ASSEMBLY CONSISTING OF A CURRENT COLLECTOR AND A SILICON ELECTRODE

The invention relates to an assembly comprising a current collector and a silicon electrode, wherein the current collector and the electrode are connected together via at least one of the surfaces thereof by an elastic polymer layer. The invention can be used in the field of lithium batteries.

DETAILED DESCRIPTION OF A PARTICULAR EMBODIMENT

The example described below illustrates the preparation of a current collector-electrode assembly represented in appendedFIG. 1, said assembly being composed of a stack comprising a current collector substrate made of aluminium coated, on one of the surfaces thereof, with a carpet of carbon nanotubes, the substrate being in contact via its coated surface with the carpet, with a first surface of an elastic polymer layer, the latter being in contact, via a second surface opposite said first face, with an electrode layer.

1) Formation of the Electrode

Firstly, an ink is prepared by mixing 0.3 g of alginate, 1.5 g of a silicon powder (having an average particle size of 310 nm), 0.1125 g of short carbon fibres (<10 μm) and 0.1125 g of carbon black of super P type.

The alginate has the function:

a) of binding the active materials with the electron conductor;

b) of making the electrode adhere to the carbon nanotubes and to the elastic polymer layer.

The silicon powder used is passivated with a layer of oxide not extending beyond 10 nm thickness, the active surface not having to exceed 30 m2/g.

Secondly, the ink thereby prepared is deposited, by spread coating, on a latex-alumina composite support to form a layer of thickness 150 μm, said layer then being dried then compressed at 1000 kg/cm2.

The latex-alumina composite support meets the following specificities:a ratio by volume of 20% latex and 80% alumina;a surface energy comprised between 20 and 60 ml/m2;the following pore distribution: [0-200 nm]=[10% and 20%]; [200 nm-600 nm]=[30% and 75%] and [>600 nm]=[5% and 60%).

Finally, thirdly, the resulting assembly is subjected to a step of carbonisation at 700° C. for 1 hour in the presence of a slightly reducing argon-H2gaseous mixture (3% by volume of H2), whereby an electrode layer remains. This carbonisation step enables the electrode to no longer contain organic compounds, which are replaced by amorphous carbon resulting from said carbonisation. The amorphous carbon derived from the carbonisation plays the role of cement for the electrode structure and thus guarantees its cohesion. During the carbonisation, the latex is also destroyed by a pyrolytic mechanism. The carbonisation step is followed by a step of sintering at 1400° C. in air for 1 hour, so as to consolidate the grains of alumina remaining from the carbonisation treatment.

2) Formation of the Stack

On the electrode layer obtained previously (of a thickness of 2 μm), a polymeric layer made of a flexible, insulating and adherent styrene-butadiene latex is deposited by sputtering.

On this polymeric layer is applied, by pressing, a current collector comprising an aluminium sheet of a thickness of 10 μm coated with a bundle of single-walled and multi-walled carbon nanotubes of a length of 10 μm, whereby the carbon nanotubes pass through the elastic polymer layer and come into contact with the electrode layer.

An assembly remains comprising a stack of layers such as represented in appendedFIG. 1, for which the references reported in this figure represent respectively the following elements:for the reference1, the complete assembly obtained at the end of this example;for the reference3, the current collector;for the reference5, the aluminium substrate belonging to the current collector;for the reference7, the bundle of carbon nanotubes starting from the substrate5and passing through the polymeric layer so as to arrive in contact with the electrode layer;for the reference9, the elastic polymer layer; andfor the reference11, the electrode layer as such.

The assembly obtained according to this example shown in a button cell type structure has been subjected to a cycling test at a capacity C/20 (it being carried out at C/20 for 5 hours then C/10 up to 4.2 V and cycling at 20° C. at C/20 at 100% of the capacity).

A button cell comprising an assembly not according to the invention has been subjected to this same cycling test, said button cell being identical to that mentioned above, except that the assembly does not comprise a polymeric layer.

This assembly not according to the invention has been formed from an electrode coated with an ink identical to that used for the example of the invention without resorting to the polymeric layer.

After drying, this electrode is punched to a diameter of 14 mm and compressed under a load of 2 tonnes. With this electrode in the form of pellet is associated a Selgard separator pellet and a Viledon separator pellet. This sandwich thereby formed is mounted in a button cell with three pressure shims and the assembly is arranged in an anhydrous glove box for the filling of electrolyte and crimping. The positive electrode is formed of lithium metal. There is no polymeric layer interposed between the electrode and the current collector.

The results of these cycling tests are reported inFIG. 2, which is a graph illustrating the evolution of the capacity C (in mA.h/g) as a function of the number of cycles N, curve a corresponding to the test carried out with the assembly according to the invention and curve b corresponding to the test carried out with the assembly not according to the invention.

For the test carried out with the assembly not according to the invention, the capacity drops very rapidly, when the number of cycles increases, which is not the case of the test carried out with the assembly according to the invention.