Arm, Vol. 40. No. 13-14. pp. 2177-2184, 1995 Ekvier Science Ltd. Printed in Great Britain @x3-4686/95 59.50 + 0.00
AN OVERVIEW OF THE RESEARCH AND DEVELOPMENT OF SOLID POLYMER ELECTROLYTE BATTERIES KAZUO MURATA Central Laboratory, Yuasa Corporation 6-6, Josai-cho, Takatsuki, Osaka 569, Japan (Received 6 April 1995) Abstract-A review has been given on the history, status and prospect of solid polymer electrolyte and batteries using it. It includes the market trend for solid polymer electrolyte batteries, the background and future prospects of solid polymer electrolyte, and the introduction of film-like solid polymer electrolyte batteries that Yuasa has developed for electronic applications. The introduction includes the performance and safety features of the batteries from the standpoint of the application for electronic devices. Key words: solid polymer electrolyte, ion conductivity, SPE battery, film battery, safety feature
INTRODUCTION Several research institutes have been developing batteries with solid polymer electrolytes for more than 15 years since Professor Wright discovered the ionic conduction in polymers in 1975, and Dr. Armand proposed the use of an ionic conductive polymer in batteries in 1978. Research efforts have been growing each year, resulting in research projects such as the United States Advanced Battery Consortium in the United States, and the national program led by NED0 in Japan. The sodium-sulfur battery, which the Ford Motor Company invented in 1966, uses an inorganic solid electrolyte, and has been under development for more than 25 years. In Japan, a large market for this battery for utility load leveling should emerge in a few years. In contrast, the solid polymer electrolyte batteries are expected to appear in the market place within 20 years after their development started. There will be two major applications for solid polymer electrolyte batteries. One is electronics devices, which have become smaller and lighter each year, the other one is electric vehicles. This paper reviews the trend of the market in which the solid polymer electrolyte batteries are expected to be used, and the present status of the development of the solid polymer electrolyte battery technology, including the both primary and secondary solid polymer electrolyte batteries Yuasa has developed.
MARKET TREND IN ELECTRONIC DEVICES Electronic devices have become quite diversified, and their power consumption rate and weight has been decreasing considerably. In fact, the batteries represent a significant fraction of the weight in many
electronic devices today. Thus, there exists a need for making batteries smaller and lighter. In addition, battery shape factors are also becoming a concern since a rectangular battery is often preferable to conventional cylindrical ones due to their more efficient use of space. Solid polymer electrolyte batteries are potentially very flexible in terms of their form factor. It is anticipated that if the power consumption of electronic devices can be further reduced, then a film-like battery will be sufficient to power these devices in the near future. Such a film-like solid polymer electrolyte battery is quite promising from the standpoint of both the battery design and the manufacturing processes. Fig. 1 illustrates the trend of the shape of the existing batteries, together with the direction which Yuasa plans to follow.
The California legislation requiring 2% of the zero emission vehicles in 1998 has stimulated car manufacturers and battery manufacturers worldwide. Today electric vehicles cannot compete with the internal combustion engine vehicles on a cost performance basis. However, government mandate with an emphasis on reducing air pollution in major cities is providing the stimulus for electric vehicles to, at least partially, displace internal combustion engine vehicles. The success of this effort is heavily dependent on advances in battery technology. Also, the overall efficiency of the utilization of petroleum is calculated to be higher by approximately 8% when the petroleum is used to generate electricity at a power station, which is then used to power the electric vehicle, rather than using the petroleum directly for the internal combustion engine vehicle. When the electricity is available from a nuclear power station, the electric vehicle is ideal.
Fig. 1. Trend of shape of batteries.
The requirements that the electric vehicle places on the batteries are more stringent than the other applications. The depth-of-discharge is deep, the discharge rate is high, and battery packs containing from 100 to 200 batteries connected in series, will require both high performance and high reliability. Therefore, a technology that can produce each cell with very little performance variation and yield a battery which withstands overcharging and overdischarging will be required. At present, it is generally believed that the leadacid battery and/or the nickel-metal hydride battery will be used as the power source for electric vehicles in this century, and the lithium battery is expected to substitute them in the future. Although each battery
system has its own advantages and disadvantages, the lithium battery is superior in the energy density. As to the safety concern, the use of the solid polymer electrolyte over a liquid electrolyte in lithium batteries should improve its safety. The operating temperature of WC, which will be required for the solid polymer electrolyte battery, should not be a problem in electric vehicles.
FEATURES OF SOLID POLYMER ELECTROLYTE BATTERY
The history of battery development can be seen as the history of the material development. However,
Plasticized SPE System Interface lmpedanc
Dry SPE System
Fig. 2. Comparison
of cell impedance.
Solid polymer electrolyte batteries
35 30 25 20 15
Li + (SPE:PC=O:lOO) --------
Li + (SPE:PC=50:50)
Li + (SPE:PC=lOO:O)
Normalized Weight 1.OOmg Programming Rate: 70 “Cknin.
0 -5 100
Temperature Fig. 3. Thermal decomposition
the use of new materials does not necessarily mean a good battery. In this regard, the solid polymer electrolyte has several features that the conventional liquid electrolyte does not have. It is generally recognized that the mechanism of ionic conduction of the present organic solid electrolyte relies on the segmental movement of the polymer chain and the concentration of carrier ions.
of various electrolytes with Li.
Therefore, no convection can occur in dry polymer electrolytes. Since diffusion of ions is slow, concentration gradients of ionic species occur in dry solid polymer electrolyte during discharge, and these concentration gradients result in an increased cell impedance. Much research effort has been made in order to improve the ionic conductivity of the solid polymer
3 MnOr + (SPE:PC=O:lOO) ___-----
MnOz + (SPE:PC=lOO:O
Normalized Weight 1.OOmg Programming Rate: 10 “Urnin.
Fig. 4. ‘Thermal decomposition
of various electrolytes with
Positive Current Collector
Solid Polymer Electrolyte
\ Negative Current Collector
\ Lithium Anode
Fig. 5. Cross-sectional view of film-like battery.
electrolytes, but the best results to date are still far lower than that of the liquid electrolytes. Another source of concern in this technology is the electrode/electrolyte interfacial impedance, which is higher than the impedance of the electrolyte itself as shown in Fig. 2. Much effort is likely to be needed to reduce this interfacial impedance in order to reduce the total impedance of the battery with the solid polymer electrolyte. Some people claim that the solid polymer electrolyte battery is safe because it does not leak electrolyte. However, every battery is, in general, designed to prevent electrolyte leakage, so this does not appear to be a positive advantage with the solid polymer electrolyte battery. High temperature stability, on the other hand, is an advantage of the solid polymer electrolyte battery. Figures 3 and 4 show thermal decomposition data of solid polymer electrolytes and liquid electrolyte in the presence of either lithium or manganese dioxide in differential scanning calorimetry. The thermal stability of the solid polymer electrolytes over the liquid electrolyte is obvious.
cessibility of these materials is questionable at this time. Another noteworthy strategy is the development of a single-ion conductive solid polymer electrolyte to prevent non-uniform distribution of the ions. Another approach is incorporation of plasticizer to enhance the ionic conductivity. Yuasa has been employing this effective approach in the development of the solid polymer electrolyte battery.
PRESENT STATUS OF SOLID POLYMER ELECTROLYTE BA’ITERY Since the beginning of the 198Os, the research on the solid polymer electrolyte battery has primarily been carried out in Europe and North America. Toward the end of the 198Os, research programs in
PROGRESS IN DEVELOPMENT OF SOLID POLYMER ELECTROLYTE For the past 15 years or so, ionically conductive polymers, with an emphasis on ethylene oxide based materials, has been the focus of much effort. The primary focus of this work was to improve the ionic conductivity of these materials by increasing segmental mobility of the solvating polymer. Two of the most common strategies employed were the use of copolymers and the addition of side chains to reduce the temperature in glass transition. Obviously, the complexation of ions by the polymer reduces the segmental mobility of the polymer, which adversely affects the ionic conduction in the polymer salt complex. Thus, a tradeoff exists between ion content and segmental mobility. Recently, a solid polymer electrolyte based on a molten salt concept has been proposed in which the charge carrier content is high without the fear of a resultant higher temperature in glass transition. However, the pro-
10 ._ -’ 10 ’
Bobbir? Type A4 size _U___L__dL_J& +i 10 i’ 10 3 Energy
Fig. 6. Performance comparison of various primary batteries.
Solid polymer electrolyte batteries
60 s ‘C d ._ .z 3
Plasticized SPE Type (Card Size)
50 40 30
10 -3 Discharge Rate
Fig. 7. Discharge characteristics at various temperatures.
other parts of the world were underway. In the 199Os, Hydro Quebec of Canada and Yuasa formed a joint venture called ACEP, which possessesrelevant patent rights, and promotes the development of the solid polymer electrolyte battery. In this shared research effort, Yuasa has been developing small solid polymer electrolyte batteries for electronics applications, while Hydro Quebec has been concentrating on the development of large solid polymer electrolyte batteries. In both cases, Yuasa and Hydro Quebec will work together to market them.
Yuasa has been developing solid polymer electrolyte batteries primarily for IC cards such as an ID card, a smart card and a tag. Yuasa plans to supply a limited quantity of the primary battery to an original equipment manufacturer in August, 1994, and to begin mass production in 1995. Figure 5 shows the structure of the battery. Figure 6 compares the performance of Yuasa solid polymer electrolyte battery with that of other primary batteries. Figure 7 compares discharge characteristics of a solid polymer electrolyte battery with those of a liquid electrolyte
Table 1. Specifications of film-like primary battery
Volumetric energy density (wh/l)
Dropping cut cell into water
A metal lid bar with a diameter of 13 mm is placed on a cell, and then pressure is gradually applied using a hydraulic press
A cell is cut into halves using scissors, then is dropped into water.
No fire No explosion
No temperature rise observed
Table 2. Safety test results of film-like primary
At 500 kg/cm2 the cell voltage suddenly began to decrease and finally reached nearly 0 V at loo0 kg/cm2.
The half at the top of the water was observed to be bubbling gently. The other half of the bottom of the tank was also observed to be bubbling gently.
Fold down a cell
Drill a hole in cell with a Smm-diameter drill
x.:. :-_ .:.. : ::::.: ......... r-l
A cell is stapled with a stapler Observe change in cell voltage and temperature
Cell temperature was observed to rise onlv bv 0.6”C
The cell voltage immediately reached 0 V when fold down. The cell voltage recovered to nearly OCV in 1Omin when the cell had been unfolded.
The cell voltage immediately reached 0 V.
The cell voltage immediately reached OV due to shorting when the cell was staoled.
SPE Plasticized SPE Liquid Electrolyte
1000/T Fig. 8. Ionic conductivity of various electrolytes.
battery at various temperatures. Table 1 summarizes the specifications of Yuasa film-like solid polymer electrolyte primary battery. Various reliability tests and safety tests have been conducted to confirm the reliability and safety of the battery. Table 2 sum-
marizes the safety test results. In all the tests, the battery never exploded or caught fire. Also, when Yuasa solid polymer electrolyte battery and a conventional button-type lithium battery using liquid electrolyte were heated on a hot plate, the buttontype battery exploded at 8O”C, but the Yuasa solid polymer electrolyte battery was intact. Following the marketing of primary batteries, Yuasa plans to market secondary batteries as well. Figure 8 compares the ionic conductivity of solid polymer electrolytes with that of liquid electrolyte. For the secondary battery for electronics applications, Yuasa uses plasticized solid polymer electrolyte. Yuasa is also working on non-plasticized solid polymer electrolyte, and is now developing a secondary battery for load leveling use in homes in the future under NEDO’s lo-year research program.
CONCLUSION The solid polymer electrolyte battery has been under development since the beginning of the 198Os, and is expected to find a market in the latter half of the 1990s. The market will probably start with electronics applications, and is expected to be followed by electric vehicles application in the 21st century.
REFERENCES 1. P. V. Wright, Br. Polym. J. 7, 319 (1975). 2. M. B. Armand, Annu. Reo. Mater. Sci. 16,245 (1986). 3. H. Onishi and T. Shimoi, NRC Report (in Japanese), 84 (1993). 4. C. A. Angell, C. Liu and E. Sanches, Nature 362, 137 (1993). 5. S. Kate, Y. Yoshihisa, K. Takeuchi and K. Murata, Power Sources 13,409 (1991).