SEPARATOR LAMINATE FOR LITHIUM SECONDARY BATTERY, ELECTRODE ASSEMBLY INCLUDING THE SAME, AND LITHIUM SECONDARY BATTERY INCLUDING THE SAME

Abstract

The present disclosure relates to a separator for lithium secondary battery, a method for manufacturing same, and a lithium secondary battery including the same. The separator laminate according to one embodiment of the present disclosure includes: a plurality of separators; and adhesive layers located between mutually adjacent separators among the plurality of separators, wherein the adhesive layers are formed along the edges of the mutually adjacent separators so as to have a separation space between the mutually adjacent separators.

Claims

1. A separator laminate for a lithium secondary battery comprising: a plurality of separators; and adhesive layers disposed between adjacent separators, wherein the adhesive layers are disposed along edges of the adjacent separators so as to have a separation space between the adjacent separators.

2. The separator laminate for a lithium secondary battery according to claim 1, wherein, the adhesive layers are in a form of a frame in which the central portion is opened.

3. The separator laminate for a lithium secondary battery according to claim 2, wherein, border portions of the separators adjacent to the adhesive layer are connected and fixed to each other, by the adhesive layer.

4. The separator laminate for a lithium secondary battery according to claim 1, wherein, each of the plurality of separators comprises one selected from the group consisting of a single-sided SRS separator, a double-sided SRS separator, a polyolefin fabric, a free-standing organic/inorganic mixed film, and a polymer-based separator.

5. The separator laminate for a lithium secondary battery according to claim 4, wherein, at least one of the plurality of separators comprises one selected from the group consisting of a single-sided SRS separator, a double-sided SRS separator, and a free-standing organic/inorganic mixed film.

6. The separator laminate for a lithium secondary battery according to claim 1, wherein, the adhesive layer comprises one selected from the group consisting of polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethylmethacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyimide, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methyl cellulose, acrylonitrile-styrene-butadiene copolymer, polyimide, and polyvinylalcohol, or a mixture of two or more thereof.

7. The separator laminate for a lithium secondary battery according to claim 1, wherein, the adhesive layer has a thickness of 0.01 μm to 100 μm.

8. An electrode assembly comprising: a negative electrode; a positive electrode; and the separator laminate of claim 1 disposed between the negative electrode and the positive electrode.

9. The electrode assembly according to claim 8, wherein, an area of the separator laminate is larger than an area of the negative electrode and an area the positive electrode and thus extended outword at four corners of the negative electrode and the positive electrode, and the adhesive layer of the separator laminate is disposed at a portion not facing the negative electrode or the positive electrode along the edge of the separator included in the separator laminate.

10. The electrode assembly according to claim 8, wherein, the negative electrode is a lithium metal negative electrode comprising: a lithium-free negative electrode, which is a Li free anode, made of only a copper current collector; or a copper current collector; and a lithium metal layer located on the copper current collector.

11. The electrode assembly according to claim 8, wherein the electrode assembly is implemented in a stack process, a lamination and stack process, a stack and folding process, a jelly-roll process, or a zigzag folding process.

12. A lithium secondary battery comprising the electrode assembly of claim 8.

13. The lithium secondary battery comprising the electrode assembly of claim 12, wherein, when charging the lithium secondary battery with a constant current of 0.1 C in a temperature range of 20 to 30° C. until reaching 4.25 V and then discharging the lithium secondary battery with a constant current of 0.5 C until reaching 3.0 V is referred to as one charge/discharge cycle, at the point of the time the capacity retention rate according to the following Equation 1 reaches 80%, n is 30 or more:
Capacity retention rate (%)=100*{Discharge capacity after n cycles}/{Discharge capacity after 1 cycle}.  [Equation 1]

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0142] FIG. 1 schematically shows a possible form of an adhesive layer and an application form thereof according to an embodiment of the present disclosure.

[0143] FIG. 2 is a schematic diagram of a unit electrode assembly according to an embodiment of the present disclosure.

[0144] FIG. 3 is a schematic diagram of a unit electrode assembly according to an embodiment of the present disclosure.

[0145] FIG. 4 is a schematic diagram of a unit electrode assembly according to an embodiment of the present disclosure.

[0146] FIG. 5 is a schematic diagram of a unit electrode assembly according to an embodiment of the present disclosure.

[0147] FIG. 6 is a schematic diagram of a unit electrode assembly according to an embodiment of the present disclosure.

[0148] FIG. 7 is a schematic diagram of a unit electrode assembly according to an embodiment of the present disclosure.

[0149] FIG. 8 is a schematic diagram of a unit electrode assembly according to an embodiment of the present disclosure.

[0150] FIG. 9 is a schematic diagram of a unit electrode assembly according to an embodiment of the present disclosure.

[0151] FIG. 10 is a schematic diagram of a unit electrode assembly according to an embodiment of the present disclosure.

[0152] FIG. 11 is a schematic diagram of a unit electrode assembly according to an embodiment of the present disclosure.

[0153] FIG. 12 shows the results of evaluating the electrochemical characteristics of each of the lithium secondary batteries of Examples 1 to 2 and Comparative Examples 1 to 3 described later.

[0154] FIGS. 13a and 13b show the observation of the separator recovered during driving of the lithium secondary battery of Comparative Example 1 described later.

[0155] FIG. 14 shows the observation of a separator recovered during driving of the lithium secondary battery of Example 1 described later.

[0156] FIG. 15 shows the observation of a separator recovered during driving of a lithium secondary battery of Example 2 described later.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0157] Hereinafter, preferred examples of the present disclosure, comparative examples, and test examples for evaluating them are described. However, the following examples are only preferred examples of the present disclosure, and the present disclosure is not limited to the following examples.

Example 1

[0158] (1) Manufacture of a Separator Laminate Having a Structure of [Single-Sided SRS Separator/Adhesive Layer/Single-Sided SRS Separator]

[0159] Two polyethylene substrates (width*length*thickness: 40 mm*60 mm*5 um, porosity 40%) were prepared, and a coating layer was formed on each surface using a dip coating method.

[0160] Specifically, a coating solution was produced by using Al.sub.2O.sub.3 powder having a D50 particle diameter of 500 nm as the inorganic particle, PVdF as the binder, and NMP (N-methyl-2-pyrrolidone) as the solvent. However, the content of the solid content in the total weight (100% by weight) of the coating solution was set to be 30 wt. %, but the volume ratio of the inorganic particles and the binder was set to be 6:1.

[0161] For each of the polyethylene substrates, only 20% of the total thickness was immersed in the coating solution for 3 minutes, which was then taken out and dried at 80° C. for 60 minutes.

[0162] Thereby, two single-sided SRS separators were obtained.

[0163] One of them was used as the upper separator and the other one was used as the lower separator.

[0164] The upper separator and the lower separator were set so that the coating layer directs the outer surface as shown in FIG. 2, and a 1 μm gap was placed between them.

[0165] An adhesive film (thickness: 1 μm) was inserted as an adhesive layer in the gap, and then laminated for 3 seconds under the condition of a temperature of 90° C. and a pressure of 5.0 MPa using a roll press device.

[0166] Here, the adhesive film was made of PVDF, and the total width*length*thickness was 40 mm*60 mm*3 um, and an opening of width*length*thickness: 35 mm*55 mm*3 um was formed in the center (see FIG. 1).

[0167] Thereby, an integrated separator laminate having a structure of [single-sided SRS separator/adhesive layer/single-sided SRS separator] was finally obtained, which was referred to as the separator laminate of Example 1.

[0168] (2) Manufacture of Lithium Secondary Battery

[0169] 35 integrated separator laminates of Example 1 were used, 15 double-sided positive electrode sheets, and 16 double-sided negative electrode sheets were used, and two single-sided positive electrode sheets were used at the outermost part, 11 bi-cells were stacked, and assembled in a stack & folding process known in the art to thereby implement a lithium secondary battery.

[0170] Here, each double-sided negative electrode sheet was a lithium free negative electrode made of only a 10 um-thick copper foil current collector. In addition, each single-sided positive electrode sheet was one in which LiNi.sub.0.6Co.sub.0.2Mn.sub.0.2 as a positive electrode active material was loaded at 4 mAh/cm.sup.2 on a 12 um-thick Al foil current collector.

[0171] As an electrolyte, a high-concentration ether-based electrolyte (3.5M LiFSI in DME) was used to evaluate a 2 Ah stack cell. 3.5M LiFSI (lithium bis(fluorosulfonyl)imide) was dissolved in 1,2-dimethoxyethane (DME), and then injected into the separator in the assembly.

Example 2

[0172] (1) Manufacture of a Separator Having a Structure of [Single-Sided SRS Separator/Adhesive Layer/Polyethylene Fabric/Adhesive Layer/Single-Sided SRS Separator]

[0173] 3 polyethylene substrates (width*length*thickness 40 mm*60 mm*5 um, porosity 40%) was prepared, and a coating layer was formed on two polyethylene substrates of them in the same manner as in Example 1 to thereby obtain two single-sided SRS separators (upper separator, lower separator) and one polyethylene fabric separator (intermediate separator).

[0174] And, the coating layer of the upper separator and the lower separators were set to direct to the outer surface as shown in FIG. 12, and an intermediate separator was interposed between the upper separator and the lower separator, but the gap between the upper separator and one surface of the intermediate separator opposite thereto was set to 1 μm, and the gap between the other surface of the lower separator and the other surface of the intermediate separator opposite thereto was set to 1 μm.

[0175] Two adhesive films identical to those used in Example 1 were prepared, an adhesive film (thickness 1 μm) was inserted into each gap between the separators, and then laminated in the same manner as in Example 1.

[0176] Thereby, a separator laminate having a structure of [single-sided SRS separator/adhesive layer/polyethylene fabric/adhesive layer/single-sided SRS separator] was finally obtained, which was referred to as the separator laminate of Example 2.

[0177] (2) Manufacture of Lithium Secondary Battery

[0178] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the separator laminate of Example 2 was used instead of the separator laminate of Example 1.

Comparative Example 1

[0179] (1) Manufacture of Double-Sided SRS Separator

[0180] One polyethylene substrate (width*length*thickness: 40 mm*60 mm*5 um, porosity 40%) was prepared and used as a substrate.

[0181] Coating layers were formed on both sides of the substrate. Specifically, in the same coating solution as in Example 1, the entire surface of the substrate was immersed for 3 minutes and then taken out, and dried at 80° C. for 60 minutes.

[0182] Thereby, a double-sided SRS separator was finally obtained, which was referred to as a separator laminate of Comparative Example 1.

[0183] (2) Manufacture of Lithium Secondary Battery

[0184] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the separator laminate of Comparative Example 1 was used instead of the separator laminate of Example 1.

Comparative Example 2

[0185] (1) Manufacture of a Separator Having a Structure of [Single-Sided SRS Separator/Single-Sided SRS Separator]

[0186] A single-sided SRS separator was prepared in the same manner as in Example 1, and then simply laminated without an adhesive film to manufacture a separator having a structure of [single-side SRS separator/single-sided SRS separator].

[0187] (2) Manufacture of Lithium Secondary Battery

[0188] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the separator of Comparative Example 2 was used instead of the separator of Example 1.

Comparative Example 3

[0189] (1) Manufacture of a Separator Having a Structure of [Single-Sided SRS Separator/Polyethylene Fabric/Single-Side SRS Separator]

[0190] A single-sided SRS separator was prepared in the same manner as in Example 1, and then simply laminated without an adhesive film using the polyethylene substrate of Example 2 to manufacture a separator having a structure of [single-sided SRS separator/polyethylene fabric/single-side SRS separator].

[0191] (2) Manufacture of Lithium Secondary Battery

[0192] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the separator of Comparative Example 3 was used instead of the separator of Example 1.

Experimental Example 1 (Evaluation of Electrochemical Properties of Lithium Secondary Battery)

[0193] Each of the separator laminates of Examples 1 to 2 and Comparative Example 1 was assembled in a stack-and-folding process, thereby enabling driving as a lithium secondary battery.

[0194] However, each of the separator laminates of Comparative Examples 2 and 3 could not be implemented as an electrode assembly including lamination such as stack & folding and lamination & folding.

[0195] Specifically, each of the separators of Comparative Examples 2 and 3 included two or more separators, but adjacent separators were independently separated, and a large number of mismatches occurred, and thus could not be implemented as the electrode assembly including lamination.

[0196] Therefore, each of the separators of Comparative Examples 2 and 3 was assembled in the form of a coin cell commonly known in the art, and implemented as a lithium secondary battery.

[0197] The discharge capacity according to the charge/discharge cycle of each lithium battery was normalized to the discharge capacity of the first cycle, and the results are shown in FIG. 2.

[0198] Charge: 0.1 C, CC/CV, 4.25V, 1/20C cut-off

[0199] Discharge: 0.5 C, CC, 3.0 V, cut-off

[0200] Here, in order to increase the reliability of the experimental results, when charge/discharge experiments for each lithium secondary battery were conducted 3 times and micro-short circuit occurred two times or more, it was indicated as “whether or not micro-short circuit occurred: presence” in in Table 1 below. Further, in the case of “the cycle number at the point where the capacity retention rate reached 80% at 0.1 C charge/0.5 C discharge”, the results obtained by the three charge/discharge experiments were arithmetically averaged and recorded in Table 1 below.

[0201] Independently from this, it was charged up to 4.25 V at 0.1 C and then discharged up to 3 V at 0.1 C under constant current (CC) conditions. The results of measuring the discharge capacity are also shown in Table 1 below.

TABLE-US-00001 TABLE 1 Cycle numbers at the point where the capacity Whether stack Cycle number at retention reached type battery whether or not the occurrence 80% during 0.1 C assembly is micro-short point of micro- 0.1 discharge charge/0.5 C possible circuit occurred short circuit capacity (Ah) discharge Example 1 Possible Absence (No micro-short 2.02 46 circuit occurred) Example 2 Possible Absence (No micro-short 2.01 41 circuit occurred) Comparative Possible Presence 14 2.02 (Evaluations is Example 1 impossible) Comparative Impossible Absence (No micro-short 0.05 37 Example 2 (Coin cell circuit occurred) (Coin cell evaluation) evaluation) Comparative Impossible Absence (No micro-short 0.05 38 Example 3 (Coin cell circuit occurred) (Coin cell evaluation) evaluation)

[0202] Referring to Table 1 and FIG. 12, it can be seen that each of the separator laminates of Examples 1 to 2 may be implemented as an electrode structure including lamination such as stack & folding as well as lamination & folding. This is because each of the separator laminates of Examples 1 to 2 includes two or more substrates, but an adhesive film is inserted each between adjacent substrates to form an integrated separator.

[0203] Further, in each of the separator laminates of Examples 1 to 2, the adhesive film is in contact with only the border portions of adjacent separators, thus partially leaving a separation space between adjacent separators. The separation space between adjacent separators induces a change in the growth direction of the metal column, inhibits micro-short circuit and ultimately secure the life of the battery.

[0204] Specifically, in each of the separator laminates of Examples 1 to 2, even if the lower separator is pierced by a metal column grown toward the positive electrode from the surface of the negative electrode, the metal column whose growth direction is changed is made to grow horizontally in the separation space between separators when the growth of the metal column is blocked by the intermediate separator (Example 2) or the upper separator (Examples 1 and 2), thereby inhibiting micro-short circuit.

[0205] On the other hand, the separator of Comparative Example 1 can be implemented as an electrode structure including lamination, such as stack & folding and lamination & folding.

[0206] However, in the separator of Comparative Example 1, an SRS separator in which an adjacent substrate and a coating layer are integrally formed does not have a structure in which a separation space is formed between the two separators, and thus, it is not possible to induce a change in the growth direction of the metal column grown from the negative electrode, and it is not possible to inhibit micro-short circuit of the lithium secondary battery to which this is applied.

[0207] Actually, according to FIG. 13, in the case of Comparative Example 1, it can be seen that after 14 cycles, a phenomenon in which the charge capacity excessively increases occurs. In addition, it can be confirmed that the lithium secondary battery to which the separator of Comparative Example 1 is applied stops driving before reaching 80% of the capacity retention rate due to a micro-short circuit occurring at the initial state of driving.

Experimental Example 2 (Observation of Whether Micro-Short Circuit Occurred)

[0208] The lithium secondary battery of Comparative Example 1 that was driven according to Experimental Example 1 allowed to stop at the 14th cycle and then decomposed to recover the separator. The separator of Comparative Example 1 thus recovered was photographed with a digital camera and a digital microscope (Dino-Lite Digital Microscope), respectively, and each photographed image is shown in FIGS. 13a and 13b.

[0209] Specifically, with respect to the recovered separator of Comparative Example 1, the both surfaces were separated based on the center of thickness, photographed with a digital camera, and shown in FIG. 13a. In the order from left to right of FIG. 13a, of the both sides of the recovered separator, it corresponds to the outside and inside of the surface in contact with the negative electrode, and the inside and outside of the surface in contact with the positive electrode.

[0210] Further, FIG. 13b is a photograph of a metal column embedded in the inside of the surface in contact with the positive electrode among both sides of the recovered separator, taken with a digital microscope.

[0211] Referring to FIGS. 13a and 13b, the double-sided SRS separator of Comparative Example 1 has only one substrate, so it is vulnerable to attack by metal columns grown from the negative electrode, and is a structure in which between the upper coating layer and the substrate, and between the upper coating layer and the substrate, are attached without any gaps, respectively, and thus, it is possible to know that there is no room for the growth direction of the metal column to change.

[0212] The lithium secondary battery of Example 1 that was driven according to Experimental Example 1 allowed to stop at the 46th cycle (when the capacity retention rate reached 80%), and then decomposed to recover the separator. The separator laminate of Example 1 thus recovered was photographed with a digital camera, and the photographed image is shown in FIGS. 14a and 14b.

[0213] Specifically, with respect to the recovered separator laminate of Example 1, the outside and inside of the first separator and the second separator in contact with the positive electrode are shown.

[0214] Referring to FIGS. 14a and 14b, it can be confirmed that in the separator laminate of Example 1, the first separator was pierced, but metal columns were grown horizontally on the surface of the second separator, and the surface in contact with the positive electrode could not be pierced.

[0215] In addition, the lithium secondary battery of Example 2, which was driven according to Experimental Example 1, was stopped in a fully charged state at the 41st cycle (when the capacity retention reached 80%), and then decomposed to recover the separator. The separator laminate of Example 2 thus recovered was photographed with a digital camera, and the photographed image is shown in FIG. 15.

[0216] Specifically, with respect to the recovered separator laminate of Example 2, the both surfaces were separated based on the intermediate separator, then photographed with a digital camera and shown in FIG. 15. In the order from left to right of FIG. 15, in the both sides of the recovered separator, it corresponds to the outside and inside of the surface in contact with the negative electrode, and the inside and outside of the surface in contact with the positive electrode.

[0217] Referring to FIG. 15, in the separator laminate of Example 2, no portion attacked by the metal column grown from the negative electrode was found.

[0218] This is inferred to be because the thickness of the separator is partially thickened by a structure in which three separators are laminated, the thickness of the separator is partially thickened, and thus has excellent mechanical strength, as compared with the case of using one separator of Comparative Example 1.

[0219] Even if the lower separator in contact with the negative electrode in the separator laminate of Example 2 was attacked by a metal column, additional growth of the metal column would have been blocked by the intermediate separator. Further, in the separator laminator of Example 2, an adhesive layer is present between the lower separator and the intermediate separator, and the adhesive layer creates a separation space between the separators, Thus, it is inferred that the metal column, which stopped further growth, does not cause a short circuit because the growth direction changes to the horizontal direction in the separation space.

[0220] Those of ordinary skill in the field to which the present disclosure belongs will be able to perform various applications and modifications within the scope of the present disclosure based on the above disclosure.