HMWO6 (M = Ta, Nb) nanocomposites

SOLID STATE ELSEVIER

Solid

Slat12 Ionics

89

(19%) 147-

IONICS

I57

Synthesis and electrochemical lithium insertion in polyaniline/HMWO, ( M = Ta, Nb) nanocomposites B.E. Koene, L.F. Nazar ” Department

r?f’Chcmistry.

Urhwrsi~

qf

Wotcrloo,

Guelph-Wutcrloo

Ccntre ji,r Gmduute

Work in Chemi.stry,

Onturin,

Cunodu

,V2L 3GI

Received 3 Novcmbcr 1995; accepted I6 November 1995

Abstract The relatively strong Bronsted acidity of the trirutilc-like layered oxides HMWO, . nH,O (M = Ta, Nb) was used to intercalatc aniline to form a bilayer of the guest species within the interlayer gap having the formulation [Aniline],,,,HMW06. Thermal treatment in air resulted in expulsion of half of the aniline, together with polymerization of the remaining aniline within the layers and formation of the novel nanocomposite, PANI,,,, HMWO,. EI’lR studies showed that the emeraldine salt form of PAN1 was present. This was consistent with measurements which showed increased conductivity for the S/cm). The electronanocomposite (1 X lo-’ S/cm>. compared to that of the insulator [Aniline],,,, HMWO, (< lo-” chemical properties of these materials for lithium insertion reactions were studied using the polymer nanocomposites as cathodes in conventional lithium cells. Electrochemical insertion of lithium was compared to lithium insertion in the oxide in the absence of the polymer. In the case of PANI,HMW06 (M = Ta, Nb), the Li diffusion coefficient was measured using the geometrical surface area, the BET surface area. and the SEIM surface area. Irrespective of the method of surface area measurement, the chemical diffusion coefficient increases for both Ta and Nb oxides upon insertion of polyaniline into the host. In the case of HTaWO, the diffusion coefficient increases by more than an order of magnitude (factor of 20). WC ascribe this increase in Li ion mobility to a decrease in the Li+ Ion interaction with the host lattice in the polymer nanocomposite. Keyw~orc1.s:

Intercalation; Nanocomposite;

Polymiline;

Tungsten oxide

1. Introduction

teristics

Transition metal oxides have been studied extensively as potential cathode materials in lithium secondary batteries as a result of their desirable charac-

Corresponding

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Tel: 5 19-888-4567. ext. 5810: Fax:

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such

as a high

positive

operating

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high theoretical power density, and very good revcrsibility with limited degradation [I]. One-problem that is often encountered with these materials is low electronic conductivity over some region of the redox range. Another limitation of oxides with respect to their use in electrochemical applications is their relatively poor lithium ion transport properties compared to other materials 121. For example, a given oxide will have a substantially lower lithium diffusion coefficient than an isostructural sulfide. The higher clectronegativity of oxygen versus sulfur re-

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suits in strong electrostatic attraction of the lithium guest ions (or other alkali) ions to the oxide ions of the lattice. thus hindering their mobility. Hence, diffusion constants for Li+ are as low as lo-‘” cmZ s-l in many oxides. One possibility for enhancing and expanding the range of the electronic conductivity of the metal oxide host material is to create a polymer/oxide nanocomposite by the intercalation of an electronically-conducting polymer into the interlayer region of a layered inorganic oxide. [3-S]. This newlyemerging class of materials has been recently rcviewed [4]. Since these materials form an intimate mixture on a “nano’‘-scale (- 10 A)>,the conduction of electrons in this material is facilitated compared to an equivalent macrocomposite. The polymer can also shuttle”, conducting electrons act as an “electron throughout the layer or between adjacent layers of the host material. The latter would allow for the possibility of three-dimensiona electron conduction. As conductive polymers are currently being investigated as cathode materials in lithium batteries [6,7]: there is also the interesting possibility that the polymer may participate in the redox reactions at the cathode. This would extend the capacity of the host material. Such approaches have inspired the development of conductive polymer/metal oxide blends such as polypyrrole/MnO, [8], and polyaniline/WO, [9]. These materials are macrocomposites, however, comprised of a physical mixture of the two components. In addition, they contain counterions for both the reduced oxide and oxidized polymer, in contrast to polymer-intercalated nancomposites in which the oxidized polymer serves as the counterion for the reduced oxide. The materials are interesting as they raise the question of the role of the conductive polymer. Most of these arc not stable to reduction beyond the neutral state, and hence in batteries in which the polymer is the cathode, the redox couple is between the oxidized polymer and the neutral form: i.e., [PANI]Y+[ClO~], c, PANI + $10; [61. Thus, reaction at the cathode involves inter/deintercalation of the (somewhat) mobile anion. Formation of the nanocomposite allows us to study Li insertion in (or in the presence of) the conductive polymer since the oxide is an immobile anion and has redox properties of its own, and determine the effect of the insertion of a polymer on the lithium diffusibility.

In the past nine years, many different polymers have been intercalated in different layered structures using a variety of insertion and polymerization methods [4]. Oxide hosts such as MOO, [5,10,11], V,05 [12,13] and FeOOH [4] for example, have been intercalated with polymers such as poly(aniline). poly(ethylene oxide), poly(phenylene vinylene) and poly(pyrrole). Other layered structures including sulfides, phosphates, layered double hydroxides and silicates have also been used as host materials for intercalated polymers. Although interest in the design of these nanocomposite materials has included their use as battery cathodes. only a very few reports [ 11,14,15] have included a study of how (or if) the conductive polymer affects lithium ion migration in the host. The latter issue is addressed here. In this paper, the insertion and subsequent polymerization of aniline within the interlayer gap of HMWO, (M = Ta, Nb) is described, which results in formation of novel nanocomposites, PAN&, HMWO,. Electrochemical insertion of lithium in these materials was examined, and compared to lithium insertion in the oxide in the absence of the polymer. Chemical diffusion measurements on both materials were carried out based upon the current pulse relaxation (CPR) t.echnique described by Basu and Worrell [16], based on the technique of Wen et al. [17]. The diffusion coefficient was measured using the geometrical surface area, the BET surface area, and the surface area was determined by taking into account the shape of the particles observed by SEM.

2. Experimental HMWO, (M = Ta,Nb) was prepared following literature methods [18]. LiMWO, was first synthesized by annealing stoichiometric amounts of WO,, M,O,, and Li,CO, at 900°C for 48 h with an intermediate grinding step. Lithium ions were exchanged for protons using 3M nitric acid. The white solids, HMWO, . nH,O (where n = 1.5) were either heated at 125°C to give the dehydrated product (n = OS), or were stirred with aniline for several days at 100°C in a sealed tube in vacua. The resulting off-white solids, [Aniline],HMWO, (M = Ta la, M = Nb II& were filtered and washed several times

with methanol. la and IIa were heated at 130°C in air for 2 days to polymerize the monomer within the layers to produce [Polyaniline], HMWO, (M = Ta Ip, M = Nb Up).

on each sample from room temperature to 800°C in a flowing atmosphere of air using a heating rate of I O”C/min. 2.2. Electrochemical

measurements

2.1. Instrumentation X-ray powder diffraction (XRD) patterns were obtained on a Siemens DSOO X-ray diffractometer equipped with a diffracted beam monochromator. using Cu Ko radiation. Samples were prepared by evaporating suspensions of the intercalated oxide onto microscope slides. This resulted in preferred orientation of the oxide layers parallel to the plane of the slide, which therefore selectively enhanced the interlayer (or 000 diffraction peaks, and suppressed the mixed hkl or hk0 diffraction peaks. Samples were scanned at a step scan rate of 0.02”/s. Infrared spectra were obtained on a Nicolet 520 Fourier transform infrared (FUR) spectrometer. Samples were prepared as KBr pellets. Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out using a Polymer Laboratories Thermal Sciences STAl500 (when it was working). DTA ‘and TGA curves were run simultaneously

2. Aniline,HTaWO

The electrochemical properties were examined using a Swagelock-type cell comprised of a lithium counter electrode, and 1.O M LiClOJpropylene carbonate as the electrolyte. The working electrode consisted of approximately 5-10 mg of the desired polymer/oxide nanocomposite, or oxide mixed with 25% (by weight) Ketjen carbon black and 2% EPDM (ethylene propylene diene monomer) binder. The cells were cycled under galvanostatic conditions at current densities ranging from 0.05-0.2 mA/cm” between preset voltage (or charge) limits using a multichannel galvanostat/potentiostat system (MACPII.~)“~. Chemical diffusion measurements on both polymer containing and pristine materials were carried out based upon the current pulse relaxation (CPR) technique described by Basu and Worrell 151. The coefficient was measured using the geometrical surface area, the BET surface area, and the surface arca found by taking into account the shape of the

6

40

30

10

2 THETA &ees~ Fig.

I. XRD

patterns (Cu Ku

radiation)

for (iI) HTaWO,;

(b) [Aniline],,,

HTaWO,;

(c) [P4NI]0,,,

HTaWO,,

obtained as oriented tilms.

LI.E. Koene, L.F.

150

Nu:ur/

Solid State Ionics 89 (1996) 147-157

particles as observed by SEM. A constant current pulse in the range of 0.2-1.0 mA for 10 s was applied to the cell to introduce lithium ions to the cathode. After curtailing the pulse, the open circuit voltage decay was measured over time until the concentration of lithium ions within the cathode was homogeneous; i.e., when the system reached equilibrium. The slope (k) of the decay voltage versus and the slope (m) of the equilibrium open l/P, circuit voltage versus x(Li) were used in the following equation to calculate the chemical diffusion coefficient, D:

(‘1 where D is the diffusion coefficient, V is the molar volume of the cathode, r is the time of current pulse, i is the current, F the Faraday’s constant, and A is the surface area of cathode. Assumptions similar to those of Yamamoto et al. [ 181 were used in the calculations.

3. Materials characterization:

results

3.J. XRD analysis XRD analysis of the materials obtained from the reaction of aniline with HTaWO, and HNbWO, host lattices gives direct evidence for the intercalation of aniline. Only the results for HTaWO, are shown in Fig. 1 since the XRD pattern for HNbWO, was very similar. The data for both structures, however, are summarized in Table 1. The upper trace in Fig. 1 shows the diffraction pattern for HTaWO, . nH,O, where IZ= 0.5, from which a basal or intcrlayer

Table 1 lnterlaycr

spacings

Material

for HMWO, Basal spacing

Layer expansion

(A)

CK,

HTaWO,.iH,O

IO.50

Anilinc,HTaWO,(Ia) PAN1 y HTaWO,(lp)

20.02 14.50

1 !.I 5.4

HNbWO,.+H,O Aniline,~HNbWO,(Ila) PANI,HNbWO,(IIp)

10.49 20.02 14.45

11.1 5.35

Scheme I. Schematic showing layers of a) [Aniline],,,, HMWO, (M = ‘k(I); Nh(I1); h) [P.4NI],,3, HMWO, (M = Ta(I); Nb(II).

spacing of 10.5 A is calculated. IJpon intercalation of aniline, the basal spacing increased to 20.02 A (Fig. 1, middle trace). If we take the “pristine” interlayer distance as that corresponding to the c axis for the unit cell of TaW0,.5 [18] (9.1 A> then this expansion of 10.9 A corresponds to two layers of aniline molecules arranged approximately perpendicular to the layers as shown in Scheme 1. This is arrangement is confirmed by preliminary molecular modelling studies using IIIOSYMTM. Upon heating Ia and IIa, the colour changed to a very dark blue-green indicating that polymerization of aniline had occurred within the layers to produce Ip and Up. Thz polymerization process is accompanied by 5.5 A decrease in interlayer spacing (compared to aniline/HT$Nb)WO,) to give interlayer spacings of about 14.5 A. This suggests that partial expulsion of aniline occurs (which is confirmed by thermal analysis, vide infra), ‘and that a “single” layer of polyaniline is formed, as depicted in Scheme I (right). 3.2. Infrared spectroscopy The 1R spectrum of the aniline-intercalated, Ia (Fig. 2, upper trace) (or IIa) contains many absorptions matching those found in the spectra of molecular anilinium salts. For example, the sharp peak at 1497 cm-’ corresponds to an NH stretch of a primary ammonium salt [20]. Moreover, the characteristic absorptions of aniline? such as primary amine NH stretches at 1300 and 1600 cm-‘, are notably absent. This indicates that there is no -‘free” aniline within the material, and hence the interstitial aniline molecules are directly bound to the hydrogen atoms within the inorganic layers. Thus, the aniline is held in by
4

2000 40bo

3oti 20bo 1000 500 Wavenumber@nil )

Fig. 2. FTIR spectm of (a) [Aniline],6, HTawOd; (b) [PANIlo.;~ HTaWO,.

Heat treatment of the aniline-intercalated samples (Ia and Ila) at 150°C for 12 h resulted in a dramatic change in color from white to dark blue-green. This was accompanied by drastic changes in the IR spectra (Fig. 2) indicating that polymerization of the interstitial aniline has occurred. Table 2 summarizes the assignments for the absorption bands observed for Ip [2 l-231. The occurrence of a strong quinoid ring deformation at 1571 cm-’ gives a qualitative indication of the oxidation state of the polymer. The relative magnitude of this peak, compared to that of the benzenoid ring deformation at 14% cm-‘, suggests that the aromatic rings within the polymer chain have substantial quinoid character. This compares closely with those of the emeraldine oxidation state, but is quite dissimilar to that found in the IR spectra for the pemigraniline or leucoemeraldine states of polyaniline 1241. Whether the emeraldine Table 2 Infrared st~~lral assignment for Ip, Relb. [2O-221 Wavenumber (cm-‘)

Assignment

1571 1496 1426 1305 1247 1144 987 903 755 659

qoinoid ring deformation benzcnoid ring dcformalion C-N stretch of 2” aromatic amine in plane aromatic C-H bending in plane aromatic C-H bending inorzanic host inorsanic host out or plane aromatic C-H bending inorganic host

,

.

.

I500

.

Wavenumher(cm-‘)

.

I

1ootl

Fig. 3. mIR spectra of (a) emeraldine base; (b) emcraldine salt; and CC) [PANI],,~~ HT~WO,.

salt or base form of the polymer is formed is more difficult to discern, although this infomlation can be obtained from comparison of the “fingerprint” region of the IR spectra (Fig. 3). We find that the IR spectra of Ip (11~) are much closer to that of the conductive emeraldine salt than the nonconductive emeraldine base. Although the same vibrations occur in both forms of polyaniline, there is a slighht shift to lower frequencies for the salt form. For example, the strong, broad C-H bending 1 I44 cm- ’ is a diagnostic band for the salt form. Furthermore, a unique and characteristic absorption is present in the near IR region from 4000-1600 cm-‘. This broad and intense absorption is due to the free-charge carrier absorption in the conductive polymer [21]. The absorbance at 755 cm-’ is characteristic of an aromatic ring with just one substituent. This will only appear for the end groups of polyaniline since all of the other units in the polymer chain should have two substituents. Therefore. the size of this peak gives an indication of the chain length of the synthesized polymer. Unfortunately, an absorption in the host lattice overlaps this peak, making any quantitative measurements difficult. Despite this, the greater intensity and breadth of this peak compared to the host material suggests that the interstitial polymer is likely made up of a low molecular mass polymer or possibly oligomers. The absorptions corresponding to the metal-oxide stretches compare very closely with those found in the hosts HTa(Nb)WO, . nH,O and LiTa(Nb)WO, as seen in Table 3. This indicates that there were no major structural phase changes or alterations in the

B.E. Kome, L.F. .Vuxw/Solid

152 Table 3 Comparison

of M-O (M = Ta,W) infrared absorptions

SlateIonics

(all values in cm-

89 (1996)

147-157

’)

Assignment

u(M=O)

v(M-O--M)

v(M-.-0)

LiTaWO, HTaWO, . I .5H 2O Anilinc,HTaWO, Polyanilinc,,HTaWO,

967 988 985 987

886 916 X80 9003

762, 668 773, 665 758. 650 755.659

bonding in the host during the insertion and polymerization reactions. For example, if the host was reduced during the oxidation of aniline to polyaniline, there would be evidence for the presence of reduced metal centres in the IR spectrum. 3.3. Thermal analysis Thermal gravimetric analysis of la and IIa (Fig. 4a) exhibits two principal weight losses. The first weight loss (endothermic transition) between 100 and 300°C corresponds to partial evaporation of aniline from the material, that gives rise to the 5.5 K decrease in interlayer spacing in the XRD pattern (vide supra). In the region of the DTA curve from 300 to 45O”C, two exothermic transitions are visible that are not accompanied by any significant weight loss. We ascribe these to phase changes related to the polymerization of the remaining aniline in the lattice, by oxidative heat treatment during the TGA scan in air. The exothermic weight loss at 600°C results from the combustion of the polyaniline that was formed. Assuming that all of the weight loss is due to removal of aniline by evaporation or combustion,

Temperature (dcg C) Fig. 4. TGA/DTA curves for [Anilinela,, HMWO, (M = T&a); (21) ~bUl3; (b) [PANll,,, HMWO, (M = Ta(Ip); Nb(llp).

determination of the total mass loss of the sample (27’S), gives rise to the formulation [Aniline],,, HMWO, .

Fig. 4b displays the TGA/DTA curve for Ip and IIp, produced by thermal treatment of the aniline-intercalated materials in air. The absence of the bilayer-aniline phase (IaJIa) is apparent from the lack of significant mass loss between 100 and 300°C. The ca. 2% weight loss between 300 and 450°C may be due to removal of a very small amount of residual unpolymerized aniline within the lattice. This process is accompanied by an exothermic transition in the DTA curve, implying that the aniline (or aniline oligomers) are removed by combustion in this step. Alternately, loss of these molecules may result in polymerization of a small remaining portion of aniline, giving rise to a small exothermic transition. The exothermic weight loss at 600°C corresponding to polyaniline combustion coincides exactly with that in Ia (IIa). The total mass loss corresponds to the formulation PAN1 o.,JHMW06, almost exactly half of the organic content of the starting aniline-containing material. This, again. is consistent with loss of a single layer of aniline from the bilayer arrangement shown in Scheme 1. We note that the cxothermic combustion of polyaniline in bulk samples of polyaniline (produced by chemical polymerization using peroxydisulfate) occurs over a much broader range than that of the interlayer polymer. This suggests that the polymer within the layers likely has a much narrower molecular mass distribution than that of the bulk polymer. Also, all of the polymer chains in Ip and IIp must be in a similar environment for the combustion to take place over such a small range of temperature. This implies that the polyaniline within the layers has a high degree of crystallinity. The results of elemental analyses (Galbraith Labs) for la, IIa, lp and IIp summarized in Table 4 confirm the quantitative results of the thermal analysis. The

Table 4 Elemental

analysis

for Ic, Id, UC, and Ild (values in mass %)

Element

Carbon

Nitrogen

limgstcn

Tantalum

Aniline, HTaWO, (ICY) PANI,HTaWO, (Id>

9.01 4.97

1.61 0.86

30.73 32.75

34.70

Anilinc,rHNbWO, (11~) PANI,$INbWO, (IId)

10.98 6.05

I .96

36.75 39.52

1.13

formulae found are AnilineO,,,_,~,,HTaWO, (Ia), PAN1 0.,~q_0.3RHTaWOh (Ip), Anilineo.70_,.,,IINbWO, (Ila), and PANI0.j,_0.39HNbWOc, (11~). The difference between the values calculated for the nitrogen and carbon analyses (shown as a range in the formulae) is within experimental error. We also note that the expected 1 : 1 molar ratio of Ta: W (calculated, 1 : 0.93) is retained after thermal treatment in air.

4. Materials

characterization:

discussion

The choice of host material for this work was dictated by the ease of introducing the aniline monomer between the layers. The trirutile layered oxides HM(v)M(“‘)O~ . nHZO (M(I) = Ta,Nb, M(II) = W 125,261, MO 1271) have been shown to readily intercalate many weak organic Lewis bases. Due to the relatively strong Brmnsted acidity of HTa(Nb) WO, compared to common layered oxides such as HZTi401), HTiNbO,, Il,,jMoO,, and HCa,Nb,O,, we concluded that it should be an ideal host for the intercalation and subsequent polymerization of weakly basic monomers such as aniline. These layered oxides are also insulators, and hence enhancement of electronic conductivity resulting from polyaniline incorporation will be quite evident. In addition, Li ion mobility in LiTaWO, has also been reported to be fairly high [25]. The host oxides LiTaWO, and LiNbWO, are isostructural, and hence their monomer and polymer nanocomposites were found to be virtually identical. These properties are not unusual considering that the chemistry of Ta and Nb is very similar. and therefore their corresponding oxides behave in the sanle manner. Although the materials are identical as far as the chLaracterization here, the two different series are treated separately

due to differences in their conductivity and electrochemistry shown later. The method employed for the insertion of aniline in these materials is slightly different from that used by Kinomura et al. [6]. The intercalation of aniline was carried out at 100°C rather than room temperature since lower temperatures led to incomplete reaction. However, it was necessary to react the aniline with the oxicle in a sealed, evacuated tube to avoid the oxidative polymerization that occurred during thermal treatment in air. The latter resulted in polymerization of aniline on the surface of the oxide rather than insertion between the layers. Upon intercalation, a bilayer of aniline molecules was formed, giving rise to a large interlayer expansion (Ad) of almost I 1 ..&. Bi-versus monolayer formation is determined by the effective charge/basal surface area ratio of the inorganic host layers, which is greater for TaWO; than for UO,PO; , for example. Thus, in the latter, intercalation of aniline results in a monolayer (Ad = 6 A). Polymerization of aniline to form bulk polyaniline is usually effected by electrochemical or chemical oxidative polymerization in solution (by either peroxydisulfate or FCC],). The polymerization of monomers within various hosts has also been accomplished by these oxidizing agents [28-321. Following these examples, the polymerization of [Aniline],,,HTa(Nb)WO, was attempted using aqueous solutions of FeCl,, CuClO, and (NH,),!!$O, at room temperature. XRD and IR data indicated that polymerization did occur in these cases; however, the 001 peaks in the XRD pattern corresponding to the interlayer distance were more than an order of magnitude less intense than those shown in Fig. 1, and were much broader. The lower intensity suggests lower crystallinity likely due lo disordered chains within the layers. The broadness of the peaks indicates a

shorter coherence length resulting from degradation of the layers. The IR spectra showed that very low-molecular weight polyanilinc was formed. It was recently demonstrated that aniline intercalated in redox-active host lattices can be polymerized by treatment with oxygen at relatively mild temperatures [33]. This was the method used in these stuclies. Oxidative polymerization of aniline proceeds by a poorly understood mechanism (irrespective of the oxidant), although it is generally agreed that the initial step involves the formation of a radical-cation intermediate. The method by which the species propagates is still not clearly understood, although various different mechanisms for this growth step have been proposed [34]. The polymerization of aniline within the HTa(Nb)WO, layers appears to occur by oxidation via a transition metal (W, Ta, or Nb) activation of oxygen, as suggested in the oxygen-induced polymerization of aniline at room temperature in Anilinium,V,O, . nH,O. Anilinium salts do not usually polymerize without any oxidizing agent other than oxygen. The polymerization of aniline within Aniline,HTaWO, can be completed under much milder conditions than described above. For example, the Aniline,HTaWO, becomes slightly blue in air after aging it in air for about one month and very dark over time. The product shows very broad diffraction peaks that suggest low crystallinity and disorder of polymer (oligomer) between tbe layers. Also, the IR spectrum of this material showed absorptions corresponding to both aniline and polyaniline indicating incomplete polymerization. The fact that polyaniline will polymerize within the spaces of a layered oxide provides some useful information on the mechanism of aniline polymerization. It is commonly thought that the mechanism involves two aniline (or aniline i-mer, where i = 1-x) radical cations reacting with each other to form a polymeric chain. This scheme may be feasible for a solution polymerization, but the probability of two radicals encountering each other within the confined space of a layered material is very unlikely. The mobility of the ionic molecules within the layers must be very low and the concentration (and lifetime) of radicals is much too small for two to interact. Therefore, a mechanism whereby only one radical cation reacts with a neutral species is more likely to be appropriate in this case.

The oxidation state of the polymer within the oxide layers is, of course, very important for the overall conductivity of the nanocomposite. The infrared spectra show that the conductive form of polyaniline, emeraldine salt, was formed in both cases (Ip and Up). This is likely due to interaction with the HTa(Nb)WO, . nI-I,0 layers which are very acidic [6]. The acidity of these materials was confirmed by the presence of anilinium absorptions in the TR spectra of la and IIa following the insertion of aniline. Formation of the emeraldine salt form of PAN1 is consistent with the room temperature conductivity of [PANI],,,HTaW06, which is approximately 3 X lo-” S/cm. This value is much less than that observed for the conductive form of PAN1 itself, but is quite comparable to values obtained for emeraldine PAN1 intercalated in insulating hosts. In these cases, the insulating host exerts a dilution effect on the conductivity of the polymer.

5. Electrochemical

properties

Since PAN1 exists in the partially oxidized from, emeraldine, both the PANI and the transition metal can participate in the reduction process. Hence, the redox behaviour of PAN1 augments the reduction cycle such that the discharge process corresponding to the introduction of lithium in the PANI,HTaW06 cathode can be written as follows: [PANI+],HTaWO,

+ yLi++

e-

@ Li,[PANl],[HTaWOC;J] w Li,+,[PANI],[HTaWO~(X+7)]. Fig. 5 shows the discharge-charge curves (cycles 2 + 50) between x=0 and x = 1 of the T.,i/[PANI+],HTaWOh cell as a function of the degree of lithium concentration (x) in the intercalated active cathode material. This plot shows that lithium can be reversibly inserted and removed from the cathode at a fairly high current density. The galvanostatic cycling was carried out at a current density of 0.1 mh/cm’. One lithium was incorporated into the host oxide lattice over a voltage range of 4.0- I .4 V. The cell voltage decreases in two steps as a function of lithium intercalation suggesting that the structure offers more than one type of intercalation site. The

H.E. Koene,

L.F. Narar/.Solicl

State Ionics

89 (1996)

147-157

1.55

:.

0.02

0.04

0.06

0.08

0.1

0.2

0.25

lime (h)

Fig. 5. Discharge-charge curves (cycles HTaWO, obnincd at 100 pA/cm’.

2 + 50) for [PANI],,,

b)

inflection of 0.3 corresponds to the filling of one of these sites. The results are consistent with both the tantalum tungsten oxide and the polyaniline being involved in the redox reactions. For a current density of 0.03-0.1 mA/cm’, the number of lithium ions reversibly inserted was relatively constant; this cell capacity was also recovered in subsequent cycles. To determine lithium ion diffusion coefficients, short pulses on the order of 1 mA were supplied to the cathode to displace the system from equilibrium. The overpotential was then measured over time as it relaxed back to the open circuit voltage value (Fig. 6a). The plot of change in voltage (AE) versus 1/v produced a straight line with slope k (Fig. 6b). The linearity of this relationship shows that the diffusion of lithium is rate limiting on the relaxation of the transient voltage. A plot of the open circuit voltage versus x(Li) resulted in a straight line with slope m (Fig. 6~). The main source of confusion in the comparison of diffusion coefficients with literature values centres on what is accepted as the area of the cathode [I 81. Most reported lithium diffusion coefficient measureTable 5 BET and SEM adjusted surface areas per gram of material Material

BET surface area (m’/g)

(m’/g>

HTaWO, PAN1 ~ HfiWO, HNbWO, PASI,HNbW06

5.67 8.24 4.48 7.45

1.02 I .48 0.82 1.35

0

0.05

0.1 l/G'2

0.15 (d"2)

0.012 0.014 0.016 0.018 0.02 0.022 0.024 0.026 xti

Fig. 6. For [PANI]0,3, HTaWO,; (a) Cllmeflt pdSC-~~O~tagC decay over time; (b) change in voltage versus I i t2; (c) plot of b,oltagc versus .k-(Li).

SEM djusted urfacc area

ments were derived assuming the surface area of material to be the geometrical surface area (i.e. area of the surface of the cathode exposed to electrolyte - for a pressed pellet = ,rrr2). This sumption may be valid for materials such as

the the the asthe

BS:‘. Koene, L.F. .\ruzar/ Solid Sratc lonics 89 (1996) 147-i57

156 Table 6 Diffusion

coefficients

at z I 0 calculated

using different

surface areas

Material

Gcomelrical (cm’/s)

BET km’/s)

HTaWO, PANI, HTiaWO, HNbWO, PAN1 ,HNbWO,

7.1 1.6 6.8 1.2

2.7x 3.0 x 3.1 x 2.0 x

x X X x

10 -* IO-” IO-’ 1o-5

alloy Li,Al studied by Wen et al. [16]. As pointed out by Yamamoto et al. 1191, this is likely not the case when working with polycrystalline materials which have significant grain boundary effects. For these materials, the actual surface area of the particles can be estimated by BET measurements. Another factor must also be taken into account since the lithium diffusion in layered materials can only occur in two dimensions between the layers and not perpendicular to the layers. Thus, an estimate of the average particle size and shape by SEM or a similar technique can be used to give a more accurate value of the actual diffusion surface area. The measurcmcnt of the BET and SEM ad.justed surface area for the materials is shown in Table 5. The surface area for the polyaniline intercalated materials is higher than those of the isolated hosts, as a result of particle fragmentation. The chemical diffusion coefficients calculated using Eq. (1) with each of the different surface area assumptions are displayed in Table 6. These were all calculated at low values of intercalated lithium z = 0. It is difficult to compare the values obtained here to any of the literature values clue to the different surface area assumptions. However, the two oxides

SEM (cm2/s) 10-l” lo- ‘j 10 I3 lo- ‘?

8.0 x 8.9X 9.2 x 5.x x

lo- I3 lo- ‘I lo- I2 10 ”

in the presence and absence of polymer can be readily compared. Irrespective of the method of surface area determination, we conclude that the chemical diffusion coefficient increases for both materials subsequent to the insertion of polyaniline into the host. In the case of HTaWO, the diffusivity increases by about an order of magnitude. The diffusion coefficients were also determined at various mol% lithium with the HTaWO, and the PANI,HTaWOh as shown in Fig. 7. Initially at z = 0, there is an order of magnitude better diffusivity for the composite material over that of the isolatcd host. However, as more lithium is inserted, the diffusivity decreases to virtually the same values as the oxide host. We propose that the polymer props the layers of the oxide apart slightly and shields the lithium ions from the polarizing effect of the oxide, thus providing a lower energy pathway for the ions to move. This effect is most pronounced at small values of z(Li), but as more lithium is inserted, the effect is suppressed.

Acknowledgements LFN thanks the NSERC (Canada) for funding this research.

References

Fig. 7. Li ion diffusion HTaWO,.

coefficients

for [PANI],,2,

HTaWO,

and

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