Patent Grant
10403903
This invention relates to the art of electrochemical cells, and more particularly, to a new and improved electrochemical cell construction that provides for increased energy density. The cell comprises a Group IA anode or negative electrode and a fluorinated carbon (CFx) cathode or positive electrode.
Electrochemical cells provide electrical energy that powers a host of electronic devices such as external and implantable medical devices. Among the many medical devices powered by electrochemical cells are leadless pacemakers. Leadless pacemakers are typically cylindrically shaped devices that are inserted into the body through a circular catheter and attached directly to heart tissue. Leadless pacemaker devices contain both the device circuitry and electrical power source in one container. In some embodiments, the electrochemical cell power source may comprise up to 80 percent of the volume of a leadless pacemaker device. In addition, leadless pacemaker devices are designed to operate within the body for an extended period of time, as long as 10 years or more. Thus, it is desirable to provide an electrochemical cell having a reduced size without sacrificing energy capacity of the cell. Such an electrochemical cell having a reduced size with increased energy density would, therefore, enable the development of less invasive miniaturized leadless pacemakers as well as other medical devices that require a relatively small electrical power source with increased capacity.
Prior art electrochemical cells designed to power miniaturized medical devices, such as a leadless pacemaker, are generally of a bobbin-type construction as illustrated in
In addition, the cathode of cylindrically shaped cells, may radially expand within the casing during discharge. Radial expansion of the cathode is not desired as the expanding cathode can interfere with the intended operation of the cell.
Thus, the electrochemical cell of the present invention is constructed having a cathode current collector of a variety of shapes and configurations designed to minimize radial expansion of the cathode and instead encourage axial expansion of the cathode within the casing.
Thus, as will be discussed in more detail hereinafter, the cathode assembly of the present invention comprises a unique structure that provides a lithium electrochemical cell with increased energy density.
The present invention provides an electrochemical cell comprising a cathode or positive electrode comprising a solid cathode active material, such as fluorinated carbon, and an anode or negative electrode comprising an active material, such as lithium or LiC6. The cathode and anode are positioned within the casing such that the anode and cathode are adjacent to each other. In comparison to other prior art lithium/fluorinated carbon cells, such as cells of a bobbin-type construction, the structure of the electrochemical cell of the present invention increases energy density. In addition, the electrochemical cell of the present invention is constructed to minimize radial expansion of the cathode during cell discharge.
The electrochemical cell of the present invention is constructed to minimize unoccupied casing volume and minimize utilization of non-electrochemically active materials, such as separator and current collector materials. For example, the electrode assembly is constructed such that the interface between the anode and cathode is oriented perpendicular to the length of the cell. This configuration, therefore, minimizes the interfacial electrode gap in comparison to prior art designs, such as a bobbin-type cell.
In another embodiment, the preferred fluorinated carbon (CFx) cathode active material is formed into a cylindrical shape that is positioned in intimate contact with a cathode current collector that comprises a variety of shapes and configurations. The various cathode and current collector constructions of the present invention increase utilization of internal casing volume and minimize potential radial expansion of the cathode active material. In an embodiment, the current collector of the present invention is constructed having a variety of angled configurations that encourage axial movement of the cathode active material during cell discharge.
In still another embodiment, the cathode current collector is constructed in a folded or corrugated configuration wherein portions of the current collector that lie on opposing sides of a fold are oriented at an angular relationship to one another. In an alternate embodiment, the cathode current collector has a helical cork screw configuration. In either case, the current collector is designed to encourage axial movement of the cathode such that the cathode active material moves towards the anode, therefore minimizing radial expansion of the cathode. Thus, the various configurations of electrochemical cells according to the present invention provide a cell having a reduced volume and increased energy density in comparison to the prior art cells, particularly those of a bobbin-type construction. Furthermore, the increased energy density of an electrochemical cell of the present invention provides for increased cell life.
The electrode assembly is activated with a nonaqueous electrolyte hermetically sealed inside the casing. This makes the present cell particularly useful for powering implantable medical devices of a relatively small size, such as a leadless cardiac pacemaker.
These and other aspects of the present invention will become more apparent to those skilled in the art by reference to the following description and the appended drawings.
Referring now to the drawings,
Preferably, the cell 10 is built in a case negative design with the casing 22 serving as the anode terminal. In addition, the cathode 18 includes a cathode active material 30 that is in electrical contact with a cathode current collector. The cathode current collector is electrically connected to a terminal pin 34 that exits the cell 10 through the header assembly 24.
In an embodiment, the anode 20 is positioned within the casing 22 distal the cathode 18 in the case-negative cell 10 design of the present invention. As shown in
A prior art cell 44 of a bobbin-type construction is shown in the cross-sectional view of
In addition, cylindrically shaped cathodes may expand in a radial direction during discharge as the cathode electrochemically reacts with the anode. Radial expansion of the cathode in a cylindrically shaped cell is generally not desired as the cathode active material may cause undesirable mechanical pressure on the interior casing surface.
Furthermore, since radial expansion of the cathode within some cylindrically shaped cell configurations is taken into account during cell design, miniaturization of the cell is generally limited because the diameter of the cylindrically shaped casing is designed to accommodate for radial expansion of the cathode.
In contrast to the design of the bobbin-type prior art cell 44 shown in
Referring to
In addition to maximizing utilization of internal casing volume, the electrochemical cell 10 of the present invention comprises a cathode current collector 46 having an angled configuration. In a preferred embodiment, the cathode current collector 46 having the preferred angled configuration is designed to minimize radial expansion and provide for axial movement of the cathode active material 30 along the imaginary axis A-A in a distal direction within the cell casing 22 during discharge. It is noted that the cathode current collector may be formed into a variety of non-limiting angled configurations.
As illustrated, the current collector 46 is oriented such that adjacent top surfaces 54, for example surfaces 54A and 54B, or adjacent bottom surfaces 56, for example surfaces 56A and 56B, that reside on opposing sides of a respective fold or crease 58 are oriented at an angular relationship with respect to each other. In an embodiment, a top fold angle θ extends between adjacent top surfaces 54A, 54B that reside on opposite sides of the fold 58. A bottom fold angle ϕ extends between adjacent bottom surfaces 56A, 56B that reside on opposite sides of a fold or crease 58. In an embodiment the top fold angle θ or the bottom fold angle ϕ may be obtuse angles. In a preferred embodiment, the top fold angle θ or the bottom fold angle ϕ may range from about 100° to about 170°. Moreover, there is at least two folds at spaced locations along the length of the current collector. The embodiment shown in
In an embodiment, the cathode current collector is designed to flex in an axial direction along the longitudinal axis A-A within the casing during cell discharge. As such, the thickness 52 of the cathode current collector 46 is designed to be relatively thin and flexible having a thickness ranging from about 0.0025 cm to about 0.25 cm. This preferred thickness is designed to promote flexure of the current collector 46 during operation of the cell 10. During cell discharge, the folded configuration of the current collector 46 is designed to relax thereby extending the current collector distal end 50 in a distal direction within the cell 10 thus encouraging axial movement of the cathode within the cell 10. In an embodiment, the current collector 46 of the present invention may comprise a plurality of perforations 62 that extend through the current collector thickness 52. These perforations 62 are designed to aid in adhesion of the cathode active material 30 to the surface of the current collector 46. In a further embodiment, the current collector 46 may be composed of a flexible metal comprising a grid foil structure, such as the Microgrid® expansion metal made by Dexmet® of South Wallingford Conn. In an embodiment, the flexible metal comprising the grid foil structure is designed to expand during cell discharge thereby moving the cathode active material 30 in an axial direction towards the anode active material 40.
In an embodiment, the cathode current collector 46 shown in
In an embodiment, the helically shaped current collectors 64, 70 shown in
In an embodiment, the anode and cathode current collectors 42, 46, 64, and 70 may be composed of a variety of metals including, but not limited to, titanium, carbon coated titanium, tantalum, platinum, gold, aluminum, cobalt nickel alloys, highly alloyed ferritic stainless steel containing molybdenum and chromium, and nickel-, chromium- and molybdenum-containing alloys, the former being preferred. The cathode active material is preferably carbon or fluorinated carbon, CFx.
Referring now to
The upper lid 78 is preferably a disc-shaped member of the same material as the casing and comprises an upper lid surface 86 and a lower surface 88 extending to a cylindrical sidewall 90. The lid 78 is in a snug fitting relationship with the proximal open end 12 of the casing 22. In that position, the upper lid surface 86 is coplanar with the proximal end of the casing sidewall 23.
As shown in
In an embodiment, the cathode 18 in the case negative cell configuration 10 is enveloped in an ionically porous polymeric material serving as the separator 28 (
Those skilled in the art will readily recognize that the cell of the present invention can also be readily adapted to primary electrochemical systems of a liquid cathode type, or a secondary cell such as a lithium ion cell having a carbonaceous negative electrode and lithium cobalt oxide positive electrode.
By way of example, in an illustrative primary cell, the anode is of an alkali metal, preferably lithium, contacted to a nickel, copper or stainless steel current collector. The preferred cathode active material is CFx. This electrochemical couple is preferably activated with an electrolyte that can be a 1.0M to 1.4M solution of BF4, LiAsF6 or LiPF6 in γ-butyrolactone.
In the secondary electrochemical cell, the anode or negative electrode comprises an anode material capable of intercalating and de-intercalating the anode active material, such as the preferred alkali metal lithium. A carbonaceous negative electrode comprising any of the various forms of carbon (e.g., coke, graphite, acetylene black, carbon black, glass carbon, etc.), which are capable of reversibly retaining the lithium species, is preferred for the anode material. A material that is suitable for the present invention is described in U.S. Pat. No. 5,443,928 to Takeuchi et al., which is assigned to the assignee of the present invention and incorporated herein by reference. Graphite is another preferred material. Regardless the form of the carbon, fibers of the carbonaceous material are particularly advantageous because they have excellent mechanical properties that permit them to be fabricated into rigid electrodes capable of withstanding degradation during repeated charge/discharge cycling. Moreover, the high surface area of carbon fibers allows for rapid charge/discharge rates.
Also in secondary systems, the positive electrode preferably comprises a lithiated material that is stable in air and readily handled. Examples of such air-stable lithiated cathode active materials include oxides, sulfides, selenides, and tellurides of such metals as vanadium, titanium, chromium, copper, molybdenum, niobium, iron, nickel, cobalt and manganese. The more preferred oxides include LiNiO2, LiMn2O4, LiCoO2, LiCo0.92Sn0.08O2, LiCo1-xNixO2, LiFePO4, and LiNixCOyMn(1-y) O2.
An electrolyte is also required to activate the anode/cathode combination in the secondary system. The composition of the electrolyte depends on the materials of construction of the anode and the cathode as well as the product application for the cell. A preferred electrolyte for a lithium ion secondary cell has a lithium salt dissolved in a solvent system of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate and propylene carbonate.
It is appreciated that various modifications to the inventive concepts described herein may be apparent to those of ordinary skill in the art without departing from the scope of the present invention as defined by the appended claims.
This application claims priority from U.S. Provisional Application Ser. No. 62/348,269, filed Jun. 10, 2016.
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