Cathode active material and secondary battery including same

Abstract

The present invention provides a cathode active material for which, in a particle size distribution (PSD) graph, there are two or more peaks, the height ratio of the upper two peaks on the basis of height being in the range of 1-1.5, thus providing a high pellet density, and accordingly achieving a high energy density.

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 including the same, and more specifically, to a positive electrode active material having a high pellet density by satisfying a specific condition in which the height ratio of the top two peaks based on height in a particle size distribution graph satisfies a specific condition, and to a secondary battery having a high energy density by including such a positive electrode active material.

[0002] Lithium iron phosphate (LFP) cathode active materials have many advantages, such as lower cost, eco-friendliness, high thermal stability at high temperatures, and excellent structural stability, as well as good cycle life compared to ternary materials like NCM, but they have the disadvantage of lower battery competitiveness due to lower energy density than ternary materials.

[0003] These drawbacks act as a barrier to securing driving range when LFP cathode active materials are used in electric vehicle batteries, making it urgent to improve low energy density.

[0004] In this regard, since the energy density of LFP cathode active materials improves when the pellet density (PD) is secured above a certain level (2.45 or higher), various studies are being conducted to realize the characteristics of high-energy-density lithium iron phosphate batteries by increasing pellet density through minimizing inter-particle voids of the cathode active material.

[0005] For example, it has been confirmed that when active materials are configured to form two peaks in a particle size distribution (PSD) graph, the pellet density is improved. However, creating such a particle size distribution inevitably requires the addition of a process to mix active material particles of different sizes, which leads to process disadvantages such as reduced productivity and increased unit costs; therefore, a solution to address this is needed.

[0006] In addition, when attempting to improve pellet density, there is a tendency for a trade-off to occur where other battery characteristics are degraded.

[0007] Therefore, there is a high need in the industry for new technologies that can improve pellet density while minimizing the degradation of battery characteristics without causing process disadvantages.

[0008] The present invention aims to solve the problems of the prior art described above and technical challenges that have been requested over time.

[0009] After conducting in-depth research and various experiments, the inventors of this application confirmed that when an active material is configured such that the particle size and particle size distribution satisfy specific conditions, high energy density can be achieved by increasing pellet density while minimizing the degradation of electrical characteristics such as charge / discharge capacity and efficiency, and that this can be preferably applied to lithium iron phosphate cathode active materials which suffer from the problem of low energy density, and thus completed the present invention.

[0010] Accordingly, the positive electrode active material of the present invention is characterized by having two or more peaks in the particle size distribution (PSD) graph, and the height ratio of the top two peaks based on height is in the range of 1 to 1.5.

[0011] Referring to the PSD graphs of Figures 1a to 1c, which show the particle size distribution of active material particles with uniform particle sizes, and Figure 2, which shows the particle size distribution of active material particles with two types of particle sizes, first, in Figures 1a to 1c, the particle size distribution is uniform, so only one peak exists in the PSD graphs, and the space (voids) between the particles is not densely filled, resulting in a low pellet density. On the other hand, in Figure 2, two peaks exist in the PSD graph, and it can be seen that small particles are filled in the voids between large particles, resulting in a higher pellet density.

[0012] According to the present invention, as defined above, there are two peaks based on height, which can be viewed as peaks corresponding to small particles and large particles according to particle size. The peak height ratio is the ratio of large particles to small particles, and as described above, is 1 to 1.5, preferably with slightly more large particles, which is a condition that can provide a high packing rate as shown in FIG. 2. The preferred ratio of large particles to small particles is 1 to 1.34, and more preferably 1.08 to 1.2.

[0013] For reference, the "peak" in the PSD graph defined in the present invention refers to the highest point of each peak rising high on the graph, which mathematically means the point where the slope becomes zero, and may exist as a single peak or multiple peaks on the graph.

[0014]

[0015] An increase in pellet density can be advantageously applied to lithium iron phosphate cathode active materials, where low energy density is a particular concern. Accordingly, in one preferred example, the cathode active material may have a composition represented by the following chemical formula 1.

[0016] Li x MP a O b (1)

[0017] In the above formula,

[0018] 0 <x≤2, 0≤a≤2, 0<b≤4;

[0019] M includes Fe and may optionally further include one or more of the following: transition metals of groups 3 to 12 excluding Fe, post-transition metals and metalloids of groups 13 to 15, alkaline earth metals, nonmetals of groups 14 to 16, lanthanides, and actinides.

[0020] In the above,

[0021] "Group 3 to 12 transition metals excluding Fe" may be, for example, Ni, Co, Mn, Sc, Ti, V, Cr, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, etc.

[0022] "Post-transition metals and metalloids in groups 13 to 15" may be, for example, Al, Ga, In, Sn, Tl, Pb, Bi, Po, B, Si, Ge, As, Sb, Te, At, etc., and

[0023] "Alkali earth metals" can be, for example, Be, Mg, Ca, Sr, Ba, Ra, etc., and

[0024] "Nonmetallic elements in groups 14 to 16" may be, for example, C, P, S, Se, etc.

[0025] A preferred cathode active material may be in the form of LiFePO4 or LiFePO4 doped with one or more of post-transition metals, metalloids, alkaline earth metals, nonmetals of groups 14 to 16, lanthanides, and actinides, and additionally, may be in the form coated with carbon, etc. as needed.

[0026]

[0027] In one specific example, particle size control of the anode active material can be achieved by adjusting process parameters in an air-flow grinder.

[0028] Air-flow grinders are widely used to achieve target particle sizes in cathode active materials. They operate on the principle that compressed air is injected into the mill by an injector to induce collisions between the particles injected sequentially, causing them to break down or finer, and then discharged out of the mill under specific conditions along the resulting airflow. Figure 4 illustrates a schematic diagram of such an air-flow grinder. Since there is no direct collision between the equipment and the particles, the possibility of wear on parts and equipment is low, and the particle size distribution of the fined particles is sharply realized compared to the products ground by other grinders.

[0029]

[0030] In the case of LFP active materials, in order to solve resistance and output problems, particles are ground into nano-sized particles using an air-flow grinder, etc., so generally, the PSD graph is processed to have a much larger number of small particles of 1 μm or less.

[0031] However, as previously explained, the size and distribution of cathode active material particles affect pellet density and ultimately energy density; consequently, it is difficult to achieve the target pellet density characteristics when using fine lithium iron phosphate powder with a high-efficiency air-flow grinder. For this reason, the industry adopts methods to achieve these characteristics through other processes, such as calcination, instead of grinding, or by appropriately mixing two particles of different sizes.

[0032] On the other hand, according to the present invention, the pellet density of the anode active material can be increased by controlling process factors in the grinding process itself.

[0033] Specifically, in the present invention, the residence time of active material particles inside the mill of the air-flow type grinder is increased to induce aggregation of the active material particles, and as a result, a combination of particles having a difference in average particle size can be created by forming some particles with larger particle sizes.

[0034] The above process control can be achieved in the grinding process of an air-flow grinder through the selective adjustment of process factors such as grinding pressure, the rotational speed of the classifier rotor, the bed level (indicating the amount of powder remaining inside the mill), and the mill internal pressure. For example, the mill internal pressure is indicated as a negative value when the pressure is adjusted to increase the rate at which the ground powder is discharged to the outside, and as a positive value when the pressure is adjusted to decrease the rate at which it is discharged to the outside. By adjusting the mill internal pressure to an appropriate positive value (weak positive pressure), the residence time of the ground powder inside the mill can be increased to induce aggregation. More specific examples regarding the control of process factors can be found in the experimental details described later.

[0035]

[0036] In one specific example, when the highest points of the two peaks are referred to as the first peak peak and the second peak peak, the horizontal axis position of the first peak peak may be located in a region greater than 1 μm, and the horizontal axis position of the second peak peak may be located in a region less than 1 μm.

[0037] In the above, the first peak can be seen as representing large particles, and the second peak as representing small particles.

[0038] Generally, Ni-based cathode active materials have a fine region of less than 1 μm, which leads to a deterioration in lifespan characteristics, so the particle size is made much larger. On the other hand, LFP cathode active materials may be advantageous to produce nano-sized fine particles due to characteristic limitations, so the above peak condition may be suitable for LFP cathode active materials where the presence of particles with a particle size of less than 1 μm may be required.

[0039] In this case, the first peak peak may have a height equal to or greater than that of the second peak peak.

[0040] Preferably, the height of the first peak peak is greater than the height of the second peak peak, so that the volume percentage of large particles represented by the first peak is slightly higher, which can help improve pellet density.

[0041]

[0042] In another specific example, the difference between the horizontal axis position of the first peak peak and the horizontal axis position of the second peak peak may be in the range of 1 to 1.5, and specifically, the horizontal axis position of the first peak peak may be located in the 1.6 to 2 μm region and the horizontal axis position of the second peak peak may be located in the 0.4 to 0.55 μm region.

[0043] The difference in the above horizontal axis positions refers to the difference calculated by comparing the particle size (in μm units) at the horizontal axis position of the first peak peak and the particle size (in μm units) at the horizontal axis position of the second peak peak. Some position differences explained later are also based on this calculation method.

[0044] If large or small particles fall outside the applicable range, an appropriate combination of particles is not achieved, which may result in a decrease in pellet density. For example, if small or large particles are too large, there may be cases where grinding is insufficient or particles are excessively aggregated, in which case appropriate grinding is required.

[0045] If the difference (gap) at the horizontal axis position is too large, the difference in particle size between large and small particles increases, causing the pellet density to decrease; therefore, it is desirable for large and small particles to exist with an appropriate range of spacing, and for each of these positions to be within the area defined above, it may be effective for improving pellet density.

[0046] The preferred difference in the horizontal axis position may be in the range of 1.24 to 1.49, and the preferred position may be in the range of 1.73 to 1.93 μm for large particles and 0.44 to 0.49 μm for small particles.

[0047]

[0048] In one preferred example, the ratio of the lowest height of the region between the top two peaks to the highest peak height may be in the range of 1.3 to 1.5.

[0049] The particles located in the region between the top two peaks above can be classified as "heavy particles" having a kind of intermediate particle size, and the presence of these heavy particles results in the largest pellet density due to high void filling compared to the cases of Figures 1a to 1c and Figure 2, as shown in Figure 3 through a combination of small and large particles.

[0050] These heavy particles may be particles belonging to the particle size range of 1 to 3 μm, for example, in the PSD graph, and if there are too few or too many, the balance of filling the voids is disrupted, making it difficult to improve pellet density. However, heavy particles are not limited to particles in the above particle size range, and may be defined as particles existing between large and small particles.

[0051] Therefore, as described above, it may be desirable for the large particle peak height / baryron peak height to be in the range of 1.3 to 1.5 so that they exist in a constant ratio while being smaller than the large particle corresponding to the highest peak.

[0052] Just as the relationship between large particles and baryons as described above can be specified, the relationship between small particles and baryons can also be specified, for example, the ratio of the lowest height of the region between the two upper peaks and the height of the second highest peak can be in the range of 1 to 1.3.

[0053] The ratio of peak heights of small and medium particles can be a desirable condition for improving pellet density, as previously mentioned.

[0054]

[0055] In one specific example, the lowest height of the region between the peaks may be 3.8 volume% or more, which can be viewed as the size of the heavy particle region, preferably 3.8 to 4.2 volume%, more preferably 3.84 to 3.98 volume%.

[0056]

[0057] In another preferred example, the height difference between the top two peaks based on the height criteria may be 1.4 volume% or less.

[0058] The difference in height of the above peaks should be as small as possible, but if the difference between the peaks is excessively small, it is difficult to achieve an improvement in pellet density, so it may preferably be 0.35 to 1.4 volume%, and more preferably 0.4 to 0.9 volume%.

[0059]

[0060] The cathode active material of the present invention has a very high pellet density even when compared to conventional cathode active materials, specifically 2.45 g / cm³ 3 It may be higher. Since such high pellet density results in high energy density, it can be very advantageously used in applications where capacity-to-weight ratio is important, such as electric vehicle batteries. The preferred range is 2.45 g / cm³. 3 Up to 2.65 g / cm³ 3 It could be.

[0061]

[0062] The present invention also relates to a secondary battery characterized by comprising the above-mentioned positive active material.

[0063] Since the composition and manufacturing method of secondary batteries are known in the art, a detailed description thereof is omitted in this specification.

[0064] As explained above, the cathode active material of the present invention is configured such that the particle size and particle size distribution satisfy specific conditions, thereby enabling high energy density through an increase in pellet density while minimizing the degradation of electrical characteristics such as charge / discharge capacity and efficiency, and can be particularly preferably applied to lithium iron phosphate cathode active materials.

[0065] FIGS. 1a to 1c are cross-sectional shapes and particle size distribution graphs showing the particle size distribution by size of active material particles having uniform particle diameters;

[0066] Figure 2 is a cross-sectional shape and a particle size distribution graph showing the particle size distribution of active material particles having large and small particle size peaks;

[0067] Figure 3 is a cross-sectional shape and a particle size distribution graph showing the particle size distribution of active material particles having particle size peaks of large, medium, and small particles;

[0068] Figure 4 is a schematic diagram showing the operating principle of an air-flow grinder;

[0069] FIGS. 5 to 9 are PSD graphs of Comparative Examples 1 to 7, Examples 2 and 6;

[0070] Fig. 10 is an SEM image of Comparative Example 6;

[0071] Figure 11 is an SEM image of Example 6.

[0072] The present invention will be described further below with reference to embodiments thereof, but the scope of the invention is not limited by them.

[0073]

[0074] [Reference Example]

[0075] A lithium iron phosphate cathode active material having a specific pellet density value was used, manufactured by the following steps.

[0076] - Step 1: Iron phosphate hydrate (FePO4xH2O), lithium carbonate (Li2CO3), glucose, polyethylene glycol 6000 (PEG6000), titanium dioxide (TiO2), and distilled water are well mixed and dispersed in a tank to a solid content of 40 to 45%.

[0077] - Step 2: The slurry obtained above is passed through a Beads Mill to an intermediate particle size (D 50 Polish so that the value becomes 0.3 to 0.7 μm.

[0078] - Step 3: The slurry polished above is dried in a spray dryer under conditions of a hot air temperature of 250°C and a centrifugal disk speed of 20,000 to 25,000 rpm.

[0079] - Step 4: The dried ivory-colored powder is filled into a firing vessel and subjected to primary and secondary sintering in a Roller Hearth Kiln (RHK) at a temperature of approximately 700–800°C under a nitrogen atmosphere for about 24 hours to obtain black powder. This powder has an intermediate particle size (D 50 ) has a value of 20 ~ 25 μm.

[0080] - Step 5: The above powder is ground in an air-flow grinder to obtain the final lithium iron phosphate cathode active material.

[0081]

[0082] [Comparative Example 1]

[0083] The same raw materials used in the reference example were used, but the conditions of the air-flow grinder were set to a grinding pressure of 4.5 to 5.0 bar, a classifier rotor of 10,000 to 11,400 rpm, a bed level of 0.2 to 0.4 kg, and a mill internal pressure setting of -30 mbar and operated. At this time, the actual value of the mill internal pressure is operated within the range of -30 to 0 mbar.

[0084]

[0085] [Comparative Example 2]

[0086] The same raw materials used in the reference example were used, and the setting values ​​for the grinding pressure, classifying rotor, and mill internal pressure in the air-flow grinder conditions of Comparative Example 1 were identical, but the bed level condition was changed to 0.4 ~ 0.7 kg and operated. At this time, the actual value of the mill internal pressure is operated within the range of -30 ~ 0 mbar.

[0087]

[0088] [Comparative Example 3]

[0089] The same raw materials used in the reference example were used, and the grinding pressure, classifying rotor, and bed level in the air-flow grinder conditions of Comparative Example 2 were identical, but the setting value of the internal mill pressure was changed to -70 mbar and operated. At this time, the actual value of the internal mill pressure is operated within the range of -70 to -40 mbar.

[0090]

[0091] [Comparative Example 4]

[0092] The same raw materials used in the reference example were used, and the setting values ​​for the grinding pressure, classifying rotor, and mill internal pressure in the air-flow grinder conditions of Comparative Example 3 were the same, but the bed level condition was changed to 0.6 to 0.9 kg and operated. At this time, the actual value of the mill internal pressure is operated within the range of -70 to -40 mbar.

[0093]

[0094] [Comparative Example 5]

[0095] The same raw materials used in the reference example were used, and the setting values ​​for the grinding pressure, bed level, and internal mill pressure in the air-flow grinder conditions of Comparative Example 4 were the same, but the classifying rotor conditions were changed to 9,000 to 9,500 rpm and operated. At this time, the actual value of the internal mill pressure is operated within the range of -70 to -40 mbar.

[0096]

[0097] [Comparative Example 6]

[0098] The same raw materials used in the reference example were used, but the conditions of the air-flow grinder were set to a grinding pressure of 3.5 to 4.0 bar, a classifier rotor of 8,500 to 9,100 rpm, a bed level of 0.4 to 0.7 kg, and a mill internal pressure setting of -30 mbar and operated. At this time, the actual value of the mill internal pressure is operated within the range of -30 to 0 mbar.

[0099]

[0100] [Comparative Example 7]

[0101] The same raw materials used in the reference example were used, and the grinding pressure and bed level in the air-flow grinder conditions of Comparative Example 6 were the same, but the setting values ​​of the classifying rotor and the internal pressure of the mill were changed to 10,000 to 11,400 rpm and -50 mbar, respectively, and the machine was operated. At this time, the actual value of the internal pressure of the mill is operated within the range of -50 to -20 mbar.

[0102]

[0103] [Example 1]

[0104] The same raw materials used in the reference example were used, and the grinding pressure, classifying rotor, and bed level conditions among the air-flow grinder conditions of Comparative Example 7 were identical, but the setting value of the internal mill pressure was changed to -35 mbar and operated. At this time, the actual value of the internal mill pressure is operated within the range of -10 to +10 mbar.

[0105]

[0106] [Example 2]

[0107] The same raw materials used in the reference example were used, and the grinding pressure, classification rotor, and bed level conditions among the air-flow grinder conditions of Example 1 were identical, but the setting value of the internal mill pressure was changed to -30 mbar and operated. At this time, the actual value of the internal mill pressure is operated within the range of -10 to +20 mbar.

[0108]

[0109] [Example 3]

[0110] The same raw materials used in the reference example were used, and the grinding pressure, classification rotor, and bed level conditions among the air-flow grinder conditions of Example 1 were identical, but the setting value of the internal mill pressure was changed to -25 mbar and operated. At this time, the actual value of the internal mill pressure is operated within the range of -10 to +25 mbar.

[0111]

[0112] [Example 4]

[0113] The same raw materials used in the reference example were used, and the grinding pressure, classification rotor, and bed level conditions among the air-flow grinder conditions of Example 1 were identical, but the setting value of the internal mill pressure was changed to -20 mbar and operated. At this time, the actual value of the internal mill pressure is operated within the range of -10 to +30 mbar.

[0114]

[0115] [Example 5]

[0116] The same raw materials used in the reference example were used, and the grinding pressure, classification rotor, and bed level conditions among the air-flow grinder conditions of Example 1 were identical, but the setting value of the internal mill pressure was changed to -15 mbar and operated. At this time, the actual value of the internal mill pressure is operated within the range of -10 to +35 mbar.

[0117]

[0118] [Example 6]

[0119] The same raw materials used in the reference example were used, and the classification rotor and bed level conditions among the air-flow grinder conditions of Example 1 were the same, but the grinding pressure and mill internal pressure were changed to 2.5 to 3.0 bar and -30 mbar, respectively, and operated. At this time, the actual value of the mill internal pressure is operated within the range of -10 to +40 mbar.

[0120]

[0121] [Example 7]

[0122] The same raw materials used in the reference example were used, and the grinding pressure, classification rotor, and bed level conditions among the air-flow grinder conditions of Example 6 were identical, but the setting value of the internal mill pressure was changed to -25 mbar and operated. At this time, the actual value of the internal mill pressure is operated within the range of -10 to +45 mbar.

[0123]

[0124] [Experimental Example 1]

[0125] The particle size distribution of lithium iron phosphate cathode active material samples prepared in Comparative Examples 1 to 7 and Examples 1 to 7 was confirmed by measuring the particle size. Based on this, the peak height, difference in peak height, ratio of peak height, peak position, and difference in peak position of large and small particles were measured and are shown in Table 1 below, and the height, difference in height, ratio of height, etc. of medium particles were measured and are shown in Table 2 below.

[0126] The particle size distribution graph was prepared under the following PSD measurement conditions.

[0127] <Measurement Conditions>

[0128] - Measuring equipment: Malvern mastersizer 3000 (Hydro MV)

[0129] - Cycle speed: 2000 rpm / sec

[0130] - Refractive index ratio: 1.692

[0131] - Equipment input solvent: Distilled water

[0132] - Sample of cell:

[0133] Obscuration low limit (concentration): 10.00%

[0134] Obscuration high limit (concentration): 15.00%

[0135] - Calculation Logic: Mie (default)

[0136] - Sample amount: 0.5000 g

[0137] - Sample Injection Dispersant: 10% Sodium Hexamethaphosphate 1 ml

[0138] - Sample input solvent: 40 ml distilled water

[0139] - Sample ultrasonic dispersion: 40 KHz, 3 min

[0140]

[0141] In addition, PSD graphs of Comparative Examples 1 to 7 and Examples 2 and 6 are shown in FIGS. 5 to 9, and SEM images of Comparative Example 6 and Example 6 are shown in FIGS. 10 and 11, respectively.

[0142]

[0143]

[0144] By referring to the results of Tables 1 and 2 above together with Figures 5 to 11, the following facts can be confirmed.

[0145]

[0146] (1) It can be seen that the positive active material of the examples satisfies all conditions such as the ratio of the heights of the top two peaks in terms of height being 1 to 1.5, the horizontal axis position of the large particle peak being located in the 1.6 to 2 μm region and the horizontal axis position of the small particle peak being located in the 0.4 to 0.55 μm region, and the difference between the horizontal axis position of the large particle peak and the horizontal axis position of the small particle peak being 1 to 1.5, but the positive active material of the comparative examples does not.

[0147]

[0148] (2) It can be seen that the positive electrode materials of the examples satisfy all conditions such as the ratio of the height of the large particle peak to the minimum height of the heavy particle peak being 1.3 to 1.5, and the ratio of the height of the small particle peak to the minimum height of the heavy particle peak being 1 to 1.3, but the positive electrode materials of the comparative examples do not.

[0149]

[0150] (3) When referring to the results of Experimental Example 2 described later, it can be seen that the positive active materials of Examples 4 to 7, particularly the positive active material of Example 6, exhibit a high pellet density (PD), and accordingly, the difference on the horizontal axis is more preferably in the range of 1.24 to 1.49, and the position on the horizontal axis is more preferably in the range of 1.73 to 1.93 μm for large particles and in the range of 0.44 to 0.49 μm for small particles.

[0151]

[0152] (4) Compared to the low pellet density of the positive active material of Comparative Example 6 (Fig. 10), the positive active material of Example 6 can be seen in the SEM image to have a high pellet density (Fig. 11) with a mixture of large, medium, and small particles.

[0153]

[0154] (5) Referring to Figure 5, it can be seen that the particle size distribution changes significantly depending on the Bed level factor. When the amount of powder to be maintained inside the mill is less than the set minimum amount (0.5 to 0.7 kg), the raw material flowing in in real time does not reach the nozzle height and is not broken down, and excessive aggregation of particles occurs.

[0155]

[0156] (6) Referring to Figure 6, it can be seen that the particle size distribution changes significantly depending on the internal pressure factor of the mill. When a large amount of negative pressure is applied inside the mill, the finely ground particles are discharged without being sufficiently retained inside the mill, and it can be seen that the region of less than 1 μm is dominant.

[0157]

[0158] (7) Referring to Figure 7, it can be seen that the particle size distribution changes significantly depending on the classifying rotor speed factor. As the classifying rotor speed increases, the crushed fine particles remain sufficiently inside the mill and form aggregates with each other, so the particle size distribution shifts from a region of less than 1 μm to a region of medium particles.

[0159]

[0160] (8) Referring to Figure 8, it can be seen that the particle size distribution changes significantly depending on the classification rotor speed and the internal pressure of the mill when the grinding pressure is lowered. When the grinding is reduced, the finely crushed particles remain sufficiently inside the mill and form aggregates with each other through the adjustment of the classification rotor speed and the internal pressure of the mill, so that the particle size distribution moves from a region of less than 1 μm to a region of medium particles with larger particle sizes.

[0161]

[0162] (9) The results of the comparative examples in Figures 5 to 8, despite the results of items (5) to (8) due to the control of process factors, do not provide the desired level of pellet density, as can be seen in the results of Table 3 which will be explained later.

[0163]

[0164] (10) Referring to Figure 9 and Table 3, which will be explained later, the pellet density changes significantly depending on the internal pressure factor of the mill when the grinding pressure is lowered. This suggests that the internal pressure of the mill alone can have a significant effect on the residence time of the crushed particles inside the mill, and that more aggregates of medium particle size were formed in the finished product of Example 1 compared to the comparative example.

[0165]

[0166] (11) Referring to FIG. 9, it can be seen that when the classification rotor speed is higher at the same grinding pressure or when the grinding pressure is lower in other combinations of the same conditions, the broken particles remain inside the mill for a longer time, which is advantageous for increasing the number of aggregates and contributes to increasing the pellet density. It can be seen that the target pellet density value reached the target level with the air-flow grinding condition combination of Example 6, and the particle size distribution is also similar. This suggests that the relationship between the shape and particle size distribution of the particles and the pellet density can be established, and it proves that in order to improve pellet density, it is desirable for the composition of the material to consist of two or more types rather than a single shape, and for a clear particle size distribution of a Trimodal shape (a Bimodal shape with an overlapping center) to be produced.

[0167]

[0168] [Experimental Example 2]

[0169] The pellet density of the lithium iron phosphate cathode active materials obtained in Comparative Examples 1 to 7 and Examples 1 to 7 was measured, and the results are shown in Table 3 below. The method for measuring pellet density is as follows.

[0170] The equipment used was the CARVER Auto pellet press 3887NE.L model. Approximately 1.0 g of the finished pulverized active material was placed in a dedicated mold, and the initial height (mm) was measured using the equipment's vernier caliper. Then, a pressure of 2.5 ton was applied to the mold for 30 seconds and then released. Afterward, the height (mm) of the sample was measured after compression using the equipment's vernier caliper.

[0171] Then, the thickness of the compressed sample was calculated according to the following formula, and

[0172] A. Sample thickness (cm) = [Sample height after compression (mm) - Initial height (mm)] * 0.1

[0173] The pellet density was calculated according to the following formula.

[0174] B. Pellet density (g / cm³) 3 ) = Weight of active material (g) / Volume of pellet (cm³) 3 ) = Weight of active material (g) / [Mold radius (cm)] 2 * 3.141592 * Sample thickness (cm)

[0175]

[0176] In addition, a secondary battery was fabricated using the above positive active material and its electrochemical properties were evaluated, and the results are shown together in Table 3 below.

[0177] Specifically, PVdF, a binder, was dissolved in N-methylpyrrolidone (NMP), and the lithium iron phosphate cathode active materials prepared in Comparative Examples 1 to 7 and Examples 1 to 7, respectively, and Super-C, a conductive material, were mixed in a weight ratio of 3:95:2 to prepare a cathode forming composition, which was then evenly coated onto an aluminum current collector and dried in a hot air dryer at 120°C to evaporate the NMP. Then, a rolling process was carried out and the cathode was manufactured by drying in a vacuum oven at 120°C for 12 hours.

[0178] An electrode assembly is fabricated by preparing the above-mentioned anode to fit the CR2032 type coin cell size, using Li metal as the cathode, and interposing a porous polyethylene separator between them.

[0179] A lithium secondary battery was fabricated by placing the above-mentioned electrode assembly inside a CR2032 type coin cell case and injecting an electrolyte in which a lithium salt was dissolved in a carbonate-based solvent.

[0180] The lithium secondary battery produced in this way was aged at room temperature for 10 hours to impregnate with electrolyte and achieve electrochemical equilibrium.

[0181] The above lithium secondary battery was evaluated using a Toscat-3100 charge / discharger and conducted in a 25°C constant temperature chamber. The voltage range was 4.3 to 2.5V, and for both charging and discharging, a current density of 0.1C was applied, followed by a 0.05C constant voltage interval, and the process was repeated a total of two times.

[0182]

[0183] First, let's examine the relationship between the "charge volume capacity" and "discharge volume capacity" in Table 3 above and the energy density.

[0184] Generally, in the battery manufacturing process, the application of positive electrode active materials inevitably involves rolling them onto electrode plates. Since pellet density is also measured by applying pressure, it allows for the prediction of the density of the positive electrode active material applied to the battery. If pellet density is low, the amount of positive electrode active material per unit volume decreases, which may result in a lower capacity per unit volume when the battery pack is finally completed. In other words, pellet density allows for the inference of the volumetric energy density of the battery pack, which corresponds to "volumetric capacity (mAh / cm²)." 3 It can be expressed as ).

[0185] Meanwhile, volumetric energy density is generally expressed as [Capacity (Ah) × Average Operating Voltage (V) × Volume (L)]. In the above, assuming the average operating voltage is the same, [Pellet density (g / cm³) 3 Energy density was compared using "volume capacity" calculated as ) × capacity (mAh / g).

[0186] Therefore, "filling volume capacity" is [pellet density (g / cm³) 3 It is calculated as ) × charging capacity (mAh / g), and "discharge volume capacity" is [pellet density (g / cm³)]. 3 It is calculated as ) × discharge capacity (mAh / g).

[0187]

[0188] Based on this, examining the results in Table 3, first, the cathode active material of the embodiments according to the present invention has a pellet density (PD) of at least 2.45 g / cm³ 3 As shown above, it can be seen that it is significantly larger than the comparative examples. This increase in pellet density (PD) directly contributes to an increase in energy density when the cathode active material is applied to a battery pack.

[0189] Furthermore, when comparing the examples and comparative examples in terms of charge volume capacity and discharge volume capacity, it can be confirmed that the volume capacity of the examples is significantly higher. From this, it can be expected that when applied to a battery (battery pack) where a rolling process is essential, a battery using the positive active material of the examples will have a significantly higher energy density.

[0190] Meanwhile, such an increase in pellet density leads to a decrease in charge / discharge capacity due to the low specific surface area, so even if the pellet density increases, the desired energy density may not be achieved due to the reduced charge / discharge capacity. However, as shown in the results of Table 3 above, the embodiments according to the present invention can achieve a result of improving energy density by minimizing the decrease in charge / discharge capacity while increasing the pellet density.

[0191] Therefore, it is important to appropriately achieve both pellet density and charge / discharge capacity of the LFP cathode active material simultaneously to increase energy density, and the pellet density of the examples can be seen as a value capable of overcoming the capacitive inferiority caused by the lower surface area compared to the comparative examples.

[0192]

[0193] 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. In the Particle Size Distribution (PSD) graph, Two or more peaks exist, and A positive electrode active material characterized by the height ratio of the top two peaks based on height being in the range of 1 to 1.

5.

2. The positive active material according to claim 1, characterized in that the positive active material has a composition represented by the following chemical formula 1: There x MP a OR b (1) In the above formula, 0 <x≤2, 0≤a≤2, 0<b≤4; M includes Fe and may optionally further include one or more of the following: transition metals of groups 3 to 12 excluding Fe, post-transition metals and metalloids of groups 13 to 15, alkaline earth metals, nonmetals of groups 14 to 16, lanthanides, and actinides.

3. The positive active material according to claim 1, characterized in that the particle size control of the positive active material is achieved by controlling process factors in an air-flow grinder.

4. An anode active material according to claim 3, characterized by inducing aggregation of the active material particles by increasing the residence time of the active material particles inside the mill of the air-flow type grinder.

5. In Paragraph 1, When the highest points of the above two peaks are referred to as the first peak peak and the second peak peak, A positive electrode active material characterized in that the horizontal axis position of the first peak peak is located in a region greater than 1 μm, and the horizontal axis position of the second peak peak is located in a region less than 1 μm.

6. The positive active material according to claim 5, characterized in that the height of the first peak peak is equal to or greater than that of the second peak peak.

7. An anode active material according to claim 5, characterized in that the difference between the horizontal axis position of the first peak peak and the horizontal axis position of the second peak peak is in the range of 1 to 1.

5.

8. A positive electrode active material according to claim 5, characterized in that the horizontal axis position of the first peak peak is located in the 1.6 to 2 μm region and the horizontal axis position of the second peak peak is located in the 0.4 to 0.55 μm region.

9. The positive active material according to claim 1, characterized in that the ratio of the lowest height of the region between the top two peaks to the highest peak is in the range of 1.3 to 1.

5.

10. A positive active material according to claim 1, characterized in that the ratio of the lowest height of the region between the top two peaks to the height of the second highest peak is in the range of 1 to 1.

3.

11. A positive active material according to claim 1, characterized in that the minimum height of the region between the peaks is 3.8 volume% or more.

12. The positive active material according to claim 1, characterized in that the height difference between the top two peaks based on height is 1.4 volume% or less.

13. In claim 1, the pellet density is 2.45 g / cm³ 3 A positive electrode active material characterized by the above.

14. A secondary battery characterized by comprising a positive electrode active material according to claim 1.

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