Cathode material composite

Abstract

The present invention relates to a cathode active material composite for a battery coated with reduced graphene oxide, specifically comprising a core portion including a cathode active material and a shell portion formed to surround the core portion, wherein the shell portion comprises one or more layers of reduced graphene oxide (rGO), thereby enabling the realization of a secondary battery with excellent electronic conductivity and energy density.

Description

Technology Field

[0001] The present invention relates to a cathode active material composite for a battery coated with reduced graphene oxide. Background Technology

[0002] With the recent increase in demand for lithium-ion batteries in transportation, the demand for high-capacity and high-energy-density batteries is surging. To develop high-capacity batteries, it is necessary to increase the thickness of the electrodes and form effective conductive paths to exchange electrons with the current collector. Among these, graphene, a next-generation material, is attracting attention for its excellent properties, including superior electronic conductivity and the ability to provide high-dimensional (high shape anisotropy) contacts.

[0003] Conventionally, there is a method for increasing the electronic conductivity of an active material using graphene that utilizes a wet coating method in which a carbon material (graphene) is dispersed in a solvent using a dispersant, and then an active material and a binder solution are added and mixed and dried.

[0004] However, even when using a highly dispersible graphene dispersion, it is difficult to maintain a uniform composition of graphene within the electrode due to the attractive forces between graphene particles during the electrode manufacturing and solvent drying processes. Consequently, a large amount of graphene material must be mixed during manufacturing to provide sufficient conductivity; however, since such a large amount of graphene material cannot contribute to the battery's capacity, there is a disadvantage of low electrode energy density. The problem to be solved

[0005] In order to solve the problems of the prior art as described above, the objective of the present invention is to provide a composite of a positive electrode active material for a battery that can improve the energy density of the electrode by achieving higher conductivity than a conventional graphene wet-coated positive electrode active material.

[0006] Another objective of the present invention is to provide a secondary battery electrode having low internal resistance and high energy density, comprising the above-described positive electrode active material composite.

[0007] Another objective of the present invention is to provide a secondary battery with improved efficiency and lifespan by including the secondary battery electrode as a positive electrode. means of solving the problem

[0008] To achieve the above objective, a battery positive active material composite according to one aspect of the present invention comprises a core portion containing a positive active material and a shell portion formed to surround the core portion, wherein the shell portion comprises one or more layers of reduced graphene oxide (rGO).

[0009] The average thickness of the shell portion may be 3 nm to 31 nm. In addition, the thickness deviation at each location of the shell portion may be within 55% of the average thickness of the shell portion.

[0010] The above reduced graphene oxide may contain 2 to 12 weight percent oxygen (O) relative to the graphene oxide, and the aspect ratio (long axis / thickness ratio) of the above reduced graphene oxide may be 500 to 20,000.

[0011] The specific surface area of ​​the above-mentioned cathode active material composite is 0.3 m² 2 / g to 5 m 2 It can be / g.

[0012] The reduced graphene oxide may be dry-coated to form one or more layers and included in the shell portion, and specifically, the reduced graphene oxide may be dry-coated using a mechano-fusion device. More specifically, the reduced graphene oxide may be dry-coated by running the mechano-fusion device at 1,000 to 10,000 rpm for 10 to 30 minutes.

[0013] A secondary battery electrode according to another aspect of the present invention comprises the positive electrode active material complex.

[0014] A secondary battery according to another aspect of the present invention comprises the secondary battery electrode as a positive electrode. Effects of the invention

[0015] The cathode active material composite for a battery according to the present invention facilitates the formation of a conductive network, thereby providing excellent electronic conductivity. Due to its low specific surface area, it can suppress the increase in internal resistance of the electrode caused by adverse reactions with the electrolyte. Furthermore, due to its excellently developed pore structure incorporated into the electrode, it has the effect of improving the electrolyte impregnation rate and enabling easy diffusion of ions.

[0016] Furthermore, a secondary battery comprising the cathode active material composite according to the present invention exhibits high efficiency due to the excellent electronic and ionic conductivity of the electrode, thereby enabling high charge and discharge capacities. Additionally, side reactions between the cathode active material and the electrolyte are suppressed, resulting in excellent battery life characteristics. Brief explanation of the drawing

[0017] FIG. 1 briefly illustrates the structure of a positive electrode active material composite for a battery according to one aspect of the present invention. FIG. 2 is a transmission electron microscope (TEM) image of various sides of the core (NCM 811) and shell (Graphene) of an electrode positive active material composite (10) according to one embodiment of the present invention. Figure 3 shows data obtained by analyzing a portion of the constituent elements of a positive electrode active material composite for a battery according to one embodiment of the present invention using energy dispersive spectroscopy (EDS). FIG. 4 is a scanning electron microscope (SEM) image showing the overall shape of a battery positive active material composite according to one embodiment of the present invention (a, Example 3), a non-coated positive active material (bare, Comparative Example 1), and a case where the positive active material is dry-coated with carbon black (c, Comparative Example 3). Figure 5 is a magnified SEM (scanning electron microscope) image of each of the surfaces of Figures 4a, b, and c. FIG. 6 shows the discharge capacity for the C-rate for Example 1, Comparative Example 1, and Comparative Example 3 of the present invention. Figure 7 shows electrochemical impedance spectroscopy (EIS) data for Example 1, Comparative Example 1, and Comparative Example 3 of the present invention. FIG. 8 is an SEM image of an electrode prepared according to one embodiment of the present invention (Example 3) and an electrode with rGO simply stirred with a positive electrode active material (Comparative Example 4). Specific details for implementing the invention

[0018] Terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted in a meaning and concept consistent with the technical spirit of the invention, based on the principle that the inventor can appropriately define the concept of the terms to best describe his invention.

[0019] Therefore, the configurations illustrated in the embodiments and manufacturing examples described in this specification are merely one preferred embodiment of the present invention and do not represent all of the technical ideas of the present invention; thus, it should be understood that various equivalents and modifications that can replace them may exist at the time of filing this application.

[0020] Hereinafter, with reference to the drawings, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the invention. However, the present invention may be embodied in various different forms and is not limited to the manufacturing examples and embodiments described herein.

[0021] FIG. 1 briefly illustrates the structure of a positive electrode active material composite for a battery according to one aspect of the present invention. Referring to FIG. 1, a positive electrode active material composite (10) for a battery according to one aspect of the present invention comprises a core portion (11) containing a positive electrode active material and a shell portion (12) formed to surround the core portion (11), wherein the shell portion (11) is characterized by having one or more layers of reduced graphene oxide (rGO) formed therein. Specifically, FIG. 4(a) is a photograph of a positive electrode active material for a battery according to one embodiment of the present invention taken using a scanning electron microscope (SEM).

[0022] In the cathode active material composite (10) of the present invention, the shell portion (12) is formed such that reduced graphene oxide flakes are physically bonded to the surface of the cathode active material by van der Waals forces, and the reduced graphene oxide has a structure in which a single layer or multiple layers are attached, forming one or more layers.

[0023] The shell portion (12) is formed after reduced graphene oxide flakes of nanoparticles are bonded to the surface of the positive active material by van der Waals forces, and the thickness of the shell portion can be formed thinly and uniformly. In the present invention, the average thickness of the shell portion of the positive active material composite refers to the thickness of the reduced graphene oxide in the normal direction from the surface of the positive active material, and refers to the average of the thicknesses measured at five or more different locations on the surface of the positive active material composite. FIG. 2 is a transmission electron microscope (TEM) image of several sides of the core portion (NCM 811) and the shell portion (Graphene) of the positive active material composite (10) for electrodes according to an embodiment of the present invention, showing the thickness of the shell portion at each side. Referring to FIG. 2a to c, it can be seen that the shell portion is partially formed with thicknesses of 15 nm, 24 nm, 23 nm, 31 nm, and 10 nm, respectively. Specifically, the average thickness of the shell is 3 nm to 31 nm, which is advantageous in terms of improved conductivity due to graphene and the battery capacity of the cathode active material. If the thickness of the shell is less than 3 nm, the electronic conductivity of the cathode active material is significantly reduced, which can lead to a significant drop in capacity when applied to a battery. If the thickness of the shell is thicker than 31 nm, the diffusion of lithium ions into the cathode active material is reduced, resulting in a problem of low capacity development.

[0024] Reduced graphene oxide included in the shell portion forms one or more layers in the form of nanoflakes through van der Waals forces, and the shell portion can be formed with a thin and uniform thickness. When the thickness deviation at each location of the shell portion is defined as the difference between the average thickness of a single cathode active material composite and the maximum thickness of the shell portion or the difference between the average thickness of a single cathode active material composite and the minimum thickness, the thickness deviation at each location of the shell portion may be within 55% of the average thickness of the shell portion. For example, referring to FIG. 2, the average thickness at five different points (15 nm, 24 nm, 23 nm, 31 nm, 10 nm) on the surface of a single cathode active material composite of FIG. 2 according to one embodiment of the present invention is 20.6 nm. At this time, the difference between the average thickness (20.6 nm) and the minimum thickness (10 nm) is 10.6 nm, which is 51.5% of the average thickness value, and the difference between the average thickness (20.6 nm) and the maximum thickness (31 nm) is 10.4 nm, which is 50.5% of the average thickness value. If the thickness deviation exceeds 55% of the average thickness, localized lithium ion and electron transfer occurs, which reduces the uniformity of the performance of the cathode active material composite and may lead to a deterioration of the battery's lifespan.

[0025] In addition, when a shell is formed by coating the surface of the core with reduced graphene oxide through dry coating, the reduced graphene oxide flake particles bonded to the maximum thickness portion of the shell are detached from the shell during the dry coating process, making it very easy to physically bond to the minimum thickness portion of the shell, and the thickness variation of the shell can be easily controlled to 55% or less of the average thickness.

[0026] The core portion of the positive electrode active material for a battery according to the present invention can be used without limitation as long as it is a positive electrode active material used in a secondary battery, for example, LiCoO2 (LCO), Li[Ni,Co,Mn]O2 (NCM), Li[Ni,Co,Al]O2 (NCA), LiMn2O4 (LMO), or LiFePO4 (LFP), and specifically, NCM 811 composed of nickel (Ni), cobalt (Co), and manganese (Mn) can be used, but it is not particularly limited as long as the reduced graphene oxide can form a shell portion on the surface of the positive electrode active material.

[0027] The shell portion of the positive electrode active material for a battery according to the present invention is formed by including reduced graphene oxide and contains carbon and oxygen. The positive electrode active material of the core portion has a hydrophilic surface due to impurities such as LiOH and Li2CO3 during the synthesis process. At this time, the oxygen contained in the reduced graphene oxide imparts hydrophilicity to the reduced graphene oxide, thereby helping the reduced graphene oxide and the positive electrode active material of the core portion to easily physically bond. Specifically, it is advantageous for the reduced graphene oxide included in the shell portion to contain 2 to 12 weight percent of oxygen (O) relative to the graphene oxide when considering uniform coating and electronic conductivity. If the amount of oxygen (O) in the reduced graphene oxide of the shell portion is less than the above range, the reduced graphene oxide may not be sufficiently attached to the surface of the positive electrode active material by physical bonding, which may cause a problem in forming the shell portion; conversely, if the amount of oxygen is greater than the above range, there may be a problem in significantly reduced electronic conductivity.

[0028] Figure 3 shows the constituent elements of a portion of a cathode active material composite for a battery according to one embodiment of the present invention, and in the embodiment, Ni, Co, and Mn are observed using NCM 811 as the cathode active material, and C and O are observed as elements of reduced graphene oxide.

[0029] The shape of reduced graphene oxide can influence the process of coating the cathode active material of the core portion. Specifically, the shape of the shell portion is determined by the shape of the reduced graphene oxide, and among the shapes of the reduced graphene oxide, the aspect ratio (ratio of the major axis to the thickness) can act as the most important factor. In the present invention, the aspect ratio (ratio of the major axis to the thickness) of the reduced graphene oxide refers to the ratio of the maximum length (width) to the thickness of the plate-like structure of the reduced graphene oxide; specifically, it refers to the ratio of the maximum length (major axis) to the thickness of the plate-like structure in the reduced graphene oxide formed as a plate-like single-layer or multi-layer structure. According to the above definition, if the aspect ratio of the reduced graphene oxide is less than 500, the cohesive force between the graphene planes becomes excessively strong, making it difficult to achieve a uniform coating, and if it exceeds 20,000 and becomes thin and wide, the path for lithium ions to bypass the graphene becomes long, making it difficult, so the aspect ratio of the reduced graphene oxide is preferred to be between 500 and 20,000.

[0030] A smaller specific surface area of ​​the cathode active material composite is advantageous because it suppresses adverse reactions with the electrolyte within the electrode. The specific surface area of ​​the cathode active material composite can be controlled by adjusting the average thickness of the shell portion. Specifically, the specific surface area of ​​the cathode active material composite tends to decrease as the average thickness of the shell portion increases, while conversely, the specific surface area of ​​the composite tends to increase as the average thickness of the shell portion decreases. The cathode active material composite for a battery according to the present invention may be formed in a shape as shown in FIG. 4a as an example, and the surface may be formed as shown in FIG. 5a (Example 3), so that the specific surface area may be significantly lower than that of the cathode active material (b, Comparative Example 1) or the case where the cathode active material is dry-coated with carbon black (c, Comparative Example 3). Specifically, the specific surface area of ​​the cathode active material composite according to one embodiment of the present invention is 0.3 m² 2 / g to 5 m 2It can be / g. Due to the characteristic that reduced graphene oxide attaches to the surface of the cathode active material in flake form and forms a shell, 0.3 m 2 It is difficult to form a cathode active material complex with a specific surface area of ​​less than / g, and a specific surface area of ​​5 m² 2 If the value exceeds / g, the frequency of side reactions with the electrolyte increases significantly, and there may be a problem in that an excessive amount of side reaction layers are formed, which increases the internal resistance of the electrode along with gas generation.

[0031] In order to form one or more layers of reduced graphene oxide on the surface of the core portion of the cathode active material and to achieve a low specific surface area of ​​the composite and high porosity when included in the electrode, it is preferred that the reduced graphene oxide be dry-coated during the process of forming the shell portion. Unlike conventional wet coating, dry coating is characterized by the fact that particles of reduced graphene oxide are bonded by electrostatic attraction to form the shell portion, and a separate dispersant is not required. Therefore, the problem of increased internal resistance of the electrode caused by residual dispersant in conventional wet coating can be resolved. In addition, in the case of dry coating, the composite has high compressive strength, which allows for higher porosity of the electrode after rolling compared to wet coating or carbon materials. High electrode porosity improves the electrolyte impregnation rate, which can accelerate the diffusion rate of lithium ions within the battery, thereby improving battery efficiency. Furthermore, as described above, dry coating also has the effect of controlling the thickness variation of the shell portion.

[0032] The method of dry-coating the core portion with reduced graphene oxide to form the shell portion is not particularly limited; however, dry-coating can be performed more efficiently when using a mechano-fusion device to dry-coate the reduced graphene oxide. A mechano-fusion device is a method of fusing nanoparticles onto the surface of large particles by applying mechanical energy to the particle surface using compression and shear force. When used in the process of forming the cathode active material composite of the present invention, it fuses reduced graphene oxide, which is a flake-shaped nanoparticle, onto the surface of the cathode active material, which is a relatively large particle corresponding to the core portion, thereby forming the cathode active material composite. When using a mechano-fusion device, the reduced graphene oxide can be pulverized into nanoparticles by compression, the reduced graphene oxide particles can be evenly dispersed across the entire surface of the cathode active material by shear force, and the cathode active material and the reduced graphene oxide can be bonded by van der Waals forces.

[0033] When performing dry coating using a meccano-fusion device, the meccano-fusion device can operate at 1,000 to 10,000 rpm for 10 to 30 minutes, and for example, operating at 6,000 to 10,000 rpm for 10 to 30 minutes may be preferred for effective dry coating.

[0034] A secondary battery electrode according to another aspect of the present invention is characterized by comprising a positive electrode active material composite according to the present invention. Due to the positive electrode active material composite, a dispersant may not be used in the secondary battery electrode. Additionally, the electrode may specifically be applied to a positive electrode by including a positive electrode active material.

[0035] A secondary battery according to another aspect of the present invention is characterized by including the secondary battery electrode as a positive electrode. The secondary battery may be, for example, a lithium secondary battery, but depending on the type of positive electrode active material, it may be applied without being particularly limited to the type of secondary battery, such as a lead-acid battery, a nickel-cadmium battery, a nickel-hydrogen battery, or a lithium-ion battery.

[0036] Hereinafter, preferred embodiments are presented to aid in understanding the present invention; however, the following embodiments are merely illustrative of the invention and the scope of the invention is not limited to the following embodiments.

[0037] [Preparation Example: Preparation of a Cathode Active Material Composite]

[0038] 20 g of NCM811 cathode active material (L&F, NIB-X10B) and 0.2 g of reduced graphene oxide (rGO) flakes (100:1 ratio) were placed in a dry coating (mechano-fusion) machine (Fristch, pulverisette 14) and coated at 8,000 rpm for 10 minutes to prepare a cathode active material composite (Example 3), which is an embodiment of the present invention. Examples 1 to 7 were prepared by varying the ratio of the cathode active material to the reduced graphene oxide to change the thickness of the shell portion. In addition, Examples 8 to 24 were prepared by changing the aspect ratio of the rGO (500 to 20,000), the thickness variation of the shell portion, and the oxygen content. Reference Example 1 and Comparative Examples 1 to 5 were also prepared by varying the composition of the shell portion or the coating method to compare the effects with the examples. The constituent materials and coating methods of the examples and comparative examples are shown in Table 1 below. In this case, in Reference Example 1 and Comparative Example of Table 1, 'aspect ratio satisfaction' means the case where the aspect ratio (long axis / thickness ratio) of the graphene is 500 to 20,000.

[0039] positive electrode active material Shell (coating material) Coating method Examples 1–24 Li[NiCoMn]O2 rGO Dry coating Reference Example 1 Li[NiCoMn]O2 rGO (aspect ratio satisfied) Wet coating Comparative Example 1 Li[NiCoMn]O2 X(bare) - Comparative Example 2 Li[NiCoMn]O2 Carbon black Wet coating Comparative Example 3 Li[NiCoMn]O2 Carbon black Dry coating Comparative Example 4 1) Li[NiCoMn]O2 rGO (aspect ratio satisfied) Electrode coating (high-pressure dispersion and simple stirring) Comparative Example 5 Li[NiCoMn]O2 GO (satisfies aspect ratio), insufficient electron transfer due to very low conductivity Dry coating

[0040] 1) In the case of the electrode coating of Comparative Example 4, a shell portion was not formed due to simple stirring.

[0042] [Experimental Example 1: Evaluation of Physical Properties of Anode Active Material Composite]

[0043] The physical properties of Examples 1 to 24 prepared according to Preparation Example 1 were measured using TEM analysis, specific surface area measuring instruments, AFM, and XPS, and the results are shown in Table 2 below.

[0044] Average thickness of the shell (nm) Specific surface area of ​​the complex (m²) 2 / g) rGO aspect ratio Shell thickness deviation (average thickness %) Oxygen content of the shell (%) Example 1 3 3.75 3379 Within 55% 2 Example 2 8 1.95 3379 Within 55% 2 Example 3 17 0.74 3379 Within 55% 2 Example 4 25 0.59 3379 Within 55% 2 Example 5 30 0.32 3379 Within 55% 2 Example 6 1.5 4.93 3379 Within 55% 2 Example 7 35 0.27 3379 Within 55% 2 Example 8 17 0.79 350 (7um, 20nm) Within 55% 2 Example 9 17 0.72 500 (5um, 10nm) Within 55% 2 Example 10 17 0.73 10000 (20um, 2nm) Within 55% 2 Example 11 17 0.71 15000 (30nm, 2nm) Within 55% 2 Example 12 17 0.74 19500 (39um, 2nm) Within 55% 2 Example 13 17 0.70 25000 (50um, 2nm) Within 55% 2 Example 14 17 0.74 3379 Within 20% 2 Example 15 17 0.74 3379 Within 25% 2 Example 16 17 0.74 3379 Within 30% 2 Example 17 17 0.74 3379 Within 45% 2 Example 18 17 0.74 3379 Within 65% 2 Example 19 17 0.74 3379 Within 55% 1 Example 20 17 0.74 3379 Within 55% 4 Example 21 17 0.74 3379 Within 55% 6 Example 22 17 0.74 3379 Within 55% 8 Example 23 17 0.74 3379 Within 55% 12 Example 24 17 0.74 3379 Within 55% 15

[0046] [Preparation Example 2: Preparation of a secondary battery cathode]

[0047] 20 g each of the Example and Comparative Examples of Preparation Example 1, 0.2040 g of a carbon nanotube and carbon black mixed NMP dispersion (based on solid content), and 0.2040 g of a PVDF-NMP binder solution (based on solid content) were weighed (solid ratio 98:1:1), and a mixed electrode slurry was prepared using a Thincky mixer at 800 rpm for 20 minutes. After casting the electrode slurry using a blade, the solvent was dried to obtain a loading amount of 16 mg / cm². 2 Obtain the electrode.

[0049] [Experimental Example 2: Evaluation of Physical Properties of Secondary Battery Cathode]

[0050] For the secondary battery cathode prepared according to Preparation Example 2, the physical properties and electrochemical characteristics of the secondary battery cathode containing the cathode active material composite of each example and comparative example were measured using the charge / discharge evaluation method and the method using a powder resistor, and the results are shown in Table 3 below. In addition, the discharge capacity with respect to the C-rate for Example 1, Comparative Example 1, and Comparative Example 3 is shown in Figure 6, and the results of the Electrochemical Impedance Spectroscopy (EIS) measurement are shown in Figure 7.

[0051] 3C High Rate Charge / Discharge Capacity (Relative to 0.1C Capacity, %) Powder conductivity (63.66 MPa, S / cm) Example 1 58.7 2.7 * 10 -2 Example 2 60.4 3.2 * 10 -2 Example 3 60.9 4.1 * 10 -2 Example 4 55.9 5.6 * 10 -2 Example 5 52.7 6.1 * 10 -2 Example 6 50.3 2.0 * 10 -2 Example 7 49.4 7.7 * 10 -2 Example 8 47.2 3.1 * 10 -2 Example 9 54.3 3.5 * 10 -2 Example 10 58.1 4.7 * 10 -2 Example 11 53.7 5.2 * 10 -2 Example 12 50.2 5.4 * 10 -2 Example 13 45.3 5.9 * 10 -2 Example 14 61.6 4.5*10 -2 Example 15 61.3 4.4*10 -2 Example 16 61.0 4.2 * 10 -2 Example 17 58.7 4.1 * 10 -2 Example 18 53.4 3.9 * 10 -2 Example 19 50.4 5.8 * 10 -2 Example 20 61.0 2.4 * 10 -2 Example 21 58.1 1.1 * 10 -2 Example 22 53.6 8.7 * 10 -3 Example 23 52.8 5.3 * 10 -3 Example 24 49.7 2.1 * 10 -3 Reference Example 1 40.3 4.8 * 10 -3 Comparative Example 1 35.3 1.1 * 10 -4 Comparative Example 2 20.4 2.2 * 10 -3 Comparative Example 3 38.7 9.5 * 10 -3 Comparative Example 4 18.1 - Comparative Example 5 3.4 3.5 * 10 -6

[0052] Referring to Table 3, it can be seen that the 3C high-rate charge / discharge capacities of Examples 1 to 24 are generally higher than those of Comparative Examples 1 to 5 and Reference Example 1. Additionally, through Figure 6, it can be seen that while the discharge capacity at 3C of Comparative Examples 1 and 3 decreases significantly, the discharge capacity retention rate of Example 1 is maintained excellently compared to the Comparative Examples.

[0053] Among the examples, regarding the aspect ratio of rGO, referring to Tables 2 and 3, the aspect ratio of rGO (350) of Example 8 and the aspect ratio of rGO (25000) of Example 13 fall outside the range of 500 to 20,000, and it can be confirmed that the high-rate charge / discharge capacity of the remaining examples within the range of 500 to 20,000 is measured to be higher than that of Examples 8 and 13. Through this, it can be seen that when the aspect ratio of rGO is 500 to 20,000, a superior high-rate charge / discharge capacity is achieved. In addition, in Example 8, it can be confirmed that as the aspect ratio increases, distortion of the lithium ion diffusion path occurs, and while the powder conductivity increases, the high-rate charge / discharge capacity decreases.

[0054] And regarding the shell thickness, referring to Tables 2 and 3, Example 6 has an average shell thickness of 1.5 nm, and Example 7 has an average shell thickness of 35 nm. Considering that the shell thickness and the composite specific surface area have a dependent relationship, when comparing Example 3 with Examples 6 and 7, where the average shell thickness falls within the range of 3 to 31 nm (17 nm) and other factors are identical, it can be confirmed that the high-rate charge / discharge capacity of Example 3 is superior. In other words, it can be seen that the battery performance is better when the average shell thickness is between 3 and 31 nm.

[0055] Referring to Tables 2 and 3 regarding the thickness variation of the shell portion, Example 18 has a shell portion thickness variation exceeding 55% (65%), and it can be confirmed that Example 3 has superior high-rate charge / discharge capacity compared to Example 18 when compared to Example 3, which has the same conditions. Through this, it can be seen that the battery performance is superior when the shell portion thickness variation is within 55%. Furthermore, in the case of Examples 14 to 16, which have superior shell portion thickness uniformity (smaller thickness variation), it can be confirmed that the high-rate charge / discharge capacity is superior compared to Example 3, which has the same other factors. This can be interpreted as being due to the fact that as the thickness uniformity of the shell portion increases, Li ions can be uniformly fluxed into the composite.

[0056] Referring to Tables 2 and 3 regarding the oxygen content of the shell, it can be seen that for Examples 20 to 24, which have high oxygen content, the powder conductivity decreases with increasing oxygen content compared to Example 3, which is under the same conditions. In the case of Example 24, which has particularly high oxygen content, it can be seen that both the powder conductivity and the high-rate charge / discharge capacity decrease significantly. Additionally, for Example 19, which has low oxygen content in the shell, it can be seen that the high-rate charge / discharge capacity decreases significantly compared to Example 3. Through this, it can be seen that the battery performance is particularly excellent when the oxygen content of the shell is 2 to 12 weight%.

[0057] Figure 7 shows the electrochemical impedance spectroscopy (EIS) data of Example 1, Comparative Example 1, and Comparative Example 3. Referring to Figure 7, it can be seen that Example 1, coated with reduced graphene oxide, has a lower semicircle size (resistance of SEI formed by the by-reaction layer + electron transfer resistance) than Comparative Example 1, which is uncoated, and Comparative Example 3, which is simply coated with carbon black. This can be interpreted as Example 1, coated with reduced graphene oxide, suppressing the formation of by-reaction products and properly forming an electron transfer pathway. Additionally, the results of Figure 7 can be interpreted as supporting the excellent discharge capacity retention effect of the example shown in Figure 6.

[0058] Figure 8 shows SEM images of Example 3(a), in which the cathode active material was dry-coated with rGO and high-speed stirred, and Comparative Example 4(b), in which the cathode active material and rGO flakes were dispersed under high pressure and simply stirred. Referring to Figure 8, it can be seen that the shape of the composite is well maintained in the case of the cathode (a) prepared according to the present invention, but when only rGO flakes are included without dry coating, it can be seen that agglomeration of the cathode active material occurs. Therefore, it can be seen that rGO must be included by coating it onto the surface of the cathode active material, rather than simply being included, to significantly contribute to the improvement of battery performance.

[0059] Although embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and it will be obvious to those skilled in the art that various modifications and variations are possible within the scope of the technical concept of the present invention as described in the claims. Explanation of the symbols

[0060] 10: Cathode active material complex 11: Core 12: Shellbu

Claims

Claim 1 A battery positive active material composite comprising: a core portion including a positive active material; and a shell portion formed to surround the core portion; wherein the shell portion includes one or more layers of reduced graphene oxide (rGO) and the aspect ratio (long axis / thickness ratio) of the reduced graphene oxide is 500 to 20,000. Claim 2 A positive electrode active material composite for a battery, wherein, in claim 1, the average thickness of the shell portion is 3 nm to 31 nm. Claim 3 A positive electrode active material composite for a battery according to claim 1, wherein the thickness deviation of the shell portion is within 55% of the average thickness of the shell portion. Claim 4 A positive electrode active material composite for a battery, wherein the reduced graphene oxide comprises 2 to 12 weight percent oxygen (O) relative to the reduced graphene oxide. Claim 5 A positive electrode active material composite for a battery according to claim 1, wherein the aspect ratio (long axis / thickness ratio) of the reduced graphene oxide is 3,379 to 19,500. Claim 6 In paragraph 1, the specific surface area is 0.3 m² 2 / g to 5 m 2 A positive electrode active material composite for batteries, with a phosphorus content of / g. Claim 7 A positive electrode active material composite for a battery, wherein the reduced graphene oxide is dry-coated to form one or more layers and is included in the shell portion. Claim 8 In claim 7, the reduced graphene oxide is a battery positive electrode active material composite, which is dry-coated using a mechano-fusion device. Claim 9 In claim 8, the reduced graphene oxide is a battery positive electrode active material composite, which is dry-coated by running a mechano-fusion device at 1,000 to 10,000 rpm for 10 to 30 minutes. Claim 10 A secondary battery electrode comprising a positive active material complex according to any one of claims 1 to 9. Claim 11 A secondary battery comprising the secondary battery electrode of claim 10 as a positive electrode.

Images