JP2017174558A - Lithium composite oxide, method for manufacturing the same, positive electrode active material for secondary battery, and secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium composite oxide which is useful as a positive electrode active material for a secondary battery.SOLUTION: A lithium composite oxide is represented by the composition formula, LiNaNiMnO(where 0.7≤x≤0.9 and x≤0.05) and has a maximum of aLi-NMR primary resonance peak in a range of 650-900 ppm. The half value width of the primary resonance peak is preferably 200-450 ppm. The lithium composite oxide is obtained by chemically inserting a lithium ion into a composite oxide represented by the composition formula, LiNaNiMnO(where 0.7≤x≤0.9 and y≤0.05).SELECTED DRAWING: Figure 3

Description

  The present invention relates to a lithium composite oxide and a method for producing the same. Furthermore, this invention relates to the positive electrode active material for secondary batteries using the said lithium complex oxide, and a secondary battery.

  Secondary batteries are mounted on many portable electronic devices such as mobile phones and notebook computers. Secondary batteries such as lithium secondary batteries are expected to be put into practical use as large batteries such as 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.

A lithium secondary battery has an electrode composed of a positive electrode and a negative electrode, and a separator or a solid electrolyte containing a non-aqueous electrolyte as main components. Both the positive electrode and the negative electrode contain a material (electrode active material) 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 so far as materials for positive electrode active materials of lithium secondary batteries. .

Li _2/3 Ni _1/3 Mn _2/3 O ₂ is obtained by exchanging sodium in the starting material Na _2/3 Ni _1/3 Mn _2/3 O ₂ with lithium, and the structure of the starting material Depending on the structure, the structure of Li _2/3 Ni _1/3 Mn _2/3 O ₂ obtained is different. For example, when Na _2/3 Ni _1/3 Mn _2/3 O ₂ having a P3 structure is used as a starting material, Li _2/3 Ni _1/3 Mn _2/3 O _{2 as} an ion exchanger has an O3 structure. Have By heat-treating _Li 2/3 _Ni 1/3 _Mn 2/3 _{O 2} of O3 structure, crystal structure changes, it is reported that the electrochemical characteristics are improved (Non-patent Document 1).

When ion exchange is carried out by heating P2 structure Na _2/3 Ni _1/3 Mn _2/3 O ₂ in a solution in which a lithium salt is dissolved in a low-boiling solvent such as methanol or ethanol, a part of sodium is obtained. An oxide represented by the composition formula (Li _z Na _{2 / 3-z} ) Ni _1/3 Mn _2/3 O ₂ remaining without being replaced with lithium is 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).

Kazuaki Chiba et al., 2014 Electrochemical Fall Conference Abstracts, 1P23 (2014) Kazuaki Chiba et al., 55th Battery Discussion Meeting, 2B18, (2014)

  The secondary battery using the lithium composite oxide of Non-Patent Document 1 as the positive electrode material has an advantage that the discharge capacity is large. However, since there is a region where the voltage drops rapidly during discharging, it may be difficult to detect the charging rate. By using the lithium composite oxide of Non-Patent Document 2, a rapid voltage drop during 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 using a negative electrode material such as graphite.

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

  The present inventors use a lithium composite oxide in which lithium is chemically inserted into a predetermined lithium sodium composite oxide as a positive electrode active material, thereby increasing the initial charge capacity of the secondary battery and increasing the capacity. As a result, the present invention was reached.

The lithium composite oxide of the present invention is represented by the composition formula Li _x Na _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 layered rock salt type crystal structure.

The lithium composite oxide of the present invention has a maximum of ⁶ Li-NMR main resonance peak 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. ^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 preferably has a peak maximum in the range of 650 to 750 ppm.

The lithium composite oxide is represented by, for example, the composition formula Li _z Na _{2 / 3-z} Ni _1/3 Mn _2/3 O ₂ (0.33 ≦ z ≦ 0.63), and the crystal structure is a layered rock salt. It is obtained by chemically inserting lithium ions into a lithium sodium composite oxide having a mold structure. The difference between the lithium amount x in the composition formula after chemical insertion of lithium ions and the lithium amount z in the composition formula before chemical insertion of lithium ions preferably satisfies 0.2 ≦ x−z ≦ 0.5.

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

  Furthermore, the present invention relates to a positive electrode active material containing the above 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 a positive electrode active material of a lithium secondary battery, a high-capacity secondary battery can be obtained.

It is a figure which shows an example of the synthetic | combination path | route of lithium complex oxide. It is a fragmentary sectional view showing an example of a secondary battery typically. It is a ⁶ Li-NMR spectra of the lithium composite oxide of Example. 2 is a waveform analysis result of ⁶ Li-NMR spectrum of the lithium composite oxide of Example 1. FIG. 4 is a waveform analysis result of ⁶ Li-NMR spectrum of the lithium composite oxide of Example 2. FIG. ⁶ is a waveform analysis result of ⁶ Li-NMR spectrum of the lithium composite oxide of Example 3. ⁶ 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 figure of a heat processing body. It is a powder X-ray-diffraction figure of the lithium composite oxide (lithium insertion body) of an Example, and the lithium composite oxide (heat processing body) of a comparative example. It is a graph of the charging / discharging test result of the secondary battery of an Example, a reference example, and a comparative example. It is a graph of the charging / discharging 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 Na _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 a 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 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 is characterized by a large initial charge capacity because the amount of lithium exceeds 2/3 by inserting lithium by chemical insertion or the like. The lithium amount x in the composition formula is preferably 0.75 or more, and more preferably 0.8 or more.

  The sodium amount y in the composition formula is preferably 0.02 or less. By reducing the residual sodium amount y, the lithium amount x increases and the charge characteristics tend to be improved. On the other hand, since it is difficult to completely replace sodium with lithium, the amount of sodium y is generally greater than zero. The amount of sodium y is preferably 0.001 to 0.02.

The lithium composite oxide may contain NiO as a byproduct phase. In the lithium composite oxide, hydrogen may exist at a part of the lithium site or the sodium site. 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. Lithium composite oxide having oxygen deficiency, represented by the composition formula _{Li x Na y H w Ni 1/3} Mn 2/3 O 2-b. The oxygen deficiency b in the composition formula is preferably 0.1 or less, more preferably 0.01 or less, and further preferably 0.005 or less.

  The crystal structure of the lithium composite oxide is preferably a layered rock salt structure. A layered structure of a coordination polyhedron in which oxygen is six-coordinated to lithium has a crystal structure represented by a space group R-3m. For example, an O3 structure in which lithium is present at the center of an octahedron composed of six oxygen atoms can be given. In addition, a spinel structure in which lithium is present at the center of a tetrahedron composed of four oxygen atoms and a P3 structure in which lithium is present at the center of a triangular prism composed of six oxygen atoms are included in part. Also 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.

The main resonance peak of ⁶ Li-NMR may be separated into a plurality of 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.

[Method for producing lithium composite oxide]
An example of the synthesis route of the 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. An ion exchanger represented by the composition formula Li _z Na _{2 / 3-z} Ni _1/3 Mn _2/3 O ₂ is obtained by ion-exchanging a part of the starting sodium. A heat-treated body can be obtained by heat-treating this ion exchanger in an oxygen-containing atmosphere. The lithium composite oxide represented by the composition formula Li _x Na _y Ni _1/3 Mn _2/3 O ₂ is obtained by chemically inserting lithium ions into the heat-treated body.

(Starting material)
Na _2/3 Ni _1/3 Mn _2/3 O ₂ having a P3 structure has a coordinated polyhedral layered structure in which oxygen is coordinated to sodium and is represented by a space group R3m. 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. For example, a sodium raw material, a nickel raw material and a manganese raw material can be produced by using Na: Ni: Mn = 2. It is obtained by weighing and mixing so as to be 1: 2, and heating in an atmosphere containing oxygen gas such as air. Since sodium is volatile during heating, the amount of sodium raw material charged may be slightly excessive.

Examples of the sodium raw material include sodium metal 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 and Na ₂ O _{2 and the} like. Of these, acetates are _preferable, CH 3 COONa is 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.

Manganese raw materials include manganese metal 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 them, manganese oxide, or the like are preferred, _Mn 2 _{O 3} is more preferable.

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

The mixing method of the raw materials is not particularly limited, and may be mixed by a wet method or a dry method using a known mixer such as a mixer. What is necessary is just to set the calcination temperature of a mixture suitably according to a raw material, Usually, about 400-900 degreeC, Preferably it is about 450-800 degreeC. The firing time may be set according to the firing temperature or the like. The cooling method is not particularly limited, and may be naturally cooled (cooled in the furnace) or gradually cooled. During cooling, sodium may be exchanged for protons 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)
An ion exchanger represented by the composition formula Li _z Na _{2 / 3-z} Ni _1/3 Mn _2/3 O ₂ is carried out by carrying out an ion exchange reaction in which sodium of the starting material obtained as described above is exchanged for lithium. Is obtained.

  The ion exchange is performed, for example, by heating the starting material and the lithium compound. As a 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 in which a starting material is added to a solution containing a lithium compound and heating (solution system), and a method in which the starting material is mixed with a lithium compound and heated (melting system).

In the ion exchanger, it is preferable that a part of sodium in the starting material remains without being exchanged with lithium. The residual sodium amount (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 more preferable. Accordingly, the lithium amount z is preferably 0.33 to 0.63, more preferably 0.45 to 0.59, and still more preferably 0.47 to 0.57. Similar to the starting material, the ion exchanger may have some sodium exchanged for protons. The ion exchanger may contain NiO as a byproduct phase.

  In order to leave sodium, there are a method of performing ion exchange in a solution and a method of reducing the amount of lithium compound used relative to the starting material. Since the control of the amount of lithium and the amount of residual sodium is easy, it is preferable to perform ion exchange in a solution.

  The ion exchange using the solution is performed, for example, by dispersing the starting material powder in a solution in which the lithium compound is dissolved and heating the solution. As the solvent, polar solvents such as water, ethanol, methanol, butanol, hexanol, propanol, tetrahydrofuran, acetone, acetonitrile, N, N-dimethylformamide, dimethyl sulfoxide, acetic acid and formic acid are preferable, and these are used alone or in combination of two or more. Can be used. In these, it is preferable to use ethanol or methanol, and it is more preferable to use methanol.

  By adjusting the amount of 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 still more preferably 1 to 2 times the molar ratio of 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 perform ion exchange by reflux heating. After the ion exchange, the product is washed with ethanol or methanol and dried to obtain an ion exchanger.

Li _2/3 Ni _1/3 Mn _2/3 O ₂ in which almost the whole amount of sodium of Na _2/3 Ni _1/3 Mn _2/3 O ₂ having a P3 structure is replaced with lithium has an O 3 structure. Li _2/3 Ni _1/3 Mn _2/3 O ₂ having an O 3 structure has a layered structure of coordination polyhedron in which oxygen is coordinated to lithium and is represented by a space group R-3m. 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 the sodium in the starting material remains without being exchanged with lithium as described above, the ion exchanger is obtained by powder X-ray diffraction in which a sodium composite oxide having a P3 structure and a lithium having an O3 structure are used. There is a peak that does not match any peak of the complex 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 having an intermediate crystal structure between them.

(Heat treatment)
A heat-treated body is 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 further preferably 400 to 700 ° C. The heat treatment atmosphere is not particularly limited, and examples include air (air atmosphere), vacuum, oxidizing atmosphere, reducing atmosphere, inert atmosphere, and the like. Among these, it is preferable to perform the heat treatment in an air atmosphere or an oxidizing atmosphere. When heat treatment is performed in an oxidizing atmosphere, the heat treatment may be performed in an oxygen atmosphere containing substantially only oxygen. What is necessary is just to set heat processing time suitably according to heat processing temperature, Usually, it is about 1 to 6 hours, Preferably it is 1 to 5 hours. Examples of the cooling method after the heat treatment include natural cooling (cooling in the furnace) and slow cooling.

  The heat treatment body has the same chemical composition as the ion exchanger before heat treatment, except that oxygen deficiency may be introduced by 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 is changed by the heat treatment, and in addition to the layered structure of the coordination polyhedron in which oxygen is coordinated to sodium and the layered structure of the coordination polyhedron in which oxygen is coordinated to lithium, oxygen is added to lithium. A 4-coordinate spinel structure is included (see FIG. 10). This is presumably because the transition metal oxide layer constituting the layered structure of the coordination polyhedron in which oxygen is six-coordinated to lithium moves to the lithium layer by heat treatment.

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

  The insertion of lithium into the heat treatment body is performed, for example, in a lithium salt solution. As a lithium salt, what was mentioned above as a lithium compound used for ion exchange is used preferably, and lithium iodide is especially preferable. Lithium iodide and other lithium salts may be used in combination. As the solvent, those described above as the solvent used for ion exchange are preferably used, and acetonitrile is particularly preferable.

  The amount of lithium insertion 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 in terms of 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 perform lithium insertion by reflux heating. After inserting lithium, the product is washed with ethanol or methanol 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 Na _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 lithium amount x in the lithium insert and the lithium amount z in the heat-treated body represented by the composition formula Li _z Na _{2 / 3-z} Ni _1/3 Mn _2/3 O ₂ , that is, the lithium insertion amount x− z preferably satisfies 0.2 ≦ x−z ≦ 0.5.

Since most of the sodium remaining in the heat-treated body is exchanged for lithium by lithium insertion, the X-ray diffraction pattern of the lithium insertion body is represented by the composition formula Li _2/3 Ni _1/3 Mn _2/3 O ₂ . This 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 treatment body, and lithium introduced into an empty site Is considered to be different in coordination state with oxygen (coordination strength, electron density, etc.). Along with this, since there are lithiums having different electronic states, the main resonance peak of ⁶ Li-NMR is broadened, and it is estimated that a plurality of phases (peaks) are observed by waveform analysis.

[Positive electrode active material for secondary battery and secondary battery]
The positive electrode active material for a secondary battery of the present invention is used for a 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, and 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 secondary batteries may contain components other than the main component. The positive electrode active material for a secondary battery may contain only one kind of the above lithium composite oxide, or may contain two or more kinds.

  The secondary battery of the present invention includes a positive electrode, a negative electrode, an electrolyte, and other battery elements as necessary, and the positive electrode contains the positive electrode active material described above. The secondary battery of the present invention can employ a battery element of a conventionally known secondary battery as it is, except that the positive electrode active material mainly composed of the lithium composite oxide is contained in the positive electrode. The secondary battery of the present invention may have any configuration of a coin type, a button type, a cylindrical type, and an all solid type.

  FIG. 2 is a partial cross-sectional view schematically showing an example of a lithium secondary battery. The lithium secondary battery 1 includes 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 disposed on the lower side, and the negative electrode terminal 2 is disposed on the upper side. An 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 lower side. Between the positive electrode 6 and the negative electrode 3, a separator 4 impregnated with an electrolytic solution that separates them from each other is interposed. The positive electrode can 7 and the negative electrode terminal 2 are electrically insulated by an insulating packing 5.

  The positive electrode 6 can be produced by preparing a positive electrode mixture by blending the above-described positive electrode active material with a conductive agent, a binder, or the like, if necessary, and crimping this to a current collector. As the current collector, stainless mesh, aluminum foil, or the like can be used. As the conductive agent, acetylene black, ketjen black, or 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. Blended.

  As the counter electrode with respect to the positive electrode, known materials that function as a negative electrode and are capable of occluding and releasing lithium can be used. As materials thereof, metallic materials such as metallic lithium and lithium alloys, graphite, MCMB (mesocarbon micro) Carbon-based materials such as beads).

  A well-known battery element can be employ | adopted for a separator, a battery container, etc. Known electrolytes and solid electrolytes can also be used as the electrolyte. For example, as an electrolytic solution, an electrolyte such as lithium perchlorate or lithium hexafluorophosphate is used in a solvent such as ethylene carbonate (EC), dimethyl carbonate (DMC), propylene carbonate (PC), or diethyl carbonate (DEC). What was dissolved can be used. In an all solid state secondary battery, as an electrolyte, for example, a polymer solid electrolyte such as a polyethylene oxide polymer compound, a polymer compound containing at least one of a polyorganosiloxane chain or a polyoxyalkylene chain, and a sulfide. A solid electrolyte, an oxide 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 rapid voltage drop during discharge. For this reason, in a region where the voltage does not drop rapidly during discharging, the charge rate when the secondary battery is mounted can be detected easily 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 using an ICP emission spectroscopic analyzer (ICPS-8000, manufactured by Shimadzu Corporation). Powder X-ray diffraction was measured by D8 ADVANCE manufactured by Bruker.

[Synthesis of starting materials]
Sodium acetate (CH ₃ COONa) powder having a purity of 98.5% or more, nickel hydroxide (II) (Ni (OH) ₂ ) powder having a purity of 99.9% or more, manganese oxide having a purity of 99.9% or more ( III) (Mn ₂ O ₃ ) was weighed so that the molar ratio Na: Ni: Mn = 0.687: 0.333: 0.667. These were dispersed in ethanol in a mortar, mixed, pelletized, and filled into a JIS standard gold crucible. Firing was performed 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 is appropriate in the chemical formula of Na _2/3 Ni _1/3 Mn _2/3 O ₂ Was confirmed. From the diffraction pattern of the X-ray diffraction pattern (see FIG. 9), it was confirmed that the obtained sample had a rhombohedral system and a layered rock salt structure of the space group R3m. The lattice constants determined by the least square method are a = 2.8865Å (error: within 0.0001Å), c = 16.781Å (error: within 0.001Å), and a 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)).

[Comparative example]
<Ion exchange>
A starting material Na _0.67 Ni _0.33 Mn _0.67 O ₂ polycrystal obtained above, lithium nitrate (LiNO ₃ ) powder with a purity of 99% or more and lithium chloride (LiCl) with a purity of 99% or more. The mixture (molar ratio 88:12) was weighed so that Na _0.67 Ni _0.33 Mn _0.67 O ₂ : (LiNO ₃ + LiCl) = 1: 7 by weight. After mixing these in a mortar, they were filled in an alumina crucible and held in an electric furnace at 260 ° C. for 1 hour in an air atmosphere to carry out a lithium ion exchange treatment. Thereafter, it was washed with hot water and naturally 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. This ion exchanger was confirmed to be valid by the chemical formula of Li _2/3 Ni _1/3 Mn _2/3 O ₂ in which almost all of the sodium in the starting material was replaced with lithium. From the diffraction pattern of the powder X-ray diffraction measurement (FIG. 9), it was confirmed that the ion exchanger had a rhombohedral crystal system and a layered rock salt structure of the space group R-3m. The lattice constants are a = 2.666Å (error: within 0.0001Å) and c = 14.470Å (error: within 0.001Å), and the known Li _2/3 Ni _1/3 Mn ₂ having the 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. Heat treatment was performed by holding in an electric furnace at 500 ° C. for 5 hours under an air atmosphere. Thereafter, the temperature was returned to room temperature by standing 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 rhombohedral system and a layered rock salt structure of the space group R-3m, as before heat treatment. The lattice constants are a = 2.8855 mm (error: within 0.0002 mm) and c = 14.238 mm (error: within 0.002 mm), and the a-axis lattice constant is increased compared with that before the heat treatment. The lattice constant of was decreased.

[Examples 1 to 4 and Reference Example]
<Ion exchange>
A solution was prepared by dissolving lithium bromide (LiBr) powder having a purity of 99.9% or more in 15 g of dehydrated methanol having a purity of 99.8%. To this solution, 1.1 g of the starting material obtained above (polycrystalline Na _0.67 Ni _0.33 Mn _0.67 O ₂ ) was added. In preparing the LiBr methanol solution, the concentration of LiBr 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). (A = 0.4 in Example 2, A = 0.8 in Example 2, A = 1.6 in Examples 3 and Reference Examples, and A = 2.0 in Example 4).

Next, the mixture was heated to reflux at 110 ° C. for 5 hours using a Dimroth cooler, and lithium ion exchange treatment was performed. Then, it wash | cleaned with methanol and air-dried and obtained the ion exchanger of the composition shown in Table 1. All 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 , where sodium in the starting material is replaced with lithium and part of the 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. Heat treatment was performed by holding in an electric furnace at 500 ° C. for 5 hours under an oxygen atmosphere. Thereafter, the temperature was returned to room temperature by standing in the furnace to obtain a heat-treated body. Each of the obtained heat-treated bodies had the same composition as the ion exchanger before the heat treatment. A sample obtained by performing the above ion exchange at A = 1.6 and carrying out the heat treatment was used as a reference example.

<Lithium insertion>
A solution was prepared by dissolving lithium iodide (LiI) powder having a purity of 99.9% or more in 15 g of acetonitrile having a purity of 99.5%. To this solution, 1.0 g of the heat-treated body obtained above was added. When preparing an 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 LiI were in a molar ratio of 1: 2. did. Subsequently, the lithium ion insertion process was implemented by heating to reflux at 140 ° C. for 10 hours using a Dimroth cooler. Then, it wash | cleaned with methanol and air-dried and obtained the lithium insertion body of the composition shown in Table 1.

[Evaluation]
< ⁶ Li-NMR>
Under the following conditions, ⁶ Li solid 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 inserting lithium were measured. . Chemical shifts were recorded as relative values to 1.0 M ⁶ LiCl aqueous solution.
Measuring device: Bruker AVANCE 300 ( ⁶ Li resonance frequency: 44 MHz)
Temperature: Room temperature Rotation speed: 50 kHz
Pulse width: 3.6 μs (π / 2 pulse)
Pulse sequence: rotor-synchronized spin-echo pulse sequence

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

  As shown in Table 1, the composition 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 lithium salt used during ion exchange increased. Correspondingly, a difference is also observed in the X-ray diffraction patterns (FIGS. 9 and 10), and the intermediate structure (O3 structure and P3 in the vicinity of 2θ (° / CuKα) = 17 ° increases as the amount of residual sodium increases. The peak of the structure which is not one of the structures was large.

In the lithium inserts in which lithium ions were chemically inserted into the heat-treated body, the residual sodium amount in the lithium inserts tended to increase as the amount of residual sodium in the ion exchanger and heat-treated body increased, but the difference was slight. It was. In addition, no clear tendency was observed in 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 all of the lithium composite oxides of Examples 1 to 4 were of the comparative example. A diffraction pattern equivalent to that of the heat-treated body (HT-O3-Li _0.67 Ni _0.33 Mn _0.67 O ₂ ) was exhibited.

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 sharp peak was observed in the vicinity of 725 ppm as compared with Examples 1 to 4, whereas in the lithium inserts of Examples 1 to 4, the main resonance in the vicinity of 500 to 1000 ppm was observed. The peak has 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 ⁶ Li-NMR spectra 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. In Examples 1 to 3, the main resonance peak was separated into two peaks, and the peak area on the low magnetic field side tended to increase as the amount of sodium in the heat-treated body before lithium insertion increased.

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 peaks to become broader. The sintered body of Example 4 (before lithium insertion) with a small amount of residual sodium had a peak maximum in the vicinity of 750 ppm. On the other hand, the sintered body of Example 1 with a small amount of residual sodium had a peak maximum near 560 ppm, and the peak maximum was shifted by a high magnetic field.

When the residual amount of sodium in the heat-treated bodies was different, no significant difference was observed in the X-ray diffraction after lithium insertion, whereas a difference was observed in the ⁶ Li-NMR peak shape. This is considered that the influence of the coordination state of oxygen with respect to lithium is large. That is, as shown in FIG. 10, due to the difference in the amount of sodium remaining in the heat-treated body before lithium insertion, the ratio of the O3 structure, the P3 structure and the intermediate structure is different, and accordingly, ⁶ Li as shown in FIG. -There are differences in NMR peak shape and chemical shift. The structure in the sintered body before lithium insertion and the coordination state of oxygen to lithium are different, so the site where lithium is inserted by chemical insertion and the coordination state of oxygen to the inserted lithium are different. ^It is thought that the difference of the peak shape of ⁶ Li-NMR was brought about.

[Production 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 (heat treated body) obtained in Reference Example and Comparative Example 1 was used as a positive electrode active material, and acetylene black as a conductive agent. Then, tetrafluoroethylene as a binder was mixed at 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 in 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 produced according to a known cell configuration / assembly method.

  Each of the fabricated lithium secondary batteries was subjected to a charge / discharge test (electrochemical lithium insertion / desorption test) 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 charge / discharge characteristics were evaluated. The charge / discharge test was started from charging (lithium desorption).

  Example 2 of lithium secondary ion battery using positive electrode active material of lithium insert of Example 3 (Example), heat-treated body before lithium insertion of Example 3 (Reference Example), and heat-treated body of Comparative Example 1 (Comparative Example) The charge / discharge test results are shown in FIG. Moreover, the charging / discharging test result of the lithium secondary ion battery which used the lithium insertion body of Examples 1-4 for the positive electrode active material is shown in FIG.

  As shown in FIG. 12, although the lithium secondary battery using the lithium composite oxide of the comparative example had a high initial discharge capacity, a rapid voltage drop occurred in the vicinity of the discharge curve capacity of 70 to 90 mAh / g. In the lithium secondary battery using the composite oxide before lithium insertion, the rapid voltage drop in the discharge curve was suppressed, but the initial discharge capacity was slightly smaller than the comparative example, and the initial charge capacity was significantly smaller. 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 rapid voltage drop was suppressed. Further, in the examples, the initial discharge capacity was significantly increased as compared with the comparative example and the reference example.

  As shown in FIG. 13, the lithium secondary battery using the lithium composite oxide other than Example 3 also had a high initial charge capacity with a voltage drop suppressed 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 is easy to detect the charging rate and has a high capacity because the voltage drop is suppressed.

Claims (13)

It is represented by the composition formula Li _x Na _y Ni _1/3 Mn _2/3 O ₂ , where 0.7 ≦ x ≦ 0.9, 0 <y ≦ 0.05,
A lithium composite oxide having a maximum of ⁶ Li-NMR main resonance peak in a range of 650 to 900 ppm and a half width of the main resonance peak of 200 to 450 ppm. 2. 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 described. 3. The lithium composite oxide according to claim 1, wherein the ⁶ Li-NMR main resonance peak can be separated into two or more peaks by waveform analysis.   4. The lithium composite oxide according to claim 1, wherein a sodium amount y in the composition formula satisfies 0.001 ≦ y ≦ 0.02.   The lithium composite oxide according to any one of claims 1 to 4, wherein the crystal structure is a layered rock salt structure. A compound oxidation represented by the composition formula Li _z Na _{2 / 3-z} Ni _1/3 Mn _2/3 O ₂ , where 0.33 ≦ z ≦ 0.63 and the crystal structure is a layered rock-salt structure The lithium composite oxide according to any one of claims 1 to 5, which is obtained by chemically inserting lithium ions into a product. It is represented by the composition formula Li _y Na _x Ni _1/3 Mn _2/3 O ₂ , where 0.7 ≦ x ≦ 0.9 and 0 <y ≦ 0.05. There,
A compound oxidation represented by the composition formula Li _z Na _{2 / 3-z} Ni _1/3 Mn _2/3 O ₂ , where 0.33 ≦ z ≦ 0.63 and the crystal structure is a layered rock-salt structure A method for producing a lithium composite oxide, wherein lithium ions are chemically inserted into a product. By treating the composite oxide represented by the composition formula Li _z Na _{2 / 3-z} Ni _1/3 Mn _2/3 O ₂ in a temperature range of 20 ° C. to 200 ° C. in a lithium salt solution, The method for producing a lithium composite oxide according to claim 7, wherein chemical insertion of lithium ions is performed.   The method for producing a lithium composite oxide according to claim 8, wherein the lithium salt contains lithium iodide.   The method for producing a lithium composite oxide according to claim 8 or 9, wherein a solvent of the lithium salt solution contains acetonitrile.   The difference between the lithium amount x in the composition formula after chemical insertion of lithium ions and the lithium amount 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 according to any one of 10 to 10.   The positive electrode active material for secondary batteries which has as a main component the lithium complex oxide of any one of Claims 1-6. Including a positive electrode, a negative electrode, and an electrolyte;
A secondary battery, wherein the positive electrode contains the positive electrode active material according to claim 12.