Chemically Preintercalated Bilayered K x V2O5· n H2O Nanobelts as a

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Chemically Preintercalated Bilayered KxV2O5·nH2O Nanobelts as a High-Performing Cathode Material for K-ion Batteries Mallory Clites, James L Hart, Mitra L. Taheri, and Ekaterina Pomerantseva ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01278 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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ACS Energy Letters

Chemically Preintercalated Bilayered KxV2O5·nH2O Nanobelts as a HighPerforming Cathode Material for K-ion Batteries Mallory Clites, James L. Hart, Mitra L. Taheri, Ekaterina Pomerantseva* Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104, USA * Corresponding Author: Phone: 215-571-4612; E-mail: [email protected] Permanent Address: 3141 Chestnut Street 27-344, Philadelphia, PA 19104 ABSTRACT Tailoring structure of the electrode material through chemical insertion of charge-carrying ions emerged as an efficient approach leading to enhanced performance of energy storage devices. Here, we for the first time report the effect of chemically preintercalated K+ ions on electrochemical charge storage properties of the bilayered vanadium oxide (δ-V2O5) as a cathode in non-aqueous K-ion batteries, a low-cost alternative to Li-ion batteries, which is attractive to large-scale energy storage. δ-K0.42V2O5·0.25H2O with the expanded interlayer spacing of 9.65 Å exhibited record high initial discharge capacity of 268 mAh·g-1 at a current rate of C/50 and 226 mAh·g-1 at a current rate of C/15. K-preintercalated bilayered vanadium oxide showed capacity retention of 74% after 50 cycles at a constant current of C/15, and 58% capacity retention when the current rate was increased from C/15 to 1C. Analysis of the mechanism of charge storage revealed that diffusion-controlled intercalation dominates over non-faradaic capacitive contribution. High electrochemical performance of δ-K0.42V2O5·0.25H2O is attributed to the facilitated diffusion of electrochemically cycled K+ ions through well-defined intercalation sites, formed by chemically preintercalated K+ ions. TOC GRAPHIC

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Li-ion batteries (LIBs) have dominated the rechargeable battery market since their commercialization in 1991. However, despite their high energy density, the limited abundance of lithium raises concerns about solely relying on LIBs for our society’s rechargeable energy storage needs. Therefore, alternative non-aqueous intercalation-based electrochemical systems that utilize ions, other than Li+ ion, are of interest. Among those, Na-ion and K-ion batteries attract increasing attention due to the high abundance of sodium and potassium in Earth’s crust and ocean water. Additionally, the singly-charged nature of Na+ and K+ ions is more favorable for diffusion in the crystal structure of the electrode material as compared to ions with higher charge that need to overcome stronger electrostatic interactions to move from the surface into the center of electrode particles. While the size and the mass of K+ ion are larger than those of Na+ ion, recently demonstrated highly reversible potassium intercalation in graphite electrodes with organic electrolyte, which cannot be achieved in case of Na+ ions, sparked interest in discovery and development of the cathode materials with capacities matching those for KC8 (279 mAg·g-1).1 In addition to the ability to use graphitic carbon as an anode, K-ion batteries have other advantages over Na-ion batteries.1 Compared to Na+ ion, K+ ion forms smaller solvated ions, leading to higher mobility in electrolytes and lower desolvation energy resulting in improved kinetics of the ion insertion at the electrode/electrolyte interface. Moreover, the low intercalation potential of K+ ion into graphite allows for the fabrication of high voltage batteries if combined with the cathode material with high potential of potassium intercalation. If additionally, the structure of the cathode material is tailored to incorporate large number of K+ ions and exhibit fast diffusion of K+ ions, a K-ion battery with high energy and high power density can be created. To date, most studies on cathodes for non-aqueous K-ion batteries have focused on Prussian blue analogues,2-8 polyanionic compounds,9-12 organic crystals13-15 and layered transition metal oxides.16-18 The latter offer open two-dimensional channels for K+ ion diffusion, which is promising for achieving high electrode performance. However, the layered oxides investigated in non-aqueous K-ion batteries so far, K0.7Fe0.5Mn0.5O216, K0.3MnO217 and P1- and P3-KxCoO218 delivered initial capacities of only 178 mAh·g-1 (voltage range of 1.5 - 4.0 V, current density of 20 mA·g-1), 136 mAh·g-1 (voltage range of 1.5 - 4.0 V, current density of 27.9 mA·g-1) and 60 mAh·g1

(voltage range of 2.0 – 3.9 V, current density of 11.8 mA·g-1), respectively. Higher charge storage

capability can be achieved by expanding interlayer spacing in layered transition 2 ACS Paragon Plus Environment

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Figure 1. Schematic illustration of the chemical pre-intercalation synthesis approach, which was used to incorporate K+ ions in the interlayer space of bilayered vanadium oxide. Decomposition of the precursor, α-V2O5 powder, in an aqueous media is carried out in the presence of K+ ions. In the sol-gel process, followed by aging, K+ ions together with water molecules are trapped between the growing bilayers of vanadium oxide, leading to the formation of δKxV2O5·nH2O phase. The positions of K+ ions and water molecules in δ-KxV2O5·nH2O are hypothetical, and structure refinement needs to be done to establish precise crystallographic site of the interlayer species.

metal oxide structure and utilizing transition metals in high oxidation states that can undergo several reduction steps. Bilayered vanadium oxide (δ-V2O5) has emerged as a high capacity cathode material for Na-ion and Mg-ion batteries.19-28 The structure of δ-V2O5 is built of double V-O layers separated by a large for oxides distance of ~11.5 Å, stabilized by water molecules and available for ion intercalation.29 Additionally, vanadium can be reduced from V5+ to V3+ by accepting two electrons, leading to high theoretical capacity of the material. Using Na-ion system, previously we have demonstrated that chemical pre-intercalation δ-V2O5 with charge-carrying ions (Na+ ions in case of Na-ion batteries) prior to the electrochemical testing leads to substantially increased capacity.30 Later we have reported that the versatile chemical pre-intercalation synthesis approach, schematically shown in Figure 1 for K+ ions, can be used to insert a range of singly- and doublycharged ions within the interlayer space of the δ-V2O5 phase.31 δ-KxV2O5·nH2O has a bilayered structure with the interlayer spacing of 9.62 Å.31 Interestingly, highly disordered K0.22V1.74O4.37·0.82H2O nanosheets, with the structure similar to the bilayered vanadium oxide, demonstrated a high capacity of 183 mAh·g-1 in half cells with aqueous electrolyte.32 However, electrochemical behavior of K-preintercalated bilayered vanadium oxide in non-aqueous K-ion cells has never been reported before. 3 ACS Paragon Plus Environment

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Here we demonstrate a record high for cathodes initial capacity of 226 mAh·g-1 at a current rate of C/15 (~20 mA·g-1) exhibited by δ-KxV2O5·nH2O nanobelts in K-ion cells with organic electrolyte. We discuss cycling and rate performance of the material and show that diffusionlimited intercalation dominates charge storage mechanism of K-preintercalated δ-V2O5·phase. Hydrothermally treated product of the aged precipitate, obtained in a sol-gel process described above, formed a single-phase K-preintercalated bilayered vanadium oxide with nanobelts morphology (Fig. 2). Well-defined δ-KxV2O5·nH2O nanobelts had width of ~30100 nm and lengths of up to 50 microns (Fig. 2a). Analysis of the EDX spectra (Fig. 2a, inset) revealed the presence of potassium in chemical composition of the material. At the same time, the absence of the chlorine peak, which is typically observed at 2.5 V, indicated that K+ ions are intercalated within the V-O framework and the excess of KCl salt, used in the synthesis process, was removed by washing. The K:V ratio in the synthesized material was determined via AAS measurements (Fig. S1) and found to be ~0.21, which corresponds to an empirical formula of K0.42V2O5·nH2O.

Figure 2. Morphology, structure and composition characterization of the K-preintercalated bilayered vanadium oxide. (a) SEM image showing well-formed nanobelts. The inset is an EDX spectrum demonstrating presence of potassium and absence of chlorine peaks. (b) XRD pattern with the interlayer spacing calculated from the position of (001) reflection. (c) TEM image of a single nanobelt showing the interlayer spacing of 9.65 Å. (d) Weight loss curve determined via TGA in air.

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Formation of the single-phase bilayered δ-K0.42V2O5·nH2O without any impurities was confirmed via XRD analysis (Fig. 2b). XRD patterns of the material exhibited peaks at 2θ of 9o and 27o, which correspond to the (001) and (003) planes of the bilayered vanadium oxide phase, respectively. The interlayer spacing, calculated from the position of (001) peak, was determined to be ~9.65 Å. TEM imaging confirmed the layered structure and interlayer spacing of δ-K0.42V2O5·nH2O (Fig. 2c). The interlayer spacing was extracted from measuring the distance between 10 successive V2O5 layers. This process was repeated at multiple locations within the nanobelt shown in Fib. 2c, giving an average value of 9.65±0.08Å, in agreement with the interlayer spacing from XRD data (Fig. 2b). Interestingly, compared to the bilayered vanadium oxide preintercalated with other alkali or alkali-earth ions (Li+, Na+, Mg2+ and Ca2+ ions),31 the peaks in the XRD pattern of δK0.42V2O5·nH2O are relatively narrow, indicating better order with a more uniform distance between bilayers. Additionally, K-preintercalated δ-V2O5 shows the smallest interlayer spacing, compared to other δ-MxV2O5·nH2O (M = Li, Na, Mg, Ca) phases.31 This finding agrees with the results observed for another family of layered materials, Mxenes,33 containing inorganic ions and water molecules in the interlayer space. Comparison of Ti 3C2 MXene phase containing various alkali and alkali-earth ions revealed that the K-containing sample had the smallest interlayer spacing. This result was ascribed to the presence of only one row of water molecules between the layers in K-preintercalated material, while Naand Mg-preintercalated Ti3C2 contained two rows of water molecules.33 Interlayer water content in δ-K0.42V2O5·nH2O was evaluated via TGA by calculating the mass loss in the temperature range of 100 - 400 oC, which have been previously attributed to the loss of water molecules from the interlayer space in bilayered vanadium oxide.21-22, 34 The mass losses of 2.16% in this temperature range corresponds to an n value of 0.25 in the empirical formula of K0.42V2O5·nH2O.

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Figure 3. Electrochemical characterization of δ-K0.42V2O5·nH2O nanobelts. (a) First charge cycle and second discharge/charge cycle curves at a current rate of C/50. (b) Cycling performance at a current rate of C/15. (c) Rate performance at current rates shown in the figure. Electrochemical testing was carried out in the voltage window of 2.0 – 4.3 V.

To get an insight into the electrochemical processes occuring in K-ion cells containing δK0.42V2O5·nH2O electrodes, a slow rate (i.e. C/50) galvanostatic discharge/charge measurements were performed (Fig. 3a). To understand whether chemically preintercalated K+ ions are electrochemically active, the cycling was started on a charge cycle. The first charge capacity of 54 mAh·g-1 corresponds to ~0.38 electrons (K0.38V2O5), indicating that only a small fraction of the preintercalated ions (~0.04 K+ ions per δ-V2O5 unit cell) cannot be electrochemically extracted. Most likely, the K+ ions remaining in the structure of bilayered vanadium oxide serve as stabilizing species, similarly to the chemically preintercalated δ-NaxV2O5·nH2O cycled in Na-ion cells30 and ZnxV2O5·nH2O cycled in aqueous Zn-ion cells.35 We refer to the following discharge/charge cycle as the second cycle (Fig. 3a) to distinguish it from the first charge cycle. Cycling at low current rate of C/50, which corresponds to ~6 mA·g-1, revealed seven small plateau regions at 2.09, 2.62, 2.94, 3.40, 3.62, 3.73, and 3.91 V vs K/K+ in the second discharge curve. On the contrary, δNaxV2O5·nH2O electrodes in Na-ion cells exhibited a second discharge curve with a sloping shape, without any noticeable plateaus.30 Pleateaus in case of K-preintercalated bilayered vanadium oxide correspond to better defined crystallographic sites for K+ ion intercalation, which can be attributed to smaller interlayer spacing in δ-K0.42V2O5·nH2O and larger size of K+ ion, compared to Na-ion system.30 In the 2.0 – 4.3 V window versus K/K+, a very high reversible capacity of 268 mAh·g-1 was obtained in the second discharge cycle (Fig. 3a).

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The discrete intercalation potentials within the charge/discharge profiles are more clearly shown in the dQ/dV profiles (Figure S2 in the Supporting Information). The plateau potentials determined from the dQ/dV profile are in good agreement with those identified from the charge/discharge profile. The cycling life of the δ-K0.42V2O5·nH2O electrode has been investigated in a galvanostatic mode at C/15 rate (Fig. 3b). The cycling behavior was highly reversible. The maximum discharge capacity of 226 mAh·g-1 at this current rate was obtained in the third discharge cycle. After 50 cycles, 74% of the highest observed capacity was retained. This capacity retention is higher than that exhibited by K0.3MnO2 and K0.7Fe0.5Mn0.5O2 cathode materials (Table 1).16-17 While P2-K0.41CoO2 and P3-K0.67CoO218 seemingly demonstrate better capacity retention than δ-K0.42V2O5·nH2O (Table 1), it is important to note that the absolute value of the capacity after 30 electrochemical cycles is more than three times higher in case of K-preintercalated bilayered oxide (191 mAh·g-1) than in case of K-containing cobalt oxides (57 mAh·g-1). Figure 3c shows results from the rate capability experiment upon cycling at different C-rates: C/15, C/10, C/5 and 1C. Figure S3 in Supporting Information shows the charge/discharge curves for each c-rate used in this rate capability study. At each current rate the cell was tested for five cycles. The capacity slowly decreases at a constant current rate, but drops more significantly when the current rate is increased. The δK0.42V2O5·nH2O electrode showed the capacity retention of 58%, when current rate increased from C/15 to 1C. The electrode recovered its high capacity above 200 mAh·g -1 when current rate was decreased from 1C to C/15 (Fig. 3c).

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Table 1. Electrochemical performance of layered transition metal oxide cathodes in non -aquoues K-ion batteries. Material

Highest discharge

Capacity retention

Capacity retention at

capacity

highest C-rate

δ-K0.42V2O5·nH2O

226 mAh·g-1

74%

58%

(this work)

(C/15 or 20 mA·g-1, 2.0 –

(50 cycles, C/15,

(from C/15 to 1C,

4.3 V)

2.0 – 4.3 V)

2.0 – 4.3 V)

58%

11%

(C/10 or 27.9 mA·g , 1.5

(50 cycles, C/10,

(from C/10 to 5C,

– 4.0 V)

1.5 – 4.0 V)

1.5 – 4.0 V)

60 mAh·g-1

95%

K0.3MnO2 17

136 mAh·g-1 -1

P2-K0.41CoO2 P3-K0.67CoO2

18

K0.7Fe0.5Mn0.5O2 16

(11.8 mA·g , 2.0 – 3.9 V) -1

(30 cycles, 11.8 mA·g ,

(from 11.8 mA·g-1 to 472

2.0 – 3.9 V)

mA·g-1, 2.0 – 3.9 V)

178 mAh·g-1 (20 mA·g , 1.5 – 4.0 V) -1

67% -1

70%

19% -1

(45 cycles, 20 mA·g ,

(from 20 mA·g-1 to 1000

1.5 – 4.0 V)

mA·g-1, 1.5 – 4.0 V)

The discharge/charge voltage profiles (Fig. 3a) have a shape with both sloping and plateau regions, suggesting that the charge storage mechanism of δ-K0.42V2O5·nH2O in K-ion cells may have contributions from both diffusion-limited and non-diffusion limited processes. In order to evaluate which mechanism is dominating, we followed previously reported approach based on the analysis of voltammetric sweep rate dependence.36-38 Cyclic voltammograms of δ-K0.42V2O5·nH2O at different sweep rates are shown in Figure S3 (Supporting Information). The current response (i) at a fixed potential (V) can be described by Equation (1): 1

𝑖 (𝑉) = 𝑘1 𝑣 + 𝑘2 𝑣 2

(1)

where ν is the sweep rate, k1ν corresponds to the capacitive effects (non-diffusion controlled processes), and k2ν1/2 corresponds to the diffusion-controlled intercalation of K+ ions. The values of k1 and k2 were determined by processing data shown in Figure S4.

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Figure 4. Evaluation of the mechanism of charge storage by δ-KxV2O5·nH2O nanobelts. (a) Cyclic voltammetry curve at a sweep rate of 0.5 mV·s-1: the total current (solid line) is obtained experimentally; the non-diffusion controlled current (shaded region) is determined using Eq. 1. (b) Comparison of the total charge stored: the stored charge is divided into non-diffusion controlled contribution and diffusion-controlled intercalation contribution.

Figure 4a shows the cyclic voltammogram for the capacitive current (shaded region) as it compares with the total measured current obtained at a sweep rate of 0.5 mV·s -1. The shape of this curve matches well with the dQ/dV profile obtained from the galvanostatic cycling (Figure S2). Through this analysis, it was found that 71% of the total charge is stored through intercalation, and 29% of the total charge corresponds to the surface-based capacitive processes (Fig. 4b). Similarly to other oxide materials, the capacitive current can be attributed to pseudocapacitive contribution, and not to double-layer capacitance.36-38 In this work, we showed high electrochemical performance of δ-K0.42V2O5·0.25H2O) as a cathode material in non-aquous K-ion batteries. K-preintercalated δ-V2O5 with the interlayer spacing of 9.65 Å demonstrated high initial discharge capacities of 268 mAhg-1 and 226 mAhg-1 at C/50 and C/15, respectively. These capacities are significantly higher than those obtained for other materials, which have been previously studied as K-ion cathode materials in cells with organic electrolyte. The high capacities obtained from this phase are attributed to the large interlayer spacing and better defined sites for electrochemically cycled K+ ions, achieved through the chemical pre-intercalation

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synthesis approach. δ-K0.42V2O5·nH2O exhibited enhanced cycleability and rate capability compared to the previously reported layered oxide cathodes in non-aqueous K-ion batteries.

AUTHOR INFORMATION Corresponding Author Tel: 215-571-4612 Fax: 215-895-6760 Email: [email protected] Author Contributions E.P. developed the concept and designed the experiments. M.C. carried out experimental work. J.H. and M.T. carried out TEM experiments. All authors contributed to writing this manuscript.

ACKNOWLEDGEMENTS We acknowledge Drexel’s Centralized Research Facilities and Bryan Byles assistance with materials characterization. We also acknowledge Adam Blickley from the Materials Electrochemistry Group for his assistance with charge storage mechanism analysis. FUNDING SOURCE We would like to thank the National Science Foundation (award number: DMR-1609272) for funding. MLT and JLH gratefully acknowledge funding from the National Science Foundation’s Major Research Instrumentation Program under award #1429661. Supporting Information Available: The Supporting Information that accompanies this manuscript includes the experimental methods in this work, atomic adsorption spectroscopy data, dQ/dV data from galvanostatic cycling, charge/discharge curves from rate capability studies, and cyclic voltammograms at increasing sweep rates.

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the electrochemistry of NaxV2O5nH2O. Physical Chemistry Chemical Physics 2011, 13, 1804718054. Lee, S. H.; DiLeo, R. A.; Marschilok, A. C.; Takeuchi, K. J.; Takeuchi, E. S., Sol Gel Based Synthesis and Electrochemistry of Magnesium Vanadium Oxide: A Promising Cathode Material for Secondary Magnesium Ion Batteries. ECS Electrochemistry Letters 2014, 3 (8), A87-A90. Moretti, A.; Maroni, F.; Osada, I.; Nobili, F.; Passerini, S., V2O5 Aerogel as a Versatile Cathode Material for Lithium and Sodium Batteries. ChemElectroChem 2015, 2 (4), 529-537. Moretti, A.; Secchiaroli, M.; Buchholz, D.; Giuli, G.; Marassi, R.; Passerini, S., Exploring the Low Voltage Behavior of V2O5 Aerogel as Intercalation Host for Sodium Ion Battery. Journal of The Electrochemical Society 2015, 162 (14), A2723-A2728. Su, D.; Wang, G., Single-Crystalline Bilayered V2O5 Nanobelts for High-Capacity Sodium-Ion Batteries. ACS Nano 2013, 7 (12), 11218-11226. Tepavcevic, S.; Liu, Y.; Zhou, D.; Lai, B.; Maser, J.; Zuo, X.; Chan, H.; Král, P.; Johnson, C. S.; Stamenkovic, V.; Markovic, N. M.; Rajh, T., Nanostructured Layered Cathode for Rechargeable Mg-Ion Batteries. ACS Nano 2015, 9 (8), 8194-8205. Tepavcevic, S.; Xiong, H.; Stamenkovic, V. R.; Zuo, X.; Balasubramanian, M.; Prakapenka, V. B.; Johnson, C. S.; Rajh, T., Nanostructured Bilayered Vanadium Oxide Electrodes for Rechargeable Sodium-Ion Batteries. ACS Nano 2012, 6 (1), 530-538. Wei, Q.; Jiang, Z.; Tan, S.; Li, Q.; Huang, L.; Yan, M.; Zhou, L.; An, Q.; Mai, L., Lattice Breathing Inhibited Layered Vanadium Oxide Ultrathin Nanobelts for Enhanced Sodium Storage. ACS Applied Materials & Interfaces 2015, 7 (33), 18211-18217. Wei, Q.; Liu, J.; Feng, W.; Sheng, J.; Tian, X.; He, L.; An, Q.; Mai, L., Hydrated vanadium pentoxide with superior sodium storage capacity. Journal of Materials Chemistry A 2015, 3 (15), 8070-8075. Yin, J.; Pelliccione, C. J.; Lee, S. H.; Takeuchi, E. S.; Takeuchi, K. J.; Marschilok, A. C., Communication—Sol-Gel Synthesized Magnesium Vanadium Oxide, MgxV2O5 · nH2O: The Role of Structural Mg2+ on Battery Performance. Journal of The Electrochemical Society 2016, 163 (9), A1941-A1943. Petkov, V.; Trikalitis, P. N.; Bozin, E. S.; Billinge, S. J. L.; Vogt, T.; Kanatzidis, M. G., Structure of V2O5·nH2O Xerogel Solved by the Atomic Pair Distribution Function Technique. Journal of the American Chemical Society 2002, 124 (34), 10157-10162. Clites, M.; Byles, B. W.; Pomerantseva, E., Effect of aging and hydrothermal treatment on electrochemical performance of chemically pre-intercalated Na-V-O nanowires for Na-ion batteries. Journal of Materials Chemistry A 2016, 4 (20), 7754-7761. Clites, M.; Pomerantseva, E., Bilayered vanadium oxides by chemical pre-intercalation of alkali and alkali-earth ions as battery electrodes. Energy Storage Materials 2018, 11 (Supplement C), 30-37. Charles, D. S.; Feygenson, M.; Page, K.; Neuefeind, J.; Xu, W.; Teng, X., Structural water engaged disordered vanadium oxide nanosheets for high capacity aqueous potassium-ion storage. Nature Communications 2017, 8, 15520-15527. Ghidiu, M.; Halim, J.; Kota, S.; Bish, D.; Gogotsi, Y.; Barsoum, M. W., Ion-Exchange and Cation Solvation Reactions in Ti3C2 MXene. Chemistry of Materials 2016, 28 (10), 3507-3514. Kristoffersen, H. H.; Metiu, H., Structure of V2O5·nH2O Xerogels. The Journal of Physical Chemistry C 2016, 120 (7), 3986-3992. Kundu, D.; Adams, B. D.; Duffort, V.; Vajargah, S. H.; Nazar, L. F., A high-capacity and long-life aqueous rechargeable zinc battery using a metal oxide intercalation cathode. 2016, 1, 16119. Augustyn, V.; Simon, P.; Dunn, B., Pseudocapacitive oxide materials for high-rate electrochemical energy storage. Energy & Environmental Science 2014, 7 (5), 1597-1614. Brezesinski, T.; Wang, J.; Tolbert, S. H.; Dunn, B., Ordered mesoporous [alpha]-MoO3 with isooriented nanocrystalline walls for thin-film pseudocapacitors. Nat Mater 2010, 9 (2), 146-151.

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(38) Wang, J.; Polleux, J.; Lim, J.; Dunn, B., Pseudocapacitive Contributions to Electrochemical Energy Storage in TiO2 (Anatase) Nanoparticles. The Journal of Physical Chemistry C 2007, 111 (40), 14925-14931.

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Chemically Preintercalated Bilayered KxV2O5·nH2O Nanobelts as a HighPerforming Cathode Material for K-ion Batteries Mallory Clites, James L. Hart, Mitra L. Taheri, Ekaterina Pomerantseva* Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104, USA * Corresponding Author: Phone: 215-571-4612; E-mail: [email protected]

Graphical Abstract

Chemically preintercalated K+ ions form well-defined intercalation sites leading to facilitated diffusion and superior material performance in K-ion batteries.

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