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

Abstract

The present invention aims to provide an Fe-based positive electrode active material that can operate at high voltage and has a higher discharge capacity, and a lithium-ion secondary battery containing the positive electrode active material. [Solution] A positive electrode active material mainly composed of lithium iron composite fluoride, The lithium iron composite fluoride is a positive electrode active material represented by the following formula (1). Li x FeF (3+x-y) (1) In equation (1), x and y are 0.4

Description

[Technical Field]

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

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

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

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

[0005] For example, Non-Patent Document 1 reports that LiFeF3 is generated during charging and discharging when ferric fluoride (FeF3) is used, and that the average discharge voltage is 3.1V. Non-Patent Document 2 reports that LiFeO2 can be expected to have a high energy density. [Prior art documents] [Non-patent 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) [Overview of the project] [Problems that the invention aims to solve]

[0007] Non-patent document 2 indicates that the actual voltage is around 2.5V, which is lower than the expected voltage. The average discharge voltage (3.1V) in non-patent document 1 is also lower than the voltage of LiFePO4, suggesting there is room for improvement to achieve even higher voltages. In addition, the discharge capacity was insufficient for Fe-based cathode active materials.

[0008] The present invention has been made to solve the above problems. An object of the present invention is to provide an Fe-based positive electrode active material that can operate at high voltage and can further increase the capacity during discharge, and a lithium ion secondary battery including the positive electrode active material. Furthermore, the present invention 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 containing a lithium-iron composite fluoride as a main component, The positive electrode active material, wherein the lithium-iron composite fluoride is represented by the following formula (1). Li x FeF (3+x-y) (1) In formula (1), x and y are numbers satisfying 0.4 < x < 1.2 and 0.01 < y < 0.5.

[0010] The positive electrode active material according to [1], compared with conventional lithium-iron composite fluorides, can reduce repulsion between ferric ions (Fe 3+ ) and lithium ions (Li + ), and is considered to have improved ionic conductivity. For this reason, the average discharge voltage is high, high-voltage operation is possible, and the capacity during discharge can be further increased. As a result, in a lithium ion secondary battery including the positive electrode active material, the number of required batteries can be reduced, contributing to cost reduction.

[0011] [2] In the formula (1), x and y are, when 0.4 < x ≦ 0.5, 0.01 < y < 0.1 or 0.2 < y < 0.5, when 0.5 < x ≦ 0.7, 0.01 < y < 0.5, when 0.7 < x < 1.0, 0.01 < y < 0.1, or 0.1 < y < 0.4 when 1.0 ≦ x < 1.2, 0.05 < y < 0.2, The positive electrode active material according to [1], which is a number satisfying the above.

[0012] The positive electrode active material described in [2] can further increase the capacity during discharge. Therefore, the capacity of a lithium-ion secondary battery containing the positive electrode active material can be increased, and the energy density can be further increased.

[0013] [3] A positive electrode active material according to [1] or [2], having a peak in the range of 20°≦2θ≦30° in the X-ray diffraction pattern.

[0014] The positive electrode active material [3] has peaks derived from the crystal structure of LiFe2F6. Therefore, it has a high average discharge voltage and can operate at high voltage.

[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 described in any of [1] to [3].

[0016] The lithium-ion secondary battery described in [4] contains a positive electrode active material described in any of [1] to [3]. This indicates that the battery can operate at a high voltage and that its discharge capacity can be increased.

[0017] [5] The lithium-ion secondary battery described in [4], wherein the dQ / dV plot during discharge of the charge-discharge cycle has a peak in the range of 3.9 to 4.0 V.

[0018] [5] The lithium-ion secondary battery described shows that the positive electrode active material undergoes a chemical reaction in the high voltage range of 3.9 to 4.0 V. This indicates that the battery can operate at higher voltages.

[0019] [6] A lithium-ion secondary battery as described in [4] or [5], having an average discharge voltage of 3.8 to 4.0 V.

[0020] The lithium-ion secondary battery described in [6] has a high average discharge voltage of 3.8 to 4.0V. This indicates that it can operate at high voltage.

[0021] [7] A method for producing a positive electrode active material mainly composed of a lithium iron composite fluoride, using lithium fluoride, ferrous fluoride, and ferric fluoride as raw materials, a method for producing a positive electrode active material, wherein the lithium iron composite fluoride is represented by the following formula (1). Li x FeF (3+x-y) (1) In formula (1), x and y are when 0.4 < x ≤ 0.5, 0.01 < y < 0.1 or 0.2 < y < 0.5, when 0.5 < x ≤ 0.7, 0.01 < y < 0.5, when 0.7 < x < 1.0, 0.01 < y < 0.1, or 0.1 < y < 0.4 when 1.0 ≤ x < 1.2, 0.05 < y < 0.2, which is a number satisfying the above.

[0022] The method for producing a positive electrode active material according to [7] can obtain a positive electrode active material that 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 required batteries can be reduced, contributing to cost reduction.

[0023] [8] The method for producing a positive electrode active material according to [7], wherein y in the formula (1) is changed according to the blending amount of the ferrous fluoride.

[0024] The method for producing a positive electrode active material according to [8] can obtain a positive electrode active material that 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 required batteries can be reduced, contributing to cost reduction. Effects of the Invention

[0025] According to the present invention, there can be provided an Fe-based positive electrode active material that can operate at a high voltage and can further increase discharge capacity, and a lithium ion secondary battery including the positive electrode active material. Brief Description of the Drawings

[0026] [Figure 1] This figure shows a portion of the X-ray diffraction pattern of a positive electrode active material according to one embodiment of the present invention. [Figure 2] This figure shows a portion of the X-ray diffraction pattern of the positive electrode active material when x in equation (1) is x = 0.4 and y is 0 ≤ y ≤ 0.4. [Figure 3] This is a schematic cross-sectional view showing a lithium-ion secondary battery according to one embodiment of the present invention. [Figure 4] This graph shows the discharge curve of a lithium-ion secondary battery containing positive electrode active material when x in equation (1) is x = 0.6 and y is 0 ≤ y ≤ 0.4. [Figure 5] This graph shows the dQ / dV plot for the charge-discharge cycle of a lithium-ion secondary battery containing positive electrode active material when x in equation (1) is x = 0.6 and y is 0 ≤ y ≤ 0.4. [Figure 6] This is a flowchart showing an example of a method for manufacturing a positive electrode active material according to one embodiment of the present invention. [Figure 7] This figure shows a portion of the X-ray diffraction pattern of the positive electrode active material when x in equation (1) is x = 0.5 and y is 0 ≤ y ≤ 0.4. [Figure 8] This figure shows a portion of the X-ray diffraction pattern of the positive electrode active material when x in equation (1) is x = 0.8 and y is 0 ≤ y ≤ 0.4. [Figure 9] This figure shows a portion of the X-ray diffraction pattern of the positive electrode active material when x in equation (1) is x = 1.0 and y is 0 ≤ y ≤ 0.4. [Figure 10] This figure shows a portion of the X-ray diffraction pattern of the positive electrode active material when x in equation (1) is x = 1.2 and y is 0 ≤ y ≤ 0.4. [Figure 11] This graph shows the charge-discharge curve of a lithium-ion secondary battery containing positive electrode active material when x in equation (1) is x = 0.4 and y is 0 ≤ y ≤ 0.4. [Figure 12] This graph shows the charge-discharge curve of a lithium-ion secondary battery containing positive electrode active material when x in equation (1) is x = 0.5 and y is 0 ≤ y ≤ 0.4. [Figure 13]This graph shows the charge-discharge curve of a lithium-ion secondary battery containing positive electrode active material when x in equation (1) is x = 0.8 and y is 0 ≤ y ≤ 0.4. [Figure 14] This graph shows the charge-discharge curve of a lithium-ion secondary battery containing positive electrode active material when x in equation (1) is x = 1.0 and y is 0 ≤ y ≤ 0.4. [Figure 15] This graph shows the charge-discharge curve of a lithium-ion secondary battery containing positive electrode active material when x in equation (1) is x = 1.2 and y is 0 ≤ y ≤ 0.4. [Figure 16] This graph shows the dQ / dV plot for the charge-discharge cycle of a lithium-ion secondary battery containing positive electrode active material when x in equation (1) is x = 0.4 and y is 0 ≤ y ≤ 0.4. [Figure 17] This graph shows the dQ / dV plot for the charge-discharge cycle of a lithium-ion secondary battery containing positive electrode active material when x in equation (1) is x = 0.5 and y is 0 ≤ y ≤ 0.4. [Figure 18] This graph shows the dQ / dV plot for the charge-discharge cycle of a lithium-ion secondary battery containing positive electrode active material when x in equation (1) is x = 0.8 and y is 0 ≤ y ≤ 0.4. [Figure 19] This graph shows the dQ / dV plot for the charge-discharge cycle of a lithium-ion secondary battery containing positive electrode active material when x in equation (1) is x = 1.0 and y is 0 ≤ y ≤ 0.4. [Figure 20] This graph shows the dQ / dV plot for the charge-discharge cycle of a lithium-ion secondary battery containing positive electrode active material when x in equation (1) is x = 1.2 and y is 0 ≤ y ≤ 0.4. [Modes for carrying out the invention]

[0027] Preferred embodiments of the present invention will be described in detail below.

[0028] [Cathode active material] The positive electrode active material of the present embodiment contains lithium iron composite fluoride as a main component, and is used for a positive electrode of a lithium ion secondary battery. The expression "containing lithium iron composite fluoride as a 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 still more preferably 99% by mass or more, based on the total mass of the positive electrode active material, and the content may be 100% by mass. The positive electrode active material may contain components other than the main component as long as the functions of the present invention are not impaired.

[0029] The positive electrode active material of the present embodiment may contain only one type of lithium iron composite fluoride, or may contain two or more types of lithium iron composite fluoride, as long as it contains lithium iron composite fluoride as a main component.

[0030] When a positive electrode active material is produced with lithium iron composite fluoride as a main component, the composition ratio (Li:Fe:F) of the entire lithium iron composite fluoride is maintained even in the obtained positive electrode active material. When a positive electrode active material obtained using lithium iron composite fluoride having such a composition as a main component is used in a secondary battery, high-voltage operation and high capacity can be achieved. In addition, the composition ratio of the lithium iron composite fluoride is adjusted to be the same as the composition ratio required for the target positive electrode active material.

[0031] (Lithium Iron Composite Fluoride) The lithium iron composite fluoride of the present embodiment is represented by the following formula (1). Li x FeF (3+x-y) (1) In formula (1), x and y are numbers satisfying 0.4 < x < 1.2 and 0.01 < y < 0.5. When x and y are within the above numerical ranges, the average discharge voltage and capacity can be increased, and a small battery having high energy density can be constructed.

[0032] In formula (1), x and y are when 0.4 < x ≤ 0.5, 0.01 < y < 0.1 or 0.2 < y < 0.5, when 0.5 < x ≤ 0.7, 0.01 < y ≤ 0.5, When 0.7<x<1.0, 0.01<y<0.1 or 0.1<y<0.4 When 1.0≦x<1.2, 0.05<y<0.2, it is preferable that x and y are numbers satisfying the above conditions. When x and y are within the above numerical ranges, the average discharge voltage and capacity can be further increased. Therefore, the capacity of a lithium ion secondary battery containing the positive electrode active material can be increased, and the energy density can be further increased.

[0033] In formula (1), x represents the molar ratio of Li to Fe. The molar ratio of Li to Fe is x:1. In formula (1), y represents ferrous ion (Fe 2+ )-derived fluoride ion (F - ). The molar ratio of Li, Fe and F is x:1:(3+x-y). The composition of the lithium iron composite fluoride can be determined by inductively coupled plasma (ICP) emission spectrometry, combustion ion chromatography, or the like.

[0034] <X-ray Diffraction (XRD) Pattern> Fig. 1 shows, as an example of the X-ray diffraction (XRD) pattern of the positive electrode active material according to the present embodiment, a part of the XRD pattern when x in formula (1) is 0.6 and y satisfies 0≦y≦0.4 (Examples 4 to 8 described later and Comparative Example 10). As shown in Fig. 1, when x in formula (1) is 0.6 and y satisfies 0≦y≦0.4, the positive electrode active material has a peak in the range of 20°≦2θ≦30°. The peak near 2θ=27° represents a peak derived from the crystal structure of LiFe2F6. This means that when x in formula (1) is 0.6 and y satisfies 0≦y≦0.4, the positive electrode active material has the same crystal structure as LiFe2F6. Furthermore, when x in formula (1) is 0.6 and y satisfies 0≦y≦0.4, the positive electrode active material is considered to be excellent in water resistance, less prone to moisture adhesion, and have a stable crystal structure. As such, it is preferable that the positive electrode active material of the present embodiment does not contain moisture.

[0035] Next, Figure 2 shows a portion of the XRD patterns when x in equation (1) is x=0.4 and y is 0≦y≦0.4 (Comparative Examples 1-6 described later). As shown in Figure 2, the positive electrode active material when x in equation (1) is x=0.4 and y is 0≦y≦0.4 has multiple peaks in the range of 20°≦2θ≦30°. The low peak around 2θ=21° represents a peak originating from the crystal structure of LiFe2F6. The peaks in the range of 22°≦2θ≦23° and the peaks in the range of 25°≦2θ≦26° represent peaks originating from the crystal structure of FeF3·3H2O. The peaks in the range of 23°≦2θ≦24° and the peak around 2θ=28° represent peaks originating from the crystal structure of FeF3·0.33H2O. Furthermore, the peaks in the range of 28°≦2θ≦29° and the peaks in the range of 29°<2θ≦30° represent peaks originating from the crystal structure of FeF3. Thus, the positive electrode active material in equation (1) where x is x=0.4 and y is 0≦y≦0.4 has peaks originating from various crystal structures, suggesting that the crystal structure changed in the atmosphere. This is thought to be because the positive electrode active material in equation (1) where x is x=0.4 and y is 0≦y≦0.4 has low water resistance and is prone to moisture adhesion.

[0036] [Lithium-ion rechargeable battery] The lithium-ion secondary battery of this embodiment comprises a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode contains a positive electrode active material mainly composed of the aforementioned lithium iron composite fluoride. The lithium-ion secondary battery of this embodiment may also include other battery elements as needed.

[0037] The lithium-ion secondary battery of this embodiment can use the same battery elements as known lithium-ion secondary batteries, except that the positive electrode contains a positive electrode active material mainly composed of the aforementioned lithium iron composite fluoride and containing carbon. The lithium-ion secondary battery of this embodiment may have any of the following configurations: coin type, button type, cylindrical type, prismatic type, or laminate type. Furthermore, the lithium-ion secondary battery of this embodiment can be applied to a wide range of applications, such as mobile devices like mobile phones and laptop computers, and in-vehicle applications.

[0038] The following description of the lithium-ion secondary battery of this embodiment will focus on a lithium-ion secondary battery using an electrolyte (coin-type lithium-ion secondary battery). Each battery element described below can also be applied to all-solid-state lithium-ion secondary batteries and semi-solid-state lithium-ion secondary batteries that do not use an electrolyte.

[0039] As shown in Figure 3, the lithium-ion secondary battery 1 of this embodiment comprises a negative electrode can (negative electrode terminal) 20, a negative electrode 3, a separator 4 impregnated with electrolyte, an insulating packing (gasket) 5, a positive electrode 2, and a positive electrode can 10.

[0040] The positive electrode can 10 is positioned below the separator 4, and the negative electrode can 20 is positioned above the separator 4. The positive electrode can 10 and the negative electrode can 20 form the outer shape of the lithium-ion secondary battery 1. Between the positive electrode can 10 and the negative electrode can 20, the positive electrode 2 and the negative electrode 3 are provided via the separator 4, which is impregnated with electrolyte. The positive electrode can 10 and the negative electrode can 20 are electrically insulated by an insulating packing 5.

[0041] The lithium-ion secondary battery 1 can be manufactured by preparing a positive electrode composite material by adding a conductive agent, a binder, etc., as needed to the positive electrode active material of this embodiment, and then pressing this composite material onto a current collector (not shown). Preferably, stainless steel mesh, aluminum foil, etc., can be used as the current collector. Preferably, carbon nanotubes (CNTs), acetylene black, ketchen black, etc., can be used as the conductive agent. Preferably, tetrafluoroethylene, polyvinylidene fluoride, etc., can be used as the binder.

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

[0043] In a lithium-ion secondary battery 1, the negative electrode 3 relative to the positive electrode 2 can be any known material that functions as a negative electrode active material and is capable of intercalating and releasing lithium, such as metallic materials like metallic lithium and lithium alloys, carbon-based materials like graphite and MCMB (mesocarbon microbeads), or silicon-based materials like silicon (Si), Si alloys, and silicon oxide. Among these, metallic lithium and graphite are preferred as the negative electrode 3.

[0044] The separator 4 and battery container (positive electrode container 10, negative electrode container 20) can use known battery components.

[0045] As the electrolyte, known electrolytes, semi-solid electrolytes, solid electrolytes, etc., can be used. As an electrolyte, for example, an electrolyte such as lithium perchlorate or lithium hexafluoride phosphate can be dissolved in a solvent such as ethylene carbonate (EC), dimethyl carbonate (DMC), propylene carbonate (PC), or diethyl carbonate (DEC) can be used.

[0046] As semi-solid and solid electrolytes, known semi-solid and solid electrolytes can be used, except for using the above-mentioned lithium iron composite fluoride as the main component and a positive electrode active material containing carbon. Examples of semi-solid electrolytes include electrolytes composed of polymer components and standard electrolytes. Examples of polymer components include polyvinylidene fluoride (PVDF) / polyethylene oxide (PEO), polyacrylonitrile (PAN) / PEO, polymethyl methacrylate (PMMA), PVDF / hexafluoropropylene (HFP), etc. Examples of standard electrolytes include 1 mol / L lithium hexafluorophosphate (LiPF6) EC / DMC solution, 1 mol / L LiPF6EC / ethyl methyl carbonate (EMC) solution, 1 mol / L LiPF6EC / DMC / EMC solution, etc.

[0047] In the case of all-solid-state lithium-ion secondary batteries, the electrolyte can be a polymer-based solid electrolyte such as a polyethylene oxide-based polymer compound, a polymer compound containing at least one of a polyorganosiloxane chain or a polyoxyalkylene chain, a sulfide-based solid electrolyte, or an oxide-based solid electrolyte.

[0048] For the positive electrode of an all-solid-state lithium-ion secondary battery, for example, in addition to the positive electrode active material, conductive agent, and binder described above, a positive electrode composite material containing a solid electrolyte can be supported on a positive electrode current collector made of aluminum, nickel, stainless steel, or the like.

[0049] In this embodiment, the lithium-ion secondary battery 1 can operate at a high voltage and increase its discharge capacity because the positive electrode 2 contains the positive electrode active material of this embodiment.

[0050] <Discharge curve of charge-discharge cycle> Figure 4 shows an example of the discharge curve in the charge-discharge cycle of the lithium-ion secondary battery of this embodiment, specifically the case where x in equation (1) is x = 0.6 and y is 0 ≤ y ≤ 0.4 (Examples 4 to 8 and Comparative Example 10, described later). The horizontal axis of the graph in Figure 4 represents the capacity (discharge capacity) of the lithium-ion secondary battery. The vertical axis of the graph in Figure 4 represents the voltage of the lithium-ion secondary battery during charge and discharge. The capacities of the lithium-ion secondary batteries shown in FIG. 4 are as follows: Comparative Example 10 (y=0 in formula (1)) is 63.7 mAh / g, Example 4 (y=0.05 in formula (1)) is 66.4 mAh / g, Example 5 (y=0.1 in formula (1)) is 72.6 mAh / g, Example 6 (y=0.2 in formula (1)) is 73.0 mAh / g, Example 7 (y=0.3 in formula (1)) is 75.5 mAh / g, and Example 8 (y=0.4 in formula (1)) is 63.8 mAh / g. As shown in FIG. 4, it can be confirmed that when x in formula (1) is x=0.6 and y satisfies 0≦y≦0.4, the discharge capacity of the positive electrode active material is improved when y satisfies 0.01<y≦0.4. This is because, in the positive electrode active material where x in formula (1) is x=0.6 and y satisfies 0≦y≦0.4, increasing y allows ferric ions (Fe 3+ ) and lithium ions (Li + ) to reduce the repulsion between them, which is considered to improve ion conductivity.

[0051] <dQ / dV plot for charge-discharge cycle> FIG. 5 shows a dQ / dV plot for the charge-discharge cycle of FIG. 4. The horizontal axis of the graph in FIG. 5 represents the voltage during the charge-discharge cycle. The vertical axis of the graph in FIG. 5 represents the value obtained by differentiating the capacity of FIG. 4 with respect to voltage (dQ / dV plot, dQdV -1 plot). The downward convex curve in FIG. 5 represents the curve during discharge.

[0052] As shown in FIG. 5, the discharge curve has a peak at 3.93 to 3.95 V. These peaks indicate that a chemical reaction occurs in the positive electrode active material at the positive electrode during discharge. This shows that the chemical reaction occurs at a high voltage of 3.93 to 3.95 V during discharge, indicating that the lithium-ion secondary battery can operate at high voltage.

[0053] In the lithium-ion secondary battery of the present embodiment, it is preferable that the dQ / dV plot during discharge in a charge-discharge cycle has a peak in the range of 3.9 to 4.0 V. A dQ / dV plot during discharge in a charge-discharge cycle having a peak within the above numerical range means that the lithium-ion secondary battery can operate at a higher voltage. In this specification, a dQ / dV plot "having a peak" means that in the dQ / dV plot, the depth is 0.1 mAhg. -1 V -1 This means that it has valleys greater than or equal to the above.

[0054] The lithium-ion secondary battery of this embodiment has a peak depth of 0.1 mAhg in the dQ / dV plot during discharge in the charge-discharge cycle. -1 V -1 It is preferable that the above is true, and 0.5mAhg -1 V -1 More preferably, the above, and 1.0mAhg -1 V -1 More preferably, the above is true, and 2.0mAhg -1 V -1 It is particularly preferable that the values ​​be above this level. If the peak depth of the dQ / dV plot is above the above lower limit, the capacity of the lithium-ion secondary battery can be increased. The maximum value of the peak depth of the dQ / dV plot is not particularly limited, but for example, 1000mAhg -1 V -1 The following are preferable. The "peak depth" is given by the depth of the valley (the absolute value of the valley bottom) in the dQ / dV plot.

[0055] [Method for manufacturing positive electrode active material] The positive electrode active material of this embodiment contains the above-mentioned lithium iron composite fluoride as its main component. As the lithium source for the lithium iron composite fluoride, known compounds such as halides such as lithium fluoride (LiF), hydroxides such as lithium hydroxide monohydrate (LiOH·H2O), carbonates such as lithium carbonate (Li2CO3), and acetates such as lithium acetate (CH3COOLi) and lithium acetate dihydrate (CH3COOLi·2H2O) can be used, and there are no particular restrictions.

[0056] As an iron source for lithium iron composite fluoride, trivalent iron is preferred over divalent iron, and ferric fluoride (FeF3) is more preferred, as it allows for high-voltage operation. Furthermore, as an iron source for lithium iron composite fluoride, it is preferable to use both ferric fluoride and ferrous fluoride (FeF2) in combination, as this allows for a higher discharge capacity.

[0057] When producing lithium iron composite fluoride, the lithium source and iron source described above are mixed and subjected to mechanical treatment (first mechanical treatment) for a predetermined time and under predetermined conditions. For example, when lithium fluoride is used as the lithium source and divalent iron (FeF2) and trivalent iron (FeF3) are used as the iron source, it is thought that the compound represented by formula (1) above is formed by the following reaction. xLiF + (1-y)FeF3 + yFeF2 → Li x FeF (3+x-y)

[0058] The value of x in the compound represented by formula (1) can be adjusted by the amount of LiF per mole of Fe. The value of y in the compound represented by formula (1) can be adjusted by the molar ratio of FeF3 to FeF2.

[0059] The specific means applied during the first mechanical processing are not particularly limited, but various means conventionally used for the purpose of grinding and mixing solid materials can be applied. Of these means, a ball mill is preferred, and a planetary ball mill is more preferred because it can sufficiently grind and mix the raw materials.

[0060] The time required for the first mechanical processing is preferably 8 to 12 hours, and more preferably 9 to 11 hours. As a condition for the first mechanical processing, the rotational speed is preferably 250 to 450 rpm, and more preferably 300 to 400 rpm. The temperature during the first mechanical process is not particularly limited and can be performed at room temperature (for example, 5-30°C). The atmosphere during the first mechanical treatment is preferably an inert gas (such as a noble gas like argon (Ar), or nitrogen (N2) gas).

[0061] Carbon is added to the lithium iron composite fluoride obtained by the first mechanical treatment and mixed, and then subjected to a mechanical treatment (second mechanical treatment) for a predetermined time and under predetermined conditions. By performing a second mechanical process, the capacity and rate characteristics of the lithium-ion secondary battery can be improved. Examples of carbon to be added include elemental carbon such as carbon nanotubes (CNTs), carbon black, and graphite, with the use of carbon nanoparticles being preferable. Examples of carbon nanoparticles include CNTs and carbon black. Among these carbon nanoparticles, CNTs are preferred from the viewpoint of further enhancing the conductivity of the positive electrode active material. The conditions for the second mechanical treatment (means, time, rotational speed, temperature, atmosphere, etc.) are the same as those for the first mechanical treatment.

[0062] It is preferable to heat-treat the composite of lithium iron composite fluoride and carbon obtained by the second mechanical treatment. By heat-treating, the crystal structure of the lithium iron composite fluoride changes, and the capacity of a lithium-ion secondary battery using this composite as a positive electrode active material can be increased. This is thought to be because the composition ratio of the LiFe2F6 crystal structure and the FeF3 crystal structure in lithium iron composite fluoride changes due to heat treatment, resulting in an increase in the LiFe2F6 crystal structure.

[0063] By heat-treating a composite of lithium iron fluoride and carbon, the capacity of a lithium-ion secondary battery containing this composite as a positive electrode active material can be increased, and the energy density can be further enhanced.

[0064] The firing temperature in the heat treatment is preferably 100 to 300°C, more preferably 150 to 250°C, and even more preferably 175 to 225°C. The firing time in the heat treatment is preferably 0.5 to 20 hours, more preferably 2 to 15 hours, and even more preferably 4 to 8 hours. The atmosphere used in heat treatment is preferably an inert gas (such as a noble gas like argon (Ar), or nitrogen (N2) gas). The pressure used in heat treatment can be atmospheric pressure (0.1013 MPa), but a low vacuum (for example, 10 MPa) is also acceptable. 2 Pa~10 5 Pa) is preferred.

[0065] The composite of the lithium iron composite fluoride and carbon described above may be used as the positive electrode active material, or a composite that has been heat-treated (heat-treated composite) may be used as the positive electrode active material. By using the obtained positive electrode active material as the positive electrode to fabricate a lithium-ion secondary battery, a battery that operates at high voltage can be obtained.

[0066] Figure 6 shows a flowchart illustrating the method for producing the positive electrode active material in this embodiment. As shown in Figure 6, the lithium iron composite fluoride of this embodiment is preferably made from lithium fluoride, ferrous fluoride, and ferric fluoride, and is represented by the above formula (1). In equation (1), the molar ratio of Li, Fe, and F is adjusted to x:1:(3+xy). In this case, it is preferable that y in equation (1) be changed by the amount of ferrous fluoride added. The meanings of each term in Figure 6 are the same as those of the terms described above. [Examples]

[0067] The following describes embodiments of the present invention, but the present invention is not limited to the following embodiments.

[0068] [Example 1] (Li 0.5 FeF 3.45 (Preparation of the compound with x=0.5 and y=0.05 in equation (1)) 0.052 g of lithium fluoride (LiF), 0.429 g of ferric fluoride (FeF3), and 0.019 g of ferrous fluoride (FeF2) were subjected to a first mechanical treatment using a planetary ball mill. The planetary ball mill used was a Fritsh Premium line PL-7. The pot and balls were made of zirconium oxide, and 50 g of 5 mm diameter balls were used in an 80 mL pot. The treatment conditions for the first mechanical treatment were 350 rpm for 10 hours. Subsequently, 0.125 g of carbon nanotubes (CNTs) were added to the pot, and a second mechanical treatment was performed to obtain a composite. The treatment conditions for the second mechanical treatment were under an Ar atmosphere, 25 °C, 350 rpm, and 10 hours. Furthermore, the obtained composite was subjected to a 10°C treatment under an argon gas atmosphere. 3 At Pa, the material was heat-treated in an oven at 200°C for 5 hours to obtain 0.5 g of positive electrode active material.

[0069] [Examples 2-12, Comparative Examples 1-24] 0.5 g of positive electrode active material was obtained in the same manner as in Example 1, except that LiF, FeF3, and FeF2 were weighed so that x and y in equation (1) were the values ​​shown in Tables 1 and 2. In Tables 1 and 2, "y=0" means that the raw material does not contain FeF2. In Table 1, all examples where y=0 are comparative examples. In Table 3, described later, examples where the discharge capacity is 60.0 mAh / g or more and the discharge capacity is greater than that when y=0 for the same value of x are designated as examples, and all others are designated as comparative examples.

[0070] [Table 1]

[0071] [Table 2]

[0072] X-ray diffraction measurements were performed on each of the obtained positive electrode active materials according to the following measurement conditions. The results are shown in Figures 1, 2, and 7-10. X-ray diffraction measurement conditions X-ray diffractometer: SmartLab, manufactured by Rigaku Corporation. X-ray source: CuKα ray (CuKα=1.5418Å) Aperture angle of the incident parallel slit: 5.0° Length of the incidence longitudinal limiting slit: 5.0 mm Aperture angle of the light-receiving parallel slit: 5.0° Kβ filter: Used Step width: 0.01° Entrance slit: 1 / 6° Light-receiving slit 1: 4.0 mm Light-receiving slit 2: 13mm

[0073] As shown in Figures 1 and 8-10, the positive electrode active materials with x = 0.6, 0.8, 1.0, and 1.2 in equation (1) match the diffraction patterns of LiFe2F6 with DB card number 01-074-2193, space group P42 / mnm, and crystal system tetragonal, indicating that they have a similar crystal structure to LiFe2F6. On the other hand, as shown in Figures 2 and 7, the positive electrode active materials for x=0.4 and 0.5 in equation (1) were found to have peaks originating from the crystal structures of FeF3·0.33H2O and FeF3·3H2O, in addition to the diffraction pattern of FeF3 with space group R-3c and trigonal crystal system (DB card number 00-061-0194). This is thought to be because the positive electrode active materials for x=0.4 and 0.5 in equation (1) have low water resistance and are prone to moisture adhesion.

[0074] [Manufacturing of lithium-ion secondary batteries] (Fabrication of the positive electrode) 80 parts by mass of the positive electrode active material obtained in Examples 1-12 and Comparative Examples 1-24, 10 parts by mass of acetylene black, and 10 parts by mass of polyvinylidene fluoride were dispersed in N-methylpyrrolidone as a solvent to prepare a slurry (positive electrode mixture) containing 80% by mass of the positive electrode active material, 10% by mass of acetylene black, and 10% by mass of polyvinylidene fluoride as solid content. This slurry was applied to aluminum foil, pressed with 15 tons of pressure, and then punched out with a 10 mm diameter punch to produce a positive electrode. At this time, the mass of the positive electrode active material was adjusted to 3.5 mg.

[0075] (Creation of coin-shaped cells) The fabricated positive electrode (10 mm in diameter) was placed on top of the positive electrode container, and a porous polyethylene film was placed on top of it as a separator, secured with a polypropylene gasket. Then, a 0.5 mm thick lithium negative electrode was placed on top, and a spacer for thickness adjustment was added. Next, a mixed solvent of ethylene carbonate and diethyl carbonate (volume ratio 5:5) containing 1 mol / L lithium hexafluoride phosphate was added between the positive electrode container and the negative electrode as a non-aqueous electrolyte solution, impregnating the separator. The negative electrode container was then placed on top and sealed to fabricate a coin-type cell (lithium-ion secondary battery).

[0076] [Battery performance evaluation] The battery performance was evaluated using the fabricated coin-type cells. Specifically, the fabricated coin-type cells were charged and discharged at a constant current of 5 mA / g (0.025C, 1C = 200 mA / g) per unit mass of positive electrode active material. The upper voltage limit during constant current charging and discharging was set at 4.25V and the lower voltage limit at 3.35V. The rest time after charging and discharging was set at 10 minutes. The discharge capacity (mAh / g) was calculated per unit mass of positive electrode active material. The discharge curves for constant current charging and discharging in each example are shown in Figures 4, 11 to 15.

[0077] From the obtained discharge curve, the horizontal axis is voltage, and the vertical axis is the value obtained by differentiating the capacitance with respect to 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 for each example are shown in Figures 5, 16 to 20.

[0078] As shown in Figures 5 and 16-20, in Examples 1-12 to which the present invention was applied, the peak voltage of the dQ / dV plot during discharge was high, ranging from 3.9V to 4.0V, confirming that the chemical reaction occurred at a high voltage specific to the compound represented by formula (1). In contrast, in Comparative Examples 1 to 24, where x and y in equation (1) are outside the scope of the present invention, the peak depth at the peak voltage of the dQ / dV plot during discharge observed around 4V was shallower compared to Examples 1 to 12, as shown in Figures 5 and 16 to 20.

[0079] Table 3 summarizes the discharge capacity values ​​during discharge based on the above results. In Table 3, the unit of the values ​​is "mAh / g".

[0080] [Table 3]

[0081] Table 4 summarizes the peak voltage values ​​during discharge based on the above results. In Table 4, the unit of measurement is "V".

[0082] [Table 4]

[0083] From the above results, it was found that the present invention provides an Fe-based positive electrode active material that can operate at high voltage and has a higher capacity during discharge, as well as a lithium-ion secondary battery containing the positive electrode active material. [Explanation of symbols]

[0084] 1…Lithium-ion rechargeable battery 2...Positive electrode 3...Negative electrode 4... Separator 5…Insulating packing (gasket) 10…Positive electrode can 20... Negative electrode can (negative electrode terminal)

Claims

1. A positive electrode active material whose main component is lithium iron composite fluoride, The lithium iron composite fluoride is a positive electrode active material represented by the following formula (1). Li ｘ FeF （３＋ｘ－ｙ） (1) In equation (1), x and y are numbers that satisfy 0.4 < x < 1.2 and 0.01 < y < 0.

5.

2. In the above equation (1), x and y are When 0.4 < x ≤ 0.5, then 0.01 < y < 0.1 or 0.2 < y < 0.

5. When 0.5 < x ≤ 0.7, then 0.01 < y < 0.

5. When 0.7 < x < 1.0, then 0.01 < y < 0.1, or 0.1 < y < 0.4 When 1.0 ≤ x < 1.2, then 0.05 < y < 0.

2. The positive electrode active material according to claim 1, which is a number that satisfies the requirements.

3. The positive electrode active material according to claim 2, wherein the X-ray diffraction pattern has a peak in the range of 20° ≤ 2θ ≤ 30°.

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 described in any one of claims 1 to 3.

5. The lithium-ion secondary battery according to claim 4, wherein the dQ / dV plot during discharge in a charge-discharge cycle has a peak in the range of 3.9 to 4.0 V.

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

7. A method for producing a positive electrode active material mainly composed of lithium iron composite fluoride, Lithium fluoride, ferrous fluoride, and ferric fluoride are used as raw materials. A method for producing a positive electrode active material, wherein the lithium iron composite fluoride is represented by the following formula (1). Li ｘ FeF （３＋ｘ－ｙ） (1) In equation (1), x and y are, When 0.4 < x ≤ 0.5, then 0.01 < y < 0.1 or 0.2 < y < 0.

5. When 0.5 < x ≤ 0.7, then 0.01 < y ≤ 0.

5. When 0.7 < x < 1.0, then 0.01 < y < 0.1, or 0.1 < y < 0.4 When 1.0 ≤ x < 1.2, then 0.05 < y < 0.

2. It is a number that satisfies the following condition.

8. A method for producing a positive electrode active material according to claim 7, wherein the amount of ferrous fluoride added changes y in formula (1).

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