SOLVENT-FREE ELECTRODE PREPARATION METHOD AND ELECTRODE FORMULATIONS LIKELY TO BE OBTAINED BY SAID METHOD

Abstract

SOLVENT-FREE ELECTRODE PREPARATION METHOD AND ELECTRODE FORMULATIONS CANNOT BE OBTAINED BY SAID METHOD. This application relates to solvent-free electrode formulations based on fluoropolymer, obtained by extrusion and / or comprising one or more co-binders including TPU, the electrodes containing them, and the corresponding electrochemical elements and accumulators. Figure for abstract: None

Description

Title of the invention: METHOD FOR PREPARING A SOLVENT-FREE ELECTRODE AND THE ELECTRODE FORMULATIONS CAPABLE OF BEING OBTAINED BY SAID PROCESS

[0001] The present invention relates to the field of energy storage, and more specifically to accumulators, in particular of the lithium type.

[0002] The operation of lithium accumulators is based on the reversible exchange of the lithium ion between a positive electrode and a negative electrode, separated by a separator containing an electrolyte, the lithium being inserted into the negative electrode during charging operation.

[0003] Typically, the electrodes consist of a metal strip onto which an electrode formulation consisting of active material and possibly binder and conductive element is applied.

[0004] Due to the constant increase in energy and battery needs, it is necessary to improve their manufacturing, to facilitate industrialization, reduce costs and improve their environmental impact.

[0005] Currently, a very significant part of the manufacturing cost of an electrode is linked to its manufacturing process, particularly for Li-ion technology. Indeed, the solvent used to prepare the ink (based on active material, conductive fillers and binder) which will be coated on the strip to design the electrode, must be evaporated. This step therefore involves the use of energy-intensive ovens.

[0006] It is also desirable to limit the use of solvents in an environmental approach.

[0007] In an effort to eliminate these solvents and reduce the manufacturing costs of electrodes, new so-called solvent-free processes are currently being developed.

[0008] Thus, WO 2015 / 161289 describes an electrode composition based on polytetrafluoroethylene (PTFE) and co-binder(s), obtained by fibrillation of PTFE by a high shear process such as air jet milling in particular.

[0009] PTFE has the particularity when it is well sheared of producing fibrils which form a network which contributes to the formation of porosities within the electrode.

[0010] However, the jet-milling step may require batch work, which is incompatible with continuous industrial operation. Furthermore, jet-milling grinding may damage fragile active materials (such as graphite, for example).

[0011] It is therefore necessary to provide a more suitable and / or easily industrializable process.

[0012] The present invention thus relates to a new solvent-free route for the improved preparation of electrode formulations, in particular intended for Li-ion technology.

[0013] According to a first object, the present invention relates to a method for preparing an electrode formulation comprising: • The preparation of a premix comprising an active electrode material and a fluoropolymer; • Mixing the premix with a co-binder; • Fibrillation of the mixture obtained by extrusion.

[0014] The term "fluoropolymer" as used herein refers to fluorinated polymers whose repeating unit is a fluorocarbon, comprising multiple carbon-fluorine bonds. Among these fluoropolymers, mention may in particular be made of polytetrafluoroethylene (PTFE) and its derivatives, such as chlorofluoroethylene, perfluoroalkoxy (PFA), polychlorotrifluoroethylene (PCTFE or PTFCE), fluorinated ethylene propylene (FEP), ethylene tetrafluoroethylene or poly(ethylene-co-tetrafluoroethylene) (ETFE), tetrafluoroethylene perfluoromethylvinylether (MFA), more particularly PTFE. Preferably, said fluoropolymers are of the fibrillable type.

[0015] The term "fibrillable" means the types of fluoropolymers which are capable of fibrillating, i.e. which can form a network of fibers in the mixture with the premix, under the extrusion conditions. The types of fluoropolymers can be of different shapes and / or grades.

[0016] The term "pre-mix" means a preliminary composition previously prepared before adding one or more additional ingredients; in this case the pre-mix comprises the mixture of the fluoropolymer and the active material, and possibly a conductive element, before subsequent addition of the co-binder. The pre-mix may also comprise one or more optional additives such as lubricants. In the context of solid electrolyte batteries, the pre-mix may also comprise solid electrolyte particles.

[0017] The active electrode material can be chosen from electrochemically active materials. It depends on the type of electrode (positive or negative) and the type of battery considered.

[0018] Thus in the case of lithium batteries, the negative electrode active material is in particular graphite, silicon, lithium, a lithium alloy or a lithiophilic material, alone or in a mixture, such as mixed SiOx / graphite active materials. The expression “lithiophilic” here defines a material having an affinity for lithium, (ie) its ability to form alloys with lithium, such as silicon, silver, zinc and magnesium.

[0019] The following active ingredients may also be mentioned:

[0020] - a titanium and niobium oxide TNO having the formula:

[0021] LixTia_yMyNbb_ ZM'ZO( (X+4a+5b) / 2)-c-dXc

[0022] where0 <x<5;0<y<l;0<z<2;l<a<5;l<b<25; 0,25 <a / b<2;0<c< 2 and 0 < d < 2; ay > 0; bz > 0;

[0023] M and M' each represent at least one element selected from the group consisting of Li, Na, K, Mg, Ca, B, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Y, Zr, Nb, Mo, Ru, Ag, Sn, Sb, Ta, W, Bi, La, Pr, Eu, Nd and Sm;

[0024] X represents at least one element selected from the group consisting of S, F, Cl and Br.

[0025] The index d represents an oxygen vacancy. The index d can be less than or equal to 0.5.

[0026] Said at least one titanium and niobium oxide may be chosen from TiNb2O7, Ti2 Nb2O9 and Ti2Nb10O29.

[0027] - a lithiated titanium oxide or a titanium oxide capable of being lithiated. LTO is chosen among the following oxides:

[0028] Lix_aMaTiy_bM'bO4_c_dXc in which 0 <x<3 ; l<y<2,5 0<a<l 0<b<l 0<c<2 et -2,5<d<2,5

[0029] M represents at least one element selected from the group consisting of Na, K, Mg, Ca, B, Mn, Fe, Co, Cr, Ni, Al, Cu, Ag, Pr, Y and La;

[0030] M' represents at least one element selected from the group consisting of B, Mo, Mn, Ce, Sn, Zr, Si, W, V, Ta, Sb, Nb, Ru, Ag, Fe, Co, Ni, Zn, Al, Cr, La, Pr, Bi, Sc, Eu, Sm, Gd, Ti, Ce, Y and Eu;

[0031] X represents at least one element selected from the group consisting of S, F, Cl and Br;

[0032] The index d represents an oxygen vacancy. The index d can be less than or equal to 0.5;

[0033] Such as Li4Ti50i2, Li2TiO3jlaramsdellite Li2Ti3O7,LiTi2O4, LixTi2O4, with 0 <x<2 et li2na2ti60i4, plus particulièrement li4_amati5_bm’bo4jpar exemple li4tibo|2 (ou li4 3ti 5 3o4).

[0034] HxTiyO4 in which 0 <x<l ; 0<y<2, et

[0035] a mixture of the compounds Lix.aMaTiy_bM'bO4.c.dXcet HxTiyO4.

[0036] Examples of lithiated titanium oxides are spinel Li4TibO|2, Li2TiO3jla ramsdellite Li2Ti3O7,LiTi2O4, LixTi2O4, with 0 <x<2 et li2na2ti60i4.

[0037] A preferred LTO compound has the formula Li4 _aMaTi5.bM'bO4.for example Li4Ti50i2 which is also written Li4 / 3Ti5 / 3O4.

[0038] The active material of the positive electrode is not particularly limited. It can be selected from the following groups or mixtures thereof:

[0039] - a compound (a) of formula LixMi_y_z_wM'yM”zM'”w02 (LM02) where M, M', M” and M”' are selected from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, W and Mo provided that at least M or M' or M'' or M”' is selected from Mn, Co, Ni, or Fe; M, M', M” and M”' being different from each other; and 0.8 <x<l,4 ; 0<y<0,5 0<z<0,5 0<w<0,2 et x+y+z+w<2,l

[0040] - a compound (b) of formula LixMn2_y_zM'yM"zO4 (LMO), where M' and M" are chosen in the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo; M' and M" being different from each other, and 1 <x<1,4 ; 0<y<0,6 0<z<0,2

[0041] - a compound (c) of formula LixFei yMyPO4(LFMP) where M is chosen from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo; and 0.8 <x<l,2 ; 0<y<0,6

[0042] - a compound (d) of formula LixMni y zM'yM”zP04 (LMP), where M' and M” are different from each other and are selected from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo, with 0.8 <x<l,2 ; 0<y<0,6 0<z<0,2

[0043] - a compound (e) of formula xLi2MnO3; (lx)LiM02 where M is at least one element chosen from Ni, Co and Mn and x <l.

[0044] - a compound (f) of formula Lii+XMO2 yFy of cubic structure where M represents at least one element selected from the group consisting of Na, K, Mg, Ca, B, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Y, Zr, Nb, Mo, Ru, Ag, Sn, Sb, Ta, W, Bi, La, Pr, Eu, Nd and Sm and where 0 < x < 0.5 and 0 < y < 1;

[0045] A conductive element may also be added for the preparation of positive electrode. It may be selected from electronically conductive materials, such as graphite, carbon black, acetylene black, soot, graphene, carbon fibers, carbon nanotubes or a mixture thereof.

[0046] The preparation of the premix can be carried out by simply mixing the constituents, typically in the form of powders, with stirring.

[0047] The term "co-binder" (also called "cobinder") means a material which makes it possible to give the electrode cohesion of the different components and its mechanical strength on the current collector, and / or to give a certain flexibility to the electrode for its implementation in a cell. More particularly, the co-binder according to the invention ensures cohesion between the fluoropolymer and the active material.

[0048] According to one embodiment, the co-binder is chosen from thermoplastic polyurethane (TPU), poly(styrene-butadiene-styrene) (SBS), poly(styrene-ethylene-butadiene-styrene) (SEBS), thermoplastic elastomers (TPE), vulcanized thermoplastics (TPV), thermoplastic copolyesters (TPC), polystyrene-b-poly(ethylene-butylene)-b-polystyrene (SEBS), butadiene-acrylonitrile copolymers also called "nitrile rubbers" (NBR), hydrogenated butadiene-acrylonitrile copolymers, also called "hydrogenated nitrile rubbers" (HNBR), elastomers, thermoplastics or ethylene-acrylate terpolymers.

[0049] More particularly, the co-binder is TPU.

[0050] “Extrusion” means a thermomechanical process in which the formulation is forced to pass through a die, under the action of pressure and heat.

[0051] The extrusion step can be adapted according to several parameters, such as the mixing temperature, the type of extruder screw profile, the type of extruder die, the rotation speed and / or the length of the screws.

[0052] According to one embodiment, the extrusion can be carried out with a single- or twin-screw extruder, co-rotating or not.

[0053] According to one embodiment, the screw profile used in the extruder is of the shearing type in order to fibrillate the fluoropolymer in the extruder. The screw profile may contain one or more mixing zones. The number of mixing zones typically depends on the number of introduction zones. The position of the mixing zones in the extruder generally depends on the number of material introduction zones. After each material introduction zone, a mixing zone may be added.

[0054] Typically, the type of screw element for shearing the material can be adapted to the type of active material contained in the premix. If the active material is sensitive to shear, it is preferable to favor low or medium shear elements. If the active material is not very sensitive to shear, it is possible to use low, medium or high shear elements.

[0055] The rotation speed of the screw is generally the same throughout the screw. It is generally recommended to rotate it between 100 rpm and 1000 rpm, in particular between 100 and 750 rpm. The rotation speed of the screw is generally adapted according to the desired material flow rate at the extruder outlet. The lower the rotation speed of the screw, the lower the output flow rates will be. Note that low rotation speeds result in longer residence times in the extruder. In such a case, if the material input flow rate is high, there may be a risk of clogging the extruder. In the case of a high screw rotation speed, the output flow rates may fluctuate if the incoming material flow rates are too low.

[0056] The extrusion step can advantageously be carried out at a temperature between 25°C and the degradation temperature of the fluoropolymer, more particularly between the melting temperature of the co-binder and the melting temperature of the fluoropolymer under the extrusion conditions, it being understood that the degradation and / or melting temperatures of the fluoropolymer under the extrusion conditions can be reduced. due to the mechanical stresses exerted. For example, for PTFE, the degradation temperature is approximately 350°C and the melting temperature is approximately 327°C, it being understood that due to the stresses exerted, the extrusion temperature is preferably less than or equal to 260°C.

[0057] According to another object, the present invention also relates to the electrode formulation capable of being obtained by the method according to the invention.

[0058] According to another object, the present invention also aims at an electrode formulation comprising: • an active electrode material; • a fluoropolymer; • thermoplastic polyurethane (TPU) as a co-binder.

[0059] The fluoropolymer is as defined above.

[0060] According to one embodiment, the aforementioned electrode formulations according to the invention may further comprise a conductive element. This is particularly the case for positive electrodes, as discussed above.

[0061] According to one embodiment, the electrode formulations according to the invention may further comprise one or more additives chosen from lubricants such as oils or waxes or graphite.

[0062] In addition, the formulations according to the invention may also comprise a carbon additive. This additive is distributed in the electrode so as to form an electronic percolating network between the active material and the current collector.

[0063] When present, the carbon additive may be up to approximately 10% (by weight), in particular from 1 to 6% (by weight) of the total content of the formulation.

[0064] Thus, according to one embodiment, the formulations according to the invention comprise (by weight): - From 80 to 98.5% active matter; From 0.1 to 5% PTFE; From 0.1 to 5% TPU; - From 0 to 5% lubricant; and - From 0 to 10% percolating carbon.

[0065] The electrode formulation according to the invention is suitable for positive or negative electrodes.

[0066] The term "negative electrode" designates when the accumulator is discharging, the electrode functioning as an anode and when the accumulator is charging, the electrode functioning as a cathode, the anode being defined as the electrode where an electrochemical oxidation reaction takes place (emission of electrons), while the cathode is the seat of the reduction. The term negative electrode also designates the electrode from which the electrons leave, and from which the cations (Li+) are released in discharge.

[0067] The term "positive electrode" designates the electrode where the electrons enter, and where the cations (Li+) arrive in discharge.

[0068] According to the invention, the electrode formulation is porous, the porosity being conferred by the fluoropolymer fibrils generated by the extrusion. This porosity makes it possible in particular, on the one hand, to accommodate the lithium metal in the porosity of the negative electrode during charging, and on the other hand to maintain mechanical strength of the electrode.

[0069] According to the invention, the term “porous” means a pore size of less than 300 nm. The pore size corresponds to the structure of the material having an organized network of channels of very small variable pore size: typically a pore size of less than 1 pm, preferably less than 300 nm. This pore size gives the electrode a particularly large active surface area per unit of electrode surface area.

[0070] According to one embodiment, the electrode has a porosity of between 10 and 60%, preferably between 15 and 35%, the porosity representing the percentage of voids in the total volume of the formulation considered. The porosity can be measured by Hg porosimetry or by Helium porosimetry in general.

[0071] According to another object, the present invention also aims at an electrode comprising the electrode formulation according to the invention shaped.

[0072] According to one embodiment, said electrode may consist of a conductive support used as a current collector which is coated with the formulation according to the invention in a shaped form.

[0073] Current collector means an element such as a pad, plate, sheet or other, made of conductive material, connected to the positive or negative electrode, and ensuring the conduction of the flow of electrons between the electrode and the terminals of the battery.

[0074] The current collector is preferably a two-dimensional conductive support such as a solid or perforated strip, made of metal, for example copper, nickel, steel, stainless steel or aluminum.

[0075] Said electrode may in particular be a Li-ion type electrode.

[0076] When it is a negative electrode of the Li-ion type, this advantageously consists of the formulation comprising PTFE, TPU and graphite formed on a current collector such as a copper foil.

[0077] When it is a positive electrode, this advantageously consists of the formulation comprising PTFE, TPU, a positive electrode active material, an electronically conductive element and a carbon additive, shaped on a current collector such as an aluminum strip.

[0078] The electrodes according to the invention can be prepared by applying or adapting conventional electrode manufacturing methodologies.

[0079] Thus typically, the formulation obtained at the end of the extrusion step is shaped for example by pressing, to obtain a self-supporting formulation which will then be laminated by calendering for example on the current collector.

[0080] According to another object, the present invention also relates to an electrochemical element comprising at least one electrode according to the invention.

[0081] The term "electrochemical element" means an elementary electrochemical cell consisting of the positive electrode / electrolyte / negative electrode assembly, making it possible to store the electrical energy supplied by a chemical reaction and to restore it in the form of current.

[0082] The chemical elements according to the invention can be adapted to different battery technologies and types of electrolytes.

[0083] Thus, according to one embodiment, the electrochemical element may be of the Lithium-ion type.

[0084] Li-ion cells are based on the reversible exchange of lithium ion between a positive electrode and a negative electrode, separated by an electrolyte, the lithium being deposited at the negative electrode during charging operation. Typically, for these accumulators, the positive electrode formulation comprises a lithiated transition metal oxide as the active material and the negative electrode formulation comprises graphite as the active material.

[0085] According to one embodiment, the electrochemical element can also be of the “solid” or even “primary Li” type.

[0086] The term “solid” designates elements with a solid electrolyte, such as oxides, halides, sulfides or a polymer.

[0087] The term “Li-primary” designates a non-rechargeable lithium element.

[0088] According to another object, the present invention also relates to a module electrochemical comprising the stacking of at least two elements according to the invention, each element being electrically connected with one or more other element(s).

[0089] The term “module” therefore designates here the assembly of several electrochemical elements, said assemblies being able to be in series and / or parallel.

[0090] Another object of the invention is also a battery comprising one or more modules according to the invention.

[0091] The term “battery” or accumulator means the assembly of several modules according to the invention.

[0092] According to one embodiment, the batteries according to the invention are accumulators whose capacity is greater than 100 mAh, typically 1 to 100 Ah. Figures

[0093] [Fig-1] [Fig. 1] represents the observation by SEM of the structure of an anode of formulation 94% Graphite / 2% PTFE / 4% TPU prepared according to the examples.

[0094] [Fig.2] [Fig.2] represents the comparison of the pore size distribution for a reference anode (represented by circles) and for anodes according to the invention (varying by their PTFE or TPU content) (samples 1, 2 and 3 represented by squares, diamonds and triangles, respectively)

[0095] [Fig.3] [Fig.3] represents the comparison of the amount of porosity for a reference anode (rounds) and for anodes according to the invention (varying in their PTFE or TPU content) (samples 1, 2 and 3: squares, diamonds and triangles, respectively). Examples

[0096] 1. Preparation of the electrodes

[0097] Electrode formulations according to the invention are prepared by making a premix of the active material (graphite) and fibrillable PTFE. Then the premix is ​​mixed with the co-binder (TPU) using a twin-screw extruder, at a temperature between 70 and 260°C and at a rotation speed between 100 and 750 rpm.

[0098] The following formulations were prepared:

[0099] [Tables 1] Parameter Formulations (% wt / wt of formulation) Pre-mix 1 Pre-mix 2 Pre-mix 3 Active Material (graphite) 90.25 94.6 93.6 Fibrillabl e binder (PTFE) 4.75 1.4 1.4 Co-binder (PTU) 5 4 5

[0100] The mixture recovered at the outlet of the extruder is then transferred to an external roller mixer to manufacture a self-supporting electrode (or pressed under a press) and for shaping the electrode. Adhesion to the strip is then obtained by colamination (by calendering) on ​​a current collector.

[0101] The SEM image in [Fig.l] shows that the PTFE fibrils are well distributed in the electrode. They form a network which contributes to the formation of porosities within the electrode.

[0102] 2. Characterization of the electrodes

[0103] 2.1 The porosity of the electrodes obtained according to Example 1 was analyzed for the different compositions, and compared to that of a reference electrode.

[0104] Porosity was measured by Hg porosimetry.

[0105] The reference electrode was produced using a solvent process. The active material (graphite), the binder (SBR) and the co-binder (CMC), all in powder form, are first mixed using a dry process using a planetary mixer. A solvent (NMP) is then added to produce an ink. This ink is then coated onto a copper current collector. The solvent is then evaporated using a system for suctioning and recycling the solvent. The formulation of the reference electrode is composed of 97% active material, 1.5% binder and 1.5% co-binder. The premix / solvent mass ratio when adding the solvent is 40 / 60.

[0106] In the graphs of [Fig.2], the reference electrode is represented by circles, and the formulations of the invention 1, 2 and 3 above are represented by squares, diamonds and triangles, respectively.

[0107] [Fig.2] illustrates the size distribution of the pores contained in the reference electrode and in the electrodes made via the present invention. Two populations of pores are observed. It appears that the size distribution of the pores contained in the electrodes made by the method described in this invention is in the range of the pore size of the reference electrode prepared in the standard route (solvent route). In addition, [Fig.2] shows that by changing the amount of PTFE and co-binder (here TPU) in the specific formulation, it is possible to control and modulate the amount of porosity manufactured but also the average pore size of the electrode.

[0108] [Fig. 3] shows the amount of porosity contained in the reference electrode prepared by solvent route and in electrodes prepared according to the method presented in the present invention. It appears that the amount of porosity of the electrodes prepared according to the method presented depends on the formulation. Furthermore, it is shown that it is possible to modulate the amount of porosity of the electrode by varying the formulation, namely the content of active material, binder and co-binder.

[0109] 2.2 Lifespan

[0110] The experiments carried out showed a lifetime greater than 30 cycles, at 60°C and in C / 5 regime.

Claims

Claims

1. A process for the solvent-free preparation of an electrode formulation comprising: • Preparing a premix comprising an electrode active material and a fluoropolymer; • Mixing the premix with thermoplastic polyurethane (TPU) as a deco-binder; • Fibrillation of the mixture obtained by extrusion.

2. Method according to claim 1 such that the extrusion is carried out with a single- or twin-screw type extruder.

3. A method according to claim 1 or 2 such that the fluoropolymer is chosen from polytetrafluoroethylene (PTFE) and its derivatives, such as chlorofluoroethylene, perfluoroalkoxy (PFA), polychlorotrifluoroethylene (PCTFE or PTFCE), fluorinated ethylene propylene (FEP), ethylene tetrafluoroethylene or poly(ethylene-co-tetrafluoroethylene) (ETFE), tetrafluoroethylene perfluoromethylvinyl ether (MFA).

4. A method according to any preceding claim such that the fluoropolymer is PTFE.

5. Method according to any one of the preceding claims such that the extrusion step is carried out at a temperature between 25°C and the degradation temperature of the fluoropolymer.

6. A solvent-free Li-ion electrode formulation comprising: • an electrode active material; • a fibrillated fluoropolymer; • thermoplastic polyurethane (TPU) as a co-binder.

7. A formulation according to claim 6 such that the fluoropolymer is as defined in claim 3 or 4.

8. A formulation according to claim 6 as further comprising a conductive element.

9. Formulation according to claim 6 or 8 such that it comprises (by weight): - From 80 to 98.5% of active material; From 0.1 to 5% of PTFE; From 0.1 to 5% of TPU; - From 0 to 5% of lubricant; and

10.

11.

12.

13. - From 0 to 10% percolating carbon. Formulation according to any one of claims 6, 8 to 9 having a porosity of between 15 and 35%. A Li-ion type electrode comprising the electrode formulation according to any one of claims 6, 8 to 10 formed on a current collector. An electrode according to claim 11, such that it comprises PTFE, TPU, graphite. Li-ion type electrochemical element comprising an electrode according to claim 11 or 12.

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