ORIGINAL PAPER

V2O5-SiO2 hybrid as anode material for aqueous rechargeablelithium batteries

Zheng Zhang1 & Hui Wang1 & Shan Ji2 & Bruno G. Pollet3 & Rongfang Wang1

Received: 23 January 2016 /Revised: 16 March 2016 /Accepted: 19 March 2016# Springer-Verlag Berlin Heidelberg 2016

Abstract V2O5-SiO2 hybrid material was fabricated by heat-treating a mixture of H2SiO3 and V2O5. SEM, TEM,XRD, and N2 isotherm analyses were performed tocharacterize the morphology and structure details ofthe as-prepared V2O5-SiO2. The possibility of usingthe as-prepared V2O5-SiO2 as anode material for aque-ous lithium-ion batteries was investigated. Potentiostaticand galvanostatic results indicated that the intercalation/de-intercalation of Li+ in this material in aqueous elec-trolyte was quasi-reversible. It was also found that a dis-charge capacity of up to 199.1 mAh g−1 was obtained at acurrent density of 50 mA g−1 in aqueous solution of 1 MLi2SO4, a value which is much higher than the available re-ported capacities of vanadium (+5) oxides in aqueouselectrolytes.

Keywords Aqueous rechargeable lithium battery . Vanadiumpentoxide . Silica . Anodematerial . Hybrid materials

Introduction

Lithium ion batteries (LIBs) used as power sources for porta-ble electronic devices and transport applications were firstproduced in the early 1990s. LIBs commonly consist of threemain components, namely, the anode, cathode, and organicliquid electrolyte. The wide potential range of organic electro-lytes allows LIBs to operate at high voltages delivering higherspecific energy densities than their aqueous counterparts.However, these organic electrolytes are highly flammable,and the use of lithium salts and separators as well as the pro-cessing for using organic electrolytes are very expensive [1]. Asa safe and cost-effective alternative, a new type of LIBs with anaqueous electrolyte was developed by Dahn’s group in 1994[2]. This type of battery uses LiMn2O4 as cathode and VO2 asanode in an aqueous electrolyte consisting of 5 M LiNO3 in0.001 M LiOH. The researchers found that the specific energydensity of the newly developed battery was 75 Wh kg−1 [2].They also observed that by using this new type of “chemistry”combination, the disadvantages of conventional LIBs, i.e., thehigh manufacturing cost and safety problems, could be elimi-nated. Although the battery’s energy capacity and cycling lifeare relatively low, the results highlighted a new path for devel-oping aqueous rechargeable lithium-ion batteries (ARLB).

It was found that oxides represent a unique class of impor-tant high capacity materials which can be used in ARLB.Among these oxides, considerable attention has been paid tovanadium oxides as a promising anode for ARLB. For exam-ple, an ARLB based on LiNi0.81Co0.19O2 as cathode andLiV3O8 as anode in 1.0 M Li2SO4 (or LiCl) electrolyte wassuccessfully developed by Köhler et al. [3]. The researchersshowed that the newly developed ARLB had a capacity of ca.45 mAh g−1 at an output cell voltage of 1.2 V, and ca. 70 % ofthe discharge capacity remained after 30 charge/discharge cy-cles. Wang et al. [4] reported that the cycling stability of

* Shan Jijissshan@126.com

* Rongfang Wangwrf38745779@126.com

1 College of Chemistry and Chemical Engineering, Northwest NormalUniversity, Lanzhou 730070, China

2 College of Chemistry and Chemical Engineering, ShenzhenUniversity, Shenzhen 518060, China

3 Eau2Energy, Nottingham, England NG14 6DX, UK

IonicsDOI 10.1007/s11581-016-1696-z

LiMn2O4/5 M LiNO3/LixV2O5 ARLB could be signifi-cantly improved by coating polypyrrole (PPy) on theanode. A recent study by Stojkovi et al. [5] showed thatthe specific capacity of LiMn2O4/saturated LiNO3/V2O5

xerogel ARLB can reach values up to 69 mAh g−1,together with improved cycling stability due to itsunique structure. Xie et al. [6, 7] prepared several va-nadium dioxides of unique morphologies via a hydro-thermal route and found that specific capacity valuescan reach up to 80 mAh g−1. Recently, there has beensome interest in [8–11] preparing and testing vanadiumoxides which were found to exhibit large specific capac-ities of up to 100 mAh g−1. Some other researcherssuch as Li et al. [12] showed that high specific capacityvalues of up to 234 mAh g−1 could be achieved. Intheir study, they found that using H2V3O8 nanowiresas anode materials could significantly enhance the per-formance of ARLB [12].

Mesoporous silicate has attracted considerable atten-tion due to its tuneable pore size, excellent stability, andhigh surface area, which makes it an ideal candidate ascatalyst, support, and electrode materials. For example,Huang et al. [13] found that when SnO2 was mixedwith SiO2, very high specific capacity values were ob-tained due to enhanced interfacial diffusion caused byhighly dispersed SiO2 and LiSi2O3 phases. Similar find-ings were also reported by Liu’s group [14]. Inspired bythe aforementioned observations and studies, we reportherein the fabrication and testing of a V2O5-SiO2 hybridmaterial, in which V2O5 blocks were dispersed incotton-like mesoporous SiO2 (acting as a high surfacehost for V2O5). The as-prepared V2O5-SiO2 exhibited ahigh specific capacity value of ca. 200 mAh g−1, whichis much higher than the specific capacity of V2O5-basedelectrode materials for ARLBs reported in the literature.

Experimental

Preparation of the samples

Na2SiO3 (853 mg) was dissolved into 5 mL H2O andstirred for 0.5 h. HCl solution (2 M, 8 mL ) was addeddrop by drop into the above mixture under constantstirring until white colloid was formed. The mixturewas filtered and washed with deionized water until pHof the filtrate was ca. 7. After that, 182 mg of commer-cial V2O5 was added in the above sample and thenground using a pestle and mortar until a homogeneousmixture was obtained. The above mixture was trans-ferred into a furnace and then heated at 150 °C in airfor 1 h and increased to 500 °C for 1 h. After coolingdown to room temperature, the V2O5-SiO2 hybrid

material was obtained. For comparison purposes, theSiO2 was prepared by drying the obtained white colloid,and V2O5 was also prepared by heat-treatment usingcommercial V2O5.

Physical characterizations

X-ray diffraction (XRD) patterns of the as-prepared materialswere recorded on a Shimadzu XD-3A (Japan) goniom-eter, using Cu-Kα radiation operating at 40 kV and35 mA. Scanning electron microscopy (SEM) imageswere obtained using a Carl Zeiss Ultra Plus by sprayingAu film. Transmission electron microscopy (TEM) mea-surements coupled with energy-dispersive spectroscopy(EDS) were carried out using a JEM-2010 ElectronMicroscope (Japan) with an acceleration voltage of200 kV. The sorption isotherms were obtained using aQuantachrome Autosorb-1 volumetric analyzer. Specificsurface areas were determined using a Brunauer-Emmett-Teller (BET), and the density functional theory(DFT) was applied for analyzing the full range of poresize distribution. The solubility products of the as-prepared V2O5-SiO2 in the electrolyte after cycling test-ing was determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES) analysis usingIRIS Intrepid II spectrometer (Thermo Fisher).

Electrochemical characterizations

Cyclic voltammograms (CVs) and galvanostatic charge/discharge experiments were carried out to evaluate theelectrochemical performance of V2O5-SiO2 in 1 mol L−1

Na2SO4 using a three-electrode cell. The working electrodewas fabricated by pasting a homogeneous slurry made ofV2O5-SiO2, carbon black, and polytetrafluoroethylene (massratio of 8:1:1) in ethylene glycol. The mixture was then rolledinto uniform slices, followed by drying at 80 °C for 6 h. Then,the slice was covered onto a 304 stainless steel plate(1 cm × 1 cm) with a tablet press. Ten milliligrams of V2O5-SiO2 was loaded on the electrode. The LiMn2O4 electrode wasfabricated in the sameway as the V2O5-SiO2 electrode, but thearea was 2 cm × 2 cm. The amount of LiMn2O4 on the elec-trode was approximately 20 mg.

A three-electrode cell and three electrodes were used,in which the reference electrode was a Hg/Hg2SO4 (satu-rated Na2SO4) electrode. All potentials are shown versusthe Hg/Hg2SO4 reference electrode. The electrolyte was a1.0 mol L−1 Li2SO4. Galvanostatic charge/discharge andcycling life tests were measured using a Neware BatteryTest System (Shenzhen Neware Technology Company,China).

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Results and discussion

The crystal structures of the as-prepared samples werecharacterized by XRD. As shown in Fig. 1, a broadpeak appears in the XRD pattern of SiO2, suggestingthat the obtained SiO2 is amorphous [15]. In the caseof V2O5 samples, all peaks are consistent with ortho-rhombic V2O5 (JCPDS files 41-1426) without exhibitingany impurity peaks [16]. After thermal treatment, XRDpatterns of V2O5-SiO2 hybrid samples show that the twophases of V2O5 and SiO2 are retained.

Figure 2 shows SEM images of SiO2, V2O5, andV2O5-SiO2 samples. As shown in Fig. 2a, the SiO2

sample displays a loose cotton-like morphology (obtain-ed without spraying the Au film). The V2O5 image inFig. 2b shows blocks of irregular shapes. Figure 2c, 2dshows the V2O5-SiO2 images at different magnifications.It can be seen that the V2O5 blocks are dispersedamong the SiO2 cotton-like structure, indicating thatSiO2 and V2O5 existed as separate phases. The mor-phology of the V2O5-SiO2 sample was also character-ized by TEM. It can be observed from Fig. 3a, 3b thatthe loose cotton-like structure of SiO2 and the V2O5

blocks are present in V2O5-SiO2. The composition ofthe V2O5-SiO2 samples was analyzed by EDS, and the

obtained spectrum is shown in Fig. 3c. The figure clear-ly shows that Si, V, and O are present; it also shows aweak sodium element signal, suggesting that a little Na+

was retained in SiO2. Quantitative elemental analysisshows that the atomic ratio of Si/V is 2.9:1, which isclose to the initial ratio in the precursors. The distribu-tion profile of Si, V, and O elements detected by TEMimages (Fig. 3d) is also present in Fig. 3e. Figure 3eshows that the positions and widths of the peaks of theO element match with those of Si and V elements, in-dicating that the Si and V are in oxide forms. From thesame figure, it can be observed that the peak shape forSi in the position from 0 to ca. 800 nm is differentfrom that of the peak shape for V, implying that SiO2

and V2O5 are separated; this observation is in very goodagreement with the SEM results.

Figure 4 shows the N2 adsorption-desorption iso-therms and the corresponding pore size distribution ofSiO2, V2O5, and V2O5-SiO2. The isotherms of SiO2 andV2O5-SiO2 in Fig. 4a are of type IV with H2 hysteresis,indicating that the two samples contained some irregularpore structures [17]. The curve of V2O5 exhibits type Icharacteristics, corresponding to the well-knownLangmuir isotherms indicating the absence of pores inthe material [18]. When V2O5 was compounded withSiO2, the pore structure was retained in the structureof V2O5-SiO2. Their pore size distributions are shownin Fig. 4b. The figure also shows the hierarchical poresin the range of micro- to meso-size existing in SiO2.The micropores and mesopores are also present in theV2O5-SiO2 samples. According to the data in the N2

adsorption-desorption isotherms, the calculated BET sur-face areas of SiO2, V2O5, and V2O5-SiO2 were found tobe 214.8, 4.1, and 180.7 m2 g−1, respectively.

Figure 5 shows CVs of V2O5-SiO2 at various scanruns. During the first cathodic scan run, two peaks areobserved, one at −0.056 V and the other at +0.257 V,corresponding to a two-step lithiation [5]. The peak at−0.90 V corresponds to hydrogen evolution. The nega-tive potential of hydrogen evolution clearly shows thatV2O5-SiO2 hybrid materials can be used as anode inARLB. It was also observed that a weak peak locatedat +0.287 V and a broad peak at +0.75 V appear in theanodic scan, corresponding to a two-step de-lithiationprocess. In total, nine (9) consecutive CV scans werecarried out, and all of them exhibited similar CV shapesto the 1st scan, indicating that the structure of V2O5-SiO2 did not significantly change during the Li+

insertion/de-insertion process. The decrease of the peakcurrent, together with a small shift of the peak potentialposition in the first 9 cycles, indicates capacity losswhich is a common feature for ARLBs. This phenome-non could be attributed to the side reactions betweenFig. 1 XRD patterns of SiO2, V2O5, and V2O5-SiO2 samples

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Li+ and the aqueous solution, as well as the initial irre-versible structure change [9].

To estimate the electrochemical performance ofV2O5-SiO2 anode, V2O5-SiO2//LiMn2O4 ARLB was as-sembled using 1.0 M Li2SO4 solution as aqueous elec-trolyte. The CVs of V2O5-SiO2 and LiMn2O4 in 1.0 MLi2SO4 aqueous solution are shown in Fig. 6. In thecase of the LiMn2O4 electrode, two anodic peaks areobserved: a very weak peak at ca. +1.15 V and theother at ca. +1.29 V, corresponding to the de-lithiationof the LiMn2O4 electrode. In the negative scan, a redoxpeak at +0.82 V was observed, corresponding to thelithiation of LiMn2O4. These findings are in good agree-ment with the two-step Li+ de-intercalation/intercalationprocess of LiMn2O4 in aqueous electrolyte [19]. ForV2O5-SiO2 anode, two redox peaks are also observedat +0.32 and −0.12 V corresponding to the Li+ insertioninto V2O5-SiO2. A broad anodic peak at +0.52 V canbe ascribed to the Li+ extraction from V2O5-SiO2. It canbe noted from the CVs that the evolution of hydrogen

in the aqueous electrolyte occurs at much lower poten-tials, i.e., at ca. −0.9 V, suggesting that V2O5-SiO2 re-mains stable during the lithium insertion/extraction pro-cess in the aqueous solution. It was found that due tothe electrode overpotentials, the oxygen and hydrogenevolution potential shifted anodically and cathodically,respectively, in turn increasing the working potentialwindow of the aqueous electrolyte. Cyclic voltammetryproperties of V2O5-SiO2 and LiMn2O4 prove that, in ourconditions, it is possible to use LiMn2O4 and V2O5-SiO2 as cathode and anode, respectively, in an aqueousLIB without causing hydrogen and oxygen evolutions inthe potential range used.

The typical galvanostatic discharge/charge curves of theV2O5-SiO2 sample in aqueous electrolyte (1 M Li2SO4) ata current density of 25 mA g−1 are shown in Fig. 7a. Thecurves obtained in aqueous electrolyte show similar galva-nostatic discharge-charge profiles with pseudo-plateaus,revealing that the lithium insertion/extraction proceeds inseveral steps. The shapes of the charge/discharge curves

Fig. 2 SEM images of a SiO2, b V2O5, and c, d V2O5-SiO2 samples

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are different from those reported by Ivana and co-workers[5]. In their work, V2O5 xerogel samples in aqueous LiNO3

solution did not display plateaus or pseudo-plateaus in thedischarge curve region due to its amorphous structure. The

pseudo-plateaus in the curve of the V2O5-SiO2 sample canbe related to the phase transitions of its crystal structure,such as the change in organic electrolyte that was reportedby Taegyeong and co-workers [16]. The initial discharge

Fig. 4 a N2 adsorption-desorption isotherms and b the corresponding pore size distribution curves of SiO2, V2O5, and V2O5-SiO2

Fig. 3 a, b TEM images, c EDS, d STEM image, and e line-scan EDS profiles of the V2O5-SiO2 sample; the scale of the red line in STEM images isconsistent with that of the horizontal axis of the line-scan EDS profiles

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and charge capacity values of the V2O5-SiO2|1.0 MLi2SO4|LiMn2O4 cell based on the weight of total anodematerial were found to be 70.4 and 70.3 mAh g−1,respectively.

To study the effect of the composition of V2O5-SiO2 onits performance, a series of V2O5-SiO2 hybrid samplescontaining various Si/V molar ratios were prepared usingthe same procedure. The corresponding galvanostatic dis-charge curves at a current density of 50 mA g−1 are pre-sented in Fig. 7b. Here, the trend of the capacity with theV/Si molar ratio is plotted in the inset of Fig. 7c. According

to a recent report by Wang and co-workers [20], SiO2 wasnot active for the Li+ intercalation/de-intercalation into theanode, thus V2O5 is the active material in V2O5-SiO2 dur-ing the charge/discharge process. The initial discharge andcharge capacity values of the V2O5-SiO2 at a current den-sity of 50 mA g−1 were calculated based on the anodeactive material of V2O5. It was observed that the initialdischarge capacity value for V2O5-SiO2 (199.1 mAh g−1)is higher than that of pure V2O5. This finding could be dueto the presence of porous SiO2 at the anode. As shown inthe SEM and TEM images, the V2O5 blocks are dispersedon the SiO2 matrix, implying that the area on the V2O5

blocks in contact with the electrolyte is increased, whichcould lead to the observed enhanced capacity. Moreover,the mesoporous SiO2 could facilitate the mass transfer dur-ing the charge/discharge process, resulting in the improvedcapacity. As shown in Fig. 7b, with the increase of thecontent of SiO2, the capacity increases and reaches a max-imum value with the Si/V molar ratio of 3:1. This indicatesthat the SiO2 support plays an important role in the im-provement of performance.

In addition, the V2O5-SiO2|1.0 M Li2SO4|LiMn2O4 cellshows higher utilization of the capacity with respect to thecases of other vanadium(+5)-oxide-based anode andLiMn2O4 as cathode in ARLB. For example, at acharging/discharging rate similar to the ones observed inthis study, the V2O5 xerogel|saturated LiNO3|LiMn2O4 cellprepared by Ivana and co-workers [5] showed the initialdischarge and charge capacity of 69 and 73 mAh g−1, re-spectively. The crystalline Li3V6O16 rod anode prepared byVivek Sahadevan et al. [11] displayed the initial discharge

Fig. 6 Cyclic voltammogramms(CVs) of V2O5-SiO2 andLiMn2O4 electrodes in 1.0 MLi2SO4 aqueous electrolyte at thescan rate of 1 mV s−1

Fig. 5 CVs of the V2O5-SiO2 hybrid with a scan rate of 10 mV s−1 atdifferent cycles

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and charge capacity of 120 and 110 mAh g−1 with theLiMn2O4 cathode in 3.0 M LiNO3 aqueous solution at acurrent density of 0.5 mAh g−1 respectively. TheLiV3O8|1.0 M Li2SO4|LiMn2O4 cell fabricated by Wanget al. [21] exhibited an initial discharge and charge capac-ity of 55.1 and 61.8 mAh g−1 at a charge/dischargecurrent density of 0.2 mA cm−2. The higher capacityof the V2O5-SiO2 hybrid can be ascribed to the largesurface area achieved by the presence of the SiO2 sup-port and short diffusion distances provided by the po-rous structure of SiO2.

The cycle performances of the V2O5-SiO2 hybrid andpure V2O5 in aqueous electrolyte are shown in Fig. 8. Itwas found that the capacities of V2O5-SiO2 and pureV2O5 rapidly decrease, maintaining 12 and 18 % of

the initial capacity after 60 cycles of charge/dischargein the aqueous electrolyte, respectively. In fact, capacitydecay upon cycling was extensively observed for nega-tive electrode materials of ARLB [3, 5, 11, 12, 22]. Itwas suggested that the loss of capacity could be causedby the transition metal ion dissolution, the phase transforma-tion of the electrode material, the chemical reaction betweenthe active material and water, or the decomposition ofthe electrolyte. In our case, the decrease in capacity ofV2O5-SiO2 is most likely to be due to the dissolution ofvanadium in aqueous solution (as observed by thechange of color of the electrolyte from colorless to lightyellow). SEM images of the electrode and the contentof vanadium in the electrolyte after cycling performancewere obtained in order to determine the dissolution ofvanadium (Fig.9). As shown in Fig. 9, irregular V2O5

blocks are observed in the SEM images of the V2O5-SiO2 (Fig. 9a) and V2O5 (Fig. 9c) electrodes prior tothe cycling test. After 60 cycling tests, it can be ob-served that V2O5 blocks disappeared as clearly shownin SEM images of the V2O5-SiO2 (Fig. 9b) and V2O5

(Fig. 9d) electrodes, indicating that most of the materialdissolved in the electrolyte during the cycling test. Tosupport this finding, it was observed that the color ofthe electrolyte changed from colorless to light yellowafter the cycling test. To further confirm this finding,the content of vanadium in the electrolyte after the cy-cling test was quantitatively analyzed by ICP-AES. Thedata showed that the vanadium content in the electro-lytes were ca. 88.5 and 85 % of its original amounts inV2O5-SiO2 (Fig. 9b) and V2O5, respectively, clearlyproving that most of the vanadium had dissolved inthe electrolyte. As previously discussed, the dispersionof the V2O5 blocks on SiO2 increases the contact areabetween V2O5 blocks and the aqueous electrolyte,which may worsen the dissolution of V2O5. This obser-vation also suggests that the porous SiO2 support canenhance the capacity of V2O5, but not benefit its cy-cling life.

Compared to the capacity decay upon cycling for thevanadium oxide as anode for ARLB such as 25 % ofthe original capacity after 100 cycles for LiV3O8/LiNi0.81Co0.19O2 [3], 89 % after 100 cycles for V2O5

xerogel/LiMn2O4 [5], ~52 % after 100 cycles for theLi3V6O16/Pt foil [11], and 72 % after 50 cycles forH2V3O8/carbon [12], the capacity decay of the V2O5-SiO2 hybrid is serious. This is due to the fact that thedispersion of V2O5 with SiO2 increases the contact areabetween the V2O5 blocks and the aqueous electrolyte,which could aggravate the dissolution of V2O5. In ad-dition, the beaker-type cell that was used in our exper-iments requires a large amount of electrolyte (ca.30 mL), which could trigger the dissolution of V ion

Fig. 7 a Galvanostatic discharge-charge profiles of V2O5-SiO2|1.0 MLi2SO4|LiMn2O4 ARLB at a current density of 25 mA g−1. bGalvanostatic discharge profiles of V2O5-SiO2|1.0 M Li2SO4|LiMn2O4

ARLB for the second cycle at a current density of 50 mA g−1; inset thetrend of the capacity of V2O5-SiO2|1.0MLi2SO4|LiMn2O4 ARLB for thesecond cycle at a current density of 50 mA g−1

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in the electrolyte [12]. However, Tang and co-workerssuccessfully improved the cycle stability of V2O5 [8]and MoO3 [23] by surface polymerization of pyrrole.Wang et al. [4] coated the LixV2O5 anode with PPy,which was shown to greatly improve the cycling

stability of the LixV2O5/LiMn2O4 cell. In the light ofthese works, further modification such as surface coat-ing on V2O5-SiO2 and lower electrolyte amount maylead to improvement in the cycling stability of V2O5-SiO2 in aqueous electrolytes.

Fig. 9 SEM images of a, b V2O5-SiO2 and c, d V2O5 electrodes before and after 60 cycles at a current density of 50 mA g−1, respectively

Fig. 8 Cycling performance of V2O5-SiO2|1.0 M Li2SO4|LiMn2O4 (a) and V2O5|1.0 M Li2SO4|LiMn2O4 ARLBs (b) at a current density of 50 mA g−1

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Conclusions

The V2O5-SiO2 hybrid was obtained via a facile thermaltreatment method using commercial V2O5 together withas-prepared SiO2. When used as the electrode materialfor ARLBs, V2O5-SiO2 showed that the intercalationand de-intercalation of lithium ions are similar to thoseof other vanadium oxide anodes in aqueous electrolytes.The porous SiO2 support can enhance the initial capac-ity of V2O5 (up to 199.1 mAh g−1) at 50 mA g−1 dueto the increased contact area between V2O5 and theelectrolyte. However, at the same time, a decrease inthe cycling life was observed. Thus, for future work,the surface coating on V2O5-SiO2 could be an effectiveway to improve both the capacity and cycling stabilityof vanadium oxide in aqueous electrolytes.

Acknowledgments Financial support from the National NaturalScience Foundation of China (grant nos. 21163018, 21363022,and 51362027) and JCYJ20140418095735600 is greatlyappreciated.

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