Patent Application
20050277024
The disclosure relates to improving the performance of batteries by reducing the internal resistance of the electrodes. In one embodiment, the present invention may be illustrated as a system and method for increasing battery performance by precoating the electrode plates with Nickel, to improve the adherence of the metal hydrides, or other electrolytic materials, to a metallic substrate.
There are a number of processes to form electrodes for use in rechargeable cells. One well known method involves sintering. Sintered positive nickel hydroxide electrodes are typically formed by sintering nickel powder at elevated temperatures to form a porous three dimensional plaque and then impregnating this porous structure with nickel salts which are subsequently converted to nickel hydroxide. The three-dimensional substrate refers to a porous substrate having at least a portion of its interior surface area in contact with electrochemically active material.
U.S. Pat. No. 2,724,733, for example, describes a sintered electrode structure for a nickel cadmium cell having nickel hydroxide as the active material for the sintered positive electrode. U.S. Pat. No. 3,826,684 also describes a nickel cadmium cell having a sintered positive nickel electrode containing additives to enhance charge acceptance. However, the manufacturing process for the sintered electrode is capital intensive. The porous three-dimensional sinter plaque structure is impregnated by immersing the plaque in a nickel salt bath and converting the nickel salt to nickel hydroxide by the reaction of the salt with caustic. To obtain an electrode containing a sufficient amount of active material, this immersion step must be repeated several times. The sintered nickel electrode therefore requires a costly, complicated and time consuming process for its fabrication.
Another process to form electrodes involves forming a paste from nickel hydroxide and other additives and depositing the paste into an appropriate three-dimensional substrate. Such three-dimensional substrates are typically formed by nickel plating polyurethane foams, non woven felts and carbon fiber mats, and are referred to in the art as sponge metal, foamed porous metal, porous metal matrix, and felt. U.S. Pat. Nos. 4,251,603 and 4,582,098 are representative of processes which use a sponge-like three-dimensional porous metal matrix to form the electrode. Specifically, U.S. Pat. No. 4,251,603 describes a battery electrode having a carrier made of a sponge-like porous metal matrix having a multiplicity of spaces throughout. An active material paste fills the matrix followed by drying, calendering and cutting to the desired size. U.S. Pat. No. 4,582,098 describes a method of spraying a pasty mixture into a porous three-dimensional metal body from both sides so as to fill the pores with the mixture. As compared to sintered electrodes, foam electrodes provide greater capacity, e.g., 1250 mAH as compared to 1100 mAH in a typical AA cell. The higher porosity of the foam material is believed to contribute to the greater capacity of the foam electrode. However, the materials and manufacturing costs for foam positive electrodes are also high.
Both of these processes for manufacturing an electrode require that a porous three-dimensional substrate be filled with active material, a process more costly and complex than a coating process. Efforts have been undertaken to provide for a rechargeable positive electrode with a two-dimensional current collector substrate supporting a layer of active material on each major surface of the substrate. A two-dimensional current collector or electrically conductive substrate refers to a substrate such as a foil sheet where the substrate thickness is not coextensive with the electrode thickness and the electrochemically active material contacts essentially the external surface area of the substrate. Volume changes that occur in the positive electrode have hampered efforts to provide satisfactory adhesion between the two-dimensional conductive substrate and the active material when the material is coated onto the substrate. As a result, the active material separates from the substrate, reducing capacity and increasing resistance. Various non-coating methods for forming a positive electrode with a two-dimensional current collector substrate have been proposed. For example, U.S. Pat. No. 3,898,099 describes an electrode sheet formed of active material and polytetrafluoroethylene (PTFE) fibers as a binder wherein the sheet is subsequently pressed onto a metal screen or foil current collector. This process is complex in that the active material electrode sheet is formed by repeated working of the paste by rolling and folding in order to form a cohesive sheet. A current collector substrate is then sandwiched between sheets to form the electrode assembly followed by additional pressure to the assembly to form the finished electrode. Another proposed solution to the adhesion problem for a positive electrode using a two-dimensional substrate is to corrugate a two-dimensional substrate and then apply a thin sintered metal layer on the surfaces of the corrugated substrate prior to coating with an active material. The required additional working of the substrate to provide a corrugated profile adds additional manufacturing costs to the electrode.
Further, when using a metal hydride (NiMH) powder to form a negative electrode, a problem exists when it is extruded onto a very smooth nickel plated steel substrate. Moreover, nickel presintering has been tried as a means for increasing the adhesion between the NiMH powder and metal substrate, but conventional NiMH plates do not have an edge prepared for the subsequent welding operation to attach the current collectors onto a coiled electrode.
It would therefore be desirable to provide a positive and negative electrode plate with lower internal resistance, to improve cell performance, better adhesion of electrolytic powders to the metal substrates, and a well-prepared edge to facilitate the connection of a coiled electrode to a current collector.
The present disclosure relates to a system and method for providing improved electrodes for use in energy storage devices. In a preferred embodiment, the method may include sintering a layer of powdered metal onto a metal substrate upon which an electrolytically active material may be disposed to create an electrode. One of the preferred substrate materials is nickel plated steel. One of the preferred powdered metals is nickel. The metal may preferably be sintered on to the metal substrate at about the glass transition temperature of the powdered metal.
In a preferred embodiment, an electrode may have a layer of sintered metal on a metal substrate over which an electrolytically active or non-active material may be placed. Such electrolytically active and non-active materials may include, for example, Ni(OH)2, which may be formed on the layer of sintered, powdered metal, sintered Nickel powder, Nickel foam or NiMH, which may be disposed by means such as extrusion, spraying, painting, etc. The present specification provides an example of the use of extrusion as a means for applying a layer of material onto a metal substrate. Those skilled in the art will recognize that a precoating method and system in accordance with the present invention may be applied to electrode manufacturing methods that do not employ extrusion. Further, such an electrode may also include wiped edges to provide a clean surface upon which to connect a current collector the electrode.
The present invention, in a preferred embodiment, relates to an electrical energy storage device and, more specifically, to rechargeable storage cells. By way of example and illustration, the present specification describes D-Cell batteries. It is noted, however, that each of the principles and discoveries mentioned herein apply with equal weight to cells having a coiled energy storage device, such as AA, AAA, C, for example, and other cells, such as those which do not use cylindrically wound coils like prismatic batteries, oval cells, etc. Exemplary energy storage devices for use in accordance with the presently disclosed system and method are described in U.S. Pat. No. 6,265,098, U.S. Pat. No. 5,667,907, U.S. Pat. No. 5,439,488, and U.S. Pat. No. 5,370,711, each of which is hereby incorporated by reference in its entirety.
In the manufacture of rechargeable energy storage cells, metal substrates may be coated with powdered electrolytic material to form an electrode.
It has been found that by sintering a layer of powdered nickel, preferably a thin layer, onto the nickel steel substrate, the adhesion of the NiMH active material is improved, providing a negative electrode with lower internal resistance than one produced without a layer of sintered nickel on the steel substrate. The layer of material deposited on the substrate may be electrolytically active or non-active. In general, the layer may comprise transition metals that provide a precoat on a nickel plated steel or pure nickel substrate. Other metals may be used for the substrate, depending on the type of electrode being produced and whether the electrode will be used on a wound, prismatic, or other type of cell. Moreover, a second layer of non-electrolytic material may be sintered onto or otherwise deposited over the sintered precoat to further improve the adhesion of electrolytic material to the substrate and improve the overall reliability and performance of the electrode. Suitable materials for either or both layers may include, without limitation, transition metals, powders and foams thereof, and preferably be selected from the group comprising nickel, zinc, zinc oxide, nickel foam, porous nickel, porous cadmium, and porous silver. Additional suitable materials may include pure nickel, copper, aluminum, and alloys comprising transition metals.
Illustrative of the present method for improving the performance of a negative electrode, a presintering composition comprising at least powdered nickel and binders is applied to the Ni plated steel substrate. Once the Ni powder is coated onto the steel substrate, it then goes through a doctor blade for wiping clean edges to facilitate the attachment of current collectors to the electrode. An exemplary device 100 for coating a substrate is shown in
A metal hydride paste is then disposed onto the nickel sintered substrate through an extruding machine. During application and processing of the metal hydride, the edges are wiped cleaned by doctor blades that may be incorporated into the extruder, although they may be provided elsewhere in the process. The nickel powder is fused to the substrate before the metal hydride powder is extruded onto the substrate, in order to prevent flaking of the active material by providing a surface upon which the metal hydride paste can adhere to, as shown in
After metal hydride is extruded onto the sintered substrate, it is dried and wound in preparation for sizing. Once it is sized it is ready for slitting and winding.
In one illustrative embodiment, a doctor blade used in an extruder in accordance with the present method may comprise pressure pad slots divided into two halves and a wiper slot also divided into two halves, as shown in
An energy storage device in accordance with the present invention may be used for storing and supplying energy in a variety of different environments and for a variety of different purposes. For example, an energy storage device in accordance with the present invention may be used for storing and supplying energy in transportation vehicles, including for example ground transportation vehicles, air transportation vehicles, water surface transportation vehicles, underwater transportation vehicles, and other transportation vehicles. An energy storage device in accordance with the present invention may be used for storing and supplying energy in communication and entertainment devices, including for example telephones, radios, and televisions, and other communication and entertainment devices. An energy storage device in accordance with the present invention may be used for storing and supplying energy in home appliances as an alternative to AC current sources or in conjunction therewith. When used with an appropriate power inverter, energy storage devices of the present invention may also provide a substitute for AC current sources when such sources are unavailable or inconvenient. The examples described in this paragraph are merely representative, not definitive.
