Negative electrode for lithium battery and lithium ion battery

Abstract

To provide a negative electrode for a lithium battery, etc., which can manufacture a lithium battery with an excellent capacity retention rate.SOLUTION: A negative electrode for a lithium battery includes a negative electrode active material having a silicon atom, and a boron-based solid electrolyte containing a boron-based compound having an organic cation and a boron cluster anion, and a bond exists between the silicon atom of the negative electrode active material and the boron atom of the boron-based compound contained in the boron-based solid electrolyte.SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to a negative electrode for a lithium battery and a lithium ion battery.

Background Art

[0002] Conventionally, all-solid-state batteries are known as lithium-ion batteries with excellent safety. Patent Document 1 discloses a negative electrode composite active material obtained by coating a negative electrode active material containing silicon with a sulfide solid electrolyte, a flexible crystal containing at least one cation selected from the group consisting of pyrrolidinium, tetraalkylammonium, and tetraalkylphosphonium, and a carborane-based anion, wherein the coating rate of the sulfide solid electrolyte on the surface of the negative electrode active material is 80% or more, and the volume ratio of the flexible crystal to the total of the sulfide solid electrolyte and the flexible crystal is 30% to 75% by volume. A negative electrode for a lithium ion battery is disclosed.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] All-solid-state batteries are required to have an excellent capacity retention rate such that the discharge capacity does not decrease even when charge and discharge are repeated. The present disclosure is made in view of the above circumstances. The problem to be solved by one embodiment of the present disclosure is to provide a negative electrode for a lithium battery that can produce a lithium battery having an excellent capacity retention rate. The problem to be solved by one embodiment of the present disclosure is to provide a lithium ion battery including the negative electrode for a lithium battery.

Means for Solving the Problems

[0005] Means for solving the above problems include the following embodiments. <1> A negative electrode for a lithium battery, comprising a negative electrode active material having a silicon atom, a boron-based solid electrolyte containing a boron-based compound having an organic cation and a boron cluster anion, and there is a bond between the silicon atom of the negative electrode active material and the boron atom of the boron-based compound contained in the boron-based solid electrolyte. <2> The negative electrode for a lithium battery according to <1> above, wherein the negative electrode active material is in direct contact with the boron-based solid electrolyte. <3> The negative electrode for a lithium battery according to <1> or <2> above, having a porosity of 10% or less. <4> The negative electrode for a lithium battery according to any one of <1> to <3> above, wherein the ratio of the content of the boron-based solid electrolyte to the content of the negative electrode active material is 0.5 to 2 on a volume basis. <5> A lithium ion battery comprising a positive electrode layer, a solid electrolyte layer, and a negative electrode layer composed of the negative electrode for a lithium ion battery according to any one of <1> to <4> above.

Advantages of the Invention

[0006] According to an embodiment of the present disclosure, a negative electrode for a lithium battery capable of manufacturing a lithium battery having an excellent capacity retention rate is provided. According to an embodiment of the present disclosure, a lithium ion battery including the negative electrode for a lithium battery is provided.

Brief Description of the Drawings

[0007]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0008] In the present disclosure, a numerical range indicated using "~" means a range including the numerical values described before and after "~" as the minimum value and the maximum value, respectively. In the numerical ranges described step by step in the present disclosure, the upper limit value or the lower limit value described in a certain numerical range may be replaced with the upper limit value or the lower limit value of the numerical range of other step-by-step descriptions. In the numerical ranges described in the present disclosure, the upper limit value or the lower limit value described in a certain numerical range may be replaced with the value shown in the examples. In the present disclosure, a combination of two or more preferred embodiments is a more preferred embodiment. In the present disclosure, the amount of each component means the total amount of a plurality of substances corresponding to each component unless otherwise specified when there are a plurality of substances corresponding to each component.

[0009] [Negative electrode for lithium battery] The negative electrode for a lithium battery of the present disclosure contains a negative electrode active material having a silicon atom and a boron-based solid electrolyte containing a boron-based compound having an organic cation and a boron cluster anion, and there is a bond between the silicon atom of the negative electrode active material and the boron atom of the boron-based compound contained in the boron-based solid electrolyte.

[0010] According to the negative electrode for a lithium battery of the present disclosure, a lithium battery having an excellent capacity retention rate can be manufactured. The reason for the above effect is not clear, but it is presumed as follows. By repeating the charge and discharge of the battery, the negative electrode active material contained in the conventional negative electrode for a lithium ion battery repeats expansion and contraction, resulting in the interfacial peeling between the negative electrode active material and the solid electrolyte, generating voids, and reducing the reaction area, thus causing a decrease in the capacity retention rate. In the negative electrode for a lithium battery of the present disclosure, there is a bond between the silicon atom of the negative electrode active material and the boron atom of the boron-based compound contained in the boron-based solid electrolyte, whereby the above interfacial peeling is suppressed, and thus it is presumed that the above effect is achieved. In the negative electrode for a lithium battery disclosed in Patent Document 1 or the like, since the negative electrode active material is coated with a sulfide solid electrolyte, it is presumed that a bond cannot be formed with the atoms of the flexible crystal and there is room for improvement in the capacity retention rate.

[0011] From the viewpoint of the capacity retention rate, the porosity of the negative electrode for a lithium battery is preferably 10% or less, more preferably 5% or less, still more preferably 1% or less, and may even be 0%. Note that the voids may be caused by the interfacial peeling of the negative electrode active material and the solid electrolyte. In the present disclosure, the porosity is measured as follows. First, a lithium-ion battery is manufactured using the negative electrode for a lithium-ion battery. Next, at 60 °C, the lithium-ion battery is charged under the conditions of 0.5 mA / cm 2 , 4.05 V cut-off, rested for 10 minutes, charged under the conditions of 0.5 mA / cm 2 , discharged under the conditions of 2.5 V cut-off and rested for 10 minutes, and the charge-discharge process is repeated 20 times with one cycle being one charge-discharge process. After the charge-discharge process, the negative electrode for a lithium-ion battery is cut, and a cross-sectional image thereof is obtained by a scanning electron microscope (SEM). The porosity is measured from the cross-sectional image using image processing software.

[0012] (Negative electrode active material) The negative electrode active material is not particularly limited as long as it has silicon atoms, and examples thereof include silicon, silicon alloy, silicon oxide, and the like.

[0013] The negative electrode active material is preferably in direct contact with the boron-based solid electrolyte. Thereby, a good bond between the silicon atom and the boron atom can be formed. For example, by using the negative electrode active material as an uncovered material, the negative electrode active material can be brought into direct contact with the boron-based solid electrolyte. In the present disclosure, "uncovered" means a state in which there is a portion not covered by an electrolyte or the like. The ratio of the uncovered region (the region where the negative electrode active material is in direct contact with the boron-based solid electrolyte) to the surface area of the negative electrode active material is preferably 50% or more, more preferably 70% or more, still more preferably 90% or more, and most preferably 100%. Whether the negative electrode active material is in direct contact with the boron-based solid electrolyte and whether it is an uncovered material are determined by XPS analysis of the cross section of the negative electrode active material.

[0014] The content ratio of the negative electrode active material to the total volume of the negative electrode for a lithium-ion battery is not particularly limited and can be 30% by volume to 60% by volume.

[0015] (boron-based solid electrolyte) -boron-based compound- It includes a boron-based compound having an organic cation and a boron cluster anion. From the viewpoint of capacity retention rate, the boron-based compound is preferably a solid matrix represented by G p A. Here, G represents an organic cation, A represents a boron cluster anion, and p is 1 or 2.

[0016] The organic cation can have at least one of an ammonium cation and a phosphonium cation. Specific examples of the ammonium cation and the phosphonium cation include the following Structures 1 to 4.

[0017]

Chemical formula

[0018] In Structures 1 to 4, R, R’, R’’, and R’’’ are each independently a substituent belonging to any of the following groups (i) to (vi). Group (i) A linear, branched, or cyclic C1-C8 alkyl or fluoroalkyl group, Group (ii) A C6-C9 aryl or fluoroaryl group, Group (iii) A linear, branched, or cyclic C1-C8 alkoxy or fluoroalkoxy group, Group (iv) A C6-C9 aryloxy or fluoroaryloxy group, Group (v) An amino group, Group (vi) A substituent containing two or more groups selected from Groups (i) to (v)

[0019] From the viewpoint of capacity retention rate, the organic cation is preferably at least one selected from the group consisting of N-methyl-N-propylpyrrolidinium (Pyr13), N-methyl-N,N-diethyl-N-propylammonium (N1223), N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium (DEME), N-methyl-N-propylpiperidinium (Pip13), N-methyl-N-(2-methoxyethyl)-pyrrolidinium (Pyr12ol), trimethylisopropylphosphonium (P111i4), methyltriethylphosphonium (P1222), methyltributylphosphonium (P1444), N-methyl-N-ethylpyrrolidinium (Pyr12), N-methyl-N-butylpyrrolidinium (Pyr14), N,N,N-triethyl-N-hexylammonium (N2226); triethylhexylphosphonium (P2226), and N-ethyl-N,N-dimethyl-N-butylammonium (N4211), more preferably at least one selected from the group consisting of Pyr14, Pyr13, and P1222, and particularly preferably Pyr14.

[0020] Examples of boron cluster anions include boranes having 6 to 12 boron atoms with a total charge of -2, carboranes having one carbon and 5 to 11 boron atoms in a cluster structure with a total charge of -1, and carboranes having two carbon atoms and 4 to 10 boron atoms in a cluster structure with a total charge of -1 or -2, etc. The boron cluster anion may have a substituent, examples of which include a halogen atom and an organic group. Examples of the organic group include an alkyl group and an alkoxy group.

[0021] The boron cluster anion may be an anion represented by any one of the following formulas (1) to (5). (1) [B y H (y-z-i) R z X i -2 , (2) [CB (y-1) H (y-z-i) R z X i - (3) [C 2 B (y-2) H (y-t-j-1) R t X j - (4) [C 2 B (y-3) H (y-t-j) R t X j - (5) [C 2 B (y-3) H (y-t-j-1) R t X j -2 In formulas (1) to (5), y is an integer in the range of 6 to 12, (z + i) is an integer in the range of 0 to y, (t + j) is an integer in the range of 0 to (y - 1), X is F, Cl, Br, I, or a combination thereof, and R is an organic group or hydrogen. Examples of the organic group include an alkyl group and an alkoxy group.

[0022] ​​​​​Specific examples of the boron-based compound include Pyr 14 CB 9 H 10 and the like.

[0023] From the viewpoint of the capacity retention rate, the content of the boron-based compound in the boron-based solid electrolyte is preferably 5 mol% to 30 mol%, and more preferably 15 mol% to 25 mol%.

[0024] - Lithium salt of boron cluster anion - From the viewpoint of the lithium ion conductivity, the boron-based solid electrolyte layer preferably contains a lithium salt of a boron cluster anion. Since the boron cluster anion has been described above, the description thereof is omitted here.

[0025] Specific examples of the lithium salt of the boron cluster anion include LiCB 9 H 10 and LiCB 11 H 12 and the like.

[0026] From the viewpoint of the lithium ion conductivity, the content of the boron-based compound in the boron-based solid electrolyte is preferably 70 mol% to 95 mol%, and more preferably 75 mol% to 85 mol%.

[0027] From the viewpoint of the capacity retention rate, the boron-based solid electrolyte is preferably a flexible crystal. In the present disclosure, the flexible crystal is defined as a substance composed of a regularly arranged three-dimensional crystal lattice and having orientational and rotational disorder at the molecular species or molecular ion level. Since the flexible crystal has both a solid form of a crystal and flexibility, it can change into a form corresponding to the fine unevenness at the interface between the electrode and the electrolyte. For example, the elastic modulus of a general flexible crystal is about 1 / 20 to 1 / 100 of that of a sulfide solid electrolyte.

[0028] From the perspective of capacity retention rate, the Young's modulus of the boron-based solid electrolyte is preferably from 0.1 GPa to 5 GPa, more preferably from 1 GPa to 2 GPa. In the present disclosure, the Young's modulus of the boron-based solid electrolyte is measured by nanoindentation.

[0029] From the perspective of capacity retention rate, the content of the boron-based solid electrolyte with respect to the total volume of the negative electrode for a lithium-ion battery is preferably from 30% by volume to 60% by volume, more preferably from 40% by volume to 50% by volume. From the perspective of capacity retention rate, the ratio of the content of the boron-based solid electrolyte to the content of the negative electrode active material (content of boron-based solid electrolyte / content of negative electrode active material) is preferably from 0.5 to 2, more preferably from 0.8 to 1.2 on a volume basis.

[0030] Examples of the shape of the boron-based solid electrolyte include particulate. The particle size of the boron-based solid electrolyte is not particularly limited and is preferably from 0.05 μm to 3.0 μm. In the present disclosure, the method for measuring the particle size is to disperse the object to be measured in a dispersion medium, measure the volume-based particle size distribution using a particle size distribution measuring device, and take the particle size corresponding to 50% of the value of the obtained volume-based cumulative particle size distribution as the particle size.

[0031] (Conductive material) The negative electrode for a lithium-ion battery can contain a conductive material. As the conductive material, conventionally known materials that can be used in the negative electrode layer provided in a lithium-ion battery can be used, and examples include carbon materials such as acetylene black, metal particles, and conductive polymers.

[0032] The content of the boron-based solid electrolyte with respect to the total volume of the negative electrode for a lithium-ion battery is not particularly limited and can be from 3% by volume to 20% by volume.

[0033] The negative electrode for a lithium-ion battery can be manufactured by mixing a negative electrode active material, a boron-based solid electrolyte, etc., preparing a negative electrode composite material, and molding this. The bond between the silicon atoms contained in the negative electrode active material and the boron atoms contained in the boron-based solid electrolyte can be formed by performing the above mixing using a molten boron-based solid electrolyte.

[0034] (Other materials) The negative electrode for a lithium-ion battery may contain materials other than the above-described materials. For example, a sulfide solid electrolyte may be contained in the electrode for a lithium-ion battery. From the viewpoint of forming a good bond between silicon atoms and boron atoms, the content rate of the sulfide solid electrolyte with respect to the total volume of the negative electrode for a lithium-ion battery is preferably 10% by volume or less, more preferably 5% by volume or less, still more preferably 1% by volume or less, and most preferably not contained.

[0035] [Lithium-ion battery] The lithium-ion battery of the present disclosure includes a positive electrode layer, a solid electrolyte layer, and a negative electrode layer. The negative electrode layer is composed of the negative electrode for a lithium-ion battery described above. The lithium-ion battery of the present disclosure may be an all-solid-state battery.

[0036] When a set of the positive electrode layer, the solid electrolyte layer, and the negative electrode layer is used as a power generation unit, the lithium-ion battery may have only one power generation unit, or may have two or more power generation units. When the lithium-ion battery has two or more power generation units, those power generation units may be connected in series or may be connected in parallel.

[0037] (Solid electrolyte layer) The solid electrolyte layer contains a solid electrolyte. As the solid electrolyte, conventionally known ones that can be used for the solid electrolyte layer included in a lithium-ion battery can be used, and examples thereof include sulfide solid electrolytes. The solid electrolyte layer may contain a binder. Examples of the binder include rubber-based binders such as styrene-butadiene rubber, and fluoride-based binders such as polyvinylidene fluoride (PVDF).

[0038] (Positive electrode layer) The positive electrode layer can contain a positive electrode active material. Examples of the positive electrode active material include oxide active materials and the like. Examples of the oxide active material include rock salt layer-structured active materials such as LiCoO 2 and the like, spinel-type active materials such as LiMn 2 O 4 and the like, olivine-type active materials such as LiFePO 4 and the like. The positive electrode layer can contain a solid electrolyte. As the solid electrolyte, those conventionally known and usable for the positive electrode layer provided in a lithium-ion battery can be used, and for example, a sulfide solid electrolyte can be mentioned. The positive electrode layer can contain a conductive material. As the conductive material, those conventionally known and usable for the positive electrode layer provided in a lithium-ion battery can be used, and examples include carbon materials such as acetylene black, metal particles, conductive polymers, and the like.

[0039] (Positive electrode current collector) The lithium-ion battery of the present disclosure may further include a positive electrode current collector. The positive electrode current collector collects current from the positive electrode layer. The positive electrode current collector is disposed at a position opposite to the solid electrolyte layer with respect to the positive electrode layer.

[0040] (Negative electrode current collector) The lithium-ion battery of the present disclosure may further include a negative electrode current collector. The negative electrode current collector collects current from the negative electrode layer. The negative electrode current collector is disposed at a position opposite to the solid electrolyte layer with respect to the negative electrode layer.

[0041] FIG. 1 is a cross-sectional view showing an embodiment of the lithium-ion battery of the present disclosure. As shown in FIG. 1, the lithium-ion battery 10 includes a positive electrode current collector 14, a positive electrode layer 11, a solid electrolyte layer 12, a negative electrode layer 13, and a positive electrode current collector 15 in this order. The lithium-ion battery of the present disclosure is not limited to FIG. 1.

Examples

[0042] Hereinafter, the present disclosure will be described in more detail with reference to examples, but the invention of the present disclosure is not limited to these examples only.

[0043] <Example 1> Pyr 14 CB 9 H 10 (boron-based compound), LiCB 9 H 10 (lithium salt of boron cluster anion), LiCB 11 H 12 (lithium salt of boron cluster anion), were weighed so as to have a molar ratio of 2:4:4, mixed with a mortar, and a mixture was obtained. Next, the mixture was transferred to a screw bottle, heated at 180 °C, and mixed in a molten state for 6 hours. After mixing, it was cooled to room temperature (25 °C) to solidify the mixture, and a boron-based solid electrolyte (Young's modulus 1.4 GPa) was obtained.

[0044] At 180 °C, the negative electrode active material (silicon), the molten boron-based solid electrolyte, and the conductive material (carbon material: VGCF) were mixed so as to have a volume ratio of 46:46:8 to obtain a negative electrode composite material.

[0045] The positive electrode active material (layered nickel oxide), the sulfide solid electrolyte (Li 2 S-P 2 S 5 -LiI-LiBr), and the conductive material (carbon material: VGCF) were mixed so as to have a volume ratio of 74:23:3 to obtain a positive electrode composite material.

[0046] 101.7 mg of the sulfide solid electrolyte was introduced into the cylinder of the press cell and press-molded at a pressure of 98 MPa to form a solid electrolyte layer. From one side of the cylinder, 29.4 g of the above positive electrode composite material was introduced and press-molded at a pressure of 98 MPa to form a positive electrode with an area of 1 cm 2 . Then, an aluminum foil was introduced and press-molded at a pressure of 196 MPa to form a positive electrode current collector on the surface of the positive electrode. From the other side of the cylinder, 8.9 g of the above negative electrode composite material was introduced and press-molded at a pressure of 196 MPa to form a negative electrode with an area of 1 cm2 A negative electrode was formed. Subsequently, a copper foil was introduced and press-molded at a pressure of 588 MPa to form a negative electrode current collector on the surface of the negative electrode. Subsequently, torque constraint was performed at a constraint pressure of 2 Nm to obtain a battery cell (lithium-ion battery).

[0047] <Comparative Example 1> A negative electrode active material (silicon), a sulfide solid electrolyte, and a conductive material were mixed so as to have a volume ratio of 46:46:8 to obtain a negative electrode composite material. A battery cell was manufactured in the same manner as in Example 1 except that this was used to form a negative electrode.

[0048] <<Confirmation of the bond between the negative electrode active material and the electrolyte>> At 60 °C, the battery cells of Example 1 and Comparative Example 1 were charged under the conditions of 0.5 mA / cm 2 , 4.05 V cut-off, rested for 10 minutes, discharged under the conditions of 0.5 mA / cm 2 , 2.5 V cut-off, and rested for 10 minutes. The charge-discharge process was repeated 20 times with one cycle consisting of charge, rest, discharge, and rest. Using X-ray photoelectron spectroscopy (XPS), the presence or absence of a bond between the negative electrode active material and the electrolyte contained in the negative electrode (described as "presence or absence of bond between negative electrode active material and electrolyte" in Table 1) was confirmed. As a result, it was confirmed that a bond exists between the silicon atoms of the negative electrode active material of Example 1 and the boron atoms of the boron-based compound contained in the boron-based solid electrolyte. On the other hand, the presence of a bond was not confirmed between the silicon atoms of the negative electrode active material of Comparative Example 1 and the atoms of the sulfide solid electrolyte.

[0049] <<Measurement of capacity retention rate>> Similar to the confirmation of the bond between the negative electrode active material and the electrolyte, the battery cells of Example 1 and Comparative Example 1 were subjected to 20 charge-discharge cycles. The discharge capacity of each cycle of the charge-discharge process was measured. After dividing the discharge capacity of each cycle by the initial discharge capacity and multiplying by 100, the capacity retention rates of the battery cells of Example 1 and Comparative Example 1 were calculated respectively. The results are summarized in Table 1.

[0050] <<Measurement of the Porosity of the Negative Electrode after Charge and Discharge Treatment>> Similar to the confirmation of the bond between the negative electrode active material and the electrolyte, charge and discharge treatment was performed 20 times on the battery cells of Example 1 and Comparative Example 1. After the charge and discharge treatment, the negative electrode was cut, and its cross-sectional image was obtained by a scanning electron microscope (SEM) (magnification: 10,000 times). The cross-sectional image of the negative electrode of Example 1 is shown in Fig. 2, and the cross-sectional image of the negative electrode of Comparative Example 1 is shown in Fig. 3. In Fig. 2, the boron-based solid electrolyte is indicated by reference numeral 21, and the negative electrode active material is indicated by reference numeral 22. In Fig. 3, the sulfide solid electrolyte is indicated by reference numeral 31, the negative electrode active material is indicated by reference numeral 32, and the void is indicated by reference numeral 33. The ratio of the voids (porosity) generated between the negative electrode active material and the electrolyte was determined by image processing software, and the results are summarized in Table 1.

[0051]

Table 1

Explanation of Reference Numerals

[0052] 10: Lithium ion battery, 11: Positive electrode layer, 12: Solid electrolyte layer, 13: Negative electrode layer, 14: Positive electrode current collector, 15: Negative electrode current collector, 21: Boron-based solid electrolyte, 22, 32: Negative electrode active material, 31: Sulfide solid electrolyte, 33: Void

Claims

1. a negative electrode active material having a silicon atom; a boron-based solid electrolyte containing a boron-based compound having an organic cation and a boron cluster anion; and a bond exists between the silicon atom of the negative electrode active material and the boron atom of the boron-based compound contained in the boron-based solid electrolyte; a negative electrode for a lithium battery having a porosity of 0.9% or more and 5% or less.

2. The negative electrode for a lithium battery according to claim 1, wherein the negative electrode active material is in direct contact with the boron-based solid electrolyte.

3. The negative electrode for a lithium battery according to claim 1 or 2, wherein the ratio of the content of the boron-based solid electrolyte to the content of the negative electrode active material is 0.5 to 2 on a volume basis.

4. A lithium ion battery comprising a positive electrode layer, a solid electrolyte layer, and a negative electrode layer composed of the lithium ion battery negative electrode according to claim 1 or 2.

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