Lithium sulfide-iron-carbon composite

Abstract

Provided is a lithium sulfide-iron-carbon composite which contains lithium, iron, sulfur and carbon as the constituent components thereof, wherein: lithium sulfide (Li2S) is contained as the main phase; the crystallite size calculated from the half-width of a diffraction peak based on the (111) plane of the Li2S obtained by powder X-ray diffraction is 50nm or less; and the Li content is 50-70 at%, the Fe content is 2-10 at%, the S content is 20-40 at% and the C content is 1-8 at% if the total content of the lithium sulfide-iron-carbon composite is 100 at%. As a result, a novel material is provided which is a compound having lithium sulfide as a main component and useful as a lithium-ion secondary battery positive electrode active material, and has a high lithium sulfide use rate, a high capacitance, favorable cycle properties and rate properties, and excellent charge / discharge properties.

Description

[Technical Field]

[0001] The present invention relates to a lithium sulfide-iron-carbon composite 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. In current lithium-ion secondary batteries, the capacity of the positive electrode has lagged behind that of 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 250-300 mAh / 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, because lithium sulfide contains lithium, alloys such as graphite and silicon can be used for the anode, dramatically expanding the range of anode options and avoiding the risk of short circuits caused by dendrite formation when metallic lithium is used. However, in battery systems using organic electrolytes, lithium sulfide elutes into the electrolyte as lithium polysulfides during charging and discharging, which migrate to the anode and segregate (see, for example, Non-Patent Document 1 below), making it difficult to achieve the high capacity inherent to lithium sulfide. Therefore, improving the performance of batteries using lithium sulfide in the cathode requires measures such as designing a cathode layer that retains the eluted lithium polysulfides within the cathode, devising an electrolyte that protects the anode, and even replacing it with a solid electrolyte that does not elute.

[0005] One possible method for suppressing the elution of lithium polysulfide 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 polysulfide compounds described in Patent Documents 1 and 2 are x Fe y S z Examples of such compounds include: lithium sulfide. The introduction of transition metal elements can suppress the release of sulfur and provide electrical conductivity, thereby improving the utilization rate of lithium sulfide. However, to improve cycle performance, it is necessary to improve the reversibility of the structure associated with repeated Li insertion and extraction. One possible approach 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 heterogeneous elements (transition metal elements, typical elements) is a lithium-iron-phosphorus-sulfur-carbon composite (see, for example, Patent Document 3). While its advantages, such as high capacity without staged pre-charge and discharge, have been reported, no examples of improved cycle performance have yet been reported. Secondary batteries for use in the aforementioned portable electronic devices, hybrid vehicles, and other devices require high capacity as well as good cycle performance and good charge / discharge performance (rate performance) under high loads. Therefore, there is a need for the development of materials that satisfy both high capacity and good cycle and rate performance. [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] 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 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 cycle characteristics and rate characteristics, as well as excellent charge and discharge characteristics. [Means for solving the problem]

[0009] The present inventors have conducted extensive research to achieve the above-mentioned objectives. As a result, they have discovered that a lithium sulfide-iron-carbon composite containing lithium sulfide (LiS) as a primary phase has a crystallite size of 50 nm or less, calculated from the half-width of the diffraction peak based on the (111) plane of LiS obtained by powder X-ray diffraction. The lithium sulfide-iron-carbon composite has a Li content of 50-70 atomic %, an Fe content of 2-10 atomic %, an S content of 20-40 atomic %, and a C content of 1-8 atomic %, where the total amount of the lithium sulfide-iron-carbon composite is 100 atomic %. This composite has improved lithium sulfide utilization, resulting in a high-capacity material. Furthermore, the formation of sulfur-iron bonds significantly reduces free sulfur, improving cycle and rate performance. This lithium sulfide-iron-carbon composite can be obtained, for example, by passing a direct current pulse current through a mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound to cause a thermal reaction by current sintering, followed by mechanical milling of the resulting product. According to this method, a mixture of lithium sulfide and iron sulfide is formed during electric current sintering, whereby the reaction at the atomic level has progressed moderately due to the heating reaction, and this mixture is then subjected to mechanical milling together with carbon, which facilitates the formation of a metastable phase in which iron atoms are incorporated into the lithium sulfide crystal lattice, and the resulting fine particle size facilitates stabilization of this metastable phase. The present invention was completed as a result of further research based on these findings. That is, the present invention includes the following configurations.

[0010] Item 1. A lithium sulfide-iron-carbon composite containing lithium, iron, sulfur, and carbon 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 A lithium sulfide-iron-carbon composite having a Li content of 48.0 to 70.0 atomic %, an Fe content of 2.0 to 10.0 atomic %, an S content of 20.0 to 40.0 atomic %, and a C content of 1.0 to 9.0 atomic %, where the total amount of the lithium sulfide-iron-carbon composite is 100 atomic %.

[0011] Item 2. The lithium sulfide-iron-carbon composite according to Item 1, wherein the abundance ratio of lithium sulfide (LiS) is 90 mol % or more based on the lithium sulfide-iron-carbon composite as estimated by Rietveld analysis.

[0012] Item 3. The lithium sulfide-iron-carbon composite according to Item 1 or 2, wherein the iron is arranged in a crystal lattice of the lithium sulfide (LiS) to form an Fe-S bond.

[0013] Item 4. The lithium sulfide-iron-carbon composite according to any one of Items 1 to 3, which is for use in a lithium ion secondary battery.

[0014] Item 5. The lithium sulfide-iron-carbon composite according to any one of Items 1 to 4, which is used as a positive electrode active material for a lithium ion secondary battery.

[0015] Item 6. A method for producing the lithium sulfide-iron-carbon composite according to any one of Items 1 to 5, (1) a direct current sintering step in which a mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound is heated and reacted by applying a direct current pulse current to the mixture; and (2) A mechanical milling step in which the product obtained in the step (1) is subjected to mechanical milling. A manufacturing method comprising:

[0016] Item 7. The production method according to Item 6, wherein steps (1) and (2) are carried out in a non-oxidizing atmosphere.

[0017] Item 8. The production method according to Item 6 or 7, wherein step (2) is carried out in the presence of a carbon-containing compound.

[0018] Item 9. A positive electrode active material for a lithium ion secondary battery, comprising the lithium sulfide-iron-carbon composite according to any one of Items 1 to 5.

[0019] Item 10. A lithium ion secondary battery comprising the positive electrode active material for lithium ion secondary batteries according to Item 9 as a constituent element.

[0020] Item 11. 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 9 and a lithium ion conductive solid electrolyte. [Effects of the Invention]

[0021] The lithium sulfide-iron-carbon composite of the present invention is in the form of finely divided particles with a crystallite size of 50 nm or less. It contains lithium sulfide (LiS) as the main phase, and the added elements are adjusted within a specific composition range. This, combined with the above, results in a high utilization rate of lithium sulfide, allowing the high capacity characteristic of lithium sulfide to be fully exhibited, and also suppresses the elution of polysulfides during lithium insertion and desorption reactions, allowing the composite to exhibit excellent cycle characteristics. Furthermore, the presence of carbon further improves the electrical conductivity, resulting in a positive electrode active material with high capacity and excellent rate characteristics.

[0022] Therefore, the lithium sulfide-iron-carbon composite 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.

[0023] 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]

[0024] [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 the samples obtained in Example 1 and Comparative Example 1. [Figure 3] 1 shows charge-discharge curves at a current density of 0.13 mA / cm2 of all-solid-state lithium-ion secondary batteries using the samples obtained in Example 1 and Comparative Example 1 as positive electrode active materials. [Figure 4] 1 shows the results of cycle characteristics of all-solid-state lithium ion secondary batteries using the samples obtained in Example 1 and Comparative Example 1 as positive electrode active materials. [Figure 5]1 shows X-ray diffraction patterns of samples obtained in Example 2 and Comparative Example 2. [Figure 6] 1 shows charge-discharge curves at a current density of 0.64 mA / cm2 of all-solid-state lithium-ion secondary batteries using the samples obtained in Example 2 and Comparative Example 2 as positive electrode active materials. [Figure 7] 1 shows charge-discharge curves at a current density of 1.3 mA / cm2 of all-solid-state lithium-ion secondary batteries using the samples obtained in Example 2 and Comparative Example 2 as positive electrode active materials. [Figure 8] 1 shows an X-ray diffraction pattern of the sample obtained in Comparative Example 3. [Figure 9] 10 shows charge-discharge curves at a current density of 0.13 mA / cm2 of an all-solid-state lithium-ion secondary battery using the sample obtained in Comparative Example 3 as a positive electrode active material. [Figure 10] 1 shows X-ray diffraction patterns of the samples obtained in Example 3 and Comparative Example 4. [Figure 11] 1 shows charge-discharge curves at a current density of 1.3 mA / cm2 of all-solid-state lithium-ion secondary batteries using the samples obtained in Example 3 and Comparative Example 4 as positive electrode active materials. DETAILED DESCRIPTION OF THE INVENTION

[0025] In this specification, when a numerical range is expressed as "A to B," it means A or more and B or less.

[0026] Furthermore, the term "containing" encompasses all of "comprise," "consist essentially of," and "consist only of."

[0027] 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.

[0028] 1. Lithium sulfide-iron-carbon composite The lithium sulfide-iron-carbon composite of the present invention is a lithium sulfide-iron-carbon composite containing lithium, iron, sulfur, and carbon 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 lithium content is 50 to 70 atomic %, the Fe content is 2 to 10 atomic %, the S content is 20 to 40 atomic %, and the C content is 1 to 8 atomic %, based on the total amount of the lithium sulfide-iron-carbon composite being 100 atomic %. The lithium sulfide-iron-carbon composite of the present invention has a high lithium sulfide utilization rate, the high capacity and excellent rate characteristics inherent to lithium sulfide, and excellent cycle characteristics due to the suppression of polysulfide elution during lithium insertion and extraction reactions. Furthermore, the presence of carbon further improves the electrical conductivity, making it a high-capacity positive electrode active material, and it is particularly useful as a positive electrode active material for lithium-ion secondary batteries.

[0029] With such a lithium sulfide-iron-carbon composite 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 0.64 mA / cm 2 The initial discharge capacity can be reduced to 50-90% at an applied current of 1.3 mA / cm 2 It is possible to make the value of the initial discharge capacity at this temperature 30 to 80%. In this specification, the presence of such a characteristic may be referred to as "having excellent rate characteristics."

[0030] In powder X-ray diffraction analysis, the lithium sulfide-iron-carbon composite of the present invention is found to have a primary phase consisting of lithium sulfide. The amount of the lithium sulfide phase is preferably 90 mol % or more, more preferably 95 mol % or more, based on the entire lithium sulfide-iron-carbon composite of the present invention (100 mol %). The upper limit of the amount of the lithium sulfide phase is not particularly limited, but is typically 100 mol %. Furthermore, the lithium sulfide-iron-carbon composite of the present invention may contain small amounts of impurities (such as the raw material compounds described below) up to 10 mol % (particularly up to 5 mol %) in addition to the lithium sulfide crystalline phase. This level of impurity content has only a limited effect on charge / discharge characteristics. The amount of impurities present in the composite is estimated using conventional Rietveld analysis of X-ray diffraction data. The Rietveld analysis is described in detail in the following non-patent document (F. Izumi and T. Ikeda, Mater Sci. Forum, 321-324, 198 (2000)).

[0031] In such a composite, iron atoms are arranged within the lithium sulfide crystal lattice to form Fe-S bonds, and the inclusion of Fe provides electrical conductivity to the interior, resulting in high electrode utilization (especially positive electrode utilization), allowing the inherent high capacity properties of lithium sulfide to be more fully exhibited. Furthermore, when the manufacturing method of the present invention described below is employed, iron-containing lithium sulfide in a metastable phase is formed, and this can be refined by mechanical milling to form submicron particles, stabilizing the inherent metastable iron-containing lithium sulfide.

[0032] In the lithium sulfide-iron-carbon composite of the present invention having the above-mentioned characteristics, it is preferable that the added element iron atoms are arranged within the lithium sulfide crystal lattice to form Fe-S bonds, which further tends to suppress the presence of free sulfur and also tends to suppress the elution of lithium polysulfides into the electrolyte upon deintercalation and insertion of Li, which then migrates to and deposits on the negative electrode, making it easier to exhibit particularly excellent cycle characteristics. The lithium sulfide-iron-carbon composite of the present invention is endowed with good electrical conductivity due to the inclusion of Fe and C, and in particular, when Fe forms Fe-S bonds within the lithium sulfide crystal lattice, the utilization rate of the interior of the lithium sulfide crystal is easily improved, making it easier to produce a material with even higher capacity and excellent rate characteristics.

[0033] Furthermore, the lithium sulfide-iron-carbon composite of the present invention preferably has a stabilized metastable phase in which iron atoms are introduced into the lithium sulfide crystal lattice, and the carbon is uniformly dispersed. The lithium sulfide-iron-carbon composite of the present invention preferably comprises submicron-sized crystallites because the metastable lithium-iron-sulfur composite exists stably. More specifically, the crystallite size of the lithium sulfide-iron-carbon composite of the present invention is preferably 50 nm or less, more preferably 40 nm or less (particularly 1 to 30 nm). When the lithium sulfide-iron-carbon composite of the present invention is produced by a manufacturing method including mechanical milling, as described below, the crystallites can be refined by mechanical milling. The crystallite size of the lithium sulfide-iron-carbon composite 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.

[0034] Furthermore, the proportions of each element in the lithium sulfide-iron-carbon composite 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 theoretical capacity estimated from the amount of Li is 600 mAh / g or more, and that the amount of Fe is sufficient to ensure electrical conductivity. However, while the inclusion of carbon facilitates improved electrical conductivity, an excessive amount of C degrades the capacity, cycle characteristics, and rate characteristics. From this perspective, it is preferable that, based on the total amount of the lithium sulfide-iron-carbon composite of the present invention formed, the Li content be 48.0 to 70.0 at.% (particularly 50.0 to 60.0 at.%), the Fe content be 2.0 to 10.0 at.% (particularly 4.0 to 9.0 at.%), the S content be 20.0 to 40.0 at.% (particularly 30.0 to 40.0 at.%), and the C content be 1.0 to 9.0 at.% (particularly 3.0 to 8.0 at.%).

[0035] 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.

[0036] 2. Manufacturing method of lithium sulfide-iron-carbon composite The lithium sulfide-iron-carbon composite of the present invention is not particularly limited, but (1) a direct current sintering step in which a mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound is heated and reacted by applying a direct current pulse current to the mixture; and (2) A mechanical milling step in which the product obtained in the step (1) is subjected to mechanical milling. According to this method, a mixture of lithium sulfide and iron sulfide can be obtained in which the reaction at the atomic level has progressed appropriately due to the overheating reaction in the electric current sintering step, and this mixture can be subjected to mechanical milling, if necessary, together with a carbon-containing compound, to refine the particles, stabilize the metastable phase in which iron atoms are incorporated into the lithium sulfide phase, and obtain a composite in which carbon is uniformly dispersed. This method will be specifically described below.

[0037] (2-1) Raw material powder In the present invention, a lithium-containing compound, an iron-containing compound, a sulfur-containing compound, and, if necessary, a carbon-containing compound are used as raw materials.

[0038] The types of lithium-containing compounds, iron-containing compounds, sulfur-containing compounds, and carbon-containing compounds are not particularly limited, and a mixture of four or more compounds containing one type each of lithium, iron, sulfur, and carbon may be used, or a compound containing two or more elements selected from lithium, iron, sulfur, and carbon may be used as part of the raw material.

[0039] These raw material compounds are preferably compounds that do not contain metal elements other than lithium and iron. Furthermore, it is preferable that elements contained in the raw material compounds other than lithium, iron, sulfur, and carbon are released or volatilized by heat treatment in a non-oxidizing atmosphere, as described below.

[0040] Specific examples of such raw material compounds include lithium-containing compounds such as lithium sulfide (LiS), lithium carbonate (LiCO), and lithium hydroxide (LiOH); iron-containing compounds such as metallic iron (Fe), iron sulfide (FeS, FeS, etc.), and iron sulfate (FeSO); sulfur-containing compounds such as sulfur (S), lithium sulfide (LiS), benzothiophene (CHS), and iron sulfide (FeS, FeS, etc.); and carbon-containing compounds such as carbon (C), lithium carbonate (LiCO), and benzothiophene (CHS). Among these, a combination of lithium sulfide (LiS), iron sulfide (FeS, FeS), and carbon (C) is particularly preferred, as it consists only of the constituent elements of the lithium sulfide-iron-carbon composite of the present invention, which is the product, and can be reacted with a minimum number of raw materials. Among these raw material compounds, the carbon is not particularly limited, and for example, graphite, mesoporous carbon, hard carbon (non-graphitizable carbon material), etc. can be used.

[0041] 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%.

[0042] The blending ratios of the raw materials, consisting of a lithium-containing compound, an iron-containing compound, a sulfur-containing compound, and a carbon-containing compound, are not particularly limited, but preferably contain enough Fe to facilitate the formation of Fe-S bonds without generating free sulfur in the final lithium sulfide-iron-carbon composite of the present invention, enough Li to ensure a theoretical capacity of about 600 mAh / g or greater as estimated from the Li amount, and enough Fe and C to ensure electrical conductivity and prevent deterioration of capacity, cycle characteristics, and rate performance. From these perspectives, the blending ratios of the raw materials are preferably adjusted so that the Li content is 48.0 to 70.0 at.% (particularly 50.0 to 60.0 at.%), the Fe content is 2.0 to 10.0 at.% (particularly 4.0 to 9.0 at.%), the S content is 20.0 to 40.0 at.% (particularly 30.0 to 40.0 at.%), and the C content is 1.0 to 9.0 at.% (particularly 3.0 to 8.0 at.%). The mixing ratio of the raw material compounds can be adjusted so that the ratio of each element contained in the raw material compounds is the same as the ratio of each element in the intended lithium sulfide-iron-carbon composite of the present invention.

[0043] 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.

[0044] (2-2) Electric current sintering process (process (1)) In the present invention, first, as the electric current sintering step, it is preferable to subject a mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound to an electric current sintering treatment. Specifically, the raw material mixture is filled into a conductive mold (conductive container), and the conductive mold (conductive container) is preferably energized with a DC pulse current (a method called a spark plasma sintering method, a pulse current sintering method, a plasma activated sintering method, etc.), thereby heating the conductive mold (conductive container) by Joule heat, heating the raw material mixture in the conductive mold (conductive container), diffusing and transferring each element, and preferably producing an intermediate in which each element is mixed at the atomic level.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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).

[0049] 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).

[0050] 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.

[0051] 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.

[0052] The pressure to be applied when pressing the raw material powder is preferably, for example, 5 to 60 MPa, and more preferably 10 to 50 MPa, from the viewpoints of easily strengthening contact between the raw material compounds, easily allowing for sufficient inter-atomic diffusion during heating, and easily allowing for sufficient inter-atomic reaction in the raw material compounds.

[0053] 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.

[0054] A specific example of the electric current sintering process of the present invention will be described below with reference to Fig. 1, which shows a schematic diagram of an electric current sintering apparatus. Note that the electric current sintering apparatus used in the present invention is not limited to the apparatus described below.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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 .

[0060] (2-3) Mechanical milling process (process (2)) The lithium sulfide-iron-carbon composite of the present invention can be obtained by subjecting the intermediate obtained by the electric current sintering treatment described above to mechanical milling, followed by mixing, pulverization, and reaction. When the intermediate obtained by the electric current sintering treatment described above is subjected to mechanical milling together with a carbon-containing compound, it is preferable to mix the obtained intermediate with the carbon-containing compound before subjecting it to mechanical milling, and then subjecting it to mechanical milling.

[0061] Mechanical milling is a method of grinding and mixing raw materials while applying mechanical energy to them, causing a reaction. This method applies mechanical impact and friction to the raw materials to grind and mix them, which causes the compounds contained in the raw materials to come into vigorous contact and become finer, making it easier to obtain a metastable phase. In the present invention, the above-described mechanical milling process forms a metastable lithium sulfide-iron-carbon composite, which is difficult to produce by heat treatment alone in the electric current sintering process, while also finely sintering the composite to ensure stable existence. Furthermore, adding an appropriate amount of carbon imparts good electrical conductivity, resulting in excellent capacity, cycle characteristics, and rate characteristics.

[0062] 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 vibration mill is preferable.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 3. Lithium-ion secondary batteries Taking advantage of the excellent properties described above, the lithium sulfide-iron-carbon composite 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 lithium sulfide-iron-carbon composite 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 cycle characteristics and rate characteristics, making it useful as a positive electrode active material for lithium ion secondary batteries. A lithium ion secondary battery that uses the lithium sulfide-iron-carbon composite 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.

[0067] 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 lithium sulfide-iron-carbon composite of the present invention is used as a positive electrode active material.

[0068] 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 above-described lithium sulfide-iron-carbon composite of the present invention is used as a positive electrode active material.

[0069] For the positive electrode, the lithium sulfide-iron-carbon composite 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. As the conductive material, for example, carbon materials such as graphite, cokes, carbon black, and acicular carbon can be used.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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 lithium sulfide-iron-carbon composite of the present invention is used as the positive electrode active material.

[0074] 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.

[0075] For the positive electrode of an all-solid-state lithium ion secondary battery, for example, the lithium sulfide-iron-carbon composite 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.

[0076] 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.

[0077] 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]

[0078] 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.

[0079] 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 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.

[0080] 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.

[0081] 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.

[0082] After cooling to room temperature, the graphite jig was transferred to a glove box with an argon gas atmosphere at a dew point of -80°C. The lithium sulfide and iron sulfide reactant was removed from the mold and ground in a mortar. Acetylene black (AB) powder was added to the lithium sulfide-iron sulfide reactant at a mass ratio of 95:5 (AB:Li / Ag). After thorough mixing in the mortar, the mixture was placed in a zirconia pot under an argon gas atmosphere and mechanically milled at 400 rpm for 15 hours using a Fritsch Japan Co., Ltd. planetary ball mill (Model P-7). The atomic percentages of the elements used in the raw materials were Li 54.4%, Fe 5.4%, S 32.6%, and C 7.6%.

[0083] As shown in Figure 2, the X-ray diffraction pattern of the obtained sample showed a peak derived from lithium sulfide as the main phase, with no other clear peaks. In other words, the abundance ratio of lithium sulfide (Li2S) estimated by Rietveld analysis was 100 mol%. Furthermore, the crystallite size estimated from the half-width of the diffraction peak based on the (111) plane of lithium sulfide was approximately 22 nm. From the above, this method successfully produced a lithium sulfide-iron-carbon composite with a main phase of lithium sulfide and a crystallite size of 50 nm or less.

[0084] The obtained lithium sulfide-iron-carbon composite 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.

[0085] For the positive electrode, the lithium sulfide-iron-carbon composite, the argyrodite-type sulfide solid electrolyte, and acetylene black were mixed in a mass ratio of 4:5:0.7 to form a positive electrode composite. The positive electrode composite / argyrodite-type sulfide solid electrolyte / lithium indium foil was pressure-molded to form a pellet battery with a diameter of 10 mm. This was used at a current density of 0.13 mA / cm. 2 ~1.3mA / cm 2 A charge-discharge test was carried out by starting charge with constant current measurement at a cutoff voltage of 1.0-3.0V.

[0086] As shown in FIG. 3, the results of the charge / discharge capacity showed that the initial discharge capacity per electrode weight was 319 mAh / g, which was higher than that of Comparative Example 1 (284 mAh / g).

[0087] As shown in Figure 4, the cycle characteristics were 2 In the case of Comparative Example 1, the capacity retention rate after 10 cycles was about 75%, which was higher than that in Comparative Example 1 (about 71%). Furthermore, at a current density of 0.25 mA / cm 2 In this case, the capacity retention rate after 10 cycles was about 40%, which was higher than that in Comparative Example 1 (about 34%).

[0088] From the above results, it was confirmed that by preparing a lithium sulfide-iron-carbon composite under the conditions employed in the present invention, a positive electrode active material with improved capacity and cycle characteristics can be obtained.

[0089] Comparative Example 1 A lithium sulfide-iron-carbon composite was produced in the same manner as in Example 1, except that the amounts of the lithium sulfide-iron sulfide reactant and acetylene black (AB) added were adjusted to a mass ratio of lithium sulfide-iron sulfide reactant:AB = 90:10. The ratios (atomic %) of the elements used in the raw materials were Li 50.2%, Fe 5.0%, S 30.1%, and C 14.7%.

[0090] The X-ray diffraction pattern of the obtained sample, as shown in Figure 2, showed a peak derived from lithium sulfide as the main phase, with no other clear peaks. 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 approximately 24 nm.

[0091] A charge-discharge test was carried out in exactly the same manner as in Example 1, except that this composite powder was used as the positive electrode active material of an all-solid-state lithium ion secondary battery.

[0092] As shown in FIG. 3, the results of the charge / discharge capacity showed that the initial discharge capacity per electrode weight was 284 mAh / g, which was lower than that in Example 1 (319 mAh / g).

[0093] As shown in Figure 4, the cycle characteristics were 2 In the case of Example 1, the capacity retention rate after 10 cycles was about 71%, which was lower than that in Example 1 (about 75%). Furthermore, at a current density of 0.25 mA / cm 2 In this case, the capacity retention rate after 10 cycles was about 34%, which was lower than that in Example 1 (about 40%).

[0094] From the above results, it was found that even if electric current sintering and mechanical milling were performed, if the carbon content was too high, the capacity and cycle characteristics were poor.

[0095] Example 2 A lithium sulfide-iron-carbon composite was produced in the same manner as in Example 1, except that the mixing ratio of lithium sulfide (LiS) and iron sulfide (FeS) was set to a molar ratio of 6:1. The ratios (atomic %) of each element used in the raw materials were Li 55.6%, Fe 4.6%, S 32.4%, and C 7.4%.

[0096] As shown in Figure 5, the X-ray diffraction pattern of the obtained sample showed a peak derived from lithium sulfide as the main phase, with no other clear peaks. In other words, the abundance ratio of lithium sulfide (Li2S) estimated by Rietveld analysis was 100 mol%. Furthermore, the crystallite size estimated from the half-width of the diffraction peak based on the (111) plane of lithium sulfide was approximately 23 nm. From the above, this method successfully produced a lithium sulfide-iron-carbon composite with a main phase of lithium sulfide and a crystallite size of 50 nm or less.

[0097] A charge-discharge test was carried out in exactly the same manner as in Example 1, except that this composite powder was used as the positive electrode active material of an all-solid-state lithium ion secondary battery.

[0098] Current density 0.64mA / cm 2 As shown in Figure 6, the initial discharge capacity per electrode weight was 135 mAh / g (current density 0.13 mA / cm 2 This was about 59% of the value (about 230 mAh / g) in the case of Comparative Example 1, which was higher than the value (76 mAh / g) in Comparative Example 2. In addition, the current density was 1.3 mA / cm 2 As shown in Figure 7, the initial discharge capacity per electrode weight was 83 mAh / g (current density 0.13 mA / cm 2 This was about 36% of the value in the case of Example 1 (about 230 mAh / g), which was higher than the value in the case of Comparative Example 2 (13 mAh / g).

[0099] From the above results, it was confirmed that by preparing a lithium sulfide-iron-carbon composite under the conditions employed in the present invention, a positive electrode active material with improved capacity and rate characteristics can be obtained.

[0100] Comparative Example 2 A lithium sulfide-iron-carbon composite was produced in the same manner as in Example 1, except that the mixing ratio of lithium sulfide (LiS) and iron sulfide (FeS) was set to a molar ratio of 6:1, and the amounts of lithium sulfide-iron sulfide reactant and acetylene black (AB) added were adjusted to a mass ratio of lithium sulfide-iron sulfide reactant:AB = 90:10. The ratios (atomic %) of each element used in the raw materials were Li 51.4%, Fe 4.3%, S 30.0%, and C 14.4%.

[0101] The X-ray diffraction pattern of the obtained sample, as shown in Figure 5, showed a peak derived from lithium sulfide as the main phase, with no other clear peaks. 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 approximately 25 nm.

[0102] A charge-discharge test was carried out in exactly the same manner as in Example 1, except that this composite powder was used as the positive electrode active material of an all-solid-state lithium ion secondary battery.

[0103] Current density 0.64mA / cm 2 As shown in Figure 6, the initial discharge capacity per electrode weight was 76 mAh / g (current density 0.13 mA / cm 2 This was about 33% of the value (about 230 mAh / g) in Example 1, which was lower than the value (135 mAh / g) in Example 2. In addition, the current density was 1.3 mA / cm 2 As shown in Figure 7, the initial discharge capacity per electrode weight was 13 mAh / g (current density 0.13 mA / cm 2 This was about 6% of the value in Example 1 (about 230 mAh / g), which was lower than the value in Example 2 (83 mAh / g).

[0104] From the above results, it was found that even if electric current sintering and mechanical milling were performed, if the carbon content was too high, the capacity and rate characteristics were poor.

[0105] Comparative Example 3 Lithium sulfide (Li2S) and iron sulfide (FeS2) (manufactured by Aldrich, model number: 778117-25G) were mixed in a mass ratio of 8:2, and then subjected to electric sintering at 600°C under the same conditions as in Example 1.

[0106] The resulting product was mixed with acetylene black (AB) powder in a mass ratio of lithium sulfide-iron sulfide reactant:AB = 95:5 to produce a composite. The ratios (atomic %) of each element used in the raw materials were Li 56.5%, Fe 2.7%, S 33.7%, and C 7.1%.

[0107] The X-ray diffraction pattern of the obtained sample is shown in Figure 8. As is clear from Figure 8, this product exhibited a peak derived from lithium sulfide as the main phase, along with small amounts of peaks attributable to LiFeS and FeS. The crystallite size estimated from the half-width of the diffraction peak based on the (111) plane of lithium sulfide was approximately 120 nm.

[0108] From the above results, it was found that even if the atomic ratio of lithium, iron, sulfur, and carbon satisfies the conditions of claim 1, a lithium sulfide-iron-carbon composite with a lithium sulfide crystallite size of 50 nm or less cannot be obtained if only electric current sintering is performed without mechanical milling.

[0109] Except for using this composite powder as a positive electrode material for a lithium secondary battery, a charge-discharge test was carried out in exactly the same manner as in Example 1. The charge-discharge characteristics are shown in Figure 9, and the initial charge-discharge capacity was a low value of 3 mAh / g or less.

[0110] From the above results, it was found that even if the atomic configuration described in claim 1 is used as a positive electrode material for a lithium secondary battery, a high capacity value cannot be obtained unless the lithium sulfide-iron-carbon composite has a crystallite size of 50 nm or less.

[0111] Example 3 A lithium sulfide-iron-carbon composite was produced in the same manner as in Example 1, except that the mixing ratio of lithium sulfide (LiS) and iron sulfide (FeS) was set to a molar ratio of 3:1. The ratios (atomic %) of each element used in the raw materials were Li 50.1%, Fe 8.4%, S 33.5%, and C 8.0%.

[0112] As shown in Figure 10, the X-ray diffraction pattern of the obtained sample showed a peak derived from lithium sulfide as the main phase, with no other clear peaks. In other words, the abundance ratio of lithium sulfide (Li2S) estimated by Rietveld analysis was 100 mol%. Furthermore, the crystallite size estimated from the half-width of the diffraction peak based on the (111) plane of lithium sulfide was approximately 17 nm. From the above, this method successfully produced a lithium sulfide-iron-carbon composite with a main phase of lithium sulfide and a crystallite size of 50 nm or less.

[0113] A charge-discharge test was carried out in exactly the same manner as in Example 1, except that this composite powder was used as the positive electrode active material of an all-solid-state lithium ion secondary battery.

[0114] Current density 1.3mA / cm 2 As shown in FIG. 11, the initial discharge capacity per electrode weight was 187 mAh / g, which was higher than that of Comparative Example 4 (163 mAh / g).

[0115] From the above results, it was confirmed that by preparing a lithium sulfide-iron-carbon composite under the conditions employed in the present invention, a positive electrode active material with improved capacity and rate characteristics can be obtained.

[0116] Comparative Example 4 A lithium sulfide-iron-carbon composite was produced in the same manner as in Example 1, except that the mixing ratio of lithium sulfide (LiS) and iron sulfide (FeS) was set to a molar ratio of 3:1, and the amounts of lithium sulfide-iron sulfide reactant and acetylene black (AB) added were adjusted to a mass ratio of lithium sulfide-iron sulfide reactant:AB = 90:10. The ratios (atomic %) of each element used in the raw materials were Li 45.8%, Fe 7.6%, S 30.6%, and C 16.0%.

[0117] The X-ray diffraction pattern of the obtained sample, as shown in Figure 10, showed a peak derived from lithium sulfide as the main phase, along with a small amount of FeS. The abundance ratio of lithium sulfide (Li2S) estimated by Rietveld analysis was 99 mol%. The crystallite size estimated from the half-width of the diffraction peak based on the (111) plane of lithium sulfide was approximately 25 nm.

[0118] A charge-discharge test was carried out in exactly the same manner as in Example 1, except that this composite powder was used as the positive electrode active material of an all-solid-state lithium ion secondary battery.

[0119] Current density 1.3mA / cm 2 As shown in FIG. 11, the initial discharge capacity per electrode weight was 163 mAh / g, which was lower than that in Example 3 (187 mAh / g).

[0120] From the above results, it was found that even if electric current sintering and mechanical milling were performed, if the carbon content was too high, the capacity and rate characteristics were poor. [Explanation of symbols]

[0121] 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. A lithium sulfide-iron-carbon composite for a lithium ion secondary battery, containing lithium, iron, sulfur, and carbon 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 50 nm or less, and A lithium sulfide-iron-carbon composite for a lithium ion secondary battery, wherein the Li content is 48.0 to 70.0 atomic %, the Fe content is 2.0 to 10.0 atomic %, the S content is 20.0 to 40.0 atomic %, and the C content is 1.0 to 9.0 atomic %, relative to 100 atomic % of the total amount of the lithium sulfide-iron-carbon composite.

2. The lithium sulfide-iron-carbon composite was used as a reference, and the lithium sulfide (Li ２ 2. The lithium sulfide-iron-carbon composite for a lithium ion secondary battery according to claim 1, wherein the abundance ratio of S) is 90 mol % or more.

3. The iron is the lithium sulfide (Li ２ 2. The lithium sulfide-iron-carbon composite for a lithium ion secondary battery according to claim 1, wherein the lithium sulfide-iron-carbon composite is arranged in a crystal lattice of the lithium sulfide-iron-carbon compound (S) to form an Fe—S bond.

4. The lithium sulfide-iron-carbon composite for lithium ion secondary batteries according to claim 1, which is used as a positive electrode active material for lithium ion secondary batteries.

5. A method for producing the lithium sulfide-iron-carbon composite for a lithium ion secondary battery according to any one of claims 1 to 4, comprising: (1) a direct current sintering step in which a mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound is heated and reacted by applying a direct current pulse current to the mixture; and (2) A mechanical milling step in which the product obtained in the step (1) is subjected to a mechanical milling treatment. A manufacturing method comprising:

6. The method according to claim 5, wherein steps (1) and (2) are carried out in a non-oxidizing atmosphere.

7. The method according to claim 5, wherein the step (2) is carried out in the presence of a carbon-containing compound.

8. A positive electrode active material for a lithium ion secondary battery, comprising the lithium sulfide-iron-carbon composite for a lithium ion secondary battery according to any one of claims 1 to 4.

9. A lithium ion secondary battery comprising the positive electrode active material for lithium ion secondary batteries according to claim 8 as a constituent element.

10. 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 8 and a lithium ion conductive solid electrolyte.

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