JP2019175630A - Positive electrode active material and lithium ion secondary battery - Google Patents

Abstract

To provide a positive electrode active material and a lithium ion secondary battery having improved cycle characteristics even at a deep charge and discharge depth.SOLUTION: A positive electrode active material 140 has a coating layer 143 made of a lithium-containing composite oxide at least part of a positive electrode active material 141. The lithium-containing composite oxide is characterized by the following composition formula (1). LiMaO(1) [1<x8, 3<y<6, Ma is at least one selected from a group consisting of Al, Si, Ga, Ge, In, Sn, and Nb]SELECTED DRAWING: Figure 2

Description

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

  Lithium ion secondary batteries have rapidly spread as a power source for mobile devices such as mobile phones. One of the reasons is the high energy density. In a device having a limited power supply space such as a mobile device, a battery having a large energy amount, that is, a high energy density is desirable even if it is small. For the same reason, lithium ion secondary batteries are attracting attention even in electric vehicles with limited power supply mounting space, and batteries with higher energy density are still required.

  Since the energy density of the battery is mainly determined by the charge / discharge capacity of the electrode active material, it is necessary to increase the capacity of the electrode active material in order to increase the energy density. As one of the means, it has been studied to increase the charge / discharge capacity of the positive electrode active material, which was conventionally about 50%, and to increase the charge / discharge capacity. However, since the charging voltage increases as the charging depth is increased, the decomposition reaction at the cathode active material / electrolyte interface is accelerated, the crystal structure of the cathode active material is unstable due to Li desorption, and transition metal ions from the cathode active material. There was a problem that the cycle characteristics deteriorated.

  In order to obtain good cycle characteristics even at a deep charge / discharge depth, for example, studies have been made to coat the surface of the positive electrode active material particles with a reaction suppression layer (Non-patent Document 1).

Chemical Communication, (2006) 544-545.

However, the characteristics are still unsatisfactory and further improvement is required.
The present invention has been made in view of the problems of the prior art, and an object thereof is to provide a positive electrode active material having improved cycle characteristics even at a deep charge / discharge depth and a lithium ion secondary battery using the same. .

In order to solve the above problems, a positive electrode active material according to the present invention is a positive electrode active material having a coating layer made of a lithium-containing composite oxide on at least a part of the positive electrode active material, the lithium-containing composite oxide Is the following composition formula (1).
Li _x MaO _y (1)
[1 <x <8, 3 <y <6, Ma is at least one selected from the group consisting of Al, Si, Ga, Ge, In, Sn, and Nb]

  According to such a configuration, the positive electrode active material is coated with a lithium-containing composite oxide having low catalytic activity, so that the reaction between the positive electrode active material and the electrolyte that occurs at a high potential is effectively suppressed, and cycle characteristics are improved. To do.

  The positive electrode active material according to the present invention preferably has an intermediate layer between the positive electrode active material and the coating layer, and the intermediate layer preferably contains a divalent metal-containing composite oxide.

  According to such a configuration, the divalent metal substituted with Li ions remains without being detached from the positive electrode active material at the time of charging, so that the laminated structure of the positive electrode active material can be stabilized and the cycle characteristics are further improved.

The divalent metal-containing composite oxide constituting the intermediate layer of the positive electrode active material according to the present invention is preferably represented by the following composition formula (2).
MbMc ₂ O ₄ (2)
[Mb is at least one selected from the group consisting of Ca, Mg, Ba, Zn, and Mc is at least one selected from the group consisting of Mn, Fe, Co, Ni, Al]

  Such a configuration is suitable for stabilizing the laminated structure of the positive electrode active material, and the cycle characteristics are further improved.

The positive electrode active material according to the present invention is preferably represented by the following composition formula (3).
Li _a Ni _b Co _c Mn _d Al _e O ₂ (3)
[In the above formula (3), 0 ≦ a ≦ 1.0, 0 ≦ b ≦ 1.0, 0 ≦ c <1.0, 0 ≦ d ≦ 1.0, 0 ≦ e ≦ 1.0, a + b + c + d + e = 1.0]

  A lithium ion secondary battery according to the present invention includes a positive electrode current collector, a positive electrode active material layer including a positive electrode active material, a negative electrode current collector, and a negative electrode active material layer including a negative electrode active material. And a separator and an electrolyte, wherein the positive electrode active material is the positive electrode active material.

  The electrolyte is preferably composed of an ionic liquid and a lithium salt.

  According to the present invention, it is possible to provide a positive electrode active material and a lithium ion secondary battery excellent in cycle characteristics even at a deep charge / discharge depth.

It is a schematic cross section of the lithium ion secondary battery in this embodiment. It is a positive electrode active material in this embodiment, Comprising: It is a schematic diagram in case a coating layer is uniform. It is a positive electrode active material in this embodiment, Comprising: It is a schematic diagram in case a coating layer is non-uniform | heterogenous. 2 is a reflected electron image obtained by a scanning electron microscope (SEM) in Example 1. FIG. 6 is a reflected electron image obtained by a scanning electron microscope (SEM) in Example 2. FIG. (This is a reflection electron image by a scanning electron microscope (SEM) in Comparative Example 1.

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

A. Lithium Ion Secondary Battery FIG. 1 is a schematic cross-sectional view showing an example of a lithium ion secondary battery according to this embodiment. As shown in FIG. 1, a lithium ion secondary battery 100 according to the present embodiment is disposed adjacent to each other between a plate-like negative electrode 20 and a plate-like positive electrode 10 facing each other, and the negative electrode 20 and the positive electrode 10. A plate-shaped separator 18, an electrolyte containing lithium ions, a case 50 for containing these in a sealed state, and one end electrically connected to the negative electrode 20 and the other The negative electrode lead 62 whose one end protrudes outside the case, and the positive electrode lead 60 whose one end is electrically connected to the positive electrode 10 and whose other end protrudes outside the case.

  The positive electrode 10 includes a positive electrode current collector 12 and a positive electrode active material layer 14 formed on the positive electrode current collector 12. The negative electrode 20 includes a negative electrode current collector 22 and a negative electrode active material layer 24 formed on the negative electrode current collector 22. The separator 18 is located between the positive electrode active material layer 14 and the negative electrode active material layer 24.

  Hereinafter, the lithium ion secondary battery according to the present embodiment will be described for each configuration.

1. The positive electrode 10 includes a positive electrode current collector 12 and a positive electrode active material layer 14 formed on the positive electrode current collector 12, and the positive electrode active material layer 14 is formed on at least a part of the positive electrode active material 141. A positive electrode active material 140 having a coating layer 143 made of a lithium-containing composite oxide is contained. Furthermore, the positive electrode active material layer 14 may contain a positive electrode conductive agent and a positive electrode binder.

(Positive electrode current collector)
The positive electrode current collector 12 may be any conductive plate material, and examples thereof include aluminum, stainless steel (SUS), nickel, iron, and titanium. Of these, aluminum and SUS are preferably used.

(Positive electrode active material)
The positive electrode active material 141 according to this embodiment refers to a lithium-containing transition metal oxide without a coating layer. The positive electrode active material 141 is not particularly limited as long as it can occlude / release lithium ions. For example, LiMn ₂ O ₄ and LiNi _0.5 Mn ₁ which are spinel positive electrode active materials _{. 5} O ₄ , LiMnO ₂ which is a layered oxide system, LiCoO ₂ , LiNiO ₂ , LiNi _0.33 Co _0.33 Mn _0.33 O ₂ , LiNi _0.8 Co _0.15 Al _0.05 O ₂ , olivine can be mentioned system positive electrode active material in which LiFePO _4, LiCoPO _4, LiNiPO _4, etc., particularly preferably a positive electrode active material of layered oxide.

  When the positive electrode active material is a layered oxide, the amount of Li insertion / extraction increases by increasing the charging depth, and the charge / discharge capacity can be improved.

  On the other hand, when the positive electrode active material is a layered oxide material, it is preferable because a material having low cycle characteristics is greatly improved at a deep charge / discharge depth.

  The particle diameter of the positive electrode active material 141 is preferably about 0.1 μm to 50 μm as the primary particle diameter. The primary particles may be granulated to form secondary particles, in which case a secondary particle diameter of about 5 μm to 100 μm is preferable.

  Although the content of the positive electrode active material 141 contained in the positive electrode active material layer 14 varies depending on the type of the positive electrode active material 141, specifically, it is preferably about 80 wt% to 99.5 wt%, more preferably. Is in the range of 90 wt% to 99.5 wt%.

(Positive electrode active material)
As shown in FIG. 2 and FIG. 3, the positive electrode active material 140 according to the present embodiment has a coating layer represented by the following composition formula (1) as an outermost layer on at least a part of the positive electrode active material. Point to. The positive electrode active material 140 has a coating layer 143 made of a lithium-containing composite oxide on at least a part of the positive electrode active material 141, and the lithium-containing composite oxide is represented by the following composition formula (1). Is.
Li _x MaO _y (1)
[1 <x <8, 3 <y <6, Ma is at least one selected from the group consisting of Al, Si, Ga, Ge, In, Sn, and Nb]

The coating layer 143 is made of a crystalline or amorphous lithium-containing composite oxide, and is preferably a compound that does not contain a 3d transition metal, for example, a general formula Li _x MaO _y (1 <x <8, 3 <y <6, Ma is represented by at least one selected from the group consisting of Al, Si, Ga, Ge, In, Sn, and Nb.

  The lithium-containing composite oxide may be crystalline or amorphous.

  When the lithium-containing composite oxide is crystalline, it can be identified by transmission electron microscope-restricted field diffraction (TEM-SAED).

  When the lithium-containing composite oxide is amorphous, transmission electron microscope-energy dispersive X-ray spectroscopy (TEM-EDX), transmission electron microscope-electron energy loss spectroscopy (TEM-EELS), inductively coupled plasma-luminescence spectroscopy Although identification is possible by analyzing the composition by analysis method (ICP-AES) or time-of-flight secondary ion mass spectrometry (TOF-SIMS), identification by TEM-EDX is particularly preferable.

  Although there is no limitation in particular in the thickness of the coating layer 143, it is preferable that they are 1 nm-500 nm.

  Further, the coating form of the coating layer 143 does not necessarily have to be the whole on the positive electrode active material 141, and a coating effect can be exhibited even if it is a part on the positive electrode active material 141 as shown in FIG. .

  Thus, by covering the positive electrode active material 141 with a lithium-containing composite oxide having low catalytic activity, the reaction between the positive electrode active material 140 and the electrolyte that occurs at a high potential is effectively suppressed, and cycle characteristics are improved. It becomes possible to make it.

(Middle layer)
Further, in order to stabilize the laminated structure of the positive electrode active material, as shown in FIGS. 2 and 3, an intermediate layer containing a divalent metal-containing composite oxide is interposed between the positive electrode active material 141 and the coating layer 143. 142 is preferable.

  Examples of the divalent metal include Ca, Mg, Ba, and Zn.

Moreover, as said bivalent metal containing complex oxide, what is represented, for example by following formula (2) is mentioned.
MbMc ₂ O ₄ (2)
[Mb is at least one selected from the group consisting of Ca, Mg, Ba, Zn, and Mc is at least one selected from the group consisting of Mn, Fe, Co, Ni, Al]

  When the divalent metal is inserted between the metal oxide layers of the positive electrode active material, the divalent metal does not desorb during charging and remains between the layers, and the stacked structure is fixed by a higher charge density. The structural change of the positive electrode active material can be suppressed and the structural stability can be enhanced.

The intermediate layer 142 is replaced with a transition metal element in the positive electrode active material depending on the heat treatment conditions, and the spinel phase MbMc ₂ O ₄ (Mb is at least one selected from the group consisting of Ca, Mg, Ba, and Zn). , Mc may be at least one selected from the group consisting of Mn, Fe, Co, Ni, and Al).

  Since this spinel phase has a small structural change due to charging and discharging and is stable, the structural change of the positive electrode active material during charging can be suppressed and the structural stability can be further improved.

  The covering form of the intermediate layer 142 is not necessarily the whole on the positive electrode active material 141, and even if it is a part on the positive electrode active material 141 as shown in FIG. Further, the covering layer 143 on the intermediate layer 142 is not necessarily required to cover the entire intermediate layer 142 and may be partially exposed.

  The intermediate layer includes time-of-flight secondary ion mass spectrometry (TOF-SIMS), inductively coupled plasma-emission spectroscopy (ICP-AES), transmission electron microscope-energy dispersive X-ray spectroscopy (TEM-EDX), limited field of view. Identification by diffraction (SAED) is possible, but identification by TEM-EDX is particularly preferable.

(Positive electrode active material layer)
The positive electrode active material layer 14 is mainly composed of a positive electrode active material 140, a positive electrode binder, and an amount of positive electrode conductive agent as required.

(Positive electrode conductive agent)
Examples of the positive electrode conductive agent include carbon black such as acetylene black and ketjen black.

(Positive electrode binder)
Examples of the positive electrode binder include polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).

2. Negative Electrode The negative electrode 20 used in the present invention has a negative electrode current collector 22 and a negative electrode active material layer 24 formed on the negative electrode current collector 22. Furthermore, the negative electrode active material layer may contain a negative electrode conductive agent and a negative electrode binder.

(Negative electrode current collector)
The material of the negative electrode current collector 22 is not particularly limited as long as it has conductivity, and examples thereof include copper, stainless steel (SUS), and nickel. Of these, copper and SUS are preferably used.

(Negative electrode active material layer)
The negative electrode active material layer 24 is mainly composed of the negative electrode active material 20, a negative electrode binder, and a negative electrode conductive agent in an amount as required.

(Negative electrode active material)
The negative electrode active material 20 is not particularly limited as long as it can occlude and release lithium ions. For example, metal lithium, lithium alloy, metal oxide, metal sulfide, metal nitride, and Examples thereof include carbon materials such as graphite. The negative electrode active material 20 may be in the form of a powder or a thin film.

(Negative electrode binder and conductive agent)
The negative electrode binder and the negative electrode conductive agent used for the negative electrode active material layer 24 are the same as the positive electrode binder and the positive electrode conductive agent used for the positive electrode active material layer, but also polyimide, polyamide, polyamideimide, carboxymethylcellulose. Or styrene butadiene rubber can be used.

3. Separator The separator 18 is not particularly limited as long as it has a function of retaining an electrolyte, and examples thereof include a porous film such as polyethylene and polypropylene, a resin nonwoven fabric, and a glass fiber nonwoven fabric.

4). Electrolyte The electrolyte used in the present embodiment is disposed so as to be in contact with the positive electrode active material having the coating layer made of the lithium-containing composite oxide, and may be an organic substance or an inorganic substance. It is highly effective in some cases.

  The organic electrolyte is not particularly limited as long as it can be disposed so as to be in contact with the positive electrode active material 140 having the coating layer 143 made of the lithium-containing composite oxide, and can be oxidatively decomposed. For example, organic electrolyte solution, polymer electrolyte, gel electrolyte, etc. can be mentioned.

As the organic electrolyte, a non-aqueous electrolyte containing a lithium salt and a non-aqueous solvent is usually used.
The lithium salt is not particularly limited as long as it is a lithium salt used in a general lithium ion secondary battery. For example, LiPF ₆ , LiBF ₄ , LiN (CF ₃ SO ₂ ) ₂ , LiCF ₃ SO ₃ , Examples include LiC (CF ₃ SO ₂ ) ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ F) ₂ and LiClO ₄ .

  The non-aqueous solvent is not particularly limited as long as it can dissolve the lithium salt. For example, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxyethane. 1,2-diethoxyethane, acetonitrile, propionitrile, tetrahydrofuran, 2-methyltetrahydrofuran, dioxane, 1,3-dioxolane, nitromethane, N, N-dimethylformamide, dimethyl sulfoxide, sulfolane, γ-butyrolactone, etc. It is done. These non-aqueous solvents may be used alone or in combination of two or more. Moreover, an ionic liquid can also be used as a non-aqueous solvent.

The polymer electrolyte contains a lithium salt and a polymer.
As a lithium salt, the thing similar to the lithium salt used for the said organic electrolyte solution can be used.
The polymer is not particularly limited as long as it can be gelled. For example, polyethylene oxide, polypropylene oxide, polyacrylonitrile, polyvinylidene fluoride (PVDF), polyurethane, polyacrylate, cellulose Etc.

5. Battery Case The case 50 is for sealing the laminate 30 and the electrolytic solution therein. The case 50 is not particularly limited as long as it can suppress 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, as the case 50, as shown in FIG. 1, a metal laminate film in which a metal foil 52 is coated with a polymer film 54 from both sides can be used. For example, an aluminum foil can be used as the metal foil 52 and a film such as polypropylene can be used as the polymer film 54. For example, the material of the outer polymer film 54 is preferably a polymer having a high melting point, such as polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is polyethylene (PE) or polypropylene (PP). Etc. are preferred.

B. Method for Producing Lithium Ion Secondary Battery A method for producing a lithium ion secondary battery 100 according to the present embodiment includes a lithium ion secondary battery 100 having a positive electrode active material layer 14, a negative electrode active material layer 24, and an organic electrolyte. It is a manufacturing method, and has a positive electrode active material production process for producing a positive electrode active material 140 having a coating layer 143 made of a lithium-containing composite oxide.

According to this embodiment, the positive electrode active material 140 having a coating layer made of a lithium-containing composite oxide can be obtained. By forming the positive electrode active material layer 14 using such a positive electrode active material 140, it is possible to suppress a decrease in cycle characteristics even under a deep charge depth, that is, under a high voltage charge.
Hereinafter, each process in the manufacturing method of the lithium secondary ion battery 100 which concerns on this embodiment is demonstrated.

(1) Positive electrode active material manufacturing process The manufacturing process of the positive electrode active material in the present embodiment includes a step of forming a bivalent metal-containing composite oxide as an intermediate layer into a positive electrode active material and a lithium-containing composite as a coating layer. An oxide coating step. Although the formation process of the intermediate layer 142 is not necessarily required, the cycle characteristics can be greatly improved by forming the intermediate layer 142.

(Intermediate layer formation process)
The intermediate layer 142 of the positive electrode active material according to this embodiment is obtained by depositing a divalent metal-containing composite oxide or hydroxide on the positive electrode active material 141 and performing a heat treatment. The deposition process may be a wet process or a dry process.

  As a wet method, for example, after putting a positive electrode active material into an alkaline solution and dropping an aqueous solution such as a divalent metal chloride while stirring, a divalent metal hydroxide is deposited on the positive electrode active material, Examples of the method include dehydration by heat treatment to obtain an oxide, and heat treatment at a high temperature of 500 ° C. or higher.

Examples of the dry method include a method in which a divalent metal-containing composite oxide is pressure-bonded to a positive electrode active material by, for example, mechanical milling.

  As heat processing temperature in this process, it is preferable to exist in the range of 500 to 900 degreeC, and it is more preferable to exist in the range of 600 to 800 degreeC. The heat treatment time in this step is preferably in the range of 0.5 hours to 3 hours, and more preferably in the range of 0.5 hours to 2 hours.

  The heat treatment atmosphere in this step is not particularly limited as long as the target intermediate layer can be formed and the cathode active material material is not deteriorated. For example, an air atmosphere, an oxygen atmosphere, a nitrogen atmosphere, Examples thereof include an argon-hydrogen atmosphere or a reduced pressure environment. Examples of the heat treatment method for the positive electrode active material include a method using a firing furnace.

(Coating layer formation process)
The coating layer 143 of the positive electrode active material according to this embodiment is obtained by mixing a positive electrode active material and a metal alkoxide solution serving as a lithium-containing composite oxide material, and then hydrolyzing the lithium-containing composite oxide on the positive electrode active material. The coating layer 143 is formed, and the coating layer 143 is crystallized by heat treatment.

  As the positive electrode active material, a material in which the intermediate layer 142 is formed by the intermediate layer forming step may be used.

  The mixing step of the metal alkoxide solution and the positive electrode active material is, for example, a method in which a solution obtained by mixing lithium ethoxide and tetraethoxysilane in a stoichiometric ratio is dropped into the positive electrode active material and mixed with a planetary mixer or the like. Can be mentioned.

  As a method for forming the coating layer 143 by hydrolysis in this step, distilled water necessary for hydrolysis is dropped into the slurry obtained by the planetary mixing treatment of the concentrated metal alkoxide and the positive electrode active material, and the planetary mixing treatment is performed again. And the like.

As heat processing temperature in this process, it is preferable to exist in the range of 120 to 900 degreeC, and it is more preferable to exist in the range of 250 to 750 degreeC.
In addition, the heat treatment time in this step is preferably in the range of 0.5 hours to 3 hours, and more preferably in the range of 0.5 hours to 1 hour.

  The heat treatment atmosphere in this step is not particularly limited as long as the target coating layer can be crystallized and does not degrade the positive electrode active material, and examples thereof include an air atmosphere, an oxygen atmosphere, and a nitrogen atmosphere. And an argon-hydrogen atmosphere and a reduced pressure environment. Examples of the heat treatment method for the positive electrode active material include a method using a firing furnace.

(2) Other processes The manufacturing method of the lithium ion secondary battery 100 according to the present embodiment is not particularly limited as long as it includes at least the positive electrode active material manufacturing process. For example, the positive electrode active material A positive electrode active material layer forming step for forming a positive electrode active material layer 14 containing a positive electrode active material having a coating layer made of a lithium-containing composite oxide obtained in the material manufacturing step, and a negative electrode active material layer 24 are formed. You may have the negative electrode active material layer formation process, the battery assembly process of assembling a lithium ion secondary battery, etc. Since these steps are the same as those in a general lithium ion secondary battery, description thereof is omitted here. Further, the obtained lithium ion secondary battery is the same as that described in the above-mentioned “A. Lithium ion secondary battery”, and thus the description thereof is omitted here.

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

  Hereinafter, the present invention will be described in more detail with reference to examples.

[Example 1]
(Manufacture of positive electrode active material)
An ethanol solution of lithium ethoxide and tetraethoxysilane was mixed and adjusted so that Li / Si = 2 and the Li concentration was 1.0 mol / L to prepare a precursor solution of a lithium-containing composite oxide. This precursor solution was added to 15 g of LiCoO ₂ powder and mixed with a planetary mixer at 2000 rpm for 1 minute. The addition amount of the precursor solution at this time was adjusted to be 0.5 wt% when the lithium-containing composite oxide composition was assumed to be Li _x SiO _{2 + x / 2} (x = Li / Si ratio). After adding 0.18 mL of distilled water to the slurry obtained by mixing and further hydrolyzing by mixing at 2000 rpm for 1 minute with a planetary mixer, it is dried at 120 ° C. in the atmosphere and 350 ° C. under vacuum. The heat treatment was performed for 30 minutes to obtain a positive electrode active material.

(Preparation of positive electrode)
The above positive electrode active material and acetylene black were mixed, and a polyvinylidene fluoride (PVDF) binder dissolved in n-methylpyrrolidone (NMP) was added to prepare a slurry. The mixing ratio of the positive electrode active material, acetylene black, and PVDF was 97.5: 1.0: 1.5 (weight ratio). The slurry was applied to an Al current collector, dried, and pressed to prepare a positive electrode.

(Production of lithium ion secondary battery)
The positive electrode and copper foil (thickness 16 μm) are cut into predetermined dimensions, respectively, and aluminum foil (width 4 mm, length 40 mm, thickness 100 μm), nickel foil (width 4 mm, length 40 mm, A thickness of 100 μm) was ultrasonically welded. Polypropylene (PP) was wrapped around this external lead terminal and thermally bonded. A polyethylene separator cut to a predetermined size is sandwiched between the positive electrode and copper foil, and these battery elements are thermocompression bonded to an aluminum laminate material composed of a polyethylene terephthalate (PET) layer, an Al layer, and a polypropylene (PP) layer. Fixed by. The separator used was a polyethylene microporous membrane. After the obtained battery element was dried, it was transferred into a glove box filled with argon gas, and a Li foil cut to a predetermined size was pasted on the copper foil. The thickness of the PET layer of the laminating agent was 12 μm, the thickness of the Al layer was 40 μm, and the thickness of the PP layer was 50 μm. Further, in the production of the battery outer package, the PP layer was disposed inside the outer package. A battery element is placed in the outer package, and a mixed solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed at a volume ratio of 3: 7, and lithium hexafluorophosphate (LiPF ₆ ) at a concentration of 1 mol / L. An appropriate amount of the electrolyte solution contained in the above was added, and the outer package was sealed in a vacuum. Thereby, the lithium ion secondary battery of Example 1 was produced.

(Identification of coating layer)
In the XRD pattern of the positive electrode active material obtained by the manufacturing process of the positive electrode active material, a diffraction pattern that can be attributed to LiCoO ₂ and a diffraction pattern that can be attributed to Li ₂ SiO ₃ were observed. ₂ SiO ₃ was found.

(Measurement of electrical characteristics)
The lithium ion secondary battery of Example 1 was charged at a constant current up to 4.5 V at a current value corresponding to 0.1 C as a current density in a constant temperature bath at 25 ° C., and then charged at a constant voltage at 4.5 V. went. Constant voltage charging was continued until the current density dropped to a value corresponding to 0.01C. Thereafter, constant current discharge was performed to 2.5 V at a current density of 0.1 C. At this time, the initial discharge capacity of Example 1 was 183.0 mAh / g.
After the measurement of the above electric characteristics, the battery was further charged at a constant current up to 4.5 V at a current value corresponding to 1.0 C, and then charged at a constant voltage at 4.5 V. Constant voltage charging was continued until the current density dropped to a value corresponding to 0.1C. Thereafter, constant current discharge was performed to 2.5 V at a current density of 1.0 C. When a cycle test was repeated for 30 cycles, assuming that the initial discharge capacity was 100%, the discharge capacity after 30 cycles was 44.9%. This value was taken as the capacity retention rate and listed in Table 1 together with the production conditions for the positive electrode active material. A high cycle characteristic indicates that the capacity retention rate is maintained at a high level, and that the durability in the charge / discharge cycle is excellent.

The improvement in the capacity retention rate is thought to be because the coating layer formed on LiCoO ₂ suppressed the reaction between the electrolytic solution and the positive electrode active material, and also suppressed elution of Co ions from LiCoO ₂ . 4 to 6 show SEM-COMPO images of Examples 1 and 2 and Comparative Example 1. FIG. LiCoO ₂ containing Co, which is a heavy metal, gives a bright contrast (Comparative Example 1), but the layers containing Si, which is a light element, cover LiCoO ₂ so that Examples 1 and 2 have a dark contrast. . Since the coating layer is nearly uniform, it can be seen that the contact with the electrolytic solution is effectively suppressed.

[Example 2]
(Manufacture of positive electrode active material)
A positive electrode active material was produced in the same manner as in Example 1 except that Li / Si = 4 and Li concentration was 1.5 mol / L in the preparation of the precursor solution of the lithium-containing composite oxide. Ion secondary batteries were fabricated and electrical characteristics were measured.

(Identification of coating layer)
In the XRD pattern of the positive electrode active material, a diffraction pattern that can be attributed to LiCoO ₂ and a diffraction pattern that can be attributed to Li ₄ SiO ₄ were observed, which revealed that the coating layer was Li ₄ SiO ₄ .

Table 1 shows the capacity retention rate. Compared with the case where the coating layer is Li ₂ SiO ₃ , it can be seen that the capacity retention rate tends to be slightly improved. This, Li movement in LiCoO ₂ interface is facilitated by the Li concentration at LiCoO ₂ interface area increases, thereby presumably because the overvoltage is more accelerated the decomposition inhibiting organic substances decreases.

[Example 3]
(Manufacture of positive electrode active material)
In the preparation of the lithium-containing composite oxide precursor solution, Li / Si = 8, Li concentration was 2.0 mol / L, and the heat treatment temperature was 600 ° C. An active material was manufactured, and a lithium ion secondary battery was manufactured and its electrical characteristics were measured.

(Identification of coating layer)
In the XRD pattern of the positive electrode active material, a diffraction pattern that can be attributed to LiCoO ₂ and a diffraction pattern that can be attributed to Li ₈ SiO ₆ were observed, which revealed that the coating layer was Li ₈ SiO ₆ .

Table 1 shows the capacity retention rate. It can be seen that even when Li ₈ SiO ₆ is coated, the capacity retention rate is improved by coating.

[Comparative Example 1]
Using LiCoO ₂ not coated with anything, a lithium ion secondary battery was produced and its electrical characteristics were measured. Table 1 shows the capacity retention rate. It can be seen that the one without the coating layer has a low capacity retention rate.

[Examples 4 to 6]
A coating layer was formed in the same manner as in Example 1 except that aluminum triisopropoxide was used instead of tetraethoxysilane and the Li / Ma ratio and heat treatment temperature were changed to the conditions shown in Table 2. The layer was identified. Here, Ma represents Al. In addition, the production of the positive electrode active material, the lithium ion secondary battery, and the evaluation of the electrical characteristics were performed in the same manner as in Example 1.

[Examples 7 to 9]
A coating layer was formed in the same manner as in Example 1 except that germanium tetraethoxide was used instead of tetraethoxysilane, and the Li / Ma ratio and the heat treatment temperature were changed to the conditions shown in Table 2, and the coating layer was formed by XRD. Was identified. Here, Ma represents Ge. In addition, the production of the positive electrode active material, the lithium ion secondary battery, and the evaluation of the electrical characteristics were performed in the same manner as in Example 1.

[Examples 10 to 11]
A coating layer was formed in the same manner as in Example 1 except that indium tripropoxide was used instead of tetraethoxysilane, and the Li / Ma ratio and the heat treatment temperature were changed to the conditions shown in Table 2. The coating layer was formed by XRD. Was identified. Here, Ma represents In. In addition, the production of the positive electrode active material, the lithium ion secondary battery, and the evaluation of the electrical characteristics were performed in the same manner as in Example 1.

[Example 12]
A coating layer was formed in the same manner as in Example 1 except that tin tetraethoxide was used instead of tetraethoxysilane, and the Li / Ma ratio and the heat treatment temperature were changed to the conditions shown in Table 2, and the coating layer was formed by XRD. Was identified. Here, Ma represents Sn. In addition, the production of the positive electrode active material, the lithium ion secondary battery, and the evaluation of the electrical characteristics were performed in the same manner as in Example 1.

[Examples 13 to 14]
A coating layer was formed in the same manner as in Example 1 except that niobium pentaethoxide was used instead of tetraethoxysilane, and the Li / Ma ratio and the heat treatment temperature were changed to the conditions shown in Table 2. The coating layer was formed by XRD. Was identified. Here, Ma represents Nb. In addition, the production of the positive electrode active material, the lithium ion secondary battery, and the evaluation of the electrical characteristics were performed in the same manner as in Example 1.

  Table 2 shows capacity retention rates of Examples 4 to 14. Even if the kinds of elements are different, the effect of improving the capacity retention rate by the coating can be seen.

[Example 15]
(Formation of intermediate layer)
10 g of LiCoO ₂ was added to 50 mL of HNO ₃ solution adjusted to a concentration of 0.1 mol / L, stirred for 10 minutes, washed with water, recovered, and dried at 100 ° C. for 2 hours. This LiCoO ₂ was added to 1000 mL of an aqueous NaOH solution adjusted to a concentration of 0.2 mol / L, and 11 mL of an aqueous 1 mol / L MgCl ₂ solution was slowly added dropwise while stirring. After stirring at room temperature for 5 hours, the resulting black powder is washed with water and collected, and heat treatment is performed in air at a temperature of 800 ° C. for 2 hours to include a composite metal oxide containing Mg ²⁺ on LiCoO ₂ An intermediate layer was formed.

(Identification of the intermediate layer)
When TEM-EDX measurement was performed on LiCoO ₂ with an intermediate layer formed, MgO of about 10 to 50 nm was distributed on the LiCoO ₂ particles, and as a result of EDX line scan, the interface between MgO and LiCoO ₂ was observed. Since the region where only Mg and Co coexist was confirmed, it was found that a composite oxide containing Mg and Co was formed.

The TEM-SAED, taking the area diffraction pattern of the coexistence region of the Mg and Co, since it gave a diffraction pattern that can be attributed to similar hexagonal and LiCoO _2, the structure of the composite oxide similar to LiCoO ₂ It was found that the structure was maintained.

Furthermore, when the amount of Mg in LiCoO ₂ particles in which the intermediate layer was formed by ICP-AES was quantified, it was found that Mg was included in a molar ratio of about 3.5% with respect to Co.

(Manufacture of positive electrode active material)
An ethanol solution of lithium ethoxide and tetraethoxysilane was mixed and adjusted so that Li / Si = 4 and Li = 1.5 mol / L to prepare a precursor solution of a lithium-containing composite oxide. This precursor solution was added to 15 g of LiCoO ₂ powder on which an intermediate layer was formed by the above method, and mixed for 1 minute at 2000 rpm with a planetary mixer. The amount of the precursor solution added at this time was 0.5 wt% when the lithium-containing composite oxide composition was assumed to be Li _x SiO _{2 + x / 2} (x = Li / Si ratio). After adding 0.18 mL of distilled water to the slurry obtained by mixing and further hydrolyzing by mixing at 2000 rpm for 1 minute with a planetary mixer, it is dried at 120 ° C. in the atmosphere and 350 ° C. under vacuum. The heat treatment was performed for 30 minutes to obtain a positive electrode active material.

  The production of the positive electrode active material layer, the production of the lithium ion secondary battery, and the measurement of the electric characteristics were the same as in Example 1. The capacity retention rate is shown in Table 3. The heat treatment temperature (A) in Table 3 indicates the heat treatment temperature when forming the intermediate layer, and the heat treatment temperature (B) indicates the heat treatment temperature when forming the coating layer.

As compared with Example 3 having only the coating layer of Li ₄ SiO ₄ , it can be seen that the capacity retention rate was greatly improved by introducing the intermediate layer.

[Comparative Examples 2 to 4]
Comparative examples 2 to 4 show the results of examining the electrical characteristics by changing the heat treatment temperature to 600 to 800 ° C. only in the intermediate layer, together with the Mg / Co ratio estimated by ICP-AES. Because there is no coating layer, all of the comparative examples have a capacity retention rate lower than that of Example 2.

The reason why the capacity retention rate has been dramatically improved by forming the coating layer together with the intermediate layer is that the intermediate layer that is destroyed by the interface reaction with the electrolytic solution is protected by coating with the Li-containing composite oxide. This is probably because the effect of stabilizing the structure of LiCoO ₂ by the intermediate layer was greatly improved.

[Examples 16 to 18]
In Example 15, the evaluation results when the elements used for the intermediate layer are Ca, Ba, and Zn are shown in Table 4 together with the Mb / Co ratio estimated by ICP-AES. Here, Mb refers to any element of Ca, Ba, and Zn. The intermediate layer is formed by the same method except that Mb chloride is used instead of MgCl ₂ . Production of the positive electrode active material, production of the positive electrode active material layer, production of the lithium ion secondary battery, and measurement of electrical characteristics were the same as in Example 1. The heat treatment temperature (A) in Table 4 indicates the heat treatment temperature when forming the intermediate layer, and the heat treatment temperature (B) indicates the heat treatment temperature when forming the coating layer.

  It can be seen that the divalent metal introduced into the intermediate layer has an effect of improving the capacity retention rate other than Mg.

[Example 19]
In Example 15, the manufacturing method of the positive electrode active material is the same except that the heat treatment temperature at the time of forming the intermediate layer is 900 ° C. The production of the positive electrode active material layer, the production of the lithium ion secondary battery, and the measurement of electric characteristics were the same as in Example 1. The heat treatment temperature (A) in Table 5 indicates the heat treatment temperature during formation of the intermediate layer, and the heat treatment temperature (B) indicates the heat treatment temperature during formation of the coating layer. Table 5 shows the capacity retention ratio together with the Mg / Co ratio estimated by ICP-AES.

(Identification of the intermediate layer)
When TEM-EDX measurement was performed on LiCoO ₂ with an intermediate layer formed, MgO of about 10 to 50 nm was distributed on the LiCoO ₂ particles, and as a result of EDX line scan, the interface between MgO and LiCoO ₂ was observed. Since the region where only Mg and Co coexist was confirmed, it was found that a composite oxide containing Mg and Co was formed.

When a limited field diffraction pattern of the coexistence region of Mg and Co was taken by TEM-SAED, a diffraction pattern that could be attributed to a cubic crystal different from LiCoO ₂ was given. As a result of combining with the composition analysis by TEM-EELS, it was found that the composite oxide was MgCo ₂ O ₄ having a spinel structure.

  It can be seen that even when the intermediate layer has a spinel structure by high-temperature heat treatment, the capacity retention rate can be improved.

[Examples 20 to 23]
In Example 15, the positive electrode active materials were LiNi _0.5 Co _0.5 O ₂ , LiNi _0.33 Co _0.33 Mn _0.33 O ₂ , LiNi _0.8 Co _0.1 Mn _0.1 O ₂ , Table 6 shows the results when LiNi _0.8 Co _0.15 Al _0.05 O ₂ is used. The manufacturing method of the positive electrode active material is the same as that of Example 15 except that the type of the positive electrode active material is changed. The production of the positive electrode active material layer, the production of the lithium ion secondary battery, and the measurement of electric characteristics were the same as in Example 1.

[Comparative Examples 5 to 8]
In Examples 21 to 23, the positive electrode active material, LiNi _0.5 Co _0.5 O ₂ , LiNi _0.33 Co _0.33 Mn _0.33 O ₂ , LiNi when the intermediate layer and the coating layer were not formed Table 6 shows capacity retention rates of _0.8 Co _0.1 Mn _0.1 O ₂ and LiNi _0.8 Co _0.15 Al _0.05 O ₂ . Except that the type of the positive electrode active material was changed, it was the same as Comparative Example 1 such as the production of the positive electrode active material layer, the production of the lithium ion secondary battery, and the measurement of electrical characteristics. The heat treatment temperature (A) in Table 6 indicates the heat treatment temperature when forming the intermediate layer, and the heat treatment temperature (B) indicates the heat treatment temperature when forming the coating layer.

  From Table 6, it can be seen that even when the type of the positive electrode active material is changed, the capacity retention rate of each positive electrode active material is improved by forming the intermediate layer and the coating layer.

DESCRIPTION OF SYMBOLS 10 ... Positive electrode 20 ... Negative electrode 12 ... Positive electrode collector 14 ... Positive electrode active material layer 140 ... Positive electrode active material material 141 ... Positive electrode active material 142 ... Intermediate | middle layer 143 ... -Coating layer 18 ... Separator 22 ... Negative electrode current collector 24 ... Negative electrode active material layer 30 ... Battery element 50 ... Case 52 ... Metal foil 54 ... Polymer film 60, 62 ... Lead 100 ... Lithium ion secondary battery.

Claims (6)

A positive electrode active material having a coating layer containing a lithium-containing composite oxide at least partially on the positive electrode active material, wherein the lithium-containing composite oxide is represented by the following composition formula (1).
LixMaOy (1)
[1 <x <8, 3 <y <6, Ma is at least one selected from the group consisting of Al, Si, Ga, Ge, In, Sn, and Nb]   The positive electrode active material according to claim 1, further comprising an intermediate layer between the positive electrode active material and the coating layer, wherein the intermediate layer includes a divalent metal-containing composite oxide. The positive electrode active material according to claim 2, wherein the divalent metal-containing composite oxide is represented by the following composition formula (2).
MbMc ₂ O ₄ (2)
[Mb is at least one selected from the group consisting of Ca, Mg, Ba, Zn, and Mc is at least one selected from the group consisting of Mn, Fe, Co, Ni, Al] The positive electrode active material according to any one of claims 1 to 3, wherein the positive electrode active material is represented by the following composition formula (3).
Li _a Ni _b Co _c Mn _d Al _e O ₂ (3)
[In the above formula (3), 0 ≦ a ≦ 1.0, 0 ≦ b ≦ 1.0, 0 ≦ c <1.0, 0 ≦ d ≦ 1.0, 0 ≦ e ≦ 1.0, a + b + c + d + e = 1.0]   A positive electrode having a positive electrode current collector, a positive electrode active material layer containing a positive electrode active material, a negative electrode current collector, a negative electrode having a negative electrode active material layer containing a negative electrode active material, the positive electrode and the 5. A positive electrode according to claim 1, comprising a separator positioned between the negative electrode, the negative electrode, the positive electrode, and an electrolyte in contact with the separator, wherein the positive electrode active material is a positive electrode according to claim 1. A lithium ion secondary battery characterized by being an active material. The lithium ion secondary battery according to claim 5, wherein the electrolyte includes an ionic liquid and a lithium salt.

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