WO2014041076A1

WO2014041076A1 - Method for operating a lithium battery involving an active material of a specific electrode

Info

Publication number: WO2014041076A1
Application number: PCT/EP2013/068915
Authority: WO
Inventors: Etienne RADVANYI; Cédric HAON; Séverine Jouanneau-Si Larbi; Willy Porcher
Original assignee: Commissariat à l'énergie atomique et aux énergies alternatives
Priority date: 2012-09-12
Filing date: 2013-09-12
Publication date: 2014-03-20
Also published as: FR2995453A1; FR2995453B1; EP2896084A1

Abstract

The invention relates to a method for operating a lithium battery including at least one electrochemical cell that contains two electrodes placed on either side of a lithium ion-conducting electrolyte. One of the electrodes contains an active material that contains an alloy of at least one metal or semi-metal element and lithium, while the other electrode is a lithium source electrode. Said method includes the following steps: a) a preliminary step for implementing a method for preparing said active material of the above-mentioned electrode, said preparation method including a step for electrochemically incorporating lithium in a molar ratio Li/M up to 5 in a material consisting of at least one metal or semi-metal element capable of forming an alloy with lithium, said step being carried out under conditions sufficient for obtaining an active electrode material having a molar ratio Li/M (M being the element M as defined above) of at least 1/3 of the molar ratio when said material is completely alloyed with lithium, by means of which said active electrode material is obtained; and b) a step of operation by cycling said battery in lithium without the total delithiation of said electrode mentioned in step a). Step b) is carried out in a capacity domain corresponding to, at most, 2/3 of the total reversible capacity of the active material of said electrode.

Description

METHOD FOR OPERATING A LITHIUM ACCUMULATOR INVOLVING A SPECIFIC ELECTRODE ACTIVE MATERIAL

DESCRIPTION

TECHNICAL AREA

The present invention relates to a method of operating a lithium battery involving a specific active material prelithiated from one of the two electrodes included in an electrochemical cell of the accumulator.

The general field of the invention can thus be defined as that of lithium accumulators and, more specifically, lithium-ion type accumulators.

Lithium batteries are increasingly being used as a stand-alone energy source, particularly in portable electronic equipment (such as mobile phones, laptops, tools), where they are gradually replacing nickel-cadmium accumulators (NiCd) and nickel metal hydride (NiMH). They are also widely used to provide the power supply needed for new micro applications, such as smart cards, sensors or other electromechanical systems.

Lithium batteries operate on the principle of insertion-deinsertion (or lithiation-delithiation) of lithium according to the following principle.

When discharging the accumulator, the lithium is removed from the negative electrode in the form of ionic Li migrates through the ionic conductive electrolyte and is interposed in the crystal lattice of the active material of the positive electrode. The passage of each Li ⁺ ion in the internal circuit of the accumulator is exactly compensated by the passage of an electron in the external circuit, thereby generating an electric current. The specific energy density released by these reactions is both proportional to the potential difference between the two electrodes and the amount of lithium that will be interposed in the active material of the positive electrode.

When charging the accumulator, the reactions occurring within the accumulator are the reverse reactions of the discharge, namely that:

the negative electrode will insert lithium into the network of the material constituting it;

the positive electrode will release lithium.

By this principle of operation, lithium batteries require two different insertion compounds to the negative electrode and the positive electrode.

The positive electrode is generally based on lithiated oxide of transition metal:

of the lamellar oxide type of formula LiM0 ₂ , where M denotes Co, Ni, Mn, Al and mixtures thereof, such as LiCoO ₂ , LiNiO ₂ , Li (Ni, Co, Mn, Al) O ₂ ; or

of the spinel structure oxide type, such as LiMn ₂ 0 ₄ .

The negative electrode may be based on a carbon material, and in particular based on graphite. Graphite has a theoretical specific capacity of the order of 372 mAh / g (corresponding to the formation of the LiCe alloy) and a practical specific capacity of the order of 320 mAh / g. However, graphite has a high degree of irreversibility during the first charge, a continuous loss of capacity in cycling and a prohibitive kinetic limitation in the case of a high charge / discharge regime (for example, for a C / 2 charge regime).

In order to improve the insertion properties of lithium into the negative electrode, researchers have focused their efforts on finding new electrode materials.

Thus, they discovered that materials or elements capable of forming an alloy with lithium are able to constitute excellent alternatives to the use of graphite.

Thus, it has been demonstrated that the insertion of silicon into a negative electrode makes it possible to significantly increase the specific practical capacity of the negative electrode related to the insertion of lithium therein, which is 320 mAh / g for a graphite electrode and 3578 mAh / g for a silicon-based electrode (corresponding to the formation of the Lii ₅ Si ₄ alloy during the insertion at room temperature of the lithium in the silicon) . Thus, by means of simple forecasts, it is possible to envisage a gain of about 40 and 35%, respectively in energy density and in mass energy, if the graphite is replaced by silicon in a conventional battery of the "lithium-ion" die. Moreover, the average operating potential window of the lithium-silicon alloy of formula Li _x Si with x ranging from 0 to 3.75 (0.7-0.05 V / Li-Li ⁺ ) higher than that graphite, avoids the formation of a metallic lithium deposit and associated risks. In addition, it is established that the formation reaction of the lithium-silicon alloy, leading to a very high specific practical capacity (of the order of 3578 mAh / g), is reversible.

Nevertheless, the use of silicon in a negative electrode of a lithium battery poses a number of difficulties.

In particular, during the formation reaction of the silicon-lithium alloy (corresponding to the insertion of lithium into the negative electrode in charge process), the volume expansion between the delithiated phase and the lithiated phase can reach values ranging from 240 to 400%. This strong expansion, followed by a contraction of the same amplitude (corresponding to the disinsertion of lithium in the negative electrode during the discharge process) can quickly lead to irreversible mechanical damage to the electrode and in particular a sputtering of said electrode and, as a result, degradation of the electrode / electrolyte interface. These phenomena can cause a rapid degradation of the electrochemical performance of the electrodes made from this type of materials. To maintain the initial electrochemical performance, it is possible to limit the cycling capacity of the electrodes, in order to limit the depth of lithiation and thus reduce the volume expansion of the latter. Thus, for example, by limiting the cycling capacity of a negative electrode active material based on silicon particles at 1200 mAh / g, this amounts to subjecting said particles to a lithiation rate ranging from 0 to 1.25. which makes it possible to obtain good electrode performance in terms of cyclability. However, for negative electrodes comprising this type of particles in combination with a split-carbon electrically conductive additive and with a polymeric binder, it is not possible to obtain sufficient electrochemical performance in terms of practical specific capacity. and coulombic efficiency (for example, greater than 99.7%) over more than one hundred cycles, which notably prevents the integration of this type of electrode in accumulators of the lithium-ion type.

The authors discovered, surprisingly, that by specifically preparing an active material of one of the electrodes (typically the negative electrode) for lithium accumulator and then applying, during the operation of this electrode material in a lithium battery, a specific cycling mode, it is possible to overcome the aforementioned drawbacks. STATEMENT OF THE INVENTION

Thus, the invention relates to a method of operating a lithium battery comprising at least one electrochemical cell comprising two electrodes (more specifically, a positive electrode and a negative electrode) disposed on either side of a lithium ion conductive electrolyte, one of the electrodes comprising an active material comprising an alloy of at least one metallic or semi-metallic M element and lithium, while the other electrode is a lithium source, said method comprising the steps following: a) a preliminary step of implementing a method for preparing said active material of the above-mentioned electrode, said method comprising a step of electrochemically incorporating lithium in a molar ratio Li / M up to In a material based on at least one metallic or semi-metallic element M capable of forming an alloy with lithium, preferably, said step being performed under conditions sufficient to obtain an electrode active material having a Li / M molar ratio (M being the element M as defined above) of at least 1/3 of the molar ratio when said material is completely alloyed with lithium, whereby said electrode active material is obtained; and b) a step of operation by cycling (s) said lithium accumulator without total delithiation of said electrode mentioned in step a), preferably, step b) being performed over a range of capacitances corresponding to at most 2/3 of the total reversible capacitance of the active material of said electrode, and preferably to at most 1/3 of the total reversible capacitance of the active material of said electrode.

Conventionally, the electrode comprising an active material comprising an alloy of at least one metallic or semimetallic element M and lithium corresponds to the negative electrode of the accumulator.

As mentioned above, step a) comprises a step of electrochemically incorporating lithium in a Li / M molar ratio of up to 5 (i.e. value is at most equal to 5) in a material based on at least one metallic or semi-metallic element M capable of forming an alloy with lithium, preferably, said step being carried out under conditions sufficient to obtain an active material electrode having a molar ratio Li / M (M being the element M as defined above) of at least 1/3 of the molar ratio when said material is completely alloyed with lithium, whereby said active material of the electrode is obtained.

More preferably, step a) is carried out under conditions sufficient to obtain an electrode active material having a molar ratio Li / M (M being the element M as defined above) of at least equal to 2/3 of the molar ratio when said material is completely alloyed with lithium.

Step a) can be described as a prelithiation step, knowing that the material obtained at the end of this step is intended to be used as an electrode active material in a lithium battery.

The above-mentioned element M is, as mentioned above, an element M capable of forming an alloy with lithium, this element possibly being either a metallic element or a semi-metallic element.

As examples of metal element, mention may be made of aluminum, tin, germanium and mixtures thereof.

As examples of semi-metallic element, mention may be made of silicon.

These elements are particularly interesting because they have a very high theoretical specific capacity (for example, of the order of 3600 mAh / g for silicon) compared to materials such as graphite commonly used to enter the negative electrode constitution. .

The material based on at least one metallic or semi-metallic element M may be in the form of only one element M (for example, pure silicon), an alloy of element (s) M or a composite material based on at least one element M.

The composite material based on at least one element M may be made of a material comprising silicon and another element chosen from carbon, tin, aluminum, germanium and mixtures thereof.

Advantageously, the material based on at least one metallic or semi-metallic element M advantageously has a total reversible capacity greater than 2500 mAh / g, specific examples corresponding to this characteristic being the following:

pure silicon (whose theoretical specific capacity is 3600 mAh / g);

-a silicon-carbon composite, the carbon content of which will not exceed 30 ~ 6 in mass

a silicon-tin composite, the tin content of which will not exceed 40 ~ 6 by weight

a silicon-aluminum composite, the aluminum content of which will not exceed 40% by mass; and

a silicon-germanium composite, the germanium content of which will not exceed 55 ~ 6 by mass.

By way of example, when the material comprises elements Mi, M ₂ , M _n in proportions y ₁ , y ₂ , y _n , each element having respectively an effective reversible capacitance of x 1, x ₂ , x _n , then the capacitance total reversible of the material corresponds to the sum ( ^χ ιγι + Χ2Υ2 + ··· + ^χ _η Υη) · H means that this total reversible capacitance does not take into account the irreversible capacity related to the parasitic reactions, as in particular the surface reactions .

The material based on at least one metallic or semi-metallic element M may be in the form of nanometric particles, for example particles having an average particle diameter of less than 200 nm.

The material based on at least one metallic or semi-metallic element M may coexist with other materials such as organic binders, electrically conductive materials and mixtures thereof.

The organic binders may be polymeric binders, advantageously chosen from electrochemically stable polymers, for example, in a window of potentials ranging from 0 to 5 V relative to Li / Li ⁺ , such polymers possibly being cellulosic polymers.

The electrically conductive materials may be carbonaceous materials, in particular carbon materials in divided form, such as spherical particles, fibers. More specifically, it may be carbon black, carbon fibers, acetylene black, carbon nanotubes and mixtures thereof.

From a practical point of view, the electrochemical incorporation step can be carried out electrostatically, that is to say by applying a current of constant intensity for a period of time necessary for the incorporation of lithium according to the desired molar ratio, i.e., a molar ratio of up to 5 and preferably so that the electrode active material has a Li / M molar ratio (M being the M component); as defined above) of at least 1/3 of the molar ratio when said material is completely alloyed with lithium.

To do this, the material based on at least one metallic or semi-metallic element M may be formed into an electrode which is placed in a device comprising a cell comprising a lithium source electrode, for example a lithium metal electrode. or any other material capable of providing electrochemically lithium, between which is disposed a lithium ion conductive electrolyte.

More particularly, it may be a liquid electrolyte comprising a lithium salt.

Thus, for example, the liquid electrolyte may comprise a solvent or mixture of carbonate-type solvents, such as ethylene carbonate, propylene carbonate, dimethyl carbonate or diethyl carbonate, and / or a solvent or a mixture of ether-type solvents, such as dimethoxyethane, dioxolane, dioxane, tetraethyleneglycoldimethylether (known by the abbreviation TEGDME) and mixtures thereof in which a lithium salt is dissolved.

By way of example, the lithium salt may be chosen from the group consisting of LiPF ₆ , LiClC 3, LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF 3 SO ₂ ) 3, LiN (C ₂ F ₅ SO ₂ ) lithium bistrifluoromethylsulfonylimide (known by the abbreviation LiTFSI) LiN [S0 ₂ CF ₃ ] ₂ and mixtures thereof.

The intensity delivered at the terminals of the device is fixed so as to generate a lithiation phenomenon of the aforementioned electrode active material until the desired molar ratio is obtained, this ratio being determined by measuring the practical specific capacity of said material.

Indeed, according to Faraday's law, the quantity of electricity Q (in Ah) which passes through the electrolyte is fixed, in galvanostatic mode, by the relation:

Q = I (t) .dt wherein:

-I denotes the imposed current (in A); and

-t denotes the duration of application of this current.

The lithium metal of the lithium source electrode is converted to Li ⁺ and releases an electron, while Li ⁺ will be incorporated into the active material of the electrode.

For a mole of Li, the charge corresponds to a faraday (1 F = 96487 ° C.mol ^_1 = 26.8 Ah.mol ^-1 ) The quantity of charges per mole of lithium reacted (Q / n) can be expressed by the following relation:

x.F = (Q / n) = (I * t) / (m / M) = (I * t * M) / m therefore x = (I * t * M) / (26, 8 * m)

in which :

-x is the number of moles of lithium reacted for one mole of active material;

-t is the duration of application of the current

(in h); M is the molar mass of the active material (in g mol ^-1 ) and m is the mass of the active material (in g).

The specific capacity of the active material (expressed in mAh.g ^-1 ) being expressed by the following value:

C = (Q / m) = (26, 8 * x * 1000) / M,

it is thus possible, by measuring this quantity during the application of the current I, to easily rise to the desired molar ratio and once reached, to stop the imposition of the current.

In other words, preferably, during the incorporation step, the materials based on at least one metallic or semi-metallic element M are lithiated in a molar ratio corresponding to the obtaining of a specific capacity. at least one-third of the total reversible capacitance of said material, preferably at least 2/3 of said total reversible capacitance (this total reversible capacitance corresponding, for a negative electrode, to the amount of electricity generated during the reversible disinsertion of lithium atoms and does not include lithium atoms, which are trapped during the first charge, especially on the surface of the active material).

The prelithic materials obtained at the end of step a) of this invention are structurally different from those obtained by the metallurgical route (in particular, by metallurgy of powders), in particular as regards their degree of crystallinity, in that they are partly amorphous. These prelithy materials are intended to be used as active materials in an electrode, and more specifically, a negative electrode for operating lithium accumulators, said accumulator comprising at least one electrochemical cell each comprising an electrode comprising such an active material. , and another electrode disposed on either side of a lithium ion conductive electrolyte.

The use of prelithy materials in a lithium battery has several advantages.

During cycling of the accumulator (i.e. during charge-discharge processes thereof), a fraction of the inserted lithium ions contributes to the growth of the electrode / electrolyte interface, forming products. of the carbonates or lithium oxide type on the surface of the constituent particles of the negative electrode. These reactions are irreversible, so that the lithium ions consumed for the formation of these products no longer participate in the cycling of the electrode and contribute to the irreversible capacity between the charge and the discharge. Prélithier the electrode active material as defined above allows to have a significant source of lithium ions in the active material of the electrode and thus improves the coulombic efficiency of the accumulator at each cycle (which corresponds the percentage of electric charge stored in the battery during charging, which is recoverable during discharge). During cycling of the accumulator with an electrode prelithiated active material, it is possible to cycle the material in a lithiation domain, where the surface variations are reduced and the stability of the electrode / electrolyte interface is improved. Indeed, if the variation of volume is linear with the rate of lithiation, the variation of surface is not it. Thus, for the same capacity (or the same amount of lithium ions inserted) cycled, the surface variation is lower for a prelithiated active material relative to a material, which is not. Thus, for example, for a silicon electrode (more precisely, composed of spherical silicon particles) cycled over a capacitance range of 1200 mAh / g, the surface area of the particles will vary less between 2400 and 3600 mAh / g ( which means, on the one hand, that the electrode has been prelithicated up to 2400 mAh / g, which corresponds to a Li / Si molar ratio of 2.5 and that the cycling of the material has been carried out on a field of molar ratio ranging from 2.5 to 3.75) between 0 and 1200 mAh / g (which corresponds to cycling said material over a range of 0 to 1.25 molar ratio).

From the foregoing, it has emerged for the inventors the importance of prelithying an electrode active material as defined above electrochemically and also the importance, once in operation in a lithium accumulator, of not delithier completely, during cycling, said electrode active material thus prelithiated. This is apparent from step b) defined above. By proceeding in this way, the electrode prepared in step a) comprises during a cycling (that is to say, during a charge-discharge process) always lithium inserted, which amounts to to say that the electrode remains lithiated permanently during the cycling of the electrodes.

As mentioned above, the step of operation by cycling (s) is carried out, preferably, over a range of capacities of up to at most 2/3 of the total reversible capacity of the active material (which corresponds to a restricted cycling domain), so that the electrode remains always lithiated.

More specifically, the step of operation by cycling (s) can be performed on a range of capacities corresponding to at most 1/3 of the total reversible capacity of the active material.

Thus, for example, for a negative electrode based on an active material consisting of nanometric particles of pure silicon, with a total reversible capacity of 3600 mAh / g, corresponding to a molar ratio of 3.75, when the silicon is completely lithiated, the step of preparing said electrode can result in an electrode having a molar ratio Li / M of 2.5. The cycling operation step (s) carried out on a range of capacities corresponding to 1/3 of the total reversible capacity of the material, thus amounts to cycling over a range of capacitances of 1200 mAh / g, which amounts to to say in other words that during a cycling: the molar ratio Li / M changes from 3.75 to 2.5 during the disinsertion; and

the Li / M molar ratio changes from 2.5 to 3.75 at the time of insertion.

The electrode is thus subjected to a restricted cycling domain (1/3 of its total reversible capacity) while being in a high lithiation domain (greater than 2/3 of its total reversible capacity).

For this type of material, in such a field, there is a small surface variation and thus a good stability of the material even if the number of cycles becomes very important. Moreover, working in a capacity range of 1200 mAh / g makes it possible to obtain a very significant gain in specific energy density compared to commonly used negative electrode active materials, such as graphite, whose accessible capacity range can not go beyond 372 mAh / g.

The operating method is adapted, as indicated above, to lithium accumulators, and in particular to lithium-ion accumulators of the lithium-ion type, in particular because the operating mode of the invention makes it possible to access to excellent coulombic efficiency.

In this case, the prelithiated electrode will respond to the same specificities as those announced above and, moreover, will advantageously have a high density of energy density, such as a density of energy density. greater than 1800 mAh / cm and a large surface density, for example, a surface density of at least 2.5 mAh / cm ² , these densities being determined on the basis of the theoretical specific capacities of the active materials used.

The lithium source electrode, for its part, can be any positive electrode as described in the part devoted to the technical field.

Other features and advantages of the invention will emerge from the additional description which follows and which relates to particular embodiments.

Of course, this additional description is only given as an illustration of the invention and does not in any way constitute a limitation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the evolution of the potential V (expressed in V relative to the Li ⁺ / Li pair) as a function of the practical practical capacity C (expressed in mAh / g) obtained during the first two cycles (respectively curve a) for the first cycle and curve b for the second cycle) for the first electrode of Example 1.

FIG. 2 is a graph showing the evolution of the potential V (expressed in V with respect to the Li ⁺ / Li pair) as a function of the practical specific capacity C (expressed in mAh / g) for the prelithiation step (curve a ) and for an insertion / desinsertion cycle over a capacity range of 2400 to 3600 mAh / g (curve b). FIGS. 3 and 4 are graphs, which respectively illustrate the evolution of the practical specific capacitance obtained in terms of insertion and insertion C (expressed in mAh / g) (curves a) and b) with the first electrode and curves c) and d) with the second electrode) as a function of the number of cycles N and the evolution of the coulombic efficiency E obtained with the first electrode (curve a) and the second electrode (curve b) as a function of the number of cycles N in the framework of Example 1.

FIG. 5 is a graph illustrating the evolution of the potential V (expressed in V with respect to the Li ⁺ / Li pair) as a function of the practical specific capacity C (expressed in mAh / g) for the prelithiation step (curve a ) and for a charge / discharge cycle over a range of capacitances from 1200 to 3600 mAh / g (curve b) for the electrode of Example 2.

FIGS. 6 and 7 are graphs, which respectively illustrate the evolution of the practical specific capacitance obtained in insertion and in C deinsertion (expressed in mAh / g) (curves a) and b) as a function of the number of cycles N and l evolution of coulombic efficiency E obtained with the abovementioned electrode as a function of the number of cycles N for example 2.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

EXAMPLE 1

In this example, two identical electrodes (called respectively, first electrode and second electrode) made of nanoscale silicon (65% by weight), carbon fibers (25% by weight) and carboxymethylcellulose (10% by weight) (this additive acts as a binding agent organic), with a basis weight of the order of 2 mg of silicon per cm ² were cycled under different conditions in a button cell with lithium metal as lithium source electrode (this electrode having a thickness of 135 μm and having a surface identical to the other electrode) in galvanostatic mode at a rate of C / 7 at room temperature. The electrolyte used is a 1M LiPF ₆ salt dissolved in an ethylene carbonate / diethylene carbonate mixture in a 1: 1 volume ratio with a polypropylene separator having a thickness of 25 microns.

More specifically, the galvanosplastic mode consists of imposing a constant current of intensity I and following the evolution of the potential V at the terminals of the button cell over time t, the measurement being done in dynamic mode. The current imposed between the two electrodes is such that the imposed current regime is equal to C / 20, ie here about 600 μΑ. A reduction current, of negative value by convention, is imposed on the button cell until the potential difference across the battery reaches a set minimum limit value and then the current is reversed to the maximum limit value set. V _max of the potential difference.

The first non-prelithiated electrode was cycled by limiting the capacity to 1200 mAh / g, the electrode is cycled at a molar ratio ranging from 0 to 1.25 for several tens of cycles. FIG. 1 illustrates the cycling of this electrode by representation only of the first two charge-discharge cycles having been implemented as illustrated in FIG. 1, representing the evolution of the potential V (expressed in V relative to the Li ⁺ / pair). Li) according to the practical specific capacity C (expressed in mAh / g).

The second electrode was cycled as follows:

firstly, a current I of 600 μΑ is imposed for a duration t of 13 hours until a Li / Si molar ratio of 2.75 is obtained, ie, in other words, a capacity of 2400 mAh / g;

in a second step, charge / discharge cycles are imposed several tens of times with a capacity limitation of 1200 mAh / g, which means, in other words, that these cycles are applied over a range of capacitances ranging from from 2400 to 3600 mAh / g, that is to say a Li / Si molar ratio ranging from 2.75 to 3.5.

The appended figure 2 illustrates the evolution of the potential V according to the specific practical capacity for the aforementioned first time and for a charge / discharge cycle over a range of capacities ranging from 2400 to 3600 mAh / g.

The two aforementioned electrodes were cycled according to the protocols described above for several tens of cycles and their electrochemical performances were compared as illustrated on FIGS. 3 and 4 in the appendix, which respectively illustrate the evolution of the practical specific capacitance obtained in charge and in discharge C (expressed in mAh / g) (curves a) and b) with the first electrode and curves c) and d) with the second electrode) as a function of the number of cycles N and the evolution of the coulombic efficiency E obtained with the first electrode (curve a) and the second electrode (curve b) as a function of the number of cycles N.

Through this first example, the following conclusions emerge:

the fact of cycling with a prelithiated electrode, which remains during cycling (that is to say here with the second electrode, makes it possible to obtain much better electrochemical performances than with the first electrode), both in terms of maintaining specific practical capacity only in terms of coulombic efficiency;

it is not necessary to proceed to the addition of a specific additive in the electrolyte to obtain such performances.

EXAMPLE 2

In this example, an electrode of the same composition and basis weight as those described in Example 1 was cycled as follows:

firstly, a current I of 600 μΑ is imposed for a duration t of 6.5 hours until a Li / Si molar ratio of 1.25 is obtained, that is to say in other words a capacity of 1200 mAh / g; in a second step, load / discharge cycles are imposed several tens of times with a capacity limitation of 2400 mAh / g, which means, in other words, that these cycles are applied over a range of capacitances ranging from from 1200 to 3600 mAh / g, which amounts to cycling the material of the negative electrode with a lithiation rate ranging from 1.25 to 3.5.

Figure 5, attached, illustrates the evolution of the potential V (in V) as a function of the specific practical capacity for the aforementioned first time and for a charge / discharge cycle over a range of capacitances ranging from 1200 to 3600 mAh / g. .

The above-mentioned electrode was cycled according to the protocols described above for several tens of cycles and its electrochemical performances were compared as illustrated in FIGS. 6 and 7 attached, which respectively illustrate the evolution of the specific practical capacity obtained. in charge and in discharge C (expressed in mAh / g) (respectively curves a) and b) as a function of the number of cycles N) and the evolution of the coulombic efficiency E obtained with the aforementioned electrode.

Thus, even when the electrodes are cycled on very large capacities, by maintaining the material of the negative electrode permanently lithiated during the cycles, it is possible to obtain excellent electrochemical performances.

Claims

1. A method of operating a lithium battery comprising at least one electrochemical cell comprising two electrodes arranged on either side of a lithium ion conductive electrolyte, one of the electrodes comprising an active material comprising an alloy of minus one element M metal or semi-metallic and lithium, while the other electrode is a lithium source, said method comprising the following steps: a) a preliminary step of implementing a method for preparing said active material of the above-mentioned electrode, said method comprising a step of electrochemically incorporating lithium in a Li / M molar ratio of up to 5 in a material based on at least one metallic or semi-metallic element M capable of forming an alloy with lithium, said step being performed under conditions sufficient to obtain an electrode active material having a molar ratio Li / M (M being the element M as defined above) of at least 1/3 of the molar ratio when said material is completely alloyed with lithium, whereby said electrode active material is obtained; and

b) a step of operation by cycling (s) said lithium accumulator without total delithiation of said electrode mentioned in step a), step b) being performed on a range of capacities corresponding to at most 2/3 of the capacity total reversible of the active material of said electrode.

2. The method of claim 1, wherein the metal element M is selected from aluminum, tin, germanium and mixtures thereof.

3. The method of claim 1, wherein the semi-metallic element M is silicon.

4. Method according to any one of the preceding claims, in which the material based on at least one metallic or semi-metallic element M is in the form of only one element M, one alloy element ( s) M or a composite material based on at least one element M.

5. Method according to claim 4, wherein the composite material based on at least one element M is a material comprising silicon and another element selected from carbon, tin, aluminum, germanium and mixtures of these.

6. Method according to any one of the preceding claims, wherein the material based on at least one metallic or semi-metallic element M has a total reversible capacitance greater than 2500 mAh / g.

7. Process according to any one of the preceding claims, in which the material based on at least one metallic or semi-metallic element M is chosen from:

pure silicon;

A method according to any one of the preceding claims, wherein the material based on at least one metallic or semi-metallic element M coexists with another material selected from organic binders, electrically conductive materials and mixtures thereof.

The process of claim 8, wherein the organic binders are polymeric binders.

The method of claim 8 or 9, wherein the electrically conductive materials are carbonaceous materials in divided form.

The method of any one of the preceding claims, wherein the step Electrochemical incorporation is carried out by galvanostatic means.

12. The method as claimed in any one of the preceding claims, in which step a) is carried out under conditions sufficient to obtain an electrode active material having a molar ratio Li / M of at least equal to 2/3 of the molar ratio when said material is completely alloyed with lithium.

13. The method as claimed in claim 1, wherein step b) is carried out over a range of capacitances corresponding to at most 1/3 of the total reversible capacitance of the active material of said electrode.