JP2023003666A - Positive electrode active material for sodium secondary battery - Google Patents

Abstract

To provide a positive electrode active material capable of improving charge and discharge cycle characteristics of a sodium secondary battery at high potential.SOLUTION: The positive electrode active material for a sodium secondary battery has a P2 type structure, includes Na, Mn, Ni, Co and O as constituting elements, and has a composition satisfying the relationship in the following formula (1). 0.30≤Co/(Mn+Ni+Co)≤0.35 ...(1)SELECTED DRAWING: None

Description

The present application discloses a positive electrode active material for a sodium secondary battery.

Non-Patent Document 1 discloses a positive electrode active material for a sodium secondary battery having a P2 type structure and a composition represented by Na _0.70 Mn _0.60 Ni _0.30 Co _0.10 O ₂ . ing.

Journal of The Electrochemical Society, 161(14), A1987-A1991 (2014)

A positive electrode active material having a P2-type structure and having a composition represented by Na _0.70 Mn _0.60 Ni _0.30 Co _0.10 O ₂ has a high potential (for example, 4.0 V vs. Na ⁺ / At a potential exceeding Na), a phase transition occurs and the P2-type structure cannot be maintained. If a sodium secondary battery configured using such a positive electrode active material is repeatedly charged and discharged in a range including a high potential, the capacity of the battery tends to decrease. That is, in the prior art, there is room for improvement in the charge-discharge cycle characteristics of sodium secondary batteries at high potentials.

As one means for solving the above problems, the present application provides
having a P2-type structure,
Containing Na, Mn, Ni, Co and O as constituent elements,
Having a composition that satisfies the relationship of the following formula (1),
A cathode active material for a sodium secondary battery is disclosed.

0.30≦Co/(Mn+Ni+Co)≦0.35 (1)

By configuring a sodium secondary battery using the positive electrode active material of the present disclosure, the charge-discharge cycle characteristics of the sodium secondary battery at high potentials are likely to be improved.

1 shows the capacity retention rate after 5 cycles of charging and discharging in the sodium potential range of 4.3 V to 1.5 V for each of the sodium secondary batteries of Examples 1 to 4 and Comparative Example 1. FIG. 4 shows X-ray diffraction patterns after maintaining at 4.3 V for each of the positive electrode active materials of Example 2 and Comparative Example 1. FIG. For each of Examples 1 to 4, the relationship between the average primary particle size of the positive electrode active material and the discharge capacity after 5 cycles of charging and discharging is shown.

1. Positive electrode active material for sodium secondary battery The positive electrode active material of the present disclosure is a positive electrode active material for a sodium secondary battery, has a P2 type structure, and contains Na, Mn, Ni, Co and O as constituent elements, It has a composition that satisfies the relationship of the following formula (1).

0.30≦Co/(Mn+Ni+Co)≦0.35 (1)

1.1 Crystal Structure The positive electrode active material of the present disclosure has a P2 type structure. Whether or not the positive electrode active material has a P2 type structure can be easily determined by X-ray diffraction measurement or the like.

1.2 Composition The positive electrode active material of the present disclosure contains Na, Mn, Ni, Co and O as constituent elements. In addition, the positive electrode active material of the present disclosure has a composition that satisfies the relationship of formula (1) above. For example, the cathode active material of the present disclosure is _{NaaMnxNiyCozO2} (0.50≤a≤1.00, 0< _x , 0< _y , 0< _z , 3.00≤4x ₊ 2y+3z≤3 .50, 0.30≦z/(x+y+z)≦0.35). a may be 0.60 or more or 0.64 or more, and may be 0.80 or less or 0.74 or less. x may be 0.40 or more or 0.46 or more, and may be 0.55 or less or 0.50 or less. z may be 0.30 or more and may be 0.35 or less.

According to new knowledge of the present inventors, in a sodium secondary battery using a positive electrode active material having the above P2 type structure, if the Co contained in the positive electrode active material is a certain amount or more, the sodium secondary battery has a high Charge-discharge cycle characteristics at potential are likely to be improved. This is thought to be because, in the positive electrode active material, the amount of Co that causes redox at high potential increases, and the phase transition of the P2-type structure is less likely to occur at high potential, that is, it becomes easier to maintain the P2-type structure at high potential. be done. However, if the amount of Co contained in the positive electrode active material is too large, it becomes difficult to maintain the P2 type structure in the positive electrode active material. In the positive electrode active material of the present disclosure, as in the above formula (1), the composition ratio (molar ratio) of Co to the total of Mn, Ni, and Co is in the range of 0.30 or more and 0.35 or less. , while maintaining the P2 type structure from low to high potentials, the charge-discharge cycle characteristics at high potentials are likely to be improved. The positive electrode active material of the present disclosure is, for example, 4.0 V vs. It can also be used in a sodium secondary battery that charges and discharges in a range containing more than Na ⁺ /Na.

1.3 Shape The shape of the positive electrode active material of the present disclosure may be, for example, particulate. The positive electrode active material may be primary particles or secondary particles formed by agglomeration of primary particles. As far as the inventors have confirmed, when the primary particle size and secondary particle size of the positive electrode active material change, the capacity of the sodium secondary battery configured using the positive electrode active material also changes. From the viewpoint of ensuring a larger capacity in a sodium secondary battery, the average primary particle size of the positive electrode active material may be less than 10 μm or 7 μm or less, and the average secondary particle size may be less than 25 μm or 20 μm or less. may The lower limits of the average primary particle size and average secondary particle size of the positive electrode active material are not particularly limited.

The average primary particle size and average secondary particle size of the positive electrode active material can be specified by observation using SEM, TEM, or the like. Specifically, an image is obtained by photographing the positive electrode active material by SEM, TEM, or the like. Among the primary particles of the positive electrode active material contained in the image, 20 or more primary particles are randomly measured for their projected area equivalent circle diameters. By averaging the number of measured values, the average primary particle size of the positive electrode active material can be specified. The same applies to the method of measuring the average secondary particle diameter, and it may be specified as the number average value of the projected area circle equivalent diameters of the secondary particles contained in the image.

2. Method for Producing Positive Electrode Active Material for Sodium Secondary Battery The positive electrode active material of the present disclosure can be produced, for example, as follows. First, a first solution containing transition metal ions (Mn ions, Ni ions and/or Co ions) and a second solution containing Na ions are prepared. The first solution may be prepared as separate solutions for each type of ion. Next, a precursor containing Na and a transition metal is synthesized by a coprecipitation method using the first solution and the second solution. Thereafter, the precursor is optionally calcined with a Na source or the like to obtain the positive electrode active material of the present disclosure.

The transition metal ion may be derived from, for example, a transition metal nitrate or its hydrate. Na ions may be derived, for example, from sodium carbonate. The solvent that constitutes the solution may be, for example, water. The solution may contain acid or alkali. The composition of the produced positive electrode active material can be adjusted by adjusting the concentration of each ion in the first solution and the second solution.

When synthesizing the precursor by the coprecipitation method, the first solution and the second solution may be added dropwise. Moreover, after synthesizing the precursor by the coprecipitation method, the precursor may be washed using a centrifuge or the like.

The firing temperature for obtaining the positive electrode active material by firing the precursor may be a temperature at which a P2 type structure is obtained, for example, it may be 700° C. or higher, or 800° C. or higher, and 900° C. or lower. good too. The sintering atmosphere may be any atmosphere as long as it is possible to obtain a composite oxide. For example, an oxygen-containing atmosphere such as an air atmosphere can be used. Other conditions such as baking time are not particularly limited.

Alternatively, the positive electrode active material of the present disclosure may be produced by methods other than the coprecipitation method (eg, solution method, solid phase reaction method, etc.).

3. Sodium Secondary Battery The technology of the present disclosure also has aspects as a sodium secondary battery. That is, the sodium secondary battery of the present disclosure can include a positive electrode containing the positive electrode active material, a negative electrode containing the negative electrode active material, and an electrolyte disposed between the positive electrode and the negative electrode. In addition to these, members such as a battery case, connection terminals, and the like, which are commonly used as batteries, may be provided.

3.1 Positive Electrode When the sodium secondary battery is an electrolyte-based battery, the positive electrode may contain, in addition to the positive electrode active materials described above, optionally other positive electrode active materials, conductive aids, binders, and the like. good. Moreover, the positive electrode may be in a state of being impregnated with the electrolytic solution. On the other hand, when the sodium secondary battery contains a solid electrolyte as an electrolyte, the positive electrode optionally contains other positive electrode active materials, solid electrolytes, conductive aids, binders, etc. in addition to the above positive electrode active materials. may As the conductive aid and the binder, those generally used for constructing the positive electrode of the sodium secondary battery may be used. The electrolytic solution and solid electrolyte will be described later. The ratio of each component contained in the positive electrode is not particularly limited.

The positive electrode may include a positive electrode current collector. That is, it may include a positive electrode active material layer containing the above positive electrode active material and the like and a positive electrode current collector. The positive electrode current collector may be made of, for example, metal foil. The method of manufacturing the positive electrode may be the same as the conventional method, except that the positive electrode active material described above is used.

3.2 Negative Electrode When the sodium secondary battery is an electrolyte-based battery, the negative electrode may optionally contain a conductive aid, a binder, and the like in addition to the negative electrode active material. Moreover, the negative electrode may be in a state of being impregnated with the electrolytic solution. On the other hand, when the sodium secondary battery contains a solid electrolyte as an electrolyte, the negative electrode may optionally contain a solid electrolyte, a conductive aid, a binder, etc. in addition to the negative electrode active material. As the negative electrode active material, a material having a base potential at which sodium ions are occluded and released as compared with the above positive electrode active material may be used. For example, an inorganic negative electrode active material such as metallic sodium may be used, or a negative electrode active material made of an organic compound may be used. As the conductive aid and the binder, those generally used for constituting the negative electrode of the sodium secondary battery may be used. The electrolytic solution and solid electrolyte will be described later. The ratio of each component contained in the negative electrode is not particularly limited.

The negative electrode may include a negative electrode current collector. That is, it may include a negative electrode active material layer containing the above negative electrode active material and the like and a negative electrode current collector. The negative electrode current collector may be made of, for example, metal foil. The method of manufacturing the negative electrode may be the same as the conventional method.

3.3 Electrolyte When the sodium secondary battery is an electrolytic solution-based battery, a solution obtained by dissolving an electrolyte in a solvent is used as the electrolytic solution. The electrolyte in this case may be, for example, a sodium compound such as _NaPF6 . The solvent may be, for example, a carbonate solvent such as ethylene carbonate (EC) or diethyl carbonate (DEC). An electrolytic solution-based sodium secondary battery can be constructed, for example, by disposing a separator between the positive electrode and the negative electrode and immersing them in an electrolytic solution. The separator in this case may be made of a resin such as polyethylene or polypropylene, non-woven fabric, or ceramic.

On the other hand, when the sodium secondary battery contains a solid electrolyte, the solid electrolyte may be an inorganic solid electrolyte or an organic solid electrolyte (polymer electrolyte). Examples of inorganic solid electrolytes include oxides such as Na _{3 Zr 2 PSi 2} O ₁₂ and _{Na 2 O - 11Al 2 O 3 ;} hydrides and borides such as Cl ₁₂ ; sulfides such as Na ₃ PS ₄ , Na ₃ SbS ₄ , Na _2.88 Sb _0.88 W _0.12 S ₄ ; fluorides such as NaPF ₆ and NaBF ₄ At least 1 type is mentioned. A sodium secondary battery containing a solid electrolyte can be constructed, for example, by disposing a layer of the solid electrolyte between the positive electrode and the negative electrode.

Hereinafter, the technology of the present disclosure will be described in more detail with reference to examples, but the technology of the present disclosure is not limited to the following examples.

1. Preparation of Sodium Secondary Battery 1.1 Example 1
1.1.1 Synthesis of Positive Electrode Active Material Manganese nitrate tetrahydrate (manufactured by Aldrich), nickel nitrate hexahydrate (manufactured by Nacalai Tesque) and cobalt nitrate (manufactured by Aldrich) were each weighed and added to pure water. A first solution was obtained by dissolving. On the other hand, sodium carbonate and ammonia water were each weighed and dissolved in pure water to obtain a second solution. The first solution and the second solution were added dropwise to synthesize a precursor by a coprecipitation method. The synthesized precursor was washed with a centrifuge. After that, the precursor and sodium carbonate were mixed and baked at 900° C. for 12 hours in an air atmosphere to synthesize a positive electrode active material.

The obtained positive electrode active material had a composition represented by Na _0.64 Mn _0.48 Ni _0.20 Co _0.32 O _2±δ . Moreover, when the obtained positive electrode active material was subjected to X-ray diffraction measurement using CuKα rays, the positive electrode active material had a P2 type structure. Furthermore, when the obtained positive electrode active material was observed with an SEM, the average primary particle size of the positive electrode active material was 3 μm, and the average secondary particle size was 15 μm.

1.1.2 Preparation of Positive Electrode The positive electrode active material, the conductive aid, and the binder were mixed in a mass ratio of positive electrode active material:conductive aid:binder=85:10:5. During the mixing, N-methyl-2-pyrrolidone (manufactured by Kishida Chemical Co., Ltd.) was used as a dispersing agent to obtain a slurry positive electrode mixture. The obtained slurry was applied onto an aluminum foil, dried, and rolled to obtain a positive electrode.

1.1.3 Preparation of Negative Electrode Metallic sodium was used as the negative electrode.

1.1.4 Preparation of Electrolyte Solution An electrolysis was prepared by dissolving NaPF ₆ as an electrolyte at a concentration of 1 mol/L in a solvent in which EC and DEC were mixed in a volume ratio of EC:DEC = 1:1. A liquid (manufactured by Kishida Chemical Co., Ltd.) was used.

1.1.5 Preparation of Separator A separator made of a mixture of polyethylene and polypropylene was used.

1.1.6 Fabrication of Coin Cell A coin cell as a sodium secondary battery for evaluation was fabricated using the positive electrode, negative electrode, electrolytic solution, and separator described above.

1.2 Example 2
A coin cell was produced in the same manner as in Example 1, except that the concentrations of the first solution and the second solution were adjusted to change the composition of the positive electrode active material. The obtained positive electrode active material had a composition represented by Na _0.68 Mn _0.50 Ni _0.20 Co _0.30 O _2±δ . Moreover, when the obtained positive electrode active material was subjected to X-ray diffraction measurement using CuKα rays, the positive electrode active material had a P2 type structure. Furthermore, when the obtained positive electrode active material was observed by SEM, the average primary particle size of the positive electrode active material was 4 μm, and the average secondary particle size was 8 μm.

1.3 Example 3
A coin cell was produced in the same manner as in Example 1, except that the concentrations of the first solution and the second solution were adjusted to change the composition of the positive electrode active material. The obtained positive electrode active material had a composition represented by Na _0.67 Mn _0.46 Ni _0.19 Co _0.35 O _2±δ . Moreover, when the obtained positive electrode active material was subjected to X-ray diffraction measurement using CuKα rays, the positive electrode active material had a P2 type structure. Furthermore, when the obtained positive electrode active material was observed with an SEM, the average primary particle size of the positive electrode active material was 7 μm, and the average secondary particle size was 20 μm.

1.4 Example 4
A coin cell was produced in the same manner as in Example 1, except that the concentrations of the first solution and the second solution were adjusted to change the composition of the positive electrode active material. The obtained positive electrode active material had a composition represented by Na _0.74 Mn _0.50 Ni _0.20 Co _0.30 O _2±δ . Moreover, when the obtained positive electrode active material was subjected to X-ray diffraction measurement using CuKα rays, the positive electrode active material had a P2 type structure. Furthermore, when the obtained positive electrode active material was observed by SEM, the average primary particle size of the positive electrode active material was 10 μm, and the average secondary particle size was 25 μm.

1.5 Comparative Example 1
A coin cell was produced in the same manner as in Example 1, except that the concentrations of the first solution and the second solution were adjusted to change the composition of the positive electrode active material. The obtained positive electrode active material had a composition represented by Na _0.66 Mn _0.60 Ni _0.30 Co _0.10 O _2±δ . Moreover, when the obtained positive electrode active material was subjected to X-ray diffraction measurement using CuKα rays, the positive electrode active material had a P2 type structure. Furthermore, when the obtained positive electrode active material was observed with an SEM, the average primary particle size of the positive electrode active material was 2 μm, and the average secondary particle size was 15 μm.

2. 2. Evaluation of Sodium Secondary Battery 2.1 Evaluation of Charge-Discharge Cycle Characteristics For each of the coin cells of Examples 1 to 4 and Comparative Example 1, charging and discharging were repeated 5 times in the sodium potential range of 4.3 V to 1.5 V. . The temperature during charging and discharging was 25°C. For each coin cell, the ratio of the discharge capacity at the 5th cycle to the discharge capacity at the 1st cycle (capacity retention rate) was calculated to evaluate the charge/discharge cycle characteristics.

2.2 Evaluation of Crystal Structure of Positive Electrode Active Material at High Potential For each of the positive electrode active materials of Example 2 and Comparative Example 1, the crystal structure when maintained at 4.3 V was confirmed.

3. Evaluation Results Evaluation results are summarized in Table 1 below and FIGS.

As is clear from the results shown in Table 1 and FIG. 1, the coin cell according to Comparative Example 1 had a capacity retention rate of 82% after charge-discharge cycles. In contrast, the coin cells according to Examples 1 to 4 had a capacity retention rate of 97% or more after charge-discharge cycles, and the charge-discharge cycle characteristics at high potential were significantly improved as compared with the coin cell according to Comparative Example 1. rice field.

As shown in FIG. 2, the positive electrode active material according to Comparative Example 1 cannot maintain the P2 type structure when maintained at 4.3 V, whereas the positive electrode active material according to Example 2 is maintained at 4.3 V. The P2-type structure could be maintained even when maintained at The positive electrode active material according to Example 2 contains a large amount of Co, which causes redox at high potentials, and is considered to be less susceptible to phase transition of the P2 type structure at high potentials. The positive electrode active materials according to Examples 1, 3 and 4 were the same as the positive electrode active material according to Example 2.

As shown in Table 1 and FIG. 3, for Examples 1 to 3 in which the average primary particle size and the average secondary particle size of the positive electrode active material are small, the average primary particle size and the average secondary particle size of the positive electrode active material are It was found that the capacity of the sodium secondary battery was higher than that of Example 4, which was large.

From the above results, it can be said that by adopting the positive electrode active material having the following structure in the sodium secondary battery, the charge-discharge cycle characteristics at high potential of the sodium secondary battery can be improved.

(1) Having a P2 type structure.
(2) Containing Na, Mn, Ni, Co and O as constituent elements and having a composition satisfying the relationship 0.30≦Co/(Mn+Ni+Co)≦0.35;

Claims (1)

having a P2-type structure,
Containing Na, Mn, Ni, Co and O as constituent elements,
Having a composition that satisfies the relationship of the following formula (1),
Positive electrode active material for sodium secondary batteries.
0.30≦Co/(Mn+Ni+Co)≦0.35 (1)

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