Positive electrode active material, method for producing the same, and non-aqueous electrolyte secondary battery

Abstract

The present invention provides a positive electrode active material which has a main phase having an O2 type layered structure that can be attributed to the space group P63mc, and which has a composition represented by composition formula LiaNabMncMdO(2±α). (In the composition formula, M represents one or more additive elements which include at least Ni and are selected from the group consisting Ni, Al, Ti, Sn, Zr, Nb, W and Mo; and a-d and α satisfy 0.7 ≤ a ≤ 1.33, 0 < b < 0.1, 0.7 < c < 0.9, 0.9 < c + d < 1.1, 1.4 ≤ c / d ≤ 12 and 0 ≤ α ≤ 0.3.)

Description

Technical Field

[0001] The present invention relates to a positive electrode active material, a method for producing the same, and a non-aqueous electrolyte secondary battery.

Background Art

[0002] Non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries have excellent characteristics such as high energy density, having a high electromotive force while being small and lightweight. Utilizing these excellent characteristics, lithium-ion secondary batteries are used in a wide range of applications such as small electronic devices such as mobile phones and laptop computers, and large electric drive devices such as electric vehicles and hybrid vehicles.

[0003] For the positive electrode of a lithium-ion secondary battery, a positive electrode active material containing Co (cobalt) is often used from the viewpoint of enhancing various battery characteristics such as discharge capacity and discharge rate characteristics. For example, in Patent Document 1, a non-aqueous electrolyte secondary battery positive electrode active material including a Li2MnO3-LiMO2 solid solution having a specific crystal structure including an O2 structure and an O3 structure, and represented by the general formula Li x Na y [Li z1 Mn z2 M* (1-z1-z2) O (2±γ) {0.67 < x < 1.1, 0 < y < 0.1, 0 < z1 < 0.33, 0.5 < z2 < 0.95, 0 ≦ γ < 0.1, M* includes at least Ni and Co, two or more metal elements} is described.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In order to increase the discharge capacity of a secondary battery, it is desirable to increase the voltage during charging. However, the O3 structure contained in the positive electrode active material of Patent Document 1 may transition to a spinel structure when the voltage during charging is increased. Therefore, there is a risk that the charge-discharge characteristics may deteriorate when charging at a high voltage. In addition, Co used in the positive electrode active material of Patent Document 1 is an element with high industrial demand, but has a small reserve as a resource and is an expensive element. Therefore, a positive electrode active material that does not contain Co, has a high discharge capacity, and has excellent discharge rate characteristics is desired.

[0006] The present invention has been made in view of such a background, and provides a positive electrode active material that can be charged at a high voltage, does not contain Co, has a high discharge capacity, and has excellent discharge rate characteristics, a method for manufacturing the same, and a non-aqueous electrolyte secondary battery using this positive electrode active material.

Means for Solving the Problems

[0007] One aspect of the present invention is a positive electrode active material used in a non-aqueous electrolyte secondary battery, having an O2-type layered structure attributable to the space group P63mc as the main phase, Li a Na b Mn c M d O (2±α) and having a composition represented by the composition formula (where M in the composition formula represents one or more additive elements selected from the group consisting of Ni, Al, Ti, Sn, Zr, Nb, W, and Mo and containing at least Ni, and a to d and α satisfy 0.7 ≤ a ≤ 1.33, 0 < b < 0.1, 0.7 < c < 0.9, 0.9 < c + d < 1.1, 4 ≤ c / d ≤ 12, 0 ≤ α ≤ 0.3).

[0008] 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, wherein the positive electrode contains the positive electrode active material of the above aspect.

[0009] Still another aspect of the present invention is a method for manufacturing a positive electrode active material of the above aspect, comprising: producing a Na-doped precursor having a crystal phase with a P2-type layered structure attributable to the space group P63 / mmc and containing Na, Mn, and additive element M; and then substituting Na atoms in the Na-doped precursor with Li atoms by ion exchange. The method is for manufacturing a positive electrode active material.

Advantages of the Invention

[0010] The positive electrode active material has a composition represented by the specific composition formula. The main phase of the positive electrode active material is an O2-type layered structure attributable to the space group P63mc. Since the O2-type layered structure has a stable crystal structure even when charged at a high voltage, charging can be performed at a high voltage. Further, the positive electrode active material having the specific composition and crystal structure does not contain Co and has a high discharge capacity. Furthermore, the positive electrode active material has excellent discharge rate characteristics, and the difference between the discharge capacity when discharging at a high current density and the discharge capacity when discharging at a low current density can be made smaller.

[0011] The positive electrode active material is used for the positive electrode of the non-aqueous electrolyte secondary battery (hereinafter referred to as "secondary battery"). Therefore, the secondary battery can be charged at a high voltage, has a high discharge capacity, and has excellent discharge rate characteristics.

[0012] In the method for manufacturing the positive electrode active material, first, a Na-doped precursor having the specific crystal structure is produced. Then, the positive electrode active material can be easily produced by substituting Na (sodium) atoms in the Na-doped precursor with Li (lithium) atoms by ion exchange.

[0013] As described above, according to the above aspect, it is possible to provide a positive electrode active material that can be charged at a high voltage, does not contain Co, has a high discharge capacity, and has excellent discharge rate characteristics, a method for manufacturing the same, and a non-aqueous electrolyte secondary battery using the positive electrode active material.

Brief Description of the Drawings

[0014] [Figure 1] FIG. 1 is an explanatory diagram showing an example of an X-ray diffraction pattern of a Na-doped precursor used for the preparation of a positive electrode active material. [Figure 2] FIG. 2 is an explanatory diagram showing the X-ray diffraction patterns of active material S1 and active material R1 in Example 1. [Figure 3] FIG. 3 is an explanatory diagram showing the X-ray diffraction patterns of active material S5 and active material S8 in Example 1. [Figure 4] FIG. 4 is a developed view showing the internal structure of the secondary battery for evaluation in Example 1. [Figure 5] FIG. 5 is an explanatory diagram showing the relationship between the value of c / d and the discharge capacity in active materials S1 to S12 and active material R4 of Example 1. [Figure 6] FIG. 6 is an explanatory diagram showing the relationship between the value of c / d and the discharge capacity ratio in active materials S1 to S12 and active material R4 of Example 1. [Figure 7] FIG. 7 is an explanatory diagram showing the relationship between the Na content and the discharge capacity in active materials S1 to S12 and active material R4 of Example 1. [Figure 8] FIG. 8 is a cross-sectional view showing a main part of the all-solid-state secondary battery in Example 2.

MODE FOR CARRYING OUT THE INVENTION

[0015] (Positive electrode active material) The positive electrode active material contains Li a Na b Mn c M d O (2±α) and has a composition represented by the following compositional formula (where M in the compositional formula represents one or more additive elements selected from the group consisting of Ni, Al, Ti, Sn, Zr, Nb, W, and Mo and containing at least Ni, and a to d and α satisfy 0.7 ≦ a ≦ 1.33, 0 < b < 0.1, 0.7 < c < 0.9, 0.9 < c + d < 1.1, 4 ≦ c / d ≦ 12, and 0 ≦ α ≦ 0.3). Further, the positive electrode active material contains, as a main phase, an O2-type layered structure attributable to the space group P63mc.

[0016] A positive electrode active material having the aforementioned specific composition formula and whose main phase is an O2-type layered structure has a high discharge capacity. Furthermore, because the O2-type layered structure has high crystal structure stability, it is less likely to change to a crystal structure with poor charge-discharge characteristics, such as a spinel structure, even when charged at high voltages. Therefore, a positive electrode active material with an O2-type layered structure as the main phase can suppress the decrease in charge-discharge capacity due to changes in crystal structure, even when charged at higher charging voltages. Moreover, it can be expected that such a positive electrode active material can improve charge-discharge cycle characteristics.

[0017] The aforementioned "main phase" refers to the crystalline phase with the highest content among the crystalline phases contained in the positive electrode active material. The content of each crystalline phase in the positive electrode active material can be calculated, for example, based on the X-ray diffraction pattern obtained by powder X-ray diffraction.

[0018] The value of 'a' in the aforementioned compositional formula, that is, the ratio of the number of moles of Li to the total number of moles of Li (lithium), Na (sodium), Mn (manganese), additive element M, and O (oxygen), is between 0.7 and 1.33. This makes it easy to improve the discharge capacity of the positive electrode. If the value of 'a' in the aforementioned compositional formula is less than 0.7, the amount of Li atoms that act as charge carriers will be insufficient, which may lead to a decrease in discharge capacity. On the other hand, if the value of 'a' in the aforementioned compositional formula is greater than 1.33, the stability of the O2-type layered structure in the positive electrode active material decreases, and a crystalline phase having a crystalline structure other than the O2-type layered structure is more likely to form. As a result, a crystalline phase having a crystalline structure other than the O2-type layered structure may become the dominant phase.

[0019] The value of b in the composition formula of the positive electrode active material, that is, the ratio of the number of moles of Na to the total number of moles of Li, Na, Mn, and additive elements M and O, is greater than 0 and less than 0.1. By doping the positive electrode active material with a small amount of Na, the discharge capacity and discharge rate characteristics of the positive electrode active material can be easily improved. From the viewpoint of more reliably obtaining these effects, the value of b in the composition formula is preferably 0.001 or greater, more preferably 0.02 or greater, and even more preferably 0.03 or greater.

[0020] On the other hand, if the Na content in the positive electrode active material is excessively high, it may lead to a decrease in discharge capacity. This problem can be easily avoided by setting the value of b in the above composition formula to less than 0.1, preferably 0.095 or less, and more preferably 0.07 or less. From the viewpoint of further increasing the discharge capacity of the positive electrode active material, the value of b in the above composition formula is preferably 0.001 or more and 0.095 or less, more preferably 0.001 or more and 0.07 or less, and even more preferably 0.03 or more and 0.07 or less.

[0021] One possible reason why doping with Na produces such effects is as follows: + is Li + It has a larger ionic radius than [another element]. Therefore, it is thought that doping the positive electrode active material with a small amount of Na expands the diffusion paths of Li within the positive electrode active material. As a result, it is thought that the discharge capacity of the positive electrode active material can be improved as Li becomes easier to diffuse within the positive electrode active material.

[0022] The positive electrode active material contains one or more additive elements M. At least Ni (nickel) is included in the additive elements M. In addition to Ni, one or more elements from among Al (aluminum), Ti (titanium), Sn (tin), Zr (zirconium), Nb (niobium), W (tungsten), and Mo (molybdenum) may also be included in the additive elements M.

[0023] The value of c in the aforementioned compositional formula, that is, the ratio of the number of moles of Mn to the total number of moles of Li, Na, Mn, additive element M, and O, is greater than 0.7 and less than 0.9. Furthermore, the value of d in the aforementioned compositional formula, that is, the ratio of the number of moles of additive element M to the total number of moles of Li, Na, Mn, additive element M, and O, is set such that the sum of the values ​​of c and d in the aforementioned compositional formula is between 0.9 and 1.1, and the ratio of the value of c to the value of d, c / d, is between 4 and 12.

[0024] By setting the sum of the values ​​of c and d in the aforementioned compositional formula, and the ratio of the value of c to the value of d (c / d), to the aforementioned specific range, the stability of the O2-type layered structure in the positive electrode active material can be increased, and the main phase of the positive electrode active material can be made to be an O2-type layered structure. If at least one of the sum of the values ​​of c and d in the aforementioned compositional formula, and the ratio of the value of c to the value of d (c / d), is outside the aforementioned specific range, the stability of the O2-type layered structure in the positive electrode active material decreases, and a crystalline phase having a crystalline structure other than the O2-type layered structure is more likely to form. As a result, there is a risk that a crystalline phase having a crystalline structure other than the O2-type layered structure will become the main phase.

[0025] Furthermore, by setting the value of c in the composition formula to the specific range, a portion of the Mn in the crystal lattice of the O2-type layered structure can be replaced with the added element M. As a result, the discharge capacity of the positive electrode active material can be easily improved.

[0026] The reason why such effects can be obtained by substituting Mn is thought to be as follows: Since the aforementioned additive element M has a larger ionic radius than Mn, substituting a portion of the Mn in the positive electrode active material with additive element M can increase the length of the a-axis in the crystal lattice of the O2-type layered structure. When the length of the a-axis is increased in this way, it is thought that the diffusion of Li atoms between the lithium layer and the lithium-containing transition metal layer in the O2-type layered structure is promoted. As a result, Li becomes easier to diffuse within the positive electrode active material, and the discharge capacity of the positive electrode active material can be improved.

[0027] The ratio c / d of the value of c to the value of d in the above composition formula is preferably 5 or more and 10 or less. In this case, the discharge capacity of the positive electrode active material can be increased, the discharge rate characteristics can be improved, and the difference between the discharge capacity when discharged at a high current density and the discharge capacity when discharged at a low current density can be reduced.

[0028] It is more preferable that the positive electrode active material has a composition in which the value of b in the composition formula (i.e., the Na content) satisfies 0.001 ≤ b ≤ 0.07, and the ratio of the value of c to the value of d, c / d, is 5 or more and 10 or less. In this case, it is possible to improve the discharge capacity itself while obtaining the effect of improving the discharge rate characteristics described above. From the viewpoint of further enhancing these effects, it is even more preferable that the positive electrode active material has a composition in which the value of b in the composition formula satisfies 0.03 ≤ b ≤ 0.07, and the ratio of the value of c to the value of d, c / d, is 5 or more and 10 or less.

[0029] The value of 2±α in the above compositional formula, that is, the ratio of the number of moles of O to the total number of moles of Li, Na, Mn, additive element M, and O, is between 1.7 and 2.3. This increases the stability of the O2-type layered structure in the positive electrode active material, allowing the main phase of the positive electrode active material to be the O2-type layered structure. If the value of α in the above compositional formula exceeds 0.3, the stability of the O2-type layered structure in the positive electrode active material decreases, and a crystalline phase with a crystalline structure other than the O2-type layered structure is more likely to form. As a result, there is a risk that a crystalline phase with a crystalline structure other than the O2-type layered structure will become the main phase.

[0030] The positive electrode active material is usually used in powder form from the viewpoint of shortening the diffusion distance of lithium ions within the positive electrode active material. The positive electrode active material is preferably composed of particles with a particle diameter of 10 μm or less. That is, the positive electrode active material may be, for example, a powder having a particle size distribution where the maximum particle diameter is 10 μm or less, or it may be an aggregate of primary particles with a particle diameter of 10 μm or less. In this case, the diffusion distance of lithium ions within the positive electrode active material can be shortened, making it easier to reduce the internal resistance in a non-aqueous electrolyte secondary battery. From the viewpoint of shortening the diffusion distance of lithium ions within the positive electrode active material, the positive electrode active material is preferably composed of primary particles with a particle diameter of 1 μm or less.

[0031] Furthermore, the particle size of the positive electrode active material, as mentioned above, can be measured based on magnified images obtained by observing the positive electrode active material using a scanning electron microscope (SEM). More specifically, for 50 or more particles of the positive electrode active material randomly selected from the magnified images observed with the SEM, the circumscribed circle for each particle is determined, and its diameter is taken as the particle size of each individual particle. Then, the maximum of these particle sizes is taken as the particle size of the positive electrode active material.

[0032] (Method for manufacturing positive electrode active material) The positive electrode active material, for example, has a crystalline phase having a P2-type layered structure that can be assigned to the space group P63 / mmc, and a Na-doped precursor containing Na, Mn, and the additive element M is prepared. Subsequently, the Na atoms in the Na-doped precursor are replaced with Li atoms by ion exchange to obtain the Na-doped precursor.

[0033] One method for preparing the Na-doped precursor is the coprecipitation method. When preparing the positive electrode active material by the coprecipitation method, first, a first solution containing Mn ions, Ni ions, and ions of additive element M other than Ni, which may be added as needed, and an alkaline second solution are prepared. The first solution can be prepared, for example, by dissolving the aforementioned metal element's nitrate, sulfate, hydroxide, carbonate, etc., in water. The concentration of each ion in the first solution can be appropriately set according to the desired composition in the positive electrode active material. As the second solution, for example, an aqueous solution of a base such as sodium carbonate or sodium hydroxide can be used.

[0034] Next, the first and second solutions prepared in this manner are mixed. This results in the formation of a precipitate containing Mn, Ni, and the added element M in the mixture of the first and second solutions. For example, if an aqueous carbonate solution such as sodium carbonate is used as the second solution, a carbonate containing Mn, Ni, and the added element M will be formed in the mixture of the first and second solutions. When mixing the first and second solutions, an alkaline third solution can be added to the mixture of the first and second solutions as needed to adjust the rate of precipitate formation. For example, aqueous ammonia can be used as the third solution.

[0035] The precipitate is Mn e M f It is preferable that the composition is represented by the composition formula of CO3 (wherein M in the composition formula is selected from the group consisting of Ni, Al, Ti, Sn, Zr, Nb, W, and Mo, and represents one or more additive elements including at least Ni, and e and f satisfy e+f=1 and 4≦e / f≦12). By forming a precipitate with such a composition during the manufacturing process of the positive electrode active material, the content of the O2-type layered structure in the final positive electrode active material can be further increased.

[0036] Next, the precipitate formed in the mixture is mixed with a compound that serves as a Na source to prepare a mixture. If necessary, a compound that serves as a Li source may be added to the mixture at this stage. The ratio of the precipitate to the Na source compound and the Li source compound should be appropriately set according to the desired composition of the positive electrode active material. Examples of Na source compounds include sodium carbonate, sodium oxide, sodium nitrate, and sodium hydroxide. Examples of Li source compounds include lithium carbonate.

[0037] By calcining the mixture obtained in this way under an oxidizing gas atmosphere, a Na-doped precursor having a P2-type layered structure as the main phase can be obtained. As the oxidizing gas, for example, air can be used. If the calcination temperature is too low, the P2-type layered structure will not be sufficiently formed in the Na-doped precursor. On the other hand, if the calcination temperature is too high, a crystalline phase having a crystalline structure other than the P2-type layered structure is more likely to be formed in the Na-doped precursor.

[0038] From the viewpoint of more easily obtaining a Na-doped precursor having a P2-type layered structure as the main phase, the firing temperature during calcination is preferably 700 to 1100°C. More preferably, the firing temperature during calcination is 800 to 1000°C. In this case, a Na-doped precursor consisting only of a P2-type layered structure can be obtained more easily. The holding time during calcination can be appropriately set from the range of 0.5 to 50 hours.

[0039] The content of Mn, additive element M, and O in the Na-doped precursor is the same as the content of Mn, additive element M, and O in the desired positive electrode active material, and the total content of Li and Na in the Na-doped precursor is the same as the total content of Li and Na in the desired positive electrode active material. It is preferable that the content of Na in the Na-doped precursor is greater than the content of Li. That is, the Na-doped precursor is, for example, Na x Li y Mn c M d O (2±α)It preferably has a composition represented by the following compositional formula (wherein, in the compositional formula, M represents one or more additive elements selected from the group consisting of Ni, Al, Ti, Sn, Zr, Nb, W, and Mo and containing at least Ni, and x, y, c, d, and α satisfy 0.5 ≦ x ≦ 1, 0 ≦ y ≦ 0.33, 0.7 < c < 0.9, 0.9 < c + d < 1.1, 4 ≦ c / d ≦ 12, and 0 ≦ α ≦ 0.3).

[0040] The content of the crystal phase having a P2-type layered structure in the Na-doped precursor is preferably 50% by mass or more, more preferably 85% by mass or more, still more preferably 90% by mass or more, particularly preferably 95% by mass or more, and most preferably 100% by mass, that is, the Na-doped precursor is composed only of the crystal phase having a P2-type layered structure. By increasing the content of the crystal phase having a P2-type layered structure, the content of the crystal phase having an O2-type layered structure in the finally obtained positive electrode active material can be easily increased.

[0041] Incidentally, the content of the P2-type layered structure in the above-described Na-doped precursor can be calculated based on, for example, an X-ray diffraction pattern obtained by the powder X-ray diffraction method.

[0042] After obtaining the Na-doped precursor as described above, the Na atoms in the Na-doped precursor are replaced with Li atoms by ion exchange. The specific method of ion exchange is not particularly limited, and for example, a method of heating in a state where the Na-doped precursor and a Li ion source are in contact with each other can be employed. In this case, as the Li ion source, a lithium salt such as lithium chloride or lithium nitrate can be used. Further, the heating temperature during ion exchange may be a temperature at which the Li ion source melts.

[0043] When a mixture of lithium salts is used as the Li ion source, the heating temperature during ion exchange is preferably 400°C or lower. In this case, the formation of the O3-type layered structure can be suppressed. From the viewpoint of further increasing the content of the O2-type layered structure in the positive electrode active material, the heating temperature during ion exchange is more preferably 350°C or lower. The heating time during ion exchange can be appropriately set from, for example, a range of 0.5 to 8 hours.

[0044] Furthermore, the Na atoms in the Na-doped precursor can be replaced with Li atoms by contacting the Na-doped precursor with an aqueous solution containing Li ions. In this case, the aqueous solution containing Li ions may be heated as needed to promote ion exchange. As the aqueous solution to be contacted with the Na-doped precursor, an aqueous solution of a lithium salt such as lithium chloride or lithium nitrate can be used. The concentration of Li ions in the aqueous solution can be appropriately set from, for example, a range of 1 to 10 mol%. The contact time between the Na-doped precursor and the aqueous solution can be appropriately set from, for example, a range of 10 to 300 hours.

[0045] The positive electrode active material can be obtained by substituting the Na ions in the Na-doped precursor with Li ions as described above. After ion exchange is complete, the obtained positive electrode active material may be crushed into a powder as needed. The method of crushing the calcined body is not particularly limited. For example, various methods can be used for crushing, such as manual crushing using a mortar and pestle or mechanical crushing using a ball mill. Furthermore, after crushing the calcined body, the powder may be classified as needed to adjust the particle size of the positive electrode active material.

[0046] (Nonaqueous electrolyte secondary battery) The positive electrode active material is configured to be used as the positive electrode in a non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, with lithium ions as the charge carrier. The non-aqueous electrolyte in the non-aqueous electrolyte secondary battery using the positive electrode active material may be liquid or solid. That is, non-aqueous electrolyte secondary batteries to which the positive electrode active material can be applied include, for example, secondary batteries using a non-aqueous electrolyte such as lithium-ion secondary batteries, and all-solid-state secondary batteries using lithium ions as the charge carrier and a solid electrolyte.

[0047] A lithium-ion secondary battery may primarily consist of a positive electrode containing the positive electrode active material, a negative electrode, a separator interposed between the positive and negative electrodes, a non-aqueous electrolyte, additives, and a case housing these components. Examples of lithium-ion secondary battery shapes include coin-type, cylindrical, stacked, and prismatic types.

[0048] The positive electrode of a lithium-ion secondary battery comprises 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, such as metal foils like copper foil, stainless steel mesh, aluminum foil, and nickel foil, as well as perforated metal, expanded metal, and metal mesh.

[0049] The positive electrode may contain a binder interposed between the positive electrode active material and the positive electrode current collector. Examples of binders include fluorine-based binders such as polyvinylidene fluoride and polytetrafluoroethylene, rubber-based binders such as styrene-butadiene rubber, olefin-based binders such as polypropylene and polyethylene, and cellulose-based binders such as carboxymethylcellulose.

[0050] Furthermore, the positive electrode may contain conductive agents or conductive additives to enhance electrical conductivity. Examples of conductive agents include graphite, carbon black, acetylene black, and coke.

[0051] The positive electrode can be manufactured, for example, by the following method. First, a paste-like positive electrode mixture containing positive electrode active material is prepared. The positive electrode mixture may optionally contain an organic solvent to disperse or dissolve solid components such as the positive electrode active material. Next, the positive electrode mixture is applied to the surface of the 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 forming the positive electrode active material layer, the positive electrode active material layer may be pressed to increase its density if necessary. The positive electrode can then be obtained.

[0052] The negative electrode of a lithium-ion secondary battery comprises a negative electrode active material and a negative electrode current collector that holds the negative electrode active material. Examples of negative electrode active materials include silicon-based active materials such as Si, Si alloys and silicon oxide, carbon materials having a graphite structure such as graphite and hard carbon, and lithium titanate (Li4Ti5O 12 Lithium oxides such as ) metallic lithium and lithium alloys can be used. In addition, the same conductor as the positive electrode current collector described above can be used as the negative electrode current collector.

[0053] The negative electrode, like the positive electrode, may contain a binder, a conductive agent, and a conductive additive. The binder, conductive agent, and conductive additive that can be used in the negative electrode are the same as those used in the positive electrode.

[0054] The method for manufacturing the negative electrode is the same as for the positive electrode. That is, first, a paste-like negative electrode mixture containing negative electrode active material is prepared. The negative electrode mixture may contain an organic solvent to disperse or dissolve solid components such as the negative electrode active material, if necessary. 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 to increase its density, if necessary. A negative electrode can be obtained by the above method.

[0055] The non-aqueous electrolyte may contain an organic solvent and an electrolyte consisting of a lithium salt. Examples of lithium salts include LiPF6. Examples of organic solvents include ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, monofluoroethylene carbonate, and methyl-2,2,2-trifluoroethyl carbonate. These organic solvents may be used individually or in combination of two or more.

[0056] The non-aqueous electrolyte preferably contains at least one organic solvent selected from ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate. These organic solvents are highly polar and can dissolve large amounts of electrolyte. Therefore, by preparing a non-aqueous electrolyte using these organic solvents, the transport fraction of the charge carrier in a lithium-ion secondary battery can be easily increased.

[0057] Furthermore, an all-solid-state secondary battery may include, for example, a positive electrode containing the positive electrode active material, a negative electrode containing the negative electrode active material, a separator interposed between the positive and negative electrodes and containing a solid electrolyte, and a case for housing these components as its main constituent elements.

[0058] The positive electrode of an all-solid-state secondary battery may be composed of, for example, a positive electrode active material, a solid electrolyte, and additives such as conductive agents, which may be added as needed. In this case, from the viewpoint of further increasing the energy density of the secondary battery, the content of the positive electrode active material in the positive electrode is preferably 60 to 99% by mass, and more preferably 70 to 95% by mass.

[0059] Examples of solid electrolytes used in the positive electrode of all-solid-state secondary batteries include lithium lanthanum zirconate, LiPON, and Li 1+X Al X Ge 2-XOxide solid electrolytes such as (PO4)3, Li-SiO glass, and Li-Al-SO glass, or sulfide solid electrolytes such as Li2S-P2S5, Li2S-SiS2, LiI-Li2S-SiS2, LiI-Si2S-P2S5, Li2S-P2S5-LiI-LiBr, LiI-Li2S-P2S5, LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, and Li2S-P2S5-GeS2 can be used. The solid electrolyte content in the positive electrode is preferably 1 to 40% by mass, and more preferably 5 to 30% by mass.

[0060] When a conductive agent and a conductive additive are added to the positive electrode, the content of the conductive agent and conductive additive can be appropriately set according to the electrical conductivity, ionic conductivity, and the desired energy density of the secondary battery. The additives used in the positive electrode of an all-solid-state secondary battery are the same as those used in the positive electrode of a lithium-ion secondary battery.

[0061] The negative electrode of an all-solid-state secondary battery may consist, for example, of a negative electrode active material, a solid electrolyte, and additives such as conductive agents added as needed. The negative electrode active material and additives used in the negative electrode of an all-solid-state secondary battery are the same as those used in the negative electrode of a lithium-ion secondary battery. Furthermore, the solid electrolyte used in the negative electrode is the same as the solid electrolyte used in the positive electrode as described above.

[0062] The separator of an all-solid-state secondary battery contains a solid electrolyte. The solid electrolyte used in the separator is the same as the solid electrolyte used in the positive electrode as described above. In addition to the solid electrolyte, the separator may contain a binder to bind the solid electrolyte particles together, if necessary. Examples of binders that can be used include fluororesins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluororubber, and thermoplastic resins such as polypropylene, polyethylene, and polyethylene terephthalate. [Examples]

[0063] (Example 1) In this example, examples of the positive electrode active material will be described while referring to FIGS. 1 to 7. The positive electrode active material (Table 1, active materials S1 to S12) in this example is Li a Na b Mn c M d O (2±α) has a composition represented by the following compositional formula (where M in the compositional formula is selected from the group consisting of Ni, Al, Ti, Sn, Zr, Nb, W, and Mo, and represents one or more additive elements containing at least Ni, and a to d and α satisfy 0.7 ≦ a ≦ 1.33, 0 < b < 0.1, 0.7 < c < 0.9, 0.9 < c + d < 1.1, 4 ≦ c / d ≦ 12, 0 ≦ α ≦ 0.3). Further, the main phase of the positive electrode active material has an O2-type layered structure attributable to the space group P63mc.

[0064] The manufacturing method of the positive electrode active material in this example is as follows. First, manganese sulfate pentahydrate as a Mn source and nickel sulfate hexahydrate as a Ni source were weighed so that the ratio c / d of the value of c to the value of d in the above compositional formula became the value shown in Table 1. These compounds were dissolved in distilled water to prepare a first solution. Separately from the first solution, an aqueous sodium carbonate solution with a concentration of 0.5 mol / L was prepared and used as the second solution.

[0065] Next, the first solution and the second solution were simultaneously dropped into a beaker to form a precipitate containing Mn and Ni in the beaker. At this time, the dropping rates of the first solution and the second solution were adjusted so that the pH of the mixed solution in the beaker was within the range of 7 to 9. After the dropping of the first solution and the second solution was completed, the mixed solution in the beaker was heated to 50°C and stirred at a rotation speed of 500 rpm for 1 hour. After the stirring was completed, the precipitate was taken out from the beaker and washed with distilled water. After washing, the precipitate was dried at a temperature of 100°C for 24 hours and then crushed using an agate mortar.

[0066] The powdered precipitate obtained as described above was mixed with sodium carbonate and lithium carbonate in a molar ratio of sodium carbonate:lithium carbonate = 4:1. This mixture was heated at 800°C for 8 hours to calcine it and obtain a Na-doped precursor. The obtained Na-doped precursor was crushed using an agate mortar and pestle to obtain a powder.

[0067] The Na-doped precursor obtained in this manner consists of a crystalline phase having a P2-type layered structure. As an example of the X-ray diffraction pattern of the Na-doped precursor, Figure 1 shows the X-ray diffraction patterns of the Na-doped precursor used to prepare the active material S1 and the Na-doped precursor used to prepare the active material R1. The X-ray diffractometer used was Rigaku Corporation's "SmartLab®," the characteristic X-ray irradiated was CuKα, the X-ray tube voltage was 40kV, and the irradiation current was 20mA. In Figure 1, the vertical axis represents diffraction intensity (relative intensity), and the horizontal axis represents the diffraction angle 2θ (unit:°).

[0068] As shown in Figure 1, the X-ray diffraction pattern of the Na-doped precursor that becomes the active material S1 shows only diffraction peaks originating from the P2-type layered structure. On the other hand, the Na-doped precursor that becomes the active material R1 shows diffraction peaks originating from the P2-type layered structure as well as small diffraction peaks originating from the O3-type layered structure.

[0069] Next, lithium nitrate and lithium chloride were mixed in a molar ratio of lithium nitrate:lithium chloride = 88:12 to prepare a Li salt mixture. This Li salt mixture was mixed with a Na-doped precursor in a molar ratio of 10:1, and then ion exchange was performed by heating at 280°C in air for 0.5 to 8 hours. After the ion exchange was completed, the reaction product was washed with distilled water, dried, and then crushed using an agate mortar to obtain the positive electrode active materials (active materials S1 to S12) shown in Table 1.

[0070] In this example, for comparison with active materials S1 to S12, active materials R1 to R4, shown in Table 1, were prepared. Active materials R1 to R4 have the compositions shown in Table 1 and contain the crystalline phases shown in Table 1. The manufacturing method for active materials R1 to R4 is the same as that for active materials S1 to S12, except that the types and ratios of raw materials were changed to achieve the compositions shown in Table 1.

[0071] Next, the composition and crystal structure of the positive electrode active material shown in Table 1 were identified.

[0072] • Composition of positive electrode active material The molar ratios of each metal element in the positive electrode active material were measured by inductively coupled plasma emission spectrometry (i.e., ICP-AES). The compositions of active materials S1-S12 and R1-R4 are as shown in the "Composition of Positive Electrode Active Material" column of Table 1.

[0073] • Crystal structure of the crystalline phase contained in the positive electrode active material The crystalline phases contained in active materials S1-S12 and R1-R4 were identified by powder X-ray diffraction. The measurement apparatus and conditions used for powder X-ray diffraction were the same as those used for the powder X-ray diffraction of the Na-doped precursor described above. As examples of the X-ray diffraction patterns of the positive electrode active materials, Figure 2 shows the X-ray diffraction patterns of active materials S1 and R1, and Figure 3 shows the X-ray diffraction patterns of active materials S5 and S8. In Figures 2 and 3, the vertical axis represents diffraction intensity (relative intensity), and the horizontal axis represents the diffraction angle 2θ (unit: °).

[0074] Based on the obtained X-ray diffraction patterns, the crystalline phase contained in each positive electrode active material was identified. As shown in Figures 2, 3, and Table 1, it was confirmed that active materials S1-S12 and R2-R4 are composed of a crystalline phase with an O2-type layered structure. Furthermore, as shown in Figure 3, in positive electrode active materials with a relatively high Na content, diffraction peaks originating from residual Na after ion exchange appeared at lower angles than the highest intensity diffraction peak.

[0075] On the other hand, as shown in Figure 2 and Table 1, it was confirmed that the main phase of the active material R1 is a T2-type layered structure, and that an O2-type layered structure is included as a secondary phase.

[0076] • Configuration and manufacturing method of evaluation secondary battery Next, a secondary battery for evaluation was fabricated using the obtained positive electrode active material. As shown in Figure 4, the secondary battery 1 in this example is a CR2032 type coin cell equipped with a positive electrode 2, a counter electrode 3, a separator 4, and a non-aqueous electrolyte 5.

[0077] More specifically, the secondary battery 1, as shown in Figure 4, has a case 11 which is a relatively small, bottomed cylindrical shape, and a top cover 12 which closes the opening of the case 11. A space is formed between the case 11 and the top cover 12. The top cover 12 is joined to the case 11 by crimping.

[0078] The space between the case 11 and the top lid 12 houses the positive electrode 2, the counter electrode 3, the separator 4, and the non-aqueous electrolyte 5. A rubber packing 15 is placed between the positive electrode 2 and the separator 4. A spacer 13 and a washer 14 are provided between the top lid 12 and the positive electrode 2. The spacer 13 is positioned to contact the positive electrode 2. The washer 14 is placed between the spacer 13 and the top lid 12. Specifically, the spacer 13 in this example is a disc-shaped stainless steel plate.

[0079] The positive electrode 2 comprises a positive electrode current collector 21 and a positive electrode active material layer 22 provided on the positive electrode current collector 21, which contains one of the positive electrode active materials S1 to S12 and R1 to R4 shown in Table 1. In this example, the positive electrode current collector 21 is specifically a disc-shaped aluminum foil with a diameter of 16 mm. The counter electrode 3 is specifically a disc-shaped metallic lithium foil.

[0080] The separator 4 is made of polypropylene and is interposed between the positive electrode 2 and the counter electrode 3. The non-aqueous electrolyte 5 is filled in at least between the positive electrode 2 and the counter electrode 3 within the secondary battery 1. In this example, the non-aqueous electrolyte 5 is a mixed solvent in which ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate are mixed in equal volumes, and LiPF6 at a concentration of 1 mol / L is dissolved.

[0081] Next, the manufacturing method of the secondary battery 1 will be described. First, polyvinylidene fluoride, used as a binder, was dissolved in N-methyl-2-pyrrolidone to prepare a binder solution with a concentration of 12% by mass. 1.0 g of positive electrode active material and 0.1178 g of carbon black, used as a conductive additive, were added to 0.49 g of this binder solution and kneaded to obtain a positive electrode composite material. This positive electrode composite material was applied to the positive electrode current collector 21 and then dried to form a positive electrode active material layer 22. Subsequently, the positive electrode active material layer 22 was pressed together with the positive electrode current collector 21 using a roll press to increase the density of the positive electrode active material layer 22 to 2.4-2.8 g / cm³. 3 It was increased to that point.

[0082] The positive electrode 2 was obtained by punching out the positive electrode active material layer 22 together with the positive electrode current collector 21 into a disc shape with a diameter of 16 mm. The counter electrode 3 was fabricated by punching out lithium foil into a disc shape.

[0083] Next, the counter electrode 3, separator 4, packing 15, positive electrode 2, spacer 13, and washer 14 were stacked in order inside the case 11, and the non-aqueous electrolyte 5 was poured into the case 11. After that, the top cover 12 was placed over the opening of the case 11 and the space between the case 11 and the top cover 12 was sealed by crimping. Thus, a secondary battery 1 was obtained.

[0084] Next, a rate characteristic test was performed using the obtained evaluation secondary battery. The rate characteristic test was performed using a charge / discharge device (Bio-Logic's "BCS-815").

[0085] • Rate characteristics test First, the secondary battery was charged in constant current-constant voltage mode at a temperature of 25°C. The current density in constant current mode was set to 1 / 10C, ​​and the system switched to constant voltage mode when the secondary battery voltage reached 4.8V. In constant voltage mode, the charging voltage was set to 4.8V, and charging continued until the current reached 0.01mA. After the current reached 0.01mA in constant voltage mode, charging and discharging were paused for 10 minutes to stabilize the potential of the secondary battery. Subsequently, the battery was discharged to 2.0V with a constant current density of either 1 / 10C or 1C, and the discharge curve at this time was obtained.

[0086] Based on the discharge curves obtained using the method described above, the discharge capacity at each current density was calculated and these values ​​are listed in the "Discharge Capacity" column of Table 1. Furthermore, the ratio of the discharge capacity at 1C to the discharge capacity at 1 / 10C was calculated and this value is listed in the "Discharge Capacity Ratio" column of Table 1. Note that "C," the unit of current density during charging and discharging, theoretically represents the current density at which the charge or discharge rate reaches 100% in one hour. That is, theoretically, a secondary battery can be completely discharged by discharging at a current density of 1C for one hour. In this example, the current density corresponding to 1C is specifically 250 mA / g.

[0087] [Table 1]

[0088] As shown in Table 1, the active materials S1 to S12 have the aforementioned specific compositions. Furthermore, the main phase of these active materials is an O2-type layered structure. When these active materials were discharged at a current density of 1 / 10C, ​​their discharge capacity was higher than that of the Co-containing active material R2. Additionally, a comparison of active materials S1 to S12 with active material R3, which does not contain Co or the additive element M, shows that substituting a portion of the Mn in the O2-type layered structure with an additive element M such as Ni significantly improved the discharge capacity.

[0089] On the other hand, as shown in Table 1, the main phase of the active material R1 is a T2-type layered structure, resulting in a lower discharge capacity compared to the active materials S1-S12, which consist of an O2-type layered structure. Because the c / d value of active material R4 was large, the effect of the added element M was insufficient. As a result, the discharge capacity of active material R4 was lower than that of active materials S1 to S12.

[0090] For active materials S1 to S12, whose main phase is an O2-type layered structure and which contain Mn and additive element M, Figure 5 shows the relationship between the c / d value and discharge capacity, and Figure 6 shows the relationship between the c / d value and discharge capacity ratio. In Figure 5, the horizontal axis represents the c / d value in the composition formula, and the vertical axis represents the discharge capacity (unit: mAh / g) when discharged at a current density of 1 / 10C. Similarly, in Figure 6, the horizontal axis represents the c / d value in the composition formula, and the vertical axis represents the discharge capacity ratio (unit: %).

[0091] As shown in Table 1, Figure 5, and Figure 6, it can be seen that active materials S2-S5 and S8-S12, whose c / d values ​​are between 5 and 10, have even higher discharge capacity and discharge capacity ratio than the other active materials.

[0092] Furthermore, Figure 7 shows the relationship between the Na content and discharge capacity in active materials S1 to S12 and active material R4. The horizontal axis of Figure 7 represents the Na content, i.e., the value of b in the aforementioned composition formula, and the vertical axis represents the discharge capacity (unit: mAh / g) when discharged at a current density of 1 / 10C. As shown in Figure 7, it can be seen that active materials S8 to S11, which have a Na molar ratio of 0.03 to 0.07, have a higher discharge capacity than the other active materials.

[0093] (Example 2) This example shows an example of an all-solid-state secondary battery using a positive electrode active material. The non-aqueous electrolyte secondary battery 102 in this example is configured as an all-solid-state secondary battery. Specifically, as shown in Figure 8, the non-aqueous electrolyte secondary battery 102 has a positive electrode 202 containing a positive electrode active material, a negative electrode 302 containing a negative electrode active material, and a separator 402 containing a solid electrolyte interposed between the positive electrode 202 and the negative electrode 302.

[0094] The positive electrode 202 contains a positive electrode active material coated with lithium niobate, a conductive additive, and a solid electrolyte. The positive electrode active material used in the positive electrode 202 in this example is the active material S5 from Example 1, the conductive additive is carbon nanofiber, and the solid electrolyte is Li3PS4.

[0095] A method for coating a positive electrode active material with lithium niobate is as follows: First, a treatment solution is prepared by mixing 30-35.5% hydrogen peroxide solution, niobium, 28-30% ammonia solution, and lithium hydroxide. After mixing this treatment solution with the positive electrode active material, the solvent is removed using an evaporator to form a lithium niobate film on the surface of the positive electrode active material.

[0096] The negative electrode 302 contains a negative electrode active material, a conductive additive, and a solid electrolyte. In this example, the negative electrode active material used in the negative electrode 302 is lithium titanate, the conductive additive is carbon nanofiber, and the solid electrolyte is Li3PS4.

[0097] Separator 402 is composed of a solid electrolyte. Specifically, the solid electrolyte used in separator 402 is Li3PS4.

[0098] In manufacturing the non-aqueous electrolyte secondary battery 102, first, a positive electrode composite material containing a positive electrode active material, a conductive additive, and a solid electrolyte, and a negative electrode composite material containing a negative electrode active material, a conductive additive, and a solid electrolyte are prepared. Next, the positive electrode composite material is placed into a press mold. Then, the positive electrode composite material in the press mold is compressed to form a positive electrode 202. Next, a separator composite material containing a solid electrolyte is placed on top of the positive electrode 202 in the press mold, and the separator composite material is compressed together with the positive electrode 202. This forms a separator 402 on top of the positive electrode 202. Then, the negative electrode composite material is placed on top of the separator 402 in the press mold, and the negative electrode composite material is compressed together with the positive electrode 202 and the separator 402. As a result, a non-aqueous electrolyte secondary battery 102 can be obtained in which the positive electrode 202, separator 402, and negative electrode 302 are sequentially stacked.

[0099] Next, the discharge capacity of the non-aqueous electrolyte secondary battery 102 was measured using the following method. First, the non-aqueous electrolyte secondary battery 102 was charged in constant current-constant voltage mode at a temperature of 25°C. The current density in constant current mode was set to 18 mA / g, and the system was switched to constant voltage mode when the secondary battery voltage reached 2.9 V. In constant voltage mode, the charging voltage was set to 2.9 V, and charging was carried out until the current value reached 0.01 mA. After the current density in constant voltage mode reached 0.01 mA, charging and discharging was paused for 10 minutes to stabilize the potential of the secondary battery. After that, the non-aqueous electrolyte secondary battery 102 was discharged in constant current-constant voltage mode, and the discharge curve at this time was obtained. The current density in constant current mode was set to 18 mA / g, and the system was switched to constant voltage mode when the secondary battery voltage reached 1.0 V. In constant voltage mode, the discharge voltage was set to 1.0 V, and discharging was carried out until the current value reached 0.01 mA.

[0100] The discharge capacity of the non-aqueous electrolyte secondary battery 102, calculated from the discharge curve, was 167 mAh / g. From this result, it can be understood that the positive electrode active material can be applied not only to lithium-ion secondary batteries but also to all-solid-state secondary batteries.

[0101] Although specific embodiments of the positive electrode active material, its manufacturing method, and non-aqueous electrolyte secondary battery according to the present invention have been described above based on the examples, the embodiments of the positive electrode active material, its manufacturing method, and non-aqueous electrolyte secondary battery according to the present invention are not limited to the embodiments described above, and the configuration can be appropriately changed without impairing the spirit of the present invention.

[0102] For example, in Example 1 described above, data was shown for an evaluation secondary battery in which metallic lithium was used as the counter electrode. From the results of these examples, it can be seen that the above-mentioned positive electrode active material is suitable for the positive electrode of a lithium-ion secondary battery. In actually constructing a lithium-ion secondary battery, a positive electrode containing the positive electrode active material of Example 1 can be combined with a negative electrode containing a negative electrode active material made of carbon, lithium oxide, etc.

Claims

1. A positive electrode active material used in non-aqueous electrolyte secondary batteries, space group P6 ３ The main phase is an O2-type layered structure that can be attributed to mc, Li ａ Na ｂ Mn ｃ M ｄ O （２±α） A positive electrode active material having a composition represented by the following compositional formula (wherein M in the compositional formula is selected from the group consisting of Ni, Al, Ti, Sn, Zr, Nb, W, and Mo, and represents one or more additive elements including at least Ni, and a to d and α satisfy 0.7 ≤ a ≤ 1.33, 0 < b < 0.1, 0.7 < c < 0.9, 0.9 < c + d < 1.1, 4 ≤ c / d ≤ 12, and 0 ≤ α ≤ 0.3).

2. The positive electrode active material according to claim 1, wherein the value of b in the composition formula satisfies 0.001 ≤ b ≤ 0.07, and the ratio of the value of c to the value of d, c / d, is 5 or more and 10 or less.

3. A non-aqueous electrolyte secondary battery having a positive electrode, a negative electrode, and a non-aqueous electrolyte, A non-aqueous electrolyte secondary battery comprising the positive electrode active material described in claim 1 or 2.

4. A method for producing a positive electrode active material according to claim 1 or 2, space group P6 ３ A Na-doped precursor was prepared that has a crystalline phase with a P2-type layered structure that can be attributed to / mmc, and contains Na, Mn, and the additive element M. A method for producing a positive electrode active material, wherein the Na atoms in the Na-doped precursor are subsequently replaced with Li atoms by ion exchange.

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