Method for manufacturing a negative electrode mixture layer containing graphite particles and sulfide solid electrolyte particles

Abstract

To provide a manufacturing method of an anode mixture for all-solid batteries capable of achieving high discharge capacity.SOLUTION: A manufacturing method of an anode mixture includes the steps of: respectively weighing a sulfide all-solid electrolyte having a form of a secondary aggregate consisting of an ion conductive compound consisting of lithium, sulfur and phosphor and graphite particles in a predetermined blending ratio; dry-mixing all the weighed graphite particles and the quantity of the weighed sulfide all-solid electrolyte in a range of 20 mass% or more and 40 mass% or less in a mixing time equal to or longer than 5 minutes; putting the remaining weighed sulfide all-solid electrolyte into a mixture, which is obtained by the dry-mixing, on a quantity basis in the range of 20 mass% or more and 40 mass% or less, ad performing dry-mixing for a mixing time within 1 minute.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a negative electrode mixture containing graphite particles and a sulfide solid electrolyte. [Background technology]

[0002] In recent years, electronic devices such as mobile phones, tablet devices, and laptop computers have become increasingly sophisticated and functional, and as a result, there is a growing demand for smaller, lighter, and higher-capacity secondary batteries for these devices. Under these circumstances, non-aqueous electrolyte (liquid-based) secondary batteries, typified by lithium secondary batteries, are rapidly gaining popularity in the field of these electronic devices because they offer higher battery voltages and higher energy densities than nickel-cadmium batteries and nickel-metal hydride batteries.

[0003] Furthermore, against the backdrop of recent environmental concerns, non-aqueous electrolyte secondary batteries have also become mainstream as motor drive power sources for electric vehicles and hybrid vehicles. However, non-aqueous electrolyte secondary batteries generally contain flammable organic solvents, which pose safety issues such as fires when abnormalities such as thermal runaway occur in the secondary battery, and immediate improvements are desired. As one technology for improving such safety issues, active research and development is being conducted on all-solid-state lithium secondary batteries that use solid electrolytes instead of organic solvent-based electrolytes.

[0004] The main components of the all-solid-state lithium secondary battery are a three-layer structure consisting of a cathode composite (layer) containing a cathode active material and a solid electrolyte, a cathode composite (layer) containing a cathode material using In metal, Li-In alloy, or carbon and a solid electrolyte, and a solid electrolyte (layer) disposed between the cathode composite and the anode composite. The solid electrolytes contained in the above-mentioned constituent materials can be broadly divided into oxide-based and sulfide-based types. A representative example of the former oxide type is Li7La3Zr2O 12 and Li5La3Nb2O 12 Examples include:

[0005] However, oxide-based solid electrolytes have poor crushability, which can prevent the formation of a dense structure during the powder compaction process in which powdered raw materials are compressed to form a solid-state lithium secondary battery. Oxide-based solid electrolytes also have issues, such as the need for high-temperature sintering to achieve high ionic conductivity. In contrast, sulfide-based solid electrolytes do not have the aforementioned drawbacks. In particular, typical sulfide-based solid electrolytes (hereinafter also referred to as LPS) having the Li7PS6, Li4P2S6, or Li3PS4 phases, as disclosed in Patent Document 1, possess the flexibility and adhesiveness inherent to sulfides, allowing them to be easily deformed during the powder compaction process. Furthermore, sulfide-based solid electrolytes have the advantage that, depending on their composition, they can achieve high ionic conductivity without heat treatment, making them a more promising material than oxide-based solid electrolytes.

[0006] The LPS is a compound synthesized using diphosphorus pentasulfide and lithium sulfide as raw materials. Its synthesis method typically involves a reaction synthesis using prolonged ball impact heat in a planetary ball mill (mechanical milling device) filled with an inert gas, or, if necessary, a heat treatment to precipitate a crystalline phase (metastable phase). Patent Document 2 also discloses a cost-effective technique for preparing a negative electrode composite using LPS synthesized by the above method, in which graphite particles, which can electrochemically insert and remove ions, are blended as an active material. This technique involves mechanically mixing graphite particles and LPS particles, and then compacting the resulting mixed powder to form a negative electrode composite layer. The LPS contained in the negative electrode composite layer serves as a Li-ion conduction path. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-155087 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-203545 Summary of the Invention [Problem to be solved by the invention]

[0008] In the above-mentioned all-solid-state lithium-ion battery (hereinafter also referred to as all-solid-state battery), it is considered preferable to increase the content of graphite particles in the negative electrode composite from the viewpoint of increasing capacity. The reason for this is that a high content of graphite particles allows the negative electrode to accept a larger number of Li ions that have migrated from the positive electrode. However, when the negative electrode composite is prepared so as to contain a large amount of graphite particles, it can sometimes be difficult to uniformly mix the graphite particles and LPS particles, depending on the type of graphite particles.

[0009] In particular, when preparing a negative electrode composite by a dry method, the mixture of graphite particles and LPS particles tends to be insufficient, and the quality of the mixture can affect the battery characteristics. Therefore, unless the mixture is improved, simply increasing the graphite particle content may not fully utilize the LPS's role as a conductive path, and the desired discharge capacity may not be achieved. The present invention has been made in consideration of the above problems, and aims to provide a method for producing a negative electrode composite for an all-solid-state battery that can achieve a high discharge capacity. [Means for solving the problem]

[0010] In order to achieve the above object, a negative electrode mixture for an all-solid-state battery according to the present invention is layer The method for producing the ion-conductive compound is composed of lithium, sulfur, and phosphorus. Primary particles with an average particle size of 0.5 to 2 μm are aggregated to form particles with an average particle size of 60 to 100 μm. Sulfide all-solid electrolyte with secondary aggregate morphology particle and, Particle size distribution D50 is 8 μm or more and 13 μm or less a step of weighing out the total amount of the weighed graphite particles and the sulfide all-solid electrolyte so as to have a predetermined blending ratio; particle and dry-mixing the weighed sulfide all-solid electrolyte solution with the weighed sulfide all-solid electrolyte solution for a mixing time of 5 minutes or more. particle and a step of adding the remainder in an amount ranging from 20% by mass to 40% by mass and dry mixing each amount for a mixing time of 1 minute or less. and a step of compacting the obtained negative electrode composite to form a negative electrode composite layer having a porosity of 3% or less. It is characterized by consisting of: [Effects of the Invention]

[0011] According to the present invention, it is possible to provide an all-solid-state battery that has a high initial charge / discharge capacity and an excellent capacity retention rate in a cycle test. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a photograph of a battery evaluation cell used in an example of the present invention. [Figure 2] FIG. 2 is a perspective view of the battery evaluation cell of FIG. 1 with the housing removed. [Figure 3] FIG. 2 is a schematic longitudinal cross-sectional view of a battery evaluation cell with the housing removed. DETAILED DESCRIPTION OF THE INVENTION

[0013] 1.Negative electrode mixture for all-solid-state batteries A negative electrode composite for an all-solid-state battery according to an embodiment of the present invention will now be described. This negative electrode composite contains graphite particles and an ionically conductive sulfide solid electrolyte. The negative electrode composite is compacted to obtain a negative electrode composite layer having a porosity of 3% or less. The porosity in the negative electrode composite layer can be determined by the following method. Specifically, a cross section of the compacted negative electrode composite layer is processed using a CP (Cross Section Polisher) device, and five arbitrarily selected fields of view from the mirror-finished cross section are imaged using an SEM. The SEM images of each field are binarized to distinguish between voids and other areas. The ratio of the area occupied by the voids to the total area of ​​the SEM image for each field is calculated, and the resulting area ratios are averaged across the five fields. This allows the porosity to be determined.

[0014] As described above, by limiting the porosity in the negative electrode composite to 3% or less, it is possible to increase the initial charge / discharge capacity of an all-solid-state battery containing the negative electrode composite layer and also improve the capacity retention rate during cycle testing. Specifically, during charge / discharge of an all-solid-state battery, Li ions move through the LPS, which acts as an ion conduction path within the negative electrode composite, and thereby intercalate into and deintercalate from the crystalline layers of the graphite particles. Therefore, as contact between the graphite particles and the surrounding LPS particles decreases, the proportion of voids at their interface (sometimes referred to as the graphite particle / LPS interface) increases. When the porosity in the negative electrode composite layer exceeds 3%, the contact area between the graphite particles and the LPS particles becomes too small, hindering efficient Li ion migration and making it difficult for Li ions to intercalate into and deintercalate from the graphite particles. In other words, excessive graphite particles that do not contribute to charge / discharge are present in the negative electrode composite layer, significantly reducing the charge / discharge capacity.

[0015] As mentioned above, the reason for the voids at the graphite particle / LPS interface is thought to be that the graphite particles and LPS particles are not mixed properly when preparing the negative electrode composite, resulting in insufficient packing of LPS particles around the graphite particles during powder compaction. In other words, LPS particles generally have very poor fluidity, making it difficult to achieve a uniform mixture when mixing graphite particles and LPS particles to prepare the negative electrode composite.

[0016] Under these circumstances, we conducted extensive research to minimize the formation of voids at the graphite particle / LPS interface and thereby reduce the porosity in the negative electrode composite layer to 3% or less. We found that by using a blend of at least two types of LPS particles—fine particles and particles larger than the fine particles—we could improve the mixability of the graphite particles and LPS particles and reduce the porosity. Specifically, the porosity can be reduced by blending the LPS particles constituting the negative electrode composite so that, of the total amount of LPS particles, the proportion of fine particles with an average particle diameter of 2 μm to 5 μm is 20% to 40% by mass, and the proportion of large particles with an average particle diameter of 40 μm to 60 μm is 20% to 40% by mass.

[0017] As described above, by using LPS particles containing at least 2-5 μm fine particles and 40-60 μm large particles, good contact between the graphite particles and LPS particles is achieved during compaction, reducing the likelihood of voids at the graphite particle / LPS interface, enabling a void ratio of 3% or less in the negative electrode composite layer. This ensures sufficient contact area between the graphite particles and solid electrolyte particles that make up the negative electrode composite layer, forming numerous ion conduction paths within the negative electrode composite layer and increasing the graphite particle utilization. As a result, the charge / discharge capacity of all-solid-state batteries can be increased, and the discharge capacity retention rate during cycle testing can also be improved.

[0018] In the negative electrode composite of the embodiment of the present invention, the graphite particles constituting the negative electrode composite further have an average flatness of 0 or more and less than 0.3, a particle size distribution D50 of 8 μm or more and 13 μm or less, and a BET value of 1 m 2 / g or more 3m 2 By using graphite particles that satisfy these requirements, the contact state between the graphite particles and the LPS particles can be further improved when the graphite particles are compacted.

[0019] If the average flattening ratio, which is a characteristic related to the shape of graphite particles, is 0.3 or higher, the material will contain many irregularly shaped graphite particles, which will hinder the smooth flow of the LPS particles when forming the negative electrode composite layer by powder compaction, making it more likely that voids will form around the graphite particles where the LPS particles are not filled. Conversely, as the average flattening ratio of graphite particles approaches zero, the shape will change from irregular to spherical, improving the packing of LPS particles around graphite particles even if the flowability of the LPS particles is poor.

[0020] The average flattening ratio of graphite particles can be calculated by the following formula 1. Here, f is the average flattening ratio, a is the major axis of a single particle, and b is the minor axis of a single particle. These radii were calculated by taking images of five randomly selected fields of view on the cross section or surface of a group of graphite particles using an SEM, measuring the major and minor axes of five randomly selected graphite particles from each SEM image, and averaging the results. [Formula 1] f=1-b / a

[0021] If the particle size distribution D50, which is a characteristic of the average particle size of graphite particles, is less than 8 μm, the graphite particles will be too fine and will be less dispersible when mixed with LPS particles, resulting in the formation of voids around the graphite particles. In other words, if the graphite particles are too fine, they will tend to aggregate, resulting in insufficient contact area with the LPS particles, resulting in an increase in LPS particles that do not conduct ions and graphite particles that do not intercalate or deintercalate Li-ions. Conversely, if the particle size distribution D50 exceeds 13 μm, the number of graphite particles per unit thickness of the negative electrode composite layer will decrease, resulting in a decrease in charge / discharge capacity. The particle size distribution D50 of graphite particles was determined using a laser diffraction method, in which a laser is irradiated onto graphite particles dispersed in a liquid and the refracted and scattered light is measured.

[0022] The BET value, which is a characteristic related to the specific surface area of ​​graphite particles, is 1m 2 If the BET value is less than 3m / g, the graphite particles have less opportunity to come into contact with the LPS particles during mixing to prepare the negative electrode composite, resulting in poor mixing, and the LPS particles are less likely to adhere to the surface of the graphite particles, making it more likely that voids will form at the graphite particle / LPS interface during powder compaction. 2 If the BET value exceeds 1 / g, excessive gas components and moisture may be adsorbed onto the surface of the graphite particles, which may cause unstable battery characteristics and may make it difficult to obtain high capacity. The BET value is obtained using the BET equation based on the gas adsorption method (nitrogen adsorption method).

[0023] Although it is possible to achieve a porosity of 5% or less in the negative electrode composite layer simply by specifying the above-mentioned graphite particle properties, such as the average flatness, particle size distribution D50, and BET value, a porosity of 3% or less is desirable to further improve the properties of all-solid-state batteries, and it has been difficult to achieve a porosity of 3% or less simply by specifying the above-mentioned graphite particle properties. Although reducing the porosity from 5% to 3% is numerically small, the voids in the negative electrode composite layer are mainly voids that occur at the graphite particle / LPS interface, and the size of the voids is often around 5 to 10 μm, so reducing the porosity from 5% to 3% has generally been extremely difficult.

[0024] For example, even when spherical graphite particles are used, small voids can remain at the interface, and so we conducted extensive research, thinking that there must be other factors besides the poor fluidity of the LPS particles that cause poor mixing between graphite and LPS particles. We then focused on the fact that when graphite and LPS particles are mixed to create the negative electrode composite, the LPS sometimes does not adhere to the surface of the graphite particles. In other words, after mixing the graphite and LPS particles, the particles sometimes separate at a microscopic level, and we thought this was the reason why the void ratio could not be reduced.

[0025] Therefore, the inventors conducted further research not only into the properties of graphite particles but also into LPS, which tends to adhere to the surface of graphite particles. They discovered that when LPS particles are synthesized by the mechanical milling method described below, they are produced in the form of secondary aggregates with particle sizes of approximately 60 to 100 μm, which is the cause of insufficient adhesion to the surface of graphite particles and insufficient mixing with graphite particles. In this case, it was thought that mixing could be improved by breaking down the LPS particles in the form of secondary aggregates. However, LPS secondary aggregates are prone to degradation when mechanically mixed or broken down to obtain single particles. For example, prolonged mixing times can result in a significant decrease in ionic conductivity.

[0026] Therefore, we prepared two types of LPS particles with different particle sizes, produced by varying the crushing time, and mixed them with graphite particles to produce a negative electrode composite. This resulted in a porosity of less than 3% in the negative electrode composite layer. Because the volume of voids at the graphite particle / LPS interface is very small, the LPS particles that fill these gaps do not necessarily have to be the entire amount of LPS particles used in the negative electrode composite. Furthermore, to adhere to the graphite particle surface, fine powder LPS particles are preferable if they adhere along the uneven surface of the graphite particle. However, even in this case, the amount needed to cover one layer of the graphite particle surface is sufficient.

[0027] Therefore, for example, by adding LPS particles to crushed graphite particles in at least two stages, it is possible to mix finer LPS particles obtained by mixing for a long period of time with larger LPS particles that are less susceptible to deterioration obtained by mixing for a short period of time with the graphite particles. This makes it possible to produce a negative electrode composite layer with low porosity at the graphite particle / LPS interface while minimizing the content of LPS particles damaged by mixing.

[0028] Specifically, it is preferable that the total amount of LPS particles used as the negative electrode composite contain 20% to 40% by mass of more pulverized fine particles with an average particle diameter of 2 μm to 5 μm, and 20% to 40% by mass of large particles with an average particle diameter of 40 μm to 60 μm, which are less damaged by mixing. By using a negative electrode composite in which LPS particles containing fine particles and large particles, each having a predetermined particle size range, are blended with graphite particles in this way, the porosity of the negative electrode composite layer after powder compaction can be kept to 3% or less, and an all-solid-state battery cell with excellent battery characteristics such as initial charge / discharge capacity can be fabricated.

[0029] 1. Method for manufacturing negative electrode composite layer Next, an embodiment of the method for producing a negative electrode mixture layer according to the present invention will be described, taking as an example a case where lithium sulfide and diphosphorus pentasulfide are used as raw materials for LPS particles.

[0030] 1.1 Graphite particles The graphite particles that make up the negative electrode composite can be either natural or artificial graphite. However, artificial graphite is preferred because it easily meets the above-mentioned average flatness, particle size distribution D50, and BET value characteristics. MCMB (Meso-Carbon MicroBeads: spherical carbon particles) are particularly preferred. On the other hand, natural graphite generally has a poor flatness due to its non-spherical shape, a wide particle size distribution D50, and a high BET value, which often results in poor compatibility with LPS particles. Amorphous carbon may be applied to the surface of the graphite particles to improve Li-ion insertion and extraction during charge and discharge. This facilitates adhesion of LPS particles to the surface of the graphite particles during mixing of the negative electrode composite, enabling higher charge and discharge capacities.

[0031] 1.2LPS particles (1) Lithium sulfide (LiS) There are no particular restrictions on the lithium sulfide (LiS) that serves as one of the raw materials for the LPS particles that make up the negative electrode composite. However, it is preferable to use an industrially produced, commercially available product, and one with a purity of 99% or higher is particularly preferred. Lithium sulfide can be produced, for example, by the method described in Japanese Patent No. 3528866. This production method involves blowing hydrogen sulfide into a feed solution containing lithium hydroxide and an aprotic organic solvent placed in a reaction vessel, causing the lithium hydroxide and hydrogen sulfide to react to produce lithium hydrosulfide, and then subjecting the resulting reaction solution containing lithium hydrosulfide to dehydrosulfidation treatment to produce lithium sulfide.

[0032] (2) Diphosphorus pentasulfide (P2S5) There are no particular restrictions on the phosphorus sulfide that is the other raw material for the LPS particles that make up the negative electrode composite, but it is preferable to use commercially available, industrially produced diphosphorus pentasulfide (P2S5), which preferably has a purity of 99% or higher.

[0033] (3) Mixing ratio When producing a typical high ion conductive sulfide, it is preferable to adjust the lithium sulfide content relative to the total lithium sulfide and diphosphorus pentasulfide used as starting materials to within a range of 30 to 95 mol%. The specific content is determined appropriately depending on the application. For example, as in the case of a 70Li2S-30P2S5 solid electrolyte produced using starting materials containing lithium sulfide and diphosphorus pentasulfide in a molar ratio of 70:30, if the lithium sulfide content is below 70 mol%, the high ion conductive sulfide becomes glassy and requires heat treatment for production. Furthermore, due to its instability in the air, handling during synthesis and storage becomes difficult. Therefore, the lithium sulfide content in the starting materials for the high ion conductive sulfide, LPS, is preferably within a range of 75 to 80 mol%.

[0034] (4) LPS synthesis LPS can be obtained by synthesizing the starting materials lithium sulfide and diphosphorus pentasulfide blended in the above-mentioned ratio. This synthesis is preferably performed using mechanical milling. Mechanical milling is a processing method in which the powder to be processed is loaded into a milling container of a mixing device along with a large number of metal or ceramic balls, which act as media that physically interact with the powder. The media move in response to the rotation and vibration of the milling container, applying physical forces such as mixing, stirring, and impact to the powder.

[0035] The mixer used in this mechanical milling method is not particularly limited as long as it can charge and mix the above-mentioned media, which perform actions such as uniform mixing and stirring on the powder to be processed, into a milling vessel, and common powder mixers such as planetary ball mills, rotary ball mills, and attritors can be used, with planetary ball mills being particularly preferred among these. This is because the random movement of balls due to planetary motion in planetary ball mills promotes uniform mixing of the powder to be processed and imparts large reaction energy to the powder, thereby very efficiently imparting a driving force that promotes the synthesis reaction.

[0036] Thus, unlike two-step processing methods in which multiple starting material powders are first mixed and then the resulting mixed powder is heat-treated, mechanical milling generates heat through the impact of rapidly moving balls, which allows multiple starting material powders to be uniformly mixed and undergo rapid localized chemical reactions simultaneously, making it suitable for producing exceptionally high ionic conductors.

[0037] In the mechanical milling method described above, it is preferable to control the atmosphere inside the milling container. For example, when filling a milling container made of stainless steel with ZrO2 on the inside, it is desirable to create an inert gas atmosphere inside the milling container. For this reason, the milling container is preferably filled in a glove box filled with an inert gas such as Ar gas, N2 gas, or He gas. However, since the mechanical milling device itself cannot usually be placed inside the glove box, after filling, the milling container is removed from the glove box and milling, such as mixing, is performed in an environment exposed to the outside air. For this reason, the lid of the milling container is preferably designed to hermetically seal the inside of the milling container, preferably with a silicone packing.

[0038] The reason why it is preferable to hermetically seal the milling container as described above is that sulfides begin to deteriorate immediately upon exposure to air, and are particularly susceptible to moisture. Even a small amount of moisture in the gas inside the milling container can cause decomposition, resulting in the generation of H2S gas. Therefore, it is preferable to seal the inert gas filling the milling container in a dry state and hermetically seal it with packing as described above to prevent leakage to the outside. Note that gas leakage to the outside can also be considered a state in which moisture-laden air from the outside air enters the milling container. To seal dry inert gas into the milling container as described above, the dew point inside the glove box where the filling operation is performed is preferably controlled to below -70°C dp, more preferably below -80°C dp (moisture content 0.5 ppm).

[0039] 1.3 Powder compaction A negative electrode composite layer for an all-solid-state battery can be produced by powder compacting the negative electrode composite obtained by mixing the graphite particles and LPS particles. In powder compacting of this negative electrode composite, it is preferable to stack the positive electrode composite, solid electrolyte, and negative electrode composite in this order into three layers and simultaneously compact the three-layer structure by applying pressure in the stacking direction. Good moldability is desirable for this powder compacting of the negative electrode composite. However, because the negative electrode composite of the present invention does not contain a resin binder, the moldability depends on the adhesiveness of the LPS particles.

[0040] To fully utilize the adhesive properties of the LPS particles, the graphite particles are preferably mixed with the LPS particles in a ratio of 50 to 70 parts by mass per 100 parts by mass of the total of the graphite particles and the LPS particles, which allows the graphite particles and the LPS particles to be compacted into a compact with excellent shape retention and no defects or cracks.

[0041] On the other hand, if the graphite particles are blended in an amount exceeding 70 parts by mass per 100 parts by mass of the graphite particles and LPS particles combined, the shape retention may be reduced, resulting in cracks. Such cracks in the compact may increase resistance and reduce charge / discharge capacity. Conversely, if the graphite particles are blended in an amount less than 50 parts by mass per 100 parts by mass of the graphite particles and LPS particles combined, the carbon content of the graphite particles in the compacted negative electrode composite layer is too low, resulting in insufficient absorption of Li ions from the positive electrode active material. This may also result in reduced charge / discharge capacity. One possible solution to this problem would be to increase the thickness of the negative electrode composite layer to compensate for the carbon content. However, increasing the thickness of the negative electrode composite layer increases resistance, making this an undesirable solution.

[0042] As mentioned above, LPS particles immediately after synthesis by mechanical milling preferably have the form of secondary aggregates (secondary particles) with an average particle size of 60–100 μm, formed by the aggregation of primary particles with an average particle size of 0.5–2 μm. Dry mixing is preferred for mixing LPS particles consisting of these secondary aggregates with graphite particles. This is because wet mixing can result in a violent reaction between the LPS and the solvent used for mixing, which can lead to deterioration, whereas dry mixing does not present such a problem. Furthermore, since synthesized LPS particles are vulnerable to friction and impact during mixing, and MCMB can also be crushed or distorted when subjected to force, which can lead to a deterioration in battery performance, a mixing method that minimizes damage to the LPS and graphite particles is preferred. Therefore, hand mixing using a mortar, as described below, is preferred. However, when using mechanical milling, such as a planetary ball mill, as in the production of LPS particles, mixing at the minimum rotation speed and for as short a time as possible is desirable.

[0043] A more preferable mixing method, due to its reduced damage potential, is the mortar-based mixing method. By adjusting the force applied to the powder and the mixing time, mixing can be achieved with minimal damage. However, when the desired ratio of LPS particles and graphite particles is added to a mortar and mixed with a pestle for more than 10 minutes, the LPS particles are easily crushed into fine powder, but they are also damaged during this process. Damage to the LPS particles reduces the inherent sulfide adhesiveness of LPS, reducing its ability to function as a binder. This can lead to the formation of numerous voids at the interface between the graphite and LPS particles when forming the anode composite layer. Furthermore, the ionic conductivity of LPS itself decreases, potentially increasing the resistance of the anode composite layer and reducing battery capacity. Furthermore, the activity of LPS particles increases when they are crushed, which can lead to degradation due to trace amounts of moisture and oxygen in the environment, reducing their shelf life and preventing storage.

[0044] Therefore, in an embodiment of the present invention, LPS particles and graphite particles are mixed using a mortar in the following manner. Specifically, first, the LPS particles and graphite particles are weighed out to achieve the desired blend ratio. Next, the total amount of the weighed graphite particles and 20% to 40% by mass of the total amount of the weighed LPS particles are placed in the mortar, and a first mixing is performed using a pestle at a speed of approximately 30 to 120 revolutions per minute, taking care not to press the mixture too hard, to ensure a uniform overall mixture. Secondary aggregates of LPS particles are easily broken down by mixing with the pestle for approximately 5 minutes, yielding primary particles and secondary aggregates (secondary particles) with particle sizes of approximately 2 μm to 5 μm. Therefore, this first mixing is performed for 5 minutes.

[0045] This makes it easier for the LPS particles to adhere to the surface of the graphite particles, allowing the surface to be covered with LPS particles. Although the LPS particles are somewhat damaged during this first mixing process, the benefits of reduced porosity at the graphite particle / LPS interface outweigh this disadvantage, and are therefore more effective in increasing capacity. However, by minimizing the ratio of fine particles to the total amount of LPS particles measured above, the adverse effects of this damage are limited to the particles that fill the voids.

[0046] After the first mixing, the remaining LPS particles are added to the mixture in the mortar in amounts ranging from 20% to 40% by mass. Each addition is then performed using the same mixing method as above, with a pestle for 1 minute. The LPS particles added during the second and subsequent mixings are less likely to be damaged due to the short mixing time. Therefore, the LPS particles added during the second and subsequent mixings have almost no adverse effect on the properties of the negative electrode composite layer. However, the short mixing time does not allow for sufficient disintegration, resulting in a particle size of approximately 40 μm to 60 μm, with secondary aggregates only slightly disintegrated.

[0047] That is, in the method for producing a negative electrode composite according to an embodiment of the present invention, a first 5-minute mixing step forms fine LPS particles with a particle diameter of 2 μm to 5 μm, which then coat the graphite particles. Then, each subsequent 1-minute mixing step mixes relatively large, less damaged LPS particles with a particle diameter of 40 μm to 60 μm with the graphite particles whose surfaces are coated with the fine LPS particles. This improves the filling of voids at the graphite particle / LPS interface by the fine LPS particles, while the relatively large LPS particles maintain their adhesiveness, resulting in a synergistic effect that reduces the porosity of the negative electrode composite layer formed by the powder compaction to 3% or less. As a result, a high-capacity all-solid-state battery can be fabricated.

[0048] The average particle size of the secondary aggregates of LPS particles (also referred to as the average aggregate size) was determined by taking SEM images of five randomly selected fields and measuring and averaging the outer diameters of five randomly selected secondary aggregates from each SEM image. The laser diffraction method, which uses a conventional laser to measure the refracted and scattered light, cannot be used because most of the water or organic solvents used react chemically with LPS and change its properties. It is also possible to calculate particle size from the BET value, but this method has problems, such as the degradation of LPS during handling.

[0049] When evaluating the charge / discharge capacity of an all-solid-state battery fabricated by laminating the above-mentioned negative electrode composite with a solid electrolyte made of LPS particles used in the negative electrode composite and a separately prepared positive electrode composite, samples of the negative electrode composite, positive electrode composite, and solid electrolyte for evaluation are compacted and measured. The thickness of the compacts compacted from each sample is preferably about 100 to 200 μm for the positive electrode composite layer and the negative electrode composite layer, and about 300 to 500 μm for the solid electrolyte layer, taking into consideration the balance between shape retention during compaction (no breaks or cracks during compaction) and resistance.

[0050] When forming a negative electrode composite layer from the negative electrode composite, for example, the negative electrode composite is filled into a mold with an inner diameter of 10 mm and pressed in the thickness direction at a pressure of approximately 2 to 4 kN to perform temporary powder compaction, followed by high-pressure powder compaction. During this high-pressure powder compaction, the positive electrode composite, solid electrolyte, and negative electrode composite are stacked in this order from the bottom up, and a pressure of 40 kN is applied in this stacking direction to simultaneously compact the three layers. This results in a three-layer integrally structured compact. An all-solid-state battery can be manufactured by incorporating the three-layer integrally structured compact thus formed into a commercially available battery cell case (sealed type: not exposed to the atmosphere). Next, the present invention will be described in more detail based on the following examples and comparative examples, but the present invention is not limited to these examples. [Example]

[0051] [Example 1] (LPS synthesis) A total of 80 g of ZrO2 balls (ball diameter 4 mm) were placed in a milling container (volume 45 ml; inner wall made of ZrO2) attached to a Fritsch mechanical milling machine (PL-7) and dried in vacuum at 100°C. This dried milling container was placed in a glove box (-80°C dp) in an Ar atmosphere and further dried overnight.

[0052] A milling vessel in the glove box was charged with 0.3828 g of lithium sulfide (LiS) manufactured by Junsei Chemical Co., Ltd. and 0.6172 g of diphosphorus pentasulfide (PS) manufactured by Sigma-Aldrich and sealed. The molar ratio of lithium sulfide to diphosphorus pentasulfide was LiS:PS = 75:25. After sealing, the milling vessel was removed from the glove box and placed in the Fritsch mechanical milling machine (PL-7) installed in a dry room (ambient temperature 22°C:-45°C dp). Mechanical milling was performed at 510 rpm for 20 hours. After mechanical milling was completed, the vessel was returned to the glove box, the lid of the vessel was opened, and the synthesized powdered LPS was removed and passed through a 100-mesh sieve.

[0053] The obtained undersieved LPS particles were analyzed using a non-exposed Raman spectrometer (Horiba, Ltd.) (microscopic laser Raman spectrometer LabRAM HR-800, light source: Ar laser (514 nm excitation), excitation power: 0.2 mW, measurement range: 100–4000 cm). -1 ), which confirmed the presence of the Li3PS4 phase. Furthermore, to confirm the degree of disintegration when the LPS particles synthesized by the mechanical milling method were mixed in a mortar, 120 mg of the LPS particles that passed through the sieve were sampled and divided into six samples. Five of these samples were placed in separate mortars and disintegrated in the mortar for 1, 3, 5, 10, and 20 minutes, respectively. The average value of five agglomerate diameters (secondary particle diameters) randomly selected from the SEM images was then calculated for each of the six samples, including these five samples and one undisintegrated sample.

[0054] As a result, the uncrushed sample in the as-synthesized state had an average particle size of 89 μm, the sample crushed for 1 minute had an average particle size of 53 μm, the sample crushed for 3 minutes had an average particle size of 19 μm, and the sample crushed for 5 minutes had an average particle size of 3 μm. On the other hand, the sample crushed for 10 minutes had re-aggregated the finely pulverized powder, reaching an average particle size of 21 μm. The sample crushed for 20 minutes had re-aggregated in the same way as the sample crushed for 10 minutes, and had an average particle size of 26 μm.

[0055] (Preparation of negative electrode mixture) The particles were placed in a mortar in an Ar atmosphere glove box (-80°C dp) with an average flatness of 0.03, particle size distribution D50 of 10.2 μm, and BET value of 1.2 m. 280 mg of graphite particles (PG11A) manufactured by Rongtan Electronic Materials Co., Ltd. (1 / g) was added, followed by 20 mg of the undersized LPS particles. The first mixing was performed using a pestle at a rotation speed of 60 revolutions per minute for 5 minutes, taking care not to compress the particles. Next, 20 mg of the undersized LPS particles was added to the mixture in the mortar after the first mixing, and the second mixing was performed using the same mortar method for 1 minute. This addition of 20 mg of LPS particles and mixing for 1 minute were repeated two more times to perform the third and fourth mixings. This produced a granular negative electrode composite as an intermediate product, with a 50:50 blend of graphite particles and LPS particles by mass.

[0056] (Preparation of positive electrode mixture) A lithium nickel composite oxide powder prepared in advance by a known technique was used as the positive electrode active material. That is, nickel oxide powder containing Ni as the main component and lithium hydroxide were mixed and fired to obtain Li 1.090 Ni 0.76 Co 0.14 Al 0.10 Lithium nickel composite oxide powder (hereinafter referred to as NCA particles) was prepared as the cathode active material represented by 02. The average particle size of the obtained NCA particles was measured by laser diffraction and found to be 10.6 μm. The specific surface area was measured by the BET method and found to be 0.16 m. 2 120 mg of these NCA particles and 80 mg of the above-mentioned undersieved LPS particles were placed in a mortar in a glove box (-80°C dp) with an Ar atmosphere, and gently mixed with a pestle at a rotation speed of 60 revolutions per minute for 10 minutes, taking care not to press the particles together.

[0057] (Preparation of powder compacts) In an Ar atmosphere glove box (-80°C dp), 60 mg of the above-mentioned undersized LPS powder was placed in a cylindrical mold with an inner diameter of 10 mm and pre-molded at 2 kN to prepare a solid electrolyte layer. 15 mg of the above-prepared granular negative electrode composite was layered on top of this solid electrolyte layer and pre-molded again at 2 kN. The resulting two-layer compact was turned over, and 20 mg of the above-prepared positive electrode composite powder was deposited on both sides of the solid electrolyte layer opposite the pre-molded negative electrode composite layer, followed by final compaction at 40 kN. This produced a three-layer integrated compact consisting of a positive electrode composite layer, a solid electrolyte layer (SE layer), and a negative electrode composite layer. This compact was placed in a sealed battery evaluation cell (KP-solidCell) manufactured by Hohsen Co., Ltd., as shown in Figures 1 and 2, and the evaluation cell was assembled by restraining it with a torque of 7 Nm. As shown in FIG. 3, this evaluation cell has an integrally structured compact 10 consisting of a negative electrode composite layer 11, a solid electrolyte layer (SE layer) 12, and a positive electrode composite layer 13, which can be pressed in the stacking direction with a predetermined restraining force by an upper electrode 21 and a lower electrode 22 within a cylindrical insulating tube 20.

[0058] (Evaluation of battery characteristics and measurement of porosity of negative electrode composite layer) The terminals of the resulting evaluation cell were connected to a charge / discharge device (HJ-SD8) manufactured by Hokuto Denko Corporation, and the initial discharge capacity was determined over a voltage range of 2.37 to 4.17 V_CC (0.1 C = 19 mAh / g), yielding a value of 124 mAh / g. Furthermore, the discharge capacity retention rate was determined by repeating charge / discharge 20 times under the same conditions as the initial charge / discharge, and the change in capacity between the initial discharge and the 20th cycle was measured, yielding a value of 92%. After evaluating the battery characteristics, the porosity of the negative electrode composite layer was determined as an average of five SEM images as described above, yielding a value of 2.4%.

[0059] [Example 2] An intermediate negative electrode composite containing graphite particles and LPS particles in a 50:50 mass ratio was prepared in the same manner as in Example 1, except that 30 mg of LPS particles were added instead of 20 mg in the first and third mixing runs and the fourth mixing run was omitted. This intermediate negative electrode composite was then used to prepare a three-layer integrated compact consisting of a positive electrode composite layer, a solid electrolyte layer (SE layer), and a negative electrode composite layer in the same manner as in Example 1. This compact was then placed in a sealed battery cell in the same manner as in Example 1 to assemble an evaluation cell. The battery characteristics were evaluated and the porosity of the negative electrode composite layer was measured in the same manner as in Example 1. The initial discharge capacity was 123 mAh / g, the discharge capacity retention rate was 91%, and the porosity of the negative electrode composite layer was 2.6%.

[0060] [Example 3] Average flatness ratio is 0.24, particle size distribution D50 is 8.7 μm, BET value is 1.2 m 2 An intermediate negative electrode composite was prepared in a 50:50 blend by mass of graphite particles and LPS particles in the same manner as in Example 1, except that graphite particles (PW8A) manufactured by Rongtan Electronic Materials Co., Ltd. with a particle size of 1 / g were used. This intermediate negative electrode composite was then used to prepare a compact having a three-layer integral structure consisting of a positive electrode composite layer, a solid electrolyte layer (SE layer), and a negative electrode composite layer in the same manner as in Example 1. This compact was then placed in a sealed battery cell in the same manner as in Example 1 to assemble an evaluation cell, and the battery characteristics were evaluated and the porosity of the negative electrode composite layer was measured in the same manner as in Example 1. The results were an initial discharge capacity of 120 mAh / g, a discharge capacity retention rate of 90%, and a porosity of the negative electrode composite layer of 2.9%.

[0061] [Comparative Example 1] Average flatness ratio is 0.42, particle size distribution D50 is 5.3 μm, BET value is 2.4 m 2An intermediate negative electrode composite was prepared in the same manner as in Example 1, except that 80 mg of graphite particles (AC1) manufactured by Rongtan Electronic Materials Co., Ltd. (each 80 mg of graphite particles) with a mass ratio of 1 / g and 80 mg of the undersized LPS particles used in Example 1 were placed in a mortar and mixed once for 10 minutes using a pestle in the same manner. This intermediate negative electrode composite was then used to prepare a three-layer integrated powder compact consisting of a positive electrode composite layer, a solid electrolyte layer (SE layer), and a negative electrode composite layer in the same manner as in Example 1. This powder compact was then placed in a sealed battery cell in the same manner as in Example 1 to assemble an evaluation cell. The battery characteristics were evaluated, and the porosity of the negative electrode composite layer was measured in the same manner as in Example 1. The initial discharge capacity was 104 mAh / g, the discharge capacity retention rate was 79%, and the porosity of the negative electrode composite layer was 6.7%.

[0062] Comparative Example 2 Instead of graphite particles (AC1), the average flatness is 0.37, the particle size distribution D50 is 11.8 μm, and the BET value is 4.4 m 2 An intermediate negative electrode composite was prepared in the same manner as in Comparative Example 1, except that graphite particles (OMAC-R) manufactured by Osaka Gas Chemicals, with a mass ratio of 1 / g, were used, in which graphite particles and LPS particles were blended at a 50:50 ratio by mass. This intermediate negative electrode composite was then used to prepare a compact having a three-layer integral structure consisting of a positive electrode composite layer, a solid electrolyte layer (SE layer), and a negative electrode composite layer, in the same manner as in Example 1. This compact was then placed in a sealed battery cell in the same manner as in Example 1 to assemble an evaluation cell, and the battery characteristics were evaluated and the porosity of the negative electrode composite layer was measured in the same manner as in Example 1. The results were an initial discharge capacity of 105 mAh / g, a discharge capacity retention rate of 80%, and a porosity of the negative electrode composite layer of 5.3%. The measurement results for Examples 1 to 3 and Comparative Examples 1 and 2 are summarized in Table 1 below.

[0063] [Table 1] [Explanation of symbols]

[0064] 10 Powder Compacts 11 Negative electrode composite layer 12 Solid electrolyte layer (SE layer) 13 Positive electrode composite layer 20 Insulating tube 21 Upper electrode 22 Lower electrode

Claims

[Claim 1] A method for producing a negative electrode composite layer, comprising the steps of: weighing out sulfide all-solid-state electrolyte particles having a form of secondary aggregates with an average particle size of 60 to 100 μm formed by aggregation of primary particles with an average particle size of 0.5 to 2 μm, the sulfide all-solid-state electrolyte particles being made of an ion-conductive compound composed of lithium, sulfur, and phosphorus; and graphite particles having a particle size distribution D50 of 8 μm to 13 μm, so as to obtain a predetermined blending ratio; dry-mixing the total amount of the weighed graphite particles with an amount of the weighed sulfide all-solid-state electrolyte particles in a range of 20% by mass to 40% by mass for a mixing time of 5 minutes or more; adding the remainder of the weighed sulfide all-solid-state electrolyte particles in an amount in a range of 20% by mass to 40% by mass increments to the dry-mixed mixture, and dry-mixing each amount for a mixing time of 1 minute or less; and compacting the obtained negative electrode composite to form a negative electrode composite layer with a porosity of 3% or less.

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