JP6995346B2 - Positive electrode material for lithium secondary batteries and its manufacturing method - Google Patents

Description

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

A lithium secondary battery, which is a non-aqueous electrolyte secondary battery, has a high energy density. Therefore, the lithium secondary battery has been put into practical use not only as a small power source for mobile phones and notebook computers, but also as a large power source for electric vehicles and the like. Particularly in recent years, there has been a demand for the development of battery materials having a significantly higher capacity than current batteries from the viewpoint of replacing gasoline in electric vehicle applications. The theoretical capacity of Li ₂ MnO ₃ is 460 mAh / g, which is higher than the capacity of lithium cobalt oxide (LiCoO ₂ ) of 150 mAh / g. Therefore, Li ₂ MnO ₃ is attracting attention as a positive electrode material for a high energy density lithium secondary battery. However, Li ₂ MnO ₃ has problems that it has a low electrochemical activity and that Li is desorbed with oxygen release at the time of initial charging, so that a sufficient charge / discharge capacity cannot be obtained.

In order to solve this problem, there are a wide range of methods for improving the electrochemical activity by combining the material represented by the chemical formula of LiMO ₂ represented by lithium cobalt oxide having a layered structure with Li ₂ MnO ₃ . 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. Shown. Further, Non-Patent Document 2 reports that by gradually increasing the charge capacity, structural changes due to oxygen release are suppressed and charge / discharge characteristics are improved. However, since LiMO ₂ has a smaller reversible capacity than Li ₂ MnO ₃ , it becomes difficult to realize a high-capacity positive electrode exceeding 300 mAh / g when combined.

Further, in Patent Document 3, by mechanically milling Li ₂ O and Mn O ₂ , Li ₂ MnO ₃ having a rock salt type structure can be synthesized instead of the conventional layered type, and Li ₂ MnO ₃ having a rock salt type structure can be synthesized. 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 the Li positive electrode material having a rock salt type structure is easier than the diffusion of Li in the Li positive electrode material having a layered structure. Therefore, it is considered that Li ₂ MnO ₃ having a rock salt type structure showed higher performance than the conventional Li positive electrode material. However, with Li ₂ MnO ₃ alone, cycle deterioration is remarkable, and longevity is an issue.

Japanese Patent Publication No. 2004-528691 Japanese Unexamined Patent Publication No. 2008-270201 Japanese Unexamined Patent Publication No. 2013-252995

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 circumstances, and an object of the present invention is to develop a positive electrode material having a high initial discharge capacity and cycle stability during charging / discharging.

The present inventors have carried out diligent research to achieve the above object. As a result, it was found that Li _{2 MnO 3 having a NaCl-type structure reported in Patent Document 3 can also be obtained by mechanically milling Li 2 MnO 3 having a} layered 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 mechanically milling Li ₂ MnO ₃ having a layered structure. Furthermore, it has been found that by mixing Li ₂ MnO ₃ having a layered structure with another Li positive electrode material at the time of milling, the capacity is further increased and the cycle stability at the time of charging / discharging 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 the 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, and has a layered structure. It has a mixing step of mixing the Li ₂ MnO ₃ having and the Li positive electrode material.

According to the present invention, it is possible to obtain a positive electrode material for a lithium secondary battery having a very high capacity and improved cycle stability during charging / discharging as compared with the conventional case.

X-ray diffraction pattern 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 charge / discharge test of the positive electrode for a lithium secondary battery produced from the complex obtained in Example 1-1. The graph which shows the result of the charge / discharge test of the positive electrode for a lithium secondary battery produced from the complex obtained in Example 1-2. The graph which shows the result of the charge / discharge test of the positive electrode for a lithium secondary battery produced from the complex obtained in Example 1-3. The graph which shows the result of the charge / discharge test of the positive electrode for a lithium secondary battery produced from the complex obtained in Example 2. FIG. The graph which shows the result of the charge / discharge test of the positive electrode for a lithium secondary battery produced from the complex obtained in Example 3. FIG. The graph which shows the result of the charge / discharge test of the positive electrode for a lithium secondary battery produced from the complex obtained in Example 4. The graph which shows the result of the charge / discharge test of the positive electrode for a lithium secondary battery produced from the complex obtained in Example 5. The graph which shows the result of the charge / discharge test of the positive electrode for a lithium secondary battery produced from the complex obtained in Example 6. The graph which shows the result of the charge / discharge test of the positive electrode for a lithium secondary battery produced from the complex obtained in Example 7. The graph which shows the result of the charge / discharge test of the positive electrode for a lithium secondary battery made from the complex obtained in the comparative example 1. FIG. The graph which shows the result of the charge / discharge test of the positive electrode for a lithium secondary battery produced from the composite obtained in Comparative Example 2.

Hereinafter, the complex of the present invention, the method for producing the complex, and the positive electrode material will be described based on the embodiments and examples. The duplicate explanation will be omitted as appropriate. Further, when "-" is described between two numerical values to indicate a numerical range, these two numerical values are also included in the numerical range.

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

The composite of the present embodiment has a low crystalline reaction product of Li ₂ MnO ₃ having a NaCl type structure and a Li positive electrode material, or Li ₂ having a highly crystalline layered structure which is a precursor of the reaction product. Contains a mixture of MnO ₃ and Li positive electrode material. The Li ₂ MnO ₃ and Li positive electrode materials can be synthesized by solid phase firing, a hydrothermal method, or a sol-gel method, but the method for synthesizing the 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, Li ₂ MnO ₃ can be synthesized by uniformly mixing a Mn oxide and a lithium compound and then firing at 400 ° C. to 1000 ° C. in the air.

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 such as sulfate, nitrate, or chloride is dropped into an alkaline aqueous solution to form a precipitate, which is then filtered and dried to remove water and Mn oxidation. You can get things. Examples of the alkaline aqueous solution include sodium hydroxide aqueous solution, sodium carbonate aqueous solution, lithium hydroxide aqueous solution, potassium hydroxide aqueous solution and the like. These alkaline aqueous solutions may be used alone or in combination of two or more.

An aqueous solution containing Mn ions is gradually added to an alkaline aqueous solution adjusted to have a pH of 9 to 14, preferably 12 to 14 at a temperature of 10 ° C to 50 ° C, preferably about 20 ° C to 30 ° C, and precipitates. It is preferable to generate an object. When two or more kinds of alkaline aqueous solutions are used, each aqueous solution may be added separately or at the same time. The shape of the Mn oxide is not particularly limited, but it is preferably in the form of powder from the viewpoint of handleability. In the above procedure, if the Mn ion is replaced with a salt containing another transition metal ion, each transition metal oxide can be obtained.

The Li positive electrode material is an oxide, and preferably contains at least one selected from Mn, Co, Ni, and Fe other than Ti, V, and tetravalent. 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. As the transition metal oxide, a commercially available product may be used, or a transition metal oxide produced by the above-mentioned method may be used.

The ratio B / A of the substance amount B of the Li positive electrode material to the substance amount A of Li ₂ MnO ₃ , that is, "the molar amount of Li ₂ MnO ₃ / the molar amount of the Li positive electrode material" is 0.5 to 2.0. Is preferable. 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. In addition to the Li ₂ MnO ₃ and Li positive electrode materials, the composite of the present embodiment includes carbon-based materials such as carbon black for the purpose of improving conductivity, and fluorides and phosphates as surface coating substances of the composite. You may have a small amount of such.

Li positive electrode materials include lithium manganate (LiMnO ₂ ), lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium spinnel manganese (LiMn ₂ O ₄ ), and lithium nickel nickel manganese (LiNi _1/2 Mn). _3/2 O ₄ ), Lithium iron oxide (LiFePO ₄ ), Lithium titanate (Li ₂ TIO ₃ , Li ₄ Ti ₅ O ₁₂ ), Li (Mn _x N _y Co _z ) O ₂ (x + y + z = 1, xyz ≠ 0) etc. can be exemplified. Among these, when a positive electrode obtained from a composite 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. As the Li positive electrode material, a commercially available product may be used, or a synthetic product may be used.

If the particle size of the complex is too small, the specific surface area becomes large, side reactions on the electrode surface are likely to occur during the charge / discharge reaction, and good electrode characteristics cannot be obtained. Further, if the particle size of the complex is too large, it takes time to diffuse the ions in the particles, and it becomes difficult to react uniformly, so that the electrode performance is deteriorated. 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 width of the diffraction peak attributed to the NaCl-type structure of the XRD measurement based on Scherrer's equation.

(Positive material for lithium secondary batteries)
The positive electrode material for the 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 a high capacity and excellent charge / discharge cycle characteristics, it is possible to increase the capacity of a lithium secondary battery using a positive electrode containing this positive electrode material.

(Manufacturing method of 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 for producing the complex of the present embodiment includes a mixing step of mixing Li ₂ MnO ₃ having a layered structure and a Li positive electrode material. In the mixing step, Li ₂ MnO having a layered structure is such that the ratio B / A of the amount of substance B in the Li positive electrode material to the amount A of Li ₂ MnO ₃ having a layered structure is 0.5 to 2.0. It is preferable to have a process of blending ₃ and the Li positive electrode material.

The Li ₂ MnO ₃ having a layered structure and the Li positive electrode material need to be mixed while applying high energy so that Li and Mn in Li ₂ MnO ₃ are cationically mixed. Therefore, it is preferable to mechanically mill the mixture of Li ₂ MnO ₃ having a layered structure and the Li positive electrode material to mix them. As the mechanical milling device, for example, a ball mill, a vibration mill, a turbo mill, a disc mill, or the like 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 atmospheric atmosphere can be adopted.

(Lithium secondary battery)
The lithium secondary battery according to the embodiment of the present invention includes a positive electrode containing the positive electrode material for the 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-state lithium secondary battery, a metallic lithium secondary battery, or the like. The basic structure of these lithium secondary batteries may be the same as that of a known lithium secondary battery, except that the positive electrode material for the lithium secondary battery of the present embodiment is used as the positive electrode active material. can. The shape of the lithium secondary battery of the present embodiment is not particularly limited, and a cylindrical shape, a square shape, or the like can be adopted.

The negative electrode may contain a negative electrode material containing lithium as a negative electrode active material, or may be composed of a negative electrode active material not containing lithium. As the negative electrode active material, a substance that reacts with lithium, such as graphite, easily crystalline carbon, difficult-to-sinter carbon, lithium metal, silicon, tin, or an alloy containing these, can be used. A negative electrode can be manufactured by supporting these negative electrode active materials on a negative electrode current collector made of Al, Cu, Ni, stainless steel, carbon, etc., using a conductive agent, a binder, or the like, if necessary. The separator is made of, for example, a polyolefin resin such as polyethylene or polypropylene, a fluororesin, nylon, an aromatic aramid, or a material such as inorganic glass. As the separator, a material in the form of a porous membrane, a non-woven fabric, or a woven fabric can be used.

In the non-aqueous electrolyte lithium secondary battery, a positive electrode mixture obtained by mixing the positive electrode material, the conductive agent, and the binder of the lithium secondary battery of the present embodiment, which is the positive electrode active material, is used as a positive electrode collection such as Al, stainless steel, or carbon cloth. A positive electrode can be manufactured by supporting it on an electric body. As the conductive agent, for example, a carbon material such as graphite, coke, carbon black, or needle-shaped carbon can be used. Non-aqueous electrolyte The electrolyte of the lithium secondary battery is a non-aqueous solvent-based electrolyte. As the solvent of this electrolytic solution, a solvent of an electrolytic solution of a known non-aqueous solvent-based secondary battery such as carbonates, ethers, nitriles, and sulfur-containing compounds can be used.

In the all-solid-state lithium secondary battery, a positive electrode mixture containing a positive electrode material, a conductive agent, a binder, a solid electrolyte, etc. of the lithium secondary battery of the present embodiment as a positive electrode active material is used as a positive electrode mixture such as Ti, Al, Ni, and stainless steel. A positive electrode can be manufactured by supporting it on a positive electrode current collector. As the conductive agent, a carbon material such as graphite, coke, carbon black, or acicular carbon can be used as in the case of the non-aqueous electrolyte lithium secondary battery. Examples of the solid electrolyte of the all-solid-state lithium secondary battery include polymer-based solid electrolytes such as polyethylene oxide-based polymer compounds, polyorganosiloxane chains, and polymer compounds containing at least one of the polyoxyalkylene chains, and sulfide-based polymers. A solid electrolyte, an oxide-based solid electrolyte, or the like can be used.

Hereinafter, the present invention will be specifically described with reference to Examples, but the present invention is not limited to these Examples.

Synthesis example: Preparation of Li ₂ MnO ₃ having a layered structure Li ₂ MnO ₃ constituting the complex was prepared by the following method. Commercially available manganese sulfate (MnSO ₄ ) was dissolved in distilled water to obtain a manganese sulfate aqueous solution having a concentration of 1 M. This manganese sulfate aqueous solution was added little by little to a sodium hydroxide aqueous solution having a concentration of 1 M to form a precipitate. The precipitate was washed with distilled water until it became neutral, and then dried in the air at 80 ° C. 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 substance of Mn in the obtained manganese oxide: the amount of substance of Li in lithium carbonate was 1: 2. This mixture was heat-treated in the air at 800 ° C. for 10 hours to obtain Li ₂ MnO ₃ having a layered structure.

Examples 1-1 to 7: Preparation of complex Li 2 MnO 3 having a layered structure and Li ₂ MnO ₃ having a layered structure are shown in Table 1 so that the mixing ratio (so-called molar ratio) becomes the value shown in Table 1. It was blended with a Li positive electrode material. The "mixing ratio (molar ratio)" in Table 1 indicates the amount of substance of Li ₂ MnO ₃ having a mixed layered structure: the amount of substance of the Li positive electrode material. Then, using a planetary ball mill, they were dry-mixed by mechanical milling treatment at 500 rpm for 10 hours to 140 hours to obtain a complex of Examples 1-1 to 7. The mechanical milling treatment was carried out at room temperature in an atmospheric atmosphere using a ZrO ₂ pot having a capacity of 80 mL and a ZrO ₂ ball.

Experimental Example 1: XRD measurement of the complex The complex obtained in Example 1-1 was subjected to XRD measurement using Cu—Kα rays 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, diffraction peaks mainly attributed to NaCl-type structures are confirmed, and some diffraction peaks attributed to layered structures or spinel-type structures are also confirmed. rice field. 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 agglomerate of particles having a primary particle diameter of about several tens of nm to 200 nm.

Experimental Example 2: Charging / discharging test of electrochemical cell (Example 1-1 to Example 7, Comparative Example 1, Comparative Example 2)
Using the complex obtained in the above example, an electrochemical cell (coin cell CR2032) was prepared by the following method, and a charge / discharge test was performed. A mixture of 84% by mass of the complex obtained in Examples 1-1 to 7 by mass, 8% by mass of acetylene black (AB), and 8% by mass of the PTFE binder was prepared and prepared into an aluminum mesh. It was closely bonded and heat-treated (under reduced pressure at 220 ° C. for 10 hours or more) to obtain a positive electrode of each example.

A metallic lithium foil having a capacity about 50 times the calculated capacity of the test electrode was used as a counter electrode. Moreover, a polypropylene microporous membrane was used as a separator. Then, a solution (1 mol / L) in which LiPF ₆ was dissolved in a mixed solution of ethylene carbonate (EC) and diethyl carbonate (DEC) (EC: DEC = 1: 1 (volume% ratio)) was used as the electrolytic solution. A charge / discharge test was performed with a current density of 10 mA / g in the range of 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 complex obtained in Example 1-1 showed an initial charge capacity of 288 mAh / g and an initial discharge capacity of 389 mAh / g. The discharge capacity retention rate after 20 cycles with respect to the initial capacity of the positive electrode (simply "maintenance rate" in Table 1) was 66%. Further, the results of charge / discharge tests using the positive electrodes of Examples 1-2 to 7 are shown in FIGS. 4 to 11, respectively. The initial capacity charge capacity and the initial discharge capacity were determined based on the charge / discharge curves shown in FIGS. 4 to 11. The results are shown in Table 1.

As shown in Table 1, good electrode characteristics were obtained in the positive electrode prepared from the complex (Examples 1-1 to 1-3) containing the Li positive electrode material containing Mn other than tetravalent. Further, it was found that the positive electrode prepared from the complex (Example 3) containing the Li positive electrode material containing Mn and Co can obtain better electrode characteristics. Furthermore, it was found that high discharge capacity and cycle stability can be obtained when the amount of substance of Li ₂ MnO ₃ : the amount of substance of the Li positive electrode material is 1: 0.5 to 1. That is, the preferred Li positive electrode materials were LiMn ₂ O ₄ and Li (Mn _0.7 Ni _0.2 Co _0.1 ) O ₂ .

Further, except that Li ₂ MnO ₃ (Comparative Example 1) or LiMn ₂ O ₄ (Comparative Example 2) having a NaCl-type structure was used instead of the complex, the same method as above was used for each Comparative Example. A positive electrode was prepared. The results of the charge / discharge test 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 complex of the present invention can be used as a positive electrode active material for a non-aqueous electrolyte secondary battery.

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 amount B of the Li positive electrode material to the substance amount 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.
A complex in which the transition metal is at least one selected from Mn, Co, Ni, and Fe other than Ti and tetravalent. 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 N _y Co _z ) O ₂ (x + y + z = 1, xyz ≠ 0). A positive electrode material for a lithium secondary battery containing the complex according to any one of claims 1 to 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.
A method for producing a complex having a mixing step of mixing Li ₂ MnO ₃ having a layered structure and the Li positive electrode material. In claim 6,
The mixing step has the layered structure so that the ratio B / A of the substance amount B of the Li positive electrode material to the substance amount A of the Li ₂ MnO ₃ having the layered structure is 0.5 to 2.0. A method for producing a composite having a process 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.
A method for producing a complex in which the transition metal is at least one selected from Mn, Co, Ni, and Fe other than Ti and tetravalent. In claim 8,
A method for producing a complex in which the Li positive electrode material is at least one of LiMn ₂ O ₄ and Li (Mn _x N _y Co _z ) O ₂ (x + y + z = 1, xyz ≠ 0).

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