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An Enhanced High-Rate Na3V2(PO4)3‑Ni2P Nanocomposite Cathode with Stable Lifetime for Sodium-Ion Batteries Jinju Song,† Sohyun Park,† Vinod Mathew, Jihyeon Gim, Sungjin Kim, Jeonggeun Jo, Seokhun Kim, Muhammad Hilmy Alfaruqi, Joseph Paul Baboo, In-Ho Kim, Sun-Ju Song, and Jaekook Kim* Department of Materials Science and Engineering, Chonnam National University, 300 Yongbongdong, Bukgu, Gwangju 500-757, South Korea S Supporting Information *
ABSTRACT: Herein, we report on a high-discharge-rate Na3V2(PO4)3-Ni2P/C (NVP-NP/C) composite cathode prepared using a polyol-based pyro synthesis for Na-ion battery applications. X-ray diffraction and electron microscopy studies established the presence of Na3V2(PO4)3 and Ni2P, respectively, in the NVP-NP/C composite. As a cathode material, the obtained NVP-NP/C composite electrode exhibits higher discharge capacities (100.8 mAhg−1 at 10.8 C and 73.9 mAhg−1 at 34 C) than the NVP/C counterpart electrode (62.7 mAhg−1 at 10.8 C and 4.7 mAhg−1 at 34 C), and the composite electrode retained 95.3% of the initial capacity even after 1500 cycles at 16 C. The enhanced performance could be attributed to the synergetic effect of the Ni2P phase and nanoscale NVP particles, which ultimately results in noticeably enhancing the electrical conductivity of the composite. The present study thus demonstrates that the Na3V2(PO4)3-Ni2P/C nanocomposite is a prospective candidate for NIB with a high power/energy density. KEYWORDS: nanocomposite, Na3V2(PO4)3, Ni2P, cathode, carbon coating, sodium ion battery
1. INTRODUCTION Na-ion batteries (NIBs) are promising for large-scale grid energy storage systems and are competent alternatives to Li-ion batteries (LIBs) owing to the cost effectiveness and abundance of sodium.1 It has therefore become required to develop a Naion battery electrode that shows structural stability, high discharge rate capability, and long cycle life.1−4 Some of the cathode materials tested until now for NIB applications include oxides like NaxMO2 (M = Mn, Fe, Co, Cr, etc.) and phosphates, NaMPO 4 (M = Mn, Fe), Na 2 FeP 2 O 7 , Na3M2(PO4)3 (M = V, Ti), which have shown promising structural stabilities and discharge rate performances.5−14 However, the development of these electrodes are still in the laboratory stages and require further improvements, or new materials with enhanced properties need to be identified before practical utilization. Among these, the NASICON (Na super ion conductor) structured materials appears promising for realizing high energy densities due to its covalently bonded three-dimensional framework with numerous interstitial spaces for the facile migration of Na+ ions. Specifically, Na3V2(PO4)3 has been demonstrated recently as a potential cathode for use in NIBs because of its high Na-ion conduction, moderate operating potential (3.4 V, vs Na+/Na), minimal variation in volume during charging/discharging, thermal stability, and high theoretical energy density (400 Wh/kg).15−17 However, efforts to address the electronically insulating characteristics that © XXXX American Chemical Society
significantly limits the electrochemical properties of Na3V2(PO4)3 have been almost restricted to carbon coating, particle downsizing, and substitution of transition-metal ions.5,18−39 For example, Kuppan et al. first reported that carbon-coated NVP can improve the durability and the rate performance, with over 30 000 electrochemical cycles, using a novel scalable precipitation method.18 Composited with graphene as a carbon network to enhance the rate performance, the electrode delivers approximately 86 mAhg−1 at 5 C.19 Nano core−shell NVP/C materials prepared by hydrothermal assisted sol−gel method have been shown to provide 94.9 mAhg−1 at 5 C.21 Interestingly, despite several electronically conducting additives including metals and phosphides being known, the improvements in the electrochemical performance was realized in NVP by forming composites with only carbon materials such as graphene, reduced graphene oxide, carbon coating, carbon nanofiber or a carbon core−shell.19−35 Particularly, metal-rich phosphides demonstrate noble-metallike catalytic activity, high chemical stability, and excellent electrical conductivity.40 Although detected as an impurity phase, metallic Fe2P was suggested to improve the electrochemical properties of LiFePO4 cathode in LIB applications.41 Received: September 13, 2016 Accepted: December 6, 2016 Published: December 6, 2016 A
DOI: 10.1021/acsami.6b11629 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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(Na:V:P:Ni). Consecutively, the imflammable liquid thinner (50 mL) was added to the precursor solution, which was then stirred for 30 min. The final solution was uniformly poured onto a hot-plate maintained at 250 °C. The solution was ignited by using an electric torch, which led to fast precipitation. After being self-extinguished, the as-prepared powders were heat-treated at 800 °C for 12 h in Ar for high sample crystallinity. Under the same preparation conditions except for the addition of nickel-acetate, Na3V2(PO4)3/C (NVP/C) composite was synthesized for comparison purposes. However, in this case, the starting materials of Na-CH3COO, V(C5H7O2)3, and H3PO4 were weighed out in the molar ratio of 3:2:3 (Na:V:P). 2.2. Structural and Physical Characterization. The powder Xray diffraction pattern of the prepared samples were measured using the PANalytical X’Pert PRO Multi Purpose X-ray Diffractometer. This diffractometer is operated at 60 kV and 55 mA and produced a Nifiltered Cu Kα radiation (λ = 1.5406 Å). The measurements were recorded in the scanning angle (2θ) range between 10° and 80° with a resolution of 0.02°. The Synchrotron X-ray powder diffraction (SXRD) data was collected at the 9B high-resolution powder diffraction beamline of Pohang Accelerator Laboratory (PAL), Korea. The synchrotron powder XRD pattern was fitted by the whole pattern using the FULLPROF program for accurate determination. The morphology and size were probed by HR-TEM and FE-SEM using an FEI Tecnai F20 Model (Korea Basic Science Institute (KBSI), Gwangju Centre) operating at 200 kV and a HITACHI S4700 Model, respectively. The carbon contents in the prepared samples were determined by an elemental analyzer (EA-1110, Thermo Quest, Italy). The particle size distributions (PSD) of the samples were measured using a laser diffraction analyzer (ELS-8000, Electrophoretic Light Scattering). The measurements were carried out on a Bio Logic Science Instrument (VSP 1075) to measure the electronic conductivities of the assembled cells. A small AC perturbation voltage of 5 mV within the frequency range of 0.01 Hz−1.0 MHz was used for the impedance measurement of the prepared cells. The conductivity measurement of the bulk samples was performed by preparing pellets (of ∼9Φ diameter) from about 100 mg of NVP/C and NVP-NP/C powder samples using a cold isostatic press (CIP). The pellets were then heated under Argon atmosphere for 5 h before coating both sides with Au paste to form the blocking electrodes. The impedance measurements were performed using a Frequency response analyzer (Wonatech, Zive MP5) in a two electrode configuration at zero dc-bias with range of 106 Hz−0.01 Hz response at signal amplitude of 50 mV. The electrical conductivities (σ) of the prepared samples were estimated on the basis of the formula σ = t/(RbA), the parameters t, Rb, and A representing the thin-film thickness, bulk material resistance, and the cross-sectional area of the pellet surface, respectively. In the impedance plot, Rb corresponds to the intercept on the real axis in the high-frequency region.48 The elemental oxidation states were examined by XPS (Thermo VG Scientific instrument, Multilab 2000 in Chonnam Center for Research Facilities) using Al Kα as the X-ray source. The spectrometer was calibrated with respect to the C 1s peak binding energy of 284.6 eV. The Synchrotron X-ray absorption Near Edge structure measurements were carried out on the BL7D beamline at the Pohang Light Source (PLS). All the spectra were collected at room temperature in the transmission mode at V K-edge. The collected data was processed using the normal method by obtaining the absorbance and analyzed using the ATHENA program. 2.3. Electrochemical Measurements. The electrode was prepared using the doctor blade method by casting a homogeneous slurry prepared by mixing an 8:1:1 weight ratio of active material (AM): Ketjen black: PVDF binder in N-methyl-2-pyrrolidone (NMP), respectively, onto an Al-foil current collector before vacuum-drying at 80 °C and pressing between stainless steel twin rollers. The resulting foil punched into circular discs before drying formed the cathode, and the active material loading of NVP and NVP-NP was 3.36 mg and 2.56 mg, respectively. The prepared cathode, sodium metal anode, a glass filter separator, and a 1 M NaPF6 electrolyte solution containing 1:1 mixture of ethylene carbonate (EC) and propylene carbonate (PC),
Precisely, the metallic Fe2P phases formed via reduction on the surface of the LiFePO4 particles at elevated temperatures tend to play the role of an electrical conductor and facilitate multidimension channel formation for charge transfer and ultimately enhance the electronic conductivity by 7 orders of magnitude in the bulk composite phase.42 In addition, Rui Qing et al., reported that the presence of nickel phosphide in a LiNixFe1−xPO4/C composite was beneficial to the overall electronic conductivity and hence contributed to superior charge/discharge capacity and cyclability.43 Importantly, there has been a recent surge in research on Ni−P-based phosphides and in particular, Ni2P electrodes for lithium-ion batteries in view of its easy availability, very low electrical resistivity (10−4 Ωcm−1).44−46 Therefore, the recent research attention of Ni2P and the premise that there are no reports on NVP-metal phosphide composite for NIB cathodes to the authors’ best knowledge thus motivate the development of a NVP-metal phosphide composite electrode for NIBs. Herein, we present the realization of a Na3V2(PO4)3-Ni2P/C composite cathode with enhanced electrochemical reactivity by utilizing a simple polyol-assisted pyro-synthetic strategy performed in open-air environment followed by annealing for sodium battery applications.47 The polyol medium plays the role of the solvent, and the conductive carbon source and the composite formation is facilitated by the addition of an appropriate molar ratio of Ni during the synthesis. A systematic effort is undertaken to investigate the exclusive contribution of Ni2P by studying the physicochemical properties of the Na3V2(PO4)3-Ni2P/C composite in comparison to those of Na3V2(PO4)3/C counterpart prepared by the same pyrosynthetic strategy.5 The other advantage of using this synthetic strategy is that the phosphoric acid (used as the phosphorus source) tend to act as the catalyst during the pyro synthesis and accelerates the fast open-air combustion of polyol fuel. The high exothermic energy released from the fuel combustion facilitates the formation of nanoparticles surface-coated by carbon.47 The prepared NVP/C cathode delivered a discharge capacity of 117 mAhg−1 and maintained 56% of the theoretical capacity at 2.67 C.5 Nevertheless, the present work is aimed at further improvement of the electrochemical properties of NVP by introducing a highly electronically conducting metal phosphide additive like Ni2P and thereby realizing a new Na3V2(PO4)3-Ni2P/C (NVP-NP/C) composite cathode. Besides the advantages mentioned earlier, the organic solvent (polyol) utilized in the present synthesis tends to limit particle growth and provide a reducing environment to stabilize the V(III) oxidation state. These factors in turn tend to favor the formation of Ni2P particles. To ensure the formation of a highly crystalline composite and hence enhanced sodium storage properties, the as-prepared composite was sintered for prolonged durations (5−12 h). In short, the present work reports on the development of a nanocrystalline NVP-NP/C composite with a thin carbon layer, carbon matrix, and electrically conducting phase-constituent by the polyol-assisted pyro synthesis for improving the electrochemical reactivity of sodium with NVP.
2. EXPERIMENTAL SECTION 2.1. Synthesis. The NVP-NP/C composite was synthesized via the polyol-assisted pyro synthesis outlined as follows: Na-acetate (NaCH3COO), vanadium acetylacetonate (V(C5H7O2)3), phosphoric acid (H3PO4), and nickel-acetate (Ni(OCOCH3)2·4H2O) were dissolved in 80 mL of tetraethylene glycol (TTEG) in the molar ratio 3:1.9:3:0.1 B
DOI: 10.1021/acsami.6b11629 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces respectively, and 2% FEC (Panaxetec) were employed to assemble 2032-type coin cells. The cells thus assembled in an Ar-filled glovebox were tested in the voltage range of 2.5−3.8 V after aging them for 12 h.
patterns, it is clear that all diffraction peaks of the NVP-NP/C sample excluding the asterisk marked peaks (*) match well with the peaks in the latter pattern. The matched peaks in both samples can be well-indexed to the reference rhombohedral Na3V2(PO4)3 (JCPDS no. 620345). More importantly, the fact that the earmarked peaks in the pattern of NVP correspond to Ni2P (JCPDS no. 65-3544) not only confirms the presence of NVP and Ni2P phases in the NVP-NP/C composite but also validates the production of this composite by utilizing the present one step pyro-synthetic strategy. In order to glean more detailed information on the structure and their phase fractions, the Rietveld refinement using the FULLPROF method on the Synchrotron XRD (SXRD) pattern of the prepared NVP-NP/C composite was performed, and the obtained pattern and refinement data are provided in Figure 1b and Table S1, respectively. The results clearly confirmed the rhombohedral and hexagonal structures of Na3V2(PO4)3 and Ni2P phases, respectively, in the composite. The lattice parameter values were determined to be a = b = 8.7264 Å, c = 21.8195 Å for NVP and a = b = 5.8725 Å, c = 3.5184 Å for Ni2P. The phase fractions of Na3V2(PO4)3 and Ni2P phases in the prepared composite were estimated to be 94.9% and 5.01%, respectively. More importantly,“goodnessof-fit“ (low Rp, Rwp, and Rexp values shown in Figure 1b, inset) clearly validates the refinement process. In order to confirm the oxidation state of vanadium, synchrotron X-ray absorption near-edge structure (XANES) spectroscopy was performed, and the spectrum for the NVPNP composite in comparison to those obtained for NVP/C, V2O3 (reference V(III) state) and VO2 (reference V(IV) state) are shown in Figure 2a. Interestingly, the XANES pattern of the NVP-NP/C sample, which is almost similar to that of the NVP/C counterpart, is closely related to the V2O3 reference spectrum and thereby indicates the average oxidation state of vanadium to be V (III). For further confirmation, the V2p and Ni2p X-ray photoelectron spectroscopy (XPS) spectra of the NVP-Ni2P/C composite is compared to those of the NVP/C sample in Figure 2b. The XPS spectra of the NVP-NP/C composite clearly confirm the presence both the V and Ni elements. The V 2p3/2 profile reveals a binding energy peak value at 517 eV (same as that in case of the NVP/C sample, Figure 2b) that is attributed to the V(III) state.49 Furthermore, the low intensity of the Ni 2p3/2 signal observed for the NVP-NP/C sample is probably
3. RESULTS AND DISCUSSION The XRD pattern of NVP-NP/C sample prepared by the present pyro synthesis in comparison with that obtained for NVP/C is presented in Figure 1a. On comparison of these
Figure 1. (a) X-ray powder diffraction pattern of nanocrystalline NVP/C and NVP-NP/C composite samples synthesized by the pyro synthesis (* refers to standard peaks of Ni2P). (b) XRD Rietveld refinement results of NVP-NP/C.
Figure 2. (a) V K-edge XANES spectra of NVP/C, NVP-NP/C composite, V2O3, and VO2. (b) XPS high-resolution spectra of the NVP/C and NVP-NP/C composite. C
DOI: 10.1021/acsami.6b11629 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 3. FE-SEM images of (a) NVP-NP/C and (b) NVP/C composite. (c) Particle size distribution of these samples obtained by pyro synthesis.
Figure 4. FE-TEM images of obtained NVP-NP/C composite at (a) low magnification; (b) the corresponding elemental mapping images for C (blue), O (yellow), Na (sky blue), V (green), Ni (pink), and P (red). (c) Low-resolution TEM image of NVP particles. (d) High-resolution (HR) TEM image of NVP particle (inset: SAED pattern). (e) HR image of Ni2P particle (inset: SAED pattern).
neously drenched carbon sources from the organic solvent (polyol) tend to prevent particle growth while providing a reducing atmosphere and maintaining V(III) oxidation states.5 All these trends are clearly observed for the NVP/C sample except that there appears to be slightly bigger nanoparticles, although the range appears to be in the same order of magnitude, as confirmed from the size distribution DLS plots (Figure 3c) of both the samples. Nevertheless, the average particle sizes of the majority of the nanoparticles mainly lie in the range of 40−80 nm. It is clear that the small amount of microsized particles coexist with agglomerated nanoparticles, suggesting the observed particles’ morphology and distribution are the same as reported in our previous work.5 FE-TEM analysis was performed on the prepared NVP-NP/ C composite sample to further understand the particle morphology and size. The presence of carbon additive/coating, Ni2P distribution, and the corresponding images are shown in Figure 4. The TEM image in Figure 4a shows nanosized particles with slight aggregation in addition to the presence of a
related to the apparently low concentration of Ni atoms. These results thus suggest the successful synthesis of the NVP-NP/C composite by the pyro-synthetic strategy and also provides clear evidence that the addition of low concentration of Ni to the NASICON structured NVP did not significantly change the V(III) state in the composite. Field-emission SEM and PSD studies were carried out for both samples to compare the differences in their particle-sizes and morphologies, as shown in Figure 3. Figure 3a shows the SEM image of the NVP-NP/C that the average particle size in the range of 40 and 70 nm are agglomerated with spherical morphology. In adidition, a small number of microsized particles are also randomly dispersed among the nanosized particles in shown Figure 4a. The microparticles can be possibly formed as a result of the sintering process at elevated temperatures. Nevertheless, the low-magnification image in Figure 3a reveals a majority of nanoparticles in the NVP-NP/C sample even after sintering at high temperatures. This clearly suggests that the homogeD
DOI: 10.1021/acsami.6b11629 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces few bulk particles. The earmarked area (pink-bordered square) indicates what appears to be the particle with relatively high crystalline features. Furthermore, the energy dispersive X-ray (EDX) elemental mapping studies were performed to confirm the distribution of each element in the composite sample. Figure 4b shows the corresponding elemental mapping images of V, P, Ni, O, and C. The images reveal that all elements except Ni are well-dispersed in the area under study. More specifically, the few Ni-containing clusters corresponding to Ni2P are probably present as hundreds nano-sized particles. For further confirmation, HRTEM studies performed in the pink square part (see panel b of Figure 4) with high mapping intensity of Ni show lattice fringes with d spacings of 0.29 and 0.28 nm corresponding to the (110) and (101) lattice planes, respectively, of Ni2P, as shown in Figure 4e. In addition, the corresponding SAED pattern (inset of Figure 4e) reveals the (211), (1−21), (1−12), (−1−21), and (−2−1−1) Miller indices identified along the [1−1−1] zone axis, indicating the formation of highly crystalline Ni2P particles in the composite. These findings consider the facts that a fair distribution of carbon is observed (Figure 4b) and that a carbon coating layer was formed on the Ni2P particle (Figure 4e). The magnified view in Figure 4c confirms the average size of the primary particles to be between 30 and 70 nm along with the carbon network formed from the organic solvent (polyol). The highresolution TEM image in Figure 4d clearly reveals distinguishable lattice fringes with d-spacings of 0.37 and 0.44 nm, respectively, which correspond to the (113) and (104) diffraction planes of the primary NVP particles. The respective selected area electron diffraction (SAED) pattern (inset of Figure 4d) reveals the well-indexed (110), (204), and (211) crystal planes of NVP. In order to confirm carbon contents, elemental analysis was performed, and the results revealed the presence of 8.31% of carbon content in the NVP-NP/C composite sample. It is worth noting that the carbon content in the NVP/C was also estimated to be comparable (9.3%). The estimated value appears to be sufficient enough to facilitate the carbon layer and network formation. These results thus suggest that the highly crystalline NVP-NP/C composite containing a carbon network/coating is not only successfully produced by the present pyro-synthetic strategy but also anticipated to increase the overall electronic conductivity in the material. The performance of the NVP/C and NVP-NP/C eletrodes were carried out by performing electrochemical measurements. Figure 5a displays the initial electrochemical profile of NVPNP/C compared to that of NVP/C synthesized by pyro synthesis. The electrodes were cycled within the potential range of 2.5 and 3.8 V at 0.08 C rate. As observed in Figure 5a, both the voltage profiles display the characteristic potential plateaus around 3.4 V of the V3+/V4+ redox couple. The NVP-NP/C composite electrodes registered a higher initial discharge capacity of 110.9 mAhg−1 compared to the NVP/C, which yielded 108.9 mAhg−1 at 0.08 C, exhibiting improved utilization because of compositing with high-conductive Ni2P. To elucidate further on the electronic contribution of Ni2P to the rate performance of the prepared composite, we examined the comparative rate capabilities in the potential range of 2.5− 3.8 V at progressive current densities starting from 0.08 to 34 C for the test cells with the NVP-NP/C and NVP/C composite electrodes. The test cells were maintained at each current rate for three cycles. The obtained results in Figure 5b indicates that the NVP-NP/C composite showed better rate performance under high current densities. Precisely, the NVP-NP/C
Figure 5. (a) Galvanostatic charge/discharge potential profiles of nanocrystalline NVP/C and NVP-NP/C composite samples in the potential range of 2.5−3.8 V at 0.08 C. (b) Comparison of the rate capabilities at various current rates between 0.08 C and 34 C. (c) Nyquist plots obtained in the frequency range between 0.01 Hz and 1.0 MHz of the NVP/C and NVP-NP/C electrodes after the initial charge cycle with the equivalent circuit (inset).
electrode indicates discharge capacities of 103.8, 100.8, and 73.9 mAhg−1 at 2.7, 10.8, and 34 C, respectively, compared to 81.7, 62.7, and 4.7 mAhg−1 at 2.7, 10.8, and 34 C, respectively, delivered by the NVP/C. It is worth noting here that the particle morphologies, particle sizes, and structural features of the NVP-NP/C composite are comparable or not significantly altered from those observed for the NVP/C cathode. Therefore, the remarkable performance of the NVP-NP/C composite, particularly, at higher current densities can be attributed to the presence of the highly electrical conducting material (Ni2P) that offers electrical connection with the NVP particles and hence lead to the overall electrochemical performance in the NVP-NP/C composite. Electrochemical impedance spectroscopy analysis on the NVP-NP/C and NVP/ C electrodes was performed to further investigate the enhanced electronic conductivity, and the respective impedance plots are compared in Figure 5c. Z-View software was used to perform the fitting for the obtained curves and the generated circuit E
DOI: 10.1021/acsami.6b11629 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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and it still shows superior rate performance (81 mAhg−1 at 30 C) and ultra cycle life (78 mAhg−1 at 5 C after 2000 cycles).31 Three-dimensional-skeleton carbon-coated NVP has a relatively large specific surface area, which can deliver 117 mAhg−1 of discharge capacity and consistent performance over time.35 The outstanding electrochemical properties in all the samples have been attributed to the nanostructured particles within the carbon layer and the embedded carbon matrix in these electrodes. However, our results expand upon past reports owing to the enhanced electrochemical performance attained by the creation of a highly electronic conductive Ni2P metal phosphide phase constituent with carbon-coated NVP in the composite. Further, the cycle performances at 1 C, in Figure 6a, reveal that discharge specific capacities of 71.79 and 100.3 mAhg−1,
model is presented in the Figure 5c inset. On comparing the semicircles observed for the prepared NVP-NP/C and NVP/C electrodes in the first charge state in the frequency range between 0.01 Hz and 1.0 MHz with a small AC perturbation voltage of 5 mV, it can be observed that the charge-transfer resistance in the former case is lower. Precisely, the Rct for NVP-NP/C electrode (8.724 Ω) is 1 order of magnitude smaller than that for the NVP/C electrode (56.42 Ω). This implies the feasibility of the Ni2P particles significantly contributing to the electronic transportation in the NVP-NP/ C composite during the electrochemical reaction. To arrive at a meanignful conclusion on the electronic conductivity contribution of Ni2P in the composite, the NVP/C and NVP-NP/ C powder samples were directly used for electrochemical impedance spectroscopy measurement, and the resulting curves are plotted in Figure S1. The Nyquist plot of the NVP/C powder sample revealed a large depressed semicircle followed by a slanted line in the high- and low-frequency regions, respectively. This trend is clearly consistent with the reponse found for LiFePO4 by Wand et al.50 Accoding to their study, the observation of the large depressed semicircle can be used to describe NVP/C as an ionic conductor with reasonable ionic conductivity and electronic insulator because of its low electronic conductivity. In contrast, the Nyquist plot of the NVP-NP/C sample is represented by a straight line that is almost parallel to the imaginary-axis of the complex impedance (Z″). This trend is clearly indicative of a capacitive behavior and is almost consistent with that of a pure electronic conductor, according to Wang et al. In general, the estimation of ionic and electronic conductivities from the AC impedance response for such mixed ionic/electronic conductors is well reported by Jamnik and Wand.50,51 Precisely, the highfrequency intercept of the real axis (Z′) corresponds to the combined effect of electronic (Re) and ionic (Ri) resistance in parallel (RiRe/Ri + Re) and the low-frequency intercept of the real axis corresponds to the electronic resistance (Re).50 Thus, in the present case, the estimated electronic conductivity of the NVP/C and NVP-NP/C powder samples are ∼2.57 X10−7 S cm−1 and ∼1.7 X10−4 S cm−1, the values being consistent with those reported for purely ionic conducting (LiFePO4) and electronically conducting (LiFe0.95Ni0.05PO4) materials, respectively.50 In addition, an attempt to fit the obtained impedance plots by modeling with two RC circuits using the Z-view program identified the characteristic frequencies as 103.5 Hz and 107.5 Hz for NVP/C and NVP-NP/C, respectively. The higher characteristic frequency is most likely influenced by the high electron conductivity in the present NVP-NP/C sample. Therefore, from these observations, it can be reasonable to conclude that the present NVP/C sample demonstrates a predominantly ion-conducting character, whereas electronic conduction is dominant in the NVP-NP/C sample. This high electronic conductivity thererfore clearly demonstrates that the Ni2P particles formed during the pyro synthesis offers electronic connectivity to the nanosized NVP particles and thereby facilitate enhanced electrochemical performances in the NVP-NP/C composite. Nevertheless, the observed electrochemical properites of the prepared electrodes are in congruence with the observed improvement in rate performances (Figure 5b). For example, the nanostructure of NVP through a double-carbon embedding process by using a facile soft chemistry delivered the specific capacity of 44 mAhg−1 in 6 s.23 The NVP/C/CMK-3 nanocomposite was synthesized by a simple, effective method,
Figure 6. Cycle performance at (a) 1 C and (b) 16 C. (c) X-ray powder diffraction pattern of NVP-NP/C composite sample after 1500 cycle at 16 C (* indicates Ni2P peaks).
corresponding to 82.8% and 93.5%, respectively, of initial discharge capacity, is maintained until the 900th cycle for the NVP-NP/C and NVP/C electrodes. The long cycle life of the NVP-NP/C composite electrode at an elevated current density of 16 C is demonstrated in Figure 6b as a high initial discharge capacity of 83.86 mAhg−1 is delivered and that 80 mAhg−1 capacity is retained even after 1500 cycles. In addition, the Coulombic efficiencies can be maintained at 100% on extended cycling. After 1500 cycles at 16 C, X-ray F
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program, (Project No. 10050477, development of separator with low thermal shrinkage and electrolyte with high ionic conductivity for Na-ion batteries) funded By the Ministry of Trade, Industry & Energy (MI, Korea).
diffraction of the cathode was performed to confirm the structural stability of the NVP-NP/C electrode. The XRD patterns, in Figure 6c, was confirmed; after long-term cycling, the electrode still maintained the original structure containing Ni2P phase. For the present composite electrode, Ni2P particles played a vital role in offering conductive pathways that facilitate electronic transport. The improvement in the electrochemical properties of the NVP-NP/C composite is derived from the synergistic effect of the NVP and Ni2P particles that enable the improvement of electronic conductivity. Thus, the present study highlights the feasibility for the successful development of a composite cathode comprising NASICON-type NVP and an electrically conducting phosphide by utilizing an open-air pyrosynthetic strategy for high energy/power sodium-ion batteries.
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4. CONCLUSIONS In summary, we report on a highly rate capable NVP-NP/C composite cathode synthesized via a pyro synthesis. XRD and HR-TEM studies confirmed the composite was composed of the NASICON-type Na3V2(PO4)3 and crystalline Ni2P phases. The refinement results revealed that the amount of the impregnated Ni2P in the NVP-NP/C was 5.01%. The XPS and XANES studies confirmed the oxidation state of V remains in V(III) state after addition of Ni. These factors appear to beneficially influence the sodium storage capacity (110.9 mAhg−1 at 0.08 C) and rate capability (73.9 mAhg−1 at 34 C) of the NVP-NP/C electrode. In addition, 95.3% of the initial capacity can be retained after 1500 cycles at 16 C, which is attractive for development of NIBs with long-term cycling stability. Therefore, the approach to develop composites containing NVP and Ni2P definitely seems promising to further the practical realization and comprehension of highly performing NIB cathodes. Moreover, the present synthetic method holds potential for developing nanostructured composite electrodes from metal phosphides for rechargeable battery applications.
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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/acsami.6b11629. Crystallographic table of the NVP-NP/C from Rietveld refinement analysis, powder AC impedance of NVP/C and NVP-NP/C at room temperature (PDF)
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REFERENCES
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: +82-62-530-1703. Fax: +82-62-530-1699. ORCID
Jaekook Kim: 0000-0002-6638-249X Author Contributions †
These authors contributed equally to this work (J.S. and S.P.)
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2014R1A2A1A10050821). This work was supported by Industrial Strategic technology development G
DOI: 10.1021/acsami.6b11629 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
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