Received 19th July 2019Accepted 23rd September 2019
DOI: 10.1039/c9ta07822c
rsc.li/materials-a
23140 | J. Mater. Chem. A, 2019, 7, 23
3O8 nanobelt electrode forsuperior aqueous and quasi-solid-state zinc ionbatteries†
Jianwei Lai,a Hui Tang, b Xiuping Zhuc and Ying Wang *a
Rechargeable zinc ion batteries (ZIBs) featuring high abundance, environmental benignity, cost
effectiveness, and intrinsic safety are regarded as high-potential grid energy storage systems, but the
developments of high-performance cathodes with large capacity, high energy density, and long-term
cyclability remain a huge challenge, owing to sluggish Zn2+ intercalation kinetics with bivalent charges in
the cathodes. Herein, we report novel NH4V3O8$1.9H2O nanobelts as advanced cathode materials in
aqueous and quasi-solid-state (QSS) ZIBs. When examined in aqueous ZIBs, these cathode materials
enable ultrafast Zn2+ diffusion and highly reversible processes, exhibiting superior electrochemical
performances with a high discharge capacity of 463 mA h g�1 at 0.1 A g�1, excellent rate capability (183
mA h g�1 even at 10 A g�1), and impressive cycling stability with a capacity retention of 81% after 2000
cycles, retaining a decent discharge capacity of 166 mA h g�1 at 10 A g�1. Moreover, the
NH4V3O8$1.9H2O electrode can deliver a high energy density of 332 W h kg�1 at a power density of 72
W kg�1 and retain an energy density of 101 W h kg�1 at a high power density of 5519 W kg�1. In addition,
the QSS flexible Zn/NH4V3O8$1.9H2O battery is investigated, showing durable cycling performance and
stable electrochemical properties under various bending states. This study shows that the
NH4V3O8$1.9H2O nanobelt cathode with high energy density and long cycle life is a potential candidate
for grid energy storage systems, and it sheds light on the rational design of novel cathodes for practical
rechargeable ZIBs.
Introduction
Environmentally benign, cost-effective, and safe energy storagematerials with giant energy density and more efficient abilitiesare urgently desired for widespread renewable power storagesystems.1–3 Among different energy storage systems, lithium-ionbatteries (LIBs) have dominated the commercial market overseveral decades, owing to their satisfactory energy density andimpressive charge–discharge performance.4–7 However, theimminent concerns of the huge-cost and insufficient lithiumnatural resources, ammable and harmful organic electrolytes,and the highly active chemical nature of lithium, hinder theirgrid-scale developments in electrochemical storage applica-tions.8–10 As such, novel rechargeable batteries with multivalentions serving as charge carriers (Zn2+, Ca2+, Mg2+, or Al3+) started
ngineering, Louisiana State University,
@lsu.edu
y of Electronic Science and Technology,
Engineering, Louisiana State University,
tion (ESI) available. See DOI:
140–23148
to receive much attention as alternative candidates, owing totheir high safety, high abundance, and cost-effective produc-tion.11–14 In particular, aqueous rechargeable ZIBs have attractedgreat attention owing to a series of advantages, including highnatural abundance, low cost, low redox potential (�0.76 V vs.the standard hydrogen electrode), high theoretical capacity (820mA h g�1), nontoxicity, and high stability in aqueous chemistryof zinc metals.15,16 Nevertheless, despite the merits of aqueousZIBs, the development of cathodes with large and stablecapacity, superior rate performance, and outstanding cyclingstability remains an ongoing challenge.16 In addition, the ever-growing demands for exible devices have promoted thedevelopment of wearable energy storage devices. Flexible QSSrechargeable batteries with unique mechanical properties arecapable of being bent, folded, or rolled without sacricing theirexcellent electrochemical performances. Such QSS batterieshave benecial impacts on a wide set of exible applications,such as medical devices (e.g., diagnosis) and smart devices (e.g.,wearable electronics).17,18 As a competitive candidate,rechargeable QSS ZIBs show numerous advantages, such aselectrolyte-leak prevention compared to their liquid counter-parts, good compatibility with the human body, cost effective-ness, and intrinsic safety. Furthermore, Zn2+ based QSSelectrolytes offer desirable exibility and high stability and
effectively eliminate the formation of zinc dendrites, showinghigh potential for exible energy storage devices.19,20
There have been some reports regarding cathode materialsfor ZIBs, including manganese oxides,21–26 Prussian blueanalogs,27,28 polyanion compounds,29,30 and vanadium-basedmaterials.31,32 However, most of the cathode materials sufferfrom limited capacity, inferior rate capability, or poor cyclingperformance. Notably, the intensive positive polarization andlarge solvation sheath of Zn2+ would result in sluggish migra-tion kinetics and poor rate performance. Nevertheless, layeredvanadium-based materials can be promising cathodes foraqueous ZIBs, owing to low cost, high abundance, high capacitywith multi-electron transfer, and favorable cycling stability witha stable structure.31 The “pillar” strategy through employingcations or lattice water in the layered vanadates has been provento greatly enhance their rate performance and cyclingstability.31–36 These “pillars” can not only facilitate Zn2+ migra-tion, but also improve the layered-structure stability duringcontinuous Zn2+ ingress/egress. To illustrate, Chen et al.prepared a NaV3O8$1.5H2O ZIB cathode, displaying a highcapacity of 380 mA h g�1 at a current density of 0.1 A g�1 and82% capacity can be retained aer 1000 cycles at 4 A g�1.37
Kundu et al. presented a Zn0.25V2O5$nH2O cathode, and itshowed 282 mA h g�1 at 0.3 A g�1 and a capacity retention of81% over 1000 cycles at 2.4 A g�1.31 We fabricated an interlayer-expanded V6O13$nH2O electrode pillared with structural water,which showed a high capacity of 395 mA h g�1 at 0.1 A g�1 andretained 87% capacity for 1000 cycles at 5 A g�1.38 Liang et al.investigated various potassium vanadates, concluding thatK2V8O21 offered 247 mA h g�1 at 0.3 A g�1 and 83% capacity wasretained at 6 A g�1 aer 300 cycles.39 Furthermore, Liang et al.studied a ake-like NH4V4O10 cathode with a high capacity of�400 mA h g�1 at 0.3 A g�1 and negligible capacity fade over1000 cycles at 10 A g�1.35 Although the strategy by employingNa+,37 K+,39 Zn2+,31 or Mg2+ cations40 as pillars can enhance thestructural stability and long-term cyclability, these cations withhigh atomic mass contribute no capacity to the ZIBs, whichwould limit the specic capacity of the cathodes. From theresearch above, it is deduced that layered hydrated ammoniumvanadium oxide can be an excellent candidate with betterelectrochemical performances in aqueous ZIBs. Specically, thelower atomic weight of NH4
+ pillars in layered hydratedammonium vanadium bronzes compared to those summarizedabove would help achieving higher theoretical capacity. More-over, the strong hydrogen bonds formed by the NH4
+ pillars andvanadium-oxide layer can enhance the layered-structurestability and thus mitigate the structural contraction/expansionduring Zn2+ migration, which contribute to long-cyclingstability.
The electrochemical properties and reaction mechanism ofvarious ammonium vanadium bronzes were investigated inrechargeable Li+, Na+, or Mg2+ ion batteries with organic elec-trolytes.41–45 For instance, Liu et al. published an NH4V3O8
nanorod cathode for use in LIBs that presented a high capacityof�327mA h g�1 at 0.03 A g�1 and impressive rate performancewith a reversible capacity of 181.8 mA h g�1 at 0.6 A g�1,respectively.41 Moreover, Hong et al. synthesized NH4V4O10
nanobelts for Mg2+ ion batteries, delivering a considerablecapacity of 174.8 mA h g�1 at 42.12 mA g�1.45 Notably, the largeinterlayer spacings of NH4V3O8 and NH4V4O10 are 8.1 and 9.4 A,respectively, which are easily accessible for Zn2+ transportation,which may become the competitive candidate for superior ZIBs.
In this study, two types of layered hydrated ammoniumvanadates are synthesized, i.e., NH4V3O8$1.9H2O (AVO-1) andNH4V4O10$1.6H2O (AVO-2) nanobelts, and they are then exam-ined as cathodes for rechargeable ZIBs. In particular, AVO-1nanobelts manifest better electrochemical properties inaqueous Zn2+ chemistry, demonstrating high potential forsuperior ZIBs. Furthermore, a QSS exible Zn/AVO-1 battery isconstructed and presents satisfactory electrochemical proper-ties under various bending states.
Results and discussion
Various layered hydrated ammonium vanadates are preparedthrough a simple hydrothermal method by tuning the concen-tration of an oxalic acid precursor. Details of the preparationmethod can be found in the ESI.† As presented in Fig. 1a andS1a,† the crystal structures of the as-synthesized powders areexamined by X-ray diffraction (XRD). The XRD pattern of AVO-1is indexed well to monoclinic (NH4)2V6O16$1.5H2O (JCPDS no.00-051-0376), while the XRD pattern of AVO-2 matches mono-clinic NH4V4O10 (JCPDS no. 00-026-0096). Previous studies showthat the most intense peak from XRD patterns of NH4V3O8
prepared by a hydrothermal method would become muchweaker as the pH of the reaction decreases and even becomeweaker than other characteristic peaks when pH approaches2.46,47 Therefore, the most intense peak of our AVO-1 may beweak compared with other peaks, since the pH of the hydro-thermal reaction is 3. In this case, the peaks in the range of 25–30� or 50–52� from AVO-1 are relatively strong, as the strongestpeak becomes weaker. In addition, the XRD pattern of AVO-1 isconsistent with that of hydrated NH4V3O8.48 The accessible largeinterplanar spacing plays a crucial part in improving the elec-trochemical properties of the cathode in ZIBs, which canprovide more ion diffusion pathways and promote rapidmigration of the electrolyte ions through the electrode. Notably,the interlayer distance of the (002) plane of AVO-1 and theinterlamellar spacing of the (001) facet of AVO-2 are 8.1 A and9.4 A, respectively, calculated based on Bragg's equation, whichare comparable to those of the previously reportedK0.5V2O5$0.76H2O,49 Na2V6O16$3H2O,50 Ca0.25V2O5$nH2O,51 zincpyrovanadate,52 etc. The crystal structure of NH4V3O8 is con-structed by distorted VO6 octahedra and VO5 square pyramidswith NH4
+ cations between the vanadium-oxide layers, forminga zigzag layered structure. The NH4
+ ions as the interlayerstabilizer can mitigate the structural change upon Zn2+ ingress.Additionally, NH4V4O10 is built of VO6 octahedrons encapsu-lating ammonium ions to construct a typical layered structure.Both scanning electron microscopy (SEM) and transmissionelectron microscopy (TEM) are employed to study their corre-sponding morphological characteristics. As shown in Fig. 1band c, AVO-1 exhibits a 3D porous urchin-like architecture witha size of �25 mm, and the high-resolution SEM image conrms
Fig. 1 (a) Powder XRD pattern of the as-synthesized AVO-1 and the inset showing the crystal structure. (b and c) SEM images, (d and e) TEMimages, and (f) corresponding TGA results of AVO-1.
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that the urchin-like structure is constructed by one-dimensionalnanobelts with widths ranging from 83 to 180 nm, which havethe tendency to form bundles. TEM further validates thenanobelt morphology of AVO-1, in accordance with the high-resolution SEM images (Fig. 1d and e). Similarly, AVO-2 alsoexhibits an urchin-like structure with a size of around 22 mm,composed of nanobelts with widths of 55–135 nm (Fig. S1b–e†).The favorable porous urchin-like architectures of both AVO-1and AVO-2 would be benecial for shortening the Zn2+ diffusionpathway, facilitating kinetics of Zn2+ insertion/extraction, andmay render excellent electrochemical performances. The crys-talline water content is calculated via the thermogravimetricanalysis that is conducted within the temperature range of 35–600 �C in an N2 atmosphere. As shown in Fig. 1f and S1f,† thelattice water content is estimated to be 1.9 per NH4V3O8 unitand 1.6 for each NH4V4O10 unit respectively, based on theircorresponding weight loss.
The electrochemical behaviors of AVO-1 and AVO-2 as ZIBcathodes are evaluated in 2032 coin cells made in an ambientatmosphere, by using a 3 M Zn(CF3SO3)2 aqueous electrolyteand metallic Zn foil anode. As shown in Fig. 2a, the rst threecyclic voltammogram (CV) curves of AVO-1 are recorded ina voltage range of 0.2–1.4 V (vs. Zn/Zn2+) at 0.1 mV s�1. Thecathode peaks located at 0.78, 0.72, and 0.5 V are observedduring the rst reduction scan, which are slightly different fromthe peak positions at 0.8, 0.74, and 0.56 V in the following cyclesof the cathodic scan. The cathode peak shi can be attributed tothe activation process of AVO-1 in the discharge state. Notably,a well-resolved cathodic peak at 0.72 V in the rst CV cycle isobserved, but it disappears in the next two cycles. Thisphenomenon occurs probably because the intercalated Zn2+
ions locate in the “dead Zn2+ sites”, which is irreversible andcannot be extracted from the AVO-1 cathode during the
23142 | J. Mater. Chem. A, 2019, 7, 23140–23148
subsequent anodic scan. A similar phenomenon can beobserved in other aqueous ZIBs employing vanadates as thecathode, such as in Na0.33V2O5.53 The initial three oxidationscans present entirely overlapping oxidation peaks located at0.69 and 0.95 V. Several reduction and oxidation peaks areevidently observed, suggesting a multistep reaction processwith respect to Zn2+ ingress/egress. Additionally, the CV prolesof initial three cycles exhibit good repeatability, except for theactivation with a slight peak shi in the rst cathodic scan,indicating desirable reversibility and structural stability of theZn/AVO-1 battery. Such CV curves are slightly different fromthose of Na2V6O16$3H2O50 and zinc pyrovanadate52 and thus itselectrochemical behavior is slightly different, which may beascribed to the variant cation pillars of NH4
+ in AVO-1. Fig. S3a†shows the rst three CV curves of the AVO-2 electrode, where theanodic peak at 0.76 V and the cathodic peak at 0.8 V are slightlydifferent from the corresponding peaks of the following twocycles. Similar to AVO-1 and other reported vanadate cathodesused in aqueous ZIBs, the peak shi is ascribed to the activationprocess during the initial cycle.33 Subsequently, a small peak at0.9 V followed by two cathodic peaks at 0.83 and 0.57 V and twoanodic peaks at 0.8 and 0.96 V can be observed. It should benoted that cathodic and anodic peaks of AVO-1 overlap morecompletely than those redox peaks of AVO-2, except for theinitial CV cycle with the activation process, indicating betterelectrochemical reversibility of the AVO-1 cathode.
Employing the galvanostatic method with a specic currentdensity of 0.1 A g�1, the electrochemical proles of AVO-1 uponZn2+ ingress/egress are shown in Fig. S2.† Remarkably, AVO-1delivers a high initial discharge capacity of 463 mA h g�1 andcharge capacity of 456 mA h g�1, with a correspondingcoulombic efficiency (CE) of 98.5%. This high initial CEsuggests good reversibility of Zn2+ intercalation/deintercalation.
Fig. 2 Electrochemical performances of the AVO-1 cathode in the potential range of 0.2–1.4 V vs. Zn/Zn2+. (a) Initial three CV curves at 0.1 mVs�1, (b) rate properties, (c) related discharge–charge profiles applied with different current densities, and (d and e) long-term cycling perfor-mances at the current densities of 5 and 10 A g�1, respectively.
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The favorable rst CE is mainly attributed to the weakenedelectrostatic interactions between the host structure and Zn2+
with the assistance of crystalline water, and its stable layeredframework pillared by NH4
+. The discharge curves exhibit twoplateau-like parts in the voltage range of 0.9–0.65 and 0.65–0.4V, while the charge curves present two plateau-like areas of 0.6–0.8 and 0.8–1.1 V, which correspond to the intercalation/dein-tercalation of Zn2+. Notably, the charge–discharge proles forthe subsequent two cycles completely overlap, implying a goodreversibility and structural stability of the AVO-1 electrode. Rateperformance in aqueous ZIBs is one of the most importantindicators to evaluate the electrochemical properties andtransport kinetics of Zn2+ during electrochemical reactions,which is essential to fulll the increasing demands for grid-scale energy storage devices. Unfortunately, a variety of cath-odes reported earlier present poor rate performances, due to theirreversible structure change and intensive polarization of Zn2+
with slow charge transfer kinetics.22,37,52 However, the as-prepared AVO-1 electrode exhibits superior rate capability when
subjected to harsh galvanostatic evaluation from 0.1 to up to 10A g�1 within a voltage window between 0.2 and 1.4 V, as illus-trated in Fig. 2b and c. The AVO-1 electrode exhibits reversiblecapacities of 463, 421, 396, 367, 357, 322, 284, 251, 220, and 183mA h g�1 at a specic current of 0.1, 0.2, 0.5, 0.8, 1, 2, 4, 6, 8, and10 A g�1, respectively. It should be noted that there is only�38% capacity decay as the applied current density is ampliedfrom 0.1 to 4 A g�1. Furthermore, a decent average-capacity of173 mA h g�1 is obtained even applied with the highest currentrate of 10 A g�1, and the associated discharge–charge time is�62 s, suggesting the ultrafast transportation kinetics andimpressive rate capability of the AVO-1 cathode. When thecurrent rate abruptly recovers to 0.1 A g�1, the dischargecapacity is restored to 420 mA h g�1, suggesting a strongtolerance ability for fast Zn2+ migration and high structuralstability. The rate behavior of AVO-2 is elucidated with appliedcurrent density starting from 0.2 to 5 A g�1, as presented inFig. S3b.† Capacities of 384, 351, 324, 308, 257, 206, 162, and123 mA h g�1 are recorded at 0.2, 0.5, 0.8, 1, 2, 3, 4, and 5 A g�1,
and the capacity can recover to 369 mA h g�1 when the currentrate goes back to 0.2 A g�1. Notably, the good rate performanceof the AVO-2 electrode is very competitive with those of thecathodes reported before.32,39,52 However, the rate capability ofAVO-2 is much inferior compared with AVO-1, suggesting thedifference of Zn2+ diffusion kinetics in these two layeredhydrated ammonium vanadates for aqueous ZIBs, which will bediscussed later. The long cycling performances of AVO-1 andAVO-2 are further examined at a high specic current of 5 A g�1
under the galvanostatic test (Fig. 2d and S3c†). The AVO-1electrode provides a high initial capacity of 268 mA h g�1 witha CE of over 94%, while AVO-2 presents a comparable dischargecapacity of 288 mA h g�1 with a CE of 95%. Remarkably,a capacity retention of over 87% is obtained for AVO-1, whileonly 65% capacity is retained for AVO-2 aer 1000 cycles,demonstrating the superior cycling stability of AVO-1 than AVO-2. More signicantly, an enduring and highly reversible capacityof 166 mA h g�1 over 2000 cycles at an exceptionally highspecic current of 10 A g�1 is achieved with 81% retainedcapacity, further demonstrating the durable long-life cyclingperformance of the AVO-1 electrode (Fig. 2e). In addition, 3 MZnSO4 aqueous electrolyte is also employed to study the cyclingproperties of AVO-1 at 5 A g�1, showing lower reversiblecapacities and severe capacity loss when compared with thecells using 3 M Zn(CF3SO3)2 aqueous solution (Fig. S4†). Thisresult is ascribed to more rapid kinetics and higher reversibilityof Zn plating/stripping in the batteries based on the aqueousZn(CF3SO3)2 electrolyte than those with the aqueous ZnSO4
Fig. 3 (a) CV profiles of the Zn/AVO-1 battery at a series of scan rates. (bthe CV data. (c) CV curve showing the capacitive contribution (gray aresponding DZn2+ vs. different Zn2+ (de)intercalation states. (f) The Ragoneaqueous ZIBs.
23144 | J. Mater. Chem. A, 2019, 7, 23140–23148
electrolyte, as bulky CF3SO3� ions are capable of passivating the
Zn anode, reducing the number of water moleculessurrounding Zn2+ and simultaneously mitigating the solvationeffect.61
To investigate the fundamental mechanism behind themarked distinction of electrochemical properties between thesetwo layered hydrated ammonium vanadates with a very similarurchin-like morphology, the charge storage kinetics are quan-tied using the CV method and galvanostatic intermittenttitration technique (GITT). The representative CV proles of theAVO-1 cathode at various sweep rates ranging from 0.1 to 1 mVs�1 are illustrated in Fig. 3a. Notably, these CV proles basicallymaintain similar shapes with the increasing scan rate, indi-cating excellent endurance for Zn2+ continuous ingress/egress.Based on the following equation i ¼ avb, the b value can beconrmed to be in the range of 0.5–1.0, where the b valueapproaching 0.5 represents a diffusion-controlled process andthe b value reaching 1.0 indicates a capacitive process. Theabove equation can be reorganized to log i¼ b log v + log a. Thelinear relationships of log i vs. log v at the peak potentials areshown in Fig. 3b. The coefficients b of peaks 1, 2, 3 and 4 aredetermined to be 0.74, 0.73, 0.83, and 0.70, respectively, mani-festing that the electrochemical reactions are controlled by bothcapacitive and diffusion-controlled processes. Moreover, thecapacitive and diffusion-limited contributions can be quanti-tatively separated to a capacitive (k1v) and a diffusion-controlledprocess (k2v
1/2) from the current response (i) at a specicpotential (V), based on the equation i(V) ¼ k1v + k2v
1/2. As
) log i versus log v plots at specific reduction/oxidation states based ona) at 0.6 mV s�1. (d) GITT profiles of the AVO-1 electrode. (e) Corre-plots of the AVO-1 electrode and previously published cathodes for
presented in Fig. 3c, �53% of the current contribution isassigned to the capacitive properties at 0.6 mV s�1, accountingfor the impressively high rate behavior of the AVO-1 electrode.As for the AVO-2 cathode, the b values of the four peaks are 0.79,0.53, 0.76, and 0.57, correspondingly, suggesting that thecapacitive and diffusion-limited behaviors co-control the chargestorage process (Fig. S5a and b†). 49% of the current is providedby the capacitive process at the same scan rate of 0.6 mV s�1 forAVO-2, which is lower than the capacitive contribution (over53%) in AVO-1 (Fig. S5c†). The GITT is used to explore thekinetics of Zn2+ migration during cycling by examining the Zn2+
diffusion coefficient (DZn2+). The DZn2+ can be determined using
the following equation: DZn2þ ¼ 4L2
ps
�DEs
DEt
�2
, where L is the Zn2+
diffusion length which is equal to the electrode thickness, s isthe time of applied current pulse, DEs is the variation of steady-state potential for the related step, and DEt is the potentialchange of a one-step GITT test aer eliminating the iR drop.54,55
As shown in Fig. 3d, e and S5d, e,† the calculated Zn2+ diffusioncoefficients of the AVO-1 electrode at each single-step GITT testduring the overall discharge process are higher than the DZn2+ ofAVO-2, suggesting faster Zn2+ diffusion in the AVO-1 electrode.The average value of DZn2+ is 1.18 � 10�8 cm2 s�1 in AVO-1,which is �2 times higher thanPlease insert5.64 � 10�9 cm2 s�1
in AVO-2 during the discharge process, supporting the betterrate capability of AVO-1 than AVO-2.
The electrochemical impedance spectroscopy (EIS) tests arecarried out to investigate the interfacial charge transfer processof AVO-1 and AVO-2, respectively (Fig. S6†). The charge-transferresistance of AVO-1 (19.57 U) is lower than that of AVO-2 (22.41U), which accounts for the better rate capability of AVO-1.Moreover, recent studies reveal that the lattice water can act asthe “lubricant” in layered vanadium-based compounds andplays a crucial role in enhancing their electrochemical perfor-mances for aqueous ZIBs.36 The more crystalline water per unitof AVO-1 compared to AVO-2 may also contribute to the betterelectrochemical properties of AVO-1. In addition, structuralchange of AVO-1 during the discharge and charge process in theaqueous Zn/AVO-1 battery is investigated through ex situ XRDcharacterization. As presented in Fig. S7,† the XRD patterns ofthe AVO-1 electrode at the pristine and charged states displaysimilar patterns, while the (002) peak from the XRD patterns atthe discharged state slightly shis from 10.82� to 10.92�. Sucha slight peak shi indicates the negligible structural change ofAVO-1 during Zn2+ intercalation, demonstrating structuralstability and reversible Zn2+ insertion/extraction. Sucha phenomenon of negligible structural change can also beobserved in the layered (NH4)2V6O16$1.5H2O cathode foraqueous ZIBs, delivering ultra-stable cycling performance.54
Notably, the well-preserved crystal structure of AVO-1 at thedischarged state implies that NH4
+ would not be expelledduring the rst discharge process, which is different from thepreviously reported NH4V4O10 cathode.56 To further explore themultistep oxidation–reduction reactions during Zn2+ ingress/egress, we obtain the X-ray photoelectron spectroscopy (XPS)spectra of AVO-1 electrodes at pristine, fully discharged, and
charged states. As shown in Fig. S8a and b,† there is no signal ofthe Zn element in pristine AVO-1, while two noticeable peaksrelated to Zn 2p3/2 and Zn 2p1/2 are detected at the fully dis-charged electrode, conrming the insertion of Zn2+ into theAVO-1 host. At the fully charged state, the corresponding peaksfrom the Zn element become weaker but do not fully disappear,resulting from partially irreversible Zn2+ intercalation, which isconsistent with the rst CV curve with partial irreversibilitypresented above (Fig. S8c†).50 The valence states of V at differentstates are also investigated (Fig. S8d–f†). At the original state, V2p3/2/V 2p1/2 peaks located at 517.54/525.23 eV are ascribed tothe V 2p3/2–V 2p1/2 spin–orbit doublet for V(V), while V 2p3/2/V2p1/2 peaks at 516.17/524.02 eV are assigned to V(IV), which mayresult from the impurity of the V2O5 precursor.33,54 At the dis-charged state, the characteristic binding energies of 517.71/525.29, 516.91/524.15, and 515.10/522.15 eV from V 2p3/2/V 2p1/2peaks are attributed to V(V), V(IV), and V(III), respectively,revealing the reduction from V(V) to V(IV)/V(III) during Zn2+
insertion. When charged back to 1.4 V, most of V(IV)/V(III) isoxidized to V(V), demonstrating highly reversible Zn2+ insertion/extraction in the AVO-1 lattice. Notably, the N element signal atthree different states can be clearly observed, indicating thatNH4
+ would not be expelled during Zn2+ intercalation (Fig. S8g–i†).
The superior Zn2+ insertion/extraction performance of theAVO-1 electrode is further reected by the Ragone plots incomparison with recently reported cathode materials (based onthe active mass of cathodes) for aqueous ZIBs, including(NH4)2V10O25$8H2O,59 ZnHCF,27 VS2,58 NaV3O8$1.5H2O,37
Zn0.25V2O5$nH2O,31 etc. As presented in Fig. 3f, the AVO-1 elec-trode achieves the highest energy density of 332 W h kg�1 ata power density of 72W kg�1, and a decent energy density of 101W h kg�1 can be realized with an ultrahigh power density of5519 W kg�1, which outperforms the ZIB cathode materialspublished before,27,31,37,52,57–59 demonstrating its huge potentialfor grid energy storage systems. It is worth mentioning that theintegral Zn/AVO-1 aqueous battery exhibits a considerableenergy density of 209 W h kg�1 at 45 W kg�1 (calculated usingthe 200% Zn anode and the cathode), which surpasses variousconventional aqueous batteries, including Pb-acid batteries(�35 W h kg�1) and aqueous Li-ion batteries (40–85 W h kg�1).Moreover, the Zn2+ storage properties of previously publishedvanadium-based electrodes in ZIBs are listed in Table S1.†Compared to the other cathodes, our AVO-1 electrode exhibitsthe highest reversible capacity of 463 mA h g�1 at 0.1 A g�1,a considerable capacity of 268 mA h g�1 with 87% retainedcapacity for 1000 cycles at 5 A g�1, and an impressive capacity of166 mA h g�1 aer 2000 cycles at 10 A g�1.
Inspired by the superiority of the QSS ZIBs and the highelectrochemical performances of the AVO-1 positive electrode inaqueous ZIBs, exible Zn/AVO-1 batteries were constructed byemploying a ZnSO4/gelatin water-based QSS electrolyte, exibleZn anode, and AVO-1 cathode (Fig. 4a). The as-assembled QSSZn/AVO-1 battery delivers high capacities of 378, 264, 203, 142,and 115 mA h g�1 at 0.1, 0.2, 0.5, 0.8, and 1 A g�1, respectively(Fig. S9†). Furthermore, the reversible discharge capacity canretain 276 mA h g�1 as the current rate goes back from 1 to 0.1 A
Fig. 4 (a) Schematic illustration of the as-assembled flexible QSS Zn/AVO-1 cell by employing flexible Zn foil as the anode, the ZnSO4/gelatin gelas the QSS electrolyte, and AVO-1 nanobelts as the cathode. (b) Long-term cycling properties with an applied current density of 0.1 A g�1 for theinitial five cycles and 0.5 A g�1 for the subsequent cycles. (c) Typical charge–discharge profiles under various bending states at 0.2 A g�1. (d) Adigital picture of four LED lights powered by three flexible QSS batteries in series.
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g�1. Notably, the electrochemical properties of the QSS Zn/AVO-1 battery are not as good as those of the aqueous counterpart,owing to the lower ionic conductivity of the gel, but QSS ZIBscan still deliver competitive electrochemical performances. Thelong-term cycling property is evaluated with an applied speciccurrent of 0.1 A g�1 for the initial ve cycles and 0.5 A g�1 forsubsequent cycles. As shown in Fig. 4b, a specic dischargecapacity of 381 mA h g�1 is achieved, consistent with the ratebehaviors at 0.1 A g�1, and the discharge capacities stabilize at�140 mA h g�1 at 0.5 A g�1 aer ve cycles with a retainedcapacity of 133 mA h g�1 aer 200 cycles, manifesting the goodcycling performance of the as-assembled QSS battery. Further-more, such a long-term cyclability of the Zn/AVO-1 cell issuperior to that of the Zn/V5O12$6H2O QSS cell,60 and compa-rable to that of the Zn/PANI cell.61 As shown in Fig. S10a,† therst discharge curve of the QSS Zn/AVO-1 battery presents twoplateau-like areas in the voltage range of 0.95–0.73 and 0.65–0.45 V, while the following charge curve shows one plateau-likepart of 0.9–1.1 V. The corresponding CV prole is relativelynarrow compared to that of aqueous cells (Fig. S10b†).Furthermore, the EIS curves of the QSS Zn/AVO-1 cell displaya higher charge-transfer resistance compared with that of theaqueous Zn/AVO-1 battery, suggested by a much wider semi-circle that represents the enlarged charge-transfer resistance(788.6 U) (Fig. S11†). In addition, water from the QSS electrolyte
23146 | J. Mater. Chem. A, 2019, 7, 23140–23148
can get involved in the electrochemical reactions, since it is oneof the main compositions in the electrolyte. As shown inFig. S12,† the XRD pattern of the Zn anode at the rst fullydischarged state in the QSS Zn/AVO-1 cell conrms the co-existence of Zn and Zn4(SO4)(OH)6$5H2O, which is consistentwith previous reports on vanadium-based cathodes for aqueousor QSS ZIBs.37,62,63 To demonstrate the high potential for exibleenergy storage devices, the galvanostatic behaviors of the QSSbattery enduring different bending states are presented inFig. 4c. The exible Zn/AVO-1 battery exhibits specic dischargecapacities of 255, 247, and 243mA h g�1 at the at state, 90� and180� bending states with an applied current of 0.2 A g�1,showing negligible capacity loss under various bending statesand maintaining similar discharge–charge proles, demon-strating desirable exibility. In addition, three QSS Zn/AVO-1cells in series can successfully power four LED lights (Fig. 4d).As such, the results above show that QSS Zn/AVO-1 batteries canserve as promising exible energy storage devices.
Conclusion
In summary, layered hydrated ammonium vanadates includingNH4V3O8$1.9H2O (AVO-1) and NH4V4O10$1.6H2O (AVO-2)nanobelts are investigated as cathodes for rechargeableaqueous and QSS ZIBs. The charge storage kinetics of the AVO-1
and AVO-2 electrodes are evaluated by employing the CVmethod and the GITT. It is found that AVO-1 exhibits largercapacitive contribution at the same scanning rate and higherDZn2+ than those of AVO-2. As expected, the AVO-1 cathodedemonstrates better electrochemical performances regardingthe rate behaviors and cycling properties. Beneting froma large interlayer spacing of 8.1 A with easily accessible chan-nels for Zn2+ diffusion, favorable nanoscale morphology, andstable NH4
+ as the interlayer “pillar”, the as-prepared AVO-1electrode exhibits a large reversible capacity of 463 mA h g�1 at0.1 A g�1, high-rate capability of up to 10 A g�1 with a consid-erable discharge capacity of 183 mA h g�1, and excellent long-cycle life (87% capacity retention aer 1000 cycles at 5 A g�1 and81% capacity retention over 2000 cycles at 10 A g�1 can beachieved). Furthermore, the investigated aqueous Zn/AVO-1battery delivers a competitive energy density of 209 W h kg�1 at45 W kg�1 (based on the 200% Zn anode and the cathode).Additionally, a QSS exible Zn/AVO-1 cell is constructed usinga gel electrolyte, in which a decent capacity of 133 mA h g�1 isobtained for 200 cycles and the electrochemical performancecan be maintained under various bending states. Our resultsdemonstrate that the NH4V3O8$1.9H2O electrode with impres-sive electrochemical properties in aqueous and QSS ZIBs holdshigh potential for grid-scale storage systems and exible energystorage devices.
Conflicts of interest
The authors declare no conict of interests.
Acknowledgements
We acknowledge the nancial support from the EconomicDevelopment Assistantship (EDA) of Louisiana State University.We thank Dr X. Sun and Prof. Q. Wu at the School of RenewableNatural Resources of Louisiana State University for the thermalgravimetric analyzer. We also thank Dr Z. Xie at the University ofTennessee for the discussion.
References
1 J. B. Goodenough, Nat. Mater., 2018, 1, 204.2 F. Wang, O. Borodin, T. Gao, X. Fan, W. Sun, F. Han,A. Faraone, J. A. Dura, K. Xu and C. Wang, Nat. Mater.,2018, 17, 543–549.
3 B. Dunn, H. Kamath and J. M. Tarascon, Science, 2011, 334,928–935.
4 M. S. Whittingham, Chem. Rev., 2004, 104, 4271–4302.5 V. Etacheri, R. Marom, R. Elazari, G. Salitra and D. Aurbach,Energy Environ. Sci., 2011, 4, 3243–3262.
6 M. H. Han, E. Gonzalo, G. Singh and T. Rojo, Energy Environ.Sci., 2015, 8, 81–102.
7 F. Wu and G. Yushin, Energy Environ. Sci., 2017, 10, 435–459.8 Q. Wang, P. Ping, X. Zhao, G. Chu, J. Sun and C. Chen, J.Power Sources, 2012, 208, 210–224.
10 H. Kim, J. Hong, K. Y. Park, H. Kim, S. W. Kim and K. Kang,Chem. Rev., 2014, 114, 11788–11827.
11 M. Song, H. Tan, D. Chao and H. Fan, Adv. Funct. Mater.,2018, 28, 1802564.
12 A. Ponrouch, C. Frontera, F. Bardeand and M. R. Palacın,Nat. Mater., 2016, 15, 169–172.
13 H. D. Yoo, I. Shterenberg, Y. Gofer, G. Gershinsky, N. Pourand D. Aurbach, Energy Environ. Sci., 2013, 6, 2265–2279.
14 M. Lin, M. Gong, B. Lu, Y. Wu, D. Wang, M. Guan, M. Angell,C. Chen, J. Yang, B. J. Hwang and H. Dai, Nature, 2015, 520,324–328.
15 C. Xu, B. Li, H. Du and F. Kang, Angew. Chem., Int. Ed., 2012,51, 933–935.
16 A. Konarov, N. Voronina, J. H. Jo, Z. Bakenov, Y. K. Sun andS. T. Myung, ACS Energy Lett., 2018, 3, 2620–2640.
17 S. Y. Lee, K. H. Choi, W. S. Choi, Y. H. Kwon, H. R. Jung,H. C. Shin and J. Y. Kim, Energy Environ. Sci., 2013, 6,2414–2423.
18 L. Dong, C. Xu, Y. Li, Z. H. Huang, F. Kang, Q. H. Yang andX. Zhao, J. Mater. Chem. A, 2016, 4, 4659–4685.
19 Q. Li, Q. Zhang, C. Liu, Z. Zhou, C. Li, B. He, P. Man, X. Wangand Y. Yao, J. Mater. Chem. A, 2019, 7, 12997–13006.
20 Y. Li, J. Fu, C. Zhong, T. Wu, Z. Chen, W. Hu, K. Amine andJ. Lu, Adv. Energy Mater., 2019, 9, 1802605.
21 N. Zhang, F. Cheng, J. Liu, L. Wang, X. Long, X. Liu, F. Li andJ. Chen, Nat. Commun., 2017, 8, 405.
22 M. H. Alfaruqi, V. Mathew, J. Gim, S. Kim, J. Song,J. P. Baboo, S. H. Choi and J. Kim, Chem. Mater., 2015, 27,3609–3620.
23 H. Pan, Y. Shao, P. Yan, Y. Cheng, K. S. Han, Z. Nie, C. Wang,J. Yang, X. Li, P. Bhattacharya, K. T. Mueller and J. Liu, Nat.Energy, 2016, 1, 16039.
24 B. Jiang, C. Xu, C.Wu, L. Dong, J. Li and F. Kang, Electrochim.Acta, 2017, 229, 422–428.
25 J. Hao, J. Mou, J. Zhang, L. Dong, W. Liu, C. Xu and F. Kang,Electrochim. Acta, 2018, 259, 170–178.
26 C. Zhu, G. Fang, J. Zhou, J. Guo, Z. Wang, C. Wang, J. Li,Y. Tang and S. Liang, J. Mater. Chem. A, 2018, 6, 9677–9683.
27 L. Zhang, L. Chen, X. Zhou and Z. Liu, Adv. Energy Mater.,2015, 5, 1400930.
28 Z. Liu, G. Pulletikurthi and F. Endres, ACS Appl. Mater.Interfaces, 2016, 8, 12158–12164.
29 G. Li, Z. Yang, Y. Jiang, C. Jin, W. Huang, X. Ding andY. Huang, Nano Energy, 2016, 25, 211–217.
30 W. Li, K. Wang, S. Cheng and K. Jiang, Energy StorageMaterials, 2018, 15, 14–21.
31 D. Kundu, B. D. Adams, V. Duffort, S. H. Vajargah andL. F. Nazar, Nat. Energy, 2016, 1, 16119.
32 Y. Cai, F. Liu, Z. Luo, G. Fang, J. Zhou, A. Pan and S. Liang,Energy Storage Materials, 2018, 13, 168–174.
33 Z. Xie, J. Lai, X. Zhu and Y. Wang, ACS Appl. Energy Mater.,2018, 1, 6401–6408.
34 Y. Yang, Y. Tang, G. Fang, L. Shan, J. Guo, W. Zhang,C. Wang, L. Wang, J. Zhou and S. Liang, Energy Environ.Sci., 2018, 11, 3157–3162.
35 B. Tang, J. Zhou, G. Fang, F. Liu, C. Zhu, C. Wang, A. Pan andS. Liang, J. Mater. Chem. A, 2019, 7, 940–945.
36 M. Yan, P. He, Y. Chen, S. Wang, Q. Wei, K. Zhao, X. Xu,Q. An, Y. Shuang, Y. Shao, K. T. Mueller, L. Mai, J. Liu andJ. Yang, Adv. Mater., 2018, 30, 1703725.
37 F. Wan, L. Zhang, X. Dai, X. Wang, Z. Niu and J. Chen, Nat.Commun., 2018, 9, 1656.
38 J. Lai, H. Zhu, X. Zhu, H. Koritala and Y. Wang, ACS Appl.Energy Mater., 2019, 2, 1988–1996.
39 B. Tang, G. Fang, J. Zhou, L. Wang, Y. Lei, C. Wang, T. Lin,Y. Tang and S. Liang, Nano Energy, 2018, 51, 579–587.
40 F. Ming, H. Liang, Y. Lei, S. Kandambeth, M. Eddaoudi andH. N. Alshareef, ACS Energy Lett., 2018, 3, 2602–2609.
41 H. Wang, Y. Ren, W. Wang, X. Huang, K. Huang, Y. Wangand S. Liu, J. Power Sources, 2012, 199, 315–321.
42 S. Sarkar, P. S. Veluri and S. Mitra, Electrochim. Acta, 2014,132, 448–456.
43 H. Fei, X. Liu, Y. Lin and M. Wei, J. Colloid Interface Sci.,2014, 428, 73–77.
44 A. Sarkar, S. Sarkar, T. Sarkar, P. Kumar, M. D. Bharadwajand S. Mitra, ACS Appl. Mater. Interfaces, 2015, 7, 17044–17053.
45 E. A. Esparcia Jr, M. S. Chae, J. D. Ocon and S. T. Hong, Chem.Mater., 2018, 30, 3690–3696.
46 D. Vernardou, M. Apostolopoulou, D. Louloudakis,N. Katsarakis and E. Koudoumas, New J. Chem., 2014, 38,2098–2104.
47 G. S. Zakharova, C. Taschner, T. Kolb, C. Jahne,A. Leonhardt, B. Buchner and R. Klingeler, Dalton Trans.,2013, 42, 4897–4902.
48 L. Q. Mai, C. S. Lao, B. Hu, J. Zhou, Y. Y. Qi, W. Chen,E. D. Gu and Z. L. Wang, J. Phys. Chem. B, 2006, 110,18138–18141.
23148 | J. Mater. Chem. A, 2019, 7, 23140–23148
49 S. Islam, M. H. Alfaruqi, D. Y. Putro, V. Soundharrajan,B. Sambandam, J. Jo, S. Park, S. Lee, V. Mathew andJ. Kim, J. Mater. Chem. A, 2019, 7, 20335–20347.
50 V. Soundharrajan, B. Sambandam, S. Kim, M. H. Alfaruqi,D. Y. Putro, J. Jo, S. Kim, V. Mathew, Y.-K. Sun and J. Kim,Nano Lett., 2018, 18, 2402–2410.
51 C. Xia, J. Guo, P. Li, X. Zhang and H. N. Alshareef, Angew.Chem., Int. Ed., 2018, 57, 3943–3948.
52 C. Xia, J. Guo, Y. Lei, H. Liang, C. Zhao and H. N. Alshareef,Adv. Mater., 2018, 30, 1705580.
53 P. He, G. Zhang, X. Liao, M. Yan, X. Xu, Q. An, J. Liu andL. Mai, Adv. Energy Mater., 2018, 8, 1702463.
54 X. Wang, B. Xi, Z. Feng, W. Chen, H. Li, Y. Jia, J. Feng, Y. Qianand S. Xiong, J. Mater. Chem. A, 2019, 7, 19130.
55 J. Ding, Z. Du, L. Gu, B. Li, L. Wang, S. Wang, Y. Gong andS. Yang, Adv. Mater., 2018, 30, 1800762.
56 G. Yang, T. Wei and C. Wang, ACS Appl. Mater. Interfaces,2018, 10, 35079–35089.
57 R. Trocoli and F. L. Mantia, ChemSusChem, 2015, 8, 481–485.58 P. He, M. Yan, G. Zhang, R. Sun, L. Chen, Q. An and L. Mai,
Adv. Energy Mater., 2017, 7, 1601920.59 T. Wei, Q. Li, G. Yang and C. Wang, J. Mater. Chem. A, 2018,
6, 20402–20410.60 N. Zhang, M. Jia, Y. Dong, Y. Wang, J. Xu, Y. Liu, L. Jiao and
F. Cheng, Adv. Funct. Mater., 2019, 29, 1807331.61 F. Wan, L. Zhang, X. Wang, S. Bi, Z. Niu and J. Chen, Adv.
Funct. Mater., 2018, 28, 1804975.62 T. Wei, Q. Li, G. Yang and C. Wang, Adv. Energy Mater., 2019,
1901480.63 D. Kundu, S. H. Vajargah, L. Wan, B. Adams, D. Prendergast
and L. F. Nazar, Energy Environ. Sci., 2018, 11, 881–892.
