Positive electrode active material for energy storage element, positive electrode for energy storage element, energy storage element, and energy storage device

Abstract

A positive electrode active material for a power storage element according to one aspect of the present invention: has an olivine crystal structure; has the surface thereof at least partially covered with carbon; and satisfies either (A) or (B). (A) The pore volume, in a range where the pore size is 60 to 200 nm which is found by BJH method from the desorption isotherm using nitrogen gas adsorption, is 0.05 cm3 / g to 0.25 cm3 / g; and the specific pore surface area, in a range where the pore size using nitrogen gas adsorption is 10 to 200 nm, is 5 m2 / g or greater. (B) The half-width ratio (200) / (131), which is the peak corresponding to the (200) plane to the peak corresponding to the (131) plane according to powder X-ray diffraction using CuKα-radiation in a state of charge, is 1.10 or lower.

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material for an electric storage device, a positive electrode for an electric storage device, an electric storage device, and an electric storage device. [Background technology]

[0002] Due to their high energy density, non-aqueous electrolyte secondary batteries, such as lithium ion secondary batteries, are widely used in electronic devices such as personal computers and communication terminals, automobiles, etc. Non-aqueous electrolyte secondary batteries generally have a pair of electrodes electrically isolated by a separator and a non-aqueous electrolyte interposed between the electrodes, and are configured to charge and discharge by transferring ions between the electrodes. In addition to non-aqueous electrolyte secondary batteries, capacitors such as lithium ion capacitors and electric double layer capacitors are also widely used as energy storage elements.

[0003] In recent years, olivine-type positive electrode active materials, which are inexpensive and highly safe, have been attracting attention as positive electrode active materials for use in the above-mentioned energy storage devices. Because of their low electronic conductivity, it has been difficult to obtain a discharge capacity close to the theoretical capacity of these olivine-type positive electrode active materials. However, a technique for coating the surface with carbon has been proposed to improve electronic conductivity (see Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-034306 Summary of the Invention [Problem to be solved by the invention]

[0005] However, when applying the olivine-type positive electrode active material to a starting battery for an automobile, etc., high-rate performance in a low-temperature environment is required, and since this high-rate performance in a low-temperature environment is affected by factors other than electronic conductivity, there is a risk that sufficient performance cannot be obtained even if the surface of the olivine-type positive electrode active material is coated with carbon.

[0006] The present invention has been made in light of the above circumstances, and an object of the present invention is to provide a positive electrode active material for an energy storage device that can increase the capacity of the energy storage device during high-rate discharge in a low-temperature environment. [Means for solving the problem]

[0007] A positive electrode active material for an energy storage device according to one aspect of the present invention has an olivine crystal structure, at least a portion of the surface of which is coated with carbon, and satisfies either (A) or (B) below. (A) The pore volume in the pore diameter range of 60 nm to 200 nm calculated by the BJH method from the desorption isotherm using nitrogen gas adsorption is 0.05 cm 3 / g or more 0.25cm 3 / g or less, and the pore specific surface area in the pore diameter range of 10 nm to 200 nm measured by nitrogen gas adsorption method is 5m 2 / g or more. (B) In a charged state, the half-width ratio (200) / (131) of the peak corresponding to the (200) plane to the peak corresponding to the (131) plane by powder X-ray diffraction using CuKα radiation is 1.10 or less. [Effects of the Invention]

[0008] A positive electrode active material for an energy storage device according to one aspect of the present invention can increase the capacity of an energy storage device during high-rate discharge in a low-temperature environment. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a perspective view showing the appearance of an energy storage device according to one embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing an energy storage device configured by assembling a plurality of energy storage elements according to one embodiment of the present invention. [Figure 3] FIG. 3 is a graph showing the relationship between pore volume and discharge capacity ratio. [Figure 4] FIG. 4 is a graph showing the relationship between the pore specific surface area and the discharge capacity ratio. [Figure 5] FIG. 5 is a graph showing the relationship between the peak half width ratio (200) / (131) and the discharge capacity ratio. [Figure 6] FIG. 6 is a graph showing the relationship between the half-width of the peak corresponding to the (131) plane and the output ratio. DETAILED DESCRIPTION OF THE INVENTION

[0010] A positive electrode active material for an energy storage device according to one aspect of the present invention has an olivine crystal structure, at least a portion of the surface of which is coated with carbon, and satisfies either (A) or (B) below. (A) The pore volume in the pore diameter range of 60 nm to 200 nm calculated by the BJH method from the desorption isotherm using nitrogen gas adsorption is 0.05 cm 3 / g or more 0.25cm 3 / g or less, and the pore specific surface area in the pore diameter range of 10 nm to 200 nm measured by nitrogen gas adsorption method is 5m 2 / g or more. (B) In a charged state, the half-width ratio (200) / (131) of the peak corresponding to the (200) plane to the peak corresponding to the (131) plane measured by powder X-ray diffraction using CuKα radiation is 1.10 or less, or in a charged state, the half-width ratio (200) / (131) of the peak corresponding to the (200) plane to the peak corresponding to the (131) plane measured by powder X-ray diffraction using CuKα radiation is 1.10 or less.

[0011] The positive electrode active material for an energy storage device has at least a portion of its surface coated with carbon, has an olivine-type crystal structure, and satisfies the above condition (A), i.e., has a pore volume and pore specific surface area within a specific range, thereby enabling the capacity of the energy storage device to be increased during high-rate discharge in a low-temperature environment. The reasons for this are presumed to be as follows: The positive electrode active material for an energy storage device has good electronic conductivity because at least a portion of its surface is coated with carbon, so the discharge capacity in a low-temperature environment tends to be dominated by the diffusibility of lithium ions. The pore diameter range of 60 nm to 200 nm is a region where non-aqueous electrolytes can easily penetrate, and the pore volume in this pore diameter range is set to 0.05 cm 3 / g or more 0.25cm 3 / g or less, and the pore specific surface area in the pore diameter range of 10 nm to 200 nm is 5m 2 / g or more, a pore structure that promotes the penetration of non-aqueous electrolyte can be obtained. As a result, it is presumed that the lithium ion diffusibility in the positive electrode mixture layer is improved. Therefore, the positive electrode active material for an energy storage device can increase the capacity of the energy storage device during high-rate discharge in a low-temperature environment.

[0012] The positive electrode active material for an energy storage device has at least a portion of its surface coated with carbon, has an olivine-type crystal structure, and satisfies the above condition (B). That is, the peak full width at half maximum ratio (200) / (131) measured by powder X-ray diffraction using CuKα radiation in a charged state falls within a specific range. This enables the energy storage device to have a high capacity during high-rate discharge in a low-temperature environment. The reason for this is presumed to be as follows. Because the positive electrode active material for an energy storage device has good electronic conductivity due to at least a portion of its surface being coated with carbon, the discharge capacity in a low-temperature environment tends to be dominated by the diffusibility of lithium ions. It is presumed that the positive electrode active material for an energy storage device has a crystal structure favorable for lithium ion diffusion in the solid phase because the peak full width at half maximum ratio (200) / (131) measured by powder X-ray diffraction using CuKα radiation in a charged state falls within a specific range. As a result, the positive electrode active material for an energy storage device has a crystal structure favorable for lithium ion diffusion in the solid phase. As a result, high lithium ion diffusibility is likely to be achieved even in a low-temperature environment. Therefore, the positive electrode active material for an electric storage device can increase the capacity of the electric storage device during high-rate discharge in a low-temperature environment.

[0013] When the positive electrode active material for an energy storage device satisfies the above condition (B), it is preferable that the half-width of the peak corresponding to the (131) plane, measured by powder X-ray diffraction using CuKα radiation in a discharged state, is 0.110 to 0.155. When the half-width of the peak corresponding to the (131) plane is within the above range, the particle size of the primary particles constituting the secondary particles is in a suitably fine range, which increases the diffusibility of lithium ions and increases the capacity of the energy storage device during high-rate discharge in a low-temperature environment.

[0014] The positive electrode active material for an electricity storage device is preferably a compound represented by the following formula 1. Life x Mn (1-x) PO4(0≦x≦1) 1 When the positive electrode active material contains iron, manganese, or a combination thereof as a transition metal, the charge / discharge capacity can be increased.

[0015] A positive electrode for an energy storage device according to one aspect of the present invention includes the positive electrode active material, and because the positive electrode for an energy storage device includes the positive electrode active material, the capacity of the energy storage device can be increased during high-rate discharge in a low-temperature environment.

[0016] An electric storage device according to one aspect of the present invention includes the positive electrode, and because the electric storage device includes a positive electrode containing the positive electrode active material, the electric storage device has excellent high-rate discharge performance in a low-temperature environment.

[0017] An electric storage device according to one aspect of the present invention includes a plurality of electric storage elements, and includes one or more of the electric storage elements. Since the electric storage device includes one or more of the electric storage elements, the electric storage device has excellent high-rate discharge performance in a low-temperature environment.

[0018] Hereinafter, a positive electrode active material for an energy storage element, a positive electrode for an energy storage element, an energy storage element, and an energy storage device according to one embodiment of the present invention will be described in detail in this order.

[0019] <Positive electrode active material for energy storage devices> The positive electrode active material for a storage battery element (hereinafter also simply referred to as the positive electrode active material) has an olivine-type crystal structure. A compound having an olivine-type crystal structure has a crystal structure that can be assigned to the space group Pnma. A crystal structure that can be assigned to the space group Pnma means that the compound has a peak that can be assigned to the space group Pnma in an X-ray diffraction diagram. An example of a compound having an olivine-type crystal structure is AMPO4 (A is an alkali metal such as Li, Na, or K, and M is a transition metal such as Fe, Mn, Co, or Ni). The compound having the olivine-type crystal structure is a polyanion salt in which oxygen elimination reaction from the crystal lattice does not easily proceed, and therefore is highly safe and inexpensive.

[0020] The positive electrode active material for an electricity storage device is preferably a compound represented by the following formula 1. Life x Mn (1-x) PO4(0≦x≦1) 1 Therefore, the positive electrode active material is lithium manganese phosphate (LiMnPO4), lithium iron phosphate (LiFePO4), lithium iron manganese phosphate (LiFex Mn 1-x It is preferably composed of PO4, 0 < x < 1), or a combination thereof. By including iron, manganese, or a combination thereof as the transition metal in the positive electrode active material, the charge-discharge capacity can be increased more.

[0021] The compound represented by Formula 1 is a phosphate compound containing manganese, iron, or a combination thereof and lithium. The compound represented by Formula 1 may contain transition metal elements other than manganese and iron and typical elements such as aluminum. However, it is preferable that the compound represented by Formula 1 is substantially composed of manganese, iron, or a combination thereof, lithium, phosphorus, and oxygen.

[0022] In Formula 1, the upper limit of x is 1, and 0.95 is preferable. Also, the lower limit of x is 0, and 0.25 is preferable. By the range of x being the above range, it has more excellent life characteristics. Note that x may be substantially 1.

[0023] The average particle diameter of the primary particles of the positive electrode active material is preferably, for example, 0.01 μm or more and 0.2 μm or less, and more preferably 0.02 μm or more and 0.1 μm or less. By setting the average particle diameter of the primary particles of the compound represented by Formula 1 within the above range, the diffusibility of lithium ions in the positive electrode binder layer is improved.

[0024] The average particle diameter of the secondary particles of the positive electrode active material is preferably, for example, 3 μm or more and 20 μm or less, and more preferably 5 μm or more and 15 μm or less. By setting the average particle diameter of the secondary particles of the compound represented by Formula 1 within the above range, manufacturing and handling become easy, and at the same time, the diffusibility of lithium ions in the positive electrode binder layer is improved.

[0025] At least a portion of the surface of the positive electrode active material is coated with carbon. By coating at least a portion of the surface of the positive electrode active material with carbon, electronic conductivity can be improved. The carbon content in the positive electrode active material is preferably 0.5% by mass or more and 5% by mass or less. By setting the carbon content within the above range, electrical conductivity can be increased, and the electrode density and, therefore, the capacity of the energy storage element can be increased.

[0026] When the positive electrode active material satisfies the above (A), the lower limit of the pore volume in the range of pore diameters of 60 nm to 200 nm determined by the BJH method from the desorption isotherm using the nitrogen gas adsorption method is 0.05 cm 3 / g, 0.10cm 3 On the other hand, the upper limit of the pore volume is preferably 0.25 cm 3 / g, 0.20cm 3 When the pore volume is within the above range, an energy storage device including a positive electrode containing the active material can have a large capacity during high-rate discharge in a low-temperature environment.

[0027] When the positive electrode active material satisfies the above (A), the lower limit of the specific surface area of ​​pores in the range of pore diameters of 10 nm to 200 nm as measured by nitrogen gas adsorption is 5 m 2 / g, 7m 2 When the pore specific surface area is within the above range, an energy storage device including a positive electrode containing the active material can have a large capacity during high-rate discharge in a low-temperature environment.

[0028] The pore volume of the positive electrode active material having a pore diameter in the range of 60 nm or more and 200 nm or less and the specific surface area of ​​the pores having a pore diameter in the range of 10 nm or more and 200 nm or less are calculated according to the following procedure. The pore volume and specific pore surface area were measured using Quantachrome's "autosorb iQ" and control analysis software "ASiQwin." 1.00 g of the lithium transition metal composite oxide sample was placed in a measurement tube and vacuum dried at 120°C for 12 hours to thoroughly remove moisture from the sample. Next, adsorption and desorption isotherms were measured using liquid nitrogen gas adsorption within a relative pressure P / P0 (P0 = approximately 770 mmHg) range of 0 to 1. The desorption isotherm was then used to calculate the pore distribution and specific pore surface area using the BJH method.

[0029] When the positive electrode active material satisfies the above condition (B), the upper limit of the half-width ratio (200) / (131) of the peak corresponding to the (200) plane to the peak corresponding to the (131) plane, measured by powder X-ray diffraction using CuKα radiation in a charged state, is 1.10, preferably 1.08. It is presumed that the upper limit of the peak half-width ratio (200) / (131) above provides a crystalline structure favorable for lithium ion diffusion within the solid phase of the positive electrode active material for energy storage devices, thereby facilitating high lithium ion diffusivity even in low-temperature environments. Therefore, the positive electrode active material can increase the capacity of energy storage devices during high-rate discharge in low-temperature environments. The lower limit of the peak half-width ratio (200) / (131) is preferably 0.95, more preferably 1.00. When the lower limit of the half width ratio (200) / (131) of the peaks is within the above range, the capacity of the energy storage element during high-rate discharge in a low-temperature environment can be increased.

[0030] When the positive electrode active material satisfies the above condition (B), the upper limit of the half-width of the peak corresponding to the (131) plane, as measured by powder X-ray diffraction using CuKα radiation in a discharged state, is preferably 0.155, more preferably 0.145. The lower limit of the half-width of the peak corresponding to the (131) plane is preferably 0.110, more preferably 0.115. When the half-width of the peak corresponding to the (131) plane is within the above range, the particle size of the primary particles constituting the secondary particles falls within a suitably fine range, thereby increasing the diffusibility of lithium ions and enabling the capacity of the energy storage device to be increased during high-rate discharge in a low-temperature environment.

[0031] When the positive electrode active material satisfies the above (B), the upper limit of the half-width of the peak corresponding to the (200) plane, as measured by powder X-ray diffraction using CuKα radiation in a charged state, is preferably 0.20. The lower limit of the half-width of the peak corresponding to the (200) plane is preferably 0.11. When the half-width of the peak corresponding to the (200) plane is within the above range, the capacity of the energy storage device during high-rate discharge in a low-temperature environment can be further increased.

[0032] The powder X-ray diffraction peaks of this positive electrode active material using CuKα radiation belong to the orthorhombic space group Pnma. The half-width of the peak corresponding to the (200) plane measured by powder X-ray diffraction using CuKα radiation is determined from the diffraction peak at 2θ = 29.7 ± 0.5° in the X-ray diffraction pattern using CuKα radiation. Similarly, the half-width of the peak corresponding to the (131) plane measured by powder X-ray diffraction using CuKα radiation is determined from the diffraction peak at 2θ = 35.6 ± 0.5°.

[0033] The half-width of the diffraction peak of the positive electrode active material is measured using an X-ray diffractometer (Rigaku, Model: MiniFlex II). Specifically, the measurement is performed under the following conditions and procedures: the radiation source is CuKα radiation, the acceleration voltage and current are 30 kV and 15 mA, respectively. The sampling width is 0.01°, the scan time is 14 min (scan speed 5.0), the divergence slit width is 0.625°, the receiving slit width is open, and the scattering slit is 8.0 mm. The obtained X-ray diffraction data is automatically analyzed using the software "PDXL" provided with the X-ray diffractometer, and the half-width is determined from the output. Note that peaks derived from Kα2 are not removed when analyzing the X-ray diffraction data. Here, "Refine background" and "Auto" are selected in the work window of the PDXL software, and refinement is performed so that the intensity error between the measured pattern and the calculated pattern is 4000 or less. This refinement involves background processing, and based on the results of subtracting the baseline, values ​​of the peak intensity and half-width of each diffraction line are obtained.

[0034] For measuring the pore volume and pore specific surface area of ​​the positive electrode active material, if the positive electrode active material is a powder before charge / discharge and before the positive electrode is fabricated, the powder is used for the measurement as is. When taking a measurement sample from the positive electrode removed by disassembling the storage element, before disassembling the storage element, discharge the positive electrode at a constant current value (0.1C) that is one-tenth of the current value that produces the same amount of electricity as the nominal capacity of the storage element when a constant current is applied to the storage element for one hour, until the voltage reaches the lower limit of the specified voltage in a 25°C environment. Disassemble the storage element, remove the positive electrode, and separate the positive electrode plate by 1 to 4 cm. 2 The positive electrode plate is cut into a sufficiently small area. Using this positive electrode plate, a storage element is assembled with a metal lithium electrode as the counter electrode. A constant current discharge is performed at a current value of 10 mA per 1 g of positive electrode mixture at 25°C until the terminal voltage reaches 2.0 V, adjusting the battery to a fully discharged state. The battery is then disassembled again, and the positive electrode is removed. The removed positive electrode is thoroughly washed with dimethyl carbonate to remove any nonaqueous electrolyte adhering to the positive electrode, and the positive electrode mixture is then collected from the positive electrode substrate after drying at room temperature for one day. The above steps up to disassembly of the battery, as well as the cleaning and drying of the positive electrode, are carried out in an argon atmosphere with a dew point of -60°C or below. The resulting mixture powder is dispersed in a solvent such as N-methylpyrrolidone (NMP) to remove the binder (PVdF, etc.) from the positive electrode mixture. The powder is then washed with dimethyl carbonate and dried, and the conductive agent is removed using air classification or the like. The positive electrode mixture thus collected is used to measure the pore volume and pore specific surface area.

[0035] When measuring the positive electrode active material in a discharged state by powder X-ray diffraction, if the positive electrode active material is a powder before charge / discharge and before the positive electrode is fabricated, it is used for the measurement as is. When a measurement sample is taken from a positive electrode taken out by disassembling an energy storage element, a positive electrode mixture taken from an energy storage element adjusted to a fully discharged state is used for measurement using a metallic lithium electrode as the counter electrode in the same procedure as in the measurement of the pore volume and pore specific surface area described above.

[0036] On the other hand, to measure the positive electrode active material in a charged state by powder X-ray diffraction, a storage element with a metallic lithium electrode as the counter electrode was adjusted to a fully discharged state using the same procedure as in the measurement of the pore volume and pore specific surface area described above, and then constant-current charging was performed at a current value of 0.1 C in an environment of 25°C until the terminal voltage reached 3.6 V, followed by constant-voltage charging at 3.6 V. The charge was terminated when the current value reached 0.02 C. The positive electrode was removed from this storage element, and the positive electrode mixture was sampled and subjected to measurement.

[0037] [Method of manufacturing the positive electrode active material for the energy storage device] The positive electrode active material can be produced, for example, by the following procedure. A mixed solution of FeSO4 and MnSO4 at an arbitrary ratio was added dropwise to a reaction vessel containing ion-exchanged water at a constant rate, while adding NaOH solution, NH3 solution, and NH2NH2 solution dropwise to maintain a constant pH. x Mn 1-x (OH) precursor is prepared. x Mn (1-x)The (OH)2 precursor is mixed with LiH2PO4 and sucrose powder in a solid phase, and then fired in a nitrogen atmosphere to produce a positive electrode active material having an olivine crystal structure and represented by the following formula 1: Life x Mn (1-x) PO4(0≦x≦1) 1

[0038] When producing a positive electrode active material that satisfies the above (A), the pore size range of 60 nm to 200 nm is a region where the non-aqueous electrolyte can easily penetrate, and the positive electrode active material has a pore volume in this pore size range of 0.05 cm 3 / g or more 0.25cm 3 / g or less, and the pore specific surface area in the pore diameter range of 10 nm to 200 nm is 5m 2 By making the pore volume at least 1 / g, it is possible to obtain a pore structure that promotes the penetration of non-aqueous electrolytes. x Mn 1-x This can be achieved by controlling the pH during preparation of the (OH)2 precursor. The pH range is preferably 8 to 11. If the pH exceeds 11, the pore size of the positive electrode active material may become too small, making it difficult for the non-aqueous electrolyte to penetrate the positive electrode active material. Furthermore, by using NH3 as a complexing agent and NH2NH2 as an antioxidant, the pore specific surface area of ​​the positive electrode active material can be adjusted to a favorable range. Without the use of NH3 and NH2NH2, the pore specific surface area of ​​the positive electrode active material may become too small, resulting in insufficient high-rate discharge performance in low-temperature environments. Thus, in this method for producing a positive electrode active material, by setting the pH within the above range and using the above NH3 and NH2NH2, a pore structure that promotes the penetration of the non-aqueous electrolyte can be obtained. As a result, it is believed that the lithium ion diffusibility in the positive electrode mixture layer is improved. Therefore, this positive electrode active material for an energy storage device can increase the capacity of an energy storage device during high-rate discharge in a low-temperature environment.

[0039] When producing a positive electrode active material that satisfies the above (B), the half-width of the peak corresponding to the (131) plane and the half-width of the peak corresponding to the (200) plane, as determined by powder X-ray diffraction using CuKα radiation in a charged state of the positive electrode active material, can be obtained by controlling the NH3 concentration. 3 More than 1mol / dm 3 The NH3 concentration is preferably 1 mol / dm or less. 3 If the NH3 concentration exceeds 0.25 mol / dm, the precursor may not be fully resolved to the desired composition. 3 If the concentration is less than 100%, uniform element distribution within a particle may not be achieved. Furthermore, by using NH2NH2 as an antioxidant for the precursor, the half-width of the peak of the positive electrode active material can be adjusted to a favorable range. Thus, in the manufacturing method of the positive electrode active material, by setting the NH3 concentration range and using the NH2NH2, a crystalline structure favorable for lithium ion diffusion within the solid phase can be obtained. As a result, high lithium ion diffusibility can be easily achieved even in low-temperature environments. Therefore, the positive electrode active material for an energy storage device can increase the capacity of the energy storage device during high-rate discharge in a low-temperature environment.

[0040] The positive electrode active material for an energy storage device can increase the capacity of the energy storage device during high-rate discharge in a low-temperature environment.

[0041] <Positive electrodes for energy storage elements> The positive electrode for an energy storage device (hereinafter also simply referred to as a positive electrode) contains the positive electrode active material. The positive electrode has a positive electrode substrate and a positive electrode mixture layer disposed on the positive electrode substrate directly or via an intermediate layer.

[0042] [Positive electrode substrate] The positive electrode substrate is electrically conductive. Examples of the material for the substrate include metals such as aluminum, titanium, tantalum, and stainless steel, or alloys thereof. Among these, aluminum and aluminum alloys are preferred in terms of the balance between high potential resistance, high conductivity, and cost. Examples of the form of the positive electrode substrate include foil and vapor-deposited film, with foil being preferred in terms of cost. In other words, aluminum foil is preferred as the positive electrode substrate. Examples of aluminum or aluminum alloys include A1085 and A3003 as specified in JIS-H4160 (2006).

[0043] The average thickness of the positive electrode substrate is preferably 5 μm or more and 50 μm or less, and more preferably 10 μm or more and 40 μm or less. By setting the average thickness of the positive electrode substrate within the above range, it is possible to increase the strength of the positive electrode substrate while increasing the energy density per volume of the energy storage device. The "average thickness of the substrate" refers to the value obtained by dividing the punched mass when a substrate of a predetermined area is punched out by the true density of the substrate and the punched area, and the same applies to the negative electrode substrate.

[0044] [Positive electrode mixture layer] The positive electrode mixture layer contains the above-described positive electrode active material.

[0045] The positive electrode mixture layer may further contain a positive electrode active material other than the above-described positive electrode active material having an olivine crystal structure. Such other positive electrode active material can be appropriately selected from known positive electrode active materials commonly used in lithium-ion secondary batteries, etc. However, the lower limit of the total content of the positive electrode active material having an olivine crystal structure relative to the total positive electrode active material contained in the positive electrode mixture layer is preferably 90 mass %, more preferably 99 mass %. In this way, by using essentially only the positive electrode active material having an olivine crystal structure as the positive electrode active material, the effects of the present invention can be further enhanced.

[0046] As a known positive electrode active material for a lithium ion secondary battery, a material capable of absorbing and releasing lithium ions is usually used. Examples of the known positive electrode active material include lithium transition metal composite oxides having an α-NaFeO2 type crystal structure, lithium transition metal composite oxides having a spinel type crystal structure, polyanion compounds, chalcogen compounds, sulfur, etc. As a lithium transition metal composite oxide having an α-NaFeO2 type crystal structure, for example, Li[Li x Ni 1-x ]O2(0≦x<0.5), Li[Li x Ni γ Co (1-x-γ) ]O2(0≦x<0.5, 0<γ<1), Li[Li x Co (1-x) ]O2(0≦x<0.5), Li[Li x Ni γ Mn (1-x-γ) ]O2(0≦x<0.5, 0<γ<1), Li[Li x Ni γ Mn β Co (1-x-γ-β) ]O2(0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1), Li[Li x Ni γ Co β Al (1-x-γ-β) ]O2 (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1). Lithium transition metal composite oxides with spinel crystal structure include Li x Mn2O4, Li x Ni γ Mn (2-γ)Examples of polyanion compounds include Li3V2(PO4)3, Li2MnSiO4, and Li2CoPO4F. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, and molybdenum dioxide. The atoms or polyanions in these materials may be partially substituted with atoms or anion species of other elements. The surfaces of these materials may be coated with other materials. In the positive electrode mixture layer, one of these materials may be used alone as a known positive electrode active material, or two or more of these materials may be used in combination. In the positive electrode mixture layer, one of these compounds may be used alone, or two or more of these compounds may be used in combination.

[0047] The content of the positive electrode active material in the positive electrode mixture layer is not particularly limited, but the lower limit is preferably 50 mass %, more preferably 80 mass %, and even more preferably 90 mass %, while the upper limit is preferably 99 mass %, more preferably 98 mass %.

[0048] The positive electrode mixture layer contains optional components such as a conductive agent, a binder, a thickener, and a filler, as required.

[0049] (Conductive agent) The conductive agent is not particularly limited as long as it is a material having electrical conductivity. Examples of such conductive agents include carbonaceous materials, metals, and conductive ceramics. Examples of carbonaceous materials include graphitized carbon, non-graphitized carbon, and graphene-based carbon. Examples of non-graphitized carbon include carbon nanofibers, pitch-based carbon fibers, and carbon black. Examples of carbon black include furnace black, acetylene black, and ketjen black. Examples of graphene-based carbon include graphene, carbon nanotubes (CNTs), and fullerenes. The conductive agent may be in the form of powder or fiber. As the conductive agent, one of these materials may be used alone, or two or more may be mixed. These materials may also be used in combination. For example, a composite of carbon black and CNTs may be used. Among these, carbon black is preferred from the viewpoints of electronic conductivity and coatability, and acetylene black is particularly preferred.

[0050] The content of the conductive agent in the positive electrode mixture layer is preferably 1% by mass to 10% by mass, more preferably 3% by mass to 9% by mass. By setting the content of the conductive agent within this range, the energy density of the energy storage element can be increased.

[0051] (binder) Examples of the binder include thermoplastic resins such as fluororesins (polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), etc.), polyethylene, polypropylene, and polyimide; elastomers such as ethylene propylene diene rubber (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), and fluororubber; and polysaccharide polymers.

[0052] The binder content in the positive electrode mixture layer is preferably 1% by mass to 10% by mass, more preferably 3% by mass to 9% by mass. By setting the binder content within this range, the positive electrode active material can be stably maintained.

[0053] (thickener) When an aqueous dispersion medium is used, the thickener used is a polysaccharide polymer such as carboxymethyl cellulose (CMC), methyl cellulose, etc. If the thickener has a functional group that reacts with lithium, it is preferable to deactivate this functional group in advance by methylation or the like.

[0054] (filler) The filler is not particularly limited, and examples of the filler include polyolefins such as polypropylene and polyethylene, inorganic oxides such as silicon dioxide, alumina, titanium dioxide, calcium oxide, strontium oxide, barium oxide, magnesium oxide, and aluminosilicates, hydroxides such as magnesium hydroxide, calcium hydroxide, and aluminum hydroxide, carbonates such as calcium carbonate, sparingly soluble ionic crystals such as calcium fluoride, barium fluoride, and barium sulfate, nitrides such as aluminum nitride and silicon nitride, substances derived from mineral resources such as talc, montmorillonite, boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite, and mica, and artificial products thereof.

[0055] The positive electrode mixture layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, and I; typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, and Ge; and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Nb, Sn, Sr, Ba, and W as components other than the positive electrode active material, conductive agent, binder, thickener, and filler.

[0056] (middle class) The intermediate layer is a coating layer on the surface of the positive electrode substrate, and contains conductive particles such as carbon particles to reduce the contact resistance between the positive electrode substrate and the positive electrode active material layer. The configuration of the intermediate layer is not particularly limited, and it can be formed, for example, from a composition containing a resin binder and conductive particles.

[0057] <Energy storage element> An energy storage element according to one embodiment of the present invention includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. Hereinafter, a non-aqueous electrolyte secondary battery will be described as an example of an energy storage element. The positive electrode and negative electrode are usually stacked or wound alternately with a separator interposed therebetween to form an electrode assembly. This electrode assembly is housed in a case, and the case is filled with a non-aqueous electrolyte. The non-aqueous electrolyte is interposed between the positive electrode and the negative electrode. The case may be a known metal case, resin case, or the like, that is typically used as a case for a non-aqueous electrolyte secondary battery.

[0058] [Positive electrode] The positive electrode of the energy storage element is as described above.

[0059] [Negative electrode] The negative electrode includes a negative electrode substrate and a negative electrode mixture layer laminated directly or indirectly on at least one surface of the negative electrode substrate. The negative electrode may also include an intermediate layer disposed between the negative electrode substrate and the negative electrode mixture layer.

[0060] (negative electrode substrate) The negative electrode substrate is conductive. Metals such as copper, nickel, stainless steel, nickel-plated steel, and aluminum, or alloys thereof, are used as the material for the negative electrode substrate. Among these, copper or a copper alloy is preferred. Examples of the negative electrode substrate include foils and vapor-deposited films, with foils being preferred from the viewpoint of cost. Therefore, copper foil or a copper alloy foil is preferred as the negative electrode substrate. Examples of copper foil include rolled copper foil and electrolytic copper foil.

[0061] The average thickness of the negative electrode substrate is preferably 2 μm to 35 μm, more preferably 3 μm to 30 μm, even more preferably 4 μm to 25 μm, and particularly preferably 5 μm to 20 μm. By setting the average thickness of the negative electrode substrate within the above range, the strength of the negative electrode substrate can be increased while increasing the energy density per volume of the secondary battery. The "average thickness of the substrate" refers to the value obtained by dividing the punched mass when a substrate of a predetermined area is punched out by the true density and punched area of ​​the substrate.

[0062] (Negative electrode mixture layer) The negative electrode mixture layer contains a negative electrode active material. The negative electrode mixture layer contains optional components such as a conductive agent, a binder, a thickener, and a filler, as needed. The optional components such as the conductive agent, the binder, the thickener, and the filler can be selected from the materials exemplified for the positive electrode above.

[0063] The negative electrode active material can be appropriately selected from known negative electrode active materials. A material capable of absorbing and releasing lithium ions is usually used as the negative electrode active material for lithium ion secondary batteries. Examples of the negative electrode active material include metallic Li; metals or semimetals such as Si and Sn; metal oxides or semimetal oxides such as Si oxide, Ti oxide, and Sn oxide; and Li4Ti5O 12 , LiTiO 2、 Examples of the material include titanium-containing oxides such as TiNbO; polyphosphate compounds; silicon carbide; and carbon materials such as graphite, non-graphitizable carbon (hard carbon), and graphitizable carbon (soft carbon). Among these materials, graphite and non-graphitizable carbon are preferred. In the negative electrode mixture layer, one of these materials may be used alone, or two or more may be used in combination.

[0064] "Graphite" refers to graphite that has an average lattice spacing (d 002 ) is a carbon material with a particle size of 0.33 nm or more and less than 0.34 nm. Examples of graphite include natural graphite and artificial graphite. "Non-graphitic carbon" refers to the carbon that has an average lattice spacing (d 002 ) is 0.34 nm or more and 0.42 nm or less. Non-graphitizable carbon includes non-graphitizable carbon and graphitizable carbon. Non-graphitizable carbon includes, for example, resin-derived materials, petroleum pitch or petroleum pitch-derived materials, petroleum coke or petroleum coke-derived materials, plant-derived materials, alcohol-derived materials, etc. "Non-graphitizable carbon" refers to a carbon material having the above d 002 The term "easily graphitizable carbon" refers to a carbon material having a particle size of 0.36 nm or more and 0.42 nm or less. 002This refers to carbon materials with a particle size of 0.34 nm or more and less than 0.36 nm.

[0065] Here, the "discharged state" of a carbon material refers to a state in which an open circuit voltage is 0.7 V or higher in a single-electrode battery using a negative electrode containing a carbon material as the negative electrode active material as the working electrode and metallic Li as the counter electrode. Since the potential of the metallic Li counter electrode in the open circuit state is approximately equal to the redox potential of Li, the open circuit voltage of the single-electrode battery is approximately equal to the potential of the negative electrode containing the carbon material relative to the redox potential of Li. In other words, an open circuit voltage of 0.7 V or higher in the single-electrode battery means that lithium ions capable of being absorbed and released during charging and discharging have been sufficiently released from the carbon material, which is the negative electrode active material.

[0066] The content of the negative electrode active material in the negative electrode mixture layer is preferably 60% by mass or more and 99% by mass or less, and more preferably 90% by mass or more and 98% by mass or less. By setting the content of the negative electrode active material within this range, both high energy density and manufacturability of the negative electrode mixture layer can be achieved.

[0067] The negative electrode mixture layer may contain typical non-metallic elements such as B, N, P, F, Cl, Br, and I; typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, Ge, Sn, Sr, and Ba; and transition metal elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, and W as components other than the negative electrode active material, conductive agent, binder, thickener, and filler.

[0068] (middle class) The intermediate layer is a coating layer on the surface of the negative electrode substrate, and contains conductive particles such as carbon particles to reduce the contact resistance between the negative electrode substrate and the negative electrode active material layer. As with the positive electrode, the configuration of the intermediate layer is not particularly limited, and it can be formed, for example, from a composition containing a resin binder and conductive particles.

[0069] [Non-aqueous electrolyte] The nonaqueous electrolyte may be a known nonaqueous electrolyte commonly used in general nonaqueous electrolyte secondary batteries (storage elements). The nonaqueous electrolyte includes a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent. The nonaqueous electrolyte may be a solid electrolyte or the like.

[0070] The nonaqueous solvent may be a known nonaqueous solvent commonly used as a nonaqueous solvent for general nonaqueous electrolytes for energy storage devices. Examples of the nonaqueous solvent include cyclic carbonates, chain carbonates, esters, ethers, amides, sulfones, lactones, and nitriles. Among these, it is preferable to use at least a cyclic carbonate or a chain carbonate, and it is more preferable to use a combination of a cyclic carbonate and a chain carbonate. When a cyclic carbonate and a chain carbonate are used in combination, the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate:chain carbonate) is not particularly limited, but is preferably, for example, 5:95 to 50:50.

[0071] Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), vinylethylene carbonate (VEC), chloroethylene carbonate, fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), styrene carbonate, catechol carbonate, 1-phenylvinylene carbonate, and 1,2-diphenylvinylene carbonate, with EC being preferred among these.

[0072] Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diphenyl carbonate, with EMC being preferred among these.

[0073] The electrolyte salt may be any known electrolyte salt commonly used in non-aqueous electrolytes for general energy storage elements, including lithium salts, sodium salts, potassium salts, magnesium salts, and onium salts, with lithium salts being preferred.

[0074] Examples of the lithium salt include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiClO4, and LiN(SO2F)2, and lithium salts having a hydrocarbon group in which hydrogen is substituted with fluorine, such as LiSO3CF3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), LiC(SO2CF3)3, and LiC(SO2C2F5)3. Among these, inorganic lithium salts are preferred, and LiPF6 is more preferred.

[0075] The lower limit of the concentration of the electrolyte salt in the nonaqueous electrolyte is 0.1 mol / dm 3 is preferred, and 0.3 mol / dm 3 More preferably, 0.5 mol / dm 3 is more preferably 0.7 mol / dm 3 On the other hand, the upper limit is not particularly limited, but is preferably 2.5 mol / dm 3 is preferred, and 2.0 mol / dm 3 More preferably, 1.5 mol / dm 3 is more preferable.

[0076] The non-aqueous electrolyte may contain other additives. Furthermore, the non-aqueous electrolyte may be a room temperature molten salt, an ionic liquid, or the like.

[0077] [Separator] The separator may be, for example, a woven fabric, a nonwoven fabric, or a porous resin film. Among these, a porous resin film is preferred from the viewpoint of strength, and a nonwoven fabric is preferred from the viewpoint of nonaqueous electrolyte retention. As the main component of the separator, a polyolefin such as polyethylene or polypropylene is preferred from the viewpoint of strength, and a polyimide or aramid is preferred from the viewpoint of resistance to oxidative decomposition. These resins may also be combined.

[0078] An inorganic layer may be disposed between the separator and the electrode. This inorganic layer is a porous layer also called a heat-resistant layer. Alternatively, a separator having an inorganic layer formed on one side of a porous resin film may be used. The inorganic layer is usually composed of inorganic particles and a binder, and may contain other components. The inorganic layer may be disposed on the surface facing the positive electrode, on the surface facing the negative electrode, or on both surfaces. Generally, the inorganic layer is preferably disposed on the surface facing the positive electrode in order to suppress denaturation due to the action of the positive electrode. On the other hand, in a storage element in which the negative electrode active material contains lithium metal, the inorganic layer is preferably disposed on the surface facing the negative electrode in order to reduce the risk of short circuiting due to the deposition of metallic lithium. Therefore, it may be preferable that the inorganic layer be disposed on both surfaces.

[0079] [Specific configuration of the energy storage element] The shape of the energy storage element of this embodiment is not particularly limited, and examples thereof include cylindrical batteries, pouch film batteries, prismatic batteries, flat batteries, coin batteries, and button batteries.

[0080] FIG. 1 shows an energy storage element 1 as an example of a prismatic battery. Note that this figure is a see-through view of the inside of a case 3. An electrode assembly 2 having a positive electrode and a negative electrode wound with a separator sandwiched between them is housed in the prismatic case 3. The positive electrode is electrically connected to a positive electrode terminal 4 via a positive electrode lead 41. The negative electrode is electrically connected to a negative electrode terminal 5 via a negative electrode lead 51. A non-aqueous electrolyte is poured into the case 3.

[0081] [Method of manufacturing an energy storage element] The energy storage element can be manufactured by a known method except for using the positive electrode active material as the positive electrode active material. The manufacturing method of the energy storage element according to this embodiment can be appropriately selected from known methods. The manufacturing method of the energy storage element includes, for example, the steps of preparing an electrode assembly, preparing a non-aqueous electrolyte, and housing the electrode assembly and the non-aqueous electrolyte in a case. The step of preparing the electrode assembly includes the steps of preparing a positive electrode and a negative electrode, and forming the electrode assembly by stacking or winding the positive electrode and the negative electrode with a separator interposed therebetween.

[0082] The step of housing the nonaqueous electrolyte in the case can be appropriately selected from known methods. For example, when a liquid nonaqueous electrolyte is used, the nonaqueous electrolyte is injected through an injection port formed in the case, and then the injection port is sealed. Details of the other components constituting the energy storage element obtained by the manufacturing method of the energy storage element are as described above.

[0083] [Other embodiments] The energy storage device according to the present invention is not limited to the above-described embodiments, and various modifications may be made without departing from the spirit of the present invention. For example, the configuration of one embodiment can be added to the configuration of another embodiment, or part of the configuration of one embodiment can be replaced with the configuration of another embodiment or well-known technology. Furthermore, part of the configuration of one embodiment can be deleted. Also, well-known technology can be added to the configuration of one embodiment.

[0084] In the above embodiment, the description has focused on the case where the energy storage element is a non-aqueous electrolyte secondary battery, but other energy storage elements may also be used. Examples of other energy storage elements include capacitors (electric double layer capacitors, lithium ion capacitors), etc. Examples of non-aqueous electrolyte secondary batteries include lithium ion non-aqueous electrolyte secondary batteries.

[0085] <Electricity storage device> An energy storage device according to one embodiment of the present invention includes a plurality of energy storage elements, and includes one or more of the above-described energy storage elements of the present invention. Furthermore, an energy storage unit can be configured by using one or more energy storage elements (cells) of the present invention, and this energy storage unit can be used to configure an energy storage device. The energy storage device can be used as a power source for automobiles such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). Furthermore, the energy storage device can be used in various power supply devices such as engine startup power supplies, auxiliary power supplies, and uninterruptible power supplies (UPS).

[0086] 2 shows an example of a power storage device 30 in which power storage units 20, each of which is an assembly of two or more electrically connected power storage elements 1, are further assembled. The power storage device 30 may include a bus bar (not shown) that electrically connects two or more power storage elements 1, and a bus bar (not shown) that electrically connects two or more power storage units 20. The power storage units 20 or the power storage device 30 may include a status monitoring device (not shown) that monitors the status of one or more power storage elements. [Example]

[0087] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0088] [Examples 1-1 to 1-10 and Comparative Examples 1-1 to 1-14] (Preparation of positive electrode active material) (1) Preparation of LiFePO4 (Examples 1-1 to 1-6 and Comparative Examples 1-1 to 1-8) 750cm 3 2 dm containing ion-exchanged water 3 In a reaction vessel, 1 mol / dm 3 The solution was added dropwise at a constant rate while maintaining the pH at the constant values ​​shown in Table 1. 3 NaOH solution and 0.5 mol / dm 3 NH3 aqueous solution and 0.5 mol / dm 3The NH2NH2 aqueous solution was added dropwise to prepare an Fe(OH)2 precursor. In Comparative Example 1-2, the pH was adjusted without adding an NH3 aqueous solution, and in Comparative Examples 1-3, 1-4, 1-10, and 1-13, the pH was adjusted without adding an NH2NH2 aqueous solution. The prepared Fe(OH)2 precursor was then solid-phase mixed with LiH2PO4 and sucrose powder. The mixture was then fired in a nitrogen atmosphere at the firing temperatures listed in Table 1 to prepare a positive electrode active material, LiFePO4, having an olivine crystal structure. (2) LiFe 0.5 Mn 0.5 Preparation of PO4 (Examples 7, 8, and Comparative Examples 9 to 11) 750cm 3 2 dm containing ion-exchanged water 3 In a reaction vessel, FeSO4 and MnSO4 were added in a molar ratio of 1:1, totaling 1 mol / dm 3 The aqueous solution was added dropwise at a constant rate while maintaining the pH at the constant values ​​shown in Table 1. 3 NaOH solution and 0.5 mol / dm 3 NH3 aqueous solution and 0.5 mol / dm 3 NH2NH2 aqueous solution was added dropwise, and Fe 0.5 Mn 0.5 The (OH)2 precursor was prepared. 0.5 Mn 0.5 The (OH) precursor was mixed with LiHPO and sucrose powder in a solid phase, and the mixture was fired in a nitrogen atmosphere at the firing temperature shown in Table 1 to obtain a positive electrode active material LiFe having an olivine crystal structure. 0.5 Mn 0.5 PO4 was produced. (3) LiFe 0.25 Mn 0.75 Preparation of PO4 (Examples 1-9, 1-10, and Comparative Examples 1-12 to 1-14) 750cm 3 2 dm containing ion-exchanged water 3 In a reaction vessel, FeSO4 and MnSO4 were added in a molar ratio of 1:3, totaling 1 mol / dm 3The solution was added dropwise at a constant rate while maintaining a constant pH of 4 mol / dm 3 NaOH solution and 0.5 mol / dm 3 NH3 aqueous solution and 0.5 mol / dm 3 NH2NH2 aqueous solution was added dropwise, and Fe 0.25 Mn 0.75 The (OH)2 precursor was prepared. 0.25 Mn 0.75 The (OH) precursor was mixed with LiHPO and sucrose powder in a solid phase, and the mixture was fired in a nitrogen atmosphere at the firing temperature shown in Table 1 to obtain a positive electrode active material LiFe having an olivine crystal structure. 0.25 Mn 0.75 PO4 was produced.

[0089] Table 1 shows the value of x, the pH during precursor preparation, the heat treatment temperature, the pore volume in the pore diameter range of 60 nm to 200 nm, and the specific surface area of ​​the pore diameter range of 10 nm to 200 nm for the positive electrode active materials of Examples 1-1 to 1-10 and Comparative Examples 1-1 to 1-14. The pore volume and specific surface area were measured according to the methods described above.

[0090] (Preparation of positive electrode) N-methylpyrrolidone (NMP) was used as the dispersion medium, and the above-mentioned positive electrode active material, acetylene black as a conductive agent, and PVdF as a binder were used. A suitable amount of NMP was added to a mixture of the positive electrode active material, conductive agent, and binder in a mass ratio of 90:5:5 to adjust the viscosity, thereby preparing a positive electrode mixture paste. Next, the above-mentioned positive electrode mixture paste was applied to both sides of an aluminum foil serving as a positive electrode substrate, leaving uncoated areas (areas where the positive electrode active material layer was not formed), and the mixture was dried at 120°C and roll-pressed to form a positive electrode mixture layer on the positive electrode substrate. The amount of the positive electrode mixture paste applied was 10 mg / cm in solids. 2 In this way, the positive electrodes of Examples 1-1 to 1-10 and Comparative Examples 1-1 to 1-14 were obtained.

[0091] (Preparation of negative electrode) Graphite was used as the negative electrode active material, SBR as the binder, and CMC as the thickener. The negative electrode active material, binder, and thickener were mixed in a mass ratio of 97:2:1, and an appropriate amount of water was added to adjust the viscosity to prepare a negative electrode mixture paste. This negative electrode mixture paste was applied to both sides of copper foil, the negative electrode substrate, leaving uncoated areas (areas where the negative electrode active material layer was not formed), and then dried to prepare a negative electrode active material layer. The negative electrode was then fabricated by roll pressing.

[0092] (Preparation of non-aqueous electrolyte) EC and EMC were mixed in a volume ratio of 3:7, and LiPF6 was added at 1 mol / dm 3 A non-aqueous electrolyte was prepared by dissolving the solution at a concentration of 100 ppm.

[0093] (Fabrication of energy storage element) Next, the positive electrode and the negative electrode were stacked via a separator made of a polyethylene substrate and an inorganic layer formed on the polyethylene substrate to prepare an electrode assembly. The inorganic layer was disposed on the surface facing the positive electrode. The electrode assembly was housed in an aluminum rectangular battery case, and a positive electrode terminal and a negative electrode terminal were attached. The nonaqueous electrolyte was poured into the case (rectangular battery case), which was then sealed to obtain the energy storage elements of Examples 1-1 to 1-10 and Comparative Examples 1-1 to 1-14.

[0094] (Capacity confirmation test) Each of the above storage elements was charged at a constant current of 0.1 C to 3.6 V at 25°C, followed by constant voltage charging at 3.6 V. The charge was terminated when the charge current reached 0.02 C. After a 10-minute pause, the element was discharged at a constant current of 0.1 C to 2.0 V at 25°C. A 10-minute pause was then allowed after discharge. The above cycle was repeated twice, and the second discharge capacity was taken as the 0.1 C capacity.

[0095] (Low temperature high rate discharge performance test: discharge capacity ratio) Each of the above storage elements was charged at a constant current of 0.1 C to 3.6 V at 25°C, followed by constant voltage charging at 3.6 V. The charging termination condition was set to 0.02 C. After a 10-minute pause, the element was discharged at a constant current of 2 C to 2.0 V at 25°C, and the "2C discharge capacity at 25°C" was measured. Next, the element was charged at a constant current of 0.1 C to 3.6 V at 25°C, followed by constant voltage charging at 3.6 V. The charging termination condition was set to 0.02 C. A 10-minute pause was then set. The element was then discharged at a constant current of 2 C to 2.0 V at 0°C, and the "2C discharge capacity at 0°C" was measured. From the 2C discharge capacity at 25°C and the 2C discharge capacity at 0°C, the "discharge capacity ratio," i.e., the percentage of the 2C discharge capacity at 0°C to the 2C discharge capacity at 25°C, was calculated as an index of low-temperature high-rate discharge performance. The values ​​are shown in Table 1. Furthermore, the relationship between the pore volume and the discharge capacity ratio in the pore diameter range of 60 nm to 200 nm is shown in Figure 3, and the relationship between the pore specific surface area and the discharge capacity ratio in the pore diameter range of 10 nm to 200 nm is shown in Figure 4.

[0096] Table 1 shows the results of the low-temperature high-rate discharge performance test.

[0097] [Table 1]

[0098] As shown in Table 1, FIGS. 3 and 4 above, the positive electrode active material has an olivine-type crystal structure, and at least a portion of the surface is covered with carbon. The pore volume in the pore diameter range of 60 nm to 200 nm, as determined by the BJH method from the desorption isotherm using the nitrogen gas adsorption method, is 0.05 cm. 3 / g or more 0.25cm 3 / g or less, and the pore specific surface area in the pore diameter range of 10 nm to 200 nm measured by nitrogen gas adsorption method is 5m 2It can be seen that Examples 1-1 to 1-10, in which the average capacitance is 1 / g or more, have larger capacities during high-rate discharge in a low-temperature environment than Comparative Examples 1-1 to 1-12.

[0099] In addition, the positive electrode active material is LiFePO4 (x=1), LiFe 0.5 Mn 0.5 PO4(x=0.5) and LiFe 0.25 Mn 0.75 When comparing the discharge capacity ratios for each of the LiFePO4 (x = 0.25) and LiFePO4 (x = 0.25), it can be seen that the larger the specific surface area of ​​the pores in the pore diameter range of 10 nm to 200 nm, the higher the discharge capacity ratio. 2 / g or more, the discharge capacity ratio was particularly excellent.

[0100] The above results demonstrate that a positive electrode active material that satisfies the above (A) can increase the capacity of an electricity storage device during high-rate discharge in a low-temperature environment.

[0101] [Examples 2-1 to 2-22 and Comparative Examples 2-1 to 2-7] (Preparation of positive electrode active material) (1) Preparation of LiFePO4 (Examples 2-1 to 2-14 and Comparative Examples 2-1 to 2-5) 750cm 3 2 dm containing ion-exchanged water 3 In a reaction vessel, 1 mol / dm 3 The solution of FeSO4 in water was added dropwise at a constant rate while maintaining the pH at a constant value. 3 NaOH solution, NH3 solution of the concentration shown in Table 2, and 0.5 mol / dm 3An NH2NH2 aqueous solution was added dropwise to prepare an Fe(OH)2 precursor. In Comparative Examples 2-1, 2-6, and 2-7, the pH was adjusted without adding an NH3 aqueous solution. The prepared Fe(OH)2 precursor was then solid-phase mixed with LiH2PO4 and sucrose powder. The carbon coating amount was 1% by mass relative to the total mass of the positive electrode active material in all of Examples 2-1 to 2-22 and Comparative Examples 2-1 to 2-7. Then, the mixture was fired in a nitrogen atmosphere at the firing temperatures listed in Table 2 to prepare a positive electrode active material LiFePO4 having an olivine crystal structure. (2) LiFe 0.5 Mn 0.5 Preparation of PO4 (Examples 2-15 to 2-18 and Comparative Example 2-6) 750cm 3 2 dm containing ion-exchanged water 3 In a reaction vessel, FeSO4 and MnSO4 were added in a molar ratio of 1:1, totaling 1 mol / dm 3 The solution was added dropwise at a constant rate while maintaining a constant pH of 4 mol / dm 3 NaOH solution, NH3 solution of the concentration shown in Table 2, and 0.5 mol / dm 3 NH2NH2 aqueous solution was added dropwise, and Fe 0.5 Mn 0.5 The (OH)2 precursor was prepared. 0.5 Mn 0.5 The (OH) precursor was mixed with LiHPO and sucrose powder in a solid phase, and the mixture was fired in a nitrogen atmosphere at the firing temperature shown in Table 2 to obtain a positive electrode active material LiFe having an olivine crystal structure. 0.5 Mn 0.5 PO4 was produced. (3) LiFe 0.25 Mn 0.75 Preparation of PO4 (Examples 2-19 to 2-22 and Comparative Example 2-7) 750cm 3 2 dm containing ion-exchanged water 3 In a reaction vessel, FeSO4 and MnSO4 were added in a molar ratio of 1:3, totaling 1 mol / dm 3The solution was added dropwise at a constant rate while maintaining a constant pH of 4 mol / dm 3 NaOH solution, NH3 solution of the concentration shown in Table 2, and 0.5 mol / dm 3 NH2NH2 aqueous solution was added dropwise, and Fe 0.25 Mn 0.75 The (OH)2 precursor was prepared. 0.25 Mn 0.75 The (OH) precursor was mixed with LiHPO and sucrose powder in a solid phase, and the mixture was fired in a nitrogen atmosphere at the firing temperature shown in Table 2 to obtain a positive electrode active material LiFe having an olivine crystal structure. 0.25 Mn 0.75 PO4 was produced.

[0102] Table 2 shows the x value, NH concentration, baking temperature, peak half-width ratio (200) / (131) measured by powder X-ray diffraction using CuKα radiation in the charged state, and the half-width of the peak corresponding to the (131) plane measured by powder X-ray diffraction using CuKα radiation in the discharged state for the positive electrode active materials of Examples 2-1 to 2-22 and Comparative Examples 2-1 to 2-7. The half-widths of the peaks were measured according to the method described above.

[0103] Using the positive electrode active materials of Examples 2-1 to 2-22 and Comparative Examples 2-1 to 2-7, energy storage elements of Examples 2-1 to 2-22 and Comparative Examples 2-1 to 2-7 were obtained in the same manner as in Example 1-1.

[0104] (Capacity confirmation test) A capacity confirmation test was carried out under the same conditions as above, and the 0.1 C capacity was measured.

[0105] (Low temperature high rate discharge performance test: discharge capacity ratio) A low-temperature high-rate discharge performance test was conducted under the same conditions as above, and the "2C discharge capacity at 25°C" and "2C discharge capacity at 0°C" were measured to determine the "discharge capacity ratio." These values ​​are shown in Table 2. The relationship between the half-width ratio (200) / (131) of the peaks of the positive electrode active material and the discharge capacity ratio is shown in FIG. 5, and the relationship between the half-width of the peak corresponding to the (131) plane of the positive electrode active material and the discharge capacity ratio is also shown in FIG. 5.

[0106] (Low temperature output performance test) The energy storage devices of Examples 2-1 to 2-22 and Comparative Examples 2-1 to 2-7 after one cycle of the capacity confirmation test were stored in a thermostatic chamber at 25°C for 3 hours, then subjected to constant-current charging at a charging current of 0.1 C up to a voltage at which the SOC (State of Charge) reached 50%, followed by constant-voltage charging at a voltage at which the SOC reached 50%. The charging was terminated when the charging current reached 0.02 C. The output power was then measured at 1 second after energization using the IV method in each thermostatic chamber at 0°C and 25°C. The ratio of the output power measured at 0°C to the output power measured at 25°C was calculated as the "output ratio."

[0107] Table 2 shows the results of the low-temperature high-rate discharge performance test and the low-temperature output performance test.

[0108] [Table 2]

[0109] As shown in Table 2, FIGS. 5 and 6 above, Examples 2-1 to 2-22, in which the positive electrode active material has an olivine crystal structure, at least a portion of the surface is coated with carbon, and in which the half-width ratio (200) / (131) of the peak corresponding to the (200) plane to the peak corresponding to the (131) plane is 1.10 or less as measured by powder X-ray diffraction using CuKα radiation in a charged state, have a superior discharge capacity ratio at 0°C to 25°C compared to Comparative Examples 2-1 to 2-7.

[0110] Furthermore, it can be seen that Examples 2-3 to 2-7, 2-10 to 2-14, 2-16, 2-17, 2-20, and 2-21, in which the half-width of the peak corresponding to the (131) plane measured by powder X-ray diffraction using CuKα radiation in the discharged state of the positive electrode active material is 0.110 or more and 0.155 or less, also have an excellent output ratio at 0°C to 25°C.

[0111] The above results demonstrate that a positive electrode active material that satisfies the above condition (B) can increase the capacity of an electricity storage device during high-rate discharge in a low-temperature environment. [Industrial Applicability]

[0112] The present invention can be applied to an energy storage element used as a power source for electronic devices such as personal computers and communication terminals, automobiles, and the like. [Explanation of symbols]

[0113] 1. Energy storage element 2 Electrode body 3 Cases 4 Positive terminal 41 Positive lead 5 Negative terminal 51 Negative lead 20 Energy storage unit 30 Energy storage device

Claims

1. It has an olivine-type crystal structure and comprises a compound represented by the following formula 1: At least a portion of the surface of the compound is coated with carbon, A positive electrode active material for a storage device that satisfies the following (B): (B) In a charged state, the half-width ratio (200) / (131) of the peak corresponding to the (200) plane to the peak corresponding to the (131) plane, measured by powder X-ray diffraction using CuKα radiation, is 0.95 or more and 1.10 or less. LiFe x Mn (1-x) PO 4 (0≦x≦1) ...1

2. 2. The positive electrode active material for a storage device according to claim 1, wherein the half width of the peak corresponding to the (131) plane measured by powder X-ray diffraction using CuKα rays in a discharged state is 0.110 or more and 0.155 or less.

3. A positive electrode for a storage device, comprising the positive electrode active material according to claim 1 or 2.

4. An electric storage element comprising the positive electrode according to claim 3 .

5. A power storage device comprising a plurality of power storage elements, and comprising one or more of the power storage elements according to claim 4.

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