Positive electrode active material, its manufacturing method, and lithium secondary battery including the same

Abstract

A positive electrode active material with improved thermal safety and a method for producing the same are provided. [Solution] The present invention relates to a cathode active material, a manufacturing method thereof, and a lithium secondary battery including the same, and provides a cathode active material comprising lithium metal oxide particles in the form of secondary particles including primary particles, wherein the surfaces of the secondary particles are plate-shaped primary particles in which the narrowest angle between the c-axis of the primary particle and a line connecting an imaginary point at the center of the primary particle and the center point of the secondary particle is 70 to 90 degrees.

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material, a method for producing the same, and a lithium secondary battery including the same. [Background technology]

[0002] Recently, there has been an explosion in IT mobile devices and small power-driven devices (e-bikes, small EVs, etc.). This is driven by the sudden increase in demand and the need for electric vehicles with a driving range of over 400 km. The development of secondary batteries with high capacity and high energy density for driving is progressing actively worldwide. It is being considered.

[0003] To manufacture such a high-capacity battery, a high-capacity positive electrode active material must be used. stomach. The currently available layered cathode active material with the highest capacity is LiNiO 2 (275mAh / g), but the structure is prone to collapse during charging and discharging, and there is a problem with oxidized water. However, the current situation is that the thermal stability is low and commercialization is difficult.

[0004] To solve this problem, it is necessary to use other stable transition metals at the unstable Ni site. (Co, Mn, etc.) must be replaced, and for this purpose, Co and Mn are replaced. A ternary NCM system was developed. However, in the case of ternary NCM, the thermal stability decreases as the Ni content increases. do. Summary of the Invention [Problem to be solved by the invention]

[0005] Therefore, the present inventors have controlled the structure of the primary particles on the surface of the layered secondary particles to We propose a method to suppress the decomposition reaction of the electrolyte in the cathode. Safety can be improved. [Means for solving the problem]

[0006] In one embodiment of the present invention, lithium metal oxide particles in the form of secondary particles comprising primary particles are the surface of the secondary particle is aligned with the c-axis of the primary particle; The narrower angle between the point and the line connecting the center point of the secondary particle is 60 to 90 degrees. A positive electrode active material is provided, which includes primary particles having a crystalline shape.

[0007] The surface of the secondary particle means the outermost part of the secondary particle, which is formed by the primary particle.

[0008] FIG. 1 is a schematic diagram of a secondary particle according to one embodiment of the present invention.

[0009] In order to define the alignment direction of primary particles, as shown in Figure 1, plate-shaped primary particles are defined as having a narrower angle of 60 to 90° between the c-axis of the primary particle and a line connecting the virtual center point of the primary particle and the center point of the secondary particle. More specifically, the narrower angle may be 70 to 90°. On the other hand, as shown in Figure 1, the c-axis refers to the long axis of the primary particle. Furthermore, primary particles having a needle-like shape with the angle being 0° or more and less than 60° are defined as having a needle-like shape. More specifically, the angle may be 0° or more and 20° or less.

[0010] The plate-like particles on the surface of the secondary particles suppress side reactions with the electrolyte, and the positive electrode active material The thermal stability of the compound can be improved.

[0011] More specifically, the radius of a circle having a radius of 50% of the radius of the secondary particle at the center point of the secondary particle is The area occupied by the plate-like primary particles is 20% or more of the area of ​​the total surface area (100% by area). More specifically, it may be 30% by area or more.

[0012] Specifically, FIG. 2 is an SEM photograph of secondary particles according to one embodiment of the present invention.

[0013] As shown in Figure 2, a circle is set based on the center of the secondary particle, and then the area inside the circle is The area of ​​the platelet-shaped primary particles can be calculated. At this time, as shown in Figure 2, the area occupied by plate-shaped primary particles is 20% or more by area. In this case, as described above, the side reaction with the electrolyte can be effectively controlled. More preferably, it may be 50% by area or more, and even more preferably, it may be 70% by area or more. The upper limit of the total area may be 100% by area of ​​plate-shaped particles, but in practice, Some needle-shaped particles are present, and plate-shaped particles may account for 95% or less by area.

[0014] The area of ​​a circle at the center point of the secondary particle, which is 50% of the radius of the secondary particle, is 100%. %, the average length of the plate-like primary particles present within the area is 750 nm to 1.2 It may be 5 μm.

[0015] The length of a plate-like primary particle means the length in the longest direction of the particle. When the thickness satisfies the above range, the desired battery characteristics can be ensured.

[0016] The inside of the secondary particle is defined by a c-axis of the primary particle and a virtual point at the center of the primary particle. and the center point of the secondary particle, the narrower angle is 0° or more and less than 60° More specifically, the primary particles may have a needle-like shape of 0 to 20°. The primary particles may be:

[0017] More specifically, inside the secondary particle, needle-shaped particles are arranged toward the center of the secondary particle. It may also have a radial structure.

[0018] The concentration gradient in the secondary particles where the nickel concentration decreases from the inside to the surface When such a structure has a specific surface area, such an internally directed structure can be formed.

[0019] In this case, the core of the secondary particle has a constant nickel concentration. This can be achieved by mixing the raw materials appropriately during the precursor manufacturing step described below. can be tightly controlled.

[0020] More specifically, the content of nickel in the metal in the secondary particles is 80 mol % or more. It is possible to obtain a high nickel content that cannot be obtained with existing positive electrode active materials containing nickel at 50 mol % or less. The nickel concentration can be increased for power performance.

[0021] In another embodiment of the present invention, a metal precursor is obtained by introducing an aqueous metal salt solution into a coprecipitation reactor. and mixing the metal precursor and the lithium source material and then calcining the mixture to obtain a positive electrode active material. wherein the step of obtaining a metal precursor by introducing an aqueous metal salt solution into the coprecipitation reactor includes: The pH is measured in the range of 1 to 30% at the end of the reaction, based on the total reaction time of 100%. A method for producing a positive electrode active material by changing conditions is provided.

[0022] More specifically, a method of changing the pH at the end of the reaction in the stage of obtaining a precursor by coprecipitation reaction. In this case, the reaction completion time is 100% of the total reaction time. This can mean 1 to 30 hours from the last end time.

[0023] More specifically, the reaction end point is the final end point of the total reaction time of 100 hours. It may be 1 to 10 hour % or 1 to 5 hour %.

[0024] This is because the shape of the plate-like primary particles on the surface of the positive electrode active material according to one embodiment of the present invention is The composition can be influenced and appropriately controlled to meet the desired specifications. Specifically, in the step of obtaining a metal precursor by introducing an aqueous metal salt solution into the coprecipitation reactor, The difference between the reaction start pH and reaction end pH may be 0.1 to 0.8.

[0025] More specifically, the reaction time is 1 to 30 hours relative to the total reaction time of 100 hours. %, the pH condition can be adjusted to be 0.1 to 0.8 higher. In this case, the plate-like particles present on the surface of the secondary particles can be formed uniformly.

[0026] More specifically, the rate of adjusting the pH condition is 0.0016 to 0.0133 pH / min. More specifically, it may be 0.0066 to 0.0133 pH / min. Good too.

[0027] Such process conditions will be explained in more detail in the examples below.

[0028] In another embodiment of the present invention, a positive electrode including the positive electrode active material according to the embodiment of the present invention described above may be used. a negative electrode including a negative electrode active material; and an electrolyte located between the positive and negative electrodes. We provide an aluminum secondary battery.

[0029] The description of the positive electrode active material is the same as that of the embodiment of the present invention described above, and therefore will be omitted. do.

[0030] The positive electrode active material layer may include a binder and a conductive material.

[0031] The binder serves to firmly adhere the positive electrode active material particles to each other and also to act as a current collector. It acts as a good adhesive to the conductive material.

[0032] The conductive material is used to impart conductivity to the electrode. Any material that conducts electrons without undergoing chemical changes can be used in the pond. It is Noh.

[0033] The negative electrode includes a current collector and a negative electrode active material layer formed on the current collector. The material layer includes a negative electrode active material.

[0034] The negative electrode active material may be a material capable of reversibly inserting / extracting lithium ions, Metals, alloys of lithium metal, materials that can be doped and dedoped with lithium, or transition metals Contains metal oxides.

[0035] The material capable of reversibly inserting / extracting lithium ions is a carbon material, Any carbon-based negative electrode active material commonly used in lithium-ion secondary batteries can be used. Typical examples include crystalline carbon, amorphous carbon, or a combination of these. can be done.

[0036] The lithium metal alloys include lithium and Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al and Sn An alloy with a metal selected from the group consisting of:

[0037] The material capable of doping and dedoping lithium includes Si, SiO x (0 <x< 2), Si-Y alloy (wherein Y is an alkali metal, alkaline earth metal, Group 13 element, Group 14 element, an element selected from the group consisting of elements, transition metals, rare earth elements, and combinations thereof; (Si is not included), Sn, SnO2, Sn-Y (where Y is an alkali metal, alkali Earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof and the like, which is an element selected from the group consisting of:

[0038] Examples of the transition metal oxide include vanadium oxide and lithium vanadium oxide. The negative electrode active material layer contains a binder and optionally further contains a conductive material. Good too.

[0039] The binder serves to firmly adhere the negative electrode active material particles to each other and also to act as a current collector. It acts as a good adhesive to the conductive material.

[0040] The conductive material is used to impart conductivity to the electrode. Any material that conducts electrons without undergoing chemical changes can be used in the pond. It is Noh.

[0041] The current collector may be made of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foil, or Foam, copper foam, conductive metal coated polymer substrate, and A combination selected from the group consisting of these may be used.

[0042] The negative electrode and the positive electrode are formed by mixing an active material, a conductive material, and a binder in a solvent to prepare an active material composition. The electrode is manufactured by coating the composition on a current collector. Since this is well known in the art, detailed explanations will be omitted here. For example, N-methylpyrrolidone can be used, but the present invention is not limited to this. do not have.

[0043] The electrolyte includes a non-aqueous organic solvent and a lithium salt.

[0044] The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move. Fulfill your share.

[0045] The lithium salt dissolves in an organic solvent and acts as a source of lithium ions in the battery. This allows basic lithium secondary battery operation and facilitates the transfer of lithium ions between the positive and negative electrodes. It is a substance that plays a role in promoting movement.

[0046] Depending on the type of lithium secondary battery, a separator may be present between the positive electrode and the negative electrode. Examples of such separators include polyethylene, polypropylene, polyvinylidene fluoride, Polyethylene / polypropylene or multilayer films of two or more layers can be used. Two-layer separator, polyethylene / polypropylene / polyethylene three-layer separator, Mixed multilayer films such as propylene / polyethylene / polypropylene triple layer separators Of course, it can be used.

[0047] Lithium secondary batteries are classified into lithium-ion batteries depending on the type of separator and electrolyte used. They are classified into lithium-ion polymer batteries and lithium polymer batteries, and are divided into They are classified into cylindrical, square, coin, pouch, etc., and are divided into bulk and thin film types depending on the size. The structure and manufacturing method of these batteries are widely known in the field. , detailed explanation will be omitted. [Effects of the Invention]

[0048] In the positive electrode active material according to one embodiment of the present invention, the plate-like surface with low reactivity comes into contact with the electrolyte, This can suppress the decomposition reaction of the electrolyte on the surface of the positive electrode. Such a reduction in side reactions of the electrolyte improves the thermal stability of the positive electrode active material. More specifically, during DSC analysis, the peak temperature increases and the total heat release decreases, resulting in thermal safety. It is possible to provide a positive electrode active material with improved properties. [Brief explanation of the drawings]

[0049] [Figure 1] FIG. 2 is a schematic diagram of a secondary particle according to one embodiment of the present invention. [Figure 2] 1 shows the shape of primary particles present on the surface of the positive electrode active material of Example 1. [Figure 3] 1 shows the shape of primary particles present on the surface of the positive electrode active material of Example 3. [Figure 4] 1 shows the shape of primary particles present on the surface of the positive electrode active material of Comparative Example 1. [Figure 5] 1 shows the cross-sectional shape of the positive electrode active material of Example 1. [Figure 6] 1 shows the cross-sectional shape of the positive electrode active material of Example 3. [Figure 7] 1 shows the cross-sectional shape of the positive electrode active material of Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0050] The following detailed description of the preferred embodiments of the present invention is provided by way of example only. The present invention is not limited by these claims, but is limited only by the scope of the claims set forth below. be defined.

[0051] (Example 1) Production of positive electrode active material containing 88 mol% Ni 1) Preparation of metal salt solutions First, NiSO4·6H2O is used as the nickel source material, and C is used as the cobalt source material. oSO4·7H2O, and MnSO4·H2O was used as the manganese source material. Two aqueous metal salt solutions with different concentrations of o and Mn were prepared. The first metal salt aqueous solution for core formation was prepared by dissolving (Ni 0.98 Co 0.01 Mn 0.01 The raw materials were mixed so as to satisfy the stoichiometric molar ratio of )(OH)2. Independently, the aqueous solution of the second metal salt for the shell formation was prepared by dissolving (Ni 0. 64 Co 0.23 Mn 0.13 )(OH)2 so as to satisfy the stoichiometric molar ratio of each of the above-mentioned elements. The ingredients were mixed.

[0052] 2) Co-precipitation process A coprecipitation reactor was prepared in which two metal salt aqueous solution supply tanks were connected in series. The first metal salt aqueous solution and the second metal salt aqueous solution were charged into a metal salt aqueous solution supply tank. Distilled water was added to the coprecipitation reactor, and the reactor temperature was maintained constant while stirring. In addition, NH4(OH) is used as a chelating agent, and NaOH solution is used as a pH adjuster. did. At this time, the initial pH in the reactor during the reaction was set to 11.2.

[0053] In this way, the pH is maintained constant and the chelating agent is supplied to the reactor. The time and amount of each metal salt solution was adjusted from two metal salt aqueous solution supply tanks. Specifically, the first metal salt aqueous solution was added at a rate of 0.4 liters / hour, and the precipitate was The coprecipitation reaction was carried out until the diameter reached approximately 11.1 μm. At this time, the flow rate was adjusted to prevent the reaction of the solution. The average residence time in the reactor is set to about 10 hours, and after the reaction reaches a steady state, Allow the reaction to stand for a period of time to achieve a denser coprecipitate. did.

[0054] Next, while changing the mixing ratio of the first metal salt aqueous solution and the second metal salt aqueous solution, The total feed solution was added at 0.4 liters / hour, and the feed rate of the first metal salt aqueous solution was 0. The feed rate of the second metal salt aqueous solution was gradually reduced to 0.35 liters / hour. The flow rate was gradually increased by 1 / hour. At this time, the average residence time of the solution in the reactor was adjusted by adjusting the flow rate. The time should be within 20 hours, and the precipitates were mixed together until the diameter reached 16.0 μm. A precipitation reaction was carried out. At this time, the pH was adjusted to a high level of 12.0 one hour before the end of the reaction. The pH was 0.0133 pH / min.

[0055] 3) Post-processing The precipitate obtained by the series of coprecipitation steps was filtered, washed with water, and then heated at 100°C. After drying in an oven for 24 hours, the composition of the whole particle was determined to be (Ni 0.88 Co 0. 095 Mn 0.025 )(OH)2, and the average particle size is 16.0 μm. An active material precursor was prepared.

[0056] 4) Firing process There is a core-shell concentration gradient, and Ni 0.88 Co 0.095 Mn 0.025 (OH)2 A precursor with the composition of ZrO2 (Aldrich, 4N, Zr concentration 3,400ppm) was used. Al(OH)3 (Aldrich, 4N, Al concentration standard 140 ppm) was uniformly Then, LiOH·H2O (Samchun Chemical, ery grade) in a molar ratio of 1:1.05, and then heated in a furnace. The mixture was charged into a furnace and fired under a flow of oxygen.

[0057] Thereafter, the mixture was naturally cooled, and then crushed and classified to produce a positive electrode active material.

[0058] (Example 2) Preparation of positive electrode active material containing 88 mol% Ni When preparing the raw material, (Ni 0.88 Co 0.095 Mn 0.025 )(OH)2 The molar ratio was adjusted to satisfy the reaction starting pH during the coprecipitation process. The pH was adjusted to a high level of 11.8 one hour before the end of the reaction. The positive electrode active material was prepared in the same manner as in Example 1, except that the rate was adjusted to 0.01 pH / min. did.

[0059] (Example 3) Preparation of positive electrode active material containing 88 mol% Ni During the coprecipitation step, the reaction starting pH was adjusted to 11.2, and the pH was adjusted to 11.2 one hour before the end of the reaction. The pH was adjusted to the range of 11.6, and the pH adjustment rate was adjusted to 0.0066 pH / min. A positive electrode active material was produced in the same manner as in Example 2, except that the above-mentioned adjustment was performed.

[0060] (Example 4) Preparation of positive electrode active material containing 88 mol% Ni During the coprecipitation step, the reaction starting pH was adjusted to 11.2, and the pH was adjusted to 11.2 one hour before the end of the reaction. The pH was adjusted to the range of 11.4, and the pH adjustment rate was adjusted to 0.0033 pH / min. A positive electrode active material was produced in the same manner as in Example 2, except that the above-mentioned adjustment was performed.

[0061] (Comparative Example 1) Production of a positive electrode active material containing 88 mol% Ni The same procedure as in Example 2 was repeated except that the pH was maintained at 11.0 during the coprecipitation step. A positive electrode active material was produced in the same manner as in the above.

[0062] [Table 1]

[0063] (Experimental Example 1) Shape analysis of the surface of the positive electrode active material (SEM) The positive electrode active materials of Examples 1, 3, and Comparative Example 1 were analyzed by SEM. The shape of the primary particles present on the surface was observed.

[0064] FIG. 2 shows the shape of the primary particles present on the surface of the positive electrode active material of Example 1, and shows the shape of the primary particles. It was confirmed that the secondary particles uniformly surrounded the entire surface of the positive electrode active material. The surface (c-axis surface) is a surface where the desorption / insertion reaction of Li does not occur, so the electric potential on the surface of the positive electrode It is expected that the decomposition reaction of the solution was suppressed.

[0065] FIG. 3 shows the shape of primary particles present on the surface of the positive electrode active material of Example 3, and shows the shape of a plate-like particle. The secondary particles and acicular particles are mixed on the surface of the positive electrode active material.

[0066] FIG. 4 shows the shape of primary particles present on the surface of the positive electrode active material of Comparative Example 1, and shows the shape of the primary particles. The secondary particles are uniformly distributed. This is the same shape as that usually observed when a positive electrode active material is produced. is similar to

[0067] (Experimental Example 2) Shape analysis of cross section of positive electrode active material (TEM) The cross sections of the positive electrode active materials of Example 1, Example 3, and Comparative Example 1 were cut using an FIB and analyzed by TEM. The shape distribution of primary particles in the cross section of the positive electrode material was observed using an analyzer.

[0068] FIG. 5 shows the cross-sectional shape of the positive electrode active material of Example 1, and FIG. 6 shows the cross-sectional shape of the positive electrode active material of Example 3. 7 shows the cross-sectional shape of the positive electrode active material of Comparative Example 1. The arrangement of the primary particles inside the positive electrode active material was acicular in all of Examples 1, 3, and Comparative Example 1. Numerous needle-shaped particles are observed, and they show a radial shape arranged toward the center of the secondary particle.

[0069] However, in the case of the arrangement of the primary particles on the surface of the positive electrode active material, Comparative Example 1 is similar to the interior and has needle-like structure. In Example 3, the primary particles exhibited plate-like morphology and the primary particles exhibited needle-like morphology. It can be confirmed that primary particles are mixed with the crystalline particles. It can be seen that the positive electrode active material of Example 1 contains more plate-shaped particles.

[0070] How many primary particles having a plate-like shape were present on the surface of the positive electrode active materials of Examples 1 to 4 and Comparative Example 1? We investigated how much of the surface area it occupies. This was confirmed by the following method.

[0071] As shown in Figure 2, the SEM photograph of the secondary particle is used as a reference, and the secondary particle at the center point of the secondary particle is The area occupied by plate-shaped particles relative to the area of ​​a circle with a radius of 50% of the primary particle (100% area) The product was calculated.

[0072] [Table 2]

[0073] (Experimental Example 3) Differential Scanning Calorimetry in the Charged State ng Calorimetry, DSC) The positive electrode active materials of Examples 1 to 4 and Comparative Example 1 were used to manufacture coin cells and charge them. The battery was disassembled and the thermal stability of the positive electrode active material was analyzed by DSC.

[0074] The slurry for the electrode plate is composed of the positive electrode, conductive material (Denka Black), binder (PVDF , KF1100) = 92.5:3.5:4 wt%, so that the solid content was about 30%. NMP (N-Methyl-2-pyrrolidone) was added to reduce the viscosity of the slurry. Adjusted. The prepared slurry was coated onto a 15 μm thick Al foil using a doctor blade method. After that, it was dried and rolled. The electrode loading was 14.6 mg / cm 2 and the rolling density is 3.1 g / cm 3 in The electrolyte was 1M LiPF in ethylene carbonate:dimethyl carbonate. : Ethyl methyl carbonate = 3:4:3 (vol%), polypropylene separator A coin cell was fabricated using a lithium anode (200µm, Honzo metal). Charging conditions are CC / CV 2.5~4.25V, 1 / 20C cut-off. there were. After that, the coin cell was disassembled in a dry room, and 10 mg of the positive electrode active material was extracted and analyzed by DSC. The analysis was carried out.

[0075] DSC analysis was performed by increasing the temperature from 25°C to 400°C at a rate of 5°C / min. After analyzing the set temperature, the peak temperature at which the calorific value becomes maximum, and the total calorific value, the results are shown in the table below. I have summarized it in 3.

[0076] (Experimental Example 4) Electrochemical Characterization The positive electrode active materials of Examples 1 to 4 and Comparative Example 1 were mixed in a 2032 coin-shaped container in the same manner as in Experimental Example 3. After manufacturing the half-cell, it was aged at room temperature (25°C) for 10 hours and then subjected to charge / discharge tests. They went on strike.

[0077] The capacity evaluation was based on a standard capacity of 215mAh / g, and the charge / discharge conditions were CC / CV2.5 to 4.2. 5V, 1 / 20C cut-off applied. Initial capacity is 0.2C charge / 0.2C discharge. It went under the conditions.

[0078] [Table 3]

[0079] From Table 3 above, it can be seen that the active materials of the examples have an increased peak temperature, a decreased total heat generation amount, and improved thermal stability. It is clear that the safety has improved.

[0080] [Table 4]

[0081] As can be seen from Table 4 above, the positive electrode active materials according to the examples of the present application have the same thermal stability as shown in Table 3. Although the difference is significantly improved, the charge / discharge capacity and efficiency are comparable to those of Comparative Example 1. We can see that.

[0082] The present invention is not limited to the above-described embodiments and may be manufactured in a variety of different forms. It is possible for a person having ordinary skill in the art to which the present invention pertains to understand the technical concept of the present invention. It is understood that the invention may be embodied in other specific forms without changing its concept or essential characteristics. Therefore, the above-described embodiments are illustrative in all respects and are not restrictive. You have to understand that it isn't.

Claims

1. lithium metal oxide particles in the form of secondary particles including primary particles, the surface of the secondary particle includes plate-like primary particles, the narrower angle between the c-axis of the primary particle and a line connecting an imaginary point at the center of the primary particle and the center point of the secondary particle being 60 to 90°; With respect to 100% of the area of ​​a circle based on 50% of the radius of the secondary particle at the center point of the secondary particle, The area occupied by the plate-like primary particles is 20% by area or more, The interior of the secondary particle contains needle-shaped primary particles in which the narrowest angle between the c-axis of the primary particle and a line connecting an imaginary point at the center of the primary particle and the center point of the secondary particle is 0° or more and less than 60°, and the area of ​​a circle at the center point of the secondary particle, which is 50% of the radius of the secondary particle, is: The average length of the plate-like primary particles present within the area is 750 nm to 1.25 μm.

2. 2. The positive electrode active material according to claim 1, wherein the content of nickel in the metal in the secondary particles is 80 mol % or more.

3. A positive electrode comprising the positive electrode active material according to claim 1 or 2; a negative electrode including a negative electrode active material; and an electrolyte located between the positive electrode and the negative electrode.

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