JP6613952B2 - Positive electrode active material, and positive electrode and lithium ion secondary battery using the same - Google Patents

Description

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

In recent years, various electric vehicles are expected to be widely used for solving environmental and energy problems. As an in-vehicle power source such as a motor driving power source that holds the key to practical application of these electric vehicles, lithium ion secondary batteries have been intensively developed. However, in order to widely disseminate lithium ion batteries as in-vehicle power supplies, it is strongly desired not only to increase the capacity, but also to realize battery deterioration suppression and high-speed charge / discharge by repeated charge / discharge.

  As a positive electrode active material for a lithium ion secondary battery, a lithium-containing metal oxide such as lithium cobaltate having a crystal structure belonging to the space group R-3m is used. However, since cobalt is a scarce resource, production costs increase. For this reason, in recent years, lithium-containing nickel-manganese oxide containing nickel and manganese is sometimes used as an inexpensive positive electrode active material. However, there have been problems such as a decrease in initial charge / discharge efficiency and a decrease in cycle characteristics as compared with conventional lithium cobalt oxide.

In order to improve this cycle characteristic, it has been proposed to coat the surface of the positive electrode active material with an oxide such as ZrO ₂ , MgO, Al ₂ O ₃ (for example, Non-Patent Document 1). However, although cycle deterioration is suppressed as compared with that before coating, it cannot be said to be sufficient. Further, in the means of coating the surface of the positive electrode active material with an oxide, the amount of the positive electrode active material per unit volume is reduced, so that the capacity per unit volume is lost.

Electrochem. Solid-State Lett. , 4, A159 2001

The present invention has been made in view of the above circumstances, enables positive electrode active materials that can suppress capacity deterioration due to repeated charge and discharge without loss of capacity per unit volume, and further have excellent initial charge and discharge efficiency, and It aims at providing the electrode and lithium ion secondary battery which use it.

In order to achieve the above object, the positive electrode active material according to the present invention has a general formula: (Li _xy Co _y ) Co _1-yz MzO ₂ (where 0.8 ≦ x ≦ 1.2, 0 .01 ≦ y ≦ 0.1, 0.0 ≦ z ≦ 0.99, and M is at least one metal element selected from the group consisting of Li, Al, Ni and Mn) A positive electrode active material having a layered rock salt structure, wherein Li _p Co _{3 -p} O ₄ (where 0.1 ≦ p <1.0) is present in a part of primary particles Is present.

According to the positive electrode active material according to the present invention, it is possible to suppress a decrease in capacity during cycling. Details of the mechanism are not clear, but Li is desorbed at the time of charging, and Co ⁺² having an ionic radius close to that of Li enters an empty Li site that causes destabilization of the structure. A part of the structure is more stable. It is considered that the deterioration of the crystal structure due to repeated charge and discharge was suppressed by becoming Li _p Co _3-p O ₄ which is a simple cubic crystal.

The positive electrode active material according to the present invention is characterized in that the average valence of the Li _p Co _{3 -p} O ₄ is 2.0 to 2.9.

The _Li p _{Co 3-p O} average valence of Co in the ₄ initial charge-discharge efficiency not only suppression of reduction capacity during the cycle is improved by a 2.0 to 2.9. In the cathode active material having a layered rock salt structure containing general Co, the average valence of Co is usually trivalent, but Li _p Co _3-p O ₄ has an average valence of 2.0 to 2.9 and the positive electrode active By being present in the material, the average valence of Co existing in the positive electrode active material can be lowered. Thereby, the changeable valence region in the charging / discharging process is increased, and Li can be more easily desorbed and inserted from the positive electrode active material. Therefore, Li desorbed in the discharge process can be efficiently reinserted into the positive electrode active material. Ni ⁺² can also enter the Li site, but becomes NiO phase and is electrochemically inactive, leading to capacity degradation. Therefore, it is desirable to replace only Co at the Li site, and the third peak intensity appears in the vicinity of 3 angstroms (Å) in the Fourier transform of k ^ 3 · χ (k) where only Co is a wide X-ray fine structure. As a result, it is presumed that not only the capacity decrease during the cycle was suppressed, but also the initial charge / discharge efficiency was improved.

ADVANTAGE OF THE INVENTION According to this invention, the positive electrode active material which can suppress the capacity | capacitance fall at the time of a cycle, an 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 active material of this embodiment. It is a schematic cross section of the positive electrode of this embodiment. It is a figure which shows the Fourier-transform of k ^ 3 * chi (k) of Co wide area X-ray absorption fine structure (EXAFS) of Example 1. FIG.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. The components 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.

(Positive electrode active material)
Hereinafter, a positive electrode active material, an electrode, and a lithium ion secondary battery will be described as one embodiment of the present invention. In addition, this invention is not limited to the following embodiment.

The positive electrode active material of this embodiment is (Li _xy Co _y ) Co _1-yz M _z O ₂ (where 0.8 ≦ x ≦ 1.2, 0.01 ≦ y ≦ 0.1, 0.0 ≦ z ≦ 0.99, where M is at least one metal element selected from the group consisting of Li, Al, Ni, and Mn). And Li _p Co _{3 -p} O ₄ is present in a part of the primary particles.

The ratio of Li _p Co _{3 -p} O ₄ can be easily confirmed by conducting a Rietveld analysis from powder X-ray diffraction data.

The average valence number of Co in the Li _p Co _{3 -p} O ₄ is 2.0 to 2.9. As a result, the average Co valence of the whole positive electrode active material is lowered, and the width of the Co valence change during charging and discharging is widened, so that not only the capacity reduction during cycling but also the initial charging and discharging efficiency can be improved. it can.

The fact that the average valence of Co in the positive electrode active material has decreased to 3 or less is measured under the same conditions by measuring XANES (Xray Absorption Near Edge Structure) near the K absorption edge of Co by X-ray absorption spectroscopy. Judgment can be easily made by confirming that the K absorption edge of Co exists on the low energy side as compared with XANES of LiCoO ₂ .

  Since only Li is substituted at the Li site of the positive electrode active material, it has a peak in the vicinity of 3 angstroms (Å) in the Fourier transform of k ^ 3 · χ (k) of the Co wide X-ray absorption fine structure (EXAFS) It is preferable.

More preferably, in the positive electrode active material, the third peak intensity appearing in the vicinity of 3 angstroms (Å) is 10% of the second peak in the Fourier transform of k ^ 3 · χ (k) of the Co wide X-ray fine structure. The third peak intensity of the Fourier transform of k 3 · χ (k) of the wide-area X-ray fine structure of the transition metal M is 10% or less of the second peak intensity.

According to such a configuration, it is possible to further suppress a decrease in capacity during the cycle.

The positive electrode active material may have carbon attached to the surface.

(Method for producing positive electrode active material)
Below, the manufacturing method of the positive electrode active material which concerns on embodiment of this invention is demonstrated. According to the method for producing a positive electrode active material according to the present embodiment, the positive electrode active material according to the present embodiment described above can be formed.

The positive electrode active material according to the present invention includes a step of extracting Li from the positive electrode active material A to form a Li deficiency with respect to the positive electrode active material A having a layered rock salt structure containing Co, and for uniformly diffusing the Li deficiency. The positive electrode active material according to the present invention is obtained by obtaining a firing step, a rapid cooling step for maintaining a structure in which Li deficiency is diffused, and a step of forming Li _p Co _3-p O ₄ by adding Co to the Li deficient portion. Can be manufactured. However, the method for producing the positive electrode active material is not limited to the method described later.

The positive electrode active material A may contain Co and the crystal structure may be a layered rock salt type structure.

Li deficiency can be formed by immersing the positive electrode active material A in an acidic aqueous solution. Although there is no limitation in particular in acidic aqueous solution, hydrochloric acid, nitric acid, and a sulfuric acid are preferable. The amount of Li deficiency can be adjusted by adjusting the acid concentration and the immersion time, and the amount of Li deficiency is adjusted within a range of less than 20%. Although the active material A in which the amount of Li is adjusted in advance may be synthesized, it is desirable that the active material A is sufficiently free from stacking faults and dislocations.

In order to uniformly diffuse Li vacancies in the primary particles of the positive electrode active material A, firing is performed in an inert atmosphere or vacuum in the range of 400 ° C to 500 ° C. Li deficiency is diffused by firing, and the lattice constant can be increased. If you often form _Li p _{Co 3-p O 4} in the vicinity of the surface it may be shorter firing time. It is possible to reduce the reaction with the electrolyte solution by forming a lot of _Li p _{Co 3-p O 4} on the surface.

In order to maintain a state in which the lattice constant is widened, rapid cooling is performed after the firing step. There is no particular limitation on the rapid cooling method, but it may be put into a low-temperature liquid such as liquid nitrogen or liquid helium. Of these, the use of liquid nitrogen is preferable.

In order to insert Co into the Li deficient site and form Li _p Co _{3 -p} O ₄ , firing is performed in an air atmosphere in the range of 400 ° C. to 800 ° C., but a higher temperature rising rate is preferable and 30 ° C./s or higher is preferable. . Since the temperature rising rate is short, Co can be preferentially introduced into the Li deficient site. A high temperature state is maintained for 10 to 60 minutes, and then heated in an atmospheric environment at 350 ° C. for 10 hours or more. The amount of Li _p Co _3-p O ₄ may be appropriately adjusted by adjusting the firing temperature and the temperature rising rate.

The positive electrode active material can be manufactured through the above steps.

A positive electrode is manufactured using the positive electrode active material. Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. The components 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.

(Positive electrode)
FIG. 2 shows a cross-sectional structure of the positive electrode 10. The positive electrode 10 includes a plate-like (film-like) positive electrode current collector 12 and a positive electrode active material layer 14 formed on the positive electrode current collector 12.

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

(Conductive aid)
Examples of carbon as the conductive assistant include carbon blacks, graphites, carbon nanotubes (CNT), and vapor grown carbon fibers (VGCF). Carbon blacks include acetylene black, oil furnace, and ketjen black. It is more preferable to include one or more kinds of carbon including carbon blacks and graphites, carbon nanotubes (CNT), vapor grown carbon fibers (VGCF) and the like.

In addition, from the viewpoint of improving rate characteristics, a conductive auxiliary agent having a primary carbon particle diameter of 10 nm to 50 nm and a specific surface area of 500 to 1500 m ² / g is preferable because it can be mixed with the active material with good dispersibility. Of course, the above numerical values are not particularly limited.

(binder)
As the binder, polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based) Fluorine rubber), aromatic polyamide, cellulose, styrene / butadiene rubber, isoprene rubber, butadiene rubber, ethylene / propylene rubber, and the like may be used. Also, thermoplastic elastomeric polymers such as styrene / butadiene / styrene block copolymers, hydrogenated products thereof, styrene / ethylene / butadiene / styrene copolymers, styrene / isoprene / styrene block copolymers, and hydrogenated products thereof. May be used. Further, syndiotactic 1,2-polybutadiene, ethylene / vinyl acetate copolymer, propylene / α-olefin (carbon number 2 to 12) copolymer and the like may be used. From the viewpoint of increasing the electrode density, the specific gravity of the polymer used as the binder is preferably greater than 1.2 g / cm ³ . Moreover, it is preferable that a weight average molecular weight is 700,000 or more from the point which makes an electrode density high and improves adhesive force.

  It is preferable that the content rate of the binder contained in the positive electrode active material layer 14 is 4-10 mass% on the basis of the mass of an active material layer. When the binder content is less than 4% by mass, there is a high possibility that the amount of the binder is too small to form a strong active material layer. Moreover, when the content rate of a binder exceeds 10 mass%, the quantity of the binder which does not contribute to an electrical capacity will increase, and possibility that it will become difficult to obtain sufficient volume energy density becomes large. In this case, particularly, when the electronic conductivity of the binder is low, the electric resistance of the active material layer increases, and there is a high possibility that a sufficient electric capacity cannot 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 of aluminum or SUS foil can be used.

Next, the configuration of the lithium secondary battery including the positive electrode described above will be described.

(Lithium ion secondary battery)
A configuration example of the lithium ion secondary battery in this embodiment is shown in FIG. A lithium ion secondary battery includes a positive electrode, a negative electrode, and a separator that can occlude and release lithium. The positive electrode, the negative electrode, and the separator are sealed in a container, and charging and discharging are performed in a state where the electrolyte is impregnated. The lithium ion secondary battery 100 mainly includes a laminate 30, a container 50 that accommodates the laminate 30 in a sealed state, and a pair of leads 60 and 62 connected to the laminate 30.

  The laminated body 30 is configured such that a pair of the positive electrode 10 and the negative electrode 20 are opposed to each other with the separator 18 interposed therebetween. The positive electrode 10 has a positive electrode active material layer 14 provided on a positive electrode current collector 12, and the negative electrode 20 has a negative electrode current collector 22 provided with a negative electrode active material layer 24. The positive electrode active material layer 14 and the negative electrode active material layer 24 are in contact with both sides of the separator 18. Furthermore, leads 60 and 62 are connected to the ends of the positive electrode current collector 12 and the negative electrode current collector 22, respectively, and the ends of the leads 60 and 62 extend to the outside of the container 50.

(Negative electrode)
The negative electrode 20 includes a plate-like negative electrode current collector 22 and a negative electrode active material layer 24 formed on the negative electrode current collector 22. As the negative electrode current collector 22, the binder, and the conductive additive, the same materials as those for the positive electrode can be used. Li foil can be used as the negative electrode active material layer. Alternatively, known negative electrode active materials for batteries can be used. Examples of the negative electrode active material include graphite, non-graphitizable carbon, graphitizable carbon, and low-temperature calcined carbon that can occlude / release (intercalate / deintercalate, or dope / dedope) lithium ions. Carbon materials, metals that can be combined with lithium such as Al, Si, Sn, etc., amorphous compounds mainly composed of oxides such as SiO, SiO ₂ , SnO ₂ , or composites of silicon oxide and silicon And particles containing lithium titanate (Li ₄ Ti ₅ O ₁₂ ) and the like. Among them, it is preferable to use graphite from the viewpoint of irreversible capacity.

(Electrolytes)
The electrolyte is impregnated into the positive electrode active material layer 14, the negative electrode active material layer 24, and the separator 18. The electrolyte is not particularly limited, and for example, in the present embodiment, an electrolytic solution containing a lithium salt (electrolyte aqueous solution, electrolyte solution using an organic solvent) can be used. However, the electrolytic solution is preferably an electrolytic solution (non-aqueous electrolytic solution) that uses an organic solvent because the electrochemical decomposition voltage is low, and thus the withstand voltage during charging is limited to be low. As the electrolytic solution, a lithium salt dissolved in a non-aqueous solvent (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.

As the organic solvent, a mixture of a cyclic carbonate and a chain carbonate can be used. Preferred examples of the cyclic carbonate include propylene carbonate, ethylene carbonate, fluoroethylene carbonate, and examples of the chain carbonate include diethyl carbonate, dimethyl carbonate, and methyl ethyl carbonate. These may be used alone or in combination of two or more at any ratio.

  In the present embodiment, the electrolyte may be a gel electrolyte obtained by adding a gelling agent in addition to liquid. Moreover, it may replace with electrolyte solution and the solid electrolyte (electrolyte which consists of a solid polymer electrolyte or an ion conductive inorganic material) may contain.

(Separator)
The separator 18 is an electrically insulating porous body, 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 a group consisting of cellulose, polyester and polypropylene. Examples thereof include a nonwoven fabric made of at least one selected constituent material.

(container)
The container 50 seals the laminated body 30 and the electrolytic solution therein. The container 50 is not particularly limited as long as it can prevent leakage of the electrolytic solution to the outside and entry of moisture and the like into the electrochemical device 100 from the outside. For example, as the container 50, as shown in FIG. 1, a metal laminate film in which a metal foil is coated with a polymer film from both sides can be used. The container 50 is also called an exterior body. Moreover, when a metal laminate film is used for an exterior body, a lithium ion secondary battery excellent in rate discharge characteristics can be obtained. The reason is not clear, but the electrode expands or contracts when lithium ions are inserted into the electrode. Since the metal laminate film follows the expansion and contraction of the electrode and does not inhibit the movement of lithium ions, it is presumed to have excellent rate discharge characteristics. For example, an aluminum foil can be used as the metal foil, and a film such as polypropylene can be used as the polymer film. For example, the material of the outer polymer film is preferably a polymer having a high melting point such as polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film is preferably polyethylene or polypropylene.

(Lead)
The leads 60 and 62 are made of a conductive material such as aluminum.

Next, a manufacturing method in the positive electrode and the lithium ion secondary battery of the present embodiment will be described.

(Production method of positive electrode)
The electrode manufacturing method according to the present embodiment includes a composite process, a slurry preparation process, an electrode application process, and a rolling process.

(Slurry production process)
Next, the composite particles composed of the positive electrode active material and the conductive additive are mixed with a binder and a solvent corresponding to the type thereof, for example, a solvent such as N-methyl-2-pyrrolidone or N, N-dimethylformamide in the case of PVDF. A slurry is prepared.

(Electrode production process)
The slurry is appropriately selected from known methods such as a doctor blade, a slot die, a nozzle, a gravure roll, and coating is performed. The amount of positive electrode supported can be adjusted by adjusting the amount of coating and the line speed. After applying the slurry, drying is performed to volatilize the solvent.

(Rolling process)
Finally, rolling is performed by a roll press to complete the positive electrode. At this time, a higher electrode density can be obtained by heating the roll and softening the binder. The roll temperature is preferably in the range of 100 ° C to 200 ° C.

(Method for producing negative electrode)
The negative electrode can be produced by a slurry production process, an electrode application process, and a rolling process in the same manner as the positive electrode. Each process can be manufactured under the same conditions as the positive electrode. In addition, a negative electrode can be obtained by press-bonding a Li foil to a current collector foil.

A lithium ion secondary battery is completed by inserting the separator between the positive electrode and the negative electrode thus obtained together with the electrolytic solution into the container 50 and sealing the inlet of the container 50.

1 is completed by welding the leads 60 and 62 to the positive electrode current collector 12 and the negative electrode current collector 22, respectively, as electrodes for lead-out.

  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
10 g of LiCo _1/3 Mn _1/3 Ni _1/3 O ₂ was weighed and immersed in 50 ml of 2% nitric acid aqueous solution for 10 hours. As a result, Li is removed from LiCo _1/3 Mn _1/3 Ni _1/3 O ₂ , and Li _0.85 Co _1/3 Mn _1/3 Ni _1/3 O ₂ is obtained from the composition analysis by ICP-AES. It was confirmed. This powder was vacuum-dried at 100 ° C. for 5 hours and then fired at 400 ° C. for 6 hours using an electric furnace in an Ar atmosphere environment. After calcination, quenching was performed by pouring into liquid nitrogen. Thereafter, the temperature was raised to 400 ° C. at a temperature rising rate of 35 ° C./min, and firing was performed in an air atmosphere for 20 minutes.
By firing, Co entered the vacant Li site of Li _0.85 Co _1/3 Mn _1/3 Ni _1/3 O ₂ and partially became Li _p Co _{3 -p} O ₄ .

The amount of Co present at the Li site (3b site) and the quantification of Li _p Co _{3 -p} O ₄ were calculated from Rietveld analysis by powder X-ray diffraction measurement. At this time, Li _p Co _{3 -p} O ₄ was subjected to Rietveld analysis assuming that the Li at the 8b site of Co ₃ O ₄ was model Li _0.5 Co _2.5 O ₄ with 0.5 substituted for Li. From the analysis results, it was confirmed that 31 wt% of Li _p Co _3-p O ₄ was present.

Also, X-ray absorption spectroscopy measurement was performed, and in the Fourier transform of k ^ 3 · χ (k) of the wide-area X-ray microstructure, Co had a third peak in the vicinity of 3Å, and the wide-area X-ray microstructure of Mn and Ni It was confirmed that the third peak intensity of the Fourier transform of k ^ 3 · χ (k) was 10% or less of the second peak intensity. Further, it was confirmed that XANES near the K absorption edge of Co shifted to a lower energy side than LiCo _1/3 Mn _1/3 Ni _1/3 O ₂ and the Co average valence decreased. From the linear relationship between the valence of each of CoO and LiCoO ₂ and the value of the K absorption edge of Co, the valence of Co calculated in this example was 2.7.

  FIG. 3 shows the Fourier transform of k ^ 3 · χ (k) of the Co wide-area X-ray absorption fine structure (EXAFS). As indicated by the arrows in FIG. 3, it was confirmed that there was a peak in the vicinity of 3 Å (Å) in the k ^ 3 · χ (k) Fourier transform of the Co wide-area X-ray absorption fine structure (EXAFS). On the other hand, it was confirmed that there was no peak in the Fourier transform of k ^ 3 · χ (k) of the wide X-ray absorption fine structure (EXAFS) of Ni and Mn.

Next, the cell for battery characteristic evaluation was produced as follows.
The positive electrode active material obtained by firing and ketjen black and polyvinylidene fluoride (PVDF) as a conductive auxiliary were mixed in a weight ratio such that the positive electrode active material: Ketjen black: PVDF = 90: 5: 5. Then, N-methyl-2-pyrrolidone (NMP) as a solvent was added to prepare a slurry, and then kneading was performed for 1 hour. Thereafter, NMP was added to adjust the viscosity to 5000 mPa · s. It apply | coated on the aluminum foil which is a collector with the doctor blade method, and dried for 10 minutes at 100 degreeC. Thereafter, rolling was performed at a linear pressure of 2 tcm ⁻¹ by a roll press heated to 100 ° C. to produce a positive electrode. The amount of positive electrode active material supported on the positive electrode was adjusted to 12 mg / cm ² . The negative electrode was prepared by pressure bonding Li foil to a Cu current collector. A separator made of a polyethylene microporous membrane was sandwiched between the positive electrode and the negative electrode to obtain a laminate (battery element). This laminated body was put into an aluminum laminated pack (a laminated sheet bag body in which two main surfaces of an aluminum foil were coated with polypropylene (PP) and polyethylene terephthalate (PET), respectively). As the electrolytic solution, ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 3: 7, and LiPF ₆ was dissolved as a supporting salt so as to be 1.5 mol / L. The above electrolyte was poured into an aluminum laminate pack containing the laminate, and then vacuum-sealed to produce a battery characteristic evaluation cell of Example 1.

(Example 2)
10 g of LiCo _1/3 Mn _1/3 Ni _1/3 O ₂ was weighed and immersed in 50 ml of 1% nitric acid aqueous solution for 10 hours. From composition analysis by ICP-AES, it was confirmed that Li _0.93 Co _1/3 Mn _1/3 Ni _1/3 O ₂ was obtained. About others, the positive electrode active material was obtained in the same process as Example 1.
Moreover, it was Co amount and quantification of _Li p _{Co 3-p O 4} and 3b-site under the same conditions as in Example 1. X-ray absorption spectroscopy measurement was performed, and in the Fourier transform of k ^ 3 · χ (k) of the wide-area X-ray microstructure, Co has a third peak near 3Å, and k ^ of the wide-area X-ray microstructure of Mn and Ni. It was confirmed that the third peak intensity of Fourier transform of 3 · χ (k) was 10% or less of the second peak intensity.
Further, it was confirmed that XANES near the K absorption edge of Co shifted to a lower energy side than LiCo _1/3 Mn _1/3 Ni _1/3 O ₂ and the Co average valence decreased. The Co valence was evaluated by the same calculation method as in Example 1, and the results are shown in Table 1. Moreover, the lithium ion secondary battery using this positive electrode active material was produced as a battery evaluation cell under the same conditions as in Example 1.

(Example 3)
10 g of LiCo _1/3 Mn _1/3 Ni _1/3 O ₂ was weighed and immersed in 50 ml of 0.5% nitric acid aqueous solution for 10 hours. It was confirmed from composition analysis by ICP-AES that Li _0.96 Co _1/3 Mn _1/3 Ni _1/3 O ₂ . A positive electrode active material was obtained in the same process as in Example 1.
Moreover, it was Co amount and quantification of _Li p _{Co 3-p O 4} and 3b-site under the same conditions as in Example 1. X-ray absorption spectroscopy measurement was performed, and in the Fourier transform of k ^ 3 · χ (k) of the wide-area X-ray microstructure, Co has a third peak near 3Å, and k ^ of the wide-area X-ray microstructure of Mn and Ni. It was confirmed that the third peak intensity of Fourier transform of 3 · χ (k) was 10% or less of the second peak intensity.
Further, it was confirmed that XANES near the K absorption edge of Co shifted to a lower energy side than LiCo _1/3 Mn _1/3 Ni _1/3 O ₂ and the Co average valence decreased. And the valence of Co was similarly evaluated by the calculation method described in Example 1, and the results are shown in Table 1. Moreover, the lithium ion secondary battery using this positive electrode active material was produced as a battery evaluation cell under the same conditions as in Example 1.

Example 4
It carried out to quenching by the same process as Example 1, and baked at 450 degreeC for 1 hour. Otherwise, a positive electrode active material was obtained in the same process as in Example 1.
Moreover, it was Co amount and quantification of _Li p _{Co 3-p O 4} and 3b-site under the same conditions as in Example 1. X-ray absorption spectroscopy measurement was performed, and in the Fourier transform of k ^ 3 · χ (k) of the wide-area X-ray microstructure, Co has a third peak near 3Å, and k ^ of the wide-area X-ray microstructure of Mn and Ni. It was confirmed that the third peak intensity of Fourier transform of 3 · χ (k) was 10% or less of the second peak intensity.
Further, it was confirmed that XANES near the K absorption edge of Co shifted to a lower energy side than LiCo _1/3 Mn _1/3 Ni _1/3 O ₂ and the Co average valence decreased. And the valence of Co was similarly evaluated by the calculation method described in Example 1, and the results are shown in Table 1. Moreover, the lithium ion secondary battery using this positive electrode active material produced the cell for battery evaluation on the same conditions as Example 1. FIG.

(Example 5)
It carried out to quenching by the same process as Example 1, and baked at 500 degreeC for 1 hour. Otherwise, a positive electrode active material was obtained in the same process as in Example 1.
Moreover, it was Co amount and quantification of _Li p _{Co 3-p O 4} and 3b-site under the same conditions as in Example 1. X-ray absorption spectroscopy measurement was performed, and in the Fourier transform of k ^ 3 · χ (k) of the wide-area X-ray microstructure, Co has a third peak near 3Å, and k ^ of the wide-area X-ray microstructure of Mn and Ni. It was confirmed that the third peak intensity of Fourier transform of 3 · χ (k) was 10% or less of the second peak intensity.
Further, it was confirmed that XANES near the K absorption edge of Co shifted to a lower energy side than LiCo _1/3 Mn _1/3 Ni _1/3 O ₂ and the Co average valence decreased. And the valence of Co was similarly evaluated by the calculation method described in Example 1, and the results are shown in Table 1. Moreover, the lithium ion secondary battery using this positive electrode active material produced the cell for battery evaluation on the same conditions as Example 1. FIG.

(Example 6)
It carried out to quenching in the same process as Example 2, and baked at 450 degreeC for 1 hour. Otherwise, a positive electrode active material was obtained in the same steps as in Example 2.
Moreover, it was Co amount and quantification of _Li p _{Co 3-p O 4} and 3b-site under the same conditions as in Example 1. X-ray absorption spectroscopy measurement was performed, and in the Fourier transform of k ^ 3 · χ (k) of the wide-area X-ray microstructure, Co has a third peak near 3Å, and k ^ of the wide-area X-ray microstructure of Mn and Ni. It was confirmed that the third peak intensity of Fourier transform of 3 · χ (k) was 10% or less of the second peak intensity.
Further, it was confirmed that XANES near the K absorption edge of Co shifted to a lower energy side than LiCo _1/3 Mn _1/3 Ni _1/3 O ₂ and the Co average valence decreased. And the valence of Co was similarly evaluated by the calculation method described in Example 1, and the results are shown in Table 1. Moreover, the lithium ion secondary battery using this positive electrode active material produced the cell for battery evaluation on the same conditions as Example 1. FIG.

(Example 7)
It carried out to quenching in the same process as Example 2, and baked at 500 degreeC for 1 hour. Otherwise, a positive electrode active material was obtained in the same steps as in Example 2.
Moreover, it was Co amount and quantification of _Li p _{Co 3-p O 4} and 3b-site under the same conditions as in Example 1. X-ray absorption spectroscopy measurement was performed, and in the Fourier transform of k ^ 3 · χ (k) of the wide-area X-ray microstructure, Co has a third peak near 3Å, and k ^ of the wide-area X-ray microstructure of Mn and Ni. It was confirmed that the third peak intensity of Fourier transform of 3 · χ (k) was 10% or less of the second peak intensity.
Further, it was confirmed that XANES near the K absorption edge of Co shifted to a lower energy side than LiCo _1/3 Mn _1/3 Ni _1/3 O ₂ and the Co average valence decreased. And the valence of Co was similarly evaluated by the calculation method described in Example 1, and the results are shown in Table 1. Moreover, the lithium ion secondary battery using this positive electrode active material produced the cell for battery evaluation on the same conditions as Example 1. FIG.

(Comparative Example 1)
Except for the step of immersing in nitric acid, everything was the same as Example 1. X-ray absorption spectroscopy measurement was performed, and the third peak was not confirmed in the vicinity of 3 mm in Mn, Co, and Ni in the Fourier transform of k ^ 3 · χ (k) of the wide-area X-ray fine structure. Co valence was evaluated in the same manner as in Example 1. The results are shown in Table 1.

(Battery evaluation method)
The evaluation cells of Examples 1 to 7 and Comparative Example 1 were charged at a constant current constant voltage up to 4.3 V at a 0.1 C rate (current value at which discharge was completed in 10 hours when constant current discharge was performed at 25 ° C.), The first charge / discharge efficiency was determined by performing one cycle of constant current discharge up to 2.8V. Thereafter, 50 cycles were performed at the 1C rate, and the capacity maintenance rate was determined (in the table, described in the item of maintenance rate after 50 cycles). As described above, Table 1 shows the results of Examples 1 to 7 and Comparative Example 1 as the battery evaluation of the initial charge / discharge efficiency and the capacity retention rate after the cycle. Note that the Li composition in the table shows the value of n of _{Li n Co 1/3 Mn 1/3 Ni 1/3} O 2 produced by deficient Li with nitric acid.

In Table 1, in Examples 1-7, the capacity | capacitance retention after 50 cycles is comparatively high compared with the comparative example 1, and the result of the initial charge / discharge efficiency is high, and Co and LiCo _1/3 present at the Li site are shown. The effect of substituting a part of Mn _1/3 Ni _1/3 O ₂ with Li _p Co _3-p O ₄ is observed.

(Examples 8 to 14)
LiCo changed _1/3 Mn _1/3 Ni _1/3 O ₂ in _{Li (Li 0.2 Ni 0.17 Co 0.07} Mn 0.56) O 2, otherwise, examples 8 to 14 respectively Corresponding to Examples 1 to 7, positive electrode active materials were produced under the same conditions as in the corresponding examples.
In the Rietveld analysis, the positive electrode active material was evaluated in the same procedure except that the peak appearing in the vicinity of 22 ° was excluded when calculating the amount of Co substitution in the Li site.
Further, X-ray absorption spectroscopy measurement was performed, and in the Fourier transform of k ^ 3 · χ (k) of the wide X-ray microstructure, Co has a third peak in the vicinity of 3Å, and k of the wide X-ray microstructure of Mn and Ni. It was confirmed that the third peak intensity of the Fourier transform of ^ 3 · χ (k) was 10% or less of the second peak intensity. In addition, the average valence of Co is reduced by shifting from XANES near the K absorption edge of Co to a lower energy side than Li (Li _0.2 Ni _0.17 Co _0.07 Mn _0.56 ) O ₂ . It was confirmed. The average valence of Co is also calculated and shown in Table 2 together with the battery characteristics. The evaluation method of the battery characteristics is the same as that in Example 1 described above.

(Comparative Example 2)
A positive electrode active material was prepared in the same manner as in Comparative Example 1 except that Li (Li _0.2 Ni _0.17 Co _0.07 Mn _0.56 ) O ₂ was used as the positive electrode active material. X-ray absorption spectroscopy measurement was performed, and the third peak was not confirmed in the vicinity of 3 mm in Mn, Co, and Ni in the Fourier transform of k ^ 3 · χ (k) of the wide-area X-ray fine structure. As a result of evaluating the valence of Co in the same manner as in Example 1, it was 3.0.

In Table 2, in Examples 8-14, the capacity retention rate after 50 cycles was higher than that of Comparative Example 2, and the results of high initial charge / discharge efficiency were shown. Even in Li (Li _0.2 Ni _0.17 Co _0.07 Mn _0.56 ) O ₂ having a high Mn ratio and a layered rock salt structure but a space group of C2 / m, a part thereof is Li _p Co _{3− p} O ₄ can be substituted, and the effect is seen.

(Examples 15 to 21)
LiCo _1/3 Mn _1/3 Ni _1/3 O ₂ is changed to LiCoO ₂ , and other than that, Examples 15 to 21 correspond to Examples 1 to 7, respectively. An active material was prepared.
Further, X-ray absorption spectroscopy measurement was performed, and in the Fourier transform of k ^ 3 · χ (k) of the wide-area X-ray fine structure, a third peak of Co appeared in the vicinity of 3Å.
It was also confirmed that XANES near the K absorption edge of Co shifted to a lower energy side than LiCoO ₂ and the Co average valence decreased. The average valence of Co is also calculated and shown in Table 3 together with the battery characteristics. The evaluation method of the battery characteristics is the same as that in Example 1 described above.

(Comparative Example 3)
All were the same as Comparative Example 1 except that LiCoO _{2 was} used as the positive electrode active material. X-ray absorption spectroscopy measurement was performed, and the third peak was not confirmed in the vicinity of 3Å in the Fourier transform of k ^ 3 · χ (k) of the wide-area X-ray fine structure.

In Table 3, in Examples 15 to 21, higher capacity retention rate after 50 cycles than Comparative Example 3, and shows the initial charge and discharge efficiency is high results, a part of LiCoO _{2 Li p Co 3-p} The effect of substitution with O ₄ is observed.

(Comparative Example 4)
LiNiO ₂ was produced in the same manner as in Example 1 and evaluated. From the powder X-ray diffraction measurement, crystallinity was deteriorated and battery characteristics were not exhibited. No effect is seen with a positive electrode material that does not contain Co.

(Comparative Example 5)
LiCo _1/3 Mn _1/3 Ni _1/3 O ₂ and Co ₃ O ₄ weighed in a weight ratio of 9: 1 were mixed in a mortar and fired at 400 ° C. for 6 hours using an electric furnace in the atmospheric environment. A positive electrode active material was obtained. Using this, a battery cell was produced and evaluated in the same manner as in Example 1.

(Comparative Example 6)
Li (Li _0.2 Ni _0.17 Co _0.07 Mn _0.56 ) O ₂ and Co ₃ O ₄ weighed at a weight ratio of 9: 1 were mixed in a mortar, and an electric furnace in an atmospheric environment was used. Firing was performed at 400 ° C. for 6 hours to obtain a positive electrode active material. Using this, a battery cell was produced and evaluated in the same manner as in Example 1.

(Comparative Example 7)
What weighed LiCoO ₂ and Co ₃ O ₄ at a weight ratio of 9: 1 was mixed in a mortar and fired at 400 ° C. for 6 hours using an electric furnace in an atmospheric environment to obtain a positive electrode active material. Using this, a battery cell was produced and evaluated in the same manner as in Example 1.

Although the retention after 50 cycles is slightly improved by mixing with Co ₃ O ₄ , there is no change in the initial charge / discharge efficiency. That is, the effect was not expressed only by mixing.

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

Claims (6)

(Li _xy Co _y ) Co _1-yz M _z O ₂ (wherein 0.8 ≦ x ≦ 1.2, 0.01 ≦ y ≦ 0.1, 0.0 ≦ z ≦ 0.99) , M is at least one metal element selected from the group consisting of Li, Al, Ni and Mn), and is a layered rock salt type positive electrode active material, and a part of the primary particles A positive electrode active material characterized by the presence of Li _p Co _{3 -p} O ₄ (wherein 0.1 ≦ p <1.0). 2. The positive electrode active material according to claim 1, wherein an average valence number of Co in the Li _p Co _{3 -p} O ₄ is 2.0 to 2.9.   The positive electrode active material has a peak in the vicinity of 3 angstroms (Å) in the Fourier transform of k ^ 3 · χ (k) of the wide-area X-ray absorption fine structure (EXAFS). Positive electrode active material. In the positive electrode active material, the third peak intensity appearing in the vicinity of 3 angstroms (に お い て) is 10% or more and 70% or less of the second peak in the k ^ 3 · χ (k) Fourier transform of the Co wide X-ray fine structure. The third peak intensity of the Fourier transform of k ^ 3 · χ (k) of the wide-area X-ray fine structure of the transition metal M is 10% or less of the second peak intensity. The positive electrode active material according to any one of 3. The positive electrode containing the positive electrode active material as described in any one of Claims 1-4. A positive electrode having the positive electrode active material according to any one of claims 1 to 4, a negative electrode having a negative electrode active material, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. A lithium ion secondary battery provided.

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