Positive electrode active material, lithium ion secondary battery, and method for producing positive electrode active material

Abstract

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, which contribute to improvement in energy efficiency.SOLUTION: A positive electrode active material is primarily composed of lithium-iron complex fluoride, and the lithium-iron complex fluoride is represented by the following formula (1). LixFeF(3+x) (1). In the formula (1), x is a number that satisfies the relationship 0.4≤x<1.5.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material, a lithium ion secondary battery, and a method for producing a positive electrode active material. [Background technology]

[0002] In recent years, research and development has been conducted on secondary 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 secondary batteries are becoming increasingly important as a power source for electric vehicles (EVs), hybrid electric vehicles (HEVs), and other vehicles.

[0003] Positive electrode active materials are attracting attention as a key component that determines the capacity of lithium-ion secondary batteries, and development is underway. One known positive electrode active material for lithium-ion secondary batteries is iron (Fe)-based lithium iron phosphate (LiFePO4), which has low resource risk. LiFePO4 has excellent cycle characteristics and safety, but its low voltage and small capacity result in a lower energy density (voltage x capacity) compared to conventionally used nickel (Ni) and cobalt (Co)-based materials. High-energy density electrode materials are required to build compact batteries, and high-voltage operation of the battery is important to achieve 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, Fe 3+ ⇔Fe 4+ ) is expected. However, Fe 4+ is very unstable and undergoes side reactions to form Fe 3+ Or, Fe 4+ This requires a lot of energy and may not be produced. 3+ However, even if the compound is used, it does not necessarily mean that high voltage operation is possible.

[0005] For example, Non-Patent Document 1 reports that the use of ferric fluoride (FeF3) produces LiFeF3 during charging and discharging, with an average discharge voltage of 3.1 V. Non-Patent Document 2 reports that LiFeO2 is expected to have a high energy density. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] F. Badway, et al., “Carbon Metal Fluoride Nanocomposites” J. Electrochem. Soc., 150(10) A1318-A1327 (2003) [Non-patent document 2] Y. Hu, et al., “A Simple, Quick and Eco-Friendly Strategy of Synthesis Nanosized α-LiFeO2 Cathode with Excellent Electrochemical Performance for Lithium-Ion Batteries” Materials, 11, 1176 (2018) Summary of the Invention [Problem to be solved by the invention]

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

[0008] The present invention has been made to solve the above-mentioned problems, and aims to provide an Fe-based positive electrode active material capable of high-voltage operation, a lithium-ion secondary battery including the positive electrode active material, and a method for producing the positive electrode active material, which in turn contribute to reducing resource risks and 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 complex fluoride, The lithium-iron composite fluoride is a positive electrode active material represented by the following formula (1): Li x FeF (3+x) (1) In formula (1), x is a number that satisfies 0.4≦x<1.5.

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

[0011] [2] The positive electrode active material according to [1], wherein in the formula (1), x satisfies 0.4≦x<1.0.

[0012] The positive electrode active material according to [2] can further increase the average discharge voltage and operate at a higher voltage, which can contribute to further cost reduction.

[0013] [3] The positive electrode active material according to [1] or [2], which has peaks in the range of 20°≦2θ<25° and 25°≦2θ≦30° in an X-ray diffraction pattern.

[0014] The positive electrode active material according to [3] has a peak derived from the crystal structure of FeF3 and a peak derived from the crystal structure of LiFe2F6.

[0015] [4] In an X-ray diffraction pattern, the diffraction intensity ratio (I) is the maximum peak intensity in the range of 25°≦2θ≦30° relative to the maximum peak intensity in the range of 20°≦2θ<25°. LFF / FeF3 ) satisfies the relationship of the following formula (3): I LFF / FeF3 =0.74x+b (3) (In formula (3), x represents a number that satisfies 0.4≦x<1.0 in formula (1), and b represents a number that satisfies b>0.46.)

[0016] The positive electrode active material according to [4] can increase the capacity of a lithium ion secondary battery containing the positive electrode active material, and can 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 according to any one of [1] to [4].

[0018] In the lithium ion secondary battery according to [5], the positive electrode contains the positive electrode active material according to any one of [1] to [4], which means that the battery can operate at a high voltage.

[0019] [6] The lithium ion secondary battery according to [5], wherein a dQ / dV plot during discharge in a charge-discharge cycle has a peak in the range of 3.77 to 4.0 V.

[0020] The lithium ion secondary battery according to [6] shows that the positive electrode active material undergoes a chemical reaction in the high voltage range of 3.77 to 4.0 V. This indicates that the battery can operate at a higher voltage.

[0021] [7] The lithium ion secondary battery according to [5] or [6], which has an average discharge voltage of 3.8 to 4.0 V.

[0022] The lithium ion secondary battery according to [7] has a high average discharge voltage of 3.8 to 4.0 V. This indicates that it can operate at high voltages.

[0023] [8] The lithium ion secondary battery according to any one of [5] to [7], wherein the electrolyte is a liquid electrolyte, a solid electrolyte, or a semi-solid electrolyte.

[0024] The lithium ion secondary battery according to [8] can be applied to any of liquid electrolytes, solid electrolytes, and semi-solid electrolytes, and therefore can be applied to various batteries.

[0025] [9] The lithium ion secondary battery according to any one of [5] to [8], wherein the negative electrode is metallic lithium or graphite.

[0026] The lithium ion secondary battery according to [9] can also be applied to batteries whose negative electrode is metallic lithium or graphite. Therefore, it can be applied to various batteries.

[0027]

[10] A method for producing a positive electrode active material according to any one of [1] to [4], comprising the steps of mixing a lithium source and an iron source, and mechanically treating the mixture to obtain a lithium-iron composite fluoride.

[0028] The method for producing a positive electrode active material according to

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

[0029]

[11] The method for producing a positive electrode active material according to

[10] , further comprising a step of subjecting the lithium-iron composite fluoride to a heat treatment.

[0030] The method for producing a positive electrode active material according to

[11] can provide a positive electrode active material that can increase the capacity of a lithium ion secondary battery and further increase the energy density, thereby enabling the capacity and energy density of a lithium ion secondary battery containing the positive electrode active material to be increased. [Effects of the Invention]

[0031] According to the present invention, it is possible to provide an Fe-based positive electrode active material capable of operating at high voltages, and a lithium ion secondary battery including the positive electrode active material. [Brief explanation of the drawings]

[0032] [Figure 1] FIG. 1 is a diagram showing an X-ray diffraction pattern of a positive electrode active material according to one embodiment of the present invention. [Figure 2] 1 is a cross-sectional view schematically showing a lithium ion secondary battery according to one embodiment of the present invention. [Figure 3] 1 is a graph showing charge / discharge curves in charge / discharge cycles of a lithium ion secondary battery according to one embodiment of the present invention. [Figure 4] 4 is a graph showing a dQ / dV plot for the charge / discharge cycle of FIG. 3. [Figure 5] 10 is a graph showing a dQ / dV plot in the charge / discharge cycle of a lithium ion secondary battery according to another embodiment of the present invention. [Figure 6] FIG. 1 is a diagram showing X-ray diffraction patterns of the positive electrode active materials of Examples 1 to 5 and Comparative Example 1. [Figure 7] 1 is a graph showing charge / discharge curves of a lithium ion secondary battery containing the positive electrode active material of Example 1. [Figure 8] 1 is a graph showing charge / discharge curves of a lithium ion secondary battery containing the positive electrode active material of Example 2. [Figure 9] 1 is a graph showing charge / discharge curves of a lithium ion secondary battery containing the positive electrode active material of Example 3. [Figure 10] 10 is a graph showing charge / discharge curves of a lithium ion secondary battery containing the positive electrode active material of Example 4. [Figure 11] 10 is a graph showing charge / discharge curves of a lithium ion secondary battery containing the positive electrode active material of Example 5. [Figure 12] 1 is a graph showing charge / discharge curves of a lithium ion secondary battery containing the positive electrode active material of Comparative Example 1. [Figure 13] 8 is a graph showing a dQ / dV plot for the charge / discharge cycle of FIG. 7. [Figure 14] 9 is a graph showing a dQ / dV plot for the charge-discharge cycle of FIG. 8. [Figure 15] 10 is a graph showing a dQ / dV plot for the charge / discharge cycle of FIG. 9. [Figure 16] 11 is a graph showing a dQ / dV plot for the charge-discharge cycle of FIG. 10. [Figure 17] 12 is a graph showing a dQ / dV plot for the charge / discharge cycle of FIG. 11. [Figure 18] 13 is a graph showing a dQ / dV plot for the charge / discharge cycle of FIG. 12. [Figure 19] FIG. 1 shows X-ray diffraction patterns of the positive electrode active materials of Examples 6 to 12. [Figure 20] 1 is a graph showing the relationship between the composition of the lithium iron composite fluorides of Examples 1 to 4, 6 to 9, and 11 to 12 and the diffraction intensity ratio (ILFF / FeF3). [Figure 21] 1 is a graph showing the charge / discharge capacity of lithium ion secondary batteries containing the positive electrode active materials of Examples 1 to 10. DETAILED DESCRIPTION OF THE INVENTION

[0033] Preferred embodiments of the present invention will now be described in detail.

[0034] [Cathode active material] The positive electrode active material of this embodiment is mainly composed of lithium-iron composite fluoride and is used in the positive electrode of a lithium-ion secondary battery. The term "mainly composed of" lithium-iron composite fluoride means that the content of lithium-iron composite fluoride is 75% by mass or more, preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 99% by mass or more, relative to the total mass of the positive electrode active material, and may be 100% by mass. The positive electrode active material may contain components other than the main component, as long as the function of the present invention is not impaired.

[0035] As long as the positive electrode active material of this embodiment is mainly composed of a lithium iron composite fluoride, it may contain only one kind of lithium iron composite fluoride, or may contain two or more kinds.

[0036] When producing a positive electrode active material mainly composed of a lithium iron composite fluoride, the composition ratio (Li:Fe:F) of the entire lithium iron composite fluoride is maintained in the obtained positive electrode active material. When a positive electrode active material obtained mainly composed of a lithium iron composite fluoride having such a composition is used in a secondary battery, high-voltage operation can be realized. Further, the composition ratio of the lithium iron composite fluoride is adjusted to be the same as the composition ratio required for the positive electrode active material to be obtained.

[0037] (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 formula (1), x is a number satisfying 0.4 ≦ x < 1.5. In formula (1), x is preferably 0.4 ≦ x < 1.0, more preferably 0.4 ≦ x ≦ 0.9, and still more preferably 0.5 ≦ x ≦ 0.8. When x is within the above numerical range, the average discharge voltage and capacity can be increased, and a small-sized battery having a high energy density can be constructed.

[0038] In formula (1), x represents the molar ratio of Li to Fe. The molar ratio of Li to Fe is x:1. Further, the molar ratio of Li to Fe to F is x:1:(3 + x). The composition of the lithium iron composite fluoride can be determined by inductively coupled plasma (ICP) emission spectrometry.

[0039] <X-ray diffraction (XRD) pattern> As an example of the X-ray diffraction (XRD) pattern of the positive electrode active material according to the present embodiment, the XRD pattern of Example 4 described later is shown in FIG. 1. As shown in FIG. 1, the positive electrode active material of the present embodiment preferably 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 the present embodiment has a crystal structure similar to that of FeF3 and LiFe2F6. The lithium iron composite fluoride of the present embodiment has an iron composition ratio of LiFeF4, and all iron is Fe 3+ Since it is composed of, when described according to the composition formula of LiFe2F6 of the space group P42 / mnm crystal system tetragonal crystal, the following formula (2) is obtained. Li y Fe2F (6+y) (2) In formula (2), y is a number satisfying 1 < y < 3.

[0040] <Return diffraction intensity ratio> In the XRD pattern, when there are peaks in the ranges of 20° ≤ 2θ < 25° and 25° ≤ 2θ ≤ 30°, the diffraction intensity ratio (I LFF / FeF3 [[ID=)) represented by the maximum peak intensity in the range of 25° ≤ 2θ ≤ 30° with respect to the maximum peak intensity in the range of 20° ≤ 2θ < 25° is preferably in the relationship of the following formula (3). I LFF / FeF3 = 0.74x + b (3) (However, in formula (3), x represents a number satisfying 0.4 ≤ x < 1.0 in the above formula (1), and b represents a number satisfying b > 0.46.)

[0041] In formula (3), x represents a number satisfying 0.4 ≤ x < 1.0 in the above formula (1), preferably 0.4 ≤ x ≤ 0.9, and more preferably 0.5 ≤ x ≤ 0.8. In formula (3), b represents a number satisfying b > 0.46, preferably b ≥ 0.48, and more preferably b ≥ 0.50. The upper limit value of b is not particularly limited, but for example, 2.0 is preferable.

[0042] When the positive electrode active material of this embodiment satisfies the relationship of formula (3), the capacity of a lithium ion secondary battery containing the positive electrode active material can be increased, and the energy density can be increased. This means that the composition ratio between the LiFe2F6 crystal structure and the FeF3 crystal structure in the lithium-iron composite fluoride contained in the positive electrode active material of this embodiment satisfies a certain relationship, and by controlling the composition ratio between the LiFe2F6 crystal structure and the FeF3 crystal structure so that a certain relationship is satisfied, the capacity of a lithium-ion secondary battery containing this positive electrode active material can be increased, and the energy density can be further increased.

[0043] The diffraction intensity ratio (I) is the ratio of the maximum peak intensity in the range of 25°≦2θ≦30° to the maximum peak intensity in the range of 20°≦2θ<25°. LFF / FeF3 ) can be determined by analyzing the XRD pattern. Here, "maximum peak intensity" refers to the peak intensity of the peak with the greatest height when there are multiple peaks in the range of 20°≦2θ<25° or in the range of 25°≦2θ≦30°. When there is only one peak in the range of 20°≦2θ<25° or in the range of 25°≦2θ≦30°, the peak intensity of that peak is the "maximum peak intensity." Each "peak intensity" is given by the peak height of the peak.

[0044] [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 containing the above-mentioned lithium-iron composite fluoride as a main component. The lithium ion secondary battery of this embodiment may include other battery components as necessary.

[0045] The lithium ion secondary battery of this embodiment can employ the same battery elements as known lithium ion secondary batteries, except that the positive electrode contains a positive electrode active material whose main component is the lithium-iron composite fluoride described above. The lithium ion secondary battery of this embodiment may be of any of a coin type, button type, cylindrical type, prismatic type, and laminate type. Furthermore, the lithium ion secondary battery of this embodiment can be used in a wide range of applications, such as mobile devices such as mobile phones and laptops, and in-vehicle applications.

[0046] The lithium ion secondary battery of this embodiment will be described below as a lithium ion secondary battery using an electrolyte (coin-type lithium ion secondary battery). The battery elements 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.

[0047] As shown in FIG. 2, the lithium ion secondary battery 1 of this embodiment includes a negative electrode can (negative electrode terminal) 20, a negative electrode 3, a separator 4 impregnated with an electrolyte, an insulating packing (gasket) 5, a positive electrode 2, and a positive electrode can 10.

[0048] The positive electrode can 10 is disposed below the separator 4, and the negative electrode can 20 is disposed above the separator 4, with the positive electrode can 10 and the negative electrode can 20 forming the outer shape of the lithium-ion secondary battery 1. A positive electrode 2 and a negative electrode 3 are provided between the positive electrode can 10 and the negative electrode can 20 via a separator 4 impregnated with an electrolyte, and the positive electrode 2 and the negative electrode 3 are separated by the separator 4. The positive electrode can 10 and the negative electrode can 20 are electrically insulated by an insulating packing 5.

[0049] The lithium ion secondary battery 1 can be manufactured by preparing a positive electrode mixture by blending a conductive agent, a binder, etc. with the positive electrode active material of this embodiment as needed, and then pressing this onto a current collector (not shown) to produce a positive electrode 2. As the current collector, preferably, stainless steel mesh, aluminum foil, etc. can be used. As the conductive agent, preferably, carbon nanotubes (CNT), acetylene black, Ketchen black, etc. can be used. As the binder, preferably, tetrafluoroethylene, polyvinylidene fluoride, etc. can be used.

[0050] 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 mass%, more preferably 90 to 99 mass%. The content of the conductive agent in the positive electrode mixture is preferably 1 to 15 mass%, more preferably 0.1 to 5 mass%. The content of the binder in the positive electrode mixture is preferably 0.1 to 10 mass%, more preferably 0.1 to 5 mass%.

[0051] In the lithium-ion secondary battery 1, the negative electrode 3 for the positive electrode 2 can be made of any known material that functions as a negative electrode active material and can absorb and release lithium, such as metallic materials such as metallic lithium and lithium alloys, carbon materials such as graphite and MCMB (mesocarbon microbeads), and silicon materials such as silicon (Si), Si alloys, and silicon oxide. Among these, metallic lithium and graphite are preferred for the negative electrode 3.

[0052] The separator 4 and the battery container (positive electrode can 10, negative electrode can 20) can be made of known battery elements.

[0053] The electrolyte may be a known electrolytic solution, semi-solid electrolyte, solid electrolyte, etc. The electrolytic solution may be, for example, a solution in which an electrolyte such as lithium perchlorate or lithium hexafluorophosphate is dissolved in a solvent such as ethylene carbonate (EC), dimethyl carbonate (DMC), propylene carbonate (PC), or diethyl carbonate (DEC).

[0054] As the semi-solid electrolyte and solid electrolyte, any known semi-solid electrolyte and solid electrolyte can be used, except for the positive electrode active material containing the above-mentioned lithium iron composite fluoride as the main component. Examples of semi-solid electrolytes include electrolytes composed of a polymer component and a standard electrolyte solution. Examples of the polymer component include polyvinylidene fluoride (PVDF) / polyethylene oxide (PEO), polyacrylonitrile (PAN) / PEO, polymethyl methacrylate (PMMA), and PVDF / hexafluoropropylene (HFP). Examples of the standard electrolyte solution include a 1 mol / L lithium hexafluorophosphate (LiPF) EC / DMC solution, a 1 mol / L LiPF EC / ethyl methyl carbonate (EMC) solution, and a 1 mol / L LiPF EC / DMC / EMC solution.

[0055] In the case of an all-solid-state lithium ion secondary battery, the electrolyte may be, for example, 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.

[0056] For the positive electrode of the all-solid-state lithium ion secondary battery, for example, a positive electrode composite containing the above-mentioned positive electrode active material, conductive agent, and binder as well as a solid electrolyte can be supported on a positive electrode current collector made of aluminum, nickel, stainless steel, or the like.

[0057] The lithium ion secondary battery 1 of this embodiment can operate at a high voltage because the positive electrode 2 contains the positive electrode active material of this embodiment.

[0058] <dQ / dV plot of charge / discharge cycle> FIG. 3 shows a graph of Example 4, which will be described later, as an example of a charge / discharge curve in the charge / discharge cycle of the lithium-ion secondary battery of this embodiment. The horizontal axis of the graph in FIG. 3 represents the capacity of the lithium-ion secondary battery. The vertical axis of the graph in FIG. 3 represents the voltage of the lithium-ion secondary battery during charge / discharge. In the charge / discharge curves in FIG. 3, the curves sloping upward to the right represent the curves during charging, and the curves sloping upward to the left represent the curves during discharging. 1st, 2nd, 3rd, and 4th in FIG. 3 represent the number of charge / discharge cycles.

[0059] The capacity of the lithium ion secondary battery shown in Figure 3 is 30 mAh / g, and the voltage of the curve when discharging at half the capacity, 15 mAh / g, is the average discharge voltage. From Figure 3, the average discharge voltage of the lithium ion secondary battery of this embodiment is about 3.8 V.

[0060] FIG. 4 shows the dQ / dV plot for the charge-discharge cycle of FIG. 3. The horizontal axis of the graph in FIG. 4 represents the voltage during the charge-discharge cycle. The vertical axis of the graph in FIG. 4 represents the value obtained by differentiating the capacity of FIG. 3 with respect to the voltage (dQ / dV plot, dQdV -1 The graph shows the graph of the charge time and the discharge time. The upward convex curve in Figure 4 represents the charge time, and the downward convex curve represents the discharge time. The 1st, 2nd, 3rd, and 4th in Figure 4 represent the number of charge / discharge cycles.

[0061] As shown in Figure 4, the curve during charging has peaks at 3.85 V and 4.1 V. The curve during discharging has peaks at 3.77 V and 4.0 V. These peaks indicate that the positive electrode active material in the positive electrode undergoes a chemical reaction during charging or discharging. This indicates that a chemical reaction occurs in the high voltage range of 3.77 to 4.0 V during discharging, and demonstrates that the lithium-ion secondary battery can operate at high voltages.

[0062] Fig. 5 shows a graph of Example 1, which will be described later, as an example of a dQ / dV plot in the charge-discharge cycle of a lithium-ion secondary battery according to another embodiment. The upward convex curve in Fig. 5 represents the curve during charging, and the downward convex curve represents the curve during discharging. 1st, 2nd, 3rd, and 4th in Fig. 5 represent the number of charge-discharge cycles.

[0063] The discharge curve of Example 1 has peaks at 3.5 V, 3.77 V, and 4.0 V. The dQ / dV value of the peak at 3.5 V is −0.0708 mAhg -1 V -1 , the peak dQ / dV value at 3.77V is -0.090mAhg -1 V -1, the peak dQ / dV value at 4.0V is -0.1634mAhg -1 V -1 The dQ / dV value of each peak represents the average value of three charge / discharge cycles excluding the first cycle. The peak height at 3.77 V is -0.090 / -0.0708 = 1.27 times the peak height at 3.5 V, and the peak height at 4.0 V is -0.1634 / -0.0708 = 2.31 times the peak height at 3.5 V. The discharge curve of Example 3 described below has a dQ / dV value of the peak at 3.5 V of -0.107 mAhg -1 V -1 , the peak dQ / dV value at 4.0V is -0.464mAhg -1 V -1 In Example 3, the peak height at 4.0 V is -0.464 / -0.107 = 4.34 times the peak height at 3.5 V. As described above, in the discharge curves in Examples 1 and 3, the peak height at 3.77 to 4.0 V is 1.2 to 4.4 times the peak height at 3.3 to 3.5 V. This indicates that during discharge, the chemical reaction at 3.77 to 4.0 V involves 1.2 to 4.4 times the amount of material occurring compared to the chemical reaction at 3.3 to 3.5 V, demonstrating that the lithium ion secondary battery can operate at high voltages.

[0064] The average discharge voltage of the lithium ion secondary battery of this embodiment is preferably 3.8 to 4.0 V, more preferably 3.8 to 3.9 V. When the average discharge voltage of the lithium ion secondary battery is equal to or greater than the above lower limit, it can operate at a higher voltage. The upper limit of the average discharge voltage of the lithium ion secondary battery is not particularly limited, but is substantially 4.0 V.

[0065] [Method of manufacturing positive electrode active material] The positive electrode active material of this embodiment contains the above-mentioned lithium-iron composite fluoride as a main component. The lithium source of the lithium-iron composite fluoride is not particularly limited, and may be any known compound, such as a halide such as lithium fluoride (LiF), a hydroxide such as lithium hydroxide monohydrate (LiOH·H2O), a carbonate such as lithium carbonate (Li2CO3), or an acetate such as lithium acetate (CHCl3COOLi) or lithium acetate dihydrate (CHCl3COOLi·2H2O).

[0066] As the iron source of the lithium iron complex fluoride, trivalent iron is preferable to divalent iron because it can be operated at a high voltage, and ferric fluoride (FeF3) is more preferable.

[0067] When producing the lithium-iron composite fluoride, the lithium source and the iron source are mixed and subjected to a mechanical treatment for a predetermined time under predetermined conditions. For example, when lithium fluoride is used as the lithium source and trivalent iron (FeF3) is used as the iron source, the compound represented by the formula (1) is thought to be produced by the following reaction: LiF+FeF3→LiFeF4 (compound in which x=1 in the formula (1))

[0068] The value of x in the compound represented by the formula (1) can be adjusted by the molar ratio of LiF to FeF3.

[0069] The specific means used in the mechanical treatment are not particularly limited, but various means conventionally used for the purpose of pulverizing and mixing solid substances can be applied. Among these means, a ball mill is preferred, and a planetary ball mill is more preferred because it can pulverize and mix the raw materials sufficiently.

[0070] The time for the mechanical treatment is, for example, preferably 8 to 12 hours, more preferably 9 to 11 hours. As a condition for carrying out the mechanical treatment, the rotation speed is preferably 250 to 450 rpm, more preferably 300 to 400 rpm. The temperature at which the mechanical treatment is carried out is not particularly limited, and the treatment can be carried out at room temperature (for example, 5 to 30°C). The atmosphere during the mechanical treatment is preferably an inert gas (a rare gas such as argon (Ar), nitrogen (N2) gas, etc.).

[0071] The lithium-iron composite fluoride obtained by the mechanical treatment is preferably subjected to a heat treatment, which changes the crystal structure of the lithium-iron composite fluoride and increases the capacity of a lithium-ion secondary battery using the lithium-iron composite fluoride as a positive electrode active material. This is thought to be because the composition ratio between the LiFe2F6 crystal structure and the FeF3 crystal structure in the lithium-iron composite fluoride changes due to the heat treatment, and the LiFe2F6 crystal structure increases.

[0072] By subjecting the lithium-iron composite fluoride to a heat treatment, a lithium-iron composite fluoride that satisfies the relationship of the above-mentioned formula (3) can be obtained, and the capacity of a lithium-ion secondary battery containing the lithium-iron composite fluoride as a positive electrode active material can be increased, and the energy density can be further increased.

[0073] 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 for the heat treatment is preferably an inert gas (a rare gas such as argon (Ar), nitrogen (N2) gas, etc.). The pressure in the heat treatment may be normal pressure (0.1013 MPa), but low vacuum (for example, 10 2 Pa~10 5 Pa) is preferred. Before the heat treatment, carbon coating, which will be described later, may be performed.

[0074] After obtaining the compound represented by formula (1) through mechanical treatment, it is preferable to coat the compound represented by formula (1) with carbon particles by pulverizing and mixing the compound with carbon particles, from the viewpoint of improving the capacity and rate characteristics. Examples of carbon particles that can be used include carbon nanotubes (CNTs), acetylene black, and Ketchen Black. Among these carbon particles, CNTs are preferred from the viewpoint of further increasing the conductivity of the positive electrode active material.

[0075] The crushing and mixing conditions for carbon coating can be the same as those for the mechanical treatment described above.

[0076] The obtained positive electrode active material is used as a positive electrode to produce a lithium ion secondary battery, whereby a battery that operates at a high voltage can be obtained. [Example]

[0077] Examples of the present invention will be described below, but the present invention is not limited to the following examples. It should be noted that Examples 4, 5, 9, and 10 are reference examples.

[0078] [Example 1] (Li 0.6 FeF 3.6 (Preparation of the compound of formula (1) where x=0.6) 0.439 g of ferric fluoride (FeF3) and 0.0606 g of lithium fluoride (LiF) were subjected to mechanical treatment using a planetary ball mill. The planetary ball mill used was a Premium line PL-7 manufactured by Fritsch. The pot and balls were made of zirconium oxide, and 50 g of 5 mm diameter balls were used in a 45 mL pot. The mechanical treatment conditions were 350 rpm and 10 hours. 0.55 g of carbon nanotubes (CNTs) were then added to the pot, and further mechanical treatment was performed to obtain a positive electrode active material. The mechanical treatment conditions for obtaining the positive electrode active material were an Ar atmosphere, 25°C, 350 rpm, and 10 hours.

[0079] The obtained positive electrode active material was subjected to X-ray diffraction measurement under the following measurement conditions, and the results are shown in FIG. <X-ray diffraction measurement conditions> X-ray diffraction equipment: Rigaku, SmartLab X-ray source: CuKα ray (CuKα=1.5418Å) Incident parallel slit aperture angle: 5.0° Length of entrance length limiting slit: 5.0 mm Receiving parallel slit aperture angle: 5.0° Kβ filter: used Step width: 0.01° Entrance slit: 1 / 6° Receiving slit 1: 4.0 mm Receiving slit 2: 13 mm

[0080] As shown in FIG. 6, the diffraction pattern of the obtained positive electrode active material matches 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), and it was found to have a crystal structure similar to that of FeF3 and LiFe2F6.

[0081] [Example 2] (Li 0.7 FeF 3.7 (Preparation of the compound of formula (1) where x=0.7) 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 mechanical treatment using a planetary ball mill. The obtained positive electrode active material was subjected to X-ray diffraction measurement under the same conditions as in Example 1. The results are shown in FIG. As shown in FIG. 6, the diffraction pattern of the obtained positive electrode active material matches 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), and it was found to have a crystal structure similar to that of FeF3 and LiFe2F6.

[0082] [Example 3] (Li 0.8 FeF 3.8 (Preparation of the compound of formula (1) where x=0.8) 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 mechanical treatment using a planetary ball mill. The obtained positive electrode active material was subjected to X-ray diffraction measurement under the same conditions as in Example 1. The results are shown in FIG. As shown in FIG. 6, the diffraction pattern of the obtained positive electrode active material matches 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), and it was found to have a crystal structure similar to that of FeF3 and LiFe2F6.

[0083] [Example 4] (Preparation of LiFeF4 (compound of formula (1) where x = 1.0)) 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 mechanical treatment using a planetary ball mill. The obtained positive electrode active material was subjected to X-ray diffraction measurement under the same conditions as in Example 1. The results are shown in FIG. As shown in FIG. 6, the diffraction pattern of the obtained positive electrode active material matches 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), and it was found to have a crystal structure similar to that of FeF3 and LiFe2F6.

[0084] [Example 5] (Li 1.2 FeF 4.2 (Preparation of the compound of formula (1) where x=1.2) 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 mechanical treatment using a planetary ball mill. The obtained positive electrode active material was subjected to X-ray diffraction measurement under the same conditions as in Example 1. The results are shown in FIG. As shown in FIG. 6, the diffraction pattern of the obtained positive electrode active material matches 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), and it was found to have a crystal structure similar to that of FeF3 and LiFe2F6.

[0085] [Comparative Example 1] (Preparation of LiFeF3) 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 mechanical treatment using a planetary ball mill. The obtained positive electrode active material was subjected to X-ray diffraction measurement under the same conditions as in Example 1. The results are shown in FIG. As shown in FIG. 6, the obtained positive electrode active material matched the diffraction pattern of LiFe2F6, DB card number 01-074-2193, space group P42 / mnm, crystalline system tetragonal, and was found to have the same crystal structure as LiFe2F6.

[0086] [Fabrication of lithium-ion secondary batteries] (Preparation of positive electrode) 80 parts by mass of the positive electrode active material obtained in Examples 1 to 5 and Comparative Example 1, 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 solids. This slurry was applied to aluminum foil, pressed at 15 tons, and then punched out with a 10 mm diameter punch to prepare a positive electrode. The mass of the positive electrode active material was adjusted to 3.5 mg.

[0087] (Fabrication of coin-type cells) The fabricated positive electrode (10 mm in diameter) was placed on top of the positive electrode can, and a porous polyethylene film was placed on top of it as a separator, held in place with a polypropylene gasket. A 0.5 mm thick Li negative electrode was then placed on top, with a spacer to adjust the thickness. A nonaqueous electrolyte solution, a mixture of ethylene carbonate and diethyl carbonate (volume ratio 5:5) containing 1 mol / L of lithium hexafluorophosphate dissolved therein, was then added between the positive electrode can and the negative electrode, impregnating the separator. The negative electrode can was then placed on top and sealed, completing a coin-type cell (lithium-ion secondary battery).

[0088] [Battery performance evaluation] The battery performance was evaluated using the produced coin-type cells. Specifically, the produced coin-type cells were subjected to constant current charging and discharging at a current value of 5 mA / g per mass of the positive electrode active material. The upper limit voltage during constant current charging and discharging was 4.25 V and the lower limit voltage was 3.35 V. The rest time after charging and discharging was 10 minutes. The charge / discharge capacity (mAh / g) was calculated per unit mass of the positive electrode active material. The charge / discharge curves of constant current charging and discharging for Examples 1 to 5 and Comparative Example 1 are shown in Figures 7 to 12.

[0089] In Examples 1 to 5 to which the present invention was applied, the energy densities were 113 Wh / kg, 136 Wh / kg, 175 Wh / kg, 108 Wh / kg, and 119 Wh / kg, respectively. Furthermore, as shown in Figures 7 to 11, in Examples 1 to 5 to which the present invention was applied, the average discharge voltages were 3.76 V, 3.79 V, 3.73 V, 3.79 V, and 3.72 V, respectively. Thus, it was confirmed that Examples 1 to 5 to which the present invention was applied had high energy densities and average discharge voltages, and that high energy density and high voltage operation were achieved. In contrast, Comparative Example 1 (LiFeF3), in which the positive electrode active material did not contain the compound represented by formula (1), had an energy density of 87 Wh / kg and an average discharge voltage of 3.64 V, as shown in FIG.

[0090] From the obtained charge / discharge curve, the horizontal axis is the voltage, and the vertical axis is the value obtained by differentiating the capacity with respect to the voltage (dQ / dV, dQdV -1 ) and a dQ / dV plot was created, and the voltage at which the chemical reaction occurred (reaction voltage) was determined from the peak voltage of the dQ / dV plot during discharge. The dQ / dV plots for Examples 1 to 5 and Comparative Example 1 are shown in Figures 13 to 18.

[0091] As shown in Figures 13 to 17, in Examples 1 to 5 to which the present invention was applied, the peak voltages of the dQ / dV plots during discharge were high, ranging from 3.77 V to 4.0 V, confirming that a chemical reaction occurred at a high voltage specific to the compound represented by formula (1). In contrast, in Comparative Example 1 (LiFeF3), in which the positive electrode active material does not contain the compound represented by formula (1), as shown in FIG. 18, no peak voltage was observed in the dQ / dV plot during discharge around 4 V, and only a broad peak was observed in the operating voltage range. 2+ / Fe 3+ In order to observe this clear reaction, it is necessary to lower the lower limit of the operating voltage range, and in Comparative Example 1, it is believed that the chemical reaction occurs at a voltage lower than the lower limit voltage of 3.35 V in this example.

[0092] [Example 6] The positive electrode active material obtained in Example 1 was subjected to 10 3 The mixture was subjected to heat treatment at 200°C for 5 hours using an oven at 1000kJ / cm². The obtained positive electrode active material was subjected to X-ray diffraction measurement under the same conditions as in Example 1. The results are shown in FIG. As shown in FIG. 19, the obtained positive electrode 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), and was found to have a crystal structure similar to that of FeF3 and LiFe2F6. Next, a coin-shaped cell was fabricated using the heat-treated positive electrode active material in the same manner as in Example 1, and the charge / discharge capacity was measured. The results are shown in Figure 21. The capacity without heat treatment is indicated by "○".

[0093] [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 FIG. As shown in FIG. 19, the obtained positive electrode 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), and was found to have a crystal structure similar to that of FeF3 and LiFe2F6. Next, a coin-type cell was fabricated using the heat-treated positive electrode active material in the same manner as in Example 1, and the charge / discharge capacity was measured. The results are shown in Figure 21. The capacity without heat treatment is indicated by "△".

[0094] [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 FIG. As shown in FIG. 19, the obtained positive electrode 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), and was found to have a crystal structure similar to that of FeF3 and LiFe2F6. Next, a coin-type cell was fabricated using the heat-treated positive electrode active material in the same manner as in Example 1, and the charge / discharge capacity was measured. The results are shown in Figure 21. The capacity without heat treatment is indicated by "□".

[0095] [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 FIG. As shown in FIG. 19, the obtained positive electrode 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), and was found to have a crystal structure similar to that of FeF3 and LiFe2F6. Next, a coin-type cell was fabricated using the heat-treated positive electrode active material in the same manner as in Example 1, and the charge / discharge capacity was measured. The results are shown in Figure 21. The capacity without heat treatment is indicated by "▽".

[0096] [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 FIG. As shown in FIG. 19, the obtained positive electrode 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), and was found to have a crystal structure similar to that of FeF3 and LiFe2F6. Next, a coin-type cell was fabricated using the heat-treated positive electrode active material in the same manner as in Example 1, and the charge / discharge capacity was measured. The results are shown in Figure 21. The capacity without heat treatment is indicated by "◇".

[0097] [Example 11] (Li 0.4 FeF 3.4 (Preparation of the compound of formula (1) where x=0.4) 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 subjected to mechanical treatment using a planetary ball mill. The obtained positive electrode active material was subjected to X-ray diffraction measurement under the same conditions as in Example 1. The results are shown in FIG. As shown in FIG. 19, the obtained positive electrode 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), and was found to have a crystal structure similar to that of FeF3 and LiFe2F6.

[0098] [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 FIG. As shown in FIG. 19, the obtained positive electrode 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), and was found to have a crystal structure similar to that of FeF3 and LiFe2F6.

[0099] (Diffraction intensity ratio of heat-treated positive electrode active material) X-ray diffraction measurements were performed on each of the heat-treated positive electrode active materials in Examples 6 to 9 and Example 12 under the same measurement conditions as in Example 1. By analyzing the obtained XRD patterns, the diffraction intensity ratio (I LFF / FeF3 ) and calculate the diffraction intensity ratio (I LFF / FeF3 The approximate curve and the approximate formula obtained from the plots of the above five points are shown in Figure 20.

[0100] (Diffraction intensity ratio of positive electrode active material without heat treatment) X-ray diffraction measurements were performed on each of the positive electrode active materials obtained in Examples 1 to 4 and Example 11 under the same measurement conditions as in Example 1. From the obtained XRD patterns, plots were made in the same manner as in the case of the "diffraction intensity ratio of the heat-treated positive electrode active material," and an approximation curve and an approximation formula were determined, which are shown in FIG.

[0101] As shown in Figure 20, the approximate curve for the case with heat treatment is the straight line shown by the thick line (I LFF / FeF3 =0.74x+0.46), and it was confirmed that the relationship of the following formula (3) was satisfied. I LFF / FeF3 =0.74x+b (3) (In formula (3), x represents a number that satisfies 0.4≦x<1.0 in formula (1), and b represents a number that satisfies b>0.46.) On the other hand, the approximate curve without heat treatment is the straight line shown by the thick line (I LFF / FeF3 =0.74x+0.46), and did not satisfy the relationship of the above formula (3). This indicates that the diffraction intensity ratio (I LFF / FeF3 ) increases, which is thought to mean that the crystal structure of LiFe2F6 in the lithium iron complex fluoride increases.

[0102] 21, it was confirmed that the charge-discharge capacity was increased by heat treatment in the lithium ion secondary batteries using the positive electrode active materials of Examples 6 to 8. In particular, in Example 6, the charge-discharge capacity was 29.6 mAhg -1 from 42.6mAhg -1 (1.44 times). On the other hand, in the lithium ion secondary batteries using the positive electrode active materials of Examples 9 and 10, the charge / discharge capacity was reduced by the heat treatment. As described above, the effect of heat treatment differed depending on the composition of the lithium-iron composite fluoride contained in the positive electrode active material (the value of x in formula (1)), and the effect of heat treatment was confirmed when 0.4≦x<1.0.

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

[0104] 1...Lithium-ion secondary 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 complex fluoride, The positive electrode active material, wherein the lithium iron composite fluoride is represented by the following formula (1): Li ｘ FeF （３＋ｘ） (1) In formula (1), x is a number that satisfies 0.4≦x<1.

0.

2. 2. The positive electrode active material according to claim 1, which has peaks in the ranges of 20°≦2θ<25° and 25°≦2θ≦30° in an X-ray diffraction pattern.

3. In the X-ray diffraction pattern, the diffraction intensity ratio (I) is expressed as the maximum peak intensity in the range of 25°≦2θ≦30° relative to the maximum peak intensity in the range of 20°≦2θ<25°. ＬＦＦ／ＦｅＦ３ 2. The positive electrode active material according to claim 1, wherein R 1 and R 2 satisfy the following formula (3): I ＬＦＦ／ＦｅＦ３ =0.74x+b (3) (In formula (3), x represents a number that satisfies 0.4≦x<1.0 in formula (1), and b represents a number that satisfies b>0.46.)

4. 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 according to any one of claims 1 to 3.

5. The lithium ion secondary battery according to claim 4, wherein a dQ / dV plot during discharge in a charge-discharge cycle has a peak in the range of 3.77 to 4.0 V.

6. The lithium ion secondary battery according to claim 4, wherein the average discharge voltage is 3.8 to 4.0 V.

7. 5. The lithium ion secondary battery according to claim 4, wherein the electrolyte is a liquid electrolyte, a solid electrolyte, or a semi-solid electrolyte.

8. 5. The lithium ion secondary battery according to claim 4, wherein the negative electrode is metallic lithium or graphite.

9. A method for producing a positive electrode active material according to any one of claims 1 to 3, A method for producing a positive electrode active material, comprising the steps of mixing a lithium source and an iron source and mechanically treating the mixture to obtain a lithium-iron composite fluoride.

10. The method for producing a positive electrode active material according to claim 9 , further comprising a step of subjecting the lithium-iron composite fluoride to a heat treatment.

11. A method for producing a positive electrode active material containing a lithium iron complex fluoride as a main component, comprising: The lithium iron composite fluoride is represented by the following formula (1): A method for producing a positive electrode active material, comprising the steps of mixing a lithium source and an iron source and mechanically treating the mixture to obtain a lithium-iron composite fluoride. Li x FeF (3+x) (1) In formula (1), x is a number that satisfies 0.4≦x<1.5.

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