Positive electrode active material and lithium secondary battery containing the same

Abstract

To provide a positive electrode active material capable of resolving low structural stability while maintaining high electrochemical properties of a high-Ni type positive electrode active material.SOLUTION: There is provided a positive electrode active material including at least nickel, cobalt, and a doping metal and including a lithium composite oxide having a layered structure capable of achieving intercalation / deintercalation of lithium. The lithium composite oxide includes secondary particles that are aggregates in which a plurality of primary particles is aggregated and a grain boundary is formed between adjacent primary particles, and the secondary particles are aggregates of primary particles exhibiting a concentration gradient in which concentration of the doping metal has a (-) slope from the grain boundary between the primary particles toward the center portion of the primary particle. When the radius of the secondary particles is referred to as R, a first concentration gradient section in which the concentration of the doping metal has a (-) slope and a second concentration gradient section in which the concentration of the doping metal has a (+) slope are present only within a depth of 0 to 0.2 R from a surface portion of the secondary particles.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a lithium secondary battery using a cathode active material in which a plurality of primary particles are aggregated to form secondary particles, and the secondary particles are provided as an aggregate of primary particles in which a concentration gradient of a doping metal is formed in a direction from the grain boundary between the primary particles toward the center of the primary particles, and a cathode including the same, which has improved electrochemical properties and stability.

Background Art

[0002] A battery stores electric power by using substances capable of undergoing an electrochemical reaction for the positive electrode and the negative electrode. As a typical example of such a battery, there is a lithium secondary battery that stores electrical energy by the difference in chemical potential when lithium ions are intercalated / deintercalated in the positive electrode and the negative electrode.

[0003] The lithium secondary battery is manufactured by using substances capable of reversible intercalation / deintercalation of lithium ions as the positive electrode and the negative electrode active materials, and filling an organic electrolyte or a polymer electrolyte between the positive electrode and the negative electrode.

[0004] As the positive electrode active material of the lithium secondary battery, lithium composite oxides are used, and examples thereof include composite oxides such as LiCoO2, LiMn2O4, LiNiO2, and LiMnO2, which are being studied.

[0005] Among the above positive electrode active materials, LiCoO2 is the most widely used because of its excellent life characteristics and charge / discharge efficiency. However, it is expensive due to the resource limitations of cobalt used as a raw material, and thus has the disadvantage of limited price competitiveness.

[0006] Lithium manganese oxides such as LiMnO2 and LiMn2O4 have the advantages of excellent thermal safety and low cost, but they have the drawbacks of low capacity and poor high-temperature performance. On the other hand, LiNiO2-based cathode active materials exhibit high discharge capacity battery characteristics, but their synthesis is difficult due to cation mixing problems between Li and transition metals, and consequently, there are significant problems with their rate characteristics.

[0007] Furthermore, depending on the degree of cation mixing, a large amount of Li byproducts are generated. Most of these Li byproducts consist of LiOH and Li2CO3 compounds, which cause gelling problems during the production of the positive electrode paste and gas generation as charging and discharging progresses after electrode production. Residual Li2CO3 not only increases the swelling phenomenon of the cell and reduces the number of cycles, but also causes the battery to swell.

[0008] To compensate for these shortcomings, the demand for high-Ni type cathode active materials with a Ni content of 50% or more has begun to increase as cathode active materials for secondary batteries. However, while such high-Ni type cathode active materials exhibit high capacity characteristics, the increased Ni content in the cathode active material leads to structural instability due to Li / Ni cation mixing. Due to this structural instability of the cathode active material, lithium secondary batteries can degrade rapidly not only at high temperatures but also at room temperature.

[0009] On the other hand, there is prior art related to positive electrode active materials for lithium secondary batteries, such as Korean Published Patent Publication No. 10-2014-0022681 (Patent Document 1).

[0010] Patent Document 1 discloses that among the compounds represented by the following chemical formula 1, the doping metal represented by Me can improve the electrochemical properties of the positive electrode active material by forming a concentration gradient that decreases from the surface to the center of the positive electrode active material particles.

[0011] [C1] Li1+x M 1-k Me k O2

[0012] However, the reality is that simply having a concentration gradient of doping metal that is limited to the surface of the positive electrode active material (secondary particles) is insufficient to simultaneously improve the electrochemical properties and structural stability of the positive electrode active material (especially in the case of high-Ni).

[0013] Therefore, the reality is that it is necessary to develop cathode active materials that complement the problems of such high-Ni type cathode active materials. [Prior art documents] [Patent Documents]

[0014] [Patent Document 1] Korean Published Patent Publication No. 10-2014-00022681 (Publication Date: 2014.02.25) [Overview of the Initiative] [Problems that the invention aims to solve]

[0015] As described above, a certain trade-off relationship may exist between certain indicators of the electrochemical properties of a positive electrode active material for lithium secondary batteries and certain indicators of its stability. Therefore, if the capacity characteristics of the positive electrode active material are excessively improved, the structural stability of the particles constituting the positive electrode active material may decrease, potentially preventing it from exhibiting stable charge and discharge performance.

[0016] Therefore, the object of the present invention is to provide a positive electrode active material that can maintain the high electrochemical properties of existing positive electrode active materials for lithium secondary batteries, particularly high-Ni type positive electrode active materials, while eliminating their low structural stability.

[0017] Specifically, an object of the present invention is to provide a positive electrode active material having improved electrochemical characteristics and stability by providing an aggregate of primary particles in which a plurality of primary particles are aggregated and a concentration gradient of a doping metal is formed in a direction from a grain boundary between the primary particles toward the center of the primary particle as secondary particles.

[0018] Another object of the present invention is to provide a lithium secondary battery using a positive electrode containing the positive electrode active material defined in the present application.

Means for Solving the Problems

[0019] According to one aspect of the present invention, there is provided a positive electrode active material including a layered lithium composite oxide containing at least nickel, cobalt, and a doping metal and capable of lithium intercalation / deintercalation.

[0020] Here, the lithium composite oxide includes secondary particles which are aggregates in which a plurality of primary particles are aggregated and a grain crystal system is formed between the adjacent primary particles, and the secondary particles are aggregates of primary particles in which the concentration of the doping metal shows a concentration gradient having a (-) slope from the grain boundary between the primary particles toward the center of the primary particle.

[0021] Here, the primary particle is represented by the following Chemical Formula 1.

[0022] [Chemical Formula 1] Li a Ni 1-(b+c+d+e) Co b M1 c M2 d M3 e O f

[0023] (Here, M1 is at least one selected from Mn and Al, M2 and M3 are each independently selected from Al, Ba, B, Ce, Cr, Mg, Mn, Mo, Na, K, P, V, Sr, Ti, W, Nb, and Zr, M1 to M3 are different from each other. (0.90≦a≦1.15, 0≦b≦0.15, 0≦c≦0.10, 0≦d≦0.025, 0≦e≦0.025, 1.0≦f≦2.0)

[0024] In one embodiment, the secondary particles may be aggregates of primary particles exhibiting a concentration gradient in which at least one concentration selected from M2 and M3 decreases from the grain boundaries between the primary particles toward the center of the primary particles.

[0025] Furthermore, there may be a first concentration gradient section where the concentration of the doping metal has a negative slope and a second concentration gradient section where the concentration of the doping metal has a positive slope, moving from the surface of the secondary particle toward the center of the secondary particle.

[0026] Furthermore, according to another aspect of the present invention, a positive electrode comprising a positive electrode active material as defined herein is provided.

[0027] Furthermore, according to another aspect of the present invention, a lithium secondary battery using the positive electrode defined in this application is provided. [Effects of the Invention]

[0028] Generally, high-Ni type positive electrode active materials used to achieve high capacity lithium secondary batteries have the problem of low stability. However, the positive electrode active material according to the present invention embodies the secondary particles constituting the positive electrode active material as aggregates of primary particles in which the concentration of the doping metal has a (-) slope from the grain boundaries between primary particles toward the center of the primary particles. This improves the low stability of high-Ni type positive electrode active materials and contributes to the stable expression of electrochemical properties.

[0029] On the other hand, in the case of high-Ni type positive electrode active materials, a large amount of Li by-products such as LiOH and Li2CO3 are present on the surface compared to positive electrode active materials with a relatively low Ni content. These Li by-products may act as a cause of gelation during the manufacture of positive electrode paste using the positive electrode active material, or as a cause of gas generation during the charging, discharging, and / or storage of lithium secondary batteries.

[0030] However, the positive electrode active material according to the present invention makes it possible to reduce the content of Li by-products both inside and outside the positive electrode active material by forming a concentration gradient with respect to the doping metal concentration not only on the surface of the secondary particles but also from the grain boundaries between the primary particles constituting the secondary particles.

[0031] Furthermore, the positive electrode active material is a region where side reactions can occur with the electrolyte during charging, discharging, and / or storage of lithium secondary batteries, not only on the surface of the secondary particles but also at the grain interfaces within the secondary particles (i.e., the crystal grain boundaries between primary particles).

[0032] In this case, the larger the surface area of ​​the positive electrode active material, the larger the area in which side reactions can occur, thus increasing the likelihood of side reactions. Such side reactions can induce a phase transformation in the lithium composite oxide crystal structure that constitutes the positive electrode active material (for example, from a layered structure to a rock salt structure). Such phase transformations in the crystal structure within the surface of the positive electrode active material have been pointed out as one of the causes of a decrease in electrochemical properties such as the lifespan characteristics of lithium secondary batteries.

[0033] The positive electrode active material according to the present invention reduces the surface area on which side reactions with the electrolyte can occur during charging, discharging, and / or storage of a lithium secondary battery. This is achieved by forming a concentration gradient with respect to the doping metal concentration not only on the surface of the secondary particles but also from the grain boundaries between the primary particles constituting the secondary particles. By reducing the possibility of side reactions with the electrolyte during charging, discharging, and / or storage of a lithium secondary battery, the charging, discharging, and / or storage stability can be improved.

[0034] As a result, the positive electrode active material according to the present invention includes secondary particles provided as aggregates of primary particles in which a doping metal concentration gradient is formed in the direction from the grain boundaries between primary particles toward the center of the primary particles. This contributes to improving not only various physical properties, including the structural stability of the positive electrode active material, but also electrochemical properties such as lifetime characteristics and efficiency characteristics.

[0035] In addition to the effects described above, the specific effects of the present invention will be described below, along with a detailed explanation of the specific matters for carrying out the invention. [Brief explanation of the drawing]

[0036] [Figure 1] This graph schematically shows the concentration gradient of secondary particles contained in the positive electrode active material according to one embodiment of the present invention. The graph shown in Figure 1 schematically shows an example of EDX analysis results obtained by line scanning from the surface of the secondary particles toward the center of the secondary particles for doping metals called M2 and / or M3. [Figure 2] This graph shows the doping metal content in secondary particles with a radius of 7.5 μm, obtained by line scanning after selecting these particles from the positive electrode active materials of Examples 1 to 7, and then performing cross-sectional processing using a FIB (Ga-ion source) to obtain cross-sectional SEM images, which were then confirmed by EDX analysis of the resulting images. [Figure 3] This graph shows the doping metal content in secondary particles with a radius of 7.5 μm, obtained by line scanning after selecting these particles from the positive electrode active materials of Examples 1 to 7, and then performing cross-sectional processing using a FIB (Ga-ion source) to obtain cross-sectional SEM images, which were then confirmed by EDX analysis of the resulting images. [Figure 4] This graph shows the doping metal content in secondary particles with a radius of 7.5 μm, obtained by line scanning after selecting these particles from the positive electrode active materials of Examples 1 to 7, and then performing cross-sectional processing using a FIB (Ga-ion source) to obtain cross-sectional SEM images, which were then confirmed by EDX analysis of the resulting images. [Figure 5] This graph shows the doping metal content in secondary particles with a radius of 7.5 μm, obtained by line scanning after selecting these particles from the positive electrode active materials of Examples 1 to 7, and then performing cross-sectional processing using a FIB (Ga-ion source) to obtain cross-sectional SEM images, which were then confirmed by EDX analysis of the resulting images. [Figure 6] This graph shows the doping metal content in secondary particles with a radius of 7.5 μm, obtained by line scanning after selecting these particles from the positive electrode active materials of Examples 1 to 7, and then performing cross-sectional processing using a FIB (Ga-ion source) to obtain cross-sectional SEM images, which were then confirmed by EDX analysis of the resulting images. [Figure 7] This graph shows the doping metal content in secondary particles with a radius of 7.5 μm, obtained by line scanning after selecting these particles from the positive electrode active materials of Examples 1 to 7, and then performing cross-sectional processing using a FIB (Ga-ion source) to obtain cross-sectional SEM images, which were then confirmed by EDX analysis of the resulting images. [Figure 8] This graph shows the doping metal content in secondary particles with a radius of 7.5 μm, obtained by line scanning after selecting these particles from the positive electrode active materials of Examples 1 to 7, and then performing cross-sectional processing using a FIB (Ga-ion source) to obtain cross-sectional SEM images, which were then confirmed by EDX analysis of the resulting images. [Figure 9] This graph shows the doping metal content in secondary particles with a radius of 7.5 μm, obtained by line scanning after selecting these particles from the positive electrode active materials of Comparative Examples 1 to 3 and performing cross-sectional processing using a FIB (Ga-ion source), followed by EDX analysis of the resulting cross-sectional SEM images. [Figure 10]This graph shows the doping metal content in secondary particles with a radius of 7.5 μm, obtained by line scanning after selecting these particles from the positive electrode active materials of Comparative Examples 1 to 3 and performing cross-sectional processing using a FIB (Ga-ion source), followed by EDX analysis of the resulting cross-sectional SEM images. [Figure 11] This graph shows the doping metal content in secondary particles with a radius of 7.5 μm, obtained by line scanning after selecting these particles from the positive electrode active materials of Comparative Examples 1 to 3 and performing cross-sectional processing using a FIB (Ga-ion source), followed by EDX analysis of the resulting cross-sectional SEM images. [Figure 12] This graph shows the doping metal content in secondary particles, confirmed by line scanning through EDX analysis of cross-sectional SEM images obtained after selecting secondary particles with a radius of 7.5 μm contained in the positive electrode active material according to Reference Example 1 and Reference Example 2 and then cross-sectionally processing them using FIB (Ga-ion source). [Figure 13] The graphs show the doping metal content in secondary particles, confirmed by line scanning through EDX analysis of cross-sectional SEM images obtained after selecting secondary particles with a radius of 7.5 μm contained in the positive electrode active materials according to Reference Examples 1 and 2 and then performing cross-sectional processing using FIB (Ga-ion source). Figures 2 to 13 are excerpts of line scanning results from the surface to a depth of 3 μm (3,000 nm) of secondary particles contained in each positive electrode active material. [Figure 14] The results of XRD analysis of the positive electrode active material according to Examples 1 to 3 are shown. [Figure 15] The results of XRD analysis of the positive electrode active material according to Examples 1 to 3 are shown. [Figure 16] The results of XRD analysis of the positive electrode active material according to Examples 1 to 3 are shown. [Modes for carrying out the invention]

[0037] For the convenience of making the present invention easier to understand, certain terms are defined herein. Unless otherwise specifically defined herein, scientific and technical terms used herein have meanings that are generally understood by those ordinary in the art. Furthermore, unless otherwise specified in the context, singular terms should be understood to include their plural forms, and plural terms should be understood to include their singular forms.

[0038] The following describes in more detail the positive electrode active material according to the present invention and the lithium secondary battery using a positive electrode containing the positive electrode active material.

[0039] positive electrode active material According to one aspect of the present invention, a positive electrode active material is provided which includes a layered lithium composite oxide capable of lithium intercalation / deintercalation.

[0040] The lithium composite oxide includes secondary particles, which are aggregates formed by the aggregation of multiple primary particles, with grain boundaries formed between adjacent primary particles.

[0041] Here, primary particles refer to a single crystal grain (grain or crystallite), and secondary particles refer to aggregates formed by the aggregation of multiple primary particles. These secondary particles are sometimes called bulk particles. Furthermore, the primary particles may be rod-shaped, elliptical, and / or circular, or amorphous.

[0042] Voids and / or grain boundaries may exist between the primary particles that constitute the secondary particles. For example, the primary particles may separate from adjacent primary particles within the secondary particle to form internal voids. Alternatively, the primary particles may form surfaces within the secondary particle by contacting the internal voids without forming grain boundaries by contacting adjacent primary particles.

[0043] On the other hand, the surface of the primary particle present on the outermost surface of the secondary particle that is exposed to the outside air forms the surface of the secondary particle.

[0044] Here, the average particle size of the primary particles is within the range of 0.1 μm to 10 μm, preferably 0.1 μm to 5 μm, thereby realizing the optimal density of the positive electrode produced using the positive electrode active material according to various embodiments of the present invention. The average particle size of the secondary particles may vary depending on the number of aggregated primary particles, but may be between 3 μm and 20 μm.

[0045] Furthermore, the lithium composite oxide may contain at least nickel, cobalt, and a doping metal. If the lithium composite oxide further contains manganese other than nickel, cobalt, and a doping metal, the lithium composite oxide may be an NCM type composite oxide. Furthermore, if the lithium composite oxide further contains aluminum other than cobalt and a doping metal, the lithium composite oxide may be an NCA type composite oxide.

[0046] As described above, the secondary particles constituting the lithium composite oxide are aggregates formed by the aggregation of multiple primary particles. In this case, the interface between adjacent primary particles or the bonding surface between adjacent primary particles can be defined as the grain boundary between primary particles. Furthermore, the grain boundary between adjacent primary particles may coincide with the surface of the primary particles forming the grain boundary.

[0047] Here, the primary particles constituting the secondary particles can exhibit a concentration gradient in which the concentration of the doping metal has a negative slope from the grain boundaries between the primary particles toward the center of the primary particles.

[0048] The change in the concentration of the doping metal within the primary particle, formed in the direction from the surface of the secondary particle toward the center of the secondary particle, may be measured by EDX analysis (line scanning) of the cross-section of the secondary particle.

[0049] If the secondary particles are aggregates of primary particles that exhibit a concentration gradient of the doping metal with a negative slope from the grain boundaries between the primary particles toward the center of the primary particles, then the EDX analysis results for the secondary particles can be confirmed to show that the concentration gradient pattern for the doping metal is repeated.

[0050] For example, when line scanning is performed on primary particles present on the outermost surface of secondary particles, from the surface of the secondary particles toward the center of the secondary particles, a concentration gradient with a negative slope of the doping metal concentration may appear from the outermost surface of the secondary particles toward the primary particles, and conversely, a concentration gradient with a positive slope of the doping metal concentration may appear within primary particles relatively closer to the center of the secondary particles. Even in such cases, with respect to the primary particles, the concentration of the doping metal within a single primary particle shows a concentration gradient with a negative slope of the negative slope from the surface of the primary particle (in this case, the surface of the primary particle means the grain boundary between multiple primary particles) toward the center of the primary particle.

[0051] On the other hand, the concentration of the doping metal shows a (-) gradient from the grain boundaries between the primary particles toward the center of the primary particles. With respect to the secondary particles, there exists a first concentration gradient section with a (-) gradient and a second concentration gradient section with a (+) gradient from the surface of the secondary particles toward the center of the secondary particles. Furthermore, the first concentration gradient section with a (-) gradient and the second concentration gradient section with a (+) gradient may be repeated from the surface of the secondary particles toward the center of the secondary particles.

[0052] Here, the ordinal numbers used to refer to concentration gradient intervals with (+) and / or (-) slopes are merely used to distinguish between concentration gradient intervals with (+) slopes and concentration gradient intervals with (-) slopes.

[0053] Furthermore, a concentration gradient with a positive (+) slope means that the concentration of the doping metal significantly increases between the start and end points of the line scanning. On the other hand, a concentration gradient with a negative (-) slope means that the concentration of the doping metal significantly decreases between the start and end points of the line scanning. For example, a significant increase or decrease in the concentration of the doping metal means that the change in the concentration of the doping metal between the start and end points of the concentration gradient interval is 2 at% or more.

[0054] A concentration gradient with a positive (+) slope indicates that the gradient with respect to the concentration of the doping metal is positive (+) between the start and end points of the line scanning, and does not consider changes in the slope due to slight differences in the concentration of the doping metal in a portion of the section between the start and end points of the line scanning.

[0055] Similarly, it should be understood that demonstrating a concentration gradient with a negative slope is sufficient if the gradient with respect to the concentration of the doping metal between the start and end points of the line scanning is negative. This means that changes in the slope due to slight differences in the concentration of the doping metal in a portion of the line scanning between the start and end points are not considered.

[0056] The primary particles that constitute the secondary particles as defined in this application are represented by the following formula 1.

[0057] [C1] Li a Ni 1-(b+c+d+e) Co b M1 c M2 d M3 e O f

[0058] (Here, M1 is at least one selected from Mn and Al. M2 and M3 are each independently selected from Al, Ba, B, Ce, Cr, Mg, Mn, Mo, Na, K, P, V, Sr, Ti, W, Nb, and Zr. M1 to M3 are different from each other. 0.90≦a≦1.15, 0≦b≦0.15, 0≦c≦0.10, 0≦d≦0.025, 0≦e≦0.025, 1.0≦f≦2.0.

[0059] The primary particles may be high-Ni type lithium composite oxides in which the concentrations (mol%) of Ni, Co, M1, M2, and M3 in the above formula 1 satisfy the following formula 1.

[0060] [Formula 1] Ni / (Ni+Co+M1+M2+M3)≧80.0

[0061] Furthermore, the primary particles may be a high-Ni / low-Co type lithium composite oxide in which the concentrations (mol%) of Ni, Co, M1, M2, and M3 in the above formula 1 satisfy formula 1, and the Co content is 10 mol% or less, preferably 5 mol% or less.

[0062] In other words, the primary particles can satisfy the following formula 2 in terms of the concentrations (mol%) of Ni, Co, M1, M2, and M3 in the above formula 1.

[0063] [Formula 2] Co / (Ni+Co+M1+M2+M3)≦5.0

[0064] In general, it is known that in lithium composite oxides containing at least Ni and Co, the increasing Ni content leads to structural instability of the lithium composite oxide due to Li / Ni cation mixing. Furthermore, it has been reported that in lithium composite oxides containing at least Ni and Co, the decreasing Co content increases the initial overpotential (resistance), inevitably leading to a decrease in rate characteristics.

[0065] However, the lithium composite oxide contained in the positive electrode active material according to one embodiment of the present invention exhibits a concentration gradient with a negative slope in the doping metal from the grain boundaries between primary particles constituting secondary particles toward the center of the primary particles, thereby mitigating and / or preventing the structural instability and rate characteristics degradation of high-Ni type or high-Ni / low-Co type lithium composite oxides.

[0066] On the other hand, in the above formula 1, M2 and / or M3 may correspond to the doping metals described above. As a result, the secondary particles may be aggregates of primary particles exhibiting a concentration gradient in which the concentration of at least one selected from M2 and M3 decreases from the grain boundaries between the primary particles toward the center of the primary particles.

[0067] Figure 1 is a schematic graph showing the concentration gradient of secondary particles contained in the positive electrode active material according to one embodiment of the present invention. The graph shown in Figure 1 schematically shows an example of EDX analysis results obtained by line scanning from the surface of the secondary particles toward the center of the secondary particles for doping metals called M2 and / or M3.

[0068] Referring to Figure 1, with respect to the secondary particles, the concentration of the doping metal can have a negative slope in the direction from the outermost surface of the secondary particles toward the center of the secondary particles. In this case, the interval showing the concentration gradient with the aforementioned negative slope will be called the first concentration gradient interval s1.

[0069] Thus, because the concentration of the doping metal has a negative slope in the direction from the outermost surface of the secondary particle toward the center of the secondary particle, a first peak (p0) for the concentration of the doping metal may appear in the graph at the position corresponding to the outermost surface of the secondary particle.

[0070] If the average particle size of the primary particles is r, the concentration gradient within the primary particles may exist within a depth of 0.2r from the grain boundary between the primary particles. If the concentration gradient within the primary particles exists to a depth greater than 0.2r from the grain boundary between the primary particles, it may lead to instability in the crystal structure within the primary particles or degrade the electrochemical properties.

[0071] On the other hand, after the first concentration gradient section (s1), there may be a concentration maintenance section (s3) in which the concentration of the doping metal is maintained for a predetermined depth.

[0072] The aforementioned concentration maintenance interval refers not only to an interval in which the concentration of the doping metal does not change significantly and is nearly uniform, but can also refer to all intervals in which the absolute value of the change in the concentration of the doping metal is 3 at%, preferably 2 at%, or less. The absolute value of the change in the concentration of the doping metal within the concentration maintenance interval may vary depending on the type of doping metal and the content of the doping metal in the primary particles, but the change in the concentration of the doping metal within the concentration maintenance interval will be an insignificant change when compared to the first concentration gradient interval (s1) and the second concentration gradient interval (s2).

[0073] Within the aforementioned concentration maintenance interval, the concentration of the doping metal can have a positive gradient toward the center of the secondary particles. In this case, the interval in which the concentration gradient with the aforementioned positive gradient appears will be called the second concentration gradient interval (s2).

[0074] As shown in Figure 1, when explained with reference to the secondary particles, it can be confirmed that the first concentration gradient section (s1), the concentration maintenance section (s3), and the second concentration gradient section (s2) repeatedly exist from the surface of the secondary particles toward the center of the secondary particles. However, even in such cases, it can be understood that the second concentration gradient section (s2) appears through a concentration gradient with a negative slope from the grain boundaries between the primary particles toward the center of the primary particles.

[0075] In other words, only if the secondary particles are aggregates of primary particles that exhibit a concentration gradient in which the concentration of the doping metal has a negative slope from the grain boundaries between the primary particles toward the center of the primary particles, a first concentration gradient section in which the concentration of the doping metal has a negative slope toward the center of the secondary particles and a second concentration gradient section in which the concentration of the doping metal has a positive slope may repeatedly exist.

[0076] On the other hand, because the concentration of the doping metal repeatedly exists in a first concentration gradient section with a negative slope and a second concentration gradient section with a positive slope from the surface of the secondary particle toward the center of the secondary particle, at least one peak (p1, p2) may appear at the position corresponding to the outermost surface of the secondary particle after the first peak (p0) for the concentration of the doping metal.

[0077] The second peak (p1), which first appears after the first peak (p0), may appear due to the existence of the first concentration gradient interval s1 after the second concentration gradient interval (s2). In this case, the second peak (p1) may be formed by the second concentration gradient interval (s2) located within a primary particle on the outermost surface of the secondary particle and the first concentration gradient interval (s1) located within a primary particle adjacent to the primary particle. In this case, the second concentration gradient interval (s2) located within a primary particle on the outermost surface of the secondary particle and the first concentration gradient interval (s1) located within a primary particle adjacent to the primary particle can be defined together as a peak region (a1), and the peak located within the peak region (a1) can be defined as the second peak (p1). In this case, the position where the second peak p1 appears may be a grain boundary formed between two adjacent primary particles.

[0078] The stability of the secondary particles on their surface can be improved by the first concentration gradient interval (s1) that exists after the first peak (p0) that appears on the surface of the secondary particles. For example, the presence of the first concentration gradient interval (s1) for the doping metal on the surface of the secondary particles can reduce the content of Li by-products on the surface of the secondary particles, or reduce the specific surface area of ​​the secondary particles, thereby reducing the possibility of side reactions with the electrolyte.

[0079] On the other hand, the primary particles exhibit a concentration gradient with a negative slope in the concentration of the doping metal from the grain boundaries between the primary particles toward the center of the primary particles. As a result, within the primary particles, a first concentration gradient section (s1), a concentration maintenance section (s3), and a second concentration gradient section (s2) sequentially exist from the center of the secondary particles toward the surface of the secondary particles. In this way, the structural stability of the primary particles can be improved by the sequential existence of the first concentration gradient section (s1), the concentration maintenance section (s3), and the second concentration gradient section (s2) within the primary particles toward the center of the secondary particles toward the surface of the secondary particles.

[0080] Furthermore, the concentration gradient pattern of the doping metal described above reduces the content of Li byproducts not only on the surface of the secondary particles but also at the grain boundaries between the primary particles, thereby reducing the exposed area inside the secondary particles and reducing the possibility of side reactions with the electrolyte.

[0081] On the other hand, there exists a first concentration gradient section (s1) with a negative gradient and a second concentration gradient section (s2) with a positive gradient, where the concentration of the doping metal decreases from the surface of the secondary particle toward the center of the secondary particle. Preferably, the first concentration gradient section (s1) and the second concentration gradient section (s2) are repeated at least twice. In this case, a concentration maintenance section (s3) may exist between the first concentration gradient section (s1) and the second concentration gradient section (s2) that are present at least within the same primary particle.

[0082] As the first concentration gradient section (s1) and the second concentration gradient section (s2) are repeated from the surface of the secondary particle toward the center of the secondary particle, peak regions are repeatedly formed along the depth direction from the surface of the secondary particle.

[0083] Furthermore, the peak region may exist not only at a position adjacent to the surface of the secondary particle, but also at a position adjacent to the center of the secondary particle or even at the center of the secondary particle. However, in order to ensure that the peak region extends to the center of the secondary particle, depending on the type of doping metal, it may be necessary to excessively increase the content of the raw material containing the doping metal introduced during the process of synthesizing the positive electrode active material. If the content of the raw material containing the doping metal introduced during the process of synthesizing the positive electrode active material becomes excessively high, the content of the doping metal in the primary particles may become unnecessarily high, or the content of the doping metal-derived compound present at least in part of the grain boundaries between the primary particles and the surface of the secondary particles may become excessively high, which may actually reduce the electrochemical properties and stability of the positive electrode active material.

[0084] Therefore, preferably, the first concentration gradient section (s1) and the second concentration gradient section (s2) are repeated at least twice, from the surface of the secondary particle toward the center of the secondary particle. However, the upper limit of the number of times the first concentration gradient section (s1) and the second concentration gradient section s2 are repeated may be appropriately adjusted considering the composition of the positive electrode active material and the type of doping metal.

[0085] For example, if the radius of the secondary particle is R, then the first concentration gradient section (s1) and the second concentration gradient section (s2) are limited to a depth of 0 to 0.2R from the surface of the secondary particle, which may be advantageous in terms of the electrochemical properties and stability of the positive electrode active material.

[0086] If the first concentration gradient section (s1) and the second concentration gradient section (s2) exist generally from the surface to the central portion of the secondary particle, it is preferable that the number of times the first concentration gradient section (s1) and the second concentration gradient section (s2) located within a depth of 0 to 0.2R from the surface of the secondary particle are repeated (or the number of times the peaks (p1, p2, ...) exist, the number of times the peak regions (a1, ...) exist) is greater than the number of times the first concentration gradient section (s1) and the second concentration gradient section (s2) located within a depth of 0.2R to R from the surface of the secondary particle are repeated (or the number of times the peaks (p1, p2, ...) exist, the number of times the peak regions (a1, ...) exist).

[0087] Furthermore, the positive electrode active material according to some embodiments of the present invention may include a coating layer that covers at least a portion of the surface of the primary particles (e.g., the grain boundaries between the primary particles) and / or the secondary particles formed by the aggregation of the primary particles.

[0088] For example, the coating layer may be present so as to cover at least a portion of the exposed surface of the primary particles. In particular, the coating layer may be present so as to cover at least a portion of the exposed surface of the primary particles that are on the outermost side of the secondary particles.

[0089] As a result, the coating layer may exist as a layer that continuously or discontinuously coats the surface of the primary particles and / or the secondary particles formed by the aggregation of the primary particles. If the coating layer exists discontinuously, it may exist in an island form.

[0090] The coating layer present in this manner can help maintain the high electrochemical properties of the positive electrode active material, particularly high-Ni type positive electrode active materials, while also contributing to overcoming their low structural stability.

[0091] Furthermore, the coating layer may exist in a solid solution form that does not form a boundary with the primary particles and / or the secondary particles formed by the aggregation of the primary particles.

[0092] The coating layer may contain at least one compound represented by the following chemical formula 2. That is, the coating layer can be defined as a region where the compound represented by the following chemical formula 2 exists.

[0093] [Case 2] Li g M4 h O i

[0094] (Here, M4 is at least one selected from Mn, Al, Co, Ti, Zr, Sr, Mg, V, B, Mo, Zn, Nb, Ba, Ca, Ta, Fe, Cr, Sn, Hf, Ce, and W. 0 ≤ g ≤ 10, 0 <h≦8、0<i≦15である。)

[0095] Furthermore, the coating layer may be in a form in which different compounds exist simultaneously within a single layer, or in a form in which the different compounds represented in Chemical Formula 2 exist in separate layers.

[0096] The compound represented by formula 2 may be physically and / or chemically bonded to the primary particles represented by formula 1. Alternatively, the compound may exist in a state where it forms a solid solution with the primary particles represented by formula 1.

[0097] The positive electrode active material according to this embodiment can enhance structural stability by including a coating layer that covers at least a portion of the surface of the primary particles (e.g., the interface between the primary particles) and / or the secondary particles formed by the aggregation of the primary particles. Furthermore, when such a positive electrode active material is used in a lithium secondary battery, the high-temperature storage stability and life characteristics of the positive electrode active material can be improved. In addition, the compound can reduce residual lithium in the positive electrode active material and act as a lithium ion transport pathway, thereby influencing the improvement of the efficiency characteristics of the lithium secondary battery.

[0098] Furthermore, depending on the circumstances, the compound may be present not only at least a portion of the interfaces between the primary particles and the surfaces of the secondary particles, but also in the internal voids formed within the secondary particles.

[0099] The aforementioned compound is a compound in which lithium and an element represented by M4 are combined, or as a compound of M4, the aforementioned compound is, for example, Li a W b O c Li a Zr b O c Li a Ti b O c Li a Ni b O c Li a B b O c , W b O c , Zr b O c Ti b O c or B b O c It may also be expressed as such. Furthermore, non-restrictive examples of the above compounds include Li2B4O7, Li3BO3, Li2B2O7, Li2B8O 13 , Li2VO3, Li3VO4, Li6Zr3O9, Li2ZrO3, Li 5.5 Zr 2.6 2O8, Li 44 Ba19 Examples include Li4Ba, Li2TiO3, LiTi7O4, and LiTi2O4. The examples given above are merely for convenience to aid understanding, and the compounds defined in this application are not limited to the examples given above.

[0100] In other embodiments, the compound may be a compound comprising lithium and at least two elements represented by M4, or further comprising a compound comprising lithium and at least two elements represented by M4. A compound comprising lithium and at least two elements represented by M4 is, for example, Li a (W / Ti) b O c Li a (W / Zr) b O c Li a (W / Ti / Zr) b O c Li a (W / Ti / B) b O c Other options are also acceptable, but the term is not necessarily limited to these.

[0101] Here, the compound can exhibit a concentration gradient that decreases from the surface of the secondary particle toward the center of the secondary particle. As a result, the concentration of the compound can decrease from the surface of the secondary particle toward the center of the secondary particle.

[0102] As described above, the compound exhibits a concentration gradient that decreases from the surface of the secondary particles toward the center of the secondary particles, thereby effectively reducing the residual lithium present on the surface of the positive electrode active material and preventing side reactions caused by unreacted residual lithium. Specifically, the residual lithium present on the surface of the secondary particles reacts with the raw material of the compound represented by formula 2 to form the compound represented by formula 2, thereby reducing the residual lithium present on the surface of the secondary particles. Furthermore, the compound can prevent a decrease in crystallinity in the inner region of the surface of the positive electrode active material. In addition, the compound can prevent the overall structure of the positive electrode active material from collapsing during the electrochemical reaction.

[0103] Furthermore, the coating layer may include a first coating layer containing at least one compound represented by the formula 2, and a second coating layer containing at least one compound represented by the formula 2, but a compound different from the compound contained in the first coating layer.

[0104] For example, the first coating layer may be present so as to cover at least a portion of the exposed surface of the primary particles that are on the outermost side of the secondary particles, and the second coating layer may be present so as to cover at least a portion of the exposed surface of the primary particles that are not covered by the first coating layer and the surface of the first coating layer.

[0105] Lithium-ion battery In yet another aspect of the present invention, a positive electrode can be provided that includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector. Here, the positive electrode active material layer may include positive electrode active materials according to various embodiments of the present invention. Therefore, since the positive electrode active material is as described above, a detailed explanation will be omitted for convenience, and only the remaining undescribed components will be described below.

[0106] The positive electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc. may be used. The positive electrode current collector may also have a thickness of 3 to 500 μm, and fine irregularities can be formed on the surface of the current collector to increase the adhesion strength of the positive electrode active material. For example, it may be used in various forms such as film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0107] The positive electrode active material layer may be manufactured by applying a positive electrode slurry composition, which includes a conductive material and, if necessary, a binder, together with the positive electrode active material, to the positive electrode current collector.

[0108] In this case, the positive electrode active material may be included in an amount of 80 to 99 wt%, more specifically 85 to 98.5 wt%, relative to the total weight of the positive electrode active material layer. When included in this content range, excellent capacity characteristics can be observed, but the material is not necessarily limited to this range.

[0109] The conductive material is used to impart conductivity to the electrodes and can be used without particular limitations as long as it has electronic conductivity in the battery without causing a chemical change. Specific examples include graphite such as natural graphite or artificial graphite, carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, summer black, and carbon fiber, metal powders or metal fibers such as copper, nickel, aluminum, and silver, conductive whiskers such as zinc oxide and potassium titanate, conductive metal oxides such as titanium oxide, or conductive polymers such as polyphenylene derivatives. One of these may be used alone or a mixture of two or more. The conductive material may be included in an amount of 0.1 to 15% by weight relative to the total weight of the positive electrode active material layer.

[0110] The binder plays a role in improving adhesion between positive electrode active material particles and adhesion between the positive electrode active material and the current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, of which one or more may be used. The binder may be included in an amount of 0.1 to 15% by weight relative to the total weight of the positive electrode active material layer.

[0111] The positive electrode may be manufactured by a conventional positive electrode manufacturing method, except that the positive electrode active material is used. Specifically, the positive electrode may be manufactured by applying a positive electrode slurry composition, prepared by dissolving or dispersing the positive electrode active material and selectively a binder and conductive material in a solvent, onto a positive electrode current collector, followed by drying and rolling.

[0112] The solvent may be any solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and one or more of these may be used individually or in mixtures of two or more. The amount of solvent used should be such that it dissolves or disperses the cathode active material, conductive material, and binder, and then provides a viscosity that allows for excellent thickness uniformity during coating for cathode manufacturing, taking into consideration the coating thickness and production yield of the slurry.

[0113] In other embodiments, the positive electrode may be manufactured by casting the positive electrode slurry composition onto a separate support, peeling it off the support, and then laminating the resulting film onto the positive electrode current collector.

[0114] Furthermore, according to yet another aspect of the present invention, an electrochemical element including the aforementioned positive electrode may be provided. Specifically, the electrochemical element may be a battery, a capacitor, or the like, and more specifically, a lithium secondary battery.

[0115] The lithium secondary battery may specifically include a positive electrode, a negative electrode positioned opposite the positive electrode, and a separator membrane and electrolyte interposed between the positive and negative electrodes. Here, since the positive electrode is as described above, for convenience, a detailed explanation will be omitted, and only the remaining components not mentioned above will be described in detail below.

[0116] The lithium secondary battery may further selectively include a battery container for housing the electrode assembly comprising the positive electrode, the negative electrode, and the separator membrane, and a sealing member for sealing the battery container.

[0117] The negative electrode may include a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector.

[0118] The negative electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy may be used. The negative electrode current collector may also typically have a thickness of 3 μm to 500 μm, and, similar to the positive electrode current collector, fine irregularities can be formed on the surface of the current collector to strengthen the bonding force of the negative electrode active material. For example, it may be used in various forms such as film, sheet, foil, net, porous material, foam, and nonwoven fabric.

[0119] The negative electrode active material layer may be manufactured by applying a negative electrode slurry composition, which includes the negative electrode active material together with a conductive material and, if necessary, a selective binder, to the negative electrode current collector.

[0120] As the negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; and SiO2. β Examples include lithium-doped and dedoped metal oxides such as (0<β<2), SnO2, vanadium oxide, and lithium vanadium oxide, or composites containing the metallic compound and carbonaceous material, such as Si-C composites or Sn-C composites. One or more of these mixtures may be used. A metallic lithium thin film may also be used as the negative electrode active material. Furthermore, low-crystalline carbon and high-crystalline carbon may all be used as the carbon material. Typical examples of low-crystalline carbon include soft carbon and hard carbon, while typical examples of high-crystalline carbon include amorphous, plate-like, flaky, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch-derived cokes.

[0121] The aforementioned negative electrode active material may be present in an amount of 80 to 99 wt% based on the total weight of the negative electrode active material layer.

[0122] The binder may be added in an amount of 0.1 to 10 wt% based on the total weight of the negative electrode active material layer, as a component that assists in bonding between the conductive material, active material, and current collector. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.

[0123] The conductive material may be added as a component to further improve the conductivity of the negative electrode active material, in an amount of 10% by weight or less, preferably 5% by weight or less, based on the total weight of the negative electrode active material layer. Such a conductive material is not particularly limited as long as it has conductivity without inducing a chemical change in the battery, and for example, graphite such as natural graphite or artificial graphite, carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black, conductive fibers such as carbon fibers and metal fibers, metal powders such as carbon fluoride, aluminum, and nickel powder, conductive whiskey such as zinc oxide and potassium titanate, conductive metal oxides such as titanium oxide, and conductive materials such as polyphenylene derivatives may be used.

[0124] In one embodiment, the negative electrode active material layer may be manufactured by coating a negative electrode slurry composition, which is prepared by dissolving or dispersing a negative electrode active material and a binder and conductive material selectively in a solvent, onto a negative electrode current collector and drying it, or by casting the negative electrode slurry composition onto a separate support, peeling it off the support, and laminating the resulting film onto the negative electrode current collector.

[0125] In other embodiments, the negative electrode active material layer may be manufactured by coating a negative electrode slurry composition, which is prepared by dissolving or dispersing a negative electrode active material and a binder and conductive material selectively in a solvent, onto a negative electrode current collector and drying it, or by casting the negative electrode slurry composition onto a separate support, peeling it off the support, and laminating the resulting film onto the negative electrode current collector.

[0126] On the other hand, in the lithium secondary battery, the separation membrane separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. It can be used without particular limitations as long as it is a membrane typically used in lithium secondary batteries, and it is especially preferable that it has low resistance to ion movement of the electrolyte and excellent electrolyte moisture absorption capacity. Specifically, porous polymer films, such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures of two or more layers thereof may be used. Alternatively, ordinary porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may be used. Furthermore, to ensure heat resistance or mechanical strength, coated separation membranes containing ceramic components or polymeric substances may be used, and may be selectively used as single-layer or multi-layer structures.

[0127] Furthermore, the electrolytes used in the present invention include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.

[0128] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0129] The organic solvent can be any solvent that serves as a medium through which ions involved in the electrochemical reaction of the battery can move, without any particular limitations. Specifically, the organic solvent may be an ester solvent such as methyl acetate, ethyl acetate, γ-butyrolactone, or ε-caprolactone; an ether solvent such as dibutyl ether or tetrahydrofuran; a ketone solvent such as cyclohexanone; an aromatic hydrocarbon solvent such as benzene or fluorobenzene; dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), or propylene carbonate (propylene Carbonate solvents such as carbonate (PC), alcohol solvents such as ethyl alcohol and isopropyl alcohol, nitriles such as R-CN (where R is a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms, and may include a double-bonded aromatic ring or ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, or sulfolanes may be used. Among these, carbonate solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant that can improve the charge and discharge performance of the battery, and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred. In this case, the electrolyte performance can be improved by mixing the cyclic carbonate and the linear carbonate in a volume ratio of about 1:1 to about 1:9.

[0130] The lithium salt may be any compound capable of providing lithium ions for use in a lithium secondary battery, without any particular limitations. Specifically, the lithium salt may be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3), LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2, etc. The concentration of the lithium salt is preferably within the range of 0.1 to 2.0 M. When the concentration of the lithium salt falls within this range, the electrolyte has appropriate conductivity and viscosity, exhibiting excellent electrolyte performance and allowing lithium ions to move effectively.

[0131] In addition to the electrolyte components, the electrolyte may further contain one or more additives for the purpose of improving battery life characteristics, suppressing the decrease in battery capacity, and improving battery discharge capacity, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphate triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, the additive may be present in an amount of 0.1 to 5% by weight relative to the total weight of the electrolyte.

[0132] As described above, the lithium secondary battery containing the positive electrode active material according to the present invention exhibits excellent discharge capacity, output characteristics, and life characteristics in a stable manner, making it useful in portable devices such as mobile phones, laptop computers, and digital cameras, as well as in the electric vehicle field, such as hybrid electric vehicles (HEVs).

[0133] The external shape of the lithium secondary battery according to the present invention is not particularly limited, but it may be cylindrical, rectangular, pouch-shaped, or coin-shaped using a can. Furthermore, the lithium secondary battery can be used not only as a battery cell used as a power source for small devices, but may also be preferably used as a unit battery in medium-to-large battery modules containing multiple battery cells.

[0134] According to yet another aspect of the present invention, a battery module and / or a battery pack including the lithium secondary battery as a unit cell can be provided.

[0135] The battery module or battery pack can be used as a power source for one or more medium-to-large devices, including power tools, electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs), or power storage systems.

[0136] The present invention will be described in more detail below with reference to examples. However, these examples are for illustrative purposes only and should not be construed as limiting the scope of the present invention.

[0137] Manufacturing Example 1. Manufacturing of positive electrode active material Example 1 Co-precipitation method for spherical Ni 0.92 Co 0.06 Mn 0.02 (OH)2 hydroxide precursor was synthesized.

[0138] Specifically, in a 90L reactor, a 1.5M aqueous solution of composite transition metal sulfuric acid, prepared by mixing nickel sulfate, cobalt sulfate, and manganese sulfate in a molar ratio of 92:6:2, was mixed with 25 wt% NaOH and 30 wt% NH4OH. The pH in the reactor was maintained at 11.5, and the reactor temperature was maintained at 60°C. In addition, an inert gas, N2, was added to the reactor to prevent oxidation of the produced precursor.

[0139] After the synthesis of the hydroxide precursor is complete, it is washed and dehydrated using a filter press (F / P) apparatus, and Ni 0.92 Co 0.06 Mn 0.02 (OH)2 hydroxide precursor was obtained.

[0140] The obtained hydroxide precursor was mixed with LiOH (Li / (Ni+Co+Mn)mol ratio=1.05), and then heated in a firing furnace under an O2 atmosphere, increasing the temperature by 2°C per minute to 700°C, followed by heat treatment at 700°C for 10 hours (first heat treatment) to obtain a lithium composite oxide.

[0141] Next, without washing the obtained lithium composite oxide with water, TiO2 weighed to a concentration of 0.05 mol% was mixed with the lithium composite oxide. Then, while maintaining an O2 atmosphere, the temperature was increased by 2°C per minute to 700°C, and then further heat-treated at 700°C for 10 hours (second heat treatment) to finally obtain the positive electrode active material.

[0142] Example 2 The cathode active material was produced in the same manner as in Example 1, except that during the second heat treatment of the lithium composite oxide, 0.05 mol% of ZrO2 was mixed instead of TiO2, the temperature was raised by 2°C per minute while maintaining an O2 atmosphere, and then further heat treatment (second heat treatment) was performed at 710°C for 10 hours.

[0143] Example 3 The cathode active material was produced in the same manner as in Example 1, except that during the second heat treatment of the lithium composite oxide, 0.1 mol% of V2O3 was mixed instead of TiO2, the temperature was raised by 2°C per minute to 720°C while maintaining an O2 atmosphere, and then further heat treatment (second heat treatment) was performed at 720°C for 10 hours.

[0144] Example 4 The cathode active material was produced in the same manner as in Example 1, except that during the second heat treatment of the lithium composite oxide, 0.2 mol% of H3BO3 was mixed instead of TiO2, the temperature was raised by 2°C per minute while maintaining an O2 atmosphere, and then further heat-treated at 720°C for 10 hours (second heat treatment).

[0145] Example 5 The cathode active material was produced in the same manner as in Example 1, except that during the second heat treatment of the lithium composite oxide, 0.2 mol% of WO3 was mixed instead of TiO2, the temperature was raised to 700°C at a rate of 2°C per minute while maintaining an O2 atmosphere, and then further heat-treated at 700°C for 10 hours (second heat treatment).

[0146] Example 6 The cathode active material was produced in the same manner as in Example 1, except that during the second heat treatment of the lithium composite oxide, 0.2 mol% of Sr(OH)2 was mixed instead of TiO2, the temperature was raised to 680°C at a rate of 2°C per minute while maintaining an O2 atmosphere, and then further heat treatment (second heat treatment) was performed at 680°C for 10 hours.

[0147] Example 7 The cathode active material was produced in the same manner as in Example 1, except that during the second heat treatment of the lithium composite oxide, 0.2 mol% Al2O3 was mixed instead of TiO2, the temperature was raised to 700°C at a rate of 2°C per minute while maintaining an O2 atmosphere, and then further heat treatment (second heat treatment) was performed at 700°C for 10 hours.

[0148] Example 8 Co-precipitation method for spherical Ni 0.92 Co 0.06 Mn 0.02 (OH)2 hydroxide precursor was synthesized.

[0149] Specifically, in a 90L reactor, a 1.5M aqueous solution of composite transition metal sulfuric acid, prepared by mixing nickel sulfate, cobalt sulfate, and manganese sulfate in a molar ratio of 95:4:1, was mixed with 25 wt% NaOH and 30 wt% NH4OH. The pH in the reactor was maintained at 11.5, and the reactor temperature was maintained at 60°C. In addition, an inert gas, N2, was added to the reactor to prevent oxidation of the produced precursor.

[0150] After the synthesis of the hydroxide precursor is complete, it is washed and dehydrated using a filter press (F / P) apparatus, and Ni 0.92 Co 0.06 Mn 0.02 (OH)2 hydroxide precursor was obtained.

[0151] The obtained hydroxide precursor was mixed with LiOH (Li / (Ni+Co+Mn)mol ratio=1.05), and then heated in a firing furnace under an O2 atmosphere, increasing the temperature by 2°C per minute to 700°C, followed by heat treatment at 700°C for 10 hours (first heat treatment) to obtain a lithium composite oxide.

[0152] Next, without washing the obtained lithium composite oxide with water, TiO2 weighed to a concentration of 0.05 mol% was mixed with the lithium composite oxide. Then, while maintaining an O2 atmosphere, the temperature was increased by 2°C per minute to 700°C, and then further heat-treated at 700°C for 10 hours (second heat treatment) to finally obtain the positive electrode active material.

[0153] Comparative Example 1 The positive electrode active material was manufactured in the same manner as in Example 1, except that TiO2 was not mixed during the second heat treatment of the lithium composite oxide.

[0154] Comparative Example 2 The cathode active material was produced in the same manner as in Example 1, except that during the second heat treatment of the lithium composite oxide, 0.05 mol% of ZrO2 was mixed instead of TiO2, the temperature was raised to 850°C at a rate of 2°C per minute while maintaining an O2 atmosphere, and then further heat treatment (second heat treatment) was performed at 850°C for 24 hours.

[0155] Comparative Example 3 During the first heat treatment, Ni 0.92 Co 0.06 Mn 0.02 A cathode active material was produced in the same manner as in Example 1, except that a (OH)2 hydroxide precursor, LiOH (Li / (Ni+Co+Mn)mol ratio=1.05), and Al2O3 (weighed and mixed to make up 0.5 mol% of the total mixture) were mixed, and then the temperature was increased by 2°C per minute to 700°C in a calcination furnace while maintaining an O2 atmosphere, and then heat-treated at 700°C for 10 hours.

[0156] Reference example 1 The cathode active material was produced in the same manner as in Example 1, except that during the second heat treatment of the lithium composite oxide, 0.1 mol% TiO2 was mixed, the temperature was raised to 350°C at a rate of 2°C per minute while maintaining an O2 atmosphere, and then further heat treatment (second heat treatment) was performed at 350°C for 10 hours.

[0157] Reference example 2 The cathode active material was produced in the same manner as in Example 1, except that 0.8 mol% TiO2 was mixed during the second heat treatment of the lithium composite oxide, the temperature was raised by 2°C per minute while maintaining an O2 atmosphere, and then further heat-treated at 720°C for 24 hours (second heat treatment).

[0158] Manufacturing Example 2: Manufacturing of Lithium-ion Rechargeable Batteries A cathode slurry was prepared by dispersing 94 wt% of each cathode active material, 3 wt% of artificial graphite, and 3 wt% of PVDF binder, as prepared according to Production Example 1, in 3.5 g of N-methyl-2-pyrrolidone (NMP). The cathode slurry was applied to a 20 μm thick aluminum (Al) thin film, which served as the cathode current collector, and dried. The cathode was then manufactured by roll pressing. The loading level of the cathode was 7 mg / cm². 2 The electrode density is 3.2 g / cm³. 3 That was the case.

[0159] A coin cell was manufactured using a commonly known manufacturing process, with lithium foil as the counter electrode relative to the positive electrode, a porous polyethylene membrane (Celgard 2300, thickness: 25 μm) as the separation membrane, and a liquid electrolyte containing ethylene carbonate and ethyl methyl carbonate mixed in a volume ratio of 3:7, with LiPF6 present at a concentration of 1.15 M.

[0160] Experimental Example 1. SEM / EDX Analysis of Metal Composite Hydroxides and Cathode Active Materials Figures 2 to 8 are graphs showing the doping metal content of secondary particles with a radius of 7.5 μm, obtained by line scanning the cross-sectional SEM images obtained after selecting secondary particles with a radius of 7.5 μm contained in the positive electrode active material of Examples 1 to 7 and cross-sectional processing using FIB (Ga-ion source). Figures 9 to 11 are graphs showing the doping metal content of secondary particles with a radius of 7.5 μm, obtained by line scanning the cross-sectional SEM images obtained after selecting secondary particles with a radius of 7.5 μm, contained in the positive electrode active material of Comparative Examples 1 to 3 and cross-sectional processing using FIB (Ga-ion source).

[0161] Furthermore, Figures 12 and 13 are graphs showing the doping metal content of secondary particles with a radius of 7.5 μm, respectively, that were selected from the positive electrode active material according to Reference Example 1 and Reference Example 2, and then cross-sectionally processed using FIB (Ga-ion source). These graphs were obtained by line scanning the cross-sectional SEM images of the secondary particles and confirmed through EDX analysis.

[0162] Figures 2 to 13 show excerpts of line scanning results from the surface of secondary particles contained in each positive electrode active material to a depth of 3 μm (3,000 nm).

[0163] For reference, Comparative Example 1 does not contain doping metal, so the line scanning graph in Figure 9 shows the baseline. Although the line scanning results for secondary particles contained in the positive electrode active material of Example 8 are not attached separately, it was confirmed that the secondary particles in the positive electrode active material of Example 8 also exhibit a concentration gradient pattern similar to that of the positive electrode active materials of Examples 1 to 7 shown in Figures 2 to 8.

[0164] [Table 1]

[0165] Referring to Figures 2 to 8, it can be confirmed that, in the positive electrode active materials according to Examples 1 to 7, if the radius of the secondary particle (7.5 μm) is R, then within a depth of 0 to 0.2R (1.5 μm) from the surface of the secondary particle, there are repeatedly two concentration gradient sections: a first concentration gradient section with a negative slope and a second concentration gradient section with a positive slope, where the concentration of the doping metal decreases from the surface of the secondary particle towards the center of the secondary particle.

[0166] On the other hand, referring to Figure 10, it can be confirmed that the secondary particles contained in the positive electrode active material of Comparative Example 2 do not have a concentration gradient section in which the concentration of the doping metal has a (+) slope from the surface of the secondary particle toward the center of the secondary particle. In other words, the secondary particles contained in the positive electrode active material of Comparative Example 2 have a concentration gradient in which the concentration of the doping metal gradually decreases from the surface of the secondary particle toward the center of the secondary particle.

[0167] Referring to Figure 11, it can be confirmed that in the case of the secondary particles contained in the positive electrode active material of Comparative Example 3, the concentration of the doping metal is maintained almost uniformly without a significant change from the surface of the secondary particles towards the center of the secondary particles.

[0168] Furthermore, it can be confirmed that, similar to the positive electrode active materials of Examples 1 to 7, the secondary particles contained in the positive electrode active material according to Reference Example 1 are limited to a depth of 0 to 0.2R (1.5 μm) from the surface of the secondary particles, and that there are a first concentration gradient section with a negative slope and a second concentration gradient section with a positive slope of the doping metal concentration from the surface of the secondary particles toward the center of the secondary particles. However, in the positive electrode active material according to Reference Example 1, the first and second concentration gradient sections do not repeat toward the center of the secondary particles.

[0169] The secondary particles contained in the positive electrode active material according to Reference Example 2, like the positive electrode active materials according to Examples 1 to 7, repeatedly exhibit a first concentration gradient section with a negative slope and a second concentration gradient section with a positive slope of the doping metal from the surface of the secondary particle towards the center of the secondary particle. However, it can be confirmed that the secondary particles contained in the positive electrode active material according to Reference Example 2 also repeatedly exhibit the first and second concentration gradient sections even in the region beyond a depth of 0.2R (1.5 μm) from the surface of the secondary particle.

[0170] Experimental Example 2. XRD Analysis of Cathode Active Material X-ray diffraction (XRD) analysis was performed on each of the positive electrode active materials produced by Production Example 1 to confirm whether there are compounds other than the lithium composite oxide having the crystal structure represented by Chemical Formula 1 among the positive electrode active materials. The XRD analysis was carried out using a Bruker D8 Advance diffractometer with Cu Kα radiation (1.540598 Å).

[0171] Figures 14 to 16 show the XRD analysis results of the positive electrode active materials according to Examples 1 to 3, respectively.

[0172] Referring to Figures 14 to 16, peaks corresponding to the crystal structures of the compounds represented by Li g Ti h O i / Li g Zr h O i / Li g V h O i (0 ≤ g ≤ 10, 0 < h ≤ 8, 0 < i ≤ 15) were observed.

[0173] That is, when the lithium composite oxide obtained through the first heat treatment is not washed with water and a raw material containing a dopant is mixed with the lithium composite oxide and then the second heat treatment is performed, it can be confirmed that the residual lithium present on the surface of the lithium composite oxide (secondary particles) obtained through the first heat treatment reacts with the raw material containing the dopant to form a compound represented by Chemical Formula 2 below. <>

[0174] [Chemical Formula 2] Li g M4 h O i

[0175] (where M4 is at least one selected from Mn, Al, Co, Ti, Zr, Sr, Mg, V, B, Mo, Zn, Nb, Ba, Ca, Ta, Fe, Cr, Sn, Hf, Ce, and W, 0 ≤ g ≤ 10, 0 <h≦8、0<i≦15である。)

[0176] Experimental Example 3. Measurement of residual lithium and specific surface area of ​​positive electrode active material. Residual lithium is measured by the amount of 0.1M HCl used to reach pH 4 by pH titration. First, 5g of each positive electrode active material produced according to Production Example 1 is placed in 100ml of DIW and stirred for 15 minutes, then filtered. Next, 50ml of the filtered solution is taken, and 0.1M HCl is added to it. The amount of HCl consumed due to the pH change is measured to determine Q1 and Q2, and the residual LiOH and Li2CO3 are calculated using the following formula.

[0177] M1=23.94(LiOH Molecular weight) M2=73.89(Li2CO3Molecular weight)

number

number

[0178] Furthermore, the BET specific surface area was calculated from the amount of nitrogen gas adsorbed under liquid nitrogen temperature (77K) using a gas adsorption specific surface area measuring device (BELSORP-miniII from MicrotaracBEL).

[0179] The results of the residual lithium and BET specific surface area measurements are shown in Table 2 below.

[0180] [Table 2]

[0181] Referring to the results in Table 2 above, it can be confirmed that the positive electrode active material in Example 1 showed a decrease in residual lithium content without an increase in specific surface area compared to Comparative Example 1.

[0182] Furthermore, in the case of the positive electrode active material according to Comparative Example 2, which has a concentration gradient in which the concentration of the doping metal gradually decreases from the surface of the secondary particles toward the center of the secondary particles, it can be confirmed that the residual lithium content is excessively high compared to Examples 1 to 7.

[0183] On the other hand, in Reference Example 1, where the first and second concentration gradient sections for the doping metal do not repeatedly exist from the surface of the secondary particle toward the center of the secondary particle, and in Reference Example 2, where the first and second concentration gradient sections repeatedly exist even in the region beyond a depth of 0.2R (1.5 μm) from the surface of the secondary particle, it can be confirmed that the effect of reducing residual lithium on the positive electrode active material according to Example 1 is negligible, and that the specific surface area actually increased.

[0184] As shown in Figure 12, in the case of the positive electrode active material according to Reference Example 1, the first concentration gradient section and the second concentration gradient section do not repeatedly exist from the surface of the secondary particles toward the center of the secondary particles, and it is expected that the effect of reducing residual lithium in the region close to the center of the secondary particles is insufficient compared to Example 1. In the case of the positive electrode active material according to Reference Example 2, the number of repetitions of the first concentration gradient section and the second concentration gradient section toward the center of the secondary particles increased due to the heat treatment which was maintained for a relatively long time, but it is expected that lithium leached out again from the lithium composite oxide in the form of an impurity, and the residual lithium content increased.

[0185] Experimental Example 4. Evaluation of the capacity and life characteristics of lithium secondary batteries The lithium secondary batteries (coin cells) manufactured in Manufacturing Example 2 were subjected to charge-discharge experiments using an electrochemical analyzer (Toyo, Toscat-3100) at 25°C, with a voltage range of 3.0V to 4.3V and a discharge rate of 0.1C, and their charge and discharge capacities were measured.

[0186] Furthermore, the same lithium secondary battery was subjected to 50 charge / discharge cycles under 1C / 1C conditions at 25°C and a drive voltage range of 3.0V to 4.4V. The ratio of the discharged capacity at the 50th cycle to the initial capacity (cycle capacity retention) was then measured.

[0187] The measurement results are shown in Table 3 below.

[0188] [Table 3]

[0189] Referring to the results in Table 3 above, it can be confirmed that the charge-discharge efficiency and cycle capacity retention rate of lithium secondary batteries using the positive electrode active materials from Examples 1 to 8 were improved compared to Comparative Example 1, in which there was no concentration gradient for doping metal within the secondary particles.

[0190] Furthermore, in Comparative Example 2, which has a concentration gradient in which the concentration of the doping metal gradually decreases from the surface of the secondary particle toward the center of the secondary particle, it can be confirmed that the cycle capacity maintenance rate improved slightly compared to Comparative Example 1, but the charge-discharge efficiency decreased instead.

[0191] Furthermore, in the cases of Reference Examples 1 and 2, where the first and second concentration gradient intervals for the doping metal are not repeatedly present from the surface of the secondary particle toward the center of the secondary particle (i.e., there are no at least two peaks (p1, p2) after the first peak (p0) for the concentration of the doping metal that appears at a position corresponding to the outermost surface of the secondary particle), or where the first and second concentration gradient intervals are excessively repeated, causing peaks due to the first and second concentration gradient intervals to exist even in the region close to the center of the secondary particle, it can be confirmed that the improvement effect on charge / discharge efficiency and / or cycle capacity maintenance rate compared to Comparative Example 1 is negligible.

[0192] Although embodiments of the present invention have been described above, any person with ordinary skill in the art can modify and change the present invention in various ways by adding, changing, deleting, or adding components, as long as it does not deviate from the spirit of the invention as described in the claims, and this can also be said to be within the scope of the rights of the present invention.

Claims

1. A positive electrode active material comprising a lithium composite oxide with a layered structure that includes at least nickel, cobalt, and a doping metal, and is capable of lithium intercalation / deintercalation, The lithium composite oxide has a nickel content of 50 mol% or more among the metal elements other than lithium. The lithium composite oxide includes secondary particles, which are aggregates in which multiple primary particles aggregate and grain boundaries are formed between adjacent primary particles. The secondary particles are aggregates of primary particles in which the concentration of the doping metal exhibits a concentration gradient with a negative slope from the grain boundaries between the primary particles toward the center of the primary particles. If the radius of the secondary particle is R, then within a depth of 0 to 0.2R from the surface of the secondary particle, there exists a first concentration gradient interval with a negative slope and a second concentration gradient interval with a positive slope of the doping metal. Cathode active material.

2. The primary particles are represented by the following formula 1, and the positive electrode active material is as described in claim 1. [Chemical formula 1] Li ａ Ni １－（ｂ＋ｃ＋ｄ＋ｅ） Co ｂ M1 ｃ M2 ｄ M3 ｅ O ｆ (Here, M1 is at least one selected from Mn and Al. M2 and M3 are each independently selected from Al, Ba, B, Ce, Cr, Mg, Mn, Mo, Na, K, P, V, Sr, Ti, W, Nb, and Zr. M1 to M3 are different from each other. (0.90 ≤ a ≤ 1.15, 0 ≤ b ≤ 0.15, 0 ≤ c ≤ 0.10, 0 ≤ d ≤ 0.025, 0 ≤ e ≤ 0.025, 1.0 ≤ f ≤ 2.0.)

3. The positive electrode active material according to claim 2, wherein the concentrations (mol%) of Ni, Co, M1, M2, and M3 among the primary particles represented by formula 1 satisfy the following formula 2. [Formula 2] Co / (Ni+Co+M1+M2+M3)≦5.0

4. The positive electrode active material according to claim 2, wherein the secondary particles are aggregates of primary particles exhibiting a concentration gradient in which at least one concentration selected from M2 and M3 decreases from the grain boundaries between the primary particles toward the center of the primary particles.

5. The positive electrode active material according to claim 1, wherein the first concentration gradient section and the second concentration gradient section are repeated at least twice from the surface of the secondary particle toward the center of the secondary particle.

6. The positive electrode active material according to claim 1, wherein a concentration maintenance interval exists between the first concentration gradient interval and the second concentration gradient interval in which the absolute value of the change in the concentration of the doping metal is 3 at% or less.

7. The positive electrode active material according to claim 1, wherein a compound represented by the following chemical formula 2 is present at the grain boundaries between the primary particles and at least a portion of the surface of the secondary particles. [Chemical 2] Li ｇ M4 ｈ O ｉ (Here, M4 is at least one selected from Mn, Al, Co, Ti, Zr, Sr, Mg, V, B, Mo, Zn, Nb, Ba, Ca, Ta, Fe, Cr, Sn, Hf, Ce, and W. (0 ≤ g ≤ 10, 0 < h ≤ 8, 0 < i ≤ 15.)

8. Li is present in at least a portion of the grain boundaries between the primary particles and the surface of the secondary particles. ２ B ４ O ７ Li ３ BO ３ Li ２ B ２ O ７ Li ２ B ８ O １３ Li ２ VO ３ Li ３ VO ４ Li ６ Zr ３ O ９ Li ２ ZrO ３ Li ５．５ Zr ２．６ 20 ８ Li ４４ Ba １９ Li ４ Ba, Li ２ TiO ３ LiTi ７ O ４ and LiTi ２ O ４ The positive electrode active material according to claim 1, wherein there is at least one compound selected from the above.

9. The positive electrode active material according to claim 1, wherein the average particle size of the primary particles is 0.1 μm to 10.0 μm.

10. The positive electrode active material according to claim 9, wherein, if the average particle size of the primary particles is r, the concentration gradient within the primary particles exists within a depth of 0 to 0.2r from the grain boundaries between the primary particles.

11. The positive electrode active material according to claim 1, wherein the average particle size of the secondary particles is 3 μm to 20 μm.

12. A positive electrode comprising the positive electrode active material according to any one of claims 1 to 11.

13. A lithium secondary battery using the positive electrode described in claim 12.

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