FR3108602A1 - Treatment of carbon particles and their use for the preparation of electrodes resistant to the oxidation of the electrolyte - Google Patents

Abstract

Treatment of carbon particles and their use for the preparation of electrodes resistant to oxidation of the electrolyte The present application relates to carbon particles coated with a protective material against oxidation of the electrolyte, and their use as a material positive electrode. The application also relates to electrochemical cells and batteries comprising such electrodes. Figure for abstract: None

Description

Treatment of carbon particles and their use for the preparation of electrodes resistant to oxidation of the electrolyte

The present invention relates to the field of energy storage, and more specifically to accumulators, in particular of the lithium type.

Rechargeable lithium-ion batteries indeed offer excellent energy and volume densities and today occupy a prominent place in the market for portable electronics, electric and hybrid vehicles and even stationary energy storage systems.

Their operation is based on the reversible exchange of the lithium ion between a positive electrode and a negative electrode, separated by an electrolyte. Solid electrolytes also offer a significant improvement in terms of safety insofar as they present a much lower risk of flammability than liquid electrolytes. Nevertheless, the electrolytes of these accumulators, such as sulphide type electrolytes, are often unstable.

Upon charging, the electrolytes break down to form a passivation layer on the surface called the "Solid-Electrolyte-Surface Interphase" (SEI). The formation of this layer has the effect of reducing the battery energy storage capacity.

In particular, the electrolytes are liable to oxidize in the presence of the conductive elements present in the positive electrodes in particular. This is the case with the carbon powder which reacts with the electrolyte (Zhang et al, ACS applied materials & interfaces , 2017, vol. 9, no 41, p. 35888-35896). This reaction is undesirable in that it consumes part of the battery capacity.

It is therefore desirable to provide an electrode material that avoids this secondary reaction, while maintaining the required electrical conductivity of the carbon particles.

The invention therefore aims in particular to provide coated carbon particles, protected from direct contact with the electrolyte, while maintaining electronic conductivity.

According to a first object, the present application relates to carbon particles, characterized in that they are coated by a layer comprising a material C1, such that C1 is chosen from the following compounds:

M(OH) _x , MO _x , A _a M(OH) _x , A _a MO _x , MY _z , A _a MY _z , M(NH ₂ ) _x

and their mixtures

Such as

A is an element chosen from alkaline earths (Li, Na, K, Mg, Ca);

M is chosen from C, B, P, S, Al, Si, transition metals (Ni, Cr, Ti, Zr, Nb, etc.) or rare earths;

Y is halogen (F, I, Cl, Br);

x is between 0.5 and 3.5;

z is between 1 and 5;

a is between 0 and 5.

The particles can be coated over all or part of their peripheral surface. According to one embodiment, they are coated over their entire peripheral surface.

According to one embodiment, the coating layer consists exclusively of material C1.

According to one embodiment, the coating layer has a thickness of less than 20 nm, in particular less than 10 nm, more preferably from 2 to 5 nm.

Typically, the electronic resistance of the layer of material C1, resulting from the resistivity of the material C1 and its thickness, is less than 0.1 Ohm.m² and/or the carbon coated with the layer of material C1 has an electronic conductivity greater than 1 mS/cm.

The carbon particles are chosen in particular from carbon nanoparticles.

A nanoparticle is understood to mean a nano-object of which at least one of the three dimensions is at the nanometric scale, or particles whose nominal diameter (or equivalent) is less than approximately 100 nm (Institut national de recherche et de sécurité ( INRS), Nanomaterials , Paris, June 2008).

The particles are not limited as to their morphology. Mention may thus be made of spherical particles, aggregates of particles or fibers or nanofibers. In particular, the particles can be chosen from carbon powder particles, such as carbon black particles, carbon fibers, or carbon nanoparticles, such as carbon nanotubes (CNT).

According to one embodiment, compound C1 is capable of reacting with an electrolyte compound, such as sulphides, so as to form a compound C2, such that the carbon coated with compound C2 has a conductivity of less than 0.1 mS /cm.

By way of material C1 suitable for the invention, mention may in particular be made of polymers of the PEO (polyethylene oxide) type, polymers of the PPC (polyprolylene carbonate) type, Si(OH) _{4 ,} Al(OH) ₃ , SiO ₂ , LiNbO ₃ , LiZrO ₂ , LiSiO ₃ H, B(OH) ₃ , H ₃ PO ₄ , NiCl ₂ , Li ₃ YCl ₃ , carbon functionalized with ketone functions C=O, alcohol C-OH carboxyl CO ₂ H, or amine C-NH ₂ .

According to another object, the present invention also relates to the method for preparing coated carbon particles according to the invention, said method comprising the step of applying the layer comprising the material C1 to said particles.

According to one embodiment, the application can be carried out by any method allowing the deposition of a thin layer, such as:

• chemical deposition: sol-gel, centrifugal coating or spin-coating, vapor phase deposition, atomic layer deposition or atomic layer deposition ALD, molecular layer deposition or molecular layer deposition MLD, or by controlled oxidation; And
• physical deposition: vacuum evaporation, cathode sputtering, pulsed laser deposition, electrohydrodynamic deposition.

Typically, the layer can be deposited by ALD or controlled oxidation of the surface of the carbon particles in a humid medium.

Controlled oxidation can be carried out in solution containing an oxidant such as peroxides (H ₂ O ₂ , HClO ₄ , KMnO ₄ ) by adapting the temperature (at 60° C. for example). This oxidation can also be carried out in an alkaline solution (based on NaOH, LiOH or KOH) under an oxygen atmosphere at 80° C. for example.

ALD or MLD consists in exposing the surface of carbon particles successively to different chemical precursors in order to obtain ultra-thin layers.

Typically, the polymers are preferentially deposited by MLD.

The compounds Si(OH) ₄ , Al(OH) ₃ , SiO ₂ , LiNbO ₃ , LiZrO ₂ , LiSiO ₃ H, B(OH) ₃ , H ₃ PO ₄ , NiCl ₂ , Li ₃ YCl ₃ are preferentially produced by ALD

The functionalizations of carbon are generally carried out by controlled oxidation in KOH at 80° C. under oxygen for 24 hours; the powder can be used after washing and drying. An additional chemical treatment based on organic compounds containing ketone functions, or associated carboxylic acids or bases, makes it possible to functionalize the surface of the carbon and to increase the number of ketone, alcohol, carboxyl or amine groups per unit of carbon surface.

According to another object, the present invention also relates to a positive electrode comprising the coated carbon particles according to the invention.

According to one embodiment, the oxidation current stabilized at 4.5 V is less than 10 μA per gram of carbon. This oxidation current is measured on an electrode containing the positive electrode active material, the solid electrolyte, the treated carbon and the binder. The positive electrode material is preferably coated with a lithiated oxide of the LiNbO ₃ type for example which makes it possible to avoid the reaction of the positive material with the electrolyte. An accumulator is assembled from the positive electrode, a negative electrode made of lithium metal and a layer of solid electrolyte separating the 2 electrodes. The accumulator is first charged at constant current, for example at a rate of C/50 up to a voltage of 4.5V. Charging is continued while maintaining the voltage at 4.5V. A steady decrease in current is observed. This decrease is composed of a decreasing exponential term and a constant corresponding to the stabilized value characteristic of the corrosion of the electrolyte. In order to measure this stabilized value, the accumulator can be maintained at 4.5V for several days; the corrosion current corresponds to the stabilized oxidation current if its value has not changed significantly after 24 or 48 hours for example.

In the context of the present invention, the positive electrode can be of any known type. The cathode generally consists of a conductive support used as a current collector which is coated with a layer containing the cathodic active material and generally also a binder and carbon as electronic conductive material. This carbonaceous additive is distributed in the electrode so as to form an electronic percolating network between all the particles of active material and the current collector.

The cathodic active material is not particularly limited. It can be chosen from the following groups or mixtures thereof:

- a compound (a) of formula Li _x M _1-yzw M' _y M'' _z M''' _w O ₂ (LMO2) where M, M', M'' and M''' are chosen from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, W and Mo provided that at least M or M' or M'' or M''' is chosen from Mn, Co, Ni, or Fe; M, M', M'' and M''' being different from each other; and 0.8≤x≤1.4; 0≤y≤0.5; 0≤z≤0.5; 0≤w≤0.2 and x+y+z+w<2.1;

- a compound ( b) of formula Li _x Mn _2-yz M' _y M'' _z O ₄ (LMO), where M' and M" are chosen from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo; M' and M" being different from each other, and 1≤x≤1.4; 0≤y≤0.6; 0≤z≤0.2;

- a compound ( c) of formula Li _x Fe _1-y M _y PO ₄ (LFMP) where M is chosen from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo; and 0.8≤x≤1.2; 0≤y≤0.6;

- a compound (d) of formula Li _x Mn _1-yz M' _y M'' _z PO ₄ (LMP), where M' and M'' are different from each other and are chosen from the group consisting of in B, Mg, Al, Si, Ca, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo, with 0.8≤x≤1.2; 0≤y≤0.6; 0≤z≤0.2;

- a compound (e ) of formula xLi ₂ MnO ₃ ; (1-x)LiMO ₂ where M is at least one element chosen from Ni, Co and Mn and x≤1.

- a compound (f) of formula Li _1+x MO _2-y F _y of cubic structure where M represents at least one element chosen from the group consisting of Na, K, Mg, Ca, B, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Y, Zr, Nb, Mo, Ru, Ag, Sn, Sb, Ta, W, Bi, La, Pr, Eu, Nd and Sm and where 0 ≤ x ≤ 0.5 and 0 ≤ y ≤ 1.

The current collector is preferably a two-dimensional conductive support such as a solid or perforated strip, based on metal, for example nickel, steel, stainless steel or aluminum, preferably aluminum.

According to another object, the present invention also relates to an electrochemical element comprising a positive electrode according to the invention.

“Electrochemical element” means an elementary electrochemical cell made up of the positive electrode/electrolyte/negative electrode assembly, allowing the electrical energy supplied by a chemical reaction to be stored and returned in the form of current.

The electrochemical element according to the invention is particularly suitable for lithium accumulators, such as Li-ion, primary Li (non-rechargeable) and Li-S accumulators. These materials can also be used in Na-ion, K-ion, Mg-ion or Ca-ion type batteries.

According to one embodiment, the electrolyte can be chosen from polymer-type or oxide-type electrolytes, generally exhibiting instability with the positive electrode during charging. Mention may thus be made of polymer-based electrolytes containing ether functions such as PEO, PPO or derived polymers which may contain traces or significant amounts of organic solvent. Mention may also be made of electrolytes based on polymer containing carbonate functions such as PEC or PPC, or based on acrylate which may also contain traces or significant amounts of organic solvent. These different polymers can contain salts such as LiTFSI, LiFSI, LiPF ₆ , LiClO ₄ .. Moreover, the electrolytes can also be chosen from the class of oxides such as LLZO garnet (Li ₇ La ₃ Zr ₂ O ₁₂ ) and its derivatives, NASICON, LLTO (Li _0.33 La _0.557 TiO ₃ ) and related phases, LiPON type compounds (Li _3.2 PO _3.8 N _0.2 ), Li ₃ OCl type anti-perovskite …

Preferably, the electrolyte is chosen from sulphide electrolytes such as Li ₃ PS ₄ , (Li ₃ PS ₄ ) _0.8 (LiI) _0.2 , as well as all of the phases [(Li ₂ S) _y (P ₂ S ₅ ) _1-y ] _(1-z) (LiX) _z (with X a halogen element; 0<y<1;0<z<1), argyrodites such as Li ₆ PS ₅ X, with X=Cl, Br , I, or Li ₇ P ₃ S ₁₁ , sulphide electrolytes having the crystallographic structure equivalent to the compound Li ₁₀ GeP ₂ S ₁₂ .

The electrolyte can also be a combination of the various compounds mentioned above.

According to another object, the present invention also relates to an electrochemical module comprising the stacking of at least two elements according to the invention, each element being electrically connected with one or more other element(s).

The term “module” therefore designates here the assembly of several electrochemical elements, said assemblies possibly being in series and/or parallel.

According to another of these objects, the invention also relates to a battery or "accumulator" comprising one or more modules according to the invention.

By “battery” is meant the assembly of several modules according to the invention. The invention preferably relates to accumulators whose capacity is greater than 100 mAh, typically 1 to 100Ah

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Figure 1 represents a schematic view of the interactions between the carbon particles and the electrolyte particles.

FIG. 2 represents the coulombic efficiency of an electrochemical element, according to the invention with treated carbon particles (curve represented by circles), compared with untreated particles (curve represented by crosses).

FIG. 3 represents the evolution of the current as a function of time during the charge at constant voltage expressed in μA per gram of carbon treated in the positive electrode.

FIG. 4 represents the evolution of the current as a function of time during the charge at constant voltage expressed in μA per gram of untreated carbon in the positive electrode.

According to Figure 1, the carbon particles (1) are represented schematically in spherical form, but it is understood that the particles are not limited to this morphology.

The particles (1) are coated on their outer peripheral surface with a layer (3) comprising the material C1. During charging, the portion of layer of material C1 in contact with the electrolyte (2) reacts so as to form a modified layer (4) comprising the material C2 produced by reaction between C1 and the electrolyte.

As shown in Figure 1, the portion of the protective layer (3) which is not in contact with the electrolyte (2) does not undergo any reaction: the carbon particles (1) in contact with each other others make it possible to maintain a conductive path between them, via their contact zone comprising the conductive material C1.

The following examples are given by way of non-limiting illustration of one embodiment of the present invention.

Examples

Example 1

The following example is intended to show the expected effect of a surface treatment of carbon powder. In this example, the treatment of the carbon aims to create carboxylic, ketone and alcohol functions containing COH functions on the surface of the carbon. The compound C1 on the surface can be written in the form COxHy.

Carbon powder, of the carbon black type, for example reference carbon C65 from Imerys or even carbon nanofibers, is used as electronic conductive additive. 1 gram of powder is placed in an aqueous solution containing potash of 1M concentration and containing 0.01g of CMC (carboxymethylcellulose) whose role is to facilitate the wetting of the carbon powder by the solution. After homogenization of the suspension, the solution is saturated with oxygen by bubbling oxygen in the solution for one hour. The solution is placed in a sealed cell to prevent the reaction of the CO ₂ in the air with the potassium hydroxide forming carbonates. Heat treatment of the cell is carried out in an oven at 80° C. for 8 hours. The suspension is then filtered and then washed with distilled water and dried at 200°C.

All of the manipulations of the powders described were carried out in a glove box under an Argon atmosphere. A positive electrode is prepared in a glove box by mixing in a mortar 100mg of NCA type material coated with a compound of the LiNbO ₃ type, 100mg of solid electrolyte of composition Li ₆ PS ₅ Cl prepared by mechanosynthesis, and 20 mg of treated carbon . The mixture is compressed in a 4cm² pelletizer under a pressure of 2t/cm²; a layer of solid electrolyte based on Li ₃ PS ₄ is then produced by introducing a quantity of electrolyte of 400 mg, the whole is again compressed to 2t/cm², then a film of lithium metal is deposited on the layer of electrolyte solid and compressed to 0.5t/cm². The assembly is then placed between 2 stainless steel manifolds and compressed to 0.5t/cm². The accumulator thus formed is sealed after soldering connections on the collectors. A charge is then carried out by applying a current corresponding to a rate of C/50 until reaching 4.5V. Then a constant voltage of 4.5V is applied to the battery for several days and the current delivered is recorded. As shown in Figure 3, the stabilized current from about 2 days corresponding to the oxidation of the electrolyte is about 5 μA/g _C .

Figure 4 illustrates the fact that the stabilized current from about 24 hours is about 50µA/g _C . This value is much higher than that corresponding to the electrode containing treated carbon.

Example 2

Complete graphite/Li ₆ PS ₅ Cl/NCA cells are assembled according to the following protocol, all of the manipulations of the powders being carried out in a glove box under an argon atmosphere. The solid electrolyte used in this example is an argyrodite of the Li ₆ PS ₅ Cl type (~1 mS/cm at room temperature). The composite mixture of NCA, sulphide electrolyte (SE) and carbonaceous additive (carbon black or carbon black, CB, as described in example 1) used as positive electrode was prepared manually in a mortar respecting the mass quantities NCA: SE: CB 70:30:3. The mixture used as negative electrode is prepared according to the same protocol from graphite and SE in mass proportions graphite:SE 2:1. The complete graphite /SE/NCA cells were prepared in specific cells, similar to pellet molds 7 mm in diameter, the pistons of which are made of stainless steel and the body of insulating material. First, 40 mg of electrolyte are introduced and then compressed under 250 MPa to form the separating electrolytic layer (SEL). The cell is then opened on one side to introduce 15 mg of the positive electrode mixture as well as an aluminum disk (serving as a current collector). The positive electrode is formed by compression at 250 MPa. 12 mg of the negative electrode mixture are then introduced onto the other face of the SEL. The entire cell is then compressed under 500 MPa for several minutes. A pressure of 250 MPa is maintained on the cell thereafter thanks to screws.

Two cells are thus assembled with the carbons untreated and treated as described in example 1.

After resting for 12 h, the cells thus assembled are then characterized by galvanostatic cycling between 2.8 and 4.1 V at ambient temperature. After performing 5 reference cycles at a reduced current of C/20, the cells are cycled for 20 cycles at C/10. Figure 2 shows the coulombic efficiency values (ratio of recovered capacity in discharge to charged capacity) obtained for these cells.

With untreated carbon, a significant excess capacity is obtained from the first cycles, and does not stabilize thereafter due to irreversible non-passivating reactions accentuated by the presence of carbon. Consequently, the coulombic efficiency is low and deteriorates during cycling, with gaps between successive cycles showing strong instabilities. With treated carbon, the coulombic efficiency value is much higher and tends to stabilize more.

Claims (12)

Carbon particles, characterized in that said particles are coated with a layer comprising a material C1, such that C1 is chosen from the following compounds:
M(OH) _x , MO _x , A _a M(OH) _x , A _a MO _x , MY _z , A _a MY _z , M(NH ₂ ) _x
and their mixtures
Such as
A is an element chosen from alkaline earths (Li, Na, K, Mg, Ca);
M is chosen from C, B, Al, P, Si, transition metals (Ni, Cr, Ti, Zr, Nb, etc.) or rare earths;
Y is halogen (F, I, Cl, Br);
x is between 0.5 and 3.5;
z is between 1 and 5;
a is between 0 and 5. Carbon particles according to claim 1 such that the coating layer has a thickness of less than 20 nm. Carbon particles according to claim 1 or 2 such that said carbon particles are in the form of CNTs or nanofibers. Carbon particles according to claim 1 or 2 such that the carbon particles have an average size of less than 100 nm. Carbon particles according to any one of the preceding claims, such that the compound C1 reacts with the sulphides to form a compound C2, such that the carbon coated with the layer of material C2 has a conductivity of less than 0.1 mS/cm. . A method of preparing coated carbon particles according to any preceding claim comprising applying C1 to said particles. Process according to Claim 6, such that the application is carried out by atomic layer deposition (ALD) or molecular layer deposition (MLD), or by controlled oxidation. Positive electrode comprising the particles according to any one of claims 1 to 5. Positive electrode according to the preceding claim, characterized in that the oxidation current stabilized at 4.5 V is less than 10 µA per gram of carbon. Electrochemical element comprising a positive electrode according to any one of claims 8 or 9. Electrochemical module comprising the stack of at least two elements according to claim 10, each element being electrically connected with one or more other element(s). Battery comprising one or more modules according to claim 11.