Lithium metal battery

Abstract

The present invention relates to 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 comprising a polymer binder, a lithium salt, and a silica-cellulose nanofiber composite is provided on at least one surface of the separator.

Description

lithium metal battery The present invention relates to a lithium metal battery. 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. 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. 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. [Prior Art Literature] [Patent Document] (Patent Document 1) Republic of Korea Patent Publication No. 10-2021-0009188 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. 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 polymer binder, a lithium salt, and a silica-cellulose nanofiber composite is provided on at least one surface of the separator. The lithium metal battery according to the present invention can realize high stability and improved lifespan by having a coating layer capable of effectively preventing adverse effects due to dendrites on at least one side of the separator. Figure 1 is a schematic diagram showing the inhibition of dendrite formation in a lithium metal negative electrode by a separator having a coating layer according to one embodiment of the present invention. FIG. 2 is a schematic diagram showing the suppression of dendrite formation in a lithium metal negative electrode by a separator having a coating layer according to one embodiment of the present invention. Figure 3 shows an SEM image of the surface of the membrane according to Comparative Example 1. Figure 4 shows an SEM image of the surface of the coating layer of the coating separator according to Example 1. Figure 5 shows an SEM image of the surface of the coating layer of the coating separator according to Example 1. Figure 6 shows the results of an experiment to confirm the wettability of a membrane according to comparative examples and examples. Figure 7 shows the results of an experiment to confirm the wettability of the separator according to comparative examples and examples with respect to the electrolyte. Figure 8 is a graph showing the life characteristics of coin cells to which a coating separator according to an example and a comparative example is applied. Figure 9 is an image of a lithium metal negative electrode of a lithium metal battery using a separator according to Comparative Example 1 after 300 cycles of charge and discharge. Figure 10 is an image of a lithium metal negative electrode after 300 charge-discharge cycles of a lithium metal battery using a coated separator according to Example 1. In this specification, when a part is said to "include" a certain component, this means that it may include other components, but not to the exclusion of other components, unless specifically stated otherwise. 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. In this specification, “%” and “part” indicating content are based on weight unless otherwise specified. 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. Hereinafter, the present invention will be described in detail. 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 polymer binder, a lithium salt, and a silica-cellulose nanofiber composite is provided on at least one surface of the separator. 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. 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. According to one embodiment of the present invention, the coating layer of the separator may be a polymer matrix composed of a polymer binder in which a lithium salt and a silica-cellulose nanofiber composite are dispersed. According to one embodiment of the present invention, the silica-cellulose nanofiber composite may be one in which silica particles are attached to the surface of cellulose nanofibers. According to one embodiment of the present invention, the cellulose nanofibers may be cellulose fibers in which cellulose chains in which hexose glucoses are linked by β-1,4 bonds form bundles and have an average diameter of nanometers among fibers linked through interchain hydrogen bonds. The cellulose nanofibers have an overall fiber shape and an average diameter thereof may be several to several tens of nm, specifically, 1 nm to 100 nm, specifically, 5 nm to 50 nm, and more specifically, 10 nm to 30 nm. In addition, the average length of the cellulose nanofibers may be several to several tens of μm, specifically, 1 μm to 100 μm, and more specifically, 5 μm to 30 μm. According to one embodiment of the present invention, the content of the silica particles may be 10 parts by weight or more and 20 parts by weight or less based on 100 parts by weight of the cellulose nanofibers. More detailed information and manufacturing method of the silica-cellulose nanofiber composite may include all of the contents disclosed in Republic of Korea Patent Publication No. 10-2020-0042220. According to one embodiment of the present invention, the content of the silica-cellulose nanofiber composite may be 10 parts by weight or more and 300 parts by weight or less with respect to 100 parts by weight of the polymer binder. Specifically, the content of the silica-cellulose nanofiber composite may be 10 parts by weight or more and 200 parts by weight or less, 10 parts by weight or more and 100 parts by weight or less, 10 parts by weight or more and 50 parts by weight or less, 20 parts by weight or more and 50 parts by weight or less, or 20 parts by weight or more and 40 parts by weight or less with respect to 100 parts by weight of the polymer binder. Within the above content range, the silica-cellulose nanofiber composite is evenly dispersed within the coating layer to form a network of nanofibers, thereby appropriately imparting wettability to the coating layer and minimizing side effects due to uneven distribution of the electrolyte. In addition, the silica-cellulose nanofiber composite may be a cation of a lithium salt in the coating layer, Li + It facilitates the movement of lithium ions while restricting the movement of anions, thereby enabling lithium ions to be evenly deposited on the lithium metal negative electrode during charging and discharging of the battery, thereby effectively suppressing the formation of dendrites. FIG. 1 is a schematic diagram illustrating the inhibition of dendrite formation in a lithium metal anode using a separator having a coating layer according to an embodiment of the present invention. The left drawing of FIG. 1 illustrates a dendrite formation process in a lithium metal battery in which the separator does not have a coating layer according to the present invention, and the right drawing illustrates a dendrite formation inhibition process in a lithium metal battery in which the separator has a coating layer according to the present invention. Specifically, the silica-cellulose nanofiber composite in the coating layer according to the present invention can suppress the formation of lithium dendrites by trapping lithium salt anions according to charge and discharge of the lithium metal battery, thereby allowing lithium ions to be evenly deposited on the lithium metal anode. FIG. 2 is a schematic diagram illustrating the inhibition of dendrite formation in a lithium metal anode by a separator having a coating layer according to an embodiment of the present invention. The left drawing of FIG. 2 illustrates the movement of lithium ions in a lithium metal battery in which the separator does not have a coating layer according to the present invention, and the right drawing illustrates the movement of lithium ions in a lithium metal battery in which the separator has a coating layer according to the present invention. Specifically, the silica-cellulose nanofiber composite in the coating layer according to the present invention can effectively improve the wettability of the coating layer with respect to the electrolyte, thereby enabling lithium ions in the electrolyte to be evenly deposited on the lithium metal anode during charge and discharge of the lithium metal battery. That is, as described above, the silica-cellulose nanofiber composite effectively inhibits the movement of anions of a lithium salt to the lithium metal anode, and further provides the separator with wettability with respect to the electrolyte, thereby inducing even movement of lithium ions of a lithium salt to the lithium metal anode, thereby inhibiting the formation of lithium dendrites. According to one embodiment of the present invention, the polymer binder may include at least one non-fluorinated polymer binder selected from the group consisting of poly(ethylene oxide) (PEO), poly(acrylonitrile) (PAN), poly(methyl metacrylate) (PMMA), poly(ethylene carbonate) (PEC), poly(propylene carbonate) (PPC), and PAN-PMMA copolymer (PAN-co-PMMA). Specifically, according to one embodiment of the present invention, the polymer binder of the coating layer may be a non-fluorinated polymer binder. The coating layer uses a non-fluorinated polymer binder instead of a fluorinated polymer binder that may decompose depending on the operation of the battery to generate hydrofluoric acid, thereby lowering the performance of the battery, thereby preventing problems caused by the fluorinated polymer binder. Furthermore, the coating layer includes a non-fluorinated polymer binder, thereby increasing the bonding strength between the separator and the lithium metal anode, and implementing high wettability with respect to the electrolyte together with the silica-cellulose nanofiber composite, thereby inducing even deposition of lithium ions onto the lithium metal anode during charge and discharge of the lithium metal battery. In addition, as described above, the polymer binder may form a polymer matrix to disperse and fix the lithium salt and the silica-cellulose nanofiber composite within the coating layer. Specifically, according to one embodiment of the present invention, the polymer binder may include a mixture of PAN and a PAN-PMMA copolymer. More specifically, according to one embodiment of the present invention, the polymer binder may include PAN and a PAN-PMMA copolymer (PAN-co-PMMA) in a weight ratio of 3:1 to 5:1. 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. 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. Furthermore, Li, which is a cation of lithium salt in the coating layer + While the movement of lithium ions is facilitated, the movement of anions is restricted by the silica-cellulose nanofiber composite, so that lithium ions can be evenly deposited on the lithium metal negative electrode during charging and discharging of the battery, thereby suppressing the formation of dendrites. To this end, the lithium salt may be present in an amount of 50 to 150 parts by weight, or 70 to 120 parts by weight, based on 100 parts by weight of the polymer binder. According to one embodiment of the present invention, the coating layer may further include a plasticizer. The plasticizer may enable smoother movement of ions within the coating layer. The plasticizer may include at least one selected from the group consisting of gamma-Butyrolactone (GBL) and succinonitrile (SN). 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. 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. 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. 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. 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. 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. 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. [Example 1] 0.8 g of PAN (Polyacrylonitrile, Mw 150,000) and 0.2 g of PAN-co-PMMA (PAN-PMMA copolymer, Mw less than 100,000) were added to 11 g of EC (Ethylene carbonate), and 2 g of succinonitrile (SN) as a plasticizer was additionally added, and the mixture was stirred and dissolved at a temperature of approximately 60°C for 4 hours. After PAN and PAN-co-PMMA were completely dissolved, 1 g of LiPF6 was added and completely dissolved to prepare a polymer solution. And, the identification number of the Republic of Korea Patent Publication No. 10-2020-0042220

[0052] 0.5 g of the silica-cellulose nanofiber composite manufactured based on the above was added to the manufactured polymer solution and dispersed for about 3 minutes using a ball mill to manufacture a coating solution. The composition ratio in the above coating solution was as shown in Table 1 below. Polymer Ethylene carbonate (solvent) LiPF6 (lithium salt) SN (plasticizer) Silica-cellulose nanofiber composite Weight % 6.5713136.5 The manufactured coating solution was cast to a thickness of 10 ㎛ on one side of a commercial separator, Celgard 2320, using a doctor blade, and then dried in a vacuum oven at a temperature of approximately 60°C for 1 hour to manufacture a coated separator. [Comparative Example 1] Only Celgard 2320, a commercial membrane, was used without a coating layer. [Comparative Example 2] A coated separator was manufactured in the same manner as in Example 1, except that alumina particles (Al2O3, 50 nm) were used instead of the silica-cellulose nanofiber composite. Figure 3 shows an SEM image of the surface of the membrane according to Comparative Example 1. Figures 4 and 5 show SEM images of the surface of the coating layer of the coated separator according to Example 1. Specifically, Figure 5 is an enlarged view of the results of Figure 4. According to Figures 4 and 5, it can be confirmed that the coating layer of the coated separator according to Example 1 has a silica-cellulose nanofiber composite dispersed therein. Figure 6 shows the results of an experiment to confirm the wettability of membranes according to comparative examples and examples. According to Figure 6, it can be confirmed that Comparative Example 1, which does not have a separate coating layer, has low surface wettability, and it can be confirmed that the coated separator of Example 1, which applied a silica-cellulose nanofiber composite, has higher wettability to moisture than the coated separator of Comparative Example 2, which applied alumina particles. Figure 7 shows the results of an experiment to confirm the wettability of the separator according to comparative examples and examples to the electrolyte. According to Figure 7, it was confirmed that the coated separator of Example 1, which applied the silica-cellulose nanofiber composite, exhibited higher wettability to the electrolyte than Comparative Examples 1 and 2. In particular, the coated separator of Example 1 exhibited the best effect of absorbing the electrolyte immediately after dropping it. [Experimental Example] 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 1M LiPF6 (EC / DEC with 10% FEC) electrolyte was injected into each electrode to manufacture them. Then, the life characteristics of the manufactured coin cells were evaluated at a rate of 1 C charge / discharge. Fig. 8 is a graph showing the life characteristics of coin cells to which coated separators according to Examples and Comparative Examples were applied. According to the results of Fig. 8, it was confirmed that the lithium metal battery to which the coated separator according to Example 1 was applied maintained a capacity similar to the initial performance even after reaching 300 cycles. In contrast, the lithium metal batteries to which the separators according to Comparative Examples 1 and 2 were applied showed a result of losing about 10% of the initial capacity after 300 cycles, and showed a tendency for the capacity decrease to decrease rapidly compared to Example 1. Fig. 9 is an image of a lithium metal anode after 300 charge / discharge cycles of a lithium metal battery using a separator according to Comparative Example 1. In addition, Fig. 10 is an image of a lithium metal anode after 300 charge / discharge cycles of a lithium metal battery using a coated separator according to Example 1. Referring to Figs. 9 and 10, it was confirmed that the lithium metal anode after charge / discharge of the lithium metal battery using the coated separator according to Example 1 had even lithium deposition, significantly suppressing the formation of lithium dendrites. In contrast, it was expected that the lithium metal anode after charge / discharge of the lithium metal battery using the separator according to Comparative Example 1 had uneven lithium deposition, partially resulting in the formation of high lithium dendrites. As shown in the results of FIGS. 8 and 10, it can be seen that the lithium metal battery using the coated separator according to Example 1 can contribute to improving the lifespan of the lithium metal battery by effectively suppressing the formation of lithium dendrites by dispersing the deposition of lithium metal on the surface of the lithium metal negative electrode due to repeated charging and discharging.

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 comprising a polymer binder, a lithium salt, and a silica-cellulose nanofiber composite is provided on at least one surface of the separator.

2. In claim 1, The above silica-cellulose nanofiber composite is a lithium metal battery in which silica particles are attached to the surface of cellulose nanofibers.

3. In claim 2, A lithium metal battery, wherein the content of the silica particles is 10 parts by weight or more and 20 parts by weight or less based on 100 parts by weight of the cellulose nanofibers.

4. In claim 1, A lithium metal battery, wherein the content of the silica-cellulose nanofiber composite is 10 parts by weight or more and 300 parts by weight or less with respect to 100 parts by weight of the polymer binder.

5. In claim 1, A lithium metal battery, wherein the polymer binder comprises at least one non-fluorinated polymer binder selected from the group consisting of poly(ethylene oxide) (PEO), poly(acrylonitrile) (PAN), poly(methyl metacrylate) (PMMA), poly(ethylene carbonate) (PEC), poly(propylene carbonate) (PPC), and PAN-PMMA copolymer (PAN-co-PMMA).

6. In claim 1, A lithium metal battery, wherein the polymer binder comprises a mixture of PAN and PAN-PMMA copolymer.

7. 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.

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