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 the technology of rechargeable lithium batteries (or secondary batteries). More specifically, it relates to an active anode 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 providing ionic conduction between the two electrodes.

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

[0004] They are still the subject of intensive research with a view to obtaining ever greater capacity, longer life, shorter charging times, lighter weight and acceptable cost price. Taking into account an additional criterion, which is becoming major in the development of this technology, particularly for the manufacture of electric vehicles, is safety. One of the pivots of this factor is the electrolyte. In Li-ion batteries, this is generally liquid and flammable, subjected to high temperatures, the risks of thermal runaway exist. Solid electrolytes have been developed, but associated with existing anode materials, they do not yet allow sufficient ionic conductivity to be achieved.

[0005] At the same time, lithium metal or Li-metal batteries, which have been revealed for many years but have not been used, in particular because of short circuits occurring through the separator and causing fires, have recently been brought back to the forefront. They would make it possible to increase the battery capacity per unit of 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 an energy of at least 350 Wh / kg (250-280 Wh / kg for the most efficient Li-ion batteries). In this technology, the constituent material The anode is made of metallic lithium and, like a Li-ion battery, lithium ions circulate from one electrode to the other. However, Li-metal batteries also suffer from the risk of ignition of the electrolyte when it is liquid, which commonly contains one or more organic solvents. During the charge and discharge cycles, dendrites form on the anode which grow towards the cathode until they cross the separator and cause a short circuit. These batteries can, however, use a solid electrolyte, which is less flammable, reducing the risk of fire. However, the available solid electrolytes have a lower ionic conductivity; they also have a high production cost, particularly due to high manufacturing technology.

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

[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 for electric vehicles in particular, with their weaknesses.

[0008] It is in this context that the present invention falls within the scope of, in addition to and with details of, technologies which are the subject of patent applications in the name of the Applicant, for example patent application FR3112655A1, with the provision of a lithium-metal-based anode material and its implementation in a battery, and advantageously in a battery comprising a liquid electrolyte which can be solidified in the form of a gel by exposure to radiation. It makes it possible to overcome the aforementioned drawbacks, in particular the problem of dendrites and that of untimely consumption of lithium, and thus to envisage development on an industrial scale of batteries with a Li-metal anode, and thus meet the demand.

[0009] This material is based on a metal such as lithium and is designed to suppress the formation of large and pointed dendrites likely to pierce the separator, while preventing the leakage and consumption of lithium metal in the electrolyte and promoting electronic transmission and the circulation of ions, 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 chosen 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 metal compound coating the core, a fraction of the nanofibers being completely immersed in the core metal and another fraction of the nanofibers having a portion immersed in the core metal and another portion passing through the protective layer to the outside.

[0011] In the following text, the present invention is set out with reference to lithium-based metal microparticles, but, as indicated above, they may comprise a core of one or more other metals such as sodium, potassium, magnesium and aluminium, whilst having the same characteristics, properties and advantages as particles having a lithium core, without departing from the scope of the invention.

[0012] It has been found that by significantly increasing the contact surface of the lithium material with the electrolyte, in accordance with the microparticles of the invention compared to an extruded lithium-metal sheet according to the known implementation of Li-metal batteries laminated on a copper sheet, or compared to an electroconductive surface on which micrometric particles of lithium metal are deposited according to the aforementioned document, the current densities are significantly reduced, only leading to the formation of small micro-dendrites which limit the harmful consequences of the dendrites, without however completely resolving the problem.It should also be noted that generally the dendrites grow perpendicular to the surface which generates them: in the case of an anode with a lithium metal sheet, 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 compared to the separator.

[0013] By microparticles is meant 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] In order to increase the performance of the anode, the metal core is crossed by electroconductive nanofibers which will greatly facilitate the transmission of electrons from the core of the particles to the anode current collector. Some of these nanofibers are completely immersed in the metal, others have a part immersed in the metal and another part passing through the protective layer to open outwards, and their multiplicity will cause a multitude of contacts between the core of the metal particles and the anode current collector. The latter is advantageously either metallic or composite comprising a polymer sheet coated with a metallic, carbon or composite electron-conducting material, for example made of carbon or glass fibers coated with conductive metal. Depending on the manufacturing, the nanofibers protrude from the protective layer by a few microns, for example 2 pm or more depending on their length.

[0015] According to a variant, nanofibers having a portion immersed in the metal and a portion passing through the protective layer towards the outside also have a portion emerging from the particles towards 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 electrically conductive nature of the nanofibers may come from one or more materials constituting them or may be provided or reinforced by a treatment, for example a surface treatment. They may thus be made of one or more electrically conductive materials, such as carbon nanofibers, and are optionally treated to increase their electrical conductivity. Alternatively, they may be 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.

[0018] Advantageously, these are carbon fibers of the VGCF (Vapor Grown Carbon Fibers) type. Carbon fibers are naturally electrically conductive, however, in order to increase their power, 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, the appropriate thickness of which is 10-100 nm, better still 20-50 nm, is applied to fibers according to a conventional method known to those skilled in the art such as chemical vapor deposition (CVD) or physical vapor deposition (PVD).

[0019] Ideally, they mostly correspond to 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 metal compound. For the purposes of the performance of a battery of the invention, this protective layer is waterproof and aims to prevent direct contact between the electrolyte and the lithium metal which creates an SEI (Solid Electrolyte Interphase) which is detrimental to the cyclability of the battery due to the high consumption of lithium. It is also very conductive of ions.

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

[0023] The exit of the nanofibers from the Li-metal core by crossing this layer is a vulnerable zone, particularly in terms of sealing. To meet this requirement, the metal compound is advantageously an ionic nitride such as a nitride of the core metal, notably 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 to be one of the best solid conductors of lithium ions. The core can be covered with microparticles by any technique known to those skilled in the art. According to a judicious variant of the invention, the method consists of chemically transforming the lithium metal on the surface of the core into lithium nitride by reacting it with a nitriding agent.It allows complete filling of the interstices formed at the exit of the nanofibers from the core, ensuring perfect sealing of the protective layer including at the sensitive parts, without creating an excess thickness which 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, comes from its strength; it in fact forms a robust shell which will withstand the mechanical stresses of the swelling and deflation of the core of the microparticles caused by the recombination of the lithium metal from the lithium ions emanating from the cathode during the charge / discharge cycles. Cracks can appear knowing that Li3N, although very hard, is brittle. Thanks to the nanofibers of the anode material of the invention, these cracks are avoided; in fact, they are extremely resistant to traction and constitute, in the protective layer, a real network reinforced with the lithium metal and Li3N, which makes it possible to obtain a real robust shell without risk of cracks.

[0025] The thickness of the protective layer results from a compromise between fineness and robustness: it is of 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 the microparticles as described above, in any of the embodiments presented. Thus, the following are included: of the invention: - an anode comprising as active material microparticles of the invention and a current collector material; - a battery comprising such an anode, a cathode compatible with the anode, a separator, a liquid, gel, gellable, solid or solvable electrolyte and external current collectors.

[0027] Thus, the invention also relates to an anode which comprises microparticles as described above in any one of the variants set out, considered alone or in combination and a current collector material. The latter may be constituted by any suitable material that a person skilled in the art will be able to choose. Copper metal strips are commonly used, for example. Advantageously, in the invention, it is constituted by an electrically conductive material which may take the form of a non-woven fabric 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 which comprises an anode whose active material comprises or consists of microparticles as described above, in any of the embodiments 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 a variant, a mixture of the particles of the invention and the electrolyte is deposited on or in the current collector material. This mixture, called semi-liquid ink, is inserted or deposited on or in the 3D porous electrically conductive current collector material to be converted into a solid gel or a solid, by exposure to heat or radiation. The 3D anode current collector is advantageously made of a non-woven fabric of carbon fibers, in particular commercially conductive carbon fibers. In order to increase their electrical conduction, they can be coated with a layer of a titanium compound which is a good conductor of electricity. It can be chosen from titanium nitride (TiN), titanium carbide (TiC) and titanium carbonitride (TiCN).

[0030] The electrolyte, whatever its form, liquid, solid, solidifiable, gel, gellable, contains the elements necessary for its function, namely to circulate the ions. This electrolyte, to ensure ionic conduction, is generally made up of at least one salt, for example, LiPF6, LITFSI, LiBF4, LiClO4 or NaFSI, of 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), .... or / and of at least one ionic liquid, it can be hybrid and made up 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 (EGP) or a polymer solid matrix electrolyte, for example PEO (polyethylene oxide), or even a combination of particles of the invention and compressed solid electrolyte particles. As indicated previously, a superiority of Li-metal anodes compared to Li-ion anodes is the ability to use a solid electrolyte. Very advantageously, a battery of the invention will be a so-called all-solid battery, i.e. operating with an electrolyte that is initially liquid and then solidified. But the use of a liquid or semi-liquid electrolyte is entirely possible.

[0031] According to an advantageous variant, the electrolyte may further contain additional lithium salts, in particular chosen from LiNO3 and LiN3, which will contribute to sealing the micro- and nano-cracks which 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 adjuvant, such as a surfactant, capable of forming around said particles a protective film completely covering the particles of the invention (for example, Li-metal nitrided with Li3N). Since Li3N is also very reactive to oxygen and water and the electrolyte is not very reactive, this second protective film helps to protect the particles from attack by humid air when handling the particles to manufacture a battery of the invention.The adjuvant is preferably liquid and is subsequently solidified thermally or by radiation to form the complementary protective film.

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

[0033] An additional advantage of an anode of the invention is its compatibility with many cathode materials. Thus, one or more suitable cathode materials may be chosen from sulfur, lithium sulfide, titanium disulfide, molybdenum disulfide, titanium disulfide, iron disulfide, the insertion materials LiCoO2 (LCO), LiNixMnyCozO2 (NMC), LiFePO4 (LFP), LiMnO2 (LMO) and LiNiCoAIO2 (NCA). Thus, the invention also relates to a battery comprising an anode of the invention and such a cathode.

[0034] The present invention is described below in more detail with the support of the following figures which illustrate the particles which 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 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 worked to produce spherical microparticles of 3 to 40 microns in diameter. After cooling, the oil is then removed by filtration before successive washes with organic solvents, such as hexane or toluene, or supercritical CO2, continuously recycled, to remove and dissolve the remaining oil. The microparticles are recovered in a container and kept under argon, then dried.

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

[0046] In the same nitriding installation still under N2, or under argon, the lithium-metal cores provided with their protective layer of Li3N are mixed with the electrolyte which is still liquid, taking into account the organic solvents or the ionic liquids which conduct ions, but which can be gelled or solidified, either thermally or by radiation subsequently, 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 humid ambient 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] According to [Fig.2], microparticles comprising a core 1 are observed spherical Li-metal in which VGCF 3 carbon nanofibers are homogeneously embedded, and a protective layer 2 tightly surrounding said core 1. This protective layer 2 is crossed by VGCF 3 carbon nanofibers which open into the electrolyte 4. The current collector is made of carbon fibers 5.

Claims

Claims

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 electroconductive 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 metal of the core and another fraction of the nanofibers having a portion immersed in the metal of the core and another portion passing through the protective layer to the outside.

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

3. Microparticles according to claim 1 or 2, characterized in that the metal of the core is lithium and the metal 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, more preferably 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 waterproof, preventing direct contact of the 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, charac- characterized 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 film of the titanium compound 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 portion immersed in the metal and a portion passing through the protective layer also have a portion emerging from the particles towards the outside.

14. An 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 consists of an electrically conductive material which can take the form of a non-woven fabric of carbon fibers, optionally coated with a layer of a titanium compound chosen from titanium nitride (TiN), titanium carbide (TiC) and titanium carbonitride (TiCN).

16. A secondary battery comprising an anode according to claim 14 or 15, a cathode compatible with the anode, a separator, a liquid, gel, gellable, 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 constituted by a combination 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 even a combination of particles of any claims 1-13 and solid electrolyte particles put into compression.

18. Battery according to claim 16 or 17, characterized in that the electrolyte additionally contains lithium salts chosen in particular 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 chosen from sulfur, lithium sulfide, titanium disulfide, molybdenum disulfide, titanium disulfide, iron disulfide, the insertion materials LiCoO2 (LCO), LiNixMnyCozO2 (NMC), LiFePO4 (LFP), LiMnO2 (LMO) and LiNiCoAIO 2 (NCA).

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