C for Sodium Ion Batteries: Controlling

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Na-Rich Na3+xV2−xNix(PO4)3/C for Sodium Ion Batteries: Controlling the Doping Site and Improving the Electrochemical Performances Hui Li,† Ying Bai,*,† Feng Wu,†,‡ Qiao Ni,† and Chuan Wu*,†,‡ †

Beijing Key Laboratory of Environmental Science and Engineering, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, People’s Republic of China ‡ Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, People’s Republic of China ABSTRACT: In order to get an element substituted into Na3V2(PO4)3/C in an appointed V site, the simple sol−gel method is used to design and prepare a series of Na-rich Na3+xV2−xNix(PO4)3/C (x = 0−0.07) compounds. To get a charge balance, the ratio of Na, V, and Ni would be changed if Ni goes into a different site. Hence, ICP is applied to probe the real stoichiometry of the as-prepared Na3+xV2−xNix(PO4)3/C (x = 0−0.07). According to the Na/V ratio from the ICP result, it indicates that Ni2+ goes to a V site, and more Na+ will be introduced into the crystal to keep the charge balance. In addition, the crystal structure changes are explored by XRD and Rietveld refinement, it is indicated from the results that Ni2+ doping does not destroy the lattice structure of Na3V2(PO4)3. When applied as Nastorage material, the electrochemical property of all Ni 2 + doped Na3+xV2−xNix(PO4)3/C composites have been significantly improved, especially for the Na3.03V1.97Ni0.03(PO4)3/C sample. For example, 107.1 mAh g−1 can be obtained at the first cycle; after 100 cycles, the capacity retention is as high as 95.5%. Moreover, when charging/discharging at a higher rate of 5 C, the capacity still remains 88.9 mAh g−1, displaying good rate performance. The good electrochemical performance is ascribed to the optimized morphology, stable crystal structure, and improved ionic conductivity. KEYWORDS: sodium ion batteries, cathode, Na3V2(PO4)3/C, nickel substitution, Na-rich

1. INTRODUCTION So far, lithium ion batteries have been widely applied to our social life, such as in portable electric devices and electric vehicles.1−5 However, due to limited and unevenly distributed lithium sources, cost will be the key problem for application of lithium ion batteries. Hence, new battery systems are being explored to relieve the pressure on the lithium resource.6−9 In recent years, due to the abundance and low cost of sodium, sodium ion batteries have gotten widespread concern.10 In addition, the electrochemical principles between sodium ion batteries and lithium ion batteries are similar. Therefore, it is very important to probe low cost sodium ion batteries to meet the development of large scale energy storage.11 To date, a number of cathode materials, for example, P2− Na 2 / 3 F e 1 / 2 Mn 1 / 2 O 2 , 1 2 , 1 3 Na x MnO 2 , 1 4 Na FePO 4 , 1 5 Na0.7Fe0.7Mn0.3O2,16 and Na3V2(PO4)3 have been studied.17−22 Among these materials, Na3V2(PO4)3, which owns an open NASICON framework, high specific energy density (∼400 Wh/kg), and high thermal stability, is considered to be a prospective cathode of sodium ion batteries.23 However, similar to other lithium metal phosphates, the electronic conductivity of Na3V2(PO4)3 is very poor, which greatly affects its electrochemical properties. In fact, many attempts have been made in order to improve this defect. Carbon coating and cation doping are two effective ways to modify the sample.24−28 However, the carbon layer covered on the surface of the sample © 2016 American Chemical Society

mainly improves the electronic conductivity of Na3V2(PO4)3 powder, but the bulk phase characteristics of Na3V2(PO4)3 are difficult modify by carbon coating. In contrast, a doping cation ion has been demonstrated to be useful for remedying the bulk phase characteristic. Hence, carbon coated with a combination of ions doping method is expected to be a better way for enhancing the electrochemical property. Due to a similar ionic radius among Ni2+ (0.072 nm), V3+ (0.074 nm), and Fe2+ (0.076 nm), Ni2+ is often chosen as a doping element for Li3V2(PO4)329 and LiFePO4.30 In this work, the partial substitution of nickel in Na3V2(PO4)3 is investigated. In order to ensure Ni2+ goes into a V site, a series of Na-rich Ni2+ doped Na3+xV2−xNix(PO4)3/C (x = 0−0.07) samples are designed and synthesized. According to Na/V ratio which is originated from ICP, it can be concluded that Ni2+ goes into a V3+ site and more Na will be introduced into the crystal to keep the charge balance. The electrochemical performances show that Na3V2(PO4)3 with low content of Ni2+ can improve its reversible capacity and cycling stability. The influences of Ni2+ on the modification of the crystal structure, as well as the reason for enhanced electrochemical properties, are explored and discussed in detail. Received: August 7, 2016 Accepted: September 26, 2016 Published: September 26, 2016 27779

DOI: 10.1021/acsami.6b09898 ACS Appl. Mater. Interfaces 2016, 8, 27779−27787

Research Article

ACS Applied Materials & Interfaces Scheme 1. Sketch Map of the Preparation of Na3+xV2−xNix(PO4)3/C (x = 0−0.07) by Sol−Gel Method

Figure 1. XRD patterns and crystal structure of Na3+xV2−xNix(PO4)3/C (x = 0−0.07). 2.3. Electrochemical Tests. Electrochemical measurements of Na3+xV2−xNix(PO4)3/C (x = 0−0.07) samples were carried out using CR2025 coin cells. Na metal was used as the counter electrode. The electrodes were made by active material, SP and PVDF with ratio of 8:1:1. Then, the electrodes were dried at 100 °C for 10 h. Afterward,10 mm diameter circular strip electrodes were cut. The mass loading of the active material on the electrode was about 2.0 mg cm−2. Electrolyte was 1 M NaClO4 in PC with 2% FEC, and glass fiber (Whatman) was used as separator.A LAND battery test system (CT2001A, Wuhan, China) was used to test galvanostatic discharge/charge measurements under the potential of 2.5−4.0 V (vs Na+/Na). Before the galvanostatic charging/discharging tests, aging the batteries over 8 h to make sure Na3+xV2−xNix(PO4)3/C (x = 0−0.07) was fully soaked by the electrolyte. The Na+ diffusion coefficient was evaluated by EIS with a frequency range from100 kHz to 10 mHz.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Sol−gel method was used to synthesize a series of Na-rich Na3+xV2−xNix(PO4)3/C, where x = 0, 0.01, 0.03, 0.05, and 0.07 (as shown in Scheme 1). NaOH, NH4VO3, NH4H2PO4, Ni(CH3COO)2, and citric acid were employed as raw materials. To synthesize Na3+xV2−xNix(PO4)3/C materials, NH4VO3 was dissolved first into distilled water and then the solution was stirred at 80 °C. After a clear solution formed, citric acid solution, NaOH solution, Ni(CH3COO)2 solution, and NH4H2PO4 solution were dropped in stoichiometric amounts into the NH4VO3 solution one by one with violent stirring at 80 °C; the gel was formed after several hours. The gel was calcined at 350 °C for 4 h in air atmosphere, afterward the precursor cooling to room temperature. After slight grounding, the precursor was reheated at 800 °C in flowing argon atmosphere for 8 h to get the final Na3+xV2−xNix(PO4)3/C. All chemicals were used directly without any further purification, and they were analytical reagents. 2.2. Characterization of the As-Prepared Samples. X-ray diffraction (XRD, D/MAX-RB) characterization with Cu Kα radiation was used to probe the crystal structure. The XRD diffraction data were refined using the FullProf program. The morphology and microstructure of the samples were probed by SEM (HITACHI S-4800). The nature of the coated carbon layer was explored by Raman spectroscopy (RAM ARAMIS), equiped with a 514 nm Ar ion laser. The microstructures of all the samples were obtained using TEM (Hitachi H-800). The carbon content was obtained by organic element analyzer (Vario EL cube). The contents of Na, V, Ni, and P were detected by ICP-OES (IRIS Intrepid II XSP). ZView software is used to simulate the equivalent circuit of EIS and calculate the parameters of the equivalent circuit.

3. RESULTS AND DISCUSSION 3.1. XRD of Ni2+ Doped Na3+xV2−xNix(PO4)3/C (x = 0− 0.07). XRD patterns of Na3+xV2−xNix(PO4)3/C (x = 0−0.07) materials are presented in Figure 1. As can be seen from XRD, both undoped sample and Ni2+ doped samples display a pure phase of Na3V2(PO4)3 with R3C ̅ space group. Each reflection, including peak positions and intensities, can be indexed to the NASICON structure. It indicates that low dose doping of Ni2+ into Na3V2(PO4)3 does not destroy the crystal structure of the material. 3.2. Identifying the Doping Site of Ni2+ in Na3V2(PO4)3. However, according to the previous reports, there are two available sites to substitute for the lithium transition metal 27780


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ACS Applied Materials & Interfaces

Table 3. ICP Results for Ni2+ Doped Na3V2(PO4)3 Samples

phosphate materials. One site is a transition metal site, and the other site is a lithium site.31,32 Similar to lithium transition metal phosphates, there may also exist two sites (Na site and V site) for Na3V2(PO4)3 to be substituted. Moreover, it is worth noting that identifying the doping site is helpful to better comprehend how the crystal structure influences the electrochemical properties of polyanion cathode materials. Thus, to identify where Ni2+ goes is very important. Scientists have done a lot of work to probe the doping site. Li33 has proposed an effective way to explore the preferred doping site in polyanion materials; the formula used is as follows:

sample

Na

V

Ni

P

Na/V

x=0 x = 0.03 x = 0.07

3.01 3.04 3.06

2 1.968 1.90

0 0.034 0.08

3 3 3

1.5045 1.544 1.611

DM1(2) = |(XM − XM1(2))/XM1(2))| + |(rM − rM1(2))/rM1(2)|

where XM and rM are electronegativity and ionic radius of the dopant and XM1(2) and rM1(2) are electronegativity and ionic radius of the substituted ion. If DM1 < DM2, the dopant prefers to go to the M1 site, while if DM1 > DM2, the dopant is inclined to occupy the M2 site. According to their formula, for Ni2+ doped Na3V2(PO4)3, DV = 0.267 and DNa = 1.376. Obviously, DNa > DV; the V site is more suitable for Ni2+ to occupy in Na3V2(PO4)3 than the Na site, as we designed it. Due to the different valence between Ni2+ and V3+, to keep the charge balance, the ratio of V, Na, and Ni will be changed differently when Ni goes into a different site. For example, one more sodium will be introduced into the crystal when one Ni replaces V because of the higher valence of V3+ than Ni2+. However, if Ni goes into a Na site, one more sodium will disappear while one Ni replaces Na. The theoretical ratios of Na, V, Ni, and P are listed in Table 1 and Table 2 for the

Figure 2. Na/V ratio for Ni2+ doped Na3V2(PO4)3 when doped at different sites and Na/V ratio for the ICP results.

data also indicate that Ni goes into a V site. And to keep charge balance, extra Na ions are introduced into the crystal; thus the formula of the Ni2+ doped sample should be Na3+xV2−xNix(PO4)3/C (x = 0−0.07). As is known, electrochemical properties of semiconductors can be influenced by low content of impurities; the Rietveld refinement is conducted to get more precise XRD analysis results. The refinement is done by changing the occupation n umbe r s of Ni an d V to ge t t he Ni 2 + d op ed Na3+xV2−xNix(PO4)3. Figure 3 displays the refinement results for Na3V2(PO4)3/C and Na3.03V1.97Ni0.03(PO4)3/C. Table 4 illustrates the structural parameters of Na3+xV2−xNix(PO4)3 phases as determined from the Rietveld refinement. As is indicated from the results, the lattice parameters of a (=b) and c and cell volume decrease with the increasing amount of Ni2+. The smaller radius of Ni2+ (0.072 nm) than V3+ (0.074 nm) may result in the slight changes of lattice parameter, which is additional evidence that Ni2+ goes into the V3+ site. It is hypothesized that the decreased volume may stablize the structure and hence facilitate the electrochemical performance at high current denisty.34−36 In addition, since both the Rp and Rwp values are below 15%, the Rietveld refinement results are credible. 3.3. Morphology of Na3+xV2−xNix(PO4)3/C. Morphology is a significant aspect to affect the electrochemical property of the sample; to observe the effects of Ni2+ on the morphology of Na3V2(PO4)3, typical SEM images of Na3+xV2−xNix(PO4)3/C (x = 0, 0.03, and 0.07) are shown in Figure 4a−c. One can see from Figure 4a that the undoped Na3V2(PO4)3/C sample particle displays large particle size with irregular shape. After doping with Ni2+, the morphology has changed. As can be seen in Figure 4b, the Na3.03V1.97Ni0.03(PO4)3/C sample is still without regular shape. However, many pores, which are randomly distributed on the particle, are observed. With the increasing Ni2+ amount, when x = 0.07, the irregular particle is composed of many nanoparticles. The changed morphology for Ni2+ doped samples should be ascribed to the formation of gas

Table 1. Theoretical Ratios of Na, V, Ni, and P in Ni2+ Doped Na3V2(PO4)3 If Ni2+ Is Doped into the V Site x x x x x

= = = = =

0 0.01 0.03 0.05 0.07

Na

V

Ni

P

Na/V

3 3.01 3.03 3.05 3.07

2 1.99 1.97 1.95 1.93

0 0.01 0.03 0.05 0.07

3 3 3 3 3

1.5 1.513 1.538 1.564 1.591

Table 2. Theoretical Ratios of Na, V, Ni, and P in Ni2+ Doped Na3V2(PO4)3 If Ni2+ Is Doped into the Na Site x x x x x

= = = = =

0 0.01 0.03 0.05 0.07

Na

V

Ni

P

Na/V

3 2.98 2.94 2.9 2.86

2 2 2 2 2

0 0.01 0.03 0.05 0.07

3 3 3 3 3

1.5 1.49 1.47 1.45 1.43

different doping sites. It can be concluded that Na/V ratio is an important index of doping site. As can be seen from Table 1 and Table 2, if Ni goes into a V site, the Na/V ratio increases with the increasing content of Ni. In contrast, Na/V ratio decreases if Ni replaces Na. To get the exact chemical position of Ni2+ doped Na3+xV2−xNix(PO4)3/C, ICP measurement is employed. The molar ratios for Na, V, and Ni for Ni2+ doped Na3+xV2−xNix(PO4)3/C are listed in Table 3. The comparison of the Na/V ratio trend for different doping sites is displayed in Figure 2; it can be found that Na, V, Ni, and P ratios of all the samples are very close to the designed ratio. One can clearly see that the Na/V ratio trend of designed samples is similar to that doped at a V site, as listed in Table 2; namely, the experiment 27781


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Figure 3. XRD Rietveld refinement results of (a)Na3V2(PO4)3/C and (b) Na3.03V1.97Ni0.03(PO4)3/C.

Table 4. Structural Parameters of Na3+xV2−xNix(PO4)3 Phases Determined from XRD Rietveld Refinement x in Na3+xV2−x Nix(PO4)3

a (=b)/Å

C/Å

V/Å3

Rp/%

Rwp/%

0 0.01 0.03 0.05 0.07

8.72940(5) 8.72840(5) 8.72635(2) 8.72582(1) 8.72469(7)

21.81302(8) 21.81102(8) 21.80241(8) 21.79512(7) 21.78979(6)

1439.51(4) 1439.15(4) 1438.76(2) 1437.92(3) 1437.02(1)

10.5 9.8 9.3 9.7 10.3

13.8 12.3 11.9 12.7 13.2

3.4. Raman Spectra of Carbon Layers. In order to better study the structural characteristic of the carbon coating layer, Raman measurement is employed. Figure 5 shows the Raman spectroscopy of Na3V2(PO4)3/C and Na3.03V1.97Ni0.03(PO4)3/ C. The two broad peaks (solid line) which are located around 1350 and around 1590 cm−1 are D-band and G-band, respectively.43−45 The D-band has some relationships with disordered carbon, while the G-band is associated with the graphitic structure, which shows the degree of graphitization. The crystallinity degree of carbon can be obtained from the peak intensity ratio between peaks D and G (ID/IG).46−50 In the present case, the ID/IG ratios of Na3V2(PO4)3/C and Na3.03V1.97Ni0.03(PO4)3/C are 1.009 and 1.087, respectively. The result demonstrates that the coatings mainly contain sp2 type carbon; hence the sample displays high electronic conductivity profiles.49,50 Furthermore, Gaussian numerical simulation is used to deconvolute the two broad peaks to four peaks (dashed line). The fitted two peaks (peak 2 which is located around 1360 cm−1 and peak 4 around 1600 cm−1) are related to sp2 type carbon; peak 1 (around ∼1200 cm−1) and peak 3 (1530 cm−1) can be assigned to sp3 type carbon. It is worthy to note that the area ratio between sp3 and sp2 (Asp3/ Asp2) is associated with the natural property of the carbon layer.45,46 From Figure 5, the Asp3/Asp2 ratios are estimated to be

(such as CO, CO2, and H2O) which is from the decomposition of Ni(CH3COO)2 when calcined at high temperature.37,38 In detail, the added Ni(CH3COO)2 will decompose to CO, CO2, and H2O; hence more gas than without Ni(CH3COO)2 added will be released and leads to porous morphology. It is worthy to note that the porous morphology would enlarge the contact area of electrode and electrolyte, and thus it is helpful for improving the electrochemical reaction kinetics, hence leading to improved electrochemical performance.39−42 HRTEM is further employed to observe the difference of microstructure between doped and undoped samples; the HRTEM images of Na3+xV2−xNix(PO4)3/C (x = 0, 0.03, and 0.07) are displayed in Figure 4d−f. It is very clear that both the doped and undoped samples are covered by a uniform carbon layer that is around 4−8 nm. It is easy to understand that a mixed conductive network can form from the carbon layer which is helpful for electron transporting, hence resulting in the improvement of electrochemical performance.24 Furthermore, according to the organic elemental analysis, the carbon content is about 5.38, 5.17, and 5.23 wt % for Na3V2(PO4)3/C, Na3.03V1.97Ni0.03(PO4)3/C, and Na3.07V1.93Ni0.07(PO4)3/C, respectively. The HRTEM image in Figure 4e displays lattice fringes with d spacing of about 6.193 and 3.730 nm, which are the (012) plane and (113) plane of Na3V2(PO4)3. 27782


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Figure 4. SEM images of Na3+xV2−xNix(PO4)3/C samples: (a) x = 0; (b) x = 0.03; (c) x = 0.07. HRTEM images of Na3+xV2−xNix(PO4)3/C samples: (d) x = 0; (e) x = 0.03; (f) x = 0.07.

Figure 5. Raman spectra of (a) Na3V2(PO4)3/C and (b) Na3.03V1.97Ni0.03(PO4)3/C.

electrode, all Ni2+ doped Na3+xV2−xNix(PO4)3/C electrodes display enhanced rate performance, especially at high current density. When charged/discharged at 0.2 C, the capacities for all samples do not show a clear difference. But Na3.03V1.97Ni0.03(PO4)3/C electrode displays remarkably enhanced electrochemical property when charged/discharged under high current density. When charged/discharged at 3 and 5 C, the capacity of Na3.03V1.97Ni0.03(PO4)3/C is 95.4 and 88.9 mAh g−1, respectively; the capacity is much higher than undoped Na3V2(PO4)3/C electrode (15.5 mAh g−1 at 2 C and 8.8 mAh g−1 at 5 C). When Na3.03V1.97Ni0.03(PO4)3/C is recycled at 3, 2, 1, 0.5, and 0.2 C, the discharge capacity of Na3.03V1.97Ni0.03(PO4)3/C can still reach 95.1, 100.2, 104, 105.6, and 107.9 mAh g−1, respectively. In comparison to the sample which is prepared by traditional sol−gel method24 or solid-state method,51 the rate performance of Ni doped Na3V2(PO4)3/C is

0.47 and 0.43 for Na3V2(PO4)3/C and Na3.03V1.97Ni0.03(PO4)3/ C, respectively, indicating that a large amount of the coated carbon in both samples exists in sp2 type. Both fittings suggest that the carbon layers lead to high electrical conduction, and the carbon layer for different samples plays the same role in the electrochemical property for both Na3V2(PO4)3/C and Na3.03V1.97Ni0.03(PO4)3/C. 3.5. Electrochemical Performances of Na3+xV2−xNix(PO4)3/C. The electrochemical properties of Na3V2(PO4)3/C and various Na3+xV2−xNix(PO4)3/C (x = 0− 0.07) cathodes are explored and displayed in Figure 6. Figure 6a displays the various Na3+xV2−xNix(PO4)3/C electrodes at 0.2 C charge and different rates discharge, which is from 0.2 to 5 C. It is clear that the rate ability of all Ni 2+ doped Na3+xV2−xNix(PO4)3/C electrodes are better than that of the undoped sample. Compared to undoped Na3V2(PO4)3/C 27783


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Figure 6. (a) Rate capability of Na3+xV2−xNix(PO4)3/C (x = 0 to 0.07) at different rates; (b) charging/discharging profiles of Na3.03V1.97Ni0.03(PO4)3/C at different rates; (c) charging/discharging profiles of Na3V2(PO4)3/C at different rates. Cycling stability of Na3+xV2−xNix(PO4)3/C at (d) 0.5 and (e) 1 C.

the initial specific capacity is lost when cycled at 0.5 C (Figure 6d). When cycled at 1 C, the initial capacity is 107.1 mAh g−1; the capacity retention is 95.5% after 100 cycles, demonstrating outstanding capacity retention. However, the undoped Na3V2(PO4)3/C can only deliver 76.4 and 24.57 mAh g−1 after 100 cycles at 0.5 and 1 C, respectively. The enhanced electrochemical property can be ascribed to the extra Na+ in the Na3V2(PO4)3 crystal. Since Ni2+ goes into a V site, more Na+ will be introduced into Na3V2(PO4)3 to keep charge balance. According to previous reports,53,54 Na ions are fully or partially located in two kinds of Na sites. One is Na(1) site, which only provides one position per formula unit, and the other one is Na(2) site, which has three positions per formula unit. Na ions in the Na(2) site will shuttle from Na3V2(PO4)3 and electrolyte while V3+/V4+ redox reaction happens. There is one empty position in the Na(2) site for Na3V2(PO4)3; hence the extra Na ion which is introduced to keep charge balance will be in the Na(2) site, where the extra Na is electrochemical active. In addition, the extra Na+ in the voids/channels may make the crystal structure more stable, which leads to better cycling performance. 3.6. EIS for Na3+xV2−xNix(PO4)3/C. From the above Rietveld refinement results, we can see that the crystal parameters decrease with increasing content of Ni2+. The decreased volume will affect the sodium ion diffusion conductivity undoubtly, hence leading to different electro-

improved a lot. In addition, Ni doped Na3V2(PO4)3/C delivers similar electrochemical performance with Mn doped Na3V2(PO4)3/C.52 It can be concluded that cation doping can both improve the specific capacity and rate performance. To explore the reason why the high rate performances of Ni2+ doped Na3V2(PO4)3/C are enhanced, the charging/discharging curves of Na3+xV2−xNix(PO4)3/C and Na3V2(PO4)3/C at different rates are presented in Figure 6b,c. As can be seen in Figure 6b, when the currents are increased from 0.2 C to 0.5, 1, 2, 3, and 5 C, the shapes of the charging/discharging profiles remain almost unchanged. A long flat plateaus is observed at 3.4 V at 0.2 C; after increasing the rate to 5 C, the flat plateaus also can be clearly observed at 3.1 V with only a little potential drop of about 0.3 V. However, when cycled at 0.5 C, no discharge plateaus can be observed, suggesting the decreased crystal structure of Ni2+ doped Na3+xV2−xNix(PO4)3/C is more stable during the charging/discharging process. With the purpose of confirming the influence of Ni2+ doping on the cycle life for the Na3V2(PO4)3/C electrodes, the cycling behaviors for the present Na3V2(PO4)3/C and various Na3+xV2−xNix(PO4)3/C samples are explored at different current densities (0.5 and 1 C), as shown in Figure 6d,e. It is easy to find out that Na3.03V1.97Ni0.03(PO4)3/C displays the best cycling capability at both 0.5 and 1 C. Na3.03V1.97Ni0.03(PO4)3/ C could deliver an initial capacity of 113.37 mAh g−1, after 100 cycles; 102.6 mAh g−1 can still be obtained and only 9.4% of 27784


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Figure 7. (a) Nyquist plots of Na3+xV2−xNix(PO4)3/C (x = 0−0.07); inset, the equivalent electrical circuit for the EIS); (b) Zre and ω−1/2 relationship.

can be deduced according to the above two equations. As displayed in Table 6, all the sodium ion diffusion coefficients for Ni2+ doped Na3+xV2−xNix(PO4)3/C are clearly higher than that of the undoped sample. In addition, Na3.03V1.97Ni0.03(PO4)3/C shows the highest sodium ion diffusion coefficient (9.39 × 10−13 cm2 s−1), which is an order higher than the undoped sample (1.21 × 10−14 cm2 s−1). The result indicates that Ni2+ can make the crystal structure more stable to buffer the stress caused by volume change in the charging/discharging process, and also Ni2+ doped samples are helpful for sodium ion diffusing. Combined with the decreased unit cell volumes of Na3+xV2−xNix(PO4)3/C (x = 0.01 to 0.07), it is easy to understand that Ni2+ doping can make the crystal structure more stable and does help to improve the mobility of Na+ in the Na3V2(PO4)3 particles. Above all, the enhanced electrochemical performance of Ni2+ doped samples could be attributed to several reasons: First, after Ni2+ replacing V3+, more Na+ will be introduced into Na3+xV2−xNix(PO4)3/C to keep the charge balance. The extra Na+ is hosted in the electrochemical active site; as a result, more Na+ can diffuse during electrochemical reaction for doped samples. In addition, the extra Na+ make the crystal structure more stable, which can improve the cycle performance. Next, the decreased volume for Ni2+ doped samples makes the structure more stable, which leads to higher Na+ diffusion coefficients and better rate performance. Additionally, the porous morphology after Ni2+ doping benefits the electrochemical property due to the enlarged contact area of electrode and electrolyte.

chemical performance. Here EIS is employed to calculate the sodium diffusion coefficient. Figure 7a displays the impedance spectra of Na3+xV2−xNix(PO4)3/C (x = 0−0.07). The semicircle in the high frequency range and straight line in the low frequency stand for the charge transfer resistance and sodium ion diffusion, respectively.55−57 The equivalent circuit to analyze the impedance spectra is displayed in the inset of Figure 7a. In the circuit, Re represents the uncompensated resistance, Rct depicts the charge transfer, CPE corresponds to passivation film capacitance and double layer capacitance, and Zw is the Warburg impedance.58 The detailed fitting results data are listed in Table 5. Notably, the Rct of Ni2+ doped Table 5. Kinetic Parameters of Na3+xV2−xNix(PO4)3/C (x = 0−0.07) Obtained from Equivalent Circuit Fitting x in Na3+xV2−x Nix(PO4)3 0 0.01 0.03 0.05 0.07

Re/Ω 4.597 4.402 3.406 3.49 4.566

Rct/Ω 351.7 275.2 230.3 266.1 286.4

Na3V2(PO4)3/C cathodes are clearly smaller than that of the undoped sample; the Na3.03V1.97Ni0.03(PO4)3/C sample shows the smallest Rct (230.3 Ω); it indicates that the charge transfer process comes more easily for Ni2+ doped materials. In addition, the sodium ion diffusion coefficient is obtained based on the following equation:59,60 D=

R2T 2 2A n F C σ

2 4 4 2 2

4. CONCLUSIONS In summary, a series of Na-rich Na3+xV2−xNix(PO4)3/C (x = 0−0.07) cathode materials are synthesized by sol−gel method. Combining theoretical Na/V ratio and the real stoichiometry which is originated from ICP, it can be concluded that Ni2+ goes into a V site. At the same time, extra Na ions are introduced into Na3V2(PO4)3 crystal to keep the charge balance. The extra Na+ will be hosted at the Na(2) site, which not only increases the active Na content but also stablizes the crystal structure. In addition, XRD refinement results demonstrate that a low content of Ni2+ doping does not

(1)

Zre = R e + R ct + σω−1/2

(2)

where R represents the gas constant, T stands for the absolute temperature, A is the surface area, n depicts the number of electrons per species during oxidization, F is the Faraday constant, C corresponds to the concentration of sodium ion (3.47 × 10−3 mol cm−3),43 and σ is the Warburg factor which can be calculated from Zre (eq 2). The relationship of Zre and ω−1/2 is displayed in Figure 6b. The sodium diffusion coefficient

Table 6. Sodium Diffusion Coefficients of Na3+xV2−xNix(PO4)3/C (x = 0−0.07) x in Na3+xV2−xNix(PO4)3 D/(cm2/s)

0 1.21 × 10−14

0.01 7.73 × 10−13 27785

0.03 9.39 × 10−13

0.05 8.71 × 10−13

0.07 6.98 × 10−13


Research Article


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destroy the crystal structure of Na3V2(PO4)3; the lattice parameters are decreased with the increasing content of Ni2+, which may make the structure more stable. The electrochemical performances are also improved for Na3+xV2−xNix(PO4)3/C (x = 0.01−0.07). The optimized Na 3.03V1.97Ni0.03(PO4)3/C delivers the initial discharge capacity of 107.1 mAh g−1, after 100 cycles at 1 C; 95.5% of the initial discharge capacity can be still obtained. When the charge/discharge rate increases from 0.2 to 5 C, the capacity decreases from 109.7 to 88.9 mAh g−1, demonstrating excellent cycle and rate capability. In this work, the strategy of designing V site doped Na3+xV2−xNix(PO4)3/C based on charge balance is also expected to be helpful in developing desired electrode materials for advanced secondary batteries.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.B.). *E-mail: [email protected] (C.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work is supported by the National Basic Research Program of China (Grant No. 2015CB251100), the Program for New Century Excellent Talents in University (Grant No. NCET-13-0033), and the Beijing Co-construction Project (Grant No. 20150939014).

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C for Sodium Ion Batteries: Controlling

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