Electrode for lithium-ion battery and method for preparing the same

Abstract

The present invention provides an electrode for a lithium-ion battery, which comprises an active layer with a linear styrene-ethylene / butylene-styrene (SEBS) copolymer as a binder. The SEBS copolymer is characterized in that: the melt flow rate (MFR) measured at 230 °C and a load of 2.16 kg is 4 g / 10 min to 220 g / 10 min; the molecular weight is less than 100,000 g / mol, especially 50,000 g / mol to 100,000 g / mol; and the styrene content is 10 wt% to 20 wt%. The present invention also provides a method for preparing the electrode and a lithium-ion battery comprising the electrode, and the electrode can be a positive electrode or a negative electrode.

Description

[0001] This application claims the benefit of European Patent Application EP 20382398.4 filed on May 13, 2020. Technical Field

[0002] The present disclosure relates to the field of rechargeable batteries. In particular, it relates to an electrode comprising a linear styrene-ethylene / butylene-styrene (SEBS) copolymer as a binder, and to a lithium-ion battery comprising said electrode. More particularly, it relates to a positive electrode and a negative electrode, and to methods for their preparation. Background Art

[0003] The increasing demands for mobility, sensing, and interconnection have driven the need for low-cost and environmentally friendly energy storage systems.

[0004] Lithium-ion batteries are a favorable technology for this purpose because, compared to other traditional electrochemical batteries, lithium-ion batteries have a higher volumetric energy density (Wh·L -1 ) and gravimetric energy density (Wh·g -1 ). In addition, lithium-ion batteries also exhibit several attractive features, such as no memory effect, low self-discharge, a large number of charge-discharge cycles, and high energy efficiency during the energy conversion process. Lithium-ion batteries are suitable for integration in portable devices (smartphones, laptops, tablets) and smart sensors / actuators, which are highly relevant to the "Internet of Things" (IoT) and contribute to the rapid implementation of the concepts of smart homes, smart cities, and biomedical care facilities. It also represents a reliable technology that can be integrated into electric vehicles and hybrid vehicles, off-grid power systems in isolated areas, and uninterruptible power supplies (UPS).

[0005] The positive electrode of a lithium-ion battery is prepared by coating a slurry on a metal current collector substrate (aluminum). The slurry consists of a mixture of an active material, a binder, a conductive additive, and a solvent.

[0006] One of the polymers that can be used as a binder belongs to the styrene block copolymer (SBC) series. Styrene-butadiene-styrene (SBS) copolymers are widely used in industry because they can maintain a high elongation rate and exhibit abrasion resistance, durability, and chemical resistance. The styrene-ethylene / butylene-styrene (SEBS) copolymer obtained by hydrogenating the SBS polymer has higher heat resistance and resistance to ultraviolet radiation, as well as improved elastic properties. In addition, the SEBS copolymer is non-toxic and has no cytotoxic effects even during natural aging.

[0007] In a recent study, it was shown that SEBS is a very attractive polymer binder for lithium-ion batteries (see R. et al., "Poly(styrene-butene / ethylene-styrene): A New Polymer Binder for High-Performance Printable Lithium-Ion Battery Electrodes", ACS Applied Energy Materials, 2018, Vol. 1, pp. 3331-3341), in which full cells were assembled with screen-printed electrodes (anode and cathode). The performance of the printed cathode cells was evaluated, showing high discharge capacities of 137 mAh g -1 (at C / 5) to 52 mAh g -1 (at 5C). The SEBS-based ink formulated for screen printing showed a uniform particle distribution, and the results also demonstrated that the SEBS-based polymer binder provided a better interfacial structure than the commonly used PVDF polymer binder. SUMMARY OF THE INVENTION

[0008] The present inventors have recognized that by using a linear styrene-ethylene / butene-styrene (SEBS) copolymer having a high melt flow rate, a relatively low molecular weight, and a styrene content as a binder in the active layer of an electrode (anode and / or cathode), a battery with better performance, particularly with improved specific capacity, can be manufactured.

[0009] Accordingly, a first aspect of the present invention relates to an electrode for a lithium-ion battery, the electrode comprising an active layer containing an electrode active material, a conductive additive, and a linear styrene-ethylene / butene-styrene (SEBS) copolymer as a binder, the SEBS copolymer being characterized in that:

[0010] - a melt flow rate (MFR) measured at 230 °C and a load of 2.16 kg is from 4 g / 10 min to 220 g / 10 min;

[0011] - a molecular weight of less than 100,000 g / mol, particularly from 50,000 g / mol to 100,000 g / mol; and

[0012] - a styrene content of from 10 wt% to 20 wt%.

[0013] A second aspect of the present invention relates to a method for preparing the above electrode, the method comprising:

[0014] a) obtaining a slurry comprising:

[0015] A linear styrene-ethylene / butylene-styrene (SEBS) copolymer as an adhesive, an electrode active material, a conductive additive, and a suitable solvent, wherein the SEBS copolymer is characterized in that:

[0016] - The melt flow rate (MFR) measured at 230 °C and a load of 2.16 kg is from 4 g / 10 min to 220 g / 10 min, such as 4 g / 10 min, 10 g / 10 min, 30 g / 10 min, 50 g / 10 min, 100 g / 10 min, or 150 g / 10 min;

[0017] - The molecular weight is less than 100,000 g / mol, especially from 50,000 g / mol to 100,000 g / mol, such as 70,000 g / mol, or 75,000 g / mol, or 80,000 g / mol; and

[0018] - The styrene content is from 10 wt% to 20 wt%, such as 15 wt%.

[0019] b) Applying the slurry onto a current collector; and

[0020] c) Drying the applied slurry to form an electrode active layer on the current collector.

[0021] The third aspect of the present invention relates to a lithium-ion battery, which includes a positive electrode, a negative electrode, a separator, and a suitable electrolyte disposed between the positive electrode and the negative electrode, wherein the positive electrode and / or the negative electrode is as described above. Description of the Drawings

[0022] Figure 1 Shows the rheological properties of samples containing SEBS C-H6180X (triangles; Example 1), samples containing SEBS C-H6110 (squares; Comparative Example 1), and samples containing SBS C-718 (circles; Comparative Example 2) prepared using Method 1: a) Response of viscosity to steady shear at 0.5 s -1 of; b) Time evolution of the storage modulus G' after steady shear stops; c) Mechanical spectra (G': hollow symbols; G": solid symbols) recorded at the end of the time evolution; d) Stress correlation of G' (hollow symbols) and G" (solid symbols). The symbols represent the average values measured for each ink at 4 different sample thicknesses.

[0023] Figure 2Rheological characterization of sample SEBS C-H6110 / M3 at different thicknesses: 800 μm (left triangle), 817 μm (circle), 820 μm (diamond), and 980 μm (upper triangle). a) Mechanical spectra recorded at the end of the time evolution (G': open symbols; G": solid symbols); b) Oscillatory strain response to large amplitude oscillatory stress at 1 Hz frequency.

[0024] Figure 3 Shows: a) Time dependence of the shear storage modulus G' measured after 60 s of steady shear cessation at a shear rate of 0.5 s -1 for inks SEBS C-H6110 / M1 (square), SEBS C-H6110 / M2 (circle), and SEBS C-H6110 / M3 (triangle); b) Mechanical spectra of inks SEBS C-H6110 / M1 (triangle), SEBS C-H6110 / M2 (circle), and SEBS C-H6110 / M3 (square) (G': open symbols; G": solid symbols); c) Stress dependence of the storage modulus (G': open symbols) and loss modulus (G": solid symbols) of samples SEBS C-H6110 / M1 (triangle), SEBS C-H6110 / M2 (circle), and SEBS C-H6110 / M3 (square) measured during a large amplitude oscillatory shear (LAOS) stress sweep.

[0025] Figure 4 SEM surface images of the positive electrodes obtained using different polymer binders and methods are shown: a) SBS C-718 / M1, b) SEBS C-H6180X / M1, c) SEBS C-H6110 / M1, d) SEBS C-H6110 / M2, and e) SEBS C-H6110 / M3.

[0026] Figure 5 Shows: a) Fifth charge / discharge cycle curves at C5 and C / 5 rates; b) Rate performance of the discharge curves at C-rates between 5C and C / 5; c) Cycle life stability at C-rate and 2C-rate; d) Coulombic efficiency as a function of the number of cycles. All measurements were carried out at room temperature on all printed positive electrodes prepared by different polymers and dispersion methods (samples SBS C-718 / M1 are represented by squares, samples SEBS C-H6180X / M1 are represented by circles, samples SEBS C-H6110 / M1 are represented by triangles, samples SEBS C-H6110 / M2 are represented by stars, and samples SEBS C-H6110 / M3 are represented by diamonds).

[0027] Figure 6Shows: a) the EIS results after cycling and the corresponding equivalent circuits, and b) the differential capacity (dQ / dV) results of all printed positive electrodes prepared by different polymers and dispersion methods (squares represent sample SBS C-718 / M1, circles represent sample SEBS C-H6180X / M1, triangles represent sample SEBS C-H6110 / M1, stars represent SEBS C-H6110 / M2, and diamonds represent sample SEBS C-H6110 / M3). Detailed Description

[0028] Unless otherwise specified, all terms used in this application shall be understood to have their ordinary meanings known in the art. Other more specific defined terms used in this application are described below and will be uniformly applicable throughout the specification and claims, unless a more extensive definition is provided by a clearly defined definition otherwise.

[0029] The term "C-rate" as used herein refers to a measure of the discharge rate of a battery relative to the maximum capacity of the battery. A 1C rate means that the discharge current will discharge the entire battery in 1 hour.

[0030] The term "specific capacity" refers to the amount of electric charge (Ah) stored and transported per unit weight of the electrode active material, usually expressed in mAh / g, where g refers to the g of the active material.

[0031] Unless otherwise specified, the term "weight percentage (wt%)" of the components of an electrode in the slurry obtained for preparing the electrode active layer refers to the percentage of each component in the total weight of the electrode active layer.

[0032] It should be noted that in this specification and the appended claims, unless the context clearly dictates otherwise, the singular forms "a", "an", and "the" include plural referents.

[0033] As described above, a first aspect of the present disclosure relates to an electrode for a lithium-ion battery, the electrode comprising an active layer, the active layer comprising a positive electrode active material, a conductive additive, and a linear styrene-ethylene / butylene-styrene (SEBS) copolymer, the SEBS copolymer being characterized in that: the melt flow rate (MFR) measured at 230 °C and a load of 2.16 kg is 4 g / 10 min to 220 g / 10 min, such as 4 g / 10 min, 10 g / 10 min, 30 g / 10 min, 50 g / 10 min, 100 g / 10 min, or 150 g / 10 min; the molecular weight is less than 100,000 g / mol, particularly 50,000 g / mol to 100,000 g / mol, such as 70,000 g / mol, or 75,000 g / mol, or 80,000 g / mol; and the styrene content is 10 wt% to 20 wt%, such as 15 wt%.

[0034] The SEBS copolymer of the present disclosure exhibits particularly high adhesion to the current collector, excellent mechanical properties, and good thermal stability, and can endow the battery with better performance when used as an adhesive for the positive electrode and / or negative electrode, especially for lithium-ion batteries.

[0035] The linear SEBS copolymer used in the present disclosure can be prepared by a method comprising the following steps:

[0036] a) In the presence of a polar modifier (such as tetrahydrofuran or bis(tetrahydrofuran)propane), styrene (S), butadiene (B), and styrene (S) are sequentially polymerized with an anionic initiator (such as n-butyllithium) in a non-polar solvent (such as cyclohexane, n-hexane, and mixtures thereof), and the reaction mode of butadiene is changed by generating a 1,2 vinyl structure in the chain to obtain a linear SBS - Li + copolymer.

[0037] b) Reacting the obtained linear SBS - Li + copolymer with a hydrophilic substance such as butylated hydroxytoluene (BHT) or methanol to obtain a linear SBS polymer; and

[0038] d) Hydrogenating the central butadiene block (B) of SBS in the presence of hydrogen and a titanium catalyst, thereby converting it into an ethylene / butylene (EB) block, and obtaining a linear styrene- b -ethylene / butylene- b -styrene block copolymer (SEBS).

[0039] In step a), in the absence of a polar modifier, mainly a 1,4 structure and a 1,2 vinyl structure with a reduced proportion (the latter being about 9 wt% to 12 wt% relative to the butadiene added) are obtained in the polybutadiene polymer block. In contrast, in the presence of a polar modifier, the proportion of the 1,2 vinyl structure increases to the level of 50 wt% to 65 wt%. In addition, the amount of styrene used in the polymerization reaction is such that the styrene content in the final SEBS is 10 wt% to 20 wt%, for example 15 wt%, and the polymerization proceeds to the extent that the total molecular weight of the SEBS copolymer is equal to or less than 100,000 g / mol, especially 50,000 g / mol to 100,000 g / mol, for example 70,000 g / mol, or 75,000 g / mol, or 80,000 g / mol. Since the MFR decreases as the MW increases, once the styrene content and the vinyl content are determined, the MW can be adjusted to obtain the desired MFR, that is, 4 g / 10 min to 220 g / 10 min.

[0040] The molecular weights of the polystyrene block, the polyethylene / butene block, and the SEBS are controlled by the ratio and amount of the monomers styrene and / or butadiene relative to the initiator in the anionic polymerization reaction.

[0041] The molecular weight is determined by gel permeation chromatography (GPC), using Mark-Houwink k = 0.0003253 and α = 0.693.

[0042] Thus, in one embodiment, the above linear SEBS copolymer is characterized in that:

[0043] - The melt flow rate (MFR) measured at 230 °C and a load of 2.16 kg is 4 g / 10 min to 220 g / 10 min, for example 4 g / 10 min, 10 g / 10 min, 30 g / 10 min, 50 g / 10 min, 100 g / 10 min or 150 g / 10 min;

[0044] - The molecular weight is less than 100,000 g / mol, especially 50,000 g / mol to 100,000 g / mol, for example 70,000 g / mol, or 75,000 g / mol, or 80,000 g / mol;

[0045] - The styrene content is 10 wt% to 20 wt%, for example 15 wt%; and

[0046] - The vinyl content is 50 wt% to 65 wt%, for example 60 wt% or 62 wt%.

[0047] In another embodiment, it can be optionally combined with one or more features of the above specific embodiments. The features of the above linear SEBS further lie in that the molecular weight of the polystyrene block (S) is equal to or less than 9000 g / mol.

[0048] Examples of commercially available SEBS with the above features are Calprene C-H6180X, Calprene C-H6181X, and Calprene C-H6182X of the Dynasol Group, and their properties are shown in Table 1 below.

[0049] Table 1

[0050]

[0051] The linear SEBS copolymer disclosed above is used as a binder in the preparation of the lithium-ion battery electrode of the present invention.

[0052] The lithium-ion battery electrode is obtained by forming an electrode active layer on a current collector. The electrode active layer contains an electrode active material, a conductive additive, and the linear SEBS copolymer disclosed above, where the electrode active material can be a positive electrode active material or a negative electrode active material.

[0053] The electrode active layer is formed by applying a slurry composition containing an electrode active material, a conductive additive, and the binder disclosed herein to the current collector, and then drying the slurry composition, that is, by evaporating the solvent.

[0054] The method of applying the slurry composition to the current collector is not particularly limited. Examples of this method include screen printing, doctor blade method, dipping method, reverse roll method, direct roll method, gravure method, extrusion method, comma direct coating, slide die coating, and brush coating method.

[0055] Examples of the drying method include: drying with warm air, hot air, or low humidity air; vacuum drying; and drying methods such as irradiation with (far) infrared rays, electron beams, etc. The drying time is usually 1 minute to 60 minutes. The drying temperature is usually 40°C to 180°C, especially 60°C to 80°C, for example 70°C. The electrode active material layer can be formed by repeatedly applying and drying the slurry composition multiple times.

[0056] Examples of the material of the current collector include metals, carbon, and conductive polymers. Metals are preferably used. Examples of the metals used for the current collector include aluminum, platinum, nickel, tantalum, titanium, stainless steel, copper, and alloys.

[0057] Among these metals, considering conductivity and pressure resistance, copper, aluminum, or aluminum alloy is preferably used.

[0058] The thickness of the current collector is preferably from 5 μm to 100 μm, more preferably from 8 μm to 70 μm, and even more preferably from 10 μm to 50 μm.

[0059] The slurry composition can be obtained by mixing an electrode active material, a conductive additive, the linear SEBS copolymer disclosed above, and a solvent.

[0060] Examples of suitable solvents include, but are not limited to, cyclopentyl methyl ether (CPME), cyclohexane, n - hexane, methylcyclohexane, benzene, toluene, tetrahydrofuran, cyclopentane, ethylbenzene, n - heptane, 1 - hexene, n - octane, n - pentane, and o - xylene.

[0061] CPME is considered a "green solvent" and is often recommended for replacing toxic solvents because of its low acute toxicity and negative mutagenicity. Since CPME has low solubility in water, it is also easily recyclable, and since its boiling point is 106 °C, the energy level required for its evaporation is low. Thus, in one embodiment, the solvent is CPME. The aim of this green method is to reduce the environmental impact associated with the use of toxic solvents and to reduce waste of resources by recycling materials.

[0062] Positive electrode

[0063] In one embodiment, optionally in combination with one or more features of the above - described embodiment, the electrode of the present invention is a positive electrode. Thus, the electrode active material is a positive electrode active material.

[0064] The role of the binder is to promote good physical cohesion between the active material and the conductive additive, improve the mechanical stability and flexibility of the electrode, and provide high adhesion of the positive electrode slurry to the substrate. In addition, it helps to form a structural network with electrical pathways, improving the diffusion of lithium ions and the transport of electrons.

[0065] In one embodiment, optionally in combination with one or more features of the above - described embodiment, relative to the total weight of the positive electrode active layer, the content of the binder in the positive electrode active layer is from 2 wt% to 25 wt%, for example 10 wt%.

[0066] The positive electrode active material acts as a lithium - ion reservoir. Examples of the positive electrode active material include, but are not limited to, carbon - coated lithium iron phosphate (C - LiFePO4), lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium cobalt phosphate (LiCoPO4), lithium manganese phosphate (LiMnPO4), lithium nickel oxide (LiNiO2), lithium nickel cobalt oxide [LiNi 1-x Co x O2 (0.2 ≤ x ≤ 0.5)], lithium nickel manganese cobalt oxide (LiNi 1 / 3 Mn 1 / 3Co 1 / 3 O2), lithium nickel manganese oxide (LiNi 0.5 Mn 0.5 O2), lithium vanadate (LiV2O5), lithium vanadium (V) oxide (LiV3O8). Among various active materials, C-LiFePO4 with an olivine structure is considered one of the most relevant materials due to its high theoretical storage capacity (170 mAh·g -1 ). In addition, it exhibits excellent thermal and chemical stability and is an environmentally "friendly" low-cost material as it contains no heavy metals or toxic metals in its composition.

[0067] Generally, the amount of the positive electrode active material in the positive electrode active layer is 45 wt% to 95 wt%, for example 80 wt%, relative to the total weight of the positive electrode active layer.

[0068] The conductive additive improves the conductivity of the active material and increases the conductivity of the positive electrode. Examples of the conductive additive include, but are not limited to, carbon black (such as carbon black C-NERGY TM Super C45 (hereinafter simply referred to as C45; Imerys G&C)), graphite, and carbon fiber. In particular, the conductive additive is graphite.

[0069] Generally, the amount of the conductive additive in the positive electrode active layer is 3 wt% to 30 wt%, for example 10 wt%, relative to the total weight of the positive electrode active layer.

[0070] Negative electrode

[0071] In another embodiment, the electrode of the present invention is a negative electrode. Then, in addition to the linear SEBS copolymer used as the binder described above, the active layer further includes a negative electrode active material and a conductive additive.

[0072] In one embodiment, optionally in combination with one or more features of the above specific embodiments, the content of the binder in the negative electrode active layer is 2 wt% to 25 wt%, for example 10 wt%, relative to the total weight of the negative electrode active layer.

[0073] Examples of the negative electrode active material include, but are not limited to, carbon that is easily graphitized, carbon that is difficult to graphitize, low-crystalline carbon (amorphous carbon) (such as pyrolytic carbon), graphite (natural graphite, artificial graphite), alloy materials formed by tin or silicon, and oxides (such as silicon oxide, tin oxide, or lithium titanate).

[0074] Generally, the amount of the negative electrode active material in the negative electrode active layer is 45 wt% to 95 wt% relative to the total weight of the negative electrode active layer.

[0075] Examples of the conductive additive include, but are not limited to, carbon black (e.g., carbon black C-NERGY TM Super C45 (hereinafter referred to as C45; Imerys G&C)), and carbon fiber. In particular, the conductive additive is carbon black.

[0076] Generally, relative to the total weight of the negative electrode active layer, the amount of the conductive additive in the negative electrode active layer is 3 wt% to 30 wt%, for example 10 wt%.

[0077] Preparation of electrodes

[0078] As described above, the second aspect of the present invention relates to a method for preparing the above electrode, the method comprising:

[0079] a) obtaining a slurry comprising:

[0080] a linear styrene-ethylene / butene-styrene (SEBS) copolymer as a binder, an electrode active material, a conductive additive, and a suitable solvent, the SEBS copolymer being characterized in that:

[0081] - the melt flow rate (MFR) measured at 230 °C and a load of 2.16 kg is 4 g / 10 min to 220 g / 10 min, for example 4 g / 10 min, 10 g / 10 min, 30 g / 10 min, 50 g / 10 min, 100 g / 10 min, or 150 g / 10 min;

[0082] - the molecular weight is less than 100000 g / mol, particularly 50000 g / mol to 100000 g / mol, for example 70000 g / mol, or 75000 g / mol, or 80000 g / mol; and

[0083] - the styrene content is 10 wt% to 20 wt%, for example 15 wt%;

[0084] b) applying the slurry onto a current collector; and

[0085] c) drying the applied slurry to form an electrode active layer on the current collector.

[0086] In particular, the vinyl content of the linear SEBS is 50 wt% to 65 wt%, for example 60 wt% or 62 wt%.

[0087] For the electrodes used in the method, all specific embodiments of the electrodes of the present invention are also specific embodiments of the method of the present invention.

[0088] In one embodiment, optionally in combination with one or more features of the above specific embodiments, the electrode is a positive electrode, and the electrode active material is a positive electrode active material.

[0089] The inventors have also recognized that by implementing the method for preparing the positive electrode disclosed herein, but adding different components in a specific order to form an aqueous slurry that will be used to obtain the positive electrode active layer, a battery with an even better specific capacity can be obtained.

[0090] Therefore, in a specific embodiment of the method for preparing the positive electrode, optionally in combination with one or more features of the above specific embodiments, the slurry of step a) is formed by the following method:

[0091] - Dissolve the SEBS copolymer in a solvent (especially CPME) under stirring to obtain a copolymer solution;

[0092] - Add the positive electrode active material and the conductive additive to the stirred copolymer solution in sequence to obtain a suspension; and

[0093] - Stir the obtained suspension to obtain a slurry.

[0094] As shown in the examples, the samples prepared by the above method show a higher charge / discharge specific capacity compared to the samples prepared by other dispersion methods. In addition, the attenuation of the capacity is also slightly lower.

[0095] Without intending to be bound by theory, it is believed that the optimized positive electrode performance may be related to the improved dispersion degree of the positive electrode components, which can be evaluated by the rheological properties of the ink used to prepare the positive electrode and the morphological characteristics of the printed positive electrode.

[0096] The positive electrode that can be obtained by the method as described above also constitutes a part of the present invention. All specific embodiments of this method are also specific embodiments of the positive electrode that can be obtained by the said method.

[0097] In another embodiment, optionally in combination with one or more features of the above specific embodiments, the electrode is a negative electrode, and the electrode active material is a negative electrode active material.

[0098] In another embodiment, optionally in combination with one or more features of the above specific embodiments, in the method for preparing the electrode disclosed above, step b) is carried out by screen printing.

[0099] Lithium-ion battery

[0100] The electrodes as described above can be used to manufacture lithium-ion batteries. Thus, as described above, the lithium-ion battery also forms part of the present invention, which includes a positive electrode, a negative electrode, a separator, and a suitable electrolyte disposed between the positive electrode and the negative electrode, wherein the positive electrode and / or the negative electrode are as described above, i.e., include the linear styrene-ethylene / butylene-styrene (SEBS) copolymer as described above.

[0101] When both the positive electrode active layer and the negative electrode active layer include the SEBS described in the present disclosure as a binder, an even greater improvement in specific capacity can be obtained. When the separator also contains the above-mentioned SEBS as a binder, this improvement is even higher.

[0102] Therefore, in a specific embodiment, the separator also contains the SEBS as described above.

[0103] Examples of the separator include: microporous films or non-woven fabrics containing polyolefin resins (such as polyethylene or polypropylene or SEBS / polyolefin composites) or aromatic polyamide resins; and porous resin coatings containing inorganic ceramic powders and impregnated with an electrolyte (an organic solvent containing a lithium salt).

[0104] In a more specific embodiment, the separator includes a polyolefin resin and the above-mentioned SEBS. The non-woven fabric made of SEBS / polyolefin compounds has the advantage of providing the required flexibility for the separator. In addition, these non-woven fabrics can be made in different thicknesses and with filaments of different diameters to achieve the required porosity. Therefore, SEBS provides flexibility and durability for the separator.

[0105] From the perspective of reducing the resistance generated by the separator in lithium-ion secondary batteries and having excellent processability when manufacturing lithium-ion secondary batteries with a porosity of 40% to 60%, the thickness of the separator is preferably 0.5 μm to 40 μm, more preferably 1 μm to 30 μm, and even more preferably 1 μm to 25 μm.

[0106] Examples of the electrolyte include solutions obtained by dissolving lithium salts in non-aqueous solvents. Examples of lithium salts include LiPF6, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiClO4, CF3SO3Li, C4F9SO3Li, CF3COOLi, (CF3CO)2NLi, (CF3SO2)2NLi, (C2F5SO2)NLi, and mixtures thereof. In particular, the lithium salt is selected from LiPF6, LiClO4, CF3SO3Li, and mixtures thereof. With respect to the electrolyte, the amount of the lithium salt is generally 1 wt% to 30 wt%, especially 5 wt% to 20 wt%.

[0107] Examples of solvents for the electrolyte include: alkyl carbonates such as dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), butylene carbonate (BC) or methyl ethyl carbonate (MEC); esters such as γ-butyrolactone or methyl formate; ethers such as 1,2-dimethoxyethane or tetrahydrofuran; sulfur-containing compounds such as sulfolane or dimethyl sulfoxide; and mixtures thereof. In particular, the solvent is selected from dimethyl carbonate, ethylene carbonate, propylene carbonate, diethyl carbonate, methyl ethyl carbonate and mixtures thereof.

[0108] The lithium ion secondary battery is obtained by superimposing a negative electrode and a positive electrode with a separator interposed therebetween, placing the resulting product in a battery container, injecting an electrolyte into the battery container, and sealing the opening of the battery container.

[0109] Throughout the specification and claims, the term "comprise" and variations of the term do not exclude other technical features, additives, ingredients or steps.

[0110] Furthermore, the term "comprising" includes the case of "consisting of".

[0111] Furthermore, the present invention encompasses all possible combinations of the specific and preferred embodiments herein.

[0112] The following examples and drawings are provided by way of illustration and are not intended to limit the present invention.

[0113] Examples

[0114] 1. Materials

[0115] C-LiFePO4 (C-LFP) obtained from Phostech Lithium was used as the active material. Carbon black particles (Super P-C45) obtained from Timcal Graphite & Carbon were used as the conductive additive. Styrene-butadiene-styrene (SBS Calprene C-718; abbreviated as C-718) and styrene-ethylene-butadiene-styrene (SEBS Calprene C-H6180X and SEBS Calprene C-H6110) provided by Dynasol were used as the polymer binder (Table 2). The cyclopentyl methyl ether (CPME) green solvent was obtained from Carlo Erba. 1M lithium hexafluorophosphate (LiPF6) (Solvionic) dissolved in a 1:1 volume / volume solution of ethylene carbonate (EC)-dimethyl carbonate (DMC) was used as the electrolyte. All reagents and materials were used as received.

[0116] Table 2: Characteristics of SBS and SEBS polymers used as polymer binders for the positive electrode.

[0117]

[0118] 2. Preparation of positive electrode

[0119] 2.1 Positive electrode ink formulation

[0120] Several ink positive electrode formulations were prepared by mixing the active material (C-LFP), polymer binder, and conductive additive (Super P-C45) with CPME as the solvent. As the polymer binder, the following copolymers were used: SBS C-718, SEBS C-H6180X, and SEBS C-H6110.

[0121] The relative amounts of the solid components used were as follows: 80% active material, 10% polymer binder, and 10% conductive additive. The total solid content, i.e., the total amount of the active material, polymer binder, and conductive additive, was 40 wt% of the ink positive electrode formulation. Thus, the content of the polymer binder was 6 wt% of the ink positive electrode formulation.

[0122] 2.2 Description of the positive electrode preparation method

[0123] Three different methods of producing the ink were evaluated.

[0124] a) Method 1

[0125] First, the copolymer used as the binder was weighed and dissolved in CPME, and magnetically stirred (1000 rpm) at room temperature for 30 minutes to prepare a binder solution with a copolymer concentration of 6 wt%. Then, the C-LFP active material and C45 conductive additive were weighed, subjected to dry stirring treatment, and manually mixed thoroughly. Then, the dry mixture was added to the copolymer solution to achieve a solid ratio of 3.5 mL solvent + 1.86 solids, where the weight of the polymer was 0.19 g. Then, the resulting ink was subjected to a new stirring treatment (1000 rpm) at room temperature for 1 hour 30 minutes. The ink that had already reached the consistency of a paste was placed in an ultrasonic bath (ATM40-3LCD) for 1 hour, and then returned to the stirring plate (1000 rpm) for a final stirring treatment at room temperature for 30 minutes.

[0126] b) Method 2

[0127] Method 2 involves preparing a dry mixture of the active material and the conductive additive as in Method 1. Then, CPME is added to the dry mixture, and the resulting suspension is continuously stirred (1000 rpm) at room temperature for 1 hour and 30 minutes. Then the copolymer is added to the suspension, and stirring is continued (1000 rpm) at room temperature for 30 minutes. Immediately afterwards, the ink is placed in an ultrasonic bath for 1 hour and then returned to the stirring plate (1000 rpm) for a final stirring treatment at room temperature for 30 minutes as well.

[0128] c) Method 3

[0129] This method does not use a dry mixing process. At the start of the preparation, the polymer is dissolved in the CPME solvent and magnetically stirred (1000 rpm) at room temperature for 30 minutes to form a copolymer solution. After that, the C-LFP active material and the C45 conductive additive are added to the stirred copolymer solution in sequence. The resulting suspension is continuously stirred (1000 rpm) at room temperature for 1 hour and 30 minutes and then ultrasonically treated for 1 hour. Then the ink is returned to the stirring plate (1000 rpm) and stirring is carried out at room temperature for another 30 minutes.

[0130] It is important to mention that the samples are labeled as POL / MX, where POL refers to the polymer using the abbreviation and model number of the polymer (SBS C-718, SEBS C-H6180X or SEBS C-H6110), and MX represents the experimental preparation method, where X represents the method number used (1, 2 or 3).

[0131] 2.3 Fabrication of the positive electrode

[0132] The ink is printed using a stainless-steel manual machine by screen printing. The adjustable substrate holder allows for x, y, and z-axis adjustment. To ensure an optimal distance of 10 mm between the grid and the substrate, the frame stably holds the grid. The grid consists of a polyester grid with 65 lines / cm -1 with a square side mesh opening of 102 μm and a wire diameter of 52 μm. A constant force of 17 N and an angle of 45° are applied between the squeegee and the printing substrate. The positive electrode is printed using an aluminum substrate and then dried (solvent evaporation) in a conventional oven (Selecta, 2000208) at 60 °C in an air atmosphere for 20 minutes. The thickness of the positive electrode is approximately 24 ± 3 μm, and the average porosity measured with a glass hydrometer is 70 ± 3%.

[0133] 3. Characterization of inks

[0134] A constant volume (1.5 ml) of sample was loaded into a stress-controlled rotational rheometer (ARG2, TA Instruments) equipped with a 40 mm diameter plate coated with sandpaper to limit wall slip. An automatic gap setting protocol (constant plate movement speed from the back-off distance to the sample thickness) was used to generate a similar flow history during sample loading. The shear geometry was tapped with water to avoid any solvent loss during the test. A new sample was used for each thickness tested in the rheometer. A conditioning step (0.5 s -1 Steady shear) induced the same shear history on each sample. The duration of this step was 60 s to ensure a steady shear viscosity reading for all samples. The sample structural recovery after flow cessation was then followed by recording the storage modulus (G') and loss modulus (G") under 1 Hz, 0.01% small amplitude oscillatory shear (SAOS). After reaching equilibrium, the mechanical spectrum was recorded by sweeping the strain from 100 Hz to 0.01 Hz under 0.03% SAOS. Finally, the response of the samples to large amplitude oscillatory shear (LAOS) was determined by sweeping the stress at a constant frequency of 1 Hz and recording the basic components (G' and G") of the cyclic strain response.

[0135] 4. Characterization of positive electrodes

[0136] The morphology of the cathode was evaluated using a scanning electron microscope (SEM) with a NanoSEM-FEI Nova200 instrument at different magnifications (25000X and 120000X) and an accelerating voltage of 10 kV.

[0137] The adhesion between the cathode and the current collector as well as the cohesion and flexibility of the cathode film were evaluated. For this purpose, a homemade “bend tester” was used with different metal bars with diameters between 10.0 mm and 1.5 mm. Each metal rod was measured three times as described in A. et al., 2015.

[0138] 5. Battery assembly and characterization

[0139] The battery assembly was carried out using a Li / C-LFP Swagelok-type half-cell in a self-made glove box filled with argon. A Whatman glass microfiber disk (10 mm in diameter, grade GF / A) soaked in the electrolyte solution was used as the separator and placed between metallic lithium (0.75 mm thick, 8 mm in diameter, 99.9% purity) and the previously prepared printed positive electrode (8 mm in diameter). The Landt CT2001A instrument was used to test the charge / discharge cycles of the half-cell. The cycles were carried out at room temperature, with a voltage range of 2.5 V to 4.2 V and a current density of C / 5 to 5C (C = 170 mA.g -1 ). The electrochemical impedance spectroscopy (EIS) was measured using an Autolab PGSTAT 12 instrument in the frequency range of 1 MHz to 10 mHz with an amplitude of 10 mV.

[0140] Example 1 and Comparative Examples 1 and 2. Rheological properties of inks - Influence of different types of polymers

[0141] To evaluate the effect of the chemical structure of the copolymer on the dispersion quality of the active material and the conductive additive in the ink (slurry), samples formulated with SEBS C-H6180X (Example 1), SEBS C-H6110 (Comparative Example 1), and SBS C-718 (Comparative Example 2) were prepared using Method 1. Figure 1 The rheological statistics calculated in the experiments using different thicknesses for each type of ink are shown.

[0142] All the tested formulations showed qualitative rheological similarities (see Figure 1 ). Figure 1 The quantitative differences between the inks were evident, indicating that the type of copolymer affects the structure of the inks prepared by a similar method.

[0143] Table 3 shows the rheological parameters extracted from the data shown in Figure 1 : the steady-state viscosity (η) at a shear rate of 0.5 s -1 , the elastic shear modulus (G0) at structural equilibrium, the critical strain γ C (the strain at which G' drops to 95% of its plateau value G p ), the yield stress σ Y (the stress at which G' = G", indicating the onset of flow), and G p / G"max.

[0144] Table 3

[0145]

[0146] Table 3 summarizes the average values and the corresponding errors of the selected rheological parameters. As shown in this table, there are significant differences in the steady shear viscosities measured during preshearing. FromFigure 1 b It can be seen that the two inks formulated with SEBS (Example 1 and Comparative Example 1) show a faster structure build-up than the ink prepared with SBS C-718 (Comparative Example 2). The latter shows greater elasticity, as indicated by the larger value of G0 reported in Table 3 and Figure 1 the value of the storage shear modulus measured at 1 Hz in the mechanical spectra plotted in c. As pointed out in the section discussing the effect of the dispersion method on the ink structure above, both a longer structure build-up and greater elasticity indicate a more open network of fractal flocs of the particles. Therefore, SBS C-718 promotes a better degree of dispersion, which is verified by the higher elastic shear modulus at the structural balance (G0) in Table 3 than that of other SEBS polymers. On the other hand, the results of the non-linear rheology experiments show different structural diagrams. The critical strain γ C at the onset of non-linear behavior and the yield stress σ Y values indicate that the ink formulated with SEBS C-H6180X is more resistant to shear stress than the other two inks (see Table 3). This greater ability to resist mechanical stress is related to better particle dispersion. Therefore, the copolymer SEBS C-H6180X is the best choice for improving the dispersion of the active material and the conductive additive. The Payne effect is used to formulate a dispersion index, which is calculated from the ratio of the plateau value G p of G' at a smaller stress amplitude to the local maximum value G"max of G" measured at a larger amplitude. For samples that have reached structural equilibrium, the ratio G p / G"max shows a saturated constant value, and clearly establishes the relationship between a larger G p / G"max value and a sample with better colloidal particle dispersion. The values of G Figure 1 / G"max calculated from the data in d are reported in Table 3. The G p / G"max ratio in Table 3 indicates that the SBS copolymer is beneficial for better dispersion of solid particles in the ink. Generally speaking, p and the rheological results reported in Table 3 clearly show that if a high degree of particle dispersion is to be achieved in the ink, the SEBS C-H6110 copolymer should not be selected. Figure 1

[0147] Comparative Examples 1, 3 and 4. Rheological properties of inks - Influence of different ink preparation methods

[0148] Figure 2 ​Describes the reproducibility of the rheological data measured with sample SEBS C-H6110 / M3 (Comparative Example 3), where four tests were performed at different sample thicknesses. A similar set of qualitative data was obtained with samples SEBS C-H6110 / M1 (Comparative Example 1; see previous section) and SEBS C-H6110 / M2 (Comparative Example 4). At the onset of steady shear, different instantaneous viscosity values were measured, which converged to a steady value after 40 s (see Figure 2 a). However, mechanical spectra reminiscent of particulate gels with flocculation, sterically stabilized particulate gels, or glassy dispersions of Brownian spheres could be measured. Figure 2 In a, G' shows a very weak frequency dependence and is larger than G", which shows a reproducible minimum between 2 Hz and 10 Hz, which within the framework of the mode-coupling theory of glasses defines the separation between the relaxation time of the particles within the cage formed by the nearest neighbors and the slower time of cage melting. Similar qualitative spectra have been recorded for NMP-based positive electrode slurries using PVDF as the binder.

[0149] However, the data shown in Figure 2 were statistically processed in order to quantitatively distinguish between samples produced according to three different experimental procedures. Figure 3 Shows the mean values and error bars of the rheological functions calculated from the data of four tests at different thicknesses. Overall, Figure 3 the data shown in

[0150] 0.5 s -1 steady shear tests were mainly performed to improve the experimental reproducibility of concentrated suspensions by shear-induced specific structures. Disregarding their results, the rheological data indicate that sample SEBS C.H6110 / M3 shows a longer structure build-up after the steady shear is stopped. Figure 3 a shows that the G' of sample SEBS C-H6110 / M1 reaches structural equilibrium at 1000 s after an initial power-law behavior typical of glassy or colloidal dispersions. In contrast, the structure of sample SEBS C-H6110 / M3 continues to recover after 2000 s and hardly reaches equilibrium within 1 hour. Sample SEBS C-H6110 / M2 shows an intermediate kinetic process.

[0151] Figure 3The mechanical spectra of the three inks plotted in b show that the shear modulus of sample SEBS C-H6110 / M3 is significantly larger than that of the other two inks at all tested frequencies. The local minimum of G", corresponding to the transition between structural relaxation processes at low frequencies and fast local motion of particles at high frequencies, is present for all samples. However, the local motion in the SEBS C-H6110 / M2 sample shifts to higher frequencies, indicating an acceleration of this relaxation process.

[0152] Figure 3 The LAOS behavior of the slurry shown in c demonstrates the Payne effect commonly seen in highly filled polymer melts or concentrated dispersions. This effect is characterized by a simultaneous decrease in the G' modulus and the G" local maximum (G" becomes larger than G') before yielding and flowing at larger stresses. In general, Figure 3 The data in c show that sample SEBS C-H6110 / M3 is more resistant to stress from the onset of nonlinear behavior and the crossover between G' and G" occurs at higher stresses. Figure 3 The rheological data in Figure 2 show that the flocculated particles constituting the structure of the slurry SEBS C-H6110 / M3 are less aggregated.

[0153] The SEBS C-H6180X / M3 is expected to behave similarly compared to the SEBS C-H6180X / M1 and SEBS C-H6180X / M2.

[0154] Example 2. Morphology and conductivity behavior of printed positive electrodes

[0155] The uniform structure of the printed cathode plays an important role in the battery performance. Therefore, the morphological uniformity of the samples was evaluated by SEM measurement ( Figure 4 ). Good particle distribution was verified in all printed cathodes, indicating that the ink properties are suitable for screen printing technology. No aggregation of components was observed in all samples.

[0156] Regardless of the polymer binder type and dispersion method, the images show a three-dimensional interconnected structure of all three battery components (binder, conductive additive and active material). The non-spherical shape of the active material particles is equally obvious in all samples, and they present a rod-like structure with non-uniform sizes on the order of 2μm. The presence of voids along the electrode surface is obvious (porosity) and is attributed to solvent evaporation and differences in particle size and shape. This porosity allows lithium ions to enter the positive electrode, increasing the interfacial area between the electrode and the electrolyte, which is beneficial for the insertion kinetics.

[0157] Figure 4 a, b and c show that once the same dispersion method is applied, the morphology does not depend on the chemical structure of the binder. Comparing the SBS samples (Figure 4 a; Comparative Example 2) and SEBS sample prepared using method 1 ( Figure 4 b and c, that is, Example 1 and Comparative Example 1, respectively), it can be concluded that the dry blending of the active material and the conductive additive resulted in a similar degree of particle dispersion. Therefore, the presence of ethylene blocks in the SEBS copolymer (absent in SBS) and the proportion of styrene do not affect the morphology of the positive electrode, because the porosity (about 70%) and microstructure of the positive electrode are the same. In addition, it was observed that the DC conductivity of different polymers dispersed in the same way was about 10 S.m -1 According to this, a good structural distribution is achieved within the different polymers studied, promoting good mechanical stability and good cohesion between the different particles.

[0158] The conductivity was measured by the four-point probe method using a power supply DC 9818 from Time Electronics and a nanovoltmeter 2182 from Keithley. In the measurement, the electrode material was deposited on an insulating substrate and the conductivity (σ e , S.cm -1 ) is calculated as follows:

[0159] σ e =I / (3.3625tV)

[0160] Where t is the thickness of the sample in cm, I is the current in amperes, and V is the voltage in volts.

[0161] on the other hand, Figure 4 c, d and e (Comparative Examples 1, 4 and 3, respectively) show significant differences in particle distribution between samples prepared using inks prepared using different preparation methods. In particular, the sample prepared using method 2 ( Figure 4 d) shows that the polymer particles cover the active and conductive particles. The presence of copolymer particles can be explained by the fact that once the particles are first dispersed in the solvent, the resulting slurry has a significant viscosity, which is not conducive to a sufficient dissolution of the copolymer. In addition, the mixing time of the copolymer in the ink is reduced compared to the other two methods. The polymer coating of the particles affects not only the mechanical properties, but also the DC conductivity, since the electrical connection between the active materials is promoted. In this particular sample, the DC conductivity is 19.6 ± 0.9 Sm -1 , which is higher than the inks prepared by different methods: SEBS C-H6110 / M1 and SEBSC-H6110 / M3 are 8.4±1.5Sm -1 and 15.7±1.0Sm -1 Nonetheless, these samples showed the same porosity of 70%.

[0162] Generally speaking, Figure 4 The results in [reference] show that the preparation method of the ink affects the structure of the positive electrode. On the contrary, the chemical properties of the binder have no obvious effect on the particle dispersion and porosity in the positive electrode.

[0163] Example 3. Electrochemical performance - Cycling behavior

[0164] The half-cell electrochemical kinetics of the printed positive electrode was evaluated by different techniques, including charge / discharge cycling ( Figure 5 ), electrochemical impedance spectroscopy, and differential capacity (dQ / dV) ( Figure 6 ). At room temperature, the charge / discharge cycling performance was evaluated at different rates from C / 5 to 5C in the voltage range of 2.5 V to 4.2 V. Figure 5 Figure a shows the charge / discharge performance results of the fifth cycle of all printed positive electrodes at 5C and C / 5 rates. There are obvious flat plateaus in the charge and discharge curves, located at about 3.2 V and 3.7 V respectively, and decrease with the increase of the C-rate. Therefore, the diffusion of lithium decreases. This flat voltage plateau is indicated by the presence of Fe 2+ and Fe 3+ phases for the redox reaction between LiFePO4 and FePO4 - .

[0165] By comparing different polymer binders, the charge / discharge performance of SEBS C-H6110 / M1 (at 5C) and the lower platform stability (the discharge capacities at 5C and C / 5 are 52 mAh·g -1 and 137 mAh·g -1 respectively). This fact proves that due to the specific interaction between the active material, conductive additive and polymer matrix, the polymer binder type effectively affects the battery rate performance. Therefore, the positive electrode prepared with SEBS C-H6180X / M1 shows the highest discharge capacity at both 5C and C / 5 rates (105 mAh·g -1 and 142 mAh·g -1), Therefore, by comparing the discharge performance of SEBS C-H6110 / M1 and SEBS C-H6180X / M1, it can be concluded that the ethylene / butene (E / B) ratio has a significant impact on battery performance, and the battery performance increases with the increase of the E / B ratio. It can also be concluded that for almost the same styrene content (25% and 30% for SBS C-718 / M1 and SEBS C-H6110 / M1 respectively), the presence of ethylene in SEBS has a negative impact on the discharge capacity at high C-rates and a positive impact at low C-rates. Compared with SBS, the presence of ethylene in SEBS results in higher polymer entanglement, promoting better coupling between the polymer and the particles (active material and conductive additive), thus improving the battery performance.

[0166] Regarding different methods for preparing inks, the conclusion is that compared with the samples prepared by other preparation methods, the sample (M3) prepared by method 3 shows a higher charge-discharge specific capacity (the charge capacity and discharge capacity are 150 mAh.g -1 and 147 mAh.g -1 ), respectively). Compared with methods M1 and M2, method M3 is characterized by adding the active material and the conductive additive to the polymer solution separately, increasing the interaction between the two kinds of particles and the polymer. The increased interaction between the polymer and the particles improves the pathway of lithium ion movement and promotes higher lithium intercalation / deintercalation. On the other hand, the results of SEBS C-H6110 / M1 show a lower plateau voltage and an increase in potential, which promotes the reduction of lithium diffusion, explaining the lower specific capacity of this sample.

[0167] The rate performance was also evaluated ( Figure 5 b), showing that the discharge capacity decreases with the increase of the C-rate. The discharge capacities of SBSC-718 / M1, SEBS C-H6180X / M1 and SEBS C-H6110 / M1 are 128.3 mAh.g -1 , 141.2 mAh.g -1 and 136.3 mAh.g -1 at C / 5, respectively, and 83.6 mAh.g -1 , 105.3 mAh.g -1 and 52.5 mAh.g -1 at 5C, respectively, indicating that the choice of the polymer matrix has an impact on battery performance, and SEBS C-H6180X / M1 shows the highest discharge capacity at different C-rates. The evaluation of different dispersion methods for a given polymer binder shows that the discharge capacities of SEBS C-H6110 / M1, SEBS C-H6110 / M2 and SEBS C-H6110 / M3 are 136.3 mAh.g-1 and 126.7 mAh / g -1 and 146.4 mAh / g -1 , and were 52.5 mAh / g -1 , 95.8 mAh / g -1 and 106.3 mAh / g -1 , respectively, at 5 C. The ink sample (SEBS C-H6110 / M3) obtained by Method 3 had a higher discharge capacity. Figure 5 b also shows the recovery cycles of different samples, demonstrating a reduced capacity fade for all samples at C / 5.

[0168] After 50 charge / discharge cycles at C-rate and 2C-rate, the cycling stability of the printed cathodes was evaluated ( Figure 5 c), and all samples showed good stability and high capacity retention at both cycling and scan rates. After 50 cycles at 2C, the capacity retentions of the cathodes with different polymer binders were 85%, 93%, and 100% for SEBS C-H6110 / M1, respectively. The capacity fade between the 2nd and 50th cycles was calculated to be 40%, 5%, and 7% for SEBS C-H6110 / M1, SBS C-718 / M1, and SEBS C-H6180X / M1, respectively.

[0169] In addition, different dispersion methods led to appropriate stability and high capacity retention of the cathodes at cycling and scan rates. Therefore, the capacity retentions of SEBS C-H6110 / M1, SEBS C-H6110 / M2, and SEBS C-H6110 / M3 after 50 cycles at 2C were 85%, 99%, and 88%, respectively, and the capacity fades between the 2nd and 50th cycles were 40%, 1%, and 1%, respectively.

[0170] Except that the SEBS C-H6110 / M2 cathode showed a Coulombic efficiency of approximately 95% at C-rate, the Coulombic efficiencies ( Figure 5 d) of all polymer binders and preparation methods were around 100%.

[0171] Therefore, the overall results indicate that SEBS C-H6180X is the most suitable polymer as a polymer binder for cathode development due to its high discharge capacity, good stability, high capacity retention, and low capacity fade. In addition, it was also concluded that the dispersion method affects the rate performance of the battery. Compared with the other two methods, Method M1 (conventional) led to lower battery rate performance ( Figure 5 a). Method M3 led to higher charge and discharge capacities of the cathode (the discharge capacities at 5C and C / 5 were 106 mAh / g -1and 147 mAh·g -1 )。The conclusion is that the slurry preparation method also affects the cathode performance, and Method 3 is the method that leads to the best results, indicating that the active material and conductive additive are preferably added to the polymer solution separately to improve the interaction with the polymer binder and obtain better battery performance.

[0172] Example 4. Electrochemical impedance spectroscopy (EIS) and dQ / dV results

[0173] Through EIS( Figure 6 a) and dQ / dV( Figure 6 b) techniques, the electrochemical characteristics of printed cathodes prepared using different polymers and methods were evaluated.

[0174] The Nyquist plots( Figure 6 a) of the printed cathodes before and after battery cycling were evaluated. The Nyquist plot is characterized by two different steps, one is a semicircle at high frequency and the other is a straight line at low frequency. In this case, the proposed equivalent circuit (inserted in Figure 6 a) was used to evaluate the kinetic parameters and reaction mechanism. The equivalent circuit and the Nyquist plot are characterized in that the semicircle located at high frequency represents the sum of different resistance contributions, such as: electrolyte (R e ), film surface (R f ) representing the migration resistance of lithium ions through the solid electrolyte interface (SEI) film located on the cathode surface, and charge transfer resistance (R ct ). On the other hand, at lower frequencies, the straight line represents the semi-infinite diffusion of the Warburg element (W) related to lithium ion diffusion. Since there is no SEI on the cathode surface, the prepared cathode evaluated before cycling does not show a Warburg line defined as 45° in the low frequency region. Another element of the equivalent circuit is the capacitance element (CPE3), which describes the different intercalation capacities in the LFP cathode material.

[0175] The experimental results fitted with the equivalent circuit showed good agreement( Figure 6 a), and the total resistance (R e ), R SEI and R ct ) calculated by R total obtained through the fitting step could be obtained. The results showed that the resistance of all samples increased after battery cycling, except for the SEBS C-H6180X / M1 sample, which decreased from 519 Ω to 321 Ω. The high R ct of the SEBS C-H6110 / M1 sample (3860 Ω before cycling and 3926 Ω after cycling) led to poor performance of the battery in charge / discharge and rate performance cycling. According to the results, SEBS C-H6110 is the polymer binder that leads to the cathode with higher R total , while ink preparation method 1 leads to the cathode with the highest Rtotal The positive electrode. From the total resistance value, it can be concluded that the polymer and the dispersion method affect the specific polymer-filler interaction, resulting in changes in the transport of ions at the solid-electrolyte interface and charge transport. The lithium-ion diffusion coefficient (D Li + ) of the active cathode material before and after cycling for all samples was calculated according to the following formula.

[0176]

[0177] Z′ = R1 + R 2，SEI + R 3，ct + σ W W -1 / 2

[0178] where R is the gas constant, T is the absolute temperature, A is the surface area of the positive electrode, n is the number of electrons per molecule during the oxidation process, F is the Faraday constant, C is the concentration of Li + , σ w is the Warburg coefficient, R1 is the electrolyte resistance, R 2,SEI is the resistance of the SEI, R 3,ct is the charge transfer resistance, and W is the angular frequency.

[0179] The results show that after cycling, due to the formation of the SEI and the impregnation of the electrolyte, the battery exhibits an increase in D Li + compared to the samples before cycling. Comparing all the samples, SEBS C-H6180X / M1 exhibits the highest D Li + (15.6x10 -16 cm 2 .s -1 ) after cycling. This indicates that the SEBS C-H6180X copolymer improves the structural network of the electrical conduction path, enabling better diffusion of lithium ions. Regarding the dispersion method, it is shown that method 2 and method 3 improve D Li + compared to method 1. Method 2 and method 3 show D Li + to be 238x10 -16 cm 2 .s -1 and 187x10 -16 cm 2 .s -1 respectively. The improvement of D Li + depends more on the appropriate selection of the dispersion method rather than the chemical structure of the copolymer.

[0180] Figure 6b shows the differential capacity (dQ / dV) curve of the printed positive electrode. Two symmetric and relatively sharp peaks are observed between 3.35 V and 3.50 V, indicating high electrochemical reversibility of lithium ions. These values correspond to the reduction (in the backward scan potential) and oxidation (in the forward scan potential) processes in the printed positive electrode due to the intercalation and deintercalation processes of lithium ions, respectively. The peak separation voltage is between 0.09 V and 0.12 V. The peak separation voltage of SEBS C-H6110 / M1 is as high as 0.12 V, and high concentration polarization and high total resistance are observed, which again proves the low battery performance of this positive electrode. On the other hand, by comparing different polymer binders, SEBS C-H6180X / M1 shows 0.09 V, strengthening its higher battery performance.

[0181] The rapid increase in current in the oxidation peak and reduction peak indicates that the printed positive electrode obtained with the sample SEBS C-H6180X shows lower polarization and better reversibility compared to other samples. In addition, the low separation of the oxidation peak and reduction peak of SEBS C-H6180X is due to the decrease in the size of the particle aggregates of the active material and the conductive additive in the polymer matrix.

[0182] List of cited references

[0183] 1. R. et al. "Poly(styrene-butene / ethylene-styrene): A New Polymer Binder for High-Performance Printable Lithium-Ion Battery Electrodes", ACS Applied Energy Materials, 2018, Vol. 1, pp. 3331-3341.

[0184] 2. A. et al. "Influence of Solvent Evaporation Rate in the Preparation of Carbon-Coated Lithium Iron Phosphate Cathode Films on Battery Performance", Energy Technology, 2016, Vol. 4, pp. 573-582.

Claims

1. An electrode for a lithium-ion battery, the electrode comprising an active layer containing an electrode active material, a conductive additive, and a linear styrene-ethylene / butylene-styrene (SEBS) copolymer as a binder; the linear styrene-ethylene / butylene-styrene copolymer is characterized in that, - the melt flow rate (MFR) measured at 230 °C and a load of 2.16 kg is from 4 g / 10 min to 220 g / 10 min; - the weight-average molecular weight is less than 100,000 g / mol; and - the styrene content is from 10 wt% to 20 wt%.

2. The electrode according to claim 1, wherein, The linear styrene-ethylene / butylene-styrene copolymer is further characterized in that the weight-average molecular weight of the polystyrene block (S) is equal to or less than 9,000 g / mol.

3. The electrode according to claim 1 or 2, which is a positive electrode, the active layer is a positive electrode active layer, and the electrode active material is a positive electrode active material.

4. The electrode according to claim 3, wherein, The amount of the binder in the positive electrode active layer is from 2 wt% to 25 wt% based on the total weight of the positive electrode active layer.

5. The electrode according to claim 4, wherein, The amount of the positive electrode active material is from 45 wt% to 95 wt% based on the total weight of the positive electrode active layer.

6. The electrode according to claim 5, wherein, The amount of the conductive additive is from 3 wt% to 30 wt% based on the total weight of the positive electrode active layer.

7. The electrode according to claim 1 or 2, which is a negative electrode, the active layer is a negative electrode active layer, and the electrode active material is a negative electrode active material.

8. The electrode according to claim 7, wherein, The amount of the binder in the negative electrode active layer is from 2 wt% to 25 wt% based on the total weight of the negative electrode active layer.

9. A method for preparing the electrode according to claim 1, the method comprising: a) obtaining a slurry comprising: a linear styrene-ethylene / butylene-styrene copolymer as a binder, an electrode active material, a conductive additive, and a suitable solvent, the linear styrene-ethylene / butylene-styrene copolymer being characterized by: - the melt flow rate (MFR) measured at 230 °C and a load of 2.16 kg is from 4 g / 10 min to 220 g / 10 min; - the weight-average molecular weight is less than 100,000 g / mol; and - the styrene content is from 10 wt% to 20 wt%; b) applying the slurry onto a current collector; and c) drying the applied slurry to form an electrode active layer on the current collector.

10. The method according to claim 9, wherein, The electrode is a positive electrode, and the electrode active material is a positive electrode active material.

11. The method according to claim 10, wherein, The slurry in step a) is formed by: - dissolving the linear styrene-ethylene / butylene-styrene copolymer in a solvent under stirring to obtain a copolymer solution; - sequentially adding a positive electrode active material and a conductive additive to the stirred copolymer solution to obtain a suspension; and - subjecting the obtained suspension to a stirring treatment to obtain a slurry.

12. The method according to claim 9, wherein, The electrode is a negative electrode, and the electrode active material is a negative electrode active material.

13. The method according to any one of claims 9 to 12, wherein, Step b) is carried out by screen printing.

14. A lithium-ion battery, which includes a positive electrode, a negative electrode, a separator, and a suitable electrolyte disposed between the positive electrode and the negative electrode, wherein, The positive electrode and / or the negative electrode is the electrode according to claim 1.

15. The lithium ion battery according to claim 14, wherein, The positive electrode is the electrode according to any one of claims 3 to 6.

16. The lithium ion battery according to claim 14, wherein, The negative electrode is the electrode according to claim 7 or 8.

Images