Positive electrode active materials for lithium secondary batteries and their preparation methods

Abstract

This invention provides a positive electrode active material for lithium secondary batteries and its preparation method, wherein the positive electrode active material exhibits improved high-temperature lifetime and resistivity increase rate. The positive electrode active material comprises a lithium boron compound coating on the powder surface of the positive electrode active material for lithium secondary batteries, wherein the peak intensity ratio of two optional peaks in the time-of-flight secondary ion mass spectrometry (ToF-SIMS) spectrum of the coating is equal to the peak intensity ratio of the corresponding peak of the LiBO2 compound within a range of ±50%.

Description

Technical Field

[0001] This invention relates to a positive electrode active material for lithium secondary batteries and a method for preparing the same. This application claims priority to Korean Patent Application No. 10-2021-0107659, filed in Korea on August 13, 2021, the disclosure of which is incorporated herein by reference. Background Technology

[0002] Rechargeable lithium-ion batteries are gaining attention as an alternative to fossil fuels. Lithium-ion batteries are commonly used in conventional handheld devices such as mobile phones, cameras, and power tools. However, recently, the application of these lithium-ion batteries has tended to expand into electric vehicles (EVs, HEVs, PHEVs), high-capacity energy storage systems (ESS), and uninterruptible power supply systems (UPS).

[0003] A lithium-ion secondary battery has a structure comprising: an electrode assembly for a single cell, the electrode assembly having a positive electrode plate and a negative electrode plate and a separator inserted between the two electrodes, each of the positive and negative electrode plates containing an active material coated on an electrode current collector; and an external material, i.e., a battery casing, which is configured to house and seal the electrode assembly and the electrolyte. Lithium transition metal composite oxides have been used as positive electrode materials for such lithium-ion secondary batteries. In particular, lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMnO2 or LiMn2O4), lithium iron phosphate (LiFePO4), or LiNiO2 are frequently used. In addition, in order to improve the low thermal stability of LiNiO2 while maintaining its excellent reversible capacity, the following materials have been used: nickel-manganese lithium metal composite oxides in which nickel is partially replaced by manganese with high thermal stability; NCM lithium transition metal composite oxides in which nickel is partially replaced by manganese and cobalt; NCA lithium transition metal composite oxides in which nickel is partially replaced by cobalt and aluminum; or NCMA lithium transition metal composite oxides in which nickel is partially replaced by cobalt, manganese and aluminum.

[0004] Among these cathode active materials, Ni-based lithium cathode materials using Ni have been prepared by the following steps: mixing a lithium source with a precursor; calcining the resulting mixture at a high temperature; and further calcining the resulting product with H3BO3 at a low temperature, allowing the surface to be coated with B. However, the problem with the above method is that it has limitations in improving the high-temperature lifetime characteristics and resistivity increase rate of the cathode active material, thus creating a need to address this issue. Summary of the Invention

[0005] Technical issues

[0006] The present invention aims to solve the problems of related technologies. Therefore, the present invention aims to provide a positive electrode active material for lithium secondary batteries and a method for preparing the same, wherein the positive electrode active material has improved high-temperature lifetime characteristics and resistance increase rate.

[0007] Technical solution

[0008] To address the aforementioned technical problems, in one aspect of the present invention, a positive electrode active material for lithium secondary batteries is provided, the positive electrode active material comprising a lithium boron compound coating on the powder surface of the positive electrode active material for lithium secondary batteries, wherein the peak intensity ratio of two optional peaks in the time-of-flight secondary ion mass spectrometry (ToF-SIMS) spectrum of the coating is equal to the peak intensity ratio of the corresponding peak of the LiBO2 compound within a range of ±50%.

[0009] Here, the peak intensity ratio of the highest peak detected in the ToF-SIMS spectrum at mass 156.85–156.95 to the highest peak detected at mass 153.05–153.15 can be 4.3 ± 50%.

[0010] Furthermore, the peak intensity ratio of the highest intensity peak detected in the ToF-SIMS spectrum at mass 182.85–182.95 to the highest intensity peak detected at mass 179.05–179.15 can be 9.9 ± 50%.

[0011] In another aspect of the present invention, a method for preparing a positive electrode active material for lithium secondary batteries is provided, the method comprising:

[0012] First firing step: Mix the lithium source with the precursor and heat-treat the resulting mixture;

[0013] Second firing step: The firing product obtained from the first firing step is mixed with the first B source, and the resulting mixture is heat-treated to form a surface protective layer on the firing product;

[0014] Water washing step: to remove unreacted lithium remaining on the surface of the calcined product obtained from the second calcination step; and

[0015] Coating step: Dry the product washed with water in the previous step, and heat-treat the resulting product together with the second source B.

[0016] The precursor may be a Ni-rich lithium transition metal oxide, and the nickel content of the Ni-rich lithium transition metal oxide may be 70 mol% or more, based on the total molar number of transition metals.

[0017] The heat treatment temperature in the first firing step can be 0.75-1.5 times the heat treatment temperature in the second firing step.

[0018] The washing step can be carried out by mixing the fired product with water at a weight ratio of 50%-200% and stirring.

[0019] The first or second B source can be selected from any one or a combination of the following: H3BO3, H4BO4, B2O3, LiBO2, Li2B4O7, B4C, AlBO2, and AlB2O4.

[0020] The contents of the first and second B sources are at a level that makes the weight ratio of B to the positive electrode active material equal to 200-5000 ppm.

[0021] Beneficial effects

[0022] According to the present invention, a positive electrode active material for lithium secondary batteries is provided, wherein the positive electrode active material has improved high-temperature lifetime characteristics, resistivity increase rate and output characteristics.

[0023] According to the method of the present invention, a positive electrode active material for lithium secondary batteries with improved high-temperature lifetime characteristics and resistivity increase rate can be obtained by controlling the coating / doping time point of B during the firing step.

[0024] In the method according to the invention, the B source is added at a different time point than the addition of the lithium source in the second firing step, thereby providing an effect of improving the reactivity of the B source with the positive electrode active material.

[0025] According to the method of the present invention, the second firing step includes a coating / doping process of element B to form a surface protective layer on the fired product of the positive electrode active material. Therefore, the element B coating formed during firing can effectively suppress the surface degradation of the positive electrode active material that may occur during the water washing step.

[0026] According to the method of the present invention, when the dried water-washed product is heat-treated together with a second B source at low temperature, the reactivity of the surface of the positive electrode active material with B changes, thereby providing a positive electrode active material with improved high-temperature lifetime characteristics, resistivity increase rate and low-temperature output characteristics. Attached Figure Description

[0027] The accompanying drawings illustrate preferred embodiments of the invention and, together with the foregoing disclosure, serve to provide a further understanding of the technical features of the invention. Therefore, the invention should not be construed as limited to the drawings.

[0028] Figure 1 This is a flowchart illustrating a method for preparing a positive electrode active material according to the present invention.

[0029] Figure 2 and Figure 3 The results of measurements using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) are shown in Examples 2 and 4, as well as Comparative Examples 2 and 4.

[0030] Figure 4 It is a graph showing the capacitance retention rate and resistance increase rate as a function of the number of cycles. Detailed Implementation

[0031] Preferred embodiments of the invention will be described in detail below with reference to the accompanying drawings. Before the description, it should be understood that the terminology used in the specification and appended claims should not be construed as limited to its general and dictionary meaning, but rather should be interpreted according to the meaning and concept corresponding to the technical aspects of the invention, based on the principle that the inventors are allowed to appropriately define the terms for the best interpretation. Therefore, the descriptions presented herein are merely preferred examples for illustrative purposes and are not intended to limit the scope of the invention; thus, it should be understood that other equivalents and variations of the invention can be given without departing from its scope.

[0032] In the following description, reference will be made to the accompanying drawings, which form part of this invention. The embodiments described in the detailed description, drawings, and claims are not intended to limit the invention. Other embodiments and variations may be made without departing from the scope of the invention. Various different constituent elements can be arranged, substituted, combined, separated, and designed with respect to the embodiments summarized herein and depicted in the drawings. It should be understood that all of these are clearly contemplated herein.

[0033] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0034] Although the invention will be described in conjunction with specific embodiments, it will be apparent to those skilled in the art that various changes and variations can be made without departing from the scope of the invention. In addition to the foregoing, it should be understood that other equivalents can be made without departing from the scope of the invention. Such variations and changes also fall within the scope of the appended claims. The scope of the invention will be defined only by the claims and the entire scope of the equivalents defined by the claims. It should also be understood that the invention is not limited to the specific embodiments. Furthermore, it should be understood that the terminology used herein is for the purpose of describing specific embodiments only and is not intended to limit the invention.

[0035] Figure 1 This is a flowchart illustrating a method for preparing a positive electrode active material according to the present invention. In the following text, reference will be made to... Figure 1A method for preparing a positive electrode active material according to the present invention will be described in more detail.

[0036] First, a first firing step (step S10) of mixing a lithium source and a precursor and performing heat treatment is carried out.

[0037] A positive electrode active material for a lithium secondary battery is prepared through the first firing step (S10). For example, an NCM-based lithium transition metal composite oxide can be prepared. Preferably, the positive electrode active material prepared according to the present invention is an NCM-based lithium transition metal composite oxide, but different types of positive electrode active materials for lithium secondary batteries can also be prepared according to the present invention.

[0038] The precursor is a raw material and can be prepared by the following steps: mixing an aqueous solution of a nickel compound, a cobalt compound, a manganese compound, etc. with an alkaline solution and reacting to obtain a precipitate as a reaction product, and then drying and heat-treating the precipitate. For example, the precursor can be represented by the following Chemical Formula 1:

[0039] [Chemical Formula 1]

[0040] Ni x1 Co y1 Mn z1 Al s1 (OH)2

[0041] where 0.7 ≤ x1 ≤ 0.99, 0 < y1 < 0.3, 0 < z1 < 0.3, and 0 ≤ s1 ≤ 0.1.

[0042] For example, the precursor can be a precursor for preparing a Ni-rich lithium transition metal oxide, and based on the total molar amount of the transition metals, the nickel content of the precursor can be 70 mol% or more.

[0043] The lithium source can be a lithium compound such as Li2CO3 or LiOH·H2O.

[0044] The fired product obtained through the first firing step (S10) can be a lithium transition metal composite oxide represented by the following Chemical Formula 2:

[0045] [Chemical Formula 2]

[0046] Li a [Ni b Co c Mn d Al e 1-f M 1 f O2

[0047] where M 1 ​represents at least one selected from the following: Zr, W, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, F, P, and S,

[0048] 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.

[0049] For example, the lithium transition metal composite oxide can be LiNi 0.86 Co 0.05 Mn 0.07 Al 0.02 O2.

[0050] In the first firing step, the B source is not mixed.

[0051] The heat treatment in the first firing step can be carried out at an oxygen concentration of 21 - 100%. The heat treatment temperature of the first firing step can be set to 0.75 - 1.5 times the heat treatment temperature of the second firing step carried out after the first firing step. The heat treatment temperature can be appropriately varied within the above range according to the content of each element in the NCM. For example, the heat treatment in the first firing step can be carried out at 500 - 900 °C. When the temperature is lower than 500 °C, the reaction may not occur sufficiently. When the temperature is higher than 900 °C, the cathode material may thermally decompose, resulting in a reduction in performance.

[0052] Next, a second firing step (step S20) is carried out in which the fired product obtained from the first firing step is mixed with the first B source and heat-treated to form a surface protection layer on the fired product. The heat treatment in the second firing step can be carried out by cooling the fired product after the heat treatment in the first firing step (step S10) and introducing the first B source into the fired product.

[0053] The method according to an embodiment of the present invention is characterized in that the B source is not mixed in the first firing step (step S10), and then the B source is introduced in two steps. The first introduction time point is the second firing step (step S20). The B source introduced in the second firing step is called the first B source.

[0054] The first B source is a coating / doping material and can be any one or a combination selected from the following: H3BO3, H4BO4, B2O3, LiBO2, Li2B4O7, B4C, AlBO2, and AlB2O4. The method of mixing the fired product obtained from the first firing step with the first B source can be carried out by a solid-phase method or a liquid-phase method. Such a solid-phase or liquid-phase method can include mixing, milling, spray drying, grinding, etc. Preferably, a dry solid-phase method is used.

[0055] The heat treatment in the second firing step can be carried out at an oxygen concentration of 21-100%. Similar to the heat treatment in the first firing step, the heat treatment in the second firing step can be carried out at 500-900°C. When the heat treatment temperature is below 500°C, it is impossible to form a surface protective layer smoothly or uniformly. When the heat treatment temperature is above 900°C, the positive electrode active material may not be able to maintain its original performance. The heat treatment temperature can be appropriately varied within the above range depending on the content of each element in the NCM. However, as mentioned above, the heat treatment temperature in the first firing step is preferably 0.75-1.5 times the heat treatment temperature in the second firing step. It has been shown that when the heat treatment temperature in the first firing step is not within the above range, even after adding the first B source and performing firing, washing, and coating in the second firing step, the high-temperature lifetime characteristics are reduced. Therefore, since it is necessary to prevent the reduction of high-temperature lifetime characteristics, the relationship between the heat treatment temperature in the first firing step and the heat treatment temperature in the second firing step is crucial.

[0056] In summary, the method according to an embodiment of the present invention is characterized in that the firing is performed in two stages (a first firing step and a second firing step) rather than in a single operation. Furthermore, the B source is not added in the first firing step, but only in the second firing step. The B source, which serves as a coating / doping material, is not introduced in the first firing step, but only in the second firing step. The first B source introduced in the second firing step causes the fired product to be coated / doped with B element to form a surface protective layer. A portion of the first B source is introduced into the fired product for doping, while another portion of the first B source is disposed on the surface of the fired product to form a B source coating, thereby serving as a surface protective layer.

[0057] According to related technologies, the B source is only introduced for coating in the final step after firing. According to the present invention, the B source is added in the second firing step at a time different from the introduction time of the lithium source. In this way, the reactivity between the B source and the positive electrode active material can be improved. The surface protective layer formed in the second firing step suppresses surface degradation of the positive electrode active material in the subsequent water washing step (S30). Therefore, it is noted that according to the method of the present invention, a surface protective layer is formed on the surface of the fired product before the water washing step (S30), thereby suppressing surface degradation of the positive electrode active material during the water washing step (S30).

[0058] The calcined product obtained through the second calcination step (S20) can be a lithium transition metal composite oxide represented by the following chemical formula 3:

[0059] [Chemical Formula 3]

[0060] Li a [Ni bCo c Mn d Al e 1-f (B,M 1 ) f O2

[0061] where M 1 represents at least one selected from the following: Zr, W, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, F, P, and S,

[0062] 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.

[0063] Then, a water washing step (step S30) is performed to remove unreacted lithium remaining on the surface of the fired product obtained from the second firing step.

[0064] Since the positive electrode active material may contain lithium compounds such as LiOH remaining thereon, such a water washing step may be required. The water washing step can be performed by mixing the fired product and water at a weight ratio of 50% - 200% and stirring. During washing, additives such as LiOH and NaOH can be introduced.

[0065] Since a surface protection layer is formed on the fired product of the positive electrode active material through the process of coating / doping with B element in the second firing step, the B element coating formed during firing can have the effect of suppressing surface deterioration of the positive electrode active material that may occur in the water washing step. Compared with the method of the related art in which the B source is introduced only in the last step after firing for coating, the method according to an embodiment of the present invention provides a remarkable effect of suppressing surface deterioration of the positive electrode active material in the water washing step. According to the related art, the surface of the fired product after water washing does not contain the surface protection layer as disclosed herein.

[0066] Thereafter, a coating step (step S40) is performed to dry the washing product obtained from the water washing step and perform heat treatment together with a second B source.

[0067] The B source is not mixed in the first firing step (step S10), but is subsequently added in two steps. The first introduction time point is the above-mentioned second firing step (step S20), and the second introduction time point is the coating step (step S40). The B source introduced in the coating step is referred to as the second B source.

[0068] ​The second B source is a coating / doping material, and can be selected from any one or a combination of the following: H3BO3, H4BO4, B2O3, LiBO2, Li2B4O7, B4C, AlBO2, and AlB2O4. The method for mixing the water-washed and dried product with the second B source can be a solid-phase method or a liquid-phase method. Such solid-phase or liquid-phase methods can include mixing, grinding, spray drying, abrasion, etc.

[0069] The heat treatment in the second firing step can be carried out at an oxygen concentration of 21-100%. The heat treatment in step S40 can be carried out at 200-400°C. When the heat treatment temperature is below 200°C, the positive electrode active material may not be able to fully realize its capacity. When the heat treatment temperature is above 400°C, the high-temperature lifetime characteristics may decrease. Therefore, the heat treatment temperature in step S40 is preferably below 400°C.

[0070] The first source may be the same as or different from the second source. However, the contents of the first and second B sources are at a level that makes the weight ratio of B to the positive electrode active material equal to 200-5,000 ppm.

[0071] When the weight ratio of B to the positive electrode active material is less than 200 ppm, there is no coating effect. When the weight ratio of B to the positive electrode active material is greater than 5,000 ppm, excessive coating may form, which is not preferred in terms of capacity and resistance. When the washed and dried product is heat-treated together with the second B source at low temperature, the reactivity of the positive electrode active material surface with B changes, thereby improving high-temperature lifetime, resistivity increase rate, and low-temperature output characteristics. Because the degradation of the positive electrode active material surface is suppressed by the surface protective layer in the water washing step (step S30), the reactivity of the second B source introduced in the coating step (S40) with the positive electrode active material surface is higher than the reactivity of the B source with the degraded positive electrode active material surface without a surface protective layer and after water washing.

[0072] The positive electrode active material prepared by the method described above according to the present invention has the following characteristics. The positive electrode active material according to the present invention comprises a lithium boron compound coating on the powder surface of a positive electrode active material for lithium secondary batteries, wherein the peak intensity ratio of two optional peaks in the time-of-flight secondary ion mass spectrometry (ToF-SIMS) spectrum of the coating is equal to the peak intensity ratio of the corresponding peak of the LiBO2 compound within the range of ±50%.

[0073] The positive electrode active material according to the present invention may comprise a lithium boron compound coating on the powder surface of a positive electrode active material for lithium secondary batteries, wherein the peak intensity ratio of two optional peaks in the time-of-flight secondary ion mass spectrometry (ToF-SIMS) spectrum of the coating may be equal to the peak intensity ratio of the corresponding peak of the LiBO2 compound within the range of ±50%.

[0074] For example, the positive electrode active material can be an NCM-type lithium transition metal composite oxide. In particular, in the case of Ni-rich lithium transition metal composite oxides with a nickel content of 70% or more, reactivity with the electrolyte should be reduced to increase stability under cyclic operating conditions. Positive electrode active materials incorporating the coating according to the invention can reduce reactivity with the electrolyte, thereby reducing gas generation at high temperatures.

[0075] Positive active materials incorporating coatings according to the present invention provide improved high-temperature lifetime, resistivity increase rate, and output characteristics.

[0076] In this paper, the coating exhibits a peak intensity ratio of 4.3 ± 50% between the highest intensity peak detected at mass 156.85–156.95 and the highest intensity peak detected at mass 153.05–153.15 in the ToF-SIMS spectrum. The expression "4.3 ± 50%" refers to the range from 4.3–2.15 (50% of 4.3) to 4.3 + 2.15, i.e., from 2.15 to 6.45.

[0077] Furthermore, the coating can exhibit a peak intensity ratio of 9.9 ± 50% between the highest intensity peak detected at mass 182.85–182.95 and the highest intensity peak detected at mass 179.05–179.15 in the ToF-SIMS spectrum. The expression "9.9 ± 50%" refers to the range from 9.9–4.95 (50% of 9.9) to 9.9 + 4.95, i.e., from 4.95 to 14.85.

[0078] In other words, the coating of the positive electrode active material according to the present invention exhibits peaks present within a specific mass range as determined by ToF-SIMS, wherein these peaks have a predetermined peak intensity ratio equal to the peak intensity ratio of the corresponding peaks in the LiBO2 compound within ±50%. When the positive electrode active material is used as the positive electrode of a lithium secondary battery, it can provide high output and high stability.

[0079] As a result, according to the present invention, the high-temperature lifetime, resistivity increase rate, and low-temperature output characteristics of the positive electrode active material can be improved by controlling the timing of B coating / doping during the firing step.

[0080] The invention will be described in more detail below with reference to embodiments.

[0081] Example:

[0082] First, LiOH·H2O, used as the lithium source, and Ni, used as the precursor, are mixed in such a way that the molar ratio of Li to transition metals (Ni, Co, Mn, Al) is 1.03:1. 0.88 Co0.05 Mn 0.07 (OH)2 + Al(OH)3, and the resulting mixture is subjected to a first calcination step at 700℃ for 5 hours to obtain LiNi. 0.86 Co 0.05 Mn 0.07 Al 0.02 O2 was used as the first calcination product. Next, the first calcination product was mixed with a first boron source, H3BO3, at a weight ratio of 100:0.86, and the resulting mixture was subjected to a second calcination step at 750°C for 5 hours. The calcined product was then mixed with water at a weight ratio of 100:100 and stirred for 5 minutes. The washed product was then filtered using a filter press and vacuum dried at 130°C. Subsequently, the dried product was mixed with a second boron source, H3BO3, at a weight ratio of 100:0.57, and the resulting mixture was heat-treated at 300°C for 4 hours to obtain a positive electrode active material with a boron-coated surface.

[0083] Comparative Example 1:

[0084] The LiOH·H2O used as the lithium source and the Ni used as the precursor are mixed in such a manner that the molar ratio of Li to transition metals (Ni, Co, Mn, Al) can be 1.03:1. 0.88 Co 0.05 Mn 0.07 The mixture of (OH)2 and Al(OH)3 was subjected to a first calcination step at 700℃ for 5 hours to obtain the first calcined product LiNi. 0.86 Co 0.05 Mn 0.07 Al 0.02 O2. Next, without adding B source, perform the second firing step at 750°C for 5 hours.

[0085] Comparative Example 2:

[0086] The LiOH·H2O used as the lithium source and the Ni used as the precursor are mixed in such a manner that the molar ratio of Li to transition metals (Ni, Co, Mn, Al) can be 1.03:1. 0.88 Co 0.05 Mn 0.07 The mixture of (OH)2 and Al(OH)3 was subjected to a first calcination step at 700℃ for 5 hours to obtain the first calcined product LiNi. 0.86 Co 0.05 Mn 0.07 Al 0.02O2. Next, without adding a B source, a second calcination step was performed at 750°C for 5 hours. The calcined product was mixed with water at a weight ratio of 100:100, and the resulting mixture was stirred for 5 minutes. Then, the washed product was filtered through a filter press and vacuum dried at 130°C. Subsequently, the dried product was mixed with H3BO3 at a weight ratio of 100:0.57 and heat-treated at 300°C for 4 hours.

[0087] Comparative Example 3:

[0088] The LiOH·H2O used as the lithium source and the Ni used as the precursor are mixed in such a manner that the molar ratio of Li to transition metals (Ni, Co, Mn, Al) can be 1.03:1. 0.88 Co 0.05 Mn 0.07 (OH)2 + Al(OH)3 are mixed, and the resulting mixture is then mixed with H3BO3 to achieve a Ni content of 100:0.91 by weight. 0.88 Co 0.05 Mn 0.07 (OH)2 and H3BO3 are mixed, and then the first firing step is carried out at 700°C for 5 hours, and the second firing step is carried out at 750°C for 5 hours.

[0089] Comparative Example 4:

[0090] The LiOH·H2O used as the lithium source and the Ni used as the precursor are mixed in such a manner that the molar ratio of Li to transition metals (Ni, Co, Mn, Al) can be 1.03:1. 0.88 Co 0.05 Mn 0.07 (OH)2 + Al(OH)3 are mixed, and the resulting mixture is then mixed with H3BO3 to achieve a Ni content of 100:0.91 by weight. 0.88 Co 0.05 Mn 0.07 (OH)₂ and H₃BO₃ were mixed, and then subjected to a first calcination step at 700°C for 5 hours, followed by a second calcination step at 750°C for 5 hours. The calcined product was mixed with water at a weight ratio of 100:100, and the resulting mixture was stirred for 5 minutes. The washed product was then filtered through a filter press and vacuum dried at 130°C. Subsequently, the dried product was mixed with H₃BO₃ at a weight ratio of 100:0.57 and heat-treated at 300°C for 4 hours.

[0091] Comparative Example 5:

[0092] The LiOH·H2O used as the lithium source and the Ni used as the precursor are mixed in such a manner that the molar ratio of Li to transition metals (Ni, Co, Mn, Al) can be 1.03:1.0.88 Co 0.05 Mn 0.07 The mixture of (OH)2 and Al(OH)3 was subjected to a first calcination step at 700°C for 5 hours to obtain the first calcined product LiNi. 0.86 Co 0.05 Mn 0.07 Al 0.02 O2. Then, the first calcined product is mixed with H3BO3 at a weight ratio of 100:0.86, and then a second calcination step is carried out at 750°C for 5 hours.

[0093] In summary, the above embodiments and comparative examples are as follows.

[0094] Example: Li + NCM(OH)2 + Al mixing → 700℃, first firing step → positive electrode active material + first B source mixing → 750℃, second firing step → water washing → drying, 130℃ → second B source mixing and coating (300℃)

[0095] Comparative Example 1 (Positive Electrode Active Material + No B Source Mixing, Two-Step Firing): Li + NCM(OH)₂ + Al Mixing → 700℃, First Firing Step → 750℃, Second Firing Step

[0096] Comparative Example 2 (Positive Electrode Active Material + Mixing without B Source, Two-Step Firing): Li + NCM(OH)2 + Al mixing → 700℃, First Firing Step → 750℃, Second Firing Step → Water Washing → Drying, 130℃ → H3BO3 Mixing and Coating (300℃)

[0097] Comparative Example 3 (Lithium source and B source are mixed in the first firing step): Li + NCM(OH)2 + Al + H3BO3 mixed → 700℃, first firing step → 750℃, second firing step

[0098] Comparative Example 4 (Lithium source and B source are mixed in the firing step): Li + NCM(OH)2 + Al + H3BO3 mixed → 700℃, first firing step → 750℃, second firing step → water washing → drying, 130℃ → H3BO3 mixing and coating (300℃).

[0099] Comparative Example 5: Li + NCM(OH)2 + Al mixture → 700℃, first firing step → positive electrode active material + H3BO3 mixture → 750℃, second firing step

[0100] The experimental items include time-varying tests, electrochemical data of coin half-cells, low-temperature output data of single cells, high-temperature lifetime data of single cells, and determination of peak values ​​detected by time-of-flight secondary ion mass spectrometry (ToF-SIMS) when a B source is added.

[0101] Results of Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS)

[0102] ToF-SIMS was performed to identify the composition of the coating. Typically, X-ray diffraction (XRD) is used to determine the composition and crystal structure of materials. However, because the thickness of the coating of the positive electrode active material is only a few nanometers to tens of nanometers, it is difficult to detect peaks by XRD. According to the present invention, the composition of a very thin coating was analyzed by ToF-SIMS. After performing ToF-SIMS, a lithium boron compound coating was shown to be additionally formed on the surface of the positive electrode active material. In other words, it was shown that a lithium boron compound was additionally formed by the method according to the present invention.

[0103] Figure 2 and Figure 3 The ToF-SIMS results for Comparative Examples 2 and 4, as well as the Examples, are shown. For comparison purposes, the results for LiBO2 are also shown.

[0104] Reference Figure 2 In this embodiment, the peak with the highest intensity among the peaks detected at mass values ​​of 156.85-156.95 (peak 1) is the peak detected at mass value of 156.9. In this embodiment, the peak with the highest intensity among the peaks detected at mass values ​​of 153.05-153.15 (peak 2) is the peak detected at mass value of 153.1. The mass value is determined by using Li... + C + C2 + and C3 + The mass value was measured using a peak-calibrated ToF-SIMS spectrum. In this embodiment, the ratio of peak 1 to peak 2 was 4.3. Even when considering detection errors and other conditions in the preparation process, according to the above embodiment, the ratio of the highest intensity peak detected at mass 156.85-156.95 to the highest intensity peak detected at mass 153.05-153.15 could still be 4.3 ± 50%.

[0105] In the examples, the peak intensity ratio of the peak detected at mass 156.9 to the peak detected at mass 153.1 was similar to the peak intensity ratio of the corresponding peak in the LiBO2 compound (i.e., the peak intensity ratio of peak 1 to peak 2 in LiBO2) within ±50%. The term "similar" means that the ratio of the examples was the same as that of LiBO2 within ±50%. In Comparative Examples 2 and 4, those peaks were observed with different patterns. In Comparative Examples 2 and 4, the ratios of peak 1 to peak 2 were 0.3 and 2.0, respectively.

[0106] Reference Figure 3In this embodiment, the highest intensity peak (peak 3) among the peaks detected at mass values ​​of 182.85-182.95 is the peak detected at mass value of 182.9. In this embodiment, the highest intensity peak (peak 4) among the peaks detected at mass values ​​of 179.05-179.15 is the peak detected at mass value of 179.1. The mass value is determined by using Li... + C + C2 + and C3 + The mass value was measured using a peak-calibrated ToF-SIMS spectrum. In this embodiment, the ratio of peak 3 to peak 4 was 9.9. Even considering detection errors and other conditions in the preparation process, according to the above embodiment, the ratio of the highest intensity peak detected at mass 182.85–182.95 to the highest intensity peak detected at mass 179.05–179.15 can still be 9.9 ± 50%.

[0107] In the examples, the peak intensity ratio of the peak detected at mass 182.9 to the peak detected at mass 179.1 was similar to the peak intensity ratio of the corresponding peak in the LiBO2 compound (i.e., the peak intensity ratio of peak 3 to peak 4 in LiBO2) within ±50%. The term "similar" means that the ratio in the examples was the same as that in LiBO2 within ±50%. In Comparative Examples 2 and 4, those peaks were observed with different patterns. In Comparative Examples 2 and 4, the ratios of peak 3 to peak 4 were 1.5 and 3.0, respectively.

[0108] As can be seen from the present invention, the high-temperature lifetime characteristics and low-temperature output characteristics are improved by introducing a lithium boron compound having the above-mentioned peak intensity characteristics into the coating.

[0109] Methods for evaluating the characteristics of coin half-cells

[0110] A positive electrode slurry was prepared by mixing various positive electrode active materials, carbon black as a conductive material, and PVDF as a binder in an N-methylpyrrolidone solvent at a weight ratio of 96:2:2 according to Examples and Comparative Examples 1 to 5. The positive electrode slurry was coated onto one surface of an aluminum current collector, dried at 130°C, and pressed to obtain a positive electrode. Lithium metal was used as the negative electrode. A porous polyethylene separator was then inserted between the positive and negative electrodes to provide an electrode assembly, which was then housed in a case, and an electrolyte was injected into the case to obtain a lithium secondary battery. In this paper, the electrolyte was prepared by dissolving 1.0 M lithium hexafluorophosphate (LiPF6) in an organic solvent containing ethylene carbonate / ethyl methyl carbonate / diethyl carbonate (EC / MC / DEC) mixed in a 3 / 4 / 3 volume ratio.

[0111] Each lithium-ion secondary battery half-cell was charged to 4.25V at 0.1C in constant current-constant voltage (CC-CV) mode at 25°C, and then discharged to 3.0V at 0.1C constant current for charge / discharge testing. During the tests, the charging capacity, discharging capacity, efficiency, and DC internal resistance (DCIR) were measured.

[0112] Methods for evaluating the characteristics of a single cell

[0113] A positive electrode slurry was prepared by mixing various positive electrode active materials, conductive materials, binders, and additives in a ratio of 97.5 / 1.0 / 1.35 / 0.15. The positive electrode slurry was coated onto an aluminum current collector, and natural graphite and artificial graphite were mixed in a specific ratio to prepare the negative electrode counter. An electrolyte was prepared with a salt concentration of 0.7 M, and lithium bis(fluorosulfonyl)imide (LiFSI, 0.3 M) was added to it to maintain lifetime. The electrolyte was injected in 100 μL, and a separator of appropriate size, manufactured by our company, was inserted between the positive and negative electrodes. The electrolyte was injected into the prepared single cell, subjected to charge / discharge cycles, and initial gases were removed in preparation for single cell evaluation.

[0114] Charge / discharge cycles were performed at 25°C. When the initial capacity was achieved, the voltage drop (ΔV) and resistance of each cell were measured simultaneously from SOC10 to SOC0 at -10°C and 2.0C. To determine high-temperature lifetime characteristics, each cell was charged to 4.2V at 0.5C constant current mode at 45°C and discharged to 3.0V at 1.0C constant current mode. At the 100th, 200th, 300th, and 400th cycles, the cells were charged to 4.2V at 0.33C constant current mode at 25°C and discharged to 3.0V at 0.33C constant current mode to achieve the same charge / discharge test. Capacity retention and resistance increase were measured.

[0115] Test results

[0116] The improvement in the calcination product of various positive electrode active materials over time was determined. The change over time was evaluated as the change in the weight of Li2CO3 remaining on the surface. Table 1 shows the results of the time-varying test for Comparative Examples 1 and 5, in which the weight ratio of Li2CO3 / calcination product was measured on day 1 (day 0), day 1 (day 1), day 2 (day 2), and day 3 (day 3).

[0117] [Table 1]

[0118]

[0119] Referring to Table 1, Comparative Example 5 shows a smaller change over time compared to Comparative Example 1. Therefore, it can be seen that Comparative Example 5, which involves adding a B source during the second firing step, provides better results in terms of change over time compared to Comparative Example 1, which does not have a B source added. According to the present invention, adding a B source during the second firing step can thus achieve improvement in terms of change over time.

[0120] Table 2 shows the electrochemical data for each coin half-cell, including charging capacity, discharging capacity, efficiency, and DCIR.

[0121] [Table 2]

[0122]

[0123] Referring to Table 2, compared with Comparative Examples 2 or 4, the embodiments according to the present invention show higher charging capacity, discharging capacity and efficiency, as well as lower DCIR.

[0124] Comparative Example 2 includes a two-step firing process, but unlike the present invention, the B source is not mixed in the second firing step. Unlike the present invention, Comparative Example 2 includes mixing the B source in the first firing step, but not in the second firing step. Therefore, when the B source is mixed in the second firing step, rather than the first, according to the present invention, the charging capacity, discharging capacity, efficiency, and DCIR can be improved. The results show that the surface resistance of the positive electrode active material in the positive electrode of the lithium secondary battery can be reduced by using the method according to the present invention. In particular, in the case of Comparative Example 4, where the B source is introduced in the firing step, it can be seen that the positive electrode active material cannot achieve its capacity, undesirably causing an increase in DCIR.

[0125] Table 3 shows the low-temperature output data for a single cell.

[0126] [Table 3]

[0127]

[0128] Referring to Table 3, compared with Comparative Example 2, the embodiments according to the present invention show improved output characteristics, including voltage drop (Δ voltage) and resistance.

[0129] Comparative Example 2 includes a two-step firing process, but unlike the present invention, the B source is not mixed in the second firing step. Therefore, when the B source is mixed in the second firing step instead of the first, according to the present invention, the low-temperature output characteristics can be improved.

[0130] Table 4 shows the high-temperature lifetime data for single cells, including capacity retention and resistance increase determined per 100 cycles, and Figure 4 It is a graph showing the capacitance retention rate and resistance increase rate as a function of the number of cycles.

[0131] [Table 4]

[0132]

[0133] Refer to Table 4 and Figure 4 It can be seen that, compared with Comparative Examples 2 or 4, the embodiments according to the present invention exhibit a higher capacitance retention rate and a smaller resistance increase rate.

[0134] Therefore, according to the present invention, when the B source is mixed in the second firing step instead of the first firing step, the high-temperature lifetime characteristics can be improved. When the above results are put together, it can be seen that the coated positive electrode active material provides improved characteristics by performing the firing process in two steps and introducing the B source and lithium source at different time points to improve the reactivity of the B source with the positive electrode active material.

[0135] The invention has been described in detail with reference to specific embodiments and accompanying drawings. However, it should be understood that the detailed description and specific examples are given by way of illustration only, as various changes and variations within the scope of the invention will become apparent to those skilled in the art from this detailed description.

Claims

1. A method for preparing a positive electrode active material for a lithium secondary battery, the method comprising: A first firing step: mixing a lithium source with a precursor and heat-treating the resulting mixture to obtain a transition metal composite oxide represented by Chemical Formula 2 below; A second firing step: mixing the fired product obtained from the first firing step with a first B source and heat-treating the resulting mixture to form a surface protective layer on the fired product; A water washing step: removing unreacted lithium remaining on the surface of the fired product obtained from the second firing step; A coating step: drying the product washed with water in the previous step and heat-treating the resulting product together with a second B source, and wherein the heat treatment temperature in the first firing step is 0.75 times to 1.5 times the heat treatment temperature in the second firing step, Chemical Formula 2: Li a [Ni b Co c Mr d Al e ] 1-f M 1 f O2 Among them, M 1 It indicates that it is selected from at least one of the following: Zr, W, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, F, P, and S. 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.

2. The method for preparing positive electrode active material for lithium secondary batteries according to claim 1, wherein, The precursor is a Ni-rich lithium transition metal oxide, and based on the total molar amount of transition metals, the nickel content of the Ni-rich lithium transition metal oxide is 70 mol% or more.

3. The method for preparing positive electrode active material for lithium secondary batteries according to claim 1, wherein, The water washing step is carried out by mixing the fired product with water at a weight ratio of 50% to 200% and stirring.

4. The method for preparing positive electrode active material for lithium secondary batteries according to claim 1, wherein, The first B source or the second B source is any one or a combination selected from the following: H3BO3, H4BO4, B2O3, LiBO2, Li2B4O7, B4C, AlBO2, and AlB2O4.

5. The method for preparing positive electrode active material for lithium secondary batteries according to claim 1, wherein, The contents of the first B source and the second B source are at a level such that the weight ratio of B / positive electrode active material is equal to 200 ppm - 5,000 ppm.

6. A positive electrode active material for a lithium secondary battery prepared by the method of claim 1, which comprises a lithium boride compound coating on the surface of the powder of the positive electrode active material for a lithium secondary battery, in, where the peak intensity ratio of two optional peaks in the time-of-flight secondary ion mass spectrometry (ToF-SIMS) spectrum of the coating is equal to the peak intensity ratio of the corresponding peaks of the LiBO2 compound within a range of ±50%.

7. The positive electrode active material for lithium secondary batteries according to claim 6, wherein, The peak intensity ratio of the peak with the highest intensity among the peaks detected at a mass of 156.85 - 156.95 in the ToF-SIMS spectrum of the coating to the peak with the highest intensity among the peaks detected at a mass of 153.05 - 153.15 is 4.3 ± 百分之50.

8. The positive electrode active material for lithium secondary batteries according to claim 6, wherein, The peak intensity ratio of the peak with the highest intensity among the peaks detected at a mass of 182.85 - 182.95 in the ToF-SIMS spectrum of the coating to the peak with the highest intensity among the peaks detected at a mass of 179.05 - 179.15 is 9.9 ± 百分之50.

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