Positive Electrode Active Material for Lithium Secondary Battery and Method for Producing the Same

Abstract

The present invention provides a positive electrode active material for a lithium secondary battery, which can improve the high-temperature life and resistance increase rate of the positive electrode active material, and a method for manufacturing the same. The positive electrode active material according to the present invention includes a lithium boron compound coating layer on the surface of a positive electrode active material powder for a lithium secondary battery, and the peak intensity ratio between two peaks in the coating layer in a ToF-SIMS spectrum is the same as the corresponding peak intensity ratio in the LiBO2 material within a range of ±50%.

Description

Technical Field

[0001] The present invention relates to a positive electrode active material for a lithium secondary battery and a method for manufacturing the same. This application claims priority based on Korean Patent Application No. 10-2021-0107659 filed on August 13, 2021, and all the contents disclosed in the specification and drawings of the said application are incorporated into this application.

Background Art

[0002] Lithium secondary batteries that can be repeatedly charged and discharged have been in the spotlight as an alternative to fossil energy. Lithium secondary batteries have been mainly used in traditional handheld devices such as mobile phones, video cameras, and power tools. However, recently, their application fields have gradually increased, such as electric vehicles (EVs, HEVs, PHEVs), large-capacity power storage devices (ESSs), and uninterruptible power supply systems (UPSs).

[0003] A lithium secondary battery includes an electrode assembly in which unit cells having a structure in which a positive electrode plate and a negative electrode plate, on which an active material is coated on a current collector, are arranged with a separator interposed therebetween, and an exterior material that hermetically stores the electrode assembly together with an electrolytic solution, that is, a battery case. As the positive electrode active material of a lithium secondary battery, lithium composite transition metal oxides are used. Among them, lithium cobalt oxide such as LiCoO2, lithium manganese oxide (such as LiMnO2 or LiMn2O4), lithium iron phosphate compound (LiFePO4), or LiNiO2, etc. are mainly used. Also, as a method for improving low thermal stability while maintaining the excellent reversible capacity of LiNiO2, nickel manganese-based lithium composite metal oxides in which part of nickel is replaced with manganese having excellent thermal stability, NCM-based lithium composite transition metal oxides replaced with manganese and cobalt, NCA-based lithium composite transition metal oxides replaced with cobalt and aluminum, and NCMA-based lithium composite transition metal oxides replaced with cobalt, manganese, and aluminum are used.

[0004] Among these, the Ni-based lithium cathode active material using Ni is manufactured by mixing a lithium source and a precursor, firing at a high temperature, and then firing again at a low temperature together with H3BO3 to coat boron (B) on the surface. However, such a manufacturing method has limitations in improving the high-temperature life and resistance increase rate of the cathode active material, so improvement is required. Summary of the Invention Problems to be Solved by the Invention

[0005] The problem to be solved by the present invention is to provide a cathode active material for a lithium secondary battery and a manufacturing method thereof that can improve the high-temperature life and resistance increase rate of the cathode active material. Means for Solving the Problems

[0006] To solve the above problems, the cathode active material according to the present invention includes a lithium boron compound coating layer on the surface of the cathode active material powder for a lithium secondary battery, and the coating layer has a peak intensity ratio between two peaks in the spectrum of ToF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry) within the range of ±50% of the corresponding peak intensity ratio in the LiBO2 substance.

[0007] Here, the coating layer may have an intensity ratio of the strongest peak among the peaks detected at mass 156.85 - 156.95 to the strongest peak among the peaks detected at mass 153.05 - 153.15 of 4.3 ± 50% in the ToF-SIMS spectrum.

[0008] And the coating layer may have an intensity ratio of the strongest peak among the peaks detected at mass 182.85 - 182.95 to the strongest peak among the peaks detected at mass 179.05 - 179.15 of 9.9 ± 50% in the ToF-SIMS spectrum.

[0009] To solve the above problems, the method for manufacturing a positive electrode active material according to the present invention includes a primary firing step of mixing a lithium source and a precursor and then performing heat treatment, and a secondary firing step of mixing a first boron source with the fired product obtained by the primary firing and performing heat treatment to form a surface protection layer on the fired product, a water washing step of removing unreacted residual lithium on the surface of the fired product obtained by the secondary firing, and a coating step of drying the water-washed product obtained by the water washing and performing heat treatment together with a second boron source.

[0010] The precursor is a Ni-based lithium transition metal oxide with a high content, and the nickel content of the precursor can be 70 mol% or more based on the total number of moles of transition metals.

[0011] The heat treatment temperature during the primary firing can be 0.75 to 1.5 times the heat treatment temperature during the secondary firing.

[0012] The water washing step can be performed by mixing the fired product and water at a weight ratio of 50 to 200% and stirring.

[0013] The first boron source or the second boron source can be used alone or in combination of one or more of H3BO3, H4BO4, B2O3, LiBO2, Li2B4O7, B4C, AlBO2, and AlB2O4.

[0014] The amounts of the first boron source and the second boron source can be such that the weight of boron (B) / the weight of the positive electrode active material is 200 to 5,000 ppm.

Advantages of the Invention

[0015] According to the present invention, a positive electrode active material for a lithium secondary battery with improved high-temperature life, resistance increase rate, and output characteristics is provided.

[0016] According to the manufacturing method of the present invention, by adjusting the timing of boron (B) coating / doping during the firing process, a positive electrode active material for a lithium secondary battery with improved high-temperature life and resistance increase rate can be manufactured.

[0017] In the manufacturing method of the present invention, by varying the input timing of the boron source from that of the lithium source while adding the boron source during the secondary firing, an effect of improving the reactivity between the boron source and the positive electrode active material can be obtained.

[0018] According to the manufacturing method of the present invention, by including a coating / doping process of boron (B) element in the secondary firing process, a surface protection layer is formed on the fired product of the positive electrode active material, so that the B element coating layer generated during firing can suppress the surface deterioration of the positive electrode active material that may occur during the water washing process.

[0019] According to the manufacturing method of the present invention, when the dried water-washed product is heat-treated at a low temperature together with the second boron source, by changing the reactivity between the surface of the positive electrode active material and boron, the high-temperature life, resistance increase rate, and low-temperature output characteristics of the positive electrode active material can be improved.

[0020] The following drawings attached to this specification illustrate embodiments of the present invention and serve to further understand the technical idea of the present invention together with the detailed description of the invention. Therefore, the present invention should not be construed as being limited only to the matters described in the drawings.

Brief Description of the Drawings

[0021]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0022] Hereinafter, embodiments of the present invention will be described with reference to the attached drawings. Prior to this, terms and words used in this specification and the claims should not be construed as being limited to their ordinary and dictionary meanings. The inventor himself must interpret them in accordance with the meaning and concept corresponding to the technical idea of the present invention in accordance with the principle that he can appropriately define the concept of the terms in order to explain the invention in the best way. Therefore, the embodiments described in this specification and the configurations shown in the drawings are merely the most desirable embodiments of the present invention and do not represent all of the technical ideas of the present invention. It must be understood that there may be various equivalents and modifications that can replace them at the time of this application.

[0023] In the following description, reference is made to the attached drawings that form part of this application. The embodiments, drawings, and claims described in the detailed description are not intended to be limiting. Other embodiments can be utilized without departing from the spirit and scope of the subject matter disclosed herein, and other changes are possible. It will be understood that the aspects of the present invention generally described herein and illustrated by the drawings can be arranged, substituted, combined, separated, and designed in various other configurations, and all of these have been duly considered herein.

[0024] Unless defined otherwise, all technical and scientific terms used in this specification have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs (hereinafter referred to as "person skilled in the art").

[0025] The present invention is not limited by the specific examples described in this application. As will be apparent to those skilled in the art, various changes and modifications can be made without departing from the spirit and scope of the present invention. In addition to those listed herein, functionally equivalent methods within the scope of this application will be apparent to those skilled in the art from the above description. Such changes and modifications are within the scope of the appended claims. Together with the entire scope of equivalents qualified by such claims, the present invention is limited only by the claims. It should be understood that the present invention is of course not limited to a specific method that can be changed. The technical terms used herein are used only for the purpose of explaining specific examples and are not intended to be restrictive.

[0026] Figure 1 is a flowchart of a method for manufacturing a positive electrode active material according to the present invention. Hereinafter, a method for manufacturing a positive electrode active material for a lithium secondary battery according to the present invention will be described with reference to Figure 1.

[0027] First, after mixing a lithium source and a precursor, a primary firing step of heat treatment is performed (S10).

[0028] Through the primary firing step (S10), a positive electrode active material for a lithium secondary battery is synthesized. For example, an NCM-based lithium composite transition metal oxide can be manufactured. Desirably, the positive electrode active material manufactured in the present invention is an NCM-based lithium composite transition metal oxide, but other types of positive electrode active materials used in lithium secondary batteries can also be manufactured by the present invention.

[0029] The precursor can be obtained by mixing an aqueous solution such as a nickel compound, a cobalt compound, and a manganese compound as raw material substances with a basic solution and reacting them to obtain a reaction precipitate, and drying and heat-treating the precipitate. For example, it can be a precursor represented by the following Chemical Formula 1.

[0030] [Chemical Formula 1] Ni x1 Co y1 Mn z1 Al s1 (OH)2 (In Chemical Formula 1, 0.7 ≦ x1 ≦ 0.99, 0 < y1 < 0.3, 0 < z1 < 0.3, and 0 ≦ s1 ≦ 0.1 may be satisfied.)

[0031] For example, the precursor may be capable of producing a high-content Ni-based lithium transition metal oxide, and the nickel content of the precursor may be 70 mol% or more based on the total number of moles of transition metals.

[0032] The lithium source may be a lithium compound such as Li2CO3 or LiOH·H2O.

[0033] The fired product produced through the first firing step (S10) may be, for example, a lithium composite transition metal oxide represented by the following Chemical Formula 2.

[0034] [Chemical Formula 2] Li a [Ni b Co c Mn d Al e 1-f M 1 f O2 (In Chemical Formula 2, M 1 is one or more selected from the group consisting of Zr, W, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, F, P, and S, and 0.8 ≦ a ≦ 1.2, 0.7 ≦ b ≦ 0.99, 0 < c < 0.3, 0 < d < 0.3, 0.01 ≦ e ≦ 0.1, and 0 ≦ f ≦ 0.1 may be satisfied.)

[0035] For example, the lithium composite transition metal oxide may be LiNi 0.86 Co 0.05 Mn 0.07 Al 0.02 O2.

[0036] In the first firing, no boron source is mixed.

[0037] ​The heat treatment during the first firing can be carried out in the range of oxygen concentration from 21% to 100%. The heat treatment temperature of the first firing can be 0.75 to 1.5 times the heat treatment temperature of the subsequent second firing. The appropriate heat treatment temperature can vary within the above range according to the NCM element content. For example, the heat treatment during the first firing can be carried out at 500 to 900 °C. If it is less than 500 °C, sufficient reaction may not occur. If it exceeds 900 °C, the performance may decrease due to thermal decomposition of the positive electrode active material.

[0038] Next, a second firing step of forming a surface protection layer on the fired product by mixing a first boron source with the fired product obtained by the first firing and performing heat treatment is carried out (S20). After the heat treatment of the first firing (S10), after cooling, a first boron source can be introduced into the fired product to perform the heat treatment of the second firing.

[0039] It is characterized in that no boron source is mixed during the first firing (S10), and then the boron source is introduced in two portions. The first introduction point is the second firing step (S20). The boron source introduced at this time is referred to as the first boron source.

[0040] The first boron source is a coating / doping raw material substance, and one or more of H3BO3, H4BO4, B2O3, LiBO2, Li2B4O7, B4C, AlBO2, and AlB2O4 can be used alone or in combination. The method of mixing the first boron source with the fired product obtained by the first firing can be carried out by a solid-phase or liquid-phase method. As the solid-phase or liquid-phase method, methods such as mixing, milling, spray drying, and grinding can be used. Desirably, it is carried out by a dry solid-phase method.

[0041] The heat treatment during the secondary firing can be carried out in an oxygen concentration range of 21 - 100%. The heat treatment of the secondary firing can also be carried out at 500 - 900°C, similar to the heat treatment of the primary firing. If it is less than 500°C, it may be difficult to form the surface protection layer smoothly or uniformly. If it is 900°C or higher, it is difficult for the cathode active material to maintain its original properties. Such a range of heat treatment temperature can be changed depending on the NCM element content. However, as described above, it is desirable that the heat treatment temperature during the primary firing is 0.75 - 1.5 times that of the heat treatment temperature during the secondary firing. When the heat treatment temperature during the primary firing is outside the above range, it was confirmed that even if the first boron source is added during the secondary firing, fired, washed with water, and coated, the high-temperature life characteristics are inferior. Therefore, the relationship condition between the heat treatment temperature during the primary firing and the heat treatment temperature during the secondary firing is of critical significance because it is for preventing the deterioration of the high-temperature life characteristics.

[0042] Thus, in the present invention, it is characterized in that the firing process is not carried out at once, but is divided into two steps: primary firing and secondary firing. Also, it is characterized in that the boron source is not added during the primary firing but is added during the secondary firing. The feature is that no boron source, which is the coating / doping raw material substance, is added in the first firing, and the boron source, which is the coating / doping raw material substance, is added only in the second firing. The first boron source introduced during the secondary firing coats / dopes the B element on the fired product to form a surface protection layer. A part of the first boron source is mixed into the interior of the fired product and doped, and the remaining part is located on the surface of the fired product to generate a boron source coating layer, and the boron source coating layer acts as the surface protection layer.

[0043] Conventionally, coating has been performed by introducing a boron source only in the final stage after firing. In the present invention, by adding a boron source during the secondary firing while differing the introduction timing from that of the lithium source, it is possible to achieve the effect of improving the reactivity between the boron source and the positive electrode active material. The surface protection layer formed by the secondary firing suppresses the surface deterioration of the positive electrode active material in the subsequent water washing step (S30). Thus, in the manufacturing method of the present invention, it should be noted that the surface protection layer is already formed on the surface of the fired product before performing the water washing step (S30), and the surface deterioration of the positive electrode active material in the water washing step (S30) is suppressed.

[0044] The fired product manufactured through the secondary firing step (S20) in this way can be, for example, a lithium composite transition metal oxide represented by the following Chemical Formula 3.

[0045] [Chemical Formula 3] Li a [Ni b Co c Mn d Al e 1-f (B, M 1 ) f O2 (In Chemical Formula 3, M 1 is one or more selected from the group consisting of Zr, W, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, F, P, and S, and 0.8 ≦ a ≦ 1.2, 0.7 ≦ b ≦ 0.99, 0 < c < 0.3, 0 < d < 0.3, 0.01 ≦ e ≦ 0.1, 0 ≦ f ≦ 0.1 may be satisfied.)

[0046] Thereafter, a water washing step is performed (S30) to remove unreacted residual lithium on the surface of the fired product by the secondary firing.

[0047] Since a lithium compound such as LiOH may remain in the positive electrode active material, a water washing step is required. The water washing step can be performed by mixing and stirring the fired product and water at a weight ratio of 50 to 200%. Additives such as LiOH and NaOH may be added during the water washing.

[0048] ​By including the process of coating / doping with boron (B) element in the secondary firing process, a surface protection layer is formed on the fired product of the positive electrode active material, and the B element coating layer generated during firing can suppress the surface deterioration of the positive electrode active material that may occur during the water washing process. Compared with the conventional case where the boron source is introduced and coated only at the final stage after firing, an excellent effect of suppressing the surface deterioration of the positive electrode active material during water washing is obtained. Conventionally, the surface of the fired product on which water washing is performed is not provided with a surface protection layer as in the present invention.

[0049] Next, a coating step is performed (S40) of drying the water-washed product by the water washing and then heat-treating it together with the second boron source.

[0050] During the first firing (S10), the boron source is not mixed, and then the boron source is added in two portions. The first addition point is the above-described secondary firing step (S20), and the second addition point is the coating step (S40). The boron source introduced at this time is used as the second boron source.

[0051] The second boron source is a coating / doping raw material substance, and one or more of H3BO3, H4BO4, B2O3, LiBO2, Li2B4O7, B4C, AlBO2, and AlB2O4 can be used alone or in combination. The method of mixing the second boron source with the dried water-washed product can be performed in a solid phase or liquid phase manner. As the solid phase or liquid phase method, methods such as mixing, milling, spray drying, and grinding can be used.

[0052] The heat treatment can be performed in the range of an oxygen concentration of 21 to 100%. The heat treatment in step S40 can be performed at 200°C to 400°C. If it is less than 200°C, there is a risk that the capacity manifestation may not proceed smoothly. If it exceeds 400°C, the high-temperature life may deteriorate, so it is desirable not to exceed 400°C.

[0053] The first boron source and the second boron source may be the same or different. However, the amounts of the first boron source and the second boron source may be such that the weight of boron (B) / the weight of the positive electrode active material is 200 to 5,000 ppm.

[0054] If the weight of boron / the weight of the positive electrode active material is less than 200 ppm, there is no coating effect. If the weight of boron / the weight of the positive electrode active material exceeds 5,000 ppm, an excessive coating layer may be generated, and the excessive coating layer is not desirable in terms of capacity and resistance. When the dried and washed product is heat-treated at a low temperature together with the second boron source, by changing the reactivity between the surface of the positive electrode active material and B, the high-temperature life, the resistance increase rate, and the low-temperature output characteristics of the positive electrode active material can be improved. Through the washing step (S30), since the surface deterioration of the positive electrode active material is suppressed by the surface protection layer, the reactivity between the second boron source introduced in the coating step (S40) and the surface of the positive electrode active material is superior to the reactivity between the boron source and the surface of the positive electrode active material that has been washed and deteriorated without the surface protection layer.

[0055] The positive electrode active material of the present invention manufactured by the above-described manufacturing method has the following characteristics. The positive electrode active material according to the present invention includes a lithium boron compound coating layer on the surface of the positive electrode active material powder for a lithium secondary battery, and in the coating layer, the peak intensity ratio between two peaks in the ToF-SIMS spectrum is the same as the corresponding peak intensity ratio in the LiBO2 substance within a range of ±50%.

[0056] The positive electrode active material according to the present invention includes a lithium boron compound coating layer on the surface of the positive electrode active material powder for a lithium secondary battery, and in the coating layer, the peak intensity ratio between two peaks in the ToF-SIMS spectrum may be the same as the corresponding peak intensity ratio in the LiBO₂ substance within a range of ±50%.

[0057] For example, the positive electrode active material may be an NCM-based lithium composite transition metal oxide. Among them, especially in the case of a high-content Ni-based lithium composite transition metal oxide with a nickel content of 70% or more, in order to increase the stability in the cycle driving environment, the reactivity with the electrolyte should be reduced. The positive electrode active material according to the present invention can reduce the reactivity with the electrolyte by including the coating layer, so that gas generation at high temperature is reduced.

[0058] The positive electrode active material according to the present invention includes the coating layer, thereby improving the high-temperature life and the resistance increase rate. Furthermore, the output characteristics are also improved.

[0059] Here, the coating layer may have an intensity ratio of the strongest peak among the peaks detected at mass 156.85 to 156.95 and the strongest peak among the peaks detected at mass 153.05 to 153.15 of 4.3 ± 50% in the ToF-SIMS spectrum. Here, "4.3 ± 50%" indicates the range between the value obtained by adding 2.15, which is 50% of 4.3, to 4.3 and the value obtained by subtracting it from 4.3. That is, it indicates 2.15 to 6.43.

[0060] And the coating layer may have an intensity ratio of the strongest peak among the peaks detected at mass 182.85 to 182.95 and the strongest peak among the peaks detected at mass 179.05 to 179.15 of 9.9 ± 50% in the ToF-SIMS spectrum. Here, "9.9 ± 50%" indicates the range between the value obtained by adding 4.95, which is 50% of 9.9, to 9.9 and the value obtained by subtracting it from 9.9. That is, it indicates 4.95 to 14.85.

[0061] That is, the coating layer of the positive electrode active material according to the present invention not only has peaks in a specific mass range obtained by ToF-SIMS analysis, but also these peaks have a predetermined peak intensity ratio, and the peak intensity ratio is the same as the corresponding peak intensity ratio in the LiBO2 substance within the range of ± 50%. When such a positive electrode active material is used for the positive electrode of a lithium secondary battery, high output and high stability can be achieved.

[0062] According to the present invention as described above, by adjusting the timing of boron (B) coating / doping in the firing process, the high-temperature life, resistance increase rate, and low-temperature output characteristics of the positive electrode active material can be improved.

[0063] Hereinafter, the present invention will be described in more detail with reference to experimental examples.

[0064] Example: LiOH·H2O as a lithium source, Ni as a precursor 0.88 Co 0.05 Mn 0.07 (OH)2 + Al(OH)3 were mixed so that the molar ratio of Li: transition metals (Ni, Co, Mn, Al) was 1.03:1, and primary fired at 700 °C for 5 hours to obtain LiNi 0.86 Co 0.05 Mn 0.07 Al 0.02 O2. Thereafter, the primary fired product and H3BO3 as the first boron source were mixed at a weight ratio of 100:0.86, and then secondary fired at 750 °C for 5 hours. The produced fired product and water were mixed at a weight ratio of 100:100 and stirred for 5 minutes. Thereafter, the washed product was filtered by a filter press and vacuum dried at 130 °C. Next, the dried product was mixed with H3BO3 as the second boron source at a weight ratio of 100:0.57 and heat treated at 300 °C for 4 hours to produce a positive electrode active material having boron (B) coated on the surface.

[0065] Comparative Example 1: LiOH·H2O as a lithium source, Ni as a precursor 0.88 Co 0.05 Mn 0.07 (OH)2 + Al(OH)3 were mixed so that the molar ratio of Li: transition metals (Ni, Co, Mn, Al) was 1.03:1, and primary fired at 700 °C for 5 hours to obtain LiNi 0.86 Co 0.05 Mn 0.07 Al 0.02 O2. Thereafter, no boron source was added, and secondary firing was performed at 750 °C for 5 hours.

[0066] Comparative Example 2: LiOH·H2O as the lithium source, Ni as the precursor 0.88 Co 0.05 Mn 0.07 (OH)2 + Al(OH)3 were mixed so that the molar ratio of Li: transition metals (Ni, Co, Mn, Al) became 1.03:1, and primary calcination was performed at 700 °C for 5 hours to obtain LiNi 0.86 Co 0.05 Mn 0.07 Al 0.02 O2. Then, without adding any boron source, secondary calcination was performed at 750 °C for 5 hours. The produced calcined product and water were mixed at a weight ratio of 100:100 and stirred for 5 minutes. Then, the washed product was filtered with a filter press and vacuum dried at 130 °C. Next, the dried product was mixed with H3BO3 at a weight ratio of 100:0.57 and heat-treated at 300 °C for 4 hours.

[0067] Comparative Example 3: LiOH·H2O as the lithium source, Ni as the precursor 0.88 Co 0.05 Mn 0.07 (OH)2 + Al(OH)3 were mixed so that the molar ratio of Li: transition metals (Ni, Co, Mn, Al) became 1.03:1, and thereto Ni 0.88 Co 0.05 Mn 0.07 (OH)2 and H3BO3 were mixed at a weight ratio of 100:0.91 and primary calcination was performed at 700 °C for 5 hours, followed by secondary calcination at 750 °C for 5 hours.

[0068] Comparative Example 4: LiOH·H2O as the lithium source, Ni as the precursor 0.88 Co 0.05 Mn 0.07 (OH)2 + Al(OH)3 were mixed so that the molar ratio of Li: transition metals (Ni, Co, Mn, Al) became 1.03:1, and thereto Ni 0.88 Co 0.05 Mn 0.07(OH)2 and H3BO3 were mixed at a weight ratio of 100:0.91 and subjected to a first firing at 700 °C for 5 hours and a second firing at 750 °C for 5 hours. The produced fired product and water were mixed at a weight ratio of 100:100 and stirred for 5 minutes. Then, the washed product was filtered using a filter press and vacuum dried at 130 °C. Next, the dried product was mixed with H3BO3 at a weight ratio of 100:0.57 and heat treated at 300 °C for 4 hours.

[0069] Comparative Example 5: LiOH·H2O as a lithium source, Ni as a precursor 0.88 Co 0.05 Mn 0.07 (OH)2 + Al(OH)3 were mixed so that the molar ratio of Li: transition metals (Ni, Co, Mn, Al) became 1.03:1, and a first firing was carried out at 700 °C for 5 hours to obtain a first fired product of LiNi 0.86 Co 0.05 Mn 0.07 Al 0.02 O2 was produced. Then, the first fired product and H3BO3 were mixed at a weight ratio of 100:0.86 and subjected to a second firing at 750 °C for 5 hours.

[0070] In short, the above-described examples and comparative examples are as follows.

[0071] Example: Li + NCM(OH)2 + Al mixture → First firing at 700 °C → Positive electrode active material + First boron source mixture → Second firing at 750 °C → Water washing → Drying at 130 °C → Mixing with the second boron source for coating (300 °C)

[0072] Comparative Example 1: (Without mixing positive electrode active material and boron source, two-stage firing): Li + NCM(OH)2 + Al mixture → First firing at 700 °C → Second firing at 750 °C

[0073] Comparative Example 2: (Without mixing positive electrode active material and boron source, two-stage firing): Li + NCM(OH)2 + Al mixture → First firing at 700 °C → Second firing at 750 °C → Water washing → Drying at 130 °C → Mixing with H3BO3 for coating (300 °C)

[0074] Comparative Example 3: (Mixing the lithium source and boron source together during the first firing): Li + NCM(OH)2 + Al + H3BO3 mixing → first firing at 700°C → second firing at 750°C

[0075] Comparative Example 4: (Mixing the lithium source and boron source together during the first firing): Li + NCM(OH)2 + Al + H3BO3 mixing → first firing at 700°C → second firing at 750°C → washing with water → drying at 130°C → mixing with H3BO3 for coating (300°C)

[0076] Comparative Example 5: Li + NCM(OH)2 + Al mixing → first firing at 700°C → mixing the cathode active material + H3BO3 → second firing at 750°C

[0077] The experimental items are the time-dependent change test, electrochemical data of the coin-type half cell, low-temperature output data of the monocell, high-temperature life data of the monocell, and confirmation of the peaks detected when adding the boron source through ToF-SIMS.

[0078] <Analysis results of ToF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry, time-of-flight secondary ion mass spectrometry)>[ The components of the coating layer were confirmed through ToF-SIMS analysis. Usually, XRD measurement is used to confirm the material composition and crystal structure. However, since the coating layer of the cathode active material has a thickness of several nm to several tens of nm, it is not easy to detect peaks by XRD measurement. In the present invention, ToF-SIMS analysis was used to confirm the components of a very thin coating layer. As a result of the ToF-SIMS analysis, in the examples of the present invention, a lithium borate compound coating layer additionally formed on the surface of the cathode active material was confirmed. That is, it was confirmed that a lithium borate compound was additionally formed by the manufacturing method of the present invention.

[0079] Figures 2 and 3 are the analysis results of the ToF-SIMS spectra of Comparative Examples 2 and 4 and the examples. For comparison, the results of LiBO2 are also shown together.

[0080] Referring to FIG. 2, in the example, among the peaks detected at mass 156.85 to 156.95, the strongest peak (peak 1) is the peak measured at mass 156.9. In the example, among the peaks detected at mass 153.05 to 153.15, the strongest peak (peak 2) is the peak measured at mass 153.1. The mass values are the measured values after calibrating the ToF-SIMS spectrum with Li + , C + , C2 + , C3 + peaks. In the example, the intensity ratio of peak 1 to peak 2 is 4.3. Even considering measurement errors and other manufacturing requirements, in the examples of the present invention, the intensity ratio between the strongest peak among the peaks detected at mass 156.85 to 156.95 and the strongest peak among the peaks detected at mass 153.05 to 153.15 can have a value of 4.3 ± 50%.

[0081] In the example, the intensity ratio between the peak measured at mass 156.9 and the peak measured at mass 153.1 is similar to the corresponding peak intensity ratio in the LiBO2 substance (i.e., the intensity ratio of peak 1 to peak 2 in the LiBO2 substance). Similar means being the same within a range of ±50%. In Comparative Examples 2 and 4, these peaks are observed in different patterns. In Comparative Example 2, the intensity ratio of peak 1 to peak 2 is 0.3, and in Comparative Example 4, the intensity ratio of peak 1 to peak 2 is 2.0.

[0082] Referring to FIG. 3, in the example, among the peaks detected at mass 182.85 to 182.95, the strongest peak (peak 3) is the peak measured at mass 182.9, and among the peaks detected at mass 179.05 to 179.15, the strongest peak (peak 4) is the peak measured at mass 179.1. The mass values are the measured values after calibrating the ToF-SIMS spectrum with Li + , C + , C2 + , C3 +This is the value measured after calibration at the peak. In the examples, the intensity ratio between peak 3 and peak 4 is 9.9. Even considering measurement errors and other manufacturing requirements, in the examples of the present invention, the intensity ratio between the strongest peak among the peaks detected at mass 182.85 - 182.95 and the strongest peak among the peaks detected at mass 179.05 - 179.15 can have a value of 9.9 ± 50%.

[0083] In the examples, the intensity ratio between the peak measured at mass 182.9 and the peak measured at mass 179.1 is similar to the corresponding peak intensity ratio in the LiBO2 substance (i.e., the intensity ratio between peak 3 and peak 4 in the LiBO2 substance). In Comparative Examples 2 and 4, these peaks are observed in different patterns. In Comparative Example 2, the intensity ratio between peak 3 and peak 4 is 1.5, and in Comparative Example 4, the intensity ratio between peak 3 and peak 4 is 3.0.

[0084] According to the present invention, it was confirmed that a lithium boron compound having such characteristics of the peak intensity ratio is included in the coating layer, and as a result, the high-temperature life characteristics and low-temperature output characteristics are improved.

[0085] <Method for evaluating characteristics of coin-type half cell> Each of the positive electrode active materials, carbon black conductive materials, and PVdF binders produced in the examples and Comparative Examples 1 - 5 was mixed in a weight ratio of 96:2:2 in an N-methylpyrrolidone solvent to produce a positive electrode slurry. After applying it to one side of an aluminum current collector, it was dried at 130°C and rolled to produce a positive electrode. Lithium metal was used as the negative electrode.

[0086] An electrode assembly was manufactured with a porous polyethylene separator interposed between the positive electrode and the negative electrode thus manufactured, and after positioning the electrode assembly inside the case, an electrolytic solution was injected into the case to manufacture a lithium secondary battery. At this time, the electrolytic solution was prepared by dissolving lithium hexafluorophosphate (LiPF6) with a concentration of 1.0 M in an organic solvent composed of ethylene carbonate / ethyl methyl carbonate / diethyl carbonate (EC / EMC / DEC mixed volume ratio = 3 / 4 / 3).

[0087] For each coin-type half cell of the lithium secondary battery manufactured in this way, at 25 °C, in the CC (Constant Current)-CV (Constant Voltage) mode, charging was carried out at 0.1C until it reached 4.25V, and discharging was carried out at a constant current of 0.1C until 3.0V, and the charge capacity, discharge capacity, efficiency, and DCIR were measured when the charge-discharge experiment was performed.

[0088] <Method for Evaluating Characteristics of Monocell> A positive electrode slurry was manufactured with the ratio of the positive electrode active material, conductive material, binder, and additive being 97.5 / 1.0 / 1.35 / 0.15. The positive electrode slurry was coated on an aluminum current collector, and a negative electrode as a counter electrode was prepared by blending natural graphite and artificial graphite in a specific ratio. The electrolytic solution was prepared with a salt concentration of 0.7 M, and LiFSI (Lithium bis(fluorosulfonyl)imide, 0.3 M) was added for life maintenance. The injection volume was calculated to be 100 μL per electrode, and a self-manufactured separator was positioned between the positive electrode and the negative electrode in an appropriate size. The prepared monocell was filled with the electrolytic solution and formation charge-discharge was carried out to remove the initial gas, preparing for the evaluation of the monocell.

[0089] Performed under temperature conditions of 25°C. When the initial capacity ended, the voltage drop (ΔVoltage) and resistance were measured while discharging at 2.0C from SOC10 to 0 at -10°C. For the measurement of high-temperature life characteristics, charging was carried out at a constant current of 0.5C until 4.2V at 45°C, and then discharging was carried out at a constant current of 1.0C until 3.0V. Every 100, 200, 300, and 400 cycles, charging was carried out at a constant current of 0.33C until 4.2V at 25°C, and then discharging was carried out at a constant current of 0.33C until 3.0V, and the capacity retention rate and resistance increase rate during charge and discharge were measured.

[0090] <Experimental Results> For the fired product of the positive electrode active material, improvement in change over time was confirmed. The change over time was evaluated by the numerical change of surface residual Li2CO3 according to time. Table 1 summarizes the results of the change over time tests for Comparative Example 1 and Comparative Example 5, showing the weight ratio of Li2CO3 / fired product measured on the first day, one day later, two days later, and three days later.

[0091]

Table 1

[0092] Referring to Table 1, the change over time of Comparative Example 5 is less than that of Comparative Example 1. Therefore, it can be seen that Comparative Example 5, in which the boron source was mixed and secondarily fired, is superior in terms of change over time compared to Comparative Example 1 in which no boron source was added at all. Also in the present invention, since the boron source is mixed and secondarily fired, the change over time is improved.

[0093] Table 2 shows electrochemical data such as charge capacity, discharge capacity, efficiency, and DCIR for the coin-type half cell.

[0094]

Table 2

[0095] Referring to Table 2, in the case of the examples of the present invention, the charge capacity, discharge capacity, and efficiency are superior, and the DCIR is low, compared to Comparative Example 2 and Comparative Example 4.

[0096] In Comparative Example 2, although the firing is carried out in two steps, it does not mix a boron source during the secondary firing as in the present invention. Comparative Example 4 does not mix a boron source during the secondary firing as in the present invention, but mixes a boron source from the primary firing. Thus, when a boron source is mixed as in the present invention, especially when it is not added during the primary firing but added during the secondary firing, there is an effect of improving the charge capacity, discharge capacity, efficiency, and DCIR. Therefore, by the manufacturing method as in the present invention, the surface resistance value of the positive electrode active material in the positive electrode of the lithium secondary battery can be reduced. In particular, in the case of Comparative Example 4 in which a boron source is added during the firing but from the primary firing, the problem that the capacity is not manifested and the DCIR increases is serious, and it can be seen that it is not appropriate.

[0097] Table 3 summarizes the low-temperature output data of the single cell.

[0098]

Table 3

[0099] Referring to Table 3, in the case of the examples of the present invention, it can be confirmed that the output characteristics such as voltage drop (Δ voltage) and resistance are improved compared to Comparative Example 2.

[0100] In Comparative Example 2, although the firing is carried out in two steps, it does not mix a boron source during the secondary firing as in the present invention. Thus, when a boron source is mixed as in the present invention, especially when it is not added during the primary firing but added during the secondary firing, there is an effect of improving the low-temperature output.

[0101] Table 4 summarizes the high-temperature life data of the single cell, that is, the data of the capacity retention rate and the resistance increase rate measured after moving to the normal temperature chamber every 100 cycles, and FIG. 4 is a graph of the capacity retention rate and the resistance increase rate according to the increase in the number of cycles.

[0102]

Table 4

[0103] 4, it can be seen that in the examples of the present invention, the capacity retention rate is higher and the resistance increase rate is lower than in Comparative Examples 2 and 4.

[0104] As described above, when a boron source is mixed in as in the manufacturing method of the present invention, and particularly when it is not added during the first firing but is added during the second firing, the high-temperature life characteristics are improved. Taking the above experimental results together, it can be understood that by dividing the firing process into two stages as in the present invention and adding the boron source at different times from the lithium source, the reactivity between the boron source and the positive electrode active material is improved, resulting in improved characteristics of the coated product.

[0105] Although the present invention has been described above with reference to limited embodiments and drawings, it goes without saying that the present invention is not limited thereto, and various modifications and variations can be made by a person having ordinary skill in the art to which the present invention pertains within the scope of the technical concept of the present invention and the equivalent scope of the claims.

Claims

1. A primary firing step of mixing a lithium source and a precursor and then performing heat treatment; A secondary firing step of forming a surface protection layer on the fired product by mixing a first boron source with the fired product obtained by the primary firing and performing heat treatment; A water washing step of removing unreacted residual lithium on the surface of the fired product obtained by the secondary firing; A coating step of drying the water-washed product obtained by the water washing and performing heat treatment together with a second boron source, including: The heat treatment temperature during the primary firing is 0.75 to 1.5 times the heat treatment temperature during the secondary firing; The heat treatment during the primary firing is performed at 500 to 900°C; The heat treatment during the secondary firing is performed at 500 to 900°C; A method for producing a positive electrode active material, wherein the amounts of the first boron source and the second boron source are such that the weight of boron / the weight of the positive electrode active material is 200 to 5,000 ppm.

2. The precursor is a nickel-based lithium transition metal oxide with a high content, and the nickel content of the precursor is 70 mol% or more based on the total number of moles of transition metals. The method for producing a positive electrode active material according to Claim 1.

3. The water washing step is performed by mixing the fired product and water in a weight ratio of 50 to 200% and stirring. The method for producing a positive electrode active material according to Claim 1.

4. The first boron source or the second boron source is H ３ BO ３ 、H ４ BO ４ 、B ２ O ３ 、LiBO ２ 、Li ２ B ４ O ７ 、B ４ C、AlBO ２ 、AlB ２ O ４ The method for producing a positive electrode active material according to claim 1, wherein one or more of them are used alone or in combination.

Images