Interlayer-Expanded V6O13·nH2O Architecture Constructed for anAdvanced Rechargeable Aqueous Zinc-Ion BatteryJianwei Lai,† Haihui Zhu,‡ Xiuping Zhu,‡ Harsha Koritala,§ and Ying Wang*,†

†Department of Mechanical & Industrial Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, United States‡Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, State College,Pennsylvania 16802, United States§Department of Chemical Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180-3522, United States

*S Supporting Information

ABSTRACT: Rechargeable aqueous zinc-ion batteries have been intensively studied as novel promising large-scale energystorage systems recently, owing to their advantages of high abundance, cost effectiveness, and high safety. However, thedevelopment of suitable cathode materials with superior performance is severely hampered by the sluggish kinetics of Zn2+ withdivalent charge in the host structure. In the present work, a highly reversible aqueous Zn2+ battery is demonstrated in aqueouselectrolyte using V6O13·nH2O hollow microflowers composed of ultrathin nanosheets. Benefiting from the synthetic merits of itsfavorable architecture and expanded interlamellar spacing that results from its structural water, the V6O13·nH2O cathodeexhibits outstanding electrochemical performances with a high reversible capacity of 395 mAh g−1 at 0.1 A g−1, superior ratecapability, and durable cycling stability with a capacity retention of 87% up to 1000 cycles. In addition, the reaction mechanismis significantly investigated in detail. This study demonstrates that the V6O13·nH2O nanostructure is emerging as a promisingcathode material for the high-potential rechargeable aqueous zinc-ion battery, and it may shed light on the water-initiatedeffective interlayer engineering strategy for the construction of high-performance cathode materials for grid-scale energy storagedevices.

KEYWORDS: aqueous zinc-ion battery, V6O13·nH2O nanosheets, interlayer engineering, energy storage,electrochemical characterization

1. INTRODUCTION

The increasing demands for environmentally friendly large-scale energy storage devices with high energy density, highsafety, and low cost have stimulated the development ofadvanced rechargeable batteries, due to the energy crisis andenvironmental pollution caused by traditional energy technol-ogy.1−6 Among various energy storage systems, lithium-ionbatteries (LIBs) have been widely used in portable electricaldevices and large-scale energy storage systems for severaldecades.7−10 However, the high cost, limited lithium resources,as well as safety concerns on flammable organic electrolytesgreatly hinder their further development.11−14 Recently,rechargeable multivalent ion (Zn2+, Mg2+, Ca2+, or Al3+)batteries have been attracting great interest, as they areconsidered as both high-potential and high-safety energystorage devices when compared to expensive and unsafe Li-ion

batteries.15−19 In this regard, rechargeable aqueous zinc-ionbatteries (ZIBs) are particularly attractive due to theirdistinctive advantages, such as low cost, low redox potentialof Zn2+/Zn (−0.76 V vs standard hydrogen electrode), highabundance, high stability in water, high theoretical capacity(820 mAh g−1), as well as high energy density (5851 mAhmL−1) in terms of zinc metals.20 The cost and environmentalpollution can be greatly reduced by replacing traditionalorganic electrolytes with aqueous-based electrolytes. Suchaqueous electrolytes can exhibit excellent ionic conductivity upto 1.0 S cm−1, superior to the organic electrolytes, and may

Received: November 30, 2018Accepted: February 4, 2019Published: February 4, 2019

Article

www.acsaem.orgCite This: ACS Appl. Energy Mater. 2019, 2, 1988−1996

© 2019 American Chemical Society 1988 DOI: 10.1021/acsaem.8b02054ACS Appl. Energy Mater. 2019, 2, 1988−1996

Dow

nloa

ded

via

LO

UIS

IAN

A S

TA

TE

UN

IV o

n M

arch

29,

201

9 at

19:

27:5

7 (U

TC

).

See

http

s://p

ubs.

acs.

org/

shar

ingg

uide

lines

for

opt

ions

on

how

to le

gitim

atel

y sh

are

publ

ishe

d ar

ticle

s.

effectively boost the high rate capabilities of aqueous ZIBs forgrid-scale renewable energy storage systems.21,22

Recently, intensive research in designing the advancedmaterials of ZIBs has been conducted for large-scale energystorage.15,16 Significant efforts and investigations havecontributed to the rapid development of reversible zinc-ionstorage electrode materials, such as manganese-based oxides,Prussian blue analogs, and vanadium-based materials. Man-ganese oxides, including α-, β-, γ-, and δ-MnO2 have beenstudied as intercalation hosts for zinc ions.23−26 Unfortunately,MnO2 suffers from poor rate performance and rapid capacityfading in aqueous electrolyte. For example, Kim et al. reporteda δ-MnO2 nanoflake cathode with a capacity of 252 mAh g−1,and only 44% of the highest capacity can be retained after 100cycles at 83 mAh g−1.26 Liu et al. reported a ZIB with improvedenergy density and cycling performance by employing MnSO4as an additive for the effective suppression of Mn2+ whencycling.27 Attempts on the Prussian blue analogs deliveredlimited capacity (∼50 mAh g−1) and suffered from O2evolution at a high operation voltage of ∼1.7 V.28

The advantages of vanadium-based materials for ZIBsinclude high specific capacity, high safety, abundance resource,and low cost. Layered vanadium oxide bronzes show enhancedcycling stability and rate capability by employing cations orstructural water as “pillars”.29−33 These pillars can effectivelyimprove the structural stability during charge−dischargeprocesses and provide a fast Zn2+ diffusion path. Mai et al.presented a V2O5·nH2O/graphene cathode with expandedinterlayer spacing (1.26 nm) for ZIBs, which delivered a highspecific capacity of ∼372 mAh g−1 at 0.3 A g−1 and retained71% of the maximum capacity after 900 cycles at 6 A g−1.34 Itshould be noted that the intercalation kinetics of Zn2+ in metaloxides are sluggish due to the intensive polarization and largesolvation sheath of Zn2+. Such good electrochemical perform-ance of the V2O5·nH2O/graphene cathode can be attributed tothe reduced effective charge in H2O-solvated Zn2+. Mai et al.also reported a sodium-stabilized vanadium oxide (Na0.33V2O5)nanowire cathode, showing a capacity of 367.1 mAh g−1 at 0.1A g−1, and 93% capacity retention could be achieved after 1000cycles at 1 A g−1.35 Nazar et al. developed a Zn0.25V2O5·nH2Opillared by Zn2+ and crystalline water, exhibiting a capacity of∼282 mAh g−1 at 0.3 A g−1 and 81% capacity retention after1000 cycles at 2.4 A g−1.29 Kim et al. constructed a LiV3O8cathode with a capacity of ∼256 mAh g−1 at 16 mAh g−1 and75% capacity retention after 65 cycles at 133 mA g−1.36 Lianget al. studied a variety of sodium vanadates, demonstrating thatNa5V12O32 displayed a high capacity of 281 mAh g−1 at 0.5 Ag−1 and maintained 71% capacity over 2000 cycles at 4 A g−1.37

They also investigated NH4V4O10 cathode with ∼400 mAh g−1

at 0.3 A g−1 with negligible capacity loss up to 1000 cycles at10 A g−1.38 Except for a few cathode candidates, most of thereported cathode materials of ZIBs are seriously hindered bylow specific capacity, poor rate capability, and limited cycle lifefor future practical applications.31−43 Therefore, there is anurgent need to develop a novel vanadium-based oxide cathodematerial with high capacity, high durability, and excellent rateperformance, which may play a crucial role in the breakthroughof achieving practical large-scale ZIBs.Owing to its high specific capacity, wide availability, and low

cost, the reaction mechanism and electrochemical performanceof mixed-valence V6O13 have been explored in organicelectrolytes for rechargeable alkali-ion-based batteries.44,45

For instance, Yu et al. reported the V6O13 nanotextiles

evaluated as a cathode for LIBs, which delivered a highcapacity of 326 mAh g−1 at 20 mA g−1 and a reversible capacityof 134 mAh g−1 at 500 mA g−1, showing superior electro-chemical performance and huge potential as high-energycathode materials.44 However, the electrochemical reactionmechanisms of V6O13·nH2O in aqueous electrolytes withmultivalent ions (e.g., Zn2+) remain still unclear. The crystalstructure of V6O13 is composed of corner and edge-sharingdistorted VO6 octahedra, which form alternating single anddouble vanadium oxide layers connected by shared cor-ners.46,47 There are three various vanadium environmentsnoted as V(1), V(2), and V(3) in the crystal structure ofV6O13. According to the valence bond sum calculations, theV(1) atoms in single layers and V(3) atoms in double layersare occupied by V4+, while the V(2) atoms in the double layersexhibit V5+. It should be noted that the crystal structure ofV6O13 is fully connected in three dimensions, even though it iseasy to understand the structure in respect of single and doublelayers. Based on the first principle calculations, previous studyreveals that the water insertion in interlayer-expanded V6O13·nH2O is ascribed to the strong intercalation of water moleculesand the lattice O ions between the single and double layers,forming hydroxyl radicals on each side. Such expandedinterlayer spacing by water intercalation exhibited good ratecapability and excellent cycling stability for lithium ionstorage.48 Moreover, V6O13 with the mixed valences of V4+

and V5+ shows high electronic conductivity at room temper-ature, which is favorable for the ultrafast charge and dischargeprocesses when applied in the battery system.44

Herein, a novel hydrated V6O13·nH2O (H-VO) material as apromising cathode has been developed for the construction ofa high-performance aqueous rechargeable zinc-ion battery byemploying an aqueous Zn(CF3SO3)2 electrolyte and zinc foilanode, as illustrated in Scheme 1. Benefiting from the

interlayer-expanded strategy through feasible water intercala-tion and the unique dandelion-like hollow architectureconstructed by ultrananosheets, the particular V6O13·nH2Ocathode can provide abundant accessible channels for fastkinetics of Zn2+ within the electrode, short transportation pathfor Zn2+ migration, and promote the rapid transfer of electrons.In addition, the in situ growth of a carbon layer on the

Scheme 1. Schematic Illustration of a RechargeableAqueous Zinc-Ion Battery Based on V6O13·nH2O Cathodeand Metallic Zn Foil Anode

ACS Applied Energy Materials Article

DOI: 10.1021/acsaem.8b02054ACS Appl. Energy Mater. 2019, 2, 1988−1996

1989

nanosheet surface of V6O13·nH2O can greatly improve itselectrical conductivity. Therefore, the novel zinc-ion batteryexhibits a high reversible capacity of 395 mAh g−1, outstandingrate capability up to 20 A g−1 with a decent capacity of 97 mAhg−1, and long-term cycling stability (87% capacity retentionafter 1000 cycles at 5A g−1).

2. RESULTS AND DISCUSSIONH-VO nanosheets were synthesized via a simple hydrothermalmethod, and the proposed synthesis mechanism is presented inFigure S1. Briefly, vanadium oxide powder first fully reactedwith oxalic acid in a molar ratio of 1:3 to form a homogeneousvanadyl oxalate hydrate (VOC2O4·nH2O) solution, and thecolor of the solution simultaneously turned dark blue. Then,the H-VO was successfully fabricated through the reactionbetween the generated VO2+ species and hydrogen peroxide.48

The morphology of as-synthesized H-VO was studied byscanning electron microscopy (SEM) and transmissionelectron microscopy (TEM). As shown in Figure 1a, the H-VO displays a hollow microflower structure with sizes of 2−7μm. The high-resolution SEM image further presents thatthese hollow structures are constructed by two-dimensionalultrathin nanosheets with widths of 50−200 nm, which areentangled together (Figure 1b). In addition, the presences ofV, O, and C in H-VO are confirmed in the energy dispersive X-ray spectrum (EDS) results, showing a carbon content of∼2.99 wt % (Figure S2). Furthermore, the TEM image inFigure 1c shows that the H-VO possesses the feature ofultrathin layers, consistent with the high-solution SEM image.From the high-resolution TEM image in Figure 1d, a singlenanosheet exhibits distinct lattice fringe areas apart from areaswithout fringe, and it could be attributed to the carbon layercovered on the surface of the nanosheet, which was formedduring the hydrothermal reaction. It should be noted that suchin situ coating of carbon on the surface of the H-VOnanosheets would be beneficial to enhance the interfacialconductivity of the sample when evaluated in aqueous ZIB.The interplanar spacing of 0.583 nm is well matched with the(200) crystalline facet of H-VO. Such a favorable hollow

structure of H-VO would be advantageous for the fast kineticsof Zn2+ intercalation and deintercalation. In order to determinethe crystalline water content of H-VO, thermogravimetricanalysis (TGA) was carried out in the temperature range of30−500 °C under the nitrogen atmosphere as displayed inFigure 1e. The total weight loss is attributed to the evaporationof physically absorbed water (30−100 °C) and lattice water(100−350 °C). Based on a weight loss of 3.5 wt %, it can beestimated that there is approximately 1.1 structural water perV6O13 unit. The crystal structure of the as-prepared H-VO wascharacterized by X-ray diffraction (XRD) as presented inFigure 1f. The XRD pattern of H-VO can be well indexed tomonoclinic V6O13 with the space group of C2/m (unit cellparameters of a = 11.9 Å, b = 3.671 Å, c = 10.122 Å, and β =100.87°; JCPDS no. 01-072-1278). Based on Bragg’s equation(d = 0.5nλ/sin θ), the diffraction peak located at lower anglesindicates a larger interlayer distance. The diffraction peak of H-VO at 2θ = 7.8° from facet (001) is attributed to the waterinsertion with expanded interlayer, and the correspondinginterlayer spacing is calculated to be 1.13 nm, which is largerthan the 0.99 nm value at 2θ = 8.9° from the standard PDFcard. The larger lattice spacing of 1.13 nm may provide morediffusion channels for the migration of Zn2+, and the expandedinterlamellar spacing is comparable to the V2O5·nH2Ocathode,34 which is more electrochemically superior topreviously reported VS2,

40 zinc pyrovanadate,41 andNa2V6O16·3H2O,

49 etc. In addition, the expanded interlayerof H-VO at (001) plane is in good agreement with a previousreport.48 After the sample was annealed at 400 °C under Arflow for 2 h, the dehydrated V6O13 (VO) was obtained. Thediffraction peak attributed to the interlayer spacing of 1.13 nmis divided into two peaks at 9.1° and 12.4°, indicating thedecreased lattice space. The peak position change suggests thatthe interlayer expansion is mainly attributed to the waterintercalation.The electrochemical performances of H-VO and VO as

cathode materials were evaluated using 3 M Zn(CF3SO3)2aqueous solution as the electrolyte and metallic zinc foil as theanode assembled in 2032 coin-type cells. The initial three

Figure 1. (a, b) SEM images of the as-synthesized H-VO. (c, d) TEM and HRTEM images of H-VO. (e) Corresponding TGA result of H-VO. (f)Powder XRD patterns of H-VO and VO.

ACS Applied Energy Materials Article

DOI: 10.1021/acsaem.8b02054ACS Appl. Energy Mater. 2019, 2, 1988−1996

1990

cyclic voltammogram (CV) curves of H-VO in the voltagerange of 0.2−1.4 V (vs Zn/Zn2+) at a scan rate of 0.1 mV s−1

are displayed in Figure 2a. During the first cathodic scan, thereare five cathode perks located at 1.07, 0.90, 0.76, 0.63, and 0.44V, and the small peak at 1.07 V may be caused by someirreversible reactions occurring in the initial cycle, which willbe discussed later. The remaining four cathode peaks areslightly different from the peak positions at 0.91, 0.77, 0.65,and 0.46 V in the subsequent cycles of the reduction scan. Thereduction peak shifting could be ascribed to the activation ofH-VO during the discharge process. From the initial threeanodic scans, they exhibit completely overlapping peaks at0.52, 0.78, 0.95, and 1.10 V. In addition, the CV curves in theinitial three cycles show good similarity and repeatability withthe exclusion of a small deviation in the first cathodic CVcurves, suggesting the good structural stability and excellentreversibility of the Zn/H-VO battery. Over four distinct pairsof reduction/oxidation peaks are clearly observed, implying afour-step reaction mechanism corresponding to the Zn2+

insertion/extraction. Such CV curves of the H-VO are quitedifferent from many reported vanadium-based cathodes foraqueous ZIBs with only two pairs of redox peaks, such asV2O5·nH2O,34 Zn2V2O7,

39 and H2V3O8.43 For the VO

electrode, five reduction peaks of the first cathodic scan at1.10, 0.78, 0.62, 0.56, and 0.50 V are noticeably different fromthree cathode peaks at 0.85, 0.65, and 0.49 V in the followingtwo cycles, and three oxidation peaks at 0.59, 0.76, and 1.00 Vcan be observed in these initial three cycles (Figure S3a).Furthermore, the selected second CV curves comparison at 0.1mV s−1 between H-VO and VO show significant differences

based on the number of peaks and corresponding peakpositions (Figure S3b).Based on such highly reversible CV curves of these two

cathodes, their cycling performances are further investigated byemploying the galvanostatic charge−discharge method. Aspresented in Figure 2b, H-VO delivers a remarkable and stableinitial discharge capacity of 383 mAh g−1 at a current density of0.1 A g−1 and corresponding charge capacity of 392 mAh g−1

with a high initial Coulombic efficiency of 97.6%. Thefavorable Coulombic efficiency implies good reversibility ofZn2+ ingress/egress. In the following 15 cycles, the specificcapacity gradually increases, which is ascribed to the activationprocess of the H-VO electrode, and then stabilizes in the rangeof 390−395 mAh g−1 with an overall Coulombic efficiencyover 99.5%. It should be noted that the H-VO can maintain thehigh and stable capacities of ∼390 mAh g−1 when applied alow current density of 0.1 A g−1 during the whole 50 cycles,which outperforms many previous reported cathodes thatsuffered from severe capacity fade under the same currentdensity in the initial few cycles,35,41,43 indicating the highreversibility and stability nature of the H-VO electrode. Basedon the high discharge capacity of 390 mAh g−1, the number ofelectrons transfer during electrochemical reaction is calculatedto be ∼7.7, corresponding to 3.85 Zn2+ migration. Notably, thevanadium ions in H-VO can be reduced to V3+ with eightelectrons transferring, so its theoretical capacity can becalculated to be 401.6 mAh g−1. As discussed above, the CVcurves of H-VO electrode show multistep Zn2+ intercalation/deintercalation behaviors (Figure 2a). According to Figure 2c,H-VO delivers a capacity of 104.2 mAh g−1 at 0.3 A g−1 in the

Figure 2. Electrochemical performances of the H-VO and VO electrodes within the potential range of 0.2−1.4 V vs Zn/Zn2+. (a) CV curves of H-VO electrode at a scan rate of 0.1 mV s−1 in the initial three cycles. (b) Cycling performance and the corresponding Coulombic efficiency of H-VOand VO electrodes at 0.1 A g−1. (c) Galvanostatic charge−discharge profiles of H-VO electrode under various current densities. (d) Ratecapabilities of H-VO and VO electrodes. (e) Long-term cycling properties of H-VO and VO electrodes at 5 A g−1.

ACS Applied Energy Materials Article

DOI: 10.1021/acsaem.8b02054ACS Appl. Energy Mater. 2019, 2, 1988−1996

1991

voltage range of 0.8−1.4 V during the discharge process,corresponding to one Zn2+ (two electrons transfer) insertion.Subsequently, H-VO displays a capacity of 278.8 mAh g−1 in0.2−0.8 V, corresponding to ∼2.77 Zn2+ (5.54 electronstransfer) intercalation. Notably, there are two V5+ and four V4+

ions for each V6O13·1.1H2O. Therefore, we speculate that twoV5+ ions are reduced to V4+ (two electrons transfer) during0.8−1.4 V, and 5.54 V4+ are further reduced to V3+ (5.54electrons transfer) during 0.2−0.8 V. Similarly, H-VO presentsa capacity of 282.8 mAh g−1 between 0.2 and 0.93 V duringcharge process, corresponding to the oxidation of 5.63 V3+ toV4+. In the voltage range of 0.93−1.4 V, a capacity of 104.3mAh g−1 is shown, relevant to the oxidation of 2 V4+ to V5+.Thus, 5.63 V3+ are oxidized to V4+ during 0.2−0.93 V, and 2V4+ are oxidized to V5+ during 0.93−1.4 V. It should be notedthat H-VO exhibits a discharge capacity of 211 mAh g−1 during0.6−1.4 V, and a discharge capacity of 175.6 mAh g−1 during0.2−0.6 V at 0.3 A g−1, which shows ∼54.6% capacitycontribution in 0.6−1.4 V, and 45.4% (<50%) capacity isobtained in 0.2−0.6 V (Figure 2c). These results are similar tothe previously reported VOG sample having a capacitycontribution of 59% in 0.6−1.6 V and 41% in 0.2−0.6 V.34

However, for the dehydrate VO electrode, although it providesa higher first discharge capacity of 434 mAh g−1 at the samecurrent density of 0.1 A g−1 due to the elimination of structuralwater, VO displays a significantly reduced initial chargecapacity of 404 mAh g−1, indicating a poorly reversibleprocess. The capacity of VO dramatically decreases to 323mAh g−1 with further charge−discharge after 20 cycles andthen gradually increases to 370 mAh g−1, which may beascribed to the activation and stabilization of VO by waterintercalation and irreversible Zn2+ as pillars trapped in theinterlayer during cycling. Such relatively unstable cyclingperformance of VO largely hinders its practicability for grid-scale energy storage devices. Moreover, the comparison oftypical galvanostatic charge−discharge curves at the selectedcycle with a current density of 0.1 A g−1 between H-VO andVO manifests a reversibly high capacity of ∼395 mAh g−1

achieved for H-VO, while an unfavorable capacity of ∼323mAh g−1 is presented for VO (Figure S4).Furthermore, the excellent rate performance is of signifi-

cance for large-scale aqueous ZIBs to address the ever-growingdemands for practical energy storage systems. However, manyprevious reported cathodes for aqueous ZIBs displayundesirable rate behaviors, which are largely hampered bythe high polarization of Zn2+ with divalent charge in the hoststructure. In the presented H-VO electrode, it showsexceptionally satisfactory capacity even under severe galvano-static tests. The comparison of rate capabilities andcorresponding charge−discharge curves with a voltage rangeof 0.2−1.4 V starting from a current density of 0.3 to 20 A g−1

are illustrated in Figure 2c, d. Extraordinarily, via increasing thecurrent density from 0.3 up to 10 A g−1, the H-VO electrodeexhibits impressive capacities of 386 and 270 mAh g−1,respectively, with only ∼30% capacity loss. Even when thehighest current rate of 20 A g−1 is applied, a considerablecapacity of 97 mAh g−1 still can be obtained. Thecorresponding discharge or charge time is as short as ∼17.5s, demonstrating ultrafast charge transfer kinetics of the H-VOelectrode and superior rate capability. More importantly, as thecurrent density abruptly returns back to 0.8 A g−1, thereversible discharge capacity restores to 364 mAh g−1, which iscomparable to the specific capacity of 372 mAh g−1 under the

same current density in previous cycles, manifesting a strongendurance for rapid Zn2+ insertion/extraction and goodstructural reversibility. In contrast, the rate behavior of VOapplied with identical increasing current rates is severelyinferior, displaying an initial discharge capacity of 398 mAh g−1

at 0.3 A g−1, drastically declining to 61 mAh g−1 at 10 A g−1

with corresponding capacity fading over 80%, and decliningfurther to 16 mAh g−1 at 20 A g−1 (Figures 2d and S5). Such asignificant rate performance difference implies that the waterintercalation in H-VO plays a vital role in remarkablyimproving the rate capability. The superior capacities of H-VO under a series of current densities probably benefit fromthe enlarged interlamellar spacing (1.13 nm), which providesabundant accessible channels and simultaneously facilitatesrapid transportation of electrolyte ions through the electrode.In addition, the unique hollow-microflower structure of H-VOassembled from ultrathin nanosheets can not only effectivelyshorten the diffusion path lengths of Zn2+ ingress/egress butalso possess the volume flexibility to buffer the volume changeduring cycling. Furthermore, H-VO with mixed valence statesof V4+ and V5+ shows metallic characteristics, which canincrease the electrical conductivity of the electrode and enablethe fast electron transfer. Therefore, the excellent rateperformance of the Zn/H-VO battery could not only realizehigh capacity but also achieve ultrafast charge/dischargeprocesses.The long-term cycling properties at a high current density of

5 A g−1 are further examined to reveal the superiority of H-VOas opposed to VO in Figures 2e and S6. H-VO delivers aninitial desirable discharge capacity of 300 mAh g−1, with acorresponding high Coulombic efficiency over 95%. Benefitingfrom the water incorporation synthetic strategy, H-VO canimpressively maintain a high capacity of 262 mAh g−1 evenafter 1000 cycles with a capacity retention of 87% under thecritical high current density, and the correlative highCoulombic efficiencies over 99% are achieved except for thefirst cycle. Furthermore, a marked distinction in cycle lifebetween H-VO and VO is clearly observed, showing that theaverage capacity of H-VO is 292 mAh g−1, while VO onlydisplays a 151 mAh g−1 average capacity. The significant gap isas large as 141 mAh g−1, almost comparable to the averagecapacity of VO. Such magnificent cycling performance shouldbe attributed to the unique architecture of the H-VO cathode,where the expanded interlayer at plane (001) through thesuccessful introduction of structural water facilitates Zn2+

diffusion. Based on the synthetic merits of H-VO, it exhibitsmore durable cycling stability without adding any vanadium solin the electrolyte or graphene support as compared to theV2O5·nH2O/graphene cathode for aqueous ZIB.34

The electrochemical kinetics of the H-VO electrode arefurther investigated through CV measurements and thegalvanostatic intermittent titration technique (GITT).50 FigureS7a presents the CV profiles of the H-VO electrode at the scanrate of 0.1 and 0.2 mV s−1. There are four pairs of redox peaksat 0.1 mV s−1, as discussed above. When the scan rate increasesto 0.2 mV s−1, the CV curves show two pairs of distinct redoxpeaks, and the other two small redox peaks become broad dueto the larger scan rate. The capacitive response can becalculated by employing the equation of i = avb, where both aand b are adjustable parameters. This equation can berearranged to log i = b log v + log a. The coefficient b valueof ∼0.5 represents a diffusion control process, while a b valueof ∼1 indicates a capacitive process. As shown in Figure S7b,

ACS Applied Energy Materials Article

DOI: 10.1021/acsaem.8b02054ACS Appl. Energy Mater. 2019, 2, 1988−1996

1992

the relationships between log i and log v are plotted, and the bvalues of the four peaks are determined to be 0.92, 1.20, 1.06,and 0.83, which suggests that the charge storage behaviors aremainly controlled by a pseudocapacitive process. Furthermore,the ratio of capacitive contribution can be decided by theequation of i = k1v+ k2v

1/2 at a specific potential, where k1v isthe capacitive effect and k2v

1/2 represents the diffusion controlprocess. At the scan rate of 0.1 mV s−1, ∼62.9% capacity isattributed to the capacitive process, which implies that thecapacitive contribution is dominant. This 62.9% capacitivecontribution of the H-VO electrode is comparable to the61.4% contribution in the previously reported VS2 at the samescan rate.40 Moreover, the Zn2+ diffusion coefficient (DZn

2+)can be calculated based on the following equation:

DL E

E4

Zn

2s

t

2

2

πτ=

ΔΔ

+ikjjjjj

y{zzzzz (1)

where L is the diffusion length (cm) of Zn2+ which is equal tothe thickness of the electrode, τ is the relaxation time (s), ΔEs

is the steady-state potential change (V) by the current pulseduring a single-step GITT experiment, and ΔEt represents thepotential change (V) during the constant current pulse of asingle-step GITT experiment after eliminating the iR drop. Asdisplayed in Figure 3, the overall calculated diffusioncoefficients of Zn2+ (DZn

2+) during the entire discharge processin the H-VO cathode are higher than the DZn

2+ values in theVO cathode, demonstrating the faster kinetics of the as-synthesized H-VO electrode. The average DZn

2+ values of H-VO in the discharge process are 1.3 times larger than those ofVO. The higher ionic diffusivity of H-VO is ascribed to itsenlarged lattice spacing (1.13 nm in (001)), which can notonly provide a more effective diffusion path for Zn2+ migrationbut also effectively reduce the electrostatic interactions in thecrystal structure of H-VO during the electrochemicalintercalation process. It is worth noting that the Zn2+ diffusioncoefficients of H-VO in the graph under different insertionstates keep stable except for the fully intercalation state,indicating the excellent stability of its structure during theingress of Zn2+. The lower DZn

2+ value when approaching 0.2 V

Figure 3. GITT curves and calculated diffusion coefficient of Zn2+ vs various Zn2+ insertion/extraction states of H-VO and VO.

Figure 4. Ex situ XRD patterns of H-VO collected at different discharge/charge states and corresponding discharge and charge curves under thecurrent density of 0.2 A g−1.

ACS Applied Energy Materials Article

DOI: 10.1021/acsaem.8b02054ACS Appl. Energy Mater. 2019, 2, 1988−1996

1993

results from the increased electrostatic repulsion in the H-VOelectrode upon Zn2+ insertion. The above GITT analysis resultcan convincingly support the superior rate capability andremarkable cycling stability of the H-VO electrode instead ofVO.The outstanding Zn2+ intercalation/deintercalation perform-

ance of the H-VO electrode is further reflected in comparisonto previously reported vanadium-based cathodes for aqueousZIBs. As presented in Table S1, our developed H-VO electrodedemonstrates the highest specific capacity of 395 mAh g−1 andthe superlative retained capacity of 262 mAh g−1 with animpressive capacity retention over 87%, which outperformsthese published cathode materials, showing its high potentialfor practical energy storage devices.Ex situ X-ray diffraction (XRD) is performed to investigate

the structural evolution and corresponding reaction mecha-nisms of the H-VO cathode upon selected charge anddischarge states during the first two cycles. As displayed inFigure 4a−c, the reflection pattern of the H-VO electrode atthe initial state can match well with the as-synthesized V6O13·nH2O, with the exclusion of the peak ascribed to the PTFEbinder at 18°. When first discharged to 0.2 V at the currentdensity of 0.2 A g−1, the characteristic diffraction peak locatedat 7.8° representing the (001) plane with an interlayer spacingof 1.13 nm shifts to a lower angle at 6.6° with a calculated d-spacing of 1.33 nm, and a series of new diffraction peaksappears at the 2θ angles of 13.1°, 17.2°, 19.6°, and 30.6°,indicating that a phase transition occurs. Notably, theinterlamellar spacing expands from 1.13 to 1.33 nm upon in-depth discharge process, which is ascribed to the insertion ofZn2+ shielded by water molecules with reduced effective chargeinto the interlayer of the H-VO. Such a favorable watershielding layer provides an electrostatic shielding effect forZn2+, which can lower the activation energy for its ingress intothe host structure, thus contributing to effectively facilitatingthe fast Zn2+ diffusion. Furthermore, the peak intensity of the(001) facet shows a dramatic increase, which manifests thechange of crystallinity initiated by the intercalation of Zn2+

during the initial discharge process. At the following fullycharged state, a slight contraction from 1.33 to 1.07 nm isobserved due to the deintercalation of Zn2+, and the new peaksinitially formed during the previous first discharge processremain with slight shifts to 12.3°, 16.9°, 20.9°, and 30.1°,respectively. As shown in Figure S8, these remaining four newpeaks can be well indexed to the pure phase ofZn3V2O7(OH)2·2H2O (JCPDS no. 01-087-0417, spacegroup: P3̅m1), revealing an irreversible phase transition fromV6O13·nH2O to ZnxV2O7(OH)2·2H2O (x > 3) during theprevious discharge process, which is in accordance with theirreversible reduction peak at 1.07 V from the first cathodic CV

curve. It should be noted that the V−O−V pillars support thezinc oxide layers to form the layered Zn3V2O7(OH)2·2H2Owith a porous crystal framework, which is advantageous forZn2+ migration in its lattice. Subsequently, when discharged asecond time to 0.8 V, the major peak of V6O13·nH2Oattributed to the (001) plane repeatedly shifts to the samelower angle of 6.6°, while the peak of Zn3V2O7(OH)2·2H2O at12.3° slightly shifts to 13.1° with the corresponding latticespacing only decreased by 6%. It is speculated that theexpanded interlayer of V6O13·nH2O is ascribed to the insertionof water-shielding Zn2+ with reduced effective charge asmentioned above, while the interlayer contraction ofZn3V2O7(OH)2·2H2O is induced by the intense electrostaticinteraction between bare Zn2+ and vanadium−oxygen layers, assupported by a related reported V2O5·nH2O/graphenecathode34 and Na2V6O16·1.63H2O cathode51 in aqueousZIBs, respectively. Reversibly, the fully charged electrode at1.4 V at the second cycle showed the same XRD pattern as theelectrode after the first cycle, suggesting the reversibletransformation between ZnaV6O13·mH2O/ZnxV2O7(OH)2·2H2O and ZnbV6O13·mH2O/ZnyV2O7(OH)2·2H2O (a > b; x> y) after the first cycle. As revealed by the ex situ XRD, theelectrochemical reaction is different between the first dischargeprocess and subsequent discharge process. Notably, thisdifference can be observed from a small, irreversible reductionpeak at 1.07 V in the initial cathodic CV curve. We speculatethat this irreversible reduction peak is so small that thedifference in the charge−discharge profiles may not beobviously observed. In addition, the structural change of H-VO after a series of cycles at fully charged states is furtherinvestigated. As displayed in Figure S9, the major peak ofV6O13·nH2O at 8.3° negatively shifts to 6.5°, while the majorpeak of Zn3V2O7(OH)2·2H2O at 12.4° positively shifts to13.1° after the first cycle, indicating a specific amount of watermolecules and Zn2+ from electrolyte may be trapped in thecrystal structure, which accounts for the structural change andgradual capacity decay. After 200 cycles, the XRD patterns ofH-VO show no obvious difference, implying the good structurepreservation with high durability, consistent with its desirablelong-term cycling stability.In consequence, the schematic illustration of Zn2+ storage

mechanism is shown in Figure 5. The correspondingelectrochemical mechanism of the Zn/H-VO rechargeablebattery is displayed as follows.Anode:

Zn Zn 2e2↔ ++ −

Cathode:

Figure 5. Schematic illustration of Zn2+ and water co-intercalation into H-VO electrode during the initial discharge process and reversible Zn2+

extraction/insertion in the subsequent process.

ACS Applied Energy Materials Article

DOI: 10.1021/acsaem.8b02054ACS Appl. Energy Mater. 2019, 2, 1988−1996

1994

n a x a x m n

m

V O H O ( )Zn 2( )e ( 2)H O

Zn V O H O Zn V O (OH) 2H O

(first discharge process)a x

6 13 22

2

6 13 2 2 7 2 2

· + + + + + − +

↔ · + ·

+ −

m m

a x b y

a x b y

Zn V O H O Zn V O (OH) 2H O Zn V O H O

Zn V O (OH) 2H O ( )Zn

2( )e (subsequent cycles)

a x b

y

6 13 2 2 7 2 2 6 13 2

2 7 2 22

· + · ↔ ·

+ · + + − −

+ + − −

+

−

3. CONCLUSIONSIn summary, we designed and developed a novel aqueousrechargeable ZIB based on the H-VO cathode with a highpotential for large-scale and cost-effective production. Owingto numerous transportation channels offered by large inter-lamellar spacing (1.13 nm) and short diffusion pathwaysbenefiting from the unique hollow microflower structure, theH-VO cathode delivers a high specific capacity of 395 mAh g−1

at 0.1 A g−1, excellent rate capability with a considerablecapacity of 97 mAh g−1 at an extremely high current density of20 A g−1, and excellent cycle durability with a high capacityretention of 87% up to 1000 cycles at 5 A g−1. Our resultsdemonstrate that the V6O13·nH2O with greatly enhancedelectrochemical performances possesses high potential forlarge-scale practical ZIB application and may shed light on thewater-initiated effective interlayer engineering strategy for theconstruction of high-performance promising cathode materialsin a zinc-ion storage system.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsaem.8b02054.

Synthesis method, materials characterization, electro-chemical characterization, synthetic scheme, EDSspectrum of H-VO, cyclic voltammograms of VO at0.1 mV s−1, CV curves comparison of H-VO and VO at0.1 mV s−1, galvanostatic discharge/charge curves of H-VO and VO at 0.1 A g−1, discharge/charge profiles ofVO at different current densities, discharge/chargecurves of H-VO and VO at 5 A g−1, CV profiles of H-VO at various scan rates and corresponding plots of log iversus log v, XRD pattern of H-VO at the fully chargedstate after the first cycle, XRD patterns of H-VO aftervarious cycles at fully charged states, and comparison ofour present work with previously reported vanadium-based cathodes (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: ywang@lsu.edu.ORCIDXiuping Zhu: 0000-0002-2767-4211Ying Wang: 0000-0003-4650-9446NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe sincerely acknowledge the Economic DevelopmentAssistantship (EDA) from the graduate school of LouisianaState University and the National Science Foundation (NSF)

(1560305) for financial support. We gratefully thank theShared Instrumentation Facilities (SIF) at Louisiana StateUniversity for material characterizations. We also thank Prof.Q. L. Wu and Dr. X. X. Sun at the School of RenewableNatural Resources of Louisiana State University for the use ofthe thermal gravimetric analyzer.

■ REFERENCES(1) Dunn, B.; Kamath, H.; Tarascon, J.-M. Electrical Energy Storagefor the Grid: A Battery of Choices. Science 2011, 334, 928.(2) Goodenough, J.; Park, K. The Li-Ion Rechargeable Battery: APerspective. J. Am. Chem. Soc. 2013, 135, 1167−1176.(3) Yang, Z.; Zhang, J.; Kintner-Meyer, M.; Lu, X.; Choi, D.;Lemmon, J.; Liu, J. Electrochemical energy storage for green grid.Chem. Rev. 2011, 111, 3577−3613.(4) Kang, B.; Ceder, G. Battery Materials for Ultrafast Charging andDischarging. Nature 2009, 458, 190.(5) Thackeray, M. M.; Wolverton, C.; Isaacs, E. D. Electrical EnergyStorage for TransportationApproaching the Limits of, and Goingbeyond, Lithium-Ion Batteries. Energy Environ. Sci. 2012, 5, 7854.(6) Liu, J.; Zhang, J.; Yang, Z.; Lemmon, J.; Imhoff, C.; Graff, G.; Li,L.; Hu, J.; Wang, C.; Xiao, J.; Xia, G.; Viswanathan, V.; Baskaran, S.;Sprenkle, V.; Li, X.; Shao, Y.; Schwenzer, B. Materials Science andMaterials Chemistry for Large Scale Electrochemical Energy Storage:From Transportation to Electrical Grid. Adv. Funct. Mater. 2013, 23,929.(7) Whittingham, M. Lithium batteries and cathode materials. Chem.Rev. 2004, 104, 4271−4302.(8) Etacheri, V.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D.Challenges in the Development of Advanced Li-ion Batteries: AReview. Energy Environ. Sci. 2011, 4, 3243−3262.(9) Kim, H.; Hong, J.; Park, K.; Kim, H.; Kim, S.; Kang, K. AqueousRechargeable Li and Na Ion Batteries. Chem. Rev. 2014, 114, 11788−11827.(10) Sun, Y.; Chen, Z.; Noh, H.; Lee, D.; Jung, H.; Ren, Y.; Wang,S.; Yoon, C.; Myung, S.; Amine, K. Nanostructured high-energycathode materials for advanced lithium batteries. Nat. Mater. 2012,11, 942.(11) Tarascon, J.; Armand, M. Issues and challenges facingrechargeable lithium batteries. Materials for Sustainable Energy.2010, 171−179.(12) Balakrishnan, P.; Ramesh, R.; Prem Kumar, T. Safetymechanisms in lithium-ion batteries. J. Power Sources 2006, 155,401−414.(13) Xu, K. Electrolytes and interphases in Li-ion batteries andbeyond. Chem. Rev. 2014, 114, 11503−11618.(14) Wang, Q.; Ping, P.; Zhao, X.; Chu, G.; Sun, J.; Chen, C.Thermal runaway caused fire and explosion of lithium ion battery. J.Power Sources 2012, 208, 210−224.(15) Song, M.; Tan, H.; Chao, D.; Fan, H. Recent Advances in Zn-Ion Batteries. Adv. Funct. Mater. 2018, 28, 1802564.(16) Fang, G.; Zhou, J.; Pan, A.; Liang, S. Recent Advances inAqueous Zinc-Ion Batteries. ACS. Energy Lett. 2018, 3, 2480−2501.(17) Yoo, H.; Shterenberg, I.; Gofer, Y.; Gershinsky, G.; Pour, N.;Aurbach, D. Mg rechargeable batteries: an on-going challenge. EnergyEnviron. Sci. 2013, 6, 2265−2279.(18) Ponrouch, A.; Frontera, C.; Barde, F.; Palacín, M. Towards acalcium-based rechargeable battery. Nat. Mater. 2016, 15, 169.(19) Lin, M.; Gong, M.; Lu, B.; Wu, Y.; Wang, D.; Guan, M.; Angell,M.; Chen, C.; Yang, J.; Hwang, B.; Dai, H. An ultrafast rechargeablealuminium-ion battery. Nature 2015, 520, 324.(20) Xu, C.; Li, B.; Du, H.; Kang, F. Energetic zinc ion chemistry:the rechargeable zinc ion battery. Angew. Chem. 2012, 124, 957−959.(21) Sun, W.; Wang, F.; Hou, S.; Yang, C.; Fan, X.; Ma, Z.; Gao, T.;Han, F.; Hu, R.; Zhu, M.; Wang, C. Zn/MnO2 Battery ChemistryWith H+ and Zn2+ Coinsertion. J. Am. Chem. Soc. 2017, 139, 9775.

ACS Applied Energy Materials Article

DOI: 10.1021/acsaem.8b02054ACS Appl. Energy Mater. 2019, 2, 1988−1996

1995

(22) Ding, J.; Du, Z.; Gu, L.; Li, B.; Wang, L.; Wang, S.; Gong, Y.;Yang, S. Ultrafast Zn2+ Intercalation and Deintercalation in VanadiumDioxide. Adv. Mater. 2018, 30, 1800762.(23) Lee, B.; Lee, H.; Kim, H.; Chung, K.; Cho, B.; Oh, S.Elucidating the intercalation mechanism of zinc ions into α-MnO2 forrechargeable zinc batteries. Chem. Commun. 2015, 51, 9265−9268.(24) Zhang, N.; Cheng, F.; Liu, J.; Wang, L.; Long, X.; Liu, X.; Li, F.;Chen, J. Rechargeable aqueous zinc-manganese dioxide batteries withhigh energy and power densities. Nat. Commun. 2017, 8, 405.(25) Alfaruqi, M.; Mathew, V.; Gim, J.; Kim, S.; Song, J.; Baboo, J.;Choi, S.; Kim, J. Electrochemically induced structural transformationin a γ-MnO2 cathode of a high capacity zinc-ion battery system.Chem. Mater. 2015, 27, 3609−3620.(26) Alfaruqi, M.; Gim, J.; Kim, S.; Song, J.; Pham, D.; Jo, J.; Xiu, L.;Mathew, V.; Kim, J. A layered δ-MnO2 nanoflake cathode with highzinc-storage capacities for eco-friendly battery applications. Electro-chem. Commun. 2015, 60, 121−125.(27) Pan, H.; Shao, Y.; Yan, P.; Cheng, Y.; Han, K.; Nie, Z.; Wang,C.; Yang, J.; Li, X.; Bhattacharya, P.; Mueller, K.; Liu, J. Reversibleaqueous zinc/manganese oxide energy storage from conversionreactions. Nat. Energy. 2016, 1, 16039.(28) Liu, Z.; Pulletikurthi, G.; Endres, F. A Prussian Blue/ZincSecondary Battery with a Bio-Ionic Liquid−Water Mixture asElectrolyte. ACS Appl. Mater. Interfaces 2016, 8, 12158−12164.(29) Kundu, D.; Adams, B.; Duffort, V.; Vajargah, S.; Nazar, L. AHigh-Capacity and Long-Life Aqueous Rechargeable Zinc Batteryusing a Metal Oxide Intercalation Cathode. Nat. Energy. 2016, 1,16119.(30) Tang, H.; Peng, Z.; Wu, L.; Xiong, F.; Pei, V.; An, Q.; Mai, L.Vanadium-Based Cathode Materials for Rechargeable MultivalentBatteries: Challenges and Opportunities. Electrochemical EnergyReviews. 2018, 1, 169.(31) Xie, Z.; Lai, J.; Zhu, X.; Wang, Y. Green Synthesis of VanadateNanobelts at Room Temperature for Superior Aqueous RechargeableZinc-Ion Batteries. ACS Appl. Energy Mater. 2018, 1, 6401−6408.(32) Yang, Y.; Tang, Y.; Fang, G.; Shan, L.; Guo, J.; Zhang, W.;Wang, C.; Wang, L.; Zhou, J.; Liang, S. Li+ intercalated V2O5· nH2Owith enlarged layer spacing and fast ion diffusion as an aqueous zinc-ion battery cathode. Energy Environ. Sci. 2018, 11, 3157−3162.(33) Ming, F.; Liang, H.; Lei, Y.; Kandambeth, S.; Eddaoudi, M.;Alshareef, H. Layered MgxV2O5·nH2O as Cathode Material for High-Performance Aqueous Zinc Ion Batteries. ACS Energy Lett. 2018, 3,2602−2609.(34) Yan, M.; He, H.; Chen, Y.; Wang, S.; Wei, Q.; Zhao, K.; Xu, X.;An, Q.; Shuang, Y.; Shao, Y.; Mueller, K.; Mai, L.; Liu, J.; Yang, J.Water-Lubricated Intercalation in V2O5· nH2O for High-Capacity andHigh-Rate Aqueous Rechargeable Zinc Batteries. Adv. Mater. 2018,30, 1703725.(35) He, P.; Zhang, G.; Liao, X.; Yan, M.; Xu, X.; An, Q.; Liu, J.;Mai, L. Sodium Ion Stabilized Vanadium Oxide Nanowire Cathodefor High-Performance Zinc-Ion Batteries. Adv. Energy Mater. 2018, 8,1702463.(36) Alfaruqi, M.; Mathew, B.; Song, J.; Kim, S.; Islam, S.; Pham, D.;Jo, J.; Kim, S.; Baboo, J.; Xiu, Z.; Lee, K.; Sun, Y.; Kim, J.Electrochemical Zinc Intercalation in Lithium Vanadium Oxide: AHigh-Capacity Zinc-Ion Battery Cathode. Chem. Mater. 2017, 29,1684−1694.(37) Guo, X.; Fang, G.; Zhang, W.; Zhou, J.; Shan, L.; Wang, L.;Wang, C.; Lin, T.; Tang, Y.; Liang, S. Mechanistic Insights of Zn2+

Storage in Sodium Vanadates. Adv. Energy Mater. 2018, 8, 1801819.(38) Tang, B.; Zhou, J.; Fang, G.; Liu, F.; Zhu, C.; Wang, C.; Pan,A.; Liang, S. Engineering the interplanar spacing of ammoniumvanadates as a high-performance aqueous zinc-ion battery cathode. J.Mater. Chem. A 2019, 7, 940.(39) Sambandam, B.; Soundharrajan, V.; Kim, S.; Alfaruqi, M.; Jo, J.;Kim, S.; Mathew, V.; Sun, Y.; Kim, J. Aqueous Rechargeable Zn-IonBatteries: An Imperishable and High-Energy Zn2V2O7 NanowireCathode through Intercalation Regulation. J. Mater. Chem. A 2018, 6,3850−3856.

(40) He, P.; Yan, M.; Zhang, G.; Sun, R.; Chen, L.; An, Q.; Mai, L.Layered VS2 Nanosheet-Based Aqueous Zn Ion Battery Cathode. Adv.Energy Mater. 2017, 7, 1601920.(41) Xia, C.; Guo, J.; Lei, Y.; Liang, H.; Zhao, C.; Alshareef, H.Rechargeable Aqueous Zinc-Ion Battery Based on Porous FrameworkZinc Pyrovanadate Intercalation Cathode. Adv. Mater. 2018, 30,1705580.(42) Tang, B.; Fang, G.; Zhou, J.; Wang, L.; Lei, Y.; Wang, C.; Lin,T.; Tang, Y.; Liang, S. Potassium Vanadates with Stable Structure andFast Ion Diffusion Channel as Cathode for Rechargeable AqueousZinc-Ion Batteries. Nano Energy 2018, 51, 579−587.(43) He, P.; Quan, Y.; Xu, X.; Yan, M.; Yang, W.; An, Q.; He, L.;Mai, L. High-Performance Aqueous Zinc−Ion Battery Based onLayered H2V3O8 Nanowire Cathode. Small 2017, 13, 1702551.(44) Ding, Y.; Wen, Y.; Wu, C.; van Aken, P. A.; Maier, J.; Yu, Y. 3DV6O13 nanotextiles assembled from interconnected nanogrooves ascathode materials for high-energy lithium ion batteries. Nano Lett.2015, 15, 1388−1394.(45) Fei, H.; Lin, Y.; Wei, M. Facile synthesis of V6O13 micro-flowersfor Li-ion and Na-ion battery cathodes with good cycling perform-ance. J. Colloid Interface Sci. 2014, 425, 1−4.(46) Meng, W.; Pigliapochi, R.; Bayley, P.; Pecher, O.; Gaultois, M.;Seymour, I.; Liang, H.; Xu, W.; Wiaderek, K.; Chapman, K.; Grey, C.P. Unraveling the complex delithiation and lithiation mechanisms ofthe high capacity cathode material V6O13. Chem. Mater. 2017, 29,5513−5524.(47) Spencer Braithwaite, J.; Richard A Catlow, C.; Harding, J.;Gale, J. A theoretical study of lithium intercalation into V6O13acombined classical, quantum mechanical approach. Phys. Chem. Chem.Phys. 2001, 3, 4052−4059.(48) Xu, N.; Ma, X.; Wang, M.; Qian, T.; Liang, J.; Yang, W.; Wang,Y.; Hu, J.; Yan, C. Stationary full li-ion batteries with interlayer-expanded V6O13 cathodes and lithiated graphite anodes. Electrochim.Acta 2016, 203, 171−177.(49) Soundharrajan, V.; Sambandam, B.; Kim, S.; Alfaruqi, M.;Putro, D.; Jo, J.; Kim, S.; Mathew, V.; Sun, Y.; Kim, J. Na2V6O16·3H2O Barnesite Nanorod: An Open Door to Display a Stable andHigh Energy for Aqueous Rechargeable Zn-Ion Batteries as Cathodes.Nano Lett. 2018, 18, 2402−2410.(50) Deiss, E. Spurious chemical diffusion coefficients of Li+ inelectrode materials evaluated with GITT. Electrochim. Acta 2005, 50,2927−2932.(51) Hu, P.; Zhu, T.; Wang, X.; Wei, X.; Yan, M.; Li, J.; Luo, W.;Yang, W.; Zhang, W.; Zhou, L.; Zhou, Z.; Mai, L. Highly DurableNa2V6O16· 1.63 H2O Nanowire Cathode for Aqueous Zinc-IonBattery. Nano Lett. 2018, 18, 1758−1763.

ACS Applied Energy Materials Article

DOI: 10.1021/acsaem.8b02054ACS Appl. Energy Mater. 2019, 2, 1988−1996

1996