Iron-containing lithium polysulfide

Abstract

In the present invention, iron-containing lithium polysulfide contains lithium, iron, and sulfur as constituent elements, wherein: lithium sulfide (Li2S) is included as the main phase; the crystallite size that is calculated from the half-width of a diffraction peak that is based on the (111) plane of the Li2S obtained by powder X-ray diffraction is 50 nm or less; and, taking the total iron-containing lithium polysulfide content to be 100 at%, the Li content is 50–70 at%, the Fe content is 2–12 at%, the S content is 20–40 at%, the C content is 0–5 at%, and the halogen content is 0–0.8 at%. In a compound that has lithium sulfide as a main component and is useful as a positive electrode active material for a lithium-ion secondary battery, said iron-containing lithium polysulfide is a material that has a high lithium sulfide use rate, high capacitance, and excellent rate properties.

Description

[Technical Field]

[0001] The present invention relates to iron-containing lithium polysulfide and a method for producing the same. [Background technology]

[0002] Due to the recent advances in performance of portable electronic devices, hybrid vehicles, etc., there is an increasing demand for higher capacity secondary batteries (especially lithium-ion secondary batteries) used in them. Current lithium-ion secondary batteries have lagged behind in increasing the capacity of the positive electrode compared to the negative electrode, and even the high-capacity Li(Ni,Mn,Co)O2-based materials that have been actively researched and developed recently only have a capacity of around 250-300mAh / g.

[0003] On the other hand, sulfur has a high theoretical capacity of approximately 1670 mAh / g, making it a promising candidate for high-capacity electrode materials. However, because elemental sulfur does not contain lithium, lithium or lithium-containing alloys must be used for the anode, limiting the range of anode options.

[0004] In contrast, lithium sulfide contains lithium, so alloys such as graphite and silicon can be used for the negative electrode, dramatically expanding the range of options for the negative electrode and avoiding the risk of short circuits caused by dendrite formation when using metallic lithium. However, lithium sulfide has poor electrical conductivity (see, for example, Non-Patent Document 1 below).

[0005] One possible method for improving electrical conductivity is to form bonds with heteroelements so that sulfur atoms are not liberated during the Li insertion and desorption reactions. To achieve this, it is necessary to prepare a compound in which a heteroelement is introduced into lithium sulfide. As the heteroelement to be added, a transition metal element that can impart conductivity to insulating lithium sulfide is suitable. For example, the Li x Fe y S zExamples of such compounds include those listed above. The introduction of such transition metal elements can suppress the liberation of sulfur elements and provide electrical conductivity, thereby improving the utilization rate of lithium sulfide. Another possible method is to introduce a typical element and form bonds between sulfur and the transition metal and the typical element to stabilize the framework structure. An example of a lithium sulfide composite incorporating multiple different elements (transition metal elements, typical elements) is a lithium-iron-phosphorus-sulfur-carbon composite (see, for example, Patent Document 3). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2010 / 084808 [Patent Document 2] International Publication No. 2015 / 037598 [Patent Document 3] International Publication No. 2016 / 080443 [Non-patent literature]

[0007] [Non-Patent Document 1] T. Takeuchi, H. Kageyama, K. Nakanishi, M. Ogawa, T. Ohta, A. Sakuda, H. Sakaebe, H. Kobayashi, and Z. Ogumi, J. Electrochem. Soc., 162, A1745 (2015). Summary of the Invention [Problem to be solved by the invention]

[0008] However, even the above-described conventional sulfides are not sufficient in terms of electrical conductivity, and for the purpose of improving the output characteristics of the battery, it is necessary to further improve the electrical conductivity and energy density, and as a result, improve the capacity and rate characteristics.

[0009] The present invention has been made in view of the current state of the prior art described above, and a main object of the present invention is to provide a novel material that is a compound containing lithium sulfide as a main component and is useful as a positive electrode active material for lithium ion secondary batteries, and that has a high utilization rate of lithium sulfide, a high capacity, and further has good rate characteristics and excellent charge / discharge characteristics. [Means for solving the problem]

[0010] The present inventors have conducted extensive research to achieve the above-mentioned object. As a result, they have discovered that an iron-containing lithium polysulfide containing lithium sulfide (LiS) as a main phase, having a crystallite size of 50 nm or less as calculated from the half-width of the diffraction peak based on the (111) plane of LiS obtained by powder X-ray diffraction, and having a Li content of 50 to 70 atomic %, an Fe content of 2 to 12 atomic %, an S content of 20 to 40 atomic %, a C content of 0 to 5 atomic %, and a halogen content of 0 to 0.8 atomic %, improves the utilization of lithium sulfide, resulting in a high-capacity material. Furthermore, the amount of impurities is low, improving electrical conductivity, and the rate characteristics are also improved due to the formation of bonds between sulfur and iron, which significantly reduces free sulfur. Iron-containing lithium polysulfide can be obtained, for example, by passing a DC pulse current through a mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound to heat and react through an electric current sintering process, then subjecting the resulting product to a first mechanical milling process, followed by a second mechanical milling process with a predetermined metal sulfide. This method not only produces a mixture of lithium sulfide and iron sulfide in which the reaction at the atomic level has progressed appropriately, but also contains a small amount of iron sulfide, resulting in improved electrical conductivity and improved capacity and rate characteristics. The present invention was completed as a result of further research based on these findings. That is, the present invention includes the following configurations.

[0011] Item 1. Iron-containing lithium polysulfide containing lithium, iron, and sulfur as constituent elements, It contains lithium sulfide (Li2S) as the main phase, The crystallite size calculated from the half-width of the diffraction peak based on the (111) plane of LiS obtained by powder X-ray diffraction is 50 nm or less, and The iron-containing lithium polysulfide has a Li content of 50 to 70 atomic %, an Fe content of 2 to 12 atomic %, an S content of 20 to 40 atomic %, a C content of 0 to 5 atomic %, and a halogen content of 0 to 0.8 atomic %, where the total amount of the iron-containing lithium polysulfide is 100 atomic %.

[0012] Item 2. The iron-containing lithium polysulfide according to Item 1, wherein the abundance ratio of lithium sulfide (LiS) is 70 mol % or more based on the iron-containing lithium polysulfide as estimated by Rietveld analysis.

[0013] Item 3. The iron-containing lithium polysulfide according to Item 1 or 2, further having an iron sulfide (FeS) phase.

[0014] 4. Electrical conductivity at 25°C is 10 -3 ~10 -6 Item 4. The iron-containing lithium polysulfide according to any one of Items 1 to 3, wherein the viscosity is 1000 s / cm.

[0015] Item 5. The iron-containing lithium polysulfide according to any one of items 1 to 4, which is for use in a lithium ion secondary battery.

[0016] Item 6. The iron-containing lithium polysulfide according to any one of Items 1 to 5, which is used as a positive electrode active material for a lithium ion secondary battery.

[0017] Item 7. A method for producing the iron-containing lithium polysulfide according to any one of items 1 to 6, (3) A mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound; a heat-treated product of the mixture; or a mechanically milled product of the heat-treated product of the mixture; and a mixture having an electrical conductivity of 10 -3 A process of mechanically milling a metal sulfide having a viscosity of S / cm or more. A manufacturing method comprising:

[0018] Item 8. Before the step (3), (2) A step of subjecting a mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound, or a heat-treated product of the mixture, to mechanical milling. Item 7. The manufacturing method according to Item 7, comprising:

[0019] Item 9. Before the step (2), (1) heating a mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound; Item 9. The manufacturing method according to item 8, comprising:

[0020] Item 10. The production method according to Item 9, wherein the step (1) is a step of passing a direct current pulse current through the mixture to heat and react the mixture.

[0021] Item 11. The method according to any one of Items 7 to 10, wherein steps (1) to (3) are carried out in a non-oxidizing atmosphere.

[0022] Item 12. A positive electrode active material for a lithium ion secondary battery, comprising the iron-containing lithium polysulfide according to any one of items 1 to 6.

[0023] Item 13. A lithium ion secondary battery comprising the positive electrode active material for lithium ion secondary batteries according to Item 12 as a constituent element.

[0024] Item 14. An all-solid-state lithium ion secondary battery comprising, as components, the positive electrode active material for a lithium ion secondary battery according to Item 13 and a lithium ion conductive solid electrolyte. [Effects of the Invention]

[0025] The iron-containing lithium polysulfide of the present invention is in the form of finely divided particles with a crystallite size of 50 nm or less, contains lithium sulfide (LiS) as a main phase, and has additive elements adjusted within a specific composition range, resulting in a low impurity content. This, combined with the above, results in a high utilization rate of lithium sulfide, fully exhibiting the high capacity properties inherent to lithium sulfide, and also improving electrical conductivity, resulting in a positive electrode active material with excellent rate characteristics.

[0026] Therefore, the iron-containing lithium polysulfide of the present invention is a highly useful substance as a positive electrode active material for lithium secondary batteries such as non-aqueous electrolyte lithium ion secondary batteries and all-solid-state lithium ion secondary batteries.

[0027] Furthermore, according to the production method of the present invention, a composite having such excellent performance can be produced relatively easily. [Brief explanation of the drawings]

[0028] [Figure 1] FIG. 1 is a schematic diagram of an example of an electric current sintering apparatus. [Figure 2] 1 shows X-ray diffraction patterns of samples obtained in Examples 1 and 2 and Comparative Examples 1 and 2. [Figure 3] 1 shows initial discharge curves at current densities of 0.13 to 1.3 mA / cm 2 of all-solid-state lithium-ion secondary batteries using the samples obtained in Examples 1 and 2 and Comparative Examples 1 and 2 as positive electrode active materials. [Figure 4] 1 shows X-ray diffraction patterns of samples obtained in Example 3 and Comparative Example 3. [Figure 5] 1 shows initial discharge curves at a current density of 0.13 to 1.3 mA / cm 2 of all-solid-state lithium-ion secondary batteries using the samples obtained in Example 3 and Comparative Example 3 as positive electrode active materials. [Figure 6] 1 shows X-ray diffraction patterns of the samples obtained in Example 4 and Comparative Example 4. [Figure 7] 1 shows initial discharge curves at a current density of 0.13 to 1.3 mA / cm 2 of all-solid-state lithium-ion secondary batteries using the samples obtained in Example 4 and Comparative Example 4 as positive electrode active materials. [Figure 8] 1 shows X-ray diffraction patterns of the samples obtained in Example 5 and Comparative Example 5. [Figure 9] 1 shows initial discharge curves at a current density of 0.13 to 1.3 mA / cm 2 of all-solid-state lithium-ion secondary batteries using the samples obtained in Example 5 and Comparative Example 5 as positive electrode active materials. [Figure 10] 1 shows X-ray diffraction patterns of the samples obtained in Example 6 and Comparative Example 6. [Figure 11]1 shows initial discharge curves at a current density of 0.13 to 1.3 mA / cm 2 of all-solid-state lithium-ion secondary batteries using the samples obtained in Example 6 and Comparative Example 6 as positive electrode active materials. DETAILED DESCRIPTION OF THE INVENTION

[0029] In this specification, when a numerical range is expressed as "A to B," it means A or more and B or less.

[0030] Furthermore, the term "containing" encompasses all of "comprise," "consist essentially of," and "consist only of."

[0031] In the present invention, the term "lithium ion secondary battery" is a concept that also encompasses "metal lithium secondary batteries" that use metallic lithium as the negative electrode material. Furthermore, in the present invention, the term "lithium ion secondary battery" refers to both "nonaqueous lithium ion secondary batteries" that use a nonaqueous electrolyte and "all-solid-state lithium ion secondary batteries" that use a solid electrolyte.

[0032] 1. Iron-containing lithium polysulfide The iron-containing lithium polysulfide of the present invention is an iron-containing lithium polysulfide containing lithium, iron, and sulfur as constituent elements, and contains lithium sulfide (LiS) as a major phase. The crystallite size calculated from the half-width of the diffraction peak based on the (111) plane of LiS obtained by powder X-ray diffraction is 50 nm or less. The iron-containing lithium polysulfide has a Li content of 50 to 70 atomic %, an Fe content of 2 to 12 atomic %, an S content of 20 to 40 atomic %, an C content of 0 to 5 atomic %, and a halogen content of 0 to 0.8 atomic %, based on the total amount of the iron-containing lithium polysulfide taken as 100 atomic %. The iron-containing lithium polysulfide of the present invention has a high lithium sulfide utilization rate, can fully exhibit the high capacity properties inherent to lithium sulfide, and has improved electrical conductivity. It is a material that can be used as a positive electrode active material with excellent rate characteristics, and is particularly useful as a positive electrode active material for lithium-ion secondary batteries.

[0033] According to such iron-containing lithium polysulfide of the present invention, for example, when a current of 0.13 mA / cm is applied, 2 When the initial discharge capacity is taken as 100%, the applied current is 1.3 mA / cm 2 It is possible to set the value of the initial discharge capacity at this temperature to 60 to 95%, preferably 70 to 90%. In this specification, the presence of such characteristics may be referred to as "having excellent rate characteristics."

[0034] The iron-containing lithium polysulfide of the present invention is found to have a major phase consisting of lithium sulfide in powder X-ray diffraction analysis. The amount of lithium sulfide phase present is preferably 70 mol% or more, more preferably 80 mol% or more, and even more preferably 85 mol% or more, based on the entire iron-containing lithium polysulfide of the present invention (100 mol%). The upper limit of the amount of lithium sulfide phase present is not particularly limited, but is typically 100 mol%. The iron-containing lithium polysulfide of the present invention may contain up to 30 mol% (particularly up to 20 mol%, and even more preferably up to 15 mol%) of other crystals in addition to the lithium sulfide crystal phase. At this level of content, the effect on charge / discharge characteristics is limited. In particular, when the iron-containing lithium polysulfide of the present invention contains a small amount (0.1 to 15 mol%, preferably 0.5 to 10 mol%) of iron sulfide (FeS) phase, the capacity and rate characteristics can be further improved. The amount present in the composite is estimated using conventional Rietveld analysis of X-ray diffraction data. The Rietveld method is described in detail in the following non-patent document (F. Izumi and T. Ikeda, Mater Sci. Forum, 321-324, 198 (2000)).

[0035] In such iron-containing lithium polysulfide of the present invention, iron atoms are arranged within the lithium sulfide crystal lattice to form Fe-S bonds, and since it contains Fe and the amount of impurities is reduced, conductivity is imparted to the interior, resulting in a high electrode utilization rate (particularly the positive electrode utilization rate), and the high capacity characteristics inherent to lithium sulfide can be more fully exhibited. Furthermore, when the manufacturing method of the present invention described below is adopted, a metastable phase of iron-containing lithium polysulfide is formed, and this can be refined by mechanical milling to form submicron particles, thereby stabilizing the inherent metastable phase of iron-containing lithium polysulfide.

[0036] In the iron-containing lithium polysulfide of the present invention having the above-mentioned characteristics, it is preferable that the iron atoms as the added element are arranged in the lithium sulfide crystal lattice to form Fe-S bonds, which makes it easier to further suppress the presence of free sulfur and also confers good electrical conductivity, making it easier to exhibit excellent capacity and rate characteristics.

[0037] Furthermore, the iron-containing lithium polysulfide of the present invention preferably has a stabilized metastable phase in which iron atoms are introduced into the lithium sulfide crystal lattice. The iron-containing lithium polysulfide of the present invention preferably comprises submicron or smaller crystallites, since the metastable phase of the iron-containing lithium polysulfide is stable. More specifically, the crystallite size of the iron-containing lithium polysulfide of the present invention is preferably 50 nm or less, more preferably 40 nm or less (particularly 1 to 30 nm). When the iron-containing lithium polysulfide of the present invention is produced by a production method including mechanical milling, as described below, the crystallites can be refined by mechanical milling. The crystallite size of the iron-containing lithium polysulfide of the present invention is a value calculated according to the Scherrer formula from the half-width of the diffraction peak based on the (111) plane, which exhibits the highest intensity of the peak of lithium sulfide observed as the main phase in powder X-ray diffraction measurement.

[0038] Furthermore, the proportions of each element in the iron-containing lithium polysulfide of the present invention are not particularly limited. However, it is preferable that the amount of Fe is sufficient to form Fe-S bonds without generating free sulfur, that the amount of Li is sufficient to provide a theoretical capacity of 600 mAh / g or more, as estimated from the amount of Li, and that the amount of Fe is sufficient to ensure electrical conductivity. Furthermore, excessive amounts of impurities, such as carbon, can degrade the capacity, cycle characteristics, and rate characteristics. From this perspective, the Li content is preferably 50 to 70 atomic % (particularly 51 to 60 atomic %), the Fe content is 2 to 12 atomic % (particularly 4 to 11 atomic %), the S content is 20 to 40 atomic % (particularly 30 to 39 atomic %), the C content is 0 to 5 atomic % (particularly 0 to 3 atomic %), and the halogen content is 0 to 0.8 atomic % (particularly 0 to 0.5 atomic %), based on the total amount of the lithium sulfide-iron-carbon composite of the present invention (100 atomic %). It is particularly preferable that the composite contains no carbon or halogen (i.e., both the C content and the halogen content are 0 atomic %).

[0039] The reason why the amount of Li was set to be the amount that would result in a theoretical capacity of 600mAh / g or more is that the oxide-based high-capacity material Li(Ni,Mn,Co)O2 has a maximum energy density of 300mAh / g x 4V = 1200Wh / kg, and it was determined that a sulfur-based material of the same level (voltage 2V) with an energy density of 600mAh / g or more would be sufficient.

[0040] The iron-containing lithium polysulfide of the present invention as described above can improve electrical conductivity. Specifically, the iron-containing lithium polysulfide of the present invention has an electrical conductivity of 10 -3 ~10 -6 S / cm, preferably 10 -3 ~3×10 -6 The electrical conductivity of the iron-containing lithium polysulfide of the present invention can be measured by compacting the powder at 300 MPa and measuring the current when a voltage is applied.

[0041] 2.Method for producing iron-containing lithium polysulfide The iron-containing lithium polysulfide of the present invention is not particularly limited, but may be (3) A mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound; a heat-treated product of the mixture; or a mechanically milled product of the heat-treated product of the mixture; and a mixture having an electrical conductivity of 10 -3 A process of mechanically milling a metal sulfide having a viscosity of S / cm or more. According to this method, by performing mechanical milling treatment with a predetermined metal sulfide, the particles are refined, and the metastable phase in which iron atoms are incorporated into the lithium sulfide phase is stabilized, and the electrical conductivity can be particularly improved.

[0042] In addition, the production method of the present invention further includes the steps of: (2) A step of subjecting a mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound, or a heat-treated product of the mixture, to mechanical milling. As a result, prior to the step (3), the particles are refined and the metastable phase in which iron atoms are incorporated into the lithium sulfide phase is stabilized, which makes it easier to improve the electrical conductivity.

[0043] In addition, the production method of the present invention further includes, before the step (2), (1) heating a mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound; This allows the reaction to proceed appropriately by heating, and the electrical conductivity can be easily further improved by the subsequent steps (2) and (3). This method will be specifically described below.

[0044] (2-1) Raw material powder In the present invention, a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound are used as raw materials.

[0045] The types of lithium-containing compounds, iron-containing compounds, and sulfur-containing compounds are not particularly limited, and three or more types of compounds containing one type each of lithium, iron, and sulfur may be mixed and used, or a compound containing two or more types of elements selected from lithium, iron, and sulfur may be used as part of the raw material.

[0046] These raw material compounds are preferably compounds that do not contain metal elements other than lithium and iron, or carbon, and it is preferable that elements other than lithium, iron, and sulfur contained in the raw material compounds are released or volatilized by heat treatment.

[0047] Specific examples of such raw material compounds include lithium-containing compounds such as lithium sulfide (LiS) and lithium hydroxide (LiOH), iron-containing compounds such as metallic iron (Fe), iron sulfides (FeS, FeS, etc.), and iron sulfate (FeSO), and sulfur-containing compounds such as sulfur (S), lithium sulfide (LiS), and iron sulfides (FeS, FeS, etc.). Among these, a combination of lithium sulfide (LiS) and iron sulfides (FeS, FeS) is particularly preferred, which consists only of the constituent elements of the iron-containing lithium polysulfide of the present invention, which is the product, and which can be reacted with a minimum number of raw materials.

[0048] There are no particular limitations on the shape of these raw material compounds, but a powder with an average particle size of approximately 0.1 to 100 μm is preferred. However, because a mechanical milling process is involved, there are no particular limitations on the size of the raw material compounds; raw material compounds with large particle sizes can be used, and the average particle size can be controlled as needed by grinding them in a mortar or other device. The average particle size of the raw material compounds is determined by measuring the particle size distribution using a dry laser diffraction / scattering method, and the value at which the cumulative frequency distribution reaches 50%.

[0049] The blending ratios of the raw materials, consisting of a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound, are not particularly limited. However, it is preferable that the final product, the iron-containing lithium polysulfide of the present invention, contains enough Fe to easily form Fe-S bonds without generating free sulfur, that the theoretical capacity estimated from the Li amount is about 600 mAh / g or more, and that the Fe content is sufficient to ensure electrical conductivity and prevent deterioration of capacity and rate characteristics. From this perspective, the blending ratios of the raw materials are preferably adjusted so that the Li content is 50 to 70 atomic % (especially 51 to 60 atomic %), the Fe content is 2 to 12 atomic % (especially 4 to 11 atomic %), the S content is 20 to 40 atomic % (especially 30 to 39 atomic %), the C content is 0 to 5 atomic % (especially 0 to 3 atomic %), and the halogen content is 0 to 0.8 atomic % (especially 0 to 0.5 atomic %). The blending ratios of the raw material compounds can be adjusted so that the ratios of the elements contained in the raw material compounds are the same as the ratios of the elements in the target iron-containing lithium polysulfide of the present invention.

[0050] The reason why the amount of Li was set to be the amount that would result in a theoretical capacity of 600mAh / g or more is that the oxide-based high-capacity material Li(Ni,Mn,Co)O2 has a maximum energy density of 300mAh / g x 4V = 1200Wh / kg, and it was determined that a sulfur-based material of the same level (voltage 2V) with an energy density of 600mAh / g or more would be sufficient.

[0051] (2-2) Process (1) When the step (1) is adopted, it is preferable to first subject a mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound to a heat treatment. Although the heating method is not particularly limited, it is preferable to perform electric sintering from the viewpoint that a mixture of lithium sulfide and iron sulfide in which the reaction at the atomic level has progressed appropriately by the heating reaction can be easily obtained.

[0052] When electric sintering is performed in step (1), specifically, the raw material mixture described above is filled into a conductive mold (conductive container), and the conductive mold (conductive container) is energized, preferably with a direct current pulse (a method known as spark plasma sintering, pulse electric current sintering, plasma activated sintering, etc.), whereby the conductive mold (conductive container) is heated by Joule heat, the raw material mixture in the conductive mold (conductive container) is heated, and the elements diffuse and migrate, making it possible to suitably produce an intermediate in which the elements are mixed together at the atomic level.

[0053] The atmosphere during the electric current sintering treatment is preferably a non-oxidizing atmosphere. Although the electric current sintering treatment may be performed under normal pressure, it is preferable to perform the electric current sintering treatment under pressure.

[0054] As a specific method, it is preferable to fill a conductive container with a raw material mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound as raw materials, and then pass a pulsed ON-OFF direct current therethrough while applying pressure, preferably in a non-oxidizing atmosphere.

[0055] The material of the conductive mold (conductive container) is not particularly limited as long as it is conductive, and in addition to those made from carbon, iron, iron oxide, aluminum, tungsten carbide, etc., those made from a mixture of carbon and / or iron oxide with silicon nitride can also be suitably used.

[0056] To further suppress oxidation of sulfides, the electric sintering treatment is preferably carried out in an inert gas atmosphere such as Ar or N2, or in a reducing atmosphere such as H2, etc. Alternatively, the electric sintering treatment can be carried out under a reduced pressure state with a sufficiently low oxygen concentration, for example, a reduced pressure of 20 Pa or less (particularly 1 to 20 Pa).

[0057] Specifically, when a container that can ensure a sufficiently sealed state is used as the conductive mold (conductive container), the container can be kept in a non-oxidizing atmosphere. Furthermore, the conductive mold (conductive container) does not need to be completely sealed. When an incompletely sealed container is used, the container can be placed in a reaction chamber, and the reaction chamber can be kept in a non-oxidizing atmosphere. This allows the heating reaction of the raw material mixture to be carried out in a non-oxidizing atmosphere. In this case, the reaction chamber is preferably kept in an inert gas atmosphere or a reducing gas atmosphere of 0.01 MPa or more (particularly 0.05 to 0.2 MPa).

[0058] By applying a DC pulse current to a conductive container filled with the above-mentioned raw material mixture, the conductive container is heated by Joule heat, and the raw materials in the container are heated, causing the starting materials to react with each other and forming an intermediate in which the atoms are mixed together. This method can produce the desired intermediate in a short time of 30 minutes or less, so that an intermediate with a composition ratio close to the raw material mixture ratio can be obtained with little loss of easily volatile elements such as Li and S.

[0059] The heating temperature in the electric current sintering step is usually preferably 400 to 1200°C, more preferably 500 to 1100°C, from the viewpoints of more thoroughly interdiffusing the constituent elements to facilitate mutual mixing at the atomic level, reducing sulfur (free sulfur) that is not bonded to the transition metals and typical elements, thereby facilitating high capacity, and easily suppressing loss due to volatilization of elements such as Li and S, thereby facilitating high capacity. The time for holding within the above heating temperature range is preferably 0 to 30 minutes, from the viewpoints of easily suppressing loss due to volatilization of elements such as Li and S, thereby facilitating high capacity, and once the temperature range is reached, the electric current can be immediately stopped (i.e., the holding time at the above heating temperature is set to 0) and the material can be allowed to cool.

[0060] The pressure to be applied when pressing the raw material powder is preferably, for example, 5 to 60 MPa, more preferably 10 to 50 MPa, from the viewpoints of making it easier to strengthen contact between the raw material compounds, making it easier to sufficiently interdiffuse atoms during heating, and making it easier to sufficiently react between atoms in the raw material compounds. Note that, when the product has a composition with a low melting point, it is also possible to heat without applying pressure (atmospheric pressure). From this viewpoint, and taking into consideration the case where the product has a composition with a low melting point, the pressure to be applied when pressing the raw material powder can be 0.1 to 60 MPa, preferably 0.1 to 50 MPa.

[0061] The device for electric current sintering is not particularly limited as long as it can heat, cool, pressurize, etc. the raw material mixture and can apply the current required for discharge. For example, a commercially available electric current sintering device (spark plasma sintering device) can be used. Such an electric current sintering device and its principle are disclosed, for example, in JP-A-10-251070.

[0062] A specific example of electric current sintering will be described below with reference to a schematic diagram of an electric current sintering apparatus shown in Fig. 1. Note that the electric current sintering apparatus is not limited to the apparatus described below.

[0063] The electric sintering apparatus 1 shown in Fig. 1 has a sintering die (electronically conductive container) 3 into which a sample 2 is loaded, and a pair of upper and lower punches 4 and 5. The punches 4 and 5 are supported by punch electrodes 6 and 7, respectively, and a pulse current can be supplied to the sample 2 loaded in the sintering die 3 via these punch electrodes 6 and 7 while applying pressure as necessary. The material of the sintering die 3 is not limited, and examples include carbon materials such as graphite.

[0064] 1, the current-carrying section including the conductive container 3, current-carrying punches 4 and 5, and punch electrodes 6 and 7 is housed in a water-cooled vacuum chamber 8, and the interior of the chamber can be adjusted to a predetermined atmosphere by an atmosphere control mechanism 15. Therefore, it is preferable to use the atmosphere control mechanism 15 to adjust the interior of the chamber to a non-oxidizing atmosphere.

[0065] The control device 12 drives and controls the pressurizing mechanism 13, the pulse power supply 11, the atmosphere control mechanism 15, the water cooling mechanisms 16 and 10, and the temperature measuring device 17. The control device 12 drives the pressurizing mechanism 13 so that the punch electrodes 6 and 7 pressurize the raw material mixture at a predetermined pressure.

[0066] The pulse current applied for heating can be, for example, a pulsed (ON-OFF switching) DC current with a pulse width of about 2 to 3 milliseconds and a frequency of about 3 to 300 Hz. The specific current value varies depending on the type and size of the conductive mold (conductive container), but it is preferable to determine the specific current value so that the temperature range described above is achieved. For example, when a graphite mold with an inner diameter of 15 mm is used, a current of 200 to 1000 A is preferred, and when a graphite mold with an inner diameter of 100 mm is used, a current of 1000 to 8000 A is preferred. During the electric sintering process, it is preferable to increase or decrease the current value while monitoring the mold temperature, thereby controlling the current value so that the desired temperature can be maintained.

[0067] In order to bring the raw material mixture into a pressurized state, for example, the raw material mixture filled in the conductive container 3 can be pressurized via punch electrodes 6 and 7 .

[0068] (2-3) Process (2) When step (2) is employed, a mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound, or a heat-treated product of the mixture, is subjected to a mechanical milling treatment to mix, pulverize, and react with each other.

[0069] When a mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound is used, the raw material powders can be mixed and then subjected to mechanical milling treatment. When a heat-treated product of the mixture is used, the product obtained in the above-mentioned step (1) can be used.

[0070] The mechanical milling method is a method of grinding and mixing raw materials while applying mechanical energy to them to cause a reaction. According to this method, by grinding and mixing the raw materials by applying mechanical impact and friction to them, the compounds contained in the raw materials come into vigorous contact with each other and are pulverized, so that a metastable phase is easily obtained. In the present invention, the above-mentioned mechanical milling treatment can form metastable iron-containing lithium polysulfide, which is difficult to produce by heat treatment alone, and can be pulverized and exist stably.

[0071] As the mechanical milling device, for example, a ball mill, a vibration mill, a turbo mill, a disk mill, etc. can be used, and among these, a ball mill is preferred.

[0072] Since the mechanical milling treatment is easy to suppress oxidation of sulfides, it is preferably carried out in a non-oxidizing atmosphere, for example, in an inert gas atmosphere such as Ar or N2, or in a reducing atmosphere such as H2, etc. Alternatively, it can be carried out under a reduced pressure state with a sufficiently low oxygen concentration, for example, a reduced pressure of 20 Pa or less (particularly 1 to 20 Pa) of oxygen partial pressure.

[0073] The rotation speed during mechanical milling is preferably 200 to 600 rpm, more preferably 250 to 550 rpm, to prevent sulfur from volatilizing and facilitate the formation of the desired composite with a high sulfur content.

[0074] The temperature during mechanical milling is preferably 200°C or lower, more preferably 20 to 100°C, in order to prevent sulfur from volatilizing and to facilitate the formation of the desired composite having a high sulfur content.

[0075] The mechanical milling time is not particularly limited, but it is preferable to carry out the mechanical milling treatment until the crystallite size of the resulting composite becomes 50 nm or less.

[0076] (2-4) Second mechanical milling step (step (3)) In step (3), a mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound; a heat-treated product of the mixture; or a mechanically milled product of the heat-treated product of the mixture; and a solution having an electrical conductivity of 10 -3 The iron-containing lithium polysulfide of the present invention can be obtained by subjecting the iron-containing lithium polysulfide to a mechanical milling treatment, mixing, pulverizing, and reacting the iron-containing lithium polysulfide with a metal sulfide having a specific surface area of ​​5 S / cm or more.

[0077] When using a mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound, the electrical conductivity must be 10 -3 In addition, when a heat-treated mixture of the above mixture is used, the electrical conductivity of the product obtained in the above step (1) can be increased to 10. -3 In addition, when a mechanically milled product of the heated product of the mixture is used, the product obtained in the above step (2) can be used.

[0078] In addition, when the above-mentioned step (1) or step (2) is adopted, the product contains an electric conductivity of 10 -3 When a metal sulfide having a conductivity of 0.5 S / cm or more is already contained in an amount to be added as described below, it is not necessary to add a new metal sulfide. In the present invention, even when a sulfide having a high electrical conductivity, such as iron sulfide, is used as the raw material, if the peak in the X-ray diffraction spectrum disappears in the milling step of step (2), that is, if mechanical milling is performed to an extent that the sulfide phase cannot be identified, or if the content of the sulfide phase decreases as a result of the milling step of step (2), it is necessary to add a metal sulfide separately.

[0079] The electrical conductivity of the usable metal sulfide is not particularly limited, but from the viewpoints of easily improving the utilization rate of lithium sulfide, easily demonstrating the inherent high capacity characteristics of lithium sulfide, and easily having excellent rate characteristics, 10 3 ~10 -3 S / cm is preferred, 10 3 ~10-2 The electrical conductivity of the metal sulfide is measured by compacting the powder at about 300 MPa and measuring the amount of current when a voltage is applied.

[0080] Metal sulfides having such electrical conductivity are not particularly limited, but examples thereof include iron-containing lithium polysulfides (LiFeS, etc.; however, these are different from the products of steps (1) and (2)), iron sulfides (FeS, FeS, etc.), molybdenum sulfides (MoS, MoS, etc.), copper sulfides (CuS, etc.), etc. These metal sulfides can be used alone or in combination of two or more.

[0081] In step (3), the amount of the metal sulfide added is not particularly limited. However, from the viewpoints of easily improving the utilization rate of lithium sulfide, easily allowing the high capacity characteristics inherent to lithium sulfide to be fully exhibited, and easily achieving excellent rate characteristics, the metal sulfide can be added in an amount of 3 to 25 mass%, preferably 5 to 20 mass%, based on 100 mass% of the total amount of the intermediate obtained in step (2) and the metal sulfide.

[0082] As the mechanical milling device, for example, a ball mill, a vibration mill, a turbo mill, a disk mill, etc. can be used, and among these, a ball mill is preferred.

[0083] Since the mechanical milling treatment is easy to suppress oxidation of sulfides, it is preferably carried out in a non-oxidizing atmosphere, for example, in an inert gas atmosphere such as Ar or N2, or in a reducing atmosphere such as H2, etc. Alternatively, it can be carried out under a reduced pressure state with a sufficiently low oxygen concentration, for example, a reduced pressure of 20 Pa or less (particularly 1 to 20 Pa) of oxygen partial pressure.

[0084] The rotation speed during mechanical milling is preferably 200 to 600 rpm, more preferably 250 to 550 rpm, to prevent sulfur from volatilizing and facilitate the formation of the desired composite with a high sulfur content.

[0085] The temperature during mechanical milling is preferably 100°C or lower, more preferably 20 to 80°C, in order to prevent sulfur from volatilizing and to facilitate the formation of the desired composite having a high sulfur content.

[0086] The mechanical milling time is not particularly limited, but it is preferable to carry out the mechanical milling treatment until the crystallite size of the resulting composite becomes 50 nm or less.

[0087] 3. Lithium-ion secondary batteries Taking advantage of the excellent properties described above, the iron-containing lithium polysulfide of the present invention can be effectively used as a positive electrode active material for lithium primary batteries, lithium ion secondary batteries (metal lithium secondary batteries, etc.), and the like. In particular, since the iron-containing lithium polysulfide of the present invention is a material containing lithium in its structure, it is a material that can be charged and discharged, and furthermore, it has a high capacity and excellent rate characteristics, making it useful as a positive electrode active material for lithium ion secondary batteries. A lithium ion secondary battery that uses the iron-containing lithium polysulfide of the present invention as a positive electrode active material may be a nonaqueous electrolyte lithium ion secondary battery that uses a nonaqueous solvent-based electrolytic solution as the electrolyte, or may be an all-solid-state lithium ion secondary battery that uses a lithium ion-conductive solid electrolyte.

[0088] The structures of the nonaqueous electrolyte lithium ion secondary battery and the all-solid-state lithium ion secondary battery can be the same as those of known lithium ion secondary batteries, except that the iron-containing lithium polysulfide of the present invention is used as the positive electrode active material.

[0089] For example, a non-aqueous electrolyte lithium ion secondary battery can have the same basic structure as a known non-aqueous electrolyte lithium ion secondary battery, except that the iron-containing lithium polysulfide of the present invention described above is used as a positive electrode active material.

[0090] For the positive electrode, the iron-containing lithium polysulfide of the present invention described above can be used as the positive electrode active material, and a positive electrode mixture containing a conductive material and a binder can be supported on a positive electrode current collector made of Al, Ni, stainless steel, carbon cloth, etc. Examples of the conductive material that can be used include carbon materials such as graphite, cokes, carbon black, and acicular carbon.

[0091] Both lithium-containing and lithium-free materials can be used for the negative electrode. For example, graphite, non-sinterable carbon, lithium metal, tin, silicon, alloys containing these, and SiO can also be used. These negative electrode active materials can also be supported on a negative electrode current collector made of Al, Cu, Ni, stainless steel, carbon, or the like, using a conductive material, binder, or the like, as needed.

[0092] The separator may be made of a material such as a polyolefin resin, such as polyethylene or polypropylene, a fluororesin, nylon, aromatic aramid, or inorganic glass, and may be in the form of a porous film, nonwoven fabric, or woven fabric.

[0093] As the solvent for the non-aqueous electrolyte, any solvent known as a solvent for non-aqueous solvent-based secondary batteries, such as a carbonate compound, an ether compound, a nitrile compound, or a sulfur-containing compound, can be used.

[0094] Furthermore, all-solid-state lithium ion secondary batteries can also have the same structure as known all-solid-state lithium ion secondary batteries, except that the iron-containing lithium polysulfide of the present invention is used as the positive electrode active material.

[0095] In this case, examples of the lithium ion conductive solid electrolyte that can be used include polymer-based solid electrolytes such as polyethylene oxide-based polymer compounds, polymer compounds containing at least one of polyorganosiloxane chains and polyoxyalkylene chains, as well as sulfide-based solid electrolytes and oxide-based solid electrolytes.

[0096] For the positive electrode of an all-solid-state lithium ion secondary battery, for example, the iron-containing lithium polysulfide of the present invention can be used as a positive electrode active material, and a positive electrode mixture containing a conductive material, a binder, and a solid electrolyte can be supported on a positive electrode current collector made of Ti, Al, Ni, stainless steel, etc. As with non-aqueous solvent-based lithium ion secondary batteries, for example, carbon materials such as graphite, cokes, carbon black, and acicular carbon can be used as the conductive material.

[0097] Both lithium-containing and lithium-free materials can be used for the negative electrode. For example, graphite, non-sinterable carbon, lithium metal, tin, silicon, alloys containing these, and SiO can be used. These negative electrode active materials can also be supported on a negative electrode current collector made of Al, Cu, Ni, stainless steel, carbon, or the like, using the above-mentioned conductive materials, binders, or the like, as needed.

[0098] There are no particular limitations on the shape of the non-aqueous electrolyte lithium ion secondary battery and the all-solid-state lithium ion secondary battery, and they may be cylindrical, prismatic, or the like. [Example]

[0099] The present invention will be specifically described below with reference to examples and comparative examples, although it goes without saying that the present invention is not limited to the following examples.

[0100] Example 1 Commercially available lithium sulfide (LiS) (manufactured by Kojundo Chemical Laboratory Co., Ltd., model number: LII06PB) and iron sulfide (FeS) (manufactured by Alfa Aesar, model number: 14024) were weighed in a glove box (dew point: -80°C) under an argon gas atmosphere so that the molar ratio of LiS:FeS was 4:1. The mixture was thoroughly mixed in a mortar and then filled into a graphite mold with an inner diameter of 15 mm.

[0101] Next, the graphite mold filled with the raw materials was placed in an electric sintering machine. The graphite mold and the electric parts including the electrode parts were placed in a vacuum chamber, which was evacuated to a vacuum (approximately 20 Pa) and then filled with high-purity argon gas (oxygen concentration approximately 0.2 ppm) up to atmospheric pressure.

[0102] Then, a DC pulse current of approximately 600 A (pulse width 2.5 ms, cycle 28.6 Hz) was applied to the raw material packed into the graphite mold while pressurizing it at approximately 30 MPa. The area around the graphite mold was heated at a temperature increase rate of approximately 200°C / min, and reached 600°C 3 minutes after the start of pulse current application. Then, the current application and pressure were immediately stopped, and the material was allowed to cool naturally.

[0103] After cooling to room temperature (25°C), the graphite jig was transferred to a glove box with an argon gas atmosphere and a dew point of -80°C. The reaction product of lithium sulfide and iron sulfide was removed from the mold and crushed in a mortar. It was then placed in a zirconia pot under an argon gas atmosphere and subjected to mechanical milling at 400 rpm for 20 hours using a planetary ball mill (model P-7) manufactured by Fritsch Japan Co., Ltd.

[0104] Then, LiFeS (commercially available LiS and FeS were mixed in a 1:1 molar ratio at room temperature (25°C) and then heated at 1000°C for 1 minute in an electric sintering machine to produce a powder with an electrical conductivity of 10 -2 S / cm (25°C)) was mixed in a mortar to form a reaction product of lithium sulfide and iron sulfide (LiFeS:LiFeS = 6:4 (mass ratio), and the mixture was placed in a zirconia pot under an argon gas atmosphere and processed by mechanical milling at 400 rpm for 1 hour using a planetary ball mill (Model P-7) manufactured by Fritsch Japan Co., Ltd.

[0105] The ratios (atomic %) of each element in the obtained sample were Li 52%, Fe 11%, and S 37%. The obtained sample did not contain carbon or halogens. In order to measure the electrical conductivity of the obtained sample, it was pressed at 300 MPa and the current value was measured when a voltage was applied. The result was 4.4 × 10 -4S / cm, which is the electrical conductivity of LiFeS before being composited with LiFeS (Comparative Example 1), 1.1 × 10 -5 This was a dramatic improvement compared to S / cm.

[0106] As shown in Figure 2, the X-ray diffraction pattern of the obtained sample showed peaks derived from low-crystalline lithium sulfide (Li2S) as the main phase, as well as peaks derived from iron sulfide (FeS). In other words, the abundance ratio of lithium sulfide (Li2S) estimated by Rietveld analysis was 96 mol % and that of iron sulfide (FeS) was 4 mol %. Furthermore, the crystallite size estimated from the half-width of the diffraction peak based on the (111) plane of lithium sulfide was 20 nm. From the above, it can be seen that this method successfully produced iron-containing lithium polysulfide whose main phase was lithium sulfide and whose crystallite size was 50 nm or less.

[0107] The obtained iron-containing lithium polysulfide was used as the positive electrode active material, indium metal as the negative electrode, and an argyrodite-type sulfide-based solid electrolyte as the electrolyte to assemble an all-solid-state lithium-ion secondary battery, and charge-discharge tests were conducted.

[0108] For the positive electrode, the above-mentioned iron-containing lithium polysulfide, argyrodite-type sulfide-based solid electrolyte, and acetylene black were mixed in a mass ratio of 4:5:1 to form the positive electrode composite. A 10 mm diameter pellet battery was fabricated by pressure molding the positive electrode composite / argyrodite-type sulfide-based solid electrolyte / lithium indium foil. A charge-discharge test was performed on this battery by starting charging with constant current measurement in the range from 3.0 V to 1.0 V. The current density during charging was 0.13 mA / cm. 2 and 0.13 to 1.3 mA / cm for discharge. 2 It was decided.

[0109] The initial (first cycle) discharge curve is shown in Figure 3. The initial discharge capacity is higher at low discharge current densities (0.13 mA / cm 2 ) at 520mAh / g and 0.25mA / cm 2 500mAh / g, 0.64mA / cm 2480mAh / g, high current density (1.3mA / cm 2 ) was 450mAh / g. In other words, at a high current density (1.3mA / cm 2 ) at a low current density (0.13 mA / cm 2 ) showed a high value of 86% of the initial discharge capacity, and it can be seen that a positive electrode active material excellent in capacity and rate characteristics was obtained.

[0110] Example 2 Iron-containing lithium polysulfide was obtained in the same manner as in Example 1, except that the mechanical milling time for Li2FeS2 with the reaction product of lithium sulfide and iron sulfide (Li8FeS5) was set to 2 hours.

[0111] The ratios (atomic %) of each element in the obtained sample were Li 52%, Fe 11%, and S 37%. The obtained sample did not contain carbon or halogens. In order to measure the electrical conductivity of the obtained sample, the powder was compacted at 300 MPa and the current value was measured when a voltage was applied. The result was 4.0 × 10 -4 S / cm, which is the electrical conductivity of LiFeS before being composited with LiFeS (Comparative Example 1), 1.1 × 10 -5 This was a dramatic improvement compared to S / cm.

[0112] As shown in Figure 2, the X-ray diffraction pattern of the obtained sample showed peaks derived from low-crystalline lithium sulfide (Li2S) as the main phase, as well as peaks derived from iron sulfide (FeS). In other words, the abundance ratio of lithium sulfide (Li2S) estimated by Rietveld analysis was 91 mol % and that of iron sulfide (FeS) was 9 mol %. Furthermore, the crystallite size estimated from the half-width of the diffraction peak based on the (111) plane of lithium sulfide was 18 nm. From the above, it can be seen that this method successfully produced iron-containing lithium polysulfide whose main phase was lithium sulfide and whose crystallite size was 50 nm or less.

[0113] The obtained iron-containing lithium polysulfide was used as the positive electrode active material, indium metal as the negative electrode, and an argyrodite-type sulfide-based solid electrolyte as the electrolyte to assemble an all-solid-state lithium-ion secondary battery, and charge-discharge tests were conducted.

[0114] For the positive electrode, the above-mentioned iron-containing lithium polysulfide, argyrodite-type sulfide-based solid electrolyte, and acetylene black were mixed in a mass ratio of 4:5:1 to form the positive electrode composite. A 10 mm diameter pellet battery was fabricated by pressure molding the positive electrode composite / argyrodite-type sulfide-based solid electrolyte / lithium indium foil. A charge-discharge test was performed on this battery by starting charging with constant current measurement in the range from 3.0 V to 1.0 V. The current density during charging was 0.13 mA / cm. 2 and 0.13 to 1.3 mA / cm for discharge. 2 It was decided.

[0115] The initial (first cycle) discharge curve is shown in Figure 3. The initial discharge capacity is higher at low discharge current densities (0.13 mA / cm 2 ) at 510mAh / g and 0.25mA / cm 2 500mAh / g, 0.64mA / cm 2 480mAh / g, high current density (1.3mA / cm 2 ) was 450mAh / g. In other words, at a high current density (1.3mA / cm 2 ) at a low current density (0.13 mA / cm 2 ) showed a high value of 88% of the initial discharge capacity, and it can be seen that a positive electrode active material excellent in capacity and rate characteristics was obtained.

[0116] Comparative Example 1 An iron-containing lithium polysulfide was obtained in the same manner as in Example 1, except that LiFeS was not added to the reaction product of lithium sulfide and iron sulfide (LiFeS), and the subsequent mechanical milling treatment was not performed. Specifically, the iron-containing lithium polysulfide was obtained as follows.

[0117] Commercially available lithium sulfide (LiS) (manufactured by Kojundo Chemical Laboratory Co., Ltd., model number: LII06PB) and iron sulfide (FeS) (manufactured by Alfa Aesar, model number: 14024) were weighed in a glove box (dew point: -80°C) under an argon gas atmosphere so that the molar ratio of LiS:FeS was 4:1. The mixture was thoroughly mixed in a mortar and then filled into a graphite mold with an inner diameter of 15 mm.

[0118] Next, the graphite mold filled with the raw materials was placed in an electric sintering machine. The graphite mold and the electric parts including the electrode parts were placed in a vacuum chamber, which was evacuated to a vacuum (approximately 20 Pa) and then filled with high-purity argon gas (oxygen concentration approximately 0.2 ppm) up to atmospheric pressure.

[0119] Then, a DC pulse current of approximately 600 A (pulse width 2.5 ms, cycle 28.6 Hz) was applied to the raw material packed into the graphite mold while pressurizing it at approximately 30 MPa. The area around the graphite mold was heated at a temperature increase rate of approximately 200°C / min, and reached 600°C 3 minutes after the start of pulse current application. Then, the current application and pressure were immediately stopped, and the material was allowed to cool naturally.

[0120] After cooling to room temperature (25°C), the graphite jig was transferred to a glove box with an argon gas atmosphere and a dew point of -80°C. The reaction product of lithium sulfide and iron sulfide was removed from the mold and crushed in a mortar. It was then placed in a zirconia pot under an argon gas atmosphere and subjected to mechanical milling at 400 rpm for 20 hours using a planetary ball mill (model P-7) manufactured by Fritsch Japan Co., Ltd.

[0121] The ratios (atomic %) of each element in the obtained sample were Li 57%, Fe 7%, and S 36%. The obtained sample did not contain carbon or halogens. In order to measure the electrical conductivity of the obtained sample, the powder was compacted at 300 MPa and the current value was measured by applying a voltage. The electrical conductivity was found to be 1.1 x 10 -5 The values ​​were low compared to Examples 1 and 2, namely, S / cm.

[0122] The X-ray diffraction pattern of the obtained sample, as shown in Figure 2, consisted of peaks derived from low-crystalline lithium sulfide (Li2S) as the main phase, with no other peaks observed. In other words, the abundance ratio of lithium sulfide (Li2S) estimated by Rietveld analysis was 100 mol%. In addition, the crystallite size estimated from the half-width of the diffraction peak based on the (111) plane of lithium sulfide was 26 nm.

[0123] The obtained iron-containing lithium polysulfide was used as the positive electrode active material, indium metal as the negative electrode, and an argyrodite-type sulfide-based solid electrolyte as the electrolyte to assemble an all-solid-state lithium-ion secondary battery, and charge-discharge tests were conducted.

[0124] For the positive electrode, the above-mentioned iron-containing lithium polysulfide, argyrodite-type sulfide-based solid electrolyte, and acetylene black were mixed in a mass ratio of 4:5:1 to form the positive electrode composite. A 10 mm diameter pellet battery was fabricated by pressure molding the positive electrode composite / argyrodite-type sulfide-based solid electrolyte / lithium indium foil. A charge-discharge test was performed on this battery by starting charging with constant current measurement in the range from 3.0 V to 1.0 V. The current density during charging was 0.13 mA / cm. 2 and 0.13 to 1.3 mA / cm for discharge. 2 It was decided.

[0125] The initial discharge curve is shown in Figure 3. The initial discharge capacity is higher at low discharge current densities (0.13 mA / cm 2 ) at 490mAh / g and 0.25mA / cm 2 430mAh / g, 0.64mA / cm 2 280mAh / g, high current density (1.3mA / cm 2 ) was 140mAh / g. In other words, at a high current density (1.3mA / cm 2 ) at a low current density (0.13 mA / cm 2 ) and a positive electrode active material with particularly excellent rate characteristics was not obtained.

[0126] Comparative Example 2 Iron-containing lithium polysulfide was obtained in the same manner as in Example 1, except that the mechanical milling time of the reaction product of lithium sulfide and iron sulfide (LiFeS) and LiFeS was set to 0 hours (the reaction product of lithium sulfide and iron sulfide (LiFeS) and LiFeS were mixed, but mechanical milling was not performed).

[0127] The ratios (atomic %) of each element in the obtained sample were Li 52%, Fe 11%, and S 37%. The obtained sample did not contain carbon or halogens. In order to measure the electrical conductivity of the obtained sample, the powder was compacted at 300 MPa and the current value was measured by applying a voltage. The electrical conductivity was found to be 1.0 × 10 -5 The values ​​were low compared to Examples 1 and 2, namely, S / cm.

[0128] The X-ray diffraction pattern of the obtained sample was composed of low-crystalline lithium sulfide (Li2S) and low-crystalline Li2FeS2, as shown in Figure 2. That is, the abundance ratio of lithium sulfide (Li2S) estimated by Rietveld analysis was 86 mol % and the abundance ratio of Li2FeS2 was 14 mol %. In addition, the crystallite size estimated from the half-width of the diffraction peak based on the (111) plane of lithium sulfide was 26 nm.

[0129] The obtained iron-containing lithium polysulfide was used as the positive electrode active material, indium metal as the negative electrode, and an argyrodite-type sulfide-based solid electrolyte as the electrolyte to assemble an all-solid-state lithium-ion secondary battery, and charge-discharge tests were conducted.

[0130] For the positive electrode, the above-mentioned iron-containing lithium polysulfide, argyrodite-type sulfide-based solid electrolyte, and acetylene black were mixed in a mass ratio of 4:5:1 to form the positive electrode composite. A 10 mm diameter pellet battery was fabricated by pressure molding the positive electrode composite / argyrodite-type sulfide-based solid electrolyte / lithium indium foil. A charge-discharge test was performed on this battery, starting with a constant current measurement within the range from the initial voltage to 1.0 V. The current density during charging was 0.13 mA / cm. 2and 0.13 to 1.3 mA / cm for discharge. 2 It was decided.

[0131] The initial (first cycle) discharge curve is shown in Figure 3. The initial discharge capacity is higher at low discharge current densities (0.13 mA / cm 2 ) at 470mAh / g and 0.25mA / cm 2 410mAh / g, 0.64mA / cm 2 380mAh / g, high current density (1.3mA / cm 2 ) was 250mAh / g. In other words, at a high current density (1.3mA / cm 2 ) at a low current density (0.13 mA / cm 2 ) and a positive electrode active material with particularly excellent rate characteristics was not obtained.

[0132] Example 3 Commercially available lithium sulfide (LiS) (manufactured by Kojundo Chemical Laboratory Co., Ltd., model number: LII06PB) and iron sulfide (FeS) (manufactured by Alfa Aesar, model number: 14024) were weighed in a glove box (dew point: -80°C) under an argon gas atmosphere so that the molar ratio of LiS:FeS was 5:1. The mixture was thoroughly mixed in a mortar and then filled into a graphite mold with an inner diameter of 15 mm.

[0133] Next, the graphite mold filled with the raw materials was placed in an electric sintering machine. The graphite mold and the electric parts including the electrode parts were placed in a vacuum chamber, which was evacuated to a vacuum (approximately 20 Pa) and then filled with high-purity argon gas (oxygen concentration approximately 0.2 ppm) up to atmospheric pressure.

[0134] Then, the raw material packed in the graphite mold was pressurized at approximately 30 MPa while a DC pulse current of approximately 600 A (pulse width 2.5 ms, cycle 28.6 Hz) was applied. The area around the graphite mold was heated at a temperature increase rate of approximately 200°C / min, and reached 600°C 3 minutes after the start of pulse current application. Immediately after that, the current application and pressure were stopped, and the material was allowed to cool naturally. 10 FeS6 was prepared.

[0135] After cooling to room temperature (25°C), the graphite jig was transferred to a glove box with an argon gas atmosphere at a dew point of -80°C, and the reaction product of lithium sulfide and iron sulfide (Li 10 The LiFeS was removed from the mold and heated at 1000°C for 1 minute in an electric sintering machine. -2 S / cm (25℃)) 10 The mixture was mixed in a mortar to a mass ratio of FeS6:Li2FeS2 = 8:2, placed in a zirconia pot under an argon gas atmosphere, and mechanically milled at 400 rpm for 2 hours using a planetary ball mill (Model P-7) manufactured by Fritsch Japan Co., Ltd. to obtain a sample.

[0136] The ratios (atomic %) of each element in the obtained sample were Li 56%, Fe 8%, and S 36%. The obtained sample did not contain carbon or halogens.

[0137] The X-ray diffraction pattern of the obtained sample was composed of low-crystalline lithium sulfide (Li2S) and iron sulfide (FeS), as shown in Figure 4. The abundance ratio of lithium sulfide (Li2S) determined by Rietveld analysis was 94 mol % and the abundance ratio of FeS was 6 mol %. The crystallite size calculated from the half-width of the diffraction peak based on the (111) plane of lithium sulfide was 23 nm.

[0138] The obtained iron-containing lithium polysulfide was used as the positive electrode active material, indium metal as the negative electrode, and an argyrodite-type sulfide-based solid electrolyte as the electrolyte to assemble an all-solid-state lithium-ion secondary battery, and charge-discharge tests were conducted.

[0139] For the positive electrode, the above-mentioned iron-containing lithium polysulfide, argyrodite-type sulfide-based solid electrolyte, and acetylene black were mixed in a mass ratio of 4:5:1 to form the positive electrode composite. A 10 mm diameter pellet battery was fabricated by pressure molding the positive electrode composite / argyrodite-type sulfide-based solid electrolyte / lithium indium foil. A charge-discharge test was performed on this battery, starting with constant current measurement within the range of 1.0 to 3.0 V. The current density for charging was 0.13 mA / cm. 2 , and 0.13 to 1.3 mA / cm for discharge 2 It was decided.

[0140] The initial (first cycle) discharge curve is shown in Figure 5. The initial discharge capacity is higher at low discharge current densities (0.13 mA / cm 2 ) at approximately 620mAh / g and 0.25mA / cm 2 Approximately 540mAh / g and 0.64mA / cm 2 At a high current density (1.3mA / cm 2 A discharge capacity of approximately 430 mAh / g was obtained at a high current density (1.3 mA / cm 2 ) discharge capacity at low current density (0.13 mA / cm 2 ) showed a high value of approximately 70%, resulting in an electrode material with excellent rate characteristics.

[0141] Comparative Example 3 The reaction product of lithium sulfide and iron sulfide (Li 10 The mechanical milling time of FeS6 and LiFeS2 was set to 0 hours (the reaction product of lithium sulfide and iron sulfide (LiFeS6)). 10 A sample was prepared in exactly the same manner as in Example 3, except that FeS6) and Li2FeS2 were mixed, but mechanical milling was not performed.

[0142] The ratios (atomic %) of the elements in the obtained sample were Li 56%, Fe 8%, and S 36%, similar to those in Example 3. The obtained sample did not contain carbon or halogens.

[0143] The X-ray diffraction pattern of the obtained sample consisted of crystalline lithium sulfide (Li2S) and crystalline Li2FeS2, as shown in Figure 4. The abundance ratio of lithium sulfide (Li2S) determined by Rietveld analysis was 87 mol % and that of Li2FeS2 was 13 mol %. The crystallite size calculated from the half-width of the diffraction peak based on the (111) plane of lithium sulfide was 106 nm.

[0144] The obtained composite powder was assembled into an all-solid-state battery in the same manner as in Example 3, and a charge-discharge test was carried out. The initial (first cycle) discharge curve is shown in Figure 5. The initial discharge capacity was 0.13 mA / cm at a low discharge current density (0.13 mA / cm 2 ) only a low discharge capacity of about 150 mAh / g was obtained at a current density of 0.25 mA / cm 2 At 140mAh / g and 0.64mA / cm 2 At high current densities (1.3mA / cm), the 2 ) only achieved a low discharge capacity of about 120 mAh / g, and Li 10 It was found that simply mixing Li2FeS2 with FeS6 does not result in an electrode material that is excellent in both capacity and rate characteristics.

[0145] Example 4 Commercially available lithium sulfide (LiS) (manufactured by Kojundo Chemical Laboratory Co., Ltd., model number: LII06PB) and iron sulfide (FeS) (Alfa Lithium sulfide and iron sulfide (manufactured by Aesar, model number: 14024) were weighed in a glove box (dew point: -80°C) under an argon gas atmosphere so that the molar ratio of LiS:FeS was 5:1. The mixture was then thoroughly mixed in a mortar and pestle. MoS (a powder mixture of MoS, Mo, and CuS in a molar ratio of 3:3:2 was heat-treated at 1000°C under an argon gas atmosphere to synthesize CuMoS, which was then immersed in 5 mol / L HCl for 1 hour to elute the Cu, producing MoS) was mixed in a mortar at room temperature (25°C) so that the reaction product of lithium sulfide and iron sulfide:MoS was 8:2 (mass ratio). The mixture was placed in a zirconia pot under an argon gas atmosphere and mechanically milled using a Fritsch Japan Co., Ltd. planetary ball mill (model P-7) at 400 rpm for 40 hours.

[0146] The ratios (atomic %) of each element in the obtained sample were Li 55%, Fe 5%, S 37%, and Mo 3%. The obtained sample did not contain carbon or halogens. In order to measure the electrical conductivity of the obtained sample, the powder was compacted at 300 MPa and the current value was measured by applying a voltage. The result was 3.5 × 10 -4 S / cm, which is the electrical conductivity of Comparative Example 4, which was not subjected to mechanical milling treatment. -8 This was a dramatic improvement compared to less than S / cm (unmeasurable).

[0147] As shown in Figure 6, the X-ray diffraction pattern of the obtained sample showed peaks derived from low-crystalline lithium sulfide (Li2S) as the main phase, with no other peaks derived from iron sulfide (FeS). In other words, the abundance ratio of lithium sulfide (Li2S) estimated by Rietveld analysis was 100 mol%. In addition, the crystallite size calculated from the half-width of the diffraction peak based on the (111) plane of lithium sulfide was 17 nm.

[0148] The obtained iron-containing lithium polysulfide was used as the positive electrode active material, indium metal as the negative electrode, and an argyrodite-type sulfide-based solid electrolyte as the electrolyte to assemble an all-solid-state lithium-ion secondary battery, and charge-discharge tests were conducted.

[0149] For the positive electrode, the above-mentioned iron-containing lithium polysulfide, argyrodite-type sulfide-based solid electrolyte, and acetylene black were mixed in a mass ratio of 4:5:1 to form the positive electrode composite. A 10 mm diameter pellet battery was fabricated by pressure molding the positive electrode composite / argyrodite-type sulfide-based solid electrolyte / lithium indium foil. A charge-discharge test was performed on this battery by starting charging with constant current measurement in the range from 3.0 V to 1.0 V. The current density during charging was 0.13 mA / cm. 2 and 0.13 to 1.3 mA / cm for discharge. 2 It was decided.

[0150] The initial (first cycle) discharge curve is shown in Figure 7. The initial discharge capacity is higher at low discharge current densities (0.13 mA / cm 2 ) at 520mAh / g and 0.25mA / cm 2 500mAh / g, 0.64mA / cm 2 480mAh / g, high current density (1.3mA / cm 2 ) was 420mAh / g. In other words, at a high current density (1.3mA / cm 2 ) at a low current density (0.13 mA / cm 2 ) showed a high value of 81% of the initial discharge capacity, and it can be seen that a positive electrode active material excellent in capacity and rate characteristics was obtained.

[0151] Comparative Example 4 A sample was prepared in exactly the same manner as in Example 4, except that the mechanical milling time of Mo6S8 with the reaction product of lithium sulfide and iron sulfide was set to 0 hours (the reaction product of lithium sulfide and iron sulfide was mixed with Mo6S8, but mechanical milling was not performed).

[0152] The ratios (atomic %) of each element in the obtained sample were Li 55%, Fe 5%, S 37%, and Mo 3%, similar to Example 4. The obtained sample did not contain carbon or halogens. In order to measure the electrical conductivity of the obtained sample, the powder was compacted at 300 MPa and the current value was measured by applying a voltage. -8 The value was dramatically lower than that of Example 4, being less than S / cm (impossible to measure).

[0153] The X-ray diffraction pattern of the obtained sample consisted of crystalline lithium sulfide (Li2S), crystalline Mo6S8, and crystalline iron sulfide (FeS), as shown in Figure 6. The abundance ratios of lithium sulfide (Li2S), Mo6S8, and FeS were determined by Rietveld analysis to be 82 mol%, 2 mol%, and 16 mol%, respectively. The crystallite size calculated from the half-width of the diffraction peak based on the (111) plane of lithium sulfide was 80 nm.

[0154] The obtained composite powder was assembled into an all-solid-state battery in the same manner as in Example 4, and a charge-discharge test was carried out. The initial (first cycle) discharge curve is shown in FIG. 7. The initial discharge capacity was 0.01 at a low discharge current density (0.13 mA / cm 2 ) only a low discharge capacity of about 110 mAh / g was obtained at a current density of 0.25 mA / cm 2 At approx. 100mAh / g and 0.64mA / cm 2 At high current densities (1.3mA / cm), the 2 ) only achieved a low discharge capacity of about 70 mAh / g.

[0155] Example 5 Commercially available lithium sulfide (LiS) (manufactured by Kojundo Chemical Laboratory Co., Ltd., model number: LII06PB) and iron sulfide (FeS) (manufactured by Alfa Aesar, model number: 14024) were weighed in an argon gas atmosphere in a glove box (dew point: -80°C) so that the molar ratio of LiS:FeS was 5:1. The mixture was then thoroughly mixed in a mortar and pestle. MoS (manufactured by Kojundo Chemical Laboratory Co., Ltd., model number: MOI06PB) was then added to the mortar at room temperature (25°C) so that the mass ratio of the reactant (LiS) and iron sulfide (FeS) was 8:2. The mixture was placed in a zirconia pot under an argon gas atmosphere and mechanically milled in a Fritsch Japan Co., Ltd. planetary ball mill (model P-7) at 400 rpm for 20 hours to prepare a sample.

[0156] The ratios (atomic %) of each element in the obtained sample were Li 54%, Fe 5%, S 38%, and Mo 3%. The obtained sample did not contain carbon or halogens. In order to measure the electrical conductivity of the obtained sample, the powder was compacted at 300 MPa and the current value was measured by applying a voltage. The result was 3.4 × 10 -3 S / cm, which is the electrical conductivity of Comparative Example 5, which was not subjected to mechanical milling treatment. -8 This was a dramatic improvement compared to less than S / cm (unmeasurable).

[0157] The X-ray diffraction pattern of the obtained sample was composed of low-crystalline lithium sulfide (LiS) and iron sulfide (FeS), as shown in Figure 8. The abundance ratios of lithium sulfide (LiS), MoS, and FeS estimated by Rietveld analysis were 91 mol%, 0 mol%, and 9 mol%, respectively. The crystallite size calculated from the half-width of the diffraction peak based on the (111) plane of lithium sulfide was 19 nm.

[0158] The obtained iron-containing lithium polysulfide was used as the positive electrode active material, indium metal as the negative electrode, and an argyrodite-type sulfide-based solid electrolyte as the electrolyte to assemble an all-solid-state lithium-ion secondary battery, and charge-discharge tests were conducted.

[0159] For the positive electrode, the above-mentioned iron-containing lithium polysulfide, argyrodite-type sulfide-based solid electrolyte, and acetylene black were mixed in a mass ratio of 4:5:1 to form the positive electrode composite. A 10 mm diameter pellet battery was fabricated by pressure molding the positive electrode composite / argyrodite-type sulfide-based solid electrolyte / lithium indium foil. A charge-discharge test was performed on this battery by starting charging with constant current measurement in the range from 3.0 V to 1.0 V. The current density during charging was 0.13 mA / cm. 2 and 0.13 to 1.3 mA / cm for discharge. 2 It was decided.

[0160] The initial (first cycle) discharge curve is shown in Figure 9. The initial discharge capacity was measured at a low discharge current density (0.13 mA / cm 2 ) at approximately 660mAh / g and 0.25mA / cm 2 Approximately 620mAh / g and 0.64mA / cm 2 Approximately 600mAh / g at high current density (1.3mA / cm 2 A discharge capacity of approximately 560 mAh / g was obtained at a high current density (1.3 mA / cm 2 ) discharge capacity at low current density (0.13 mA / cm 2 ) showed a high value of approximately 86%, resulting in an electrode material with excellent rate characteristics.

[0161] Comparative Example 5 A sample was prepared in exactly the same manner as in Example 5, except that the mechanical milling treatment time of MoS with the reaction product of lithium sulfide and iron sulfide was set to 0 hours (the reaction product of lithium sulfide and iron sulfide was mixed with MoS, but mechanical milling treatment was not performed).

[0162] The ratios (atomic %) of each element in the obtained sample were Li 54%, Fe 5%, S 38%, and Mo 3%, similar to Example 5. The obtained sample did not contain carbon or halogens. In order to measure the electrical conductivity of the obtained sample, the powder was compacted at 300 MPa and the current value was measured by applying a voltage. -8The value was less than S / cm (impossible to measure) and was dramatically lower than that of Example 5.

[0163] The X-ray diffraction pattern of the obtained sample was composed of crystalline lithium sulfide (Li2S), iron sulfide (FeS), and MoS2, as shown in Figure 8. The abundance ratio of lithium sulfide (Li2S) was 77 mol%, the abundance ratio of Mo6S8 was 8 mol%, and the abundance ratio of iron sulfide (FeS) was 15 mol%, as determined by Rietveld analysis. The crystallite size calculated from the half-width of the diffraction peak based on the (111) plane of lithium sulfide was 84 nm.

[0164] The obtained composite powder was assembled into an all-solid-state battery in the same manner as in Example 5, and a charge-discharge test was carried out. The results are shown in FIG. 9. The initial discharge capacity was 0.1% at a low discharge current density (0.13 mA / cm 2 ) only a low discharge capacity of about 3 mAh / g was obtained at a current density of 0.25 mA / cm 2 Even at approximately 3mAh / g and 0.64mA / cm 2 Even at high current densities (1.3mA / cm), the 2 ) only a low discharge capacity of about 2 mAh / g was obtained, indicating that simply mixing MoS2 does not result in an electrode material with excellent capacity and rate characteristics.

[0165] Example 6 Commercially available lithium sulfide (LiS) (manufactured by Kojundo Chemical Laboratory Co., Ltd., model number: LII06PB) and iron sulfide (FeS) (manufactured by Alfa Aesar, model number: 14024) were weighed out in a molar ratio of LiS:FeS = 5:1 and thoroughly mixed in a mortar. After that, CuS (manufactured by Fujifilm Wako Pure Chemical Corporation, model number: 034-04462) was added to the mortar at room temperature (25 °C) to achieve a mass ratio of lithium sulfide and iron sulfide to CuS = 8:2. The mixture was placed in a zirconia pot under an argon gas atmosphere and mechanically milled for 40 hours using a Fritsch Japan Co., Ltd. planetary ball mill (model P-7) at 400 rpm to prepare a sample.

[0166] The ratios (atomic %) of each element in the obtained sample were Li 54%, Fe 5%, S 37%, and Cu 4%. The obtained sample did not contain carbon or halogens. In order to measure the electrical conductivity of the obtained sample, the powder was compacted at 300 MPa and the current value was measured by applying a voltage. The electrical conductivity was found to be 1.6 × 10 -6 S / cm, which is the electrical conductivity of Comparative Example 6, which was not subjected to mechanical milling treatment. -8 This was a dramatic improvement compared to less than S / cm (unmeasurable).

[0167] The X-ray diffraction pattern of the obtained sample was composed of low-crystalline lithium sulfide (Li2S), as shown in Figure 10. The abundance ratio of lithium sulfide (Li2S) determined by Rietveld analysis was 100 mol %. The crystallite size calculated from the half-width of the diffraction peak based on the (111) plane of lithium sulfide was 23 nm.

[0168] The obtained iron-containing lithium polysulfide was used as the positive electrode active material, indium metal as the negative electrode, and an argyrodite-type sulfide-based solid electrolyte as the electrolyte to assemble an all-solid-state lithium-ion secondary battery, and charge-discharge tests were conducted.

[0169] For the positive electrode, the above-mentioned iron-containing lithium polysulfide, argyrodite-type sulfide-based solid electrolyte, and acetylene black were mixed in a mass ratio of 4:5:1 to form the positive electrode composite. A 10 mm diameter pellet battery was fabricated by pressure molding the positive electrode composite / argyrodite-type sulfide-based solid electrolyte / lithium indium foil. A charge-discharge test was performed on this battery by starting charging with constant current measurement in the range from 3.0 V to 1.0 V. The current density during charging was 0.13 mA / cm. 2 and 0.13 to 1.3 mA / cm for discharge. 2 It was decided.

[0170] The initial (first cycle) discharge curve is shown in Figure 11. The initial discharge capacity was measured at a low discharge current density (0.13 mA / cm 2 ) at approximately 550mAh / g and 0.25mA / cm 2Approximately 490mAh / g, 0.64mA / cm 2 Approximately 450mAh / g, 1.3mA / cm 2 A discharge capacity of approximately 380 mAh / g was obtained at a high current density (1.3 mA / cm 2 ) discharge capacity at low current density (0.13 mA / cm 2 ) showed a high value of approximately 70%, resulting in an electrode material with excellent rate characteristics.

[0171] Comparative Example 6 A sample was prepared in exactly the same manner as in Example 6, except that the mechanical milling treatment time of CuS with the reaction product of lithium sulfide and iron sulfide was 0 hours (the reaction product of lithium sulfide and iron sulfide was mixed with CuS, but mechanical milling treatment was not performed).

[0172] The ratios (atomic %) of each element in the obtained sample were Li 54%, Fe 5%, S 37%, and Cu 4%, similar to Example 6. The obtained sample did not contain carbon or halogens. In order to measure the electrical conductivity of the obtained sample, the powder was compacted at 300 MPa and the current value was measured by applying a voltage. -8 The value was dramatically lower than that of Example 6, being less than S / cm (impossible to measure).

[0173] The X-ray diffraction pattern of the obtained sample consisted of crystalline lithium sulfide (LiS), iron sulfide (FeS), and CuS, as shown in Figure 10. The abundance ratio of lithium sulfide (LiS), CuS, and iron sulfide (FeS) determined by Rietveld analysis was 73 mol%, 12 mol%, and 15 mol%, respectively. The crystallite size calculated from the half-width of the diffraction peak based on the (111) plane of lithium sulfide was 86 nm.

[0174] The obtained composite powder was assembled into an all-solid-state battery in the same manner as in Example 6, and a charge-discharge test was carried out. The results are shown in FIG. 11. The initial discharge capacity was 0.1% at a low discharge current density (0.13 mA / cm 2) only a low discharge capacity of about 7 mAh / g was obtained at a current density of 0.25 mA / cm 2 Even at approximately 4mAh / g and 0.64mA / cm 2 Even at approximately 3mAh / g and 1.3mA / cm 2 In other words, it was found that simply mixing CuS with the reaction product of lithium sulfide and iron sulfide does not result in an electrode material with excellent capacity or rate characteristics. [Explanation of symbols]

[0175] 1 Electric current sintering equipment 2. Sample 3 Die (conductive container) 4, 5 Electrical punch 6,7 Punch electrode 8 Water-cooled Vacuum Chamber 9 Cooling channel 10, 16 Water cooling mechanism 11 Sintering power supply 12 Control device 13 Pressure mechanism 14 Position measurement mechanism 15 Atmosphere control mechanism 17 Temperature measuring device

Claims

1. An iron-containing lithium polysulfide for use as a positive electrode active material for a lithium ion secondary battery, the iron-containing lithium polysulfide containing lithium, iron, and sulfur as constituent elements, Lithium sulfide (Li ２ S) as a main phase, Li obtained by powder X-ray diffraction ２ The crystallite size calculated from the half-width of the diffraction peak based on the (111) plane of S is 23 nm or less, and The iron-containing lithium polysulfide for use as a positive electrode active material for a lithium ion secondary battery has a Li content of 50 to 70 atomic %, an Fe content of 2 to 12 atomic %, an S content of 20 to 40 atomic %, a C content of 0 to 5 atomic %, and a halogen content of 0 to 0.8 atomic %, where the total amount of the iron-containing lithium polysulfide is taken as 100 atomic %.

2. The amount of lithium sulfide (Li) estimated by Rietveld analysis based on the iron-containing lithium polysulfide was ２ 2. The iron-containing lithium polysulfide for a positive electrode active material of a lithium ion secondary battery according to claim 1, wherein the abundance ratio of iron-containing lithium polysulfide is 70 mol % or more.

3. The iron-containing lithium polysulfide for use as a positive electrode active material for a lithium ion secondary battery according to claim 1 , further comprising an iron sulfide (FeS) phase.

4. Electrical conductivity at 25°C is 10 －３ ~10 －６ 2. The iron-containing lithium polysulfide for use as a positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the surface roughness of the lithium polysulfide is 0.5 S / cm.

5. A method for producing the iron-containing lithium polysulfide for use as a positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 4, comprising: (3) A mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound; a heat-treated product of the mixture; or a mechanically milled product of the heat-treated product of the mixture; and a mixture having an electrical conductivity of 10 －３ A step of mechanically milling a metal sulfide having a viscosity of 0.5 S / cm or more. A manufacturing method comprising:

6. Before the step (3), (2) A step of subjecting a mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound, or a heat-treated product of the mixture, to mechanical milling. The method of claim 5 , comprising:

7. Before the step (2), (1) a step of heating a mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound; The method of claim 6 , comprising:

8. The method according to claim 7, wherein the step (1) is a step of passing a direct current pulse current through the mixture to heat and react the mixture.

9. The method according to claim 7, wherein steps (1) to (3) are carried out in a non-oxidizing atmosphere.

10. A positive electrode active material for a lithium ion secondary battery, comprising the iron-containing lithium polysulfide for a positive electrode active material of a lithium ion secondary battery according to any one of claims 1 to 4.

11. A lithium ion secondary battery comprising the positive electrode active material for lithium ion secondary batteries according to claim 10 as a constituent element.

12. An all-solid-state lithium ion secondary battery comprising, as components, the positive electrode active material for a lithium ion secondary battery according to claim 10 and a lithium ion conductive solid electrolyte.

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