Na2V6O16Â·3H2O Barnesite Nanorod: An Open-Door to Display a

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Na2V6O16·3H2O Barnesite Nanorod: An Open-Door to Display a Stable and High-Energy for Aqueous Rechargeable Zn-Ion Batteries as Cathode Vaiyapuri Soundharrajan, Balaji Sambandam, Sungjin Kim, Muhammad Hilmy Alfaruqi, Dimas Yunianto Putro, Jeonggeun Jo, Seokhun Kim, Vinod Mathew, Yang-Kook Sun, and Jaekook Kim Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b05403 • Publication Date (Web): 23 Mar 2018 Downloaded from http://pubs.acs.org on March 23, 2018

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Na2V6O16·3H2O Barnesite Nanorod: An Open-Door to Display a Stable and High-Energy for Aqueous Rechargeable Zn-Ion Batteries as Cathode Vaiyapuri Soundharrajana‡, Balaji Sambandama‡, Sungjin Kim a, Muhammad H. Alfaruqi a, Dimas Yunianto Putroa, Jeonggeun Jo a, Seokhun Kim a, Vinod Mathew a, Yang-Kook Sunb and Jaekook Kim a* a

Department of Materials Science and Engineering, Chonnam National University, Gwangju 500-757, South Korea.

b

Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea.

ABSTRACT. Owing to their safety and low cost, aqueous rechargeable Zn-ion batteries (ARZIBs) are currently more feasible for grid-scale applications, as compared to their alkalicounterparts such as lithium and sodium ion batteries (LIBs, SIBs), for both aqueous and nonaqueous systems. However, the materials used in ARZIBs have a poor rate capability and inadequate cycle lifespan, serving as a major handicap for long-term storage applications. Here, we report vanadium-based Na2V6O16·3H2O nanorods employed as a positive electrode for ARZIBs, which display superior electrochemical Zn storage properties. A reversible Zn2+ ion (de)intercalation reaction describing the storage mechanism is revealed using in situ synchrotron

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X-ray diffraction technique. This cathode material delivers a very high rate capability and high capacity retention of more than 80% over 1,000 cycles, at a current rate of 40C (1C = 361 mA g−1). The battery offers a specific energy of 90 W·h kg−1 at a specific power of 15.8 KW kg−1 , enlightening the material advantages for an eco-friendly atmosphere.

KEYWORDS. layer structured metal oxide; aqueous Zn-ion batteries; high capacity; prolonged cycle lifespan; high energy.

The development of environmentally friendly large-scale energy storage systems (ESSs) with high safety, high energy density, and low cost has recently been a prime goal for researchers. Currently, lithium-ion batteries (LIBs) dominate the field of portable electrical devices such as mobile phones and laptops, which are essential devices in our daily lives. However, the demand for grid storage, electric vehicles and hybrid electric vehicles is increasing, for which lithium cannot meet the eventual requirements, owing to the toxicity, safety hazard, and cost issues arising from organic electrolytes.1–3 Batteries based on aqueous electrolytes seem to be a good choice for grid-scale storage as a substitute for flammable organic electrolytes. Aqueous rechargeable batteries have been attracting significant attention, as aqueous electrolytes are inexpensive, easy to assemble, and have a higher ionic conductivity by several orders of magnitude than those of organic electrolytes, resulting in a high rate capability and high power density.4–12 Recently, aqueous sodium-ion batteries and aqueous Zn ion batteries (ZIBs) have been studied extensively owing to their low cost, high capacity, good electronic conductivity, and elemental abundance.13–18 In particular, ZIBs are considered as a vital contender for possible grid-scale renewable EESs. Zinc displays certain benefits over Li and Na ion batteries as an anode material. In particular, zinc is abundant in nature and possesses a high theoretical capacity


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(820 mAh g-1), high energy density (5851 mAh mL−1) and a low negative potential (−0.762 V vs. SHE).19,20 In addition, a zinc-based redox couple involves two-electron transfer during the charge / discharge processes, which will enable a large storage capacity in comparison to Li or Na ions batteries, which operate through a single-electron transfer.21 However, considering the demands for ZIBs, the availability of cathode materials to store the divalent Zn2+ ions is limited, and has its own electrochemical disadvantages such as rapid capacity fading, low capacity and poor rate capability. For example, the most frequently studied cathode materials based on manganese-based oxides such as δ-MnO2,22 α-MnO2,23 and αMn2O3,24 cannot meet the expectations of a stable electrode material owing to the Jahn–Teller distortion effect, upon repeated electrochemical cycling accompanied by Mn2+ dissolution resulted in drastic capacity fading. Recently, Pan et al.25 demonstrated a method to overcome the fading problem in α-MnO2 by adding MnSO4 salt as an electrolyte additive, which suppresses the Mn2+ dissolution in the electrolyte, yielding an admirable rate capability and a very good capacity retention of 92% after 5,000 cycles; this indicates a stable chemical conversion reaction mechanism between the cathode and proton. In addition, Li et al.26 originally reported that NASICON-structured Na3V2(PO4)3 (NVP) as the host for Zn2+ ions showed a good electrochemical stability, while He et al.27 documented the possibility of a reversible insertion/extraction of Zn2+ in VS2, a layered transition-metal chalcogenide. However, the capacity of the former was poor (~97 mAh g−1 at 0.5 C). The recent advances and the applications of layered vanadates such as LiV3O8 (LVO),28 Zn0.25V2O5⋅nH2O,29 V2O5. nH2O30 and H2V3O831 for ZIBs and their suitable electrochemical properties motivated the exploration of layered sodium vanadate as an electrode material for ZIBs.


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Na2V6O16·3H2O (NVO, Barnesite) exhibits a characteristic layered structure comprised of V3O8 layers and interstitial hydrated Na ions. In a monoclinic NVO system, the V3O8 layer is composed of VO6 octahedra and edge-sharing V2O8 square pyramid units with hydrated Na ions situated amongst the layers.32 Moreover, sodium vanadates, owing to their typical layered structure and structural flexibility, have served as a guest species to store Li+ and Na+ ions with good electrochemical properties and became a ubiquitous electrode material for LIBs and SIBs.33,34 It is considered that Na+ in Na2V6O16·2.36H2O is situated at the octahedral sites between the layers of vanadium and oxygen atoms. The vacant tetrahedral sites are available for occupation by Li ions.35 In specific, considering that the ionic radii of Li+ (0.74 Å) and Zn2+ (0.76 Å) are almost equal and even smaller than the Na+ ion (0.99 Å), the volume expansion during the Zn2+ intercalation in to the NVO electrode will be same as that of Li+. Hence, we demonstrate that Na2V6O16·xH2O with ample vacancies in the structure stores divalent metal ions and can be used as a high-capacity positive electrode material in aqueous ZIBs with very stable cycling performance and rate capability. The results open the possibilities of using a wider family of alkali-earth metal vanadium oxide bronze hydrated compounds as an intercalation system for ZIBs. Results and Discussion. The NVO powder obtained via a rapid micro-wave technique (see supporting information) was taken for physicochemical studies. The powder X-ray diffraction pattern of an as-synthesized NVO sample in Figure 1a shows well indexed reflections with a standard profile (JCPDS No:16-0601) in a typical monoclinic Na2V6O16·3H2O symmetry with the P2/m space group. Further, the lattice water in the pristine sample is confirmed by TG analysis as shown in Figure S1, supplementary file. SEM image of the NVO sample provides the intrinsic morphology, in which clear 1D nanorods are projected in Figure 1b. It should be


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pointed out that these nanorods appear to be moderately assembled together to form nanobundles with a calculated average length of 500 nm. Furthermore, low- and high-resolution TEM micrographs were recorded to verify the formation of nanobundles via stacking of nanorods and

Figure 1. (a) PXRD pattern for as-prepared Na2V6O16. 3H2O, (b) high resolution-SEM image of NVO nanorods, (c) HR-TEM image with clear lattice fringes and (d) crystal structure of Na2V6O16. x H2O. Inset in (c): TEM image at low magnification. the results are presented in in Figure 1c. The low resolution image in Figure 1c inset, clearly reflects the dense component to be surrounded by lighter components, in a core–shell-like structure, and thereby provides no evidence for the formation of any bundle-type structures. The measured average width of this unique structure is less than 100 nm and the length exceed over 500 nm. However, the high-resolution image in Figure 1c of a single nanorod reveals clearly


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distinguishable lattice fringe regions separated from a major portion of ‘fringe-free’ domains. This observation indicate that these nanorods are composed of alternate crystalline and amorphous domains. The lattice features reflect the structural arrangements and high crystallinity of the NVO sample; in the high-resolution TEM image, the measured stripe distance, d001 = 0.71 nm, closely corresponds to the (001) plane of the monoclinic Na2V6O16·3H2O. In addition, the crystal structure of Na2V6O16 with known amount of water molecules (2H2O), as described by Bachmann and Barnes,36 is projected in Figure 1d, which confirms the growth direction of the nanorods along the crystallographic a axis. This observation forecasts that the storage of Zn-ions in the NVO nanorods can occur along the same a axis and is elaborately discussed in the later section. Further, the detailed X-ray photoelectron spectroscopy (XPS) analysis confirming the intrinsic elemental compositions of the NVO material is provided in Figure S2. The electrochemical zinc-storage properties were examined for the fabricated Zn/NVO coin cells with a 1 M aqueous ZnSO4 electrolyte. Figure 2a shows the initial three cycles of CV plots for the NVO electrode within a voltage window of 0.4–1.4 V vs Zn/Zn2+ at a scan speed of 0.1 mV s−1. In the forward scan, we observe a small shoulder (0.91 V) followed by two sharp peaks at 0.746 and 0.514 V, demonstrating the electrochemical intercalation of Zn2+ into the layered structure. In the reverse scan, two distinct peaks at 0.75 and 1.002 V continued by a shoulder (1.17 V) replicate the corresponding de-intercalation of Zn2+ ions from the layered framework. The origin of the peaks could be due to continuous reductions from V5+ to lower oxidation states in the forward scan, and the converse may occur in the reverse scan. Interestingly, the reduction sweep of first cycle in the CV profile is slightly different from the rest of the cycles in terms of their peak positions, however, the oxidation sweep holds almost similar peaks positions for the entire measurement. The shifting of the reduction peaks in the first cycle can be related to the


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gradual activation of the fresh electrode, as observed in spinel ZnMn2O4 cathode for ZIBs.37 Moreover, the succeeding CV curves display good reproducibility and similarity after the first (a)

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Figure 2. (a) CV profiles for NVO cathode, (b) selected discharge/charge patterns for NVO cathode for ARZIBs at 5C for initial 400 cycles and 2C for 100 cycles, (c) corresponding cyclability span for 500 cycles. cycle, demonstrating the good electrochemical reversibility of the electrode. The galvanostatic discharge/charge evolution of Na2V6O16·3H2O nanostructured electrode between 0.4 and 1.4 V for first two cycles at 5C and 401st and 500th cycles at 2C, respectively, is


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represented in Figure 2b. The 1C rate was defined as 361 mA g−1, based on the average capacity achieved at a moderate current density of 100 mA g−1 (see 0.28C in Figure 3a) . The NVO electrode delivers an initial specific discharge capacity of 266.6 mAh g−1 (insertion of approximately 3.29 Zn per unit formula including water molecules, according to the Faraday equation) and sequential specific charge capacity of 268 mAh g−1 with a Columbic efficiency, CE% slightly greater than 100 after the end of the first cycle. It is clearly seen that the discharge and charge curves revealed distinct plateaus very similar to the electrochemical study, as evidenced from the CV measurement. Interestingly, as shown in the Figure 2b, the trend of the voltage curves remains almost unchanged even in the 401st cycle when the current rate is lowered to 2C. The corresponding cycling performance of the NVO electrode at two different current rates (5C for 400 cycles followed by 2C for 100 cycles) is provided in Figure 2c. The NVO electrode provides initial discharge capacities of 266.6 and 279.2 mAh g−1, respectively, for 5 and 2C rates, at the 1st and the 401st cycles, and reversible specific capacities of 250 and 262 mAh g−1, at the same rates, after the 400th and 500th cycles. This indicates a significant reversibility of the NVO electrode, even after returning from the high current testing rate. Additionally, the electrode was tested under moderate applied current rates of 0.5 and 3C with detailed discussion thereby given in Figure S3. The high rate performance is vital criteria for the practical realizations of ZIBs when applied for grid storage applications. However, to the author’s best knowledge, the rate capability of the reported electrode candidates for ARZIBs were found lacking. In our context, the NVO nanorods unveiled an exceptionally steady capacity under severe discharge–charge testing conditions. As


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demonstrated in Figure 3a, NVO electrode is assessed at different rates from 0.28 to 55.4C and then returned to 1.38C. The NVO electrode supplied average discharge capacities of 361 (5 cycles), 341 (10 cycles), 319 (10 cycles), 301 (10 cycles), 277 (10 cycles), 230 (15 cycles), 174 (15 cycles), and 113.95 (15 cycles) mAh g−1 at the rates of 0.28, 1.38, 2.77, 4.15, 8.3, 16.62, (a)

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Figure 3. (a) C-rate studies for NVO cathode at different current rates, (b) cyclability comparison of three different current rates of 15, 25 and 40C, respectively, runs for 300, 500 and 1,000 cycles.


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27.7, and 55.4C, respectively. In particular, when the rate is returned to 1.38C, the electrode is able to recover a reversible capacity of 318 mAh g−1 even after the extremely high current testing (after the completion of 100 cycles), demonstrating exceptional rate competence of the NVO nanorod electrode. This indicates that the zinc (de)intercalation process did not appear to be obstructed at high current rates. In specific, a drop in initial Coulombic efficiency (CE) is observed beyond a high applied current rate of 16.62 C. For example, when the C rates are altered from 16.62 C to 27.7 C and 27.7 C to 55.4 C, the initial CE dropped to 90% and 88%, respectively. The low irreversibility in the initial cycle at high C rates can be related to the large difference in the consecutive current rates (of one order of magnitude) applied abruptly on the electrode although further studies are needed to ascertain this. The corresponding dischargecharge patterns for the C- rate performance are given in Figure S4. In consideration of the practical realization of large-scale zinc storage systems, the electrodes were subjected to severe testing conditions. The cyclability tests for prolonged cycles were conducted for high current rates to monitor the electrochemical stability of the NVO cathode. A comparison of the cycling performances at 15, 25 and 40C is illustrated in Figure 3b. The maximum capacities of 240, 214 and 152 mAh g−1 are reached at the 50th, 54th and 157th cycles, respectively, for 15, 25 and 40C. The reversible discharge capacities of 228, 182 and 128.5 mAh g−1 are retained, for the three different applied current rates of 15, 25 and 40C, respectively, for after 300, 500 and 1,000 cycles. This signifies preeminent capacity retentions of 95, 85 and 84.5% at their respective rates of 15, 25 and 40C. It is evident that the specific capacity increase during the initial 150 cycles at 40 C to realize a maximum activation, thanks to a periodic activation process upon cycling. The general reason account for the increasing capacity for the initial few cycles is related to the large number of active sites at the interface that tends to


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enhance the interfacial zinc storage on repeated cycling. In addition, at discharge/charge rates as high as 40C, the ingress/egress of Zn2+ ions into/from the NVO gallery occurs at a rapid pace within a very short time period. The guest ions penetration ability requires definite time to attain complete equilibrium. Therefore, an increasing number of electrochemical sites are penetrated by the Zn2+ ions thereby leading to higher specific capacities on repeated cycling. Therefore, greater the applied current density, greater the increase in initial zinc storage capacities and longer the period of cycles required for electrode activation. For example, this activation process is observed at comparatively lower current density (~ 5C) for several tens of cycles (Figure 2c) as the electrode reaches maximum capacity after 25th cycle, but not like high applied current densities (Figure 3b), where they require few hundreds of cycles to attain stable capacities. Moreover, the electrode exhibits a continual cycling performance with slight decay upon prolonged cycling up to 1,000 cycles. In addition, yet another long term cyclability of 1,000 cycles for an intermediate current rate of 30C is shown in Figure S5. It is to be noted that very few materials have been acknowledged to reveal superior rate proficiency and long-standing ability. To the best of the authors' knowledge, the NVO electrode delivers one among the best electrochemical performance for ARZIBs. In order to establish the electrochemical mechanism during discharge/charge processes, in situ synchrotron XRD analyses were performed. In Figure 4 panel (a) exhibits the 1st discharge/charge profile for NVO electrode with selected in situ XRD scan numbers while the remaining panels from (b) to (e) represent selected XRD 2 regions. In detail, at scan 1, before electrochemical reaction begins (at OCV), the NVO reflections pattern are indexed well with those observed for the as-prepared sample (JCPDS No.:16-0601). After the discharge reaction is initiated, all the reflections start to shift including the mother plane (001). When the electrode


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reaches 0.78 V (scan no 10), a new set of planes is observed at the 2 values of 8.06, 12.2, 16.2, 21.2, 32.9, and 58.5°, which propagate very quickly over the scans. These new planes are well matched with Zn4(OH)6 SO4.5H2O (JCPDS No.: 78-0246), a layered material, formed during the electrochemical discharge process, which was precipitated from the electrolyte on the electrode surface. The peaks intensities, especially for the mother plane (001), compete over the NVO planes (panel (b)). Beyond 10 scans, the mother plane of NVO are slightly shifted toward higher scanning angles (2) while the remaining characteristic reflections undergo gradual shifts toward lower 2 values, as clearly denoted in panels (c) and (d). Clear observation of all planes of Zn4(OH)6 SO4.5H2O are noted during the discharge process. At the end of scan no. 34 or complete discharge reaction, a reasonable decrease in the interlayer spacing of the mother plane of NVO from 7.92 to 7.88 Å suggests the intercalation of Zn2+ ions on the corresponding reflection. This reduction in the inter-planar spacing indicates an improvement in the structural coordination owing to the strong electrostatic interaction between the intercalated zinc ions and the (V3O8)- layers. In general, such layered-type compounds tend to accommodate Li+/Na+ ions between consecutive layers via weak van der Waals interaction.28,38,39 A clear lower angle (negative shift) shift is observed in other planes of NVO; for example, (204) reflection in panel (d), undergoes a gradual negative shift (towards lower 2 values) during the discharge reaction. It should be noted here that the overlap of the (204) reflection with the strong stainless-steel (SS, current collector) peak make it difficult to observe the former reflection in the initial few scans. However, beyond scan no. 10, the negatively shifted (204) reflection becomes visible as a shoulder and further transforms into a high intensity peak at full discharge (scan no. 34). A reasonable and maximum shift of 0.8° is noted for this reflection. This expansion of inter-layer space is raised from the reduction of V5+ ions and thereby increase the M-M bond distance


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between the layers.28 Interestingly, there is no evidence for sodium ions being extracted out from the crystal structure as these ions are quite stable and alleviate the structure during cycling. This is further supported by related published works.34,40 The observation of a new phase, including various features of peak shifting and peak intensity growth upon discharging, started to reverse during subsequent charging, as clearly observed from the in situ synchrotron XRD patterns (scan nos. 35 to 85) in Figure 4. This indicates a clear reversible (de)intercalation mechanism for the NVO electrode. Interestingly, the new phase, zinc hydroxide sulfate, observed during the discharge process could undergo dissolution while charging. The origin of formation and dissolution of this new phase may require further studies, which will be undertaken in the near future. However, Lee et al.41 described the role of pH evolution during the electrochemical reaction and demonstrated the formation of layered zinc hydroxide sulfate as a precipitate on the surface of the electrode. Similarly, there is the possibility for vanadium dissolution during the discharge process and resultant increase in the electrolyte pH, and hence the precipitation on the surface of the electrode. A reverse mechanism may happen in their counterpart charging process. The typical migration path of the Zn-ions in the NVO gallery is predicted by using the VESTA 3D crystal structure program, after introducing Zn-ions into the layered framework (Figure S6). Na2V6O16. XH2O, in general, shows both layered and tunnel characteristics along the crystallographic a and c axis, respectively, for parallel and perpendicular directions to that of the (001) plane. For example, the zinc ion (de)intercalation in (001) plane along b axis gives sufficient space for Zn-ion to travel across the layer of the gallery. The incoming zinc ions can be accommodated in the square pyramidal sites (O4-O4 distance in VO5 units is about 4 Å between two V3O8 layers) between consecutive V3O8 layers as Na+ ions almost occupy the available


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Figure 4. (a) Electrochemical discharge-charge profiles cycled within 0.4−1.4 V at a current rate of 300 mA g−1. In situ XRD profiles obtained within selected scanning angle (2θ) domains of (b) 6-20°, (c) 24-35°, (d) 43-53°, and (e) 57-63°. 

represents a new phase of Zn4 (OH)6

SO4. 5H2O. octahedral sites and thereby stabilize the overall structure.35 The intercalation of Zn-ions thus clearly influences the synchrotron XRD investigation as the (001) plane is gradually shifted towards higher 2 values during the entire period of discharge (Figure 4b). The migration of Zn ions in yet another plane, (204) and a maximum negative shift (towards lower 2 values as in Figure 4d) observed during the discharge process may account for the elongation along the corresponding inter-planar distance after the entire discharge reaction. However, the expansion of the inter-plane is comparatively 4 times lesser than that of the (001) plane. However, the


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tunnel network observed clearly along the c axis (Figure S7) is occupied by Na ions, and this feature tend to hinder the mobility of the incoming Zn-ions. Hence, it is reasonable to conclude that the present layered structure appear to significantly impact the (de)intercalation mechanism in the nanorods based cathode. It is highly worthwhile to carry out the ex situ XRD; indeed, a significantly different environment from the in-situ study, to further confirm the electrochemical mechanism. The XRD patterns of before and after (1st charge) electrochemical reactions exhibit a similar pattern, confirming the reversible (de)intercalation mechanism, whereas the discharge pattern is completely different from the counterparts (Figure 5a). The observation of predominate planes, which correspond to zinc hydroxide sulfate formed during the electrochemical reaction as a precipitate on the surface of the electrode, summarizes the in situ analysis. However, the features of the NVO planes are utterly suppressed by the zinc hydroxide sulfate planes in the ex situ analysis. Thus, the observation from in situ and ex situ depths run parallel, giving strong evidence for the proposed mechanisms. The electrochemical mechanism is further supported by the ex situ TEM analysis, as given in Figure 5b. The precipitation of zinc hydroxide sulfate, upon discharging at 0.4 V is confirmed through the high-resolution TEM images in Figure 5b(i and ii), as uniform particles with average size less than 10 nm are observed on the surface of the NVO nanorods. Interestingly, no such particles of zinc salt are observed at the end of the charge process (Figure 5b (iii) and (iv)), thereby supporting the abovementioned mechanism. The corresponding elemental mapping images in Figures S8 and S9 also confirm the formation of the constituents at 0.4/1.4 V charge/discharge processes, respectively. As expected, the sulfur element is observed in the EDX mapping and STEM elemental mapping images of the discharged product (Figure S8). In


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Figure 5. (a) Ex situ XRD array for NVO cathode at discharge and charge depths. For a comparison, XRD pattern of the electrode before the electrochemical reaction is given at the bottom, (b) ex situ TEM low magnification images for (i) 1st discharge, (iii) 1st charge and high magnification images at (ii) 1st discharge, (iv) first charge. The arrow marks in b(ii) indicate a formation of new zinc phase. addition, SEM image in Figure S8h reveals the composite electrode morphology before the electrochemical reaction. Further, TEM line profile in Figure S10, confirming the elements of zinc hydroxidesulfate, added advantage for analyzing the small particles. Though the formation of zinc hydroxidesulfate is not quite new, its role on electrochemical reaction is not well defined.8,41–43 Lee et al.41 monitored the formation of the sulfate precipitate during the cycling of a -MnO2 cathode for ZIB via measuring the pH studies. They concluded that manganese dissolution increased the pH of the electrolyte and hence led to the formation of zinc hyrdroxidesulfate precipitate on the electrode surface. Recently, the Nazar group studying


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V3O7. H2O electrodes for ZIBs proposed that the hydroxidesulfate precipitate formation is due to the side reaction between the dissolved oxygen in the electrolyte and available species (like Zn2+, SO42- and H2O).44 Their mechanism of hydroxidesulfate precipitate formation was explained on the basis of electro-deposition of zinc hydroxidesulfate on a polypyrrole film from ZnSO4 aqueous solution at 0 V against SCE, which corresponds to 1 V against Zn2+/Zn system. From the in situ XRD profile (Figure 4b), the formation of this compound is roughly identified around 0.78 V (between the scan numbers 10-15). It should also be noted that the formation of hydroxidesulfate precipitate was confirmed not only from the electrochemically cycled Zn-V3O7. nH2O electrode after few cycles at discharge point, but also from the surface of the Zn-Zn symmetry cell, thereby inferring that it is formed as a result of a side reaction. These observations lead one to conclude that the capacity contribution due to the formation/dissolution of zinc hydroxide sulfate is insignificant. However, further systematic studies are necessary to identify the real contribution in conjunction with NVO redox electrochemical reaction. Interestingly, Xia et al.43 suggested that the formation of hydroxide sulfate precipitate on the surface of a zinc pyrovanadate electrode can stabilize the battery material under long-term cycling. Hence, electrochemical measurements were performed for the electrode at elevated temperatures of 60 and 75 C (Figure S11) and the formation of the precipitate in these cases were analyzed separately. At 60 C, the NVO electrode supplies a discharge capacity of 325 mAh g-1 at 4.15C (1500 mA g-1), which is higher than the room temperature measurement (Figure 3a). Although the temperature influences the capacity, in general, the stability of the electrode is abrupt. However, a drastic capacity fading is noted while the electrode was cycled at 75 C. The registered capacity is close to the value at room temperature measurement. The abrupt capacity fading in both cases confirms the instability of the electrode, as the ex situ XRD


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of the electrodes show reduced formation of the precipitate as in the form of Zn4(OH)6 SO4. 0.5H2O (JCPDS No.: 44-0674). This new phase is might be occurred by elimination of further water molecules from Zn4(OH)6 SO4. 5H2O phase during vacuum drying the post-mortem electrode under 120 C. Especially, the formation of surface precipitate reduced drastically when raised the temperature from 60 to 75 C (Figure S12a). This further supports that the quantity of surface precipitation is greatly diminished at 75 C through ex situ TEM image (Figure S12b). These observations clearly support the finding that the stability of the electrode is influenced by the precipitate formed on the surface of the electrode during electrochemical cycling. The multi-step vanadium based redox zinc (de)intercalation reaction was supported by ex situ XPS. Figure 6a shows V2p scan of the discharged product, in which 2p3/2 energy level is deconvoluted to produce peaks at 514.9, 516.18, 517.4, and 518.7 eV, respectively for +3, +4, +5, and a satellite peak of +4 oxidation states.45,46 On the contrary, the deconvoluted energy level in the charged state are represented by the peaks at 516.1, 517.3, and 518.7 eV, respectively for +4, +5, and the satellite peak of +4 oxidation states, revealing a similar environment to that before the electrochemical reaction (as-prepared). Correspondingly, Zn2p finger prints at 1021.8 (2p3/2) and 1044.9 (2p1/2) eV are highly dominated in discharge process, which is absent before the reaction and slightly apparent in the charge process (Figure 6a). The reflection of Zn features in the latter case may not be avoided during the ex situ program. In addition, the remarks of sulfate precipitation/dissolution through the electrochemical reaction was further validated with the findings from ex situ XRD/TEM by analyzing lower binding energy region from ex situ XPS. The discharge profile at 0.4 V in Figure S13 shows a clear peak at 169 eV, corresponding to the SO4- species probably from Zn4(OH)6 SO4.5H2O, which is absent after the subsequent charge


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process.47 As expected, the latter pattern clearly is much similar to that recorded before the electrochemical reaction (Figures S13c and a). Overall, the in-situ and ex-situ structural and microstructural characterizations clearly indicate that the electrochemical discharge reaction mechanism of the present Na2V6O16·3H2O electrode with zinc can be explained on the basis of Zn-intercalation/de-intercalation into/from the layered host in addition to the zinc hydroxide sulfate precipitation/dissolution at the electrode surface. The Ragone plot in Figure 6b can be used to evaluate the practical suitability of the NVO

Figure 6. (a) Ex situ XPS for V2p and Zn2p at 1st discharge (0.4V) and 1st charge (1.4 V). For a comparison, pristine (as-prepared) sample XPS pattern are given in the bottom, (b) Ragone plot for various ARZIBs candidates along with NVO cathode. material along with reported cathodes for ARZIBs. This electrode delivers a specific energy of 287 W·h kg−1 at the specific power of 79 W kg−1 based on the active mass of the cathode material. The energy loss for the present NVO material is very low; at two different specific power values as high as 7900 and 15800 W kg−1, high specific energies of 138.25 and 90 Wh kg−1, respectively, are achieved. Thus, the performance of the present NVO cathode competes


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well with those of the Zn0.25V2O5 cathode, which was demonstrated to show minimal energy loss among the ARZIB cathodes of LiV3O8,28 Mn2O3,24 VS2,27 NVP,26 H2V3O8,31 and Zn3V2O7(OH)2·2H2O,43 reported so far. In summary, a layered Na2V6O16. 3H2O cathode has been developed for a new high-energy zincion battery system, which achieved a remarkable stability over 1,000 cycles with a high reversible capacity of 128 mAh g−1 at a current rate of 40C. At a very high specific power of 15.8 KW kg−1, a high specific energy of 90 W·h kg−1 is achieved, which enables the practical applicability of the NVO cathode for ARZIBs. A series of in-situ/ex-situ investigations revealed that the electrochemical mechanism contributing to such high specific energies in this system can be attributed to the Zn-ion intercalation/de-intercalation reaction. The present study thus opens a wide door for aqueous rechargeable batteries with Zn metallic anode and aqueous ZnSO4 electrolyte without any active carbon support such graphene, graphene oxide for eco-friendly, low-cost, and high-stability cathode electrodes for next-generation grid-scale energy storage applications. ASSOCIATED CONTENT Supporting Information. Supporting Information contains material and methods, electrochemical evolution, TGA and XPS, cyclability at 0.5/3C and 30C, Zn ions travels through 3D crystallographic views, high temperature electrochemical studies, elemental mapping for discharge/charge depths, discharge profiles for C-rate and ex situ XPS.

AUTHOR INFORMATION


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Corresponding Author *[email protected]; Fax: +82-62-530-1699; Tel: +82-62-530-17. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was mainly supported by the Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013M3A6B1078875). This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2017R1A2A1A17069397). REFERENCES (1)

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Layered Na2V6O16·3H2O nanorod cathode, utilized for aqueous rechargable Zn-ion batteries, displays high reversible capacities (250 mA h g−1 at 5C after 400 cycles), exceptional rate capabilities (114 mA h g−1 at 55.4C), long cyclespan (1000 cycles at 30 and 40C) and specific energies of 287 (90) Wh Kg-1 at specific powers of 79 (15800) W kg-1.


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Na2V6O16Â·3H2O Barnesite Nanorod: An Open-Door to Display a

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