JP2019087435A - Positive electrode material for lithium secondary battery and manufacturing method thereof - Google Patents

Abstract

To provide a composite used for a positive electrode material for a non-aqueous electrolyte secondary battery, which has high capacity and can be stably charged and discharged.SOLUTION: A composite includes an LiMnOhaving an NaCl type structure and an Li positive electrode material including transition metal other than tetravalent Mn. The ratio B/A of the substance mass B of the Li positive electrode material to the substance mass A of LiMnOis 0.5 to 2.0. The Li positive electrode material is preferably at least one of LiMnOand Li(MnNiCo)O(x+y+z=1, xyz≠0).SELECTED DRAWING: Figure 2

Description

  The present invention relates to a positive electrode material for a non-aqueous electrolyte secondary battery and a method for producing the same.

Lithium secondary batteries, which are non-aqueous electrolyte secondary batteries, have high energy density. For this reason, lithium secondary batteries are put to practical use as small power supplies such as mobile phones and notebook computers, and also large power supplies such as electric vehicles. Particularly in recent years, development of a battery material having a much higher capacity than current batteries has been required from the viewpoint of gasoline substitution in electric vehicle applications. The theoretical capacity of Li ₂ MnO ₃ is 460 mAh / g, which is higher than the capacity of 150 mAh / g of lithium cobaltate (LiCoO ₂ ). For this reason, Li ₂ MnO ₃ attracts attention as a positive electrode material for high energy density lithium secondary batteries. However, Li ₂ MnO ₃ has problems of low electrochemical activity and the fact that sufficient charge and discharge capacity can not be obtained because Li is desorbed while releasing oxygen during initial charge.

In order to solve this problem, there is a wide range of methods for improving the electrochemical activity by combining Li ₂ MnO ₃ with a material represented by the chemical formula of LiMO ₂ represented by lithium cobaltate having a layered structure. It is being considered. According to Patent Document 1, Patent Document 2, Non-Patent Document 1, and Non-Patent Document 2, the material represented by the chemical formula of Li ₂ MnO ₃ -LiMO ₂ has improved activity and a reversible capacity of 250 mAh / g or more. It shows. Further, Non-Patent Document 2 reports that the charge and discharge characteristics are improved by suppressing the structural change due to the release of oxygen by gradually increasing the charge capacity. However, since LiMO ₂ has a smaller reversible capacity compared to Li ₂ MnO ₃ , realization of a high capacity positive electrode exceeding 300 mAh / g becomes difficult when complexed.

Further, in Patent Document 3, by mechanical milling of Li ₂ O and MnO ₂ , Li ₂ MnO ₃ having a rock salt type structure instead of the conventional layered type can be synthesized, and Li ₂ MnO ₃ having a rock salt type structure Has been reported to exhibit a high initial discharge capacity of 280 mAh / g. It is reported in Non-Patent Document 3 that the diffusion of Li in a rock salt type Li positive electrode material is easier than the diffusion of Li in a layered structure Li positive electrode material. For this reason, it is considered that Li ₂ MnO ₃ having a rock salt type structure exhibited high performance as compared with the conventional Li positive electrode material. However, with Li ₂ MnO ₃ alone, cycle deterioration is remarkable, and prolonging the life is a problem.

Japanese Patent Publication No. 2004-528691 JP, 2008-270201, A JP, 2013-252995, A

C. S. Johnson, N. Li, C. Lefief, J. T. Vaughey, M. M. Thackeray, Chem. Mater., 20, 6095-6106 (2008) A. Ito, D. Li, Y. Ohsawa, Y. Sato, Journal of Power Sources, 183, 344-346 (2008) J. Lee, A. Urban, X. Li, D. Su, G. Hautier, G. Ceder, Science, 343, 519-522 (2014)

  The present invention has been made in view of the above-described circumstances, and an object thereof is to develop a positive electrode material having a high initial discharge capacity and cycle stability at the time of charge and discharge.

The present inventors have intensively studied to achieve the above object. As a result, it has been found that Li ₂ MnO ₃ having a NaCl type structure reported in Patent Document 3 can also be obtained by mechanical milling of Li ₂ MnO ₃ having a layer type structure. Then, it was confirmed that a high initial charge / discharge capacity can be obtained by Li ₂ MnO ₃ having a NaCl type structure obtained by mechanical milling of Li ₂ MnO ₃ having a layer type structure. Furthermore, it has been found that by mixing Li ₂ MnO ₃ having a layered structure with other Li positive electrode materials at the time of milling, the capacity is further increased and the cycle stability at the time of charge and discharge is also improved.

The composite of the present invention contains Li ₂ MnO ₃ having a NaCl type structure and a Li positive electrode material containing a transition metal other than tetravalent Mn. The positive electrode material for a lithium secondary battery of the present invention contains the composite of the present invention. The method for producing a composite of the present invention is a method for producing a composite containing Li ₂ MnO ₃ having a NaCl type structure and a Li positive electrode material containing a transition metal other than tetravalent Mn, which has a layered structure. and Li ₂ MnO ₃ having, having a mixing step of mixing a Li cathode material.

  According to the present invention, it is possible to obtain a positive electrode material for a lithium secondary battery with very high capacity and improved cycle stability at the time of charge and discharge as compared with the prior art.

The X-ray-diffraction figure of the complex obtained in Example 1-1. Transmission electron microscope image of the complex obtained in Example 1-1. The graph which shows the result of the charging / discharging test of the positive electrode for lithium secondary batteries produced from the complex obtained in Example 1-1. The graph which shows the result of the charging / discharging test of the positive electrode for lithium secondary batteries produced from the complex obtained in Example 1-2. The graph which shows the result of the charging / discharging test of the positive electrode for lithium secondary batteries produced from the complex obtained in Example 1-3. 7 is a graph showing the results of charge / discharge tests of a positive electrode for a lithium secondary battery produced from the composite obtained in Example 2. 4 is a graph showing the results of charge / discharge tests of a positive electrode for a lithium secondary battery produced from the composite obtained in Example 3. 8 is a graph showing the results of charge / discharge tests of a positive electrode for a lithium secondary battery produced from the composite obtained in Example 4. 7 is a graph showing the results of charge / discharge tests of a positive electrode for a lithium secondary battery produced from the composite obtained in Example 5. 8 is a graph showing the results of charge / discharge tests of a positive electrode for a lithium secondary battery produced from the composite obtained in Example 6. 7 is a graph showing the results of charge / discharge tests of a positive electrode for a lithium secondary battery produced from the composite obtained in Example 7. 5 is a graph showing the results of charge / discharge tests of a positive electrode for a lithium secondary battery produced from the composite obtained in Comparative Example 1. The graph which shows the result of the charging / discharging test of the positive electrode for lithium secondary batteries produced from the complex obtained by the comparative example 2. FIG.

  Hereinafter, the composite of the present invention, the method for producing the composite, and the positive electrode material will be described based on embodiments and examples. In addition, duplication explanation is omitted suitably. Moreover, when describing "-" between two numerical values and expressing a numerical range, these two numerical values are also included in a numerical range.

(Complex containing Li ₂ MnO ₃ having NaCl type structure)
The composite according to the embodiment of the present invention includes Li ₂ MnO ₃ having a NaCl type structure and a Li positive electrode material. Since Li ₂ MnO ₃ has a NaCl type structure, the conduction of Li ions is facilitated compared to the conventional layered type Li ₂ MnO ₃ . The Li positive electrode material is a substance that contains Li, and when used as a positive electrode of a lithium secondary battery, can release and insert Li ions during charge and discharge. The Li positive electrode material contains a transition metal other than tetravalent Mn. Since this transition metal is other than tetravalent Mn in Li ₂ MnO ₃ , activation at the time of initial charge becomes easy, and high capacity can be easily obtained in the subsequent charge and discharge cycles.

The composite of this embodiment is a low crystalline reactant of Li ₂ MnO ₃ having a NaCl type structure with a Li positive electrode material, or a Li ₂ having a highly crystalline layered structure which is a precursor of this reactant. It contains a mixture of MnO ₃ and Li positive electrode material. Li ₂ MnO ₃ and Li positive electrode materials can be synthesized by solid phase firing, hydrothermal method, or sol-gel method, but the synthesis method of Li ₂ MnO ₃ and Li positive electrode materials is not particularly limited. Li ₂ MnO ₃ can be synthesized by a firing method. For example, after uniformly mixed Mn oxide and the lithium compound, Li ₂ MnO ₃ can be synthesized by calcining in air at 400 ° C. to 1000 ° C..

The Mn oxide used for the synthesis of Li ₂ MnO ₃ can be synthesized by the precipitation method. Specifically, an aqueous solution containing Mn ions, for example, an aqueous solution of sulfate, nitrate, chloride or the like is dropped into an aqueous alkaline solution to form a precipitate, which is then filtered and dried to remove water, thereby removing Mn oxide. The thing is obtained. Examples of the aqueous alkali solution include sodium hydroxide aqueous solution, sodium carbonate aqueous solution, lithium hydroxide aqueous solution, potassium hydroxide aqueous solution and the like. These aqueous alkali solutions may be used alone or in combination of two or more.

  An aqueous solution containing Mn ions is added little by little to an aqueous alkali solution adjusted to have a pH of 9 to 14, preferably 12 to 14 at a temperature of 10 ° C. to 50 ° C., preferably 20 ° C. to 30 ° C. It is preferable to produce a product. When using 2 or more types of alkaline aqueous solution, each aqueous solution may be separately added, and may be added simultaneously. The form of the Mn oxide is not particularly limited, but is preferably in the form of powder from the viewpoint of handling. In the above procedure, replacing each Mn ion with a salt containing another transition metal ion gives each transition metal oxide.

  The Li positive electrode material is preferably an oxide and contains at least one selected from Ti, V, Mn other than tetravalent Mn, Co, Ni, and Fe. The Li positive electrode material can be synthesized by a firing method. For example, if the Li positive electrode material is an oxide, the Li positive electrode material can be obtained by uniformly mixing the transition metal oxide and the lithium compound and then firing at 600 ° C. to 1000 ° C. A commercial item may be used for a transition metal oxide, and the transition metal oxide produced by the above-mentioned method may be used.

The ratio B / A of the amount of substance B of Li cathode material for substances amount A of Li ₂ MnO _3, i.e. "molar amount of a molar amount / Li cathode material Li ₂ MnO _3" is a 0.5 to 2.0 Is preferred. The composite of the present embodiment can be used as a positive electrode material such as a positive electrode active material for a lithium secondary battery. The composite of the present embodiment is not limited to Li ₂ MnO ₃ and Li positive electrode materials, for the purpose of improving conductivity, carbon-based materials such as carbon black, and fluoride or phosphate as a surface covering material of the composite. You may have a small amount etc.

As a lithium positive electrode material, lithium manganate (LiMnO ₂ ), lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), spinel manganese lithium (LiMn ₂ O ₄ ), spinel nickel manganese lithium (LiNi _1/2 Mn) _3/2 O ₄ ), lithium iron phosphate (LiFePO ₄ ), lithium titanate (Li ₂ TiO ₃ , Li ₄ Ti ₅ O ₁₂ ), Li (Mn _x Ni _y Co _z ) O ₂ (x + y + z = 1, xyz) ≠ 0) etc. can be illustrated. Among these, when the positive electrode obtained from the complex of Li ₂ MnO ₃ and spinel manganese lithium is used, the charge / discharge capacity and cycle characteristics of the lithium secondary battery can be improved. A commercially available Li positive electrode material may be used, or a synthetic product may be used.

  If the particle size of the composite is too small, the specific surface area is increased, side reactions on the electrode surface are likely to occur during charge and discharge reaction, and good electrode characteristics can not be obtained. In addition, when the particle size of the composite is too large, it takes time to diffuse the ions in the particles and it becomes difficult to react uniformly, resulting in a decrease in electrode performance. In order to obtain good cycle characteristics of the electrode, the particle size is preferably 100 nm to 2 μm, and particularly preferably about 1 μm. The particle size can be calculated from the TEM image of the sample. The crystallite size of the complex is preferably 50 nm to 90 nm. The crystallite size of the complex can be calculated from the half value width of the diffraction peak attributed to the NaCl type structure of the XRD measurement, based on the Scherrer equation.

(Positive material for lithium secondary battery)
The positive electrode material for a lithium secondary battery according to the embodiment of the present invention contains the composite according to the embodiment of the present invention. Since this positive electrode material has high capacity and excellent charge / discharge cycle characteristics, it is possible to increase the capacity of a lithium secondary battery using the positive electrode containing this positive electrode material.

(Method of manufacturing complex)
The method for producing a composite according to an embodiment of the present invention is a method for producing a composite containing Li ₂ MnO ₃ having a NaCl type structure and a Li positive electrode material containing a transition metal other than tetravalent Mn. The method of producing a composite body of the present embodiment, the Li ₂ MnO ₃ having a layered structure, and a mixing step of mixing a Li cathode material. Mixing step, so that the ratio B / A of the substance amount B in Li cathode material for substances amount A of Li ₂ MnO ₃ having a layered structure is 0.5 to 2.0, Li ₂ MnO having a layered structure It is preferable to have the process of mix | blending ₃ and Li positive electrode material.

It is necessary to mix Li ₂ MnO ₃ having a layered structure and Li positive electrode material combination while adding high energy so that Li and Mn in Li ₂ MnO ₃ perform cation mixing. Therefore, it is preferable to mechanically mill and mix a blend of Li ₂ MnO ₃ having a layered structure and a Li positive electrode material combination. As a mechanical milling device, for example, a ball mill, a vibration mill, a turbo mill, and a disc mill can be used. Among these, mixing using a ball mill is preferable. The atmosphere at the time of mixing is not particularly limited, and, for example, an inert gas atmosphere such as Ar or N ₂ or an air atmosphere can be adopted.

(Lithium rechargeable battery)
The lithium secondary battery according to the embodiment of the present invention includes a positive electrode including the positive electrode material for a lithium secondary battery of the present embodiment as a positive electrode active material, a negative electrode, an electrolyte, and a separator. The lithium secondary battery is, for example, a non-aqueous electrolyte lithium secondary battery, an all solid lithium secondary battery, a metal lithium secondary battery or the like. The basic structure of these lithium secondary batteries may be similar to that of a known lithium secondary battery except that the positive electrode material for a lithium secondary battery of this embodiment is used as a positive electrode active material. it can. The shape of the lithium secondary battery of the present embodiment is also not particularly limited, and a shape such as a cylindrical shape or a square shape can be adopted.

  The negative electrode may contain a lithium-containing negative electrode material as a negative electrode active material, or may be composed of a lithium-free negative electrode active material. As the negative electrode active material, for example, a substance which reacts with lithium, such as graphite, microcrystalline carbon, hardly sinterable carbon, lithium metal, silicon, tin, or an alloy containing any of these, can be used. A negative electrode can be produced by supporting these negative electrode active materials on a negative electrode current collector made of Al, Cu, Ni, stainless steel, carbon or the like using a conductive agent, a binder or the like as necessary. The separator is made of, for example, a material such as a polyolefin resin such as polyethylene or polypropylene, a fluorine resin, nylon, an aromatic aramid, or an inorganic glass. The separator can use a material in the form of a porous membrane, a non-woven fabric, or a woven fabric.

  In a non-aqueous electrolyte lithium secondary battery, a positive electrode material such as Al, stainless steel, or carbon cloth, which is a positive electrode active material of the lithium secondary battery of the present embodiment which is a positive electrode active material, a conductive agent, and a binder are mixed. The positive electrode can be manufactured by supporting the current collector. As a conductive agent, carbon materials, such as graphite, cokes, carbon black, or needle-like carbon, can be used, for example. The electrolyte of the non-aqueous electrolyte lithium secondary battery is a non-aqueous solvent-based electrolyte. As a solvent of this electrolyte solution, the solvent of electrolyte solution of well-known nonaqueous solvent type secondary batteries, such as carbonates, ethers, nitriles, and a sulfur-containing compound, can be used.

  In the all solid lithium secondary battery, a positive electrode material containing the positive electrode material of the lithium secondary battery of the present embodiment as a positive electrode active material, a conductive agent, a binder, a solid electrolyte and the like, Ti, Al, Ni, stainless steel, etc. The positive electrode can be manufactured by supporting it on the positive electrode current collector. As in the case of the non-aqueous electrolyte lithium secondary battery, a carbon material such as graphite, cokes, carbon black or needle-like carbon can be used as the conductive agent. The solid electrolyte of the all solid lithium secondary battery includes, for example, polymer based solid electrolytes such as polyethylene oxide type polymer compounds, polymer compounds containing at least one of polyorganosiloxane chain and polyoxyalkylene chain, sulfide type A solid electrolyte, an oxide-based solid electrolyte, or the like can be used.

  EXAMPLES Hereinafter, the present invention will be specifically described by way of examples, but the present invention is not limited to these examples.

Synthesis Example: The Li ₂ MnO ₃ constituting the manufacturing complex of Li ₂ MnO ₃ having a layered structure was prepared in the following manner. Commercially available manganese sulfate (MnSO ₄ ) was dissolved in distilled water to obtain a 1 M aqueous solution of manganese sulfate. The manganese sulfate aqueous solution was added little by little to a 1 M aqueous sodium hydroxide solution to form a precipitate. The precipitate was washed with distilled water until it became neutral, and then dried at 80 ° C. in the atmosphere to obtain manganese oxide (Mn ₃ O ₄ ). The obtained manganese oxide and commercially available lithium carbonate (LiCO ₃ ) were mixed so that the amount of the substance of Mn in the obtained manganese oxide: the amount of the substance of Li in lithium carbonate would be 1: 2. The mixture was heat treated in air at 800 ° C. for 10 hours to obtain Li ₂ MnO ₃ having a layered structure.

Examples 1-1 to 7 Preparation of a Composite Li ₂ MnO ₃ having a layered structure and listed in Table 1 such that the mixing ratio (so-called molar ratio) has the value described in Table 1 It compounded with Li positive electrode material. In addition, "mixing ratio (molar ratio)" in Table 1 indicates the amount of substance of Li ₂ MnO ₃ having a layered structure compounded: the amount of substance of Li positive electrode material. Thereafter, dry mixing was performed by mechanical milling at 500 rpm for 10 hours to 140 hours using a planetary ball mill to obtain the composites of Example 1-1 to Example 7. The mechanical milling was performed at room temperature in an air atmosphere using an 80 mL volume of a ZrO ₂ pot and a ZrO ₂ ball.

Experimental Example 1: XRD Measurement of Complex The complex obtained in Example 1-1 was subjected to XRD measurement using a Cu-Kα ray having a wavelength of 0.15405 nm. The results are shown in FIG. According to FIG. 1, although the crystallinity of this complex is very low, a diffraction peak attributed mainly to the NaCl type structure is confirmed, and a diffraction peak attributed to the layered structure or the spinel type structure is partially confirmed. The That is, the main component of this complex was a low crystalline reaction product of Li ₂ MnO ₃ having a NaCl type structure and a Li positive electrode material. A TEM image of this complex is shown in FIG. As shown in FIG. 2, it was confirmed that the complex is an aggregate of particles having a primary particle diameter of about several tens of nm to about 200 nm.

Experimental Example 2: Charge / Discharge Test of Electrochemical Cell (Examples 1-1 to 7, Comparative Examples 1 and 2)
An electrochemical cell (coin cell CR2032) was produced by the following method using the composite obtained in the above example, and a charge and discharge test was performed. Example 1-1 A mixture comprising 84% by mass of the composite as an active material obtained in Example 1, 8% by mass of acetylene black (AB) and 8% by mass of a PTFE binder is prepared, and an aluminum mesh is prepared. The electrodes were closely bonded and heat-treated (under reduced pressure, at 220 ° C. for 10 hours or more) to obtain the positive electrode of each example.

A metallic lithium foil having a capacity of about 50 times the test electrode calculated capacity was used as a counter electrode. In addition, a microporous polypropylene membrane was used as a separator. Then, a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (EC: DEC = 1: 1 ( volume% ratio)) to those obtained by dissolving LiPF ₆ to (1 mol / L) was used as the electrolyte. The charge / discharge test was performed at a current density of 10 mA / g at a cutoff potential of 1.5 V to 4.8 V. The results are shown in FIG.

  As shown in FIG. 3, the positive electrode using the composite obtained in Example 1-1 exhibited an initial charge capacity of 288 mAh / g and an initial discharge capacity of 389 mAh / g. In addition, the discharge capacity retention ratio after 20 cycles with respect to the initial capacity of the positive electrode (simply, the "maintenance ratio" in Table 1) was 66%. Moreover, the result of the charging / discharging test using the positive electrode of Example 1-2 to Example 7 is each shown to FIGS. 4-11. The initial capacity charge capacity and the initial discharge capacity were determined based on the charge and discharge curves shown in FIGS. 4 to 11. The results are shown in Table 1.

As shown in Table 1, the positive electrode produced from the composite (Example 1-1 to Example 1-3) containing the Li positive electrode material containing Mn other than tetravalent obtained good electrode characteristics. In addition, it was found that the positive electrode produced from the composite (Example 3) containing the Li positive electrode material containing Mn and Co has better electrode characteristics. Furthermore, it has been found that high discharge capacity and cycle stability can be obtained when the substance mass of Li ₂ MnO ₃ : the substance mass of Li positive electrode material is 1: 0.5-1. That is, preferable Li positive electrode materials were LiMn ₂ O ₄ and Li (Mn _0.7 Ni _0.2 Co _0.1 ) O ₂ .

Moreover, except that Li ₂ MnO ₃ (Comparative Example 1) having a NaCl type structure (Comparative Example 1) or LiMn ₂ O ₄ (Comparative Example 2) was used in place of the complex, each Comparative Example was prepared in the same manner as above. The positive electrode was produced. The results of charge and discharge tests using the positive electrodes of Comparative Example 1 and Comparative Example 2 are shown in FIGS. 12 and 13, respectively. As shown in FIG. 12, in Comparative Example 1, the initial charge capacity was 432 mAh / g and the initial discharge capacity was 314 mAh / g. As shown in FIG. 13, in Comparative Example 2, the initial charge capacity was 105 mAh / g and the initial discharge capacity was 332 mAh / g.

  The composite of the present invention can be used as a positive electrode active material for non-aqueous electrolyte secondary batteries.

Claims (9)

A composite containing Li ₂ MnO ₃ having a NaCl type structure and a Li positive electrode material containing a transition metal other than tetravalent Mn. In claim 1,
A composite in which the ratio B / A of the substance mass B of the Li positive electrode material to the substance mass A of the Li ₂ MnO ₃ is 0.5 to 2.0. In claim 1 or 2,
The Li positive electrode material is an oxide,
The complex, wherein the transition metal is at least one selected from Ti, Mn other than tetravalent, Co, Ni, and Fe. In claim 3,
A complex in which the Li positive electrode material is at least one of LiMn ₂ O ₄ and Li (Mn _x Ni _y Co _z ) O ₂ (x + y + z = 1, xyz ≠ 0).   The positive electrode material for lithium secondary batteries containing the composite as described in any one of Claims 1-4. A method for producing a composite containing Li ₂ MnO ₃ having a NaCl type structure and a Li positive electrode material containing a transition metal other than tetravalent Mn,
And Li ₂ MnO ₃ having a layered structure, manufacturing method of a composite body having a mixing step of mixing the Li cathode material. In claim 6,
The mixing step has the layered structure such that the ratio B / A of the mass B of the Li positive electrode material to the mass A of Li ₂ MnO ₃ having the layered structure is 0.5 to 2.0. A method for producing a composite, comprising the step of blending Li ₂ MnO ₃ and the Li positive electrode material. In claim 6 or 7,
The Li positive electrode material is an oxide,
The method for producing a complex, wherein the transition metal is at least one selected from Ti, Mn other than tetravalent, Co, Ni, and Fe. In claim 8,
The method for producing a complex, wherein the Li positive electrode material is at least one of LiMn ₂ O ₄ and Li (Mn _x Ni _y Co _z ) O ₂ (x + y + z = 1, xyz ≠ 0).

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