Patent Description:
Silicon, germanium, tin, and many other materials are attractive active materials because of their high electrochemical capacity. For example, silicon has a theoretical capacity of about <NUM> mAh/g, which corresponds to the Li. Yet, many of these materials are not widely used in commercial lithium ion batteries. One reason is that some of these materials exhibit substantial changes in volume during cycling. For example, silicon swells by as much as <NUM>% when charged to its theoretical capacity. Volume changes of this magnitude can cause substantial stresses in the active material structures, resulting in fractures and pulverization, loss of electrical and mechanical connections within the electrode, and capacity fading.

Conventional electrodes include polymer binders that are used to hold active materials on the substrate. Most polymer binders are not sufficiently elastic to accommodate the large swelling of some high capacity materials. As a result, active material particles tend to separate from each other and the current collector. Overall, there is a need for improved applications of high capacity active materials in battery electrodes that minimize the drawbacks described above.

The foregoing discussion of the prior art derives from <CIT> and <CIT> in which the inventors propose addressing the elasticity and swelling problems of prior art materials by providing electrochemically active electrode materials comprising a high surface area template containing a metal silicide and a layer of high capacity active material deposited over the template. The template reportedly serves as a mechanical support for the active material and/or an electrical conductor between the active material and, for example, a substrate. According to the inventors, due to the high surface area of the template, even a thin layer of the active material can provide sufficient active material loading and corresponding electrode capacity per surface area. As such, the thickness of the active material layer may be maintained sufficiently small to be below its fracture threshold to preserve its structural integrity during battery cycling. The thickness and/or composition of the active layer may also be specifically profiled to reduce swelling near the substrate interface and preserve the interface connection.

<CIT> discloses a battery anode comprised of a coated metallic nanowire array.

<CIT> discloses a battery technology, more specifically a current collector, which can be generally used in a secondary battery, and an electrode using the same.

<CIT> discloses an anode including a plurality of metal fibers with a three-dimensional (3D) network structure, and a silicon-containing layer having a thickness of about <NUM> or less formed on a surface of and inside the 3D network structure of the plurality of metal fibers and a lithium battery including the same.

<CIT> discloses a process for making a valve metal material useful for forming electrolytic devices comprising the steps of: establishing multiple tantalum or niobium components in a billet of a ductile material; working the billet to a series of reduction steps to form said tantalum or niobium components into elongated elements; cutting the resulting elongated elements and leaching the ductile metal from the elements; washing and mixing the cut elements; and forming the cut elements into a sheet. The resulting sheet may be formed into anodes and cathodes and assembled to form a wet electrolytic capacitor.

The present invention overcomes the aforesaid and other disadvantages of the prior art by providing electrodes formed of extremely fine filaments of the valve metal tantalum or other valve metals produced following the teachings of my prior <CIT> and <CIT>.

In my prior <CIT>, I disclose an approach to the production of extremely fine valve metal filaments, such as tantalum, for capacitor use. The benefits of fine filaments relative to fine powders are higher purity, lower cost, uniformity of cross section, and ease of dielectric infiltration, while still maintaining high surface area for anodization. The uniformity of cross section results in capacitors with high specific energy density, lower ESR and ESL, and less sensitivity to forming voltage and sintering temperature as compared to fine powder compacts.

As disclosed in my aforesaid '<NUM> U. patent, valve metal filaments, preferably tantalum, are fabricated by combining filaments of the valve metal with a ductile metal so as to form a billet. The second, ductile metal is different from the metal that forms the filaments. The filaments are substantially parallel, and are separated from each other and from the billet surface by the second, ductile metal. The billet is reduced by conventional means--e.g., extrusion and wire drawing--to the point where the filament diameter is in the range of <NUM> to <NUM> microns in diameter. At that point, the second, ductile metal is removed, preferably by leaching in mineral acids, leaving the valve metal filaments intact. The filaments are suitable for use in tantalum capacitor fabrication.

Other patents involving valve metal filaments and fibers, their fabrication, or articles made therefrom include <CIT>), <CIT>), <CIT>), <CIT>), <CIT>), <CIT>), <CIT>), <CIT>), <CIT>), <CIT>), and <CIT>).

See also my earlier <CIT> in which I describe a process for fabrication of fine-valve metal filaments for use as porous metal compacts used in the manufacture of electrolytic capacitors. According to my '<NUM> U. patent, a metal billet consisting of multiple filaments of a valve metal, preferably tantalum, is contained within and spaced apart by a ductile metal, preferably copper. The billet is reduced by conventional means, such as extrusion and wire drawing, the resulting composite product is cut into lengths, and the ductile metal separating the valve metal components is removed by leaching in acid. A similar compaction technique has been proposed to fabricate composites by providing continuous layers of tantalum and copper sheets layered together in a jellyroll. The jellyroll is then reduced to a small size by extrusion and drawing. Starting with sheets of tantalum and copper offers advantages over working with filaments. However, at reduced sizes, the copper cannot readily be leached out due to the presence of the continuous tantalum layers.

Also, in my prior <CIT>, I describe improvements over the prior art much as described in my '<NUM> U. patent by creating one or more open slots in the starting billet stage and filling the slots with ductile metal prior to extrusion and drawing. After extrusion and drawing to small size, the slots remain. As a result, the ductile metal readily may be leached and removed from between the tantalum layers. The resulting product is a series of compacted tantalum layers each progressively of smaller width. In one embodiment of the invention, continuous layers of tantalum and copper are layered together in a jellyroll and formed into a billet which is circular in cross-section, and the slots are concentrically evenly spaced radially around the billet. The resulting product is a series of concentric split tubes each progressively of smaller diameter towards the center.

As described in my '<NUM> patent, employing a foil or sheet of tantalum as opposed to filaments greatly simplifies assembly of the billet. Employing sheet tantalum also ensures greater uniformity since the thickness of the starting sheet can be controlled more readily than using a multiple of separate filaments. This in turn produces substantially more uniform capacitor material resulting in substantially higher values of CV/g. See also my prior <CIT> and <CIT>.

I have now found that electrodes formed of extremely fine valve metal filaments as described in my aforesaid U. Patents advantageously may be employed as electrode material for high capacity rechargeable batteries, particularly lithium ion rechargeable batteries.

The present invention provides a method according to claim <NUM> of forming an electrode substrate useful for forming a lithium ion battery comprising the steps of:.

In one embodiment the valve metal is selected from the group consisting of tantalum, niobium, an alloy of tantalum, an alloy of niobium, hafnium, titanium and aluminum.

In another embodiment the filaments have a thickness of less than about <NUM>-<NUM> microns, preferably below about <NUM> micron.

The electrochemically active material comprises silicon nanoparticles, germanium or tin.

In still yet another embodiment, the electrically active electrode material is formed into an anode.

Further features and advantages of the present invention will be seen from the following detailed description, taken in conjunction with the accompanying drawings, wherein,.

Referring to <FIG>, the production process starts with the fabrication of valve metal filaments, preferably tantalum, by combining filaments or wires of tantalum with a ductile material, such as copper to form a billet at step <NUM>. The billet is then sealed in an extrusion can in step <NUM>, and extruded and drawn in step <NUM> following the teachings of my '<NUM> U. The extruded and drawn filaments are then cut or chopped into short segments, typically <NUM>/16th-<NUM>/4th inch long at a chopping station <NUM>. Preferably the cut filaments all have approximately the same length. Actually, the more uniform the filament, the better. The chopped filaments are then passed to an etching station <NUM> where the ductile metal is leached away using a suitable acid. For example, where copper is the ductile metal, the etchant may comprise nitric acid.

Etching in acid removes the copper from between the tantalum filaments. After etching, one is left with a plurality of short filaments of tantalum. The tantalum filaments are then washed in water in a washing station <NUM>, and the wash water is partially decanted to leave a slurry of tantalum filaments in water. The slurry of tantalum filaments in water is then cast as a thin sheet using, for example, a Doctor Blade at casting station <NUM>. Excess water is removed, for example, by rolling at a rolling station <NUM>. The resulting mat is then further compressed and dried at a drying station <NUM>.

As an alternative to "Doctor Blade formation", the thin sheet may be formed by spray casting the slurry onto to a substrate, excess water removed and the resulting mat pressed and dried as before.

There results a highly porous thin sheet of tantalum filaments substantially uniform in thickness.

As reported in my aforesaid PCT application, an aqueous slurry of chopped filaments will adhere together sufficiently so that the fibers may be cast as a sheet which can be pressed and dried into a stable mat. This is surprising in that the metal filaments themselves do not absorb water. Notwithstanding, as long as the filaments are not substantially thicker than about <NUM> microns, they will adhere together. On the other hand, if the filaments are much larger than about <NUM> microns, they will not form a stable mat or sheet. Thus, it is preferred that the filaments have a thickness of less than about <NUM> microns, and preferably below <NUM> micron thick. To ensure an even distribution of the filaments, and thus ensure production of a uniform mat, the slurry preferably is subjected to vigorous mixing by mechanical stirring or vibration.

The density of the resulting tantalum mat may be varied simply by changing the final thickness of the mat.

Also, if desired, multiple layers may be stacked to form thicker mats <NUM> that may be desired, for example, for high density applications.

The resulting tantalum mat comprises a porous mat of sub-micron size tantalum filaments in contact with one another, whereby to form a conductive mat.

Referring to <FIG>, mats <NUM> of electrode material produced by the process above described are then coated with a suitable electrochemical active material at coating station <NUM>. For example, in the case of a lithium ion battery, the electrode material forming the mats should be coated with electrochemically active material such as silicon nanoparticles that take and release lithium ions during cycling of the lithium ion cell. The coated mats are then assembled in a stack at an assembly station <NUM>, between separator sheets <NUM> to form positive (anode) and negative (cathode) electrodes <NUM>, <NUM>. The electrodes <NUM>, <NUM> and separator sheets <NUM> are wound together in a jelly roll and inserted in the case <NUM> with a positive tab <NUM> and a negative tab <NUM> extending from the jelly roll in an assembly station <NUM>. The tabs can then be welded to exposed portions of the electrode substrates, and the case filled with electrolyte and the case sealed. The result is a high capacity rechargeable battery in which the electrode material comprises extremely ductile fine metal filaments capable of repeatably charging and drain without adverse affect.

<FIG> and <FIG> are SEM photographs of Ta filament at different magnification.

Claim 1:
A method of forming an electrically active electrode material for use with a lithium ion cell, wherein the electrically active electrode material consists of an electrically conductive substrate material formed of filaments of a valve metal not larger than about <NUM> microns in cross section, which filaments are directly adhered together without an adhesive to form a stable sheet or mat, and wherein the filaments of the sheet or mat are directly coated with silicon nanoparticles, germanium or tin,
characterized by the steps in sequence of:
(a) establishing multiple components of a valve metal in a billet of a ductile material;
(b) working the billet to a series of reduction steps to form said valve metal components into elongated elements;
(c) cutting the elongated elements from step (b) into filaments not larger than about <NUM> microns in cross section, and leaching the ductile material from the elements;
(d) washing the cut elements from step (c) with water to form a slurry in which the filaments are evenly distributed;
(e) forming the cut elements from step (d) by directly casting the cut elements into a stable sheet or mat without an adhesive; and
(f) directly coating the sheet or mat resulting from step (e) with said silicon nanoparticles, germanium or tin.