Positive electrode active material and lithium ion secondary battery

Abstract

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, which contributes to energy efficiency.SOLUTION: A positive electrode active material is mainly composed of lithium iron complex fluoride, and the lithium iron complex fluoride is represented by the following formula (1). LixFeF(3+x) (1). In formula (1), x is a number that satisfies 0.5<x<1.5.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[0002] In recent years, research and development has been conducted on 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 and a lithium-ion secondary battery including the positive electrode active material, which will in turn reduce resource risks and contribute to cost reduction.

Means for Solving the Problem

[0009] To achieve the above object, the present invention provides the following means. [1] A positive electrode active material mainly composed of a lithium iron composite fluoride, wherein the lithium iron composite fluoride is represented by the following formula (1), and is a positive electrode active material. Li x FeF (3+x) (1) In formula (1), x is a number satisfying 0.5 < x < 1.5.

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

[0011] [2] The positive electrode active material according to [1], wherein in the formula (1), x satisfies 0.6 ≤ x ≤ 1.0.

[0012] [2] The positive electrode active material can further increase the average discharge voltage and operate at a higher voltage. Therefore, it 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 the range of 25° ≤ 2θ ≤ 30° in the X-ray diffraction pattern.

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

[0015] [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 [1] to [3].

[0016] In the lithium ion secondary battery according to [4], the positive electrode contains the positive electrode active material according to any one of [1] to [3], which means that the battery can operate at a high voltage.

[0017] [5] The lithium ion secondary battery according to [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.

[0018] The lithium ion secondary battery according to [5] 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.

[0019] [6] The lithium ion secondary battery according to [4] or [5], wherein the height of the peak between 3.77 and 4 V in a dQ / dV plot during discharge in a charge-discharge cycle is 1.2 to 4.4 times the height of the peak between 3.3 and 3.5 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 any one of [4] to [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 [4] 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 [4] 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. [Effects of the Invention]

[0027] 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]

[0028] [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. DETAILED DESCRIPTION OF THE INVENTION

[0029] Preferred embodiments of the present invention will now be described in detail.

[0030] [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.

[0031] As long as the positive electrode active material of this embodiment contains lithium iron composite fluoride as the main component, it may contain only one kind of lithium iron composite fluoride, or may contain two or more kinds.

[0032] When the positive electrode active material is produced using lithium iron composite fluoride as the main component, the composition ratio (Li:Fe:F) of the entire lithium iron composite fluoride is maintained in the obtained positive electrode active material. When the positive electrode active material obtained using 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 so as to be the same as the composition ratio required for the positive electrode active material to be obtained.

[0033] (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 0.5 < x < 1.5. In formula (1), x is preferably 0.6 ≦ x ≦ 1.2, more preferably 0.6 ≦ x ≦ 1.0, and still more preferably 0.7 ≦ x ≦ 1.0. When x is within the above numerical range, the average discharge voltage can be increased and operation at a higher voltage can be achieved. For this reason, a small-sized battery having a high energy density can be constructed.

[0034] In formula (1), x represents the molar ratio of Li and Fe. The molar ratio of Li and Fe is x:1. Further, the molar ratio of Li, Fe, and F is x:1:(3 + x). The composition of the lithium iron composite fluoride can be determined by inductively coupled plasma (ICP) emission spectrometry.

[0035] <X-ray diffraction (XRD) pattern> As an example of the X-ray diffraction (XRD) pattern of the positive electrode active material according to this 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 this 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 this embodiment has a crystal structure similar to that of FeF3 and LiFe2F6. The lithium iron composite fluoride of this embodiment has an iron composition ratio of all Fe 3+ Since it is composed of, when described according to the composition formula of LiFe2F6 in 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.

[0036] [Lithium-ion secondary battery] The lithium-ion secondary battery of this embodiment includes a positive electrode, a negative electrode, and an electrolyte, and the positive electrode contains a positive electrode active material mainly composed of the above-mentioned lithium iron composite fluoride. The lithium-ion secondary battery of this embodiment may include other battery elements as necessary.

[0037] For the lithium-ion secondary battery of this embodiment, the battery elements of a known lithium-ion secondary battery can be directly adopted except that the positive electrode contains a positive electrode active material mainly composed of the above-mentioned lithium iron composite fluoride. The lithium-ion secondary battery of this embodiment may have any configuration of coin type, button type, cylindrical type, rectangular type, or laminate type. Further, the lithium-ion secondary battery of this embodiment can be applied to a wide range of uses such as for mobile devices such as mobile phones and notebook computers, and for in-vehicle use.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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%.

[0043] 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.

[0044] The separator 4 and the battery container (positive electrode can 10, negative electrode can 20) can be made of known battery elements.

[0045] 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).

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] <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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] [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).

[0058] 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.

[0059] 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))

[0060] The value of x in the compound represented by the formula (1) can be adjusted by the molar ratio of LiF to FeF3.

[0061] 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.

[0062] 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.).

[0063] 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.

[0064] The crushing and mixing conditions for carbon coating can be the same as those for the mechanical treatment described above.

[0065] 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]

[0066] Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0067] [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.

[0068] 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

[0069] As shown in FIG. 6, the diffraction patterns of the obtained positive electrode active material match those of FeF3 (DB card number 00-061-0194, space group R-3c, trigonal crystal system) and 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.

[0070] [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 patterns of the obtained positive electrode active material match those of FeF3 (DB card number 00-061-0194, space group R-3c, trigonal crystal system) and 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.

[0071] [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 patterns of the obtained positive electrode active material match those of FeF3 (DB card number 00-061-0194, space group R-3c, trigonal crystal system) and 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.

[0072] [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 patterns of the obtained positive electrode active material match those of FeF3 (DB card number 00-061-0194, space group R-3c, trigonal crystal system) and 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.

[0073] [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 patterns of the obtained positive electrode active material match those of FeF3 (DB card number 00-061-0194, space group R-3c, trigonal crystal system) and 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.

[0074] [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.

[0075] [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.

[0076] (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).

[0077] [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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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. [Explanation of symbols]

[0082] 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): <h2 style=";text-align:left;direction:ltr">Li<h2 style=";text-align:left;direction:ltr"> ｘ <h2 style=";text-align:left;direction:ltr"> FeF<h2 style=";text-align:left;direction:ltr"> （３＋ｘ） <h2 style=";text-align:left;direction:ltr"> (1) In formula (1), x is a number that satisfies 0.5<x<1.

5.

2. The positive electrode active material according to claim 1 , wherein in formula (1), x satisfies 0.6≦x≦1.

0.

3. 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.

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 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 in a dQ / dV plot during discharge of a charge / discharge cycle.

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

8. 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.

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

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