JP6090085B2 - Positive electrode active material, method for producing positive electrode active material, and lithium battery - Google Patents

Description

  The present invention relates to a positive electrode active material capable of increasing the discharge capacity of a battery.

One type of positive electrode active material for lithium batteries is a polyanionic compound represented by Li _a MXO ₄ (M is Ni, Co, Mn, etc., X is P, Si, Ti, etc.). Research has been done. For example, according to Patent Document 1, an electrochemical device having excellent discharge capacity and rate characteristics can be formed by supporting polyanionic compound particles having a predetermined average primary particle size on carbon particles.

  According to Patent Documents 2 and 3, in order to improve the performance of the active material, it is effective to carry an alloy on the active material or to provide an alloy phase.

JP 2010-86772 A Japanese Patent No. 3578992 JP 2006-185716 A

However, as a result of diligent research conducted by the present inventor, it has been found that when a TiO ₄ compound is used as a positive electrode active material of a battery among the polyanion compounds described above, a sufficient discharge capacity may not be obtained. .

  Then, this invention makes it a subject to provide the positive electrode active material which can increase the discharge capacity of a battery.

The present inventor has made the following ideas and knowledge through earnest research.
(1) The conventional Li ₂ MTiO ₄ -based composite oxide has a lithium ion diffusion path due to its structure, but lithium is not sufficiently desorbed, and only a real capacity of about half the theoretical capacity is obtained as a positive electrode active material. It is not done. This is probably because the diffusion coefficient of lithium ions is low and the diffusion path of lithium is not sufficient.
(2) In the composite oxide, lithium should move by two, but in the conventional technique, only one electron is reacted. In order to reach a two-electron reaction, it is considered necessary to secure a wide diffusion path because of the structure.
(3) By supporting metal particles on the composite oxide and improving the electron conductivity, a wide diffusion path can be secured and the capacity can be increased.
(4) On the other hand, when the metal particles are exposed to a high potential during charging of the battery, they may react with the electrolyte and elute. In order to suppress elution of the metal particles, it is effective to enclose the metal particles in the composite oxide particles.
(5) In order to encapsulate the metal particles in the composite oxide particles, the metal particles may be generated in the composite oxide synthesis process.

The present invention has been completed based on the above ideas and findings. That is,
1st this invention is a positive electrode active material which has a Ni-Co alloy in the particle | grains of complex oxide which belongs to the space group fm-3m represented by following formula (1).
Li _a Ni _x Co _y Mn _z TiO _b (1)

  In the present invention, “a composite oxide belonging to the space group fm-3m represented by the following formula (1)” is a composite oxide having an irregular rock salt structure composed of Li, Co, Ni, Mn, Ti, and O. Say. That is, as long as such a structure is adopted, the values of a, b, x, y, and z in the formula (1) are not particularly limited. For example, a composite oxide of a = 2, b = 4, x = 1/3, y = 1/3, and z = 1/3 can be mentioned, but is not limited thereto.

  “Composite oxide particles” are preferably particles made of composite oxides, but strictly speaking, particles in which Li, Mn, Ti, and O that do not constitute an irregular rock salt structure are partially confirmed. There may be. That is, when XRD measurement is performed on the positive electrode active material, it is only necessary to confirm a diffraction peak derived from an irregular rock salt structure and a diffraction peak derived from a Ni—Co alloy described later.

  “Having a Ni—Co alloy in the composite oxide particles” means that the Ni—Co alloy is present at least in the particles. That is, the Ni—Co alloy exposed on the surface of the composite oxide particle may exist.

  In the present invention, the presence of the “Ni—Co alloy” can be confirmed by the presence or absence of a diffraction peak when XRD measurement is performed on the positive electrode active material. Further, “having a Ni—Co alloy in the composite oxide particles” can be confirmed by the following method. That is, after applying the positive electrode active material to the positive electrode of the electrolyte battery, the XRD diffraction peak of the positive electrode before and after battery charging and the XRD diffraction peak of the positive electrode before and after battery discharge are confirmed. When the peak derived from the Ni-Co alloy can be confirmed together with the peak derived from the composite oxide before and after charging / discharging, the Ni-Co alloy is not eluted into the electrolyte during charging / discharging of the battery, that is, It can be judged that the composite oxide particles have a Ni-Co alloy.

  2nd this invention is a lithium battery provided with the positive electrode active material which concerns on 1st this invention.

  In the present invention, the “lithium battery” is not particularly limited as long as the positive electrode active material according to the present invention is used in the positive electrode.

According to a third aspect of the present invention, a mixture containing Ni and Co in excess as compared with a composite oxide belonging to the space group fm-3m represented by the following formula (1) is fired in the presence of a reducing agent. A method for producing a positive electrode active material, wherein the composite oxide particles and the Ni—Co alloy are generated, and the Ni—Co alloy is supported in the composite oxide particles.
Li _a Ni _x Co _y Mn _z TiO _b (1)

  In the present invention, “… a mixture containing Ni and Co in excess compared to the composite oxide” means the composition ratio of Li, Ni, Co, Mn, Ti and O that can form an irregular rock salt structure and the mixture. By comparing the composition ratio, it means that Ni and Co are excessively contained in the mixture. By firing such a mixture “in the presence of a reducing agent”, a Ni—Co alloy can be produced while synthesizing the composite oxide, and at least a part of the Ni—Co alloy is contained in the composite oxide particles. Can be easily included.

  In the third aspect of the present invention, an organic substance derived from an organic acid can be used as the reducing agent. The “organic substance” only needs to function as a reducing agent, and may be a compound containing carbon in the molecule. For example, when a composite oxide is synthesized by a solution method, a residue derived from an organic acid added at the time of mixing raw materials, or a residue derived from an organic acid salt when an organic acid salt is used as a raw material It can be used as “organic matter”.

  In the positive electrode active material according to the present invention, the Ni—Co alloy is contained in the predetermined composite oxide particles. Thereby, the electron conductivity can be improved to ensure a wide diffusion path, while the elution of the alloy during the battery reaction can be appropriately suppressed. That is, according to the present invention, it is possible to provide a positive electrode active material capable of increasing the discharge capacity of a battery.

It is the schematic for demonstrating the positive electrode active material which concerns on this invention. It is a figure which shows the XRD measurement result about each sample which concerns on an Example and a comparative example. It is a SEM observation result of the sample which concerns on an Example, (A) is a secondary electron image, (B) is a reflected electron image. It is a figure which shows the charging / discharging test result of a battery at the time of using the sample which concerns on an Example as a positive electrode active material. It is a figure which shows the charging / discharging test result of a battery in case the sample which concerns on a comparative example is used as a positive electrode active material. It is a figure which shows the XRD measurement result after charging / discharging about the sample which concerns on an Example.

1. Cathode Active Material The cathode active material according to the present invention is characterized by having a Ni—Co alloy in the composite oxide particles belonging to the space group fm-3m represented by the following formula (1).
Li _a Ni _x Co _y Mn _z TiO _b (1)

  FIG. 1 schematically shows a positive electrode active material 10 according to an embodiment of the present invention. As shown in FIG. 1, the positive electrode active material 10 has a Ni—Co alloy 2 in a composite oxide particle 1. In the present invention, it is sufficient that at least the Ni—Co alloy 2 is present in the composite oxide particle 1, and as shown in FIG. 1, a part of the Ni—Co alloy 2 is exposed on the surface of the composite oxide particle 1. You may do it.

1.1. Composite oxide particles 1
The composite oxide particle 1 is a composite oxide particle belonging to the space group fm-3m represented by the above formula (1). The complex oxide only needs to have an irregular rock salt structure composed of Li, Co, Ni, Mn, Ti, and O. As long as such a structure is adopted, the values of a, b, x, y, and z in the formula (1) are not particularly limited. For example, a composite oxide of a = 2, b = 4, x = 1/3, y = 1/3, and z = 1/3 can be mentioned, but is not limited thereto. Usually, a = 1.5 to 2.5, b = 3 to 4, x = 0 to 1, y = 0 to 1, and z = 0 to 1.

  In the present invention, the composite oxide particle 1 may be any material as long as a diffraction peak derived from an irregular rock salt structure can be confirmed when XRD measurement is performed on the positive electrode active material 10, and a diffraction peak derived from an irregular rock salt structure. It is preferred that only be confirmed.

  The size of the composite oxide particle 1 is not particularly limited, but the primary particle diameter is preferably 10 nm to 1 μm, more preferably 10 nm to 500 nm. The size of the composite oxide particle 1 can be adjusted according to synthesis conditions and the like.

1.2. Ni-Co alloy 2
The positive electrode active material 10 has a Ni—Co alloy 2 in the composite oxide particle 1. The Ni—Co alloy 2 only needs to be present in at least the particles 1, and the size and form thereof are not limited. For example, when a positive electrode active material is produced by a solution method to be described later, a particulate Ni—Co alloy can be precipitated. In this case, the primary particle diameter is preferably 10 nm to 300 nm, more preferably 10 nm to 100 nm. As described above, a part of the Ni—Co alloy 2 may be exposed on the surface of the composite oxide particle 1.

  The content of the Ni—Co alloy in the positive electrode active material 10 is not particularly limited as long as the effect of the present invention is exhibited.

  As described above, in the positive electrode active material 10, the Ni—Co alloy 2 is included in the composite oxide particles 1. Thereby, while it is possible to improve the electron conductivity and secure a wide diffusion path, it is possible to appropriately suppress the elution of the alloy 2 during the battery reaction, and to increase the discharge capacity of the battery.

2. Lithium battery This invention also has the side as a lithium battery. That is, it is a lithium battery provided with the above-described positive electrode active material according to the present invention. The configuration of the lithium battery is not particularly limited as long as the positive electrode active material according to the present invention is used in the positive electrode, and other configurations such as an electrolyte, a negative electrode, and a current collector are not limited. As for other configurations, it is possible to adopt the configurations conventionally applied to lithium batteries as they are. As described above, since the positive electrode active material according to the present invention can suppress the elution of the Ni—Co alloy during battery charging / discharging, the positive electrode active material is applied to a lithium battery using an electrolytic solution (particularly a nonaqueous electrolytic solution). Is also applicable.

3. Method for Producing Cathode Active Material The cathode active material according to the present invention can be easily produced by a characteristic production method as described below, for example. That is, in the method for producing a positive electrode active material according to the present invention, a mixture containing an excessive amount of Ni and Co as compared with a composite oxide belonging to the space group fm-3m represented by the following formula (1) is coexistent with a reducing agent. The composite oxide particles and the Ni—Co alloy are produced by firing at a lower temperature, and the Ni—Co alloy is supported in the composite oxide particles.
Li _a Ni _x Co _y Mn _z TiO _b (1)

  That is, in the present invention, by using a mixture containing Ni and Co in excess of the composition ratio of the composite oxide, and firing while reducing the Ni-Co source of the mixture, the composite oxide is synthesized and synthesized. A Ni—Co alloy is produced during the sintering process. This makes it possible to easily manufacture a positive electrode active material in which a Ni—Co alloy is taken into composite oxide particles.

  The mixture may contain at least a Li source, a Ni source, a Co source, a Mn source, and a Ti source. Any reducing agent may be used as long as it can convert a Ni source and a Co source into a Ni—Co alloy, and various organic substances can be used. Hereinafter, the case where a positive electrode active material is manufactured by a solution method will be described.

  When the positive electrode active material is synthesized by the solution method, for example, a nitric acid solution can be used as the solution, various acetates can be used as the Li source, Ni source, Co source, and Mn source, and oxidation can be performed as the Ti source. Titanium can be used. As already explained, the composition ratio in the mixture is such that the final amount of the positive electrode active material contains an excessive amount of Ni source and Co source so that a predetermined amount of a complex oxide having a random rock salt structure and a Ni—Co alloy are included. What is necessary is just to adjust suitably so that it may become.

  Here, when a positive electrode active material is synthesized by a solution method, various raw materials can be easily dissolved by adding an organic acid to the solution. However, various raw materials may be dispersed without being dissolved. The organic acid can also function as a reducing agent in the firing step. Or when various acetates are used as a raw material as mentioned above, the said acetic acid can also function as a reducing agent in a baking process.

  That is, in the solution method, after the gel is obtained by stirring and drying a mixed dispersion solution obtained by mixing various raw materials, the gel is calcined at a temperature lower than the normal calcination temperature or at normal calcination. The organic matter is intentionally left by calcination in a shorter time than the time. By firing the powder in which the organic matter remains as it is, a desired composite oxide can be synthesized while the organic matter functions as a reducing agent to produce a Ni—Co alloy.

  The calcination should just be the conditions which can leave organic substance, removing a water | moisture content, For example, it is preferable to set it as the heat processing for about 5 hours at 200 degreeC by air atmosphere. On the other hand, the firing is not particularly limited with respect to the firing atmosphere, firing temperature, and firing time, as long as the positive electrode active material according to the present invention can be produced. For example, the positive electrode active material according to the present invention can be produced by baking at 800 ° C. to 1000 ° C. for about 3 hours to 24 hours, preferably at 900 ° C. for about 10 hours in an inert gas atmosphere such as argon. It is.

  The production method according to the present invention is not limited to the above-described solution method. For example, in synthesizing the composite oxide described above by solid-phase reaction, a powder mixture containing an excessive amount of Ni source and Co source is prepared, and further, solid organic matter etc. is further mixed therein, followed by firing. It can be said that the formation of the Ni—Co alloy and the synthesis of the desired composite oxide can proceed simultaneously. However, from the viewpoint of easily producing a positive electrode active material excellent in homogeneity at a low temperature and obtaining a fine positive electrode active material suitable for application to a battery, a positive electrode active material is produced by a solution method. Is preferred.

Hereinafter, although the positive electrode active material which concerns on this invention is explained in full detail based on an Example, this invention is not limited to the following specific forms. In the examples, a Ni—Co alloy was generated during the synthesis of Li ₂ Ni _1/3 Co _1/3 Mn _1/3 TiO ₄ and the alloy was supported in the composite oxide particles.

1. Production of positive electrode active material 1.1. Cathode Active Material According to Example A cathode active material according to the example was obtained by the following synthesis procedure.
(1) Solution A was obtained by dissolving lithium acetate, manganese acetate, nickel acetate, cobalt acetate, and glycolic acid in an aqueous nitric acid solution. At this time, nickel acetate and cobalt acetate were excessively dissolved so that Ni and Co were in excess of the composite oxide composition.
(2) Titanium oxide was dispersed in the obtained solution A to obtain a solution B.
(3) The solution B was stirred while being kept at 80 ° C., and gelled while removing water.
(4) The obtained gel was recovered and heat-treated in the atmosphere at 200 ° C. for 5 hours to obtain a powder in which organic matter remained while removing moisture.
(5) The obtained powder was fired at 900 ° C. for 10 hours in an argon atmosphere to obtain a positive electrode active material.

1.2. Positive electrode active material according to comparative example The positive electrode active material according to the comparative example was obtained by the following synthesis procedure.
(1) Lithium acetate, manganese acetate, nickel acetate, cobalt acetate, and glycolic acid were dissolved in an aqueous nitric acid solution to obtain a solution A ′.
(2) Titanium oxide was dispersed in the obtained solution A to obtain a solution B ′. In the solution B ′, the composition ratio of the various raw materials was made to match that of Li ₂ Ni _1/3 Co _1/3 Mn _1/3 TiO ₄ .
(3) The solution B was stirred while being kept at 80 ° C., and gelled while removing water.
(4) The obtained gel was collected and heat-treated in the atmosphere at 350 ° C. for 10 hours to remove residual organic substances together with moisture to obtain a powder.
(5) The obtained powder was fired at 900 ° C. for 10 hours in an argon atmosphere to obtain a positive electrode active material.

2. Evaluation of positive electrode active material 2.1. XRD Measurement Results FIG. 2 shows XRD diffraction patterns of positive electrode active materials according to examples and comparative examples. As is clear from FIG. 2, the positive electrode active material according to the example was able to confirm the diffraction peak derived from the Ni—Co alloy together with the diffraction peak derived from the composite oxide belonging to the space group fm-3m. That is, Ni and Co are excessively contained, and organic substances are intentionally left in the calcination of the gel, so that the Ni—Co source can be reduced at the time of firing, and the Ni—Co alloy is included in the positive electrode active material. It can be said that it was possible. On the other hand, only the diffraction peak derived from the composite oxide was confirmed in the positive electrode active material according to the comparative example. That is, it can be said that the Ni—Co alloy could not be generated in the positive electrode active material because the organic substances were completely removed by calcination of the gel.

2.2. SEM Observation Results FIG. 3 shows an SEM image of the positive electrode active material according to the example. 3A shows a secondary electron image, and FIG. 3B shows a reflected electron image. Table 1 below shows the EDX measurement results (spectrum 1 and 2) in the white spots that can be confirmed in FIG. 3B and the EDX measurement results (spectrums 3 and 4) in other regions.

  From the results in FIG. 3 and Table 1, it can be said that the white spot portion in FIG. 3B has a higher ratio of Ni and Co compared to other regions, and Ni—Co alloy is present. That is, the positive electrode active material according to the example includes a Ni—Co alloy in the composite oxide synthesis process, thereby containing the Ni—Co alloy in the composite oxide particles, and a part of the alloy. It can be seen that is exposed on the surface of the particles. That is, it was confirmed that the positive electrode active material according to the example had a structure as shown in FIG.

2.3. Charge / Discharge Test Results Coin cells were produced using the positive electrode active materials according to Examples and Comparative Examples and each member shown below, and a charge / discharge test was performed. In the charge / discharge test, the battery was charged in a constant current mode with an upper limit of 4.8 V, and then discharged to 2 V to obtain a discharge capacity. The results according to the example are shown in FIG. 4, and the results according to the comparative example are shown in FIG.

(Positive electrode layer)
The ratio of the positive electrode active material, the conductive assistant, and the binder contained in the layer was positive electrode active material: acetylene black: PVdF = 85: 10: 5 (mass ratio). As a dispersant, N-methyl-2-pyrrolidone (manufactured by Nacalai Tesque) was used.
(Positive electrode current collector)
Aluminum foil was used.
(Nonaqueous electrolyte)
A mixed solvent in which EC: DMC = 1: 1 (volume ratio) was blended with LiPF ₆ as an electrolyte at 1 mol / L was used.
(Negative electrode layer)
Metallic lithium was used.
(Negative electrode current collector)
SUS was used.
(Casing)
A SUS 2032 type coin cell was used.

  As apparent from FIGS. 4 and 5, the battery using the positive electrode active material according to the example increased the discharge capacity about twice as much as that according to the comparative example, and a discharge capacity of 200 mAh / g was obtained. The remarkable effect by having supported the Ni-Co alloy in the composite oxide particle in the positive electrode active material was confirmed.

2.4. Confirmation that Ni-Co alloy is contained in composite oxide particles The XRD diffraction pattern of the positive electrode before and after the charge / discharge test was confirmed. The results are shown in FIG.

  As is clear from FIG. 6, a peak attributable to the Ni—Co alloy was observed before and after charging and discharging. That is, it was shown that the Ni—Co alloy was not eluted into the electrolyte during charge / discharge. This confirms that the Ni—Co alloy is present in the composite oxide particles.

  As described above, the specific configuration and remarkable effects of the positive electrode active material according to the present invention could be confirmed by Examples.

  The present invention can be widely used as a positive electrode active material for lithium batteries and the like. According to the present invention, the discharge capacity of a battery can be increased.

DESCRIPTION OF SYMBOLS 1 Composite oxide 2 Ni-Co alloy 10 Positive electrode active material

Claims (4)

A positive electrode active material comprising a Ni—Co alloy in a composite oxide particle belonging to the space group fm-3m represented by the following formula (1).
Li _a Ni _x Co _y Mn _z TiO _b (1)
(In the formula (1), a = 1.5 to 2.5, b = 3 to 4, x = 0 to 1, y = 0 to 1, and z = 0 to 1.) A lithium battery comprising the positive electrode active material according to claim 1. By firing a mixture containing Ni and Co in excess in the presence of a reducing agent as compared with a composite oxide belonging to the space group fm-3m represented by the following formula (1), the composite oxide particles and A method for producing a positive electrode active material, wherein a Ni—Co alloy is produced and a Ni—Co alloy is supported in particles of the composite oxide.
Li _a Ni _x Co _y Mn _z TiO _b (1)
(In the formula (1), a = 1.5 to 2.5, b = 3 to 4, x = 0 to 1, y = 0 to 1, and z = 0 to 1.) The manufacturing method of Claim 3 whose said reducing agent is the organic substance derived from an organic acid.