JP6792836B2 - Lithium composite oxide for positive electrode active material and its manufacturing method, positive electrode active material for lithium secondary battery and lithium secondary battery - Google Patents

Description

The present invention relates to a lithium composite oxide used as a positive electrode active material of a lithium secondary battery and a method for producing the same. Furthermore, the present invention relates to a positive active material and a lithium secondary battery for a lithium secondary battery using the lithium composite oxide.

Secondary batteries are installed in many portable electronic devices such as mobile phones and laptop computers. Secondary batteries such as lithium secondary batteries are also expected to be put into practical use as large batteries for hybrid vehicles and power load leveling systems, and their importance is increasing. Development of a secondary battery with higher capacity and longer life is required for practical use as a large battery.

The main components of a lithium secondary battery are an electrode composed of a positive electrode and a negative electrode, and a separator or a solid electrolyte containing a non-aqueous electrolyte solution. Both the positive electrode and the negative electrode contain a material (active material for electrodes) capable of reversibly occluding and releasing lithium. The crystal structure and electrochemical properties of lithium nickel manganese oxide having a composition of Li _2/3 Ni _1/3 Mn _2/3 O ₂ have been investigated as a material for the positive electrode active material of a lithium secondary battery. ..

Li _2/3 Ni _1/3 Mn _2/3 O ₂ is obtained by exchanging sodium of Na _2/3 Ni _1/3 Mn _2/3 O ₂ which is a starting material with lithium, and the structure of the starting material. The structure of the obtained Li _2/3 Ni _1/3 Mn _2/3 O ₂ is different depending on the above. For example, when Na _2/3 Ni _1/3 Mn _2/3 O ₂ having a P3 structure is used as a starting material, the ion exchanger Li _2/3 Ni _1/3 Mn _2/3 O ₂ has an O3 structure. Has. It has been reported that heat treatment of Li _2/3 Ni _1/3 Mn _2/3 O ₂ having an O3 structure changes the crystal structure and improves the electrochemical properties (Non-Patent Document 1).

When Na _2/3 Ni _1/3 Mn _2/3 O ₂ having a P3 structure is heated in a solution in which a lithium salt is dissolved in a low boiling solvent such as methanol or ethanol to perform ion exchange, a part of sodium is released. An oxide having a composition formula (Li _z Na _{2 / 3-z} ) Ni _1/3 Mn _2/3 O ₂ remaining without being exchanged with lithium can be obtained. It has been reported that a secondary battery using a material obtained by heat-treating this sodium residual oxide (lithium-sodium composite oxide) as a positive electrode active material suppresses a rapid voltage drop during discharge (when lithium is inserted). (Non-Patent Document 2).

Kazuki Chiba et al., 2014 Electrochemical Autumn Meeting Abstracts, 1P23 (2014) Kazuki Chiba et al., 55th Battery Discussion Meeting Abstracts, 2B18, (2014)

A secondary battery using the lithium composite oxide of Non-Patent Document 1 as a positive electrode material has an advantage of having a large discharge capacity. However, since there is a region where the voltage drops sharply during discharging, it may be difficult to detect the charge rate. By using the lithium composite oxide of Non-Patent Document 2, a sudden voltage drop at the time of discharge can be suppressed. However, since the secondary battery using this positive electrode material has a small charge capacity, it is difficult to realize a high charge / discharge capacity when a negative electrode material such as graphite is used.

As described above, there is room for further improvement in the conventional positive electrode active material using the lithium composite oxide. The present invention has been made in view of the above, and an object of the present invention is to provide a lithium composite oxide useful as a positive electrode active material.

By using a lithium composite oxide in which lithium is chemically inserted into a predetermined lithium sodium composite oxide as a positive electrode active material, the present inventors can increase the initial charge capacity of the secondary battery and increase the capacity. We found that there was, and came to the present invention.

The lithium composite oxide of the present invention is represented by the composition formula Li _x N _y Ni _1/3 Mn _2/3 O ₂ . In the formula, 0.7 ≦ x ≦ 0.9 and 0 <y ≦ 0.05. The amount of sodium y in the composition formula preferably satisfies 0.001 ≦ y ≦ 0.002. The lithium composite oxide preferably has a crystal structure having a layered rock salt structure.

The lithium composite oxide of the present invention has a maximum of the main resonance peak of ⁶ Li-NMR in the range of 650 to 900 ppm. The half width of the main resonance peak is preferably 200 to 450 ppm. The lithium composite oxide is preferably one in which the main resonance peak of ⁶ Li-NMR can be separated into two or more peaks by waveform analysis. ^{6 When} the main resonance peak of Li-NMR is separated into one or more peaks by waveform analysis, at least one of the peaks obtained by waveform analysis preferably has a peak maximum in the range of 650 to 750 ppm.

The above lithium composite oxide is, for example, expressed by a composition formula _{Li z Na 2/3-z} Ni 1/3 Mn 2/3 O 2 (0.33 ≦ z ≦ 0.63), the crystal structure is layered rock salt It is obtained by chemically inserting lithium ions into a lithium-sodium composite oxide having a mold structure. It is preferable that the difference between the amount of lithium x in the composition formula after the chemical insertion of lithium ions and the amount of lithium z in the composition formula before the chemical insertion of lithium ions satisfies 0.2 ≦ x−z ≦ 0.5.

The chemical insertion of lithium ions is carried out, for example, by treating a lithium sodium composite oxide in a lithium salt solution such as lithium iodide at 20 ° C to 200 ° C. Acetonitrile or the like is preferably used as the solvent for the lithium salt solution.

Furthermore, the present invention relates to the above-mentioned positive electrode active material containing a lithium composite oxide, and a secondary battery using the positive electrode active material as a positive electrode material.

By using the lithium composite oxide of the present invention as the positive electrode active material of the lithium secondary battery, a high capacity secondary battery can be obtained.

It is a figure which shows an example of the synthetic pathway of a lithium composite oxide. It is a partial cross-sectional view schematically showing an example of a secondary battery. ⁶ Li-NMR spectrum of the lithium composite oxide of the example. It is a waveform analysis result of the ^6- Li-NMR spectrum of the lithium composite oxide of Example 1. It is a waveform analysis result of the ^6- Li-NMR spectrum of the lithium composite oxide of Example 2. It is a waveform analysis result of the ^6- Li-NMR spectrum of the lithium composite oxide of Example 3. It is a waveform analysis result of ⁶ Li-NMR spectrum of the lithium composite oxide of Example 4. It is a ⁶ Li-NMR spectrum before and after lithium insertion of a lithium composite oxide. It is a powder X-ray diffraction pattern of a starting material and an ion exchanger. It is a powder X-ray diffraction pattern of a heat-treated body. It is a powder X-ray diffraction pattern of the lithium composite oxide (lithium insert) of the example and the lithium composite oxide (heat-treated body) of the comparative example. It is a graph of the charge / discharge test result of the secondary battery of an Example, a reference example and a comparative example. It is a graph of the charge / discharge test result of the secondary battery of an Example.

[Lithium sodium composite oxide]
The lithium composite oxide of the present invention has a composition represented by the formula Li _x N _y Ni _1/3 Mn _2/3 O ₂ . In the formula, 0.7 ≦ x ≦ 0.9 and 0 <y ≦ 0.05. The lithium composite oxide of the present invention is suitably used as a positive electrode active material for a lithium secondary battery.

A general composite oxide containing Li, Na, Ni and Mn is represented by the composition formula (Li _p Na _{2 / 3-p} ) (Ni _q Mn _1-q ) O ₂ , and a total of 1 mol of nickel and manganese. The total amount of lithium and sodium is 2/3 mol, which is a lithium-deficient system. On the other hand, the lithium composite oxide of the present invention has a feature that the initial charge capacity is large because the amount of lithium exceeds 2/3 by inserting lithium by chemical insertion or the like. The amount of lithium x in the above composition formula is preferably 0.75 or more, more preferably 0.8 or more.

The sodium amount y in the above composition formula is preferably 0.02 or less. By reducing the residual sodium amount y, the lithium amount x tends to increase and the charging characteristics tend to improve. On the other hand, since it is difficult to completely replace sodium with lithium, the amount of sodium y is generally larger than 0. The amount of sodium y is preferably 0.001 to 0.02.

The lithium composite oxide may contain NiO as a by-product phase. The lithium composite oxide may have hydrogen present in a part of lithium sites or sodium sites. If hydrogen is present in a part of the lithium sites or sodium site, lithium composite oxide represented by the composition formula _{Li x Na y H a Ni 1/3} Mn 2/3 O 2. The amount of hydrogen a in the composition formula is preferably 0.01 or less, more preferably 0.005 or less, and even more preferably 0.001 or less. The lithium composite oxide may have an oxygen deficiency. The lithium composite oxide having an oxygen deficiency is represented by the composition formula Li _x N _y H _w Ni _1/3 Mn _2/3 O _2-b . The oxygen deficiency amount b in the composition formula is preferably 0.1 or less, more preferably 0.01 or less, and even more preferably 0.005 or less.

The crystal structure of the lithium composite oxide is preferably a layered rock salt type structure. The layered structure of the coordinated polyhedron in which oxygen is coordinated to lithium has a crystal structure represented by the space group R-3 m. For example, there is an O3 structure in which lithium is present in the center of an octahedron composed of six oxygen atoms. In addition, a spinel structure in which lithium is present in the center of a tetrahedron composed of four oxygen atoms and a P3 structure in which lithium is present in the center of a triangular prism composed of six oxygen atoms are partially included. May be good. Lithium may be present in the transition metal oxide layer as well as in the transition metal oxide layer.

Lithium composite oxide of the present ^invention, the spectral shape of the 6 Li-NMR is characterized, in the range of ^{650～900Ppm,} having a maximum of the main resonance peak of 6 Li-NMR. This main resonance peak is broad, and it is considered that lithium phases having different oxygen coordination states are mixed. The main resonance peak having a maximum in the range of 650 to 900 ppm preferably has a half width of 200 to 450 ppm, more preferably 270 to 430 ppm, and even more preferably 300 to 420 ppm.

⁶ The main resonance peak of Li-NMR may be separated into a plurality of peaks by waveform analysis. At least one of the peaks obtained by the waveform analysis preferably has a peak maximum in the range of 650 to 750 ppm.

[Manufacturing method of lithium composite oxide]
An example of the synthetic route of the above lithium composite oxide is shown in FIG. In the synthesis method shown in FIG. 1, Na _2/3 Ni _1/3 Mn _2/3 O ₂ having a P3 structure is used as a starting material. By ion exchange some of the sodium of the starting materials in lithium ion exchangers represented by the composition formula _{Li z Na 2/3-z} Ni 1/3 Mn 2/3 O 2 can be obtained. A heat-treated body can be obtained by heat-treating this ion exchanger in an oxygen-containing atmosphere. By chemically inserting lithium ions into the heat-treated body, the above-mentioned lithium composite oxide represented by the composition formula Li _x N _y Ni _1/3 Mn _2/3 O ₂ can be obtained.

(Starting substance)
Na _2/3 Ni _1/3 Mn _2/3 O ₂ having a P3 structure has a layered structure of a coordinated polyhedron in which oxygen is coordinated with sodium, and is represented by a space group R3 m. Sodium exists in the center of a triangular prism composed of six oxygen atoms, and there are three transition metal oxide layers per unit cell. The starting material may contain NiO as a by-product phase.

Na _2/3 Ni _1/3 Mn _2/3 O ₂ having a P3 structure as a starting material can be produced by a known method, and for example, a sodium raw material, a nickel raw material and a manganese raw material can be produced as Na: Ni: Mn = 2. It is obtained by weighing and mixing in a ratio of 1: 2 and heating in an atmosphere in which oxygen gas is present, such as in air. Since sodium easily volatilizes when heated, the amount of sodium raw material charged may be slightly excessive.

Examples of the sodium raw material include metallic sodium and sodium compounds. Examples of sodium compounds include acetates such as CH ₃ COONa and CH ₃ COONa · 3H ₂ O; nitrates such as NaNO ₃ ; carbonates such as Na ₂ CO ₃ ; hydroxides such as NaOH; Na ₂ O, Na ₂ O. Oxides of the _second magnitude can be mentioned. Of these, acetate is preferred, and CH ₃ COONa is more preferred.

Nickel raw materials include metallic nickel and nickel compounds. Examples of the nickel compound include oxides such as NiO; hydroxides such as NiOH, Ni (OH) ₂ , and NiOOH. Among these, nickel hydroxide is preferable, and Ni (OH) ₂ is more preferable.

Examples of the manganese raw material include metallic manganese and manganese compounds. Examples of the manganese compound include oxides such as MnO, Mn ₂ O ₃ , Mn ₃ O ₄ , and MnO ₂ ; hydroxides such as MnOH and MnOOH. Among these, manganese oxide and the like are preferable, and Mn ₂ O ₃ is more preferable.

A compound containing two or more of sodium, nickel and manganese can also be used in the production of the starting material. Such material, NaMnO ₂ and sodium manganese oxide, sodium nickel oxide such as NaNiO _2, include manganese-nickel hydroxide and the like.

The method of mixing the raw materials is not particularly limited, and the raw materials may be mixed wet or dry using, for example, a known mixer such as a mixer. The firing temperature of the mixture may be appropriately set according to the raw material, and is usually about 400 to 900 ° C., preferably about 450 to 800 ° C. The firing time may be set according to the firing temperature and the like. The cooling method is not particularly limited, and usually, natural cooling (cooling in a furnace) or slow cooling may be used. During cooling, sodium may be exchanged for protons of moisture in the air. When sodium is exchanged for protons, the starting material has a composition formula of Na _{2 / 3-v} H _v Ni _1/3 Mn _2/3 O ₂ . Generally, v is 0.1 or less.

(Ion exchange)
By carrying out the ion exchange reaction replacing the sodium of the starting material obtained by the above-mentioned lithium ion exchangers represented by the composition formula _{Li z Na 2/3-z} Ni 1/3 Mn 2/3 O 2 Is obtained.

Ion exchange is carried out, for example, by heating the starting material and the lithium compound. As the lithium compound used for ion exchange, lithium salts such as lithium nitrate, lithium chloride, lithium bromide, lithium hydroxide and lithium iodide are preferable, and these can be used alone or in combination of two or more. Examples of the heating method include a method of adding a starting substance to a solution containing a lithium compound and heating (solution system), and a method of mixing the starting substance with a lithium compound and heating (melting system).

In the ion exchanger, it is preferable that a part of sodium in the starting material remains without being exchanged for lithium. The amount of residual sodium (2 / 3-z) in the composition formula Li _z Na _{2 / 3-z} Ni _1/3 Mn _2/3 O ₂ is preferably 0.04 to 0.37, preferably 0.08 to 0.25. Is more preferable, and 0.1 to 0.2 is even more preferable. Along with this, the amount of lithium z is preferably 0.33 to 0.63, more preferably 0.45 to 0.59, and even more preferably 0.47 to 0.57. Like the starting material, the ion exchanger may have a portion of sodium exchanged for protons. Further, the ion exchanger may contain NiO as a by-product phase.

Examples of residual sodium include a method of performing ion exchange in a solution and a method of reducing the amount of the lithium compound used as a starting material. Since it is easy to control the amount of lithium and the amount of residual sodium, it is preferable to perform ion exchange in the solution.

Ion exchange using a solution is carried out, for example, by dispersing the powder of the starting material in a solution in which a lithium compound is dissolved and heating the powder. The solvent is preferably a polar solvent such as water, ethanol, methanol, butanol, hexanol, propanol, tetrahydrofuran, acetone, acetonitrile, N, N-dimethylformamide, dimethyl sulfoxide, acetic acid, formic acid, etc. Can be used. Among these, ethanol or methanol is preferably used, and methanol is more preferable.

By adjusting the amount of the lithium compound used and the reaction temperature, the amount of lithium and the amount of residual sodium in the ion exchanger can be adjusted. The amount of the lithium compound used is preferably 0.1 to 3 times, more preferably 0.5 to 2.5 times, and even more preferably 1 to 2 times the molar ratio with respect to the starting material. The temperature of the ion exchange treatment is usually 50 to 300 ° C, preferably 60 to 200 ° C. The treatment time is usually 1 to 60 hours, preferably 3 to 24 hours. From the viewpoint of keeping the treatment temperature constant, it is preferable to carry out ion exchange by reflux heating. After ion exchange, the product is washed with ethanol, methanol, or the like and dried to obtain an ion exchanger.

Li _2/3 Ni _1/3 Mn _2/3 O ₂ having a P3 structure in which substantially all of the sodium in Na _2/3 Ni _1/3 Mn _2/3 O ₂ has been exchanged for lithium has an O3 structure. Li _2/3 Ni _1/3 Mn _2/3 O ₂ having an O3 structure has a layered structure of a coordinated polyhedron in which oxygen is coordinated to lithium, and is represented by a space group R-3 m. Lithium exists in the center of an octahedron composed of six oxygen atoms, and there are three transition metal oxide layers per unit cell.

On the other hand, when a part of sodium in the starting material remains without being exchanged for lithium as described above, the ion exchanger is a sodium composite oxide having a P3 structure and lithium having an O3 structure in powder X-ray diffraction. There is a peak that does not match any of the peaks of the composite oxide (see FIG. 9). That is, it is considered that the ion exchanger in which a part of sodium remains is not a simple mixture of a sodium composite oxide having a P3 structure and a lithium composite oxide having an O3 structure, but has an intermediate crystal structure between the two.

(Heat treatment)
A heat-treated body can be obtained by heat-treating the ion exchanger obtained as described above. The heat treatment temperature is preferably 300 to 800 ° C., more preferably 350 to 750 ° C., and even more preferably 400 to 700 ° C. The heat treatment atmosphere is not particularly limited, and examples thereof include an atmosphere (air atmosphere), a vacuum, an oxidizing atmosphere, a reducing atmosphere, and an inert atmosphere. Among these, it is preferable to perform the heat treatment in an air atmosphere or an oxidizing atmosphere. When the heat treatment is performed in an oxidizing atmosphere, the heat treatment may be performed in an oxygen atmosphere containing substantially only oxygen. The heat treatment time may be appropriately set according to the heat treatment temperature, and is usually about 1 to 6 hours, preferably 1 to 5 hours. Examples of the cooling method after the heat treatment include natural cooling (cooling in a furnace) and slow cooling.

The heat-treated body has the same chemical composition as the ion exchanger before the heat treatment, except that oxygen deficiency may be introduced by the heat treatment. That is, the ratio of Li, Na, Ni and Mn in the heat-treated body is substantially the same as the ratio in the ion exchanger. On the other hand, the crystal structure was changed by the heat treatment, and in addition to the layered structure of the coordinated polyhedron in which oxygen was 6-coordinated to sodium and the layered structure of the coordinated polyhedron in which oxygen was 6-coordinated to lithium, oxygen was added to lithium. It contains a 4-coordinated spinel structure (see FIG. 10). It is considered that this is because the transition metal oxide layer forming the layered structure of the coordinated polyhedron in which oxygen is coordinated to lithium is moved to the lithium layer by the heat treatment.

(Lithium insertion)
By chemically inserting lithium into the heat-treated body obtained as described above, a lithium insert represented by the composition formula Li _x N _y Ni _1/3 Mn _2/3 O ₂ can be obtained. In the lithium insertion process, sodium in the heat-treated body is replaced with lithium, and lithium is inserted into the empty site. Therefore, the lithium amount x of the lithium insert represented by Li _x N _y Ni _1/3 Mn _2/3 O ₂ is larger than 2/3. The sodium amount y is smaller than the sodium amount (2 / 3-z) of the heat-treated body before lithium insertion.

Insertion of lithium into the heat-treated body is performed, for example, in a lithium salt solution. As the lithium salt, the above-mentioned lithium compound is preferably used as the lithium compound used for ion exchange, and among them, lithium iodide is preferable. Lithium iodide and other lithium salts may be used in combination. As the solvent, the above-mentioned solvent is preferably used as the solvent used for ion exchange, and acetonitrile is particularly preferable.

The amount of lithium inserted can be adjusted by adjusting the amount of lithium salt used and the reaction temperature. The amount of the lithium salt used is preferably 0.5 to 5 times, more preferably 1 to 3 times, the molar ratio with respect to the heat-treated body. The temperature of the lithium insertion treatment is preferably 20 to 200 ° C, more preferably 50 to 180 ° C. The treatment time is usually 1 to 60 hours, preferably 3 to 24 hours. From the viewpoint of keeping the treatment temperature constant, it is preferable to insert lithium by reflux heating. After inserting lithium, the product is washed with ethanol, methanol or the like and dried to obtain a lithium insert.

As described above, the lithium amount x of the lithium insert represented by the composition formula Li _x N _y Ni _1/3 Mn _2/3 O ₂ is larger than 2/3 and is 0.7 to 0.9. The difference between the amount of lithium x, the composition formula _{Li z Na 2/3-z} Ni 1/3 Mn 2/3 O 2 with lithium amount z of heat treatment bodies represented in the lithium insertion material, i.e. the insertion amount of lithium x- It is preferable that z satisfies 0.2 ≦ x−z ≦ 0.5.

Since most of the sodium remaining in the heat-treated body is replaced with lithium by lithium insertion, the X-ray diffraction pattern of the lithium insert is represented by the composition formula Li _2/3 Ni _1/3 Mn _2/3 O ₂ . It is similar to the X-ray diffraction pattern of the lithium composite oxide having an O3 structure. On the other hand, lithium introduced by ion exchange of Na _2/3 Ni _1/3 Mn _2/3 O _{2 having} a P3 structure, lithium introduced by ion exchange of residual sodium in the heat-treated body, and lithium introduced into an empty site. Is considered to be different from oxygen in the coordination state (coordination strength, electron density, etc.). Along with this, it is estimated that the main resonance peak of ⁶ Li-NMR becomes broad due to the presence of lithium having different electronic states, and a plurality of phases (peaks) are observed by waveform analysis.

[Positive electrode active material for secondary batteries and secondary batteries]
The positive electrode active material for a secondary battery of the present invention is used for the positive electrode of a lithium secondary battery, and contains the above lithium composite oxide (lithium insert) as a main component. The content of the lithium composite oxide in the positive electrode active material is preferably 51% by weight or more, more preferably 70% by weight or more, still more preferably 90% by weight or more. As long as the function of the present invention is not impaired, the positive electrode active material for a secondary battery may contain components other than the main component. The positive electrode active material for a secondary battery may contain only one type of the above lithium composite oxide, or may contain two or more types.

The secondary battery of the present invention includes a positive electrode, a negative electrode, an electrolyte, and if necessary, other battery elements, and the positive electrode contains the above-mentioned positive electrode active material. In the secondary battery of the present invention, the battery elements of the conventionally known secondary battery can be adopted as they are, except that the positive electrode contains the positive electrode active material containing the lithium composite oxide as a main component. The secondary battery of the present invention may have any of a coin type, a button type, a cylindrical type, and an all-solid type configuration.

FIG. 2 is a partial cross-sectional view schematically showing an example of a lithium secondary battery. The lithium secondary battery 1 is composed of a negative electrode terminal 2, a negative electrode 3, a separator 4 impregnated with an electrolytic solution, an insulating packing 5, a positive electrode 6, and a positive electrode can 7. In the form shown in FIG. 2, the positive electrode can 7 is arranged on the lower side, and the negative electrode terminal 2 is arranged on the upper side. The outer shape of the lithium secondary battery 1 is formed by the positive electrode can 7 and the negative electrode terminal 2.

Between the positive electrode can 7 and the negative electrode terminal 2, the positive electrode 6 and the negative electrode 3 are provided in layers in order from the bottom. A separator 4 impregnated with an electrolytic solution that separates the positive electrode 6 and the negative electrode 3 is interposed between the positive electrode 6 and the negative electrode 3. The positive electrode can 7 and the negative electrode terminal 2 are electrically insulated by the insulating packing 5.

The positive electrode 6 can be produced by blending the above-mentioned positive electrode active material with a conductive agent, a binder, or the like as necessary to prepare a positive electrode mixture, which is then pressure-bonded to a current collector. As the current collector, a stainless mesh, aluminum foil, or the like can be used. As the conductive agent, acetylene black, Ketjen black and the like can be used. As the binder, tetrafluoroethylene, polyvinylidene fluoride or the like can be used. The composition of the positive electrode active material, the conductive agent, the binder and the like in the positive electrode mixture is not particularly limited. For example, the conductive agent is about 1 to 30% by weight (preferably 5 to 25% by weight), the binder is 0 to 30% by weight (preferably 3 to 10% by weight), and the balance is the positive electrode active material. It is mixed.

As the counter electrode to the positive electrode, a known material that functions as a negative electrode and can occlude and release lithium can be adopted, and the material thereof is a metallic material such as metallic lithium or lithium alloy, graphite, MCMB (mesocarbon micro). Examples include carbon-based materials such as beads).

Known battery elements can be used for the separator, battery container, and the like. As the electrolyte, a known electrolyte, a solid electrolyte, or the like can be adopted. For example, as the electrolytic solution, an electrolyte such as lithium perchlorate or lithium hexafluorophosphate is used as a solvent such as ethylene carbonate (EC), dimethyl carbonate (DMC), propylene carbonate (PC), and diethyl carbonate (DEC). A dissolved product can be used. In the all-solid type secondary battery, as the electrolyte, for example, a polymer-based solid electrolyte such as a polyethylene oxide-based polymer compound, a polymer compound containing at least one of a polyorganosiloxane chain or a polyoxyalkylene chain, and a sulfide. A system-based solid electrolyte, an oxide-based solid electrolyte, or the like can be used.

By using the lithium composite oxide of the present invention as the positive electrode active material, in addition to being able to increase the capacity of the secondary battery, it is possible to suppress a sudden voltage drop during discharge. Therefore, in a region where the voltage does not drop sharply during discharging, it is possible to easily detect the charge rate when the secondary battery is mounted and at low cost.

Examples of the present invention will be described below, but the present invention is not limited to these examples. In the following, the composition of the oxide was analyzed by an ICP emission spectrophotometer (ICPS-8000, manufactured by Shimadzu Corporation). Powder X-ray diffraction was measured by Bruker's D8 ADVANCE.

[Synthesis of starting material]
Sodium acetate (CH ₃ COONa) powder with a purity of 98.5% or more, nickel (II) hydroxide (Ni (OH) ₂ ) powder with a purity of 99.9% or more, and manganese oxide with a purity of 99.9% or more ( III) (Mn ₂ O ₃ ) was weighed so that the molar ratio was Na: Ni: Mn = 0.687: 0.333: 0.667. These were dispersed in ethanol in a mortar, mixed, pelletized, and filled in a JIS standard gold crucible. It was fired at 650 ° C. for 10 hours in an oxygen atmosphere using a tubular electric furnace.

The chemical composition of the obtained sample is Na: Ni: Mn = 0.67: 0.33: 0.67, and the chemical formula of Na _2/3 Ni _1/3 Mn _2/3 O ₂ is valid. Was confirmed. From the diffraction pattern of the X-ray diffraction pattern (see FIG. 9), it was confirmed that the obtained sample had a layered rock salt structure of a space group R3 m in a rhombic crystal system. The lattice constants obtained by the least squares method are a = 2.8865 Å (error: within 0.0001 Å) and c = 16.781 Å (error: within 0.001 Å), and known Na _{2 /} having a P3 structure. It was in good agreement with the value of ₃ Ni _1/3 Mn _2/3 O ₂ (see, for example, Z. Lu et al., Chem. Mater., 12, 3583 (2000)).

[Comparison example]
<Ion exchange>
The starting material Na _0.67 Ni _0.33 Mn _0.67 O ₂ polycrystal obtained above, lithium nitrate (LiNO ₃ ) powder having a purity of 99% or more, and lithium chloride (LiCl) having a purity of 99% or more. The mixture (molar ratio 88:12) was weighed so that the weight ratio was Na _0.67 Ni _0.33 Mn _0.67 O ₂ : (LiNO ₃ + LiCl) = 1: 7. After mixing these in a mortar, they were filled in an alumina crucible and kept in an electric furnace at 260 ° C. in an air atmosphere for 1 hour to carry out a lithium ion exchange treatment. Then, it was washed with hot water and air-dried to obtain an ion exchanger.

The chemical composition of the ion exchanger was Na: Li: Ni: Mn: = 0.0018: 0.66: 0.33: 0.66. In this ion exchanger, almost all of the sodium in the starting material was replaced with lithium, and it was confirmed that the chemical formula of Li _2/3 Ni _1/3 Mn _2/3 O ₂ was valid. From the diffraction pattern of the powder X-ray diffraction measurement (FIG. 9), it was confirmed that the ion exchanger was a rhombic crystal system and had a layered rock salt structure of the space group R-3 m. The lattice constants are a = 2.8666 Å (error: within 0.0001 Å) and c = 14.470 Å (error: within 0.001 Å), and known Li _2/3 Ni _1/3 Mn ₂ having an O3 structure. It was in good agreement with the value of _{/ 3} O ₂ (see, for example, Z. Lu et al., Chem. Mater., 12, 3583 (2000)).

<Heat treatment>
The polycrystal of the ion exchanger obtained above was pulverized and filled in an alumina crucible. The heat treatment was carried out by holding the mixture in an electric furnace at 500 ° C. for 5 hours in an air atmosphere. Then, the temperature was returned to room temperature by allowing to cool in the furnace to obtain a heat-treated body.

The chemical composition of the heat-treated body was Na: Li: Ni: Mn: = 0.0019: 0.66: 0.33: 0.68, and the composition of the ion exchanger before the heat treatment was maintained. From the diffraction pattern of the powder X-ray diffraction measurement (see FIG. 10), it was found that the heat-treated body had a layered rock salt structure having a space group R-3 m in a rhombic crystal system as in the case before the heat treatment. The lattice constants are a = 2.8855 Å (error: within 0.0002 Å) and c = 14.238 Å (error: within 0.002 Å), and the lattice constant of the a-axis increases compared to before the heat treatment, and the c-axis The lattice constant of was decreasing.

[Examples 1 to 4 and reference example]
<Ion exchange>
A solution was prepared in which 15 g of dehydrated methanol having a purity of 99.8% was dissolved in lithium bromide (LiBr) powder having a purity of 99.9% or more. 1.1 g of the starting material (polycrystal of Na _0.67 Ni _0.33 Mn _0.67 O ₂ ) obtained above was added to this solution. In the preparation of the LiBr methanol solution, the LiBr concentration was adjusted so that the starting materials Na _0.67 Ni _0.33 Mn _0.67 O ₂ and LiBr had a molar ratio of 1: A (Example 1). In Example 2, A = 0.8 in Example 2, A = 1.6 in Example 3 and Reference Example, and A = 2.0 in Example 4).

Next, a lithium ion exchange treatment was carried out by reflux heating at 110 ° C. for 5 hours using a Dimroth condenser. Then, it was washed with methanol and air-dried to obtain an ion exchanger having the composition shown in Table 1. Each of the ion exchangers can be represented by the composition formula L _{2 / 3-z} Na _z Ni _1/3 Mn _2/3 O ₂ , in which sodium in the starting material is replaced with lithium and a part of sodium is replaced. It remained without being.

<Heat treatment>
The polycrystal of the ion exchanger obtained above was pulverized, and the pulverized product was filled in an alumina crucible. The heat treatment was carried out by holding the mixture in an electric furnace at 500 ° C. for 5 hours in an oxygen atmosphere. Then, the temperature was returned to room temperature by allowing to cool in the furnace to obtain a heat-treated body. All of the obtained heat-treated bodies had the same composition as the ion exchanger before the heat treatment. The above-mentioned ion exchange was carried out at A = 1.6, and the heat treatment was carried out as a reference example.

<Lithium insertion>
A solution was prepared in which lithium iodide (LiI) powder having a purity of 99.9% or more was dissolved in 15 g of acetonitrile having a purity of 99.5%. 1.0 g of the heat-treated body obtained above was added to this solution. When preparing the acetonitrile solution of LiI, the LiI concentration was adjusted so that the heat-treated L _{2 / 3-z} Na _z Ni _1/3 Mn _2/3 O ₂ and Li I had a molar ratio of 1: 2. did. Next, a lithium ion insertion treatment was carried out by reflux heating at 140 ° C. for 10 hours using a Dimroth condenser. Then, it was washed with methanol and air-dried to obtain a lithium insert having the composition shown in Table 1.

[Evaluation]
< ⁶ Li-NMR>
Under the following conditions, ⁶ Li solid-state NMR ( ⁶ Li MAS-NMR) of the lithium composite oxides obtained in Examples 1 to 4 and the heat-treated bodies of Examples 1 and 4 before lithium insertion was measured. .. Chemical shifts were recorded as relative to 1.0 M aqueous ⁶ LiCl solution.
Measuring device: Bruker AVANCE 300 ( ⁶ Li resonance frequency: 44 MHz)
Temperature: Room temperature Rotation speed: 50kHz
Pulse width: 3.6 μs (π / 2 pulse)
Pulse sequence: rotor-synchronized spin-echo pulse sequence

Table 1 shows the composition of the lithium composite oxides of Examples 1 to 4, and the peak (maximum) and peak width at half maximum of ⁶ Li-NMR. The ⁶ Li-NMR spectra of the lithium composite oxides of Examples 1 to 4 and Comparative Example 1 are shown in FIG. Further, the ⁶ Li-NMR spectra of the lithium composite oxides of Examples 1 and 4 before and after lithium insertion are shown in FIG. 8 together with the ⁶ Li-NMR spectra of the lithium composite oxide of Comparative Example 1. The starting material Na _2/3 Ni _1/3 Mn _2/3 O ₂ and the powder X-ray diffraction pattern of the ion exchanger are shown in FIG. 9, the powder X-ray diffraction pattern of the heat-treated body is shown in FIG. 10, and the lithium insert powder X is shown. The line diffraction pattern is shown in FIG.

As shown in Table 1, the compositions of the ion-exchanger and the heat-treated body tended to increase the amount of lithium and decrease the amount of sodium as the amount A of the lithium salt used during the ion exchange increased. Correspondingly, there is a difference in the X-ray diffraction patterns (FIGS. 9 and 10), and the larger the residual amount of sodium, the more the intermediate structure (O3 structure and P3) near 2θ (° / CuKα) = 17 °. The peak of the structure (structure that is neither of the structures) was large.

In the lithium insert in which lithium ions were chemically inserted into the heat-treated body, the larger the amount of residual sodium in the ion exchanger and the heat-treated body, the larger the amount of residual sodium in the lithium insert tended to be, but the difference was small. Ion. In addition, there was no clear tendency for the amount of lithium in the lithium insert. In the X-ray diffraction pattern (FIG. 11), the peak of the intermediate structure disappeared and a slight difference was observed in the diffraction peak angle, but the lithium composite oxides of Examples 1 to 4 were all comparative examples. The diffraction pattern equivalent to that of the heat-treated body (HT-O3-Li _0.67 Ni _0.33 Mn _0.67 O ₂ ) was shown.

On the other hand, a clear difference was observed in the ⁶ Li-NMR spectrum (FIG. 3) of the lithium insert. In the sintered body of Comparative Example 1, a sharper peak was observed at around 725 ppm as compared with Examples 1 to 4, whereas in the lithium inserts of Examples 1 to 4, the main resonance was around 500 to 1000 ppm. The peak had a broad shape, and in Examples 1 to 3, a shoulder was observed on the low magnetic field side of the main resonance peak.

Waveform analysis (decomposition) by functional decomposition of the ⁶ Li-NMR spectrum of the lithium inserts of Examples 1 to 4 was performed by Dmfit software, and the peak position, peak height, peak width and peak area ratio were calculated. The analysis results are shown in FIGS. 4 to 7. In Examples 1 to 3, the main resonance peak was separated into two peaks, and the larger the amount of sodium in the heat-treated body before lithium insertion, the larger the peak area on the low magnetic field side tended to be.

In the ⁶ Li-NMR spectrum (broken line in FIG. 8) of the heat-treated body before lithium insertion, a strong peak is observed on the high magnetic field side as in Comparative Example 1, whereas the peak intensity of the low magnetic field is large after lithium insertion. There was a tendency for the peak to become broader. The sintered body of Example 4 (before lithium insertion) having a small amount of residual sodium had a peak maximum around 750 ppm. On the other hand, the sintered body of Example 1 having a small amount of residual sodium had a peak maximum near 560 ppm, and the peak maximum was shifted to a high magnetic field.

When the residual amount of sodium in the heat-treated body was different, there was no significant difference in the X-ray diffraction after lithium insertion, whereas there was a difference in the peak shape of ⁶ Li-NMR. It is considered that this is largely influenced by the coordination state of oxygen on lithium. That is, as shown in FIG. 10, the ratios of the O3 structure, the P3 structure and the intermediate structure are different due to the difference in the residual amount of sodium in the heat-treated body before lithium insertion, and as shown in FIG. 8, ⁶ Li -There are differences in the peak shape and chemical shift of NMR. Due to the difference in the structure of the sintered body before lithium insertion and the coordination state of oxygen to lithium, the site where lithium is inserted by chemical insertion and the coordination state of oxygen to the inserted lithium are different. ^{6 It} is considered that the difference in the peak shape of Li-NMR was caused.

[Manufacturing and evaluation of lithium secondary batteries]
Each of the lithium composite oxide (lithium insert) obtained in Examples 1 to 4 and the lithium composite oxide (heated body) obtained in Reference Example and Comparative Example 1 was used as a positive electrode active material, and acetylene black was used as a conductive agent. , Tetrafluoroethylene as a binder was mixed so as to have a weight ratio of 5: 5: 1 and pressure-bonded to an Al mesh to prepare an electrode. For each electrode, a 1M solution prepared by dissolving lithium metal as a counter electrode and lithium hexafluorophosphate in a mixed solvent (volume ratio 1: 2) of ethylene carbonate (EC) and dimethyl carbonate (DMC) is used as an electrolytic solution. A lithium secondary battery (coin-type cell) was produced. The battery was manufactured according to a known cell configuration / assembly method.

A charge / discharge test (electrochemical lithium insertion / removal test) was performed on each of the manufactured lithium secondary batteries at a current density of 15 mA / g and a cutoff potential of 4.8 V to 2.0 V under a temperature condition of 25 ° C. The charging / discharging characteristics were evaluated. The charge / discharge test started with charging (lithium desorption).

A lithium secondary ion battery using the lithium insert of Example 3 (Example), the heat-treated body before lithium insertion of Example 3 (reference example), and the heat-treated body of Comparative Example 1 (Comparative Example) as the positive electrode active material. The charge / discharge test results are shown in FIG. Further, FIG. 13 shows the charge / discharge test results of the lithium secondary ion battery using the lithium inserts of Examples 1 to 4 as the positive electrode active material.

As shown in FIG. 12, the lithium secondary battery using the lithium composite oxide of the comparative example has a high initial discharge capacity, but a sharp voltage drop occurs in the vicinity of the capacity of the discharge curve of 70 to 90 mAh / g. The lithium secondary battery using the composite oxide before lithium insertion suppressed a sharp voltage drop in the discharge curve, but the initial discharge capacity was slightly smaller and the initial charge capacity was significantly smaller than in the comparative example. It was. On the other hand, the lithium secondary battery using the composite oxide after inserting lithium had a discharge curve similar to that of the reference example, and a sudden voltage drop was suppressed. In the embodiment the initial charging capacity was significantly increased in comparison with Comparative Examples and Reference Examples.

As shown in FIG. 13, the lithium secondary battery using the lithium composite oxide other than Example 3 also had a voltage drop suppressed and a high initial charge capacity as in Example 3. From these results, it can be seen that the secondary battery using the lithium composite oxide of the present invention as the positive electrode active material has a high capacity and is easy to detect the charge rate because the voltage drop is suppressed.

Claims (12)

A lithium composite oxide used as a positive electrode active material for lithium secondary batteries.
The composition formula is represented by Li _x Na _y Ni _1/3 Mn _2/3 O ₂ , and in the formula, 0.75 ≦ x ≦ 0.90 and 0 <y ≦ 0.05.
A lithium composite oxide for a positive electrode active material having a maximum of ⁶ Li-NMR main resonance peaks in the range of 650 to 900 ppm, a half width of the main resonance peaks of 200 to 450 ppm, and an O3 structure. According to claim 1, when the main resonance peak of ⁶ Li-NMR is separated into one or more peaks by waveform analysis, at least one of the peaks obtained by waveform analysis has a peak maximum in the range of 650 to 750 ppm. The lithium composite oxide for the positive electrode active material described. The lithium composite oxide for a positive electrode active material according to claim 1 or 2, wherein the main resonance peak of ⁶ Li-NMR can be separated into two or more peaks by waveform analysis. The lithium composite oxide for a positive electrode active material according to any one of claims 1 to 3, wherein the sodium amount y in the composition formula satisfies 0.001 ≦ y ≦ 0.02. A method for producing a lithium composite oxide used as a positive electrode active material for a lithium secondary battery.
The lithium composite oxide is represented by the composition formula _{Li x Na y Ni 1/3 Mn 2/3} O 2, wherein Ri 0.75 ≦ x ≦ 0.90,0 <y ≦ 0.05 der ,
The composition formula is represented by Li _z Na _{2 / 3-z} Ni _1/3 Mn _2/3 O ₂ , 0.33 ≤ z ≤ 0.63 in the formula, and the crystal structure is a layered rock salt type complex oxidation. A method for producing a lithium composite oxide for a positive electrode active material , which comprises chemically inserting lithium ions into a product. The composite oxide represented by the composition formula Li _z Na _{2 / 3-z} Ni _1/3 Mn _2/3 O ₂ is treated in a lithium salt solution in a temperature range of 20 ° C. to 200 ° C. to obtain the above. The method for producing a lithium composite oxide for a positive electrode active material according to claim 5, wherein chemical insertion of lithium ions is carried out. The method for producing a lithium composite oxide for a positive electrode active material according to claim 6, wherein the lithium salt contains lithium iodide. The method for producing a lithium composite oxide for a positive electrode active material according to claim 6 or 7, wherein the solvent of the lithium salt solution contains acetonitrile. Claim 5 that the difference between the amount of lithium x in the composition formula after chemical insertion of lithium ions and the amount of lithium z in the composition formula before chemical insertion of lithium ions satisfies 0.2 ≦ x−z ≦ 0.5. The method for producing a lithium composite oxide for a positive electrode active material according to any one of 8 to 8. Composition formula Na _2/3 Ni _1/3 Mn _2/3 O _{2 The} starting material sodium represented by P3 structure is exchanged for lithium for ion exchange, and the obtained ion exchanger is heat-treated at 300 to 800 ° C. by to produce the composite oxide represented by the compositional formula _{Li z Na 2/3-z} Ni 1/3 Mn 2/3 O 2, as claimed in any one of claims 5-9 positive A method for producing a lithium composite oxide for an active material . A positive electrode active material for a lithium secondary battery containing the lithium composite oxide according to any one of claims 1 to 4 as a main component. Includes positive, negative, and electrolyte
A lithium secondary battery in which the positive electrode contains the positive electrode active material according to claim 11.