Anode for lithium-ion battery

Abstract

The present invention relates to an anode for a Li-ion secondary battery, which is covered with a protective film based on fluorinated copolymer(s). The present invention also relates to a method for producing this anode. The present invention also relates to a Li-ion secondary battery comprising an anode according to the invention. The present invention finally relates to the use of fluorinated copolymer(s) as a film covering an anode of a lithium-ion battery comprising a negative electrode active material.

Description

Technical Field

[0001] The present invention generally relates to the field of electrical energy storage in Li-ion type lithium secondary batteries. More specifically, the present invention relates to an anode for a Li-ion secondary battery, which includes a protective film based on a fluorinated copolymer(s). The present invention also relates to a method for preparing this anode. Finally, the present invention relates to a Li-ion secondary battery including the anode according to the present invention.

Background Art

[0002] Conventional lithium-ion batteries include a flammable liquid electrolyte based on a solvent and a lithium salt. The use of this type of battery is increasing not only in the field of electronic consumer goods such as computers, tablets or mobile phones (smartphones), but also in the field of transportation, especially train transportation. However, improving the safety and reducing the manufacturing cost of these lithium batteries have become major issues. These batteries use a graphite anode into which Li ions are inserted during charging. The resulting compound is C6Li, that is, 1 Li equivalent per 72 g of graphite.

[0003] It has long been known that the replacement of graphite with a Li metal anode can enable a significant increase in energy density. However, the main problem regarding the use of an anode made of lithium metal is related to the formation of Li dendrites during the reduction of the metal, which can cause low Coulomb efficiency, an increase in battery volume, an acceleration of electrolyte decomposition, perforation of the separator, and thus short circuit and thermal runaway.

[0004] It is possible to stabilize the interface using additives, or to repair the deposits by means of agents acting as shielding agents or by mechanically blocking the growth with ceramics.

[0005] The stabilization of the Li anode would be advantageous in both solvent / salt-based batteries and solid polymer electrolyte (SPE)-based batteries that do not use liquid solvents, avoiding the use of flammable liquid components in conventional Li-ion batteries and enabling the manufacture of thinner and perhaps more flexible batteries.

[0006] Thus, a solution is proposed in the publication “Dendrite-Free Lithium Deposition for Lithium Metal Anodes with Interconnected Microsphere Protection” by Yong-Gun Lee et al., Chem. Mater., 2017, 29(14), pp. 5906-5914. This document describes a Li metal anode protected by microspheres of polystyrene-co-divinylbenzene P(S-DVB). These microspheres induce ions to specific regions and apply mechanical pressure to dendrites during their growth.

[0007] In another approach proposed by Y. Liu and Y. Cui in the publication “Lithium Metal Anodes: A Recipe for Protection” published in Joule 1, 2017, pp. 643-650, a protective layer can be formed during battery operation to form a SEI (solid electrolyte interface). This interface is an ion conductor formed by a mixture of Li2S6 and P2S5 (denoted as LSPS) and complexed to yield specific polymer species in dimethoxyethane (DME). This additive is reduced at the surface of the Li metal, giving a high-density layer of Li3PS4. The amorphous nature of this material is extremely important for good protection. The SEI is uniform and compact without a high defect density.

[0008] The above two methods are complex to implement. The microspehere must have a high degree of uniformity in size and cross-linking to prevent weak regions where dendrites can apply pressure. The structure of Li3PS4 must also be well-controlled to provide the best protection. This control is not easy. Furthermore, this structure is very sensitive to moisture and oxygen, and its handling is difficult.

[0009] In the publication "Dendrte-Free Lithium Deposition via Self-Healing Electrostatic Shield Mechanism" published in J. Am.Chem.Soc.2013,135,135,4450-4456, Ding et al. proposed an electrostatic self-healing mechanism that reduces the formation of lithium dendrites by cations, and the reduction potential of these cations is lower than that of Li / Li + (Rb,Cs). These cations accumulate near the initial Li clusters to create an electrostatic shield. This shield discourages the growth of new lithium atoms on existing clusters. This means that the lithium layer is of better quality and smoother. The disadvantages of this method are the low Coulombic efficiency due to an insufficient SEI (solid electrolyte interface), and the cost and availability of Rb and Cs.

[0010] Another solution for stabilizing the interface with the anode is to coat the electrode with a polymer or solid layer. The publication "Improved Cycling Stability of Lithium ELectrodes in Rechargeable Lithium Batteries" by I.S. Kang et al. published in J. Electrochem.Soc.2014,161,A53-A57 describes the application of a layer of poly(3,4-ethylenedioxythiophene)-co-poly(ethylene glycol) copolymer on a Li metal anode.

[0011] Document KR20110082289 describes a Li-ion battery in which a polymer film containing lithium powder is disposed between an electrode and a separator. The film is prepared by applying a solution in which the lithium metal is dispersed in a polymer binder solution onto the electrode and then performing a drying process. The binder solution is prepared by dissolving at least one of a fluorine-based polymer, an acrylic polymer, SBR (styrene-butadiene) rubber, and a polyacrylonitrile-based polymer in a non-aqueous solvent.

[0012] However, these approaches are based on confinement and do not change the essential behavior of dendrite growth.

[0013] Therefore, it is necessary to provide an anode for a Li-ion secondary battery that is protected by an effective means against dendrite formation and is suitable for industrial implementation.

Prior Art Documents

Patent Documents

[0014]

Patent Document 1

Non-Patent Documents

[0015]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0016] Therefore, an object of the present invention is to solve at least one of the drawbacks of the prior art, namely, the formation of dendrites, which is particularly notable in the case of particularly high charge / discharge rates.

[0017] The present invention also aims to provide a method for manufacturing these anodes stabilized by the deposition of a fluorinated copolymer film. Finally, the present invention aims to provide a Li-ion secondary battery including a stabilized anode.

Means for Solving the Problems

[0018] The present invention relates first to a negative electrode (or anode) for a lithium-ion battery, including a current collector and a layer of a negative electrode active material covered with a film of a fluorinated copolymer. The term "fluorinated copolymer" includes the copolymers and terpolymers described below.

[0019] Characteristically, the film includes at least one of the following copolymers. - A copolymer of formula P(VDF-TrFE) containing vinylidene fluoride units and trifluoroethylene units, or - A copolymer of formula P(VDF-TFE) containing vinylidene fluoride units and tetrafluoroethylene units, or - A fluorinated terpolymer selected from the following - A terpolymer of formula P(VDF-TrFE-CTFE) containing vinylidene fluoride units, trifluoroethylene units and chlorotrifluoroethylene units, - A terpolymer of formula P(VDF-TrFE-CTFE) containing vinylidene fluoride units, trifluoroethylene units and chlorotrifluoroethylene units, - A terpolymer of formula P(VDF-TrFE-HFP) containing vinylidene fluoride units, trifluoroethylene units and hexafluoropropylene units, - A terpolymer of formula P(VDF-TFE-CFE) containing vinylidene fluoride units, tetrafluoroethylene units and 1,1-chlorofluoroethylene units, - A terpolymer of formula P(VDF-TFE-CTFE) containing vinylidene fluoride units, tetrafluoroethylene units and chlorotrifluoroethylene units, - A terpolymer of formula P(VDF-TFE-HFP) containing vinylidene fluoride units, tetrafluoroethylene units and hexafluoropropylene units

[0020] According to various embodiments, the anode has the following characteristics, which are combined as necessary.

[0021] According to one embodiment, the film comprises a blend of a P(VDF-TrFE) copolymer and a P(VDF-TFE) copolymer.

[0022] According to one embodiment, the film comprises a blend of a P(VDF-TrFE) copolymer and one of the terpolymers.

[0023] According to one embodiment, the film comprises a blend of a P(VDF-TFE) copolymer and one of the terpolymers.

[0024] According to one embodiment, the film comprises a blend of two terpolymers selected from those listed above.

[0025] The negative electrode active material is an alkali metal. According to one embodiment, the negative electrode active material is lithium.

[0026] According to one embodiment, the film has a thickness in the range of 1 to 14 μm, preferably 1 to 10 μm, more preferably 2 to 10 μm, including the limit values.

[0027] According to one embodiment, the film has a density in the range of 1.2 to 2 g / cm 3 of.

[0028] The present invention also relates to a method for manufacturing a negative electrode comprising a layer of a negative electrode active material covered with a film of the fluorinated copolymer(s). This method includes a step of depositing or forming a film of the fluorinated copolymer(s) on the surface of the negative electrode active material.

[0029] According to one embodiment, the formation of the film is carried out via a solvent route by evaporating the solvent from a solution of the fluorinated copolymer(s).

[0030] According to one embodiment, the deposition of the film is carried out via a drying route consisting of preparing a fluorinated film on a suitable support and transferring this fluorinated film onto the layer of the negative electrode active material.

[0031] Another main subject of the present invention is a Li-ion secondary battery comprising a negative electrode, a positive electrode and an electrolyte.

[0032] Another main subject of the present invention is - a copolymer of formula P(VDF-TrFE) comprising vinylidene fluoride units and trifluoroethylene units, - a copolymer of formula P(VDF-TFE) comprising vinylidene fluoride units and tetrafluoroethylene units, and - A fluorinated terpolymer selected from a terpolymer of formula P(VDF-TrFE-CFE) containing vinylidene fluoride units, trifluoroethylene units and chlorofluoroethylene units, a terpolymer of formula P(VDF-TrFE-CTFE) containing vinylidene fluoride units, trifluoroethylene units and chlorotrifluoroethylene units, a terpolymer of formula P(VDf-TrFE-HFP) containing vinylidene fluoride units, trifluoroethylene units and hexafluoropropylene units, a terpolymer of formula P(VDF-TFE-CFE) containing vinylidene fluoride units, tetrafluoroethylene units and chlorofluoroethylene units, a terpolymer of formula P(VDF-TFE-CTFE) containing vinylidene fluoride units, tetrafluoroethylene units and chlorotrifluoroethylene units, and a terpolymer of formula P(VDf-TFE-HFP) containing vinylidene fluoride units, tetrafluoroethylene units and hexafluoropropylene units Or a blend of a P(VDF-TrFE) copolymer and a P(VDF-TFE) copolymer, a blend of a P(VDF-TrFE) copolymer and one of the terpolymers, a blend of a P(VDF-TFE) copolymer and one of the terpolymers, and a blend of two terpolymers among those listed above, for use as a film covering a lithium-ion battery anode comprising a negative electrode active material for inhibiting the formation of lithium dendrites in the negative electrode active material.

[0033] The present invention makes it possible to overcome the drawbacks of the prior art. More specifically, the present invention provides an anode stabilized by the presence of a film of fluorinated copolymer(s) deposited on the surface of the negative electrode active material, the film inhibiting the formation of dendrites in the negative electrode active material. The present invention is particularly suitable for the manufacture of lithium-ion secondary batteries, the negative electrode active material of which is lithium. The stability of this electrode improves the performance of the battery and extends its service life.

Mode for Carrying Out the Invention

[0034] The present invention will now be described in more detail and in a non-limiting manner in the following description.

[0035] According to a first aspect, the present invention relates to a negative electrode for a lithium-ion battery including a current collector and a layer of a negative electrode active material covered with a film of a fluorinated copolymer (s). Characteristically, the film is a copolymer of formula P(VDF-TrFE) containing vinylidene fluoride units and trifluoroethylene units, a copolymer of formula P(VDF-TFE) containing vinylidene fluoride units and tetrafluoroethylene units, or a terpolymer of formula P(VDF-TrFE-CFE) containing vinylidene fluoride units, trifluoroethylene units and 1,1-chlorofluoroethylene units, a terpolymer of formula P(VDF-TrFE-CTFE) containing vinylidene fluoride units, trifluoroethylene units and chlorotrifluoroethylene units, a terpolymer of formula P(VDf-TrFE-HFP) containing vinylidene fluoride units, trifluoroethylene units and hexafluoropropylene units, a terpolymer of formula P(VDF-TFE-CFE) containing vinylidene fluoride units, tetrafluoroethylene units and chlorofluoroethylene units, a terpolymer of formula P(VDF-TFE-CTFE) containing vinylidene fluoride units, tetrafluoroethylene units and chlorotrifluoroethylene units, and a terpolymer of formula P(VDf-TFE-HFP) containing vinylidene fluoride units, tetrafluoroethylene units and hexafluoropropylene units, or a blend of one of the copolymers and one of the terpolymers, or a blend of two copolymers, or a blend of two terpolymers.

[0036] The negative electrode active material can be an alkali metal, an alkaline earth metal or an element of the boron group. The negative electrode active material claimed in the present invention is an alkali metal.

[0037] According to various embodiments, the anode has the following characteristics, which are combined as necessary.

[0038] According to one embodiment, the negative electrode active material is lithium.

[0039] According to one embodiment, the film has a thickness in the range of 1 to 14 μm, preferably in the range of 1 to 10 μm, more preferably in the range of 2 to 10 μm, including the limit values. The thickness of the fluorinated film must be limited to avoid causing interfacial impedance that is disadvantageous to the power operation of the battery. In a film that is too thick, the diffusion of lithium becomes slow.

[0040] According to one embodiment, the fluorinated film has a density in the range of 1.2 to 2 g / cm 3 of.

[0041] According to one embodiment, the fluorinated film is a porous film having a pore diameter between 100 nm and 1 μm.

[0042] The presence of a fluorinated film on the layer of the negative electrode active material can prevent or at least greatly reduce the formation of lithium ion dendrites. The targeted copolymer or terpolymer is necessarily polar and is the FE phase (the ferroelectric phase of the copolymer) and / or the RFE phase (the relaxor ferroelectric phase of the terpolymer), which are similar to the beta phase obtained with PVDF under very special conditions (for example, by stretching the film, dissolving it in a specific solvent, and slow evaporation or adding additives (these solutions are not compatible with industrially available methods)). Without wishing to go into technical details, the applicant believes that the polarity of the fluorinated film obtained from the described copolymers and terpolymers is sufficient to enable the orientation of lithium cations in a plane parallel to the current collector and the film of Li metal, rather than in a vertical direction that would favor the formation of dendrites.

[0043] Therefore, the present invention is based on the use of a fluorinated copolymer or terpolymer, or a blend thereof, in the form of a film deposited on the surface of the negative electrode active material. The term "fluorinated" is understood to mean a polymer containing -F groups.

[0044] Preferably, the copolymers and terpolymers are ferroelectric polymers or relaxor ferroelectric polymers. Ferroelectric copolymers or terpolymers have a large hysteresis of the polarization curve (charge vs. applied field) with high retention (typically on the order of 20 V / μm, or even 50 V / μm) and high remanent polarization (typically on the order of 60 mC / m 2 ). Relaxor terpolymers or copolymers have low retention (typically less than 10 V / μm), low remanent polarization (typically less than 20 mC / m 2 ), or no remanent polarization at all, high saturation polarization (typically on the order of 60 mC / m 2 or even 70 mC / m 2 ) and a maximum dielectric permittivity of the dielectric as a function of temperature that depends on the frequency of the electric field.

[0045] Furthermore, another advantage is the higher dielectric permittivity of the copolymers, especially terpolymers, which allows for more significant polarization at the interface with lithium and more significant dissociation of the electrolyte.

[0046] The relative dielectric permittivity of the copolymers and terpolymers at 23°C and 1 kHz is greater than 10. The maximum dielectric permittivity as a function of temperature is at least 30, or even 40. The relative dielectric permittivity can be measured by dielectric spectroscopy.

[0047] The Curie temperature of the P(VDF-TrFE) copolymer is between 50 and 140°C. The Curie temperature of the polymers of the present invention can be measured by differential scanning calorimetry or dielectric spectroscopy.

[0048] According to one embodiment, in a copolymer of the formula P(VDF-TrFE), the proportion of units derived from trifluoroethylene is less than 55 mol% relative to the total of the units derived from vinylidene fluoride and the units derived from trifluoroethylene, and more than 18 mol%.

[0049] According to one embodiment, in the copolymer of formula P(VDF-TFE), the proportion of units derived from tetrafluoroethylene is less than 60 mol% and more than 10 mol% with respect to the total of units derived from vinylidene fluoride and units derived from tetrafluoroethylene.

[0050] The copolymers and terpolymers of the present invention can be produced by using any known method such as emulsion polymerization, microemulsion polymerization, suspension polymerization, and solution polymerization. For terpolymers, it is particularly preferred to use the method described in document WO2010 / 116105. This method makes it possible to obtain high molecular weight and appropriately structured terpolymers.

[0051] According to one embodiment, in the context of this patent application, the weight average molar mass, also referred to as the "molecular weight" (Mw) of the terpolymer, has a value of 200,000 to 1,500,000 g / mol, preferably 250,000 to 1,000,000 g / mol, and more preferably 300,000 to 700,000 g / mol.

[0052] The latter can be adjusted by modifying specific parameters of the method such as the temperature in the reactor or by adding a mobile agent.

[0053] The molecular weight distribution can be estimated by SEC (size exclusion chromatography) using a set of three columns with increasing porosity and dimethylformamide (DMF) as the eluent. The stationary phase is a styrene-DVB gel. The detection method is based on the measurement of the refractive index, and the calibration is performed with polystyrene standards. The sample is dissolved in DMF at 0.5 g / l and filtered through a 0.45 μm nylon filter.

[0054] The molecular weight can also be evaluated by measuring the melt flow index (MFI) at 230 °C under a load of 10 kg according to ASTM D1238 (ISO 1133). The MFI is between 0.1 and 100, preferably between 0.5 and 50, and more specifically between 1 and 10.

[0055] Also, the molecular weight can be characterized by measuring the viscosity in solution according to standard ISO 1628. Methyl ethyl ketone (MEK) is a preferred solvent for the terpolymer for the determination of the viscosity index.

[0056] More generally, the molar composition of the terpolymers of the present invention can be determined by various means. Conventional methods for elemental analysis of carbon, fluorine and chlorine or bromine elements result in a system of two or three independent equations with two independent unknowns (e.g., %VDF and %TrFE, %Y = 100 - (%VDF + %TrFE)), which allows the composition to be clearly calculated by the weight of the polymer, from which the molar composition is inferred.

[0057] Use can also be made of multinuclear (in this example proton ( 1 H) and fluorine ( 19 F)) NMR techniques by analysis of a solution of the polymer in a suitable deuterated solvent. The NMR spectra are recorded on an FT-NMR spectrometer fitted with a multinuclear probe. Next, the specific signals given by the different monomers are located in the spectra produced according to either nucleus. Thus, for example, the TrFE (CFH=CF2) unit gives a specific signal characteristic of the CFH group (around 5 ppm) in proton NMR. The same is true for the CH2 group of VDF (broad unresolved peak centred around 3 ppm). The relative abundance of the two monomers, i.e. the VDF / TrFE molar ratio, is obtained from the relative integral of the two signals.

[0058] Combining the relative integrals of the various signals obtained by proton NMR and fluorine NMR results in a system of equations from which the molar concentrations of the various monomer units are obtained by its resolution.

[0059] Finally, elemental analysis for heteroatoms such as chlorine or bromine can be combined with NMR analysis. Thus, the content of CTFE or CFE can be determined by measuring the chlorine content by elemental analysis.

[0060] Thus, one of ordinary skill in the art can utilize various methods or combinations of methods to determine the composition of the terpolymers of the present invention with certainty and the required accuracy.

[0061] According to one embodiment, the molar ratio of VDF units to TrFE units or TFE units in the terpolymer has a value of 85 / 15 to 30 / 70, preferably a value of 75 / 25 to 40 / 60.

[0062] According to one embodiment, the proportion of units derived from a CFE monomer, an HFP monomer or a CTFE monomer is 1 to 15 mol%, more preferably 1 to 12 mol%, based on all the units of the terpolymer.

[0063] According to one embodiment, the fluorinated copolymer or terpolymer can include additional units introduced during radical polymerization, between 0.1 and 10 mol%, preferably between 0.2 and 8 mol%, and particularly between 0.5 and 5 mol%. These additional units make it possible to improve certain specific properties of the film, such as adhesion, by introducing phosphonate-type functional units such as acids, alcohols, glycidyl, and trifluoromethacrylic acid, without degrading its electroactive properties.

[0064] According to one embodiment, the fluorinated copolymer or polymer can be functionalized. That is, after the polymerization step, it is chemically modified to introduce chemical functional groups that enable crosslinking of the film, for example, by a terpolymer having an azide functional group along the polymer chain, or that improve the adhesion of the film on the electrode, for example, by a terpolymer having associative functional groups such as imidazolidinyl, triazolyl, triazinyl, bisureyl, and ureidopyrimidyl groups.

[0065] Copolymers of the formula P(VDF-TrFE) or P(VDF-TFE) are compatible with the terpolymers listed above and have a Curie temperature different from that of the terpolymers. The term "compatible" is understood to mean that a blend of two polymers forms a homogeneous phase having a single glass transition temperature.

[0066] According to one embodiment, when the fluorinated film deposited on the negative electrode active material is formed of a blend of a copolymer and a terpolymer, they are present in a weight ratio of 50:50 to 1:99, preferably 45:55 to 1:99, more particularly preferably 40:60 to 5:95.

[0067] According to an embodiment, the film also includes a compatible polymer of the acrylic and / or methacrylic type. These have the effect of stabilizing the film, promoting adhesion, or enabling crosslinking.

[0068] According to a second aspect, the present invention relates to a method for manufacturing a negative electrode including a layer of a negative electrode active material covered with a film of the fluorinated copolymer(s). This method includes a step of depositing or forming a film of the fluorinated copolymer(s) on the surface of the negative electrode active material.

[0069] According to one embodiment, the deposition of the film is carried out via a solvent route. The solvent - route method consists of dissolving the fluorinated copolymer and / or the fluorinated terpolymer in a solvent or a mixture of solvents that does not damage the anode, particularly when the anode is made of Li metal. In particular, many solvents, including ketones or esters, damage Li metal by chemical reaction.

[0070] Copolymers and terpolymers based on vinylidene fluoride and trifluoroethylene are soluble in a number of solvents. Unlike PVDF homopolymers, they crystallize easily and give a polar (ferroelectric or relaxor ferroelectric) phase from the solution. In this way, it becomes possible to easily, rapidly, and inexpensively produce a deposit that inhibits dendrite formation.

[0071] The solvents used in the present invention are selected from the group consisting of carbonates, carbamates, nitriles, amides, sulfoxides (such as dimethyl sulfoxide), sulfolane, nitromethane, 1,3-dimethyl-2-imidazolidinone, 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone, 3-methyl-2-oxazolidinone, and mixtures thereof.

[0072] Among the nitriles, for example, acetonitrile, pyruvonitrile, propionitrile, methoxypropionitrile, dimethylaminopropionitrile, butyronitrile, isobutyronitrile, valeronitrile, pivalonitrile, isovaleronitrile, glutaronitrile, methoxyglutaronitrile, 2-methylglutaronitrile, 3-methylglutaronitrile, adiponitrile, malononitrile, and mixtures thereof can be mentioned.

[0073] Among the carbonates, for example, ethylene carbonate (EC) (CAS: 96-49-1), propylene carbonate (PC) (CAS: 108-32-7), butylene carbonate (BC) (CAS: 4437-85-8), dimethyl carbonate (DMC) (CAS: 616-38-6), diethyl carbonate (DEC) (CAS: 105-58-8), methyl ethyl carbonate (EMC) (CAS: 623-53-0), diphenyl carbonate (CAS: 102-09-0), methyl phenyl carbonate (MPC) (CAS: 13509-27-8), dipropyl carbonate (DPC) (CAS: 623-96-1), methyl propyl carbonate (MPC) (CAS: 1333-41-1), ethyl propyl carbonate (EPC), vinylene carbonate (VC) (CAS: 872-36-6), fluoroethylene carbonate (FEC) (CAS: 114435-02-8), trifluoropropylene carbonate (CAS: 167951-80-6) or mixtures thereof can be mentioned.

[0074] These solvents advantageously have a dielectric constant of 1 or more over a temperature range between 0 and 100 °C, preferably between 10 and 80 °C, and advantageously between 15 and 70 °C. The measurement of the dielectric constant of the dielectric can be carried out using a Sefelec LCR 819 LCR meter that enables the measurement of capacitance proportional to the dielectric constant.

[0075] Following the step of depositing a solution of a copolymer and / or a terpolymer and a solvent (s) in film form on the negative electrode active material, a step of evaporating the solvent (drying the film) is carried out. After drying, according to one embodiment, the film has a thickness in the range of 2 to 14 μm, preferably 2 to 10 micrometers, including the limit values.

[0076] According to one embodiment, the deposition of the film is carried out via a drying route that consists of preparing a fluorinated film on a suitable support and transferring this fluorinated film onto a layer of the negative electrode active material. The transfer of the fluorinated film is carried out by any mechanical method, which can be carried out by roller or lamination and is subsequently pressed using a heat source not exceeding 100 °C.

[0077] According to one embodiment, the solution contains a co-crosslinking agent such as a bifunctional or polyfunctional (meth)acrylic monomer, a bifunctional or polyfunctional primary amine, a bifunctional or polyfunctional compound having an azide functional group, an organic peroxide, a bifunctional or polyfunctional allyl compound, etc. at the point of reactive double bonds. After deposition in film form, the method according to the invention includes a step of crosslinking the film thermally between 30 °C and 200 °C, preferably between 50 °C and 180 °C, particularly between 60 °C and 160 °C, or by radiation, preferably by ultraviolet radiation, particularly ultraviolet radiation between wavelengths 250 and 405 nm. After crosslinking the film, the latter is insoluble in some or all of these solvents used to manufacture the solution, and the use of the electrolyte is no longer limited by the solubility of the film, thus giving a special advantage to the crosslinked film.

[0078] Another subject of the present invention is a lithium-ion secondary battery including a negative electrode, a positive electrode, and an electrolyte.

Examples

[0079] The following examples illustrate the scope of the present invention without limitation.

[0080] The samples were heated to 100 °C, maintained at this temperature for 3 minutes, and their thermal histories were erased. Then, after cooling the products to -80 °C, they were heated to 200 °C at a rate between 5 °C / min and 20 °C / min.

[0081] [Example 1 (Comparison)] A button battery was assembled using a Li metal anode, a 25-μm thick PP separator, and a cathode containing 2% PVDF, 5% Ketjen Black, and 93% NMC111 (LiNi 0.33 Mn 0.33 Co 0.32 O2). The electrolyte is 1 M LiFSI as a mixture with EMC / EC (volume ratio 7 / 3).

[0082] Cycling is performed at 2C during charge and discharge between 3 and 4.2 volts over 300 cycles.

[0083] [Example 2] A solution of a P(VDF-TrFE) copolymer with a molar composition of 80% VDF and 20% TrFE in an EC / EMC mixture with a volume ratio of 3 / 7 was prepared. Next, after drying, an amount of liquid sufficient to obtain a 4-micron film was deposited on a Li metal foil inside a glove box. Relatively rapid evaporation of the copolymer solution occurred.

[0084] A button battery was assembled using a Li anode treated with a fluoropolymer, a 25-μm thick PP separator, and a cathode containing 2% PVDF, 5% Ketjen Black, and 93% NMC111 (LiNi 0.33 Mn 0.33 Co 0.32 O2). The electrolyte is 1 M LiFSI as a mixture with EMC / EC (volume ratio 7 / 3).

[0085] Cycling is performed at 2C during charging and discharging between 3 and 4.2 volts over 300 cycles.

[0086] [Example 3] These two button cells were disassembled in a glove box, and the Li anode was examined by a scanning electron microscope (SEM). The anode treated with the fluorinated copolymer of Example 2 has very few dendrite beginnings (the size is less than 50 nm), and the surface density is low.

[0087] The untreated anode from Comparative Example 1 has dendrites, and its surface density is much larger, and the length is between several nm and about 1 micron.

[0088] [Example 4] A dimethoxyethane solution of P(VDF-TrFE-CTFE) terpolymer is prepared. The molar ratio of VDF units to TrFE units is equal to 67%, and the molar ratio of CTFE is equal to 8%. Next, an amount of liquid sufficient to obtain a non-porous 4 μm film after drying is deposited on a Li metal foil in a glove box. Relatively rapid evaporation of the copolymer solution occurs.

[0089] A button cell is assembled using the Li anode treated with the fluoropolymer, a 25 μm thick PP separator, and a cathode containing 2% PVDF, 5% Ketjen Black, and 93% NMC111 (LiNi 0.33 Mn 0.33 Co 0.32 O2). The electrolyte is 1M LiFSI as a mixture with EMC / EC (volume ratio 7 / 3).

[0090] Cycling is performed at 2C during charging and discharging between 3 and 4.2 volts over 100 cycles.

[0091] [Example 5] The button battery of Example 4 is disassembled in a glove box, and the Li anode is examined with a scanning electron microscope (SEM). The anode treated with the fluorinated terpolymer of Example 4 has very few dendrite beginnings (with a size of less than 12 - 13 nm) and a very low surface density. The examination indicates that the terpolymer seems to be more effective than the copolymer of Example 2.

[0092] [Example 6] A 1,3 - dioxolane solution of a P(VDF - TrFE) copolymer with a molar composition of 80% VDF and 20% TrFE is prepared. Next, it is deposited on a glass support, dried for the time necessary to remove the solvent, and then the fluorinated copolymer film is peeled off and mechanically applied to a Li metal piece in a glove box. A pressure of 1 MPa is applied to the assembly at a temperature of 60 °C to obtain an operable fluorinated film - Li anode composite. The Li anode treated with the fluoropolymer, a 25 - μm - thick PP separator, and a cathode containing 2% PVDF, 5% Ketjen Black, and 93% NMC111 (LiNi 0.33 Mn 0.33 Co 0.32 O2) are used to assemble a button battery. The electrolyte is 1M LiFSI as a mixture with EMC / EC (volume ratio 7 / 3). Cycling is performed at 2C in charging and discharging between 3 and 4.2 volts over 100 cycles.

[0093] The button battery is disassembled in a glove box, and the Li anode is inspected with a scanning electron microscope (SEM). The treated anode has very few dendrite beginnings (with a size of less than 50 nm) and has a low surface density.

Claims

1. An anode for a lithium-ion battery, comprising a negative electrode active material, wherein the anode is - A terpolymer of formula P(VDF-TrFE-CFE) containing vinylidene fluoride units, trifluoroethylene units and chlorofluoroethylene units, a terpolymer of formula P(VDF-TrFE-CTFE) containing vinylidene fluoride units, trifluoroethylene units and chlorotrifluoroethylene units, a terpolymer of formula P(VDF-TrFE-HFP) containing vinylidene fluoride units, trifluoroethylene units and hexafluoropropylene units, a terpolymer of formula P(VDF-TFE-CFE) containing vinylidene fluoride units, tetrafluoroethylene units and chlorofluoroethylene units, a terpolymer of formula P(VDF-TFE-CTFE) containing vinylidene fluoride units, tetrafluoroethylene units and chlorotrifluoroethylene units, a terpolymer of formula P(VDF-TFE-HFP) containing vinylidene fluoride units, tetrafluoroethylene units and hexafluoropropylene units, a fluorinated terpolymer selected from At least one terpolymer selected from Or a blend selected from blends of two terpolymers among said terpolymers Covered with a film containing Wherein the negative electrode active material is an alkali metal, Anode.

2. The anode according to claim 1, wherein the negative electrode active material is lithium.

3. The anode according to claim 1 or 2, wherein the film has a thickness in the range of 1 to 14 μm.

4. The film has a density in the range of 1.2 to 2 g / cm ３ The anode according to any one of claims 1 to 3.

5. The anode according to any one of claims 1 to 4, wherein the molar ratio of VDF units to TrFE units or TFE units in the terpolymer has a value of 85 / 15 to 30 / 70.

6. The anode according to any one of claims 1 to 5, wherein the proportion of units derived from CFE monomer, HFP monomer or CTFE monomer is 1 to 15 mol% based on all units of the terpolymer.

7. A method for manufacturing a negative electrode comprising a layer of a negative electrode active material covered with a film of a fluorinated terpolymer according to any one of claims 1 to 6, the method comprising the step of depositing a film of the fluorinated terpolymer on the surface of the negative electrode active material.

8. The method according to claim 7, wherein the deposition of the film is carried out by a solvent route.

9. The method according to claim 7, comprising the step of dissolving the fluorinated terpolymer according to any one of claims 1 to 6 in a solvent selected from the group consisting of carbonates, carbamates, nitriles, amides, sulfoxides, sulfolanes, nitromethane, 1,3-dimethyl-2-imidazolidinone, 1,3-dimethyl-3,4,5,6-tetrahydro-2(1,H)-pyrimidinone, 3-methyl-2-oxazolidinone, and mixtures thereof.

10. The method according to claim 7, wherein the deposition of the film is carried out by a drying path.

11. A lithium-ion secondary battery having an anode, a positive electrode, and an electrolyte according to any one of claims 1 to 6.

12. - A terpolymer of the formula P(VDF-TrFE-CFE) containing vinylidene fluoride units, trifluoroethylene units, and chlorofluoroethylene units, a terpolymer of the formula P(VDF-TrFE-CTFE) containing vinylidene fluoride units, trifluoroethylene units, and chlorotrifluoroethylene units, a terpolymer of the formula P(VDF-TrFE-HFP) containing vinylidene fluoride units, trifluoroethylene units, and hexafluoropropylene units, a terpolymer of the formula P(VDF-TFE-CFE) containing vinylidene fluoride units, tetrafluoroethylene units, and chlorofluoroethylene units, a terpolymer of the formula P(VDF-TFE-CTFE) containing vinylidene fluoride units, tetrafluoroethylene units, and chlorotrifluoroethylene units, a terpolymer of the formula P(VDF-TFE-HFP) containing vinylidene fluoride units, tetrafluoroethylene units, and hexafluoropropylene units selected from terpolymers selected from or a blend selected from blends of two of said terpolymers Use as a film for covering an anode for a lithium-ion battery containing a negative electrode active material, for inhibiting the formation of lithium dendrites in the negative electrode active material.