SOLID-STATE BATTERIES

Abstract: A solid electrolyte battery composed of a molded cath­ ode comprising a solid electrolyte and one member selected from the compounds of the chemical formulas TiX₂, Nb₂X₅, WX₃ and MoX₃ (wherein X is an oxygen or sulfur atom), a mol­ ded Li⁺ -ion or Cu⁺ -ion conductive solid electrolyte, and a re­ versible lithium or copper anode.

Description: SOLID-STATE BATTERIES This invention relates to solid-state batteries and, more particularly, relates to entirely solid primary and secondary batteries. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a drawing illustrating the construction of a battery in which the collector metal according to this invention serves also as a battery case and a terminal. Fig. 2 is a drawing illustrating the construction of a battery in which the battery elements are enclosed with a photo- or thermo-setting resin according to this invention. Fig. 3 is a graph representing the relationship between the type of cathodic substance and the discharge potential in a battery employing a Li+-ion conductive solid electrolyte. Fig. 4 is a graph representing the effect of the type of cathodic substance on the relationship between the discharge current density and the flat voltage in a battery employing a Li -ion conductive solid electrolyte. Fig. 5 is a graph representing the change in discharge capacity accompanying the repeated charge-discharge cycles in a battery employing a Li+-ion conductive solid electrolyte. Fig. 6 is a graph representing the effect of the collector material as well as the anode material on the self-discharge ratio in a battery employing a Li+ -ion conductive solid electrolyte. Fig. 7 is a graph representing the relationship between the discharge current density and the flat voltage in a battery employing a Cu -ion conductive solid electrolyte. Fig. 8 is a graph representing the change in discharge capacity accompanying the repeated charge-discharge cycles in a battery employing a Cu+-ion conductive solid electrolyte. Fig. 9 is a graph representing the relationship between the open circuit voltage and the flat voltage when the mixing ratio of copper powder to Cu2S is varied. Fig. 10 is a graph representing the change in final voltage with the repeated charge-discharge cycles. All commerciallized batteries employing a solid electrolyte are primary ones. These primary batteries generally employ as the solid electrolyte a Li -ion or Ag -ion conductive substance. These substances, however, have a disadvantage of forming discharge products of high electric resistance which cause the battery voltage to decline in direct proportion to the amount of discharge. For instance, if an iodine-containing compound is used in the cathode, there is formed Lil or AgI upon discharge. On the other hand, studies have recently been advanced in the direction of lithium secondary batteries employing sulfides or oxides of tetra- to hexa-valent transition metals as cathodes. Most of these lithium batteries, however, employ organic electrolytes, those employing solid electrolytes being very few. In making secondary batteries of the lithium anode type, the most important problem is how to prevent lithium from depositing in mistlike form. In general, when a solid electrolyte is employed in a cell, the mistlike or dendrite metal deposition presents a more serious problem than in the case of a liquid electrolyte. This problem seems to be a major cause which hinders the solid-state batteries from rapid commercialization. An object of this invention is to provide a repeatedly usable battery which is made of inexpensive materials and in which the formation of discharge products having a high electric resistance, which is one of the disadvantages of conventional solid electrolyte batteries, is eliminated by utilizing an intercalation substance such as a sulfide or oxide of Ti, Nb, W, or Mo as the cathode in combination with a reversible anode. When titanium disulfide is used as the cathodic active substance and lithium as the anodic active substance, the charge and discharge reactions are represented by the formula: XLi + TiS2 + e ( Li TiS2 The constructional elements of the present battery are entirely in solid state. For this reason, it is easy not only to arrange each cell in series, as contrasted with a wet-type battery, but also to construct a battery in such a way that the battery elements are enclosed with a photo- or thermo-setting resin by dipping and succeeding curing. Examples of the above constructions are shown in Fig. 1 and Fig. 2. These are batteries in series arrangement of unit cells. A single unit structure is, of course, possible. Fig. 1 shows a construction in which the collector metal serves also as the battery case and a terminal. Fig. 2 shows another consturction in which the battery is enclosed with a photo- or thermo-setting resin. In these figures 1 is a cathodic active substance which is made of a molded mixture of a solid electrolyte with 70 to 90% of a sulfide or oxide of Ti, Nb, W, or Mo. 2 is a press-molded solid electrolyte comprising a highly Li -ion conductive substance of the type of nLiI. EMI4.1 (a double salt of lithium iodide and a n-alkylpyridinium iodide, R being an n-alkyl group), Li3N, or mLiI#nLi2S.P2S5, or a highly Cu -ion conductive compound of RbCu4I1 5C13 5 or (K1/4 Rb3,4)Cu4I1#5Cl3#5. 3 is a reversible metal anode comprising a sheet of lithium-aluminum alloy or a molded mixture thereof with an electrolyte when a Li+-ion conductive solid electrolyte is used, or a molded mixture of copper or Cu2S with an electrolyte when a Cu+-ion conductive solid electrolyte is used. 4 is a cathode collector comprising a highly anticorrosive stainless steel of the ferrite type containing 30% or more of chromium; carbon, gold, palladium or platinum. In the examples shown in Figs. 1 and 2, a conductive graphite paint is used to serve both as the cathode collector and the binder between each cell. In the example of Fig. 1, the cathode collector only for the end cell is a high-chrome stainless steel which serves as both the battery cap and the cathode terminal. In the example of Fig. 2, collector 4' consisting of a conductive graphite paint or a stainless steel screen which is pressed against the surface of the cathodic active substance 1 is used to the end cell. 6 is a copper anode collector provided on the anodic substance 3 by vacuum deposition, spattering, or flame spraying. In the example shown in Fig. 1, a copper anode collector 7 for the end cell (the cell at the lowest position) is used to serve as both the battery cap and the terminal. 8 is a packing material used only in the example of Fig. 1 for the purpose of insulating the cathode from the anode and enclosing the battery elements in the case. 9 and 10 used in the example of Fig. 2 are lead wires electrically connected to cathode corrector 4', and anode collector 6, respectively. 11 is an outside layer of a photoor thermo-setting resin. Both Figs. 1 and 2 are examples in which three cells are connected in series. As described in the foregoing, the present invention makes it possible to provide a secondary battery in which the formation of discharge products having a high electric resistance, which is one of the disadvantages of conventional solid electrolyte batteries, is eliminated by using as the cathodic active substance a substance which permits the easy interlayer transfer of Li ion or Cu ion during the charge and discharge periods and which is increased in conductivity by the insertion of these ions, and using a reversible metal anode in combination with said cathode. The constructional elements of the present solid electrolyte battery are entirely of inexpensive solid substances. Consequently, there is no fear of electrolyte leakage, as contrasted with other batteries using a liquid substance; moreover, it is easy to provide a series arrangement and to reduce the overall thickness of the battery; and it is also possible to enclose the battery elements with a photo- or thermosetting resin by dipping and subsequent curing, as shown in an example of Fig. 2. Example 1 One of the examples in which a Li+-ion conductive electrolyte is employed is shown below. The electrolyte EMI7.1 is formed by the contact between a cathodic active substance and an anode metal sheet. However, it is of course possible to construct a battery by separately preparing the electrolyte as described later in Example 3, disposing a layer of the resulting electrolyte between the layers of cathodic and anodic active substances, and subjecting the assembly to press-molding. A cathodic active substance was prepared by press-molding 3 mmol of TiS2, Nb2S5, WS3, TiO2, Nub205, or WO3 into a disc of 18 mm in diameter under a pressure of 300 MPa (megapascal). The disc was coated, on the side counter to the anode, with a complex, EMI7.2 (liquid), formed by the addition of iodine to n-butylpyridinium iodide. The coated side was pressure-welded to a Li-Al alloy sheet to obtain a single cell of the construction type of Fig. 1. For comparison, a reference cell of the conventional type (comparative example) closely resembling the above cell was prepared by molding a mixture of 1/8 mmol of a complex, EMI7.3 formed by the addition of iodine, and 20% of silica gel to obtain a molded cathode, and jointing the molded cathode to a lithium anode by pressure welding. The performance characteristics of each single cell were as shown in Figs. 3 to 5. In the figures, the Arabic numerals refer to sulfides or oxides in the following way: 1 TiS2, 1' TiO2, 2 Nb2S5, 2' Nub205, 3 WS3, 3' WO3, 5 the conventional-type cell. Fig. 3 shows the change in voltage when the cell is discharged at 30 WA. Fig. 4 shows the relationship between the discharge current density and the flat voltage. Fig. 5 shows the change in discharge capacity with the increase in the number of chargedischarge cycles when the cell is discharged to 1 V at 30 pA and charged to 3 V at the same amperage. From Figs. 3 and 4, it is seen that the conventional cell shows a large voltage change with the discharge, the change being in direct proportion to the amount of discharged electricity and the discharge current density, indicating clearly that the discharge voltage is controlled by the electric resistance of the discharge products, whereas the cell of this invention showed better flatness of the voltage change. Even in the cell of this invention a critical current density was observed at about 500 ijA/cm2. This seems to be originated from a low ionic conductivity of the electrolyte, i.e. 10 5 10 4 S/cm. If an electrolyte of high conductivity is used, the critical current density will become larger, as shown later in Example 3. When a sulfide was used in the cathodic active substance, the charge and discharge polarization was small, because as compared with the case of oxide, the interlayer transfer of Li -ion is easier in the case of sulfide. In any event, it was found that the present cell withstands about 300 cycles of charge and discharge. Example 2 In Example 1, a high-chrome stainless steel was used as the cathode collector and a lithiumaluminum alloy (10 atom % of Al) was used as the anode. In the present Example, the effect of the cathode collector material and that of the anode material were examined. In a manner similar to that in Example 1, a number of single cells were prepared by using 3 mmol of TiS2 as cathode substance and various types of cathode collector materials shown in Fig. 6. Another cell of the conventional type was prepared by using a cathode collector of high-chrome stainless steel and a lithium anode. The effect of the material of the cathode collector was most markedly manifested on the selfdischarge ratio among the performance characteristics, as shown in Fig. 6. The self-discharge ratio was determined after the cell had been left standing at 700C for one month. Among the collector materials, high-chrome stainless steel, graphite paste, platinum, palladium, and gold showed a small self-discharge ratio of 1% or less. When used as anode, lithium showed apparently a larger self-discharge ratio. This is due to the partial short circuit caused by the misty deposition of lithium which took place during the first charge after the determination of initial discharge capacity. It was found that in a cell employing a lithium-aluminum alloy (10 atom e of aluminum), such a phenomenon is difficult to take place and, hence, the cell withstands the repeated charge-discharge cycles as shown in Fig. 5. Example 3 Examples of single cells in which a Cu -ion conductive electrolyte was used are described below. Cathodic active substances were prepared by adding 100 mg of RbCu4I1 5C13 5 to 5 mmol of TiS2, Nb2S5, WS3, MOS3, TiO2, Nb2O5, WO3, or MoO3. An anodic active substance was prepared by intermixing 288 mg of powdered electrolytic copper, 192 mg of Cu2S and 120 mg of the electrolyte. The anodic active substance in layer form and the cathodic active substance in layer form were placed facing each other with a layer of 0.8 g of the electrolyte between them. The assembly was press-molded to form an integrated three-layered disc of 18 mm in diameter. The collector metals were flame-sprayed over respective electrodes and lead wires were attached to both collectors. The resulting assembly of cell elements was enclosed with an epoxy resin, as in the constructional example of Fig. 2. For the purpose of examining the effect of anode material, another enclosed cell was similarly prepared using a cathodic active substance containing TiS2 and an anodic active substance comprising a mixture of 480 mg of powdered copper and 120 mg of the electrolyte. The performance characteristics of these cells were as shown in Figs. 7 and 8. The meaning of the Arabic numeral is the same as in Example 1, except that 4, 4' and 5 represent MoS3, MoO3 and the copper anode, respectively. Fig. 7 represents the relationship between the discharge current density and the flat voltage. Fig. 8 shows the change in discharge capacity with the repetition of charge-discharge cycle, one cycle consisting of the discharge down to 0.3 V at 1 mA and the charge up to 0.65 V at 1 mA. The Cu+ -ion conductive solid electrolyte has a conductivity of 10 2S/cm or above and permits the charge and discharge at higher amperage as compared with the case of Li+-ion conductive electrolytes. However, because of the larger ionic radius of Cu+ -ion than that of Li+ -ion, the Cu+-ion transfer per mol of the cathodic active substance is about 0.5 mol with TiS2 and Nb2S5 which exhibited largest capacities and about 0.2 mol with WO3 and MoO3 which exhibited least capacities, as contrasted with a maximum of 0.8 mol of Li -ion in Example 1. An anodic active substance containing about 40% of Cu2S was effective in markedly increasing the charge-discharge cycle life without much affecting the voltage characteristics. Example 4 For the purpose of examining the discharge polarization at the anode and its effect on the cycle life, several single cells were constructed by using TiS2 as the cathodic active substance, RbCu4Il SC13 5 as the electrolyte, and mixtures of powdered copper and Cu2S in varied ratios. In Fig. 9 are shown the open circuit voltage and the flat voltage during the discharge period when the cell was discharged for 20 hours and then charged for 20 hours both at 1 mA. Fig 10 represents the change in final voltage with the repetition of charge-discharge cycle under the above-noted conditions. From the above results, it is seen that as compared with an anode of powdered copper alone, those containing up to 40% by weight of Cu2S are more effective in decreasing the discharge polarization and increasing the cycle life.

Claims: CLAIMS 1. A solid electrolyte battery composed of a molded cathode comprising a solid electrolyte and one member selected from the compounds of the chemical formulas TiX2, Nb2X5, WX3 and MoX3 (wherein X is an oxygen or sulfur atom), a molded Li -ion or Cu -ion conductive solid electrolyte, and a reversible lithium or copper anode. 2. A solid electrolyte battery according to Claim 1, wherein a lithium-aluminum alloy is used as the reversible lithium anode. 3. A solid electrolyte battery according to Claim 1, wherein a molded mixture of copper, copper (I) sulfide and a solid electrolyte is used as the reversible copper anode. 4. A solid electrolyte battery according to Claim 1, wherein at least one member selected from the group consisting of ferrite-type stainless steels of 30% or more in chromium content, carbon, gold, palladium and platinum is used as the cathode collector. 5. A solid electrolyte battery according to Claim 1 or 3, wherein copper is used as the anode collector.

Inventors: NINOMIYA, YOSHITO, SEKIDO, SATOSHI, SOTOMURA, TADASHI
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1982
Main classification: H01M10
IPC codes: H01M10, H01M4