Alkali metal evaporation device, pre-lithiation method and pre-lithiation negative electrode

Abstract

The invention provides an alkali metal evaporation device which comprises an unwinding mechanism, a winding chamber for accommodating the winding mechanism, and an evaporation chamber which is arranged between the unwinding mechanism and the winding chamber and is used for accommodating an alkali metal evaporation mechanism, wherein an atmosphere separation mechanism is provided between the winding chamber and the vapor deposition chamber, and the winding chamber is provided with a decompression mechanism capable of moving the winding chamber while maintaining an internal decompression state. In addition, the present invention also provides a pre-lithiation method implemented using the alkali metal vapor deposition apparatus, a pre-lithiated negative electrode obtained by the pre-lithiation method, and a lithium ion secondary battery including the pre-lithiated negative electrode.

Description

Technical Field

[0001] This invention relates to an apparatus and a method for vapor deposition of alkali metals on a flexible substrate (typically a mesh substrate). A typical example of a vapor deposition apparatus is one that performs vapor deposition in a roll-to-roll manner. The vapor deposition apparatus can be used, for example, in the production of thin-film solar cells, thin-film batteries, and flexible displays. Background Technology

[0002] In recent years, secondary batteries utilizing energy storage technologies based on alkali metals such as lithium have attracted considerable attention. Among them, lithium-ion batteries, with their high energy density, are being used in electronic devices and electric vehicles.

[0003] Graphite is the primary anode material used in lithium-ion batteries; however, its theoretical capacity is only 372 mAh / g. To meet the increasing energy density demands of lithium-ion batteries in recent years, anode materials with higher capacity are desired. Therefore, silicon-based anodes have attracted attention in recent years. The theoretical capacity of pure silicon is 3579 mAh / g (Li₂O₃). 15 Si4) and silicon monoxide (SiO) have a theoretical capacity of 2680 mAh / g, which is higher than that of graphite.

[0004] However, in the actual use of silicon-based anodes, due to the large volume expansion and contraction of silicon as an active material particle during charging and discharging, there are problems that need to be overcome, such as particle breakage, damage to the interface film, and peeling from the current collector. Therefore, silicon-based anodes made by mixing silicon and graphite are generally used.

[0005] Furthermore, a problem with silicon-based anodes is that they have a larger initial irreversible capacity compared to graphite anodes. This is because during the initial charging process, lithium ions react with the electrolyte or SiO material to form a solid electrolyte interface (SEI) layer mainly composed of lithium silicate (Li4SiO4) and lithium oxide (Li2O). The initial coulombic efficiency (ICE: the ratio of initial discharge capacity to initial charge capacity during the first charge-discharge cycle, where a larger irreversible capacity corresponds to a smaller initial coulombic efficiency) of graphite anodes is 90-95%, while the ICE of silicon-based anodes is 70-90%, lower than that of graphite. To address this issue, a technique called pre-lithiation has been proposed in the manufacturing process of lithium-ion batteries (see Patent Document 1).

[0006] Pre-lithiation technology refers to the process of adding irreversible lithium to the negative electrode before the battery manufacturing process is complete. Pre-lithiation technology can improve the energy density of lithium-ion batteries and extend their cycle life. Irreversible capacity refers to the lithium capacity consumed by the solid-electrolyte interlayer (SEI) formed on the negative electrode. This is mainly due to the decomposition of the electrolyte during the initial charging process, and the formation of the SEI is usually unavoidable in rechargeable lithium batteries.

[0007] For example, the following pre-lithiation methods have been reported.

[0008] (1) Pre-lithiation technology of attaching or transferring lithium metal foil to the surface of the negative electrode (see Patent Document 2 and Patent Document 3).

[0009] (2) A technique of pre-lithiating the negative electrode by placing it in an electrolyte tank and reacting it with lithium metal from the counter electrode (see Patent Document 4).

[0010] (3) A technique of pre-lithiation by immersing the negative electrode in a strongly reducing lithium-containing solution and performing a direct redox reaction (see Patent Document 5).

[0011] (4) A technique for prelithiation by coating a printable lithium composition containing surface-stabilized lithium metal powder (SLMP) onto the surface of the negative electrode (see Patent Document 6 and Patent Document 7).

[0012] (5) A technique for pre-lithiation of a negative electrode material by reacting it with a lithium metal source in the process of forming a negative electrode powder material or a process of forming a negative electrode mixture (see Patent Documents 8, 9 and 10).

[0013] The physical vapor deposition (PVD) technology used here is an advantageous production method in the thin film field. It can accurately fill the amount of lithium required for pre-lithiation. Furthermore, the highly active lithium metal is not easily affected by atmospheric gases because it is processed under vacuum. Therefore, this technology is expected to be a method to improve the energy density of lithium-ion batteries.

[0014] However, the surface of the anode after pre-lithiation is unstable, and the pre-lithiation effect is reduced due to the influence of a small amount of moisture and / or oxygen in the drying chamber.

[0015] Various pre-lithiation methods are known, and the electrodes need to be carefully handled in the post-processing steps. However, since this is a technical secret in battery production, there are very few reports on related processing.

[0016] For example, in Patent Document 11, in order to prevent the resin film treated in a vacuum from absorbing oxygen and moisture before being placed in the apparatus for the next process, the wound resin film is placed in a storage container under reduced pressure, and then dry gas is introduced into the storage container for storage, wherein the treatment time in the atmosphere before being transferred to a dedicated storage container is controlled to be within 60 minutes.

[0017] However, pre-lithiated anodes made by doping lithium metal are highly sensitive to atmospheric gases. In lithium battery manufacturing, especially in processes affected by moisture, manufacturing equipment is typically installed in a drying chamber. However, pre-lithiated anodes are still sensitive to atmospheric gases, thus requiring strict operational management within the drying chamber.

[0018] Existing technical documents

[0019] Patent documents

[0020] Patent Document 1: Japanese Patent Application Publication No. 2020-534654

[0021] Patent Document 2: WO2023 / 146254A1

[0022] Patent Document 3: US20220052307A1

[0023] Patent Document 4: KR10-2022-0142638A1

[0024] Patent Document 5: KR10-2023-0128605A1

[0025] Patent Document 6: WO2022 / 216460A1

[0026] Patent Document 7: US20240113274A1

[0027] Patent Document 8: US20230411598A1

[0028] Patent Document 9: US20200227723A1

[0029] Patent Document 10: US20160126543A1

[0030] Patent Document 11: Japanese Patent Application Publication No. 2007-120777 Summary of the Invention

[0031] [The technical problem to be solved by this invention]

[0032] As mentioned above, the anode after pre-lithiation is unstable because active lithium metal is used in the pre-lithiation process.

[0033] Therefore, the object of the present invention is to provide an alkali metal vapor deposition apparatus that can suppress the performance degradation of a negative electrode (e.g., a pre-lithiated negative electrode) formed by vapor deposition of alkali metal on a negative electrode substrate and thereby manufacture a high-safety lithium-ion secondary battery including the negative electrode.

[0034] [Technical Solution]

[0035] To achieve the above-mentioned objectives, the present invention adopts the following technical solution.

[0036] The alkali metal vapor deposition apparatus of the present invention includes an unwinding mechanism, a winding chamber accommodating the winding mechanism, and a vapor deposition chamber accommodating the alkali metal vapor deposition mechanism between the unwinding mechanism and the winding chamber. An atmosphere separation mechanism is provided between the winding chamber and the vapor deposition chamber, and the winding chamber is provided with a depressurization mechanism capable of moving the winding chamber while maintaining an internal depressurization state or a mechanism capable of moving the winding chamber while maintaining an internal inert atmosphere.

[0037] According to a preferred embodiment of the invention, the internal depressurization state of the winding chamber indicates that the total pressure is in the range of 1.0E-5 Pa to 10 Pa, the oxygen partial pressure is in the range of 1.0E-7 Pa to 1.0E-3 Pa, and the water pressure is in the range of 1.0E-7 Pa to 1.3E-3 Pa.

[0038] According to a preferred embodiment of the present invention, the pressure reduction mechanism is a cooling trap, or an exhaust pump such as a turbomolecular pump, a dry vacuum pump, or a cryogenic pump, wherein the coolant used in the cooling trap is a coolant capable of achieving a temperature of -40°C or lower.

[0039] According to a further preferred embodiment of the invention, the alkali metal vapor deposition apparatus is further configured with a mechanism for unwinding and winding a mask to pattern the alkali metal film formed by vapor deposition.

[0040] According to a further preferred embodiment of the present invention, the alkali metal is sodium or lithium.

[0041] According to a preferred embodiment of the present invention, the inert atmosphere includes a rare gas atmosphere, a nitrogen atmosphere, or other inert atmospheres.

[0042] The present invention also relates to a pre-lithiation method, which is carried out by evaporating lithium on a negative electrode substrate using an alkali metal evaporation apparatus as described above, wherein the negative electrode is a graphite negative electrode or a silicon-based negative electrode.

[0043] The present invention further relates to a pre-lithiated anode, which is prepared by using the pre-lithiation method described above.

[0044] The present invention further relates to a lithium-ion secondary battery, comprising a positive electrode, a negative electrode and a separator, wherein the negative electrode is a pre-lithiated negative electrode as described above.

[0045] [Technical Effects]

[0046] By using the alkali metal vapor deposition apparatus of the present invention to perform a pre-lithiation process, the performance degradation of the obtained pre-lithiation anode can be suppressed, thereby manufacturing a high-safety lithium-ion secondary battery including the pre-lithiation anode. Attached Figure Description

[0047] Figure 1 The initial coulombic efficiency (ICE) of half-cells fabricated under various conditions using pre-lithiated electrodes from Reference Examples 1-2 and Comparative Examples 1-4 is shown.

[0048] Figure 2 The initial coulombic efficiency (ICE) of half-cells using pre-lithiated electrodes from Examples 1-3 and Comparative Examples 5-6, maintained for 1 day under different pressure atmospheres, is shown.

[0049] Figure 3 The initial coulombic efficiency (ICE) of half-cells using pre-lithiated electrodes maintained for 1 day at different oxygen partial pressures, as shown in Examples 1-2 and Comparative Examples 7-9, is displayed.

[0050] Figure 4 The changes in the initial coulombic efficiency (ICE) of the pre-lithiated anode stored in dry air (the same atmosphere as Comparative Example 2) and vacuum atmosphere (the same atmosphere as Example 2) over time are shown.

[0051] Figure 5 This is a schematic diagram of an example of the vapor deposition apparatus of the present invention.

[0052] Figure 6 This is a schematic diagram of two exemplary winding chambers that can be used in the vapor deposition apparatus of the present invention. Detailed Implementation

[0053] The present invention will be described below through specific embodiments.

[0054] During the initial charging process of a lithium-ion secondary battery, lithium ions react with the electrolyte or negative electrode material to form a solid-electrolyte interlayer (SEI), leading to a decrease in the initial coulombic efficiency of the negative electrode. To suppress this decrease, metallic lithium can be pre-filled onto the negative electrode (the so-called "pre-lithiation technology"). Pre-lithiation technology is generally divided into direct pre-lithiation technology and indirect pre-lithiation technology. Direct pre-lithiation technology refers to the direct deposition of lithium onto the negative electrode. As an indirect pre-lithiation technology, known techniques include depositing lithium onto a support substrate with a release material and then transferring the lithium onto the surface of the negative electrode (see Patent Document 2), and depositing lithium onto a separator and then assembling the separator and the negative electrode into a battery cell.

[0055] It should be noted that this invention relates to direct pre-lithiation technology. Specifically, this invention employs physical vapor deposition (PVD) technology to directly deposit metallic lithium on a negative electrode (e.g., a graphite or silicon-based negative electrode) to precisely control the amount of lithium required for pre-lithiation. However, the inventors discovered in their research that the desired high initial coulombic efficiency cannot be achieved solely through physical vapor deposition (PVD). Further research revealed that while depositing lithium on the negative electrode increases the lithium content, the added lithium on the negative electrode surface may not exert the expected effect. Although the underlying reasons are not fully understood, it is speculated that lithium deposited on the negative electrode surface has high reactivity and readily reacts with moisture and oxygen in the air, thus preventing its participation in the battery's charge-discharge cycle. Therefore, the inventors have developed a novel alkali metal evaporation device that can keep the deposited electrode under reduced pressure or inert atmosphere before battery assembly to avoid interference from the atmospheric environment. Thus, the pre-lithiated negative electrode prepared using this alkali metal evaporation device can achieve the desired high initial coulombic efficiency.

[0056] The alkali metal vapor deposition apparatus of the present invention includes an unwinding mechanism, a winding chamber accommodating the winding mechanism, and a vapor deposition chamber accommodating the alkali metal vapor deposition mechanism between the unwinding mechanism and the winding chamber. An atmosphere separation mechanism is provided between the winding chamber and the vapor deposition chamber, and the winding chamber is equipped with a depressurization mechanism that can move the winding chamber while maintaining an internal depressurization state or a mechanism that can move the winding chamber while maintaining an internal inert atmosphere.

[0057] Figure 5 The vapor deposition apparatus shown (left) is a single-sided film forming machine, including a vapor deposition chamber and a winding chamber. A vapor deposition mechanism is installed in the vapor deposition chamber, and a winding mechanism is installed in the winding chamber. An atmosphere separation mechanism is provided between the vapor deposition chamber and the winding chamber so that the winding chamber can be separated from the vapor deposition chamber in an airtight manner. The winding chamber is equipped with a decompression mechanism that can move the winding chamber to the next process while maintaining the internal decompression state. Figure 5 (Right) is a schematic diagram showing the processing apparatus for the next step. Additionally, the alkali metal vapor deposition apparatus of the present invention can also be a double-sided film deposition machine.

[0058] like Figure 6As shown, the depressurization mechanism may include a cooling trap (e.g., a liquid nitrogen trap) installed within the winding chamber (left side) or an exhaust pump (e.g., a turbomolecular pump, a dry vacuum pump, or a cryogenic pump) connected to the winding chamber (right side) to maintain a depressurized (or vacuum) state inside the winding chamber, particularly to maintain low moisture and oxygen partial pressures. The depressurization state inside the winding chamber includes a total pressure in the range of 1.0E-5 Pa to 10 Pa, preferably 2.0E-5 Pa to 0.1 Pa; an oxygen partial pressure in the range of 1.0E-7 Pa to 1.0E-3 Pa, preferably 2.0E-7 Pa to 5.0E-4 Pa; and a moisture pressure in the range of 1.0E-7 Pa to 1.3E-3 Pa, preferably 1.0E-6 Pa to 5.0E-4 Pa. By controlling the total pressure, oxygen partial pressure, and moisture pressure within the winding chamber within their respective ranges, the pre-lithiated electrode can be protected from interference by oxygen and moisture in the air, thereby maintaining its high initial coulombic efficiency, i.e., maintaining the desired pre-lithiation effect.

[0059] Preferably, a cooling trap is used as the depressurization mechanism. When a cooling trap is installed in the winding chamber, gas can be continuously discharged from the winding chamber even without power supply, allowing the winding chamber to be moved while maintaining an internal depressurization state without the need for an additional power supply mechanism. As the coolant used in the cooling trap, to maintain the water venting rate, a coolant capable of achieving temperatures of -40°C or lower is desirable, such as a mixture of dry ice and an organic solvent (e.g., ethanol or acetone) or liquid nitrogen. Furthermore, a mixture of dry ice and a mixed solvent including water and methanol can also be used as the aforementioned coolant.

[0060] Alkali metals include lithium, sodium, potassium, rubidium, cesium, or francium, with sodium or lithium being preferred.

[0061] like Figure 5 As shown, the alkali metal vapor deposition apparatus is also equipped with a mechanism for unwinding and winding the mask to pattern the alkali metal film formed by vapor deposition.

[0062] The process or method for preparing a pre-lithiated negative electrode using the alkali metal evaporation apparatus of the present invention includes the following steps:

[0063] 1) Deposit lithium metal on the negative electrode substrate to form a lithium film with a thickness of 0.1 μm to 10 μm, for example, 1 μm to 3 μm;

[0064] 2) Activate the atmosphere separation mechanism to separate the atmospheres of the vapor deposition chamber and the winding chamber;

[0065] 3) Open the vapor deposition chamber to the atmosphere;

[0066] 4) Use a slitting machine to cut the lithium-deposited negative electrode substrate between the evaporation chamber and the atmosphere separation mechanism (during this period, the inside of the winding chamber is kept under reduced pressure);

[0067] 5) Use a transfer machine to transport the winding chamber to the next process;

[0068] 6) Internal maintenance and filling of the vapor deposition source with lithium metal;

[0069] 7) Set up the next winding chamber and exhaust (or evacuate) the vapor deposition chamber and the winding chamber.

[0070] Preferably, in the above process or method, the winding chamber is kept under reduced pressure or in an inert atmosphere throughout the entire process after step 2) and before step 5).

[0071] The present invention also provides a pre-lithiation method, which is carried out by depositing lithium on a negative electrode substrate using an alkali metal evaporation device. The negative electrode is a graphite negative electrode or a silicon-based negative electrode. Silicon-based negative electrodes (also known as silicon-containing negative electrodes) include silicon-oxygen negative electrodes, silicon-carbon negative electrodes, etc., wherein silicon-carbon negative electrodes include negative electrodes formed by mixing silicon and graphite.

[0072] The present invention also provides a pre-lithiated anode prepared by using the pre-lithiation method.

[0073] The present invention also provides a lithium-ion secondary battery including the pre-lithiated negative electrode.

[0074] Example

[0075] [Stability of the pre-lithiated electrode]

[0076] Half-cells using graphite materials were fabricated, and pre-lithiation evaluation was conducted.

[0077] <Pre-lithiation treatment via lithium evaporation>

[0078] The electrode substrate used for pre-lithiation was a graphite electrode substrate (graphite layer / copper foil) purchased from Shenzhen Youyan Technology Co., Ltd. The evaporation apparatus consisted of a glove box, a transfer chamber, and a deposition chamber. The target electrode substrate was placed on a sample holder in the glove box and transported to the deposition chamber via the transfer chamber. Lithium metal in a crucible placed in the deposition chamber was heated with a lamp heater, and lithium evaporation was performed while the sample holder was rotated. The deposition rate and thickness were controlled using a Quartz Crystal Microbalance (QCM) film thickness monitor at a rate of 50 Å / s. After evaporating the lithium, the electrode substrate was evaporated, and then heat-treated at 130°C using a lamp heater to complete the activation treatment of the pre-lithiation reaction.

[0079] <Making a Half-Battery>

[0080] After the pre-lithiated graphite electrode substrate is stored in an electrode storage box, it is transferred to a glove box for battery production. The processing electrode is a working electrode cut to a size of φ14mm, and the counter electrode is a lithium foil (50μm) cut to a size of φ16mm. The electrolyte is a substance (manufactured by DoDo Chem) in which LiPF6 is dissolved at a ratio of 1M (mol / L) in an equal volume mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). The separator is a porous material made of polypropylene (manufactured by Celgard, #2325). These are used to manufacture button batteries.

[0081] Evaluation of half-cell (battery performance)

[0082] The initial discharge conditions were constant current discharge conditions with a low current C / 10 (1C represents the current that can fully discharge the cell in 1 hour) and a cutoff voltage of 0.01V. The initial charging conditions were constant current charging conditions with a low current C / 10 and a cutoff voltage of 1.5V. The initial coulombic efficiency (ICE) of each half-cell was evaluated. The ICE of the fabricated half-cell was calculated using the following formula.

[0083] ICE (%) = (Initial charge capacity / Initial discharge capacity) × 100

[0084] (Note: The original battery (full cell) started from charging, but this time the half cell starts from discharging, so ICE is calculated according to the formula above.)

[0085] <Reference Example 1>

[0086] The ICE results for the half-cell fabricated with electrodes without pre-lithiation treatment showed an 85.8%.

[0087] <Reference Example 2>

[0088] After pre-lithiation treatment with a lithium vapor deposition film thickness of 1.7 μm, a half-cell was fabricated without electrode preservation, and the results showed an ICE of 102.6%. The ICE exceeding 100% is due to the presence of unconsumed active lithium during the formation of the solid electrolyte interlayer (SEI). In subsequent embodiments, the pre-lithiation lithium film thickness was set to 1.7 μm.

[0089] <Comparative Example 1>

[0090] After pre-lithiation treatment, the electrode was stored for one day in an atmosphere at room temperature and 20% humidity (in a winter room without humidity control). As a result, the ICE dropped significantly to 87.5%, showing a value approximately the same as that of Reference Example 1. It is believed that the pre-lithiation effect was completely lost through the reaction of the electrode with moisture in the atmosphere.

[0091] <Comparative Example 2>

[0092] After pre-lithiation, the electrode was stored for one day in a glove box equipped with a moisture removal filter and in an atmosphere of circulating dry air (dew point -50°C, moisture concentration approximately 40 ppm, oxygen concentration 20%). The ICE value dropped significantly to 87.5%, showing a value approximately the same as in Reference Example 1. This result indicates that, in addition to moisture, the pre-lithiation electrode is also affected by oxygen.

[0093] <Comparative Example 3>

[0094] After pre-lithiation, the sample was stored for one day in a glove box equipped with a moisture removal filter and circulating argon atmosphere (dew point -50°C, oxygen concentration 0.01–0.1%). The ICE value dropped significantly to 86.0%, showing a value approximately the same as in Reference Example 1. This indicates that although the oxygen concentration was lower than in Comparative Example 2, further reduction in oxygen concentration is necessary to maintain the effectiveness of pre-lithiation.

[0095] <Comparative Example 4>

[0096] After pre-lithiation, the sample was stored for one day in a glove box equipped with moisture and oxygen removal filters in a circulating argon atmosphere (moisture concentration less than 0.1 ppm, oxygen concentration 1-10 ppm). The ICE result was 94.0%. Although this value is slightly lower than that of Reference Example 1, a high ICE retention performance was achieved by storing the sample in an argon atmosphere with reduced moisture and oxygen concentrations.

[0097] The initial coulombic efficiency (ICE) of half-cells prepared under various conditions in Reference Examples 1-2 and Comparative Examples 1-4 is summarized in [the table below]. Figure 1 middle.

[0098] Next, to obtain better preservation performance of the pre-lithiated electrode, the pre-lithiated electrode was stored in a vacuum container and evaluated. Additionally, the exhaust speed of the vacuum pump was adjusted to regulate the vacuum pressure during preservation.

[0099] <Example 1>

[0100] After pre-lithiation, the pre-lithiated electrode was stored for one day in a high vacuum atmosphere of 2.2E-5 Pa generated using a turbomolecular pump (TMP). The results showed that the ICE was 101.1%, comparable to Reference Example 2, indicating high ICE retention performance.

[0101] <Example 2>

[0102] After pre-lithiation, the pre-lithiated electrode was stored for one day in a medium vacuum atmosphere of 0.14 Pa created using a dry vacuum pump (DRP). The results showed that the ICE was 100.4%, achieving the same high ICE retention performance as in Example 1.

[0103] <Example 3>

[0104] After pre-lithiation, the pre-lithiated electrode was stored in a low vacuum atmosphere of 10 Pa for 1 day by adjusting the pipe resistance (flow conductance) between the dry vacuum pump (DRP) and the electrode storage box. The results showed that the ICE was 100.5%, and the same high ICE retention performance was obtained as in Example 1.

[0105] <Comparative Example 5>

[0106] After pre-lithiation, the pre-lithiated electrode was stored in a low vacuum atmosphere of 20 Pa for 1 day by adjusting the pipe resistance (flow conductance) between the dry vacuum pump (DRP) and the electrode storage box. The results showed that the ICE was 92.6%, and a decrease in ICE was observed.

[0107] <Comparative Example 6>

[0108] After pre-lithiation, the pre-lithiated electrode was stored in a low vacuum atmosphere of 30 Pa for 1 day by adjusting the pipe resistance (flow conductance) between the dry vacuum pump (DRP) and the electrode storage box. The results showed that the ICE was 86.7%, and a greater ICE decrease was observed than in Comparative Example 5.

[0109] The initial coulombic efficiencies of the half-cells obtained after storing the pre-lithiated electrodes under different pressures for one day in Examples 1-3 and Comparative Examples 5-6 are shown in the figures. Figure 2 middle.

[0110] Next, in order to assess the effect of the remaining gas in the vacuum, the evaluation was carried out by controlling the partial pressure of the gas.

[0111] Clearly, dew point (moisture concentration) control is necessary when handling lithium metal. For example, the safety handling precautions in the safety data sheet of Honjo Metal, Co., Ltd. state that to maintain the metallic luster of lithium, processing should be carried out in a dry atmosphere with a dew point below -30°C. A moisture concentration at a dew point of -30°C at room temperature is equivalent to 376 ppm, and the pressure ratio of total pressure to moisture pressure is 0.0376%. That is, the moisture pressure in a total pressure of 10 Pa is 3.76E-3 Pa, and it is desirable to store the lithium metal in an atmosphere at least below this moisture pressure.

[0112] Furthermore, when dry air is evacuated to 10 Pa using a vacuum pump, and the oxygen concentration is 20% of that at atmospheric pressure, the oxygen partial pressure is 2 Pa. However, compared to the polar molecule H₂O, the nonpolar molecule oxygen is more easily expelled, and the actual oxygen partial pressure is lower than 2 Pa. However, due to the working principle of a manometer, partial pressure cannot be measured at high pressures such as 10 Pa.

[0113] Therefore, using oxygen partial pressure as the research object, the ICE retention performance was evaluated by introducing oxygen to control the oxygen partial pressure in the storage atmosphere. The partial pressure was analyzed using a ULVAC Qulee (CGM-051). The mass fractions of moisture, oxygen, and argon were monitored using gauges 18, 32, and 40, respectively.

[0114] <Comparative Example 7>

[0115] After pre-lithiation, the atmosphere was regulated by introducing a mixture of oxygen and argon after venting using a turbomolecular pump (TMP). The vacuum level during storage was 2.9E-3 Pa for oxygen, 1.0E-1 Pa for argon, 1.4E-5 Pa for H₂O, and a total pressure of 0.1 Pa. Storage under this atmosphere for one day resulted in an ICE of 92.0%, indicating a decrease in ICE.

[0116] <Comparative Example 8>

[0117] After pre-lithiation, the atmosphere was purged using a turbomolecular pump (TMP) and then regulated by introducing oxygen. The vacuum conditions during storage were: oxygen partial pressure 2.7E-2 Pa, argon partial pressure 3.2E-8 Pa (lower limit of measurement 1.0E-7 Pa), H₂O partial pressure 1.1E-5 Pa, and total pressure 0.03 Pa. Storage under this atmosphere for one day resulted in an ICE of 86.3%, a significant decrease (almost identical to the value in Reference Example 1).

[0118] <Comparative Example 9>

[0119] After pre-lithiation, the vacuum atmosphere was conditioned by introducing oxygen after venting using a turbomolecular pump (TMP). Furthermore, the TMP rotation speed was set to 50% to reduce the venting rate. The vacuum conditions during storage were: oxygen partial pressure 1.6E⁻¹ Pa, argon partial pressure 1.3E⁻⁶ Pa, H₂O partial pressure 5.1E⁻⁵ Pa, and total pressure 1.7 Pa. Storage under this atmosphere for one day resulted in an ICE of 84.2%, indicating a decrease in ICE (almost the same value as in Reference Example 1).

[0120] The initial coulombic efficiencies of the half-cells in Examples 1-2 and Comparative Examples 7-9 after storing the pre-lithiated electrodes under different oxygen partial pressures for one day are shown in the figures. Figure 3 middle.

[0121] Next, the changes in ICE over storage time were verified under dry air (the same atmosphere as Comparative Example 2) and vacuum atmosphere (the same atmosphere as Example 2). Figure 4 The results showed that after storage in a dry air atmosphere for 3 hours, the ICE was 88.7%, a significant decrease. On the other hand, after storage in a vacuum atmosphere for 3 days, the ICE was 102.0%, remaining unchanged.

[0122] Based on the above results, to prevent the pre-lithiation treated electrode from deteriorating in performance (i.e., suppressing the decrease in ICE) and proceeding to the next process, atmosphere control is necessary, but this is difficult to achieve by simply improving the atmosphere of the drying chamber. Therefore, to maintain the ICE of the pre-lithiation treated electrode, it is necessary to store the pre-lithiation electrode in a reduced pressure atmosphere, with the storage pressure ranging from 1.0E-5 Pa to 10 Pa (the minimum total pressure is the minimum pressure that can be exhausted by TMP), the oxygen partial pressure ranging from 1.0E-7 Pa to 1.0E-3 Pa (the minimum oxygen partial pressure is the lower limit that can be measured by the partial pressure system), and the moisture pressure ranging from 1E-7 Pa to 3.8E-3 Pa (the minimum moisture pressure is the lower limit that can be measured by the partial pressure system).

Claims

1. An alkali metal vapor deposition apparatus, comprising an unwinding mechanism, a winding chamber accommodating the winding mechanism, and a deposition chamber accommodating the alkali metal vapor deposition mechanism between the unwinding mechanism and the winding chamber. An atmosphere separation mechanism is provided between the winding chamber and the evaporation chamber, and the winding chamber is equipped with a decompression mechanism that can move the winding chamber while maintaining an internal decompression state or a mechanism that can move the winding chamber while maintaining an internal inert atmosphere.

2. The alkali metal vapor deposition apparatus as described in claim 1, wherein, The internal decompression state of the winding chamber indicates that the total pressure is in the range of 1.0E-5 Pa to 10 Pa, the oxygen partial pressure is in the range of 1.0E-7 Pa to 1.0E-3 Pa, and the water pressure is in the range of 1.0E-7 Pa to 1.3E-3 Pa.

3. The alkali metal vapor deposition apparatus as described in claim 1 or 2, wherein, The pressure reduction mechanism is a cooling trap or an exhaust pump.

4. The alkali metal vapor deposition apparatus as described in claim 3, wherein, The coolant in the cooling trap is a coolant capable of achieving a temperature of -40°C or lower.

5. The alkali metal vapor deposition apparatus as described in claim 3, wherein, The exhaust pump is a turbomolecular pump, a dry vacuum pump, or a cryogenic pump.

6. The alkali metal vapor deposition apparatus according to any one of claims 1 to 5, wherein, The alkali metal vapor deposition apparatus is also equipped with a mechanism for unwinding and winding a mask to pattern the alkali metal film formed by vapor deposition.

7. The alkali metal vapor deposition apparatus according to any one of claims 1 to 6, wherein, The alkali metal is sodium or lithium.

8. The alkali metal vapor deposition apparatus according to any one of claims 1 to 7, wherein, The inert atmosphere includes a rare gas atmosphere or a nitrogen atmosphere.

9. A pre-lithiation method, which is carried out by depositing lithium on a negative electrode substrate using an alkali metal vapor deposition apparatus as described in any one of claims 1 to 8.

10. The pre-lithiation method as described in claim 9, wherein, The negative electrode is a graphite negative electrode or a silicon-based negative electrode.

11. A pre-lithiated negative electrode, which is made by using the pre-lithiation method as described in claim 9 or 10.

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