Positive electrode active material and lithium ion secondary battery

Abstract

The purpose of the present invention is to provide: an Fe-based positive electrode active material capable of high-voltage operation; and a lithium ion secondary battery comprising the positive electrode active material. In addition, the positive electrode active material and the lithium ion secondary battery contribute to high efficiency of energy. This positive electrode active material contains a lithium-iron complex fluoride as a main component and contains carbon. The lithium-iron complex fluoride is represented by formula (1). LixFeF (3 + x) (1) In formula (1), x is a number satisfying 0.4 < = x < = 1.2.

Description

Technical Field

[0001] This invention relates to positive electrode active materials and lithium-ion secondary batteries. Background Technology

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

[0003] The positive electrode active material, as a crucial component determining the capacity of lithium-ion secondary batteries, has attracted attention, and its development is progressing. One example of a positive electrode active material used in lithium-ion secondary batteries is lithium iron phosphate (LiFePO4), known for its low resource risk. LiFePO4 exhibits excellent cycle characteristics and safety, but suffers from low voltage and small capacity, resulting in a lower energy density (voltage × capacity) compared to previously used nickel (Ni) and cobalt (Co) based materials. To construct small batteries, high-energy-density electrode materials are required, and high-voltage operation is essential for achieving high energy density.

[0004] With the aim of increasing the voltage of batteries using materials containing elements with low resource risk, there has been a long-standing expectation to utilize expensive transition metals (e.g., not...). , but However, Fe 4+ It is very unstable and will turn into Fe due to side reactions. 3+ Or, Fe 4+ It requires a great deal of energy and is sometimes impossible to generate. Therefore, even using Fe... 3+ Even if a compound is used as a positive electrode active material, it may not be able to operate at high voltage.

[0005] For example, Non-Patent Literature 1 reports the generation of LiFeF3 during charge-discharge using iron fluoride (FeF3) with an average discharge voltage of 3.1V. Non-Patent Literature 2 reports the expectation of high energy density for LiFeO2.

[0006] Existing technical documents Non-patent literature Non-patent literature 1: F. Badway, et al., “Carbon Metal Fluoride Nanocomposites” J. Electrochem. Soc., 150(10) A1318-A1327 (2003) Non-patent literature 2: Y. Hu, et al., “A Simple, Quick and Eco-Friendly Strategy of Synthesis Nanosized α -LiFeO2 Cathode with Excellent ElectrochemicalPerformance for Lithium-Ion Batteries” Materials, 11, 1176 (2018) Summary of the Invention

[0007] The technical problem that the invention aims to solve In Non-Patent Literature 2, the actual voltage is around 2.5V, which is lower than the expected voltage. The average discharge voltage (3.1V) in Non-Patent Literature 1 is also lower than the voltage of LiFePO4, indicating room for improvement in order to achieve higher voltages.

[0008] This invention was made to solve the aforementioned technical problems, and its purpose is to provide an Fe-based positive electrode active material capable of operating at high voltage and a lithium-ion secondary battery containing the positive electrode active material. Furthermore, it further reduces resource risks and contributes to cost reduction.

[0009] Technical solutions for solving technical problems To achieve the above objectives, the present invention provides the following technical solutions.

[0010] [1] A positive electrode active material, which is mainly composed of lithium iron fluoride composite and contains carbon. The lithium iron fluoride composite is 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.

[0011] [1] The positive electrode active material involved has a high average discharge voltage and can operate at high voltage. Therefore, in lithium-ion secondary batteries containing this positive electrode active material, the number of batteries required can be reduced, which helps to reduce costs.

[0012] [2] According to the positive electrode active material described in [1], the mass ratio (M1:M2) of the lithium iron composite fluoride to the carbon (M2) is 90:10 to 60:40.

[0013] [2] The positive electrode active material involved can increase the capacity of lithium-ion secondary batteries containing the positive electrode active material and further improve the energy density.

[0014] [3] According to the positive electrode active material described in [1] or [2], wherein the mass ratio (M1:M2) of the lithium iron composite fluoride to the mass of the carbon is 90:10 to 80:20.

[0015] [3] The positive electrode active material involved can increase the capacity of lithium-ion secondary batteries containing the positive electrode active material and further improve the energy density.

[0016] [4] The positive electrode active material according to any one of [1] to [3], wherein the carbon is carbon nanotube or carbon black.

[0017] [4] The positive electrode active material involved can increase the capacity of lithium-ion secondary batteries containing the positive electrode active material and further improve the energy density.

[0018] [5] A lithium-ion secondary battery comprising a positive electrode, a negative electrode and an electrolyte, wherein the positive electrode contains any one of [1] to [4] a positive electrode active material.

[0019] [5] The positive electrode of the lithium-ion secondary battery involved contains any one of the positive electrode active materials described in [1] to [4]. This means that the battery can operate at a high voltage.

[0020] [6] According to the lithium-ion secondary battery described in [5], the dQ / dV curve (plot) during discharge of the charge-discharge cycle has a peak in the range of 3.94 to 4.01 V.

[0021] [6] The lithium-ion secondary batteries involved showed that the positive electrode active material underwent a chemical reaction in a high voltage range of 3.94–4.01 V. This indicates that the battery can operate at a higher voltage.

[0022] [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 is mainly composed of lithium iron fluoride composite and contains carbon. The lithium iron fluoride composite is represented by the following formula (1). The lithium-ion secondary battery exhibits a peak in the dQ / dV curve during discharge in the range of 3.94–4.01V during charge-discharge cycles.

[0023] Li x FeF (3+x) (1) In equation (1), x is a number that satisfies 0.4≤x≤1.2.

[0024] [7] The lithium-ion secondary batteries involved have a high average discharge voltage and are capable of operating at high voltage. Therefore, the number of batteries required can be reduced, which helps to reduce costs.

[0025] Invention Effects According to the present invention, an Fe-based positive electrode active material capable of operating at high voltage and a lithium-ion secondary battery comprising the positive electrode active material can be provided. Attached Figure Description

[0026] Figure 1 This is a diagram showing a portion of the X-ray diffraction pattern of a positive electrode active material according to one embodiment of the present invention.

[0027] Figure 2 This is a diagram showing a portion of the X-ray diffraction pattern of a positive electrode active material according to another embodiment of the present invention.

[0028] Figure 3 This is a schematic cross-sectional view of a lithium-ion secondary battery according to one embodiment of the present invention.

[0029] Figure 4 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 14.

[0030] Figure 5 This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Example 14 during charge-discharge cycles.

[0031] Figure 6 This is a flowchart illustrating an example of a method for manufacturing a positive electrode active material according to one embodiment of the present invention.

[0032] Figure 7 This is a diagram showing the X-ray diffraction patterns of the positive electrode active materials of Examples 1-5 and Comparative Example 1.

[0033] Figure 8 This is a diagram showing the X-ray diffraction patterns of the positive electrode active materials of Examples 6-12.

[0034] Figure 9 This is a partial X-ray diffraction pattern of the positive electrode active material of Example 8.

[0035] Figure 10 This is a partial X-ray diffraction pattern of the positive electrode active material of Example 9.

[0036] Figure 11 This is a partial X-ray diffraction pattern of the positive electrode active material of Example 13.

[0037] Figure 12 This is a partial X-ray diffraction pattern of the positive electrode active material of Example 15.

[0038] Figure 13 This is a partial X-ray diffraction pattern of the positive electrode active material of Example 16.

[0039] Figure 14 This is a partial X-ray diffraction pattern of the positive electrode active material of Example 17.

[0040] Figure 15 This is a partial X-ray diffraction pattern of the positive electrode active material of Example 18.

[0041] Figure 16 This is a partial X-ray diffraction pattern of the positive electrode active material of Example 19.

[0042] Figure 17 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 1.

[0043] Figure 18 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 2.

[0044] Figure 19 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 3.

[0045] Figure 20 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 4.

[0046] Figure 21 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 5.

[0047] Figure 22 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 6.

[0048] Figure 23 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 7.

[0049] Figure 24 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 8.

[0050] Figure 25 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 9.

[0051] Figure 26 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 10.

[0052] Figure 27 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 12.

[0053] Figure 28 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 13.

[0054] Figure 29 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 15.

[0055] Figure 30 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 16.

[0056] Figure 31 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 17.

[0057] Figure 32 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 18.

[0058] Figure 33 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 19.

[0059] Figure 34 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Comparative Example 1.

[0060] Figure 35 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Comparative Example 2.

[0061] Figure 36 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Comparative Example 3.

[0062] Figure 37 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Comparative Example 4.

[0063] Figure 38 This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Example 1 during charge-discharge cycles.

[0064] Figure 39This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Example 2 during charge-discharge cycles.

[0065] Figure 40 This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Example 3 during charge-discharge cycles.

[0066] Figure 41 This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Example 4 during charge-discharge cycles.

[0067] Figure 42 This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Example 5 during charge-discharge cycles.

[0068] Figure 43 This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Example 6 during charge-discharge cycles.

[0069] Figure 44 This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Example 7 during charge-discharge cycles.

[0070] Figure 45 This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Example 8 during charge-discharge cycles.

[0071] Figure 46 This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Example 9 during charge-discharge cycles.

[0072] Figure 47 This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Example 10 during charge-discharge cycles.

[0073] Figure 48 This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Example 12 during charge-discharge cycles.

[0074] Figure 49 This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Example 13 during charge-discharge cycles.

[0075] Figure 50 This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Example 15 during charge-discharge cycles.

[0076] Figure 51This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Example 16 during charge-discharge cycles.

[0077] Figure 52 This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Example 17 during charge-discharge cycles.

[0078] Figure 53 This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Example 18 during charge-discharge cycles.

[0079] Figure 54 This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Example 19 during charge-discharge cycles.

[0080] Figure 55 This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Comparative Example 1 during charge-discharge cycles.

[0081] Figure 56 This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Comparative Example 2 during charge-discharge cycles.

[0082] Figure 57 This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Comparative Example 3 during charge-discharge cycles.

[0083] Figure 58 This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Comparative Example 4 during charge-discharge cycles.

[0084] Figure 59 This is a graph showing the charge-discharge curves of a lithium-ion secondary battery containing the positive electrode active material of Example 20.

[0085] Figure 60 This is a graph showing the dQ / dV curves of a lithium-ion secondary battery containing the positive electrode active material of Example 20 during charge-discharge cycles.

[0086] Explanation of reference numerals in the attached figures 1. Lithium-ion secondary battery 2 Positive electrode 3 Negative electrode 4. Isolation components 5. Insulating seals (gaskets) 10 Positive electrode container 20 Negative electrode container (negative electrode terminal). Detailed Implementation

[0087] The preferred embodiments of the present invention will now be described in detail.

[0088] [Positive electrode active material] The positive electrode active material of this embodiment is mainly composed of lithium iron fluoride composite and also contains carbon. This positive electrode active material is used as the positive electrode of a lithium-ion secondary battery. Using lithium iron fluoride composite as the "main component" means that, relative to the total mass of the positive electrode active material, the content of lithium iron fluoride composite 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. As long as it does not impair the function of the present invention, the positive electrode active material may also contain components other than the main component and carbon.

[0089] In this embodiment, the positive electrode active material can contain only one type of lithium iron fluoride or two or more types of lithium iron fluoride, as long as the main component is lithium iron fluoride composite fluoride.

[0090] When the positive electrode active material is manufactured with lithium iron fluoride composite as the main component, the overall composition ratio (Li:Fe:F) of the lithium iron fluoride composite is maintained in the obtained positive electrode active material. When the positive electrode active material obtained with lithium iron fluoride composite as the main component having this composition is used in a secondary battery, high-voltage operation can be achieved. Furthermore, the composition ratio of the lithium iron fluoride composite can be adjusted to be the same as the required composition ratio of the desired positive electrode active material.

[0091] (Lithium iron fluoride) The lithium iron composite fluoride of this embodiment is represented by the following formula (1).

[0092] Li x FeF (3+x) (1) In equation (1), x is a number that satisfies 0.4 ≤ x ≤ 1.2. In equation (1), x is preferably 0.5 ≤ x ≤ 1.0, and more preferably 0.6 ≤ x ≤ 0.9. If x is within the above numerical range, the average discharge voltage and capacity can be improved, and a small battery with high energy density can be constructed.

[0093] In equation (1), x represents the molar ratio of Li to Fe. The molar ratio of Li to Fe is x:1. In addition, the molar ratio of Li, Fe and F is x:1:(3+x).

[0094] The composition of lithium iron fluoride composites can be determined by inductively coupled plasma atomic emission spectrometry (ICP) and combustion ion chromatography.

[0095] (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 obtaining the effects of the present invention more appropriately, carbon nanotubes or carbon black are preferred as these elemental carbons.

[0096] In this embodiment, the carbon content, expressed as the mass ratio (M1:M2) of the lithium iron fluoride composite (M1) to the carbon (M2), is preferably 90:10 to 60:40, more preferably 90:10 to 70:30, and even more preferably 90:10 to 80:20. If the mass ratio (M1:M2) is within the above range, the capacity of the lithium-ion secondary battery can be increased, further improving the energy density.

[0097] <X-ray diffraction (XRD) patterns> Figure 1 As an example of the X-ray diffraction (XRD) pattern of the positive electrode active material involved in this embodiment, a portion of the XRD pattern of Example 7 described later is shown. Figure 1 As shown, the positive electrode active material in Example 7 is at 20°≤2 θ The range <25° and 25°≤2 θ Peaks are present in the range ≤30°. Peaks are present in the range ≤20°. θ Peaks in the range <25° indicate peaks originating from the crystal structure of FeF3 as trivalent iron. 25° ≤ 2 θ Peaks in the range ≤30° indicate peaks originating from the crystal structure of LiFe2F6. This means that the positive electrode active material of Example 7 has the same crystal structure as FeF3 and LiFe2F6.

[0098] then, Figure 2 This represents a portion of the XRD pattern of Example 14, described later. For example... Figure 2 As shown, in the positive electrode active material of Example 14, 20° ≤ 2 θ The peaks in the range of <25° disappeared. This means that the positive electrode active material of Example 14 has become a single-phase state with only the same crystal structure as LiFe2F6.

[0099] Thus, by controlling the crystal structure of the lithium iron composite fluoride contained in the positive electrode active material of this embodiment to a single-phase state, the capacity of the lithium-ion secondary battery containing this positive electrode active material can be further increased, and the energy density can be further improved.

[0100] [Lithium-ion rechargeable battery] The lithium-ion secondary battery of this embodiment includes a positive electrode, a negative electrode, and an electrolyte. The positive electrode contains a positive electrode active material with the aforementioned lithium iron composite fluoride as its main component. The lithium-ion secondary battery of this embodiment may include other battery elements as needed.

[0101] The lithium-ion secondary battery of this embodiment, except that the positive electrode contains a positive electrode active material mainly composed of the aforementioned lithium iron composite fluoride and containing carbon, can directly utilize known lithium-ion secondary battery elements. The lithium-ion secondary battery of this embodiment can have any structure among coin-shaped, button-shaped, cylindrical, square, and laminated types. Furthermore, the lithium-ion secondary battery of this embodiment can be applied to a wide range of applications, including mobile devices such as mobile phones and laptops, and automotive applications.

[0102] The following description concerns the lithium-ion secondary battery using an electrolyte (coin-type lithium-ion secondary battery) according to this embodiment. 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.

[0103] like Figure 3 As shown, the lithium-ion secondary battery 1 of this embodiment includes a negative electrode can (negative terminal) 20, a negative electrode 3, an electrolyte-impregnated separator 4, an insulating sealant (gasket) 5, a positive electrode 2, and a positive electrode can 10.

[0104] The positive electrode container 10 is disposed on the lower side of the separator 4, and the negative electrode container 20 is disposed on the upper side of the separator 4, forming the shape of the lithium-ion secondary battery 1. A positive electrode 2 and a negative electrode 3 are disposed between the positive electrode container 10 and the negative electrode container 20, separated by the separator 4 which is impregnated with electrolyte. The positive electrode 2 and the negative electrode 3 are electrically insulated from each other by an insulating seal 5.

[0105] In the lithium-ion secondary battery 1, the positive electrode active material of this embodiment can be combined with conductive agents, binders, etc. to prepare a positive electrode mixture as needed, and then pressed onto the current collector (not shown) to produce the positive electrode 2.

[0106] As a current collector, stainless steel mesh, aluminum foil, etc. are preferred. As a conductive agent, carbon nanotubes (CNTs), acetylene black, Ketjen black, etc. are preferred. As a binder, tetrafluoroethylene, polyvinylidene fluoride, etc. are preferred.

[0107] The combination 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 75-100% by mass, more preferably 90-99% by mass. The content of the conductive agent in the positive electrode mixture is preferably 1-15% by mass, more preferably 0.1-5% by mass. The content of the binder in the positive electrode mixture is preferably 0.1-10% by mass, more preferably 0.1-5% by mass.

[0108] In a lithium-ion secondary battery 1, the negative electrode 3, which is the opposite of the positive electrode 2, can be made of known materials that function as the negative electrode active material and can absorb and release lithium, such as metallic materials like lithium metal and lithium alloys; carbon-based materials like graphite and MCMB (mesophase carbon microspheres); 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.

[0109] The separator 4 and the battery container (positive electrode container 10, negative electrode container 20) can adopt known battery elements.

[0110] As the electrolyte, known electrolytes, semi-solid electrolytes, and solid electrolytes can be used. For example, electrolytes prepared by dissolving electrolytes such as lithium perchlorate and lithium hexafluorophosphate in solvents such as ethylene carbonate (EC), dimethyl carbonate (DMC), propylene carbonate (PC), and diethyl carbonate (DEC) can be used.

[0111] In addition to using a positive electrode active material that is mainly composed of the aforementioned lithium iron composite fluoride and contains carbon, known semi-solid electrolytes and solid electrolytes can also be used as semi-solid electrolytes and solid electrolytes.

[0112] Examples of semi-solid electrolytes include those composed of polymer components and standard electrolytes. Examples of polymer components include polyvinylidene fluoride (PVDF) / ethylene oxide (PEO), polyacrylonitrile (PAN) / PEO, polymethyl methacrylate (PMMA), and PVDF / hexafluoropropylene (HFP). Examples of standard electrolytes include 1 mol / L lithium hexafluorophosphate (LiPF6) EC / DMC solution, 1 mol / L LiPF6EC / ethyl methyl carbonate (EMC) solution, and 1 mol / L LiPF6EC / DMC / EMC solution.

[0113] In the case of all-solid-state lithium-ion secondary batteries, solid electrolytes such as polyethylene oxide-based polymeric compounds, polymeric solid electrolytes containing at least one of polyorganosiloxane chains or polyoxyethylene chains, sulfide-based solid electrolytes, and oxide-based solid electrolytes can be used as electrolytes.

[0114] Regarding the positive electrode of an all-solid-state lithium-ion secondary battery, for example, a positive electrode compound containing a solid electrolyte, in addition to the aforementioned positive electrode active material, conductive agent, and binder, can be loaded onto a positive electrode current collector such as aluminum, nickel, or stainless steel.

[0115] The lithium-ion secondary battery 1 of this embodiment is able to operate at high voltage because the positive electrode 2 contains the positive electrode active material of this embodiment.

[0116] <dQ / dV curves for charge-discharge cycles> Figure 4 In the figure, as an example of the charge-discharge curve in the charge-discharge cycle of the lithium-ion secondary battery of this embodiment, a graph with respect to Example 14 described later is shown. Figure 4 The horizontal axis of the graph represents the capacity of the lithium-ion secondary battery. Figure 4 The vertical axis of the graph represents the voltage of the lithium-ion secondary battery during charging and discharging. Figure 4 In the charge / discharge curves, the curve in the upper right corner represents the curve during charging, and the curve in the lower right corner represents the curve during discharging.

[0117] Figure 4 The lithium-ion secondary battery shown has a capacity of 56.2 mAh / g.

[0118] Figure 5 express Figure 4 The dQ / dV curve during charge-discharge cycles. Figure 5 The horizontal axis of the graph represents the voltage during charge-discharge cycles. Figure 5 The vertical axis of the graph represents the voltage versus the curve. Figure 4 The value obtained by differentiating the capacity (dQ / dV curve, dQdV) -1 curve). Figure 5 The upward-convex curve represents the curve during charging, and the downward-convex curve represents the curve during discharging.

[0119] like Figure 5 As shown, the charging curve has a peak at 4.01V, and the discharging curve has a peak at 3.94V. These peaks indicate that a chemical reaction occurred in the positive electrode active material during charging or discharging. This indicates that a chemical reaction occurred at a high voltage of 3.94V during discharging, demonstrating that the lithium-ion secondary battery can operate at high voltage.

[0120] The lithium-ion secondary battery of this embodiment preferably has a peak in the dQ / dV curve during discharge in the range of 3.94 to 4.01 V during charge-discharge cycles. The presence of a peak in the dQ / dV curve during discharge within this range indicates that the lithium-ion secondary battery can operate at higher voltages.

[0121] It should be noted that in this specification, "having a peak" in the dQ / dV curve means having a height (or "depth" in the case of the discharge curve) of 40 mAh g. -1 V -1 The above peaks (or valleys in the case of the discharge curve) are considered valleys.

[0122] In this embodiment, the lithium-ion secondary battery preferably has a peak depth of 40 mAh g during the discharge phase of a charge-discharge cycle, representing the peak depth of the dQ / dV curve.-1 V -1 The above, more preferably 100mAhg -1 V -1 The above is further preferred to be 200mAhg. -1 V -1 The above is particularly preferred for 500mAh g. -1 V -1 The above applies. If the peak depth of the dQ / dV curve is above the aforementioned lower limit, the capacity of the lithium-ion secondary battery can be further improved. The maximum value of the peak depth of the dQ / dV curve is not particularly limited; for example, 5000 Ahg is preferred. -1 V -1 the following.

[0123] It should be noted that "peak depth" is given by the depth of the valley in the dQ / dV curve (the absolute value of the value at the bottom of the valley).

[0124] [Manufacturing method of positive electrode active material] The positive electrode active material in this embodiment is mainly composed of the aforementioned lithium iron composite fluoride and contains carbon. As the lithium source for the lithium iron composite fluoride, known compounds such as halides like lithium fluoride (LiF), hydroxides like lithium hydroxide monohydrate (LiOH·H2O), carbonates like lithium carbonate (Li2CO3), and acetates like lithium acetate (CH3COOLi) and lithium acetate dihydrate (CH3COOLi·2H2O) can be used, without particular limitation.

[0125] As the iron source for lithium iron fluoride composites, from the perspective of being able to operate at high voltage, trivalent iron is preferred over divalent iron, and iron fluoride (FeF3) is even more preferred.

[0126] In manufacturing lithium-iron composite fluorides, the aforementioned lithium source and iron source are mixed and mechanically treated (first mechanical treatment) under specified time and conditions. For example, when using lithium fluoride as the lithium source and trivalent iron (FeF3) as the iron source, it is believed that the compound shown in formula (1) can be obtained through the following reaction.

[0127] LiF + FeF3 → LiFeF4 (the compound in formula (1) where x = 1) The value of x in the compound shown in formula (1) can be adjusted by the molar ratio of LiF to FeF3.

[0128] The specific means used in the first mechanical processing is not particularly limited, but various means previously used for the purpose of crushing and mixing solid materials can be applied. Among these means, a ball mill is preferred; from the perspective of being able to fully crush and mix the raw materials, a planetary ball mill is more preferred.

[0129] The time for performing the first mechanical treatment is preferably 8 to 12 hours, more preferably 9 to 11 hours.

[0130] As a condition for performing the first mechanical processing, the rotational speed is preferably 250 to 450 rpm, more preferably 300 to 400 rpm.

[0131] There is no particular temperature limit for the first mechanical treatment; it can be carried out at room temperature (e.g., 5–30°C).

[0132] The atmosphere for the first mechanical treatment is preferably an inert gas (such as rare gases like argon (Ar) or nitrogen (N2)).

[0133] Carbon is added to and mixed in the lithium iron fluoride obtained by the first mechanical treatment, and then mechanically treated (second mechanical treatment) for a specified time and under specified conditions.

[0134] By performing a second mechanical treatment, the capacity and rate characteristics of lithium-ion secondary batteries can be improved.

[0135] As the added carbon, examples of elemental carbon mentioned above are possible, with carbon microparticles being preferred. Examples of carbon microparticles include carbon nanotubes (CNTs) and carbon black. Even among these carbon microparticles, CNTs are preferred from the viewpoint of further improving the conductivity of the positive electrode active material.

[0136] The conditions (methods, time, rotational speed, temperature, atmosphere, etc.) for the second mechanical treatment are the same as those for the first mechanical treatment.

[0137] Preferably, the lithium-iron composite fluoride and carbon composite obtained through the second mechanical treatment is subjected to heat treatment. By performing heat treatment, the crystal structure in the lithium-iron composite fluoride changes, which can increase the capacity of lithium-ion secondary batteries using this composite as the positive electrode active material.

[0138] This can be attributed to the change in the crystal structure ratio of LiFe2F6 to FeF3 in the lithium-iron composite fluoride through heat treatment, and the increase in the crystal structure of LiFe2F6.

[0139] By subjecting the lithium iron composite fluoride and carbon composite to heat treatment, the capacity of lithium-ion secondary batteries containing this composite as the positive electrode active material can be further increased, and the energy density can be further improved.

[0140] The firing temperature during heat treatment is preferably 100–300°C, more preferably 150–250°C, and even more preferably 175–225°C.

[0141] 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.

[0142] The atmosphere used in heat treatment is preferably an inert gas (such as rare gases like argon (Ar) or nitrogen (N2)).

[0143] The pressure during heat treatment can be atmospheric pressure (0.1013 MPa), preferably low vacuum (e.g., 10...). 2 Pa~10 5 Pa).

[0144] The above-mentioned lithium iron composite fluoride and carbon composite can be used as the positive electrode active material, or the material of the composite that has been heat-treated (the heat-treated composite) can be used as the positive electrode active material.

[0145] By using the obtained positive electrode active material as the positive electrode to make lithium-ion secondary batteries, it is possible to obtain batteries that operate at high voltage.

[0146] The method for manufacturing the positive electrode active material according to this embodiment is illustrated in the flowchart. Figure 6 . Figure 6 The meanings of the terms in this document are the same as those of the terms mentioned above.

[0147] Example The following describes embodiments of the present invention, but the present invention is not limited to the following embodiments.

[0148] [Example 1] (Li) 0.6 FeF 3.6 (Preparation of compounds with x = 0.6 in formula (1) and complexes with CNT = 10% by mass) The first mechanical treatment of 0.439 g of iron fluoride (FeF3) and 0.0606 g of lithium fluoride (LiF) was performed using a planetary ball mill. The planetary ball mill used was a Fritsch Premium line PL-7. The cylinder and grinding balls were made of zirconium oxide, and 50 g of 5 mm diameter grinding balls were used in an 80 mL cylinder. The first mechanical treatment conditions were 350 rpm for 10 hours. Then, 0.55 g of carbon nanotubes (CNTs) were added to the cylinder for a second mechanical treatment to obtain the positive electrode active material. The second mechanical treatment conditions were Ar atmosphere, 25 °C, 350 rpm for 10 hours.

[0149] The obtained positive electrode active material was subjected to X-ray diffraction analysis under the following conditions. The results are presented below. Figure 7 .

[0150] Conditions for X-ray Diffraction Measurement X-ray diffraction equipment: Manufactured by Rigaku Corporation, SmartLab X-ray source: Cu K α X-rays (Cu K) α =1.5418Å) The opening angle of the incident parallel slit: 5.0° Incident length limits slit length: 5.0 mm The opening angle of the parallel slit receiving light is 5.0°. K β Filter: Use Step width: 0.01° Entrance slit: 1 / 6° Light-receiving slit 1: 4.0mm Light-receiving slit 2: 13mm like Figure 7 As shown, the obtained positive electrode active material is consistent with the diffraction pattern of FeF3 (space group R-3c, trigonal crystal system) with DB card number 00-061-0194 and the diffraction pattern of LiFe2F6 (space group P42 / mnm, tetragonal crystal system) with DB card number 01-074-2193, indicating that it has the same crystal structure as FeF3 and LiFe2F6.

[0151] [Example 2] (Li) 0.7 FeF 3.7 (Preparation of compounds with x = 0.7 in formula (1) and complexes with CNT = 10% by mass) The positive electrode active material was otherwise obtained by first mechanical treatment of 0.431 g of iron fluoride (FeF3) and 0.0693 g of lithium fluoride (LiF) using a planetary ball mill apparatus, in the same manner as in Example 1.

[0152] 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 below. Figure 7 .

[0153] like Figure 7 As shown, the obtained positive electrode active material is consistent with the diffraction pattern of FeF3 (space group R-3c, trigonal crystal system) with DB card number 00-061-0194 and the diffraction pattern of LiFe2F6 (space group P42 / mnm, tetragonal crystal system) with DB card number 01-074-2193, indicating that it has the same crystal structure as FeF3 and LiFe2F6.

[0154] [Example 3] (Li) 0.8 FeF 3.8(Preparation of compounds with x = 0.8 in formula (1) and complexes with CNT = 10% by mass) The positive electrode active material was otherwise obtained by first mechanical treatment of 0.422 g of iron fluoride (FeF3) and 0.078 g of lithium fluoride (LiF) using a planetary ball mill apparatus, in the same manner as in Example 1.

[0155] 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 below. Figure 7 .

[0156] like Figure 7 As shown, the obtained positive electrode active material is consistent with the diffraction pattern of FeF3 (space group R-3c, trigonal crystal system) with DB card number 00-061-0194 and the diffraction pattern of LiFe2F6 (space group P42 / mnm, tetragonal crystal system) with DB card number 01-074-2193, indicating that it has the same crystal structure as FeF3 and LiFe2F6.

[0157] [Example 4] (Preparation of LiFeF4 (the compound in formula (1) with x = 1.0) and CNT = 10% by mass) The positive electrode active material was otherwise obtained by first mechanical treatment of 0.407 g of iron fluoride (FeF3) and 0.0934 g of lithium fluoride (LiF) using a planetary ball mill apparatus, in the same manner as in Example 1.

[0158] 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 below. Figure 7 .

[0159] like Figure 7 As shown, the obtained positive electrode active material is consistent with the diffraction pattern of FeF3 (space group R-3c, trigonal crystal system) with DB card number 00-061-0194 and the diffraction pattern of LiFe2F6 (space group P42 / mnm, tetragonal crystal system) with DB card number 01-074-2193, indicating that it has the same crystal structure as FeF3 and LiFe2F6.

[0160] [Example 5] (Li) 1.2 FeF 4.2 (Preparation of compounds with x = 1.2 in formula (1) and complexes with CNT = 10% by mass) The positive electrode active material was otherwise obtained by first mechanical treatment of 0.391 g of iron fluoride (FeF3) and 0.108 g of lithium fluoride (LiF) using a planetary ball mill apparatus, in the same manner as in Example 1.

[0161] 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 below. Figure 7 .

[0162] like Figure 7 As shown, the obtained positive electrode active material is consistent with the diffraction pattern of FeF3 (space group R-3c, trigonal crystal system) with DB card number 00-061-0194 and the diffraction pattern of LiFe2F6 (space group P42 / mnm, tetragonal crystal system) with DB card number 01-074-2193, indicating that it has the same crystal structure as FeF3 and LiFe2F6.

[0163] [Comparative Example 1] (Preparation of a complex of LiFeF3 and CNT = 10% by mass) The positive electrode active material was otherwise obtained by first mechanical treatment of 0.392 g of ferrous fluoride (FeF2) and 0.108 g of lithium fluoride (LiF) using a planetary ball mill apparatus, in the same manner as in Example 1.

[0164] 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 below. Figure 7 .

[0165] like Figure 7 As shown, the obtained positive electrode active material has the same diffraction pattern as LiFe2F6 with space group P42 / mnm and tetragonal crystal system (DB card number 01-074-2193), and has the same crystal structure as LiFe2F6.

[0166] [Example 6] For the positive electrode active material obtained in Example 1, under an argon atmosphere, at 10 3 Pa was subjected to heat treatment at 200°C for 5 hours using an oven.

[0167] 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 below. Figure 8 .

[0168] like Figure 8 As shown, the obtained positive electrode active material is consistent with the diffraction pattern of FeF3 (space group R-3c, trigonal crystal system) with DB card number 00-061-0194 and the diffraction pattern of LiFe2F6 (space group P42 / mnm, tetragonal crystal system) with DB card number 01-074-2193, indicating that it has the same crystal structure as FeF3 and LiFe2F6.

[0169] [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 X-ray diffraction measurements were performed on the obtained positive electrode active material under the same conditions as in Example 1. The results are shown below. Figure 8 .

[0170] like Figure 8 As shown, the obtained positive electrode active material is consistent with the diffraction pattern of FeF3 (space group R-3c, trigonal crystal system) with DB card number 00-061-0194 and the diffraction pattern of LiFe2F6 (space group P42 / mnm, tetragonal crystal system) with DB card number 01-074-2193, indicating that it has the same crystal structure as FeF3 and LiFe2F6.

[0171] Additionally, a portion of the XRD patterns in the results (20°≤2) θ (Magnified view ≤30°) shown in Figure 1 .

[0172] like Figure 1 As shown, the obtained positive electrode active material has peaks from the crystal structure of FeF3 and peaks from the crystal structure of LiFe2F6.

[0173] [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 X-ray diffraction measurements were performed on the obtained positive electrode active material under the same conditions as in Example 1. The results are shown below. Figure 8 .

[0174] like Figure 8 As shown, the obtained positive electrode active material is consistent with the diffraction pattern of FeF3 (space group R-3c, trigonal crystal system) with DB card number 00-061-0194 and the diffraction pattern of LiFe2F6 (space group P42 / mnm, tetragonal crystal system) with DB card number 01-074-2193, indicating that it has the same crystal structure as FeF3 and LiFe2F6.

[0175] Additionally, a portion of the XRD patterns in the results (20°≤2) θ (Magnified view ≤30°) shown in Figure 9 .

[0176] like Figure 9 As shown, the obtained positive electrode active material has peaks from the crystal structure of FeF3 and peaks from the crystal structure of LiFe2F6.

[0177] [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 X-ray diffraction measurements were performed on the obtained positive electrode active material under the same conditions as in Example 1. The results are shown below. Figure 8 .

[0178] like Figure 8 As shown, the obtained positive electrode active material is consistent with the diffraction pattern of FeF3 (space group R-3c, trigonal crystal system) with DB card number 00-061-0194 and the diffraction pattern of LiFe2F6 (space group P42 / mnm, tetragonal crystal system) with DB card number 01-074-2193, indicating that it has the same crystal structure as FeF3 and LiFe2F6.

[0179] Additionally, a portion of the XRD patterns in the results (20°≤2) θ (Magnified view ≤30°) shown in Figure 10 .

[0180] like Figure 10 As shown, the obtained positive electrode active material has peaks from the crystal structure of FeF3 and peaks from the crystal structure of LiFe2F6.

[0181] [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 X-ray diffraction measurements were performed on the obtained positive electrode active material under the same conditions as in Example 1. The results are shown below. Figure 8 .

[0182] like Figure 8 As shown, the obtained positive electrode active material is consistent with the diffraction pattern of FeF3 (space group R-3c, trigonal crystal system) with DB card number 00-061-0194 and the diffraction pattern of LiFe2F6 (space group P42 / mnm, tetragonal crystal system) with DB card number 01-074-2193, indicating that it has the same crystal structure as FeF3 and LiFe2F6.

[0183] [Example 11] (Li) 0.4 FeF 3.4 (Preparation of compounds with x = 0.4 in formula (1) and complexes with CNT = 10% by mass) The positive electrode active material was obtained by mechanically processing 0.458 g of iron fluoride (FeF3) and 0.0421 g of lithium fluoride (LiF) using a planetary ball mill apparatus, otherwise the same as in Example 1.

[0184] 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 below. Figure 8 .

[0185] like Figure 8 As shown, the obtained positive electrode active material is consistent with the diffraction pattern of FeF3 (space group R-3c, trigonal crystal system) with DB card number 00-061-0194 and the diffraction pattern of LiFe2F6 (space group P42 / mnm, tetragonal crystal system) with DB card number 01-074-2193, indicating that it has the same crystal structure as FeF3 and LiFe2F6.

[0186] [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 X-ray diffraction measurements were performed on the obtained positive electrode active material under the same conditions as in Example 1. The results are shown below. Figure 8 .

[0187] like Figure 8 As shown, the obtained positive electrode active material is consistent with the diffraction pattern of FeF3 (space group R-3c, trigonal crystal system) with DB card number 00-061-0194 and the diffraction pattern of LiFe2F6 (space group P42 / mnm, tetragonal crystal system) with DB card number 01-074-2193, indicating that it has the same crystal structure as FeF3 and LiFe2F6.

[0188] [Example 13] (Li) 0.6 FeF 3.6 (Preparation of compounds with x = 0.6 in formula (1) and complexes with CNT = 20% by mass) The amount of CNT added into the cylinder was 0.125g, and otherwise, the positive electrode active material was obtained in the same manner as in Example 6.

[0189] 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 below. Figure 11 .

[0190] [Example 14] (Li) 0.7 FeF 3.7 (Preparation of compounds with x = 0.7 in formula (1) and complexes with CNT = 20% by mass) The amount of CNT added into the cylinder was 0.125g, and otherwise, the positive electrode active material was obtained in the same manner as in Example 7.

[0191] 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 from the results (20° ≤ 2) was analyzed. θ (Magnified view ≤30°) shown in Figure 2 .

[0192] like Figure 2 As shown, the peaks of the obtained positive electrode active material from the FeF3 crystal structure disappear, and it has peaks from the LiFe2F6 crystal structure.

[0193] [Example 15] (Li) 0.8 FeF 3.8 (Preparation of compounds with x = 0.8 in formula (1) and complexes with CNT = 20% by mass) The amount of CNT added to the cylinder was 0.125g, and otherwise, the positive electrode active material was obtained in the same manner as in Example 8.

[0194] 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 from the results (20° ≤ 2) was analyzed. θ (Magnified view ≤30°) shown in Figure 12 .

[0195] like Figure 12 As shown, the peaks of the obtained positive electrode active material from the FeF3 crystal structure disappear, and it has peaks from the LiFe2F6 crystal structure.

[0196] [Example 16] (Li) 0.9 FeF 3.9 (Preparation of compounds with x = 0.9 in formula (1) and complexes with CNT = 20% by mass) The first mechanical treatment of 0.414 g of iron fluoride (FeF3) and 0.0857 g of lithium fluoride (LiF) was performed using a planetary ball mill apparatus, so that the amount of CNT added into the cylinder was 0.125 g. Otherwise, the positive electrode active material was obtained in the same manner as in Example 1.

[0197] The obtained positive electrode active material was subjected to heat treatment under the same conditions as in Example 6, and X-ray diffraction analysis was performed on the obtained positive electrode active material under the same conditions as in Example 1. A portion of the XRD pattern (20° ≤ 2) from the results was analyzed. θ (Magnified view ≤30°) shown in Figure 13 .

[0198] like Figure 13 As shown, the peaks of the obtained positive electrode active material from the FeF3 crystal structure disappear, and it has peaks from the LiFe2F6 crystal structure.

[0199] [Example 17] (Preparation of LiFeF4 (the compound in formula (1) with x = 1.0) and CNT = 20% by mass) The amount of CNT added into the cylinder was 0.125g, and otherwise, the positive electrode active material was obtained in the same manner as in Example 9.

[0200] 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 from the results (20° ≤ 2) was analyzed. θ (Magnified view ≤30°) shown in Figure 14 .

[0201] like Figure 14 As shown, the peaks of the obtained positive electrode active material from the FeF3 crystal structure disappear, and it has peaks from the LiFe2F6 crystal structure.

[0202] [Example 18] (Li) 1.1 FeF 4.1 (Preparation of compounds with x = 1.1 in formula (1) and complexes with CNT = 20% by mass) The first mechanical treatment of 0.399 g of iron fluoride (FeF3) and 0.101 g of lithium fluoride (LiF) was performed using a planetary ball mill apparatus, so that the amount of CNT added into the cylinder was 0.125 g. Otherwise, the positive electrode active material was obtained in the same manner as in Example 1.

[0203] The obtained positive electrode active material was subjected to heat treatment under the same conditions as in Example 6, and X-ray diffraction analysis was performed on the obtained positive electrode active material under the same conditions as in Example 1. A portion of the XRD pattern (20° ≤ 2) from the results was analyzed. θ (Magnified view ≤30°) shown in Figure 15 .

[0204] like Figure 15 As shown, the peaks of the obtained positive electrode active material from the FeF3 crystal structure disappear, and it has peaks from the LiFe2F6 crystal structure.

[0205] [Example 19] (Li) 1.2 FeF 4.2 (Preparation of compounds with x = 1.2 in formula (1) and complexes with CNT = 20% by mass) The amount of CNT added into the cylinder was 0.125g, otherwise, the positive electrode active material was obtained in the same manner as in Example 10.

[0206] 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 from the results (20° ≤ 2) was analyzed. θ (Magnified view ≤30°) shown in Figure 16 .

[0207] like Figure 16 As shown, the peaks of the obtained positive electrode active material from the FeF3 crystal structure disappear, and it has peaks from the LiFe2F6 crystal structure.

[0208] [Comparative Example 2] (Li) 0.2 FeF 3.2 (Preparation of compounds with x = 0.2 in formula (1) and complexes with CNT = 10% by mass) The first mechanical treatment of 0.478 g of iron fluoride (FeF3) and 0.022 g of lithium fluoride (LiF) was performed using a planetary ball mill apparatus, and then heat treatment was carried out under the same conditions as in Example 6 to obtain the positive electrode active material.

[0209] [Comparative Example 3] (Li) 1.4 FeF 4.4 (Preparation of compounds with x = 1.4 in formula (1) and complexes with CNT = 10% by mass) The first mechanical treatment of 0.378 g of iron fluoride (FeF3) and 0.122 g of lithium fluoride (LiF) was performed using a planetary ball mill apparatus, and then heat treatment was carried out under the same conditions as in Example 6 to obtain the positive electrode active material.

[0210] [Comparative Example 4] (Li) 1.6 FeF 4.6 (Preparation of compounds with x = 1.6 in formula (1) and complexes with CNT = 10% by mass) The first mechanical treatment of 0.366 g of iron fluoride (FeF3) and 0.134 g of lithium fluoride (LiF) was performed using a planetary ball mill apparatus, and then heat treatment was carried out under the same conditions as in Example 6 to obtain the positive electrode active material.

[0211] [The fabrication of lithium-ion secondary batteries] (The production 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 positive electrode active material, 10% by mass of acetylene black, and 10% by mass of polyvinylidene fluoride as solid components. This slurry was coated onto aluminum foil, pressed at 15 tons, and then punched using a 10mm diameter punch to produce the positive electrode. At this time, adjustments were made to ensure that the mass of the positive electrode active material was 3.5 mg.

[0212] (Making of a coin-shaped battery) The prepared positive electrode (10 mm in diameter) is placed on a positive electrode container, and a porous polyethylene film is placed on top as a separator, which is then pressed down with a polypropylene gasket. Next, a 0.5 mm thick Li negative electrode is placed, along with a thickness adjustment spacer. Then, as a non-aqueous electrolyte solution, a mixed solvent of ethylene carbonate and diethyl carbonate (volume ratio 5:5) containing 1 mol / L lithium hexafluorophosphate is added between the positive and negative electrodes to impregnate the separator. The negative electrode container is then covered and sealed to produce a coin-shaped battery (lithium-ion secondary battery).

[0213] [Battery performance evaluation] The performance of the fabricated coin-shaped batteries was evaluated. Specifically, the fabricated coin-shaped batteries were subjected to constant current charging and discharging at a current of 5 mA / g per unit mass of the positive electrode active material. The upper limit voltage for constant current charging and discharging was set to 4.25V, and the lower limit voltage was set to 3.35V. The rest time after charging and discharging was set to 10 minutes. The charge / discharge capacity (mAh / g) was calculated per unit mass of the positive electrode active material. The charge / discharge curves of Examples 1-10, Examples 12-19, and Comparative Examples 1-4 under constant current charging and discharging are shown below. Figure 4 , Figures 17-37 .

[0214] Based on the obtained charge-discharge curves, the horizontal axis is set to voltage, and the vertical axis is set to the value obtained by differentiating the voltage with respect to the capacity (dQ / dV, dQdV). -1 dQ / dV curves were constructed, and the voltage at which the chemical reaction occurred (reaction voltage) was determined based on the peak voltage of the dQ / dV curve during discharge. The dQ / dV curves for Examples 1-10, Examples 12-19, and Comparative Examples 1-4 are shown below. Figure 5 , Figures 38-58 .

[0215] like Figure 5 , Figures 38-54As shown, it can be confirmed that the peak voltage of the dQ / dV curves during discharge of Examples 1 to 10, 12 to 19 of the present invention is as high as 3.94V to 4.01V, and a chemical reaction occurred at the inherent high voltage of the compound shown in Formula (1).

[0216] In contrast, such as Figure 55 As shown, Comparative Example 1 (LiFeF3), whose positive electrode active material does not contain the compound shown in formula (1), did not show a peak voltage of the dQ / dV curve during discharge near 4V, and only showed a broad peak in the operating voltage range. It is believed that this is to observe Fe... 2+ / Fe 3+ A clear reaction requires lowering the lower limit of the operating voltage range. In Comparative Example 1, a chemical reaction occurred at a voltage lower than the lower limit voltage of 3.35V of this embodiment.

[0217] In addition, x in equation (1) refers to comparative examples 2 to 4, which are outside the scope of this invention. Figures 56-58 As shown, the peak depth of the dQ / dV curve at the peak voltage observed near 4V during discharge is shallower than that of Examples 6-10 and Example 12.

[0218] [Example 20] (Preparation of the complex of LiFeF4 (the compound in formula (1) with x = 1.0) 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 CNT.

[0219] Using the obtained positive electrode active material, coin-type batteries (lithium-ion secondary batteries) were fabricated in the same manner as in Examples 1-19, and battery performance was evaluated. Charge-discharge curves under constant current are shown below. Figure 59 Furthermore, based on the obtained charge-discharge curves, dQ / dV curves were prepared in the same manner as in Examples 1-19. The results are shown below. Figure 60 .

[0220] like Figure 59 As shown, it can be confirmed that in the lithium-ion secondary battery using the positive electrode active material of Example 20 with CB as carbon, the discharge capacity is 53.6 mAh g. -1 This shows a discharge capacity of 51 mAh g compared to the lithium-ion secondary battery using the positive electrode active material of Example 17 with CNTs as carbon. -1 Equal value. Additionally, such as Figure 60 As shown, it can be confirmed that the peak voltage of the dQ / dV curve during discharge is as high as 3.960V, and a chemical reaction occurred due to the inherent high voltage of the compound shown in formula (1).

[0221] The peak voltage during charging and discharging, the peak height (or peak depth) at the peak voltage, and the discharge capacity values ​​from the above results are summarized in Tables 1 to 4.

[0222] [Table 1]

[0223] [Table 2]

[0224] [Table 3]

[0225] [Table 4]

[0226] As can be seen from the above results, according to the present invention, an Fe-based positive electrode active material capable of operating at high voltage and a lithium-ion secondary battery containing the positive electrode active material can be provided.

[0227] It is also known that by subjecting the positive electrode active material of the present invention to heat treatment, the capacity of the lithium-ion secondary battery containing the positive electrode active material can be increased.

Claims

1. A positive electrode active material, characterized in that, It is mainly composed of lithium iron fluoride composites and also contains carbon. The lithium iron fluoride composite is 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 positive electrode active material according to claim 1, wherein, The mass ratio of the lithium iron fluoride composite M1 to the carbon M2 is 90:10 to 60:

40.

3. The positive electrode active material according to claim 1, wherein, The mass ratio of the lithium iron fluoride composite M1 to the carbon M2 is 90:10 to 80:

20.

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

5. A lithium-ion secondary battery, characterized in that, It comprises 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 4.

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

7. A lithium-ion secondary battery, characterized in that, It has a positive electrode, a negative electrode, and an electrolyte. The positive electrode contains a positive electrode active material. The positive electrode active material is mainly composed of lithium iron fluoride composite and contains carbon. The lithium iron fluoride composite is 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. The dQ / dV curve of the lithium-ion secondary battery during charge-discharge cycles has a peak in the range of 3.94 to 4.01 V.

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