Title of Invention

GASTIGHT SEALED NICKEL / METAL HYDRIDE CELL FOR THE ELECTROCHEMICAL STORAGE OF ENERGY

Abstract

Gastight sealed nickel/metal hydride cell for the electrochemical storage of energy, with at least one positive nickel oxide electrode and at least one negative electrode storing hydrogen, wherein a hydrophilic separator is arranged between the positive and negative electrode, together with an alkaline electrolyte or an alkaline electrolyte mixture, i*, characterised in that, in the sequence of several positive and negative electrodes, every second electrode is divided into two parts, and that one or more negative electrodes is or are provided with a gas-permeable hydrophobic transport element to transport the gases of the cell atmosphere.

Full Text

FORM 2 THE PATENTS ACT 1970 [39 OF 1970] COMPLETE SPECIFICATION [See Section 10 and Rule 13] "GASTIGHT SEALED NICKEL/METAL HYDRIDE CELL FOR THE ELECTROCHEMICAL STORAGE OF ENERGY" DEUTSCHE AUTOMOBILGESELLSCHAFT MBH, a German company of Gifhorner Strasse 57, D-38112 Braunschweig, Germany, The following specification particularly describes the nature of the invention and the manner in which it is to be performed:- The invention relates to a gastight cell for the electrochemical storage of energy, having at least one positive nickel oxide electrode and at least one hydrogen-storing negative electrode, a hydrophilic separator being arranged between the positive and negative electrodes, and having an alkaline electrolyte or an alkaline electrolyte mixture. (Storage) batteries for storing electrical energy in the form of chemical energy, which can then be removed again as electrical energy, have been known since the end of the last century. Even today, the lead storage battery is still in widespread use. In such a battery, the electrodes or plates comprise the active material, which is the actual energy store, and a lead support (grid), which holds the active material. There are also batteries with alkaline aqueous electrolytes. All these galvanic elements substantially comprise the energy-storing electrodes of positive and negative polarity, the electrolyte, the separator between the electrodes, the cell or battery vessel and the current-carrying, connective inactive parts, such as for example the supply and discharge lines for the current to and from the electrodes. These also include substrate material, current discharge lugs, poles, pole bridges, pole screws, washers and pole connectors. In a gastight nickel/metal hydride cell, the negative and positive electrodes are arranged alternately next to one another, for example in a prismatic or cuboidal housing, and are separated from one another by a separator. On account of the normally absent freely mobile excess of electrolyte, the electrodes are each of the electrodes is in contact with the cell atmosphere, i.e. the gas space of the cell. The gases which are evolved during charging of the cell pass into this gas space and, in quiescent phases of the cell, react at the negative electrodes. For this purpose, the gases have to diffuse into the electrodes. By way of example, the hydrogen is reincorporated in the lattice of the storage alloy of the negative electrode, until an equilibrium.state is reached. Cells of the generic type are described in EP 0 460 424 Bl, EP 0 460 425 Bl and DE 39 29 306 C2. A problem of these cells is that in quiescent phases charge balancing in all the negative electrodes of the cell is not possible, and an excess pressure of hydrogen and oxygen continues to obtain. In fact, auxiliary electrodes or special multilayer electrodes are required for charge balancing. Therefore, the object of the present invention is to provide a cell of the abovementioned type in which charge balancing is possible in the negative electrodes and the excess pressure is reduced with the minimum possible outlay. The solution consists in a cell having the features of claim 1. Therefore, according to the invention, it is provided that one or more negative electrodes are provided with a gas-permeable, hydrophobic transport element for transporting the gases of the cell atmosphere. The gases of the cell atmosphere, namely hydrogen and oxygen,' fill these transport elements and reach the pores in the negative electrodes, which are only partially filled with electrolyte. There, the oxygen is quickly reduced or reacts with the hydrogen, which is present in excess, to form water. The gaseous hydrogen reacts with the storing alloy until thermodynamic equilibrium is reached. In this way, charge balancing is achieved in all the negative electrodes of the cell in quiescent phases, and the excess gas pressure is reduced. It is even possible to reverse the polarity of the cell, since the hydrogen which now evolves at the positive electrode reaches the transport elements and therefore the negative electrodes via the gas phase, and is then oxidized. This sequence of events can be detected by means of a discharge current, e.g. approximately -0.2 V cell voltage, which flows for an unlimited time. Advantageous refinements will emerge from the subclaims. In the case of cells with high load-bearing capacities and electrode thicknesses of less than 0.5 mm, the outer negative electrodes are preferably flanked by a transport element. Consequently, the balancing operations described above take place relatively slowly. When using cells with a thicker electrode, it is advantageous if one or more negative electrodes are split into two parts, the two parts being separated from one another by a hydrophobic, gas-permeable transport element. As a result, the balancing operations which have been described above take place relatively quickly. Preferably, in the sequence of a plurality of positive and negative electrodes, every second negative electrode is split into two parts. The two parts of the split negative electrodes advantageously have half the thickness or half the capacitance of the unsplit negative electrodes. The transport element is, for example, a hydrophobic nonwoven layer, preferably comprising electrolyte-repelling polypropylene fibers. The positive electrodes are, for example, nickel oxide electrodes, preferably fibrous-structure framework electrodes, while the negative electrodes are hydrogen-storing electrodes. The separators preferably comprise polyamide fiber nonwoven or hydrophilic polypropylene fiber nonwoven. A preferred embodiment of the cell according to the invention is diagrammatically depicted in Figure 1. The cell 1 has a prismatic housing 2 with a positive pole 3 and a negative pole 4. In the housing 2 there are positive nickel oxide electrodes 5 and negative electrodes 6a, 6b comprising a hydrogen-storage alloy, which are in each case separated from one another by a separator 7 . All the electrodes are in communication with the gas space 8. Every second negative electrode 6b comprises two half part electrodes 10, 11 which are separated from one another by a gas-permeable transport element 12 in the form of a hydrophobic nonwoven layer. The negative electrode is preferably equipped with a specifically set hydrophobic/hydrophilic balance. For this purpose, the active compound is obtainable from a paste which is composed of a dry fraction and a liquid fraction. In addition to a hydrogen-storage alloy and PTFE, the dry fraction also contains soot; the liquid fraction contains water and an alcohol with 3-6 C atoms, the particles of the storage alloy being covered with PTFE in the manner of fibrils. The addition of soot is important to make the mixture easier to process. The addition of soot makes the mixture pasty and able to flow. In the electrode, the soot promotes the electrical contact on a microscale (up to approximately 500 pm) , i.e. it spans the distances and provides electrical contact between the openings or pores in the substrate material. These distances or openings, with a size of up to 500 um, cannot be bridged by the particles in the paste or the active compound which can be obtained therefrom, since they generally have a diameter of only approximately 10 to 100 um. Furthermore, the soot serves as an oxygen getter for protecting the oxygen-sensitive storage alloy. The PTFE is responsible for the hydrophobic properties of the electrode and enables the three-phase boundary to be set. The reduction of the oxygen and the release and uptake of the hydrogen in the working cell take place in the only partially wetted pores. The PTFE is also responsible for enabling the paste to flow and hold together in the mixing or shaping process. The alcohol is in turn responsible for the temporary wetting of the PTFE powder, since otherwise there would be no distribution through fibrillation in the mixing process. The method for producing the electrode according to the invention is very simple. The components are mixed in a mixer until a cohesive paste is formed. The paste is shaped and combined with the metallic substrate material (for example expanded metal, fabric, grid, perforated sheet) of the electrode. This is an extraordinarily simple process sequence. The electrode according to the invention means that it is also no longer necessary to use a PTFE dispersion with a high wetting agent content, which has to be removed by decomposition at elevated temperature (300°C), damaging the storage alloy, in conventional plastic-bonded storage electrodes. The dry fraction contains 85-95 parts of the storage alloy, approximately 2-10 parts of soot and 3-8 parts of PTFE. The liquid fraction contains 30-70 parts by volume of water and approximately 70-30 parts by volume of the alcohol. Alcohols with a boiling point of the order of magnitude of approximately 100°C/ i.e. for example n-butanol or n-propanol, are particularly suitable. Furthermore, polyethylene glycol may be included in the liquid fraction. The PTFE component means that the finished electrode can only be wetted by lye with extreme difficulty. Therefore, to achieve a sufficient uptake of electrolyte, a polyethylene glycol can be supplied with the make-up water. The proportion of polyethylene glycol is 0.05-0.2% (based on the dry fraction). It is preferable to use a polyethylene glycol with a molecular weight of between 105 and 5 x 106 g/mol. The alcohol used is preferably n-propanol or n~butanol. Depending on the soot content, the ratio of the dry fraction to the liquid fraction is between 4:1 and 6:1, based on mass. The electrode according to the invention is preferably used in an alkaline storage battery with positive nickel oxide electrode. The electrode according to the invention is produced by rolling a dough-like paste onto a structured metal substrate, such as for example an expanded metal or grid mesh. The dough-like paste is prefabricated in a first production step by a mixing and kneading process. The solid and liquid components are mixed in a kneading machine until a cohesive paste is formed, for example in a stable domestic kneading machine. The PTFE particles are fibrillated by the hard compound grains and hold the paste together. The electrode is shaped either by manual rolling or in a rolling train. Either a sheet is produced and is combined with the substrate after drying or the kneaded compound is applied directly to the substrate and is then dried. WE CLAIM:- 1. Gastight sealed nickel/metal hydride cell for the electrochemical storage of energy, with at least one positive nickel oxide electrode and at least one negative electrode storing hydrogen, wherein a hydrophilic separator is arranged between the positive and negative electrode, together with an alkaline electrolyte or an alkaline electrolyte mixture, i*, characterised in that, in the sequence of several positive and negative electrodes, every second electrode is divided into two parts, and that one or more negative electrodes is or are provided with a gas-permeable hydrophobic transport element to transport the gases of the cell atmosphere. 2. Cell as claimed in claim 1, wherein the outer negative electrodes of an electrode package of n positive and n+1 negative electrodes are each flanked by a transport element. 3. Cell as claimed in claim 1, wherein one or more negative electrodes is or are divided into two parts, wherein the two parts are separated from one another by a hydrophobic gas-permeable transport element. 4. Cell as claimed in claim 1, wherein the two parts of the divided negative electrodes have half the thickness and half the capacity of the undivided negative electrodes. 5. Cell as claimed in any of the preceding claims, wherein the transport element is a hydrophobic mat layer, preferably made of electrolyte-repellent polypropylene fibres. 6. Cell as claimed in any of the preceding claims, wherein the positive electrodes are fibre-structured framework electrodes. 7. Cell as claimed in any of the preceding claims, wherein the separators are made of polyamide fibre mat or hydrophilic polypropylene fibre mat. 8. Cell as claimed in any of the preceding claims, wherein it has an electrode with storage capacity for hydrogen, with a metallic carrier material to which is applied an active material, wherein the active material is obtainable from a paste comprised of a dry component and a liquid component, wherein the dry component comprises a mixture of a powdery storage aljoy for hydrogen, carbon black and polytetrafluoroethylene (PTFE), wherein the particles of the storage alloy are coated with PTFE in fibrillated form, and the liquid component is comprised of a mixture of water and an alcohol with 3 to 6-atoms. 9. Cell as claimed in claim 8, wherein the dry component contains 85 to 95 parts of the storage alloy for hydrogen, 2 to 10 parts carbon black, and 3 to 8 parts PTFE. 10. Cell as claimed in any of claims 8 and 9, wherein the liquid component contains 30 to 70 parts by volume of water and 70 to 30 parts by volume of the alcohol, together with 0.05 to 0.2% (based on the dry component)PEG. 11. Cell as claimed in any of claims 8 to 10, wherein the liquid component also contains polyethylene glycol (PEG). 12. Cell as claimed in any of claims 8 to 11, wherein the mass ratio between the dry component and the liquid component is 4:1 to 6:1. Dated this 7th day of January, 2005. [JAYANTA PAL] OF REMFRY & SAGAR ATTORNEY FOR THE APPLICANTS

Documents:

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in-pct-2002-00024-mum-cancelled pages(13-6-2005).pdf

in-pct-2002-00024-mum-claims(granted)-(13-6-2005).doc

in-pct-2002-00024-mum-claims(granted)-(13-6-2005).pdf

IN-PCT-2002-00024-MUM-CORRESPONDENCE(1-5-2009).pdf

in-pct-2002-00024-mum-correspondence(23-3-2006).pdf

in-pct-2002-00024-mum-correspondence(ipo)-(28-6-2004).pdf

in-pct-2002-00024-mum-drawing(13-6-2005).pdf

in-pct-2002-00024-mum-form 13(1-5-2009).pdf

in-pct-2002-00024-mum-form 19(19-4-2004).pdf

in-pct-2002-00024-mum-form 1a(13-6-2005).pdf

in-pct-2002-00024-mum-form 2(granted)-(13-6-2005).doc

in-pct-2002-00024-mum-form 2(granted)-(13-6-2005).pdf

in-pct-2002-00024-mum-form 3(11-6-2005).pdf

in-pct-2002-00024-mum-form 3(7-1-2002).pdf

in-pct-2002-00024-mum-form 5(13-6-2005).pdf

in-pct-2002-00024-mum-form 5(7-1-2002).pdf

in-pct-2002-00024-mum-petition under rule 137(13-6-2005).pdf

in-pct-2002-00024-mum-power of authority(11-6-2005).pdf

in-pct-2002-00024-mum-power of authority(18-1-2002).pdf