Lithium-ion secondary battery

Abstract

Provided is a lithium-ion secondary battery with improved battery life. A lithium-ion secondary battery comprising an electrode and an electrolyte solution, wherein the electrode contains an electrode active material containing more than 50 mass% of sulfur, the electrode contains carbon nanotubes, when the content (mass%) of carbon nanotubes in the electrode is ACNT and the volume (mm3) of the electrode is VEL, ACNT and VEL satisfy formula (1) below, and the electrolyte solution contains a fluorine compound. (1) ACNT × VEL > 0.5

Description

Lithium-ion rechargeable battery

[0001] This invention relates to a lithium-ion secondary battery.

[0002] Lithium-ion rechargeable batteries have a large charge and discharge capacity and are primarily used as batteries for portable electronic devices. Furthermore, their use in electric vehicles is increasing, and improvements in performance are expected.

[0003] Patent Document 1 describes a lithium-ion secondary battery that uses a sulfur-based active material, obtained by calcining a raw material containing a polymer with methacrylonitrile as a monomer component and sulfur, as the electrode active material. It has also been proposed to increase the battery capacity of a lithium-ion secondary battery by using a material that can intercept and release more lithium ions, such as silicon (Si) or tin (Sn), as the negative electrode active material.

[0004] Japanese Patent Publication No. 2020-167144

[0005] However, there is still a need to improve the battery life of lithium-ion secondary batteries. Furthermore, the above-mentioned materials proposed as negative electrode active materials have the problem of poor cycle characteristics when repeatedly charged and discharged due to large volume changes associated with the intercalation and release of lithium ions. In addition, carbon materials such as graphite and hard carbon are also used, but their theoretical capacity is already being reached, and significant capacity improvements are not expected.

[0006] The present invention aims to provide an electrode active material that can improve battery life, an electrode for a lithium-ion secondary battery using the electrode active material, and a lithium-ion secondary battery with improved battery life using the electrode.

[0007] The present invention relates to the following lithium-ion secondary battery: A lithium-ion secondary battery comprising electrodes and an electrolyte, wherein the electrodes contain an electrode active material containing more than 50% by mass of sulfur, the electrodes contain carbon nanotubes, and the carbon nanotube content (by mass) in the electrodes is A CNT , volume of the electrode (mm 3 ) is V ELWhen it is A CNT and V EL satisfy the following formula (1), and the electrolyte contains a fluorine compound, a lithium ion secondary battery. (1) A CNT ×V EL > 0.5

[0008] According to the present invention, it is possible to provide an electrode active material capable of improving battery life, an electrode for a lithium ion secondary battery using the electrode active material, and a lithium ion secondary battery having improved battery life using the electrode.

[0009] In this specification, "cycle characteristics" refers to the characteristics in which the charge-discharge capacity of a secondary battery is maintained despite repeated charge-discharge. Therefore, with repeated charge-discharge, a secondary battery with a large degree of decrease in charge-discharge capacity and a low capacity retention rate has poor cycle characteristics, while conversely, a secondary battery with a small degree of decrease in charge-discharge capacity and a high capacity retention rate has excellent cycle characteristics. Further, excellent cycle characteristics are synonymous with an improved battery life of the secondary battery, and poor cycle characteristics are synonymous with a decreased battery life of the secondary battery.

[0010] Hereinafter, the configuration of one embodiment of the present invention will be described in detail. Regarding the description of numerical ranges, the upper and lower limit numerical values related to "above", "below", "more than", "less than", etc. can be arbitrarily combined numerical values, and the numerical values in the examples can also be used as these upper and lower limits. Further, a numerical range shown as including the lower limit or the upper limit is construed as simultaneously disclosing a numerical range not including the upper limit or the lower limit as long as it does not conflict with the gist of this specification, and conversely, a numerical range shown as not including the lower limit or the upper limit is construed as simultaneously disclosing a numerical range including the lower limit or the upper limit as long as it does not conflict with the gist of this specification.

[0011] One embodiment of the present invention is a lithium ion secondary battery including an electrode and an electrolyte, wherein the electrode includes an electrode active material containing more than 50% by mass of sulfur, the electrode includes carbon nanotubes, and the content (% by mass) of carbon nanotubes in the electrode is A CNT , the volume of the electrode (mm 3) is V EL If A CNT and V EL The following formula (1) satisfies the lithium-ion secondary battery, wherein the electrolyte contains a fluorine compound. (1) A CNT ×V EL >0.5

[0012] While not intended to be constrained by theory, the following is a possible reason why the lithium-ion secondary battery of this embodiment can improve battery life. Specifically, in the lithium-ion secondary battery of this embodiment, the carbon nanotube (CNT) content in the electrode material and the volume of the electrode satisfy a predetermined relationship, which is thought to contribute to an improvement in capacity retention rate and thus improve battery life.

[0013] Here, the technical significance of equation (1) is considered as follows. That is, equation (1) is a technical lower limit standard that uniquely provides the minimum limit that must be met by the carbon nanotube content in response to the increasing connectivity requirements of the electron conduction network that arise with the expansion of the volume of the positive electrode composite electrode. For more details, see V EL When the coefficient is large, the conductive path length increases in the thickness direction and in-plane direction, and the number of contact points also increases, thus increasing the risk of disruption of the electronic network. In contrast, A CNT Fluorine is the main effector responsible for three-dimensional percolation and contact maintenance between primary particles, and the coupling density increases in proportion to its content. Furthermore, electrolytes containing fluorine compounds tend to form a passive film with a high fluorine content at the positive electrode interface, increasing the contact resistance at the conductive additive interface. This increase in interfacial resistance relatively increases the risk of electron network fragmentation, making it even more important to ensure a lower limit on the amount of CNTs commensurate with the increase in volume. Therefore, equation (1) is an empirical and reproducible technical threshold for ensuring a minimum degree of electron coupling commensurate with the geometric expansion of the electrode.

[0014] Note V EL It is considered that V is a quantity that is constrained by the design and cannot be set independently for at least the following reasons. Firstly, V EL This is constrained by the capacity target and volumetric energy density. That is, given the target capacity and volumetric energy density in the same cell design, the active material area load and thickness are linked, VEL This is determined. Here, if the thickness is increased excessively, the diffusion length and electron path length will increase, and the internal resistance will increase, V EL It is not possible to freely increase or decrease V. Secondly, EL This is constrained by the increased interfacial resistance due to the fluorine compound-containing electrolyte. In other words, while fluorine compounds provide high oxidation resistance and stable film formation, they tend to increase the contact resistance at the conductive additive interface. And V EL The larger A is, the more the cumulative effect of this interfacial resistance becomes apparent, CNT The need arises to increase the degree of three-dimensional connectivity, and the lower limit of equation (1) functions as a practically essential condition. Therefore, equation (1) is equal to V determined under design constraints. EL This provides a lower bound index for ensuring electronic connectivity.

[0015] A CNT It is preferable that the amount is less than 4.00% by mass.

[0016] Preferably, the average length of the carbon nanotubes is greater than 1 μm, and the average diameter is less than 100 nm.

[0017] This configuration is thought to facilitate the formation of a three-dimensional conductive network.

[0018] Preferably, the electrode active material further includes a metal compound containing at least one selected from the group consisting of iron, molybdenum, vanadium, and titanium.

[0019] The aforementioned metal compound is preferably an iron compound.

[0020] The coating density of the electrode (mg / cm²) 2 Let D be A CNT It is preferable that D and A satisfy the following equation: (2) D × A CNT >0.25

[0021] Electrode coating density and A CNT By making the product of these two values ​​exceed a predetermined value, it is believed that the effects of adding CNTs can be fully realized.

[0022] The coating density of the electrode (mg / cm²) 2When D is denoted as 2.50 mg / cm³, then D is 2.50 mg / cm³. 2 It is preferable that it be greater than.

[0023] The volume of the electrolyte is V LYT (mL) and the coating density of the electrode (mg / cm³) 2 Let D be A CNT D and V LYT It is preferable that the following equation is satisfied: (3) D × A CNT / V LYT >0.89

[0024] The coating density of the electrode, and A CNT It is believed that the effects of adding CNTs can be fully realized by ensuring that the volume of the electrolyte satisfies the above equation.

[0025] <Definition> "Electrode" refers to a layer composed of electrode material, i.e., an electrode material layer. In this specification, an electrode may be formed on the surface of a current collector, or it may not include a current collector. The term "electrode" refers to the electrode material layer itself, and does not include the current collector even if the electrode material layer is formed on the surface of a current collector.

[0026] "Electrode coating density" refers to the mass per unit area (mg / cm³) of the electrode material layer. 2 )

[0027] "Electrode volume" refers to the volume of the electrode material layer (mm²). 3 This means the volume of the electrode material layer, for example, the volume including any voids present in the electrode material layer. The volume of the electrode may normally change during charging and discharging due to the expansion or contraction of the electrode material layer, but in this specification, the volume of the electrode refers to the volume of the electrode during discharge. Generally, the electrode material layer of the positive electrode tends to expand during discharge, and the electrode material layer of the negative electrode tends to contract during discharge.

[0028] "Electrode material" is a general term for the materials that make up the electrode layer. Examples of electrode materials include electrode active materials, conductive additives, binders, and so on.

[0029] "Electrode active material" refers to one of the electrode materials in a battery, specifically the substance responsible for the oxidation-reduction reaction that generates electricity in a lithium-ion secondary battery. Electrode active materials include positive electrode active material and negative electrode active material.

[0030] "Particles" refers to a state in which the electrode active material has been finely ground to a degree suitable for mixing with other materials for the purposes of the present invention. The size of such particles constituting the electrode active material is not particularly limited as long as it allows the above mixing to be carried out suitably. For example, when "particles" are expressed in terms of median diameter, they can be in the range of 1 nm to 1000 μm.

[0031] Unless otherwise specified, "particle size" is expressed as the median diameter (d50).

[0032] A carbon nanotube (CNT) is an allotrope of carbon, a carbon material formed by a single-layer or multi-layer coaxial tube structure of graphene sheets. The length of a carbon nanotube is measured as the length of the CNT along its fiber axis, and the diameter of a carbon nanotube is measured as the width of the CNT in a direction perpendicular to its fiber axis. The average length of a carbon nanotube is determined by photographing the CNTs with a transmission or scanning electron microscope, measuring the lengths of 50 CNTs along their fiber axes, and taking the arithmetic mean. The average diameter of a carbon nanotube is also determined in the same way as the average length of the CNTs, by photography, measurement, and arithmetic mean.

[0033] "Volume of electrolyte" refers to the total volume of the electrolyte solution, including the solute. The unit is mL.

[0034] "Discharge capacity on the 12th cycle" refers to the discharge capacity (mAh / g) on ​​the 12th cycle, counting the first discharge as the initial charge-discharge cycle, and repeating the cycle of charging, discharging, and charging again.

[0035] "Discharge capacity at the 150th cycle" refers to the discharge capacity (mAh / g) at the 150th cycle when repeatedly charging and discharging, as described above.

[0036] <Measurement Method> The "elemental amount" is measured by the method described in the examples. For example, this method is applied to sulfur, carbon, hydrogen, nitrogen, etc.

[0037] The "particle size distribution" is measured using a laser diffraction / scattering particle size distribution analyzer (PSA1090L manufactured by Anton Paar) with water as the dispersion medium.

[0038] "Median diameter" is the volume-based cumulative 50% diameter (d50) in the particle size distribution, and can be measured by the method described in the Examples section below.

[0039] <Lithium-ion secondary battery> The lithium-ion secondary battery of this embodiment will be described below.

[0040] The lithium-ion secondary battery of this embodiment comprises predetermined electrodes and an electrolyte.

[0041] (Electrode) The electrode of this embodiment comprises a predetermined electrode active material and carbon nanotubes (CNTs). The electrode can be used as the positive or negative electrode of a lithium-ion secondary battery, and is particularly preferred to be used as the positive electrode.

[0042] [Electrode Active Material] The electrode active material of this embodiment contains more than 50% by mass of sulfur. The electrode active material may also contain CNTs. The electrode active material may further contain carbon, hydrogen, nitrogen, metal compounds, etc.

[0043] <<Amount of Sulfur Element>> From the viewpoint of improving battery life, the amount of sulfur element in the electrode active material is preferably more than 51.0% by mass, more preferably more than 55.0% by mass, even more preferably more than 56.0% by mass, even more preferably more than 57.0% by mass, even more preferably more than 58.0% by mass, even more preferably more than 59.0% by mass, even more preferably more than 60.0% by mass, even more preferably more than 61.0% by mass, even more preferably more than 62.0% by mass, and even more preferably more than 63.0% by mass. There is no particular upper limit on the amount of sulfur element, but it is usually 80.0% by mass, and may also be 70.0% by mass.

[0044] The electrode active material may contain elements other than sulfur. Examples of such elements include carbon, hydrogen, and nitrogen.

[0045] <<Carbon Element Amount>> From the viewpoint of improving battery life, the carbon element amount is preferably more than 5.0% by mass, more preferably more than 10.0% by mass, and even more preferably more than 15.0% by mass. On the other hand, the amount of this element is preferably less than 30.0% by mass, more preferably less than 25.0% by mass, and even more preferably less than 23.0% by mass.

[0046] <<Hydrogen Element Amount>> During the production of the electrode active material, hydrogen is released outside the system as hydrogen sulfide through the calcination of raw materials containing organic compounds and sulfur. For this reason, the hydrogen element amount of the electrode active material is preferably less than 1.0 mass%, more preferably less than 0.9 mass%, even more preferably less than 0.8 mass%, even more preferably less than 0.7 mass%, even more preferably 0.6 mass% or less, and even more preferably 0.5 mass% or less. When it is less than 1.0 mass%, there is a tendency for the calcination (sulfidation reaction) to be sufficient. Therefore, in this case, there is a tendency for the charge / discharge capacity to improve.

[0047] <<Nitrogen Element Amount>> The amount of nitrogen element (mass%) in the electrode active material can be 0 mass% if no nitrogen source is used in the raw materials. For example, when preparing the raw materials by the WET method, if a solvent containing nitrogen atoms is used, nitrogen elements can be detected.

[0048] <Metal Compounds> The electrode active material may include metal compounds. Examples of such metal compounds include those containing at least one selected from the group consisting of iron, molybdenum, vanadium, and titanium. The metal compound is preferably an iron compound. One or more metal compounds can be used.

[0049] In the case where the electrode active material of this embodiment contains the metal compound, the amount of metal element (mass%) in the electrode active material is preferably more than 10.0 mass%, more preferably more than 15.0 mass%, and even more preferably more than 20.0 mass%, from the viewpoint of improving the performance of the electrode and / or battery. On the other hand, the amount of the same element is preferably less than 30.0 mass%, more preferably less than 25.0 mass%, and even more preferably less than 24.0 mass%.

[0050] ≪CNT≫ Carbon nanotubes (CNTs) are carbon materials in which graphene sheets are arranged in a single-wall or multi-wall coaxial tubular structure. A single-wall carbon nanotube is a carbon nanotube consisting of a single layer of graphene sheet, and is also called a single-wall carbon nanotube (SWCNT). A multi-wall carbon nanotube is a carbon nanotube consisting of two or three or more layers of graphene sheet, and is also called a multi-walled carbon nanotube (MWCNT). Both single-wall CNTs and multi-wall CNTs can be used, but it is preferable that they include single-wall carbon nanotubes. CNTs can be used individually or in combination of two or more types.

[0051] In this embodiment, the CNTs may be included in the electrode. For example, the CNTs may be included in the electrode active material, or they may be included in the electrode material constituting the electrode material layer as a conductive additive. Alternatively, the CNTs may be included in both the electrode active material and the electrode material other than the electrode active material.

[0052] From the viewpoint of improving battery life, it is preferable that the average length of the CNTs is above a predetermined value and the average diameter is below a predetermined value. This is because it is thought that the conductivity of the electrode active material can be improved. The average length is preferably greater than 1 μm, more preferably greater than 1.5 μm, and even more preferably 2 μm or more. There is no particular upper limit to the average length, and it may be 100 μm, 50 μm, or 20 μm. The average diameter is preferably less than 100 nm, more preferably less than 50 nm, even more preferably less than 10 nm, even more preferably less than 5 nm, even more preferably less than 3 nm, and even more preferably less than 2 nm. There is no particular lower limit to the average diameter, but it is usually around 1 nm.

[0053] The aspect ratio of the CNT is preferably greater than 10, more preferably greater than 100, even more preferably greater than 1000, and also preferably less than 100000, more preferably less than 50000, and even more preferably less than 10000.

[0054] CNT has a specific surface area of ​​200 m².2 Preferably, it is 2400m or more. 2 It is preferable that the amount is less than or equal to / g. The specific surface area is 400 m². 2 More preferably 500 m / g or more, and even more preferably 500 m 2 / g or more, more preferably 600m 2 It is 1 / g or more. On the other hand, the specific surface area is 2000 m². 2 It is more preferable to have less than / g, and even more preferable to have 1800m 2 It is less than or equal to / g. The specific surface area is measured using the BET multipoint method.

[0055] CNTs preferably have a G / D ratio of 10 or higher. A G / D ratio of 20 or higher is more preferable, 30 or higher is even more preferable, and 40 or higher is even more preferable. While there is no particular upper limit to the G / D ratio, a ratio of 90 or higher indicates a CNT with extremely few defects. Here, the G / D ratio is the ratio of representative Raman shift peaks in the Raman spectrum of a carbon material (CNT), and more specifically, the ratio of the G-band peak derived from the graphite structure to the D-band peak derived from defects. The above Raman spectrum was obtained using a RAMANtouch (excitation wavelength λ = 532 nm, grating: 1200 gr / mm, resolution: 1.2 cm) manufactured by Nanophoton Inc. -1 This was measured using ).

[0056] The amount of metal impurities in the CNTs is preferably 5% by mass or less. Preferably, the amount of metal impurities is 3% by mass or less, more preferably 2% by mass or less, and even more preferably 1% by mass or less. The less metal impurities there are, the better, but for example, 0.1% by mass is considered to be a sufficiently low amount of metal impurities. The amount of metal impurities is measured by inductively coupled high-frequency plasma atomic emission spectroscopy (ICP-AES).

[0057] <<Median Diameter>> The electrode active material is preferably composed of particles, and its size is preferably suitable for electrode manufacturing. From the viewpoint of improving battery life, the preferred range for the particle size of the electrode active material is a median diameter (d50) of more than 1.0 μm and less than 40.0 μm. The median diameter is more preferably more than 1.5 μm, even more preferably more than 2.0 μm, and even more preferably more than 3.0 μm. Furthermore, the median diameter is more preferably less than 30.0 μm, even more preferably less than 25.0 μm, even more preferably less than 20.0 μm, even more preferably less than 15.0 μm, even more preferably less than 10.0 μm, and even more preferably less than 8.0 μm. The median diameter can be measured by the method described in the Examples section below.

[0058] [Other electrode materials] The electrode of this embodiment may include other electrode materials in addition to the electrode active material described above.

[0059] Other electrode materials include CNTs, which are essential electrode materials when the electrode active material does not contain CNTs. On the other hand, when the electrode active material contains CNTs, CNTs are optional electrode materials. CNTs used as optional electrode materials can be the same as those used when included in the electrode active material. CNTs added as electrode materials other than the electrode active material are added as conductive additives.

[0060] Furthermore, other electrode materials include those listed in the section on manufacturing methods below. All of these electrode materials can be used to construct electrodes in the same manner as described in that section. For example, when using the electrode as a positive electrode, the conductive additive, binder, current collector, etc., listed in the section on manufacturing methods below can be used in the same manner as described in that section to form the positive electrode. Similarly, when using the electrode as a negative electrode, the conductive additive, binder, current collector, etc., listed in the section on manufacturing methods below can be used in the same manner as described in that section to form the negative electrode. Thus, these explanations in the section on manufacturing methods below can be considered as explanations of the electrodes in this embodiment.

[0061] [A CNT ] A CNTThis represents the carbon nanotube content (mass%) in the electrode, i.e., the total amount of CNTs contained in the electrode. For example, if CNTs are present in both the electrode active material and the electrode material other than the electrode active material, this represents the sum of the content of both.

[0062] A CNT For example, it is preferable that it be less than 4.00% by mass. CNT The amount is preferably less than 3.00% by mass, more preferably less than 2.00% by mass, even more preferably less than 1.00% by mass, even more preferably less than 0.90% by mass, even more preferably less than 0.80% by mass, even more preferably less than 0.70% by mass, even more preferably less than 0.60% by mass, even more preferably less than 0.55% by mass, and even more preferably 0.52% by mass or less. On the other hand, the content is preferably more than 0.04% by mass, more preferably more than 0.05% by mass, even more preferably more than 0.06% by mass, even more preferably 0.07% by mass or more, even more preferably more than 0.10% by mass, even more preferably more than 0.15% by mass, even more preferably more than 0.20% by mass, even more preferably more than 0.25% by mass, even more preferably more than 0.30% by mass, even more preferably 0.40% by mass or more, and even more preferably more than 0.50% by mass.

[0063] [Formula (1)] The electrode of this embodiment has a carbon nanotube content (mass%) in the electrode of A CNT , electrode volume (mm 3 ) is V EL If A CNT and V EL The following equation (1) is satisfied. Here, A CNT This means "carbon nanotube content in the electrode / 1% by mass", V EL This is "electrode volume / 1 mm 3 This means "A". Therefore, A CNT and V EL Both are treated as dimensionless quantities, and equation (1) becomes a dimensionless evaluation formula that does not depend on the unit system. (1) A CNT ×V EL >0.5

[0064] The right-hand side of equation (1) is more preferably 1.0, even more preferably 1.5, even more preferably 2.0, even more preferably 2.1, even more preferably 2.8, and even more preferably 3.0. For reference, the upper limit of the value on the left-hand side of equation (1) can be assumed to be around 50.

[0065] The value of the left side of equation (1) is A CNT and V EL The values ​​of these parameters can be adjusted by increasing or decreasing them.

[0066] [DC 12 DC 150 ] DC 12 This is the discharge capacity after the 12th cycle. The discharge capacity after the 12th cycle is the discharge capacity after 12 charge-discharge cycles with a discharge termination voltage of 1.0V and a charge termination voltage of 3.0V, following the manufacturing of the electrodes and battery. In other words, it is the 12th discharge when repeating the cycle of 1st discharge, 1st charge, 2nd discharge, 2nd charge. Also, DC 150 This is the discharge capacity at the 150th cycle when charging and discharging are repeated in a similar manner. In the case of discharge, when discharged with a constant current (for example, a current value equivalent to 50 mA per gram of positive electrode active material), the voltage of 3.0 V will eventually drop to 1.0 V. The total time (h) during which the voltage drops from 3.0 V to 1.0 V is measured and multiplied by the current flowed (mA) to obtain the capacity (mAh), and dividing this by the weight of the active material gives the specific capacity (mAh / g). On the other hand, in the case of charging, the voltage increases due to charging with a constant current, and charging ends when it finally reaches 3.0 V. The discharge capacity at the 150th cycle (DC) will be described later. 150 The same applies to ).

[0067] DC 12The (mAh / g) is preferably greater than 750 mAh / g. The discharge capacity is more preferably greater than 760 mAh / g, even more preferably 765 mAh / g or more, even more preferably greater than 770 mAh / g, and even more preferably greater than 780 mAh / g. There is no particular upper limit to the initial discharge capacity, and the higher the better. Therefore, there is little point in mentioning the upper limit of the initial discharge capacity, but as a reference value, it can usually be assumed to be, for example, around 1000 mAh / g.

[0068] DC 150 The (mAh / g) is preferably greater than 700 mAh / g. The discharge capacity is more preferably greater than 750 mAh / g, even more preferably greater than 780 mAh / g, even more preferably 800 mAh / g or more, and even more preferably greater than 810 mAh / g. There is no particular upper limit to the initial discharge capacity, and the higher the better. Therefore, there is little point in mentioning the upper limit of the initial discharge capacity, but as a reference value, it can usually be assumed to be, for example, around 1000 mAh / g.

[0069] The discharge capacity when the electrode of this embodiment is used as the positive electrode is determined by the configuration of the positive electrode, and can be measured using a negative electrode and electrolyte that are within the technically common sense range that can be used as a lithium-ion secondary battery (i.e., Li will not be depleted), and that allow the performance related to the discharge capacity of the positive electrode to be fully exhibited. For example, for the negative electrode, it is preferable to use an amount of lithium (mol) that is preferably 2 times or more, more preferably 5 times or more, even more preferably 10 times or more, and even more preferably 50 times or more, the amount of sulfur (mol) in the positive electrode. Also, for example, for the electrolyte, if the amount of electrolyte (μL) is preferably 10 times or more, more preferably 20 times or more, and even more preferably 50 times or more, the discharge capacity of the positive electrode can be fully exhibited, leading to a longer battery life. On the other hand, considering the energy density of the battery, it is preferable that the amount of electrolyte be small. For example, the amount of electrolyte (μL) is preferably 5 times or less, more preferably 3 times or less, and even more preferably 1 time or less, relative to the amount of sulfur (mg) in the positive electrode. Here, the volume V of the electrolyte LYT (mL) refers to the total volume of the electrolyte solution containing the solute.

[0070] In this embodiment, the electrode, when used as the positive electrode, has a discharge capacity of DC at the 12th discharge. 12 (mAh / g), discharge capacity at 150th discharge is DC 150 If it is (mAh / g), DC 12 DC for 150 The percentage is preferably over 90%.

[0071] The percentage is more preferably greater than 92%, even more preferably greater than 95%, even more preferably greater than 98%, even more preferably greater than 99%, even more preferably 100% or more, even more preferably greater than 100%, even more preferably greater than 102%, even more preferably greater than 104%, and even more preferably greater than 106%. The upper limit of the above percentage may exceed 100%, and as a reference value, it can be assumed to be around 110%.

[0072] [Formula (2)] The electrode of this embodiment has an electrode coating density (mg / cm³). 2 Let D be A CNT It is preferable that and D satisfy the following equation. Here, A CNT ' is the carbon nanotube content in the electrode / 1% by mass, and D is the coating density / 1 mg / cm 2 This means "A". Therefore, A CNT Both D and A are treated as dimensionless quantities, and equation (2) becomes a dimensionless evaluation formula that does not depend on the unit system. (2) D × A CNT >0.25

[0073] The right-hand side of equation (2) is more preferably 1.20, even more preferably 1.30, even more preferably 1.40, even more preferably 1.50, even more preferably 1.60, even more preferably 1.70, and even more preferably 1.80. The upper limit of the value on the left-hand side of equation (2) can be assumed to be approximately 60.00, or approximately 30.00, or approximately 10.00, or approximately 5.00, as a reference value.

[0074] ≪Electrode Coating Density≫ The electrode coating density (D) is the mass per unit area (mg / cm³) of the electrode material layer constituting the electrode. 2 It is represented as follows:

[0075] The coating density of the electrode (mg / cm 2 ) is preferably more than 2.50 mg / cm 2 , more preferably more than 2.80 mg / cm 2 , even more preferably more than 3.00 mg / cm 2 , even more preferably more than 3.30 mg / cm 2 , even more preferably more than 3.50 mg / cm 2 . There is no particular limitation on the upper limit of the coating density of the electrode. The higher, the better, but it is naturally restricted by the form of formula (3). Therefore, it is meaningless to mention the upper limit of the coating density. However, usually, the coating density of the electrode can be assumed to be about 15.0 mg / cm 2 .

[0076] The value on the left side of formula (2) can be adjusted by increasing or decreasing the values of D and A CNT respectively.

[0077] (Electrolyte) The electrolyte of this embodiment contains a fluorine compound. Except for containing a fluorine compound, the electrolyte can be the same as a normal one. Therefore, such an electrolyte contains a solvent, a lithium salt, and optionally an additive. Here, the fluorine compound refers to a compound containing a fluorine atom. The fluorine compound may be a solvent, a lithium salt, or an additive, but it is preferably a solvent. Preferred fluorine compounds include fluoroethylene carbonate (FEC), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), methyl 2,2,2-trifluoroethyl carbonate (FEMC), bis 2,2,2-trifluoroethyl carbonate (TFEC), ethyl difluoroacetate, etc. The fluorine compound can be used alone or in combination of two or more.

[0078] [Solvent] As solvents, organic solvents such as ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS), and diethyl sulfone (ESE) can be used. The solvent can be used alone or in mixtures of two or more types.

[0079] Preferred solvents include EC, EMC, DEC, and FEC, with EMC and FEC being more preferred, and a combination of EMC and FEC being even more preferred.

[0080] When using two or more solvents, their mixing ratio can be determined by conventional methods in the art. For example, when using EMC and FEC, the EMC:FEC ratio is preferably in the range of 6:4 to 8:2, more preferably in the range of 6.5:3.5 to 7.5 to 2.5, and particularly preferably 7:3. When using EC and DEC, the EC:DEC ratio is preferably in the range of 4:6 to 6:4, more preferably in the range of 4.5:5.5 to 5.5 to 4.5, and particularly preferably 5:5.

[0081] [Lithium Salts] Lithium salts that can be used include LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiAsF6 (lithium hexafluoroarsenate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluoro(oxalato)borate), LiBOB (lithium bis(oxalate)borate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluoro(oxalato)phosphate), and LiTFOP (lithium tetrafluoro(oxalato)phosphate). Of these, LiFSI and LiTFSI are preferred, and LiTFSI is more preferred. Lithium salts can be used individually or in combination of two or more types.

[0082] The concentration of lithium salt in the electrolyte can be determined by conventional methods in the art. The electrolyte concentration is preferably, for example, 0.5 M or more and 1.5 M or less. More preferably, it is 0.7 M or more, and even more preferably 0.9 M or more. On the other hand, it is more preferably, it is 1.3 M or less, and even more preferably 1.1 M or less.

[0083] [Additives] As additives, vinylene carbonate (VC), fluoroethylene carbonate (FEC), vinylethylene carbonate (VEC), succinonitrile (SN), adiponitrile (AND), 1,3-propensultone (PST), sulfonic acid ester cyclic quaternary ammonium salts, tris(trimethylsilyl) phosphate (TMSP), tris(trimethylsilyl) borate (TMSB), etc. can be used. Of these, VC and FEC are preferred, and FEC is more preferred. Additives can be used individually or in mixtures of two or more types.

[0084] The additive content is preferably within the range of 0% by mass or more and 30% by mass or less relative to the total mass of the electrolyte. The amount of additive used may be, for example, 20% by mass or less, 10% by mass or less, or 5% by mass or less relative to the total mass of the electrolyte. Furthermore, the additive may not be included at all.

[0085] It should be noted that there is some overlap between the solvent and additives that make up the electrolyte. For example, FEC can be both a solvent and an additive. However, in the case of an electrolyte containing FEC, as long as FEC is added in a manner that is consistent with the description of the solvent content above, or in a manner that is consistent with the description of the additive content above, and the other components are also consistent with the description of the electrolyte content above, it does not matter whether FEC was added as a solvent or as an additive. The same applies to other components if they can be both a solvent and an additive.

[0086] [Preparation of Electrolyte] The electrolyte can be prepared according to the conventional methods of the art, as long as it contains a fluorine compound. The electrolyte can be prepared, for example, by uniformly mixing a solvent, a lithium salt, and optionally an additive. In this case, there are no particular restrictions on the order in which the materials are added.

[0087] [Volume of electrolyte] Volume of electrolyte (V LYT ) can vary depending on the size of the battery, and therefore cannot be specified in general terms; it is sufficient to use at least the minimum amount necessary to bring out the performance of the electrode active material and allow the battery to function adequately. LYT This will be explained in more detail below.

[0088] (Other Materials Constituting the Lithium-Ion Secondary Battery) The lithium-ion secondary battery of this embodiment can be configured by using the materials described in the column of the manufacturing method below in the same manner as described in the same column. That is, when using the electrode for the lithium-ion secondary battery as the positive electrode, the negative electrode, electrolyte, separator, etc. described in the column of the manufacturing method below can be used in the same manner as described in the same column to configure the lithium-ion secondary battery. On the other hand, when using the electrode for the lithium-ion secondary battery as the negative electrode, the positive electrode, electrolyte, separator, etc. described in the column of the manufacturing method below can be used in the same manner as described in the same column to configure the lithium-ion secondary battery. Therefore, these descriptions in the column of the manufacturing method below can be taken into consideration as the description of this lithium-ion secondary battery.

[0089] (Formula (3)) For the lithium-ion secondary battery of this embodiment, when the volume (mL) of the electrolyte is V LYT and the coating density (mg / cm 2 ) of the electrode is D, it is preferable that A CNT and D and V LYT satisfy the following formula. Here, A CNT means "carbon nanotube content in the electrode / 1 mass%", D means "coating density / 1 mg / cm 2 ", and V LYT means "volume of the electrolyte / 1 mL". Therefore, A CNT and D and V LYT are all treated as dimensionless quantities, and formula (3) becomes a dimensionless evaluation formula that does not depend on the unit system. (3) D × A CNT / V LYT > 0.89

[0090] The right side of formula (3) is more preferably 1.00, further preferably 2.00, further preferably 4.50, further preferably 5.00, further preferably 5.50, further preferably 6.00, further preferably 6.40, further preferably 6.50. The upper limit of the value on the left side of formula (3) can also be assumed to be about 60.00, or about 30.00, or about 20.00, or about 10.00 as a reference value.

[0091] ≪ V LYT≫ Volume V of electrolyte LYT Regarding (mL), in the case of the coin-type battery presented in the examples, it is preferably 0.10 mL or more, more preferably 0.12 mL or more, and even more preferably 0.15 mL or more. On the other hand, V is preferably 0.40 mL or less, more preferably 0.35 mL or less, even more preferably 0.30 mL or less, and even more preferably 0.28 mL or less.

[0092] The value on the left side of equation (3) is D × A CNT The value and V LYT The values ​​of these parameters can be adjusted by increasing or decreasing them.

[0093] (Applications) The lithium-ion secondary battery of this embodiment is useful as a lithium-ion secondary battery with improved battery life and can be used as a battery for portable information terminals such as smartphones and notebook personal computers, portable electronic devices such as music players and digital cameras, medical devices, and next-generation clean energy vehicles such as hybrid electric vehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHEVs).

[0094] <Manufacturing Method> The manufacturing method of the lithium-ion secondary battery of this embodiment will be described below in order.

[0095] (Manufacturing of electrode active material) Electrode active material can be manufactured, for example, by the steps of (1) mixing sulfur, an organic compound, and CNTs (where applicable) to obtain a calcination raw material, (2) calcining the calcination raw material to obtain a calcined product, and (3) crushing the calcined product into particles to obtain electrode active material.

[0096] [Ingredients] The ingredients are described below.

[0097] ≪Organic Compounds≫ Organic compounds are compounds containing at least carbon atoms and hydrogen atoms, and are not particularly limited as long as they incorporate sulfur to form an organic sulfur compound when calcined with sulfur in a non-oxidizing atmosphere in the presence of a carbon material. Furthermore, organic compounds may also contain heteroatoms such as nitrogen atoms and sulfur atoms. Specific examples of organic compounds include polymers of unsaturated chain hydrocarbon monomers and condensates of substituted aromatic hydrocarbons and sulfur chloride. Organic compounds can be used individually or in combination of two or more.

[0098] Polymers of unsaturated-chain hydrocarbon monomers Examples of polymers of unsaturated-chain hydrocarbon monomers include resins such as acrylic resins, as well as diene rubbers. One or more polymers of unsaturated-chain hydrocarbon monomers can be used.

[0099] Examples of acrylic resins include polymers obtained by polymerizing monomers containing at least one selected from the group consisting of acrylate compounds represented by the following chemical formula (1); polymers obtained by polymerizing monomers containing at least one selected from the group consisting of acrylate compounds represented by the following chemical formula (1); polymers obtained by polymerizing monomers containing at least one selected from the group consisting of acrylate compounds represented by the following chemical formula (1) and at least one selected from the group consisting of diacrylate compounds represented by the following chemical formula (2); and polymers obtained by polymerizing polymers containing at least one selected from the group consisting of acrylate compounds represented by the following chemical formula (1) and at least one selected from the group consisting of diacrylate compounds represented by the following chemical formula (2). One or more types of acrylic resins can be used. CH2=C(R 11 ) COOR 12 (1) (Here, R 11 R is a hydrogen atom or a methyl group, 12 (This is an alkyl group.) CH2=C(R 21 )COO-Y-OCO(R 22 ) C = CH2 (2) (where R21 and R 22 (wherein Y is the same or different hydrogen atom or methyl group, Y is a linear hydrocarbylene group, which may have at least one substituent selected from the group consisting of hydroxyl groups and alkyl groups, and the carbon skeleton constituting the hydrocarbylene group may have ether bonds with oxygen atoms. However, when there are two or more such ether bonds, there are always two or more carbon atoms interposed between adjacent oxygen atoms.)

[0100] In chemical formula (1), R 11 A methyl group is preferred, R 12 The group is preferably an alkyl group having 1 to 6 carbon atoms, more preferably an alkyl group having 1 to 4 carbon atoms, and among these, a methyl group, an n-butyl group, an i-butyl group, or a t-butyl group is preferred. Examples of compounds represented by chemical formula (1) include methyl (meth)acrylate and butyl (meth)acrylate, and more preferably methyl methacrylate (MMA) and butyl methacrylate. Here, "(meth)acrylate" in methyl (meth)acrylate and butyl (meth)acrylate means either "acrylate" or "methacrylate" (the same applies hereinafter). A further preferred example of a compound represented by chemical formula (1) is methyl methacrylate.

[0101] In chemical formula (2), R 21 and R 22 In all cases, a methyl group is preferred. The number of carbon atoms in the hydrocarbylene group (linear chain) of Y is preferably 2 to 6, and more preferably 2 or 3. The number of substituents on Y is preferably 1 to 4, and more preferably 1 or 2. The substituents on Y are preferably one or more substituents selected from the group consisting of a hydroxyl group and an alkyl group having 1 to 4 carbon atoms, and a methyl group is preferred as the alkyl group having 1 to 4 carbon atoms. When the carbon skeleton of Y has an ether bond with an oxygen atom, for example, the portion corresponding to -Y-O- is preferably represented by the following chemical formula (3) (however, substituents on Y are not considered in chemical formula (3)). -(CH2)l - (CH2CH2O) m -(CH2CH2CH2O) n - (3) (Here, l is between 0 and 6, m is between 0 and 3, and n is between 0 and 2. However, l, m, and n cannot be 0 at the same time.)

[0102] In chemical formula (3), it is preferable that l is 1, 2, 3, 4, 5, or 6, and m and n are 0; or that m is 1, 2, or 3, and l and n are 0; or that n is 1 or 2, and l and m are 0.

[0103] Examples of compounds represented by chemical formula (2) include ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, neopentin glycol di(meth)acrylate, and glycerin di(meth)acrylate. Of these, ethylene glycol dimethacrylate (EGDMA) is preferred.

[0104] Preferred examples of acrylic resins include homopolymers of methyl (meth)acrylate, homopolymers of butyl (meth)acrylate, copolymers of methyl (meth)acrylate and ethylene glycol di(meth)acrylate, and copolymers of butyl (meth)acrylate and ethylene glycol di(meth)acrylate. Of these, methacrylate-type acrylic resins are preferred. A more preferred example of an acrylic resin is a copolymer of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA).

[0105] In this embodiment, the acrylic resin is preferably in the form of fine particles. Here, fine particles refer to particles with a particle diameter of 300.0 μm or less. The particle diameter is preferably 270.0 μm or less, more preferably 200.0 μm or less, even more preferably 100.0 μm or less, even more preferably 50.0 μm or less, even more preferably 20.0 μm or less, even more preferably 15.0 μm or less, even more preferably 13.0 μm or less, even more preferably 10.0 μm or less, and even more preferably 6.0 μm or less. On the other hand, the lower limit of the particle diameter is not particularly limited, but is usually, for example, 0.1 μm or more, and preferably 1.0 μm or more. The particle diameter is the value (median diameter) measured by the particle size distribution analyzer PSA1090L manufactured by Anton Paar.

[0106] The acrylic resin may be spherical fine particles or porous fine particles. If the acrylic resin is porous, its oil absorption capacity is preferably 100 mL / 100 g or more, more preferably 110 mL / 100 g or more, even more preferably 120 mL / 100 g or more, even more preferably 130 mL / 100 g or more, and even more preferably 140 mL / 100 g or more. The oil absorption capacity is a value measured in accordance with JIS K 5101-13-2:2004. More specifically, it can be measured by the method described in paragraph 0069 of Japanese Patent Application Publication No. 2017-88501.

[0107] As long as the acrylic resin has the above structure, the Mw is not particularly limited. However, the Mw of acrylic resin is usually in the range of 2,000 to 1,500,000. Mw is a value measured by gel permeation chromatography (GPC) (calibrated with polystyrene).

[0108] Acrylic resins are commercially available or can be manufactured by conventional methods that are within the knowledge of those skilled in the art. Examples of commercially available acrylic resins include those manufactured by Sekisui Chemical Co., Ltd.

[0109] Examples of diene rubbers include natural rubber, isoprene rubber, and butadiene rubber such as high-cis polybutadiene rubber. Diene rubbers are commercially available or can be manufactured by conventional methods within the scope of the knowledge of those skilled in the art.

[0110] Condensates of Substituted Aromatic Hydrocarbons and Sulfur Chloride Examples of condensates of substituted aromatic hydrocarbons and sulfur chloride include condensates of alkylphenols and sulfur chloride. Specific examples of alkylphenol-sulfur chloride condensates include Takkirol V200, TS3108, and TS3109 manufactured by Taoka Chemical Industries, Ltd., and Vultac3 manufactured by Arkema. One or more condensates of substituted aromatic hydrocarbons and sulfur chloride can be used.

[0111] <<CNT>> The above-mentioned CNTs can be used. CNTs can be used not only as a calcination raw material, but also as a conductive additive, as described below. Therefore, the amount of CNTs to be added as a calcination raw material is the content A in the electrode. CNT As long as the formulation is such that it falls within the aforementioned range, it is not particularly limited. There is no particular restriction on whether CNTs are incorporated as a calcination raw material or as a conductive additive, and an appropriate A CNT It is not particularly limited as long as the goal is achieved.

[0112] <<Carbon Materials Other Than CNTs>> Any carbon material other than CNTs commonly used in this field can be used. Preferably, the carbon material has a graphite structure. Preferably, the carbon material is conductive. Examples of such carbon materials include porous carbon materials such as activated carbon, graphite, carbon black, acetylene black, Ketjenblack, as well as carbon fibers such as carbon fiber, vapor-grown carbon fiber (VGCF), and carbon nanofiber, and nanocarbon materials in shapes other than carbon fiber such as graphene and fullerene. Of these, carbon fibers such as carbon fiber, vapor-grown carbon fiber (VGCF), and carbon nanofiber are preferred. One or more types of such carbon materials can be used.

[0113] From the viewpoint of improving battery life, the content of carbon material excluding CNTs is preferably less than 5 parts by mass per 100 parts by mass of the organic compound. More preferably, the content is less than 1 part by mass, and even more preferably less than 0.5 parts by mass. On the other hand, the content is preferably greater than 0.05 parts by mass, more preferably greater than 0.07 parts by mass, and even more preferably 0.10 parts by mass or more. On the other hand, if the carbon material contains CNTs, the CNT content is the content of CNTs in the electrode A CNT It is within the range.

[0114] <Sulfur> Various forms of sulfur can be used, such as powdered sulfur, insoluble sulfur, precipitated sulfur, and colloidal sulfur, but precipitated sulfur and colloidal sulfur are preferred. One or more types of sulfur can be used.

[0115] From the viewpoint of improving the performance of electrodes and / or batteries, the sulfur content in the calcined raw materials is preferably more than 50 parts by mass, more preferably more than 100 parts by mass, even more preferably more than 200 parts by mass, even more preferably more than 300 parts by mass, even more preferably more than 400 parts by mass, and even more preferably 500 parts by mass or more, per 100 parts by mass of organic compound. A content of more than 50 parts by mass tends to improve battery life. On the other hand, there is no particular upper limit for the sulfur content, but it is preferably less than 1000 parts by mass, more preferably less than 900 parts by mass, even more preferably less than 800 parts by mass, and even more preferably less than 700 parts by mass. A content of less than 1000 parts by mass tends to be cost-advantageous. In this specification, "cycle characteristics" refers to the characteristic that the charge / discharge capacity of a secondary battery is maintained despite repeated charging and discharging. Therefore, secondary batteries that experience a large decrease in charge / discharge capacity and have a low capacity retention rate with repeated charging and discharging have poor cycle characteristics, while secondary batteries that experience a small decrease in charge / discharge capacity and have a high capacity retention rate have superior cycle characteristics.

[0116] While various allotropes of sulfur can be used, it is preferable to use sulfur S8, which is solid at room temperature and pressure, and more preferably elemental sulfur S8.

[0117] <<Raw Metal Compounds>> When the electrode active material contains a metal compound comprising at least one selected from the group consisting of iron, molybdenum, vanadium, and titanium, a raw metal compound containing the corresponding metal is used as the calcination raw material.

[0118] 《Raw Iron Compounds》 When the metal is iron, the raw iron compounds include iron compounds containing divalent or trivalent iron ions, but are not particularly limited as long as they decompose during calcination and react with sulfur to produce iron disulfide, and various types can be used. Examples of raw iron compounds include iron salts and iron complexes. Examples of iron salts include both organic iron salts and inorganic iron salts. On the other hand, iron complexes include neutral iron complexes and salts of iron complex ions (iron complex salts). Of these, organic iron salts, inorganic iron salts, or neutral iron complexes are preferred. One or more raw iron compounds can be used.

[0119] Examples of iron organic acid salts include divalent iron (Fe 2+ ) and organic acid salts, or trivalent iron (Fe 3+ Examples include salts of iron and organic acids. Of these, salts of divalent iron and organic acids are preferred. The organic acids are not particularly limited, but include those having a carboxyl group (-COOH) or a sulfo group (-SO3H), but those having a carboxyl group are preferred. Specific examples of organic acids include fatty acids, oxalic acid, tartaric acid, citric acid, malic acid, succinic acid, etc. Specific examples of fatty acids include acetic acid, propionic acid, butyric acid, etc., which have 1 to 6 carbon atoms. Of these, acetic acid and oxalic acid are preferred. Preferred examples of iron organic acid salts include iron(II) acetate and iron(II) oxalate. These may also be hydrates. One or more types of iron organic acid salts can be used.

[0120] Examples of inorganic iron salts include divalent iron (Fe 2+) and inorganic acid salts, or trivalent iron (Fe 3+ Examples include salts of inorganic acids. Specifically, examples of inorganic acids include hydrochloric acid, sulfuric acid, and nitric acid. Of these, nitric acid is preferred. Preferred examples of inorganic iron salts include iron(II) chloride, iron(III) chloride, iron(II) sulfate, iron(III) sulfate, iron(II) nitrate, and iron(III) nitrate. These may also be hydrates. One or more types of inorganic iron salts can be used.

[0121] Examples of iron complexes include divalent iron (Fe 2+ ) complexes, and trivalent iron (Fe 3+ Examples of iron complexes include those of the following: The iron complex may be in the form of a neutral complex or a complex salt. The ligands that coordinate to the iron ion are not particularly limited and include, for example, chlorine atoms, halogen atoms such as bromine atoms, cyano groups, dicyclopentadienyl groups, and N,N'-bis(salicylidene)ethylenediamine. Examples of iron complexes include potassium hexacyanoferrate(II) ([Fe(CN)6]K4), potassium hexacyanoferrate(III) ([Fe(CN)6]K3), sodium tetrachloroferrate(III) ([FeCl4]Na), dicyclopentadienyl iron(II) (ferrocene), and N,N'-bis(salicylidene)ethylenediaminetoferrate(III) chloride. One or more iron complexes can be used.

[0122] 《Molybdenum Compounds as Raw Materials》 When the metal is molybdenum, examples of molybdenum compounds as raw materials include molybdenum trioxide (VI), sodium molybdate (VI), hexaammonium heptamolybdate (VI), diammonium molybdate (VI), calcium molybdate (VI), molybdic acid (VI), phosphomolybdic acid (VI), and molybdenum disulfide (VI). One or more types of molybdenum compounds as raw materials can be used.

[0123] <Vanadium Compounds as Raw Materials> When the metal is vanadium, examples of vanadium compounds as raw materials include vanadium pentoxide (V), ammonium metavanadate (V), vanadium oxytrichloride (V), sodium metavanadate (V), potassium vanadate (V), sodium vanadate (V), vanadium tetrachloride (IV), vanadium oxysulfate (IV), vanadium oxydichloride (IV), vanadium oxide (IV), vanadium trichloride (IV), vanadium oxide (III), and hexavanadium decantoxide (IV, V). One or more types of vanadium compounds can be used as raw materials.

[0124] 《Raw Titanium Compounds》 When the metal is titanium, examples of raw titanium compounds include titanium oxide, titanium dioxide, titanium dioxide, titanium trioxide, and titanium tetrachloride. One or more raw titanium compounds can be used.

[0125] <Content of raw material metal compounds> From the viewpoint of improving the performance of electrodes and / or batteries, the content of raw material metal compounds in the calcination raw materials is preferably 50 parts by mass or more and 300 parts by mass or less per 100 parts by mass of the organic compound. The content is more preferably more than 50 parts by mass, even more preferably more than 60 parts by mass, even more preferably more than 70 parts by mass, and even more preferably more than 75 parts by mass. On the other hand, the content is more preferably less than 250 parts by mass, even more preferably less than 200 parts by mass, even more preferably less than 150 parts by mass, and even more preferably 100 parts by mass or less.

[0126] 《Median Diameter of Raw Metal Compounds》 When using raw metal compounds as calcination raw materials, it is preferable to crush them beforehand. The median diameter (d50) of the raw metal compound is preferably 12.00 μm or less, and more preferably 10.00 μm or less, more preferably 8.00 μm or less, even more preferably 6.00 μm or less, even more preferably 4.00 μm or less, and even more preferably 3.00 μm or less. On the other hand, there is no particular limit to the lower limit of the median diameter, but it is usually around 0.10 μm or more, and may be around 1.00 μm or 2.00 μm. The median diameter can be measured by the method described above.

[0127] 《Specific Surface Area of ​​Raw Metal Compounds》 The specific surface area of ​​raw metal compounds is 1.0 m². 2 It is preferable that the amount be 2.0 m / g or more, and more preferably 2.0 m / g or more. 2 / g or more, more preferably 3.0m 2 / g or more, more preferably 4.0m 2 / g or more, more preferably 4.5m 2 It is 1 / g or more. On the other hand, there is no particular limit on the upper limit of the specific surface area, but it is usually 40.0 m². 2 It is approximately 1 / g or less, and 20.0m 2 It may be less than or equal to 10.0 m 2 It may be around 1 / g or less. The specific surface area can be measured using a fully automatic specific surface area measuring device, Macorb (HM-model 1201, manufactured by Mountec Co., Ltd.).

[0128] The raw material metal compounds having the median diameter or specific surface area described above can be prepared by conventional methods, for example, by grinding the metal compound in a grinder. As such a grinder, for example, one manufactured by Nippon Analytical Industry Co., Ltd. (e.g., JFC-2000) can be used.

[0129] <<Other Materials>> The raw materials may, as desired, include other materials commonly used in this field.

[0130] [Mixing Process (1)] The mixing process is a process for preparing the calcination raw materials. The mixing process can be carried out by mixing sulfur, an organic compound, and other optional components. The other optional components include CNTs when they are included in the electrode active material.

[0131] The above mixing can be carried out by conventional methods and is not particularly limited as long as the method ensures that these components are thoroughly mixed. In this embodiment, at least the following methods, such as mixing by the wet method, mixing by the dry method, or mixing by monomer polymerization, can be listed as preferred mixing methods.

[0132] ≪WET Method≫ In this embodiment, the WET method, in preparing the raw materials, includes the steps of: (a-1) adding an organic compound and other optional components excluding sulfur to a solvent such as an organic solvent to obtain a mixture; (a-2) removing the solvent from the mixture to obtain a dry mixture; and (a-3) mixing the dry mixture with sulfur.

[0133] In step (a-1), the method of adding the organic compound and other optional components to the organic solvent is not particularly limited, as long as they can be mixed to obtain a mixture. For example, (1) the organic compound and other optional components may be added to the organic solvent and mixed simultaneously; (2) the organic compound may be added to the organic solvent and mixed, and then other optional components may be added and mixed afterward; or (3) the organic compound may be added to the organic solvent and mixed, and then other optional components may be added and mixed afterward.

[0134] In step (a-1), any organic solvent commonly used in this field can be used. Examples of such solvents include N-methyl-2-pyrrolidone, N,N-dimethylformaldehyde, alcohols, hexane, water, acetone, ethers such as tetrahydrofuran, etc. Furthermore, it is preferable that the organic solvent dissolves organic compounds, as this contributes to good mixing. One or more of these solvents can be used.

[0135] Step (a-1) can be carried out, for example, by stirring in a container such as a beaker.

[0136] In step (a-2), the removal of organic solvents can be carried out by conventional methods. For example, this removal can be performed by subjecting the mixture from step (a-1) to drying methods such as heating and drying, vacuum drying, or vacuum heating and drying.

[0137] It is preferable to grind the resulting dry mixture before proceeding to the next step. This is because it is expected that the mixing in step (a-3) will be carried out more effectively.

[0138] In step (a-3), the mixing of the dry mixture and sulfur can be carried out by conventional methods, such as using a blender.

[0139] When CNTs or raw material metal compounds are used as optional components, the CNTs or raw material metal compounds can be mixed in step (a-1), and the order in which the CNTs or raw material metal compounds are added to the organic solvent is not particularly limited, as with other raw materials, and they may be added in any order.

[0140] <<DRY Method>> In this embodiment, the DRY method is a method that, in preparing the raw materials, includes (b-1) a step of mixing the organic compound, sulfur, and other optional components, all in powder form.

[0141] Here, "powder" refers to a state in which each solid raw material has been finely ground to a degree suitable for mixing under the purpose of this embodiment. The size of each particle constituting the powder is not particularly limited as long as mixing is carried out appropriately, but is usually in the range of, for example, 1 μm to 40 μm. From the viewpoint of improving the performance of electrodes and / or batteries, the size of the particles is preferably 2 μm or more, more preferably 3 μm or more, even more preferably 4 μm or more, preferably 30 μm or less, even more preferably 20 μm or less, even more preferably 15 μm or less, and even more preferably 10 μm or less in median diameter. The median diameter can be measured by the method described above.

[0142] This mixing can be carried out by conventional methods, for example, in the same manner as the mixing in step (a-3) above. Furthermore, if a raw material metal compound is added to the raw materials, it can be mixed together with the other materials.

[0143] <<Monomer Polymerization Method>> In this embodiment, the monomer polymerization method includes the following steps in preparing the raw materials: (c-1) Dispersing other optional components, excluding sulfur, in the raw material monomer of an organic compound beforehand, and then subjecting the raw material monomer to a polymerization reaction to obtain an optional component-dispersed polymer; (c-2) Mixing the optional component-dispersed polymer with sulfur.

[0144] A polymer with arbitrary components dispersed is an organic compound in which arbitrary linear segments are dispersed inside or on the surface of the polymer. Such arbitrary components can include carbon nanotubes (CNTs) and raw material metal compounds. A polymer with arbitrary components dispersed can be prepared by first dispersing the arbitrary components in a raw material monomer of an organic compound, and then subjecting the monomer to a polymerization reaction. This polymerization reaction can be carried out by conventional methods.

[0145] Specific examples of optional component-dispersed polymers include, for example, acrylic resins in which iron ion-containing compounds and CNTs are dispersed in an acrylic resin, and which are CNT and iron ion-containing compound-dispersed acrylic resins. Preferred examples of acrylic resins used for this purpose include homopolymers of methyl (meth)acrylate and copolymers of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA). Preferred examples of iron ion-containing compounds used for this purpose include iron(II) oxalate.

[0146] It is preferable to pulverize the resulting optional component-dispersed polymer before proceeding to the next step. This is because it is expected that the mixing in step (c-2) will be carried out more effectively.

[0147] Step (c-2) can be carried out in the same manner as step (a-3) above.

[0148] In all methods, including the wet method, dry method, and monomer polymerization method, it is desirable to thoroughly mix the raw materials beforehand in preparation for firing.

[0149] The raw materials thus obtained may be used as is in the next firing process, or, if desired, they may be molded into pellets before being used in the next process.

[0150] [Firing Process (2)] The firing process is a process of firing the firing material obtained above. Firing can be carried out by conventional methods, for example, by heating the firing material at a predetermined heating rate until it reaches a predetermined temperature, maintaining that temperature for a predetermined time, and then allowing it to cool naturally.

[0151] <Non-oxidizing atmosphere> It is preferable to carry out the firing under a non-oxidizing atmosphere. A non-oxidizing atmosphere is an atmosphere that substantially does not contain oxygen and is used to suppress oxidative degradation of the constituent components and excessive thermal decomposition. Specifically, this refers to an inert gas atmosphere such as nitrogen or argon, a sulfur gas atmosphere, an ammonia gas atmosphere, etc. Therefore, firing can be suitably carried out, for example, in a quartz tube under an inert gas atmosphere.

[0152] <<Heating Rate>> The heating rate is preferably in the range of 50°C / h to 500°C / h. The heating rate is preferably 80°C / h or higher, more preferably 100°C / h or higher, and even more preferably 120°C / h or higher. On the other hand, the heating rate is more preferably 400°C / h or lower, even more preferably 350°C / h or lower, and even more preferably 300°C / h or lower. When the heating rate is within this range, it tends to be easier to achieve the objective of improving battery life.

[0153] ≪Firing Temperature and Time≫ The firing temperature is the temperature after the raw material has been heated up and is maintained for a certain period of time for the firing of the raw material. The temperature is preferably in the range of over 250°C and under 550°C. A temperature above 250°C tends to prevent insufficient sulfidation reaction and prevent a decrease in the charge / discharge capacity of the target product. On the other hand, a temperature below 550°C tends to prevent the decomposition of the raw material and prevent a decrease in yield and charge / discharge capacity. A temperature above 300°C is more preferable, over 350°C is even more preferable, and 370°C or higher is even more preferable. On the other hand, a temperature below 500°C is more preferable, and under 450°C is even more preferable.

[0154] From the viewpoint of improving the performance of the electrodes and / or batteries, the firing temperature in the firing process is preferably higher than the temperature at which the raw material metal compound undergoes thermal decomposition.

[0155] The time for maintaining the firing temperature can be set appropriately depending on the type of raw material, firing temperature, etc., but it is preferable to maintain it for at least 1 hour and no more than 6 hours. Maintaining it for at least 1 hour tends to allow the firing to proceed sufficiently, while maintaining it for at least 6 hours tends to prevent excessive thermal decomposition of the constituent components.

[0156] <<Equipment>> Firing can be carried out, for example, by a muffle furnace, or by using a continuous device such as a twin-screw extruder. When using a continuous device, there is the advantage that sulfur-based electrode active material can be produced continuously in a series of operations, such as kneading, crushing, mixing, and firing the raw materials within the device.

[0157] A muffle furnace is a type of furnace designed to prevent contamination of the sample by having the heat source (heater) enclosed by a heat plate or similar partition so that it is not exposed inside the furnace. One example of a muffle furnace has a heater at the bottom of the furnace, which is enclosed by a heat plate. A lid is installed on the front of the furnace, and the structure is designed to maintain an inert gas atmosphere inside the furnace. A thermocouple is attached to the lid so that the temperature inside the furnace can be measured during firing. Inside the furnace, there are two trays, for example, an upper and a lower tray, which are rectangular reaction vessels made of stainless steel for firing the raw materials.

[0158] The furnace is designed to continuously supply and discharge gas (for example, an inert gas such as argon (Ar) gas) from the outside through gas inlet pipes and gas outlet pipes. The gas outlet pipes are connected to a trap tank containing, for example, an aqueous sodium hydroxide solution. The exhaust gas that is about to leave the muffle furnace through the gas outlet pipes passes through the aqueous sodium hydroxide solution in the trap tank before being released to the outside. Therefore, even if the exhaust gas contains hydrogen sulfide gas generated by the reaction, it is neutralized by the aqueous sodium hydroxide solution and removed from the exhaust gas.

[0159] [Residue Removal Process] The processed material obtained after calcination may contain unreacted sulfur that has precipitated from sublimated sulfur during calcination. Since these residues can degrade the cycle characteristics, it is desirable to remove them as much as possible if present. Residue removal can be carried out by conventional methods such as vacuum heating and drying, hot air drying, and solvent washing.

[0160] [Grinding and Classification] The obtained electrode active material is preferably ground to a predetermined particle size and classified to obtain particles of a size suitable for electrode manufacturing. The preferred particle size range for the electrode active material is as described above.

[0161] Grinding can be carried out by conventional methods, for example, by using a grinder such as a cutter mill or jet mill and subjecting the material to grinding treatment under predetermined conditions. The grinding conditions vary depending on the mill used, but for example, when using a cutter mill (for example, the Free Speed ​​Mill FS-20 manufactured by Labonect Co., Ltd.), the processing can be performed at a rotation speed of 20,000 rpm to 30,000 rpm for 1 second to 30 seconds. When using a dry jet mill (for example, the NanoJet Mizer NJ-30 manufactured by Aisin Nano Technologies Co., Ltd.), the processing speed can be 1 g / min to 3 g / min and the grinding pressure can be 0.5 MPa to 2.0 MPa. Classification can be carried out, for example, using a sieve.

[0162] Furthermore, in the firing method using the twin-screw extruder described earlier, the shearing during kneading allows for the simultaneous production of the electrode active material and the pulverization of the produced electrode active material into particles.

[0163] (Manufacturing of electrodes) The electrodes of this embodiment can be manufactured by preparing an electrode material containing the electrode active material obtained above using a conventional method, and then manufacturing an electrode material layer made of the electrode material. In other words, the electrode of this embodiment is the electrode material layer itself made of the electrode material, and does not include a current collector. The electrode of this embodiment may be formed on a current collector, or it may consist only of an electrode material layer without a current collector. The electrode can be obtained in the same way as when manufacturing electrodes for general lithium-ion secondary batteries using the electrode active material described above. However, since the electrodes of this embodiment always contain CNTs, if the electrode active material does not contain CNTs, it must contain CNTs as a conductive additive as described below.

[0164] [When using electrode active material as positive electrode active material] In this case, the positive electrode according to this embodiment can be manufactured in the same manner as a general positive electrode for lithium-ion secondary batteries, except that the electrode active material is used as the positive electrode active material and the positive electrode always contains CNTs. For example, the positive electrode can be manufactured by mixing the electrode active material with a conductive additive, a binder, and a solvent to prepare a paste-like positive electrode material, molding the positive electrode material, and drying it. Alternatively, if the lithium-ion secondary battery has a current collector, the positive electrode can be manufactured by applying the positive electrode material to the current collector and then drying it. Or, the positive electrode can also be manufactured, for example, by kneading the electrode active material with a conductive additive, a binder, and a small amount of solvent using a mortar and pestle, forming it into a film, and then pressing it onto the current collector using a press.

[0165] <<Conductive Additives>> Any conductive additive commonly used in this field can be used, and conductive carbon materials among those mentioned above can also be used. More specifically, examples include vapor-grown carbon fiber (VGCF), carbon powder, carbon black (CB), acetylene black (AB), Ketjenblack (KB), graphite, CNTs, or fine powders of metals that are stable at the positive electrode potential, such as aluminum or titanium. In addition, one or more types of conductive additives can be used. However, since the electrode in this embodiment always contains CNTs, if the electrode active material does not contain CNTs, then CNTs must be included as the conductive additive.

[0166] Examples of binders include polyvinylidene difluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyimide (PI), polyamideimide (PAI), carboxymethylcellulose (CMC), polyvinyl chloride (PVC), acrylic resin, methacrylic resin (PMA), polyacrylonitrile (PAN), modified polyphenylene oxide (PPO), polyethylene oxide (PEO), polyethylene (PE), and polypropylene (PP). One or more types of binders may be used.

[0167] <Solvent> Examples of solvents include N-methyl-2-pyrrolidone, N,N-dimethylformaldehyde, alcohols, hexane, and water. One or more solvents may be used.

[0168] <<Formulation Amount>> The proportions of the materials constituting these positive electrodes are not particularly limited, but for example, it is preferable to blend 2 to 100 parts by mass of conductive additive, 2 to 50 parts by mass of binder, and an appropriate amount of solvent with 100 parts by mass of electrode active material.

[0169] <Current Collector> As the current collector, one that is commonly used for the positive electrode of a lithium-ion secondary battery can be used. For example, examples of current collectors include metal foils such as aluminum foil, aluminum mesh, perforated aluminum sheet, expanded aluminum sheet, stainless steel foil, stainless steel mesh, perforated stainless steel sheet, expanded stainless steel sheet, foamed nickel, nickel nonwoven fabric, copper foil, copper mesh, perforated copper sheet, expanded copper sheet, titanium foil, titanium mesh, as well as carbon nonwoven fabric, carbon woven fabric, etc. Among these, current collectors containing metal foil are preferred. One type of current collector may be used, or two or more types may be used in combination. The surface of the current collector may be coated with carbon or the like. A specific example of a current collector whose surface is coated with carbon or the like is, for example, carbon-coated aluminum foil. In this case, the current collector includes the carbon-coated portion.

[0170] [When using electrode active material as negative electrode active material] In this case, the negative electrode according to this embodiment can be manufactured in the same manner as a general negative electrode for lithium-ion secondary batteries, except that the electrode active material is used as the negative electrode active material and the negative electrode always contains CNTs. For example, the negative electrode can be manufactured by mixing the electrode active material with a conductive additive, a binder, and a solvent to prepare a paste-like negative electrode material, molding the negative electrode material, and drying it. Alternatively, if the lithium-ion secondary battery has a current collector, the negative electrode can be manufactured by applying the negative electrode material to the current collector and then drying it. Or, the negative electrode can also be manufactured, for example, by kneading the electrode active material with a conductive additive, a binder, and a small amount of solvent using a mortar and pestle, forming it into a film, and then pressing it onto the current collector using a press.

[0171] The conductive additive, binder, and solvent can be the same as those used in the above case where the electrode active material is the positive electrode active material, and the current collector can also be the same.

[0172] (Manufacturing of Lithium-ion Secondary Batteries) The lithium-ion secondary battery of this embodiment can be manufactured in the same manner as a general lithium-ion secondary battery, except that the electrodes obtained above and an electrolyte containing a fluorine compound are used.

[0173] [When using the positive electrode obtained above] The lithium-ion secondary battery of this embodiment can be manufactured by conventional methods using the positive electrode obtained above, a negative electrode, an electrolyte containing a fluorine compound, and optionally, components such as a separator.

[0174] <<Negative Electrode>> As negative electrode materials, known materials such as metallic lithium, carbon-based materials such as graphite, silicon-based materials such as silicon thin films, and alloy-based materials such as copper-tin and cobalt-tin can be used. When using lithium-free materials as negative electrode materials, for example, carbon-based materials, silicon-based materials, or alloy-based materials from the above-mentioned negative electrode materials, it is advantageous in that it is less likely to cause short circuits between the positive and negative electrodes due to the generation of dendrites. However, when these lithium-free negative electrode materials are used in combination with the positive electrode of this embodiment, neither the positive nor the negative electrode contains lithium. For this reason, lithium pre-doping treatment is required to pre-insert lithium into either the negative electrode or the positive electrode, or into both. For lithium pre-doping, known methods can be followed. For example, when doping lithium into the negative electrode, one method is to assemble a half-cell using metallic lithium as the counter electrode and insert lithium by electrolytic doping, or by adhesive pre-doping, in which metallic lithium foil is attached to the electrode and left in an electrolyte solution, utilizing the diffusion of lithium into the electrode to insert lithium. Furthermore, the electrolytic doping method described above can also be used when pre-doping lithium into the positive electrode. As a lithium-free negative electrode material, silicon-based materials, which are particularly high-capacity negative electrode materials, are preferred, and among them, thin-film silicon, which has a thin electrode thickness and is advantageous in terms of capacity per unit volume, is more preferred.

[0175] ≪Electrolyte≫ The electrolyte can be the one described above.

[0176] <Separator> In addition to the negative electrode, positive electrode, and electrolyte described above, lithium-ion secondary batteries may also include components such as separators. The separator is interposed between the positive electrode and the negative electrode, allowing the movement of ions between the positive and negative electrodes while preventing internal short circuits between them. If the lithium-ion secondary battery is a sealed type, the separator is also required to have the function of holding the electrolyte. As the separator, it is preferable to use a thin-walled, microporous or nonwoven membrane made of polyethylene, polypropylene, polyacrylonitrile, aramid, polyimide, cellulose, glass, or the like.

[0177] <Shape> The shape of lithium-ion secondary batteries is not particularly limited and can be in various shapes such as cylindrical, stacked, coin-type, laminated, and button-type.

[0178] [When using the negative electrode obtained above] The lithium-ion secondary battery of this embodiment can be manufactured by conventional methods using the negative electrode obtained above, a positive electrode, an electrolyte containing a fluorine compound, and optionally, components such as a separator.

[0179] <Positive Electrode> The positive electrode material is not particularly limited as long as it is a transition metal oxide or solid solution oxide containing lithium, or a substance that can electrochemically intercept and release lithium ions. Examples of transition metal oxides containing lithium include Li-Co composite oxides such as LiCoO2, LiNi x Co y Mn z Examples include Li-Ni-Co-Mn composite oxides such as O2, Li-Ni composite oxides such as LiNiO2, or Li-Mn composite oxides such as LiMn2O4. As a solid solution oxide, for example, Li a Mn x Co y Ni z O2 (1.150≦a≦1.430, 0.450≦x≦0.600, 0.100≦y≦0.150, 0.200≦z≦0.280), LiMn x Co y Ni z O2 (0.300≦x≦0.850, 0.100≦y≦0.300, 0.100≦z≦0.300), LiMn1.5 Ni 0.5 Examples include O4. These compounds can be used individually or in combination of two or more.

[0180] <Other> The electrolyte, separator, and lithium-ion secondary battery can also be the same as those used when "using the positive electrode obtained above."

[0181] The present invention will be described based on examples, but the present invention is not limited to the examples.

[0182] The chemicals and materials used in the examples and comparative examples are summarized below. These were purified according to conventional methods as necessary.

[0183] <Materials used in the test> Acrylate compound (1): Methyl methacrylate (Methyl methacrylate manufactured by Tokyo Chemical Industry Co., Ltd., purity > 99.8%) Diacrylate compound (2): Ethylene glycol dimethacrylate (Ethylene glycol dimethacrylate manufactured by Tokyo Chemical Industry Co., Ltd., purity > 97.0%) Raw material metal compound (iron compound): Iron(II) oxalate dihydrate (Iron(II) oxalate dihydrate manufactured by Kanto Chemical Co., Ltd., special grade) Carbon nanotube (CNT): CNT dispersion (SWCNT dispersion manufactured by Kusumoto Chemical Co., Ltd., dispersion medium: N-methyl-2-pyrrolidone, average fiber diameter: 1.6 nm, average fiber length: 2-10 μm, specific surface area: 800-1600 m 2 ( / g, G / D ratio: 40 or more, metal impurity amount: 1% by mass or less) Sulfur: Precipitated sulfur manufactured by Tsurumi Chemical Industries, Ltd. Conductive additive 1: CNT (SWCNT dispersion manufactured by Kusumoto Chemicals, Ltd. as described above) Conductive additive 2: Acetylene black (HS-100 manufactured by Denka Co., Ltd.) Conductive additive 3: Vapor-grown carbon fiber (VGCF manufactured by Showa Denko K.K.) Binder: Acrylic resin (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., average molecular weight 2700-7500) LiTFSI: Bis(trifluoromethanesulfonyl)imide lithium EMC: Ethyl methyl carbonate FEC: Fluoroethylene carbonate

[0184] Manufacturing Example 1 (Fine Grinding of Raw Metal Compounds) Before use as a raw material, the raw metal compound was ground for 10 minutes using a cryogenic grinder (JFC-2000 manufactured by Nippon Analytical Industry Co., Ltd.).

[0185] Manufacturing Example 2 (Preparation of Metal Compound Equidispersed Polymer by Monomer Polymerization Method) Before preparing the calcination raw materials, a metal compound equidispersed polymer was prepared in which raw material metal compounds and CNTs were dispersed in an organic compound by monomer polymerization. The preparation of the metal compound equidispersed polymer was carried out as follows: According to the formulation in Table 1, 80 parts by mass of acrylate compound (1) and 20 parts by mass of diacrylate compound (2) were mixed, and then 100 parts of raw material metal compound and 0.5 to 1.01 parts of CNTs were dispersed to prepare a mixture, which was then subjected to a polymerization reaction. The obtained metal compound equidispersed polymer was an acrylic resin in which CNTs and raw material metal compounds were dispersed inside and on its surface. The polymerization conversion rate was 100%.

[0186] <Preparation of electrode active material> (Preparation of calcination raw material) The metal compound homodisperse polymer obtained above and sulfur were mixed using a blender to obtain the raw material for calcination.

[0187] (Reaction apparatus) A muffle furnace was used for calcining the raw materials. The muffle furnace is as described above.

[0188] (Casturing Process) First, with the calcination raw materials contained in a stainless steel reaction vessel tray, the atmosphere inside the muffle furnace was replaced three times with Ar gas using a vacuum pump. Then, while continuously supplying Ar gas at a flow rate of 100 mL / min through the gas introduction pipe, heating of the muffle furnace was started 30 minutes after the start of supply. The temperature was increased at a heating rate of 5°C / min, and when the temperature of the calcination raw materials reached the calcination temperature shown in Table 1, the heat treatment was carried out for 2 hours while maintaining that temperature. Next, while adjusting the flow rate of Ar gas, the calcined product was allowed to cool naturally to 25°C in an Ar gas atmosphere, and then the calcined product was removed from the muffle furnace.

[0189] (Removal of unreacted sulfur) To remove unreacted sulfur (elemental sulfur in a free state) remaining in the product after the calcination process, the following steps were performed. Specifically, the calcined material was crushed in a mortar, the crushed material was placed in a glass tube oven, and heated at 250°C for 3 hours under vacuum suction to obtain an electrode active material from which unreacted sulfur had been removed (or which contained only a trace amount of unreacted sulfur). The heating rate was 10°C / min.

[0190] (Grinding process) The calcined material from which unreacted sulfur had been removed was ground using a cutter mill (Free Speed ​​Mill, FS-20, manufactured by Labonect Co., Ltd.).

[0191] (Classification process) To remove coarse particles from the calcined material after crushing, the electrode active material was obtained by classification using a 32 μm mesh stainless steel sieve.

[0192] <Physical Properties of Electrode Active Materials> The following properties were investigated for the electrode active materials obtained above.

[0193] (Elemental Analysis) The elemental amounts of carbon, hydrogen, nitrogen, and sulfur were calculated as the mass ratio (%) of each element in the total amount of electrode active material from the mass measured using an Elementar vario MICRO cube fully automated elemental analyzer. The results are shown in Table 1.

[0194] (Particle size distribution, median diameter) The particle size distribution was measured using a laser diffraction / scattering particle size distribution analyzer (PSA1090L manufactured by Anton Paar) with water as the dispersion medium, and a particle size distribution curve was obtained. From the particle size distribution curve, the volume-based cumulative 50% diameter (d50) was defined as the median diameter (μm).

[0195] (CNT content) The CNT content in the electrode active material was calculated from the ratio of the mass of the obtained electrode active material to the mass of CNTs in the electrode active material. The mass of CNTs in the electrode active material was calculated assuming that the CNTs did not undergo reactions such as sulfidation or decomposition during firing and that their weight remained unchanged before and after firing.

[0196] The results are shown in Table 1 below.

[0197]

[0198] <Fabrication of Lithium-ion Secondary Batteries> Lithium-ion secondary batteries were fabricated as follows.

[0199] Example 1 (Positive Electrode) As described in Table 2, the electrode active material from Manufacturing Example B obtained above, conductive additive 2 (acetylene black), conductive additive 3 (vapor-grown carbon fiber), and binder (acrylic resin) were weighed in a mass ratio of active material:conductive additive 2:conductive additive 3:binder = 85:5:5:5. The weighed materials were placed in a container, and a uniform electrode slurry was prepared by stirring and mixing using a rotation-orbit mixer (ARE-250 manufactured by Thinky Co., Ltd.) with milliQ water as a dispersant. The prepared electrode slurry was applied to 17 μm aluminum foil using an applicator with a slit width of 100 μm, and the electrode, which was compressed using a roll press, was heated in a dryer at 120°C for 3 hours. After drying, the electrode (positive electrode) was obtained by punching it out to a diameter of φ11 mm. Subsequently, the mass of the electrode was measured, and the amount of active material in the electrode was calculated from the above ratio.

[0200] (Negative electrode) A metallic lithium foil (14 mm in diameter, 500 μm thick, disc-shaped, manufactured by Honjo Metal Co., Ltd.) was used as the negative electrode. A stainless steel sheet was used as the negative electrode current collector.

[0201] (Electrolyte) As the electrolyte, a non-aqueous electrolyte was used, prepared by dissolving LiTFSI (bis(trifluoromethanesulfonyl)imide lithium) in a mixed solvent of EMC (ethyl methyl carbonate) and FEC (fluoroethylene carbonate). The EMC and FEC were mixed in a volume ratio of 7:3. The concentration of LiTFSI in the electrolyte was 1.0 mol / L.

[0202] (Lithium-ion secondary battery) A coin cell was fabricated using the above positive and negative electrodes. Specifically, in a dry room, a separator (Celgard 2400, 25 μm thick polypropylene microporous film) and a glass nonwoven fabric filter (440 μm thick, GA100, ADVANTEC) were sandwiched between the positive and negative electrodes to form an electrode body battery. This electrode body battery was housed in a battery case made of stainless steel (CR2032 type coin cell battery component, manufactured by Hosen Co., Ltd.). The above electrolyte was injected into the battery case. The amount was 0.28 mL in terms of electrolyte volume. The battery case was sealed with a crimping machine to obtain a lithium-ion secondary battery.

[0203] Examples 2 and 3, and Comparative Example 1: Lithium-ion secondary batteries of each example and comparative example were obtained by processing the corresponding materials in the same manner as in Example 1, according to the description in Table 2. However, in the preparation of the electrode slurry, instead of 85 parts by mass of electrode active material, in Example 2, 84.6 parts by mass of electrode active material and 0.4 parts by mass of conductive additive 1 were used, and in Example 3, 84.75 parts by mass of electrode active material and 0.25 parts by mass of conductive additive 1 were used. Here, the amount of conductive additive 1 is the amount added as the net amount of CNTs.

[0204] <Evaluation of Lithium-ion Secondary Batteries> (Discharge Capacity, Capacity Retention Rate) For the coin-type lithium-ion secondary batteries prepared in each example and comparative example, under test temperature conditions of 30°C, the positive electrode active material was charged and discharged at a current value equivalent to 50 mA for the first 10 discharges, and then charged and discharged at a current value equivalent to 100 mA from the 10th charge onward. Here, "10th discharge" refers to the 10th discharge in a cycle where the first discharge is considered the initial charge-discharge, followed by the first charge, second discharge, and second charge. In all charge-discharge cycles, the discharge termination voltage was 1.0 V and the charge termination voltage was 3.0 V. The discharge capacity at the 12th and 150th discharges (mAh / g) were observed. These measurements were performed using a battery performance evaluation device (BLS system, manufactured by Keisokuki Center Co., Ltd.).

[0205] 12th discharge capacity DC 12The initial capacity was defined as (mAh / g). A larger initial capacity indicates a larger charge / discharge capacity for the lithium-ion secondary battery, which is considered preferable. The discharge capacity DC was also determined after 150 cycles. 150 (mAh / g) and the 12th discharge capacity DC 12 The capacity retention rate (%) was calculated from (mAh / g) using the following formula. A higher capacity retention rate indicates that the lithium-ion secondary battery has superior cycle characteristics. Capacity retention rate (%) = (DC 150 / DC 12 ) × 100

[0206] The results are shown in Table 2 below.

[0207]

[0208] The examples that satisfy formula (1) show high effectiveness, while the comparative examples that do not satisfy formula (1) actually show deterioration. This confirms the technical significance of the present invention.

[0209] Furthermore, in the examples and comparative examples, the conductive additive, binder, and negative electrode were used under common conditions, clearly demonstrating that the configuration of the present invention is the main factor in the manifestation of the effect. Although it is practically difficult to demonstrate this for all combinations, the group of materials used in the present invention are all common materials for lithium secondary batteries, and it has not been confirmed that the effect is lost when other materials are used. Therefore, it has general applicability that can be inferred to other material systems, and the technical effect of the present invention is reaffirmed.

[0210] <Embodiments> Preferred embodiments are shown below.

[0211] <1> A lithium-ion secondary battery comprising an electrode and an electrolyte, wherein the electrode contains an electrode active material containing sulfur in an amount of more than 50% by mass, preferably more than 51.0% by mass, more preferably more than 55.0% by mass, even more preferably more than 56.0% by mass, even more preferably more than 57.0% by mass, even more preferably more than 58.0% by mass, even more preferably more than 59.0% by mass, even more preferably more than 60.0% by mass, even more preferably more than 61.0% by mass, even more preferably more than 62.0% by mass, and even more preferably more than 63.0% by mass, wherein the electrode contains carbon nanotubes, preferably more than 0.25% by mass, more preferably more than 0.30% by mass, even more preferably 0.40% by mass or more, and even more preferably more than 0.50% by mass, and the carbon nanotube content (by mass) in the electrode is A CNT , volume of the electrode (mm 3 ) is V EL If A CNT and V EL A lithium-ion secondary battery that satisfies the following formula (1), or preferably the right-hand side of formula (1) is 1.0, more preferably 1.5, even more preferably 2.0, even more preferably 2.1, even more preferably 2.8, and even more preferably 3.0, wherein the electrolyte contains a fluorine compound. (1) A CNT ×V EL >0.5 <2> The above A CNTA lithium-ion secondary battery according to <1> above, wherein the amount is less than 4.00% by mass, preferably less than 3.00% by mass, more preferably less than 2.00% by mass, even more preferably less than 1.00% by mass, even more preferably less than 0.90% by mass, even more preferably less than 0.80% by mass, even more preferably less than 0.70% by mass, even more preferably less than 0.60% by mass, even more preferably less than 0.55% by mass, and even more preferably 0.52% by mass or less. <3> A lithium-ion secondary battery according to <1> or <2> above, wherein the average length of the carbon nanotubes is greater than 1 μm, preferably greater than 1.5 μm, more preferably 2 μm or more, and the average diameter is less than 100 nm, preferably less than 50 nm, more preferably less than 10 nm, even more preferably less than 5 nm, even more preferably less than 3 nm, and even more preferably less than 2 nm. <4> A lithium-ion secondary battery according to any one of <1> to <3> above, wherein the electrode active material further comprises a metal compound containing at least one selected from the group consisting of iron, molybdenum, vanadium, and titanium. <5> The lithium-ion secondary battery according to <4> above, wherein the metal compound is an iron compound. <6> Coating density of the electrode (mg / cm 2 Let D be A CNT A lithium-ion secondary battery according to any one of the above <1> to <5>, wherein D and satisfy the following formula, or preferably the right-hand side of formula (2) is 1.20, more preferably 1.30, even more preferably 1.40, even more preferably 1.50, even more preferably 1.60, even more preferably 1.70, and even more preferably 1.80. (2) D × A CNT >0.25 <7> Coating density of the electrode (mg / cm³) 2 When D is 2.50 mg / cm³, 2 More preferably 2.80 mg / cm 2 More preferably 3.00 mg / cm 2 More preferably 3.30 mg / cm² 2 More preferably 3.50 mg / cm 2 A lithium-ion secondary battery as described in any one of the above items <1> to <6>, which is greater than the above. <8> The volume of the electrolyte is V LYT(mL) and the coating density of the electrode (mg / cm³) 2 Let D be A CNT D and V LYT A lithium-ion secondary battery according to any one of the above <1> to <7>, wherein the following formula is satisfied, or preferably the right-hand side of formula (3) is 1.00, more preferably 2.00, even more preferably 4.50, even more preferably 5.00, even more preferably 5.50, even more preferably 6.00, even more preferably 6.40, and even more preferably 6.50. (3) D × A CNT / V LYT >0.89 <9> A lithium-ion secondary battery according to any one of the above <1> to <8>, wherein the electrode active material is an organic sulfur compound. <10> A lithium-ion secondary battery according to <9>, wherein the carbon nanotube is contained in the electrode active material which is the organic sulfur compound.

Claims

1. A lithium-ion secondary battery comprising electrodes and an electrolyte, wherein the electrodes contain an electrode active material containing more than 50% by mass of sulfur, the electrodes contain carbon nanotubes, and the carbon nanotube content (by mass) in the electrodes is A CNT , volume of the electrode (mm 3 ) is V EL If A CNT and V EL A lithium-ion secondary battery that satisfies the following formula (1), wherein the electrolyte contains a fluorine compound. (1) A CNT ×V EL >0.5 2. The lithium-ion secondary battery according to claim 1, wherein the right-hand side of equation (1) is 2.

0.

3. A CNT However, the lithium-ion secondary battery according to claim 1 or 2, wherein the amount is less than 4.00% by mass.

4. The lithium-ion secondary battery according to any one of claims 1 to 3, wherein the average length of the carbon nanotubes is greater than 1 μm and the average diameter is less than 100 nm.

5. The lithium-ion secondary battery according to any one of claims 1 to 4, wherein the electrode active material further comprises a metal compound containing at least one selected from the group consisting of iron, molybdenum, vanadium, and titanium.

6. The lithium-ion secondary battery according to claim 5, wherein the metal compound is an iron compound.

7. When the coating density (mg / cm 2 ) of the electrode is D, the lithium-ion secondary battery according to any one of claims 1 to 6, wherein A CNT and D satisfy the following formula. (2) D × A CNT > 0.25 8. The lithium-ion secondary battery according to claim 7, wherein the right-hand side of equation (2) is 1.

20.

9. Coating density of the electrode (mg / cm²) 2 When D is 2.50 mg / cm³, 2 A lithium-ion secondary battery according to any one of claims 1 to 8, which is superior to the above.

10. The volume of the electrolyte is V LYT (mL) and the coating density of the electrode (mg / cm³) 2 Let D be A CNT D and V LYT A lithium-ion secondary battery according to any one of claims 1 to 9, wherein D × A satisfies the following formula. (3) D × A CNT / V LYT >0.89 11. The lithium-ion secondary battery according to claim 10, wherein the right-hand side of equation (3) is 4.

50.

12. The lithium-ion secondary battery according to any one of claims 1 to 11, wherein the electrode active material is an organic sulfur compound.

13. The lithium-ion secondary battery according to claim 12, wherein the carbon nanotube is contained in the electrode active material which is the organic sulfur compound.

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