KR20220001085A - Secondary battery, a secondary battery module including the same and a device including the same - Google Patents

Abstract

The present invention provides a secondary battery, a battery module including the same, and a device, wherein the secondary battery includes a jelly roll in which a first electrode assembly including at least one first unit cell and a second electrode assembly including at least one second unit cell are stacked, the first electrode assembly is positioned in the outermost layer of the jelly roll, the second electrode assembly is positioned inside the outermost layer, the first unit cell includes a first negative electrode mixture, the second unit cell includes a second negative electrode mixture, the first negative electrode mixture includes a graphite-based negative electrode active material, and the second negative electrode mixture includes a negative electrode active material containing a silicon-based material.

Description

Secondary battery, secondary battery module and device including the same

The present invention relates to a secondary battery, a secondary battery module and a device including the same.

Recently, the demand for high-energy-density cells is increasing to improve the mileage of EVs. In particular, silicon-based materials (a mixture of silicon oxide and carbon) are attracting attention as anode materials for high-energy-density cells because they can produce a high capacity of 30,000 mA/g.

However, the silicon-based material causes excessive volume expansion during charging/discharging, which causes detachment of the outermost anode and distortion of the appearance.

In addition, in the case of an outermost electrode in a cell including a plurality of stacked electrodes, since charging/discharging is performed on only one side of the electrode, the electrode positioned at the outermost portion is increased due to volume expansion during charging/discharging of the silicon-based active material. There is a problem in that the electrode is bent and detached from the electrode.

An object of the present invention is to improve the problem of cell distortion and the detachment of the outermost electrode of the cell caused by the expansion of the silicon-based active material during charging/discharging by using a specific graphite-based active material with controlled volume expansion.

In addition, the cell resistance is prevented from increasing by controlling the binder content while suppressing cell distortion caused by volume expansion of the silicon-based active material.

In addition, by controlling the electrode thickness of the cell and the number of unit cells, it is to suppress detachment of the outermost electrode of the cell and to improve the energy density per unit volume of the cell.

An embodiment of the present invention includes a jelly roll in which a first electrode assembly including at least one first unit cell; and a second electrode assembly including at least one or more second unit cells; The electrode assembly is located in the outermost layer of the jelly roll, the second electrode assembly is located inside the outermost layer, the first unit cell includes a first negative electrode mixture, and the second unit cell contains a second negative electrode mixture. Including, wherein the first negative electrode mixture includes a graphite-based negative active material, and the second negative electrode mixture includes a negative electrode active material containing a silicon-based material, to provide a secondary battery.

The graphite-based negative active material of the first negative electrode mixture contains artificial graphite and natural graphite, and the content of the artificial graphite may be the same as or higher than that of the natural graphite.

The negative active material of the second negative electrode mixture may include 0.1 to 35% by weight of a silicon-based material and 65 to 99.9% by weight of a graphite-based material, based on the total weight.

The first negative electrode mixture may include 2.5 to 6% by weight of the first binder based on the total weight of the solid content.

The first binder may include 1 to 3% by weight of a thickener and 1.5 to 3% by weight of the binder, based on the total weight of the solid content of the first negative electrode mixture.

The jelly roll may be a stack type, a lamination/stack type, or a stack/folding type jelly roll.

In the secondary battery, an increase rate of the thickness after charging (SOC100) with respect to the thickness before charging (shipment state, SOC30) may be 3.5% or less.

The number of the first unit cells may be 3.9 to 10.5% of the total number of the first unit cells and the second unit cells.

Another embodiment provides a battery module including the secondary battery as a unit battery.

Another embodiment provides a device including the battery module as a power source.

It is possible to improve the outermost desorption phenomenon due to volume expansion of the silicon-based active material asymmetrically occurring in the outermost electrode of the cell.

In addition, cell distortion caused by volume expansion of the silicon-based active material can be physically controlled.

1 is a schematic diagram of a jelly roll before/after charging and discharging according to the prior art.
2 is a schematic diagram of a jelly roll before/after charging and discharging according to an embodiment of the present invention.
3 is a photograph evaluating whether or not the outermost electrode is detached when the secondary batteries prepared in Example 1 and Comparative Examples 1 to 3 are fully charged.

Advantages and features of the present invention and methods of achieving them will become apparent with reference to the embodiments described below in detail in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but will be implemented in a variety of different forms, and only these embodiments allow the disclosure of the present invention to be complete, and common knowledge in the technical field to which the present invention belongs It is provided to fully inform the possessor of the scope of the invention, and the present invention is only defined by the scope of the claims. With reference to the accompanying drawings will be described in detail for the implementation of the present invention. Irrespective of the drawings, like reference numbers refer to like elements, and "and/or" includes each and every combination of one or more of the recited items.

Unless otherwise defined, all terms (including technical and scientific terms) used in this specification may be used with meanings commonly understood by those of ordinary skill in the art to which the present invention belongs. In the entire specification, when a part "includes" a certain component, this means that other components may be further included, rather than excluding other components, unless otherwise stated. The singular also includes the plural, unless the phrase specifically states otherwise.

In this specification, when a part of a layer, film, region, plate, etc. is “on” or “on” another part, this includes not only the case where it is “directly on” the other part, but also the case where there is another part in between. do.

In the present specification, a unit cell is manufactured by stacking a positive electrode, a separator, and a negative electrode, an electrode assembly is manufactured by stacking at least one unit cell, and a jelly roll means manufactured by stacking at least one or more electrode assemblies.

An embodiment of the present invention includes a jelly roll in which a first electrode assembly including at least one first unit cell; and a second electrode assembly including at least one or more second unit cells; The electrode assembly is located in the outermost layer of the jelly roll, the second electrode assembly is located inside the outermost layer, the first unit cell includes a first negative electrode mixture, and the second unit cell contains a second negative electrode mixture. and, wherein the first negative electrode mixture includes a graphite-based negative electrode active material, and the second negative electrode mixture includes a negative electrode active material containing a silicon-based material.

Silicon-based materials are used as anode active materials for high-energy-density cells, but excessive volume expansion occurs during charging/discharging, causing detachment of the outermost anode and distortion of appearance. There is a problem that the density is reduced again. In particular, in the case of the outermost electrode assembly of the jelly roll, only one side of the electrode (the electrode facing the inside of the jelly roll) is charged/discharged, so when a silicon-based material with a large volume expansion during charging/discharging is used, the electrode assembly is asymmetrical. It expands and bends in one direction, causing a phenomenon in which the electrode active material is detached. In the present invention, in order to solve this problem, a first unit cell containing a graphite-based active material having low volume expansion is laminated on the outermost layer of the jelly roll, and a silicon-based layer is placed inside the outermost layer (intermediate layer of the jelly roll). By stacking the second unit cell including the anode active material containing the material, it is possible to achieve high energy density while remarkably improving the problem of detachment of the outermost electrode active material and cell distortion.

In addition, even if the second unit cell expands due to volume expansion of the silicon-based material as the first unit cell is stacked on the upper and lower portions of the second unit cell by stacking the first electrode assembly on the outermost layer The volume expansion in the Z-axis (thickness direction) of the silicon-based material is uniformly suppressed by the physical pressure of the first unit cell stacked on the top/bottom of the outermost shell.

In the secondary battery according to an embodiment of the present invention, as described above, the first electrode assembly having relatively little volume expansion is placed in the outermost layer (the upper outermost layer and the lower outermost layer) of the jelly roll, and the volume is expanded By locating this large second electrode assembly inside the jelly roll, the detachment of the outermost electrode can be improved, and the cell energy density can be improved.

The first electrode assembly may include at least one or more first unit cells, and may include 1 to 3, preferably 1 to 2, more preferably 1 first unit cells. In addition, the second electrode assembly may include at least one or more second unit cells, for example, 31 to 50, preferably 33 to 50, more preferably 33 to 48 second unit cells. . In addition, the first unit cell is 3.9 to 10.5%, preferably 4.2 to 10% or 4.5 to 6.5%, more preferably 4.5 to 6.0% or 4.5 to 5.3% with respect to the total number of the first and second unit cells. It can be stacked as appropriate.

When the total number of unit cells is excessively small, the capacity loss due to the use of the entire amount of the graphite-based material as the active material in the first unit cell is greater compared to the thickness reduction effect due to suppression of detachment of the outermost electrode. On the other hand, if the number of unit cells is excessively increased, the cells may become thick and handling may not be easy in the manufacturing process.

On the other hand, the first unit cell and the second unit cell are bi-cells (anode-separator-cathode-separator-positive electrode, cathode-separator-positive electrode-separator- negative electrode), or may be a monocell (anode-separator-cathode). On the other hand, the bicell and monocell are examples, and since a large number of bicells and monocells are possible, the present invention is not limited thereto.

The first unit cell includes a first negative electrode mixture containing a graphite-based negative electrode active material, wherein the graphite-based negative electrode active material contains artificial graphite and natural graphite at the same time, and the content of the artificial graphite is the same as the content of natural graphite or higher.

In general, natural graphite has the characteristic of expanding during charging and discharging compared to artificial graphite and has somewhat lower hardness. It is analyzed that charging/discharging is performed only on the viewing electrode), and the above-described characteristics of natural graphite are further deepened by applying a high-energy-density cell. Accordingly, as the content of artificial graphite in the graphite-based negative active material increases to more than an excessive amount, volume expansion is reduced, cell energy density is increased, and detachment of the outermost electrode can be suppressed.

Meanwhile, the graphite-based negative active material may have a particle size of 8 to 20 μm, and may be amorphous, plate-like, flake, spherical, or fibrous, but the present invention is not limited thereto.

The graphite-based negative active material included in the first negative electrode mixture contains artificial graphite and natural graphite in a weight ratio of 9.5:0.5 to 4:6, preferably 9:1 to 5:5, preferably 9:1 to 6:4 by weight. can be included as

The first negative electrode mixture may further include a first binder, and the first binder includes a thickener and a binder.

The first binder serves to well adhere the negative active material particles to each other and also to adhere the negative active material to the current collector. Water-based binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene , polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof, specifically The binder may include a binder made of carboxyl methyl cellulose (CMC), styrene-butadiene rubber (SBR), and a mixture thereof.

The first binder may be included in an amount of 2.5 to 6 wt%, preferably 2.5 to 5.2 wt%, more preferably 2.5 to 4.5 wt%, based on the total weight of the first negative electrode mixture. Specifically, the first binder includes a thickener and a binder, and 1 to 3% by weight of the thickener and 1.5 to 3% by weight of the binder, preferably 1 to 2.5% by weight of the thickener and 1.5 to 1.5% by weight of the binder, based on the total weight of the first negative electrode mixture solid content 2.7% by weight, more preferably 1-2% by weight of a thickener and 1.5-2.5% by weight of a binder.

In the jelly roll according to an embodiment of the present invention, the second unit cell includes a second negative electrode mixture including a silicon-based negative active material, 0.1 to 35% by weight of the silicon-based negative active material based on the total weight of the negative active material, preferably is 1.0 to 30% by weight, more preferably 3.0 to 25% by weight, and a negative electrode active material other than a silicon-based material may be used with the balance being mixed.

As the silicon-based material, Si, SiO _x (0<x<2), Si-Q alloy (wherein Q is alkali metal, alkaline earth metal, group 13 element, group 14 element, group 15 element, group 16 element, transition metal , an element selected from the group consisting of rare earth elements and combinations thereof, and not Si), a Si-carbon composite, or at least one of these and SiO ₂ may be mixed and used. Preferably it may be Si or SiO _x (0<x<2), more preferably SiO _x (0<x<2).

The negative active material other than the silicon-based material may be used without limitation as long as it is a generally used negative active material, preferably natural graphite, artificial graphite, or a graphite-based material that is a combination thereof.

In addition, as a non-limiting example, the second negative electrode mixture may include 0.1 to 35% by weight of a silicon-based material and 65 to 99.9% by weight of a graphite-based material based on the total weight of the negative active material. Specifically, 5 to 35% by weight of the silicon-based material and 65 to 95% by weight of the graphite-based material, preferably 10 to 35% by weight of the silicon-based material and 65 to 90% by weight of the graphite-based material. Accordingly, it is possible to implement a high energy density cell.

In one embodiment, the second negative electrode mixture may further include a second binder, and the second binder includes a thickener and a binder. The second binder may use the same material as the first binder. The second binder may be included in an amount of 2.5 to 8% by weight, preferably 2.5 to 6% by weight, and more preferably 2.5 to 4.5% by weight based on the total weight of the second negative electrode mixture. Specifically, the second binder includes a thickener and a binder, and 1 to 3% by weight of the thickener and 1.5 to 3% by weight of the binder, preferably 1 to 2.5% by weight of the thickener and 1.5 to 1.5% by weight of the binder, based on the total weight of the second negative electrode mixture solid content 2.7 wt %, more preferably 1.1 to 1.8 wt % of a thickener and 2.0 to 2.5 wt % of a binder.

Meanwhile, in one embodiment, the first negative electrode mixture and the second negative electrode mixture may satisfy Relational Equation 1 below.

[Relational Expression 1]

0.6 < B ₁ /B ₂ < 1.0

In Relation 1, B ₁ and B _{2 are the weight ratio of the binder excluding the thickener in the first binder (B 1} ) and the binder excluding the thickener in the second binder with respect to the total solids of the first negative electrode mixture and the second negative electrode mixture, respectively. is the weight ratio of (B ₂ ).

In Relation 1, 0.65 < B ₁ /B ₂ < 1, preferably 0.75 < B ₁ /B ₂ < 0.95, more preferably 0.8 < B ₁ /B ₂ < 0.9.

Since the second unit cell uses a silicon-based negative active material having a large volume expansion, a binder of a certain amount or more is required. Unlike the outermost electrode (the first unit cell), the second unit cell is symmetrically charged on both sides, so there is practically no energy density loss due to electrode detachment, but lifespan may be reduced due to internal delamination during operation because it can On the other hand, since the first unit cell uses a graphite-based active material with a low expansion rate, and among them, 50% by weight or more of artificial graphite is used. Thus, it is possible to improve resistance and energy density.

On the other hand, as the amount of the binder is small, the capacity can be increased by relatively increasing the content of the active material, and there is an effect of improving the output according to the decrease of the resistance. However, it is generally well known that a certain amount of binder is required to increase the adhesiveness of the active material, the conductive material, and the current collector. However, in the present invention, by stacking the silicon-based active material-containing second unit cell and the graphite-based active material-containing first unit cell at a specific position of the jelly roll, when the content of the binder contained in each unit cell is adjusted within the range of Relation 1 It was confirmed that optimal battery performance could be realized. Specifically, by satisfying the above content range, it is possible to prevent the problem of output and capacity loss occurring due to too many binders in any unit cell, and also the binding between the active material, the conductive material, and the current collector because the binder in any unit cell is too small. This weakening can improve the problem of increased resistance or deteriorated lifespan.

The first negative electrode mixture and the second negative electrode mixture may each further include a conductive material, and may be 0.1 to 5% by weight, preferably 0.1 to 3% by weight, based on the total weight of each negative electrode mixture. The conductive material is used to impart conductivity to the electrode, and any electronically conductive material may be used without causing a chemical change in the configured battery. Examples of the conductive material include carbon-based materials such as graphite, carbon black, acetylene black, ketjen black, and carbon fiber; Metal-based substances, such as metal powders, such as copper, nickel, aluminum, and silver, or a metal fiber; conductive polymers such as polyphenylene derivatives; Alternatively, a conductive material including a mixture thereof may be used.

Meanwhile, the positive electrode included in the first unit cell and the second unit cell is preferably a composite oxide of lithium and a metal selected from cobalt, manganese, nickel, and combinations thereof, but the present invention is not limited thereto. .

In addition, the separator may be, for example, selected from glass fiber, polyester, polyethylene, polypropylene, polytetrafluoroethylene, or a combination thereof, and may be in the form of a nonwoven fabric or a woven fabric. For example, a polyolefin-based polymer separator such as polyethylene or polypropylene may be mainly used for a lithium secondary battery, and a separator coated with a composition containing a ceramic component or a polymer material may be used to secure heat resistance or mechanical strength, optionally It can be used in a single-layer or multi-layer structure, and if a separator known in the art is used, it may be used, but the present invention is not limited thereto.

The jelly roll according to an embodiment of the present invention may be prepared by stacking a plurality of electrode assemblies and inserting them into a case, and then injecting an electrolyte.

The plurality of electrode assemblies include the first electrode assembly and the second electrode assembly, which are as described above.

The case may be formed by laminating an insulating layer, an adhesive layer, and a metal thin film, and the case may be adopted that is commonly used in the field, and there is no limitation in the appearance according to the use of the battery, for example, using a can Cylindrical, prismatic, pouch (pouch) type or coin (coin) type, etc., preferably, may be a pouch type. As a non-limiting example, the metal thin film may include aluminum (Al) and the like, thereby securing mechanical rigidity of the case and blocking moisture and oxygen from the outside.

In addition, the electrolyte may be injected and accommodated in the case, and the electrolyte may impregnate the first and second electrode assemblies.

The electrolyte includes an organic solvent and a lithium salt. The organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move, and for example, a carbonate-based, ester-based, ether-based, ketone-based, alcohol-based or aprotic solvent may be used. The organic solvents may be used alone or in mixture of two or more, and when two or more kinds are mixed and used, the mixing ratio may be appropriately adjusted according to the desired battery performance. On the other hand, if an organic solvent known in the art is used, it may be used, but the present invention is not limited thereto. In addition, the lithium salt is a material that dissolves in an organic solvent, acts as a source of lithium ions in the battery, enables the basic operation of a lithium secondary battery, and promotes movement of lithium ions between the positive electrode and the negative electrode. As the lithium salt, a known material may be used at a concentration suitable for the purpose. On the other hand, the electrolyte may further include a known solvent to improve charge/discharge characteristics, flame retardancy characteristics, etc., if necessary, and may include a known additive.

The jelly roll may be a stack type, a lamination/stack type, or a stack/folding type jelly roll, but the present invention is not limited thereto.

The secondary battery according to an embodiment of the present invention has improved cell distortion and the problem of cell distortion caused by the expansion of the silicon-based active material during charging/discharging. The increase rate of the thickness after charging (SOC100) may be 3.5% or less, preferably 3.4% or less. In this case, the charging condition may be, as a non-limiting example, 0.3C, 4.2V constant current charging, and 0.05C cutoff constant voltage charging condition.

In addition, the secondary battery may have a cell energy density of 700 Wh/L or more, preferably 710 Wh/L or more, and more preferably 720 Wh/L or more. Accordingly, the present invention can provide a secondary battery having a high energy density, and by including all the configurations of the present invention, it is possible to effectively suppress detachment of the outermost electrode despite achieving a high energy density.

Another embodiment provides a battery module including the secondary battery as a unit battery.

Another embodiment provides a device or mobility including the battery module as a power source.

The secondary battery according to the present invention may be used not only in a battery cell used as a power source for a small device, but may also be preferably used as a unit cell in a medium or large-sized battery module including a plurality of battery cells. Preferred examples of the mid-to-large device include, but are not limited to, an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, and a power storage system.

Hereinafter, preferred examples and comparative examples of the present invention will be described. However, the following examples are only preferred examples of the present invention, and the present invention is not limited thereto.

Example

(Example 1)

<Unit cell manufacturing>

The graphite-based active material (natural graphite: artificial graphite = 3:7 weight ratio), the first binder (CMC thickener: SBR binder = 1.2: 1.5 weight ratio) and the conductive material (carbon black) were mixed in a weight ratio of 94.3: 2.7: 3 A first negative electrode was prepared using the negative electrode mixture.

Anode active material (SiOx (0<x<2): artificial graphite = 11:89 weight ratio), second binder (CMC thickener: SBR binder = 1.5: 2.0 weight ratio) and conductive material (CNT) in a weight ratio of 96.4: 3.5: 0.1 A second negative electrode was prepared using the negative electrode mixture mixed with

For the positive electrode and the separator, a positive electrode and PE separator prepared by mixing NCM811 positive electrode active material, binder (PVdF), and conductive material (CNT) in a weight ratio of 98.1:1.3:0.6 by weight were used.

A first unit cell was prepared by stacking the prepared first negative electrode, a separator, and a positive electrode, and a second unit cell was prepared using a second negative electrode instead of the first negative electrode. In this case, the thickness of the prepared first unit cell was 0.163 mm, and the thickness of the second unit cell was 0.134 mm.

<Production of Jelly Roll>

Using the prepared first electrode assembly composed of one first unit cell and the second electrode assembly composed of 40 second unit cells, the outermost layers of the jelly roll (top and bottom outermost layers) as shown in the jelly roll schematic diagram of FIG. ), placing a total of two, one first electrode assembly, respectively, and stacking the second electrode assembly on the inner side of the outermost layer.

<Secondary battery manufacturing>

A secondary battery was manufactured by inserting the prepared jelly roll into a battery case (pouch type), injecting electrolyte, and then welding. At this time, as the electrolyte, lithium salt 1M LiPF ₆ was mixed with an organic solvent (EC:EMC:DEC=25:45:30 by volume), and 5% by volume of the electrolyte additive FEC was used.

evaluation example

[Evaluation Example 1] : Evaluation of cell thickness change, cell energy density per volume and detachment of the outermost electrode

(Comparative Example 1)

A jelly roll and a secondary battery were manufactured in the same manner as in Example 1, except that a second electrode assembly composed of one second unit cell was laminated on the upper and lower portions of the outermost layer of the jelly roll. .

(Comparative Example 2)

Using the prepared first electrode assembly consisting of one first unit cell and the second electrode assembly consisting of one second unit cell, the top and bottom of the outermost layer of the jelly roll are respectively laminated with the first electrode assembly, and the inner The second electrode assembly and the first electrode assembly were alternately stacked. Except that, in the same manner as in Example 1, jelly rolls and secondary batteries were manufactured.

(Comparative Example 3)

A jelly roll and a secondary battery were manufactured in the same manner as in Example 1, except that a first electrode assembly composed of 40 first unit cells was stacked instead of a second electrode assembly stacked on the inside of the outermost layer of the jelly roll. did

(Assessment Methods)

* Cell thickness and energy density measurement

Constant current charging was performed with a current of 0.3C rate until the voltage reached 4.2V, and then, while maintaining 4.2V in the constant voltage mode, cut-off was performed at the current of 0.05C rate to perform constant voltage charging. Thereafter, the discharge capacity (Ah) and energy (Wh) are measured by discharging at a constant current of 0.3C rate until the voltage reaches 2.5V, and the volume-energy density is calculated by measuring the volume of each battery in the 4.2V state of charge. Calculated. The results are shown in Table 1 below.

At this time, the thickness of the cell without charging (shipment state, SOC30) and the thickness of the fully charged cell (SOC100) were measured, and the results are shown in Table 1 below.

*Evaluation of detachment of the outermost electrode

A constant current was applied to the secondary battery until the battery voltage reached 4.2V at a current of 0.3C rate in the charger and discharger in the chamber where the temperature was maintained at 25℃. When the battery voltage reached 4.2V, the current reached the 0.05C rate. A constant voltage was applied until charging was performed.

The thickness of the secondary battery before and after charging was measured, and the electrode (outermost electrode) of the first unit cell charged by disassembling the charged cell was left at room temperature (25° C.) for 10 minutes without a separate washing process. Thus, the state of the adhesive surface with the substrate surface active material layer was visually confirmed, and the results are summarized in Table 1 and FIG. 3 below.

Cell Energy Density (Wh/L) Cell thickness (mm) number of stacks
(positive/negative) Detachment of the outermost electrode before charging after full charge Increase (%) Comparative Example 1 719.0 (-9.2) 13.96 14.52 4 41/42 O Example 1 728.1 13.78 14.24 3.3 41/42 X Comparative Example 2 698.8 (-29.3) 14.30 14.79 3.4 41/42 X Comparative Example 3 671.4 (-56.7) 14.83 15.33 3.4 41/42 X

Referring to Table 1 and Figure 3, Comparative Example 1 is a secondary battery comprising only the second unit cell containing the silicon-based active material in the jelly roll, compared to Example 1, the second unit cell in the outermost layer of the jelly roll It was confirmed that the outermost electrode was detached as the layers were stacked. In addition, Comparative Example 1 showed a result in which the energy density was decreased (-9.2 Wh/L) than that of Example 1 because the cell thickness was greatly increased despite the content of the silicon-based material the most.

The outermost layer of the jelly roll in Comparative Example 2 is the same as in Example 1 in that the first unit cells are laminated, but there is a difference in that the first unit cells and the second unit cells are alternately laminated inside the jelly roll. Accordingly, the cell thickness expansion and the effect of inhibiting detachment of the outermost electrode were the same as in Example 1, but the cell energy density was significantly reduced.

In Comparative Example 3, contrary to Comparative Example 1, the jellyroll was composed of only the first unit cell, and there was no problem of detachment of the outermost electrode because a silicon-based active material was not used, but the cell energy density was significantly reduced.

[Evaluation Example 2] : Evaluation of interfacial adhesion between the negative electrode active material layer and the current collector and detachment of the outermost electrode according to the change in the binder content

(Examples 2-4)

When preparing the first negative electrode, the graphite-based active material, the first binder, and the conductive material were prepared in a weight ratio of 93.5 to 94.8: 3.5 to 2.2: 3 (100% by weight in total), and the contents of the thickener and the binder in the first binder are shown in Table 2 A unit cell, a jelly roll, and a secondary battery were prepared in the same manner as in Example 1, except that it was prepared as described above.

(Assessment Methods)

*Evaluation of interfacial adhesion between the negative electrode active material layer and the current collector

The first negative electrode prepared in Examples 1-4 was cut to 18 mm wide / 150 mm long, and after attaching an 18 mm wide tape to the foil layer of the negative electrode, it was allowed to be sufficiently adhered with a roller having a load of 2 kg. The active material layer of the negative electrode was attached to one side of the tensile tester using double-sided tape. The tape attached to the foil was fastened to the opposite side of the tensile tester, and the adhesive force was measured, and the results are summarized in Table 2 below.

*Evaluation of detachment of the outermost electrode

It proceeded in the same manner as in Evaluation Example 1, and the results are shown in Table 2 below.

Binder content (CMC:SBR weight ratio) Adhesion (N) Detachment of the outermost electrode Example 2 3.5 (1.5:2.0) 0.538 X Example 3 3.2 (1.2:2.0) 0.533 X Example 1 2.7 (1.2:1.5) 0.415 X Example 4 2.2 (1.2:1.0) 0.388 O

(In Table 2, the binder content is a weight % with respect to the total weight of the solid content of the negative electrode mixture used for manufacturing the first negative electrode of the first unit cell)

Referring to Table 2, when the content of the first binder in the first unit cell negative electrode mixture was 2.2% by weight or less, the interfacial adhesion of the outermost electrode was reduced, and thus the outermost electrode was detached. Specifically, there was little difference in adhesion according to the content of CMC used as a thickener (CMC also acts as a binder but plays a large role as a thickener), but there was a large difference in adhesion according to the content of SBR used as a binder. In addition, it was found that the outermost electrode detachment occurred when the binder (SBR) content was 1 wt% or less.

[Evaluation Example 3] : Evaluation of cell thickness change, cell energy density per volume, and outermost electrode detachment according to the change of the graphite-based active material of the first unit cell

(Examples 5-7)

A unit cell, a jelly roll, and a secondary battery were prepared in the same manner as in Example 1, except that the natural graphite:artificial graphite weight ratio of the graphite-based active material was prepared as shown in Table 3 during the preparation of the first negative electrode.

(Assessment Methods)

Cell thickness, energy density (Energy density) measurement, and outermost electrode detachment evaluation was performed in the same manner as in Evaluation Example 1, and the results are shown in Table 3 below.

Artificial: natural graphite
(weight ratio) Cell Energy Density (Wh/L) Cell thickness (mm) Detachment of the outermost electrode before charging after full charge Increase (%) Example 5 9:1 730.0 13.74 14.20 3.3 X Example 1 7:3 728.1 13.78 14.24 3.3 X Example 6 5:5 726.0 13.81 14.28 3.4 X Example 7 3:7 709.9 14.13 14.95 5.8 O

Referring to Table 3, when the content of artificial graphite in the negative electrode active material in the first unit cell is 50% by weight or more, which is the preferred range of the present invention, the detachment of the outermost electrode can be more effectively suppressed, and the content of artificial graphite can be increased. The cell energy density may also be slightly improved.

In general, artificial graphite has lower adhesive strength than natural graphite (because artificial graphite has a coarse particle surface and the binder flows between the particle surfaces to reduce the effective amount of binder), in the present invention, binder (SBR) 1 weight It can be seen that the SBR is sufficient to prevent detachment of the outermost electrode at a content of more than %, preferably 1.5% by weight or more. In addition, natural graphite has good adhesion compared to artificial graphite, but has a high expansion rate, and when realizing a high energy density cell, the expansion characteristic of natural graphite exerts a higher influence than the adhesive property, affecting cell adhesion and detachment of the outermost electrode. suggest that you are crazy Therefore, it was found that the outermost unit cell of the jelly roll contains artificial graphite among the graphite-based active materials in an amount equal to or greater than that of natural graphite, in terms of suppressing detachment of the outermost electrode of the high energy cell.

[Evaluation Example 4] : Evaluation of cell energy density and detachment of the outermost electrode according to the change in the total number of unit cells

(Examples 8-9)

A jelly roll and a secondary battery were manufactured in the same manner as in Example 1, except that the number of the first unit cell and the second unit cell was prepared as shown in Table 4 below.

(Comparative Examples 4 to 5)

A jelly roll and a secondary battery were manufactured in the same manner as in Comparative Example 1, except that the number of the first unit cell and the second unit cell was prepared as shown in Table 4 below.

(Assessment Methods)

Cell thickness, cell energy density and outermost electrode detachment were evaluated in the same manner as in Evaluation Example 1, and the results are shown in Table 4 below.

number of unit cells cell thickness
(before filling, mm) Cell Energy Density (Wh/L) Detachment of the outermost electrode first unit cell
(Count/ %) second unit cell total Example 1 2 pieces / 4.8% 40 pieces 42 / 100% 13.78 729.2 X Example 8 2 pieces / 5.6% 34 36 / 100% 11.81 715.5 X Example 9 2 pieces / 6.3% 30 pieces 32 / 100% 10.51 662.0 X Comparative Example 1 - 42 pieces 42 / 100% 13.96 719.0 O Comparative Example 4 - 36 pieces 36 / 100% 11.98 715.1 O Comparative Example 5 - 32 pieces 32 / 100% 10.66 711.0 O

Referring to Table 4, Comparative Examples are secondary batteries composed only of second unit cells using a silicon-based active material, and when a high energy density cell is manufactured by changing the number of unit cells in the range of 32 to 40, detachment of the outermost electrode The phenomenon could not be suppressed.

On the other hand, in Examples 1 and 8, it was confirmed that the cell energy density was equal to or higher than that of Comparative Example, but the detachment of the outermost electrode did not occur.

On the other hand, in the case of Example 9, when the total number of unit cells is small, it can be seen that the capacity loss caused by using the graphite-based material instead of the silicon-based electrode as the outermost electrode is larger than the thickness reduction effect due to suppression of desorption. Therefore, when the graphite-based first unit cell is placed on the outermost electrode layer (outermost upper and lower total two) and the remaining silicon-based second unit cells are stacked inside, the total number of unit cells is 36 or more. It is preferable because energy density cells can be manufactured.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, but may be manufactured in a variety of different forms, and those of ordinary skill in the art to which the present invention pertains will appreciate the technical spirit of the present invention. However, it will be understood that the invention may be embodied in other specific forms without changing essential features. Therefore, it should be understood that the embodiments described above are illustrative in all respects and not restrictive.

Claims (10)

A first electrode assembly including at least one first unit cell; and a second electrode assembly including at least one or more second unit cells;
The first electrode assembly is located on the outermost layer of the jelly roll and the second electrode assembly is located on the inside of the outermost layer,
The first unit cell includes a first negative electrode mixture and the second unit cell includes a second negative electrode mixture,
The first negative electrode mixture includes a graphite-based negative active material and the second negative electrode mixture includes a negative electrode active material containing a silicon-based material, a secondary battery. The secondary battery of claim 1, wherein the graphite-based negative active material of the first negative electrode mixture contains artificial graphite and natural graphite, and the content of the artificial graphite is equal to or higher than that of the natural graphite. The secondary battery of claim 1 , wherein the negative active material of the second negative electrode mixture comprises 0.1 to 35% by weight of a silicon-based material and 65 to 99.9% by weight of a graphite-based material, based on a total weight. The secondary battery of claim 1 , wherein the first negative electrode mixture comprises 2.5 to 6% by weight of the first binder based on the total weight of the solid content. The secondary battery of claim 1 , wherein the first binder comprises 1 to 3% by weight of a thickener and 1.5 to 3% by weight of the binder, based on the total weight of the solid content of the first negative electrode mixture. The secondary battery of claim 1, wherein the jelly roll is a stack type, lamination/stack type, or stack/folding type jelly roll. The secondary battery according to claim 1, wherein an increase rate of the thickness after charging (SOC100) with respect to the thickness before charging (shipment state, SOC30) is 3.5% or less. The secondary battery of claim 1, wherein the number of the first unit cells is 3.9 to 10.5% of the total number of the first unit cells and the second unit cells. A battery module comprising the secondary battery of any one of claims 1 to 8 as a unit battery. A device comprising the battery module of claim 9 as a power source.

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