Negative electrode for lithium ion secondary battery and secondary battery comprising same

Abstract

The present invention relates to a negative electrode for a lithium ion secondary battery and a secondary battery comprising the same, the negative electrode comprising: a current collector: a negative electrode mixture layer formed on at least one surface of the current collector, in which the negative electrode mixture layer is divided into a plurality of regions on a plane, the regions include a first region composed of a first negative electrode mixture and a second region composed of a second negative electrode mixture, and the thickness of the negative electrode mixture layer of the first region is thicker than the thickness of the negative electrode mixture layer of the second region based on discharge.

Description

Technical Field

[0001] The present invention relates to a negative electrode for a lithium ion secondary battery and a secondary battery including the negative electrode. Background Art

[0002] In recent years, with the increasing attention to environmental problems, the exhaust gas emitted by vehicles using fossil fuels such as gasoline or diesel has been designated as one of the main causes of air pollution. As a means to replace such vehicles, extensive research is being conducted on electric vehicles (EVs), hybrid electric vehicles (HEVs), etc.

[0003] Meanwhile, as a power source for such electric vehicles (EVs), hybrid electric vehicles (HEVs), etc., lithium secondary batteries having high discharge voltage and power stability are mainly used. In addition, with the increasing demand for high-energy secondary batteries having high energy density, active development and research are being conducted on high-capacity negative electrodes for such secondary batteries.

[0004] In recent years, in order to realize a secondary battery having high capacity and high energy density, a negative electrode mixture composition in which a silicon-based active material having a high capacity per unit weight is mixed with graphite, which is usually used as a negative electrode active material, has been developed.

[0005] The silicon-based active material as described above is effective in improving the energy density due to its large capacity, but has a disadvantage in that it has a larger volume expansion ratio than graphite during charge and discharge. Therefore, although the energy density can be significantly increased as the content of the silicon-based active material increases, the thickness of the battery increases during charging of the battery, and thus various problems caused by the application of the silicon-based active material occur, for example, an increase in the surface pressure of the battery module and the battery pack.

[0006] Figure 1 Conceptually shows the thickness expansion of the negative electrode according to the charging of the battery as described above. As Figure 1 shown in (a) of, in the negative electrode 10 containing a silicon-based active material as an active material, the negative electrode mixture layer 3 having a predetermined thickness expands in volume according to the charging process of the battery, as Figure 1 shown in (b) of, thereby increasing the thickness of the negative electrode mixture layer 4.

[0007] Therefore, there is a strong need for a technology that can control the problems caused by volume expansion during charge and discharge when applying a high-capacity silicon-based active material to improve the energy density of a lithium ion secondary battery. Summary of the Invention

[0008] (I) Technical Problem to be Solved

[0009] An object of the present invention is to provide a negative electrode for a lithium secondary battery, which uses a silicon-based active material as a negative electrode active material in the negative electrode of a lithium ion secondary battery to improve the energy density while solving the problems caused by the volume expansion of the negative electrode during charge and discharge due to the use of the silicon-based active material, and a secondary battery including the negative electrode.

[0010] (II) Technical solution

[0011] An object of the present invention is to provide a negative electrode for a lithium secondary battery, the negative electrode including: a current collector; a negative electrode mixture layer formed on at least one surface of the current collector, wherein the negative electrode mixture layer is divided into a plurality of regions in a plane, the regions including a first region having a first negative electrode mixture composition and a second region having a second negative electrode mixture composition different from the first negative electrode mixture composition, and the thicknesses of the negative electrode mixture layers of the first region and the second region based on discharge are different from each other.

[0012] The second region may be n (n is a natural number greater than 1), and the first region may be n + 1.

[0013] The first region may be two or more, and the second region may be located between adjacent first regions.

[0014] The regions may be three or more and an odd number, and the first region may be located in the end regions of the regions.

[0015] In the first region and the second region, the thickness of the negative electrode mixture layer of the second region may be thicker than the thickness of the negative electrode mixture layer of the first region.

[0016] The negative electrode mixture layer of the first region may contain a silicon-based active material, and the negative electrode mixture layer of the second region may not contain a silicon-based active material or the content of the silicon-based active material contained therein is less than the content of the silicon-based active material contained in the negative electrode mixture layer of the first region.

[0017] The silicon-based active material may be at least one selected from SiO x (0 ≤ x < 2), Si-C composite, and Si-Y alloy (where Y is an element selected from alkali metals, alkaline earth metals, transition metals, Group 13 elements, Group 14 elements, rare earth elements, and combinations thereof).

[0018] The content of the silicon-based active material in the negative electrode mixture layer of the first region may be 1-30% by weight, and the content of the silicon-based active material in the negative electrode mixture layer of the second region may be 10% by weight or less.

[0019] The negative electrode may contain a carbon-based active material.

[0020] For at least one of the negative electrodes, the thickness difference between the first region and the second region based on full charge may be within 10% of the thickness of the first region.

[0021] Another aspect of the present invention provides a secondary battery including any one of the above negative electrodes.

[0022] The thickness difference between the first region and the second region of the negative electrode based on full charge may be within 10% of the thickness of the first region.

[0023] (III) Beneficial Effects

[0024] According to a specific embodiment of the present invention, by including a silicon-based active material, the energy density can be increased, and at the same time, the problem of the thickness increase of the negative electrode caused by the volume expansion of the silicon-based active material during charging and the resulting increase in surface pressure can be controlled.

[0025] In addition, according to a specific embodiment of the present invention, due to the voids caused by the electrode step difference, the migration channel of the electrolyte is ensured, the electrode wettability is improved, and the migration of lithium ions can be increased. Therefore, a further increase in power can be expected. Description of the Drawings

[0026] Figure 1 is a cross-sectional view showing a negative electrode including a silicon-based active material, schematically showing the concept that the charging of the battery causes the silicon-based active material to expand and the thickness of the negative electrode to increase.

[0027] Figure 2 is a cross-sectional view schematically showing a negative electrode according to an example of the present invention, schematically showing a negative electrode in which a negative electrode mixture layer having a step difference with different thicknesses between regions including two or more regions is formed on a current collector.

[0028] Figure 3 Conceptually shows Figure 2 the negative electrode in which the thickness of the electrode mixture layer in a partial region increases due to the expansion of the silicon-based active material during charging in the negative electrode shown in

[0029] Figure 4 Schematically shows Figure 2 a plan view of the negative electrode according to the present invention shown in

[0030] Figure 5FIG. is a cross-sectional view schematically showing a negative electrode according to another example of the present invention, schematically showing a negative electrode in which the thickness of a negative electrode mixture layer is different between two or more regions, resulting in a step difference, and the width of a first region is greater than the width of a second region. BEST MODE

[0031] An object of the present invention is to provide a negative electrode that includes a silicon-based active material as a negative electrode active material to achieve a high energy density and can solve the problem caused by an increase in the thickness of the battery due to the volume expansion of the silicon-based negative electrode active material during the charging process of the battery.

[0032] Hereinafter, the negative electrode according to the present invention will be specifically described with reference to the drawings.

[0033] Figure 2 Schematically showing the negative electrode provided in the present invention, as Figure 2 shown, the negative electrode 10 according to the present invention includes a negative electrode mixture layer on one or both sides of the negative electrode current collector 2. Figure 2 An example of forming a negative electrode mixture layer only on one side of the negative electrode current collector 2 is shown, but it is not limited thereto, and a negative electrode mixture layer may be formed on both sides of the negative electrode current collector 2.

[0034] The negative electrode mixture layer may be formed to have a plurality of regions on a plane. That is, the negative electrode mixture layer may be divided into a plurality of regions along one direction on the plane of the negative electrode current collector 2. For example, as Figure 2 shown, the negative electrode mixture layer may be divided into a plurality of regions from one end of the negative electrode current collector 2 toward the other end facing each other, and the first region 5 and the second region 7 may be alternately formed.

[0035] At this time, the first region 5 and the second region 7 may form a negative electrode mixture layer of a negative electrode mixture composition having different compositions according to each region. Specifically, the negative electrode mixture layer formed in the first region 5 and the negative electrode mixture layer formed in the second region 7 may be mixture layers having different volume expansion characteristics during charging of the battery. For example, the negative electrode mixture layer in the first region 5 may have a relatively large volume expansion characteristic during charging, and the negative electrode mixture layer in the second region 7 may have a relatively small volume expansion characteristic during charging.

[0036] Furthermore, the negative electrode mixture layer in the first region 5 and the negative electrode mixture layer in the second region 7 may have different thicknesses in consideration of the volume expansion characteristics of each negative electrode mixture layer. That is, the negative electrode mixture layer in the first region 5 with a larger volume expansion characteristic may be formed with a thinner thickness (d1), and the negative electrode mixture layer in the second region 7 with a smaller volume expansion characteristic may be formed with a thickness (d2) thicker than the thickness (d1) of the negative electrode mixture layer in the first region 5. More specifically, the thickness (d2) of the negative electrode mixture layer in the second region 7 may be set in consideration of the degree of increase in thickness (d1') of the negative electrode mixture layer in the first region 5 due to volume expansion during battery charging.

[0037] As described above, by changing the thicknesses of the negative electrode mixture layer in the first region 5 and the negative electrode mixture layer in the second region 7, and forming the negative electrode mixture layer in the second region 7 with the maximum thickness (d1') that the negative electrode mixture layer in the first region 5 has during charging, therefore, as Figure 3 shown, even if the thickness (d1') of the negative electrode mixture layer in the first region 5 increases due to volume expansion during the charging process, the thickness (d2) of the negative electrode mixture layer in the second region 7 can be maintained, so that the thickness change of the overall negative electrode 10 can be minimized. Finally, problems caused by an increase in the surface pressure applied to the battery case, module case, and battery pack can be prevented.

[0038] As described above, when the surface pressure increases, the module and / or battery pack may expand. If the above-mentioned module and battery pack cannot withstand the expansion, the structure of the module and / or battery pack will eventually collapse, which may damage the sealing performance. Further, in the case of the battery cell unit, if there is no pressure provided by the module and battery pack, the adhesion of the electrodes in the battery may decrease, which may lead to a reduction in performance, such as an increase in resistance, a decrease in capacity performance, and a decrease in lifespan.

[0039] Therefore, a silicon-based active material for improving the energy density may be included in the negative electrode mixture layer in the first region 5, and the silicon-based negative electrode active material may not be included in the negative electrode mixture layer in the second region 7.

[0040] As Figure 2 shown, the first region 5 and the second region 7 may be alternately formed. In addition, the negative electrode mixture layer in the second region 7 with a smaller thickness change may be provided at both side edge portions of the negative electrode current collector 2. As described above, by providing the negative electrode mixture layer in the second region 7 at the edge portion, that is, the end region, even if the negative electrode mixture layer in the first region 5 expands in volume, expansion in the thickness direction can be induced, and volume expansion in the width direction can be suppressed.

[0041] More specifically, the sum of the first region and the second region, that is, the total number of regions of the negative electrode mixture layer can be 3 or more and an odd number. Further, when the number of negative electrode mixture layers in the first region 5 is n (n is a natural number), the number of negative electrode mixture layers in the second region 7 can be n + 1. Therefore, the first region 5 can be provided between adjacent second regions 7.

[0042] The widths of the negative electrode mixture layers in the respective regions of the negative electrode according to the present invention may be the same or different. For example, as Figure 4 shown, the width (w1) of the first region and the width (w2) of the second region may be the same. As Figure 5 shown, the width (w1) of the first region may be less than the width (w2) of the second region.

[0043] If necessary, a negative electrode mixture layer of a third region may be further included. The negative electrode mixture layer of the third region may contain a silicon-based negative electrode active material, and the content of the silicon-based negative electrode active material contained in the negative electrode mixture layer of the third region is not particularly limited. For example, when the content of the silicon-based active material contained in the negative electrode mixture layer of the third region is less than the content of the silicon-based negative electrode active material contained in the negative electrode mixture layer of the first region, it can be controlled by forming the thickness of the negative electrode mixture layer of the third region to be thicker so that the final thickness according to the increase in thickness during charging reaches the thickness of the second negative electrode mixture layer. On the other hand, when the content of the silicon-based active material contained in the negative electrode mixture layer of the third region is greater than the content of the silicon-based negative electrode active material contained in the negative electrode mixture layer of the first region, it can be controlled by forming the thickness of the negative electrode mixture layer of the third region to be thinner so that the final thickness according to the increase in thickness during charging reaches the thickness of the second negative electrode mixture layer.

[0044] In the first region and the second region, the difference (d1' - d2') between the thickness (d1') of the first region and the thickness (d2') of the second region at full charge may be 0 (i.e., the case where the thicknesses of the first region and the second region are the same) or greater than 0, and may be less than the difference (d1 - d2) between the thickness (d1) of the first region and the thickness (d2) of the second region before charging. At this time, d1 and d2 may be different, the thickness of d1 may be greater than d2, and d1 and d1' may be the same. Expressing this in an equation is as shown in the following equation (1).

[0045] 0 ≤ (d1' - d2') / (d1 - d2) < 1 (1)

[0046] More specifically, the equation (1) may be 0 or more and 0.3 or less, 0 or more and 0.5 or less, or 0 or more and 0.7 or less.

[0047] In the present invention, based on full charge, the thickness of the negative electrode mixture layer in the first region is preferably the same as that of the negative electrode mixture layer in the second region, and it is more preferable that the difference in thickness between the negative electrode mixture layer in the first region and the negative electrode mixture layer in the second region is smaller. More specifically, although not limited thereto, based on the thickness of the negative electrode mixture layer in the second region, the difference in thickness can be, for example, 10% or less, specifically, or can be 5% or less, and more specifically can be 3% or less.

[0048] When the thicknesses of the first region and the second region become the same due to charging, the formula (1) becomes 0, and it can also be represented by the following formula (2).

[0049] Initial thickness of the first region (d1) × (1 + thickness increase rate of the first region at full charge) = Initial thickness of the second region (d2) × (1 + thickness increase rate of the second region at full charge) (2)

[0050] In the present invention, although not limited thereto, the silicon-based active material can be selected from SiO x (0 ≤ x < 2), Si-C composite, and Si-Y alloy (where Y is an element selected from alkali metals, alkaline earth metals, transition metals, Group 13 elements, Group 14 elements, rare earth elements, and combinations thereof), and more specifically, it can be SiO x .

[0051] In the present invention, although not limited thereto, the negative electrode mixture layer in the first region may contain 30 - 1 wt% of the silicon-based active material.

[0052] Furthermore, the negative electrode mixture layer in the first region may further contain a carbon-based negative electrode active material. The carbon-based negative electrode active material is not particularly limited as long as it is commonly used as a negative electrode active material. For example, it can be one or more selected from artificial graphite, natural graphite, and graphitized mesophase carbon microspheres. More specifically, artificial graphite can be used, or artificial graphite and natural graphite can be used in combination.

[0053] In addition, the negative electrode mixture layer in the first region may contain an adhesive for bonding between the negative electrode active materials and for bonding between the negative electrode current collector and the negative electrode mixture layer. The adhesive can be at least one selected from rubber-based adhesives and water-soluble polymer-based adhesives.

[0054] The rubber-based adhesive can use a dispersible adhesive that is insoluble in water-based solvents such as water but can be smoothly dispersed in water-based solvents. For example, it can include at least one selected from styrene butadiene rubber (SBR), hydrogenated nitrile butadiene rubber (HNBR), acrylonitrile butadiene rubber, acrylic rubber, butyl rubber, and fluoro rubber. Specifically, in terms of easy dispersion and excellent phase stability, it can include at least one selected from styrene butadiene rubber and hydrogenated nitrile butadiene rubber. More specifically, it can include styrene butadiene rubber.

[0055] In addition, the water-soluble polymer-based adhesive of the adhesive is an adhesive that can be dissolved in water-based solvents such as water and can include at least one selected from polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyethylene glycol (PEG), polyacrylonitrile (PAN), polyacrylamide (PAM), and carboxymethyl cellulose (CMC).

[0056] The adhesive is not limited thereto. The content of the adhesive can be 20% by weight or less, for example, it can be 0.1% by weight or more, 0.5% by weight or more, 1% by weight or more, 1.5% by weight or more, 2% by weight or more, or 3% by weight or more, and can be 20% by weight or less, 15% by weight or less, 10% by weight or less, 7% by weight or less, 5% by weight or less, relative to the total weight of the negative electrode mixture layer in the first region. In addition, the rubber-based adhesive and the water-soluble polymer-based adhesive can be used in combination. In this case, the rubber-based adhesive and the water-soluble polymer-based adhesive can be included in a weight ratio of 0.1 to 99:99 to 1 independently of each other. For example, they can be included in various weight ratios such as 1 to 95:95 to 1, 25 to 75:75 to 25, etc.

[0057] Furthermore, the negative electrode mixture layer in the first region may contain a conductive material to improve conductivity. When the conductive material is included, although not particularly limited, for example, one or more selected from graphite, carbon black, carbon nanotubes, metal powders, and conductive oxides may be used. More specifically, carbon nanotubes may be used, and the volume expansion of the silicon-based active material can be more effectively prevented from causing peeling.

[0058] Relative to the total weight of the negative electrode mixture layer in the first region, the content of the conductive material may be 20 wt% or less. For example, it may be 0.5 wt% or more, 0.7 wt% or more, 1 wt% or more, 1.2 wt% or more, or 1.5 wt% or more, and may be 20 wt% or less, 15 wt% or less, 10 wt% or less, 7 wt% or less, or 5 wt% or less.

[0059] In addition, the negative electrode mixture layer in the second region may contain a carbon-based negative electrode active material, a binder, and may further contain a conductive material as needed. The negative electrode active material, binder, and conductive material are the same as those described above, so the description is omitted.

[0060] The composition of the negative electrode mixture in the negative electrode mixture layer in the second region is not limited thereto, and the negative electrode active material, binder, and conductive material as needed may be included in the same content as the composition of the negative electrode mixture layer in the first region.

[0061] In addition, the negative electrode mixture layer in the second region may contain a carbon-based active material such as graphite as the negative electrode active material, and may not contain a silicon-based active material. When a silicon-based active material is included, the negative electrode mixture layer in the second region may contain the silicon-based active material in an amount less than the content of the silicon-based active material contained in the negative electrode mixture layer in the first region. For example, the content of the silicon-based active material in the negative electrode mixture layer in the second region may be 10 wt% or less.

[0062] In addition, when a negative electrode mixture layer in the third region is included, the components contained in the negative electrode mixture layer in the first region may be included, and the content of the silicon-based negative electrode active material contained in the negative electrode mixture layer in the first region may be adjusted. In addition, the negative electrode mixture layer in the third region may contain a silicon-based active material having a volume expansion rate different from the volume expansion rate of the silicon-based negative electrode active material contained in the first region.

[0063] The negative electrode of the present invention as described above can improve the energy density by including a silicon-based active material, and at the same time, can control the problem of the increase in the thickness of the negative electrode caused by the volume expansion of the silicon-based active material during charging and the resulting increase in the surface pressure.

[0064] Furthermore, a secondary battery including the negative electrode of the present invention can prevent problems caused by an increase in surface pressure due to a change in the thickness of the negative electrode caused by the volume expansion of the active material during the charge and discharge of the battery, even when a silicon-based active material is included as the negative electrode active material in the negative electrode. Detailed Description of the Invention

[0065] Hereinafter, the present invention will be described in more detail with reference to examples. The following examples are merely examples of the present invention, and the present invention is not limited thereto.

[0066] Example 1

[0067] Prepare a first negative electrode mixture slurry composed of 20% by weight of Si, 76% by weight of graphite as a carbon-based negative electrode active material, 1% by weight of a conductive material, and 3% by weight of a binder. In addition, prepare a second negative electrode mixture slurry composed of 96% by weight of graphite as a carbon-based negative electrode active material, 1% by weight of a conductive material, and 3% by weight of a binder.

[0068] Coat one side of a copper foil (width 5 cm × length 6 cm) as a negative electrode current collector alternately with the first negative electrode mixture slurry and the second negative electrode mixture slurry in the longitudinal direction, thereby manufacturing a negative electrode in which the respective negative electrode slurry coatings formed by the first negative electrode mixture slurry and the second negative electrode mixture slurry are alternately arranged in a striped pattern.

[0069] At this time, the striped negative electrode mixture coatings according to the first negative electrode mixture slurry and the second negative electrode mixture slurry are uniformly formed with a width of 1 cm each, and a total of 5 stripes are formed.

[0070] Dry and calender the negative electrode mixture coating to manufacture a negative electrode. Measure the thickness of the mixture layer of the manufactured negative electrode. The thickness of the first region according to the first negative electrode mixture coating is 110 µm, and the thickness of the second region according to the second negative electrode mixture coating is 132 µm.

[0071] Use the manufactured negative electrode to manufacture a battery, perform a full charge (fully charged), then disassemble the battery, measure the thickness of the negative electrode swollen by the charge, and calculate the thickness increase rate of each region and show it in Table 1 below.

[0072] [Table 1]

[0073]

[0074] As can be seen from Table 1, when calculated based on the thicknesses before and after full charge, the thickness increase rate of the first region is 50% [=(165 - 110)×100 / 110], and the thickness increase rate of the second region is 25% [=(165 - 132)×100 / 132]. When using the thickness increase rates of the respective regions calculated as above to calculate Equation 1, it can be seen that the equation of Equation 1 is satisfied. Initial thickness of the first region (d1)×(1 + thickness increase rate of the first region at full charge) = Initial thickness of the second region (d2)×(1 + thickness increase rate of the second region at full charge) (1).

[0075] 110μm×(1 + 0.5) = 132μm×(1 + 0.25)

[0076] [Description of Reference Numerals]

[0077] 2: Negative current collector

[0078] 3: Negative electrode mixture layer

[0079] 4: Volume-expanded negative electrode mixture layer

[0080] 5: Negative electrode mixture layer of the first region

[0081] 7: Negative electrode mixture layer of the second region

[0082] 10: Negative electrode

Claims

1. A negative electrode, comprising: Current collector; A negative electrode mixture layer formed on at least one surface of the current collector, wherein the negative electrode mixture layer is divided into a plurality of regions in a plane, the regions including a first region having a first negative electrode mixture composition and a second region having a second negative electrode mixture composition different from the first negative electrode mixture composition, the thicknesses of the negative electrode mixture layer of the first region and the negative electrode mixture layer of the second region with respect to discharge are different from each other.

2. The negative electrode according to claim 1, wherein, There are n second regions and n + 1 first regions, where n is a natural number greater than 1.

3. The negative electrode according to claim 1, wherein There are two or more first regions, and the second regions are located between adjacent first regions.

4. The negative electrode according to claim 1, wherein, The number of the regions is three or more and odd, and the first regions are located in the end regions of the negative electrode mixture layer.

5. The negative electrode according to claim 1, wherein Among the first region and the second region, the thickness of the negative electrode mixture layer of the second region is thicker than that of the negative electrode mixture layer of the first region.

6. The negative electrode according to claim 5, wherein, The negative electrode mixture layer of the first region contains a silicon-based active material, and the negative electrode mixture layer of the second region does not contain a silicon-based active material.

7. The negative electrode according to claim 5, wherein, The negative electrode mixture layer of the first region contains a silicon-based active material, and the negative electrode mixture layer of the second region contains a silicon-based active material in an amount less than that contained in the negative electrode mixture layer of the first region.

8. The negative electrode according to claim 6, wherein, The silicon-based active material is at least one selected from SiO x , where 0≤x<2; Si-C composite; and Si-Y alloy, wherein Y is an element selected from alkali metals, alkaline earth metals, transition metals, Group 13 elements, Group 14 elements, rare earth elements, and combinations thereof.

9. The negative electrode according to claim 6, wherein, The content of the silicon-based active material in the negative electrode mixture layer of the first region is 1 to 30% by weight, and the content of the silicon-based active material in the negative electrode mixture layer of the second region is 10% by weight or less.

10. The negative electrode according to claim 1, wherein, The negative electrode mixture layer contains a carbon-based active material.

11. The negative electrode according to any one of claims 1 to 10, wherein, The difference in thickness between the first region and the second region with respect to full charge is within 10% of the thickness of the first region.

12. A secondary battery comprising the negative electrode according to any one of claims 1 to 10.

13. The secondary battery according to claim 11, wherein, The difference in thickness between the first region and the second region of the negative electrode with respect to full charge is within 10% of the thickness of the first region.

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