratesfrom 0.5 to 3 C. With increasing current rate, the dischargecapacities of the two materials decrease gradually, indicating adiffusion-controlled kinetic process for the electrode reaction.37,38

It is observed that the h-Ni2P electrode shows a lower fading ratethan the Ni2P electrode. At a high rate of 3 C, the specific chargecapacity 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 cyclingis regained for h-Ni2P, but for Ni2P, the capacity just shows a highinitial value and then decreases markedly. The cyclability ofh-Ni2P is significantly improved, especially at the high rate,suggesting that the hierarchical structure and the carbon networkin the composite facilitate charge transportation at high rates.Figure 4b also gives the Coulombic efficiency of h-Ni2P and Ni2Pelectrodes at different rates. It obviously demonstrates that theh-Ni2P electrode has a higher initial Coulombic efficiency thanthe 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 muchlower than that of h-Ni2P. At a rate of 1 C, though the Coulombicefficiency of h-Ni2P fluctuates, it still remaines stable and near100%, which accounts for the effect of the carbon network andhierarchical structure on the improvement of the conductivityand electrode stability. In addition, the addition of OAm and thereaction temperature will also influence the cycling performance(see Figure S2, Supporting Information). However, the opti-mized synthetic strategy for better electrochemical performancestill employs 4 mmol of OAm as the surfactant agent at 320 �C asmentioned in the Experimental Section.

Figure 5 shows the CV curves of h-Ni2P and Ni2P electrodesperformed over the potential range of 0�3.0 V (vs Li+/Li) at a

scanning rate of 0.1 mV s�1. The reduction peak of h-Ni2P with amaximum at 0.6 V corresponds to the decomposition of Ni2Pinto metallic Ni and the formation of amorphous Li3P and a solidelectrolyte interface (SEI). For the Ni2P electrode, this peakshifts to 0.4 V. The two oxidation peaks of h-Ni2P located atabout 1.36 and 2.32 V can be attributed to the decomposition ofSEI and Li3P, respectively.

19,39,40 Compared withNi2P, the redoxpeaks of h-Ni2P are more intensive, suggesting that a larger

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

Figure 4. (a) Cycling performance of h-Ni2P and Ni2P electrodes atdifferent charge�discharge rates from the 2nd cycle to the 50th cycle.(b) Rate performance and Coulombic efficiency of h-Ni2P and Ni2Pelectrodes.

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amount of Li+ reacts with Ni2P due to the improved conductivity.In addition, the anodic peaks, which can be indexed to thereversible reaction of metallic Ni to Ni2P, are quite obvious andvery intensive, indicating a sufficient oxidation reaction processduring the anodic cycling. The enlarged curve of the Ni2Pelectrode, as shown in the inset of Figure 5a, shows that theanodic peak is located at about 2.28 V. Therefore, the separationbetween the reduction and oxidation peaks (ΔU) of h-Ni2Pdecreases as compared to that of Ni2P, indicating weaker polar-ization and better reversibility. This is because the high electronicconductive carbon in h-Ni2P is beneficial for the diffusion of Liions. In the second and third cycles, as shown in Figure 5b, thearea of the anodic peaks of Ni2P is already quite small. Contrarily,the areas of the cathodic and anodic peaks of h-Ni2P are closerthan those of Ni2P. The smaller the difference in areas of cathodicand anodic peaks, the higher the Coulombic efficiency of theelectrode. Briefly, the carbon network in h-Ni2P facilitates thereversible reaction Ni2P + 3Li+ + 3e� T Li3P + 2Ni, confirmingagain weaker polarization and better reversibility.

Figure 6 displays the typical charge�discharge curves ofh-Ni2P and Ni2P electrodes. The initial discharge capacity ofh-Ni2P is as high as 994.5 mA h g�1, which is much higher thanthat of Ni2P (653.9 mA h g�1). The extra capacity of theelectrodes compared with the theoretic capacity of Ni2P(526 mA h g�1 for h-Ni2P spheres containing 8.78 wt % carbon)resulted from the formation of an SEI during the first dischargingprocess.40 After the first cycle, the discharge capacity of h-Ni2P

decreases slightly and remains at about 580 mA h g�1. For theNi2P electrode, the initial discharge capacity is 653.9 mA h g�1,which decreases markedly. Overall, the main reason for theimprovement of cycling performance of h-Ni2P is introducingthe amorphous carbon network. As is known to all, a poorconductive SEI film will form during the cycling. The SEI film isa gel-like polymer which contains LiF, Li2CO3, and lithium alkylcarbonate (ROCO2Li),

39 so more SEI will lead to poorer con-ductivity. However, for the h-Ni2P electrode, the amorphouscarbon is able to keep the Ni2P nanoparticles electrically con-nected as a network and thus facilitates charge transportation.3,24,41

In addition, during the initial discharge process of both theelectrodes, a broad plateau between 1.5 and 1.0 V is ascribed tothe formation of an intermediate compound, besides a broadplateau corresponding to the decomposition of Ni2P into metallicNi as mentioned above. For the Ni2P electrode, the plateaubetween 1.5 and 1.0 V is quite flat, indicating that the formation ofan intermediate compound is dominant and controls the wholereduction process. Also, no obvious plateau between 1.0 and 0.5 Vis detected. Therefore, there exists insufficiency of the reductionreaction of the further transformation into Li3P and Ni. Com-pared withNi2P, the h-Ni2P electrode shows a slope in the voltagerange of 1.0�1.5 V in the initial charge curve, related to theoxidation ofNi0 toNi1+ or Ni2+. Also, an obvious plateau between1.0 and 0.5 V is observed, which corresponds to the furtherreaction into Li3P andmetallic Ni. This process has been depictedas a two-step reaction process.42 Furthermore, as shown inFigure 6, there exists a well-pronounced voltage step near 1.0 V,

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 to3.0 V. The inset of (a) is the enlarged image corresponding to the dashedsquare region.

Figure 6. Discharge�charge curves for (a) h-Ni2P and (b) Ni2Pelectrodes.

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which has been described traditionally as the dividing linebetween the insertion process and the conversion process.2

To determine whether the reaction occurred during the initialdischarge process, ex situ XRD patterns of the h-Ni2P electrodeat different discharge potentials at C/60 are investigated asshown in Figure 7 by labeling some significant voltages duringthe discharge process (Figure 5a). With decreasing potential, adecrease in the intensity of the main Ni2P phase is observed. Anew set of Bragg peaks is illustrated by black dots, tilted squares,and inverted triangles. To accurately observe the growth of theintermediate phase, selected powder diagrams have been magni-fied in the angular region (2θ = 40�50�; see the inset ofFigure 7b). When the potential reaches 2.0 V, there is no obviousstructural change. At the cutoff potential of 1.4 V, besides thepeaks of the Ni substrate, there is a broad convex peak around2θ = 43�44�, where the main hexagonal structured Ni5P2 islocated. After the potential reaches 1.0 V, a new set of Braggpeaks appear (illustrated by plus signs in Figure 7b). Thesepeaks can be indexed to Ni12P5, which has been reported as theintermediate compound with a tetragonal structure and latticeconstants of a = b = 8.646 Å and c = 5.070 Å.14Meanwhile, somepeaks of Ni5P2 disappear. After the electrode is fully discharged,pursuing the lithiation results in the disappearance of Ni2P atthe expense of the formation of hexagonal Li3P (Figure 7a).What is more, it is considered that Li3P mainly exhibits anamorphous structure;43,44 thus, a relatively wide diffraction peakappears at 2θ = 25�35�, where the main diffraction peaks of Li3P

are all located. Interestingly, the Ni:P ratio of Ni5P2 and Ni12P5 is2.5:1 and 2.4:1, respectively. That is to say, in the initial stage ofthe first discharge, lithiation will consume a large amount of P3�

to form Li3P, thus resulting in the formation of Ni5P2, whichdeviates from the stoichiometric ratio of 2:1 more obviously.After the lithiation process kinetically maintains stability, metallicNi0 begins to form through diffusion, thus resulting in the Ni:Pratio becoming small again.

To obtain a more exact and straightforward investigation ofelectrochemical process, TEM/HRTEM measurements wereperformed on the cycled electrodes. When the h-Ni2P electrodeis fully discharged, the initial particles transform into an agglom-erate of crystallized 5 nm particles embedded in a crystallizedlithiated 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, thenanoparticles and matrix are attributed to metallic Ni and theLi3P phase, respectively. The Ni2P + 3Li+ + 3e� f Li3P + 2Niconversion reaction expected in the first discharge from theplateau in Figure 6 is thus confirmed by ex situ XRD andHRTEM analysis. Parts b and c of Figure 8 give the TEM imagesof the fully discharged Ni2P and h-Ni2P electrodes at the fifthcycle. The Ni2P network is disintegrated after discharge andthe particles are aggregated severely for the Ni2P electrode(Figure 8b). As shown in Figure 8c, however, the networkstructure of h-Ni2P is undamaged, indicating that the Ni2Pnanoparticles are stabilized by the carbon filling. In addition,

Figure 7. Ex situ XRD patterns of h-Ni2P at different potentials of thefirst discharge cycled between 0.02 and 3 V at a C/60 rate: (a) region of2θ = 20�80�, (b) enlarged pattern of the region between 2θ = 40� and2θ = 50� in (a).

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

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the nanocrystalline Ni particles still exhibit good dispersity,which can be attributed to the hierarchical structure to bufferthe particle aggregation and volume expansion.

The SEM images (Supporting Information, Figure S3) of thefully charged h-Ni2P and Ni2P electrodes are analyzed after 50cycles. For the Ni2P electrode, the nanoparticles are pulverizedand aggregated severely after cycling. However, graininess ofh-Ni2P is still visible, though there is slight aggregation, indicatingthat the h-Ni2P electrode is stabilized by the carbon network. Thisaccounts 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 ofh-Ni2P is built after introduction of the amorphous carbonnetwork and formation of the hierarchical structure.

’CONCLUSIONS

Hierarchical nanostructured Ni2P (h-Ni2P) spheres weresuccessfully synthesized via a one-pot reaction of an organic-phase strategy in a stirred solution of Ni(acac)2 as the metalprecursor and TOP as the phosphorus source by using OAm asthe surfactant. The h-Ni2P spheres composed of ordered nano-particles show better cycling performance than the Ni2P spheresprepared by using OA as the surfactant, especially at highcharge�discharge rate. This is attributed to the larger specificsurface area of h-Ni2P, leading to a sufficient contact area forNi2P/electrolyte, a shortened diffusion length of Li ions, and anenhanced reactivity of the electrode reaction during cycling. Inaddition, the amorphous carbon and the hierarchical structure ofthe nanoparticle network can buffer the volume expansion andthus increase the electrode stability during cycling. Therefore, theh-Ni2P spheres synthesized through this facile strategy are apotential anode material for lithium ion batteries.

’ASSOCIATED CONTENT

bS Supporting Information. TEM images and cycling per-formance of Ni2P synthesized under different conditions andSEM images of Ni2P electrodes after cycling. This material isavailable 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:tujp@zju.edu.cn.

’ACKNOWLEDGMENT

The assistance ofMr. Xin-ting Cong andDr.Wan-zhenHuangfor TEM analysis is grateful acknowledged.

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