Negative electrode structure and method for manufacturing a negative electrode structure

Abstract

To extend the lifespan of lithium-ion secondary batteries. [Solution] To achieve the above objective, a negative electrode structure according to one embodiment of the present invention is a negative electrode structure applied to the negative electrode of a lithium-ion secondary battery. The negative electrode structure comprises 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 Li 1-x-y Bi x Mg y (0

Description

[Technical Field]

[0001] The present invention relates to a negative electrode structure applied to lithium-ion secondary batteries and a method for manufacturing the negative electrode structure. [Background technology]

[0002] With the advancement of mobile devices such as cell phones and smartphones, lithium-ion secondary batteries used in these devices are attracting attention. In such lithium batteries, a lithium metal layer is formed on the negative electrode current collector as a negative electrode structure (see, for example, Patent Document 1). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2012-017478 [Overview of the project] [Problems that the invention aims to solve]

[0004] In recent years, all-solid-state batteries, in which the electrolyte is made of a solid material, have attracted attention among the lithium-ion secondary batteries mentioned above. In such all-solid-state batteries, attempts have been made to increase the volumetric energy density (W·h / L) of the battery by, for example, reducing the thickness of the lithium metal layer.

[0005] However, reducing the thickness of the lithium metal layer can lead to problems such as dendrite growth from the lithium metal layer towards the solid electrolyte layer, or delamination between the lithium metal layer and the solid electrolyte layer, which can shorten the lifespan of the lithium-ion secondary battery due to degradation of the lithium metal layer.

[0006] In view of the above circumstances, the object of the present invention is to provide a negative electrode structure and a method for manufacturing the same that enables longer lifespan in lithium-ion secondary batteries. [Means for solving the problem]

[0007] To achieve the above objective, a negative electrode structure according to one embodiment of the present invention is a negative electrode structure applied to the negative electrode of a lithium-ion secondary battery. The negative electrode structure comprises a negative electrode current collector and a lithium alloy layer formed on the negative electrode current collector. The stoichiometric ratio 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) And, The thickness of the lithium alloy layer is between 1 μm and 20 μm.

[0008] Such a negative electrode structure would enable longer lifespan in lithium-ion secondary batteries.

[0009] In the above-described negative electrode structure, the Young's modulus of the lithium alloy layer may be 4 GPa or more and 30 GPa or less.

[0010] Such a negative electrode structure would enable longer lifespan in lithium-ion secondary batteries.

[0011] In the above-described negative electrode structure, the relative density of the lithium alloy layer may be 80% or more.

[0012] Such a negative electrode structure would enable longer lifespan in lithium-ion secondary batteries.

[0013] In the above-described negative electrode structure, a layer with a relatively high bismuth content may be formed in the lithium alloy layer.

[0014] Such a negative electrode structure would enable longer lifespan in lithium-ion secondary batteries.

[0015] To achieve the above object, a method for manufacturing a negative electrode structure according to one embodiment of the present invention is a method for manufacturing a negative electrode structure applied to the negative electrode of a lithium-ion secondary battery. In the method for manufacturing a negative electrode structure, either a lithium-containing layer or a modification layer is formed on a negative electrode current collector. After forming the above one, the other of the lithium-containing layer and the modification layer is formed on the above one layer. As the modification layer, a magnesium layer, a bismuth layer, a layer containing lithium and magnesium, or a layer containing magnesium and bismuth is used. The stoichiometric ratio of the alloy layer formed by combining the lithium-containing layer and the modification layer formed on the negative electrode current collector is Li 1-x-y Bi x Mg y (0 < x + y ≤ 0.124, 0 ≤ x ≤ 0.024, 0 < y ≤ 0.10) and the thickness of the alloy layer is 1 μm or more and 20 μm or less.

[0016] With such a manufacturing method, the service life can be extended in a lithium-ion secondary battery.

[0017] In the above method for manufacturing a negative electrode structure, magnesium may be contained in the lithium-containing layer.

[0018] With such a manufacturing method, the service life can be extended in a lithium-ion secondary battery.

[0019] In the above method for manufacturing a negative electrode structure, when a layer containing magnesium and bismuth is used as the modification layer, after forming a film of either magnesium or bismuth, a film of the other of magnesium and bismuth may be formed.

[0020] With such a manufacturing method, the service life can be extended in a lithium-ion secondary battery.

Advantages of the Invention

[0021] According to the present invention, a negative electrode structure that enables longer lifespan in lithium-ion secondary batteries and a method for manufacturing the same are provided. [Brief explanation of the drawing]

[0022] [Figure 1] This is a schematic cross-sectional view showing an example of a negative electrode structure according to this embodiment. [Figure 2] This is a schematic cross-sectional view showing an example of a method for manufacturing the negative electrode structure according to this embodiment. [Figure 3] This is a schematic cross-sectional view showing another example of a method for manufacturing the negative electrode structure according to this embodiment. [Figure 4] This is a schematic cross-sectional view showing yet another example of a method for manufacturing the negative electrode structure according to this embodiment. [Figure 5] This is a schematic cross-sectional view showing a high-concentration bismuth layer within a lithium alloy layer. [Figure 6] This graph shows an example of the effects of this embodiment. [Modes for carrying out the invention]

[0023] Embodiments of the present invention will be described below with reference to the drawings. Furthermore, identical components or components having the same function may be denoted by the same reference numeral, and after describing such components, their descriptions may be omitted as appropriate. Also, the numerical values ​​shown below are illustrative and not limiting to this example.

[0024] Figure 1 is a schematic cross-sectional view showing an example of a negative electrode structure according to this embodiment. The negative electrode structure 1 shown in Figure 1 is applied, for example, to the negative electrode of an all-solid-state lithium-ion secondary battery. In the example in Figure 1, in addition to the negative electrode structure 1, a solid electrolyte layer 30 in contact with the negative electrode structure 1 is shown.

[0025] The negative electrode structure 1 includes a lithium alloy layer 10 and a negative electrode current collector 20. The lithium alloy layer 10 functions as a lithium source of the lithium ion secondary battery. The lithium alloy layer 10 is formed on the negative electrode current collector 20. Also, the lithium alloy layer 10 is provided between the negative electrode current collector 20 and the solid electrolyte layer 30.

[0026] The lithium alloy layer 10 contains magnesium (Mg) in addition to lithium (Li). Further, the lithium alloy layer 10 may contain bismuth (Bi). Here, the stoichiometric ratio of the lithium alloy layer 10 is Li 1-x-y Bi x Mg y (0 < x + y ≤ 0.124, 0 ≤ x ≤ 0.024, 0 < y ≤ 0.10) and is represented by. The numbers at the lower right of each element symbol are 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. Also, the thickness of the lithium alloy layer 10 is 1 μm or more and 20 μm or less.

[0027] In the lithium alloy layer 10, if the thickness is 1 μm or more and 20 μm or less, but x + y > 0.124, or x > 0.024, or y > 0.10, the desired life of the lithium ion secondary battery cannot be obtained, which is not preferable.

[0028] Also, 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), if the thickness of the lithium alloy layer 10 is less than 1 μm, a continuous film cannot be formed, which is not preferable, and if the thickness of the lithium alloy layer 10 is greater than 20 μm, the productivity becomes poor, which is not preferable.

[0029] Furthermore, the thickness of the lithium alloy layer 10 is more preferably 1 μm to 10 μm, and even more preferably 1 μm to 5 μm. This is preferable because reducing the thickness of the negative electrode structure 1 further improves the volumetric energy density (W·h / L) as a lithium-ion secondary battery.

[0030] Furthermore, the Young's modulus of the lithium alloy layer 10 is between 4 GPa and 30 GPa. The Young's modulus is measured, for example, by nanoindentation. If the Young's modulus of the lithium alloy layer 10 is less than 4 GPa, there is a risk of short circuits occurring due to dendrite growth, which is undesirable. If the Young's modulus of the lithium alloy layer 10 is greater than 30 GPa, the lithium alloy layer 10 becomes less moldable, and there is a risk of poor contact with the solid electrolyte, which is also undesirable. More preferably, the Young's modulus of the lithium alloy layer 10 is between 10 GPa and 25 GPa, and even more preferably between 15 GPa and 20 GPa.

[0031] Furthermore, the relative density of the lithium alloy layer ((measured density / theoretical density) × 100 (%)) is 80% or higher. The measured density can be determined, for example, from film thickness measurement and ICP emission spectroscopy. If the relative density of the lithium alloy layer 10 is less than 80%, the effective utilization rate of lithium will decrease, which is undesirable. Moreover, the relative density of the lithium alloy layer 10 is more preferably 85% or higher, and even more preferably 90% or higher.

[0032] In the lithium alloy layer 10, the presence of magnesium in the lithium improves the mechanical strength of the lithium alloy layer 10 compared to, for example, a case where the lithium alloy layer 10 is composed of pure lithium. Furthermore, the presence of bismuth in the lithium alloy layer 10 improves the wettability of the lithium alloy layer 10 to the solid electrolyte layer 30 compared to, for example, a case where the lithium alloy layer 10 is composed of pure lithium. In addition, the presence of magnesium and at least one of bismuth in the lithium alloy layer 10 increases the electrode potential of the lithium-ion secondary battery and suppresses the reduction (decomposition) of the solid electrolyte layer 30 compared to, for example, a case where the lithium alloy layer 10 is composed of pure lithium. Note that these effects are illustrative, and the effects caused by the addition of magnesium or bismuth to lithium are not limited to those described above.

[0033] As a result, 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 incorporating the negative electrode structure 1 is greatly improved.

[0034] The negative electrode current collector 20 is, for example, a copper (Cu) foil, a nickel (Ni) foil, an iron (Fe) foil, or an alloy foil containing at least two of copper, nickel, and iron. The negative electrode current collector 20 may also be a stainless steel (SUS) foil. The solid electrolyte layer 30 is, for example, a sulfide-based solid electrolyte layer. Examples of sulfide-based materials include Li3PS4, Li6PS5X (X = any of Cl, Br, or I), and Li 10 GeP2S 12 These are some examples.

[0035] An example of a method for manufacturing the negative electrode structure 1 is shown below.

[0036] In this embodiment, either a lithium-containing layer or a modification layer is formed on the negative electrode current collector, and after one of these is formed, the other of the lithium-containing layer or the modification layer is formed on the other. As the modification layer, a magnesium layer, a bismuth layer, a layer containing lithium and magnesium, or a layer containing magnesium and bismuth can be used. Here, the lithium-containing layer may contain magnesium. Furthermore, when a layer containing magnesium and bismuth is used as the modification layer, the material of either magnesium or bismuth may be formed first, and then the material of the other of magnesium or bismuth may be formed.

[0037] Figures 2(a) to 2(d) are schematic cross-sectional views showing an example of a method for manufacturing a negative electrode structure according to this embodiment. In this embodiment, for example, sputtering is applied as the film deposition method for layer formation. In the sputtering method, a sputtering target made of the respective metal or alloy is used. The film deposition method is not limited to sputtering; vapor deposition may also be used.

[0038] For example, as shown in Figure 2(a), a magnesium layer 121 is formed on the negative electrode current collector 20. The thickness of the magnesium layer 121 is between 50 nm and 600 nm. Next, as shown in Figure 2(b), a bismuth layer 122 is formed on the magnesium layer 121. The thickness of the bismuth layer 122 is between 50 nm and 200 nm. As a result, a modified layer 120 containing magnesium and bismuth is formed on the negative electrode current collector 20.

[0039] Here, the bismuth layer 122 may be formed on the negative electrode current collector 20 first, and then the magnesium layer 121 may be formed on top of the bismuth layer 122. That is, the order in which the magnesium layer 121 and the bismuth layer 122 are stacked on the negative electrode current collector 20 may be either magnesium layer 121 first or bismuth layer 122 first. Alternatively, the modification layer 120 may be formed all at once using an alloy target containing magnesium and bismuth. Furthermore, either the formation of the magnesium layer 121 or the formation of the bismuth layer 122 may be omitted.

[0040] Next, as shown in Figure 2(c), a lithium-containing layer 110 is formed on the modified layer 120. For example, a pure lithium metal layer is used as the lithium-containing layer 110 shown in Figure 2(c). The thickness of the lithium-containing layer 110 is between 1 μm and 20 μm.

[0041] Subsequently, the components of the modification layer 120 diffuse into the lithium-containing layer 110 (diffusion at room temperature), and as shown in Figure 2(d), a lithium alloy layer 10 is formed on the negative electrode current collector 20. This forms the negative electrode structure 1.

[0042] Figures 3(a) to 3(d) are schematic cross-sectional views showing another example of a method for manufacturing the negative electrode structure according to this embodiment.

[0043] For example, as shown in Figure 3(a), a lithium-containing layer 110 is formed on the negative electrode current collector 20. As the lithium-containing layer 110 shown in Figure 3(a), for example, a pure lithium metal layer is used. The thickness of the lithium-containing layer 110 is 1 μm or more and 20 μm or less. Next, as shown in Figure 3(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 shown in Figure 3(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. As a result, a modified layer 120 containing magnesium and bismuth is formed on the lithium-containing layer 110.

[0044] Here, the bismuth layer 122 may be formed on the lithium-containing layer 110, and then the magnesium layer 121 may be formed on the bismuth layer 122. That is, the order in which the magnesium layer 121 and the bismuth layer 122 are laminated on the lithium-containing layer 110 may be either magnesium layer 121 first or bismuth layer 122 first. Alternatively, the modification layer 120 may be formed all at once using an alloy target containing magnesium and bismuth. Furthermore, either the formation of the magnesium layer 121 or the formation of the bismuth layer 122 may be omitted.

[0045] Subsequently, the components of the modification layer 120 diffuse into the lithium-containing layer 110 (diffusion at room temperature), and as shown in Figure 3(d), a lithium alloy layer 10 is formed on the negative electrode current collector 20. This forms the negative electrode structure 1.

[0046] Figures 4(a) to 4(c) are schematic cross-sectional views showing yet another example of a method for manufacturing the negative electrode structure according to this embodiment.

[0047] For example, as shown in Figure 4(a), a lithium-containing layer 111 is formed on the negative electrode current collector 20. The lithium-containing layer 111 shown in Figure 4(a) is, for example, a layer in which magnesium is added to pure lithium. For example, the lithium-containing layer 111 is formed using an alloy target containing lithium and magnesium. The thickness of the lithium-containing layer 111 is between 1 μm and 20 μm. Next, as shown in Figure 4(b), a bismuth layer 122 is formed on the lithium-containing layer 111. The thickness of the bismuth layer 122 is between 50 nm and 200 nm. This forms a modified layer of bismuth 122 on the lithium-containing layer 111.

[0048] Here, the order in which the lithium-containing layer 111 and the bismuth layer 122 are laminated on the negative electrode current collector 20 is such that the lithium-containing layer 111 may be laid first, or the bismuth layer 122 may be laid first. Also, the formation of the bismuth layer 122 may be omitted.

[0049] Subsequently, the components of the bismuth layer 122 diffuse into the lithium-containing layer 111 (diffusion at room temperature), and as shown in Figure 4(c), a lithium alloy layer 10 is formed on the negative electrode current collector 20. This forms the negative electrode structure 1.

[0050] The diffusion of magnesium or bismuth in lithium-containing layer 110, and the diffusion of bismuth in lithium-containing layer 111, have been confirmed, for example, by EDX analysis (energy-dispersive X-ray analysis) of the cross-section of lithium alloy layer 10. Furthermore, the chemical hypothesis of lithium alloy layer 10 is determined by the thickness (μm), atomic weight (g / mol), and volume density (g / cm³) of each of the lithium-containing layers 110, 111, magnesium layer 121, and bismuth layer 122. 3 The conversion is performed using the following formula. Here, the atomic weights of Li, Mg, and Bi are assumed to be 6.941, 24.305, and 208.98 g / mol, respectively, and their volume densities are assumed to be 0.534, 1.738, and 9.78 g / cm³, respectively. 3 It was calculated as follows.

[0051] Furthermore, bismuth does not diffuse as easily into the lithium metal layer as magnesium. Therefore, a layer with a relatively high bismuth content (atom%) may be formed in the lithium alloy layer 10. For example, after the film formation process shown in Figures 2(a) to 2(c), a bismuth-high concentration layer 1220 with a relatively high bismuth content may be formed near the negative electrode current collector 20, as shown in Figure 5(a). Also, after the film formation process shown in Figures 3(a) to 3(c), a bismuth-high concentration layer 1220 may be formed near the surface of the lithium alloy layer 10 on the opposite side of the negative electrode current collector 20, as shown in Figure 5(b). These configurations have been confirmed by EDX analysis of the cross-section of the lithium alloy layer 10.

[0052] Figure 6 is a graph illustrating an example of the effects of this embodiment. The horizontal axis represents film thickness (nm), and the vertical axis represents the lifetime of the evaluation sample (hours).

[0053] The evaluation sample uses a laminate consisting of: Layer A: current collector (SUS foil) / Layer B: pure Li layer (thickness 100 μm) / Layer C: sulfide solid electrolyte layer / Layer D: pure Li layer (thickness 5 μm) / Layer E: modification layer / Layer F: current collector (SUS foil). 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 laminated to them. As mentioned above, the lamination order of Layers D and E may also be reversed.

[0054] The evaluation involves performing charge-discharge cycle tests on this laminate. For example, +0.3 mA / cm 2 The current set to -0.3mA / cm² is applied. 2 The current set to a specific value is applied and then the voltage is measured, alternating between the two states every 60 minutes. The time when the voltage becomes unstable and drops sharply, or when it swings to a large value, is defined as the lifespan of the evaluation sample.

[0055] In Figure 6, the lifetime of a device without the E layer and without the modification layer is shown by a dashed line as a comparative example. In this case, the lifetime is approximately 250 hours.

[0056] In contrast, when magnesium (Mg) was laminated as a modification layer at 50 nm, 100 nm, 200 nm, or 600 nm (x=0.009, 0.018, 0.036, or 0.100), it was found that the lifespan was improved compared to the comparative example in all cases. Similarly, when bismuth (Bi) was laminated as a modification layer at 50 nm, 100 nm, or 200 nm (y=0.006, 0.012, or 0.024), it was found that the lifespan was improved compared to the comparative example in all cases. Furthermore, when magnesium (Mg) / bismuth (Bi) was laminated at 200 nm (100 nm each) as a modification layer, it was found that the lifespan was also longer than the comparative example. For example, when magnesium (Mg) / bismuth (Bi) was laminated at 200 nm as a modification layer, a lifespan extension of more than 5 times was achieved.

[0057] Although embodiments of the present invention have been described above, it goes without saying that the present invention is not limited to the embodiments described above and can be modified in various ways. Each embodiment is not necessarily an independent form and can be combined to the extent that it is technically possible. [Explanation of Symbols]

[0058] 1...Negative electrode structure 10…Lithium alloy layer 20...Negative electrode current collector 30...Solid electrolyte layer 110, 111... Lithium-containing layer 120...Modification layer 121…Magnesium layer 122... Bismuth layer 1220... High-concentration bismuth layer

Claims

1. A negative electrode structure applied to the negative electrode of a lithium-ion secondary battery, Negative electrode current collector and The lithium alloy layer formed on the negative electrode current collector and It is equipped with, The stoichiometric ratio 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) And, The thickness of the lithium alloy layer is 1 μm or more and 20 μm or less. Negative electrode structure.

2. A negative electrode structure according to claim 1, The Young's modulus of the lithium alloy layer is 4 GPa or more and 30 GPa or less. Negative electrode structure.

3. A negative electrode structure according to claim 1, The relative density of the lithium alloy layer is 80% or more. Negative electrode structure.

4. A negative electrode structure according to claim 1, A layer with a relatively high bismuth content was formed in the aforementioned lithium alloy layer. Negative electrode structure.

5. A method for manufacturing a negative electrode structure applied to the negative electrode of a lithium-ion secondary battery, A lithium-containing layer and a modification layer are formed on the negative electrode current collector. After forming one of the above layers, the other of either the lithium-containing layer or the modification layer is formed on the other layer. The modified layer may be a magnesium layer, a bismuth layer, a layer containing lithium and magnesium, or a layer containing magnesium and bismuth. The stoichiometric ratio of the alloy layer formed on the negative electrode current collector, which is the combination of 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) And, The thickness of the alloy layer is 1 μm or more and 20 μm or less. A method for manufacturing a negative electrode structure.

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

7. A method for manufacturing a negative electrode structure according to claim 5, When a layer containing magnesium and bismuth is used as the modification layer, one of the materials, magnesium or bismuth, is deposited first, and then the other material, magnesium or bismuth, is deposited. A method for manufacturing a negative electrode structure.

Images