NEW GENERATION LITHIUM-ION BATTERY AND THE ASSOCIATED MANUFACTURING PROCESS

Description

technical field

[0001] The present invention belongs to the technical field of new generation high-energy batteries.

[0002] The invention relates to a sulfur-based cathode for lithium-ion batteries, known as LIB. The invention further relates to a sulfur-based cathode for lithium-sulfur batteries.

[0003] Finally, the invention relates to the manufacturing process of such a cathode and such a battery. Prior art

[0004] Among the various systems for storing energy in chemical form, the lithium-ion battery, or LIB, has become dominant over the last thirty years due to the energy and power it can deliver. This technology has developed extensively and is used in almost all portable electronic applications as well as in the latest electric vehicles, to the detriment of other rechargeable batteries.

[0005] Until now, most commercial lithium-ion batteries have used graphite as the anode material. Graphite has the advantage of being an inexpensive and very abundant material. It also has high electrical conductivity and a theoretical specific capacity of 370 mAh / g.

[0006] Cathodes are primarily found in layered lithium-ion batteries of the LiMo2 type, where M is a metal such as nickel, iron, cobalt, manganese, or a mixture of these metals. For example, olivine-based cathodes (LiMPO4) or spinel-based cathodes (LiM2O4) are commonly used. Among these cathode materials, LiNi1 / 3Mn1 / 3Co1 / 3O2 is the most widely used due to its lower cost, lower toxicity, and improved cycling stability, even at high temperatures. This cathode has a theoretical specific capacity of approximately 100 mAh / g.

[0007] Today, LIB technology has reached 87% of its commercially feasible energy density limit; acquiring the remaining 13% will be costly and slow with very limited returns on investment.

[0008] The current environmental and energy context and the limitations of Lithium-ion batteries are motivating the search for new battery technologies that are less expensive, have a smaller ecological impact, do not require the use of problematic and increasingly scarce metals such as cobalt, and deliver higher power and energy densities.

[0009] To increase the energy density of these electrochemical systems, one solution is to maximize lithium storage in the host materials. This can be achieved by increasing the active material per unit area and by using materials with high storage capacity. This involves increasing the capacity, namely the number of charges (mAh) per unit weight and / or volume.

[0010] One of the leading and most promising technologies being considered for the next generations of batteries remains Lithium Silicon-Sulfur technology.

[0011] Sulfur (S) and Silicon (Si) are two attractive electrode materials for the next generation of batteries because of their abundance, high theoretical capacity and low cost.

[0012] For the positive electrode also called cathode, Sulfur has the advantage of a high specific capacity (1675 mAh / g), six to ten times greater than that of the cathodes currently used in commercial batteries.

[0013] For the negative electrode, also called the anode, Silicon has the advantage of a high specific capacity (3579 mAh / g), ten times greater than that of the anodes currently used in commercial batteries.

[0014] However, during the discharge and charge cycle, the insertion of lithium into sulfur and silicon causes significant volumetric and therefore morphological variations of approximately 80% and 300%, respectively. These variations induce strong mechanical stresses leading to the pulverization of the active material, resulting in a loss of capacity.

[0015] Furthermore, for the Sulfur cathode, the reduction of elemental Sulfur leads to the formation of an intermediate chain of Lithium polysulfides (Li2Sx, 3 <x<8) qui sont solubles dans les électrolytes organiques usuels, provoquant une dégradation de la capacité à long terme, ralentissant la cinétique de réaction pendant l'exécution et l'autodécharge de la batterie.

[0016] Another problem with Sulfur and Silicon electrodes lies in their low electronic conductivity, which generally requires the addition of a significant amount of electronic conductor to ensure proper electronic percolation of the electrode.

[0017] This constitutes a technological hurdle because maintaining the nominal specific capacitance of such electrodes remains a major unsolved problem.

[0018] Current commercial electrode manufacturing processes require the use of polymeric conductive binders and / or carbon-based conductive additives. These binders and additives involve the use of polluting and toxic chemical compounds.

[0019] The present invention aims to overcome, at least in part, the disadvantages of using sulfur and silicon in next-generation batteries.

[0020] One aim of the invention is, moreover, to propose a sulfur-based cathode: exhibiting a higher specific energy than state-of-the-art sulfur-based cathodes, and / or having a longer lifespan than state-of-the-art sulfur-based cathodes, and / or not reducing, or reducing little or less than state-of-the-art sulfur-based cathodes, during successive charge and discharge cycles, the charging efficiency and / or the coulombic efficiency of a battery in which such a cathode is used, and / or not causing, or reducing little or less than state-of-the-art sulfur-based cathodes, during successive charge and discharge cycles, self-discharge of the accumulator and / or irreversible loss of capacity of a battery in which such a cathode is used.

[0021] One aim of the invention is, moreover, to propose a lithium silicon-sulfur type battery: aiming to overcome, at least in part, the disadvantages of LIBs, and / or having a specific energy greater than 270 Wh / kg and / or having a lifespan of at least 2000 charge / discharge cycles.

[0022] One aim of the invention is, moreover, to propose a method for manufacturing a lithium silicon-sulfur type battery: aiming to overcome, at least in part, the drawbacks of the prior art LIB manufacturing process, and / or not requiring the use of binder or additive during electrode manufacturing. Presentation of the invention

[0023] For this purpose, a cathode for Lithium-Ion batteries is proposed, comprising: a layer of a conductive material, preferably aluminum, arranged to collect the current flowing through the cathode, called the cathode substrate; a layer of carbon nanotubes (CNTs) aligned in electrical contact with the cathode substrate and extending, preferably from one of their two ends, primarily perpendicular to the cathode substrate; solid sulfur that coats, at least partially, an outer wall of the CNTs; and a solid layer of lithium sulfate (Li₂SO₄), called the outer Li₂SO₄ layer, covering, at least partially, preferably completely, the CNT layer so as to form a stack of layers in which the CNT layer is situated between the cathode substrate and the outer Li₂SO₄ layer; and / or solid lithium sulfate (Li₂SO₄) encapsulating and / or enveloping, at least partially part, the solid Sulfur that lines the external walls of the CNTs.

[0024] Preferably, solid sulfur forms a continuous or discontinuous layer and / or nanoparticles. The layer and / or the solid sulfur nanoparticles have a thickness or, respectively, a diameter of less than 20 nm, preferably less than 10 nm. Preferably, a solid sulfur thickness of less than 20 nm does not significantly increase the strength of the layer and / or the solid sulfur nanoparticles. Preferably, this effect is enhanced for a sulfur thickness of less than 10 nm.

[0025] Preferably, Li2SO4 is permeable to the electrolyte.

[0026] Preferably, Li2SO4 is impermeable to polysulfides.

[0027] Preferably, the outer layer of Li₂SO₄ and the Li₂SO₄ covering the solid sulfur have the effect of retaining or sequestering the polysulfides, formed by the reduction of solid sulfur when the cathode is operating, within the cathode. In other words, the outer layer of Li₂SO₄ and the Li₂SO₄ covering the solid sulfur have the effect of preventing the polysulfides from diffusing out of the cathode.

[0028] Preferably, Lithium Sulfate (Li 2 SO 4) further coats and / or envelops, at least in part, preferably only in part, the NTCs lining the external walls of the NTCs.

[0029] Preferably, the outer layer of Li 2 SO 4 has a thickness of less than 10 nm, preferably less than 8 nm, and / or greater than 3 nm.

[0030] Li₂SO₄, encapsulating at least partially solid sulfur and / or enveloping at least partially CNTs, can form a continuous or discontinuous film, called the internal Li₂SO₄ film layer. The internal Li₂SO₄ film can extend, at least partially, into the CNT layer. Preferably, the internal film is a thin film. The thin film may have a thickness of less than 10 nm, preferably less than 8 nm.

[0031] Preferably, solid sulfur can be any of the known allotropic forms of solid sulfur. Solid sulfur can have the formula Sn, where n is the number of sulfur atoms. Preferably, solid sulfur can have a cyclic structure. By way of non-limiting example, solid sulfur can have the formula S6, S8, S9, S10, S12, S18, or S20. Preferably, solid sulfur has the formula S8.

[0032] According to the invention, a layer can be understood to be a film.

[0033] Preferably, the NTC layer covers and / or is in contact, preferably direct, with the cathode substrate. Preferably, the cathode substrate is covered and / or is in contact, preferably direct, with a face, surface, or side of the NTC layer located on the opposite side to the face, surface, or side of the NTC layer in contact with the outer Li2SO4 layer.

[0034] Preferably, the cathode does not include polymer binder or carbon additives.

[0035] Preferably, the Li2SO4 layer has the effect of preventing the diffusion of Sulfur out of the cathode and / or the formation of polysulfide intermediate products.

[0036] Preferably, the solid layer of Li2SO4 covers and / or is in contact, preferably directly, with the other end of the NTCs and / or with a face or surface or side, called the external face or surface or side, of the NTC layer located on the opposite side to the face or surface or side of the NTC layer in contact with the cathode substrate.

[0037] The Li2SO4 layer can form a coating around the NTC layer.

[0038] Preferably, the Li₂SO₄ solid layer forms an external layer or interface of the cathode. Preferably, the external layer or surface formed by the Li₂SO₄ solid layer is intended to be in contact with a porous element or an electrolyte-soaked separator in a lithium-ion cell.

[0039] Preferably, the Li2SO4 layer has a thickness greater than 25 nm, preferably even more than 50 nm, more preferably than 75 nm and even more preferably than 100 nm and / or less than 2000 nm, preferably even more than 1000 nm, more preferably than 750 nm and even more preferably than 500 nm.

[0040] Preferably, the cathode NTCs: have a length greater than 10 µm, preferably 30 µm and / or less than 500 µm, preferably 250 µm, and / or comprise between two and ten walls and / or preferably have a diameter between 4 and 16 nm, preferably have between three and five walls and / or preferably have a diameter between 5 and 8 nm, and / or have a density between 1.10 11< and 1.10 12< tubes / cm 2< .

[0041] The effect of NTCs is to increase the specific surface area of ​​the electrode and / or to ensure rapid charge transfer between the conductive material layer and the sulfur nanoparticles and / or to give the electrode better resistance to volume expansion without undergoing degradation and / or to provide a highly conductive physical support.

[0042] Preferably, a mass charge of Sulfur of the S 8 Sulfur nanoparticles is greater than 1.5 mg / cm 2< .

[0043] In the case of solid sulfur particles, the particles will swell and shrink significantly during successive charge / discharge cycles. According to the invention, the mass charge of sulfur nanoparticles is arranged so that the sulfur nanoparticles are sufficiently spaced due to their swelling. During discharge, the solid sulfur dissolves in the electrolyte and then undergoes reduction, ultimately precipitating as lithium sulfide (Li₂S) in a cathode. The precipitation of Li₂S occurs at the interface with the electrolyte. The partial molar volume of Li₂S is 2.768 × 10⁻⁵ m³ / mol, while the partial molar volume of solid sulfur (S₈) is 1.239 × 10⁻⁴ m³ / mol. Considering that each mole of solid Sulfur can dissociate into eight moles of Li2S, the complete conversion of one mole of S8 into Li2S occupies 76% more space.The low density of sulfur nanoparticles means that their size varies little during successive charge / discharge cycles.

[0044] According to the invention, a Lithium-Ion battery, called LIB, is also proposed, comprising the cathode according to the invention.

[0045] Preferably, LIB does not include polymer binder or carbon additives.

[0046] Preferably, the LIB according to the invention is said to be "new generation".

[0047] The coupling of these two high-capacity electrodes, a Silicon electrode (3579 mAh / g) and a Sulfur electrode (1675 mAh / g), makes it possible to obtain an average cell voltage of 2.15 V and a theoretical specific energy of 1982 Wh / kg (based on the two electrodes), which is an order of magnitude higher than that of traditional LIBs.

[0048] Preferably, the LIB includes: an anode based on, or comprising, Silicon nanoparticles, preferably grafted onto CNTs, a porous element disposed between the cathode and the anode.

[0049] Preferably, the anode does not contain polymer binder or carbon additives.

[0050] Preferably, the silicon nanoparticles are composed of amorphous silicon. Preferably, the silicon nanoparticles have a size greater than 1 nm, preferably 2 nm, preferably even greater than 5 nm and / or less than 50 nm, preferably 40 nm, preferably even greater than 30 nm.

[0051] Preferably, the porous element is a membrane. The porous element can be formed from a stack of two distinct layers.

[0052] Preferably, the LIB anode includes: a layer of a conductive material, preferably copper, arranged to collect the current flowing through the anode, called the anode substrate, a layer of Carbon Nanotubes (CNTs) aligned in electrical contact with the anode substrate and extending, preferably from one of their ends, mainly perpendicular to said anode substrate, Silicon nanoparticles which coat, at least, preferably only, the outer wall of at least a part of the CNTs, preferably the outer wall of the CNTs.

[0053] Preferably, the NTC layer covers and / or is in contact, preferably direct, with the anode substrate. Preferably, the anode substrate is covered and / or is in contact, preferably direct, with a face, surface, or side of the NTC layer located on the opposite side to the face, surface, or side—referred to as the external face, surface, or side—of the NTC layer intended to be in contact with the porous element.

[0054] Preferably, the porous element covers and / or is in contact, preferably direct, with the other end of the NTCs and / or with the outer face of the NTC layer located on the opposite side to the face or surface or side of the NTC layer in contact with the cathode substrate.

[0055] Preferably, the size of the Silicon nanoparticles is greater than 2 nm, preferably even more than 5 nm and / or is less than 50 nm, preferably even more than 40 nm and even more preferably than 30 nm.

[0056] Preferably, the silicon mass charge of the silicon nanoparticles in the anode is greater than or equal to 2 mg / cm². Preferably, the silicon mass charge of the silicon nanoparticles in the anode is greater than or equal to 0.01 mg / cm², more preferably 0.1 mg / cm², more preferably 1 mg / cm², more preferably 2 mg / cm², even more preferably 2.5 mg / cm², particularly preferably 3 mg / cm², most preferably 3.5 mg / cm², and most preferably 4 mg / cm². Preferably, the silicon mass charge of the silicon nanoparticles in the anode is less than or equal to 5 mg / cm². Preferably, the mass charge of Silicon of the Silicon nanoparticles of the anode is equal to 4.5 mg / cm 2< .

[0057] According to the invention, a method for manufacturing a cathode is also proposed, preferably a cathode according to the invention, more preferably a cathode for, or intended to be integrated into, a Lithium-Ion Battery (LIB), more preferably into an LIB according to the invention. The method comprises the steps of: to obtain, or provide, an electrode, referred to as the electrode obtaining step, comprising a layer of a conductive material, arranged to collect the current flowing through the cathode, referred to as the electrode substrate, from which extends, mainly perpendicularly, a layer of carbon nanotubes (CNTs) aligned in electrical contact with the electrode substrate, then to coat or deposit on a face or surface or a side of the CNT layer, located on the side opposite a face or surface or a side, referred to as the outer face or surface or side, of the CNT layer in electrical contact with the electrode substrate, with a solution, referred to as the sulfur solution, comprising solid sulfur to form a coating, or to coat or coat or graft or line or form a coating, with solid sulfur which lines, at least part of, an outer wall of the CNTs,then cover or deposit on the outer face of the NTCs layer a solution comprising dissolved Lithium Sulfate (Li2SO4) to form a solid layer of solid Lithium Sulfate (Li2SO4) covering the NTCs layer.

[0058] Preferably, the step of coating the face of the NTC layer, located on the opposite side to the face of the NTC layer in electrical contact with the electrode substrate, with the Li2SO4 solution can consist of depositing a drop or a film or a strand of the Li2SO4 solution on the face of the NTC layer, located on the opposite side to the face of the NTC layer in electrical contact with the electrode substrate.

[0059] Preferably, the Li2SO4 solution is an aqueous solution of Li2SO4.

[0060] Preferably, the process includes, prior to the step of forming the solid Li₂SO₄ layer, a step of drying the sulfur solution coating, at least partially, the outer wall of the NTCs. Preferably, the process includes, prior to the step of drying the sulfur solution, a step of rinsing the cathode, preferably a step of rinsing the sulfur solution deposited on the outer face of the NTC layer.

[0061] Preferably, the process includes, subsequent to the step of coating the face of the NTC layer with the solution comprising dissolved Li2SO4, a step of drying the Li2SO4. Preferably, the process includes, prior to the step of drying the Li2SO4.

[0062] Preferably, the step of covering the outer face of the NTCs layer with the sulfur solution and / or with the solution comprising dissolved Li 2 SO 4 includes a step of penetrating said solutions into the NTCs layer towards the substrate, preferably even down to the substrate.

[0063] Preferably, the step of penetrating the sulfur solution into the NTC layer has the effect of encapsulating and / or enveloping, at least in part, the solid sulfur that lines the external walls of the NTCs with solid lithium sulfate (Li2SO4).

[0064] Preferably, the sulfur solution further comprises a solid sulfur powder, which may contain or be made up of solid sulfur nanoparticles, a polar solvent, preferably aprotic or protic, preferably isopropyl alcohol and / or N-methyl-2-pyrrolidone, and carbon disulfide.

[0065] These solvents are given as examples. There are other suitable solvents that a person skilled in the art will be able to choose.

[0066] Preferably, the volume ratio, VSCS / VSP, between a volume of carbon disulfide, denoted VSCS, and a volume of polar solvent, denoted VSP, is between 10 and 30%. Preferably, the volume ratio VSCS / VSP is greater than or equal to 10%, preferably 12%, preferably 14%, preferably 15%, more preferably 16%, more preferably 18%, and most preferably 20%. Preferably, the volume ratio VSCS / VSP is less than or equal to 30%, preferably 28%, preferably 26%, preferably 25%, more preferably 24%, more preferably 22%, and most preferably 20%.

[0067] Preferably, the process for manufacturing a cathode includes the step, known as plasma treatment, of treating the CNTs of the electrode, preferably to treat the electrode, with a cold or athermal plasma and / or water vapor; the plasma treatment step is carried out prior to the step of coating the CNT layer with the sulfur solution and subsequent to the step of obtaining the electrode.

[0068] The plasma treatment step creates defects on the outer wall of the CNTs. For example, the treatment step can be performed by ion irradiation.

[0069] Preferably, the plasma treatment step further includes an acceleration, by polarization, of all or part of the species present in the plasma towards the electrode.

[0070] Preferably, the electrode preparation step includes a step of synthesizing the NTCs on the electrode substrate by hot filament chemical vapor deposition, said synthesis step comprising: the arrangement of the electrode substrate equidistant from four aligned hot filaments, preferably longitudinally, a gas sweep, including precursors parallel to the hot filaments.

[0071] Preferably, the gas comprising the precursors includes gaseous hydrogen and gaseous carbonaceous precursors, such as, for example, methane and / or acetylene, ethylene, propan-2-ol, and carbon monoxide.

[0072] Preferably, the process includes, prior to the NTC synthesis step, a step of depositing a layer, called a barrier layer, with a thickness of between 5 and 80 nm, preferably between 10 and 50 nm, of Aluminium oxide (Al 2 O 3) deposited on the electrode substrate and a layer of Iron, called a catalyst layer, deposited on the barrier layer, with a thickness of between 1 and 30 nm.

[0073] The manufacturing process for the cathode according to the invention is particularly suitable, preferably even specially designed, for implementing the cathode according to the invention. Thus, any feature of the manufacturing process for the cathode according to the invention can be integrated into the cathode according to the invention and vice versa.

[0074] According to the invention, a method for manufacturing a Lithium-Ion battery, referred to as LIB, is also proposed, comprising the steps of: implement the manufacturing process of a cathode according to the invention, obtain or supply an anode for the LIB, assemble the LIB by interposing a porous element between the cathode and the anode.

[0075] Preferably, the manufacturing process of a LIB includes the step of sealing, in an oxygen-free medium, the cathode, the porous element and the anode to form the LIB.

[0076] Preferably, the manufacturing process of a LIB includes the step of soaking the porous element with electrolyte.

[0077] The manufacturing process for the LIB according to the invention is particularly well-suited, and preferably specially designed, for implementing the LIB according to the invention. Thus, any feature of the manufacturing process for the LIB according to the invention can be incorporated into the LIB according to the invention, and vice versa. Description of the figures

[0078] Other advantages and features of the invention will become apparent upon reading the detailed description of implementations and embodiments, which are by no means limiting, and the following attached drawings: [ Fig. 1 ] there FIGURE 1 illustrates three scanning electron microscopy images of three nanostructured current collectors according to the invention, whose vertically aligned carbon nanotube mats have thicknesses of 30, 60 and 150 µm, [ Fig. 2 ] there FIGURE 2 is a transmission electron microscopy image of carbon nanotubes of an embodiment of a cathode in which the carbon nanotubes are coated with solid sulfur, and a schematic representation of such an electrode in which the carbon nanotubes are coated with solid sulfur, [ Fig. 3 ] there FIGURE 3is a scanning electron microscopy image of a portion of the carbon nanotube film coated with silicon nanoparticles of an embodiment of an anode according to the invention and a schematic representation of such an anode, [ Fig. 4 ] there FIGURE 4 is a transmission electron microscopy image of carbon nanotubes of an embodiment of a cathode in which the carbon nanotubes are coated with a solid layer of solid Lithium Sulfate and a schematic representation of a cathode according to the invention, [ Fig. 5 ] there FIGURE 5 is a schematic representation of a Lithium-ion battery according to the invention, [ Fig. 6 ] there FIGURE 6 is a graph illustrating the evolution of the specific capacitance and surface capacitance of cathode 1 against metallic lithium according to the invention for 500 discharge cycles at a discharge rate of C / 3, [ Fig. 7 ] there FIGURE 7is a graph illustrating the evolution of the specific capacitance and surface capacitance of cathode 1 against metallic lithium according to the invention for 1000 discharge cycles at a discharge rate of 2C, [ Fig. 8 ] there FIGURE 8 is a graph illustrating the specific capacity of anodes 3 according to the invention having different mass charges of Silicon, against metallic lithium, for 6 discharge cycles at a discharge rate of C / 20, [ Fig. 9 ] there FIGURE 9 is a graph illustrating the evolution of the specific capacity and Coulombic efficiency of anode 3 against metallic lithium according to the invention for 2000 discharge cycles at a discharge rate of C / 20 and C / 5, [ Fig. 10 ] there FIGURE 10 is a graph illustrating the specific capacity and surface capacity of a Lithium-ion battery according to the invention at a charge rate of C / 20, [ Fig. 11 ] there FIGURE 11is a graph illustrating the energy density of a Lithium-ion battery according to the invention at a charge rate of C / 20. Description of the implementation methods

[0079] The embodiments described below are not exhaustive; variants of the invention may include, in particular, a selection of the described features, isolated from the other described features (even if this selection is isolated within a sentence containing these other features), provided that this selection of features is sufficient to confer a technical advantage or to differentiate the invention from the prior art. This selection includes at least one feature, preferably a functional feature without structural details, or with only a portion of the structural details if this portion alone is sufficient to confer a technical advantage or to differentiate the invention from the prior art.

[0080] With reference to FIGURES 1 to 11 , an embodiment of the invention is described.

[0081] With reference to FIGURES 1 to 5A cathode 1 according to the invention for Lithium-Ion Batteries (LIBs) is presented. The cathode 1 comprises a layer 41 of a conductive material arranged to collect the current flowing through the cathode 1, referred to as the substrate 41 of the cathode 1. In this embodiment, the conductive material is aluminum. The cathode comprises a layer 51 of carbon nanotubes (CNTs) 9 aligned in electrical contact with the substrate 41 of the cathode 1. The CNTs 9 extend, from one of their ends, referred to as the proximal end, primarily perpendicularly to the substrate 41 of the cathode 1. The CNTs 9 according to the invention are multi-walled and comprise at least one wall.

[0082] With reference to FIGURES 1 to 5 The NTCs 9 comprise between three and five walls with a diameter between 5 and 8 nm. The NTCs 9, according to the embodiment, have a length between 30 and 200 µm.

[0083] Cathode 1 comprises solid Sulfur 61 which lines, at least in part, an outer wall of the NTCs 9. As illustrated on the FIGURE 2 Solid Sulfur 61 can form a discontinuous thin film. However, depending on the deposition conditions, it can also form a nearly continuous film. Solid Sulfur 61 has a thickness of approximately 5 to 10 nm, depending on the embodiment. The thinness of Solid Sulfur 61 on the surface of the CNTs 9 makes it difficult to image by scanning electron microscopy. According to a non-limiting embodiment, Solid Sulfur 61 is Sulfur S 8 or cyclooctasulfur, forming a discontinuous thin film.

[0084] The cathode 1 also includes a solid layer 7 of solid Lithium Sulfate (Li2SO4), referred to as the outer layer 7 of Li2SO4, covering the layer 51 of NTCs9 so as to form a stack of layers in which the layer 51 of NTCs9 is located between the substrate 41 of cathode 1 and the outer layer 7 of Li2SO4. The outer layer 7 covers the other end, referred to as the distal end, of the NTCs9. This layer 7 has the effect of preventing the diffusion of Lithium polysulfides, formed by the reduction of elemental Sulfur, into the electrolyte during cycling, away from the cathode 1, when the latter is operating within a LIB. Advantageously, according to the embodiment, the solid layer 7 is continuous and covers the layer 51 of NTCs 9. The thickness of the outer layer 7 is estimated to be between 1000 and 2000 nm according to the embodiment.

[0085] Although, by adjusting the Li₂SO₄ deposition conditions, cathode 1 may comprise only the outer layer 7 of Li₂SO₄, cathode 1 preferably also comprises Li₂SO₄, referred to as a protective film 8, encapsulating and / or enveloping, at least partially, the solid sulfur 61 that lines the outer walls of the NTCs 9. The protective film 8 has an estimated thickness of between 3 and 8 nm, depending on the embodiment. The protective film 8 of Li₂SO₄ prevents the diffusion of lithium polysulfides into the electrolyte contained in cathode 1 when the latter is operating within a LIB.

[0086] With reference to the schematic representation of the FIGURE 4The protective film 8 of Li₂SO₄ is shown as forming a continuous layer completely coating the solid sulfur 61 and the NTCs 9. However, depending on the deposition conditions of the Li₂SO₄, the protective film 8 of Li₂SO₄ can be quasi-continuous or discontinuous. It can form solid plates 8 of Li₂SO₄ or a discontinuous solid film 8 of Li₂SO₄ partially coating the NTCs 9 and partially coating the solid sulfur 61. In this case, although offering less protection against the diffusion of lithium polysulfides into the electrolyte, its effect is still significant. The discontinuous solid film 8 of Li₂SO₄, partially coating the CNTs 9 and partially the solid Sulfur 61, has the effect of encapsulating and / or enveloping, at least partially, the solid Sulfur 61 that lines the external walls of the CNTs 9. With reference to the transmission electron microscopy image of the FIGURE 4, we can observe the protective film 8 of Li 2 SO 4 forming a continuous layer completely coating the NTCs 9.

[0087] The cathode 1 according to the invention has a mass charge of solid Sulfur 61 which is greater than 1.5 mg / cm². The mass charge of solid Sulfur 61 of the cathode 1 illustrated in the FIGURE 2is estimated at 2 mg / cm². The mass loading is adjustable by modifying the implementation conditions of cathode 1 as described in the section relating to the process according to the invention. Increasing the mass loading improves the performance of cathode 1 and therefore of a LIB comprising such a cathode 1. There is an upper limit to the mass loading per unit area that allows for improved performance of cathode 1. This limit is determined by the density of the NTCs 9 on the substrate 41, i.e., the distance between the NTCs 9, the length and / or thickness of the NTCs 9, and by the volumetric expansion of the solid sulfur 61 during the charging / discharging cycles.

[0088] With reference to the FIGURE 6The evolution of the specific capacity and surface capacity of cathode 1 according to the invention is presented for 500 discharge cycles at a discharge rate of C / 3. It is noted that cathode 1 exhibits a specific capacity stabilizing at a value greater than 500 mAh.g⁻¹. The performance of cathode 1 according to the invention, for a discharge rate of C / 3, is therefore 2.6 times greater than commercial Li-ion batteries. With reference to the FIGURE 7 The evolution of the specific capacity and surface capacity of cathode 1 according to the invention is presented for 1000 discharge cycles at a discharge rate of 2C. It is observed that cathode 1 exhibits a specific capacity stabilizing at a higher value of approximately 400 mAh.g⁻¹. The performance of cathode 1 according to the invention, for a discharge rate of 2C, is therefore 2.2 times greater than that of commercial Li-ion batteries.

[0089] Depending on the embodiment, with reference to the FIGURE 3 , it is also described an anode 3 based on 62 silicon nanoparticles (NPs).

[0090] The anode 3 comprises a layer 42 of a conductive material arranged to collect the current flowing through the anode 3, referred to as the substrate 42 of the anode 3. The conductive material is copper in this embodiment. The anode 3 comprises a layer 52 of NTCs 9 aligned in electrical contact with the substrate 42 of the anode 3. The NTCs 9 extend, from one of their ends, primarily perpendicularly to the substrate 42 of the anode 3.

[0091] The NPs 62 of the anode 3 line the outer wall of the NTCs 9. The size of the Silicon NPs 62 is between 5 and 30 nm depending on the embodiment.

[0092] The anode 3 according to the invention preferably has a silicon mass charge greater than 2 mg / cm². The mass charge is adjustable by modifying the operating conditions of the anode 3 as described in the section relating to the process according to the invention. With reference to the FIGURE 8 The specific capacity of anodes 3 according to the invention, having silicon mass densities of 0.2, 2.5, and 4 mg / cm², is illustrated. It is observed that increasing the mass density improves the specific capacity of anode 3 and therefore its performance, and thus that of a LIB 10 comprising such anode 3. As mentioned for cathode 1, there is an upper limit to the mass density per unit area that allows for improved performance of anode 3.

[0093] With reference to the FIGURE 9The evolution of the specific capacity and Coulombic efficiency of anode 3 according to the invention, having a silicon mass charge of 0.03 mg / cm², is presented for 2000 discharge cycles at a discharge rate of C / 20 and C / 5. It is observed that anode 3 exhibits a specific capacity stabilizing at a value greater than 1250 mAh.g⁻¹. The Coulombic efficiency exhibits a stable value of approximately 100%. The performance of anode 3 according to the invention, for a discharge rate of C / 5, is therefore 3.6 times greater than that of commercial Li-ion batteries.

[0094] Depending on the embodiment, with reference to the FIGURE 5 A LIB 10 comprising cathode 1 according to the invention is also described. Although cathode 1 according to the invention can be mounted in a LIB with any anode, according to the embodiment, cathode 1 is mounted with anode 3.

[0095] According to the embodiment, the LIB 10 is a CR2032 button cell comprising a porous element 111, 112 formed by a stack 11 of two layers 111, 112 interposed between the anode 3 and the cathode 1. The stack 11 comprises a layer 111, or spacer, which is made of glass microfiber, sold under the trade name Glass Microfibre Filter by the manufacturer Whatman®, with a thickness of 675 µm, a diameter of 16.5 mm and a retention threshold of 2.7 µm. The other layer 112 of the stack 11, or separator, 25 µm thick, is made of microporous polypropylene sold under the trade name Celgard 2400 by the manufacturer Celgard®.

[0096] THE FIGURES 10 And 11illustrate the performance of the LIB 10 according to the invention. The sulfur and silicon concentration of cathode 1 and anode 3 is 1 mg / cm². The cycles are performed at a charge rate of C / 20. A stable discharge capacity exceeding 300 mAh / g is obtained after 100 cycles. The LIB 10 exhibits an energy density of approximately 750 Wh.kg⁻¹. This energy density value is three times higher than that of commercial lithium-ion batteries.

[0097] According to this embodiment, it is also an embodiment of a method for manufacturing a cathode 1 according to the invention. The method comprises the step of obtaining an electrode 15, referred to as the electrode 15 obtaining step, as shown in the FIGURE 1 This step may involve obtaining electrode 15 or purchasing a commercial electrode 15. With reference to the FIGURE 1The electrode 15 includes a layer 41 of a conductive material, Aluminium according to the embodiment, arranged to collect the current flowing through the cathode 1, called substrate 41 of the electrode 15. The electrode 15 includes a layer 51 of aligned NTCs 9 extending, mainly perpendicularly, from the substrate 41. The NTCs 9 are in electrical contact, by one of their ends, with the substrate 41 of the electrode 15.

[0098] The process further comprises the step of coating the outer face 14 of the layer 51 of NTCs 9 with a sulfur solution comprising solid sulfur to form the solid sulfur coating 61 lining the outer wall of the NTCs 9 as illustrated in the FIGURE 2 The outer face 14 of the layer 51 of NTCs 9 is located on the opposite side to the face of the layer 51 of NTCs 9 in electrical contact with the substrate 41 of the electrode 15.

[0099] The process includes the step of coating the outer face 14 of the layer 51 of NTCs 9 with a solution comprising Li 2 SO 4 to form the layer 8 of Li 2 SO 4, i.e. the protective film 8 of Li 2 SO 4, covering the layer 51 of NTCs 9. According to the non-limiting embodiment, the solution comprising Li 2 SO 4 is an aqueous solution of distilled water and isopropyl alcohol, in which the Li 2 SO 4 is dissolved at a concentration of 1 mol.l -1.

[0100] According to the non-limiting embodiment, the step of coating the outer face 14 of the layer 51 of NTCs 9 with the solution comprising Li 2 SO 4 includes the infiltration of the solution comprising Li 2 SO 4 into the layer 51 of NTCs 9. The infiltration of the solution comprising Li 2 SO 4 into the layer 51 of NTCs 9 allows the formation of a solid film, which may be discontinuous, 8 of Li 2 SO 4 coating, at least in part, the NTCs 9 and the solid Sulfur 61.

[0101] According to the embodiment, the step of forming the solid sulfur coating 61 lining the outer wall of the NTCs 9 includes infiltrating the sulfur solution into the layer 51 of the NTCs 9. The sulfur solution consists of sulfur S 8 (Sigma Aldrich) or cyclooctasulfur in a solution comprising carbon disulfide (CS 2) with isopropyl alcohol (IPA) or with N-methyl-2-pyrrolidone (NMP) in a volume ratio between (7:3) and (9:1). The NTCs 9 are then washed repeatedly with dilute sulfur solutions. The NTCs 9 are subsequently dried, for example in an oven, at 40 °C for 4 to 8 hours.

[0102] According to a non-limiting embodiment, a manufacturing process for the anode 3 is proposed. The process for obtaining the electrode 15 is described above. The NPs 62 are deposited onto the NTCs 9 by chemical vapor deposition (CVD) using a gas stream containing a dilute silane precursor (SiH4) mixed with dihydrogen (H2). The substrate 42 is loaded into a CVD reactor preheated to 540°C, which is swept with H2 at a flow rate of 30 cubic centimeters per minute (sccm). Alternatively, a gas mixture of SiH4 at a concentration of 1% diluted in nitrogen (N2) can be used. In this case, it is possible to operate down to ambient pressure.

[0103] After 3 to 10 minutes of heating the substrate 42, SiH4 is introduced into the CVD reactor at a flow rate of 10 sccm, and a tungsten filament placed at the H2 inlet of the reactor is heated to approximately 1000°C (at a power of 70 to 80 W). Atomic hydrogen is thus generated to etch the excess silicon on the surface of the NTCs 9 and improve the deposition and homogeneity of the NPs 62 along the entire length of the NTCs 9.

[0104] The pressure inside the reactor is maintained at 5 mbar with a SiH4 partial pressure between 1 and 5 mbar. The synthesis time is between 1 and 15 minutes depending on the desired Silicon 62 nanoparticle size. At the end of the process, the resulting electrode is moved to a cooling zone within the reactor and removed after complete cooling. The diameter of the 62 nanoparticles ranges from 5 nm to 30 nm, with deposition times between 3 and 15 minutes. Beyond 10 minutes, a silicon layer begins to form. The homogeneity and density of the 62 nanoparticle deposit can be adjusted by modifying the parameters associated with the deposition time, gas flow rate, and hot filament power.

[0105] In one embodiment, a defect generation step on the NTCs 9 is proposed. In the non-limiting embodiment presented, this step is performed by plasma treatment. Electrode 15, prior to the fabrication of the anode 3, cathode 1, or LIB 10, is exposed to a non-thermal water vapor plasma at a pressure of 2 mbar in a CVD reactor, for example, by Plasma Enhanced Chemical Vapor Deposition (PECVD). The PECVD reactor has a three-electrode configuration. A non-thermal water vapor plasma is ignited between two multi-hole graphite electrodes located above electrode 15. The two graphite electrodes are 2.5 cm apart. A third electrode located below electrode 15, acting as a support, is connected to the negative terminal of the power supply, which is equipped with a voltmeter.When the power supply is off, it acts as a floating voltmeter; otherwise, it can be used to generate controllable power by applying a negative bias to obtain a controllable ionic flux and ionic energy striking electrode 15.

[0106] Steam is injected at a flow rate of 10 sccm and the pressure inside the reactor is around 2 mbar. A plasma is created by applying a voltage between 650 and 700 V between the two graphite electrodes of the reactor.

[0107] After the plasma was turned on, the voltage applied to the anode of the PECVD reactor was kept almost constant between 400 and 420 Volts with a current intensity between the two multi-hole electrodes of the order of 0.20 to 0.24 A. With this set of plasma parameters, the experiments were continued by changing the exposure time which varied from 2 to 10 minutes.

[0108] A potential difference between the second and third electrodes was added by applying a power of 1 to 5 W to the third electrode connected to electrode 15, which allows better control of the flow and energy of the ions bombarding electrode 15.

[0109] The size and density of the defects created are controllable via the plasma exposure time and plasma parameters (voltage and current density).

[0110] According to the embodiment, a manufacturing process for a LIB 10 is also proposed. The manufacturing process for the LIB 10 includes the step of obtaining an anode 3. This step may consist of procuring the anode 3 or purchasing a commercial anode 3. The manufacturing process further includes the step of assembling the LIB 10 by interposing the stack 11 of the two layers 111, 112 between the cathode 1 and the anode 3.

[0111] According to a non-limiting embodiment, LIB 10 comprises an electrolyte solution containing 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSi) and 0.25 M lithium nitrate (LiNO3) in 1,3-dioxolane:1,2-dimethoxyethane (DOL:DME) in a 1:1 volume ratio. Prelithiation of anode 3 is achieved by cycling it with metallic lithium in a button cell configuration. A piece of lithium, acting as an additional lithium source during discharge, can be placed on anode 3. The mass of silicon in the anode is balanced with the mass of sulfur in the cathode.

[0112] The LIB 10 is manufactured in a glove box under an argon atmosphere. The lithium anode 3 is placed on the negative casing and covered by the two layers 111 and 112. The cathode 1 is placed on the separator 112 and covered by a stainless steel spacer. A stainless steel spring is placed on the spacer to provide sufficient pressure and contact between all the components. The LIB 10 is then sealed in a crimp-on button cell.

[0113] An alternative step to the pre-lithiation step described above for cathode 1 is also proposed. This alternative step consists of coating the NTCs 9 with the sulfur solution during the step of the process according to the invention, which involves coating them with Lithium Sulfide (Li₂S). The Li₂S cathode avoids the silicon pre-lithiation step. Furthermore, Li₂S has the advantage of a lower density (1.66 g.cm⁻³) than sulfur (2.07 g.cm⁻³), so it is not necessary to reserve additional empty space to accommodate potential volume expansion during cycling.

[0114] According to a non-limiting embodiment, the step of obtaining electrode 15 may include a synthesis of electrode 15.

[0115] The synthesis process can be carried out on commercial thin sheets of aluminium (Al) or copper (Cu), used as substrate 41, 42, with a thickness between 10 and 50 µm.

[0116] A layer of aluminum oxide (Al₂O₃), between 10 and 50 nm thick, followed by a layer of iron (Fe), between 1 and 10 nm thick, are successively deposited onto substrates 41 and 42 in an electron beam evaporator. The deposition (of aluminum oxide and iron) is carried out under high vacuum (on the order of 10⁻⁸ mbar). During deposition, the temperature of substrates 41 and 42 is maintained at 300°C. The substrate heating process is essential for several reasons: 1) it ensures the degassing of all impurities from the substrates, 2) it allows the diffusion of atoms onto the surface of the pre-deposited alumina layer, and 3) it promotes the adhesion of the multilayers to the substrates and also improves the adhesion of the CNTs during their subsequent growth. Other catalysts, such as Cobalt, Nickel or alloys, can be used.The initial thickness and chemical nature of the catalyst determine the morphology of the resulting NTCs 9 (tube diameter and number of walls). Furthermore, by varying the CVD deposition parameters (pretreatment time between 1 and 5 minutes and / or gas composition and pressure), the total surface area of ​​the NTCs 9 and their density can be controlled, and consequently, the porosity (inter-tube distance) of the resulting vertically aligned NTCs 9 mat.

[0117] Next, the NTCs 9 are synthesized in a four-hot-filament chemical vapor deposition (4HF-CVD) reactor. The main difference between a standard thermal CVD reactor and the 4HF-CVD reactor lies in the addition of four tungsten filaments at the inlet of each gas inlet. The presence of four tungsten filaments (two at each gas inlet) allows for the growth of aligned NTCs 9 with a greater degree of uniformity in diameter and length, and, most importantly, their synthesis over large areas.

[0118] The substrate 41, 42 coated with catalyst layers (Al₂O₃ and Fe) is held in a cooling zone until the reactor reaches a temperature of 600°C and a pressure of 8 mbar. The substrate 41, 42 coated with catalyst layers (Al₂O₃ and Fe) is then moved to the deposition zone where no significant thermal gradient is present.

[0119] A pretreatment of the catalysts under activated gaseous hydrogen (or atomic hydrogen) is carried out for 1 to 5 minutes. The gaseous hydrogen flows along two dedicated tungsten filaments heated to over 2000°C (for an applied power of 400 to 500 W) at a flow rate of 50 to 75 sccm and a reactor pressure of 5 to 12 mbar. The atomic / activated hydrogen reduces the iron catalyst layer and creates point defects in the underlying Al₂O₃ layer, which act as trapping sites for the formation of catalyst nanoparticles.

[0120] The actual growth of the 9-CNTs is achieved by exposing the pretreated substrate to a CH4 / H2 gas mixture at flow rates of 50 and 20 sccm, respectively, and at a reactor pressure between 10 and 15 mbar. The growth time ranges from 5 to 120 minutes, allowing the CNT heights to reach between 7 µm and 200 µm. The CH4 flows along two dedicated, fed filaments. The role of atomic hydrogen generation in this step is to prevent excessive deposition of amorphous carbon during the growth of the 9-CNTs. The growth time is proportional to the desired final length of the 9-CNTs while the catalyst is active, i.e., to the height of the resulting aligned 9-CNT mat.

[0121] After the growth of the NTCs 9, the electrodes 15 are moved to the cooling zone at the end of the growth process and kept there until the reactor has completely cooled.

[0122] In general, the diameter, number of walls and density of the NTCs 9 can be adjusted via the initial thickness of the catalyst layer, its chemical nature (Fe, Co, Ni or alloys), the flow and composition of the gas precursors, the pressure and duration of the pretreatment step.

[0123] Of course, the invention is not limited to the examples just described and many modifications can be made to these examples without departing from the scope of the invention.

[0124] Furthermore, the different features, forms, variants and embodiments of the invention can be associated with each other in various combinations insofar as they are not incompatible or mutually exclusive.

Claims

1. A cathode (1) for a lithium-ion battery, comprising: - a layer (41) of a conductive material, arranged to collect the current flowing through the cathode, referred to as the substrate of the cathode, - a layer (51) of aligned carbon nanotubes (CNTs) (9) in electrical contact with the substrate of the cathode and mainly extending perpendicularly to the substrate of the cathode, - solid sulfur (61) which at least partially coats an outer wall of the CNTs, and - a solid layer (7) of solid lithium sulfate (Li2SO4), which layer is referred to as the outer layer of Li2SO4, covering the layer of CNTs so as to form a stack of layers, wherein the layer of CNTs is located between the substrate of the cathode and the outer layer of Li2SO4.

2. The cathode (1) according to claim 1, comprising solid lithium sulfate (Li2SO4) (8) encapsulating and / or enveloping, at least partially, the solid sulfur (61) which coats the outer walls of the CNTs (9).

3. The cathode (1) according to claim 1 or 2, wherein a mass loading of solid sulfur (61) is greater than 1.5 mg / cm2.

4. The cathode (1) according to any one of claims 1 to 3, wherein the outer layer of Li2SO4 has a thickness of less than 10 nm and greater than 3 nm.

5. A lithium-ion battery (10), referred to as LIB, comprising the cathode (1) according to any one of claims 1 to 4.

6. The LIB (10) according to claim 5, comprising: - an anode (3) based on silicon nanoparticles (62), - a porous element (111, 112) arranged between the cathode (1) and the anode.

7. The LIB (10) according to claim 6, wherein the anode (3) comprises: - a layer (42) of a conductive material, referred to as the substrate of the anode, arranged to collect the current flowing through the anode, - a layer (52) of aligned carbon nanotubes (CNTs) (9) in electrical contact with the substrate of the anode and mainly extending perpendicularly to said substrate of the anode, - silicon nanoparticles (62) which coat the outer wall of the CNTs.

8. The LIB (10) according to claim 6 or 7, wherein a silicon mass loading of the silicon nanoparticles (62) of the anode (3) is greater than 2 mg / cm2.

9. A method for manufacturing a cathode (1), said method comprising the steps of: - obtaining an electrode (15), referred to as the electrode obtaining step, said electrode comprising a layer (41) of a conductive material, arranged to collect the current flowing through the cathode, referred to as the substrate of the electrode, from which extends, mainly perpendicularly, a layer (51) of aligned carbon nanotubes (CNTs) (9) in electrical contact with the substrate of the electrode, then - covering one face (14), referred to as the outer face, of the layer of CNTs, located on the side opposite to a face of the layer of CNTs in electrical contact with the substrate of the electrode, with a solution, referred to as the sulfur solution, comprising solid sulfur (61) to form a coating of solid sulfur which at least partially coats an outer wall of the CNTs, and then - covering the outer face of the layer of CNTs with a solution comprising dissolved lithium sulfate (Li2SO4) to form a layer (7) of solid lithium sulfate (Li2SO4) covering the layer of CNTs.

10. The method according to claim 9, wherein the sulfur solution comprises a solid sulfur powder, a polar solvent and carbon disulfide, a volume ratio between the carbon disulfide and the polar solvent is between 10 and 30 %.

11. The method according to claim 9 or 10, comprising a step, referred to as plasma treatment, of treating the CNTs (9) of the electrode (15) with a cold plasma and / or water vapor; the plasma treatment step is carried out prior to the step of covering the layer of CNTs with the sulfur solution and subsequent to the electrode obtaining step.

12. The method according to the preceding claim, wherein the plasma treatment step further comprises acceleration, by polarization, of all or some of the species present in the plasma toward the electrode (15).

13. The method according to any one of claims 9 to 12, wherein the step of obtaining the electrode (15) comprises a step of synthesizing the CNTs (9) on the substrate (41, 42) of the electrode by hot-filament chemical vapor deposition, said synthesis step comprises: - arranging the substrate of the electrode equidistant from four aligned hot filaments, - flushing a gas, comprising precursors, parallel to the hot filaments.

14. The method according to the preceding claim, comprising, prior to the step of synthesizing the CNTs (9), a step of depositing a layer, referred to as the barrier layer, of aluminum oxide (Al2O3) with a thickness of between 5 and 80 nm on the substrate of the electrode and an iron layer, referred to as the catalyst layer, deposited on the barrier layer, with a thickness of between 1 and 30 nm.

15. A method for manufacturing a lithium-ion battery (10), referred to as LIB, comprising the steps of: - implementing the method for manufacturing a cathode (1) according to any one of claims 9 to 14, - obtaining an anode (3) for the LIB, - assembling the LIB by interposing a porous element (11) between the cathode and the anode.