Additive for lithium ion secondary battery electrode and lithium ion secondary battery electrode

Abstract

To provide an additive which can be added to an electrode of a lithium ion secondary battery to improve its characteristics.SOLUTION: An additive for a lithium ion secondary battery electrode, comprising a composite oxide having a garnet-type or garnet-like crystal structure containing elements Li, La, Zr, and O, wherein a part of the elements Li is substituted with an element A which is different from the elements and can form a Li site vacancy, a ratio of the Li site vacancies is 40% or more and 80% or less, and an ionic conductivity at room temperature is 1 * 10 - 5S / cm or more.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an additive that can be added to an electrode of a lithium ion secondary battery to improve the characteristics thereof. [Background technology]

[0002] Lithium-ion batteries are used not only as small power sources for portable electronic devices such as personal computers and smartphones, but also as large stationary emergency power sources. In recent years, lithium ion batteries have been developed to have large capacity, high-speed charge / discharge characteristics, good cycle characteristics, and excellent safety, and are suitable for use in electric vehicles, hybrid electric vehicles, and the like.

[0003] Patent Document 1 discloses that a lithium ion battery in which a garnet-type oxide is mixed in the positive electrode material can further improve cycle characteristics and thermal stability.

[0004] Garnet-type oxide Li7La3Zr2O containing the elements Li, La, Zr, and O 12 The crystal structure of LLZ (hereafter referred to as LLZ) is mainly cubic and tetragonal. It is known that by substituting certain elements constituting LLZ with other elements, the cubic LLZ becomes stable and has high ionic conductivity. Non-Patent Document 1 reports that in the crystal structure of LLZ, the element Li occupies two types of crystallographic sites, Li1 and Li2, located at the tetrahedral 24d site and the distorted octahedral 96h site, respectively, with the Li site occupancy rates of Li1 = 0.97(7) and Li2 = 0.349, with the standard deviations indicated in parentheses. It has also been reported that the conduction pathway of Li ions in LLZ migrates via the Li1 site along the Li2 → Li1 → Li2 site, forming a three-dimensional network of Li ion migration pathways within the LLZ structure.

[0005] In Non-Patent Document 2, when Ga is substituted into LLZ, Ga substitutes for a part of the Li1 position of element Li, and Li+ and Ga 3+ It has been reported that by introducing Li vacancies due to the difference in charge, a cubic LLZ with high ionic conductivity can be formed. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-113655

[0007] [Non-Patent Document 1] J.Awaka et al.,Chem.Lett.,2011.40.60-62 [Non-patent document 2] C.Bernuy-Lopez et al.,Chem.Mater.,2014.26.3610-3617 Summary of the Invention [Problem to be solved by the invention]

[0008] The lithium ion battery described in Patent Document 1 has excellent high-temperature cycle characteristics and thermal stability of the positive electrode, but its input / output characteristics are insufficient, and the inventors' investigations have revealed that the discharge voltage and charge / discharge capacity decrease significantly, particularly when the current value during charge / discharge is increased (during high-rate charge / discharge). Improving the rate characteristics of lithium-ion batteries is highly desirable, as it leads to shorter charging times and discharge characteristics that can withstand large loads. Currently, rate characteristics are improved by adjusting the particle size of the positive electrode material and the thickness of the positive electrode coating on the current collector. However, reducing the particle size of the positive electrode material to improve rate characteristics reduces the packing density, resulting in a decrease in battery capacity. Furthermore, reducing the thickness of the positive electrode coating on the current collector to improve rate characteristics also results in a decrease in battery capacity. Therefore, batteries with excellent rate characteristics, even with a conventional positive electrode structure, are desired.

[0009] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an additive material for lithium ion secondary battery electrodes, which, when added to the electrodes of a lithium ion secondary battery, enables the manufacture of a lithium ion secondary battery having excellent input / output characteristics in which the discharge voltage and charge / discharge capacity do not easily decrease even when the battery is charged and discharged at high speed, i.e., excellent rate characteristics. [Means for solving the problem]

[0010] In order to solve the above problems, the present inventors have developed a composite oxide having a garnet-type or garnet-like crystal structure containing the elements Li, La, Zr, and O, in which a portion of the elements is substituted with an element A different from the elements mentioned above, the oxide has a Li-site vacancy ratio of 40% to 80%, and the oxide has an ionic conductivity of 1×10 at room temperature. -5 The present inventors have found that when a composite oxide having a specific conductivity of 0.5 S / cm or more is added to an electrode of a lithium ion secondary battery, the battery has excellent charge / discharge characteristics during high-speed charge / discharge, i.e., the rate characteristics are significantly improved, and have completed the present invention.

[0011] The gist of the present invention is as follows.

[0012] (1) A composite oxide having a garnet-type or garnet-like crystal structure containing the elements Li, La, Zr, and O, in which a portion of element Li is substituted with an element A that is different from the elements mentioned above and can form Li-site vacancies, and has a Li-site vacancy ratio of 40% to 80% and an ionic conductivity of 1×10 at room temperature. -5 An additive for a lithium ion secondary battery electrode, characterized by having a specific resistance of S / cm or more. (2) The additive for a lithium ion secondary battery electrode according to (1), characterized in that the average particle diameter D50 of the composite oxide is 1 / 2 or less of the thickness of the lithium ion secondary battery electrode. (3) Ionic conductivity at room temperature is 5×10 -4 The additive for a lithium ion secondary battery electrode according to (1) or (2) above, characterized in that it has a specific resistance of 0.5 S / cm or more. (4) The additive for a lithium ion secondary battery electrode according to any one of (1) to (3), wherein the element A has d electrons and is in a cationic state with an octahedral coordination selectivity of 50 kJ / mol or more in stabilization by the ligand field of an oxygen anion, and the molar ratio A / La of the element A to La is 0.01 or more and 0.50 or less. (5) The additive for a lithium ion secondary battery electrode according to any one of (1) to (4) above, further comprising an element B which replaces a part of the element La in the crystal structure. (6) The additive for a lithium ion secondary battery electrode according to any one of (1) to (5), wherein the element A is in a cationic state in which the octahedral coordination selectivity in stabilization by the ligand field of an oxygen anion is 60 kJ / mol or more. (7) The additive for a lithium ion secondary battery electrode according to any one of (1) to (6) above, wherein the element A is contained in the form of a divalent cation. (8) The additive for a lithium ion secondary battery electrode according to any one of (1) to (7), wherein the lattice constant of the crystal structure is 12.93 Å or more. (9) A lithium ion secondary battery comprising the additive for a lithium ion secondary battery electrode according to any one of (1) to (8). (10) A positive electrode sheet for a lithium ion secondary battery, containing dispersed therein the additive for a lithium ion secondary battery electrode according to any one of (1) to (8). (11) A lithium ion secondary battery characterized by using the positive electrode sheet of (10) above. [Effects of the Invention]

[0013] According to the present invention, it is possible to provide an additive for a lithium ion secondary battery electrode, which reduces the interfacial reaction resistance between the lithium ion secondary battery electrode and the electrolyte solution and thereby reduces the voltage drop, and therefore has particularly excellent input / output characteristics during high-speed charge / discharge, that is, makes it possible to manufacture a lithium secondary battery electrode with excellent rate characteristics. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic explanatory diagram showing a presumed mechanism of the effect of the electrode additive for lithium ion secondary batteries of the present invention. [Figure 2] 1 is a schematic diagram showing that the interfacial reaction resistance R2 is reduced by the dielectric polarization action of the additive particles (shown as LLZ particles in the figure) of the present invention present at the interface between the active material particles (shown as positive electrode particles in the figure) of the positive electrode of a lithium ion secondary battery and the electrolyte. DETAILED DESCRIPTION OF THE INVENTION

[0015] The additive material for the lithium ion secondary battery electrode used in this embodiment is a composite oxide having a garnet-type or garnet-type-like crystal structure containing the elements Li, La, Zr, and O, in which a portion of the element Li is substituted with an element A that is different from the element Li and can form Li-site vacancies, and has a Li-site vacancy ratio of 40% to 80% and an ionic conductivity of 1×10 at room temperature. -5 It is characterized by being S / cm or more.

[0016] First, the site will be described. A site is a crystallographically equivalent lattice position. If an atom exists at that lattice position, the site is said to be occupied and is called an occupied site, and if no atom exists at that lattice position, it is called a vacant site. For example, LLZ has five occupied sites, called the Li1 site, Li2 site, La site, Zr site, and O site. These are sometimes called the tetrahedral 24d site, the distorted octahedral 96h site, the dodecahedral 24c site, the octahedral 16a site, and the 96h site. In an ideal garnet structure, Li2 can be considered to be located at two equivalent positions (96h) slightly offset from the center of the LiO6 hexahedron within the distorted octahedron, or at the center (48g). Furthermore, Rietveld analysis of powder diffraction patterns can be used to identify the occupied sites along with the crystal's space group. Furthermore, the statistical proportion of a given atom occupying a given site is called the site occupancy, or simply the occupancy rate. The proportion of vacancies relative to the total lattice sites that a given atom can occupy is called the vacancy fraction.

[0017] As shown in Figure 1(a), a voltage drop ΔV occurs during charging and discharging of a lithium-ion secondary battery due to the battery's internal resistance R. Therefore, as the current I increases (high-speed charging and discharging), a large voltage drop occurs according to Ohm's law. As a result, the discharge voltage and charge / discharge capacity decrease significantly during high-speed charging and discharging. Therefore, to improve high-speed charging and discharging characteristics (rate characteristics), it is necessary to reduce the internal resistance R. In this case, if we focus on the positive electrode of a lithium-ion secondary battery, its internal resistance is made up of the positive electrode internal resistance, interfacial reaction resistance, and electrolyte resistance, as shown in Figure 1(b). That is, as shown in FIG. 2, the additive material of the lithium ion secondary battery electrode of the present invention is brought into contact with the active material of the electrode, and thereby the Li + Ion vacancies and Li + The dielectric polarization caused by ions + The electrostatic adsorption of ions and solvate molecules causes an interfacial reaction, which reduces the interfacial reaction resistance. This reduces the rate of decrease in discharge voltage and charge / discharge capacity during high-speed charge / discharge, improving input / output characteristics.

[0018] Substitution with element A generates vacancies at the Li sites due to the difference in charge between element A and Li, and the increase in Li-site vacancies in the crystal structure increases dielectric polarization and reduces interfacial reaction resistance. However, if the amount of Li-site vacancies (vacancy rate) is too high, the Li content decreases, resulting in too few conductive carrier ions and a decrease in electrical conductivity. Therefore, if the amount of Li-site vacancies exceeds a certain level, ionic conductivity decreases. Therefore, the amount of Li-site vacancies is preferably 40% to 80%, more preferably 50% to 80%, and even more preferably 53% to 78%. Since Li ions are inserted and removed between the electrode and the electrolyte via the additive material of the lithium ion secondary battery electrode, the mobility of lithium ions during charging and discharging of the lithium ion secondary battery depends on the lithium ion conductivity of the electrolyte, and therefore, it is preferable that the lithium ion conductivity of the additive material of the lithium ion secondary battery electrode of the present invention is high. Specifically, it is preferable that the lithium ion conductivity of the additive material of the lithium ion secondary battery electrode of the present invention is high. -5 S / cm or more, more preferably 5×10 -4 S / cm or more, and more preferably 1×10 -3 S / cm or more. While good lithium ion mobility can be used to design high-power batteries, low ionic conductivity leads to poor power performance due to the rate-limiting effect of the solid electrolyte.

[0019] From the above, it is sufficient for the additive material in a lithium-ion secondary battery electrode to be in contact with the active material of the electrode. To prevent a decrease in the active material content in the electrode, it is preferable for the additive material particles in the lithium-ion secondary battery electrode to be fine. Using fine, particulate additives increases the number of contact points with the active material, thereby achieving a more efficient effect of the present invention. Specifically, it is more preferable for the average particle size D50 to be half or less of the thickness of the lithium-ion secondary battery electrode. Similarly, it is more preferable for the average particle size D50 (De) of the additive material in a lithium-ion secondary battery electrode to be greater than the average particle size D50 (Dc) of the positive electrode material (positive electrode active material) of the lithium-ion battery in a De / Dc ratio of 0.01 or more and 1.0 or less. Therefore, while smaller particle sizes are preferable, particle sizes of 100 nm or less may be difficult to manufacture and may result in poor yields, making them impractical. Whether the average particle size D50 of the additive material in the lithium ion secondary battery electrode is equal to or less than half the thickness of the lithium ion secondary battery electrode can be confirmed, for example, by ultrasonically dispersing the additive material using an ultrasonic homogenizer or the like, and then measuring the particle size distribution using a particle size distribution measuring device, thereby confirming the average particle size D50 of the additive material (cumulative 50% on a volume basis).

[0020] The Li ions in the crystalline structure of the additive material for lithium-ion secondary battery electrodes occupy two types of crystallographic sites, Li1 and Li2 sites, which are located in tetrahedral and distorted octahedral sites, respectively. The conduction pathway of Li ions in the LLZ is via the Li1 site and along the Li2 → Li1 → Li2 site, forming a three-dimensional network of Li ion migration pathways within the LLZ structure. Substituting an element into LLZ introduces Li vacancies, stabilizing the cubic crystal and resulting in high lithium ion conductivity. However, if the substitution element is located at the Li1 site as in Non-Patent Document 2, the free movement of Li along the Li2-Li1-Li2 site may be hindered, potentially resulting in a decrease in conductivity. Therefore, by including element A in a cationic state with an octahedral coordination selectivity of 50 kJ / mol or more, element A can be selectively substituted into the Li2 site, which is an octahedral site, and the crystalline structure can be stabilized by the substitution that stabilizes the coordination.

[0021] This optimizes the ratio of Li charge carriers and Li vacancies without inhibiting the free movement of Li ions along the Li2 → Li1 → Li2 sites, resulting in high ionic conductivity and stable properties with high dielectric polarization, and it also increases the Li vacancy concentration while ensuring Li ion pathways compared to conventional substitution methods. Therefore, increasing the substitution amount of element A increases ionic conductivity, but if the substitution amount is too high, the Li content decreases and the number of conductive carrier ions becomes too few, so if the content of element A exceeds a certain level, ionic conductivity decreases. Specific examples of the element A include Al. 3+ , Ga 3+ , Mg 2+ , Co 2+ , V 3+ , Mn 3+ , Ni 2+ , Cu 2+ The element may be one or two elements selected from the group consisting of:

[0022] However, the content of element A, expressed as a molar ratio A / La relative to La, is 0.01 or more and 0.50 or less. If the content of element A, expressed as a molar ratio A / La relative to La, is less than 0.01, sufficient conductivity may not be stably obtained for the reasons mentioned above. On the other hand, if the content of element A, expressed as a molar ratio A / La relative to La, exceeds 0.50, sufficient conductivity may not be obtained because the number of Li ions that serve as conduction carriers becomes too small or the substituted ions interfere with the Li ion conduction path, as explained above. The content of element A, expressed as a molar ratio A / La relative to La, is more preferably 0.01 or more and 0.35 or less, and even more preferably 0.01 or more and 0.20 or less.

[0023] The element A is in a cationic state with an octahedral coordination selectivity of 50 kJ / mol or more, that is, the electron state (particularly the number of d electrons) and valence satisfy the above conditions. 2-When comparing the stabilization by the crystal field (ligand field) of tetrahedral and octahedral coordination, the octahedral coordination has a larger stabilization energy, which indicates superior octahedral selectivity, and in fact, it tends to occupy octahedral sites (positions) in oxide crystals. It is known that oxides of typical metal elements (electropositive elements) form ionic crystals (for example, Encyclopedia of Chemistry 3, 16th edition, condensed version, March 10, 1974).

[0024] In this system, when the octahedral coordination selectivity (the difference in coordination stabilization energy between octahedral and tetrahedral coordination) is 50 kJ / mol or more, the ionic conductivity is stably increased, and the effects of the present invention are significantly achieved. This is because, as mentioned above, the substitution has octahedral coordination selectivity, allowing element A to be effectively substituted at the Li site, which is an octahedral site. Therefore, when the octahedral coordination selectivity is less than 50 kJ / mol, high ionic conductivity cannot be obtained. It is more preferable that element A contained in the lithium ion conductive oxide material is in a cationic state with an octahedral coordination selectivity of 60 kJ / mol or more in stabilization by the ligand field of oxygen anions. When the octahedral coordination selectivity of element A is 60 kJ / mol or more, the element A tends to be fixed at the Li2 site, which is an octahedral site, more strongly. Furthermore, the coordination stabilization makes it difficult for the metal ions of element A to move, stabilizing the crystal structure. This results in less inhibition of the free movement of Li, resulting in a more stable and higher ionic conductivity.

[0025] The above can be similarly expressed in terms of the coordination stabilization energy (crystal field stabilization energy, CFSE) and the octahedral site preference energy (OSPE). When expressed in terms of the crystal field splitting parameter (10Dq), an OSPE (Δ10Dq) of 2 or greater indicates octahedral coordination preference.

[0026] The additive material for the lithium ion secondary battery electrode of the present invention is a composite oxide having a garnet-type or garnet-like crystal structure. The garnet-type crystal structure has a space group Ia-3d, and the garnet-like crystal structure has a space group I41 / acd. The basic composition of a composite oxide with a garnet-type crystal structure containing the elements Li, La, Zr, and O is Li7La3Zr2O 12 and the sites necessary to improve the ionic conductivity are substituted.

[0027] When the additive material for a lithium ion secondary battery electrode further contains an element B (B is one or more elements selected from Ca, Ba, Sr, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) that replaces a part of the element La, higher ionic conductivity can be obtained and the following effects can be obtained. However, the ionic radius of element B is 0.8 or more relative to that of La. If an element has an ionic radius of 0.8 or less relative to the ionic radius of La, the ionic conductivity may tend to decrease, so the ionic radius is preferably 0.9 or more, and more preferably 1.0 or more. The effect of the present invention can be easily achieved by adding element B to the additive material of the lithium ion secondary battery electrode, because the crystal lattice of the additive material is expanded, thereby minimizing the decrease in Li ion conductivity. If the molar ratio B / La to La greatly exceeds 0.15, the lithium ion conductivity may tend to decrease, and the ratio is preferably 0.01 or more and 0.13 or less, and more preferably 0.02 or more and 0.1 or less. From the viewpoint of expanding the crystal lattice, element B is Ba. 2+ or Sr 2+ It is preferable to use Sr 2+ It is more preferable to use

[0028] In the present invention, as described above, the element A contained in the additive material of the lithium ion secondary battery electrode is contained so as to replace a part of the element Li. By replacing a part of the element Li with the element A, the electrode becomes more stable and has a higher ionic conductivity. Whether or not the element A has substituted a part of the element Li in the garnet-type or garnet-like crystal structure can be confirmed by, for example, performing X-ray diffraction measurement using synchrotron radiation and then performing Rietveld analysis using RIETAN-FP based on the obtained X-ray diffraction pattern. It is more preferable that element A contained in the additive material for the electrode of a lithium ion secondary battery is contained in the state of a divalent cation. It is believed that the difference in charge between Li and element A creates vacancies at the Li sites in the crystal structure, facilitating the movement of Li ions (facilitating hopping conduction of Li ions), thereby improving ionic conductivity. In other words, this makes it easier to achieve the effects of the present invention. Whether element A is contained in the form of a divalent cation or not can be determined by, for example, measuring the amount of X-ray absorption at an energy near the absorption edge of element A using XAFS (X-ray Absorption Fine Structure). This can confirm the valence and coordination number of element A.

[0029] Furthermore, when the additive material for the lithium ion secondary battery electrode further contains an element C (C is one or more elements selected from Sc, Y, Nb, Ta, In, Ge, Sn, and Te) which replaces a part of the element Zr in the crystal structure of the additive material, a higher ionic conductivity can be obtained and the following effects can be obtained. The effect of the present invention can be easily obtained by adding the element C to the additive material of the lithium ion secondary battery electrode. 4+ It is believed that the difference in charge between element C and element B causes Li ions or Li vacancies to form at the Li sites in the crystal structure, which facilitates the movement of Li ions and improves ionic conductivity. In other words, this makes it easier to achieve the effects of the present invention. The content of element C, expressed as a molar ratio C / La relative to La, is preferably 0.01 or more and 0.50 or less. If the content of element C, expressed as a molar ratio C / La relative to La, is less than 0.01, sufficient conductivity may not be stably obtained for the reasons mentioned above. On the other hand, if the content of element C, expressed as a molar ratio C / La relative to La, exceeds 0.50, the amount of Li ions that serve as conductive carriers may be too large or too small, as explained above, and sufficient conductivity may not be obtained. The content of element C, expressed as a molar ratio C / La relative to La, is more preferably 0.01 or more and 0.35 or less, and even more preferably 0.01 or more and 0.20 or less. As the element C, Nb is used from the viewpoint of electrical conductivity. 5+ or Ta 5+ It is more preferable to use

[0030] The additive material for the electrode of a lithium ion secondary battery preferably has a crystal structure with a lattice constant of 12.93 Å or more. Garnet-type or garnet-like crystal structures tend to achieve higher lithium ion conductivity as the lattice constant increases. In other words, this makes it easier to achieve the effects of the present invention. The additive for the lithium ion secondary battery electrode of the present invention may be added together with other additives, and in particular, when added together with zirconium oxide composite oxides containing zirconium oxide, yttrium, scandium, or other rare earth metal ions, the rate characteristics become more excellent.

[0031] The additive material for the lithium ion secondary battery electrode of the present invention may be produced by any production method as long as it satisfies the above-mentioned requirements of the present invention, for example, a gas phase synthesis method such as PVD or CVD, a solid phase reaction method, a spray pyrolysis method, or a wet method such as a coprecipitation method or a sol-gel method.

[0032] As an example of a method for producing the additive material for the lithium ion secondary battery electrode of the present invention, a synthesis example by solid-state reaction method can be shown below. As raw materials, sulfates, oxides, carbonates, hydroxides, nitrates, acetates, oxalates, halides, etc. of the constituent elements can be used. For mixing, a planetary mill, a bead mill, a ball mill, or the like can be used. The calcination temperature of the mixture varies depending on the constituent components and the composition (for example, depending on the substituted metal ions), but is preferably 900°C or higher and 1200°C or lower. The calcination time varies depending on the constituent components and the blending (for example, depending on the substituted metal ion), but is preferably 5 hours or more and 20 hours or less. The calcination atmosphere is an atmosphere with an appropriate oxygen partial pressure depending on the constituent components and the composition, such as air, nitrogen, argon, or nitrogen or argon with an adjusted oxygen partial pressure. The powder obtained by calcination may be used as is, or may be further pulverized. For pulverization, a planetary mill, a bead mill, a ball mill, or the like may be used. The particle size after pulverization should be adjusted to an appropriate particle size depending on the application, and typically, a D50 of 0.3 μm or more and 20 μm or less is preferred.

[0033] As another example of a method for producing the additive material for the lithium ion secondary battery electrode of the present invention, a synthesis example by spray pyrolysis can be shown below. Any raw material may be used as long as it can be dissolved in an aqueous solution or an organic solvent. For example, carbonates, hydroxides, nitrates, acetates, oxalates, halides, metal alkoxides, etc. of the constituent elements can be used. The solvent may be water, an alkane, an alkanecarboxylic acid and / or an alcohol. The temperature of the spray pyrolysis varies depending on the constituent components, the formulation, and the solvent concentration (for example, depending on the substituted metal ion), but is usually 200°C or higher and 1200°C or lower. The particle size of the droplets when sprayed is preferably 0.01 μm or more and 100 μm or less. The carrier gas atmosphere is an atmosphere with an appropriate oxygen partial pressure depending on the constituent components and composition, such as air, nitrogen or argon, or nitrogen or argon with an adjusted oxygen partial pressure. The powder obtained by spray pyrolysis may be used as it is, or may be further pulverized. When pulverizing, a planetary mill, a bead mill, a ball mill, etc. can be used. The particle size after pulverization is preferably adjusted to an appropriate particle size according to the application. Usually, D50 is preferably 0.3 μm or more and 20 μm or less.

[0034] The lithium-ion secondary battery manufactured using the additive for the lithium-ion secondary battery electrode of the present invention includes a positive electrode having a positive electrode active material that occludes and releases lithium, a negative electrode having a negative electrode active material that occludes and releases lithium, an electrolyte that is interposed between the positive electrode and the negative electrode and conducts lithium, a separator provided between the positive electrode and the negative electrode, and the additive for the lithium-ion secondary battery electrode is present in at least one of the positive electrode and the negative electrode. For example, the additive for the lithium-ion secondary battery electrode may be present in the positive electrode or may be present in the negative electrode. By doing so, the interfacial reaction resistance of the electrolyte with the positive electrode or the negative electrode becomes small, and the input / output characteristics can be further improved. In the present embodiment, for the sake of convenience of explanation, the case where the additive for the lithium-ion secondary battery electrode is provided in the positive electrode will be mainly described.

[0035] The positive electrode of the lithium-ion secondary battery manufactured using the additive of the present invention is, for example, a paste-like positive electrode material obtained by mixing a positive electrode active material, an additive for the lithium-ion secondary battery electrode, a conductive material, and a binder, adding an appropriate solvent, and applying and drying it on the surface of a current collector, and compressing it as necessary to increase the electrode density to form a positive electrode sheet. As the positive electrode active material, sulfides containing transition metal elements, oxides containing lithium and transition metal elements, etc. can be used. Specifically, transition metal sulfides such as TiS2, TiS3, MoS3, FeS2, Li (1-x) MnO2 (0 < x < 1, etc., the same below), Li (1-x) lithium manganese composite oxides such as Mn2O4, Li (1-x) lithium cobalt composite oxides such as CoO₂, Li (1-x)Lithium nickel composite oxides such as NiO2, lithium vanadium composite oxides such as LiV2O3, and transition metal oxides such as V2O5 can be used. Among these, lithium transition metal composite oxides, for example, LiCoO2, LiNiO2, LiMnO2, LiV2O3, and LiNi 0.80 Co 0.15 Al 0.05 O2, LiNi 0.50 Co 0.20 Mn 0.30 O2, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 and the like are preferred.

[0036] The conductive material for the lithium ion secondary battery produced using the additive of the present invention is not particularly limited as long as it is an electron-conductive material that does not adversely affect the battery performance of the positive electrode, and examples thereof include graphite such as natural graphite (scale graphite, flake graphite) and artificial graphite, acetylene black, carbon black, ketjen black, carbon whisker, needle coke, carbon fiber, and metals (copper, nickel, aluminum, silver, gold, etc.), or a mixture of two or more of these. Of these, carbon black and acetylene black are preferred as the conductive material from the viewpoints of electron conductivity and coatability.

[0037] The binder serves to bind the active material particles and conductive material particles together, and may be, for example, a fluorine-containing resin such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), or fluorine rubber, or a thermoplastic resin such as polypropylene or polyethylene, or ethylene-propylene-diene mer (EPDM), sulfonated EPDM, or natural butyl rubber (NBR), either alone or as a mixture of two or more. Also usable are aqueous binders such as cellulose-based binders or aqueous dispersions of styrene-butadiene rubber (SBR).

[0038] Examples of solvents that can be used to disperse the positive electrode active material, conductive material, and binder include organic solvents such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, N,N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran.

[0039] Examples of application methods include roller coating such as an applicator roll, screen coating, doctor blade method, spin coating, and bar coater, and any of these can be used to obtain a desired thickness and shape.

[0040] Examples of current collectors include aluminum, titanium, stainless steel, nickel, iron, calcined carbon, conductive polymers, and conductive glass, as well as aluminum or copper whose surfaces have been treated with carbon, nickel, titanium, or silver to improve adhesion, conductivity, and oxidation resistance. These surfaces can also be subjected to oxidation treatment. Current collectors may be in the form of foils, films, sheets, nets, punched or expanded materials, laths, porous materials, foams, or fiber aggregates. The thickness of the current collector is, for example, 1 to 500 μm.

[0041] The negative electrode of the lithium ion secondary battery produced using the additive of the present invention may be formed, for example, by mixing a negative electrode active material, a conductive material, and a binder, adding an appropriate solvent to form a paste-like negative electrode material, applying it to the surface of a current collector, drying it, and compressing it to increase the electrode density as needed. Examples of the negative electrode active material include inorganic compounds such as lithium, lithium alloys, and tin compounds, carbonaceous materials capable of absorbing and releasing lithium ions, and conductive polymers. The conductive material, binder, solvent, etc. used in the negative electrode may be the same as those exemplified for the positive electrode. The negative electrode current collector can be made of copper, nickel, stainless steel, titanium, aluminum, baked carbon, conductive polymers, conductive glass, Al-Cd alloys, etc., or, for example, copper whose surface has been treated with carbon, nickel, titanium, silver, etc. to improve adhesion, conductivity, and reduction resistance. These surfaces can also be oxidized. The shape of the current collector can be the same as that of the positive electrode.

[0042] The electrolyte solution for the lithium-ion secondary battery manufactured using the additive of the present invention can be a nonaqueous electrolyte solution containing a supporting salt. Examples of solvents for the nonaqueous electrolyte solution include carbonates, esters, ethers, nitriles, furans, sulfolanes, and dioxolanes, which can be used alone or in combination. Specific examples of carbonates include cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, and chloroethylene carbonate; linear carbonates such as dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate, methyl-t-butyl carbonate, di-i-propyl carbonate, and t-butyl-i-propyl carbonate; cyclic esters such as γ-butyl lactone and γ-valerolactone; and linear esters such as methyl formate, methyl acetate, ethyl acetate, and methyl butyrate. Among these, a combination of a cyclic carbonate and a chain carbonate is preferred, as this combination not only provides excellent cycle characteristics, which represent the battery characteristics during repeated charge and discharge, but also allows for a well-balanced viscosity of the electrolyte, the electric capacity of the resulting battery, and the battery output.

[0043] Examples of supporting salts contained in lithium-ion secondary batteries manufactured using the additive of the present invention include LiPF6, LiClO4, LiAsF6, LiBF4, LiSO3CF3, and LiN(SO3CF3)2. The concentration of this supporting salt in the nonaqueous electrolyte is preferably 0.1 mol / L or more and 5 mol / L or less, and more preferably 0.5 mol / L or more and 2 mol / L or less. When the concentration of the supporting salt is 0.1 mol / L or more, a sufficient current density can be obtained, and when it is 5 mol / L or less, the electrolyte can be more stabilized.

[0044] A lithium ion secondary battery manufactured using the additive of the present invention may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it has a composition that can withstand the range of use of the lithium secondary battery, and examples thereof include polymer nonwoven fabrics such as polypropylene nonwoven fabrics and polyphenylene sulfide nonwoven fabrics, and thin microporous films of olefin resins such as polyethylene and polypropylene. These may be used alone or in combination.

[0045] The shape of the lithium ion secondary battery produced using the additive of the present invention is not particularly limited, and examples thereof include coin type, button type, sheet type, laminated type, cylindrical type, flat type, square type, etc. Furthermore, a plurality of such lithium secondary batteries may be connected in series to be applied to large batteries used in electric vehicles, etc.

[0046] As described above, the positive electrode material for lithium ion secondary batteries using the additive for lithium ion secondary battery electrodes of the present invention reduces the interfacial reaction resistance between the positive electrode and the electrolyte, thereby reducing the voltage drop. Therefore, by using the additive of the present invention, it is possible to produce a lithium ion secondary battery with high output characteristics. [Example]

[0047] The present invention will be described below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0048] (Preparation of additives for lithium-ion secondary battery electrodes) Li2CO3, Al2O3, Ga2O3, MgO, VO3, Mn2O3, CuO, NiO, La(OH)3, SrCO3, BaO, and ZrO2 were used as raw materials for the additives of lithium-ion secondary battery electrodes. These raw material powders were weighed and mixed to obtain the additives with the compositions shown in Tables 1 to 3. The raw materials were mixed as follows: the weighed raw material powders were placed in a plastic container along with zirconia balls, mixed in a ball mill in ethanol for 15 hours, and then dried to obtain a raw material mixture. Note that the starting composition contained more Li2CO3 than the target composition to account for the partial loss of Li during sintering. Therefore, even though the starting composition does not satisfy the charge compensation (charge conservation law) of the garnet-type crystal structure, it is understood that the resulting composition satisfies charge compensation after sintering due to the loss of Li and / or oxygen deficiency. It should be noted that there is substantially no loss of elements other than Li during firing, and the charged composition ratio is basically maintained even after firing. The resulting raw material mixture was fired in an MgO heat-resistant container in air or a nitriding atmosphere at 1100°C for 10 hours. Further firing was performed in an MgO heat-resistant container in a nitriding atmosphere at 1100°C for 4 hours to obtain a pre-fired product. This pre-fired product was placed in a nylon pot together with zirconia balls, ground in ethanol for 2 hours in a planetary ball mill, and then dried to obtain additives Nos. 1 to 71 for lithium-ion secondary battery electrodes (corresponding to the additives of Examples 1 to 67 and Comparative Examples 2 to 5 shown in Tables 1 to 3, respectively). When the particle size of the synthesized powder was measured using additive No. 66 (Example 66) as a representative example, the average particle size D50 was 1.7 μm.

[0049] (Comparative Example 1) The positive electrode active material is a lithium transition metal composite oxide, LiNi, with an average particle size D50 of about 10 μm. 1 / 3 Co 1 / 3 Mn 1 / 3O2 was used. Timcal's KS6 and Super-P products were used as conductive additives, and Kureha's KF Polymer (a solution of PVdF dissolved in N-methylpyrrolidone) was used as the binder. The positive electrode active material, KS6, Super-P, and binder were weighed out in a weight ratio of 94:1:2:3, and NMP was added and mixed to prepare a positive electrode slurry. The resulting slurry was applied to an aluminum current collector using the doctor blade method, dried, punched into a 13 mm diameter disk, and pressed to prepare a positive electrode (without additives).

[0050] (Examples 1 to 67 and Comparative Examples 2 to 5) Positive electrodes were fabricated using the lithium ion secondary battery electrode additives Nos. 1 to 71 synthesized by the above method. The positive electrode active material was a lithium transition metal composite oxide, LiNi, with an average particle size D50 of about 10 μm. 1 / 3 Co 1 / 3 Mn 1 / 3 O2 was used. Timcal's trade names KS6 and Super-P were used as the conductive additive, and Kureha's trade name KF Polymer (a solution of PVdF dissolved in N-methylpyrrolidone) was used as the binder. The weight ratio of "positive electrode active material: additive: KS6: Super-P: binder" was weighed out in a ratio of 94:1:1:2:3, and NMP was added and kneaded to prepare a positive electrode slurry. The resulting slurry was applied to an aluminum current collector using the doctor blade method, dried, punched into a disk with a diameter of 13 mm, and pressed to prepare a positive electrode (positive electrode sheet). Using Example 66 as a representative example, the thickness of the positive electrode sheet was measured and found to be 90 μm.

[0051] (Coin cell assembly) The positive electrode, negative electrode, electrolyte, and separator were made, in that order, from metallic lithium cut into a disk shape, a 1 mol / l solution of the solute LiPF6 in a solvent made by mixing ethylene carbonate and diethyl carbonate in a volume ratio of 3:7, and a microporous polypropylene membrane.A coin-type battery CR2032 type (diameter 20 mm, height 3.2 mm) was assembled and battery evaluation measurements were performed.

[0052] (Rate characteristic evaluation) The coin-type lithium secondary battery thus fabricated was initially activated in a thermostatic chamber at 30°C. Charging was performed at a rate of 0.1C using a constant current / constant voltage CCCV with an upper cutoff voltage of 4.23V. Discharging was performed at a rate of 0.1C with a lower discharge voltage limit of 3.0V. Under these conditions, charging and discharging were repeated three times for initial activation. Next, the coin-type lithium secondary batteries after the initial activation were charged at a constant current and constant voltage of 4.23 V at a rate of 0.1 C with an upper cutoff voltage of 4.23 V, and discharged at a rate of 3 C with a lower discharge voltage limit of 3.0 V. The discharge capacity of each example and comparative example at this time was divided by the discharge capacity of comparative example 1 (no additive added) to calculate the rate characteristics (%). In other words, Rate characteristic (%) = (discharge capacity of each example or comparative example at discharge rate of 3C and lower limit voltage of 3.0V) ÷ (discharge capacity of comparative example 1 (no additives added) at discharge rate of 3C and lower limit voltage of 3.0V).

[0053] [Crystal structure analysis] Using an X-ray diffractometer (Rigaku smartlab), X-ray diffraction measurements were performed on the synthesized powders of the lithium-ion secondary battery electrode additives (Nos. 1 to 71), and X-ray diffraction patterns were obtained. It was confirmed that the obtained X-ray diffraction pattern closely matched the X-ray diffraction pattern No. 4422259 (Li7La3Zr2O12, space group: Ia-3d(230)) in the CSD (Cambridge Structural Database). Therefore, it can be concluded that the lithium-ion secondary battery electrode additives (Nos. 1 to 71) have a garnet-type crystal structure or a garnet-like crystal structure. Based on the obtained X-ray diffraction patterns, Rietveld analysis was performed using Fujio Izumi's multipurpose pattern fitting system RIETAN-FP to calculate the lattice constants of each sample, and the values ​​shown in Tables 1 to 3 were obtained. Furthermore, synchrotron radiation was used to perform X-ray diffraction measurements. Based on the obtained X-ray diffraction pattern, Rietveld analysis was performed using RIETAN-FP to confirm the substitution of element A at the Li site. By refining the occupancy rate at each atomic site, the Li occupancy rate could be determined. Since the occupancy rate is the statistical proportion of an atom occupying a site, the proportion of vacancies where no atoms exist was calculated from this, and the Li site vacancy rates shown in Tables 1 to 3 were obtained. Note that although RIETAN-FP was used for the Rietveld analysis, other calculation software can also be used. In addition, the valence and coordination number of element A can be confirmed by measuring the amount of X-ray absorption at energies near the absorption edge of element A using XAFS (X-ray Absorption Fine Structure). The specific method of Rietveld analysis by Fujio Izumi is shown in Fujio Izumi's book "Practical Powder X-ray Analysis," second edition.

[0054] The ionic conductivity of the obtained lithium-ion secondary battery electrode additive materials (Nos. 1 to 71) was measured by preparing sintered bodies. A binder was added to the lithium-ion secondary battery electrode additive material, which was then placed in a mold with a diameter of 15 mm and pressed to a thickness of approximately 11.5 mm to obtain a compact of the lithium-ion secondary battery electrode additive material. The compact was covered with calcined powder of the same composition as the compact, and fired at 1200°C for 4 hours in air or a nitriding atmosphere to obtain a sintered body of the lithium-ion secondary battery electrode additive material (Nos. 1 to 71).

[0055] The top and bottom surfaces of the sintered bodies (Nos. 1 to 71) of the additive material for the lithium ion secondary battery electrode were polished, and then various evaluations and measurements shown below were carried out.

[0056] Relative Density After measuring the mass of each sintered compact (Nos. 1 to 71) of the additive material for the lithium-ion secondary battery electrode, the length of each side or the diameter and thickness of each sintered compact of the additive material for the lithium-ion secondary battery electrode were measured at several locations using a vernier caliper and a micrometer, and the average values ​​were calculated. Using these measurements, the volume of Nos. 1 to 71 was calculated, and the apparent density was calculated. Furthermore, the theoretical density of each composition was calculated, and the apparent density was divided by the theoretical density and multiplied by 100 to calculate the relative density, and the values ​​shown in Tables 1 to 3 were obtained.

[0057] [Lithium ion conductivity measurement] After coating both sides of the samples (the polished sintered bodies obtained above) with Au by sputtering, AC impedance measurements were performed at room temperature using an electrochemical measurement system manufactured by N4L (Newtons4th Ltd) to calculate the lithium ion conductivity. As a result, the lithium ion conductivity values ​​shown in Tables 1 to 3 were obtained for each sample.

[0058] [Stability evaluation] The sample (sintered body sample) whose lithium ion conductivity was measured above was also heat-treated in air at 720°C for 10 hours, and then the ion conductivity was measured in the same manner as above. The conductivity after heat treatment was divided by the conductivity before heat treatment, and the result was multiplied by 100 to calculate the rate of decrease in conductivity due to heat treatment, which was evaluated as the stability of the crystal structure. As a result, the values ​​shown in Tables 1 to 3 were obtained.

[0059] [Table 1]

[0060] [Table 2]

[0061] [Table 3]

[0062] As shown in Tables 1 to 3, in Examples 1 to 67 and Comparative Examples 2 to 5, the conductivity was 1×10 -5 By adding an additive for lithium-ion secondary battery electrodes with a capacity of S / cm or higher to the positive electrode, the rate characteristics of both are improved.

[0063] Furthermore, as shown in Tables 1 to 3, in Examples 1 to 67 and Comparative Examples 2 to 5, when the molar ratio of element A to La was in the range of 0.01 or more and 0.50 or less and the octahedral selectivity of element A was 50 kJ / mol or more, the stability was 90% or more, indicating high stability. In Tables 1 to 3, the values ​​in the A / La column are shown with the second decimal place as the significant figure.

[0064] Furthermore, as shown in Tables 1 to 3, when Mg was substituted, the density improved and the relative density reached 88% or more, and when Sr was added, the lattice constant was expanded and higher ionic conductivity was observed.

[0065] Furthermore, when the octahedral selectivity was 60 kJ / mol or higher, the stability was 95% or higher in all cases, indicating higher stability.

[0066] The additives of the lithium ion secondary battery electrodes of Examples 1 to 67 have an ionic conductivity of 1×10 -5 However, in Examples 60 to 67 in which Sr is added and the A element is a divalent cation, the conductivity is 1 × 10 -3 The rate characteristics were 135% or more at efficiencies of 1000 S / cm or more, and higher ionic conductivity and output characteristics were observed. Therefore, by adding the additives of the lithium ion secondary battery electrodes of Examples 1 to 67 to the electrodes, it is possible to manufacture lithium ion secondary batteries with excellent input / output characteristics in which the discharge voltage and charge / discharge capacity do not easily decrease even when charged and discharged at high speed.

[0067] In this example, the average particle size D50 of the additive material was less than half the thickness of the lithium-ion secondary battery electrode. However, the effects of the present invention were confirmed to some extent even when the average particle size D50 of the additive material was three-quarters the thickness of the lithium-ion secondary battery electrode. However, more significant effects were obtained when the average particle size D50 of the additive material was less than half the thickness of the lithium-ion secondary battery electrode. It is believed that when the average particle size D50 of the additive material was less than half the thickness of the lithium-ion secondary battery electrode, the number of contact points between the additive material and the active material increased, resulting in better input / output characteristics. Furthermore, when the average particle size D50 of the additive material exceeded the thickness of the lithium-ion secondary battery electrode, the separator could be broken, causing a short circuit, and reducing the battery assembly yield.

[0068] Furthermore, when a zirconium oxide composite oxide containing zirconium oxide, yttrium, scandium, or other rare earth metal ions was added in addition to the additives of the examples, the rate characteristics were further improved.

[0069] In this example, the additive of the present invention was added to the electrode at 1 mass %, but the effect of the present invention was also observed at 0.1 mass %, and a more pronounced effect was obtained by increasing the amount added. However, increasing the amount added reduces the amount of active material, which in turn reduces the battery capacity, so adding 10 mass % or more may not be practical.

[0070] Furthermore, in addition to the additives of the examples, the rate characteristics were further improved by adding additives of Ta and Nb as representative examples of element C, each with a molar ratio C / La to La of 0.20.

[0071] In this example, the additive was added to the positive electrode, but the same effect was obtained when it was added to the negative electrode. The effect was even greater when it was added to both electrodes.

Claims

1. An electrode for a lithium ion secondary battery using an electrolyte solution, the electrode comprises an active material and an additive; the additive material is made of a composite oxide having a garnet-type or garnet-type-like crystal structure containing the elements Li, La, Zr, and O, and element Li is partially substituted with element A which is different from the elements and can form Li-site vacancies, and a molar ratio A / La of element A to La in the additive material is 0.01 or more and 0.50 or less, The element A is Co. ２＋ , V ３＋ , Mn ３＋ , Ni ２＋ , and Cu ２＋ and one or two selected from the group consisting of: The additive has a Li site vacancy rate of 40% or more and 80% or less due to the substitution of the element A, and has an ionic conductivity of 1×10 at room temperature. －５ S / cm or more, and Li ＋ Ion vacancies and Li ＋ It can cause dielectric polarization by ions, A lithium ion secondary battery electrode using an electrolyte, characterized in that the proportion of the additive at the interface between the electrolyte and the active material of the electrode is 0.1 mass % or more and 1 mass % or less in the electrode.

2. 2. The lithium ion secondary battery electrode according to claim 1, wherein the average particle diameter D50 of the composite oxide forming the additive is equal to or less than half the thickness of the electrode.

3. The additive has an ionic conductivity of 5×10 at room temperature. －４ 3. The lithium ion secondary battery electrode according to claim 1, wherein the electrical conductivity is 25 S / cm or more.

4. 4. The lithium ion secondary battery electrode according to claim 1, wherein the additive further contains an element B which is formed by substituting a part of the element La in the crystal structure.

5. The lithium ion secondary battery electrode according to any one of claims 1 to 4, characterized in that the element A in the additive is in a state of a cation in which a regular octahedral coordination selectivity in stabilization by a ligand field of an oxygen anion is 60 kJ / mol or more.

6. 6. The lithium ion secondary battery electrode according to claim 1, wherein the additive contains the element A in the form of a divalent cation.

7. 7. The lithium ion secondary battery electrode according to claim 1, wherein the lattice constant of the crystal structure of the additive is 12.93 Å or more.

8. A lithium ion secondary battery using an electrolyte solution comprising the electrode according to any one of claims 1 to 7.

9. An additive material to be added to an electrode of a lithium ion secondary battery using an electrolyte solution, the additive material comprising a composite oxide having a garnet-type or garnet-like crystal structure containing the elements Li, La, Zr, and O, A part of the element Li is substituted with an element A different from the element A and capable of forming Li-site vacancies, and the Li-site vacancy ratio is 40% or more and 80% or less, and the ionic conductivity at room temperature is 1×10 －５ S / cm or more, The element A is Al ３＋ , Ga ３＋ , and Mg ２＋ 1. An additive for an electrode of a lithium ion secondary battery, characterized in that it is contained in the form of one or two cations selected from the group consisting of:

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