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Electrochimica Acta 109 (2013) 835– 842
Contents lists available at ScienceDirect
Electrochimica Acta
jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta
he LiFe(1−x)VxPO4/C composite synthesized by gel-combustionethod, with improved rate capability and cycle life in aerated
queous solutions
ilica Vujkovic a, Dragana Jugovic b, Miodrag Mitric c, Ivana Stojkovica,ikola Cvjeticanina, Slavko Mentusa,∗,1
University of Belgrade, Faculty of Physical Chemistry, P.O. Box 137, Studentski trg 12-16, 11158 Belgrade, SerbiaInstitute of Technical Sciences of SASA, Knez Mihajlova 35, 11000 Belgrade, SerbiaVinca Institute of Nuclear Science, University of Belgrade, 11000 Belgrade, Serbia
r t i c l e i n f o
rticle history:eceived 15 May 2013eceived in revised form 28 July 2013ccepted 28 July 2013
a b s t r a c t
The nitrate-(glycine + malonic acid)-assisted gel-combustion process, followed by a heat treatment at750 ◦C under reductive atmosphere, was used as a fast and effective way to synthesize vanadium dopedolivine incorporated in carbon matrix, of general formula LiFe(1−x)VxPO4/C. The two-phased Rietveldrefinement confirmed that vanadium incorporation into olivine structure was complete. The heating
eywords:oulombic capacityithium iron phosphateate capabilityechargeable lithium batteriesanadium doping
under reduction atmosphere caused the formation of iron phosphide to some extent, the concentrationwas determined by Rietveld analysis. The capacity and rate capability of these composites were testedby both cyclic voltammetry and galvanostatic cycling. Specifically, the average discharging capacities ofthe composite with x = 0.055, determined in an saturated aqueous LiNO3 solution equilibrated with air,at the rates of 1, 10 and 100 C, amounted to 91, 73 and 35 mAh g−1, respectively, with no perceivablecapacity fade.
© 2013 Elsevier Ltd. All rights reserved.
. Introduction
Thanks to its high theoretical capacity of 170 mAh g−1, a very flatlateau (3.5 V vs. Li+/Li), environmental appropriateness and lowost, LiFePO4 presents a very desirable cathodic material of Li ionatteries [1]. In pioneering studies, its low conductivity of nearly0−9 S cm−1 was considered to be an obstacle in achieving theoret-
cal capacity [1,2]. Later, various ways were applied successfully tovercome this disadvantage: coating the particles with electron-cally conductive agent such as carbon [3–11] or Fe2P [12–14],article size reduction [3,15], doping with supervalent cations16–27], and doping by anions [28,29]. A particular improvementn the rate capability and capacity retention of LiFePO4/C composite
as achieved by vanadium doping [18–27].Li et al. [27] were the first who investigated the Li-ion battery
ith aqueous electrolyte (ARLB). However, early studies in aqueousolutions indicated low capacity utilization and very pronouncedapacity fading during cycling tests [30,31], due to the dissolution of
∗ Corresponding author at: Serbian Academy of Sciences and Arts, Knez Mihajlova5, 11158 Belgrade, Serbia. Tel.: +381 11 2187133; fax: +381 11 2187133.
E-mail address: [email protected] (S. Mentus).1 ISE member.
013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.07.219
the electrode materials in electrolytes. Not much far ago, Manickamet al. [32] published the use of LiFePO4 in aqueous media, althoughwith poor characteristics. Luo et al. [33] have also emphasizedthe influence of the chemical reaction of electrode materials withboth water and O2 on the capacity fading in LiTi2(PO4)3/aqueousLi2SO4/LiFePO4 battery. It has been suggested that preparation ofolivine composites with thick carbon shell may impede dissolutionreactions with OH− and O2 and reduce capacity fading [34,35].
Having in mind the literature reports about the positive effectof vanadium doping on the rate performance observed in organicelectrolytes [19–27], and potential protecting action of carbon shellagainst corrosion in aerated aqueous electrolyte solutions [33,34],we synthesized vanadium doped LiFePO4/C composites, in order toinvestigate its behavior in aerated aqueous electrolyte. For compar-ison purposes, undoped LiFePO4/C composite, reported previously[35], was also used. The synthesis was carried out by simple, nitrate-(glycine + malonic acid) gel-combustion route, which formed in situa large amount of carbon [35]. The heat treatment in reductionatmosphere finished the synthesis procedure. The XRD-analysisevidenced that this way of synthesis enabled to obtain true V-doped
LiFePO4 olivine phase, accompanied by some amount of Fe2P,while carbon content was evidenced and measured by thermo-gravimetry. Since the vanadium doped olivines were studied so farexclusively in organic electrolyte [18,19,21–26], for the purposes
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36 M. Vujkovic et al. / Electroch
f comparison, we checked briefly the electrochemical behaviorf this material in organic electrolyte, 1 M LiClO4 in propylene car-onate (PC), and an expected improvement relative to the undopediFePO4 olivine was clearly demonstrated. However, the substan-ial part of this study relates to the electrochemical behavior of thislectrode material in aqueous LiNO3 solution, which is importantrom the aspect of aqueous lithium-ion batteries (ARLB). In aqueouslectrolyte, the obtained composite displayed actually much higherate tolerance than in organic electrolyte. In addition, thanks to ahick in situ formed protecting C layer, it displayed, contrary to theecent reports [33,34], a negligible capacity fading in aqueous LiNO3lectrolyte solutions saturated by air.
. Experimental
The samples were synthesized by gel-combustion processeported elsewhere [35], using lithium nitrate, iron (II) oxalateehydrate (synthesized as described elsewhere [18,22]), ammo-ium dihydrogen phosphate, as a source of lithium, iron andhosphate, respectively. The reactants were dissolved in deion-
zed water in the mole ratios to satisfy the targeted stoichiometryxpressed by the formula LiFe(1−x)VxPO4/C, with x = 0.00, 0.03 and.05, heated to 80 ◦C and mixed using a magnetic stirrer. Then,lycine was added into the reaction mixture to provide glycines. nitrate molar ratio of 2:1, and subsequently, malonic acid wasdded in an amount of 60 wt% of the expected mass of olivine.o obtain doped samples, corresponding amount of ammoniumonovanadate was added, what caused the color of the solution
o change from yellow to green one. After removing the major-ty of water by evaporation, the gelled precursor was heated tonitiate the auto-combustion, yielding a flocculent product. Theombustion product was heated in a quartz tubular furnace at00 ◦C for 3 h and calcined at 750 ◦C for 6 h, under reducing atmo-phere (Ar/H2 = 95/5). The samples obtained were denoted as LFP/C,FVP(i)/C and LFVP(ii)/C, for x = 0.00, 0.03 and 0.05, respectively.
The VO2 powder prepared by hydrothermal method was useds active component of counter electrode for the galvanostaticxperiments in aqueous electrolyte solution. Its synthesis andlectrochemical behavior are described elsewhere [36]. The con-iderable stoichiometric excess of VO2 was used, to provide thatiFePO4/C composite presents the main resistive element, i.e.,etermines the behavior of the assembled cell as the whole.
X-ray diffraction data were collected on the Philips PW 1050iffractometer with Cu-K�1,2 radiation (Ni filter) at the room tem-erature. Measurements were done in 2� range 10–110◦ with stepize of 0.02◦ and counting time of 14 s per step.
The morphology of the synthesized powders was analyzedy field emission scanning electron microscopy (FESEM-TESCANira3 XMU).The carbon content in the composites was determined by means
f thermobalance TA SDT Model 2090 whereas the electrical con-uctivity of samples was measured by means of the ac bridgeayne Kerr B224 at the fixed frequency of 1 kHz. For electrical
onductivity measurements examined powders were pressed intoellets and both contact surfaces of the pellets were coated by silveraste in order to achieve a good electrical contact.
For electrochemical investigations, the working electrodeas made from the active material (75%), carbon black (20%),oly(vinylidene fluoride) (PVDF) as a binder (5%) and a N-methyl-pyrrolidone as a solvent. The resulting suspension was homog-nized in an ultrasonic bath and deposited on electronically
onducting support. The electrode was dried at 120 ◦C for 4 h.ypical loading was 5 mg cm−2, accounting with the complete com-osition of the electrode material. Somewhat modified weightatio, 85:10:5, and the same drying procedure, were used to prepareActa 109 (2013) 835– 842
VO2 electrode. The Coulombic capacity values (mAh g−1) presentedin this paper for all samples are calculated per gram of pure olivinephase.
Cyclic voltammetry experiments were carried out with a deviceGamry PCI4/300 Potentiostat/Galvanostat. The three electrode cellconsisted of a working electrode, a wide platinum foil as a counterelectrode, and a saturated calomel electrode (SCE) as a referenceone. The electrolyte solution was saturated LiNO3 aqueous solution.As published previously [37], lithium nitrate solution is electro-chemically inert within a voltage window of at least 2 V.
Galvanostatic charging/discharging experiments in saturatedLiNO3 aqueous solution were carried out in a two-electrodearrangement, involving V-LiFePO4/C as a working electrode andVO2 in a stoichiometric excess, as a counter electrode, using thesoftware-controlled battery testing device Arbin BT-2042. Thestainless steel plates were used as the current collectors for bothpositive and negative electrode. The cell was assembled in roomatmosphere, and cycled in the voltage window between 0.01 and1.2 V.
3. Results and discussion
3.1. Structure, composition, morphology and electricalconductivity
X-ray powder diffraction patterns were used for phase identifi-cation and structural analysis. The two-phase Rietveld refinementswere performed on XRD data to quantify the amounts of the presentcrystalline phases. The composite LiFePO4/C synthesized for thepurposes of a previous study [35] was also subjected to this analysis.
Crystal structure refinements were based on the Rietveld fullprofile method [38] using the Koalariet computing program basedon a fundamental parameters convolution approach to generateline profiles [39]. The structure of LiFePO4 phase has been refinedin the space group Pnma (D2h
16) in olivine type with the follow-ing crystallographic positions: Li+ ions in crystallographic position4a [0, 0, 0] with local symmetry 1; Fe2+ and P5+ ions occupied twodifferent crystallographic 4c positions [x, 1/4, z] with local symme-try m; O2− ions occupied three different crystallographic positions:additional two 4c positions and one general 8d position [x, y, z] withlocal symmetry 1.
Throughout the refinement, the presumption was checked thatin olivine LiFePO4 phase vanadium ions substituted only iron ions.The Rietveld refinement results indicated a noticeable decreaseof primitive cell volume of olivine phase with the enlargementof vanadium concentration. Having in mind that V3+ ionic radiusis somewhat smaller than Fe2+ one (rVI(Fe2+ HS) = 0.77 A andrVI(V3+) = 0.64 A [40]), this was the indication that vanadium ionswere successfully incorporated in the olivine lattice. In addition,the refined values of vanadium concentration matched very wellthe targeted compositions of the powders, within the limits of theexperimental error.
Apart of the reflections characteristic of olivine phase, in theXRD diagrams of vanadium doped samples, in Fig. 1, the reflectionsat 2� of 40.3, 44.2, and 47.2, degrees were observed too, whichcorrespond to the lines (1 1 1), (2 1 0) and (2 0 1), respectively, ofFe2P, indicating its appearance as a second phase. The structure ofFe2P phase was refined in the space group P62m (D3h
3) with thefollowing crystallographic positions: Fe atoms occupied two crys-tallographic positions 3f [x, 0, 0] and 3 g [x, 0, 1/2] with the samelocal symmetry mm; P atoms occupied two crystallographic pos-
itions: 1b [0, 0, 1/2] with local symmetry 6m2 and 2c [1/3, 2/3,0] with local symmetry 6. These data evidenced that vanadiumdoping did not alter the lattice parameters of the Fe2P phase, i.e., itappeared a completely separate phase.
M. Vujkovic et al. / Electrochimica
Fig. 1. The observed (.), calculated (−), and the difference between the observedand calculated (bottom) X-ray diffraction data of the investigated samples takenait
t
rVw%o
t room temperature. Vertical markers below the diffraction patterns indicate pos-tions of possible Bragg reflections for olivine type LiFePO4 (up) and Fe2P structureype (down).
In Fig. 1 the observed X-ray diffraction profiles were presentedogether with the calculated ones.
The main results of the final Rietveld refinements were summa-ized in Tables 1 and 2. The values for both microstrain and strain in
-doped samples were refined to zero, within the limits of the error,hile for the undoped sample, a residual microstrain of 0.20(4)was obtained, which denotes that vanadium ions stabilized thelivine structure. The Rietveld refinement also showed additional
Acta 109 (2013) 835– 842 837
electron density on the lithium sites indicating so-called “anti-site”defect in which a Li ion (on the M1 site) and a Fe ion (on the M2 site)are interchanged. For that reason, during the refinements it wasallowed for Fe ions to occupy Li site (M1) as well as M2 site. Thisapproach resulted in the decrease of R values, and the best fit wasobtained for the values of occupation numbers given in Table 1. Thisanti-site disorder (ca. 1–2 mol %) is believed to be intrinsic prop-erty of olivine LiFePO4 [41]. However, X-ray diffraction refinementis not very sensitive on lithium or vacancy occupation of crystallo-graphic positions, since lithium has a small X-ray scattering factor.This means that charge compensation by vacancies on the lithiumsite can accompany vanadium incorporation in olivine lattice. Hereone should mention that charge compensation caused by vanadiumdoping into olivine was claimed to occur also either by mediationof electrons [22], or, for low temperature synthesis, by vacant Fe2+
ion sites [24].Both refined and fixed fractional atomic coordinates were used
for the calculation of all relevant bond distances and bond anglesthat enabled us to determine coordination polyhedra. Some rele-vant bond distances were presented in Table 2. The incorporationof vanadium resulted in the increase of Li O distances, whichmay make easier deinsertion/insertion of lithium from/into olivinestructure. The shorter P O distance, observed in the doped samples,indicated a more stable framework of olivine structure.
The amount of Fe2P as a second phase increased on rising dopantconcentration, Table 1. According to Zhao et al. [42], by appropriatefuel vs. nitrate mole ratio equal to 4, the formation of Fe2P phaseduring the synthesis of LiFePO4 was suppressed. Since we used fuelto nitrate mole ratio close to this appropriate one, the formation ofphosphide phase in our case was most probably due to the ther-mal treatment of the samples in reducing Ar + 5% H2 atmosphere,as reported by Rho et al. [13] and Omenya et al. [24]. Since this fea-ture was not observed when we synthesized undoped LiFePO4/Ccomposite, [35] and the amount of phosphide was commensurableto the concentration of vanadium (Table 1), we may conclude thatvanadium catalyzed the formation of phosphides under reducingatmosphere conditions.
Accounting with the fact that during the synthesis of V-dopedsamples a part of iron and phosphorus was lost on account ofthe formation of Fe2P, the actual mole fraction of vanadiumbecame somewhat higher than the targeted one. Based on boththe XRD analysis and input concentrations, the approximate com-positions of the obtained vanadium-doped olivine samples wereLiFe0.968V0.032PO4 and LiFe0.945V0.055PO4. Based on the thermo-gravimetric determination [35], the undoped olivine contained13.4% of carbon, while doped samples contained almost equalmass fractions of carbon amounting to 10.0%. Thus, the actualcomposition of our samples LFP/C, LVFP(i)/C and LVFP(ii)/C,was LiFePO4/13.4%C, (LiFe0.968V0.032PO4 + 8.0 wt.% Fe2P)/10%C, and(LiFe0.945V0.055PO4+ 10.8 wt.% Fe2P)/10%C, respectively.
Under electron microscope, at low magnification, the obtainedproducts appeared as highly porous sponge with flat pore walls.Such morphology was because of a partial liquefaction and evolu-tion of gas bubbles during gel-combustion procedure. However, athigh magnification of pore walls, as shown in Fig. 2, one may per-ceive that one deals with the agglomerates of fine uniform particleswith the mean diameter of 20 nm. Apparently, the amount of dopedvanadium did not affect the intrinsic morphology of the samples.On the basis of SEM images only, it was impossible to distin-guish between different phases present in the composite powder.According to Fig. 1, in all three samples olivine type LiFePO4 wasobtained as a major phase. There was no evidence for the forma-
tion of crystalline carbon and the internal amorphous carbon couldbe considered as a constituent contributing to the background ofthe XRD diagram. Therefore, Fig. 2 may be interpreted as displayingfine olivine particles imbedded in the carbon matrix.
838 M. Vujkovic et al. / Electrochimica Acta 109 (2013) 835– 842
Table 1The final results of the two-phase Rietveld structural refinement of the synthesized powders.
Targeting compositions LFP/C LFVP(i)/C LFVP(ii)/C
Latticeparameters (Å)
LiFePO4 LiFePO4 Fe2P LiFePO4 Fe2Pa = 10.3270(9) a = 10.3094(3) a = 5.8683(3) a = 10.2909(6) a = 5.8684(4)b = 6.0090(5) b = 5.9965(2) b = 5.8683(3) b = 5.9874(4) b = 5.8684(4)c = 4.6968(5) c = 4.6970(2) c = 3.4601(3) c = 4.7000(3) c = 3.4591(4)
Primitive cell volume (Å3) V = 291.46(5) V = 290.37(2) V = 103.19(1) V = 289.60(3) V = 103.16(2)wt.% 100(0) 92.0(2) 8.0(2) 89.8(2) 10.2(2)Li site occ. by Fe 0.022(9) 0.030(7) 0.020(8)Fe site occ. by V – 0.03(2) 0.045(16)R factor (%) Rwp = 5.85 Rwp = 6.26 Rwp = 7.55
VFP(i
m1dcdtc
3
L
TM
T
Fig. 2. FESEM micrographs of L
The electric conductivity of LFP/C, LVFP(i)/C and LVFP(ii)/C,easured at room temperature, amounted to 1.43 × 10−3 S cm−1,
.68 × 10−3 S cm−1 and 1.7 × 10−3 S cm−1, respectively. Vanadium-oped samples displayed somewhat higher value of electriconductivity in spite of lower carbon content (13.4 vs. 10 wt.%). Thisifference may be described to the presence of Fe2P which reducedhe band gap of LiFePO4 [43], while itself presented a metallic typeonductor [12–14].
.2. Electrochemical behavior of V-doped LiFePO4/C composites
The electrochemical investigations of vanadium dopediFePO4/C composites were carried out by cyclic voltammetry
able 2 O bond distances.
Sample M O bond LFP/CLength [Å]
LFVP(i)/CLength [Å]
LFVP(ii)/CLength [Å]
Fe–O(1) 2.2160 2.1853 2.1529Fe–O(2) 2.0913 2.1183 2.0540Fe–O(3) × 2 2.2425 2.2480 2.2293Fe–O(3)′ × 2 2.0814 2.0651 2.0892(Fe–O)ave 2.1592 2.1550 2.1406Li–O(1) × 2 2.1587 2.1942 2.1861Li–O(2) × 2 2.0772 2.0787 2.1076Li–O(3) × 2 2.1665 2.1705 2.1682(Li–O)ave 2.1341 2.1478 2.1539P–O(1) 1.5271 1.5033 1.5282P–O(2) 1.5891 1.5304 1.5751P–O(3) × 2 1.5527 1.5572 1.5352(P–O)ave 1.5554 1.5369 1.5434
he averaged values are boldfaced.
)/C (left) and LVFP(ii)/C (right).
and glavanostatic charging/discharging methods. Prior to theinvestigations in aqueous electrolytes, the tests in the electrolytewith organic solvent, propylene carbonate (PC) were performed,in order to compare the performance of the obtained samples tothat of the composites of similar composition, investigated thusfar exclusively in PC based organic electrolytes.
3.2.1. Cyclic voltammetry in the 1 M LiClO4/PC electrolyte solutionFig. 3 presents the cyclic voltammograms recorded in 1 M
LiClO4/PC electrolyte solution at various scan rates. One can clearlyseen that all samples exhibited a pair of anodic and cathodic peaks,corresponding the Fe2+/Fe3+ redox process in olivine. The absenceof any other redox peaks in vanadium doped samples, unlikelyto the case described in refs. [23,24], confirmed the results ofXRD analysis that vanadium was completely incorporated into thehost lattice. The crucial indicator of electrochemical reversibil-
ity – peak potential difference (Ep,a − Ep,c)/2, observed at variousscan rates is presented in Table 3, for all the three samples. Forundoped sample, the peak separation was practically identical toTable 3The peak potential differences ((Ep,a − Ep,c)/2) at various scan rates calculated fromcyclic voltammograms of LFP/C, LFeVP(i)/C, and LFVP(ii)/C recorded in LiClO4/PC.
Scan rate/mV s−1 (Ep,a − Ep,c)/2 (mV)
LFP/C LFVP(i)/C LFVP(ii)/C
1 316.8 120.4 184.35 579.5 223.5 392.810 876.7 299.6 530.0
M. Vujkovic et al. / Electrochimica Acta 109 (2013) 835– 842 839
i)/C (c
ttatmaersidwoass
3s
cd(ttee[ir
Fc
Fig. 3. Cyclic voltammograms of LFP/C (a), LFeVP(i)/C (b) and LFVP(i
hat published previously [35]. A significantly reduced peak separa-ion and increased current response of V-doped samples indicated
significantly faster kinetics of intercalation/deintercalation reac-ions. With the increase in scan rate, the effect of V doping became
ore pronounced, especially for LVFP(i)/C sample. Namely, even at scan rate of 20 mV s−1, which is unusually high for any organiclectrolyte [35], the LVFP(i)/C composite maintained well definedeversible peaks inside of the potential window of electrochemicaltability of the electrolyte. From only cyclic voltammetry, accord-ng to the peak separation criteria, the sample with lower V contentisplayed apparently the maximum performance. This disagreesith the previous studies where the maximum performance was
bserved at V concentration of 5 mol %[23] and 7 mol % V [22],nd may be due to a combined effect of high participation of initu formed carbon and Fe2P. However in aqueous solution, as pre-ented in next sections, the behavior returned to the expected one.
.2.2. Galvanostatic measurement in 1 M LiClO4/PC electrolyteolution
In 1 M LiClO4/PC electrolyte solution, the galvanostaticharge/discharge curves of the LFP/C and LVFP(i)/C samples, Fig. 4,isplayed very flat charge and discharge voltage plateaus at 3.5 Vvs. Li+/Li) and 3.36 V (vs. Li+/Li), respectively, characteristic forwo-phase equilibrium FePO4/LiFePO4 [1,2,44]. This figure illus-rates that the incorporation of vanadium into the olivine structurenabled to achieve, in comparison to undoped olivine, a moreffective utilization of active material, as also reported elsewhere
22–26]. Since all samples displayed similar electronic conductiv-ty of ∼10−3 S cm−1, the differences in electrochemical behavioreflected primarily the effect of the dopant.ig. 4. The initial charge/discharge curves for un-doped LFP/C and doped LFVP(i)/Composites measured at the rate C/3 in 1 M LiClO4/PC.
) in 1 M LiClO4/PC solution, recorded at various scan rates (mV s−1).
One may note that LVFP(i)/C showed better rate performancesthan LVFP(ii)/C, which might be attributed to the presence of theFe2P phase in higher concentration in the later sample (Table 1), i.e.,to the more expressed blocking effect of Fe2P toward the lithiumtransport through the composite/electrolyte boundary.
The best capacity value registered in this study for V-dopedcomposites at the rate 1 C, 140 mA h−1, was somewhat smallerin comparison to that reported in the literature, amounting to150 mA h−1 [20,24]. This difference might be attributed, most prob-ably, to the impeding effect of an excessive amount of non-olivinephases (carbon and iron phosphide). However, as we emphasizedin the introductory part, high carbon content is very desirable in thesense of prolongation of cycle life in aerated aqueous electrolyte,as demonstrated in the next sections.
Fig. 5 shows the dependence of capacity on the number of charg-ing/discharging cycles measured either at constant rate of 1 C (left)or at various rates in the range 1–5 C (right). Specifically, underhigh current rates, the differences in capacity of undoped anddoped samples became particularly significant in favor of dopedsamples. The Rietveld refinement showed Li O bond lengtheningwith increased dopant concentration. Therefore, the improvementof electrochemical properties of LiFePO4 in organic electrolyte viavanadium doping may be explained by an accelerated diffusion ofLi ions thanks to the space effects. In conclusion, although not max-imum performance of V doped samples reported by other authorsfor organic electrolyte solutions [22,23] was not achieved by thissynthesis route, the benefits of V doping were clearly confirmed.
3.2.3. Cyclic voltammetry in aerated aqueous LiNO3 electrolytesolution
The comparative cyclic voltammograms of LFP/C, LVFP(i)/C andLVFP(ii)/C in aqueous LiNO3 solution in standard air atmosphereat a common scan rate of 1 mV s−1 (left), and at various scan ratesranging 5–100 mV s−1, are shown in Fig. 6. Upon comparison withthe analogous diagrams recorded in organic electrolyte shown inFig. 3, one may conclude that vanadium doped LiFePO4/C compos-
ite displayed significantly enlarged current densities at each scanrate and reduced separation of the peak potentials (the later arepresented in quantitative manner in Table 4). Almost completepeaks may be registered at unusually high scanning rate of evenTable 4The peak potential differences ((Ep,a − Ep,c)/2) at various scan rate calculated fromcyclic voltammograms of LFP/C, LFeVP(i)/C and LFVP(ii)/C recorded in LiNO3.
(Ep,a − Ep,c)/2 (mV)
Scan rate/mVs−1 LFP/C LVFP(i)/C LVFP(ii)/C
1 55 39 295 89 75 6610 120 96 90
840 M. Vujkovic et al. / Electrochimica Acta 109 (2013) 835– 842
Fig. 5. Cyclic performance of LFP/C, LFeVP(i)/C and LFVP(ii)/C, in 1 M LiClO4/PC at common rate of 1 C (a) and different high charge/discharge rates of 1 C–5 C (b).
F ii)/C i(
3oF
[Ltt
Frs
ig. 6. The comparison of cyclic voltammograms of LFP/C [35], LFeVP(i)/C and LFVP(b) and LFVP(ii)/C (c) in saturated aqueous LiNO3 solution at various scan rates.
00 mV s−1. It should be pointed out that, contrary to the case ofrganic electrolyte, in aqueous electrolyte the higher content ofe2P in LVFP(ii)/C composite does not cause the kinetic problems.
The CV profiles of LFP/C in aqueous LiNO3 solution from the ref.35] were used in order to compare them with the new data for
VFP(i)/C and LVFP(ii)/C. These CV profiles were used to constructhe dependence of the peak current density on the square root ofhe scan rate, shown in Fig. 7.ig. 7. The relation between the current density peaks and square root of the scanate of LFP/C, LFeVP(i)/C and LFVP(ii)/C samples tested in saturated aqueous LiNO3
olution.
n LiNO3 at a common scan rate of 1 mV s−1 (a); Cyclic voltammograms of LFeVP(i)/C
The linear dependence in Fig. 7 indicates the applicability ofRandles–Sevcik equation
ip = 0.4463nF(
nF
RT
)1/2ACv1/2D1/2 (1)
For peak current, ip, to be expressed in amperes, the concentra-tion of lithium, C = CLi, should be in mol cm−3, the real surface areaexposed to the electrolyte in cm2, chemical diffusion coefficient oflithium through the solid phase, D = DLi, in cm2 s−1, and sweep rate,v, in V s−1.
In the present case, Eq. (1) may be adapted by omitting the num-ber of electrons, n, being equal to 1. This equation is often used in theliterature to calculate the chemical diffusion coefficient of lithium[45–49]. For materials with identical both Li concentration andactive surface area the difference in slopes given by Eq. (1) indicatesexclusively the difference in diffusion coefficients of lithium in thesolid phase. The slopes for LFP/C, LFV(i)/C and LFV(ii)/C, presentedin Fig. 7, amount in anodic current region to 73, 107 and 130 A g−1
(V s−1)−1/2, and they are practically identical to the correspondingslopes in cathodic current region. Thanks to the fact that the con-sidered composites were synthesized under identical conditions,one may expect that the correlation between the squared slopesreflect the correlation between the lithium diffusion coefficients.
The slope of the dependence (1) may be somewhat affected
by a significant contribution of charge transfer resistance (Rct).However, peak potential separation is much more sensitive to themagnitude of Rct, as derived by Matsuda and Ayabe [50]. Accordingto the theory they derived, for a negligible value of Rct, the peak
M. Vujkovic et al. / Electrochimica Acta 109 (2013) 835– 842 841
F rate o5 us cha
po
ctos
stepeTcatsi[[
3e
LtwtsaascDro551dsioacr
ig. 8. (a) Coulombic capacity versus cycle number of LVFP(ii)/C composite at the
0th charge/discharge curves. (b) Coulombic capacity versus cycle number at vario
otential separation does not exceed much 59 mV, while, a growthf Rct causes an increase of this peak potential separation infinitely.
A mutual comparison of the slopes presented in Fig. 7 indicates alear improvement of diffusivity of lithium with increasing concen-ration of vanadium. Simultaneously, the improvement in the ratef charge transfer manifesting itself as a decrease of peak potentialeparation may be observed, as presented in Table 4.
The slope of the plot of the current density versus square root ofcan rate in Fig. 7, was about three to four times higher comparedo the slope of the samples of the same composition in organiclectrolyte (which may be determined from Fig. 3). In addition,eak potential separation in aqueous electrolyte (Table 4) is sev-ral times lower than in organic electrolyte for identical scan rates.hus one may conclude that both charge transfer and diffusion pro-esses of Li ion intercalation in solid olivine phase are much faster inqueous than in organic environment. A reasonable explanation ofhis difference is the assumption of the appearance of a passivatingolid electrolyte interface (SEI) in organic PC environment], whichs evidenced many times at the boundary Li-insertion material/PC44,51,52], and which can not appear in an aqueous environment51,52].
.2.4. Galvanostatic measurements in aerated aqueous LiNO3lectrolyte solutions
The cyclic galvanostatic measurements were performed forVFP(ii)/C in LiNO3 aqueous solution under rising current rate, inhe voltage range between 0.01 V and 1.2 V vs. VO2, and the resultsere presented in Fig. 8. The charging/discharging rate was ini-
ially 1 C for first 50 cycles (Fig. 8a), and then it was increasedtepwise up to even 100 C (Fig. 8b). The inset in Fig. 8a shows thectual charge/discharge profiles for 1st, 2nd, 3rd and 50th cyclet 1 C. The shape of charge/discharge curves did not change sub-tantially with the number of cycles, indicating stable coulombicapacity. The initial discharge capacity was 99.6 mAh g−1 at 1 C.uring galvanostatic cycling, after a small initial capacity loss, a
ather stable value of roughly 91 mAh g−1 was measured through-ut 50 cycles. At high and very high rates of 2, 3, 5, 10, 20, 40,0 and 100 C (Fig. 8b), the mean capacity was 85, 78, 76, 74, 55,0, 47 and 35 mAh g−1, respectively, and after return to the rate
C, the capacity recovered itself completely. Apparently, vanadiumoping improved the rate capability in saturated aqueous LiNO3olution in comparison to the undoped LFP/C composite reportedn ref. [35]. The very good high rate performance of LVFP(ii)/C,
bserved in saturated aqueous LiNO3 electrolyte solution, indicatedfast lithium-ion diffusivity of this material. In addition, higherarbon content in composite powder obviously prevented the sideeactions of active phase with dissolved OH− and O2 species in
f 1 C, within a common cut-off cell voltage 0.01–1.2 V; inset: the 1st, 2nd, 3rd andrge/discharge rates. The capacity is calculated per one gram of LVFP(ii).
aqueous electrolyte, being responsible for a rapid capacity fading[33,34].
4. Conclusions
Fast and simple malonic acid-assisted glycine-nitrate gel-combustion process was used to synthesize successfully thevanadium doped olivine LiFePO4/C composites with high con-tent (10 and 13.4 wt.%) of in situ-formed carbon. The two-phasedRietveld refinement revealed complete vanadium incorporationinto the olivine structure. A part of iron was lost on account of theformation of iron phosphide phase, which, as a metallic conductor,supported the electronic conductivity of samples originating fromcarbon. In accordance with the advantages of vanadium doping,evidenced in organic electrolyte, both high initial capacity of Li-ionintercalation/deintercalation in aqueous electrolytes (among thehighest ones reported for aqueous solutions), and significant rateimprovement were evidenced. For instance, appreciable capacityretention of nearly 35 mAh g−1 was measured at the rate of even100 C. Simultaneously, thanks to a solid carbon protection layer,capacity fade caused by olivine reactions with dissolved oxygen,being the highest obstacle for its practical application, was almosteliminated. This opens the way of undisturbed use of LiFePO4 inaqueous Li-ion batteries.
Acknowledgments
The Ministry of Education, Science and Technological Develop-ment of the Republic of Serbia supported this work (Project No.III 45014 – M.V., I.S., N.C., and S.M., Project No. III 45004 – D.J.,and Project No. III 45015 – M. M.). S.M. is grateful to The Ser-bian Academy of Science and Arts for supporting this investigationthrough the project “Electrocatalysis in the contemporary processof energy conversion”.
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