Hollow−Cuboid Li3VO4/C as High-Performance Anodes for Lithium-Ion BatteriesChangkun Zhang,† Chaofeng Liu,† Xihui Nan,† Huanqiao Song,† Yaguang Liu,† Cuiping Zhang,†
and Guozhong Cao*,†,‡
†Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China‡Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195, United States
*S Supporting Information
ABSTRACT: Li3VO4 has been demonstrated to be a promising anodematerial for lithium-ion batteries with a low, safe voltage and large capacity.However, its poor electronic conductivity hinders its practical applicationparticularly at a high rate. This work reports that Li3VO4 coated with carbonwas synthesized by a one-pot, two-step method with F127 ((PEO)100−(PPO)65−(PEO)100) as both template and carbon source, yielding amicrocuboid structure. The resulting Li3VO4/C cuboid shows a stable capacityof 415 mAh g−1 at 0.5 C and excellent capacity stability at high rates (e.g., 92%capacity retention after 1000 cycles at 10 C = 4 A g−1). The lithiation/delithiation process of Li3VO4/C was studied by ex situ X-ray diffraction andRaman spectroscopy, which confirmed that Li3VO4/C underwent a reversibleintercalation reaction during discharge/charge processes. The excellentelectrochemical performance is attributed largely to the unique microhollowstructure. The voids inside hollow structure can not only provide more spaceto accommodate volume change during discharge/charge processes but also allow the lithium ions insertion and extraction fromboth outside and inside the hollow structure with a much larger surface area or more reaction sites and shorten the lithium ionsdiffusion distance, which leads to smaller overpotential and faster reaction kinetics. Carbon derived from F127 through pyrolysiscoats Li3VO4 conformably and thus offers good electrical conduction. The results in this work provide convincing evidence thatthe significant potential of hollow−cuboid Li3VO4/C for high-power batteries.
The development of high-energy and high-density rechargeablebatteries with good safety and long cycle life is imperative forthe next generation of portable electronics and hybrid and allelectric cars.1−4 In spite of the great success in commercializa-tion and the widespread applications, the current lithium ionbatteries (LIBs) with graphite as the anode generally sufferfrom poor performance in fast charge/discharge processes withpotential safety concerns.1,5 Alternative anode materials withhigh storage capacity and high rate capacity are needed; inaddition, the alternate anode must also meet safety require-ments. Recently, Li3VO4 (LVO) has been studied as analternate anode material for next-generation LIBs.6,7 LVOconsists of corner-sharing VO4 and LiO4 tetrahedrons.
7 LVOhas small volume changes during lithiation/delithiationprocesses and fast diffusion of lithium ions.8,9 In comparisonwith other anodic materials under intensive study, lithium ionscan be intercalated to LVO between 0.5 and 1.0 V vs Li/Li+, forexample, appreciably lower than that of Li4Ti5O12 (∼1.56 V vsLi/Li+), and has a high theoretical capacity of 394 mAh g−1,corresponding to x = 2 in Li3+xVO4 with the end dischargingvoltage of 0.2 V.9 However, the pristine LVO exhibited poor
rate capacity because of its low electrical conductivity (∼5.8×10−6 S cm−1 for LVO10). Hybridization with carbon has provedas an effective way to enhance electrical conductivity, leading tohigh performance.9,11−15 For example, carbon-encapsulatedsynthesized LVO presented exceedingly good rate capability (areversible capability of 450, 340, and 106 mAh g−1 at 0.1, 10,and 80 C, respectively) and long cycling performance (80%capacity retention after 2000 cycles at 10 C).11 Other carbonmaterials such as graphite,13 carbon nanotube,15 andgraphene9,14,16 have also been reported to coat on LVO. Shiet al.9 synthesized a hollow LVO/graphene microboxcomposites by sol−gel method, and the composites showed areversible capacity of 378 mAh g−1 at 0.1 C rate. Hollow LVO/carbon nanotube has also been reported,15 though the high rateperformance was not attained.This paper reports a one-pot, two-step method for the
synthesis of hollow LVO/C microcuboid composites withexcellent lithium ion storage properties. The PluronicF127 was
Received: October 15, 2015Accepted: December 10, 2015Published: December 10, 2015
not only selected as template to form hollow structure, but alsoserved as carbon source in this method. The synthesis wasaccomplished via a sol−gel method and can potentially bescalable to large quantities for industrial production. Theresulting unique hollow LVO/C microcuboid compositeexhibits high capability of 481 mA g−1 at 0.1 C. When at 5and 20 C, it can yield a reversible capability of 329 and 230 mAh g−1 respectively. After 1000 cycles at 10 C, the hollow LVO/C material can deliver 92% capacity retention. The synthesismechanism and the relationship between the nano-/micro-structure and much enhanced electrochemical properties of theresulting LVO/C cuboid composite anode have been discussed.
2. EXPERIMENTAL SECTION2.1. Material Synthesis and Characterization. The LVO/C
samples were synthesized by a sol−gel method. The PluronicF127block copolymer was first dissolved in water and ethanol solution.Then, 0.41 g of V2O5 (0.225 mol L−1, TianJin·FUChemical, >99.0%)and 0.568 g of LiOH·H2O (1.35 mol L−1, Sinopharm ChemicalReagent, >98.0%) were added, and the mixture was then sonicated for10 min and stirred for 5 h at room temperature. The solution pH is12.3; we did not control the pH during the process. The precursor
(LVO-F127) was dried and collected at 55 °C. The precursors weresubsequently treated at 450, 600, and 750 °C, respectively, for 2 h in 1atm Ar (>99.999%) to form LVO/C-450, LVO/C-600, and LVO/C-750 composites. The F127 concentration in the solution was set as 0.5,1.0, and 5.0% (w/v) respectively. LVO without F127 template was alsosynthesized, and the annealing process was set at 600 °C for 2 h in Ar.
The crystalline structure of the samples was detected by X-raydiffraction (XRD, MXP21 VAHF) using Cu Kα radiation (λ = 1.5418Å). The morphologies were characterized by the field-emissionscanning electron microscopy (SEM) and transmission electronmicroscopy (TEM, JEOL JEM-2010). The surface area was testedby N2 adsorption−desorption analyses (ASAP 2020 HD88).Thermogravimetry/differential scanning calorimetry (TG/DSC) wasconducted on a simultaneous thermal analyzer (STA 449F3,NETZSCH) from 25 to 600 °C at 10 °C min−1. X-ray photoelectronspectra (XPS) were recorded on an X-ray photoelectron spectrometer(ESCALAB 250Xi) referencing the C 1s peak to 284.6 eV. Ramanspectra were collected with a Horiba JOBIN YVON Raman system(LabRAM HR Evolution) using an argon-ion laser (532 nm) as theexcitation source.
2.2. Electrochemical Characterization. Electrochemical per-formances were evaluated with standard CR2032 coin cells. Theelectrode consisted of 75 wt % active material, 20 wt % acetylene black,and 5 wt % sodium carboxymethyl cellulose (CMC) as binder on Cu
Figure 1. Schematic diagram of the synthesis procedure of the hollow LVO/C microcuboid composite.
Figure 2. (a) XRD patterns of the different composites. (b) TG/DSC curves of the LVO/C-600. (c) V 2p XPS spectra of the LVO/C-600. (d) C 1sXPS of the LVO/C-600.
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foil with a loading mass of 1 mg cm−2. Then, the electrode was dried at120 °C for 12h. The separator was NKK TF4840, and the electrolytewas 1 M LiPF6 solution in ethylene carbonate (EC)/dimethylcarbonate (DMC) (EC/DMC = 1:1 in volume). All cells wereassembled in a glovebox filled with water and oxygen contents < 0.1ppm, using Li metal as the counter and reference electrodes. Thegalvanostatic charging/discharging measurements were conducted ona LAND (Wuhan, China) automatic battery tester. Electrochemicalimpedance spectroscopy (EIS) and cyclic voltammetry (CV) wascarried out on a Solartron Instrument. The frequency of EISmeasurement ranged from 100 kHz to 0.1 Hz.
3. RESULTS AND DISCUSSIONFigure 1 shows the schematic diagram of the synthesisprocedure. The PluronicF127 copolymer has a central blockof poly(propylene oxide), PPO, and end blocks of poly-(ethylene oxide), PEO, resulting in the structure (PEO)100−(PPO)65−(PEO)100. In the sol−gel method, the F127 was firstdissolved in water and ethanol solution. Then, V2O5 and LiOHwere added; the yellow mixture turned to a white dispersionafter sonication for 10 min and was stirred further for 5 h atroom temperature. The precursor LVO-F127 was dried at 55°C. The XRD pattern shown in Figure 2a reveals that theprecursor LVO-F127 has already transformed into LVOcompletely (Powder Diffraction File No. 38-1247, JCPDS).The total reaction equation was as follows:16
+ → +− −V O 2OH 2VO H O2 5 3 2 (1)
+ → +LiVO 2LiOH Li VO H O3 3 4 2 (2)
The TGA result at O2 atmosphere (Figure S1) reveals thatthe template F127 decomposed above 350 °C. The precursorwas subsequently treated at high temperatures for 2 h to formLVO/C composites. The XRD analysis in Figure 2a shows thatall the samples could be the LVO orthorhombic phase, withoutany preferential crystal orientation, thus suggesting apolycrystalline material. In addition, there is no detectableshift of XRD peaks, indicating that the different annealingtemperature did introduce neither impurities nor ionic defects.In Figure 2b, the slow mass decreasing from the TG at O2 can
be attributed to the carbon oxidation along with the increasingof temperature. The carbon content in the LVO/C-600composite is about 7.1 wt %, which corresponds to theendothermic DSC peak at about 350 °C.Figure 2c,d shows the V 2p and C 1s XPS spectra of LVO/C-
600. The peak at 525.5 and 517.6 eV can be ascribed to V 2p1/2and V 2p3/2 electrons for V in the pentavalent state, whichmatches well with a previous report for pure LVO.13 The peakat 523.9 eV corresponds to an X-ray satellite of O 1s.17
Combined with the XRD and XPS results, we can conclude thatthe residual carbon in Ar gas at 1 atm would not reduce thevanadium ions from pentavalent to tetravalent state. The resultsare different from the findings reported in our previous paper,11
which can be attributed to the different synthesis method. Inthe present study, the LVO phase has already formed beforeheat treatment, whereas in our earlier work, the in situ carboncoating through reduction of organic molecules was introducedduring the synthesis and formation of LVO with vanadium(IV)acetylacetonate and lithium hydroxide as precursors.11 A smallamount of tetravalent vanadium ions could be found in otherpentavalent vanadium oxides from metal doping such as Mo−LiV3O8
18 and Ni,19 Sn20 doping V2O5, and insert atmosphereannealing such as N2-annealed V2O5 xerogel.
21 There are threepeaks at 284.6, 286.2, and 289.5 eV for C 1s. The first peakarises from CC on the surface of LVO, and the other twosuggest the existence of C−O and CO, respectively,indicating the formation of carbonated species.22,23
The morphologies of the composites were characterizedusing SEM and TEM systems. In Figure S-2a, withoutsurfactant F127 as template, the pristine LVO particles havean irregular shape with a particle size of 0.8−2.0 μm. With a lowconcentration of F127 (0.5 w/v %), the synthesized LVO/Cparticles are of approximately 350−500 nm, which is muchsmaller than that of pristine LVO, and some particlesaggregated to form sheets (Figure S-3a,b). Small particle sizein LVO/C with a large surface area would facilitate lithium iontransport. Figure S-4 shows the results of nitrogen sorptionanalyses; the specific surface area was found to be 11.9 and 2.7m2 g−1 for LVO/C-600 and LVO samples. With an increased
Figure 3. (a−c) SEM images of the LVO/C-600 sample; (d) HRTEM image of the LVO/C-600 sample.
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concentration of F127 (1.0 w/v %, Figure S3c), more LVO/Csheets were formed. Some sheets turned to form a porousmicrohollow structure. When the concentration of F127 wasincreased further to 5.0 w/v %, only a hollow microcuboidstructured LVO with size of ∼13 × 5 μm2 formed with a wallthickness of 450 nm (Figure 3a−c). The above results clearlyindicate that the presence of surfactant F127 plays an importantrole in the formation of cuboid structure. Annealing at 450 and600 °C in Ar has little effect on the morphologies of the LVO/
C composites (Figure S-5a); however, annealing at 750 °Cresulted in a little breaking of some hollow LVO cuboids(Figure S-5b). From SEM images of sample LVO/C-600 inFigure 3a−c, one can see that the hollow LVO/C microcuboidis assembled by small primary particles of 400 nm in diameter.The HRTEM image of LVO/C-600 composite (Figure 3d)shows the lattice space of 0.39 nm, which can be assigned to the(011) plane.
Figure 4. Top row: SEM images and EDX results. Bottom row: element mapping of C, O, and V for the LVO/C-600 sample.
Figure 5. (a and b) Discharge and charge curves of LVO and LVO/C-600 electrodes for the first and second cycles at 0.1 C. (c and d) CV curves ofthe LVO and LVO/C-600 electrodes for the first and second cycles with the scan rate of 0.2 mV s−1.
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The introduction of F127 has resulted in the formation ofmuch smaller LVO particles, suggesting that the presence ofF127 promoted the nucleation of LVO. F127 may serve as aninitial nucleation site and may also retard the diffusion ofgrowth species. The concentration of surfactant F127 used inthe present study for the synthesis of LVO is much lower thanthe critical micelle formation concentration.24,25 Although thesubsequent solvent evaporation concentrates the solution inwater and F127 and LVO species, the LVO/C with low F127concentration (0.5 and 1.0 w/v %) did not receive the hollowstructure (Figure S-4a−c), which confirmed that the LVOphase has already formed during the stirring process. At lowF127 concentration, the F127 in the solution favors LVOnucleation, resulting in small particle. With increasing F127concentration, more PPO groups of F127 interacted withhydrophobic LVO, inducing the particles to assemble sheet andcuboid structures, and the subsequent solvent evaporationstrengthens this interaction. The hollow−cuboid structure hascollapsed when annealing the LVO-F127 under air atmosphere(Figure S-4d) where F127 has been totally removed, whichshows that F127 has important effects to form the hollow−cuboid structure. The phenomenon is different from othermetal oxides because of the easy synthesis of LVO at roomtemperature.26,27
The energy-dispersive X-ray (EDX) mapping images of C, O,and V elements shown in Figure 4 were used to analyze theelementary distribution in LVO/C-600 sample. The elementalmapping images in Figure 4 display the existence and uniformdistribution of C, O, and V elements, which indicates that C isdistributed uniformly the LVO/C. It is demonstrated in thiswork that F127 can be taken not only as template to formhollow/sheet structure LVO but also as carbon source coatedonto the LVO uniformly.The coin cells were overcharged between 0.2 and 3.0 V vs
Li/Li+ at 40 mA g−1 (0.1 C). Galvanostatic discharge andcharge curves shown in Figure 5a,b demonstrated that thelithium ion intercalation occurs mainly in a voltage range of 1.0and 0.5 V vs Li/Li+. From the charge/discharging flat differenceof the electrodes, the LVO/C-600 has polarization lower thanthat of the pristine LVO especially at second cycle. The lowerpolarization can be derived from the carbon coated onto theLVO uniformly in the LVO/C-600 sample. The first dischargecapacities of the cells with LVO and LVO/C-600 were 469 and604 mAh g−1. The LVO/C-600 electrode delivered a higherdischarge capacity than the bare LVO. At second cycle, thedischarge capacities of LVO and LVO/C-600 electrodes are326 and 481 mAh g−1, which are lower than the initialdischarge, resulting from the formation of the solid electrolyteinterface (SEI) film. The phenomenon can also be seen inother anode materials such as SnO2,
28 CoO,29 and CoSx.30 The
discharge capacity of LVO/C-600 is higher than the theoreticalvalue (394 mAh g−1). Although LVO/C composites in otherworks also have high capacity beyond the theoreticalvalue,14,31,32 there is no explanation given. Jian et al. consideredthat the higher capacity of the LVO/graphene was contributedby graphene that was also taken as conductive agent in theelectrode.16 It is likely a result of acetylene black’scontribution,33 which has been discussed in our previouswork.11
Figure 5c,d shows the CV curves of the electrodes for thefirst and second cycles at 0.2 mV s−1. In the first cycle of LVO/C-600, two reduction peaks were found at 0.56 and 0.36 V,which are attributed to the insertion of Li+ into LVO and the
formation of solid electrolyte interphase (SEI),9,13 and twooxidation peaks at 1.09 and 1.35 V, corresponding to theoxidation of LVO. In the second cycle, the oxidation peaksshifted from 1.09 to 1.10 V and from 1.35 to 1.34 V, and thetwo reduction peaks shifted from 0.56 to 0.77 V and from 0.36to 0.52 V. These changes are usually ascribed to the occurrenceof side reactions on the electrode surfaces and interfaces due tothe SEI formation as well as phase transformation,16 whichcorrespond to the differences of the charging/dischargingcurves (Figure 5a,b). However, for the pristine LVO electrode,only one pair of broad redox peaks was found in the first cycleat ∼0.45 V for reduction and 1.09 V for oxidation. However,two pairs of redox peaks appeared in the second cycle, verysimilar to that of LVO/C-600; all four peaks appeared at almostidentical voltage positions as those of LVO/C-600. It might berelated to the formation of the SEI films and the insertioninstability in the initial stage. In other works, CV curves showvarious amounts of peaks in the first cycle, but all show twopeaks in the second cycle that correspond to 2 Li+ insertion inLVO.9,11,12,16 It should be noted, however, that LVO/C-600exhibits higher current density suggesting that the LVO/C-600electrode has better kinetic properties and better rateperformance.To investigate the structural reversibility of LVO/C upon
lithium ions insertion/extraction, we recorded ex situ XRD andRaman measurements on the LVO/C-600 electrode at differentdischarge/charge states. As shown in Figure 6a, the maindiffraction peaks of LVO can be observed for the electrode atopen voltage (2.95 V). When discharged to 1.0 V, XRD patternremains the same as that obtained at open voltage, showing
Figure 6. (a) Ex situ XRD patterns. (b) Raman spectra ofelectrochemically cycled LVO/C-600 electrode during the firstdischarge/charge process.
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maintenance of the LVO structure. The original LVO peaksbecome much weaker with the increase of depth of discharge.At 0.2 V, the LVO structure has changed significantly; however,no new phase was found, which is different from the previousworks, probably because of the instability of the new phase atambient conditions.6,12,14 In the following charge, all diffractionpeaks of LVO were detected except for the peak at about 37.5.Meanwhile, the recovered peaks when charging to 3.0 Vbecome a little broader, indicating decreased crystallization ofthe original structure.8,12
The Raman spectra shown in Figure 6b also confirmed thatthere is little change for the LVO-600 during the firstdischarge/charge. The band at about 815 cm−1 can beattributed to symmetric stretching of (VO4
3−), whereas theband at around 780 cm−1 can be attributed to asymmetricstretching of (VO4
3−).34 As shown in Table S1, the ratio of thepeak intensity at around 815 and 780 cm−1 decreased withdifferent depth of discharge and recovered when charged to 3.0V. The peak of asymmetric stretching of VO4
3− also positivelyshifted from 815 to 822 cm−1 when discharged to 0.2 V thencame back to 819 cm−1 when recharged to 3.0 V, which can beconducted in the changing of the stretching vibration of VO4
3−
due to lithium ion insertion/extraction.35
The electrochemical performance of the electrode wasmeasured at different current densities. In Figure 7a, thebattery cells were cycled under different rates of 0.1−50 C. TheLVO/C-600 electrode exhibited the higher capacity than thebare LVO electrode especially at a high rate. It can be seen thatdischarge capacities of LVO/C-600 electrode were about 420,402, 371, 329, 280, and 230 mAh g−1 at discharge rates of 0.5,1, 2, 5, 10, and 20 C, respectively. For the LVO electrode, thecapacities were found to be only 260, 213, 176, 123, 87, and 52
mAh g−1 at the corresponding discharging rates. At a currentdensity as high as 50 C (20 A g−1), the capacity of LVO/C-600electrode remained at 145 mA h g−1, which is three times morethan that of LVO (40 mAh g−1). In Figure S-6, the capacity ofLVO/C-750 electrode is similar to LVO/C-600 electrode atlow rate; however, it has fast decay at a high rate. The betterperformance of LVO/C-600 might be attributed to thehomogeneous integration of the LVO with the carbon at 600°C.30 The electrochemical performance of the LVO/C-600shows distinct improvement compared with those reported inthe literature6,9,11,12,14,16,31 (Table S-2).Figure 7c shows the cycling performance of LVO/C-600
electrode at 0.5 C; LVO/C-600 exhibits very stable perform-ance after the first cycle. The discharge capacity is 415 mAh g−1
after 50 cycles, and the Coulombic efficiency remains constantclose to 100%. The discharge capacity is slightly higher thancharge capacity, though the difference is significantly smallerthan our earlier results and diminishes with the increasing cyclenumbers. We do not have a definitely explanation yet; however,it is likely due to two possible mechanisms and requires moreresearch. The first one is probably attributable to a little moreLi+ insertion than extraction during cycling; the LVO/Ccomposite may consist of either lithium ion vacancies in LVOor some lithium ions inserted and retained in carbon coating.The second is due to some irreversible reaction with impuritiesin electrolyte. Furthermore, as shown in Figure 7d, both LVOand LVO/C-600 electrodes exhibit long stable and reversiblecycling property even at a high rate of 10 C (4 A g−1). A goodcycling performance of the LVO/C-600 electrode was obtainedwith 92% capacity retention after 1000 cycles, which is superiorto that reported for vanadium-based anode materials.12
Figure 7. (a) Discharge capacities of the electrodes at various rates. (b) Galvanostatic discharge and charge curves of LVO/C-600 at different rates.(c) Capacity retention and Coulombic efficiency of LVO/C-600 at 0.5 C. (d) Capacity retention and Coulombic efficiency of the LVO and LVO/C-600 at 10 C (4 A g−1).
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EIS measurement has been carried out between 100 kHz and0.1 Hz at 2.54 V vs Li/Li+. The Nyquist plots (Figure 8a) showthat the charge-transfer resistance (Rct) of LVO/C-600 (65 Ω)is much smaller than that of the carbon-free pristine LVO (150Ω), suggesting that the LVO/C-600 electrode has fasterkinetics for lithium ions insertion/extraction. The lithiumdiffusion coefficient, D, could be calculated from the Warburgregion using the following equation:12
σ=D R T A n F C/22 2 2 4 402 2
(3)
where R is the gas constant, T is the absolute temperature, F isthe Faraday constant, n is the number of electrons transferredper molecule, A is the active surface area of the electrode (0.50cm2), C0 is the concentration of lithium ions in the cathode (9.8× 10−3 mol cm−3), D is the apparent ion diffusion coefficient,and σ is the Warburg factor, which is relative to Z′. From theslope of the lines in the inset of Figure 8b can be obtained12
σω′ = + + −Z R RD L1/2
(4)
Using the value(s) of eq 3 (and eq 4), the lithium diffusioncoefficient D of LVO/C is calculated to be 2.17 × 10−16 and1.75 × 10−15 cm2 s−1 (Table 1) for the LVO and LVO/C-600
electrodes, respectively. The lithium diffusion coefficients of theLVO/C-600 composite have increased by an order ofmagnitude compared to that of carbon-free LVO.11 Theseresults show that the LVO/C-600 composite presents a smallercharge-transfer resistance and higher lithium diffusion coef-ficient, which are favorable for improving the electrochemicalperformance that is in accordance with electrochemical results.The superior battery performance of LVO/C-600 composite
is attributed to its unique structural characteristics. First, thecarbonization of F127 provides a continuous electron pathwayin the LVO/C-600. Second, the LVO/C-600 hollow has theopen and porous structure favoring emission, strain/stress, andretaining good structural stability during the discharge/chargeprocess. Third, the LVO/C-600 composite exists as smaller
primary particles that generate larger surface area and surfaceenergy.
4. CONCLUSIONSThe carbon-coated LVO hollow microcuboid material wassynthesized by a one-pot, two-step method. F127 was taken astemplate to form hollow LVO microcuboid structure and thenpyrolyzed to uniformly coat carbon onto LVO, offeringexcellent electrical conductivity throughout the composite.The resultant LVO/C-600 composite demonstrated excellentlithium-ion storage property with capacities of 481 mAh g−1 at0.1 C, 366 mA h g−1 at 10 C, and 145 mA h g−1 even at 50 C(20 A g−1). After 1000 cycles at 10 C, the sample maintained92% of the initial reversible capacity. The voids inside thehollow structure provided more space to accommodate volumechanges and shorten the lithium ions diffusion distance, whichled to small overpotential and fast reaction kinetics. Thisstrategy is facile and effective and readily extended to thepreparation of other carbon-coated lithium transition metaloxides to enhance their lithium-ion storage properties andbattery performances.
■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.5b09810.
TG curve of F127 in O2; SEM images of the pristineLVO, LVO-F127, and Li3VO4/C with 0.5 w/v % F127and 1 w/v % F127 annealed in Ar at 600 °C; SEMimages of the Li3VO4/C with 1 w/v % F127 annealed inAr at 600 °C and 5 w/v % F127 annealed in Air at 600°C; N2 adsorption−desorption isotherm of the pristineLVO and LVO/C-600 samples; SEM images of theLi3VO4 with LVO/C-450 and LVO/C-750 samples; peakintensity of RM at around 815 and 780 cm−1; rateperformance of LVO, LVO/C-450, and LVO/C-750;comparison of the electrochemical performances withprecious works. (PDF)
Figure 8. (a) Nyquist plots for the electrodes after three cycles (the ac amplitude was 5 mV and at 2.54 V vs Li/Li+). Inset shows the equivalentcircuit and the partial enlarged EIS at high frequencies. (b) Linear fitting of Z′ vs ω−1/2 relationship.
Table 1. Discharge Capacity, Rct, and D of the LVO andLVO/C-600 Samples
samples capacity at 0.1 C (mAh g−1) Rct (Ω) D (cm2 s−1)
LVO 469 150 2.17 × 10−16
LVO/C-600 604 65 1.75 × 10−15
ACS Applied Materials & Interfaces Research Article
This work was supported by the “Thousands Talents” programfor pioneer researcher China, Postdoctoral Science Foundationof China (2015M570987), the National Science Foundation ofChina (51374029 and 91433102), Program for New CenturyExcellent Talents in University (NCET-13-0668), andFundamental Research Funds for the Central Universities(FRF-TP-14-008C1).
■ REFERENCES(1) Yoo, H. D.; Markevich, E.; Salitra, G.; Sharon, D.; Aurbach, D.On the Challenge of Developing Advanced Technologies forElectrochemical Energy Storage and Conversion. Mater. Today 2014,17 (3), 110−121.(2) Whittingham, M. S. Ultimate Limits to Intercalation Reactionsfor Lithium Batteries. Chem. Rev. 2014, 114 (23), 11414−11443.(3) Masse,́ R.; Uchaker, E.; Cao, G. Beyond Li-Ion: ElectrodeMaterials for Sodium- and Magnesium-Ion Batteries. Sci. China Mater.2015, 58 (9), 715−766.(4) Liu, C.; Neale, Z. G.; Cao, G., Understanding ElectrochemicalPotentials of Cathode Materials in Rechargeable Batteries. Mater.Today 2015, 10.1016/j.mattod.2015.10.009.(5) Zhang, M.; Wang, T.; Cao, G. Promises and Challenges of Tin-Based Compounds as Anode Materials for Lithium-Ion Batteries. Int.Mater. Rev. 2015, 60 (6), 330−352.(6) Li, H.; Liu, X.; Zhai, T.; Li, D.; Zhou, H. Li3VO4: A PromisingInsertion Anode Material for Lithium-Ion Batteries. Adv. Energy Mater.2013, 3 (4), 428−432.(7) Kim, W.-T.; Jeong, Y. U.; Lee, Y. J.; Kim, Y. J.; Song, J. H.Synthesis and Lithium Intercalation Properties of Li3VO4 as a NewAnode Material for Secondary Lithium Batteries. J. Power Sources 2013,244, 557−560.(8) Ni, S.; Lv, X.; Ma, J.; Yang, X.; Zhang, L. ElectrochemicalCharacteristics of Lithium Vanadate, Li3VO4 as a New Sort of AnodeMaterial for Li-Ion Batteries. J. Power Sources 2014, 248, 122−129.(9) Shi, Y.; Wang, J.-Z.; Chou, S.-L.; Wexler, D.; Li, H.-J.; Ozawa, K.;Liu, H.-K.; Wu, Y.-P. Hollow Structured Li3VO4 Wrapped withGraphene Nanosheets in Situ Prepared by a One-Pot Template-FreeMethod as an Anode for Lithium-Ion Batteries. Nano Lett. 2013, 13(10), 4715−4720.(10) Fu, Q.; Du, F.; Bian, X.; Wang, Y.; Yan, X.; Zhang, Y.; Zhu, K.;Chen, G.; Wang, C.; Wei, Y. Electrochemical Performance andThermal Stability of Li1.18Co0.15Ni0.15Mn0.52O2 Surface Coated withthe Ionic Conductor Li3VO4. J. Mater. Chem. A 2014, 2 (20), 7555−7562.(11) Zhang, C.; Song, H.; Liu, C.; Liu, Y.; Zhang, C.; Nan, X.; Cao,G. Fast and Reversible Li Ion Insertion in Carbon-EncapsulatedLi3VO4 as Anode for Lithium-Ion Battery. Adv. Funct. Mater. 2015, 25(23), 3497−3504.(12) Liang, Z.; Lin, Z.; Zhao, Y.; Dong, Y.; Kuang, Q.; Lin, X.; Liu,X.; Yan, D. New Understanding of Li3VO4/C as Potential Anode forLi-Ion Batteries: Preparation, Structure Characterization and LithiumInsertion Mechanism. J. Power Sources 2015, 274 (0), 345−354.(13) Liang, Z.; Zhao, Y.; Ouyang, L.; Dong, Y.; Kuang, Q.; Lin, X.;Liu, X.; Yan, D. Synthesis of Carbon-Coated Li3VO4 and Its HighElectrochemical Performance as Anode Material for Lithium-IonBatteries. J. Power Sources 2014, 252 (0), 244−247.(14) Liu, J.; Lu, P.-J.; Liang, S.; Liu, J.; Wang, W.; Lei, M.; Tang, S.;Yang, Q. Ultrathin Li3VO4 Nanoribbon/Graphene Sandwich-LikeNanostructures with Ultrahigh Lithium Ion Storage Properties. NanoEnergy 2015, 12 (0), 709−724.(15) Li, Q.; Sheng, J.; Wei, Q.; An, Q.; Wei, X.; Zhang, P.; Mai, L. AUnique Hollow Li3VO4/Carbon Nanotube Composite Anode forHigh Rate Long-Life Lithium-Ion Batteries. Nanoscale 2014, 6 (19),11072−11077.(16) Jian, Z.; Zheng, M.; Liang, Y.; Zhang, X.; Gheytani, S.; Lan, Y.;Shi, Y.; Yao, Y. Li3VO4 Anchored Graphene Nanosheets for Long-Life
and High-Rate Lithium-Ion Batteries. Chem. Commun. 2015, 51, 229−231.(17) Chen, L.; Jiang, X.; Wang, N.; Yue, J.; Qian, Y.; Yang, J., Surface-Amorphous and Oxygen-Deficient Li3VO4−Δ as a Promising AnodeMaterial for Lithium-Ion Batteries. Advanced Science 2015, 2 (9),10.1002/advs.201500090.(18) Song, H.; Liu, Y.; Zhang, C.; Liu, C.; Cao, G. Mo-Doped LiV3O8
Nanorod-Assembled Nanosheets as a High Performance CathodeMaterial for Lithium Ion Batteries. J. Mater. Chem. A 2015, 3 (7),3547−3558.(19) Zheng, Y.-Z.; Ding, H.; Uchaker, E.; Tao, X.; Chen, J.-F.; Zhang,Q.; Cao, G. Nickel-Mediated Polyol Synthesis of Hierarchical V2O5
Hollow Microspheres with Enhanced Lithium Storage Properties. J.Mater. Chem. A 2015, 3 (5), 1979−1985.(20) Li, Y.; Yao, J.; Uchaker, E.; Zhang, M.; Tian, J.; Liu, X.; Cao, G.Sn-Doped V2O5 Film with Enhanced Lithium-Ion Storage Perform-ance. J. Phys. Chem. C 2013, 117 (45), 23507−23514.(21) Liu, D.; Liu, Y.; Pan, A.; Nagle, K. P.; Seidler, G. T.; Jeong, Y.-H.; Cao, G. Enhanced Lithium-Ion Intercalation Properties of V2O5
Xerogel Electrodes with Surface Defects. J. Phys. Chem. C 2011, 115(11), 4959−4965.(22) Chen, D.; Jiang, Z.; Geng, J.; Wang, Q.; Yang, D. Carbon andNitrogen Co-Doped TiO2 with Enhanced Visible-Light PhotocatalyticActivity. Ind. Eng. Chem. Res. 2007, 46 (9), 2741−2746.(23) Cai, Z.; Xu, L.; Yan, M.; Han, C.; He, L.; Hercule, K. M.; Niu,C.; Yuan, Z.; Xu, W.; Qu, L.; Zhao, K.; Mai, L. Manganese Oxide/Carbon Yolk−Shell Nanorod Anodes for High Capacity LithiumBatteries. Nano Lett. 2015, 15 (1), 738−744.(24) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Micellization ofPoly(Ethylene Oxide)-Poly(Propylene Oxide)-Poly(Ethylene Oxide)Triblock Copolymers in Aqueous-Solutions - Thermodynamics ofCopolymer Association. Macromolecules 1994, 27 (9), 2414−2425.(25) Brinker, C. J.; Lu, Y.; Sellinger, A.; Fan, H. Evaporation-InducedSelf-Assembly: Nanostructures Made Easy. Adv. Mater. 1999, 11 (7),579−585.(26) Ming, H.; Ming, J.; Oh, S.-M.; Tian, S.; Zhou, Q.; Huang, H.;Sun, Y.-K.; Zheng, J. Surfactant-Assisted Synthesis of Fe2O3 Nano-particles and F-Doped Carbon Modification toward an ImprovedFe2O3@CFx/LiNi0.5Mn1.5O4 Battery. ACS Appl. Mater. Interfaces 2014,6 (17), 15499−15509.(27) Veliscek-Carolan, J.; Knott, R.; Hanley, T. Effects of PrecursorSolution Aging and Other Parameters on Synthesis of OrderedMesoporous Titania Powders. J. Phys. Chem. C 2015, 119 (13), 7172−7183.(28) Wang, X.; Li, Z.; Zhang, Z.; Li, Q.; Guo, E.; Wang, C.; Yin, L.Mo-Doped SnO2 Mesoporous Hollow Structured Spheres as AnodeMaterials for High-Performance Lithium Ion Batteries. Nanoscale2015, 7 (8), 3604−3613.(29) Guan, H.; Wang, X.; Li, H.; Zhi, C.; Zhai, T.; Bando, Y.;Golberg, D. Coo Octahedral Nanocages for High-PerformanceLithium Ion Batteries. Chem. Commun. 2012, 48 (40), 4878−4880.(30) Wu, R.; Wang, D. P.; Rui, X.; Liu, B.; Zhou, K.; Law, A. W. K.;Yan, Q.; Wei, J.; Chen, Z. In-Situ Formation of Hollow HybridsComposed of Cobalt Sulfides Embedded within Porous CarbonPolyhedra/Carbon Nanotubes for High-Performance Lithium-IonBatteries. Adv. Mater. 2015, 27 (19), 3038−3044.(31) Li, Q.; Wei, Q.; Wang, Q.; Luo, W.; An, Q.; Xu, Y.; Niu, C.;Tang, C.; Mai, L. Self-Template Synthesis of Hollow Shell-ControlledLi3VO4 as a High-Performance Anode for Lithium-Ion Batteries. J.Mater. Chem. A 2015, 3 (37), 18839−18842.(32) Ni, S.; Zhang, J.; Lv, X.; Yang, X.; Zhang, L. SuperiorElectrochemical Performance of Li3VO4/NiO/Ni Electrode Via aCoordinated Electrochemical Reconstruction. J. Power Sources 2015,291 (0), 95−101.(33) Fan, C.; Xu, Z. Lithium Storage Function of Acetylene Black inthe Negative Electrodes of Lithium Ion Batteries. Tansu Jishu 2007, 1(01), 19−21.
ACS Applied Materials & Interfaces Research Article
(34) Tao, Y.; Yi, D.; Li, J. Electrochemical Formation of CrystallineLi3VO4/Li4SiO4 Solid Solutions Film. Solid State Ionics 2008, 179 (40),2396−2398.(35) Burba, C. M.; Frech, R. Raman and Ftir Spectroscopic Study ofLixFePO4 (0 ≤ X ≤ 1). J. Electrochem. Soc. 2004, 151 (7), A1032−A1038.
ACS Applied Materials & Interfaces Research Article
