Solid State Ionics 118 (1999) 141–147

Charging characteristics of Li V O11x 3 8

*Jin Kawakita , Takashi Miura, Tomiya KishiDepartment of Applied Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi 3-14-1, Kouhoku-ku,

Yokohama 223-8522, Japan

Received 29 September 1998; accepted 25 October 1998

Abstract

Charging characteristics of Li V O was investigated using XRD and FT-IR measurements. The delithiated product,11x 3 8

Li V O was prepared by both chemical and electrochemical lithium extraction from fully lithiated samples, Li V O .42y 3 8 4.2 3 8

The experimental results showed that chemical insertion and extraction were reversible reactions. In the case ofelectrochemical lithium extraction, the crystal structure of the delithiated product (x 5 0.5) was returned to that of theoriginal material (x 5 0.2) though the reaction route was not reversible. 1999 Elsevier Science B.V. All rights reserved.

Keywords: Vanadate; Lithium; Extraction; Chemical; Electrochemical

1. Introduction erties mainly during lithium insertion under normalexperimental conditions. Some of them [3–9,12]

Lithium vanadium oxide, Li V O is one of the presented the curves of open circuit potential (OCP)11x 3 8

most interesting candidates for active cathode materi- versus composition described by the formulaals of the lithium secondary batteries. The excellent Li V O . The others [3,5,7,10–12] characterized11x 3 8

suitability as an intercalation host is provided by the the crystal structures of the lithiated vanadates bylayered structure refined by Wadsley [1]. The feature X-ray diffraction (XRD) and infrared (IR) spectro-

1is that the adjacent layers are connected by Li ions scopic analyses. Picciotto et al. [11] determined the22octahedrally coordinated by O ions though almost crystal structures of the original and a lithiated

all the vanadium components exist in pentavalent product, i.e. Li V O and Li V O . The authors1.2 3 8 4 3 8Vstate, i.e. V . The remaining vanadium components [12] refined lattice constants of the chemically and

are present in tetravalent state owing to occupation electrochemically lithiated Li V O with various x11x 3 81of the tetrahedral sites by excess Li ions, as values.

represented by x in Li V O . There were many reports [3,5,7,9,10,13–15] on11x 3 8

¨Since Besenhard and Schollhorn [2] found that the cyclic voltammetry and the galvanostatic dis-lithium insertion reaction was able to occur con- charge and charge tests on one or many cycles atcerning this oxide, many researchers [3–12] have various current densities, in addition to the structuralinvestigated its electrochemical and structural prop- reversibility before and after cycling. There are few

investigations [5,11], however, concerning the rela-*Corresponding author. tions between the crystal structure and open circuit

0167-2738/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved.PI I : S0167-2738( 98 )00437-8

142 J. Kawakita et al. / Solid State Ionics 118 (1999) 141 –147

potential changes during charging (i.e. lithium ex- A delithiated product, Li V O was obtained4.22y 3 8

traction) process. by both chemical and electrochemical methods. InIn this study, the authors focused on the lithium the chemical method, 200 mg of a powder sample of

extraction behaviour of Li V O . Delithiation of Li V O was dispersed into an appropriate volume11x 3 8 4.2 3 8

Li V O was carried out using both chemical and of 0.125M Br /CHCl solution (Br , Kanto chemi-4.2 3 8 2 3 2

electrochemical methods. The OCPs and crystal cal co., inc.) used as an oxidant, referring to Picciot-structures of delithiated products with various com- to’s report [11]. The suspension was stirred at roompositions were characterized and compared with temperature for 7 days under argon atmosphere. LiBrthose of lithiated products. as a by-product was removed by way of thoroughly

repeated decantation using ethanol or chloroform.Then, the powder sample of Li V O was ob-4.22y 3 8

2. Experimental tained by evaporating ethanol or chloroform underevacuated atmosphere. In the electrochemical meth-

Li V O as the starting material was prepared by od, a cell described above was also used.1.2 3 8

high temperature synthesis of Li CO and V O as Li V O was obtained by galvanostatic charge at2 3 2 5 4.22y 3 822described elsewhere in detail [16]. 1 0.1 mAcm . Note that in the following part not

A fully lithiated product, Li V O was obtained Li V O but Li V O is used to represent the4.2 3 8 4.22y 3 8 11x 3 8

by both chemical and electrochemical methods. In composition of the delithiated product as well as thethe chemical method, a powder sample of Li V O lithiated.1.2 3 8

( , 38 mm in particle size) was dispersed into excess The crystal structures of lithiated /delithiated sam-0.46M n-butyl lithium/n-hexane solution (Kanto ples were characterized by powder X-ray diffractionchemical co., inc.) used as a reductant. The suspen- (XRD) and Fourier transform infrared (FT-IR)sion was stirred at room temperature for 7 days spectroscopic measurements. The diffractometer andunder argon atmosphere. An unreacting reagent was spectrometer used were Rigaku RINT-1300 (CuKaremoved by a way of repeated decantation using radiation with a Ni filter) and Bio-Rad FTS-165n-hexane. All the procedures were carried out in the (KBr disk method), respectively. In this paper, theglove box filled with an argon gas. Then, the powder rest potential was regarded as the quasi open circuitsample of Li V O was obtained by evaporating potential (OCP) when its change was within 0.0014.2 3 8

21n-hexane under evacuated atmosphere. The lithium Vh after electrochemical experiments. The OCPscontent of the obtained samples was determined by of the chemically lithiated /delithiated samples wereelemental analysis for Li and V using atomic absorp- also measured using the same preparation proceduretion analysis (Hitachi, 180-55 AAS). In the electro- as described above.chemical method, a cylindrical glass cell with thethree electrodes was used. The working electrodewas a press-formed pellet of the mixed powder of 3. Results and discussionLi V O , acetylene black (Denka Black, Denki1.2 3 8

Kagakukogyo) and poly(tetrafluoroethylene) (PTFE, 3.1. Chemical lithium extractionMitsui-Du Pont) in a weight ratio of 80:15:5. Metal-lic lithium rods (Aldrich, 3.2 mm in diameter) were Fig. 1 shows the OCP versus composition of theused as the counter and reference electrodes. The compound, Li V O , obtained by chemical lithium11x 3 8

electrolyte was 1 M LiClO /PC solution (Mitsubishi insertion into Li V O and chemical extraction4 1.2 3 8

Chemical, Battery Grade). Li V O was obtained from Li V O . The potential plateau is observed4.2 3 8 4.2 3 8

by passing a predetermined quantity of electricity near 2.7 V for 1.5 , x , 3.0 in both two chemicalthrough the electrochemical cell. Galvanostatic dis- methods. On the other hand, the potential gap ischarge was carried out at 258C at a current density of considerable large between lithiated and delithiated

222 0.1 mAcm with a galvanostat (Toho Technical samples with the small content of lithium, especiallyResearch, PS-08) controlled by a personal computer near x 5 0.2. This phenomenon is discussed later.(EPSON, PC-286VF). All these procedures were XRD patterns of Li V O obtained by chemical11x 3 8

carried out in the glove box filled with an argon gas. lithium insertion are compared in Fig. 2. Broad peaks

J. Kawakita et al. / Solid State Ionics 118 (1999) 141 –147 143

x 5 0.2 and lithiated samples, for example at x 5 0.9,as seen in Fig. 2a and 2b. For 0.2 , x , 1.5, all thepeaks of each pattern were ascribed to Li V O11x 3 8

(indexed in JCPDS card No. 18-754). Beyond x 5

1.5, many new diffraction lines appeared in thepattern, for example at x 5 2.6, as seen in Fig. 2c. Inparticular, a new peak is clearly observed in thehigher angle (near 148) than the original peak. Allthe new lines were attributed to the Li V O phase4 3 8

found by Picciotto et al. [11]. For 1.5 , x , 3.2, thesecond Li V O phase existed in addition to the4 3 8

original LiV O one. Beyond x 5 3.2, only the3 8

diffraction lines assigned to the Li V O phase were4 3 8

observed, for example at x 5 3.3, as shown in Fig.2d. Lithium insertion limit of the Li V O phase was4 3 8

around x 5 3.3 in this study.XRD patterns of Li V O obtained by chemical11x 3 8

lithium extraction are compared in Fig. 3. DuringFig. 1. OCP plots of Li V O by chemical methods, (d: lithium extraction, the structure of a delithiated11x 3 8

extraction, and j: insertion). compound changed from the Li V O phase (e.g. at4 3 8

x 5 3.3, Fig. 3a) to the original LiV O one (x 5 1.5,3 8

Fig. 3c) via coexistence state of these two phases(x 5 0.1, Fig. 3b). This structural change was revers-ible, and so the phase of Li V O was dependent11x 3 8

on the x value regardless of chemical methods to

Fig. 2. XRD patterns of Li V O during chemical lithium11x 3 8

insertion (a: x 5 0.2 as original material, b: x 5 0.9, c: x 5 2.6, andd: x 5 3.3).

near 128 and 268 in 2u are due to Mylar film, whichcovers the sample during the XRD measurement. Aslithium insertion proceeded below x 5 1.5, shifting Fig. 3. XRD patterns of Li V O during chemical lithium11x 3 8of the peak position (2u ) or broadening of some extraction, (a: x 5 3.3, b: x 5 1.5, c: x 5 0.1, and d: x 5 0.2 asdiffraction lines was observed between original at original material).

144 J. Kawakita et al. / Solid State Ionics 118 (1999) 141 –147

vary the content of lithium. The Li V O phase peaks per a phase was within 10 in a pattern. At any1.2 3 8

x, a and b axes in a phase have the characteristicformed by lithium extraction, however, has thevalue and no obvious difference between insertiondifferent relative intensity, I of the XRD patternhkl

and extraction of lithium. The similar observationcompared with the original Li V O , as seen in Fig.1.2 3 8

was also confirmed to some extent with respect to3c and 3d, respectively. For example, the intensitythe c axis and b angle at almost all the x values. Atratio of (020) and (103) plane, I /I of the020 103

the small content of lithium for x , 0.5, however, bdelithiated product was larger than that of theangles of the delithiated products are wider than thatoriginal compound. This observation had been ex-of the original material corresponding to the b angleplained by a slight modification of the parent struc-in the literature [1,11]. This means that sufficientture by Picciotto et al. [11]. It might be alsochemical extraction caused a further distortion of theconsidered that the specific plane did not orientcrystal structure, which might be related to a lowerpreferentially because the particle became isotropicpotential of the delithiated material than the originalowing to contraction and expansion of layers duringone.lithium insertion and extraction in addition to a

IR spectra of Li V O obtained by chemicalphysical contact between the particles by stirring 11x 3 8

lithium insertion are compared in Fig. 5. Accordingduring chemical reaction.to Kera’s report [17], absorption bands near 1000,Lattice constants were calculated from XRD pat-

21terns of Li V O obtained by chemical methods. 950 and 730 cm were due to stretching vibrations,11x 3 8

Fig. 4 shows their dependence on x in Li V O . n, of the bond between vanadium and oxygen atoms.11x 3 8

Lattice constants were not plotted because of their Furthermore, first two bands and the last werelittle reliability, if the number of distinguishable ascribed to n(V5 O) and n(V-O-V), respectively, as

described in Fig. 5a. During lithium insertion, n(V-O-V) declined and n(V5 O) shifted to the lowerwavenumber because of decrease in the bond

Fig. 4. Dependence of lattice constants on x in Li V O obtained11x 3 8

by chemical methods, (j: Li V O phase upon delithiation, d:4 3 8

LiV O phase upon delithiation, h: Li V O phase lithiation, s: Fig. 5. IR spectra of Li V O during chemical lithium insertion,3 8 4 3 8 11x 3 8

LiV O phase upon lithiation, 1 : by Wadsley [1], and 3 : by (a: x 5 0.2 as original material, b: x 5 0.9, c: x 5 2.6, and d:3 8

Picciotto et al. [11]). x 5 3.3).

J. Kawakita et al. / Solid State Ionics 118 (1999) 141 –147 145

strength by reduction of vanadium component, for atoms caused by the distortion of the structureexample at x 5 0.9 as shown in Fig. 5c. In the describe above.two-phase region (for example at x 5 2.6, Fig. 5c),new bands appeared at the wavenumber lower than 3.2. Electrochemical lithium extraction

21850 cm in addition to already present bands. Thesenew bands became obvious at the higher content of Fig. 7 shows discharge and charge curves of

22lithium, for example at x 5 3.3, as shown in Fig. 5d. Li V O at 60.1 mAcm together with the OCP1.2 3 8

This was a spectrum of the Li V O phase with the versus x in Li V O obtained by electrochemical4 3 8 11x 3 8

composition of Li V O because only one phase lithium insertion and extraction. Upon lithiation, a4.3 3 8

was observed in the XRD pattern at x 5 3.3 (see Fig. potential gap between OCPs and discharge curve is2d). large for x . 1.5 and OCPs keep almost constant

IR spectra of Li V O obtained by chemical near 2.65 V for x . 2.0. On the other hand, it is11x 3 8

lithium extraction are compared in Fig. 6. The result negligibly small upon delithiation and a potentialof IR spectroscopy showed that during lithium plateau is not observed on OCPs. Furthermore, manyextraction the Li V O phase was changed to the steps of the discharge curve disappear in the charg-4 3 8

original LiV O phase via co-existence state of these ing curve. It was suggested that a reaction mecha-3 8

two phases, as seen in Fig. 6a, 6c, and 6b, respec- nism of lithium insertion was different from that oftively. Compared with the original material, how- extraction when electrochemical methods were used,ever, in the spectrum of a delithiated product at though lithium insertion /extraction was the revers-

21x 5 0.1, n(V5 O) near 1000 cm shifts to the ible reaction in the case of chemical methods. Abouthigher wavenumber and shoulder-like n(V5 O) near 0.5 mole of lithium was remained in the compound if

21970 cm is observed as a clear peak, as shown in a charging potential was limited to 3.6 V corre-Fig. 6c and 6d. This indicated the presence of sponding to the OCP of the starting material,different bond length between vanadium and oxygen Li V O . In order to recover the rest capacity, one1.2 3 8

has to go on charging up to 3.8 V presumably

22Fig. 6. IR spectra of Li V O during chemical lithium ex- Fig. 7. Discharge and charge curves at 60.1 mAcm and OCP11x 3 8

traction, (a: x 5 3.3, b: x 5 1.5, c: x 5 0.1, and d: x 5 0.2 as plots of Li V O by electrochemical methods, (m, ———:11x 3 8

original material). extraction, and ., – –: insertion).

146 J. Kawakita et al. / Solid State Ionics 118 (1999) 141 –147

1because some of the Li inserted in the lattice are22strongly bound to O [18].

XRD patterns of Li V O obtained by electro-11x 3 8

chemical lithium insertion are compared in Fig. 8. Insome patterns, diffraction lines near 388 and 448 in2u are due to a silver paste used for the electriccontact between a working electrode and a copperlead. The result of the XRD measurement indicatedthat electrochemical lithium insertion proceededsimilar to the case by a chemical method. TheLiV O phase was retained for 0.2 , x , 1.5, for3 8

example at x 5 0.7 as shown in Fig. 8a and 8b. Boththe LiV O and Li V O phases coexisted for 1.5 ,3 8 4 3 8

x , 3.2, and then the Li V O phase was observed4 3 8

for x . 3.2, for example at x 5 2.7 and 3.6, as shownin Fig. 8c and 8d, respectively.

XRD patterns of Li V O obtained by electro-11x 3 8

chemical lithium extraction are compared in Fig. 9.Even if the amount of extracted lithium was con-siderably small, the Li V O phase disappeared and4 3 8

only the LiV O phase was observed in the pattern,3 8 Fig. 9. XRD patterns of Li V O during electrochemical lithium11x 3 8for example at x 5 2.7, as shown in Fig. 9b. Then, extraction, (a: x 5 3.6, b: x 5 2.7, c: x 5 0.5, and d: x 5 0.2 aslithium extraction from Li V O proceeded as the original material).11x 3 8

single LiV O phase. At x 5 0.5, the delithiated3 8

product has the pattern similar to the original materi-al including the relative intensities, as shown in Fig.9c and 9d, respectively. Some of the diffraction linesin one pattern shift slightly corresponding to thecontent of lithium, compared to another pattern.Accordingly, the crystal structure returned to theoriginal one by electrochemical lithium extractionthough extraction traced along a reaction mechanismdifferent from the simple reverse reaction of inser-tion. This result may be explained by the hypothesisthat available sites for lithium occupation upondelithiation are different from those upon lithiation.At the early stage of delithiation, the Li V O phase4 3 8

was changed to the single LiV O phase with a high3 8

lithium content such as Li V O (x 5 2.5). Proba-3.5 3 81bly, Li ions in the latter phase occupy kinetically

tetrahedral sites which are not available duringlithiation. This is because these sites provided byphase transition from Li V O phase to LiV O one4 3 8 3 8

might have site energies slightly different fromFig. 8. XRD patterns of Li V O during electrochemical lithium11x 3 8tetrahedral sites in the original LiV O phase beforeinsertion, (a: x 5 0.2 as original material, b: x 5 0.7, c: x 5 2.7, 3 8

and d: x 5 3.6). lithium insertion.

J. Kawakita et al. / Solid State Ionics 118 (1999) 141 –147 147

case of lithiation. For x , 0.5, however, a structuraldistortion of the delithiated product became largerthan the original material, presumably leading to thelowering of OCPs.

In the early step of electrochemical lithium ex-traction from Li V O , the Li V O phase dis-4.2 3 8 4 3 8

appeared and only the LiV O phase appeared.3 8

Subsequent lithium extraction reaction proceeded inthe single LiV O phase up to about x 5 0.5 if the3 8

upper potential limit of charging is 3.6 V. Therefore,electrochemical extraction had an irreversible re-action mechanism against electrochemical insertion.However, the structure of the delithiated product atx 5 0.5 returned to that consistent with the originalmaterial.

References

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Fig. 10. Dependence of lattice constants on x in Li V O 77) 267.11x 3 8

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[18] S. Panero, M. Pasquali, G. Pistoia, J. Electrochem. Soc. 130LiV O one via co-existence of these two phases in3 8(1983) 1225.respective specific ranges of x values, contrary to the