Silicon active material, negative electrode active material layer, all-solid-state battery, and method for producing silicon active material

Abstract

This silicon active material is porous silicon particles having pores, and the amount of pores having a pore diameter of 30 nm or less is 0.2 cm3 / g or more, the ratio of the amount of pores having a pore diameter of 30 nm or less accounting for the amount of pores having a pore diameter of 100 nm or less is 80% or more, and the amount of oxygen is 10 mass% or less.

Description

Silicon-based active material, negative electrode active material layer, all-solid-state battery, and method for manufacturing silicon-based active material

[0001] The present disclosure relates to a silicon-based active material used in an electricity storage device, a negative electrode active material layer, an all-solid-state battery, and a method for producing a silicon-based active material.

[0002] Patent Literature 1 discloses an all-solid-state battery using a silicon-based active material as a negative electrode active material. Since silicon has a higher theoretical capacity than graphite, the use of a silicon-based active material is expected to increase the capacity of power storage devices such as all-solid-state batteries.

[0003] Japanese Patent Application Laid-Open No. 2020-004685

[0004] Silicon has the property of expanding significantly when it absorbs charge carriers such as lithium ions. In an energy storage device, excessive volume change in the negative electrode during charging and discharging can reduce the durability of the device.

[0005] The silicon-based active material of the present disclosure is a porous silicon particle having pores, and the amount of pores having pore diameters of 30 nm or less is 0.2 cm ３ / g or more, the proportion of pores with a diameter of 30 nm or less to the total number of pores with a diameter of 100 nm or less is 80% or more, and the oxygen content is 10 mass % or less.

[0006] In one embodiment of the silicon-based active material, the proportion of pores having a pore diameter of more than 3 nm to the proportion of pores having a pore diameter of 30 nm or less is 90% or more. ３ / g or less.

[0007] One embodiment of the silicon-based active material has a specific surface area of ​​100 m ２ / g or more 250m ２ In one embodiment of the silicon-based active material, the average particle size is 1 μm or more and 3 μm or less.

[0008] In one embodiment of the silicon-based active material, the particle aspect ratio is 1 or more and 1.7 or less. ...

[0009] The all-solid-state battery that solves the above problem includes the above-mentioned negative electrode active material. The method for producing a silicon-based active material that solves the above problem includes a reduction step of bringing a Si raw material containing SiO into contact with Mg vapor under reduced pressure and under conditions where the Mg vapor pressure is equal to or lower than the equilibrium pressure of the following reaction formula (1), thereby obtaining an intermediate product containing Si and MgO, and a washing step of removing MgO from the intermediate product.

[0010]

[0011] One aspect of the cleaning step includes a first cleaning step and a second cleaning step, in which the first cleaning step is a step of generating a magnesium salt from MgO using an acid, and the second cleaning step is a step of washing away the magnesium salt using a rinse liquid.

[0012] One embodiment of the rinse liquid is an acidic aqueous solution having a pH of 6 or less.

[0013] According to the present invention, it is possible to realize an electricity storage device that has a high capacity and in which the volume change of the negative electrode during charging and discharging is small.

[0014] Fig. 1 is a schematic cross-sectional view of a negative electrode. Fig. 2 is a temperature-Mg vapor pressure curve of reaction formula (1). Fig. 3 is an explanatory diagram of the reduction process. Fig. 4 is a schematic cross-sectional view of an all-solid-state battery.

[0015] An embodiment of the present invention will be described below with reference to the drawings. The silicon-based active material of this embodiment is used, for example, as a negative electrode active material capable of absorbing and releasing charge carriers such as lithium ions in the negative electrode of a power storage device. The power storage device is, for example, a secondary battery such as a nickel-metal hydride secondary battery or a lithium-ion secondary battery. The power storage device may also be an all-solid-state battery or an electric double layer capacitor. One example of the silicon-based active material is an active material for an all-solid-state battery used in the negative electrode of the all-solid-state battery.

[0016] <Silicon-Based Active Material> The silicon-based active material contains silicon as a main component and is made of porous silicon particles having three-dimensionally connected pores.

[0017] [Composition] The silicon content of the silicon-based active material is, for example, 60% by mass or more, preferably 80% by mass or more, and more preferably 90% by mass or more. The silicon content is, for example, 100% by mass or less. The silicon content refers to the mass ratio of silicon atoms contained in the silicon-based active material.

[0018] The silicon-based active material contains oxygen in a range where the oxygen content is 0% by mass or 10% by mass or less. The oxygen content is preferably 9.5% by mass or less, and more preferably 9.2% by mass or less. The oxygen content is, for example, 0% by mass or more, 0.5% by mass or more, or 2% by mass or more. The oxygen content refers to the mass ratio of oxygen atoms contained in the silicon-based active material.

[0019] The amount of oxygen can be controlled, for example, by adjusting the specific surface area of ​​the silicon-based active material. Oxides are formed on the surfaces of particles of the silicon-based active material. Therefore, the amount of oxygen tends to decrease as the specific surface area of ​​the particles decreases. The amount of oxygen can also be controlled by adjusting the manufacturing conditions. As will be described in detail later, the amount of oxygen can be controlled by adjusting the processing conditions of the first and second cleaning steps included in the manufacturing method of the silicon-based active material.

[0020] The silicon-based active material may contain other components in addition to silicon and oxygen. Examples of such other components include magnesium compounds. The proportion of such other components is, for example, 0% by mass or more and 10% by mass or less, and preferably 0% by mass or more and 5% by mass or less. An example of a silicon-based active material has a proportion of the above impurities of 0% by mass.

[0021] [Shape] The average particle diameter of the silicon-based active material is, for example, 1 μm or more and 3 μm or less. The average particle diameter of the silicon-based active material is preferably 1.5 μm or more. The average particle diameter of the silicon-based active material is preferably 2.5 μm or less. In this specification, the "average particle diameter" refers to the median diameter (d ５０ ) means

[0022] The silicon-based active material particles preferably have a shape close to a perfect sphere. More specifically, the aspect ratio of the silicon-based active material particles is, for example, 1 to 2, preferably 1 to 1.7, and more preferably 1 to 1.5. The aspect ratio is, for example, an average value.

[0023] 1 is a schematic diagram of a negative electrode 20 of a power storage device. The negative electrode 20 includes a negative electrode current collector 21 and a negative electrode active material layer 22 formed on one surface of the negative electrode current collector 21. The negative electrode active material layer 22 is formed by applying a slurry-like negative electrode mixture, which is obtained by mixing components constituting the negative electrode active material layer 22, such as a silicon-based active material, with a dispersion medium, onto the surface of the negative electrode current collector 21 and then drying the mixture.

[0024] Here, as shown in the enlarged view of FIG. 1 , when the silicon-based active material particles P have a shape with a large aspect ratio, the silicon-based active material particles are stacked on the negative electrode current collector 21 with the direction of their major axes aligned with the surface direction of the negative electrode current collector 21. In this case, when charge carriers L, such as lithium ions, travel a long distance around the silicon-based active material particles P as they move in the thickness direction of the negative electrode active material layer 22, the distance required for the charge carriers L to travel around the silicon-based active material particles P increases. In contrast, when the silicon-based active material particles P have a shape close to a perfect sphere, i.e., a shape with a small aspect ratio, the distance required for the charge carriers L to travel around the silicon-based active material particles P can be shortened. This shortens the conduction path of the charge carriers in the thickness direction of the negative electrode active material layer 22, thereby improving the ionic conductivity of the negative electrode active material layer 22.

[0025] [Pore] In this specification, the pore volume means the pore volume per unit mass calculated based on the BJH (Barret-Joyner-Halenda) method.

[0026] The total pore volume of the silicon-based active material is, for example, 0.1 cm ３ / g or more, preferably 0.2 cm ３ / g or more, more preferably 0.3 cm ３ The total pore volume is, for example, 0.8 cm ３ / g or less, preferably 0.6 cm ３ / g or less, more preferably 0.4 cm３ / g or less.

[0027] The silicon-based active material has pores with a pore diameter of 100 nm or less. The amount of pores with a pore diameter of 100 nm or less is, for example, 0.2 cm ３ / g or more 0.8cm ３ The amount of pores with a diameter of 100 nm or less is preferably 0.2 cm ３ / g or more, more preferably 0.3 cm ３ The amount of pores with a pore diameter of 100 nm or less is, for example, 0.8 cm ３ / g or less, preferably 0.6 cm ３ / g or less, more preferably 0.4 cm ３ The proportion of the amount of pores having a pore diameter of 100 nm or less to the total amount of pores is, for example, 90% or more, preferably 95% or more, and more preferably 99% or more.

[0028] The silicon-based active material has pores with a pore diameter of 30 nm or less. The amount of pores with a pore diameter of 30 nm or less is 0.2 cm ３ / g or more, preferably 0.3 cm ３ / g or more, more preferably 0.35 cm ３ The amount of pores with a pore diameter of 100 nm or less is, for example, 0.8 cm ３ / g or less, preferably 0.6 cm ３ / g or less, more preferably 0.4 cm ３ / g or less.

[0029] In the silicon-based active material, the majority of pores with a pore diameter of 100 nm or less are pores with a pore diameter of 30 nm or less. The ratio R (30 / 100) of the amount of pores with a pore diameter of 30 nm or less to the amount of pores with a pore diameter of 100 nm or less is 80% or more, preferably 85% or more. The ratio R (30 / 100) is, for example, 100% or less, 95% or less, or 90% or less.

[0030] In the silicon-based active material, it is preferable that the majority of pores with a pore diameter of 30 nm or less are pores with a pore diameter of more than 3 nm. In other words, it is preferable that the silicon-based active material has few micropores with a pore diameter of 3 nm or less. The ratio R (3-30 / 30) of the amount of pores with a pore diameter of more than 3 nm to the amount of pores with a pore diameter of 30 nm or less is, for example, 90% or more, preferably 95% or more. The ratio R (3-30 / 30) is, for example, 100% or less. In addition, the amount of pores with a pore diameter of 3 nm or less is, for example, 0.05 cm ３ / g or less, preferably 0.02 cm ３ / g or less, and more preferably 0.015 cm ３ / g or less. The amount of pores with a pore diameter of 3 nm or less is, for example, 0 cm ３ / g or more.

[0031] In the silicon-based active material, the peak value of the pore size distribution is, for example, 5 nm or more, preferably 8 nm or more, and more preferably 10 nm or more. The peak value is, for example, 50 nm or less, preferably 30 nm or less, and more preferably 20 nm or less. The peak value is the value of the main peak in the pore distribution plot based on the BJH method, and can be considered to be the pore size of the main pores possessed by the silicon-based active material. Furthermore, it is preferable that the silicon-based active material has only one peak formed in the pore distribution plot based on the BJH method.

[0032] [Specific Surface Area] In this specification, the specific surface area of ​​a silicon-based active material is measured by the BET (Brunauer-Emmett-Teller) method. ２ It refers to the BET surface area per unit mass as measured by adsorption.

[0033] The specific surface area of ​​the silicon-based active material is, for example, 100 m ２ / g or more 250m ２ The specific surface area of ​​the silicon-based active material is preferably 100 m ２ / g or more, more preferably 150m ２ The specific surface area of ​​the silicon-based active material is preferably 200 m ２ / g or less, more preferably 150m ２ / g or less.

[0034] <Method for manufacturing silicon-based active material> The method for manufacturing a silicon-based active material includes a reduction step and a washing step described below. [Reduction step] (Si raw material) The reduction step is a step of bringing a raw material containing silicon oxide (hereinafter referred to as Si raw material) into contact with Mg vapor to reduce the silicon oxide, thereby obtaining an intermediate product containing Si and MgO through a reduction reaction. Examples of the Si raw material include SiO, SiO ２ As the Si raw material, SiO is preferably used. The Si raw material may contain components other than silicon oxide. The proportion of silicon oxide in the Si raw material is, for example, 50 mass % or more, preferably 90 mass % or more, and more preferably 99 mass % or more.

[0035] The Si raw material is preferably in powder form. In this case, the average particle size of the Si raw material is, for example, 3 μm or less, preferably 1 μm or less, and the average particle size of the Si raw material is, for example, 0.5 μm or more.

[0036] The Si raw material powder can be obtained by, for example, performing a pulverization process to pulverize a block of Si raw material using a pulverization device such as a bead mill. The pulverization process is preferably performed under conditions that allow the block of Si raw material to be volume-pulverized. For example, in the first half of the pulverization process, pulverized particles generated by surface pulverization using frictional force or shear force are further pulverized in the second half of the pulverization process under conditions that allow impact force or compression force to be volume-pulverized. By performing the pulverization process under conditions that allow volume pulverization, the aspect ratio of the particles of the Si raw material can be reduced. By using a Si raw material with a small aspect ratio, the aspect ratio of the particles of the finally obtained silicon-based active material can be reduced.

[0037] (Pressure Conditions) The first pressure condition for the reduction reaction is a reduced pressure. The reduced pressure is a pressure of less than 1 atmosphere (101,325 Pa). The reduced pressure is, for example, preferably 100 Pa or less, and more preferably 20 Pa or less. As the pressure in the reduction step decreases, Mg vapor is more likely to diffuse within the reaction system. This increases the chance of contact between the Si raw material and Mg vapor, and as a result, the reduction reaction in the reduction step is more likely to proceed. Furthermore, since the reduction reaction is more likely to proceed, the temperature required to proceed with the reduction step can be lowered.

[0038] The second pressure condition for the reduction reaction is a Mg vapor pressure equal to or lower than the equilibrium pressure of the following reaction formula (1).

[0039]

[0040] Reaction formula (1) is Mg ２ This shows a reversible decomposition reaction in which Si is decomposed into Mg vapor and Si. The temperature-Mg vapor pressure curve of reaction formula (1) is shown in Figure 2. The Mg vapor pressure below the equilibrium pressure of reaction formula (1) is lower than that of curve 1. At Mg vapor pressures below the equilibrium pressure of reaction formula (1), Mg ２ The forward reaction of decomposing Si into Mg vapor and Si proceeds, and the Mg vapor and Si are converted into Mg ２ The reverse reaction to produce Si does not proceed, or proceeds very little. Therefore, the reduction reaction produces SiO or SiO ２ The Si generated from the reaction with Mg vapor produces Mg ２ The generation of Si can be suppressed.

[0041] (Method for generating Mg vapor) As long as it is possible to bring Mg vapor into contact with the Si raw material so as to satisfy the second pressure condition, the method for generating Mg vapor is not particularly limited. For example, a method for generating Mg vapor may be used in which Mg vapor is generated by heating a reducing agent serving as an Mg source, such as an Mg alloy or metallic Mg.

[0042] Among these, a method using an Mg alloy (hereinafter referred to as a low equilibrium pressure Mg alloy) in which the equilibrium pressure of the reaction to generate Mg vapor is equal to or lower than the equilibrium pressure of reaction formula (1) is particularly preferred. In the reaction system in which Mg vapor is generated from the low equilibrium pressure Mg alloy, the Mg vapor pressure does not exceed the equilibrium pressure of reaction formula (1). Therefore, the reaction to generate Mg vapor from the low equilibrium pressure Mg alloy and the reaction in which Mg vapor is brought into contact with the Si raw material can be carried out in the same reaction system. In this case, the equipment used in the reduction step can be simplified.

[0043] Examples of low equilibrium pressure Mg alloys include Mg ２ Si, MgCa alloy, MgCu ２ , MgNi ２ , and MgSn alloys. Table 1 below shows the equilibrium pressures at temperatures of 600°C, 700°C, and 800°C in the reaction that generates Mg vapor from a low equilibrium pressure Mg alloy. The Mg vapor pressure values ​​in Table 1 are expressed in common logarithms. ２ The value for Si is the equilibrium pressure for reaction (1).

[0044] Among low equilibrium pressure Mg alloys, those in which the equilibrium pressure of the reaction that generates Mg vapor is close to the equilibrium pressure of reaction formula (1) are preferred.

[0045]

[0046] For reference, Table 1 also shows the equilibrium pressure of the reaction generating Mg vapor from metallic Mg at each of the above temperatures. As shown in Table 1, the equilibrium pressure of the reaction generating Mg vapor from metallic Mg is higher than the equilibrium pressure of reaction formula (1). Therefore, the Mg vapor pressure in the reaction system generating Mg vapor from metallic Mg is higher than the equilibrium pressure of reaction formula (1). When using a method for generating Mg vapor from metallic Mg, a process is performed to reduce the Mg vapor pressure in the reaction system. For example, a metal that bonds more easily with Mg than Si, such as Ca, Cu, Sn, or Ni, is separately placed in the reaction system. The same applies when using a method for generating Mg vapor from an Mg alloy, in which the equilibrium pressure of the reaction generating Mg vapor exceeds the equilibrium pressure of reaction formula (1).

[0047] The amount of reducing material used as a Mg source, such as an Mg alloy or metallic Mg, is determined based on the amount of SiO contained in the Si raw material. ２ In other words, the amount of the reducing agent is such that all of the SiO or SiO contained in the Si raw material is reduced. ２ For example, the amount of SiO or SiO contained in the Si raw material is more than the amount required for reducing the amount of SiO. ２ The amount of reducing agent that generates 1 molar equivalent of Mg vapor per mole of Mg is defined as the reference amount. In this case, the amount of reducing agent charged is 1 time or more of the reference amount, preferably 1.1 times or more, more preferably 1.2 times or more, and even more preferably 1.3 times or more. In addition, the amount of reducing agent charged is, for example, 2 times or less of the reference amount.

[0048] (Temperature Conditions) The temperature of the reduction reaction is the temperature at which Mg vapor exists below the equilibrium pressure of reaction formula (1). By adjusting the temperature of the reduction reaction, the pore size distribution of the silicon-based active material can be controlled. As the temperature of the reduction reaction increases, the pores formed in the silicon-based active material tend to become larger. Therefore, in order to increase the number of pores with a pore diameter of 30 nm or less, it is preferable to suppress the temperature of the reduction reaction. Furthermore, in order to reduce the number of micropores with a pore diameter of 3 nm or less, it is preferable not to suppress the temperature of the reduction reaction too much.

[0049] The temperature of the reduction reaction is, for example, 900°C or lower, preferably 800°C or lower, and more preferably 700°C or lower. By adjusting the temperature of the reduction reaction in this manner, it is possible to increase the number of pores with a pore diameter of 30 nm or less. The temperature of the reduction reaction is, for example, 500°C or higher, preferably 600°C or higher, and more preferably 650°C or higher. By adjusting the temperature of the reduction reaction in this manner, it is possible to reduce the number of pores with a pore diameter of 3 nm or less.

[0050] (Reaction Time) The reaction time of the reduction reaction can be appropriately selected depending on the pressure conditions and the temperature conditions, and is, for example, 3 hours or more and 24 hours or less.

[0051] (Specific Example of Reduction Step) A specific example of the reduction step will be described with reference to FIG. 3 . The stainless steel reaction vessel 10 is a vessel configured to allow gas to circulate between the inside and outside of the vessel. A raw material tray 11 and a reduction tray 12 are accommodated within the reaction vessel 10. The raw material tray 11 is placed on a leg 11a that stands upright from the bottom of the reaction vessel 10. The raw material tray 11 is a mesh-like tray that is breathable. A powdered Si raw material A is placed in the raw material tray 11.

[0052] The reduction tray 12 is disposed below the raw material tray 11 at the bottom of the reaction vessel 10. A powdered low equilibrium pressure Mg alloy B is disposed in the reduction tray 12. Therefore, in the reaction vessel 10, the Si raw material A and the low equilibrium pressure Mg alloy B are disposed in a non-contact state.

[0053] The reaction vessel 10 containing the Si source A and the low equilibrium pressure Mg alloy B is placed in a vacuum furnace 13. The vacuum furnace 13 is then depressurized and heated to a temperature at which Mg vapor is generated from the low equilibrium pressure Mg alloy B. The Mg vapor generated from the low equilibrium pressure Mg alloy B is diffused in the reaction vessel 10. When the Mg vapor comes into contact with the Si source A in the source tray 11, SiO or SiO contained in the Si source A is evaporated. ２ After a predetermined reaction time has elapsed, the reaction vessel 10 is removed from the vacuum furnace 13, and the intermediate product containing Si and MgO produced in the raw material tray 11 of the reaction vessel 10 is collected.

[0054] <Washing Step> The washing step includes a first washing step and a second washing step. Through the first washing step and the second washing step, a silicon-based active material is obtained.

[0055] The first washing step is a step of treating the intermediate product obtained in the reduction step with an acid. In the first washing step, magnesium salts are produced from MgO contained in the intermediate product, and the produced magnesium salts are washed away. The acid used in the first washing step is not particularly limited, and any acid capable of producing magnesium salts from MgO may be used. Examples of acids used in the first washing step include hydrochloric acid, nitric acid, and sulfuric acid. The concentration of the acid used in the first washing step can be appropriately selected depending on the type of acid. As an example, the reaction formula when hydrochloric acid is used is shown below.

[0056] MgO+2HCl→MgCl ２ +H ２ The second cleaning step is a step of rinsing away magnesium salts and the like remaining in the first cleaning step using a rinse solution. The rinse solution is water or an acidic aqueous solution having a pH of 6 or less. The acid contained in the acidic aqueous solution is preferably an acid that can be removed with an alcohol such as ethanol, for example, an organic acid such as acetic acid. An example of the acidic aqueous solution is a 1% aqueous acetic acid solution. If necessary, after the second cleaning step, further cleaning treatment using an alcohol such as ethanol may be performed.

[0057] As described above, the oxygen content of the resulting silicon-based active material can be controlled by adjusting the processing conditions of the first and second cleaning steps. For example, when the processing temperature is 0°C in one or both of the first and second cleaning steps, the oxygen content of the resulting silicon-based active material is lower than when processing is performed at room temperature. Furthermore, when an acidic aqueous solution is used as the rinse solution in the second cleaning step, the oxygen content of the resulting silicon-based active material is lower than when water is used.

[0058] <All-Solid-State Battery> An example of an all-solid-state battery to which the silicon-based active material of this embodiment is applied will be described. As shown in FIG. 4 , the all-solid-state battery 30 includes a battery element 31. The all-solid-state battery 30 may further include an exterior body (not shown). The battery element 31 may be housed in the exterior body. The exterior body may be a metal case or the like, or may be a pouch made of an aluminum laminate film or the like.

[0059] The battery element 31 includes a positive electrode 32, a solid electrolyte layer 33, and a negative electrode 34. The negative electrode 34 is, for example, the above-described negative electrode 20, and includes the silicon-based active material of this embodiment. The solid electrolyte layer 33 blocks electronic conduction and conducts ions between the positive electrode 32 and the negative electrode 34. The positive electrode 32 and the solid electrolyte layer 33 are not particularly limited, and a known positive electrode and a known solid electrolyte layer that are applied to all-solid-state batteries can be used.

[0060] The battery element 31 is formed, for example, by stacking individually formed positive electrodes 32, solid electrolyte layers 33, and negative electrodes 34, and then pressing them in the stacking direction. The all-solid-state battery 30 may be a single cell or a stacked battery in which a plurality of battery elements 31 are stacked. The stacked battery may be a monopolar stacked battery (a parallel-connected stacked battery) or a bipolar stacked battery (a series-connected stacked battery). The shape of the all-solid-state battery 30 may be, for example, a coin type, a laminate type, a cylindrical type, or a prismatic type.

[0061] <Function> Next, the function of this embodiment will be described. The particles of the silicon-based active material are porous silicon having pores and a skeleton portion that separates the pores. When porous silicon expands due to the absorption of charge carriers such as lithium ions, it can expand inward to fill the pores. Therefore, compared to solid silicon, porous silicon undergoes a smaller volume change accompanied by an increase in its external shape. Therefore, by using porous silicon as the negative electrode active material, it is possible to reduce the volume change of the negative electrode during charge and discharge.

[0062] Here, in order to fully exert the effect of suppressing the volume change of the negative electrode based on the porous silicon, it is necessary to maintain the pores and the skeletal portion against the volume expansion of the porous silicon that accompanies charge and discharge. In other words, stress such as expansion pressure during volume expansion acts on the skeletal portion, which may cause the skeletal portion to break and the pores to collapse. In this case, the porous silicon cannot expand inward to fill the pores, and therefore cannot exert the effect of suppressing the volume change of the negative electrode during charge and discharge.

[0063] In the silicon-based active material of this embodiment, pores are formed so as to satisfy the following conditions 1 and 2: Condition 1: The amount of pores with a pore diameter of 30 nm or less is 0.2 cm ３ / g or more.

[0064] Condition 2: The proportion of pores with a pore diameter of 30 nm or less to the total pore diameter of 100 nm or less is 80% or more. In other words, the silicon-based active material of this embodiment has a high proportion of fine pores with a pore diameter of 30 nm or less and a low proportion of large pores with a pore diameter of more than 30 nm. In this case, the skeletal portion can be formed thick while maintaining the total pore volume. This prevents the skeletal portion from breaking and the pores from being crushed due to stress such as expansion pressure during volume expansion. As a result, the effect of suppressing volume change of the negative electrode based on porous silicon can be more reliably obtained.

[0065] Furthermore, the silicon-based active material of this embodiment has an oxygen content of 10% by mass or less. When fine pores that satisfy the above conditions 1 and 2 are formed, the specific surface area of ​​the porous silicon increases. When the specific surface area increases, the amount of silicon oxide produced by surface oxidation during production, etc. increases, resulting in a decrease in the capacity of the silicon-based active material. By setting the oxygen content to 10% by mass or less, it is possible to suppress the decrease in capacity caused by an increase in the specific surface area.

[0066] Furthermore, the silicon-based active material of this embodiment has a high proportion of fine pores with a pore diameter of 30 nm or less and a low proportion of large pores with a pore diameter of more than 30 nm. In this case, the skeletal portion can be formed thick while maintaining the total pore volume. This prevents the skeletal portion of the silicon-based active material from being crushed and the pores from being crushed by the pressing pressure during battery production.

[0067] <Effects> According to this embodiment, the following effects can be obtained. (1) The silicon-based active material has pores that satisfy the above conditions 1 and 2, and the oxygen content is 10 mass % or less. By using the silicon-based active material having the above configuration as the negative electrode active material of an electricity storage device, it is possible to realize an electricity storage device that has a high capacity and in which the volume change of the negative electrode during charging and discharging is small.

[0068] (2) The silicon-based active material has a ratio of pores with a diameter of more than 3 nm to pores with a diameter of 30 nm or less of 90% or more, and the ratio of pores with a diameter of 3 nm or less to pores with a diameter of 0.05 cm ３ / g or less. In other words, the pores of the silicon-based active material are mostly pores with a pore diameter of 3 nm or more and 30 nm or less, and there are few fine pores with a pore diameter of 30 nm or less. In this case, the effect of suppressing the volume change of the negative electrode during charge and discharge can be more significantly obtained.

[0069] (3) The silicon-based active material has a specific surface area of ​​100 m ２ / g or more 250m ２ In this case, the effect of suppressing the capacity decrease based on the oxygen content being 10 mass % or less can be more significantly obtained.

[0070] (4) The silicon-based active material has an average particle diameter of 1 μm or more and 3 μm or less. In this case, the effect of (1) above is more pronounced. (5) The silicon-based active material has a particle aspect ratio of 1 or more and 1.7 or less. In this case, the distance required for a charge carrier to wrap around the silicon-based active material particle in the negative electrode active material layer can be shortened. This shortens the conduction path of charge carriers in the thickness direction of the negative electrode active material layer, thereby improving the ionic conductivity of the negative electrode active material layer.

[0071] (6) The silicon-based active material is an active material for an all-solid-state battery. In an all-solid-state battery, a high pressing pressure is applied to bond the positive electrode active material layer of the positive electrode 32, the solid electrolyte layer 33, and the negative electrode active material layer of the negative electrode 34 together. Therefore, during the manufacturing process, the skeleton of the silicon-based active material may be broken, causing the pores to collapse. Therefore, when used as an active material for an all-solid-state battery, it is particularly effective to suppress the volume change of the negative electrode during charge and discharge based on the pores that satisfy the above conditions 1 and 2.

[0072] Example 1: Preparation of porous silicon (Mg ２Synthesis of Si) Si powder (30 g) of 300 μm or less and metallic Mg powder (53.5 g) of 180 μm or less were mixed. The resulting mixture was placed in a stainless steel container with a lid and heated at 600° C. for 6 hours in an Ar atmosphere to obtain powdered Mg. ２ Si was obtained.

[0073] (Reduction Step) As shown in FIG. 3, a Si raw material is placed in a raw material tray 11 in a stainless steel reaction vessel 10, and Mg ２ Si was placed in the reaction vessel 10. The reaction vessel 10 was placed in a vacuum furnace 13, and the vacuum furnace 13 was heated at 650°C for 12 hours while evacuating with a rotary pump. The pressure inside the vacuum furnace 13 was 1 Pa. After the heat treatment, the powdery intermediate product was collected from the raw material tray 11. The raw material used was SiO powder (30 g) with an average particle size of 1.0 μm and an aspect ratio of 1.68. The amount of Mg alloy charged was 36.5 g.

[0074] (Washing step) The intermediate product was added to a 15% by mass HCl aqueous solution and stirred for 20 hours or more while maintaining the temperature at 0° C., and then the solid content was collected by filtration. Next, the collected solid content was washed successively with a 1% acetic acid aqueous solution and ethanol while maintaining the temperature at 0° C., and then vacuum dried at 120° C. for 12 hours to obtain powdery porous silicon.

[0075] Examples 2 to 5: Preparation of porous silicon Example 2 Porous silicon was obtained in the same manner as in Example 1, except that the heat treatment temperature was changed to 700°C.

[0076] Example 3 Porous silicon was obtained in the same manner as in Example 1, except that the heat treatment temperature was changed to 680°C.

[0077] (Example 4) The Si raw material was SiO ２ Porous silicon was obtained in the same manner as in Example 1, except that the powder was changed to the above.

[0078] (Example 5) The Si raw material was SiO ２ Porous silicon was obtained in the same manner as in Example 1, except that the powder was changed to the above.

[0079] Example 6 Porous silicon was obtained in the same manner as in Example 1, except that the heat treatment temperature was changed to 750°C.

[0080] <Comparative Examples 1 to 6: Preparation of Porous Silicon> (Comparative Example 1) Mg pulverized to an average particle size of 3.0 μm ２ The Si powder was fired in a 5% oxygen atmosphere to decompose it into MgO and Si. The resulting mixture of MgO and Si was subjected to the same washing process as in Example 1 to obtain powdered porous silicon.

[0081] (Comparative Example 2) The Si raw material was SiO ２ Porous silicon was obtained in the same manner as in Example 1, except that the powder was changed to that of Example 1 and the heat treatment temperature was changed to 650°C.

[0082] (Comparative Example 3) The Si raw material was SiO ２ Porous silicon was obtained in the same manner as in Example 1, except that the powder was changed to that of Example 1, that coarse particles were removed by sieving in air after acid washing and drying, and that the heat treatment temperature was changed to 650°C.

[0083] (Comparative Example 4) The Si raw material was a SiO ２ Porous silicon was obtained in the same manner as in Example 1, except that the powder was changed to that of Example 1, that coarse particles were removed by sieving in air after acid washing and drying, and that the heat treatment temperature was changed to 600°C.

[0084] (Comparative Example 5) The Si raw material was SiO ２ Porous silicon was obtained in the same manner as in Example 1, except that the powder was changed to that of Example 1 and the heat treatment temperature was changed to 650°C.

[0085] (Comparative Example 6) The Si raw material was SiO ２ Porous silicon was obtained in the same manner as in Example 1, except that the powder was changed to that of Example 1 and the heat treatment temperature was changed to 650°C.

[0086] <Pore analysis> Using a specific surface area / pore distribution analyzer, the nitrogen adsorption / desorption isotherms of the porous silicon particles of each example and each comparative example were measured by gas adsorption. Based on the obtained nitrogen adsorption / desorption isotherms, a pore distribution plot was created using the BJH method and the BET method, and the peak value of the pore diameter peak, the amount of pores by size, and the specific surface area (BET value) were determined. The results are shown in Tables 2 and 3. The peak value is the value of the main peak of the pore distribution plot.

[0087] <Measurement of Oxygen Amount> The oxygen amount of the porous silicon particles of each example and each comparative example was measured using an oxygen / nitrogen / hydrogen analyzer manufactured by LECO Corp. The results are shown in Table 3.

[0088] <Evaluation of Battery Characteristics> (Preparation of Negative Electrode Mixture) Butyl butyrate, a 5 mass % butyl butyrate solution of a polyvinylidene fluoride (PVDF) binder, vapor grown carbon fiber (VGCF) as a conductive additive, electrode active material particles of each example, and Li as a sulfide solid electrolyte were placed in a polypropylene container. ２ S-P ２ S ５ The glass ceramic was added and stirred for 30 seconds using an ultrasonic disperser (UH-50 manufactured by SMT Co., Ltd.). Next, the container was shaken for 30 minutes using a shaker (TTM-1 manufactured by Shibata Scientific Co., Ltd.) to obtain a negative electrode composite slurry.

[0089] (Formation of negative electrode active material layer) The obtained negative electrode mixture slurry was applied onto a copper (Cu) foil serving as a negative electrode current collector layer by a blade method using an applicator, and dried on a hot plate heated to 100°C for 30 minutes, thereby forming a negative electrode active material layer on the negative electrode current collector layer.

[0090] (Formation of Solid Electrolyte Layer) A polypropylene container was charged with heptane, a 5 mass % heptane solution of a butylene rubber (BR) binder, and Li as a sulfide solid electrolyte. ２ SP ２ S ５ The glass ceramic was added and stirred for 30 seconds using an ultrasonic disperser (UH-50 manufactured by SMT Co., Ltd.) The container was then shaken for 30 minutes using a shaker (TTM-1 manufactured by Shibata Scientific Co., Ltd.) to obtain a solid electrolyte slurry.

[0091] The obtained solid electrolyte slurry was applied onto an aluminum (Al) foil serving as a release sheet by a blade method using an applicator, and then dried on a hot plate heated to 100° C. for 30 minutes to form a solid electrolyte layer. A plurality of solid electrolyte layers were produced.

[0092] (Preparation of Positive Electrode Mixture) A polypropylene container was charged with butyl butyrate, a 5 mass % butyl butyrate solution of a PVDF-based binder, and LiNi particles having an average particle size of 6 μm as a positive electrode active material. １／３ Co １／３ Mn １／３ O ２ , Li as a sulfide solid electrolyte ２ S-P ２ S ５ The glass ceramic and VGCF as a conductive additive were added to a container and stirred for 30 seconds with an ultrasonic disperser (UH-50 manufactured by SMT Co., Ltd.). The container was then shaken for 3 minutes with a shaker (TTM-1 manufactured by Shibata Scientific Co., Ltd.), further stirred for 30 seconds with the ultrasonic disperser, and then shaken for 3 minutes with the shaker to obtain a slurry-like positive electrode composite (positive electrode composite slurry).

[0093] (Formation of Positive Electrode Active Material Layer) The obtained positive electrode mixture slurry was applied onto an Al foil serving as a positive electrode current collector layer by a blade method using an applicator, and the coating was dried on a hot plate heated to 100°C for 30 minutes, thereby forming a positive electrode active material layer on the positive electrode current collector layer.

[0094] (Preparation of Evaluation Battery) A positive electrode current collector layer, a positive electrode active material layer, and a first solid electrolyte layer were laminated in this order. This laminate was set in a roll press and pressed at a pressure of 100 kN / cm and a temperature of 165°C to obtain a positive electrode laminate.

[0095] The negative electrode current collector layer, the negative electrode active material layer, and the second solid electrolyte layer were laminated in this order, and the laminate was set in a roll press and pressed at a pressure of 60 kN / cm and a temperature of 25°C to obtain a negative electrode laminate.

[0096] Furthermore, the aluminum foil serving as a release sheet was peeled off from the surfaces of the solid electrolyte layers of the positive electrode laminate and the negative electrode laminate, and then the aluminum foil serving as a release sheet was peeled off from the third solid electrolyte layer.

[0097] The positive electrode laminate and the negative electrode laminate were stacked one on top of the other so that the solid electrolyte layer sides of each laminate faced the third solid electrolyte layer, and the stack was placed in a planar uniaxial press and pre-pressed at 100 MPa and 25° C. for 10 seconds. Finally, the stack was placed in a planar uniaxial press and pressed for 1 minute at a pressure of 200 MPa and a press temperature of 120° C. This produced a battery for evaluation.

[0098] (Measurement of initial charge / discharge efficiency and confining pressure fluctuation) An initial charge / discharge test was performed on each of the obtained evaluation batteries while they were constrained at a predetermined confining pressure using a constraining jig. The initial charge / discharge test conditions included constant current-constant voltage charging at a current equivalent to 0.1 C up to 4.55 V, followed by constant current discharging at a current equivalent to 0.1 C down to 3.0 V. In the initial charge / discharge test, the initial charge capacity (mAh) and initial discharge capacity (mAh) were measured, and the initial charge efficiency was calculated based on the following formula. The results are shown in Table 3.

[0099] Initial charge efficiency (%) = initial discharge capacity / initial charge capacity × 100. Furthermore, during the initial charge / discharge test, the confining pressure of the evaluation battery was monitored using a load cell. Based on the measured values ​​of the confining pressure before charge / discharge and the confining pressure at the time when the charge per unit weight of Si reached 1000 mAh / g during the initial charge, the increase in confining pressure from the state before charge / discharge was calculated. Then, the normalized confining pressure fluctuation (hereinafter referred to as confining pressure fluctuation) was calculated based on the following formula. The results are shown in Table 3.

[0100] Confining pressure fluctuation (MPa / mAh) = Confining pressure increase (ΔMPa) / Charging capacity (mAh)

[0101]

[0102]

[0103] As shown in Table 3, Comparative Example 1 is an example in which the amount of pores with a pore diameter of 30 nm or less is small, and does not satisfy Conditions 1 and 2. On the other hand, Examples 1 to 6 and Comparative Examples 2 to 6 are examples in which Conditions 1 and 2 are satisfied.

[0104] Condition 1: The amount of pores with a pore diameter of 30 nm or less is 0.2 cm ３ Condition 2: The proportion of pores with a diameter of 30 nm or less to the total number of pores with a diameter of 100 nm or less is 80% or more.

[0105] When Comparative Example 1 was used, the value of the confining pressure fluctuation was 0.361. In contrast, when Examples 1 to 6 and Comparative Examples 2 to 6 were used, the value of the confining pressure fluctuation was less than 0.3, and the confining pressure fluctuation was significantly reduced. The reduction in the confining pressure fluctuation means that the volume change of the negative electrode during charge and discharge is suppressed. From these results, it can be seen that by using porous silicon having pores that satisfy the above conditions 1 and 2 as the negative electrode active material, the volume change of the negative electrode during charge and discharge can be suppressed.

[0106] Next, Examples 1 to 6 and Comparative Examples 2 to 6 differ in the amount of oxygen. In Examples 1 to 6, the amount of oxygen is 10% by mass or less, and in Comparative Examples 2 to 6, the amount of oxygen is greater than 10% by mass. The initial charge-discharge efficiency when Comparative Examples 2 to 6 are used is 71.7 to 76.3. In contrast, the initial charge-discharge efficiency when Examples 1 to 6 are used is 80.4 to 87.1, which is significantly higher. From these results, it can be seen that by using porous silicon having pores that satisfy Conditions 1 and 2 and an oxygen amount of 10% by mass or less as the negative electrode active material, it is possible to obtain both the effect of suppressing volume change of the negative electrode during charge and discharge and the effect of increasing capacity.

[0107] A...Si raw material B...Low equilibrium pressure Mg alloy L...Charge carrier P...Silicon-based active material particles 10...Reaction vessel 11...Raw material tray 11a...Leg 12...Reduction tray 13...Vacuum furnace 20...Anode 21...Anode current collector 22...Anode active material layer 30...All-solid-state battery 31...Battery element 32...Cathode 33...Solid electrolyte layer 34...Anode

Claims

1. Porous silicon particles with pores, the amount of pores with a diameter of 30 nm or less is 0.2 cm ３ / g or more, a ratio of pores with a pore diameter of 30 nm or less to pores with a pore diameter of 100 nm or less is 80% or more, and an oxygen content is 10 mass % or less.

2. The silicon-based active material according to claim 1, wherein the proportion of pores having a diameter of more than 3 nm to the total number of pores having a diameter of 30 nm or less is 90% or more.

3. The amount of pores with a diameter of 3 nm or less is 0.05 cm ３ The silicon-based active material according to claim 1 or 2, wherein the SiO 2 content is 0.15 / g or less.

4. The specific surface area is 100m ２ / g or more 250m ２ The silicon-based active material according to any one of claims 1 to 3, wherein the silicon-based active material has a molecular weight of 1 / g or less.

5. The silicon-based active material according to any one of claims 1 to 4, wherein the average particle size is 1 μm or more and 3 μm or less.

6. The silicon-based active material according to any one of claims 1 to 5, wherein the particle aspect ratio is 1 or more and 1.7 or less.

7. A negative electrode active material layer containing the silicon-based active material according to any one of claims 1 to 6.

8. An all-solid-state battery comprising the negative electrode active material layer according to claim 7.

9. A method for producing a silicon-based active material according to any one of claims 1 to 6, comprising: a reduction step of bringing a Si raw material containing SiO into contact with Mg vapor under reduced pressure and under conditions of an Mg vapor pressure equal to or lower than the equilibrium pressure of the following reaction formula (1), thereby obtaining an intermediate product containing Si and MgO; and a washing step of removing MgO from the intermediate product.

10. The method for producing a silicon-based active material according to claim 9, wherein the cleaning step includes a first cleaning step and a second cleaning step, the first cleaning step being a step of producing a magnesium salt from MgO using an acid, and the second cleaning step being a step of washing away the magnesium salt using a rinse liquid.

11. The method for producing a silicon-based active material according to claim 9 or 10, wherein the rinse liquid is an acidic aqueous solution having a pH of 6 or less.

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