JP2008218126A - Method of evaluating positive electrode active material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method capable of quickly quantifying and evaluating the coating status of a surface-coated positive electrode active material at high precision. <P>SOLUTION: The method of evaluating the positive electrode active material includes a surface treatment process of dissolving the surface of a positive electrode active material whose component density changes from the surface into the inside into an acid solution and a computation process of analyzing elements in the acid solution into which the surface of the positive electrode active material is dissolved and computing the coverage from the results of analysis. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for evaluating a positive electrode active material. More specifically, the present invention relates to a method for evaluating a positive electrode active material that can easily quantify a surface state in a short time and perform highly accurate evaluation.

  With the increase in functionality and performance of portable devices, the power consumption of portable devices is increasing, and a further increase in capacity is required for batteries serving as power sources. In particular, a secondary battery having a high energy density is strongly demanded from the viewpoint of economy and reduction in size and weight of portable devices. As typical secondary batteries, lead storage batteries, alkaline storage batteries, lithium ion secondary batteries, and the like are known.

  Among secondary batteries, lithium ion secondary batteries in particular have advantages such as high output and high energy density. The lithium ion secondary battery includes, for example, a positive electrode capable of reversibly inserting and removing lithium ions, a negative electrode, a nonaqueous electrolytic solution or a gel electrolyte, and a solid electrolyte.

In addition to lithium cobaltate (LiCoO ₂ ) capable of intercalating and deintercalating lithium ions, lithium nickelate (LiNiO ₂ ) or these lithium-containing transitions can be used as positive electrode materials for lithium ion secondary batteries. A composite oxide in which a metal element is partially substituted for a metal compound is used. In addition, lithium manganate (LiMn ₂ O ₄ ) having a spinel structure has high energy density and high voltage, and thus is often used and is being developed as an inexpensive material having high voltage.

  Usually, in a lithium ion secondary battery, a lithium cobaltate is used for the positive electrode and a carbon material is used for the negative electrode, and the operating voltage is used in the range of 4.2V to 2.5V. In the unit cell, the terminal voltage can be increased to 4.2 V largely due to excellent electrochemical stability such as a non-aqueous electrolyte material or a separator.

  At present, in a lithium ion secondary battery operating at a maximum of 4.2 V, the positive electrode active material such as lithium cobaltate used in the lithium ion secondary battery uses only about 60% of its theoretical capacity. For lithium ion secondary batteries, in addition to cost reduction, higher energy density, higher reliability, and longer life are desired.

  As a method for improving these characteristics, particularly the energy density of the battery, there is a method of setting a high upper limit voltage for charging. When the charging voltage is increased, more lithium is intercalated / deintercalated from the lithium composite oxide, which is the positive electrode active material, so that the capacity can be increased.

  In principle, it is possible to utilize the remaining capacity by increasing the charging voltage. Actually, for example, as disclosed in Patent Document 1, it is known that high energy density can be realized by setting the voltage during charging to 4.25 V or more.

International Publication No. 03/019713 Pamphlet

By the way, in a secondary battery with a high charging voltage, Li _x CoO ₂ (0 <x ≦ 1.0), Li _x NiO ₂ (0 <x ≦ 1.0), etc. have high potential, stability, and long life. Therefore, it is most promising as a positive electrode active material used for a battery. In particular, a positive electrode active material mainly composed of LiCoO ₂ is a positive electrode active material exhibiting a high potential, and it is expected that the use of this positive electrode active material for a battery increases the charging voltage and the energy density. In a battery with a high charging voltage, there are problems that the charge / discharge cycle life is reduced and the high temperature characteristics are deteriorated.

In addition, lithium nickel composite oxide represented by LiNiO ₂ and lithium nickel composite oxide in which a part of nickel (Ni) is substituted with cobalt (Co) or manganese (Mn) are more stable at a higher potential than LiCoO _2. However, it is disadvantageous for increasing the energy density by lowering the discharge potential and lowering the volume density compared to LiCoO ₂ .

Therefore, in order to stabilize the active material mainly composed of LiCoO ₂ , different elements such as aluminum (Al), magnesium (Mg), zirconium (Zr), and titanium (Ti) are dissolved (for example, patent documents). 2), use a small amount of LiMn _1/3 Co _1/3 Ni _1/3 O _{2 and the} like, and coat the LiCoO ₂ surface with spinel lithium manganate, lithium spinel titanate, nickel cobalt composite oxide Have been proposed (see, for example, Patent Document 3 and Patent Document 4).

JP 2004-30391 A

JP 2000-164214 A JP 2002-151078 A

  In addition, it has been proposed to stabilize the active material by treatment with a halogen element such as fluorine (F) or sulfur (S) or a chalcogen element instead of substitution or coating with a metal element. (For example, see Patent Documents 5 to 7)

Japanese Patent No. 3141858 Japanese Patent No. 3157413 JP 2005-11688 A

  As a means for quantitatively determining the surface composition of the surface-coated positive electrode active material or the degree of coating, conventionally, photoelectrons emitted by the photoelectric effect using XPS (X-ray Photoelectron Spectroscopy) are detected, By analyzing the energy of the photoelectrons, a method of measuring the coverage of the conductive polymer material to the composite oxide on the surface of the electrode layer per occupied area (for example, see Patent Document 8) By chromaticity measurement in the Lab method, A method for evaluating the state of the surface coating (see, for example, Patent Document 9) has been proposed.

JP 2002-8638 A

JP 2002-50355 A

  However, the method using XPS (X-ray Photoelectron Spectroscopy) as described in Patent Document 8 requires a large-scale apparatus and the measurement itself takes a long time. It is unsuitable for targeting samples. There is also a method of observing the composition of the powder surface by EDX (Energy Dispersive X-ray Analysis), but as a means for quantifying the coating state, only the information on the outermost surface is obtained, so the information is insufficient.

  Accordingly, an object of the present invention is to provide a method for evaluating a positive electrode active material, which can easily quantify the coating state of the surface-coated positive electrode active material in a short time and can perform highly accurate evaluation.

In order to solve the above-described problems, the present invention provides:
A surface treatment step of dissolving the surface of the positive electrode active material in an acid solution;
A method for evaluating a positive electrode active material, comprising: an elemental analysis on an acid solution in which the surface of the positive electrode active material is dissolved by a surface treatment step, and a calculation step of calculating a coverage from the measurement result. .

  In this invention, since the surface of the positive electrode active material is dissolved in an acid solution and elemental analysis is performed, and the coverage is calculated from the result, the surface state of the positive electrode active material can be easily quantified in a short time. Then, by classifying the positive electrode active material based on this coverage, highly accurate evaluation is possible.

  According to the present invention, the positive electrode active material can be easily quantified by the coverage in a short time, and the positive electrode active material can be evaluated with high accuracy by classifying the positive electrode active material based on the coverage. .

  First, a method for evaluating a positive electrode active material according to an embodiment of the present invention will be described. The positive electrode active material evaluation method according to one embodiment of the present invention includes a surface treatment step in which the surface of the positive electrode active material is dissolved in an acid solution, and an acid solution in which the surface of the positive electrode active material is dissolved in the surface treatment step. And a calculation step of performing elemental analysis and calculating the coverage from the measurement result. Hereinafter, the surface treatment process and the calculation process will be described in detail.

  The surface treatment step is performed, for example, by dissolving the surface of the positive electrode active material having a change in the concentration of elements constituting from the surface toward the inside in the acid solution. Specifically, for example, at least part of a lithium composite oxide containing lithium (Li) and one or more transition metal elements M1 as main elements is made of an oxide containing one or more metal elements M2. The surface of the positive electrode active material provided with the coating layer is dissolved in an acid solution at each dissolution time (n1 to nK: K is an integer of 2 or more).

Here, examples of the transition metal group element M1 include a 3d transition metal element and a 4d transition metal element. More specifically, for example, cobalt (Co), nickel (Ni), manganese (Mn), iron (Fe ) And the like. Examples of the lithium composite oxide containing lithium (Li) and the transition metal element M1 as main elements include polyoxides such as αNaFe type oxide, spinel type oxide, and olivine type oxide. For example, cobalt Lithium oxide (LiCoO ₂ ), lithium nickelate (LiNO ₂ ), lithium manganate (LiMn ₂ O ₄ ), lithium iron phosphate (LiFePO ₄ ), etc. Examples thereof include lithium composite oxides whose average composition is represented by Chemical Formula 3.

(Chemical formula 1)
Li _{(1 + p)} Co _(1-q) M _q O _(2-y) X _z
(However, in chemical formula 1, M is magnesium (Mg), aluminum (Al), boron (B), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), nickel. (Ni), copper (Cu), zinc (Zn), molybdenum (Mo), tin (Sn), tungsten (W), zirconium (Zr), yttrium (Y), niobium (Nb), calcium (Ca) and strontium At least one member selected from the group consisting of (Sr), and p, q, y, and z are −0.10 ≦ p ≦ 0.10, 0 ≦ q <0.3, and −0.10 ≦ y ≦. 0.20 and 0 ≦ z ≦ 0.1.)
(Chemical formula 2)
Li _{(1 + p)} Ni _(1-q) M _q O _(2-y) X _z
(However, in chemical formula 2, M is magnesium (Mg), aluminum (Al), boron (B), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), copper (Cu), zinc (Zn), molybdenum (Mo), tin (Sn), tungsten (W), zirconium (Zr), yttrium (Y), niobium (Nb), calcium (Ca) and strontium At least one member selected from the group consisting of (Sr), and p, q, y, and z are −0.10 ≦ p ≦ 0.10, 0 ≦ q <0.3, and −0.10 ≦ y ≦. 0.20 and 0 ≦ z ≦ 0.1.)
(Chemical formula 3)
Li _a M _b PO ₄
(However, in Chemical Formula 3, M represents at least one element selected from Groups 2 to 15. a and b are 0 ≦ a ≦ 2.0 and 0.5 ≦ b ≦ 2.0. It is a value within the range.)

  Examples of the metal element M2 contained in the oxide of the coating layer include nickel (Ni), manganese (Mn), magnesium (Mg), aluminum (Al), boron (B), cobalt (Co), and titanium (Ti). , Vanadium (V), chromium (Cr), iron (Fe), copper (Cu), zinc (Zn), molybdenum (Mo), tin (Sn), calcium (Ca), strontium (Sr), tungsten (W) , Yttrium (Y), zirconium (Zr) and the like.

Different lithium composite oxides provided with a coating layer made of an oxide containing at least one metal element M2 are converted into acid solutions such as hydrochloric acid (HCl) solution, nitric acid (HNO ₃ ) solution, etc. The surface treatment is carried out so as to dissolve at each dissolution time (n1 to nK: K is an integer of 2 or more).

  Next, the calculation process will be described. In the calculation step, elemental analysis is performed on the acid solution in which the surface of the positive electrode active material is dissolved, and the coverage is calculated from the measurement result.

More specifically, in the calculation process, for example, after performing the surface treatment process, elemental analysis with lithium (Li), transition metal element M1, and metal element M2 dissolved in the acid solution as measurement objects is performed for each dissolution time. Based on the measurement results, the dissolution amount x% and the coverage rate y are calculated by the following equations 1 and 2, respectively, and the calculated K dissolution amounts x% and the coverage rate y are linearly approximated, A function is obtained, and a coverage y1 with respect to an arbitrary dissolution amount x1% is calculated from the obtained linear function.
(Formula 1)
x% = [(“Mass of Li + Mass of M1 + M2 of Mass” / “Mass of Positive Electrode Active Material Before Surface Treatment”) × 100] (%)
(Formula 2)
y = (“M2 mole fraction” / “M1 mole fraction + M2 mole fraction”)

  Examples of elemental analysis include elemental analysis such as ICP emission analysis (Inductively Coupled Plasma emission spectrometry), atomic absorption spectrometry (Atomic Absorption Spectrometry), etc. Among them, ICP emission analysis is preferable. This is because ICP emission analysis can perform analysis with higher sensitivity. Further, in the ICP emission analysis, a sample dissolved in an acid solution can be used as it is as a sample for elemental analysis, and all elements can be analyzed in one analysis. In the elemental analysis, for example, the supernatant of the acid solution in which the surface of the positive electrode active material is dissolved is collected as a sample, and the elemental analysis is performed on lithium (Li), the transition metal element M1, and the metal element M2. And the mass of Li, the mass of M1, the mass of M2, the molar fraction of M1 and M2 are calculated | required from the measured value, and the dissolution amount x% and the coverage y can be calculated | required from Formula 1 and Formula 2.

  The dissolution amount x% and the coverage rate y are measured by collecting samples from the respective acid solutions in which the positive electrode active material is dissolved at different dissolution times (n1 to nK: K is an integer of 2 or more). , K dissolution amount x%, coverage value y is calculated. If the dissolution time is too long, the difference in the coverage y is not clear, and the dissolution amount x% hardly increases. Therefore, 0 to 10 minutes is preferable. For example, when surface treatment is performed at three different dissolution times, such as 1 minute, 2 minutes, and 4 minutes, K is 3, and the surface treatment is performed at a dissolution time different from 1 minute, 2 minutes, and 4 minutes. Three x% and y values are calculated from the three samples performed.

  Next, by plotting the calculated K amount of dissolution x% and the coverage rate y, and making a graph with the dissolution amount x% as the x-axis and the coverage rate y as the y-axis, a linear function is obtained by linear approximation. And the coverage y1 with respect to an arbitrary dissolution amount x1% is calculated from the obtained linear function. Then, the positive electrode active materials are classified based on the obtained coverage y1.

  Here, as arbitrary dissolution amount x1%, 0.1%-2% are preferable, and 0.5% is especially preferable. This is because if the arbitrary amount of dissolution x1 is lower than 0.1%, in addition to detecting a trace component that is not coated, the slope of the linear function becomes large, resulting in variations in measurement. This is because if the arbitrary dissolution amount x1% is larger than 2%, a large amount of lithium composite oxide as a core material is dissolved out, and it becomes difficult to obtain a difference in coverage.

  As described above, the surface state of the surface-covered positive electrode active material can be quantified based on the obtained coverage rate y1, and the surface-covered positive electrode active material is classified based on the coverage rate y1. High evaluation is possible.

  Conventionally, as a method of evaluating the surface-coated positive electrode active material, a method of observing the bonding state of the surface elements of the positive electrode active material by XPS (X-ray Photoelectron Spectroscopy), or EDX (Energy Dispersive X-ray). Analysis), a method for confirming the elemental composition of the particle surface layer has been proposed.

  However, quantifying and evaluating the coating state of the surface-covered positive electrode active material with a conventional method requires a long time, so there are differences in the coating state due to subtle temperature differences in the manufacturing process and surface treatment conditions. It was difficult to easily quantify and evaluate the difference in coating state due to slight differences. In addition, since the conventional method only obtains information on the outermost surface, the information is insufficient as a means for quantifying the state of the coating.

  On the other hand, in the positive electrode active material evaluation method according to an embodiment of the present invention, the coverage ratio is calculated by dissolving the surface of the positive electrode active material particles in an acid solution, which is then subjected to ICP emission spectroscopy (Inductively Coupled Plasma emission spectrometry) or Measurement is performed using elemental analysis such as atomic absorption spectrometry, and the coverage is calculated from the measurement results, so that the coating state can be quantified easily and in a short time.

  And since this coverage can also quantify the difference in the coating state due to subtle temperature differences in the manufacturing process and the difference in the coating state due to slightly different surface treatment conditions, the positive electrode active material based on this coverage By classifying, it is possible to easily perform a highly accurate evaluation in a short time.

  Next, an example of a nonaqueous electrolyte battery using the positive electrode active material evaluated in this way will be described with reference to the drawings. FIG. 1 shows a structure of an example of a non-aqueous electrolyte battery using a positive electrode active material evaluated by a positive electrode active material evaluation method according to an embodiment of the present invention. This battery is, for example, a nonaqueous electrolyte secondary battery, for example, a lithium ion secondary battery.

  As shown in FIG. 1, this secondary battery is a so-called cylindrical type, and a strip-shaped positive electrode 21 and a strip-shaped negative electrode 22 are interposed in a substantially hollow cylindrical battery can 11 via a separator 23. The wound electrode body 20 is wound. The battery can 11 is made of, for example, iron (Fe) plated with nickel (Ni), and has one end closed and the other end open. Inside the battery can 11, a pair of insulating plates 12 and 13 are arranged perpendicular to the winding peripheral surface so as to sandwich the winding electrode body 20.

  At the open end of the battery can 11, a battery lid 14, a safety valve mechanism 15 provided inside the battery lid 14 and a heat sensitive resistance element (Positive Temperature Coefficient; PTC element) 16 are interposed via a gasket 17. It is attached by caulking, and the inside of the battery can 11 is sealed. The battery lid 14 is made of, for example, the same material as the battery can 11.

  The safety valve mechanism 15 is electrically connected to the battery lid 14 via the heat sensitive resistance element 16, and the disk plate 15A is reversed when the internal pressure of the battery exceeds a certain level due to an internal short circuit or external heating. Thus, the electrical connection between the battery lid 14 and the wound electrode body 20 is cut off. When the temperature rises, the heat sensitive resistance element 16 limits the current by increasing the resistance value and prevents abnormal heat generation due to a large current. The gasket 17 is made of, for example, an insulating material, and asphalt is applied to the surface.

  The wound electrode body 20 is wound around a center pin 24, for example. A positive electrode lead 25 made of aluminum (Al) or the like is connected to the positive electrode 21 of the spirally wound electrode body 20, and a negative electrode lead 26 made of nickel (Ni) or the like is connected to the negative electrode 22. The positive electrode lead 25 is electrically connected to the battery lid 14 by being welded to the safety valve mechanism 15, and the negative electrode lead 26 is welded to and electrically connected to the battery can 11.

  FIG. 2 shows an enlarged part of the spirally wound electrode body 20 shown in FIG. The positive electrode 21 includes, for example, a positive electrode current collector 21A having a pair of opposed surfaces, and a positive electrode active material layer 21B provided on both surfaces of the positive electrode current collector 21A. In addition, you may make it have the area | region where the positive electrode active material layer 21B exists only in the single side | surface of 21 A of positive electrode collectors. The positive electrode current collector 21A is made of, for example, a metal foil such as an aluminum (Al) foil.

  The positive electrode active material layer 21B includes, for example, a positive electrode active material, and may include a conductive agent such as carbon black or graphite and a binder such as polyvinylidene fluoride as necessary.

  As the positive electrode active material, for example, when the positive electrode active material evaluation method according to one embodiment of the present invention is used, it is preferable to use a positive electrode active material having a coverage y1 of 0.30 or more. This is because when the coverage y1 is smaller than 0.30, the effect of stabilizing the active material is small and sufficient characteristics cannot be obtained. For example, if the surface coating with spinel lithium manganate, spinel lithium titanate, or nickel cobalt composite oxide is insufficient with respect to the positive electrode core material, the coverage is easily lowered. This is because the coating rate decreases in order to detect sensitively the increase in the dissolution amount of cobalt (Co) caused by the core material coming out on the surface due to insufficient adsorption of the coating material. Conceivable.

  On the contrary, if the coating of the coating material is sufficient, the dissolution of cobalt (Co) is lowered, so that an increase in the coverage can be expected. Further, the covering ratio easily changes depending on the firing conditions. When a certain amount of coating material is coated, if the firing temperature is increased, the tendency of the coverage to decrease is confirmed. This is because, by firing at a higher temperature, nickel (Ni) diffuses into the cobalt (Co) core material, and as a result, the core material cobalt (Co) is exposed to the surface, so that the positive electrode active material It is conceivable that the stabilization effect of this will decrease. The firing temperature is preferably 850 ° C. to 900 ° C. so that the core material can be sufficiently coated and the diffusion of nickel (Ni) into the core material is minimized. If the coverage at that time is 0.30 or more, sufficient characteristics can be obtained.

  The negative electrode 22 includes, for example, a negative electrode current collector 22A having a pair of opposed surfaces and a negative electrode active material layer 22B provided on both surfaces of the negative electrode current collector 22A. In addition, you may make it have the area | region where the negative electrode active material layer 22B exists only in the single side | surface of 22 A of negative electrode collectors. The anode current collector 22A is made of a metal foil such as a copper (Cu) foil.

  The negative electrode active material layer 22B includes, for example, a negative electrode active material, and may include a binder such as polyvinylidene fluoride as necessary. The negative electrode active material includes a negative electrode material capable of inserting and extracting lithium (Li).

Examples of the negative electrode material include carbon materials, metal compounds, oxides, sulfides, lithium nitrides such as LiN ₃ , lithium metals, metals that form alloys with lithium, and polymer materials.

  Examples of the carbon material include non-graphitizable carbon, graphitizable carbon, graphite, pyrolytic carbons, cokes, glassy carbons, organic polymer compound fired bodies, carbon fibers, and activated carbon. Among these, examples of coke include pitch coke, needle coke, and petroleum coke. An organic polymer compound fired body refers to a carbonized material obtained by firing a polymer material such as phenol resin or furan resin at an appropriate temperature, and part of it is non-graphitizable carbon or graphitizable carbon. Some are classified as: In addition, examples of the polymer material include polyacetylene and polypyrrole.

  Among the negative electrode materials, those having a charge / discharge potential relatively close to lithium metal are preferable. This is because the lower the charge / discharge potential of the negative electrode 22, the easier it is to increase the energy density of the battery. Among these, a carbon material is preferable because a change in crystal structure that occurs during charge / discharge is very small, a high charge / discharge capacity can be obtained, and good cycle characteristics can be obtained. In particular, graphite is preferable because it has a high electrochemical equivalent and can provide a high energy density. Moreover, non-graphitizable carbon is preferable because excellent cycle characteristics can be obtained.

  Examples of the negative electrode material include lithium metal alone, a metal element or metalloid element simple substance, alloy or compound capable of forming an alloy with lithium. These are preferable because a high energy density can be obtained. In particular, when used together with a carbon material, a high energy density can be obtained, and an excellent cycle characteristic can be obtained. Note that in this specification, alloys include those composed of one or more metal elements and one or more metalloid elements in addition to those composed of two or more metal elements. The structures include solid solutions, eutectics (eutectic mixtures), intermetallic compounds, or those in which two or more of them coexist.

Such metal elements or metalloid elements include tin (Sn), lead (Pb), aluminum (Al), indium (In), silicon (Si), zinc (Zn), antimony (Sb), bismuth (Bi). ), Cadmium (Cd), magnesium (Mg), boron (B), gallium (Ga), germanium (Ge), arsenic (As), silver (Ag), zirconium (Zr), yttrium (Y) or hafnium (Hf) ). These alloys or compounds, for example, those represented by the chemical formula Ma _s Mb _t Li _u or a chemical formula Ma _p Mc _q Md _r,. In these chemical formulas, Ma represents at least one of a metal element and a metalloid element capable of forming an alloy with lithium (Li), and Mb is a metal element other than lithium (Li) and Ma and a metalloid element. At least one kind is represented, Mc represents at least one kind of nonmetallic elements, and Md represents at least one kind of metal elements and metalloid elements other than Ma. The values of s, t, u, p, q, and r are s> 0, t ≧ 0, u ≧ 0, p> 0, q> 0, and r ≧ 0, respectively.

Among these, a simple substance, alloy or compound of Group 4B metal element or semimetal element in the short-period type periodic table is preferable, and silicon (Si) or tin (Sn), or an alloy or compound thereof is particularly preferable. These may be crystalline or amorphous. In addition, inorganic compounds that do not contain lithium (Li), such as MnO ₂ , V ₂ O ₅ , V ₆ O ₁₃ , NiS, and MoS, can also be used for any of the positive and negative electrodes.

  As the separator 23, for example, a polyethylene porous film, a polypropylene porous film, a synthetic resin nonwoven fabric, or the like can be used. The separator 23 is impregnated with an electrolytic solution that is a liquid electrolyte.

  As the electrolytic solution, a nonaqueous electrolytic solution in which an electrolyte salt is dissolved in a nonaqueous solvent can be used. A nonaqueous solvent and an electrolyte salt are appropriately combined, and any nonaqueous solvent can be used as long as it is used for this type of battery. For example, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4 -Methyl-1,3 dioxolane, diethyl ether, sulfolane, methyl sulfolane, acetonitrile, propionitrile, anisole, acetate ester, butyrate ester, propionate ester, etc., and one or more of these can be used .

A lithium salt can be used as the electrolyte salt. Examples of the lithium salt include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ , LiClO ₄ , LiB (C ₆ H ₅ ) ₄ , LiCH ₃ SO ₃ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , LiAlCl ₄ , LiSiF ₆ , LiCl, LiBF ₂ (ox) [lithium difluorooxalate borate], LiBOB (lithium bisoxalate borate), LiBr, etc. are suitable, and among these, Any one kind or a mixture of two or more kinds is used. Among them, LiPF ₆ is preferable because it can obtain high ion conductivity and can improve cycle characteristics.

  This secondary battery can be manufactured, for example, as described below. First, for example, a positive electrode active material, a conductive agent, and a binder are mixed to prepare a positive electrode mixture, and the positive electrode mixture is dispersed in a solvent such as N-methylpyrrolidone to obtain a positive electrode mixture slurry. . Subsequently, the positive electrode mixture slurry is applied to the positive electrode current collector 21A and the solvent is dried. Then, the positive electrode active material layer 21B is formed by compression molding using a roll press or the like, and the positive electrode 21 is manufactured.

  Further, for example, a negative electrode active material and a binder are mixed to prepare a negative electrode mixture, and the negative electrode mixture is dispersed in a solvent such as N-methylpyrrolidone to obtain a negative electrode mixture slurry. Subsequently, the negative electrode mixture slurry is applied to the negative electrode current collector 22A, and the solvent is dried. Then, the negative electrode active material layer 22B is formed by compression molding using a roll press or the like, and the negative electrode 22 is manufactured.

  Next, the positive electrode lead 25 is attached to the positive electrode current collector 21A by welding or the like, and the negative electrode lead 26 is attached to the negative electrode current collector 22A by welding or the like. After that, the positive electrode 21 and the negative electrode 22 are wound through the separator 23, and the tip of the positive electrode lead 25 is welded to the safety valve mechanism 15, and the tip of the negative electrode lead 26 is welded to the battery can 11. The positive electrode 21 and the negative electrode 22 are sandwiched between a pair of insulating plates 12 and 13 and stored in the battery can 11.

  After the positive electrode 21 and the negative electrode 22 are accommodated in the battery can 11, the electrolyte solution described above is injected into the battery can 11 and impregnated in the separator 23. After that, the battery lid 14, the safety valve mechanism 15, and the heat sensitive resistance element 16 are fixed to the opening end of the battery can 11 by caulking through the gasket 17. As described above, the secondary battery shown in FIG. 1 can be manufactured.

  EXAMPLES Hereinafter, the present invention will be specifically described with reference to examples. However, the present invention is not limited to these examples. In Examples, specific examples in which Samples 1 to 14 produced as described below were evaluated using the positive electrode active material evaluation method of the present invention will be described.

<Sample 1>
First, a positive electrode active material was produced as follows. As composite oxide particles, lithium cobaltate powder having an average composition of Li _1.03 CoO ₂ and an average particle diameter of 13 μm measured by a laser scattering method is prepared, and lithium carbonate (Li ₂ CO _{3) is used} as a raw material for the coating layer. ) Powder, nickel hydroxide (Ni (OH) ₂ ) powder and manganese carbonate (MnCO ₃ ) powder at a molar ratio of Li ₂ CO ₃ : Ni (OH) ₂ : MnCO ₃ = 1: 1: 1. A mixed precursor powder was prepared.

Next, the precursor powder is added to 100 parts by mass of this lithium cobaltate powder so as to be 10 parts by mass in terms of LiNi _0.5 Mn _0.5 O ₂ , and 1 part by using 100 parts by mass of pure water at 25 ° C. After stirring and dispersing over time, it was dried under reduced pressure at 70 ° C. to form a precursor layer on the surface of the composite oxide particles. Subsequently, this was heated at a rate of 3 ° C./min, held at 800 ° C. for 3 hours, and then gradually cooled to form a coating layer, whereby a positive electrode active material of Sample 1 was obtained.

In addition, when the powder X-ray diffraction pattern using CuKα was measured for this positive electrode active material powder, the obtained pattern was a diffraction peak corresponding to LiCoO ₂ having a layered rock salt structure, and nickel (Ni ), A diffraction peak considered to correspond to a coating material of manganese (Mn) was obtained, and no impurities were confirmed.

<Sample 2>
The positive electrode active material of Sample 2 was prepared in the same manner as Sample 1 except that the same precursor powder as Sample 1 was pulverized with a ball mill device until the average particle size became 2 μm or less and mixed with the composite oxide particles. .

<Sample 3>
The positive electrode active material of Sample 3 was prepared by performing the same surface coating treatment as Sample 2 except that the temperature at which the heat treatment was performed was changed to 900 ° C.

<Sample 4>
10 parts by mass of LiNi _0.5 Mn _0.5 O ₂ obtained by pulverizing the same precursor powder as in sample 1 until the average particle size is 2 μm or less with respect to 100 parts by mass of composite oxide particles as in sample 1 Then, a precursor layer was formed by processing with a mechano-fusion apparatus, and then a coating layer was formed by performing the same heat treatment as in Sample 1, and a positive electrode active material of Sample 4 was produced.

<Sample 5>
A positive electrode active material of Sample 5 was produced in the same manner as Sample 1, except that the heat treatment temperature was 750 ° C.

<Sample 6>
A positive electrode active material of Sample 6 was produced in the same manner as Sample 1, except that the temperature increase rate during the heat treatment was 10 ° C./min and the holding time at 800 ° C. was 2 hours.

<Sample 7>
As the composite oxide particles, a composite oxide powder having an average composition of Li _1.03 Co _0.98 Al _0.01 Mg _0.01 O ₂ was used, and a precursor powder similar to that of Sample 1 was pulverized with a ball mill device until the average particle size became 2 μm or less. A positive electrode active material of Sample 7 was produced in the same manner as Sample 4, except that it was mixed with the composite oxide particles.

<Sample 8>
100 parts by mass of the same composite oxide particles as sample 7 and 10 parts by mass of lithium nickel manganate powder having an average composition of Li _1.03 Ni _0.5 Mn _0.5 O ₂ and an average particle diameter of 3 μm were processed by a mechanofusion apparatus. Then, after forming the precursor layer, a coating layer was formed by performing a heat treatment at a heating rate of 3 ° C./min, a heat treatment temperature of 750 ° C., and a holding time of 3 hours, and a positive electrode active material of Sample 8 was produced.

<Sample 9>
100 parts by mass of the same composite oxide particles as in sample 8 and 5 parts by mass of lithium nickel manganate powder having an average composition of Li _1.03 Ni _0.5 Mn _0.5 O ₂ and an average particle diameter of 3 μm were processed by a mechanofusion apparatus. After forming the precursor layer, a coating layer was formed by performing a heat treatment at a heating rate of 3 ° C./min, a heat treatment temperature of 750 ° C., and a holding time of 3 hours, and a positive electrode active material of Sample 9 was produced.

<Sample 10>
100 parts by mass of the same composite oxide particles as in sample 8 and 2 parts by mass of lithium nickel manganate powder having an average composition of Li _1.03 Ni _0.5 Mn _0.5 O ₂ and an average particle diameter of 3 μm were processed by a mechanofusion apparatus. After forming the precursor layer, a coating layer was formed by performing a heat treatment at a heating rate of 3 ° C./min, a heat treatment temperature of 750 ° C., and a holding time of 3 hours, and a positive electrode active material of Sample 10 was produced.

<Sample 11>
A positive electrode active material of Sample 11 was produced in the same manner as Sample 1, except that the maximum heat treatment temperature was 1000 ° C.

<Sample 12>
100 parts by mass of the same composite oxide particles as in sample 8 and 1 part by mass of lithium nickel manganate powder having an average composition of Li _1.03 Ni _0.5Mn0.5 O ₂ and an average particle diameter of 3 μm were _{processed by a mechanofusion} apparatus. Then, after forming a precursor layer, a coating layer was formed by performing a heat treatment at a heating rate of 3 ° C./min, a heat treatment temperature of 750 ° C., and a holding time of 3 hours, and a positive electrode active material of Sample 12 was produced.

<Sample 13>
A positive electrode active material of Sample 13 was produced in the same manner as Sample 10, except that the maximum heat treatment temperature was 950 ° C. The surface coverage was 0.25.

<Sample 14>
A positive electrode active material of Sample 14 was produced in the same manner as Sample 10, except that the maximum heat treatment temperature was 900 ° C.

  Next, the positive electrode active material coverage was determined for each of the positive electrode active materials of Samples 1 to 14 as described below.

  First, the surface treatment process of the positive electrode active material was performed, and the measurement sample was adjusted. The positive electrode active material particles were precisely weighed and placed in a beaker. Next, 10 ml of a commercially available 0.01 mol / l HCl solution (for volume analysis manufactured by Wako) was added to the beaker with a whole pipette, and the mixture was stirred for a certain time with a magnetic stirrer.

  Next, after quantitatively collecting the supernatant with a disposable syringe, a syringe filter (manufactured by ADVANTEC, PTFE 0.2 mm) was attached to the syringe and filtered into a volumetric flask. Next, it was made up with diluted HCl solution. (10-fold dilution) The above operation was performed with dissolution times of 1 minute, 2 minutes, and 4 minutes, and three measurement samples having different dissolution times were obtained.

  Next, ICP measurement (product name: ICPS-8100, manufactured by SHIMADZU) was performed on the three measurement samples using lithium (Li), nickel (Ni), cobalt (Co), and manganese (Mn) as measurement targets. Then, based on the measurement results, the dissolved mass for each element and the molar fraction for each element are calculated. (Equation 1): x% = [(“Li mass + M1 mass + M2 mass” / “Surface treatment” Mass of previous positive electrode active material ”)] × 100 (%), (formula 2); y = [(“ M2 mole fraction ”/“ M1 mole fraction + M2 mole fraction ”)] x% and coverage y were determined. Here, the dissolved mass for each element was calculated from the measured value (ppm value) and the diluted concentration. The mole fraction for each element was calculated by dividing the measured value (ppm value) of each element by the atomic weight of the element. In addition, the graph which plotted the amount of melt | dissolution with respect to melt | dissolution time is shown in FIG. In FIG. 3, point a indicates the amount of dissolution with respect to the dissolution time of 1 minute. Point b indicates the amount dissolved for a dissolution time of 2 minutes. Point c indicates the amount dissolved for a dissolution time of 4 minutes.

  Next, as shown in FIG. 4, the obtained three dissolution amounts x% and coverage y are plotted as point a ′, point b ′, and point c ′, and point a ′, point b ′, point By connecting c ′ with a straight line and performing a linear approximation to obtain a linear function, a linear function y = −0.156x + 0.401 was derived. The value of y with respect to x = 0.5 was calculated from this linear function y = −0.156x + 0.401, and this value of y was defined as the coverage ratio y1.

  Next, using the positive electrode active materials of Sample 1 to Sample 14, as described below, a non-aqueous electrolyte secondary battery as shown in FIG. 1 was produced, and the produced non-aqueous electrolyte secondary battery was Discharge energy and load characteristics were measured.

  First, 86% by mass of the positive electrode active material powder, 10% by mass of graphite as a conductive agent, and 4% by mass of polyvinylidene fluoride as a binder are mixed and dispersed in N-methyl-2-pyrrolidone as a solvent. After forming a positive electrode mixture slurry, uniformly apply to both surfaces of the positive electrode current collector 21A made of a strip-shaped aluminum foil having a thickness of 20 μm, dry, and compression-mold with a roll press to form the positive electrode active material layer 21B. Thus, the positive electrode 21 was produced. After that, an aluminum positive electrode lead 25 was attached to one end of the positive electrode current collector 21A.

  In addition, 90% by mass of artificial graphite powder as a negative electrode active material and 10% by mass of polyvinylidene fluoride as a binder are mixed and dispersed in N-methyl-2-pyrrolidone as a solvent to form a negative electrode mixture slurry. After that, the negative electrode 22 was produced by uniformly applying and drying both sides of the negative electrode current collector 22A made of a strip-shaped copper foil having a thickness of 10 μm, and compression forming with a roll press to form the negative electrode active material layer 22B. . At that time, the amounts of the positive electrode active material and the negative electrode active material are adjusted so that the open circuit voltage at the time of full charge is 4.4 V, and the capacity of the negative electrode 22 is represented by a capacity component due to insertion and extraction of lithium. Designed. After that, a nickel negative electrode lead 26 was attached to one end of the negative electrode current collector 22A.

  Next, the produced positive electrode 21 and negative electrode 22 were wound many times through a separator 23 made of a porous polyolefin film, to produce a wound electrode body 20. Subsequently, the wound electrode body 20 is sandwiched between the pair of insulating plates 12 and 13, the negative electrode lead 26 is welded to the battery can 11, and the positive electrode lead 25 is welded to the safety valve mechanism 15. It was stored inside a plated iron battery can 11.

After that, by injecting an electrolyte into the battery can 11 and caulking the battery can 11 through the gasket 17, the safety valve mechanism 15, the heat sensitive resistance element 16 and the battery lid 14 are fixed, and the outer diameter is 18 mm. A cylindrical secondary battery having a height of 65 mm was obtained. As the electrolytic solution, a solution obtained by dissolving LiPF ₆ as an electrolyte salt to 1.0 mol / l in a solvent in which ethylene carbonate and dimethyl carbonate were mixed at an equal volume ratio was used.

  The produced nonaqueous electrolyte secondary battery was subjected to a charge / discharge test as described below in a temperature environment of 55 ° C., and the discharge energy and cycle characteristics were measured.

  Charging was performed at a constant current of 1000 mA until the battery voltage reached 4.4 V, and then constant voltage charging was performed at a constant voltage until the total charging time reached 2.5 hours, resulting in a fully charged state. . In discharging, a constant current was discharged at a constant current of 800 mA until the battery voltage reached 3.0 V, and the battery was completely discharged.

  In the charge / discharge test, the discharge energy at the first cycle was defined as discharge energy.

  For the cycle characteristics, the capacity retention rate of the discharge capacity at the 200th cycle in the above-described charge / discharge test with respect to the initial discharge capacity was determined by (“discharge capacity at the 200th cycle” / “initial capacity”) × 100%. The initial capacity is obtained from the discharge capacity at the first cycle in the charge / discharge test.

  In the above charge / discharge test, the discharge voltage was 2000 mA constant current at a constant current of 2000 mA until the battery voltage reached 3.0 V, and the first cycle 2000 mA discharge capacity and the discharge current of 800 mA constant battery voltage. The ratio of the 800 mA discharge capacity in the first cycle when the constant current discharge was performed until the voltage reached 3.0 V was obtained as the load characteristic.

  Table 1 shows the measured coverage ratios of Sample 1 to Sample 14, and the measured discharge energy, load characteristics, and capacity retention rate of the nonaqueous electrolyte secondary batteries produced using Sample 1 to Sample 14.

  As shown in Table 1, in the non-aqueous electrolyte secondary battery using Sample 1 to Sample 14, the capacity retention rate of the non-aqueous electrolyte secondary battery using Sample 1 to Sample 10 having a coverage of 0.31 or more is the sample. The positive electrode active materials of Sample 1 to Sample 10 having a coating rate of 0.31 or more were evaluated to be good as compared with 11 to Sample 14. That is, the difference in coating state due to temperature difference in the manufacturing process and the difference in coating state due to different surface treatment conditions are quantified depending on the coverage, and the positive electrode active material is good based on this coverage It was confirmed that the positive electrode active material can be evaluated with high accuracy by classifying it as not good.

  The present invention is not limited to the above-described embodiments of the present invention, and various modifications and applications are possible without departing from the spirit of the present invention. For example, in the above-described embodiment, the surface treatment process is performed by changing the dissolution time, and the amount of dissolution x% and the coverage rate y are calculated. However, the surface treatment is performed by changing the concentration of the acid solution while keeping the dissolution time constant. A process may be performed and the amount of dissolution x% and the coverage rate y may be calculated. Further, in the above-described embodiment, the case where the positive electrode active material having a change in the element concentration formed from the surface toward the inside is described, but the present invention changes to the element concentration formed from the surface toward the inside. It can also be applied to confirming that there is no change in the element concentration for the positive electrode active material having no.

  Furthermore, for example, as an example of a non-aqueous electrolyte battery using a positive electrode active material evaluated by the above-described positive electrode active material evaluation method, a non-aqueous electrolyte battery having an electrolyte as an electrolyte has been described, but the present invention is not limited thereto. is not.

  For example, a gel electrolyte can be used as the electrolyte. The gel electrolyte is in the form of a so-called gel containing an electrolytic solution and a polymer compound that serves as a holding body that holds the electrolytic solution. As the matrix of the gel electrolyte, various polymer compounds can be used as long as they can gel by absorbing the nonaqueous electrolytic solution. Examples of the polymer compound include fluorine-based polymers such as poly (vinylidene fluoride) and poly (vinylidene fluoride-co-hexafluoropropylene), and ether-based polymers such as poly (ethylene oxide) and the same crosslinked product. Poly (acrylonitrile) can also be used. In particular, it is desirable to use a fluorine-based polymer from the viewpoint of redox stability.

  Furthermore, for example, a solid electrolyte containing an electrolyte salt can be used as the electrolyte. As the solid electrolyte, any inorganic solid electrolyte or polymer solid electrolyte can be used as long as the material has lithium ion conductivity. Examples of the inorganic solid electrolyte include lithium nitride and lithium iodide. The polymer solid electrolyte is composed of an electrolyte salt and a polymer compound that dissolves the electrolyte salt. The polymer compound is an ether polymer such as poly (ethylene oxide) or a crosslinked product thereof, poly (methacrylate) ester, or acrylate. Etc. can be used alone or in a molecule or in a mixture.

  Furthermore, the battery shape is not particularly limited. In addition to the cylindrical shape, various shapes such as a square shape, a coin shape, a button shape, and a laminate seal shape can be used.

  Furthermore, the manufacturing method of the electrode of a negative electrode and a positive electrode is not ask | required. A method in which a known binder or conductive material is added to the material and a solvent is added for coating, a method in which a known binder or the like is added to the material and heated to apply, a material alone or a conductive material or a binder. A method of producing a molded body electrode on the current collector by mixing with an adhesive and performing a process such as molding can be used, but is not limited thereto. More specifically, it can be prepared by mixing with a binder, an organic solvent or the like to form a slurry, and then applying and drying on a current collector. Alternatively, regardless of the presence or absence of the binder, it is also possible to produce a strong electrode by pressure molding while applying heat to the active material.

  Furthermore, as a method for manufacturing the battery, in addition to a manufacturing method in which a positive electrode and a negative electrode are wound around a core through a separator, a stacking method in which electrodes and a separator are sequentially stacked can be employed.

1 is a cross-sectional view showing an example of the structure of a nonaqueous electrolyte battery using a positive electrode active material evaluated by a positive electrode active material evaluation method according to an embodiment of the present invention. It is sectional drawing which expands and represents a part of winding electrode body shown in FIG. It is a graph which shows the relationship of the dissolution amount with respect to dissolution time. It is a graph which shows the relationship of the coverage with respect to dissolution amount.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Battery can 12, 13 ... Insulating plate 14 ... Battery cover 15 ... Safety valve mechanism 16 ... Thermal resistance element 17 ... Gasket 20 ... Winding electrode body 21 ... Positive electrode 21A ... Positive electrode current collector 21B ... Positive electrode active material layer 22 ... Negative electrode 22A ... Negative electrode current collector 22B ... Negative electrode active material layer 23 ... Separator 24 ... Center pin 25 ... Positive electrode lead 26 ... Negative electrode lead

Claims (7)

A surface treatment step of dissolving the surface of the positive electrode active material in an acid solution;
And a calculation step of performing elemental analysis on the acid solution in which the surface of the positive electrode active material is dissolved by the surface treatment step and calculating a coverage from the measurement result, and a method for evaluating the positive electrode active material . The method for evaluating a positive electrode active material according to claim 1, further comprising a classification step of classifying the positive electrode active material based on the coverage. The method for evaluating a positive electrode active material according to claim 1, wherein the elemental analysis is performed by ICP emission spectrometry (Inductively Coupled Plasma emission spectrometry). The positive electrode active material includes a coating layer made of an oxide containing one or more metal elements M2 on at least a part of a lithium composite oxide containing lithium (Li) and one or more transition metal elements M1. It is provided,
The surface treatment step is performed such that the surface of the positive electrode active material is dissolved in an acid solution at different K dissolution times (n1 to nK: K is an integer of 2 or more),
In the calculation step, the lithium (Li), the transition metal element M1, and the metal are respectively added to the K acid solutions in which the surface of the positive electrode active material is dissolved in the dissolution times by the surface treatment step. Perform elemental analysis with the element M2 as the measurement target, calculate K dissolution amount x% and coverage y from the measurement results of the elemental analysis using the following formulas 1 and 2, respectively, and calculate the calculated K dissolution amount The positive electrode active material according to claim 1, wherein a linear function is obtained from x% and coverage y by linear approximation, and the coverage y1 with respect to an arbitrary dissolved amount x1% is calculated from the obtained linear function. Evaluation method.
(Formula 1)
x% = [(“Mass of Li + Mass of M1 + M2 of Mass” / “Mass of Positive Electrode Active Material Before Surface Treatment”)] × 100 (%)
(Formula 2)
y = [("M2 mole fraction" / "M1 mole fraction + M2 mole fraction")] The positive electrode active material evaluation method according to claim 4, wherein x1% is 0.1% to 2%. 5. The method for evaluating a positive electrode active material according to claim 4, wherein the transition metal element M1 is at least one of cobalt (Co), nickel (Ni), manganese (Mn), and iron (Fe). . 5. The method for evaluating a positive electrode active material according to claim 4, wherein the metal element M2 is at least one of nickel (Ni) and manganese (Mn).

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