Ni2P Nanosheets on Carbon Cloth: An Efficient Flexible Electrode for

3 hours ago - Synopsis. Ni2P Ns/CC is efficient for sodium-ion batteries, achieving a capacity of 399 mA h g−1 at 0.2 A g−1 after 100 cycles. At 2...
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Ni2P Nanosheets on Carbon Cloth: An Efficient Flexible Electrode for Sodium-Ion Batteries Yuan Wang,†,‡ Qi Pan,† Kun Jia,† Huanbo Wang,§ Jiajia Gao,† Chunliu Xu,† Yanjun Zhong,† Abdulmohsen Ali Alshehri,⊥ Khalid Ahmad Alzahrani,⊥ Xiaodong Guo,*,† and Xuping Sun*,‡ †

School of Chemical Engineering, Sichuan University, Chengdu, 610065 Sichuan, China School of Environment and Resource, Southwest University of Science and Technology, Mianyang, 621010 Sichuan, China ‡ Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu, 610054 Sichuan, China ⊥ Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia

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S Supporting Information *

hybrid with a 3D yolk−shell-like structure as an anode for SIBs, which delivers a reversible capacity of 181 mA h g−1 at 0.2 A g−1 after 100 cycles.24 This reveals that the architecture combining the advantages of nanostructure and microstructure can provide a superior electrochemical stability. In addition, the monodisperse Ni2P immobilized on nitrogen- and phosphoruscodoped carbon as an anode was reported by Shi et al., and it delivers a capacity of 361 mA h g−1 at 0.1 A g−1 after 300 cycles.25 Although TMPs combined with the advanced carbon-based material could be a promising electrode for SIBs, the complex technological process limits their wide use. Recently, selfsupported electrodes have been paid much attention for SIBs, especially the nanostructure directly developing on a substrate as a flexible electrode.27−29 These advanced design strategies offer us an efficient additive-free electrode.30 We thus anticipate an enhanced electrochemical performance of SIBs by developing Ni2P on carbon cloth, which, however, has not been explored so far. Herein, we present Ni2P nanosheets on carbon cloth (Ni2P Ns/CC; see the Supporting Information for preparative details) as an efficient 3D anode for SIBs. Such a Ni2P Ns/CC electrode delivers a high capacity of 399 mA h g−1 at 0.2 A g−1 after 100 cycles. It still delivers 72 mA h g−1 even at 2 A g−1 after 1000 cycles with good capacity retention. The structure of such a Ni2P Ns/CC gives a strong bond with a current collector and reduces the diffusion pathway of Na+ during the sodiation and desodiation processes. Figure S1 exhibits the X-ray diffraction (XRD) patterns of Ni2P Ns/CC and its precursor Ni(OH)2 Ns/CC products. The precursor shows the characteristics of Ni(OH)2 (JCPDS 140117). After phosphidation, the diffraction peaks of XRD are indexed to the characteristics of Ni2P (JCPDS 89-2742). This indicates the successful conversion of Ni(OH)2 to Ni2P after phosphidation treatment. In addition, a couple of peaks at around 25° and 45° for the two patterns are attributed to a CC substrate (JCPDS 26-1077). Figure 1a shows a low-magnification scanning electron microscopy (SEM) image of Ni(OH)2 Ns/CC, which reveals that CC is fully covered with Ni(OH)2 structures. The magnified SEM image (Figure 1a, inset)

ABSTRACT: Transition-metal phosphides have been increasingly investigated because of their high theoretical specific capacity and low potential for sodium storage. Herein, we describe the development of Ni2P nanosheets on carbon cloth (Ni2P Ns/CC), which behaves as a flexible 3D anode for sodium-ion batteries. Such a Ni2P Ns/CC delivers a high capacity of 399 mA h g−1 at 0.2 A g−1. At 2 A g−1, it still delivers 72 mA h g−1 even after 1000 cycles. The impressive performance is attributed to such a self-supported structure. Moreover, a possible conversion reaction mechanism is also carefully revealed.

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eveloping advanced electrochemical energy-storage technologies has become increasingly important because of the utilization of clean and renewable energy from wind, solar, and wave energy.1,2 Sodium-ion batteries (SIBs) exhibit a more promising prospect than costly lithium-ion batteries (LIBs) because of their similar chemistry to LIBs and abundant sodium resources.3,4 Traditional anode materials for SIBs, such as hard carbon, are regarded as promising candidates for large-scale energy-storage systems, but the limited sodium storage makes it unsuitable for advanced electronic equipment.5,6 Therefore, numerous novel anode materials that possess higher theoretical capacity, including metal oxides,7,8 sulfides,9,10 and phosphides,11,12 were applied for SIBs. Among them, transitionmetal phosphides (TMPs) distinguish themselves with their moderate intercalation potentials and low polarization.13 However, a big problem of unsatisfactory capacity retention still needs to be solved, which results from the huge volume expansion and loss of electrical contact of active materials during the sodiation and desodiation processes.14−16 To address this challenge, various strategies have been utilized to enhance their cycling stability including different elements, morphology design, carbon introduction, etc.16−20 Nickel phosphides (NiPx), as a typical kind of phosphide, have drawn great attention because of their impressive ability for LIBs21−24 but show unsatisfactory electrochemical performance for SIBs.24−26 This is due to the larger Na+ radius (2.04 Å) than that of Li+ (1.52 Å), leading to the more sluggish kinetics and larger volume change. Wu et al. reported a graphene/Ni2P © XXXX American Chemical Society

Received: February 15, 2019

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DOI: 10.1021/acs.inorgchem.9b00451 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Electrochemical performances of the Ni2P Ns/CC electrode for SIBs. (a) CV curves at 0.1 mV s−1. (b) Discharge−charge voltage profiles at 0.2 A g−1 between 3.0 and 0.01 V versus Na/Na+. (c) Rate performance. (d) Cycling performance at 0.2 A g−1 for 100 cycles.

discharge curve, one prominent cathodic peak appeared at 0.58 V but disappeared in subsequent cycles, which could be associated with the decomposition of Ni2P and the formation of a solid electrolyte interface (SEI) film. As for anodic sweep, one broad peak appearing at 0.3−1.0 V could be attributed to the extraction of Na+. In subsequent sweeps, the main cathodic peak shifted to 1.05 V, which could be ascribed to drastic modification of the texture or structure.16,36 The following peak appeared at 0.3 V, which may be related to the generated Na3P. In addition, the overlapped curves in the subsequent sweeps imply the excellent cycle performance of such an electrode. Figure 2b shows the discharge−charge voltage profiles of the Ni2P Ns/CC electrode at 0.2 A g−1 in the initial three cycles. In the first discharge curve, an obvious plateau in the voltage range of 0.4−0.8 V is observed corresponding to the formation of a SEI film. Two plateaus at 1.1 and 0.3 V in the subsequent discharge curves also match well with those in the CV curves. The rate performance of the Ni2P Ns/CC electrode is investigated by charging and discharging under various current densities ranging from 0.1 to 2 A g−1. As shown in Figure 2c, the average capacities of 514, 432, 312, 152, and 68 mA h g−1 are observed at current densities of 0.1, 0.2, 0.5, 1, and 2 A g−1, respectively. When backed up to 0.1 A g−1, a reversible capacity of 490 mAh g−1 can be obtained. Notably, the Ni2P Ns/CC electrode presents an excellent cycling stability. A capacity of 399 mA h g−1 is observed with a capacity retention of 90% over 100 cycles at 0.2 A g−1 (Figure 2d). Moreover, it still retains a capacity of 153 mA h g−1 at 0.5 A after 600 cycles (Figure S4a) and a capacity of 72 mA h g−1 even at 2A g−1 after 1000 cycles (Figure S4b). A large loss capacity in the initial discharge process is due to formation of the SEI layer. Figure S5 shows the SEM and TEM images of Ni2P after being fully discharged to 0.01 V. It is clear that agglomeration is observed on the surface of a nanosheet (Figure S5a), which can be attributed to an SEI layer. Moreover, an SEI layer is carefully observed in the edge of a nanosheet (Figure S5b), further indicating formation of the SEI layer. For comparison, the Ni2P particles are mixed with binder poly(vinylidene fluoride) in an N-methylpyrrolidone solvent, and the resulting slurry is painted on bare CC (Ni2P particle/ CC). Figures S6 and S7 show the XRD pattern and SEM image of the Ni2P particles, indicating the successful phosphidation

Figure 1. SEM images of (a) Ni(OH)2 Ns/CC and (b) Ni2P Ns/CC. (c) TEM image for the Ni2P nanosheet. (d) HRTEM image and (e) SAED pattern taken from Ni2P. (f) Elemental mapping of the Ni2P nanosheet.

indicates the morphology of their nanosheets. Compared with Ni(OH)2 Ns/CC, almost no obvious changes in the structure can be observed for Ni2P Ns/CC (Figures 1b and S2). The transmission electron microscopy (TEM) image is shown in Figure 1c and further confirms that the phosphided product exhibits the structure of a nanosheet. The high-resolution TEM (HRTEM) image (Figure 1d) displays a clear lattice with a d spacing of 0.203 nm, which matches well with that of the (201) plane of Ni2P. The corresponding selected-area electron diffraction (SAED) pattern (Figure 1e) shows four diffraction rings indexed to the (111), (201), (210), and (300) planes of the Ni2P phase. The TEM-based energy-dispersive spectrometry elemental mapping images (Figure 1f) display that the P and Ni elements are uniformly distributed all over the surface of the nanosheet. The surface chemical composition of the Ni2P Ns was investigated by X-ray photoelectron spectroscopy (XPS). The high-resolution Ni 2p spectrum (Figure S3a) displays six distinct peaks. The peaks at around 853.5 and 870.8 eV are assigned to Ni 2p3/2 and Ni 2p1/2 of Ni−P, and the peaks at 856.9 and 874.7 eV are attributed to Ni 2p3/2 and Ni 2p1/2 of Ni−O.16,24,31 In addition, the peaks at around 861.9 and 879.3 eV are assigned to the satellite peaks.31,32 The high-resolution P 2p spectrum (Figure S3b) shows two prominent peaks that can be deconvoluted into four peaks for 129.7, 130.7, 133.8, and 134.7 eV corresponding to P 2p3/2 and P 2p1/2 of P−Ni as well as P 2p3/2 and P 2p1/2 of P−O.24,33 The O species could be due to superficial oxidation caused by the prolonged exposure of TMPs to air.33−35 The electrochemical behaviors of Ni2P Ns/CC are first investigated by cyclic voltammetry (CV). Figure 2a shows a typical CV curve of the Ni2P Ns/CC electrode at 0.1 mV s−1 in a potential range of 0.01−3.0 V versus Na/Na+. In the initial B

DOI: 10.1021/acs.inorgchem.9b00451 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry and morphology of the microsphere formed by a nanosheet. The carbon content of the electrodes is analyzed by thermal gravimetry analysis (TGA). Figure S8 shows the TGA profiles of two samples, indicating that the carbon content of Ni2P Ns/ CC and Ni2P particle/CC are 88.3 and 87.4 wt %, respectively. As an anode for SIBs, Ni2P particle/CC delivers a poor rate performance (Figure S9a), and only a capacity of 47 mA h g−1 is obtained after 100 cycles at 0.2 A g−1 (Figure S9b). The electrochemical performances of Ni2P without CC are also tested. As shown in Figure S10, the electrode of the Ni2P particle on copper foil exhibits an unsatisfactory rate performance (Figure S10a) and a low capacity of 40 mAh g−1 at 0.2 A g−1 (Figure S10b). Second-cycle CV curves of Ni2P particle/CC and Ni2P Ns/CC are illustrated in Figure S11. This shows that the loop of Ni2P Ns/CC has a larger area, further indicating its larger discharge−charge capacity. Figure S12 shows two Nyquist plots of Ni2P Ns/CC and Ni2P particle/CC, revealing a lower chargetransfer resistance of the Ni2P Ns/CC electrode. A more detailed comparison about other nickel-based phosphide materials for SIBs is listed in Table S1. All results strongly confirm the superior electrochemical performance of Ni2P Ns/ CC. Moreover, bare CC as an anode is also tested for SIBs, and it provides almost no capacity during the electrochemical process (Figure S13). To investigate the chemical reaction process, in situ electrochemical impedance spectroscopy (EIS) was conducted during the first cycling. Figure 3a shows the EIS spectra of the

Ex situ XRD is performed to further investigate the possible reaction mechanism. Figure 3c exhibits the XRD patterns of the Ni2P Ns/CC electrode at different states of charge in the initial cycle. During the sodiation process, no distinct new phase is observed until the Ni2P Ns/CC electrode discharges to 0.3 V, where the Na3P phases appear and the Ni2P phases disappear. After being fully discharged to 0.01 V, the peak of Na3P becomes more distinct. In the desodiation process, the obvious Ni2P phases are observed at 0.3 V, where the Na3P phases still remain. When charged to 0.8 V, the Na3P phases disappear and the XRD patterns show the characteristics of Ni2P, indicating excellent stability. It is hard to detect all products in the cycled electrode by XRD measurement. Therefore, HRTEM and SAED are also carried out. Figure 3d shows the HRTEM and SAED images of the Ni2P Ns/CC electrode after being fully discharged to 0.01 V. They display a few lattices with d spacings of 0.196 and 0.380 nm, corresponding to the (104) and (102) planes of Na3P. The SAED pattern (Figure 3d, inset) exhibits four diffraction rings indexed to the (103), (203), and (302) planes of Na3P and the (220) plane of Ni, indicating the full conversion of Ni2P. Accordingly, the reaction mechanism may be simulated as follows: Ni 2P + 3Na + + 3e− → Na3P + 2Ni

(1)

The CV curves of the Ni2P Ns/CC electrode at different scan rates are shown in Figure 4a to investigate the reaction kinetics.

Figure 4. (a) CV curves at different scan rates of the Ni2P Ns/CC electrode from 0.1 to 5 mV s−1. (b) Contribution ratio of the capacitiveand diffusion-controlled currents at different scan rates.

Clearly, the locations of the cathodic and anodic peaks shift in opposite directions with scan rate increases from 0.1 to 5 mV s−1, indicating an increased reaction overpotential. The capacitive effect is qualitatively analyzed according to the relationship of current (i) and scan rate (v) from the CV curves:

Figure 3. (a) In situ EIS spectral evolution of the Ni2P Ns/CC electrode at different charge−discharge potentials. (b) Calculated Rct values at different charge−discharge potentials and the equivalent circuit model (inset). (c) Ex situ XRD patterns of a Ni2P Ns/CC anode at various charge−discharge potentials. (d) HRTEM and SAED images of the Ni2P Ns/CC electrode at an initial discharge to 0.01 V.

i = avb

(2)

For this equation, both a and b are constants. b is between 0.5 and 1.0. It is well-known that b is close to 0.5, indicating a diffusion-controlled process. When b approaches 1, this indicates a surface capacitive process.39−41 Figure S14 presents the relationship between log i and log v for both the cathodic and anodic peaks to determine the value of b. This delivers a b value of 0.80 for the cathodic peak and of 0.87 for the anodic peak, indicating that a surface-capacitance-dominated process occupies prominent character. The pseudocapacitance effect as a result of surface adsorption gives more pathways for Na+ from the surface to the Ni2P bulk. Furthermore, the relationship of the capacitive- and diffusion-controlled contributions at various scan rates was also expressed according to

Ni2P Ns/CC electrode at different states of discharge and charge, and their spectra are fitted by an equivalent electrical circuit presented in the inset of Figure 3b. All of the fitted charge-transfer reaction resistances (Rct) are also summarized in Figure 3b. It is noted that Rct increased gradually from 153 Ω in the sodiated state at 2.0 V to 353 Ω in the completely sodiated state at 0.01 V. This is due to the formation of a new phase with poor electron conductivity and growth of the SEI layer.37,38 During the desodiation process, Rct starts to obviously decrease, which could be caused by the decomposition of a new phase and the SEI layer.37

i (V) = k1v1/2 + k 2v C

(3) DOI: 10.1021/acs.inorgchem.9b00451 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry i (V)/v1/2 = k1 + k 2v1/2

(4) Ellis, B. L.; Nazar, L. F. Sodium and Sodium-Ion Energy Storage Batteries. Curr. Opin. Solid State Mater. Sci. 2012, 16, 168−177. (5) Zhou, D.; Peer, M.; Yang, Z.; Pol, V. G.; Key, F. D.; Jorne, J.; Foley, H. C.; Johnson, C. S. Long Cycle Life Microporous Spherical Carbon Anodes for Sodium-Ion Batteries Derived from Furfuryl Alcohol. J. Mater. Chem. A 2016, 4, 6271−6275. (6) Wang, P.; Zhu, X.; Wang, Q.; Xu, X.; Zhou, X.; Bao, J. KelpDerived hard carbons as Advanced Anode Materials for Sodium-Ion Batteries. J. Mater. Chem. A 2017, 5, 5761−5769. (7) Kim, K.; Ali, G.; Chung, K. Y.; Yoon, C. S.; Yashiro, H.; Sun, Y.; Lu, J.; Amine, K.; Myung, S. Anatase Titania Nanorods as an Intercalation Anode Material for Rechargeable Sodium Batteries. Nano Lett. 2014, 14, 416−422. (8) Longoni, G.; Pena Cabrera, R. L.; Polizzi, S.; D'Arienzo, M.; Mari, C. M.; Cui, Y.; Ruffo, R. Shape-Controlled TiO2 Nanocrystals for NaIon Battery Electrodes: The Role of Different Exposed Crystal Facets on the Electrochemical Properties. Nano Lett. 2017, 17, 992−1000. (9) Wu, L.; Lu, H.; Xiao, L.; Qian, J.; Ai, X.; Yang, H.; Cao, Y. HighCapacity Antimony Sulphide Nanoparticle Decorated Graphene Composite as Anode for Sodium-Ion Batteries. J. Mater. Chem. A 2014, 2, 16424−16428. (10) Hu, Z.; Wang, L.; Zhang, K.; Wang, J.; Cheng, F.; Tao, Z.; Chen, J. Pyrite FeS2 for High-Rate and Long-Life Rechargeable Sodium Batteries. Angew. Chem., Int. Ed. 2014, 53, 12794−12798. (11) 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. (12) Li, W.; Chou, S.; Wang, J.; Kim, J. H.; Liu, H.; Dou, S. Sn4+xP3 @ Amorphous Sn-P Composites as Anodes for Sodium-Ion Batteries with Low Cost, High Capacity, Long Life, and Superior Rate Capability. Adv. Mater. 2014, 26, 4037−4042. (13) Wang, X.; Kim, H.; Xiao, Y.; Sun, Y. Nanostructured Metal Phosphide-Based Materials for Electrochemical Energy Storage. J. Mater. Chem. A 2016, 4, 14915−14931. (14) Li, W.; Chou, S.; Wang, J.; Liu, H.; Dou, S. A New, Cheap, and Productive FeP Anode Material for Sodium-Ion Batteries. Chem. Commun. 2015, 51, 3682−3685. (15) Wang, Y.; Wu, C.; Wu, Z.; Cui, G.; Xie, F.; Guo, X.; Sun, X. FeP Nanorod Arrays on Carbon Cloth: A High-Performance Anode for Sodium-Ion Batteries. Chem. Commun. 2018, 54, 9341−9344. (16) Miao, X.; Yin, R.; Ge, X.; Li, Z.; Yin, L. Ni2P@Carbon Core-Shell Nanoparticle-Arched 3D Interconnected Graphene Aerogel Architectures as Anodes for High-Performance Sodium-Ion Batteries. Small 2017, 13, 1702138. (17) Li, Z.; Zhang, L.; Ge, X.; Li, C.; Dong, S.; Wang, C.; Yin, L. Coreshell Structured CoP/FeP Porous Microcubes Interconnected by Reduced Graphene Oxide as High Performance Anodes for Sodium Ion Batteries. Nano Energy 2017, 32, 494−502. (18) Liu, J.; Kopold, P.; Wu, C.; van Aken, P. A.; Maier, J.; Yu, Y. Uniform Yolk−Shell Sn4P3@C Nanospheres as High-Capacity and Cycle-Stable Anode Materials for Sodium-Ion Batteries. Energy Environ. Sci. 2015, 8, 3531−3538. (19) Kim, Y.; Kim, Y.; Choi, A.; Woo, S.; Mok, D.; Choi, N.; Jung, Y. S.; Ryu, J. H.; Oh, S. M.; Lee, K. T. Tin Phosphide as a Promising Anode Material for Na-Ion Batteries. Adv. Mater. 2014, 26, 4139−4144. (20) Li, Q.; Li, Z.; Zhang, Z.; Li, C.; Ma, J.; Wang, C.; Ge, X.; Dong, S.; Yin, L. Low-Temperature Solution-Based Phosphorization Reaction Route to Sn4P3/Reduced Graphene Oxide Nanohybrids as Anodes for Sodium Ion Batteries. Adv. Energy Mater. 2016, 6, 1600376. (21) Lou, P.; Cui, Z.; Jia, Z.; Sun, J.; Tan, Y.; Guo, X. Monodispersed Carbon-Coated Cubic NiP2 Nanoparticles Anchored on Carbon Nanotubes as Ultra-Long-Life Anodes for Reversible Lithium Storage. ACS Nano 2017, 11, 3705−3715. (22) Lu, Y.; Wang, X.; Mai, Y.; Xiang, J.; Zhang, H.; Li, L.; Gu, C.; Tu, J.; Mao, S. X. Ni2P/Graphene Sheets as Anode Materials with Enhanced Electrochemical Properties versus Lithium. J. Phys. Chem. C 2012, 116, 22217−22225. (23) Luo, Z.; Zhang, Y.; Zhang, C.; Tan, H. T.; Li, Z.; Abutaha, A.; Wu, X.; Xiong, Q.; Khor, K. A.; Hippalgaonkar, K.; Xu, J.; Hng, H. H.;

(4)

In eq 3, k1v1/2 and k2v correspond to the current contributions from the diffusion-controlled process and surface capacitive effects, respectively. The parameters k1 and k2 are easily determined from eq 4. Figure 4b clearly shows that the capacitive-controlled process occurs in the reaction process at various scan rates. The capacitive contribution increases gradually with an increase of the scan rate from 27% at 0.1 mV s−1 to 59% at 5 mV s−1. This demonstrates that the capacitive process is predominant at higher rate and the pseudocapacitance proportion effectively enhances the electrochemical performance.42 In conclusion, Ni2P Ns/CC is experimentally proven to be an efficient 3D anode for SIBs. Ni2P Ns/CC achieves a reversible capacity of 399 mA h g−1 at 0.2 A g−1 with an excellent capacity retention of 90% after 100 cycles. A control experiment also demonstrates the superior electrochemical performance of such a self-supported anode. Moreover, various techniques, including TEM, SAED, and ex situ XRD, are conducted to reveal the sodium-storage mechanism of Ni2P. This work not only provides us an attractive electrode material for SIBs but would open up exciting new avenues to the rational design of TMPs for related applications.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00451. Experimental section, XRD patterns, XPS spectra, SEM and TEM images, TGA profiles, electrochemical performances, CV curves, Nyquist plots, b values, and Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.G.). *E-mail: [email protected] (X.S.). ORCID

Huanbo Wang: 0000-0003-1104-5688 Yanjun Zhong: 0000-0002-5057-3144 Xiaodong Guo: 0000-0003-0376-7760 Xuping Sun: 0000-0002-5326-3838 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 (Grants 21575137 and 21878915) and the Youth Foundation of Sichuan University (Grant 2017SCU04a08).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.9b00451 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.9b00451 Inorg. Chem. XXXX, XXX, XXX−XXX