JP2016076454A - Positive electrode active material for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode active material capable of developing a high potential and inhibiting a metal element from being eluted, and also to provide a lithium ion secondary battery which is excellent in capacity and high temperature life.SOLUTION: A positive electrode active material for a lithium ion secondary battery includes: a composite oxide with Li (lithium) and transition metal containing at least Mn (manganese); and a surface oxide which is an oxide having a metal element of a bivalence or more. In the composite oxide, a surface layer part (B) is fluorinated and a central part (C) is not fluorinated. The surface oxide (A) exists on the surface of the composite oxide and is fluorinated.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a positive electrode active material for a lithium ion secondary battery and a lithium ion secondary battery using the same.

  Lithium ion secondary batteries are widely used as compact power sources for portable information terminals and the like because of their high energy density. In recent years, lithium ion secondary batteries have begun to be used as large-scale power sources used in vehicle applications such as electric vehicles and hybrid electric vehicles, or stationary industrial applications such as power storage.

  When used as a large power source, a large number of lithium ion secondary batteries are used. The batteries can be used in multiple series as needed. Although the voltage of the conventional lithium ion secondary battery is around 4 V, a higher voltage lithium ion secondary battery is required as a battery having a higher energy density or for the purpose of reducing the number of batteries in series.

  In a high voltage lithium ion secondary battery, a positive electrode active material that stably expresses a high potential of 4.5 V or more on the basis of metallic lithium is used for the positive electrode.

Conventionally, a positive electrode active material having a layered rock salt type crystal structure represented by the general formula LiMO ₂ (M is a transition metal mainly composed of Co, Ni, or Mn) is used at an upper limit potential of around 4.3V. It has been. In recent years, attempts have been made to raise the upper limit potential of the positive electrode active material.

On the other hand, as a positive electrode active material that stably develops a potential of 4.5 V or more, Mn represented by the general formula LiMn _{2 -X} M _X O ₄ (M is Ni, Co, Cr, Fe, Cu, etc.) A spinel type complex oxide partially substituted with a transition metal is known. The spinel-type composite oxide expresses a potential of about 4 V due to Mn oxidation-reduction, but stably expresses a potential of 4.5 V or more due to the transition metal oxidation-reduction by the above-described transition metal substitution. In particular, it is known that a spinel complex oxide in which a part of Mn is substituted with Ni exhibits particularly stable performance.

  On the other hand, high-voltage lithium ion secondary batteries are generally known to have a problem of short high-temperature life (life when operated at a high temperature (generally 50 ° C. to 40 ° C. to 60 ° C.)). Yes. It is well known that one of the main causes is the progress of oxidative decomposition of the electrolyte solution on the surface of the positive electrode at a high potential.

  It is also known that another main cause of this problem is the elution of transition metal elements from a high potential positive electrode active material. The elution of the metal element degrades the positive electrode active material, and the eluted metal element is deposited on the negative electrode, thereby reducing the performance of the negative electrode. As the positive electrode potential is higher, the elution amount tends to increase.

  In the spinel-type complex oxide in a charged state, for example, the problem that Mn elutes into the electrolytic solution in a high temperature environment of 50 ° C. is well known. Even in the transition metal-substituted spinel composite oxide that expresses 4.5 V or higher, the elution amount of the metal element is remarkably increased because of its high potential.

  As a prior art for this problem, for example, there are the following techniques. Patent Document 1 discloses a spinel complex oxide in which a specific cation is substituted. However, the effect of suppressing elution of metal elements is not always sufficient. Patent Document 2 discloses a spinel complex oxide in which a part of oxygen is substituted with fluorine as an anion. However, there is a problem that the capacity is remarkably lowered as the fluorine substitution amount is increased, and the performance as an active material is lowered. In addition, the effect of suppressing elution of metal elements in the high potential positive electrode is not sufficiently clear.

  Another prior art for this problem is related to the surface of the positive electrode active material. For example, Patent Document 3 discloses a positive electrode active material coated with a fluorine compound. However, the reaction between the positive electrode active material and the lithium ions is hindered due to a decrease in the diffusibility of lithium ions in the coating layer, and there is a problem that the performance as an active material such as high load characteristics decreases. Patent Document 4 discloses a positive electrode active material provided with a fluorine-substituted spinel layer containing a crystalline metal halide. However, the effect of suppressing elution of metal elements is not always sufficient.

JP 2005-322480 A JP 2002-75366 A Special table 2008-536285 gazette JP 2000-128539 A

  As described above, the high voltage lithium ion secondary battery has a problem that the high temperature life is short because the transition metal element is eluted from the positive electrode active material. In the prior art, this problem cannot be solved sufficiently, and the capacity may be reduced.

  An object of the present invention is to provide a positive electrode active material capable of expressing a high potential and suppressing elution of a metal element, and a lithium ion secondary battery excellent in capacity and high-temperature life.

  The positive electrode active material for a lithium ion secondary battery according to the present invention has the following characteristics. A composite oxide of Li (lithium) and a transition metal containing at least Mn (manganese) and a surface oxide that is an oxide having a divalent or higher valent metal element are included. In the composite oxide, the surface portion is fluorinated and the central portion is not fluorinated. The surface oxide exists on the surface of the composite oxide and is fluorinated.

  ADVANTAGE OF THE INVENTION According to this invention, the positive electrode active material which can express a high electric potential and can suppress elution of a metal element can be provided. By using this positive electrode active material, it is possible to provide a lithium ion secondary battery having excellent capacity and high temperature life.

It is a photograph of the SEM image of the cross section of a positive electrode active material, and is a figure which shows the example of the measurement location which calculated | required the ratio of the structural element of a positive electrode active material. The schematic diagram of the lamination cell for positive electrode charge. 1 is a schematic diagram of a laminate type lithium ion secondary battery. FIG. The figure which shows the element ratio in the surface oxide of the positive electrode active material, the surface layer part of complex oxide, and the center part of complex oxide in Example 1 (positive electrode active material 4A) and Comparative Example 1 (positive electrode active material 4Z) . In Example 2 (positive electrode active material 5N) and Comparative Example 2 (positive electrode active material ZN and positive electrode active material FN), the surface oxide of the positive electrode active material, the surface layer portion of the composite oxide, and the central portion of the composite oxide FIG. The figure which shows the element ratio in the surface oxide of the positive electrode active material, the surface layer part of complex oxide, and the center part of complex oxide in Example 2 (positive electrode active material 5T) and Comparative Example 2 (positive electrode active material ZT) . The figure which shows the element ratio in the surface oxide of the positive electrode active material, the surface layer part of complex oxide, and the center part of complex oxide in Example 3 (positive electrode active material 5G) and Comparative Example 3 (positive electrode active material ZG) . The figure which shows the element ratio in the surface oxide of the positive electrode active material, the surface layer part of complex oxide, and the center part of complex oxide in Example 4 (positive electrode active material 5M) and Comparative Example 4 (positive electrode active material ZM) . The figure which shows the element ratio in the surface oxide of the positive electrode active material, the surface layer part of complex oxide, and the center part of complex oxide in Example 4 (positive electrode active material 5E) and Comparative Example 4 (positive electrode active material ZE) . The figure which shows the element ratio in the surface oxide of the positive electrode active material, the surface layer part of complex oxide, and the center part of complex oxide in Example 5 (positive electrode active material 5C) and Comparative Example 5 (positive electrode active material ZC) . The figure which shows the element ratio in the surface oxide of the positive electrode active material, the surface layer part of complex oxide, and the center part of complex oxide in Example 5 (positive electrode active material 5U) and Comparative Example 5 (positive electrode active material ZU) .

  The positive electrode active material for a lithium ion secondary battery according to the present invention is a surface oxide which is a composite oxide of Li (lithium) and a transition metal containing at least Mn (manganese), and an oxide having a divalent or higher metal element. The composite oxide is characterized in that the surface layer portion is fluorinated, the central portion is not fluorinated, and the surface oxide exists on the surface of the composite oxide and is fluorinated. .

  The characteristics of the positive electrode active material for a lithium ion secondary battery according to the present invention will be described in detail below.

  The elution of metal ions from the high potential positive electrode active material proceeds at the interface between the positive electrode active material and the non-aqueous electrolyte. This reaction is said to involve water present as impurities in the electrolyte and a small amount of fluorine ions derived from a fluorine compound that is a lithium salt.

  One of the features of the positive electrode active material according to the present invention is that the surface layer portion of the composite oxide that acts as the positive electrode active material is fluorinated. That is, fluorine having a higher electronegativity than oxygen exists in the surface layer portion of the composite oxide, and this fluorine is bonded to the transition metal. Thereby, it is considered that the binding force between the transition metal and the anion becomes stronger, and the elution of the transition metal is suppressed. Note that it is not necessary that the entire surface portion of the composite oxide is fluorinated. That is, in the composite oxide, a part of the surface layer portion may be fluorinated, and the fluorinated surface layer portion may not completely cover the composite oxide. Further, the central portion of the composite oxide is not fluorinated.

  Another feature of the positive electrode active material according to the present invention is that an oxide having a divalent or higher valent metal element is present on the surface of the composite oxide, and this oxide (hereinafter referred to as “surface oxide”) is fluorine. It is becoming. One of the actions of the surface oxide is a physical action that prevents direct contact between the positive electrode active material and the electrolytic solution. Furthermore, it is presumed that there is an action to prevent the elution reaction of the positive electrode active material by reacting with moisture and fluorine ions instead of the positive electrode active material. However, there is a possibility that the metal element constituting the surface oxide is eluted by this reaction with moisture or fluorine ions. Therefore, the surface oxide is fluorinated. By fluorinating the surface oxide to some extent, it is presumed that the binding force between the metal element and the anion becomes strong, and the above action is exhibited while suppressing the elution of the metal element.

  Note that the surface oxide does not necessarily have to cover the entire surface of the composite oxide. When the surface oxide is present so as to cover a part of the surface of the composite oxide, the insertion of lithium ions into the composite oxide is compared to the case where the surface oxide is present so as to cover the entire surface of the composite oxide. -Desorption is not hindered, and an increase in battery resistance and a decrease in capacity can be prevented.

  The fluorination of an oxide means that an oxide fluorine compound is present. For example, when a part of oxygen in the oxide is replaced with fluorine, the oxide is fluorinated.

  The above actions and effects become more remarkable as the working potential of the positive electrode active material is higher, specifically at 4.5 V or higher. At such a high potential, it is considered that both the surface layer portion of the composite oxide and the surface oxide are fluorinated, thereby obtaining an effect of suppressing the elution of the metal from the positive electrode active material. In other words, when at least one of the surface layer portion of the composite oxide and the surface oxide (for example, the surface oxide) is not fluorinated, elution of the metal from the non-fluorinated one (surface oxide) There is a risk that will concentrate.

  Here, the composite oxide which acts as a positive electrode active material for the positive electrode active material for a lithium ion secondary battery according to the present invention will be described.

The positive electrode active material according to the present invention has a composite oxide of lithium (Li) and a transition metal containing at least manganese (Mn), and the composite oxide exhibits capacity. The composite oxide is not particularly limited as long as Li and Mn are included. For example, the composite oxide may be a composite oxide having a layered rock salt type crystal structure represented by a general formula LiMO ₂ (M includes Mn and includes one or both of Co and Ni). A spinel-type composite oxide containing Mn as a main constituent element stably expresses a potential of 4.5 V or more, and thus is more desirable as a positive electrode active material.

The spinel type complex oxide has a cubic spinel type crystal structure represented by the general formula LiMn ₂ O ₄ , and stably expresses a discharge potential of about 4 V by oxidation and reduction of Mn. Here, by substituting a part of Mn with a specific transition metal element, an oxidation-reduction potential of 4.5 V or more is stably expressed. As such a transition metal element, Ni, Cr, Fe, Co, Cu and the like are known.

In particular, a spinel complex oxide represented by the general formula LiNi _x Mn _2−x O _{4 in} which a part of Mn is substituted with Ni exhibits a large and stable capacity in the vicinity of 4.6V. The capacity near 4.6V increases with increasing x in the above general formula, and reaches a theoretical maximum at x = 0.5.

The above-described spinel complex oxide substituted with Ni used for the positive electrode active material according to the present invention has the general formula Li _a Ni _x Mn _y M _z O _{4 + α} (M is Ti, Ge, Mg, Fe, Co, and Cu). At least one of 0.3 ≦ x ≦ 0.55, 1.2 ≦ y ≦ 1.6, 0 ≦ Z ≦ 0.4, 1.9 ≦ x + y + z ≦ 2.05, 0.95 ≦ a ≦ 1.1, −0.2 ≦ α ≦ 0.1).

  When the substitution amount x of Ni is less than 0.3, the ratio of transition metal elements that develop capacity at less than 4.5 V is large, which is not always desirable. If x exceeds 0.55, there is a risk of heterogeneous formation. For this reason, it is desirable that 0.3 ≦ x ≦ 0.55.

  The element M is at least one of Ti, Ge, Mg, Fe, Co, and Cu, and is not necessarily included in the positive electrode active material (z = 0 may be satisfied). The action of the element M varies depending on the type of element. Ti, Ge, and Mg do not necessarily contribute to the development of capacity at 4.5 V or higher by their own redox, but stabilize the crystal structure and have some effect on the suppression of transition metal elution. I can expect. Fe, Co, and Cu can contribute to the development of capacity at 4.5 V or more in addition to stabilization of the crystal structure. The suitable substitution amount z of the element M is not necessarily constant depending on the kind of element and the substitution amount of other elements, but if it exceeds 0.4, there is a risk of heterogeneous formation. For this reason, it is desirable that 0 ≦ Z ≦ 0.4.

  The element M may contain a metal element other than the above elements.

  The value of the composition ratio y of Mn is preferably 1.2 ≦ y ≦ 1.6 in order to maintain the structure of the spinel complex oxide.

In the above general formula Li _a Ni _x Mn _y M _z O _{4 + α} , the sum of the composition ratios of Ni, Mn, and element M (x + y + z) is preferably around 2 which is the same as LiMn ₂ O _4. 1.9 ≦ x + y + z ≦ 2.05 in some cases. If the value of x + y + z is out of the above range, a different phase may be generated, which is not preferable.

  The value of a is a value indicating the non-stoichiometry of Li during production. If a is less than 0.95, the capacity may decrease, and if it exceeds 1.1, a foreign phase may be generated. For this reason, it is desirable that 0.95 ≦ a ≦ 1.1.

  The value of α is a value indicating the non-stoichiometry of oxygen during production. Outside the range of −0.2 ≦ α ≦ 0.1, a heterogeneous phase may be generated, which is not preferable.

In the positive electrode active material according to the present invention, the surface layer portion of the composite oxide that acts as the positive electrode active material is fluorinated. When the composite oxide is represented by the general formula Li _a Ni _x Mn _y M _z O _{4 + α} , the complex oxide having a fluorinated surface layer portion is like Li _a Ni _x Mn _y M _z O _{4 + α-β} F _β . In addition, it can be expressed that a part of oxygen is substituted with fluorine. The amount of fluorine is affected by the measurement method, measurement location, and measurement range, but in the above general formula, β is preferably in the range of 0.5 or less. The higher the amount of fluorine, the higher the effect of suppressing the elution of metal can be expected.However, if the amount of fluorine is too large, the spinel structure may not be maintained. There is a risk that the performance as an active material may be reduced due to a decrease or the like. For this reason, it is desirable that 0 <β ≦ 0.5.

  In the positive electrode active material according to the present invention, the surface portion of the composite oxide is fluorinated, and the central portion of the composite oxide is not fluorinated. If the fluorination is performed up to the center of the composite oxide, the performance as an active material such as a decrease in capacity may be reduced. For this reason, only the surface portion of the composite oxide is fluorinated, and the central portion is not fluorinated.

  Here, the surface oxide which exists in the surface of complex oxide is demonstrated about the positive electrode active material for lithium ion secondary batteries by this invention.

  In the positive electrode active material according to the present invention, an oxide (surface oxide) having a bivalent or higher valent metal element exists on the surface of the composite oxide, and the surface oxide is fluorinated. The presence of the fluorinated surface oxide on the surface of the composite oxide is considered to prevent direct contact between the positive electrode active material and the electrolytic solution and to suppress elution of metal elements from the positive electrode active material. If the surface of the composite oxide is coated with fluoride instead of oxide, the binding force between the fluoride fluorine and lithium ions is too strong, which inhibits the diffusion of lithium ions and improves the performance as an active material. May fall. Furthermore, there is a possibility that the reaction with the fluorine ions in the electrolyte does not proceed with the fluoride and the effect of preventing the elution reaction of the positive electrode active material cannot be obtained.

  When the metal element constituting the surface oxide is monovalent, the stability as an oxide is inferior. The divalent or higher-valent metal element constituting the surface oxide is not particularly limited, but may contain at least one of Al, Ti, Ge, Y, Zr, Nb, In, Sn, and Ta. preferable. One reason for this is considered to be high stability as an oxide and high binding energy with fluorine.

  It is more preferable that the surface oxide is a composite of the above-described metal element and lithium and is fluorinated. That is, the surface oxide is preferably lithiated and fluorinated. When the surface oxide is lithiated, an effect of increasing the diffusion rate of lithium in the surface oxide or an effect of increasing the amount of diffused lithium can be expected. The ratio of lithium to metal element in the surface oxide need not be stoichiometric.

  The lithiation of the surface oxide means that a surface oxide lithium compound is present. For example, the surface oxide is lithiated by replacing a part of oxygen in the surface oxide with lithium.

  As the metal element constituting the lithiated and fluorinated surface oxide, Nb, Ta, or Ti is preferable. The reason for this is considered that the stability as an oxide and the binding energy with fluorine are high, and that lithiation is easy to some extent.

  The positive electrode active material according to the present invention can be obtained, for example, by the following method.

  The positive electrode active material according to the present invention can be obtained by preparing a composite oxide, providing a surface oxide on the surface thereof, and then fluorinating these. Preferably, the surface oxide is lithiated with or after fluorination. In addition, after fluorinating the surface of the produced composite oxide, it can be obtained by providing a fluorinated surface oxide on the surface. In addition, after fluorinating the surface of the produced composite oxide, the surface oxide can be obtained by providing a surface oxide on the surface and then fluorinating the surface oxide. In the following, a method is described in which a composite oxide is prepared, a surface oxide is provided on the surface thereof, and then these are fluorinated.

  The composite oxide can be obtained by a method similar to a general method for synthesizing inorganic compounds. The composite oxide can be obtained by weighing the raw materials so as to obtain a desired element ratio, mixing them uniformly, and heat-treating them. A crushing or granulating step may be included.

  As the compound as a raw material, a suitable oxide, hydroxide, chloride, nitrate, carbonate or the like of each element can be used. A compound containing two or more elements can also be used as a raw material. The compound as the raw material can be obtained, for example, by making a solution in which a transition metal element such as Mn or Ni is dissolved weakly alkaline and precipitating it as a composite hydroxide. Alternatively, it can be obtained by spray drying a solution in which a metal element as a raw material is dissolved.

  You may repeat the process of mixing of a raw material and heat processing as needed. The mixing conditions and heat treatment conditions at that time can be appropriately selected. In addition, when repeating mixing and heat treatment, raw materials may be added as appropriate so that the desired composition ratio is obtained in the final heat treatment. For example, a composite oxide having a desired composition can be obtained by mixing Mn and Ni raw materials and heat-treating them to form oxides, and adding lithium raw materials thereto and performing heat treatment at a lower temperature.

  The method of providing the surface oxide on the composite oxide is not limited. In the case of the liquid phase method, the composite oxide may be poured into an aqueous solution in which the raw material for the surface oxide is dissolved, and then the pH may be adjusted to precipitate the surface oxide, or the aqueous solution may be sprayed and dried. Also good. Alternatively, the composite oxide can be put into an organic solution in which a metal alkoxide is dissolved and stirred, and the solvent can be removed by evaporation. In the case of the gas phase method, the raw material of the surface oxide may be introduced into the reaction vessel of the fluidized bed into which the composite oxide is charged, and reacted to be deposited. In any method, a desired surface oxide may be obtained by performing oxidation treatment or heat treatment from a state in which a surface oxide that has not been completely reacted is provided.

  The method for the fluorination treatment is not limited. These oxides can be fluorinated by subjecting the composite oxide provided with a surface oxide to heat treatment with ammonium fluoride or acidic ammonium fluoride.

  Further, the surface oxide can be lithiated together with the fluorination of the oxide by heat treatment of the composite oxide and lithium fluoride.

  The form and composition of the positive electrode active material according to the present invention can be known by performing an appropriate pretreatment on the positive electrode active material or the positive electrode using the positive electrode active material and performing instrumental analysis. The positive electrode active material in the battery can be known by disassembling the battery in an inert atmosphere, taking out the positive electrode, performing an appropriate pretreatment, and conducting the same instrumental analysis.

  The positive electrode for analysis is obtained by washing the positive electrode taken out of the battery with an organic solvent having the same component as the electrolytic solution or acetone. Furthermore, the mixture part containing the active material is sampled from the positive electrode, the component derived from the electrolyte on the surface of the binder and the positive electrode active material is removed with an organic solvent such as N-methyl-2-pyrrolidone (NMP), and the solid powder component is removed. Take out. The conductive agent and the positive electrode active material can be easily distinguished by means such as morphological observation by a scanning electron microscope (SEM) and composition analysis by energy dispersive X-ray spectroscopic analysis (EDX).

  The composition of the positive electrode active material, composite oxide and its surface layer, and surface oxide according to the present invention can be determined by X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), X-ray fluorescence (XRF) analysis, or It can also be known by means such as secondary ion mass spectrometry (SIMS).

  In order to know the form and composition of the positive electrode active material of the present invention, it is one of the preferred means that the analysis range of the cross section of the active material is extremely narrowed to the submicron order or less and elemental analysis is performed by AES or SIMS. is there.

  In order to observe the cross section of the active material, there is a method in which the positive electrode active material is embedded in a resin and cut. Alternatively, after the positive electrode active material is hardened, it can be cut, and in some cases, there is a method of cutting in the form of a positive electrode.

  Next, a configuration example of the lithium ion secondary battery according to the present invention will be described. The lithium ion secondary battery according to the present invention includes a positive electrode, a negative electrode, and an electrolytic solution.

  A positive electrode has the positive electrode active material of this invention, for example, is produced in the following procedures. A positive electrode active material and particles of a conductive agent such as carbon black (CB) are mixed, and a solution in which a binder is dissolved is added thereto, mixed and stirred to prepare a positive electrode mixture slurry. The slurry is applied to a positive electrode current collector such as an aluminum foil and dried, and then the aluminum foil is subjected to molding such as pressing or cutting to a desired size to produce a positive electrode. In addition, let the one end part of a positive electrode electrical power collector be an uncoated part which does not apply a positive mix slurry.

  The material of the binder is not particularly limited. A known binder such as a fluorine resin such as polyvinylidene fluoride, a cellulose polymer, a styrene resin, or an acrylic resin can be used. Depending on the type of material, the binder can be dissolved in a solvent such as water or NMP and used as a solution.

  The negative electrode active material used for the negative electrode is not particularly limited. Metal lithium, various carbon materials, metal lithium, lithium titanate, an oxide such as tin or silicon, a metal alloyed with lithium such as tin or silicon, or a composite material of these materials can be used. In order to increase the battery voltage, a carbon material such as graphite, graphitizable carbon, or non-graphitizable carbon having a relatively low potential can be used for the negative electrode.

  When using a powdery negative electrode active material, a negative electrode is produced as follows, for example. A negative electrode active material, a solution in which a binder is dissolved, and a conductive agent such as CB, if necessary, are weighed and mixed so as to obtain a desired mixture composition, thereby preparing a negative electrode mixture slurry. The slurry is applied to a negative electrode current collector such as a copper foil and dried, and then the copper foil is subjected to molding such as pressing or cutting to a desired size to produce a negative electrode. One end of the negative electrode current collector is an uncoated portion where the negative electrode mixture slurry is not applied.

  The electrolytic solution is not particularly limited, and a nonaqueous electrolytic solution in which a lithium salt is dissolved in a nonaqueous solvent, which is used in a conventional lithium ion secondary battery, can be used.

As the lithium salt, LiClO ₄ , LiCF ₃ SO ₃ , LiPF ₆ , LiBF ₄ , or LiAsF ₆ can be used alone or in combination of two or more.

  As the non-aqueous solvent, various cyclic carbonates and chain carbonates can be used. For example, ethylene carbonate, propylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, or the like can be used. Alternatively, a derivative obtained by substituting a part of hydrogen of carbonate with fluorine or the like, which is more resistant to oxidation, can also be used. Furthermore, various additives can be added to the non-aqueous electrolyte as long as the object of the present invention is not hindered. For example, vinylene carbonate can be added to improve battery life, or flame retardancy can be imparted. For this purpose, phosphate esters and the like can be added. As the non-aqueous solvent, an ionic liquid that is a salt that is liquid at room temperature, such as imidazolium / fluorosulfonylimide, can also be used.

  The lithium ion secondary battery of the present invention is produced using the positive electrode, negative electrode, and electrolytic solution described above. The lithium ion secondary battery of the present invention can have a button shape, a cylindrical shape, a square shape, a laminate shape, or the like.

  The cylindrical battery is manufactured as follows, for example. The positive electrode and the negative electrode are cut into strips, and terminals for taking out current are provided in the uncoated part. A separator is sandwiched between the positive electrode and the negative electrode, and this is wound into a cylindrical shape to produce an electrode group, which is then stored in a cylindrical container made of SUS or aluminum. A non-aqueous electrolyte is injected into a container containing the electrode group in dry air or in an inert gas atmosphere, and the container is sealed to produce a cylindrical lithium ion secondary battery.

  As the separator, a porous resin insulating film such as polyethylene, polypropylene, or aramid, or an inorganic compound layer such as alumina provided thereon can be used.

  The square battery is manufactured as follows, for example. In the winding in the above-described cylindrical battery manufacturing procedure, the winding axis is biaxial, and an elliptical electrode group is manufactured. Thereafter, this electrode group is housed in a rectangular container, and is sealed after injecting an electrolytic solution, similarly to a cylindrical battery.

  In cylindrical and prismatic batteries, an electrode group in which a separator, a positive electrode, a separator, a negative electrode, and a separator are stacked in this order can be used instead of winding.

  A laminate-type battery is produced, for example, as follows. The laminated electrode group is stored in a bag-like aluminum laminate sheet lined with an insulating sheet such as polyethylene or polypropylene. With the electrode terminal protruding from the opening, the electrolyte is injected into the bag of the aluminum laminate sheet, and then the opening is sealed.

  The use of the lithium ion secondary battery according to the present invention is not particularly limited. For example, power sources for power sources such as electric vehicles and hybrid electric vehicles, industrial equipment such as elevators equipped with a system that recovers at least a part of kinetic energy, power sources for power storage systems for business use and home use, solar power, It can be used as various large power sources such as a power source for a natural energy power generation system such as wind power. Moreover, it can also be used as various small power sources such as portable devices, information devices, household electric devices, and electric tools.

  Hereinafter, detailed examples of the positive electrode active material for a lithium ion secondary battery according to the present invention will be shown and described in detail. However, the present invention is not limited to the examples described below.

In Example 1, a positive electrode active material 4A in which the composition of the composite oxide was Li _1.06 Mn _1.94 O ₄ and the surface oxide was a fluorinated and lithiated titanium oxide was produced.

(Preparation of positive electrode active material)
Manganese dioxide (MnO ₂ ) and lithium carbonate (Li ₂ CO ₃ ) as raw materials were weighed and wet-mixed with pure water using a planetary pulverizer. After drying the mixture, it was placed in an alumina crucible and baked in an electric furnace at 800 ° C. for 20 hours in an air atmosphere. The fired product was pulverized to obtain a composite oxide.

  Titanium isopropoxide was used as a raw material for the surface oxide. The obtained composite oxide and isopropyl alcohol in which 3% by weight of titanium isopropoxide was dissolved in the composite oxide were added to the flask. These were decompressed while stirring in a 50 ° C. warm bath, and the alcohol was evaporated to dryness. The obtained powder was taken out from the flask and dried in air at 80 ° C.

  This powder obtained by drying was mixed with lithium fluoride and lithium hydroxide. The mixture was put in an alumina crucible and baked in an air atmosphere at 700 ° C. for 5 hours in an electric furnace to obtain a positive electrode active material 4A.

[Comparative Example 1]
As Comparative Example 1, a positive electrode active material 4Z having a composition of Li _1.06 Mn _1.94 O _3.9 F _0.1 and having no surface oxide on the surface of the composite oxide was produced. In addition to MnO ₂ and Li ₂ CO ₃ used in Example 1, lithium fluoride (LiF) was used to obtain a composite oxide in the same manner as in Example 1. This composite oxide was designated as a positive electrode active material 4Z.

(Preparation of positive electrode)
After mixing 6 wt% of CB as a conductive agent with 88 wt% of the positive electrode active material, an NMP solution containing 6 wt% of an acrylic binder as a binder was added and mixed to prepare a positive electrode mixture slurry. This slurry was applied to one side of an aluminum foil (positive electrode current collector). After drying the slurry, the aluminum foil was cut and compression molded with a press, and an aluminum terminal was welded to the uncoated part to produce a positive electrode for battery evaluation. Separately, a positive electrode for analysis and a positive electrode for dissolution test were also produced. The positive electrode for analysis and dissolution test was prepared by punching the aluminum foil after drying the slurry into a 20 mm diameter and then compression molding.

(Preparation of negative electrode)
As a negative electrode material, 92% by weight of artificial graphite and a solution of 8% by weight of PVDF (polyvinylidene fluoride) dissolved in NMP were mixed to prepare a negative electrode mixture slurry. This slurry was applied to one side of a copper foil (negative electrode current collector). After drying the slurry, the copper foil was cut and compression-molded with a press, and a nickel terminal was welded to the uncoated part to produce a negative electrode.

(Elemental analysis of positive electrode active material)
In elemental analysis of the positive electrode active material, a cross section of the positive electrode for analysis is processed by ion milling, and the cross section of the positive electrode active material is measured by AES (electron gun acceleration voltage 10 kV, current 10 nA, beam diameter of about 25 nm). The ratio of the constituent elements of was determined.

  FIG. 1 is a photograph of an SEM image of a cross section of a positive electrode active material, showing an example of a measurement location where the ratio of constituent elements of the positive electrode active material was obtained. The measurement was performed at three locations: the surface oxide (A) present on the outermost surface of the positive electrode active material (the surface of the composite oxide), the surface layer portion (B) of the composite oxide, and the central portion (C) of the composite oxide. .

(Elution test of positive electrode)
In the elution test of the positive electrode, a laminate cell for charging the positive electrode was prepared and the positive electrode for the elution test was charged, and the positive electrode for the elution test after charging was immersed in a high-temperature electrolytic solution and eluted into the electrolytic solution. The amount of metal ions was measured.

  FIG. 2 is a schematic view of the produced positive electrode charging laminate cell. On a current collector foil 11 made of aluminum, a positive electrode 12 for a dissolution test having a diameter of 20 mm, a porous separator 13 made of polypropylene having a thickness of 30 μm, a metal lithium foil 14 and a current collector foil 15 made of copper in this order. Laminated. This laminate was sandwiched between 6 cm square laminate sheets 16 lined with polypropylene, and the current collector foils 11 and 15 protruded from the laminate sheet 16 so that the three sides of the laminate sheet 16 were sealed. After injecting a non-aqueous electrolyte into the bag-shaped laminate sheet 16 and impregnating the electrolyte with the electrode and the separator by reducing the pressure, the bottom side (one side not sealed) of the laminate sheet 16 is sealed, A laminate cell for charging the positive electrode was produced.

  For this laminate cell, constant current and constant voltage charge with a charge / discharge current of 0.2 CA in time, a charge upper limit voltage of 4.3 V and a total charge time of 6 hours, and a constant current discharge with a discharge lower limit voltage of 3.5 V After repeating the above three times, constant current and constant voltage charging was performed with a charging / discharging current of 0.2 CA, an upper limit voltage of 4.3 V, and a total charging time of 6 hours.

Thereafter, the cell was disassembled, and the charged positive electrode was taken out. The taken-out positive electrode and 5 cm ³ of the electrolytic solution were sealed in a Teflon (registered trademark) sealed container and stored in an environment of 50 ° C. for 7 days. The electrolytic solution after storage was analyzed by high frequency induction plasma spectroscopy (ICP), the concentration of metal elements excluding Li constituting the positive electrode active material was measured, and the total amount (mol) of metal elements in the electrolytic solution was determined. And the elution amount (mol / g) of the metal element per weight (g) of the positive electrode active material was determined.

As the electrolytic solution, a nonaqueous electrolytic solution in which 1 mol / dm ^{3 of} lithium hexafluorophosphate as a lithium salt was dissolved in a nonaqueous mixed solvent in which ethylene carbonate and dimethyl carbonate were mixed at a volume ratio of 3: 7 was used.

(Production of battery)
FIG. 3 is a schematic diagram of the manufactured laminate-type lithium ion secondary battery. A positive electrode 22 having a coating portion of 40 mm × 27 mm, a polypropylene porous separator 23 having a thickness of 30 μm, and a negative electrode 27 having a coating portion of 42 mm × 30 mm were laminated in this order. The laminate is sandwiched between 7 cm square laminate sheets 26 lined with polypropylene, and a negative electrode terminal 28 made of nickel and a positive electrode terminal 29 made of aluminum protrude from the laminate sheet 26 so that the bottom side of the laminate sheet 26 (terminal 28 , 29 except for the side opposite to the side from which 29 protrudes. An electrolyte solution was poured into the bag-shaped laminate sheet 26, and the electrode and the separator were impregnated with the electrolyte solution under reduced pressure, and then the bottom side was sealed to produce a laminate type battery. As the batteries, a battery using the positive electrode active material 4A of Example 1 and a battery using the positive electrode active material 4Z of Comparative Example 1 were produced.

As the electrolytic solution, a nonaqueous electrolytic solution in which 1 mol / dm ^{3 of} lithium hexafluorophosphate as a lithium salt was dissolved in a nonaqueous mixed solvent in which ethylene carbonate and dimethyl carbonate were mixed at a volume ratio of 3: 7 was used.

(Charge / discharge test and high temperature life test)
The prepared battery was subjected to a charge / discharge test and a high temperature life test.

  The charging conditions of the charging / discharging test are constant current and constant voltage charging, in which the charging current is a time rate of 0.2 CA, the charging upper limit voltage is 4.2 V, and the total charging time is 6 hours. The discharge conditions are constant current discharge with a discharge current of 0.2 CA and a discharge lower limit voltage of 3V. This charge / discharge cycle was repeated five times, and the discharge capacity at the fifth cycle was determined as the battery capacity. Next, as a rate test, after charging under this charging condition, a constant current discharge with a discharge current of 1 CA and a discharge lower limit voltage of 3 V was performed, the battery capacity was measured, and the ratio to the battery capacity after discharge at 0.2 CA was determined. The volume ratio was obtained.

  Subsequently, a high temperature life test was conducted. After charging under the above charging conditions, the battery was stored in an environment of 50 ° C. for 7 days. Then, after 3V constant current discharge at 0.2CA at room temperature, charge and discharge are performed in one cycle under the same conditions as the measurement of the battery capacity. The discharge capacity at this time is defined as the battery capacity after the high temperature life test, and the high temperature life test. The ratio with the previous battery capacity was determined as the maintenance rate. If the maintenance rate is high, the high temperature life is prolonged.

  FIG. 4 is a diagram showing the results of elemental analysis of the positive electrode active material in Example 1 and Comparative Example 1, in the surface oxide of the positive electrode active material, the surface layer portion of the composite oxide, and the central portion of the composite oxide. It is a figure which shows an element ratio (mol%).

  In the positive electrode active material 4A of Example 1, fluorine and lithium were detected from the surface oxide containing titanium. Further, although fluorine was detected from the surface layer portion of the composite oxide, fluorine was not detected from the central portion. Therefore, it was confirmed that the positive electrode active material 4A was a positive electrode active material according to the present invention.

  On the other hand, the positive electrode active material 4Z of Comparative Example 1 did not have a surface oxide, and fluorine was detected from both the surface layer portion and the central portion of the composite oxide, confirming that it was not the positive electrode active material according to the present invention. .

  Table 1 shows the amount of elution of metal elements of the positive electrode active material 4A of Example 1 and the positive electrode active material 4Z of Comparative Example 1, and the battery capacity and 1CA of a lithium ion secondary battery produced using these positive electrode active materials. The capacity ratio during discharge and the maintenance rate after the high-temperature life test are shown.

  The positive electrode active material 4A of Example 1 has a smaller amount of metal element elution than the positive electrode active material 4Z of Comparative Example 1. As a result, the battery of Example 1 manufactured using the positive electrode active material 4A had an effect that the maintenance rate was higher than that of the battery of Comparative Example 1 manufactured using the positive electrode active material 4Z. Moreover, the battery of Example 1 had the effect that the battery capacity was high compared with the battery of Comparative Example 1. The reason is considered that the positive electrode active material 4Z is entirely fluorinated while the positive electrode active material 4A is not fluorinated at the center.

In Example 2, a positive electrode active material 5N having a composite oxide composition of LiNi _0.46 Mn _1.54 O ₄ , a positive electrode active material 5T having LiNi _0.45 Mn _1.35 Ti _0.2 O ₄ , and Was made. The surface oxide is a fluorinated and lithiated niobium oxide for both positive electrode active materials.

A method for producing the positive electrode active material 5N and the positive electrode active material 5T will be described. First, a raw material for a composite oxide of metal elements excluding lithium was prepared. For the positive electrode active material 5N, MnO ₂ and nickel oxide (NiO) were weighed and mixed. For the positive electrode active material 5T, MnO ₂ , nickel oxide, and titanium oxide (TiO ₂ ) were weighed and mixed. Each of these mixtures was placed in an alumina crucible and fired in an electric furnace at 1000 ° C. for 12 hours in an air atmosphere to obtain a composite oxide material of the positive electrode active material 5N and the positive electrode active material 5T. The raw materials for each composite oxide and Li ₂ CO ₃ were weighed and mixed, and calcined at 800 ° C. for 20 hours in an air atmosphere. The fired product was pulverized to obtain a composite oxide of the positive electrode active material 5N and the positive electrode active material 5T.

  Pentaethoxyniobium was used as a raw material for the surface oxide. 2 wt% of pentaethoxyniobium was dissolved in ethyl alcohol with respect to each of the obtained composite oxides. Hereinafter, a positive electrode active material 5N and a positive electrode active material 5T were produced in the same manner as in Example 1 using the obtained composite oxides and ethyl alcohol in which pentaethoxyniobium was dissolved.

[Comparative Example 2]
As Comparative Example 2, three types of positive electrode active materials having no surface oxide on the surface of the composite oxide were prepared.

A composite oxide having the same composition as LiNi _0.46 Mn _1.54 O ₄ as the positive electrode active material 5N was produced in the same manner as in Example 2, and this was used as the positive electrode active material ZN.

A composite oxide having a composition of LiNi _0.46 Mn _1.54 O _3.9 F _0.1 was prepared in the same manner as in Example 2 using LiF as a raw material, and this was used as a positive electrode active material FN.

A composite oxide having the same composition LiNi _0.45 Mn _1.35 Ti _0.2 O ₄ as the positive electrode active material 5T was produced in the same manner as in Example 2, and this was used as the positive electrode active material ZT.

(Elution test of positive electrode)
The elution test of the positive electrode was performed in the same manner as in Example 1 except that the charge upper limit voltage was set to 4.9V.

(Production of battery)
Using the produced positive electrode active materials of Example 2 and Comparative Example 2, laminate type lithium ion secondary batteries were produced in the same manner as in Example 1.

(Charge / discharge test and high temperature life test)
The charge / discharge test and the high-temperature life test were performed in the same manner as in Example 1 except that the upper limit voltage of charge was 4.8V.

  5 and 6 are diagrams showing the results of elemental analysis of the positive electrode active material in Example 2 and Comparative Example 2, and the surface oxide of the positive electrode active material, the surface layer portion of the composite oxide, and the center of the composite oxide It is a figure which shows the element ratio (mol%) in a part. FIG. 5 shows the results of the positive electrode active material 5N, the positive electrode active material ZN, and the positive electrode active material FN, and FIG. 6 shows the results of the positive electrode active material 5T and the positive electrode active material ZT.

  In the positive electrode active material 5N and the positive electrode active material 5T of Example 2, fluorine and lithium were detected from the surface oxide containing niobium. Further, although fluorine was detected from the surface layer portion of the composite oxide, fluorine was not detected from the central portion. Therefore, it was confirmed that the positive electrode active material 5N and the positive electrode active material 5T are positive electrode active materials according to the present invention.

  On the other hand, the positive electrode active material ZN, the positive electrode active material FN, and the positive electrode active material ZT of Comparative Example 2 have no surface oxide, and no fluorine is detected from the positive electrode active material ZN and the positive electrode active material ZT. In the active material FN, fluorine was detected from both the surface layer portion and the central portion of the composite oxide. Therefore, it was confirmed that the positive electrode active material ZN, the positive electrode active material FN, and the positive electrode active material ZT were not positive electrode active materials according to the present invention.

  Table 2 shows the amount of elution of metal elements of the positive electrode active material 5N and the positive electrode active material 5T of Example 2 and the positive electrode active material ZN, the positive electrode active material FN, and the positive electrode active material ZT of Comparative Example 2, and these The battery capacity of the lithium ion secondary battery produced using a positive electrode active material, the capacity ratio at the time of 1CA discharge, and the maintenance factor after a high temperature life test are shown.

  The positive electrode active material 5N and the positive electrode active material 5T of Example 2 have less metal element elution than the positive electrode active material FN with the smallest elution amount in Comparative Example 2. As a result, the battery of Example 2 manufactured using the positive electrode active materials 5N and 5T has an effect that the maintenance rate is higher than the battery of Comparative Example 2 manufactured using the positive electrode active materials ZN, FN, and ZT. Obtained.

  Moreover, the battery of Example 2 produced using the positive electrode active materials 5N and 5T had the effect that the battery capacity was high compared with the battery of the comparative example 2 produced using the positive electrode active material FN. The reason is considered that the positive electrode active material FN is entirely fluorinated, whereas the positive electrode active materials 5N and 5T are not fluorinated at the center. In addition, the battery of Example 2 manufactured using the positive electrode active materials 5N and 5T has a surface oxide that does not exhibit capacity compared to the battery of Comparative Example 2 manufactured using the positive electrode active materials ZN and ZT. Although the battery capacity is slightly small because it exists in the substance, the capacity ratio is about the same, and the effect that the maintenance ratio is high is obtained.

In Example 3, a positive electrode active material 5G having a composite oxide composition of LiNi _0.45 Mn _1.45 Ge _0.1 O ₄ was produced. The surface oxide is fluorinated aluminum oxide.

A method for producing the positive electrode active material 5G will be described. The method for producing the positive electrode active material 5G is the same as the method for producing the positive electrode active material 5N and the positive electrode active material 5T in Example 2. First, MnO ₂ , nickel oxide, and germanium oxide (GeO ₂ ) were weighed and mixed. This mixture was put in an alumina crucible and fired in an electric furnace at 1000 ° C. for 12 hours in an air atmosphere to obtain a composite oxide material of the positive electrode active material 5G. This composite oxide raw material and Li ₂ CO ₃ were weighed and mixed, and calcined at 800 ° C. for 20 hours in an air atmosphere. The fired product was pulverized to obtain a composite oxide of the positive electrode active material 5G.

  Aluminum triisopropoxide was used as a raw material for the surface oxide. Using the obtained composite oxide and 1.5% by weight of aluminum triisopropoxide based on the composite oxide, a powder dried by the same procedure as in Example 1 was obtained.

  This powder obtained by drying was mixed with lithium fluoride and lithium hydroxide. The mixture was placed in an alumina crucible and fired in an electric furnace at 700 ° C. for 5 hours in an air atmosphere. The fired product was washed with distilled water and dried, and then heat treated again at 400 ° C. for 12 hours in an inert atmosphere to obtain a positive electrode active material 5G.

[Comparative Example 3]
As Comparative Example 3, a positive electrode active material ZG in which the surface of the composite oxide having the same composition as in Example 3 was coated with aluminum fluoride was produced. That is, the positive electrode active material ZG has a coating portion of aluminum fluoride on the surface of the composite oxide.

  An aluminum nitrate aqueous solution and a composite oxide produced in the same manner as in Example 3 were added to a beaker, and an ammonium fluoride aqueous solution was gradually added while stirring them at a temperature of 80 ° C. Then, after stirring at 80 degreeC for 24 hours, it filtered and the powdery material was obtained. This powder was washed with distilled water and dried, and then heat-treated at 400 ° C. in an inert atmosphere to prepare a positive electrode active material ZG.

(Elemental analysis of positive electrode active material)
Only in the elemental analysis of the surface oxide of Example 3 and the coating part of Comparative Example 3, since the thickness of both was thin, the element ratio was measured by AES from the active material surface.

(Production of battery)
Using the produced positive electrode active materials of Example 3 and Comparative Example 3, laminate type lithium ion secondary batteries were produced in the same manner as in Example 2.

(Charge / discharge test and high temperature life test)
The charge / discharge test and the high-temperature life test were performed in the same manner as in Example 2.

  FIG. 7 is a diagram showing the results of elemental analysis of the positive electrode active material in Example 3 and Comparative Example 3, in which the surface oxide of the positive electrode active material (coating portion in Comparative Example 3), the surface layer portion of the composite oxide, It is a figure which shows the element ratio (mol%) in the center part of complex oxide.

  In the positive electrode active material 5G of Example 3, fluorine was detected from the surface oxide containing aluminum, and lithium was hardly detected. Further, although fluorine was detected from the surface layer portion of the composite oxide, fluorine was not detected from the central portion. Therefore, it was confirmed that the positive electrode active material 5G was a positive electrode active material according to the present invention.

  On the other hand, in the positive electrode active material ZG of Comparative Example 3, oxygen was detected in impurities from the coating portion, but the coating portion was composed of aluminum and fluorine, and there was no surface oxide, so that the positive electrode active material according to the present invention was used. It was confirmed that it was not a substance.

  Table 3 shows the amount of elution of metal elements of the positive electrode active material 5G of Example 3 and the positive electrode active material ZG of Comparative Example 3, and the battery capacity, 1CA of a lithium ion secondary battery produced using these positive electrode active materials The capacity ratio during discharge and the maintenance rate after the high-temperature life test are shown.

  The positive electrode active material 5G of Example 3 has less metal element elution than the positive electrode active material ZG of Comparative Example 3. As a result, the battery of Example 3 manufactured using the positive electrode active material 5G had an effect that the maintenance rate was higher than that of the battery of Comparative Example 3 manufactured using the positive electrode active material ZG. In addition, the battery of Example 3 had an effect that both the battery capacity and the capacity ratio were higher than those of Comparative Example 3. This is presumably because the surface oxide of the positive electrode active material 5G is superior in lithium ion diffusibility to the coating portion of the positive electrode active material ZG.

In Example 4, a positive electrode active material 5M having a composite oxide composition of LiNi _0.45 Mn _1.5 Mg _0.05 O ₄ and a positive electrode having LiNi _0.4 Mn _1.4 Fe _0.2 O ₄ Active material 5E was produced. The surface oxide is a fluorinated and lithiated titanium oxide for both positive electrode active materials.

A method for producing the positive electrode active material 5M and the positive electrode active material 5E will be described. For the positive electrode active material 5M, MnO ₂ , nickel oxide, and magnesium oxide (MgO) are used, and for the positive electrode active material 5E, MnO ₂ , nickel oxide, and iron oxide (Fe ₂ O ₃ ) are used. The raw material of each composite oxide was obtained by the same method as that described above, and each composite oxide was obtained by the same method as in Example 2.

  Using these composite oxides, a surface oxide was provided on the composite oxide in the same manner as in Example 1 to produce a positive electrode active material 5M and a positive electrode active material 5E.

[Comparative Example 4]
As Comparative Example 4, the same composite oxide as in Example 4 was used, and two types of positive electrode active materials that were fluorinated and lithiated without providing a surface oxide on the surface of the composite oxide were produced.

  A composite oxide having the same composition as that of the positive electrode active material 5M of Example 4, lithium fluoride, and lithium hydroxide were mixed. The mixture was put in an alumina crucible and fired at 700 ° C. for 5 hours in an air atmosphere to obtain a positive electrode active material ZM.

  Similarly, a positive electrode active material ZE was obtained using the same composite oxide as the positive electrode active material 5E of Example 4.

(Elution test of positive electrode)
The elution test of the positive electrode was performed in the same manner as in Example 2.

(Production of battery)
Using the produced positive electrode active materials of Example 4 and Comparative Example 4, laminate type lithium ion secondary batteries were produced in the same manner as in Example 2.

(Charge / discharge test and high temperature life test)
The charge / discharge test and the high-temperature life test were performed in the same manner as in Example 2.

  8 and 9 are diagrams showing the results of elemental analysis of the positive electrode active material in Example 4 and Comparative Example 4, and are the surface oxide of the positive electrode active material, the surface layer portion of the composite oxide, and the center of the composite oxide. It is a figure which shows the element ratio (mol%) in a part. FIG. 8 shows the results of the positive electrode active material 5M and the positive electrode active material ZM, and FIG. 9 shows the results of the positive electrode active material 5E and the positive electrode active material ZE, respectively.

  In the positive electrode active material 5M and the positive electrode active material 5E of Example 4, fluorine and lithium were detected from the surface oxide containing titanium. Further, although fluorine was detected from the surface layer portion of the composite oxide, fluorine was not detected from the central portion. Therefore, it was confirmed that the positive electrode active material 5M and the positive electrode active material 5E are positive electrode active materials according to the present invention.

  In FIG. 8, the magnesium plot is not drawn so as to overlap with the plots of other elements, but in the positive electrode active material 5M of Example 4, magnesium is detected from the surface layer portion and the central portion of the composite oxide. Magnesium was not detected from the surface oxide.

  On the other hand, in the positive electrode active material ZM and the positive electrode active material ZE of Comparative Example 4, fluorine was detected from the surface layer portion of the composite oxide, and fluorine was not detected from the center portion. However, the positive electrode active material ZM and the positive electrode active material ZE do not have a surface oxide. Therefore, it was confirmed that the positive electrode active material ZM and the positive electrode active material ZE were not the positive electrode active material according to the present invention.

  Table 4 uses the positive electrode active material 5M and positive electrode active material 5E of Example 4 and the amount of elution of metal elements of the positive electrode active material ZM and positive electrode active material ZE of Comparative Example 4, and these positive electrode active materials. The battery capacity of the lithium ion secondary battery produced in this way, the capacity ratio during 1 CA discharge, and the maintenance rate after the high-temperature life test are shown.

  The positive electrode active material 5M and the positive electrode active material 5E of Example 4 both have a smaller amount of metal element elution than the positive electrode active material ZM and the positive electrode active material ZE of Comparative Example 4. As a result, the battery of Example 4 manufactured using the positive electrode active materials 5M and 5E has an effect that the maintenance rate is higher than the battery of Comparative Example 4 manufactured using the positive electrode active materials ZM and ZE. It was. Moreover, the battery of Example 4 had the effect that the battery capacity was comparable or slightly higher than the battery of Comparative Example 4, and the capacity ratio was slightly higher.

In Example 5, the positive electrode active material 5C having a composite oxide composition of LiNi _0.4 Mn _1.4 Co _0.2 O ₄ and the positive electrode having LiNi _0.4 Mn _1.5 Cu _0.1 O ₄ An active material 5U was prepared. The surface oxide is fluorinated and lithiated tantalum oxide for both positive electrode active materials.

A method for producing the positive electrode active material 5C and the positive electrode active material 5U will be described. For positive electrode active material 5C, MnO ₂ , nickel oxide, and cobalt oxide (Co ₂ O ₃ ) are used, and for positive electrode active material 5U, MnO ₂ , nickel oxide, and copper oxide (CuO) are used. The raw material of each composite oxide was obtained by the same method as that described above, and each composite oxide was obtained by the same method as in Example 2.

  Pentaethoxytantalum was used as a raw material for the surface oxide. Using the obtained composite oxide and 2 wt% pentaethoxytantalum with respect to the composite oxide, a surface oxide was provided on the composite oxide in the same manner as in Example 2, and fluorination treatment and lithiation treatment were performed. The positive electrode active material 5C and the positive electrode active material 5U were produced.

[Comparative Example 5]
As Comparative Example 5, the same composite oxide as in Example 5 was used, and the same surface oxide of tantalum oxide as in Example 5 was provided on the surface of the composite oxide, but the surface oxide was subjected to fluorination treatment and lithiation treatment. Two types of positive electrode active materials not subjected to the above were prepared.

  The same composite oxide as that of the positive electrode active material 5C of Example 5 was produced, and a treatment for providing a surface oxide was performed in the same manner as in Example 5 (the fluorination treatment and lithiation treatment were not performed) to produce the positive electrode active material ZC did.

  Similarly, a positive electrode active material ZU was produced using the same composite oxide as the positive electrode active material 5U of Example 5.

(Elution test of positive electrode)
The elution test of the positive electrode was performed in the same manner as in Example 2 except that the upper limit voltage for charging was set to 5.0V.

(Production of battery)
Using the produced positive electrode active materials of Example 5 and Comparative Example 5, laminated lithium ion secondary batteries were produced in the same manner as in Example 2.
(Charge / discharge test and high temperature life test)
The charge / discharge test and the high-temperature life test were performed in the same manner as in Example 2 except that the upper limit charge voltage was 4.9V.

  10 and 11 are diagrams showing the results of elemental analysis of the positive electrode active material in Example 5 and Comparative Example 5, and the surface oxide of the positive electrode active material, the surface layer portion of the composite oxide, and the center of the composite oxide It is a figure which shows the element ratio (mol%) in a part. FIG. 10 shows the results of the positive electrode active material 5C and the positive electrode active material ZC, and FIG. 11 shows the results of the positive electrode active material 5U and the positive electrode active material ZU.

  In the positive electrode active material 5C and the positive electrode active material 5U of Example 5, fluorine and lithium were detected from the surface oxide containing tantalum. Further, although fluorine was detected from the surface layer portion of the composite oxide, fluorine was not detected from the central portion. Therefore, it was confirmed that the positive electrode active material 5C and the positive electrode active material 5U were positive electrode active materials according to the present invention.

  On the other hand, in the positive electrode active material ZC and the positive electrode active material ZU of Comparative Example 5, no fluorine was detected in any of the surface oxide containing tantalum and the surface layer portion of the composite oxide. Therefore, it was confirmed that the positive electrode active material ZC and the positive electrode active material ZU were not the positive electrode active material according to the present invention.

  In Table 5, the amount of elution of metal elements of the positive electrode active material 5C and the positive electrode active material 5U of Example 5 and the positive electrode active material ZC and the positive electrode active material ZU of Comparative Example 5 and these positive electrode active materials are used. The battery capacity of the lithium ion secondary battery produced in this way, the capacity ratio during 1 CA discharge, and the maintenance rate after the high-temperature life test are shown.

  Compared with the positive electrode active material ZC and the positive electrode active material ZU of Comparative Example 5, both the positive electrode active material 5C and the positive electrode active material 5U of Example 5 have a smaller amount of metal element elution. As a result, the battery of Example 5 manufactured using the positive electrode active materials 5C and 5U has an effect that the maintenance rate is higher than the battery of Comparative Example 5 manufactured using the positive electrode active materials ZC and ZU. It was. Moreover, the battery of Example 5 had the effect that the battery capacity and the capacity ratio were comparable or slightly higher than those of the battery of Comparative Example 5.

  As shown in the above examples, according to the present invention, it is possible to provide a positive electrode active material capable of expressing a high potential and suppressing elution of a metal element, and a lithium ion secondary battery excellent in capacity and high-temperature life. Can do.

  DESCRIPTION OF SYMBOLS 11 ... Current collecting foil made of aluminum, 12 ... Positive electrode for dissolution test, 13 ... Porous separator, 14 ... Metal lithium foil, 15 ... Current collecting foil made of copper, 16 ... Laminate sheet, 22 ... Positive electrode, 23 ... Porous Separator, 26 ... laminate sheet, 27 ... negative electrode, 28 ... negative electrode terminal, 29 ... positive electrode terminal.

Claims (7)

A composite oxide of Li and a transition metal containing at least Mn;
And a surface oxide that is an oxide having a bivalent or higher valent metal element,
The composite oxide has a fluorinated surface portion, a fluorinated center portion,
The surface oxide exists on the surface of the composite oxide and is fluorinated.
A positive electrode active material for a lithium ion secondary battery.   The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the composite oxide is a spinel composite oxide containing Mn. The composite oxide has a general formula Li _a Ni _x Mn _y MzO _{4 + α} (M is at least one of Ti, Ge, Mg, Fe, Co, and Cu, 0.3 ≦ x ≦ 0.55, 1 .2 ≦ y ≦ 1.6, 0 ≦ Z ≦ 0.4, 1.9 ≦ x + y + z ≦ 2.05, 0.95 ≦ a ≦ 1.1, −0.2 ≦ α ≦ 0.1) The positive electrode active material for lithium ion secondary batteries according to claim 2.   The lithium ion secondary battery according to claim 2, wherein the surface oxide includes at least one of Al, Ti, Ge, Y, Zr, Nb, In, Sn, and Ta as the metal element. Positive electrode active material.   The positive electrode active material for a lithium ion secondary battery according to claim 4, wherein the surface oxide is lithiated.   The positive electrode active material for a lithium ion secondary battery according to claim 2, wherein the surface oxide includes at least one of Nb, Ta, and Ti as the metal element and is lithiated. A positive electrode, a negative electrode, and an electrolytic solution;
The said positive electrode has a positive electrode active material for lithium ion secondary batteries of any one of Claims 1-6 as a positive electrode active material, The lithium ion secondary battery characterized by the above-mentioned.

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