New Energy and Materials Laboratory (NEML), Department of Chemistry & Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials,udan University, Shanghai 200433, ChinaCollege of Chemistry and Chemical Engineer, Jiangxi Normal University, Nanchang 330022, China
r t i c l e i n f o
rticle history:eceived 12 August 2013eceived in revised form 23 October 2013ccepted 14 November 2013vailable online 26 November 2013
a b s t r a c t
A nanocomposite of molybdenum trioxide (MoO3) nanobelts coated with polypyrrole was prepared as ananode material for aqueous rechargeable sodium batteries (ARSBs). When nanowire Na0.35MnO2 is usedas the cathode, the ARSB can deliver an energy density of 20 Wh kg−1 at 80 W kg−1 and even maintain 18Wh kg−1 at 2.6 kW kg−1 in 0.5 mol L−1 Na2SO4 aqueous electrolyte, suggesting a good rate capability thatcan be comparable with supercapacitors. In addition, its cycling behavior is greatly improved comparedwith the virginal MoO . This will provide a new direction to explore non-carbon anode materials for
eywords:queous rechargeable sodium battery
ARSB)nodeathodeolypyrrole
3
ARSBs with excellent electrochemical performance. This good performance exhibits that this battery willbe a promising candidate for the storage of solar and wind energies.
Energy storage systems are of great importance for the extensiveeployment of renewable energy, due to the intermittent naturef solar and wind energies [1]. In the case of lithium ion bat-ery, there is growing concern about cost and limitation of lithiumerrestrial reserves for large-scale electrical energy storage appli-ation besides the safety and reliability problem [2]. Therefore, newtrategies toward replacing lithium ion batteries with energy stor-ge systems based on more Earth-abundant elements are neededecause of limited lithium sources [3].
Sodium ion batteries (SIBs) have recently received interest as low cost alternative to lithium ion batteries for large-scale elec-ric storage applications, because of the high availability of sodiumources, and the similar chemistry of sodium and lithium [4]. How-ver, it is difficult to find a suitable host material for reversiblea-ion storage because Na+ ion (0.102 nm) are about 40% larger in
adius than Li+ ion (0.076 nm) [5]. Hence, finding and optimizinguitable electrode materials are crucial for the development of Na-on batteries. Recently, a large variety of host materials, such as
layered oxides [6,7], polyanion fluorophosphates [8], hexacyano-ferrate [9,10] and organic polymers [11], has been demonstratedas sodium ion insertion cathodes, with certain redox capacity andcycling life. In contrast, few anode materials have been reported.For anode materials, most of works are focused on hard carbonsince the large Na+ ions are difficult to insert into graphitic layers[2]. Furthermore, only a few studies have reported the sodium stor-age properties of metal oxides including Fe3O4 and �-Fe2O3 [12],Sb2O4 thin film [13] and TiO2 nanotubes [14]. However, most ofthem exhibit relatively low capacity and poor cycling performance.
In the case of batteries based on aqueous systems, they are muchsafer than those based on organic electrolytes [15–19], and areregarded as promising candidates for large-scale energy storage.Formerly we found that a layered NaMnO2 can be a good cathodematerial for aqueous rechargeable sodium batteries (ARSBs) sinceits cycling behavior is very excellent, with only slight capacity fad-ing in 0.5 mol L−1 Na2SO4 aqueous solution after 10000 cycles evenwhen oxygen is not removed [20].
Among many metal oxides investigated as electrode materi-als [21], molybdenum oxide (MoO3) has attracted great interestas an anode material, due to a superior theoretical specific capac-
ity of nearly 1111 mAh g−1, which is about three times of thatfor graphite [22]. Moreover, MoO3 is a layered structure whichis formed by stacking bilayer sheets of MoO6 octahedra with vander Waals forces [23]. These structures are precisely suitable for
Y. Liu et al. / Electrochimica Acta 116 (2014) 512– 517 513
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ig. 1. (a) SEM and (b) TEM micrographs of the virginal MoO3 nanobelts, (c) TEManowires.
nsertion/removal of small ions such as H+ and K+. However, itsow electronic conductivity and structural collapse in an aqueouslectrolyte are detrimental to high-rate and long-term cycling per-ormance in electrochemical devices [24]. To improve the energyensity of ARSBs, a new direction is to replace the hard carbonaterials with oxides.Here we found that a polypyrrole (PPy) coating layer on the sur-
ace of the MoO3 nanobelts can be a good anode material for Na+
on host. Combining this anode with a cathode from Na0.35MnO2anowires, an ARSB is built up by using 0.5 mol L−1 Na2SO4 aque-us solutions as the electrolyte. It presents good rate capability asell as excellent cycling performance.
. Experimental
.1. Preparation of anode materials
All reagents were analytical grade. The MoO3 nanobelts wererepared by a hydrothermal method [25]. In a typical synthesis,.21 g of Na2MoO4·2H2O and 0.6 g of NaCl were dissolved in 40 mLeionized water. After being gently stirred for 10 min, 3 mol L−1
Cl was added into the solution with stirring to reach a pH of 2t room temperature. The reaction solution was then transferrednto a 100 mL Teflon-lined stainless steel autoclave and kept in anven at 180 ◦C for 24 h. The autoclave was left to cool naturallyo room temperature; the obtained precipitate was collected byuction filtration, washed several times with deionized water andbsolute ethanol, and dried at 80 ◦C under vacuum overnight to getoO3 nanobelts.The PPy@MoO3 nanocomposite was prepared through the poly-
erization of pyrrole (Py) on the MoO3 nanobelts surface usingodium dodecylbenzenesulfonate (NaDBS) as a surfactant and FeCl3s an oxidant [26]. Briefly, 0.2 g MoO3 was dispersed into a flask con-
aining 160 mL 0.05 mol L−1 aqueous solution of NaDBS and stirredor 0.5 h, then 0.1 mL pyrrole and 10 ml 0.2 M FeCl3 solution weredded into the aqueous solution. The mixture was kept at 0 ◦C for
h under stirring. Finally, the obtained composite was filtered and
graph of the PPy@MoO3 nanocomposite and (d) SEM micrograph of Na0.35MnO2
washed with water and ethanol, respectively, and then dried undervacuum at 40 ◦C overnight.
The Na0.35MnO2 nanowire was prepared by a hydrothermalmethod, which has been described in our former report [27].Typically, a nanowire birnessite-MnO2 (�-MnO2) was at firsthydrothermally synthesized by the mixed solution of MnSO4,(NH4)2S2O8 and (NH4)2SO4 in a molar ratio of 1:1:4 at 140 ◦C. Sub-sequently, 0.2 g of the as-prepared �-MnO2 powder was dispersedin 35 mL 5 mol L−1 NaOH aqueous solution, and then the solutionwas placed in a 50 ml Teflon-lined autoclave. The autoclave washeated at 205 ◦C for 48 h. After that, the precipitated powder wasfiltered, washed with water three times, and then dried at 60 ◦C.
2.2. Characterization of the prepared anode materials
Crystal structures of the prepared MoO3 and PPy@MoO3nanocomposite were characterized by X-ray powder diffraction(XRD) using a Bruker Analytical X-ray Systems with CuK� radiationsource filtered by a nickel thin plate. Scanning electron micrographs(SEM) and transmission electron micrographs (TEM) were obtainedon a Philips XL30 scanning electron microscope and a JEOL JEM-2010 transmission electron microscope, respectively.
For electrochemical tests, the anode was prepared by pressinga powdered mixture of the sample (PPy@MoO3 or virginal MoO3),acetylene black, and poly (tetrafluoroethylene) (PTFE) in a weightratio of 8:1:1. The obtained sheet was punched into small diskwith about 2 mg in mass, 0.64 cm2 in area and 0.4 mm in thick-ness. Finally, these disks were pressed onto Ni-grids at a pressureof 15 MPa and then dried under vacuum at 120 ◦C for 12 h. TheNa0.35MnO2 nanowire cathode was prepared by the same way asthe anode. The aqueous Na2SO4 solution of 0.5 mol L−1 was used asthe electrolyte.
Electrochemical impedance spectroscopy (EIS) was carried out
with an electrochemical work station (CHI604 C, Chenhua Ltd. Co.,Shanghai, China). The amplitude of the alternating current per-turbation was 5 mV and the frequency range was 105 to 10−3 Hz.Galvanostatic charge and discharge between -0.7 and 0.4 V (vs. SCE)
514 Y. Liu et al. / Electrochimica Ac
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ig. 2. (a) XRD patterns and (b) thermogravimetic analysis of the virginal MoO3
anobelts and the PPy@MoO3 nanocomposite under air.
as performed with a three-electrode electrochemical cell by using saturated calomel electrode (SCE) and a nickel mesh as the ref-rence and the counter electrodes, respectively. A two-electrodeell consisting of the cathode and the anode with a distance ofbout 1 cm was used to test the electrochemical properties ofhe ARSB based on the PPy@MoO3/0.5 mol L−1 Na2SO4 aqueousolution/Na0.35MnO2 and the MoO3//Na0.35MnO2. All electrochem-cal measurements were performed at ambient temperature.
. Results and discussion
The prepared ˛-MoO3 presents one-dimensional nanobeltshich are about 5 �m in length and 300 nm in width (Fig. 1a and
). TEM micrograph (Fig. 1c) of the PPy@MoO3 nanocompositelearly demonstrates the presence of a polypyrrole skin coveringhe ˛-MoO3 nanobelts and the thickness of PPy coating is about0-80 nm. SEM micrograph of Na0.35MnO2 is shown in Fig. 1d. Itan be observed that the obtained Na0.35MnO2 presents a nanowiretructure which is about 30 nm in diameter.
The X-ray diffraction (XRD) patterns of the virginal MoO3 andPy@MoO3 nanocomposite are shown in Fig. 2a. All the diffractioneaks can be indexed as the orthorhombic MoO3 phase with the
attice parameters a = 0.396 nm, b = 1.386 nm, and c = 0.370 nm, inood agreement with the literature values (JCPDS file No.05-0508).urthermore, the intensities of the (020), (040) and (060) diffractioneaks of the virginal MoO3 nanobelts are stronger than those of thether peaks, which is different from those in the standard spectrumf JCPDS file No.05-0508 and the reported �-MoO3 [28,29], imply-
ng that there is a layered crystal structure or a highly anisotropicrowth for the prepared nanobelts [30,31]. The peak intensity ofhe PPy@MoO3 nanocomposite is obviously weaker than that of theirginal MoO3 nanobelts. This can be mainly attributed to the PPy
ta 116 (2014) 512– 517
coating [32]. However, their peak positions are the same as thoseof the virginal MoO3 nanobelts. As for the (021) diffraction peak ofthe nanocomposite, its intensity is stronger than that of the (040)diffraction peak indicating the existence of semi-crystalline PPy atabout 2� = 27o [33]. TG curve (Fig. 2b) of the PPy@MoO3 nanocom-posite shows its weight loss slope. The weight loss below 100 ◦Ccan be attributed to the loss of physical adsorbed water. The masspercentage of PPy in the nanocomposite was estimated to be about60% by the weight loss above 200 ◦C.
CV curves of the prepared PPy@MoO3 nanocomposite and theNa0.35MnO2 nanowire at the scan rate of 1 mV s−1 in 0.5 mol L−1
Na2SO4 aqueous electrolyte (Fig. 3a) show that the two sets ofredox potentials for the nanocomposite are situated at -0.25/-0.08 Vand -0.49/-0.34 V (vs. SCE), respectively, which corresponds to thereversible intercalation/deintercalation of Na+ ions into/from theMoO3 host [28]. This means that the PPy@MoO3 nanocompositewill be stable as an anode material for ARSBs. As to the redox poten-tials for the Na0.35MnO2, there are two clearly separated sharpoxidation peaks at 0.72 V and 0.57 V (vs. SCE), respectively, whichare consistent with the deintercalation of Na+ ions from the hoststructure in aqueous electrolytes [35]. However, there is only onereduction peak located at 0.31 V (vs. SCE), which is probably dueto the overlapping of the two reduction peaks. When the scan rateincreases from 1 to 40 mV s−1 (Fig. 3b), the separation betweenthe redox peaks increases due to the increase of overpotentials.However, even at the high scan rate of 40 mV s−1, the redox peaksare still evident. This suggests that the nanocomposite will presentgood rate capability [34]. As to the reason, it can be reflected inFig. 3c, which shows that the PPy@MoO3 nanocomposite presentsa much smaller charge transfer resistance than the virginal MoO3without PPy coating. Evidently, this is due to the PPy coating, whichis a well-known conductive polymer from the doping of anions [35].The PPy coating can partially act as a conductive binder to increasethe contact between particles; therefore, the particle-to-particleresistance and the charge transfer resistance are decreased [16,32].
The galvanostatic charge-discharge curves for the PPy@MoO3electrodes (Fig. 4a) show that its reversible capacity is 33 mAh g−1
which is higher than that of reported literatures [36,41] in aque-ous systems and that for Na0.35MnO2 is 39 mAh g−1 at the currentdensity of 400 mA g−1. As to the nanocomposite, there is no evi-dent voltage plateau. In the case of the nanowire Na0.35MnO2, thereare two weak voltage plateaus, at about 0.5 and 0.7 V, respectively.When the weight ratio of the Na0.35MnO2 to the nanocompos-ite was fixed at about 1:1, an ARSB system consisting of thenanocomposite anode and the nanowire Na0.35MnO2 cathode using0.5 mol L−1 Na2SO4 electrolyte was built up. Results from Fig. 4apresent that the electrochemical window of this ARSB can be from0 to 1.7 V without irreversible reactions, which is in agreementwith the above CV results from Fig. 3a. In the charge and dischargecurves for the full cells, there is a slight plateau indicating the redoxreaction.
The satisfactory cycling behavior of the ARSB consisting ofPPy@MoO3/0.5 mol L−1 Na2SO4/Na0.35MnO2 at 550 mA g−1 (basedon the weight of the anode) is shown in Fig. 4b. The nanocom-posite evidently exhibits an improved cycling behavior comparedwith the virginal MoO3 nanobelts. After 1000 cycles, there is only21% capacity loss and the main capacity loss happens at the initialcycles. The Coulombic efficiency for the initial cycles is not veryhigh, which is the main reason why the weight ratio for the cath-ode and anode is 1:1. Since Na0.35MnO2 has a very high Coulombicefficiency, it is perhaps because some intercalated sodium ions inthe anode could not be completely deintercalated [37–39]. After
the first several cycles the efficiency is almost 100%, which is sim-ilar to that for lithium ion batteries. In contrast, the virginal MoO3nanobelts present a rapid capacity fading. After 50 cycles, the capac-ity retention is less than 30%. It is believed that the Mo ions dissolve
Y. Liu et al. / Electrochimica Acta 116 (2014) 512– 517 515
Fig. 3. (a) CV curves of the nanocomposite of MoO3 coated with PPy and thenanowire Na0.35MnO2 at scan rate of 1 mV s−1, (b) CV curves of the nanocompositeat
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(c)
Fig. 4. (a) Charge-discharge curves of the nanocomposite anode, the nanowireNa0.35MnO2 cathode and the ARSB system consisting of the two electrodes, (b)cycling behavior of the MoO3//Na0.35MnO2 and PPy@MoO3//Na0.35MnO2 full cells
t different scan rates and (c) Nyquist plots of the virginal MoO3//Na0.35MnO2 andhe PPy@MoO3//Na0.35MnO2 in 0.5 mol L−1 Na2SO4 aqueous electrolyte.
n the electrolyte due to the large size of Na+ ions during theharging-discharging process [40]. As for the nanocomposite, thePy coating cannot only inhibit the dissolution of molybdenum ionsut also buffer the possible volume changes during the cycling pro-ess [32,41]. As a result, better cycling behavior is achieved for the
anocomposite of MoO3 coated with PPy.
The Ragone plots of the ARSBs system based on the nanocom-osite and Na0.35MnO2 are shown in Fig. 4c. All the data were
at 550 mA g−1 and (c) Ragone plots of the ARSBs system using 0.5 mol L−1 Na2SO4
aqueous solution.
calculated on the basis of the total mass of the two active elec-trode materials. The energy density for the ARSBs consisting of thevirginal MoO3 is 24 Wh·kg−1 at power density of 48 W·kg−1 andfades to 5.3 Wh·kg−1 at 2.1 kW·kg−1. In contrast, the energy den-sity of the ARSBs consisting of the PPy@MoO3 nanocomposite is 20
Wh·kg−1 at power density of 80 W·kg−1, which is lower than thatfor the virginal MoO3. This is mainly due to the existence of PPyin the nanocomposite, which can be proved by TG curves (Fig. 2b).
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16 Y. Liu et al. / Electrochim
owever, it keeps an excellent rate behavior with an energy den-ity of 18 W·h·kg−1 even at 2.6 kW kg−1, which is superior to theirginal MoO3 for ARSBs. Obviously, this good rate behavior is dueo the PPy coating, which is consistent with the results from thebove CV curves (Fig. 3b). In addition, this kind of excellent rateapability can be compared with the reported supercapacitors inqueous electrolytes [42–44], which is very rare for battery sys-ems. In the case of the cathode, it has a good rate capability dueo the nanostructure [37,45]. As to the anode material, it is knownhat MoO3 has a good electronic conductivity as a semiconductor.ere a conductive PPy coating is further introduced to decrease
he charge transfer resistance as mentioned above to get a goodate capability. Since both anode and cathode materials have goodate capability, the ARSB presents excellent rate behavior, which isuperior to the previous reports.
. Conclusion
In this work, we have developed an aqueous rechargeableodium battery (ARSB) system based on a nanocomposite of MoO3oated with PPy and Na0.35MnO2 in 0.5 mol L−1 Na2SO4 aqueouslectrolyte. The charge and discharge voltage range for this ARSB isrom 0 to 1.7 V. As a result, the PPy coating provides not only goodycling performance but also excellent rate capability for the ARSB.ven at 2.6 kW kg−1, its energy density can be 18 Wh kg−1, 90% ofhat at power density of 80 W·kg−1. This provides a new directiono explore non-carbon anode materials for ARSBs with excellentlectrochemical performance.
cknowledgement
Financial supports from NSFC (21073046) and STCSM12JC1401200) are gratefully appreciated.
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