An overview of the research and development of solid polymer electrolyte batteries

An overview of the research and development of solid polymer electrolyte batteries

Electrochimica Acta 45 (2000) 1501 – 1508 www.elsevier.nl/locate/electacta An overview of the research and development of solid polymer electrolyte b...

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Electrochimica Acta 45 (2000) 1501 – 1508 www.elsevier.nl/locate/electacta

An overview of the research and development of solid polymer electrolyte batteries Kazuo Murata *, Shuichi Izuchi, Youetsu Yoshihisa Yuasa Corporation, 6 -6, Josai-cho, Takatsuki, Osaka 569 -0065, Japan Received 1 October 1998; received in revised form 29 March 1999

Abstract The research and development of solid polymer electrolyte (SPE) began when Wright found ion conductivity in a PEO-alkaline metal ion complex in 1975. The conductivity then was 1 × 10 − 7 S cm − 1 at room temperature. A lithium polymer battery has features such as flexibility in the shape of a cell design, leak proof of electrolyte, high safety, etc., but poses the challenge of how close its electrical performance can be made to that of a liquid electrolyte cell. Therefore, various efforts have so far been made to improve the ionic conductivity of the SPE. Recently, such efforts have also included the development of gelled SPE and porous SPE, especially in consideration of its practical application, in particular the use at low temperature. The ionic conductivity of such SPEs now reaches 1 ×10 − 3 S cm − 1 at room temperature. This paper reviews the history and the present status of the research and development of SPE and the lithium polymer battery, and presents an outlook of the future research and development activities. The paper also introduces the history of the improvement of primary and lithium polymer secondary batteries using SPE at the Yuasa Corporation, with the performance and the applications of its present commercial products, and presents their future outlook. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Solid polymer electrolyte (SPE); Lithium battery; Dry SPE; Gelled SPE; Porous SPE

1. Introduction Since Wright [1] discovered ionic conductivity in a PEO/Na+ complex in 1975, the research and development effort has become quite active on solid polymer electrolyte (SPE), in particular for improvement of the ionic conductivity. Ever since Armand [2] proposed the application of SPE to lithium batteries, the research and development effort has been made throughout the world, the United States, Japan and Europe in particular. In these countries, such work is quite active in national projects such as USABC in the USA, NEDO in Japan and JOULE in Europe. * Corresponding author.

Recently, batteries have been especially required for superior electrical performance in response to the need for the miniature power supply for consumer applications, so the gelled SPE and the porous SPE are expected to play a major role. The work in this field is therefore very popular. Also, lithium batteries using the gelled SPE are being developed enthusiastically, and among them Yuasa has already commercialized a primary type and is now working toward commercialization of the secondary version also.

2. The history of development of SPE SPE can be classified into the following three types: dry SPE, gelled SPE and porous SPE. Fig. 1 shows the history of development of various types of SPE and

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Fig. 1. History of SPE development.

Fig. 2. History of ionic conductivity’s improvement.

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Fig. 3. Gelled SPE models.

Fig. 2 shows how their ionic conductivity has improved.

2.1. The history of de6elopment of the dry SPE There are two means to increase the ionic conductivity of the dry SPE: (I) suppression of crystallization of polymer chains to improve polymer chain mobility; (II) increase in the carrier concentration. The suppression of crystallization of polymer chains to improve polymer chain mobility can be realized by (i) cross-linking; (ii) co-polymerization; (iii) comb formation (side chains and dendritic polymers); (iv) polymer alloy (including IPN: Inter Penetrating Network), and (v) inorganic filler blend. Among these, combinations of (i) crosslinking; (ii) co-polymerization, and (iii) comb formation are useful for obtaining a high ionic conductivity. In the combination of (i) cross-linking and (ii) co-polymerization, Cheradame et al. [3] in 1984 proved an ionic conductivity of about 5 ×10 − 5 S cm − 1 at 25°C by cross-linking block co-polymers of EO and PO, and in the comb formation, Hall et al. [4] in 1986 proved 2 ×10 − 4 S cm − 1 at 25°C by adding PEO side chains to polysyloxane chains, and Watanabe et al. [5] in 1993 extended this method by synthesizing a dendritic polymer by attaching PEO chains to glycidyl ether side chains. The increase of the carrier concentration can be realized by (i) use of highly dissociable salts, and (ii) increase in salt concentration. The highly dissociable salts involves one with anions not fixed in the polymer, and the other with anions fixed in the polymer (single

ion conductor). For the one with anions not fixed in the polymer, salts having a small lattice energy, large anions and plasticity to the polymer have been studied, and Vallee et al. [6] in 1992 proved 4 × 10 − 5 S cm − 1 at 25°C with a Li trifluorosulfonyl imide/PEO system, drastically improving from 1 × 10 − 7 S cm − 1 at 25°C with a NaI/PEO system in 1975. For the one with anions fixed in the polymer (single ion conductor), Kobayash et al. [7] in 1985 proved 1× 10 − 7 S cm − 1 at 25°C with a carboxylate system, Bendabah et al. [8] in 1995 proved 6 × 10 − 7 S cm − 1 at 25°C with a sulfonate system, and Fujinami et al. [9] in 1997 proved 2 ×10 − 5 S cm − 1 at 25°C with a siloxy aluminate system. The increase in salt concentration involves the polymer-insalt, which solidifies molten salts with certain polymers. In this connection, Angell et al. and Watanabe et al. announced 1 ×10 − 3 S cm − 1 at 25°C in 1993 [10,11].

2.2. The history of de6elopment of the gelled SPE The gelled SPE has been developed along the following two concepts: (I) the gelled SPE by physical crosslinking, and (II) the gelled SPE by chemical cross-linking. For the physical cross-linking gelled SPE, Feuillade et al. [12] in 1975 began research with a PAN system, and later, Tsuchida et al. [13] in 1983 proved 1× 10 − 3 S cm − 1 at 25°C with a PVDF system, and Iijima et al. [14] in 1985 proved 1 × 10 − 3 S cm − 1 at 25°C with a PMMA system. For the chemical crosslinking gelled SPE, Feuillade et al. [12] in 1975 began research with a P(VDF-HFP) cross-linking system, and

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Morita et al. [15] in 1990 proved 1 ×10 − 3 S cm − 1 at 25°C with a PEO system. Fig. 3 shows a model of the two types of the gelled SPE. The chemical cross-linking gelled SPE seems thermally stable over time, but the physical cross-linking one tends to swell and dissolve with heat (DH), and leaks solution from the structure with time (Dt).

2.3. The history of de6elopment of the porous SPE For the porous SPE Gozdz et al. [16] in 1995 proved 1× 10 − 3 S cm − 1 at 25°C with a P(VDF-HFP) system. In the future, a micro-phase separation type SPE which lies between the chemical cross-linking gelled SPE and the porous SPE could also be developed.

3. The history of development organizations of lithium polymer batteries The application of dry SPE to a lithium battery started when the announcement by Armand, Domain University, was made in 1978 [2], and then ANVAR (France), Elf V Acquitaine (France) and Hydro Quebec (Canada) studied in a joint program and announced the development of a 3 V secondary battery in 1985 [17]. Later Hydro Quebec acquired the intellectual properties jointly owned by the three organizations, and in 1990 transferred these properties to a company called ACEP (Canada), a joint venture company between Hydro Quebec and Yuasa Corporation to administer such properties. Since then, Hydro Quebec and Yuasa have been jointly developing lithium polymer batteries under the partnership. In addition to this, Hydro Quebec was involved in a joint development program with 3M (USA) to develop a lithium polymer battery for electric vehicles under the USABC project. In this program, a lithium polymer battery of 119 Ah was developed in 1998 [18]. Also, the Yuasa Corporation is developing a lithium polymer battery under the NEDO project for

Fig. 4. Energy density comparison (including package).

distributed energy storage use. In Europe, a lithium polymer battery is being developed under the JOULE project. The application of the gelled SPE to a lithium secondary battery started when Feuillade et al. [12] in 1975 applied a physical cross-linking gelled SPE of a PAN system to a lithium secondary battery. In 1990, Mead (USA) announced the development of a lithium secondary battery using the chemical cross-linking gelled SPE of a PEO system [19]. After succeeding this work, Valence (USA) is now developing a lithium polymer battery. In Japan, Yuasa commercialized a primary lithium polymer battery in 1996, and is now working toward commercialization of a secondary version also. Since the the gelled SPE by chemical cross-linling has the following three features it is suitable for battery use; (i) high electrolyte liquid retention; (ii) high productivity, and (iii) high mechanical strength. In 1996 Bellcore (USA) announced the development of a lithium secondary battery using the porous SPE of a P(VDF-HFP) system [20]. Much effort has been made to commercialize this battery, but to date the effort has not been as successful as expected. Along with the effort to increase the ionic conductivity of the SPE, work is also being carried out to decrease the impedance at the interface between the SPE and the active material, in order to improve the battery performance.

4. Development of lithium polymer batteries in Yuasa In Yuasa, both primary and secondary lithium polymer batteries have been developed primarily for consumer electronics devices. The primary battery is of Li/gelled SPE/MnO2, and the secondary one is of C/ gelled SPE/LiCoO2.

4.1. Primary battery In the development of the primary lithium polymer batteries, efforts were primarily concentrated on an increase in the energy density and an improvement of the high-rate discharge performance by decreasing the internal impedance during 1994 and 1998. The increase in the energy density has been accomplished by an increase in the filling density of the positive active material, a decrease in the thickness of a lithium foil, and a decrease in the sealing space. The successful decrease in the sealing space has also enabled Yuasa to design, in 1998, a very small battery with a volume of only 0.2 ml which is 1/5 that of the smallest battery developed in 1994. As a result, the Yuasa primary lithium polymer battery shows an energy density comparable with that of a coin-type lithium primary battery with liquid electrolyte as shown in Fig. 4, in spite of the

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Fig. 5. Discharge characteristics comparison at various temperatures.

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Fig. 5 compares the Yuasa lithium polymer primary battery, model CS3603 (nominal capacity: 25 mAh, size: 22 ×29×0.3 mm), with a coin-type battery (CR1616: 50 mAh) with liquid electrolyte in performance at various discharge rates and temperatures. The superiority of Yuasa lithium polymer battery is particularly obvious in high-rate discharge at low temperature. This is due to the fact that the Yuasa lithium polymer battery has a larger electrode surface because of its extreme thinness, and the reduced interfacial impedance between the positive electrode and the SPE, because the positive active material was mixed with conductive additives and gelled SPE before assembling into a battery. Thus, with an optimum positive active material composition coupled with the advantage of easy film forming inherent with the lithium polymer battery, a thin lithium polymer primary battery with good performance has become possible. Finally, Fig. 6 compares the Yuasa lithium polymer primary battery with a coin-type battery with liquid electrolyte in capacity retention during storage at 60°C. The Yuasa lithium polymer battery shows better capacity retention. In battery production, efficient production is now possible using a reliable sealing technology and a printing technology.

4.2. Secondary battery

Fig. 6. Self discharge characteristics comparison.

Fig. 7. Cycle life characteristics comparison.

fact that the Yuasa primary lithium polymer battery has a thickness of only 0.3 mm. This thickness is applicable to the IC card of the ISO standard (0.75 mm thick). The improvement of the high-rate discharge performance was accomplished by a reduction in the interfacial impedance between the SPE and the positive active material. This impedance has been reduced by 1998 to half of that in 1994.

Unlike the conventional lithium-ion secondary battery with liquid electrolyte, the lithium polymer secondary battery has no free electrolyte, so it can be assembled into a plastic film package, rather than a metal hard case. This is quite advantageous for designing a thin battery with a large surface area. Yuasa has developed two models of lithium polymer secondary batteries: 3.6 V – 400 mAh and 54 × 75×2.2 mm in size, and 3.6 V – 1750 mAh and 147 ×206×1.2 mm in size. An improvement of the charge/discharge cycle life and an increase in the energy density were the main challenges in the development. The improvement of the charge/discharge cycle life was accomplished by improving the composition and production process of both the positive and negative electrodes, together with the improvement of the electrolyte composition. By improving the conductive additives in the positive electrode and the composition of SPE contained in the positive and negative electrodes, the 1998 life cycle performance has drastically improved since that of 1994, as shown in Fig. 7, which is favorably compared with that of a liquid electrolyte type lithium-ion battery. The increase in the energy density was accomplished by an increase in the filling density of active material through an improved manufacturing process for the electrodes, to obtain an energy density of 180 Wh/l, which is, however, still lower when compared with 220 – 260 Wh/1 of a prismatic liquid electrolyte type

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the liquid electrolyte type lithium-ion battery, nothing happened during these tests with the polymer battery, except bulging in over-charge testing.

5. Summary of the status and future outlook

5.1. SPE

Fig. 8. Discharge rate characteristics comparison at 25°C.

lithium-ion battery. With further improvements in the filling density of the active materials, coupled with the use of a graphite negative electrode having a flat discharge voltage, the energy density is expected to reach one comparable with that of the liquid electrolyte type battery. Another issue is the need for an improvement of discharge performance at a high rate. Fig. 8 compares discharge performance of the Yuasa lithium-ion polymer battery with that of a corresponding liquid electrolyte type battery having the same non-graphitized carbon negative electrode as that of the Yuasa lithiumion polymer battery. The Yuasa lithium-ion polymer battery shows somewhat lower performance at a 1 h-rate discharge. This is due to the fact that the interfacial impedance between the electrode and the electrolyte is still higher with the polymer battery. Further effort will be made to reduce this interfacial impedance to improve the discharge performance of the polymer battery at a high-rate discharge. A major merit of the lithium polymer battery is its inherent safety over the liquid electrolyte type lithiumion battery. Table 1 summarizes safety test results of the Yuasa lithium polymer secondary battery. Unlike

The improvement of the performance of the dry SPE has been carried out along the two approaches: (I) enhancement of the mobility of the polymer chains, and (II) an increase in the carrier density. With the combination of the above (I) and (II), the ionic conductivity at room temperature is now about 1× 10 − 4 S cm − 1, while with the polymer-in-salt based on the above (II), it is now about 1× 10 − 3 S cm − 1, and with the singleion conductor approach, it is about 1 ×10 − 6 S cm − 1. In considering the use of a battery at a lower temperature, 1× 10 − 3 S cm − 1 is probably needed for SPE, except for the single-ion conductor type, which probably requires 1×10 − 4 S cm − 1. Although the polymerin-salt now shows 1 × 10 − 3 S cm − 1 at room temperature, it tends to crystallize at lower temperature, which prevents it from practical use. Therefore, the future trend could be forecast as follows. 1. A high mobility polymer. 1.1 Increasing the mobility of the interpenetrated polymer in IPN. 1.2 Increasing the mobility of the polymers in the dendritic polymer system. 2. A single ion conductor with increasing number of dissociating lithium ions. 3. The polymer-in-salt polymer with molten salt. 3.1 Preventing crystallization below −10°C. 3.2 Molten salt with a wide stability window. Recently, studies on the porous SPE having similar performance as liquid electrolyte are active. Consider-

Table 1 Safety test results of Yuasa gel type secondary battery and liquid electrolyte batterya Test items

Lithium ion polymer secondary battery (size: 54×75×2.2 mm, 400 mAh)

Lithium ion battery (liquid) (Yuasa prismatic) (size: 23×49×8 mm, 600 mAh)

Nail prick test

Nothing happened (temp. rise within 20°C) Nothing happened

Explosion, smoke, electrolyte leakage (temp. rise about 250°C) Explosion, electrolyte leakage

Nothing happened (temp. rise within 20°C) Bulged to 6 mm (temp. rise within 20°C)

Electrolyte leakage (temp. rise about 100°C) Explosion, electrolyte leakage (temp. rise about 100°C)

Heating test with hot plate (200°C) see notes (2) Short circuit test Abnormal charge test (600%)

a

Notes: (1) all were bare cells without the protection circuit; (2) cells were sandwiched between a preheated (200°C) hot plate and metal plate.

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ing leak-proof and safety, however, SPE should preferentially have a homogeneous macro-structure, which physically prevents separation of the matrix polymer and the electrolyte. For leak-proof in particular, retention of sufficient osmosis pressure is important. In designing a gelled SPE with a good ionic conductivity, taking the above into consideration, a structure with a large domain (a matrix with a long molecular chain length and coarse cross-linking) composed of polymers having less interaction with lithium ion, or a micro-phase separation type, is favorable. In addition, a gelled structure composed of the matrix polymer which has affinity with anions and solvents but is repellent to lithium ions might bring about an ionic conductivity comparable or superior to that of liquid electrolyte, if successful.

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provement of the electrolyte composition to increase the wettability of the active material and the electrolyte; (ii) increase of the active material surface area; (iii) prevention of the formation of reaction products at the interface, and (iv) elastic gelled SPE to cope with expansion and contraction of active material. The improvement of the electrolyte composition for better wettability may include addition of a low molecular-weight polymer to the dry SPE, and increase of the ratio of solution for the gelled SPE. In addition, good electrical contact between active material particles, and between the active material and the current collector is also important for good battery performance.

5.2.3. Stable performance (with respect to the secondary battery)

5.2. Battery with SPE The features of the lithium polymer battery include high safety, leak-proof and flexibility in designing (thin, large area, shape, etc.). The success of commercial products lies in whether a commercial battery can be designed with acceptable performance including a high energy density, especially at high-rate discharge at low temperature, and stable performance as represented by good recovery of the capacity after standing for a long time, maintaining the features mentioned above.

5.2.1. Energy density An increase in the energy density can possibly be realized in the following three approaches: (i) increasing the filling density of active material; (ii) use of high-capacity active material, and (iii) high packaging density. (i) Increasing the filling density of active material may include the application of a compression filling process. (ii) Use of high-capacity active material may include the use of polysulfides positive active material and metallic lithium negative active material. A method to prevent diffusion of depolymerized active material in the polymerization/depolymerization type active material such as polysulfide and dendrite formation at the metallic lithium electrode is essential to realize the long life of a battery using such active materials. (iii) High packaging density requires minimization of the package space including the sealing part, taking advantage of inherent safety and leakproof property of the polymer battery. 5.2.2. High-rate discharge performance at low temperature The reduction in the interfacial impedance between the active material and the electrolyte is paramount. For this, perhaps there will be four approaches: (i) modification of the active material surface and im-

5.2.3.1. Capacity reco6ery after standing. A battery is required to recover its full capacity when recharged even after standing for a long time. At present, however, some capacity is lost even when the battery is charged after standing for a long time, both in the polymer battery and the liquid electrolyte battery. Clarification of the mechanism and its solution are essential. 5.2.3.2. Decrease of self discharge. Both the polymer battery and the liquid electrolyte battery now show a relatively large self discharge at high temperatures such as 60°C. Improvement is necessary. 5.2.3.3. Cyclic life performance. The present lithium polymer secondary battery using non-graphitized carbon as the negative electrode shows a cyclic life performance comparable to that of the liquid electrolyte type. For the future, the life of the battery using graphite as the negative active material should also be improved.

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