WO2003030282A2

WO2003030282A2 - Electrically conductive layer of a positive electrode

Info

Publication number: WO2003030282A2
Application number: PCT/AT2002/000280
Authority: WO
Inventors: Martha Maly-Schreiber; Adam Whitehead
Original assignee: Funktionswerkstoffe Forschungs- Und Entwicklungs Gmbh
Priority date: 2001-09-28
Filing date: 2002-09-25
Publication date: 2003-04-10
Also published as: AT409973B; WO2003030282A3; ATA15432001A; AU2002340614A1

Abstract

The invention relates to a flexible electrically conductive layer of a planar, positive electrode, in particular of an Li or Li-ion battery. The invention is provided with a support comprising an electrically conductive coating that consists of an alloy with the composition NiMYPx, wherein:Y </= 0.3, 0.01 </= x </= 0.5 and M is selected from the group containing Mo, W, Cr. V, Sn, Co. This permits the simple and cost-effective use of said layer, even for electric potentials in the range of 4V and above (vis-à-vis Li/Li<+>).

Description

Current-conducting layer of a positive electrode

The invention relates to a flexible current-conducting layer of a flat positive electrode of an alkali or alkali ion battery, in particular a Li or Li ion battery, consisting of a carrier with an at least partially electrically coated surface. Furthermore, the invention also relates to a method for producing such a current-conducting layer.

Primary batteries, which receive the energy through the manufacturing process and cannot be recharged because the electrolyte decomposes the negative pole, have been known for a long time and are used for a wide variety of applications. For example, primary Li cells and primary Li-ion cells are mainly used in small portable electronic devices, such as watches, hearing aids, cameras and the like, owing to their high gravimetric and volumetric energy density. Secondary batteries, which receive the energy only after production by formatting or charging and represent reversible systems that are reversible in their charging / discharging behavior with regard to both their electrochemistry and the structure of their electrodes, have only slowly become available to them in recent years Primary batteries coming up, so far always comparably lower gravimetric and volumetric energy densities also used for small, portable electronic devices of the type mentioned, since they can usually be recharged in the range of a thousand times and more often and are therefore, in spite of the initially higher costs, altogether much cheaper than primary batteries. Secondary batteries can also contain alkali metals, alkali alloys or alkali ions (such as in particular lithium).

The positive electroactive material of alkaline batteries or alkaline ion batteries, such as Li or Li ion cells, can consist of various compounds which react with the alkali metal or lithium to provide an acceptable voltage - see see for example J. Desilvestro and O. Haas, J. Electrochem. Soc, 137 (1), 5C (1990) or S. Megahed, J. Power Sources, 51, 79 (1994) or M. Winter, JO Besenhard, ME Spahr and P. Noväk, Adv. Mater., 10 (10 ), 725 (1998). For use in secondary batteries, the reactions taking place on the positive and negative electrodes must, as mentioned, be at least largely reversible. Since in recent years, the industry has increasingly been demanding single cells with higher voltages, so that reasonable operating voltages are already available with one or at least a few batteries, positive electrode materials, such as LiMnO ₂ , LiNiO ₂ , LiCoO ₂ and Li The like is becoming increasingly important. Such positive electrodes reversibly store lithium at 4.2 V compared to Li / Li ⁺ . In addition, experiments with positive electrodes with even higher positive potentials are already being carried out in research and development.

Typically, cathodes are produced by attaching electroactive material in powder form with suitable additives which increase the conductivity (for example carbon black, graphite powder, metal powder and the like) by means of a polymer binder (for example PVDF, EPDM, PTFE and the like) on an electrical conductor. For positive electrodes that work at electrode voltages> 3.5 V compared to Li / Li ⁺ , the current conductor nowadays usually consists of an aluminum foil, since only a few materials are stable at such electrode voltages. In addition to aluminum, these include, for example, Au, Ti, Mo, W, Pt or stainless steel of certain types. Au and Pt are too expensive for common uses and the other examples mentioned are not suitable for simple application in a sufficient layer thickness (for example from aqueous solutions). With regard to the aluminum used primarily in foil form for current conductors of the type of interest here, it should be mentioned as a disadvantage that this is chemically unstable in electrolytes which contain certain salts (for example UCF ₃ SO) - however, such salts have certain positive properties in the battery system , so that it would be desirable to find other more stable materials that are inexpensive and easy to handle, especially for the current-conducting layer of positive electrodes of Li or Li-ion batteries, but also suitable for other alkali metals , In addition to aluminum foils as the base material for positive electrodes of the type mentioned, arrangements of the type mentioned at the outset are also known in which non-conductive, flat polymer fabrics (woven or knitted) per se, for example by immersion in aqueous solutions with carbon in various forms (carbon black, graphite or the like) can be coated. However, after the conductivity of carbon is far below that of conventional metals (copper for example, has an at least hundred times higher conductivity than pyrolitic graphite and by a factor of 10 ⁵ higher conductivity than carbon black) are necessary to ensure extremely thick carbon coatings to provide sufficient for this purpose power line ,

From AT 408 288 B, the coating of layer-wise and / or locally processed with different weave density, also used as a current-conducting layer, polymer materials with different metal coatings and other conductive substances, such as carbon, is known, but it is specifically about locally different storage densities of the electroactive material including any additives and the coating applied to the conductivity on the non-conductive plastic threads of the polymer fabric consists only of the known materials according to the prior art, which have already been mentioned above.

On the basis of the prior art mentioned and the disadvantages mentioned above in this regard, the object of the present invention is to improve a flexible current-conducting layer of the type mentioned at the outset in such a way that the disadvantages mentioned are avoided and in particular that it is an easy-to-install, inexpensive electrically conductive layer Coating for the surface of the support can be specified, which is particularly stable even at high electrode potentials (for example in the range of 4 V and above against Li / Li ⁺ ). The actual cell voltage can also be <3.5 V, for example, while the positive electrode is nevertheless at a correspondingly high potential compared to, for example, Li / Li ⁺ . For example, if a negative electrode is used that works at a potential> 1 V compared to Li / Li ⁺ (for example, some called "high capacity carbons") and this is used together with a LiMn ₂ O ₄ cathode (with about 4 to 4.2 V compared to Li / Li ⁺ ), the cell voltage is less than 3.2 V compared to Li / Li ⁺ - Nevertheless, the potential prevailing at the positive electrode is above 4 V, which in the manner described makes difficulties with the current-conducting layers according to the prior art.

According to the present invention, the above-mentioned object is achieved in the case of a current-conducting layer of the type mentioned in the introduction in that the electrically conductive coating of the surface of the carrier essentially consists of an alloy of the composition NiM _y Pχ - with: y <0.3

0.01 <x <0.5 and

M consists of the group: Mo, W, Cr, V, Sn, Co. This alloy can also contain inevitable impurities in trace form during production, without significant effects on the positive behavior being ascertainable.

As an approach for explaining the positive effects of the described features of the invention, a consideration of the usual materials or those previously used for such coatings or of their behavior in the present context can be assumed: Ni, Co, Ag and most other metallic elements show strong ones Oxidation currents at high potentials compared to Li / Li ⁺ in non-aqueous electrolytes. In principle, these currents can result from the oxidation of the electrolytes or the oxidation of the metal. However, since Li battery electrolytes are generally stable to relatively high potentials on other surfaces (Pt, glazed carbon, oxides) and a certain pitting can be observed on the metallic surfaces, these oxidation currents can almost certainly cause the oxidation and dissolution of the metal ( for example in the form of Ag = Ag ⁺ + e ^" , Cu = Cu ⁺ + e ^" etc.). Electrodes made of aluminum foils are stable in a wide variety of Li-battery electrolytes, since there is a very thin oxide film on the aluminum surface. The thin oxide film is thick enough around one Preventing further oxidation of the aluminum is insoluble in the electrolyte and does not require an excessively high additional electrical resistance. If, however, the surface is scratched or otherwise damaged or an incompatible electrolyte is used or too high potential is applied, pitting will also occur on the aluminum surface, which will only be stopped again if the aluminum with the electrolyte is reformed with an oxide layer (or in some cases a fluoride layer - depending on the electrolyte).

Some types of stainless steel are highly stable, but pure iron and pure chrome are relatively unstable. The stainless steel apparently also forms a protective layer in the electrolyte by partially dissolving the more soluble components and reacting them with the electrolyte (the reaction of stainless steel with water typically leaves a chromium and oxygen-rich surface layer). Accordingly, the introduction of the phosphorus portion described into the alloy can obviously promote the corrosion resistance by forming a similar “passivation layer” at elevated potentials. That means that a low current flow can be expected in accordance with the solution of some nickel on the surface and oxidation of the remaining phosphorus, according to which the layer of oxidized phosphorus or the oxidized phosphorus-nickel layer on the surface is much more resistant to further oxidation than the underlying alloy, which prevents further reaction with the electrolyte mentioned elements such as W and Mo, which can also form or support such a passivation layer. This passivation process is also favored by the stability of W and Mo, for example, at high potentials (the pure metals are reactive and cannot be obtained from aqueous solution) en coated, but are easily passivated - similar to aluminum - in the described manner).

The specified parameter ranges have proven their worth for the purposes of the present invention - a further increase in the phosphorus portion would increase the corrosion resistance, but at the same time undesirably also the electrical see resistance. Furthermore, the insertion of the M materials mentioned increases the electrical resistance and can make the reversible incorporation slower and more difficult to control. For this reason, a Ni-P alloy (without the addition of M materials), for example, can also be preferred for some systems. In general, larger additions of M materials not only significantly increase the electrical resistance in the manner described, but of course also significantly increase the costs, which can be very relevant for such mass products.

In a particularly preferred embodiment of the invention it is provided that at least one further conductive layer, preferably made of graphite, conductive carbon black or another carbon compound or of Au or Pt, is arranged between the carrier surface and the electrically conductive coating. These layers serve mainly to facilitate the production or to improve and simplify the actual coating and can be applied in a very thin form as a starting layer in a variety of suitable ways. This starting layer should also be stable with respect to the potentials to be expected, so that damage to the actual conductive layer above it cannot lead to oxidation and / or dissolution of the starting layer.

According to a further preferred embodiment of the invention, it is provided that a further cover layer made of non-porous carbon is applied over the electrically conductive coating, which offers an additional protective function - after the actual power conduction takes place in the alloy layer, this carbon layer is not thick and actual closed critical.

In a particularly preferred embodiment of the invention, the actual carrier consists of a three-dimensionally woven or knitted polymer material, the fibers of which have an electrically conductive coating (as is known per se in general form from AT 408.288 B mentioned at the beginning). The woven threads of this polymeric material preferably consist of fibers of a polymer from the following group: polyester, silicone rubber, polyethylene, polypropylene, ethylene-tetrafluoroethylene copolymer, polytetrafluoroethylene len and polyvinylidene fluoride. This results in a flexible, particularly light, flat support which has an advantageous large surface area with easy penetrability for the electrolyte or other electroactive materials and additives. Such materials are inexpensive to manufacture and can be cut into various shapes and sizes without any problems, so that the electrodes can be used for a wide variety of batteries. In addition to such woven or knitted fabrics as a carrier for the conductive layer, so-called randomly woven mats (non-woven fiber fabrics) made of polymer fibers of random orientation or thin polymer films with perforations or similar flat structures can also be used for this purpose, which also applies, for example, to the use of such current-conducting layers for extremely thin, flat batteries tailored to the respective application.

The method according to the invention which is particularly advantageous for producing a flexible current-conducting layer of the type described above is characterized in that the electrically conductive coating is carried out chemically or electrochemically or by a combination of these two process steps, which enables simple and inexpensive production, which also enables the use of the invention of mass products makes it economical.

The invention is explained in more detail below with the aid of two examples. 1 used for explanation shows the first cycle of an aluminum foil current arrester according to the prior art and of a current collector fabric coated with nickel alloy according to Example 1 according to the present invention in an electrolyte composed of 1 mol dm ^"3 LiCF ₃ SO ₃ / ethylene carbonate / Diethyl carbonate at a scanning speed of 0.2 mV s ^"1 and FIG. 2 shows a corresponding second cycle, FIGS. 3 and 4 show the same for example 2 with a different electrolyte at different current densities.

Example 1:

Before activation, a polyester fabric was cleaned with an alkaline solution using a palladium-based activator and then rinsed. Then the active fourth fabric, through 600 s immersion in a bath at 65-70 ° C with the following composition, chemically coated with an alloy.

Table 1 - Bath composition:

A pH of 6 was adjusted by adding sodium hydroxide.

A simple resistance measurement showed that the fabric was electrically conductive over the entire area. In addition, investigations with the scanning electron microscope (SEM) showed that the tissue was coated uniformly. The ratio of the atoms Ni: P to 88:12 was determined by EDX analysis, this corresponds to an alloy composition of NiP ₀ .ι -

Gravimetric measurements showed that the loading of the alloy was 1.5 mg cm ^'2 on the tissue with a mass of 8.0 mg cm ^"2 .

In a glove box filled with argon, button cells were produced using electrodes from the coated fabric (~ 1 cm ^"2 geometrical area), lithium as counterelectrode and an inert separator. The electrodes were examined at 30 ° C. by means of cyclic voltammetry Scanning speed was 0.2 mV s ^"1 from the quiescent voltage to 4.3 V vs. Li / Li ⁺ and then repetitively between 2.3 V and 4.3 V vs. Li / Li ⁺ .

With an electrolyte of 1 mol dm ^"3 LiCF ₃ SO ₃ in ethylene carbonate / diethyl carbonate, electrodes made of aluminum foil showed a behavior which is illustrated in FIGS. 1 and 2 is. In Fig. 1 it can be seen that the aluminum foil was “activated ^{, Λ} (eg: passivating film was removed) when the potential was reduced to 4.3 V vs. Li / Li rose. This led to a high current flow until the passivating film regressed at low potentials.

In contrast, the nickel phosphorus alloy showed a permanent passivating behavior.

These effects are more pronounced when you consider the behavior of the current arrester during the second cycle. The aluminum foil showed a significantly higher, irreversible current flow compared to the nickel phosphor alloy. Electrodes made of nickel phosphorus alloy showed a lower current flow in the 2nd cycle than in the first cycle.

For this reason, current conductors made of nickel-phosphorus alloys according to the present invention even show a higher stability in this electrolyte than the conventional aluminum foil current collectors.

Example 2:

A polyester fabric was coated with an alloy after activation with a palladium-based activator (as in Example 1). The activated tissue was immersed in a bath of the following composition at 60 ° C. for 1,800 seconds

Table 2 - Bath composition:

A pH of 9 was set by adding sodium hydroxide.

A silver-colored metallic film on the fabric was then visually detectable. Corresponding measurements confirmed the electrical conductivity of the surface.

An atomic ratio of the elements Ni: P: W of 82: 14: 4 was determined by EDX analysis, which corresponds to an alloy composition NiW ₀ , osPo.i7.

The electrodes were again tested in button cells, but now with an electrolyte based on LiCI0 ₄ . 3 shows data of the first cycle of the current-conducting layer according to the invention and of aluminum foil. 4 shows the associated second cycle. It should be noted that both FIGS. 3 and 4 show different current density scales from FIGS. 1 and 2.

Aluminum foil is much more stable in this electrolyte than that discussed in Example 1. Both the Ni-W-P alloy and the aluminum foil showed irreversible currents. It should be noted here that the surface of the Ni-WP alloy (applied to the plastic fabric in the manner described) was very much larger than that of the aluminum foil, so that it can be assumed that, based on the "electrochemical surface" instead of the " geometric surface "the alloy is more stable than the aluminum foil.

3 and 4 that both the aluminum foil and the arrangement according to the invention were more stable in the second cycle than in the first.

The added come in the Example 2 "M ^Λ component increases while costs and complexity of the coating process, reduces the deposition rate and increases the electrical resistance, but offers the advantage of increasing the stability of the current-conducting layer under the given conditions (voltage, electrolyte composition ... .).

claims:

Claims

claims:

1. Flexible current-conducting layer of a flat positive electrode of an alkali or alkali ion battery, in particular a Li or Li-ion battery, consisting of a carrier with an at least partially electrically conductive coated surface, characterized in that the electrically conductive Coating the surface of the carrier essentially from an alloy of the composition

NiM _y P _x - with: y <0.3

0.01 <x <0.5 and

M consists of the group: Mo, W, Cr, V, Sn, Co.

2. Flexible current-conducting layer according to claim 1, characterized in that at least one further conducting layer, preferably made of graphite, conductive carbon black or another carbon compound or of Au or Pt, is arranged between the carrier surface and the electrically conductive coating.

3. Flexible current-conducting layer according to claim 1 or 2, characterized in that a cover layer made of non-porous carbon is applied over the electrically conductive coating.

4. Flexible current-conducting layer according to one or more of claims 1 to 3, characterized in that the carrier consists of a three-dimensionally woven or knitted polymer material, the fibers of which have the electrically conductive coating.

5. Flexible current-conducting layer according to one or more of claims 1 to 3, characterized in that the carrier consists of a perforated polymer film.

6. Flexible current-conducting layer according to one or more of claims 1 to 3, characterized in that the support consists of a random fiber mat (non-woven fiber fabric) made of polymer fibers.

7. A method for producing a flexible current-conducting layer according to one or more of claims 1 to 6, characterized in that the electrically conductive coating is carried out chemically or electrochemically or by a combination of these two process steps.