JP2017188424A - Positive electrode active material for lithium ion secondary battery and lithium ion secondary battery positive electrode using the same, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery excellent in discharge capacity and cycle characteristics.SOLUTION: A positive electrode active material for a lithium ion secondary battery contains: a first compound represented by chemical formula Li(NiMa)O(where 0.95≤x≤1.05 and 0.70≤y≤0.95 are satisfied, and Ma represents at least one element selected from Co, Mn, V, Ti, Fe, Zr, Nb, Mo, Al, and W); and a second compound represented by chemical formula LiVOPO. In the positive electrode active material, W>5.0°C is satisfied where W is a full width at half maximum of an exothermic peak obtained between 150°C and 260°C by differential scanning calorimetry (DSC) performed on a mixture of the first compound and the second compound under a condition of 5°C/min.SELECTED DRAWING: Figure 1

Description

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

Compared with lithium cobalt oxide LiCoO ₂ , which is a typical existing active material for lithium ion secondary batteries, lithium nickel composite oxides using Ni, Co, Mn, Ni, Co, Al, etc. are more charged and discharged. It is known that capacity can be obtained.

  However, the lithium-nickel composite oxide has a lower thermal decomposition starting temperature than lithium cobaltate, and furthermore, the Ni valence becomes tetravalent at the time of charging and the stability of the crystal structure is lowered, so that the deterioration due to the cycle increases. In addition, since the positive electrode active material during charging causes desorption of Li ions and generation of electrons, a large amount of heat is generated in the vicinity of the active material, and it is assumed that energy corresponding to the thermal decomposition start temperature is partially generated. In addition, deterioration due to heat is also expected.

In order to improve the instability problem of such lithium-nickel composite oxides, a technique for combining with a material having excellent thermal stability such as a spinel compound such as LiMn ₂ O ₄ and a phosphate compound such as LiFePO ₄ is known. It has been.

  In Patent Document 1, an effort is made to improve reliability by combining a lithium nickel composite oxide and a spinel compound or a phosphoric acid compound having excellent thermal stability and forming a multilayer. However, although the thermal stability of the entire electrode can be improved by the method of Patent Document 1, the effect is greatly weakened in a fine region around the active material and the cycle characteristics are not improved. Moreover, in patent document 1, since the occupation rate of the spinel compound and phosphoric acid compound in the whole electrode is comparatively high, there also exists a subject that discharge capacity becomes low. Therefore, further increase in capacity and improvement in cycle characteristics of a mixed positive electrode active material including a lithium nickel composite oxide and a material having excellent thermal stability are required.

JP 2007-48744 A

  This invention is made | formed in view of the subject which the said prior art has, and aims at providing the positive electrode active material excellent in discharge capacity and cycling characteristics, the positive electrode using the same, and a lithium ion secondary battery. .

In order to achieve the above object, a positive electrode active material for a lithium ion secondary battery according to the present invention has a chemical formula Li _x (Ni _y Ma _1-y ) O ₂ (0.95 ≦ x ≦ 1.05, 0.70). ≦ y ≦ 0.95, Ma is at least one element selected from Co, Mn, V, Ti, Fe, Zr, Nb, Mo, Al, and W), and a chemical formula LiVOPO ₄ In a DSC analysis (differential scanning calorimetry) in which a mixture of the first compound and the second compound was measured under the condition of 5 ° C./min, When the half-value width of the exothermic peak at 0 ° C. is W, W> 5.0 ° C.

  In the positive electrode active material having such a structure, since the second compound having excellent thermal stability is mixed, rapid heat generation during charging is suppressed, so that distortion of the crystal structure is suppressed, and transition metals such as Ni and Co are suppressed. Thus, the valence fluctuation of the active material becomes small, and the crystal structure of the active material can be stabilized. As a result, it is speculated that elution of transition metals and desorption of oxygen are suppressed, and excellent cycle characteristics can be exhibited.

  The second compound according to the present invention is preferably contained in an amount of 1 to 30 wt% with respect to the sum of the weight of the first compound and the weight of the second compound.

  When the content of the second compound satisfies the above range, the first compound having a high capacity and the second compound having excellent thermal stability are mixed in a well-balanced manner, so that high cycle characteristics and an excellent discharge capacity are obtained. It can be compatible.

  It is preferable that at least a part of the surface of the primary particles of the first compound according to the present invention is coated with the second compound to form a coating layer.

Compared with the second compound, the first compound tends to generate heat on the low temperature side in DSC analysis (differential scanning calorimetry), has a large amount of heat generation, and tends to have a high heat generation rate. When the surface of the first compound is coated with the second compound, the instability factor of the first compound acts directly by the special function of the second compound, and rapid heat generation It is thought that it suppresses. As a result, structural destabilization of the active material is further suppressed, and excellent cycle characteristics are exhibited. Although this mechanism is not necessarily clear, it is assumed that V (vanadium) of the second compound, LiVOPO ₄ , is caused by flexibility due to its ability to take various valences.

  The primary particles of the first compound aggregate to form secondary particles, and at least a part of the surface of the secondary particles is coated with the second compound to form a coating layer. preferable.

  When the first compound forms secondary particles and the second compound is present on at least a part of the surface of the secondary particles, a positive electrode active material excellent in energy density and cycle characteristics can be obtained.

  The positive electrode active material for a lithium ion secondary battery is preferably contained in a positive electrode.

  According to this configuration, a positive electrode having excellent cycle characteristics can be obtained.

  The lithium ion secondary battery according to the present invention preferably includes a positive electrode, a negative electrode having a negative electrode active material, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte.

  According to such a configuration, a lithium ion secondary battery having excellent cycle characteristics can be obtained.

  ADVANTAGE OF THE INVENTION According to this invention, the positive electrode active material excellent in discharge capacity and cycling characteristics, the positive electrode using the same, and a lithium ion secondary battery can be provided.

It is a schematic cross section of a lithium ion secondary battery provided with the positive electrode and negative electrode which concern on one Embodiment of this invention. It is a DSC measurement result of the positive electrode active material which concerns on one Embodiment of this invention.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. In addition, this invention is not limited to the following embodiment. The constituent elements described below include those that can be easily assumed by those skilled in the art and those that are substantially the same. Furthermore, the constituent elements described below can be appropriately combined.

(Lithium ion secondary battery)
FIG. 1 is a schematic cross-sectional view showing a lithium ion secondary battery according to this embodiment. As shown in FIG. 1, the lithium ion secondary battery 100 mainly includes a laminate 30, a case 50 that accommodates the laminate 30 in a sealed state, and a pair of leads 60 and 62 connected to the laminate 30. ing.

  The laminated body 30 is configured such that a pair of a positive electrode 10 and a negative electrode 20 are disposed to face each other with a separator 18 interposed therebetween. The positive electrode 10 is obtained by providing a positive electrode active material layer 14 on a plate-like (film-like) positive electrode current collector 12. The negative electrode 20 is obtained by providing a negative electrode active material layer 24 on a plate-like (film-like) negative electrode current collector 22. The main surface of the positive electrode active material layer 14 and the main surface of the negative electrode active material layer 24 are in contact with the main surface of the separator 18. Leads 62 and 60 are connected to the end portions of the positive electrode current collector 12 and the negative electrode current collector 22, respectively, and the end portions of the leads 60 and 62 extend to the outside of the case 50.

  In the following description, either one or both of the positive electrode active material and the negative electrode active material are collectively referred to as an active material, and either one or both of the positive electrode active material layer and the negative electrode active material layer are collectively referred to. In some cases, one or both of the positive electrode and the negative electrode is collectively referred to as an electrode.

(Positive electrode active material layer)
The positive electrode active material layer 14 is mainly composed of a positive electrode active material, a binder, and an amount of the positive electrode conductive additive as required.

(Positive electrode active material)
The positive electrode active material of this embodiment has the chemical formula Li _x (Ni _y Ma _1-y ) O ₂ (0.95 ≦ x ≦ 1.05, 0.70 ≦ y ≦ 0.95, Ma is Co, Mn, V And at least one element selected from Ti, Fe, Zr, Nb, Mo, Al, and W), and a second compound represented by the chemical formula LiVOPO ₄ , In DSC analysis (differential scanning calorimetry) in which the mixture of the first compound and the second compound was measured under the condition of 5 ° C./min, when the half-value width of the exothermic peak from 150 ° C. to 260 ° C. is W, W > 5.0 ° C.

  In the positive electrode active material of this configuration, since the second compound having excellent thermal stability is mixed, rapid heat generation during charging is suppressed, so that distortion of the crystal structure is suppressed, and transition metals such as Ni and Co are suppressed. Thus, the valence fluctuation of the active material becomes small, and the crystal structure of the active material can be stabilized. As a result, it is speculated that elution of transition metals and desorption of oxygen are suppressed, and excellent cycle characteristics can be exhibited.

(Measurement of DSC analysis (differential scanning thermal analysis))
DSC analysis (differential scanning calorimetry) is measured by the following method. First, an electrode including a positive electrode active material, a conductive additive, and polyvinylidene fluoride (PVDF) according to the present embodiment is prepared. The electrode is produced by the method described later. Next, after weighing a predetermined amount of this electrode, it is placed in an aluminum container, a predetermined amount of electrolyte is added, and the same is covered with an aluminum lid and caulked. This measurement container is set in a DSC analyzer (Thermo Plus DSC8230 manufactured by RIGAKU) and measured at a heating rate of 5 ° C./min. Of the peaks obtained by DSC measurement, the full width at half maximum is calculated for a peak appearing in the range of 150 ° C. to 260 ° C.

  It is preferable to contain 1-30 wt% of the 2nd compound of this embodiment with respect to the sum total of the weight of a 1st compound, and the weight of a 2nd compound. Moreover, as the range, it is more preferable that it is 2.5-30.0 wt%. This further improves cycle characteristics. Furthermore, it is more preferable that it is 5-30.0 wt%, and, thereby, cycling characteristics improve further.

  When the content of the second compound satisfies the above range, the high-capacity first compound and the second compound having excellent thermal stability are mixed in a well-balanced manner, so that both high cycle characteristics and excellent discharge capacity are compatible. Can do.

  It is preferable that at least a part of the surface of the primary particles of the first compound of the present embodiment is coated with the second compound to form a coating layer.

  Compared to the second compound, the first compound tends to generate heat on the low temperature side in DSC analysis (differential scanning calorimetry), has a large amount of heat generation, and tends to have a higher heat generation rate. When the surface of the first compound is coated with the second compound, the instability factor of the first compound acts directly by the special function of the second compound and suppresses rapid heat generation it is conceivable that. As a result, structural destabilization of the active material is further suppressed, and excellent cycle characteristics are exhibited.

As a method of coating the second compound LiVOPO ₄ on the surface of the first compound Li _x (Ni _y Ma _1-y ) O ₂ , dry mixing using a pot mill containing zirconia balls or alumina balls, or a rough machine is used. Examples thereof include dry mixing, coating using a fluidized bed apparatus, and particle combination using mechanofusion. Moreover, the coating stability can also be improved by performing an appropriate heat treatment after the coating treatment. The heat treatment condition is preferably a temperature range of 300 ° C. to 500 ° C., and the atmosphere is preferably in air or an oxygen atmosphere.

  In the first compound of the present embodiment, the primary particles aggregate to form secondary particles, and at least a part of the surface of the secondary particles is coated with the second compound to form a coating layer. Is preferred.

  In addition, since the first compound forms secondary particles, a positive electrode active material excellent in energy density and cycle characteristics can be obtained.

(Positive electrode current collector)
The positive electrode current collector 12 may be a conductive plate material, and for example, a metal thin plate (metal foil) such as aluminum, an alloy thereof, or stainless steel can be used.

(Positive electrode binder)
The binder is not particularly limited as long as the positive electrode active material and the conductive material can be bound to the current collector 12, and a known binder can be used. Examples thereof include fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and vinylidene fluoride-hexafluoropropylene copolymer.

(Positive electrode conductive aid)
The positive electrode conductive auxiliary agent is not particularly limited as long as it improves the conductivity of the positive electrode active material layer 14, and a known conductive auxiliary agent can be used. Examples thereof include carbon-based materials such as graphite, carbon black, and graphene, metal fine powders such as copper, nickel, stainless steel, and iron, a mixture of carbon materials and metal fine powders, and conductive oxides such as ITO.

(Negative electrode active material layer)
The negative electrode active material layer 24 is mainly composed of a negative electrode active material, a binder, and a conductive auxiliary agent in an amount as required.

(Negative electrode active material)
As the negative electrode active material, for example, carbon materials such as graphite capable of inserting and extracting lithium ions, non-graphitizable carbon, graphitizable carbon, low-temperature calcined carbon, and lithium such as Al, Si, Sn, etc. Examples thereof include a metal, an amorphous compound mainly composed of an oxide such as SiO, SiO ₂ , SnO ₂ , and Fe ₂ O _3, and particles containing Li ₄ Ti ₅ O _{12 and the} like.

(Negative electrode current collector)
The negative electrode current collector 22 may be any conductive plate material, and for example, a metal thin plate (metal foil) of copper, nickel, stainless steel, or an alloy thereof can be used.

(Negative electrode binder)
In addition to the above-described materials exemplified as the positive electrode binder, cellulose, styrene / butadiene rubber, ethylene / propylene rubber, polyimide resin, polyamideimide resin, polyacrylic acid, or the like may be used as the binder.

(Negative conductive auxiliary)
A negative electrode conductive support agent is not specifically limited, A well-known conductive support agent can be used. Examples thereof include carbon materials such as pyrolytic carbon such as carbon black, cokes, glassy carbons, organic polymer compound fired materials, carbon fibers, and activated carbon. Moreover, you may add negative electrode active material materials, such as non-graphitizable carbon, easily graphitizable carbon, and graphite, changing a shape.

  With the components described above, the electrode can be produced by a commonly used method. For example, a paint containing an active material (a positive electrode active material or a negative electrode active material), a binder (a positive electrode binder or a negative electrode binder), a solvent, and a conductive additive (a positive electrode conductive additive or a negative electrode conductive aid) is placed on the current collector. It can manufacture by apply | coating and removing the solvent in the coating material apply | coated on the electrical power collector.

  As the solvent, for example, N methyl-2-pyrrolidone, N, N-dimethylformamide, water or the like can be used.

  There is no restriction | limiting in particular as an application | coating method, The method employ | adopted when producing an electrode normally can be used. Examples thereof include a slit die coating method and a doctor blade method.

  The method for removing the solvent in the paint applied on the current collector is not particularly limited, and the current collector applied with the paint may be dried, for example, in an atmosphere of 80 ° C. to 150 ° C.

  Then, the electrode on which the active material layer has been formed in this way may be then pressed by a roll press device or the like, if necessary. The linear pressure of the roll press can be, for example, 100 to 2,000 kgf / cm.

  Next, other components of the lithium ion secondary battery 100 will be described.

(Separator)
The separator 18 only needs to be formed of an electrically insulating porous structure, for example, a single layer of a film made of polyethylene, polypropylene or polyolefin, a stretched film of a laminate or a mixture of the above resins, or cellulose, polyester And a fiber nonwoven fabric made of at least one constituent material selected from the group consisting of polypropylene.

(Electrolytes)
The electrolyte is contained in the positive electrode active material layer 14, the negative electrode active material layer 24, and the separator 18. The electrolyte is not particularly limited. For example, in the present embodiment, an electrolyte containing a lithium salt can be used. However, the electrolyte aqueous solution is preferably an electrolyte that uses an organic solvent because the electrochemical decomposition voltage is low and the withstand voltage during charging is limited to be low. As the electrolyte, a lithium salt dissolved in an organic solvent is preferably used. Examples of the lithium salt include LiPF ₆ , LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiCF ₃ , CF ₂ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ , A salt such as LiN (CF ₃ CF ₂ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiN (CF ₃ CF ₂ CO) ₂ , or LiBOB can be used. In addition, these salts may be used individually by 1 type, and may use 2 or more types together.

  Moreover, as an organic solvent, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate etc. are mentioned preferably, for example. These may be used alone or in combination of two or more at any ratio.

  In the present embodiment, an example of the electrolyte has been described, but a gel electrolyte to which a gelling agent is added may be used. Moreover, it can replace with these electrolytes and can also use a solid electrolyte.

(Case)
The case 50 seals the laminated body 30 and the electrolyte therein. The case 50 is not particularly limited as long as it can prevent leakage of the electrolyte to the outside and entry of moisture and the like into the lithium ion secondary battery 100 from the outside. For example, as the case 50, as shown in FIG. 1, a metal laminate film in which a metal foil 52 is coated with a polymer film 54 from both sides can be used. As the metal foil 52, for example, an aluminum foil can be used, and as the synthetic resin film 54, a film such as polypropylene can be used. For example, the material of the outer polymer film 54 is preferably a polymer having a high melting point such as polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is preferably polyethylene or polypropylene.

(Lead)
The leads 60 and 62 are made of a conductive material such as aluminum. Then, the leads 62 and 60 are respectively welded to the positive electrode current collector 12 and the negative electrode current collector 22 by a known method, and a separator is provided between the positive electrode active material layer 14 of the positive electrode 10 and the negative electrode active material layer 24 of the negative electrode 20. 18 may be inserted into the case 50 together with the electrolytic solution with the 18 interposed therebetween, and the entrance of the case 50 may be sealed.

  The lithium ion secondary battery of the present embodiment preferably includes a positive electrode, a negative electrode having a negative electrode active material, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte.

  A lithium ion secondary battery excellent in discharge capacity and cycle characteristics can be obtained by combining the positive electrode with an electrolyte and a negative electrode.

  The preferred embodiment of the present invention has been described above, but the present invention is not limited to the above embodiment. For example, the lithium ion secondary battery is not limited to the shape shown in FIG. It may be a type or the like.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

Example 1
(Preparation of positive electrode active material)
For Li _1.00 (Ni _0.85 Co _0.10 Al _0.05 ) O ₂ 100 g forming secondary particles as the first compound, LiVOPO ₄ as the second compound, After adding the second compound so that the weight of the second compound is 5.0% by weight with respect to the sum of the weight of the second compound and the weight of the second compound, using a mechanofusion made by Hosokawa Micron The particle composite treatment was performed. Subsequently, it heat-processed for 10 minutes on 350 degreeC conditions in oxygen stream, and the positive electrode active material of Example 1 was obtained.

(Preparation of positive electrode)
A mixture of the positive electrode active material of Example 1, polyvinylidene fluoride (PVDF) as a binder, and acetylene black was dispersed in N-methyl-2-pyrrolidone (NMP) as a solvent to prepare a slurry. Note that the slurry was prepared so that the weight ratio of the positive electrode active material, acetylene black, and PVDF was 92: 3: 5 in the slurry. This slurry was applied on an aluminum foil as a positive electrode current collector, dried, and then rolled to obtain a positive electrode on which an active material layer containing a positive electrode active material was formed.

(Preparation of negative electrode)
A mixture of graphite as a negative electrode active material, styrene / butadiene rubber and cellulose as a binder, and carbon black as a conductive additive was dispersed in pure water as a solvent to prepare a slurry. This slurry was applied on a copper foil as a negative electrode current collector, dried, and then rolled to obtain a negative electrode on which an active material layer containing a negative electrode active material was formed.

(Production of evaluation cell)
The positive electrode and negative electrode prepared as described above and a separator made of a polyethylene porous film are cut into predetermined dimensions, and laminated in such a sequence as negative electrode, separator, positive electrode, separator, and negative electrode to form four negative electrodes and three positive electrodes. did. This laminate was put in an aluminum laminate pack, an electrolyte solution was injected, and then vacuum-sealed to produce a lithium ion secondary battery using the positive electrode active material of Example 1. As the electrolyte solution, a solution obtained by dissolving LiPF ₆ at a concentration of 1 M (1 mol / L) in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) was used. The volume ratio of EC to DEC in the mixed solvent was EC: DEC = 30: 70.

(Example 2)
An evaluation cell was produced in the same manner as in Example 1 except that the weight ratio of the second compound was 2.5% by weight.

(Example 3)
An evaluation cell was produced in the same manner as in Example 1 except that the weight ratio of the second compound was 7.5% by weight.

Example 4
An evaluation cell was produced in the same manner as in Example 1 except that the weight ratio of the second compound was 1.0% by weight.

(Example 5)
An evaluation cell was produced in the same manner as in Example 1 except that the heavy travel expense ratio of the second compound was 10.0% by weight.

(Example 6)
An evaluation cell was produced in the same manner as in Example 1 except that the weight ratio of the second compound was changed to 15.0% by weight.

(Example 7)
An evaluation cell was produced in the same manner as in Example 1 except that the weight ratio of the second compound was changed to 30.0% by weight.

(Example 8)
An evaluation cell was produced in the same manner as in Example 1 except that the first compound was changed to Li _1.00 (Ni _0.80 Co _0.10 Mn _0.10 ) O ₂ .

Example 9
Except for changing the first compound to Li _1.00 (Ni _0.80 Co _0.10 Mn _0.10 ) O ₂ and changing the weight ratio of the second compound to 2.5% by weight. An evaluation cell was prepared in the same manner as in Example 1.

(Example 10)
Except for changing the first compound to Li _1.00 (Ni _0.80 Co _0.10 Mn _0.10 ) O ₂ and changing the weight ratio of the second compound to 7.5% by weight. An evaluation cell was prepared in the same manner as in Example 1.

(Example 11)
Example 1 except that the first compound was changed to Li _1.00 (Ni _0.80 Co _0.10 Mn _0.10 ) O ₂ and the weight ratio of the second compound was 1.0 weight. An evaluation cell was produced in the same manner as in Example 1.

Example 12
Except that the first compound was changed to Li _1.00 (Ni _0.80 Co _0.10 Mn _0.10 ) O ₂ and the weight ratio of the second compound was 10.0% by weight. An evaluation cell was produced in the same manner as in Example 1.

(Example 13)
Except for changing the first compound to Li _1.00 (Ni _0.80 Co _0.10 Mn _0.10 ) O ₂ and changing the weight ratio of the second compound to 15.0 wt%. An evaluation cell was prepared in the same manner as in Example 1.

(Example 14)
Except that the first compound was changed to Li _1.00 (Ni _0.80 Co _0.10 Mn _0.10 ) O ₂ and the weight ratio of the second compound was changed to 30.0% by weight. An evaluation cell was prepared in the same manner as in Example 1.

(Example 15)
An evaluation cell was prepared in the same manner as in Example 1 except that the mechanofusion treatment and heat treatment of the first compound and the second compound were not performed.

(Example 16)
The same method as in Example 1 except that the mechanofusion treatment and the heat treatment of the first compound and the second compound were not carried out and the weight ratio of the second compound was changed to 2.5% by weight. An evaluation cell was produced.

(Example 17)
In the same manner as in Example 1, except that the mechanofusion treatment and heat treatment of the first compound and the second compound were not carried out, and the weight ratio of the second compound was changed to 7.5% by weight. An evaluation cell was produced.

(Example 18)
The same method as in Example 1 except that the mechanofusion treatment and heat treatment of the first compound and the second compound were not carried out, and the weight ratio of the second compound was changed to 1.0% by weight. An evaluation cell was produced.

(Example 19)
The same method as in Example 1 except that the mechanofusion treatment and the heat treatment of the first compound and the second compound were not carried out and the weight ratio of the second compound was changed to 10.0% by weight. An evaluation cell was produced.

(Example 20)
The same method as in Example 1 except that the mechanofusion treatment and the heat treatment of the first compound and the second compound were not carried out and the weight ratio of the second compound was changed to 15.0% by weight. An evaluation cell was produced.

(Example 21)
The same method as in Example 1 except that the mechanofusion treatment and heat treatment of the first compound and the second compound were not carried out and the weight ratio of the second compound was changed to 30.0% by weight. An evaluation cell was produced.

(Example 22)
An evaluation cell was produced in the same manner as in Example 1 except that the particle form of the first compound was not primary particles but primary particles.

(Example 23)
For evaluation in the same manner as in Example 1, except that the particle form of the first compound is not secondary particles but primary particles, and the weight ratio of the second compound is changed to 1.0% by weight. A cell was produced.

(Example 24)
For evaluation in the same manner as in Example 1 except that the particle form of the first compound is not secondary particles but primary particles, and the weight ratio of the second compound is changed to 30.0% by weight. A cell was produced.

(Example 25)
An evaluation cell was produced in the same manner as in Example 1 except that the weight ratio of the second compound was changed to 50% by weight.

(Example 26)
Except for changing the first compound to Li _1.00 (Ni _0.80 Co _0.10 Mn _0.10 ) O ₂ and changing the weight ratio of the second compound to 40.0 wt%. An evaluation cell was prepared in the same manner as in Example 1.

(Comparative Example 1)
An evaluation cell was produced in the same manner as in Example 1 except that the second compound was not added.

(Comparative Example 2)
In the same manner as in Example 1 except that the first compound was changed to Li _1.00 (Ni _0.80 Co _0.10 Mn _0.10 ) O ₂ and the second compound was not added. An evaluation cell was produced.

(Measurement of DSC analysis (differential scanning thermal analysis))
DSC measurement was performed using the positive electrode and the electrolyte solution prepared in Examples and Comparative Examples. First, 10 mg of the electrode was weighed, put into an aluminum container, 1 μl of an electrolyte solution was added, and then an aluminum lid was put on and caulked. This measurement container was set in a DSC analyzer (Thermo Plus DSC8230 manufactured by RIGAKU) and measured at a heating rate of 5 ° C./min. Next, the full width at half maximum was calculated from the peak obtained by DSC measurement. FIG. 2 shows the DSC peaks of Example 1 and Comparative Example 1.

(Measurement of 0.1C discharge capacity)
About the lithium ion secondary battery for evaluation produced in the Example and the comparative example, the discharge rate was 0.1 C (constant current discharge was performed at 25 degreeC) using the secondary battery charge / discharge test apparatus (made by Hokuto Denko Co., Ltd.) The discharge capacity per unit weight of the active material (unit: mAh / g) was measured when the current value was set to end discharge in 10 hours.

(Evaluation method of cycle characteristics)
About the lithium ion secondary battery for evaluation produced by the Example and the comparative example, the cycle characteristic was measured in the environment of 25 degreeC using the secondary battery charging / discharging test apparatus (made by Hokuto Denko Co., Ltd.). Charge / discharge cycle of constant current and constant voltage charge to 4.2V at 0.5C, constant current discharge to 2.8V at 1C, repeat capacity cycle after 100 cycles, measure capacity retention after 100 cycles, cycle characteristics as cycle retention ratio It was evaluated as (unit:%). In addition, the sample measured by n = 5 about each level, respectively, and made the average value the evaluation value.

(Judgment of results)
The discharge capacity of 0.1 C constant current discharge is 170 mAh / g or more and the capacity maintenance rate after 100 cycles is 80% or more, “good”, the discharge capacity of 0.1 C constant current discharge is less than 170 mAh / g, and Those having a capacity retention rate of less than 80% after 100 cycles were judged as “bad”.

  Table 1 shows the first compound, the second compound and the amount added, 0.1 C discharge capacity, and cycle retention.

  As shown in Table 1, since the lithium ion secondary batteries of Examples 1 to 24 had a DSC peak half width within a predetermined range, the effect of the present invention was confirmed that the capacity retention rate after 100 cycles was excellent. On the other hand, since the DSC peak half width was outside the predetermined range in Comparative Examples 1 and 2, the desired characteristics could not be obtained. Moreover, when 1-30 wt% of 2nd compounds were contained with respect to the sum total of a 1st compound and a 2nd compound, it was confirmed that especially outstanding cycling characteristics and discharge capacity can be obtained.

  DESCRIPTION OF SYMBOLS 10 ... Positive electrode, 12 ... Positive electrode collector, 14 ... Positive electrode active material layer, 18 ... Separator, 20 ... Negative electrode, 22 ... Negative electrode collector, 24 ... Negative electrode Active material layer, 30 ... laminate, 50 ... case, 52 ... metal foil, 54 ... polymer film, 60, 62 ... lead, 100 ... lithium ion secondary battery.

Claims (6)

Chemical formula Li _x (Ni _y Ma _1-y ) O ₂ (0.95 ≦ x ≦ 1.05, 0.70 ≦ y ≦ 0.95, Ma is Co, Mn, V, Ti, Fe, Zr, Nb, At least one element selected from Mo, Al, and W) and a second compound represented by the chemical formula LiVOPO ₄ , the first compound and the second compound In DSC analysis (differential scanning calorimetry) of a mixture of compounds measured at 5 ° C./min, lithium ions satisfying W> 5.0 ° C. where W is the half width of an exothermic peak from 150 ° C. to 260 ° C. Positive electrode active material for secondary battery.   2. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the second compound is contained in an amount of 1 to 30 wt% with respect to the sum of the weight of the first compound and the weight of the second compound.   The positive electrode active material for a lithium ion secondary battery according to claim 1 or 2, wherein at least a part of the surface of primary particles of the first compound is coated with the second compound to form a coating layer.   The primary particles of the first compound aggregate to form secondary particles, and at least a part of the surface of the secondary particles is coated with the second compound to form a coating layer. Or the positive electrode active material for lithium ion secondary batteries of 2.   The positive electrode for lithium ion secondary batteries containing the positive electrode active material as described in any one of Claims 1 thru | or 4. A lithium ion secondary comprising the positive electrode for a lithium ion secondary battery according to claim 5, a negative electrode having a negative electrode active material, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte. battery.

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