阅读说明：本技术 锂金属二次电池及其制造方法 (Lithium metal secondary battery and method for manufacturing same ) 是由 崔白范 具滋训 金玟旭 李相均 于 2018-12-21 设计创作，主要内容包括：提供了一种确保电极-隔膜的粘合强度的锂金属二次电池及其制造方法。根据本公开的锂金属二次电池包括负极、隔膜和正极,所述负极包括锂金属箔作为负极材料,其中在锂金属箔的表面上形成纳米压印图案结构,所述锂金属箔的表面是负极的面对隔膜的表面,并且负极与隔膜彼此粘合。(Provided are a lithium metal secondary battery ensuring electrode-separator adhesive strength and a method of manufacturing the same. The lithium metal secondary battery according to the present disclosure includes an anode including a lithium metal foil as an anode material, wherein a nanoimprint pattern structure is formed on a surface of the lithium metal foil, the surface of the lithium metal foil is a surface of the anode facing a separator, and the anode and the separator are adhered to each other, a separator, and a cathode.)

1. A lithium metal secondary battery comprising:

a negative electrode including a lithium metal foil as a negative electrode material,

wherein a nanoimprint pattern structure is formed on a surface of the lithium metal foil, the surface of the lithium metal foil is a surface of the negative electrode facing the separator, and the negative electrode and the separator are adhered to each other.

2. The lithium metal secondary battery according to claim 1, wherein the separator is filled in the pattern structure to form a physical bond between the anode and the separator.

3. The lithium metal secondary battery according to claim 2, wherein the physical bond is formed when the separator is filled into the pattern structure by deformation.

4. The lithium metal secondary battery according to claim 2, wherein the physical bond is formed when a separator adhesive applied to a surface of the separator is filled into the pattern structure.

5. A method of manufacturing a lithium metal secondary battery, comprising:

stacking and laminating a negative electrode, a separator and a positive electrode, the negative electrode including a lithium metal foil as a negative electrode material, wherein a nanoimprint pattern structure is formed on a surface of the lithium metal foil, the surface of the lithium metal foil being a surface of the negative electrode facing the separator; and

and bonding the negative electrode and the separator.

6. The method of manufacturing a lithium metal secondary battery according to claim 5, wherein in the bonding, the separator is filled in a pattern structure to form a physical bond between the anode and the separator.

7. The method of manufacturing a lithium metal secondary battery according to claim 5, wherein the adhesion of the negative electrode and the separator is formed in lamination, or is formed by first laminating the negative electrode and the separator to manufacture a negative electrode-separator adhesion structure, or manufacturing a negative electrode-separator adhesion structure, and then laminating the positive electrode; the lamination load was 10 kgf.

8. The method of manufacturing a lithium metal secondary battery according to claim 6, wherein the physical bonding is formed when the separator is filled into the pattern structure due to deformation.

9. The method of manufacturing a lithium metal secondary battery according to claim 6, wherein the physical bond is formed when a separator adhesive applied to a surface of a separator is filled into a pattern structure.

10. The method of manufacturing a lithium metal secondary battery according to claim 5, wherein forming a pattern structure comprises directly applying pressure to a surface of the lithium metal foil using a pattern mold.

11. The method of manufacturing a lithium metal secondary battery according to claim 5, wherein a distance between patterns in the pattern structure is not more than 1.60 μm.

Technical Field

The present disclosure relates to a lithium metal secondary battery using lithium (Li) metal as an anode material without a separate anode active material, and a method of manufacturing the same. The present application claims priority from korean patent application No. 10-2017-0180546, filed in korea at 27.12.2017, and korean patent application No. 10-2018-0163554, filed in korea at 17.12.2018, the disclosures of which are incorporated herein by reference.

Background

Secondary batteries can be repeatedly charged, and are attracting attention as a substitute for fuel energy. The secondary battery is mainly used in conventional hand-held devices such as a cellular phone, a video camera, and a power tool. Recently, the application range of the secondary battery tends to be gradually expanded to electric vehicles (EV, HEV, PHEV), large-capacity Energy Storage Systems (ESS), and uninterruptible power supply systems (UPS).

The secondary battery includes: an electrode assembly including a positive electrode, a negative electrode, and a separator interposed therebetween; and an electrolyte electrochemically reacting with the active materials coated on the positive and negative electrodes. A typical secondary battery is a lithium ion secondary battery in which electrochemical reactions occur in a positive electrode and a negative electrode due to reactions of lithium ions as working ions during charge and discharge. Conventional lithium ion secondary batteries employ lamination during assembly to achieve adhesive strength between electrodes and separators within an electrode assembly. Lamination is the process of thermally bonding the separator and the electrodes. The lamination bonds the separator and the electrode stacked on each other by heat, and as a result, the bonding strength between the separator and the electrode is improved. The rough surface shape of conventional electrodes including active materials, conductive materials, and binders makes it easy to form an electrode-separator bond by lamination with a separator.

Recently, in order to improve the energy density of lithium ion secondary batteries, it is highly required to develop next-generation secondary batteries directly using lithium metal foil as a negative electrode material without a separate negative electrode active material. Lithium metal has a high ionization tendency and low density, as well as a very low standard electrode potential and a very high specific capacity. Although lithium metal has problems such as internal short circuits of the battery due to growth of lithium dendrites and the risk of explosion that may occur due to exposure to moisture, the highest energy density can be achieved if the problems are solved, and lithium metal is worth further research due to such advantages.

However, when a lithium metal foil having a flat smooth surface is used as the negative electrode and forms a bonding interface with the separator, physical bonding achieved by the rough active material surface shape of the conventional lithium ion secondary battery cannot be obtained as desired due to shape deformation of (the binder in) the separator, but only chemical bonding by electrostatic attraction can be relied on. Therefore, in the assembly process of a lithium sulfur battery, a lithium air battery, and an all-solid battery to which a lithium metal foil may be applied, adhesion lower than the electrode-separator adhesion strength obtained in a conventional lithium ion secondary battery may be formed. This limits the assembly workability of the next-generation secondary battery, resulting in defects such as separation and bending tolerances.

Fig. 1 is a diagram illustrating a problem when a lithium metal foil having a flat smooth surface is used as a negative electrode and is bonded to a separator.

As shown in fig. 1 (a), it is assumed that a lithium metal foil negative electrode 1 having a flat smooth surface, a separator 2, and a positive electrode 3 are stacked and laminated to form a single cell 4 as shown in (b). In general, the cathode 3 including the PVDF-based binder and the active material of the metal oxide having a high elastic modulus forms stronger interfacial adhesion with the separator 2 than the lithium metal foil anode 1 having a flat and smooth surface. Therefore, as shown in (c), defects such as warping occur in the single cells 4 due to the difference in adhesive strength between the negative and positive electrodes and the separator. In the case where the difference between the adhesion degrees of the anode, cathode and separator is large, if the single cell 4 is bent too much due to the difference in properties between the anode and cathode after lamination, the lithium metal foil anode 1 having a flat and smooth surface and low in adhesion strength is highly likely to be separated as shown in (d).

Meanwhile, in the case of an all-solid battery, in some cases, a lamination pressure is applied to achieve electrode-separator (electrolyte layer) adhesion of unit cells. Due to the hard (highly elastic) active material, the soft separator (electrolyte layer) may be partially damaged, thereby causing a short circuit. In order to solve this problem, patent document 1 proposes the following electrode design: even if a strong lamination pressure is applied, the interface of the electrode toward the separator (electrolyte layer) has less active material component to prevent electrical short-circuiting of the separator (electrolyte layer). However, this method is difficult to technically implement and economically inefficient, and cannot be applied to a lithium metal all-solid battery that does not use an anode active material.

[ relevant documents ]

[ patent document ]

(patent document 1) JP2011-124028A

Disclosure of Invention

Technical problem

The present disclosure is directed to providing a lithium metal secondary battery that ensures electrode-separator adhesive strength.

The present disclosure is also directed to providing a method of manufacturing a lithium metal secondary battery that ensures electrode-separator adhesive strength.

Technical scheme

In order to achieve the above object, a lithium metal secondary battery according to the present disclosure includes an anode, a separator, and a cathode, the anode including a lithium metal foil as an anode material, wherein a nanoimprint pattern structure is formed on a surface of the anode facing the separator, i.e., a surface of the lithium metal foil, and the anode and the separator are adhered to each other.

Here, preferably, the separator is filled in the pattern structure to form a physical bond between the anode and the separator.

In this case, the physical bond may be formed when the separator is filled in the pattern structure by being deformed, and may also be formed when the separator adhesive applied to the surface of the separator is filled in the pattern structure.

Preferably, the lithium metal foil has a thickness of 20 to 40 μm and a height of the surface pattern structure of 50 to 500 nm.

Preferably, the adhesive strength between the anode and the separator may be 3 times or more higher than that of the conventional art when the same lamination load is used.

In order to achieve another object, a method of manufacturing a lithium metal secondary battery according to the present disclosure includes: stacking and laminating a negative electrode including a lithium metal foil as a negative electrode material, a separator, and a positive electrode, wherein a nanoimprint pattern structure is formed on a surface of the negative electrode facing the separator, i.e., a surface of the lithium metal foil; and bonding the negative electrode and the separator.

The adhesion of the anode and the separator may be formed in lamination, or may be formed by first laminating the anode and the separator to make an anode-separator adhesion structure, or making an anode-separator adhesion structure, and then laminating the cathode; the lamination load may be 10 kgf.

Forming the patterned structure includes directly applying pressure to the surface of the lithium metal foil using a patterned mold. Preferably, the pattern mold may form a nanopattern having a height of 50 to 500 nm. Therefore, it is preferable to manufacture the pattern mold by a bottom-up method, such as self-assembly, to use the organic mold manufactured by duplicating the pattern mold.

Preferably, the distance between the patterns in the pattern structure is not more than 1.60 μm.

The lithium metal secondary battery according to the present disclosure may have a single cell and a bicell manufactured by lamination as unit cells, and may be implemented as a stacked battery by stacking the unit cells, folding the unit cells in a zigzag shape, and stacking and folding the unit cells.

Advantageous effects

According to the present disclosure, when the anode includes a lithium metal foil as an anode material, a physical bond is formed between the anode and the separator by shape deformation of the separator, thereby ensuring close adhesion between the anode and the separator. By the improved interfacial adhesion between the anode and the separator, stress that may occur in the anode/cathode may be avoided, thereby preventing the bending of the unit cell and the separation of the anode. The lithium metal foil having a surface pattern structure forms a strong bond with the separator by physical bonding during the lamination process, thereby improving assembly workability.

The lithium metal secondary battery according to the present disclosure may have a single cell and a double cell as unit cells, and may be implemented by stacking the unit cells, folding the unit cells in a zigzag shape, and stacking and folding the unit cells. Various types of secondary batteries can be manufactured regardless of the type of unit cells, and the improved anode-separator adhesion strength allows the properties of both unit cells and stacked batteries to be significantly improved.

Drawings

Detailed Description

Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. Before the description, it should be understood that the terms or words used in the specification and the appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present disclosure on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation. Therefore, the embodiments described herein and the illustrations shown in the drawings are only embodiments of the present disclosure and are not intended to fully describe the technical aspects of the present disclosure, so it should be understood that various other equivalent forms and modifications may be possible in implementing the invention.

In the embodiments described below, it should be understood that the lithium metal secondary battery covers any battery using lithium ions as working ions and including lithium metal foil as a negative electrode material, even though the name varies according to the kind of electrolyte or separator used in the lithium metal secondary battery, the kind of package used to package the lithium metal secondary battery, and the internal or external structure of the lithium metal secondary battery.

In addition, the lithium metal secondary battery is not limited to the number of components. Therefore, the lithium metal secondary battery should be construed as a unit cell including an assembly of a cathode/separator/anode and an electrolyte in a packaging material, as well as an assembly of unit cells, a module including an assembly connected in series and/or parallel, a battery pack including an assembly connected in series and/or parallel, and a battery system including a battery pack connected in series and/or parallel.

The present disclosure proposes the use of a surface pattern structure of lithium metal foil to improve electrode-separator adhesion. A surface pattern structure was formed on the lithium metal foil using a nanoimprint technique. The surface pattern structure is formed by directly applying pressure to the surface of the lithium metal foil using a pattern mold. The lithium metal foil having a surface pattern structure forms a bond with the separator through physical bonding during the lamination process, thereby improving assembly workability.

When lithium metal foil is used as a negative electrode material, an interfacial bond having a lower viscosity is formed with the separator as compared with a positive electrode using a positive electrode active material. In order to solve the problem, the inventors verified the effect of the nanoimprint pattern structure formed on the surface of the lithium metal foil and proposed the present disclosure through extensive studies on various ways of improving adhesion, such as applying a separator adhesive on the surface of the lithium metal foil or the surface of the separator, performing corona, RIE, and acid treatments on the surface of the lithium metal foil or the surface of the separator to improve charge properties, and designing the surface roughness of the surface of the lithium metal foil or the surface of the separator to improve physical adhesion (anchoring).

The present disclosure relates to a lithium metal secondary battery. The lithium metal secondary battery of the present disclosure includes a lithium metal foil as an anode material, and is the same as a conventional lithium metal secondary battery in that it includes an anode having an anode material, a separator, and a cathode, and a method of manufacturing the same. However, the lithium metal secondary battery of the present disclosure and the conventional lithium metal secondary battery have different surface shapes of anode materials, and are laminated differently from the conventional method for manufacturing the lithium metal secondary battery.

Fig. 2 illustrates a lithium metal foil negative electrode-separator bonding structure included in a lithium metal secondary battery according to the present disclosure. Fig. 3 illustrates another example of a lithium metal foil negative electrode-separator bonding structure included in a lithium metal secondary battery according to the present disclosure. First, as shown in (a) of fig. 2, a lithium metal foil negative electrode 10 having a surface pattern structure 12 and a separator 20 are prepared and laminated to manufacture a lithium metal foil negative electrode-separator bonding structure 22 as shown in (b), and positive electrodes are laminated together to manufacture an electrode assembly, which is then put into a packaging material (e.g., a pouch case), and then injected with an electrolyte and sealed to manufacture a lithium metal secondary battery. The lithium metal secondary battery has a pattern structure on a surface of the anode 10 facing the separator 20, i.e., a surface of the lithium metal foil, and has adhesiveness between the anode 10 and the separator 20.

A nanoimprint technique is applied to form the surface pattern structure 12 on the lithium metal foil. The surface pattern structure 12 is formed by directly applying pressure to the surface of the lithium metal foil using a pattern mold. When the separator 20 is bonded to the lithium metal foil negative electrode 10 having the surface pattern structure 12, the separator 20 is filled into the surface pattern structure 12 by shape deformation. Therefore, adhesion is formed between the anode 10 and the separator 20 by physical bonding during lamination, thereby improving assembly workability.

In this case, when the separator 20 is filled into the surface pattern structure 12 by deformation, a physical bond may be formed, and as shown in fig. 3, when the separator adhesive layer 18 applied to the surface of the separator 20 is filled into the surface pattern structure 12, a physical bond may be formed. In this case, the membrane adhesive layer 18 may be partially filled into the surface pattern structure 12, or may form a conformal layer on the surface of the surface pattern structure 12, or may be present only on the highest protruding region of the surface pattern structure 12.

Preferably, there is no gap between separator 20 and lithium metal foil negative electrode 10, and when separator binder layer 18 is present, there is no gap between separator 20, separator binder layer 18, and lithium metal foil negative electrode 10.

Separator 20 may be a polyolefin-based polymer, such as PE and PP, and separator adhesive layer 18 may be a PVDF-based adhesive additionally formed and provided on separator 20.

As described below, the method of forming the surface pattern on the lithium metal foil negative electrode 10 may be an imprinting method using pressure, such as nanoimprinting. In this case, the lithium metal is pressed down when subjected to pressure, and the modulus and density of the material increase, and the lithium metal becomes rigid. A better bonding interface may be formed when further including a relatively soft separator adhesive layer 18, as opposed to rigid lithium metal. Referring back to fig. 2, when a lithium metal foil is used as an anode material, the present disclosure uses a lithium metal foil rolled to a thickness d of 100 μm or less to achieve the energy density of a battery. The thickness d is calculated by the average distance between the two outermost surfaces of the lithium metal foil. The lithium metal foil used as the anode material may or may not include a current collector. When the lithium metal foil includes a current collector, the lithium metal may be formed, for example, 10 to 100 μm thick on both surfaces of a copper foil current collector, for example, 5 to 20 μm thick. When the lithium metal foil does not include a current collector, the lithium metal foil may be rolled alone to 100 μm or less without a separate current collector. The height h of the surface pattern structure 12 is 1 μm or less. The height h corresponds to the height of the protrusion or the depth of the groove, and is calculated from the average height of the degree to which the surface pattern structure 12 protrudes or is recessed from the lithium metal foil. The lithium metal foil thickness exceeding 100 μm is unstable for achieving high energy density due to the increase in thickness and volume when manufacturing the stacked battery. Preferably, the thickness ranges from 20 to 40 μm. In this case, the height h of the surface pattern structure 12 is more preferably 50 to 1000 nm. Lithium metal foil thicknesses of less than 20 μm are difficult to handle and are problematic with respect to structural rigidity. When a metal current collector (e.g., copper foil) is applied, the electrode may be manufactured by rolling different types of metals between lithium and the current collector, but when a lithium metal foil having a thickness of less than 20 μm is applied, there is a fear that the lithium metal is damaged upon rolling. In addition, when a battery is manufactured using a lithium metal foil as a negative electrode, in practical use, in the case where the reversible thickness of lithium metal varies during charge and discharge, if the thickness of lithium metal is very small, structural stability cannot be secured. A lithium metal foil thickness of more than 40 μm is undesirable from the viewpoint of energy density due to the increase in thickness and volume when manufacturing a stacked battery. The height of the surface pattern structure of less than 50nm is not sufficient to have sufficient physical bonding between the negative electrode and the separator. It is difficult to expect that the lithium metal having the surface pattern height of less than 50nm is pressed with the separator adhesive layer formed on the surface of the separator fabric by the agglomeration of the separator adhesive particles having the size of several tens or hundreds of nm to form physical adhesion (anchoring). A surface pattern height of more than 1000nm is undesirable because the separator adhesive layer on the surface of the separator cannot be densely filled in the surface pattern structure. When excessive pressure, temperature and rate are applied to form a physical bond, wrinkles, cracks or separation may occur on the electrode surface. As a result of experiments to produce a 120nm high grating structure on a 40 μm thick rolled lithium metal foil by DVD-R structure stimulation, as described in the following experimental examples, it can be seen that the adhesion is significantly improved without charge surface treatment.

The nanoimprint pattern mold for manufacturing a lithium metal secondary battery according to the present disclosure has convex portions or concave portions having a height of 50 to 1000 nm. The grooves may be formed in the reverse shape of the convex portions on the surface of the lithium metal foil by directly pressing the convex portions against the lithium metal foil, and the protrusions may be formed in the reverse shape of the concave portions on the surface of the lithium metal foil by pushing the lithium metal foil into the concave portions. The convex portion or the concave portion may be in the shape of a column, a cone, or the like, and preferably has a tapered shape that narrows upward toward the top of the convex portion and downward toward the bottom of the concave portion, because it is easy to insert the separator into the surface structure of the lithium metal foil to be formed later. Therefore, according to the manufacturing method, the convex portion or the concave portion of the pattern mold is preferably conical, and may have a triangular pyramid, a quadrangular pyramid/a rectangular pyramid, a conical shape, or the like.

Most preferably, the separator is densely filled in the surface structure of the lithium metal foil by shape deformation to form a completely tight bond without a gap between the lithium metal foil and the separator. When the surface structure of the lithium metal foil is a convex portion, the separator has a concave portion fitting the convex portion, thereby obtaining a joined sectional structure as if they were joined to each other at the corresponding position; also, when the surface structure of the lithium metal foil is a concave portion, the separator has a convex portion fitting the concave portion, thereby obtaining a joined sectional structure as if they were joined to each other at the corresponding position. In particular, it should be noted that the surface structure of the lithium metal foil is intentionally formed by nanoimprinting, but the shape deformation of the separator is achieved by the lamination pressure. In order to obtain a completely tight bond, the appropriate shape, height and lamination pressure of the surface structure (taking into account the lamination load and the area on which the load acts) have to be selected.

A master mold (master mold) of silicon or quartz manufactured by an etching technique, i.e., a so-called top-down method generally used to manufacture a pattern mold, or an organic mold manufactured by duplicating the master mold can be manufactured only on the order of micrometers, a minimum pattern size (or height) is 10 to 15 μm regardless of how small the pattern is, and such a mold is unstable for implementing the present disclosure. The present disclosure proposes to manufacture a pattern mold by a bottom-up method, such as self-assembly, to use an organic mold manufactured by replicating the pattern mold. In particular, it is desirable to use an organic material of PDMS, ETPTA, polyurethane, and PFPE, and to use a soft mold by duplicating a pattern mold manufactured by a bottom-up method. With the mold, a surface pattern structure having a height of 50 to 1000nm, i.e., a nano-scale micro pattern, may be formed on the lithium metal foil.

For example, a method of forming a pattern mold of a triangular pyramid having a height of 50 to 1000nm is described below.

1) A single layer of silica or Polystyrene (PS) particles having a size of 1 μm or less is formed in a Hexagonal Close Packed (HCP) structure by self-assembly on the surface of a glass substrate, a polymer film, or a metal foil.

2) The organic mold was fabricated by PDMS, ETPTA, epoxy, or PFPE replication using the HCP structure monolayer surface as a master mold. After applying an organic substance to the master mold and removing the substrate, the silica or polystyrene particles may be removed (etched) according to surface properties (e.g., surface energy) of the material of the organic mold used to form a triangular pyramid nano-pattern of a relief or an intaglio. The use of high molecular weight ETPTA or PFPE can produce hard molds compared to the use of PDMS or low molecular weight ETPTA, and is suitable for nanoimprint applications.

3) Accordingly, triangular pyramid nanopatterns of different sizes may be formed on the surface of the lithium metal foil by nanoimprinting using PFPE in relief or intaglio. Preferably, a triangular pyramid nanopattern having a height of 50 to 1000nm is formed.

Meanwhile, the shape of the concave portion or the convex portion of the pattern mold is not necessarily limited to the above example. The pattern may include an island-shaped pattern isolated from other patterns in four directions, such as a cone shape or a pillar shape, and a line and space pattern having a repeated stripe-shaped pattern extending in one direction and spaced apart from the other patterns by a predetermined distance. For example, the pattern may have a repetition of ridges and furrows.

Fig. 4 is a graph illustrating an improved effect when a lithium metal foil having a surface pattern structure is used as a negative electrode and is bonded to a separator according to the present disclosure.

As shown in (a) of fig. 4, the lithium metal foil negative electrode 10 having the surface pattern structure 12, the separator 20, and the positive electrode 30 are stacked and laminated to form a single cell 40 as shown in (b). Obviously, a separator adhesive may be applied to separator 20 as an auxiliary bonding means.

The cathode 30, which generally uses a PVDF-based binder and a metal oxide active material having a high elastic modulus, forms good interfacial adhesion with the separator 20. If a lithium metal foil having a flat and smooth surface is used as the negative electrode, the adhesion of the negative electrode to the separator will be poor. However, since the present disclosure uses the anode 10 having the surface pattern structure 12 on the surface of the lithium metal foil, the separator itself and/or the separator binder are filled in the surface pattern structure 12, and the anode 10 and the separator 20 are joined to each other, thereby physically improving the interfacial adhesion between the anode 10 and the separator 20. Therefore, even if stress that may occur in the negative/positive electrodes acts as indicated by the arrow (c), the stress is offset, thereby preventing the bending of the unit cell 40 and the negative electrode from separating.

< Experimental example >

To manufacture the experimental samples according to the present disclosure, nanoimprint pressing was performed on the surface of the lithium metal foil using DVD-R (LG Electronics, R4.7) as a template. As described previously, it is desirable to manufacture and use the pattern mold by the bottom-up method, but it can be seen that pattern transfer can be accomplished using a readily available DVD-R as the pattern mold, and the effect thereof has been confirmed.

Fig. 5 is a photograph of a process of preparing an experimental example sample according to the present disclosure. FIG. 6 is a cross-sectional view of a DVD-R for nanoimprinting.

First, a DVD-R (R4.7, LG Electronics) as shown in (a) of fig. 5 was prepared and disassembled to remove the organic dye and the aluminum layer to prepare a polycarbonate having a grating structure (to be used as a pattern mold). As shown in FIG. 6, the distance d between the track pitches of DVD-R_t740nm, track pitch height h_t120nm, track pitch width w_tIs 320 nm. Using DVD-R as a template at 300kgf/cm²Was pressed on the surface of a 40 μm thick rolled lithium metal foil for 1 minute, and as shown in fig. 5 (b), a diffraction phenomenon of the DVD-R surface was also observed on the surface of the lithium metal foil. This indicates that the grating structure (including the ridges and grooves) of the DVD-R was transferred onto the surface of the lithium metal foil to form a pattern on the surface of the lithium metal foil. Therefore, it can be seen that when pressure is directly applied to the surface of the lithium metal foil using the pattern mold as proposed in the present disclosure, the pattern of the pattern mold can be transferred onto the surface of the lithium metal foil.

Fig. 7 is a photograph of a general lithium metal foil having a flat smooth surface as a comparative example.

As shown in FIG. 7, a grating structure (height h) having an optical disk (DVD-R) on the surface is prepared_t120nm) (experimental examples of the present disclosure) and a general lithium metal foil (comparative example), and were laminated with a separator of a lithium ion secondary battery, respectively, to manufacture an electrode-separator bonding structure.

Each lithium metal foil was 15mm wide and 50mm long. For the lamination, roll lamination was used, and a load of 10kgf was applied at a rate of 300mm/sec under a temperature condition of 60 ℃. For the comparative examples, lamination loads of 100kgf and 250kgf were prepared.

Comparative adhesion strength tests were performed on the electrode-separator adhesive structures of the experimental examples and comparative examples of the present disclosure. The adhesive strength was measured by the usual 90 ° peel test and the rate was 100 mm/min.

Fig. 8 is a graph showing the 90 ° peel test results of the experimental examples and comparative examples of the present disclosure.

Referring to fig. 8, when the comparative example under the lamination load of 10kgf is designated as 100% adhesive strength, the adhesive strength increases to 185% and 192% as the lamination load increases to 100kgf and 250 kgf. In contrast, in the case of the experimental examples of the present disclosure, an adhesive strength of 320% was obtained at a lamination load of 10 kgf. As described above, the experimental examples of the present disclosure may have an adhesive strength 3 times or more higher than that of the comparative examples under the same lamination load. Even if the lamination load of the comparative examples was increased 10 times and 25 times, high adhesive strength could not be achieved.

As described above, it can be seen that the lithium metal foil having the nanoimprint pattern structure on the surface according to the experimental examples of the present disclosure has higher adhesive strength with the separator than the general lithium metal foil used as the comparative example, even at a low lamination pressure, and good adhesive strength results can be obtained.

Fig. 9 is a graph showing 90 ° peel test results of another experimental example and comparative example of the present disclosure.

The sample manufacturing and testing methods were similar to the experimental example used to obtain the graph of fig. 8.

This test evaluates the change in adhesion strength with the pattern spacing on the surface of the lithium metal foil. Here, the pattern spacing is the distance between patterns, and also refers to the distance d between track pitches as previously shown in FIG. 6_t。

The pattern intervals were set to 0.00. mu.m, 0.32. mu.m, 0.74. mu.m, and 1.60. mu.m. The pattern interval of 0.00 μm represents the lithium metal foil having no pattern, and may correspond to a comparative example showing the results of fig. 8. The pattern interval of 0.32 μm represents the lithium metal foil for pattern transfer using a blu-ray disc as a template. The pattern interval of 0.74 μm represents the lithium metal foil for pattern transfer using DVD-R as a template as shown in the experimental example of fig. 8. The maximum pattern interval of 1.60 μm represents a lithium metal foil for pattern transfer using a CD-R as a template.

Each lithium metal foil was 15mm wide and 50mm long, and for lamination, lamination was performed using a roller, and a load of 10kgf was applied at a rate of 300mm/sec under a temperature condition of 60 ℃.

Comparative adhesion strength tests were performed on the electrode-separator adhesive structures of the experimental examples and comparative examples of the present disclosure. The adhesive strength was measured by the usual 90 ° peel test and the rate was 100 mm/min.

Referring to fig. 9, when a comparative example under a lamination load of 10kgf is designated as 100% adhesive strength, the experimental examples of the present disclosure having pattern intervals of 0.32 μm and 0.74 μm reached adhesive strength of 300% or more, and the adhesive strength improvement effect was apparent. It has been found that the experimental examples of the present disclosure having a pattern interval of 1.60 μm achieve an adhesive strength of about 150% and have a higher adhesive strength than the comparative examples.

After manufacturing an electrode assembly including an electrode-separator bonding structure, the battery stiffness of a secondary battery including the electrode assembly was measured. There was no significant difference between the presence or absence of the pattern. Also, there is no significant difference between the different pattern spacings. This is because, as expected, the cell stiffness is primarily dependent on the stiffness of the positive electrode itself, not the negative electrode stack bond strength.

It is determined whether there is separation of the separator at the edge in the electrode-separator bonding structure. In the case of the comparative example having a pattern interval of 0.00 μm (i.e., no pattern), edge separation was observed. The experimental examples of the present disclosure having pattern intervals of 0.32 μm and 0.74 μm had no edge separation. Therefore, it can be seen that when a pattern is formed on the lithium metal foil according to the present disclosure, the adhesive strength with the separator is higher, and the effect of preventing separation at the edges is better. However, even if the lithium metal foil has a pattern, edge separation was observed in the sample having a pattern interval of 1.60 μm. Therefore, in terms of preventing edge separation, it is desirable that the pattern interval is not too large when forming a pattern on the lithium metal foil. For example, it is desirable that the pattern interval is not more than 1.60 μm.

Hereinafter, a method of manufacturing a lithium metal secondary battery according to the present disclosure will be described in detail with reference to fig. 10 based on the above-described configuration.

Fig. 10 is a flowchart illustrating a method of manufacturing a lithium metal secondary battery according to an embodiment of the present disclosure.

A method of manufacturing a lithium metal secondary battery according to the present disclosure starts with preparing a lithium metal foil, and forming a nanoimprint pattern structure on a surface facing a separator to manufacture an anode (s 1). In the case of a single-sided negative electrode, a surface pattern structure may be formed on one surface of the lithium metal foil; in the case of a double-sided negative electrode, a surface pattern structure may be formed on both sides of the lithium metal foil. Two single-sided negative electrodes having a surface pattern structure on one side may be bonded and used as a double-sided negative electrode.

The pattern structure is formed by a nanoimprint technique that directly applies pressure to the surface of the lithium metal foil using the previously described pattern mold. The conditions of the pattern mold for forming the surface pattern structure having a height of 50 to 1000nm, i.e., the nano-scale micro pattern, are described above.

Subsequently, the separator and the positive electrode are stacked and laminated on the negative electrode prepared in step s1 (s 2).

In this case, the anode and the separator are first laminated to make an anode-separator bonded structure, and then the anodes are laminated together to form an assembly. The negative electrode, separator, and positive electrode may be laminated together to form an assembly. In any case, the lamination load may be 10 kgf. When the shape of the separator is deformed by the lamination pressure, the separator is filled in the pattern structure, thereby forming a physical bond between the anode and the separator.

In the above experimental results (FIG. 8), the height h was formed on the rolled lithium metal foil of 40 μm thickness as a result of the structural stimulation by DVD-R_tAs a result of experiments for a grating structure of 120nm, it was found that the adhesion can be greatly improved without performing a charge surface treatment, but if necessary, in order to further improve the adhesion strength, a method of improving the adhesion, in which a separator adhesive is applied on the surface of the lithium metal foil or the surface of the separator, and the charge property is improved by corona, RIE and acid treatment on the surface of the lithium metal foil or the surface of the separator, may also be used.

The battery manufactured in this step may be a single cell 40 having a negative electrode 10-separator 20-positive electrode 30 structure as shown in fig. 4 (b), an a-type bicell 140 having a positive electrode 30 '-separator 20' -negative electrode 10 '-separator 20' -positive electrode 30 'structure as shown in fig. 11 (a), or a C-type bicell 240 having a negative electrode 10' -separator 20 '-positive electrode 30' -separator 20 '-negative electrode 10' structure as shown in fig. 11 (b). Fig. 11 shows an example in which the negative electrode 10 'is a double-sided negative electrode and the positive electrode 30' is a double-sided positive electrode.

When the pattern structure is formed on the surface of the lithium metal foil facing the separator according to the present disclosure, the condition of applying the lamination pressure may be reduced. Referring to the experimental results of fig. 8, as described above, good anode-separator adhesion strength can be ensured at low pressure.

The lithium metal secondary battery according to the present disclosure may have the single cell 40 and the bicells 140 and 240 manufactured by the above-described method as unit cells, and may be implemented as a stacked battery by stacking the unit cells, folding the unit cells in a zigzag shape, and stacking and folding the unit cells. According to the present disclosure, not only the performance of the unit cell itself but also the performance of a stacked battery using the unit cell may be improved due to improved anode-separator adhesion strength.

Fig. 12 is a view illustrating a lithium metal secondary battery according to still another embodiment of the present disclosure.

Referring to fig. 12, a double-sided negative electrode 10 'and a double-sided positive electrode 30' are prepared and assembled with a folded separator 20 ″ folded in a zigzag shape to manufacture a zigzag-folded battery 340. In general, when zigzag folding is performed, separation occurring between the anode and the folded separator may be more serious than the problem in the single cell described with reference to fig. 1 due to the difference in adhesive strength between the anode and the cathode. According to the present disclosure, although the lithium metal secondary battery is manufactured in a zigzag-folded form, the improved anode-separator adhesion strength reduces the possibility of separation and significantly reduces the defects of the zigzag-folded stacked battery.

As described above, the present disclosure manufactures a negative electrode through a simultaneous nanopattern transfer process by previously forming a pattern mold having a nano-scale protrusion and depression pattern and pressing the pattern mold on the surface of a lithium metal foil. This can rapidly form nano-patterns in large quantities and is very suitable for mass production of lithium metal secondary batteries.

The surface pattern structure formed by transfer printing is determined only by the shape of the pattern mold, and thus there are no other unpredictable process variables. The bottom-up method can form a very fine surface pattern structure having a height of 50 to 1000nm, and thus, a small thickness of 40 μm or less is desired in terms of energy density to simultaneously transfer the pattern to a suitable ratio of the lithium metal foil. In addition, the pressing technique can basically be used and implemented by very inexpensive equipment.

Meanwhile, in the battery process, the application of pressure must follow the following time sequence: rolling the electrode (in the present disclosure, making the thickness of the lithium metal foil 20 to 40 μm) before manufacturing the unit cell, a unit cell assembly process (fig. 4 (a) and (b) and fig. 10 (s 2)), a stacked cell assembly process (e.g., fig. 11), and a jig formation (J/F) activation process before releasing the final product.

Contrary to the present disclosure, if a lithium metal foil having a flat smooth surface is used, it is necessary to strongly apply pressure in steps (c) and (c) in order to obtain the performance of the unit cell/the stacked cell. Defects such as cracks, tears, warps, and undulations that may occur in the manufactured electrode or separator (electrolyte layer in the case of an all-solid battery) are not considered. However, according to the present disclosure, an electrode-separator (electrolyte layer) high adhesion interface may be formed by applying a low pressure in steps (c) and (c). Therefore, defects that may occur due to the application of high pressure may be reduced.

The method according to the present disclosure is easier and economically more efficient than techniques aimed at reducing the active material content in the electrode as mentioned in the prior art. In addition, the method according to the present disclosure may additionally have the following advantages.

The pressure application in the steps (r), (c) and (c) cannot be omitted, but only the minimum pressure required for the process (to prevent the electrode separator from separating and bending tolerance) may be applied. This makes it easy to remove the gas generated in the J/F activation step.

Meanwhile, if very high pressure, temperature and rate are applied in the assembly process to ensure the performance of the unit cell, tight adhesion between the electrode and the separator may be formed due to the shape change of the polymer binder (e.g., PVDF), and strong electrode-separator adhesion strength may be obtained. However, this increases the necessary time for electrolyte filling during pre-aging after the assembly process and before the activation process, resulting in increased process costs. According to the present disclosure, the electrode-separator adhesive strength is increased by applying a minimum pressure without excessive pressure, thereby facilitating electrolyte filling in pre-aging after injection.

Although the present disclosure has been described above with respect to a limited number of embodiments and drawings, the present disclosure is not limited thereto, and it will be apparent to those skilled in the art that various modifications and changes may be made within the technical aspects of the present disclosure and the equivalent scope of the appended claims.