Cathode active material and secondary battery including same

Abstract

The present invention provides a cathode active material characterized by comprising a transition metal and having a hardness index defined by the following formula in the range of 15 to 27. [Formula I] In the formula, D: D50 (㎛) of cathode active material particles analyzed from PSD; G: Crystal grain size (nm); C: c-axis lattice constant (Å) analyzed from XRD; and R: R-factor analyzed from XRD.

Description

positive electrode active material and secondary battery containing the same

[0001] The present invention relates to a positive electrode active material and a secondary battery containing the same, and more specifically, to a positive electrode active material comprising a transition metal and satisfying a specific numerical range of a Hardness Index defined by a c-axis lattice constant, crystal grain size, etc., and a secondary battery containing the same.

[0002] As the battery market, such as electric vehicles, has been growing rapidly recently, the demand for lithium-ion batteries is also on the rise.

[0003] Among the required characteristics of these lithium secondary batteries for electric vehicles, output characteristics are related to the charging speed and instantaneous acceleration capability of the electric vehicle, and various characteristics such as short charging time and long driving range are required.

[0004] Therefore, there is a high need for lithium secondary batteries with excellent output characteristics and electrochemical characteristics.

[0005] In general, much research is underway to improve the output characteristics of the key materials of lithium-ion batteries—the cathode, anode, separator, and electrolyte—and among them, research on cathode active materials, the most critical component, is particularly active.

[0006] Representative cathode active materials requiring high power output and high durability include lithium transition metal oxides with a high nickel (Ni) content, and based on this, high capacity can be achieved to increase the mileage of an electric vehicle on a single charge.

[0007] However, due to the unstable nature of Ni, as its content increases, residual lithium byproducts such as Li2CO3 and LiOH are generated during the active material calcination process, and anisotropic shrinkage / expansion occurs during charging and discharging, leading to deterioration of the crystal structure and causing problems such as gelation of the slurry and reduced battery stability.

[0008] To improve the characteristics of cathode active materials that deteriorate due to these problems, a method to improve particle strength has been proposed. Through this, it is possible to improve the lifespan of secondary batteries by minimizing the occurrence of fine powder and cracks during the pressing process, which corresponds to the rolling process in the manufacturing process of electrode plates.

[0009] In this regard, in order to bring about an improvement in particle strength, it is first required to clearly identify the correlation between the physical factors of the cathode active material and the particle strength.

[0010] The present invention aims to solve the problems of the prior art described above and technical challenges that have been requested over time.

[0011] After conducting in-depth research and various experiments, the inventors of the present application were able to create a new relationship equation that can identify the correlation between certain factors of the cathode active material and particle breakage before solving the problem of material property degradation caused by particle breakage during the rolling process. They confirmed that the above-mentioned problem can be solved by exhibiting a desired particle strength when the value defined in this relationship equation satisfies a specific numerical range, and thus completed the present invention.

[0012] Accordingly, the positive electrode active material according to the present invention comprises a transition metal and is characterized by having a Hardness Index defined by the following formula in the range of 15 to 27.

[0013]

[0014] In the above formula,

[0015] D: D50 (㎛) of the positive active material particles analyzed from PSD (particle size distribution);

[0016] G: Grain size (nm);

[0017] C: c-axis lattice constant (Å) analyzed from XRD (X-ray diffraction); and

[0018] R: R-factor analyzed from XRD.

[0019]

[0020] In the equation defining the Hardness Index above ('Hardness Index Equation'), each factor directly or indirectly affects the particle strengths, and it can be confirmed from the experimental details described later that excellent battery characteristics, particularly outstanding lifespan characteristics, can be obtained only when the Hardness Index derived from the interrelationships of these factors satisfies specific range conditions.

[0021]

[0022] The 'particle strength' defined in this invention can be understood as the opposite concept of 'particle breakage'.

[0023] Specifically, in the case of polycrystalline cathode active materials with many grain boundaries, the large specific surface area resulting from the numerous grain boundaries and the fine particles generated therefrom leads to an increase in reaction sites with the electrolyte, and tends to have lower lifespan characteristics compared to single-crystal materials.

[0024] As previously explained, since such positive electrode active materials can improve the lifespan of secondary batteries by minimizing the occurrence of fine particles and cracks during the rolling process in the manufacturing process of electrode plates through an increase in particle strength, particle strength can be defined by evaluating the degree of fine particle generation (particle breakage) during pressing in this way.

[0025] For example, after weighing a specific amount of cathode active material powder, the particle size distribution can be obtained by pressing it with a pressure of 4 tons (4 MPa) using a Pellet Die (Carver; #3619 13mm pellet Die) and measuring it with an analyzer (Malvern; PSA Analyzer), and the particle strength can be determined from the percentage of the fine particle region of 1 μm or less.

[0026] Figure 1 shows the particle size distribution (PSD) before and after pressing the cathode active material as described above, and in particular, shows an enlarged view of the changes in the fine particle region of 1 μm or less. Referring to Figure 1, it can be seen that the volume of the powder in the fine particle region of 1 μm or less increased significantly after pressing. Therefore, the particle strength can be indirectly estimated based on the volume fraction of the fine particle region of 1 μm or less; thus, it can be determined that if the volume fraction of the fine particle region increases due to particle breakage, the particle strength is low, and conversely, if the volume fraction of the fine particle region decreases, the particle strength is high. This can also be confirmed in the last item of the results in Table 1, which will be explained later.

[0027]

[0028] In the above Hardness Index formula, first, regarding D50 ('D'), theoretically, grain strength tends to decrease as D50 increases, because the larger the grain size, the greater the possibility of internal defects (grain boundaries, cavities, etc.) forming.

[0029] Second, regarding the grain size ('G'), it can be expected that the grain strength increases as the grain size decreases, because as the grain size decreases, there are more surfaces capable of absorbing external forces. However, if the grain size is excessively small, the grain strength may start to decrease as the likelihood of fracture along the grain boundaries increases due to an increase in grain boundaries. In one preferred example, the grain size may be in the range of 149 to 194 nm.

[0030] Third, regarding the c-axis lattice constant ('C'), based on the potential energy curve shown in Figure 2, if the c-axis lattice constant becomes smaller than the optimal distance between atoms, the bonding force weakens due to interatomic repulsion, which causes a decrease in particle strength. On the other hand, if the c-axis lattice constant becomes excessively large, the interatomic bonding force weakens, which also causes a decrease in particle strength.

[0031] The inventors of the present application examined the correlation between the volume fraction of the fine region of 1 μm or less and particle strength, i.e., particle fracture, with respect to various factors, and in particular, confirmed that the c-axis lattice constant has a high correlation. Figure 3a shows a graph showing the correlation between the c-axis lattice constant and particle fracture, Figure 3b shows a graph showing the correlation between crystal grain size and particle fracture, and Figure 3c shows a graph showing the correlation between D50 and particle fracture. In these graphs, it can be seen that the correlation of the c-axis lattice constant is significantly higher.

[0032] In one preferred example, the c-axis lattice constant may be in the range of 14.2096 to 14.2467 Å.

[0033] Fourth, regarding the R-factor ('R'), the R-factor represents hexagonal ordering, or crystallinity, and a smaller value indicates higher crystallinity. Generally, one might expect that higher crystallinity—that is, a smaller R-factor—would lead to stronger grain strength; however, this appears to apply only to cases involving single crystals. In practice, since typical cathode active materials are polycrystalline with crystal structures in various directions, a complex intertwining of these crystal structures may actually be advantageous in terms of grain strength. Therefore, conversely, lowering crystallinity—that is, increasing the R-factor—may result in improved grain strength. However, if crystallinity becomes too low, the number of defects within the grain increases, which can have an adverse effect on grain strength.

[0034] In one preferred example, the R-factor may be in the range of 0.499 to 0.564.

[0035]

[0036] The control of these various factors (D50, grain size, c-axis lattice constant, R-factor) can also be achieved through partial changes or modifications in the manufacturing process of the cathode active material. For example, increasing the calcination temperature tends to decrease the c-axis lattice constant and increase the grain size. Additionally, as the Li / M ratio increases, the c-axis lattice constant decreases, and as the rolling intensity during the process increases, the D50 and grain size tend to decrease. Thus, these factors can be controlled by various elements.

[0037] When the Hardness Index formula defined by the above factors is applied to the experimental results described later, a graph as shown in Fig. 4 is obtained.

[0038] Referring to Fig. 4, there is a very high correlation with grain breakage (R) in the Hardness Index range of 15 to 27. 2It can be seen that it has (=0.9882). In particular, it can be confirmed that it exhibits the lowest grain fracture, that is, the highest grain strength, in the range of Hardness Index 16 to 22.

[0039]

[0040] The positive active material of the present invention may preferably have a composition including Ni as a transition metal.

[0041] As previously explained, while Ni is an essential transition metal for realizing high-content secondary batteries, it has disadvantages such as the generation of residual lithium byproducts and the degradation of the crystal structure. In this invention, however, it is highly suitable as a cathode active material that can improve electrochemical properties, such as lifespan characteristics, by increasing particle strength.

[0042] This high content of Ni may preferably be 50 mol% or more, more preferably 80 to 98 mol%, based on the total amount of transition metals.

[0043] The positive active material of the present invention may additionally include transition metals such as Co and Mn in addition to Ni, and as a non-limiting example, may have a composition such as that of Chemical Formula 1 below.

[0044] Li a Ni x Co y Mn z O 2+b (1)

[0045] In the above equation, 0 <a≤1.2, 0≤b≤0.2, 0.5<x<1, 0<y<0.5, 0<z<0.5, 0≤2-(a+x+y+z)≤0.2 이다

[0046]

[0047] In one preferred example, the (003) plane / (104) plane peak ratio measured during XRD analysis may be in the range of 2.08 to 2.13.

[0048] In layered structural materials with high Ni content, Ni 2+ is Li+ Due to its low diffusivity and similar atomic radius, it can diffuse into the Li layer. In this case, Li in the Li layer + and Ni 2+ As the degree of mixing of cations occurs, the layered structure changes into a spinel structure as the degree of mixing increases, degrading electrochemical properties by decreasing the size of the (003) peak and increasing the size of the (104) peak. Accordingly, the degree of mixing I (003) / I (104) It is expressed as a ratio, and as the intensity ratio increases, crystallinity increases and the degree of cation mixing decreases. In other words, as the peak ratio increases, the spinel structure becomes smaller and the grain strength increases due to increased layered structure crystallinity; however, excessive increase in crystallinity (greater than 2.13) may actually weaken the grain strength due to the formation of clear boundaries between crystal structures.

[0049] In another preferred example, the ratio (c / a) of the c-axis lattice constant and the a-axis lattice constant measured during XRD analysis may be in the range of 4.9390 to 4.9483.

[0050] The c / a ratio is an indicator of the degree of development of the two-dimensional planar structure within the layered structure; as the value increases, the diffusivity of lithium may be improved due to the increase in the 6c-site interlayer, but in terms of particle strength, the particle strength may be good when the 6c-site interlayer is present in an appropriate ratio.

[0051] The desirable range of the above (003) plane / (104) plane peak ratio and c / a ratio is also demonstrated in the experimental results described later.

[0052]

[0053] The present invention also provides a secondary battery characterized by including the above positive active material.

[0054] Since the composition of secondary batteries and the method of manufacturing them are known in the art, a detailed description thereof is omitted in this specification.

[0055] As explained above, the positive electrode active material according to the present invention satisfies the condition of a specific numerical range for the value obtained from the Hardness Index formula, thereby minimizing particle breakage caused by excellent particle strength during the rolling process and enabling the desired excellent battery characteristics, particularly significantly superior lifespan characteristics.

[0056] Figure 1 is a graph showing the particle size distribution (PSD) of the positive electrode active material before and after pressing;

[0057] Figure 2 is a graph showing the change in potential energy according to the interatomic distance;

[0058] Figure 3a is a graph showing the correlation between the c-axis lattice constant and particle breakage;

[0059] Figure 3b is a graph showing the correlation between grain size and grain fracture;

[0060] Figure 3c is a graph showing the correlation between D50 and particle breakage;

[0061] FIG. 4 is a graph showing the correlation between the Hardness Index and particle breakage according to the present invention;

[0062] Fig. 5a is an SEM image of the NCM precursor of Example 1;

[0063] FIG. 5b is an SEM image of the positive electrode active material of Example 1;

[0064] Fig. 6a is an SEM image of the NCM precursor of Example 3;

[0065] Figure 6b is an SEM image of the positive electrode active material of Example 3.

[0066] The present invention will be described further below with reference to embodiments thereof, but the scope of the invention is not limited by them.

[0067]

[0068] Example 1

[0069] A metal salt aqueous solution was prepared by mixing NiSO4, CoSO4, and MnSO4 in a molar ratio of 96:2:2 using distilled water from which internal dissolved oxygen had been removed by bubbling with nitrogen gas for more than 2 hours.

[0070] The above aqueous metal salt solution was introduced into a reactor at 60°C, and a co-precipitation reaction was carried out using NaOH and NH3·H2O as precipitating agents and chelating agents at 400 rpm and pH 11.0–11.5. In the co-precipitation reaction for 31 hours, Ni as an NCM precursor 0.96 Co 0.02 Mn 0.02 (OH)2 was prepared.

[0071] The above NCM precursor and LiOH·H2O were blended using a mixer at a molar ratio of Li / Me 1.02, and doping sources Al(OH)3 and ZrO2 were blended at 0.1 mol and 0.15 mol, respectively, based on the active material, at 2,000 rpm for 20 minutes. The blended mixture was calcined at 710°C at a heating rate of 5°C / min for 24 hours to produce Li(NCM)O2.

[0072] To remove residual lithium, washing was performed within a range of 65 to 75% solid content, and after stirring for 1 minute, moisture was removed using a pump for 5 minutes. The washed product was dried in a vacuum oven for 8 hours. Finally, the dried washed product was mixed with H3BO3 at a concentration of 0.06 molar based on the active material, and the mixture was calcined at 300°C at a heating rate of 5°C / min for 12 hours to produce a positive electrode active material.

[0073]

[0074] Example 2

[0075] A positive electrode active material was prepared in the same manner as in Example 1, except that the molar ratio of NiSO4, CoSO4, and MnSO4 was 83:11:6 and the calcination temperature was 780℃.

[0076]

[0077] Example 3

[0078] A positive electrode active material was prepared in the same manner as in Example 1, except that the molar ratio of NiSO4, CoSO4, and MnSO4 was 89:7:4 and the calcination temperature was 745℃.

[0079]

[0080] Comparative Example 1

[0081] A positive electrode active material was prepared in the same manner as in Example 1, except that the calcination temperature was set to 725℃.

[0082]

[0083] Comparative Example 2

[0084] A positive electrode active material was prepared in the same manner as in Example 1, except that the calcination temperature was set to 695℃.

[0085]

[0086] Experimental Example 1

[0087] The NCM precursor and cathode active material prepared in Examples 1 and 3, respectively, were attached to a metal sample mold and coated with Au using a coater. Then, measurements were taken at a magnification of 10,000 using a JEOL JSM-IT500 instrument under the conditions of Resolution: 1.0 nm 10 kV, Electron gun: Cold-cathode field emission type electron gun, Detector: SE (BSE), and acceleration voltage 10 kV, and the results are shown in Figures 5 and 6.

[0088] FIG. 5a is an SEM image of the NCM precursor of Example 1, and FIG. 5b is an SEM image of the positive electrode active material of Example 1. Correspondingly, FIG. 6a is an SEM image of the NCM precursor of Example 3, and FIG. 6b is an SEM image of the positive electrode active material of Example 3.

[0089] Referring to these drawings, it can be seen that the NCM precursor and cathode active material of Example 3, with a Ni content of 89%, have larger primary particle sizes and larger crystal grain sizes compared to the NCM precursor and cathode active material of Example 1, with a Ni content of 96%, due to the relatively higher calcination temperature. On the other hand, as described below, excellent high-temperature life characteristics are exhibited due to the relatively low Ni content, low Hardness Index, and high particle strength.

[0090]

[0091] Experimental Example 2

[0092] The following measurements were performed on the cathode active materials prepared in Examples 1 to 3 and Comparative Examples 1 and 2, respectively, and the results are shown in Table 1 below.

[0093] First, for XRD measurement, a Bruker instrument (XRD measuring instrument) was used to obtain crystal structure information of the cathode active material, and measurements were taken in the range of 10° to 120° at a scan rate of 2° / min.

[0094] For BET measurement, after weighing the active material, pretreatment was performed by cooling at 100°C / 1 hour and 300°C / 2 hours using Micromeritics’ VacPrep 061 instrument, and then BET was measured using nitrogen as the adsorbent gas in the company’s Tristar II 3020 instrument.

[0095]

[0096] In Table 1 above, the Hardness Index is It was calculated from (D: D50 of the positive active material particles in PSD (㎛), G: crystal grain size (nm), C: c-axis lattice constant of XRD (Å), R: R-factor in XRD).

[0097] The following facts can be confirmed from the contents of Table 1 above.

[0098]

[0099] First, the positive active materials of the examples all have a Hardness Index in the range of 15 to 27, whereas the positive active materials of the comparative examples each fall outside the above range.

[0100]

[0101] Second, as requirements for providing the above Hardness Index, it can be seen that the c-axis lattice constant is 14.2096 to 14.2467 Å, ​​the crystal grain size is 149 to 194 nm, and the R-factor is 0.499 to 0.564. When the ranges of the above lattice constant, crystal grain size, and R-factor are simultaneously satisfied with the appropriate range of the Hardness Index, the grain strength can be improved.

[0102]

[0103] Third, it can be confirmed that the positive active materials of the examples satisfying the above range of Hardness Index have a significantly smaller proportion of particles smaller than 1 μm due to particle size breakage when compared to the positive active materials of the comparative examples, that is, they have high particle strength.

[0104]

[0105] Fourth, it can be confirmed that the (003) plane / (104) plane peak ratio measured during XRD analysis of the cathode active materials falls within the range of 2.08 to 2.13, and the ratio of the c-axis lattice constant to the a-axis lattice constant (c / a) falls within the range of 4.9390 to 4.9483. It can be confirmed that the above ranges for the (003) plane / (104) plane peak ratio and the ratio of the lattice constant (c / a) are not satisfied alone, but are satisfied simultaneously with the Hardness Index to have high particle strength. For example, in the case of Comparative Example 2, the (003) plane / (104) plane peak ratio is 2.11, which falls within the appropriate range (2.08 to 2.13), but the Hardness Index is 13.5, which falls outside the appropriate range (15 to 27), thus showing low particle strength.

[0106]

[0107] Experimental Example 3

[0108] 2.03 g of binder prepared by dissolving 11.8 wt% of PVdF in 1-Methyl-2-Prrolidinone (NMP), 0.18 g of conductive material (Super C-65), and 1 g of NMP were mixed in a first mixing step at 1200 rpm for 3 minutes. Subsequently, 12 g of the cathode active material and 1 g of NMP prepared in the above examples and comparative examples, respectively, were added and mixed under the same conditions as the first mixing step to prepare a slurry. The prepared slurry was cast onto an Al foil, dried in an oven at 120°C for 20 minutes, and pressed twice, followed by drying in a vacuum oven at 120°C for 1 hour to obtain a concentration of 16.3 to 17.3 mg / cm². 2 A phosphorus electrode plate (anode) was manufactured.

[0109] After assembling a CR2032 coin-type half-cell (coin cell) based on the electrode plate in a moisture-controlled dry room, it was aged at room temperature for 12 hours to impregnate with electrolyte and achieve electrochemical equilibrium.

[0110] Coin cells were evaluated using a PNE-CTS proAnalyzer charge / discharger. First, formation was performed in a constant temperature chamber at 25°C, and rate characteristics and lifespan evaluations were conducted in a constant temperature chamber at 45°C. For the formation evaluation, a current density of 0.2C was applied in the voltage range of 4.3 to 2.5V, followed by a Constant-Voltage section down to 0.005C, and charging and discharging were performed a total of once. After the formation section was completed, a rate evaluation was performed once at a charging current density of 0.5C and a discharging current density of 1.0C, and a lifespan evaluation was performed more than 50 times at the same current densities of 0.5 / 1.0C.

[0111] The results are shown in Table 2 below.

[0112]

[0113] As shown in Table 2 above, the batteries of Examples 1 to 3 have excellent overall battery characteristics, and in particular, it can be confirmed that their lifespan characteristics are significantly superior when compared to the batteries of Comparative Examples 1 and 2.

[0114]

[0115] The present invention is not limited to the above embodiments and can be manufactured in various different forms, and those skilled in the art will understand that the invention can be implemented in other specific forms without changing the technical concept or essential features of the invention. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive.

Claims

1. Contains transition metals, and A positive electrode active material characterized by a Hardness Index defined by the following formula ranging from 15 to 27: In the above formula, D: D50 (㎛) of the positive active material particles analyzed from PSD (particle size distribution); G: Grain size (nm); C: c-axis lattice constant (Å) analyzed from XRD (X-ray diffraction); and R: R-factor analyzed from XRD.

2. The positive active material according to claim 1, characterized in that the c-axis lattice constant (C) is 14.2096 to 14.2467 Å.

3. The positive active material according to claim 1, characterized in that the crystal grain size (G) is 149 to 194 nm.

4. The positive active material according to claim 1, characterized in that the R-factor (R) is 0.499 to 0.

564.

5. The positive electrode active material according to claim 1, characterized by including Ni as a transition metal.

6. A positive electrode active material according to claim 5, characterized in that the Ni content is 80 to 98 mol% based on the total amount of transition metal.

7. The positive electrode active material according to claim 1, characterized in that the (003) plane / (104) plane peak ratio measured during XRD analysis is 2.08 to 2.

13.

8. The positive electrode active material according to claim 1, characterized in that the ratio (c / a) of the c-axis lattice constant to the a-axis lattice constant measured during XRD analysis is 4.9390 to 4.9483.

9. A secondary battery characterized by including a positive electrode active material according to claim 1.

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