Method for manufacturing protective film

Abstract

This invention provides a method for manufacturing a protective film in which the formation of irregularities on the metallic lithium surface due to the aggregation of lithium oxide is suppressed when oxygen is introduced during the sputtering process. [Solution] A method for manufacturing a protective film, comprising: (A) step A of forming a first layer including an oxide solid electrolyte layer on metallic lithium by sputtering; and (B) step B of forming a second layer including an oxide solid electrolyte layer on the first layer by sputtering, wherein the introduced gas (a) introduced in step A does not contain oxygen, and the introduced gas (b) introduced in step B contains oxygen.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a protective film for a metallic lithium negative electrode.

Background Art

[0002] Metallic lithium secondary batteries have a high energy density and are widely applied in the fields of electricity, electronics, communication, computer industry, etc. In order to improve the battery performance of metallic lithium secondary batteries, various studies and developments have been made, and it has been devised to form a film on the lithium surface to prevent a direct reaction between a non-aqueous electrolyte and a lithium metal layer.

[0003] Patent Document 1 discloses forming an aluminum oxide layer and a carbon layer on metallic lithium by a sputtering method.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, the gas introduced in the sputtering process is only argon gas. In such a case, there is a risk that oxygen may desorb from an oxide solid electrolyte such as aluminum oxide during the sputtering process. Due to the desorption of oxygen, there is a possibility of forming a film that does not have electrical insulation, an unstable SEI film is formed, dendrites are likely to grow, and there is a problem of inducing an internal short circuit.

[0006] Furthermore, when oxygen is introduced to metallic lithium and a sputtering process is performed to suppress the deoxygenation of oxide solid electrolytes, the surface of the metallic lithium is exposed to oxygen plasma, leading to localized oxidation of the metallic lithium and the aggregation of lithium oxide. This aggregation creates irregularities on the metallic lithium surface, resulting in uneven deposition of lithium ions, which facilitates dendrite growth, induces internal short circuits, and deteriorates the cycle characteristics.

[0007] This invention was made in view of solving the above-mentioned problems, and aims to provide a method for manufacturing a protective film in which the formation of irregularities on the metallic lithium surface due to the aggregation of lithium oxide is suppressed when oxygen is introduced and a sputtering process is performed. [Means for solving the problem]

[0008] The inventors of the present invention conducted diligent studies to solve the above problems and found that a method for manufacturing a protective film, comprising the steps of forming a first layer containing an oxide solid electrolyte layer on metallic lithium by sputtering in the absence of oxygen, and forming a second layer containing an oxide solid electrolyte layer on the first layer by sputtering in the presence of oxygen, solves the above problems. The present invention was completed by further studies based on this finding and includes the following embodiments.

[0009] Section 1. A method for manufacturing a protective film, (A) Step A, in which a first layer containing an oxide solid electrolyte layer is formed on metallic lithium by sputtering, (B) Step B, in which a second layer containing an oxide solid electrolyte layer is formed on the first layer by sputtering, Includes, The introduced gas (a) in step A does not contain oxygen. A manufacturing method wherein the introduced gas (b) introduced in step B contains oxygen. Section 2. The introduced gas (a) is an inert gas, The manufacturing method according to item 1, wherein the introduced gas (b) is a mixed gas of an inert gas and oxygen. Section 3. The manufacturing method according to item 1 or 2, wherein the introduced gas (b) contains 5% by volume or more of oxygen. Section 4. The manufacturing method according to any one of claims 1 to 3, wherein the preceding layer comprises LLZO. Section 5. The manufacturing method according to any one of claims 1 to 4, wherein the target used for forming the second layer is the same as the target used for forming the first layer. Section 6. Furthermore, (C) The step C may include forming one or more intervening layers containing an oxide solid electrolyte layer on the second layer by sputtering, (D) Step D, in which a third layer including an oxide solid electrolyte layer is formed on the second layer or the intervening layer by sputtering, Includes, The target used for forming the intervening layer and the third layer is the same as the target used for forming the first layer and the second layer. The introduced gas (c) introduced in step C may contain oxygen. The manufacturing method according to item 5, wherein the oxygen concentration of the introduced gas (d) introduced in step D is higher than the oxygen concentrations of the introduced gas (b) and the introduced gas (c). Section 7. The manufacturing method according to any one of claims 1 to 6, wherein the sputtering method in step A and / or step B is a sputtering method using dielectric-coupled plasma. [Effects of the Invention]

[0010] According to the present invention, even when oxygen is introduced and the sputtering process is performed, a protective film can be manufactured in which the formation of irregularities on the metallic lithium surface due to the aggregation of lithium oxide is suppressed. [Brief explanation of the drawing]

[0011] [Figure 1]This is a diagram showing Structural Example 1 of the protective film manufactured according to the present invention. [Figure 2] This is a diagram showing Structural Example 2 of the protective film manufactured according to the present invention. [Figure 3] This is a diagram showing the effect of the protective film according to the present invention on the aggregation of lithium oxide on the surface of metallic lithium. [Figure 4] This is a diagram showing the cycle characteristics when metallic lithium having the protective film according to the present invention is used as a lithium negative electrode.

Embodiments for Carrying Out the Invention

[0012] <Definition of Terms> In the present invention, "including" and "containing" are concepts that encompass any of "comprise", "consist essentially of", and "consist of".

[0013] In the present invention, the "protective film" is a layer structure formed on metallic lithium. When metallic lithium is used as a lithium electrode, the protective film generates a stable passive film (SEI) and prevents a direct reaction between the non-aqueous electrolyte and the lithium metal layer.

[0014] In the present invention, the "sputtering method" refers to a method for manufacturing a thin film similar to physical vapor deposition. In the sputtering method, a coating target and a film material (target) are installed in a sealed processing chamber, and an introduction gas such as argon is introduced. Then, a voltage is applied to ionize the introduction gas and make it collide with the target. Thereby, the surface atoms of the target are ejected and reach the substrate, forming a thin film. Depending on the technique, the sputtering method is classified into a diode sputtering method, a magnetron sputtering method, a DC sputtering method, an RF sputtering method, etc.

[0015] In the present invention, the "oxide solid electrolyte layer" refers to a layer structure included in the protective film manufactured according to the present invention, and contains at least one type of oxide solid electrolyte. The first layer, second layer, intervening layer, and third layer of the present invention each contain an oxide solid electrolyte layer and may be substantially composed of an oxide solid electrolyte layer.

[0016] In this invention, when a numerical range is indicated as "A to B", the numerical range means "greater than or equal to A and less than or equal to B".

[0017] 1. Method for manufacturing protective film The present invention is a method for manufacturing a protective film, (A) Step A, in which a first layer containing an oxide solid electrolyte layer is formed on metallic lithium by sputtering, (B) Step B, in which a second layer containing an oxide solid electrolyte layer is formed on the first layer by sputtering, Includes, The introduced gas (a) in step A does not contain oxygen. The manufacturing method includes a method in which the introduced gas (b) introduced in step B contains oxygen.

[0018] Figure 1 shows an example of the structure of a protective film manufactured according to the present invention. In Figure 1, the protective film 1a manufactured according to the present invention is formed on metallic lithium 11 and comprises, in this order, a first layer 12 containing an oxide solid electrolyte layer and a second layer 13 containing an oxide solid electrolyte layer on the metallic lithium 11.

[0019] In step A, an oxide solid electrolyte layer is formed on metallic lithium using an oxygen-free introduced gas, thereby suppressing localized oxidation of the metallic lithium surface by exposing it to oxygen plasma. Furthermore, in step B, an oxide solid electrolyte layer is further formed on the oxide solid electrolyte layer formed in the absence of oxygen using an oxygen-containing introduced gas, thereby suppressing the detachment of oxygen from the oxide solid electrolyte.

[0020] To suppress the generation of by-reaction products, the introduction gas used in the present invention preferably contains an inert gas. Examples of inert gases include noble gases such as helium (He), neon (Ne), argon (Ar), krypton (Kr), or xenon (Xe), and nitrogen. These may be used individually or as mixtures thereof. It is preferable that 95% or more by volume of the oxygen-free component of the introduction gas used in the present invention be an inert gas, more preferably 98% or more by volume be an inert gas, and even more preferably 99% or more by volume be an inert gas. It is particularly preferable that the introduction gas (a) introduced in step A consists substantially of an inert gas, and the introduction gas (b) introduced in step B consists substantially of a mixture of an inert gas and oxygen. Furthermore, because it is inexpensive, it is preferable to use argon as the inert gas.

[0021] To suppress the detachment of oxygen from the oxide solid electrolyte, the introduced gas (b) introduced in step B preferably contains 3% or more by volume of oxygen, and more preferably 5% or more by volume. On the other hand, excessive oxygen supply significantly reduces the film deposition rate, so the introduced gas (b) preferably contains 70% or less by volume of oxygen, and more preferably 50% or less by volume.

[0022] The target used in the present invention includes an oxide solid electrolyte. The oxide solid electrolyte contained in the target is not particularly limited, but for example, lithium lanthanum zirconate (LLZO:Li7La3Zr2O 12 ), Lithium aluminum titanium phosphate (LATP:Li 1+x Al x Ti 2-x (PO4)3) Lithium phosphorus oxynitride (LiPON:Li 5+x P2O 6-x N 1+x) and the like can be used. These may be used individually or as a mixture. The target preferably contains LLZO because it has thermal and chemical stability against reaction with metallic lithium. In particular, the first layer of the oxide solid electrolyte layer that is in direct contact with metallic lithium preferably contains LLZO.

[0023] In the present invention, the target used for forming the second layer of the oxide solid electrolyte layer may be the same as or different from the target used for the first layer. Using a different target is preferable because it allows for the imparting of arbitrary properties such as electrical efficiency and stability. On the other hand, when the target used for forming the second layer is the same as the target used for the first layer, it is preferable because it is advantageous in terms of cost and the manufacturing time of the protective film can be shortened.

[0024] In the present invention, the thicknesses of the first and second layers are not particularly limited and may be appropriately changed according to the composition of the target used and the application. For example, when metallic lithium is used as the lithium electrode, the thickness of the first layer is preferably 0.5 nm to 40 nm, and more preferably 1 nm to 20 nm. The thickness of the second layer is preferably 2 nm to 120 nm, and more preferably 5 nm to 80 nm.

[0025] The present invention may include a step of forming a layer containing an oxide solid electrolyte layer on a second layer formed by the method described above, using the same target as used for forming the first and second layers, by sputtering. The number of layers is not particularly limited, and the protective film produced by the present invention may have a multilayer structure. Therefore, the present invention is (C) The step C may include forming one or more intervening layers containing an oxide solid electrolyte layer on the second layer by sputtering, (D) The step D includes forming a third layer containing an oxide solid electrolyte layer on the second layer or the intervening layer by sputtering, The manufacturing method includes a method in which the target used for forming the intervening layer and the third layer is the same as the target used for forming the first layer and the second layer.

[0026] Figure 2 shows an example of the structure of a protective film manufactured according to the present invention. In Figure 2, the protective film 1b manufactured according to the present invention is formed on metallic lithium 11 and comprises, in this order, a first layer 12 containing an oxide solid electrolyte layer, a second layer 13 containing an oxide solid electrolyte layer, and a third layer 15 containing an oxide solid electrolyte layer on the metallic lithium 11. The protective film 1b may also optionally include an intervening layer 14 containing an oxide solid electrolyte layer between the second layer 13 containing an oxide solid electrolyte layer and the third layer 15 containing an oxide solid electrolyte layer.

[0027] The intervening layer is located between the second and third layers. When forming the intervening layer, it is preferable that the introduced gas (c) contains an inert gas. Furthermore, in order to suppress the detachment of oxygen from the oxide solid electrolyte, it is preferable that the introduced gas (c) contains oxygen.

[0028] In step D, which forms the third layer, it is preferable that the oxygen concentration of the introduced gas (d) is higher than that of the introduced gases (b) and (c). In this case, when the target surface is excessively oxidized and the first layer is formed on the next metallic lithium, the decrease in the oxygen composition of the solid electrolyte on the target surface is suppressed, and the stability of the first layer is improved.

[0029] In a further embodiment, when performing step C to form an intervening layer and step D to form a third layer, it is preferable to perform step C multiple times while gradually increasing the oxygen concentration of the introduced gas (c) to form a multilayer intervening layer before forming the third layer. In this case, the protective film can be manufactured using the same equipment and target, simply by changing the oxygen concentration of the introduced gas, thereby shortening the manufacturing time of the protective film.

[0030] In the present invention, the thickness of the intervening layer and the third layer is not particularly limited and may be appropriately changed according to the composition of the target used and the application. For example, when metallic lithium is used as the lithium electrode, the total thickness of the second layer, intervening layer and third layer is preferably 2 nm to 120 nm, and more preferably 5 nm to 80 nm.

[0031] In the present invention, the sputtering method in step A and / or step B may be a sputtering method using dielectric-coupled plasma (ICP sputtering method). When implementing the ICP sputtering method, the apparatus used may be, for example, an apparatus disclosed in Japanese Patent Publication 6264248. The ICP sputtering method is preferable because the plasma diffuses throughout the processing chamber, allowing for uniform film formation and a high film formation rate. However, due to the diffusion of the plasma, there is a high possibility that the metallic lithium will be exposed to oxygen plasma. Therefore, the effect of the present invention is enhanced by ensuring that the introduced gas (a) introduced in step A, which forms the first layer, does not contain oxygen. Furthermore, in step B, which forms the second layer by the ICP sputtering method, oxidation of the solid electrolyte on the target surface is promoted. Therefore, when forming the first layer on the next metallic lithium, the decrease in the oxygen composition of the solid electrolyte on the target surface is suppressed, and the stability of the first layer is improved.

[0032] Similarly, when performing process C and / or process D, the sputtering method may be ICP sputtering. Therefore, the sputtering method for any one of processes A, B, C, and D may be ICP sputtering, and it is preferable that the sputtering method for all processes A, B, C, and D is ICP sputtering.

[0033] The present invention further includes a lithium electrode comprising metallic lithium on which a protective film manufactured according to the present invention is formed. Such an electrode has excellent cycle characteristics and suppresses the decrease in discharge capacity when repeated charging and discharging occurs. [Examples]

[0034] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way to these examples.

[0035] Example 1 The effect of a protective film in suppressing surface irregularities on the metallic lithium surface caused by the aggregation of lithium oxide was evaluated. The protective film was formed as follows. First, metallic lithium and LLZO were placed in the processing chamber of a sputtering apparatus. Argon gas was introduced into the processing chamber, a voltage was applied to generate argon plasma, and a negative voltage was applied to the metallic lithium to deposit a first layer containing a solid electrolyte layer. Furthermore, a mixed gas containing 95 volume% argon gas and 5 volume% oxygen gas was introduced, and a second layer containing a solid electrolyte layer was deposited in the same manner. As a comparative example, a protective film was formed on metallic lithium under the same conditions as the second layer in Example 1. The metallic surface of the metallic lithium was photographed using a scanning electron microscope (JEOL Ltd., JCM-7000). The results are shown in Figure 3.

[0036] In the comparative example, lithium oxide aggregates on the surface of the metallic lithium, whereas the formation of the protective film according to the present invention suppresses the aggregation of lithium oxide and inhibits the formation of irregularities on the metallic lithium surface.

[0037] Example 2 The metallic lithium with a protective film, manufactured in Example 1, was used as the lithium anode, and its cycle characteristics were evaluated by repeatedly charging and discharging it. As a comparative example, the cycle characteristics of metallic lithium without a protective film were evaluated as the lithium anode. The results are shown in Figure 4.

[0038] When using metallic lithium having a protective film according to the present invention, it can be seen that the cycle characteristics of the lithium anode are improved. [Explanation of Symbols]

[0039] 1a Protective film (Structure example 1) 1b Protective film (Structure example 2) 11. Lithium metal 12. First layer containing oxide solid electrolyte layer 13. Second layer containing oxide solid electrolyte layer 14 Interlayer containing an oxide solid electrolyte layer 15 Third layer containing oxide solid electrolyte layer

Claims

1. A method for manufacturing a protective film, (A) Step A, in which a first layer containing an oxide solid electrolyte layer is formed on metallic lithium by sputtering, (B) Step B, in which a second layer containing an oxide solid electrolyte layer is formed on the first layer by sputtering, Includes, The introduced gas (a) in step A does not contain oxygen. A manufacturing method wherein the introduced gas (b) introduced in step B contains oxygen.

2. The introduced gas (a) is an inert gas, The manufacturing method according to claim 1, wherein the introduced gas (b) is a mixed gas of an inert gas and oxygen.

3. The manufacturing method according to claim 1, wherein the introduced gas (b) contains 5% by volume or more of oxygen.

4. The manufacturing method according to claim 1, wherein the first layer comprises LLZO.

5. The manufacturing method according to claim 1, wherein the target used for forming the second layer is the same as the target used for forming the first layer.

6. Furthermore, (C) The process may include step C of forming one or more intervening layers containing an oxide solid electrolyte layer on the second layer by sputtering, (D) Step D, in which a third layer including an oxide solid electrolyte layer is formed on the second layer or the intervening layer by sputtering. Includes, The target used for forming the intervening layer and the third layer is the same as the target used for forming the first layer and the second layer. The introduced gas (c) in step C may contain oxygen. The manufacturing method according to claim 5, wherein the oxygen concentration of the introduced gas (d) introduced in step D is higher than the oxygen concentrations of the introduced gas (b) and the introduced gas (c).

7. The manufacturing method according to claim 1, wherein the sputtering method in step A and / or step B is a sputtering method using dielectric-coupled plasma.

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