Negative electrode for secondary battery, method of manufacturing the same, and secondary battery using the same
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- NAT INST FOR MATERIALS SCI
- Filing Date
- 2023-09-13
- Publication Date
- 2026-07-30
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Abstract
Description
[Technical field]
[0001] The present invention relates to a negative electrode for a secondary battery, a method for producing the same, and a secondary battery using the same. [Background technology]
[0002] Lithium metal anodes are an important component technology not only as an electrode technology that further increases the energy density of lithium-ion batteries, but also in next-generation energy storage technologies such as lithium-sulfur batteries and lithium-air batteries.
[0003] A lithium metal film with controlled grain size has been reported as a negative electrode for a secondary battery (see, for example, Patent Document 1). Patent Document 1 provides a negative electrode for a secondary battery having a lithium metal film made of crystal grains with a grain size of more than 15 μm and not more than 100 μm. Furthermore, Patent Document 1 describes that the crystal grains of such a lithium metal film are <101> However, Patent Document 1 does not disclose the density, and since the manufacturing process involves dropping molten lithium metal onto a current collector and applying pressure to produce a lithium metal film, it is desirable to be able to manufacture the film by a roll-to-roll method in order to increase the area.
[0004] Recently, a technique for producing a lithium metal film by vapor deposition has been reported (see, for example, Non-Patent Document 1). According to Non-Patent Document 1, although the lithium metal film produced by vapor deposition has a reduced impurity concentration, it still contains numerous voids, and further improvement is required. [Prior art documents] [Patent documents]
[0005] [Patent Document 1] JP 2022-37341 A [Non-patent literature]
[0006] [Non-Patent Document 1] Alec S. Ho et al.,ACS Energy Lett.,2022,7,3,1120-1124 Summary of the Invention [Problem to be solved by the invention]
[0007] In view of the above, an object of the present invention is to provide a metallic negative electrode for a secondary battery using a lithium metal film that improves battery characteristics, a method for producing the same, and a secondary battery using the same. [Means for solving the problem]
[0008] The negative electrode for a secondary battery of the present invention comprises a current collector and a lithium metal film located on the current collector, and the relative density of the lithium metal film (the actual density of the lithium metal film is less than the theoretical density of lithium metal (0.534 g / cm 3 ) is 96% or more, and the lithium metal film is made of crystal grains having a grain size in the range of 5 μm or more and 15 μm or less, thereby solving the above-mentioned problem. The lithium metal film may have a relative density of 98% or more. The lithium metal film may be composed of crystal grains having a size in the range of 10 μm to 15 μm. The lithium metal film is <001> or <101> It may be made up of crystal grains oriented in a direction. The lithium metal film is <111> It may further contain crystal grains oriented in the direction. The lithium metal film may have a thickness in the range of 1 μm to 100 μm. The current collector may be at least one selected from the group consisting of a copper (Cu) plate, a nickel (Ni) plate, a beryllium (Be) plate, an iron (Fe) plate, a chromium (Cr) plate, a manganese (Mn) plate, a molybdenum (Mo) plate, a niobium (Nb) plate, a tantalum (Ta) plate, a vanadium (V) plate, a zirconium (Zr) plate, and an alloy plate thereof. The lithium metal film may further include a film selected from the group consisting of an inorganic compound film, an organic compound film, a carbon film, a porous film, and a composite film thereof. The method for producing the negative electrode for a secondary battery according to the present invention includes using a lithium metal material as a deposition source, setting the substrate temperature to 255 K or less, and forming a lithium metal film on a current collector as a substrate by a vacuum deposition method, thereby solving the above-mentioned problem. The substrate temperature may be in the range of 90K to 225K. The substrate temperature may be in the range of 110K to 190K. The current collector may be transported by a roll-to-roll method. The secondary battery according to the present invention comprises a positive electrode, a negative electrode, and an electrolyte, and the negative electrode is the above-mentioned negative electrode, thereby solving the above-mentioned problems. The electrolyte may have lithium ion conductivity. A separator may be provided between the positive electrode and the negative electrode. The positive electrode may be an air electrode. Effect of the Invention
[0009] The negative electrode for a secondary battery of the present invention is composed of crystal grains having a grain size within the specific range described above and uses a lithium metal film having a relative density of 96% or more. Therefore, when such a lithium metal film is used in the negative electrode of a secondary battery, abnormal growth of lithium that accompanies charging and discharging is suppressed, and various secondary batteries with improved battery characteristics can be provided.
[0010] The method for producing a secondary battery negative electrode of the present invention can produce the above-mentioned secondary battery negative electrode by simply performing vacuum deposition at a substrate temperature of 255 K or less. A large-area secondary battery negative electrode can be easily provided by simply transporting a current collector by roll-to-roll, which is advantageous for practical use. [Brief description of the drawings]
[0011] [Figure 1] Schematic diagram showing a negative electrode for a secondary battery according to the present invention. [Diagram 2] FIG. 1 is a schematic diagram showing details of a lithium metal film constituting a negative electrode for a secondary battery according to the present invention. [Diagram 3] Schematic diagram showing a vacuum deposition device that uses the roll-to-roll method [Figure 4] Schematic diagram showing a lithium ion secondary battery of the present invention. [Diagram 5] Schematic diagram showing a vacuum deposition apparatus [Figure 6] Schematic diagram of electrochemically evaluating the density of lithium metal anodes [Figure 7] Schematic diagram explaining the cyclic dissolution-precipitation test [Figure 8] EBSD images of the negative electrode surfaces of Examples 1 and 6 [Figure 9] EBSD image of the negative electrode surface of Example 7 [Figure 10] Graph showing the grain size distribution of crystal grains on the negative electrode surface in Example 1 [Figure 11] FIG. 1 shows an SEM image of the negative electrode surface of Example 1 after a certain amount of dissolution. [Figure 12] FIG. 1 shows an SEM image of the negative electrode surface of Example 6 after a certain amount of dissolution. [Figure 13] FIG. 1 shows an SEM image of the negative electrode surface of Example 7 after a certain amount of dissolution. [Figure 14] FIG. 1 shows an SEM image of the negative electrode surface after deposition in Example 7. [Figure 15] Graph showing dissolution characteristics of the negative electrode surface in Example 1 [Figure 16] A diagram showing the potential characteristics when dissolution and deposition were repeated using the negative electrode of Example 1. [Figure 17] FIG. 1 shows cycle characteristics using negative electrodes of Examples 1, 6, and 7. [Figure 18] Schematic diagram explaining why the Coulomb efficiency is less than 100% [Figure 19] A diagram illustrating lithium loss on the deposited lithium metal film side. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note that like elements are given like reference numerals and their description will be omitted. (Embodiment 1) In the first embodiment, the negative electrode for a secondary battery and a method for producing the same of the present invention will be described in detail.
[0013] FIG. 1 is a schematic diagram showing a negative electrode for a secondary battery according to the present invention. FIG. 2 is a schematic diagram showing details of the lithium metal film constituting the negative electrode for a secondary battery according to the present invention.
[0014] The negative electrode 100 for a secondary battery according to the present invention includes a current collector 110 and a lithium metal film 120 disposed thereon. Here, the actual density of the lithium metal film 120 is the theoretical density (0.534 g / cm 3 ) (hereinafter referred to as relative density) is 96% or more, and the lithium metal film 120 is made of crystal grains having an average grain size in the range of 5 μm to 15 μm. By controlling the grain size of the crystal grains of the lithium metal film 120 in this way and focusing on the film density, it is possible to provide a secondary battery negative electrode in which abnormal growth of lithium due to charging and discharging is suppressed, and an excellent secondary battery using the same.
[0015] The average grain size of the crystal grains is preferably 10 μm or more and 15 μm or less, which can improve the battery characteristics of the secondary battery. The average grain size of the crystal grains is more preferably 12 μm or more and 14 μm or less, which can further improve the relative density and the battery characteristics of the secondary battery.
[0016] The lithium metal film 120 is <001> Grains oriented in the direction 210 or <101> The grains 220 may be oriented in the direction <111> The film may further include crystal grains 230 oriented in the direction as long as the orientation of the entire film is not affected.
[0017] As shown in Fig. 2, the lithium metal film 120 is composed of a plurality of crystal grains 210, 220, and 230. For simplicity, only three types of crystal grains are shown in Fig. 2, but the present invention is not limited to this. In this specification, the average grain size of the crystal grains is analyzed and calculated by image analysis software (HKL CHANNEL5, HKL Tango, ver.5.12.72.0, Oxford Instruments Co., Ltd.) attached to an electron backscatter diffraction (EBSD) measurement device.
[0018] In the present specification, "lithium metal film <001> "Composed of crystal grains oriented in the direction" means that, within the range of EBSD observation, <001> This means that crystal grains 210 oriented in the direction account for 60% or more. <101> The same can be said for crystal grains oriented in the direction.
[0019] The lithium metal film 120 preferably has a thickness in the range of 1 μm to 100 μm. This allows it to function as a negative electrode. The lithium metal film 120 more preferably has a thickness in the range of 5 μm to 70 μm. This allows a negative electrode for a secondary battery to be provided with good yield. The lithium metal film 120 even more preferably has a thickness in the range of 20 μm to 60 μm. This allows the characteristics of the secondary battery to be improved.
[0020] The relative density of the lithium metal film 120 is not particularly limited to an upper limit as long as it is 96% or more, and may be 100%. However, since it is practically difficult to obtain a lithium metal film having the same density as the theoretical density, it may be less than 100%. This reduces voids, organic matter, and other inclusions as much as possible, further suppressing abnormal growth of lithium during charging and discharging, and providing a secondary battery with excellent battery characteristics. The density of the lithium metal film 120 is more preferably 98% or more.
[0021] The mechanism by which the relative density of the lithium metal film becomes 96% or more is that at a film formation temperature exceeding 255K, the crystal grains grow three-dimensionally in the early stage of film formation, and voids (gaps) are likely to occur at the grain boundaries, but by setting the temperature at 255K or less, the crystal growth changes to two-dimensional growth, and growth without gaps between the crystal grains is realized. Furthermore, since the linear expansion coefficient of lithium is larger near room temperature than other typical metals, the film is formed at a low temperature, and the crystal grains tend to expand while the temperature is raised to room temperature after film formation, and strong compressive stress is applied between the crystal grains, which is presumably also contributing to the high density. It is believed that by using the manufacturing method of the present invention described later and lowering the substrate temperature to 255K or less in the vacuum deposition method, not only are the crystal grains limited to a range of 5 μm to 15 μm, but voids between the crystal grains are also removed.
[0022] The current collector 110 can be any metal that does not form an intermetallic compound with lithium metal, and is illustratively selected from the group consisting of copper (Cu) plate, nickel (Ni) plate, beryllium (Be) plate, iron (Fe) plate, chromium (Cr) plate, manganese (Mn) plate, molybdenum (Mo) plate, niobium (Nb) plate, tantalum (Ta) plate, vanadium (V) plate, zirconium (Zr) plate, and alloy plates thereof. These metal plates will not react with the lithium metal film 120 in the manufacturing method described below. Among them, the use of Cu plate or Ni plate is preferable because it allows the secondary battery negative electrode to be provided at low cost.
[0023] The current collector 110 preferably has a thickness in the range of 1 μm to 50 μm inclusive. If the thickness is in this range, the current collector 110 can support the lithium metal film 120 and supply a current to the lithium metal film 120.
[0024] The lithium metal film 120 may have a film thereon selected from the group consisting of inorganic compound films, organic compound films, carbon films, porous films, and composite films thereof. These films are inactive to the active lithium metal film 120, and therefore have the function of making the ion concentration distribution on the surface of the lithium metal film 120 uniform, suppressing localized lithium charging and dendrite generation, and improving the characteristics of the secondary battery. Depending on the material selected, they may also function as a separator to protect the lithium metal film 120.
[0025] The inorganic compound film is, for example, a film of an oxide or sulfide of at least one element selected from the group consisting of lithium (Li), calcium (Ca), sodium (Na), magnesium (Mg), beryllium (Be), potassium (K), silicon (Si), phosphorus (P), boron (B) and aluminum (Al). Representative examples include Li2O, SiO2, P2O5, Li2S, etc.
[0026] Moreover, the inorganic compound film may be boron nitride (BN), molybdenum sulfide (MoS2), aluminum fluoride (AlF3), zinc oxide (ZnO), or the like.
[0027] The organic compound is, for example, a polymer film selected from the group consisting of polyethylene oxide (PEO), polypropylene oxide (PPO), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyethyleneimine (PEI), polydimethylsiloxane (PDMS), poly 3,4 ethylenedioxythiophene (PEDOT), aluminum alkoxide (alcone), Nafion, polyethylene glycol diacrylate (PEGDA), polyurea (PolyUrea), and derivatives thereof.
[0028] The carbon film is, for example, a film of carbon selected from the group consisting of carbon black, graphite, carbon nanotubes, graphene, carbon nanofibers, fullerene or a derivative thereof, and diamond-like carbon (DLC).
[0029] The porous membrane is a woven or nonwoven fabric, and is illustratively made of at least one resin selected from the group consisting of polyethylene resin, polypropylene resin, polyvinyl alcohol resin, polyethylene terephthalate resin, polyacrylonitrile resin, polyimide resin, and aramid resin, or made of glass fiber. These can function as a separator.
[0030] Such a film on the lithium metal film 120 preferably has a thickness in the range of 50 nm to 1 μm. When a plurality of the above-mentioned films are laminated, the total thickness should be in the range of 50 nm to 1 μm.
[0031] Next, a method for producing the negative electrode for a secondary battery according to the present invention will be described. The negative electrode for a secondary battery of the present invention is manufactured by a vacuum deposition method. Specifically, the method for manufacturing a negative electrode for a secondary battery of the present invention includes using a lithium metal material as a deposition source, setting the substrate temperature to 255 K or less, and forming a lithium metal film on a current collector as a substrate by a vacuum deposition method. The inventors of the present application have found that by controlling the substrate temperature to 255 K or less, a lithium metal film can be formed with a theoretical density (0.534 g / cm 3 It has been found that a lithium metal film having a ratio (relative density) of 96% or more to the surface area of the lithium metal oxide and having crystal grains with grain sizes in the range of 5 μm to 15 μm can be obtained.
[0032] The manufacturing method of the present invention may be carried out using a vacuum deposition apparatus equipped with a cooling mechanism. The degree of vacuum in the vacuum deposition apparatus is, for example, 1×10 -9 Pa or more 1×10 -6 Pa or less, 1×10 -8 Pa or more 1×10 -7 A range of Pa or less may be adopted.
[0033] The current collector may be placed on a substrate holder of a vacuum deposition apparatus. The current collector is as described with reference to FIG. 1 and is therefore omitted. The substrate holder is cooled by a refrigerant and is set to 255K or less. Such a refrigerant may be, for example, a mixed solvent of an organic solvent selected from the group consisting of liquid nitrogen, liquid helium, liquid hydrogen, ethanol, ether, ethyl ether, acetone, and acetonitrile and dry ice, a mixed refrigerant of zinc chloride and ice, or the like. Among these, liquid nitrogen is preferred from the viewpoints of ease of availability and ease of handling.
[0034] The substrate temperature is not particularly limited as long as it is set to 255K or less, and depends on the coolant used, but from the viewpoint of cost, it is preferably above the boiling point of liquid nitrogen (77K). The substrate temperature preferably falls within the range of 90K to 225K. Within this range, the relative density of the lithium crystal grains is 96% or more, <001> Direction or <101> A lithium metal film containing crystal grains oriented in the direction can be obtained. The substrate temperature is more preferably 100K or lower, and under these conditions, a lithium metal film having a relative density of 98% or higher can be obtained, thereby improving the characteristics of the secondary battery.
[0035] The lithium metal material, which is the deposition source, may be a lump of lithium metal, and preferably has a purity of 99.8% or more. The lithium metal material is placed in a metal crucible that is not reactive with lithium and does not form an intermetallic compound, and heated to a temperature exceeding the melting point (180.5°C). The higher the heating temperature of the metal crucible, the faster the deposition rate, and the heating temperature may be set according to the desired deposition rate. For example, the heating temperature of the metal crucible may be in the range of 200°C to 800°C, and is preferably in the range of 500°C to 700°C, taking into consideration the film quality of the lithium metal film and the deposition rate.
[0036] The current collector may be transported by a roll-to-roll method, which allows for a large area. Figure 3 shows an exemplary vacuum deposition apparatus using the roll-to-roll method.
[0037] FIG. 3 is a schematic diagram showing a vacuum deposition apparatus employing a roll-to-roll method.
[0038] The vacuum deposition apparatus 300 includes a carry-in chamber 310, a film formation chamber 320, and an unloading chamber 330. The carry-in chamber 310 is adapted to be fitted with an unwinding roll 301 around which the current collector 110, which is a substrate, is wound, and is connected to a vacuum pump (not shown), so that the inside can be depressurized.
[0039] The film-forming chamber 320 includes a metal crucible 303 in which lithium metal material 302 is placed, a heater 304 for heating the metal crucible 303, and a film-forming roll 306 equipped with a cooling mechanism 305. The film-forming chamber 320 is connected to a vacuum pump (not shown) and the inside can be depressurized.
[0040] The unloading chamber 330 is adapted to be fitted with a take-up roll 307 for taking up the current collector 110 having the lithium metal film formed in the film-forming chamber 320, and is connected to a vacuum pump (not shown) so that the inside can be depressurized.
[0041] Unwinding roll 301, film-forming roll 306, and take-up roll 307 constitute a transport mechanism that transports current collector 110 in a roll-to-roll manner via guide rolls. A spare chamber for adjusting the pressure difference and / or a treatment chamber for pre-treatment of the current collector, post-treatment of the formed lithium metal film, and application of a protective film may be provided between carry-in chamber 310, film-forming chamber 320, and carry-out chamber 330.
[0042] In this manner, a negative electrode for a secondary battery having a lithium metal film on the current collector is obtained. The lithium metal film may further include the inorganic compound film, organic compound film, carbon film, porous film, composite film thereof, or the like.
[0043] (Embodiment 2) In the second embodiment, a secondary battery using the negative electrode for a secondary battery of the present invention described in the first embodiment will be described.
[0044] FIG. 4 is a schematic diagram showing a lithium ion secondary battery of the present invention.
[0045] The secondary battery 400 of the present invention includes an anode 100, a cathode 410, and an electrolyte 420 sandwiched between them. Here, the anode 100 is the anode 100 for a secondary battery of the present invention described with reference to FIG. 1, and therefore a description thereof will be omitted.
[0046] The positive electrode 410 further includes a current collector 430 and a positive electrode active material 440 located thereon. The current collector 430 is not particularly limited as long as it can supply current, and examples that can be used include stainless steel, aluminum (Al), an aluminum alloy, titanium, a carbon sheet, an indium tin oxide (ITO) substrate, and an antimony tin oxide (ATO) substrate.
[0047] The positive electrode active material 440 may be an active material commonly used in lithium ion secondary batteries, and may, for example, be lithium cobalt oxide (LCO), nickel-cobalt-lithium manganese oxide ternary material (NCM), nickel-cobalt-lithium aluminum oxide ternary material (NCA), lithium manganese oxide (LMO), lithium iron phosphate (LFP), or lithium iron silicate (LFS). 1 / 3 Mn 1 / 3 Co 1 / 3 O2, LiNi 0.5 Mn 0.5 O2, Li2MnO3-LiMO2(M=Co, Ni, Mn), Li 1+x Mn 2-x O4, Li(MnAl)2O4, LiMn 1.5 Ni 0.5 O4, LiMnPO4, LiFePO4, LiCoPO4, Li2FePO4F, Li2FeSiO4, etc.
[0048] The positive electrode active material 440 may contain a conductive material, a binding agent (binder), a thickener, and a dispersion medium as necessary. The conductive material may be carbon black such as Ketjen Black (registered trademark) or acetylene black, activated carbon, graphite, carbon fiber, carbon nanotubes, carbon nanohorns, mesoporous carbon, or the like. The binder may be polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), or the like.
[0049] The positive electrode 410 is obtained by mixing the positive electrode active material 440 with, if necessary, a conductive material, a binder, a thickener, and a dispersion medium, applying the mixture onto the current collector 430, and drying it.
[0050] The electrolyte 420 is not particularly limited as long as it has lithium ion conductivity, but is preferably an electrolyte solution containing a solvent and a lithium salt dissolved therein.
[0051] The solvent may be a commonly used lithium ion conductive solvent. For example, cyclic carbonates such as propylene carbonate (PC) and ethylene carbonate (EC), chain carbonates such as diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC), cyclic carboxylates such as γ-butyrolactone (GBL), γ-valerolactone (GVL), and α-methyl-γ-butyrolactone, chain carboxylates such as methyl acetate, ethyl acetate, methyl propionate, ethyl butyrate, butyl butyrate, and isobutyl propionate, ethers such as ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether, ionic liquids, water, and mixtures of two or more of these may be used.
[0052] The lithium salt may be any lithium salt commonly used in lithium ion secondary batteries, such as lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium bistrifluoromethanesulfonylamide (LiTFSA) [(CF3SO2)2NLi], lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), lithium bisfluorosulfonylimide (LiFSI), or lithium halides.
[0053] Although not shown in FIG. 4, a separator may be provided between the negative electrode 100 and the positive electrode 410. The separator is a porous membrane that is a woven or nonwoven fabric, and is illustratively made of at least one resin selected from the group consisting of polyethylene resin, polypropylene resin, polyvinyl alcohol resin, polyethylene terephthalate resin, polyacrylonitrile resin, polyimide resin, and aramid resin, or made of glass fiber. As described with reference to FIG. 1, when the negative electrode 100 has a porous membrane on the lithium metal membrane, it is not necessary to provide a separator.
[0054] Although the lithium ion secondary battery 400 has been described in FIG. 4, it is also possible to provide a lithium air secondary battery by using an air electrode as the positive electrode 410. In this case, the positive electrode active material 440 may be a porous carbon material such as carbon black, such as Ketjen Black (registered trademark) or acetylene black, activated carbon, graphite, carbon fiber, carbon nanotube, carbon nanohorn, or mesoporous carbon. Here, a conductive material, a binder, or a thickener may be added together with the porous carbon material. If the porous carbon material is self-supporting, the current collector 430 may be omitted.
[0055] 4 is enclosed in an exterior body made of a polymer film such as polyethylene or polycarbonate, or a metal film such as aluminum or nickel, to provide a secondary battery having a shape such as a coin type, a cylindrical type, a square type, a sheet type, etc. Such a secondary battery can be used in various batteries for electronic devices, automobiles, etc.
[0056] The present invention will now be described in detail with reference to specific examples, but it should be noted that the present invention is not limited to these examples. EXAMPLES
[0057] [Example 1 to Example 6] In Examples 1 to 6, a lithium metal film was formed on a Cu foil (manufactured by Nilaco Corporation, thickness 80 μm) as a current collector using the vacuum deposition apparatus shown in FIG. 5 under the conditions shown in Table 1 to produce a negative electrode for a secondary battery.
[0058] FIG. 5 is a schematic diagram showing a vacuum deposition apparatus.
[0059] The vacuum deposition apparatus 500 includes a metal crucible 303 in which a lithium metal material 302 is placed, a heater 304 for heating the metal crucible 303, and a cooling mechanism 305. The apparatus also includes a substrate holder 510 for holding a Cu foil. The apparatus is connected to a vacuum pump (not shown). The internal base vacuum level is 10 -8 Pa or more 10 -7 The pressure was reduced to below 1 Pa. Substrate holder 510 was rotatable, and liquid nitrogen was introduced to serve as cooling mechanism 305. The lithium metal film of Example 6 was formed in the same manner except that the cooling mechanism was not operated.
[0060] [Table 1]
[0061] The negative electrodes (lithium metal film / Cu foil) of Examples 1 to 6 thus obtained were observed by electron backscatter diffraction (EBSD) using a scanning electron microscope (SEM, manufactured by JEOL Ltd., JSM-7800F) equipped with an EBSD detector. The average particle size was calculated from the obtained images. These results are shown in Figures 8 and 10 and Table 2.
[0062] The quality of the lithium bulk of the negative electrodes of Examples 1 to 6 was evaluated. Specifically, the surface of the negative electrodes of Examples 1 to 6 was charged with an electrolyte solution (electrolyte concentration: 1 mol / L) in which LiTFSI (lithium bis(trifluoromethanesulfonyl)imide) was dissolved in tetraethylene glycol dimethyl ether to a charge of 0.5 mAh / cm. 2 After dissolving only the SiO2, the specimen was observed using an SEM. The results are shown in Figures 11 and 12.
[0063] Next, electrochemical dissolution and deposition test cells were manufactured using the negative electrodes of Examples 1 to 6, and their charge and discharge characteristics were measured. The specific manufacturing procedure is as follows. The negative electrodes of Examples 1 to 6 were punched out into a circle with a diameter of 14.7 mm. Instead of a positive electrode, a Cu foil 610 (FIG. 6) punched out to a diameter of 16 mm was used, and it was devised so that lithium would not protrude from the Cu on the positive electrode side even if the above-mentioned negative electrode was slightly misaligned during the assembly of the test cell.
[0064] The electrodes obtained as the positive and negative electrodes were laminated in a laminate-type battery case with a polyethylene (PE) separator 620 (Figure 6) (manufactured by Double Scope Co., Ltd.) as a separator, and an electrolyte solution (electrolyte concentration: 4 mol / L, liquid volume: 37 μL / cm) in which LiFSI (lithium bis(trifluoromethanesulfonyl)imide) was dissolved in dimethoxyethane was used as the electrolyte. 2 ) was injected to obtain CR2032 coin cells (coin cells of Examples 1 to 6). The electrodes were each connected to a charge-discharge tester (580 Battery Tester manufactured by Scribner Associates).
[0065] FIG. 6 is a schematic diagram showing the electrochemical evaluation of the density of a lithium metal negative electrode.
[0066] The density of the lithium metal film was calculated using the coin cells of Examples 1 to 6 obtained in this manner. FIG. 6(A) shows the cross-sectional appearance of the coin cell before electrodeposition, and FIG. 6(B) shows the cross-sectional appearance of the coin cell after electrodeposition. First, as shown in FIG. 6, a test was carried out in which the lithium metal film 120 was completely dissolved and electrodeposited on the opposing Cu foil 610. The charge amount Q required for complete dissolution was PallThe density ρ of the lithium metal film was calculated from the capacity (mAh) using the following formula. ρ=3.6×(Q Pall / A)×(M / F)×1 / T (1) where F is the Faraday constant (C / mol), M is the molar mass of lithium (g / mol), and A is the area of the dissolved lithium (cm 2 ), and T is the physical thickness of the lithium film (cm). T was measured using a digital micrometer (Mitutoyo Corporation, MDH-25MC) in a glove box before the dissolution and charge-discharge tests. The results are shown in Figure 15 and Table 2.
[0067] FIG. 7 is a schematic diagram illustrating a repetitive dissolution-precipitation test.
[0068] The cycle characteristics of the coin cells of Examples 1 to 6 were evaluated, and the Coulombic efficiency CE was calculated. Fig. 7(A) shows the cross-sectional appearance of the coin cell before the charge-discharge test, Fig. 7(B) shows the cross-sectional appearance of the coin cell during discharge, and Fig. 7(C) shows the cross-sectional appearance of the coin cell after it was fully charged again. As shown in Fig. 7, at a current density of 1.0 mA / cm 2 , circulation capacity 4mAh / cm 2 (Lithium metal film thickness equivalent to 19.28μm), cut-off potential ±1.0V, 4-hour discharge and 4-hour charge were repeated. P and the quantity of electricity Q during discharge S Measure the Coulombic efficiency CE(Q S / Q P × 100%) was calculated. The results are shown in Figures 16 and 17 and Table 3.
[0069] [Example 7] In Example 7, a secondary battery negative electrode was produced using a commercially available extruded and rolled lithium metal film (manufactured by Honjo Metals, thickness: 50 μm). First, the surface of the commercially available lithium metal film was observed using EBSD. Next, the commercially available lithium metal film was bonded to a Cu foil to form a negative electrode, and a coin cell was produced in the same manner as in Examples 1 to 6. The quality of the lithium bulk was evaluated, the charge / discharge characteristics were measured, and SEM observation was performed. These results are shown in Figures 9, 13, 14, 16, and 17.
[0070] The above results will be summarized. FIG. 8 is a diagram showing EBSD images of the surfaces of the negative electrodes of Examples 1 and 3. FIG. 9 is a diagram showing an EBSD image of the surface of the negative electrode of Example 7.
[0071] Figures 8 and 9 show a crystal grain mapping image and an inverse pole point (IPF) map. Although Figures 8 and 9 are shown in grayscale, in reality, the plane index of the crystal plane perpendicular to the direction perpendicular to the lithium metal film surface, which is the negative electrode surface (Z-axis direction), is calculated and displayed according to a color key.
[0072] According to FIG. 8(A), the lithium metal film of the negative electrode of Example 1 is <001> According to FIG. 8(B), the grains in Example 3 are oriented in the direction of the crystal grains. <101> Although not shown, the crystal grains in Examples 2 and 4 were also oriented in the same direction. <001> Oriented grains or <101> The majority of the crystal grains are oriented in one direction. <001> Preferred orientation, or <101> It had a preferred orientation.
[0073] According to FIG. 9, the crystal grains of the lithium metal film of Example 7 that had been subjected to extrusion and rolling were random, and no preferred orientation was confirmed.
[0074] In addition, when the average grain size was calculated from the crystal grain mapping image of FIG. 8, the average grain size of the crystal grains in the lithium metal films of Examples 1 to 6 was within the range of 5 μm to 15 μm. On the other hand, the average grain size of the crystal grains in the lithium metal film of Example 7, calculated from the crystal grain mapping image of FIG. 9, was significantly larger, being within the range of 100 μm to 1000 μm. In addition, the lithium metal film manufactured by the method described in Patent Document 1 also consists of crystal grains larger than 15 μm. These facts show that a lithium metal film with a small average grain size can be obtained by adopting a vacuum deposition method in which the substrate temperature is set to room temperature or lower.
[0075] FIG. 10 is a graph showing the grain size distribution of the crystal grains on the surface of the negative electrode of Example 1.
[0076] 10, it was found that the crystal grain size had a peak between 10 μm and 20 μm. Although not shown, the crystal grains on the negative electrode surface of Examples 2 to 6 also showed a grain size distribution similar to that of FIG.
[0077] FIG. 11 is a diagram showing an SEM image of the negative electrode of Example 1 after a certain amount of the surface of the negative electrode was dissolved. FIG. 12 is a diagram showing an SEM image of the negative electrode of Example 6 after a certain amount of the surface of the negative electrode was dissolved. FIG. 13 is a diagram showing an SEM image of the negative electrode of Example 7 after a certain amount of the surface of the negative electrode was dissolved. FIG. 14 is a diagram showing an SEM image of the negative electrode surface of Example 7 after deposition.
[0078] According to FIG. 11, no etch pits were observed in the lithium metal film of Example 1 formed at 93K. Although not shown, the lithium metal films of Examples 2 to 4 formed at 255K or less also had a similar appearance. If such a lithium metal film without etch pits is used for the negative electrode, abnormal growth of lithium during charging can be suppressed, and battery characteristics can be improved. On the other hand, according to FIG. 12, in the lithium metal film of Example 6 formed at 293K, irregular pits of several μm in size were confirmed in the crystal grains and at the crystal grain boundaries, resulting in low density and indicating that there is a possibility that lithium may grow abnormally from the pits during electrodeposition.
[0079] According to FIG. 13, in the lithium metal film of Example 7 subjected to extrusion and rolling, many large etch pits of 10 μm to 30 μm appeared along the extrusion and rolling directions, and as shown in FIG. 14, even after lithium deposition, the etch pits were not filled, and abnormal growth of lithium was observed starting from the etch pits.
[0080] FIG. 15 is a graph showing the dissolution characteristics of the lithium negative electrode surface of Example 1.
[0081] The density of the lithium metal film of Example 1, calculated using the results of FIG. 15 and formula (1), was 98.77%. As shown in Table 2, it was found that the densities of the lithium metal films of Examples 2 to 4, which were formed at 255K or lower, all satisfied 96% or more. On the other hand, the densities of the lithium metal films of Examples 5 and 6, which were formed at temperatures higher than 270K, were low, being less than 96%. It was confirmed that the density of the lithium metal film of Example 7, which was subjected to extrusion and rolling, was low, being 84 to 94%, and varied.
[0082] Here, the difference between the lithium metal film of the negative electrode of the present invention and those described in Patent Document 1 and Non-Patent Document 1 will be explained. The lithium metal film of the negative electrode described in Patent Document 1 is composed of crystal grains having a grain size exceeding 15 μm, and does not have a grain size controlled in the range of 5 μm to 15 μm. Naturally, it can be said that the relative density of the lithium metal film of the negative electrode described in Patent Document 1 is not 96% or more. Moreover, according to Non-Patent Document 1, the deposition conditions of the lithium metal film are the same as those of Example 6 in which the substrate was not heated, and it can be said that a lithium metal film having a relative density of 96% or more was not obtained.
[0083] [Table 2]
[0084] FIG. 16 is a graph showing potential characteristics when the negative electrodes of Examples 1 and 7 were used for repeated dissolution and deposition.
[0085] FIG. 16(A) shows the case where the negative electrode of Example 1 was used, and FIG. 16(B) shows the case where the negative electrode of Example 7 was used. The numbers marked with "#" in the figures indicate the number of cycles, and the potential characteristics are extracted and shown approximately every 10 cycles.
[0086] In the figure, the horizontal axis indicates the charge density, and the vertical axis indicates the voltage of the battery cell. It was found that the potential characteristics in FIG. 16(A) have smaller variations than those in FIG. 16(B), and the overvoltage also tends to be smaller. Although not shown, the charge / discharge characteristics of the coin cells using the negative electrodes of Examples 2 to 4 were similar to those in FIG. 16(A). In other words, in the case of evaporated films, the potential characteristics were not dependent on the film formation temperature, and no significant differences were obtained.
[0087] FIG. 17 is a graph showing cycle characteristics when the negative electrodes of Examples 1, 6 and 7 are used.
[0088] In the figure, the horizontal axis is the number of cycles, and the vertical axis of the upper row is the amount of electricity Q during charging. P The axis in the lower row is the Coulomb efficiency. The negative electrode of Example 7 showed a decrease in the amount of electricity after only 68 cycles, and the negative electrode of Example 6 showed a decrease in the amount of electricity after 76 cycles. On the other hand, the negative electrode of Example 1, which was formed at 255K or less, did not show a decrease in the amount of electricity until 89 cycles, and it was found that it had excellent cycle characteristics and a long life. This result corresponds to the presence or absence of etch pits and the abnormal growth of lithium that occurs from the etch pits, as described with reference to Figures 10 to 12. Although not shown, coin cells using the negative electrodes of Examples 2 and 3 also showed similar cycle characteristics. Here, various data obtained from the charge and discharge tests using the negative electrodes of Examples 1, 3, 5 and 6 are summarized in Table 3.
[0089] [Table 3]
[0090] FIG. 18 is a diagram for explaining a schematic reason why the coulomb efficiency is less than 100%. FIG. 19 is a diagram illustrating the lithium loss on the evaporated lithium metal film side.
[0091] Fig. 18(A) is a cross-section of a coin cell at the beginning of charging, showing how dendritic growth portion 1810 is formed on Cu foil 610. Fig. 18(B) is a cross-section of a coin cell at the middle of charging, showing how dendritic growth portion 1810 is separated from Cu foil 610. Fig. 18(C) shows the cross-section of the coin cell when fully charged. Fig. 19(A) shows the cross-section of the coin cell when fully charged, continuing from Fig. 18(C), and Fig. 19(B) shows the cross-section of the coin cell when discharging.
[0092] The quantity of electricity Q in Figure 17 P The behavior of 1710 is shown in Q P is the specified 4mAh / cm 2 The number of cycles at which the L " I will call it "C L indicates the number of cycles at which, during repeated dissolution and deposition of a film thickness (B) that is about 40% of the initial physical film thickness (A), the dendritic growth portions 1810 detach from the electrode during dissolution (become electrochemically inactivated) as shown in FIG. 18(B) and accumulate to completely use up the surplus lithium (121) shown in FIG. 7(B).
[0093] As shown in FIG. 18, the deactivation of the dendritic growth portion 1810 on the opposing Cu foil 610 contributes to the measured Coulomb efficiency CE, and the Coulomb efficiency CE is always Q S P Therefore, the Coulombic efficiency CE is less than 100%. The quality of the lithium metal dissolved from the lithium metal negative electrode 100 and electrodeposited on the opposing Cu foil 610 does not depend on the substrate temperature during deposition of the lithium metal film 120, and the average Coulombic efficiency CE is generally about 99.0%, as shown in Table 3.
[0094] Therefore, first, the lithium loss film thickness for each cycle is calculated as follows: (100-CE) / 100×B(19.28μm) When the total loss C was calculated by integrating it up to the limit cycle, it was found that there was a loss D that was not reflected in the measurement of the Coulomb efficiency CE obtained from ABC.
[0095] As shown in FIG. 19, when lithium electrodeposited on the opposing Cu foil 610 is dissolved and electrodeposited on the original evaporated lithium metal, the dendritic growth portion 1810 is generated, and the total amount of lithium deactivated at the next dissolution is shown up to the limit cycle. Assuming an actual battery equipped with a lithium metal negative electrode, D is rather an index showing the quality of the lithium metal negative electrode, and can be said to be more essential. This D, or D / A normalized by the initial film thickness A, is preferably small, and for example, D / A may be 0.001 or more and 0.3 or less. In fact, the D / A of the negative electrodes of Examples 1 and 3 formed at 255K or less satisfies 0.001 or more and 0.3 or less, and is clearly lower than that of the negative electrodes of Examples 5 and 6 formed at a temperature higher than 270K. It was confirmed that the D / A of the negative electrodes of Examples 2 and 3 also satisfies 0.001 or more and 0.3 or less.
[0096] Furthermore, from the loss D, the Coulomb efficiency CE2 of dissolution and deposition on the negative electrode 100 side having the evaporated lithium metal film 120 is CE2=(BD / C L ) / B×100 (%) It can be easily calculated from. For reference, the calculated Coulombic efficiency CE2 is also shown in Table 3. According to Table 3, the Coulombic efficiency CE2 of the negative electrodes of Examples 1 and 3 formed at 255K or less is improved to 99.11% or more, compared to that of the negative electrodes of Examples 5 and 6 formed at a temperature higher than 270K. It is known that an improvement of 0.1% or more in the 99% range contributes greatly to the realization of a lithium metal negative electrode with a long cycle life.
[0097] From the above results, it was shown that by using a lithium metal material as a deposition source, setting the substrate temperature to 255K or less, and forming a lithium metal film on a current collector as a substrate by vacuum deposition, a negative electrode for a secondary battery can be obtained that includes a current collector and a lithium metal film positioned thereon, the lithium metal film being made of crystal grains having a grain size in the range of 5μm to 15μm, and a secondary battery with excellent battery characteristics such as cycle characteristics and Coulombic efficiency can be provided. In particular, it was shown that by ensuring a relative density of the lithium metal film of 96% or more, abnormal growth of lithium is suppressed, further improving the battery characteristics. [Industrial Applicability]
[0098] The negative electrode for secondary batteries of the present invention uses a lithium metal film having a specific particle size and a density of 96% or more, and can provide various secondary batteries with excellent characteristics. Such a negative electrode is manufactured by a vacuum deposition method in which the substrate temperature is controlled to 255 K or less, and can be easily made into a large area by roll-to-roll processing, which is advantageous for practical use. [Explanation of symbols]
[0099] 100 Negative electrode for secondary batteries 110, 430 Current collector 120 Lithium metal film 121 Surplus Lithium 210, 220, 230 Grains 300, 500 Vacuum deposition equipment 310 Loading Room 320 Deposition chamber 330 Unloading room 301 Unwinding roll 302 Lithium Metal Materials 303 Metal crucible 304 Heater 305 Cooling mechanism 306 Coating roll 307 Winding Roll 400 Secondary battery 410 Positive electrode 420 Electrolyte 440 Cathode active material 510 PCB Holder 610 Cu foil 620 Separator 1810 Dendritic growth area
Claims
1. Current collector and, A lithium metal film located on the current collector and Equipped with, The relative density of the lithium metal film (the theoretical density of lithium metal at the actual density of the lithium metal film (0.534 g / cm³) ３ The percentage of the ratio to ) is 96% or higher. The lithium metal film is a negative electrode for a secondary battery, comprising crystal grains having a particle size in the range of 5 μm to 15 μm.
2. The negative electrode for a secondary battery according to claim 1, wherein the relative density of the lithium metal film is 98% or more.
3. The negative electrode for a secondary battery according to claim 1, wherein the lithium metal film consists of crystal grains having a particle size in the range of 10 μm to 15 μm.
4. The negative electrode for a secondary battery according to claim 1, wherein the lithium metal film consists of crystal grains oriented in the <001> or <101> direction.
5. The negative electrode for a secondary battery according to claim 4, wherein the lithium metal film further contains crystal grains oriented in the <111> direction.
6. The negative electrode for a secondary battery according to claim 1, wherein the lithium metal film has a film thickness in the range of 1 μm to 100 μm.
7. The negative electrode for a secondary battery according to claim 1, wherein the current collector is selected from at least one of the group consisting of copper (Cu) plate, nickel (Ni) plate, beryllium (Be) plate, iron (Fe) plate, chromium (Cr) plate, manganese (Mn) plate, molybdenum (Mo) plate, niobium (Nb) plate, tantalum (Ta) plate, vanadium (V) plate, zirconium (Zr) plate, and alloy plates thereof.
8. The negative electrode for a secondary battery according to claim 1, further comprising a film selected from the group consisting of an inorganic compound film, an organic compound film, a carbon film, a porous film, and a composite film thereof, on the lithium metal film.
9. A method for manufacturing a negative electrode for a secondary battery according to any one of claims 1 to 8, comprising using a lithium metal material as a deposition source, keeping the substrate temperature below 255 K, and forming a lithium metal film on a current collector as a substrate by vacuum deposition.
10. The method according to claim 9, wherein the substrate temperature satisfies a temperature range of 90K to 225K.
11. The method according to claim 10, wherein the substrate temperature satisfies a temperature range of 110K to 190K.
12. The method according to claim 9, wherein the current collector is conveyed in a roll-to-roll manner.
13. A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode is a negative electrode for a secondary battery according to any one of claims 1 to 8.
14. The secondary battery according to claim 13, wherein the electrolyte has lithium-ion conductivity.
15. The secondary battery according to claim 13, further comprising a separator between the positive electrode and the negative electrode.
16. The secondary battery according to claim 13, wherein the positive electrode is an air electrode.