EP1649530A2

EP1649530A2 - Method for preparing an electrode from a porous material, resulting electrode and corresponding electrochemical system

Info

Publication number: EP1649530A2
Application number: EP04737895A
Authority: EP
Inventors: Karim Zaghib; Abdelbast Guerfi; Patrick Charest; Robert Kostecki; Kimio Kinoshita; Michel Armand
Original assignee: Hydro Quebec
Current assignee: Hydro Quebec
Priority date: 2003-06-25
Filing date: 2004-06-25
Publication date: 2006-04-26
Also published as: US8828103B2; JP5514248B2; CA2432397A1; JP5342746B2; EP2475032A3; JP2012199241A; WO2004114437A2; CA2528847C; CA2528847A1; CA2848580C; WO2004114437A3; US8048561B2; EP2475032B1; US9768441B2; US20140374265A1; JP2007505444A; ES2693619T3; CA2848580A1; US20070154805A1; US20120017429A1

Abstract

The invention concerns a method for preparing electrodes from a porous material to obtain electrodes useful in electrochemical systems and which exhibit at least one of the following properties: high mAh/gram capacity, high mAh/litre capacity, good cyclability, low self-discharge rate, and good environmental tolerance.

Description

PROCESS FOR THE PREPARATION OF ELECTRODE FROM A

POROUS MATERIAL, ELECTRODE THUS OBTAINED AND

CORRESPONDING ELECTROCHEMICAL SYSTEM

Technical field of the invention

The subject of the present invention is methods of preparing electrodes from a porous material, in particular the methods of preparing electrodes involving the preparation of an alloy and those in which the electrodes are at least partially covered. of carbon.

The present invention also relates to the electrodes obtained from a porous material or containing a porous material, in particular negative electrodes for lithium micro batteries which contain a porous silicon.

Another object of the present invention consists of any electrochemical system containing at least one electrode obtained from a porous material or containing a porous material, and more particularly electrochemical systems containing micro batteries consisting of at least one electrode according to the invention.

Description of the state of the art ₍

Newly developed polymer electrolyte generators use metallic lithium, sometimes sodium, or other alkali metals, as anode strips. The alkali metals are malleable and can be used in the form of thin films as mentioned in patents CA-A-2,099,526 and CA-A-2,099,524.

However, the use of metallic lithium or other alkali metals presents, in certain cases of extreme use, eg at temperatures above 100 ° Celsius, risks of fusion of lithium or alkali metal and destruction of the cell. electrochemical. In addition, under forced conditions of electrochemical cycling, the formation of dendrites, for example lithium dendrites can occur, in particular if the charging currents are too high. The formation of dendrites has many disadvantages. While the same alloy, when operating at a more anodic potential, for example at a potential whose value is between +300 to 450 mVolts for lithium aluminum vs lithium, does not allow the deposition of lithium or growth dendritic.

The use of alloys of alkali metals, in particular lithium, is thus described in patent US-A-4,489,143, in the case of generators operating in molten salt media.

In organic media, and more particularly in polymeric media, where the thicknesses of the electrode films are less than 100 micrometers, it becomes very difficult to use alloyed anode strips (also called alloy-based strips). Indeed, the lithium intermetallic compounds which can be used as anodes, such as LiAl, Li ₂ iSi ₅ , Li ₂ iSn ₅ , Li ₂₂ Pb ₅ and others, are hard and brittle and cannot be laminated like lithium or weakly like lithium ally. It is also mentioned in patent C A-A-1,222,543, that these anodes can be produced in thin films by producing composites consisting of powders of the intermetallic compound bonded by the polymer electrolyte, or also in US patent -A-4,590,840, that it is possible under certain conditions to pre-lithiate the strip of the host metal of the anode, by chemically treating the surface or by electrochemically charging part of the strip.

However, these techniques, although functional under certain conditions, use reactive materials, and the pre-inserted alloys are often pyrophoric or pose difficulties in implementation and performance optimization. When the anodes are prepared in the discharged state, one of the major difficulties to be overcome comes from the large volume variation resulting from the formation of the alloy which causes significant stresses on the structure.

When one seeks to form the alloy from a strip of non-lithiated host metal during assembly or after assembly of a generator with polymer electrolyte, the volume expansion of the structure in the direction the thickness of the strips could only be compensated by an appropriate design of the cell, by accommodating for example the increase in total thickness of the overlapping strips, all the more that in the direction of the thickness, the variation is very short and therefore much more negligible.

Known technologies for the realization of micro batteries in an electrochemical system are described in the conference, lth ABA BRNO 2000, Advanced Batteries and Accumulators, June 28.8-1.9, 2000 Brno University of technology, Antoninska 1, Brmno, Czech Republic http: / /www.aba-brno.cz/aba2000/part 1 / 13- bludska.pdf. and in the reference: Bull. Master. Science., Vol.26, No.7, December 2003, pp.673-681 http://www.ias.ac.in/matersci/bmsdec2003/673.pdf, and require the import of the micro battery in the system.

There was therefore a need for new materials capable of being used as a component of an electrode and devoid of one or more of the drawbacks of the materials traditionally used in this application.

There was more particularly a need for a new electrode material having at least one of the following properties:

- a large capacity in mAh / gram; - a large capacity in mAh / liter;

- good cyclability;

- a low self-discharge rate; and

- good environmental tolerance.

There was also a great need for new electrode materials suitable for microtechnology such as micro-batteries.

There was also a need for electrodes with very little cracking at the end of their manufacture and this, for the sake of longevity. Brief description of the drawings

Figure 1-4 shows the mechanism for inserting lithium into silicon by a method according to the invention with the formation of a Li _x Si _y alloy.

Figure 2-4 shows the electrochemical formation mechanism, according to an embodiment of the invention, of a porous silicon which serves as an anode for micro batteries.

Figure 3-4 relates to the manufacture of a micro battery according to the invention from an electrode based on porous silicon coated with carbon. The role of this carbon is to establish an electrochemical bridge between silicon and lithium. In addition, carbon provides the electronic conductivity of the negative electrode based on porous silicon. It is also used for the adhesion of silicon particles during volume expansion.

The different stages of this manufacturing are as follows:

1- deposition of thin layers of photo-resist;

2- placement of a mask, then passage of a UV beam to crosslink the desired areas;

3- Dissolution of non-crosslinked areas by chemical pickling;

4- Carbonization of the photo-resist not pickled (which will form the anode in the micro battery);

5- Introduction of the electrolyte, followed by the cathode; 6- Cutting in the pickled areas to have micro batteries.

Figure 4-4 relates to an optical image of a carbon structure obtained by laser pyrolysis of a layer of photoresist. The graph at the top represents the Raman spectrum of the carbon obtained.

Summary of the invention

Methods of preparing an electrode for an electrochemical system from a porous material make it possible to prepare electrodes for an electrochemical system exhibiting very advantageous properties with regard to their physicochemical performance but also mechanical. The technology presented makes it possible to create micro batteries, directly in the electrochemical circuits.

General description of the invention

A first object of the present invention consists of a method for preparing an electrode for an electrochemical system from a porous material. Preferably, the porous material used has a porosity, measured according to the mercury method hereinafter referred to, which varies from 1 to 99%, limits included. Even more advantageously, the porosity of the material varies from 20 to 80%, limits included.

According to an advantageous embodiment of the invention, the porous material used is such that the average size of the pores therein varies from 1 nanometer to 1 micrometer, limits included. Even more preferably, the size of the pores varies from 10 to 250 nanometers, limits included.

According to another advantageous embodiment, the dispersion of the pores is substantially uniform. Preferably, this distribution is chosen so that its d50 is between 100 and 150 nanometers. >

The pores are advantageously located on the surface of the porous material and extend through said porous material. Preferably, the pores have a depth between 1 micrometer and 3 millimeters and said porous material a thickness between 2 micrometers and 3.5 millimeters. It is desirable that the majority of the pores present in the porous material do not pass through all of the porous material right through. This would result in weakening the electrode film.

According to another preferred embodiment, said porous material is chosen from materials capable of forming an alloy with an alkali metal. Thus, the porous material can be chosen from the group consisting of silicon, tin, aluminum, silver, gold, platinum and mixtures of at least two of these materials put in porous form.

The preparation of the alloy is carried out chemically and / or electrochemically. It has been surprisingly found that particularly interesting results are obtained when the porosity rate of the material used to form the electrode is such that the cavities of the porous material can absorb the volume extension generated during the formation of the alloy. with the alkali metal.

According to a particular embodiment of the invention, an anode is prepared from a porous silicon.

An anode according to the invention can thus be obtained by forming an alloy from at least one source of porous silicon and from at least one alkali metal chosen from the group consisting of Li, Na, Ca and mixtures at least two of these.

Advantageously, an anode is prepared from a porous silicon, the porosity of which, measured according to the mercury porosimeter method, varies from 5 to 95% by volume, limits included. Even more advantageously, the porosity of the silicon used is approximately 75% by volume.

The porous silicon used as porous material is obtained from a source of silicon chosen from the group consisting of: silicon waffles, silicon wafers, silicon films and mixtures of at least 2 of these.

Preferably, the porous silicon used as the porous material is obtained from a single crystal silicon.

According to an advantageous embodiment of the invention, the porous silicon is obtained from a source of silicon, by electrochemical treatment, in a bath comprising at least one salt, said salt preferably being chosen from the group consisting by NHxFy with X being 4 or 5 and Y being 1 or 2, more preferably still the salt chosen is NH ₄ F.

For example, the treatment of the silicon source contains at least one salt in solution, which is preferably a mixture of H ₂ S0 ₄ , NH ₄ F and H ₂ 0, and at least one non-aqueous solvent. which is preferably an alcohol or a ketone, the non-aqueous solvent (s) is (are) preferably chosen from the group consisting of methanol, ethanol and acetone and mixtures of at least two of these. Such a bath advantageously contains, by volume:

- 5 to 20% methanol; and

- 75 to 20% of H ₂ S0 ₄ .

Preferably, the alloy is based on porous silicon and it is in the form Si _x Li _y , with x representing a number between 1 and 5, and y representing a number between 5 and 21. More preferably still, in the alloy, x represents approximately 4 and y represents approximately 21.

According to an advantageous embodiment of the invention, the alloy formed is of the Si _x Li _{y type} and it is obtained electrochemically by contacting a source of silicon with lithium and / or metallic lithium in the form of strips or waffles, at a temperature between 40 and 100 ° Celsius, preferably at a temperature of about 80 ° Celsius.

The duration of the contacting of the silicon source, with lithium and / or metallic lithium in the form of strips or waffles, is between 1 and 12 hours, preferably said duration is approximately 3 hours.

A second object of the present invention consists of the electrodes obtained by the implementation of a method according to any one of the methods described in the first object of the invention.

An advantageous sub-family of anodes according to the invention consists of anodes containing at least 60% by mass and preferably at least 40% of a porous material, preferably porous silicon.

Another particularly interesting sub-family consists of the anodes, which are at least partially covered with carbon.

The anodes and cathodes according to the invention have the advantage of being substantially free of cracks. A third object of the present invention consists of electrochemical systems such as those comprising at least one electrode that obtained by any of the preparation methods defined in the first object of the invention or as defined in the second subject of the invention.

As a preferred example of particularly preferred electrochemical systems, mention may be made of the batteries in which the electrolyte. is of type, liquid, gel, or polymer.

Preferably, in these batteries, the cathode is of the LiCo0 ₂ , LiFeP0 ₄ , LiNi0 ₂ , LiNio, ₅ Mno, 5θ ₂ , LiNio, ₃₃ Coo, ₃₃ Mn _{0) 33} 0 _{2 type} , and the cathode is preferably of the 1 to 5 Volts.

Rechargeable type batteries offering particularly significant performance are thus obtained, preferably they are of the lithium ion type.

A subfamily of batteries of the invention of interest consists of those in the form of micro batteries, preferably having dimensions of between 1 mm 2 and 10 cm 2, and which exhibit at least one of the following electrochemical properties:

an electrochemical capacity greater than 1 μWh

: a cyclability greater than 500, preferably greater than 1000 cycles;

. a self-discharge rate of less than 5%, preferably less than 4%, more preferably still less than 3%; and

. a lifetime, measured according to the storage test carried out under ambient conditions, which is greater than 3 years, preferably greater than 5 years.

A fourth object of the present invention is constituted by the use of the electrodes, and preferably the anodes of the invention, in an electrochemical system.

Preferably, the anode is used as a negative electrode for micro lithium batteries. A fifth object of the present invention consists of the methods of manufacturing an electrode, based on porous silicon and covered at least partially with carbon, obtained by thermal pyrolysis of a layer of polymer, preferably deposited in a thin layer on a support preferably insulating in porous silicon such as Si3N4. The pyrolysis of the polymer is advantageously carried out at a temperature between 600 and 1,100 ° C and, preferably, for a duration between 30 minutes and 3 hours.

According to another variant of the invention, during the implementation of the method for manufacturing an electrode based on porous silicon and covered at least partially with carbon, laser pyrolysis is carried out of a preferably deposited polymer layer as a thin layer on a silicon support (insulator). The beam used preferably provides an intensity of between 10 and 100 milliwatts and it is preferably placed at a distance of between 0.5 micrometers and 1 millimeter from the silicon support. The photoresist layer carbonizes by laser pyrolysis, exposing the layer to it. This implies the conversion of the C-H-0 functions into carbon. Preferably, the exposure is carried out for a period of between 1 second and one minute. Preferably, the silicon support consists of a single crystal of silicon and it has a thickness of between 100 microns and 3 millimeters.

A sixth object of the present invention consists of the electrodes obtained by implementing one of the methods defined in the fourth object of the invention.

A seventh object of the present invention consists of electrochemical systems comprising at least one electrode according to the fifth object of the invention.

Description of preferred modes of the invention

The present invention relates to the use of a porous material in a micro battery. More particularly, the invention relates to an electrochemical generator including a negative electrode comprising a host porous metal, in particular silicon. The host metal strip being intended to subsequently constitute a negative electrode and having the property of absorbing lateral expansion and substantially preventing the change in the plane of the porous metal during the formation of alloy between the host metal and the alkali metal.

In particular, after formation of the host material lithium alloy, the alloy cracks during electrochemical activity. The possibility of having a volume expansion plays a major role in the integrity of the electrode.

Porous silicon is thus advantageously used in this technology as the active material constituting the anode for the li-ion battery. The theoretical capacity of porous silicon is 1970 mAh grams and 2280 mAh / 1.

The volume extension associated with the silicon and lithium alloy is preferably between 30 and 40%. Thus the cavities formed in the porous silicon serve to compensate for the volume expansion of the alloy based on Li and Si.

The mechanism of inserting chemical or electrochemical lithium into porous silicon, according to an embodiment of the invention, is illustrated in Figure 1-4.

The empty space generated by the porosity of the silicon is occupied by the volume extension of the alloy Si _x Li _y , with x varying from 1 to 5 and y varying from 4 to 21. Preferably, the alloy has the formula Li ₂₁ Si ₅ .

Preparation of porous silicon

In the context of the processes for preparing porous silicon according to the invention, a mixture of NH ₄ F is advantageously used to dissolve Si and Si0 ₂ present as impurities.

The porous silicon is obtained electrochemically in an electrolyte based on NH ₄ F (50%) + H ₂ 0 + Methanol in a ratio of (2: 2: 1), the addition of methanol avoids the formation of l on the surface. The porosity rate is calculated according to the lithium intercalation rate which is proportional to the volume extension of the alloy If _x Li _y . The porosity is measured by the mercury method described in the reference: The Powder Porosity Characterization The bat NYS College of Ceramics at Alfred University, June 18, 2002, http://nyscc.alfred.edu/external/ppc/ppc.html.

This technique is described in more detail in the field of semiconductors http://etd.caltech.edu/etd/available/etd-08062002-192958/unrestricted/Chapter3.pdf. in chapter 3 of the Ph.D thesis entitled Effects OF SURFACE MODIFICATION ON CHARGECARPJER DYNAMICS AT SEMICONDUCTEUR INTERFACES by Agnes Juang, 2003, California Institute of Technology Pasadena, California.

It has also been surprisingly found that micro batteries, according to the invention, using microelectrodes containing carbon and based on porous silicon can be manufactured by different original techniques hereinafter explained in detail.

These are thermal or laser pyrolysis of H or O type heteroatoms present on the surface of the silica which have been deposited as thin layers on the insulator also called the "insulator".

Both techniques involve the conversion of C-H-0 functions to carbon, but they differ in the procedure by the formation of microstructures which form the microelectrodes in the micro battery. In both techniques, it is preferable to start from a commercial grade of silicon called dense silicon available in the form of waffles.

According to an advantageous approach, the semiconductor preparation techniques involve the "patteming" of carbon by the photolithography methods involving a photo mask which is used to "model" the electrode structures which can be interdigitalized surfaces of the electrodes.

First technique for preparing an electrode based on porous silicon

A conventional "waffle" transfer silicon with an insulation layer can serve as a substrate for microelectrodes. According to this innovative approach, Carbon electrodes are formed from regular photo-resistors by heat treatment (usually at temperatures of 600 to 1,100 ° Celsius in an inert atmosphere for one hour) which carbonizes them and which makes the photo-resistance electrically conductive.

Electrochemically active electrode materials can be selectively deposited on carbons by electrochemical methods, and for some applications, the carbon itself can serve as the electrode.

The process which is used to fabricate the structures of the microelectrodes implements a succession of steps. In the first step, a thin layer of Si ₃ N ₄ (around 100 nm) is deposited by chemical vapor deposition (CVD), which serves as an insulator to separate the conductive silicon "wafer" from the carbon structure. Subsequent steps involving spin coating, "patteming" and photo-resistance pyrolysis are used to form the final carbon structure. Both negative and positive photoresist can be used to form the carbonaceous conductive micro-electrodes.

These techniques are described in the references: Abs 253, IMLB 12 Meeting, 2004 The Electrochemical Society, Journal of Power Sources, Volume: 89, Issue: 1, July, 2000 and Applied Physics Letters Vol 84 (18) pp. 3456-3458. May 3, 2004.

Second technique for preparing an electrode based on porous silicon

The second approach does not involve the use of a photo mask. Indeed, we only use a narrow laser beam with "path", controlled to move along a specific path "path". The control of the movement of the laser beam on the surface of the photoresistor by computer control allows the preparation of a wide variety of microelectrode devices comprising channels. The intensity of the power of the laser vapor is controlled in order to avoid the vaporization of the photo-resistance, instead of its conversion to carbon, and this also minimizes the loss of carbon by laser ablation. In a subsequent step, the unreacted photo resistors in certain areas which are not exposed to laser vapor are dissolved to leave only the carbonaceous microelectrodes on the silicon waffle. The radiation from the laser beam is capable of converting the photoresist polymer to carbon. Similarly, results comparable to those obtained by thermal pyrolysis, are obtained by implementing a Raman spectrum.

The document How Semiconductors are made E. Reichmanis and O. Nalamasu, Bell Labs, Lucent Technologies, Intersil, 20.06.2003, Intersil Corporation Headquarters and Elantec Product Group, 675 Trade Zone Blvd, Milpitas, CA 95035 describes silicon waffles (and their method of preparation) usable in the context of the present invention.

Martin Key's document, SU-8 Photosensitive Epoxy, CNM, Campus UAB, Bellaterra 08193, Barcelona, Spai (http://www.cnm.es/projects/microdets/index.html) illustrates methods for filing a carbon-based polymer on silicon waffles.

The document Direct Measurement of the Reaction Front in Chemically Amplified Photoresists, E. Reichmanis and O. Nalamasu, Bell Labs, Lucent Technologies, Sciences, 297, 349 (2002) describes methods commonly used to attack selected areas of photo-resistances.

The content of these three documents is incorporated by reference into the present request.

Examples: carbon microstructures formed by photoresistor laser pyrolysis are described below.

A photoresistor (Oir 897-101, Olin Corp., Norwalk, CT) was used to produce a thin film of an organic precursor on an Si substrate. An integrated Raman microscope Labram system manufactured by the ISA Horiba group was used to laser pyrolysis of photoresistor, and also to analyze the structure of the carbon product.

The excitation wavelength was either supplied by internal HeNe (632 nm), a 20 mW laser or by an external Ar-ion (514nm), 2 W Laser. The power of the laser beam was adjusted to the desired levels with neutral filters of various variable optical density. The size of the laser beam on the surface of the sample can be varied from 1.6 to a few hundred microns; and it is controlled by the characteristics of the optical microscopes and the distance between the sample and the objective lenses. The diameter of the laser beam applied in our experiments was 5 microns. To control the position of the sample relative to the laser beam, a motorized scanning XY microscope, 0.1 micron resolution was used. The exposure time of the photoresistor to the laser beam was controlled either by the XY scanning speed or by a digital shutter laser beam (model 845 HP by Newport Corp.), which was used in the static experiments.

Controlling the movement of the photo resistance sample - If by the computer program it is possible to design a wide variety of microelectrodes in the form of rows.

The power density of the laser beam must be controlled in order to avoid vaporization of the photoresistor without converting it to carbon or to minimize the loss of carbon by laser ablation.

Four layers of a positive resistance photo were coated on an Si waffle and then baked at 150 degrees Celsius. An optical image which illustrates the result obtained by laser pyrolysis for the production of carbon structure from a positive photoresistor is shown in Figure 4-4. A laser operating at 632 nm, and with a beam size of 5 micrometers and a power of 8 mW, was used to produce the modeled carbons. A computer program was used to control the movement of the sample to form the carbon configurations. The speed of movement of the motorized XY sample was 8 mm per second. The width of the imprint in the interdigital structure is approximately 20 micrometers. It is slightly wider than the carbon bond that connects them because each finger has been exposed to the laser beam twice. The Raman spectrum of carbon positioned on one of the fingers is shown in the same Figure 4-4. It is remarkably similar to that obtained by standard thermal pyrolysis of the same photoresist at 1,000 Celsius. These results Preliminary studies demonstrate that it has easily obtained micro carbon electrodes having a Raman spectrum comparable to those obtained by heat treatment.

A new high-performance technology based on the use of the laser is thus proposed for the preparation of capacities of a size suitable for small electronic devices.

This new method of the invention called - direct laser lithography (DDL) - makes it possible to produce micro electrodes from organic and inorganic precursors, suitable for Li-ion batteries and creation, from any type of substrate. , fully rechargeable functional micro batteries.

The versatility of DLL technology allows the tailor-made production of micro-power sources which can be distributed and integrated directly into electronic components.

In addition, the DLL technology does not require a photo mask to achieve the desired configuration for the microelectrodes.

As a result, DDL can produce a micro battery design faster than conventional photolithography. The micro batteries obtained offer improved specific energy and power due to their reduced weight and volume, when the electronic substrate becomes part of the elements of the battery.

The cathode can be prepared from a target of cathode material preferably chosen from the group consisting of LiCo02, LiMn ₂ 0 ₄ , LiMn _1/3 Ni _1/3 Cθι _{/ 3} 0 ₂ , LiMnι _{/ 2} Niι _{/ 2} 0 ₂ , LiMP0 ₄ (M = Fe, Co, Ni, Mn) and mixtures of at least two of these, preferably the material target is pressed, the laser is applied to the target at powers that can vary from 20mW to 2W to create the porous material constituting the cathode which is then etched from the target by laser and deposited on the half porous Si / carbon / electrolyte stack. A second technique for preparing the cathode by laser is carried out using a compound in pasty form formed from a mixture of cathode powder and a carrier solution which is preferably toluene, heptane or a mixture of these. The pasty solution is coated on a support plate which is preferably made of glass and placed at 100 μm from the substrate (silicon or other). The laser beam of UV radiation is applied through the support plate and the cathode is projected onto the substrate by pyrolysis.

Although the present invention has been described using specific implementations, it is understood that several variations and modifications may be grafted onto said implementations, and the present invention aims to cover such modifications, uses or adaptations of the present invention following, in general, the principles of the invention and including any variation of the present description which will become known or conventional in the field of activity in which the present invention is found, and which can be applied to the essential elements mentioned above, in accordance with the scope of the following claims.

Claims

claims

1. Method for preparing an electrode for an electrochemical system from a porous material.

2. Method for preparing an electrode according to claim 1, in which the porosity of said porous material, measured according to the mercury method, varies from 1 to

99%, terminals included-.

3. A method of preparing an electrode according to claim 2, wherein the porosity of said material varies from 20 to 80%, limits included.

4. Method for preparing an electrode according to any one of claims 1 to 3, in which the average pore size in said porous material varies from

1 nanometer to 1 micrometer, terminals included.

5. Method for preparing an electrode according to claim 4, in which the pore size varies from 10 to 250 nanometers, limits included.

6. A method of preparing an electrode according to any one of claims 1 to 5, wherein the dispersion of the pores is substantially uniform, preferably the distribution of the pores is such that its d50 is between 100 and 150 nanometers.

7. A method of preparing an electrode, according to any one of claims 1 to 6, wherein the pores are located on the surface of the porous material and extend through said porous material; preferably the pores have a depth of between 1 micrometer and 3 millimeters, and said porous material a thickness of between 2 micrometers and 3.5 millimeters.

8. A method of preparing an electrode according to claim 7, wherein the pores do not pass through the porous material right through.

9. Method for preparing an electrode according to any one of claims

1 to 8, wherein said porous material is capable of forming an alloy with an alkali metal.

10. Method for preparing an electrode according to any one of claims

1 to 9, in which the porous material is chosen from the group consisting of silicon, tin, aluminum, silver, gold, platinum and mixtures of at least two of these materials put in porous form .

11. A method of preparing an electrode according to claim 9 or 10, wherein the preparation of the alloy is done chemically and / or electrochemically.

12. A method of preparing an electrode according to claim 11, wherein the porosity rate of the material used to form the electrode is such that the cavities of the porous material can absorb the volume expansion generated during the formation of the alloy with alkali metal.

13. A method of preparing an electrode which is an anode, according to any one of claims 1 to 12, wherein the porous material is a porous silicon.

14. A method of preparing an anode according to claim 13, wherein said anode is obtained by forming an alloy from at least one source of porous silicon and at least one alkali metal selected from the group consisting through

Li, Na, Ca and mixtures of at least two of these.

15. Process for the preparation of an anode, according to claim 14, in which the anode is based on a porous silicon, the porosity of which, measured according to the mercury porosimeter method, varies from 5 to 95% by volume. , terminals included.

16. Process for the preparation of an anode, according to claim 15, in which the porosity is approximately 75% by volume.

17. Process for the preparation of an anode, according to any one of claims 14 to 16, in which the porous silicon used as porous material is obtained from a source of silicon chosen from the group consisting of: waffles of silicon, silicon wafers, silicon films and mixtures of at least

2 of these.

18. A method of preparing an anode according to any one of claims 14 to 17, wherein the porous silicon used as porous material is obtained from a single crystal silicon.

19. A method of preparing an anode according to any one of claims 13 to 18, in which the porous silicon is obtained from a source of silicon, by electrochemical treatment, in a bath comprising at least one salt, said salt preferably being chosen from the group consisting of NHχF _γ with X being 4 or 5 and Y being 1 or 2, more preferably still the salt chosen is NH ₄ F.

20. A method of preparing an anode according to claim 19, wherein the bath used for the treatment of the silicon source contains at least one salt in solution, which is preferably a mixture of H ₂ S0 ₄ , NH F and H ₂ 0, and at least one non-aqueous solvent which is preferably an alcohol or a ketone, the non-aqueous solvent (s) being preferably chosen from the group consisting of methanol, ethanol, l acetone and mixtures of at least 2 of these.

21. Process for the preparation of an anode according to claim 20, in which the bath contains by volume of:

- 10 to 60% of NH ₄ F;

- 5 to 20% methanol; and

- 75 to 20% of H ₂ SO ₄ . ^'

22. Process for the preparation of an anode according to any one of claims 14 to 21, in which the alloy based on porous silicon is in the form Si _x Li _y , with x representing a number between 1 and 5, and representing there a number between 5 and 21.

23. The process for preparing an anode according to claim 22, in which x represents approximately 4 and y represents approximately 21.

24. A method of preparing an anode according to any one of claims 19 to 23, in which the alloy formed is of the Si _x Li _{y type} and it is obtained electrochemically by contacting a source of silicon with lithium and / or metallic lithium in the form of strips or waffles, at a temperature between 40 and 100 ° Celsius, preferably at a temperature of about 80 °

Celsius.

25. A method of preparing an anode according to claim 24, in which the duration of the contacting of the source of silicon and of metallic lithium is between 1 and 12 hours, preferably said duration is approximately 3 hours. .

26. Anode obtained by the implementation of a method according to any one of claims 1 to 25.

27. Anode characterized in that it contains at least 60% by mass and preferably at least 40% of a porous material, preferably porous silicon.

28. Anode according to claim 27, covered at least partially with carbon.

29. Anode according to claim 27 or 28, substantially free from cracks.

30. An electrochemical system comprising at least one anode as defined in any one of claims 26 to 29, at least one cathode and at least one electrolyte.

31. The electrochemical system in the form of a battery according to claim 30, in which the electrolyte is of the liquid, gel, or polymer type.

32. The electrochemical system according to claim 31 which is a battery in which the cathode is of the LiCo0 ₂ , LiFeP0 ₄ , LiNi0 ₂ , LiNio, ₅ Mno, ₅ 0 ₂ , LiNio, ₃₃ Cθo, ₃₃ Mno _{> 33} 0 _{2 type} , and the cathode is preferably 1 to 5 Volts.

33. Battery according to claim 32 of rechargeable type, preferably of lithium ion type.

34. Battery according to claim 33, in the form of micro-battery, preferably having dimensions of between 1 mm2 and 10cm2, and which have at least one of the following electrochemical properties: -

- electrochemical performance:

. an electrochemical capacity greater than 1 μWh

a cyclability greater than 500, preferably greater than 1000 cycles;

. a longer lifespan, according to the storage test carried out under ambient conditions, greater than 3 years, preferably greater than 5 years.

35. Use of an anode according to any one of claims 26 to 29, in an electrochemical system.

36. Use according to claim 33 as a negative electrode for micro lithium batteries.

37. Method for manufacturing an electrode based on porous silicon and covered at least partially with carbon by thermal pyrolysis of a polymer layer preferably deposited in a thin layer on a support preferably insulating in porous silicon such as Si3N4, the pyrolysis of the polymer being. preferably conducted at a temperature between 600 and 1,100 ° C and preferably for a period between 30 minutes and 3 hours.

38. Method for manufacturing an electrode based on porous silicon and covered at least partially with carbon, by laser pyrolysis of a polymer layer preferably deposited as a thin layer on a silicon support (insulator), the laser beam having preferably an intensity between 10 and 100 milliwatts and being placed, preferably, at a distance between 0.5 micrometers and 1 millimeter of the silicon support, preferably for a period of between 1 second and one minute.

39. The method of claim 37 or 38, wherein the silicon support is constituted by a single crystal of silicon and it has a thickness between 100 microns and 3 millimeters.

40. An electrode obtained by implementing one of the methods defined in any one of claims 36 to 39.

41. An electrochemical system comprising at least one electrode according to claim 40.

42. Process for the preparation of an electrode according to any one of claims

1 to 13 which is a cathode prepared from a target of cathode material, preferably said target is chosen from the group consisting of LiCo02, LiMn204, LiMnl / 3Nil / 3Col / 302, LiMnl / 2Nil / 202, LiMP04 ( M = Fe, Co, Ni, Mn) and mixtures of at least two of these, preferably the material target is pressed, the laser is applied to the target at powers that can vary from 20mW to 2W to create the porous material constituting the cathode which is then stripped from the target by laser and deposited on a half porous Si battery carbon / electrolyte.

43. A method of preparing an electrode according to any one of claims 1 to 13 which is a cathode prepared from a compound in pasty form formed from a mixture of a cathode powder with a carrier solution, preferably toluene, heptane or a mixture of at least 2 of these; the pasty solution is coated on a support plate (preferably made of glass) placed 100 μm from the substrate, preferably silicon; the laser beam of UV radiation is applied through the support plate and the cathode is projected onto the substrate by pyrolysis.