JP2016105359A - Positive electrode active material and lithium ion secondary battery arranged by use thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a positive electrode active material for a lithium ion secondary battery, which exhibits high cycle characteristics when raising a charging voltage to increase a charge depth for the purpose of materializing a lithium ion secondary battery with a higher performance; and a lithium ion secondary battery arranged by use of such a positive electrode active material.SOLUTION: A positive electrode active material of the present invention comprises: a positive electrode active material base material including a lithium complex oxide; and a coating layer covering a part of the positive electrode active material base material. The positive electrode active material base material shows paramagnetism. The coating layer is of a ferromagnetic oxide.SELECTED DRAWING: Figure 1

Description

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

  Lithium ion secondary batteries are widely used in portable devices and the like because of their large capacity per unit volume and weight, and research and development for higher capacity applications such as electric vehicles are being actively promoted.

  A lithium ion secondary battery mainly includes a positive electrode, a negative electrode, and a liquid electrolyte disposed between the positive electrode and the negative electrode. Electrode active materials for the positive electrode and the negative electrode are materials capable of inserting and removing lithium ions, and in order to increase the capacity of lithium ion secondary batteries, the search for materials having a larger energy density has been actively pursued.

A lithium ion secondary battery using a lithium metal composite oxide, particularly a lithium cobalt composite oxide (LiCoO ₂ ), which is relatively easy to synthesize, as a positive electrode material can obtain a high voltage of 4V and has a high energy density. As practical use is progressing.

  However, since the lithium cobalt composite oxide uses a cobalt compound that is expensive and has limited resources as a raw material, it causes an increase in the cost of the battery. For this reason, search of positive electrode active material materials other than lithium cobalt complex oxide is also advanced.

Newly proposed materials as positive electrode active materials for lithium ion secondary batteries include lithium manganese composite oxide (LiMn ₂ O ₄ ) using manganese, which is cheaper than cobalt, and lithium nickel composite oxide using nickel. (LiNiO ₂ ).

  Lithium-manganese composite oxide is excellent in thermal stability because the raw material is inexpensive and the crystal structure is stable when lithium is desorbed during charging, but the theoretical capacity is only about half that of lithium-cobalt composite oxide. For this reason, it has a drawback that it is difficult to meet the demand for a large capacity lithium ion secondary battery. Further, when the temperature is 45 ° C. or higher, there is a problem that the self-discharge is intense and the cycle characteristics are deteriorated.

On the other hand, lithium nickel composite oxide (LiNiO ₂ ) has a higher theoretical capacity than lithium cobalt composite oxide and is one of the promising positive electrode active material materials because it is inexpensive. However, lithium in a charged state was extracted. There is a drawback that the thermal stability is inferior because the state stability is low.

In order to achieve both the thermal stability and the high charge / discharge capacity of the lithium nickel composite oxide, it is possible to use a Ni material such as LiNi _1/3 Co _1/3 Mn _1/3 O ₂ or LiNi _0.5 Mn _0.5 O _2. Attempts have been made to replace parts with elements such as Co, Al, Mn, and Cu, and certain effects have been reported.

  Furthermore, in order to improve stability, a method has been proposed in which the positive electrode active material is covered with a different compound to prevent direct contact between the positive electrode active material and the electrolytic solution. For example, Patent Document 1 proposes coating the surface of lithium manganate oxide with an oxide or hydroxide of Mg, Zn, Fe, Co, Ni, Sn. In secondary batteries using such a lithium manganate oxide as a positive electrode active material, elution of manganese from the lithium manganate oxide is suppressed, and an improvement in capacity retention rate after high-temperature storage is seen.

  Patent Document 2 discloses at least one selected from the group consisting of titanium oxide, alumina, zinc oxide, chromium oxide, lithium oxide, nickel oxide, copper oxide, and iron oxide between the particles of the lithium-containing composite oxide. A positive electrode for a non-aqueous electrolyte secondary battery is disclosed in which the metal oxide is dispersed, and the cycle characteristics after 50 cycles are improved.

  In any of the prior arts, the rate characteristics tend to be reduced by covering the periphery of the positive electrode active material with a different metal compound, and in particular, the cycle characteristics when the charge voltage is increased to increase the charge depth in order to increase the charge / discharge capacity. Is not enough. This is considered to be caused by the increase in impedance due to the elution of the active material into the electrolyte as the charge voltage increases.

JP 2000-040512 A Japanese Patent Laid-Open No. 2004-6301

  An object of this invention is to obtain the positive electrode active material which shows a high cycle characteristic, and a lithium ion secondary battery using the same, when charging voltage is raised and charging depth is raised.

  In order to achieve the above object, a positive electrode active material according to the present invention has a positive electrode active material base material containing a lithium composite oxide, and a coating layer covering a part of the positive electrode active material base material, The positive electrode active material base material is paramagnetic, and the coating layer is a ferromagnetic oxide.

  By preventing direct contact between the positive electrode active material matrix and the electrolyte, decomposition of the electrolyte at high charging potential is suppressed, and elution of the positive electrode active material matrix into the electrolyte is prevented, increasing impedance. Suppress. As a result, deterioration of the battery capacity can be suppressed.

  Especially in this invention, the effect that the cycling characteristics when it is set as high charge potential is excellent is shown. Since the positive electrode active material base material is paramagnetic and the coating layer material is ferromagnetic, the adhesion is strong and the effect of suppressing deterioration continues for a long time. Therefore, it is considered that a high cycle characteristic improvement effect at a high charge potential is exhibited.

  Furthermore, in the positive electrode active material according to the present invention, the coating amount of the coating layer with respect to the positive electrode active material base material is preferably 0.15 to 10.0% by mass. By doing in this way, the cycle characteristic when it is set as high charge potential is excellent, keeping the discharge capacity per battery weight high.

  Furthermore, it is preferable that the coating amount of the coating layer with respect to a positive electrode active material base material is 0.25-5.0 mass% about the positive electrode active material which concerns on this invention. By doing in this way, the cycle characteristic when it is set as high charge potential is excellent, keeping the discharge capacity per battery weight still higher.

  In the positive electrode active material according to the present invention, it is preferable that the positive electrode active material base material constitutes secondary particles, and the coating layer covers the secondary particles. By doing in this way, the cycle characteristic when it is set as high charge potential is excellent, keeping the discharge capacity per battery weight still higher.

  In the positive electrode active material according to the present invention, the coating layer is preferably composed of particles containing a ferromagnetic oxide, and the particles are preferably 1/10 or less of the average particle diameter (D50) of the base material. By doing in this way, it becomes possible to coat | cover the surface of an active material particle uniformly, and the cycling characteristics when it is set as a high charge potential are excellent.

  Furthermore, in the lithium ion secondary battery according to the present invention, the saturation magnetization of the coating layer is preferably 10 emu / g or more. By doing in this way, the high cycle characteristic when it is set as a high charge potential is acquired, and the effect continues over a long term.

Furthermore, in the lithium ion secondary battery according to the present invention, the positive electrode active material base material is manganese lithium oxide, nickel lithium oxide, or LiMPO ₄ (M represents at least one element selected from Fe, Mn, and Ni). It is preferable to contain any of these. Manganese lithium oxide includes a solid solution of LiMn ₂ O ₄ , LiMnO ₂ , Li ₂ MnO ₃ , Li ₂ MnO ₃ and LiMO ₂ (M = Mn, Ni, Co) in excess of Li. Nickel lithium oxides include LiNiO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNi _0.5 Mn _0.5 O _2, etc., a part of Ni such as Co, Al, Mn, Cu, etc. Including those substituted with these elements.

Further, in the lithium ion secondary battery according to the present invention, the coating layer has Fe ₃ O ₄ , γ-Fe ₂ O ₃ , LiFe ₅ O ₈ , ferrite (MnFe ₂ O ₄ , NiFe ₂ O ₄ , CoFe ₂ O _4, etc.). It is preferable that at least one of these is included. By doing in this way, the cycle characteristic when it is set as high charge potential is excellent, keeping the discharge capacity per battery weight still higher.

  Furthermore, it is preferable that the lithium ion secondary battery which concerns on this invention contains said positive electrode active material.

  According to the present invention, when the charging voltage is increased to increase the charging depth, a positive electrode active material for a lithium ion secondary battery exhibiting high cycle characteristics and a lithium ion secondary battery using the same can be obtained.

It is a schematic cross section of the positive electrode active material preform | base_material covered with the coating layer of this embodiment. It is a schematic cross section of a lithium ion secondary battery provided with the positive electrode active material of this embodiment.

  Hereinafter, preferred embodiments of the present invention will be described. 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.

  As shown in FIG. 1, the positive electrode active material of this embodiment covers at least a part of the surface of a positive electrode active material base material 1 containing a lithium composite oxide exhibiting paramagnetism with a coating layer 2 that is a ferromagnetic oxide. It is a thing.

  By having the coating layer 2, direct contact between the positive electrode active material base material 1 and the electrolytic solution is prevented, decomposition of the electrolytic solution at a high charging potential is suppressed, and the electrolytic solution of the positive electrode active material base material 1. Prevents elution into and suppresses increase in impedance. As a result, the present invention exhibits an effect that the cycle characteristics are particularly excellent when the charge potential is high. Since the positive electrode active material base material 1 is paramagnetic and the material of the coating layer 2 is ferromagnetic, adhesion is strong, and the effect of suppressing deterioration continues for a long time. Therefore, it is considered that a high cycle characteristic improvement effect at a high charge potential is exhibited.

  Furthermore, the present inventors have surprisingly found that the rate characteristic at a high charging potential is also improved as an effect of the present invention.

(Positive electrode active material base material)
The positive electrode active material base material 1 of this embodiment is a lithium composite oxide having, for example, a layered rock salt type, spinel type, or olivine type crystal structure, and exhibits paramagnetism. Among these, it is preferable to contain any of manganese lithium oxide, nickel lithium oxide, or LiMPO ₄ (M represents at least one element selected from Fe, Mn, and Ni). Manganese lithium oxide includes a solid solution of LiMn ₂ O ₄ , LiMnO ₂ , Li ₂ MnO ₃ , Li ₂ MnO ₃ and LiMO ₂ (M = Mn, Ni, Co) in excess of Li. Nickel lithium oxides include LiNiO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNi _0.5 Mn _0.5 O _2, etc., a part of Ni such as Co, Al, Mn, Cu, etc. Including those substituted with these elements.

  The average particle diameter of primary particles of the positive electrode active material base material 1 is preferably 0.05 μm or more and 10 μm or less. A lithium ion secondary battery using such a positive electrode active material has a high capacity. When an active material having an average primary particle size of more than 0.05 μm is used, the powder tends to be easily handled, and when an active material of less than 10 μm is used, the capacity tends not to be impaired. More preferably, the average temporary particle size is 0.07 μm or more and 3 μm or less.

  Further, the positive electrode active material base material preferably forms secondary particles, and the average secondary particle diameter is preferably 3 μm or more and 50 μm or less. When the average secondary particle diameter is 3 μm or more, the electrode density tends to increase when it is used as an electrode, and when it is 50 μm or less, a flat electrode sheet tends to be easily produced.

(Coating layer)
The coating layer 2 of this embodiment contains a ferromagnetic oxide. By doing in this way, the effect that the high cycle characteristic in a high charge potential is excellent is shown.

  The saturation magnetization of the coating layer 2 is preferably 10 emu / g or more. By doing so, the adhesion between the positive electrode active material base material 1 and the coating layer 2 becomes stronger, and high cycle characteristics at a high charge potential can be obtained.

  An apparatus for measuring the saturation magnetization of the electrode material is not particularly limited, but a VSM (Vibrating Sample Magnetometer) is preferably used. VSM is a device that vibrates a sample in a magnetic field at a constant frequency and amplitude and measures its magnetization. When the sample is vibrated at the measurement site of the vibrating sample magnetometer and a magnetic field is applied to the sample, magnetic lines of force are generated in synchronization with the vibration, and the amount of magnetization is measured. The amount of magnetization with respect to the magnetic field is plotted, and the amount of magnetization when searching for magnetic field = 0 is the saturation magnetization of the ferromagnetic material. Although the magnetization of the ferromagnetic material remains as residual magnetization (saturation magnetization) even when the magnetic field = 0, the magnetization of the paramagnetic material and the antiferromagnetic material becomes 0 when the magnetic field = 0, and is clearly distinguished from the ferromagnetic material.

Specific examples of the ferromagnetic oxide material used as the coating layer 2 include various iron oxides, ferrite containing iron oxide as a main component and various substitution elements, and chromium dioxide. Ferrite is classified into spinel type, garnet type such as Y ₃ Fe ₅ O _12, and magnetoplumbite type such as SrFe ₁₂ O ₁₉ and BaFe ₁₂ O _{19 in} terms of crystal structure. Among these, spinel type ferrites such as Fe ₃ O ₄ , γ-Fe ₂ O ₃ , LiFe ₅ O ₈ , MnFe ₂ O ₄ , NiFe ₂ O ₄ , and CoFe ₂ O ₄ are preferably used in the present invention.

  A part of the coating layer 2 may contain a substance other than the ferromagnetic oxide, and the content is preferably less than 10% by mass.

  The coating layer 2 may be only attached to the positive electrode active material base material 1, or may partially react on the surface of the base material to form a new interface layer. Further, the coverage is not particularly limited as long as the effect is generated, but the surface of the positive electrode active material base material 1 is preferably 75% or more, and more preferably 90% or more.

  The coating amount on the positive electrode active material base material 1 is preferably 0.15 to 10.0% by mass, and more preferably 0.5 to 5.0% by mass. If it is 0.15 mass% or more, the effect of this embodiment tends to increase, and if it is 10.0 mass% or less, the capacity per battery weight tends not to be impaired.

(Method for producing positive electrode active material)
The method of synthesizing the positive electrode active material base material 1 is not particularly limited, and may be synthesized by various methods such as a solid-phase reaction method, a method of firing the material through precipitation from a solution, a spray combustion method, and a molten salt method. Can do. For example, in the case of producing by a solid phase reaction method, LiOH · H ₂ O, Li ₂ CO _{3 and the} like serving as a lithium source, and hydroxides and oxides of the respective metals as the respective metal sources are intended positive electrodes They can be synthesized by mixing at a ratio corresponding to the composition of the active material base material and firing in an oxygen stream or in air at a temperature of about 700 to 1000 ° C. for about 10 to 20 hours.

  As a method of coating the positive electrode active material base material 1, a dry layer method is used in which a coating layer material is prepared in advance by a solid phase method, a sol-gel method, or a coprecipitation method, and the coating layer material is coated on the active material by a mechanochemical method such as a ball mill. Preferably by the method.

  Further, a wet method in which an active material base material is mixed and stirred in a raw material solution to be a coating material can also be used. As the wet method, a coprecipitation method or a sol-gel method can be used, and a target coating layer can be obtained by applying a heat treatment after adhering to the secondary particle surface.

  The coating layer 2 of the present invention is preferably composed of particles containing a ferromagnetic oxide, and the average secondary particle diameter (D50) of the particles is the average secondary particle diameter (D50) of the positive electrode active material base material secondary particles. Is preferably 1/10 or less, more preferably 1/100 or less. By doing in this way, uniform coating becomes possible, and the cycle characteristics when the charge potential is high are excellent.

(Lithium ion secondary battery)
A positive electrode and a lithium ion secondary battery using the positive electrode active material according to the present invention will be described.

  As shown in FIG. 2, the lithium ion secondary battery 100 mainly includes a stacked body 30, a case 50 that accommodates the stacked body 30 in a sealed state, and a pair of leads 60 and 62 connected to the stacked body 30.

  The laminated body 30 is configured such that the positive electrode 10 and the negative electrode 20 are disposed to face each other with the separator 18 interposed therebetween. The positive electrode 10 is a product in which a positive electrode active material layer 14 is provided on a positive electrode current collector 12. The negative electrode 20 is a product in which a negative electrode active material layer 24 is provided on a negative electrode current collector 22. 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. Leads 60 and 62 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.

  Here, as the positive electrode current collector 12 of the positive electrode 10, for example, an aluminum foil or the like can be used. The positive electrode active material layer 14 is a layer containing the above-described active material particles, a binder, and a conductive material added as necessary. Examples of the conductive material added as necessary include carbon blacks, carbon materials, and conductive oxides such as ITO.

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

  Such a positive electrode is formed by a known method, for example, an electrode active material including the active material particles 1 described above, or an active material particle 1, a binder, and a conductive material, and a solvent corresponding to their type, for example, N in the case of PVDF. -Slurry added to a solvent such as methyl-2-pyrrolidone or N, N-dimethylformamide is applied to the surface of the positive electrode current collector 12 and dried.

  As the negative electrode current collector 22, a copper foil or the like can be used. Moreover, as the negative electrode active material layer 24, the thing containing a negative electrode active material, a electrically conductive material, and a binder can be used. It does not specifically limit as a electrically conductive material, A carbon material, a metal powder, etc. can be used. As the binder used for the negative electrode, fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and tetrafluoroethylene-hexafluoropropylene copolymer (FEP) can be used.

Examples of the negative electrode active material include carbon materials such as graphite and non-graphitizable carbon, metals that can be combined with lithium such as Al, Si, and Sn, silicon oxides such as SiO ₂ and SiO, and other SnO ₂ Examples thereof include particles containing metal oxides, amorphous compounds mainly composed of these oxides, lithium titanate (Li ₄ Ti ₅ O ₁₂ ), and the like.

  The manufacturing method of the negative electrode 20 should just adjust slurry and apply | coat to a collector like the manufacturing method of the positive electrode 10. FIG.

The electrolytic solution is not particularly limited. For example, in the present embodiment, an electrolytic solution containing a lithium salt in an organic solvent can be used. Examples of the lithium _salt, LiPF _6, LiClO 4, salts of LiBF ₄ or the like can be used. In addition, these salts may be used individually by 1 type, and may use 2 or more types together.

  Preferred examples of the organic solvent include propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, and the like. These may be used alone or in combination of two or more at any ratio.

  The separator 18 is at least one selected from the group consisting of a monolayer of a porous film made of polyethylene, polypropylene or polyolefin, a stretched film of a laminate or a mixture of the above resins, or cellulose, polyester and polypropylene. A fiber nonwoven fabric made of a constituent material can be used.

  The case 50 seals the laminated body 30 and the electrolytic solution therein. The case 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 lithium ion secondary battery 100 from the outside. For example, a metal laminate film can be used. .

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

  The active material of this embodiment can also be used as an electrode material for electrochemical devices other than lithium ion secondary batteries. As such an electrochemical element, a secondary battery other than a lithium ion secondary battery, such as a metallic lithium secondary battery (which uses an electrode containing the composite particles of the present invention as a cathode and metallic lithium as an anode). And electrochemical capacitors such as lithium capacitors.

Example 1
(Creation of positive electrode active material)
LiNi _0.8 Co _0.15 Al _0.05 O ₂ was used as the paramagnetic positive electrode active material base material. Secondary particles are not formed, and the D50 average particle size of the primary particles is 0.85 μm.
As the coating layer, MgFe ₂ O ₄ particles having a D50 average particle diameter of 0.25 μm were used. It was 8.4 emu / g when saturation magnetization was calculated | required using VSM (Toei Kogyo Co., Ltd. make, VSM-P7 type | mold).
The finally obtained positive electrode active material has MgFe ₂ O ₄ particles added to the positive electrode active material base material so as to form a coating layer of 15% by mass of MgFe ₂ O _4, together with stabilized zirconia beads having a diameter of 1 μm. The mixing process was performed in a pot mill at 200 rpm for 3 hours.
The positive electrode active material powder subjected to the mixing treatment was subjected to X-ray diffraction and SEM observation, and it was confirmed that MgFe ₂ O ₄ particles were uniformly coated on the surface of the positive electrode active material base material particles.
In addition, quantitative analysis of Li, Ni, Co, Al, Mg, and Fe was performed using an ICP emission analyzer. The number of moles was calculated from the quantitative values of Li, Ni, Co, and Al, and the weight of LiNi _0.8 Co _0.15 Al _0.05 O ₂ was calculated by adding oxygen in an amount equal to the total number of moles. The number of moles was calculated from the quantitative values of Mg and Fe, and the weight of MgFe ₂ O ₄ was calculated by adding 4/3 times the total number of moles of oxygen. As a result, it was found that the mixed positive electrode active material powder contained 15.0% by mass of MgFe ₂ O ₄ as a coating layer with respect to LiNi _0.8 Co _0.15 Al _0.05 O _2. It was.
The D50 average particle diameter was calculated by image analysis using a scanning electron microscope. Thereafter, this method is used unless otherwise specified.

  A coating material for positive electrode was prepared by mixing a coated active material, a conductive additive, and a solvent containing a binder. The positive electrode coating material was applied to an aluminum foil (thickness 22 μm) as a current collector by a doctor blade method, dried at 110 ° C., and rolled. This obtained the positive electrode comprised from a positive electrode active material layer and a collector. As the conductive assistant, ketjen black (manufactured by Lion Corporation, EC600JD) and acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd., FX-35) were used. As a solvent containing a binder, N-methyl-2-pyrrolidone (manufactured by Kureha Chemical Industry Co., Ltd., KF7305) in which PVDF was dissolved was used.

(Half-cell creation)
The produced positive electrode and separator (polyolefin microporous membrane) were cut into predetermined dimensions. The negative electrode used was a copper foil current collector with a lithium foil attached thereto. The positive electrode was provided with a portion to which no electrode paint was applied in order to weld the external lead terminal. A positive electrode, a negative electrode, and a separator were laminated in this order. The positive electrode and the negative electrode were ultrasonically welded with an aluminum foil (width 4 mm, length 40 mm, thickness 80 μm) and nickel foil (width 4 mm, length 40 mm, thickness 80 μm) as external lead terminals, respectively. Polypropylene (PP) grafted with maleic anhydride was wrapped around this external lead terminal and thermally bonded. An aluminum laminate material composed of a PET (polyethylene terephthalate) layer, an Al layer, and a PP (polypropylene) layer was used as a battery outer package enclosing a battery element in which a positive electrode, a negative electrode, and a separator were laminated. The thickness of the PET layer is 12 μm, the thickness of the Al layer is 40 μm, and the thickness of the PP layer is 50 μm. A battery element was put into the outer package, an appropriate amount of electrolyte was added, and the outer package was vacuum heat sealed to produce a lithium ion secondary battery of Example 1. In addition, as electrolyte solution, what dissolved LiPF6 with the density | concentration of 1M (mol / L) in the mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) was used. The volume ratio of EC to DMC in the mixed solvent was EC: DMC = 3: 7.

(Charge / discharge rate measurement)
After charging the half cell of Example 1 under a predetermined condition in a 25 ° C. environment, the charge capacity and the discharge capacity (unit: mAh / g) were measured by discharging. In this measurement, the theoretical capacity of LiNi _0.8 Co _0.15 Al _0.05 O _{2 as} the positive electrode active material was set to 200 mAh / g. The charging is performed until the upper limit charging voltage is 4.5 V (VS. Li ^/ Li ⁺ ), the charging rate is 0.1 C, the positive electrode voltage reaches the upper limit charging voltage, and the charging current is attenuated to 1/20 C. It was. For the discharge, the lower limit discharge voltage was 2.8 V (VS. Li ^/ Li ⁺ ), the discharge rate was 0.1 C, and this was the 0.1 C discharge capacity. 0.1 C is a current value at which the discharge is terminated by a constant current discharge for 10 hours.
Next, after charging at a charge rate of 0.5 C, discharging was performed at a discharge rate of 2 C, which was defined as a 2 C discharge capacity. For evaluation of the rate characteristics, 2C discharge capacity / 0.1C discharge capacity was calculated.

(Cycle measurement)
Next, the cycle characteristics were measured under the condition of 0.5 C for both the charge rate and the discharge rate. The upper limit charging voltage and the lower limit discharging voltage were 4.5 V and 2.8 V, respectively, as in the soot discharge rate measurement. In order to evaluate the cycle characteristics, the discharge capacity at the 50th cycle / the discharge capacity at the first cycle was calculated.

(Example 2)
A half cell was prepared in the same manner as in Example 1 except that the amount of MgFe ₂ O ₄ particles with respect to the positive electrode active material base material was 0.10% by mass, and charge / discharge rate measurement and cycle characteristic evaluation were performed.

(Example 3)
A half cell was prepared in the same manner as in Example 1 except that the amount of MgFe ₂ O ₄ particles with respect to the positive electrode active material base material was 0.15 mass%, and charge / discharge rate measurement and cycle characteristic evaluation were performed.

Example 4
A half cell was prepared in the same manner as in Example 1 except that the amount of MgFe ₂ O ₄ particles relative to the positive electrode active material base material was 10.0% by mass, and charge / discharge rate measurement and cycle characteristic evaluation were performed.

(Example 5)
A half cell was prepared in the same manner as in Example 1 except that the amount of MgFe ₂ O ₄ particles relative to the positive electrode active material base material was 0.25% by mass, and charge / discharge rate measurement and cycle characteristic evaluation were performed.

(Example 6)
A half cell was prepared in the same manner as in Example 1 except that the amount of MgFe ₂ O ₄ particles with respect to the positive electrode active material base material was 5.0 mass%, and charge / discharge rate measurement and cycle characteristic evaluation were performed.

(Example 7)
LiNi _0.8 Co _0.15 Al _0.05 O ₂ forming secondary particles was used as the positive electrode active material base material. The particle diameter of the primary particles was around 0.8 μm, and the D50 average particle diameter of the secondary particles was 12.7 μm.
Example 1 except that MgFe ₂ O ₄ particles having a D50 average particle diameter of 1.75 μm were used as the coating layer and the amount of MgFe ₂ O ₄ particles was 2.5 mass% with respect to the positive electrode active material base material. Similarly, a half cell was prepared, and charge / discharge rate measurement and cycle characteristic evaluation were performed.

(Example 8)
A half cell was prepared in the same manner as in Example 7 except that MgFe ₂ O ₄ particles having a D50 average particle diameter of 1.27 μm were used as the coating layer, and charge / discharge rate measurement and cycle characteristic evaluation were performed.

Example 9
A half cell was prepared in the same manner as in Example 8 except that CuFe ₂ O ₄ particles having a D50 average particle diameter of 1.10 μm were used as the coating layer, and charge / discharge rate measurement and cycle characteristic evaluation were performed.

(Example 10)
A half cell was prepared in the same manner as in Example 8 except that Fe ₃ O ₄ particles having a D50 average particle diameter of 0.065 μm were used as the coating layer, and charge / discharge rate measurement and cycle characteristic evaluation were performed.

(Example 11)
A half cell was prepared in the same manner as in Example 8 except that γ-Fe ₂ O ₃ particles having a D50 average particle size of 0.080 μm were used as the coating layer, and charge / discharge rate measurement and cycle characteristic evaluation were performed.

(Example 12)
A half cell was prepared in the same manner as in Example 8 except that LiFe ₅ O ₈ particles having a D50 average particle size of 0.095 μm were used as the coating layer, and charge / discharge rate measurement and cycle characteristic evaluation were performed.

(Example 13)
A half cell was prepared in the same manner as in Example 8 except that MnFe ₂ O ₄ particles having a D50 average particle diameter of 0.85 μm were used as the coating layer, and charge / discharge rate measurement and cycle characteristic evaluation were performed.

(Example 14)
A half cell was prepared in the same manner as in Example 8 except that NiFe ₂ O ₄ particles having a D50 average particle diameter of 0.77 μm were used as the coating layer, and charge / discharge rate measurement and cycle characteristic evaluation were performed.

(Example 15)
A half cell was prepared as in Example 8 except that CoFe ₂ O ₄ particles having an average D50 particle diameter of 0.83 μm were used as the coating layer, and charge / discharge rate measurement and cycle characteristic evaluation were performed.

The same as Example 8 except that the coating layer was a mixture of Fe ₃ O ₄ particles having a D50 average particle size of 0.065 μm and LiFe ₅ O ₈ particles having a 0.095 μm mass ratio of 1: 1. A half cell was prepared, and charge / discharge rate measurement and cycle characteristic evaluation were performed.

(Example 17)
LiNi _1/3 Co _1/3 Mn _1/3 O ₂ forming secondary particles was used as a paramagnetic positive electrode active material base material. The particle diameter of the primary particles was around 0.9 μm, and the D50 average particle diameter of the secondary particles was 6.1 μm. A half cell was prepared as in Example 10 using Fe ₃ O ₄ particles having a D50 average particle diameter of 0.065 μm as the coating layer, and charge / discharge rate measurement and cycle characteristic evaluation were performed. In this measurement, the upper limit charging voltage is 4.5 V (VS. Li ^/ Li ⁺ ), the lower limit discharging voltage is 2.8 V (VS. Li ^/ Li ⁺ ), and LiNi _1/3 Co _1/3 that is the positive electrode active material. The charge / discharge current was set with the theoretical capacity of Mn _1/3 O ₂ being 185 mAh / g.

(Example 18)
As the paramagnetic positive electrode active material base material, LiMn ₂ O ₄ forming secondary particles was used. The particle diameter of the primary particles was around 0.7 μm, and the D50 average particle diameter of the secondary particles was 3.6 μm. A half cell was prepared as in Example 10 using Fe ₃ O ₄ particles having a D50 average particle diameter of 0.065 μm as the coating layer, and charge / discharge rate measurement and cycle characteristic evaluation were performed. In this measurement, the upper limit charging voltage is 4.5 V (VS. Li ^/ Li ⁺ ), the lower limit discharge voltage is 3.5 V (VS. Li ^/ Li ⁺ ), and the theoretical capacity of LiMn ₂ O ₄ that is the positive electrode active material is The charge / discharge current was set as 148 mAh / g.

(Example 19)
Li ₂ MnO ₃ —LiMO ₂ -based solid solution forming secondary particles as a paramagnetic positive electrode active material base material, 20 (Li ₂ MnO ₃ ) -80 (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) was used. The particle diameter of the primary particles was around 0.8 μm, and the D50 average particle diameter of the secondary particles was 18.5 μm. A half cell was prepared as in Example 10 using Fe ₃ O ₄ particles having a D50 average particle diameter of 0.065 μm as the coating layer, and charge / discharge rate measurement and cycle characteristic evaluation were performed. In this measurement, the upper limit charging voltage is 4.7 V (VS. Li / Li ⁺ ), the lower limit discharging voltage is 2.0 V (VS. Li / Li ⁺ ), and 20 (Li ₂ MnO ₃ ) − which is a positive electrode active material. The charge / discharge current was set with a theoretical capacity of 80 (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) being 250 mAh / g.

(Example 20)
LiFePO ₄ forming secondary particles was used as the paramagnetic positive electrode active material base material. The particle size of the primary particles was around 0.6 μm, and the D50 average particle size of the secondary particles was 4.2 μm. A half cell was prepared as in Example 10 using Fe ₃ O ₄ particles having a D50 average particle diameter of 0.065 μm as the coating layer, and charge / discharge rate measurement and cycle characteristic evaluation were performed. In this measurement, the upper limit charging voltage is 4.2 V (VS. Li ^/ Li ⁺ ), the lower limit discharging voltage is 2.5 V (VS. Li / Li ⁺ ), and the theoretical capacity of LiFePO ₄ as the positive electrode active material is 169 mAh / The charge / discharge current was set as g.

(Comparative Example 1)
LiCoO ₂ that is a diamagnetic material was used as a positive electrode active material base material. This formed secondary particles, and the particle size of the primary particles was around 1.1 μm, and the D50 average particle size of the secondary particles was 12.5 μm. As the coating layer, α-Fe ₂ O ₃ particles having a D50 average particle size of 0.075 μm were used. This saturation magnetization was 0, confirming that it was paramagnetic. Half cells were prepared in the same manner as in Example 10, and charge / discharge rate measurement and cycle characteristic evaluation were performed. In this measurement, the upper limit charging voltage is 4.4 V (VS. Li ^/ Li ⁺ ), the lower limit discharge voltage is 2.8 V (VS. Li / Li ⁺ ), and the theoretical capacity of LiCoO ₂ as the positive electrode active material is 137 mAh / The charge / discharge current was set as g.

(Comparative Example 2)
As the covering layer, Fe ₃ O ₄ particles having a D50 average particle diameter of 0.065 μm were used, a half cell was prepared in the same manner as in Comparative Example 1, and charge / discharge rate measurement and cycle characteristic evaluation were performed.

(Comparative Example 3)
A half cell was prepared in the same manner as in Example 7 except that the coating layer was not provided, and charge / discharge rate measurement and cycle characteristic evaluation were performed.

(Comparative Example 4)
A half cell was prepared in the same manner as in Example 7 except that α-Fe ₂ O ₃ particles having a D50 average particle diameter of 0.075 μm were used as the coating layer, and charge / discharge rate measurement and cycle characteristic evaluation were performed.

(Comparative Example 5)
LiNi _1/3 Co _1/3 Mn _1/3 O ₂ used in Example 16 was used as the positive electrode active material base material, and α-Fe ₂ O ₃ particles having a D50 average particle size of 0.075 μm were used as the coating layer. A half cell was prepared in the same manner as in Example 16 except that the charge / discharge rate measurement and cycle characteristic evaluation were performed.

The results are shown in Table 1. The saturation magnetization of the coating layer shown in the table is a value at 25 ° C.

Examples 1-2 using paramagnetic LiNi _0.8 Co _0.15 Al _0.05 O ₂ as the positive electrode active material base material and ferromagnetic MgFe ₂ O ₄ as the coating layer were used as the positive electrode active material base material. Compared with Comparative Example 1 using diamagnetic LiCoO ₂ and paramagnetic α-Fe ₂ O ₃ as the coating layer, the capacity retention was higher, and the upper limit charging voltage was 4.5 V (VS.Li/ It can be seen that the cycle characteristics are superior to those of Li ⁺ ). Furthermore, a higher 2C discharge capacity / 0.1 C discharge capacity is shown, and it can be seen that the rate characteristics at a high charge potential are also improved by the present invention. Further, even when compared with Comparative Example 2 using the Fe ₃ O ₄ of ferromagnetic as a covering layer, Examples 1-2 show a more excellent cycle characteristics and rate characteristics.

Further, a _LiNi 0.8 _Co 0.15 _{Al 0.05} O ₂ paramagnetic used as the positive electrode active material matrix, even when compared with Comparative Example 3 not provided with the coating layer, Examples 1-2 and more Excellent cycle and rate characteristics.

Paramagnetic LiNi _0.8 Co _0.15 Al _0.05 O ₂ and LiNi _1/3 Co _1/3 Mn _1/3 O ₂ are used as the positive electrode active material base material, and paramagnetic α is used as the coating layer. Even when compared with Comparative Examples 4 to 5 using —Fe ₂ O ₃ , Examples 1 to 2 show more excellent cycle characteristics and rate characteristics. By comparing these Comparative Examples 1 to 5 and Examples 1 and 2, the effect of the present invention using the paramagnetic positive electrode active material base material and the ferromagnetic coating layer is clear.

  Examples 3 to 4 in which the coating amount of the coating layer with respect to the positive electrode active material base material was 0.15 to 10.0% by mass showed higher cycle characteristics and rate characteristics than Examples 1 and 2. Moreover, Examples 5-6 which made the coating amount of the coating layer with respect to a positive electrode active material base material 0.25-5.0 mass% showed a still higher cycle characteristic and rate characteristic compared with Examples 3-4. Yes.

  Example 7 in which the positive electrode active material base material forms secondary particles shows higher cycle characteristics and rate characteristics than Examples 1-6. Further, Example 8 in which the average secondary particle diameter (D50) of the coating layer is 1/10 or less of the average secondary particle diameter (D50) of the positive electrode active material base material secondary particles is compared with Example 7, and It shows high cycle characteristics and rate characteristics.

Example 9 using CuFe ₂ O ₄ having a saturation magnetization of 10 emu / g or more as the coating layer is more compared to Example 8 using MgFe ₂ O ₄ having a saturation magnetization of 8.4 emu / g as the coating layer. It shows high cycle characteristics and rate characteristics.

Examples 10 to 15 using Fe ₃ O ₄ , γ-Fe ₂ O ₃ , LiFe ₅ O ₈ , MnFe ₂ O ₄ , NiFe ₂ O ₄ , and CoFe ₂ O ₄ exhibiting ferromagnetism as the coating layer are as follows: As a result, the cycle characteristics and rate characteristics superior to those of Example 9 using CuFe ₂ O ₄ are shown. Moreover, as a paramagnetic positive electrode active material base material, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiMn ₂ O _4, 20 (Li ₂ MnO ₃ ) -80 (LiNi _1/3 Co _1/3 Mn _{1 / 3} ) In Examples 17 to 20 using LiFePO ₄ , cycle characteristics and rates equivalent to those in Example 10 using LiNi _0.8 Co _0.15 Al _0.05 O ₂ as the paramagnetic positive electrode active material base material are also used. It turns out that the characteristic is shown.

Example 16 using a mixture of Fe ₃ O ₄ exhibiting ferromagnetism and LiFe ₅ O ₈ as the coating layer also shows cycle characteristics and rate characteristics equivalent to those of Example 10, and a plurality of types of coating layer materials are used. It can be seen that the effects of the present invention can be obtained even if used.

  As a result of the above, according to the implementation of the present invention, it was found that a positive electrode active material for a lithium ion secondary battery exhibiting high cycle characteristics can be obtained when the charging voltage is increased and the charging depth is increased.

  By using the positive electrode active material for a lithium ion secondary battery that exhibits high cycle characteristics when the charging voltage is increased and the charging depth is increased according to the present invention, a higher performance lithium ion secondary battery can be obtained. This is suitably used as a power source for portable electronic devices, and is also used as an electric vehicle, household and industrial storage battery.

DESCRIPTION OF SYMBOLS 1 ... Positive electrode active material base material, 2 ... Coating layer, 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 ... Laminated body 50 ... Exterior body 62 ... Positive electrode lead 60 ... Negative electrode lead 100 ... Lithium ion secondary battery

Claims (9)

A positive electrode active material matrix containing a lithium composite oxide;
A coating layer covering a part of the positive electrode active material base material,
The positive electrode active material base material exhibits paramagnetism,
The positive electrode active material, wherein the coating layer is a ferromagnetic oxide.   The positive electrode active material according to claim 1, wherein a coating amount of the coating layer with respect to the positive electrode active material base material is 0.15 to 10.0 mass%.   The positive electrode active material according to claim 1, wherein a coating amount of the coating layer with respect to the positive electrode active material base material is 0.25 to 5.0 mass%.   The positive electrode active material according to claim 1, wherein the positive electrode active material base material constitutes secondary particles, and the coating layer covers the secondary particles. .   The said coating layer consists of particle | grains containing a ferromagnetic oxide, The particle | grains are 1/10 or less of the average particle diameter (D50) of the said base material, The any one of Claims 1-4 characterized by the above-mentioned. The positive electrode active material described in 1. The positive electrode active material according to claim 1, wherein the covering layer has a saturation magnetization of 10 emu / g or more. The positive electrode active material base material includes any one of manganese lithium oxide, nickel lithium oxide, and LiMPO ₄ (M represents at least one element selected from Fe, Mn, and Ni). The positive electrode active material of any one of Claims. The covering layer is _{Fe 3 O 4, γ-Fe} 2 O 3, LiFe 5 O 8, MnFe 2 O 4, NiFe 2 O 4, any one of the preceding claims comprising at least one CoFe ₂ O ₄ The positive electrode active material according to Item.   A positive electrode having the positive electrode active material according to any one of claims 1 to 8, 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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