Lithium insertion in molybdenum and vanadium oxide

Lithium insertion in molybdenum and vanadium oxide films C. Julien, B. Yebka and S. Ziolkiewicz Laboratoire des Milieux Désordonnés et Hétérogènes, URA 800 Université Pierre et Marie Curie, 4 place Jussieu, 75252 Paris cedex 05, France A. Doi Department of Materials Science and Engineering, Nagoya Institute of Technology Gokiso-chu, Showa-ku, Nagoya 466, Japan

Thin-films of transition-metal oxides have been grown and investigated for their use as positive electrodes in rechargeable lithium microbatteries. Molybdenum and vanadium oxide compounds such as MoO3, V2O5, and V6O13 are considered. The lithium insertion characteristics are reported for different degrees in the structure and morphology of flash-evaporated and rfsputtered films. Electrochemical features of thin-film cells are compared with those of bulk materials. Thermodynamics and kinetics of Li/M-O cells have been studied as a function of growth conditions of thin-film cathodes. These growth parameters have been optimized for obtaining the best potentialcomposition curves from the practical point of view. Considering both ionic and electronic contributions to the charge/discharge behavior, the electrochemical features are discussed from the point of view of energy diagrams. Long term cycling has been investigated for each type of cell.

INTRODUCTION With the tendency in microelectronics towards decreasing in the size of active elements the critical distribution of electric power becomes a major problem [1]. Low power requirements in some microdevices and their capability of integration have brought about the use of all solid state thin-film batteries. By preparing electrochemical cells with thin-film architecture, extensive opportunities arise for their utilization in extremely diverse fields of technology, where small dimensions, high specific-energy ratings, reliable performance and long lives are required. Applications especially adapted to these batteries are: the monolithic hybridization with CMOS random access memories, combination with solar cells, sensor technology, etc. In principle, a solid-state microbattery is a rechargeable lithium cell, which consists of three active layers: a fast-ion conductor thin film (solid electrolyte) sandwiched between two thin-film electrodes. The ion source (anode) is a lithium-metal thin film and the cathode is an intercalation-type material. Various kinds of transition-metal oxides have been investigated as materials for lithium insertion and extraction processes, which are very important electrochemical reactions for active materials in rechargeable lithium batteries. Characteristics of cathode materials are listed in Table I.

Electrochemical Society Proceedings Volume 97-24

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Many semiconducting oxides are candidate-materials for positive electrodes in lithium microbatteries and electrochromic systems [2]. But the literature dealing with their electrochemical properties is remarkably sparse. We consider films of molybdenum oxides and vanadium oxides such as MoO3, V2O5, and V6O13. The common feature of lithium intercalation of all host materials found so far is the structural environment of available empty sites to host Li ions. In this connection, materials with different degrees of disorder in their structure can be grown which are interesting as intercalation hosts of lithium applicable to rechargeable batteries. In this paper we report a series of measurements involving lithium intercalation in molybdenum and vanadium oxide films grown with different conditions. The potentialcomposition curves of Li//M-O cells have been measured and thermodynamics are studied from both fundamental and practical point of view. Considering both ionic and electronic contributions to the charge/discharge behavior, the electrochemical features are discussed from the point of view of energy diagrams. Table I. Lithium stoichiometry and voltage range in various bulk compounds. Compound

Stoichiometry

Voltage (V)

Specific capacity (mAh g-1)

LixMoO3 LixMoO2.765 LixV2O5 LixV6O13

0≤x≤1.5 0≤x≤1.5 0≤x≤3 0≤x≤8

3.0-1.5 3.0-1.3 3.5-1.8 3.0-1.8

178 120 142 416

EXPERIMENTAL Transition-metal oxide films were grown using flash evaporation and r.f. sputtering techniques. The influence of the different growth conditions has been investigated on the crystallinity of these films using the flash evaporation method allows the control of the rate deposition and avoids any decomposition of the starting material before evaporation. A variety of samples was obtained by changing the substrate temperature (Ts) and the annealing process conditions. Flash-evaporated samples were grown from pre-baked oxide powders using a home-made apparatus with a vacuum of below 10-3 Pa. Sputtered MoO3 samples were reactively deposited from a molybdenum target in an oxygen plus argon atmosphere under a 0.8 Pa total pressure using a Balzers BAE-250 deposition system with rf-power of 100 W. Compositional and structural studies were performed with photoelectron spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman scattering (RS) experiments. Discharge curves of Li/Li+/M-O cells using MoO3, V2O5 and V6O13 cathode films were carried out at current densities 2-10 µA cm-2 using a Mac-Pile system. The measured voltages of thin-film microbatteries were in the range 1.5-3.5 V. RESULTS AND DISCUSSION 1. Flash-evaporated MoO3 films Molybdenum trioxide films with an orthorhombic structure have been grown using flash and thermal evaporation techniques for investigating the influence of the different growth


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conditions on the crystallinity of these films [3-5]. Scanning electron micrographs of MoO3 films deposited on silica substrates show that for an amorphous film grown at Ts=30 °C one can observe different types of areas: (i) zones which may be responsible of the broad bands recorded in the X-ray diffraction diagram and (ii) cracks in the MoO3 film due to the high stress induced by a cooled substrate. Film deposited at 120 °C is a mixture of amorphous and crystalline phases. The crystalline part includes two types of crystal geometry, i.e., elongated and short (may be monoclinic) crystals. Film deposited at 250 °C exhibits clearly the formation of elongated micro-crystallites. It is interesting to note that this film displays the layered structure of the orthorhombic MoO3 phase. The RS spectra of MoO3 films exhibit a lower signal strength than that in crystal, but the recorded broad bands are located in the vicinity of the frequencies of the vibrational modes of the crystal. Vibrational analysis shows that the polymorphism of these films can be deduced from RS spectra. The RS spectrum of a film deposited on silicon substrate maintained at 120 °C displays the stretching modes attributed to the -MoO3 phase. It can be concluded that all the structural investigations on MoO3 films converge towards similar results, describing the layered structure, -MoO3, of films grown at above Ts=250 °C. Fig. 1 shows the discharge profiles for Li//MoO3 thin film batteries as a function of the degree of lithium insertion. Discharges were carried out at current density 2 µA/cm 2. The measured voltages are in the range 1.6-3.2 V. From the electrochemical features we may make some general remarks that are (i) an initial voltage of 3.2 V was measured for MoO3 thin-film cathode cells, which is higher than that recorded on the galvanic cell using crystalline cathode [6], (ii) the cell voltage decreases continuously as a function of the degree of Li inserted, (iii) the steadily behavior is a function of the structural arrangement in the film and thus, depends on the substrate deposition temperature; (iv) no voltage plateau occurs during the discharge, and (v) 1.5 Li per mole of oxide is inserted in the host material. The electrochemical process seems to be a classical intercalation mechanism for the lithium ions with a simultaneous injection of ions and electrons in the framework forming a homogeneous lithium molybdenum bronze without clustering. It is remarked the reduction process accompanied by coloration of MoO3 films. As shown in Fig. 1 (curve a) the cell voltage decreases significantly at high lithium intercalation (x≥1) for the MoO3 film deposited at room temperature. The voltage drop is less for film grown at Ts=120 °C (curve b). First, this suggests that Li diffusion may be anisotropic and limited by grain boundary effects which affect the discharge curve. The discharge curve of the cell fabricated with the MoO3 film deposited at 250 °C (curve b) is quite stable. This may be attributed to the unique layered structure of -MoO3 with large grain size in film. A second possibility is the presence of the mixed - phase which may reduce the standard potential. A third explanation may be oxygen-defects in the host structure involving a lower Fermi level in such a semiconducting film. This possibility can explain the easy intercalation despite the wide gap of these materials. Kinetic parameters of Li+-ions in LixMoO3 films in the composition range 0≤x≤1.5 are evaluated during the relaxation period of the discharge. In an intercalation compound the chemical diffusion coefficient, D*, is given by D*=DoW, where Do is the component diffusion and W is the thermodynamic factor [7]. The results show that D* and W are strongly dependent on the composition in LixMoO3 and the morphology of the film. The chemical diffusion coefficients of Li ions in LixMoO3 film deposited at 250 °C (curve c) is at least one


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order of magnitude higher than that for the MoO3 film grown at Ts=120 °C (curve b). The chemical diffusion coefficient of Li+ ions in amorphous film is lower than 10-12 cm2 s-1 (curve a). For MoO3 film prepared at Ts=120 °C a maximum value of 5×10-12 cm2 s-1 is obtained at x=0.8 for D*, which follows a quadratic law with the Li composition. The chemical diffusion coefficient behavior in MoO3 film deposited at 250 °C differs from the previous one. We observe that D* has an almost constant value of 1.5×10-11 cm2 s-1 in the range 0.1≤x≤1.2 and increases at higher concentration of intercalated ions. We tentatively attribute this complex behavior to the polycrystalline state of MoO3 films grown at high substrate temperature for which the enhancement factor has a large value. In this case, the intercalation process is partly controlled by the number of ion occupancies in the host lattice of the crystallite in the film.

FIG. 1. Discharge profiles of Li//MoO3 cells as a function of degree of Li inserted in electrodes grown by the flash-evaporation technique. Substrate temperature during MoO3 deposition was (a) 30, (b) 120, and (c) 250 °C. The thermodynamic factor of MoO3 thin film varies from 50 to 800 in the compositional range 0.2≤x≤1.5. We observe a linear dependence of the thermodynamic factor, W, which may be associated with the decrease of electronic mobility in Li xMoO3 films. The linear dependence of W may be also associated with the oxygen-defects in the host lattice. It is a fact that numerous of such defects exist in the film structure, even when the crystallinity is improved by different conditions of preparation. Here, we remark that W is two orders of magnitude higher than the thermodynamic factor in MoO 3 crystal [7]. Considering that a MoO3 film is an oxygen-deficient material, the model of charge transport in internal defectmaterial can be applied [8]. Defects are Li-interstitials, Li*, and conduction electrons, e', for example. It has been shown [35] that, in a solid solution where no internal defect reactions occur, the thermodynamic factor is related to the defect concentration (if dilute defects exist)


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and follows a linear variation with the degree of intercalation. The large increase of W may be also associated with the decrease of the electronic mobility in Li xMoO3 film. 2. Sputtered MoO3 films The discharge profiles of Li//MoO3 cells using rf-sputtered films exhibit almost similar features. Fig. 2 reports the discharge curves as a function of the lithium inserted of such MoO3 electrodes. As for flash-evaporated electrodes, the initial voltage is around 3 V and the discharge appears to be smooth without any potential steps, as the amount of Li intercalated increases, as expected for an amorphous material. Almost 1.5 Li per MoO 3 mole were inserted in this condition corresponding to the reduction of molybdenum atoms to a pentavalent state. However, the cell potential does not vary much as a function of oxygen flow during MoO3 deposition in the range 1.75≤≤8 sccm.

FIG. 2. Discharge profiles of Li//MoO3 cells as a function of degree of Li inserted in electrodes grown by rf sputtering. Oxygen flow during MoO3 deposition was (a) 1.75, (b) 2, and (c) 8 sccm. The high insertion capability has been confirmed also in kinetics measurements. An amorphous film grown at =1.75 sccm displays low chemical diffusion coefficients for the Li ions of 3×10-14 cm2 s-1 in the compositional range 0.4≤x≤1.5 as shown in Fig. 3. D* increases by two orders of magnitude in films grown at higher oxygen partial pressure. A value D*=2×10-12 cm2 s-1 is obtained for =8 sccm. The features can be explained by considering that small organized regions are present in rf-MoO3 films. This short order favors the lithium diffusion in the film structure and is subjected to lattice volume expansion during the first stage of Li insertion as shown in Fig. 3 (curve c).


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FIG. 3. Chemical diffusion coefficient of Li inserted in electrodes grown by rf sputtering. Oxygen flow was: (a) 1.75, (b) 2, and (c) 8 sccm during MoO3 deposition. 3. Flash-evaporated V2O5 films V2O5 is an orange-red semiconducting material of orthorhombic structure composed of VO6 octahedra joined partly on edge and partly on corner. This creates an open structure with empty sites able to accommodate small atoms. Films of vanadium pentoxide have been obtained by different techniques including flash evaporation [9] and rf-magnetron sputtering [10]. Amorphous V2O5 grown by flash evaporation is more homogeneous and exhibits higher conductivities than vapor deposited films [11]. Structural investigations of flash-evaporated V2O5 show that films deposited at room temperature are rather amorphous, whereas V2O5 films grown on silicon substrate maintained at above 160 °C are polycrystalline and exhibit a well-resolved Raman spectrum. When an asdeposited V2O5 film is thermally annealed in O2 ambient at 300 °C during 6 h, the structural disorder disappears and the Raman spectrum exhibits similar features than the V2O5 crystalline phase. It is worth to note that the nature of the substrate plays an important role in the growth of V2O5 films. A (100) oriented silicon substrate provides well-layered structured films with a good crystallinity. This is a good example of "van der Waals epitaxy" where the layered compound grows perfectly on the silicon substrate. Discharge curves as a function of degree of lithium insertion for Li//V2O5 thin film microbatteries are shown in Fig. 4. The electrochemical properties of active-cathode films have been investigated as a function of the growth conditions, namely the substrate temperature. Crystalline V2O5 has been thoroughly characterized as electrode material in lithium batteries [12]. During the first discharge several plateaus appear in the compositional range 0≤x≤3 related to the formation of new ordered Li xV2O5 structure, and these features are retained in the subsequent charges and discharges as long as the voltage is limited to 2.2 V. The flash-evaporated films behave higher voltage vs. Li than crystalline V2O5. The discharge curve of a cell having amorphous cathodic material (Fig. 4a) exhibits a lower voltage than for the cell having a polycrystalline V2O5 film as cathode (Fig. 4b). However, considering the ex-


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perimental error, it seems that the films can uptake about 3 Li per V2O5 mole at discharge potentials down to 2.1 V. The V2O5 electrode grown at Ts=250°C exhibits interesting electrochemical properties for a microbattery application. The high voltage region is comparable to that crystalline V2O5 in the range 0≤x≤1. The voltage curve is less steep at x>1 and the discharge occurs steadily down to 2.6 V at x=2.7.

FIG. 4. Discharge curves of Li//V2O5 cells as a function of degree of lithium inserted. Active cathode films were grown on substrate maintained at temperature (a) Ts=25 °C and (b) Ts=250 °C. Kinetics of Li+-ions have been measured in Li xV2O5 films grown at Ts=250 °C. In the compositional range 0≤x≤1, the chemical diffusion coefficient of Li has an almost constant value 3×10-13 cm2 s-1. By comparison, Baudry et al. [13] reported diffusion coefficient of 2.5×10-12 cm2 s-1 for Li+ ions in V2O5 thin films grown by evaporation as determined from impedance measurements on an electrochromic window at 3 V vs. Li, whereas Bates et al. [14] have shown that the diffusion coefficient varies from 4×10-15 to 2.6×10-12 cm2 s-1 in the range 0.29≤x≤2.6 for dc-magnetron sputtered V2O5 films. The thermodynamic factor obtained from kinetic data exhibits an almost linear variation on x with an ion-ion interaction factor of 15. At x=1, the high value of W can be interpreted by the generation of an internal potential gradient asserted due to the displacement of the electronic charges and Li-ions inserted concentration profiles. The secondary performance of a cell using a V2O5 film as cathode and a polymeric gel as electrolyte has been investigated. Typical discharge-charge behavior as a function of cell capacity for cycling between 3.5 and 1.0 V is shown in Fig. 5. These curves were recorded using a thin-film cathode grown at Ts=250 °C. The film performs for cycling even for a large depth of discharge with a current density 15 µA cm-2. 4. Flash-evaporated V6O13 films V6O13 is a black material of monoclinic structure. The black color is an indication of a broad electron energy band in the visible range. The breadth of the band suggests it is not


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fully occupied and so the material has relatively high intrinsic conductivity. If we think of V6O13 as deficient in oxygen compared to the V2O5 structure, the oxygen vacancies (holes) may be considered to form the conduction band. The lower oxygen content requires that, compared to V2O5, more of the VO6 octahedra be joined on edge than corners. This results in a more open structure with a three-dimensional network of channels for intercalation. As a result, V6O13 can accommodate 8 Li per formula unit [15].

FIG. 5. Charge-discharge profile vs. cell capacity recorded with a current density 15 µA cm-2 using a V2O5 thin-film cathode grown at Ts=250 °C. Flash-evaporated V6O13 thin films were grown from pre-synthesized polycrystalline powder. This method allows the control of the rate deposition and avoids any decomposition of the starting material. Different varieties of samples are obtained by changing the temperature of silicon substrates and the annealing process conditions. We have measured the structural, optical and electrical properties of these films and how these properties are affected by different thin-film preparation conditions. The film crystallinity was studied using Raman scattering spectroscopy. The RS spectrum of as-deposited films is amorphous-like whereas films annealed in Ar ambient at 300 °C exhibit well-resolved Raman lines. The influence of the growth conditions on the crystallinity of V6O13 films is seen from the shape and the frequency shift of the bands attributed to libration modes in the low-frequency region. Electrical conductivity of V6O13 thin films has been investigated as a function of the growth conditions. An amorphous V6O13 film deposited at 25 °C has a conductivity of 5×10-5 S cm-1 with the activation energy of 0.27 eV. The conductivity of V6O13 is believed to result from electron hopping between V4+ and V5+ sites. V6O13 can be written V25+V44+O132-, where the electron conduction is governed by a small polaron mechanism. For films annealed in either Ar or O2 ambient the conductivity is more than two orders of magnitude higher than for asdeposited films and one observes a slight decrease of the activation energy.


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FIG. 6. Discharge curves of Li//V6O13 cells as a function of degree of Li inserted. Active cathode films were grown with various conditions: (a) grown at Ts=25 °C and annealed at 300 °C in Ar, (b) grown at Ts=250 °C and annealed at 300 °C in Ar, and (c) as deposited at Ts=250 °C. Fig. 6 shows the discharge profiles of Li//V6O13 thin-film cells as a function of degree of lithium inserted. These discharge measurements have been carried out using a current density of 5 µA cm-2. The measured cell voltages are in the range 3.3-2.5 V. It is worth to note the influence of the substrate temperature on the electrochemical behavior of the active cathode films. A film grown at Ts=250 °C displays a monotonic behavior of the discharge curve in the range 0≤x≤6, whereas the cell voltage for a film grown at Ts=25 °C decreases more rapidly as the Li+-ions are inserted in the host material. Furthermore, we observe the appearance of a voltage plateau for the film annealed at 300 °C but the different phases recorded during the discharge of a cell with crystalline V6O13 are not observed in these experiments. This behavior can be attributed to the difference in the electronic band structure. Here, there are localized states situated in the band-gap and their density varies with the film crystallinity giving rise to different electrochemical features. In LixV6O13 films grown at Ts=25 °C the chemical diffusion coefficient remains constant over the compositional range 0≤x≤6 with a mean value of 10-13 cm2 s-1. This value is about five orders of magnitude lower than the values obtained for the stoichiometric crystalline phase. All the diffusivity data are listed in Table II. This is reasonable. In other systems, such as LixInSe [16] and LiMn2O4 [17] the same result has been obtained. As the diffusion path for lithium ions passes through the cavities, occupation of the pyramidal sites will change the diffusion coefficient in polycrystalline film grown at Ts=250 °C. The difference between the oxide prepared in crystalline form and the film could thus be ascribed to differences in crystal perfection, i.e., static disorder or short chains with undistorted cavities.


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Table II. Chemical diffusion coefficient of lithium in thin-film cathodes. Compound LixMoO3 LixV2O5 LixV6O13

Diffusion coefficient (cm2 s-1)

Deposition Ts (°C)

at x=0.2

at x=0.5

at xmax

120 250 25 25 250

1×10-12 1.8×10-11 3×10-13 4×10-13 1.2×10-13

2×10-12 1.2×10-11 3×10-13 3×10-13 1.2×10-13

1.5×10-12 5×10-11 2×10-13 2×10-13 1×10-15

Fig. 7 displays the cell capacity as a function of the cycle number of Li/polymer/V-O microbatteries for cycling between 3.5 and 1.5 V at current density of 15 µA cm-2. As illustrated in this figure after the third cycle of discharge/charge of a Li//V2O5 thin-film cell there is a loss of about 12% of its initial capacity and then we observe a slow but continuous decrease in the amount of lithium inserted into and extracted from the film cathode, possibly due to further irreversible structural changes. For cycling between 3.5 and 1.5 V the capacity loss reaches a level of about 0.9% per cycle. The capacity loss in a Li//V6O13 thin-film cell is smaller than for Li//V2O5. We observe a continuous decrease with a loss level of about only 0.3% per cycle.

FIG. 7. Capacity vs. cycle number of Li/polymer/V-O microbatteries for cycling in the voltage range 3.5-1.5 V at current density 15 µA cm-2. (a) Li//V2O5 and (b) Li//V6O13 cell.

CONCLUDING REMARKS 1. The growth of flash-evaporated V2O5 on (100) oriented silicon substrate provides welllayered structured films with a good crystallinity. This is a good example of a "van der Waals


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epitaxy" where the layered compound grows perfectly on the silicon substrate. Flashevaporated V6O13 films are grown by the control of the rate deposition which avoids any decomposition of the starting material. Different varieties of samples were obtained by changing the temperature of silicon substrates and the annealing process conditions. 2. Electrochemical cells using Mo and V oxide films as cathode have the general features: (i) initial voltages are in the range of 3.0-3.3 V which are higher than those recorded on the galvanic cell using crystalline cathodes, (ii) the cell voltage decreases continuously as a function of the degree of Li inserted, (iii) the steadily behavior is a function of the structural arrangement in the film and thus, depends on the substrate deposition temperature, (iv) no voltage plateau occurs during the discharge. The electrochemical process seems to be a classical intercalation mechanism for the lithium ions. The reduction process is accompanied by coloration of MoO3 and V2O5 films. These results suggest that the Li + diffusion may be anisotropic and limited by grain boundary effects which affect the discharge curve. The discharge curve of the cell fabricated with a film deposited at 250 °C is quite stable over cycling. This may be attributed to the unique layered structure of -MoO3 and V2O5 with large grain size in the films.

FIG. 8. Cell voltage vs. stored charge, Q, per film thickness and area for lithium microbatteries using MoO3, V2O5, and V6O13 cathode material. 3. The cell voltage remains high with films because donating electrons are trapped in the localized band located in the band gap of disordered semiconductors. The cell voltage can be tune with the disorder induced by the formation of vacancies. MoO 3 films are interesting as potential electroactive materials for either solid-state lithium microbatteries or electrochromic devices. 4. A comparison of the investigated Li microcells is shown in Fig. 8. V6O13 films have the highest capacity (≈0.6 C µm-1 cm-2) with a good cycleability better than V2O5 film that makes


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V6O13 film much more attractive for secondary microbatteries (see Fig. 7). The charge density was 0.34 C µm-1 cm-2 for the flash-evaporated MoO3 film and 0.23 C µm-1 cm-2 for the sputtered one. This difference in charge density speaks in favor of a larger porosity in the flash-evaporated film than in the sputtered one, which, in fact, is the expected result. The maximum value of Q was 0.4 C µm-1 cm-2 for the flash-evaporated V2O5 film. ACKNOWLEDGEMENTS This work was partially supported by the Indo-French Centre for the Promotion of Advanced Research (IFCPAR) under grant No.1408-2. REFERENCES 1. C. Julien, in: Handbook of Solid State Chemistry, Ed. by P.J. Gellings and H.J.M. Bouwmeester (CRC Press, 1997), p. 371. 2. C. Julien and G.A. Nazri, Solid State Batteries: Materials Design and Optimization, Kluwer Acad. Publ., Boston, 1994. 3. C. Julien, O.M. Hussain, L. El-Farh and M. Balkanski, Solid State Ionics 53/56, 400 (1992). 4. C. Julien, L. El-Farh, M. Balkanski, O.M. Hussain and G.A. Nazri, Appl. Surf. Sci. 65/66, 325 (1993). 5. C. Julien, G.A. Nazri, J.P. Guesdon, A. Gorenstein, A. Khelfa and O.M. Hussain, Solid State Ionics 73, 319 (1994). 6. C. Julien and G.A. Nazri, Solid State Ionics 68, 111 (1994). 7. W. Weppner and R.A. Huggins, J. Electrochem. Soc. 124, 1569 (1977). 8. J. Maier, Mater. Res. Soc. Symp. Proc. 210, 499 (1991). 9. L. Murawski, C. Gledel, C. Sanchez, J. Livage and J.P. Audiere, J. Non-Cryst. Solids 89, 98 (1987). 10. K. West, B. Zachau-Christiansen, S.V. Skaarup and F.W. Poulsen, Solid State Ionics 57, 41 (1992). 11. C. Julien, A. Khelfa, J.P. Guesdon, V. Tuncheva, and F. Gendron, Ionics 3, 1 (1997). 12. J.M. Cocciantelli, J.P. Doumerc, M. Pouchard, M. Broussely and J. Labat, J. Power Sources 34, 103 (1991). 13. P. Baudry, M.A. Aegerteer, D. Deroo and B. Valla, J. Electrochem. Soc. 138, 460 (1991). 14. J.B. Bates, G.R. Gruzalski, N.J. Dudney, C.F. Luck and X. Yu, Solid State Ionics 70-71, 619 (1994). 15. K. West, B. Zachau-Christiansen and T. Jacobsen, Electrochim. Acta 28, 1829 (1983). 16. C. Julien, I. Samaras, M. Tsakiri, P. Dzwonkowski and M. Balkanski, Mater. Sci. Eng. B 3, 25 (1989). 17. C. Liquan and J. Schoonman, Solid State Ionics 67, 17 (1994).


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