Positive electrode materials for lithium batteries based on VOPO 4

Ž .Solid State Ionics 140 2001 209–221www.elsevier.comrlocaterssi

Positive electrode materials for lithium batteries based on VOPO4

N. Dupre, J. Gaubicher, T. Le Mercier, G. Wallez, J. Angenault, M. Quarton)´Laboratoire de Cristallochimie du Solide, UniÕersite Pierre et Marie Curie-Paris VI, Case 176, 4 place Jussieu,´

F-75252 Paris Cedex 05, France

Received 7 November 2000; received in revised form 19 January 2001; accepted 21 January 2001

Abstract

A detailed electrochemical study of Li insertion in thea-form of VOPO and the optimization of the cycling performance4

are presented. The redox process occurs in one step close to 3.76 V, along with a phase transition. In order to improve theintercalation kinetics, the electronic conductivity was optimized by introducing a mixed valency, and the ionic conductivitywas favored by introducing ‘pillaring’ molecules or ions in the interlayer space. In this way, the electrochemical behaviors

Ž .of a-VOPO P2H O, a-Na VOPO ,a-K VOPO anda-Mg VOPO 0FxF1 have been studied: the hydrate compound4 2 x 4 x 4 x 4Ž .shows a good specific capacity 100 mA hrg at a Cr5 regime , but a poor cyclability was mainly because of water

Ž q. Ž q.oxidation. The inserted large alkaline ions improved the cyclability up to 80 cycles Na or over 100 cycles K .Enhancements of the VOPO specific capacity have been obtained as well by mechanical ball-millings.q2001 Elsevier4

Science B.V. All rights reserved.

Keywords: Lithium batteries; Intercalation; Cathode; Vanadium; Phosphate

1. Introduction

Owing to the low density and the high reductivepower of lithium metal, a great deal of interest hasbeen focused on the development of lithium batteriesas power sources for portable devices or electricvehicles. So far, investigations have been mainlydevoted to oxides as electrode intercalation com-

Ž .pounds. VOXO compounds XsS, P, As present4

as well open 2D or 3D frameworks that are suitablefor lithium intercalation process. In addition, thesecompounds present attractive theoretical specific ca-

) Corresponding author. Tel.:q33-1-44-27-55-44; fax:q33-1-44-27-25-48.

Ž .E-mail address: [email protected] M. Quarton .

Žpacities from 135 to 165 mA hrg, depending on the.X element . The present study mainly deals with the

a-form of VOPO , whose performance as lithium4

intercalation material will be assessed and optimized.This compound is already used in the chemical

industry as a catalyst for the synthesis of maleicw xanhydride from n-butane or n-butene 1 . Some

chemical intercalations of alkaline and alkaline–earthw xions have been performed 2–4 . Moreover, it was

w xshown that some ‘large’ molecules, like pyridine 5 ,w xaliphatic carboxylic acids 6 or polyoxyethylene

w xcompounds 7 , are likely to intercalate in the inter-layer space. A previous work has shown thata-VOPO P2H O is a mixed protonic–electronic con-4 2

ductor with dominant protonic component at roomtemperature, where the conductivity, determined by

0167-2738r01r$ - see front matterq2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0167-2738 01 00818-9

( )N. Dupre et al.rSolid State Ionics 140 2001 209–221´210

impedance spectroscopy, is of the order of 10y5 Sy1 w xcm 8 . The existence of a mixed conduction has

allowed the use ofa-VOPO P2H O as an electrode4 2

able to reversibly exchange protons with a solid-stateprotonic conductor. As a consequence of its lamellarstructure, thea-form appears to be a promisingelectrode material for lithium batteries.

However, the hydrous and anhydrousa-forms ofVOPO have never been considered as host materi-4

als for lithium batteries despite their good theoreticalspecific capacities, 135 and 165 mA hrg, respec-tively. Some of us have shown that theb-VOXO4w x w x9,10 series, especiallyb-Li VOPO 11 , are in-0.92 4

teresting for their possible use as positive electrodematerials. The former work shows that the lithiumintercalation–deintercalation kinetics inb-VOPO is4

rather low, but its optimization led to better perfor-mance. Both structures of thea- andb-forms consistof chains of distorted VO octahedra connected by6

PO tetrahedra. The VO –PO connectivity, how-4 6 4

ever, is different insofar as thea-form presents aŽlamellar structure with a quadratic framework space

. w xgroup P4rn 12 , whereas that of theb-form is 3DŽ . w xorthorhombic space groupPnma 13 . Thus, lithium

diffusion should be more favored in thea-formbecause of its more open structure. Herein, we shallstudy the lithium electrochemical intercalation inVOPO forms along with the improvement of the4

cycling performance. In this way, a VIVrV V mixedvalence and ‘pillaring’ species will be introduced, inorder to enhance the electronic conductivity and theionic conductivity, respectively. Ball-milling has alsobeen performed in order to study the influence of thegrain size on thea-VOPO performance.4

2. Experimental

a-VOPO , its intercalated derivatives andb-4

VOPO were prepared by simple and cheap methods4w x1 . a-VOPO was obtained by dehydration ofa-4

Ž y1.VOPO P2H O at 2508C 1008C h for 12 h under4 2

flowing argon.a-VOPO P2H O was synthesized in4 2Ž .an aqueous way: V O 99.6%, Aldrich and H PO2 5 3 4

Ž . Ž85%, Carlo Erba in excess molar ratio VrPs.1:7.3 were mixed and heated at 1008C overnight

under reflux. Then, the suspension was filtered,washed with water and acetone, and finally air-dried.Water content was measured by thermogravimetric

analysis under flowing argon, using a Setaram 92instrument.

The obtained yellow-green powder ofa-VOPO P4

2H O contains, systematically, some VIV ions, the2

presence of which has been confirmed by ESR spec-tra. The resulting positive charge deficit in the struc-ture is balanced by the presence of protons, asevidenced by1H NMR spectra. These VIV ions,which are also present withina-VOPO upon dehy-4

w xdration 12 , tend, on one hand, to lower the theoreti-cal specific capacity but, on the other hand, toincrease the electronic conductivity. In order to vary

IV V Žthe V rV ratio, different volumes of HNO 133 .and 9 cm for 10 g of VOPOP2H O samples have4 2

been added during the synthesis ofa-VOPO P2H O.4 2

The chemical analysis by redox titration of the vana-dium phosphate gives the following results: VIVrV V

3 Ž .s2.5% for 1 cm HNO VrHNO s3.33 and3 33 Ž .0.5% for 9 cm HNO VrHNO s0.37 . The ob-3 3

tained samples differ from one another in color. As aŽ .matter of fact, thea-VOPO or a-VOPO P2H O4 4 2

containing VIV ions becomes more and more greenas the VIVrV V ratio increases. The ESR analysisconfirms the previous result: for a green powder, astrong coupling of the VIV ions is observed, whilethe hyperfine structure is visible for a yellow pow-der. Under ambient atmosphere, the yellow-greenpowder gets more and more green in several days. Athermal treatment at 1508C in air drives back to ayellow powder, as the VIV ions are reoxidized to theV V state.

ESR spectra were recorded at 77 K on a BrukerŽ .ESP 300 E, operating at 9.3 GHz X-band . A field

modulation of 100 kHz, a modulation width of 10 Gand a microwave power of 10 mW were used.1HNMR experiments were performed on a Bruker MSL300 NMR spectrometer. Conventional spectra wereobtained at 300 MHz. Samples were spun at 12 kHzin zirconia rotors using a double-bearing probehead.

Insertion of ‘pillaring’ cations into the hydrateda-form containing 2.5% of VIV has been performedat room temperature using stoichiometric amounts of

Ža 0.25 or 0.50 M solution of alkaline iodide NaI,. Ž .KI or alkaline-earth iodide MgI over 2 or 3 days2

w xin 95% ethanolrwater or water 3 . The obtainedpowders were filtered, washed with ethanol or waterto remove any excess iodine, then air-dried. Theanhydrous compounds, strongly amorphized, were

( )N. Dupre et al.rSolid State Ionics 140 2001 209–221´ 211

obtained upon heating the hydrates at 2508Covernight under flowing argon. The amounts of in-serted sodium, potassium and magnesium have beenmonitored by emission spectroscopy. Six sampleshave been synthesized and studied, corresponding tothe following formulae: Na VOPO , Na VOPO ,0.13 4 0.23 4

Na VOPO , K VOPO , K VOPO and Mg -0.32 4 0.30 4 0.51 4 0.1

VOPO .4

b-VOPO was prepared upon reaction of stoichoi-4

metric amounts of V O and NH H PO at 7008C2 5 4 2 4w x14 .

Pellets for electrical conductivity measurementswere obtained by pressing the powder in a cylindri-cal matrix. The two opposite sides of the pellets werecoated with platinum lacquer. A HP 4284 impe-dancemeter was used for AC conductivity measure-ments on the pellets placed under argon flow. Thecomplex impedance plot shows a circle-shaped curvefrom which the conductivity was calculated usingcomplex nonlinear least-squares fitting. Electronicconductivity measurements were monitored by aTacussel PGS201T potentiostat using platinum elec-

w xtrodes acting as blocking electrodes 15 . Conductivi-ties were determined in the frequency range 20Hz–1000 kHz.

Powders were ball-milled with a planetary ballmill using zirconia bowls and balls. The ball vs.powder weight ratio was kept to 50. The particle sizeof the ground materials was measured by light diffu-sion using a Hariba capa-300 granulometer. Thepowder samples were put in ethanol and submitted toan ultrasonic dispersion in order to obtain a nicesuspension.

Electrochemical measurements were performedboth in galvanostatic and potentiodynamic modes

w xusing Swagelocke-type cells 16 and McPile sys-w xtem 17 with Li metal as negative and reference

electrode. The active material was mixed with 20%Ž .acetylene black carbon Strem Chemicals and 5%

Ž .PVDF Aldrich as binder. An aluminum diskŽ 2.surface area: 1.54 cm was then coated with aslurry made by adding cyclopentanone to the previ-ously obtained mixture, and air-dried at ambienttemperature for a few hours. A 1 M LiClO solution4

Žin ethylenecarbonaterdimethylenecarbonate ECr.DMC, Merck was used as electrolyte. The cell

components were transferred in argon atmosphereprior to the assembling process.

X-ray powder diffraction patterns were obtainedon random powder samples or on positive electrodemixtures using a Philips PW 1050 goniometer withCuK radiation.a

3. Electrochemical study of a-VOPO4

The X-ray powder diffraction pattern of the anhy-Ž .drous compound Fig. 1 matches those published by

w xLadwig 18 and are different from those reportedw xelsewhere by Tachez et al. 19 because only a few

reflections are common. Owing to the low crys-tallinity of the material inherent to the synthesismode, the XRD patterns show very broad peaks, andwe were unable to do a satisfying refinement of thelattice parameters. Other syntheses ofa-VOPO4

made between 2508C and 6008C for 12–150 h gavethe same results. Electronic diffraction experiments,however, have shown thata and b cell parameters

˚Ž . Ž6.17 A are close to those obtained by Tachez 6.20˚ .A . Moreover, electronic microscopy experimentshave shown a lamellar morphology of the powdergrains. The X-ray powder diffraction patterns showthat the 001 peak shifts progressively fromds7.11A for a-VOPO P2H O at ambient temperature to4 2

˚ Ž .ds4.11 A for a-VOPO at 2508C Fig. 1 .4

The DTA–TGA results show thata-VOPO P4

2H O dehydrates in two steps: the loss of the first2

water molecule occurs from 458C onwards, and thesecond loss occurs over 1008C. a-VOPO is stable4

ŽFig. 1. X-ray powder diffraction patterns fora-VOPO obtained4.after dehydration ofa-VOPO P2H O at 2508C and for electro-4 2

chemically intercalateda-Li VOPO .0.3 4


Ž . IV Ž . IVFig. 2. Cyclic voltamograms in potentiodynamic mode fora-VOPO ; A 2.5% of V and B 0.5% of V .4

under argon atmosphere until 7008C. At the time ofhydration of a-VOPO , the XRD peaks that are4

shifted to 2u low values account for an increase ofthe c parameter.

Furthermore, notations A and B respectively cor-respond to 2.5% and 0.5% VIVrV V ratios within thea-VOPO structure. The incremental capacity curves4

Ž . Ž .for the Lira-VOPO A and Lira-VOPO B sys-4 4

tems, obtained in potentiodynamic mode with a typi-cal rate of"10 mVr5 h, between 3.2 and 4.1 V, arereported in Fig. 2. The reduction–oxidation processoccurs mainly in one step at 3.76 V. The relativeposition of the reduction and oxidation peaks sug-gests that lithium intercalation induces a phase tran-

Žsition. This result is supported by XRD studies Fig..1 : new peaks appear on the corresponding diagram.

The l-dependent peaks shift strongly due to theincrease of the interplanar distance. The absence ofcurrent plateau on the chronoamperograms obtained

Ž .for a-VOPO A suggests that the phase transition4

kinetics is mainly controlled by diffusion of chargecarriers. Thus, the performance of the Lira-VOPO4

system should be enhanced upon increasing the elec-tronic and ionic conductivities. In the case ofa-

Ž . IVVOPO B , containing 0.5% V , the electroche-4

mical behavior appears to be similar to that ofŽ . Ž IV .a-VOPO A 2.5% V , but the broadening of the4

incremental capacity peaks suggests, however, alower kinetics. In order to check this assumption,electronic conductivity measurements have been per-formed. The results are reported in Fig. 3: the elec-

Ž .tronic conductivity ofa-VOPO A , extrapolated at4

room temperature, is roughly equal to 2.1=10y9 Scmy1, that is to say, 10 times greater than that of

Ž .a-VOPO B . The conductivity experiments showed4

that the overall conductivity is nearly equal to that ofthe purely electronic, indicating a low mobility of theprotons. The electrochemical behaviors ofa-

Ž . Ž .Fig. 3. Electronic conductivities of VOPO A and VOPO B vs.4 4

temperature.


Ž . Ž IV . Ž . Ž IV .Fig. 4. Galvanostatic voltage curve for the first cycles fora-Li VOPO A 2.5% of V and fora-Li VOPO B 0.5% of V at Cr20x 4 x 4

regime between 2.8 and 4.1 V.

Ž . Ž .VOPO A and a-VOPO B have been studied in4 4

galvanostatic mode at Cr20 regime. The first cyclesare presented in Fig. 4. Lithium intercalation in

Ž .a-VOPO A appears as a potential plateau, which4q Ž .involves more Li ions fora-VOPO A than for4

Ž . Ža-VOPO B . Moreover, the polarization voltage4

difference between the reduction and oxidation. Ž . Ž .curves for a-VOPO B 0.51 V at xs0.15 is4

Ž . Žlarger than that fora-VOPO A 0.23 V at xs4. IV0.15 . This increase of the polarization, as the V

Ž .amount decreases, confirms thata-VOPO B pre-4

sents a lower kinetics towards lithium uptakerre-moval. The specific capacities in discharge for theŽ . Ž .A and B samples are gathered in Fig. 5. Theincrease in the VIVrV V ratio improves the kineticsand the specific capacity by as much as 40 mA hrg

Fig. 5. Specific capacity vs. number of cycles at Cr20 for a-VOPO , including different proportions of VIV .4


at Cr20. Tests with higher concentrations of VIV

defects did not show any improvement of their elec-trochemical performance that, we believe, dependson ionic diffusion through the interlayer space. Asseen in Fig. 4, the specific capacities obtained fora-VOPO are much smaller than the theoretical4

value: the reversible intercalation does not exceed0.3 Liqrformula unit. In addition, a significant ca-pacity fading is observed on cycling.

4. Electrochemical study of derived phases froma-VOPO4

The intercalation potential corresponding to theV VrV IV couple is very attractive for possible appli-cations, but the kinetics of the redox process remainstoo low for possible use in solid-state batteries. Inorder to further improve the Lira-VOPO rate capa-4

bilities, we have tried to facilitate the lithium diffu-sion within the interlayer space. For this purpose, wehave studied the cycling behavior ofa-VOPO as a4

Ž q q 2q.function of different species H O, Na , K , Mg2

that have been initially intercalated to maintain anotable gap between the structural planes.

First, we have investigated the electrochemicalbehavior of a-VOPO P2H O because the water4 2

w xmolecules increase the interlayer space 20 , and thus

w xthe ionic conductivity 4 . We have studied theLirVOPO P2H O system for a fixed VIVrV V ratio4 2Ž .2.5% , both in potentiodynamic and galvanostatic

Ž .modes Cr20 and Cr5 . The electrochemical behav-ior of this system appears to be more complex thanthat of Lira-VOPO with a six-step intercalation4

Ž .process Fig. 6 . For each of the six reduction steps,the shape of the chronoamperometric responses evi-dences a two-phase equilibrium with an interfaceprogression that limits the structural transformationkinetics. For example, for the third intercalation stepŽ .Fig. 7 , the reduction current increases slightly whenstepping from 3.75 to 3.74 V, and very significantlyat the next step to 3.73 V, constituting a plateau vs.time. The polarization drops from 0.14 V forxs0.5upon the first cycle to 0.09 V upon the second cycleŽ .Fig. 8 . The first lithium intercalation–deintercala-tion cycle might lead to slight structural bendingsandror alter the grain surface state. Thus, a forma-tion of the material occurs that makes the followingcycles easier. As shown in Fig. 8, the amount ofelectrons involved upon the first deintercalation is

Žlarger than during the first intercalationysy0.06.at the end of the deintercalation . This extra capacity

Ž .is related to the oxidation peak above 3.9 V Fig. 6 .The responsible mechanism could be the oxidationof initial V IV defects with a simultaneous waterprotons deintercalation. Such a mechanism, however,

Fig. 6. Cyclic voltamogram in potentiodynamic mode ofa-VOPO P2H O.4 2


Fig. 7. Chronoamperograms corresponding to the third step of the first reduction ofa-VOPO P2H O.4 2

does not account for 0.06 electron, since the VIVrV V

ratio is only about 2.5%. The two water moleculeswithin a-VOPO P2H O are weakly bonded. Thus,4 2

because the OrO2y redox potential is roughly equal2

to 1.2 V vs. HqrH , one may expect oxidation of2

interplanar water molecules to occur close to 4 V vs.LiqrLi 8. This water decomposition may involve upto four electrons per molecule ofa-VOPO P2H O.4 2

To check on this hypothesis, a cell has beenstarted on oxidation up to 4.3 V with a Cr20 regime,and then cycled within a 4.3–2.8-V potential win-dow. The first cycles have been reported in Fig. 9:the first deintercalation process corresponds to 0.45electronrmolecule ofa-VOPO P2H O. Actually, it4 2

appears that a larger amount could have been dein-tercalated if for instance, the upper limit of the

ŽFig. 8. Galvanostatic voltage curve for the two first cycles fora-VOPO P2H O at Cr20 regime ysmole of electronsrmole of active4 2.material .


ŽFig. 9. Galvanostatic voltage curve for the first cycles fora-VOPO P2H O at Cr20 regime, oxidation startysmole of electronsrmole4 2.of active material .

potential window had been higher. A meticulousobservation of the cells upon cycling shows thepresence of bubbles at the cell ferrules, but only forhydrate compounds. This fact must be correlated tothe H O oxidation that generates gaseous oxygen. In2

order to check that water oxidation is responsible fora part of the capacity fading, cells have been cycled

within a 3.9–2.8-V potential window. The obtainedŽ .specific capacity at Cr5 is lower about 70 mA hrg

Žthan for a 4.1–2.8-V potential window about 105. ŽmA hrg , but the cycle life is clearly improved over.50 cycles . Within a 4.1–2.8-V potential window,

the Lira-VOPO P2H O system can sustain 20 cy-4 2Ž .cles with a fairly good capacity retention Fig. 10 .

Fig. 10. Specific capacity vs. number of cycles at Cr5 for a-VOPO anda-VOPO P2H O with 2.5% of VIV .4 4 2


Thus, after the first oxidation, the deintercalationprocess fory-0 is certainly more complex, involv-

q q Žing both Li and H species no extra capacitycorresponding to proton reduction has been observed

.above 2.8 V . In comparison with VOPO , the kinet-4

ics of the redox process for the Lira-VOPO P2H O4 2

system is improved: a capacity of 105 mA hrg isŽ .obtained at a Cr5 regime Fig. 10 , along with a

Žsignificant reduction in the polarization Figs. 4 and.8 . After 20 cycles, the system undergoes a capacity

fading that remains, however, higher than that of theLira-VOPO system.4

In order to improve the capacity retention oncycling, sodium, potassium and magnesium ions wereinserted in VOPO prior to electrochemical tests.4

The two criteria that have led the choice of thesepillaring cations were their low or moderate weightŽ .in order to keep a high massive capacity and their

q ˚ q ˚ 2q ˚Ž .size Na : 1.02 A, K : 1.38 A, Mg : 0.72 A ,q ˚Ž .comparable to that of the Li 0.76 A or higher

w x21 .ŽThe six obtained samples Na VOPO , Na -0.13 4 0.23

VOPO , Na VOPO , K VOPO , K VOPO4 0.32 4 0.30 4 0.51 4.and Mg VOPO were tested in galvanostatic mode0.10 4Ž .at a Cr5 regime Fig. 11 and Table 1 . In compari-

son with Lira-VOPO , it appears clearly that the4

cyclability of these systems has been improved.Moreover, for the sodium compounds, the lifetimeand the specific capacity are increased notably. Thepoor results of the magnesium compound might beexplained in terms of size and charge of the Mg2q

ion: because it is slightly smaller and has double thecharge of the Liq cation, it binds itself strongly tothe structural planes of the bulk, and thus reduces thelithium intercalation possibilities. In the case of thesodium and potassium compounds, as for the hydrate

Ž .form a-VOPO P2H O , galvanostatic experiments4 2

started on oxidation show that a certain amount ofApillaring cationsB is deintercalated. The deinterca-lated amounts, however, are very different:ysy0.23 for Na VOPO and y sy0.03 for0.32 4

K VOPO . This difference between the behavior0.30 4

of sodium and potassium compounds stems from thesizes of Naq and Kq: the Kq ions, bigger than Naq,are more stable in the interlayer space. Moreover, thespecific capacity decrease in comparison witha-VOPO suggests that the Kq ions hinder the lithium4

diffusion. This phenomenon can be the result of theopen interlayer space being too wide that leads to astrong electrostatic attraction of the Liq ions to thestructural planes. On the contrary, the Naq ions playtheir Ainterplanar pillarB role without hindering the

Fig. 11. Specific capacity vs. number of cycles for Na VOPO , ball-milled Na VOPO , Na VOPO , K VOPO , Mg VOPO0.13 4 0.23 4 0.32 4 0.30 4 0.10 4Ž .anda-VOPO A at Cr5 regime.4


Table 1Specific capacity, cycle life and polarization values fora-VOPO and its intercalated derivates4

ŽCompound Specific capacity for the Cyclability 25% 10th cycle-specific PolarizationŽ . . Ž .10th cycle mA hrg capacity loss, in cycles V

Ž . Ž .a-VOPO B 26 Cr20 f30 0.514Ž . Ž .a-VOPO A 45 Cr20 f35 0.234

Ž .43 Cr5 f35 0.24Ž . Ž .a-VOPO P2H O A 105 Cr5 f30 0.094 2

Ž .a-Na VOPO 58 Cr5 f90 0.190.13 4Ž .a-Na VOPO 72 Cr5 f70 0.230.32 4Ž .a-K VOPO 34 Cr5 )100 f0.300.30 4Ž .a-K VOPO 15 Cr5 )100 0.400.51 4Ž .a-Mg VOPO 5 Cr5 )100 f0.340.10 4Ž .‘Hard’ ball-milled 72 Cr5 )100 0.16

a-Na VOPO0.32 4Ž .‘Softly’ ball-milled 68 Cr5 )100 0.15

a-Na VOPO0.32 4Ž .Ball-milled 98 Cr5 )100 0.12

a-Na VOPO0.23 4

lithium diffusion, even if a great amount of them isexchanged for Liq ions during the cycling.

5. Influence of ball-milling

ŽBall-milling of powders grinding time from 15 to.120 min were performed in order to further improve

the performance of the poor conductive LirVOPO4

system. The X-ray powder diffraction patterns, re-ported in Fig. 12, show that a loss of crystallinityoccurs upon ball-milling fora-VOPO . This effect,4

however, is partially avoided when hexane is addedprior to ball-milling. The ball-milling effect on theparticle size has been investigated by electronic mi-croscopy. The size of a nonmilled particle is about10 mm, whereas that of ball-milled particles drops toabout 1mm, whatever the ball-milling time is; how-

Fig. 12. Evolution of the X-ray powder diffraction patterns fora-VOPO with the ball-milling conditions.4


Fig. 13. Specific capacity vs. number of cycles fora-VOPO at a Cr5 regime and forb-VOPO at a Cr10 regime, before and after 15 min4 4

ball-milling.

ever, there is no real improvement in the cyclability.Ž .The best specific capacity value 80 mA hrg was

obtained for the material ball-milled in hexane for 15Ž .min, but the capacity curve drops very fast Fig. 13 .

In order to understand the reason for the signifi-cant capacity fading ofa-VOPO , we have studied4

the b-VOPO phase in the same conditions. In this4

way, the grain size as well as the homogeneity of thepositive mixture have been optimized by ball-mill-ings of b-VOPO . Unlike the a-form, the XRD4

patterns do not show any loss of crystallinity uponŽ .ball-milling Fig. 14 . Whatever the ball-milling con-

ditions are, there are few differences with respect toŽthe particle size roughly 2.3mm for a nonball-milled

Fig. 14. X-ray powder diffraction patterns forb-Li VOPO before and after 15 and 60 min ball-millings.0.92 4


.material and 1.4mm for a ball-milled material . Theincrease of the specific capacity from 9 up to 25 mAhrg for the 15-min-ball-milled material at a nominalregime of Cr10 shows the effect of the grain size

Ž .reduction Fig. 13 .Both the a- and theb-form show an improved

specific capacity upon grain size reduction. The dif-ference between the behaviors ofa-VOPO and4

b-VOPO might come from their different states of4

crystallinity: after being ball-milled,a-VOPO shows4Ž .a high amorphization and thus a very rough surface

along with a reduced cyclability. So, it seems thatball-milling on the a-form leads to a prematuredageing process because of its less-resistant 2D struc-ture. b-VOPO retains a good crystallinity along4

with a good cyclability because of its more-resistant3D structure.

Ball-millings have been performed as well onpillared compounds to reduce the grain size, to im-prove the mixture homogeneity and to enhance boththe specific capacity and the cyclability. Ball-mill-ings without carbon black and PVDF yield poor

Ž .electrochemical performance 20 mA hrg , the posi-tive mixture ball-milled material shows no signifi-

Žcant enhancement of the specific capacity 55 mA.hrg at Cr5 . The effect of grain size reduction is

probably balanced by the ‘material destruction’ dueto a hard ball-milling. Experiments on 2-h-AsoftlyBball-milled Na VOPO and Na VOPO have0.32 4 0.23 4

been performed and lead, respectively, to specificcapacities of 65 mA hrg during 60 cycles at Cr5and 85 mA hrg during 100 cycles at Cr5. This

Žresult confirms the two effects of ball-milling i.e..grain size reduction and ‘material destruction’ : the

‘soft’ ball-milling leads to a reduced grain size andavoids too considerable material destruction.

6. Conclusions

Ž V .The chemical presence of reducible V andŽ .structural 2D open structure features ofa-VOPO4

make it a potential material for lithium batteriespositive electrode. The electrochemical study partlyconfirms this analysis: the intercalation–deintercala-tion process occurs in one step close to 3.76 V vs.Li 8. The performance of this compound, however,remains moderate, and the main part of the presentstudy deals with its optimization.

The introduction of a VVrV IV mixed valencyinduces an increase in the electronic conductivityand, correlatively, in the kinetics of the phase trans-formation associated with lithium intercalation–dein-tercalation. In order to facilitate the ionic diffusion,‘pillaring’ species have been introduced into theinterlayer space. The interplanar water improves thespecific capacity, but its oxidation above 3.9 V leadsto a drop of the capacity after 20 cycles and, conse-quently, restrains to a cycling in the 2.8–3.9-V po-tential window. The sodium ions insertion into theinterlayer space leads to the best results, even if alarge part of them are exchanged for lithium oncycling. A comparative study with other ‘pillaring’

Ž q 2q.ions K and Mg shows the essential role ofsteric and Coulombian factors.

The grain size reduction obtained by ball-millingimproves slightly the specific capacity, but the elec-trodes show a poor cycle life. The comparison withb-VOPO suggests that this phenomenon results from4

the a-VOPO 2D structure, which is significantly4

perturbed upon ball-milling as opposed to that of the3D b-VOPO . It follows that with the formation of4

many more defects trapping lithium permanently, animportant capacity fading occurs from the first cy-cles.

References

w x Ž .1 E. Bordes, P. Courtine, J. Catal. 57 1979 236.w x2 R. Pozas, S. Madueno, S. Bruque, L. Moreno-Real, M.

Martinez-Lara, C. Criado, J. Ramos-Barrado, Solid StateŽ .Ionics 51 1992 79.

w x3 A.J. Jacobson, J.W. Johnson, J.F. Brody, J.C. Scanlon, J.T.Ž .Lewandowski, Inorg. Chem. 24 1985 1782.

w x Ž .4 V. Zima, L. Benes, R. Siskova, Solid State Ionics 67 1994277.

w x5 J. Johnson, A. Jacobson, J. Brody, S. Rich, Inorg. Chem. 21Ž .1982 3820.

w x6 L. Benes, J. Votinsky, J. Kalousova, K. Handlir, Inorg.Ž .Chim. Acta 176 1990 255.

w x7 K. Melanova, L. Benes, V. Zima, R. Vahalova, Chem.Ž .Mater. 11 1999 2173.

w x8 V. Zima, M. Vlcek, L. Benes, M. Casciola, L. Massinelli, R.Ž .Palombari, Chem. Mater. 8 1996 2505.

w x9 J. Gaubicher, Y. Chabre, J. Angenault, A. Lautie, M. Quar-´Ž .ton, J. Alloys Compd. 262 1997 34.

w x10 J. Gaubicher, These Universite Pierre et Marie Curie-Paris` ´Ž .VI 1998 .

w x11 J. Gaubicher, T. Le Mercier, Y. Chabre, J. Angenault, A.Ž .Lautie, M. Quarton, J. Electrochem. Soc. 146 1999 4375.´


w x Ž .12 B. Jordan, C. Calvo, Can. J. Chem. 51 1973 2621.w x Ž .13 R. Gopal, C. Calvo, J. Solid State Chem. 5 1972 432.w x Ž .14 E. Bordes, P. Courtine, G. Pannetier, Ann. Chim. 8 1973

105.w x15 C. Wagner, 7th International Committee of Thermodynamics

Ž .and Electrochemical Kinetics, Lindau 1955 .w x16 D. Guyomard, J.M. Tarascon, J. Electrochem. Soc. 139

Ž .1992 937.w x17 C. Mouget, Y. Chabre, Multichannel Potentiostat Galvanostat

AMac PileB, licensed from CNRS and UFJ-Grenoble to Bio-Logic, 1 Av. de l’Europe, F-38640 Claix.

w x Ž .18 G. Ladwig, Z. Anorg. Allg. Chem. 338 1968 266.w x19 M. Tachez, F. Theobald, E. Bordes, J. Sol. State. Chem. 40

Ž .1981 280.w x20 F. Benabdelouahab, J.C. Volta, R. Olier, N. Guilhaume, F.

Ž .Lefebvre, J. Catal. 134 1992 151.w x Ž .21 R.D. Shannon, Acta Crystallogr., A 32 1976 751.

Positive electrode materials for lithium batteries based on VOPO 4

Documents

Transcript of Positive electrode materials for lithium batteries based on VOPO 4