Additives for lithium-ion secondary battery electrodes

Abstract

To achieve an additive that 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 is a composite oxide having a garnet-type or garnet-like crystal structure containing elements of Li, La, Zr, and O, in which a portion of the element Li is substituted with an element A which is different from the elements and can form Li-site vacancies, has a Li-site vacancy ratio of 40% or more and 80% or less, and has an ionic conductivity of 1×10-5 S / cm or more at room temperature.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] This invention relates to an additive that can be added to the electrodes of a lithium-ion secondary battery to improve its properties. [Background technology]

[0002] Lithium-ion batteries are used not only as small power sources for personal computers and portable electronic devices such as smartphones, but also as large, stationary emergency power sources. In recent years, lithium-ion batteries have seen significant development towards high capacity, fast charge / discharge characteristics, good cycle performance, and superior safety, making them ideal for use in electric vehicles, hybrid electric vehicles, and other applications.

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

[0004] Li7La3Zr2O is a garnet-type oxide containing the elements Li, La, Zr, and O. 12 The crystal structure of LLZ (hereinafter referred to as LLZ) mainly consists of cubic and tetragonal crystals. It is known that by substituting certain elements constituting LLZ with other elements, cubic LLZ becomes stable and exhibits high ionic conductivity. Non-patent Literature 1 reports that in the crystal structure of LLZ, the element Li occupies two types of crystallographic sites, Li1 and Li2 sites, located at the tetrahedral 24d site and the distorted octahedral 96h site, respectively, and that the occupancy rates of each Li site are Li1=0.97(7) and Li2=0.349, with the values ​​in parentheses indicating the standard deviation. Furthermore, it has been reported that the conduction pathway of Li ions in LLZ moves along the Li2→Li1→Li2 site via the Li1 site, and that a three-dimensional network of Li ion movement pathways is formed within the LLZ structure.

[0005] Non-patent document 2 states that when Ga is substituted into LLZ, Ga replaces a portion of the Li1 position of element Li, and Li+ and Ga 3+ It has been reported that a cubic LLZ having high ionic conductivity can be formed by introducing Li vacancies due to the difference in the charges of

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

[0007]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] The lithium ion battery described in Patent Document 1 above is excellent in high temperature cycle characteristics and the thermal stability of the positive electrode, but its input / output characteristics are not sufficient. In particular, when the current value during charge / discharge is increased (during high-speed charge / discharge), it has become clear from the examination results of the present inventors that the discharge voltage and the charge / discharge capacity are significantly reduced. Improving the rate characteristics of a lithium ion battery leads to shortening of the charging time and discharge characteristics that can handle a large load, and is desired. Currently, the rate characteristics are improved by devising the particle size of the positive electrode material, the thickness of the positive electrode coating on the current collector, etc. However, if the particle size of the positive electrode material is reduced to improve the rate characteristics, the packing density decreases, leading to a decrease in the battery capacity. Also, even if the thickness of the positive electrode coating on the current collector is reduced to improve the rate characteristics, a decrease in the battery capacity is caused. Therefore, a battery having excellent rate characteristics even with a normal positive electrode structure is desired.

[0009] The object of the present invention has been made in view of the above problems, and is to provide an additive for lithium-ion secondary battery electrodes that, when added to the electrodes of a lithium-ion secondary battery, can be used to manufacture a lithium-ion secondary battery with excellent input / output characteristics, i.e., a lithium-ion secondary battery with excellent rate characteristics, in which the discharge voltage and charge / discharge capacity do not easily decrease even when subjected to high-speed charging and discharging. [Means for solving the problem]

[0010] To solve the above problems, the present inventors have provided a composite oxide having a garnet-type or garnet-type similar crystal structure containing the elements Li, La, Zr, and O, wherein a portion of the elements is substituted with an element A different from the elements, and the Li site vacancy ratio is 40% to 80%, and the ionic conductivity at room temperature is 1 × 10⁻⁶ -5 We discovered that adding a composite oxide with a charge ratio of S / cm or higher to the electrodes of a lithium-ion secondary battery results in excellent charge-discharge characteristics during high-speed charging and discharging, i.e., a significant improvement in rate characteristics, thus completing the present invention.

[0011] The gist of this invention is as follows:

[0012] (1) A composite oxide having a garnet-type or garnet-type similar crystal structure containing the elements Li, La, Zr, and O, wherein a portion of the element Li is substituted with element A, which can form Li site vacancies unlike the aforementioned elements, and has a Li site vacancy ratio of 40% to 80%, and has an ionic conductivity of 1 × 10⁻¹⁶ at room temperature. -5 An additive for lithium-ion secondary battery electrodes, characterized by having a density of S / cm or higher. (2) The additive for lithium-ion secondary battery electrodes according to (1), characterized in that the average particle size 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 An additive for lithium-ion secondary battery electrodes according to (1) or (2) above, characterized in that it is S / cm or higher. (4) The additive for lithium-ion secondary battery electrodes according to any one of (1) to (3), characterized in that element A has d electrons and is in a cation state in which the octahedral coordination selectivity in the stabilization of the oxygen anion by the ligand field is 50 kJ / mol or more, and the molar ratio of element A to La, A / La, is 0.01 or more and 0.50 or less. (5) The additive for lithium-ion secondary battery electrodes according to any one of (1) to (4) above, further comprising element B obtained by substituting a part of element La in the crystal structure. (6) The additive for lithium-ion secondary battery electrodes according to any one of (1) to (5), characterized in that element A is in a cation state in which the octahedral coordination selectivity in the stabilization of the oxygen anion by the ligand field is 60 kJ / mol or more. (7) An additive for lithium-ion secondary battery electrodes according to any one of (1) to (6) above, characterized in that element A is included in the form of a divalent cation. (8) An additive for lithium-ion secondary battery electrodes according to any one of (1) to (7), characterized in that the lattice constant of the crystal structure is 12.93 Å or greater. (9) A lithium-ion secondary battery characterized by containing an additive for lithium-ion secondary battery electrodes as described in any of (1) to (8) above. (10) A positive electrode sheet for a lithium-ion secondary battery containing in a dispersed manner any of the lithium-ion secondary battery electrode additives described in (1) to (8) above. (11) A lithium-ion secondary battery characterized by using the positive electrode sheet described in (10) above. [Effects of the Invention]

[0013] According to the present invention, the interfacial reaction resistance between the lithium-ion secondary battery electrode and the electrolyte is reduced, resulting in a smaller voltage drop. Therefore, it is possible to manufacture lithium-ion secondary battery electrodes that have particularly excellent input / output characteristics during high-speed charging and discharging, i.e., lithium-ion secondary battery electrodes with excellent rate characteristics. [Brief explanation of the drawing]

[0014] [Figure 1] This is a schematic diagram illustrating the mechanism for estimating the effect of the lithium-ion secondary battery electrode additive of the present invention. [Figure 2] This schematic diagram illustrates how the interfacial reaction resistance R2 is reduced by the dielectric polarization effect of the additive particles of the present invention (shown as LLZ particles in the diagram) interposed at the interface between the active material particles of the lithium-ion secondary battery cathode (shown as cathode particles in the diagram) and the electrolyte. [Modes for carrying out 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 similar crystal structure containing the elements Li, La, Zr, and O, wherein a portion of the elements is replaced by element A, which is different from the elements and can form Li site vacancies, and the Li site vacancy ratio is 40% to 80%, and the ionic conductivity at room temperature is 1 × 10⁻⁶ -5 Characterized by being S / cm or higher.

[0016] First, let's explain what a site is. A site is a crystallographically equivalent lattice position. When an atom exists at a lattice position, the site is said to be occupied and is called an occupied site; when no atom exists at that position, it is called a vacant site. For example, there are five occupied sites in LLZ, which are called Li1 site, Li2 site, La site, Zr site, and O site, respectively. In some cases, they are called tetrahedral 24d site, distorted octahedral 96h site, dodecahedral 24c site, octahedral 16a site, 96h site, etc. In an ideal garnet structure, Li2 can be regarded as located at two equivalent positions (96h) or the center (48g) slightly deviated from the center of the LiO6 hexahedron within the distorted octahedron. Also, by using Rietveld analysis of the powder diffraction pattern, etc., it is possible to specify the occupied sites together with the space group of the crystal. Furthermore, the statistical ratio of an atom occupying its site is called the site occupancy rate, or simply the occupancy rate, and the ratio of vacancies to all lattice points that an atom can occupy is also called the vacancy ratio.

[0017] Here, as shown in Fig. 1(a), during charge and discharge in a lithium-ion secondary battery, a voltage drop ΔV occurs due to the internal resistance R of the battery. Therefore, when the current I increases (during high-rate charge and discharge), a large voltage drop occurs according to Ohm's law. Thus, during high-rate charge and discharge, the discharge voltage and charge-discharge capacity are significantly reduced. Therefore, in order to improve the high-rate charge and discharge characteristics (rate characteristics), it is necessary to reduce the internal resistance R. At that time, if we focus on the positive electrode in a lithium-ion secondary battery, its internal resistance becomes the positive electrode internal resistance, interfacial reaction resistance, and electrolyte resistance as shown in Fig. 1(b). That is, as shown in Fig. 2, the additive for the lithium-ion secondary battery electrode of the present invention, by contacting with the active material of the electrode, the Li + ion vacancies and the Li + ions cause dielectric polarization, electrostatically adsorb Li + ions and solvated molecules, and an interfacial reaction occurs, thereby reducing the interfacial reaction resistance. Therefore, the reduction rate of the discharge voltage and charge-discharge capacity during high-rate charge and discharge is reduced, and the input / output characteristics can be improved.

[0018] By substituting with element A, vacancies are created at the Li sites due to the charge difference between element A and Li, increasing the number of Li site vacancies in the crystal structure. This increases dielectric polarization and reduces interfacial reaction resistance. However, if the amount of Li site vacancies (vacancy ratio) is too high, the Li content decreases, resulting in too few conductive carrier ions and a decrease in conductivity. Therefore, if the amount of Li site vacancies exceeds a certain level, the ionic conductivity decreases. For this reason, 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 into and removed between the electrode and the electrolyte via the additive in 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. Therefore, it is preferable that the lithium-ion conductivity of the additive in the lithium-ion secondary battery electrode of the present invention is high. Specifically, at room temperature, 1 × 10⁻⁶ -5 S / cm or more, more preferably 5 × 10 -4 S / cm or more, and more preferably 1 × 10 -3 It is S / cm or higher. Improved lithium-ion mobility allows for the design of high-power batteries. On the other hand, low ionic conductivity results in the solid electrolyte becoming the rate-limiting factor, leading to poor power output characteristics.

[0019] From the above, it is sufficient for the additive material of the lithium-ion secondary battery electrode to be in contact with the active material of the electrode, and in order to prevent a decrease in the active material content within the electrode, it is preferable that the particles of the additive material of the lithium-ion secondary battery electrode be fine. If the additive material is fine, the number of contact points with the active material will increase, and the effects of the present invention will be obtained more efficiently. Specifically, it is more preferable that the average particle size D50 is 1 / 2 or less of the thickness of the lithium-ion secondary battery electrode. Similarly, it is more preferable that the average particle size D50 (De) of the additive material of the lithium-ion secondary battery electrode be between 0.01 and 1.0 in terms of De / Dc ratio compared to the average particle size D50 (Dc) of the positive electrode material (positive electrode active material) of the lithium-ion battery. Therefore, the smaller the particle size, the better, but particle sizes of 100 nm or less may be difficult to manufacture or may result in poor yield, making them impractical in some cases. Whether the average particle size D50 of the additive material in lithium-ion secondary battery electrodes is less than or equal to half the thickness of the lithium-ion secondary battery electrode can be confirmed, for example, by ultrasonically dispersing the material using an ultrasonic homogenizer and then measuring the particle size distribution using a particle size distribution analyzer. This allows for the confirmation of the average particle size D50 of the additive material (cumulative 50% based on volume).

[0020] In the crystalline structure of the additive for the lithium-ion secondary battery electrode, Li ions occupy two types of crystallographic sites, Li1 and Li2 sites, located in tetrahedral and distorted octahedral sites, respectively. The conduction pathway of Li ions in the LLZ moves along the Li2→Li1→Li2 site via the Li1 site, forming a three-dimensional network of Li ion movement pathways within the LLZ structure. By substituting elements in the LLZ (Lithium-Likely Zinc Zone), Li vacancies are introduced, stabilizing the cubic crystal structure and resulting in high lithium-ion conductivity. However, as described in Non-Patent Document 2, if the substituted element is located at the Li1 position, it may inhibit the free movement of Li along the Li2-Li1-Li2 site, potentially leading to a decrease in conductivity. Therefore, by including element A in a cation state with an octahedral coordination selectivity of 50 kJ / mol or more, element A can be selectively substituted at the Li2 site, which is an octahedral site, and the crystal structure can be stabilized by the coordination-stabilized substitution.

[0021] This allows for the optimization of the ratio of Li charge carriers and Li vacancies without hindering the free movement of Li ions along the Li2→Li1→Li2 site, resulting in high dielectric polarization, high ionic conductivity, and stable properties. Furthermore, compared to conventional substitution methods, it is possible to increase the Li vacancy concentration while securing the Li ion pathway. Therefore, increasing the amount of element A substitution increases ionic conductivity, but if the substitution amount is too high, the Li content decreases, resulting in too few conduction carrier ions. Consequently, if the content of element A exceeds a certain level, ionic conductivity decreases. The specific element A is preferably Al 3+ , Ga 3+ Mg 2+ Co 2+ , V 3+ Mn 3+ Ni 2+ Cu 2+ One or two elements can be selected from the following.

[0022] However, the content of element A is between 0.01 and 0.50 in molar ratio A / La to La. If the content of element A is less than 0.01 in molar ratio A / La to La, sufficient conductivity may not be stably obtained for the reasons mentioned above. On the other hand, if the content of element A is greater than 0.50 in molar ratio A / La to La, sufficient conductivity may not be obtained, as explained above, due to too few Li ions acting as conduction carriers or because substitution ions interfere with the Li ion conduction pathway. The content of element A is more preferably between 0.01 and 0.35 in molar ratio A / La to La, and even more preferably between 0.01 and 0.20.

[0023] The element A is in a cation state with an octahedral coordination selectivity of 50 kJ / mol or more, that is, an electronic state (especially the number of d electrons) and valency that satisfy the above conditions. Oxygen anion O 2-When comparing the stabilization by the crystal field (ligand field) between tetrahedral and octahedral coordination, the octahedral coordination exhibits a higher stabilization energy, indicating superior octahedral selectivity. Indeed, oxide crystals tend to occupy octahedral sites (positions). It is also known that oxides of typical metallic elements (positive elements) form ionic crystals (for example, Chemical Dictionary 3, March 10, 1974, 16th printing of the abridged edition).

[0024] In this system, if the octahedral coordination selectivity (the difference in coordination stabilization energy between octahedral and tetrahedral coordination) is 50 kJ / mol or higher, the ionic conductivity increases stably, and the effects of the present invention are significantly obtained. This is because, as mentioned above, since the substitution has octahedral coordination selectivity, element A can be effectively substituted at the Li2 site, which is an octahedral site. Therefore, if the octahedral coordination selectivity is less than 50 kJ / mol, a large ionic conductivity cannot be obtained. It is more preferable that element A contained in the lithium-ion conductive oxide material is in a cation state in which the octahedral coordination selectivity in the stabilization by the ligand field of the oxygen anion is 60 kJ / mol or higher. When the octahedral coordination selectivity of element A is 60 kJ / mol or higher, it tends to be fixed to the octahedral Li2 sites, and coordination stabilization makes it more difficult for the metal ions of element A to move, thus stabilizing the crystal structure. As a result, it does not inhibit the free movement of Li as much, and can be made more stable and have higher ionic conductivity.

[0025] Similar conclusions can be drawn from the coordination stabilization energy (crystal field stabilization energy CFSE) and the octahedral site selectivity energy (OSPE). Expressed in terms of the crystal field splitting parameter (10Dq), an OSPE (Δ10Dq) of 2 or greater indicates octahedral coordination selectivity.

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

[0027] If the additive material for lithium-ion secondary battery electrodes further contains element B (where 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) obtained by substituting a portion of 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 greater relative to La. If an element has an ionic radius of 0.8 or less relative to the ionic radius of La, the ionic conductivity tends to decrease, so it is preferably 0.9 or greater, and more preferably 1.0 or greater. Adding element B to the additive material of the lithium-ion secondary battery electrode makes it easier to obtain the effects of the present invention. This is because expanding the crystal lattice of the additive material minimizes the decrease in Li-ion conductivity. When the molar ratio B / La to La significantly exceeds 0.15, the lithium ion conductivity tends to decrease. Preferably, it is between 0.01 and 0.13, and more preferably between 0.02 and 0.1. Furthermore, regarding element B, from the viewpoint of expanding the crystal lattice, Ba 2+ or Sr 2+ It is preferable to use Sr 2+ Using this is preferable.

[0028] In this invention, as described above, element A contained in the additive material of the lithium-ion secondary battery electrode is included in such a way that it partially replaces element Li. By partially replacing element Li, element A becomes more stable and the ionic conductivity increases. Whether element A substitutes for a portion of element Li in a garnet-type or garnet-type similar crystal structure can be determined, for example, by performing X-ray diffraction measurements using synchrotron radiation and then conducting Rietveld analysis using RIETAN-FP based on the obtained X-ray diffraction pattern to confirm the substitution of element A at the Li position. It is more preferable that element A, contained in the additive for lithium-ion secondary battery electrodes, is present in the form of a divalent cation. The difference in charge between Li and element A creates vacancies in the Li sites within the crystal structure, making it easier for Li ions to move (facilitating Li ion hopping conduction), and thus improving ionic conductivity. In other words, this makes it easier to obtain the effects of the present invention. Whether or not element A is present in a divalent cation state can be determined, for example, by measuring the amount of X-ray absorption near the absorption edge of element A using XAFS (X-ray Absorption Fine Structure), thereby confirming the valence and coordination number of element A.

[0029] Furthermore, if the additive material for lithium-ion secondary battery electrodes further contains element C (where C is one or more elements selected from Sc, Y, Nb, Ta, In, Ge, Sn, and Te) obtained by substituting a portion of element Zr in the crystal structure, a higher ionic conductivity can be obtained, or the following effects can be obtained. The element C is added to the additive material of the lithium-ion secondary battery electrode, making it easier to obtain the effects of the present invention. 4+ It is believed that the difference in charge between Li and element C creates Li ions or Li vacancies at Li sites in the crystal structure, making it easier for Li ions to move and improving ionic conductivity. In other words, this makes it easier to obtain the effects of the present invention. The content of element C is preferably between 0.01 and 0.50 in molar ratio C / La to La. If the content of element C is less than 0.01 in molar ratio C / La to La, sufficient conductivity may not be stably obtained for the reasons mentioned above. On the other hand, if the content of element C is greater than 0.50 in molar ratio C / La to La, as explained above, there may be too many or too few Li ions that act as conduction carriers, resulting in insufficient conductivity. The content of element C is more preferably between 0.01 and 0.35 in molar ratio C / La to La, and even more preferably between 0.01 and 0.20. Furthermore, as for element C, Nb is chosen from the viewpoint of conductivity. 5+ Or Ta 5+ Using this is preferable.

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

[0031] The method for producing the additive material for lithium-ion secondary battery electrodes of the present invention can be any method as long as it satisfies the requirements of the present invention. Examples include gas-phase synthesis methods such as PVD and CVD, solid-phase reaction methods, spray pyrolysis methods, and wet methods such as coprecipitation and sol-gel methods.

[0032] As an example of a method for producing additives for lithium-ion secondary battery electrodes of the present invention, a synthesis example using a solid-phase reaction method can be shown below. The raw materials can include sulfates, oxides, carbonates, hydroxides, nitrates, acetates, oxalates, halides, and other constituent elements. For mixing, planetary mills, bead mills, ball mills, etc., can be used. The calcination temperature of the mixture varies depending on the components and formulation (for example, depending on the substituted metal ions), but it is preferably between 900°C and 1200°C. The calcination time varies depending on the constituent components and formulation (for example, depending on the substituted metal ions), but is preferably between 5 hours and 20 hours. The calcination atmosphere shall be one in which the oxygen partial pressure is appropriately adjusted depending on the constituent components and formulation. For example, it may be an atmosphere such as air, nitrogen or argon, or nitrogen or argon with adjusted oxygen partial pressure. The powder obtained by calcination can be used as is, or it may be further ground. When grinding, planetary mills, bead mills, ball mills, etc., can be used. The particle size after grinding should be adjusted to an appropriate size according to the application, but generally, a D50 particle size of 0.3 μm to 20 μm is preferable.

[0033] Furthermore, as another example of a method for producing additives for lithium-ion secondary battery electrodes according to the present invention, a synthesis example using spray pyrolysis can be shown below. Any raw material that can be dissolved in an aqueous solution or organic solvent may be used, but for example, carbonates, hydroxides, nitrates, acetates, oxalates, halides, metal alkoxides, etc. of constituent elements can be used. The solvent can be water, alkanes, alkanecarboxylic acids, and / or alcohols. The temperature of spray pyrolysis varies depending on the constituent components, formulation, and solvent concentration (for example, depending on the substituted metal ions), but is usually between 200°C and 1200°C. The particle size of the droplets during spraying is preferably between 0.01 μm and 100 μm. The carrier gas atmosphere should be one where the oxygen partial pressure is appropriately adjusted depending on the components and composition. For example, this could be air, nitrogen or argon, or nitrogen or argon with 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, the 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 electrolytic solution 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 electrolytic solution 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 one provided with the additive for the lithium ion secondary battery electrode 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, applying it to the surface of a current collector and drying it, 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 hereinafter), 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 such as LiCoO2, LiNiO2, LiMnO2, LiV2O3, and LiNi 0.80 Co 0.15 Al 0.05 O2, LiLiLi 0.50 Co 0.20 Mn 0.30 O2, LiLiLi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 is preferred.

[0036] The conductive material for lithium-ion secondary batteries manufactured using the additives of the present invention is not particularly limited as long as it is an electronically conductive material that does not adversely affect the battery performance of the positive electrode. For example, one or more of the following can be used: graphite such as natural graphite (scaly graphite, flake graphite) or artificial graphite, acetylene black, carbon black, Ketjen black, carbon whiskers, needle coke, carbon fiber, or a mixture of two or more metals (copper, nickel, aluminum, silver, gold, etc.). Among these, carbon black and acetylene black are preferred as conductive materials from the viewpoint of electronic conductivity and coating properties.

[0037] The binder plays the role of binding the active material particles and conductive material particles together. For example, fluororesins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and fluororubber, or thermoplastic resins such as polypropylene and polyethylene, ethylene-propylene-diemer (EPDM), sulfonated EPDM, and natural butyl rubber (NBR) can be used individually or as a mixture of two or more. Water-based binders such as cellulose-based or aqueous dispersions of styrene-butadiene rubber (SBR) can also be used.

[0038] As a solvent for dispersing the positive electrode active material, conductive material, and binder, organic solvents such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, N,N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran can be used.

[0039] Coating methods include roller coating using applicator rolls, screen coating, doctor blade method, spin coating, and bar coating, and any thickness and shape can be achieved using any of these methods.

[0040] Current collectors can be made from materials such as aluminum, titanium, stainless steel, nickel, iron, calcined carbon, conductive polymers, and conductive glass. Additionally, aluminum and copper can be treated with carbon, nickel, titanium, or silver to improve adhesion, conductivity, and oxidation resistance. These materials can also be oxidized. Current collector shapes include foil, film, sheet, net, punched or expanded materials, lath, porous materials, foams, and fiber clusters. For example, current collectors with a thickness of 1 to 500 μm are commonly used.

[0041] The negative electrode of a lithium-ion secondary battery manufactured 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 a suitable solvent to form a paste-like negative electrode material, applying and drying it on the surface of a current collector, and compressing it as needed to increase the electrode density. Examples of negative electrode active materials include inorganic compounds such as lithium, lithium alloys, and tin compounds, carbonaceous materials capable of intercalating and releasing lithium ions, and conductive polymers. Furthermore, the conductive material, binder, solvent, etc. used in the negative electrode can be the same as those exemplified for the positive electrode. The negative electrode current collector can be made of materials such as copper, nickel, stainless steel, titanium, aluminum, calcined carbon, conductive polymers, conductive glass, and Al-Cd alloys. Additionally, materials such as copper with surfaces treated with carbon, nickel, titanium, or silver can be used to improve adhesion, conductivity, and reduction resistance. These materials can also be oxidized. The shape of the current collector can be the same as that of the positive electrode.

[0042] As the electrolyte for a lithium-ion secondary battery manufactured using the additives of the present invention, a non-aqueous electrolyte containing a support salt can be used. Examples of solvents for the non-aqueous electrolyte include carbonates, esters, ethers, nitriles, furans, sulfolanes, and dioxolanes, which can be used individually or in combination. Specifically, 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. Of these, a combination of cyclic carbonates and chain carbonates is preferred. This combination not only provides excellent cycle characteristics, which represent the battery's characteristics during repeated charge and discharge cycles, but also allows for a balanced relationship between the viscosity of the electrolyte, the electrical capacity of the resulting battery, and the battery output.

[0043] Examples of support salts contained in lithium-ion secondary batteries manufactured using the additives of the present invention include LiPF6, LiClO4, LiAsF6, LiBF4, LiSO3CF3, and LiN(SO3CF3)2. The concentration of this support salt in the non-aqueous 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. A support salt concentration of 0.1 mol / L or more allows for a sufficient current density to be obtained, and a concentration of 5 mol / L or less allows for greater stability of the electrolyte.

[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 operating range of the lithium secondary battery, but examples include polymer nonwoven fabrics such as polypropylene nonwoven fabric or polyphenylene sulfide nonwoven fabric, and thin microporous membranes of olefin resins such as polyethylene or polypropylene. These may be used individually or in combination.

[0045] The shape of the lithium-ion secondary battery manufactured using the additive of the present invention is not particularly limited, but examples include coin-type, button-type, sheet-type, stacked-type, cylindrical, flat-type, and prismatic-type batteries. Furthermore, multiple such lithium-ion secondary batteries may be connected in series to create larger batteries for use in electric vehicles and the like.

[0046] As described above, the lithium-ion secondary battery positive electrode material using the additive for lithium-ion secondary battery electrodes of the present invention has a reduced interfacial reaction resistance between the positive electrode and the electrolyte, resulting in a smaller voltage drop. Therefore, by using the additive for the present invention, it is possible to manufacture a lithium-ion secondary battery with high output characteristics. [Examples]

[0047] The present invention will be described below with reference to examples and comparative examples. However, the present invention is not limited to the examples. Furthermore, Examples 1 to 12 below are for reference only.

[0048] (Preparation of additives for lithium-ion secondary battery electrodes) Li2CO3, Al2O3, Ga2O3, MgO, V2O3, Mn2O3, CuO, NiO, La(OH)3, SrCO3, BaO, and ZrO2 were used as raw materials for additives in lithium-ion secondary battery electrodes. These raw material powders were weighed and mixed to produce additive compositions shown in Tables 1-3. The mixing of each raw material was carried out as follows: the weighed raw material powders were placed in a poly container together with zirconia balls, ball-milled in ethanol for 15 hours, and then dried to obtain the raw material mixture. Note that the initial composition contains more Li2CO3 than the target composition to account for some loss of Li during calcination. Therefore, even though the initial composition does not satisfy the charge compensation (charge conservation law) of the garnet-type crystal structure, it is understood that the composition will satisfy the charge compensation after calcination due to the loss of Li and / or oxygen deficiency. Furthermore, there is virtually no loss of elements other than Li during firing, and the initial composition ratio is essentially maintained even after firing. The obtained raw material mixture was calcined on an MgO heat-resistant container at 1100°C for 10 hours in an air or nitriding atmosphere. Further calcination was performed on an MgO heat-resistant container at 1100°C for 4 hours in a nitriding atmosphere to obtain a calcined product. This calcined product was placed in a nylon pot together with zirconia balls, pulverized in ethanol using a planetary ball mill for 2 hours, and then dried to obtain additives No. 1 to 71 for lithium-ion secondary battery electrodes (corresponding to the additives in Examples 1 to 67 and Comparative Examples 2 to 5 shown in Tables 1 to 3). As a representative example, the particle size of the synthesized powder of additive No. 66 (Example 66) was measured, and the average particle size D50 was 1.7 μm.

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

[0050] (Examples 1-67 and Comparative Examples 2-5) A positive electrode was fabricated using additives No. 1 to 71 of the lithium-ion secondary battery electrode synthesized by the above method. As the positive electrode active material, LiNi was used, which is a lithium transition metal composite oxide with an average particle size D50 of approximately 10 μm. 1 / 3 Co 1 / 3 Mn 1 / 3 O2 was used. Timcal's product names KS6 and Super-P were used as conductive additives, and Kureha's product name KF Polymer (a solution of PVdF dissolved in N-methylpyrrolidone) was used as a binder. The positive electrode active material, additives, KS6, Super-P, and binder were weighed in a weight ratio of 94:1:1:2:3, and NMP was added and kneaded to prepare a positive electrode slurry. The obtained slurry was applied to an aluminum current collector using the doctor blade method, dried, punched out into a 13 mm diameter disc, and then pressed to produce a positive electrode (positive electrode sheet). The thickness of the positive electrode sheet was measured to be 90 μm, using Example 66 as a representative example.

[0051] (Coin cell assembly) For the positive electrode, negative electrode, electrolyte, and separator, respectively, we used disc-shaped pieces of metallic lithium, a solvent of ethylene carbonate and diethyl carbonate mixed in a volume ratio of 3:7 with 1 mol / l of solute LiPF6 dissolved in it, and a microporous membrane made of polypropylene. We then assembled a CR2032 type coin cell (20 mm in diameter, 3.2 mm in height) and performed battery evaluation measurements.

[0052] (Rate characteristic evaluation) The fabricated coin-type lithium secondary battery underwent initial activation in a constant temperature bath at 30°C. Charging was performed at a rate of 0.1C with a constant current / constant voltage CCCV (Cell-Cold-Voltage Charger) and 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. The initial activation was performed by repeating the charging and discharging cycles three times under these conditions. Next, the coin-type lithium secondary batteries, after initial activation, were charged at a rate of 0.1C and a constant current / constant voltage with an upper cutoff voltage of 4.23V. Discharge was performed at a rate of 3C and a lower discharge voltage limit of 3.0V. The rate characteristics (%) were calculated by dividing the discharge capacity of each example and comparative example by the discharge capacity of Comparative Example 1 (without additives). In other words, The rate characteristic (%) is given by (discharge capacity of each example and comparative example with a discharge rate of 3C and a discharge lower limit voltage of 3.0V) ÷ (discharge capacity of Comparative Example 1 (no additives) with a discharge rate of 3C and a discharge lower limit voltage of 3.0V).

[0053] [Crystal structure analysis] X-ray diffraction measurements were performed on the synthetic powders of the lithium-ion secondary battery electrode additives (No. 1-71) using an X-ray diffractometer (Rigaku smartlab), and X-ray diffraction patterns were obtained. It was confirmed that the obtained X-ray diffraction patterns were in close agreement with the CSD (Cambridge Structural Database) X-ray diffraction pattern No. 4422259 (Li7La3Zr2O12, space group: Ia-3d(230)). Therefore, it can be concluded that the lithium-ion secondary battery electrode additives (No. 1-71) have a garnet-type crystal structure or a garnet-type similar crystal structure. Furthermore, based on the obtained X-ray diffraction patterns, Rietveld analysis was performed using Fujio Izumi's multi-purpose pattern fitting system RIETAN-FP to calculate the lattice constants of each sample, resulting in the values ​​shown in Tables 1-3. Furthermore, by performing X-ray diffraction measurements using synchrotron radiation and then conducting Rietveld analysis using RIETAN-FP based on the obtained X-ray diffraction patterns, the substitution of element A to the element Li position could be confirmed, and the Li occupancy rate could be determined by refining the occupancy rate at each site of the atom. Since the occupancy rate is the statistical proportion of a site occupied by a given atom, the proportion of vacancies where no atoms exist was calculated from this, and the Li site vacancy rates shown in Tables 1-3 were obtained. Note that RIETAN-FP was used for the Rietveld analysis, but other calculation software can also be used. Furthermore, by measuring the amount of X-ray absorption near the absorption edge of element A using XAFS (X-ray Absorption Fine Structure), the valency and coordination number of element A can be confirmed. The specific method of Rietveld analysis by Fujio Izumi is described in "Practical Powder X-ray Analysis," 2nd edition, by Fujio Izumi.

[0054] The ionic conductivity of the obtained lithium-ion secondary battery electrode additives (No. 1 to 71) was measured by preparing sintered bodies. A binder was added to the lithium-ion secondary battery electrode additive and placed in a 15 mm diameter mold. The mixture was then press-molded to a thickness of approximately 11.5 mm to obtain a molded body of the lithium-ion secondary battery electrode additive. The molded body was covered with calcined powder of the same composition as the molded body and fired at 1200°C for 4 hours in air or a nitriding atmosphere to obtain sintered bodies of the lithium-ion secondary battery electrode additive (No. 1 to 71).

[0055] After polishing the upper and lower surfaces of the sintered bodies (No. 1 to 71) of the additive material for lithium-ion secondary battery electrodes, the following various evaluations and measurements were performed.

[0056] [Relative density] After measuring the mass of the sintered bodies (No. 1-71) of the additive material for lithium-ion secondary battery electrodes, the length or diameter and thickness of each side of the sintered body were measured at several points using calipers and a micrometer, and the average value was calculated. Using these measurements, the volume of No. 1-71 was calculated, and the apparent density was calculated. Furthermore, after calculating the theoretical density of each composition, the apparent density was divided by the theoretical density and multiplied by 100 to calculate the relative density, resulting in the values ​​shown in Tables 1-3.

[0057] [Measurement of lithium-ion conductivity] After coating both sides of the sample (the polished sintered body obtained above) with Au sputtering, AC impedance measurements were performed at room temperature using an electrochemical measurement system manufactured by N4L (Newtons4th Ltd), and the lithium-ion conductivity was calculated. As a result, the lithium-ion conductivity of each sample was obtained as shown in Tables 1 to 3.

[0058] [Stability evaluation] The lithium-ion conductivity of the sample (sintered sample) used to measure lithium-ion conductivity was then measured again in the same manner after heat treatment at 720°C for 10 hours in air. The conductivity after heat treatment was divided by the conductivity before heat treatment and multiplied by 100 to calculate the percentage decrease in conductivity due to heat treatment, which was then evaluated as the stability of the crystal structure. The results are shown in Tables 1 to 3.

[0059] [Table 1]

[0060] [Table 2]

[0061] [Table 3]

[0062] As shown in Tables 1-3, from Examples 1-67 and Comparative Examples 2-5, the conductivity is 1 × 10⁻⁶. -5 By adding an additive for lithium-ion secondary battery electrodes with a rate of S / cm or higher to the positive electrode, the rate characteristics are improved in both cases.

[0063] Furthermore, as shown in Tables 1-3, Examples 1-67 and Comparative Examples 2-5 showed that when the molar ratio of element A to La was within the range of 0.01 to 0.50, and the octahedral selectivity of element A was 50 kJ / mol or higher, the stability was 90% or higher, indicating high stability. In Tables 1-3, the values ​​in the A / La column are given with two decimal places as significant figures.

[0064] Furthermore, as shown in Tables 1-3, when Mg was substituted, the density improved in all cases, showing a relative density of 88% or higher. In addition, when Sr was added, the lattice constant was expanded in all cases, 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] Furthermore, the additives for the lithium-ion secondary battery electrodes in Examples 1 to 67 have an ionic conductivity of 1 × 10⁻⁶. -5 Although it has a conductivity of S / cm or higher, in Examples 60-67, where Sr is added and element A is a divalent cation, the conductivity is 1 × 10⁻⁶. -3 It exhibited a rate characteristic of 135% or higher at S / cm or above, and showed higher ionic conductivity and output characteristics. Therefore, by adding the additives for 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 subjected to high-speed charging and discharging.

[0067] In this embodiment, the average particle size D50 of the additive material was less than half the electrode thickness of the lithium-ion secondary battery. However, the effects of the present invention were also confirmed to some extent when the average particle size D50 of the additive material was 3 / 4 of the electrode thickness. However, a more significant effect was obtained when the average particle size D50 of the additive material was less than half the electrode thickness of the lithium-ion secondary battery. It is thought that when the average particle size D50 of the additive material is less than half the electrode thickness of the lithium-ion secondary battery, the number of contact points between the additive material and the active material increases, resulting in better input / output characteristics. Furthermore, when the average particle size D50 of the additive material exceeds the electrode thickness of the lithium-ion secondary battery, it can break the separator and cause a short circuit, which can reduce the assembly yield of the battery.

[0068] Furthermore, when the additives were added together with a zirconium oxide composite oxide containing zirconium oxide, yttrium, scandium, and other rare earth metal ions, in addition to the additives in the examples, the rate properties were further improved.

[0069] Furthermore, in this embodiment, the additive of the present invention was added to the electrode at a concentration of 1 mass%, but the effects of the present invention were also observed with a concentration of 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, thus reducing the battery capacity, so adding 10 mass% or more may not be practical in some cases.

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

[0071] In this embodiment, the additive was added to the positive electrode, but a similar 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. A composite oxide having a garnet-type or garnet-type similar crystal structure containing the elements Li, La, Zr, and O, which is an additive for use in electrodes of lithium-ion secondary batteries that use an electrolyte, Unlike the aforementioned elements, element A is capable of forming Li-site vacancies, and a portion of element Li is substituted with element A, resulting in a Li-site vacancy ratio of 40% to 80%, and an ionic conductivity of 1 × 10⁻¹⁶ at room temperature. －５ S / cm or more, The additive for lithium-ion secondary battery electrodes is characterized in that element A is one or two selected from the group consisting of Co²⁺, V³⁺, Mn³⁺, Ni²⁺, and Cu²⁺, and is in a cation state having d electrons, and exhibiting an octahedral coordination selectivity of 50 kJ / mol or more in the stabilization of the oxygen anion by the ligand field.

2. The additive for lithium-ion secondary battery electrodes according to claim 1, characterized in that the average particle size D50 of the composite oxide is 1 / 2 or less of the thickness of the lithium-ion secondary battery electrode.

3. The ionic conductivity at room temperature is 5 × 10⁻⁶. －４ An additive for lithium-ion secondary battery electrodes according to claim 1 or 2, characterized in that it is S / cm or higher.

4. The additive for lithium-ion secondary battery electrodes according to any one of claims 1 to 3, characterized in that the molar ratio A / La of element A to La is 0.01 or more and 0.50 or less.

5. The additive for lithium-ion secondary battery electrodes according to any one of claims 1 to 4, further comprising element B obtained by substituting a portion of element La in the aforementioned crystal structure.

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

7. The additive for lithium-ion secondary battery electrodes according to any one of claims 1 to 6, characterized in that element A is included in the form of a divalent cation.

8. The additive for lithium-ion secondary battery electrodes according to any one of claims 1 to 7, characterized in that the lattice constant of the crystal structure is 12.93 Å or greater.

9. A lithium-ion secondary battery using an electrolyte characterized by containing an additive for lithium-ion secondary battery electrodes according to any one of claims 1 to 8.

10. A positive electrode sheet for a lithium-ion secondary battery, using an electrolyte containing a dispersed additive for lithium-ion secondary battery electrodes according to any one of claims 1 to 8.

11. A lithium-ion secondary battery using an electrolyte characterized by using the positive electrode sheet of claim 10.

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