Precursor for electrode mixture, electrode mixture, electrode mixture sheet, battery electrode, and rechargeable battery

Abstract

[Problem] The purpose of the present invention is to provide: a precursor for an electrode mixture, from which an electrode mixture that achieves both a fibrillated state and high dispersibility of PTFE can be obtained; and an electrode mixture, an electrode mixture sheet, a battery electrode, and a rechargeable battery, each of which uses said precursor. [Solution] PTFE and an active material for an electrode are mixed and kneaded in advance to produce a precursor 30 for an electrode mixture with a high concentration of the PTFE that is in a fibrillated state suitable for sheet formation. Next, an active material for an electrode is added to the precursor 30 for an electrode mixture to obtain a desired electrode mixture 40. As a result, the PTFE can be uniformly dispersed while being maintained in the fibrillated state suitable for sheet formation. In this manner, an electrode mixture sheet 50 having high strength and flexibility can be produced. Furthermore, the electrode mixture sheet 50, a battery electrode 80, and a rechargeable battery 100 having reduced variation in distribution of an active material for an electrode and uniform characteristics can be manufactured with high yield.

Description

Precursors for electrode mixtures, electrode mixtures, electrode mixture sheets, electrodes for batteries, and secondary batteries

[0001] The present invention relates to an electrode mixture precursor that forms the basis of electrode materials for secondary batteries, and to an electrode mixture, an electrode mixture sheet, a battery electrode, and a secondary battery using this electrode mixture precursor.

[0002] In recent years, there has been a rapid increase in demand for rechargeable secondary batteries used in mobile devices, various wireless devices, electric vehicles, stationary power supplies, and secondary power storage systems. In such secondary batteries, the electrodes used (battery electrodes) determine the charge and discharge characteristics. In the general manufacturing method of battery electrodes, electrode active material and binder are first dissolved and dispersed in water or a solvent to form an electrode slurry. Next, this electrode slurry is applied to a current collector, and the water and solvent are removed by heat or other means to form the battery electrode. However, this wet manufacturing method requires the evaporation and removal of water and solvent from the electrode slurry after application, which is inferior in terms of economy and productivity, as well as raising concerns about worker safety and adverse effects on the environment.

[0003] Regarding this problem, for example, [Patent Document 1] and [Patent Document 2] disclose technologies relating to battery electrodes and secondary batteries manufactured by a dry method that does not use electrode slurry. In these [Patent Document 1] and [Patent Document 2], polytetrafluoroethylene, which becomes fibrous (fibrillated) when shear force is applied, is given as an example of a binder when forming the electrode mixture into a sheet by dry method.

[0004] Japanese Patent Publication No. 2024-505952, International Publication No. 2023 / 054713, Brochure

[0005] Typically, the proportion of polytetrafluoroethylene in an electrode mixture is several wt%, which is small compared to the amount of electrode active material. Therefore, as described in [Patent Document 1] and [Patent Document 2], if several wt% of polytetrafluoroethylene is added to a large amount of electrode active material and mixed all at once, the dispersibility of polytetrafluoroethylene is poor, which can lead to variations in battery characteristics and a decrease in yield. Furthermore, as mixing progresses, the fibrillation of polytetrafluoroethylene progresses, but there is a range in this fibrillation state that is suitable for sheet formation, and exceeding this range reduces the flexibility of the sheet. Therefore, if dispersibility is prioritized, the fibrillation state may exceed the range suitable for sheet formation, and there is a problem in that it is difficult to establish mixing conditions that balance both fibrillation state and dispersibility.

[0006] The present invention has been made in view of the above circumstances, and aims to provide an electrode mixture precursor that can obtain an electrode mixture that achieves both the fibrillation state of polytetrafluoroethylene and high dispersibility, as well as an electrode mixture, an electrode mixture sheet, an electrode for a battery, and a secondary battery using the same.

[0007] The present invention solves the above problems by providing an electrode mixture precursor 30 characterized by (1) containing an electrode active material for a positive or negative electrode and stretchable polytetrafluoroethylene, wherein the polytetrafluoroethylene content is 30 wt% to 90 wt%. (2) having a tensile strength of 3.0 N / mm when formed into a sheet. ２ ~9.0 N / mm ２ The above problem is solved by providing the electrode mixture precursor 30 described in (1) above, characterized in that (3) contains an electrode active material for a positive or negative electrode and the electrode mixture precursor 30 described in (1) or (2) above, wherein the stretchable polytetrafluoroethylene is 0.1 wt% to 10.0 wt% in total and has a tensile strength of 0.5 N / mm ２ ~2.0 N / mm ２The above problem is solved by providing an electrode mixture 40 characterized in that... (4) The above problem is solved by providing an electrode mixture sheet 50 made of the electrode mixture 40 described in (3) above in sheet form... (5) The above problem is solved by providing a battery electrode 80 formed by bonding the electrode mixture sheet 50 described in (4) above with a current collector... (6) The above problem is solved by providing a secondary battery 100 having a negative electrode, a negative electrode terminal electrically connected to the negative electrode, a positive electrode, a positive electrode terminal electrically connected to the positive electrode, and an electrolyte for transferring ions between the negative electrode and the positive electrode, wherein one or both of the negative electrode and the positive electrode are the battery electrode 80 described in (5) above...

[0008] This invention involves pre-mixing polytetrafluoroethylene and an electrode active material to produce an electrode mixture precursor 30 in which polytetrafluoroethylene is in a fibrillated state suitable for sheet formation at a high concentration. Then, the electrode active material is added to this electrode mixture precursor 30 to produce an electrode mixture 40 with a desired electrode active material concentration. This allows for uniform dispersion of polytetrafluoroethylene. Furthermore, since the polytetrafluoroethylene in the electrode mixture precursor 30 is already in a fibrillated state suitable for sheet formation, the polytetrafluoroethylene can be homogeneously dispersed while maintaining a suitable fibrillated state. This makes it possible to produce an electrode mixture sheet 50 with high strength and flexibility. In addition, due to the high dispersibility, there is little unevenness in the electrode active material, and electrode mixture sheets 50, battery electrodes 80, and secondary batteries 100 with homogeneous properties can be manufactured with a high yield.

[0009] This is a diagram of the manufacturing process of the present invention. This is a graph showing the relationship between the PTFE content and tensile strength of the electrode mixture precursor. This is a table showing the results of the flexibility assessment of the electrode mixture. This is a graph showing the relationship between the PTFE content and tensile strength of the electrode mixture precursor. This is a graph showing the relationship between the PTFE content and tensile strength of the electrode mixture. This is a graph showing the relationship between the PTFE content and Coulomb efficiency of the electrode mixture. This is a graph showing the relationship between the angle of repose and tap density and tensile strength of the electrode mixture precursor.

[0010] The electrode mixture precursor 30, electrode mixture 40, electrode mixture sheet 50, battery electrode 80, and secondary battery 100 according to the present invention will be described with reference to the drawings. Here, Figure 1 is a manufacturing process diagram relating to the electrode mixture precursor 30, electrode mixture 40, electrode mixture sheet 50, battery electrode 80, and secondary battery 100 according to the present invention.

[0011] First, the electrode mixture precursor 30 according to the present invention is composed of stretchable polytetrafluoroethylene (hereinafter referred to as PTFE) and an electrode active material for the positive or negative electrode. Here, stretchable PTFE refers to a PTFE resin that undergoes fibrous formation (fibrillation) when shear force is applied, and examples include emulsion polymerized PTFE fine powder. Furthermore, there are no particular limitations on the PTFE used as long as it is stretchable; for example, it may be a mixture in which part of the PTFE is fibrillated and the rest is in particulate form or the like and has not yet undergone fibrillation, or it may be PTFE that has not undergone fibrillation at all. In other words, any PTFE that undergoes fibrous formation (fibrillation) when a certain shear force is applied can be used. Furthermore, this stretchable PTFE functions as a binder for electrode active materials, and the fibrillated PTFE, created by kneading, entangles with the powder components of the electrode active material, thereby binding and fixing the electrode active material. This makes it possible to form the powder components (electrode active material, etc.) into sheets using dry rolling or other processes.

[0012] Furthermore, the PTFE suitable for the present invention preferably has an average primary particle diameter of 100 nm to 450 nm, and in the aggregated state, it is particularly preferable that it has an average primary particle diameter of 10 μm to 1000 μm, which is approximately the same as the particle diameter of the electrode active material including the aggregates. In this configuration where the particle diameter of the aggregated PTFE and the electrode active material (including the aggregates) are approximately the same, the dispersibility of the PTFE and the electrode active material is high, and mixing in powder form can be easily and uniformly performed. The average primary particle diameter can be obtained by measuring the particle in any direction using an electron microscope image, and the average particle diameter in the aggregated state can be measured and obtained in accordance with the Japanese Industrial Standard JIS K 6891.

[0013] Further, the electrode active material constituting the electrode binder precursor 30 is not particularly limited, and well-known electrode active materials for positive electrodes or negative electrodes can be used. Also, as the shape of the electrode active material, conventionally used shapes such as spherical, plate-like, needle-like, columnar,块状, elliptical, etc. can be utilized, and each may exist as primary particles or as an aggregate. Furthermore, those with different chemical compositions or different particle shapes may be used alone or in combination of two or more different ones. Among them, it is most preferable to use spherical or granular ones with a particle diameter of 1 μm to 30 μm.

[0014] Further, as the electrode active material for the positive electrode, for example, in the case of a lithium ion battery, any active material that can reversibly allow lithium ions to enter and exit can be used without particular limitation. Specific examples include lithium-nickel composite oxides, lithium-cobalt composite oxides, lithium-manganese composite oxides, lithium-nickel-manganese composite oxides, lithium-nickel-cobalt composite oxides, lithium-nickel-aluminum composite oxides, lithium-nickel-cobalt-aluminum composite oxides, lithium-nickel-manganese-cobalt composite oxides, lithium-nickel-manganese-aluminum composite oxides, lithium-nickel-cobalt-manganese-aluminum composite oxides, etc., composite oxides of lithium and transition metals, TiS ２ , FeS, MoS ２ and other transition metal sulfides, MnO, V ２ O ５ , V ６ O １３ 、TiO ２Examples include transition metal oxides and olivine-type lithium phosphate oxides. Olivine-type lithium phosphate oxides, for example, contain at least one element from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, Nb, and Fe, along with lithium, phosphorus, and oxygen. These compounds may also have some elements partially substituted with other elements to improve their properties. Among these, lithium-nickel composite oxides are preferred as positive electrode active materials, and more preferably, these lithium-nickel composite oxides have the formula: LiNi Ｘ M1 Ｙ M2 Ｚ O ２ (M1 and M2 are preferably at least one metallic element from among Al, B, alkali metals, alkaline earth metals, and transition metals, and are expressed as 0.8 ≤ X ≤ 1.0, 0 ≤ Y ≤ 0.2, and 0 ≤ Z ≤ 0.2).

[0015] Furthermore, in addition to graphite, any well-known material can be used as the electrode active material for the negative electrode without any particular limitations. For example, if it does not have conductivity, metal oxide-based active material particles or silicon-based active material particles can be used, and it is particularly preferable to use metal oxide-based negative electrode active material particles. As for these metal oxide-based negative electrode active material particles, for example, titanium oxide can be used. As for this titanium oxide, there is no particular limitation as long as it is capable of intercalating and deintercalating lithium, for example, spinel-type lithium titanate, ramsdellite-type lithium titanate, titanium-containing metal composite oxide, or titanium dioxide (TiO) having a monoclinic crystal structure. ２ (B)), as well as anatase-type titanium dioxide, etc., can be used. Furthermore, as spinel-type lithium titanate, Li ４ +xTi ５ O １２ (x changes in the range of -1 ≤ x ≤ 3 due to the charge-discharge reaction) is one example. In addition, as a ramsdelite type lithium titanate, Li ２ +yTi ３ O ７ (Y changes in the range of -1 ≤ y ≤ 3 due to the charge-discharge reaction), and so on. Also, TiO ２(B) and as anatase titanium dioxide, Li １ +zTiO ２ (where z changes in the range of -1 ≤ z ≤ 0 due to charge-discharge reactions), and the like. Further, as the titanium-containing metal composite oxide, a metal composite oxide containing at least one element selected from the group consisting of Ti and P, V, Sn, Cu, Ni, and Fe, and the like can be mentioned. Examples of such metal composite oxides containing at least one element selected from the group consisting of Ti and P, V, Sn, Cu, Ni, and Fe include, for example, TiO ２ -P ２ O ５ , TiO ２ -V ２ O ５ , TiO ２ -P ２ O ５ -SnO ２ , TiO ２ -P ２ O ５ -MeO (Me is at least one element selected from the group consisting of Cu, Ni, and Fe), and the like. Such metal composite oxides preferably have a microstructure in which the crystallinity is low and the crystalline phase and the amorphous phase coexist, or the amorphous phase exists alone, because this further improves the cycle performance. Further, it is preferable to use those containing at least a part of a carbonaceous material or a silicon-containing compound.

[0016] Also, in the precursor 30 for an electrode binder according to the present invention, in addition to the above electrode active material, a conductive auxiliary agent can be added for the purpose of improving electrical characteristics. As this conductive auxiliary agent, any well-known conductive material conventionally used can be used. For example, various graphites, carbon black particles such as acetylene black and ketjen black, carbon nanotubes, carbon nanofibers, fullerenes, conductive metal materials, and the like can be mentioned. These may be used alone or in combination of two or more.

[0017] Furthermore, in addition to the above-described configuration, the electrode mixture precursor 30 according to the present invention may also contain other thermoplastic resins to enhance the film strength and flexibility when formed into an electrode mixture sheet. Examples of thermoplastic resins that can be used include polypropylene, polyethylene, polystyrene, polyethylene terephthalate, polyethylene oxide, and polyvinylidene fluoride. These may be used individually or in combination of two or more.

[0018] The electrode mixture precursor 30 according to the present invention is formed by dry mixing and kneading the above composition (precursor kneading step S102). This kneading is performed while applying shear force. By kneading while applying shear force, the stretchable PTFE becomes fibrous (fibrillated) and entangles with powder components such as electrode active material, binding and fixing them. In order to fibrillate the PTFE while improving its dispersibility, it is necessary to increase the kneading strength (rotation speed, filling amount, kneading time, etc.). However, if kneading is performed excessively, the fibrillated PTFE will aggregate, reducing its flexibility and preventing the formation of a good electrode mixture sheet 50. Therefore, it is important to find kneading conditions that allow the PTFE to fibrillate appropriately while ensuring dispersibility. In this regard, the electrode mixture precursor 30 according to the present invention has a PTFE content of 30 wt% to 90 wt%, which is extremely high compared to when the electrode mixture is mixed. Therefore, PTFE disperses homogeneously in a relatively short time, and the mixing conditions can be set by focusing on the fibrillation state of the PTFE. This makes it easy to set the mixing conditions.

[0019] Furthermore, well-known mixing equipment such as W-type mixers, V-type mixers, drum-type mixers, ribbon mixers, conical screw-type mixers, single-shaft mixers, twin-shaft mixers, MixMuller®, agitator mixers, planetary mixers, Henschel mixers, and high-speed mixers can be used for mixing and kneading.

[0020] Furthermore, there are no particular limitations on the shape of the electrode mixture precursor 30 produced by kneading, and it can be in various forms such as powder, granules, lumps, films, or fibers. However, from the viewpoint of the miscibility, dispersibility, and uniformity of the material when forming the electrode mixture 40 later, it is particularly preferable to use a powder that has been crushed to 1 mm or less. The electrode mixture precursor 30 produced in this way may be used to produce the electrode mixture 40 as shown below, or the electrode mixture precursor 30 itself may be sold to battery manufacturers, etc.

[0021] Next, the electrode mixture 40 according to the present invention will be described. The electrode mixture 40 according to the present invention is prepared by adding an electrode active material to the electrode mixture precursor 30, mixing and kneading it to a content that allows for sufficient operation as a battery electrode 80. In addition, conductive additives, well-known additives, etc., are added and mixed and kneaded as needed. Due to the addition of these electrode active materials, the content of stretchable PTFE is relatively reduced, and the total PTFE content in the electrode mixture 40 is in the range of 0.1 wt% to 10.0 wt%. Furthermore, as will be described later, the tensile strength when this electrode mixture 40 is made into a sheet is 0.5 N / mm ２ ~2.0 N / mm ２ Take it.

[0022] Furthermore, since the electrode mixture precursor 30 is pre-mixed with PTFE and electrode active material and contains the same electrode active material, it has higher dispersibility with respect to the electrode active material than PTFE alone. For this reason, the electrode mixture 40 according to the present invention can uniformly disperse PTFE throughout in a short time by using the electrode mixture precursor 30 containing the same electrode active material. In addition, since the PTFE in the electrode mixture precursor 30 is already in a fibrillated state suitable for sheet formation, by optimizing the mixing and kneading conditions during the manufacture of the electrode mixture 40, the PTFE can be uniformly dispersed while maintaining the suitable fibrillated state of the PTFE. This makes it possible to produce an electrode mixture sheet 50 with high strength and flexibility. Moreover, by pre-dispersing PTFE in the electrode mixture precursor 30 and manufacturing the electrode mixture 40 using this electrode mixture precursor 30, the dispersion efficiency is increased, and the total mixing and kneading time can be shortened compared to mixing and kneading PTFE and a large amount of electrode active material all at once. Furthermore, the same well-known mixing equipment used for preparing the electrode mixture precursor 30 can be used for mixing the electrode mixture 40 (electrode mixture mixing step S104).

[0023] Next, the electrode mixture sheet 50 according to the present invention will be described. The electrode mixture sheet 50 according to the present invention is a sheet-like molded body formed by dry rolling or the like from the electrode mixture 40 described above. For example, the electrode mixture 40 is thinned by extrusion molding or calender rolls, and then further rolled using a roll press, flat plate press, calender roll machine, etc. The rolled sheet may be further uniaxially stretched or biaxially stretched, and these processes may be repeated multiple times. The thickness of the electrode mixture sheet 50 varies depending on the battery electrode 80 and secondary battery 100 to be manufactured, but is generally 50 μm to 300 μm. As described above, the electrode mixture sheet 50 is manufactured based on an electrode mixture precursor 30 and an electrode mixture 40 in which the fibrillation state of PTFE is suitable, so it becomes a sheet with good flexibility and strength in which the fibrillation state is suitable (sheet formation process S106).

[0024] Next, the battery electrode 80 according to the present invention will be described. The battery electrode 80 according to the present invention is constructed by laminating a current collector onto the electrode mixture sheet 50 described above. Here, the current collector is appropriately selected depending on whether it is used as the positive electrode or the negative electrode, and the type of electrode active material contained in the electrode mixture sheet 50, but for example, metals such as copper, nickel, titanium, aluminum, and stainless steel, or their alloys, or carbon materials such as carbon paper can be used. Furthermore, the shape of the current collector can be foil-shaped, film-shaped, plate-shaped, or coil-shaped, but a metal foil shape is common and most preferred. In addition, the most common method for forming the battery electrode 80 is to laminate the electrode mixture sheet 50 onto the current collector using an adhesive, but it is not limited to this, and any existing method can be used as long as it is possible to laminate the current collector and the electrode mixture sheet 50 (electrode formation step S108). Furthermore, since the battery electrode 80 according to the present invention is made from an electrode mixture sheet 50 in which an electrode mixture 40 in which PTFE and electrode active material are homogeneously formed via an electrode mixture precursor 30 is formed into a sheet, there is little variation in electrical characteristics and it has high charge and discharge characteristics.

[0025] Next, the secondary battery 100 according to the present invention will be described. The secondary battery 100 according to the present invention, like well-known secondary batteries, has a negative electrode, a negative electrode terminal electrically connected to the negative electrode, a positive electrode, a positive electrode terminal electrically connected to the positive electrode, and an electrolyte for transferring ions between the negative electrode and the positive electrode. Basically, multiple negative electrodes and positive electrodes are stacked alternately and housed in a battery case. One or both of these negative and positive electrodes are composed of the battery electrode 80 according to the present invention. The electrolyte may be liquid or solid. In the case of a liquid, a well-known separator is sandwiched between the negative electrode and the positive electrode, and the liquid electrolyte is filled into the electrode portion. In the case of a solid electrolyte, a solid electrolyte is placed between the negative electrode and the positive electrode. Each negative electrode is electrically connected to a negative electrode terminal and partially exposed from the battery case. Each positive electrode is electrically connected to a positive electrode terminal and partially exposed from the battery case. The secondary battery 100 is charged and discharged through these positive and negative electrode terminals. Furthermore, since the electrode electrode 80 of the secondary battery 100 according to the present invention uses the electrode mixture sheet 50 which has high charge and discharge characteristics with little variation as described above, it has excellent electrical characteristics and the secondary battery 100 can be manufactured with a high yield (battery manufacturing process S110).

[0026] As an electrode active material for the negative electrode, 50 wt% of spherical graphite with a particle size of 7 μm and 50 wt% of PTFE fine powder with a particle size of 400 μm, which is produced by emulsion polymerization as a stretchable PTFE, were kneaded in a batch-type twin-screw kneader at 20°C, 10 Hz, and for 10 minutes to prepare an electrode mixture precursor 30.

[0027] [Comparative Example 1] An electrode mixture precursor was prepared in the same manner as in [Example 1], except that non-stretchable PTFE produced by suspension polymerization was used instead of stretchable PTFE.

[0028] Then, the electrode mixture precursor 30 of [Example 1] and the electrode mixture precursor of [Comparative Example 1] were rolled between two rollers with a clearance of 400 μm maintained at 80°C to form sheets. As a result, the material from [Example 1] formed into a sheet without any problems, but the material from [Comparative Example 1] crumbled and did not form into a sheet. Therefore, from these results, it can be seen that stretchable PTFE is necessary for sheet formation.

[0029] An electrode mixture precursor 30 was prepared in the same manner as in [Example 1], except that the electrode active material was 70 wt% and the stretchable PTFE was 30 wt%.

[0030] An electrode mixture precursor 30 was prepared in the same manner as in [Example 1], except that the electrode active material was 10 wt% and the stretchable PTFE was 90 wt%.

[0031] [Comparative Example 2] An electrode mixture precursor was prepared in the same manner as in [Example 1], except that the electrode active material was 80 wt% and the stretchable PTFE was 20 wt%.

[0032] [Comparative Example 3] An electrode mixture precursor was prepared in the same manner as in [Example 1], except that the electrode active material was 90 wt% and the stretchable PTFE was 10 wt%.

[0033] Then, the electrode mixture precursors 30 from [Examples 1] to [Examples 3] and the electrode mixture precursors from [Comparative Example 2] and [Comparative Example 3] were formed into sheets using the method described above to obtain precursor sheets with a thickness of 380 μm to 400 μm. The tensile strength of these precursor sheets was measured using a Tensilon® manufactured by A&D Company, Limited at a speed of 50 mm / min. The results are shown in Figure 2. In Figure 2, the horizontal axis represents the PTFE content, and the vertical axis represents the tensile strength. From Figure 2, it can be seen that the tensile strength increases as the PTFE content increases. The effective PTFE content will be discussed later.

[0034] In Example 2, the electrode mixture precursor 30 was mixed with the electrode active material in a ratio of 10 wt% and 90 wt%, and the mixture was kneaded under the same conditions as when the electrode mixture precursor was kneaded to prepare the electrode mixture 40. In this Example 4, the total PTFE content of the electrode mixture 40 is 3 wt%.

[0035] The electrode mixture precursor 30 from [Example 1] was mixed with the electrode active material in a ratio of 6 wt% and 94 wt%, and the electrode mixture 40 was prepared under the same conditions as in [Example 4]. The PTFE content of the electrode mixture 40 in [Example 5] is 3 wt% in total.

[0036] The electrode mixture precursor 30 from [Example 3] was mixed with the electrode active material in a ratio of 3.3 wt% and 96.7 wt%, and the electrode mixture 40 was prepared under the same conditions as in [Example 4]. The total PTFE content of the electrode mixture 40 in [Example 6] is 3 wt%.

[0037] The electrode mixture precursor 30 from [Example 1] was mixed with the electrode active material in a ratio of 0.2 wt% and 99.8 wt%, and the electrode mixture 40 was prepared under the same conditions as in [Example 4]. The PTFE content of the electrode mixture 40 in [Example 7] was 0.1 wt% in total.

[0038] The electrode mixture precursor 30 from [Example 1] was mixed with the electrode active material in a ratio of 20 wt% and 80 wt%, and the electrode mixture 40 was prepared under the same conditions as in [Example 4]. The PTFE content of the electrode mixture 40 in [Example 8] was 10 wt% in total.

[0039] [Comparative Example 4] The electrode mixture precursor from [Comparative Example 2] was mixed in a ratio of 10 wt% and the electrode active material in a ratio of 90 wt%, and an electrode mixture was prepared under the same conditions as in [Example 4]. The total PTFE content of this electrode mixture in [Comparative Example 4] was 2 wt%.

[0040] [Comparative Example 5] The electrode mixture precursor from [Comparative Example 3] was mixed in a ratio of 10 wt% and the electrode active material in a ratio of 90 wt%, and an electrode mixture was prepared under the same conditions as in [Example 4]. The total PTFE content of this electrode mixture in [Comparative Example 5] was 1 wt%.

[0041] [Comparative Example 6] The electrode active material from [Example 1] was used at 1 wt%, and the stretchable PTFE was used at 99 wt%. The electrode mixture was prepared by kneading the materials in one go under the same conditions as when the precursor for the electrode mixture was kneaded (without going through the precursor for the electrode mixture).

[0042] [Comparative Example 7] The electrode active material from [Example 1] was used in an amount of 15 wt%, and the stretchable PTFE was used in an amount of 85 wt%. The electrode mixture was prepared by kneading the materials in one step (without going through an electrode mixture precursor) in the same manner as in [Comparative Example 6].

[0043] Then, the electrode mixtures 40 of [Examples 4] to [Examples 8] were prepared into electrode mixture sheets 50 with a film thickness of 380 μm to 400 μm using the same conditions as for preparing the precursor sheets described above. Similarly, the electrode mixtures of [Comparative Examples 4] to [Comparative Examples 7] were prepared into electrode mixture sheets with a film thickness of 380 μm to 400 μm using the same method.

[0044] Next, the flexibility of these electrode mixture sheets was evaluated by checking for cracking when wrapped around a φ4 round rod. The results are shown in Figure 3. In Figure 3, a circle (○) is used to indicate that no cracking occurred, and an "X" (×) is used to indicate that cracking occurred and the flexibility was insufficient. From Figure 3, it can be seen that the electrode mixture sheets 50 of [Examples 4] to [Examples 8] and the electrode mixture sheets of [Comparative Examples 4] to [Comparative Examples 6] did not crack and had sufficient flexibility, but [Comparative Example 7], in which 15 wt% of the electrode active material was kneaded all at once, cracked and did not have sufficient flexibility.

[0045] Next, the tensile strength of the electrode mixture sheets, excluding [Comparative Example 7], was measured under the same conditions as the precursor sheets. The results are shown in Figures 4 and 5. In Figure 4, the horizontal axis shows the PTFE content of the electrode mixture precursor used, and the vertical axis shows the tensile strength. Figure 5 shows the same data as in Figure 4, but with the total PTFE content on the horizontal axis.

[0046] As shown in Figure 4, the electrode mixture sheets 50 (marked with a circle in Figure 5) of Examples 4 to 8, which used an electrode mixture precursor 30 with a PTFE content of 30 wt% to 90 wt%, have a tensile strength of 0.5 N / mm². ２ While favorable results were obtained, the electrode mixture sheets (marked with ● in Figure 5) of Comparative Examples 4 and 5, with PTFE content of 10 wt% and 20 wt%, had a tensile strength of 0.5 N / mm². ２It was found that the strength was low when the material was formed into a sheet because it did not meet the requirement. Furthermore, from Figure 5, the electrode mixture sheets 50 (marked with a circle in Figure 5) of [Examples 4] to [Example 8] using the electrode mixture precursor 30 according to the present invention had a tensile strength of 0.5 N / mm when the content of the electrode mixture 40 was in the range of 0.1 wt% to 10.0 wt%. ２ Excellent results exceeding the previous limit were obtained. Furthermore, as mentioned above, the electrode mixture sheets of [Comparative Example 4] and [Comparative Example 5] (marked with ● in Figure 5) had a tensile strength of 0.5 N / mm². ２ It was found that the strength was low when the material was formed into a sheet because it did not meet the requirement. Furthermore, the electrode mixture sheet of [Example 6] (marked with a triangle in Figure 5), in which the electrode active material and PTFE were mixed all at once without using an electrode mixture precursor, also had a tensile strength of 0.5 N / mm². ２ It was found that the strength of the sheet was low when it did not meet the requirement. From these findings, it can be seen that when the electrode mixture precursor 30 according to the present invention, which has a PTFE content in the range of 30 wt% to 90 wt%, is used in the electrode mixture 40, a sheet with sufficient strength can be obtained with a final PTFE content in the range of 0.1 wt% to 10.0 wt%.

[0047] Next, a copper foil current collector sheet with a film thickness of 10 μm was attached to the electrode mixture sheets 50 of [Examples 4] to [Examples 8] to form battery electrodes 80. Then, multiple battery electrodes 80, which serve as negative electrodes, and separately manufactured positive electrodes were alternately stacked with a separator in between. Next, the negative electrode terminals were connected to the battery electrodes 80, and the positive electrode terminals were connected to the positive electrodes. These were then housed in a predetermined case and filled with a well-known electrolyte to form a secondary battery 100. Secondary batteries were also manufactured in the same manner for [Comparative Examples 4] to [Comparative Examples 6]. The charge-discharge characteristics of these secondary batteries 100 were measured, and the Coulomb efficiencies of [Examples 4] to [Examples 8] and [Comparative Examples 4] to [Comparative Examples 6] were calculated. The results are shown in Figure 6. In Figure 6, the horizontal axis shows the total PTFE content, and the vertical axis shows the Coulomb efficiency.

[0048] As shown in Figure 6, the secondary batteries 100 (marked with circles in Figure 6) made based on [Examples 4] to [Examples 8] using electrode mixture precursor 30 with a PTFE content of 30 wt% to 90 wt% all showed good Coulomb efficiencies of 90% or more. In contrast, the secondary batteries (marked with black dots in Figure 6) made based on [Comparative Example 4] and [Comparative Example 5] with PTFE content of 10 wt% and 20 wt%, respectively, and the secondary battery (marked with a triangle in Figure 6) made based on [Example 6] in which the electrode active material and PTFE were mixed all at once, both showed low Coulomb efficiencies of less than 90%. From this, it can be seen that the battery electrode 80 and secondary battery 100 made based on the electrode mixture precursor 30 according to the present invention have good dispersibility of the electrode mixture components (electrode active material, etc.) and have high charge / discharge characteristics.

[0049] Next, the properties when the material is formed into a sheet are thought to be related to the angle of repose and tap density of the electrode mixture precursor 30 in powder form. Therefore, we will evaluate how the angle of repose and tap density of the electrode mixture precursor 30 affect the sheet. The angle of repose is the maximum angle of the slope formed when granular material is stacked. Tap density is the maximum density when powdered material is placed in a container and vibrated.

[0050] Here, we conduct an experiment to investigate the relationship between the angle of repose and tap density when the mixing time of the electrode mixture precursor 30 is varied. First, 30 wt% of electrode active material (graphite) and 70 wt% of stretchable PTFE were dispersed using a planetary mixer under conditions of a stirring speed of 5 m / sec and a stirring time of 60 sec. Next, the stirring speed was increased to 30 m / sec and the mixture was kneaded to obtain the electrode mixture precursor. The mixing time was set to 0 sec (no mixing), 30 sec, and until the temperature of the mixture (electrode mixture precursor) rose to 40°C.

[0051] Furthermore, electrode compound precursors were obtained under the same conditions as above, using 10 wt% electrode active material and 90 wt% stretchable PTFE. The angle of repose (°) and tap density (g / mm³) of these electrode compound precursors were then determined. ３The following were measured. The angle of repose was measured by supplying the electrode mixture precursor onto a cylindrical molded body of a certain diameter and measuring the inclination angle of the cone formed during the deposition process. The tap density was measured by first placing the electrode mixture precursor to be measured into a predetermined container and tapping this container until the volume of the electrode mixture precursor no longer changed. The tap density was then calculated from the apparent volume and weight of the electrode mixture precursor at that time. Furthermore, the particle size distribution of these electrode mixture precursors was measured using Microtrac under wet measurement conditions.

[0052] As a result, the angle of repose increased with increasing mixing time and reached equilibrium at a predetermined value. Furthermore, the angle of repose tended to increase with increasing PTFE content. Similarly, the tap density decreased with increasing mixing time and reached equilibrium at a predetermined value. Again, the tap density tended to decrease with increasing PTFE content. However, no clear correlation was found between mixing time and PTFE content and particle size distribution.

[0053] Next, 3 wt% of these electrode mixture precursors were mixed with 97 wt% electrode active material (graphite) and dispersed using a planetary mixer at a stirring speed of 5 m / sec and a stirring time of 60 sec. Then, the stirring speed was increased to 30 m / sec and the mixture was kneaded until the temperature of the mixture rose to 40°C to obtain the electrode mixture. The obtained electrode mixture was then rolled into a sheet by passing it through two rollers with a clearance of 100 μm, maintained at 80°C. However, the electrode mixture using the electrode mixture precursor with a PTFE concentration of 90 wt% that was kneaded until the temperature rose to 40°C could not be formed into a sheet. Next, the film thickness of the electrode mixture sheet was measured using a microcaliper. The tensile strength of the electrode mixture sheet was also measured using a Tensilon tensile strength meter at a speed of 50 mm / min. As a result, it was found that electrode mixture precursors with a large angle of repose and high tap density could produce electrode mixture sheets with thicker film thickness and higher tensile strength under the same conditions.

[0054] [Comparative Example 8] Using the same mixing ratio as in [Example 1], the electrode active material was 50 wt% and the stretchable PTFE was 50 wt%, and the mixing conditions were adjusted to prepare an electrode mixture precursor with an angle of repose of 40°. Next, this electrode mixture precursor was mixed with the electrode active material in a ratio of 6 wt% and 94 wt%, and an electrode mixture was prepared under the same conditions as in [Examples 4] to [Examples 6] to obtain an electrode mixture sheet with a film thickness of 380 μm to 400 μm. The total PTFE content of this electrode mixture (electrode mixture sheet) was 3 wt%. Next, the tensile strength of this electrode mixture sheet was measured under the same conditions as in [Examples 4] to [Examples 6]. The relationship between the angle of repose and tensile strength was then compared with that of [Examples 4] to [Examples 6], which had the same PTFE content of 3 wt%. Furthermore, the angle of repose of the electrode mixture precursor 30 from [Example 2] used in [Example 4] was 60°, the angle of repose of the electrode mixture precursor 30 from [Example 1] used in [Example 5] was 70°, and the angle of repose of the electrode mixture precursor 30 from [Example 3] used in [Example 6] was 50°. The results are shown in Figure 7(a). In Figure 7(a), the horizontal axis represents the angle of repose of the electrode mixture precursor, and the vertical axis represents the tensile strength of the electrode mixture sheet using it.

[0055] From Figure 7(a), the tensile strength of the electrode mixture sheet of [Comparative Example 8] with an angle of repose of 40° is 0.32 N / mm². ２ This indicates that the value for [Example 4] to [Example 6], where the angle of repose is 50° to 70°, is 0.95 N / mm². ２ ~1.33 N / mm ２ It showed a lower value compared to the previous value. This is thought to be because when the angle of repose is small, the material is difficult to combine, and sufficient strength cannot be obtained when it is formed into a sheet. From this, it can be seen that the angle of repose of the electrode mixture precursor 30 needs to be 50° or higher.

[0056] [Comparative Example 9] The tap density was adjusted to 0.4 g / cm³ by adjusting the mixing conditions. ３An electrode mixture sheet with a total PTFE content of 3 wt% and a film thickness of 380 μm to 400 μm was prepared in the same manner as in [Comparative Example 8], except that the electrode mixture precursor was used. Next, the tensile strength of the electrode mixture sheet of [Comparative Example 9] was measured under the same conditions as in [Examples 4] to [Examples 6] above. The relationship between tap density and tensile strength was then compared with that of [Examples 4] to [Examples 6], which also contained 3 wt% PTFE. The tap density of the electrode mixture precursor 30 of [Example 2] used in [Example 4] was 0.8 g / cm³. ３ Therefore, the tap density of the electrode mixture precursor 30 from [Example 1] used in [Example 5] was 1.0 g / cm³. ３ Therefore, the tap density of the electrode mixture precursor 30 from [Example 3] used in [Example 6] was 0.5 g / cm³. ３ The results are shown in Figure 7(b). Here, the horizontal axis of Figure 7(b) represents the tap density of the electrode mixture precursor, and the vertical axis represents the tensile strength of the electrode mixture sheet using it.

[0057] From Figure 7(b), the tap density is 0.4 g / cm³. ３ The tensile strength of the electrode mixture sheet in [Comparative Example 9] is 0.29 N / mm². ２ This indicates a tap density of 0.5 g / cm³. ３ ~1.0 g / cm ３ The values ​​for [Examples 4] to [Examples 6] are 0.95 N / mm². ２ ~1.33 N / mm ２ It showed a lower value compared to [another value]. This is thought to be because when the tap density is low, the density decreases when it is formed into a sheet, and sufficient strength cannot be obtained. From this, the tap density of the electrode mixture precursor 30 is 0.5 g / cm³. ３ The above points are all that are necessary.

[0058] Furthermore, the tap density of the electrode mixture precursor in [Comparative Example 8] is 1.0 g / cm³. ３ 0.5 g / cm ３ The above conditions are met, and the angle of repose of the electrode mixture precursor in [Comparative Example 9] is 70°, which satisfies the condition of 50° or more. From this, it can be concluded that the electrode mixture precursor 30 has an angle of repose of 50° or more and a tap density of 0.5 g / cm³. ３ Both of the above conditions must be met.

[0059] As described above, the present invention prepares an electrode mixture precursor 30 by pre-mixing PTFE and an electrode active material to produce an electrode mixture precursor 30 in which PTFE is in a fibrillated state suitable for sheet formation at a high concentration. Then, an electrode active material is added to this electrode mixture precursor 30 to produce an electrode mixture 40 with a desired electrode active material concentration. Here, as mentioned above, the electrode mixture precursor 30 is pre-mixed with PTFE and an electrode active material and contains the same electrode active material as when the electrode mixture 40 is made, so its dispersibility with respect to the electrode active material is higher than when PTFE is used alone. For this reason, PTFE can be uniformly dispersed throughout in a short time. Furthermore, as mentioned above, the PTFE in the electrode mixture precursor 30 is in a fibrillated state suitable for sheet formation, so by optimizing the mixing and kneading conditions during the production of the electrode mixture 40, the PTFE can be uniformly dispersed while maintaining the suitable fibrillated state of PTFE. This makes it possible to produce an electrode mixture sheet 50 with high strength and flexibility. Furthermore, due to its high dispersibility, there is little bias in the electrode active material, making it possible to manufacture electrode mixture sheets 50, battery electrodes 80, and secondary batteries 100 with homogeneous properties at a high yield.

[0060] Furthermore, the materials, manufacturing methods, procedures, and production sequence of the electrode mixture precursor 30, electrode mixture 40, electrode mixture sheet 50, battery electrode 80, and secondary battery 100 shown in this example are merely examples. Necessary procedures may be added as appropriate, and the invention can be modified without departing from the spirit of the present invention.

[0061] 30 Precursors for electrode mixtures 40 Electrode mixtures 50 Electrode mixture sheets 80 Electrodes for batteries 100 Secondary batteries

Claims

1. An electrode mixture precursor characterized by containing an electrode active material for a positive or negative electrode and stretchable polytetrafluoroethylene, wherein the polytetrafluoroethylene content is 30 wt% to 90 wt%.

2. The tensile strength when formed into a sheet is 3.0 N / mm². ２ ~9.0 N / mm ２ The precursor for electrode mixture according to claim 1, characterized in that it is the same as the one described in claim 1.

3. The electrode mixture contains an electrode active material for a positive or negative electrode and an electrode precursor according to claim 1 or claim 2, wherein the stretchable polytetrafluoroethylene is 0.1 wt% to 10.0 wt% in total and has a tensile strength of 0.5 N / mm ２ ~2.0 N / mm ２ An electrode mixture characterized by being...

4. An electrode mixture sheet comprising the electrode mixture described in claim 3.

5. A battery electrode comprising an electrode mixture sheet and a current collector bonded together as described in claim 4.

6. A secondary battery comprising a negative electrode, a negative electrode terminal electrically connected to the negative electrode, a positive electrode, a positive electrode terminal electrically connected to the positive electrode, and an electrolyte for transferring ions between the negative electrode and the positive electrode, wherein one or both of the negative electrode and the positive electrode are battery electrodes according to claim 5.

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