JP2013101883A - Positive electrode active material for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode active material excellent in cycle characteristics and high in safety.SOLUTION: A positive electrode active material for a lithium ion secondary battery includes: a compound A belonging to a Pnma space group and represented as chemical formula: LiMnFeMPO(in the formula, the relationships of 0<x≤1.2, 0<α<1.2, 0<β<1.2 and 0≤γ<1.2 are satisfied, and M represents Mg, Al, Ti or the like); and a compound B belonging to a Pnma space group and different from the compound A. When a lithium ion secondary battery manufactured using a positive electrode including the positive electrode active material is fully charged and an X-ray diffraction pattern of the positive electrode active material is measured, an intensity ratio I/Ibetween a peak intensity Iin a (200) face of a compound A comprising the compound A from which Li is desorbed, belonging to a Pnma space group and represented as chemical formula: LiMnFeMPO(in the formula, the relationships of 0≤y<0.06 and x>y are satisfied) and a peak intensity I in a (200) face of the compound B is more than 0.0001 and less than 1, and an average particle size of primary particles constituting the positive electrode active material is less than 500 nm.

Description

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

Lithium ion secondary batteries are widely used in electronic devices because of their light weight, high voltage, and excellent discharge capacity. An example of a material that has been put into practical use as a positive electrode active material of a lithium ion secondary battery is lithium cobaltate. However, since cobalt is a rare metal and expensive, development of a positive electrode active material aimed at decobalting has been developed. In recent years, the development of layered nickel-based oxides typified by LiNiO ₂ and spinel manganese-based positive electrode active materials typified by LiMn ₂ O ₄ has been promoted. However, the layered nickel-based oxide has a charge product inferior in thermal stability as compared with lithium cobalt oxide, and has a problem in safety particularly during overcharge. On the other hand, although the spinel manganese-based positive electrode active material has excellent thermal stability of the charge product and high safety, its practical capacity is about 100 mAh / g, which is smaller than that of lithium cobalt oxide.

Under such circumstances, in recent years, lithium-containing composite oxides represented by a chemical formula LiMPO ₄ (wherein M is a transition metal element such as Mn and Fe), which is collectively referred to as an olivine-based compound, attracted attention. Yes. This is because the specific weight energy density and specific capacity energy density of the compound exceed that of the spinel manganese system and have excellent safety.

However, although the positive electrode active material can be expected to have cycle stability of the compound itself, there is a problem that the positive electrode active material is greatly expanded and contracted due to insertion and desorption of Li accompanying charge / discharge. For example, in (Non-Patent Document 1), the lattice volume change rate of LiFePO ₄ is about 6.8% before and after charging. It is known that LiMnPO ₄ or the like belonging to the same olivine system has a larger lattice volume change rate before and after charging. When such a positive electrode active material is used, when the number of cycles is increased, the binding property between the olivine compound and the current collector or conductive material may be reduced. When the binding property is lowered, there is a possibility that the conductive path between the positive electrode active material and the conductive material is broken, the battery capacity is lowered, and the battery life is shortened.

  On the other hand, in (patent document 1), the report which suppresses expansion and contraction by element substitution is made. However, even if the expansion and contraction is suppressed by element substitution, the entire primary particle continues to expand and contract, and it is inevitable that the binding property with the binder decreases with each cycle. In addition, there is a problem that the cycle characteristics are deteriorated because the primary particles have high resistance.

  Patent Document 2 discloses a method of improving the binding property between the positive electrode active material and the current collector by including an acrylonitrile copolymer in the binder. However, there is no description of a method for suppressing changes in the lattice constant of the positive electrode active material itself.

  Therefore, a technique for suppressing the expansion and contraction of the positive electrode active material and improving the cycle characteristics has not been reported yet.

J. Electrochem. Soc., 148 (2001) A224

JP 2011-77030 A JP 2010-272272 A

In view of the above-described conventional situation, the present invention has a chemical formula LiMPO ₄ (generally referred to as an olivine-based compound) having excellent cycle characteristics, low cost, high safety, and long battery life. And a lithium ion secondary battery using the active material, and a positive electrode active material represented by a transition metal element such as Fe.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors found that a compound belonging to the same Pnma space group is present in or around the primary particles of the olivine-based compound in the positive electrode active material, Primary particles constituting a positive electrode active material by introducing a partially inactive olivine compound that does not desorb Li even when fully charged, and defining the proportion of such inactive olivine compound Alternatively, the present inventors have found that expansion / contraction during charging / discharging of the whole positive electrode active material is alleviated and expansion / contraction of the positive electrode or battery is suppressed, and as a result, cycle characteristics are improved, and the present invention has been completed.

That is, the present invention belongs to the Pnma space group, and has the chemical formula Li _x Mn _α Fe _β M _γ PO ₄ (where 0 <x ≦ 1.2, 0 <α <1.2, 0 <β <1.2 , 0 ≦ γ <1.2, where M is one or more elements selected from the group consisting of Mg, Al, Ti, Co, Ni, Cu, and Zn) and the Pnma space group A positive electrode active material for a lithium ion secondary battery comprising a compound B different from the compound A, wherein the lithium ion secondary battery using a positive electrode having the positive electrode active material is fully charged; When the X-ray diffraction pattern of the substance was measured, the chemical formula Li _y Mn _α Fe _β M _γ PO ₄ (where 0 ≦ y <0.06, Where x> y) and diffraction caused by the crystal structure of compound A ′ And over click group, and diffraction peaks attributed to the crystal structure of the compound B is observed, in which the peak intensity I (200) plane of the compound B, compound peak intensity of (200) plane of the A 'I ₀ The I / I ₀ intensity ratio is 0.0001 <I / I ₀ <1, and the average particle size of the primary particles constituting the positive electrode active material is less than 500 nm.

  According to the present invention, the primary particles constituting the positive electrode active material or the entire positive electrode active material is relaxed in expansion / contraction during charging / discharging, and as a result, the expansion / contraction of the positive electrode is suppressed. A secondary battery can be provided. In addition, a lithium ion secondary battery having characteristics such as low cost, large capacity, high safety, and long battery life can be provided. Problems, configurations, and effects other than those described above will be clarified by the following description of embodiments.

2 is a diagram showing an X-ray diffraction pattern of a positive electrode active material obtained in Example 1 and X-ray diffraction patterns of compounds A and A ′. FIG.

Hereinafter, the present invention will be described in detail.
The positive electrode active material of the present invention belongs to the Pnma space group and has the chemical formula Li _x Mn _α Fe _β M _γ PO ₄ (where 0 <x ≦ 1.2, 0 <α <1.2, 0 <β <1 .2, 0 ≦ γ <1.2, where M is one or more elements selected from the group consisting of Mg, Al, Ti, Co, Ni, Cu and Zn), and the Pnma space The compound B belongs to the group and is different from the compound A.

When a lithium ion secondary battery using a positive electrode having a positive electrode active material is fully charged and the X-ray diffraction pattern of the positive electrode active material is measured, Li belongs to the Pnma space group from which Li is desorbed. A diffraction peak group resulting from the crystal structure of compound A ′ represented by the chemical formula Li _y Mn _α Fe _β M _γ PO ₄ (where 0 ≦ y <0.06, where x>y); A diffraction peak group due to the crystal structure of the compound B is observed, and at this time, the intensity ratio I / I between the peak intensity I of the (200) plane of the compound B and the peak intensity I ₀ of the (200) plane of the compound A ′. ₀ is 0.0001 <I / I ₀ <1, and the average particle size of primary particles constituting the positive electrode active material is less than 500 nm. Since compound B does not expand and contract during insertion and release of Li to and from the active material accompanying charge and discharge, the compound B has good binding properties with the binder even after a plurality of cycles. In addition, since it belongs to the same Pnma space group as the compound A, which is the parent phase, and the lattice constant is close, the lattice matching with the parent phase is good, and the cycle stability of the parent phase A, Li diffusibility, charge / discharge capacity can be improved. The adverse effect of is small.

  In addition, as a full charge condition at the time of measuring an X-ray diffraction pattern, for example, a positive electrode having a positive electrode active material and a lithium ion secondary battery using metallic lithium as a negative electrode is used. It is possible to employ a condition in which charging is performed at a maximum of 25 hours up to 4.3 V in terms of a potential difference from metallic lithium at a value of 1/10 of the current value that can be charged.

  The compound B may be partially contained in the primary particles composed of the compound A, and the primary particles composed of the compound A and other primary particles composed of the compound B gather to form a positive electrode active material. May be. Alternatively, both cases may be mixed.

  The lattice constant of Compound B in the Pnma space group is smaller than the lattice constant of Compound A, the a-axis direction is 10.20 to 10.33 inches, the b-axis direction is 5.98 to 6.08 inches, and the c-axis direction is 4. It is preferable that it is 70-4.82. In addition, it is preferable that the compound B has a diffraction angle of the (200) plane that is different from both the diffraction angles of the (200) plane of the compound A and the compound A ′, and Li does not desorb in a fully charged state. Such a compound B is obtained by subjecting a positive electrode containing compound A, compound A ′, and a combination thereof as a constituent material to the following treatment, so that a part of the active material in the positive electrode is brought into a region where Li elimination is inhibited. It can be formed by changing.

Specifically, for example, a positive electrode mixture slurry containing a conductive material such as Compound A, acetylene black, and a binder such as polyvinylidene fluoride is prepared, and this slurry is applied to a current collector and dried. Thus, a positive electrode is produced. Then, the lithium ion secondary battery using this positive electrode is fully charged and the positive electrode after compound A is converted to compound A ′ is heated by heating at 25 ° C. to 240 ° C. in an electrolyte containing H ₂ O. Thus, compound B can be formed. During the heat treatment, the temperature of the electrolytic solution may be kept constant or may be treated while being changed within a range of 25 ° C to 240 ° C. A particularly preferable condition is a heat treatment for 20 days in the above electrolytic solution at 80 ° C. When the heating temperature is lower than 80 ° C., the treatment time tends to be longer than 20 days, that is, the synthesis of Compound B takes a long time at a low temperature. On the other hand, when heating at a temperature exceeding 150 ° C., compound B can be synthesized in a relatively short time, but due to the reaction of the electrolyte / electrolyte, a substance other than compound B is produced on the electrode, Since there exists a possibility that the resistance of an electrode may become high, heating temperature becomes like this. Preferably it is 25 to 150 degreeC, Most preferably, it is 50 to 80 degreeC.

Further, as described above, the compound A may be heat-treated in the electrolytic solution in a fully charged state, but the lithium ion secondary according to the present invention may be performed even if the heat treatment is performed in the electrolytic solution in a state that is not fully charged. A positive electrode active material for a battery can be obtained. However, in order to obtain the predetermined peak intensity ratio I / I ₀ obtained by holding in a fully charged state by a heat treatment in a state where it is not fully charged, it is longer or at a higher temperature than in a fully charged state. However, there is a tendency to require conditions that combine them.

Furthermore, in the above-described example, a part of the compound A is converted to the compound B by heating for simplification of material synthesis. However, the present invention is not limited to this, and the compound B may be synthesized by other methods. That is, the detailed mechanism by which compound B is synthesized from compound A by heat treatment is not clear, but PF ₅ , LiF, HF, PF ₃ O derived from electrolytes, electrolytes, additives, or their mutual reaction products. , C ₂ H ₄ , CO ₂ , C ₂ H ₅ F, C ₂ H ₅ OCOOPF _{4 and the} like and a chemical reaction with compound A result in the formation of compound B in the particles of compound A. From the above, by chemically treating Compound A using each of the above-mentioned substances or combinations thereof, Compound B can be produced in the particles of Compound A, or Compound B can be synthesized separately. Compound B synthesized separately can subsequently be mixed with Compound A to obtain the positive electrode active material of the present invention.

Regardless of which synthesis method is used, a positive electrode active material in which compound A and compound B are contained inside primary particles or primary particles of compound A and primary particles of compound B are mixed, If the measurement result of the X-ray diffraction pattern of the positive electrode active material after charging satisfies the condition of 0.0001 <I / I ₀ <1 with respect to the peak intensity ratio, the lithium ion secondary battery according to the present invention Included in the positive electrode active material.

Moreover, a lithium phosphate compound can further be included in the surface of the positive electrode active material for a lithium ion secondary battery of the present invention as necessary. Thereby, battery characteristics can be improved. As the lithium phosphate compound, Li ₃ PO ₄ , Li ₄ P ₂ O ₇ , LiPO ₃ , or a combination of two or more of these can be used. The content of the lithium phosphate compound in the positive electrode active material is preferably less than 2% by weight with respect to Compound A.

  Furthermore, it is preferable to have a compound containing carbon as a conductivity improving additive on the surface of the primary particles of the positive electrode active material containing the compounds A, A ′ and the compound B. Examples of the raw material for the conductivity improving additive are not particularly limited as long as the conductivity can be improved, but saccharides such as sucrose, citric acid, sucrose, graphite, Or carbon black, such as acetylene black, these combinations, etc. can be mentioned. The content of the conductivity enhancing additive is preferably between 1.5 wt% and 10 wt% with respect to Compound A.

  A lithium ion secondary battery can be obtained by combining the positive electrode having the positive electrode active material as described above and a commonly used negative electrode, electrolyte, separator, container, and the like.

  The negative electrode active material used for the negative electrode is not particularly limited as long as the material can occlude and release lithium ions. Examples thereof include artificial graphite, natural graphite, non-graphitizable carbons, metal oxides, metal nitrides, activated carbon and the like. Any of these may be used alone or in admixture of two or more.

  The negative electrode active material is mixed with a binder, a thickener, a conductive material, a solvent, etc. as necessary to prepare a negative electrode mixture slurry, which is then applied to a current collector and dried to obtain a desired A negative electrode can be produced by cutting into a shape or the like.

  As a separator constituting the lithium ion secondary battery, for example, a porous sheet or a nonwoven fabric made of a polyolefin such as polyethylene or polypropylene can be used.

A conventional general configuration can be adopted as the electrolyte of the lithium ion secondary battery. Usually, the electrolytic solution contains an electrolyte such as LiPF ₆ , an additive such as vinylene carbonate (hereinafter, VC), and a solvent such as a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC). The type of solvent, the type and composition of the electrolyte, the type of additive, and the like are not particularly limited as long as they are substances used for lithium ion secondary batteries. Examples of the electrolyte include LiPF ₆ , Lithium salts such as LiBF ₄ and LiClO ₄ , or combinations thereof can be used, and among these, it is particularly preferable to use LiPF ₆ as a part of the electrolyte. The concentration of the electrolyte is not particularly limited as long as the lithium salt is contained in the electrolytic solution, but is preferably 0.4 mol / L to 2.0 mol / L in the electrolytic solution.

The solvent of the electrolytic solution is not particularly limited as long as it is a solvent generally used for lithium ion secondary batteries. For example, EC, EMC, dimethyl carbonate (hereinafter, DMC), diethyl carbonate (hereinafter, DEC) are applicable. An organic solvent such as these or a combination thereof can be used. The additive is not particularly limited as long as it is an additive used for a lithium ion secondary battery. VC, unsaturated sultone represented by the chemical formula C ₃ H ₄ O ₃ S, or a combination thereof Can be used.

  By using the above components, lithium ion secondary batteries having various shapes such as a coin shape, a cylindrical shape, a square shape, and an aluminum laminate sheet shape can be assembled.

  Hereinafter, the present invention will be described in more detail based on examples and comparative examples. The present invention is not limited by these examples unless it exceeds the gist.

Example 1
(1) Synthesis of Compound A A beaker was placed in an oil bath of silicone oil heated on a magnetic stirrer, and in the beaker, a solvent A in which H ₃ PO ₄ was added to a triethanolamine solvent was added with a stirrer. Heated with stirring. Subsequently, an Fe source solution in which iron (II) sulfate heptahydrate is dissolved in a solution obtained by adding H ₃ PO ₄ to water, and lithium acetate dihydrate and manganese (II) sulfate pentahydrate in water A mixed source solution in which a hydrate was dissolved was prepared, and the solution was added dropwise to the solvent A at the same time.

  The temperature of the solvent after dropping was kept at 150 ° C. At this time, the molar ratio of each element in the source solution was adjusted so that the composition ratio of the synthesized olivine compound was Li: Mn: Fe: P = 1: 0.8: 0.2: 1. For the adjustment of the molar ratio, the composition ratio by high frequency induction plasma emission spectroscopy (ICP-AES analysis) was referred. After completion of the dropwise addition, stirring was continued while maintaining the solvent at 150 ° C. for another hour, and then sealed in a PTFE autoclave and heated and pressurized at 200 ° C.

  After heating and pressing, a precipitate was obtained at the bottom of the autoclave. The precipitate was filtered, washed thoroughly with ethanol, and then dried at 120 ° C. for 2 hours or more to obtain Compound A. The average particle size of the primary particles of Compound A was 50 nm.

  About the obtained compound A, the phase was identified by the powder X-ray diffraction method. As a result of the measurement, it was confirmed that the diffraction pattern of Compound A belongs to the olivine structure (space group Pnma).

(2) Production of Positive Electrode Using Compound A Subsequently, 12 parts by weight of sucrose as a conductivity improving additive was added to 100 parts by weight of Compound A, and pulverized with acetone for 33 hours. Thereafter, the sample was heated at a temperature of 700 ° C. in an Ar atmosphere to prepare a composite material in which the conductivity enhancing additive was formed on the surface of the primary particles of Compound A. The average particle size of the primary particles of the composite material was about 50 nm.

  A slurry was prepared by weighing 80 parts by weight of the composite material, 10 parts by weight of acetylene black as a conductive material, and 10 parts by weight of a binder. As the binder, any binder can be used as long as the binding property can be ensured. For example, it is preferable to use polyvinylidene fluoride, acrylonitrile-based copolymer, or the like. This slurry was applied onto an aluminum foil as a current collector to produce a positive electrode. In this case, when the aluminum foil used had an average roughness Ra of 0.3 μm or more as the surface roughness, the binding property was particularly excellent and preferred. However, in the present invention, the aluminum foil with Ra of 0.3 μm or less was used. But the effect was recognized.

(3) Formation of Compound B Using the above positive electrode containing Compound A and a negative electrode of metallic lithium, a bipolar model cell having high airtightness was produced. The positive electrode used was molded into a circular shape of 15 mmφ. Further, a laminated separator of polypropylene and polyethylene having a thickness of 30 μm was used as the separator. After the bipolar model cell was fully charged, only the positive electrode was taken out and heated at 80 ° C. for 20 days in an electrolytic solution containing H ₂ O (hereinafter referred to as treatment 1). As an electrolytic solution, a solution in which LiPF _{6 as} an electrolyte and vinylene carbonate (hereinafter, VC) as an additive are dissolved in a mixed solvent of ethylene carbonate (hereinafter, EC) and ethyl methyl carbonate (hereinafter, EMC). Using.

  As a condition for fully charging the bipolar model cell, the capacity of the positive electrode at full charge is 1/10 of the current value that can be charged in 1 hour, and converted to a potential difference from metallic lithium up to 4.3V , A constant current constant voltage charging condition (hereinafter referred to as charging condition 1) was adopted with an upper limit of 25 hours. At this time, if the current value becomes 1 / 100th of the current value (hereinafter referred to as 1C current value) that can be estimated that the capacity of the positive electrode at full charge can be charged in 1 hour, it will be 25 hours Charging stopped without reaching.

  The ratio of Compound B, which is a region formed by treatment 1 and in which Li desorption was inhibited, was measured using a powder X-ray diffraction method in the primary particles of the positive electrode active material. That is, quantitative evaluation was performed based on the peak intensity ratio between the (200) plane of the compound A ′ belonging to the Pnma space group and the (200) plane of the compound B from which Li was eliminated from the compound A. CuKα rays were used as the X-ray source.

The X-ray diffraction pattern of the positive electrode active material after treatment 1 was measured. The result is shown in FIG. For comparison, an X-ray diffraction pattern of Compound A not subjected to Treatment 1 and Compound A (that is, Compound A ′) charged under charging condition 1 without treatment 1 is also shown. The lattice constant of compound A is 10.40Å, 6.05Å and 4.72Å in the a, b and c axis directions, respectively, whereas the lattice constant of compound B is 10.0Å in the a, b and c axis directions, respectively. From the fact that it was 25Å, 6.03Å and 4.77Å, it became clear that Compound B is a substance different from Compound A. The intensity ratio I / I ₀ between the peak intensity I of the (200) plane of compound B and the peak intensity I ₀ of the (200) plane of compound A ′ was 0.1.

(4) Characteristics of lithium ion secondary battery After washing the positive electrode, the charge / discharge characteristics of the lithium ion secondary battery were evaluated. As the charging condition, the above-described charging condition 1 is used. As the discharging condition, a positive battery having a positive electrode active material is used, and a lithium ion secondary battery model using metallic lithium as the negative electrode is used. A condition of constant current discharge (hereinafter referred to as discharge condition 1) was used with an upper limit of 25 hours up to 2.0 V in terms of a potential difference from metallic lithium at a value of 1/10 of the value. The evaluation of capacity shows a value after initialization by repeating discharge and charging for 3 cycles using a bipolar model cell in which the metallic lithium of the negative electrode is replaced. As a result of the measurement, the discharge capacity was 145 mAh / g.

  After initialization, charge at a constant current and constant voltage up to a voltage of 4.4V at a current value that is half of the 1C current value, and stop charging when the current value becomes 1/20 of the 1C current value. A charge / discharge cycle in which constant current discharge was performed up to 2 V at a current value that was a half of the 1C current value was repeated 100 times (hereinafter referred to as cycle condition 1). The discharge capacity at the second cycle obtained at that time and the capacity ratio at the 100th cycle were measured. As a result, the capacity ratio was 97.2%. The above results are shown in Tables 1 and 2.

(Example 2)
The synthesis method of Compound A and the production method of the positive electrode using Compound A were the same as in Example 1, but the composition ratio of Compound A after synthesis was Li: Mn: Fe: Mg: P = 1: 0.8. : 0.17: 0.03: 1 Mg nitrate was further dissolved in the mixed source solution, and the molar ratio of each element in the source solution was adjusted. The average particle size of the primary particles of Compound A was 50 nm. Thereafter, the battery composition and the like were all the same, and the positive electrode after full charge was heated at 80 ° C. for 20 days in the electrolyte.

As a result of measuring the X-ray diffraction pattern of the positive electrode active material after the heat treatment, the lattice constant of Compound B was 10.23 mm, 6.00 mm and 4.75 mm in the a, b and c axis directions, respectively. The intensity ratio I / I ₀ between the peak intensity I of the (200) plane of compound B and the peak intensity I ₀ of the (200) plane of compound A ′ was 0.1.

  The lithium ion secondary battery using the positive electrode was evaluated for characteristics in the same manner as in Example 1. As a result, the discharge capacity was 148 mAh / g. Furthermore, as a result of measuring the capacity ratio after repeating 100 cycles of charge and discharge under the cycle condition 1 described in Example 1, it was 97.4%. The above results are shown in Tables 1 and 2.

(Example 3)
The method for synthesizing compound A and the method for producing the positive electrode using compound A were the same as in Example 1, but the composition ratio of compound A after synthesis was Li: Mn: Fe: Zn: P = 1: 0.8. : Zn nitrate was further dissolved in the mixed source solution so as to be 0.17: 0.03: 1, and the molar ratio of each element in the source solution was adjusted. The average particle size of the primary particles of Compound A was 50 nm. Thereafter, the battery composition and the like were all the same, and the positive electrode after full charge was heated in an electrolytic solution at 80 ° C. for 20 days.

As a result of measuring the X-ray diffraction pattern of the positive electrode active material after the heat treatment, the lattice constant of Compound B was 10.23 mm, 6.08 mm and 4.82 mm in the a, b and c axis directions, respectively. The intensity ratio I / I ₀ between the peak intensity I of the (200) plane of compound B and the peak intensity I ₀ of the (200) plane of compound A ′ was 0.1.

  Moreover, when the characteristic evaluation was performed similarly to Example 1 about the lithium ion secondary battery using the said positive electrode, the discharge capacity was 151 mAh / g. Furthermore, as a result of measuring the capacity ratio after repeating 100 cycles of charge and discharge under the cycle condition 1 described in Example 1, it was 96.9%. The above results are shown in Tables 1 and 2.

Example 4
The method for synthesizing compound A and the method for producing the positive electrode using compound A were the same as in Example 1, but the composition ratio of compound A after synthesis was Li: Mn: Fe: P = 1: 0.1: 0. The molar ratio of each element in the source solution was adjusted to 9: 1. The average particle size of the primary particles of Compound A was 50 nm. Thereafter, the battery composition and the like were all the same, and the positive electrode after full charge was heated in an electrolytic solution at 80 ° C. for 20 days.

As a result of measuring the X-ray diffraction pattern of the positive electrode active material after the heat treatment, the lattice constant of Compound B was 10.33Å, 6.00Å and 4.70Å in the a, b and c axis directions, respectively. Further, the intensity ratio I / I ₀ between the peak intensity I of the (200) plane of Compound B and the peak intensity I ₀ of the (200) plane of Compound A ′ was 0.01.

  Moreover, when the characteristic evaluation was performed similarly to Example 1 about the lithium ion secondary battery using the said positive electrode, the discharge capacity was 156 mAh / g. Furthermore, as a result of measuring the capacity ratio after repeating 100 cycles of charge and discharge under the cycle condition 1 described in Example 1, it was 96.9%. The above results are shown in Tables 1 and 2.

(Example 5)
After compound A was synthesized in the same manner as in Example 1, compound A was added to ion-exchanged water having a pH of 10.0 by adding LiOH as a pH adjuster and stirred for 1 hour. Thereafter, ion exchange water in which diammonium hydrogen phosphate was dissolved was added to the aqueous solution, and the mixture was further stirred for 1 hour and then dried (hereinafter, treatment 2). The molar ratio of each element in the source solution was adjusted so that the composition ratio of Compound A was Li: Mn: Fe: P = 1: 0.8: 0.2: 1. The average particle size of the primary particles of Compound A was 50 nm. The lithium phosphate compound contained on the surface of Compound A was Li ₃ PO ₄ . The amounts of LiOH and diammonium hydrogen phosphate in the aqueous solution were adjusted so that the amount of Li ₃ PO ₄ was 1% by weight with respect to Compound A. A positive electrode was produced in the same manner as in Example 1. Thereafter, the battery composition and the like were all the same, and the positive electrode after full charge was heated at 80 ° C. for 20 days in the electrolyte.

As a result of measuring the X-ray diffraction pattern of the positive electrode active material after the heat treatment, the lattice constant of Compound B was 10.25０．２, 6.03Å and 4.77Å in the a, b and c axis directions, respectively. The intensity ratio I / I ₀ between the peak intensity I of the (200) plane of compound B and the peak intensity I ₀ of the (200) plane of compound A ′ was 0.02.

Moreover, when the characteristic evaluation was performed similarly to Example 1 about the lithium ion secondary battery using the said positive electrode, the discharge capacity was 145 mAh / g. Furthermore, as a result of measuring the capacity ratio after repeating 100 cycles of charge and discharge under the cycle condition 1 described in Example 1, it was 97.5%. The same strength ratio I / I ₀ was obtained when the above treatment 2 was applied to the positive electrode after ball milling. The above results are shown in Tables 1 and 2.

(Example 6)
After compound A was synthesized in the same manner as in Example 1, lithium phosphate compound (Li ₃ PO ₄ ) was added to compound A in the same manner as in Example 5. In addition, Mg nitrate is further dissolved in the mixed source solution so that the composition ratio of Compound A is Li: Mn: Fe: Mg: P = 1: 0.82: 0.15: 0.03: 1, The molar ratio of each element in the source solution was adjusted. The average particle size of the primary particles of Compound A was 50 nm. Further, the amounts of LiOH and diammonium hydrogen phosphate in the aqueous solution were adjusted so that the amount of Li ₃ PO ₄ contained on the surface of Compound A was 0.5% by weight with respect to Compound A. A positive electrode was produced in the same manner as in Example 1. Thereafter, the battery composition and the like were all the same, and the positive electrode after full charge was heated in an electrolytic solution at 80 ° C. for 20 days.

As a result of measuring the X-ray diffraction pattern of the positive electrode active material after the heat treatment, the lattice constant of Compound B was 10.20 mm, 5.98 mm and 4.72 mm in the a, b and c axis directions, respectively. The intensity ratio I / I ₀ between the peak intensity I of the (200) plane of compound B and the peak intensity I ₀ of the (200) plane of compound A ′ was 0.05.

Moreover, when the characteristic evaluation was performed similarly to Example 1 about the lithium ion secondary battery using the said positive electrode, the discharge capacity was 147 mAh / g. Furthermore, as a result of measuring the capacity ratio after repeating 100 cycles of charging and discharging under the cycle condition 1 described in Example 1, it was 97.6%. The same strength ratio I / I ₀ was obtained when the above treatment 2 was applied to the positive electrode after ball milling. The above results are shown in Tables 1 and 2.

(Comparative Example 1)
Compound A was synthesized with the same synthesis method and composition ratio as in Example 1. The average particle size of the primary particles of Compound A was 50 nm. A positive electrode was produced in the same manner as in Example 1. Thereafter, the battery composition and the like were all the same, and the positive electrode after full charge was heated at 80 ° C. for 160 days in an electrolytic solution in order to increase the amount of compound B produced.

As a result of measuring the X-ray diffraction pattern of the positive electrode active material after the heat treatment, the lattice constant of Compound B was 10.24 mm, 6.03 mm and 4.78 mm in the a, b and c axis directions, respectively. Further, the intensity ratio I / I ₀ between the peak intensity I of the (200) plane of Compound B and the peak intensity I ₀ of the (200) plane of Compound A ′ was 1.

  The lithium ion secondary battery using the positive electrode was evaluated for characteristics in the same manner as in Example 1. As a result, the discharge capacity was 48 mAh / g. Furthermore, as a result of measuring the capacity ratio after repeating 100 cycles of charge and discharge under the cycle condition 1 described in Example 1, it was 96.4%. The above results are shown in Tables 1 and 2.

(Comparative Example 2)
Compound A was synthesized with the same synthesis method and composition ratio as in Example 1. The average particle size of the primary particles of Compound A was 50 nm. A positive electrode was produced in the same manner as in Example 1. Thereafter, the battery composition and the like were all the same, and in order to reduce the amount of compound B produced, the fully charged positive electrode was heated at 50 ° C. for 5 days in an electrolytic solution.

As a result of measuring the X-ray diffraction pattern of the positive electrode active material after the heat treatment, the lattice constant of Compound B was 10.25０．２, 6.03Å and 4.77Å in the a, b and c axis directions, respectively. The intensity ratio I / I ₀ between the peak intensity I of the (200) plane of compound B and the peak intensity I ₀ of the (200) plane of compound A ′ was 0.0001.

  The lithium ion secondary battery using the positive electrode was evaluated for characteristics in the same manner as in Example 1. As a result, the discharge capacity was 148 mAh / g. Furthermore, as a result of measuring the capacity | capacitance ratio after repeating charging / discharging 100 cycles on the cycle conditions 1 as described in Example 1, it was 94.3%. The above results are shown in Tables 1 and 2.

(Comparative Example 3)
In this comparative example 3, the characteristics when the compound B was not generated were evaluated. Other configurations and conditions were the same as in Example 1. As a result, the discharge capacity was 151 mAh / g. Furthermore, as a result of measuring the capacity ratio after repeating 100 cycles of charge and discharge under the cycle condition 1 described in Example 1, it was 93.1%. The above results are shown in Tables 1 and 2. In addition, the horizontal bar in a table | surface means having not measured since there is no meaning in measuring.

(Comparative Example 4)
In this Comparative Example 4, in order to increase the average particle size of the primary particles of Compound A, when compound A was synthesized, the composition ratio was made the same as in Example 1, and heating in a PTFE autoclave was performed. The pressing temperature was 230 ° C. As a result, the average particle size of the primary particles was 500 nm. Then, a positive electrode was produced in the same manner as in Example 1, and then the battery composition and the like were all the same, and the fully charged positive electrode was heated in an electrolytic solution for 20 hours at 80 ° C. in order to produce Compound B. .

As a result of measuring the X-ray diffraction pattern of the positive electrode active material after the heat treatment, the lattice constant of Compound B was 10.25０．２, 6.03Å and 4.76Å in the a, b and c axis directions, respectively. The intensity ratio I / I ₀ between the peak intensity I of the (200) plane of compound B and the peak intensity I ₀ of the (200) plane of compound A ′ was 0.007.

  Moreover, when the characteristic evaluation was performed similarly to Example 1 about the lithium ion secondary battery using the said positive electrode, the discharge capacity was 100 mAh / g. Furthermore, as a result of measuring the capacity ratio after repeating 100 cycles of charge and discharge under the cycle condition 1 described in Example 1, it was 94.2%. The above results are shown in Tables 1 and 2.

(Discussion)
In the examples and comparative examples of the present invention, only the positive electrode is taken out from the bipolar model cell and the treatment 1 is performed in the electrolytic solution. However, it is not always necessary to take out the positive electrode from the cell and perform the treatment 1, The same effect was obtained even when the treatment 1 was performed while being placed in a battery in which an electrolytic solution containing H ₂ O was used. The same effect was obtained even when the negative electrode was not made of metallic lithium.

From the results of Examples 1 to 6 and Comparative Examples 1 and 2, it was found that both the discharge characteristics and the cycle characteristics were excellent when 0.0001 <I / I ₀ <1. On the other hand, when I / I ₀ exceeds 1, it has been found that the discharge capacity decreases. This is presumably because the capacity utilization rate decreased because the proportion of compound B in which lithium insertion / extraction did not occur increased. Further, from the result of Comparative Example 2, it was revealed that the capacity retention rate after 100 cycles is lower than that of each Example when the value is less than 0.0001. This means that the effect of the present invention is hardly obtained when the proportion of compound B decreases. In particular, Comparative Example 3, in which the peak intensity I of Compound B was not observed, had the lowest capacity retention rate among all the samples.

  In Examples 2 and 3, examples using Mg or Zn as M were shown. Even when Al, Ti, Co, Ni or Cu was selected as M, the cycle characteristics were improved by introducing Compound B.

  From the results of Examples 1 to 6, it was found that the lattice volume of Compound B was smaller than that of Compound A. Although the lattice volume of compound A shrinks with charging, when the lattice constant of compound B is smaller than that of compound A, the mismatch between the lattices of compound A and compound B is relaxed along with the discharge, leading to primary particles due to expansion and contraction. It is considered that the damage is reduced.

Furthermore, as is clear from the comparison between Example 1 and Example 4, good results were obtained even if the ratio of Fe and Mn was changed within the range of 0.0001 <I / I ₀ <1. . Further, the positive electrode of Example 4 was further heat-treated at 80 ° C. for a longer time, and X-ray diffraction measurement of the positive electrode active material was performed after 4 months. As a result, Compound B appeared as a main peak in the X-ray diffraction pattern. As a result of ICP analysis of the substance, the total ratio of Mn and Fe with respect to P in the chemical formula was decreased as compared with A 'which is a compound obtained by eliminating Li from Compound A or A. This degree of reduction changes by changing the heat treatment conditions, and it has been found that the longer the heat treatment time and the higher the heat treatment temperature, the greater the degree of reduction. When the same experiment was performed after the electrode was once exposed to the atmosphere and dried, the degree of decrease was increased. As described above, the desired I / I ₀ ratio could be obtained in a shorter time depending on the temperature conditions and the conditions for drying the electrodes in the atmosphere. However, these effects limited the effects obtained in the present invention. It was never done.

From the analysis results of the samples treated under various conditions as described above, the composition of Compound B produced when the treatment in the present invention was performed could be defined. As a result, in the chemical formula, Li _δ Fe _ε PO ₄ (where 0 <δ ≦ 1.2, 0 <ε <1), Li _κ Fe _λ M _ρ PO ₄ (where 0 <κ ≦ 1.2). , 0 <λ <1.2, 0 <ρ <1.2, λ + ρ <1.2), and Li _σ Mn _τ Fe _ω M _ζ PO ₄ (where 0 <σ ≦ 1.2, 0 <τ <1.2, 0 <ω <1.2, 0 <ζ <1.2, 0 <τ + ω + ζ <1.2), more specifically, Li _2−β Fe _β PO ₄ , Li _{2−β− γ} Fe _β M _γ PO _4, and Li _{(where, 0 <i ≦ α) 2} -α + i Mn α-i Fe β M γ PO 4 was found to be.

  Furthermore, from comparison between Example 1 and Comparative Example 4, it was found that the characteristics were excellent when the average particle size of the primary particles containing Compound A and Compound B was less than 500 nm. This is presumably because the smaller the primary particles, the higher the strain relaxation and the capacity utilization.

  The present invention is greatly characterized in that the compound B having the composition ratio does not desorb Li. That is, by performing a special treatment in the present invention, a part of the olivine structure has a crystal structure in which Fe is partly Li or partly Li is dissolved in the Fe site, and part of elements of Mn and Fe sites. A domain in which Li is partly dissolved in a structure in which a deficient domain or an element such as Mn or Fe is deficient is formed, and a domain in which the elimination of Li is inhibited by this is formed in a primary particle or a secondary Introduce into the particles. In other words, only a part of the compound A or the compound A ′ is converted to the compound B by the special treatment in the present invention. Thereby, it has a great feature in that it has the effect of maintaining the binding property between the olivine compound and other positive electrode structural members, which decreases with an increase in the number of cycles.

Further, Examples 5 and 6 show cases where a lithium phosphate compound is further provided on the surface of the positive electrode active material for a lithium ion secondary battery in the present invention. Even in this case, in the X-ray diffraction pattern of the positive electrode active material, good results are obtained when the intensity ratio I / I ₀ between the peak intensity I and the peak intensity I ₀ is within the range of 0.0001 <I / I ₀ <1. Obtained. In particular, it was found that Examples 5 and 6 had the highest capacity retention rate and were preferable.

In Examples 5 and 6, Li ₃ PO ₄ was used as the lithium phosphate compound, but in addition, excellent battery characteristics were obtained when Li ₄ P ₂ O ₇ and LiPO ₃ were used. In Examples 5 and 6, LiOH and diammonium hydrogen phosphate were used to contain the lithium phosphate compound in the active material, but the method of inclusion is not limited to the above.

  The content of the lithium phosphate compound is not limited to Examples 5 to 6 and can be set as appropriate. When the content is less than 2% by weight with respect to the weight of the compound A, the discharge capacity is high, preferable. Although this cause is not certain, it is thought that Li diffusion from the positive electrode active material and electronic conduction were inhibited by the lithium phosphate compound.

  Although it does not specifically limit as a use of the positive electrode active material obtained by this invention, For example, it can use for a lithium ion secondary battery etc.

  The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the present invention has substantially the same configuration as the technical idea described in the claims of the present invention, and any device that exhibits the same function and effect is the present invention. It is included in the technical scope of the invention.

Claims (11)

It belongs to the Pnma space group and has the chemical formula Li _x Mn _α Fe _β M _γ PO ₄ (where 0 <x ≦ 1.2, 0 <α <1.2, 0 <β <1.2, 0 ≦ γ <1 .2, M is one or more elements selected from the group consisting of Mg, Al, Ti, Co, Ni, Cu and Zn),
A compound B belonging to the Pnma space group and different from the compound A;
A positive electrode active material for a lithium ion secondary battery comprising:
When a lithium ion secondary battery using a positive electrode having the positive electrode active material is fully charged and an X-ray diffraction pattern of the positive electrode active material is measured,
A compound represented by the chemical formula Li _y Mn _α Fe _β M _γ PO ₄ (where 0 ≦ y <0.06, where x> y) belonging to the Pnma space group, from which Li is eliminated from the compound A A group of diffraction peaks due to the crystal structure of A ′;
A diffraction peak group due to the crystal structure of Compound B is observed;
At that time, the intensity ratio I / I ₀ between the peak intensity I of the (200) plane of the compound B and the peak intensity I ₀ of the (200) plane of the compound A ′ is 0.0001 <I / I ₀ <1. Yes,
The positive electrode active material for a lithium ion secondary battery, wherein an average particle size of primary particles constituting the positive electrode active material is less than 500 nm.   The lattice constant of Compound B in the Pnma space group is 10.20 to 10.33 inches in the a-axis direction, 5.98 to 6.08 inches in the b-axis direction, and 4.70 to 4.82 inches in the c-axis direction. The positive electrode active material for lithium ion secondary batteries according to 1.   The compound B has a diffraction angle of the (200) plane different from both of the diffraction angles of the (200) plane of the compound A and the compound A ′, and Li does not desorb in a fully charged state. Positive electrode active material for lithium ion secondary batteries. Compound B includes Li _δ Fe _ε PO ₄ (where 0 <δ ≦ 1.2, 0 <ε <1), Li _κ Fe _λ M _ρ PO ₄ (where 0 <κ ≦ 1.2, 0 <Λ <1.2, 0 <ρ <1.2, λ + ρ <1.2), and Li _σ Mn _τ Fe _ω M _ζ PO ₄ (where 0 <σ ≦ 1.2, 0 <τ <1 The compound represented by one or more chemical formulas selected from the group of .2, 0 <ω <1.2, 0 <ζ <1.2, 0 <τ + ω + ζ <1.2) The positive electrode active material for lithium ion secondary batteries in any one. Compound B contains Li _2-β Fe _β PO ₄ , Li _2-β-γ Fe _{βM γ} PO ₄ , and Li _{2-α + i} Mn _α-i Fe _{βM γ} PO ₄ (where 0 <i ≦ α The positive electrode active material for lithium ion secondary batteries according to any one of claims 1 to 4, which is a compound represented by one or more chemical formulas selected from the group of   Compound B has a different crystal structure from Compound A and Compound A ′, the lattice volume of Compound B is smaller than that of Compound A, and the lattice constant in the Pnma space group is 10.20Å to 10.33Å in the a-axis direction, The b-axis direction is 5.98 to 6.08 mm, the c-axis direction is 4.70 to 4.82 mm, and the total ratio of Mn, Fe and M elements to P in the chemical formula is determined by the compound A and the compound A ′. The positive electrode active material for a lithium ion secondary battery according to claim 4, wherein the positive electrode active material is a small compound as compared with the above.   The said positive electrode active material for lithium ion secondary batteries which further contains a lithium phosphate compound on the surface of the positive electrode active material for lithium ion secondary batteries in any one of Claims 1-6. The positive electrode active material for a lithium ion secondary battery according to claim ₇ , wherein the lithium phosphate compound is one or more selected from the group consisting of Li ₃ PO ₄ , Li ₄ P ₂ O ₇ , and LiPO ₃ .   As a fully charged condition, a lithium ion secondary battery using a positive electrode having a positive electrode active material and metallic lithium as a negative electrode is obtained by using a metal at a value that is 1/10 of the current value that can charge the capacity of the positive electrode in one hour. The positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 8, which is charged up to 4.3V in terms of a potential difference from lithium, with an upper limit of 25 hours.   The positive electrode for lithium ion secondary batteries which has the positive electrode active material for lithium ion secondary batteries in any one of Claims 1-9.   The lithium ion secondary battery using the positive electrode for lithium ion secondary batteries of Claim 10 as a positive electrode.

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