Graphene Sheets as Anode Materials with Enhanced

Article pubs.acs.org/JPCC

Ni2P/Graphene Sheets as Anode Materials with Enhanced Electrochemical Properties versus Lithium Yi Lu,† Xiuli Wang,*,† Yongjin Mai,† Jiayuan Xiang,‡ Heng Zhang,† Lu Li,† Changdong Gu,† Jiangping Tu,*,† and Scott X. Mao§ †

State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡ Narada Power Source Co., Ltd., Hangzhou 311105, China § Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States S Supporting Information *

ABSTRACT: Hybridizing Ni2P/graphene sheet composite is successfully accomplished via a one-pot solvothermal method. As anode materials for lithium-ion batteries, the Ni2P spheres with sizes of 10−30 nm can effectively prevent the agglomeration of graphene sheets. In turn, the graphene sheets with good electrical conductivity serve as a conducting network for fast electron transfer between the active materials and charge collector, as well as buffered spaces to accommodate the volume expansion/contraction during cycling. The cyclic stability and rate capability of Ni2P are significantly improved after the incorporation of graphene sheets. After 50 cycles, the Ni2P/graphene sheet hybrid delivers a capacity of 450 mA h g−1 and 360 mA h g−1 at a current density of 54.2 and 542 mA g−1, respectively. The voltage hysteresis of Ni2P with and without graphene sheets is also discussed. The incorporation of graphene sheets can partly decrease the voltage polarization, and modify the thickness of solid electrolyte interface (SEI) film.

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INTRODUCTION Since the pioneering work of Nazar and his co-workers on anode materials of transition metal phosphides,1 many studies have been carried out to investigate the reaction mechanism of these new material systems due to their high capacities and low polarization.2−7 Similar to transition metal oxides, these phosphides are also subject to a conversion reaction mechanism,4,8 which makes them far from commercial applications. Among transition metal phosphides, Ni−P systems have drew great attention because of their promising utilizations as anodes for lithium-ion batteries.9−16 It has been reported that the phases with Ni/P > 1 were found to be electrochemically inactive in powder form,8 which would have a strong effect on their cycling performances. However, much effort has been made to restore such activity by designing the electrode. Our recent results exhibited that Ni3P electrodeposited onto Ni foam delivered 400 mA h g−1 after 20 cycles;17 Ni2P wrapped by amorphous carbon layer had high initial Coulombic efficiency and good cyclic performance;18−20 a single phase of Ni5P4/C composite also enhanced the electrochemical activity, leading to a high reversible capacity.21 In spite of enhancing the cyclic stability, these nickel phosphides still fall short to be suitable for practical application © 2012 American Chemical Society

due to their poor capacity retention at high current densities. In addition, just like transition metal oxides, another obstacle for the application of NixP (x > 1) is strongly challenged by an unacceptable, in terms of round-trip energy density loss, large voltage hysteresis between the discharge and charge steps. Although Gillot et al.9 have reported that NiP2 exhibits the lowest polarization due to its metallic character and reacts with Li+ over the lowest and narrowest potential range, NixP (x > 1) nanostructure still possesses a voltage hysteresis of about 0.7− 1.2 V.18,19 Graphene, mother of all graphitic forms, is considered as a promising anode material due to its superior electrical conductivity, high surface-to-volume ratio and abundance of raw materials. Recently, metal oxide anode materials, which combined with graphene sheets have been reported to display high specific capacity and excellent cyclic stability compared to naked particles.22−26 The graphene sheets in the above structure acted as not only a buffer zone of volume change of the active materials but also a good electron transfer medium. Received: July 26, 2012 Revised: September 10, 2012 Published: September 25, 2012 22217

dx.doi.org/10.1021/jp3073987 | J. Phys. Chem. C 2012, 116, 22217−22225

The Journal of Physical Chemistry C

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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 LAND battery program-control test system at current densities of 54.2 and 542 mA g−1 in the voltage range of 0.02−3.0 V at room temperature (25 ± 1 °C). Cyclic voltammetry (CV) was performed on CHI660C electrochemical workstation at a scan rate of 0.1 mV s−1 from 0 to 3.0 V (versus Li/Li+). Electrochemical impedance spectrum (EIS) measurements were carried out using a CHI660D electrochemical workstation over a frequency range from 100 kHz to 10 mHz.

In this context, with the help of graphene sheets, the activity and cycling performance of Ni2P anode material may be enhanced. Importantly, the introduced graphene sheets can also affect the thermodynamic and kinetic properties thereby affect the voltage hysteresis of Ni2P electrode. To the best of our knowledge, there are few reports on the Ni2P/graphene sheet hybrid with improved electrochemical performance. Herein, the Ni2P/graphene sheet hybrid was prepared by a one-pot solvothermal method, using the as-prepared graphene sheets, nickel acetylacetonate (Ni(acac)2) as a metal precursor, and trioctylphosphine (TOP) as a phosphorus source. The incorporation of graphene sheets plays an important role in the improved cyclic performance and the reduction of voltage hysteresis.

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RESULTS AND DISCUSSION Material Characterization. Figure 1 is employed to certify the formation of graphene sheet reduced from GO. Figure 1a

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EXPERIMENTAL SECTION Graphene oxide (GO) was synthesized by chemical exfoliation of flake graphite powder by a modified Hummers method as originally presented by Kovtyukhova.27 The obtained precursor i.e., GO was collected by freezing drying and then calcined in a tube furnace at 500 °C for 2 h in flowing argon. As a result, the graphene sheets were obtained. The Ni2P/graphene sheet hybrid was prepared based on a one-pot synthetic strategy. In a typical process, a stock solution containing 1 mmol of Ni(acac)2 (ca. 95%, Sinopharm Chemical Reagent Co.), 7 mmol of TOP (ca. 97%, J&K Chemical Ltd.) and 4 mmol of oleic acid (OA, d = 0.85 g mL−1, Sinopharm Chemical Reagent Co.) were prepared under magnetic stirring. Then 8 mg graphene sheets were dispersed in 7 mL of N,N-dimethylformamide (DMF, ca. 99.5%, Sinopharm Chemical Reagent Co.) with sonication and magnetic stirring successively. After 1 h, the stock solution was very slowly injected (∼0.5 mL min−1) into the stirring DMF solution using a syringe pump. Then the mixed solution was heated to 300 °C and maintained at this temperature for 2 h under argon gas. After cooling to room temperature, the black product was precipitated out by adding the mixture of hexane and ethanol, and separated by centrifugation. The precipitation was washed several times and dried in vacuum. For comparison, the Ni2P particles were synthesized with the same process except the addition of graphene sheets. The structure and morphology of Ni2P and Ni2P/graphene sheet hybrid were characterized by X-ray diffraction (XRD, Philips PC-APD with Cu Kα radiation), field emission scanning electron microscopy (FESEM, S-4800), transmission electron microscopy (TEM, JEOL JEM-2010F, and Tecnai G2 F20) and X-ray photoelectron spectroscopy (XPS, PHI 5700). Thermo gravimetric (TG) analysis of Ni2P/graphene sheet was measured on a Netzsch-STA 449C apparatus in the temperature range of 30−630 °C at a heating rate of 10 °C min−1 in air. Electrochemical performances of Ni2P and Ni2P/graphene sheet hybrid were investigated with coin-type cells (CR 2025). The working electrodes were prepared via a slurry coating procedure. The slurry, which consisted of 85 wt % assynthesized powder, 10 wt % acetylene black, and 5 wt % polyvinylidene fluorides (PVDF), was dissolved in Nmethylpyrrolidinone (NMP) and incorporated on nickel foam with 12 mm in diameter. After being dried at 90 °C for 24 h in a vacuum, the samples were pressed under a pressure of 20 MPa. Test cells were assembled in an argon-filled glovebox with the metallic lithium foil as both the reference and counter electrodes, 1 M LiPF6 in ethylene carbonate (EC)−dimethyl

Figure 1. C 1s XPS spectra of (a) GO and (b) graphene in the hybrid reduced by hydrazine vapor.

shows a high-resolution C 1s XPS spectrum of GO. Five types of carbon which correspond to carbon atoms in different functional groups appear clearly. The C 1s peak of graphite is observed at 284.6 eV, C 1s of C−OH at 285.7 eV, C 1s of epoxy at 286.7 eV, C 1s of CO at 288.0 eV, and C 1s of C(O)O at 289.1 eV, respectively. Figure 1b shows XPS spectrum of C 1s of the reduced GO. The graphene sheet reduced by calcination exhibits the same oxygen-containing functionalities, but their intensities are much smaller than GO, confirming that most of the epoxide, hydroxyl and carboxyl functional groups are successfully removed. 22218



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XRD patterns of Ni2P and Ni2P/graphene sheet hybrid powders are shown in Figure 2a. All the diffraction peaks of the

Figure 3. SEM images of (a) Ni2P and (b) Ni2P/graphene sheet hybrid. Figure hybrid, hybrid, hybrid,

In the hybrid material, the Ni2P spheres with sizes of 10−30 nm selectively and densely grew onto the graphene sheets (Figure 3b). It is believed that between the Ni2P and graphene sheet, both chemisorptions and van der Waals interactions exist at many defective sites and pristine regions of the graphene sheets. Additionally, the defective sites located on the surface of graphene sheets effectively hinder diffusion, recrystallization and the growth of Ni2P grains, resulting in smaller sizes of Ni2P spheres compared with the bare ones. According to the previous works, Ni2P phase can be obtained mainly by utilizing TOA as the solvent.18,28,30 However, it seems that TOA is not an ideal solvent for the reduced GO, since the Ni2P spheres are scattering nonuniformly on the graphene sheets (Figure S1, see Supporting Information). This may be ascribed to the poor solubility of graphene sheet in TOA, thus resulting in poor interactions between the Ni2P and graphene sheet. It is suggested that DMF is a better solvent for graphene.31 Therefore, a dense and uniform distribution of Ni2P spheres on the graphene sheet can be accomplished (Figure 3b). Figure 4a shows the TEM image of Ni2P/graphene sheet hybrid. It can be seen that the hollow Ni2P spheres with a size of about 30 nm are uniformly distributed on the 2D graphene sheets. The selected area electron diffraction (SAED) pattern in the inset reveals well-crystallinity of the hybrid, and all the diffraction points can be ascribed to the Ni2P phase. It seems that the graphene sheet is quite clear without any gray shadows, indicating that there is no uncontrolled resembling on the graphene sheet.32 The Ni2P nanospheres on the surface of graphene sheets acting as spacers ensure to efficiently prevent the closely restacking of sheets, avoiding the loss of high active

2. (a) XRD patterns of pure Ni2P and Ni2P/graphene sheet where the inset is the TG result of Ni2P/graphene sheet and (b) the wide-scan XPS spectrum of Ni2P/graphene sheet where the insets are the Ni 2p and P 2p XPS spectra.

powder without graphene sheets can be ascribed to Ni2P (JCPDS 03−0953). Compared to the XRD pattern of pure Ni2P, an additional small and low broad diffraction peak appears at about 25°, corresponding to the (002) diffraction peak of the disorderedly stacked graphene sheets. These results suggest that the hybrid material is composed of graphene sheets and Ni2P. The inset is the TG curve of Ni2P/graphene sheet hybrid, which indicates that the graphene sheets are burn out when the temperature is higher than 300 °C. It is assumed that the remains are Ni2P and thus the weight loss percent of 13% can be assigned to the graphene sheets. It is worth noticing that above 600 °C, the total weight increases again, which can be attributed to the oxidation of Ni2P in the hybrid material.21 The wide-scan XPS spectrum of the hybrid material is shown in Figure 2b. Typical XPS spectra of Ni 2p and P 2p region of Ni2P are shown in the insets. The peaks at 852.9 and 871.2 eV are assigned to Ni 2p3/2 and Ni 2p1/2 energy levels, respectively. The Ni 2p binding energies for Ni2P are very close to those reported in literatures.28,29 For P 2p region, the prominent peak at 130.4 eV arises from the P 2p3/2 and P 2p1/2 doublet that has very close binding energies. These results also confirm the formation of Ni2P in the hybrid. Figure 3a shows the morphology of Ni2P grown in free solution, which is composed of spheres with sizes of 40−60 nm. 22219



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but a multilayer of about 7−10 layers. Figure 4c shows a high resolution TEM (HRTEM) image of a single Ni2P sphere. The regularity of the lattice image of the sphere indicates that it is a single crystal. The space between the adjacent planes is 2.21 Ǻ , corresponding to the (111) plane of Ni2P. Electrochemical Performance. Figure 5 shows the first and 50th galvanostatic charge−discharge curves of the Ni2P/

Figure 5. Discharge/charge profiles for (a) Ni2P/graphene sheet hybrid and (b) pure Ni2P electrodes.

graphene sheet hybrid and Ni2P electrodes ranging from 0.02 to 3.0 V at a current density of 54.2 mA g−1. The first discharge capacity of Ni2P/graphene sheet hybrid is 973.9 mA h g−1 (Figure 5a), which is much higher than that of Ni2P (849.7 mA h g−1, Figure 5b). The extra capacity compared with the theoretic capacity (542 mA h g−1) resulted from the formation of solid electrolyte interface (SEI) during the first discharge process.37 Also, it is suggested that the irreversible capacity between the first discharge and charge is mainly due to the SEI film, which forms during the low potential range.38 These gellike polymer film will lead to a poor conductivity. The incomplete decomposition of SEI film inducing a huge voltage hysteresis is always the main reason for the low Coulombic efficiency and large capacity losses (which will be discussed below). However, it is well-known that Ni nanograins can act as the catalyst toward the formation/dissolution of the organic film. So, it is reasonable to deduce that the highly conductive graphene sheets exhibit superiority to improve kinetics of the electrochemical lithiation/delitiation process, thereby to facilitate the formation of Ni nanograins, which can help to partially dissolve the formed SEI film, and then to decrease the

Figure 4. TEM images of (a) Ni2P/graphene sheet hybrid, the inset is corresponding SAED pattern and (b) graphene sheet of the corresponding region marked in red on (a, c) the HRTEM image of a single Ni2P nanosphere.

surface.33,34 In addition, these Ni2P nanospheres possess the hollow structure, which is similar to the previous reports.35,36 The cross sectional TEM image of the graphene sheet at the edge of the hybrid (marked by a square frame in Figure 4a) is used to further characterize the stacking sheet structure as shown in Figure 4b. The graphene sheet reveals the rippled and crumpled paper-like morphology, which is not a single layer, 22220



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Figure 6. (a) Cyclic performance of Ni2P/graphene sheet hybrid and pure Ni2P electrodes at a current density of 54.2 mA g−1, (b) cycling performance of Ni2P/graphene sheet hybrid at a current density of 542 mA g−1, (c) rate performance of Ni2P/graphene sheet hybrid electrode, and (d) Nyquist plots of the Ni2P/graphene sheet hybrid and Ni2P electrodes.

the reversible capacities are still maintained at 414.8 and 364.3 mA h g−1, respectively. When the current density returns to the initial 54.2 mA g−1 after 20 cycles, the Ni2P/graphene sheet hybrid recovers 75.2% of its second capacity, indicating the stable structure of the electrode. As expected, the enhancement of the electronic conductivity is essential to improve the rate performance of electrode. During our experiment, we also synthesized another hybrid with ∼20 wt % of graphene sheets (Figure S2, see Supporting Information). It is acknowledged that incorporating graphene sheets to the active material would sacrifice the capacity. Thus, although an excellent cyclic performance is obtained, the specific discharge capacity of the hybrid electrode is very low (Figure S3, see Supporting Information). Figure 6d shows the EIS spectra of Ni2P and Ni2P/graphene sheet hybrid electrodes. Two depressed semicircles are observed for both electrodes, including the one located in high frequency ranges assigned to surface film resistance and the one located in medium frequency ranges assigned to charge transfer impedance.39,40 Obviously, the diameter of the second semicircle for the Ni2P/graphene sheet hybrid is smaller than that for Ni2P, revealing lower charge transfer impedance. This phenomenon indicates that the electronic conductivity of Ni2P is improved after the incorporation of graphene sheets because they are good electronic conductor which can keep good

capacity loss upon the subsequent charge/discharge processes. The initial Coulombic efficiency of Ni2P/graphene sheet hybrid is also improved by 58.4% compared to 54.2% of Ni2P, suggesting a better utilization of active material for the hybrid electrode. After 50 cycles, the Ni2P/graphene sheet hybrid still has a higher discharge specific capacity than the Ni2P electrode, indicating the incorporation of graphene sheets could overwhelmingly enhance the Coulombic efficiency. Figure 6a compares the cyclic performances of Ni2P and Ni2P/graphene sheet hybrid electrodes. It can be seen that the reversible capacity of Ni2P electrode decreases from 466.9 to 184.2 mA h g−1 after 50 cycles at a current density of 54.2 mA g−1, corresponding to 39.5% capacity retention. In contrast, the reversible capacity of Ni2P/graphene sheet hybrid slightly decreases in the first five cycles and then keeps a stable value and reaches 449.9 mA h g−1 after 50 cycles, corresponding to 77.8% capacity retention, which has been greatly improved for Ni2P by incorporating graphene sheets. Therefore, 76.4% of the second capacity (360 mA h g−1) has been maintained even at a high current density of 542 mA g−1 as shown in Figure 6b. Another advantage of the Ni2P/graphene sheet hybrid electrode is its rate performance. As shown in Figure 6c, the reversible capacity of Ni2P/graphene sheet hybrid is stable at 555.8 mA h g−1 after 5 cycles at a current density of 54.2 mA g−1. When the current density increases to 271 and 542 mA g−1, 22221



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electronic wetting of the anchored Ni2P spheres. Also, it is worth mentioning that the above lithium storage performance is not superior to that of Ni2P/C nanotubes,20 but it exhibits a significant enhancement in the capacity retention of Ni2P/ graphene sheet hybrid at a high current density compared to previous reports.18,19,30 Thermodynamic and Kinetic Properties. The overall Li+ uptake/extraction to/from Ni2P can be written as follows, which is also subject to a conversion process like transition metal oxides:23,41−44 Ni 2P + 3Li+ + 3e− ↔ 3Ni + Li3P

(1)

As shown in Figure 5a, the first discharge voltage-capacity curve of Ni2P electrode exhibits three distinct sloping voltage ranges. Given to the Ni2P electrode would suffer an insertion process,20,30 the capacity above 0.8 V can be attributed to the Li+ insertion. The sloping voltage capacity between 0.3 and 0.8 V corresponds to the conversion reaction from initial Ni2P to Ni/Li3P nanocomposite. The capacity below 0.3 V can be attributed to the reduction of electrolyte and the formation of SEI film. The low-voltage capacity is nested in either the pseudocapacitive character of the in situ made polymeric/gel film45 or an interfacial mechanism, which has been explained by Maier et al.46−48 Therefore, to form a stable SEI film is indispensible to keep these metallic Ni nanoparticles electrically connected and to facilitate the transfer of Li+. Because of the large specific area of graphene sheets, there would be plenty of SEI film forming during the first discharge process, which could result in a low initial Coulombic efficiency.22,25,32,49,50 To optimize the initial Coulombic efficiency and cycling performance by tuning the SEI formation, different cutoff voltages of the Ni2P/graphene sheet hybrid electrode are presented in Figure 7. As shown in Figure 7a, when the cutoff voltage increases from 0.02 to 0.1 V, the initial Coulombic efficiency and capacity retention decrease to 56.7% and 39.8%, respectively. Furthermore, comparing the three cutoff voltages of 0.1, 0.2, and 0.3 V, we observe the worst capacity retention for the cutoff voltage between 0.3 and 3 V. In short, none of the scanned voltage ranges is as good capacity retention as the capacity retention of Ni2P-graphene sheet/Li cell after cycled over the full voltage range of 0.02−3 V, thus suggesting that the low-voltage process below 0.3 V is beneficial to the cell capacity retention. That is to say the formation of SEI film can prevent the metallic Ni nanoparticles from pulverization and keep them electrically connected. To get a straight insight, TEM images of three half cells cycled at different cutoff voltages after the 10th discharge process are provided in the insets of Figure 7. It can be seen that after discharged to 0.1 V, the metallic Ni nanoparticles are dispersed uniformly on the graphene sheets, although there still exist a small amount of aggregated particles after pulverization. However, we can observe a severe pulverization after the cell discharged to 0.3 V for 10 cycles. Interestingly, the amount of the aggregated nanoparticles reduces greatly, which probably indicates that after 10 times of cycling, the formed metallic nanoparticles have dropped off without effective wrapping by polymeric/gel films.45 Figure 8 displays the TEM images of the Ni2P/graphene sheet hybrid and Ni2P electrodes after the 30th cycles. As shown in Figure 8a, many crystalline Ni nanoparticles are scattering on the graphene sheets. With the help of HRTEM image, dense nanoparticles with sizes of 3−5 nm can be observed (Figure 8b). They are still anchored on the graphene sheets and no obvious aggregation of nanoparticles is observed,

Figure 7. Discharge/charge curves for a Ni2P/graphene sheet hybrid cell cycled at a current density of 54.2 mA g−1 between (a) 0.1−3 V, (b) 0.2−3 V, and (c) 0.3−3 V, the insets are the TEM images of Ni2P/ graphene sheet hybrid electrode after 10th discharge.

although they have undergone 30 times of volume expansion/ contraction associated with the lithium insertion and extraction process, highlighting the structure stability of Ni2P/graphene sheet hybrid electrode. However, as shown in Figure 8c, aggregation and broken particles are nonuniformly loaded in the pure Ni2P electrode. It is purposed that the capacity decay could be ascribed to the loss of the electrical contact of active particles owing to volume variation and microstructure instability as a consequence of phase transformation reaction during the lithium uptake and extraction.39,51 Even some primary nanoparticles do not adequately react with lithium, implying the poor kinetics of lithium intercalation. What’s 22222



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Voltage hysteresis of electrode materials refers to the phenomenon that discharge potential is lower than charge potential.40 Different reasons lead to the polarization during discharge/charge process, and the total polarization results in the voltage hysteresis. The voltage hysteresis, which is usually found in conversion reaction materials, produces a huge roundtrip inefficiency, resulting in a associated energy losses.8 Despite the fact that the graphene sheets possess a different lithium storage mechanism, the voltage hysteresis curve is the sum of the polarization of both charging and discharging at galvanostatic mode and can be used to reflect the kinetic property of storing lithium by different mechanisms of electrode materials. From Figure 9, parts a and b, it is obtained by subtracting the discharge curve at the second cycle from the charge curve at the first cycle after normalization. For the capacity normalization used here, 0 refers to the full delithiation state charged to 3.0 V and 1.0 means the starting of charging. As shown in Figure 9c, the voltage polarization of Ni2P electrode is about 0.6−1.2 V in all lithiation/delithiation ranges. Compared to the Ni2P electrode, the Ni2P/graphene sheet hybrid exhibits an obvious reduction of voltage polarization. Interestingly, the overall voltage polarization of the graphene sheet electrode decreases obviously after the incorporation of Ni2P and the voltage hysteresis curve of the Ni2P/graphene sheet hybrid exhibits the same characters as the Ni2P electrode. A possible reason is that the reduced graphene sheets contain many dangling bonds, surface defects and vacancies, which are considered as the active sites for the formation of SEI film or banding with lithium. The kinetics of lithium intercalation is limited by the thick SEI film because all Li+ in an electrolyte solution must cross this film before the formation of the lithium-intercalation graphite compounds. Also, the banding between active sites and lithium would be the activated processes, which could lead to hysteresis.23 The anchored Ni2P nanospheres cover and remove some active sites, leading to an obvious reduction of voltage polarization. However, in the hybrid electrode, the voltage polarization resulting from Ni2P is more obvious compared with the graphene sheets modified by Ni2P nanospheres and thus the voltage polarization of Ni2P dominates the whole charge/discharge process. The voltage hysteresis of Ni2P/graphene sheet hybrid is lower than that of pure Ni2P and graphene sheets, indicating that the hybridizing strategy makes a synergistic effect on the Ni2P and graphene sheets. It is considered that the decrease of voltage hysteresis is attributed to the coverage and elimination of surface defects and vacancies of graphene sheets by anchored Ni2P nanospheres. For another, as we have talked above, it is suggested that the modification of SEI formation by incorporating graphene sheets is just based on the synergistic effect. The anchored Ni2P nanospheres can prevent the formation of excess SEI films by covering and removing some active sites. Meanwhile, the SEI films on the surface of Ni2P spheres can also be modified due to the high conductive graphene sheets, which lead to the enhancement of the kinetics of lithium intercalation for Ni2P. The fast lithium transportation can reduce the chance to obtain a thick SEI film and the voltage hysteresis. Table 1 summarizes the voltage polarization values of different Ni2P composites. It can be seen that except Ni2P/C nanotubes exhibiting a relatively lower voltage polarization, all the composites possess similar voltage polarizations, suggesting that the graphene sheets indeed can partly decrease the voltage polarization by improving the lithium kinetics.

Figure 8. TEM images of the cells after 30 cycles of discharge/charge for Ni2P/graphene sheet electrode (a) at a low magnification and (b) a high magnification and for pure Ni2P electrode (c).

more, the multilayered organic-like films with a thickness of about 10 nm, namely the SEI films, are disorderedly patterned in the Ni2P electrode (Figure 8c). In contrast, it shows a relatively thin SEI layer after 30 cycles for the Ni2P/graphene sheet hybrid (Figure 8b). It can also certify that the graphene sheets introduced to Ni2P can effectively modify the SEI film to facilitate Li+ transportation and keep the decomposed nanoparticle electrically connected (which will be discussed below). 22223



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of Ni2P are significantly improved by the incorporation of graphene sheets because of the establishment of good electronic contact between the active nanospheres and fast Li+ and electron transportation during discharge−charge process. The Ni2P/graphene sheet hybrid delivers a capacity of 450 and 360 mA h g−1 after 50 cycles at current densities of 54.2 and 542 mA g−1, respectively, and also exhibits good rate capability. In addition, the hybridizing strategy makes a synergistic effect on the Ni2P nanospheres and the incorporated graphene sheets by partially decreasing the voltage polarization and mutually modifying the SEI film, thus resulting in a good cyclic performance.

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ASSOCIATED CONTENT

S Supporting Information *

SEM images of Ni2P/reduced graphene oxide synthesized by using different solvents; TEM images and cycling performance of Ni2P/graphene sheet-20%. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-571-87952856. Fax: +86-571-87952573. Email: [email protected] (X.L.W.); [email protected], tujplab@ zju.edu.cn (J.P.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51271169), Fundamental Research Funds for the Central Universities (2011QNA4006), and Key Science and Technology Innovation Team of Zhejiang Province (2010R50013). The authors also thank Dr. Li’na Wang, Mr. Wei Huang, and Xinting Cong for help in operating the TEM.

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Figure 9. Voltage/capacity traces for (a) Ni2P/graphene sheet hybrid and (b) pure Ni2P and (c) comparison of voltage hysteresis curves of graphene sheet thermal reduced from GO, Ni2P and Ni2P/graphene sheet hybrid electrode at a current density of 54.2 mA g−1.

Table 1. Comparison of the Voltage Hysteresis for Different Ni2P Composites Ni2P composite voltage hysteresis (V)

Ni2P/C nanotubes20

Ni2P/C spheres18

Ni2P nanowires30

this work

0.4−0.8

0.6−1.0

0.5−0.9

0.5−0.9

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CONCLUSIONS The Ni2P/graphene sheet hybrid is synthesized of which Ni2P spheres are up to 10−30 nm in sizes and homogeneously anchored on conducting graphene sheets. As an anode material for lithium-ion batteries, the cyclic stability and rate capability 22224



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