Positive electrode active material and lithium-ion secondary battery

Abstract

The present invention aims to provide an Fe-based positive electrode active material capable of high-voltage operation, and a lithium-ion secondary battery containing the positive electrode active material. Such a positive electrode active material and lithium-ion secondary battery contribute to energy efficiency. [Solution] A positive electrode active material mainly composed of lithium iron composite fluoride and containing carbon, The lithium iron composite fluoride is a positive electrode active material represented by the following formula (1). Li x FeF (3+x) (1) In equation (1), x is a number that satisfies 0.4 ≤ x ≤ 1.2.

Description

[Technical Field]

[0001] This invention relates to a positive electrode active material and a lithium-ion secondary battery. [Background technology]

[0002] In recent years, research and development has been conducted on rechargeable batteries that contribute to energy efficiency, in order to ensure that more people have access to affordable, reliable, sustainable, and advanced energy. In particular, lithium-ion rechargeable batteries are becoming increasingly important as a power source for electric vehicles (EVs), hybrid electric vehicles (HEVs), and other similar vehicles.

[0003] The positive electrode active material is attracting attention as a crucial component that determines the capacity of lithium-ion secondary batteries, and its development is progressing. As a positive electrode active material used in lithium-ion secondary batteries, for example, iron (Fe)-based lithium iron phosphate (LiFePO4), which has low resource risk, is known. LiFePO4 has excellent cycle characteristics and safety, but because of its low voltage and small capacity, its energy density (voltage × capacity), which is expressed as the product of voltage and capacity, is small compared to conventionally used nickel (Ni) and cobalt (Co)-based materials. High energy density electrode materials are required to construct small batteries, and high voltage operation of the battery is important for achieving high energy density.

[0004] To increase the voltage of batteries using materials containing elements with low resource risk, the use of high-value transition metals (e.g., Fe 2+ ⇔Fe 3+ Instead, fate Fe 3+ ⇔Fe 4+ ) is expected. However, Fe 4+ It is very unstable, and side reactions occur with Fe 3+ And, or, Fe 4+ This requires a very large amount of energy and may not be generated. Therefore, Fe is used as the positive electrode active material. 3+ Even when using these compounds, high-voltage operation is not always possible.

[0005] For example, Non-Patent Document 1 reports that the use of ferric fluoride (FeF3) results in the formation of LiFeF3 during charge and discharge, and the average discharge voltage is 3.1V. Non-Patent Document 2 reports that LiFeO2 can be expected to have a high energy density.

Prior Art Documents

Non-Patent Documents

[0006]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] In Non-Patent Document 2, the actual voltage is about 2.5V, which is lower than the expected voltage. The average discharge voltage (3.1V) of Non-Patent Document 1 is also lower than that of LiFePO4, and there is room for improvement for further increasing the voltage.

[0008] The present invention has been made to solve the above problems, and an object thereof is to provide an Fe-based positive electrode active material capable of operating at a high voltage and a lithium ion secondary battery including the positive electrode active material. And it further reduces the resource risk and contributes to cost reduction.

Means for Solving the Problem

[0009] In order to achieve the above object, the present invention provides the following means. [1] A positive electrode active material mainly composed of lithium iron composite fluoride and containing carbon, where the lithium iron composite fluoride is represented by the following formula (1), and it is a positive electrode active material. Li x FeF (3+x) (1) In formula (1), x is a number satisfying 0.4 ≦ x ≦ 1.2.

[0010] [1] The positive electrode active material has a high average discharge voltage and can operate at a high voltage. Therefore, in a lithium ion secondary battery containing this positive electrode active material, the number of required batteries can be reduced, contributing to cost reduction.

[0011] [2] The positive electrode active material according to [1], wherein the mass ratio (M1:M2) of the mass (M1) of the lithium iron composite fluoride to the mass (M2) of the carbon is 90:10 to 60:40.

[0012] [2] The positive electrode active material according to [2] can increase the capacity of a lithium ion secondary battery containing this positive electrode active material and can further increase the energy density.

[0013] [3] The positive electrode active material according to [1] or [2], wherein the mass ratio (M1:M2) of the mass (M1) of the lithium iron composite fluoride to the mass (M2) of the carbon is 90:10 to 80:20.

[0014] [3] The positive electrode active material according to [3] can increase the capacity of a lithium ion secondary battery containing this positive electrode active material and can further increase the energy density.

[0015] [4] The positive electrode active material according to any one of [1] to [3], wherein the carbon is carbon nanotubes or carbon black.

[0016] The positive electrode active material described in [4] can increase the capacity of a lithium-ion secondary battery containing the positive electrode active material and further increase the energy density.

[0017] [5] A lithium-ion secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode contains the positive electrode active material described in any of [1] to [4].

[0018] The lithium-ion secondary battery described in [5] has a positive electrode containing the positive electrode active material described in any of [1] to [4]. This indicates that the battery can operate at high voltage.

[0019] [6] The lithium-ion secondary battery described in [5], wherein the dQ / dV plot during discharge of the charge-discharge cycle has a peak in the range of 3.94 to 4.01 V.

[0020] The lithium-ion secondary battery described in [6] shows that the positive electrode active material undergoes a chemical reaction in the high voltage range of 3.94 to 4.01 V. This indicates that the battery can operate at higher voltages.

[0021] [7] A lithium-ion secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, The positive electrode contains a positive electrode active material, The positive electrode active material mainly consists of lithium iron composite fluoride and contains carbon. The aforementioned lithium iron composite fluoride is represented by the following formula (1), A lithium-ion secondary battery in which the dQ / dV plot during the discharge phase of a charge-discharge cycle has a peak in the range of 3.94 to 4.01V. Li x FeF (3+x) (1) In equation (1), x is a number that satisfies 0.4 ≤ x ≤ 1.2.

[0022] The lithium-ion secondary battery described in [7] has a high average discharge voltage and can operate at high voltage. Therefore, it can reduce the number of batteries required and contribute to cost reduction. [Effects of the Invention]

[0023] According to the present invention, it is possible to provide an Fe-based positive electrode active material capable of high-voltage operation and a lithium-ion secondary battery containing the positive electrode active material. [Brief explanation of the drawing]

[0024] [Figure 1] This figure shows a portion of the X-ray diffraction pattern of a positive electrode active material according to one embodiment of the present invention. [Figure 2] This figure shows a portion of the X-ray diffraction pattern of a positive electrode active material according to another embodiment of the present invention. [Figure 3] This is a schematic cross-sectional view showing a lithium-ion secondary battery according to one embodiment of the present invention. [Figure 4] This graph shows the charge and discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 14. [Figure 5] This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Example 14. [Figure 6] This is a flowchart showing an example of a method for manufacturing a positive electrode active material according to one embodiment of the present invention. [Figure 7] This figure shows the X-ray diffraction patterns of the positive electrode active materials of Examples 1-5 and Comparative Example 1. [Figure 8] This figure shows the X-ray diffraction patterns of the positive electrode active materials of Examples 6 to 12. [Figure 9] This figure shows a portion of the X-ray diffraction pattern of the positive electrode active material in Example 8. [Figure 10] This figure shows a portion of the X-ray diffraction pattern of the positive electrode active material in Example 9. [Figure 11] This figure shows a portion of the X-ray diffraction pattern of the positive electrode active material in Example 13. [Figure 12] This figure shows a portion of the X-ray diffraction pattern of the positive electrode active material in Example 15. [Figure 13] This figure shows a portion of the X-ray diffraction pattern of the positive electrode active material in Example 16. [Figure 14] This figure shows a portion of the X-ray diffraction pattern of the positive electrode active material in Example 17. [Figure 15] This figure shows a portion of the X-ray diffraction pattern of the positive electrode active material in Example 18. [Figure 16] This figure shows a portion of the X-ray diffraction pattern of the positive electrode active material in Example 19. [Figure 17] This graph shows the charge and discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 1. [Figure 18] This graph shows the charge and discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 2. [Figure 19] This graph shows the charge-discharge curve of a lithium-ion secondary battery containing the positive electrode active material of Example 3. [Figure 20] This graph shows the charge-discharge curve of a lithium-ion secondary battery containing the positive electrode active material of Example 4. [Figure 21] This graph shows the charge and discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 5. [Figure 22] This graph shows the charge-discharge curve of a lithium-ion secondary battery containing the positive electrode active material of Example 6. [Figure 23] This graph shows the charge and discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 7. [Figure 24] This graph shows the charge and discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 8. [Figure 25] This graph shows the charge and discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 9. [Figure 26] This graph shows the charge and discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 10. [Figure 27] This graph shows the charge-discharge curve of a lithium-ion secondary battery containing the positive electrode active material of Example 12. [Figure 28] This graph shows the charge-discharge curve of a lithium-ion secondary battery containing the positive electrode active material of Example 13. [Figure 29]This graph shows the charge-discharge curve of a lithium-ion secondary battery containing the positive electrode active material of Example 15. [Figure 30] This graph shows the charge and discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 16. [Figure 31] This graph shows the charge and discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 17. [Figure 32] This graph shows the charge-discharge curve of a lithium-ion secondary battery containing the positive electrode active material of Example 18. [Figure 33] This graph shows the charge and discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 19. [Figure 34] This graph shows the charge-discharge curve of a lithium-ion secondary battery containing the positive electrode active material of Comparative Example 1. [Figure 35] This graph shows the charge and discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Comparative Example 2. [Figure 36] This graph shows the charge-discharge curve of a lithium-ion secondary battery containing the positive electrode active material of Comparative Example 3. [Figure 37] This graph shows the charge and discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Comparative Example 4. [Figure 38] This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Example 1. [Figure 39] This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Example 2. [Figure 40] This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Example 3. [Figure 41] This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Example 4. [Figure 42]This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Example 5. [Figure 43] This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Example 6. [Figure 44] This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Example 7. [Figure 45] This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Example 8. [Figure 46] This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Example 9. [Figure 47] This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Example 10. [Figure 48] This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Example 12. [Figure 49] This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Example 13. [Figure 50] This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Example 15. [Figure 51] This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Example 16. [Figure 52] This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Example 17. [Figure 53] This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Example 18. [Figure 54]This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Example 19. [Figure 55] This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Comparative Example 1. [Figure 56] This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Comparative Example 2. [Figure 57] This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Comparative Example 3. [Figure 58] This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Comparative Example 4. [Figure 59] This graph shows the charge-discharge curve of a lithium-ion secondary battery containing the positive electrode active material of Example 20. [Figure 60] This graph shows the dQ / dV plot during the charge-discharge cycle of a lithium-ion secondary battery containing the positive electrode active material of Example 20. [Modes for carrying out the invention]

[0025] Preferred embodiments of the present invention will be described in detail below.

[0026] [Cathode active material] The positive electrode active material of this embodiment mainly consists of lithium iron composite fluoride and contains carbon. The positive electrode active material of this embodiment is used as the positive electrode of a lithium-ion secondary battery. "Mainly consisting of lithium iron composite fluoride" means that the content of lithium iron composite fluoride is 50% by mass or more, preferably 60% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more, relative to the total mass of the positive electrode active material. The positive electrode active material may contain components other than the main component and carbon, as long as it does not impair the function of the present invention.

[0027] The positive electrode active material of this embodiment may contain only one type of lithium iron composite fluoride, or it may contain two or more types, as long as it is mainly composed of lithium iron composite fluoride.

[0028] When a positive electrode active material is manufactured using lithium iron composite fluoride as the main component, the overall composition ratio (Li:Fe:F) of the lithium iron composite fluoride is maintained in the resulting positive electrode active material. When a positive electrode active material obtained using lithium iron composite fluoride with such a composition as the main component is used in a secondary battery, high-voltage operation can be achieved. Furthermore, the composition ratio of the lithium iron composite fluoride is adjusted to be the same as the composition ratio required for the desired positive electrode active material.

[0029] (Lithium iron composite fluoride) The lithium iron composite fluoride of this embodiment is represented by the following formula (1). Li x FeF (3+x) (1) In equation (1), x is a number satisfying 0.4 ≤ x ≤ 1.2. Preferably, x is 0.5 ≤ x ≤ 1.0, and more preferably 0.6 ≤ x ≤ 0.9. When x is within the above numerical range, the average discharge voltage and capacity can be increased, and a small battery with high energy density can be constructed.

[0030] In equation (1), x represents the molar ratio of Li to Fe. The molar ratio of Li to Fe is x:1. Also, the molar ratio of Li, Fe, and F is x:1:(3+x). The composition of lithium iron composite fluoride can be determined by methods such as inductively coupled plasma (ICP) emission spectroscopy and combustion ion chromatography.

[0031] (carbon) In this embodiment, carbon refers to elemental carbon. Examples of elemental carbon include carbon nanotubes, carbon black, graphite, and diamond. From the viewpoint of more favorably obtaining the effects of the present invention, carbon nanotubes or carbon black are preferred among these elemental carbons.

[0032] The carbon content in this embodiment is the mass ratio (M1:M2) of the mass (M1) of lithium iron composite fluoride to the mass (M2) of carbon, and 90:10 to 60:40 is preferable, 90:10 to 70:30 is more preferable, and 90:10 to 80:20 is even more preferable. When the mass ratio (M1:M2) is within the above numerical range, the capacity of the lithium-ion secondary battery can be increased, and the energy density can be further increased.

[0033] <X-ray diffraction (XRD) pattern> FIG. 1 shows a part of the XRD pattern of Example 7 described later as an example of the X-ray diffraction (XRD) pattern of the positive electrode active material according to this embodiment. As shown in FIG. 1, the positive electrode active material of Example 7 has peaks in the ranges of 20° ≤ 2θ < 25° and 25° ≤ 2θ ≤ 30°, respectively. The peak in the range of 20° ≤ 2θ < 25° represents a peak derived from the crystal structure of FeF3 which is trivalent iron. The peak in the range of 25° ≤ 2θ ≤ 30° represents a peak derived from the crystal structure of LiFe2F6. This means that the positive electrode active material of Example 7 has a crystal structure similar to that of FeF3 and LiFe2F6.

[0034] Next, FIG. 2 shows a part of the XRD pattern of Example 14 described later. As shown in FIG. 2, in the positive electrode active material of Example 14, the peak in the range of 20° ≤ 2θ < 25° has disappeared. This means that the positive electrode active material of Example 14 is in a single-phase state having only a crystal structure similar to that of LiFe2F6. Thus, by controlling the crystal structure in the lithium iron composite fluoride contained in the positive electrode active material of this embodiment to be in a single-phase state, the capacity of the lithium-ion secondary battery containing the positive electrode active material can be made larger, and the energy density can be further increased.

[0035] [Lithium-ion secondary battery] The lithium-ion secondary battery of this embodiment includes a positive electrode, a negative electrode, and an electrolyte, and the positive electrode contains a positive electrode active material mainly composed of the above-mentioned lithium iron composite fluoride. The lithium-ion secondary battery of this embodiment may include other battery elements as necessary.

[0036] The lithium-ion secondary battery of this embodiment can use the same battery elements as known lithium-ion secondary batteries, except that the positive electrode contains a positive electrode active material mainly composed of the aforementioned lithium iron composite fluoride and containing carbon. The lithium-ion secondary battery of this embodiment may have any of the following configurations: coin type, button type, cylindrical type, prismatic type, or laminate type. Furthermore, the lithium-ion secondary battery of this embodiment can be applied to a wide range of applications, such as mobile devices like mobile phones and laptop computers, and in-vehicle applications.

[0037] The following description of the lithium-ion secondary battery of this embodiment will focus on a lithium-ion secondary battery using an electrolyte (coin-type lithium-ion secondary battery). Each battery element described below can also be applied to all-solid-state lithium-ion secondary batteries and semi-solid-state lithium-ion secondary batteries that do not use an electrolyte.

[0038] As shown in Figure 3, the lithium-ion secondary battery 1 of this embodiment comprises a negative electrode can (negative electrode terminal) 20, a negative electrode 3, a separator 4 impregnated with electrolyte, an insulating packing (gasket) 5, a positive electrode 2, and a positive electrode can 10.

[0039] The positive electrode can 10 is positioned below the separator 4, and the negative electrode can 20 is positioned above the separator 4. The positive electrode can 10 and the negative electrode can 20 form the outer shape of the lithium-ion secondary battery 1. Between the positive electrode can 10 and the negative electrode can 20, the positive electrode 2 and the negative electrode 3 are provided via the separator 4, which is impregnated with electrolyte. The positive electrode can 10 and the negative electrode can 20 are electrically insulated by an insulating packing 5.

[0040] The lithium-ion secondary battery 1 can be manufactured by preparing a positive electrode composite material by adding a conductive agent, a binder, etc., as needed to the positive electrode active material of this embodiment, and then pressing this composite material onto a current collector (not shown). Preferably, stainless steel mesh, aluminum foil, etc., can be used as the current collector. Preferably, carbon nanotubes (CNTs), acetylene black, ketchen black, etc., can be used as the conductive agent. Preferably, tetrafluoroethylene, polyvinylidene fluoride, etc., can be used as the binder.

[0041] The composition of the positive electrode active material, conductive agent, and binder in the positive electrode mixture is not particularly limited. The content of the positive electrode active material in the positive electrode mixture is preferably 75 to 100% by mass, and more preferably 90 to 99% by mass. The content of the conductive agent in the positive electrode mixture is preferably 1 to 15% by mass, and more preferably 0.1 to 5% by mass. The content of the binder in the positive electrode mixture is preferably 0.1 to 10% by mass, and more preferably 0.1 to 5% by mass.

[0042] In a lithium-ion secondary battery 1, the negative electrode 3 relative to the positive electrode 2 can be any known material that functions as a negative electrode active material and is capable of intercalating and releasing lithium, such as metallic materials like metallic lithium and lithium alloys, carbon-based materials like graphite and MCMB (mesocarbon microbeads), or silicon-based materials like silicon (Si), Si alloys, and silicon oxide. Among these, metallic lithium and graphite are preferred as the negative electrode 3.

[0043] The separator 4 and battery container (positive electrode container 10, negative electrode container 20) can use known battery components.

[0044] As the electrolyte, known electrolytes, semi-solid electrolytes, solid electrolytes, etc., can be used. As an electrolyte, for example, an electrolyte such as lithium perchlorate or lithium hexafluoride phosphate can be dissolved in a solvent such as ethylene carbonate (EC), dimethyl carbonate (DMC), propylene carbonate (PC), or diethyl carbonate (DEC) can be used.

[0045] As semi-solid and solid electrolytes, known semi-solid and solid electrolytes can be used, except for using the above-mentioned lithium iron composite fluoride as the main component and a positive electrode active material containing carbon. Examples of semi-solid electrolytes include electrolytes composed of polymer components and standard electrolytes. Examples of polymer components include polyvinylidene fluoride (PVDF) / polyethylene oxide (PEO), polyacrylonitrile (PAN) / PEO, polymethyl methacrylate (PMMA), PVDF / hexafluoropropylene (HFP), etc. Examples of standard electrolytes include 1 mol / L lithium hexafluorophosphate (LiPF6) EC / DMC solution, 1 mol / L LiPF6EC / ethyl methyl carbonate (EMC) solution, 1 mol / L LiPF6EC / DMC / EMC solution, etc.

[0046] In the case of all-solid-state lithium-ion secondary batteries, the electrolyte can be a polymer-based solid electrolyte such as a polyethylene oxide-based polymer compound, a polymer compound containing at least one of a polyorganosiloxane chain or a polyoxyalkylene chain, a sulfide-based solid electrolyte, or an oxide-based solid electrolyte.

[0047] For the positive electrode of an all-solid-state lithium-ion secondary battery, for example, in addition to the positive electrode active material, conductive agent, and binder described above, a positive electrode composite material containing a solid electrolyte can be supported on a positive electrode current collector made of aluminum, nickel, stainless steel, or the like.

[0048] The lithium-ion secondary battery 1 of this embodiment can operate at high voltage because the positive electrode 2 contains the positive electrode active material of this embodiment.

[0049] <dQ / dV plot of charge / discharge cycles> Figure 4 shows an example of a charge-discharge curve for the lithium-ion secondary battery of this embodiment, specifically for Embodiment 14, which will be described later. The horizontal axis of the graph in Figure 4 represents the capacity of the lithium-ion secondary battery. The vertical axis of the graph in Figure 4 represents the voltage of the lithium-ion secondary battery during charging and discharging. In the charge-discharge curve in Figure 4, the upward-sloping curve represents the curve during charging, and the downward-sloping curve represents the curve during discharging. The lithium-ion secondary battery shown in Figure 4 has a capacity of 56.2 mAh / g.

[0050] Figure 5 shows the dQ / dV plot for the charge-discharge cycle in Figure 4. The horizontal axis of the graph in Figure 5 represents the voltage during the charge-discharge cycle. The vertical axis of the graph in Figure 5 represents the value obtained by differentiating the capacitance with respect to voltage in Figure 4 (dQ / dV plot, dQdV). -1 The plot is shown. In Figure 5, the upward-convex curve represents the curve during charging, and the downward-convex curve represents the curve during discharging.

[0051] As shown in Figure 5, the charging curve has a peak at 4.01V. The discharging curve has a peak at 3.94V. These peaks indicate that a chemical reaction is occurring in the positive electrode active material during charging or discharging. This indicates that a chemical reaction is occurring at a high voltage of 3.94V during discharging, demonstrating that the lithium-ion secondary battery can operate at high voltage.

[0052] In this embodiment, it is preferable that the dQ / dV plot during discharge in a charge-discharge cycle has a peak in the range of 3.94 to 4.01V. Having a peak in the dQ / dV plot during discharge in a charge-discharge cycle within the above numerical range means that the lithium-ion secondary battery can operate at a higher voltage. In this specification, a dQ / dV plot having a "peak" means that the height (or "depth" in the case of a discharge plot) of the dQ / dV plot is 40 mAhg. -1 V -1 This means that it has peaks (or valleys in the case of a plot during discharge) of the above magnitude.

[0053] The lithium-ion secondary battery of this embodiment has a peak depth of 40 mAhg in the dQ / dV plot during discharge in the charge-discharge cycle. -1 V -1 Preferably, it should be 100mAhg -1 V -1 It is more preferable that the above be the case, and 200mAhg -1 V -1More preferably, the above, and 500mAhg -1 V -1 It is particularly preferable that the values ​​be greater than or equal to the above lower limit. If the peak depth of the dQ / dV plot is greater than or equal to the above lower limit, the capacity of the lithium-ion secondary battery can be increased. The maximum value of the peak depth of the dQ / dV plot is not particularly limited, but for example, 5000 Ahg -1 V -1 The following are preferable. The "peak depth" is given by the depth of the valley (the absolute value of the valley bottom) in the dQ / dV plot.

[0054] [Method for manufacturing positive electrode active material] The positive electrode active material of this embodiment mainly consists of the above-mentioned lithium iron composite fluoride and contains carbon. As the lithium source for the lithium iron composite fluoride, known compounds such as halides such as lithium fluoride (LiF), hydroxides such as lithium hydroxide monohydrate (LiOH·H2O), carbonates such as lithium carbonate (Li2CO3), and acetates such as lithium acetate (CH3COOLi) and lithium acetate dihydrate (CH3COOLi·2H2O) can be used, and there are no particular restrictions.

[0055] As an iron source for lithium iron composite fluoride, trivalent iron is preferred over divalent iron, and ferric fluoride (FeF3) is more preferred, because it can be operated at high voltage.

[0056] When producing lithium iron composite fluoride, the lithium source and iron source described above are mixed and subjected to mechanical treatment (first mechanical treatment) for a predetermined time and under predetermined conditions. For example, when lithium fluoride is used as the lithium source and trivalent iron (FeF3) is used as the iron source, it is thought that the compound represented by formula (1) above is formed by the following reaction. LiF + FeF3 → LiFeF4 (compound where x=1 in equation (1) above)

[0057] The value of x in the compound represented by formula (1) can be adjusted by the molar ratio of LiF to FeF3.

[0058] The specific means applied during the first mechanical processing are not particularly limited, but various means conventionally used for the purpose of grinding and mixing solid materials can be applied. Of these means, a ball mill is preferred, and a planetary ball mill is more preferred because it can sufficiently grind and mix the raw materials.

[0059] The time required for the first mechanical processing is preferably 8 to 12 hours, and more preferably 9 to 11 hours. As a condition for the first mechanical processing, the rotational speed is preferably 250 to 450 rpm, and more preferably 300 to 400 rpm. The temperature during the first mechanical process is not particularly limited and can be performed at room temperature (for example, 5-30°C). The atmosphere during the first mechanical treatment is preferably an inert gas (such as a noble gas like argon (Ar), or nitrogen (N2) gas).

[0060] The lithium iron composite fluoride obtained by the first mechanical treatment is mixed with carbon and subjected to a mechanical treatment (second mechanical treatment) for a predetermined time and under predetermined conditions. By performing a second mechanical process, the capacity and rate characteristics of the lithium-ion secondary battery can be improved. Examples of carbon to be added include elemental carbon as described above, and carbon nanoparticles are preferred. Examples of carbon nanoparticles include carbon nanotubes (CNTs) and carbon black. Among these carbon nanoparticles, CNTs are preferred from the viewpoint of further improving the conductivity of the positive electrode active material. The conditions for the second mechanical treatment (means, time, rotational speed, temperature, atmosphere, etc.) are the same as those for the first mechanical treatment.

[0061] It is preferable to heat-treat the composite of lithium iron composite fluoride and carbon obtained by the second mechanical treatment. By heat-treating, the crystal structure of the lithium iron composite fluoride changes, and the capacity of a lithium-ion secondary battery using this composite as a positive electrode active material can be increased. This is thought to be because the composition ratio of the LiFe2F6 crystal structure and the FeF3 crystal structure in lithium iron composite fluoride changes due to heat treatment, resulting in an increase in the LiFe2F6 crystal structure.

[0062] By heat-treating a composite of lithium iron composite fluoride and carbon, the capacity of a lithium-ion secondary battery containing this composite as a positive electrode active material can be increased, and the energy density can be further enhanced.

[0063] The firing temperature in the heat treatment is preferably 100 to 300°C, more preferably 150 to 250°C, and even more preferably 175 to 225°C. The firing time in the heat treatment is preferably 0.5 to 20 hours, more preferably 2 to 15 hours, and even more preferably 4 to 8 hours. The atmosphere used in heat treatment is preferably an inert gas (such as a noble gas like argon (Ar), or nitrogen (N2) gas). The pressure used in heat treatment can be atmospheric pressure (0.1013 MPa), but a low vacuum (for example, 10 MPa) is also acceptable. 2 Pa~10 5 Pa) is preferred.

[0064] The composite of the lithium iron composite fluoride and carbon described above may be used as the positive electrode active material, or a composite that has been heat-treated (heat-treated composite) may be used as the positive electrode active material. By using the obtained positive electrode active material as the positive electrode to fabricate a lithium-ion secondary battery, a battery that operates at high voltage can be obtained.

[0065] Figure 6 shows a flowchart illustrating the method for producing the positive electrode active material of this embodiment. The meanings of the terms in Figure 6 are the same as those described above. [Examples]

[0066] The following describes embodiments of the present invention, but the present invention is not limited to the following embodiments.

[0067] [Example 1] (Li0.6 FeF 3.6 (Preparation of a complex with compound x=0.6 in formula (1), containing CNT=10% by mass) 0.439 g of ferric fluoride (FeF3) and 0.0606 g of lithium fluoride (LiF) were subjected to a first mechanical treatment using a planetary ball mill. The planetary ball mill used was a Fritsh Premium line PL-7. The pot and balls were made of zirconium oxide, and 50 g of 5 mm diameter balls were used in an 80 mL pot. The treatment conditions for the first mechanical treatment were 350 rpm for 10 hours. Subsequently, 0.55 g of carbon nanotubes (CNTs) were added to the pot, and a second mechanical treatment was performed to obtain the cathode active material. The treatment conditions for the second mechanical treatment were under an Ar atmosphere, at 25°C, 350 rpm, and for 10 hours.

[0068] The obtained positive electrode active material was subjected to X-ray diffraction measurements according to the following measurement conditions. The results are shown in Figure 7. X-ray diffraction measurement conditions X-ray diffractometer: SmartLab, manufactured by Rigaku Corporation. X-ray source: CuKα ray (CuKα=1.5418Å) Aperture angle of the incident parallel slit: 5.0° Length of the incidence longitudinal limiting slit: 5.0 mm Aperture angle of the light-receiving parallel slit: 5.0° Kβ filter: Used Step width: 0.01° Entrance slit: 1 / 6° Light-receiving slit 1: 4.0 mm Light-receiving slit 2: 13mm

[0069] As shown in Figure 7, the obtained cathode active material matched both the diffraction pattern of FeF3 (DB card number 00-061-0194, space group R-3c, trigonal crystal system) and the diffraction pattern of LiFe2F6 (DB card number 01-074-2193, space group P42 / mnm, tetragonal crystal system), indicating that it has a similar crystal structure to FeF3 and LiFe2F6.

[0070] [Example 2] (Li 0.7 FeF 3.7 (Preparation of a complex with compound x=0.7 in formula (1), containing 10% by mass of CNTs) A positive electrode active material was obtained in the same manner as in Example 1, except that 0.431 g of ferric fluoride (FeF3) and 0.0693 g of lithium fluoride (LiF) were subjected to a first mechanical treatment using a planetary ball mill apparatus. The obtained positive electrode active material was subjected to X-ray diffraction measurements under the same conditions as in Example 1. The results are shown in Figure 7. As shown in Figure 7, the obtained cathode active material matched both the diffraction pattern of FeF3 (DB card number 00-061-0194, space group R-3c, trigonal crystal system) and the diffraction pattern of LiFe2F6 (DB card number 01-074-2193, space group P42 / mnm, tetragonal crystal system), indicating that it has a similar crystal structure to FeF3 and LiFe2F6.

[0071] [Example 3] (Li 0.8 FeF 3.8 (Preparation of a complex with compound x=0.8 in formula (1), CNT=10 mass%) A positive electrode active material was obtained in the same manner as in Example 1, except that 0.422 g of ferric fluoride (FeF3) and 0.078 g of lithium fluoride (LiF) were subjected to a first mechanical treatment using a planetary ball mill apparatus. The obtained positive electrode active material was subjected to X-ray diffraction measurements under the same conditions as in Example 1. The results are shown in Figure 7. As shown in Figure 7, the obtained cathode active material matched both the diffraction pattern of FeF3 (DB card number 00-061-0194, space group R-3c, trigonal crystal system) and the diffraction pattern of LiFe2F6 (DB card number 01-074-2193, space group P42 / mnm, tetragonal crystal system), indicating that it has a similar crystal structure to FeF3 and LiFe2F6.

[0072] [Example 4] (Preparation of a complex containing LiFeF4 (compound x=1.0 in formula (1)) and CNT=10% by mass) A positive electrode active material was obtained in the same manner as in Example 1, except that 0.407 g of ferric fluoride (FeF3) and 0.0934 g of lithium fluoride (LiF) were subjected to a first mechanical treatment using a planetary ball mill apparatus. The obtained positive electrode active material was subjected to X-ray diffraction measurements under the same conditions as in Example 1. The results are shown in Figure 7. As shown in Figure 7, the obtained cathode active material matched both the diffraction pattern of FeF3 (DB card number 00-061-0194, space group R-3c, trigonal crystal system) and the diffraction pattern of LiFe2F6 (DB card number 01-074-2193, space group P42 / mnm, tetragonal crystal system), indicating that it has a similar crystal structure to FeF3 and LiFe2F6.

[0073] [Example 5] (Li 1.2 FeF 4.2 (Preparation of a complex with compound x=1.2 in formula (1), CNT=10 mass%) A positive electrode active material was obtained in the same manner as in Example 1, except that 0.391 g of ferric fluoride (FeF3) and 0.108 g of lithium fluoride (LiF) were subjected to a first mechanical treatment using a planetary ball mill apparatus. The obtained positive electrode active material was subjected to X-ray diffraction measurements under the same conditions as in Example 1. The results are shown in Figure 7. As shown in Figure 7, the obtained cathode active material matched both the diffraction pattern of FeF3 (DB card number 00-061-0194, space group R-3c, trigonal crystal system) and the diffraction pattern of LiFe2F6 (DB card number 01-074-2193, space group P42 / mnm, tetragonal crystal system), indicating that it has a similar crystal structure to FeF3 and LiFe2F6.

[0074] [Comparative Example 1] (Preparation of a composite of LiFeF3 and CNTs = 10% by mass) A positive electrode active material was obtained in the same manner as in Example 1, except that 0.392 g of ferrous fluoride (FeF2) and 0.108 g of lithium fluoride (LiF) were subjected to a first mechanical treatment using a planetary ball mill apparatus. The obtained positive electrode active material was subjected to X-ray diffraction measurements under the same conditions as in Example 1. The results are shown in Figure 7. As shown in Figure 7, the obtained cathode active material matched the diffraction pattern of LiFe2F6 with DB card number 01-074-2193, space group P42 / mnm, and crystal system tetragonal, indicating that it has a similar crystal structure to LiFe2F6.

[0075] [Example 6] The positive electrode active material obtained in Example 1 was subjected to 10 minutes of exposure to an argon gas atmosphere. 3 The sample was heat-treated in an oven at 200°C for 5 hours using Pa. The obtained positive electrode active material was subjected to X-ray diffraction measurements under the same conditions as in Example 1. The results are shown in Figure 8. As shown in Figure 8, the obtained cathode active material matched both the diffraction pattern of FeF3 (DB card number 00-061-0194, space group R-3c, trigonal crystal system) and the diffraction pattern of LiFe2F6 (DB card number 01-074-2193, space group P42 / mnm, tetragonal crystal system), indicating that it has a similar crystal structure to FeF3 and LiFe2F6.

[0076] [Example 7] The positive electrode active material obtained in Example 2 was subjected to heat treatment under the same conditions as in Example 6, and the resulting positive electrode active material was subjected to X-ray diffraction measurement under the same conditions as in Example 1. The results are shown in Figure 8. As shown in Figure 8, the obtained cathode active material matched both the diffraction pattern of FeF3 (DB card number 00-061-0194, space group R-3c, trigonal crystal system) and the diffraction pattern of LiFe2F6 (DB card number 01-074-2193, space group P42 / mnm, tetragonal crystal system), indicating that it has a similar crystal structure to FeF3 and LiFe2F6. Furthermore, a portion of the XRD pattern (enlarged view for 20°≦2θ≦30°) from the results is shown in Figure 1. As shown in Figure 1, the obtained positive electrode active material was found to have peaks originating from the crystal structure of FeF3 and peaks originating from the crystal structure of LiFe2F6.

[0077] [Example 8] The positive electrode active material obtained in Example 3 was subjected to heat treatment under the same conditions as in Example 6, and the resulting positive electrode active material was subjected to X-ray diffraction measurement under the same conditions as in Example 1. The results are shown in Figure 8. As shown in Figure 8, the obtained cathode active material matched both the diffraction pattern of FeF3 (DB card number 00-061-0194, space group R-3c, trigonal crystal system) and the diffraction pattern of LiFe2F6 (DB card number 01-074-2193, space group P42 / mnm, tetragonal crystal system), indicating that it has a similar crystal structure to FeF3 and LiFe2F6. Furthermore, a portion of the XRD pattern (enlarged view for 20°≦2θ≦30°) from the results is shown in Figure 9. As shown in Figure 9, the obtained cathode active material was found to have peaks originating from the crystal structure of FeF3 and peaks originating from the crystal structure of LiFe2F6.

[0078] [Example 9] The positive electrode active material obtained in Example 4 was subjected to heat treatment under the same conditions as in Example 6, and the resulting positive electrode active material was subjected to X-ray diffraction measurement under the same conditions as in Example 1. The results are shown in Figure 8. As shown in Figure 8, the obtained cathode active material matched both the diffraction pattern of FeF3 (DB card number 00-061-0194, space group R-3c, trigonal crystal system) and the diffraction pattern of LiFe2F6 (DB card number 01-074-2193, space group P42 / mnm, tetragonal crystal system), indicating that it has a similar crystal structure to FeF3 and LiFe2F6. Furthermore, a portion of the XRD pattern (enlarged view for 20°≦2θ≦30°) from the results is shown in Figure 10. As shown in Figure 10, the obtained cathode active material was found to have peaks originating from the crystal structure of FeF3 and peaks originating from the crystal structure of LiFe2F6.

[0079] [Example 10] The positive electrode active material obtained in Example 5 was subjected to heat treatment under the same conditions as in Example 6, and the resulting positive electrode active material was subjected to X-ray diffraction measurement under the same conditions as in Example 1. The results are shown in Figure 8. As shown in Figure 8, the obtained cathode active material matched both the diffraction pattern of FeF3 (DB card number 00-061-0194, space group R-3c, trigonal crystal system) and the diffraction pattern of LiFe2F6 (DB card number 01-074-2193, space group P42 / mnm, tetragonal crystal system), indicating that it has a similar crystal structure to FeF3 and LiFe2F6.

[0080] [Example 11] (Li 0.4 FeF 3.4 (Preparation of a complex with compound x=0.4 in formula (1), CNT=10 mass%) A positive electrode active material was obtained in the same manner as in Example 1, except that 0.458 g of ferric fluoride (FeF3) and 0.0421 g of lithium fluoride (LiF) were mechanically processed using a planetary ball mill. The obtained positive electrode active material was subjected to X-ray diffraction measurements under the same conditions as in Example 1. The results are shown in Figure 8. As shown in Figure 8, the obtained cathode active material matched both the diffraction pattern of FeF3 (DB card number 00-061-0194, space group R-3c, trigonal crystal system) and the diffraction pattern of LiFe2F6 (DB card number 01-074-2193, space group P42 / mnm, tetragonal crystal system), indicating that it has a similar crystal structure to FeF3 and LiFe2F6.

[0081] [Example 12] The positive electrode active material obtained in Example 11 was subjected to heat treatment under the same conditions as in Example 6, and the resulting positive electrode active material was subjected to X-ray diffraction measurement under the same conditions as in Example 1. The results are shown in Figure 8. As shown in Figure 8, the obtained cathode active material matched both the diffraction pattern of FeF3 (DB card number 00-061-0194, space group R-3c, trigonal crystal system) and the diffraction pattern of LiFe2F6 (DB card number 01-074-2193, space group P42 / mnm, tetragonal crystal system), indicating that it has a similar crystal structure to FeF3 and LiFe2F6.

[0082] [Example 13] (Li 0.6 FeF 3.6 (Preparation of a complex with compound x=0.6 in formula (1), containing 20% ​​by mass of CNTs) The positive electrode active material was obtained in the same manner as in Example 6, except that the amount of CNTs added to the pot was 0.125 g. The obtained positive electrode active material was subjected to X-ray diffraction measurements under the same conditions as in Example 1. The results are shown in Figure 11.

[0083] [Example 14] (Li 0.7 FeF 3.7 (Preparation of a complex with compound x=0.7 in formula (1), containing 20% ​​by mass of CNTs) The positive electrode active material was obtained in the same manner as in Example 7, except that the amount of CNTs added to the pot was 0.125 g. The obtained positive electrode active material was subjected to X-ray diffraction measurements under the same conditions as in Example 1. A portion of the XRD pattern (enlarged view at 20°≦2θ≦30°) is shown in Figure 2. As shown in Figure 2, the obtained positive electrode active material was found to have peaks originating from the crystal structure of LiFe2F6, with the peaks originating from the crystal structure of FeF3 disappearing.

[0084] [Example 15] (Li 0.8 FeF 3.8 (Preparation of a complex with compound x=0.8 in formula (1), CNT=20 mass%) The positive electrode active material was obtained in the same manner as in Example 8, except that the amount of CNTs added to the pot was 0.125 g. The obtained positive electrode active material was subjected to X-ray diffraction measurements under the same conditions as in Example 1. A portion of the XRD pattern (enlarged view at 20°≦2θ≦30°) is shown in Figure 12. As shown in Figure 12, it was found that the obtained cathode active material had peaks originating from the crystal structure of LiFe2F6, while the peaks originating from the crystal structure of FeF3 disappeared.

[0085] [Example 16] (Li 0.9 FeF 3.9 (Preparation of a complex with compound x=0.9 in formula (1), containing 20% ​​by mass of CNTs) A positive electrode active material was obtained in the same manner as in Example 1, except that 0.414 g of ferric fluoride (FeF3) and 0.0857 g of lithium fluoride (LiF) were subjected to a first mechanical treatment using a planetary ball mill apparatus, and the amount of CNTs added to the pot was 0.125 g. The obtained positive electrode active material was subjected to heat treatment under the same conditions as in Example 6, and the resulting positive electrode active material was subjected to X-ray diffraction measurement under the same conditions as in Example 1. A portion of the XRD pattern (enlarged view at 20°≦2θ≦30°) is shown in Figure 13. As shown in Figure 13, it was found that the obtained positive electrode active material had peaks originating from the crystal structure of LiFe2F6, while the peaks originating from the crystal structure of FeF3 disappeared.

[0086] [Example 17] (Preparation of a complex containing LiFeF4 (compound x=1.0 in formula (1)) and CNT=20% by mass) The positive electrode active material was obtained in the same manner as in Example 9, except that the amount of CNTs added to the pot was 0.125 g. The obtained positive electrode active material was subjected to X-ray diffraction measurements under the same conditions as in Example 1. A portion of the XRD pattern (enlarged view at 20°≦2θ≦30°) is shown in Figure 14. As shown in Figure 14, it was found that the obtained cathode active material lacked the peak originating from the crystal structure of FeF3 and instead possessed a peak originating from the crystal structure of LiFe2F6.

[0087] [Example 18] (Li 1.1 FeF 4.1 (Preparation of a complex with compound x=1.1 in formula (1), containing 20% ​​by mass of CNTs) A positive electrode active material was obtained in the same manner as in Example 1, except that 0.399 g of ferric fluoride (FeF3) and 0.101 g of lithium fluoride (LiF) were subjected to a first mechanical treatment using a planetary ball mill apparatus, and the amount of CNTs added to the pot was 0.125 g. The obtained positive electrode active material was subjected to heat treatment under the same conditions as in Example 6, and the resulting positive electrode active material was subjected to X-ray diffraction measurement under the same conditions as in Example 1. A portion of the XRD pattern (enlarged view at 20°≦2θ≦30°) is shown in Figure 15. As shown in Figure 15, it was found that the obtained cathode active material had peaks originating from the crystal structure of LiFe2F6, with the peaks originating from the crystal structure of LiFe2F6 disappearing.

[0088] [Example 19] (Li 1.2 FeF 4.2 (Preparation of a complex with compound x=1.2 in formula (1), containing 20% ​​by mass of CNTs) The positive electrode active material was obtained in the same manner as in Example 10, except that the amount of CNTs added to the pot was 0.125 g. The obtained positive electrode active material was subjected to X-ray diffraction measurements under the same conditions as in Example 1. A portion of the XRD pattern (enlarged view at 20°≦2θ≦30°) is shown in Figure 16. As shown in Figure 16, it was found that the obtained positive electrode active material lacked the peak originating from the crystal structure of FeF3 and instead possessed a peak originating from the crystal structure of LiFe2F6.

[0089] [Comparative Example 2] (Li 0.2 FeF 3.2 (Preparation of a complex with compound x=0.2 in formula (1) and CNT=10 mass%) A positive electrode active material was obtained by heat treatment under the same conditions as in Example 6, except that 0.478 g of ferric fluoride (FeF3) and 0.022 g of lithium fluoride (LiF) were subjected to a first mechanical treatment using a planetary ball mill apparatus.

[0090] [Comparative Example 3] (Li 1.4 FeF 4.4 (Preparation of a complex with compound x=1.4 in formula (1), containing CNT=10% by mass) A positive electrode active material was obtained by heat treatment under the same conditions as in Example 6, except that 0.378 g of ferric fluoride (FeF3) and 0.122 g of lithium fluoride (LiF) were subjected to a first mechanical treatment using a planetary ball mill apparatus.

[0091] [Comparative Example 4] (Li 1.6 FeF 4.6 (Preparation of a complex with compound x=1.6 in formula (1), containing 10% by mass of CNTs) A positive electrode active material was obtained by heat treatment under the same conditions as in Example 6, except that 0.366 g of ferric fluoride (FeF3) and 0.134 g of lithium fluoride (LiF) were subjected to a first mechanical treatment using a planetary ball mill apparatus.

[0092] [Manufacturing of lithium-ion secondary batteries] (Fabrication of the positive electrode) 80 parts by mass of the positive electrode active material obtained in Examples 1-19 and Comparative Examples 1-4, 10 parts by mass of acetylene black, and 10 parts by mass of polyvinylidene fluoride were dispersed in N-methylpyrrolidone as a solvent to prepare a slurry (positive electrode mixture) containing 80% by mass of the positive electrode active material, 10% by mass of acetylene black, and 10% by mass of polyvinylidene fluoride as solid content. This slurry was applied to aluminum foil, pressed with 15 tons of pressure, and then punched out with a 10 mm diameter punch to produce a positive electrode. At this time, the mass of the positive electrode active material was adjusted to 3.5 mg.

[0093] (Creation of coin-shaped cells) The fabricated positive electrode (10 mm in diameter) was placed on top of the positive electrode container, and a porous polyethylene film was placed on top of it as a separator, secured with a polypropylene gasket. Then, a 0.5 mm thick lithium negative electrode was placed on top, and a spacer for thickness adjustment was added. Next, a mixed solvent of ethylene carbonate and diethyl carbonate (volume ratio 5:5) containing 1 mol / L lithium hexafluoride phosphate was added between the positive electrode container and the negative electrode as a non-aqueous electrolyte solution, impregnating the separator. The negative electrode container was then placed on top and sealed to fabricate a coin-type cell (lithium-ion secondary battery).

[0094] [Battery performance evaluation] The battery performance was evaluated using the fabricated coin-type cells. Specifically, the fabricated coin-type cells were charged and discharged at a constant current of 5 mA / g per unit mass of positive electrode active material. The upper voltage limit during constant current charging and discharging was set to 4.25 V and the lower voltage limit to 3.35 V. The rest time after charging and discharging was set to 10 minutes. The charge / discharge capacity (mAh / g) was calculated per unit mass of positive electrode active material. The charge / discharge curves for constant current charging and discharging in Examples 1-10, Examples 12-19, and Comparative Examples 1-4 are shown in Figures 4, 17-37.

[0095] From the obtained charge-discharge curve, the horizontal axis is voltage, and the vertical axis is the value obtained by differentiating the capacity with respect to voltage (dQ / dV, dQdV). -1 As a result, a dQ / dV plot was created, and the voltage at which the chemical reaction was occurring (reaction voltage) was determined from the peak voltage of the dQ / dV plot during discharge. The dQ / dV plots for Examples 1-10, Examples 12-19, and Comparative Examples 1-4 are shown in Figures 5, 38-58.

[0096] As shown in Figures 5 and 38 to 54, in Examples 1 to 10 and 12 to 19 to which the present invention was applied, the peak voltage of the dQ / dV plot during discharge was high, ranging from 3.94V to 4.01V, confirming that the chemical reaction occurred at a high voltage characteristic of the compound represented by formula (1). In contrast, Comparative Example 1 (LiFeF3), which does not contain the compound represented by formula (1) as the positive electrode active material, did not show a peak voltage in the dQ / dV plot during discharge around 4V, as shown in Figure 55, and only showed a broad peak in the operating voltage range. 2+ / Fe 3+ In order to observe a clear reaction, it is necessary to lower the lower limit of the operating voltage range, and in Comparative Example 1, it is considered that the chemical reaction occurred at a voltage lower than the lower limit voltage of 3.35V in this embodiment. Furthermore, in Comparative Examples 2-4, where x in equation (1) is outside the scope of the present invention, the peak depth at the peak voltage of the dQ / dV plot observed around 4V during discharge was shallower compared to Examples 6-10 and Example 12, as shown in Figures 56-58.

[0097] [Example 20] (Preparation of a complex with LiFeF4 (compound x=1.0 in formula (1)) and CB=20% by mass) The positive electrode active material was obtained in the same manner as in Example 17, except that carbon black (CB) was used instead of CNTs. Using the obtained positive electrode active material, coin-type cells (lithium-ion secondary batteries) were fabricated in the same manner as in Examples 1 to 19, and their battery performance was evaluated. The charge-discharge curves under constant current are shown in Figure 59. In addition, a dQ / dV plot was created from the obtained charge-discharge curves, in the same manner as in Examples 1 to 19. The results are shown in Figure 60. As shown in Figure 59, the lithium-ion secondary battery using the positive electrode active material of Example 20, which uses CB as carbon, had a discharge capacity of 53.6 mAhg. -1 Therefore, the discharge capacity of the lithium-ion secondary battery using the positive electrode active material of Example 17, in which CNTs were used as carbon, was 51 mAhg. -1 It was confirmed that it showed values ​​equivalent to those of the previous test. Furthermore, as shown in Figure 60, the peak voltage of the dQ / dV plot during discharge was high at 3.960V, confirming that the chemical reaction occurred at a high voltage characteristic of the compound represented by equation (1).

[0098] Tables 1 to 4 summarize the peak voltage, peak height (or peak depth) at the peak voltage, and discharge capacity values ​​during charging and discharging based on the above results.

[0099] [Table 1]

[0100] [Table 2]

[0101] [Table 3]

[0102] [Table 4]

[0103] From the above results, it was found that the present invention can provide an Fe-based positive electrode active material capable of high-voltage operation and a lithium-ion secondary battery containing the positive electrode active material. Furthermore, it was found that the capacity of a lithium-ion secondary battery containing the positive electrode active material of the present invention can be increased by subjecting the positive electrode active material to heat treatment. [Explanation of symbols]

[0104] 1…Lithium-ion rechargeable battery 2...Positive electrode 3...Negative electrode 4... Separator 5…Insulating packing (gasket) 10…Positive electrode can 20... Negative electrode can (negative electrode terminal)

Claims

1. A positive electrode active material mainly composed of lithium iron composite fluoride and containing carbon, The lithium iron composite fluoride is a positive electrode active material represented by the following formula (1). Li ｘ FeF （３＋ｘ） (1) In equation (1), x is a number that satisfies 0.4 ≤ x ≤ 1.

2.

2. The mass (M) of the aforementioned lithium iron composite fluoride １ ) and the mass of the carbon (M ２ ) and the mass ratio (M １ : M ２ The positive electrode active material according to claim 1, wherein the ratio is 90:10 to 60:

40.

3. The mass (M １ of the lithium iron composite fluoride and the mass (M ２ of the carbon have a mass ratio (M １ : M ２ ) of 90:10 to 80:

20. The positive electrode active material according to claim 1.

4. The positive electrode active material according to claim 1, wherein the carbon is a carbon nanotube or carbon black.

5. A lithium-ion secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode contains the positive electrode active material described in any one of claims 1 to 4.

6. The lithium-ion secondary battery according to claim 5, wherein the dQ / dV plot during discharge in a charge-discharge cycle has a peak in the range of 3.94 to 4.01 V.

7. A lithium-ion secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, The positive electrode contains a positive electrode active material, The positive electrode active material mainly consists of lithium iron composite fluoride and contains carbon. The aforementioned lithium iron composite fluoride is represented by the following formula (1), A lithium-ion secondary battery in which the dQ / dV plot during the discharge phase of a charge-discharge cycle has a peak in the range of 3.94 to 4.01 V. Li ｘ FeF （３＋ｘ） (1) In equation (1), x is a number that satisfies 0.4 ≤ x ≤ 1.2.

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