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MoO3−x thin film batteries
Solid State Ionics 144 Ž2001. 59–64 www.elsevier.comrlocaterssi
Characteristics of LirMoO 3yx thin film batteries Hideaki Ohtsuka) , Yoji Sakurai NTT Telecommunications Energy Laboratories, Tokai-mura, Naka-gun, Ibaraki, 319-1193, Japan Received 14 December 2000; received in revised form 11 May 2001; accepted 20 May 2001
Abstract Solid-state lithium batteries fabricated by thin film technology are attracting attention as micro-batteries. The thin film battery structure means that they have a small discharge capacity, therefore, it is necessary to increase the cell capacity per unit area of the cathode film for practical use. We fabricated thin film batteries with fairly thick MoO 3yx cathode films, which had a larger specific capacity than other cathode films Že.g., LiMn 2 O4 film.. We then examined the cell performance of our thin film batteries. When the discharge current density was 10 mArcm2 , the batteries had discharge capacities of 290 mA hrcm2 per unit area, and this value is the highest among typical thin film batteries reported to date. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Solid-state battery; Thin film battery; Lithium battery; Solid electrolyte; RF sputtering; Magnetron sputtering; MoO 3
1. Introduction The demand for compact and high energy density batteries is constantly increasing with the miniaturization of microelectronics and advances in portable devices. Therefore, lithium and lithium-ion batteries have been studied and the latter are being produced commercially. All-solid-state batteries have attracted considerable attention, because of their potential for flexibility, safety and further miniaturization. There has been particular interest in solid-state lithium batteries fabricated with thin film technology w1–6x, because such solid-state devices with them can be miniaturized, and also similar dry processes can be used to fabricate these batteries.
) Corresponding author. Tel.: q81-29-287-7538; fax: q81-29287-7863. E-mail address: [email protected] ŽH. Ohtsuka..
We have already reported thin film lithium batteries with Li 2 O–V2 O5 –SiO 2 thin films as their solid electrolyte w7–9x. However, thin film batteries generally have a small discharge capacity due to their structure, namely, where the cathode and anode consist of a small quantity of thin film. Therefore, it is necessary to increase the cell capacity per unit area of cathode film for practical use. To achieve this, it is necessary to use cathode materials that have a high specific capacity or to make the cathode films thick. The cathode films of the thin film batteries reported to date are almost less than 1 mm thick. Therefore, it is possible to increase cell capacity by making the cathode film thicker. MoO 3y x cathode films have larger specific capacities than other cathode films Že.g., LiMn 2 O4 film.. We previously reported a LirMoO 3y x thin film battery, in which the MoO 3y x cathode film thickness was 1.0 mm w7x. To increase the cell capacity per unit area, we fabricated LirMoO 3y x thin film batter-
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H. Ohtsuka, Y. Sakurair Solid State Ionics 144 (2001) 59–64
ies containing MoO 3y x cathode films that were thicker than 1 mm. In this paper, we report the cell performance of thin film batteries that employ MoO 3y x as their cathode active material.
2. Experimental procedure
Fig. 1. Rough sketch of thin film battery.
2.1. Cathode and solid electrolyte film fabrication We fabricated the MoO 3y x films using the conventional RF sputtering method, as previously reported w7x. We used RF sputtering equipment ŽModel: SBR-1104, ULVAC. for the fabrication. We prepared the target by cold pressing and sintering powdered MoO 3 ŽKanto Chemical reagent grade. at 720 8C for 8 h. After completing this process, we ground the product and formed it into a disk 100 mm in diameter and about 5 mm thick. The sputtering conditions are shown in Table 1. We deposited the MoO 3y x films in an Ar gas atmosphere at a sputtering gas pressure of 4.0 Pa, an RF power of 200 W and a substrate temperature of 100 8C. The deposition rate was 1.16 mmrh. We controlled the film thickness by regulating the sputtering time. We examined the crystalline structure of the sputtered films using an X-ray diffractometer ŽModel: Rad-rX, Rigaku. with graphite monochromatized Cu-K a radiation. We calibrated the diffraction angle using Si powder as a standard. We deposited Li 2 O–V2 O5 –SiO 2 thin films by the RF magnetron sputtering method. We have reported the thin film fabrication conditions in detail elsewhere w7,10x. For the fabrication, we used planar magnetron type RF sputtering equipment ŽModel: SPL-210H, ANELVA.. We established a constant 10-ccm flow of Ar and O 2 into the vacuum chamber to maintain a 1:1 ArrO 2 ratio at a constant pressure of 2.0 Pa. We deposited the Li 2 O–V2 O5 –SiO 2 films
Table 1 Sputtering conditions for MoO 3y x thin film Target Atmosphere Gas pressure RF power Substrate temperature
MoO 3 Ar 4.0 Pa 200 W 100 8C
in an Ar–O 2 gas atmosphere at an RF power of 100 W and with a substrate temperature of about 50 8C. The deposition rate was 167 nmrh. The ionic conductivity of the film was 1.0 = 10y6 Srcm and the electronic conductivity was less than 2 = 10y1 0 Srcm, as previously reported w7x. 2.2. Fabrication of Li r MoO3 y x thin film batteries and measurements We fabricated LirMoO 3y x thin film batteries by successively depositing MoO 3yx cathode film, Li 2 O–V2 O5 –SiO 2 solid electrolyte film and Li anode film on stainless steel substrates. We deposited the Li film on top of the Li 2 O–V2 O5 –SiO 2 film by the vacuum evaporation of lithium metal in a stainless steel crucible at a pressure of 4–8 = 10y4 Pa. We measured the thickness of the cathode and the electrolyte films by using a surface profile measuring system ŽModel: Dektak 3030, Veeco Sloan Tech. Div... We estimated the Li film thickness by using the thickness monitor of a quartz oscillator. Fig. 1 shows a rough sketch of the cell structure. The thickness of the cathode, the electrolyte and the anode films of a typical cell were 4.66, 1.0 and about 8 mm, respectively. The effective electrode area of the cell was 0.49 cm2 . It was limited by the smaller electrode area, which was the anode area in the cell. We placed the thin film batteries in sealed bottles containing desiccants and then cycled them at a constant current between 1.5 and 3.5 V, by using charge–discharge apparatus ŽHokuto-denko. operated under computer control. We examined the effect of discharge current density on discharge capacity from approximately the 20th to the 30th cycle in the cycling test. We changed the discharge current density to 2, 5, 10, 20 or 50 mArcm2 , but kept the charge current density a constant value of 10 mArcm2 in the current dependence test.
H. Ohtsuka, Y. Sakurair Solid State Ionics 144 (2001) 59–64
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3. Results and discussion Fig. 2 shows the X-ray diffraction pattern of MoO 3y x thin film deposited on a SiO 2 glass substrate. We observed only a few diffraction lines and the positions of these peaks coincide with those of the Ž010., Ž020. and Ž030. peaks of Mo 9 O 26 ŽJCPDS card No. 5-441, JCPDS: Joint Committee on Powder Diffraction Standards.. Therefore, we believe that the crystalline phase of the film was Mo 9 O 26 and that the crystallographic Ž010. plane of the film was oriented parallel to the substrate surface. The film was dark blue, which indicates that the film had been reduced and this is a reasonable assumption since we deposited the film in an Ar atmosphere. Fig. 3 shows the discharge curves of a typical cell at the 1st, 10th and 21st cycles with a discharge current density of 10 mArcm2 . The open circuit voltage ŽOCV. of the fabricated cell used for the figure was 2.7 V. In the first discharge curve, there are two gently sloping regions at 2.7–2.6 and at 2.0–1.7 V. The discharge curves after the 2nd cycle are S-shaped, as seen in the 10th and 21st cycles in this figure. These average voltages are 2.3 V. The discharge capacities of the 1st, 10th and 21st cycles were 398, 318 and 289 mA hrcm2 , respectively. Fig. 4 shows the relationship between the discharge capacity at 10 mArcm2 and the cycle number. The left vertical axis corresponds to discharge capacity per unit area and the right vertical axis indicates discharge capacity per volume of 1 cm2 = 1 mm. The discharge capacity decreased rapidly during
Fig. 2. X-ray diffraction pattern of MoO 3y x film.
Fig. 3. Discharge curves of LirMoO 3y x thin film battery. Cycle numbers are indicated.
the first 10 cycles, to about 80% of its initial discharge capacity. Then, it decreased slowly with further cycling. After the 30th cycle, the fading rate of the discharge capacity was less than 0.4% per cycle. At the 40th cycle, the discharge capacity per unit area and per 1 cm2 = 1 mm volume were 263 mA hrcm2 and 56.4 mA hrcm2 mm, respectively. This shows that the battery has good cycle performance. It is reported that LirMoO 3 batteries with liquid electrolytes have 1.5 electronsrmol of MoO 3 and the discharge capacity is 279 mA hrg, when cut off voltage is 1.5 V w11–13x. The discharge curves of the 2nd and subsequent cycles are different from that of the first cycle, and the capacities become smaller than that of the first cycle and are about 140 mA hrg w11,14x. As regards reduced Mo oxide, Mo 18 O52 w15x, non-aqueous lithium batteries with Mo 18 O52 as their cathode have 1.2–1.5 electronsr mol of Mo at the first discharge, but the discharge
Fig. 4. Discharge capacity change with cycle number.
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H. Ohtsuka, Y. Sakurair Solid State Ionics 144 (2001) 59–64
Fig. 5. Discharge and charge curves of LirMoO 3y x thin film battery at the 20th cycle.
capacities after the 2nd cycle become smaller than that of the initial cycle, as found with MoO 3 . The maximum recharge efficiency is 95% for a Mo 18 O52 cathode, in contrast to 75% for MoO 3 w15x. With Mo 8 O 23 , non-aqueous lithium batteries have 1.0 electronrmol of Mo and a recharge efficiency of above 75% is obtained at 0.75 electronrmol of Mo w16x. If we assume that the density of the MoO 3y x thin film is as large as that of Mo 9 O 26 and is 4.7 grcm3 w17x, the first discharge capacity per unit area, 398 mA hrcm2 , is equivalent to 182 mA hrg of the discharge capacity per unit weight. Similarly, the calculated 10th and 20th discharge capacities per unit weight are 145 and 133 mA hrg, respectively. The first discharge capacity of 398 mA hrcm2 Ž182 mA hrg. is equivalent to 0.98 electronrmol of Mo. The stable cycling capacity is 290 mA hrcm2 Ž133 mA hrg. and equivalent to 0.72 electronrmol of Mo. The first discharge capacity of 0.98 electronrMo is smaller than those of MoO 3 and Mo 18 O52 mentioned above. We think that the cause of the small first discharge capacity is the partial short-circuit that occurs accidentally during cell fabrication. However, the relationship between the discharge capacity and the cycle number of the thin film battery resembles those of LirMoO 3 or LirMo 18 O52 batteries with liquid electrolytes. The stable cycling capacity of this thin film battery is 290 mA hrcm2 Ž133 mA hrg. and as large as that of a LirMoO 3 battery with liquid electrolyte.
Therefore, the thin film battery has good cathode utilization over extended cycling. We believe that smooth lithium diffusion in the MoO 3y x film, as well as the high electronic conductivity of the film w3x, plays an important role in sustaining the longterm cycling performance even for the thick film. The energy density per unit area of the battery is calculated to be 667 mW hrcm2 from the cycling capacity Ž290mA hrcm2 . and the average cell voltage Ž2.3 V.. Fig. 5 shows the discharge and charge curves at the 20th cycle. The discharge and the charge capacities were 291 and 304 mA hrcm2 , respectively. The charge capacity was 1.04 times larger than the discharge capacity. This means that the battery exhibits a slight self-discharge, but this self-discharge is much smaller than that of our previously reported thin film battery w7x. Both batteries have the same structure and the same solid electrolyte film, therefore, we think the self-discharge to be the result of a slight internal short-circuit caused by dust-induced pinholes or defects in the electrolyte film. We examined the effect of discharge current density on discharge capacity. Fig. 6 shows the discharge curves for various discharge current densities, which are shown by the numbers in the figure. This figure makes the following points clear: Ž1. the discharge curves at 2 and 5 mArcm2 are almost the same, Ž2. when we increase the current density to 20 and 50 mArcm2 , the cell voltages of the discharge curves decrease, Ž3. the discharge capacity, at which the cell voltage decreases rapidly, becomes smaller
Fig. 6. Discharge curves for various discharge currents. Numbers show current density.
H. Ohtsuka, Y. Sakurair Solid State Ionics 144 (2001) 59–64
with increasing current density. As regards point Ž2., the decrease in the cell voltage in the discharge curves indicates the cell over-voltage, which is caused mainly by the internal impedance of the cell. The over-voltages of the cell for various current densities are shown in Table 2. The over-voltages at the beginning of discharge correspond to the ohmic-loss caused by the internal cell impedance and increase with current density. Point Ž3. shows that some parts of the cathode film remain unused owing to the slow lithium diffusion in the cathode film when the discharge current density is large. The relationship between discharge current density and discharge capacity is shown in Fig. 7 and Table 3. In this figure, as in Fig. 4, the left vertical axis is discharge capacity per unit area and the right vertical axis is discharge capacity per volume of 1 cm2 = 1 mm. This figure also shows data for our previously reported battery w7x, which had 1.00-mm thick cathode film. When the current density was 20 mArcm2 , the discharge capacity of the battery with the 1.00-mm thick cathode film was 60 mA hrcm2 Žfor unit area., or 60 mA hrcm2 mm Žfor volume.. In contrast, the discharge capacity of the thin film battery with 4.66-mm thick cathode film was 229 mA hrcm2 for unit area and 49 mA hrcm2 mm for a volume of 1 cm2 = 1 mm. The unit area discharge capacity of the latter battery is 3.8 times larger than that of the former battery, although the cathode film thickness of the latter is 4.66 times larger than that of the former. As regards the discharge capacity in relation to volume, that of the latter is 81.7% that of the former. This shows that, unlike the thin cathode film, the thick cathode film is only partly used when there is a large current discharge. When the current densities were 2, 5, 10, 20 and 50 mArcm2 , the
Table 2 Over-voltage for discharge current density Current density ŽmArcm2 .
Over-voltage at the beginning of discharge ŽV.
2 5 10 20 50
0.015 0.040 0.050 0.185 0.440
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Fig. 7. Relationship between current density and discharge capacity of thin film batteries.
discharge capacities of the battery with thick cathode film were 310, 312, 289, 229 and 135 mA hrcm2 , respectively. Above a current density of 10 mArcm2 , the discharge capacity of the battery fell rapidly, and the cathode utilization of the battery decreased. For batteries with porous cathodes and liquid electrolytes, the electrolytes permeate through the cathode pores, and the cathodes are wholly used in the reaction. In contrast, the solid electrolytes of thin film batteries cannot permeate cathodes and the cathodes are used from the electrolyte side to the opposite side in order, because thin film batteries have a layered structure consisting of cathodes, electrolytes and anodes. Therefore, cathode utilization decreases with increasing discharge current density owing to the lithium diffusion velocity in the cathode. As there is little difference between the discharge capacTable 3 Discharge capacity of thin film batteries Current density ŽmArcm2 .
2.0 5.0 10.0 20.0 50.0
Discharge capacity Discharge capacity with 4.66-mm cathode with 1.00-mm cathode ŽmA hr cm2 .
ŽmA hr cm2 mm.
310 312 289 229 135
67 67 62 49 29
ŽmA hr cm2 .
ŽmA hr cm2 mm.
60
60
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H. Ohtsuka, Y. Sakurair Solid State Ionics 144 (2001) 59–64
ities at a current density of 5 mArcm2 and at a current density of 2 mArcm2 , the discharge capacities are close to the maximum capacity.
ance and encouragement. They also thank the members of their group for helpful discussions.
4. Conclusion
References
To increase the cell capacity per unit area, we fabricated thin film batteries with fairly thick MoO 3y x cathode film. Our thin film battery with 4.66-mm thick cathode film showed an initial discharge capacity of 398 mA hrcm2 and a cycling discharge capacity of 290 mA hrcm2 per unit area, when the discharge current density was 10 mArcm2 . This value is the highest among those of reported thin film batteries. Furthermore, the battery showed good cycling performance. Increasing the cathode film thickness reduced the cathode utilization slightly at a high discharge current density, but the discharge capacity increased. Thin film batteries are expected to become micro-energy sources as a result of the advantages of thin film. However, many issues must be discussed before they can be used practically, including the use of wet-protective films, the optimum film thickness of each component film, and multi-layered batteries.
w1x K. Kanehori, K. Matsumoto, K. Miyauchi, T. Kudo, Solid State Ionics 9–10 Ž1983. 1445. w2x A. Levasseur, M. Menetrier, R. Dormoy, G. Meunier, Mater. Sci. Eng., B 3 Ž1989. 5. w3x J.B. Bates, G.R. Gruzalski, N.J. Dudney, C.F. Luck, X.-H. Yu, S.D. Jones, Solid State Technol. 36 Ž1993. 59. w4x J.B. Bates, N.J. Dudney, D.C. Lubben, G.R. Gruzalski, B.S. Kwak, X. Yu, R.A. Zuhr, J. Power Sources 54 Ž1995. 58. w5x B. Wang, J.B. Bates, F.X. Hart, B.C. Sales, R.A. Zuhr, J.D. Robertson, J. Electrochem. Soc. 143 Ž1996. 3203. w6x Y.S. Park, S.H. Lee, B.I. Lee, S.K. Joo, Electrochem. SolidState Lett. 2 Ž1999. 58. w7x H. Ohtsuka, J. Yamaki, Solid State Ionics 35 Ž1989. 201. w8x H. Ohtsuka, S. Okada, J. Yamaki, Solid State Ionics 40–41 Ž1990. 964. w9x H. Ohtsuka, T. Shodai, Y. Sakurai, Micromechatronics 43 Ž1999. 44 Žin Japanese.. w10x H. Ohtsuka, J. Yamaki, Jpn. J. Appl. Phys. 28 Ž1989. 2264. w11x T. Tsuchida, K. Shinoda, Y. Hino, H. Takayanagi, I. Yoshioka, Extended Abstracts 3rd Int. Meeting on Lithium Batteries, Kyoto, Japan, May 27–30, 1986, p. 293. w12x N. Margalit, J. Electrochem. Soc. 121 Ž1974. 1460. w13x T. Komura, S. Ichinose, K. Takahashi, Denki Kagaku 61 Ž1993. 1298 Žin Japanese.. w14x T. Tsumura, M. Inagaki, Solid State Ionics 104 Ž1997. 183. w15x G. Pistoia, C. Temperoni, P. Cignini, M. Icovi, S. Panero, J. Electroanal. Chem. 108 Ž1980. 169. w16x P. Fiordiponti, G. Pistoia, C. Temperoni, M. Icovi, S. Panero, J. Electroanal. Chem. 108 Ž1980. 181. w17x L. Kihlborg, Acta Chem. Scand. 13 Ž1959. 954.
Acknowledgements The authors express their gratitude to Dr. I. Yamada and Dr. K. Komatsu for their continuous guid-
Characteristics of LirMoO 3yx thin film batteries Hideaki Ohtsuka) , Yoji Sakurai NTT Telecommunications Energy Laboratories, Tokai-mura, Naka-gun, Ibaraki, 319-1193, Japan Received 14 December 2000; received in revised form 11 May 2001; accepted 20 May 2001
Abstract Solid-state lithium batteries fabricated by thin film technology are attracting attention as micro-batteries. The thin film battery structure means that they have a small discharge capacity, therefore, it is necessary to increase the cell capacity per unit area of the cathode film for practical use. We fabricated thin film batteries with fairly thick MoO 3yx cathode films, which had a larger specific capacity than other cathode films Že.g., LiMn 2 O4 film.. We then examined the cell performance of our thin film batteries. When the discharge current density was 10 mArcm2 , the batteries had discharge capacities of 290 mA hrcm2 per unit area, and this value is the highest among typical thin film batteries reported to date. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Solid-state battery; Thin film battery; Lithium battery; Solid electrolyte; RF sputtering; Magnetron sputtering; MoO 3
1. Introduction The demand for compact and high energy density batteries is constantly increasing with the miniaturization of microelectronics and advances in portable devices. Therefore, lithium and lithium-ion batteries have been studied and the latter are being produced commercially. All-solid-state batteries have attracted considerable attention, because of their potential for flexibility, safety and further miniaturization. There has been particular interest in solid-state lithium batteries fabricated with thin film technology w1–6x, because such solid-state devices with them can be miniaturized, and also similar dry processes can be used to fabricate these batteries.
) Corresponding author. Tel.: q81-29-287-7538; fax: q81-29287-7863. E-mail address: [email protected] ŽH. Ohtsuka..
We have already reported thin film lithium batteries with Li 2 O–V2 O5 –SiO 2 thin films as their solid electrolyte w7–9x. However, thin film batteries generally have a small discharge capacity due to their structure, namely, where the cathode and anode consist of a small quantity of thin film. Therefore, it is necessary to increase the cell capacity per unit area of cathode film for practical use. To achieve this, it is necessary to use cathode materials that have a high specific capacity or to make the cathode films thick. The cathode films of the thin film batteries reported to date are almost less than 1 mm thick. Therefore, it is possible to increase cell capacity by making the cathode film thicker. MoO 3y x cathode films have larger specific capacities than other cathode films Že.g., LiMn 2 O4 film.. We previously reported a LirMoO 3y x thin film battery, in which the MoO 3y x cathode film thickness was 1.0 mm w7x. To increase the cell capacity per unit area, we fabricated LirMoO 3y x thin film batter-
0167-2738r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 Ž 0 1 . 0 0 8 8 9 - X
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H. Ohtsuka, Y. Sakurair Solid State Ionics 144 (2001) 59–64
ies containing MoO 3y x cathode films that were thicker than 1 mm. In this paper, we report the cell performance of thin film batteries that employ MoO 3y x as their cathode active material.
2. Experimental procedure
Fig. 1. Rough sketch of thin film battery.
2.1. Cathode and solid electrolyte film fabrication We fabricated the MoO 3y x films using the conventional RF sputtering method, as previously reported w7x. We used RF sputtering equipment ŽModel: SBR-1104, ULVAC. for the fabrication. We prepared the target by cold pressing and sintering powdered MoO 3 ŽKanto Chemical reagent grade. at 720 8C for 8 h. After completing this process, we ground the product and formed it into a disk 100 mm in diameter and about 5 mm thick. The sputtering conditions are shown in Table 1. We deposited the MoO 3y x films in an Ar gas atmosphere at a sputtering gas pressure of 4.0 Pa, an RF power of 200 W and a substrate temperature of 100 8C. The deposition rate was 1.16 mmrh. We controlled the film thickness by regulating the sputtering time. We examined the crystalline structure of the sputtered films using an X-ray diffractometer ŽModel: Rad-rX, Rigaku. with graphite monochromatized Cu-K a radiation. We calibrated the diffraction angle using Si powder as a standard. We deposited Li 2 O–V2 O5 –SiO 2 thin films by the RF magnetron sputtering method. We have reported the thin film fabrication conditions in detail elsewhere w7,10x. For the fabrication, we used planar magnetron type RF sputtering equipment ŽModel: SPL-210H, ANELVA.. We established a constant 10-ccm flow of Ar and O 2 into the vacuum chamber to maintain a 1:1 ArrO 2 ratio at a constant pressure of 2.0 Pa. We deposited the Li 2 O–V2 O5 –SiO 2 films
Table 1 Sputtering conditions for MoO 3y x thin film Target Atmosphere Gas pressure RF power Substrate temperature
MoO 3 Ar 4.0 Pa 200 W 100 8C
in an Ar–O 2 gas atmosphere at an RF power of 100 W and with a substrate temperature of about 50 8C. The deposition rate was 167 nmrh. The ionic conductivity of the film was 1.0 = 10y6 Srcm and the electronic conductivity was less than 2 = 10y1 0 Srcm, as previously reported w7x. 2.2. Fabrication of Li r MoO3 y x thin film batteries and measurements We fabricated LirMoO 3y x thin film batteries by successively depositing MoO 3yx cathode film, Li 2 O–V2 O5 –SiO 2 solid electrolyte film and Li anode film on stainless steel substrates. We deposited the Li film on top of the Li 2 O–V2 O5 –SiO 2 film by the vacuum evaporation of lithium metal in a stainless steel crucible at a pressure of 4–8 = 10y4 Pa. We measured the thickness of the cathode and the electrolyte films by using a surface profile measuring system ŽModel: Dektak 3030, Veeco Sloan Tech. Div... We estimated the Li film thickness by using the thickness monitor of a quartz oscillator. Fig. 1 shows a rough sketch of the cell structure. The thickness of the cathode, the electrolyte and the anode films of a typical cell were 4.66, 1.0 and about 8 mm, respectively. The effective electrode area of the cell was 0.49 cm2 . It was limited by the smaller electrode area, which was the anode area in the cell. We placed the thin film batteries in sealed bottles containing desiccants and then cycled them at a constant current between 1.5 and 3.5 V, by using charge–discharge apparatus ŽHokuto-denko. operated under computer control. We examined the effect of discharge current density on discharge capacity from approximately the 20th to the 30th cycle in the cycling test. We changed the discharge current density to 2, 5, 10, 20 or 50 mArcm2 , but kept the charge current density a constant value of 10 mArcm2 in the current dependence test.
H. Ohtsuka, Y. Sakurair Solid State Ionics 144 (2001) 59–64
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3. Results and discussion Fig. 2 shows the X-ray diffraction pattern of MoO 3y x thin film deposited on a SiO 2 glass substrate. We observed only a few diffraction lines and the positions of these peaks coincide with those of the Ž010., Ž020. and Ž030. peaks of Mo 9 O 26 ŽJCPDS card No. 5-441, JCPDS: Joint Committee on Powder Diffraction Standards.. Therefore, we believe that the crystalline phase of the film was Mo 9 O 26 and that the crystallographic Ž010. plane of the film was oriented parallel to the substrate surface. The film was dark blue, which indicates that the film had been reduced and this is a reasonable assumption since we deposited the film in an Ar atmosphere. Fig. 3 shows the discharge curves of a typical cell at the 1st, 10th and 21st cycles with a discharge current density of 10 mArcm2 . The open circuit voltage ŽOCV. of the fabricated cell used for the figure was 2.7 V. In the first discharge curve, there are two gently sloping regions at 2.7–2.6 and at 2.0–1.7 V. The discharge curves after the 2nd cycle are S-shaped, as seen in the 10th and 21st cycles in this figure. These average voltages are 2.3 V. The discharge capacities of the 1st, 10th and 21st cycles were 398, 318 and 289 mA hrcm2 , respectively. Fig. 4 shows the relationship between the discharge capacity at 10 mArcm2 and the cycle number. The left vertical axis corresponds to discharge capacity per unit area and the right vertical axis indicates discharge capacity per volume of 1 cm2 = 1 mm. The discharge capacity decreased rapidly during
Fig. 2. X-ray diffraction pattern of MoO 3y x film.
Fig. 3. Discharge curves of LirMoO 3y x thin film battery. Cycle numbers are indicated.
the first 10 cycles, to about 80% of its initial discharge capacity. Then, it decreased slowly with further cycling. After the 30th cycle, the fading rate of the discharge capacity was less than 0.4% per cycle. At the 40th cycle, the discharge capacity per unit area and per 1 cm2 = 1 mm volume were 263 mA hrcm2 and 56.4 mA hrcm2 mm, respectively. This shows that the battery has good cycle performance. It is reported that LirMoO 3 batteries with liquid electrolytes have 1.5 electronsrmol of MoO 3 and the discharge capacity is 279 mA hrg, when cut off voltage is 1.5 V w11–13x. The discharge curves of the 2nd and subsequent cycles are different from that of the first cycle, and the capacities become smaller than that of the first cycle and are about 140 mA hrg w11,14x. As regards reduced Mo oxide, Mo 18 O52 w15x, non-aqueous lithium batteries with Mo 18 O52 as their cathode have 1.2–1.5 electronsr mol of Mo at the first discharge, but the discharge
Fig. 4. Discharge capacity change with cycle number.
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H. Ohtsuka, Y. Sakurair Solid State Ionics 144 (2001) 59–64
Fig. 5. Discharge and charge curves of LirMoO 3y x thin film battery at the 20th cycle.
capacities after the 2nd cycle become smaller than that of the initial cycle, as found with MoO 3 . The maximum recharge efficiency is 95% for a Mo 18 O52 cathode, in contrast to 75% for MoO 3 w15x. With Mo 8 O 23 , non-aqueous lithium batteries have 1.0 electronrmol of Mo and a recharge efficiency of above 75% is obtained at 0.75 electronrmol of Mo w16x. If we assume that the density of the MoO 3y x thin film is as large as that of Mo 9 O 26 and is 4.7 grcm3 w17x, the first discharge capacity per unit area, 398 mA hrcm2 , is equivalent to 182 mA hrg of the discharge capacity per unit weight. Similarly, the calculated 10th and 20th discharge capacities per unit weight are 145 and 133 mA hrg, respectively. The first discharge capacity of 398 mA hrcm2 Ž182 mA hrg. is equivalent to 0.98 electronrmol of Mo. The stable cycling capacity is 290 mA hrcm2 Ž133 mA hrg. and equivalent to 0.72 electronrmol of Mo. The first discharge capacity of 0.98 electronrMo is smaller than those of MoO 3 and Mo 18 O52 mentioned above. We think that the cause of the small first discharge capacity is the partial short-circuit that occurs accidentally during cell fabrication. However, the relationship between the discharge capacity and the cycle number of the thin film battery resembles those of LirMoO 3 or LirMo 18 O52 batteries with liquid electrolytes. The stable cycling capacity of this thin film battery is 290 mA hrcm2 Ž133 mA hrg. and as large as that of a LirMoO 3 battery with liquid electrolyte.
Therefore, the thin film battery has good cathode utilization over extended cycling. We believe that smooth lithium diffusion in the MoO 3y x film, as well as the high electronic conductivity of the film w3x, plays an important role in sustaining the longterm cycling performance even for the thick film. The energy density per unit area of the battery is calculated to be 667 mW hrcm2 from the cycling capacity Ž290mA hrcm2 . and the average cell voltage Ž2.3 V.. Fig. 5 shows the discharge and charge curves at the 20th cycle. The discharge and the charge capacities were 291 and 304 mA hrcm2 , respectively. The charge capacity was 1.04 times larger than the discharge capacity. This means that the battery exhibits a slight self-discharge, but this self-discharge is much smaller than that of our previously reported thin film battery w7x. Both batteries have the same structure and the same solid electrolyte film, therefore, we think the self-discharge to be the result of a slight internal short-circuit caused by dust-induced pinholes or defects in the electrolyte film. We examined the effect of discharge current density on discharge capacity. Fig. 6 shows the discharge curves for various discharge current densities, which are shown by the numbers in the figure. This figure makes the following points clear: Ž1. the discharge curves at 2 and 5 mArcm2 are almost the same, Ž2. when we increase the current density to 20 and 50 mArcm2 , the cell voltages of the discharge curves decrease, Ž3. the discharge capacity, at which the cell voltage decreases rapidly, becomes smaller
Fig. 6. Discharge curves for various discharge currents. Numbers show current density.
H. Ohtsuka, Y. Sakurair Solid State Ionics 144 (2001) 59–64
with increasing current density. As regards point Ž2., the decrease in the cell voltage in the discharge curves indicates the cell over-voltage, which is caused mainly by the internal impedance of the cell. The over-voltages of the cell for various current densities are shown in Table 2. The over-voltages at the beginning of discharge correspond to the ohmic-loss caused by the internal cell impedance and increase with current density. Point Ž3. shows that some parts of the cathode film remain unused owing to the slow lithium diffusion in the cathode film when the discharge current density is large. The relationship between discharge current density and discharge capacity is shown in Fig. 7 and Table 3. In this figure, as in Fig. 4, the left vertical axis is discharge capacity per unit area and the right vertical axis is discharge capacity per volume of 1 cm2 = 1 mm. This figure also shows data for our previously reported battery w7x, which had 1.00-mm thick cathode film. When the current density was 20 mArcm2 , the discharge capacity of the battery with the 1.00-mm thick cathode film was 60 mA hrcm2 Žfor unit area., or 60 mA hrcm2 mm Žfor volume.. In contrast, the discharge capacity of the thin film battery with 4.66-mm thick cathode film was 229 mA hrcm2 for unit area and 49 mA hrcm2 mm for a volume of 1 cm2 = 1 mm. The unit area discharge capacity of the latter battery is 3.8 times larger than that of the former battery, although the cathode film thickness of the latter is 4.66 times larger than that of the former. As regards the discharge capacity in relation to volume, that of the latter is 81.7% that of the former. This shows that, unlike the thin cathode film, the thick cathode film is only partly used when there is a large current discharge. When the current densities were 2, 5, 10, 20 and 50 mArcm2 , the
Table 2 Over-voltage for discharge current density Current density ŽmArcm2 .
Over-voltage at the beginning of discharge ŽV.
2 5 10 20 50
0.015 0.040 0.050 0.185 0.440
63
Fig. 7. Relationship between current density and discharge capacity of thin film batteries.
discharge capacities of the battery with thick cathode film were 310, 312, 289, 229 and 135 mA hrcm2 , respectively. Above a current density of 10 mArcm2 , the discharge capacity of the battery fell rapidly, and the cathode utilization of the battery decreased. For batteries with porous cathodes and liquid electrolytes, the electrolytes permeate through the cathode pores, and the cathodes are wholly used in the reaction. In contrast, the solid electrolytes of thin film batteries cannot permeate cathodes and the cathodes are used from the electrolyte side to the opposite side in order, because thin film batteries have a layered structure consisting of cathodes, electrolytes and anodes. Therefore, cathode utilization decreases with increasing discharge current density owing to the lithium diffusion velocity in the cathode. As there is little difference between the discharge capacTable 3 Discharge capacity of thin film batteries Current density ŽmArcm2 .
2.0 5.0 10.0 20.0 50.0
Discharge capacity Discharge capacity with 4.66-mm cathode with 1.00-mm cathode ŽmA hr cm2 .
ŽmA hr cm2 mm.
310 312 289 229 135
67 67 62 49 29
ŽmA hr cm2 .
ŽmA hr cm2 mm.
60
60
64
H. Ohtsuka, Y. Sakurair Solid State Ionics 144 (2001) 59–64
ities at a current density of 5 mArcm2 and at a current density of 2 mArcm2 , the discharge capacities are close to the maximum capacity.
ance and encouragement. They also thank the members of their group for helpful discussions.
4. Conclusion
References
To increase the cell capacity per unit area, we fabricated thin film batteries with fairly thick MoO 3y x cathode film. Our thin film battery with 4.66-mm thick cathode film showed an initial discharge capacity of 398 mA hrcm2 and a cycling discharge capacity of 290 mA hrcm2 per unit area, when the discharge current density was 10 mArcm2 . This value is the highest among those of reported thin film batteries. Furthermore, the battery showed good cycling performance. Increasing the cathode film thickness reduced the cathode utilization slightly at a high discharge current density, but the discharge capacity increased. Thin film batteries are expected to become micro-energy sources as a result of the advantages of thin film. However, many issues must be discussed before they can be used practically, including the use of wet-protective films, the optimum film thickness of each component film, and multi-layered batteries.
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Acknowledgements The authors express their gratitude to Dr. I. Yamada and Dr. K. Komatsu for their continuous guid-