WO2020050269A1 - 硫化物系化合物粒子、固体電解質及びリチウム二次電池 - Google Patents
硫化物系化合物粒子、固体電解質及びリチウム二次電池 Download PDFInfo
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- H01M10/0561—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of inorganic materials only
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- H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
- H01M4/136—Electrodes based on inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy
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- H01M4/36—Selection of substances as active materials, active masses, active liquids
- H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
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- H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
- H01M4/628—Inhibitors, e.g. gassing inhibitors, corrosion inhibitors
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- C01P2002/70—Crystal-structural characteristics defined by measured X-ray, neutron or electron diffraction data
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- C01P2004/60—Particles characterised by their size
- C01P2004/61—Micrometer sized, i.e. from 1-100 micrometer
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- H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
- H01M2004/027—Negative electrodes
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- H01M2004/028—Positive electrodes
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- H01M2300/0065—Solid electrolytes
- H01M2300/0068—Solid electrolytes inorganic
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Definitions
- the present invention relates to sulfide-based compound particles that can be suitably used as a solid electrolyte such as a lithium secondary battery.
- An all-solid-state lithium secondary battery using a sulfide-based solid electrolyte that uses lithium sulfide (Li 2 S) or the like as a starting material does not use a flammable organic solvent, so that a safety device can be simplified. Not only can it be made excellent in manufacturing cost and productivity, but also it has a feature that a high voltage can be achieved by stacking it in series in a cell. In addition, since this type of solid electrolyte does not move except lithium ions, it is expected that safety and durability are improved, for example, a side reaction due to the transfer of anions does not occur.
- Patent Literature 1 discloses a compound having an aldirodite type crystal structure and represented by Li 7-x-2y PS 6-xy Cl x as a new sulfide-based solid electrolyte for a lithium ion battery. (1): It contains a compound represented by Li 7-x-2y PS 6-xy Cl x , and in the composition formula, 0.8 ⁇ x ⁇ 1.7, 0 ⁇ y ⁇ ⁇ 0. A sulfide-based solid electrolyte for a lithium ion battery, which satisfies 0.25 + 0.5 is disclosed.
- Patent Literature 2 examples include a lithium sulfide (Li 2 S) powder and a diphosphorus pentasulfide (Li 2 S) powder as a method for producing a solid electrolyte including a compound having lithium, phosphorus, sulfur, and halogen and having an aldirodite-type crystal structure.
- a production method is disclosed in which P 2 S 5 ) powder and lithium chloride (LiCl) powder are mixed, heated at 300 ° C. for 4 hours while flowing hydrogen sulfide gas, and then further heated at 500 ° C. for 4 hours.
- Patent Literature 3 discloses a method for producing a sulfide solid electrolyte capable of reducing the residual amount of elemental sulfur, in which an electrolyte raw material containing at least Li 2 S and P 2 S 5 and elemental sulfur are contained in a container.
- the raw material composition comprising the electrolyte raw material and elemental sulfur is made amorphous
- the amorphizing step of synthesizing the sulfide solid electrolyte material
- a heat treatment step of heat-treating the sulfide solid electrolyte material under an inert atmosphere at a temperature equal to or higher than the melting point of elemental sulfur and lower than the generation temperature of the low Li ion conductive phase, that is, at a temperature of 300 ° C. or lower;
- a method is disclosed.
- JP 2016-24874 A JP 2018-67552A JP-A-2018-80095, Claim 1, paragraph 0028, etc.
- Sulfide-based solid electrolyte materials as described above especially sulfide-based compounds having an aldirodite-type crystal structure, have a high ionic conductivity, but may generate hydrogen sulfide gas when exposed to atmospheric moisture. Therefore, there is a problem that it is necessary to handle the gas in a limited environment such as a dry room where an inert gas having an extremely low dew point is constantly supplied.
- the particle size of the sulfide-based compound particles having an aldirodite-type crystal structure is reduced to 50 ⁇ m or less, which is preferable as a solid electrolyte for a lithium secondary battery, the generation of hydrogen sulfide gas tends to increase significantly. There was a problem that handling was even more difficult.
- the present invention relates to sulfide-based compound particles having an aldirodite-type crystal structure containing atomized lithium (Li), phosphorus (P), sulfur (S), and halogen (Ha). Another object of the present invention is to provide new sulfide-based compound particles capable of suppressing generation of hydrogen sulfide gas.
- the present invention provides a sulfide-based compound particle having an aldirodite-type crystal structure containing lithium (Li), phosphorus (P), sulfur (S), and halogen (Ha), D50 by volume particle size distribution obtained by measuring by a laser diffraction scattering type particle size distribution measuring method is 50 ⁇ m or less, A sulfide-based compound particle in which the occupancy of sulfur (S) and halogen (Ha) in the S3 (4a) site calculated by neutron diffraction measurement is 85% or more is proposed.
- the sulfide-based compound particles proposed by the present invention can suppress the generation of hydrogen sulfide gas even when exposed to moisture in the atmosphere.
- generation of hydrogen sulfide gas can be suppressed even if the particle size is reduced to 50 ⁇ m or less, which is preferable for a solid electrolyte of a lithium secondary battery. Therefore, it can be industrially effectively used as a solid electrolyte such as a lithium secondary battery.
- the sulfide-based compound particles according to an example of the embodiment of the present invention are aldirodite containing lithium (Li), phosphorus (P), sulfur (S), and halogen (Ha). It is a sulfide-based compound particle having a type crystal structure.
- Sulfide compounds having an aldirodite-type crystal structure generally have high ionic conductivity as described above, but have a problem of generating hydrogen sulfide when they come into contact with atmospheric moisture.
- the generation of hydrogen sulfide can be suppressed by reducing the number of sulfur vacancies at the sulfur site, in other words, by increasing the occupancy of sulfur (S) and halogen (Ha) at the sulfur site.
- the sulfide having an aldirodite-type crystal structure constituting the present sulfide-based compound particles is a sulfide having crystallinity, that is, a sulfur-containing compound, and is a sulfide-based compound having an aldirodite-type crystal structure.
- the “aldirodite-type crystal structure” is a crystal structure of a group of compounds derived from a mineral represented by a chemical formula: Ag 8 GeS 6 .
- the sulfide-based solid electrolyte means a solid electrolyte made of a sulfur-containing compound, and the solid electrolyte is a film (so-called SEI (Solid Electrolyte Interphase)) generated at an electrode material interface in an initial charge / discharge reaction or the like after battery production. ), But not a solid having Li ion conductivity that can be used as a substitute for an electrolyte and a separator in battery design.
- the sulfide-based compound particles may contain an element other than lithium (Li), phosphorus (P), sulfur (S), and halogen (Ha).
- Li lithium
- P phosphorus
- S sulfur
- Ha halogen
- the present sulfide-based compound particles contain a crystal phase having the above-mentioned aldirodite type crystal structure as a main phase, even if they are composed of a single phase of the crystal phase, a different phase (hereinafter referred to as “heterophase”) ").
- a different phase hereinafter referred to as “heterophase”
- the different phase include Li 3 PS 4 and lithium halide.
- the “main phase” means a compound having the largest content ratio (mol ratio) among the compounds constituting the particles (the same applies hereinafter), and whether or not the main phase is determined by X-ray diffraction The content ratio can be calculated and determined by analyzing the (XRD) pattern.
- the content ratio of the crystal phase having the aldirodite-type crystal structure is preferably at least 60% by mass, and more preferably at least 70% by mass, based on all the crystal phases constituting the present sulfide-based compound particles. Preferably, it is preferably at least 80% by mass, and more preferably at least 90% by mass.
- halogen (Ha) element contained in the present sulfide compound particles examples include fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). Any combination of the above may be used. Above all, as compared with chlorine (Cl), bromine (Br) and iodine (I) have a large ionic radius and easily deform the crystal, so that sulfur deficiency is easily generated. Therefore, it can be considered that the use of Br or I or both of them can further enjoy the effects of the present invention.
- a composition formula (1) Li 7-x PS 6-x Ha x (Ha is one or more kinds) Halogen element, 0.2 ⁇ x ⁇ 2.0).
- x indicating the molar ratio of the halogen element is larger than 0.2
- the aldirodite-type crystal structure is stable near room temperature, and high ionic conductivity can be secured.
- a PS 4 structure which is the basic skeleton of Arujirodaito type crystal structure is preferable because it is possible to increase the conductivity of lithium ions.
- “x” is preferably greater than 0.2 and 2.0 or less, more preferably 0.4 or more and 1.7 or less, and especially 0.5 or more or 1.65 or less. Particularly preferred.
- “x” is preferably 1.4 or more, more preferably 1.5 or more, and especially 1.55 or more. More preferably, it is the above.
- the halogen (Ha) is a combination of a plurality of elements (for example, Cl and Br)
- “x” in the composition formula (1) is a total value of the molar ratio of each element.
- the total molar ratio x of Cl and Br preferably satisfies 1.0 ⁇ x ⁇ 1.8.
- x in the above composition formula (1) is preferably greater than 1.0 and 1.8 or less, especially 1.1 or more, or 1.7 or less, among which More preferably, it is 1.2 or more or 1.6 or less.
- the ratio (z / y) of the molar ratio z of Br to the molar ratio y of Cl is preferably 0.1 to 10.
- the ratio of the molar ratio of Br to the molar ratio of Cl (z / y) is 0.1 or more, the present sulfide-based compound particles have a low modulus of elasticity. It is preferable if the content is equal to or less than the value because high ionic conductivity is obtained. From such a viewpoint, the (z / y) is preferably 0.1 to 10, more preferably 0.2 or more, and especially preferably 0.3 or more.
- the present sulfide-based compound particles may contain a substance other than lithium (Li), phosphorus (P), sulfur (S) and halogen (Ha), for example, inevitable impurities.
- the content thereof is preferably less than 5 mol%, more preferably less than 3 mol%, particularly preferably less than 1 mol% of the present sulfide-based compound particles, from the viewpoint that the effect on performance is low.
- the sulfide compound having an aldirodite type crystal structure constituting the present sulfide compound particles has an occupancy of 85% of sulfur (S) and halogen (Ha) in the S3 (4a) site calculated by neutron diffraction measurement. Those described above are preferred. According to the neutron diffraction measurement, the content, that is, the occupancy of the element at each site (position) in the crystal structure can be measured.
- the aldirodite type crystal structure has sites called S1 (16e) site, S2 (4c) site, and S3 (4a) site as sulfur sites, that is, sites occupied by sulfur.
- the S1 site is a site constituting the PS 4 unit
- the S2 site is a site close to the PS 4 unit
- the S3 site is a site farthest from the PS 4 unit.
- the S3 site is considered to be the most susceptible to the external environment and is likely to cause sulfur deficiency.
- neutron diffraction measurement and examination revealed that the S3 site among the S1 to S3 sites was the most susceptible to sulfur deficiency, and that a correlation between sulfur deficiency and hydrogen sulfide gas generation was observed. Therefore, it was found that the generation of hydrogen sulfide gas can be suppressed by reducing the amount of defects at the S3 site.
- the occupancy of sulfur (S) and halogen (Ha) in the S3 (4a) site is defined, and it is interpreted that the closer the occupancy is to 100%, the smaller the sulfur deficiency in the S3 site. I decided that. Then, when actually measured, it was found that there was a correlation between the occupancy and the generation of hydrogen sulfide gas. From the above points, it is preferable that the sulfide-based compound has an occupancy of 85% or more of sulfur (S) and halogen (Ha) in the S3 (4a) site calculated by neutron diffraction measurement, and particularly 87 % Or more, more preferably 90% or more, and particularly preferably 95% or more. Although the upper limit is ideally 100%, it can be considered that it is actually about 99%.
- the present sulfide-based compound particles have a D50 (referred to as “average particle size (D50)” or “D50”) by volume particle size distribution measured by a laser diffraction / scattering particle size distribution measurement method of 50 ⁇ m or less. preferable.
- D50 average particle size
- D50 the sulfide-based compound particles easily enter the gap between the active material and the gap between the solid electrolyte used in combination, and the contact point and the contact area are reduced. It is preferable because it becomes larger.
- the average particle size (D50) of the present sulfide compound particles is preferably 50 ⁇ m or less, more preferably 0.1 ⁇ m or more, among them 0.3 ⁇ m or more, or 20 ⁇ m or less, among which 0.5 ⁇ m or more or 10 ⁇ m.
- the thickness be 0.5 ⁇ m or more or 5 ⁇ m or less.
- the average particle diameter (D50) of the present sulfide-based compound particles when the present sulfide-based compound particles are added into the electrode is the average particle diameter (D50) of the positive electrode active material or the average particle diameter (D50) of the negative electrode active material. It is preferably from 1 to 100%.
- the average particle diameter (D50) of the present sulfide-based compound particles is 1% or more of the average particle diameter (D50) of the active material, it is preferable because the active material can be filled without gaps. On the other hand, if it is 100% or less, the filling rate of the electrode can be increased, and this is preferable from the viewpoint of increasing the energy density of the battery.
- the average particle diameter (D50) of the present sulfide-based compound particles is preferably 1 to 100% of the average particle diameter (D50) of the active material, more preferably 3% or more, or 50% or less, and among them. More preferably, it is 5% or more or 30% or less.
- the present sulfide-based compound particles are obtained, for example, by adding elemental sulfur to a solid electrolyte raw material and mixing to obtain a mixture (“mixing step”), and converting the obtained mixture to an inert gas or hydrogen sulfide gas (H 2 S gas). ), And calcining at a temperature higher than 300 ° C. (“firing step”).
- mixing step a mixture
- inert gas or hydrogen sulfide gas H 2 S gas
- firing step calcining at a temperature higher than 300 ° C.
- the method for producing the present sulfide-based compound particles as long as the method includes the mixing step and the firing step, it is possible to arbitrarily add another treatment or another step. For example, it is possible to add processing such as stirring, crushing, and classification between the mixing step and the firing step, and to add processing such as stirring, crushing, and classification after the firing step.
- ⁇ Mixing process> it is preferable to obtain a mixture by mixing the solid electrolyte raw material and elemental sulfur.
- sulfur (S) gas can be generated from the elemental sulfur during firing, even when firing in an inert gas atmosphere.
- S sulfur
- a sufficient sulfur (S) partial pressure can be secured in the firing atmosphere. Therefore, even if hydrogen sulfide gas is not flowed, the same solid-phase reaction and crystal growth as in the case of flowing hydrogen sulfide gas can be caused, and as a result, the ionic conductivity of the solid electrolyte as a product is reduced. Can be secured.
- elemental sulfur has the property of sublimation, and even at a temperature lower than the melting point, generation of sulfur (S) gas based on a solid-gas equilibrium reaction can be expected. In addition, generation of sulfur (S) gas by a liquid-gas equilibrium reaction can be expected. Therefore, the decrease in the partial pressure of sulfur (S) in the firing atmosphere can be more effectively compensated over a wide temperature range, and the ion equivalent to that when hydrogen sulfide gas is passed can be obtained without flowing hydrogen sulfide gas. Conductivity can be more effectively ensured.
- the solid electrolyte raw material is a raw material of a substance containing an element constituting the sulfide-based solid electrolyte to be produced, and contains a substance containing lithium (Li), a substance containing sulfur (S), and a substance containing phosphorus (P). And a substance containing halogen (Ha).
- the substance containing lithium (Li) includes, for example, lithium compounds such as lithium sulfide (Li 2 S), lithium oxide (Li 2 O), lithium carbonate (Li 2 CO 3 ), and lithium metal alone.
- Examples of the substance containing phosphorus (P) include phosphorus sulfide such as diphosphorus trisulfide (P 2 S 3 ) and diphosphorus pentasulfide (P 2 S 5 ), and sodium phosphate (Na 3 PO 4 ). Examples thereof include a phosphorus compound and phosphorus alone.
- the substance containing sulfur (S) include the above-described lithium sulfide and phosphorus sulfide.
- Examples of the substance containing Ha include one or more selected from the group consisting of fluorine (F), chlorine (Cl), bromine (Br), and iodine (I) as Ha (halogen). And sodium (Na), lithium (Li), boron (B), aluminum (Al), silicon (Si), phosphorus (P), sulfur (S), germanium (Ge), arsenic (As), selenium (Se), a compound with one or more elements selected from the group consisting of tin (Sn), antimony (Sb), tellurium (Te), lead (Pb) and bismuth (Bi), or Examples of the compound include a compound in which oxygen or sulfur is further bonded to the compound.
- LiF, LiCl, LiBr, lithium halide such as LiI, PF 3, PF 5, PCl 3, PCl 5, POCl 3, PBr 3, POBr 3, PI 3, P 2 Cl 4, P 2 halogenated phosphorus I 4 such, SF 2, SF 4, SF 6, S 2 F 10, SCl 2, S 2 Cl 2, sulfur halide such as S 2 Br 2, NaI, NaF , NaCl, halogen such as NaBr
- lithium halide such as LiI, PF 3, PF 5, PCl 3, PCl 5, POCl 3, PBr 3, POBr 3, PI 3, P 2 Cl 4, P 2 halogenated phosphorus I 4 such, SF 2, SF 4, SF 6, S 2 F 10, SCl 2, S 2 Cl 2, sulfur halide such as S 2 Br 2, NaI, NaF , NaCl, halogen such as NaBr
- sodium halides, boron halides such as BCl 3 , BBr 3 , and BI 3 , and one or more of
- the elemental sulfur to be added may be any elemental sulfur that is not a sulfur compound, and is usually a solid, for example, a powder.
- the elemental sulfur to be added is ⁇ sulfur (oblique sulfur, melting point 112.8 ° C., boiling point 444.6 ° C.), ⁇ sulfur (monoclinic sulfur, melting point 119.6 ° C., boiling point 444.6 ° C.), ⁇ It may be sulfur (monoclinic sulfur, melting point 106.8 ° C., boiling point 444.6 ° C.) or any other sulfur allotrope.
- the elemental sulfur is added separately for the purpose of generating sulfur gas, and is different from the solid electrolyte raw material.
- the elemental sulfur may contain moisture and other impurities as long as the elemental sulfur is mixed with the solid electrolyte raw material and does not cause deterioration in characteristics.
- the content of the above-mentioned elemental sulfur impurity is preferably 3% by mass or less, since the influence of the property deterioration is small, and more preferably 1% by mass or less.
- the amount of single sulfur added is preferably at least 5 wt% of the total amount of the mixture of the solid electrolyte raw material and the single sulfur.
- elemental sulfur By adding 5% by weight or more of elemental sulfur of the total amount of the mixture, elemental sulfur can be arranged in the entire mixture without special mixing and dispersion treatment, and the elemental sulfur is vaporized. The resulting uneven distribution of sulfur (S) gas is suppressed, and the same ionic conductivity as when hydrogen sulfide gas is flown can be secured without flowing hydrogen sulfide gas during firing.
- the mixing amount of the elemental sulfur is too large, not only is the amount of the obtained solid electrolyte less economical, but also the amount of gaseous sulfur re-precipitated by cooling when it is discharged together with the exhaust gas, etc. And the risk of device blockage increases. Therefore, it is preferable that the amount of elemental sulfur added is 20 wt% or less of the total amount of the mixture. From this viewpoint, the mixing amount of the elemental sulfur is preferably 5 wt% or more of the total amount of the mixture, more preferably 20 wt% or less, especially 15 wt% or less, and even more preferably 10 wt% or less. .
- the timing of mixing single sulfur with the solid electrolyte raw material may be any time before firing.
- single sulfur may be mixed, or single sulfur may be mixed simultaneously with mixing of the solid electrolyte raw materials.
- any of the solid electrolyte raw materials for example, lithium sulfide, phosphorus sulfide and any part of the halogen compound with elemental sulfur
- the remaining solid electrolyte raw material for example, the remaining lithium sulfide, phosphorus sulfide and the halogen compound May be mixed.
- the method of mixing the solid electrolyte raw materials is not particularly limited. For example, they may be mixed in a ball mill, bead mill, homogenizer, mortar, or the like. However, if mixing is performed by using the mechanical milling method and the melt quenching method, and excessive kinetic energy is applied to the mixture, part or all of the sulfur or the sulfur compound added in the mixing step will be vaporized. It is preferable not to employ the mechanical milling method and the melt quenching method.
- Solid electrolyte raw materials such as lithium sulfide and phosphorus sulfide, are extremely unstable in the air, react with moisture, decompose, generate hydrogen sulfide gas, or oxidize, so gloves replaced with an inert gas atmosphere It is preferable to carry out the mixing step in a box or the like.
- the mixture obtained in the mixing step may be subjected to drying, stirring, washing, sizing, classification, etc., if necessary, and then supplied to the firing step.
- the mixture obtained in the mixing step may be fired at a temperature higher than 300 ° C.
- a sulfide having crystallinity can be produced.
- the sulfur partial pressure in the vicinity of the fired sample can be further increased as compared with the case of firing while flowing hydrogen sulfide gas, so that the occurrence of sulfur deficiency can be further suppressed and the electron conductivity increases. Can be further suppressed.
- the firing temperature at this time means the product temperature, and can be measured, for example, by inserting a thermocouple into the fired product.
- the container into which the raw materials are put during firing may be a container with a lid or a container without a lid, but it is preferable that the container is not an airtight container such as a sealed tube but a gas through which gas inside and outside the container can flow. This is because the surplus elemental sulfur among the added elemental sulfur can be more easily released out of the system, that is, outside the container.
- the container in which the raw material is put at the time of firing includes, for example, a sagger made of a material such as carbon, alumina, zirconia, or SiC.
- the firing is preferably performed while passing an inert gas or a hydrogen sulfide gas (H 2 S).
- an inert gas or a hydrogen sulfide gas H 2 S
- the above-described production method in which elemental sulfur is mixed with the solid electrolyte raw material and fired can obtain an ion conductivity equivalent to that in the case of firing by flowing hydrogen sulfide gas without flowing hydrogen sulfide gas. Since it has characteristics that can be achieved, it is more preferable to perform firing while passing an inert gas as much as possible without flowing hydrogen sulfide gas (H 2 S) in consideration of the cost of manufacturing equipment.
- the volume ratio of the hydrogen sulfide gas to the whole gas to be circulated is preferably 50% or less, more preferably 30% or less, more preferably 20% or less, and further more preferably 10% or less. , 0% (without using hydrogen sulfide gas).
- the inert gas include a nitrogen gas, an argon gas, and a helium gas.
- the sintering temperature that is, the maximum temperature of the product at the time of sintering may be higher than 300 ° C., especially 700 ° C. or less, and among them, it is desired to be 400 ° C. or more or 600 ° C. or less. It is more preferable from the viewpoint of causing a chemical reaction.
- the firing time that is, the time for heating to a temperature higher than 300 ° C., may be such that the solid phase reaction or the crystallization reaction of the mixture proceeds sufficiently, and it is preferable to appropriately adjust the mixing state of the mixture or the firing temperature. Typically, it is preferably 1 hour to 10 hours, more preferably 2 hours or more or 6 hours or less.
- the rate of temperature rise during firing is preferably 300 ° C./hr or less from the viewpoint of reducing the reaction unevenness, and taking into account the viewpoint of maintaining the firing efficiency, it is particularly 50 ° C./hr or more or 250 ° C./hr or less.
- the temperature is more preferably 100 ° C / hr or more or 200 ° C / hr or less.
- multi-stage baking may be performed in which the temperature is raised and the temperature is maintained repeatedly.
- the temperature is raised to 300 to 500 ° C. at a heating rate of 50 to 500 ° C./hr, preferably 100 ° C./hr or more or 300 ° C./hr or less, and the temperature is maintained for 1 to 20 hours.
- the temperature is raised to a temperature not lower than the melting point of the added elemental sulfur, for example, 350 to 700 ° C, and the temperature is raised for 1 to 10 hours.
- maintain can be mentioned.
- the residual amount of elemental sulfur after firing is set to 50 wt% or less before firing.
- the remaining amount is 50 wt% or less, of the added elemental sulfur, much of the extra elemental sulfur that does not contribute to Li ion conduction can be driven out of the system, and the amount of impurities in the solid electrolyte can be effectively reduced. , And the ionic conductivity can be further increased.
- the remaining amount is preferably set to 50 wt% or less, and more preferably 40 wt% or less.
- the remaining amount is preferably 30 wt% or less, particularly preferably 20 wt% or less.
- the remaining amount of the elemental sulfur was determined by measuring the weight (g) of the added elemental sulfur and the weight loss (g) of the mixture before and after calcination. Weight loss) / weight of the added elemental sulfur can be calculated and determined.
- an aldirodite-type crystal structure is formed at the time of firing, so that it is not necessary to apply strong mixing energy in the mixing step to generate the aldirodite-type crystal structure.
- an aldirodite type crystal structure may be generated at a stage before firing.
- mechanical stress may be applied using a pulverizer such as a planetary ball mill, a vibration mill, a rolling mill, a kneader, or the like. If so thus applying mechanical stress may cause a PS 4 structure which is the basic skeleton of Arujirodaito type crystal structure.
- the present sulfide-based compound particles can be used as a solid electrolyte layer of an all-solid-state lithium secondary battery or a solid electrolyte mixed with a positive electrode / negative electrode mixture.
- an all-solid-state lithium secondary battery can be formed by forming a layer containing the present sulfide-based compound particles between the positive electrode and the negative electrode.
- the shape of the battery include a laminate type, a cylindrical type, and a square type.
- the present sulfide-based compound particles have few sulfur deficiencies and are crystals with high integrity, so they are excellent in moisture resistance, and there is little characteristic deterioration even when handled in dry air, for example, even in a dry room. Assembly work of an all-solid-state lithium secondary battery can be performed.
- the layer containing the present sulfide-based compound particles was, for example, a method in which a slurry composed of the present sulfide-based compound particles, a binder, and a solvent was dropped on a substrate, and a method of scraping off with a doctor blade or the like, and the substrate was brought into contact with the slurry
- the film can be manufactured by a method of forming a coating film by a method of cutting with an air knife, a screen printing method or the like, and then removing the solvent through heating and drying.
- the powder of the present sulfide-based compound particles can be produced by pressing into a green compact by pressing or the like, and then processing it appropriately.
- the layer containing the present sulfide-based compound particles preferably has a porosity of 50% or less, more preferably 30% or less, and particularly preferably 20% or less.
- the thickness of the layer containing the present sulfide-based compound particles is typically preferably from 5 to 300 ⁇ m, and more preferably from 10 ⁇ m or more and 100 ⁇ m or less, from the balance between short circuit prevention and volume capacity density. preferable.
- the present sulfide-based compound particles and another solid electrolyte can be used as a solid electrolyte layer in which the present sulfide-based compound particles and another solid electrolyte are mixed. It can be used in combination with any of amorphous (glass), glass ceramics, and crystalline materials. Specific examples of the sulfide-based solid electrolyte include Li 2 SP 2 S 5 , Li 4 P 2 S 6 , and Li 7 P 3 S 11 .
- the solid electrolyte to be combined may be a non-sulfide, for example, an oxide-based solid electrolyte.
- a positive electrode material used as a positive electrode active material of a lithium secondary battery can be appropriately used.
- a positive electrode active material containing lithium specifically, a spinel-type lithium transition metal compound, a lithium metal oxide having a layered structure, and the like can be given.
- the positive electrode material may include a conductive material or another material in addition to the positive electrode active material to form a positive electrode mixture.
- a negative electrode material used as a negative electrode active material of a lithium secondary battery can be appropriately used.
- the present sulfide-based compound particles are electrochemically stable, so that graphite and artificial graphite charge and discharge at lithium metal or a lower potential (about 0.1 V vs Li + / Li) comparable to lithium metal.
- Carbon-based materials such as natural graphite and non-graphitizable carbon (hard carbon) can be used. Therefore, the energy density of the all-solid-state lithium secondary battery can be significantly improved. Further, silicon or tin which is promising as a high capacity material can be used as an active material.
- the electrolytic solution and the active material react with charge and discharge, causing corrosion on the surface of the active material.
- the present sulfide-based compound particles are used as the electrolyte of the lithium secondary battery and silicon or tin is used as the negative electrode active material, such a corrosion reaction does not occur, so that the durability of the battery can be improved.
- a conductive material or another material may be included in addition to the negative electrode active material to form a negative electrode mixture.
- a solid electrolyte according to an example of the embodiment of the present invention (referred to as “the present solid electrolyte”) is a solid electrolyte having the above-mentioned present sulfide compound.
- the present solid electrolyte may be composed of only the above-mentioned sulfide-based compound particles, or may be a mixture of the present sulfide-based compound particles and other compound particles.
- the content ratio of the present sulfide compound particles is 50% by mass or more, particularly 80% by mass or more, particularly 90% by mass or more, and particularly 99% by mass or more (including 100% by mass). preferable.
- the solid electrolyte preferably has a D50 of 50 ⁇ m or less based on a volume particle size distribution measured by a laser diffraction / scattering type particle size distribution measuring method.
- the solid electrolyte has a D50 of 50 ⁇ m or less, the solid electrolyte easily enters the gap between the active material and the solid electrolyte used in combination, and the contact point and the contact area are preferably increased.
- D50 is 0.1 ⁇ m or more, it is more preferable because the resistance is not increased due to an increase in the surface area of the solid electrolyte and mixing with the active material is not difficult.
- the average particle diameter (D50) of the present solid electrolyte is preferably 50 ⁇ m or less, more preferably 0.1 ⁇ m or more, among which 0.3 ⁇ m or more, or 20 ⁇ m or less, among which 0.5 ⁇ m or more or 10 ⁇ m or less.
- the thickness be 0.5 ⁇ m or more or 5 ⁇ m or less.
- Example 1 Lithium sulfide (Li 2 S) powder, diphosphorus pentasulfide (P 2 S 5 ) powder, and lithium chloride (LiCl) such that the composition of the compound having the aldirodite type crystal structure is Li 5.4 PS 4.4 Cl 0.8 Br 0.8. ) Powder and lithium bromide (LiBr) powder were each weighed to a total amount of 5 g, crushed and mixed in a ball mill for 15 hours, and then an amount of simple sulfur powder corresponding to 5 wt% of the whole mixed powder.
- the sample was crushed with a ball mill, and sized with a sieve having an opening of 53 ⁇ m to obtain a powdery sample.
- the weighing, mixing, setting in the electric furnace, taking out from the electric furnace, crushing, and sizing are all performed in a glove box replaced with a sufficiently dried Ar gas (dew point ⁇ 60 ° C. or lower).
- a compound powder (sample) represented by a composition formula: Li 5.4 PS 4.4 Cl 0.8 Br 0.8 was obtained.
- the residual amount of elemental sulfur after firing was 0 wt%.
- Example 2 A compound powder (sample) represented by the composition formula: Li 5.4 PS 4.4 Cl 0.8 Br 0.8 was obtained in the same manner as in Example 1 except that the elemental sulfur powder was not added in Example 1.
- Example 2 In Example 1, the lithium sulfide (Li 2 S) powder, the diphosphorus pentasulfide (P 2 S 5 ) powder, and the chloride were used such that the composition of the compound having the aldirodite type crystal structure was Li 5.8 PS 4.8 Cl 1.2.
- a compound powder (sample) represented by a composition formula: Li 5.8 PS 4.8 Cl 1.2 was obtained in the same manner as in Example 1 except that lithium (LiCl) powder was mixed.
- Diffraction peaks other than the peak derived from the aldirodite type crystal structure or the peak attributed to the internal standard Si powder were taken as heterophase peaks.
- Data of PDF No. 00-034-0688 was used for identification of a peak derived from the aldirodite type crystal structure.
- the Rietveld analysis was carried out with the analysis software “RIETAN-FP v2.8.3” using the XRD data measured under the above conditions. At this time, the validity index was R wp ⁇ 10 and S ⁇ 2.0.
- Table 1 shows the composition of the compound having an aldirodite type crystal structure and the occupancy of the S3 site, which were calculated from the compounding ratio of the charged raw material compounds in the compound powders (samples) obtained in Examples and Comparative Examples.
- ⁇ D50> About the compound powder (sample) obtained by the Example and the comparative example, the sample (powder) was used using the automatic sample feeder ("Microtorac SDC" by Microtrack Bell Co., Ltd.) for a laser diffraction particle size distribution measuring device. After pouring into a water-soluble solvent and irradiating a 40 W ultrasonic wave a plurality of times for 360 seconds at a flow rate of 40%, the particle size distribution was measured using a laser diffraction particle size distribution analyzer “MT3000II” manufactured by Microtrac Bell Co., Ltd. D50 was measured from the obtained chart of volume-based particle size distribution.
- the number of times of ultrasonic irradiation was the number of times until the rate of change of D50 before and after ultrasonic irradiation became 8% or less.
- the water-soluble solvent at the time of measurement was passed through a 60 ⁇ m filter, the solvent refractive index was 1.33, the particle permeability condition was transmitted, the particle refractive index was 2.46, the shape was non-spherical, and the measurement range was 0.133. ⁇ 704.0 ⁇ m, the measurement time was 30 seconds, and the average value measured twice was taken as each value.
- the sealed bag containing the sample was opened in the thermo-hygro chamber and the sample was quickly placed in the separable flask.
- the sample was placed in a separable flask and the concentration of hydrogen sulfide generated from immediately after sealing the flask until 60 minutes elapsed was measured with a hydrogen sulfide sensor (GX-2009 manufactured by Riken Keiki) after 60 minutes. Then, the volume of hydrogen sulfide was calculated from the hydrogen sulfide concentration after a lapse of 60 minutes to determine the amount of hydrogen sulfide generated.
- the ionic conductivity was measured at room temperature (25 ° C.) using a Solartron 1255B (manufactured by Toyo Technica Co., Ltd.) under the conditions of a measurement frequency of 0.1 Hz to 1 MHz and an ionic conductivity (AC impedance method). mS / cm). The results are shown in Table 1.
- the positive electrode mixture powder is prepared by mixing a positive electrode active material powder, a solid electrolyte powder, and a conductive additive (acetylene black) powder in a mortar at a mass ratio of 60: 37: 3, and is uniaxial press-molded at 20 MPa. Thus, a positive electrode mixture pellet was obtained.
- the negative electrode mixture powder was prepared by mixing a graphite powder and a solid electrolyte powder in a mortar at a mass ratio of 64:36.
- the negative electrode mixture powder After the negative electrode mixture powder is placed thereon, it is closed with a negative electrode (made of SUS) and uniaxially formed at 550 MPa, and a positive electrode mixture having a thickness of about 100 ⁇ m, a solid electrolyte layer having a thickness of about 300 ⁇ m, and a solid electrolyte layer having a thickness of about 20 ⁇ m are formed.
- An all-solid-state battery cell having a three-layer structure of a negative electrode mixture was produced. At this time, the production of the all-solid-state battery cell was performed in a glove box replaced with argon gas having a dew point temperature of ⁇ 60 ° C.
- the battery characteristics were measured by placing all the solid-state battery cells in an environmental tester kept at 25 ° C. and connecting to a charge / discharge measuring device. The battery was charged and discharged with 1 mA as 1 C. The battery was charged to 4.5 V at 0.1 C by the CC-CV method to obtain an initial charge capacity. Discharging was performed by a CC method at 0.1 C up to 2.5 V to obtain an initial discharge capacity. Each of the discharge capacities when discharged to 2.5 V at 0.1 C was 160 mAh / g or more. It can be considered that a practical discharge capacity was able to be exhibited because the solid electrolyte secured ionic conductivity that was practical.
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Abstract
Description
レーザー回折散乱式粒度分布測定法により測定して得られる体積粒度分布によるD50が50μm以下であり、
中性子回折測定により算出されるS3(4a)サイトにおける、硫黄(S)及びハロゲン(Ha)の占有率が85%以上である硫化物系化合物粒子を提案する。
本発明の実施形態の一例に係る硫化物系化合物粒子(「本硫化物系化合物粒子」と称する)は、リチウム(Li)、リン(P)、硫黄(S)及びハロゲン(Ha)を含むアルジロダイト型結晶構造を有する硫化物系化合物粒子である。
硫化物系固体電解質とは、硫黄含有化合物からなる固体電解質の意味であり、当該固体電解質とは、電池製造後の初回充放電反応等で電極材界面に生じる膜(所謂SEI(Solid Electrolyte Interphase))ではなく、電池設計に際し、電解液及びセパレーターの代替として用いることが可能なLiイオン伝導性を有する固体のことを指す。
当該異相としては、例えばLi3PS4、ハロゲン化リチウムなどを挙げることができる。
なお、「主相」とは、当該粒子を構成する化合物のうち、最も含有割合(mol比率)の大きな化合物の意味であり(後も同様)、主相であるか否かは、X線回折(XRD)パターンの解析によって含有割合を算出して判定することができる。よって、アルジロダイト型結晶構造を有する結晶相の含有割合は、本硫化物系化合物粒子を構成する全結晶相に対して、60質量%以上であることが好ましく、中でも70質量%以上であることが好ましく、中でも80質量%以上であることが好ましく、その中でも90質量%以上であることが好ましい。
前記組成式(1)において、ハロゲン元素のモル比を示す「x」が0.2より大きければ、室温近傍でアルジロダイト型結晶構造が安定であり、高いイオン伝導率を確保することができ、2.0以下であれば、アルジロダイト型結晶構造の基本骨格であるPS4構造を生成しやすく、リチウムイオンの伝導性を高めることができるため好ましい。
かかる観点から、「x」は0.2より大きく且つ2.0以下であるのが好ましく、中でも0.4以上或いは1.7以下、その中でも0.5以上或いは1.65以下であるのが特に好ましい。
他方、ハロゲンを多く含むほど、硫黄欠損をより一層低減することができるから、かかる観点からは、「x」は1.4以上であるのが好ましく、中でも1.5以上、その中でも1.55以上であるのがさらに好ましい。
なお、ハロゲン(Ha)が複数の元素(例えばClとBr)の組み合わせの場合、上記組成式(1)における「x」は、各元素のモル比の合計値である。
Clのモル比をy、Brのモル比をzとして、Cl及びBrの合計モル比x(=y+z)が1.0より大きく且つ1.8以下であれば、イオン伝導率をさらに高めることができるから好ましい。特にxが1.8以下であれば、異相の生成を制御することができ、イオン伝導率の低下を抑えることができる。
かかる観点から、HaがCl及びBrの場合、上記組成式(1)におけるxは1.0より大きく且つ1.8以下であるのが好ましく、中でも1.1以上或いは1.7以下、その中でも1.2以上或いは1.6以下であるのがさらに好ましい。
Clのモル比に対するBrのモル比の割合(z/y)が0.1以上であれば、本硫化物系化合物粒子が低弾性率となるから好ましく、他方、当該(z/y)が10以下であれば、高いイオン伝導率なものとなるから好ましい。
かかる観点から、当該(z/y)は0.1~10であるのが好ましく、中でも0.2以上或いは5以下、その中でも0.3以上或いは3以下であるのがさらに好ましい。
本硫化物系化合物粒子を構成するアルジロダイト型結晶構造を有する硫化物系化合物は、中性子回折測定により算出されるS3(4a)サイトにおける、硫黄(S)及びハロゲン(Ha)の占有率が85%以上であるものが好ましい。
中性子回折測定によれば、結晶構造中の各サイト(位置)における元素の含有率すなわち占有率を測定することができる。
当該アルジロダイト型結晶構造は、硫黄サイトすなわち硫黄が占有するサイトとして、S1(16e)サイト、S2(4c)サイト、S3(4a)サイトと称されるサイトを有している。S1サイトは、PS4ユニットを構成するサイトであり、S2サイトは、PS4ユニットに位置が近いサイトであり、S3サイトはPS4ユニットから一番離れているサイトである。これら硫黄サイトの中で、S3サイトが最も外部環境の影響を受けやすく、硫黄欠損を生じ易いと考えられる。事実、中性子回折測定を行って検討したところ、S1~S3サイトの中でS3サイトが最も硫黄欠損を生じ易く、且つ、硫黄欠損と硫化水素ガス発生との相関が認められることが分かった。よって、S3サイトにおける欠損量を少なくすることにより、硫化水素ガス発生を抑えることができることが分かった。
但し、S3サイトを構成する元素のうち、硫黄(S)のほかに、硫黄(S)の置換元素として何がどれくらい占有するかが分かっていれば、S3サイトの欠損量つまりS3サイトの構成元素のうちの意図していない元素の量を測定することができる。しかし、それが分かっていない状況では、当該欠損量を測定することは難しい。そのため、本発明では、S3(4a)サイトにおける、硫黄(S)及びハロゲン(Ha)の占有率を規定し、当該占有率が100%に近いほど、S3サイトの硫黄欠損量が少ないと解釈することとした。そして実際に測定してみると、当該占有率と硫化水素ガス発生との相関が認められることが分かった。
以上の点から、上記硫化物系化合物は、中性子回折測定により算出されるS3(4a)サイトにおける、硫黄(S)及びハロゲン(Ha)の占有率が85%以上であるものが好ましく、中でも87%以上、その中でも90%以上、その中でも95%以上であるのがさらに好ましい。なお、上限値は理想的には100%であるが、実際には99%程度であると考えることができる。
本硫化物系化合物粒子は、レーザー回折散乱式粒度分布測定法により測定して得られる体積粒度分布によるD50(「平均粒径(D50)」又は「D50」と称する)が50μm以下であるのが好ましい。
本硫化物系化合物粒子のD50が50μm以下であれば、活物質との隙間、及び、組み合わせて用いる固体電解質との隙間などに本硫化物系化合物粒子が入りやすくなり、接触点及び接触面積が大きくなるから好ましい。他方、D50が0.1μm以上であれば、本硫化物系化合物粒子からなる粉末全体の表面積が増えることによる抵抗増大や、活物質との混合が困難となることがないから、より好ましい。
かかる観点から、本硫化物系化合物粒子の平均粒径(D50)は50μm以下であるのが好ましく、中でも0.1μm以上、その中でも0.3μm以上或いは20μm以下、その中でも0.5μm以上或いは10μm以下、その中でも特に0.5μm以上或いは5μm以下であるのがさらに好ましい。
本硫化物系化合物粒子の平均粒径(D50)が、活物質の平均粒径(D50)の1%以上であれば、活物質間を隙間なく埋めることができるため好ましい。他方、100%以下であれば、電極の充填率を高めることができるので、電池の高エネルギー密度化の観点から好ましい。
かかる観点から、本硫化物系化合物粒子の平均粒径(D50)は、活物質の平均粒径(D50)の1~100%であるのが好ましく、中でも3%以上或いは50%以下、その中でも5%以上或いは30%以下であるのがさらに好ましい。
次に本硫化物系化合物粒子の製造方法の一例について説明する。但し、本硫化物系化合物粒子の製造方法は、ここで説明する本硫化物系化合物粒子の製造方法に限定されるものではない。
本工程では、固体電解質原料と、単体の硫黄とを混合して混合物を得ることが好ましい。
焼成する前に、固体電解質原料に、単体の硫黄を混合することにより、焼成時に当該単体の硫黄から硫黄(S)ガスを発生させることができ、たとえ不活性ガス雰囲気で焼成した場合であっても、焼成雰囲気において十分な硫黄(S)分圧を確保することができる。従って、硫化水素ガスを流通させなくても、硫化水素ガスを流通させた場合と同等の固相反応及び結晶成長を生じさせることができるため、結果として生成物である固体電解質のイオン伝導率を確保することができる。
中でも、単体の硫黄は、昇華する性質を有しており、融点よりも低温であっても固体-気体の平衡反応に基づいた硫黄(S)ガスの発生が期待でき、さらに融点以上の温度では、液体-気体の平衡反応による硫黄(S)ガスの発生が期待できる。したがって、広い温度範囲において焼成雰囲気における硫黄(S)分圧の低下をより一層効果的に補うことができ、硫化水素ガスを流通させなくても、硫化水素ガスを流通させた場合と同等のイオン伝導率をより効果的に確保することができる。
固体電解質原料とは、製造する硫化物系固体電解質を構成する元素を含む物質の原料となるものであり、リチウム(Li)を含有する物質、硫黄(S)を含有する物質、リン(P)を含有する物質、及び、ハロゲン(Ha)を含有する物質である。
前記リン(P)を含有する物質としては、例えば三硫化二リン(P2S3)、五硫化二リン(P2S5)等の硫化リン、リン酸ナトリウム(Na3PO4)等のリン化合物、及びリン単体等を挙げることができる。
前記硫黄(S)を含有する物質としては、上記硫化リチウム、硫化リンなどを挙げることができる。
添加する単体の硫黄は、硫黄化合物でない硫黄単体であればよく、固体例えば粉体であるのが通常である。
また、添加する単体の硫黄は、α硫黄(斜方硫黄、融点112.8℃、沸点444.6℃)、β硫黄(単斜硫黄、融点119.6℃、沸点444.6℃)、γ硫黄(単斜硫黄、融点106.8℃、沸点444.6℃)、その他硫黄同素体のいずれであってもよい。
なお、ここでいう単体の硫黄は、硫黄ガスを発生させる目的で別途添加するものであり、固体電解質原料とは異なる。
その際、上記単体硫黄の不純物の含有量は、典型的には3質量%以下であれば、特性劣化の影響が少ないため好ましく、1質量%以下であることがより好ましい。
前記混合物の合計量の5wt%以上の単体の硫黄を加えることにより、特別な混合分散処理を行わなくとも、前記混合物全体に単体の硫黄を配置することができ、単体の硫黄が気化することによって得られる硫黄(S)ガスの偏在が抑制され、焼成時に硫化水素ガスを流通させなくても、硫化水素ガスを流通させた場合と同等のイオン伝導率を確保することができる。他方、単体の硫黄の混合量が多過ぎると、得られる固体電解質の量が減り経済的でないだけでなく、揮発したガス状硫黄が排気ガス等とともに排出される際、冷却により再析出する量が多くなり、装置の閉塞のリスクが高まる。そのため、添加する単体の硫黄は、前記混合物の合計量の20wt%以下であるのが好ましい。
かかる観点から、単体の硫黄の混合量は、前記混合物の合計量の5wt%以上であるのが好ましく、中でも20wt%以下、その中でも15wt%以下、その中でも10wt%以下であるのがより一層好ましい。
但し、メカニカルミリング法及び溶融急冷法を採用して混合し、混合物に過剰な運動エネルギーを掛けると、混合工程の段階で添加した硫黄もしくは硫黄化合物の一部、または全部が気化してしまうため、これらメカニカルミリング法及び溶融急冷法は採用しないことが好ましい。
本工程では、混合工程で得られた混合物を300℃より高温で焼成すればよい。
単体の硫黄を固体電解質原料に混合して300℃より高温で焼成することにより、結晶性を有する硫化物を製造することができる。さらには硫化水素ガスを流通させながら焼成する場合に比べて、焼成試料近傍の硫黄分圧をより一層高めることができるため、硫黄欠損が生じるのをより抑えることができ、電子伝導性が高くなるのをより一層抑えることができる。
かかる観点から、焼成時に原料を入れる容器は、例えばカーボン、アルミナ、ジルコニア、SiCなどの材料からなる匣鉢などを挙げることができる。
かかる観点から、流通させるガス全体に対する硫化水素ガスの体積比率は、50%以下であることが好ましく、中でも30%以下、その中でも20%以下、さらにその中でも10%以下であるのがより一層好ましく、0%である(硫化水素ガスを使用しない)のがさらに一層好ましい。
この際、不活性ガスとしては、窒素ガス、アルゴンガス、ヘリウムガスを挙げることができる。
焼成時間、すなわち、300℃より高温に加熱する時間は、混合物の固相反応又は結晶化反応が十分進行する程度であればよく、混合物の混合状態又は焼成温度により適宜調整するのが好ましい。典型的には1時間~10時間が好ましく、中でも2時間以上或いは6時間以下であるのがさらに好ましい。
例えば、昇温速度50~500℃/hr、好ましくは100℃/hr以上或いは300℃/hr以下で、300~500℃まで昇温し、当該温度を1~20時間保持した後、昇温速度50~500℃/hr、好ましくは100℃/hr以上或いは300℃/hr以下で、添加した単体の硫黄の融点以上の温度、例えば350~700℃まで昇温し、当該温度を1~10時間保持するように焼成する方法を挙げることができる。
このように多段階焼成することにより、結晶性が高い固体電解質を製造することができるばかりか、余分な単体の硫黄を系外により一層確実に追い出すことができるから、これらの残留を防いで、イオン伝導率をより一層高めることができる。
当該残量が50wt%以下であれば、添加した単体の硫黄のうち、Liイオン伝導に寄与しない余分な単体の硫黄の多くを系外に追い出すことができ、固体電解質中の不純物量を効果的に低下させることができ、イオン伝導率をさらに高めることができるから、好ましい。
かかる観点から、当該残量を50wt%以下とするのが好ましく、中でも40wt%以下とするのがさらに好ましい。
単体の硫黄を添加した場合に、硫黄の融点以上の温度で焼成することにより、固体電解質の焼成体の細孔部等にトラップされた余剰の硫黄の大部分を追い出すことができるので、その場合、当該残量は30wt%以下、中でも20wt%以下となるのが好ましい。
単体の硫黄の残量は、添加した単体の硫黄の重量(g)と焼成前後の混合物の重量減少量(g)を測定し、100×(添加した単体の硫黄の重量-焼成前後の混合物の重量減少量)/添加した単体の硫黄の重量、を計算して求めることができる。
アルジロダイト型結晶構造を焼成前の段階で生じさせる方法としては、遊星ボールミル、振動ミル、転動ミル等の粉砕機、混練機等を使用して、機械的応力を加えるようにすればよい。このように機械的応力を加えるようにすれば、アルジロダイト型結晶構造の基本骨格であるPS4構造を生じさせることができる。
本硫化物系化合物粒子は、全固体型リチウム二次電池の固体電解質層、又は、正極・負極合材に混合する固体電解質として使用することができる。
電池の形状としては、例えばラミネート型、円筒型および角型等を挙げることができる。
この際、本硫化物系化合物粒子は、硫黄欠損が少なく、完全性の高い結晶であるため、耐湿性に優れており、乾燥空気中で取り扱っても特性劣化が少ないため、例えばドライルームなどでも全固体型リチウム二次電池の組立作業を行うことができる。
ここで、空隙率は、例えば液相法(アルキメデス法)で求めた、本硫化物系化合物粒子を含む層の真密度と見かけの密度から、下記に示す関係式により算出することができる。
空隙率(%)=(真密度-見かけの密度)÷真密度×100
正極材は、正極活物質のほかに、導電化材或いはさらに他の材料を含ませて正極合材としてもよい。
負極材についても、負極活物質のほかに、導電材或いはさらに他の材料を含ませて負極合材としてもよい。
本発明の実施形態の一例に係る固体電解質(「本固体電解質」と称する)は、上記本硫化物系化合物を有する固体電解質である。
但し、本固体電解質において、上記の本硫化物系化合物粒子の含有割合は50質量%以上、中でも80質量%以上、中でも90質量%以上、中でも99質量%以上(100質量%を含む)のが好ましい。
本固体電解質のD50が50μm以下であれば、活物質との隙間、及び、組み合わせて用いる固体電解質との隙間などに本固体電解質が入りやすくなり、接触点及び接触面積が大きくなるから好ましい。他方、D50が0.1μm以上であれば、本固体電解質の表面積が増えることによる抵抗増大、及び活物質との混合が困難となることがないからより好ましい。
かかる観点から、本固体電解質の平均粒径(D50)は50μm以下であるのが好ましく、中でも0.1μm以上、その中でも0.3μm以上或いは20μm以下、その中でも0.5μm以上或いは10μm以下、その中でも特に0.5μm以上或いは5μm以下であるのがさらに好ましい。
本発明において「X~Y」(X、Yは任意の数字)と記載した場合、特に断らない限り「X以上Y以下」の意と共に、「好ましくはXより大きい」又は「好ましくはYより小さい」の意も包含する。
また、「X以上」又は「X≦」(Xは任意の数字)と記載した場合、「Xより大きいことが好ましい」旨の意図を包含し、「Y以下」又は「Y≧」(Yは任意の数字)と記載した場合、「Yより小さいことが好ましい」旨の意図を包含する。
アルジロダイト型結晶構造を有する化合物の組成がLi5.4PS4.4Cl0.8Br0.8となるように、硫化リチウム(Li2S)粉末と、五硫化二リン(P2S5)粉末と、塩化リチウム(LiCl)粉末と、臭化リチウム(LiBr)粉末とを、全量で5gとなるようにそれぞれ秤量し、ボールミルで15時間粉砕混合を行った後、混合粉全体の5wt%に相当する量の単体硫黄粉末(α硫黄、不純物含有量0.5質量%、融点112.8℃、沸点444.6℃)0.26gを加えて乳鉢で混合して混合粉末を得た。
そして、得られた混合粉末をカーボン製の容器(40mm×30mm×20mm、非気密性)の80体積%まで充填し、これを管状電気炉にて、Arガス(Ar100体積%、硫化水素ガス0体積%)を1.0l/min流通させながら300℃(品温)で4時間加熱した後、さらに500℃(品温)で4時間加熱した。昇降温速度は200℃/hrとした。その後、試料をボールミルで解砕し、目開き53μmの篩いで整粒して粉末状のサンプルを得た。
この際、前記秤量、混合、電気炉へのセット、電気炉からの取り出し、解砕及び整粒作業は全て、十分に乾燥されたArガス(露点-60℃以下)で置換されたグローブボックス内で実施し、組成式:Li5.4PS4.4Cl0.8Br0.8で示される化合物粉末(サンプル)を得た。焼成後における単体硫黄の残量は0wt%であった。
実施例1において、単体硫黄粉末を加えなかった点、及び、Arガスに代えてH2Sガスを流通させながら加熱した点以外、実施例1と同様にして、組成式:Li5.4PS4.4Cl0.8Br0.8で示される化合物粉末(サンプル)を得た。
実施例1において、単体硫黄粉末を加えなかった点以外、実施例1と同様にして、組成式:Li5.4PS4.4Cl0.8Br0.8で示される化合物粉末(サンプル)を得た。
実施例1において、アルジロダイト型結晶構造を有する化合物の組成がLi5.8PS4.8Cl1.2となるように、硫化リチウム(Li2S)粉末と、五硫化二リン(P2S5)粉末と、塩化リチウム(LiCl)粉末とを混合した以外、実施例1と同様にして、組成式:Li5.8PS4.8Cl1.2で示される化合物粉末(サンプル)を得た。
実施例・比較例で得られた化合物粉末(サンプル)を全溶解してICP発光分析法により元素組成を測定した。その結果、仕込み原料化合物の配合比と概ね一致していることを確認した。
実施例・比較例で得られた化合物粉末(サンプル)をX線回折法(XRD、Cu線源)で分析し、X線回折パターンを得て、各位置におけるピーク強度(counts)を測定した。リガク社製のXRD装置「Smart Lab」を用いて、大気中で走査軸:2θ/θ、走査範囲:10~140deg、ステップ幅0.01deg、走査速度1deg/minの条件の下で行った。内部標準としてSi粉末(和光純薬工業製、純度99.9%)を5wt%混合し、角度補正に用いた。
アルジロダイト型結晶構造に由来するピークもしくは内部標準用Si粉末に帰属されるピーク以外の回折ピークを異相ピークとした。アルジロダイト型結晶構造に由来するピークの同定には、PDF番号00-034-0688のデータを用いた。
アルジロダイト型結晶構造に由来するピークのうち回折角2θ=24.9°~26.3°の位置に出現するピークの強度に対する、異相ピークのうち、最も高いピーク強度の比率を調べた。そして、異相ピークが存在しないか、或は、当該比率が0.04未満の場合は、アルジロダイト型結晶構造の「単相」であると判定し、当該比率が0.04以上である場合には、「異相あり」と判定した。
実施例・比較例で得られた化合物粉末(サンプル)のXRDデータを用いて、下記に示すリートベルト解析を実施した。
実施例・比較例で得た化合物粉末(サンプル)を、大強度陽子加速器施設J-PARCセンターのBL20にて、出力300kW、ダブルフレーム(DF)、2時間/サンプルの条件で中性子回折測定を行った。得られた中性子回折データを、解析ソフト「Z-Rietveld」にて解析した。この際、妥当性の指標は、Rwp<10、S<2.0とした。
X線回折測定、X線リートベルト解析および中性子回折の結果を総合して、サンプルがアルジロダイト型結晶構造からなる化合物であると判断した。ICP発光分析法により組成を定量した結果、仕込み原料化合物の配合比から算出した組成式:Li5.4PS4.4Cl0.8Br0.8またはLi5.8PS4.8Cl1.2と概ね一致していた。表1には、実施例・比較例で得られた化合物粉末(サンプル)の、仕込み原料化合物の配合比から算出したアルジロダイト型結晶構造からなる化合物の組成と、S3サイトの占有率を示した。
実施例及び比較例で得られた化合物粉末(サンプル)について、レーザー回折粒子径分布測定装置用自動試料供給機(マイクロトラック・ベル株式会社製「Microtorac SDC」)を用い、サンプル(粉体)を水溶性溶媒に投入し、40%の流速中、40Wの超音波を360秒間複数回照射した後、マイクロトラック・ベル株式会社製レーザー回折粒度分布測定機「MT3000II」を用いて粒度分布を測定し、得られた体積基準粒度分布のチャートからD50を測定した。
超音波の照射回数は、超音波照射前後におけるD50の変化率が8%以下となるまでの回数とした。
なお、測定の際の水溶性溶媒は60μmのフィルターを通し、溶媒屈折率を1.33、粒子透過性条件を透過、粒子屈折率2.46、形状を非球形とし、測定レンジを0.133~704.0μm、測定時間を30秒とし、2回測定した平均値をそれぞれの値とした。
実施例・比較例で得た化合物粉末(サンプル)を、十分に乾燥されたArガス(露点-60℃以下)で置換されたグローブボックス内で50mgずつ秤量し、ラミネートフィルムで密閉された袋に入れた。その後、乾燥空気と大気を混合することで調整した露点-30℃雰囲気で室温(25℃)に保たれた恒温恒湿槽の中に、容量1500cm3のガラス製のセパラブルフラスコを入れ、セパラブルフラスコの内部が恒温恒湿槽内の環境と同一になるまで保持してから、サンプルが入った密閉袋を恒温恒湿槽の中で開封し、素早くセパラブルフラスコにサンプルを配置した。サンプルをセパラブルフラスコに配置し、前記フラスコを密閉した直後から60分経過までに発生した硫化水素について、60分後に硫化水素センサー(理研計器製GX-2009)にて硫化水素濃度を測定した。そして、60分経過後の硫化水素濃度から硫化水素の体積を算出して硫化水素発生量を求めた。
実施例・比較例で得た化合物粉末(サンプル)を、十分に乾燥されたArガス(露点-60℃以下)で置換されたグローブボックス内で一軸加圧成形し、さらにCIP(冷間等方圧加圧装置)にて200MPaで直径10mm、厚み約4~5mmのペレットを作製した。更にペレット上下両面に電極としてのカーボンペーストを塗布した後、180℃で30分間の熱処理を行い、イオン伝導率測定用サンプルを作製した。
イオン伝導率測定は、室温(25℃)にて、東陽テクニカ社製の装置である、ソーラトロン1255Bを用いて、測定周波数0.1Hz~1MHzの条件下、交流インピーダンス法にて、イオン伝導率(mS/cm)を測定した。結果を表1に示した。
実施例1、2でそれぞれ得た化合物粉末(サンプル)のみを固体電解質として用いて正極合材、負極合材を調製し、全固体電池を作製して、それぞれ電池特性評価(初回充放電容量)を行った。
正極活物質として、層状化合物であるLiNi0.5Co0.2Mn0.3O2(NCM)粉末(D50=6.7μm)を用い、負極活物質としてグラファイト(D50=20μm)を用い、固体電解質粉末として実施例で得たサンプルを用いた。
正極合材粉末は、正極活物質粉末、固体電解質粉末及び導電助剤(アセチレンブラック)粉末を、質量比で60:37:3の割合で乳鉢混合することで調製し、20MPaで1軸プレス成型して正極合材ペレットを得た。
負極合材粉末は、グラファイト粉末と固体電解質粉末を、質量比で64:36の割合で乳鉢混合することで調製した。
上下を開口したポリプロピレン製の円筒(開口径10.5mm、高さ18mm)の下側開口部を正極電極(SUS製)で閉塞し、正極電極上に正極合材ペレットを載せた。その上から実施例1で得た粉末固体電解質を載せて、180MPaにて1軸プレスし正極合材と固体電解質層を形成した。その上から負極合材粉末を載せた後、負極電極(SUS製)で閉塞して550MPaにて1軸成形し、およそ100μm厚の正極合材、およそ300μm厚の固体電解質層、およそ20μm厚の負極合材の3層構造からなる全固体電池セルを作製した。この際、上記全固体電池セルの作製においては、露点温度-60℃のアルゴンガスで置換されたグローブボックス内で行った。
電池特性測定は、25℃に保たれた環境試験機内に全固体電池セルを入れて充放電測定装置に接続して評価した。1mAを1Cとして電池の充放電を行った。0.1Cで4.5VまでCC-CV方式で充電し、初回充電容量を得た。放電は0.1Cで2.5VまでCC方式で行い初回放電容量を得た。
0.1Cで2.5Vまで放電した際の放電容量はいずれも160mAh/g以上であった。固体電解質が実用可能なイオン伝導性を確保しているため、実用的な放電容量を発現できたと考えることができる。
Claims (6)
- リチウム(Li)、リン(P)、硫黄(S)及びハロゲン(Ha)を含むアルジロダイト型結晶構造を有する硫化物系化合物粒子であって、
レーザー回折散乱式粒度分布測定法により測定して得られる体積粒度分布によるD50が50μm以下であり、中性子回折測定により算出されるS3(4a)サイトにおける、硫黄(S)及びハロゲン(Ha)の占有率が85%以上である硫化物系化合物粒子。 - ハロゲン(Ha)として、フッ素(F)、塩素(Cl)、臭素(Br)及びヨウ素(I)のうちの一種又は二種以上を含む、請求項1に記載の硫化物系化合物粒子。
- 請求項1又は2に記載の硫化物系化合物粒子を含有する固体電解質。
- レーザー回折散乱式粒度分布測定法により測定して得られる体積粒度分布によるD50が50μm以下であることを特徴とする請求項3に記載の固体電解質。
- 請求項3又は4に記載の固体電解質と、正極活物質及び/又は負極活物質とを含むことを特徴とするリチウム二次電池用の電極材。
- 請求項3又は4に記載の固体電解質を含む層を備えたリチウム二次電池。
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WO2023234359A1 (ja) * | 2022-06-01 | 2023-12-07 | 富士フイルム株式会社 | 全固体二次電池用電極組成物、全固体二次電池用電極シート、及び全固体二次電池、並びに、全固体二次電池用電極シート及び全固体二次電池の製造方法 |
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