RECHARGEABLE NI/ZN CELL WITH SOLUBLE ZINC ANODES

Abstract: To provide a Ni/Zn cell where cycling does not result in thickering the lowest edges of negative current collectors and also in accumulating incoherent particles of zinc at the bottom of cell container, a rechargeable Ni/Zn cell is provided with soluble zinc anodes, comprising a stack of alternately arranged negative and positive plates, whereby adjacent plates are arranged with a fixed distance to each other to allow free movement of electrolyte and zinc particles between the plates.

Description: Rechargeable Ni/Zn cell with soluble zinc anodes The description refers to rechargeable Ni/Zn cells with soluble zinc anodes. The Ni/Zn cells with soluble zinc anodes are in an early stage of development at present. Particular features of these cells make them attractive as source of energy for electric vehicles. Works concerning this field have been focused mainly on electrolysis of alkali zincate solutions. The state of the art is described e.g. in the following references Ref 1 : Journal of Power Sources, 34 /1991/, pp 243-255 Ref 2: Journal of Power Sources, 5 /19807, pp 235-244 Ref 3: Chimiczeskije Istoczniki Toka, Moskwa 1968, pp 304-333 Difficulties appeared at attempts to maintain proper quality of zinc layer during pronounced cell cycling. The difficulties start with the fact that deep discharges cause some limited negative active mass shedding. Cycling results in thickering the lowest edges of negative current collectors and also in accumulating incoherent particles of zinc at the bottom of cell container. It is an object of the invention to provide a Ni/Zn cell where the described drawbacks can be avoided. The technical solution consists in providing a rechargeable Ni/Zn cell with soluble zinc anodes, comprising a stack of alternately arranged negative and positive plates, whereby adjacent plates are arranged with a fixed distance to each other to allow free movement of electrolyte and zinc particles between the plates. The described technical solution concerns the way of oxidizing and dissolving the shedded, incoherent zinc particles. Oxidizing and dissolving the undesired residues is strictly connected with constructions and the way of charging of a cell utilizing such collectors. According to one preferred aspect of the invention the distance between adjacent plates is maintained by means of spacing elements fixed at least to one of said plates. So a defined space can be reproduced between the plates easily. Said spacing elements can be spacing slugs fastened in holes made in one of the positive or negative plates. So a constant distance between conducting parts of the negative current collectors and surfaces of adjacent positive plates is maintained with the help of said spacing slugs. Those slugs can easily be produced and mounted to the plates. This construction is economic and a free flow of electrolyte and particles is guaranteed. According to a further aspect of the invention crosswise to said stack of positive and negative plates a third auxiliary electrode is positioned. This additional electrode enables to avoid the drawbacks of the state of the art especially during charging. With advantage said auxiliary electrode is being alternately electrically parallel connected with a positive and negative terminal of the cell during charge. This results in greater oxidizing of zinc at the lowest edges of current collectors and connections with negative terminal, and also in oxidizing and dissolving some zinc particles which have already fallen down to the bottom of the cell and/or connections with positive terminal. This is improved in that said auxiliary electrode is permeable for electrolyte. So electrolyte-circulation is improved and colletion of zinc particles possible. Usually said stack is arranged such that in operation said plates are arranged in a vertical position and said third auxiliary electrode is positioned horizontally under said stack. So falling particles are slummed down by electrolyte. For practical use said stack and said third auxiliary electrode are arranged inside a closed housing and electrical positive and negative terminals are located outside said housing and connected to said plates respectively. Furthermore a third electrical terminal is located outside said housing and connected to said auxiliary electrode. So a useful and improved Ni/Zn cell is provided where cycling does not result in thickering of the lowest edges of negative current collectors and also not in an accumulation of incoherent particles of zinc at the bottom of cell container. Furthermore the Ni/Zn cell can be produced economic and easy and is reliable in operation. Further features and advantages become obvious from the following description of embodiments in connection with the drawings. It is shown in Figure 1 a schematic crossection l-l of figure 2 of a negative plate current collector according to one embodiment of the invention; Figure 2 a schematic front view of the negative plate current collector according to Fig. 1 and Figure 3 a schematic crossection of cells container according to an embodiment of the invention. The main body of the current collector is made of flat, smooth plate (1 ). In the plate there are drilled holes (3). The holes serve as places where spacing slugs (2) are fastened. The notion "spacing slugs", means cylindrical, small elements/diameter about one millimetre made of an insulator. These slugs stand out of the collectors surface. The main role of spacing slugs is to maintain constant distance between conducting parts of adjacent plates. Number of spacing slugs depends on rigidity of introduced positive plates. Practically, one slug is sufficient to secure one square centimetre of adjacent plates. Described construction of current collectors enables unrestricted movement of incoherent zinc particles. Constant distance between adjacent plates can bs also maintained with the help of specially shaped separators. They can be corrugated, perforated type or similar to, described above, negative current collectors construction (with small spacing slugs). The main principles are: to maintain distance between plates, to restrict movement of incoherent zinc particles as few as possible. An ideal solution is when there is nothing between the adjacent plates (apart from electrolyte). Practically, however, plates must be mechanically supported. A secondary cell utilizing such current collectors should consist of even number of negative and odd number of positive plates. Under the stack of plates, at the bottom of the cell, there is located a third, auxiliary electrode (5). During charge, the third electrode is being alternately parallely connected with positive and negative terminal of the cell. Duration of these connections can vary in wide ranges, depending on construction and size of particular cell. The action of the auxiliary electrode should result in greater oxidizing of zinc at the lowest edges of current collectors (connections with negative terminal), and also in oxidizing and dissolving some zinc particles which have already fallen down to the bottom of the cell (connections with positive terminal). Duration of particular connections should be enough to fulfil both of these conditions. Practically, connections duration should be equal and should last about twenty seconds each. Elvolving gases should be catalitically recombined. It is the best if arrangement responsible for the auxiliary electrode connections is a part of charging device. The main body of current collector should be made of smooth plate (1 ). The spacing slugs can be made of polyethylene or similar material (2). The third, auxiliary electrode (5) should be made of nickel net. It must be connected with terminal located outside the cell container (4). The auxiliary electrode should be located above the bottom of the cell container (to separate mud falling from positives). The whole container and, at the same time, location of auxiliary electrode are shown on figure 3. Device responsible for the third electrode connections should be a part of charging device. Charging of prototype cells were performed with the help of unit based on Polish patent application No P-316861 ("Thyrystor tripping method and a charching device based on such system"). The unit was supplemented with simple electronic "timer". Electrolyte was made of 8M KOH solution, supersaturated with zinc oxide (about 1.9M ZnO solution). Such supersaturated zincate solution was stabilized by about 0.6% addition of potassium silicate. Positives were sintered and nickel felt type. In the main body of current collector (1 ) there are drilled holes (3), where spacing slugs (2) are fastened. In the main body of current collector (1 ), there are distributed spacing slugs (2). The nickel net auxiliary electrode (5) is connected with current terminal (4). Auxiliary electrode is located above the bottom of cells container (to separate mud falling from positives). The invention allows the possibility of reduction in amount of cadmium being used in battery industry today. From economic point of view construction of large and, at the same time, fairly cheap cells is possible.

Claims: Patent claims. 1. Rechargeable Ni/Zn cell with soluble zinc anodes, comprising a stack of alternately arranged negative and positive plates, characterized . in that adjacent plates are arranged with a fixed distance to each other to allow free movement of electrolyte and zinc particles between the plates. 2. Rechargeable Ni/Zn cell according to claim 1 , characterized in that the distance between adjacent plates is maintained by means of spacing elements fixed at least to one of said plates. 3. Rechargeable Ni/Zn cell according to claim 2, characterized in that said spacing elements are spacing slugs fastened in holes made in the negative plates. 4. Rechargeable Ni/Zn cell according to claim 2, characterized in that a constant distance between conducting parts of the negative current collectors and surfaces of adjacent positive plates is maintained with the help of said spacing slugs. 5. Rechargeable Ni/Zn cell according to claim 1 , characterized in that crosswise to said stack a third auxiliary electrode is positioned. 6. Rechargeable Ni/Zn cell according to claim 5, characterized in that said auxiliary electrode is being alternately electrically parallel connected with a positive and negative terminal of the cell during charge. 7. Rechargeable Ni/Zn cell according to claim 5, characterized in that said auxiliary electrode is permeable for electrolyte. 8. Rechargeable Ni/Zn cell according to claim 1 , characterized in that said stack is arranged such that in operation said plates are arranged in a vertical position and said third auxiliary electrode is positioned horizontally under said stack. . Rechargeable Ni/Zn cell according to claim 8, characterized in that said stack and said third auxiliary electrode are arranged inside a closed housing and electrical positive and negative terminals are located outside said housing and connected to said plates respectively. 10. Rechargeable Ni/Zn cell according to claim 8, characterized in that a third electrical terminal is located outside said housing and connected to said auxiliary electrode.

Inventors: SOLOPA WALDEMAR, SOLOPA, WALDEMAR
Applicants: ACCUMULATORENWERKE HOPPECKE, SOLOPA WALDEMAR, SOLOPA, WALDEMAR
Application year: 2001
Main classification: H01M10
IPC codes: H01M2, H01M6, H01M10