WO2019101869A1 - Passivated composite material for electrode - Google Patents

Abstract

The present invention relates to a passivated alloy, to the use thereof as a negative electrode, and to the method for producing same.

Description

 Passivated composite material for electrode

TECHNICAL AREA

The technical field to which the invention relates is that of metal alloys based on silicon and tin used as negative electrode materials (anode) lithium-ion battery. The invention also relates to the technical field of processes for manufacturing metal alloys based on silicon and tin. More specifically, the invention relates to the protection of the negative electrode and the electrolyte by the formation of a stable interphase, electrode / electrolyte, according to a process carried out in aqueous solution.

STATE OF THE ART

Integration into new storage systems for Li-ion or ionic storage batteries requires higher energy densities and higher power, more activity over a wide range of temperatures, and more reliability with longer time periods. cycling and calendar life sufficiently long depending on the intended applications (stationary, space, transport).

For negative electrodes, tin or silicon-based intermetallic materials exhibit high energy densities (600 to 4000 Ah / kg) but have significant aging problems related to several phenomena:

mechanical degradation due to volume variations during the formation of Li _x Sn or Li _x Si alloys leading to fracturing of the electrode and a short lifetime due to the loss of contacts (loss of electronic percolation);

 a chemical degradation which occurs at the electrode-electrolyte interface according to several phenomena:

 simple electrolyte / electrode reaction accompanied by the formation of a solid passivation layer rendered unstable by the mechanical degradation associated with volume variations;

 - reduction of solvents (EC, PC, DMC, ...) due to conduction electron transfers to the electrolyte resulting in the release of toxic gases;

reduction of the salt of the electrolyte (LiPF ₆ ) resulting in the release of toxic fluorinated gases and deposits of Li metal (dentrites). The volume expansions are controlled and absorbed by the use of intermetallic composites (such as Ni-Ti-Sn, Si / Si, Si / Ni ₃ Sn ₄ / Al / C) with deformable matrices.

 The Li-ion, Na-ion, accumulators integrated in future devices (electric or hybrid vehicles, stationary storage linked to renewable energies, space applications) must associate high energy densities, fast charges, activity in areas of wider temperatures, sufficiently long cycling and calendar durations (depending on applications) under maximum safety conditions.

Negative silicon-based electrode materials are well placed for their high lithiation capacity based on the formation of Li _x Si (x ~ 3.5) alloys. The natural abundance of silicon, its low atomic mass and its non-toxicity are determining factors. However, important problems remain related to its low electrical conductivity and the formation of unstable electrode / electrolyte solid interfaces. Both of these factors lead to limited lifetimes.

 The improvement of the conductivity of the composite is achieved through the introduction of high conductivity, active (Al, Sn) or inactive (Ni) metal species that improve the electronic percolation.

EP2 242 129 discloses alloys based on tin, silicon and carbon and containing a crystalline phase M-Sn, M being an inert metal.

The use of stable surface films, electronic insulators and conductors of Li ⁺ ions at the electrode / electrolyte interface, can passivate the surface of the electrode and prevent direct contact between the aprotic electrolyte and the surface of the electrode . This film to be stable must be composed of chemical species that are stable with respect to reduction and therefore have an electrochemical potential lower than the LUMO (unoccupied molecular orbital of lower energy) aprotic electrolytes.

It is therefore sought to protect a lithium-ion battery negative electrode material having a mass electrochemical capacity greater than 1000 mAh / g, a charged capacity capacitance of at least 1800 mAh / cm ³ , and that a lifetime in high cycling. A high cycle life means that the accumulator can be subjected to at least 180 cycles, preferably at least 200 cycles without its capacity falling by more than 20% from its initial capacity. SUMMARY OF THE INVENTION

The invention thus relates to the protection of the negative electrode (intermetallic enriched in silicon) and the electrolyte by forming an electrode / electrolyte interphase according to a process for impregnating alkaline phosphates in aqueous solution.

Controlled formation of the interphase and its coupling increases the lifespan in cycling, to reduce losses to the 1 ^st cycle and improve safety while maintaining the energy densities and power.

More specifically, the subject of the invention is a passivated alloy, said alloy comprising tin, silicon and carbon, and containing a crystalline phase M-Sn, M being an inert metal, characterized in that the alloy is protected by a superficial layer. passivation.

The term "superficial passivation layer" is intended to mean a thin, stable, electrically insulating and ionically conductive layer that can be applied to a material in order to prevent direct contact of said material with its environment. Typically, said passivation layer can be likened to a so-called solid interface / electrolyte layer (SEI).

The passivation layer may be chosen from the layers usually used to protect the intermetallics.

 This ionic conductive electronic insulating interphase avoids the reduction of the salt of the electrolyte and solvents thus preventing the deposition of metallic lithium (dendrites) and the release of toxic gases (fluorides, hydrocarbons). The process, carried out in aqueous solution of non-toxic and stable alkaline phosphates, makes it possible to control the size and the nature of the interphase formed (phosphates, phosphides). With the intermetallic composite materials enriched in silicon, the attachment of this layer is generally favored by the presence of tin in a small proportion.

The protection of the composite electrode vis-à-vis the electrolytic environment during galvanostatic cycles of charge / discharge is provided through a coating of alkaline phosphates, the reducing properties of nickel to ensure a good bonding between the layer protection and composite while ensuring better contact with the current collector through the formation of ternary phosphides, identified by Mossbauer spectroscopy of ¹¹⁹ Sn, Ni ₂ SnP type used as flexible solder in semiconductors

According to one embodiment, the passivation layer is a layer based on phosphate, in particular alkaline phosphate.

Alkaline phosphates (Li, Na) associated with transition metals (Fe, Mn, Co, Ni) with electrochemical potentials lower than the LUMO of aprotic electrolytes are stable with respect to reduction. By inductive effect the M-O-P or M-M'-P bonds promote the attachment of the layer to the negative electrode. These chemical species are characterized by various techniques such as Nuclear Magnetic Resonance (NMR), X-ray Photoelectron Spectroscopy (XPS), Mossbauer Spectrometry (SM).

An inert metal being defined as a metal not forming lithiated compounds having a significant amount of lithium, that is to say greater than 0.05Li / M, at the formation potential of the SiLix compounds (potential greater than 50 mV per relative to the Li ⁺ / Lii pair).

According to another object, the invention relates to a passivated alloy, said alloy comprising tin and silicon comprising:

a) a nanocrystalline phase containing at least 50 atomic% of the silicon element Si ⁰ ; b) a nanocrystalline phase containing a tin compound;

c) a nanocrystalline phase consisting of tin element Sn °; characterized in that it comprises a superficial passivation layer.

According to one embodiment, the nanocrystalline phase a) and / or silicon-containing particles are at least partially covered by the nanocrystalline phase b) and / or the nanocrystalline phase c).

According to one embodiment, the nanocrystalline phase a) and / or at least one particle containing silicon are at least partially covered by a tin-rich nanocrystalline phase, a tin-rich phase being present if, on at least one concentration profile measured using a transmission electron microscope, the alloy has a peak concentration such that the Sn _max / Max ratio (Sn1, Sn2) is greater than 1.1, preferably greater than 1.3; where Sn _max denotes the mass concentration at the peak concentration peak of tin; Sn1 and Sn2 denote the mass concentrations tin at the two bases of the peak; and Max (Sn1, Sn2) represents the maximum value of Sn1 and Sn2; the concentration profile originating from a point distant from a distance of between 20 and 100 nm from the center of the silicon-containing particles; the center of the silicon-containing particles being defined as the point corresponding to the center of the highest silicon concentration zones measured by Transmission Electron Microscopy (TEM) imaging.

According to one embodiment, the amount of tin Sn ° in the nanocrystalline phase c) represents up to 50%, preferably from 5 to 50%, preferably from 10 to 40% of the total amount of tin present in the alloy.

According to one embodiment, the amount of tin Sn ° in the nanocrystalline phase c) represents less than 5%, preferably less than 3% of the total amount of tin present in the alloy.

According to one embodiment, the alloy has the formula Si _a Sn _b C _c M _m where

M represents at least one inert metal, Al or a mixture thereof;

 0, 20 <a <0, 80;

 0, 05 <b <0, 40;

 0, 05 <c <0, 50;

 0, 01 <m <0, 30;

a + b> 0.45;

a + b + c + m = 1.

According to one embodiment, the mass percentage of the amount of phase containing at least 95% of silicon in phase a) is greater than 30%, preferably greater than 40% of the total weight percentage of silicon contained in the alloy.

According to one embodiment, the alloy does not comprise an SiC phase.

According to one embodiment, the alloy does not comprise crystalline carbon.

According to one embodiment, the alloy comprises the element Al present in an atomic fraction of less than 0.05, preferably 0.02 to 0.03 relative to the alloy. According to one embodiment, the size of the silicon-containing particles is 20 to 200 nm, preferably 20 to 100 nm, preferably 20 to 50 nm.

The invention also relates to an electrode comprising a passivated alloy according to the invention.

The invention also relates to a lithium-ion battery comprising at least one negative electrode which is an electrode according to the invention.

The invention also relates to a process for manufacturing a passivated alloy comprising silicon and tin, comprising the step of passivating said alloy by forming a surface layer on said alloy.

Typically, said method comprises contacting the alloy with an aqueous solution of alkali metal phosphate optionally hydrated.

The experimental conditions of formation of the passivation layer (concentration of the aqueous solution, pH, temperature, time) can be adjusted according to the type of the alloy.

Typically, the alloys are mixed with said aqueous passivation solution, for example by suspending or immersing the material in said aqueous solution or by applying and / or spreading said solution on said material, at a temperature between 20 and 80 ° C, for example placed in a thermostatic bath.

 Suspending of the powders in the aqueous solution can be carried out by controlled stirring under a gaseous flow (nitrogen flow through a porous ceramic or ultrasound to prevent agglomeration of the particles. between 2 and 6 hours) makes it possible to control the thickness of the layer.

 After reaction, the passivated composite can be recovered by filtration and then dried under vacuum, for example at 80 ° C for 12 hours.

Said passivation solution may be a solution of sodium phosphate, lithium or potassium, optionally monohydrate. It typically has an acidic pH, generally between 1, 5 and 4, especially about 2.

Typically, the passivation solution may be prepared from alkaline phosphates (for example NaH ₂ PO ₄ , H ₂ O) for a concentration varying between 40 and 140 g / l. After dissolving the alkaline phosphate in water, the solution is acidified with an acid such as orthophosphoric acid (H ₃ PO ₄ ) until the desired pH is obtained.

According to one embodiment, said method comprises the prior step of preparing said alloy. According to one embodiment, said process for preparing the alloy comprises the steps of:

a) preparing a pre-alloy from tin and at least one inert metal, or aluminum or a mixture thereof;

b) adding carbon, silicon and optionally inert metal to the preal alloy of step a);

c) synthesis of the alloy.

This method solves the problem of the total decomposition of the tin Sn ⁵ pre-alloy and makes it possible to obtain a powder of a tin-silicon alloy having a particle size sufficiently fine to be used as an active ingredient of a negative electrode of a lithium-ion battery.

According to one embodiment, the specific surface area of the carbon is between 20 and 500 mm ² / g, preferably between 20 and 100 mm ² / g.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1: Mossbauer spectra of composite C (a) and passivated composite C (b).

 Figure 2: Galvanostatic curves of composite C and passivated composite C.

 FIG. 3: Cycling resistance and coulombic efficiency of the composite C and the passivated composite C.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The manufacturing method according to the invention makes it possible to manufacture a passivated alloy based on silicon and tin. The process according to the invention makes it possible to form on the surface a passivation interface based on alkaline phosphates of the SEI type. The alloy of silicon and tin is described in EP 2,242,129. More specifically, it solves the problem of the tendency of tin to create large agglomerates, which can not be used later in the process. manufacture of a negative electrode active material of lithium battery.

 An alloy may be defined as a metal product obtained by incorporating one or more elements into a metal. The elements of an alloy react together to form a system consisting of several phases.

In order to prepare the alloy, a pre-alloy comprising tin and at least one inert metal or Al or a mixture thereof is prepared in a first step. An inert metal being defined as a metal not forming lithiated compounds having a significant amount of lithium, that is to say greater than 0.05Li / M, at the formation potential of the SiLix compounds (potential greater than 50 mV per relationship to the Li ⁺ > / Li ° couple).

Preferably, the inert metal is selected from the group consisting of Ni, Fe, Cu, Co, Mn, Ti, Zr, Y, Zn, Mo, Nb, V, W, Ag, Au, In, Mg, Pd, Pt, rare earths, lanthanides, or a mixture of these.

Tin is mixed with the inert metal (s) or Al or a mixture thereof. Preferably, tin and the inert metals and / or Al are in powder form. The pre-alloy can be obtained thermally or by mechanosynthesis, for example by grinding the powder mixture in a milling mill. In a preferred embodiment, the inert metal is nickel. The manufacture of the pre-alloy makes it possible to limit the agglomeration of the tin.

Preferably, the pre-alloy has the formula Ni _{3 + x} Sn ₄ with x ranging from 0.2 to 0.6; preferably between 0.3 and 0.5. It is possible, for example, to make a pre-alloy of formula Ni _{3 + x} Sn ₄ by metallurgy of nickel powders (Cerac 99.9%, particle size <45 μm) and tin (Alfa-Aesar 99.8%, particle size <45 μm). pm). The nickel and tin powders are pressed into pellets which are then placed in an argon sealed silica ampoule and annealed for about a week at about 700 ° C. Other pre-alloys can be envisaged, such as Cu ₆ Sn ₅ and Fe ₅ oSn ₅ o.

In a second step, the final alloy is made, for example by mechanosynthesis, by adding the remaining constituent chemical elements of the alloy, including silicon, with carbon in the finely divided state. The silicon used may have a particle size of less than or equal to about 1 micron.

In a particular embodiment, the carbon used has a BET specific surface area of between 20 and 500 m ² / g, preferably between 20 and 50 m ² / g. Below 20 m ² / g, there are not enough carbon / metal powder interfaces to prevent agglomeration of the particles. Beyond 500 m ² / g, the irreversible capacity of the alloy is too great. The carbon may for example be in the form of carbon black. Ordinary graphite such as that used in the negative electrodes of a conventional lithium-ion battery and carbon fibers are not sufficient for the process because of their too low specific surface area or unsuitable form factor. In the absence of carbon in the finely divided state, the alloy obtained is in the form of beads several millimeters in diameter which is extremely difficult to reduce in the form of a fine powder. The X-ray diffraction analysis (XRD) and the estimation of the amount of each phase by refinement by the Rietveld method shows that in the absence of carbon in the divided state, the tin-based prealloy completely decomposed into nanocrystalline tin. Transmission electron microscopy (TEM) assays confirm the presence of nearly pure tin. Pure tin coalesces and adheres to the walls of the container used for mechanosynthesis. The addition of finely divided carbon acts as a lubricant and anti-caking agent during the final alloy preparation step. It has the effect of reducing the decomposition of the pre-alloy into pure tin. It thus makes it possible to limit the formation of pure tin resulting from the decomposition of the tin pre-alloy, which causes a large agglomeration of the particles. In the presence of finely divided carbon, the M-Sn phase used as a precursor decomposes slightly. X-ray and transmission scanning electron microscopy can be observed to be a very thin layer of tin around the silicon nanocrystallites. The Applicant believes that this type of nanostructure is beneficial for the cycling performance because the tin makes it possible to accommodate the interface between the silicon, the surface of which will increase by 150% under load, and the other potentially inert phases whose surface should not or slightly vary. This type of nanostructure is more mechanically stable in cycles of swelling / deflation of the active ingredient.

At the end of this second stage of synthesis of the alloy, the final alloy containing the following phases is obtained: a nanocrystalline phase consisting of pre-alloy, a nanocrystalline phase consisting of silicon element containing at least 50 atomic%. The presence of particles of a compound containing silicon and nickel is also observed by TEM.

In a particular embodiment, the amount of silicon in the nanocrystalline phase consisting of the silicon element is greater than 50 atomic% of the total amount of silicon contained in the alloy.

In one embodiment, the tin-containing phases are nanocrystalline, i.e. the size of the crystallites is less than 100 nm, preferably less than 50 nm.

In one embodiment, the alloy does not include an X-ray diffractable (XRD) measurable SiC phase.

In one embodiment, the alloy does not include carbon in crystalline form.

The process according to the invention can be advantageously used to manufacture a passivated alloy, the alloy being of formula Si _a Sn _n C _c M _m where M represents at least one inert metal or Al or a mixture thereof; and wherein:

 0.20 <a <0.80; preferably 0.20 <a <0.50, more preferably 0.30 <a <0.50. Silicon is the main element at the origin of high mass capacities; too little silicon leads to a low capacitance and too much silicon leads to a low lifetime due to loss of electrical contacts due to the swelling and semiconductor nature of the silicon.

 0.05 <b <0.40; preferably 0.10 <b <0.30, more preferably 0.15 <b <0.25. Tin participates in the capacity. It also has the advantage of being very conductive and ductile which helps to maintain the electrical contacts during swelling and deflation. Nevertheless, pure tin tends to coalesce, which also poses problems of lifetime.

0.05 <c <0.50; preferably 0.10 <c <0.40, more preferably 0.10 <c <0.25. Carbon serves as a lubricant, anti-caking agent and minimizes the reaction between silicon and the M-Sn phase. It makes it possible to obtain fine powders that can be easily used, and on the other hand, it helps to form a rich tin phase around silicon-containing particles. The presence of the tin-rich phase makes it easier to accommodate changes in silicon volume during charge and discharge cycles of the accumulator. The quantity of tin Sn ° pure being sufficiently reduced so that the phenomena of coalescence do not occur.

 0.01 <m <0.30; preferably 0.05 <m <0.25, more preferably 0.10 <m <0.25. The inert metal serves on the one hand to limit the overall volume expansion of the alloy and on the other hand to ensure good electronic conduction inside the alloy particles. Nevertheless, too much of a quantity penalizes the capacity.

The addition of aluminum in the alloy makes it possible to improve the properties in cycling. It also participates in the capacity (up to 1 Li / atom compared to 4.4 Li for Si and Sn and 0.17 Li for graphite). Preferably, the atomic fraction of aluminum is less than 0.05, preferably 0.02 to 0.03 relative to the alloy. Above 0.05, the capacity and the lifetime of the alloy are reduced. a + b> 0.45. This condition makes it possible to have enough Si and Sn to ensure a high capacity.

The alloy has a high electrochemical capacity of at least 800 mAh / g and a high cycle life. It can therefore advantageously be used as the electrochemically active material of a negative electrode of a lithium battery.

EXAMPLES

The alloys are prepared by application or adaptation of the method described in EP 2,242,129. The silicon-based composite electrode materials are passivated with an aqueous solution of alkaline phosphates (Li, Na) more or less acidified (1,5). <pH <4). The passivation solution is prepared from alkaline phosphates (NaH ₂ PO ₄ , H ₂ O for example) for a variable concentration between 40 and 140 g / l. After dissolving the alkaline phosphate in water, the solution is acidified with orthophosphoric acid (H ₃ PO ₄ ) until the desired pH is obtained (between pH = 1.5 and pH = 4).

For the formation of the protective layer the composite materials are mixed with the aqueous solution of phosphates and placed in a bath thermostated between 20 and 80 ° C. Suspending the powders in the aqueous solution is carried out by controlled agitation of the bubbling type under a gaseous flow (nitrogen flow through a porous ceramic or ultrasound in order to avoid the agglomeration of the particles. 2 and 6 h) allows to control the thickness of the layer. After reaction, the passivated composite is recovered by filtration and then dried under vacuum at 80 ° C. for 12 hours.

For example, a characterization of the passivated composite material by ³¹ P NMR makes it possible to demonstrate the formation of the chemical species polyphosphates attributable to the attachment of the passivation layer.

 The product thus obtained is then used as electrode material. a) Protection of nanostructured Si

The passivation of the silicon nanoparticles was carried out in a solution of NaH ₂ PO ₄ or ϋH ₂ RO ₄ (90 g / L) acidified to pH = 2 (with H ₃ PO ₄ ) for 4 h at 40 ° C. The product obtained by filtration is washed with distilled water and then dried at 80 ° C. for 12 hours.

 These materials (Si and Si passivated) were electrodeposited in the form of inks of composition: active material (70%) + carbon Y50A (18%) + CMC (12%), coated on a copper collector and electrochemically tested. galvanostatic regime.

[Cycling conditions: ¹ cycle: C / 50, C / 20 [0-1, 5 V], ^2nd cycle: C / 20, C / 20 [0.07 to 1, V 5], ^3rd cycle : C / 5 (with CV ^* C / 50), C / 20 [0.07-1.5 V], cycling: C / 5 (with CV C / 50), C / 5 [0.07-1, 5 V], ^* Constant Voltage: potential set at 0.07 V and current relaxation limited to C / 50] The results are shown in Table 1.

Table 1: Electrochemical data of electrodes based on nanoparticles of Si and passivated Si.

Electrodes based on passivated silicon nanoparticles show a better cycling performance than that based on silicon nanoparticles. Stabilization of the capacity is observed after the first 80 cycles. In addition, the increase in coulombic efficiency is significant since more than 99.9% is obtained at 80 cycles, but it remains progressive. Passivation is therefore effective but the strong irreversibility for the first 50 cycles does not allow to use directly this anode material under this form. The integration of passivated silicon in composites is impossible with the synthesis method used which destroys the passivation layer, thus justifying the chemical treatment of the composite as a whole. b) Composite A protection

The composite A (Si / Ni-Ti-Sn) was prepared by grinding (Fritsch P7 planetary mill, 500 rpm, 8 h) of the mixture Ti ₆ Sn ₅ + Ni _3.6 Sn ₄ + Ni + Ti + Si in stoichiometric quantities corresponding to the composition Nio ₁₃ Tio ₁₃ Sno ₀₇ Sio ₆₇ ·

Characterization by X-ray diffraction revealed the presence of intermetallic Ni _x Ti _y Sn _z Si _{v components} , poorly crystallized bSn and Si. The Mossbauer spectroscopy of ¹¹⁹ Sn confirms the presence of Ni _x Ti _y Sn _z Si _v intermetallic components and bSn. Taking into account the values of their Lamb Mossbauer factors (0.05 for bSn and 0.20 for intermetallic species), their proportions are estimated at 17% and 83%, respectively.

The passivation of the composite A was carried out according to the method described above. The Mossbauer spectroscopy of ¹¹⁹ Sn shows that the Ni _x Ti _y Sn _z Si _v intermetallic sub-spectrum has not been modified and that that of bSn has been partially transformed into a tin phosphide species Sn ₃ P ₄ which is attributed to the attachment of the protective layer by Sn-P type interactions.

These two materials were electrodeposited in the form of inks of active material composition (70%) + carbon Y50A (18%) + CMC (12%) coated on a copper collector and tested in electrochemistry [Cycling conditions: 1 ^st cycle: C / 50, C / 20 [0-1, 5 V], ^2nd cycle: C / 20, C / 20 [0.07 to 1, 5V], ^3rd cycle: C / 5 (with CV ^* C / 50), C / 20 [0.07-1 5V], cycling: C / 5 (with CV C / 50), C / 5 [0.07-1, 5V], ^* Constant Voltage: potential set at 0, .07 V and current relaxation limited to C / 50]. The results are shown in Table 2.

Table 2: Electrochemical data for composite A and passivated composite A. For this example, the passivation conditions were not optimized (presence of tin phosphates IV type SnR ₂ 0 ₇ and subsistence of b Sn). The presence of electrochemically active tin phosphate IV is undesirable for the stability of the passivation layer. However, there is a decrease in polarization and a small increase in coulombic efficiency. c) Protection of composite B

The composite B (Si / Ni-Ti-Sn) was prepared by milling (planetary mill Fritsch P7, 500 revolutions / min, 8 h) of the mixture 0.5 [Ni _{0, 27Tio,} 26Sn _{0, i 4SiO,} 34] + 0.5 Si corresponding to the composition Nio, i3Tio 3Sno, o7SiO, 67 ·

The dispersion composite matrix Ni _0, 27Ti0 _, 26Sn _0, i.sub.4Si0, 34 was previously synthesized by grinding (Fritsch P7 planetary mill _, 500rpm _, 8h) of the 0.25 mixture.

Ni _0.27 Tio, 23Sn _0.5 o + 0.75 Nio ^ Tio, 27Sio, 47-

Components Nio ^ ^ Tio Sno.so and _{Nio, 27Tio,} 27Sio, 47 have been previously synthesized respectively from the mixture 0.67 Ni _3.5 Sn ₄ + 0.33 Ti ₆ Sn ₅ (planetary mill Fritsch P7, 500 rpm, 3 h) and 0.27 Ni + 0.27 Ti + 0.47 Si (Fritsch P7 planetary mill, 500 rpm, 12 h).

Characterization by X-ray diffraction revealed the presence of intermetallic Ni _x Ti _y Sn _z Si _{v components} , poorly crystallized bSn and Si. The Mossbauer spectroscopy of ¹¹⁹ Sn confirms the presence of Ni _x Ti _y Sn _z Si _v intermetallic components and bSn. Taking into account the values of their Lamb Mossbauer factors (0.05 for bSn and 0.20 for intermetallic species), their proportions are evaluated as 16% and 84%, respectively.

The passivation of the composite B was carried out according to the method described previously. Mossbauer spectroscopy of ¹¹⁹ Sn (Figure 7b) shows that the Ni _x Ti _y Sn _z Si _v intermetallic sub-spectrum has not been modified and that that of bSn has been partially transformed into a phosphate tin II species attributable to attachment of the protective layer by Sn-OP type interactions. The presence of residual bSn shows that the passivation conditions can be further improved.

These two materials were electrodeposited in the form of inks of active material composition (70%) + carbon Y50A (18%) + CMC (12%) coated on a copper collector and tested in electrochemistry [Cycling conditions: 1 ^st cycle: C / 50, C / 20 [0-1, 5V], ^2nd cycle: C / 20, C / 20 [0.07 to 1, 5V], ^3rd cycle: C / 5 (with CV ^* C / 50), C / 20 [0.07-1, 5V], cycling: C / 5 (with CV C / 50), C / 5 [0.07-1, 5V], ^* Constant Voltage: potential set at 0.07 V and current relaxation limited to C / 50]. The results are shown in Table 3.

Table 3: Electrochemical data of composite B and passivated composite B.

Without modifying the cycling capacity, passivation clearly improves coulombic efficiency (> 99.5%). d) Protection of composite B '

The composite B '(Si / Ni-Ti-Sn) Ni _{0, i 3} TiO _{, i 3} Si _0, o 7 SiO, 67, of the same composition as the composite B was prepared according to the same process from the mixture 0.50 [ Nio _, 27 Tio _, 26Sno _, 14 Sio _, 34] + 0.5 Si. For this composite B 'the silicon used to carry out synthesis and enrichment is nanometric.

Characterization by X-ray diffraction revealed the presence of intermetallic Ni _x Ti _y Sn _z Si _{v components} , poorly crystallized bSn and Si. The Mossbauer spectroscopy of ¹¹⁹ Sn confirms the presence of Ni _x Ti _y Sn _z Si _v intermetallic components and bSn. Taking into account the values of their Lamb Mossbauer factors (0.05 for bSn and 0.20 for intermetallic species), their proportions are evaluated as 50% and 47% respectively.

The passivation of the composite B 'was carried out according to the method described previously. The Mossbauer spectroscopy of ¹¹⁹ Sn shows that the intermetallic sub-spectrum Ni _x Ti _y Sn _z Si _v is little modified and that that of b Sn has been totally transformed into a phosphate tin II species attributable to the attachment of the layer. protection by Sn-OP type interactions.

These two materials were electrodeposited in the form of inks of active material composition (70%) + carbon Y50A (18%) + CMC (12%) coated on a copper collector and tested in electrochemistry [Cycling conditions: 1 ^st cycle: C / 50, C / 20 [0-1, 5V], ^2nd cycle: C / 20, C / 20 [0.07 to 1, 5V], ^3rd cycle: C / 5 (with CV ^* C / 50), C / 20 [0.07 to 1, 5V], cycling: C / 5 (with CV C / 50), C / 5 [0.07-1, 5V], ^* Constant Voltage: potential set at 0.07 V and current relaxation limited to C / 50]. The results are shown in Table 4.

Table 4: Electrochemical data of composite B 'and passivated composite B'.

Without significantly modifying the cycling capacity, passivation significantly improves cycling stability and coulombic efficiency (> 99.5%). e) Protection of the composite C

Composite C (Si / Ni ₃ Sn ₄ / Al / C) was prepared by grinding (Fritsch P7 planetary mill, 600 rpm, 20 h) of the mixture Ni _3.6 Sn ₄ + Si + Al + C in amounts stoichiometric composition corresponding to _Nio, Sno _{i3, i5 Sio 26Alo,} o4Co ₄ 2 · characterization by X-ray diffraction enabled to highlight the presence of components Ni ₃ Sn _4, BSN and if poorly crystallized. Mossbauer spectroscopy of ¹¹⁹ Sn (Figure 1a) confirms the presence of Ni ₃ Sn ₄ and bSn and a small proportion of tin oxide (IV). Taking into account the values of the Lamb Mossbauer factors, it also makes it possible to evaluate their proportions as 74%, 25% and 1% respectively.

The passivation of the composite C was carried out according to the method described previously. Mossbauer spectroscopy of ¹¹⁹ Sn (Figure 1b) shows that the intermetallic sub-spectrum Ni ₃ Sn ₄ has not been modified and that of bSn has been transformed into a tin oxide (II) species which is attributed to attachment of the protective layer by Sn-OP type interactions.

These two materials were electrodeposited in the form of inks of active material composition (55%) + carbon Y50A (25%) + CMC (20%) coated on a copper collector and tested in electrochemistry [Cycling conditions: 1 ^st formation cycle with lithiation at 2C followed by floating in C / 100, C / 20 delithiation, potential window 1, 5 V - 0.05 V, then C / 5 cycling, potential window 1, 5 V - 0.07 V; electrolyte EC / PC / 3D ™, 2% VC, 10% FEC, 1 M LiPF ₆ ]. The results are shown in Figures 2 and 3 and Table 5.

Table 5: Electrochemical data of composite C and passivated composite C.

The protection of the composite C does not change the nature of the material. The surface layer causes a slight decrease in the cycling capacity (5%), a significant decrease in loss in Cycle ¹ (37% to 22%) and a sharp increase in coulombic efficiency (99.3 % to 99.96-99.98%) (Figure 3). The decrease in cycling capacity results from the involvement of tin in the passivation layer, resulting in a marked improvement in coulombic efficiency.

The passivation layer minimizes electrode / electrolyte interactions. Thus, by XPS (C 1 s, F 1 s and P 2p spectra), a significant decrease in the thickness of the SEI can be observed regardless of the number of cycles showing an inhibition of the degradation process of the solvents into carbonates and of the salt. LiPF ₆ in LiF and phosphates. These observations were confirmed by TOF-SIMS.

conclusions

The invention makes it possible to create a stable interphase between a composite negative electrode based on silicon enriched intermetallics and an aprotic electrolyte of the LiPF ₆ type, in order to avoid deteriorations accelerating the aging processes.

 This interface, insulating electronic, ionic conductive is performed by impregnation of alkaline phosphates in aqueous solution, non-toxic and stable.

The examples given concern intermetallic composites with silicon as the main active element. Silicon having a low electronic conductivity of the active (Al, Sn) or inactive (Ni) conducting elements is introduced into the composite. In the examples presented the introduction of these elements is important since it plays a role in the electronic conductivity of the material and for the attachment of the passivation layer. Nickel plays a multiple role, it improves the electronic conductivity and as a reduction catalyst it intervenes on the nature of the electrode / passiv coupling. Introduced by an enrichment of Ni ₃ Sn ₄ in solid solution (Ni _{3 + n} Sn ₄ ) n excess nickel atoms are released during grinding. Their presence in greater or lesser amounts (0 <n <0.6) and the passivation temperature can modify the nature of the electrode / passivation snap (Sn-P or Sn-OP). The presence of tin can also be interesting since directly participating in the attachment of the protective layer it makes it possible to highlight the chemical species involved and to characterize them by Môssbauer spectrometry of ¹¹⁹ Sn thanks to their characteristic hyperfine parameters:

phosphide species 2 <d <2.50 mms ¹ and 0 <D <1.5 mms ¹

Phosphate species 2.50 <d <3.50 mms ¹ and 0.5 <D <1.5 mms ¹

These elements Ni, Sn can play an important role in the case of certain types of batteries by promoting the formation of Ni ₂ SnP conductive layers, flexible soldering semiconductors, to promote the interface with the current collector. Other metallic elements than those presented in the examples can be used. Their natures and their particular can control the properties of passivated composites.

Finally, in general, the experimental conditions of formation of the passivation layer, concentration of the aqueous solution, pH, temperature, reaction time must be adapted to the electrode material and to the intended application.

Claims

1. Passivated alloy, such that said alloy comprises tin, silicon and carbon, and containing a crystalline phase M-Sn, M being an inert metal, characterized in that it comprises a superficial layer of passivation.

2. Passivated alloy, such as said alloy comprises tin and silicon, comprising: a) a nanocrystalline phase containing at least 50 atomic% of the silicon element Si ⁰ ;

 b) a nanocrystalline phase containing a tin compound;

 c) a nanocrystalline phase consisting of tin element Sn °, characterized in that it comprises a superficial layer of passivation.

3. Passivated alloy according to any one of the preceding claims, such that the passivation layer is a phosphate-based layer.

4. Passivated alloy according to any one of the preceding claims, such that the passivation layer is an alkaline phosphate layer.

5. An alloy passivated according to one of the preceding claims, such that the alloy is of the formula Si _a C _b Sn _c M _m wherein M represents at least one inert metal, Al or a mixture thereof;

 0, 20 <a <0, 80;

 0, 05 <b <0, 40;

 0, 05 <c <0, 50;

 0, 01 <m <0, 30;

a + b> 0.45;

a + b + c + m = 1.

An electrode comprising a passivated alloy according to one of claims 1 to 5.

Lithium ion battery comprising at least one negative electrode which is an electrode according to claim 6.

A process for producing a passivated alloy comprising silicon and tin comprising the step of forming a passivation surface layer on said alloy.

9. The method of claim 8 comprising contacting the alloy with an aqueous solution of alkali metal phosphate optionally hydrated.