JP7797056B2 - Iron-containing lithium polysulfide - Google Patents

Description

The present invention relates to iron-containing lithium polysulfide and a method for producing the same.

Due to the recent advances in performance of portable electronic devices, hybrid vehicles, etc., secondary batteries (especially lithium-ion secondary batteries) used in these devices are increasingly required to have higher capacities. Current lithium-ion secondary batteries have lagged behind in increasing the capacity of their positive electrodes compared to their negative electrodes, 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.

On the other hand, sulfur has a high theoretical capacity of approximately 1670 mAh/g, making it a promising candidate for a high-capacity electrode material. However, because elemental sulfur does not contain lithium, lithium or an alloy containing lithium must be used for the negative electrode, which has the disadvantage of limiting the range of options for the negative electrode.

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 using metallic lithium. However, lithium sulfide has poor electrical conductivity (see, for example, Non-Patent Document 1 below).

One possible method for improving electrical conductivity is to form bonds with heteroelements so that sulfur atoms are not liberated during Li insertion and desorption reactions. To achieve this, it is necessary to prepare a compound in which a heteroelement is incorporated into lithium sulfide. Suitable heteroelements to be added are transition metal elements that can impart electrical conductivity to insulating lithium sulfide, and examples of such compounds include Li _x Fe _y S _z , as described in Patent Documents 1 and 2. The introduction of such transition metal elements can suppress the liberation of sulfur elements and impart 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 heteroelements (transition metal elements, typical elements) is a lithium-iron-phosphorus-sulfur-carbon composite (see, for example, Patent Document 3).

International Publication No. 2010/084808 International Publication No. 2015/037598 International Publication No. 2016/080443

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).

However, even the conventional sulfides mentioned above do not have sufficient electrical conductivity, and there is a need to further improve electrical conductivity and energy density, thereby improving the capacity and rate characteristics, in order to improve the output characteristics of batteries.

The present invention was made in consideration of the current state of the prior art described above, and its main purpose is to provide a novel material that is a compound primarily composed of lithium sulfide 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, good rate characteristics, and excellent charge/discharge characteristics.

The present inventors have conducted extensive research to achieve the above-mentioned object. As a result, they have found that an iron-containing lithium polysulfide containing lithium sulfide (Li ₂ S) 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 Li ₂ S 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.

Item 1. Iron-containing lithium polysulfide containing lithium, iron, and sulfur as constituent elements,
It contains lithium sulfide (Li ₂ S) as a main phase,
The crystallite size calculated from the half-width of the diffraction peak based on the (111) plane of Li ₂ S 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 %.

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

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

Item 4. The iron-containing lithium polysulfide according to any one of Items 1 to 3, having an electrical conductivity of 10 ⁻³ to 10 ⁻⁶ S/cm at 25° C.

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

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

Item 7. A method for producing the iron-containing lithium polysulfide according to any one of Items 1 to 6, comprising:
(3) A manufacturing method comprising a step of mechanically milling 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, together with a metal sulfide having an electrical conductivity of 10 ⁻³ S/cm or more.

Item 8. Before the step (3),
(2) The method according to item 7, further comprising 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 9. Before the step (2),
(1) The method according to item 8, comprising a step of heating a mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound.

Item 10. The manufacturing method described in Item 9, wherein step (1) is a step of passing a direct current pulse current through the mixture to heat and react the mixture.

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

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

Item 13. A lithium ion secondary battery comprising the positive electrode active material for a lithium ion secondary battery described in Item 12 as a constituent element.

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.

The iron-containing lithium polysulfide of the present invention is a fine particle having a crystallite size of 50 nm or less, contains lithium sulfide (Li ₂ S) as a main phase, and has additive elements adjusted within a specific composition range, resulting in a small amount of impurities. This, combined with the above, results in a high utilization rate of lithium sulfide, fully exhibiting the high capacity characteristics inherent to lithium sulfide, and improving electrical conductivity, resulting in a positive electrode active material with excellent rate characteristics.

For this reason, 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.

Furthermore, according to the manufacturing method of the present invention, composites with such excellent performance can be produced relatively easily.

FIG. 1 is a schematic diagram of an example of an electric current sintering apparatus. 1 shows X-ray diffraction patterns of samples obtained in Examples 1 and 2 and Comparative Examples 1 and 2. 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 Examples 1 and 2 and Comparative Examples 1 and 2 as positive electrode active materials. 1 shows X-ray diffraction patterns of samples obtained in Example 3 and Comparative Example 3. 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. 1 shows X-ray diffraction patterns of samples obtained in Example 4 and Comparative Example 4. 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. 1 shows X-ray diffraction patterns of samples obtained in Example 5 and Comparative Example 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 5 and Comparative Example 5 as positive electrode active materials. 1 shows X-ray diffraction patterns of samples obtained in Example 6 and Comparative Example 6. 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.

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

In addition, "containing" includes all of "comprise," "consist essentially of," and "consist only of."

In this 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 this invention, "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.

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 (Li ₂ S) as a major phase. The crystallite size, calculated from the half-width of the diffraction peak based on the (111) plane of Li ₂ S obtained by powder X-ray diffraction, is 50 nm or less. The total amount of the iron-containing lithium polysulfide is taken as 100 atomic %, and the Li content is 50 to 70 atomic %, the Fe content is 2 to 12 atomic %, the S content is 20 to 40 atomic %, the C content is 0 to 5 atomic %, and the halogen content is 0 to 0.8 atomic %. This 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 serves 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.

With such iron-containing lithium polysulfide of the present invention, for example, when the initial discharge capacity at an applied current of 0.13 mA/ ^cm2 is taken as 100%, the initial discharge capacity at an applied current of 1.3 mA/ ^cm2 can be set to 60 to 95%, preferably 70 to 90%. In this specification, the presence of such characteristics is sometimes referred to as "having excellent rate characteristics."

In powder X-ray diffraction analysis, the iron-containing lithium polysulfide of the present invention is found to have a primary phase of lithium sulfide. 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 total amount of the 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%. Furthermore, 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 impact 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, it is possible to further improve capacity and rate characteristics. 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)).

In this type of 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 high electrode utilization (particularly positive electrode utilization), and the high capacity characteristics inherent to lithium sulfide can be more fully utilized. Furthermore, when the manufacturing method of the present invention described below is adopted, a metastable phase of iron-containing lithium polysulfide is formed, which can be refined into submicron particles by mechanical milling, thereby stabilizing the inherent metastable phase of iron-containing lithium polysulfide.

In the iron-containing lithium polysulfide 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 suppresses the presence of free sulfur and provides good electrical conductivity, making it easier to exhibit excellent capacity and rate characteristics.

Furthermore, the iron-containing lithium polysulfide of the present invention preferably has a stabilized metastable phase in which iron atoms are incorporated into the lithium sulfide crystal lattice. Because the metastable phase of iron-containing lithium polysulfide is stable, the iron-containing lithium polysulfide of the present invention preferably consists of submicron or smaller crystallites. More specifically, the crystallite size of the iron-containing lithium polysulfide of the present invention is preferably 50 nm or less, and 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 manufacturing 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 using Scherrer's 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.

Furthermore, while the proportions of each element present in the iron-containing lithium polysulfide of the present invention are not particularly limited, it is preferable that the amount of Fe present is sufficient to form Fe-S bonds without generating free sulfur, that the amount of Li present is sufficient to provide a theoretical capacity of 600 mAh/g or more as estimated from the amount of Li present, and that the amount of Fe present 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, 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 50 to 70 atomic % (particularly 51 to 60 atomic %), the Fe content be 2 to 12 atomic % (particularly 4 to 11 atomic %), the S content be 20 to 40 atomic % (particularly 30 to 39 atomic %), the C content be 0 to 5 atomic % (particularly 0 to 3 atomic %), and the halogen content be 0 to 0.8 atomic % (particularly 0 to 0.5 atomic %), assuming that the total amount of the lithium sulfide-iron-carbon composite of the present invention formed is 100 atomic %. Among these, it is particularly preferable that the material contains no carbon or halogen (the carbon content and halogen content are both 0 atomic %).

The reason why the amount of Li was set to an amount that would result in a theoretical capacity of 600 mAh/g or more is that the oxide-based high-capacity material Li(Ni, Mn, Co) _O2 has a maximum energy density of 300 mAh/g × 4 V = 1200 Wh/kg, and it was determined that a sulfur-based material (voltage 2 V) with an energy density of 600 mAh/g or more would be sufficient.

The iron-containing lithium polysulfide of the present invention as described above can have improved electrical conductivity, and specifically, the electrical conductivity of the iron-containing lithium polysulfide of the present invention can be set to 10 ⁻³ to 10 ⁻⁶ S/cm, preferably 10 ⁻³ to 3×10 ⁻⁶ S/cm, at 25° C. The electrical conductivity of the iron-containing lithium polysulfide of the present invention is measured by compacting the powder at 300 MPa and measuring the current when a voltage is applied.

2. Method for Producing Iron-Containing Lithium Polysulfide The iron-containing lithium polysulfide of the present invention is not particularly limited, but may be produced by
(3) The lithium-containing compound can be obtained by a production method including a step of mechanically milling 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 metal sulfide having an electrical conductivity of ^{10 −3} S/cm or more. According to this method, by performing mechanical milling with the predetermined metal sulfide, the particles are refined, and a metastable phase in which iron atoms are incorporated into the lithium sulfide phase is stabilized, while the electrical conductivity can be particularly improved.

In addition, the production method of the present invention further includes the steps of:
(2) It is preferable to include 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, whereby the particles are refined prior to the step (3), and the metastable phase in which iron atoms are incorporated into the lithium sulfide phase is stabilized, making it easier to improve electrical conductivity.

In addition, the production method of the present invention further includes, before the step (2),
It is preferable to include a step (1) of heating a mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound, whereby the reaction can be moderately promoted 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.

(2-1) Raw Material Powders In the present invention, a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound are used as raw materials.

There are no particular limitations on the types of lithium-containing compounds, iron-containing compounds, and sulfur-containing compounds; a mixture of three or more compounds containing one type each of lithium, iron, and sulfur can be used, or a compound containing two or more of lithium, iron, and sulfur simultaneously can be used as part of the raw material.

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

Specific examples of such raw material compounds include lithium-containing compounds such as lithium sulfide (Li ₂ S) and lithium hydroxide (LiOH), iron-containing compounds such as metallic iron (Fe), iron sulfide (FeS, FeS ₂ , etc.), and iron sulfate (FeSO ₄ ), and sulfur-containing compounds such as sulfur (S), lithium sulfide (Li ₂ S), and iron sulfide (FeS, FeS ₂ , etc.). Among these, a combination of lithium sulfide (Li ₂ S) and iron sulfide (FeS, FeS 2 ) is particularly preferred, as it consists only of the constituent elements of the iron-containing lithium polysulfide product of the present invention and can be reacted with _a minimum number of raw materials.

There are no particular restrictions 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 restrictions on the size of the raw material compounds; raw material compounds with larger particle sizes can be used, and the average particle size can be controlled, if necessary, 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 finding the value at which the cumulative frequency distribution reaches 50%.

While there are no particular limitations on the blending ratios of the raw materials consisting of lithium-containing compounds, iron-containing compounds, and sulfur-containing compounds, it is preferable that the final product, the iron-containing lithium polysulfide of the present invention, contains a sufficient amount of Fe to facilitate the formation of Fe-S bonds without generating free sulfur, a sufficient amount of Li to ensure a theoretical capacity estimated from the amount of Li of approximately 600 mAh/g or greater, and a sufficient amount of Fe to ensure electrical conductivity and prevent degradation of capacity and rate characteristics. From these perspectives, it is preferable to adjust the blending ratios of the raw materials so that the Li content is 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 %). 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 target iron-containing lithium polysulfide of the present invention.

The reason why the amount of Li was set to an amount that would result in a theoretical capacity of 600 mAh/g or more is that the oxide-based high-capacity material Li(Ni, Mn, Co) _O2 has a maximum energy density of 300 mAh/g × 4 V = 1200 Wh/kg, and it was determined that a sulfur-based material (voltage 2 V) with an energy density of 600 mAh/g or more would be sufficient.

(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.

When electric sintering is performed in step (1), specifically, the raw material mixture described above is filled into a conductive mold (conductive container), and a current, preferably a direct current pulse, is passed through the conductive mold (conductive container) (a method known as spark plasma sintering, pulsed electric current sintering, plasma activated sintering, etc.). This causes the conductive mold (conductive container) to heat due to Joule heat, heating the raw material mixture within the conductive mold (conductive container). This causes the elements to diffuse and migrate, resulting in the production of an intermediate in which the elements are mixed together at the atomic level.

It is preferable that the atmosphere during this electric sintering process be a non-oxidizing atmosphere. Furthermore, electric sintering can be performed under normal pressure, but it is preferable to perform the electric sintering process under pressure.

A specific method involves filling a conductive container with a raw material mixture containing the raw materials, i.e., lithium-containing compound, iron-containing compound, and sulfur-containing compound, and then passing a pulsed ON-OFF direct current through it while applying pressure, preferably in a non-oxidizing atmosphere.

The material of the conductive mold (conductive container) is not particularly limited as long as it is conductive, and materials made from carbon, iron, iron oxide, aluminum, tungsten carbide, etc., as well as mixtures of carbon and/or iron oxide with silicon nitride, etc., can also be suitably used.

In order to further suppress oxidation of sulfides, the electric sintering treatment is preferably carried out in an inert gas atmosphere such as Ar or N ₂ or in a reducing atmosphere such as H _2. Alternatively, the 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).

Specific processing involves using a container that can ensure a sufficiently sealed state as the conductive mold (conductive container), and creating a non-oxidizing atmosphere within the container. 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 within a reaction chamber, creating a non-oxidizing atmosphere within the reaction chamber. This allows the heating reaction of the raw material mixture described above to occur in a non-oxidizing atmosphere. In this case, it is preferable to create an inert gas atmosphere or reducing gas atmosphere of 0.01 MPa or higher (particularly 0.05 to 0.2 MPa) within the reaction chamber.

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

The heating temperature during the electric current sintering process is typically preferably 400 to 1200°C, more preferably 500 to 1100°C, from the viewpoints of more fully interdiffusing the constituent elements and facilitating their intermixing at the atomic level, reducing sulfur not bonded to transition metals and typical elements (free sulfur), achieving high capacity, and suppressing losses 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 suppressing losses due to volatilization of elements such as Li and S, thereby facilitating high capacity. Once the temperature range is reached, the 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.

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 making it easier to strengthen contact between the raw material compounds, making it easier to facilitate sufficient atomic interdiffusion during heating, and making it easier to ensure sufficient reaction between atoms in the raw material compounds. Note that if the product has a composition with a low melting point, it may be possible to heat without applying pressure (atmospheric pressure). From this perspective, and taking into account the case where the product has a composition with a low melting point, the pressure to be applied when pressing the raw material powder may be 0.1 to 60 MPa, preferably 0.1 to 50 MPa.

The equipment used for electric current sintering is not particularly limited, as long as it is capable of heating, cooling, pressurizing, etc. the raw material mixture and can apply the current necessary for discharge. For example, a commercially available electric current sintering device (spark plasma sintering device) can be used. Such electric current sintering devices and their principles are disclosed, for example, in JP 10-251070 A.

Below, we will explain a specific example of electric sintering, with reference to Figure 1, which shows a schematic diagram of an electric sintering apparatus. Note that the electric sintering apparatus is not limited to the apparatus described below.

The electric current sintering apparatus 1 shown in Figure 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 pulse current can be supplied to the sample 2 loaded into 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.

In the electric sintering apparatus shown in Figure 1, the electric current section, including the conductive container 3, electric current punches 4 and 5, and punch electrodes 6 and 7, is housed in a water-cooled vacuum chamber 8, and the atmosphere inside 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 atmosphere inside the chamber to a non-oxidizing atmosphere.

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

The pulse current applied for heating can be, for example, a pulsed (ON-OFF switching) DC current with a pulse width of approximately 2-3 milliseconds and a frequency of approximately 3-300 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 using a graphite mold with an inner diameter of 15 mm, a current of 200-1000 A is preferable, and when using a graphite mold with an inner diameter of 100 mm, a current of 1000-8000 A is preferable. 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.

To pressurize the raw material mixture, for example, the raw material mixture filled in the conductive container 3 described above can be pressurized via punch electrodes 6 and 7.

(2-3) Process (2)
When the 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.

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

Mechanical milling is a method in which raw materials are milled and mixed while mechanical energy is applied, causing a reaction. This method mills and mixes the raw materials by subjecting them to mechanical impact and friction, 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 mechanical milling process described above forms metastable iron-containing lithium polysulfide, which is difficult to produce by heat treatment alone, while also finely pulverizing it so that it exists stably.

Mechanical milling devices that can be used include, for example, ball mills, vibration mills, turbo mills, and disc mills, with ball mills being preferred.

Since mechanical milling is easy to suppress oxidation of sulfides, it is preferable to perform it in a non-oxidizing atmosphere, for example, in an inert gas atmosphere such as Ar or N ₂ , or in a reducing atmosphere such as H _2. In addition, it can also be performed 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).

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 a composite with the desired high sulfur content.

The temperature during mechanical milling is preferably 200°C or less, more preferably 20 to 100°C, to prevent sulfur from volatilizing and facilitate the formation of a composite with the desired high sulfur content.

There is no particular limitation on the mechanical milling time, but it is preferable to perform the mechanical milling process until the crystallite size of the resulting composite is 50 nm or less.

(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 mixture, and a metal sulfide having an electrical conductivity of 10 ⁻³ S/cm or more are subjected to mechanical milling, followed by mixing, pulverization, and reaction, thereby obtaining the iron-containing lithium polysulfide of the present invention.

When using a mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound, the raw material powders can be mixed and then subjected to mechanical milling with a metal sulfide having an electrical conductivity of 10 ⁻³ S/cm or more. When using a heat-treated product of the mixture, the product obtained in the above-mentioned step (1) and a metal sulfide having an electrical conductivity of 10 ⁻³ S/cm or more can be used. When using a mechanically milled product of the heat-treated product of the mixture, the product obtained in the above-mentioned step (2) can be used.

In addition, when the above-described steps (1) and (2) are employed, if the product already contains a metal sulfide having an electrical conductivity of 10 ⁻³ S/cm or more in the amount to be added 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 a 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 the 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.

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 allowing the high capacity characteristics inherent to lithium sulfide to be fully exhibited, and easily providing excellent rate characteristics, it is preferably ¹⁰ to ¹⁰ S/cm, and more preferably ¹⁰ to ¹⁰ S/cm. 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.

Metal sulfides having such electrical conductivity are not particularly limited, but examples thereof include iron-containing lithium polysulfides ( _Li2FeS2 , etc.; however, different from the products of steps (1) and (2)), iron sulfides (FeS, _{FeS2 ,} etc.), molybdenum sulfides _{( MoS2} , _Mo6S8 , etc.), copper sulfides (CuS, etc.), etc. These metal sulfides can be used alone or in combination of two or more.

In step (3), the amount of the metal sulfide added is not particularly limited. However, from the viewpoint of easily improving the utilization rate of lithium sulfide, easily fully demonstrating the high capacity characteristics inherent to lithium sulfide, and easily achieving excellent rate characteristics, the metal sulfide can be added in an amount of 3 to 25% by mass, preferably 5 to 20% by mass, of the total amount of the intermediate obtained in step (2) and the metal sulfide, taken as 100% by mass.

Mechanical milling devices that can be used include, for example, ball mills, vibration mills, turbo mills, and disc mills, with ball mills being preferred.

Since mechanical milling is easy to suppress oxidation of sulfides, it is preferable to perform it in a non-oxidizing atmosphere, for example, in an inert gas atmosphere such as Ar or N ₂ , or in a reducing atmosphere such as H _2. In addition, it can also be performed 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).

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 a composite with the desired high sulfur content.

The temperature during mechanical milling is preferably 100°C or less, more preferably 20 to 80°C, to prevent sulfur from volatilizing and facilitate the formation of a composite with the desired high sulfur content.

There is no particular limitation on the mechanical milling time, but it is preferable to perform the mechanical milling process until the crystallite size of the resulting composite is 50 nm or less.

3. Lithium-ion secondary battery 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 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 using 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.

The structure of non-aqueous electrolyte lithium-ion secondary batteries and all-solid-state lithium-ion secondary batteries can be the same as that 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.

For example, with regard to non-aqueous electrolyte lithium ion secondary batteries, the basic structure can be the same as that of known non-aqueous electrolyte lithium ion secondary batteries, except that the iron-containing lithium polysulfide of the present invention described above is used as the positive electrode active material.

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 such as Al, Ni, stainless steel, or carbon cloth. Examples of conductive materials that can be used include carbon materials such as graphite, cokes, carbon black, and acicular carbon.

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, etc., using conductive materials, binders, etc., as needed.

Separators can be made of materials such as polyolefin resins such as polyethylene and polypropylene, fluororesin, nylon, aromatic aramid, and inorganic glass, and can be in the form of porous membranes, nonwoven fabrics, woven fabrics, etc.

As the solvent for the non-aqueous electrolyte, solvents known as solvents for non-aqueous solvent-based secondary batteries, such as carbonate compounds, ether compounds, nitrile compounds, and sulfur-containing compounds, can be used.

In addition, all-solid-state lithium-ion secondary batteries can 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.

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

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 the 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, or the like. As with non-aqueous solvent-based lithium-ion secondary batteries, carbon materials such as graphite, cokes, carbon black, and acicular carbon can be used as the conductive material.

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, etc., using the aforementioned conductive materials and binders, as needed.

There are no particular limitations on the shape of non-aqueous electrolyte lithium-ion secondary batteries and all-solid-state lithium-ion secondary batteries, and they may be cylindrical, rectangular, etc.

The present invention will be specifically explained below using examples and comparative examples. However, it goes without saying that the present invention is not limited to the following examples.

Example 1
Commercially available lithium sulfide (Li ₂ S) (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 Li ₂ S:FeS was 4:1, and the mixture was thoroughly mixed in a mortar and then filled into a graphite mold having an inner diameter of 15 mm.

The graphite mold filled with the raw materials was then placed in an electric sintering machine. The graphite mold and the electric parts, including the electrode parts, were housed 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.

The raw material packed into the graphite mold was then pressurized at approximately 30 MPa while a DC pulse current of approximately 600 A (pulse width 2.5 milliseconds, frequency 28.6 Hz) was applied. The area around the graphite mold was heated at a temperature increase rate of approximately 200°C/min, reaching 600°C three minutes after the start of pulse current application. The current application and pressure were then immediately stopped, and the material was allowed to cool naturally.

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 mechanically milled at 400 rpm for 20 hours using a planetary ball mill (model P-7) manufactured by Fritsch Japan Co., Ltd.

Then, at room temperature (25°C), Li ₂ FeS ₂ (prepared by mixing commercially available Li ₂ S and FeS in a 1:1 molar ratio and then heating in an electric sintering machine at 1000°C for 1 minute, electrical conductivity 10 ⁻² S/cm (25°C)) was mixed in a mortar with a reaction product of lithium sulfide and iron sulfide (Li ₈ FeS ₅ ):Li ₂ FeS ₂ = 6:4 (mass ratio), 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.

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. Furthermore, the electrical conductivity of the obtained sample was measured by compacting it at 300 MPa and measuring the current value when a voltage was applied. The result was 4.4 × 10 ⁻⁴ S/cm, which was a dramatic improvement over the electrical conductivity of Li ₈ FeS ₅ (Comparative Example 1) before being composited with Li ₂ FeS ₂ , which was 1.1 × 10 ⁻⁵ S/cm.

As shown in Figure 2, the X-ray diffraction pattern of the obtained sample showed a peak derived from low-crystalline lithium sulfide (Li ₂ S) as the main phase, and also a peak derived from iron sulfide (FeS). That is, the abundance ratio of lithium sulfide (Li ₂ S) estimated by Rietveld analysis was 96 mol%, and the abundance ratio of iron sulfide (FeS) was 4 mol%. In addition, 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, this method made it possible to produce an iron-containing lithium polysulfide whose main phase was lithium sulfide and whose crystallite size was 50 nm or less.

An all-solid-state lithium-ion secondary battery was assembled using the obtained iron-containing lithium polysulfide as the positive electrode active material, indium metal as the negative electrode, and an argyrodite-type sulfide-based solid electrolyte as the electrolyte, and charge/discharge tests were conducted.

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 a 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 conducted with this battery, starting with constant current measurement within the range of 3.0 V to 1.0 V. The current density was 0.13 mA/ ^cm² for charging and 0.13 to 1.3 mA/ ^cm² for discharging.

As shown in Figure 3, the initial (first cycle) discharge curve shows that the initial discharge capacity was 520 mAh/g when the discharge current density was low (0.13 mA/cm ² ), 500 mAh/g when it was 0.25 mA/cm ² , 480 mAh/g when it was 0.64 mA/cm ² , and 450 mAh/g when it was high current density (1.3 mA/cm ² ). In other words, the initial discharge capacity at high current density (1.3 mA/cm ² ) was as high as 86% of the initial discharge capacity at low current density (0.13 mA/cm ² ), and it can be seen that a positive electrode active material with excellent capacity and rate characteristics was obtained.

Example 2
An 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 (Li ₈ FeS ₅ ) and Li ₂ FeS ₂ was set to 2 hours.

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. Furthermore, to measure the electrical conductivity of the obtained sample, the powder was compacted at 300 MPa and the current value when a voltage was applied was measured. The result was 4.0 × 10 ⁻⁴ S/cm, which was a dramatic improvement over the electrical conductivity of Li ₈ FeS ₅ (Comparative Example 1) before being composited with Li ₂ FeS ₂ , which was 1.1 × 10 ⁻⁵ S/cm.

As shown in Figure 2, the X-ray diffraction pattern of the obtained sample showed a peak derived from low-crystalline lithium sulfide ( _Li2S ) as the main phase, and also a peak derived from iron sulfide (FeS). That is, the abundance ratio of lithium sulfide ( _Li2S ) estimated by Rietveld analysis was 91 mol%, and the abundance ratio of iron sulfide (FeS) was 9 mol%. In addition, 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, this method made it possible to produce iron-containing lithium polysulfide whose main phase was lithium sulfide and whose crystallite size was 50 nm or less.

An all-solid-state lithium-ion secondary battery was assembled using the obtained iron-containing lithium polysulfide as the positive electrode active material, indium metal as the negative electrode, and an argyrodite-type sulfide-based solid electrolyte as the electrolyte, and charge/discharge tests were conducted.

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 a 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 conducted with this battery, starting with constant current measurement within the range of 3.0 V to 1.0 V. The current density was 0.13 mA/ ^cm² for charging and 0.13 to 1.3 mA/ ^cm² for discharging.

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

Comparative Example 1
An iron-containing lithium _polysulfide was obtained in the same manner as in Example 1 _, except that _Li2FeS2 was not added to the reaction product ( _Li8FeS5 ) of lithium sulfide and iron sulfide, and the subsequent mechanical milling treatment was not performed. Specifically, the iron-containing lithium polysulfide was obtained as follows.

Commercially available lithium sulfide (Li ₂ S) (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 Li ₂ S:FeS was 4:1, and the mixture was thoroughly mixed in a mortar and then filled into a graphite mold having an inner diameter of 15 mm.

The graphite mold filled with the raw materials was then placed in an electric sintering machine. The graphite mold and the electric parts, including the electrode parts, were housed 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.

The raw material packed into the graphite mold was then pressurized at approximately 30 MPa while a DC pulse current of approximately 600 A (pulse width 2.5 milliseconds, frequency 28.6 Hz) was applied. The area around the graphite mold was heated at a temperature increase rate of approximately 200°C/min, reaching 600°C three minutes after the start of pulse current application. The current application and pressure were then immediately stopped, and the material was allowed to cool naturally.

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 mechanically milled at 400 rpm for 20 hours using a planetary ball mill (model P-7) manufactured by Fritsch Japan Co., Ltd.

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, resulting in a value of 1.1 × 10 ^-5 S/cm, which was lower than that of Examples 1 and 2.

As shown in Figure 2, the X-ray diffraction pattern of the obtained sample consisted of peaks derived from low-crystalline lithium sulfide ( _Li2S ) as the main phase, and no other peaks were observed. That is, 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.

An all-solid-state lithium-ion secondary battery was assembled using the obtained iron-containing lithium polysulfide as the positive electrode active material, indium metal as the negative electrode, and an argyrodite-type sulfide-based solid electrolyte as the electrolyte, and charge/discharge tests were conducted.

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 a 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 conducted with this battery, starting with constant current measurement within the range of 3.0 V to 1.0 V. The current density was 0.13 mA/ ^cm² for charging and 0.13 to 1.3 mA/ ^cm² for discharging.

As shown in Figure 3, the initial discharge curve shows that the initial discharge capacity was 490 mAh/g when the discharge current density was low (0.13 mA/cm ² ), 430 mAh/g when it was 0.25 mA/cm ² , 280 mAh/g when it was 0.64 mA/cm ² , and 140 mAh/g when it was high current density (1.3 mA/cm ² ). In other words, the initial discharge capacity at high current density (1.3 mA/cm ² ) was only 28% of the initial discharge capacity at low current density (0.13 mA/cm ² ), and a positive electrode active material with particularly excellent rate characteristics was not obtained.

Comparative Example 2
An 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 (Li 8 FeS 5 ) and} Li ₂ FeS ₂ was set to 0 hours (the reaction product of lithium sulfide and iron sulfide (Li 8 FeS 5 ) and Li ₂ FeS ₂ were mixed, but mechanical milling was not performed).

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, resulting in a value of 1.0 × 10 ^-5 S/cm, which was lower than that of Examples 1 and 2.

The X-ray diffraction pattern of the obtained sample consisted 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.

An all-solid-state lithium-ion secondary battery was assembled using the obtained iron-containing lithium polysulfide as the positive electrode active material, indium metal as the negative electrode, and an argyrodite-type sulfide-based solid electrolyte as the electrolyte, and charge/discharge tests were conducted.

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 a 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 conducted on this battery by starting charging with a constant current measurement within the range from the initial voltage to 1.0 V. The current density was 0.13 mA/ ^cm² for charging and 0.13 to 1.3 mA/ ^cm² for discharging.

As shown in Figure 3, the initial (first cycle) discharge curves show that the initial discharge capacity was 470 mAh/g when the discharge current density was low (0.13 mA/cm ² ), 410 mAh/g when it was 0.25 mA/cm ² , 380 mAh/g when it was 0.64 mA/cm ² , and 250 mAh/g when it was high current density (1.3 mA/cm ² ). In other words, the initial discharge capacity at high current density (1.3 mA/cm ² ) was only 54% of the initial discharge capacity at low current density (0.13 mA/cm ² ), and a positive electrode active material with particularly excellent rate characteristics was not obtained.

Example 3
Commercially available lithium sulfide (Li ₂ S) (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) in an argon gas atmosphere so that the molar ratio of Li ₂ S:FeS was 5:1, and the mixture was thoroughly mixed in a mortar and then filled into a graphite mold having an inner diameter of 15 mm.

The graphite mold filled with the raw materials was then placed in an electric sintering machine. The graphite mold and the electric parts, including the electrode parts, were housed 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.

The graphite mold was then filled with raw materials and 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 graphite mold was heated at a temperature increase rate of approximately 200°C/min, and the temperature reached 600°C 3 minutes after the start of pulse current application. The current application and pressure were then immediately stopped, and the _mixture was allowed to cool naturally to produce _LiFeS6 .

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 (Li ₁₀ FeS ₆ ) was removed from the mold, and Li ₂ FeS ₂ (prepared by mixing commercially available Li ₂ S and FeS in a 1:1 molar ratio and then heating in an electric sintering machine at 1000°C for 1 minute, with an electrical conductivity of 10 ⁻² S/cm (25°C)) was mixed in a mortar to give a Li ₁₀ FeS ₆ :Li ₂ FeS ₂ = 8:2 (mass ratio). The mixture was placed in a zirconia pot under an argon gas atmosphere and subjected to mechanical milling at 400 rpm for 2 hours using a planetary ball mill (model P-7) manufactured by Fritsch Japan Co., Ltd. to obtain a sample.

The atomic percentages of each element in the obtained sample were Li 56%, Fe 8%, and S 36%. The obtained sample did not contain any carbon or halogens.

The X-ray diffraction pattern of the obtained sample consisted of low-crystalline lithium sulfide ( _Li2S ) and iron sulfide (FeS), as shown in Figure 4. The abundance ratio of lithium sulfide ( _Li2S ) was 94 mol% and the abundance ratio of FeS was 6 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 23 nm.

An all-solid-state lithium-ion secondary battery was assembled using the obtained iron-containing lithium polysulfide as the positive electrode active material, indium metal as the negative electrode, and an argyrodite-type sulfide-based solid electrolyte as the electrolyte, and charge/discharge tests were conducted.

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 a 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 conducted with 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 for discharging it was 0.13 to 1.3 mA/cm ² .

As shown in Figure 5, the initial discharge curve (first cycle) shows that the initial discharge capacity was approximately 620 mAh/g at a low discharge current density (0.13 mA/cm ² ), approximately 540 mAh/g at 0.25 mA/cm ² , approximately 510 mAh/g at 0.64 mA/cm ² , and approximately 430 mAh/g at a high current density (1.3 mA/cm ² ). That is, the discharge capacity at a high current density (1.3 mA/cm ² ) was approximately 70% of that at a low current density (0.13 mA/cm ² ), indicating that an electrode material with excellent rate characteristics was obtained.

Comparative Example 3
A sample was prepared in exactly _{the same manner as in Example 3, except that the mechanical milling time of the reaction product of lithium sulfide and iron sulfide (Li 10 FeS 6 ) and} Li ₂ FeS ₂ was set to 0 hours (the reaction product of lithium sulfide and iron sulfide (Li 10 FeS 6 ) and Li ₂ FeS ₂ were mixed, but mechanical milling was not performed).

The ratios (atomic %) of each element in the obtained sample were Li 56%, Fe 8%, and S 36%, the same as in Example 3. The obtained sample did not contain carbon or halogens.

The X-ray diffraction pattern of the obtained sample consisted of crystalline lithium sulfide ( _Li2S ) and crystalline _Li2FeS2 , as shown in Figure 4, and the abundance ratio of lithium sulfide ( _Li2S ) determined by Rietveld analysis was 87 mol% and the abundance ratio of _Li2FeS2 was 13 mol%. In addition, the crystallite size calculated from _the half-width of the diffraction peak based on _the (111) plane of lithium sulfide was 106 nm.

The obtained composite powder was assembled into an all-solid-state battery and subjected to a charge-discharge test in the same manner as in Example 3. The initial (first cycle) discharge curve is shown in Figure 5, and the initial discharge capacity was only a low discharge capacity of about 150 mAh/g at a low discharge current density (0.13 mA/ ^{cm2 )} , about 140 mAh/g at a current density of 0.25 mA/ ^cm2 , about 130 mAh/ _g at a current density of 0.64 mA/cm2, and about 120 mAh/ _g at a high current density (1.3 mA/ ^cm2 ). This indicates that an electrode material excellent in both capacity and rate characteristics cannot be obtained by simply mixing _Li10FeS6 with _Li2FeS2 .

Example 4
Commercially available lithium sulfide (Li ₂ S) (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 Li ₂ S:FeS was 5:1. After thorough mixing in a mortar, the mixture was quenched at room temperature (25°C) to produce Mo ₆ S ₈ (a powder of MoS ₂ , Mo, and CuS mixed in a molar ratio of 3:3:2 was heat-treated at 1000°C under an argon gas atmosphere to synthesize Cu ₂ Mo ₆ S ₈ , which was then immersed in 5 mol/L HCl for 1 hour to elute the Cu, producing Mo ₆ S ₈ ). This was then used to prepare a reaction product of lithium sulfide and iron sulfide: Mo ₆ S _8. The mixture was mixed in a mortar so that the mass ratio was 8:2, and the mixture was placed in a zirconia pot under an argon gas atmosphere. The mixture was then processed by mechanical milling at 400 rpm for 40 hours using a planetary ball mill (model P-7) manufactured by Fritsch Japan Co., Ltd.

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. Furthermore, to measure the electrical conductivity of the obtained sample, the powder was compacted at 300 MPa and measured by applying a voltage to measure the current value. The result was 3.5 × 10 ⁻⁴ S/cm, which was dramatically improved compared to the electrical conductivity of Comparative Example 4, which was not subjected to mechanical milling, which was less than 10 ⁻⁸ S/cm (immeasurable).

As shown in Figure 6, the X-ray diffraction pattern of the obtained sample showed a peak derived from low-crystalline lithium sulfide ( _Li2S ) as the main phase, and no other peak derived from iron sulfide (FeS). That is, 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.

An all-solid-state lithium-ion secondary battery was assembled using the obtained iron-containing lithium polysulfide as the positive electrode active material, indium metal as the negative electrode, and an argyrodite-type sulfide-based solid electrolyte as the electrolyte, and charge/discharge tests were conducted.

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 a 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 conducted with this battery, starting with constant current measurement within the range of 3.0 V to 1.0 V. The current density was 0.13 mA/ ^cm² for charging and 0.13 to 1.3 mA/ ^cm² for discharging.

As shown in Figure 7, the initial discharge capacity was 520 mAh/g at a low discharge current density (0.13 mA/cm ² ), 500 mAh/g at 0.25 mA/cm ² , 480 mAh/g at 0.64 mA/cm ² , and 420 mAh/g at a high current density (1.3 mA/cm ² ). In other words, the initial discharge capacity at a high current density (1.3 mA/cm ² ) was as high as 81% of the initial discharge capacity at a low current density (0.13 mA/cm ² ), and it can be seen that a positive electrode active material with excellent capacity and rate characteristics was obtained.

Comparative Example 4
A sample was prepared in exactly the same manner as in Example 4, except that the mechanical milling time of the reaction product of lithium sulfide and iron sulfide with Mo ₆ S ₈ was set to 0 hours (the reaction product of lithium sulfide and iron sulfide was mixed with Mo ₆ S ₈ , but mechanical milling was not performed).

The ratios (atomic %) of each element in the obtained sample were Li 55%, Fe 5%, S 37%, and Mo 3%, similar to those in 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. The value was less than 10 ⁻⁸ S/cm (immeasurable), which was dramatically lower than that in Example 4.

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.

The obtained composite powder was assembled into an all-solid-state battery and subjected to a charge-discharge test in the same manner as in Example 4. The initial (first cycle) discharge curve is shown in Figure 7, and the initial discharge capacity was only a low discharge capacity of about 110 mAh/g at a low discharge current density (0.13 mA/ ^cm2 ), about 100 mAh/g at a current density of 0.25 mA/ ^cm2 , about 90 mAh/g at a current density of 0.64 mA/ ^cm2 , and about 70 mAh/g at a high current density (1.3 mA/ ^cm2 ).

Example 5
Commercially available lithium sulfide (Li ₂ S) (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 Li ₂ S:FeS was 5:1, and then thoroughly mixed in a mortar. MoS ₂ (manufactured by Kojundo Chemical Laboratory Co., Ltd., model number: MOI06PB) was then mixed in a mortar at room temperature (25°C) so that the reaction product of lithium sulfide and iron sulfide:MoS ₂ was 8:2 (mass ratio). The mixture was placed in a zirconia pot under an argon gas atmosphere and mechanically milled for 20 hours at 400 rpm using a planetary ball mill (model P-7) manufactured by Fritsch Japan Co., Ltd. to prepare a sample.

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. Furthermore, to measure the electrical conductivity of the obtained sample, the powder was compacted at 300 MPa and measured by applying a voltage to measure the current value. The result was 3.4 × 10 ⁻³ S/cm, which was dramatically improved compared to the electrical conductivity of Comparative Example 5, which was not subjected to mechanical milling, which was less than 10 ⁻⁸ S/cm (immeasurable).

The X-ray diffraction pattern of the obtained sample consisted of low-crystalline lithium sulfide ( _Li2S ) and iron sulfide (FeS), as shown in Figure 8, and the abundance ratio of lithium sulfide ( _Li2S ) estimated by Rietveld analysis was 91 mol%, the abundance ratio of _MoS2 was 0 mol%, and the abundance ratio of iron sulfide (FeS) was 9 mol%. The crystallite size calculated from the half-width of the diffraction peak based on the (111) plane of lithium sulfide was 19 nm.

An all-solid-state lithium-ion secondary battery was assembled using the obtained iron-containing lithium polysulfide as the positive electrode active material, indium metal as the negative electrode, and an argyrodite-type sulfide-based solid electrolyte as the electrolyte, and charge/discharge tests were conducted.

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 a 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 conducted with this battery, starting with constant current measurement within the range of 3.0 V to 1.0 V. The current density was 0.13 mA/ ^cm² for charging and 0.13 to 1.3 mA/ ^cm² for discharging.

The initial (first cycle) discharge curve is shown in Figure 9, and the initial discharge capacity was approximately 660 mAh/g at a low discharge current density (0.13 mA/cm ² ), approximately 620 mAh/g at 0.25 mA/cm ² , approximately 600 mAh/g at 0.64 mA/cm ² , and approximately 560 mAh/g at a high current density (1.3 mA/cm ² ). That is, the discharge capacity at a high current density (1.3 mA/cm ² ) was as high as approximately 86% of that at a low current density (0.13 mA/cm ² ), indicating that an electrode material with excellent rate characteristics was obtained.

Comparative Example 5
A sample was prepared in exactly the same manner as in Example 5, except that the mechanical milling treatment time of _MoS2 with the reaction product of lithium sulfide and iron sulfide was set to 0 hours (the reaction product of lithium sulfide and iron sulfide and _MoS2 were mixed, but mechanical milling treatment was not performed).

The ratios (atomic %) of each element in the obtained sample were Li 54%, Fe 5%, S 38%, and Mo 3%, similar to those in 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. The value was less than 10 ⁻⁸ S/cm (immeasurable), which was dramatically lower than that in Example 5.

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 ), _Mo6S8 , _and iron sulfide (FeS) determined by Rietveld analysis was 77 mol%, 8 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 84 nm.

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 performed. The results are shown in Figure 9, and the initial discharge capacity was only low, about 3 mAh/g, at a low discharge current density (0.13 mA/cm ² ), about 3 mAh/g at a current density of 0.25 mA/cm ² , about 2 mAh/g at a current density of 0.64 mA/cm ² , and about 2 mAh/g at a high current density (1.3 mA/cm ² ). This indicates that an electrode material excellent in both capacity and rate characteristics cannot be obtained by simply mixing MoS ₂ .

Example 6
Commercially available lithium sulfide (Li ₂ S) (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 Li ₂ S:FeS = 5:1 and thoroughly mixed in a mortar. Then, at room temperature (25°C), CuS (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., model number: 034-04462) was added to the mortar to achieve a mass ratio of lithium sulfide and iron sulfide reactant:CuS = 8:2. The mixture was placed in a zirconia pot under an argon gas atmosphere and subjected to mechanical milling at 400 rpm for 40 hours using a planetary ball mill (model P-7) manufactured by Fritsch Japan Co., Ltd. to prepare a sample.

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. Furthermore, to measure the electrical conductivity of the obtained sample, the powder was compacted at 300 MPa and measured by applying a voltage to measure the current value. The result was 1.6 × 10 ⁻⁶ S/cm, which was dramatically improved compared to the electrical conductivity of Comparative Example 6, which was not subjected to mechanical milling, which was less than 10 ⁻⁸ S/cm (immeasurable).

The X-ray diffraction pattern of the obtained sample was composed of low-crystalline lithium sulfide (Li ₂ S), and the abundance ratio of lithium sulfide (Li ₂ S) determined by Rietveld analysis was 100 mol % as shown in Figure 10. The crystallite size calculated from the half-width of the diffraction peak based on the (111) plane of lithium sulfide was 23 nm.

An all-solid-state lithium-ion secondary battery was assembled using the obtained iron-containing lithium polysulfide as the positive electrode active material, indium metal as the negative electrode, and an argyrodite-type sulfide-based solid electrolyte as the electrolyte, and charge/discharge tests were conducted.

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 a 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 conducted with this battery, starting with constant current measurement within the range of 3.0 V to 1.0 V. The current density was 0.13 mA/ ^cm² for charging and 0.13 to 1.3 mA/ ^cm² for discharging.

The initial (first cycle) discharge curve is shown in Figure 11, and the initial discharge capacity was approximately 550 mAh/g at a low discharge current density (0.13 mA/cm ² ), approximately 490 mAh/g at 0.25 mA/cm ² , approximately 450 mAh/g at 0.64 mA/cm ² , and approximately 380 mAh/g at 1.3 mA/cm ² . In other words, the discharge capacity at a high current density (1.3 mA/cm ² ) was as high as approximately 70% of that at a low current density (0.13 mA/cm ² ), indicating that an electrode material with excellent rate characteristics was obtained.

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 set to 0 hours (the reaction product of lithium sulfide and iron sulfide and CuS were mixed, but mechanical milling treatment was not performed).

The ratios (atomic %) of each element in the obtained sample were Li 54%, Fe 5%, S 37%, and Cu 4%, similar to those in 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. The value was less than 10 ⁻⁸ S/cm (immeasurable), which was dramatically lower than that in Example 6.

The X-ray diffraction pattern of the obtained sample was composed of crystalline lithium sulfide ( _Li2S ), iron sulfide (FeS), and CuS, as shown in Figure 10. The abundance ratio of lithium sulfide ( _Li2S ), 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.

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 conducted. The results are shown in Figure 11. The initial discharge capacity was only a low discharge capacity of about 7 mAh/g at a low discharge current density (0.13 mA/ ^cm2 ), and was also low at a current density of about 4 mAh/g ^at 0.25 mA/cm2, about 3 mAh/g at 0.64 mA/ ^cm2 , and about 3 mAh/g at 1.3 mA/ ^cm2 . 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.

1 Electric current sintering apparatus 2 Sample 3 Die (conductive container)
4, 5: Current-carrying punches 6, 7: Punch electrode 8: Water-cooled vacuum chamber 9: Cooling water passages 10, 16: Water cooling mechanism 11: Sintering power source 12: Control device 13: Pressurizing mechanism 14: Position measurement mechanism 15: Atmosphere control mechanism 17: Temperature measurement device

Claims (12)

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,
It contains lithium sulfide (Li ₂ S) as a main phase,
The crystallite size calculated from the half-width of the diffraction peak based on the (111) plane of Li ₂ S obtained by powder X-ray diffraction 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 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 lithium sulfide (Li ₂ S) is 70 mol % or more based on the iron-containing lithium polysulfide as estimated by Rietveld analysis. 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. 2. The iron-containing lithium polysulfide for use as a positive electrode active material in a lithium ion secondary battery according to claim 1, which has an electrical conductivity of 10 ⁻³ to 10 ⁻⁶ S/cm at 25° C. 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 the steps of:
(3) A manufacturing method comprising a step of mechanically milling 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, together with a metal sulfide having an electrical conductivity of 10 ⁻³ S/cm or more. Before the step (3),
(2) The manufacturing method according to claim 5 , further comprising 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. Before the step (2),
The method of claim 6 , comprising the step of: (1) heating a mixture containing a lithium-containing compound, an iron-containing compound, and a sulfur-containing compound. 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. The method according to claim 7 , wherein steps (1) to (3) are carried out in a non-oxidizing atmosphere. A positive electrode active material for a lithium ion secondary battery, comprising the iron-containing lithium polysulfide for a positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 4 . A lithium ion secondary battery comprising the positive electrode active material for lithium ion secondary batteries according to claim 10 as a constituent element. 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.