Negative electrodes based on silicon and fluorinated additive

Abstract

The present application relates to the use of fluorinated additive for preparing a silicon-based negative electrode, as well as to the corresponding electrodes and to Li-ion electrochemical elements comprising same.

Description

[0001] DESCRIPTION

[0002] TITLE: Negative electrodes based on silicon and fluorinated additive

[0003] The present invention relates to the field of energy storage, and more specifically to lithium accumulators.

[0004] The operation of lithium-ion batteries is based on the reversible exchange of lithium ions between a positive electrode and a negative electrode, separated by an electrolyte, with lithium being stored at the negative electrode during charging.

[0005] Rechargeable lithium-ion batteries offer excellent energy and volume densities compared to other electrochemical energy storage technologies and now occupy a leading position in the market for portable electronics, electric and hybrid vehicles and stationary energy storage systems.

[0006] It is therefore desirable to optimize the energy density of Li-ion batteries, for example by increasing the amount of lithium that can be stored at the negative electrode. Among the configurations considered to improve the energy densities of electrochemical cells, the electrochemically active negative electrode materials may include silicon, which has a capacity ten times greater than that of graphite. However, the intercalation / deintercalation of lithium within silicon is accompanied by a strong volume expansion, of the order of 300%.This expansion will cause the degradation of the Li-ion cell: i) degradation of the integrity of the electrode which leads to a reduction in the use of the electrode, ii) fracture of the electrode-electrolyte interface (or SEI for "Solid Electrolyte Interphase") which leads to the continuous formation of degradation products and consumption of the electrolyte, and iii) application of mechanical stresses on the entire battery and degradation of the other components.

[0007] In fact, the cycle life of silicon-based negative electrode Li-ion batteries is insufficient and it is crucial to improve it.

[0008] For this, the stability of the interface of the negative electrode and the electrolyte must be increased.

[0009] It is therefore desirable to provide a negative electrode whose structure and composition make it possible to increase the quantity of lithium stored and the reversibility of this storage while controlling thickness variations. Among the most frequently cited approaches, the use of silicon at the nanometric scale, the use of partially oxidized silicon (SiO x , 0 <x<1 ) ou l’utilisation de composite silicium - carbone (Si-C) sont reportées prometteuses.

[0010] EP 3 618 150 relates to a negative electrode based on particles whose core is made of SiO x , coated with a layer comprising carbon or a polymer, and a fluorinated material. The preparation of this type of particle is often complex and leads to a material whose cost makes large-scale deployment complex.

[0011] EP 3 471 177 describes a negative electrode comprising composite particles comprising a carbonaceous material, silicon and LiF, the carbonaceous phase of which comprises mixed Si-LiF particles dispersed uniformly or not in said carbonaceous phase. This approach therefore requires careful control of each step: 1) coating (e.g. of Si by LiF), 2) dispersion (e.g. of Si-LiF particles in the carbonaceous phase) and 3) coating of the carbonaceous phase containing the Si-LiF particles.

[0012] Currently, electrodes for lithium batteries are generally manufactured from a composition containing at least one electrochemically active material, at least one binder and optionally at least one electronically conductive additive, coated on a current collector. The invention therefore aims in particular to provide an improved electrode composition for a negative electrode for Li-ion batteries.

[0013] According to a first object, the invention thus aims at a negative electrode composition comprising:

[0014] As active material: particles comprising silicon;

[0015] At least one binder;

[0016] Characterized in that said composition further comprises at least one fluorinated additive distributed uniformly between the particles of active material, binder and optional conductive material within said composition.

[0017] More particularly, the invention relates to a negative electrode composition comprising:

[0018] As active ingredient:

[0019] - particles comprising silicon and

[0020] - possibly graphite particles;

[0021] - possibly a conductive material;

[0022] - at least one binder; characterized in that said composition further comprises at least one fluorinated additive distributed uniformly between the particles of active material, binder and optional conductive material within said composition. The term "negative electrode" designates, when the accumulator is in discharge, the electrode functioning as an anode, the anode being defined as the electrode where an electrochemical oxidation reaction takes place (emission of electrons). The term negative electrode also designates the electrode from which the electrons leave, and from which the cations (Li+) are released in discharge.

[0023] According to the invention, the distribution of the additive is uniform within the composition. This means a distribution of the fluorinated additive within the composition such that the additive is distributed homogeneously and non-preferentially between the particles of active material, binder and possible conductor. Typically, such a distribution is free of agglomerate, the term "agglomerate" designating the assembly of several primary particles of additive and measuring several times the size of these, typically with a diameter greater than 10 μm.

[0024] An example of the structure of this distribution is illustrated in Figure 6.

[0025] Active material refers to electrochemically active (or electroactive) lithiophilic material, i.e. capable of storing lithium, for example by intercalation of lithium and / or formation of lithium alloy.

[0026] According to the invention, the active material is silicon-based. It therefore comprises particles of material comprising silicon. According to one embodiment, said particles are chosen from silicon-carbon composite (Si-C) particles and silicon oxide SiO particles. x where 0 <x<2.

[0027] According to one embodiment, the active material may also comprise graphite particles.

[0028] Here, the term "binder" means the agents used to give the composition and / or the electrode the cohesion of the different components and its mechanical strength and adhesion to the current collector. Examples of binders include polymers such as polyethylene oxide (PEO), polyvinylidene fluoride (PVdF) and its copolymers such as polyvinylidene fluoride and hexafluoropropylene copolymer (PVDF-HFP), polytetrafluoroethylene (PTFE) and its copolymers, polyacrylonitrile (PAN), poly(methyl)- or (butyl)methacrylate, polyvinyl chloride (PVC), poly(vinyl formal), polyester, block polyetheramides, polymers of acrylic acid, methacrylic acid, acrylamide, itaconic acid, sulfonic acid, elastomers such as poly(styrene / butadiene) (SBR) and hydrogenated butadiene-acetonitrile copolymers (HNBR), and cellulose compounds such as carboxymethyl cellulose (CMC).

[0029] Preferably, the binder may be selected from carboxymethylcellulose (CMC), styrene-butadiene (SBR), lithiated polyacrylic acid (LiPAA) and non-lithiated polyacrylic acid (PAA, PAAH or PAAN). The term "conductive material" typically refers to an electronic conductor, such as a carbon material, such as, for example, graphite, carbon black, acetylene black, soot, graphene, carbon nanotubes (CNT) or a mixture thereof. According to one embodiment, the conductive material is selected from carbon black and carbon nanotubes.

[0030] According to one embodiment, the fluorinated additive can be chosen from LiF or MgF2.

[0031] According to one embodiment, the electrode composition advantageously comprises Si-C particles, and the additive MgF2.

[0032] According to an alternative embodiment, the composition comprises particles of SiOx, where x is as defined above, and LiF.

[0033] According to one embodiment, the composition comprises from 0.05% to 5% (by weight) of the fluorinated additive, in particular from 0.1 to 3% (by weight), said percentage being relative to the total weight of the constituents of the composition.

[0034] The composition may further comprise other constituents such as one or more solvents and / or additives aimed at improving the electrochemical properties of the electrode.

[0035] According to one embodiment, the electrode composition according to the invention comprises the aforementioned ingredients, in liquid suspension, typically called “ink”.

[0036] Said electrode composition is then a mixture of particles of active material, binder, additive, and possible conductor as defined previously, in suspension, organic or aqueous.

[0037] The nature of the organic or aqueous solvent generally depends on the nature of the binder used. Preferably, the suspension is aqueous, in particular when the binder is chosen from carboxymethylcellulose (CMC), styrene-butadiene (SBR), lithiated polyacrylic acid (LiPAA) and non-lithiated polyacrylic acid (PAA, PAAH or PAAN).

[0038] Typically, the formulation is an organic suspension, in N-methyl-2-pyrrolidone when PVdF is used as a binder.

[0039] According to another embodiment, said composition may also be solvent-free.

[0040] According to another object, the present invention also relates to a negative electrode comprising a current collector on which is coated on at least one of its faces a coating comprising the composition according to the invention.

[0041] Typically, said coating essentially corresponds to the composition after drying of the ink (i.e. after evaporation of the solvent. It therefore comprises the same constituents as the ink, with the exception of the solvent generally evaporated during drying. The term "composition" used here therefore encompasses the suspension (i.e.) the ink as well as the coating obtained after drying of the ink.

[0042] Generally, the coating thickness depends on 1) the material used and 2) the application. The coating thickness per side after solvent evaporation is typically between 5 and 200 pm.

[0043] The term "current collector" means an element such as a pad, plate, sheet or other, of 2D or 3D structure, made of conductive material, connected to the positive or negative electrode, and ensuring the conduction of the flow of electrons between the electrodes and the terminals of the battery. The current collector is preferably a two-dimensional conductive support such as a solid or perforated strip, made of metal, for example copper, nickel, steel, stainless steel or aluminum. Said collector at the negative electrode is generally in the form of a copper strip.

[0044] The current collector / coating assembly constituting the electrode is typically obtained by applying an electrode composition according to the invention to said current collector.

[0045] Thus, according to another object, the present invention also aims at a method for preparing the electrode according to the invention, said method comprising

[0046] - the deposition of said electrode composition according to the invention on said current collector, and

[0047] - a drying step.

[0048] According to one embodiment, the method also comprises the step of prior preparation of an electrode composition according to the invention in aqueous or organic suspension, by mixing its constituents in aqueous or organic suspension, in particular with stirring. This step can typically be carried out over a sufficiently long time (generally from one to several hours) with a high shear rate mixer or a planetary mixer, for example until no evolution of the ink is observed, typically the absence of agglomerate to the naked eye. This step generally results in a uniform distribution without agglomerate.

[0049] The deposition step is typically carried out by coating the electrode composition on all or part of at least one of the faces of the collector. This can be carried out manually or automatically by film application process or by spraying. The drying step aims to remove the solvent contained in the electrode composition, and is typically carried out by heating, generally until its complete evaporation at a suitable temperature, typically between 50°C and 200°C, under vacuum or not and in a heated enclosure or using any other relevant technique such as, for example, infrared radiation. Advantageously, the presence of the fluorinated additive according to the invention does not substantially modify the usual process for preparing said electrode composition.

[0050] Typically, the fluorinated additive is dissolved or dispersed in the suspension.

[0051] At the end of the drying stage, the coating appears in the form of a film adhering to the current collector.

[0052] The electrode thus obtained can then be calendered in order to reduce its porosity, that is to say by reducing the space between the different constituents of the electrode.

[0053] The invention also relates to an electrochemical element comprising the negative electrode according to the invention.

[0054] The term "electrochemical element" means an elementary electrochemical cell, capable of storing the electrical energy provided by a chemical reaction and restoring it in the form of current.

[0055] The electrochemical element according to the invention is typically a Li-ion element.

[0056] Thus, the invention also relates to a Li-ion type electrochemical element comprising:

[0057] - a negative electrode according to the invention;

[0058] - a positive electrode;

[0059] - a separator;

[0060] - an electrolyte.

[0061] Within the scope of the present invention, the positive electrode may be of any known type suitable for Li-ion cells.

[0062] The term "positive electrode" refers to the electrode where electrons enter, and where cations (Li + ) in landfill.

[0063] The positive electrode generally consists of a conductive support used as a current collector which is coated on at least one of its faces with the positive electrode formulation, which typically contains at least one positive electrode active material, an electronically conductive material and an optional binder.

[0064] The active material of the positive electrode is not particularly limited. It can be selected from the following groups:

[0065] - a compound (a) of formula LixMi.y. z.w M'y M” z M”' w O2(LMO2) and of so-called “lamellar” or “lamellar oxide” structure where M, M', M” and M'” are selected 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 with the proviso that at least M or M' or M” or M'” is selected 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 et x+y+z+w<2,1 ; - un composé (b) de formule Li x Mn2-y-zM'yM"zO4 (LMO4) and of so-called "spinel" structure, 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 ;

[0066] - a compound (c) of formula LixFei.yM y PO4(LFMP) and of so-called “olivine” structure 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 ;

[0067] - a compound (d) of formula Li x Mni.y. z M' y M” z PO4 (LMP), where M' and M” are different from each other and are selected from the group consisting of 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; 0,0<z<0,2 ;

[0068] - a compound (e) of formula xLi2MnOs; (1 -x)LiMO2 of so-called “lamellar” or “lamellar oxide” structure where M is at least one element chosen from Ni, Co and Mn and where 0 <x< 1 ;

[0069] - a compound (f) of formula Lii +x MO 2-y F y of cubic structure where M represents at least one element selected 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,

[0070] - a compound (g) of formula Lii +x Vi. y M y PO4F zwhere, 0 <x<1 ; 0<y<0,5; 0,8<z<1 ,2; et M représente au moins un élément choisi dans le groupe constitué de Ti, Al, Mg, Mn, Fe, Co, Y, Cr, Cu, Ni, or Zr,

[0071] - a compound (h) of formula LiNii. y.z Mn y Co z O2with 0 <y<1 , 0<z<1 et y+z<1 , aussi désigné Li(Ni,Mn,Co)O2,

[0072] - and a mixture of a) to h).

[0073] According to one embodiment, the active material of the positive electrode comprises, as active material, a lamellar oxide, such as LiNii. y.z Mn y Co z O2with 0 <y< 1 , 0<z<1 et y+z<1 .

[0074] The positive electrode electronic conductive material is generally selected from graphite, carbon black, acetylene black, soot, graphene, carbon nanotubes or a mixture thereof.

[0075] The electrolyte may be liquid and include a lithium salt dissolved in an organic solvent. This lithium salt may be selected from lithium perchlorate UCIO4, lithium hexafluorophosphate LiPF6, lithium tetrafluoroborate LiBF4, lithium hexafluoroarsenate LiAsF6, lithium hexafluoroantimonate LiSbF6, lithium trifluoromethanesulfonate ÜCF3SO3, lithium bis(fluorosulfonyl)imide Li(FSO2)2N (LiFSI), lithium trifluoromethanesulfonimide LiN(CFsSO2)2(LiTFSI), lithium trifluoromethanesulfonemethide LiC(CF3SO2)3 (LiTFSM), lithium bisperfluoroethylsulfonimide LiN(C2F5SO2)2 (LiBETI), lithium 4,5-dicyano-2-(trifluoromethyl)imidazolide (LiTDI), lithium bis(oxalatoborate) (LiBOB), lithium difluoro(oxalato)borate (LIDFOB), lithium tris(pentafluoroethyl)trifluorophosphate LiPF3(CF2CF3)3 (LiFAP) and mixtures thereof.

[0076] The solvent for the electrolyte may be selected from saturated cyclic carbonates, unsaturated cyclic carbonates, linear carbonates, alkyl esters, ethers or cyclic esters, such as lactones and mixtures thereof.

[0077] The electrolyte can also be in the form of a gel obtained by impregnating a polymer with a liquid mixture comprising at least one lithium salt and an organic solvent.

[0078] The separator is intended to prevent short circuits, while being permeable to lithium ions. It can be made of a non-woven fabric or a polymer film. The separator can be made of a layer of polypropylene (PP), polyethylene (PE), polytetrafluoroethylene (PTFE), polyacrylonitrile (PAN), polyester such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), cellulose, polyimide, glass fibers or a mixture of layers of different natures. The polymers mentioned can be coated with a ceramic layer and / or polyvinylidene difluoride (PVdF) or polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) or acrylates.

[0079] The lithium-ion cell is manufactured in a conventional manner. At least one cathode, at least one separator, and at least one anode are stacked on top of each other. The assembly can be rolled up to form a cylindrical electrochemical bundle. The electrodes can also be stacked to form a planar electrochemical bundle. A connecting piece is attached to an edge of the cathode not covered with active material. It is connected to a current output terminal. The anode can be electrically connected to the cell container. Conversely, the cathode can be connected to the cell container, and the anode to a current output terminal. After being inserted into the cell container, the electrochemical bundle is impregnated with electrolyte. The cell is then sealed.The element may also be conventionally equipped with a safety valve causing the element container to open in the event that the internal pressure of the element exceeds a predetermined value.

[0080] According to another object, the present invention also relates to an electrochemical module comprising the stack of at least two elements according to the invention, each element being electrically connected with one or more other element(s).

[0081] The term "module" therefore designates here the assembly of several electrochemical elements, said assemblies being able to be in series and / or parallel. Another object of the invention is still a battery comprising one or more modules according to the invention.

[0082] The term “battery” or accumulator means the assembly of several modules according to the invention.

[0083] The invention will be described more precisely, in an illustrative manner, with reference to the Figures and examples given below.

[0084] Figures

[0085] Figure 1 illustrates the normalized evolution of the capacity recorded at C / 5 - D / 5 (charge in 5h - discharge in 5h), at room temperature, where curve 1 corresponds to a SiO-based electrode x +graphite (LiPAA binder) with LiF additive and curve 2 corresponds to a SiOx+graphite (LiPAA binder) based electrode without additive. The arrow represents a 5-fold increase in the number of cycles before reaching 80% retention of the reference capacity.

[0086] Figure 2 illustrates the normalized evolution of the capacity at C / 5 - D / 5, at room temperature, with curve 1 corresponding to a SiO-based electrode x +graphite / / Li° with the LiF additive, and curve 2 corresponding to an electrode based on SiO x+graphite / / Li° without additive. The arrow represents a 15% increase in the number of cycles for the electrode with LiF additive compared to the electrode without additive.

[0087] Figure 3 illustrates the normalized evolution of the capacity at C / 5 - D / 5, room temperature, where curve 1 corresponds to a Si-C+graphite / / Li° electrode with the MgF2 additive, and curve 2 corresponds to a Si-C+graphite / / Li° electrode without additive. The arrow represents a 28% increase in the number of cycles for the electrode with MgF2 additive compared to the electrode without additive.

[0088] Figure 4 illustrates the normalized evolution of the capacity at C / 3 - D / 3, room temperature, where curve 1 corresponds to an NMC811 cell / / composite Si-C + graphite with MgF2 in the electrode and curve 2 corresponds to an NMC811 cell / / composite Si- C + graphite without MgF2 in the electrode.

[0089] Figure 5 illustrates the normalized evolution of the internal resistance, where curve 1 corresponds to an NMC81 1 / / Si-C + graphite composite cell with MgF2 in the electrode and curve 2 corresponds to an NMC811 / / Si-C + graphite composite cell without MgF2 in the electrode.

[0090] Figure 6 shows the structure of the electrode composition according to one embodiment of the invention, where the composition comprises Si-C particles (1) and graphite particles (4) as active materials, MgF2(2) as an electrode additive, binder (3). According to this embodiment, the composition does not contain a conductive material; however, it is understood that when a conductive material is present, the conductive material particles do not affect the uniform distribution of the additive within the structure of said composition.

[0091] Examples

[0092] 1 - Preparation of inks and electrodes

[0093] Ink formulations according to the invention were prepared according to the milk steps with the active ingredients SiOx or Si-C. The percentages indicated represent the percentages by weight of the ingredients added at the step considered, by weight relative to the total weight of the ingredients of the suspension.

[0094] In the table above, 0 <x<2, de préférence x est égal à environ 1 .

[0095] The term "stirring" implies "until complete dissolution or homogeneous distribution to the naked eye". The ink suspensions thus prepared in steps 1 a-1 f are then coated onto a copper foil, and the whole is subjected to 80°C for a period typically between a few minutes and one hour, until the water evaporates. The evaporation of the water can be identified by weighing, when the mass no longer varies over time when the drying time is increased.

[0096] The calendering of the electrode thus prepared is carried out by passing the electrode between two rollers in order to reduce the porosity of the electrode to the desired value.

[0097] 2- Distribution within the electrode

[0098] The electrode after drying can be characterized by scanning electron microscopy (SEM) in order to show the distribution of the additive within the electrode so that it is uniform and without agglomerates, the additive being located between the particles of active materials without however completely covering them.

[0099] 3- Highlighting electrochemical properties

[0100] Button cells were prepared to perform electrochemical characterization measurements with a Li metal counter electrode:

[0101] - the previously prepared electrode is cut to the correct diameter in order to be inserted into the button cell holder,

[0102] - a polymer separator, typically polypropylene, and a standard electrolyte comprising a lithium salt (LiPF6) dissolved in a mixture of carbonates are added, as well as the lithium metal disc which here acts as a counter-electrode.

[0103] Once the button cell is sealed, it is connected to a potentiostat type cycling system to generate a current of a few pA to a few mA which will depend on the capacity (mAh) of the battery tested and the desired current regime (C / X),

[0104] By means of a + and - current application system, charge and discharge cycles can thus be carried out.

[0105] Thus, a discharge capacity as a function of a number of cycles is determined.

[0106] The start of cycling begins with a temperature training cycle (1 erpoint of the curves), then aging (for the study of the lifespan) is carried out at ambient T°C. “Control cycles”, carried out at slower cycling speeds, can be inserted during aging.

Claims

CLAIMS 1. Negative electrode composition comprising: as active material: - particles comprising silicon; - at least one binder; - optionally a conductive material, characterized in that said composition further comprises at least one fluorinated additive distributed uniformly between the particles of active material, binder and optional conductive material within said composition.

2. Composition according to claim 1 such that the particles comprising silicon are chosen from silicon-carbon composite (Si-C) particles and silicon oxide SiO particles x where 0 <x<2.

3. Composition according to claim 1 or 2 such that the fluorinated additive is chosen from LiF or MgF2.

4. Composition according to any one of the preceding claims such that it comprises particles of Si-C and MgF2.

5. Composition according to any one of claims 1 to 3 such that it comprises SiO particles x where 0 <x<2 et LiF.

6. Composition according to any one of the preceding claims such that said active material further comprises graphite particles.

7. Composition according to any one of the preceding claims such that it further comprises a conductive material.

8. Composition according to any one of the preceding claims such as comprising from 0.05% to 5% (by weight) of the fluorinated additive, in particular from 0.1 to 3% (by weight), said percentage being relative to the total weight of the constituents of said composition.

9. Composition according to any one of the preceding claims such that the binder is chosen from carboxymethylcellulose (CMC), styrene-butadiene (SBR), lithiated polyacrylic acid (LiPAA) and non-lithiated polyacrylic acid (PAA, PAAH or PAAN).

10. Composition according to any one of the preceding claims such that the conductive material is chosen from carbon black and carbon nanotubes.

11. Composition according to any one of the preceding claims in aqueous or organic suspension.

12. Negative electrode comprising a current collector on which is coated on at least one of its faces a coating comprising the composition according to any one of the preceding claims.

13. Method for preparing the electrode according to claim 12 comprising a step of depositing a composition as defined according to one of claims 1 to 9 on said current collector, and a drying step.

14. Method according to claim 13 comprising the prior step of mixing the constituents of the composition defined according to one of claims 1 to 11 in aqueous or organic suspension.

15. Li-ion type electrochemical element comprising: - a negative electrode according to claim 12; - a positive electrode; - a separator; - an electrolyte.

16. Electrochemical element according to claim 15, such that the positive electrode comprises as active material a lamellar oxide, such as UNii.y. z MnyCOzO2with 0

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