Russian Journal of Applied Chemistry, Vol. 77, No. 5, 2004, pp. 763!769. Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 5, 2004, pp. 767!773. Original Russian Text Copyright + 2004 by Kuksenko.
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APPLIED ELECTROCHEMISTRY ÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ AND CORROSION PROTECTION OF METALS
Effect of How Copper Oxide Is Obtained on the Behavior of Copper Oxide Electrodes in Lithium-Ion Power Cells S. P. Kuksenko Vernadsky Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine, Kiev, Ukraine
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Received December 2, 2003
Abstract The electrochemical parameters of copper oxide electrodes in hermetically sealed lithium power cells were studied in relation to routes of copper oxide preparation, with the aim to diminish the share of side reactions.
Owing to high specific energy parameters, working voltage, and very low level of self-discharge, copper oxide3lithium (COL) power cells have long attracted the attention of development engineers and consumers as an alternative to silver3zinc (SZ) and mercury3zinc (MZ) power cells of cylindrical and button designs of various sizes [1, 2]. Recently, interest has been aroused in improvement of the fabrication technology of COL power cells in view of the increasing requirements to their electrical characteristics [335].
suitable for fabrication of lithium power cells with a voltage of 1.5 V) involves a number of experimental difficulties because of the necessity for taking into account side reductive processes, which may occur at these electrodes under load (in contrast to oxidative processes at the cathodes of power cells with a voltage of 3 V, which occur at a higher rate under open-circuit conditions [13, 14]). Differing degrees of suppression of side processes can account for the discrepancies in the results of previous experiments.
As the discharge capacity, the manner in which the voltage varies in the course of discharge, and the output power of COL power cells largely depend on how and under what conditions copper oxide is obtained, it is necessary to continue the search for industrially acceptable methods for preparing copper oxide and also for compositions of cathode pastes on its base that would ensure the required electrical parameters of the cells in various operation modes.
In the present study, an attempt was made to examine the electrochemical characteristics of electrodes based on copper oxide obtained by different methods, with a minimized contribution from side reactions and electrodes operating in a mode as close as possible to that in lithium power cells working under real conditions. EXPERIMENTAL
Presently, a wide variety of methods for obtaining copper oxide are known. These methods differ in the compositions of the starting substances and in conditions of their chemical and thermal treatment. However, there is no unambiguous published evidence that would make it possible to estimate the electrochemical parameters of copper oxide prepared by one or another technique as the active component of the positive electrode of COL power cells. Moreover, there are quite opposite assessments of the discharge characteristics of electrodes based on copper oxide obtained using the same starting substance under identical treatment conditions [3312].
(1) Thermal decomposition of Cu(NO3)2. After preliminary drying at 105oC in a vacuum for 8 h in order to remove water of crystallization, copper nitrate was subjected to thermal treatment in air at 400oC for 8 h.
Studying the electrochemical behavior of electrodes based on copper oxide (or on other active materials
(3) Thermal decomposition of CuCO3 . Cu(OH)2. Copper hydroxycarbonate was dried in a vacuum at
For fabricating copper oxide electrodes we chose copper oxide samples obtained by the following methods.
(2) Oxidation of Cu2O. Copper(I) oxide was heated in an oxygen atmosphere to 400oC and kept at this temperature for 1 h.
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105oC for 8 h in order to remove water of crystallization and then heated in air to 400oC and kept at this temperature for 4 h.
a graphite electrode without binder in the same electrolyte, the cathodic wave appeared in the first cycle only at 0.8 V relative to Li/Li+ [21].
(3a) The resulting copper oxide was additionally thermally treated in air at 800oC for 1 h.
In this study, the necessary electrical conductivity was ensured by mixing the copper oxide samples under study with powdered graphite of C1 brand in a weight ratio of 97 : 3 (which constituted approximately 92 : 8 v/v). Copper oxide electrodes without binder were compacted into pellets under a pressure of 2300 kg cm32. The absence of a binder did not impair the strength of the pellets and made it possible to raise the capacity of the copper oxide electrodes because of the increased content of the active substance. The pellets were placed in an evacuated cell and kept there at 300oC for 1 h in the atmosphere of dry argon.
(3b) Copper oxide was thermally treated at 800oC for 4 h. (4) Oxidation of spherical copper particles in pure oxygen at 1000oC. (4a) The resulting copper oxide was additionally thermally treated in air at 800oC for 1 h. The completeness of conversion of the starting substances into copper oxide was verified by X-ray diffraction analysis, with monitoring the strongest lines. Commonly, an electrically conducting additive, acetylene black [6 38] or graphite [3, 10, 11], is introduced in the electrode paste with copper oxide characterized by high electrical resistance [11]. In a number of studies [3, 6 38], a binder, Teflon, was also introduced into the paste (in [7], a carbon black hydrophobized with Teflon served as a filler); in [10, 11], no binder was used. It should be kept in mind, when choosing materials for an electrically conducting additive and a binder and deciding upon the amount of these, that these materials may be electrochemically active in the range of copper oxide reduction potentials. Indeed, studies of the cathodic behavior of various carbon materials point to their inherent electrochemical activity [15]. The discharge capacity of such electrodes is the higher, the greater the specific surface area of carbon, and it mainly depends on the structure of the amorphous region, rather than on the crystal structure. Use of a Teflon-containing binder in experiments concerned with the behavior of copper oxide electrodes cannot be considered justified because of the ability of this binder to be electrochemically reduced at potentials of 2.131.8 V relative to the lithium reference electrode Li/Li+ under loads of 1310 mA cm32 to give amorphous carbon [16 318]. An electrode made of a mixture of carbon black with 15 wt % Teflon discharged in a 1 M solution of LiClO4 in propylene carbonate (PC) to a potential of 1.0 V with an output capacity of 120 mA h g31, whereas an electrode without Teflon produced only 8 mA h g31 under the same conditions [15]. A wave at a potential of 2.1 V was observed in the potentiodynamic curve of an electrode composed of a mixture of graphite with Teflon, measured in a 1 M solution of LiClO4 in a mixture of PC and 1.2-dimethoxyethane (DME) in the first cathodic cycle. In [20], with
The electrochemical behavior of copper oxide electrodes was studied in hermetically sealed cells with a lithium anode. In doing so, it is necessary to take into account the fact that, in the course of discharge, the copper oxide electrode grows in size (swells) because the volume of discharge products somewhat exceeds the initial volume of the electrode (theoretically, by 78%). The thickness of the electrode pellets was chosen so that the cells did not overcome their overall dimensions in discharge. In a hermetically sealed cell, the copper oxide electrode experiences a deficiency of electrolyte, in contrast to the case of tests in an electrochemical cell with an excess of electrolyte [6 38]. The procedure used in this study makes it possible to assess the electrochemical characteristics of copper oxide electrodes under the conditions close to those in practical use of COL power cells. The anodes were fabricated as disks of rolled lithium. To diminish the contribution of the anode to the voltage drop across the cells in the pulsed mode of its discharge, an alloy of lithium with aluminum was formed on the anode surface, using the procedure described in [22]. The cells were fabricated with an excess of the anode capacity. Previously, most of studies concerned with the electrochemical behavior of copper oxide electrodes have been performed with electrolytes containing PC (1 M LiClO4/PC [6, 7] and 1 M LiClO4/PC + DME [11]), which are the most readily available. According to [23], PC in 1 M LiClO4/PC is reduced at potentials more negative than 1.3 V; at the same time, there have been numerous reports that a PC + DME mixed electrolyte is stable on a graphite electrode down to 0.8 V relative to Li/Li+ (see, e.g., [21]). It should be taken into account that oxygen adsorbed by the electrically conducting carbon additive (more by acetylene black, than by graphite) and copper oxide, and also
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dissolved in the electrolyte, is reduced at potentials of 2.0 31.8 V relative to Li/Li+ unless special measures to remove it are taken. At potentials more negative than 1.3 V relative to Li/Li+, water that was not removed in preparing the electrolyte undergoes reduction [23]. When the discharge behavior of copper oxide electrodes was studied in electrochemical cells with an excess of electrolyte [6 38], this had to lead to increased share of side reactions associated with reduction of oxygen and water dissolved in the electrolyte, in contrast to studies in which electrodes were tested in hermetically sealed lithium cells under the conditions of electrolyte deficiency [10, 11]. Thorough drying of the electrolyte and reliable sealing of a cell are also necessary because dehydrated copper oxide readily adsorbs water. In this study, a 1.4 M solution of LiClO4 in a mixture of PC and DME (42 : 58 w/w) and water content of less than 30 ppm was used as an electrolyte. The electrolyte was subjected to an additional drying by a previously described procedure [24] and bubbled-through with dry argon to remove dissolved oxygen.
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U, V
U, V
U, V
3b 3b`
The cells were assembled in a glove box filled with dry argon. The results reported here were obtained with cells 9.5 mm in diameter and 2.10 mm high. Discharge curves of the cells were measured after 1 month of aging. The discharge was done through a continuous load of 30 kW with a pulsed load of 1 kW connected in parallel for 1 s once in 24 h. This procedure rules out any influence of side processes that occur in tests on a continuous high load (when the active cathode substance is reduced at lower potentials). To determine how the components of the total voltage drop across the cell in a pulse, DUS, vary during discharge across a continuous load, the voltage drops across the cell were recorded as follows: at the instant of switch-on of the load, as a sum of ohmic components of the polarization resistances of both the electrodes and voltage drop in the interelectrode space, DUr; 1 ms after the switch-on of the load, as a sum of activation polarizations of the electrodes, DUa. The difference between DUS and DUr was regarded as a sum of the diffusion polarizations of both the electrodes, DUd, at the end of the 1-s period of time. As the cells were fabricated with an excess of lithium and the type of the anode polarization had to be invariable in the course of the discharge, the relative changes in the electrochemical parameters of the cell were determined during the continuous discharge with the copper oxide electrodes.
occur at the copper oxide electrodes, cells with a graphite positive electrode containing no copper oxide were fabricated under the same conditions and discharged through a load of 30 kW. The capacity of such cells in discharge to a voltage of 1.0 V was less than 1.2 mA h (the corresponding discharge curve is represented by a dotted line in Fig. 1).
The copper oxide electrodes used contain graphite as an electrically conducting additive. For this reason, to evaluate the contribution of side processes that
Figure 1 shows the discharge curves and dependences of the cell voltage at the end of a load pulse on the duration of discharge for power cells with cop-
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U, V
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Fig. 1. (1! 4, 3a, 3b, 4a) Discharge curves of lithium power cells with a copper oxide cathode and (1`34 `, 3a`, 3b`, 4a `) dependences of the cell voltage at the end of a load pulse on the duration of a continuous discharge. Cell size type 921, constant load 30 kW, pulsed load 1 kW s. (U) Voltage and (t) duration of continuous discharge. Digits at curves: methods used to produce copper oxide. Dotted line: discharge curve of a cell with a graphite cathode; dashed lines: discharge curve of a cell with a Cu2O cathode (upper) and dependence of the cell voltage by the end of a load pulse on the duration of a continuous discharge (lower).
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per oxide electrodes for which copper oxide was prepared using procedures 13 4a. The discharge capacity of a cell with copper oxide produced by method 1 was 42 mA h in discharge to a final voltage of 1.2 V, and 46 mA h, to 1.0 V. The utilization factor (UF) for copper oxide was 77 and 83%, respectively. This result is in agreement with the UF of 80% [11] obtained for copper oxide under similar conditions (in hermetically sealed cells of size type 1142 with a copper oxide electrode composed of a mixture of copper oxide obtained by decomposition of copper nitrate and 5 wt % graphite without a binder), but contradicts the data of [6]. According to [6], a copper oxide electrode composed of a mixture of copper oxide (also produced by decomposition of copper nitrate) and 33 wt % acetylene black and 17 wt % fluoroplastic binder is characterized by a UF of 40% for copper oxide in the case of its electrochemical reduction in an excess of electrolyte. Discharge curve 1 is characterized by a relatively prolonged transition portion before that of a stable working voltage, despite the low open-circuit voltage (OCV) of 1.98 V. The dependence of the cell voltage at the end of the pulse on the duration of a continuous discharge (curve 1) shows a decrease in the pulsed voltage during the first several tens of hours, with the subsequent slow rise till approximately half of the total duration of discharge. The further run of the curve of the pulsed voltage corresponds to that of the discharge curve. Of the 12 methods for obtaining copper oxide, studied in [6], the best was a short-time (1 to 3 h) oxidation of Cu2O in an atmosphere of oxygen at 300 3500oC. The thus obtained copper oxide had a UF as high as 92%, but the discharge characteristic of the electrode made of this material exhibited a prolonged transition region before a stable discharge plateau (with low average level of potentials of about 1.35 V). In the present study, copper oxide obtained using the same technique as that in [6] (method 2) differed only slightly in its electrochemical characteristics from copper oxide prepared by method 1. Its UF was found to be 79% in the case of cell discharge to 1.2 V (cell capacity was 43.5 mA h) and 89% in discharge to 1.0 V (49 mA h). In discharge curve 2, as also in curve 1, the voltage reaches a stable level in a relatively long time; curve 2` shows, similarly to curve 1`, a decrease in the pulsed voltage in the first half of the discharge. Under the same testing conditions, Cu2O was reduced in the case of a continuous load at the same potentials as copper(II) oxide synthesized from this material, but the cell with a Cu2O cathode exhibited no dip in the curve for pulsed voltage (upper and lower dashed lines in Fig. 1, respectively) and was
characterized by a lower OCV: 1.90 V, compared to 2.12 for the cell with a cathode made of copper oxide produced by method 2. As follows from curves 3 and 3`, the cell with copper oxide prepared by method 3 is distinguished by an extremely high initial discharge voltage in discharge on a continuous load (the OCV of the cell was 2.56 V), a dip in curve 2 before the plateau, and the corresponding deep dip in curve 2` for pulsed discharge. The cell capacity was relatively low: 38 and 41 mA h in discharge to 1.2 and 1.0 V, respectively. An X-ray phase analysis of the copper oxide produced by method 3 did not reveal presence of any other phases. It is noteworthy that, under the experimental conditions of [6], the electrode based on copper oxide produced by thermal decomposition of copper hydroxycarbonate at 300oC in the course of 76 h also discharged at high initial potentials, but exhibited no stable discharge plateau and had high capacity, more than 90% of the theoretical value (calculated for the reaction of copper oxide reduction). An even higher discharge capacity (99% of the theoretical value in discharge of the electrode to 1.0 V relative to Li/Li+ with a current of 0.5 mA) was observed [7] for copper oxide produced by thermal decomposition of Cu(OH)2 at 300oC for 2 h. The discharge curve of the electrode with copper oxide of this kind was also distinguished by a sloping run during the entire course of discharge. Prolonged heating of copper oxide samples decreased the discharge capacity and the reduction potentials of copper oxide electrodes. This was attributed to changes in nonstoichiometry upon removal of residual OH groups from the active form of copper oxide. Curves 3a and 3b indicate that, under the experimental conditions, a prolonged thermal treatment of copper oxide at high temperature (1 and 4 h at 800oC, respectively) improved the electrochemical parameters of copper oxide electrodes. The continuous discharge of cells with thermally treated electrodes proceeded at higher stable voltage (with relatively low initial voltage). The cell capacity increased to, respectively, 43 and 45 mA h (in discharge to 1.2 V), and the OCV decreased to 2.16 and 2.06 V. However, the strongest positive influence was exerted by thermal treatment of copper oxide produced by method 3 on the dependences of the cell voltage by the end of a load pulse on the duration of a continuous discharge (curves 3a`, 3b`). The average level at which these curves ran increased substantially and the dip in the pulsed voltage
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in their initial portions decreased dramatically (more so for curve 3b`). As the temperature and duration of thermal treatment increased, the peaks in the X-ray diffraction patterns of copper oxide samples became narrower and higher, which indicated that the grain size increased and the crystal lattice perfection was improved. The positive influence of a prolonged thermal treatment of copper oxide on the specific capacity of copper oxide was observed in [9]: the specific capacity of electrodes based on industrial copper oxide increased from 150 3300 to 550 mA h g31 after its thermal treatment in air for 16 h. A correlation was revealed between the ratio of CuO and Cu2O peak intensities for copper oxide samples and the specific capacity of the electrodes. The specific volume capacity of the electrodes based on industrial copper oxide increased by 50370% on heating copper oxide samples to 600 3 800oC followed by fast cooling (quenching) [4]. This was attributed to the appearance of an oxygen nonstoichiometry. The relatively high electrochemical parameters in a continuous discharge were observed for a cell with copper oxide produced by high-temperature oxidation of spherical copper particles (method 4). The cell had a relatively stable discharge voltage with an average value of 1.40 V (Fig. 1, curve 4) and high capacity of 48 and 51 mA h in discharge to 1.2 and 1.0 V, respectively. The OCV of the cell was 2.05 V. It is noteworthy that the discharge of a cell with copper oxide produced by controlled oxidation of copper particles (of a certain size and shape) at 400oC [10] proceeded at an average voltage of 1.4 V (at the same current density) and high output capacity, but the discharge curve differed from that obtained in this study in the longer time in which the stable voltage plateau was reached and higher value of OCV of 2.3 V. Under other experimental conditions, a copper oxide electrode made of a copper powder oxidized in air at 400oC for 1 h discharged at an average potential of 1.35 V and a UF of about 78%, and upon oxidation for 10 h, at 1.2 V and UF of 50% [6]. As can be seen from curve 4 ` in Fig. 1, short-time oxidation of spherical copper particles by method 4 also leads (as also do methods 133) to a dip in the curve for the pulsed voltage in the initial stage of discharge of a COL cell through a continuous load. Figure 2 shows how the overall voltage drop under pulsed load and its components vary in discharge of the given cell through a continuous load. It can be seen that, in the initial stage, the major contribution to the overall voltage drop comes from the diffusion RUSSIAN JOURNAL OF APPLIED CHEMISTRY
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DU5(d,r,a), V
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Fig. 2. Variation of the overall voltage drop and its components (diffusion-related, ohmic, activated) DU5(d, r, a) under pulsed load in discharge through a continuous load of a cell with copper oxide produced by method 4. (t) Duration of continuous discharge. (1) DU5, (2) DUd, (3) DUr, and (4) DUa.
component, which grows until the cell discharges by approximately 10% relative to the total duration of discharge. Also grows during this period of time the ohmic component, and the activation component does not change its sign. Further, the diffusion and activation components decrease and, after 1/4 discharge of the cell, remain virtually unchanged till 4/5 of the total discharge duration is reached. In the process, the ohmic component makes the most pronounced contribution to the overall voltage drop. In the final stage of discharge, the diffusion component again grows and, at the end of the discharge process, much exceeds the activation and ohmic components. Curve 4a` in Fig. 1 shows that additional thermal treatment of copper oxide results in that both the depth and the duration of the dip in the curve for the pulsed voltage decrease substantially. Further discharge of the cell was more stable and proceeded with a higher pulsed voltage. The cell with such a copper oxide was distinguished by a lower OCV of 1.90 V and higher discharge capacity of 50 and 52 mA h in discharge to 1.2 and 1.0 V, respectively. In discharge to 1.2 V, the specific discharge capacity of the copper oxide electrode was 625 mA h g31, and the UF of copper oxide, 95%. The cell voltage in continuous discharge rapidly reached a stable level (Fig. 1, curve 4a) at a relatively high average discharge voltage of 1.43 V. In addition, curve 4a is characterized by a rather strong bend at the end of discharge (the capacity in discharge to a final voltage of 1.35 V was 44 mA h, i.e., 88% of the capacity in discharge to 1.2 V). It was found that a minor dip in the curve for pulsed voltage in the initial stage of a continuous discharge is also associated with the corresponding change in the diffusion component of the overall voltage drop. In the plateau in the dependence of the pulsed voltage on the duration of a continuous discharge, the diffusion and ohmic components were No. 5
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virtually the same. The X-ray diffraction pattern of a copper oxide sample subjected to an additional thermal treatment showed no peaks of copper(I) oxide, which were observed in the X-ray diffraction pattern of the initial copper oxide sample. Discharge curves of COL cells are distinguished by a common feature: the longer the delay of the initial voltage in a continuous discharge, the deeper the dip in the curve describing the dependence of the pulsed voltage on the duration of the continuous discharge. This phenomenon is associated with the state of the surface of copper oxide, which depends on the method by which it is produced and on the mode of its further thermal treatment. In a discharge of a copper oxide electrode, lithium cations are introduced into the monoclinic lattice of copper oxide to give I CuII 1 3 xCuxLixO [8]. The reaction occurs at places of contact between copper oxide particles and particles of the distributed current lead (graphite in the given case). This process is preceded by reduction of surface groups, which are inevitably formed when copper oxide is obtained and processed using the methods described here. Possibly, residual OH groups can catalyze reduction of Teflon from a Teflon-containing carbon black and reduction of PC. This probably accounts for the high capacity and discharge potentials of continuously loaded electrodes with copper oxide produced at a relatively low temperature and short time of thermal treatment [6, 7]. In electrochemical reduction of surface groups, a layer of a lithium-conducting solid electrolyte grows on copper oxide particles and the diffusion of lithium cations across this layer is slow. Also slow is the diffusion of lithium cations into the particles via formaI tion of an intermediate compound CuII 1 3 xCuxLixO, whose subsequent reduction gives Cu and Li2O or Cu2O and Li2O [8] and results in disintegration of the primary film of the solid electrolyte under the action of mechanical stresses. In the same range of potentials, Cu2O enters into an electrochemical reaction (being reduced to Cu and Li2O). The rise in the diffusion polarization of the electrode in the final stage of the discharge is due to exhaustion of the electrolyte within the micropores of the swollen cathode. According to [7], the removal of chemically bound water and oxygen from copper oxide samples obtained from Cu(OH)2 continues at above 500oC. Controlled oxidation of spherical copper particles at a high temperature in pure oxygen rules out any involvement of water molecules in the synthesis process. Hydrogen-containing groups on the surface of copper oxide particles presumably appear in the con-
tact of these particles with the ambient atmosphere. Prolonged high-temperature treatment of copper oxide samples in air largely removes the surface groups, improves the crystal perfection, and gives rise to a mosaic structure of grains, which facilitates diffusion of lithium cations into the particles in the course of electrochemical reduction. This leads to a high and stable voltage in discharge through a continuous load and to a pronounced stabilization of the pulsed voltage. Additional ways to completely eliminate the dip in the curve for the pulsed voltage in the initial stage of discharge are to be sought for. Preliminary experiments demonstrated that one of the possible ways is incorporation of lithium into copper oxide in hightemperature treatment with lithium carbonate. CONCLUSIONS (1) In studying the electrochemical parameters of copper oxide electrodes, it is necessary to take into account the influence of side reactions that involve the electrically conducting carbon additive, Teflon binder, oxygen, and water. (2) Under testing conditions that are close to real working conditions of copper oxide3lithium power cells, the highest electrochemical parameters are ensured by copper oxide produced by high-temperature oxidation of spherical copper particles in pure oxygen. (3) Prolonged high-temperature treatment of copper oxide in air improves the electrical parameters of power cells, but fails to completely eliminate the dip in the curve for the pulsed voltage in the initial stage of discharge through a continuous load. REFERENCES 1. Skundin, A.M. and Nizhnikovskii, E.A., Elektron. Komponenty, 2001, no. 4, pp. 27331. 2. Kuksenko, S.P., Lugovoi, V.P., and Prokopenko, V.T., Zh. Prikl. Khim., 1997, vol. 70, no. 6, pp. 9573960. 3. Chmilenko, N.A., Prisyazhnyi, V.D., Tkalenko, D.A., et al., in Fundamental’nye problemy e/kh energetiki: Materialy IV Mezhdunarodnoi konferentsii (Fundamental Problems of Electrochemical Power Engineering: Proc. IV Int. Conf.), Saratov, 1999, pp. 136 3138. 4. Belonenko, S.A., Yalyushev, N.I., and Pugachev, A.Yu., in Litievye istochniki toka: Materialy VI Mezhdunarodnoi konferentsii (Lithium Power Cells: Proc. VI Int. Conf.), Novocherkassk, 2000, p. 48. 5. Grugeon, S., Laruelle, S., Herrera-Urbina, R., et al., J. Electrochem. Soc., 2001, vol. 148, no. 4, pp. A2853 A292.
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