Nickel phosphide nanosheets supported on reduced graphene oxide

Feb 21, 2019 - The higher discharge capacity and better cycling stability in comparison with these of the pure Ni2P verify that compositing reduced gr...
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Nickel Phosphide Nanosheets Supported on Reduced Graphene Oxide for Enhanced Aluminum-Ion Batteries Jiguo Tu, Mingyong Wang,* Xiang Xiao, Haiping Lei, and Shuqiang Jiao* State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, PR China

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ABSTRACT: The electrochemical behavior of nickel phosphide nanosheets supported on reduced graphene oxide is first explored as cathode material for aluminum-ion batteries. Ni2P/rGO nanosheets are prepared through a hydrothermal method combined with a subsequent phosphorization process. Ni2P/rGO nanosheets deliver a high first discharge capacity of 274.5 mAh g−1 at 100 mA g−1, which remain at 73.0 mAh g−1 with a Coulombic efficiency of 93.5% after 500 cycles. And even at a higher current density of 200 mA g−1, the cathode still presents the favorable discharge capacity of 60.9 mAh g−1 and Coulombic efficiency of 94.5% over 3000 cycles. The higher discharge capacity and better cycling stability in comparison with these of the pure Ni2P verify that compositing reduced graphene oxide is an effective strategy to enhance the capacity and cycling stability of Ni2P for aluminum-ion batteries. Furthermore, the energy storage mechanism of Ni2P as cathode for aluminum-ion batteries is rigorously investigated, showing that the incorporation of Al3+ into Ni2P can generate AlmNinP and elemental Ni. KEYWORDS: Nickel phosphide, Reduced graphene oxide, Redox mechanism, Diffusion coefficient, Aluminum-ion batteries

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INTRODUCTION As the third most abundant element in the earth’s crust, aluminum possesses ultrahigh theoretical specific capacity (volumetric: 8040 mAh cm−3, gravimetric: 2980 mAh g−1), which make it practicable for electric vehicles and large-scale energy storage. Despite these advantages, they usually suffer from numerous issues due to low charge/discharge voltage, poor cycle life, and electrode disintegration, which would limit their application in energy storage devices.1−5 In consideration of the growing demand for energy storage, the novel highperformance cathode materials are urgently required for the rechargeable aluminum-ion batteries (RAIBs). Graphite, a type of classical intercalation/deintercalation material, has attracted great attention since both Dai’s and Jiao’s groups and showed well-defined discharge voltage plateaus and high specific capacity.6−11 As an alternative to graphite, the conversion type materials, like oxides,12 sulfides,13−17 and chloride,18,19 have been also widely researched as prospective cathode materials, due to their capability to possess higher capacity. Transition metal phosphides (MxPy, where M = Fe, Co, Ni, Cu, Mn, etc.) with metalloid properties, high activity, and superior conductivity have been considered as high-performance catalysts and electrode materials for hydrogen evolution reaction (HER),20−26 oxygen evolution reaction (OER),27,28 and Li-ion batteries (LIBs).29−31 Nickel phosphides (NixPy), a type of the most important transition metal phosphides, have attracted extensive interest in recent years.32−35 In view of the molar ratio of Ni/P, the NixPy compounds can be segmented into metal-rich phases (x/y ≥ 1, e.g., NiP, Ni2P, Ni3P, Ni5P4, © XXXX American Chemical Society

Ni7P3, Ni8P3, and Ni12P5) and phosphorus-rich phases (x/y < 1, e.g., NiP2 and NiP3). The metal-rich phosphides, especially Ni2P, have been extensively researched, because they exhibit better chemical stability and thermal stability than the phosphorus-rich phosphides. Ni2P adopts the same hexagonal structure with Fe2P (space group: P6̅2m) (Figure 1a).36 The

Figure 1. (a) Crystal structure of Ni2P with a single unit cell. (b) Top-down view of the (001) surface with two unit cells stacked on top of one another. (c) Top-down view of the (010) surface. Received: November 22, 2018 Revised: February 17, 2019 Published: February 21, 2019 A

DOI: 10.1021/acssuschemeng.8b06063 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 2. (a) XRD patterns of the as-prepared Ni2P and Ni2P/rGO. (b−d) High-resolution XPS spectra of the (b) Ni 2p, (c) P 2p and (d) C 1s of Ni2P/rGO.

of Al3+ into nanosheets-like Ni2P can generate AlmNinP and elemental Ni.

shortest bond distance of Ni−Ni is 2.605 Å (Figure 1b), which is very close to that in metal Ni (2.490 Å), thus showing very strong metallic nature. It has been applied as an attractive anode material in place of graphite for the next-generation LIBs owing to its high gravimetric and volumetric capacity associated with the low polarization and good cycling stability.37−40 However, pure Ni2P suffers from rapid capacity fading and limited cycling life because of the intrinsic poor conductivity during the charge/discharge process. To address the problem and advance the kinetic properties of Ni2P, compositing the active materials with the highly conductive carbon materials is beneficial to enhance electronic conductivity, thereby improving the conductive paths of the active materials. The carbon materials can also relieve the aggregation and pulverization and thereby improve the structure integrity of the active materials during the continuous charge−discharge cycles.41−43 Therefore, it can be anticipated that the composite electrodes can effectively improve the electrochemical activity and structure integrity, thus resulting in a great improvement in specific capacity and capacity retention during cycling. Herein, we report a facile strategy to synthesize the nickel phosphide nanosheets supported on reduced graphene oxide (Ni2P/rGO nanosheets) by a hydrothermal reaction combined with the subsequent phosphorization process. Ni2P/rGO can reveal an initial discharge capacity of about 274.5 mAh g−1 at 100 mA g−1. Meanwhile, even at 200 mA g−1, the reversible capacity is larger than 60 mAh g−1 over 3000 cycles. Furthermore, the detailed characterization was conducted to acquire an understanding about the reversible energy storage mechanism of this new type material as cathode for Al-ion batteries. It is observed for the first time that the incorporation

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EXPERIMENTAL SECTION

Preparation of Hydroxide Precursor/rGO. In a typical hydrothermal process, 10 mmol of Ni(CH3COO)2·4H2O (99.0%) and 30 mmol of urea (99.0%) were dissolved into deionized water (140 mL) in a 500 mL beaker under magnetic stirring for 30 min to form transparent solution I. At the same time, 60 mg of graphene oxide (GO) was dispersed into 60 mL of deionized water by ultrasound for 30 min to obtain homogeneous solution II. Then, solution II was transferred to solution I under magnetic stirring for 30 min. Thereafter, the resulted suspension was transferred into a 250 mL autoclave and heated at 120 °C for 8 h. After that, the obtained precipitate was washed with distilled water and absolute ethanol and dried under oven at 80 °C for 12 h. Then, the hydroxide precursor/ reduced graphene oxide (rGO) was obtained. Similarly, the synthesis of hydroxide precursor was almost the same with hydroxide precursor/rGO, without using GO for hydrothermal preparation. Preparation of Ni2P/rGO Nanosheets. The hydroxide precursor/rGO was mixed with NaH2PO2 (99.0%) with a Ni/P molar ratio of 1:5 in an alumina boat. The alumina boat was placed in a tube furnace and calcined at 300 °C for 1 h in Ar atmosphere. The asprepared powders were washed with distilled water and absolute ethanol and then dried in an oven at 80 °C for 12 h. Eventually Ni2P/ rGO nanosheets were obtained. The preparation of Ni2P was under the same process. Fabrication of Al-Ion Batteries. The working electrodes were prepared by mixing active materials, acetylene black, and poly(vinylidene difluoride) (PVDF) with a mass ratio of 60:30:10 in Nmethyl-2-pyrrolidone (NMP) solvent under magnetic stirring and then pasted onto a pure tantalum (Ta, 20 μm thickness, 20 × 20 mm) foil. Afterward, the as-obtained electrodes were dried in an oven at 80 °C over 12 h. And the mass loading of the active material in the B

DOI: 10.1021/acssuschemeng.8b06063 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering electrode was equal to ∼0.5 mg cm−2. Moreover, the electrolyte was prepared with a molar ratio of anhydrous AlCl3 (99.0%) and 1-ethyl3-methylimidazalium chloride ([EMIm]Cl, 98.0%) of 1.3:1 in a glovebox with high-purity argon atmosphere. Before assembly, the Al foil (50 μm thickness, 25 mm × 25 mm) and Mo foil (20 μm thickness, 6 mm × 60 mm) were conducted by ultrasonic cleaning for 10 min. Mo foil adhered to cathode was used as the lead. Cathode, Al anode, and glass fiber (GF/A) separator from Whatman were fabricated with Al plastic film and dried in an oven at 80 °C. The pouch cells were further injected with the as-prepared ionic liquid electrolyte in a glovebox filled with high-purity argon atmosphere. Material Characterization. X-ray diffractmetry (XRD, Rigaku, SmartLab) was used to determine the structural changes of the samples. X-ray photoelectron spectroscopy (XPS, Kratos, AXIS Ultra DLD) was adopted to conduct the valence changes of the elements. Field-emission scanning electron microscopy (FESEM, JEOL, JSM6701F) with energy dispersive X-ray spectroscopy (EDX), and high resolution transmission electron microscopy (HRTEM, JEOL, JSM2010) with selected-area electron diffraction (SAED) were used to characterize the morphologies and microstructures. Raman spectra of the electrolytes at different states were obtained on a LabRAM HR Evolution spectrometer using a He−Cd laser (325 nm). Electrochemical Measurements. Galvanostatic charge−discharge tests were conducted using Neware BTS-53 tester (Shenzhen, China) in the potential range of 0.01−2.2 V (vs Al3+/Al) at various current densities. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were carried out with CHI 660E electrochemical workstation (Shanghai, China). CV was performed at a scan rate of 0.5 mV s−1 in the potential range of 0.01− 2.3 V (vs Al3+/Al). EIS was recorded at a stable open-circuit potential using amplitude of 5 mV in the frequency range of 100 kHz-100 mHz at different states.

in Ni2P. The Ni2+ peaks at 855.6 and 873.6 eV resulted from the oxidized Ni due to superficial oxidation in air. Two broad peaks at 861.0 and 879.5 eV can be denoted as the satellite peaks. Moreover, in the P 2p XPS spectrum (Figure 2c), the peaks at 129.5 and 130.3 eV can be put down to P in Ni2P, and the P5+ peaks at 133.2 and 134.2 eV can be attributed to oxidized P species due to air exposure. The above XPS results are exactly consistent with the reported data in some literatures.33,34,40,44 Moreover, the high-resolution XPS spectra of the Ni 2p and P 2p of the as-prepared Ni2P exhibit the same spectra with Ni2P/rGO (Figure S2). On the basis of the above XPS results of the Ni 2p and P 2p spectra, it can be also observed that the binding energy for Ni in Ni2P/rGO (853.1 eV) is positively transferred relative to that of metal Ni (852.6 eV), meanwhile the binding energy for P 2p3/2 (129.5 eV) is negatively transferred relative to that of the elemental P (130.2 eV), verifying the strong electron interaction between Ni and P. Previous research has confirmed the electron transfer from Ni atoms to adjoining P atoms in Ni2P, making Ni atom with partial positive charge denoted as Niδ+ (0 < δ < 1) and P atom with partial negative charge denoted as Pδ− (0 < δ < 1).32,45,46 The C 1s XPS spectrum of Ni2P/rGO is shown in Figure 2d. It can be observed that three resolved peaks are ascribed to the sp2-hybridized C−C and oxygenated functional groups (C−O and CO), signifying the reduction of GO.47 SEM, TEM, HRTEM, and SAED were used to prove unique morphologies and structures of the as-prepared Ni2P and Ni2P/rGO. We found that Ni2P consists of numerous nanosheets (Figure S3a) and tiny Ni2P nanosheets are firmly adhered to the wrinkled rGO to form Ni2P/rGO composite (Figure S3b). TEM images (Figure 3a, c) clarify that Ni2P/

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RESULTS AND DISCUSSION Ni2P/rGO was synthesized through a hydrothermal reaction combined with the subsequent phosphorization process. First, the hydroxide precursor supported on reduced graphene oxide was prepared using urea, Ni2+ cations and graphene oxide by a facile hydrothermal method. Then Ni2P/rGO was phosphorized by thermal decomposition of NaH2PO2 under Ar atmosphere. For comparison, Ni2P was also fabricated under the same condition, without using GO for hydrothermal preparation. The morphology and structure of the precursor were observed by SEM and TEM, as presented in Figure S1. The hydroxide precursor is comprised of transparent ultrathin nanosheets. And the hydroxide precursor/reduced graphene oxide displays flowerlike ultrathin nanosheets incorporated on a transparent reduced graphene oxide film. Ni2P and Ni2P/rGO were then obtained by annealing the hydroxide precursor and NaH2PO2 at 300 °C for 1 h in Ar atmosphere. XRD measurement was adopted to identify the structure of both samples. As displayed in Figure 2a, all the diffraction peaks of the as-prepared samples can be in perfectly agreement with the hexagonal Ni2P phase with space group of P6̅2m (Powder Diffraction File No. 89−2742, Joint Committee on Powder Diffraction Standards (JCPDS), [year]). No other impurities are discovered, demonstrating the successful preparation of Ni2P. The electronic states of the as-prepared Ni2P and Ni2P/rGO powders were investigated by X-ray photoelectron spectroscopy (XPS). The high-resolution XPS spectra of the Ni 2p, P 2p, and C 1s of Ni2P/rGO nanosheets were shown in Figure 2b− d. In the Ni 2p XPS spectrum (Figure 2b), Ni 2p3/2 and Ni 2p1/2 are both divided into three components. The peaks centered at 853.1 and 870.6 eV can be ascribed to metallic Ni

Figure 3. (a, c) TEM images, (b, d) HRTEM images and SAED patterns of the as-prepared (a, b) Ni2P and (c, d) Ni2P/rGO nanosheets (Inset: SAED pattern).

rGO composite is made of Ni2P nanosheets with a size of 50− 100 m supported on a reduced graphene oxide matrix. The EDX results embedded in Figure 3a, c verify that the atom ratio of Ni and P is about 2.05, further implying the successful synthesis of Ni2P. The relevant HRTEM image in Figure 3b reveals the lattice fringe with uniform interlayer space of 0.338 nm, corresponding to the (001) lattice plane of hexagonal Ni2P. Moreover, HRTEM image Ni2P/rGO composite (Figure 3d) represents the lattice fringe with an interlayer space of 0.508 nm, corresponding to (100) of hexagonal Ni2P, which well matches up with the above XRD results. Furthermore, the C

DOI: 10.1021/acssuschemeng.8b06063 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 4. (a, b) First charge−discharge curve of the (a) Ni2P and (b) Ni2P/rGO electrodes at 100 mA g−1. (c−f) High-resolution XPS spectra of (c, e) Ni 2p and (d, f) P 2p of Ni2P/rGO after (c, d) charge and (e, f) discharge.

decomposition of electrolyte and a SEI formation.15,49,50 It also exhibits a very high initial discharge capacity of 223.5 and 274.5 mAh g−1, respectively. After the charge and discharge cycle, the electrolyte may undergo significantly changes in composition. Raman spectra of electrolyte at different states were measured, as exhibited in Figure S4. All the electrolyte samples display the similar Raman shift of AlCl4− (352 cm−1), Al2Cl7− (314 and 435 cm−1), and EMI+ (600 cm−1). Through fitting the area of peaks according to AlCl4− and Al2Cl7−, IAlCl4−/IAl2Cl7− is gradually decreased with the continuous charge−discharged cycles (Table S1). It indeed implies that the electrolyte decomposes as the increased cycles. Moreover, it becomes more acidic, which would increase irreversible capacity, thus not conducive to high Coulombic efficiency.

selected area electron diffraction (SAED) images show clear polycrystal nanostructure. Notably, as shown from the crystal structure in Figure 1c, the channel size is less than 2.209 Å. However, it has been reported that the AlCl4− anions possessed the size of ∼5.28 Å,48 so unlike the intercalation/deintercalation mechanism of graphite cathode reported in previous literatures,6−11 AlCl4− anions will hardly intercalate/deintercalate into/from the crystal structure of Ni2P. It also suggests that a conversion reaction may occur during the charge−discharge process. The first charge−discharge curve of the Ni2P and Ni2P/rGO electrodes at 100 mA g−1 is displayed in Figure 4a,b. Note that there is a long charge potential plateau at about 1.9 V, which is ascribed to dissociation or stabilization process such as D

DOI: 10.1021/acssuschemeng.8b06063 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 5. Elemental SEM-EDX maps of Ni, P, Al, and Cl of the (a) charged and (b) discharged Ni2P.

For purpose of verifying the energy storage mechanism of this new type of cathode material for rechargeable Al batteries, specifically the discharge plateaus and the recharging process, the as-prepared materials were characterized in more detail. As illustrated in the XRD patterns in Figure S5, it can be observed that both samples contain Ni2P phase. After discharging, intensity of Ni2P peak is much lower than that after charging, indicating that Ni2P is partly reduced during discharge process. It also turns out that the sample can return to the original phase in the recharge process. As follows from Figure 4c−f, the XPS spectra of Ni 2p and P 2p of Ni2P/rGO nanosheets were characterized in different states to understand the energy storage mechanism during the charge−discharge process. After charge, the Ni 2p spectrum shows six peaks (Figure 4c). The peaks at 853.4 and 871.0 eV are resulted from metallic Ni in Ni2P. The diminished Ni2+ peaks at 856.2 and 874.4 eV are attributed to the incomplete reacted oxidized Ni. And two broad peaks at 861.4 and 879.8 eV are due to the satellite peaks. After discharge (Figure 4e), it exhibits the similar Ni 2p peaks, except for the enhanced metallic Ni peaks in contrast to that of the charged sample.

Simultaneously, the P 2p spectrum can be resolved into four peaks at 129.8, 130.6, 133.8, and 134.5 eV, implying the unchanged valence state of P in spite of the charge/discharge process. Through fitting the area of Ni peaks (853.4 and 856.2 eV), what is noteworthy is that I(853.4)/I(856.2) after discharging (0.683) is higher than that after charging (0.608), implying the relatively stronger Ni peak (853.4 eV) in discharged Ni2P/rGO sample compared with that in charged sample. Accordingly, it can be deduced that Ni2P is partially reduced to metallic Ni upon the discharge. Furthermore, the elemental SEM-EDX maps of the charged/ discharged samples (Figure 5) demonstrated that Ni, P, Al, and Cl were evenly distributed over charged and discharged Ni2P samples. It is noteworthy from EDX spectrum in Figure 5b that the discharged sample presents stronger Al and weaker Ni signals compared to these signals of the charged sample, also confirmed in Tables S2 and S3. It can be seen from Figure S6 that the Al 2p peak of the discharged sample is distinctly stronger than that of the charged sample, clearly stating that Ni2P is deteriorated and Al is incorporated into the Ni2P phase during discharged process. Meanwhile, Cl signal is much E

DOI: 10.1021/acssuschemeng.8b06063 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 6. Schematic illustration of the energy storage mechanism of the Ni2P cathode for Al-ion batteries.

In the charge process:

weaker than Al signal in Figure 5b, implying that Cl is not involved in Ni2P phase. The remaining Cl element observed are attributed to irreversible side reaction between Cl− and sp2 carbon at high potential.50,51 The above results confirm the incorporation of Al3+ cations into the Ni2P phase, rather than AlCl4− anions reported by some previous research.6−11,52,53 Furthermore, Al3+ is resulted from the decomposition of Al2Cl7− in the discharge process, and the released Al3+ and AlCl4− can recombine into Al2Cl7− in the charge process. In addition, the electrochemical reaction mechanism was further identified by TEM images (Figure S7). The fully charged sample shows a polycrystal pattern, basically the same as the structure of the pristine Ni2P. After discharge, it can be determined from the SAED pattern that metallic Ni phase generates, disclosing the phase-transition process. Therefore, we can reasonably infer that the incorporation of Al3+ into Ni2P phase generates AlmNinP and elemental Ni. To better identify the energy storage mechanism, the CV measurements of the Ni2P and Ni2P/rGO electrodes were carried out at a scan rate of 0.5 mV s−1, as presented in Figure S8. The curves of both electrodes are pretty much the same. For the Ni2P/rGO electrode (Figure S8b), there are three oxidation peaks at 0.99 V (A), 1.35 V (B), and 1.80 V (C), and two reduction peaks at 0.53 (A′) and 0.85 (B′) V. The main oxidation peak appeared at 0.99 V is ascribed to the oxidation reaction of Ni2P, and two reduction peaks are corresponding to a multistep electrochemical reduction reaction of Ni2P with Al3+, which matches well with the voltage plateaus. As the number of cycles increases, the cathodic peaks shifted negatively, and the anodic peaks shifted positively. Thereinto, the peaks of B and B′ will disappear which can be due to the oxidized Ni on the surface of Ni2P consumed completely. At the same time, the cathodic C peak, resulting from the stabilization of Al complex ions, gradually decreases and tends to disappear, similar to our previous work.54 As is well-known, the potential difference (Δφ) between the oxidation and reduction peaks is associated with the polarization and reversibility of the redox reaction.55−57 Compared with Δφ of Ni2P (0.65 V), Ni2P/rGO exhibits a marked decrease in potential difference (0.46 V), signifying less polarization and superior reversibility. The results also manifest that reduced graphene oxide is conducive to the enhancement of energy density of Ni2P for Al-ion battery. On the basis of the above observations and discussion, the energy storage mechanism of the Ni2P cathode for Al-ion batteries can be presented, as demonstrated in Figure 6. The reactions during charge/discharge cycles are summarized below:

Al mNi nP + (2 − n)Ni − me− → Ni 2P + m Al3 +

(1)

In the discharge process: Ni 2P + m Al3 + + me− → Al mNi nP + (2 − n)Ni

(2)

To demonstrate the reaction kinetics of both electrodes, the electrochemical impedance spectroscopy (EIS) measurements were performed. The Nyquist plots of the Ni2P and Ni2P/rGO electrodes at different states were presented in Figure S9a,b. The both cycled cathodes show almost the same ohmic impedance (Rs) and charge transfer resistance (Rct) as the electrodes without any charge−discharge cycles. Simultaneously, due to the high active surface area and conductive network of rGO, the Ni2P/rGO apparently displays lower Rct compared to the pure Ni2P electrode, indicating a fast electron transfer and Al complex ions diffusion between the interface of the Ni2P/rGO and electrolyte. Furthermore, the diffusion coefficient (D) of both electrodes by EIS can be precisely calculated based on the following equations:58−60 Ä É2 1 ÅÅÅÅ RT ÑÑÑÑ D = ÅÅ 2 2 Ñ (3) 2 ÅÅÇ F n ACσ ÑÑÑÖ Z Re = K + σω−1/2

(4) −1

−1

In eq 3, R is the gas constant (8.314 J K mol ); T is the absolute temperature (298 K); F is the Faraday constant (96485 C mol−1); n is the number of transferred electrons; A is the active surface area of the cathode (4.0 cm2); C is the concentration of Al ions in the cathode electrode (∼ 1.65 × 10−2 mol cm−3); σ is the Warburg coefficient, determined by the slope of the real resistance ZRe vs ω−1/2 in low frequency region (ω = 2πf). The relationship between ZRe and ω−1/2 in the low frequency region were displayed in Figure S9c,d. It can be observed that the slope of the Ni2P/rGO electrode is lower than that of the pure Ni2P electrode, signifying a faster ionic diffusion for Ni2P/rGO. In addition, the slope valves of both electrodes after charge are lower than that after discharge, confirming the increased Al-ion diffusion coefficient after charge. Consequently, the Al-ion diffusion coefficient of the Ni2P and Ni2P/rGO electrodes is calculated as 4.08 × 10−16 and 5.88 × 10−16 cm2 s−1 for the first charge process, respectively, and 2.12 × 10−16 and 2.86 × 10−16 cm2 s−1 over 100 cycles (Table S4). From what has been discussed above, we can find that the both Ni2P samples show variational morphologies, structures, and kinetic characteristics, and consequently, it is important to study the charge−discharge behavior and cycling performance. F

DOI: 10.1021/acssuschemeng.8b06063 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 7. Galvanostatic charge−discharge curves of the (a) Ni2P and (b) Ni2P/rGO electrodes at a higher current density of 200 mA g−1. (c) The long-term cycling performance of the Ni2P and Ni2P/rGO electrodes at a higher current density of 200 mA g−1.

in Figure 7a,b. As the number of cycles increases, this phenomenon about voltage increase gradually disappears. Actually, it is consistent with the behaviors of CV curves in Figure S8. It can be resulted from the reduction process of the oxidized Ni on the surface of Ni2P, and it may also be due to the decreased polarization ascribed to the phase change of the active material during the discharge process. Moreover, the charge−discharge curves and cycling stability of acetylene black were measured at 200 mA g−1, as displayed in Figure S12. It can be seen that acetylene black exhibits the capacitive behavior. However, the capacity is only ∼20 mAh g−1, which is obviously lower than that of Ni2P. Therefore, the capacity of Al−Ni2P/rGO cell at the potential range from 0.6 to 0.01 V should be resulted from Ni2P rather than acetylene black. Moreover, the discharge process of Al−Ni2P/rGO cell at the potential range from 0.6 to 0.01 V is corresponding to electrochemical reduction reaction of Ni2P with Al3+ , confirmed by the data from CV in Figure S8. The long-term cycling stabilities of both electrodes are further evaluated at 200 mA g−1, displayed in Figure 7c. It can be calculated that the first discharge capacity is 201.1 and 187.4 mAh g−1, respectively, with a Coulombic efficiency of 74.1 and 70.7%. The pure Ni2P electrode delivers the capacity of 31.4 mAh g−1 over 1300 cycles, with a Coulombic efficiency of 97.1%. Importantly, the discharge capacity of Ni2P/rGO electrode can remain at 60.9 mAh g−1 even over 3000 cycles, with a Coulombic efficiency 94.5%, verifying the superior cycling stability. As one can see, the higher capacity and better cycling stability of the Ni2P/rGO electrode can be ascribed to the uniform morphology and incorporated structure with Ni2P nanosheets supported on a reduced graphene oxide matrix. Additionally, the uniform reduced graphene oxide distribution can be propitious to the improved electronic conductivity and high active surface area, and thus enhance Al-ion diffusion behavior.

The galvanostatic charge−discharge curves from cycles 2−4 of the Ni2P and Ni2P/rGO electrodes were measured at 100 mA g−1, as shown in Figure S10a,c. Compared with the initial charge/discharge curve, it is expressly observed from the charge/discharge curves from cycles 2−4 that it has two charge potential plateaus (∼0.90 and 1.75 V vs Al3+/Al) and two discharge plateaus (∼0.65 V and below 0.55 V vs Al3+/Al). Figure S11 shows the voltage profiles over different cycles of the Ni2P and Ni2P/rGO electrodes at a current density of 100 mA g−1. As the cycles increase, the discharge capacities decrease obviously, and the charge plateau at about 1.75 V and discharge plateau at about 0.65 V gradually disappear. Moreover, Figure S10b,d shows the cycling stability of both electrodes at 100 mA g−1. It can be calculated that the second discharge capacity of the Ni2P/rGO electrode is 187.5 mAh g−1, and the capacity eventually remains at 73.0 mAh g−1 over 500 cycles, with a Coulombic efficiency of 93.5%. By contrast, the pure Ni2P electrode presents much lower capacity, cycling stability, and Coulombic efficiency. Additionally, at a higher current density of 200 mA g−1, the electrochemical performance of the Ni2P and Ni2P/rGO electrodes was further measured. It can be observed from the charge−discharge curves from cycles 2−4 in Figure 7a,b that both electrodes exhibit similar charge−discharge behaviors and two apparent discharge plateaus. The second discharge capacity the Ni2P and Ni2P/rGO electrodes is 163.9 and 154.0 mAh g−1, respectively. It can be also observed that the overpotential is a bit high, mainly attributed to the electrochemical polarization caused by the lower electrochemical reaction speed relative to electron motion speed and a voltage drop across the internal resistance of the electrode. After being composited with rGO, it could be found that the active material displayed a decreased overpotential, confirmed by charge−discharge curves and CV curves. It is noteworthy that there appears unusual voltage increase in the discharging curve G

DOI: 10.1021/acssuschemeng.8b06063 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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CONCLUSIONS In this work, the nickel phosphide nanosheets supported on reduced graphene oxide (Ni2P/rGO nanosheets) are synthesized through a hydrothermal method combined with a subsequent phosphorization process, which delivers a very high first discharge capacity of ∼274.5 mAh g−1 at 100 mA g−1 and remains at 73.0 mAh g−1 with a Coulombic efficiency of 93.5% over 500 cycles. It is also of importance to observe that the Ni2P/rGO electrode demonstrates a discharge capacity of 60.9 mAh g−1 at a higher current density of 200 mA g−1 even over 3000 cycles, with a Coulombic efficiency of 94.5%, verifying the superior long-term cycling stability. Importantly, the energy storage mechanism of the Ni2P as cathode for Alion batteries is rigorously investigated that the incorporation of Al3+ into nanosheets-like Ni2P can generate AlmNinP and elemental Ni. Accordingly, this work will inspire the promising future of transition metal phosphides as advanced cathode materials for Al-ion batteries.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b06063. SEM images, TEM images of hydroxide precursor and hydroxide precursor/reduced graphene oxide; XPS spectra of the original, charged and discharged Ni2P; Raman spectra of the electrolyte at different state; XRD patterns and TEM images with SAED patterns of the charged and discharged Ni2P; CV curves, Nyquist plots and charge−discharge curves of Ni2P and Ni2P/rGO; electrochemical performance of acetylene black; EDX element analysis results of the charged and discharged Ni2P; Al-ion diffusion coefficient of the Ni2P and Ni2P/ rGO samples at different states (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.W.). *E-mail: [email protected] (S.J.). ORCID

Jiguo Tu: 0000-0003-1118-7897 Mingyong Wang: 0000-0002-5246-163X Shuqiang Jiao: 0000-0001-9600-752X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (51804022, 51725401) and the Fundamental Research Funds for the Central Universities (FRF-TP-17-002C2).

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DOI: 10.1021/acssuschemeng.8b06063 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX