A zinc-silver oxide dry cell comprises a zinc anode, a silver oxide cathode, an immobilized body interposed between the anode and the cathode, and a metallic silver layer formed at least on the surface of the anode side of the cathode in which the metallic silver layer is formed through a reduction reaction resulting from a light exposed to the surface of the cathode. The cathode of the dry cell requires no graphite for an electroconductive ingredient, making it possible to increase an amount of silver oxide which is active ingredient for the cathode and to consequently increase a discharge capacity correspondingly.
What we claim is:
1. An alkaline dry cell comprising: an anode containing zinc as an active ingredient; a cathode containing a silver oxide as an active ingredient; an immobilized body of an alkaline electrolyte interposed between said anode and said cathode; and a metallic silver layer formed at least on the surface of the anode side of said cathode by reduction of the silver oxide through a photochemical reaction.
2. An alkaline dry cell according to claim 1, in which said metallic silver layer is formed by irradiating the surface of the cathode with a light having a wavelength of 1 to 700 nm.
3. An alkaline dry cell according to claim 1, in which said metallic silver layer is formed by irradiating the surface of the cathode with a light having a wavelength of 300 to 400 nm.
4. An alkaline dry cell according to claim 1, in which said cathode contains no graphite for an electroconductive ingredient.
BACKGROUND OF THE INVENTION
This invention relates to an improved alkaline dry cell comprising in combination a zinc anode, a silver oxide cathode and an alkaline electrolyte, and in particular an improvement in a cathode for the alkaline dry cell.
A conventional dry cell contains a silver oxide (Ag 2 O) as an active ingredient. In such a conventional dry cell, a silver oxide is a poor non-conductor showing, for example, a greater resistance of about 10 8 Ωcm. For this reason, an electroconductive ingredient such as graphite is added to a silver oxide, as shown in U.S. Pat. No. 3,427,204 so as to enhance electroconductivity. In order to obtain necessary electroconductivity from the standpoint of the characteristic of the dry cell, use is generally made of a cathode obtained by compressing a composition comprising 95 parts of silver oxide, 4.5 parts of graphite and 0.5 part of a binder under a pressure of 5 to 10 ton to form a pellet or a compressed coherent mass. The use of the graphite reduces an amount of silver oxide by that extent, when viewed based on the whole weight of the composition, resulting in a decreased discharge capacity. Therefore, difficulty is encountered in attaining high performance of a compact button type cell of closed variety in particular. Graphite shows a resistance of about 10.sup. -2 Ωcm. Since a great difference of resistance is observed between the graphite and the silver oxide, it is necessary to form a pellet by uniformly mixing each ingredient of the composition and compressing the mixture under a pressure. In actual practice, however, many difficulties are involved, since the graphite is smaller in true specific gravity than the silver oxide.
SUMMARY OF THE INVENTION
An object of this invention is to provide a zinc-silver oxide dry cell having a great discharge capacity.
Another object of this invention is to provide an alkaline dry cell including a silver oxide cathode depolarizer pellet of good conductivity and having no electroconductive ingredient such as graphite.
Another object of this invention is to provide a method for easily imparting an electroconductivity to a cathode pellet without lowering a discharge capacity.
According to this invention there is provided an alkaline primary cell comprising an anode containing zinc as an active ingredient, a cathode containing a silver oxide as an active ingredient, an immobilized body of an alkaline electrolyte interposed between the anode and the cathode, and a metallic silver layer formed at least on the surface of anode side of the cathode. The metallic silver layer is formed through a photochemical reaction resulting from the reduction of the silver oxide by a light. That is, the metallic silver layer is formed at least on the surface of the anode side of the cathode by irradiation with a light having a wavelength of 1 to 700 nm, preferably 300 to 400 nm.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a cross-sectional view showing an alkaline primary cell according to one embodiment of this invention; and
FIGS. 2 and 3 are characteristic curves each showing a comparison in discharge characteristic between a zinc-silver oxide cell according to this invention and a conventional counterpart.
DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENT
An alkaline dry cell according to this invention will be explained below by referring to FIG. 1.
In FIG. 1, (1) shows a cup-like cathode vessel made of a nickel-plated iron plate.
Within the cathode vessel 1 is formed a cathode pellet 2 which is formed by charging a mixture of 99.5 parts by weight of a silver oxide as an active ingredient and 0.5 parts of a polystylene resin as a binder into the cathode vessel 1 and compressing it. An immobilized body 3 made of a nonwoven polyamide fiber cloth impregnated with an electrolyte of an aqueous potassium hydroxide solution is formed through a cellophane separator 4 on the cathode pellet 2 and a gelatinized zinc anode 5 is formed on the immobilized body 3. A metallic silver layer 6 is formed at least on that surface of the cathode pellet 2 where it contacts with the cellophane separator 4. The metallic silver layer 6 is formed by irradiating the outer surface of the cathode pellet 2 for about 10 to 15 minutes with an ultraviolet ray with a wavelength of 300 to 400 nm using an ultraviolet lamp. 7 shows an anode cap made of a steel plate and having a nickel-plated outer surface and a tin-or copper-plated inner surface. The anode cap 7 is sealed by a sealing gasket 8 made of polyamide.
An alkaline dry cell using, instead of such a silver layer bearing cathode pellet 2, a conventional cathode pellet made of a silver oxide, graphite and binder is substantially the same as an alkaline dry cell of 11.6 mm in diameter and 5.4 mm in height which is called G13 type under the Japanese Industrial Standard (JIS).
The following Table indicates a comparison in properties between the alkaline dry cell according to this invention (FIG. 1) and a conventional alkaline dry cell in which a cathode pellet contains graphite and has no metallic silver at the surface.
Table______________________________________ This invention prior artComparison item (A) (B)______________________________________Composition silver oxide 99.5 parts 95 partsof electroconductivecathode pellet ingredient (graphite) 0 4.5 parts binder (polystyrene resin) 0.5 part 0.5 partWeight of cathode pellet 1.2 g 1.0 gWeight of silver oxide in cathodepellet composition 1.194 g 0.95 gTheoretical electrical capacity 275.8 mAH 219.5 mAHElectrical resistance from the topsurface of the cathode pellet up tothe outer surface of the cathodevessel* 6 to 8 Ω 8 to 10 ΩOpen circuit voltage 1.58 V 1.58 VShort-circuit current 0.71 A 0.64 AImpedance (1K.Z AC frequence) 2.0 Ω 2.3 ΩDischarge 20° C 5000Ωcontinuous 253 mAH 191 mAHcapacity discharge(Total End 20° C 500 Ωcontinuous 201 MAH 155 mAHVoltage 1.2 V discharge 20° C 150 Ωcontinuous 175 mAH 140mAH discharge______________________________________ *A nickel-plated steel rod with a diameter of 3 mm was contacted under a pressure of 5 kg/cm.sup.2 with the central top surface portion of a cathode pellet with a diameter of 11 mm and a thickness of 2 mm and electrical resistance between the bottom of a cathode vessel and the stee rod was measured using a DC resistance meter.
As will be evident from the Table, the cathode pellet of this invention shows a smaller electrical resistance than that in the conventional alkaline dry cell and, in consequence, the electroconductivity is excellent. It is also evident that, as compared with the conventional alkaline dry cell, the alkaline dry cell of this invention provides increased short-circuit current and increased discharge capacity of about 25 to 30%.
FIGS. 2 and 3 show a comparison in discharge characteristics between the alkaline dry cell (A) of this invention and that (B) of the conventional alkaline dry cell (see Table). FIG. 2 shows a continuous discharge at a temperature of 20° C. and a resistance of 5000 Ω and FIG. 3 shows a continuous discharge at a temperature of 20° C. and a resistance of 500 Ω. From these Figures it will appreciated that the alkaline dry cell of this invention exhibits excellent discharge characteristics in comparison with the conventional counterpart. Since according to this invention the metallic silver layer formed by irradiating with a light at least the surface of the anode side of the cathode pellet serves as an electroconductive ingredient, it is unnecessary to add graphite to the silver oxide as with the case with the conventional cathode pellet. This makes it possible to increase an amount of silver oxide, which is an active ingredient for the cathode, as compared with that required in the prior art. As a result, the discharge capacity can be correspondingly increased according to this invention.