Living substance

Abstract

To provide an active material in which expansion during insertion of a metal ion such as Li ion is suppressed.SOLUTION: An active material includes a silicon clathrate type crystal phase, includes an Na element, an Si element, and an M element which is a metal element having a larger ion radius than Si element, and the ratio of the M element with respect to the total of the Si element and the M element is 0.1 atm% or more and 5 atm% or less.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to an active material in which expansion during insertion of metal ions such as Li ions is suppressed.

Background Art

[0002] In recent years, battery development has been actively carried out. For example, in the automotive industry, the development of batteries used in electric vehicles or hybrid vehicles has been advanced. Also, Si is known as an active material used in batteries.

[0003] Although not an invention related to batteries, Patent Document 1 discloses a method for manufacturing a type II Si-based clathrate containing Na, which includes a positive pressure heat treatment step of mixing Si powder, Ge powder, and Na and heating them at a temperature of 650°C or higher to form a compound composed of Si, Ge, and Na, and a negative pressure heat treatment step of heating the compound composed of Si, Ge, and Na formed by the positive pressure heat treatment step at a temperature of 300°C or higher and 450°C or lower for 2 hours or more and 72 hours or less under a negative pressure of 10 -2 Pa or less.

[0004] Patent Document 2 discloses an electrode active material containing a clathrate compound including a crystal lattice and a guest substance encapsulated in the crystal lattice, and Patent Document 3 discloses an electrode active material containing the clathrate compound and a predetermined amount of a carbonaceous substance dispersed in the electrode active material. Patent Documents 2 and 3 disclose that the guest substance includes at least one selected from the group consisting of barium (Ba), calcium (Ca), and lithium (Li), and the crystal lattice includes at least one selected from the group consisting of gallium (Ga), aluminum (Al), indium (In), silver (Ag), gold (Au), copper (Cu), nickel (Ni), and cobalt (Co), and at least one selected from the group consisting of silicon (Si) and tin (Sn).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0006] Si has a large theoretical capacity and is effective for increasing the energy density of batteries. On the other hand, Si has a large volume change during charge and discharge. Therefore, an active material with a small volume change during charge and discharge is required. In particular, an active material in which expansion during insertion of metal ions such as Li ions is suppressed is required.

[0007] The present disclosure has been made in view of the above circumstances, and the main object thereof is to provide an active material in which expansion during insertion of metal ions such as Li ions is suppressed.

Means for Solving the Problems

[0008] In order to solve the above problems, in the present disclosure, there is provided an active material having a silicon class rate type crystal phase and containing an Na element, an Si element, and an M element which is a metal element having an ionic radius larger than that of the Si element, and the ratio of the M element to the total of the Si element and the M element is 0.1 atm% or more and 5 atm% or less.

[0009] According to the present disclosure, by including the M element, an active material in which expansion during insertion of metal ions is suppressed can be obtained.

[0010] In the above disclosure, the M element may contain at least one of Ge, Ga, and Al.

[0011] In the above disclosure, the M element may contain at least Ge.

[0012] In the above disclosure, the proportion of the M element may be 0.5 atm% or more and 3 atm% or less.

[0013] In the above disclosure, the active material is Na w Me x M y+z Si 136-z (Me is a metal element other than the Na element and the M element, and w, x, y, and z satisfy 0 < w, 0 ≦ x, 0 ≦ y, x + y ≦ w, 0 < w + x + y ≦ 24, 0.136 ≦ z ≦ 6.8) and may have a composition represented thereby.

[0014] In the above disclosure, w may be 15 or less.

Advantages of the Invention

[0015] In the present disclosure, there is an effect that an active material in which expansion during insertion of metal ions is suppressed can be provided.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0017] Hereinafter, the active material in the present disclosure will be described.

[0018] The active material in the present disclosure has a silicon clathrate-type crystal phase and contains Na element, Si element, and M element which is a metal element having an ionic radius larger than that of Si element. The ratio of the M element to the total of the Si element and the M element is 0.1 atm% or more and 5 atm% or less. In the following description, a compound (active material) having a silicon clathrate-type crystal phase may sometimes be described as a silicon clathrate compound.

[0019] According to the present disclosure, by containing the M element, an active material in which expansion during insertion of metal ions is suppressed can be obtained.

[0020] A compound having a silicon clathrate-type crystal phase has a cage-type structure for the framework atoms, and metal ions such as Li ions can enter therein. Further, even when metal ions enter, the amount of expansion is small and the volume change due to charge and discharge is small. Specific examples will be described below. Examples of the silicon clathrate-type crystal phase include silicon clathrate type I and type II crystal phases. In the silicon clathrate type I and type II crystal phases, as shown in FIGS. 1(a) and (b), polyhedra containing pentagons or hexagons are formed by a plurality of Si elements. The polyhedron has a space inside that can enclose metal ions such as Li ions. By inserting metal ions into this space, the volume change due to charge and discharge can be suppressed. Further, the silicon clathrate type I and type II crystal phases have an advantage that the crystal structure is likely to be maintained even when charge and discharge are repeated because they have a space inside that can enclose metal ions. On the other hand, ordinary Si particles have a diamond-type crystal phase. In the diamond-type crystal phase, as shown in FIG. 1(c), tetrahedrons are formed by a plurality of Si elements. Since the tetrahedron does not have a space inside that can enclose metal ions such as Li ions, the volume change due to charge and discharge is large. Therefore, the application of silicon clathrate type I and type II to batteries is being studied.

[0021] Compounds with silicon clathrate crystal phases have a small expansion amount even when metal ions are inserted into the cage structure. However, if the amount of metal ions inserted is increased in order to increase the capacity of the battery, the amount of voids in the cage structure may not be sufficient, and the active material may expand.

[0022] In view of the above situation, the present inventors have conducted extensive research and found that the expansion during insertion of metal ions can be further suppressed by adding an M element, which is a metal element having an ionic radius larger than that of the Si element, to a silicon clathrate compound containing Na and Si elements. The reason for this is presumed to be as follows. It is presumed that the skeleton size (cage size) increases as a result of a part of the Si element constituting the cage structure being replaced with the M element, and the space capable of containing metal ions becomes larger. As a result, it is presumed that the amount of expansion caused by the insertion of metal ions into the space can be further reduced.

[0023] In addition, Patent Document 1 discloses a method for producing a silicon clathrate compound having a silicon clathrate II type crystal phase containing Na, Si, and Ge elements. However, Patent Document 1 does not disclose anything about the function of the above-mentioned silicon clathrate compound as an active material of a battery.

[0024] In addition, Patent Document 2 and Patent Document 3 disclose the use of a silicon clathrate compound as an active material for a lithium ion battery. However, neither of these documents discloses a silicon clathrate compound containing Na element. Hereinafter, the active material in this disclosure will be described in detail.

[0025] 1.Active material The active material in the present disclosure contains at least Na element, Si element, and M element. The M element is a metal element with an ionic radius larger than that of the Si element. Here, since the Si element usually has a tetravalent ionic radius of 0.40 Å, the ionic radius of the M element is larger than 0.40 Å. The ionic radius of the M element may be, for example, larger than 0.40 Å, 0.41 Å or more, may be larger than 0.41 Å, or may be 0.50 Å or more. Also, the ionic radius of the M element may be, for example, 0.70 Å or less, or 0.65 Å or less. It is preferable that the ionic radius of the M element is close to that of the Si element.

[0026] As the M element, for example, it is preferable that the period or group in the periodic table is close to that of the Si element. Examples of the M element include Al element (ionic radius: 0.535 Å), Ga element (ionic radius: 0.62 Å), and Ge element (ionic radius: 0.53 Å). In the present disclosure, it is preferable that the M element is the Ga element or the Ge element, and more preferably the Ge element. Note that the M element may be one of the above-described elements, or may contain two or more elements.

[0027] The ratio of the M element to the total of the Si element and the M element in the active material is, for example, 0.1 atm% or more, and may be 0.5 atm% or more. Also, the above ratio may be, for example, 5 atm% or less, may be 3 atm% or less, or may be 1 atm% or less. The above ratio of the M element can be measured, for example, by X-ray photoelectron spectroscopy (XPS). If the ratio of the M element is too large, the M element may not form a solid solution substitution with Si, and there is a possibility of forming a clathrate structure between M-M.

[0028] The active material in the present disclosure preferably contains M element as a framework atom. In this case, the active material in the present disclosure can be regarded as a clathrate compound in which a part of the Si element, which is the framework atom, is substituted with the M element with respect to a silicon clathrate compound containing only the Si element as the framework atom. Also, the M element can be regarded as a foreign element with respect to the Si element. Further, the active material in the present disclosure may contain the M element as a framework atom and further as a guest atom.

[0029] The active material in the present disclosure has a silicon clathrate-type crystal phase. Examples of the silicon clathrate-type crystal phase include a silicon clathrate type I crystal phase and a silicon clathrate type II crystal phase. In the present disclosure, it is preferable that the active material contains at least one of the silicon clathrate type I and type II crystal phases. Also, the active material may have a silicon clathrate type I or type II crystal phase as the main phase. Here, "having a silicon clathrate type I or type II crystal phase as the main phase" means that any peak belonging to the silicon clathrate type I or type II crystal phase is the peak with the highest diffraction intensity among the peaks observed by X-ray diffraction measurement. Also, the active material may have a silicon clathrate type I or type II crystal phase as a single phase. On the other hand, the active material may have both the silicon clathrate type I and type II crystal phases. Conversely, the active material may have a silicon clathrate type I crystal phase and not have a silicon clathrate type II crystal phase, or may have a silicon clathrate type II crystal phase and not have a silicon clathrate type I crystal phase.

[0030] The crystal phase of silicon class rate I usually belongs to the space group (Pm-3n). Also, the crystal phase of silicon class rate II usually belongs to the space group (Fd-3m). Note that the crystal phases of silicon class rate I and II usually contain at least the Na element, the Si element, and the M element which is a metal element with an ionic radius larger than that of the Si element. Also, it may or may not contain the Me element which is a metal element other than the Na element and the M element.

[0031] The crystal phase of silicon class rate II has typical peaks at positions of 2θ = 20.09°, 21.00°, 26.51°, 31.72°, 36.26°, 53.01° in X-ray diffraction measurement using CuKα radiation. These peak positions may shift back and forth within a range of ±1.00°, may shift back and forth within a range of ±0.50°, or may shift back and forth within a range of ±0.30°. Note that when metal ions such as lithium ions are inserted into the crystal phase of silicon class rate II, peak shift may occur. Therefore, it is preferable to perform XRD measurement in a state where no metal ions are inserted.

[0032] The crystal phase of silicon class rate I has typical peaks at positions of 2θ = 19.44°, 21.32°, 30.33°, 31.60°, 32.82°, 36.29°, 52.39°, 55.49° in X-ray diffraction measurement using CuKα radiation. These peak positions may shift back and forth within a range of ±1.00°, may shift back and forth within a range of ±0.50°, or may shift back and forth within a range of ±0.30°. Note that when metal ions such as lithium ions are inserted into the crystal phase of silicon class rate I, peak shift may occur. Similar to the crystal phase of silicon class rate II, it is preferable to perform XRD measurement in a state where no metal ions are inserted.

[0033] Here, the diffraction intensity of the peak at 2θ = 26.51° ± 1.00° in the crystal phase of silicon class rate II is I Aand set the diffraction intensity of the peak at 2θ = 32.82° ± 1.00° in the crystal phase of silicon class rate I to I B For example, when the active material has a crystal phase of silicon class rate II, I A The ratio of I B to I B / I A value) may be, for example, 1 or less, 0.8 or less, 0.6 or less, 0.4 or less, 0.2 or less, 0.1 or less. On the other hand, the value of I B / I A may be 0 or greater than 0. Also, when the active material has a crystal phase of silicon class rate I, the ratio of I B to I A (I A / I B value) may be within the same range as the value of I B / I A described above.

[0034] The active material in the present disclosure may have a composition represented by, for example, Na w Me x M y+z Si 136-z (where M is a metal element having a larger ionic radius than the Si element, Me is a metal element other than the Na element and the M element, and w, x, y, and z satisfy 0 < w, 0 ≦ x, 0 ≦ y, x + y ≦ w, 0 < w + x + y ≦ 24, 0.136 ≦ z ≦ 6.8). In the above composition, w may be, for example, 15 or less, and may be 10 or less. On the other hand, w may be, for example, 1 or more, 3 or more, and 5 or more. If the value of w is within the above range, the active material will be one in which expansion during the insertion of metal ions (carrier ions) is more suppressed. This is considered to be because the Na content is low and the space for inserting carrier ions such as Li ions becomes large. Me is not particularly limited as long as it is an element other than the Na element and the M element, that is, an element other than the Na element and having an ionic radius equal to or less than that of the Si element. Also, the active material in the present disclosure is Na w Me x M y+z Si46-z (M is a metal element with an ionic radius larger than that of the Si element, Me is a metal element other than the Na element and the M element, and w, x, y, and z satisfy 0 < w, 0 ≦ x, 0 ≦ y, x + y ≦ w, 0 < w + x + y < 8, 0.046 ≦ z ≦ 2.3) and may have a composition represented thereby.

[0035] Examples of the shape of the active material in the present disclosure include particulate. The average primary particle diameter of the active material is, for example, 50 nm or more, may be 100 nm or more, and may be 150 nm or more. On the other hand, the average primary particle diameter of the active material is, for example, 3000 nm or less, may be 1500 nm or less, and may be 1000 nm or less. Further, the average secondary particle diameter of the active material is, for example, 1 μm or more, may be 2 μm or more, may be 5 μm or more, and may be 7 μm or more. On the other hand, the average secondary particle diameter of the active material is, for example, 60 μm or less, and may be 40 μm or less. The average primary particle diameter and the average secondary particle diameter can be determined, for example, by observation using SEM. The number of samples is preferably large, for example, 20 or more, may be 50 or more, and may be 100 or more. The average primary particle diameter and the average secondary particle diameter can be appropriately adjusted, for example, by appropriately changing the manufacturing conditions of the active material or performing a classification process.

[0036] Note that, as shown in Reference Example 5 described later, when a system containing a P element (ionic radius: 0.38 Å) with an ionic radius smaller than that of the Si element is simulated instead of the M element in the silicon clathrate compound, it is suggested that the system containing the P element is more stable and less likely to expand compared to the system containing only the Si element as the framework atom. From this, it is suggested that even when an element with a slightly smaller ionic radius than the Si element is included, the expansion of the active material during the insertion of metal ions can be suppressed. Therefore, in the present disclosure, an active material having a clathrate-type crystal phase, containing a Na element, a Si element, and an Mf element which is a non-metallic element with an ionic radius of 0.38 Å or more, and the ratio of the Mf element to the total of the Si element and the Mf element is 0.1 atm% or more and 5 atm% or less can also be provided. The ionic radius of the Mf element is, for example, 0.38 Å or more, and may be 0.40 Å or more. Further, the ionic radius of the Mf element may be, for example, 0.70 Å or less, and may be 0.65 Å or less. Examples of the Mf element include P (phosphorus).

[0037] 2. Method for manufacturing active material The method for manufacturing the active material in the present disclosure is not particularly limited. For example, a first heat treatment step of performing heat treatment on a mixture containing Si simple substance, Na simple substance, and M simple substance (metal M) to synthesize a NaMSi compound having a zintl phase, and a second heat treatment step of performing heat treatment on the NaMSi compound under reduced pressure to desorb Na can be mentioned.

[0038] In the first heat treatment step, the ratios of elemental Si, elemental Na, and elemental M are not particularly limited. For example, with respect to a total of 100 mole parts of elemental Si and elemental M, elemental M may be, for example, 0.1 mole part or more, and may be 0.5 mole part or more. Also, with respect to a total of 100 mole parts of elemental Si and elemental M, elemental M may be, for example, 5 mole parts or less, and may be 1 mole part or less. Further, with respect to a total of 1 mole part of elemental Si and elemental M, elemental Na may be, for example, 0.8 mole part or more, may be 1 mole part or more, and may be 1.1 mole part or more. On the other hand, with respect to a total of 1 mole part of elemental Si and elemental M, elemental Na may be, for example, 1.5 mole parts or less, may be 1.3 mole parts or less, and may be 1.2 mole parts or less.

[0039] The heat treatment temperature in the first heat treatment step is, for example, 500°C or higher and 1000°C or lower. Also, the heat treatment time in the first heat treatment step is, for example, 1 hour or longer and 50 hours or shorter. In particular, by performing the heat treatment under at least one of the conditions of about 700°C (for example, 650°C or higher and 750°C or lower) and about 20 hours (for example, 15 hours or longer and 25 hours or shorter), a desired active material is likely to be obtained.

[0040] In the second heat treatment step, the pressure during heat treatment is, for example, 10 Pa or less, may be 1 Pa or less, or may be 0.1 Pa or less. Also, the heat treatment temperature in the second heat treatment step is, for example, 300 °C or higher, may be 350 °C or higher. Also, the heat treatment temperature is, for example, 650 °C or lower. Also, the heat treatment time in the second heat treatment step is, for example, 30 minutes or longer. Also, the heat treatment time is, for example, 25 hours or shorter, or may be 20 hours or shorter. In the second heat treatment step, the heat treatment conditions may be changed. For example, after heat treatment under at least one of the conditions of about 340 °C (for example, 300 °C or higher and 400 °C or lower) and about 15 hours (for example, 12 hours or longer and 18 hours or shorter), heat treatment is performed under at least one of the conditions of about 430 °C (for example, 400 °C or higher and 500 °C or lower) and about 6 hours (for example, 3 hours or longer and 9 hours or shorter), whereby the desired active material can be easily obtained. Also, in the second heat treatment step performed by changing the heat treatment conditions, after the heat treatment at about 430 °C for about 6 hours, the NaMSi compound is once cooled (for example, to less than 340 °C), and then heat treatment may be performed under at least one of the conditions of about 430 °C (for example, 400 °C or higher and 500 °C or lower) and about 1 hour (for example, 30 minutes or longer and 2 hours or shorter). With such a method, Na can be more easily desorbed from the NaMSi compound. Since Na has a large atomic radius, it is considered difficult to extract it from the cage-type structure, but in the present disclosure, it is presumed that the heat treatment temperature can be lowered because the cage can be enlarged by containing the M element. The active material in the present disclosure can be obtained by the second heat treatment step. Note that the method for desorbing Na is not limited to heat treatment under vacuum.

[0041] The method for producing the active material in the present disclosure may optionally have a pulverization step of pulverizing the active material. Examples of the pulverization method include a pulverization method using any of a mortar, a ball mill, a jet mill, and a bead mill.

[0042] 3. Use The active material in the present disclosure is usually used in a battery. The active material in the present disclosure may be a negative electrode active material or a positive electrode active material, but the former is preferred. In the present disclosure, for example, a battery having a positive electrode layer, an electrolyte layer, and a negative electrode layer in this order in the thickness direction can be provided, and the negative electrode layer contains the above-described active material.

[0043] The negative electrode layer is a layer containing at least a negative electrode active material. The negative electrode layer may contain only a silicon clathrate compound as the negative electrode active material, or may further contain other active materials. In the latter case, the proportion of the silicon clathrate compound in all the negative electrode active materials may be, for example, 50% by weight or more, 70% by weight or more, or 90% by weight or more. Also, the proportion of the negative electrode active material in the negative electrode layer is, for example, 20% by weight or more and 80% by weight or less.

[0044] In addition, the negative electrode layer may contain at least one of the electrolyte, conductive material, and binder described below as needed. As the conductive material and binder, known ones used in batteries can be used. The thickness of the negative electrode layer is, for example, 0.1 μm or more and 1000 μm or less.

[0045] The positive electrode layer is a layer containing at least a positive electrode active material. In addition, the positive electrode layer may contain at least one of the electrolyte, conductive material, and binder described below as needed. Since the content of the conductive material, binder, the proportion of the positive electrode active material in the positive electrode layer, and the thickness of the positive electrode layer can be the same as those described in the section of the negative electrode layer above, the description here is omitted.

[0046] The electrolyte layer is a layer formed between the positive electrode layer and the negative electrode layer and contains at least an electrolyte. The electrolyte may be a solid electrolyte, a liquid electrolyte (electrolyte solution), or a mixture thereof. Among them, the electrolyte is preferably a solid electrolyte. In an all-solid-state battery having a solid electrolyte layer as the electrolyte layer, it is preferable to further include a restraining jig that applies a restraining pressure along the thickness direction to the positive electrode layer, the electrolyte layer, and the negative electrode layer. When the negative electrode active material expands, it is necessary to increase the restraining pressure of the battery, and the weight of the restraining jig increases. In the present disclosure, since the expansion of the active material can be suppressed, an increase in the restraining pressure of the battery can be suppressed, and the weight of the restraining jig can be reduced.

[0047] Typical examples of the solid electrolyte include inorganic solid electrolytes such as sulfide solid electrolytes, oxide solid electrolytes, nitride solid electrolytes, and halide solid electrolytes; and organic polymer electrolytes such as polymer electrolytes.

[0048] The battery in the present disclosure has at least the negative electrode layer, the positive electrode layer, and the electrolyte layer described above. Further, it usually has a positive electrode current collector that collects current from the positive electrode layer and a negative electrode current collector that collects current from the negative electrode layer. Also, the battery in the present disclosure may further include a restraining jig that applies a restraining pressure along the thickness direction to the positive electrode layer, the electrolyte layer, and the negative electrode layer. As the restraining jig, a known jig can be used. The restraining pressure is, for example, 0.1 MPa or more, and may be 1 MPa or more, or may be 5 MPa or more. On the other hand, the restraining pressure is, for example, 100 MPa or less, and may be 50 MPa or less, or may be 20 MPa or less.

[0049] The battery in the present disclosure is usually a battery in which metal ions are conducted between the positive electrode layer and the negative electrode layer. Examples of such batteries include lithium ion batteries, sodium ion batteries, potassium ion batteries, magnesium ion batteries, and calcium ion batteries. The battery in the present disclosure may be a liquid battery in which the electrolyte layer contains an electrolytic solution, or may be an all-solid-state battery in which the electrolyte layer contains a solid electrolyte. The battery in the present disclosure may be a primary battery or a secondary battery, but is preferably a secondary battery. This is because it can be repeatedly charged and discharged, and is useful, for example, as an in-vehicle battery.

[0050] The present disclosure is not limited to the above-described embodiments. The above-described embodiments are merely examples, and anything that has substantially the same configuration as the technical idea described in the claims of the present disclosure and exhibits similar effects is included in the technical scope of the present disclosure. EXAMPLES

[0051] [Example 1] (Synthesis of negative electrode active material) A mixture of Si (purity: 99.999%), Ge, and Na (purity: 99.5%) was weighed in a molar ratio of Si:Ge:Na = 0.999:0.001:1.1, and the mixture was placed in a boron nitride crucible and sealed under an Ar atmosphere. In the mixture, the ratio of Ge to the total of Si and Ge was 0.1 atm%. Then, heat treatment was performed at 700 ° C. for 20 hours. As a result, a lump of NaGeSi compound (a NaSi compound substituted with Ge element, a compound having a Zintl phase) was obtained. The obtained NaGeSi compound was crushed and heat treated under vacuum (about 0.1 Pa) at 340 ° C. for 15 hours, and then heat treated at 430 ° C. for 6 hours to cause a Na desorption reaction. A silicon clathrate powder was obtained by the above procedure. The obtained silicon clathrate powder was crushed in a mortar. A particulate negative electrode active material was obtained by the above procedure.

[0052] (Preparation of evaluation battery) Dispersion medium (butyl butyrate), binder (butyl butyrate solution in which 5% by weight of polyvinylidene fluoride is dissolved) 0.3 g, positive electrode active material coated with lithium niobate (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 ) 2 g, solid electrolyte (Li 2 S-P 2 S 5 -based glass ceramic) 0.3 g, conductive material (VGCF (vapor grown carbon fiber)) 0.03 g were shaken in a polypropylene shaker (manufactured by Shibata Scientific Co., Ltd., product name TTM-1) for 3 minutes, and further stirred with an ultrasonic disperser for 30 seconds to prepare a paste for the positive electrode layer. The paste for the positive electrode layer was applied onto a positive electrode current collector (aluminum foil) by a blade method using an applicator, and then dried on a hot plate adjusted to 100 °C for 30 minutes. Thus, a positive electrode structure having a positive electrode layer and a positive electrode current collector was obtained.

[0053] Dispersion medium (butyl butyrate), binder (butyl butyrate solution in which 5% by weight of polyvinylidene fluoride is dissolved) 0.32 g, the above negative electrode active material particles 0.8 g, solid electrolyte (Li 2 S-P 2 S 5 -based glass ceramic) 0.6 g, conductive material (VGCF (vapor grown carbon fiber)) 0.06 g were shaken in a polypropylene shaker (manufactured by Shibata Scientific Co., Ltd., product name TTM-1) for 3 minutes, and further stirred with an ultrasonic disperser for 30 seconds to prepare a paste for the negative electrode layer. The paste for the negative electrode layer was applied onto a negative electrode current collector (copper foil) by a blade method using an applicator, and then dried on a hot plate adjusted to 100 °C for 30 minutes. Thus, a negative electrode structure having a negative electrode layer and a negative electrode current collector was obtained.

[0054] Dispersion medium (heptane), binder (heptane solution in which 5% by weight of butadiene rubber is dissolved) 0.05 g, solid electrolyte (Li containing lithium iodide 2 S-P 2 S 50.4 g of the glass ceramic system was placed in a polypropylene container and stirred for 30 seconds using an ultrasonic disperser. Thereafter, the polypropylene container was shaken for 30 minutes using a shaker to prepare a paste for the solid electrolyte layer. The paste for the solid electrolyte layer was applied to an aluminum foil as a substrate by the blade method using an applicator, and then dried for 30 minutes on a hot plate heated to 100 °C to prepare a solid electrolyte layer.

[0055] Next, a negative electrode structure, a solid electrolyte layer, and a positive electrode structure were laminated in this order, and the obtained laminate was pressed under the conditions of 130 °C, 200 MPa, and 3 minutes to obtain an evaluation battery.

[0056] [Examples 2 to 3] A negative electrode active material and an evaluation battery were obtained in the same manner as in Example 1, except that the molar ratio of the mixture was adjusted so that the ratio of Ge to the total of Si and Ge was 1 atm% (Example 2) and 5 atm% (Example 3).

[0057] [Examples 4 to 6] A negative electrode active material and an evaluation battery were obtained in the same manner as in Examples 1 to 3, except that elemental Ga was used instead of elemental Ge.

[0058] [Comparative Example 1] A negative electrode active material and an evaluation battery were obtained in the same manner as in Example 1, except that elemental Si and elemental Na were weighed at a molar ratio of elemental Si:elemental Na = 1:1.1 and the heat treatment conditions under vacuum were 450 °C and 6 hours.

[0059] [Comparative Example 2] A negative electrode active material and an evaluation battery were obtained in the same manner as in Comparative Example 1, except that the heat treatment conditions under vacuum were 450 °C and 5 hours.

[0060] [Evaluation 1] (XRD measurement) X-ray diffraction (XRD) measurements using CuKα radiation were performed on the negative electrode active materials obtained in Examples 1 to 6 and Comparative Examples 1 and 2. The results of Examples 2 and 5 and Comparative Examples 1 and 2 are shown in FIGS. 2(a) to (d). As shown in FIGS. 2(a) and (b), in Example 2 and Comparative Example 1, typical peaks of the silicon-class rate II-type crystal phase were confirmed as the main phase at positions near 2θ = 20.09°, 21.00°, 26.51°, 31.72°, 36.26°, and 53.01°, and typical peaks of the silicon-class rate I-type crystal phase (for example, near 2θ = 32.82°) were also confirmed. Although not shown, similar peaks to those in Example 2 were also confirmed in Examples 1 and 3.

[0061] As shown in FIGS. 2(c) and (d), in Example 5 and Comparative Example 2, typical peaks of the silicon-class rate I-type crystal phase were confirmed near 2θ = 19.44°, 21.32°, 30.33°, 31.60°, 32.82°, 36.29°, 52.39°, and 55.49°. Although not shown, similar peaks to those in Example 5 were also confirmed in Examples 4 and 6.

[0062] (Evaluation by lattice constant) The XRD patterns obtained in Examples 1 to 6 and Comparative Examples 1 and 2 were subjected to Rietveld analysis to determine the lattice constants. In Examples 1 to 3 and Comparative Example 1, the XRD pattern of the II-type crystal phase obtained as the main phase in the XRD pattern was subjected to Rietveld analysis to determine the lattice constant. In Examples 4 to 6 and Comparative Example 2, the XRD pattern of the I-type crystal phase was subjected to Rietveld analysis to determine the lattice constant. The results are shown in FIGS. 3(a) and (b). FIG. 3(a) shows a graph of the ratio of the lattice constants of Examples 1 to 3 when the lattice constant of Comparative Example 1 is set to 1, and FIG. 3(b) shows a graph of the ratio of the lattice constants of Examples 4 to 6 when the lattice constant of Comparative Example 2 is set to 1. From the results of FIGS. 3(a) and (b), it was confirmed that the lattice constant increased due to substitution with different elements. This suggests that substitution with different elements increases the size of the Si cage, results in a low density before Li insertion, and a low expansion amount even when Li is inserted.

[0063] (Evaluation of compressive stress) The evaluation battery was restrained using a restraining tool capable of measuring the restraining pressure with a load cell, and placed in a desiccator to evaluate the restraining pressure. The obtained evaluation battery was restrained at a pressure of 5 MPa, and a constant current was applied at 0.1 C to a voltage of 4.55 V, and the initial charge was performed. The restraining pressure of the battery at that time was monitored, and the restraining pressure at the time of full charge was used for comparison. The results are shown in Table 1. The values ​​of the restraining pressure in Examples 1 to 3 are relative values ​​when the result of Comparative Example 1 is set to 1.00, and the values ​​of the restraining pressure in Examples 4 to 6 are relative values ​​when the result of Comparative Example 2 is set to 1.00. It was confirmed that the increase in the restraining pressure can be suppressed by the substitution of a different element.

[0064] [Table 1]

[0065] [Reference examples 1~5] Using the nudged elastic band method, the formation energy and Na diffusion barrier of an unsubstituted silicon clathrate compound and a clathrate compound substituted with 2.2 atm% of a foreign element were calculated. The results are shown in Table 2. Since the diffusion barrier of Na element is lowered when a foreign element is doped, it was inferred that expansion becomes difficult when lithium is inserted. It was also suggested that the compound substituted with P is more stable than the unsubstituted one.

[0066] [Table 2]

[0067] [Example 7] A negative electrode active material and a battery for evaluation were obtained in the same manner as in Example 1, except that in the Na desorption reaction, heat treatment was performed in vacuum (about 0.1 Pa) at 375°C for 14 hours, and then at 450°C for 6 hours.

[0068] [Example 8] A negative electrode active material and an evaluation battery were obtained in the same manner as in Example 7, except that the molar ratio of the mixture was adjusted so that the ratio of Ge to the total of Si and Ge was 0.5 atm%.

[0069] [Example 9] Negative electrode active material particles and an evaluation battery were obtained in the same manner as in Example 7, except that the molar ratio of the mixture was adjusted so that the ratio of Ge to the total of Si and Ge was 1 atm%, and the heat treatment at 450 °C for 6 hours in the Na desorption reaction was changed to a heat treatment at 430 °C for 6 hours.

[0070] [Example 10] Negative electrode active material particles and an evaluation battery were obtained in the same manner as in Example 7, except that the molar ratio of the mixture was adjusted so that the ratio of Ge to the total of Si and Ge was 1 atm%.

[0071] [Example 11] A negative electrode active material and an evaluation battery were obtained in the same manner as in Example 10, except that in the Na desorption reaction, after heat treatment at 450 °C for 6 hours, the negative electrode active material was cooled and then heat treatment was performed at 450 °C for 1 hour.

[0072] [Example 12] A negative electrode active material and an evaluation battery were obtained in the same manner as in Example 7, except that the molar ratio of the mixture was adjusted so that the ratio of Ge to the total of Si and Ge was 3 atm%, and in the Na desorption reaction, heat treatment was performed under vacuum (about 0.1 Pa) at 340 °C for 14 hours, followed by heat treatment at 450 °C for 6 hours.

[0073] [Comparative Example 3] A negative electrode active material and an evaluation battery were obtained in the same manner as in Comparative Example 1, except that the Na desorption reaction was performed as follows. Heat treatment was performed under vacuum (about 0.1 Pa) at 340 °C for 20 hours, followed by heat treatment at 430 °C for 6 hours. The negative electrode active material was cooled and then heat treatment was performed at 430 °C for 1 hour.

[0074] [Comparative Example 4] The molar ratio of the mixture was adjusted so that the ratio of Ge to the total of Si and Ge was 10 atm%, and the heat treatment in the Na desorption reaction was carried out in the same manner as in Example 7 except that it was only heat treatment under vacuum (about 0.1 Pa) at 400 °C for 20 hours, to obtain a negative electrode active material and an evaluation battery.

[0075] [Evaluation 2] (SEM-EDX measurement) SEM-EDX (scanning electron microscope - energy dispersive X-ray spectroscopy) measurement was performed on the negative electrode active materials obtained in Examples 7 to 12 and Comparative Examples 3 to 4, and Na / Si was measured. Na / Si was calculated from the atomic number ratio based on the signal intensity ratio of Na and Si by EDX analysis. The amount of Na was determined from Na / Si. The results are shown in Table 3.

[0076] (Evaluation of the restraint pressure) The restraint pressure was evaluated in the same manner as in Evaluation 1. The results are shown in Table 3. The restraint pressure was evaluated as a relative value when the result of Comparative Example 3 was taken as 1.00.

[0077]

Table 3

[0078] From the substitution amount and the amount of Na shown in Table 3, it can be seen that the negative electrode active material of Example 7 has a composition of Na 11.7 Ge 0.136 Si 135.864 . This composition corresponds to the composition in Na w Me x M y+z Si 136-z where M is Ge, w is 11.7, x is 0, y is 0, and z is 0.136. Also, the negative electrode active material of the above Example 1 (Ge substitution) having the same element substitution amount as in Example 7 and manufactured by the same method is presumed to have the same composition as in Example 7. Further, the negative electrode active material of Example 4 (Ga substitution) is presumed to have the same composition as in Example 8 except for the type of M element.

[0079] Similar to Examples 1 to 6 above, it was confirmed that an increase in the restraint pressure can be suppressed by substituting different elements. Further, from Examples 9 to 11, it was confirmed that the smaller the amount of Na, the more the increase in the restraint pressure can be suppressed. This is presumably because the Na content is low and the space into which Li ions are inserted becomes large. Further, when the lattice constants were determined by XRD measurement of the negative electrode active materials of Examples 7 to 12 and Comparative Example 3 in the same manner as in Evaluation 1, it was confirmed that the lattice constant increased due to substitution of different elements in all of the examples.

Claims

1. An active material for use in a lithium ion secondary battery, It has a silicon clathrate type crystal phase, containing an Na element, an Si element, and an M element (M is at least one of Al and P); The active material has a ratio of the M element to the total of the Si element and the M element of 0.1 atm % or more and 5 atm % or less.

2. The active material according to claim 1 , wherein M comprises at least Al.

3. The active material is Na ｗ Me ｘ M ｙ＋ｚ S １３６－ｚ 3. The active material according to claim 1, having a composition represented by the formula: (Me is a metal element other than Na element and M element, and w, x, y, and z satisfy 0<w, 0≦x, 0≦y, x+y≦w, 0<w+x+y≦24, 0.136≦z≦6.8).

4. The active material of claim 3 , wherein w is 15 or less.

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