Polymer solid electrolyte and a polymer solid electrolyte battery

Abstract: A polymer solid electrolyte comprises a crosslinked polyether in which a salt is dissolved, and which has a crosslinked structure of the formula: where R stands for an inert group other than a hydrogen atom. It is suitable as an electrolyte and a separator for a battery, especially a lithium battery which can be recharged. Batteries including such electrolyte are also disclosed.

Description: This invention relates to a polymer solid electrolyte comprising a particular cross-linked polyether and a battery made by employing such a polymer solid electrolyte. A great deal of research work has been performed, and is under way, for exploring the possibility of using polymer solid electrolytes in batteries, displays, etc. Referring by way of example to the use of a polymer solid electrolyte in a lithium battery, it is interposed between a positive electrode and a negative electrode. There have been proposed a variety of types of materials for polymer solid electrolytes, including various polyethylene oxides, polyphosphazenes and polyamino acids. Solid electrolyte batteries have a number of advantages. For example, they are leakage-free, have a high energy density, are inexpensive, and are easy to form in a layered structure. The materials which have so far been tested more extensively than any others are certain linear polyethers. The materials, however, have a high melting point (about 60°C) and can be used only at a temperature above their melting point, since at a temperature below their melting point, the molten compound undergoes crystallization and fails to exhibit a satisfactorily-high ionic conductivity. Various crosslinked polyethers have been proposed to replace the linear polyethers. These materials have a low melting point. Even at or around room temperature, therefore, the molten salt hardly undergoes any crystallization and shows a relatively good ionic conductivity. Trifunctional polyethers which are crosslinked with diisocyanates, such as tolylene 2,4-diisocyanate and hexamethylene diisocyanate, are, among others, suitable for use on an industrial basis. They are easy to produce and thin films of them are easily formed. They are expected to be particularly useful as electrolytes for rechargeable batteries having lithium anodes. A number of problems, however, arise from the use of any such crosslinked material in a common type of battery in which an intercalation type metal compound is used as a cathode, and lithium as an anode. The first problem resides in a reduction of performance which occurs during storage of the battery. The active hydrogen in the polymer solid electrolyte reacts with lithium during storage, and thereby raises its internal resistance resulting in a lowering of its capacity. In this context, "active hydrogen" means hydrogen of an -OH or -NH group, which reacts directly with an alkali or alkaline earth metal, or its ions. If a polyether crosslinked with isocyanate is used as the electrolyte, the "active hydrogen" is the hydrogen in the urethane bond (NHC00). The second problem resides in the poor charging and discharging repeatability of the battery. This is apparently due to the fact that they polymer solid electrolyte is a bi-ionic conductor. If a common type of battery as hereinabove mentioned is discharged, anions (lithium ions) move from the anode to the cathode and lithium is intercalated in the cathode. The anions in the polymer electrolyte (e.g. perchloric acid ions in a battery containing molten lithium perchlorate) move toward the anode. However, the anode does not accept the anions which therefore cause polarization in the electrolyte layer in the vicinity of the anode. Then, if the battery is charged, the cations which have been intercalated in the cathode can reversibly move to the anode through the electrolyte, but the anions which have polarized in the vicinity of the anode can return to their place of origin only very slowly. As a result, the ions in the polymer have a low degree of reversibility leading to a great reduction in battery capacity on repeated charging and discharging. Attempts have, therefore, been made to synthesize a single-ionic mobile polymer solid electrolyte in which only the cations are mobile, while the anions are fixed to the polymer chain. They have, however, been unsuccessful because of a great reduction in ionic conductivity, or for other reasons. It is an object of this invention to provide a polymer solid electrolyte which can be stored for a long time without lowering its quality, and a battery which includes a polymer solid electrolyte and can maintain its proper performance over a long period. It is another object of this invention to provide a polymer solid electrolyte battery which permits repeated charging and discharging without suffering from any substantial reduction in capacity. According to the invention we therefore provide a polymer solid electrolyte comprising a cross-linked polyether, of which the polyether has a structure of the formula: where R stands for an inert group other than a hydrogen atom and M stands for an element of Group I of the periodic table other than a hydrogen atom. We also provide a battery including such an electrolyte. The polyether may be cross-linked with a urethane bond. An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: Fig. 1 is a partly cutaway view of a test cell used for testing a polymer solid electrolyte embodying this invention; Fig. 2 is a graph comparing batteries embodying this invention and known in the art with respect to their capacity varying during storage; and Fig. 3 is a graph comparing the results of charging and discharging cycle life tests conducted on the batteries embodying this invention and known in the art. In Fig. 1, 1 is a 0.1mm thick layer of lithium, 2 is a teflon ring, 3 is a battery test sheet having a lower polymer portion and un upper portion formed from an active substance defining an anode, 4 is a titanium current collector, 5 is a teflon washer for thickness adjustment, 6 is a hermetic seal, 7 is an anode current collector, 8 is a cell, and 9 is a cell cover. Although the active hydrogen in a polymer solid electrolyte lowers the performance of the battery during its storage, it is possible to obtain a polymer solid electrolyte having a long storage life if the active hydrogen is replaced. Various methods can be employed for making a sheet of polymer solid electrolyte according to this invention. According to one method, one surface of a sheet of polyether crosslinked with isocyanate is coated with a reactant solution, so that the substitution of lithium may occur at only one surface thereof. According to another method, the whole sheet is dipped in a reactant solution so that the substitution of lithium may occur at both surfaces of the sheet, and a hydrogen atom, or alkyl group is, then, substituted for the lithium at one of its surfaces. According to yet another method, the substitution of lithium is caused to occur to both surfaces of the sheet and a polyether crosslinked with isocyanate is then formed on one of its surfaces. These methods are merely illustrative of a variety of methods which can be employed for making a sheet of an electrolyte according to this invention having one surface at which the substitution of lithium occurs, while no such substitution occurs at the other surface thereof. The invention will now be described in further detail with reference to several examples thereof. In the following examples, (of which examples 5 and 6 are outside the scope of this invention), the working atmosphere was composed of dry air having a relative humidity not exceeding - 50%, and the polyethers were of the triol type containing an ethylene oxide structure and a propylene oxide structure in a structural ratio of 8:2 and having a molecular weight of about 3000, unless otherwise noted. Prior to use, they were dried at a temperature of 100°C in a vacuum, while being stirred by a magnetic stirrer in a three necked flask. A solution of lithium perchlorate and polyether was prepared as will hereinafter be described. The polyether to be dissolved was added to an acetone solution containing one part by weight of lithium perchlorate for 10 parts by weight of polyether, and the solution was heated at a temperature of 100°C in a vacuum, while being stirred by a magnetic stirrer in a three-necked flask, until no distillate formed. A paste of an active material forming the cathode was prepared as will hereinafter be described. Eleven parts of a solution of lithium perchlorate and polyether, two parts of dimethylacetamide, an equivalent amount of hexamethylene diisocyanate and 0.006 part of di-n-butyltin diacetate were fully mixed to form a first mixture. Nine parts of a vanadium oxide powder and one part of acetylene black were fully mixed and their mixture was dried. The latter mixture was added to the first mixture and they were fully kneaded together to form a paste. The paste was used immediately after its preparation. EXAMPLE 1 Five parts of a dry polyether, one part of dimethylacetamide, an equivalent amount of hexamethylene diisocyanate and 0.003 part of di-n-butyltin diacetate were fully mixed together. The mixture was coated on a 50 nm (micron) nonwoven fabric of polypropylene and it was left to stand at a temperature of 80°C for 16 hours in an argon gas atmosphere to form a sheet. The sheet was dipped in a 0.1 mol/l n-hexane solution of n-butyl lithium in an argon gas atmosphere and was left therein for three hours. Then, it was transferred into a 0.02 wt.% n-hexane solution of methyl iodide and after one hour of immersion, it was washed with n-hexane and the n-hexane was thereafter removed by volatilization. Then, the sheet was dipped in a 0.04 mol/l acetone solution of lithium perchlorate in a totally closed vessel and was left therein for 20 minutes, whereby lithium perchlorate was dissolved in the sheet. Then, the paste of the active material forming the cathode,which had been prepared as hereinabove described, was coated on the upper surface of the sheet. The sheet was left to stand at a temperature of 80°C for 16 hours in an argon gas atmosphere and was, then, dried at the same temperature for 48 hours in a vacuum. Then, the sheet was incorporated into a battery test cell, which will hereinafter be referred to as "Cell I". EXAMPLE 2 Two parts of dimethylacetamide, an equivalent amount of hexamethylene diisocyanate, 0.006 part of di-n-butyltin diacetate and two parts of methyl iodide were fully mixed in 11 parts of a solution of lithium perchlorate and polyether. The mixture was coated on a 50 nm (micron) nonwoven fabric of polypropylene and it was left to stand at a temperature of 80°C for 16 hours in an argon gas atmosphere to form a sheet. The paste as hereinabove described was coated on the upper surface of the sheet. The sheet was left at a temperature of 80°C for 16 hours in an argon gas atmosphere and was dried. Then, it was incorporated into a battery test cell, which will hereinafter be referred to as "Cell II". EXAMPLE 3 Five parts of a dry polyether, one part of dimethylacetamide, an equivalent amount of hexamethylene diisocyanate and 0.003 part of di-n-butyltin diacetate were fully mixed together. The mixture was coated on a 50 nm (micron) nonwoven fabric of polypropylene and it was left at a temperature of 80°C for 16 hours in an argon gas atmosphere to form a sheet. The sheet was dipped in a 0.1 mol/l n-hexane solution of n-butyl lithium in an argon gas atmosphere and was left therein for three hours. Then, it was washed with n-hexane and the n-hexane was removed by volatilization. Then, the sheet was tippet in a 0.04 mol/l acetone solution of lithium perchlorate in a totally closed vessel and was left therein for 20 minutes, whereby lithium perchlorate was dissolved in the sheet. The paste as hereinabove described was coated on the upper surface of the sheet. The sheet was left to stand at a temperature of 80°C for 16 hours in an argon gas atmosphere and was dried at the same temperature or 48 hours in a vacuum. It was incorporated into a battery test cell, which will hereinafter be referred to as "Cell III". EXAMPLE 4 Five parts of a dry polyether, one part of dimethylacetamide, an equivalent amount of hexamethylene diisocyanate and 0.003 part of di-n-butyltin diacetate were fully mixed together. The mixture was coated on a 50 nm (micron) nonwoven fabric of polypropylene and it was left to stand at a temperature of 80°C for 16 hours in an argon gas atmosphere to form a sheet. The sheet was dipped in a 0.1 mol/l n-hexane solution of n-butyl lithium in an argon gas atmosphere and was left therein for three hours. Then, it was washed with n-hexane and the n-hexane was removed by volatilization. The sheet was, then, dipped in a 0.04 mol/l acetone solution of lithium perchlorate in a totally closed vessel and was left therein for 20 minutes, whereby lithium perchlorate was dissolved in the sheet. Two parts of dimethylacetamide, an equivalent amount of hexamethylene diisocyanate and 0.006 part of di-n-butyltin diacetate were fully mixed in 11 parts of a solution of lithium perchlorate and polyether. The mixture was applied to the upper surface of the sheet to form a thin layer thereon and the sheet was left to stand at a temperature of 80°C for 16 hours in an argon gas atmosphere. The paste as hereinabove described was coated on the upper surface of the sheet. Then, the sheet was left to stand at a temperature of 80°C for 16 hours in an argon gas atmosphere and was dried at the same temperature for 48 hours in a vacuum. It was incorporated into a battery test cell, which will hereinafter be referred to as "Cell IV". EXAMPLE 5 A part of the hydroxyl groups in the same polyether as had been employed in the foregoing examples were esterified and neutralized with lithium chloride to form -SO₃Li groups. The terminal -OH and -SO₃Li groups in the polyether had a ratio of 9:1. Lithium perchlorate was dissolved in the polyether by the method which had been employed in the foregoing examples. Two parts of dimethylacetamide, an equivalent amount of hexamethylene diisocyanate and 0.006 part of din-butyltin diacetate were fully mixed in 11 parts of a solution of lithium perchlorate and polyether. The mixture was coated on a 50 nm (micron) nonwoven fabric of polypropylene and it was left to stand at a temperature of 80°C for 16 hours in an argon gas atmosphere to form a sheet. Then, the paste as hereinabove described was coated on the upper surface of the sheet. The sheet was left to stand at a temperature of 80°C for 16 hours in an argon gas atmosphere and was dried at the same temperature for 48 hours in a vacuum. It was incorporated into a battery test cell, which will hereinafter be referred to as "Cell V". EXAMPLE 6 An equivalent amount of hexamethylene diisocyanate, 0.006 part of di-n-butyltin diacetate, and a solution which had been prepared by dissolving 4 parts of a substance of the formula C₁₂H₂₅(CH₂CH₂O)₄SO₃Li in four parts of dimethylacetamide were fully mixed in 11 parts of a solution of lithium perchlorate and polyether. The mixture was coated on a 50 nm (micron) nonwoven fabric of polypropylene and it was left to stand at a temperature of 80°C for 16 hours in an argon gas atmosphere to form a sheet. The paste as hereinabove described was coated on the upper surface of the sheet and the sheet was left to stand at a temperature of 80°C for 16 hours in an argon gas atmosphere. Then, the sheet was dried at the same temperature for 48 hours in a vacuum and was incorporated into a battery test cell, which will hereinafter be referred to as "Cell VI". A comparative cell was prepared in accordance with the prior art. Two parts of dimethylacetamide, an equivalent amount of hexamethylene diisocyanate and 0.006 part of di-n-butyltin diacetate were fully mixed in 11 parts of a solution of lithium perchlorate and polyether. The mixture was coated on a 50 micron nonwoven fabric of polypropylene and was left to stand at a temperature of 80°C for 16 hours in an argon gas atmosphere to form a sheet. The paste as hereinbefore described was coated on the upper surface of the sheet and the sheet was left to stand at a temperature of 80°C for 16 hours in an argon gas atmosphere. Then, it was dried at the same temperature for 48 hours in a vacuum and was incorporated into a battery test cell, which will hereinafter be referred to as "Cell VII". Thirty cells of each of the seven types I to VII were prepared and were tested for their variation of capacity during storage and their charging and discharging cycle life. Twenty cells of each type were tested for variation of capacity during storage. They were placed in a thermostatic oven of the recirculation type having a temperature of 80°C. Five cells were taken out of the oven and tested upon lapse of each of 0, 7.5, 15, and 30 days. The test was conducted by a constant current discharge of 13 µA (10 µA/cm²) at a temperature of 80°C until a final discharging voltage of 2 V was reached. The results are shown in FIGURE 2. Ten cells of each type were tested for their cycle life. They were placed in a thermostatic oven of the recirculation type having a temperature of 80°C and were caused to discharge a current of 52 µA (40 µA/cm²), while they were charged with a current of 26 µA (20 µA/cm²), until a final discharging voltage of 2.0 V and a final charging voltage of 4.0 V were reached. The results are shown in FIGURE 3. The results shown in FIGURES 2 and 3 confirm that the batteries of this invention are superior to the conventional one. As is obvious from the foregoing description, this invention provides a polymer solid electrolyte which can be stored for a long time without suffering from any reduction of quality, and a battery which does not lower its performance for a long time. It also provides a polymer solid electrolyte battery which can be charged and discharged for a large number of times repeatedly without having any substantial reduction of capacity.

Claims: A polymer solid electrolyte comprising a cross-linked polyether, of which the polyether has a structure of the formula: where R stands for an inert group other than a hydrogen atom and M stands for an element of Group I of the periodic table other than a hydrogen atom. A battery including an electrolyte according to claim 1.

Inventors: NODA TOMOHIKO, YOSHIHISA YOUETSU, NODA, TOMOHIKO, YOSHIHISA, YOUETSU
Applicants: YUASA BATTERY CO LTD, YUASA CORPORATION
Application year: 1988
Main classification: H01M10
IPC codes: H01M10