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] The positive electrode active material is attracting attention as an important component that determines the capacity of lithium-ion secondary batteries, and development is underway. For example, iron (Fe)-based lithium iron phosphate (LiFePO 4 ) is known. 4 Although it has excellent cycle characteristics and safety, its low voltage and small capacity mean that its energy density (voltage x capacity), expressed as the product of voltage and capacity, is small compared to conventionally used nickel (Ni) and cobalt (Co)-based materials. To build small batteries, electrode materials with high energy density are required, and high voltage operation of the battery is important to achieve high energy density.

[0004] In order 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 generated. 3+However, even if such a compound is used, it does not necessarily mean that it can be operated at high voltages.

[0005] For example, Non-Patent Document 1 describes ferric fluoride (FeF 3 ) is used, which allows LiFeF 3 It has been reported that the average discharge voltage is 3.1 V. 2 It has been reported that high energy density can be expected. [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 4 However, the voltage is lower than that of the conventional 10V power supplies, and there is room for improvement in order to increase the voltage further.

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

Means for Solving the Problems

[0009] In order 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] The positive electrode active material according to [1] has a high average discharge voltage and can operate at a high voltage. Therefore, in a lithium ion secondary battery including 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 formula (1), x satisfies 0.6 ≤ x ≤ 1.0.

[0012] The positive electrode active material according to [2] can further increase the average discharge voltage and can 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 in the range of 25° ≤ 2θ ≤ 30° in the X-ray diffraction pattern.

[0014] [3] The positive electrode active material according to [3] has a peak derived from the crystal structure of FeF 3 and a peak derived from the crystal structure of LiFe 2 F 6

[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 shows that the battery can operate at a higher voltage.

[0019] [6] The lithium ion secondary battery according to [4] or [5], wherein the peak height from 3.77 to 4 V in a dQ / dV plot during discharge in a charge-discharge cycle is 1.2 to 4.4 times that of the peak height from 3.3 to 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 shows 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 electrodes are made of metallic lithium or graphite. Therefore, the present invention can be applied to various batteries. Effect 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 description of the drawings]

[0028] [Figure 1] FIG. 2 is a diagram showing an X-ray diffraction pattern of a positive electrode active material according to one embodiment of the present invention. [Diagram 2] 1 is a cross-sectional view showing a schematic diagram of a lithium-ion secondary battery according to one embodiment of the present invention. [Diagram 3] 1 is a graph showing charge / discharge curves in a charge / discharge cycle of a lithium ion secondary battery according to an embodiment of the present invention. [Figure 4] 4 is a graph showing a dQ / dV plot for the charge / discharge cycle of FIG. 3. [Diagram 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. 2 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] 2 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] 1 is a graph showing charge / discharge curves of a lithium ion secondary battery containing the positive electrode active material of Example 4. [Figure 11] 1 is a graph showing charge / discharge curves of a lithium ion secondary battery containing the positive electrode active material of Example 5. [Figure 12] 2 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 dQ / dV plots for the charge / discharge cycle of FIG. 7. [Figure 14] 9 is a graph showing dQ / dV plots for the charge / discharge cycle of FIG. 8. [Figure 15] 10 is a graph showing dQ / dV plots for the charge / discharge cycle of FIG. [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 dQ / dV plots for the charge / discharge cycle of FIG. 11. [Figure 18] 13 is a graph showing dQ / dV plots for the charge / discharge cycle of FIG. 12. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

[0030] [Cathode active material] The cathode active material of this embodiment contains lithium iron composite fluoride as the main component and is used for the cathode of a lithium-ion secondary battery. "Containing lithium iron composite fluoride as the main component" 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, based on the total mass of the cathode active material, and it may be 100% by mass. As long as the functions of the present invention are not impaired, the cathode active material may contain components other than the main component.

[0031] As long as the cathode 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 two or more kinds.

[0032] When a cathode 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 cathode active material. When a cathode active material obtained using lithium iron composite fluoride having such a composition as the main component is used in a secondary battery, high-voltage operation can be realized. Also, the composition ratio of the lithium iron composite fluoride is adjusted to be the same as the composition ratio required for the intended cathode active material.

[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 even 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. Therefore, a small battery having a high energy density can be constructed.

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

[0035] <X-ray diffraction (XRD) pattern> Fig. 1 shows an X-ray diffraction (XRD) pattern of a positive electrode active material according to this embodiment, which is an XRD pattern of Example 4 described later. 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 trivalent iron FeF 3 . The peak in the range of 25° ≤ 2θ ≤ 30° represents a peak derived from the crystal structure of LiFe 2 F 6 . This means that the positive electrode active material of this embodiment has a crystal structure similar to that of FeF 3 and LiFe 2 F 6 . The lithium iron composite fluoride of this embodiment has an iron composition ratio of all Fe 4 in the composition ratio of LiFeF 3+ , and thus, when described according to the composition formula of space group P42 / mnm tetragonal crystal LiFe 2 F 6 , it becomes the following formula (2). Li y Fe 2 F (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-described lithium iron composite fluoride. The lithium-ion secondary battery of this embodiment may include other battery elements as necessary.

[0037] The lithium ion secondary battery of this embodiment can use the battery elements of known lithium ion secondary batteries as they are, except that the positive electrode contains the above-mentioned lithium iron composite fluoride-based positive electrode active material. The lithium ion secondary battery of this embodiment may be of any of the following types: coin type, button type, cylindrical type, square type, and laminate type. The lithium ion secondary battery of this embodiment can be used in a wide range of applications, 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). Each battery element described below can be similarly applied to an all-solid lithium ion secondary battery or a semi-solid lithium ion secondary battery that does 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 mixing a conductive agent, a binder, etc. with the positive electrode active material of this embodiment as necessary, and 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 may be made of any of the known materials that function 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), 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) may 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, an electrolyte such as lithium perchlorate or lithium hexafluorophosphate 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 or solid electrolyte, any known semi-solid electrolyte or solid electrolyte can be used other than the positive electrode active material containing the above-mentioned lithium iron composite fluoride as the main component. An example of the semi-solid electrolyte is an electrolyte composed of a polymer component and a standard electrolyte. Examples of the polymer component include polyvinylidene fluoride (PVDF) / polyethylene oxide (PEO), polyacrylonitrile (PAN) / PEO, polymethyl methacrylate (PMMA), and PVDF / hexafluoropropylene (HFP). An example of the standard electrolyte is 1 mol / L lithium hexafluorophosphate (LiPF 6 ) EC / DMC solution, 1mol / L LiPF 6 EC / Ethyl methyl carbonate (EMC) solution, 1mol / L LiPF 6 Examples include EC / DMC / EMC solutions.

[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 mixture containing a solid electrolyte in addition to the above-mentioned positive electrode active material, conductive agent, and binder 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 described below 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 charging and discharging. In the charge / discharge curves in Fig. 3, the curves sloping to the right represent the curves during charging, and the curves sloping 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 Fig. 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 Fig. 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 in 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 4. The upward convex curve in FIG. 4 represents the curve during charging, and the downward convex curve represents the curve during discharging. The 1st, 2nd, 3rd, and 4th in FIG. 4 represent the number of charge / discharge cycles.

[0053] As shown in FIG. 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 undergoes a chemical reaction in the positive electrode 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 that the lithium ion secondary battery can operate at high voltages.

[0054] Fig. 5 shows a graph of Example 1 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 that at 3.5 V, and the peak height at 4.0 V is -0.1634 / -0.0708 = 2.31 times that at 3.5 V. The discharge curve of Example 3 described later has a peak dQ / dV value 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 that at 3.5 V. Thus, in the curves during discharge in Examples 1 and 3, the peak height at 3.77 to 4.0 V is 1.2 to 4.4 times that at 3.3 to 3.5 V. This indicates that the chemical reaction at 3.77 to 4.0 V during discharge occurs with 1.2 to 4.4 times the amount of material as compared with the chemical reaction at 3.3 to 3.5 V, and indicates that the lithium ion secondary battery can operate at high voltage.

[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 higher than the lower limit, the battery 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 complex fluoride as a main component. The lithium source of the lithium iron complex fluoride may be a halide such as lithium fluoride (LiF), lithium hydroxide monohydrate (LiOH·H 2 O), lithium carbonate (Li 2 CO 3 ), lithium acetate (CH 3 COOLi), lithium acetate dihydrate (CH 3 COOLi·2H 2 It is possible to use known compounds such as acetates of the above-mentioned compounds without any particular limitation.

[0058] As the iron source for the lithium iron complex fluoride, trivalent iron is preferable to divalent iron because it can be operated at high voltage. 3 ) is more preferred.

[0059] When producing a lithium iron composite fluoride, the above-mentioned lithium source and iron source are mixed and subjected to a mechanical treatment for a predetermined time under predetermined conditions. For example, lithium fluoride is used as the lithium source and trivalent iron (FeF 3 ) is used, the compound represented by the above formula (1) is thought to be produced by the following reaction. LiF+FeF 3 → LifeF 4 (Compound in which x=1 in the above formula (1))

[0060] The value of x in the compound represented by the formula (1) is LiF and FeF 3 The amount of the fluorine-containing compound can be adjusted by changing the molar ratio of the fluorine-containing compound to the fluorine-containing compound.

[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 preferable, and a planetary ball mill is more preferable 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, and more preferably 9 to 11 hours. As a condition for carrying out the mechanical treatment, the rotation speed is preferably 250 to 450 rpm, and 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 mechanical processing is inert gas (noble gas such as argon (Ar) or nitrogen (N 2 ) gas, etc.) are preferred.

[0063] After obtaining the compound represented by formula (1) by mechanical treatment, it is preferable to coat the compound represented by formula (1) with carbon fine particles by pulverizing and mixing the compound with carbon fine particles in order to improve the capacity and rate characteristics. As the carbon fine particles, for example, carbon nanotubes (CNT), acetylene black, Ketchen black, etc. can be used. Among these carbon fine particles, CNT is preferable in order to further increase 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. EXAMPLES

[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 compound of formula (1) where x=0.6) Ferric fluoride (FeF 30.439g of ZnO (ZnO) and 0.0606g 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 Fritsh. The pot and balls were made of zirconium oxide, and 50g of balls with a diameter of 5mm were used in a 45mL pot. The treatment conditions for the mechanical treatment were 350 rpm and 10 hours. Then, 0.55g of carbon nanotubes (CNTs) were added to the pot, and further mechanical treatment was performed to obtain a positive electrode active material. The treatment conditions for the mechanical treatment to obtain the positive electrode active material were under 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 longitudinal limiting slit: 5.0mm Receiving parallel slit aperture angle: 5.0° Kβ filter: Used Step width: 0.01° Entrance slit: 1 / 6° Receiving slit 1: 4.0mm Receiving slit 2: 13 mm

[0069] As shown in FIG. 6, the obtained positive electrode active material is FeF with DB card number 00-061-0194, space group R-3c, and trigonal crystal system. 3 and DB card number 01-074-2193, space group P42 / mnm, crystal system tetragonal LiFe 2 F 6 The diffraction pattern of FeF 3 and Life 2 F 6 It was found to have a similar crystal structure to that of

[0070] [Example 2] (Li0.7 FeF 3.7 (Preparation of compound of formula (1) where x=0.7) Ferric fluoride (FeF 3 A positive electrode active material was obtained in the same manner as in Example 1, except that 0.431 g of the sintered body (sintered body) 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 obtained positive electrode active material is FeF with DB card number 00-061-0194, space group R-3c, and trigonal crystal system. 3 and DB card number 01-074-2193, space group P42 / mnm, crystal system tetragonal LiFe 2 F 6 The diffraction pattern of FeF 3 and Life 2 F 6 It was found to have a similar crystal structure to that of

[0071] [Example 3] (Li 0.8 FeF 3.8 (Preparation of compound of formula (1) where x=0.8) Ferric fluoride (FeF 3 A positive electrode active material was obtained in the same manner as in Example 1, except that 0.422 g of the sintered material (CuO) and 0.078 g of lithium fluoride (LiF) were subjected to mechanical processing 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 is FeF with DB card number 00-061-0194, space group R-3c, and trigonal crystal system. 3 and DB card number 01-074-2193, space group P42 / mnm, crystal system tetragonal LiFe 2 F 6 The diffraction pattern of FeF 3 and Life 2 F 6 It was found to have a similar crystal structure to that of

[0072] [Example 4] (LiFeF 4 (Preparation of compound of formula (1) where x=1.0) Ferric fluoride (FeF 3 A positive electrode active material was obtained in the same manner as in Example 1, except that 0.407 g of the sintered body (sintered body) 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 obtained positive electrode active material is FeF with DB card number 00-061-0194, space group R-3c, and trigonal crystal system. 3 and DB card number 01-074-2193, space group P42 / mnm, crystal system tetragonal LiFe 2 F 6 The diffraction pattern of FeF 3 and Life 2 F 6 It was found to have a similar crystal structure to that of

[0073] [Example 5] (Li 1.2 FeF 4.2 (Preparation of compound of formula (1) where x=1.2) Ferric fluoride (FeF 3 A positive electrode active material was obtained in the same manner as in Example 1, except that 0.391 g of the sintered material (CuO) 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 is FeF with DB card number 00-061-0194, space group R-3c, and trigonal crystal system. 3 and DB card number 01-074-2193, space group P42 / mnm, crystal system tetragonal LiFe 2 F 6 The diffraction pattern of FeF 3 and Life 2 F 6 It was found to have a similar crystal structure to that of

[0074] [Comparative Example 1] (LiFeF 3 Preparation of Ferrous fluoride (FeF 2 A positive electrode active material was obtained in the same manner as in Example 1, except that 0.392 g of the sintered material (CuO) 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 is LiFe with DB card number 01-074-2193, space group P42 / mnm, and tetragonal crystal system. 2 F 6 The diffraction pattern of LiFe 2 F 6 It was found to have a similar crystal structure to that of

[0075] [Preparation 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 solid contents. This slurry was applied onto an aluminum foil, pressed at 15 tons, and then punched out with a punch having a diameter of 10 mm to prepare a positive electrode. At this time, the mass of the positive electrode active material was adjusted to 3.5 mg.

[0076] (Fabrication of coin-type cells) The prepared positive electrode (diameter 10 mm) was placed on the positive electrode can, and a porous polyethylene film was placed on top of it as a separator, and held down with a polypropylene gasket. Then, a 0.5 mm thick Li negative electrode was placed, and a spacer for adjusting the thickness was placed. Then, a mixed solvent (volume ratio 5:5) of ethylene carbonate and diethyl carbonate in which 1 mol / L of lithium hexafluorophosphate was dissolved was added between the positive electrode can and the negative electrode as a non-aqueous electrolyte solution, impregnated into the separator, and the negative electrode can was placed on top and sealed to prepare a coin-type cell (lithium ion secondary battery).

[0077] [Battery performance evaluation] The battery performance was evaluated using the produced coin-type cell. Specifically, the produced coin-type cell was charged and discharged at a constant current with 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 and discharge capacity (mAh / g) was calculated per unit mass of the positive electrode active material. Charge and discharge curves of constant current charging and discharging in 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 is applied, the energy densities were 113Wh / kg, 136Wh / kg, 175Wh / kg, 108Wh / kg, and 119Wh / kg, respectively. In addition, as shown in Figures 7 to 11, in Examples 1 to 5 to which the present invention is applied, the average discharge voltages were 3.76V, 3.79V, 3.73V, 3.79V, and 3.72V, respectively. Thus, it was confirmed that Examples 1 to 5 to which the present invention is applied had high energy density and average discharge voltage, and thus achieved high energy density and high voltage operation. In contrast, the positive electrode active material of Comparative Example 1 (LiFeF 3 ) had an energy density of 87Wh / kg and an average discharge voltage of 3.64V, as shown in Figure 12.

[0079] From the obtained charge / discharge curve, the horizontal axis is voltage, and the vertical axis is capacity differentiated by voltage (dQ / dV, dQdV -1), 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 of 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 in 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, and it was confirmed that a chemical reaction occurred at a high voltage specific to the compound represented by formula (1). In contrast, the positive electrode active material of Comparative Example 1 (LiFeF 3 As shown in FIG. 18, the dQ / dV plot of the Fe-Fe alloy did not show a peak voltage at around 4 V during discharge, and only showed a broad peak in the operating voltage range. 2+ / Fe 3+ In order to observe a clear reaction, it is necessary to lower the lower limit of the operating voltage range, and in Comparative Example 1, it is 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 is apparent 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 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 the formula (1), x satisfies 0.6≦x≦1.

0.

3. 2. The positive electrode active material according to claim 1, having 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 that 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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