KR102568790B1 - Cathodes active material for lithium ion batteries, and the batteries with them - Google Patents

Abstract

Provided is a lithium-ion secondary battery having a lithium cobalt-based positive electrode active material having improved discharge capacity and a positive electrode manufactured using the positive electrode active material. The lithium ion secondary battery 1 includes a positive electrode 3 having positive electrode active material particles, an electrolyte 9 and a negative electrode 5, and at least a portion of the positive electrode active material particles have a (111) plane as a main surface in at least one direction. It has a plate-like crystal structure and also a spinel-type crystal structure, and the elemental composition of the positive electrode active material particle is represented by LiCo _2-x Ni _x O ₄ (provided that 0 < x < 2).

Description

Cathode active material particles, lithium ion secondary battery using the same, and manufacturing method of cathode active material particles {Cathodes active material for lithium ion batteries, and the batteries with them}

A method for manufacturing a lithium ion secondary battery and a cathode active material is disclosed.

Materials having a spinel structure are known as positive electrode active materials for lithium ion secondary batteries, but those showing practical capacity are limited to materials containing manganese or substituents thereof.

For example, in Patent Document 1, lithium manganese composite oxide (LiMn ₂ O ₄ etc.) is used as a cathode active material, and a lithium ion secondary battery in which carbonaceous fine particles are used as an anode active material is disclosed.

In addition, Patent Document 2 discloses a method for producing a positive electrode active material made of LiMn ₂ O ₄ or the like.

[Prior art literature]

[Patent Literature]

[Patent Document 1] Japanese Unexamined Publication No. 2012-199025

[Patent Document 2] Japanese Unexamined Publication No. 2011-81926

Among the above-mentioned materials, spinel materials that exhibit practical capacity when used in batteries are currently limited to materials containing manganese and some substituents thereof.

An object of the present invention is to provide a lithium-ion secondary battery including a lithium cobalt-based spinel-type positive electrode active material having improved discharge capacity and a positive electrode manufactured using the positive electrode active material.

The lithium ion secondary battery disclosed herein may include a positive electrode including positive electrode active material particles, an electrolyte, and a negative electrode. At least some of the particles of the positive electrode active material have a plate-shaped crystal structure in which the (111) plane is a main surface in at least one direction, and also a spinel-type crystal structure,

The elemental composition of the positive electrode active material particle is represented by LiCo _{2 - x} Ni _x O ₄ (provided that 0<x<2).

When the positive electrode active material particles disclosed herein are used for a positive electrode, the discharge capacity of a lithium ion secondary battery can be greatly improved.

1 is a cross-sectional view schematically showing a lithium ion secondary battery according to an embodiment disclosed herein.
FIG. 2 is a diagram showing a photograph by electron diffraction of a lithium cobalt nickel composite oxide related to Example 5. FIG.
3 is a diagram showing measurement results according to an X-ray diffraction (XRD) method for positive electrode active material particles related to Examples and Reference Examples disclosed herein.
4 is a view showing a photograph taken by a transmission electron microscope (TEM) of a lithium cobalt composite oxide related to Reference Example 1;
5 is a diagram showing a photograph taken by TEM of a lithium cobalt nickel composite oxide related to Example 5;
6 is a diagram showing discharge characteristics of positive electrode active materials according to Examples 4 and 5 and Reference Example 1;

<Configuration of Cathode Active Material Particles and Lithium Ion Secondary Battery>

1 is a cross-sectional view schematically showing a lithium ion secondary battery according to an embodiment disclosed herein. In addition, what was shown in FIG. 1 is an example of the lithium ion secondary battery of this embodiment, and the actual battery structure is not limited to this.

As shown in FIG. 1, the lithium ion secondary battery 1 disclosed herein includes a positive electrode 3 having positive electrode active material particles, a negative electrode 5, and an electrolyte filled between the positive electrode 3 and the negative electrode 5. (9), a separator 7 disposed between the positive electrode 3 and the negative electrode 5 to divide the electrolyte 9, and the positive electrode 3, the negative electrode 5, the separator 7 and the electrolyte 9 It may include a container 11 for accommodating.

The electrolyte 9 is a material from which lithium ions can be eluted, and may be in a liquid or gel state. When the lithium ion secondary battery 1 is an all solid state battery, the electrolyte 9 may be solid. As the constituent material of the electrolyte 9, for example, ethylene carbonate (EC) and propylene carbonate (PC) in which lithium salt is dissolved can be used as a liquid, and NASICON-type oxide (Li _x Al _y Ti _z (PO ₄ ) ₃ ), perovskite-type oxide (La _x Li _y TiO _{3 )} , sulfide ₍ Li ₁₀ GeP ₂ S _{12 ), and sulfide solid electrolyte (Li 3 PS 4} , Li ₇ P ₃ S ₁₁ , Li ₆ PS ₅ Cl, Li ₁₀ GeP ₂ S ₁₂ , etc.).

The separator 7 is made of a material that does not pass electrons but allows lithium ions to pass therethrough. As the material of the separator 7, a known material such as a microporous membrane made of polyethylene or polypropylene can be used.

The negative electrode 5 may include, for example, a current collector made of copper foil or the like, and a mixture of a negative electrode active material and a binder applied to the current collector. As the negative electrode active material, known materials such as graphite and hard carbon can be used.

The cathode 3 may include a particulate cathode active material. Specifically, the positive electrode 3 may include a current collector such as an aluminum foil, and a mixture of positive electrode active material particles coated on the current collector and a known binder.

The positive electrode active material particle of the present embodiment may include a plate-shaped crystal structure in which the (111) plane serves as a main surface in at least one direction. In this specification, the main surface refers to two broad faces facing inward and outward in a plate-like crystal. The planar shape of the crystal structure of the positive electrode active material is hexagonal.

In addition, the cathode active material particle has a spinel-type crystal structure, and the elemental composition of the cathode active material particle is represented by LiCo _2-x Ni _x O ₄ (provided that 0<x<2). That is, the positive electrode active material particles of the present embodiment are made of a material in which a part of cobalt in LiCo ₂ O ₄ is substituted with nickel. In addition, the positive active material particles may have a single crystal structure.

In the positive electrode active material particle of the present embodiment, the theoretical charge/discharge potential can be increased and the theoretical capacity can be increased by substituting a part of cobalt with nickel. In addition, since it has the above-described crystal structure, in the positive electrode active material particle of the present embodiment, the exposed area of the (111) plane through which lithium ions are released and inserted is wider than that of LiCo ₂ O ₄ having a cubic crystal structure. all. For this reason, in the case of using the positive electrode active material of the present embodiment, the discharge capacity of the lithium ion secondary battery can be increased compared to the case where LiCo ₂ O ₄ is used as the positive electrode active material. In addition, when using the positive electrode active material of the present embodiment, an improvement in charge/discharge rate can be expected.

In the elemental composition of the positive electrode active material particle (LiCo _{2 - x} Ni _x O ₄ ), when x is in the range of 0<x<2, the discharge capacity of the battery can be improved, but when x is greater than or equal to 0.8 and less than 1.2, the value of x=0 It is possible to increase the charge/discharge potential and at the same time sufficiently increase the discharge capacity compared to the case. If x = 1, the discharge capacity can be greatly increased.

When x is less than 0.8, as x decreases, the proportion of the portion having a cubic structure among the crystals of the positive electrode active material particle increases (ie, the proportion of the portion having a plate-like structure decreases), and when x is 1.2 or more, the proportion of x increases. As it grows, the plate-like structure and also the spinel-type crystal structure in the crystals of the positive electrode active material particles collapse. The reason why discharge capacity can be greatly increased in the case of x = 1 is that in this case, almost all of the positive active material particles have a plate-like crystal structure.

In addition, the average particle diameter of the cathode active material particles is not particularly limited, but may be 1000 nm or less. When the average particle diameter of the cathode active material particles is 1000 nm or less, the contact area between the cathode active material particles and the liquid electrolyte 9 can be increased compared to the case where the average particle diameter exceeds 1000 nm, and at the same time, lithium ions are introduced into the particles. It is possible to reduce the movement distance when moving to the inner electrode material. For this reason, the discharge capacity of the lithium ion secondary battery including the positive electrode active material particles can be further improved.

In addition, when the average particle diameter of the positive electrode active material particles is 500 nm or less, the contact area between the positive electrode active material particles and the liquid electrolyte 9 can be increased. In addition, the measurement of the said average particle diameter is performed using a particle size distribution analyzer. For example, the particle size can be directly analyzed by observation using a TEM. When the positive electrode active material particles are plate-shaped, the particle size varies greatly in the plane direction and the thickness direction, but in the case of the positive electrode active material of the present embodiment, the particle size is determined by the size in the plane direction.

In addition, when the positive electrode active material particles included in the positive electrode have a uniform particle diameter, variation in battery characteristics can be reduced. In addition, the cathode active material may be uniformly dispersed and applied on the current collector.

A positive electrode including the positive electrode active material particles of the present embodiment can be used as a positive electrode of a lithium ion secondary battery.

The manufacturing method of the positive electrode active material particles of the present embodiment is not particularly limited as long as it is a method capable of realizing the crystal structure described above, but may be, for example, a method using a supercritical fluid.

The method using the supercritical fluid includes the steps of reacting a material containing cobalt, a material containing nickel, and a material containing lithium in a supercritical fluid to produce a metal composite oxide containing lithium, cobalt, and nickel; and A step of mixing and reacting the metal composite oxide with a material containing lithium, which has a plate-like crystal structure in which the (111) plane is a main surface in at least one direction and also a spinel-type crystal structure, and has an elemental composition of LiCo _{2 - x} Ni _x O ₄ (provided that 0<x<2) may include a step of preparing a positive electrode active material particle.

In the step of reacting the metal composite oxide with a material containing lithium, heat treatment at, for example, 250° C. to 450° C. may be performed for about 1 hour. In addition, as the material containing the cobalt, for example, a hydrate of cobalt nitrate (Co(NO ₃ ) ₂ 6H ₂ O), etc. is used, and as the material containing the nickel, a hydrate of nickel nitrate (Ni( NO ₃ ) ₂ 6H ₂ O) and the like are used. As the material containing the lithium, for example, a hydrate of lithium hydroxide (LiOH·H ₂ O) or the like is used.

The above-mentioned supercritical state can be achieved by making the inside of the reaction vessel into a high-temperature and high-pressure condition of about 20 MPa to 50 MPa at 250° C. to 500° C. using a device for supercritical fluid.

According to this method, uniform nano-sized cathode active material particles can be produced.

Example

<Preparation of Cathode Active Material Particles>

First, solid hydrates Co(NO ₃ ) ₂ 6H ₂ O, Ni(NO ₃ ) ₂ 6H ₂ O, and LiOH H ₂ O were dissolved in ethanol to a predetermined concentration, and then stirred and mixed at 50°C. did The concentration of cobalt nitrate in ethanol was adjusted to 0.5 M, the concentration of nickel nitrate to 0.5 M, and the concentration of lithium hydroxide to 3 M.

Next, this mixed solution was sealed in a container of a supercritical synthesis apparatus (manufactured by AKICO), and then the above-mentioned metal compounds were reacted with each other for 40 minutes under conditions of 300°C and 10 MPa. Under these conditions, the solvent ethanol becomes a supercritical fluid.

Here, the cobalt:nickel molar ratio in the mixture was 9:1 as Example 1, 4:1 as Example 2, 7:3 as Example 3, and 3:2 as Example 2. What was set as Example 4 was made, 1: 1 was set as Example 5, and what was set as 2: 3 was set as Example 6.

Then, the powder obtained by evaporating off the solvent was mixed with LiOH·H ₂ O, and heat treatment was performed at 650° C. for 5 hours to obtain desired positive electrode active material particles. If the composition of the positive electrode active material particle is represented by LiCo _{2 - x} Ni _x O ₄ , x=0.2 in Example 1, x=0.4 in Example 2, x=0.6 in Example 3, x=0.8 in Example 4, In Example 5, x = 1.0, and in Example 6, x = 1.2.

In addition, in the first step, a mixture of a cobalt nitrate solution and a lithium hydroxide solution was reacted in a supercritical fluid without adding a nickel nitrate solution, and then heat treatment was performed in the same manner as in Examples 1 to 6 to obtain a composition of LiCo ₂ O ₄ Cathode active material particles having the same were prepared and used as Reference Example 1.

In order to confirm the crystal structure of the positive electrode active material particles related to Example 5 obtained as described above, analysis was performed according to the electron diffraction method. In addition, in order to confirm the crystal structure of the positive electrode active material particles related to Examples 1 to 6 and Reference Example 1, analysis according to the XRD method was performed. In addition, in the above composition formula, the positive electrode _active material particles in the case of x = 0 (Reference Example 1; _{LiCo 2} O ₄ ) and in the case of x = 1 ₍ Example 5; LiCo _1.0 Ni _1.0 O _{4 )} were respectively TEM was taken to confirm the appearance of the particles.

(electrochemical evaluation of battery)

Among the positive electrode active material particles obtained in the above process, an evaluation cell including a positive electrode including the positive electrode active material particles related to Reference Example 1 and Examples 4 and 5 was prepared, and electrochemical evaluation of the evaluation cell was performed. . Specifically, the positive electrode active material particles, acetylene black, and polytetrafluoroethylene (PTFE) were mixed in a mortar in a weight ratio of 80:10:10, and the obtained material was used as a positive electrode material.

A lithium metal foil was used as an anode material. In the evaluation cell, the positive electrode material and the negative electrode material were each fixed to a metal mesh, and an electrolyte solution (1M LiClO ₄ in EC:DEC (1:1 volume)) was sealed in a container. Using this evaluation cell, a charge/discharge test was conducted at a current density of 0.1 C from 4.7 V to a cutoff voltage of 2.7 V.

(measurement result)

FIG. 2 is a diagram showing a photograph by electron diffraction of a lithium cobalt nickel composite oxide related to Example 5. FIG. As shown in the same figure, in the range where x of the composition formula LiCo _{2 - x} Ni _x O ₄ is 1.0, the positive electrode active material particle exhibits 6-fold symmetry, and the 6-fold symmetry of the positive electrode active material particle This indicates that the plate surface has a [111] face of a spinel-type crystal structure or a [001] face of a layered rock salt crystal structure.

3 is a diagram showing measurement results according to the XRD method of positive electrode active material particles related to Examples 1 to 6 and Reference Example 1; As shown in the figure, an intensity peak can be seen at a general diffraction angle (abscissa axis in FIG. 2) in the range where x is 0 to 1.2 of the composition formula LiCo _{2 -x} Ni _x O ₄ , and x is 0 to 1.2. It was confirmed that the positive electrode active material particles had a spinel-type crystal structure within the range. However, LiCo _{0 . 8} Ni _{1 .} In ₂ O ₄ , the intensity peak at the position where the diffraction angle is around 20° was significantly increased, and it was found that there was a difference in the crystal structure from the case where x was less than 1.2.

From the combination of the results of FIGS. 2 and 3, it can be confirmed that the surface of the plate of the lithium cobalt nickel composite oxide (i.e., the positive electrode active material particle) related to Example 5 has a [111] plane with a spinel-type crystal structure.

4 is a diagram showing a photograph taken by a transmission electron microscope (TEM) of a lithium cobalt composite oxide related to Reference Example 1, and FIG. 5 is a photograph taken by a TEM of a lithium cobalt nickel composite oxide related to Example 5. It is a drawing that represents

From the photomicrograph shown in FIG. 4 , it was confirmed that many of the positive electrode active material particles having a composition of LiCo ₂ O ₄ had a cubic crystal structure indicated by symbol A. In contrast, as shown in FIG. 5, the composition is LiCo _{1 . 0} Ni _{1 .} It was confirmed that most of the positive electrode active material particles of ₀ O ₄ were shaped like hexagonal plates indicated by symbol B. It is considered that the change in crystal structure from cubic to plate-like was caused by the addition of nickel.

6 is a diagram showing discharge characteristics of the positive electrode active materials according to Examples 4 and 5 and Reference Example 1. FIG. From the results shown in the figure, it can be seen that when the composition of the positive electrode active material particles is LiCo _0.8 Ni _1.2 O ₄ , the discharge potential is higher and the discharge capacity of the battery is higher than when the composition of the positive electrode active material particles is LiCo ₂ O ₄ . there was. The composition of the positive electrode active material particles is LiCo _{1 . 0} Ni _{1 . 0} O ₄ , the composition of the positive electrode active material particles is LiCo _{0 . 8} Ni _{1 .} It was also confirmed that the discharge capacity was much higher than in the case of ₂ O ₄ .

As described above, the positive electrode active material particles according to one example of the present disclosure can be applied to a positive electrode of a lithium ion secondary battery or an electrode such as a lithium ion capacitor.

1: lithium ion secondary battery 3: positive electrode
5: cathode 7: separator
9: electrolyte 11: container

Claims (9)

Including a positive electrode, an electrolyte and a negative electrode including positive electrode active material particles,
At least some of the particles of the positive electrode active material have a plate-shaped crystal structure in which the (111) plane is a main surface in at least one direction, and also a spinel-type crystal structure,
The elemental composition of the positive active material particles is LiCo _{2 - x} Ni _x O ₄ (however, 0 < x < 2) represented by the lithium ion secondary battery. According to claim 1,
x in the above formula representing the elemental composition of the positive electrode active material particles is 0.8 or more and less than 1.2, a lithium ion secondary battery. According to claim 1 or 2,
In the above formula representing the elemental composition of the positive electrode active material particles, x is 1, a lithium ion secondary battery. According to claim 1,
The average particle diameter of the positive electrode active material particles is 1000 nm or less, a lithium ion secondary battery. According to claim 4,
The average particle diameter of the positive electrode active material particles is 500 nm or less, a lithium ion secondary battery. According to claim 1,
The positive electrode active material particle has a single crystal structure, a lithium ion secondary battery. (111) A positive electrode active material having a plate-like crystal structure in which the plane becomes a main surface in at least one direction and a spinel-type crystal structure, and having an elemental composition represented by LiCo _{2 - x} Ni _x O ₄ (where 0 < x < 2) particle. According to claim 7,
The positive electrode active material particle in which x in the above formula representing the elemental composition is 0.8 or more and less than 1.2. generating a metal composite oxide containing cobalt, nickel, and lithium by reacting a material containing cobalt, a material containing nickel, and a material containing lithium in a supercritical fluid;
A step of mixing the metal composite oxide and a material containing lithium and reacting by heat treatment at 250 to 450 ° C, having a plate-shaped crystal structure in which the (111) plane becomes a main surface in at least one direction and also a spinel-type crystal structure, A method of manufacturing a positive electrode active material particle comprising the step of preparing a positive electrode active material particle whose elemental composition is represented by LiCo _2-x Ni _x O ₄ (provided that 0<x<2).

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