The present invention generally relates to electrochemical cells and, more particularly, to a current collector and seal assembly for an electrochemical cell.
Alkaline electrochemical cells typically include a cylindrical steel can having a closed end, an open end, and side walls extending therebetween. The can contains electrochemically active materials which include a positive electrode, commonly referred to as the cathode, comprising manganese dioxide as the active material, and a negative electrode, commonly referred to as the anode, comprising zinc powder. Conventional cells often employ a bobbin-type construction in which the anode is centrally located and is surrounded by a tubularly shaped cathode which contacts the can walls. A separator is generally located between the anode and cathode, and an alkaline electrolyte solution simultaneously contacts, the cathode, the anode, and the separator.
Conventional electrochemical cells have a conductive current collector, which is typically in the shape of an elongated brass nail, inserted into the anode active material, and a seal assembly which provides closure to the open end of the steel can. The current collector generally extends through an opening, i.e., aperture, provided in the seal and provides an electrical connection between the anode and an outer conductive cover which serves as the negative terminal. The current collector, seal, and an inner metal cover are usually preassembled together to form what is commonly referred to as a current collector and seal assembly. The seal has a centrally located opening through which the current collector protrudes, and a sealant is typically disposed between the current collector and seal. The outer perimeter of the seal and the central portion of the seal which surrounds the centrally located opening are usually reinforced by a thickening of the seal's material. Between the outer perimeter and central portion, is a diaphragm which often has a thinned section for providing a stress concentration pressure release vent for allowing the seal to rupture when the cell's internal pressure exceeds a predetermined limit, to thereby vent high pressure gases from within the cell.
The reinforced central portion of the seal that surrounds and defines the centrally located opening is commonly referred to as the "hub." The current collector is inserted through the opening in the hub so that an interference fit exists between the seal hub and the collector. In some commercially available cells, the diameter of the collector nail is usually greater than the inside diameter of the opening to create an interference fit so that electrolyte cannot escape from the cell along the surface of the collector. Often, the interference fit results in the creation of tangential tension which, if excessive, may exceed the seal hub's material strength and cause the seal to split and allow electrolyte solution to escape. On the other hand, if the interference fit is insufficient, electrolyte solution may escape between the collector and the seal.
A number of current collector and seal assemblies are known for maintaining a sealed closure between the seal and current collector. One approach uses a plastic coated metallic sleeve located on the interior surface of the seal for compressing the plastic against the collector to prevent leakage of electrolyte. Yet, another approach discloses the use of an inner metal cover, in which the collector is inserted through the seal's central opening so that the upstanding wall of the seal hub which surrounds the collector is forced outward against the inner metal cover. However, the current collector exerts tangential tension against the seal's upstanding wall forming the hub. Additionally, one commercially available battery has employed a flat metallic ring around the seal body so that the seal is compressed between the collector and ring as the collector is forced through the seal's central opening. Many of the above assemblies result in the creation of tangential tension in the seal which, when exposed to potassium hydroxide, may result in stress corrosion cracking which may allow for electrolyte leakage. In an attempt to minimize the likelihood of stress corrosion cracking, prior approaches often require coating the seal with a protectant such as asphalt.
A more recent approach is disclosed in U.S. Pat. No. 5,422,201, entitled "CURRENT COLLECTOR ASSEMBLY FOR AN ELECTROCHEMICAL CELL," which is incorporated herein by reference. The aforementioned patent discloses the use of a compression means in the form of a tubularly shaped metallic component having one end flared radially outward. The metallic component is inserted around the upstanding wall forming the hub of the seal to compress a central opening formed in the hub. With the tubular metallic component in place, the current collector nail is driven upward through the compressed opening from the bottom side. In doing so, the compressed opening is forcibly increased in diameter by an amount that prevents the creation of tangential tension in the seal's hub, yet allows for compression of the hub against the current collector. The flared end of the tubular shaped metallic component creates a shear edge against the seal's vent; however, the metallic component is orientation sensitive and, therefore, may be difficult to handle and assemble with modern cell manufacturing equipment.
While known current collector assemblies have been used for many years, there exists a need for an improved current collector and seal assembly that is easy to assemble, imparts little or no tangential tension on the seal's hub to avoid hub splitting, and provides sufficient radial compressive stress and tangential compressive stress on the seal's hub.