JP7747974B2 - Positive electrode active material, its manufacturing method, and non-aqueous electrolyte secondary battery - Google Patents

Description

The present invention relates to a positive electrode active material, a method for producing the same, and a nonaqueous electrolyte secondary battery.

Non-aqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, can easily achieve high energy density. Taking advantage of these characteristics, non-aqueous electrolyte secondary batteries have recently been used in a wide range of applications, including small electronic devices such as mobile phones and laptop computers, and large electric drive systems such as electric and hybrid vehicles.

Positive electrode active materials for nonaqueous electrolyte secondary batteries include _{LiNi0.33Mn0.33Co0.33O2} (commonly known as NMC111), which contains Li (lithium), Ni (nickel), _Mn (manganese), and Co (cobalt), _{and LiNi0.8Co0.15Al0.05O2} (commonly known as NCA), _which contains Li, Ni, Co _, and _Al (aluminum). However, there is a strong demand _for positive electrode active materials with even higher discharge capacities than these positive electrode active materials. Furthermore, because these positive electrode active materials contain Co, a rare element, there is a relatively high risk of raw material costs rising due to various reasons, such as changes in supply and demand.

As an example of a Co-free positive electrode active material, Patent Document 1 describes a positive electrode active material for a non-aqueous electrolyte secondary battery, which is composed of a lithium transition metal composite oxide represented by the general formula Li _a Ni _b Mn _1-b W _c O ₂ (where 1<a<1.2, 0.5≦b≦0.7, 0<c≦0.02).

JP 2012-43637 A

If the positive and negative electrodes of a non-aqueous electrolyte secondary battery are short-circuited for some reason, the temperature of the non-aqueous electrolyte secondary battery may rise due to Joule heat. Furthermore, an excessive rise in the temperature of a non-aqueous electrolyte secondary battery may lead to decomposition of the positive electrode active material. From this perspective, there is a demand for a positive electrode active material with even higher thermal stability than the positive electrode active material described in Patent Document 1.

The present invention was made in light of this background, and aims to provide a positive electrode active material that can reduce raw material costs, has a high discharge capacity, and is excellent in thermal stability and output characteristics, a method for producing the same, and a nonaqueous electrolyte secondary battery that uses this positive electrode active material.

One aspect of the present invention is a positive electrode active material for use in a nonaqueous electrolyte secondary battery, comprising:
It has a composition represented by a composition formula of Li _a Ni _b Mn _c W _d Mg _e O _f (wherein a to f in the composition formula satisfy the relationships 0.9<a<1.2, 0.45<b<0.6, 0.25<c<0.5, 0<d<0.06, 0<e<0.1, 1.9<f<2.1),
The positive electrode active material has a layered rock salt type crystal structure that can be assigned to the space group R-3m.

Another aspect of the present invention is a non-aqueous electrolyte secondary battery having a positive electrode, a negative electrode, and a non-aqueous electrolyte,
The positive electrode is a non-aqueous electrolyte secondary battery containing the positive electrode active material of the above embodiment.

The positive electrode active material has the specific composition and crystalline structure. Because the positive electrode active material does not contain Co, the risk of rising raw material costs can be reduced. Furthermore, in the positive electrode active material, some of the Mn in the Li-Ni-Mn-based positive electrode active material is replaced with W and Mg. In this way, by adding W and Mg to the Li-Ni-Mn-based positive electrode active material, it is possible to improve the thermal stability of the positive electrode active material while ensuring high discharge capacity and excellent output characteristics.

As a result, the positive electrode active material has a high discharge capacity, excellent thermal stability and output characteristics, and can easily reduce raw material costs.

Furthermore, the nonaqueous electrolyte secondary battery of the above embodiment has a positive electrode containing the specific positive electrode active material. Therefore, the nonaqueous electrolyte secondary battery can easily reduce raw material costs, has a high discharge capacity, and is excellent in thermal stability and output characteristics.

FIG. 1 is a development view showing the internal structure of a secondary battery according to an embodiment of the present invention.

(Cathode active material)
The positive electrode active material has a composition represented by a composition formula of Li _a Ni _b Mn _c W _d Mg _e O _f (wherein a to f in the composition formula satisfy the relationships 0.9<a<1.2, 0.45<b<0.6, 0.25<c<0.5, 0<d<0.06, 0<e<0.1, and 1.9<f<2.1).

The value of a in the composition formula, i.e., the molar ratio of Li to the total number of moles of Li, Ni, Mn, W, Mg, and O, is greater than 0.9 and less than 1.2. This makes it easier to maintain the crystalline structure of the crystalline phase during charge and discharge, and sufficient diffusion paths for lithium ions can be formed within the positive electrode active material. As a result, the discharge capacity and output characteristics of the positive electrode active material can be improved.

If the value of a in the composition formula is 0.9 or less, a phenomenon known as cation mixing, in which lithium ion sites are replaced by transition metal ions in the crystal lattice of the positive electrode active material, is likely to occur. Therefore, in this case, it becomes difficult to form sufficient diffusion paths for lithium ions in the positive electrode active material, which may result in a decrease in discharge capacity and output characteristics. Furthermore, if the value of a in the composition formula is 1.2 or more, a lithium-excess layered rock-salt crystal structure belonging to the space group C2/m is likely to form in the positive electrode active material, which may result in a decrease in output characteristics.

The value of d in the composition formula, i.e., the molar ratio of W to the total number of moles of Li, Ni, Mn, W, Mg, and O, is greater than 0 and less than 0.06. Furthermore, the value of e in the composition formula, i.e., the molar ratio of Mg to the total number of moles of Li, Ni, Mn, W, Mg, and O, is greater than 0 and less than 0.1. By adding both W and Mg to the positive electrode active material and setting the contents of these elements within the specified ranges, it is possible to improve the thermal stability of the positive electrode active material without impairing the discharge capacity and output characteristics.

If the value of d in the composition formula is 0, that is, if, for example, only Mg is added to the positive electrode active material, this may result in a decrease in discharge capacity and output characteristics. From the perspective of more reliably obtaining the effects of using W and Mg in combination, the value of d in the composition formula is preferably 0.005 or greater, and more preferably 0.01 or greater.

If the value of d in the composition formula is 0.06 or greater, a crystalline phase having a crystalline structure other than the specific layered rock salt crystalline structure is likely to form in the positive electrode active material, which may result in a decrease in discharge capacity and thermal stability.

Furthermore, if the value of e in the composition formula is 0, that is, if, for example, only W is added to the positive electrode active material out of W and Mg, the thermal stability of the positive electrode active material may be reduced. From the perspective of more reliably obtaining the effects of the combined use of W and Mg, the value of e in the composition formula is preferably 0.005 or greater, and more preferably 0.01 or greater.

On the other hand, if the value of e in the composition formula is 0.1 or greater, a crystalline phase having a crystalline structure other than the specific layered rock salt crystalline structure is more likely to form in the positive electrode active material, which may result in a decrease in discharge capacity.

The values of b, c, and f in the composition formula, i.e., the molar ratio of Ni, Mn, or O to the total number of moles of Li, Ni, Mn, W, Mg, and O, are each within a range that satisfies the above relationship. If the values of b, c, and f in the composition formula do not satisfy the above relationship, a crystalline phase having a crystalline structure other than the specific layered rock salt-type crystalline structure is likely to form in the positive electrode active material, which may result in a decrease in discharge capacity.

From the viewpoint of further increasing discharge capacity while ensuring excellent thermal stability and output characteristics, the value of b in the composition formula is preferably greater than 0.5 and less than 0.6. That is, the positive electrode active material preferably has a composition represented by a composition formula of Li _a Ni _b Mn _c W _d Mg _e O _f (wherein a to f in the composition formula satisfy the relationships 0.9<a<1.2, 0.5<b<0.6, 0.25<c<0.5, 0<d<0.06, 0<e<0.1, and 1.9<f<2.1).

From the same viewpoint, the value of c in the composition formula is preferably greater than 0.25 and less than 0.42. In other words, the positive electrode active material preferably has a composition represented by a composition formula of Li _a Ni _b Mn _c W _d Mg _e O _f (wherein a to f in the composition formula satisfy the relationships 0.9<a<1.2, 0.45<b<0.6, 0.25<c<0.42, 0<d<0.06, 0<e<0.1, and 1.9<f<2.1). The positive electrode active material more preferably has a composition represented by the formula Li _a Ni _b Mn _c W _d Mg _e O _f (wherein a to f in the formula satisfy the relationships 0.9<a<1.2, 0.5<b<0.6, 0.25<c<0.42, 0<d<0.06, 0<e<0.1, and 1.9<f<2.1), and even more preferably has a composition represented by the formula Li _a Ni _b Mn _c W _d Mg _e O _f (wherein a to f in the formula satisfy the relationships 0.9<a<1.2, 0.5<b<0.6, 0.25<c<0.42, 0.01<d<0.06, 0<e<0.08, and 1.9<f<2.1).

The length of the a-axis in the crystal lattice of the positive electrode active material is preferably 2.880 Å or more and 2.900 Å or less, and the length of the c-axis is preferably 14.290 Å or more and 14.360 Å or less. In this case, a positive electrode active material with excellent thermal stability and output characteristics can be more reliably obtained. From the same viewpoint, the positive electrode active material preferably has a crystal structure in which the ratio w 64 /w 44 of the half-width w ₆₄ of the peak appearing in a diffraction angle range of 64.5±1° to the half-width w ₄₄ of the peak appearing in a diffraction angle range of _44.4 ± ₁ ° in an X-ray diffraction pattern obtained by the θ-2θ method is 1.10 or more and 1.30 or less.

The positive electrode active material may contain a crystalline phase having a crystalline structure other than the specific layered rock salt crystal structure, as long as the above-mentioned effects are not impaired. The content of the crystalline phase having a crystalline structure other than the specific layered rock salt crystal structure is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 90% by mass or less.

The content of the crystalline phases in the positive electrode active material can be calculated based on the X-ray diffraction pattern obtained by powder X-ray diffraction. More specifically, the scanning range of the diffraction angle 2θ in powder X-ray diffraction is 10° to 90°, the scanning speed is 2°/min, the sampling width is 0.02°, and the X-ray source is CuKα radiation. By performing Rietveld analysis based on the X-ray diffraction pattern obtained under these measurement conditions, it is possible to identify the type of crystalline phase in the positive electrode active material and calculate the content of each crystalline phase.

The positive electrode active material is typically used in powder form to shorten the diffusion distance of lithium ions in the positive electrode active material. It is preferable that the positive electrode active material be composed of particles with a particle diameter of 10 μm or less. That is, the positive electrode active material may be composed of, for example, primary particles having a particle diameter distribution with a maximum particle diameter of 10 μm or less, or may be secondary particles formed by aggregation of primary particles and having a particle diameter distribution with a maximum particle diameter of 10 μm or less. In this case, the diffusion distance of lithium ions in the positive electrode active material can be shortened, and the internal resistance of the non-aqueous electrolyte secondary battery can be more easily reduced. To shorten the diffusion distance of lithium ions in the positive electrode active material, it is preferable that the positive electrode active material be composed of primary particles with a particle diameter of 1 μm or less.

The particle size of the primary particles of the positive electrode active material described above can be measured based on a magnified photograph obtained by observing the positive electrode active material using a scanning electron microscope (SEM). More specifically, 50 or more particles of the positive electrode active material are randomly selected from the magnified photograph observed with the SEM, and the circumscribing circle for each particle is determined, and the diameter is taken as the particle size of the individual particle. The maximum value of these particle diameters is then taken as the particle size of the positive electrode active material. The particle size of the secondary particles of the positive electrode active material described above can be measured using a laser scattering particle size distribution analyzer.

(Method for producing positive electrode active material)
The positive electrode active material can be produced by, for example, a solid-phase method. Specifically, a mixture containing a Li source compound, a Ni source compound, a Mn source compound, a W source compound, and a Mg source compound is prepared, and then the mixture is fired in an oxidizing gas atmosphere. The ratio of each compound in the mixture can be appropriately set depending on the composition formula of the desired crystalline phase in the positive electrode active material.

The mixture is preferably in the form of a powder. By forming the mixture into a powder, the raw material compounds can be uniformly dispersed within the mixture. As a result, the compositional bias of the fired body after firing can be further reduced, and the content of the crystalline phase can be further increased.

When preparing a powdered mixture, the raw material compounds may be pulverized as needed. Pulverization methods that can be used include manual pulverization in a mortar and mortar, and mechanical pulverization using a ball mill, etc.

When preparing a powdered mixture, the particle size of the mixture may be adjusted by classifying the raw material compounds using a sieve or the like. By using these methods to make the particle size of the mixture as small as possible, the raw material compounds can be dispersed more uniformly. As a result, the compositional bias of the fired body after firing can be reduced and the content of the crystalline phase can be increased.

After preparing the mixture, the mixture can be fired in an oxidizing gas atmosphere to obtain a fired positive electrode active material containing the specific crystalline phase. The oxidizing gas can be, for example, air. The firing temperature can be set appropriately from a range of 800 to 1100°C. The holding time during firing can be set appropriately from a range of 0.5 to 50 hours.

After the firing is complete, the resulting fired body can be crushed to obtain powdered positive electrode active material. The method for crushing the fired body is not particularly limited. For example, various methods can be used, such as manual crushing in a mortar or mechanical crushing using a ball mill or the like. After crushing the fired body, the powder can be classified as needed to adjust the particle size of the positive electrode active material.

Coprecipitation may also be used as a method for producing the positive electrode active material. When producing the positive electrode active material by coprecipitation, for example, an alkaline reaction solution, a raw material solution containing ions of two or more metal elements excluding Li among the metal elements constituting the positive electrode active material, and an alkaline pH-adjusting solution are prepared. The raw material solution containing the ions is added dropwise to the reaction solution, and at the same time, a pH-adjusting solution is added dropwise to adjust the pH of the reaction solution after the raw material solution has been added dropwise. This causes a precursor containing the aforementioned metal element to precipitate in the reaction solution. The concentration of each ion in the raw material solution may be appropriately set depending on the composition formula of the desired crystalline phase in the positive electrode active material.

Next, the precursor is mixed with a Li source compound and other raw materials, if necessary, to produce a mixture. The mixture is fired in an oxidizing gas atmosphere to obtain a fired cathode active material containing the specific crystalline phase. The oxidizing gas can be, for example, air. The firing temperature can be set appropriately within the range of 800 to 1100°C. The firing holding time can be set appropriately within the range of 0.5 to 50 hours.

After the firing is complete, the resulting fired body can be crushed to obtain powdered positive electrode active material. The method for crushing the fired body is not particularly limited. For example, various methods can be used, such as manual crushing in a mortar or mechanical crushing using a ball mill or the like. After crushing the fired body, the powder can be classified as needed to adjust the particle size of the positive electrode active material.

In the method for producing the positive electrode active material, a precursor containing all metal elements in the composition except for Li is prepared by a coprecipitation method, and the precursor is mixed with a compound containing Li to prepare a mixture;
The positive electrode active material is preferably obtained by firing the mixture in an oxidizing gas atmosphere.

In this way, by using the coprecipitation method to prepare a precursor containing all metal elements except Li, it is possible to achieve a more uniform distribution of each metal element in the precursor. Then, by mixing this precursor with a Li source compound and then firing the mixture, a more uniform positive electrode active material can be obtained. Furthermore, this method can improve the discharge capacity, output characteristics, and thermal stability of the positive electrode active material in a more balanced manner.

When preparing a precursor containing all metal elements except Li using the coprecipitation method, one or more types of raw material solutions may be added dropwise to the reaction solution. However, depending on the combination of compounds dissolved in the raw material solutions, some of the compounds may be less soluble in the raw material solutions, making it difficult to adjust the ratio of metal elements in the precursor to the desired range. To more reliably avoid such problems, it is preferable to prepare two or more types of raw material solutions. By preparing two or more types of raw material solutions, compounds containing metal elements can be easily dissolved in each raw material solution. As a result, it is easier to adjust the ratio of metal elements in the precursor to the desired range.

When two or more kinds of raw material solutions are prepared, for example, one of the raw material solutions may be dropped into the other raw material solution, and then the other raw material solution may be dropped into the reaction solution. More specifically, when producing the precursor, an alkaline reaction solution, a first raw material solution containing W, and a second raw material solution containing metal elements other than Li and W in the composition are prepared,
adding the first raw material solution dropwise to the second raw material solution to prepare a mixed solution;
Thereafter, the mixed solution can be added dropwise to the reaction solution to prepare the precursor.

In addition, for example, all of the raw material solutions may be dropped into the reaction solution at the same time. More specifically, when producing the precursor, an alkaline reaction solution, a first raw material solution containing W, and a second raw material solution containing metal elements other than Li and W in the composition are prepared,
The precursor can also be prepared by simultaneously dropping the first raw material solution and the second raw material solution into the reaction solution.

Furthermore, in addition to the raw material solution and the reaction solution, an alkaline pH-adjusted solution may be prepared, and the pH-adjusted solution may be added dropwise to the reaction solution together with the first raw material solution, the second raw material solution, or the mixed solution to produce the precursor. In this case, changes in the pH of the reaction solution caused by the addition of the raw material solution can be suppressed, making it possible to more reliably produce a precursor having the desired composition.

(Nonaqueous electrolyte secondary battery)
The positive electrode active material is used in non-aqueous electrolyte secondary batteries, and is particularly suitable for lithium-ion secondary batteries. The secondary battery can include, as its main components, a positive electrode containing the positive electrode active material, a negative electrode, a separator, a non-aqueous electrolyte, an additive, and a case for accommodating these components. The secondary battery can be, for example, coin-shaped, cylindrical, stacked, or rectangular.

The positive electrode of a secondary battery has a positive electrode active material and a positive electrode current collector that holds the positive electrode active material. Various conductors can be used as the positive electrode current collector, including metal foils such as copper foil, stainless steel mesh, aluminum foil, and nickel foil, as well as punched metal, expanded metal, and metal mesh.

The positive electrode may contain a binder interposed between the positive electrode active material and the positive electrode current collector. Examples of binders that can be used include fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluororubber, and thermoplastic resins such as polypropylene, polyethylene, and polyethylene terephthalate.

The positive electrode may also contain a conductive agent or conductive aid to enhance electrical conductivity. Examples of conductive agents that can be used include graphite, carbon black, acetylene black, and cokes.

The positive electrode can be produced, for example, by the following method. First, a paste-like positive electrode mixture containing the positive electrode active material is produced. The positive electrode mixture may contain an organic solvent for dispersing or dissolving solid components such as the positive electrode active material, as needed. Next, the positive electrode mixture is applied to the surface of a positive electrode current collector and then dried to form a positive electrode active material layer on the surface of the positive electrode current collector. After the positive electrode active material layer is formed, it may be pressed, if necessary, to increase the density of the positive electrode active material layer. This completes the process of producing a positive electrode.

The negative electrode of the secondary battery includes a negative electrode active material and a negative electrode current collector that holds the negative electrode active material. Examples of the negative electrode active material include carbon materials with a _graphite structure, such as graphite and hard carbon, _and lithium-based oxides, such as lithium titanate ( _Li4Ti5O12 ). The negative electrode current collector can be made of the same conductor as the positive electrode current collector.

The negative electrode, like the positive electrode, may contain a binder, conductive agent, and conductive additive. The binders, conductive agents, and conductive additives that can be used in the negative electrode are the same as those in the positive electrode.

The method for producing the negative electrode is the same as for the positive electrode. That is, first, a paste-like negative electrode mixture containing the negative electrode active material is produced. The negative electrode mixture may contain an organic solvent for dispersing or dissolving solid components such as the negative electrode active material, as needed. Next, the negative electrode mixture is applied to the surface of the negative electrode current collector and then dried to form a negative electrode active material layer on the surface of the negative electrode current collector. After forming the negative electrode active material layer, the negative electrode active material layer may be pressed, as needed, to increase the density of the negative electrode active material layer. This completes the process of producing the negative electrode.

The non-aqueous electrolyte solution may contain an organic solvent and an electrolyte made of a lithium salt. Examples of the lithium salt include _LiPF6 . The organic solvent may be at least one selected from the group consisting of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate. These organic solvents have high polarity and can dissolve large amounts of electrolyte. Therefore, by using these organic solvents as the non-aqueous electrolyte solution, the transport number of charge carriers, such as lithium ions, in the secondary battery can be easily increased.

Examples of the positive electrode active material and a nonaqueous electrolyte secondary battery including the positive electrode active material will now be described. The positive electrode active material in this example has a composition represented by the formula Li _a Ni _b Mn _c W _d Mg _e O _f (where a to f in the formula satisfy the relationships 0.9<a<1.2, 0.45<b<0.6, 0.25<c<0.5, 0<d<0.06, 0<e<0.1, and 1.9<f<2.1). The positive electrode active material also has a layered rock salt crystal structure that can be assigned to the space group R-3m.

The positive electrode active materials in this example (test materials S1 to S9) specifically have the compositions shown in Table 1. Note that test materials R1 to R8 in Table 1 are positive electrode active materials for comparison with test materials S1 to S9. The method for producing the positive electrode active materials in this example is described below.

(Test material S1)
200 mL of distilled water was placed in a 2 L reaction vessel, and sodium hydroxide was dissolved therein to prepare a reaction solution with a pH of 12. Separately from this reaction solution, a raw material solution was prepared by dissolving nickel sulfate and manganese sulfate in distilled water, and a pH-adjusted solution was prepared by mixing an aqueous sodium hydroxide solution with aqueous ammonia.

Next, the reaction solution was heated to 50°C. While maintaining the reaction solution temperature, the raw material solution and pH-adjusted solution were simultaneously added dropwise to the reaction solution while stirring, thereby precipitating a precursor containing Ni and Mn in the reaction solution.

The precursor thus obtained was mixed with LiOH as a Li source, _WO3 as a W source, and MgO as an Mg source to prepare a mixture. This mixture was fired in air at 950°C for 5 hours and then annealed at 700°C for 12 hours to obtain a lump of positive electrode active material. The lump of positive electrode active material was crushed using a mortar to obtain a powdered positive electrode active material (test material S1). The positive electrode active material prepared by the above-mentioned method is indicated by the symbol "M1" in the "Preparation Method" column of Table 1.

(Test materials R1 to R3)
Test materials R1 to R3 have the same structure as test material S1, except that they do not contain at least one of the elements W and Mg. The method for producing test materials R1 to R3 is the same as the method for producing test material S1, except that _WO3 and/or MgO is not added to the precursor.

(Test materials S2 to S9)
200 mL of distilled water was placed in a 2 L reaction vessel, and sodium hydroxide was dissolved therein to prepare a reaction solution with a pH of 12. Separately from this reaction solution, a first raw material solution was prepared by dissolving ammonium tungstate in distilled water, a second raw material solution was prepared by dissolving nickel sulfate, manganese sulfate, and magnesium sulfate in distilled water, and a pH-adjusted solution was prepared by mixing an aqueous sodium hydroxide solution and aqueous ammonia.

Next, the reaction solution was heated to 50°C. While maintaining the reaction solution temperature, the first raw material solution, the second raw material solution, and the pH adjustment solution were simultaneously added dropwise to the reaction solution while stirring, thereby precipitating a precursor containing Ni, Mn, W, and Mg in the reaction solution.

Next, LiOH was mixed with the precursor as a Li source to prepare a mixture. This mixture was fired in air at 950°C for 5 hours, and then annealed at 700°C for 12 hours to obtain a lump of positive electrode active material. The lump of positive electrode active material was crushed using a mortar to obtain powdered positive electrode active material (test materials S2 to S9). Note that the positive electrode active material prepared by the above-mentioned method is indicated by the symbol "M2" in the "Preparation Method" column of Table 1.

(Test materials R4 to R8)
Test materials R4 to R8 have the same structure as test materials S2 to S9, except that they do not contain at least one of the elements W and Mg. The method for producing test materials R4 to R8 is the same as the method for producing test materials S2 to S9, except that _WO3 and/or MgO is not added to the precursor.

Next, the composition and crystal structure of test materials S1 to S9 and test materials R1 to S8 were identified.

Composition of Positive Electrode Active Material The molar ratio of each metal element in the positive electrode active material was measured by inductively coupled plasma atomic emission spectrometry (ICP-AES). The composition of each test material when the molar ratio of oxygen atoms was set to 2 was as shown in the "Composition of Positive Electrode Active Material" column in Table 1.

Crystal structure of the positive electrode active material: The crystal structure of the positive electrode active material was identified and its lattice constant was measured by powder X-ray diffraction. The X-ray diffraction apparatus used was a "SmartLab (registered trademark)" manufactured by Rigaku Corporation, and the characteristic X-rays used were CuKα rays. The scanning range of the diffraction angle 2θ was 10° to 90°, the scanning speed was 2°/min, and the sampling width was 0.02°.

The crystalline structures of the positive electrode active materials were identified based on the obtained X-ray diffraction patterns. Test materials S1 to S9 and test materials R1 to R8 all had a layered rock-salt crystalline structure that could be assigned to the space group R-3m. Table 1 shows the lengths of the a-axis and c-axis in the crystal lattice of each positive electrode active material. Table 2 also shows the content of the layered rock-salt crystalline structure in each positive electrode active material, and the ratio _w64 / _w44 of the half-width _w64 of the peak appearing in a diffraction angle range of _64.5 ±1° to the half-width w44 of the peak appearing in a diffraction angle range of 44.4±1°.

Next, a nonaqueous electrolyte secondary battery (test cell) was fabricated using the obtained positive electrode active material. As shown in Figure 1, the nonaqueous electrolyte secondary battery 1 of this example is a CR2032 coin battery including a positive electrode 2, a negative electrode 3, a separator 4, and a nonaqueous electrolyte 5.

More specifically, the secondary battery 1 has a cylindrical case 11 with a relatively small height and a bottom, and a top lid 12 that closes the opening of the case 11. A space is formed between the case 11 and the top lid 12. The top lid 12 is joined to the case 11 by crimping.

The space between the case 11 and the top lid 12 contains a positive electrode 2, a negative electrode 3, a separator 4, and a non-aqueous electrolyte 5. A rubber gasket 15 is disposed between the positive electrode 2 and the separator 4. A spacer 13 and a washer 14 are disposed between the top lid 12 and the positive electrode 2. The spacer 13 is disposed so as to abut against the positive electrode 2. The washer 14 is disposed between the spacer 13 and the top lid 12. In this example, the spacer 13 is specifically a disk-shaped stainless steel plate.

The positive electrode 2 has a positive electrode current collector 21 and a positive electrode active material layer 22 provided on the positive electrode current collector 21, the positive electrode active material layer 22 containing one of the positive electrode active materials shown in Table 1. In this example, the positive electrode current collector 21 is specifically a disk-shaped aluminum foil with a diameter of 16 mm. The negative electrode 3 is specifically a disk-shaped lithium metal foil.

The separator 4 is made of polypropylene and is interposed between the positive electrode 2 and the negative electrode 3. A nonaqueous electrolyte 5 is filled at least between the positive electrode 2 and the negative electrode 3 in the secondary battery 1. Specifically, the nonaqueous electrolyte 5 in this example is made of an ethylene carbonate-based organic solvent containing _LiPF6 at a concentration of 1 mol/L.

Next, the method for fabricating the secondary battery 1 will be described. First, a positive electrode active material, graphite as a conductive additive, and polyvinylidene fluoride as a binder were mixed in a mass ratio of positive electrode active material:conductive additive:binder = 85:10:5 to obtain a positive electrode mixture. This positive electrode mixture was applied to a positive electrode current collector 21 to form a positive electrode active material layer 22. The thickness of the positive electrode active material layer 22 immediately after application was approximately 220 μm.

Next, the positive electrode active material layer 22 on the positive electrode current collector 21 was pressed at a pressure of approximately 100 MPa. The positive electrode active material layer 22 and the positive electrode current collector 21 were then punched out into a disk with a diameter of 16 mm, thereby obtaining a positive electrode 2. Furthermore, a negative electrode 3 was produced by punching out a lithium foil into a disk shape.

Next, the negative electrode 3, separator 4, gasket 15, positive electrode 2, spacer 13, and washer 14 were stacked in this order inside the case 11, and the non-aqueous electrolyte 5 was poured into the case 11. After that, the top lid 12 was placed over the opening of the case 11 and crimped to seal the space between the case 11 and the top lid 12. In this way, the secondary battery 1 was obtained.

Next, the resulting battery was used to measure the discharge capacity and evaluate its output characteristics. A charge/discharge device (Bio-Logic's "BCS-815") was used to measure the discharge capacity and evaluate its output characteristics.

Discharge Capacity Measurement First, the secondary battery was charged in constant current mode at a temperature of 25°C. The current density in the constant current mode was 11 mA/g, and the end-of-charge voltage was 4.5 V. After charging was completed, the battery was discharged to 2.5 V at a constant current density of 11 mA/g, and a discharge curve was obtained. The discharge capacity of the secondary battery was calculated based on the discharge curve obtained in this manner. Table 2 shows the discharge capacity of the secondary battery using each positive electrode active material.

Evaluation of Output Characteristics After adjusting the open circuit voltage (i.e., OCV) of the secondary battery to 4.4 V, the secondary battery was discharged so that the discharge current was equivalent to 5 C (specifically, approximately 10 mA), and the voltage of the secondary battery was measured during discharge. The DC resistance of the secondary battery was calculated by dividing the potential difference between the voltage of the secondary battery 0.1 seconds after the start of discharge and the voltage of the secondary battery 10 seconds after the start of discharge by the discharge current. The DC resistance values of the secondary battery calculated in this manner are shown in the "DC Resistance" column of Table 2.

Evaluation of Thermal Stability First, the secondary battery was charged in a constant current-constant voltage mode at a temperature of 25°C. The current density in the constant current mode was 11 mA/g. The end-of-charge voltage in the constant voltage mode was 4.5 V, and an appropriate charging current was applied for 5 hours after the voltage of the secondary battery reached the end-of-charge voltage to maintain the end-of-charge voltage.

After charging was complete, the secondary battery was disassembled in a glove box filled with argon, and the positive electrode was removed. This positive electrode was washed with dimethyl carbonate and then dried in the glove box. The positive electrode active material layer was sampled from the dried positive electrode and subjected to differential scanning calorimetry. The conditions for differential scanning calorimetry were as follows:

Measurement start temperature: room temperature Measurement end temperature: 500°C
Temperature increase rate: 10°C/min

The peak temperatures of the exothermic peaks appearing in the DSC curve obtained by differential scanning calorimetry and the heat release values at those exothermic peaks are shown in Table 2.

As shown in Table 1, test materials S1 to S8 have compositions represented by the specific composition formula described above, and also have a layered rock-salt crystal structure represented by the space group R-3m. Therefore, as shown in Table 2, these test materials have excellent thermal stability and output characteristics.

In contrast, test material R1, which has a Ni content similar to that of test materials S1 to S8 but does not contain either W or Mg, has higher DC resistance and inferior output characteristics than test materials S1 to S8. Furthermore, test material R1 generates more heat than test materials S1 to S8 and has inferior thermal stability.

Test material R2, which has a Ni content similar to that of test materials S1 to S8 but does not contain Mg, has a lower exothermic peak temperature than test materials S1 to S8 and may decompose at a relatively low temperature. Furthermore, test material R2 generates a large amount of heat and has poor thermal stability.

Test material R4, which has a Ni content similar to that of test materials S1 to S8 but does not contain Mg, generates more heat and has poorer thermal stability than test materials S1 to S8.

Test materials R3 and R5, which have a Ni content similar to that of test materials S1 to S8 but do not contain W, have higher DC resistance and inferior output characteristics than test materials S1 to S8.

In addition, test material S9 has a composition represented by the specific composition formula and a layered rock-salt crystal structure represented by the space group R-3m. Therefore, test material S9 has excellent thermal stability and output characteristics.

In contrast, test material R6, which has a Ni content similar to that of test material S9 but does not contain either W or Mg, has higher DC resistance and inferior output characteristics than test material S9. Furthermore, test material R6 has a lower heat generation peak temperature than test material S9, and there is a risk of decomposition at a relatively low temperature. Furthermore, test material R6 generates a large amount of heat and has poor thermal stability.

Test material R7, which has a Ni content similar to that of test material S9 but does not contain Mg, generates more heat and has poorer thermal stability than test material S9.

Test material R8, which has a Ni content similar to that of test material S9 but does not contain W, has higher DC resistance and inferior output characteristics compared to test material S9.

As described above, by comparing test materials S1-S8 with test materials R1-R5, which have similar Ni contents, and test material S9 with test materials R6-R8, it can be seen that adding both W and Mg to a Li-Ni-Mn-based positive electrode active material can improve discharge capacity, output characteristics, and thermal stability in a balanced manner.

The specific configurations of the positive electrode active material and nonaqueous electrolyte secondary battery according to the present invention are not limited to those of the above-described examples, and can be modified as appropriate within the scope of the present invention.

1 Nonaqueous electrolyte secondary battery 2 Positive electrode 21 Positive electrode current collector 22 Positive electrode active material layer 3 Negative electrode 4 Separator 5 Nonaqueous electrolyte

Claims (6)

A positive electrode active material for use in a non-aqueous electrolyte secondary battery,
It has a composition represented by a composition formula of Li _a Ni _b Mn _c W _d Mg _e O _f (wherein a to f in the composition formula satisfy the relationships 0.9<a<1.2, 0.45<b<0.6, 0.25<c<0.5, 0<d<0.06, 0<e<0.1, 1.9<f<2.1),
A positive electrode active material having a layered rock salt crystal structure that can be assigned to space group R-3m. The positive electrode active material according to claim 1, wherein the length of the a-axis of the crystal lattice of the positive electrode active material is 2.880 Å or more and 2.900 Å or less, and the length of the c-axis is 14.290 Å or more and 14.360 Å or less. A non-aqueous electrolyte secondary battery having a positive electrode, a negative electrode, and a non-aqueous electrolyte,
A non-aqueous electrolyte secondary battery, wherein the positive electrode comprises the positive electrode active material according to claim 1 or 2. 3. The method for producing a positive electrode active material according to claim 1 or 2,
A precursor containing all metal elements in the composition except for Li is prepared by a coprecipitation method;
mixing the precursor with a compound containing Li to prepare a mixture;
The method for producing a positive electrode active material includes calcining the mixture in an oxidizing gas atmosphere to obtain the positive electrode active material. When preparing the precursor, an alkaline reaction solution, a first raw material solution containing W, and a second raw material solution containing metal elements other than Li and W in the composition are prepared;
The method for producing a positive electrode active material according to claim 4 , wherein the precursor is produced by simultaneously dropping the first raw material solution and the second raw material solution into the reaction solution. When preparing the precursor, a pH-adjusted solution exhibiting alkaline properties is further prepared,
The method for producing a positive electrode active material according to claim 5 , wherein the precursor is produced by adding the pH-adjusted solution dropwise to the reaction solution together with the first raw material solution and the second raw material solution.