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Journal of Power Sources 325 (2016) 185e193
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Potentiodynamic and galvanostatic testing of NaFe0.95V0.05PO4/C composite in aqueous NaNO3 solution, and the properties of aqueous Na1.2V3O8/NaNO3/NaFe0.95V0.05PO4/C battery Milica Vujkovi c a, Slavko Mentus a, b, * a b
University of Belgrade, Faculty of Physical Chemistry, Studentski trg 12, 11158 Belgrade, Serbia The Serbian Academy of Sciences and Arts, Knez Mihajlova 35, 11158 Belgrade, Serbia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
NaFe0.95V0.05PO4/C was obtained by sodiation of LiFe0.95V0.05PO4/C in aqueous solution. Better sodiation than lithiation kinetics was evidenced in nitrate solution. The charge storage mechanism shifted towards pseudocapacitance one. Using NaFe0.95V0.05PO4/C in a Na-ion battery, both high rate and long life were evidenced.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 February 2016 Received in revised form 24 May 2016 Accepted 7 June 2016
The NaFe0.95V0.05PO4/C composite is synthesized by electrochemical ion displacement from LiFe0.95V0.05PO4/C composite in aqueous NaNO3 solution. A coulombic capacity amounting to ~105 and ~82 mAh g1 at sodiation/desodiation rate of 500 and 5000 mAg1, respectively, is evidenced. For the sake of comparison the same investigations is performed with LiFe0.95V0.05PO4/C composite in LiNO3 solution, and better capacity retention and rate performance is evidenced for NaFe0.95V0.05PO4/C one. This advancement is found to be due a higher participation of pseudocapacity in the sodiation/desodiation charge storage process. An aqueous battery composed of NaFe0.95V0.05PO4/C cathode, belt-like Na1.2V3O8 anode and NaNO3 solution as an electrolyte, tested galvanostatically, displays long-life performance with only 10% of capacity fade after 1000 charge/discharge cycles. © 2016 Elsevier B.V. All rights reserved.
Keywords: Aqueous batteries Cathode material Olivine Pseudocapacitance Sodium-ion battery
1. Introduction Higher abundance and lower price of sodium against lithium mineral resources intensified recently the development of Na-ion
* Corresponding author. The Serbian Academy of Sciences and Arts, Knez Mihajlova 35, 11158 Belgrade, Serbia. E-mail address: [email protected] (S. Mentus). http://dx.doi.org/10.1016/j.jpowsour.2016.06.031 0378-7753/© 2016 Elsevier B.V. All rights reserved.
batteries as potential, environmentally more friendly, competitors to Li-ion batteries [1e4]. High sodium intercalation ability was found for numerous materials including various oxides [5e9], phosphates [10e15], fluorophosphates [16,17], cyanoferrates [18e20] in both organic [5e9,13,16,17] and aqueous electrolytes [9e12,19e21]. Although ionic radius of Naþ ions is somewhat higher, certain compound structures enabled even faster diffusivity of sodium than lithium ions [9,10,21,22].
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A first attempt to made an aqueous Na-ion full cell was published by Okada [23]. The cell consisted of NaTi2(PO4)3 anode and Na0.44MnO2 cathode, and displayed very short cycle life. However, recent publications indicated very promising behavior of this battery types [24e29]. In fact, the cathode/anode pairs NaTi2(PO4)3/ Na0.44MnO2 [24,25], Zn/Na0.95MnO2 [26], NaTi2(PO4)3/Na2NiFe(CN)6 [27], NaTi2(PO4)3/NaFePO4 [28], Na3Ti2(PO4)3/FePO4 [29] delivered outstanding electrochemical performance in aqueous solutions of sodium salts. Olivine LiFePO4 has been intensively studied as environmentally very acceptable electrode material [30e33]. Electrochemical behavior of NaFePO4 olivine [28,34,35] attracted attention as well. Its direct chemical synthesis resulted in electrochemically inactive maricite phase [34,36]. Thus, sodiation of LiFePO4 from either organic [13,36] or aqueous electrolytes [10,37], was proposed as a suitable way to obtain electroactive olivine NaFePO4 phase. Contrary to a great number of studies of lithium intercalation in LiFePO4, a scant number of studies was aimed to sodium intercalation behavior of NaFePO4 [34,38e42]. Unlike the direct LiFePO4eFePO4 phase transition during lithium deinsertion, the NaFePO4eFePO4 phase transition occurs indirectly, via the formation of the intermediate Na~0.7FePO4 phase, which buffers a large mismatch strain between sodiated and desodiated phase [34,42,43]. Apart of bulk faradic processes in solids, which contribute predominantly to the charge storage capacity of intercalates materials, some surface processes can also participate in the charge storage [44e49]. It was shown that the pseudocapacitance contribution becomes significant on transition from micro-to nanodispersed materials, where also morphology, crystallinity, and charging/discharging rate play role. A pronounced contribution of pseudocapacitance has been found in nanodispersed TiO2 [44,50e52], V2O5 [53,54], Li4Ti5O12 [46] and LiFePO4 [55]. Wang et al. [44] clearly showed that the pseudocapacitance of TiO2 improved its rate performance and may be adjusted as a matter of choice between high energy and high power materials. Pseudocapacitance was found to be main part of capacitance of V2O5 porous micro-/nanotubes at scan rates above 60 mV s1, whereas the diffusion-limited bulk Liþ storage dominated in the coulombic capacity of V2O5 micro-/nanorods at scan rates ranging from 20 to 300 mV s1. By studying the lithium intercalation behavior of graphene-supported LiFePO4/C, in an organic electrolyte, within the potential range 2.5e4.3 V (vs. Liþ/Li), Wang et al. [55] suggested that the capacity of nanosized LiFePO4 observed above 3.5 V (vs. Li/Liþ) and below 3.4 V (vs. Li/Liþ) originated from a pseudocapacitance, rather than from a single-phase diffusioncontrolled Li intercalation. Higher pseudocapacitance contribution was predicted in Na-ion than in Li-ion intercalation systems [46]. The present study continues our previous investigations of various nanostructured materials for possible application in lithium and sodium aqueous rechargeable batteries [10,56e58]. First of all, by analogy with the sodiation of LiFePO4 [10], we proved that the potentiodynamic sodiation of its vanadium doped form, LiFe0.95V0.05PO4/C, described in Ref. [57], is feasible too. Further to this, for sodiation/desodiation reactions of NaFe0.95V0.05PO4/C better rate performance was evidenced in comparison to lithiation/delithiation reactions of LiFe0.95V0.05PO4/ C composite. The explanation of better performance was provided through the analysis of surface charge contribution in overall charge storage processes. Finally, we assembled a Na1.2V3O8/ NaNO3/NaFe0.95V0.05PO4/C aqueous battery and confirmed its both high rate and long-life performance.
2. Experimental 2.1. Synthesis procedures The synthesis of LiFe0.95V0.05PO4/C composite, as a precursor of NaFe0.95V0.05PO4/C composite, was described in detail in our previous paper [57]. In fact, the powdery LiFePO4/C, doped with 5% of vanadium by partial replacement of Fe, was obtained by malonicassisted gel combustion procedure. Shortly, certain amounts of lithium nitrate, iron (II) oxalate-dehydrate and ammonium dihydrogen phosphate were dissolved in deionized water at a molar ratio corresponding to the targeted LiFe0.95V0.05PO4 stoichiometry. Then, glycine (glycine:nitrate ¼ 2:1), malonic acid (~60 wt% of the expected mass of olivine) and ammonium monovanadate (corresponding the 5 mol% of vanadium) were also added. The mixture was transformed into a gel, by heating at 80 C under constant mixing, and such obtained gelled precursor was subjected to autoignition which happened at roughly 200 C. The flocculent combustion product was transferred into a quartz tube furnace and heated first at 400 C (3 h) and then at 750 C (6 h), under reducing atmosphere composed of 5 vol% of H2 in Ar, yielding LiFe0.95V0.05PO4/C powder. In spite of ~5% of incorporated vanadium, original olivine structure was kept, as confirmed by XRD (X-ray diffractometer Philips 1050, working with CuKa radiation). The carbon fraction, amounting to ~13%, was determined by its combustion within a thermogravimetric device (TA SDT Model 2090). Furthermore, by potentiodynamic cycling of LiFe0.95V0.05PO4/C in a saturated aqueous solution of NaNO3, the olivine NaFe0.95V0.05PO4/C electrode was successfully obtained. Nanobelt-like Na1.2V3O8 was synthesized by precipitation, from an aqueous solution of V2O5, H2O2 and NaOH, as described in Ref. [58]. Dry precipitate annealed at 400 C was subjected to X-Ray diffractometry of powder, which confirmed layered structure of sodium vanadate. Versatile insertion capability in various aqueous electrolytic solutions, including NaNO3 one, was already evidenced for this compound [58]. 2.2. The electrochemical experiments To make working electrodes for electrochemical measurements, LiFe0.95V0.05PO4/C (or Na1.2V3O8) powder, carbon black (Vulcan XC72, Cabot Corp) and poly(vinilydene fluoride PVDF) binder, were mixed in an 85:10:5 wt ratio. Then, N-methyl-2-pyrrolidone was added to obtain liquid suspensions. Suspensions were homogenized in an ultrasonic bath and, in droplets, transferred onto glassy carbon support, and dried at 120 C for 4 h under vacuum. The loading of dry composites was 3 mg cm2, however, the results presented in this study relate to the active (carbon-free) fraction only. Such obtained working electrodes, a platinum foil as a counter electrode and a saturated calomel electrode (SCE) were assembled in the three electrode cell. The electrolyte was saturated aqueous solution of NaNO3, equilibrated with air. Both cyclic voltammetry and chronopotentiometric measurements were carried out in a three electrode cell by using a Gamry PCI4/300 Potentiostat/Galvanostat. Cyclic voltammograms (CV’s) were performed at various scan rates ranging from 1 mV s1 up to 400 mV s1. The galvanostatic charging/discharging experiments were conducted in a two-electrode cell, involving NaFe0.95V0.05PO4/C as cathode and Na1.2V3O8 as anode, homogeneously distributed on stainless disc-shaped plates, and saturated aqueous NaNO3 solution as electrolyte. The mass ratio of anode and cathode loading was 1:1. The cell was composed under room conditions. An Arbin BT-2042 device was used for galvanostatic cycling tests, which
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were carried out within the voltage range 0.01e1.2 V, at various current densities in the range 100e4000 mA g1. 3. Results and discussion 3.1. The synthesis of NaFe0.95V0.05PO4/C composite by potentiodynamic sodiation of LiFe0.95V0.05PO4/C, and its investigation in NaNO3 solutions by cyclic voltammetry Previously we demonstrated that olivine LiFePO4 within a LiFePO4/C composite may be completely sodiated by potentiodynamic cycling in an aqueous NaNO3 solution [10], offering an easy way to produce NaFePO4 form, capable of fast sodiation/desodiation reactions. In addition to this, we evidenced that the kinetics of lithiation/delithiation reactions of LiFePO4/C composite may be significantly improved by incorporation up to ~5% vanadium into olivine lattice [57] Such obtained LiFe0.95V0.05PO4/C composite, containing also ~8% of Fe2P phase and ~11% of carbon, was found to be a very promising cathode material for lithium aqueous rechargeable batteries. These findings opened the question whether LiFe0.95V0.05PO4/13%C composite, synthesized for the purposes of this study, may be electrochemically transformed into sodium intercalate, in a manner applied to its undoped analogue LiFePO4/C [10]. Several successive cyclovoltammograms of LiFe0.95V0.05PO4/C in aqueous NaNO3 solution are presented in Fig. 1. A progressive transformation of LiFe0.95V0.05PO4/C to NaFe0.95V0.05PO4/C is clearly visible during the first five cycles (Fig. 1a). Two adjacent anodic peaks and single cathodic peak, which (as measured at scan rate 10 mV s1) are positioned at ~0.034 V,~0.24 V, ~0.13 V vs. SCE, respectively, were observed. Such a cyclovoltammogram type is well-documented in the literature related to NaFePO4 in both organic [32,37,43] and aqueous electrolytes [10,37]. Namely, desodiation оf NaFePO4 occurs through an intermediate phase formation NaFePO4 / Na~0.7FePO4 which transforms further to a completely desodiated product Na~0.7FePO4 / FePO4, while the sodiation occurs in a unique step FePO4 / NaFePO4 [34,39,42]. The first and the second phase transition during the desodiation are accompanied by ~3% and ~13% difference in unit cell volume, respectively, what means higher kinetic barrier for the second transition. During the sodiation, these transitions, and the kinetic barriers as well, appear in the reverse order. Thus the overvoltage required for first phase transition covers the second one, and, as a result, only one reduction peak was observed. By analogy we may conclude that in Fig. 1, the two anodic peaks correspond to the phase transitions NaFe0.95V0.05PO4 / Na~0.7Fe0.95V0.05PO4 and Na~0.7Fe0.95V0.05PO4 / Fe0.95V0.05PO4, respectively, while the
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reduction peak corresponds to a unique reverse phase transition Fe0.95V0.05PO4 / NaFe0.95V0.05PO4 [34,39,42]. As far as the potentiodynamic LiFePO4 delithiation/lithiation is concerned, the phase transition LiFePO4 / FePO4 causes the unit cell volume change of only 6%, and thus unique current peaks are observed in both anodic and cathodic polarization directions [57]. The overlapping of CV curves during repetition of potentodynamic cycles (Fig. 1b) and stability of their main features at various scan rates, even up to very high ones (Fig. 1c), indicate an excellent reversibility of sodiation/desodiation reactions, as well as a good structural stability of this composite. This behavior may be attributed not only to thermodynamic stability of olivine structure, but also to a thick carbon layer around olivine particles which prevents the reaction with aqueous solution leading to the loss in capacity [56]. The dependence of the peak current density (ip) on the square root of the scan rate (v1/2) calculated from the CVs of NaFe0.95V0.05PO4/C in NaNO3 is linear (Fig. 2a). Linear dependence was also obtained for sodiation/desodiation of undoped NaFePO4/C composite as well as for lithiation/delithiation of LiFePO4/C composites [10]. In order to obtain a comparative survey of both lithiation and sodiation reaction kinetics of MFe1xVxPO4/C olivines (M ¼ Li or Na), and their previously reported [10] undoped analogues, in aqueous electrolytes, the absolute values of slopes of Ip-v1/2 plots are calculated and summarized in Table 1. Higher slope means faster reactions. Comparison of intercalation-deintercallation kinetics of undoped and vanadiumdoped olivine samples, either in lithium or sodium solutions, reveals faster processes in doped samples. In a study performed by Whittingham group [59], it was evidenced that, for a soft-chemistry synthesized V-doped olivine LiFePO4, the improvement of electrochemical performance originates from the lowering of diffusion barrier for Liþ ions. Namely by X-ray diffractometry investigations, they found that the effective cross-sectional area for the LiO6 octahedral faces, responsible for Liþ ion diffusion, is larger for doped than undoped sample. It is reasonable that such explanation may apply to faster sodiation/desodiation precesses of vanadiumdoped olivine sample. A further comparison of the data in Table 1 shows faster cathodic reactions with Naþ than with Liþ ions. The slopes for desodiation are slightly lower than those for delithiation, however, this is simply the consequence of spreading of desodiation charge along the two anodic steps. Thus, the slopes of Ip-v1/2 plots alone are not a quite representative criteria for the comparison of sodiation vs. lithiation kinetics. Instead, the integration of CVs, giving the amount of stored charge, could be a more reliable
Fig. 1. Cyclic voltammograms of vanadium-doped olivine in aqueous NaNO3 solution measured at 10 mV s1: a) electrochemical displacement of Liþ with Naþ ions b) the demonstration of cyclic stability of formed NaFe0.95V0.05PO4/C composite (6th15th cycle), c) the cyclic voltammograms recorded at very high scan rates.
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Fig. 2. (a) Current peak maxima of NaFe0.95V0.05PO4/C in NaNO3 solution in the function of the square root of scan rate (b) the specific capacity obtained by integration of CVs of MFe0.95V0.05PO4/C (M ¼ Li and Na) in LiNO3 and NaNO3, respectively.
Table 1 The survey of the slopes of the Ip-v1/2 dependence for MFe0.95V0.05PO4 (M ¼ Li or Na). Composite
Slope e anodic scan (A g1/V1/2 s1/2)
Slope e cathodic scan (A g1/V1/2 s1/2)
LiFePO4 in LiNO3 NaFePO4in NaNO3
73.4 82 1st peak 102 2nd peak 134 90.21st peak 98.02nd peak
73.1 138.3
LiFe0.95V0.05PO4/C in LiNO3 NaFe0.95V0.05PO4/C in NaNO3
measure for comparison. The coulombic capacity, calculated from the surfaces under CV peaks for various scanning rates, is presented in Fig. 2b. At low scan rates, 5 and 10 mV s1, similar capacity of was measured in both LiNO3 and NaNO3 solutions. However, at higher scanning rates, significantly better capacity retention was observed during the sodiation/desodiation than during the lithiation/delithiation reactions. At an unusually high scan rate of 300 mV s1, about 83% of the initial capacity (registered at 5 mV s1) was retained, which is significantly higher from ~43% observed for lithiation/delithiation reactions. This contradicts to the expectations based on the ratio of ionic radii. Namely, one may expect that smaller and more mobile Liþ ions may response faster to the potentiodynamic polarization. This contradiction raised the question about the role of pseudocapacitance current, which regularly accompanies intercalation current under potentiodynamic conditions. This question is considered in the next section. 3.2. Surface versus bulk electrochemical processes of MFe0.95V0.05PO4/C (M ¼ Li or Na) composite in aqueous electrolytic solutions In cyclic voltammetry, faradaic current is superposed to capacitance current, originating from double layer capacitance and from pseudocapacitance, if surface processes requiring charge transfer take place [50e53]. Relative contribution of faradaic current in the total current depends not only on the polarization rate but also on the structure of material [44,46,50e55], its morphology [54] and particle size [44]. In order to analyze relative contribution of faradaic and capacitance current in lithium and sodium intercalation/deintercalation processes, we used as the criteria the dependence I ¼ avb (where v is the scan rate and a and b are the system specific parameters), [44,51]. Namely the exponent value b~0.5 is characteristic for faradaic processes, characteristic of diffusion limitation, while
132 154.5
b z 1 is characteristic of surface (double layer and pseudocapacitance) processes [44,51]. The electrochemical behavior of LiFe0.95V0.05PO4/C composite in aqueous LiNO3 solution was investigated in detail in our previous paper [57]. In Fig. 3a, cyclic voltammograms recorded in that system within a broad range of scan rates, 1 mV s1 to 100 mV s1 were presented. Using peak currents, from the slope of log I vs. log(v) plot (Fig. 3b) b exponent was calculated to amount to ~0.5 for both anodic and cathodic peaks. This result indicated a preferably bulk faradaic behavior of LiFe0.95V0.05PO4/C composite. However, the shape of the frontal parts of both cathodic and anodic peaks, (linear, instead of exponential rise), as well as a considerable peak potentials separation, rising with the rise in scanning rate, contradict to a classic reversible behavior. The described behavior corresponds, in fact, to a model of electrochemical process controlled by purely ohmic resistance, proposed by Dahn [60] and applied in our ref. [11], as well. Analogous peak shape was demonstrated for reversible conducting (metal) phase formation [61]. Somewhat different behavior in the case of sodiation/desodiation processes of NaFe0.95V0.05PO4/C, follow from the shapes of cyclovoltammograms, given in Fig. 3c. In this case, the straight line connects frontal sides of cathodic peaks, however that may not be stated surely for anodic peaks, since they are distanced mutually along current axis. Differences were observed also when the b values of the I ¼ avb equation were determined. The corresponding slopes of the log I ¼ f(log v) dependence derived from the peak currents is shown in Fig. 3d. The slopes were found to lie in the range 0.7e1. Similar b values, ~0.5 for delithiation and ~0.7e0.8 for desodiation were reported for Li4Ti5O12 spinel in an organic electrolyte [46], indicating pronounced pseudocapacitance contribution in the last case. In Fig. 4, for sodiation/desodiation reactions of NaFe0.95V0.05PO4/ C, the values of exponent b, calculated at several fixed potentials,
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Fig. 3. Cyclic voltammograms (a,c) and the logarithm of the height peak current versus logarithm of the scan rate (b,d) for MFe0.95V0.05PO4/C (M ¼ Li and Na) measured in aqueous LiNO3 and NaNO3 solution at various scan rates, respectively. The CVs in LiNO3 solution obtained formerly [57] for the scan rates up to 50 mV s1 do not differ from the corresponding ones in Fig 3a.
are presented in function of potential. Observing the cathodic process (sodiation) the b-values found in the peak potential region correspond to a bulk intercalation reaction, while, at the potentials outside of the peak region, a pseudocapacitive behavior prevail. However, b values measured for anodic, desodiation direction, although having slight minima at peak potentials, show preferably pseudocapacitive behavior. This finding actualized the need for more accurate determination of relative contributions of bulk and surface charging/discharging processes. This analysis may be carried out by means of the equation [44e51]:where the first addend (C1v) on the right side corresponds to the pseudocapacitance current (theoretically obeying linear dependence of peak current on the scan rate) and the second addend corresponds to bulk faradaic reaction (theoretically obeying linear peak current dependence on square root of scan rate). To simplify its use, Eq. (1) may be expressed in the following form:
. ip v1=2 ¼ C1 v1=2 þC2 Fig. 4. The values of exponent b at various potentials during anodic (square) and cathodic (circle) potentiodynamic scans of NaFe0.95V0.05PO4/C in an aqueous NaNO3 solution.
I ¼ Ic þ If ¼ C1 v þ C2 v1=2
(1)
(2)
This linear function of v1/2, gives C2 as the intercept on the ordinate axis, and C1 as the slope. Upon determination of C1 and C2, by means of Eq. (1), one may calculate Ic and If values separately in function of scan rate for fixed potentials, and they are given in Fig. 6 for both cathodic and anodic scans for NaFe0.95V0.05PO4/C in NaNO3 solution. As expected on the basis of its linear dependence on scan rate, with the increase of the scan rate, the contribution of pseudocapacitance current increases much faster than the bulk intercalation
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Fig. 5. Surface pseudocapacitance current (square) and bulk current (circle) in function of scan rate, calculated for the redox peaks of NaFe0.95V0.05PO4/C in NaNO3 solution.
Fig. 6. Chronopotentiometric curves of NaFe0.95V0.05PO4/C in an aqueous NaNO3 solution at various current rates (a) and corresponding coulombic capacity in function of the charging/discharging rate (b).
one (Fig. 5). During sodiation, in the region of peak current, the pseudocapacitance contribution relative to the bulk current begins to became significant at very high scan rates (Fig. 5a), while during both steps of desodiation, it is significant even at lower scan rates of 5 and 10 mV s1 (Fig. 5b) overcoming significantly the bulk current at higher scan rates (Fig. 5b,c). The fraction of capacitance current for the cathodic and the two anodic peak potentials of CV’s of NaFe0.95V0.05PO4/C in NaNO3 solutions, at various scan rates, are summarized in Table 2. For the sake of comparison, corresponding values are calculated also for lithiation/delithiation processes of LiFe0.95V0.05PO4/C in LiNO3 and
Table 2 Pseudocapacity current fractions in total charging/discharging current calculated at various scan rates using the Eq. (2). Scan rate mV s1
5 10 20 30 50 100 150 200 300
Ic/(IcþId) (NaFe0.95V0.95PO4)
Ic/(IcþId) (LiFe0.95V0.95PO4)
c
a1
a2
c
a
0.069 0.095 0.13 0.15 0.19 0.25 0.29 0.32 0.36
0.53 0.61 0.69 0.73 0.78 0.83 0.86 0.87 0.90
0.29 0.37 0.46 0.51 0.57 0.65 0.70 0.73 0.77
0.033 0.040 0.051 0.071 0.085 0.097 0.107
0.053 0.064 0.081 0.11 0.13 0.15 0.16
they are also presented in Table 2.
3.3. Chronopotentiometry considerations This section contains the results of galvanostatic investigation of the coulombic capacity of NaFe0.95V0.05PO4/C composite in airequilibrated NaNO3 solution, examined at various charging/discharging rates. Fig. 6a shows the galvanostatic charging/discharging curves obtained at rates from 154 mA g1 up to even 5000 mA g1 (i.e. from 1C to ~33 C, accounting with the theoretical capacity of NaFePO4). Desodiation capacity amounted to 127.3, 107.5, 98.9, 91.7, 86.7, 83.3 and 81.9 mAh g1 while the sodiation capacity amounted to 101, 103.6, 97.5, 90, 85.8, 83.3 and 83.3 mAh g1, at current densities 154, 500, 1000, 2000, 3000, 4000 and 5000 mA g1, respectively. At very high current densities capacity remains practically unchanged, which confirms the pseudo-capacitance domination in the charge storage behavior (Fig. 6b). The voltage plateaus, which accompany phase transition during intercalation/deintercalation processes, are not clearly defined. A shortening of plateaus characteristic for two-phase contribution during intercalation/deintercalation processes with the transition from microparticles to nanoparticles is already reported several times [53,55]. This is consequence of both less pronounced phase boundary in nanoparticles and more pronounced role of surface against bulk processes. This may be expected to be even more pronounced in aqueous solutions, where capacity utilization is
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ordinarily lower than in organic solutions. For nano-LiFe0.95V0.05PO4/C composite, we found previously [57] clearly visible plateau in organic electrolyte, but very biased one in aqueous electrolyte. The positions of these plateaus, barely visible in Fig. 6a may be exposed by differentiation of chronoamperometric curves, using coulombic capacity (Q, mAh g1) as a dependent, and potential as an independent variable, as done in Fig. 7. The comparison of anodic (Fig. 7b) and cathodic dQ/dV curves (Fig. 7c) clearly reveals the differences between sodium intercalation and deintercalation processes. Sharp and high peak in Fig. 7c, characterizes a phase transition based coulombic capacity [47,62] during cathodic Fe0.95V0.05PO4/C / NaFe0.95V0.05PO4/C transition. Interestingly, in this presentation, two peaks in the dQ/dV plots can be seen even for the cathodic scan (Fig. 7a) but only at low current rates (500 mA g1). Investigating pure olivine NaFePO4, Fang et al. [37] showed also two steps during both desodiation and sodiation. Always present deviation of the dQ/dV cathodic baseline from zero, which is especially pronounced in the negative potential region, indicates the existence of pseudocapacitance processes in the whole potential region investigated. As far as the anodic polarization direction is concerned, the broad anodic peaks, together with a permanent deviation of baseline from zero [47,62] confirm a notable role of pseudocapacitance processes during both NaFe0.95V0.05PO4/C / Na~0.7Fe0.95V0.05PO4/C and Na0.7Fe0.95V0.05PO4/C / NaFe0.95V0.05PO4/C phase transitions. As expected, the peak height of both anodic and cathodic dQ/dV curves decreases with the increase of current rate, indicating decreasing contribution of “inner” (bulk diffusion) versus “outer” (surface pseudocapacitance) charge, in terms used by Trasatti et al. [63] One can see that this decrease is more pronounced for cathodic curve (about 62%) than that for anodic one (~30% for the first and ~54% for the second peak), which is in accordance with the measurements presented in Fig. 5. In comparison to the recently reported nasicon type compound, NaTi2(PO4)3 [11], in a common NaNO3 solution, the NaFe0.95V0.05PO4/C composite delivers higher charging/discharging capacity. For instance, the desodiation capacity of olivine at a rate of 500 mA g1 amounted to 103 mAh g1, while corresponding value for nasicon NaTi2(PO4)3 amounted only to ~70 mAh g1. Accordingly, one can say that sodiated form of vanadium doped olivine presents a very promising cathode material for development of sodium rechargeable aqueous batteries. To explain the origin of pseudocapacitance, it can be said that during cathodic polarization, a part of Naþ I ions, involved in the first Fe0.05V0.95PO4 / Na~0.7Fe0.95V0.05PO4 phase transition, probably occupies bulk intercalation positions of olivine, while the other part of Naþ ions, involved in the second II Na~0.7Fe0.95V0.05PO4 / NaFe0.95V0.05PO4 transition, occupies the
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surface sites. Accordingly, in the reverse anodic scan the first anodic peak should originate from pseudocapacitance, as also proved here. However, it was shown that even the second anodic peak involves high fraction of surface charge. We may speculate that this is due to a partial redistribution of Naþ ions during the first anodic NaFePO4 / Na~0.7Fe0.95V0.05PO4 phase transition. The second Na~0.7Fe0.95V0.05PO4 / FePO4 phase transition in principle may take place only after the completion of the first anodic redox process. In fact, during anodic scan all Naþ ions in the NaFe0.95V0.05PO4 bulk are forced by polarization to move towards the electrolyte, but during the first desodiation step, a part of ions, due to the energy insufficiency, remains in the olivine depth. Therefore, it is possible that such part of Naþ ions only reach the surface sites, emptied by the release of Naþ II ions into solution. This may contribute to the pseudocapacitance behavior of the second desodiation step, Na~0.7Fe0.95V0.05PO4 / NaFePO4. Another eligible explanation could be found in the morphology of olivine. It was shown that delithiation of LiFePO4 causes the cracks and high porosity of the FePO4 layer [64] which can improve the kinetics. Since the strain and volume changes are significantly higher in the case of sodium storage, these effects could be more pronounced [38] and may contribute to the higher pseudocapacitance relative to the lithium storage. 3.4. Aqueous rechargeable Na1.2V3O8/sat.NaNO3/NaFe0.95V0.05PO4/C battery Versatile insertion capability of Na1.2V3O8 nanobelts towards various ions, including Naþ ions too, was successfully confirmed in Ref. [58]. Intercalation of sodium ions into Na1.2V3O8 is predominantly one step process, while the desodiation goes through several steps. Interestingly, both NaFe0.95V0.05PO4/C and Na1.2V3O8 showed higher potential barrier for desodiation than for sodiation. That indicates generally strong interaction of the Naþ ions with the host structure. Although the redox peaks of Na1.2V3O8 are spread over broad potential interval, low negative potential of sodiation of Na1.2V3O8 [58], together with a relatively fast sodiation/desodiation kinetics and good cyclic stability, offer the possibility of its use as the anode of an aqueous battery. In NaNO3 solution, the reversible potential of Na1.2V3O8 is positioned at 0.67 V vs. SCE [58] and the reversible potential of NaFe0.95V0.05PO4/C is positioned at 0.24 V vs. SCE (Fig. 1b). An aqueous-type sodium-ion battery, composed from Na1.2V3O8 as the anode, NaFe0.95V0.05PO4/C as the cathode and an aqueous NaNO3 solution as the electrolyte, was assembled. The charge/discharge curves of this battery, obtained after 50 charge/discharge cycles in the voltage range from 0.01 to 1.2 V, were presented in Fig. 8a. According to our measurements, at
Fig. 7. First derivation of chronoamperometric curves (dQ/dV) in function of potential for NaFe0.95V0.05PO4/C in NaNO3 solution at a charging/discharging rate 500 mA g1 (a), and the same dependence at various charging/discharging rates for separately presented desodiation (b) and sodiation (c) process.
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Fig. 8. (a) The charge/discharge curves measured at rate 100 mA g1, and (b) the specific discharge capacity versus cycle number of aqueous Na1.2V3O8/NaNO3/NaFe0.95V0.05PO4/C battery. The numbers inserted in Fig. 8b are the discharging rates in units mA g1, c) the cyclic stability of Na1.2V3O8/NaNO3/LiFe0.95V0.05PO4/C battery.
100 mA g1 the capacity of Na1.2V3O8 in the investigated aqueous nitrate solution is 110 mAh g1, which is almost equal to the capacity of NaFe0.95V0.05PO4/C composite. As visible in Ref. [58], its capacity is remarkably dependent on the discharging rate, and at 500 mA g1 amounts to 50 mAh g1 only. Thus, in the diagram shown in Fig. 8b, at high charging/discharging rates, the capacity of battery is limited exclusively by Na1.2V3O8 anode. The invariability of the curves shape during long cycling indicates a good compatibility of these materials as well as a good reversibility of their sodiation/desodiation reactions during cycling in a two electrode arrangement. The initial charge and discharge capacity was found to be ~120.6 mAh g1 and ~102 mAh g1, respectively. After 50 charging/discharging cycles at a common rate of 100 mA g1, these values were slightly increased amounting to 124.7 mAh g1 and 106.8 mAh g1, respectively. Discharge capacity decreases with the increase of the scan rate from 100 mA g1 to 5000 mA g1, which is a common behavior of all batteries due to the polarization losses. It must be bear in mind that equal masses of Na1.2V3O8 and NaFe0.95V0.05PO4/C, and equal mass percents of conductivity supporting carbon black were used. The cyclic stability of Na1.2V3O8/NaNO3/LiFe0.95V0.05PO4/C battery, presented in Fig. 8c, shows an ultra-long cycling capability of this aqueous system up to 1000 charge/discharge cycles. Its initial coulombic capacity amounted to ~90 mAh g1. After 300 charging/ discharging cycles, the capacity retained its initial value, while after 1000 charging/discharging cycles, discharge capacity retained 90% of the initial value. Although the results of Section 3.2 indicate the influence of pseudocapacitance, the results outlined in Table 2 evidence that, in overall battery capacitance presented in Fig. 8, one deals with dominant role of intercalation/deintercalation processes. 4. Conclusions The LiFe0.95V0.05PO4/C composite, obtained by malonic-assisted gel-combustion procedure, was successfully transformed into its sodiated form, NaFe0.95V0.05PO4/C, by potentiodynamic cycling in an aqueous NaNO3 solution. Further potentiodynamic cycling in the same solution confirmed fast kinetics of sodiation/desodiation reaction, faster than in it undoped analogue, NaFePO4. Like to the NaFePO4 in NaNO3 solution, the sodiation/desodiation process of vanadium doped material proceeds through intermediate phase, causing the split of anodic peaks of cyclovoltammograms. The sodium and lithium charge storage capacities in this material are similar at lower scan rates, while at high scan rates the coulombic capacity of sodiated form prevailed considerably that of lithiated form. The analysis of kinetic data revealed that the excess capacity
stems primarily from pseudocapacitance. High sodium storage capacity of NaFe0.95V0.05PO4/C composite (~100 mAh g1 at rate of 500 mA g1) was found chronopotentiometrically in a threeelectrode arrangement. The aqueous sodium-ion battery with the composition Na1.2V3O8/NaNO3/NaFe0.95V0.05PO4/C was assembled and its coulombic capacity was found to be ~100 mAh g1 at rate of 100 mA g1. After 300 charge/discharge cycles the capacity kept the value equal to the initial one, while 90% capacity retention was observed after 1000 charge/discharge cycles, at mean voltage 0.5 V. Acknowledgements This work was supported by Ministry of Education, Science and Technological Development of the Republic of Serbia through the project III45014. S.M. is also indebted to the Research Fund of Serbian Academy of Sciences and Arts (F190) which supported the project “Electrocatalysis in contemporary processes of energy conversion”. References les, T. Rojo, [1] V. Palomeras, P. Serras, I. Villaluenga, K. Hueso, J. Carretero-Gonza Energy Environ. Sci. 5 (2012) 5884e5901. [2] B.L. Ellis, L. Nazar, Curr. Opin. Solid State Mater. Sci. 16 (2012) 168e177. [3] M.D. Slater, D. Kim, E. Lee, C.S. Johnson, Adv. Funct. Mater. 23 (2013) 947e958. [4] C. Masquelier, L. Croguennec, Chem. Rev. 113 (2013) 6552e6591. [5] S. Komaba, C. Takei, T. Nakayama, A. Ogata, N. Yabuuchi, Electrochem. Commun. 12 (2010) 355e358. [6] D. Buchholz, L.G. Chagas, M. Winter, S. Passerini, Electrochim. Acta 110 (2013) 208e213. [7] D. Yuan, X. Hu, J. Qian, F. Pei, F. Wu, R. Mao, X. Ai, H. Yang, Y. Cao, Electrochim. Acta 116 (2014) 300e305. [8] J.J. Ding, Y.N. Zhou, Q. Sun, X.Q. Yu, X.Q. Yang, Z.W. Fu, Electrochim. Acta 87 (2013) 388e393. [9] K. Kim, C. Yua, C.S. Yoom, S. Kim, Y.-J. Kim, Y.-K. Sun, S.-T. Myunga, Nano Energy 12 (2015) 725e734. [10] M. Vujkovi c, S. Mentus, J. Power Sources 247 (2014) 184e188. [11] M. Vujkovi c, M. Mitri c, S. Mentus, J. Power Sources 288 (2015) 176e186. [12] G. Pang, C. Yuan, P. Nie, B. Ding, J. Zhu, X. Zhang, Nanoscale 6 (2014) 6328e6334. [13] S.-M. Oh, S.T. Myung, J. Hassoun, B. Scrosati, Y.K. Sun, Electrochem. Commun. 22 (2012) 149e152. [14] H. Kim, I. Park, D.H. Seo, S. Lee, S.W. Kim, W.J. Kwon, Y.U. Park, C.S. Kim, S. Jeon, K. Kang, J. Am. Chem. Soc. 134 (2012) 10369e10372. [15] M. Hose, H. Nakayama, K. Nobuhara, H. Yamaguchi, S. Nakanishi, H. Iba, J. Power Sources 234 (2013) 175e179. [16] W. Song, X. Ji, Z. Wu, Y. Yang, Z. Zhou, F. Li, Q. Chen, C. Banks, J. Power Sources 256 (2014) 258e263. [17] W. Song, X. Ji, Z. Wu, Y. Zhu, F. Li, Y. Yao, C. Banks, RSC Adv. 4 (2014) 11375e11383. [18] X. Wu, Y. Luo, M. Sun, J. Qian, Y. Cao, C. Ai, H. Yang, Nano Energy 13 (2015) 117e123. [19] C.D. Wessells, S.V. Peddada, R.A. Huggins, Y. Cui, Nano Lett. 11 (2011) 5421e5425. [20] Y. You, X. Yu, Y. Yin, K.W. Nam, Y. Guo, Nano Res. 8 (2015) 117e128.
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Potentiodynamic and galvanostatic testing of NaFe0.95V0.05PO4/C composite in aqueous NaNO3 solution, and the properties of aqueous Na1.2V3O8/NaNO3/NaFe0.95V0.05PO4/C battery Milica Vujkovi c a, Slavko Mentus a, b, * a b
University of Belgrade, Faculty of Physical Chemistry, Studentski trg 12, 11158 Belgrade, Serbia The Serbian Academy of Sciences and Arts, Knez Mihajlova 35, 11158 Belgrade, Serbia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
NaFe0.95V0.05PO4/C was obtained by sodiation of LiFe0.95V0.05PO4/C in aqueous solution. Better sodiation than lithiation kinetics was evidenced in nitrate solution. The charge storage mechanism shifted towards pseudocapacitance one. Using NaFe0.95V0.05PO4/C in a Na-ion battery, both high rate and long life were evidenced.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 February 2016 Received in revised form 24 May 2016 Accepted 7 June 2016
The NaFe0.95V0.05PO4/C composite is synthesized by electrochemical ion displacement from LiFe0.95V0.05PO4/C composite in aqueous NaNO3 solution. A coulombic capacity amounting to ~105 and ~82 mAh g1 at sodiation/desodiation rate of 500 and 5000 mAg1, respectively, is evidenced. For the sake of comparison the same investigations is performed with LiFe0.95V0.05PO4/C composite in LiNO3 solution, and better capacity retention and rate performance is evidenced for NaFe0.95V0.05PO4/C one. This advancement is found to be due a higher participation of pseudocapacity in the sodiation/desodiation charge storage process. An aqueous battery composed of NaFe0.95V0.05PO4/C cathode, belt-like Na1.2V3O8 anode and NaNO3 solution as an electrolyte, tested galvanostatically, displays long-life performance with only 10% of capacity fade after 1000 charge/discharge cycles. © 2016 Elsevier B.V. All rights reserved.
Keywords: Aqueous batteries Cathode material Olivine Pseudocapacitance Sodium-ion battery
1. Introduction Higher abundance and lower price of sodium against lithium mineral resources intensified recently the development of Na-ion
* Corresponding author. The Serbian Academy of Sciences and Arts, Knez Mihajlova 35, 11158 Belgrade, Serbia. E-mail address: [email protected] (S. Mentus). http://dx.doi.org/10.1016/j.jpowsour.2016.06.031 0378-7753/© 2016 Elsevier B.V. All rights reserved.
batteries as potential, environmentally more friendly, competitors to Li-ion batteries [1e4]. High sodium intercalation ability was found for numerous materials including various oxides [5e9], phosphates [10e15], fluorophosphates [16,17], cyanoferrates [18e20] in both organic [5e9,13,16,17] and aqueous electrolytes [9e12,19e21]. Although ionic radius of Naþ ions is somewhat higher, certain compound structures enabled even faster diffusivity of sodium than lithium ions [9,10,21,22].
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A first attempt to made an aqueous Na-ion full cell was published by Okada [23]. The cell consisted of NaTi2(PO4)3 anode and Na0.44MnO2 cathode, and displayed very short cycle life. However, recent publications indicated very promising behavior of this battery types [24e29]. In fact, the cathode/anode pairs NaTi2(PO4)3/ Na0.44MnO2 [24,25], Zn/Na0.95MnO2 [26], NaTi2(PO4)3/Na2NiFe(CN)6 [27], NaTi2(PO4)3/NaFePO4 [28], Na3Ti2(PO4)3/FePO4 [29] delivered outstanding electrochemical performance in aqueous solutions of sodium salts. Olivine LiFePO4 has been intensively studied as environmentally very acceptable electrode material [30e33]. Electrochemical behavior of NaFePO4 olivine [28,34,35] attracted attention as well. Its direct chemical synthesis resulted in electrochemically inactive maricite phase [34,36]. Thus, sodiation of LiFePO4 from either organic [13,36] or aqueous electrolytes [10,37], was proposed as a suitable way to obtain electroactive olivine NaFePO4 phase. Contrary to a great number of studies of lithium intercalation in LiFePO4, a scant number of studies was aimed to sodium intercalation behavior of NaFePO4 [34,38e42]. Unlike the direct LiFePO4eFePO4 phase transition during lithium deinsertion, the NaFePO4eFePO4 phase transition occurs indirectly, via the formation of the intermediate Na~0.7FePO4 phase, which buffers a large mismatch strain between sodiated and desodiated phase [34,42,43]. Apart of bulk faradic processes in solids, which contribute predominantly to the charge storage capacity of intercalates materials, some surface processes can also participate in the charge storage [44e49]. It was shown that the pseudocapacitance contribution becomes significant on transition from micro-to nanodispersed materials, where also morphology, crystallinity, and charging/discharging rate play role. A pronounced contribution of pseudocapacitance has been found in nanodispersed TiO2 [44,50e52], V2O5 [53,54], Li4Ti5O12 [46] and LiFePO4 [55]. Wang et al. [44] clearly showed that the pseudocapacitance of TiO2 improved its rate performance and may be adjusted as a matter of choice between high energy and high power materials. Pseudocapacitance was found to be main part of capacitance of V2O5 porous micro-/nanotubes at scan rates above 60 mV s1, whereas the diffusion-limited bulk Liþ storage dominated in the coulombic capacity of V2O5 micro-/nanorods at scan rates ranging from 20 to 300 mV s1. By studying the lithium intercalation behavior of graphene-supported LiFePO4/C, in an organic electrolyte, within the potential range 2.5e4.3 V (vs. Liþ/Li), Wang et al. [55] suggested that the capacity of nanosized LiFePO4 observed above 3.5 V (vs. Li/Liþ) and below 3.4 V (vs. Li/Liþ) originated from a pseudocapacitance, rather than from a single-phase diffusioncontrolled Li intercalation. Higher pseudocapacitance contribution was predicted in Na-ion than in Li-ion intercalation systems [46]. The present study continues our previous investigations of various nanostructured materials for possible application in lithium and sodium aqueous rechargeable batteries [10,56e58]. First of all, by analogy with the sodiation of LiFePO4 [10], we proved that the potentiodynamic sodiation of its vanadium doped form, LiFe0.95V0.05PO4/C, described in Ref. [57], is feasible too. Further to this, for sodiation/desodiation reactions of NaFe0.95V0.05PO4/C better rate performance was evidenced in comparison to lithiation/delithiation reactions of LiFe0.95V0.05PO4/ C composite. The explanation of better performance was provided through the analysis of surface charge contribution in overall charge storage processes. Finally, we assembled a Na1.2V3O8/ NaNO3/NaFe0.95V0.05PO4/C aqueous battery and confirmed its both high rate and long-life performance.
2. Experimental 2.1. Synthesis procedures The synthesis of LiFe0.95V0.05PO4/C composite, as a precursor of NaFe0.95V0.05PO4/C composite, was described in detail in our previous paper [57]. In fact, the powdery LiFePO4/C, doped with 5% of vanadium by partial replacement of Fe, was obtained by malonicassisted gel combustion procedure. Shortly, certain amounts of lithium nitrate, iron (II) oxalate-dehydrate and ammonium dihydrogen phosphate were dissolved in deionized water at a molar ratio corresponding to the targeted LiFe0.95V0.05PO4 stoichiometry. Then, glycine (glycine:nitrate ¼ 2:1), malonic acid (~60 wt% of the expected mass of olivine) and ammonium monovanadate (corresponding the 5 mol% of vanadium) were also added. The mixture was transformed into a gel, by heating at 80 C under constant mixing, and such obtained gelled precursor was subjected to autoignition which happened at roughly 200 C. The flocculent combustion product was transferred into a quartz tube furnace and heated first at 400 C (3 h) and then at 750 C (6 h), under reducing atmosphere composed of 5 vol% of H2 in Ar, yielding LiFe0.95V0.05PO4/C powder. In spite of ~5% of incorporated vanadium, original olivine structure was kept, as confirmed by XRD (X-ray diffractometer Philips 1050, working with CuKa radiation). The carbon fraction, amounting to ~13%, was determined by its combustion within a thermogravimetric device (TA SDT Model 2090). Furthermore, by potentiodynamic cycling of LiFe0.95V0.05PO4/C in a saturated aqueous solution of NaNO3, the olivine NaFe0.95V0.05PO4/C electrode was successfully obtained. Nanobelt-like Na1.2V3O8 was synthesized by precipitation, from an aqueous solution of V2O5, H2O2 and NaOH, as described in Ref. [58]. Dry precipitate annealed at 400 C was subjected to X-Ray diffractometry of powder, which confirmed layered structure of sodium vanadate. Versatile insertion capability in various aqueous electrolytic solutions, including NaNO3 one, was already evidenced for this compound [58]. 2.2. The electrochemical experiments To make working electrodes for electrochemical measurements, LiFe0.95V0.05PO4/C (or Na1.2V3O8) powder, carbon black (Vulcan XC72, Cabot Corp) and poly(vinilydene fluoride PVDF) binder, were mixed in an 85:10:5 wt ratio. Then, N-methyl-2-pyrrolidone was added to obtain liquid suspensions. Suspensions were homogenized in an ultrasonic bath and, in droplets, transferred onto glassy carbon support, and dried at 120 C for 4 h under vacuum. The loading of dry composites was 3 mg cm2, however, the results presented in this study relate to the active (carbon-free) fraction only. Such obtained working electrodes, a platinum foil as a counter electrode and a saturated calomel electrode (SCE) were assembled in the three electrode cell. The electrolyte was saturated aqueous solution of NaNO3, equilibrated with air. Both cyclic voltammetry and chronopotentiometric measurements were carried out in a three electrode cell by using a Gamry PCI4/300 Potentiostat/Galvanostat. Cyclic voltammograms (CV’s) were performed at various scan rates ranging from 1 mV s1 up to 400 mV s1. The galvanostatic charging/discharging experiments were conducted in a two-electrode cell, involving NaFe0.95V0.05PO4/C as cathode and Na1.2V3O8 as anode, homogeneously distributed on stainless disc-shaped plates, and saturated aqueous NaNO3 solution as electrolyte. The mass ratio of anode and cathode loading was 1:1. The cell was composed under room conditions. An Arbin BT-2042 device was used for galvanostatic cycling tests, which
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were carried out within the voltage range 0.01e1.2 V, at various current densities in the range 100e4000 mA g1. 3. Results and discussion 3.1. The synthesis of NaFe0.95V0.05PO4/C composite by potentiodynamic sodiation of LiFe0.95V0.05PO4/C, and its investigation in NaNO3 solutions by cyclic voltammetry Previously we demonstrated that olivine LiFePO4 within a LiFePO4/C composite may be completely sodiated by potentiodynamic cycling in an aqueous NaNO3 solution [10], offering an easy way to produce NaFePO4 form, capable of fast sodiation/desodiation reactions. In addition to this, we evidenced that the kinetics of lithiation/delithiation reactions of LiFePO4/C composite may be significantly improved by incorporation up to ~5% vanadium into olivine lattice [57] Such obtained LiFe0.95V0.05PO4/C composite, containing also ~8% of Fe2P phase and ~11% of carbon, was found to be a very promising cathode material for lithium aqueous rechargeable batteries. These findings opened the question whether LiFe0.95V0.05PO4/13%C composite, synthesized for the purposes of this study, may be electrochemically transformed into sodium intercalate, in a manner applied to its undoped analogue LiFePO4/C [10]. Several successive cyclovoltammograms of LiFe0.95V0.05PO4/C in aqueous NaNO3 solution are presented in Fig. 1. A progressive transformation of LiFe0.95V0.05PO4/C to NaFe0.95V0.05PO4/C is clearly visible during the first five cycles (Fig. 1a). Two adjacent anodic peaks and single cathodic peak, which (as measured at scan rate 10 mV s1) are positioned at ~0.034 V,~0.24 V, ~0.13 V vs. SCE, respectively, were observed. Such a cyclovoltammogram type is well-documented in the literature related to NaFePO4 in both organic [32,37,43] and aqueous electrolytes [10,37]. Namely, desodiation оf NaFePO4 occurs through an intermediate phase formation NaFePO4 / Na~0.7FePO4 which transforms further to a completely desodiated product Na~0.7FePO4 / FePO4, while the sodiation occurs in a unique step FePO4 / NaFePO4 [34,39,42]. The first and the second phase transition during the desodiation are accompanied by ~3% and ~13% difference in unit cell volume, respectively, what means higher kinetic barrier for the second transition. During the sodiation, these transitions, and the kinetic barriers as well, appear in the reverse order. Thus the overvoltage required for first phase transition covers the second one, and, as a result, only one reduction peak was observed. By analogy we may conclude that in Fig. 1, the two anodic peaks correspond to the phase transitions NaFe0.95V0.05PO4 / Na~0.7Fe0.95V0.05PO4 and Na~0.7Fe0.95V0.05PO4 / Fe0.95V0.05PO4, respectively, while the
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reduction peak corresponds to a unique reverse phase transition Fe0.95V0.05PO4 / NaFe0.95V0.05PO4 [34,39,42]. As far as the potentiodynamic LiFePO4 delithiation/lithiation is concerned, the phase transition LiFePO4 / FePO4 causes the unit cell volume change of only 6%, and thus unique current peaks are observed in both anodic and cathodic polarization directions [57]. The overlapping of CV curves during repetition of potentodynamic cycles (Fig. 1b) and stability of their main features at various scan rates, even up to very high ones (Fig. 1c), indicate an excellent reversibility of sodiation/desodiation reactions, as well as a good structural stability of this composite. This behavior may be attributed not only to thermodynamic stability of olivine structure, but also to a thick carbon layer around olivine particles which prevents the reaction with aqueous solution leading to the loss in capacity [56]. The dependence of the peak current density (ip) on the square root of the scan rate (v1/2) calculated from the CVs of NaFe0.95V0.05PO4/C in NaNO3 is linear (Fig. 2a). Linear dependence was also obtained for sodiation/desodiation of undoped NaFePO4/C composite as well as for lithiation/delithiation of LiFePO4/C composites [10]. In order to obtain a comparative survey of both lithiation and sodiation reaction kinetics of MFe1xVxPO4/C olivines (M ¼ Li or Na), and their previously reported [10] undoped analogues, in aqueous electrolytes, the absolute values of slopes of Ip-v1/2 plots are calculated and summarized in Table 1. Higher slope means faster reactions. Comparison of intercalation-deintercallation kinetics of undoped and vanadiumdoped olivine samples, either in lithium or sodium solutions, reveals faster processes in doped samples. In a study performed by Whittingham group [59], it was evidenced that, for a soft-chemistry synthesized V-doped olivine LiFePO4, the improvement of electrochemical performance originates from the lowering of diffusion barrier for Liþ ions. Namely by X-ray diffractometry investigations, they found that the effective cross-sectional area for the LiO6 octahedral faces, responsible for Liþ ion diffusion, is larger for doped than undoped sample. It is reasonable that such explanation may apply to faster sodiation/desodiation precesses of vanadiumdoped olivine sample. A further comparison of the data in Table 1 shows faster cathodic reactions with Naþ than with Liþ ions. The slopes for desodiation are slightly lower than those for delithiation, however, this is simply the consequence of spreading of desodiation charge along the two anodic steps. Thus, the slopes of Ip-v1/2 plots alone are not a quite representative criteria for the comparison of sodiation vs. lithiation kinetics. Instead, the integration of CVs, giving the amount of stored charge, could be a more reliable
Fig. 1. Cyclic voltammograms of vanadium-doped olivine in aqueous NaNO3 solution measured at 10 mV s1: a) electrochemical displacement of Liþ with Naþ ions b) the demonstration of cyclic stability of formed NaFe0.95V0.05PO4/C composite (6th15th cycle), c) the cyclic voltammograms recorded at very high scan rates.
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Fig. 2. (a) Current peak maxima of NaFe0.95V0.05PO4/C in NaNO3 solution in the function of the square root of scan rate (b) the specific capacity obtained by integration of CVs of MFe0.95V0.05PO4/C (M ¼ Li and Na) in LiNO3 and NaNO3, respectively.
Table 1 The survey of the slopes of the Ip-v1/2 dependence for MFe0.95V0.05PO4 (M ¼ Li or Na). Composite
Slope e anodic scan (A g1/V1/2 s1/2)
Slope e cathodic scan (A g1/V1/2 s1/2)
LiFePO4 in LiNO3 NaFePO4in NaNO3
73.4 82 1st peak 102 2nd peak 134 90.21st peak 98.02nd peak
73.1 138.3
LiFe0.95V0.05PO4/C in LiNO3 NaFe0.95V0.05PO4/C in NaNO3
measure for comparison. The coulombic capacity, calculated from the surfaces under CV peaks for various scanning rates, is presented in Fig. 2b. At low scan rates, 5 and 10 mV s1, similar capacity of was measured in both LiNO3 and NaNO3 solutions. However, at higher scanning rates, significantly better capacity retention was observed during the sodiation/desodiation than during the lithiation/delithiation reactions. At an unusually high scan rate of 300 mV s1, about 83% of the initial capacity (registered at 5 mV s1) was retained, which is significantly higher from ~43% observed for lithiation/delithiation reactions. This contradicts to the expectations based on the ratio of ionic radii. Namely, one may expect that smaller and more mobile Liþ ions may response faster to the potentiodynamic polarization. This contradiction raised the question about the role of pseudocapacitance current, which regularly accompanies intercalation current under potentiodynamic conditions. This question is considered in the next section. 3.2. Surface versus bulk electrochemical processes of MFe0.95V0.05PO4/C (M ¼ Li or Na) composite in aqueous electrolytic solutions In cyclic voltammetry, faradaic current is superposed to capacitance current, originating from double layer capacitance and from pseudocapacitance, if surface processes requiring charge transfer take place [50e53]. Relative contribution of faradaic current in the total current depends not only on the polarization rate but also on the structure of material [44,46,50e55], its morphology [54] and particle size [44]. In order to analyze relative contribution of faradaic and capacitance current in lithium and sodium intercalation/deintercalation processes, we used as the criteria the dependence I ¼ avb (where v is the scan rate and a and b are the system specific parameters), [44,51]. Namely the exponent value b~0.5 is characteristic for faradaic processes, characteristic of diffusion limitation, while
132 154.5
b z 1 is characteristic of surface (double layer and pseudocapacitance) processes [44,51]. The electrochemical behavior of LiFe0.95V0.05PO4/C composite in aqueous LiNO3 solution was investigated in detail in our previous paper [57]. In Fig. 3a, cyclic voltammograms recorded in that system within a broad range of scan rates, 1 mV s1 to 100 mV s1 were presented. Using peak currents, from the slope of log I vs. log(v) plot (Fig. 3b) b exponent was calculated to amount to ~0.5 for both anodic and cathodic peaks. This result indicated a preferably bulk faradaic behavior of LiFe0.95V0.05PO4/C composite. However, the shape of the frontal parts of both cathodic and anodic peaks, (linear, instead of exponential rise), as well as a considerable peak potentials separation, rising with the rise in scanning rate, contradict to a classic reversible behavior. The described behavior corresponds, in fact, to a model of electrochemical process controlled by purely ohmic resistance, proposed by Dahn [60] and applied in our ref. [11], as well. Analogous peak shape was demonstrated for reversible conducting (metal) phase formation [61]. Somewhat different behavior in the case of sodiation/desodiation processes of NaFe0.95V0.05PO4/C, follow from the shapes of cyclovoltammograms, given in Fig. 3c. In this case, the straight line connects frontal sides of cathodic peaks, however that may not be stated surely for anodic peaks, since they are distanced mutually along current axis. Differences were observed also when the b values of the I ¼ avb equation were determined. The corresponding slopes of the log I ¼ f(log v) dependence derived from the peak currents is shown in Fig. 3d. The slopes were found to lie in the range 0.7e1. Similar b values, ~0.5 for delithiation and ~0.7e0.8 for desodiation were reported for Li4Ti5O12 spinel in an organic electrolyte [46], indicating pronounced pseudocapacitance contribution in the last case. In Fig. 4, for sodiation/desodiation reactions of NaFe0.95V0.05PO4/ C, the values of exponent b, calculated at several fixed potentials,
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Fig. 3. Cyclic voltammograms (a,c) and the logarithm of the height peak current versus logarithm of the scan rate (b,d) for MFe0.95V0.05PO4/C (M ¼ Li and Na) measured in aqueous LiNO3 and NaNO3 solution at various scan rates, respectively. The CVs in LiNO3 solution obtained formerly [57] for the scan rates up to 50 mV s1 do not differ from the corresponding ones in Fig 3a.
are presented in function of potential. Observing the cathodic process (sodiation) the b-values found in the peak potential region correspond to a bulk intercalation reaction, while, at the potentials outside of the peak region, a pseudocapacitive behavior prevail. However, b values measured for anodic, desodiation direction, although having slight minima at peak potentials, show preferably pseudocapacitive behavior. This finding actualized the need for more accurate determination of relative contributions of bulk and surface charging/discharging processes. This analysis may be carried out by means of the equation [44e51]:where the first addend (C1v) on the right side corresponds to the pseudocapacitance current (theoretically obeying linear dependence of peak current on the scan rate) and the second addend corresponds to bulk faradaic reaction (theoretically obeying linear peak current dependence on square root of scan rate). To simplify its use, Eq. (1) may be expressed in the following form:
. ip v1=2 ¼ C1 v1=2 þC2 Fig. 4. The values of exponent b at various potentials during anodic (square) and cathodic (circle) potentiodynamic scans of NaFe0.95V0.05PO4/C in an aqueous NaNO3 solution.
I ¼ Ic þ If ¼ C1 v þ C2 v1=2
(1)
(2)
This linear function of v1/2, gives C2 as the intercept on the ordinate axis, and C1 as the slope. Upon determination of C1 and C2, by means of Eq. (1), one may calculate Ic and If values separately in function of scan rate for fixed potentials, and they are given in Fig. 6 for both cathodic and anodic scans for NaFe0.95V0.05PO4/C in NaNO3 solution. As expected on the basis of its linear dependence on scan rate, with the increase of the scan rate, the contribution of pseudocapacitance current increases much faster than the bulk intercalation
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Fig. 5. Surface pseudocapacitance current (square) and bulk current (circle) in function of scan rate, calculated for the redox peaks of NaFe0.95V0.05PO4/C in NaNO3 solution.
Fig. 6. Chronopotentiometric curves of NaFe0.95V0.05PO4/C in an aqueous NaNO3 solution at various current rates (a) and corresponding coulombic capacity in function of the charging/discharging rate (b).
one (Fig. 5). During sodiation, in the region of peak current, the pseudocapacitance contribution relative to the bulk current begins to became significant at very high scan rates (Fig. 5a), while during both steps of desodiation, it is significant even at lower scan rates of 5 and 10 mV s1 (Fig. 5b) overcoming significantly the bulk current at higher scan rates (Fig. 5b,c). The fraction of capacitance current for the cathodic and the two anodic peak potentials of CV’s of NaFe0.95V0.05PO4/C in NaNO3 solutions, at various scan rates, are summarized in Table 2. For the sake of comparison, corresponding values are calculated also for lithiation/delithiation processes of LiFe0.95V0.05PO4/C in LiNO3 and
Table 2 Pseudocapacity current fractions in total charging/discharging current calculated at various scan rates using the Eq. (2). Scan rate mV s1
5 10 20 30 50 100 150 200 300
Ic/(IcþId) (NaFe0.95V0.95PO4)
Ic/(IcþId) (LiFe0.95V0.95PO4)
c
a1
a2
c
a
0.069 0.095 0.13 0.15 0.19 0.25 0.29 0.32 0.36
0.53 0.61 0.69 0.73 0.78 0.83 0.86 0.87 0.90
0.29 0.37 0.46 0.51 0.57 0.65 0.70 0.73 0.77
0.033 0.040 0.051 0.071 0.085 0.097 0.107
0.053 0.064 0.081 0.11 0.13 0.15 0.16
they are also presented in Table 2.
3.3. Chronopotentiometry considerations This section contains the results of galvanostatic investigation of the coulombic capacity of NaFe0.95V0.05PO4/C composite in airequilibrated NaNO3 solution, examined at various charging/discharging rates. Fig. 6a shows the galvanostatic charging/discharging curves obtained at rates from 154 mA g1 up to even 5000 mA g1 (i.e. from 1C to ~33 C, accounting with the theoretical capacity of NaFePO4). Desodiation capacity amounted to 127.3, 107.5, 98.9, 91.7, 86.7, 83.3 and 81.9 mAh g1 while the sodiation capacity amounted to 101, 103.6, 97.5, 90, 85.8, 83.3 and 83.3 mAh g1, at current densities 154, 500, 1000, 2000, 3000, 4000 and 5000 mA g1, respectively. At very high current densities capacity remains practically unchanged, which confirms the pseudo-capacitance domination in the charge storage behavior (Fig. 6b). The voltage plateaus, which accompany phase transition during intercalation/deintercalation processes, are not clearly defined. A shortening of plateaus characteristic for two-phase contribution during intercalation/deintercalation processes with the transition from microparticles to nanoparticles is already reported several times [53,55]. This is consequence of both less pronounced phase boundary in nanoparticles and more pronounced role of surface against bulk processes. This may be expected to be even more pronounced in aqueous solutions, where capacity utilization is
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ordinarily lower than in organic solutions. For nano-LiFe0.95V0.05PO4/C composite, we found previously [57] clearly visible plateau in organic electrolyte, but very biased one in aqueous electrolyte. The positions of these plateaus, barely visible in Fig. 6a may be exposed by differentiation of chronoamperometric curves, using coulombic capacity (Q, mAh g1) as a dependent, and potential as an independent variable, as done in Fig. 7. The comparison of anodic (Fig. 7b) and cathodic dQ/dV curves (Fig. 7c) clearly reveals the differences between sodium intercalation and deintercalation processes. Sharp and high peak in Fig. 7c, characterizes a phase transition based coulombic capacity [47,62] during cathodic Fe0.95V0.05PO4/C / NaFe0.95V0.05PO4/C transition. Interestingly, in this presentation, two peaks in the dQ/dV plots can be seen even for the cathodic scan (Fig. 7a) but only at low current rates (500 mA g1). Investigating pure olivine NaFePO4, Fang et al. [37] showed also two steps during both desodiation and sodiation. Always present deviation of the dQ/dV cathodic baseline from zero, which is especially pronounced in the negative potential region, indicates the existence of pseudocapacitance processes in the whole potential region investigated. As far as the anodic polarization direction is concerned, the broad anodic peaks, together with a permanent deviation of baseline from zero [47,62] confirm a notable role of pseudocapacitance processes during both NaFe0.95V0.05PO4/C / Na~0.7Fe0.95V0.05PO4/C and Na0.7Fe0.95V0.05PO4/C / NaFe0.95V0.05PO4/C phase transitions. As expected, the peak height of both anodic and cathodic dQ/dV curves decreases with the increase of current rate, indicating decreasing contribution of “inner” (bulk diffusion) versus “outer” (surface pseudocapacitance) charge, in terms used by Trasatti et al. [63] One can see that this decrease is more pronounced for cathodic curve (about 62%) than that for anodic one (~30% for the first and ~54% for the second peak), which is in accordance with the measurements presented in Fig. 5. In comparison to the recently reported nasicon type compound, NaTi2(PO4)3 [11], in a common NaNO3 solution, the NaFe0.95V0.05PO4/C composite delivers higher charging/discharging capacity. For instance, the desodiation capacity of olivine at a rate of 500 mA g1 amounted to 103 mAh g1, while corresponding value for nasicon NaTi2(PO4)3 amounted only to ~70 mAh g1. Accordingly, one can say that sodiated form of vanadium doped olivine presents a very promising cathode material for development of sodium rechargeable aqueous batteries. To explain the origin of pseudocapacitance, it can be said that during cathodic polarization, a part of Naþ I ions, involved in the first Fe0.05V0.95PO4 / Na~0.7Fe0.95V0.05PO4 phase transition, probably occupies bulk intercalation positions of olivine, while the other part of Naþ ions, involved in the second II Na~0.7Fe0.95V0.05PO4 / NaFe0.95V0.05PO4 transition, occupies the
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surface sites. Accordingly, in the reverse anodic scan the first anodic peak should originate from pseudocapacitance, as also proved here. However, it was shown that even the second anodic peak involves high fraction of surface charge. We may speculate that this is due to a partial redistribution of Naþ ions during the first anodic NaFePO4 / Na~0.7Fe0.95V0.05PO4 phase transition. The second Na~0.7Fe0.95V0.05PO4 / FePO4 phase transition in principle may take place only after the completion of the first anodic redox process. In fact, during anodic scan all Naþ ions in the NaFe0.95V0.05PO4 bulk are forced by polarization to move towards the electrolyte, but during the first desodiation step, a part of ions, due to the energy insufficiency, remains in the olivine depth. Therefore, it is possible that such part of Naþ ions only reach the surface sites, emptied by the release of Naþ II ions into solution. This may contribute to the pseudocapacitance behavior of the second desodiation step, Na~0.7Fe0.95V0.05PO4 / NaFePO4. Another eligible explanation could be found in the morphology of olivine. It was shown that delithiation of LiFePO4 causes the cracks and high porosity of the FePO4 layer [64] which can improve the kinetics. Since the strain and volume changes are significantly higher in the case of sodium storage, these effects could be more pronounced [38] and may contribute to the higher pseudocapacitance relative to the lithium storage. 3.4. Aqueous rechargeable Na1.2V3O8/sat.NaNO3/NaFe0.95V0.05PO4/C battery Versatile insertion capability of Na1.2V3O8 nanobelts towards various ions, including Naþ ions too, was successfully confirmed in Ref. [58]. Intercalation of sodium ions into Na1.2V3O8 is predominantly one step process, while the desodiation goes through several steps. Interestingly, both NaFe0.95V0.05PO4/C and Na1.2V3O8 showed higher potential barrier for desodiation than for sodiation. That indicates generally strong interaction of the Naþ ions with the host structure. Although the redox peaks of Na1.2V3O8 are spread over broad potential interval, low negative potential of sodiation of Na1.2V3O8 [58], together with a relatively fast sodiation/desodiation kinetics and good cyclic stability, offer the possibility of its use as the anode of an aqueous battery. In NaNO3 solution, the reversible potential of Na1.2V3O8 is positioned at 0.67 V vs. SCE [58] and the reversible potential of NaFe0.95V0.05PO4/C is positioned at 0.24 V vs. SCE (Fig. 1b). An aqueous-type sodium-ion battery, composed from Na1.2V3O8 as the anode, NaFe0.95V0.05PO4/C as the cathode and an aqueous NaNO3 solution as the electrolyte, was assembled. The charge/discharge curves of this battery, obtained after 50 charge/discharge cycles in the voltage range from 0.01 to 1.2 V, were presented in Fig. 8a. According to our measurements, at
Fig. 7. First derivation of chronoamperometric curves (dQ/dV) in function of potential for NaFe0.95V0.05PO4/C in NaNO3 solution at a charging/discharging rate 500 mA g1 (a), and the same dependence at various charging/discharging rates for separately presented desodiation (b) and sodiation (c) process.
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Fig. 8. (a) The charge/discharge curves measured at rate 100 mA g1, and (b) the specific discharge capacity versus cycle number of aqueous Na1.2V3O8/NaNO3/NaFe0.95V0.05PO4/C battery. The numbers inserted in Fig. 8b are the discharging rates in units mA g1, c) the cyclic stability of Na1.2V3O8/NaNO3/LiFe0.95V0.05PO4/C battery.
100 mA g1 the capacity of Na1.2V3O8 in the investigated aqueous nitrate solution is 110 mAh g1, which is almost equal to the capacity of NaFe0.95V0.05PO4/C composite. As visible in Ref. [58], its capacity is remarkably dependent on the discharging rate, and at 500 mA g1 amounts to 50 mAh g1 only. Thus, in the diagram shown in Fig. 8b, at high charging/discharging rates, the capacity of battery is limited exclusively by Na1.2V3O8 anode. The invariability of the curves shape during long cycling indicates a good compatibility of these materials as well as a good reversibility of their sodiation/desodiation reactions during cycling in a two electrode arrangement. The initial charge and discharge capacity was found to be ~120.6 mAh g1 and ~102 mAh g1, respectively. After 50 charging/discharging cycles at a common rate of 100 mA g1, these values were slightly increased amounting to 124.7 mAh g1 and 106.8 mAh g1, respectively. Discharge capacity decreases with the increase of the scan rate from 100 mA g1 to 5000 mA g1, which is a common behavior of all batteries due to the polarization losses. It must be bear in mind that equal masses of Na1.2V3O8 and NaFe0.95V0.05PO4/C, and equal mass percents of conductivity supporting carbon black were used. The cyclic stability of Na1.2V3O8/NaNO3/LiFe0.95V0.05PO4/C battery, presented in Fig. 8c, shows an ultra-long cycling capability of this aqueous system up to 1000 charge/discharge cycles. Its initial coulombic capacity amounted to ~90 mAh g1. After 300 charging/ discharging cycles, the capacity retained its initial value, while after 1000 charging/discharging cycles, discharge capacity retained 90% of the initial value. Although the results of Section 3.2 indicate the influence of pseudocapacitance, the results outlined in Table 2 evidence that, in overall battery capacitance presented in Fig. 8, one deals with dominant role of intercalation/deintercalation processes. 4. Conclusions The LiFe0.95V0.05PO4/C composite, obtained by malonic-assisted gel-combustion procedure, was successfully transformed into its sodiated form, NaFe0.95V0.05PO4/C, by potentiodynamic cycling in an aqueous NaNO3 solution. Further potentiodynamic cycling in the same solution confirmed fast kinetics of sodiation/desodiation reaction, faster than in it undoped analogue, NaFePO4. Like to the NaFePO4 in NaNO3 solution, the sodiation/desodiation process of vanadium doped material proceeds through intermediate phase, causing the split of anodic peaks of cyclovoltammograms. The sodium and lithium charge storage capacities in this material are similar at lower scan rates, while at high scan rates the coulombic capacity of sodiated form prevailed considerably that of lithiated form. The analysis of kinetic data revealed that the excess capacity
stems primarily from pseudocapacitance. High sodium storage capacity of NaFe0.95V0.05PO4/C composite (~100 mAh g1 at rate of 500 mA g1) was found chronopotentiometrically in a threeelectrode arrangement. The aqueous sodium-ion battery with the composition Na1.2V3O8/NaNO3/NaFe0.95V0.05PO4/C was assembled and its coulombic capacity was found to be ~100 mAh g1 at rate of 100 mA g1. After 300 charge/discharge cycles the capacity kept the value equal to the initial one, while 90% capacity retention was observed after 1000 charge/discharge cycles, at mean voltage 0.5 V. Acknowledgements This work was supported by Ministry of Education, Science and Technological Development of the Republic of Serbia through the project III45014. S.M. is also indebted to the Research Fund of Serbian Academy of Sciences and Arts (F190) which supported the project “Electrocatalysis in contemporary processes of energy conversion”. References les, T. Rojo, [1] V. Palomeras, P. Serras, I. Villaluenga, K. Hueso, J. Carretero-Gonza Energy Environ. Sci. 5 (2012) 5884e5901. [2] B.L. Ellis, L. Nazar, Curr. Opin. Solid State Mater. Sci. 16 (2012) 168e177. [3] M.D. Slater, D. Kim, E. Lee, C.S. Johnson, Adv. Funct. Mater. 23 (2013) 947e958. [4] C. Masquelier, L. Croguennec, Chem. Rev. 113 (2013) 6552e6591. [5] S. Komaba, C. Takei, T. Nakayama, A. Ogata, N. Yabuuchi, Electrochem. Commun. 12 (2010) 355e358. [6] D. Buchholz, L.G. Chagas, M. Winter, S. Passerini, Electrochim. Acta 110 (2013) 208e213. [7] D. Yuan, X. Hu, J. Qian, F. Pei, F. Wu, R. Mao, X. Ai, H. Yang, Y. Cao, Electrochim. Acta 116 (2014) 300e305. [8] J.J. Ding, Y.N. Zhou, Q. Sun, X.Q. Yu, X.Q. Yang, Z.W. Fu, Electrochim. Acta 87 (2013) 388e393. [9] K. Kim, C. Yua, C.S. Yoom, S. Kim, Y.-J. Kim, Y.-K. Sun, S.-T. Myunga, Nano Energy 12 (2015) 725e734. [10] M. Vujkovi c, S. Mentus, J. Power Sources 247 (2014) 184e188. [11] M. Vujkovi c, M. Mitri c, S. Mentus, J. Power Sources 288 (2015) 176e186. [12] G. Pang, C. Yuan, P. Nie, B. Ding, J. Zhu, X. Zhang, Nanoscale 6 (2014) 6328e6334. [13] S.-M. Oh, S.T. Myung, J. Hassoun, B. Scrosati, Y.K. Sun, Electrochem. Commun. 22 (2012) 149e152. [14] H. Kim, I. Park, D.H. Seo, S. Lee, S.W. Kim, W.J. Kwon, Y.U. Park, C.S. Kim, S. Jeon, K. Kang, J. Am. Chem. Soc. 134 (2012) 10369e10372. [15] M. Hose, H. Nakayama, K. Nobuhara, H. Yamaguchi, S. Nakanishi, H. Iba, J. Power Sources 234 (2013) 175e179. [16] W. Song, X. Ji, Z. Wu, Y. Yang, Z. Zhou, F. Li, Q. Chen, C. Banks, J. Power Sources 256 (2014) 258e263. [17] W. Song, X. Ji, Z. Wu, Y. Zhu, F. Li, Y. Yao, C. Banks, RSC Adv. 4 (2014) 11375e11383. [18] X. Wu, Y. Luo, M. Sun, J. Qian, Y. Cao, C. Ai, H. Yang, Nano Energy 13 (2015) 117e123. [19] C.D. Wessells, S.V. Peddada, R.A. Huggins, Y. Cui, Nano Lett. 11 (2011) 5421e5425. [20] Y. You, X. Yu, Y. Yin, K.W. Nam, Y. Guo, Nano Res. 8 (2015) 117e128.
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