extraction characteristics of a novel double

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Good lithium-ion insertion/extraction characteristics of a novel double metal doped hexa-vanadate compounds used in an inorganic aqueous solution Najeeb ur Rehman Lashari, Mingshu Zhao, Qingyang Zheng, Huilin Gong, and Xiaoping Song Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02041 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on July 31, 2018

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Good lithium-ion insertion/extraction characteristics of a novel double metal doped hexa-vanadate compounds used in an inorganic aqueous solution Najeeb ur Rehman Lashari†, Mingshu Zhao*†, Qingyang Zheng‡, Huilin Gong§, Xiaoping Song† † School of Science, Key Laboratory of Shaanxi for Advanced Functional Materials and Mesoscopic Physics, MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, Xi’an Jiaotong University, Xi’an, 710049, Shaanxi, China; ‡ Xi’an High Tech Res Inst, Xi’an 710025, Shaanxi, China; §Xi’an Jiaotong University, Affiliated Hosp 1, Dept Pathol, Xi’an 710061, Shaanxi, China.

ABSTRACT: We report potassium doped NaV6O15 anode with enhanced electrochemical performance, used for aqueous rechargeable lithium ion battery. Different Na1-xKxV6O15 (x=0, 0.1, 0.2, 0.3) compounds are prepared and characterized using X-ray diffraction patterns ensuring potassium doping. The electrochemical performances of the various potassium doped anodes NaV 6O15, Na0.9K0.1V6O15, Na0.8K0.2V6O15, and Na0.7K0.3V6O15 are evaluated by cyclic voltammetry and galvanostactic charge/discharge methods. The results suggest that potassium-doping has a positive effect on the electrochemical performance of aqueous rechargeable lithium ion battery. The anode Na0.8K0.2V6O15 is found to be optimized potassium doped anode materials for aqueous rechargeable lithium ion battery. The Na0.8K0.2V6O15 anode displays enhanced cycling and rate performances, an initial specific capacity of 218 mAhg -1 and 133 mAhg-1 is delivered after 50 cycles (61% capacity retention) at the current density of 100 mAg -1. The potassium doping has induced enhanced interlayer spacing in the layered structure of NaV6O15 due to potassium ions having larger ionic radii than sodium. This enhanced interlayer spacing provides wider channels for lithium-ion intercalation/extraction, which in turn increases lithiumion diffusion coefficient. The lithium-ion diffusion coefficients for NKVO-2 at 0.09, −0.26 and −0.68 V vs. saturated calomel electrode (SCE) were calculated as 1.53x10-11, 1.29x10-11 and 8.90x10-12 cm2s-1, respectively.

1. INTRODUCTION Traditional Li-ion batteries (LIB) since it’s commercialization have been widely used for diverse applications such as; mobile phones, laptop, portable electronics, electric vehicles and even large-scale energy storage systems as well 1-3. Since the growing concerns of operational safety, environmental hazards and high cost of manufacturing lithium ion batteries, the researchers have shifted their attention to adopt alternatives being safer, ecofriendly and cost-effective energy storage systems. The aqueous rechargeable lithium ion battery (ARLB) is a potential candidate, 4 having advantages of safer and ecofriendly operations at reasonable cost. With the invention of ARLB, much attention has been diverted to find the efficient electrode materials for ARLB. Numerous commercially Li ion intercalated materials such as LiCoO2 5, LiMn2O4 6, LiFePO4 7, LiMnPO4 8 and LiNi1/3Co1/3Mn1/3O 9 are efficiently performing cathode materials for ARLB. The various reported anode materials such as oxides (VO2(B) 10, layered γ-LiV3O8 11, H2V3O8 12, V2O5 13, and TiO2 14), polyanionic compounds (TiP2O7 and Na super ionic conductor (Nasicon)-type LiTi2(PO4)3) 15, and organic compounds (polypyrrole and polyamides) 16 redox reaction of which are in narrow potential window of hydrogen evolution. Based on earlier studies, majority of anode materials demonstrated

significant capacity fading during galvanostatic charge discharge cycling along with limited discharge capacity when compared to LIB. The studies on ARLB anode materials have established the reasons for poor capacity retention, which are : dissolution of active material in aqueous electrolyte 17, proton insertion resulting in irreversible structural transformation 18, occurrence of side reactions leading to formation of decomposition of water 19 and structural degradation caused by insertion and extraction during charge and discharge process 20. Among these materials, vanadium oxides and their derivatives have been considered as one of the potential candidates due to high specific energy, rate capability, high capacity, optimized voltage window and feasible availability of vanadium 21-24. The vanadium oxides possess layered structures, with lower possibility of proton insertion. Zhou et al.25 and Nair et al.26 replaced sodium ions (Na+) with lithium ion (Li+) in LiV3O8, which possess layered structure of γ-LiV3O8 crystallographically. The studies of sodium vanadate in LIB have shown that the Na1+xV3O8 compound possesses more resilient structure, with lesser vanadium dissolution that of γ- LiV3O8 27. Adding large cations (e.g. Na+, K+, and Ag+) into vanadium oxygen layers can effectively improve structural stability and enhance lithium ion storage kinetics due to the pillar effect and enhanced layer spacing caused

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by large cations 28-30. These vanadates are very stable in saturated LiNO3 and have shown better electrochemical performance as compared to vanadates used in other electrolytes i.e LiCl 26 and Li2SO431. The expansion of interlayer spacing caused by doping of larger cations lead to enhanced electrochemical properties, the same effect was reported from Na3V2(PO4)3 when Na was partially replaced with large ion K 20 . Furthermore, these hexa-vanadated have been exploited as potential cathode materials in sodium ion batteries and have shown better electrochemical performance32-34.

impressed in saturated LiNO3 solution used as electrolyte in three neck bottle. The galvanostatic charge-discharge tests of the ARLBs were performed using battery test system (Arbin BT2000). The prepared anode material, the cyclic voltammogram (CV) curves between -1 to 1V vs. standard calomel electrode (SCE) at 1 mV s-1 were obtained by an electrochemical work station (Ametek VMC-4) from the aqueous electrolyte. Lithium ion diffusion coefficient was calculated using CV curves tested at multiple scan rate of 1,2,3, and 5mV/s using Randles–Sevcik equation (Equation.1)

Here, by exploiting the interlayer spacing expansion, we investigated mixed ion hexa-vanadate (MIHV) compounds as anode for ARLB. The compound is prepared through hydrothermal process followed by annealing. The Na+ in MIHV compound is replaced with larger cation K+. The mobility of lithium ion is increased as measured by lithium ion diffusion coefficient (DLi+). The structure of the article is as follow: in section II, we present material methods, in which we demonstrate increased by partially replacing Na+ with larger cation i.e. K+ in structure of NaV6O15 as Na1-xKxV6O15(x=0, 0.1, 0.2, 0.3) termed as NVO, NKVO-1, NKVO-2, & NKVO-3. K+ doping results in improved electrochemical performance by introducing wider interlayer spacing for lithium ion intercalation/ extraction in the anode materials. The surplus-doping have shown negative effect, while NKVO-2 is the ideal K+-doping compound which display the premium cycling stability and rate performance.

3. RESULTS AND DISCUSSION

2. MATERIAL AND METHODS The mixed ion vanadate compounds are prepared by hydrothermal process followed by annealing described in our earlier work 35 . Briefly, 1 mM NH4VO3 and appropriate molar ratios of salt chlorides are dissolved in 30ml distilled water at 338.15K. Once the light-yellow solution is formed, the pH of solution is adjusted to 2 using HCl (34% concentrated). The resulting solution is further stirred for 2h at room temperature. Final orange solution is transferred to Teflon lined autoclave, sealed and put for heating at 453.15K for 8h with heating rate of 275.15K/min. Resulting green-brown precursor is washed and dried with water as well as ethanol for several times and dried under vacuum at 353.15K overnight. For the sake to improve the crystallinity of the compound its annealed at 723.15K for 4h in air using tubular furnace. The mixed ion hexa-vanadate compound materials were characterized by X-ray diffraction (XRD) analysis using Bruker D8Adavanced diffractometer from 10 to 70o. The lattice parameters were calculated by Rietveld analysis using the fullprof suite software. The morphologies were obtained using Scanning Electron Microscopy (JEOL JEM-2100). Transmission electron Microscopy (TEM) and High-resolution Transmission Electron Microscopy (HR-TEM) is used to confirm the phase direction, Gatan Digital Micrograph 3.7.4 is used calculate fringe spacing. The working electrode (WE) was prepared using synthesized vanadate compounds, acetylene black and poly-vinylidenefluoride (PVDF) binder in a weight ratio of 7:2:1 were mixed and dispersed in Nmethyl-2-pyrrolidone (NMP) solution to make the slurry. The mixed slurry was coated on Nickel mesh and dried in a vacuum oven at 373.15K for 12 h. The three-electrode cell were constructed using Nickel mesh cathode as counter electrode (CE), the saturated calomel electrode (0.242 vs SHE/V) as reference electrode (RE). All three electrodes were

Figure 1(a) Structure views of pristine NVO, NKVO-1, NKVO2, and NKVO-3. (b) XRD pattern and its Rietveld refinement of the pristine NVO, (c) XRD pattern of the NVO, NKVO-1, NKVO-2 & NKVO-3. Figure.1(a) shows the crystal structure of doped compounds (NVO, NKVO-1, NKVO-2 and NKVO-3. The crystal structure shows quadruple VO6 octa-hedra and VO5 distorted trigonal bipyramids sharing the edges and corners to form a continuous [V6O15] nlayer. Two VO6 octa-hedra at different layers act as pillars to connect the layers by corner-shared oxygen, this array of V-O polyhedra forms rectangular tunnels in which Na+ is found 36. By replacing Na+ having radius 1.02Å with K+ having radius of 1.3 Å, can get increased interlayer spacing of lattice due to larger ionic radii of doped K+ (33% change in radii) which result in wider tunnel spacing. This structure facilitates lithium ions mobility, which may result in high possibility of Li+ insertion and extraction without structural distortion 22, 37. Recent studies have also reported some interesting results indicating that the intrinsic electron conductivity of materials could be improved by slightly distorting the transition metal sites 38, 39 . Figure 1 (b) shows the XRD pattern of the pristine NVO and the Rietveld refinement of the data. As shown in Figure1(b), the differences between the Ical and Iobs curves are minimal, indicat-


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ing the reliability of the refinement results. NVO has been reported to have a monoclinic A2/m(12) structure. All the atomic parameters for the pristine NVO are given in Table 1, Furthermore same atomic parameters have been used to carryout Rietveld-refinement of all K-doped compounds as well.

Table 01, The atomic parameters of the pristine NVO according to the Rietveld refinement of the XRD pattern. The R factors of the structure refinement Rwp 0.38%, Rp 0.59%. According to the refinement data, the lattice parameters Atoms x Y z Occupancy V3 0.11900 0.00000 0.11700 0.50000 O4 0.22400 0.00000 0.26500 0.50000 O7 0.46400 0.00000 0.39600 0.50000 O1 0.04900 0.00000 0.81300 0.50000 V2 0.41100 0.00000 0.28900 0.50000 V1 0.10300 0.00000 0.33700 0.50000 O5 0.26900 0.00000 0.10700 0.50000 Na1 0.41400 0.00000 0.99500 0.25000 O2 0.07900 0.00000 0.63700 0.50000 O6 0.42100 0.00000 0.75400 0.50000 O3 0.22200 0.00000 0.43800 0.50000 Figure 1(c) displays the XRD patterns of the prepared anodes doped with and without K. The shift in XRD peaks corresponding to NVO, NKVO-1, NKVO-2 and NKVO-3 are shown in inset of Figure.1(c). The mixed ion hexa-vanadate compounds exhibited alike XRD patterns to the pristine NVO, consistent with the crystal face index of the diffraction peaks of (JCPDS No: 77-0146). All the compounds have monoclinic structure with the space group: A2/m(12), and no extra peaks were observed. Inset of Figure.1(c) clearly depicts the diffraction peaks being shifted to reduced angle corresponding to induced lattice strain caused by change in inter-planar spacings due doping of K+. This ensures K+ doping into the interior lattice without forming side phase, causing change in lattice parameters. Lattice parameters calculated using XRD Rietveld refinement are summarized in Table 2.

Figure 2.SEM images of the (a) NVO, (b) NKVO-1, (c) NKVO2 and (d) NKVO-3 scale in all figures is 1µm. The SEM images of the all compounds have been presented to demonstrate the change in particle size of NVO prior to doping. The particle of NVO without K doping Fig. 3(a) shows rough morphology with irregular distribution of micro/nan rods. Fig. 3(b), (c) and (d) shows micromorphology of K doped anode. Fig.3 (b) and (c) clearly shows vanishing of micro rods with increase in K doping. The particle size is slightly increased, more single crystal micro rods starts to appear on the same scale. Fig.3(d) show the particle size with highest amount of K doping. The particle size increased from nanoscale to microscale. The changes observed in the morphology of four anode SEM images are consistent with the shifted diffraction peaks and surge in peak intensity of the four anode materials. It is known that, small particle size can provide a short diffusion pathway for lithium ion insertion-extraction from the host materials 40. Therefore, the sample with an appropriate amount of K-doping may contribute to good electrochemical performance by providing appropriate diffusion pathway form lithium ion insertion and extraction.40.

Table 02 Rietveld refined cell parameters of the NVO, NKVO1, NKVO-2 and NKVO-3. Sample a, Å b, Å c, Å V, (Å) NVO 10.0345 3.5899 15.4604 525.5568 NKVO-1 10.0504 3.5950 15.4498 526.7195 NKVO-2 10.0839 3.6099 15.3772 528.2397 NKVO-3 10.0733 3.6059 15.4320 528.8565

Figure 3. Cyclic voltammograms of the (a) NVO, (b) NKVO-1, (c) NKVO-2 and (d) NKVO-3 at a scan rate of 0.1 mVs-1 Figure. 3 shows the current and voltage curves of the initial eight cycles in the voltage window of -1 to 1 versus standard calomel electrode (SCE). Figure. 3 (a-d) shows analogous CV form. The CV curves of NVO in Figure. 3(a) displayed three reduction peaks (0.13, -0.257V and -0.62V) between -1 and 1V.


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The consecutive oxidation peaks are positioned at 0.2 V, -0.13 V, and -0.35 V in all anodes (Figure. 3(a-d)). The consistency of oxidation peak in all anodes suggests that K-doping into NVO does not alter the electrochemical behavior This multistep Li-ion intercalation and deintercalation process in aqueous solution is similar to that in the organic electrolyte 41, 42. Furthermore, oxidation and reduction peaks are well agreement with multi-step charge-discharge profile (Figure.6(c)), this correspondence is attributed to the diverse occupation sites of the Li ions in the host43.

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mAhg-1,129 mAhg-1 at 0.1A g-1, 0.5A g-1, 1A g-1, 2A g-1 & 3A g-1 respectively. The NKVO-2 displayed the highest performance so far delivering initial capacity of 218 mAhg-1 and retained capacity when cycled at all current rates. It’s interesting to note that NKVO-2 showed no capacity loss when cycled 0.1A g-1 after 25 cycles at high current rates when compare with NKVO-3. The over doped NKVO-3 delivered initial 213 mAhg1 but showed capacity fading on successive cycling, delivered lower capacity at all rates, similar trends have also been reported with over doping 20, 45. To emphasize and ensuring rate performance, the capacity retention of all electrodes at several current rates is summarized in terms of percentage as shown in Figure. 4(c). Since, K-doping improves the capacity retention than un doped pristine NVO, while NKVO-2 provides the highest capacity retention. Therefore, the NKVO-2 is selected as the optimized K-doped anode for ARLB. Figure. 4(d) shows the symmetry in shape of first charge/discharge curves tested at different current rates of the optimized anode (NKVO-2). The three charge/ discharge plateaus in the voltage profiles resembles to the Li ions intercalation/deintercalation in the layered materials, which is in good agreement with the CV curves (Figure. 3(c)). Furthermore, no change in plateaus at higher discharge current rates further verified the effect of K-doping.

Figure 4. (a)Cycling performances, (b) Rate performance, (c) Percentage capacity retention of the NVO NKVO-1, NKVO-2 and NKVO-3 electrodes and (d) Charge-Discharge curves of NKVO-2 at various current rates Figure 4 shows the cyclic capacity, rate performance, percentage of capacity retention and charge-discharge curves of anodes. Figure 4(a) shows discharge capacities are presented for the NVO, NKVO-1, NKVO-2 and NKVO-3 electrodes at 100mAg1 for 50 cycles. The discharge capacities of all anode materials are progressively decaying over cycling. The NKVO-2 electrode delivers initial capacity of 218 mAhg-1 at the first cycle and then gradually drops 133 mAhg-1 at 50th cycle. The capacity retention rate of NKVO-2 electrode is calculated to be 61% of its preliminary discharge capacity. The discharge capacity reported here is higher than all previously reported vanadate as anode materials, such as NaV6O15 31, 35, Na1.16V3O8 44 and Na2V6O16.H2O 25. The NKVO-2 electrode performs well with appropriate doping when compare with the highly doped NKVO-3 electrode; which delivers an initial discharge capacity of 217 mAhg-1 and faded to 109 mAhg-1 at 50th cycles. The lower capacity retention of highly doped NKVO-3 electrode is associated to the over doping of K. Similar phenomenon of low performance with over doping has also been reported, though it need be investigated further 20, 45. The NVO and NKVO-1 electrodes delivered initial discharge capacities of 215 mAh g-1 and 214 mAh g-1, respectively. Figure.4(b) shows the rate performance of the prepared electrode (NVO, NKVO-1, NKVO-2, and NKVO-3) charging and discharging for 5 cycles at rate of each 0.1A g-1, 0.5A g-1, 1A g-1, 2A g-1, and 3Ag-1. The pristine NVO in Figure 4(b) delivered initial capacity of 215mAhg-1 at 0.1A g-1 and decreased sharply and suggesting lower capacity retention when cycled back at 0.1A g-1 after 25cycles at different current rates of 0.5A g-1, 1A g-1, 2A g-1 & 3A g-1. However, all mixed ion hexa vanadate compound showed higher performance compared to the pristine sample. The doped NKVO-1 showed capacity of 204 mAhg-1, 180 mAhg-1,171 mAhg-1,147

Figure 5. (a) and (b) are the respective TEM and HRTEM images of optimum performing NKVO-2 anodes before charge and discharge cycle and (c) and (d) shows same as (a) and (b), respectively, but after 50 cycles of charge and discharge cycle. Figure. 5 shows the TEM and HRTEM images of NKVO-2 before and after 50 cycles of charge-discharge. Figure 5(a) shows the TEM image of NKVO-2 anode, whose particle size (few microns), is consistent with SEM image (Figure 2(c)) of anode before charge and discharge cycling. Figure 5(b) shows the HRTEM of NKVO-2 relates to the (2 1 -5) lattice direction of monoclinic NVO by using FFT calculated fringe spacing of 0.23 nm. After cycling, the NKVO-2 exhibited similar particle size and reserved its original morphology as shown in Figure 5(c). The structure displayed in Figure 5(c) suggests consistency with the structure in Figure 5(a), which means insertion and extraction of Li-ion does not affect the anode’s morphology. Figure 5(d) shows the HRTEM image of NKVO-2 after 50 cycles. The fringe spacing of 0.239 nm is noticeable, which relates to (4 0 -4) plane demonstrating the monoclinic phase of NVO. Based on this outcomes, it can be concluded that no phase transitions are involved during lithiation process 36.


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The lithium ion diffusion coefficient was calculated using the Randles-Sevcik equation 46, 1/2 * 1/2 i p  (2.96 x105 )n3/2 ADLi  C0V

(1)

where ip is peak current in Amperes, n is electrons per molecule lithium ion (n=1), V is scan rate of cyclic voltammetry (Vs-1), C is concentration of lithium ion. A is the geometric area of the electrode (cm2).

Figure 6. (a) Cyclic voltammograms of Na0.8K0.2V6O15 (NKVO-2) in saturated LiNO3 aqueous electrolytes at scan rates of 1,2,3 & 5mVs-1 vs SCE (b) Relationship between the peak currents ip and V1/2 at multiple rates of NKVO-2. (c) Lithium ion concentration as function of discharge plateaus for NKVO-2.

Figure 6(a) shows the impact of different scan rates on the redox behaviors of Na0.8K0.2V6O15 anode (NKVO-2) in saturated LiNO3 aqueous electrolyte. CV curves at scan rate of 1,2,3and 5mV s-1, were recorded. One can see that the curve shape of the anodic and cathodic peaks are identical, however, precise linear sweep voltammogram is also observed with the increased scan rate. Figure 6(b) suggests the relationship between oxidation and reduction peak currents (I-I’, II-II’ & III-III’) and the square root of the voltage scan rate (V1/2). The peak current comes in direct proportional with V1/2, suggesting the occurrence of diffusion-controlled Li+ deintercalation/intercalation processes in anode 47. When the anode of the NKVO-2 was discharged in aqueous solution of LiNO3 voltammetric measurements evidenced three well-defined processes in the potential range 1 V/1 V vs. SCE at 0.092 V, -0.26 V and -0.68V these processes well ascribed to the progressive filling of particular sites M of the b-structure by Li+ ions.36 These three successive insertion steps correspond to the Li contents 0 < x # 0.33, 0.33 < x # 0.66 and 0.66 < x # 1.66 respectively in Li x-NKVO., the discharge capacities were observed to be 38.36 mAhg-1, 73.4 mAhg-1 and 157.8 mAhg-1, and the corresponding values of x were 0.26, 0.52 and 1.11, respectively, as shown in Figure 6(c), whereas the molar volume (Vm)of NVKO-2 crystal is 52.8cm3 mol-1. Therefore, the lithium-ion concentration of different insertions could be calculated as follows: (2) C  x / Vm Where C is concentration of Lithium ions, x is value of LixN0.8K0.2V6O15 and Vm is molar volume of NKVO-2 crystal. The diffusion coefficients (DLi+) for NVO, NKVO-1, NKVO-2 and NKVO-3 have been calculated based on relationship of i p and V1/2 using Equation.1 and listed in in Table 3. The results showed that the diffusion coefficient of Li ion was significantly increased with appropriate amount of Na-doping, and the NKVO-2 electrode possessed the largest Li diffusion coefficient.

Table 03 Calculated lithium ion diffusion co-efficient for doped and unpdoped electrodes Sample DLi+/cm2s-1 DLi+/cm2s-1 DLi+/cm2s-1 I’ peak II’ Peak III’ Peak NVO 1.13x10-11 6.54x10-12 1.11x10-12 NKVO-1 2.30x10-11 1.38x10-11 9.93x10-12 NKVO-2 1.53x10-11 1.29x10-11 8.90x10-12 NKVO-3 2.89x10-12 3.35x10-11 3.01x10-11

4. CONCLUSION In conclusion, we synthesized four anodes with potassium doping (Na1-xKxV6O15 x=0, 0.1,0.2,0.3). exploiting interlayer spacing to obtain optimized K doped anode for ARLB. The anodes (NVO, NKVO-1, NKVO-2 and NKVO-3) were synthesized via hydrothermal process followed by annealing. The single-phase doping of K ion was characterized by XRD, which came in well agreement with SEM images. The characterization techniques (SEM and XRD) showed changes in both particle size and the shifted peak intensity when K was substituted in NVO. CV and galvanostatic charge/discharge were applied to further ensure the effect of different K doping amounts based on the electrochemical performances. The lithium intercalation/deintercalation phenomenon was observed to consistent for all anodes, however electrochemical performances were changed with various doping concentration of K. The cycling and rate performances of different K doped anodes showed superior rate performance due to wider interlayer spacing when compared with un-doped NVO. In terms of cyclic performance, the NKVO-2 showed stable performance amongst all mixed ion vanadate by delivering initial capacity of 218 mAhg-1 and retaining 133 mAhg-1 (61%) after 50 cycles. The optimized performance of NKVO-2 is attributed to higher lithium ion diffusion.


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AUTHOR INFORMATION Corresponding Author: *Phone: +86-029-82663034, +86029-82667872. E-mail: [email protected]. Notes: The authors declare no competing ﬁnancial interest. †International Society of Electrochemistry member.

ACKNOWLEDGMENT The authors acknowledge the National Natural Science Funds of China (51302214), Xi 'an Science and Technology Plan Project (CXY1438(4)) and Shaanxi Province Key Scientific and Technological Innovation Team Plan (2013KCT-05).

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13.Zhang, Z.; Wang, H.; Ji, S.; Pollet, B. G.; Wang, R. F., V2O5-SiO2 hybrid as anode material for aqueous rechargeable lithium batteries. Ionics 2016, 22, (9), 15931601. 14.Yu, Y.; Sun, D.; Wang, H.; Wang, H. Y., Electrochemical Properties of Rutile TiO2 Nanorod Array in Lithium Hydroxide Solution. Nanoscale Research Letters 2016, 11. 15.He, Z. X.; Jiang, Y. Q.; Zhu, J.; Li, Y. H.; Jiang, Z.; Zhou, H. Z.; Meng, W.; Wang, L.; Dai, L., Boosting the performance of LiTi2(PO4)(3)/C anode for aqueous lithium ion battery by Sn doping on Ti sites. Journal of Alloys and Compounds 2018, 731, 32-38. 16.Liang, Y. L.; Jing, Y.; Gheytani, S.; Lee, K. Y.; Liu, P.; Facchetti, A.; Yao, Y., Universal quinone electrodes for long cycle life aqueous rechargeable batteries. Nature Materials 2017, 16, (8), 841-+. 17.Sun, K.; Juarez, D. A.; Huang, H.; Jung, E.; Dillon, S. J., Aqueous lithium ion batteries on paper substrates. Journal of Power Sources 2014, 248, (Supplement C), 582-587. 18.Wang, H.; Zeng, Y.; Huang, K.; Liu, S.; Chen, L., Improvement of cycle performance of lithium ion cell LiMn2O4/LixV2O5 with aqueous solution electrolyte by polypyrrole coating on anode. Electrochimica Acta 2007, 52, (15), 5102-5107. 19.Wessells, C.; Huggins, R. A.; Cui, Y., Recent results on aqueous electrolyte cells. Journal of Power Sources 2011, 196, (5), 2884-2888. 20.Lim, S. J.; Han, D. W.; Nam, D. H.; Hong, K. S.; Eom, J. Y.; Ryu, W. H.; Kwon, H. S., Structural enhancement of Na3V2(PO4)(3)/C composite cathode materials by pillar ion doping for high power and long cycle life sodium-ion batteries. Journal of Materials Chemistry A 2014, 2, (46), 19623-19632. 21.Chernova, N. A.; Roppolo, M.; Dillon, A. C.; Whittingham, M. S., Layered vanadium and molybdenum oxides: batteries and electrochromics. Journal of Materials Chemistry 2009, 19, (17), 2526-2552. 22.Dong, Y.; Li, S.; Zhao, K.; Han, C.; Chen, W.; Wang, B.; Wang, L.; Xu, B.; Wei, Q.; Zhang, L.; Xu, X.; Mai, L., Hierarchical zigzag Na1.25V3O8 nanowires with topotactically encoded superior performance for sodium-ion battery cathodes. Energy & Environmental Science 2015, 8, (4), 1267-1275. 23.Wei, L.; Zhao, T.; Zeng, L.; Zhou, X.; Zeng, Y., Copper nanoparticle-deposited graphite felt electrodes for all vanadium redox flow batteries. Applied Energy 2016, 180, 386-391. 24.Wei, L.; Zhao, T.; Xu, Q.; Zhou, X.; Zhang, Z., In-situ investigation of hydrogen evolution behavior in vanadium redox flow batteries. Applied Energy 2017, 190, 1112-1118. 25.Zhou, D.; Liu, S.; Wang, H.; Yan, G., Na2V6O16·0.14H2O nanowires as a novel anode material for aqueous rechargeable lithium battery with good cycling performance. Journal of Power Sources 2013, 227, (Supplement C), 111-117. 26.Nair, V. S.; Cheah, Y. L.; Madhavi, S., Symmetric Aqueous Rechargeable Lithium Battery Using Na1.16V3O8 Nanobelts Electrodes for Safe High Volume Energy Storage Applications. Journal of the Electrochemical Society 2014, 161, (3), A256-A263.


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27.Kawakita, J.; Miura, T.; Kishi, T., Comparison of Na1+xV3O8 with Li1+xV3O8 as lithium insertion host. Solid State Ionics 1999, 124, (1), 21-28. 28.Xu, Y.; Han, X.; Zheng, L.; Yan, W.; Xie, Y., Pillar effect on cyclability enhancement for aqueous lithium ion batteries: a new material of β-vanadium bronze M0.33V2O5 (M = Ag, Na) nanowires. Journal of Materials Chemistry 2011, 21, (38), 14466-14472. 29.Xu, Y.; Han, X.; Zheng, L.; Wei, S.; Xie, Y., First investigation on charge-discharge reaction mechanism of aqueous lithium ion batteries: a new anode material of Ag2V4O11 nanobelts. Dalton Transactions 2011, 40, (40), 10751-10757. 30.Bai, L.; Zhu, J.; Zhang, X.; Xie, Y., Reducing hydrated protons co-intercalation to enhance cycling stability of CuV2O5 nanobelts: a new anode material for aqueous lithium ion batteries. Journal of Materials Chemistry 2012, 22, (33), 16957-16963. 31.Sun, D.; Jin, G.; Wang, H.; Liu, P.; Ren, Y.; Jiang, Y.; Tang, Y.; Huang, X., Aqueous rechargeable lithium batteries using NaV6O15 nanoflakes as high performance anodes. Journal of Materials Chemistry 2014, 2, (32), 12999-13005. 32.He, H.; Zeng, X.; Wang, H.; Chen, N.; Sun, D.; Tang, Y.; Huang, X.; Pan, Y., NaV6O15 Nanoflakes with Good Cycling Stability as a Cathode for Sodium Ion Battery. Journal of The Electrochemical Society 2014, 162, (1). 33.Hu, F.; Jiang, W.; Dong, Y.; Lai, X.; Xiao, L.; Wu, X., Synthesis and electrochemical performance of NaV6O15 microflowers for lithium and sodium ion batteries. RSC Advances 2017, 7, (47), 29481-29488. 34.Wang, X.; Liu, Q.; Wang, H.; Jiang, D.; Chang, Y.; Zhang, T.; Zhang, B.; Mou, H.; Jiang, Y., PVP-modulated synthesis of NaV6O15 nanorods as cathode materials for high-capacity sodium-ion batteries. Journal of Materials Science 2016, 51, (19), 8986-8994. 35.Zhao, M.; Zhang, W.; Qu, F.; Wang, F.; Song, X., Good discharge capacities of NaV6O15 material for an aqueous rechargeable lithium battery. Electrochimica Acta 2014, 138, 187-192. 36.Baddour-Hadjean, R.; Bach, S.; Emery, N.; PereiraRamos, J. P., The peculiar structural behaviour of [small beta]-Na0.33V2O5 upon electrochemical lithium insertion. Journal of Materials Chemistry 2011, 21, (30), 11296-11305. 37.Semenenko, D. A.; Itkis, D. M.; Pomerantseva, E. A.; Goodilin, E. A.; Kulova, T. L.; Skundin, A. M.; Tretyakov, Y. D., LixV2O5 nanobelts for high capacity lithium-ion

battery cathodes. Electrochemistry Communications 2010, 12, (9), 1154-1157. 38.Kulka, A.; Baster, D.; Dudek, M.; Kiełbasa, M.; Milewska, A.; Zając, W.; Świerczek, K.; Molenda, J., Electrochemical properties of chemically modified phosphoolivines as cathode materials for Li-ion batteries. Journal of Power Sources 2013, 244, 565-569. 39.Yan, J.; Yuan, W.; Tang, Z.-Y.; Xie, H.; Mao, W.-F.; Ma, L., Synthesis and electrochemical performance of Li3V2(PO4)3−xClx/C cathode materials for lithium-ion batteries. Journal of Power Sources 2012, 209, 251-256. 40.Liu, D.; Cao, G., Engineering nanostructured electrodes and fabrication of film electrodes for efficient lithium ion intercalation. Energy & Environmental Science 2010, 3, (9), 1218-1237. 41.Niu, C.; Liu, X.; Meng, J.; Xu, L.; Yan, M.; Wang, X.; Zhang, G.; Liu, Z.; Xu, X.; Mai, L., Three dimensional V2O5/NaV6O15 hierarchical heterostructures: Controlled synthesis and synergistic effect investigated by in situ X-ray diffraction. Nano Energy 2016, 27, (Supplement C), 147156. 42. Lu, Y.; Wu, J.; Liu, J.; Lei, M.; Tang, S.; Lu, P.; Yang, L.; Yang, H.; Yang, Q., Facile Synthesis of Na0.33V2O5 Nanosheet-Graphene Hybrids as Ultrahigh Performance Cathode Materials for Lithium Ion Batteries. ACS Applied Materials & Interfaces 2015, 7, (31), 1743317440. 43.Wang, H.; Wang, W.; Ren, Y.; Huang, K.; Liu, S., A new cathode material Na2V6O16·xH2O nanowire for lithium ion battery. Journal of Power Sources 2012, 199, (Supplement C), 263-269. 44.Nair, V. S.; Sreejith, S.; Joshi, H.; Zhao, Y.; West, A.; Madhavi, S., The fabrication of LiMn2O4 and Na1.16V3O8 based full cell aqueous rechargeable battery to power portable wearable electronics devices. Materials & Design 2016, 93, 291-296. 45.Li, K.; Cao, L.; Huang, Z.; Chen, L.; Chen, Z.; Fu, C., Novel cathode materials LixNa2−xV2O6 (x = 2, 1.4, 1, 0) for high-performance lithium-ion batteries. Journal of Power Sources 2017, 344, 25-31. 46.Mi, C. H.; Zhang, X. G.; Li, H. L., Electrochemical behaviors of solid LiFePO4 and Li0.99Nb0.01FePO4 in Li2SO4 aqueous electrolyte. Journal of Electroanalytical Chemistry 2007, 602, (2), 245-254. 47.Allen, J.; Larry, R., Electrochemical Methods: Fundamentals and Applications John Wiley & Sons. Inc., The United State of America 2001.


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Graphic abstract Crystal structure of various hexa-vanadate compounds prepared with various contents of K doping Na1-xKxV6O15 (x=0,0.1,0.2 & 0.3). Percentage of capacity retention of pristine NVO, NKVO-1, NKVO-2 and NKVO-3 at current rates of 0.1A,0.5A,1A,2and 3A.


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Energy & Fuels

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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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Energy & Fuels

Atoms V3 O4 O7 O1 V2 V1 O5 Na1 O2 O6 O3

x 0.11900

Y

z

Occupancy

0.00000

0.11700

0.50000

0.22400

0.00000

0.26500

0.50000

0.46400

0.00000

0.39600

0.50000

0.04900

0.00000

0.81300

0.50000

0.41100

0.00000

0.28900

0.50000

0.10300

0.00000

0.33700

0.50000

0.26900

0.00000

0.10700

0.50000

0.41400

0.00000

0.99500

0.25000

0.07900

0.00000

0.63700

0.50000

0.42100

0.00000

0.75400

0.50000

0.22200

0.00000

0.43800

0.50000


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Sample

a, Å

b, Å

c, Å

V, (Å)

NVO

10.0345

3.5899

15.4604

525.5568

NKVO-1

10.0504

3.5950

15.4498

526.7195

NKVO-2

10.0839

3.6099

15.3772

528.2397

NKVO-3

10.0733

3.6059

15.4320

528.8565


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Energy & Fuels

DLi+/cm2s-1

DLi+/cm2s-1

DLi+/cm2s-1

I’ peak

II’ Peak

III’ Peak

NVO

1.13x10-11

6.54x10-12

1.11x10-12

NKVO-1

2.30x10-11

1.38x10-11

9.93x10-12

NKVO-2

1.53x10-11

1.29x10-11

8.90x10-12

NKVO-3

2.89x10-12

3.35x10-11

3.01x10-11

Sample


extraction characteristics of a novel double

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