Positive electrode active material and lithium secondary battery comprising same

Abstract

The present invention relates to a positive electrode active material and a lithium secondary battery comprising same and, more particularly, to a polycrystalline positive electrode active material having improved stability through shape control of primary particles, and a lithium secondary battery comprising same.

Description

positive electrode active material and lithium secondary battery containing the same

[0001] The present invention relates to a positive electrode active material and a lithium secondary battery including the same, and more specifically, the present invention relates to a positive electrode active material with a polycrystalline structure in which stability is improved through shape control of primary particles and a lithium secondary battery including the same.

[0002]

[0003] A battery stores electrical power by using materials capable of electrochemical reactions at the positive and negative electrodes. A representative example of such a battery is the lithium secondary battery, which stores electrical energy based on the difference in chemical potential when lithium ions intercalate or deintercalate at the positive and negative electrodes.

[0004] The above lithium secondary battery is manufactured by using materials capable of reversible intercalation / deintercalation of lithium ions as positive and negative active materials, and by filling an organic electrolyte or a polymer electrolyte between the positive and negative electrodes.

[0005] Lithium transition metal oxides are used as cathode active materials for lithium secondary batteries, and examples of such composite oxides are being studied include LiCoO2, LiMn2O4, LiNiO2, and LiMnO2.

[0006] Among the aforementioned cathode active materials, LiCoO2 is the most widely used due to its excellent lifespan characteristics and charge / discharge efficiency; however, it has the disadvantage of limited price competitiveness because it is expensive due to the resource limitations of cobalt used as a raw material.

[0007] Lithium manganese oxides such as LiMnO2 and LiMn2O4 have the advantages of excellent thermal stability and low cost, but they have the problem of low capacity and poor high-temperature characteristics. In addition, LiNiO2-based cathode active materials have the advantage of exhibiting high discharge capacity, but they are difficult to synthesize due to active cation mixing of Li and Ni, and the rate characteristics and lifespan characteristics of the synthesized cathode active materials are very low.

[0008] Accordingly, in order to improve low rate and lifetime characteristics while maintaining the high reversible capacity of LiNiO2, lithium transition metal oxides of the ternary type, such as NCM (Ni-Co-Mn) and NCA (Ni-Co-Al), or the quaternary type, such as NCMA (Ni-Co-Mn-Al), in which some of the nickel is substituted with cobalt, manganese, and / or aluminum, have been developed. Since the reversible capacity decreases as the nickel content in these ternary or quaternary type lithium transition metal oxides decreases, research to increase the nickel content in lithium transition metal oxides has been actively conducted recently.

[0009] However, as the nickel content in the lithium transition metal oxide increases, there is a problem in that the mixing of cations within the crystal structure increases, leading to reduced stability, or the content of unreacted lithium impurities such as LiOH and Li2CO3 on the surface increases. As the content of lithium impurities remaining on the surface of the lithium transition metal oxide increases, gas generation and swelling phenomena may be accelerated in the lithium secondary battery using the lithium transition metal oxide as a positive electrode active material. Furthermore, as the content of lithium impurities remaining on the surface of the lithium transition metal oxide increases, there is a problem in that the paste composition becomes gelled due to the lithium impurities when preparing a paste for forming a positive electrode active material layer using the lithium transition metal oxide.

[0010] Accordingly, there are various attempts to reduce the content of lithium impurities remaining on the surface of the lithium transition metal oxide and to reduce side reactions between the surface of the lithium transition metal oxide and the electrolyte through surface coating of the lithium transition metal oxide.

[0011] In addition, when a conductive coating layer is formed on the surface of the lithium transition metal oxide, it can contribute to improving rate capability by enhancing the ion or electron transport capacity mediated by the lithium transition metal oxide compared to when a non-conductive coating layer is formed on the surface of the lithium transition metal oxide. However, unlike a non-conductive coating layer, the electrochemical properties of a conductive coating layer change significantly due to the continuity and / or thickness of the coating layer. Therefore, if a non-uniform coating layer is formed on the surface of the lithium transition metal oxide, or if the coating material exists in the form of fine particles on the surface of the lithium transition metal oxide, it may be difficult to exhibit stable electrochemical properties for a long time due to the conductivity variation between particles.

[0012] Meanwhile, most lithium transition metal oxides have a polycrystalline structure in which multiple primary particles are aggregated into secondary particles, and the specific surface area increases as the number of primary particles constituting the secondary particles increases. As the specific surface area of ​​the lithium transition metal oxide increases, if strain caused by random volume contraction / expansion of the primary particles due to repeated charging / discharging accumulates, cracks (intergranular cracks) may occur between adjacent primary particles. Intergranular cracks present within the secondary particles reduce connectivity between adjacent primary particles, thereby interfering with the lithium ion transport mechanism. Furthermore, as the number of intergranular cracks present within the secondary particles increases, the exposed surface area of ​​the primary particles increases, leading to increased side reactions with the electrolyte and consequently a rapid decrease in the stability of the cathode active material.

[0013]

[0014] In the lithium secondary battery market, the growth of lithium secondary batteries for electric vehicles is acting as a driving force, and accordingly, the demand for cathode active materials used in lithium secondary batteries is also continuously increasing.

[0015] For example, in the past, lithium secondary batteries using lithium iron phosphate (LFP) were mainly used for the sake of ensuring safety, but recently, there has been a growing trend of using nickel-based lithium transition metal oxides, which have a higher energy capacity per unit weight compared to LFP (of course, relatively cheaper LFP is still used to reduce costs).

[0016] In addition, nickel-based lithium transition metal oxides, which are mainly used as cathode active materials for high-capacity lithium secondary batteries, generally have compositions of ternary types such as NCM (Ni-Co-Mn) and NCA (Ni-Co-Al) or quaternary types such as NCMA (Ni-Co-Mn-Al).

[0017] Accordingly, cathode active materials used in higher-performance lithium secondary batteries need to satisfy both the stability and reliability appropriately expected even under harsher operating conditions.

[0018] Conventionally, a non-conductive or conductive coating layer was formed on the surface of the lithium transition metal oxide to mitigate problems arising from an increased nickel content in the lithium transition metal oxide. However, considering electrochemical properties, lithium transition metal oxides with a conductive coating layer tend to be preferred over those with a non-conductive coating layer.

[0019] However, as mentioned above, since the electrochemical properties of a conductive coating layer vary significantly depending on the continuity and / or thickness of the coating layer, if a non-uniform coating layer is formed on the surface of the lithium transition metal oxide or if the coating material exists on the surface of the lithium transition metal oxide in the form of fine particles, it is often difficult to exhibit stable electrochemical properties over a long period due to conductivity variations between particles. The excellent continuity of the conductive coating layer means that the conductive coating layer covers the entire surface of the lithium transition metal oxide.

[0020] For example, to improve the continuity of the conductive coating layer and reduce the variation of the conductive coating layer between particles, the content of the coating raw material introduced in the coating process for the lithium transition metal oxide must be increased. However, if the content of the coating raw material introduced in the coating process for the lithium transition metal oxide is increased, the variation of the conductive coating layer between particles may instead increase, or the conductive coating layer may become thicker overall. If the conductive coating layer becomes excessively thick, it may reduce the ion or electron transport capacity mediated by the lithium transition metal oxide.

[0021] In addition, as mentioned above, in order to prevent a decrease in the lifespan and stability of ternary or quaternary type lithium transition metal oxides, it may be considered to induce the growth of primary particles constituting the lithium transition metal oxides; however, if the growth of primary particles is induced, the likelihood of intragranular cracks occurring within the primary particles may increase. If intragranular cracks exist within the primary particles, or if intragranular cracks accumulate within the primary particles due to repeated charging and discharging, the lifespan of the lithium secondary battery using the positive electrode active material is bound to decrease rapidly.

[0022] Accordingly, the present invention aims to provide a positive electrode active material capable of improving resistance to strain caused by random volume contraction / expansion of primary particles during repeated charging / discharging by controlling the shape of primary particles through dual doping of a heterogeneous element on the lithium transition metal oxide.

[0023] In addition, the present invention aims to provide a positive electrode active material that minimizes the degradation of electrochemical properties due to surface modification and improves stability, such as lifespan characteristics at room temperature and high temperature, by forming a conductive coating layer that does not alter the surface kinetic properties of a polycrystalline lithium transition metal oxide along with controlling the shape of primary particles through heat treatment of a mixture of a precursor of the lithium transition metal oxide and a co-doping source at a relatively low temperature prior to calcination of the lithium transition metal oxide.

[0024] In addition, another objective of the present invention is to provide a lithium secondary battery using the positive active material defined herein.

[0025] The objects of the present invention are not limited to those mentioned above, and other unmentioned objects and advantages of the present invention may be understood from the following description and will be more clearly understood by the embodiments of the present invention. Furthermore, it will be readily apparent that the objects and advantages of the present invention can be realized by the means and combinations thereof set forth in the claims.

[0026]

[0027] The present invention for solving the aforementioned technical problem includes the following invention.

[0028] [1] A positive electrode active material comprising a lithium transition metal oxide capable of lithium intercalation / deintercalation, wherein the lithium transition metal oxide has a secondary particle form in which a plurality of primary particles are aggregated, and the lithium transition metal oxide is co-doped with aluminum and boron, and when the distance from the center of the secondary particle to the surface of the secondary particle is r, the distance (d) from the center of the secondary particle is (1 / 3)r <d≤r인 상기 2차 입자의 표면부 내 존재하는 1차 입자의 평균 종횡비는 2.5 이상 5.0 미만인 양극 활물질.

[0029] The lithium transition metal oxide has a secondary particle form in which a plurality of primary particles are aggregated, and the lithium transition metal oxide is characterized by being co-doped with aluminum and boron. When the lithium transition metal oxide is co-doped with aluminum and boron under predetermined conditions, a change in the aspect ratio of the primary particles constituting the secondary particles can be realized.

[0030] [2] The average aspect ratio of primary particles existing within the center of the secondary particle, where the distance (d) from the center of the secondary particle is 0 ≤ d ≤ (1 / 3)r, is (1 / 3)r <d≤r인 상기 2차 입자의 표면부 내 존재하는 1차 입자의 평균 종횡비보다 작은, [1]에 기재된 양극 활물질.

[0031] [3] The average aspect ratio of the primary particles present in the center of the secondary particles is 1.5 or less, as described in [1] or [2].

[0032] [4] An anode active material described in any one of [1] to [3], wherein the aspect ratio of the primary particle above exhibits a gradient increasing from the center of the secondary particle toward the surface of the secondary particle.

[0033] [5] The above lithium transition metal oxide is a positive active material described in any one of [1] to [4], represented by the following chemical formula 1.

[0034] [Chemical Formula 1]

[0035] Li a Ni1-(b+c+d+e)Co b Mn c M1 d M2 e O2

[0036] In the above chemical formula 1,

[0037] M1 is Al and B,

[0038] M2 is at least one selected from Na, K, Mg, Ca, Sr, Ba, Rb, Ce, Hf, Ta, Cr, F, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, Mo, W, P, Ge, Nd, Gd and Cu, and

[0039] 0.85≤a≤1.15, 0≤b≤0.20, 0≤c≤0.20, 0 <d≤0.10, 0≤e≤0.10, 0<b+c≤0.40이다.

[0040] [6] The average crystallite size of the lithium transition metal oxide is 75 nm or more and less than 100 nm. [1] to [5] The positive active material described in any one of [1] to [5].

[0041] [7] A positive active material described in any one of [1] to [6], wherein the full width at half-maximum (FWHM) of the peak corresponding to the (104) crystal plane calculated from the diffraction spectrum obtained by X-ray diffraction (XRD) analysis using Cu-kα rays for the positive active material is 0.170° to 0.190°.

[0042] [8] An anode active material described in any one of [1] to [7], wherein the average particle size of the primary particles is 0.1 μm to 1.0 μm.

[0043] [9] An anode active material described in any one of [1] to [8], wherein the average particle size of the secondary particles is 2.0 μm to 16.0 μm.

[0044]

[0010] A positive active material described in any one of [1] to [9], wherein the sum of the concentrations (mol%) of aluminum and boron present at the interface between the primary particles located inside the secondary particles is greater than the sum of the concentrations (mol%) of aluminum and boron present inside the primary particles.

[0045]

[0011] A positive active material described in any one of [1] to

[0010] , wherein a coating layer containing aluminum and boron exists on the surface of the secondary particle.

[0046]

[0012] A positive active material described in any one of [1] to

[0011] , wherein a coating layer comprising aluminum and boron exists at the interface or gap between the primary particles located inside the secondary particles.

[0047] In addition, the present invention includes other embodiments as follows.

[0048]

[0013] A positive electrode comprising a positive electrode active material described in any one of [1] to

[0012] .

[0049]

[0014] A lithium secondary battery using the positive electrode described in

[0013] .

[0050]

[0051] According to the present invention, intergranular cracks resulting from the accumulation of strain caused by random volume shrinkage / expansion of primary particles during repeated charging / discharging can be suppressed and / or mitigated by controlling the shape of primary particles constituting a lithium transition metal oxide without inducing the growth of primary particles through high-temperature calcination. Through this, the lifespan characteristics of a lithium secondary battery using the positive electrode active material at room temperature and high temperature can be improved.

[0052] In addition, according to the present invention, by forming a conductive coating layer that does not alter the surface kinetic properties of a polycrystalline lithium transition metal oxide along with controlling the shape of primary particles through heat treatment of a mixture of a precursor of the lithium transition metal oxide and a co-doping source at a relatively low temperature prior to calcination of the lithium transition metal oxide, the degradation of electrochemical properties due to surface modification can be minimized.

[0053] In addition to the effects described above, the specific effects of the present invention are described together with the specific details for implementing the invention below.

[0054]

[0055] Figure 1 is a cross-sectional SEM image of a lithium transition metal oxide according to Example 1.

[0056] Figure 2 is a cross-sectional SEM image of a lithium transition metal oxide according to Example 2.

[0057] Figure 3 is a cross-sectional SEM image of a lithium transition metal oxide according to Example 3.

[0058] Figure 4 is a cross-sectional SEM image of a lithium transition metal oxide according to Comparative Example 1.

[0059] Figure 5 is a cross-sectional SEM image of a lithium transition metal oxide according to Comparative Example 2.

[0060] Figure 6 is a cross-sectional SEM image of a lithium transition metal oxide according to Comparative Example 3.

[0061] Figure 7 is a cross-sectional SEM image of a lithium transition metal oxide according to Comparative Example 4.

[0062] Figure 8 is a cross-sectional SEM image of a lithium transition metal oxide according to Comparative Example 5.

[0063]

[0064] For convenience, specific terms are defined herein to facilitate a better understanding of the present invention. Unless otherwise defined herein, scientific and technical terms used in this invention shall have the meanings generally understood by those skilled in the art. Furthermore, unless specifically indicated in the context, terms in their singular form shall be understood to include their plural form, and terms in their plural form shall be understood to include their singular form.

[0065]

[0066] positive electrode active material

[0067] The positive electrode active material herein comprises a lithium transition metal oxide capable of reversible intercalation / deintercalation of lithium ions.

[0068] The above lithium transition metal oxide is a complex metal oxide capable of lithium ion intercalation / deintercalation and has a layered crystal structure belonging to the R-3m space group. The above lithium transition metal oxide having a layered crystal structure exhibits a specific peak in the region where 2θ is 18° to 20° among the rotation patterns obtained from XRD analysis.

[0069] The lithium transition metal oxide comprises at least lithium and a transition metal. The transition metal may comprise at least one, at least two, or all selected from nickel, cobalt, and manganese.

[0070] Preferably, the lithium transition metal oxide may be a lithium nickel-based composite oxide containing nickel. Additionally, the lithium transition metal oxide may be a lithium nickel-based composite oxide containing nickel and cobalt.

[0071] In order to improve low-rate characteristics and lifetime characteristics while maintaining the high reversible capacity of LiNiO2, the lithium nickel-based composite oxide may be a lithium transition metal oxide of the ternary type, such as so-called NCM (Ni-Co-Mn) and NCA (Ni-Co-Al), or the quaternary type, such as NCMA (Ni-Co-Mn-Al), in which a portion of the nickel is substituted with cobalt, manganese, and / or aluminum. The ternary or quaternary type lithium transition metal oxide may further include dopants other than nickel, cobalt, manganese, and aluminum.

[0072] In another embodiment, the lithium transition metal oxide may be a cobalt-free type lithium transition metal oxide that does not contain cobalt in the bulk particles. The cobalt-free type lithium transition metal oxide may further include dopants other than nickel, cobalt, and manganese.

[0073] The lithium transition metal oxide above has a secondary particle form in which a plurality of primary particles are aggregated. Therefore, the lithium transition metal oxide above can be referred to as a secondary particle.

[0074] The primary particles constituting the secondary particles may have a spherical shape, a rod shape, an elliptical shape, and / or an irregular shape. Additionally, primary particles of various shapes may exist within the same cathode active material unless specifically intended during the manufacturing process. Furthermore, the primary particles refer to particle units that do not appear to have grain boundaries when observed using a scanning electron microscope at a magnification of 5,000 to 20,000 times.

[0075] The average particle size of the primary particle may be 1 μm or less, 0.1 μm or more and 1 μm or less, 0.2 μm or more and 1 μm or less, 0.3 μm or more and 1 μm or less, or 0.4 μm or more and 0.8 μm or less. In this case, the average particle size of the primary particle may be the average value of the length in the major axis direction and the length in the minor axis direction of the primary particle ([major axis length + minor axis length] / 2). The average value of the length in the major axis direction and the length in the minor axis direction of the primary particle ([major axis length + minor axis length] / 2) may be calculated from a cross-sectional SEM image of the secondary particle.

[0076] The lithium transition metal oxide can form a stable polycrystalline structure by satisfying the above range for the average particle size of the primary particle. For example, if the average particle size of the primary particle constituting the secondary particle is greater than 1 μm, the lithium transition metal oxide is likely to have a single particle consisting of one primary particle and / or a pseudo-single particle form in which 30 or fewer primary particles are aggregated. In this case, the likelihood of intragranular cracks occurring within the primary particle may increase. As intragranular cracks are present within the primary particle or accumulate within the primary particle due to repeated charging and discharging, the lifespan of the lithium secondary battery using the positive electrode active material may rapidly decrease.

[0077] In order to distinguish polycrystalline structures from monocrystalline and pseudo-monocrystalline forms, a secondary particle composed of 50 or more, or 100 or more, primary particles may be defined as a polycrystalline structure.

[0078] In addition, for primary particles lying on a straight line crossing the center of the lithium transition metal oxide in the direction of the major axis in a cross-sectional SEM image of the lithium transition metal oxide having a polycrystalline structure, the grain boundary density calculated by the following Equation 1 may be 0.95 or higher.

[0079] [Equation 1]

[0080] Grain boundary density = (Sum of the number of interfaces between primary grains lying on the straight line / Sum of the number of primary grains lying on the straight line)

[0081] As the grain boundary density calculated by Equation 1 above converges to 1, the number of primary particles constituting the lithium transition metal oxide increases, and as the grain boundary density calculated by Equation 1 above converges to 0, the number of primary particles constituting the lithium transition metal oxide decreases. When the grain boundary density calculated by Equation 1 above is less than 0.95, the lithium transition metal oxide is highly likely to have a single-particle or pseudo-single-particle form rather than a polycrystalline structure.

[0082] The average particle size (D) of the above secondary particles 50 ) may be 2.0μm or more and 16.0μm or less, 2.0μm or more and 14.0μm or less, 2.0μm or more and 12.0μm or less, 3.0μm or more and 16.0μm or less, 3.0μm or more and 14.0μm or less, 3.0μm or more and 12.0μm or less, 4.0μm or more and 16.0μm or less, 4.0μm or more and 14.0μm or less, or 4.0μm or more and 12.0μm or less. The average particle size (D) of the secondary particles above. 50 By satisfying the above range, the positive active material can have an appropriate level of energy density per unit volume. The average particle size (D) of the secondary particles. 50 ) may vary depending on the number of primary particles constituting the secondary particles.

[0083] The average particle size of the above secondary particles can be measured using the laser diffraction method. For example, after dispersing the secondary particles in a dispersion medium, they can be introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac MT 3000), irradiated with ultrasound of about 28 kHz at an output of 60 W, obtained a volume cumulative particle size distribution graph, and then measured by determining the particle size corresponding to 50% of the volume cumulative amount.

[0084] In this document, "particle size" is used interchangeably with "particle diameter" or "particle size," and unless otherwise defined, all "average particle size" refers to the intermediate volume-based particle size determined by laser diffraction.

[0085] Unless otherwise defined, as used herein, the term "surface of the particle" refers to a region relatively close to the "foremost surface" of the particle, and the term "center of the particle" refers to a region relatively closer to the "center" of the particle than the said "surface".

[0086] Accordingly, the "surface portion of the unit particle (primary particle)" refers to an area relatively close to the "foremost surface" of the unit particle (primary particle), and the "center of the unit particle (primary particle)" refers to an area relatively closer to the "exact center" of the unit particle (primary particle) than to the "surface portion."

[0087] Likewise, "surface portion of the secondary particle" refers to an area relatively close to the "foremost surface" of the secondary particle, and "center of the secondary particle" refers to an area relatively closer to the "center" of the secondary particle than to the "surface portion."

[0088] In this case, the region within any particle excluding the "surface" can be defined as the "center of the particle."

[0089] More specifically, when r is denoted as the half-diameter of the lithium transition metal oxide (distance from the center of the secondary particle to the surface of the secondary particle) measured from a cross-sectional SEM image of the lithium transition metal oxide in the present invention, the region where the distance (d) from the center of the secondary particle is 0 ≤ d ≤ (1 / 3)r is the center portion of the secondary particle, and the region where the distance (d) from the center of the secondary particle is (1 / 3)r <d혇인 영역을 상기 2차 입자의 표면부(surface portion)으로 정의할 수 있다. 상기 표면부의 최외곽은 상기 2차 입자의 최표면에 대응한다. 따라서, 상기 2차 입자의 표면부는 상기 2차 입자의 중심으로부터의 거리(d)가 (1 / 3)r<d인 지점부터 상기 2차 입자의 최표면(d=r)까지의 영역에 대응한다.

[0090] The secondary particle may have a shape that is not perfectly spherical, such as an elliptical shape. Alternatively, irregularities may exist on the surface of the secondary particle. In this case, the diameter of the secondary particle can be calculated from the average value of the major axis length and minor axis length of the lithium transition metal oxide measured from the cross-sectional SEM image of the secondary particle. Half of the average value of the major axis length and minor axis length of the lithium transition metal oxide measured from the cross-sectional SEM image of the secondary particle can be used as the half-diameter (r) of the secondary particle.

[0091] The lithium transition metal oxide defined herein has a secondary particle form in which a plurality of primary particles are aggregated, and the lithium transition metal oxide is characterized by being co-doped with aluminum and boron, and when the lithium transition metal oxide is co-doped with aluminum and boron under predetermined conditions, a change in the aspect ratio of the primary particles constituting the secondary particles can be realized.

[0092] The aspect ratio of the primary particle above refers to the ratio of the major axis length to the minor axis length (major axis length / minor axis length) of the primary particle calculated from the cross-sectional SEM image of the primary particle. The major axis length of the primary particle corresponds to the c-axis length, and the minor axis length may correspond to the a-axis length.

[0093] For example, when r is the distance from the center of the secondary particle to the surface of the secondary particle, the average aspect ratio of the primary particle existing within the center of the secondary particle where the distance (d) from the center of the secondary particle is 0 ≤ d ≤ (1 / 3)r is such that the distance (d) from the center of the secondary particle is (1 / 3)r <d≤r인 상기 2차 입자의 표면부 내 존재하는 1차 입자의 평균 종횡비보다 작을 수 있다.

[0094] The change in the aspect ratio of the aforementioned primary particles can be expressed by co-doping of aluminum and boron within the primary particles under certain conditions.

[0095] That is, aluminum strengthens metal-oxygen bonds (MO bonds) to suppress excessive grain growth and intergranular aggregation, and relieves structural stress within the crystal, while boron is present in the oxygen lattice or at grain boundaries to correct local defect sites and induces anisotropic growth in the long axis direction at the surface of primary grains, thereby forming an aspect ratio gradient between the center and the surface.

[0096] Accordingly, the average aspect ratio of the primary particles present in the center of the secondary particles is 1.5 or less, and the average aspect ratio of the primary particles present in the surface portion of the secondary particles may be 2.5 or more and less than 5.0.

[0097] The aspect ratio of the primary particle can exhibit a gradient that increases from the center of the secondary particle toward the surface of the secondary particle, thereby improving resistance to strain caused by random volume contraction / expansion of the primary particle during repeated charging / discharging.

[0098] The average aspect ratio of primary particles present in the center of the secondary particles can be calculated as the average value of the aspect ratio of the selected primary particles after selecting 10% or more, 20% or more, 30% or more, 40% or more, 50% or more, or 60% or more of the primary particles present in the center of the secondary particles from the cross-sectional SEM image of the secondary particles, or as the average value of the aspect ratio of all primary particles present in the center of the secondary particles.

[0099] If the cross-sectional shape of the primary particle is substantially circular or if there is no significant difference between the major axis length and the minor axis length, the aspect ratio of the primary particle is calculated as 1. The average aspect ratio of the primary particle present in the center of the secondary particle is 1.5 or less, 1.4 or less, or 1.35 or less. If the average aspect ratio of the primary particle present in the center of the secondary particle is greater than 1.5, the growth of the primary particle in the major axis direction within the center of the secondary particle is excessively induced, making it difficult to exhibit a gradient increasing from the center of the secondary particle toward the surface of the secondary particle.

[0100] The average aspect ratio of primary particles present within the surface portion of the secondary particles can be calculated as the average value of the aspect ratio of the selected primary particles after selecting 10% or more, 20% or more, 30% or more, 40% or more, 50% or more, or 60% or more of the primary particles present within the surface portion of the secondary particles from the cross-sectional SEM image of the secondary particles, or as the average value of the aspect ratio of all primary particles present within the surface portion of the secondary particles.

[0101] The average aspect ratio of the primary particles present within the surface portion of the secondary particles may be 2.5 or more and less than 5.0, 2.5 or more and less than 4.5, 2.5 or more and less than 4.0, or 2.8 or more and less than 3.8.

[0102] If the average aspect ratio of the primary particles present in the surface portion of the secondary particles is less than 2.5, the growth of the primary particles in the long axis direction in the surface portion of the secondary particles is insufficient, making it difficult to sufficiently improve resistance to strain caused by random volume shrinkage / expansion of the primary particles during repeated charging / discharging. On the other hand, if the average aspect ratio of the primary particles present in the surface portion of the secondary particles is 5.0 or higher, the specific surface area of ​​the secondary particles increases, which may increase side reactions with the electrolyte.

[0103] The above lithium transition metal oxide can be represented by the following chemical formula 1.

[0104] Li a Ni1-(b+c+d+e)Co b Mn c M1 d M2 e O2

[0105] In the above chemical formula 1, M1 is Al and B, and M2 is at least one selected from Na, K, Mg, Ca, Sr, Ba, Rb, Ce, Hf, Ta, Cr, F, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, Mo, W, P, Ge, Nd, Gd, and Cu, wherein 0.85≤a≤1.15, 0≤b≤0.20, 0≤c≤0.20, 0 <d≤0.10, 0≤e≤0.10, 0<b+c≤0.40이다.

[0106] In the above chemical formula 1, a, which represents the ratio of lithium to all metal elements other than lithium in the lithium transition metal oxide, may be 0.85 or more, 0.90 or more, 0.95 or more, 0.96 or more, 0.97 or more, 0.98 or more, 0.99 or more, 1.00 or more, 1.01 or more, 1.02 or more, 1.03 or more, 1.04 or more, or 1.05 or more. Additionally, a may be 1.15 or less, 1.14 or less, 1.13 or less, 1.12 or less, 1.11 or less, 1.10 or less, 1.09 or less, 1.08 or less, 1.07 or less, 1.06 or less, or 1.05 or less. The upper and lower limits of the mole fraction of lithium relative to all elements other than lithium among the above lithium transition metal oxides can be appropriately selected within a range that satisfies the definition described above.

[0107] If a in the above chemical formula 1 is less than 0.85, the capacity of the positive electrode active material containing the lithium transition metal oxide represented by the above chemical formula 1 may decrease. On the other hand, if a in the above chemical formula 1 is greater than 1.15, phase separation may occur due to the excess lithium and manganese present in the lithium transition metal oxide, and an impurity phase belonging to a space group other than the R-3m space group may occur.

[0108] In the above chemical formula 1, 1-(b+c+d+e), representing the mole fraction of nickel relative to all metal elements other than lithium in the lithium transition metal oxide, may be 0.50 or more, 0.60 or more, 0.70 or more, 0.80 or more, or 0.90 or more. Additionally, 1-(b+c+d+e) may be less than 1.0, 0.99 or less, 0.98 or less, 0.97 or less, or 0.96 or less. The upper and lower limits of the mole fraction of nickel relative to all elements other than lithium in the lithium transition metal oxide may be appropriately selected within a range that satisfies the definition described above. When the mole fraction of nickel in the lithium transition metal oxide satisfies the above range, a stable crystal structure can be formed and high energy density can be exhibited.

[0109] In the above chemical formula 1, b, representing the mole fraction of cobalt relative to all metal elements other than lithium in the lithium transition metal oxide, may be 0.20 or less, 0.19 or less, 0.18 or less, 0.17 or less, 0.16 or less, 0.15 or less, 0.14 or less, 0.13 or less, 0.12 or less, 0.11 or less, 0.10 or less, 0.09 or less, 0.08 or less, 0.07 or less, 0.06 or less, 0.05 or less, 0.04 or less, 0.03 or less, or 0.02 or less. If the lithium transition metal oxide is a cobalt-free type lithium transition metal oxide, b may be 0. If the lithium transition metal oxide contains cobalt, b may be greater than 0, 0.01 or more, or 0.02 or more. The upper and lower limits of the mole fraction of cobalt relative to all elements other than lithium in the above lithium transition metal oxide can be appropriately selected within a range that satisfies the definition described above. When the mole fraction of cobalt in the above lithium transition metal oxide satisfies the above range, a stable crystal structure can be formed and good output characteristics can be exhibited.

[0110] In the above chemical formula 1, c, representing the mole fraction of manganese relative to all metal elements other than lithium in the lithium transition metal oxide, may be 0.20 or less, 0.19 or less, 0.18 or less, 0.17 or less, 0.16 or less, 0.15 or less, 0.14 or less, 0.13 or less, 0.12 or less, 0.11 or less, 0.10 or less, 0.09 or less, 0.08 or less, 0.07 or less, 0.06 or less, 0.05 or less, 0.04 or less, 0.03 or less, or 0.02 or less. If the lithium transition metal oxide contains manganese, c may be greater than 0, 0.01 or more, or 0.02 or more. The upper and lower limits of the mole fraction of manganese relative to all elements other than lithium in the lithium transition metal oxide may be appropriately selected within a range that satisfies the definition described above. When the mole fraction of manganese among the above lithium transition metal oxides satisfies the above range, a stable crystal structure can be formed.

[0111] When the lithium transition metal oxide contains a dopant M2 other than M1, e in Chemical Formula 1 is greater than 0. Additionally, when the lithium transition metal oxide contains a dopant M2, e, which represents the mole fraction of the dopant relative to the total metal element other than lithium in the lithium transition metal oxide, may be 0.10 or less, 0.05 or less, 0.04 or less, 0.03 or less, 0.02 or less, or 0.01 or less.

[0112] When the above lithium transition metal oxide optionally includes a dopant, the dopant may include at least one selected from Na, K, Mg, Ca, Sr, Ba, Rb, Ce, Hf, Ta, Cr, F, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, Mo, W, P, Ge, Nd, Gd, and Cu, preferably at least one selected from Mg, Ca, Ti, Zr, Mo, W, and P, more preferably at least one selected from Ca, Ti, Zr, and W.

[0113] The type and combination of the above dopants can be appropriately selected within a range that does not negatively affect the electrochemical properties and stability of the above positive active material.

[0114] In the above chemical formula 1, d, which represents the mole fraction of aluminum and boron relative to the total metal elements other than lithium in the lithium transition metal oxide, may be 0.10 or less, 0.05 or less, 0.04 or less, 0.03 or less, 0.02 or less, or 0.01 or less.

[0115] As described above, the lithium transition metal oxide co-doped with aluminum (Al) and boron (B) under certain conditions can form a structural gradient in which the aspect ratio of the primary particles constituting the secondary particles gradually changes.

[0116] The aspect ratio gradient formed in this way also affects the microcrystalline growth behavior during the high-temperature sintering step, thereby stabilizing the average crystallite size of the co-doped lithium transition metal oxide to a range of 75 nm or more and less than 100 nm, preferably 75 nm or more and less than 90 nm.

[0117] The above average crystallite size can be quantitatively analyzed through X-ray diffraction analysis (XRD) by Cu-Kα X-rays on the lithium transition metal oxide. Specifically, the average crystallite size can be calculated through Rietveld refinement on the X-ray diffraction pattern in the 2θ=10°~120° region obtained through XRD analysis of the cathode active material containing the lithium transition metal oxide.

[0118] If the average crystallite size of the lithium transition metal oxide defined herein is less than 75 nm or greater than 100 nm, the improvement effect on electrochemical properties, such as rate characteristics of a lithium secondary battery using the lithium transition metal oxide as a positive electrode active material, may be negligible. In addition, if the average crystallite size is greater than 100 nm, polarization may occur as resistance increases, and cracks within the particles may occur due to the polarization phenomenon, which ultimately causes a reduction in the lifespan of the positive electrode active material.

[0119] In addition, the full width at half-maximum (FWHM) of the peak corresponding to the (104) crystal plane calculated from the diffraction spectrum obtained through X-ray diffraction (XRD) analysis using Cu-kα rays for the anode active material may be 0.170° to 0.190°.

[0120] The peak in the diffraction spectrum obtained through X-ray diffraction (XRD) analysis using Cu-kα rays for the above positive active material is a peak specific to the (104) crystal plane of the lithium transition metal oxide having an R-3m layered crystal structure, and exhibits the strongest peak intensity in the 2θ=44.5±1.0° region.

[0121] The FWHM (104) of the lithium transition metal oxide co-doped with aluminum and boron under specified conditions has a value of 0.170° to 0.190°, preferably 0.174° to 0.185°, more preferably 0.1742° to 0.1821°.

[0122] The fact that the lithium transition metal oxide co-doped with aluminum and boron under specified conditions exhibits a FWHM (104) in the range of 0.170° to 0.190° means that the co-doping of aluminum and boron did not have a significant effect on the deformation of the crystal structure of the (104) plane. Additionally, it may mean that the diffraction peaks of the (104) plane can appear relatively sharp and symmetrical compared to undoped or single-doped lithium transition metal oxides.

[0123] Under specified conditions, the lithium transition metal oxide co-doped with aluminum and boron exhibits a FWHM (104) in the range of 0.170° to 0.190°, thereby allowing microstrain within the crystal to be appropriately relieved and the coordination environment of transition metal ions within the crystallites to be maintained homogeneously.

[0124] In particular, aluminum can improve structural stability by strengthening oxygen-metal bonds, and boron can suppress cation mixing and improve crystallinity by being present in the oxygen lattice or at grain boundaries and correcting lattice defects.

[0125] Therefore, co-doped lithium transition metal oxides having a FWHM (104) value within the aforementioned range can prevent structural collapse during the charging / discharging process by suppressing inhomogeneity of the crystal structure or stress accumulation, and as a result, can achieve excellent lifetime characteristics and output characteristics.

[0126] A coating layer may be present on at least a portion of the surface of the lithium transition metal oxide. The coating layer may be present in the form of islands on a portion of the surface of the lithium transition metal oxide. Additionally, the coating layer may be present on the surface of the primary particle and / or the secondary particle. The coating layer may diffuse from the surface portion of the secondary particle toward the center of the secondary particle along the grain boundaries between the primary particles.

[0127] The coating layer located inside the secondary particle may exist at the interface or gap between the primary particles. The interface between the primary particles refers to a grain boundary formed by neighboring primary particles coming into contact with each other. The gap between the primary particles refers to the space between neighboring primary particles that do not come into direct contact with each other, and may also be referred to as a pore.

[0128] The coating layer may include aluminum and boron. By forming a coating layer containing aluminum and boron on the surface of the lithium transition metal oxide, side reactions between the lithium transition metal oxide and the electrolyte can be reduced, thereby improving the stability and lifespan of a lithium secondary battery using the lithium transition metal oxide defined herein as a positive electrode active material.

[0129] The coating layer may comprise at least one oxide selected from aluminum oxide, lithium-aluminum oxide, lithium-boron oxide, and lithium-aluminum-boron oxide. Preferably, the coating layer may comprise lithium-aluminum-boron oxide, or may comprise at least one oxide selected from aluminum oxide, lithium-aluminum oxide, and lithium-boron oxide, and lithium-aluminum-boron oxide. Additionally, it is preferable that the coating layer does not comprise boron oxide that is not complexed with lithium (e.g., HBO2, which is a reaction intermediate of a boron-containing raw material).

[0130] The oxide present in the above coating layer can be represented by the following chemical formula 3.

[0131] [Chemical Formula 3]

[0132] Li g M3 h O i

[0133] In the above Chemical Formula 3, M3 is at least one selected from Al and B, 0≤g≤8, 0≤h≤15, 2≤i≤20, excluding the case where g and h are simultaneously 0. Additionally, in the above Chemical Formula 3, g, h, and i represent numbers determined from the stoichiometric ratio according to the valence (oxidation number) of M3. For example, g, h, and i can be appropriately selected within the ranges of 0≤g≤8, 0≤h≤8, and 2≤i≤13, respectively. Furthermore, in the above Chemical Formula 3, when M3 is B, g is not 0.

[0134] More specifically, the oxide represented by the above chemical formula 3 is Li g Al h O i , Li g B h O i , Li g( Al / B) h O i , or Al h O i It may be, and non-limiting examples include Al2O3, Li5AlO4, LiBO2, Li2B4O7, Li3BO3, and Li2B5AlO 10 , LiB4Al7O 17 There are , Li3AlB2O6, etc.

[0135] The coating layer may exist on the surface of the lithium transition metal oxide in the form of a film having an average film thickness of 1 nm to 300 nm, 10 nm to 200 nm, or 30 nm to 100 nm. The thickness of the coating layer may be measured through EDX analysis of the coating elements based on a cross-sectional SEM image of the lithium transition metal oxide. The fact that the coating layer exists in the form of a film should be distinguished from the case where the oxide constituting the coating layer is dispersed and attached to the surface of the lithium transition metal oxide in the form of individual particles.

[0136] In addition, the coating layer may be formed in an island shape that discontinuously occupies the surface of the lithium transition metal oxide.

[0137]

[0138] lithium secondary battery

[0139] According to another aspect of the present invention, an anode may be provided comprising an anode current collector and an anode active material layer formed on the anode current collector. Herein, the anode active material layer may include an anode active material according to various embodiments of the present invention. Accordingly, since the anode active material is the same as previously described, a specific description is omitted for convenience, and only the remaining unmentioned components will be described below.

[0140] The above positive current collector is not particularly limited as long as it is conductive without causing chemical changes in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface treated with carbon, nickel, titanium, silver, etc. may be used. In addition, the above positive current collector may typically have a thickness of 3 μm to 500 μm, and fine irregularities may be formed on the surface of the current collector to increase the adhesion of the positive active material. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0141] The above positive active material layer can be manufactured by applying a positive slurry composition, which includes a conductive material and optionally a binder together with the positive active material, to the positive current collector.

[0142] At this time, the anode active material may be included in an amount of 80 wt% to 99 wt%, more specifically 85 wt% to 98.5 wt%, based on the total weight of the anode slurry for forming the anode active material layer. Excellent capacity characteristics may be exhibited when included within the above-mentioned content range, but it is not necessarily limited thereto.

[0143] The above conductive material is used to impart conductivity to the electrode, and in the battery being constructed, it may be used without special limitations as long as it possesses electronic conductivity without causing chemical changes. 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, thermal black, and carbon fiber; metal powder or metal fiber such as copper, nickel, aluminum, or silver; conductive whiskey such as zinc oxide or potassium titanate; conductive metal oxide such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and one of these alone or a mixture of two or more may be used. The above conductive material may be included in an amount of 0.1 wt% to 15 wt% relative to the total weight of the anode slurry for forming the anode active material layer.

[0144] The above binder serves to improve adhesion between positive active material particles and adhesion between the positive 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, and one of these alone or a mixture of two or more may be used. The above binder may be included in an amount of 0.1 wt% to 15 wt% based on the total weight of the positive slurry for forming the positive active material layer.

[0145] The above-described anode can be manufactured according to a conventional anode manufacturing method, except for using the above-described anode active material. Specifically, it can be manufactured by applying an anode slurry composition, prepared by dissolving or dispersing the above-described anode active material and optionally a binder and a conductive material in a solvent, onto an anode current collector, and then drying and rolling.

[0146] The above solvent may be a solvent commonly used in the relevant technical field, and may include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and one of these alone or a mixture of two or more may be used. The amount of the above solvent used is sufficient if it has a viscosity that dissolves or disperses the anode active material, conductive material, and binder, taking into account the coating thickness of the slurry and the manufacturing yield, and subsequently provides excellent thickness uniformity when coated for anode manufacturing.

[0147] In addition, in another embodiment, the anode may be manufactured by casting the anode slurry composition onto a separate support and then laminating the film obtained by peeling off from the support onto an anode current collector.

[0148] In addition, according to another aspect of the present invention, an electrochemical device comprising the anode described above may be provided. Specifically, the electrochemical device may be a battery, a capacitor, etc., and more specifically, a lithium secondary battery.

[0149] Specifically, the above lithium secondary battery may include a positive electrode, a negative electrode positioned opposite to the positive electrode, and a separator and an electrolyte interposed between the positive electrode and the negative electrode. Here, since the positive electrode is the same as previously described, a detailed description is omitted for convenience, and only the remaining components not previously mentioned are described in detail below.

[0150] The above lithium secondary battery may optionally further include a battery container that accommodates the electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

[0151] The above cathode may include a cathode current collector and a cathode active material layer located on the cathode current collector.

[0152] The above-mentioned negative current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and 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. In addition, the above-mentioned negative current collector may typically have a thickness of 3 μm to 500 μm, and, similar to the positive current collector, fine irregularities may be formed on the surface of the current collector to strengthen the bonding strength of the negative active material. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0153] The above-mentioned cathode active material layer can be manufactured by applying a cathode slurry composition, which includes a conductive material and optionally a binder together with the above-mentioned cathode active material, to the above-mentioned cathode current collector.

[0154] As the above-mentioned 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 βExamples include metal oxides capable of doping and dedoping lithium, such as (0 < β < 2), SnO2, vanadium oxide, and lithium vanadium oxide; or composites comprising the metal compound and carbonaceous material, such as Si-C composites or Sn-C composites, and any one or more of these may be used. Additionally, a metallic lithium thin film may be used as the negative electrode active material. Furthermore, the carbon material may include low-crystallinity carbon and high-crystallinity carbon. Representative examples of low-crystallinity carbon include soft carbon and hard carbon, while representative examples of high-crystallinity carbon include amorphous, plate-like, flake-like, 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.

[0155] The above-mentioned cathode active material may be included in an amount of 80 wt% to 99 wt% based on the total weight of the cathode slurry for forming the cathode active material layer.

[0156] The above binder is a component that assists in the bonding between the conductive material, the active material, and the current collector, and can typically be added in an amount of 0.1 wt% to 10 wt% based on the total weight of the cathode slurry for forming the cathode active material layer. 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.

[0157] The above conductive material is a component for further improving the conductivity of the negative electrode active material, and may be added in an amount of 10 wt% or less, preferably 5 wt% or less, based on the total weight of the negative electrode slurry for forming the negative electrode active material layer. Such conductive material is not particularly limited as long as it is conductive without causing chemical changes 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, thermal black; conductive fibers such as carbon fiber or metal fiber; metal powder such as carbon fluoride, aluminum, or nickel powder; conductive whiskey such as zinc oxide or potassium titanate; conductive metal oxide such as titanium oxide; conductive materials such as polyphenylene derivatives, etc. may be used.

[0158] In one embodiment, the negative active material layer may be manufactured by applying a negative slurry composition, prepared by dissolving or dispersing a negative active material and optionally a binder and a conductive material in a solvent, onto a negative current collector and drying it, or by casting the negative slurry composition onto a separate support and then laminating the film obtained by peeling it off from the support onto a negative current collector.

[0159] Meanwhile, in the above-mentioned lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a pathway for the movement of lithium ions. Any separator typically used in lithium secondary batteries may be used without special limitations, and it is particularly desirable that it has low resistance to the movement of electrolyte ions and excellent electrolyte moisture retention capacity. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin-based polymer like ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof may be used. In addition, a conventional porous nonwoven fabric, such as a nonwoven fabric made of high-melting-point glass fiber or polyethylene terephthalate fiber, may be used. Furthermore, a coated separator containing ceramic components or polymer materials may be used to ensure heat resistance or mechanical strength, and may optionally be used in a single-layer or multi-layer structure.

[0160] In addition, the electrolytes used in the present invention may include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc., which are usable when manufacturing lithium secondary batteries, but are not limited to these.

[0161] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0162] The above organic solvent may be used without special restrictions as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the above organic solvent may include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether or tetrahydrofuran; ketone-based solvents such as cyclohexanone; and aromatic hydrocarbon-based solvents such as benzene and fluorobenzene. Carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol-based solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (where R is a straight-chain, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms and may include a double bond, a directional ring, or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes may be used. Among these, a carbonate-based solvent is preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate, etc.) having high ionic conductivity and high dielectric constant that can improve the charge / discharge performance of the battery, and a low-viscosity linear carbonate-based compound (e.g., ethylmethyl carbonate, dimethyl carbonate or diethyl carbonate, etc.) is more preferred.In this case, using a mixture of cyclic carbonate and chain carbonate in a volume ratio of about 1:1 to about 1:9 can result in excellent performance of the electrolyte.

[0163] The above lithium salt can be used without special restrictions as long as it is a compound capable of providing lithium ions used in lithium secondary batteries. Specifically, the above lithium salt is LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2) 2. LiCl, LiI, or LiB(C2O4)2, etc., may be used. It is preferable to use the lithium salt within the range of 0.1 M to 2.0 M. When the concentration of the lithium salt falls within the above range, the electrolyte has appropriate conductivity and viscosity, so it can exhibit excellent electrolyte performance and lithium ions can move effectively.

[0164] When the electrolyte used in the present invention is a solid electrolyte, for example, solid inorganic electrolytes such as sulfide-based solid electrolytes, oxide-based solid electrolytes, nitride-based solid electrolytes, halide-based solid electrolytes, etc. may be used, and preferably, sulfide-based solid electrolytes may be used.

[0165] As a material for a sulfide-based solid electrolyte, a solid electrolyte containing Li, an element X (wherein X is at least one selected from P, As, Sb, Si, Ge, Sn, B, Al, Ga, and In) and S may be used. Examples of the above-mentioned sulfide-based solid electrolyte materials include Li2S-P2S5, Li2S-P2S-LiX (wherein X is a halogen element such as I or Cl), Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, and Li2S-P2S5-Z m S n (Here, m and n are integers, and Z is Ge, Zn, or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li p MO q (Here, p and q are integers, and M is P, Si, Ge, B, Al, Ga, or In) etc.

[0166] The solid electrolyte, preferably a sulfide-based solid electrolyte, may be amorphous or crystalline, or may be in a mixed state of amorphous and crystalline.

[0167] Li7La3Zr2O is a material for oxide-based solid electrolytes. 12 , Li 7-x La3Zr 1-x Nb x O 12 , Li 7-3x La3Zr2Al x O 12 , Li 3x La 2 / 3-x TiO3, Li 1+x Al x Ti 2-x (PO4)3, Li 1+x Al x Ge 2-x (PO4)3, Li3PO4, Li 3+x PO4-x N x (LiPON), Li 2+2x Zn 1-x There are GeO4 (LISICON), etc.

[0168] The aforementioned solid electrolyte may be disposed as a separate layer (solid electrolyte layer) between the anode and the cathode. Additionally, the solid electrolyte may be partially included within the anode active material layer of the anode independently of the solid electrolyte layer, or the solid electrolyte may be partially included within the cathode active material layer of the cathode independently of the solid electrolyte layer.

[0169] In addition to the above electrolyte components, the above electrolyte may further include one or more additives for the purpose of improving the lifespan characteristics of the battery, suppressing the decrease in battery capacity, and improving the discharge capacity of the battery, such as, for example, haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, triamide hexaphosphate, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, the additive may be included in an amount of 0.1 wt% to 5 wt% based on the total weight of the electrolyte.

[0170] As described above, since the lithium secondary battery containing the positive electrode active material according to the present invention stably exhibits excellent discharge capacity, output characteristics, and lifespan characteristics, it is useful in portable devices such as mobile phones, laptop computers, and digital cameras, as well as in the field of electric vehicles such as hybrid electric vehicles (HEVs).

[0171] The external shape of the lithium secondary battery according to the present invention is not subject to any particular limitations, but may be cylindrical, prismatic, pouch, or coin-shaped using a can. In addition, the lithium secondary battery can be used not only as a battery cell used as a power source for small devices, but can also preferably be used as a unit cell in a medium-to-large battery module containing a plurality of battery cells.

[0172] According to another aspect of the present invention, a battery module comprising the lithium secondary battery as a unit cell and / or a battery pack comprising the same may be provided.

[0173] The battery module or the battery pack may be used as a power source for one or more medium-to-large devices, including a power tool; an electric vehicle (EV), a hybrid electric vehicle, and a plug-in hybrid electric vehicle (PHEV); or a power storage system.

[0174]

[0175] The present invention will be described in more detail below through examples. However, these examples are intended solely to illustrate the present invention and should not be interpreted as limiting the scope of the present invention.

[0176]

[0177] Preparation Example 1. Preparation of positive electrode active material

[0178] Example 1

[0179] (a) Preparation of precursors

[0180] Spherical Ni by the co-precipitation method 0.92 Co 0.04 Mn 0.04(OH)2 hydroxide precursors were synthesized. Specifically, 50 wt% NaOH (aq) and 18 wt% NH4OH (aq) were added to a 1.5 M complex transition metal sulfuric acid aqueous solution in which nickel sulfate, cobalt sulfate, and manganese sulfate were mixed in a molar ratio of 92:4:4 in a 90 L reactor.

[0181] The pH inside the reactor was maintained at 12.0 and the temperature at 40°C, and the precursor synthesis reaction was carried out for 24 hours while introducing N2 gas into the reactor. After the reaction was complete, the precursor was separated, washed with deionized water, and dried at 120°C for 10 hours, and the volume-based average particle size (D) measured by laser diffraction analysis 50 Ni with a ) of 14.4μm 0.92 Co 0.04 Mn 0.04 (OH)2 hydroxide precursor was obtained.

[0182] (b) First heat treatment - Preparation of primary intermediate product

[0183] Ni obtained in step (a) above 0.92 Co 0.04 Mn 0.04 A mixture was prepared by mixing a (OH)2 hydroxide precursor and LiOH (Li / (Ni+Co+Mn) molar ratio = 1.04). Subsequently, the mixture was heat-treated in a furnace under an O2 atmosphere at 400°C for 8 hours to obtain a primary intermediate product.

[0184] (c) Second heat treatment - Preparation of secondary intermediate product

[0185] The primary intermediate product obtained in step (b) above was heat-treated in a furnace at 730°C in an O2 atmosphere for 10 hours to obtain a secondary intermediate product.

[0186] (d) Third heat treatment - Final product manufacturing

[0187] A mixture was prepared by mixing the secondary intermediate product obtained in step (c) above, Al2O3 (weighed so that the aluminum content is 0.5 mol% relative to the total metal elements excluding lithium in the intermediate product), and H3BO3 (weighed so that the boron content is 0.5 mol% relative to the total metal elements excluding lithium in the intermediate product). Subsequently, the mixture was heat-treated in a furnace at 730°C in an O2 atmosphere for 10 hours to obtain a final product.

[0188]

[0189] Example 2

[0190] A positive electrode active material was prepared in the same manner as in Example 1, except that the first heat treatment temperature of step (b) above was set to 450°C.

[0191]

[0192] Example 3

[0193] A positive electrode active material was prepared in the same manner as in Example 1, except that the first heat treatment temperature of step (b) above was set to 500℃.

[0194]

[0195] Comparative Example 1

[0196] A positive electrode active material was prepared in the same manner as in Example 1, except that the first heat treatment temperature of step (b) above was set to 300℃.

[0197]

[0198] Comparative Example 2

[0199] A positive electrode active material was prepared in the same manner as in Example 1, except that the first heat treatment temperature of step (b) above was set to 600℃.

[0200]

[0201] Comparative Example 3

[0202] A positive electrode active material was prepared in the same manner as in Example 1, except that a mixture was prepared by mixing the secondary intermediate product obtained in step (c) above and Al2O3 (weighed such that the aluminum content is 0.5 mol% relative to the total metal elements excluding lithium in the intermediate product), and the mixture was heat-treated in a furnace at 730°C in an O2 atmosphere for 10 hours to obtain a final product.

[0203]

[0204] Comparative Example 4

[0205] A positive electrode active material was prepared in the same manner as in Example 1, except that a mixture was prepared by mixing the secondary intermediate product obtained in step (c) above and H3BO3 (weighed so that the boron content was 0.5 mol% relative to the total metal elements excluding lithium in the intermediate product), and the mixture was heat-treated in a furnace at 730°C in an O2 atmosphere for 10 hours to obtain a final product.

[0206]

[0207] Comparative Example 5

[0208] (a) Preparation of precursors

[0209] Spherical Ni by the co-precipitation method 0.92 Co 0.04 Mn 0.04 (OH)2 hydroxide precursors were synthesized. Specifically, 50 wt% NaOH (aq) and 18 wt% NH4OH (aq) were added to a 1.5 M complex transition metal sulfuric acid aqueous solution in which nickel sulfate, cobalt sulfate, and manganese sulfate were mixed in a molar ratio of 92:4:4 in a 90 L reactor.

[0210] The pH inside the reactor was maintained at 12.0 and the temperature at 40°C, and the precursor synthesis reaction was carried out for 24 hours while introducing N2 gas into the reactor. After the reaction was complete, the precursor was separated, washed with deionized water, and dried at 120°C for 10 hours, and the volume-based average particle size (D) measured by laser diffraction analysis 50 Ni with a ) of 14.4μm 0.92 Co 0.04 Mn 0.04 (OH)2 hydroxide precursor was obtained.

[0211] (b) First heat treatment - Preparation of primary intermediate product

[0212] Ni obtained in step (a) above 0.92 Co 0.04 Mn 0.04 A mixture was prepared by mixing a (OH)2 hydroxide precursor and LiOH (Li / (Ni+Co+Mn) molar ratio = 1.04). Subsequently, the mixture was heat-treated in a furnace under an O2 atmosphere at 730°C for 10 hours to obtain a primary intermediate product.

[0213] (c) Second heat treatment - Final product manufacturing

[0214] A mixture was prepared by mixing the primary intermediate product obtained in step (c) above, Al2O3 (weighed so that the aluminum content is 0.5 mol% relative to the total metal elements excluding lithium in the intermediate product), and H3BO3 (weighed so that the boron content is 0.5 mol% relative to the total metal elements excluding lithium in the intermediate product). Subsequently, the mixture was heat-treated in a furnace at 730°C in an O2 atmosphere for 10 hours to obtain a final product.

[0215]

[0216] Preparation Example 2. Preparation of a lithium secondary battery (half-cell)

[0217] A positive electrode slurry was prepared by dispersing 94 wt% of the positive electrode active material prepared according to Preparation Example 1, 3 wt% of carbon black, and 3 wt% of PVDF binder in 30 g of N-methyl-2-pyrrolidone (NMP). The positive electrode slurry was uniformly coated onto an aluminum thin film with a thickness of 15 μm and vacuum dried at 135°C to produce a positive electrode for a lithium secondary battery.

[0218] A half-cell was prepared using a lithium foil as the counter electrode for the above anode, a porous polyethylene membrane (Celgard 2300, thickness: 25 μm) as the separator, and an electrolyte in which LiPF6 is present at a concentration of 1.15 M in a solvent mixed with ethylene carbonate and ethyl methyl carbonate in a volume ratio of 3:7.

[0219]

[0220] Experimental Example 1. Analysis of particle shape of cathode active material

[0221] For each of the cathode active materials prepared in Preparation Example 1, the lithium transition metal oxide was cross-sectionally treated using an ion-milling device to obtain a cross-sectional SEM image, and the change in aspect ratio of the primary particles observed from the cross-sectional SEM image was confirmed using an image analysis program (Image-Pro image analysis software for SEM) (see FIGS. 1 to 8).

[0222] In addition, for each lithium transition metal oxide (secondary particle), when r is the distance from the center of the secondary particle to the surface of the secondary particle, the average aspect ratio of all primary particles existing within the center of the secondary particle where the distance (d) from the center of the secondary particle is 0 ≤ d ≤ (1 / 3)r, and the distance (d) from the center of the secondary particle is (1 / 3)r <d≤r인 상기 2차 입자의 표면부 내 존재하는 전체 1차 입자의 평균 종횡비를 계산하였다.

[0223] The aspect ratio of the primary particle was calculated as the ratio of the major axis length to the minor axis length (major axis length / minor axis length), with the longest straight line passing through the center of the primary particle in the cross-sectional SEM image being the major axis length (nm) and the shortest straight line passing through the center of the primary particle being the minor axis length (nm). If the cross-sectional shape of the primary particle is substantially circular, the aspect ratio of the primary particle was calculated as 1.

[0224] The above calculation results are shown in Table 1 below.

[0225] Average aspect ratio of center part Average aspect ratio of surface part Example 11.35 3.74 Example 21.32 3.03 Example 31.33 2.86 Comparative Example 11.18 1.97 Comparative Example 21.20 2.42 Comparative Example 31.12 2.46 Comparative Example 41.19 1.24 Comparative Example 51.32 1.28

[0226] Referring to the results in Table 1, when the hydroxide precursor is pre-thermal treated at a low temperature (400–500°C) before being calcined at a relatively high temperature (700°C or higher), an aspect ratio gradient of primary particles is formed between the center and the surface of the lithium transition metal oxide due to the co-doping of aluminum and boron, and it can be confirmed that the average aspect ratio of primary particles within the surface of the lithium transition metal oxide is 2.5 or higher.

[0227] However, if the pre-thermal treatment temperature is excessively low (300°C) or excessively high (600°C), it can be observed that the difference in aspect ratio of primary particles between the center and the surface of the lithium transition metal oxide decreases. If the pre-thermal treatment temperature is excessively low (300°C), it is expected that the primary particles within the surface of the lithium transition metal oxide did not grow sufficiently in the long axis direction as the dissolved H3BO3 causes the primary particles to grow in random directions. On the other hand, if the pre-thermal treatment temperature is excessively high (600°C), dissolution of LiOH occurs, inducing growth of the primary particles in the short axis direction, and accordingly, it can be observed that the aspect ratio of the primary particles within the surface of the lithium transition metal oxide decreases.

[0228] Meanwhile, even if the hydroxide precursor is pre-thermal treated at a low temperature (400–500°C) before being calcined at a relatively high temperature (700°C or higher), if aluminum is used alone, the growth of primary particles within the surface portion of the lithium transition metal oxide may be insufficient.

[0229] In addition, even if the hydroxide precursor is pre-thermal treated at a low temperature (400–500°C) before being calcined at a relatively high temperature (700°C or higher), it can be confirmed that when boron is used alone, almost no aspect ratio gradient of the primary particles is formed between the center and the surface of the lithium transition metal oxide. When the hydroxide precursor is not pre-thermal treated at a low temperature (400–500°C) before being calcined at a relatively high temperature (700°C or higher), it can be confirmed that almost no aspect ratio gradient of the primary particles is formed between the center and the surface of the lithium transition metal oxide even when aluminum and boron are co-doped.

[0230]

[0231] Experimental Example 2. XRD Analysis of Anode Active Material

[0232] X-ray diffraction (XRD) analysis was performed on each positive electrode active material prepared according to Preparation Example 1 to calculate the average crystallite size and c-axis length of the lithium transition metal oxide contained in the positive electrode active material.

[0233] XRD analysis was performed using a Bruker D8 Advance diffractometer with Cu-Kα radiation (1.540598 Å).

[0234] The full width at half-maximum (FWHM) of the peak corresponding to the (104) crystal plane (2θ=44.5±1.0° region) of the R-3m layered crystal structure was calculated from the diffraction spectrum obtained through X-ray diffraction (XRD) analysis using Cu-kα rays of the above positive active material.

[0235] The above average crystallite size was calculated from a straight line obtained by plotting the diffraction angle θ (rad) and full width β (rad) within the 2θ=10°~120° region on a coordinate plane with sinθ on the horizontal axis and βcosθ on the vertical axis through Rietveld Refinement of the diffraction spectrum obtained through X-ray diffraction (XRD) analysis using Cu-kα rays for the above anode active material.

[0236] The results of the XRD analysis above are shown in Table 2 below.

[0237] Classification FWHM (10⁴) (°) Crystalline Size (nm) Example 1 0.17 4280.9 Example 20.18 2180.2 Example 30.17 897 9.8 Comparative Example 10.17 0985.1 Comparative Example 20.17 277 3.1 Comparative Example 30.15 6110 3.5 Comparative Example 40.17 527 2.2 Comparative Example 50.17 37 2.8

[0238] In addition, from the diffraction spectra of the anode active materials according to Examples 1 to 3, Li2B4O7, LiAlO2, LiAl5O5, LiAlB2O5 and Li2AlB5O 10 A specific peak could be observed. Through this, it can be expected that a coating layer containing aluminum and boron was formed on the lithium transition metal oxide (the interface or gap between the surface of the secondary particle and the primary particle located inside the secondary particle).

[0239]

[0240] Experimental Example 3. Evaluation of Electrochemical Characteristics of a Lithium Secondary Battery (Half-Cell)

[0241] For the lithium secondary battery (half-cell) prepared in Preparation Example 2, the initial charge capacity, initial discharge capacity, initial efficiency, and 2.0C / 0.1C rate characteristics were measured through charge / discharge experiments using an electrochemical analyzer (Toyo, Toscat-3100) at 25℃, a voltage range of 3.0V to 4.3V, and a discharge rate of 2.0C / 0.1C.

[0242] In addition, for the same lithium secondary battery (half-cell), 50 charge / discharge cycles were performed using an electrochemical analyzer (Toyo, Toscat-3100) under conditions of 45°C, voltage range 3.0V to 4.3V, and 1C / 1C, and the ratio of the discharge capacity at the 50th cycle to the initial capacity (cycle capacity retention rate) was measured.

[0243] The above measurement results are shown in Table 3 below.

[0244] Classification Charging Capacity (mAh / g) Discharging Capacity (mAh / g) Initial Efficiency (%) 2.0C / 0.CRate (%) Capacity Retention Rate (%) Example 1 236.6 209 88.3 78.4 91.3 Example 2 238.7 213.5 89.4 80.5 90.9 Example 3 230.3 201.3 87.4 79.1 90.3 Comparative Example 1 228.6 196.1 85.8 73.3 87.1 Comparative Example 2 231.1 202.1 87.5 74.9 87.2 Comparative Example 3 235.4 207.4 88.1 75.6 86.1 Comparative Example 4 237.1 210.4 88.7 75.2 86.3 Comparative Example 5226.6195.486.274.586.5

[0245] When comparing the results of Examples 1 to 3 with the results of Comparative Examples 1 to 5, it can be seen that the capacity characteristics, initial efficiency, and rate characteristics were hardly degraded even after co-doping of aluminum and boron as defined herein, and rather, the discharge capacity and rate characteristics were improved.

[0246] In addition, it can be confirmed that the high temperature (45°C) capacity retention rate of the lithium secondary battery using the positive electrode active material according to Examples 1 to 3 is significantly improved compared to the lithium secondary battery using the positive electrode active material according to Comparative Examples 1 to 5. Through the above results, it can be confirmed that an aspect ratio gradient of primary particles is formed between the center and the surface of the lithium transition metal oxide due to the co-doping of aluminum and boron, and that if the average aspect ratio of primary particles within the surface of the lithium transition metal oxide is 2.5 or higher, it can contribute to improving the stability of the positive electrode active material with a polycrystalline structure.

[0247]

[0248] Although embodiments of the present invention have been described above, those skilled in the art may modify and change the present invention in various ways by adding, changing, deleting, or adding components, etc., without departing from the spirit of the present invention as described in the claims, and such modifications and changes are also to be included within the scope of the rights of the present invention.

Claims

1. A positive electrode active material comprising a lithium transition metal oxide capable of lithium intercalation / deintercalation, The above lithium transition metal oxide has a secondary particle form in which a plurality of primary particles are aggregated, and The above lithium transition metal oxide is co-doped with aluminum and boron, and Let r be the distance from the center of the secondary particle to the surface of the secondary particle, and let (d) be the distance from the center of the secondary particle as (1 / 3)r <d≤r인 상기 2차 입자의 표면부 내 존재하는 1차 입자의 평균 종횡비는 2.5 이상 5.0 미만인, Positive active material.

2. In Paragraph 1, The average aspect ratio of the primary particles existing within the center of the secondary particles, where the distance (d) from the center of the secondary particles is 0 ≤ d ≤ (1 / 3)r, is such that the distance (d) from the center of the secondary particles is (1 / 3)r <d≤r인 상기 2차 입자의 표면부 내 존재하는 1차 입자의 평균 종횡비보다 작은, Positive active material.

3. In Paragraph 2, The average aspect ratio of the primary particles present within the center of the secondary particles is 1.5 or less, Positive active material.

4. In Paragraph 1, The aspect ratio of the primary particle above exhibits a gradient that increases in the direction from the center of the secondary particle toward the surface of the secondary particle, Positive active material.

5. In Paragraph 1, The above lithium transition metal oxide is represented by the following chemical formula 1, Cathode active material: [Chemical Formula 1] Li a Ni1-(b+c+d+e)Co b Mr c M1 d M2 e O2 In the above chemical formula 1, M1 is Al and B, M2 is at least one selected from Na, K, Mg, Ca, Sr, Ba, Rb, Ce, Hf, Ta, Cr, F, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, Mo, W, P, Ge, Nd, Gd and Cu, and 0.85≤a≤1.15, 0≤b≤0.20, 0≤c≤0.20, 0 <d≤0.10, 0≤e≤0.10, 0<b+c≤0.40이다.

6. In Paragraph 1, The average crystallite size of the above lithium transition metal oxide is 75 nm or more and less than 100 nm, Positive active material.

7. In Paragraph 1, The full width at half-maximum (FWHM) of the peak corresponding to the (104) crystal plane, calculated from the diffraction spectrum obtained through X-ray diffraction (XRD) analysis using Cu-kα rays for the above positive active material, is 0.170° to 0.190°, Positive active material.

8. In Paragraph 1, The average particle size of the primary particles is 0.1 μm to 1.0 μm, Positive active material.

9. In Paragraph 1, The average particle size of the above secondary particles is 2.0 μm to 16.0 μm, Positive active material.

10. In Paragraph 1, The sum of the concentrations (mol%) of aluminum and boron present at the interface between the primary particles located inside the secondary particles is greater than the sum of the concentrations (mol%) of aluminum and boron present inside the primary particles. Positive active material.

11. In Paragraph 1, A coating layer containing aluminum and boron existing on the surface of the above secondary particles, Positive active material.

12. In Paragraph 1, A coating layer containing aluminum and boron existing at the interface or gap between the primary particles located inside the secondary particles, Positive active material.

13. A positive electrode comprising a positive electrode active material according to any one of paragraphs 1 to 12.

14. A lithium secondary battery using a positive electrode according to Paragraph 13.

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