Metallic microparticles for battery anode

Abstract

Metallic microparticles for battery anode The invention relates to metallic microparticles for secondary battery anode and their uses in an anode and a battery, said particles comprising a core of at least one or more metals selected from lithium, sodium, potassium, magnesium and aluminum, on the one hand, and of electrically conductive nanofibers, on the other hand, and a protective layer of an ion-conducting metallic compound coating the core, a fraction of the nanofibers being completely immersed in the core metal and another fraction of the nanofibers having a part immersed in the core metal and another part passing through the protective layer to the outside.

Description

Title of the invention: Metallic microparticles for battery anode

[0001] The present invention relates to lithium rechargeable battery (or secondary battery) technology. More specifically, it relates to an anode active material and its applications in a rechargeable battery.

[0002] Lithium batteries include lithium-ion (Li-ion) batteries and lithium-metal (Li-metal) batteries. They generally consist of a negative electrode (anode) with an anode current collector, a positive electrode (cathode) with a cathode current collector, an anode-cathode separator, and an electrolyte ensuring ionic conduction between the two electrodes.

[0003] Given their advanced development, Li-ion batteries are the most commonly used. The main materials that dominate the market for Li-ion battery anodes are graphite and silicon-bonded graphite. Lithium is present only in the form of Li+ ions, which circulate from one electrode to the other within the electrolyte, which consists of lithium salts such as LiPF6, LiTFSI (formula LiC2NO4F6S2), LiBF4, or LiClO4 in an organic solvent.

[0004] They are still the subject of intensive research aimed at achieving ever greater capacity, longer lifespan, shorter charging times, lighter weight, and an acceptable production cost. An additional criterion, which is becoming crucial in the development of this technology, particularly for the manufacture of electric vehicles, is safety. One of the key aspects of this factor is the electrolyte. In Li-ion batteries, the electrolyte is generally liquid and flammable; subjected to high temperatures, the risk of thermal runaway exists. Solid electrolytes have been developed, but when combined with existing anode materials, they do not yet allow for sufficient ionic conductivity.

[0005] In parallel, lithium-metal or Li-metal batteries, which have been known for many years but not exploited, notably due to short circuits occurring through the separator and causing fires, have recently been brought back into the spotlight. They would make it possible to increase the battery capacity per unit weight. Indeed, the theoretical capacity of lithium metal is 10 times greater than that of graphite (3861 mAh / g for Li-metal and 372 mAh / g for graphite), and leads to batteries capable of providing energy of at least 350 Wh / kg (250-280 Wh / kg for the highest-performing Li-ion batteries). In this technology, the constituent material The anode is made of metallic lithium, and like a Li-ion battery, lithium ions flow from one electrode to the other. However, Li-metal batteries also suffer from the risk of electrolyte ignition when it is liquid, as it commonly contains one or more organic solvents. During charge and discharge cycles, dendrites form on the anode and grow towards the cathode, eventually passing through the separator and causing a short circuit. These batteries can, however, use a solid electrolyte, which is less flammable, thus reducing the risk of fire. However, available solid electrolytes have lower ionic conductivity and are also expensive, primarily due to the complex manufacturing process.

[0006] According to document WO02 / 21632A1, an anode material for Li-metal batteries is known, based on metallic lithium particles deposited, for example, by sintering, compression, or molding, onto an aluminum or copper foil. The authors have demonstrated that the use of such an anode in a battery comprising a sintered LiCoO2 cathode and a lithium hexafluorophosphate (LiPF6) IM electrolyte in an ethylene carbonate / dimethyl carbonate (1:1) mixture would eliminate the formation of dendrites compared to an anode consisting of a lithium foil.

[0007] It must be noted that to date, Li-metal battery technology is not yet mature and Li-ion batteries remain the most capable of meeting the exponential demand of electric vehicles in particular, with their weaknesses.

[0008] It is in this context that the present invention is situated, complementing and refining technologies that are the subject of patent applications in the name of the Applicant, for example, patent application FR3112655A1, with the introduction of a lithium-metal anode material and its implementation in a battery, and advantageously in a battery comprising a liquid electrolyte that solidifies into a gel upon exposure to radiation. It makes it possible to overcome the aforementioned drawbacks, in particular the problem of dendrites and that of premature lithium consumption, and thus to consider the industrial-scale development of batteries with a Li-metal anode, thereby meeting the demand.

[0009] This material is based on a metal such as lithium and is designed to suppress the formation of large and sharp dendrites that could pierce the separator, while preventing the leakage and consumption of lithium metal in the electrolyte and promoting electronic transmission and ion circulation, thus increasing the performance of a battery equipped with it.

[0010] Thus, a first object of the invention lies in metallic microparticles for secondary battery anode, said microparticles comprising a core of at least one or more metals selected from lithium, sodium, potassium, the magnesium and aluminum, on the one hand, and electrically conductive nanofibers, on the other hand, and a protective layer of an ion-conducting metallic compound coating the core, a fraction of the nanofibers being completely immersed in the core metal and another fraction of the nanofibers having a part immersed in the core metal and another part passing through the protective layer to the outside.

[0011] In the following text, the present invention is described with reference to lithium-based metallic microparticles, but, as indicated above, they may have a core of one or other metals such as sodium, potassium, magnesium and aluminum, while exhibiting the same characteristics, properties and advantages as particles having a lithium core, without departing from the scope of the invention.

[0012] It has been observed that by significantly increasing the contact area of ​​the lithium material with the electrolyte, in accordance with the microparticles of the invention compared to a lithium-metal sheet extruded according to the known implementation of Li-metal batteries laminated onto a copper sheet, or compared to an electroconductive surface on which micrometric particles of lithium metal are deposited according to the aforementioned document, current densities are significantly reduced, resulting only in the formation of small micro-dendrites which limit the harmful consequences of dendrites, without however completely solving the problem.It should also be noted that generally dendrites grow perpendicularly to the surface that generates them: in the case of an anode with a lithium metal foil, they are oriented towards the separator with a risk of perforation of the latter and therefore of inter-electrode short circuits, whereas with the microparticles of the invention, they are not all oriented in the same direction and this phenomenon becomes insignificant in relation to the separator.

[0013] By microparticles, we mean particles having a size of 0.1–100 pm, and according to the invention, they are advantageously essentially spherical with regard to the aforementioned dendrite orientation phenomenon; this also gives them excellent flowability. In a preferred embodiment, the particles are spherical; they may have a diameter of 0.5–100 pm, preferably 3–40 pm, and even better 10–20 pm. Compared to a metallic lithium sheet, the exposed specific surface area can be multiplied by at least 10³.

[0014] To increase the performance of the anode, the metal core is traversed by electrically conductive nanofibers that greatly facilitate the transmission of electrons from the particle core to the anode current collector. Some of these nanofibers are completely immersed in the metal, others have one part immersed in the metal and another part passing through the protective layer to emerge to the outside, and their multiplicity will result in a multitude of Contacts occur between the core of the metallic particles and the anode current collector. The latter is advantageously either metallic or composite, comprising a polymer sheet coated with a metallic, carbon-based, or composite electron-conducting material, for example, made of carbon or glass fibers coated with a conductive metal. Depending on the manufacturing process, the nanofibers protrude from the protective layer by a few microns, for example, 2 µm or more, depending on their length.

[0015] According to one variant, nanofibers having a part immersed in the metal and a part passing through the protective layer to the outside also have a part emerging from the particles to the outside.

[0016] During the manufacture of the particles, an example of which will be described later in the text, all or part of the nanofibers are distributed in the molten metal, preferably in a homogeneous manner.

[0017] The electrical conductivity of nanofibers can originate from one or more of their constituent materials or be imparted or enhanced by a treatment, for example, a surface treatment. They can thus be made of one or more electrically conductive materials, such as carbon nanofibers, and optionally are treated to increase their electrical conductivity. Alternatively, they can be made of one or more non-conductive or weakly conductive materials, such as glass or Kevlar nanofibers, and are treated to make them electrically conductive or to increase their electrical conductivity.

[0018] Advantageously, these are VGCF (Vapor Grown Carbon Fibers) type carbon fibers. Carbon fibers are naturally electrically conductive; however, in order to increase their conductivity, they are advantageously coated with a film of one or more conductive compounds. Among these compounds, titanium compounds are preferred, and in particular titanium nitride (TiN), titanium carbide (TiC), or titanium carbonitride (TiCN). This film, with an appropriate thickness of 10–100 nm, or even better, 20–50 nm, is applied to the fibers by a conventional process known to those skilled in the art, such as chemical vapor deposition (CVD) or physical vapor deposition (PVD).

[0019] Ideally, they mostly meet the dimensions of a diameter of 1-240 nm and a length of 0.5-10 pm.

[0020] In order to optimize electrical circulation, a preferred content of nanofibers, whether coated with a conductive film or not, is 1-15% by weight relative to the weight of the microparticles.

[0021] The microparticles of the invention consist of a core coated with a protective layer of a metallic compound. For the purposes of the performance of a battery of the invention, this protective layer is sealed and designed to prevent direct contact The interaction between the electrolyte and the lithium metal creates a solid electrolyte interphase (SEI) that is detrimental to the battery's cycle life due to high lithium consumption. Furthermore, it is highly conductive of ions.

[0022] The airtightness of the protective layer is paramount, as mentioned previously, but also given the very high reactivity and flammability of lithium metal upon the slightest contact with oxygen or water, and therefore with humid air. It must be handled in an anhydrous room, particularly during the manufacture and use of the microparticles.

[0023] The exit of the nanofibers from the Li-metal core through this layer is a vulnerable area, particularly in terms of sealing. To meet this requirement, the metallic compound is advantageously an ionic nitride, such as a nitride of the core metal, specifically chosen from lithium nitride (Li3N), sodium nitride (NaN3), potassium nitride (K3N), magnesium nitride (Mg3N2), and aluminum nitride (Ain); more preferably, lithium nitride (Li3N) is considered one of the best solid conductors of lithium ions. The coating of the microparticle core can be achieved by any technique known to those skilled in the art. According to a practical embodiment of the invention, the process consists of chemically transforming the lithium metal on the core surface into lithium nitride by reacting it with a nitriding agent.It allows for complete filling of the gaps formed at the exit of the nanofibers from the core, ensuring perfect sealing of the protective layer, including in sensitive areas, without creating an excess thickness that could drown the nanofibers protruding outside the particles and thus compromise their electrical conductivity.

[0024] Another advantage of this protective layer, preferably made of nitride, and even better of lithium nitride, lies in its strength; it forms a robust shell that withstands the mechanical stresses of the swelling and shrinking of the microparticle core caused by the recombination of lithium metal from the lithium ions emanating from the cathode during charge / discharge cycles. Cracks can appear, given that Li3N, although very hard, is brittle. Thanks to the nanofibers of the anode material of the invention, these cracks are avoided; indeed, they are extremely tensile-resistant and constitute, within the protective layer, a true reinforced network with the lithium metal and Li3N, thus providing a truly robust shell without the risk of cracking.

[0025] The thickness of the protective layer results from a compromise between thinness and robustness: it is on the order of 5-1000 nm, preferably 10-600 nm. It is of course determined according to the diameter of the particles.

[0026] The invention also relates to the applications of microparticles as described above, in any of the modes presented. Thus, they are part of of the invention: - an anode comprising, as active material, microparticles of the invention and a current-collecting material; - a battery comprising such an anode, a cathode compatible with the anode, a separator, a liquid, gel, gelable, solid or solubilizable electrolyte and external current collectors.

[0027] Thus, the invention also relates to an anode comprising microparticles as described above in any of the specified variants, considered alone or in combination, and a current-collecting material. The latter may be made of any suitable material that a person skilled in the art would be able to choose. Copper foil is commonly used, for example. Advantageously, in the invention, it consists of an electroconductive material that may take the form of a nonwoven of carbon fibers, optionally coated with a layer of a titanium compound chosen from titanium nitride (TiN), titanium carbide (TiC), and titanium carbonitride (TiCN).

[0028] The invention also relates to a secondary battery comprising an anode whose active material comprises or consists of microparticles as described above, in any of the modes presented. It further comprises a cathode compatible with the anode, a separator, a liquid, gel or solid electrolyte and external current collectors.

[0029] According to one embodiment, a mixture of the particles of the invention and the electrolyte is deposited on or in the current-collecting material. This mixture, called semi-liquid ink, is inserted or deposited on or in the 3D porous electroconductive current-collecting material to be converted into a solid gel or solid by exposure to heat or radiation. The 3D anode current collector is advantageously made of a carbon fiber nonwoven, in particular commercially available conductive carbon fibers. To increase their electrical conductivity, they can be coated with a layer of a titanium compound that is a good conductor of electricity. This compound can be selected from titanium nitride (TiN), titanium carbide (TiC), and titanium carbonitride (TiCN).

[0030] The electrolyte, whatever its form—liquid, solid, solidifiable, gel, or gelifiable—contains the elements necessary for its function, namely, to circulate ions. To ensure ionic conduction, this electrolyte generally consists of at least one salt, for example, LiPF6, LITFSI, LiBF4, LiClO4, or NaFSI; at least one organic solvent of this salt, for example, propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), tetraethylene glycol dimethyl ether (TEGDME), 1,3-dioxolane (DOL, DIOX), ...; and / or at least one ionic liquid. It may be a hybrid and consist of an association of solid electrolyte particles, for example, LLZO (lithium lanthanum zirconium oxides), LLZTO (lithium lanthanum tantalum zirconium oxides); in a polymer gel matrix (PGM) or a solid polymer matrix electrolyte, for example PEO (polyethylene oxide), or even a combination of particles of the invention and solid electrolyte particles under compression. As mentioned previously, a superiority of Li-metal anodes over Li-ion anodes is the ability to use a solid electrolyte. Advantageously, a battery of the invention will be an all-solid-state battery, that is, one operating with an electrolyte that is initially liquid and then solidifies. However, the use of a liquid or semi-liquid electrolyte is entirely feasible.

[0031] According to an advantageous embodiment, the electrolyte may further contain additional lithium salts, in particular selected from LiNO3 and LiN3, which will contribute to sealing micro- and nano-cracks that could form on or in the protective shell of the microparticles, by chemical reaction with the lithium. Alternatively or in addition, the electrolyte may contain a film-forming agent, such as a surfactant, capable of forming a protective film around said particles, completely covering the particles of the invention (for example, Li-metal nitride with Li3N). Since Li3N is also highly reactive to oxygen and water, and the electrolyte is relatively unreactive, this second protective film helps to protect the particles from attack by humid air during handling of the particles to manufacture a battery of the invention.The additive is preferably liquid and is subsequently solidified thermally or by radiation to form the additional protective film.

[0032] In operation, according to the aforementioned advantageous mode, the particles of the invention have a double protection, a first solid protective layer in Li3N through which the conductive nanofibers pass and a second layer in electrolyte first liquid then solidified in which the nanofibers are bathed.

[0033] An additional advantage of an anode of the invention is its compatibility with many cathode materials. Thus, one or more suitable cathode materials can be chosen from sulfur, lithium sulfide, titanium disulfide, molybdenum disulfide, titanium disulfide, iron disulfide, and the insertion materials LiCoO2 (LCO), LiNiCoO2 (NMC), LiFePO4 (LFP), LiMnO2 (LMO), and LiNiCoAlO2 (NCA). Therefore, the invention also relates to a battery comprising an anode of the invention and such a cathode.

[0034] The present invention is described in more detail below with reference to the following figures which illustrate the particles that are the subject of the invention, and according to which:

[0035] [Fig. 1] is an image obtained by scanning electron microscopy (SEM) combined with a focused ion beam (SEM-FIB) representing a cross-section of a microparticle of the invention;

[0036] [Fig.2] represents by a diagram the anode electrode material of the invention in a solidified electrolyte in which:

[0037] 1 indicates the lithium-metal core,

[0038] 2 indicates the Li3N layer;

[0039] 3 indicates VGCF microfibers;

[0040] 4 indicates the gelled electrolyte;

[0041] 5 indicates the carbon fibers of the 3D current collector.

[0042] Method for manufacturing microparticles of the invention,

[0043] The microparticles of the invention can be manufactured by melting under argon and stirring, at 220°C, a mixture of lithium metal in a mineral or paraffinic oil and VGCF type carbon fibers at a rate of 1-5% by weight relative to the weight of lithium metal.

[0044] The mixture is then processed to produce spherical microparticles with a diameter of 3 to 40 microns. After cooling, the oil is removed by filtration before undergoing successive washes with organic solvents, such as hexane or toluene, or supercritical CO2, which are continuously recycled, to remove and dissolve the remaining oil. The microparticles are collected in a container and held under argon, then dried.

[0045] They are then surface nitrided in a fluidized bed with pressurized nitrogen (N2) circulation or by immersion in liquid nitrogen or nitriding agents at a temperature, for example, of 80°C, for an exposure time ranging from a few minutes to several hours depending on the nitriding agent, in order to achieve a dense peripheral Li3N sealing layer thickness of 10-600 nm. The spherical shape of the lithium metal cores facilitates obtaining a protective layer of homogeneous thickness and without porosity, ensuring absolute sealing.

[0046] In the same nitriding installation, still under N2, or under argon, the lithium-metal cores with their Li3N protective layer are mixed with the still liquid electrolyte, taking into account organic solvents or ion-conducting ionic liquids, but gelable or solidifiable, either thermally or by radiation thereafter, to produce the anode active material ink which will be deposited on or in the current collectors, thus during the manufacture of the cells, the particles will have been protected on all their peripheral surfaces from the ambient humid air which can react with the Li3N. Microparticle of the invention

[0047] [Fig. 1] is a photograph of a particle taken with a scanning electron microscope (SEM-FIB).

[0048] Microparticles of the invention as an anode material in an electrolyte, with the current collector

[0049] In accordance with [Fig.2], microparticles comprising a core 1 are observed a spherical Li-metal core in which VGCF 3 carbon nanofibers are homogeneously embedded, and a protective layer 2 hermetically surrounding said core 1. This protective layer 2 is traversed by VGCF 3 carbon nanofibers which open into the electrolyte 4. The current collector is made of carbon fibers 5.

Claims

Demands

1. Metallic microparticles for secondary battery anode, characterized in that they comprise a core of at least one or more metals selected from lithium, sodium, potassium, magnesium and aluminum, on the one hand, and electrically conductive nanofibers, on the other hand, and a protective layer of an ion-conducting metallic compound coating the core, a fraction of the nanofibers being completely immersed in the core metal and another fraction of the nanofibers having a part immersed in the core metal and another part passing through the protective layer to the outside.

2. Microparticles according to claim 1, characterized in that the ion-conducting metallic compound is a nitride of the core metal.

3. Microparticles according to claim 1 or 2, characterized in that the core metal is lithium and the metallic compound is lithium nitride (Li3N).

4. Microparticles according to any one of claims 1-3, characterized in that they are spherical and have a diameter of 0.5-100 pm, preferably 3-40 pm, better still 10-20 pm.

5. Microparticles according to any one of claims 1-4, characterized in that the core metal is lithium and the protective layer is sealed, preventing direct contact of lithium with the electrolyte.

6. Microparticles according to any one of claims 1-5, characterized in that the thickness of the protective layer is 5-1000 nm, preferably 10-600 nm.

7. Microparticles according to any one of claims 1-6, characterized in that the electrically conductive nanofibers have a diameter of 1-240 nm and a length of 0.5-10 pm.

8. Microparticles according to any one of claims 1-7, characterized in that the nanofibers are made of one or more electrically conductive materials, such as carbon nanofibers, and are optionally treated to increase their electrical conductivity.

9. Microparticles according to any one of claims 1-7, characterized in that the nanofibers are made of one or more non- or weakly electrically conductive materials, such as glass or Kevlar nanofibers, and are treated to make them electrically conductive or to increase their electrical conductivity.

10. Microparticles according to any one of claims 1-8, characterized distinguished in that the nanofibers are carbon nanofibers and are coated with a film of a titanium compound selected from titanium nitride (TiN), titanium carbide (TiC) and titanium carbonitride (TiCN).

11. Microparticles according to claim 10, characterized in that the titanium compound film has a thickness of 10-100 nm, preferably 20-50 nm.

12. Microparticles according to any one of claims 1-11, characterized in that the nanofiber content is 1-15% by weight relative to the weight of the microparticles.

13. Microparticles according to any one of claims 1-12, characterized in that nanofibers having a part immersed in the metal and a part passing through the protective layer also have a part emerging from the particles outwards.

14. Anode comprising microparticles according to any one of claims 1-13 and a current collector material.

15. Anode according to claim 14, characterized in that the current collector material is made of an electroconductive material which can take the form of a non-woven carbon fiber, optionally coated with a layer of a titanium compound selected from titanium nitride (TiN), titanium carbide (TiC) and titanium carbonitride (TiCN).

16. Secondary battery comprising an anode according to claim 14 or 15, a cathode compatible with the anode, a separator, a liquid, gel, gelable, solid or solidifiable electrolyte and external current collectors.

17. Battery according to claim 16, characterized in that the electrolyte contains lithium salts, such as LiPF6, LiTFSI (of formula LiC2 NO4F6S2), LiBF4, LiClO4 or NaFSI, at least one organic solvent of this salt, such as propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), tetraethylene glycol dimethyl ether (TEGDME), 1,3-dioxolane (DOL, DIOX), and / or at least one ionic liquid, or may be hybrid and consist of an association of solid electrolyte particles, such as LLZO (lithium lanthanum zirconium oxides), LLZTO (lithium lanthanum tantalum zirconium oxides), in a polymer gel matrix (EGP) or a polymer solid matrix electrolyte, for example PEO, or an association of particles of any one of the claims 1-13 and solid electrolyte particles placed under compression.

18. Battery according to claim 16 or 17, characterized in that the electrolyte contains additional lithium salts, in particular selected from LiNO3 and LiN3.

19. Battery according to claims 17 and 18, characterized in that the electrolyte also contains in addition a surfactant film-forming adjuvant.

20. Battery according to any one of claims 16-19, characterized in that the cathode is made of a material selected from sulfur, lithium sulfide, titanium disulfide, molybdenum disulfide, titanium disulfide, iron disulfide, LiCoO2 (LCO), LiNiCoO2 (NMC), LiFePO4 (LFP), LiMnO2 (LMO) and LiNiCoAIO2 (NCA) insertion materials.

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