Positive electrode for nonaqueous electrolyte secondary battery and battery using same

Abstract

A positive electrode 5 for a nonaqueous electrolyte secondary battery according to the present disclosure comprises a positive electrode active material and a plurality of types of binders. The plurality of types of binders include a first binder that has the largest surface free energy among the plurality of types of binders and a second binder that has the smallest surface free energy among the plurality of types of binders. The ratio of the total mass of the plurality of types of binders to the mass of the positive electrode active material is 0.4-3.0%. The difference between the surface free energy of the first binder and the surface free energy of the second binder is not less than 5 mN / m. The ratio of the mass of the second binder to the total mass of the plurality of types of binders is more than 10.0% and not more than 70.0%.

Description

Positive electrode for non-aqueous electrolyte secondary batteries and batteries using the same 【0001】 This disclosure relates to a positive electrode for a non-aqueous electrolyte secondary battery and a battery using the same. 【0002】 The electrodes of non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, undergo volume changes during charging and discharging. This volume change is one of the causes of performance degradation in non-aqueous electrolyte secondary batteries with repeated charging and discharging. The binder contained in the electrode provides flexibility to the electrode and suppresses the performance degradation of non-aqueous electrolyte secondary batteries caused by the volume change of the electrode. Resin materials are usually used as the binder. 【0003】 Patent Document 1 discloses a positive electrode composite layer containing vinylidene fluoride-chlorotrifluoroethylene copolymer as a binder. 【0004】 Patent Document 2 discloses that the lower region of the electrode active material layer includes a first non-rubber binder but does not include a rubber binder, and the upper region of the electrode active material layer includes a second non-rubber binder and a rubber binder. 【0005】 International Publication No. 2014 / 208272, JP 2023-541161 【0006】 High-capacity non-aqueous electrolyte secondary batteries have a thick positive electrode active material layer. When the positive electrode active material layer is thick, cracks are likely to occur in the positive electrode active material layer during the winding process. On the other hand, if the binder content is increased too much to suppress cracking of the positive electrode active material layer, the resistance of the positive electrode increases significantly, which is undesirable. 【0007】 The purpose of this disclosure is to provide a technology that can suppress cracking of the positive electrode active material layer and suppress an increase in the resistance of the positive electrode. 【0008】This disclosure provides a positive electrode for a non-aqueous electrolyte secondary battery, comprising a positive electrode active material and a plurality of types of binders, wherein the plurality of types of binders include a first binder having the largest surface free energy among the plurality of types of binders and a second binder having the smallest surface free energy among the plurality of types of binders, the ratio of the total mass of the plurality of types of binders to the mass of the positive electrode active material is 0.4% or more and 3.0% or less, the difference between the surface free energy of the first binder and the surface free energy of the second binder is 5 mN / m or more, and the ratio of the mass of the second binder to the total mass of the plurality of types of binders is greater than 10.0% and 70.0% or less. 【0009】 According to this disclosure, it is possible to suppress cracking of the positive electrode active material layer and suppress an increase in the resistance of the positive electrode at the same time. 【0010】 Figure 1 is a cross-sectional view of the positive electrode for a non-aqueous electrolyte secondary battery in Embodiment 1. Figure 2 is a schematic cross-sectional view showing an example of a non-aqueous electrolyte secondary battery in Embodiment 2. 【0011】 The embodiments of this disclosure will be described below with reference to the drawings. This disclosure is not limited to the embodiments described below. 【0012】 (Embodiment 1) Figure 1 is a cross-sectional view of the positive electrode for a non-aqueous electrolyte secondary battery in Embodiment 1. The positive electrode 5 for the non-aqueous electrolyte secondary battery comprises a positive electrode current collector 5a and a positive electrode active material layer 5b. The positive electrode active material layer 5b is supported by the positive electrode current collector 5a. Hereinafter, the positive electrode 5 for the non-aqueous electrolyte secondary battery may be simply referred to as "positive electrode 5". 【0013】The positive electrode active material layer 5b contains a positive electrode active material and multiple types of binders. The multiple types of binders include a first binder and a second binder. The first binder is the binder with the largest surface free energy among the multiple types of binders. The second binder is the binder with the smallest surface free energy among the multiple types of binders. The difference between the surface free energy of the first binder and the surface free energy of the second binder is 5 mN / m or more. The ratio of the total mass of the multiple types of binders to the mass of the positive electrode active material (hereinafter sometimes referred to as "ratio A") is 0.4% or more and 3.0% or less. The ratio of the mass of the second binder to the total mass of the multiple types of binders (hereinafter sometimes referred to as "ratio B") is greater than 10.0% and 70.0% or less. 【0014】 By using two or more binders with large differences in surface free energy and adjusting the amount of each binder within an appropriate range, it is possible to suppress cracking of the positive electrode active material layer 5b and suppress an increase in the resistance of the positive electrode 5 simultaneously. 【0015】 A difference of 5 mN / m or more between the surface free energy of the first binder and the surface free energy of the second binder means that the strength of the interaction between the first binder and other materials such as the positive electrode active material is significantly different from the strength of the interaction between the second binder and other materials such as the positive electrode active material. In this case, it is possible to select a first binder that is highly effective in suppressing the increase in resistance and a second binder that is highly effective in suppressing cracking. In other words, using two or more binders with a large difference in surface free energy is advantageous in achieving both the suppression of cracking in the positive electrode active material layer 5b and the suppression of the increase in resistance of the positive electrode 5. If only one type of binder or two or more binders with similar surface free energies are used, it is difficult to achieve both the suppression of cracking and the suppression of the increase in resistance, even if the amount of binder is appropriately adjusted. 【0016】Furthermore, even if the difference in surface free energy is sufficiently large, if the ratio A of the total mass of multiple types of binders to the mass of the positive electrode active material is too small, the effect of suppressing cracking of the positive electrode 5 during the winding process will not be sufficiently obtained. If the ratio A is too large, the effect of suppressing cracking will be obtained, but the resistance of the positive electrode 5 will increase significantly. Even if the difference in surface free energy is sufficiently large, if the ratio B of the mass of the second binder to the total mass of multiple types of binders is too small, the effect of suppressing cracking of the positive electrode 5 during the winding process will not be sufficiently obtained. If the ratio B is too large, the effect of suppressing cracking will be obtained, but the resistance of the positive electrode 5 will increase significantly. 【0017】 The difference between the surface free energy of the first binder and the surface free energy of the second binder is preferably 8 mN / m or more. The upper limit of the difference between the surface free energy of the first binder and the surface free energy of the second binder is not particularly limited, for example, 20 mN / m. The difference between the surface free energy of the first binder and the surface free energy of the second binder may be 5 mN / m or more and 20 mN / m or less, and preferably 8 mN / m or more and 15 mN / m or less. 【0018】 The ratio A of the total mass of multiple types of binders to the mass of the positive electrode active material is preferably 0.4% to 2.0%. 【0019】 The ratio B of the mass of the second binder to the total mass of multiple types of binders is preferably 20.0% to 70.0%, and more preferably 20.0% to 60.0%. 【0020】Multiple types of binders contained in the positive electrode active material layer 5b can be extracted from the positive electrode active material layer 5b using a solvent such as NMP that can dissolve multiple types of binders. The mass of the positive electrode active material and conductive material contained in the positive electrode active material layer 5b can be measured by the following method: Dissolve the binders in a solvent such as NMP to separate the binders from the positive electrode active material and conductive material. Then, separate the positive electrode active material and conductive material using a centrifuge. By removing the solvent, the mass of the positive electrode active material and conductive material can be measured. The presence or absence of positive electrode active material in the solution obtained after centrifugation can be confirmed by ICP (Inductively Coupled Plasma) emission spectroscopy. In addition, by measuring the total mass of the multiple types of binders extracted, ratio A can be calculated. Multiple types of binders can be separated from each other using column chromatography or the like. By measuring the mass of the separated second binder, ratio B can be calculated. 【0021】 After separating multiple types of binders into individual components, each binder is formed into a film. The surface free energy of each binder can be measured using the resin films produced from each binder. Specifically, the static contact angle of the resin film surface at 25°C for water, ethylene glycol, and diiodomethane is measured. The static contact angle for each liquid, along with the dispersion, polarity, and hydrogen bonding terms of the surface free energy of each liquid, are introduced into the "Hata-Kitazaki extended Hawkes equation," and the simultaneous equations are solved. This allows the surface free energy of the resin film to be calculated. The surface free energy of the resin film can be considered as the surface free energy of the binder. The dispersion, polarity, and hydrogen bonding terms of the surface free energy of each liquid are literature values. 【0022】 In this embodiment, the surface free energy of the first binder is greater than the surface free energy of the second binder. 【0023】The second binder may be a polymer containing an olefin skeleton. The olefin skeleton is the part that imparts flexibility to the polymer. By including an olefin skeleton in the polymer as the second binder, flexibility can be imparted to the positive electrode active material layer 5b. As a result, cracking of the positive electrode active material layer 5b during the winding process can be suppressed. "Olefin skeleton" refers to repeating units derived from olefins such as ethylene, propylene, and butadiene. The olefin skeleton may or may not contain unsaturated bonds. 【0024】 The polymer as the second binder may further contain other skeletons besides the olefin skeleton. In other words, the polymer as the second binder may be a copolymer. In this case, the copolymer may contain, in addition to the first repeating unit (olefin skeleton) derived from the olefin, a second repeating unit (other skeletons besides the olefin skeleton) that improves the adhesion between the second binder and other materials such as the positive electrode active material. The second repeating unit may contain at least one selected from the group consisting of carboxyl groups, amino groups, amide groups, epoxy groups, ester groups, nitrile groups, hydroxyl groups, and acrylic groups. With such a configuration, it is possible to improve the adhesion between material particles contained in the positive electrode active material layer 5b while imparting flexibility to the positive electrode active material layer 5b. 【0025】 The elongation at break of the resin film made from the second binder is, for example, 200% or more. The high elongation at break of the resin film made from the second binder indicates that the second binder has excellent flexibility. The inclusion of the second binder in the positive electrode active material layer 5b suppresses cracking of the positive electrode active material layer 5b during the winding process. There is no particular upper limit to the elongation at break of the resin film made from the second binder, and it is, for example, 500%. The elongation at break (tensile elongation) of the resin film can be measured by the method specified in Japanese Industrial Standard (JIS) K7161:2014. 【0026】Specific examples of the first binder include polyvinylidene fluoride (PVDF), hydrogenated nitrile butadiene rubber (H-NBR), acrylic resin, styrene butadiene rubber (SBR), polyimide resin, aramid resin, urethane resin, silicone resin, and epoxy resin. 【0027】 Specific examples of the second binder include polyvinylidene fluoride (PVDF), hydrogenated nitrile butadiene rubber (H-NBR), acrylic resin, styrene butadiene rubber (SBR), polyimide resin, aramid resin, urethane resin, silicone resin, and epoxy resin. The second binder may also be a resin with rubber elasticity, such as hydrogenated nitrile butadiene rubber (H-NBR). 【0028】 In this embodiment, the amount of positive electrode active material layer 5b coated is, for example, 250 g / m². 2 More than 400g / m 2 The following applies. In other words, the positive electrode active material layer 5b may be formed to be thick. The thicker the positive electrode active material layer 5b, the more likely it is to crack in the positive electrode active material layer 5b during the winding process. Therefore, it is desirable to apply the technology of this disclosure to a positive electrode 5 equipped with a thick positive electrode active material layer 5b. Note that the positive electrode active material layer 5b is provided on both sides of the positive electrode current collector 5a, but "amount of coating of positive electrode active material layer 5b" in this specification means the amount of coating of one layer of positive electrode active material layer 5b. 【0029】 The positive electrode active material layer 5b may contain a lithium-containing transition metal oxide containing Li, Ni, and M as the positive electrode active material. M is at least one selected from the group consisting of Al, Mn, and Co. The particles of the lithium-containing transition metal oxide tend to be larger than the particles of lithium-containing transition metal phosphates such as lithium iron phosphate. The larger the particles of the active material, that is, the smaller the specific surface area of ​​the particles of the active material, the less binder is required. Therefore, when the positive electrode active material contains a lithium-containing transition metal oxide, the total mass of the binder in the positive electrode active material layer 5b can be reduced. 【0030】However, the type of positive electrode active material is not particularly limited. Possible positive electrode active materials included in the positive electrode active material layer 5b include lithium-containing transition metal oxides, lithium-containing transition metal phosphates, transition metal fluorides, polyanionic materials, fluorinated polyanionic materials, transition metal sulfides, transition metal oxysulfides, and transition metal oxynitrides. When lithium-containing transition metal oxides or lithium-containing transition metal phosphates are used as the positive electrode active material, the manufacturing cost of the battery can be reduced and the average discharge voltage can be increased. Examples of lithium-containing transition metal oxides include lithium cobalt oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, and lithium nickel manganese oxide. Examples of lithium-containing transition metal phosphates include lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, and lithium nickel phosphate. 【0031】 The positive electrode active material layer 5b may further contain a conductive material. Examples of conductive materials include carbon black such as acetylene black. The ratio of the mass of carbon black to the mass of the second binder is, for example, 190% to 600%, and preferably 260% to 400%. With such a configuration, it is easy to achieve both suppression of cracking of the positive electrode active material layer 5b and suppression of an increase in the resistance of the positive electrode 5. 【0032】 The positive electrode active material layer 5b may contain conductive materials other than carbon black. Examples of conductive materials other than carbon black include fibrous carbon materials. The ratio of the mass of the fibrous carbon material to the mass of the second binder is, for example, 75% to 350%, and preferably 130% to 200%. With such a configuration, it is easy to achieve both suppression of cracking of the positive electrode active material layer 5b and suppression of an increase in the resistance of the positive electrode 5. 【0033】Examples of fibrous carbon materials include carbon nanotubes (CNTs) and vapor-phase carbon fibers. Among these, carbon nanotubes are desirable because they can impart high conductivity to the positive electrode active material layer 5b with only a small amount. It is known that carbon nanotubes exist in two forms: single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). Multi-walled carbon nanotubes are preferable from a mass production standpoint because they are less expensive. Single-walled carbon nanotubes are preferable because they have higher conductivity. 【0034】 As the positive electrode current collector 5a, a sheet or film made of a metallic material such as aluminum, aluminum alloy, stainless steel, titanium, or titanium alloy may be used. The sheet or film may be porous or non-porous. As the sheet or film, metal foil, metal mesh, etc., may be used. A carbon material may be coated on the surface of the positive electrode current collector 5a as a conductive auxiliary material. 【0035】 The positive electrode 5 of this embodiment can be manufactured by the following method: A positive electrode slurry is prepared by mixing the positive electrode active material, the first binder, the second binder, the conductive material, and the solvent. The positive electrode slurry is applied to the positive electrode current collector 5a to form a coating film. The positive electrode 5 is obtained by drying the coating film to remove the solvent. 【0036】 (Modification) The surface free energy of the first binder may be less than the surface free energy of the second binder. In this case, the first binder may be polyvinylidene fluoride (PVDF), and the second binder may be aramid resin, urethane resin, polyimide resin, epoxy resin, etc. 【0037】(Embodiment 2) Fig. 2 is a schematic cross-sectional view showing an example of a non-aqueous electrolyte secondary battery in Embodiment 2. The non-aqueous electrolyte secondary battery 100 includes a container 1, an electrode group 4, and an electrolyte solution (not shown). The electrode group 4 has a wound structure. The electrode group 4 is housed in the container 1. The electrode group 4 has a positive electrode 5, a negative electrode 6, and a pair of separators 7. The electrode group 4 is impregnated with the electrolyte solution. The opening of the container 1 is closed by a sealing plate 2. One end of a positive electrode lead 5c is connected to the positive electrode 5. The other end of the positive electrode lead 5c is connected to the back surface of the sealing plate 2. An insulating packing 3 is arranged around the sealing plate 2. The negative electrode 6 has a negative electrode current collector 6a and a negative electrode active material layer 6b. One end of a negative electrode lead 6c is connected to the negative electrode 6. The other end of the negative electrode lead 6c is connected to the bottom surface of the container 1. Insulating rings 8 are arranged on the upper and lower surfaces of the electrode group 4, respectively. 【0038】 The configuration of the positive electrode 5 is as described in Embodiment 1. By using the positive electrode 5 described in Embodiment 1, it is possible to achieve both suppression of cracking of the positive electrode active material layer 5b and suppression of an increase in the resistance of the positive electrode 5. 【0039】 As the negative electrode current collector 6a, a sheet or film made of a metal material such as stainless steel, nickel, nickel alloy, copper, or copper alloy can be used. The sheet or film may be porous or non-porous. As the sheet or film, a metal foil, a metal mesh, etc. are used. A carbon material may be coated on the surface of the negative electrode current collector 6a as a conductive auxiliary material. 【0040】 The negative electrode active material layer 6b contains a negative electrode active material. The negative electrode active material can be a material having the ability to occlude and release lithium ions. The negative electrode active material includes, for example, at least one selected from the group consisting of a carbon material and a material capable of forming an alloy with lithium. Examples of the carbon material include graphite. Examples of the material capable of forming an alloy with lithium include silicon, silicon-containing oxide, tin, zinc alloy, bismuth, germanium, etc. One kind selected from these negative electrode active materials may be used, or two or more kinds may be used in combination. 【0041】The negative electrode active material layer 6b may contain at least one selected from the group consisting of graphite and silicon as the negative electrode active material. Only graphite may be included in the negative electrode active material layer 6b as the negative electrode active material. Graphite is recommended because it is less likely to deteriorate even when charged and discharged repeatedly at a deep depth. Carbon materials other than graphite may be used as the negative electrode active material. Silicon shows a larger capacity than graphite, so it is advantageous for increasing the capacity of the non-aqueous electrolyte secondary battery 100. 【0042】 The negative electrode active material layer 6b may contain other materials such as a conductive material and a binder. As the conductive material, the materials that can be used for the positive electrode active material layer 5b can also be used for the negative electrode active material layer 6b. As the binder, resin materials such as polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, polytetrafluoroethylene, carboxymethyl cellulose, polyacrylic acid, styrene-butadiene copolymer rubber, polypropylene, polyethylene, and polyimide can be used. 【0043】 The electrolytic solution is a non-aqueous electrolyte impregnated in the positive electrode 5, the negative electrode 6, and the separator 7. The electrolytic solution may fill the internal space of the container 1. Due to the function of the electrolytic solution, lithium ions can move between the positive electrode 5 and the negative electrode 6. 【0044】 The electrolytic solution contains a non-aqueous solvent and a lithium salt. 【0045】 Examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, cyclic ethers, chain ethers, nitriles, amides, and the like. One kind selected from these solvents may be used, or two or more kinds may be used in combination. 【0046】Examples of lithium salts include lithium hexafluoride phosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bisperfluoroethylsulfonylimide (LiN(SO2C2F5)2), LiAsF6, LiCF3SO3, and lithium difluoro(oxalato)borate. One of these lithium salts may be used, or two or more may be used in combination. 【0047】 The separator 7 is lithium ion conductive. The material of the separator 7 is not particularly limited as long as the passage of lithium ions is permitted. The material of the separator 7 may be at least one selected from the group consisting of gel electrolytes, ion exchange resin membranes, semipermeable membranes, and porous membranes. If the separator 7 is made of these materials, the safety of the non-aqueous electrolyte secondary battery 100 can be sufficiently ensured. Examples of gel electrolytes include gel electrolytes containing fluororesins such as PVDF. Examples of ion exchange resin membranes include cation exchange membranes and anion exchange membranes. Examples of porous membranes include porous membranes made of polyolefin resin and porous membranes containing glass paper obtained by weaving glass fibers into a nonwoven fabric. 【0048】 Container 1 is, for example, a metal container such as aluminum or stainless steel. Container 1 may have a cylindrical shape or a rectangular tube shape. 【0049】 The electrode group 4 may be wound in a cylindrical shape or in an elliptical shape. 【0050】 The shape of the non-aqueous electrolyte secondary battery 100 is not limited to a cylindrical shape. Various shapes such as coin-shaped, prismatic, sheet-shaped, button-shaped, flat, and stacked can be used for the non-aqueous electrolyte secondary battery 100. 【0051】 (Other Embodiments) (Note) The above description of embodiments discloses the following technologies. 【0052】(Technical 1) A positive electrode for a non-aqueous electrolyte secondary battery comprising a positive electrode active material and a plurality of types of binders, wherein the plurality of types of binders include a first binder having the largest surface free energy among the plurality of types of binders and a second binder having the smallest surface free energy among the plurality of types of binders, the ratio of the total mass of the plurality of types of binders to the mass of the positive electrode active material is 0.4% or more and 3.0% or less, the difference between the surface free energy of the first binder and the surface free energy of the second binder is 5 mN / m or more, and the ratio of the mass of the second binder to the total mass of the plurality of types of binders is greater than 10.0% and 70.0% or less. 【0053】 According to this disclosure, it is possible to suppress cracking of the positive electrode active material layer and suppress an increase in the resistance of the positive electrode at the same time. 【0054】 (Technology 2) The positive electrode for a non-aqueous electrolyte secondary battery according to Technology 1, wherein the second binder is a polymer containing an olefin skeleton. By including an olefin skeleton in the polymer as the second binder, flexibility can be imparted to the positive electrode active material layer. 【0055】 (Technology 3) A positive electrode for a non-aqueous electrolyte secondary battery according to Technology 1 or 2, wherein the elongation at break of the resin film made from the second binder is 200% or more. The fact that the resin film made from the second binder has a large elongation at break means that the second binder has excellent flexibility. 【0056】 (Technical 4) The coating amount of the positive electrode active material layer containing the positive electrode active material and the multiple types of binders is 250 g / m². 2 More than 400g / m 2 A positive electrode for a non-aqueous electrolyte secondary battery as described in any one of the following technologies 1 to 3. The thicker the positive electrode active material layer, the more likely cracks are to occur in the positive electrode active material layer during the winding process. Therefore, it is desirable to apply the technology of this disclosure to a positive electrode equipped with a thick positive electrode active material layer. 【0057】(Technical 5) A positive electrode for a non-aqueous electrolyte secondary battery according to any one of Technical 1 to 4, wherein the positive electrode active material comprises a lithium-containing transition metal oxide containing Li, Ni, and M, and M is at least one selected from the group consisting of Al, Mn, and Co. When the positive electrode active material contains a lithium-containing transition metal oxide, the total mass of the binder in the positive electrode active material layer can be reduced. 【0058】 (Technology 6) A positive electrode for a non-aqueous electrolyte secondary battery according to any one of Technology 1 to 5, further comprising carbon black as a conductive material, wherein the ratio of the mass of the carbon black to the mass of the second binder is 190% or more and 600% or less. With such a configuration, it is easy to achieve both suppression of cracking of the positive electrode active material layer and suppression of an increase in the resistance of the positive electrode. 【0059】 (Technical 7) A positive electrode for a non-aqueous electrolyte secondary battery according to any one of Technical 1 to 5, further comprising a fibrous carbon material as a conductive material, wherein the ratio of the mass of the fibrous carbon material to the mass of the second binder is 75% or more and 350% or less. With such a configuration, it is easy to achieve both suppression of cracking of the positive electrode active material layer and suppression of an increase in the resistance of the positive electrode. 【0060】 (Technology 8) A positive electrode for a non-aqueous electrolyte secondary battery according to Technology 7, wherein the fibrous carbon material contains carbon nanotubes. Carbon nanotubes are desirable because they can impart high conductivity to the positive electrode active material layer in small amounts. 【0061】 (Technical 9) A non-aqueous electrolyte secondary battery comprising a positive electrode for a non-aqueous electrolyte secondary battery as described in any one of Technical 1 to 8. 【0062】[Measurement of Surface Free Energy of Binder] A solution was prepared by mixing polyvinylidene fluoride (PVDF) and N-methylpyrrolidone (NMP). The solution was applied to the surface of a glass plate by spin coating to form a coating film. The coating film was dried to obtain a PVDF resin film. After leaving the resin film in air at 25°C for 12 hours, the static contact angles of the resin film surface at 25°C for water, ethylene glycol, and diiodomethane were measured. A contact angle meter (DropMaster DM-501, Kyowa Interface Science Co., Ltd.) was used to measure the static contact angle. When measuring the static contact angle, the smallest possible droplets were prepared so that the droplets did not climb up the needle. The static contact angle was calculated using the θ / 2 method with images taken 5 seconds after the droplet adhered to the surface of the resin film. 【0063】 Using the same method, resin films were prepared using two types of hydrogenated nitrile butadiene rubber (H-NBR1 and H-NBR2) and acrylic resin, and the static contact angles of these resin films were measured. 【0064】 Subsequently, the surface free energy of the resin film was calculated using the method described earlier. The results are shown in Table 1. 【0065】 The difference between the hydrogenated nitrile butadiene rubber labeled "H-NBR1" and the hydrogenated nitrile butadiene rubber labeled "H-NBR2" lies in the ratio of the mass of repeating units (olefin skeleton) derived from butadiene to the mass of the polymer. In H-NBR1, the ratio of the mass of repeating units derived from butadiene to the mass of the polymer was 70%, while the ratio of the mass of repeating units derived from acrylonitrile to the mass of the polymer was 30%. In H-NBR2, the ratio of the mass of repeating units derived from butadiene to the mass of the polymer was 40%, while the ratio of the mass of repeating units derived from acrylonitrile to the mass of the polymer was 60%. 【0066】The weight-average molecular weight of PVDF was 1,000,000. The weight-average molecular weight of H-NBR1 and H-NBR2 were both 1,200,000. The weight-average molecular weight of acrylic resin was 650,000. 【0067】 [Measurement of Elongation at Breaking of Resin Film] Binder solutions were prepared by dissolving each binder in a solvent. The binder solution was applied to a PET film to form a coating film with a thickness of 50 μm. The coating film was then dried and peeled off the PET film. This obtained a resin film. The resin film was then cut into pieces with a width of 1 cm and a length of 15 cm to obtain test specimens. A tensile test was performed using a tensile testing machine (Orientec Co., Ltd., Tensilon® UCT-100) under the conditions of a measurement temperature of 25°C, a chuck distance of 50 mm, and a tensile speed of 50 mm / min. The rate of change in length from the start of measurement until the test specimen broke was considered as the elongation at breaking. The results are shown in Table 1. 【0068】 【0069】 As shown in Table 1, the elongation at break of the resin film made from PVDF was small. In contrast, the elongation at break of the resin films made from H-NBR1, H-NBR2, and acrylic resin was large. However, the elongation at break of the resin film made from H-NBR2 was smaller than that of the resin film made from H-NBR1. In other words, even with the same H-NBR, the more repeating units (olefin skeleton) derived from butadiene there are, the higher the rubber elasticity imparted to the H-NBR. 【0070】 Furthermore, even with the same H-NBR, the surface free energy changes depending on the ratio of the mass of repeating units (olefin backbone) derived from butadiene to the mass of the polymer. H-NBR1 showed a surface free energy relatively close to that of acrylic resin. In contrast, H-NBR2 showed a surface free energy relatively close to that of PVDF. 【0071】 [Fabrication of positive electrode] LiNi 0.6 Co 0.2 Mn 0.2The cathode active material particles having a composition of O₂(NCM), carbon black (CB), PVDF, and NMP were mixed and stirred to prepare a cathode slurry. The mass ratio of the cathode active material particles, CB, and PVDF was cathode active material: CB: PVDF = 100: 1: 1. The cathode slurry was applied to both sides of an aluminum foil to form a coating film, and after drying the coating film, it was rolled. Thereby, a strip-shaped cathode of Comparative Example 1 was obtained. The coating amount of each cathode active material layer in the cathode of Comparative Example 1 was 300 g / m 2 was. 【0072】 (Comparative Examples 2 to 5 and Examples 1 to 14) Using the materials shown in Table 2, cathodes of Comparative Examples 2 to 5 and Examples 1 to 14 were produced by the same method as in Comparative Example 1. In Example 13, lithium iron phosphate (LFP) was used instead of NCM. In Examples 2, 4, 6, 8, and 9, carbon nanotubes (CNT) were used instead of carbon black (CB). The coating amount of the cathode active material layer was 300 g / m 2 in Comparative Examples 1 to 5 and Examples 1 to 13, and 380 g / m 2 in Example 14. 【0073】 In Comparative Example 3, Comparative Example 4, and Examples 1 to 14, the binder having a relatively large surface free energy is the "first binder", and the binder having a relatively small surface free energy is the "second binder". 【0074】 【0075】 [Measurement of resistance of cathode active material layer] Using an electrode resistance measuring instrument (manufactured by Hioki Electric Co., Ltd., RM2610), the resistance of the cathode active material layer in the cathodes of the examples and comparative examples was measured. The measurement current was set to 100 μA, and the voltage range was set to 0.5 V. The results are shown in Table 3. 【0076】 [Inspection of cracks in cathode active material layer] The cathodes of the examples and comparative examples were cut into test pieces having a size of 2 cm in width and 5 cm in length. The test piece was wound around a metal rod with a diameter of φ4.0 mm. Using a laser microscope (manufactured by Keyence Corporation, VK9700), the outer peripheral surface of the test piece was observed. The results are shown in Table 3. The criteria for judging "cracks in the active material layer" in Table 3 were as follows. 【0077】 ○: No cracks were found. △: Minor cracks were found. ×: Cracks were found. 【0078】 【0079】 In Table 3, the "Binder / Active Material" column represents the ratio of the total mass of the binder to the mass of the positive electrode active material. The "Difference in Surface Free Energy" column represents the difference between the surface free energy of the first binder and the surface free energy of the second binder. The "Second Binder / Binder" column represents the ratio of the mass of the second binder to the total mass of the binders. The "Elongation at Break of Second Binder" column represents the elongation at break of the resin film made using the second binder. The "Conductive Material (CB) / Second Binder" column represents the ratio of the mass of carbon black to the mass of the second binder. The "Conductive Material (CNT) / Second Binder" column represents the ratio of the mass of carbon nanotubes to the mass of the second binder. 【0080】 The positive electrodes of Comparative Examples 1, 2, and 5 contain only one type of binder. Therefore, in Table 3, the "Elongation at Break of Second Binder" column for Comparative Examples 1, 2, and 5 refers to the elongation at break of the first binder. 【0081】 The positive electrode of Comparative Example 1 contained only PVDF as a binder. As a result, it was not possible to impart sufficient flexibility to the positive electrode active material layer, and cracks occurred in the positive electrode active material layer when the positive electrode of Comparative Example 2 was wound. 【0082】 The positive electrode of Comparative Example 2 contained only H-NBR1 as a binder. H-NBR1 is a resin with rubber elasticity, which can impart flexibility to the positive electrode. Therefore, no cracks occurred in the positive electrode active material layer when the positive electrode of Comparative Example 2 was wound. However, the resistance of the active material layer in the positive electrode of Comparative Example 2 was very high. In other words, H-NBR1 was disadvantageous in terms of reducing the resistance of the positive electrode. 【0083】The positive electrode of Comparative Example 3 contained PVDF and H-NBR2 in a mass ratio of 0.5:0.5. H-NBR2, like H-NBR1, is a resin with rubber elasticity and can impart flexibility to the positive electrode. However, as can be seen from Table 1, the difference between the surface free energy of PVDF and the surface free energy of H-NBR2 was "4", which was small. Therefore, when the positive electrode of Comparative Example 2 was wound, cracks occurred in the positive electrode active material layer. In other words, H-NBR2 could not impart sufficient flexibility to the positive electrode. 【0084】 The positive electrode of Comparative Example 4 contained PVDF and H-NBR1 in a mass ratio of 0.9:0.1. H-NBR1 is a resin with rubber elasticity and can impart flexibility to the positive electrode. However, because the ratio of the mass of H-NBR1 (second binder) to the total mass of the binder was low, cracks occurred in the positive electrode active material layer when the positive electrode of Comparative Example 4 was wound. 【0085】 The positive electrode of Comparative Example 5 contained only H-NBR2 as a binder. H-NBR2 is a resin with rubber elasticity, which can impart flexibility to the positive electrode. Therefore, no cracks occurred in the positive electrode active material layer when the positive electrode of Comparative Example 5 was wound. However, the resistance of the active material layer in the positive electrode of Comparative Example 5 was high. In other words, H-NBR2 was disadvantageous from the viewpoint of reducing the resistance of the positive electrode. 【0086】 As can be seen by comparing Comparative Example 2 and Comparative Example 5, H-NBR1 excelled at imparting flexibility to the positive electrode, but significantly increased resistance. H-NBR2 was somewhat inferior in its ability to impart flexibility to the positive electrode, but was superior to H-NBR1 in suppressing the increase in resistance. 【0087】 The positive electrodes of Comparative Examples 2 and 5 both contained a sufficient amount of conductive material, but exhibited high resistance due to the binder. 【0088】In contrast, the positive electrodes of Examples 1 to 14 contained a first binder and a second binder. The first binder was PVDF. The second binder was H-NBR1 or acrylic resin. The difference between the surface free energy of the first binder and the surface free energy of the second binder was 5 mN / m or more. The ratio of the mass of the second binder to the total mass of the binder was 50% or 70%. The ratio of the total mass of the binder to the mass of the positive electrode active material was 0.4, 1, 1.5, 2, or 2.4. According to Examples 1 to 14, it was possible to suppress cracking of the positive electrode active material layer and suppress the increase in the resistance of the positive electrode at the same time. 【0089】 As can be seen from the sections on "Conductive Material (CB) / Second Binder" and "Resistance of Active Material Layer," the resistance decreased as the ratio of the mass of carbon black to the mass of the second binder increased. However, the amount of carbon black contained in the positive electrode of Example 7 was slightly too much, causing minute cracks in the positive electrode active material layer when the positive electrode of Example 7 was wound. Similarly, as can be seen from the sections on "Conductive Material (CNT) / Second Binder" and "Resistance of Active Material Layer," the resistance decreased as the ratio of the mass of carbon nanotubes to the mass of the second binder increased. The amount of carbon nanotubes contained in the positive electrode of Example 8 was slightly too much, causing minute cracks in the positive electrode active material layer when the positive electrode of Example 8 was wound. In other words, it was difficult to suppress cracking of the positive electrode active material layer and suppress the increase in the resistance of the positive electrode at the same time by increasing both the amount of binder and the amount of conductive material. 【0090】 According to the sections on "Conductive material (CB) / Second binder" and "Resistance of active material layer," it is desirable that the ratio of the mass of carbon black to the mass of the second binder be between 190% and 600%. According to the sections on "Conductive material (CNT) / Second binder" and "Resistance of active material layer," it is desirable that the ratio of the mass of fibrous carbon material such as carbon nanotubes to the mass of the second binder be between 75% and 350%. 【0091】 The technology disclosed herein is useful, for example, in lithium-ion secondary batteries.

Claims

1. A positive electrode for a non-aqueous electrolyte secondary battery, comprising: a positive electrode active material; and a plurality of types of binders, wherein the plurality of types of binders include a first binder having the largest surface free energy among the plurality of types of binders and a second binder having the smallest surface free energy among the plurality of types of binders, the ratio of the total mass of the plurality of types of binders to the mass of the positive electrode active material is 0.4% or more and 3.0% or less, the difference between the surface free energy of the first binder and the surface free energy of the second binder is 5 mN / m or more, and the ratio of the mass of the second binder to the total mass of the plurality of types of binders is greater than 10.0% and 70.0% or less.

2. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the second binder is a polymer containing an olefin skeleton.

3. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the elongation at break of the resin film made from the second binder is 200% or more.

4. The coating amount of the positive electrode active material layer containing the positive electrode active material and the multiple types of binders is 250 g / m². 2 More than 400g / m 2 The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 is as follows:

5. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the positive electrode active material comprises a lithium-containing transition metal oxide containing Li, Ni, and M, and M is at least one selected from the group consisting of Al, Mn, and Co.

6. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, further comprising carbon black as a conductive material, wherein the ratio of the mass of the carbon black to the mass of the second binder is 190% or more and 600% or less.

7. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, further comprising a fibrous carbon material as a conductive material, wherein the ratio of the mass of the fibrous carbon material to the mass of the second binder is 75% or more and 350% or less.

8. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 7, wherein the fibrous carbon material includes carbon nanotubes.

9. A non-aqueous electrolyte secondary battery comprising the positive electrode for a non-aqueous electrolyte secondary battery as described in claim 1.

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