Lithium metal battery

Abstract

The present invention provides a lithium metal battery comprising: a positive electrode; a lithium metal negative electrode; and a separator provided between the positive electrode and the lithium metal negative electrode, wherein the separator has a coating layer provided on at least one surface thereof, the coating layer including: a binder containing PAN and a fluorinated polymer; and inorganic particles.

Description

lithium metal battery

[0001] The present invention relates to a lithium metal battery.

[0002] Conventional lithium secondary batteries utilize a carbon-based anode and operate by transporting lithium ions through a liquid electrolyte containing a lithium salt dissolved in an organic solvent. In contrast, lithium metal batteries utilize anodes consisting solely of a current collector or a layer of lithium metal deposited on the current collector. During charging, lithium ions are reduced and deposited from the anode to lithium metal, and during discharge, the lithium metal is oxidized to lithium ions.

[0003] Lithium metal, used in lithium metal batteries, has emerged as a material capable of producing high-capacity batteries due to its high electrical capacity per unit mass. However, lithium metal forms dendrites during the deposition and desorption of lithium ions, which can damage the separator and cause an electrical short between the anode and cathode.

[0004] Therefore, in order to effectively operate high-capacity lithium metal batteries, a technology that can effectively prevent adverse effects caused by dendrites in lithium metal batteries is needed.

[0005] [Prior Art Literature]

[0006] [Patent Document]

[0007] (Patent Document 1) Republic of Korea Patent Publication No. 10-2021-0009188

[0008] The present invention aims to provide a lithium metal battery capable of effectively suppressing the problem of battery performance degradation due to the occurrence of dendrites in a lithium metal battery.

[0009] One embodiment of the present invention provides a lithium metal battery comprising: a positive electrode; a lithium metal negative electrode; and a separator provided between the positive electrode and the lithium metal negative electrode, wherein a coating layer including a binder including PAN and a fluorinated polymer and an inorganic particle is provided on at least one surface of the separator.

[0010] The lithium metal battery according to the present invention can achieve high stability by including a coating layer on at least one surface of the separator that effectively prevents adverse effects due to dendrites. Furthermore, the separator of the lithium metal battery according to the present invention possesses excellent ionic conductivity, high mechanical strength, and thermal stability, thereby enabling an improved lifespan of the lithium metal battery.

[0011] In this specification, when a part is said to "include" a certain component, this does not mean that it excludes other components, but rather that it may include other components, unless specifically stated otherwise.

[0012] In this specification, when it is said that a member is located "on" another member, this includes not only cases where the member is in contact with the other member, but also cases where another member exists between the two members.

[0013] In this specification, “%” and “part” indicating content are based on weight unless otherwise specified.

[0014] The lithium metal battery according to the present invention is distinguished from a lithium ion battery. Specifically, there is a significant difference in that a lithium metal battery uses lithium metal as an anode active material, while a lithium ion battery uses a non-lithium material (e.g., graphite) as an anode active material. Furthermore, an electrolyte suitable for a lithium metal battery has low reactivity with lithium metal and does not corrode lithium metal. On the other hand, an electrolyte suitable for a lithium ion battery can react with a non-lithium anode active material to form a solid-electrolyte interphase (SEI) layer. The SEI layer can play a positive role in controlling the thermodynamic chemical reactions occurring in a lithium ion battery. As mentioned above, lithium metal batteries and lithium ion batteries have significantly different operating characteristics due to the difference in anode active materials, and thus, the problems that need to be solved are bound to be different. Therefore, lithium metal batteries and lithium ion batteries cannot help but have different separator properties and electrolyte properties required for each, and in most cases, the same effect cannot be expected when applying components used in lithium ion batteries to lithium metal batteries. Therefore, it can be said that lithium metal batteries and lithium ion batteries are completely different technical fields.

[0015] Hereinafter, the present invention will be described in detail.

[0016] One embodiment of the present invention provides a lithium metal battery comprising: a positive electrode; a lithium metal negative electrode; and a separator provided between the positive electrode and the lithium metal negative electrode, wherein a coating layer including a binder including PAN and a fluorinated polymer and an inorganic particle is provided on at least one surface of the separator.

[0017] According to one embodiment of the present invention, the positive electrode may include a positive electrode active material including at least one selected from the group consisting of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, and lithium manganese oxide. The positive electrode may include a binder, the positive electrode active material, and a conductive material. However, the present invention is not limited thereto, and the composition and manufacturing method of the positive electrode may apply technical contents widely known in the art.

[0018] According to one embodiment of the present invention, the lithium metal negative electrode may be a lithium metal thin film, and may be provided in a form in which a lithium metal layer is provided on the negative electrode current collector.

[0019] According to one embodiment of the present invention, the coating layer of the separator is characterized by using PAN and a fluorinated polymer together as a binder material. Specifically, PAN (polyacrylonitrile) has the characteristics of having excellent oxidation stability and high ionic conductivity, but has a problem of low adhesion to commercial polymer separators. Therefore, by applying a fluorinated polymer with high adhesion to commercial polymer separators together with PAN as a binder material, the adhesion between the coating layer and the separator can be secured, and the oxidation stability and ionic conductivity can be improved due to the binder material of the coating layer. In order to more significantly implement such an effect, the weight ratio of PAN and PVDF in the coating layer may be 3.5:6.5 to 6.5:3.5. Specifically, the weight ratio of PAN and PVDF in the coating layer may be 1:1 to 6.5:3.5.

[0020] According to one embodiment of the present invention, the fluorinated polymer may include at least one selected from the group consisting of poly(vinyl fluoride) (PVF), poly(vinylidene fluoride) (PVDF), and poly(ethylene-co-tetrafluoroethylene) (ETFE). Specifically, the fluorinated polymer may be PVDF.

[0021] According to one embodiment of the present invention, the coating layer includes inorganic particles, and the inorganic particles can impart thermal stability and mechanical strength to the separator. In addition, the inorganic particles can inhibit the movement of anions toward the negative electrode during the movement of lithium ions during charge and discharge of the lithium metal battery, thereby enabling lithium ions to be evenly deposited on the lithium metal negative electrode, thereby suppressing the formation of dendrites.

[0022] According to one embodiment of the present invention, the content of the inorganic particles may be 150 parts by weight to 400 parts by weight based on 100 parts by weight of the total binder. Specifically, the content of the inorganic particles may be 200 parts by weight to 400 parts by weight, or 250 parts by weight to 350 parts by weight based on 100 parts by weight of the total binder. Within the above content range, the inorganic particles can sufficiently improve the thermal stability and mechanical strength of the separator, and further effectively suppress dendrite formation. If the content of the inorganic particles exceeds the above range, the inorganic particles may fall off from the binder, preventing the shape of the coating layer from being maintained, and may also cause a decrease in the ionic conductivity of the separator. In addition, if the content of the inorganic particles is less than the above range, the dendrite suppression and / or the separator from dendrites may not be effectively prevented, and further, the improvement in mechanical strength may be minimal.

[0023] According to one embodiment of the present invention, the inorganic particles may include at least one of α-Al2O3 and γ-Al2O3. Specifically, the inorganic particles may include γ-Al2O3. γ-Al2O3 may be more advantageous than α-Al2O3 because it has a lower manufacturing temperature and thus a relatively larger surface area.

[0024] According to one embodiment of the present invention, the inorganic particles include at least one of α-Al2O3 and γ-Al2O3, and may have a moisture content of less than 3 wt%. Specifically, the inorganic particles can have a very low moisture content as described above by removing hydroxyl groups on the surface through heat treatment. In the case of γ-Al2O3, a large amount of hydroxyl groups exist on the surface compared to α-Al2O3, so that uniform dispersion in a coating solution for producing a coating layer may be difficult, and there may be a possibility of peeling off from the coating layer, and therefore, it may be advantageous to remove hydroxyl groups on the surface through heat treatment. For example, the γ-Al2O3 before heat treatment has a moisture content of about 10 wt%, but can have a low moisture content of less than 3 wt% by removing hydroxyl groups through heat treatment. Specifically, when γ-Al2O3 is heat-treated, the hydroxyl groups on the surface are removed, but the surface area increases, and since it has a relatively low crystallinity compared to α-Al2O3, when a coating layer is formed using this, the penetration of the electrolyte into the coating layer can be improved, and the interfacial resistance with the lithium negative electrode in contact with the coating layer can be lowered, so that a better battery life can be exhibited.

[0025] According to one embodiment of the present invention, the particle size of the inorganic particles may be 10 nm to 100 nm, or 20 nm to 50 nm. The particle size of the inorganic particles is D 50The particle size may be within the range of the above-mentioned inorganic particles, and the measurement method thereof may be measured using methods and devices widely known in the art. When the particle size of the above-mentioned inorganic particles is within the range, the uniform movement of lithium ions to the lithium metal negative electrode through the coating layer and the movement of lithium ions are improved during battery charging, thereby improving the battery life.

[0026] According to one embodiment of the present invention, the coating layer may be provided on one side of the separator facing the lithium metal negative electrode. The coating layer is provided on the side of the separator facing the lithium metal negative electrode, thereby effectively preventing damage to the separator due to dendrite formed at the lithium metal negative electrode, thereby improving the stability and lifespan of the lithium metal battery.

[0027] According to one embodiment of the present invention, the coating layer may further include a lithium salt. The lithium salt may be selected from LiFSI (lithium bis(fluorosulfonyl) imide), LiTFSI (lithium bis(trifluoromethane sulfonyl) imide), LiPF6, LiBF4, LiAsF6, LiClO4, LiCsPF6, LiNO3, LiPO2F2, LiBr, LiBOB (lithium bis(oxalato) borate), LiDFOB (lithium difluoro(oxalate) borate), LiCF3SO3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiC(SO2CF3)3, LiN(SO3CF3)2, LiC4F9SO3, LiAlCl4, and LiTfl (lithium trifluoromethanesulfonate). The lithium salt may play a role in facilitating the movement of lithium ions within the coating layer. The lithium salt may be present in an amount of 5 to 50 parts by weight, or 10 to 30 parts by weight, based on 100 parts by weight of the total binder.

[0028] According to one embodiment of the present invention, the thickness of the coating layer may be 1 μm to 20 μm. Specifically, the thickness of the coating layer may be 1 μm to 15 μm, or 2 μm to 10 μm. Within the above thickness range, the effect of the coating layer described above can be optimized and implemented.

[0029] According to one embodiment of the present invention, the separator may be a porous separator manufactured using a highly porous / permeable material, such as felt, paper, or a microporous plastic film. The porous separator must be resistant to attack by the electrolyte and other battery components under a battery potential. For example, the porous separator may be a porous membrane made of glass, plastic, or ceramic. In addition, the porous separator may be a polymer separator having a porous or microporous network structure to confine the non-aqueous electrolyte. However, the present invention is not limited thereto, and any material commonly used in the art may be applied.

[0030] According to one embodiment of the present invention, the lithium metal battery may further include a lithium salt and an electrolyte provided between the positive electrode and the lithium metal negative electrode. The lithium salt included in the electrolyte may be the same as described above.

[0031] According to one embodiment of the present invention, the electrolyte may include a non-aqueous organic solvent. The non-aqueous organic solvent may be selected from the group consisting of glyme solvents and ether solvents. The glyme solvent may be selected from the group consisting of 1,2-dimethoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, dimethylene glycol dimethyl ether, trimethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, polyethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and triethylene glycol diethyl ether. In addition, the ether solvent may be selected from the group consisting of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl ether, and dibutyl ether.

[0032] According to one embodiment of the present invention, the electrolyte may include an organic solid electrolyte and / or an inorganic solid electrolyte. The organic solid electrolyte may be selected from the group consisting of polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyester sulfides, polyvinyl alcohols, and polyvinylidene fluoride. In addition, the inorganic solid electrolyte may be selected from the group consisting of Li4SiO4, Li3N, LiI, Li5NI2, Li3N-LiI-LiOH, Li2SiS3, Li4SiO4-LiILiOH, and Li3PO4-Li2S-SiS2. However, the organic solid electrolyte and the inorganic solid electrolyte are not limited thereto, and various materials applied in the art or newly developed materials may be applied without limitation.

[0033] According to one embodiment of the present invention, the lithium metal battery may be a coin-shaped, square-shaped, pouch-shaped or thin-film-shaped battery.

[0034] Hereinafter, the present invention will be described in detail using examples. However, the examples according to the present invention may be modified in various ways, and the scope of the present invention is not limited to the examples described below. The examples in this specification are provided to more fully explain the present invention to those of ordinary skill in the art.

[0035] [Example 1]

[0036] 1.0 g of PAN (Polyacrylonitrile, Mw 150,000) was added to 12.0 g of NMP (N-Methyl-2-pyrrolidone) and dissolved while stirring at a temperature of approximately 70°C for 4 hours. After PAN was completely dissolved, 0.24 g of (Lithium bis(trifluoromethanesulphonyl)imide) was added and completely dissolved to prepare a PAN solution.

[0037] Then, 1.0 g of PVDF (Polyvinylidene fluoride, Mw 545,000) was added to 12.0 g of NMP (N-Methyl-2-pyrrolidone) and dissolved while stirring at a temperature of about 70°C for 4 hours. After PAN was completely dissolved, about 0.16 g of (Lithium bis(trifluoromethanesulphonyl)imide) was added and completely dissolved to prepare a PVDF solution.

[0038] Then, the prepared PAN solution and PVDF solution were sufficiently mixed by stirring at a temperature of about 70°C for about 1 hour to prepare a PAN / PVDF solution.

[0039] 6.0 g of α-Al2O3 (aluminum oxide, 50 nm) was added to the prepared PAN / PVDF solution and stirred at a stirring speed of approximately 2,000 rpm for 20 minutes to prepare a PAN / PVDF coating solution in which α-Al2O3 was dispersed. At this time, the weight ratio of the binder components, PAN and PVDF, was 1:1, and the lithium salt LiTFSi was added at 1.0 M based on the total binder, and the content of α-Al2O3 was 300% with respect to the total binder weight.

[0040] The manufactured PAN / PVDF coating solution was cast to a thickness of 28 ㎛ on one side of a commercial separator, Celgard 3501 (25 ㎛), using a doctor blade, and then dried in a vacuum oven at a temperature of approximately 70 ℃ for 2 hours to manufacture a coated separator with a final thickness of 30 ㎛.

[0041] [Example 2]

[0042] A coated separator was manufactured in the same manner as in Example 1, except that the weight ratio of the binder components, PAN and PVDF, was adjusted to 6.5:3.5.

[0043] [Example 3]

[0044] A coated separator was manufactured in the same manner as in Example 1, except that the weight ratio of the binder components, PAN and PVDF, was adjusted to 3.5:6.5.

[0045] [Example 4]

[0046] A coating membrane was manufactured in the same manner as in Example 1, except that γ-Al2O3 (moisture content of approximately 2 wt%), which had been heat-treated at 500°C for 1 hour to remove surface hydroxyl groups, was used instead of α-Al2O3 as an inorganic particle.

[0047] [Example 5]

[0048] A coating membrane was manufactured in the same manner as in Example 2, except that γ-Al2O3 (moisture content of approximately 2 wt%), which had been heat-treated at 500°C for 1 hour to remove surface hydroxyl groups, was used instead of α-Al2O3 as an inorganic particle.

[0049] [Example 6]

[0050] A coated separator was manufactured in the same manner as in Example 1, except that the amount of inorganic particles, α-Al2O3, was adjusted to 150% of the total binder weight.

[0051] [Comparative Example 1]

[0052] A coated separator was manufactured in the same manner as in Example 1, except that only PAN was used as a binder material.

[0053] [Comparative Example 2]

[0054] A coated separator was manufactured in the same manner as in Example 1, except that the inorganic particles α-Al2O3 were not applied.

[0055] [Comparative Example 3]

[0056] Only Cegard 3501, a commercial membrane, was used without a coating layer.

[0057] The components and contents of the coating layer in the above examples and comparative examples are summarized in Table 1 below.

[0058] Division Input amount [g] Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Comparative Example 1 Comparative Example 2 Comparative Example 3 PAN 1.001.300.701.001.301.000.41.6-PVDF 1.000.701.301.000.701.001.60.4-LiTF Si 0.400.430.380.400.430.400.360.45-Al 2 O 3 6.00(α) 6.00(α) 6.00(α) 6.00(γ) 6.00(γ) 3.00(α) 6.00(α) 6.00(α) -

[0059] Furthermore, the properties of the coated separator in the examples and comparative examples were measured as follows, and the results were as shown in Table 2 below.

[0060] [tensile strength]

[0061] The material properties were evaluated using an Ultra-Thin Tester (UTM). The test specimens were cut to 10 × 50 mm in size according to ASTM D882, and the thickness was measured at 50 mm / hr. The tensile strength of the specimens was expressed as the tensile strength at break (MPa) value from the stress-strain curve.

[0062] [Thermal stability: shrinkage]

[0063] The change in length before and after shrinkage was measured according to high temperature conditions (140 ℃, 20 minutes) based on the MD direction of the membrane.

[0064] [Coin cell manufacturing]

[0065] Coin cells of the size of CR2032 (diameter 20 mm, thickness 32 mm) were manufactured and electrochemically evaluated. NCM622 was used as the positive electrode and Li metal foil as the negative electrode, and the coated separators according to the examples and comparative examples were applied, and the sizes of these were 14Φ, 16Φ, and 19Φ, respectively. 30 ㎕ of 1 M LiPF6 electrolyte was injected into each electrode to manufacture them.

[0066] [Impedance]

[0067] A method of measuring resistance by applying an AC voltage to the manufactured coin cell was used, and Electrochemical Impedance Spectroscopy (EIS) was measured and interpreted as a Nyquist plot. The frequency was set from 1 MHz to 1 Hz, the voltage range was -5 V to 5 V, and the amplitude was 10 mV. The measured Rs is the resistance of the electrolyte and represents the ionic conductivity characteristics of the electrolyte, and Rct is the charge transfer resistance and represents the charge transfer characteristics at the electrode interface.

[0068] [Charge / Discharge Evaluation]

[0069] Using the manufactured coin cells, the charge-discharge performance of the batteries was evaluated by constantly increasing the current in CC (Constant Current) mode. The charge C-rate was evaluated at 0.5C and the discharge C-rate at 1.0C, and the lifespan of the lithium metal battery according to the coated separator was evaluated by measuring the degree of decrease in the discharge retention rate over 300 cycles.

[0070] Classification Input amount [g] Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Comparative Example 1 Comparative Example 2 Comparative Example 3 Mechanical properties Tensile strength [Mpa] 136 133 132 131 132 121 128 138 104 Thermal stability Shrinkage rate [%] 10 119 111 22 59 1348 Impedance Rs [Ω] 5.1 4.2 5.7 3.5 2.9 6.8 10.5 3.8 3.3 Rct [Ω] 17 16 20 15 132 6 32 1443 Battery life (at 300 cycles) Discharge retention rate [%] 9 39 59 39 79 888 dead dead 80

[0071] In the case of Examples 1 to 6, it was confirmed that the tensile strength and thermal stability were greatly improved compared to Comparative Example 3, which did not apply the coated separator, and it was confirmed that the charge transfer resistance (Rct) was greatly reduced, which greatly improved the battery efficiency. In particular, in the case of Examples 1 to 5, in which the inorganic particles were applied at 300% of the binder weight, it was confirmed that they exhibited a shrinkage rate of about 10% and showed very high thermal stability, and it was confirmed that they exhibited a discharge retention rate of over 90%, which greatly extended the life of the lithium metal battery. This may mean that the coating layer of the separator effectively suppressed the formation of dendrites and protected the separator from the formed dendrites. In addition, Comparative Example 1, in which the PVDF content of the coating layer was excessively high, the battery stopped operating around the 100th cycle due to the high interfacial resistance, and Comparative Example 2, in which the PAN content of the coating layer was excessively high, the battery stopped operating around the 50th cycle due to the detachment phenomenon of the separator and the coating layer.

Claims

1. A positive electrode; a lithium metal negative electrode; and a separator provided between the positive electrode and the lithium metal negative electrode, A lithium metal battery, wherein a coating layer including a binder containing PAN (polyacrylonitrile) and a fluorinated polymer, and inorganic particles is provided on at least one surface of the separator.

2. In claim 1, A lithium metal battery, wherein the weight ratio of PAN and fluorinated polymer in the coating layer is 3.5:6.5 to 6.5:3.

5.

3. In claim 1, A lithium metal battery, wherein the fluorinated polymer comprises at least one selected from the group consisting of poly(vinyl fluoride) (PVF), poly(vinylidene fluoride) (PVDF), and poly(ethylene-co-tetrafluoroethylene) (ETFE).

4. In claim 1, A lithium metal battery, wherein the coating layer is provided on one side of the separator facing the lithium metal negative electrode.

5. In claim 1, A lithium metal battery, wherein the content of the above-mentioned inorganic particles is 150 to 400 parts by weight based on 100 parts by weight of the total binder.

6. In claim 1, A lithium metal battery, wherein the inorganic particles include at least one of α-Al2O3 and γ-Al2O3.

7. In claim 6, A lithium metal battery, wherein the above-mentioned inorganic particles have a moisture content of less than 3 wt%.