Lithium metal secondary batteries

Abstract

The present invention provides a lithium metal secondary battery having a solid electrolyte layer between the positive electrode and the negative electrode, the negative electrode having a negative electrode current collector and a protective layer, the protective layer containing a metal capable of alloying with lithium, and a volumetric capacity density of 1000 mAh / L or more.

Description

Technical Field

[0001] This invention relates to a lithium metal secondary battery. Background Technology

[0002] Based on the perspectives of rapidly popularizing portable information and communication devices and reducing CO2 emissions, a lithium metal secondary battery for electric and hybrid vehicles is being developed.

[0003] As a lithium metal secondary battery, for example, one lithium metal secondary battery is known, which includes a negative electrode having a negative current collector, a positive electrode, and a solid electrolyte layer (see Patent Document 1).

[0004] [Previous Technical Documents]

[0005] (Patent Documents)

[0006] Patent Document 1: Japanese Patent Application Publication No. 2016-12495 Summary of the Invention

[0007] [The problem the invention aims to solve]

[0008] However, during the charging of lithium metal rechargeable batteries, lithium metal segregates between the negative electrode current collector and the solid electrolyte layer, and lithium metal dendrites grow. As a result, the solid electrolyte layer cracks or the negative electrode current collector peels off, thereby reducing the durability of the lithium metal rechargeable battery.

[0009] The purpose of this invention is to provide a lithium metal secondary battery that can improve durability.

[0010] [Technical means to solve the problem]

[0011] One embodiment of the present invention is a lithium metal secondary battery having a solid electrolyte layer between a positive electrode and a negative electrode, wherein the negative electrode has a negative electrode current collector and a protective layer, the protective layer comprising a metal capable of alloying with lithium, and having a volumetric capacity density of 1000 mAh / L or more.

[0012] Optionally, the aforementioned protective layer may further comprise the aforementioned metal capable of alloying with lithium and the lithium alloy.

[0013] Optionally, the aforementioned negative electrode may further have a lithium metal layer between the aforementioned negative electrode current collector and the aforementioned protective layer.

[0014] Optionally, the ten-point average roughness (Rz) of the aforementioned negative electrode current collector is above 1.0 μm and below 3.0 μm.

[0015] Optionally, in the aforementioned protective layer, the metal capable of alloying with lithium is selected from one or more of the group consisting of antimony, bismuth, and tin.

[0016] Optionally, the thickness of the aforementioned protective layer when fully charged is 0.2 μm or more and 5 μm or less.

[0017] (The effect of the invention)

[0018] According to the present invention, a lithium metal secondary battery with improved durability can be provided. Attached Figure Description

[0019] Figure 1 This is a cross-sectional view illustrating an example of a lithium metal secondary battery according to this embodiment.

[0020] Figure 2 This is a cross-sectional view illustrating another example of a lithium metal secondary battery according to this embodiment.

[0021] Figure 3 It is a cross-sectional view of a lithium-ion battery when a zinc plating layer is formed instead of a protective layer.

[0022] Figure 4 This is a graph illustrating an example of the relationship between the voltage and charging capacity of the lithium metal secondary battery according to this embodiment.

[0023] Figure 5 This is an example of a graph showing the relationship between voltage and charging capacity of a lithium-ion battery without a protective layer.

[0024] Figure 6 This is a cross-sectional scanning electron microscope (SEM) image of the all-solid-state lithium metal secondary battery of Example 1 when fully charged.

[0025] Figure 7 This is a cross-sectional SEM image of the fully charged all-solid-state lithium metal secondary battery of Comparative Example 2. Detailed Implementation

[0026] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0027] Figure 1 An example of a lithium metal secondary battery according to this embodiment is illustrated in the figure.

[0028] The lithium metal secondary battery 10 has a solid electrolyte layer 13 between a positive electrode 11 and a negative electrode 12. Here, the positive electrode 11 has a positive electrode current collector 11a and a positive electrode composite material layer 11b. The negative electrode 12 has a negative electrode current collector 12a, a lithium metal layer 12b, and a protective layer 12c. The protective layer 12c contains a metal capable of alloying with lithium and has a volumetric capacity density of 1000 mAh / L or higher. If the volumetric capacity density of the protective layer 12c is less than 1000 mAh / L, the durability of the lithium metal secondary battery 10 will decrease.

[0029] During charging, lithium metal is deposited at the negative electrode 12 in the lithium metal secondary battery 10, and lithium ions are dissolved from the negative electrode 12 during discharging. Therefore, in the initial state, the negative electrode 12 of the lithium metal secondary battery 10 may not have a lithium metal layer 12b (see reference). Figure 2 In this case, the lithium metal secondary battery 10 is charged before use. As a result, the lithium-alloying metal expands and alloys with lithium, causing the protective layer 12c to peel off from the negative electrode current collector 12a. Consequently, lithium metal precipitates between the negative electrode current collector 12a and the protective layer 12c, forming a lithium metal layer 12b. Therefore, the growth of lithium metal dendrites is suppressed, and the surface of the solid electrolyte layer 13 is protected by the protective layer 12c.

[0030] In contrast, for example, if a zinc plating layer 12c' is formed on the negative electrode current collector 12a, then because the zinc plating layer 12c' not only has high adhesion to the negative electrode current collector 12a, but also has electronic conductivity, the lithium metal layer 12b will precipitate from the surface of the zinc plating layer 12c' (see reference). Figure 3 That is, the surface of the solid electrolyte layer 13 is not protected by the zinc plating layer 12c'. Here, the volumetric capacity density of the zinc plating layer 12c' is less than 1000 mAh / L.

[0031] Here, the volumetric capacity density of the protective layer 12c is calculated by dividing the charging capacity of the lithium metal secondary battery 10 at the initial charging voltage of 3.5V by the volume of the protective layer 12c after charging and discharging (see reference). Figure 4 Furthermore, in the absence of a protective layer 12c, the lithium metal secondary battery 10 exhibits almost no capacity even when the initial charging voltage reaches 3.5V (see reference). Figure 5 Furthermore, the volume of the protective layer 12c is calculated by multiplying the thickness of the protective layer 12c when fully charged (described later) by the projected area of ​​the negative current collector 12a.

[0032] Furthermore, in its initial state, the protective layer 12c can be either a layer containing only a metal capable of alloying with lithium (e.g., a plating layer) or a layer containing an alloy of lithium with a metal capable of alloying with lithium.

[0033] As for metals that can be alloyed with lithium, there are no particular limitations as long as the volumetric capacity density of the protective layer 12c reaches 1000mAh / L or more. For example, antimony, bismuth, tin, etc. can be listed, and two or more can be used together.

[0034] The protective layer 12c may also contain an alloy of metals other than lithium that can be alloyed with lithium.

[0035] As for the metal that can be alloyed with lithium and the metal other than lithium, there are no particular limitations as long as the volumetric capacity density of the protective layer 12c reaches 1000 mAh / L or more. Examples include Cu6Sn5, Cu2Sb, SnSb, SbBi, etc.

[0036] The thickness of the protective layer 12c when fully charged is preferably 0.2 μm or more and 5 μm or less, and more preferably 1 μm or more and 3 μm or less. If the thickness of the protective layer 12c when fully charged is 0.2 μm or more and 5 μm or less, the durability of the lithium metal secondary battery 10 will be improved.

[0037] The ten-point average roughness (Rz) of the negative electrode current collector 12a is preferably 1.0 μm or more and 3.0 μm or less, and more preferably 1.5 μm or more and 2.5 μm or less. If the ten-point average roughness (Rz) of the negative electrode current collector 12a is 1.0 μm or more, lithium metal deposited in the negative electrode current collector 12a is easily retained; if it is 3.0 μm or less, the solid electrolyte layer 13 is less prone to cracking. Therefore, if the ten-point average roughness (Rz) of the negative electrode current collector 12a is 1.0 μm or more and 3.0 μm or less, the durability of the lithium metal secondary battery 10 will be improved.

[0038] As a negative electrode current collector 12a, there are no particular limitations; for example, copper foil can be listed.

[0039] The thickness of the negative electrode current collector 12a is not particularly limited, for example, it is 6μm or more and 18μm or less.

[0040] The relative density of the lithium metal layer 12b when fully charged is preferably 60% or more, and more preferably 65% ​​or more. If the relative density of the lithium metal layer 12b when fully charged is 60% or more, the durability of the lithium metal secondary battery 10 will be improved.

[0041] Here, the relative density of the lithium metal layer 12b when fully charged is calculated using the following formula.

[0042] (Relative density of lithium metal layer 12b when fully charged [%)) = (Theoretical deposition thickness of lithium metal [μm]) / (Maximum thickness of lithium metal layer 12b when fully charged [μm]) × 100

[0043] (Theoretical lithium metal deposition thickness [μm]) = (Residual capacity of negative electrode 12 when fully charged [mAh]) / (Theoretical capacity density of lithium metal [mAh / g]) / (Theoretical density of lithium metal [g / cm³]) 3 Projected area of ​​negative current collector 12a [cm²] 2 ])×10 4

[0044] (Residual capacity of negative electrode 12 when fully charged [mAh]) = (Charging capacity during initial charge [mAh]) - (Capacity when the voltage reaches 3.5V during initial charge [mAh])

[0045] The thickness of the lithium metal layer 12b is not particularly limited, for example, it is more than 5 μm and less than 50 μm.

[0046] There are no particular limitations on the manufacturing method of the negative electrode 12. For example, a method that can be alloyed with lithium can be plated on the negative electrode current collector 12a.

[0047] As a positive current collector 11a, there are no special limitations; for example, aluminum foil can be listed.

[0048] The thickness of the positive current collector 11a is not particularly limited, for example, it is more than 10 μm and less than 20 μm.

[0049] The positive electrode composite material layer 11b contains positive electrode active material and may also contain other components.

[0050] As a positive electrode active material, there are no particular limitations as long as it can adsorb and release lithium ions, such as lithium composite oxides.

[0051] As a lithium composite oxide, there are no particular limitations; for example, LiCoO2 and Li(Ni) could be listed. 5 / 10 Co 2 / 10 Mn 3 / 10 O2, Li(Ni) 6 / 10 Co 2 / 10 Mn 2 / 10 O2, Li(Ni) 8 / 10 Co 1 / 10 Mn 1 / 10 O2, Li(Ni) 0.8 Co 0.15 Al 0.05 O2, Li(Ni) 1 / 6 Co 4 / 6Mn 1 / 6 O2, Li(Ni) 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiCoO4, LiMn2O4, LiNiO2, LiFePO4, etc., and two or more can be used together.

[0052] The content of the positive electrode active material in the positive electrode composite layer 11b is not particularly limited, for example, it is more than 70% by mass and less than 85% by mass.

[0053] Other components include, for example, solid electrolytes, conductive additives, and binders.

[0054] The thickness of the positive electrode composite layer 11b is not particularly limited, for example, it is above 70 μm and below 90 μm.

[0055] The solid electrolyte constituting the solid electrolyte layer 13 is not particularly limited as long as it has lithium-ion conductivity; for example, oxide-based electrolytes and sulfide-based electrolytes can be listed. Among these, sulfide-based electrolytes are preferred because their reactivity with lithium metal increases the effectiveness of the protective layer 12c.

[0056] The thickness of the solid electrolyte layer 13 is not particularly limited, for example, it is more than 15 μm and less than 100 μm.

[0057] Furthermore, the lithium metal secondary battery 10 can be manufactured using known methods.

[0058] The embodiments of the present invention have been described above, but the present invention is not limited to the above embodiments. Within the scope of the spirit of the present invention, the above embodiments may be appropriately modified.

[0059] [Example]

[0060] The following describes embodiments of the present invention, but the present invention is not limited to these embodiments.

[0061] [Making the negative electrode]

[0062] Three negative electrode current collectors with different ten-point average roughness (Rz) were prepared (Table 1). Here, the Rz of the negative electrode current collectors was measured using a VHX digital microscope (manufactured by Keyence).

[0063] [Table 1]

[0064] Negative current collector type Rz[μm] A Rolled copper foil 1.0 B Electrolytic copper foil 2.5 C Electrolytic copper foil 1.7

[0065] Next, a protective layer (coating) of specified thickness and weight per unit area (refer to Table 2) is deposited onto the negative current collector to obtain the negative electrode. Here, the thickness of the protective layer is the average of the measurements from five points on the cross-sectional SEM image of the negative electrode. Furthermore, the weight per unit area of ​​the protective layer is quantified using inductively coupled plasma (ICP) luminescence spectrophotometry.

[0066] [Table 2]

[0067]

[0068]

[0069] [Examples 1-7, Comparative Examples 1-3]

[0070] The following procedures are performed in a glove box with an Ar atmosphere where the dew point is -70°C and the oxygen concentration is below 1 ppm.

[0071] <The Making of Positive Electrode>

[0072] Li(Ni) will be used as the positive electrode active material 0.6 Co 0.2 Mn 0.2 O2, thio-LISICON (Li) as a solid electrolyte 3.25 Ge 0.25 P 0.75 S4), acetylene black as a conductive additive, styrene-butadiene rubber (SBR) as a binder, and butyl butyrate as a solvent were added to a rotary mixer and stirred at 2000 rpm for three minutes. Afterward, a degassing treatment was performed for one minute, thus preparing the coating solution for the positive electrode composite layer. At this point, the mass ratio of the positive electrode active material, solid electrolyte, conductive additive, and binder was set to 75:22:3:3.

[0073] A coating solution for the positive electrode composite layer is cast onto an aluminum foil serving as the positive electrode current collector. After removing the solvent by heating to 60°C, the mixture is rolled to form a layer with a density of 3.1 g / cc and a unit area weight of 26 mg / cm³. 2 A positive electrode composite material layer is obtained to obtain the positive electrode.

[0074] <Fabrication of Solid Electrolyte Layer>

[0075] Using a 10mm diameter zirconium tube, thio-LISICON (Li) as a solid electrolyte was formed under a molding pressure of 150MPa. 3.25 Ge 0.25 P 0.75 S4) Pressing and forming to obtain a solid electrolyte layer with a diameter of 10 mm.

[0076] <Fabrication of an all-solid-state lithium metal secondary battery>

[0077] With a solid electrolyte layer sandwiched between a 10mm diameter positive electrode and a 10mm diameter negative electrode (refer to Table 3), a pressure forming process is performed at a forming pressure of 1000MPa to bond the interface between the protective layer and the solid electrolyte layer, and the interface between the positive electrode composite material layer and the solid electrolyte layer, thus obtaining an all-solid-state lithium metal secondary battery. Furthermore, in Comparative Example 1, a negative electrode current collector A without a protective layer was used as the negative electrode.

[0078] [Initial capacitance and initial resistance]

[0079] Three cycles of constant current (CC)-constant voltage (CV) charging and CC discharging were performed on the all-solid-state lithium metal secondary battery, with the discharge capacity of the first cycle set as the initial capacity. Here, the CC charging and discharging of the first and second cycles were performed at 60°C and 0.3 mA. CV charging was performed for 1 hour after the voltage reached 4.3V. The discharge termination voltage was set to prevent the release of Li ions from the protective layer (see Table 3). In the third cycle, similar to the first and second cycles, CC-CV charging was performed, and the initial resistance was measured using AC impedance spectroscopy in a 60°C thermostatic bath. Here, the initial resistance was set at a frequency of 1×10⁻⁶. 3 The resistance value of the real axis at Hz.

[0080] [Centricity and resistance after durability]

[0081] After measuring the initial resistance, CC discharge and CC-CV charging were performed in the same manner as in the first and second cycles, and the discharge capacity of the tenth cycle was set as the endurance capacity. Additionally, in the eleventh cycle, the endurance resistance was measured in the same manner as in the third cycle.

[0082] The capacitance retention rate and resistance rise rate can be calculated using the following formulas.

[0083] Capacity retention rate [%] = (Capacity after durability) / (Initial capacity) × 100

[0084] Resistance rise rate [%] = (resistance after durability) / (initial resistance) × 100

[0085] [Thickness of the protective layer when fully charged]

[0086] Based on the cross-sectional SEM image of a fully charged all-solid-state lithium metal secondary battery with measured endurance resistance (reference). Figure 6 , Figure 7 ) Calculate the thickness of the protective layer when fully charged. Here, the thickness of the protective layer when fully charged is the average of the measurements from five points in the cross-sectional SEM image.

[0087] [Volume density of the protective layer]

[0088] The volumetric capacity density of the protective layer is calculated by dividing the charging capacity at 3.5V during the first CC-CV charging cycle by the volume of the protective layer after the charge-discharge cycle. Here, the volume of the protective layer is calculated by multiplying the thickness of the protective layer when fully charged by the projected area of ​​the negative current collector.

[0089] [Relative density of lithium metal layer when fully charged]

[0090] Based on the cross-sectional SEM image of a fully charged all-solid-state lithium metal secondary battery with measured endurance resistance (reference). Figure 6, Figure 7 Find the maximum thickness of the lithium metal layer when fully charged. Then, use the following formula to calculate the relative density of the lithium metal layer when fully charged.

[0091] (Relative density of lithium metal layer at full charge [%)) = (Theoretical deposition thickness of lithium metal [μm]) / (Maximum thickness of lithium metal layer at full charge [μm]) × 100

[0092] (Theoretical deposition thickness of lithium metal [μm]) = (Residual negative electrode capacity at full charge [mAh]) / (Theoretical capacity density of lithium metal [mAh / g]) / (Theoretical density of lithium metal [g / cm³]) 3 Projected area of ​​negative current collector [cm²] 2 ])×10 4

[0093] (Residual negative electrode capacity at full charge [mAh]) = (Charging capacity of the first cycle [mAh]) - (Capacity when the voltage reaches 3.5V during the first cycle of CC-CV charging [mAh])

[0094] Table 3 shows the evaluation results of the initial capacity, initial resistance, endurance capacity, endurance resistance, capacity retention, and resistance rise rate of the all-solid-state lithium metal secondary battery.

[0095] [Table 3]

[0096]

[0097]

[0098] Table 4 shows the evaluation results of the protective layer thickness, volumetric capacity density of the protective layer, and relative density of the lithium metal layer when fully charged for all-solid-state lithium metal secondary batteries.

[0099] [Table 4]

[0100]

[0101] As shown in Tables 3 and 4, the all-solid-state lithium metal secondary batteries of Examples 1 to 7 have high capacity retention and low resistance rise rate, that is, high durability.

[0102] In contrast, the all-solid-state lithium metal secondary battery of Comparative Example 1, due to the absence of a protective layer at the negative electrode and the lack of lithium ion alloying, exhibits a larger initial capacity but reduced durability. The all-solid-state lithium metal secondary batteries of Comparative Examples 2 and 3, with protective layer volumetric capacity densities of 749 mAh / L and 841 mAh / L respectively, show a lithium metal layer forming between the protective layer and the solid electrolyte layer, resulting in reduced durability. Here, the low volumetric capacity density of the protective layer and the low relative density of the lithium metal layer at full charge also suggest the formation of a lithium metal layer between the protective layer and the solid electrolyte layer.

[0103] Figure Labels

[0104] 10: Lithium metal secondary batteries

[0105] 11: Positive electrode

[0106] 11a: Positive current collector

[0107] 11b: Positive electrode composite material layer

[0108] 12: Negative electrode

[0109] 12a: Negative current collector

[0110] 12b: Lithium metal layer

[0111] 12c: Protective layer

[0112] 13: Solid electrolyte layer

Claims

1. A lithium metal secondary battery having a solid electrolyte layer between a positive electrode and a negative electrode, the aforementioned negative electrode has a negative electrode current collector and a protective layer, the relative density of a lithium metal layer at full charge is 65% or more, the aforementioned protective layer contains a metal capable of alloying with lithium, the volumetric capacity density is 1000 mAh / L or more, the volumetric capacity density of the aforementioned protective layer is a value obtained by dividing the charge capacity at the time when the voltage of the aforementioned lithium metal secondary battery reaches 3.5 V at the time of initial constant current constant voltage charging by the volume of the aforementioned protective layer, the volume of the aforementioned protective layer is a value obtained by multiplying the thickness of the aforementioned protective layer at full charge by the projected area of the aforementioned negative electrode current collector.

2. The lithium metal secondary battery of claim 1, wherein, the aforementioned protective layer further contains an alloy of the aforementioned metal capable of alloying with lithium and lithium.

3. The lithium metal secondary battery of claim 1, wherein, the aforementioned negative electrode further has a lithium metal layer between the aforementioned negative electrode current collector and the aforementioned protective layer.

4. The lithium metal secondary battery of claim 1, wherein, the ten-point average roughness (Rz) of the aforementioned negative electrode current collector is 1.0 μm or more and 3.0 μm or less.

5. The lithium metal secondary battery of claim 1, wherein, the aforementioned metal capable of alloying with lithium is one or more selected from the group consisting of antimony, bismuth, and tin.

6. The lithium metal secondary battery of claim 1, wherein, the thickness of the aforementioned protective layer at full charge is 0.2 μm or more and 5 μm or less.

7. The lithium metal secondary battery of claim 5, wherein, the aforementioned metal capable of alloying with lithium is antimony and / or bismuth.

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