Negative electrode mixture, method for producing negative electrode mixture, and secondary battery

Abstract

To disclose a negative electrode mixture capable of improving a cycle characteristic of a secondary battery.SOLUTION: A negative electrode mixture of the present disclosure, contains: an active material secondary particle; and a sulfide solid electrolyte. The active material secondary particle contains: a plurality of silicon particles; and a polymer for binding the silicon particles to each other. The polymer has a characteristic of 0.37≤εA / εB (here, εA is a bending distortion of a compact A, and the compact A is formed by the polymer and the sulfide solid electrolyte, and includes 20 vol.% of the polymer, and has a filling factor of 90%. εB is a bending distortion of a compact B, and the compact B is formed by the sulfide solid electrolyte, and is a compact having a filling factor of 90%).SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present application discloses a negative electrode mixture, a method for producing the negative electrode mixture, and a secondary battery. [Background technology]

[0002] Patent Document 1 discloses an anode layer used in an all-solid-state battery, which has active material secondary particles and a sulfide solid electrolyte, the active material secondary particles including a plurality of particles containing Si element or Sn element and a binder, and the anode layer has a porosity of 15% or less. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-121557 Summary of the Invention [Problem to be solved by the invention]

[0004] Secondary batteries having Si as the negative electrode active material still have room for improvement in terms of cycle characteristics. [Means for solving the problem]

[0005] The present application discloses the following aspects as means for solving the above problems. <Aspect 1> a negative electrode composite material comprising active material secondary particles and a sulfide solid electrolyte; the active material secondary particles include a plurality of silicon particles and a polymer that binds the silicon particles together, The polymer has the following properties: Characteristics: 0.37≦ε A / ε B ε A : Bending strain of molded body A Molded body A: A molded body made of the polymer and the sulfide solid electrolyte, containing 20% ​​by volume of the polymer and having a filling rate of 90%. ε B : Bending strain of molded body B Compact B: Compact made of the sulfide solid electrolyte and having a filling rate of 90% having Negative electrode composite material. <Aspect 2> The negative electrode composite of Aspect 1, The polymer is a fluorine-based polymer. Negative electrode composite material. <Aspect 3> The negative electrode composite of Aspect 1 or 2, The polymer is a polyvinylidene fluoride polymer. Negative electrode composite material. <Aspect 4> A method for producing a negative electrode composite, Mixing at least a plurality of silicon particles and a polymer to obtain active material secondary particles; and Mixing at least the active material secondary particles with a sulfide solid electrolyte to obtain a negative electrode composite, The polymer has the following characteristics: Characteristics: 0.37≦ε A / ε B ε A : Bending strain of molded body A Molded body A: A molded body made of the polymer and the sulfide solid electrolyte, containing 20% ​​by volume of the polymer and having a filling rate of 90%. ε B : Bending strain of molded body B Compact B: Compact made of the sulfide solid electrolyte and having a filling rate of 90% Use one having Manufacturing method. <Aspect 5> A secondary battery having a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer, The negative electrode active material layer has the negative electrode mixture according to any one of embodiments 1 to 3. Secondary battery. [Effects of the Invention]

[0006] The negative electrode mixture of the present disclosure can improve the cycle characteristics of a secondary battery. [Brief explanation of the drawings]

[0007] [Figure 1] 2 is a schematic diagram illustrating an example of secondary particles of an active material and a sulfide solid electrolyte contained in a negative electrode mixture. [Figure 2] 1 shows a schematic diagram of an example of the configuration of a secondary battery. [Figure 3] 1 shows the relationship between εA / εB and the capacity retention rate. DETAILED DESCRIPTION OF THE INVENTION

[0008] 1.Negative electrode composite material FIG. 1 shows a schematic configuration of a negative electrode composite 1 according to one embodiment. As shown in FIG. 1, the negative electrode composite 1 includes active material secondary particles 1a and a sulfide solid electrolyte 1b. The active material secondary particles 1a include a plurality of silicon particles 1ax and a polymer 1ay that binds the silicon particles 1ax together. The polymer 1ay has a molecular weight of 0.37≦ε A / ε B Here, the ε A is the bending strain of the molded body A, the molded body A is made of the polymer 1ay and the sulfide solid electrolyte 1b, contains the polymer 1ay at 20 volume % and has a filling rate of 90%, and the ε B is the bending strain of the compact B, which is made of the sulfide solid electrolyte 1b and has a packing rate of 90%.

[0009] 1.1 Active material secondary particles The active material secondary particles 1a include a plurality of silicon particles 1ax and a polymer 1ay that binds the silicon particles 1ax together. The polymer 1ay has a molecular weight of 0.37≦ε A / ε B It has the following characteristics.

[0010] 1.1.1 Silicon particles The chemical composition of the silicon particles 1ax is not particularly limited. The ratio of Si element to all elements contained in the silicon particles 1ax may be, for example, 50 mol% to 100 mol%, 70 mol% to 100 mol%, or 90 mol% to 100 mol%. The silicon particles 1ax may contain other elements in addition to Si element. The other elements may be at least one of Li element, Sn element, Fe element, Co element, Ni element, Ti element, Cr element, Al element, B element, and P element. When the silicon particles 1ax are obtained by removing Li from Li-Si alloy particles, the silicon particles 1ax may contain Si element and Li element. Furthermore, the silicon particles 1ax may contain impurities such as oxides. The silicon particles 1ax may be amorphous or crystalline.

[0011] The silicon particles 1ax may be porous. When the silicon particles 1ax are porous and have voids, the voids can absorb the expansion of the silicon, reducing the increase in the confining pressure of the secondary battery and further improving the cycle characteristics of the secondary battery. The silicon particles 1ax may be, for example, particles containing nanoporous silicon. Nanoporous silicon refers to silicon containing multiple pores with pore diameters on the nanometer order (less than 1000 nm, preferably 100 nm or less). The silicon particles 1ax may be porous silicon particles containing pores with a diameter of 55 nm or less. Pores with a diameter of 55 nm or less are difficult to crush even when pressed. In other words, porous silicon particles containing pores with a diameter of 55 nm or less tend to maintain their porous state even after pressing. For example, 1 g of the silicon particles 1ax may contain pores with a diameter of 55 nm or less at a ratio of 0.21 cc / g to 0.30 cc / g. The lower limit may be 0.22 cc / g or more, or 0.23 cc / g or more, and the upper limit may be 0.28 cc / g or less, or 0.26 cc / g or less. The amount of pores with a diameter of 55 nm or less contained in the silicon particles 1ax can be determined from the pore size distribution by, for example, a nitrogen gas adsorption method or a DFT method.

[0012] The porosity of the silicon particles 1ax is not particularly limited. The porosity of the silicon particles 1ax may be, for example, 0% or more and 80% or less. The lower limit may be 1% or more, 5% or more, 10% or more, or 20% or more, and the upper limit may be 70% or less, 60% or less, 50% or less, 40% or less, or 30% or less. The porosity of the silicon particles 1ax can be determined, for example, by observation using a scanning electron microscope (SEM). A large number of samples is preferable, for example, 100 or more. The porosity may be an average value determined from these samples.

[0013] The size of the silicon particles 1ax is not particularly limited. The particle diameter D1 of the silicon particles 1ax may be, for example, 10 nm or more and 10 μm or less. The lower limit may be 30 nm or more, 50 nm or more, 100 nm or more, or 150 nm or more, and the upper limit may be 5 μm or less, 3 μm or less, 2 μm or less, or 1 μm or less. The particle diameter D1 of the silicon particles 1ax can be determined by observation using an electron microscope such as an SEM, and is calculated as the number average of the maximum Feret diameters of multiple particles. A large number of samples is preferable, for example, 20 or more, 50 or more, or even 100 or more. The particle diameter D1 of the silicon particles 1ax can be appropriately adjusted, for example, by changing the manufacturing conditions of the silicon particles or performing a classification process.

[0014] The shape of the silicon particles 1ax is not particularly limited. For example, the silicon particles 1ax may be spherical or non-spherical.

[0015] The number of silicon particles 1ax contained in one active material secondary particle 1a is not particularly limited. The number of silicon particles 1ax contained in one active material secondary particle 1a may be 2 or more and 10,000 or less. The lower limit may be 5 or more, 10 or more, 50 or more, or 100 or more, and the upper limit may be 5,000 or less, 1,000 or less, or 500 or less.

[0016] 1.1.2 Polymers The polymer 1ay has the function of binding the silicon particles 1ax together. The location of the polymer in the active material secondary particles 1a is not particularly limited. The polymer 1ay may be present on the surface side of the active material secondary particles 1a, on the center side, or in both locations. The polymer 1ay may also be exposed on the surface of the active material secondary particles 1a.

[0017] The type of polymer 1ay is not particularly limited as long as it has the properties described below. Whether polymer 1ay has the properties described below depends on the type of monomer unit constituting polymer 1ay and the molecular weight of polymer 1ay. In this embodiment, a known polymer having the properties described below may be selected and used as polymer 1ay. Polymer 1ay may be, for example, at least one selected from butadiene rubber (BR)-based polymers, butylene rubber (IIR)-based polymers, acrylate butadiene rubber (ABR)-based polymers, styrene butadiene rubber (SBR)-based polymers, polyvinylidene fluoride (PVdF)-based polymers, polytetrafluoroethylene (PTFE)-based polymers, polyacrylic acid-based polymers, polyacrylic acid ester-based polymers, etc. In particular, high performance is likely to be exhibited when polymer 1ay is a fluorine-based polymer, especially a polyvinylidene fluoride (PVdF)-based polymer. Fluorine-based polymers have low reactivity with the sulfide solid electrolyte 1b described below. Therefore, deterioration of the sulfide solid electrolyte 1b due to a reaction between the polymer 1ay and the sulfide solid electrolyte 1b can be suppressed. Also, from the viewpoint of ion conductivity, a fluorine-based polymer is advantageous. The polymer 1ay may be a copolymer. For example, a polyvinylidene fluoride (PVdF)-based polymer may have units derived from a monomer other than VdF in addition to units derived from VdF. Only one type of polymer 1ay may be used alone, or two or more types may be used in combination.

[0018] The polymer 1ay has the following properties. When the active material secondary particles 1a contain the polymer 1ay having these properties, contact between the active material secondary particles 1a and the sulfide solid electrolyte 1b is facilitated, improving the bonding strength between the active material secondary particles 1a and the sulfide solid electrolyte 1b. Furthermore, since the polymer 1ay itself has deformability, it is believed that peeling between the active material secondary particles 1a and the sulfide solid electrolyte 1b is unlikely to occur. In other words, even if the volume of the active material secondary particles 1a (particularly the silicon particles 1ax) in the negative electrode composite 1 changes during charge and discharge, peeling between the active material secondary particles 1a and the sulfide solid electrolyte 1b is unlikely to occur, and ion conduction paths and the like are believed to be maintained. As a result, the cycle characteristics (capacity retention rate) of a secondary battery formed from the polymer 1ay are improved.

[0019] Characteristics: 0.37≦ε A / ε B ε A : Bending strain of molded body A Molded body A: A molded body made of the polymer and the sulfide solid electrolyte, containing 20% ​​by volume of the polymer and having a filling rate of 90%. ε B : Bending strain of molded body B Compact B: Compact made of the sulfide solid electrolyte and having a filling rate of 90%

[0020] In this application, three compacts with different packing ratios (= true density / bulk density) are prepared, the bending strain of each compact is measured, and from the relationship between the three measured bending strains and the packing ratio, an approximation curve is used to identify the bending strain at a packing ratio of 90%, which is regarded as "the bending strain of compact A" and "the bending strain of compact B." Specifically, this is as follows.

[0021] The "bending strain of molded body A" is determined as follows. First, polymer 1ay is dissolved in butyl butyrate to prepare a polymer solution. The polymer solution is weighed so that the sulfide solid electrolyte 1b and polymer 1ay have a volume ratio of 80:20, and then butyl butyrate is added to prepare a solution of approximately 1.5 cc so that the solid content is 28% by weight. This solution is mixed for 9 minutes using a shaker and 1 minute 30 seconds using an ultrasonic homogenizer to obtain a slurry. The slurry is cast onto a glass petri dish, dried on a hot plate at 120°C, and then crushed to obtain a mixed powder of polymer 1ay and sulfide solid electrolyte 1b. The mixed powder is press-molded to obtain a molded body for bending tests. Three types of molded bodies with different filling rates are prepared by adjusting the weight of the mixed powder and the pressing pressure. Specifically, three compacts were prepared: one with a filling rate of 70%, a length of 20 mm, a width of 2 mm, and a thickness of 1 mm; one with a filling rate of 77%, a length of 20 mm, a width of 2 mm, and a thickness of 1 mm; and one with a filling rate of 82%, a length of 20 mm, a width of 2 mm, and a thickness of 1 mm. Each compact was subjected to a three-point bending test, as described below, to determine its bending strain. The relationship between these three bending strains and filling rates was plotted on a graph, and an approximate curve was drawn using linear approximation to determine the bending strain at a filling rate of 90%, which was considered to be the "bending strain of compact A."

[0022] The "bending strain of compact B" is determined as follows. First, a compact for a bending test is obtained by press-molding sulfide solid electrolyte 1b. Three types of compacts with different filling rates are prepared by adjusting the weighing value of sulfide solid electrolyte 1b and the pressing pressure. Specifically, three compacts are prepared: a compact with a filling rate of 60%, a length of 20 mm, a width of 2 mm, and a thickness of 1 mm; a compact with a filling rate of 73%, a length of 20 mm, a width of 2 mm, and a thickness of 1 mm; and a compact with a filling rate of 78%, a length of 20 mm, a width of 2 mm, and a thickness of 1 mm. A three-point bending test, as described below, is performed on each compact to determine its respective bending strain. The relationship between these three bending strains and filling rates is plotted on a graph, and an approximate curve is drawn using linear approximation to determine the bending strain at a filling rate of 90%, which is considered to be the "bending strain of compact B."

[0023] "Bending strain" refers to bending strain measured by a three-point bending test in accordance with JIS K7171:2016. Specifically, a molded body (20 mm long, 2 mm wide, 1 mm thick) is prepared using the procedure described above, and the molded body is set in a bending tester with a support distance of 18.5 mm, and a three-point bending test is performed. The bending test span is 1 mm. The bending test speed is 0.05 mm / min.

[0024] As mentioned above, the polymer 1ay has a 0.37≦ε A / ε B It has the following characteristics: ε A / ε B The upper limit is not particularly limited, and may be, for example, 1.00 or less, 0.95 or less, 0.90 or less, 0.85 or less, 0.80 or less, 0.75 or less, 0.70 or less, or 0.65 or less.

[0025] 1.1.3 Other ingredients The active material secondary particles 1a may contain other components in addition to the silicon particles 1ax and polymer 1ay. The active material secondary particles 1a may contain more than 50% by mass and 100% by mass or less of the silicon particles 1ax and polymer 1ay in total, 70% by mass or more and 100% by mass or less, 90% by mass or more and 100% by mass or less, or 95% by mass or more and 100% by mass or less. Furthermore, the active material secondary particles 1a may contain 50% by mass or more and 99% by mass or less of the silicon particles 1ax, 70% by mass or more and 99% by mass or less, or 90% by mass or more and 99% by mass or less of the polymer 1ay, and 1% by mass or more and 50% by mass or less, 1% by mass or more and 30% by mass or less, or 1% by mass or more and 10% by mass or less of the polymer 1ay.

[0026] 1.1.4 Size of active material secondary particles The active material secondary particles 1a can be said to be formed by aggregating multiple silicon particles 1ax via polymer 1ay. The average particle diameter of the active material secondary particles 1a is not particularly limited. The average particle diameter of the active material secondary particles 1a may be 100 nm or more, 1 μm or more, 2 μm or more, or 3 μm or more, or may be 100 μm or less, 50 μm or less, 20 μm or less, 15 μm or less, or 10 μm or less. The average particle diameter of the active material secondary particles 1a can be determined by observation with an electron microscope such as an SEM, and is determined, for example, as the average value of the maximum Feret diameters of multiple secondary particles. A large number of samples is preferably used, for example, 20 or more, or may be 50 or more, or may be 100 or more. Alternatively, the average particle diameter (D50) of the active material secondary particles 1a measured by extracting only the active material secondary particles 1a from the negative electrode composite 1 may be 100 nm or more, 1 μm or more, 2 μm or more, or 3 μm or more, or may be 100 μm or less, 50 μm or less, 20 μm or less, 15 μm or less, or 10 μm or less. Note that the average particle diameter D50 referred to in this application is the particle diameter (median diameter) at 50% cumulative value in a volume-based particle size distribution determined by a laser diffraction / scattering method.

[0027] 1.1.5 Manufacturing method of active material secondary particles The active material secondary particles 1a can be produced, for example, by mixing silicon particles 1ax, polymer 1ay, and optional components. There are no particular limitations on the mixing method, and the mixture may be wet mixed using a solvent or dry mixed without using a solvent. There are also no particular limitations on the mixing means, and the mixture may be mechanically mixed using various mixing devices or may be mixed manually.

[0028] 1.2 Sulfide solid electrolyte The sulfide solid electrolyte 1b may be a glass-based sulfide solid electrolyte (sulfide glass), a glass-ceramic-based sulfide solid electrolyte, or a crystalline sulfide solid electrolyte. The sulfide glass is amorphous. The sulfide glass may have a glass transition temperature (Tg). When the sulfide solid electrolyte 1b has a crystalline phase, examples of the crystalline phase include a Thio-LISICON-type crystalline phase, an LGPS-type crystalline phase, and an argyrodite-type crystalline phase. The sulfide solid electrolyte 1b may contain, for example, a Li element, an X element (X is at least one of P, As, Sb, Si, Ge, Sn, B, Al, Ga, and In), and an S element. The sulfide solid electrolyte 1b may further contain at least one of an O element and a halogen element. The sulfide solid electrolyte 1b may contain an S element as a main anion element. Examples of the sulfide solid electrolyte 1b include Li2S-P2S5, Li2S-P2S5-LiI, Li2S-P2S5-GeS2, Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-P2S5-LiI-LiBr, Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, Li2S-P2S5-ZmSn (where m and n are positive numbers, and Z is Ge, Zn, or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, and Li2S-SiS2-Li x MO y (where x and y are positive numbers, and M is any of P, Si, Ge, B, Al, Ga, and In.) The composition of the sulfide solid electrolyte 1b is not particularly limited, but examples include xLi2S·(100-x)P2S5 (70≦x≦80), yLiI·zLiBr·(100-yz)(xLi2S·(1-x)P2S5) (0.7≦x≦0.8, 0≦y≦30, 0≦z≦30), etc. Alternatively, the sulfide solid electrolyte 1b may be a sulfide solid electrolyte represented by the general formula: Li 4-x Ge 1-x P xIt may have a composition represented by S4(0 < x < 1). In the above general formula, at least a part of Ge may be substituted with at least one of Sb, Si, Sn, B, Al, Ga, In, Ti, Zr, V, and Nb. In the above general formula, at least a part of P may be substituted with at least one of Sb, Si, Sn, B, Al, Ga, In, Ti, Zr, V, and Nb. In the above general formula, a part of Li may be substituted with at least one of Na, K, Mg, Ca, and Zn. In the above general formula, a part of S may be substituted with a halogen (at least one of F, Cl, Br, and I). Alternatively, the sulfide solid electrolyte 1b is Li 7-a PS 6-a X a (X is at least one of Cl, Br, and I, and a is a number of 0 or more and 2 or less) and may have a composition represented thereby. a may be 0, or may be greater than 0. In the latter case, a may be 0.1 or more, may be 0.5 or more, or may be 1 or more. Also, a may be 1.8 or less, or may be 1.5 or less. The sulfide solid electrolyte 1b may be, for example, particulate. The average particle diameter (D50) of the sulfide solid electrolyte 1b may be, for example, 10 nm or more and 10 μm or less.

[0029] 1.3 Other Components The negative electrode composite 1 includes at least the active material secondary particles 1a and the sulfide solid electrolyte 1b. The negative electrode composite 1 may also optionally include other components. Examples of the other components include various solid and liquid components. For example, the negative electrode composite 1 may include other active materials in addition to the active material secondary particles 1a and the sulfide solid electrolyte 1b. The proportion of the silicon particles 1ax to the total active materials included in the negative electrode composite 1 may be more than 50% by mass and 100% by mass or less, 70% by mass or more and 100% by mass or less, or 90% by mass or more and 100% by mass or less. The other active materials that may be included in the negative electrode composite 1 may be, for example, at least one selected from graphite, lithium, and the like. The negative electrode composite 1 may also include other electrolytes in addition to the active material secondary particles 1a and the sulfide solid electrolyte 1b. The other electrolytes may be solid electrolytes other than the sulfide solid electrolyte 1b. The proportion of the sulfide solid electrolyte 1b in the total electrolyte contained in the negative electrode mixture 1 may be more than 50% by mass and 100% by mass or less, 70% by mass or more and 100% by mass or less, or 90% by mass or more and 100% by mass or less. The other electrolyte that may be contained in the negative electrode mixture 1 may be, for example, an oxide solid electrolyte or an ionic solid electrolyte (e.g., one containing Li, Y, and a halogen element as constituent elements). The negative electrode mixture 1 may also contain a conductive additive in addition to the active material secondary particles 1a and the sulfide solid electrolyte 1b. Any conductive additive known as a battery conductive material may be used as the conductive additive. The negative electrode mixture 1 may also contain a binder in addition to the active material secondary particles 1a and the sulfide solid electrolyte 1b. Any binder known as a battery binder may be used as the binder. The amount of other components that may be contained in the negative electrode mixture 1 is not particularly limited and may be determined appropriately depending on the desired battery performance, etc.

[0030] 2. Manufacturing method of negative electrode mixture The technology of the present disclosure also has an aspect as a method for producing a negative electrode composite. That is, a method for producing a negative electrode composite 1 according to one embodiment includes (1) mixing at least a plurality of silicon particles 1ax and a polymer 1ay to obtain active material secondary particles 1a, and (2) mixing at least the active material secondary particles 1a and a sulfide solid electrolyte 1b to obtain the negative electrode composite 1. Here, in this embodiment, the polymer 1ay is mixed with the above-mentioned 0.37≦ε A / ε B That is, in this embodiment, before producing the active material secondary particles 1a, a combination of the polymer 1ay having the above-mentioned properties and the sulfide solid electrolyte 1b is selected. For example, the sulfide solid electrolyte 1b to be combined with the active material secondary particles 1a is prepared, and the bending strain ε of the molded body B is measured as described above. B While specifying the bending strain ε of molded body A, multiple types of polymers are prepared, and for each polymer, the bending strain ε of molded body A is determined as described above. A " and identify the identified ε B and multiple ε A Based on each of and, ε A / ε B Calculate the calculated ε A / ε B A combination of the sulfide solid electrolyte 1b and the polymer 1ay is selected such that the value of (a) is 0.37 or more, the selected polymer 1ay is used to prepare active material secondary particles 1a, and the selected sulfide solid electrolyte 1b and the active material secondary particles 1a are used to prepare the negative electrode composite 1.

[0031] 3. Secondary battery FIG. 2 schematically illustrates the configuration of a secondary battery 100 according to one embodiment. As illustrated in FIG. 2, the secondary battery 100 includes a positive electrode active material layer 20, an electrolyte layer 30, and a negative electrode active material layer 40. In the battery 100 according to one embodiment, the negative electrode active material layer 40 includes the negative electrode composite 1 described above. Problems related to cycle characteristics tend to become apparent when the secondary battery includes a solid electrolyte. In contrast, in the secondary battery 100, the negative electrode active material layer 40 includes predetermined active material secondary particles 1a along with a sulfide solid electrolyte 1b, thereby improving the cycle characteristics as described above. In the secondary battery 100, in addition to the negative electrode active material layer 40 including a sulfide solid electrolyte, one or both of the positive electrode active material layer 20 and the electrolyte layer 30 may also include a sulfide solid electrolyte. Alternatively, in the secondary battery 100, all of the positive electrode active material layer 20, the electrolyte layer 30, and the negative electrode active material layer 40 may also include a sulfide solid electrolyte. Furthermore, the secondary battery 100 may be a solid-state battery. A solid-state battery refers to a battery in which the electrolyte having carrier ion conductivity is mainly composed of a solid electrolyte. However, a liquid component may be included in part. Alternatively, the secondary battery 100 may be an all-solid-state battery that is substantially free of liquid components. The secondary battery 100 may have a general battery configuration. For example, as shown in FIG. 2 , the secondary battery 100 may include a positive electrode active material layer 20, an electrolyte layer 30, an anode active material layer 40, a positive electrode current collector 10 in contact with the positive electrode active material layer 20, and an anode current collector 50 in contact with the anode active material layer 40. The configuration of the secondary battery 100 other than the anode active material layer 40 is the same as that of a conventional battery, and may employ a configuration such as that described in Patent Document 1 (JP 2019-121557 A), for example. The method for manufacturing the secondary battery 100 may also be the same as that of a conventional battery, except for the use of the anode composite 1. [Example]

[0032] The technology of the present disclosure will be described in more detail below with reference to examples, but the technology of the present disclosure is not limited to the following examples.

[0033] 1. Polymer Characterization by Bending Test 1.1 Preparation of bending test specimens 1.1.1 PVdF-based polymer 1 PVdF-based polymer 1 was dissolved in butyl butyrate to obtain a polymer solution. The sulfide electrolyte (described below) and the polymer solution were weighed so that the volume ratio of the sulfide solid electrolyte to the polymer after drying was 80:20. Then, butyl butyrate was added as a solvent to prepare a solution of approximately 1.5 cc, with a solid content of 28% by weight. This was mixed for 9 minutes using a shaker and 1 minute 30 seconds using an ultrasonic disperser to obtain a slurry. The slurry was cast onto a glass dish, dried on a hot plate at 120 °C, and crushed in a mortar to obtain a mixed powder of PVdF-based polymer 1 and the sulfide solid electrolyte. The mixed powder was weighed and press-molded in a jig to obtain a molded body (length 20 mm, width 2 mm, thickness 1 mm) for bending tests. Three molded bodies with different filling rates were obtained by varying the weight of the mixed powder and the pressing pressure from 0.5 to 3.5 tons. One of the compacts had a filling rate of 70%, one had a filling rate of 77%, and one had a filling rate of 82%.

[0034] 1.1.2 PVdF-based polymers 2-5 PVdF polymers 2 to 5 were prepared, each having a different copolymerization component amount and molecular weight from the above PVdF polymer 1. For each of polymers 2 to 5, three molded articles with different filling rates were obtained in the same manner as above.

[0035] 1.1.3 BR polymers Using a BR polymer, three molded articles with different filling rates were obtained in the same manner as above.

[0036] Hereinafter, a molded article containing the above-mentioned PVdF-based polymer or BR-based polymer will be referred to as a "polymer molded article."

[0037] 1.1.4 Sulfide solid electrolyte 0.550 g of Li2S (Furuuchi Chemical), 0.887 g of P2S5 (Aldrich), 0.285 g of LiI (Nippo Chemical), and 0.277 g of LiBr (Kojundo Chemical) were mixed in an agate mortar for 5 minutes. 4 g of n-heptane (dehydrated grade, Kanto Chemical) was added to the resulting mixture, and the mixture was mechanically milled for 40 hours using a planetary ball mill to obtain a sulfide solid electrolyte. The resulting sulfide solid electrolyte was further mechanically milled for 20 hours to obtain finer particles. After fine-graining, the particles were weighed and press-molded in a jig to obtain compacts (20 mm long, 2 mm wide, and 1 mm thick) for bending tests. Three compacts with different filling rates were obtained by varying the weight of the sulfide solid electrolyte and the pressing pressure from 50 to 400 MPa. One compact was prepared so that its filling rate was 60%, one compact was prepared so that its filling rate was 73%, and one compact was prepared so that its filling rate was 78%.

[0038] Hereinafter, the molded body made of the sulfide solid electrolyte will be referred to as a "solid electrolyte molded body."

[0039] 1.2 Bending test For each polymer, three polymer molded bodies with different filling rates were prepared as described above, and a three-point bending test was performed on each to determine the bending strain. The measured values ​​were plotted with the filling rate on the horizontal axis and the bending strain on the vertical axis, and an approximate curve was drawn from the three-point plot using linear approximation to determine the bending strain ε at a filling rate of 90%. A (90% bending strain ε A On the other hand, the bending strain ε at a filling rate of 90% was also determined for the above-mentioned solid electrolyte compact. B (90% bending strain ε B ) was identified.

[0040] 1.3 ε A / ε B Calculation of 90% bending strain ε of each polymer molding A and the 90% bending strain ε of the solid electrolyte compact B Relative to ε A / ε BThe results are shown in Table 1 below.

[0041] [Table 1]

[0042] 2. Evaluation of battery cycle characteristics Active material secondary particles were prepared using each of the above polymers, the active material secondary particles were used to prepare a negative electrode composite, the negative electrode composite was used to prepare a secondary battery, and the cycle characteristics of the secondary battery were evaluated.

[0043] 2.1 Comparative Example 1 2.1.1 Preparation of the positive electrode A positive electrode slurry was prepared by mixing an NCM-based positive electrode active material, a sulfide-based solid electrolyte, vapor-grown carbon fiber, a PVdF-based binder, and butyl butyrate using an ultrasonic disperser. The mass ratio of the NCM-based positive electrode active material, sulfide-based solid electrolyte, vapor-grown carbon fiber, and PVdF-based binder was 100:16:2:0.75. This positive electrode slurry was applied to an Al foil (positive electrode current collector) using a blade method and dried on a hot plate at 100°C for 30 minutes, resulting in a positive electrode with a positive electrode active material layer on the Al foil surface.

[0044] 2.1.2 Preparation of the negative electrode The PVDF-based polymer 1 was dissolved in butyl butyrate to obtain a polymer solution. The polymer solution and Si particles (particle size 0.5 μm, manufactured by Kojundo Chemical Co., Ltd.) were weighed out so that the mass ratio of Si particles to polymer was 100:8, and the mixture was stirred using an ultrasonic disperser and a shaker to obtain a negative electrode slurry. The negative electrode slurry was cast onto a glass petri dish, dried on a hot plate, and crushed in a mortar to obtain active material secondary particles. The obtained active material secondary particles were secondary particles (aggregated particles) consisting of multiple Si particles and a polymer that bound the Si particles together. The active material secondary particles, sulfide-based solid electrolyte, vapor-grown carbon fiber, BR-based binder, mesitylene, and dibutyl ether were stirred and mixed using an ultrasonic disperser and a stirrer to obtain a negative electrode slurry in which the negative electrode mixture was dispersed in the solvent. The mass ratio of Si particles:polymer:sulfide-based solid electrolyte:vapor-grown carbon fiber:BR-based binder was 100:8:77.6:8.4:1.5. This negative electrode slurry was applied to a Ni foil serving as a negative electrode current collector foil by a blade method, and the resulting coating was dried on a hot plate at 100°C for 30 minutes to obtain a negative electrode having a negative electrode active material layer on the surface of the Ni foil.

[0045] 2.1.3 Preparation of solid electrolyte layer A solid electrolyte slurry was obtained by stirring a sulfide-based solid electrolyte, a PVdF-based binder, and butyl butyrate using an ultrasonic disperser. The mass ratio of the sulfide-based solid electrolyte to the PVdF-based binder was 99.6:0.4. This solid electrolyte slurry was applied to an Al foil using a blade method and dried on a hot plate at 100°C for 30 minutes to form a solid electrolyte layer on the Al foil.

[0046] 2.1.4 Fabrication of the positive electrode laminate The positive electrode active material layer and the solid electrolyte layer were laminated and pressed with a roll press at a pressure of 50 kN / cm and a temperature of 160°C. After that, the Al foil was peeled off from the solid electrolyte layer and a 1 cm 2 By punching out the sheet into a size of 100 mm, a positive electrode laminate having a structure of Al foil / positive electrode active material layer / solid electrolyte layer was obtained.

[0047] 2.1.5 Fabrication of negative electrode laminate The negative electrode active material layer and the solid electrolyte layer were laminated and pressed with a roll press at a pressure of 50 kN / cm, and then the Al foil of the solid electrolyte layer was peeled off to obtain a first laminate having a Ni foil / negative electrode active material layer / solid electrolyte layer configuration. Further, an additional solid electrolyte layer was laminated on the solid electrolyte layer side of the first laminate, and pre-pressed with a flat uniaxial press at a pressure of 100 MPa and a temperature of 25°C, after which the Al foil was peeled off from the solid electrolyte layer and a 1.08 cm 2 A second laminate having a structure of Ni foil / negative electrode active material layer / solid electrolyte layer / solid electrolyte layer was obtained by punching out the laminate to a size of 1. This second laminate was used as the negative electrode laminate.

[0048] 2.1.6 Fabrication of the battery stack The above positive electrode laminate and negative electrode laminate were stacked and pressed with a flat uniaxial press at a pressure of 200 MPa and a temperature of 135°C to obtain a battery laminate having a configuration of Ni foil / negative electrode active material layer / solid electrolyte layer / solid electrolyte layer / solid electrolyte layer / positive electrode active material layer / Al foil.

[0049] 2.1.7 Battery Stack Constraints and Initial Charge / Discharge The battery stack was sandwiched between two restraint plates, and these two restraint plates were fastened with fasteners at a restraint pressure of 1 MPa to fix the distance between the two restraint plates. (1) 1 / 10C, ​​constant current charging up to 4.55V (2) Then, charge to 4.55V, with a constant voltage of 1 / 100C. (3) Then, discharge at a constant current of 1C to 3V. (4) Then, discharge at a constant voltage of 3V to a final current of 1 / 100C. (5) Then, charge at a constant current of 1 / 3C to 4.35V. Furthermore, after that, (6) Constant voltage charging up to 4.35V, final current 1 / 100C (7) Then, discharge at a constant current of 1 / 3C to 3V. (8) Then, discharge at a constant voltage of 3V to a final current of 1 / 100C. I did this twice.

[0050] 2.1.8 Evaluation of cycle characteristics through durability tests The battery stack after the above charge / discharge cycles was subjected to a durability test, and the cycle characteristics were evaluated by calculating the capacity retention rate. Specifically, 150 charge / discharge cycles were performed under the following conditions, and the ratio of the discharge capacity C2 after 150 cycles to the discharge capacity C1 at the first cycle was calculated and used as the capacity retention rate. Charging: 2C, constant current charging up to 4.17V Discharge: 2C, constant current discharge down to 3.14V Capacity retention rate (%): [Discharge capacity C2 / Discharge capacity C1] × 100

[0051] 2.2 Examples 1 to 5 Batteries were produced in the same manner as in Comparative Example 1, except that the above-mentioned PVdF-based polymers 2 to 5 or BR-based polymer were used instead of PVdF-based polymer 1 as the polymer constituting the active material secondary particles, and the cycle characteristics were evaluated.

[0052] 2.3 Evaluation results Figure 3 shows the characteristic ε of the polymer that makes up the active material secondary particles. A / ε B As shown in FIG. 3, the polymer constituting the active material secondary particles has a capacitance of 0.37≦ε A / ε BIt can be seen that when a polymer having these properties is used, the capacity retention rate of the secondary battery is improved and the battery has excellent cycle characteristics. When the active material secondary particles contain a polymer having these properties, contact between the active material secondary particles and the sulfide solid electrolyte is facilitated, improving the bonding strength between the active material secondary particles and the sulfide solid electrolyte. Furthermore, since the polymer itself has deformability, it is believed that peeling between the active material secondary particles and the sulfide solid electrolyte is less likely to occur. In other words, even if the volume of the active material secondary particles (especially Si particles) in the negative electrode composite changes during charge and discharge, peeling between the active material secondary particles and the sulfide solid electrolyte is less likely to occur, and ion conduction paths and the like are believed to be maintained. As a result, it is believed that the cycle characteristics (capacity retention rate) of the secondary battery are improved. From the above results, it can be said that the cycle characteristics of a secondary battery are improved by constructing a secondary battery using a negative electrode composite that satisfies the following conditions. (1) The negative electrode mixture contains secondary particles of an active material and a sulfide solid electrolyte. (2) The secondary particles of the active material include a plurality of silicon particles and a polymer that binds the silicon particles together. (3) The polymer has a ε of 0.37≦ε A / ε B where ε A is the bending strain of the molded body A, which is made of the polymer and the sulfide solid electrolyte, contains the polymer at 20% by volume, and has a filling rate of 90%, and ε B is the bending strain of compact B, which is made of the sulfide solid electrolyte and has a packing rate of 90%. [Explanation of symbols]

[0053] 1 Negative electrode composite material 1a Active material secondary particles 1ax silicon particles 1ay polymer 1b Sulfide solid electrolyte 100 Secondary battery 10 Positive electrode current collector 20 Cathode active material layer 30 Electrolyte layer 40 Negative active material layer 50 Negative current collector

Claims

1. a negative electrode composite material comprising active material secondary particles and a sulfide solid electrolyte; the active material secondary particles include a plurality of silicon particles and a fluorine-based polymer that binds the silicon particles together, The fluorine-based polymer has the following characteristics: Characteristics: 0.37≦ε Ａ / ε Ｂ ≦1.00 ε Ａ : Bending strain of molded body A Molded body A: A molded body made of the fluoropolymer and the sulfide solid electrolyte, containing 20% ​​by volume of the fluoropolymer and having a filling rate of 90%. ε Ｂ : Bending strain of molded body B Molded body B: A molded body made of the sulfide solid electrolyte and having a filling rate of 90% having Negative electrode composite material.

2. The negative electrode mixture according to claim 1, The active material secondary particles are The silicon particles are contained in an amount of 70% by mass or more and 99% by mass or less, and The fluorine-based polymer is contained in an amount of 1% by mass or more and 30% by mass or less. Negative electrode composite material.

3. The negative electrode mixture according to claim 1, The fluorine-based polymer is a polyvinylidene fluoride-based polymer. Negative electrode composite material.

4. A method for producing a negative electrode composite, Mixing at least a plurality of silicon particles and a fluorine-based polymer to obtain active material secondary particles; and Mixing at least the active material secondary particles with a sulfide solid electrolyte to obtain a negative electrode composite, The fluorine-based polymer has the following characteristics: Characteristics: 0.37≦ε Ａ / ε Ｂ ≦1.00 ε Ａ : Bending strain of molded body A Molded body A: A molded body made of the fluoropolymer and the sulfide solid electrolyte, containing 20% ​​by volume of the fluoropolymer and having a filling rate of 90%. ε Ｂ : Bending strain of molded body B Molded body B: A molded body made of the sulfide solid electrolyte and having a filling rate of 90% Use one having Manufacturing method.

5. A secondary battery having a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer, The negative electrode active material layer comprises the negative electrode mixture according to any one of claims 1 to 3. Secondary battery.

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