Negative electrode structure and method for manufacturing negative electrode structure

Abstract

A negative electrode structure according to one embodiment of the present invention is applied to a negative electrode of a lithium ion secondary battery. The negative electrode structure has a negative electrode current collector and a lithium alloy layer formed on the negative electrode current collector. The stoichiometric ratio of the lithium alloy layer is Li1-x-yBixMgy (0 < x + y < = 0.124, 0 < = x < = 0.024, and 0 < y < = 0.10), and the thickness of the lithium alloy layer is 1 [mu] m to 20 [mu] m (inclusive). The negative electrode structure can prolong the service life of a lithium ion secondary battery.

Description

Technical Field

[0001] This invention relates to a negative electrode structure for use in lithium-ion secondary batteries and a method for manufacturing the negative electrode structure. Background Technology

[0002] With the development of mobile devices such as mobile phones and smartphones, the lithium-ion rechargeable batteries installed in these devices have attracted much attention. In such lithium batteries, a lithium metal layer, serving as the negative electrode structure, is formed in the negative electrode current collector (for example, see Patent Document 1).

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent document 1: Japanese Patent Application Publication No. 2012-017478. Summary of the Invention

[0006] The problem the invention aims to solve

[0007] In recent years, among the aforementioned lithium-ion secondary batteries, all-solid-state batteries with a solid electrolyte have attracted considerable attention. In such all-solid-state batteries, for example, attempts have been made to reduce the thickness of the lithium metal layer to increase the battery's volumetric energy density (W·h / L).

[0008] However, if the thickness of the lithium metal layer is reduced, dendrite growth may occur from the lithium metal layer to the solid electrolyte layer, or delamination may occur between the lithium metal layer and the solid electrolyte layer, sometimes accompanied by degradation of the lithium metal layer, resulting in a shorter lifespan for the lithium-ion secondary battery.

[0009] In view of the above, the object of the present invention is to provide a negative electrode structure and a method thereof that can extend the lifespan of lithium-ion secondary batteries.

[0010] Solution for solving the problem

[0011] To achieve the above objectives, one aspect of the negative electrode structure of the present invention is a negative electrode structure applied to the negative electrode of a lithium-ion secondary battery.

[0012] The negative electrode structure has a negative electrode current collector and a lithium alloy layer formed in the negative electrode current collector.

[0013] The stoichiometry of the above lithium alloy layer is Li 1-x-y Bi x Mg y (0<x+y≤0.124, 0≤x≤0.024, 0<y≤0.10),

[0014] The thickness of the aforementioned lithium alloy layer is more than 1 μm and less than 20 μm.

[0015] Such a negative electrode structure can extend the lifespan of lithium-ion secondary batteries.

[0016] In the above-mentioned negative electrode structure, the Young's modulus of the lithium alloy layer can be above 4 GPa and below 30 GPa.

[0017] Such a negative electrode structure can extend the lifespan of lithium-ion secondary batteries.

[0018] In the aforementioned negative electrode structure, the relative density of the lithium alloy layer can be above 80%.

[0019] Such a negative electrode structure can extend the lifespan of lithium-ion secondary batteries.

[0020] In the aforementioned negative electrode structure, a layer with a relatively high bismuth content can be formed in the aforementioned lithium alloy layer.

[0021] Such a negative electrode structure can extend the lifespan of lithium-ion secondary batteries.

[0022] To achieve the above objectives, one method of manufacturing a negative electrode structure according to the present invention is a method of manufacturing a negative electrode structure applied to the negative electrode of a lithium-ion secondary battery.

[0023] In the manufacturing method of the negative electrode structure, any one of a lithium-containing layer and a modification layer is formed on the negative electrode current collector.

[0024] After the above-mentioned layer is formed, another layer, either the lithium-containing layer or the modification layer, is formed on top of the above-mentioned layer.

[0025] As the aforementioned modifying layer, a magnesium layer, a bismuth layer, a layer containing lithium and magnesium, or a layer containing magnesium and bismuth are used.

[0026] The stoichiometric ratio of the alloy layer formed on the aforementioned negative electrode current collector, which combines the aforementioned lithium-containing layer and the aforementioned modification layer, is Li 1-x-y Bi x Mg y (0<x+y≤0.124, 0≤x≤0.024, 0<y≤0.10),

[0027] The thickness of the aforementioned alloy layer is more than 1 μm and less than 20 μm.

[0028] If such a manufacturing method is adopted, the lifespan of lithium-ion secondary batteries can be extended.

[0029] In the above-described method for manufacturing the negative electrode structure, the lithium-containing layer may contain magnesium.

[0030] If such a manufacturing method is adopted, the lifespan of lithium-ion secondary batteries can be extended.

[0031] In the above-mentioned method for manufacturing the negative electrode structure,

[0032] When using a layer containing magnesium and bismuth as the above-mentioned modification layer, the other material can be formed into a film after either magnesium or bismuth is formed.

[0033] If such a manufacturing method is adopted, the lifespan of lithium-ion secondary batteries can be extended.

[0034] Invention Effects

[0035] According to the present invention, a negative electrode structure capable of extending the lifespan of lithium-ion secondary batteries and a method thereof can be provided. Attached Figure Description

[0036] Figure 1 This is a cross-sectional schematic diagram showing an example of the negative electrode structure of this embodiment.

[0037] Figure 2 This is a cross-sectional schematic diagram illustrating an example of a method for manufacturing the negative electrode structure according to this embodiment.

[0038] Figure 3 This is a cross-sectional schematic diagram illustrating another example of a method for manufacturing the negative electrode structure according to this embodiment.

[0039] Figure 4 This is a cross-sectional schematic diagram illustrating yet another example of a method for manufacturing the negative electrode structure according to this embodiment.

[0040] Figure 5 This is a schematic cross-sectional view showing a high-concentration bismuth layer in a lithium alloy layer.

[0041] Figure 6 This is a diagram illustrating an example of the effects of this embodiment. Detailed Implementation

[0042] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Furthermore, sometimes the same reference numerals are used to refer to the same components or components having the same function, and sometimes descriptions of the component are appropriately omitted after its description. In addition, the numerical values ​​shown below are examples and are not limited to these examples.

[0043] Figure 1 This is a cross-sectional schematic diagram showing an example of the negative electrode structure of this embodiment. Figure 1 The negative electrode structure 1 shown can, for example, be applied to the negative electrode of an all-solid-state lithium-ion secondary battery. Figure 1 In the example, in addition to the negative electrode structure 1, a solid electrolyte layer 30 in contact with the negative electrode structure 1 is also shown.

[0044] The negative electrode structure 1 has a lithium alloy layer 10 and a negative electrode current collector 20. The lithium alloy layer 10 functions as a lithium source for the lithium-ion secondary battery. The lithium alloy layer 10 is formed in the negative electrode current collector 20. Furthermore, the lithium alloy layer 10 is disposed between the negative electrode current collector 20 and the solid electrolyte layer 30.

[0045] In addition to lithium (Li), the lithium alloy layer 10 also contains magnesium (Mg). Furthermore, the lithium alloy layer 10 may also contain bismuth (Bi). Here, the stoichiometry of the lithium alloy layer 10 is determined by Li... 1-x-y Bi x Mg y (0 < x + y ≤ 0.124, 0 ≤ x ≤ 0.024, 0 < y ≤ 0.10) is used. The number to the right of each element symbol is defined as the equivalent, or the ratio of the number of moles in the lithium alloy layer 10, or the ratio of the number of atoms in the lithium alloy layer 10. In addition, the thickness of the lithium alloy layer 10 is more than 1 μm and less than 20 μm.

[0046] Even if the thickness of the lithium alloy layer 10 is more than 1 μm and less than 20 μm, the desired lifespan as a lithium-ion secondary battery cannot be obtained when x+y>0.124 or x>0.024 or y>0.10, so it is not preferred.

[0047] Furthermore, in the lithium alloy layer 10, even if the stoichiometric ratio is Li 1-x-y Bi x Mg y (0<x+y≤0.124,0≤x≤0.024,0<y≤0.10), when the thickness of the lithium alloy layer 10 is less than 1μm, a continuous film will not be formed, so it is not preferred. When the thickness of the lithium alloy layer 10 is greater than 20μm, the productivity will decrease, so it is not preferred.

[0048] Furthermore, the thickness of the lithium alloy layer 10 is more preferably 1 μm or more and 10 μm or less, and even more preferably 1 μm or more and 5 μm or less. By reducing the thickness of the negative electrode structure 1, the volumetric energy density (W·h / L) of the lithium-ion secondary battery is further improved, which is therefore preferred.

[0049] Furthermore, the Young's modulus of the lithium alloy layer 10 is 4 GPa or more and 30 GPa or less. The Young's modulus can be measured, for example, by nanoindentation. When the Young's modulus of the lithium alloy layer 10 is less than 4 GPa, the lithium alloy layer 10 may experience short circuits due to dendrite growth, which is therefore undesirable. When the Young's modulus of the lithium alloy layer 10 is greater than 30 GPa, the formability of the lithium alloy layer 10 decreases, and poor contact with the solid electrolyte may occur, which is also undesirable. More preferably, the Young's modulus of the lithium alloy layer 10 is 10 GPa or more and 25 GPa or less, and even more preferably, it is 15 GPa or more and 20 GPa or less.

[0050] Furthermore, the relative density of the lithium alloy layer ((measured density / theoretical density) × 100 (%)) is 80% or more. The measured density can be determined, for example, by film thickness measurement and ICP emission spectroscopy analysis. When the relative density of the lithium alloy layer 10 is less than 80%, the effective utilization rate of lithium decreases, which is therefore undesirable. Moreover, the relative density of the lithium alloy layer 10 is more preferably 85% or more, and even more preferably 90% or more.

[0051] In the lithium alloy layer 10, by mixing magnesium into lithium, the mechanical strength of the lithium alloy layer 10 is mainly improved compared to the case where the lithium alloy layer 10 is composed of, for example, pure lithium. Furthermore, by mixing bismuth into lithium, the wettability of the lithium alloy layer 10 to the solid electrolyte layer 30 is mainly improved compared to the case where the lithium alloy layer 10 is composed of, for example, pure lithium. Moreover, by mixing at least one of magnesium and bismuth into lithium, the electrode potential of the lithium-ion secondary battery increases, and the reduction (decomposition) of the solid electrolyte layer 30 is suppressed compared to the case where the lithium alloy layer 10 is composed of, for example, pure lithium. These effects are examples, and the effects achieved by adding magnesium or bismuth to lithium are not limited to those described above.

[0052] Therefore, in the negative electrode structure 1 of this embodiment, dendrite growth from the lithium alloy layer 10 to the solid electrolyte layer 30 is suppressed, or void formation within the lithium alloy layer 10 is suppressed, or delamination between the lithium alloy layer 10 and the solid electrolyte layer 30 is suppressed, or interfacial chemical reactions between the lithium alloy layer 10 and the solid electrolyte layer 30 are suppressed. As a result, the lifespan of the lithium-ion secondary battery assembled with the negative electrode structure 1 is significantly improved.

[0053] The negative electrode current collector 20 is, for example, copper (Cu) foil, nickel (Ni) foil, iron (Fe) foil, or an alloy foil containing at least two of copper, nickel, and iron. The negative electrode current collector 20 can also be stainless steel (SUS) foil. Furthermore, the solid electrolyte layer 30 is, for example, a sulfide-based solid electrolyte layer. Examples of sulfide systems include Li3PS4, Li6PS5X (X = any one of Cl, Br, and I), and Li... 10 GeP2S 12 wait.

[0054] The following is an example of a method for manufacturing the negative electrode structure 1.

[0055] In this embodiment, either a lithium-containing layer or a modification layer is formed on the negative electrode current collector. After forming this layer, the other layer, either a lithium-containing layer or a modification layer, is formed on this layer. As the modification layer, a magnesium layer, a bismuth layer, a layer containing lithium and magnesium, or a layer containing magnesium and bismuth are used. Here, the lithium-containing layer may also contain magnesium. Furthermore, when using a layer containing magnesium and bismuth as the modification layer, the other material can be formed after either magnesium or bismuth has been formed.

[0056] Figure 2 (a)~ Figure 2 (d) is a cross-sectional schematic diagram illustrating an example of a method for manufacturing the negative electrode structure according to this embodiment. In this embodiment, sputtering, for example, can be used as the film-forming method for the forming layer. In sputtering, a sputtering target made of various metals or alloys can be used. The film-forming method is not limited to sputtering; vapor deposition can also be used.

[0057] For example, such as Figure 2 As shown in (a), a magnesium layer 121 is formed on the negative electrode current collector 20. The thickness of the magnesium layer 121 is 50 nm or more and 600 nm or less. Next, as... Figure 2 As shown in (b), a bismuth layer 122 is formed on the magnesium layer 121. The thickness of the bismuth layer 122 is 50 nm or more and 200 nm or less. Thus, a modification layer 120 containing magnesium and bismuth is formed on the negative electrode current collector 20.

[0058] Here, a magnesium layer 121 can be formed on the bismuth layer 122 after the bismuth layer 122 is formed on the negative electrode current collector 20. That is, the order in which the magnesium layer 121 and the bismuth layer 122 are stacked on the negative electrode current collector 20 can be either magnesium layer 121 first or bismuth layer 122 first. Alternatively, an alloy target containing magnesium and bismuth can be used to form the modification layer 120 in one step. Alternatively, either the formation of the magnesium layer 121 or the formation of the bismuth layer 122 can be omitted.

[0059] Next, as Figure 2 As shown in (c), a lithium-containing layer 110 is formed on the modification layer 120. Figure 2 The lithium-containing layer 110 shown in (c) can be, for example, a pure lithium metal layer. The thickness of the lithium-containing layer 110 is 1 μm or more and 20 μm or less.

[0060] Then, the components of the modification layer 120 diffuse (at room temperature) into the lithium-containing layer 110, such as... Figure 2 As shown in (d), a lithium alloy layer 10 is formed on the negative electrode current collector 20. Thus, the negative electrode structure 1 is formed.

[0061] Figure 3 (a)~ Figure 3(d) is a cross-sectional schematic diagram showing another example of a method for manufacturing the negative electrode structure of this embodiment.

[0062] For example, such as Figure 3 As shown in (a), a lithium-containing layer 110 is formed on the negative electrode current collector 20. Figure 3 The lithium-containing layer 110 shown in (a) can be, for example, a pure lithium metal layer. The film thickness of the lithium-containing layer 110 is 1 μm or more and 20 μm or less. Next, as... Figure 3 As shown in (b), a magnesium layer 121 is formed on the lithium-containing layer 110. The thickness of the magnesium layer 121 is 50 nm or more and 600 nm or less. Next, as... Figure 3 As shown in (c), a bismuth layer 122 is formed on the magnesium layer 121. The thickness of the bismuth layer 122 is 50 nm or more and 200 nm or less. Thus, a modification layer 120 containing magnesium and bismuth is formed on the lithium-containing layer 110.

[0063] Alternatively, a bismuth layer 122 can be formed on the lithium-containing layer 110, followed by the formation of a magnesium layer 121 on the bismuth layer 122. That is, the order in which the magnesium layer 121 and the bismuth layer 122 are stacked on the lithium-containing layer 110 can be either magnesium layer 121 first or bismuth layer 122 first. Furthermore, an alloy target containing magnesium and bismuth can be used to form the modification layer 120 in a single step. Alternatively, either the formation of the magnesium layer 121 or the formation of the bismuth layer 122 can be omitted.

[0064] Then, the components of the modification layer 120 diffuse (at room temperature) into the lithium-containing layer 110, such as... Figure 3 As shown in (d), a lithium alloy layer 10 is formed on the negative electrode current collector 20. Thus, the negative electrode structure 1 is formed.

[0065] Figure 4 (a)~ Figure 4 (c) is a cross-sectional schematic diagram showing another example of a method for manufacturing the negative electrode structure of this embodiment.

[0066] For example, such as Figure 4 As shown in (a), a lithium-containing layer 111 is formed on the negative electrode current collector 20. Figure 4 The lithium-containing layer 111 shown in (a) can be, for example, a layer in which magnesium is added to pure lithium. The lithium-containing layer 111 can be formed using, for example, an alloy target containing lithium and magnesium. The film thickness of the lithium-containing layer 111 is 1 μm or more and 20 μm or less. Next, as... Figure 4 As shown in (b), a bismuth layer 122 is formed on the lithium-containing layer 111. The thickness of the bismuth layer 122 is 50 nm or more and 200 nm or less. Thus, a modification layer of the bismuth layer 122 is formed on the lithium-containing layer 111.

[0067] Here, the order in which the lithium-containing layer 111 and the bismuth layer 122 are stacked on the negative electrode current collector 20 can be either lithium-containing layer 111 first or bismuth layer 122 first. Alternatively, the formation of the bismuth layer 122 can be omitted.

[0068] Then, the composition of the bismuth layer 122 diffuses (at room temperature) into the lithium-containing layer 111, such as... Figure 4 As shown in (c), a lithium alloy layer 10 is formed on the negative electrode current collector 20. Thus, the negative electrode structure 1 is formed.

[0069] The diffusion of magnesium or bismuth into the lithium-containing layer 110, and the diffusion of bismuth into the lithium-containing layer 111, can be confirmed by, for example, EDX (energy-dispersive X-ray diffraction) analysis of a cross-section of the lithium alloy layer 10. Furthermore, the stoichiometry of the lithium alloy layer 10 can be determined using the thickness (μm), atomic weight (g / mol), and bulk density (g / cm³) of each of the lithium-containing layers 110 and 111, the magnesium layer 121, and the bismuth layer 122. 3 The atomic weights of Li, Mg, and Bi are set to 6.941, 24.305, and 208.98 g / mol, respectively, and their bulk densities are set to 0.534, 1.738, and 9.78 g / cm³, respectively. 3 , and thus perform calculations.

[0070] Furthermore, bismuth is less likely to diffuse within lithium metal layers compared to magnesium. Therefore, layers with a relatively high bismuth content (atom%) are sometimes formed in the lithium alloy layer 10. For example, after… Figure 2 (a)~ Figure 2 In the case of film formation process (c), such as Figure 5 As shown in (a), a high-concentration bismuth layer 1220 with a relatively high bismuth content is sometimes formed near the negative electrode current collector 20. Furthermore, after... Figure 3 (a)~ Figure 3 In the case of film formation process (c), such as Figure 5 As shown in (b), a high-concentration bismuth layer 1220 is sometimes formed near the surface of the lithium alloy layer 10 on the side opposite to the negative electrode current collector 20. Such a structure can be confirmed by EDX analysis of the cross-section of the lithium alloy layer 10.

[0071] Figure 6 This is a graph illustrating an example of the effect of this embodiment. The horizontal axis represents film thickness (nm), and the vertical axis represents the lifetime (hours) of the evaluation sample.

[0072] As the evaluation sample, a laminate of layer A: current collector (SUS foil) / layer B: pure Li layer (100 μm thick) / layer C: sulfide solid electrolyte layer / layer D: pure Li layer (5 μm thick) / layer E: modification layer / layer F: current collector (SUS foil) is used. Here, layers A and B correspond to the counter electrode side, and layers D, E, and F correspond to the working electrode side. For example, layers D, E, and F are formed by the manufacturing method of this embodiment, and layers A, B, and C are bonded thereon. In addition, as mentioned above, the stacking order of layers D and E is sometimes reversed.

[0073] In the evaluation, charge-discharge cycle tests were performed on the laminate. For example, the test was repeated alternately every 60 minutes at a setting of +0.3 mA / cm. 2 The applied current is set to -0.3 mA / cm. 2 The current is applied, and the voltage is measured. The time it takes for the voltage to become unstable due to repeated charging and discharging, and to drop rapidly or deviate significantly from the normal range, is used as the evaluation lifetime of the sample.

[0074] exist Figure 6 In the diagram, as a comparative example, the lifetime is shown by dashed lines when the E layer is omitted and no decoration layer is set. The lifetime in this case is approximately 250 (hours).

[0075] In contrast, when magnesium (Mg) layers of 50 nm, 100 nm, 200 nm, or 600 nm (x = 0.009, 0.018, 0.036, or 0.100) were layered as modification layers, the lifetime was found to be improved compared to the comparative example in all cases. Furthermore, when bismuth (Bi) layers of 50 nm, 100 nm, or 200 nm (y = 0.006, 0.012, or 0.024) were layered as modification layers, the lifetime was also found to be improved compared to the comparative example in all cases. Moreover, when 200 nm (100 nm each) magnesium (Mg) / bismuth (Bi) layers were layered as modification layers, the lifetime was also found to be longer than the comparative example. For example, when 200 nm magnesium (Mg) / bismuth (Bi) layers were layered as modification layers, a lifetime increase of approximately 5 times or more was achieved.

[0076] The embodiments of the present invention have been described above, but the present invention is not limited to the embodiments described above, and various modifications can be made. The embodiments are not limited to individual methods, and can be combined as long as it is technically possible.

[0077] Explanation of reference numerals in the attached figures

[0078] 1: Negative electrode structure;

[0079] 10: Lithium alloy layer;

[0080] 20: Negative electrode current collector;

[0081] 30: Solid electrolyte layer;

[0082] 110, 111: Lithium-containing layer;

[0083] 120: Modification layer;

[0084] 121: Magnesium layer;

[0085] 122: Bismuth layer;

[0086] 1220: High-concentration bismuth layer.

Claims

1. A negative electrode structure used as the negative electrode in a lithium-ion secondary battery. The negative electrode structure has a negative electrode current collector and a lithium alloy layer formed in the negative electrode current collector. The stoichiometry of the lithium alloy layer is Li 1-x-y Bi x Mg y (0<x+y≤0.124, 0≤x≤0.024, 0<y≤0.10), The thickness of the lithium alloy layer is greater than 1 μm and less than 20 μm.

2. The negative electrode structure according to claim 1, wherein, The Young's modulus of the lithium alloy layer is above 4 GPa and below 30 GPa.

3. The negative electrode structure according to claim 1, wherein, The relative density of the lithium alloy layer is above 80%.

4. The negative electrode structure according to claim 1, wherein, A layer with a relatively high bismuth content is formed in the lithium alloy layer.

5. A method for manufacturing a negative electrode structure, said negative electrode structure being used as the negative electrode of a lithium-ion secondary battery. Form any one of a lithium-containing layer and a modification layer on the negative electrode current collector. After the first layer is formed, another layer, either the lithium-containing layer or the modification layer, is formed on top of the first layer. As the modifying layer, a magnesium layer, a bismuth layer, a layer containing lithium and magnesium, or a layer containing magnesium and bismuth are used. The stoichiometric ratio of the alloy layer formed on the negative electrode current collector, which combines the lithium-containing layer and the modification layer, is Li 1-x-y Bi x Mg y (0<x+y≤0.124, 0≤x≤0.024, 0<y≤0.10), The thickness of the alloy layer is greater than 1 μm and less than 20 μm.

6. The method for manufacturing the negative electrode structure according to claim 5, wherein, The lithium-containing layer contains magnesium.

7. The method for manufacturing the negative electrode structure according to claim 5, wherein, When using a layer containing magnesium and bismuth as the modifying layer, after forming a film of either magnesium or bismuth, a film of the other material is formed.

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