EP2118007A2

EP2118007A2 - Mixed lithium silicates

Info

Publication number: EP2118007A2
Application number: EP08761851A
Authority: EP
Inventors: Gérard QUOIRIN; Jean-Marie Tarascon; Christian Masquelier; Charles Delacourt; Philippe Poizot; Francis Taulelle
Original assignee: Centre National de la Recherche Scientifique CNRS
Current assignee: Centre National de la Recherche Scientifique CNRS
Priority date: 2007-02-09
Filing date: 2008-02-07
Publication date: 2009-11-18
Also published as: JP2010517913A; FR2912398A1; CA2677711A1; WO2008107571A3; CN101652321A; FR2912398B1; WO2008107571A2

Abstract

The invention relates to mixed lithium silicates, wherein the mixed silicates have the following formula Li<SUB>2</SUB>M<SUP>II</SUP>

Description

Lithium mixed silicates

The present invention relates to mixed lithium silicates, their preparation and their use as electrode active material.

It is known to use mixed silicates of Li and a transition metal as a positive electrode active material. Thus, EP-I 134 826 describes the preparation of mixed silicates by a process consisting in mixing the solid precursors of the silicate, in stoichiometric proportions, in a high-energy mill, and in carrying out a heat treatment at a temperature of at least 800 ⁰ C under controlled atmosphere. Such a process is therefore complicated to implement and unattractive because of the large amount of energy used. In addition, it provides the silicate in the form of particles of uncontrolled size, relatively large, of the order of 20 microns to 200 microns.

The object of the invention is to provide a process for the preparation of mixed silicates of Li and a transition metal M (Fe, Co, Mn, Ni) which does not have the disadvantages of the method of the prior art. Accordingly, the present invention relates to a process for the preparation of mixed lithium silicates and transition metal, the silicates obtained, and their use as electrode active material in various electrochemical devices. The method of the present invention is a process for preparing a compound of the formula Li ₂ M ¹¹ _J i _X) M _X ¹¹¹ SiO ₄ (OH) _x (I) wherein 0 <x <1 and M is Fe, Co, Mn or Ni, characterized in that it consists in: a) preparing an aqueous mixture of precursors of the compound (I); b) bringing the aqueous mixture to a temperature Tp between 60 ^° C. and boiling under atmospheric pressure and maintain it at this temperature until the complete formation and precipitation of the compound (I), and c) separating the precipitate formed and dry it; Being heard that : the aqueous precursor solution is prepared by introducing into the water the precursor of M, the precursor of Si and the precursor of Li; the pH of the precursor solution is adjusted to a value of 10 to 14; steps a) and b) are carried out with stirring.

Complete formation of the precipitate is detected by X-ray diffraction analysis on the compound taken from the reaction mixture. In a particular embodiment, quantities of precursors are used such that the respective concentrations are as follows: 0.07 <[M ¹¹ ] <0.13, 0.07 <[Si] <0.13, and 0, 7 <[Li] <1.3, in mol. L ^"1 , more preferably 0.09 <[M ¹¹ ] <0.11, 0.09 <[Si] <0.11, and 1 <[Li] <1.3, in mol. L ^{" 1} .

The precursors of the silicate are chosen from compounds that are soluble in water. The aqueous mixture of silicate precursors is advantageously prepared at room temperature or at a higher temperature, depending on the solubility of the precursors.

The precursors are preferably used in the form of a powder. The grain size is not critical.

However, for small reactor volumes (below

I L), it is advantageous to use powders having a grain size of less than 2 mm.

In a preferred embodiment, the precursor mixture is prepared by successively adding in water the M precursor, then the Si precursor and then the Li precursor. The precursor solution must be at a pH between 10 and 14. during steps a) and b). If necessary, the pH can be adjusted to this value by adding a base to the reaction medium, for example NaOH or KOH.

As a precursor of iron, can be used a sulfate of Fe ^11, Fe ¹¹ chloride, oxalate Fe ¹¹ or Fe ¹¹ acetate, each in hydrated form or not.

As a precursor of manganese can be used a sulfate Mn ^11, Mn ¹¹ a nitrate, a chloride of Mn ^11, a Mn ¹¹ oxalate, or an Mn ¹¹ acetate, each in hydrated form or not.

As a precursor of nickel, one can use a sulfate of ¹¹ Ni, ¹¹ Ni chloride, oxalate Ni ^11, or acetate of Ni ^11, each in hydrated form or not.

As cobalt precursor can be used a sulfate of Co ^11, Co ¹¹ a nitrate, a chloride ¹¹ Co, ¹¹ Co oxalate, or acetate of Co ^11, each in hydrated form or not. The mixed silicate comprises the transition metal M in M ¹¹ form and in M ¹¹¹ form. The proportion of M ¹¹¹ , that is to say the value of x, depends inter alia on the oxidizable nature of M and the oxidoreductive properties of the species present in the reaction medium. Thus, a mixed silicate of Fe or Ni may contain a high content of Fe ¹¹¹ or Ni ¹¹¹ respectively. To limit the M ¹¹¹ content, it is advantageous to introduce a reducing agent into the aqueous precursor mixture. Ascorbic acid or hydrazine can be used as the reducing agent. It is also possible to use as a reducing agent a powder of the metal M. It is advantageous to use an iron or nickel powder having a small particle size.

The aqueous precursor solution may simultaneously contain the metal powder and a reducing agent selected from ascorbic acid and hydrazine.

When the precursor solution contains ascorbic acid or hydrazine, the ascorbic acid or hydrazine content is advantageously from 0.007 to 0.013 mol. L ^-1 , preferably from 0.009 to 0.011 mol L ^-1 . When the precursor solution contains metal powder M, it is preferable that the metal content in the precursor mixture does not exceed the precursor salt concentration of the metal. Preferably, the powder content of the metal M is of the same order as the concentration of metal precursor salt, that is to say a number of moles of insoluble metal per unit volume of aqueous medium such as 0.07 < [M ⁰ ] <0.13. 4 f. . . . . .

The precursor of Si is selected from those compounds of Si which are soluble in water and in which Si is at oxidation state IV. As examples of Si precursors, mention may be made of sodium metasilicate, SiCl ₄ , Si (OH) ₄ or SiCl ₂ F ₂ , optionally in their hydrated form.

The Li precursor is a soluble compound preferably chosen from LiOH, LiCl, LiNO ₃ , Li ₂ SO ₄ , C ₂ H ₃ LiO ₂ , Li ₂ C ₂ O ₄ and Li ₂ CO ₃ , each in hydrated form or not. It is particularly advantageous to use a precursor of Li whose counterion is OH or a counterion identical to that of the precursor salt of the metal M.

In a particular embodiment, the water constituting the solvent of the aqueous mixture is degassed before the introduction of the precursors. Degassing is particularly useful when M is an easily oxidizable metal such as Fe or Ni. Degassing can be carried out by passing an inert gas (eg N ₂ or Ar) in water with gentle agitation for at least 20 min, preferably at room temperature. During step b), the temperature Tp is preferably between 80 ^° C. and the boiling temperature.

During step c), the precipitate formed is separated by conventional means. Centrifugation is particularly appropriate. After separation, the precipitate is washed with a solvent to remove residual reagents and by-products that may be formed. The residues and by-products soluble in water are removed by washing with a polar solvent preferably protic, for example water, ethanol or methanol. A second wash with a polar solvent miscible with more volatile water allows the removal of water. As the polar solvent, for example, acetonitrile, acetone, or ethanol can be used.

It is desirable to carry out drying in an inert atmosphere, particularly when the M element of the mixed silicate is Fe or Ni. The inert atmosphere can be obtained by drying under nitrogen or argon. Vacuum drying can also be carried out. The silicates obtained by the process described above constitute another subject of the invention.

A silicate according to the invention corresponds to the formula Li ₂ M ^II ₍ i- _X ) M ^III _x SiO ₄ (OH) _x (I) in which O _{x x} l 1 and M is Fe, Co, Mn or Ni, and it is characterized in that it is in the form of substantially spherical and non-coalesced particles having a size of 400 to 600 nm; it has a structure in the Pna2i space group and possibly in the Pccn space group.

When M is Fe, the silicate corresponds to the formula Li ₂ Fe ^ _d - _X ) Fe ¹¹¹ _X SiO ₄ (OH) _x and has a ⁷ Li NMR spectrum which shows 3 components at 11, -8 and -83, respectively ppm. When x <0.6, the silicate structure of Fe and Li is in the Pna2χ space group. When x> 0.6, the structure of the Fe and Li silicate is in the Pna2i space group or in the Pccn space group.

When M is Mn, the silicate corresponds to the formula Li ₂ Mn ¹¹ SiO ₄ and its structure is in the space group Pna2i. When M is Ni, the silicate corresponds to the formula Li ₂ Ni ¹¹ _CL - _X ) Ni ¹¹¹ _X SiO ₄ (OH) _x and its structure is in the space group Pna2i.

When M is Co, the silicate corresponds to the formula Li ₂ Co ¹¹ SiO ₄ and its structure is in the space group Pna 2i. A mixed silicate according to the present invention is particularly useful as a positive electrode active material in an electrochemical device, in particular in a rechargeable lithium ion battery, or a lithium battery with a polymer electrolyte. In a particular embodiment, a positive electrode for an electrochemical device is constituted by a composite material that contains:

A silicate according to the present invention,

A binder conferring a mechanical strength, a compound conferring electronic conduction,

Optionally a compound imparting ionic conductivity.

The silicate content is preferably from 80 to 90% by weight. The binder content is preferably less than 10% by weight. The content of compound conferring electronic conduction is preferably between 5 and 15% by weight. The ionically conductive compound content is preferably less than 5% by weight.

The binder may be a non-solvating polymer, a solvating polymer or a mixture of solvating polymer and non-solvating polymer. It may further contain one or more aprotic polar liquid compounds.

The non-solvating polymer may be chosen from homopolymers and copolymers of vinylidene fluoride, copolymers of ethylene, of propylene and of a diene, homopolymers and copolymers of tetrafluoroethylene, homopolymers and copolymers of N-vinylpyrrolidone, homopolymers and copolymers of acrylonitrile and ¹ homopolymers and copolymers of methacrylonitrile ..

The solvating polymer may be chosen for example from polyethers of linear structure, comb or block, forming or not a network, based on poly (ethylene oxide); copolymers containing the ethylene oxide or propylene oxide or allylglycidyl ether unit; polyphosphazenes; crosslinked networks based on polyethylene glycol crosslinked with isocyanates; copolymers of oxyethylene and epichlorohydrin ^1; and networks obtained by polycondensation and bearing groups that allow the incorporation of crosslinkable groups. The aprotic polar compound may be chosen from linear or cyclic carbonates, linear or cyclic ethers, linear or cyclic esters, linear or cyclic sulphones, sulphonamides and nitriles.

The compound conferring electronic conduction may be chosen for example from carbon blacks, graphites, carbon fibers, carbon nanowires, or carbon nanotubes. The compound conferring ionic conduction is a lithium salt, advantageously chosen from LiClO ₄ , LiPF ₆ ,

LiAsF ₆ , LiBF ₄ , LiR ₁ -SO ₃ , LiCH ₃ SO ₃ , lithium bis-perfluoroalkyl sulfonimides, lithium bis- or tri-perfluorosulfonyl-methides.

A composite positive electrode according to the invention may be prepared by mixing the silicate, a binder in a suitable solvent, a material conferring electronic conduction, and optionally a lithium salt, by spreading the mixture obtained on a metal disc serving as a collector ( for example an aluminum disk), then evaporating the solvent under an inert atmosphere. The solvent is chosen according to the binder used. A positive electrode can be further elaborated by extruding a mixture of its constituents.

A positive electrode according to the invention can be used in a battery whose operation is ensured by the reversible circulation of lithium ions in the electrolyte between the positive electrode and the negative electrode. Another object of the present invention is therefore a battery in which the electrolyte comprises a lithium salt dissolved in a solvent, the positive electrode being an electrode according to the present invention.

In a battery according to the invention, the electrolyte comprises at least one lithium salt in solution in a solvent, said salt being able to be chosen, for example and without limitation, from among the lithium salts mentioned above as constituent of the composite material electrode.

The solvent of the electrolyte may consist of one or more aprotic polar compounds chosen from linear or cyclic alkyl carbonates, linear or cyclic ethers, linear or cyclic esters, linear or cyclic sulphones, sulphonamides and nitriles. . The solvent of the electrolyte may further be a solvating polymer, selected for example from among those mentioned above as constituent of the electrode composite material. The solvent of the electrolyte may further be a mixture of a polar aprotic liquid compound selected from aprotic polar compounds and a solvating polymer.

The solvent of the electrolyte may also be a mixture of an aprotic polar compound or a solvating polymer, and a non-solvating polar polymer comprising units containing at least one heteroatom selected from sulfur, oxygen, oxygen and the like. nitrogen and fluorine.

The negative electrode of the battery may consist of lithium metal or a lithium alloy which may be chosen from β-LiAl, γ-LiAl, Li-Pb alloys (for example Li ₇ Pb ₂ ), Li-Cd- Pb, Li-Sn, Li-Sn-Cd, Li-Sn in various matrices, in particular oxygen matrices or metal matrices (for example Cu, Ni, Fe, Fe-C), Li-Al-Mn. The battery is then called "lithium battery".

The negative electrode of the battery may further consist of a composite material comprising a binder and a material capable of reversibly inserting lithium ions with low redox potential (hereinafter referred to as insertion material). The battery is then called "lithium ion battery". The insertion material may be chosen from carbon materials, natural or synthetic. These carbonaceous materials may be for example a petroleum coke, a graphite, a graphite whisker, a carbon fiber, a micro-grain carbon meso (usually referred to as meso carbon micro bead), a pitch coke (usually referred to as pitch coke), a needle coke (usually referred to as needle coke). The insert material may further be selected from oxides such as for example Li _x MoO _2, Li _x WO _2, Li _x Fe ₂ O _3, Li ₄ Ti ₅ O _2, Li _x TiO ₂ or from sulfides such as for example LigMoβSβ and LiTiS ₂ or among oxysulfides. Can also be used compounds for storing siblement reversible lithium at low potential, such as amorphous vanadates (e.g. Li _x NiVo _4), nitrides (e.g., Li _2, ₆ -χCθo, ₄ N, Li _{2+ x} FeN ₂ , Li _{7 + x} MnN ₄ ), phosphides (for example Lig- _x VP ₄ ), arsenides (for example Li ₉ _ _x VAs ₄ ) and reversible decomposition oxides (for example CoO, CuO, Cu ₂ O). The binder is an electrochemically stable organic binder in the operating range of the negative electrode. By way of example, there may be mentioned homopolymers of polyvinylidene fluoride or an ethylene propylene diene copolymer. When a silicate according to the present application is used as electrode active material, it has electrochemical performances similar to or better than those presented by the silicates of the prior art obtained by the solid process which requires treatment. thermal at high temperatures and is particularly harmful in terms of energy consumption.

The present invention is described in more detail with the aid of the following examples which are given by way of illustration, but to which it is however not limited.

The various examples were carried out using the following products: ascorbic acid (As.Ac) marketed by Acros Organics under the name L (+) -Ascorbic acid (99%, 50-81-7), particle size ≈2 mm . iron sulphate FeSO ₄ , 7H ₂ O sold by Aldrich) under the name Iron (II) sulfate heptahydrate (99%, 7782-63-0), particle size "2 mm. sodium metasilicate Na ₂ SiO ₃ , 5 H ₂ O sold by Acros Organics under the name Sodium metasilicate penldhydrid (10213-79-3), particle size ≈2 mm. lithium hydroxide LiOH, H ₂ O, sold by Alla Aesar under the name Lithium hydroxide monohydrate (98%, 1310-66-3), particle size ≈2 mm. iron chloride FeCl ₂ , 4 H ₂ O sold by Aldrich under the name Iron (II) chloride tetrahydrate (99%, 13478-10-9), particle size ≈2 mm. manganese sulphate MnSO ₄ , H ₂ O sold by Acros under the name Manganese (II) sulfate monohydrate (99%, 10034-96-5), particle size ≈2 mm. nickel sulphate NiSO ₄ , 6 H ₂ O sold by Acros under the name Nickel (II) sulfate hexahydrate (98%, 10101-97-0), particle size ≈2 mm. Iron sold by Fluka under the name Iron (99%, 7439-89-6), particle size 35, 6 μm.

FeO sold by Alfa Aesar under the name Iron (II) oxide (99.5%, 345-25-1), particle size ≈2 mm. Li ₂ SiO ₃ marketed by Alfa Aesar under the name Lithium metasilicate (10102-24-6, 99.5%), particle size ≈2 mm.

Example 1 Preparation of Li ₂ Fe ¹¹ _CL - _X ) Fe ¹¹¹ _Jc SiO ₄ (OH) _x

The synthesis of the mixed Fe and Li silicate was carried out in a reactor equipped with a reflux system, a stirring system and a device allowing the passage of argon in the medium.

In 250 ml of distilled water which had been degassed for 1 hour with argon and kept at 20 ^° C., the following compounds were introduced successively under constant stirring and with argon bubbling, 10 min apart. : ascorbic acid (As.Ac) 0.2201 g

iron sulphate FeSO ₄ , 7 H ₂ O 5.5602 g sodium metasilicate Na ₂ SiO ₃ , 5H ₂ O 5.2505 g - lithium hydioxydide LiOH, H ₂ O 12.7175 g

Then, the reaction mixture was brought to 100 ^° C. in the course of 1 hour, this temperature was maintained for 24 hours, and then the reaction mixture was allowed to return to room temperature. At room temperature, the supernatant was removed by centrifugation, the compound was washed with acetone, and the resulting compound was dried. Example 2 Preparation of Li ₂ Fe II ₍ i- _x ₎ Fe ¹¹¹ _X SiO ₄ (OH) _:

The procedure of Example 1 was repeated using the following compounds: ascorbic acid (As.Ac) 0, 2201 g Iron metal 1, 3987 g - iron sulphate FeSO ₄ , 7 H ₂ O 5, 5602 g metasilicate of sodium Na ₂ SiOa, 5H ₂ O 5, 2505 g lithium hydroxide LiOH, H ₂ O 12, 7175 g

Metallic iron was introduced at the same time as ascorbic acid.

Example 3 Preparation of Li ₂ Fe ¹¹ ₍₁ - _X) Fe ¹¹¹ _X SiO ₄ (OH) _:

The procedure of Example 1 was repeated using the following compounds: ascorbic acid (As.Ac) 0, 2201 g

Iron metal 1, 3987 g - iron chloride FeCl ₂ , 4 H ₂ O 5, 5602 g - sodium metasilicate Na ₂ SiO ₃ , 5H ₂ O 5, 2505 g lithium hydroxide LiOH, H ₂ O 12, 7175 g

Metallic iron was introduced at the same time as ascorbic acid. Iron chloride was introduced in place of iron sulphate.

Example 4

Preparation of Li ₂ Fe (<ii-- ^χ _{_x))} ^J Fe "χSiO ₄ (OH) was repeated the procedure of Example 3, using the same compounds in the same amounts, but grinding the prior sodium metasilicate and lithium hydroxide to obtain a smaller particle size. Example 5 Preparation of Li ₂ Fe ¹¹ ₍₁ - _X) Fe ¹¹¹ _Jc SiO ₄ (OH) _x

The procedure of Example 4 was repeated, but the final drying of the Fe and Li silicate under vacuum was carried out.

Example 6 Characterization of the silicates of Examples 1 to 5

Microscopic analysis FIG. 1 represents two views by scanning electron microscope (SEM) of the compound obtained according to the method of example 1. It shows that the compound obtained is in the form of unsintered particles, monodisperse in size (from 400 to 600 nm), and of regular shape substantially spherical. The result is identical for the compounds obtained according to Examples 2 to 5.

Characterization by Mössbauer spectroscopy of ⁵⁷ Fe

It makes it possible to determine the level of Fe (III), that is to say the value of x for each of the compounds prepared according to Examples 1 to 5.

FIGS. 2a, 3a, 4a, 5a and 6a respectively represent the Mossbauer spectra of the iron of each of the silicates of Examples 1 to 5. The spectrum obtained is the sum of two characteristic doublets of Fe (II) and Fe (III) in coordination with 4. The spectra are simulated with two components to describe the Fe (III) doublet and a single component for Fe (II). In each of the figures: the curve (a) represents the experimental spectrum, the curve (b) represents the calculated spectrum, - the curve (c) represents the doublet Fe (II), the curve (d) represents the doublet Fe (III ) # 1, curve (e) represents the doublet Fe (III) # 2.

The quadrupole difference and the isomeric displacement are constant for all the examples, namely: Fe (II): 0.95 mm. s ^"1 and 2.6 mm s ^{' 1} ,

- Fe (III) #l: 0.3 mm ^s' l and 1.1 mm. s ^"1 ,

Fe (III) # 2: 0.3 mm. s ^"1 and 0.7 mm s ^{' 1} . Only the ratio of the intensities of the Fe (III) / Fe (II) doublets, from which the value of x can be determined, varies.

Structural Analysis The X-ray powder diffractogram was recorded on a Bruker D8 diffractometer with a cobalt anticathode.

FIGS. 2c, 3c, 4c, 5c and 6c respectively show the X-ray diffractogram of each of the silicates of Examples 1 to 5. In these figures, (a) corresponds to the positions of the Bragg peaks for indexing in the group. of space Pna2i, and (b) corresponds to the positions of the Bragg peaks for indexing in the space group Pccn. The values of the mesh parameters are given in the table below.

The structure is of type Pna2i. For a value x greater than 60%, the crystal is built by twinning which affects the diffractogram of X-rays. In this case, it will be necessary to adopt the space group Pccn. However, the description of the phase by the Pna2i space group is possible. This twinning is the consequence of the superposition of 3 lattice planes belonging to the Pna2i space group in the (001) direction with a rotation of 0 °, 120 ° and 240 °. An apparent space group is then formed which is visible in X-ray diffraction under the space group Pccn.

NMR of ⁷ Li

The ⁷ Li NMR signal was measured by a Nuclear Magnetic Resonance spectrometer with a 4.7 Tesla Bruker magnet equipped with an Appolo Tecmag console. The acquisition was performed on a 2.5 mm Bruker probe by the magic angle spinning method at 33 kHz with a 2.5 mm diameter Zirconium rotor. The measurement was performed by a 90 ° pulse sequence followed by the acquisition and a relaxation time of 250 μs. FIGS. 2b, 3b, 4b, 5b and 6b respectively show the ⁷ Li NMR spectra of each of the silicates of Examples 1 to 5. The spectra are analyzed as the superposition of three sub-spectra corresponding to three sites ⁷ Li at 11, -8 and -83 ppm (relative to the chemical shift of ⁷ Li in a solution of saturated lithium chloride). curve (a) represents the experimental NMR ⁷ Li spectrum, - curve (b) represents the calculated spectrum, curve (c) represents the lithium site # 1, curve (d) represents the lithium site # 2, the curve (e) represents the lithium site # 3.

These three sites describe three different environments for lithium.

The proportion of these sites, as a function of the amount of Fe ¹¹¹ (or the "x" rate), is part of a Bernoulli statistical distribution with 3 elements:

The lithium site ratios of the five examples have been reported in Figure 7, in which:

(# 1) represents the absolute area of the lithium 1 site, (# 2) represents the absolute area of the lithium 2 site, (# 3) represents the absolute area of the lithium site 3, - the projection on the axis of x, the position of the numbers 1, 2, 3, 4 and 5 inscribed at the top of the figure indicates the value of x respectively for each of the silicates prepared in Examples 1 to 5.

T, a correspondence of the occupations of the Lithium sites on the Bernoulli distribution gives the value x, measured by

Môssbaucr. LG Nuclear Magnetic Resonance Spectrum of

⁷ Li to assign a signature to each material: the presence of three lithium site identifies the compound. In addition, this method makes it possible to assign for any synthesis the value of x in the formula of the compound

Li ₂ Fe ²⁺ _U x, Fe ³⁺ _x Si0 ₄ (OH) _x.

The characteristics of the five silicates prepared according to Examples 1 to 5 are reported in the table below. They show that the OH content decreases from ¹ toward the 5 ^th example. The lowest level is obtained with iron chloride as precursor, in the presence of a reducing agent (ascorbic acid) and Fe ⁰ , using small particle size reagents obtained by grinding commercial products, and performing vacuum drying.

B0850FR

Electrochemical tests

The electrochemical test was carried out in an electrochemical cell of the Swagelok® type assembled in a dry box under a controlled atmosphere. This cell comprises a positive electrode whose active material is a Fe and Li silicate obtained by the method of Example 5, a negative electrode consisting of a 1 cm ² metal lithium foil, and a fiber sheet. of borosilicate glass (type GF / D, supplied by the company Whatman) impregnated with a solution of LiPF _δ IM in a mixture of ethyl carbonate / dimethyl carbonate ECrDMC (1: 1 by mass), said impregnated sheet being placed between the two electrodes.

The material constituting the positive electrode is prepared by mixing the silicate with carbon "Ketjen Black" (compressed to 50%, 99.9% pure, marketed by Alfa) in amounts such that the carbon represents 16.7 % by weight of the electrode. The mixing is carried out by an intimate grinding of 5 minutes by mechanical grinding with an SPEX 8000 type grinder. A galvanostatic measurement was carried out by subjecting the electrochemical cell to charge discharge cycles on a "Macpile" type automatic cycler (Biology Co , SA, Claix, France) at 75 ° C and a C / 37 regime.

Figure 8 shows the cycling curves and the lithium level y. It shows a charge at 3 V during the first cycle and 2.8 V for subsequent cycles. 90% of lithium is extracted with a reversibility on 0.8 lithium. The polarization of the battery due to the material is low. The capacity of the electrode is related to the value of x. Indeed, the material will be electrochemically active only on (1-x) lithium. On the other hand, the presence of hydroxyl groups is likely to generate water molecules, harmful to the operation of the battery.

Example 7 (comparative)

A solid Fe and Li silicate was prepared according to the prior art, and the silicate obtained was characterized. Silicate preparation

According to Example 2 of EP-I 134 826, a mixture of 71.8 g of FeO and 89.9 g of Li ₂ SiO ₃ was subjected to high energy milling in a ball mill for 4 hours, then the resulting mixture was transferred to a quartz bulb in which iron shot was added.

The ampoule kept at 200 ^° C. was placed under secondary vacuum and sealed. Then, it was brought to 800 ⁰ C and maintained at this temperature for 4 hours. After cooling, the bulb was opened under argon and the shot was removed magnetically.

Microscopic analysis

Figure 9 shows two SEM micrographs of the compound obtained. It shows that the compound has a large disparity in size and morphology. It consists of sintered and agglomerated particles of heterogeneous form, whose size is between 5 microns and 200 microns.

Nuclear analysis

The ⁷ Li NMR signal was measured by a Nuclear Magnetic Resonance spectrometer with a Bruker magnet of 4.7

Tesla equipped with Appolo Tecmag console. The acquisition was carried out on a 2.5 mm Bruker probe by the method of the magic angle rotation at 33 kHz with a rotor in

Zirconium 2.5 mm in diameter. The measurement was performed by a 90 ° pulse sequence followed by the acquisition and a relaxation time of 250 μs.

The NMR spectrum Li ⁷ is shown in FIG. 10, in which: the inset represents an enlarged view of the central peak - the curve (a) represents the experimental NMR ⁷ Li spectrum, the curve (b) the calculated spectrum, the curve (c) represents the lithium site # 1, the curve (d) represents the lithium site # 2.

The NMR spectrum shows the existence of two lithium sites at -6 and -64 ppm. The integration of these sites gives a distribution of 1 and 2 lithium respectively.

Structural Analysis

The X-ray diffractogram shown in Figure 11 was recorded on a Bruker D8 powder diffractometer with a cobalt anticathode. It shows that the compound has a very good crystallinity with a great abundance of diffraction peaks. According to the literature, this diffractogram corresponds to a structure of the Pmn2i space group type (a = 6.26 Å, b = 5.32 Å, c = 5.01 Å).

Electrochemical test II was carried out according to the procedure described in Example 6, replacing the silicate according to the invention by the same amount of silicate prepared according to the prior art as described in the present example.

Figure 12 shows the cycling curves and the y-rate of lithium at 75 ° C and a C / 37 regime. It shows a charge at 3 V during the first cycle and 2.8 V for subsequent cycles. 45% of the lithium is extracted with a reversibility on 0.35 lithium.

Comparison with the characteristics of Fe and Li silicates according to the invention

The comparison of the characteristics determined for an Fe and Li silicate according to the invention and a silicate according to the prior art shows that:

The grain structure of the silicate of the invention is different from that of the silicate of the prior art;

The silicate grains of the invention are thinner and more regular than the silicate grains of the prior art;

The performances in terms of extraction / electrochemical insertion of lithium ions of the silicate of the invention are clearly better than those of the silicate of the prior art

The NMR ⁷ Li spectrum shows that the sites and the chemical shift of the lithium in a silicate of the prior art are different from the sites and the displacement of lithium in a silicate according to the present invention, which shows that the respective environment lithium is different. Example 8 Li ₂ Mn ¹¹ SiO ₄

Preparation

The synthesis of the mixed Mn and Li silicate was carried out in a reactor equipped with a reflux system, and a stirring system. In 250 ml of distilled water maintained at 20 ^° C., the following compounds are introduced with constant stirring, 10 min apart, in succession: manganese sulphate MnSO ₄ , H ₂ O ₄ , 1832 g sodium metasilicate Na ₂ SiO ₃ , 5H ₂ O 5.2505 g - lithium hydroxide LiOH, H ₂ O 12.7175 g

Then, the reaction mixture was brought to 100 ^° C. over a period of 1 h, this temperature was maintained for 24 h, and then the reaction mixture was allowed to return to room temperature. At room temperature, the supernatant was removed by centrifugation, the compound was washed with acetone, and the resulting compound was dried.

Structural Analysis

The X-ray diffractogram shown in Figure 13 was recorded on a Philips powder diffractometer with a copper anticathode. It shows that the compound has a very good crystallinity. The diffractogram can be indexed with the Pna2i space group with the mesh parameters a - 10.801 (8) Å, b = 6.318 (7) Å and c = 4.959 (4) Å. Electrochemical test

It was carried out according to the procedure described in Example 6, replacing the FeLi silicate according to the invention with the same amount of MnLi silicate according to the present example. Figure 14 shows the cycling curves and the lithium level y. The average charge and discharge potential is 3.5 V. 1.2 lithium is extracted with a reversibility on 0.8 lithium which decreases rapidly with the number of cycles. The extraction of more than 1 lithium in this compound is possible because the manganese can be at the oxidation state + IV. Example 9 Li ₂ Ni ¹¹ SiO ₄

The synthesis of the mixed silicate of Ni and Li was carried out in a reactor equipped with a reflux system, a stirring system and a device allowing the passage of argon in the medium. In 250 ml of distilled water maintained at 20 ^° C., the following compounds are introduced, with constant stirring, 10 min apart:

nickel sulphate NiSO ₄ , 6H ₂ O 6,5713 g

sodium metasilicate Na ₂ SiO ₃ , 5H ₂ O 5.2505 g - lithium hydroxide LiOH, H ₂ O 12.7175

Then, the reaction mixture was brought to 100 ^° C. over a period of 1 h, this temperature was maintained for 24 h, and then the reaction mixture was allowed to return to room temperature. At room temperature, the supernatant was removed by centrifugation, the compound was washed with acetone, and the resulting compound was dried under argon.

Example 10 Li ₂ CoSiO ₄

The synthesis of the mixed Co and Li silicate was carried out in a reactor equipped with a reflux system, and a stirring system.

In 250 ml of distilled water maintained at 20 ^° C., the following compounds were introduced with constant stirring, at 10 min intervals:

cobalt sulfate CoSO ₄ , 7H ₂ O 7.0982 g - sodium metasilicate Na ₂ SiO ₃ , 5H ₂ O 5.2505 g

lithium hydroxide LiOH, H ₂ O 12.7175

Then, the reaction mixture was heated to 100 ° C over 1 hour, maintained at this temperature for 24 hours, and the reaction mixture was allowed to warm to room temperature.

At room temperature, the supernatant was removed by centrifugation, the compound was washed with acetone, and the resulting compound was dried.

Claims

claims

1. A process for preparing a compound having the formula Li ₂ M ¹¹ _(- _X) M _X ¹¹¹ SiO ₄ (OH) _x (I) wherein O≤x≤l and M is Fe, Co, Mn or Ni, characterized in that it consists in: a) preparing an aqueous mixture of precursors of the compound

(D; b) bringing the aqueous mixture to a temperature Tp between 60 ^° C. and boiling under atmospheric pressure and maintain it at this temperature until the complete formation and precipitation of the compound (I), and c) separate the precipitate formed and dry it; it being understood that: the aqueous precursor solution is prepared by introducing into the water the precursor of M, the precursor of Si and the precursor of Li; the pH of the precursor solution is adjusted to a value of 10 to 14; steps a) and b) are carried out with stirring.

2. Method according to claim 1, characterized in that the aqueous mixture of silicate precursors is prepared at room temperature.

3. Process according to claim 1, characterized in that the precursor mixture is prepared by successively adding to the water the precursor of M, then the precursor of Si and then the precursor of Li.

4. Method according to claim 1, characterized in that the pH is adjusted to a value of 10 to 14 by adding a base to the reaction medium.

5. Method according to claim 1, characterized in that the respective concentrations of precursors in the aqueous mixture, expressed in mol. L ^"1 , are as follows: 0.07 <[M ¹¹ ] <0.13, 0.07 <[Si] <0.13, 0.7 <[Li] <1.30.

6. Process according to claim 5, characterized in that the respective concentrations of precursors in the aqueous mixture, expressed in mol. L ^"1 , are as follows: 0.09 <[M ¹¹ ] <0.11, 0.09 <[Si] <0.11, and 1 <[Li] <1.3.

7. Process according to claim 1, characterized in that the iron precursor and the nickel precursor are chosen from sulphate, chloride, oxalate, and acetate, respectively of Fe ¹¹ or Ni ¹¹ , each of which is hydrated form or not.

8. Process according to claim 1, characterized in that the manganese precursor and the cobalt precursor are chosen from sulphate, nitrate, chloride, oxalate and acetate, respectively of Mn ¹¹ or of Co ¹¹ , each in hydrated form or not.

9. The method of claim 7, characterized in that the aqueous mixture of precursors contains a reducing agent.

10. The method of claim 9, characterized in that the reducing agent comprises Fe ⁰ when M is Fe or Ni ⁰ when M is Ni.

11. Method according to one of claims 9 or 10, characterized in that the reducing agent comprises ascorbic acid or hydrazine.

12. The method of claim 11, characterized in that the content of ascorbic acid or hydrazine is 0.007 to 0.013 mol. L ^"1 .

13. The method of claim 10, characterized in SEAC qquuee iinn tthhee tteenneeuurr mmééttaall ,, iinn mmooll .. LL ^"λ is such that 0.07 <[Fe ^0] <0.13 or 0.07 <[Ni ^0] <0.13.

14. Process according to claim 1, characterized in that the precursor of Si is chosen from compounds of Si which are soluble in water and in which Si is at oxidation state IV.

15. The method of claim 14, characterized in that the precursor of Si is selected from sodium metasilicate, SiCl ₄ , Si (OH) ₄ and SiCl ₂ F ₂ , optionally in their hydrated form.

16. The method of claim 1, characterized in that the Li precursor is a _^ soluble compound preferably selected from LiOH, LiCl, LiNO _3, Li ₂ SO _4, C ₂ H ₃ Li _{2 O,} Li ₂ C ₂ O ₄ and Li ₂ CO ₃ , each in hydrated form or not.

17. Process according to claim 7, characterized in that the water constituting the solvent of the aqueous mixture is degassed before the precursors are introduced.

18. The method of claim 17, characterized in that the degassing is carried out by passing an inert gas in water with moderate stirring for at least 20 min.

19. The method of claim 1, characterized in that step b) is conducted at a temperature Tp between 80 ⁰ C and the boiling temperature of ^1.

20. Process according to claim 1, characterized in that, during step c), the precipitate formed is separated by centrifugation.

21. Process according to claim 1, characterized in that, after separation, the precipitate is washed with a protic polar solvent in order to eliminate the residual reactants and the by-products which may be formed, and then with the aid of a polar solvent miscible with water more volatile than water to remove water.

22. The method of claim 1, characterized in that the drying is carried out under an inert atmosphere, or under vacuum.

23. Silicate of formula Li ₂ M ¹¹ , ₁ - _X ) M ¹¹¹ _X SiO ₄ (OH) _x (I) in which O≤x≤l and M is Fe, Co, Mn or Ni, characterized in that: it is in the form of substantially spherical and non-coalesced particles having a size of 400 to 600 nm; it has a structure in the Pna2χ space group and possibly in the Pccn space group.

The silicate according to claim 23, wherein it corresponds to the formula Li ₂ Fe ¹¹ _-X ) Fe ¹¹¹ _X SiO ₄ (OH) _x and has a ⁷ Li NMR spectrum which shows 3 components respectively at 11 , -8 and -83 ppm.

Silicate according to claim 24, characterized in that x <0.6 and its structure is in the Pna2i space group.

Silicate according to Claim 24, characterized in that x> 0.6 and its structure is in the Pna2χ space group or in the Pccn space group.

Silicate according to Claim 23, characterized in that it corresponds to the formula Li ₂ M ¹¹ SiO ₄ in which M is Mn or Co and its structure is in the Pna2χ space group.

28. Silicate according to claim 23, characterized in that it corresponds to the formula Li ₂ Ni ¹¹ ( ₁ - _X ) Ni ¹¹¹ _X SiO ₄ (OH) _x and its structure is in the space group Pna2i.

29. A positive electrode active material for an electrochemical device, characterized in that it consists of a silicate according to any one of claims 23 to 28.

30. Positive electrode for an electrochemical device, characterized in that it comprises a silicate according to any one of claims 23 to 28 as an active ingredient.

31. Electrode according to claim 30, characterized in that it consists of a composite material which contains, in addition to said silicate: a binder conferring a mechanical strength, A compound conferring electronic conduction,

Optionally a compound imparting ionic conductivity.

32. Battery comprising a positive electrode and a negative electrode separated by an electrolyte comprising a lithium salt dissolved in a solvent, characterized in that the positive electrode is an electrode according to one of claims 30 or 31.

33. Battery according to claim 32, characterized in that the negative electrode is constituted by lithium metal or by a lithium alloy.

34. The battery as claimed in claim 32, characterized in that the negative electrode consists of a composite material comprising a binder and a material capable of reversibly inserting lithium ions with a low redox potential.