KR20130136934A - Anode active material for secondary battery - Google Patents

Abstract

A cathode active material for secondary battery of the present invention contains materials (a) with relatively good rolling characteristics and improved electrode alignment in the rolling process and (b) materials with relatively poor rolling characteristic and a low change in the electrode alignment in the rolling process to improve rolling characteristics and reduce a change in the electrode alignment after rolling, thereby obtaining a cathode active material for secondary battery with enhanced output and lifetime characteristics. [Reference numerals] (AA) Output (W);(BB) Example 1;(CC) Comparative example 1;(DD) Comparative example 2

Description

Anode Active Material for Secondary Battery

The present invention includes a negative electrode active material for a secondary battery, which includes a material having a relatively good rolling property and improving the electrode orientation during the rolling process and a material having a relatively poor rolling property and a small change in the electrode orientation during the rolling process. It is related with the negative electrode active material for secondary batteries which is characterized by excellent rolling property and little change of orientation degree after rolling, and excellent in output and lifetime characteristics.

Depletion of fossil fuels and environmental degradation issues are emerging, and many researchers are working on alternative energy development. As part of such alternative energy, secondary batteries are also being researched in various fields. In addition to existing portable devices, the field is expanding to include automotive batteries, power storage batteries, and the like.

Components constituting such a battery may be typically classified into a positive electrode, a negative electrode, an electrolyte, a separator, and the like. Among them, the most influential part of the battery can be said to be the positive electrode and the negative electrode where the electrochemical reaction takes place.

In particular, a lithium secondary battery, as the name suggests, is a battery using Li, which has a high energy density and a light weight, but has a disadvantage in that dendrites can be easily formed and have low safety.

Specifically, storage of electricity occurs through the process of entering Li ions from the anode into the cathode during charging. In this process, Li ions from the anode at the beginning of charging enter the cathode through the electrolyte, and a classification phenomenon occurs at the interface between the materials, leading to overvoltage. At this time, if there are insufficient ions that can move relative to the amount of current flowing, Li is precipitated due to overvoltage. The lithium precipitation is generated not only by the movement of lithium ions but also by electrical resistance, and closely related to the porosity of the electrode in the case of the movement of ions. The higher the permeability, the greater the mobility of Li ions, but the lower the electrical contact surface, it is necessary to properly adjust, but the situation is very difficult. In particular, high permeability also implies problems leading to low energy density. As a result, secondary batteries, which were first commercialized using Li-metal as a negative electrode, failed due to safety problems.

Accordingly, many researchers are studying materials that can be used as a negative electrode active material, and various materials such as graphite and silicon are known to be used as a negative electrode, and are actually used.

However, in order to increase the capacity of the battery, a higher density of the negative electrode mixture layer is required, and for this purpose, high pressure rolling is required. In this case, some materials have a manufacturing problem that the rolling itself is not good and cannot be rolled to a desired density, or peeling and / or dropping of the mixture layer occurs. In addition, some materials are well rolled to a desired density, so there is no manufacturing problem, but after the rolling, the side that occludes / discharges the lithium ions is oriented in the plane direction of the current collector, making it difficult to be exposed to the electrolyte, thereby decreasing the diffusion of lithium ions. In addition, not only the high rate discharge characteristic is lowered, but also the expansion / contraction in the c-axis direction (thickness direction) during charge and discharge is reflected by the change in the thickness of the mixture layer, thereby deteriorating the service life characteristics.

Therefore, there is an increasing demand for a negative electrode active material having excellent output and life characteristics while satisfying a desired electrode density.

SUMMARY OF THE INVENTION Accordingly, the present invention has been made to solve the above-mentioned problems of the prior art and the technical problems required from the past.

After extensive research and various experiments, the inventors of the present application surprisingly output the negative electrode active material as a negative electrode active material for a secondary battery, when a negative electrode active material is formed of a combination of two kinds of materials having different rolling characteristics and electrode orientation changes during the rolling process. It confirmed that the after-life characteristic was excellent, and came to complete this invention.

The negative electrode active material according to the present invention includes a material (a) having relatively good rolling properties and improving electrode orientation in the rolling process and a material (b) having relatively low rolling properties and little change in electrode orientation in the rolling process. It is characterized in that the rolling characteristics are good and the change in the degree of orientation after rolling is small, so that the output and life characteristics are excellent.

As described above, when a material (a) having a relatively good rolling property and improving the electrode orientation in the rolling process and a material (b) having a relatively poor rolling property and little change in the electrode orientation in the rolling process are included, To compensate for the shortcomings, rolling can be performed to a similar degree as that of the material (a), and the change of the degree of orientation is also reduced due to the material (b) to favor the life characteristics. This is because the material (b) serves as a kind of support between the material (a), thereby preventing the material from being oriented in the plane direction of the current collector.

The said rolling characteristic shows the extent to which rolling is carried out, Specifically, as for the rolling characteristic of the said substance (a), the ratio of the short diameter to the long diameter of the particle | grains before rolling is from 1: 1 to 1: 0.5, and the magnitude | size of the short diameter after rolling is shown. In the reduced range, it may change from 1: 0.8 to 1: 0.1. In addition, the rolling properties of the material (b) are in the range of 1: 0.9 to 1: 0.09 in a range in which the ratio of the short diameter to the long diameter of the particles before rolling is from 1: 1 to 1: 0.1 and the size of the short diameter is reduced after rolling. Changeable.

The improved degree of orientation of the material (a) may include that the surface on which the lithium ions move is oriented in the plane direction of the current collector while some particles are deformed during rolling.

In the material (a), it is preferable that the degree of orientation of the electrode after rolling is changed in the range of 0 to 40 degrees with respect to the surface of the current collector in terms of average crystal layer orientation of the particles.

In the case of manufacturing the electrode using only the material (a), since the surface that occludes / discharges the lithium ions after rolling is oriented in the plane direction of the current collector and is difficult to be exposed to the electrolyte, the diffusion of lithium ions is lowered and high rate discharge is achieved. In addition to the deterioration of the characteristics, the expansion / contraction in the c-axis direction (thickness direction) accumulates during charge and discharge, resulting in a large change in the thickness of the mixture layer. Can be.

According to the present invention, the material (b), which is not easily rolled, serves as a support between the materials (a), thereby preventing the particles of the material (a) from being oriented in the plane direction of the current collector. Therefore, it is relatively easy for the surface that occludes / discharges lithium ions of the material (a) to be exposed to the electrolyte solution, thereby improving high rate discharge characteristics, and changing the thickness of the mixture layer because the c-axis direction is different for each particle during charging and discharging. Since the is relatively small, the life characteristics can be improved.

In the present invention, any substance may be used as long as the substance (a) and the substance (b) satisfy the above conditions, but in a preferred embodiment, the substance (a) is spherical natural graphite, and the substance (b) ) Is a plate-like artificial graphite.

In general, in the case of natural graphite, since graphitization proceeds sufficiently to the particle surface layer, electrostatic repulsion between particles is strong and slipperiness is very large. Therefore, the rolling characteristics are excellent.

On the other hand, as described above, there is a problem of orientation in the plane direction of the current collector after rolling, and since the orientation is particularly present in the plate-like natural graphite, in order to prevent this, spherical natural graphite that has been spherical to the plate-like natural graphite is prevented. It is more advantageous to use.

However, even in the case of the spherical natural graphite, since it is impossible to control all the particles in the form of a perfect sphere, in the case of high-density rolling, a phenomenon occurs in which the orientation of the current collector is accompanied by deformation of some particles.

In general, in the case of artificial graphite, the particle surface layer is in a state close to the non-graphitized amorphous state. The amorphous surface layer is less slippery due to less electrostatic repulsion between particles unique to the graphite layer structure. Therefore, rolling is not performed well and since the change of form is few, orientation is not made easily.

The artificial graphite used as the substance (b) is not particularly limited in its form, and may be used in any form such as plate, sphere, fibrous or block. Preferably, plate-shaped artificial graphite is mentioned.

Specifically, the particle size (D50) of the spherical natural graphite is preferably 15 to 20 μm, more preferably 16 to 18 μm.

In addition, the particle size (D50) of the plate-like artificial graphite is preferably 5 to 10 m, more preferably 6 to 8 m.

The material (a) and the material (b) is preferably mixed in a ratio of 6: 4 to 8.5: 1.5 by weight.

If the ratio of the material (a) exceeds 8.5 it is difficult to expect the effect due to the mixing of the material (b), on the contrary, if less than 6, the rolling is not so good that it is difficult to meet the desired porosity is preferably in the above range. For the same reason as above, the substance (a) and the substance (b) are more preferably mixed in a ratio of 7: 3 to 8: 2 based on the weight.

Through various experiments, the inventors have found that the case where the material (a) and the material (b) are mixed at the above ratios shows much better output and life characteristics than the case where the other materials are mixed at other ratios.

The present invention also provides a negative electrode for a secondary battery comprising the negative electrode active material.

The negative electrode for a secondary battery is manufactured by coating, drying, and compressing a negative electrode material including the active material combination and a binder on an electrode current collector, and optionally, components such as a conductive material and a filler may be further included.

The current collector for the cathode is generally made of a thickness of 3 to 500 ㎛. Such an anode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and may be formed of a material such as copper, stainless steel, aluminum, nickel, titanium, fired carbon, surface of copper or stainless steel A surface treated with carbon, nickel, titanium, silver or the like, an aluminum-cadmium alloy, or the like can be used. The current collector may have fine irregularities on the surface thereof to increase the adhesive force of the negative electrode active material, and various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric are possible.

Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), cellulose, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene rubber, fluorine rubber, various copolymers, high molecular weight polyolefins such as polyolefin, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, Vinyl alcohol, and the like.

The conductive material is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, and examples thereof include graphite such as natural graphite and artificial graphite; Carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fiber and metal fiber; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskeys such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used. Concrete examples of commercially available conductive materials include acetylene black series such as Chevron Chemical Company, Denka Singapore Private Limited, Gulf Oil Company, etc.), Ketjenblack, EC (Armak Company), Vulcan XC-72 (Cabot Company), and Super P (Timcal).

In some cases, a filler may optionally be added as a component that inhibits the expansion of the electrode. Such a filler is not particularly limited as long as it is a fibrous material without causing a chemical change in the battery, and examples thereof include olefin polymers such as polyethylene and polypropylene; Fibrous materials such as glass fibers and carbon fibers are used.

In addition, other components such as a viscosity adjusting agent, an adhesion promoter and the like may be further included as a selective or a combination of two or more.

The viscosity adjusting agent may be added up to 30% by weight based on the total weight of the electrode mixture, so as to control the viscosity of the electrode mixture so that the mixing process of the electrode mixture and the coating process on the collector may be easy. Examples of such viscosity modifiers include carboxymethylcellulose, polyvinylidene fluoride and the like, but are not limited thereto. In some cases, the above-described solvent may play a role as a viscosity adjusting agent.

The adhesion promoter may be added in an amount of 10% by weight or less based on the binder, for example, oxalic acid, adipic acid, Formic acid, acrylic acid derivatives, itaconic acid derivatives, and the like.

In addition, the present invention provides a lithium secondary battery including the negative electrode.

The lithium secondary battery has a structure in which a lithium salt-containing non-aqueous electrolyte is impregnated into an electrode assembly having a separator interposed between a positive electrode and a negative electrode.

For example, the positive electrode may be manufactured by coating and drying a positive electrode active material on a positive electrode current collector, and may further include a binder and a conductive material and, as necessary, components described above with respect to the structure of the positive electrode.

The cathode current collector generally has a thickness of 3 to 500 mu m. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical change in the battery. For example, the surface of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel Surface treated with carbon, nickel, titanium, silver, or the like can be used. The positive electrode current collector may have fine unevenness formed on the surface thereof to increase the adhesive force of the positive electrode active material as in the case of the negative electrode current collector. Alternatively, the positive electrode current collector may have various properties such as a film, sheet, foil, net, porous body, Form is possible.

The positive electrode active material is not particularly limited as long as it is a material capable of releasing and inhaling lithium during charging and discharging, for example, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium cobalt- Manganese oxides, lithium cobalt-nickel oxides, lithium nickel-manganese oxides, lithium nickel-manganese-cobalt oxides, lithium iron-phosphate oxides, and the like, and some transition metals include aluminum, magnesium, titanium, and the like. Substituted materials may be used.

The binder, the conductive material and the filler added as necessary are the same as those described for the negative electrode.

The separator is an insulating thin film interposed between the anode and the cathode and having high ion permeability and mechanical strength. The pore diameter of the separator is generally 0.01 to 10 mu m and the thickness is generally 5 to 300 mu m. As such a separation membrane, for example, a sheet or a nonwoven fabric made of an olefin-based polymer such as polypropylene which is chemically resistant and hydrophobic, glass fiber, polyethylene or the like is used.

In some cases, a gel polymer electrolyte may be coated on the separator to increase the stability of the battery. Representative of such gel polymers are polyethylene oxide, polyvinylidene fluoride, polyacrylonitrile and the like. When a solid electrolyte such as a polymer is used as an electrolyte, the solid electrolyte may also serve as a separation membrane.

The lithium salt-containing non-aqueous electrolyte is composed of an organic solvent electrolyte and a lithium salt.

As said electrolyte solution, N-methyl- 2-pyrrolidinone, a propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, Gamma-butylo lactone, 1,2-dimethoxy ethane, 1,2-diethoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethylsulfoxide, 1,3-dioxolon, 4 -Methyl-1,3-dioxene, diethyl ether, formamide, dimethylformamide, dioxolon, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxy methane, dioxolon derivatives Aprotic organic solvents such as sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate and ethyl propionate can be used.

Examples of the organic solid electrolyte include a polymer electrolyte such as a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, an agitation lysine, a polyester sulfide, a polyvinyl alcohol, a polyvinylidene fluoride, A polymer containing an ionic dissociation group and the like 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 and sulfates of Li such as Li ₄ SiO ₄ -LiI-LiOH and Li ₃ PO ₄ -Li ₂ S-SiS ₂ can be used.

The lithium salt is a material that is readily soluble in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO 4, LiBF 4, LiB 10 Cl 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 3 Li, LiSCN, LiC (CF 3 SO 2) 3, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, 4-phenylborate, imide, and the like can be used.

For the purpose of improving the charge / discharge characteristics and the flame retardancy, the electrolytic solution is preferably mixed with an organic solvent such as pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, Benzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, . In some cases, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride may be further added to impart nonflammability, or a carbon dioxide gas may be further added to improve high-temperature storage characteristics.

In a preferred _{embodiment, LiPF 6, LiClO 4, LiBF} 4, LiN (SO 2 CF 3) 2 , such as a lithium salt, a highly dielectric solvent of DEC, DMC or EMC Fig solvent cyclic carbonate and a low viscosity of the EC or PC of And then adding it to a mixed solvent of linear carbonate to prepare a lithium salt-containing non-aqueous electrolyte.

The secondary battery according to the present invention includes a plurality of battery cells used as a power source for a medium and large-sized device that can be used for a battery cell used as a power source of a small device, high temperature stability, long cycle characteristics, And may be suitably used as a unit cell in a middle- or large-sized battery module.

Preferred examples of the above medium to large devices include a power tool that is powered by an electric motor and moves; An electric vehicle including an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), and the like; An electric motorcycle including an electric bike (E-bike) and an electric scooter (E-scooter); An electric golf cart, and the like, but the present invention is not limited thereto.

As described above, the negative electrode active material of the present invention is composed of a combination of two kinds of negative electrode active materials that vary rolling characteristics and change characteristics of electrode orientation, so that a secondary battery having excellent output characteristics and lifetime characteristics can be provided.

1 is a graph showing a change in output according to SOC of a battery prepared by applying the negative electrode mixture slurry of the exemplary embodiments and comparative examples of the present invention;

Hereinafter, the present invention will be described in further detail with reference to the following examples. However, the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

 ≪ Example 1 >

Graphite powder was prepared by mixing the spherical natural graphite having an average particle size of 16 μm and the plate-shaped artificial graphite having an average particle size of 6 μm at a ratio of 8: 2 by weight, and the graphite powder was mixed with PVDF as a binder and 8: 2. After mixing in a weight ratio, the mixture was added to NMP to prepare a negative electrode mixture slurry.

≪ Comparative Example 1 &

A negative electrode mixture slurry was prepared in the same manner as in Example 1, except that natural graphite and artificial graphite were mixed in a ratio of 9: 1 by weight.

Comparative Example 2

A negative electrode mixture slurry was prepared in the same manner as in Example 1 except that natural graphite and artificial graphite were mixed at a ratio of 5: 5 by weight.

<Experimental Example 1>

After applying the negative electrode mixture slurry according to Example 1 and Comparative Examples 1 to 2 on a 10 μm thick copper foil, dried and roll press to a thickness of 60 μm to prepare a negative electrode, respectively An electrode assembly including a negative electrode was prepared. Using a 1M LiPF ₆ EC / DEC solution as the electrolyte, a battery containing each electrode assembly was prepared.

Each of the batteries prepared above was charged and discharged from SOC 10 to SOC 100, and the change in output at each SOC was measured for each 10 units, and the results are shown in Table 1 and FIG. 1.

Example 1 Comparative Example 1 Comparative Example 2 SOC 10 306 306 307.02 SOC 20 919.02 905.659 909.84 SOC 30 1241.34 1194.42 1202.58 SOC 40 1456.56 1367.82 1375.98 SOC 50 1605.48 1505.52 1513.68 SOC 60 1737.06 1633.02 1633.02 SOC 70 1882.92 1731.96 1735.02 SOC 80 2029.8 1841.1 1846.2 SOC 90 2138.94 1985.94 1997.16 SOC 100 2377.62 2150.16 2193

Referring to Table 1 and Figure 1, the battery produced by applying the negative electrode mixture slurry of Example 1 according to the present invention, the output is significantly improved compared to the battery prepared by applying the negative electrode mixture slurry of Comparative Examples 1 and 2 It can be confirmed. This is because spherical natural graphite, which has relatively good rolling properties and improves electrode orientation in the rolling process, and plate-shaped artificial graphite, which has relatively poor rolling properties and little change in electrode orientation in the rolling process, is included in a specific range of ratios. It shows that there is an effect that the change in the degree of orientation after rolling is small and the output of the battery can be improved while being good.

<Experimental Example 2>

The initial capacity of each of the batteries prepared in Experimental Example 1 was measured, and after performing 200 cycle charge / discharge between 2.5V and 4.2V under conditions of 1C and 45 degrees Celsius, the remaining capacity was measured. The retention rate was calculated and the results are shown in Table 2 below.

Initial capacity (mAh) 200 cycle capacity (mAh) Capacity retention rate (%) Example 1 21.45 20.987 97.85 Comparative Example 1 22.25 21.3 95.73 Comparative Example 2 21.64 20.822 96.22

Referring to Table 2, the battery prepared by applying the negative electrode mixture slurry of Example 1 according to the present invention, it can be confirmed that the capacity retention rate is improved compared to the battery prepared by applying the negative electrode mixture slurry of Comparative Examples 1 and 2 have. This is because spherical natural graphite, which has relatively good rolling properties and improves electrode orientation in the rolling process, and plate-shaped artificial graphite, which has relatively poor rolling properties and little change in electrode orientation in the rolling process, is included in a specific range of ratios. It shows that there is an effect that the change in the degree of orientation after rolling is small and the life characteristics of the battery can be improved.

Those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims (14)

As a negative electrode active material for a secondary battery, rolling includes a material (a) having a relatively good rolling property and improving the electrode orientation in the rolling process and a material (b) having a relatively poor rolling property and a small change in the electrode orientation in the rolling process. A negative electrode active material for secondary batteries, characterized in that the characteristics are good and the change in orientation after rolling is small, and thus the output and life characteristics are excellent. 2. The rolling properties of the material (a) according to claim 1, wherein the ratio of the short diameter to the long diameter of the particles before rolling is from 1: 1 to 1: 0.5 in a range in which the size of the short diameter is reduced after rolling. A negative electrode active material for a secondary battery, characterized by being changed to a range of 0.1. The rolling property of the material (b) according to claim 1, wherein the ratio of the short diameter to the long diameter of the particles before rolling is from 1: 1 to 1: 0.1 in the range where the size of the short diameter is reduced after rolling from 1: 0.9 to 1: 0.09. A negative electrode active material for a secondary battery, characterized in that is changed to the range. The negative electrode active material of claim 1, wherein the degree of orientation of the electrode after rolling is changed in a range of 0 to 40 degrees with respect to the surface of the current collector in terms of average crystal layer orientation of the particles. The negative electrode active material of claim 1, wherein the material (a) is spherical natural graphite and the material (b) is plate-shaped artificial graphite. The negative active material of claim 5, wherein the spherical natural graphite has a particle size (D50) of 15 to 20 µm. 7. The negative active material of claim 6, wherein the spherical natural graphite has a particle size (D50) of 16 to 18 µm. The negative electrode active material of claim 5, wherein the particle size (D50) of the plate-shaped artificial graphite is 5 to 10 μm. The negative active material of claim 8, wherein the particle size (D50) of the plate-shaped artificial graphite is 6 to 8 μm. The negative active material of claim 1, wherein the material (a) and the material (b) are mixed at a ratio of 6: 4 to 8.5: 1.5 by weight. The negative electrode for secondary batteries of any one of Claims 1-10 containing the other negative electrode active material for secondary batteries. A lithium secondary battery comprising the secondary battery negative electrode according to claim 11. The lithium secondary battery of claim 12, wherein the lithium secondary battery is used as a unit cell of a battery module which is a power source of a medium and large device. The lithium secondary battery of claim 13, wherein the medium-to-large size device is an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, or a system for storing power.

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