Secondary battery separator, production method for same, and secondary battery

Abstract

According to the present invention, a secondary battery separator includes: a porous substrate that has a first surface and a second surface that is on the reverse side from the first surface; and a polysaccharide that is adhered to the porous substrate. The polysaccharide includes a sulfo group.

Description

Secondary battery separator, manufacturing method thereof, and secondary battery

[0001] The present disclosure relates primarily to a separator for a secondary battery.

[0002] In secondary batteries containing lithium-ion conductive non-aqueous electrolytes, the amount of metal ions eluted from the positive electrode tends to increase as the end-of-charge voltage increases. When the eluted metal ions reach the negative electrode by migration, they precipitate as metal at the negative electrode, which reduces the safety and load characteristics of the battery.

[0003] On the other hand, Patent Document 1 proposes a binder for connecting electrode materials of an electrochemical device, which is characterized by containing a pH adjusting functional agent that neutralizes bases in an aqueous system and a metal cross-linking thickener that cross-links with metal ions derived from the electrode active material in the aqueous system to form a hydrophobic gel. Patent Document 1 aims to provide a binder that can be prepared using water as a solvent and without alkalizing the electrode slurry, even when an electrode active material that is very susceptible to hydrolysis is used.

[0004] Patent Document 2 proposes a nanocomposite material comprising nano objects made of at least one first type of electron-conducting material and nano or submicron objects made of at least one second material different from the first material, wherein the nanocomposite material each comprises a nanostructure consisting of a three-dimensional network of nano objects made of at least one first type of electron-conducting material that are bound to and maintained by polysaccharides, and the nano or submicron objects made of at least one second material different from the first material self-assemble around the network and are bound to the nano objects made of at least one first type of electron-conducting material by the polysaccharides, and the nanostructures are uniformly dispersed throughout the material.

[0005] Patent Document 3 describes a negative electrode active material for a non-aqueous secondary battery in which first and second organic components are adsorbed and coated on graphite particles for the negative electrode of a non-aqueous secondary battery using aggregated graphite particles, and the aggregated graphite particles have an apparent density of 0.70 g / cm3 as measured by a tapping method. 3As described above, we have proposed a negative electrode active material for a non-aqueous secondary battery, which is characterized by having an ash content of 0.5% by mass or less, a total of 100 ppm or less of Fe, Cu, and Zn determined by acid extraction ICP analysis, and containing 0.1 to 2% by mass of one or more selected from polysaccharides, polyuronides, gelatin, casein, and water-soluble polyethers as a first component, and 0.001 to 0.5% by mass of one or more selected from monosaccharides, disaccharides, oligosaccharides, amino acids, gallic acid, tannin, saccharin and its salts, and butynediol as a second component.

[0006] Patent Document 4 proposes an electrode characterized by being constructed using a mixture containing a metal oxide or metal hydroxide as an active material, a copolymer of one or more sugars including glucose, glucuronic acid, rhamnose, mannose, galactose, or a salt thereof, at least a portion of which contains a 2-position substitution, a 3-position substitution, or a 5-position substitution, and a copolymer of maleic acid, its anhydride, or a salt thereof, and at least one unsaturated monomer.

[0007] International Publication No. 2020 / 095466 Patent Publication No. 2016-504707 Patent Publication No. 2011-134572 Patent Publication No. 2003-308840

[0008] Coating the separator with an organic polymer containing anionic functional groups capable of trapping metal ions is expected to suppress metal deposition at the negative electrode. However, it is difficult to uniformly coat the separator surface with an organic polymer containing highly polar anionic functional groups. Even if coating is possible, the organic polymer tends to aggregate, blocking the separator pores with the organic polymer, which tends to degrade the battery's load characteristics.

[0009] One aspect of the present disclosure relates to a separator for a secondary battery, comprising: a porous substrate having a first surface and a second surface opposite the first surface; and a polysaccharide adhered to the porous substrate, wherein the polysaccharide has a sulfo group.

[0010] Another aspect of the present disclosure relates to a secondary battery including a positive electrode, a negative electrode, a lithium ion conductive non-aqueous electrolyte, and a separator interposed between the positive electrode and the negative electrode, wherein the separator is the above-described secondary battery separator.

[0011] Yet another aspect of the present disclosure relates to a method for manufacturing a separator for a secondary battery, the method including the steps of: preparing a porous substrate having a first surface and a second surface opposite the first surface; preparing a solution of a polysaccharide having a sulfo group; applying the solution to the first surface of the porous substrate; and drying the porous substrate to which the solution has been applied.

[0012] The polysaccharide having sulfo groups attached to the porous substrate has a high ability to trap metal ions, is less likely to clog the pores of the porous substrate, and improves the wettability of the separator with respect to the non-aqueous electrolyte, thereby suppressing metal deposition at the negative electrode and improving the load characteristics of the secondary battery.

[0013] The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0014] 1 is a longitudinal cross-sectional view schematically illustrating an internal structure of a secondary battery according to an embodiment of the present disclosure.

[0015] Below, embodiments of a secondary battery separator according to the present disclosure and a secondary battery using the same will be described using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the expression "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits for specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not greater than the upper limit.

[0016] The present disclosure also encompasses combinations of two or more features arbitrarily selected from the appended claims, i.e., two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0017] A. Secondary Battery Separator The secondary battery separator according to the present disclosure includes a porous substrate and a polysaccharide attached to the porous substrate. The polysaccharide has one or more sulfo groups (-SO3X or -SO3 - Since polysaccharides contain many hydrophilic groups and have a relatively large molecular weight, they are hardly dissolved in the non-aqueous electrolyte in the battery and can remain attached to the porous substrate for a long period of time without peeling off.

[0018] The sulfo group is classified into an acid type (-SO3X (where X is H)), a salt type (-SO3X (where X is a cation)) and an anion type (-SO3 - The cation contained in the sulfo group as X may be a metal ion or an organic cation, and may be an ammonium ion (NH + ) is also acceptable.

[0019] The metal ion may be a cation of an alkali metal, alkaline earth metal, or the like, such as barium, calcium, magnesium, potassium, sodium, or lithium.

[0020] The multiple sulfo groups contained in a polysaccharide molecule may be of the same type or different types. For example, a polysaccharide having salt-type sulfo groups may have two or more types of sulfo groups with different cations. In addition, multiple types of polysaccharides with different molecular structures may be used in combination and attached to the porous substrate, or only one type of polysaccharide may be attached to the porous substrate.

[0021] Hereinafter, polysaccharides having sulfo groups will also be referred to as "polysaccharides (S)."

[0022] The porous substrate has a first surface and a second surface opposite the first surface. In other words, the porous substrate may be in the form of a membrane, a sheet, or a film. The porous substrate may be a stretched film, a nonwoven fabric, a woven fabric, or the like.

[0023] A separator for a secondary battery comprising a porous substrate and a polysaccharide (S) attached to the porous substrate has the effect of suppressing metal deposition at the negative electrode and can improve the load characteristics of the secondary battery. The reasons for this are thought to be as follows.

[0024] First, polysaccharide (S) can act as a barrier that inhibits the movement of metal ions between electrodes. The sulfo groups of polysaccharide (S) efficiently trap impurities and metal ions eluted from the positive electrode. The sulfo groups have a high ability to trap metal ions.

[0025] Second, the polysaccharide (S) has the property of being easily attached to a porous substrate, while being less likely to clog the pores of the porous substrate. The polysaccharide (S) exhibits high solubility in a predetermined hydrophilic solvent, making it easy to attach it thinly and uniformly along the inner walls of the pores of the porous substrate. Therefore, it is easy to prevent the polysaccharide (S) from blocking the openings of the pores or filling the pores with the polysaccharide (S). The highly ionic sulfo group is thought to contribute to the high solubility of the polysaccharide (S).

[0026] Third, polysaccharides (S) tend to improve the wettability of the porous substrate (i.e., separator) with respect to the non-aqueous electrolyte. While polysaccharides (S) are hardly soluble in non-aqueous electrolytes, they have a higher affinity with non-aqueous electrolytes than porous substrates generally used as separators in secondary batteries. A separator made of a porous substrate with polysaccharides (S) attached thereto has higher wettability with non-aqueous electrolytes than a separator that does not contain polysaccharides (S). As a result, the load characteristics of a secondary battery using a separator made of a porous substrate with polysaccharides (S) attached thereto are improved compared to a secondary battery using a separator that does not contain polysaccharides (S).

[0027] The effect of polysaccharide (S) in suppressing metal ion migration is particularly pronounced when impurity metals such as copper and iron are present in a secondary battery. When the impurity metals are exposed to the positive electrode potential, metal ions leach from the impurity metals into the non-aqueous electrolyte. Furthermore, in secondary batteries with a high upper limit voltage, metal ions may also leach from the active material particles in the positive electrode. For example, a secondary battery with an upper limit voltage exceeding 4.3 V has a high positive electrode potential, and the positive electrode active material particles contain metal components (often transition metals), which can lead to leach of metal ions. The metal ions leach into the non-aqueous electrolyte migrate from the positive electrode side to the negative electrode side and precipitate as impurity metals. In contrast, when polysaccharide (S) is contained in the separator, the migration of leachable metal ions between electrodes is significantly suppressed. Hereinafter, metal ions that can precipitate as impurity metals at the negative electrode are also referred to as impurity metal ions.

[0028] The polysaccharide (S) in the separator is fixed to the porous substrate, while impurity metal ions eluted into the non-aqueous electrolyte on the positive electrode side pass through the separator as they move to the negative electrode side. This increases the probability that the impurity metal ions are trapped by the polysaccharide (S). The trapped metal ions are fixed to the porous substrate in the same manner as the polysaccharide (S), limiting their freedom of movement. This significantly suppresses the movement of impurity metal ions from the positive electrode side to the negative electrode side.

[0029] The air permeability of the secondary battery separator according to the present disclosure (i.e., a separator comprising a porous substrate and a polysaccharide (S) attached thereto (hereinafter also referred to as "separator (S)")) measured by the method specified in JIS P 8117 may be, for example, 100 seconds / 100 mL or more and 500 seconds / 100 mL or less, or 400 seconds / 100 mL or less. Such a low air permeability can be easily ensured because the polysaccharide (S) does not easily block the pores of the porous substrate. In general, the smaller the air permeability value, the larger the pore volume of the separator tends to be.

[0030] Polysaccharides are a general term for polymers having a structure in which multiple monosaccharide molecules are bonded via glycosidic bonds. Although there are no particular limitations on the polysaccharides, from the viewpoint of ensuring ease of production of the separator, it is desirable for the polysaccharides to have the property of being soluble in a mixed solvent of water and ethanol in a volume ratio of 50:50, for example.

[0031] Examples of usable basic structures of polysaccharides include aldose, ketose, pyranose, and furanose. Monosaccharide molecules (monomers) constituting polysaccharides include triose, tetrose, pentose, hexose, and heptose. Among these, aldopentose, ketopentose, aldohexose, and ketohexose are desirable, and galactose, which is classified as an aldohexose, can be used. For example, polysaccharide (S) may have a galactose polymer skeleton. Polysaccharide (S) may also be obtained by sulfate esterification of polymers of these monosaccharides.

[0032] Polysaccharides that originally have sulfo groups (e.g., carrageenan) may be used, or polysaccharides (S) may be obtained by sulfate esterification of pectin, alginic acid, pullulan, mannan, xanthan gum, guar gum, starch, glycogen, chitin, dextran, agarose, carrageenan, heparin, hyaluronic acid, glucomannan, gum arabic, gelatin, tremel gum, etc. Among these, carrageenan is preferably used. Carrageenan is classified into types such as kappa, iota, and lambda, and any of these may be used.

[0033] The content of carrageenan contained in the polysaccharide (S) to be attached to the porous substrate (i.e., contained in the separator) may be, for example, 70% by mass or more, or the carrageenan may be 100%.

[0034] The presence of polysaccharide (S) in a separator can be confirmed simply by analyzing the infrared absorption spectrum obtained by FT-IR measurement of the separator. Various polysaccharides (S) exhibit their own unique spectra. In the spectra, peaks attributable to, for example, monosaccharide molecular species (e.g., the C-O bond contained in the galactose skeleton in carrageenan), S=O bonds, C-O-S bonds, etc. are observed.

[0035] Furthermore, whether the separator contains sulfo groups that bond to polysaccharides can be quantitatively confirmed by elemental analysis (e.g., IPC (inductively coupled plasma optical emission spectroscopy), SEM-EDX (scanning electron microscope-energy dispersive X-ray analysis), etc.).

[0036] The number of moles of sulfo groups contained in the polysaccharide (S) per unit mass is, for example, 1.0 × 10 -6 mol / g or more, and 1.0×10 -4 mol / g or more, and may be 1.0 × 10 -3 mol / g or more 1.0×10 -2 Polysaccharides with a high sulfo group content have the ability to trap more metal ions, have excellent solubility in a mixed solvent of water and ethanol, and are likely to be thinly and uniformly attached along the inner walls of the pores of the porous substrate.

[0037] The amount of polysaccharide (S) attached to the porous substrate per apparent unit area (hereinafter also referred to as the "area density of polysaccharide (S)") is, for example, 1.0 × 10 -5 g / m 2 Above 1.0 x 10 2 g / m 2 is less than or equal to 1.0 × 10 -4 g / m 2 Above 1.0 x 10 1 g / m 2 It may be less than 1.0 × 10 -3 g / m 2 1.0g / m or more 2 The apparent unit area is one unit (1 m) of the area enclosed by the outline of the projected image of the porous substrate when viewed from the normal direction of the first surface and the second surface of the porous substrate. 2 ) means

[0038] To determine the areal density of the polysaccharide (S), a sample of a predetermined size is first cut from the separator and dried by heating at 60°C for at least one hour, after which the dry mass W1 is determined. Next, the dried sample is immersed in a 50:50 volumetric water / ethanol mixed solvent (20°C to 30°C) for one hour, then thoroughly washed with a 50:50 volumetric water / ethanol mixed solvent, and dried by heating at 60°C for at least one hour, after which the dry mass W2 is determined. The polysaccharide (S) is virtually completely removed by immersion in the mixed solvent and washing with the mixed solvent. The areal density of the polysaccharide (S) is then determined from the dry masses W1 and W2 and the size (apparent area) of the sample.

[0039] The areal density of the polysaccharide (S) may be determined from a separator sample taken out of a secondary battery in a fully discharged state (SOC = 0%). In this case, the separator may be thoroughly washed with dimethyl carbonate (DMC) and then vacuum-dried before use as a sample.

[0040] In the separator (S), the distribution of the polysaccharide (S) may be varied in the thickness direction of the porous substrate. For example, a larger amount of the polysaccharide (S) may be distributed near the surface facing the positive electrode, which is the source of metal ion elution. This shortens the migration distance of metal ions eluted from the positive electrode, further reducing the probability that the metal ions will reach the negative electrode.

[0041] Specifically, when the porous substrate is divided into a first region on the first surface side (positive electrode side) and a second region on the second surface side (negative electrode side) having the same thickness, the content C1 of polysaccharide contained in the first region may be greater than the content C2 of polysaccharide contained in the second region. In this case, the mass ratio C1 / C2 of the content C1 to the content C2 is greater than 1, and may be 1.1 or greater, 1.2 or greater, or 1.5 or greater. In other words, when the thickness of the separator (S) or porous substrate (S) is T, the presence probability P1 of polysaccharide (S) present in the first region from the first surface to 0.5T (the center in the thickness direction) and the presence probability P2 of polysaccharide (S) present in the second region from the position 0.5T (the center in the thickness direction) to the second surface satisfy 1 < P1 / P2, and P1 / P2 may be 1.2 or greater, or 1.5 or greater.

[0042] The polysaccharide (S) presence probabilities P1 and P2 may be measured on a cross section obtained by cutting the separator along the thickness direction. In this case, the separator may be filled with a thermosetting resin and cured. For example, a cross-sectional sample of the separator is obtained using a CP (cross-section polisher) method, an FIB (focused ion beam) method, or the like. The P1 / P2 ratio of the polysaccharide (S) presence probabilities P1 and P2 can be determined by analyzing the separator cross-sectional sample using SEM and EDX. For example, the amount of sulfur element contained in the first region and the amount of sulfur element contained in the second region may be quantified from a sulfur element map obtained from EDX analysis data of the cross-sectional sample, and the ratio of the obtained values ​​may be used as the P1 / P2 ratio. P1 and P2 are measured in an observation field of 20 μm or more along the surface direction of the separator. P1 and P2 may also be average values ​​of P1 and P2 determined in multiple (e.g., three or more) observation fields.

[0043] <Method for manufacturing separator> Next, an example of a method for manufacturing a separator for a secondary battery (separator (S)) according to an embodiment of the present disclosure will be described. The manufacturing method includes a step (I) of preparing a porous substrate having a first surface and a second surface opposite to the first surface, a step (II) of preparing a solution of a polysaccharide having a plurality of sulfo groups (hereinafter also referred to as a "polysaccharide solution"), a coating step (III) of applying the polysaccharide solution to the porous substrate, and a drying step (IV) of drying the porous substrate to which the polysaccharide solution has been applied (i.e., a step of obtaining a separator).

[0044] (I) Step of Preparing Porous Substrate As the porous substrate, for example, a porous sheet commonly used as a separator for a secondary battery (particularly a lithium ion battery), such as a stretched film (or a microporous thin film), a nonwoven fabric, or a woven fabric, can be used. The porous sheet has appropriate mechanical strength and insulating properties.

[0045] The porous sheet may be made of a polyolefin such as polypropylene or polyethylene. The porous substrate may have a heat-resistant insulating layer on at least one of the first and second surfaces. The heat-resistant insulating layer may contain an inorganic oxide filler as a main component (e.g., 80% by mass or more) or a heat-resistant resin as a main component (e.g., 40% by mass or more). The heat-resistant resin may be a polyamide resin such as aromatic polyamide (aramid), a polyimide resin, or a polyamide-imide resin.

[0046] The thickness of the porous substrate is not particularly limited, but may be, for example, 1 to 50 μm, or 5 to 30 μm.

[0047] (II) Step of Preparing a Polysaccharide Solution The polysaccharide solution is prepared by mixing the polysaccharide (S) with a solvent and dissolving the polysaccharide (S) in the solvent. The polysaccharide solution may further contain additives other than the solvent, such as alcohol, a phosphorus compound, a boron compound, or a sulfur compound. The polysaccharide solution may contain, for example, a mixed solvent of water and ethanol and the polysaccharide (S) dissolved in such a mixed solvent, but is not limited to this.

[0048] The solvent is preferably water, alcohol (e.g., ethanol), or a mixed solvent of water and alcohol, but is not particularly limited as long as it can dissolve the polysaccharide (S). Ethers such as tetrahydrofuran, amides such as dimethylformamide, ketones such as cyclohexanone, N-methyl-2-pyrrolidone (NMP), mixed solvents thereof, and the like may also be used as at least a portion of the solvent. Of these, a mixed solvent of water and ethanol is preferable, and the volume ratio of water to ethanol is, for example, water / ethanol = 20 / 80 to 80 / 20, or may be water / ethanol = 40 / 60 to 60 / 40, or may be water / ethanol = 45 / 55 to 55 / 45.

[0049] (III) Coating Step The method for coating the porous substrate with the polysaccharide solution is not particularly limited. For example, coating methods using various coaters, dipping methods, spraying methods, etc. Examples of coaters that can be used include bar coaters, gravure coaters, blade coaters, roll coaters, comma coaters, die coaters, and lip coaters.

[0050] In the coating step, the polysaccharide solution may be applied only to the first surface of the porous substrate. For example, the polysaccharide solution may be applied only to the first surface side of the porous substrate using various coaters, or the polysaccharide solution may be sprayed only to the first surface side of the porous substrate. According to such a method, the content C1 of the polysaccharide (S) contained in the first region on the first surface side of the porous substrate can be made larger than the content C2 of the polysaccharide (S) contained in the second region on the second surface side of the porous substrate. By controlling the amount of the polysaccharide solution applied and / or controlling the drying conditions described below, the ratio of the content C1 to the content C2: C1 / C2 may be controlled to 1.1 or more.

[0051] (IV) Drying Step In the drying step, the porous substrate coated with the polysaccharide solution is dried to complete the separator. When the C1 / C2 ratio is controlled as described above, the polysaccharide (S) may be migrated to the first surface side together with the solvent by appropriately controlling the drying conditions. As a result, the polysaccharide (S) is unevenly distributed on the first surface side.

[0052] After the drying step, the separator may be rolled. Rolling can provide a separator with high flatness. The porous substrate coated with the polysaccharide solution may be rolled while being heated with a heated roll at a temperature lower than the melting point of the material of the porous substrate, thereby simultaneously performing drying and rolling.

[0053] B. Secondary Batteries Secondary batteries according to embodiments of the present disclosure include lithium-ion secondary batteries that use a material that reversibly absorbs and releases lithium ions as a negative electrode active material, lithium secondary batteries in which lithium metal precipitates at the negative electrode during charging and dissolves during discharge, and nonaqueous electrolyte secondary batteries such as solid batteries containing gel or solid electrolytes. These secondary batteries include a positive electrode, a negative electrode, a lithium-ion conductive nonaqueous electrolyte, and the above-described secondary battery separator (separator (S)) interposed between the positive electrode and the negative electrode. When the C1 / C2 ratio is controlled as described above, the separator is disposed so that its first surface faces the positive electrode. The nonaqueous electrolyte may be liquid (i.e., an electrolytic solution) as a whole, or the electrolytic solution may be held in a matrix material and used as a solid or gel electrolyte.

[0054] The secondary battery according to the present disclosure may have an end-of-charge voltage set to 4.3 V or higher, further 4.4 V or higher, or even 4.5 V or higher. Secondary batteries having such end-of-charge voltages (i.e., upper limit voltages) generally tend to have an increased amount of metal ions eluted from the positive electrode. On the other hand, the secondary battery according to the present disclosure includes a separator (S) containing a polysaccharide having a sulfo group, which reduces the probability that eluted metal ions will reach the negative electrode, thereby significantly suppressing metal deposition at the negative electrode.

[0055] The configuration of a secondary battery will be specifically described below using a lithium ion secondary battery as an example.

[0056] [Positive Electrode] The positive electrode includes, for example, a positive electrode current collector and a positive electrode active material layer. The positive electrode active material layer is supported on one or both surfaces of the positive electrode current collector.

[0057] The positive electrode active material layer is, for example, a positive electrode mixture layer made of a positive electrode mixture. The positive electrode mixture contains a positive electrode active material as an essential component, and may contain optional components such as a binder, a conductive material, a thickener, etc.

[0058] The positive electrode active material layer can be formed, for example, by applying a positive electrode slurry, in which a positive electrode mixture is dispersed in a dispersion medium, to the surface of a positive electrode current collector and drying the applied film. The dried coating film may be rolled, if necessary.

[0059] As the positive electrode current collector, a sheet-like conductive material (metal foil, mesh, net, punched sheet, etc.) is used. Among these, metal foil is preferred. Examples of the material for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium. The thickness of the positive electrode current collector is not particularly limited, but may be, for example, 1 to 50 μm, or 5 to 30 μm.

[0060] The thickness of the positive electrode active material layer is not particularly limited, but may be, for example, 30 μm to 400 μm, or 50 μm to 250 μm. A single positive electrode active material layer may be formed by a plurality of layers having different morphologies. For example, two or more layers containing active material particles having different average particle sizes may be stacked, or two or more layers having different types or compositions of positive electrode active materials may be stacked.

[0061] The average particle diameter of the particles of the positive electrode active material is, for example, 3 μm or more and 30 μm or less, and may be 5 μm or more and 25 μm or less. Here, the average particle diameter is the median diameter (D 50 The active material particles can be separated and collected from the positive electrode. For example, an LA-750 manufactured by Horiba Ltd. can be used as the measuring device.

[0062] The positive electrode active material may contain a lithium-containing transition metal oxide. From the viewpoint of increasing capacity, the lithium-containing transition metal oxide preferably contains lithium nickel oxide (composite oxide N) containing lithium and Ni and having a layered rock salt crystal structure. The proportion of the composite oxide N in the positive electrode active material may be, for example, 70 mass % or more, 90 mass % or more, or even 95 mass % or more. The proportion of Ni in the metal elements other than Li contained in the composite oxide N may be 50 atomic % or more.

[0063] The composite oxide N is, for example, a compound represented by the formula (1): Li α Ni ｘ１ M1 ｘ２ M2 （１－ｘ１－ｘ２） O ２＋β Here, element M1 is at least one selected from the group consisting of V, Co, and Mn. Element M2 is at least one selected from the group consisting of Mg, Al, Ca, Ti, Cu, Zn, and Nb. However, formula (1) satisfies the following conditions: 0.9≦α≦1.1, −0.05≦β≦0.05, 0.5≦x1<1, 0≦x2≦0.5, 0<1−x1−x2≦0.5. α increases or decreases with charge and discharge.

[0064] The composite oxide N contains Ni and may contain at least one element selected from the group consisting of Co, Mn, and Al as the element M1 and the element M2. Co, Mn, and Al contribute to stabilizing the crystal structure of the composite oxide N.

[0065] From the viewpoint of reducing costs and increasing capacity, the proportion of Co in the metal elements other than Li contained in the composite oxide N is preferably 0 atomic % or more and 20 atomic % or less, and more preferably 0 atomic % or more and 15 atomic % or less.

[0066] The proportion of Mn in the metal elements other than Li may be 30 atomic % or less, or 20 atomic % or less, or 1 atomic % or more, or 3 atomic % or more, or 5 atomic % or more.

[0067] The proportion of Al in the metal elements other than Li may be 10 atomic % or less, or 5 atomic % or less. The proportion of Al in the metal elements other than Li may be 1 atomic % or more, or 3 atomic % or more, or 5 atomic % or more.

[0068] The composite oxide N is, for example, a compound represented by the formula (2): Li α Ni(1-y1-y2-y3-z)Co ｙ１ Mn ｙ２ Al ｙ３ M ｚ O ２＋β It can be expressed as follows. Mn and / or Al contribute to stabilizing the crystal structure of the composite oxide N with a reduced Co content. The element M is an element other than Li, Ni, Co, Mn, Al, and oxygen, and may be at least one selected from the group consisting of Ti, Zr, Nb, Mo, W, Fe, Zn, B, Si, Mg, Ca, Sr, Sc, and Y. However, formula (2) satisfies 0.9≦α≦1.1, -0.05≦β≦0.05, 0≦y1≦0.1, 0≦y2≦0.6, 0≦y3≦0.1, and 0≦z≦0.10. 1-y1-y2-y3-z (=v), which indicates the atomic ratio of Ni, is, for example, 0.4 or more, and may be 0.6 or more, 0.8 or more, 0.85 or more, 0.90 or more, or 0.95 or more. Furthermore, v, which indicates the atomic ratio of Ni, may be 0.98 or less, or may be 0.95 or less.

[0069] When the composite oxide N contains Co, the proportion of Co among metal elements other than Li may be 1.5 atomic % or less. Reducing the Co content of the composite oxide N and increasing the Ni content is cost-effective and ensures high capacity. Meanwhile, such Co-free or low-Co-content composite oxide N generally tends to elute metal ions. In contrast, the secondary battery according to the present disclosure includes a separator (S) containing a polysaccharide having a sulfo group, which reduces the probability that eluted metal ions will reach the negative electrode, significantly suppressing metal deposition at the negative electrode.

[0070] Examples of conductive materials that can be optionally contained in the positive electrode active material layer include carbon nanotubes (CNTs), carbon fibers other than CNTs, and conductive particles (for example, carbon black and graphite).

[0071] [Negative Electrode] The negative electrode includes at least a negative electrode current collector, for example, a negative electrode current collector and a negative electrode active material layer. The negative electrode active material layer is supported on one or both surfaces of the negative electrode current collector.

[0072] The negative electrode active material layer may be a negative electrode mixture layer composed of a negative electrode mixture. The negative electrode mixture layer is in the form of a membrane or film. The negative electrode mixture contains particles of the negative electrode active material as an essential component, and may contain optional components such as a binder, a conductive agent, and a thickener. Alternatively, a lithium metal foil or a lithium alloy foil may be attached to the negative electrode current collector as the negative electrode active material layer.

[0073] The negative electrode mixture layer can be formed, for example, by applying a negative electrode slurry, which is a negative electrode mixture containing particles of a negative electrode active material, a binder, etc., dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the applied film. The dried coating film may be rolled, if necessary.

[0074] As the negative electrode current collector, a sheet-like conductive material (metal foil, mesh, net, punched sheet, etc.) is used. Among these, metal foil is preferred. Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy. The thickness of the negative electrode current collector is not particularly limited, but may be, for example, 1 to 50 μm, or 5 to 30 μm.

[0075] Negative electrode active materials include materials that electrochemically absorb and release lithium ions, lithium metal, lithium alloys, etc. Materials that electrochemically absorb and release lithium ions include carbon materials and alloy-based materials. Examples of carbon materials include graphite, easily graphitized carbon (soft carbon), and hardly graphitized carbon (hard carbon). Of these, graphite is preferred because of its excellent charge / discharge stability and low irreversible capacity. Examples of alloy-based materials include those containing at least one metal that can form an alloy with lithium, such as silicon, tin, silicon alloys, tin alloys, and silicon compounds. Silicon oxide, tin oxide, etc. may also be used.

[0076] Examples of alloy-based materials containing silicon include a lithium ion conductive phase and a composite material in which silicon particles are dispersed in the lithium ion conductive phase. Examples of lithium ion conductive phases that can be used include a silicon oxide phase, a silicate phase, and a carbon phase. The silicon oxide phase may contain silicon dioxide as the main component (e.g., 95 to 100% by mass). Among these, a composite material composed of a silicate phase and silicon particles dispersed in the silicate phase is preferred because it has a high capacity and a small irreversible capacity. Furthermore, a lithium silicate phase (a silicate phase containing lithium) is preferred because it has a small irreversible capacity and a high initial charge / discharge efficiency.

[0077] The lithium silicate phase may be an oxide phase containing lithium (Li), silicon (Si), and oxygen (O), and may contain other elements. The atomic ratio of O to Si in the lithium silicate phase, O / Si, is, for example, greater than 2 and less than 4. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase, Li / Si, is, for example, greater than 0 and less than 4. The lithium silicate phase has the formula: Li ２ｚ SiO ２＋ｚ(0<z<2). Preferably, z satisfies the relationship 0<z<1, and more preferably z=1 / 2. Examples of elements other than Li, Si, and O that can be contained in the lithium silicate phase include iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), and aluminum (Al).

[0078] The carbon phase may be composed of, for example, amorphous carbon with low crystallinity (i.e., amorphous carbon). The amorphous carbon may be, for example, hard carbon, soft carbon, or other.

[0079] As the binder, for example, a resin material is used. Examples of the binder include polyacrylic acid, polyacrylic acid salts and derivatives thereof, fluororesins, polyolefin resins, polyamide resins, polyimide resins, acrylic resins, vinyl resins, rubber particles, etc. One type of binder may be used alone, or two or more types may be used in combination.

[0080] Examples of conductive materials include carbon nanotubes (CNTs), carbon fibers other than CNTs, and conductive particles (for example, carbon black and graphite).

[0081] Examples of thickeners include carboxymethyl cellulose (CMC) and modified products thereof (including salts such as the Na salt), cellulose derivatives such as methyl cellulose (cellulose ethers, etc.); saponified polymers having vinyl acetate units such as polyvinyl alcohol; and polyethers (polyalkylene oxides such as polyethylene oxide, etc.).

[0082] [Non-aqueous electrolyte] The non-aqueous electrolyte may be a liquid electrolyte (electrolytic solution), a gel electrolyte, or a solid electrolyte. The liquid electrolyte (electrolytic solution) is, for example, an electrolytic solution containing a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the electrolytic solution is, for example, 0.5 mol / L or more and 2 mol / L or less. The electrolytic solution may contain known additives.

[0083] The gel electrolyte contains a lithium salt and a matrix polymer, or a lithium salt, a non-aqueous solvent, and a matrix polymer. The matrix polymer is, for example, a polymer material that absorbs the non-aqueous solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, polyether resin, and polyethylene oxide.

[0084] The solid electrolyte may be an inorganic solid electrolyte, such as a material known in all-solid-state lithium ion secondary batteries (e.g., oxide-based solid electrolytes, sulfide-based solid electrolytes, halide-based solid electrolytes, etc.).

[0085] Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, and cyclic carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0086] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 etc.), lithium salts of fluorine-containing acids (LiPF6, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2 etc.), lithium salts of fluorine-containing acid imides (LiN(SO ２ F) ２ , LiN(CFSO)2, LiN(CFSO)(CFSO), LiN(CFSO)2, etc.), lithium halides (LiCl, LiBr, LiI, etc.), etc. The lithium salt may be used alone or in combination of two or more.

[0087] An example of the structure of a secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween is housed in an exterior body together with an electrolyte solution. However, this is not limited thereto, and other forms of electrode groups may also be applied. For example, a stacked electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween may also be used. The shape of the secondary battery is also not limited, and may be, for example, a cylindrical type, a prismatic type, a coin type, a button type, a laminate type, or the like.

[0088] The structure of the secondary battery will be described below with reference to Fig. 1. Fig. 1 is a longitudinal cross-sectional view of a cylindrical nonaqueous secondary battery 10 that is an example of this embodiment. However, the present disclosure is not limited to the following configuration.

[0089] The secondary battery 10 includes an electrode group 18, an electrolyte (not shown), and a cylindrical battery can 22 with a bottom that accommodates these. A sealing body 11 is crimped to the opening of the battery can 22 via a gasket 21, thereby sealing the battery. The sealing body 11 includes a valve body 12, a metal plate 13, and an annular insulating member 14 interposed between the valve body 12 and the metal plate 13. The valve body 12 and the metal plate 13 are connected to each other at their respective centers. A positive electrode lead 15a extending from a positive electrode plate 15 is connected to the metal plate 13. Thus, the valve body 12 functions as an external terminal for the positive electrode. A negative electrode lead 16a extending from a negative electrode plate 16 is connected to the inner bottom surface of the battery can 22. An annular groove 22a is formed near the open end of the battery can 22. A first insulating plate 23 is disposed between one end face of the electrode group 18 and the annular groove portion 22a. A second insulating plate 24 is disposed between the other end face of the electrode group 18 and the bottom of the battery can 22. The electrode group 18 is formed by winding a positive electrode plate 15 and a negative electrode plate 16 with a separator 17 interposed therebetween.

[0090] Hereinafter, the present disclosure will be specifically described based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0091] EXAMPLES The present invention will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0092] Example 1 A lithium ion secondary battery was fabricated and evaluated according to the following procedure.

[0093] (1) Preparation of Positive Electrode 100 parts by mass of positive electrode active material particles, 1 part by mass of acetylene black, 1 part by mass of polyvinylidene fluoride, and an appropriate amount of NMP were mixed to obtain a positive electrode slurry. Next, the positive electrode slurry was applied to one side of an aluminum foil, the coating was dried, and then rolled to form a positive electrode active material layer (thickness 95 μm, density 3.6 g / cm) on one side of the aluminum foil. ３ ) was formed to obtain a positive electrode. 0.7 Mn 0.3 O2 was used.

[0094] The positive electrode was cut into a predetermined shape, and the positive electrode active material layer was partially peeled off to obtain a positive electrode for evaluation. The positive electrode had a 60 mm × 40 mm power generation area with the positive electrode active material layer and a 10 mm × 10 mm connection area without the positive electrode active material layer. A positive electrode tab lead was connected to the connection area.

[0095] (2) Preparation of the negative electrode: A lithium metal foil was attached to one side of an electrolytic copper foil (current collector) to prepare a negative electrode. The negative electrode was cut into the same shape as the positive electrode, and the lithium metal foil on the connection area was peeled off. After that, a negative electrode tab lead was connected to the connection area.

[0096] (3) Separator (I) Step of Preparing Porous Substrate A 12 μm-thick polyethylene microporous thin film (air permeability 108 sec / 100 mL) was prepared as a porous substrate. The microporous thin film is a biaxially stretched film that is commonly used as a separator for lithium-ion secondary batteries.

[0097] (II) Step of preparing a polysaccharide solution A polysaccharide solution containing 0.5 parts by mass of carrageenan (κ-carrageenan), 49.5 parts by mass of water, and 50 parts by mass of ethanol was prepared. The κ-carrageenan was completely dissolved in the polysaccharide solution.

[0098] (III) Coating step: A polysaccharide solution was sprayed onto only one surface (i.e., the first surface) of the microporous thin film. This allowed the content C1 of polysaccharide (S) contained in the first region on the first surface side of the microporous thin film to be greater than the content C2 of polysaccharide (S) contained in the second region on the second surface side.

[0099] (IV) Drying Step In the drying step, the porous substrate coated with the polysaccharide solution was placed on a mounting substrate with the first surface facing upward and the second surface facing upward, and dried at 60°C for 3 hours to complete a separator with the polysaccharide (S) attached.

[0100] The resulting separator had an air permeability of 128 seconds / 100 mL, and its IR spectrum was measured by FT-IR, revealing various peaks attributable to κ-carrageenan, including C—O—S bonds and S═O bonds. Furthermore, elemental analysis by IPC detected sulfur, confirming the presence of sulfo groups.

[0101] As the permeation rate of the electrolyte into the microporous thin film before the polysaccharide (S) was attached and the separator with the polysaccharide (S) attached, 0.03 mL of propylene carbonate (PC) was dropped and the time until the electrolyte permeated was measured. When the permeation time of PC into the microporous thin film before the polysaccharide (S) was attached was taken as 100, the permeation time of PC into the separator with the polysaccharide (S) attached was 50, and it was confirmed that the wettability or permeability of the separator with the electrolyte was increased by approximately two times due to the attachment of the polysaccharide (S).

[0102] (4) Preparation of Electrolyte Solution An electrolyte solution was prepared by dissolving LiPF6 at a concentration of 1 mol / L in a mixed solvent of fluoroethylene carbonate (FEC) and dimethyl carbonate (DMC) in a volume ratio of 20:80.

[0103] (5) Preparation of Evaluation Battery A battery with a design capacity of 114 mAh was prepared using a positive electrode, a negative electrode, and the above-mentioned evaluation separator. The positive electrode and the negative electrode were stacked between the positive electrode active material layer and the lithium metal foil with the evaluation separator interposed therebetween to obtain an electrode plate assembly. At this time, the first surface faced the positive electrode active material layer. The electrode plate assembly was placed in a 1.2 cm 3The battery was then housed in an envelope-shaped case made of laminate film with both ends open, along with the electrolyte solution. Each tab lead was led out from one of the openings, and the opening was sealed. The battery was then left standing under a reduced pressure of 0.02 MPa for 3 minutes, and then returned to atmospheric pressure. This cycle was repeated twice to impregnate the positive electrode active material layer and separator with the electrolyte. Finally, the other opening was sealed, yielding an evaluation battery A1 of Example 1. The evaluation battery was fabricated in a dry air atmosphere with a dew point of −60°C or lower.

[0104] (6) Battery Evaluation <Elution of Metal Ions> The evaluation battery was clamped between a pair of 10 x 5 cm stainless steel clamps (thickness: 6 mm) and pressurized and fixed at 3.2 MPa, and charged and discharged in a thermostatic chamber at 25°C under the following conditions.

[0105] First, the battery was charged at a constant current of 0.3 C (1 C is the current value required to discharge the design capacity in 1 hour) at a high temperature of 55°C until the battery voltage reached 4.5 V, and then charged at a constant voltage of 4.5 V for 3 days. Next, the battery was discharged at a constant current of 0.3 C at room temperature until the battery voltage reached 2.5 V, and then charged at a constant current of 0.3 C until the battery voltage reached 4.5 V. The battery was then charged at a constant voltage of less than 0.05 C. The battery was then discharged at a constant current of 0.3 C until the battery voltage reached 2.5 V. The battery was left standing in an open circuit for 20 minutes between charges and discharges.

[0106] Thereafter, the battery was disassembled and the amount of metal deposited on the surface of the negative electrode was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

[0107] <Load Characteristics> Next, another evaluation battery A1 was prepared, fixed with a clamp in the same manner as above, and charged and discharged in a thermostatic chamber at 25°C under the following conditions, and the ratio of the discharge capacity at 1 C to the discharge capacity at 0.1 C was calculated as the load characteristics (%). A higher value indicates better load characteristics.

[0108] First, the battery was charged at a constant current of 0.3 C until the battery voltage reached 4.5 V, and then at a constant voltage of 4.5 V until the current value was less than 0.05 C. Next, the battery was discharged at a constant current of 0.1 C until the battery voltage reached 2.5 V. Subsequently, the battery was charged at a constant current of 0.3 C until the battery voltage reached 4.5 V, and then at a constant voltage of 4.5 V until the current value was less than 0.05 C. Next, the battery was discharged at a constant current of 1 C until the battery voltage reached 2.5 V. The battery was left standing in an open circuit for 20 minutes between charges and discharges.

[0109] The amount of deposited metal and the load characteristics are shown in Table 1. Note that both the amount of deposited metal and the load characteristics are relative values ​​(%) when the value in B1 is set to 100%.

[0110] Comparative Example 1 A battery B1 was produced and evaluated in the same manner as in Example 1, except that no polysaccharide (S) was attached to the porous substrate and a 12 μm thick polyethylene microporous thin film (air permeability 108 sec / 100 mL) was used as the separator.

[0111] Comparative Example 2 An attempt was made to attach lithium polyacrylate, instead of polysaccharide (S), to the porous substrate using the same process as in Example 1, but the lithium polyacrylate could not be uniformly dissolved in the solvent, making it difficult to apply it uniformly to the microporous thin film.

[0112]

[0113] Comparing batteries A1 and B1, it can be seen that the use of a separator with polysaccharide (S) attached significantly reduces the amount of metal ions deposited. This is presumably because most of the metal ions eluted from the positive electrode are trapped by the sulfo groups of polysaccharide (S).

[0114] Furthermore, a comparison of batteries A1 and B1 reveals that the use of a separator with polysaccharide (S) attached significantly improves load characteristics. This is presumably because the polysaccharide (S) is attached thinly and uniformly along the inner walls of the separator pores without blocking them, significantly improving the wettability of the separator to the electrolyte.

[0115] The secondary battery separator according to the present disclosure and the secondary battery including the same are useful as main power sources for mobile communication devices, portable electronic devices, electric vehicles, and the like.

[0116] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0117] 10: Secondary battery, 11: Sealing body, 12: Valve body, 13: Metal plate, 14: Insulating member, 15: Positive electrode plate, 15a: Positive electrode lead, 16: Negative electrode plate, 16a: Negative electrode lead, 17: Separator, 18: Electrode group, 21: Gasket, 22: Battery can, 22a: Groove portion, 23: First insulating plate, 24: Second insulating plate

Claims

1. a porous substrate having a first surface and a second surface opposite the first surface; a polysaccharide attached to the porous substrate; The secondary battery separator, wherein the polysaccharide has a sulfo group.

2. The secondary battery separator according to claim 1 , wherein the polysaccharide is attached to at least the first surface.

3. 2. The separator for a secondary battery according to claim 1, wherein the separator has an air permeability measured by the method specified in JIS P 8117 of 100 seconds / 100 mL or more and 500 seconds / 100 mL or less.

4. 4. The secondary battery separator according to claim 1, wherein the polysaccharide has a galactose polymer skeleton.

5. The secondary battery separator according to any one of claims 1 to 3, wherein the polysaccharide includes carrageenan.

6. The number of moles of the sulfo group contained in the polysaccharide per unit mass is 1.0 × 10 -6 4. The secondary battery separator according to claim 1, wherein the molecular weight of the secondary battery separator is 1000 mol / g or more.

7. The amount of the polysaccharide attached to the porous substrate per apparent unit area is 1.0 × 10 -5 g / m 2 Above 1.0 x 10 2 g / m 2 The method according to any one of claims 1 to 3, wherein Separator for secondary batteries.

8. When the porous substrate is divided into a first region on the first surface side and a second region on the second surface side having the same thickness, 4. The secondary battery separator according to claim 1, wherein a content C1 of the polysaccharide contained in the first region is greater than a content C2 of the polysaccharide contained in the second region.

9. The separator for a secondary battery according to claim 8 , wherein a mass ratio C1 / C2 of the content C1 to the content C2 is 1.1 or more.

10. a positive electrode, a negative electrode, a lithium ion conductive non-aqueous electrolyte, and a separator interposed between the positive electrode and the negative electrode; The separator is a secondary battery separator according to any one of claims 1 to 3, The secondary battery, wherein the first surface faces the positive electrode.

11. The secondary battery according to claim 10, wherein the end-of-charge voltage is 4.3 V or higher.

12. the positive electrode contains a lithium-containing transition metal oxide as active material particles, the lithium-containing transition metal oxide includes lithium nickel oxide that contains lithium and Ni and has a layered rock salt crystal structure, 11. The secondary battery according to claim 10, wherein the proportion of Ni in the metal elements other than Li contained in the lithium nickel oxide is 50 atomic % or more.

13. 13. The positive electrode for a secondary battery according to claim 12, wherein when the lithium nickel oxide contains Co, the proportion of Co in the metal elements other than Li is 2.0 atomic % or less.

14. providing a porous substrate having a first surface and a second surface opposite the first surface; providing a solution of a polysaccharide having sulfo groups; a coating step of coating the solution onto the porous substrate; a drying step of drying the porous substrate to which the solution has been applied; A method for manufacturing a separator for a secondary battery, comprising:

15. 15. The method for producing a secondary battery separator according to claim 14, wherein the solution contains a mixed solvent of water and ethanol, and the polysaccharide dissolved in the mixed solvent.

16. The method for producing a secondary battery separator according to claim 14 , wherein the solution is applied to the first surface of the porous substrate in the applying step.