KR20220136602A - Positive electrode and lithium secondary battery - Google Patents

Abstract

The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery including the same. The positive electrode includes a first mixture layer and a second mixture layer on a current collector, wherein pre-rolling of the first mixture layer is performed under specific conditions before forming the second mixture layer, so that the rolling density and thickness of the first mixture layer can be maintained uniformly while the surface of the second mixture layer positioned at the outermost part is flattened, thereby highly realizing the safety of a battery even when a metal object such as a nail penetrates an electrode from the outside.

Description

Positive electrode and lithium secondary battery for lithium secondary battery

The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery having a first mixture layer and a second mixture layer, and specifically, to a lithium secondary battery with improved safety due to a low penetration of the first mixture layer due to the second mixture layer It relates to a positive electrode for a battery and a lithium secondary battery.

As technology development and demand for mobile devices increase, the demand for secondary batteries as an energy source is rapidly increasing. Among these secondary batteries, a lithium secondary battery having a high energy density and operating potential, a long cycle life, and a low self-discharge rate has been commercialized and widely used.

Recently, as lithium secondary batteries are used as power sources for medium and large devices such as electric vehicles, high capacity, high energy density, and low cost of lithium secondary batteries are further required. Studies for using Fe and the like are being actively conducted.

One of the main research tasks of such a lithium secondary battery is to improve the safety of the battery using the high-capacity and high-output electrode active material while implementing the same. A lithium transition metal composite oxide is used as a cathode active material for a lithium secondary battery, and among them, a lithium cobalt composite metal oxide of LiCoO ₂ having a high operating voltage and excellent capacity characteristics is mainly used. However, LiCoO ₂ has very poor thermal properties due to destabilization of the crystal structure due to lithium removal and is expensive, so there is a limit to its mass use as a power source in fields such as electric vehicles.

As a material for replacing LiCoO ₂ , lithium manganese composite metal oxide (LiMnO ₂ or LiMn ₂ O ₄ , etc.), lithium iron phosphate compound (LiFePO ₄ etc.) or lithium nickel composite metal oxide (LiNiO ₂ etc.), etc. have been developed. Among them, research and development of lithium-nickel composite metal oxide, which has a high reversible capacity of about 200 mAh/g, and is easy to implement in a large-capacity battery, is being actively studied. However, LiNiO ₂ has poor thermal stability compared to LiCoO ₂ , and when an internal short circuit occurs in a charged state due to external pressure or the like, the positive electrode active material itself is decomposed, resulting in rupture and ignition of the battery.

Accordingly, as a method for improving low thermal stability while maintaining excellent reversible capacity of LiNiO ₂ , a method of substituting a part of nickel (Ni) with cobalt (Co) or manganese (Mn) has been proposed. However, in the case of LiNi _1-α Co _α O ₂ (α=0.1~0.3) in which a part of nickel is substituted with cobalt, it shows excellent charge/discharge characteristics and lifespan characteristics, but has a problem with low thermal stability. In the case of a nickel-manganese-based lithium composite metal oxide in which a part of Ni is substituted with Mn having excellent thermal stability and a nickel-cobalt-manganese-based lithium composite metal oxide substituted with Mn and Co (hereinafter simply referred to as 'NCM-based lithium oxide') Although it has the advantage of relatively excellent cycle characteristics and thermal stability, the penetration resistance is low, so that an internal short circuit does not occur when a metal object such as a nail penetrates. have.

Accordingly, Korean Patent Application Laid-Open No. 2019-0047203 discloses a technology for securing battery safety by interposing an overcharge prevention layer between a positive electrode current collector and a positive electrode active material layer to increase resistance during overcharge to block charging current. However, in the electrode having the overcharge prevention layer as described above, the positive active material layer formed on the overcharge prevention layer is rolled in order to prevent damage to the separator due to the detached active material particles while increasing the energy density of the battery. Because of the non-uniform rolling density and thickness of the overcharge prevention layer due to the positive active material of the positive electrode, there is a limit in that the penetration resistance of the positive electrode is reduced.

Therefore, while realizing the surface planarization of the layer containing the positive electrode active material, when a metal object such as a nail penetrates the electrode from the outside, the development of a technology for a positive electrode for a lithium secondary battery with excellent safety so that heat or ignition due to overcurrent does not occur This is a necessary situation.

Republic of Korea Patent Publication No. 10-2019-0047203

Accordingly, it is an object of the present invention to provide a positive electrode for a lithium secondary battery with high safety so that heat or ignition due to overcurrent does not occur when a metal object such as a nail penetrates the electrode from the outside while the surface of the layer including the positive electrode active material is flat; To provide a lithium secondary battery including the same.

In order to solve the above-mentioned problem,

The present invention in one embodiment,

It has a structure in which a current collector, a first mixture layer including a first active material, and a second mixture layer including a second active material are sequentially stacked,

The first mixture layer provides a positive electrode for a lithium secondary battery having a thickness deviation of ±10% or less at any three points within a unit area (10 cm × 10 cm).

In this case, when the cross-sectional area of the first mixture layer is analyzed in the thickness direction of the positive electrode, the cross-sectional area ratio of the region where the first mixture layer penetrates due to the second active material contained in the second mixture layer is the maximum thickness of the first mixture layer may be 5% or less of the total area based on .

Also, the average thickness of the first mixture layer may be 0.1 μm to 20 μm.

In addition, the first active material included in the first mixture layer may include lithium iron phosphate represented by the following Chemical Formula 1:

[Formula 1]

Li _1+x Fe _1-y M ¹ _y (PO _4-z )X _z

In Formula 1, M ¹ is at least one selected from Al, Mg, and Ti, X is at least one selected from F, S, and N, and x, y and z are each -0.5≤x≤+0.5, 0 ≤y≤0.5, 0≤z≤0.1.

In addition, the second active material included in the second mixture layer may include a lithium nickel composite oxide represented by the following Chemical Formula 2:

[Formula 2]

Li _x [Ni _y Co _z Mn _w M ² _v ]O _u

In Formula 2, M ² is W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and at least one element selected from the group consisting of Mo, and x, y, z, w and v are respectively 1.0≤x≤1.30, 0.1≤y<1, 0.1<z≤0.6, 0.1<w ≤0.6, 0≤v≤0.2, 1.5≤u≤5.

In addition, the average size of the first active material may be 0.01 μm to 10 μm, and the average size of the second active material may be 1 μm to 20 μm.

In addition, the first active material and the second active material have an elliptical particle shape, the sphericity (R ₁ ) of the first active material is 1.5 to 2.0, and the sphericity (R ₂ ) of the second active material is 1.1 to 1.5, The sphericity (R ₁ ) of the first active material may be greater than the sphericity (R ₂ ) of the second active material.

In addition, the present invention in one embodiment,

A first mixture layer forming step of forming a first mixture layer on the current collector by using the slurry containing the first active material;

a first rolling step of rolling the formed first mixture layer;

a second mixture layer forming step of forming a second mixture layer on the rolled first mixture layer using a slurry containing a second active material; and

Including; a second rolling step of rolling the formed second mixture layer;

The first mixture layer provides a method of manufacturing a positive electrode for a lithium secondary battery having a thickness deviation of ±10% or less at any three points within a unit area (10 cm × 10 cm).

In this case, the first mixture layer may satisfy the thickness change rate condition of Equation 1 below:

[Equation 1] D ² _min /D ¹ _max × 100 ≥ 70%

In Equation 1, D ¹ _max represents the maximum thickness of the first mixture layer after the first rolling step, and D ² _min represents the minimum thickness of the first mixture layer after the second rolling step.

In addition, the first rolling step is performed at a speed of 5 to 60 m/s, the second rolling step is performed at a speed of 20 to 80 m/s, the second rolling step is performed at a speed faster than the first rolling step can be performed.

In addition, the first rolling step may be performed at a temperature of 50 to 100 ℃, the second rolling step may be performed at a temperature of 10 to 40 ℃.

Further, the present invention, in one embodiment, the above-described positive electrode of the present invention; cathode; and a separator positioned between the positive electrode and the negative electrode.

The positive electrode for a lithium secondary battery according to the present invention is provided with a first mixture layer and a second mixture layer on a current collector, and by performing pre-rolling of the first mixture layer under specific conditions before forming the second mixture layer, the outermost Since the rolling density and thickness of the first mixture layer can be uniformly maintained while the surface of the second mixture layer positioned is flat, the safety of the battery can be highly realized even when a metal object such as a nail penetrates the electrode from the outside.

1 is a schematic diagram showing a method of manufacturing a positive electrode for a lithium secondary battery according to the present invention.

Since the present invention can have various changes and can have various embodiments, specific embodiments will be described in detail in the detailed description.

However, this is not intended to limit the present invention to specific embodiments, and should be understood to include all modifications, equivalents and substitutes included in the spirit and scope of the present invention.

In the present invention, terms such as "comprising" or "having" are intended to designate that the features, numbers, steps, operations, components, parts, or combinations thereof described in the specification exist, but one or more other features It is to be understood that this does not preclude the possibility of the presence or addition of numbers, steps, operations, components, parts, or combinations thereof.

Also, in the present invention, when a part of a layer, film, region, plate, etc. is described as being “on” another part, this includes not only cases where it is “directly on” another part, but also a case where there is another part in between. . Conversely, when a part of a layer, film, region, plate, etc. is described as being “under” another part, this includes not only cases where the other part is “directly under” but also cases where another part is in between. In addition, in the present application, “on” may include the case of being disposed not only on the upper part but also on the lower part.

In addition, in the present invention, the sphericity (R) " may be defined as L/S, where "L" represents the major axis of the particle, such as the longest length, and "S" represents the minor axis, such as the shortest length, of the particle. That is, the sphericity can be defined as the ratio of the major axis/minor axis of the particle, and the closer the sphericity is to 1, the smaller the difference between the major and minor diameters of the particle, and the particle may have a shape close to a spherical shape.

Hereinafter, the present invention will be described in more detail.

Anode for lithium secondary battery

The present invention in one embodiment,

It has a structure in which a current collector, a first mixture layer including a first active material, and a second mixture layer including a second active material are sequentially stacked,

The first mixture layer provides a positive electrode for a lithium secondary battery having a thickness deviation of ±10% or less at any three points within a unit area (10 cm × 10 cm).

The positive electrode for a lithium secondary battery according to the present invention has a structure in which a first mixture layer and a second mixture layer are sequentially stacked on a current collector, and a second mixture layer is positioned on the outermost surface, wherein the first mixture layer is a second mixture layer The average thickness is uniform because the second active material included in the layer does not directly penetrate or the conductive material or binder included in the second mixture layer does not penetrate.

As an example, since the first mixture layer has a uniform thickness, the thickness deviation for any three points within a unit area (10 cm Х 10 cm) may be ±10% or less, specifically, ±8% or less; ±5% or less; ±3% or less; It may be less than or equal to ±1%.

As another example, when the cross section of the positive electrode in the thickness direction of the positive electrode is analyzed by a scanning electron microscope (SEM), etc., the first mixture layer may penetrate the first mixture layer due to the second active material contained in the second mixture layer. The cross-sectional area ratio of the region may be 10% or less of the total area calculated based on the maximum thickness of the first mixture layer, specifically 5% or less; 3% or less; 1% or less; 0.8% or less; Alternatively, it may be 0.5% or less, and in some cases, since there is no penetration area due to the second active material, the cross-sectional area ratio of the area infiltrated by the first mixture layer may be 0%.

According to the present invention, by controlling the thickness deviation of the first mixture layer and/or the penetration area ratio of the second active material in the cross-sectional area of the first mixture layer to the above ranges, when a metal object such as a nail passes through the positive electrode from the outside, the overcurrent High safety can be given so that heat or ignition does not occur.

In this case, the first mixture layer may include a first active material, and, if necessary, may include a first conductive material and a first binder together with the first active material. The first active material may include a lithium iron phosphate compound represented by the following Chemical Formula 1:

[Formula 1]

Li _1+x Fe _1-y M ¹ _y (PO _4-z )X _z

In Formula 1,

M ¹ is at least one selected from Al, Mg and Ti,

X is at least one selected from F, S and N,

x, y and z are -0.5≤x≤+0.5, 0≤y≤0.5, 0≤z≤0.1, respectively.

Specifically, the first active material is a lithium iron phosphate compound represented by Chemical Formula 1 and is selected from the group consisting of LiFePO ₄ , Li(Fe,Al)PO ₄ , Li(Fe,Mg)PO ₄ and Li(Fe,Ti)PO ₄ . It may include one or more selected compounds, and more specifically, LiFePO ₄ may be used.

The lithium iron phosphate compound represented by Formula 1 may have an olivine structure. Lithium iron phosphate having an olivine structure shrinks in volume as lithium escapes from an overcharge voltage of about 4.5 V or more. The mixture layer 121 acts as an insulating layer, thereby increasing the resistance of the first mixture layer 121 and blocking the charging current to reach the overcharge termination voltage.

In addition, the first mixture layer may include at least one selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, and carbon fiber as the first conductive material. For example, the first conductive material may include acetylene black.

In addition, the first binder is polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (polyvinylidenefluoride), polyacrylonitrile (polyacrylonitrile), polymethyl methacrylate ( polymethylmethacrylate) and one or more resins selected from the group consisting of copolymers thereof. As an example, the first binder may include polyvinylidenefluoride.

In addition, the first mixture layer may include 80 to 98 parts by weight of the first active material, 1 to 10 parts by weight of the first conductive material, and 1 to 10 parts by weight of the first binder based on 100 parts by weight of the total. As an example, the first mixture layer may include 84 to 96 parts by weight of the first active material, 2 to 8 parts by weight of the first conductive material, and 2 to 8 parts by weight of the first binder based on 100 parts by weight of the total, As another example, based on 100 parts by weight of the total, 88 to 96 parts by weight of the first active material, 2 to 6 parts by weight of the first conductive material, and 2 to 6 parts by weight of the first binder may be included.

In addition, the average thickness of the first mixture layer may be 0.1 μm to 20 μm, specifically, 0.1 μm to 15 μm; 0.1 μm to 10 μm; 2 μm to 10 μm; 4 μm to 10 μm; or 5 μm to 9 μm.

In addition, the second mixture layer is formed on the first mixture layer formed on the current collector, and includes a second active material, and if necessary, may include a second conductive material and a second binder together with the second active material. have. In this case, the second active material may include a lithium nickel composite oxide represented by the following Chemical Formula 2:

[Formula 2]

Li _x [Ni _y Co _z Mn _w M ² _v ]O _u

M ² is W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and It is at least one element selected from the group consisting of Mo,

x, y, z, w and v are 1.0≤x≤1.30, 0.1≤y<1, 0.1<z≤0.6, 0.1<w≤0.6, 0≤v≤0.2, 1.5≤u≤5, respectively.

The second active material is not particularly limited as long as it is a lithium metal composite oxide represented by Formula 2, and may be applied. Specifically, the second active material is LiNiO ₂ , LiNi _0.5 Co _0.5 O ₂ , LiNi _0.6 Co _0.4 O ₂ , LiNi _1/3 Co _1/3 Al _1/3 O ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNi _0.8 Co _0.1 Mn _0.1 O ₂ , and LiNi _0.7 Co _0.1 Mn _0.1 Al _0.1 O ₂ It may include one or more compounds selected from the group consisting of.

As an example, the cathode active material is a lithium nickel cobalt oxide represented by Formula 1, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNi _0.8 Co _0.1 Mn _0.1 O ₂ , or LiNi _0.7 Co _0.1 Mn _0.1 Al _0.1 O ₂ may be used individually or in combination, respectively.

Furthermore, the size and shape of the above-described first and second active materials may be uniformly adjusted. Specifically, the first active material and the second active material may have an elliptical particle shape, and in this case, the sphericity of each particle may be different from each other according to energy applied to each active material when the mixture layer is manufactured. Specifically, the first active material may have _{a sphericity (R 1 )} greater than that of the second active material (R 2 ) by being pressurized slowly at a low speed at a high temperature. For example, the sphericity (R ₁ ) of the first active material may be 1.5 to 2.0, specifically, 1.6 to 1.8. In contrast, the second active material is pressurized at a high speed at room temperature to have a sphericity (R ₂ ) of 1.1 to 1.5, specifically, a low sphericity (R) of 1.2 to 1.4.

In addition, particle sizes of the first active material and the second active material may be controlled to be constant. Specifically, the average size of the active material may be 0.01 μm to 10 μm in the case of the first active material, and more specifically, 1 μm to 10 μm; 0.5 μm to 8 μm; 0.01 μm to 5 μm; 0.01 μm to 5 μm; 1 μm to 6 μm; It may be 3 μm to 7 μm. In addition, in the case of the second active material, the average size may be 1 μm to 20 μm, more specifically, 1 μm to 20 μm; 1 μm to 20 μm; 1 μm to 20 μm; or 1 μm to 20 μm.

In addition, the second mixture layer may include at least one selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, and carbon fiber as the second conductive material. For example, the first conductive material may include acetylene black.

In addition, the second binder is polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (polyvinylidenefluoride), polyacrylonitrile (polyacrylonitrile), polymethyl methacrylate ( polymethylmethacrylate) and one or more resins selected from the group consisting of copolymers thereof. As an example, the second binder may include polyvinylidenefluoride.

In addition, the second mixture layer may include 80 to 98 parts by weight of the second active material, 1 to 10 parts by weight of the second conductive material, and 1 to 10 parts by weight of the second binder based on 100 parts by weight of the total. As an example, the second mixture layer may include 84 to 96 parts by weight of the second active material, 2 to 8 parts by weight of the second conductive material, and 2 to 8 parts by weight of the second binder based on 100 parts by weight of the total, As another example, based on 100 parts by weight of the total, 88 to 96 parts by weight of the second active material, 2 to 6 parts by weight of the second conductive material, and 2 to 6 parts by weight of the second binder may be included.

In addition, the average thickness of the second mixture layer is not particularly limited, and may specifically be 50 μm to 300 μm, more specifically 100 μm to 200 μm; 80 μm to 150 μm; 120 μm to 170 μm; 150 μm to 300 μm; 200 μm to 300 μm; or 150 μm to 190 μm.

On the other hand, the positive electrode 100 for a lithium secondary battery according to the present invention may be used as a current collector having high conductivity without causing a chemical change in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, etc. may be used, and in the case of aluminum or stainless steel, a surface treated with carbon, nickel, titanium, silver, or the like may be used. In addition, the current collector may increase the adhesion of the positive electrode active material by forming fine concavities and convexities on the surface, and various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven body are possible. In addition, the average thickness of the current collector may be appropriately applied in a range of 3 to 500 μm in consideration of the conductivity and total thickness of the positive electrode 100 to be manufactured.

The positive electrode for a lithium secondary battery according to the present invention includes a first mixture layer and a second mixture layer on a current collector as described above, while flattening the surface of the second mixture layer located at the outermost layer of the first mixture layer. By maintaining the rolling density and thickness uniformly, the safety of the battery is improved, so that heat or ignition due to overcurrent may not occur when a metal body passes through the electrode from the outside, and there is an advantage in that excellent battery performance can be realized.

Manufacturing method of positive electrode for lithium secondary battery

In addition, the present invention in one embodiment,

A first mixture layer forming step of forming a first mixture layer on the current collector by using the slurry containing the first active material;

a first rolling step of rolling the formed first mixture layer;

a second mixture layer forming step of forming a second mixture layer on the rolled first mixture layer using a slurry containing a second active material; and

Including; a second rolling step of rolling the formed second mixture layer;

The first mixture layer provides a method of manufacturing a positive electrode for a lithium secondary battery having a thickness deviation of ±10% or less at any three points within a unit area (10 cm × 10 cm).

In the method for manufacturing a positive electrode for a lithium secondary battery according to the present invention, a first mixture layer and a second mixture layer are sequentially formed on a current collector so that a second mixture layer is located at the outermost layer, and the second mixture layer is formed. The thickness uniformity of the first mixture layer may be improved by performing two rolling processes of previously performing rolling of the first mixture layer using a roll press and performing secondary rolling after forming the second mixture layer. .

As an example, the positive electrode for a lithium secondary battery manufactured by the method for manufacturing a positive electrode for a lithium secondary battery may satisfy the condition of Equation 1 below when the cross section of the positive electrode in the thickness direction of the positive electrode is analyzed with a scanning electron microscope (SEM), etc. have:

[Equation 1]

D ² _min /D ¹ _max × 100 ≥ 70%

In Equation 1, D ¹ _max represents the minimum thickness of the first mixture layer after the first rolling step, and D ² _min represents the maximum thickness of the first mixture layer after the second rolling step.

Equation 1 represents the rate of change in thickness according to the two rolling processes of the first mixture layer, and when the cross-section of the anode is analyzed after the first rolling step and the second rolling step are performed, the first mixture layer after the second rolling step The higher the ratio of the minimum thickness (D ² _min ) of the first mixture layer to the maximum thickness (D ¹ _max ) of the first mixture layer after the first rolling step, the smaller the thickness deviation of the first mixture layer. Specifically, since the first mixture layer has a very high uniformity and a small deviation between the minimum thickness and the maximum thickness, the condition of Equation 1 is 75% or more (eg, D ² _min /D ¹ _max × 100 ≥ 75%), 80% or more (eg D ² _min /D ¹ _max × 100 ≥ 80%), 85% or more (eg D ² _min /D ¹ _max × 100 ≥ 85%), 90% or more (eg D ² _min / D ¹ _max × 100 ≥ 90%), or 80 to 95% (eg, 80% ≤ D ² _min /D ¹ _max × 100 ≤ 95%).

Here, in the first rolling step, the influence of the second active material on the uniformity of the interface when the second mixture layer is rolled may be alleviated by increasing the rolling density while making the thickness of the first mixture layer uniform. To this end, the first rolling step may be performed at a low speed at a high temperature. Specifically, the first rolling step is performed at a temperature of 50°C to 100°C, more specifically 60°C to 100°C, 75°C to 100°C, 85°C to 100°C, 50°C to 90°C, 60°C to 80°C. , or at a temperature of 65°C to 90°C. In addition, the first rolling step may be performed at a rolling speed of 5 m/s to 60 m/s, specifically 5 m/s to 15 m/s, 5 m/s to 20 m/s, 5 m/s to 45 m/s, 8 m/s to 15 m/s, 10 m/s to 30 m/s, 35 m/s to 50 m/s, 25 m/s to 50 m/s or 30 It may be performed at a rolling speed of m/s to 55 m/s.

In addition, the second rolling step may be performed at a relatively low temperature and high speed compared to the first rolling step. Specifically, the second rolling step is at room temperature, specifically at a temperature of 10 °C to 30 °C, more specifically at 10 °C to 20 °C, 15 °C to 25 °C, 22 °C to 28 °C, 20 °C to 30 °C or It may be carried out at a temperature of 18 °C to 25 °C. In addition, the second rolling step may be performed at a rolling speed of 20 m/s to 70 m/s, specifically 25 m/s to 60 m/s, 35 m/s to 50 m/s, 50 It may be performed at a rolling speed of m/s to 70 m/s, 55 m/s to 65 m/s or 60 m/s to 70 m/s.

In addition, the first rolling step and the second rolling step may be performed under a pressure condition of 5 MPa to 20 MPa, respectively, and specifically, 5 MPa to 15 MPa, 5 MPa to 10 MPa, 10 MPa to 20 MPa, respectively , 15 MPa to 20 MPa or 8 MPa to 14 MPa under pressure conditions. At this time, the first rolling step is performed under the same pressure condition as the second rolling step in order to optimize the rolling density of the first mixture layer, or is 2 times or less than the second rolling step, for example, 1.5 times or less of the pressure of the second rolling step, It can be carried out under high pressure conditions of 1.2 times or less, or 1.1 times to 1.4 times.

According to the present invention, since the rolling density of the first mixture layer can be uniformly formed in an appropriate range by successively performing the first rolling step and the second rolling step under the rolling conditions as described above, the penetration resistance of the anode with non-uniform rolling density is Not only can it be prevented from being reduced, but also the thickness non-uniformity of the first mixture layer due to the second active material of the second mixture layer can be improved, so that when a metal object such as a nail penetrates the electrode, the current collector is exposed to the outside. ignition can be suppressed.

lithium secondary battery

In addition, the present invention in one embodiment,

the positive electrode according to the present invention described above; cathode; and a separator positioned between the positive electrode and the negative electrode.

The lithium secondary battery according to the present invention may include the positive electrode and the negative electrode of the present invention described above, and have a structure in which the positive electrode and the negative electrode are impregnated in a lithium salt-containing electrolyte.

Here, the negative electrode is manufactured by coating, drying, and pressing the negative electrode active material on the negative electrode current collector, and as necessary, the conductive material, organic binder polymer, filler, etc. as described above may be optionally further included.

In addition, the negative active material is, for example, graphite having a completely layered crystal structure such as natural graphite, and soft carbon having a low crystallinity layered crystal structure (graphene structure; a structure in which hexagonal honeycomb planes of carbon are arranged in layers). and carbon and graphite materials, such as hard carbon, artificial graphite, expanded graphite, carbon fiber, non-graphitizable carbon, carbon black, carbon nanotube, fullerene, activated carbon, etc., in which these structures are mixed with amorphous parts; LixFe ₂ O ₃ (0≤x≤1), LixWO ₂ (0≤x≤1), SnxMe1-xMe'yOz (Me: Mn, Fe, Pb, Ge; Me', Al, B, P, Si, periodic table metal complex oxides such as Group 1, Group 2, and Group 3 elements, halogen, 0<x≤1;1≤y≤3;1≤z≤8); lithium metal; lithium alloy; silicon-based alloys; tin-based alloys; SnO, SnO ₂ , PbO, PbO ₂ , Pb ₂ O ₃ , Pb ₃ O ₄ , Sb ₂ O ₃ , Sb ₂ O ₄ , Sb ₂ O ₅ , GeO, GeO ₂ , Bi ₂ O ₃ , Bi ₂ O ₄ and metal oxides such as Bi ₂ O ₅ ; conductive polymers such as polyacetylene; Li-Co-Ni-based materials; titanium oxide; Lithium titanium oxide and the like can be used.

As an example, the negative electrode active material may include both graphite and silicon (Si)-containing particles, and the graphite may include any one or more of natural graphite having a layered crystal structure and artificial graphite having an isotropic structure. The silicon (Si)-containing particles are particles including silicon (Si) as a main component as a metal component, and are oxidized with silicon (Si) particles, silicon oxide (SiO ₂ ) particles, or the silicon (Si) particles. It may include a mixture of silicon (SiO ₂ ) particles.

In this case, the negative active material is 80 to 95 parts by weight of graphite based on 100 parts by weight of the total; and 1 to 20 parts by weight of silicon (Si)-containing particles. The present invention can improve the charge capacity per unit mass while reducing lithium consumption and irreversible capacity loss during initial charging and discharging of the battery by adjusting the content of graphite and silicon (Si)-containing particles contained in the negative electrode active material to the above range. have.

In addition, the negative electrode mixture layer may have an average thickness of 100 μm to 200 μm, specifically, 100 μm to 180 μm, 100 μm to 150 μm, 120 μm to 200 μm, 140 μm to 200 μm, or 140 μm to It may have an average thickness of 160 μm.

In addition, the negative electrode current collector is not particularly limited as long as it has high conductivity without causing a chemical change in the battery. For example, copper, stainless steel, nickel, titanium, sintered carbon, etc. may be used, and copper However, in the case of stainless steel, a surface treated with carbon, nickel, titanium, silver, etc. may be used. In addition, the negative electrode current collector, like the positive electrode current collector, may form fine irregularities on the surface to strengthen the bonding force with the negative electrode active material, and may have various forms such as films, sheets, foils, nets, porous bodies, foams, nonwovens, etc. It is possible. In addition, the average thickness of the negative electrode current collector may be appropriately applied in the range of 3 to 500 μm in consideration of the conductivity and total thickness of the negative electrode to be manufactured.

In addition, the separator is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The separation membrane is not particularly limited as long as it is conventionally used in the art, and specifically, chemical resistance and hydrophobic polypropylene; glass fiber; Alternatively, a sheet or non-woven fabric made of polyethylene may be used, and in some cases, a composite separator in which inorganic particles/organic particles are coated with an organic binder polymer on a porous polymer substrate such as the sheet or non-woven fabric may be used. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as a separator. In addition, the membrane may have an average pore diameter of 0.01 to 10 μm, and an average thickness of 5 to 300 μm.

Meanwhile, the positive electrode and the negative electrode may be wound in the form of a jelly roll and stored in a cylindrical battery, a prismatic battery, or a pouch-type battery, or may be stored in a pouch-type battery in a folding or stack-and-folding form, but is not limited thereto.

In addition, the lithium salt-containing electrolyte according to the present invention may consist of an electrolyte and a lithium salt, and as the electrolyte, a non-aqueous organic solvent, an organic solid electrolyte, an inorganic solid electrolyte, and the like may be used.

As the non-aqueous organic solvent, for example, N-methyl-2-pyrrolidinone, ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dime ethoxyethane, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, Methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether, pyropion An aprotic organic solvent such as methyl acid or ethyl propionate may be used.

Examples of the organic solid electrolyte include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphoric acid ester polymers, poly agitation lysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, ions A polymer material containing a dissociating group, etc. may be used.

Examples of the inorganic solid electrolyte include Li ₃ N, LiI, Li ₅ Ni ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Nitrides, halides, sulfates, etc. of Li such as Li ₄ SiO ₄ -LiI-LiOH, Li ₃ PO ₄ -Li ₂ S-SiS ₂ and the like may be used.

The lithium salt is a material readily soluble in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB10Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF ₆ , LiSbF ₆ , LiAlCl ₄ , CH ₃ SO ₃ Li, (CF ₃ SO ₂ ) ₂ NLi, lithium chloroborane, lithium lower aliphatic carboxylate, lithium 4-phenylboronate, imide, or the like may be used.

In addition, for the purpose of improving charge/discharge characteristics, flame retardancy, etc. in the electrolyte solution, for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphate triamide, nitro Benzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride, etc. may be added. have. In some cases, in order to impart incombustibility, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride may be further included, and carbon dioxide gas may be further included to improve high-temperature storage characteristics, and FEC (Fluoro-Ethylene Carbonate) ), PRS (propene sultone), and the like may be further included.

battery module

Furthermore, in one embodiment, the present invention provides a battery module including the above-described secondary battery as a unit cell, and provides a battery pack including the battery module.

The battery pack may be used as a power source for a medium or large device requiring high temperature stability, long cycle characteristics, and high rate characteristics, and specific examples of the medium or large device include a power tool that is powered by an omniscient motor; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and the like; electric two-wheeled vehicles including electric bicycles (E-bikes) and electric scooters (E-scooter); electric golf carts; and a power storage system, and more specifically, a hybrid electric vehicle (HEV), but is not limited thereto.

Hereinafter, the present invention will be described in more detail by way of Examples and Experimental Examples.

However, the following Examples and Experimental Examples are merely illustrative of the present invention, and the content of the present invention is not limited to the following Examples and Experimental Examples.

Examples 1-4 and Comparative Examples 1-5. Preparation of positive electrode for lithium secondary battery

88 parts by weight of LiFePO ₄ (average size: 1±0.1 μm) as the first active material, 10 parts by weight of PVdF as a binder, and 2 parts by weight of carbon black as a conductive material were weighed and mixed in N-methylpyrrolidone (NMP) solvent to prepare 1 A slurry for a mixture layer was prepared.

Separately, 95 parts by weight of LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (average size: 15±0.2 μm) as a second active material, 2 parts by weight of PVdF as a binder, and 3 parts by weight of carbon black as a conductive material were weighed, and N-methylpyrrolol A slurry for a second mixture layer was prepared by mixing in a Don (NMP) solvent.

The slurry for the first mixture layer was applied to the aluminum foil, dried, and then a first rolling step was performed to form a first mixture layer (average thickness: 8 μm). A positive electrode for a lithium secondary battery was manufactured by continuously applying a slurry for a second mixture layer on the first mixture layer, dried, and then performing a second rolling step. At this time, by analyzing the cross section of the first mixture layer immediately after the first rolling step and the second rolling step are performed, respectively, the maximum thickness (D ¹ _max ) of the first mixture layer after the first rolling step and after the second rolling step The minimum thickness (D ² _min ) of the first mixture layer was measured, and the ratio of the measured result values (D ² _min /D ¹ _max ) and the conditions for performing the first rolling step and the second rolling step are shown in Table 1 below. It was.

first rolling step 2nd rolling step D ² _min /D ¹ _max temperature
[℃] pressure
[MPa] speed
[m/s] temperature
[℃] pressure
[MPa] speed
[m/s] Example 1 70±1 15 10 25±1 15 30 0.94 Example 2 50±1 15 10 25±1 15 30 0.85 Example 3 70±1 15 10 25±1 10 30 0.93 Example 4 70±1 15 40 25±1 15 40 0.92 Comparative Example 1 25±1 15 10 25±1 15 30 0.59 Comparative Example 2 70±1 2 10 25±1 15 30 0.63 Comparative Example 3 70±1 30 10 25±1 15 30 0.88 Comparative Example 4 70±1 15 3 25±1 15 30 0.81 Comparative Example 5 70±1 15 70 25±1 15 30 0.56

experimental example.

In order to evaluate the performance of the positive electrode for a lithium secondary battery according to the present invention, the following experiment was performed.

A) Flatness analysis of the first composite layer

① Using a transmission electron microscope (TEM), the thickness of the first mixture layer was measured at 3 arbitrary points within the unit area (10 cm Х 10 cm) of the anode surface prepared in Examples 1 to 4 and Comparative Examples 1 to 5. were measured and their average value was calculated.

In addition, ② the anodes prepared in Examples 1 to 4 and Comparative Examples 1 to 5 were cut in the thickness direction, and then a scanning electron microscope (SEM) analysis was performed on the cut surface. Using the analyzed cross-sectional image of the positive electrode, the area infiltrated by the second mixture layer due to the second active material among the total area calculated based on the longest thickness of the first mixture layer included in each positive electrode is calculated to calculate the first mixture layer The area ratio of the second mixture layer penetrating the was calculated, and the results are shown in Table 2 below.

B) Nail Penetration Test

A lithium secondary battery was manufactured using each positive electrode prepared in Examples and Comparative Examples. Specifically, as an anode active material, natural graphite, a carbon black conductive material, and a PVDF binder are mixed in an N-methylpyrrolidone solvent in a weight ratio of 85:10:5 to prepare a slurry for forming an anode, and this is applied to a copper foil. A negative electrode was prepared. An electrode assembly was prepared by laminating a separator (thickness: about 16 μm) made of a porous polyethylene (PE) film between each positive electrode prepared in Examples and Comparative Examples and the negative electrode prepared above. After the prepared electrode assembly was placed inside the battery case, an electrolyte was injected into the case to prepare a lithium secondary battery. At this time, the electrolyte is ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate (mixed volume ratio of EC / DMC / EMC 3 / 4 / 3) in an organic solvent consisting of 1.0M concentration of lithium hexafluorophosphate (LiPF ₆ ) by dissolving prepared.

For the manufactured lithium secondary battery, it was evaluated whether or not ignition occurred when a metal body with a diameter of 3 mm was dropped at a speed of 80 mm/sec to penetrate the cell in the same manner as the PV8450 certification conditions, and the results are shown in Table 2 below.

C) Cycle life performance evaluation

In the same manner as the nail penetration test, a lithium secondary battery was manufactured using the positive electrodes prepared in Examples and Comparative Examples, respectively. 100 charge/discharge (n=100) and 200 charge/discharge (n=200) conditions for each manufactured lithium secondary battery at 25°C with a final charge voltage of 4.25V, a final discharge voltage of 2.5V, and 0.5C/0.5C. While carrying out the capacity retention rate (Capacity Retention [%]) was measured. At this time, the capacity retention rate was calculated using Equation 2 below, and the results are shown in Table 2 below:

[Equation 2]

Capacity retention rate (%) = (discharge capacity at n charge/discharge/discharge capacity at one charge/discharge)Х100

Uniformity of the first mixture layer ignited
(Pass/Test) Capacity retention rate during charging and discharging average thickness Permeation area ratio of the second mixture layer 100 times (n=100) 200 times (n=200) Example 1 7.94 μm ≤0.5% 6P/6T 98.3 95.3 Example 2 8.05 μm ≤0.5% 6P/6T 98.7 96.0 Example 3 7.96 μm ≤0.6% 6P/6T 97.1 95.7 Example 4 8.10 μm ≤0.8% 6P/6T 98.9 96.1 Comparative Example 1 8.62 μm 8% 1P/6T 96.8 92.1 Comparative Example 2 8.73 μm 11% 3P/6T 92.2 81.5 Comparative Example 3 7.89 μm 2% 4P/6T 91.0 84.8 Comparative Example 4 7.64 μm 6% 3P/6T 95.8 89.3 Comparative Example 5 8.15 μm 14% 0P/6T 94.6 90.6

Referring to Table 2, in the positive electrode of the embodiment according to the present invention, the average thickness of the first mixture layer was 7.91 to 8.10 μm, and the deviation was 1.25% or less, and the penetration area ratio of the second mixture layer was less than 1%. This means that the thickness uniformity of the first mixture layer provided in the positive electrode of the present invention is high.

In addition, it can be seen that the battery having the positive electrode of the embodiment does not generate heat or ignition when a metal object such as a nail passes through the electrode from the outside, thereby improving the safety of the battery.

Furthermore, it was found that the battery having the positive electrode of the embodiment has improved electrical performance and has a high capacity retention rate of 97% or more and 95% or more, respectively, when charging and discharging 100 times and 200 times. On the other hand, in the batteries having the positive electrodes of Comparative Examples 3 and 4, the uniformity of the first mixture layer was high, but the porosity of the mixture layer was significantly reduced, and thus, it was confirmed that the electrical performance of the battery was deteriorated.

From these results, the positive electrode for a lithium secondary battery according to the present invention includes a first mixture layer and a second mixture layer on a current collector, but by performing pre-rolling of the first mixture layer under specific conditions before forming the second mixture layer. , since the rolling density and thickness of the first mixture layer can be maintained uniformly while the surface of the second mixture layer located at the outermost layer is flattened, the safety of the battery is high even when a metal object such as a nail penetrates the electrode from the outside. can be implemented

Although the above has been described with reference to the preferred embodiment of the present invention, those skilled in the art or those having ordinary knowledge in the technical field do not depart from the spirit and technical scope of the present invention described in the claims to be described later. It will be understood that various modifications and variations of the present invention may be made within the scope thereof.

Accordingly, the technical scope of the present invention is not limited to the content described in the detailed description of the specification, but should be defined by the claims.

100: Manufacturing method of a positive electrode for a lithium secondary battery
10: current collector
110: slurry for the first mixture layer
110': first composite layer
120: Slurry for the second mixture layer
120': second composite layer
130: first active material
140: second active material

Claims (12)

It has a structure in which a current collector, a first mixture layer including a first active material, and a second mixture layer including a second active material are sequentially stacked,
The first composite layer has a thickness deviation of ±10% or less for any three points within a unit area (10 cm × 10 cm) of a positive electrode for a lithium secondary battery.
According to claim 1,
When analyzing the cross-sectional area of the first mixture layer in the thickness direction of the positive electrode, the ratio of the cross-sectional area of the region where the first mixture layer penetrates due to the second active material contained in the second mixture layer is determined based on the maximum thickness of the first mixture layer. A positive electrode for a lithium secondary battery that is 5% or less of the total area.
According to claim 1,
An average thickness of the first composite layer is 0.1 μm to 20 μm, a positive electrode for a lithium secondary battery.
According to claim 1,
The first active material is a positive electrode for a lithium secondary battery comprising lithium iron phosphate represented by the following Chemical Formula 1:
[Formula 1]
Li _1+x Fe _1-y M ¹ _y (PO _4-z )X _z
In Formula 1,
M ¹ is at least one selected from Al, Mg and Ti, and X is at least one selected from F, S, and N;
x, y and z are -0.5≤x≤+0.5, 0≤y≤0.5, 0≤z≤0.1, respectively.
According to claim 1,
The second active material is a positive electrode for a lithium secondary battery comprising a lithium nickel composite oxide represented by the following Chemical Formula 2:
[Formula 2]
Li _x [Ni _y Co _z Mn _w M ² _v ]O _u
In Formula 2,
M ² is W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and It is at least one element selected from the group consisting of Mo,
x, y, z, w and v are 1.0≤x≤1.30, 0.1≤y<1, 0.1<z≤0.6, 0.1<w≤0.6, 0≤v≤0.2, 1.5≤u≤5, respectively.
According to claim 1,
The average size of the first active material is 0.01 μm to 10 μm,
The second active material has an average size of 1 µm to 20 µm, a positive electrode for a lithium secondary battery.
According to claim 1,
The first active material and the second active material have an elliptical particle shape,
The sphericity (R ₁ ) of the first active material is 1.5 to 2.0,
The sphericity (R ₂ ) of the second active material is 1.1 to 1.5,
A positive electrode for a lithium secondary battery, characterized in that the sphericity (R ₁ ) of the first active material is greater than that of the second active material (R ₂ ).
A first mixture layer forming step of forming a first mixture layer on the current collector by using the slurry containing the first active material;
a first rolling step of rolling the formed first mixture layer;
a second mixture layer forming step of forming a second mixture layer on the rolled first mixture layer using a slurry containing a second active material; and
Including; a second rolling step of rolling the formed second mixture layer;
The method of manufacturing a positive electrode for a lithium secondary battery in which the first mixture layer has a thickness deviation of ±10% or less at any three points within a unit area (10 cm × 10 cm).
9. The method of claim 8,
The first composite layer is a method of manufacturing a positive electrode for a lithium secondary battery that satisfies the thickness change rate condition of Equation 1:
[Equation 1]
D ² _min /D ¹ _max × 100 ≥ 70%
In Equation 1 above,
D ¹ _max represents the maximum thickness of the first mixture layer after the first rolling step,
D ² _min represents the minimum thickness of the first mixture layer after the second rolling step.
9. The method of claim 8,
The first rolling step is performed at a speed of 5 to 60 m/s,
The second rolling step is performed at a speed of 20 to 80 m/s,
A method of manufacturing a positive electrode for a lithium secondary battery in which the second rolling step is performed at a faster speed than the first rolling step.
9. The method of claim 8,
The first rolling step is carried out at a temperature of 50 ℃ to 100 ℃ and
The second rolling step is a method of manufacturing a positive electrode for a lithium secondary battery performed at a temperature of 10 ℃ to 40 ℃.
The positive electrode according to claim 1; cathode; and a separator positioned between the positive electrode and the negative electrode.

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