Positive electrode active material and lithium secondary battery containing the same

Abstract

To provide a positive electrode active material having improved electrochemical properties and stability.SOLUTION: A positive electrode active material is represented by a following chemical formula 1, and contains a lithium composite oxide capable of achieving lithium intercalation / deintercalation, and an average ratio b / a of a major axis length b of the voids to a minor axis length a of the voids observed in a cross-sectional SEM (scanning electron microscope) image of the lithium composite oxide is 1 to 3. [Chemical formula 1] LiwNi1-(x+y+z)CoxM1yM2zO2+α (M1 is at least one selected from Mn and Al, M2 is at least one selected from Mn, P, Sr, Ba, B, Ti, Zr, Al, Hf, Ta, Mg, V, Zn, Si, Y, Sn, Ge, Nb, W and Cu, M1 and M2 are elements different from each other, and 0.5≤w≤1.5, 0≤x≤0.50, 0≤y≤0.20, 0≤z≤0.20, and 0≤α≤0.02 are satisfied).SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a lithium secondary battery using a positive electrode active material and a positive electrode containing the positive electrode active material. More specifically, the present invention controls the area and shape of voids in a lithium composite oxide contained in the positive electrode active material by adjusting the ratio of ammonia and caustic soda used during a coprecipitation reaction for synthesizing a precursor of the positive electrode active material, thereby improving the electrochemical properties and stability of the positive electrode active material and the lithium secondary battery using the positive electrode containing the positive electrode active material.

Background Art

[0002] A battery stores electric power by using substances capable of electrochemical reactions 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 due to 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 negative electrode active materials, and filling an organic electrolyte or a polymer electrolyte between the positive electrode and the negative electrode.

[0004] Lithium composite oxides are used as the positive electrode active material of the lithium secondary battery. 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 excellent in life characteristics and charge / discharge efficiency and is the most widely used. However, due to the limited resources of cobalt used as a raw material, it is expensive, so it 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 price, but they have problems such as small capacity and poor high-temperature characteristics. In addition, LiNiO2-based cathode active materials exhibit battery characteristics with high discharge capacity, but due to the cation mixing problem between Li and transition metals, synthesis is difficult, and thus there are significant problems in rate characteristics.

[0007] In addition, a large amount of Li impurities are generated according to the degree of deepening of such cation mixing, and most of these Li impurities consist of compounds of LiOH and Li2CO3. Therefore, it causes problems such as gelling during the production of the cathode paste and gas generation during charge and discharge after the production of the electrode. Residual Li2CO3 not only increases the swelling phenomenon of the cell and reduces the cycle life but also causes the battery to bulge.

[0008] In addition, fine voids may exist in the lithium composite oxide constituting the cathode active material. By the existence of voids in the lithium composite oxide, the electrolyte can pass through, and thereby, it is possible to exhibit the electrochemical characteristics of the lithium composite oxide. However, if there are too many voids in the lithium composite oxide (usually, the porosity measured from the cross-sectional SEM (scanning electron microscope) image is mentioned as an index), conversely, the possibility of side reactions between the lithium composite oxide and the electrolyte increases, and the stability may decrease.

[0009] Thus, conventionally, attempts have been made to balance the electrochemical characteristics and stability of the lithium composite oxide by controlling the porosity in the lithium composite oxide. However, since the shapes of the voids observed from the cross-sectional SEM image of the lithium composite oxide are various, simply controlling the porosity has a limit that it only improves the stability of the lithium composite oxide. Summary of the Invention Problems to be Solved by the Invention

[0010] The present invention aims to provide a positive electrode active material with improved electrochemical properties and stability in order to solve the various problems of conventional positive electrode active materials for lithium secondary batteries.

[0011] In particular, the applicant has confirmed that the area and shape of voids in the lithium composite oxide can be controlled by adjusting the ratio of ammonia and caustic soda used in the coprecipitation reaction to synthesize the precursor of the lithium composite oxide constituting the positive electrode active material. Thus, by actively controlling the area and shape of the voids, rather than merely controlling the porosity or average diameter of the voids in the lithium composite oxide, the electrochemical properties and stability of the positive electrode active material can be further improved.

[0012] Accordingly, the present invention aims to provide a positive electrode active material in which electrochemical properties and stability are improved by controlling the area and shape of voids in the lithium composite oxide contained in the positive electrode active material by adjusting the ratio of ammonia and caustic soda used in the coprecipitation reaction for synthesizing the precursor of the positive electrode active material.

[0013] Furthermore, the present invention aims to provide a positive electrode containing the positive electrode active material defined in this application.

[0014] Furthermore, the present invention aims to provide a lithium secondary battery that uses the positive electrode as defined in this application. [Means for solving the problem]

[0015] According to one aspect of the present invention, a positive electrode active material is provided which includes a lithium composite oxide represented by the following chemical formula 1 and capable of lithium intercalation / deintercalation.

[0016] [Chemical formula 1] Li w Ni 1-(x+y+z) Co x M1 y M2z O 2+α

[0017] (Here, M1 is at least one selected from Mn or Al, M2 is at least one selected from Mn, P, Sr, Ba, B, Ti, Zr, Al, Hf, Ta, Mg, V, Zn, Si, Y, Sn, Ge, Nb, W, and Cu, M1 and M2 are different elements from each other, 0.5 ≦ w ≦ 1.5, 0 ≦ x ≦ 0.50, 0 ≦ y ≦ 0.20, 0 ≦ z ≦ 0.20, 0 ≦ α ≦ 0.02)

[0018] At this time, the average value of the ratio b / a of the long-axis length b to the short-axis length a of the voids observed from the cross-sectional SEM (scanning electron microscope) image of the lithium composite oxide may be 1 to 3.

[0019] Also, when the average particle size D50 of the lithium composite oxide is denoted as d, the average value of the long-axis length b of the voids observed from the cross-sectional SEM (scanning electron microscope) image of the lithium composite oxide may be less than 0.15d. That is, it is preferable that the average value of the long-axis length b of the voids is less than 15% of the average particle size D50 of the lithium composite oxide. Here, the average particle size D50 of the lithium composite oxide indicates the average particle size D50 of the lithium composite oxide as secondary particles.

[0020] Furthermore, the surface of the lithium composite oxide may further contain an alloy oxide represented by the following Chemical Formula 2 at least in part.

[0021] [Chemical Formula 2] Li a M3 b O c

[0022] (Here, M3 is at least one selected from Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, Ce, V, Ba, Ta, Sn, Hf, Gd, and Nd. 0 ≤ a ≤ 10, 0 <b≦8、2≦c≦13である)

[0023] Furthermore, according to another aspect of the present invention, a positive electrode containing the aforementioned positive electrode active material is provided.

[0024] Furthermore, according to yet another aspect of the present invention, a lithium secondary battery using the aforementioned positive electrode is provided. [Effects of the Invention]

[0025] According to the present invention, by adjusting the ratio of ammonia and caustic soda used in the coprecipitation reaction to synthesize the precursor of the lithium composite oxide constituting the positive electrode active material, it is possible to obtain the lithium composite oxide with controlled void area and void shape. By using a positive electrode active material containing the lithium composite oxide with controlled void area and void shape in this way, it is possible to improve various electrochemical properties such as capacity characteristics, life characteristics, and charge / discharge efficiency characteristics, which are important indicators for evaluating the performance of lithium secondary batteries.

[0026] Furthermore, as mentioned above, when the area and shape of the voids are controlled, and the porosity in the lithium composite oxide is controlled simultaneously, a better synergistic effect of the electrochemical properties can be expected.

[0027] While conventional methods for controlling the porosity in the lithium composite oxide have been able to improve stability by suppressing side reactions between the lithium composite oxide and the electrolyte, they have limitations in terms of improving the particle strength of the lithium composite oxide and suppressing crack formation during charging and discharging.

[0028] However, as introduced in this invention, by controlling the void area and void shape by adjusting the ratio of ammonia and caustic soda used in the coprecipitation reaction, the particle strength and crack formation suppression ability of the lithium composite oxide can be improved. Similarly, by simultaneously controlling the void area and void shape in the lithium composite oxide along with the void ratio, a synergistic effect on the stability of the lithium composite oxide can be expected. [Modes for carrying out the invention]

[0029] The positive electrode active material, the positive electrode containing the positive electrode active material, and the lithium secondary battery using the positive electrode according to the present invention will be described in more detail below.

[0030] positive electrode active material According to one aspect of the present invention, a positive electrode active material is provided which includes a lithium composite oxide capable of lithium intercalation / deintercalation.

[0031] The lithium composite oxide may be oxide particles in single crystal or polycrystalline form, but is preferably in polycrystalline form. A lithium composite oxide in polycrystalline form means an aggregate containing primary particles and secondary particles formed by the aggregation of multiple primary particles.

[0032] The primary particle refers to a single grain (grain or crystallite), and the secondary particle refers to an aggregate formed by the aggregation of multiple primary particles. Voids and / or grain boundaries may exist between the primary particles that make up the secondary particle.

[0033] For example, the primary particles can form internal voids within the secondary particles by separating them from adjacent primary particles. Furthermore, the primary particles can form surfaces within the secondary particles by contacting the internal voids, rather than forming grain boundaries by contacting adjacent primary particles.

[0034] Furthermore, 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.

[0035] Here, the average particle size D50 of the primary particles is within the range of 0.1 μm to 5 μm, preferably 0.1 μm to 3 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. Furthermore, the average particle size D50 of the secondary particles may vary depending on the number of aggregated primary particles, but may be between 5 μm and 20 μm.

[0036] Furthermore, the primary particles and / or secondary particles may have a rod-like, elliptical, and / or irregular shape.

[0037] Here, the lithium composite oxide is represented by the following chemical formula 1.

[0038] [Chemical formula 1] Li w Ni 1-(x+y+z) Co x M1 y M2 z O 2+α

[0039] (Here, M1 is at least one selected from Mn or Al. M2 is at least one selected from Mn, P, Sr, Ba, B, Ti, Zr, Al, Hf, Ta, Mg, V, Zn, Si, Y, Sn, Ge, Nb, W, and Cu. M1 and M2 are different elements. (0.5 ≤ w ≤ 1.5, 0 ≤ x ≤ 0.50, 0 ≤ y ≤ 0.20, 0 ≤ z ≤ 0.20, 0 ≤ α ≤ 0.02)

[0040] In this case, the lithium composite oxide may be a lithium composite oxide having a layered crystalline structure containing at least Ni and Co. Furthermore, the lithium composite oxide is preferably a high-Ni type lithium composite oxide in which x+y+z in the chemical formula 1 is 0.40 or less, preferably 0.20 or less.

[0041] In one embodiment, the positive electrode active material according to the present invention includes a lithium composite oxide in which the area and shape of the voids are controlled. By using the positive electrode active material containing the lithium composite oxide, various electrochemical properties such as capacity characteristics, life characteristics, and charge / discharge efficiency characteristics, which are important indicators for evaluating the performance of a lithium secondary battery, can be improved.

[0042] Void-related indices such as void area, void shape, and void ratio in the lithium composite oxide can be measured from cross-sectional scanning electron microscope (SEM) images of the lithium composite oxide.

[0043] Specifically, when the average particle size D50 of the lithium composite oxide is denoted as d, the average value of the major axis length b of the voids observed from the cross-sectional SEM (scanning electron microscope) image of the lithium composite oxide may be less than 0.15d, preferably less than 0.137d. That is, it is preferable that the average value of the major axis length b of the voids is less than 15% of the average particle size D50 of the lithium composite oxide. Here, the average particle size D50 of the lithium composite oxide refers to the average particle size D50 of the lithium composite oxide as secondary particles.

[0044] Furthermore, the average value of the long axis length b of the voids in the lithium composite oxide is less than 15% of the average particle size D50 of the lithium composite oxide, and the ratio b / a of the long axis length b of the voids to the short axis length a of the voids is adjusted to be between 1 and 3.

[0045] In this case, the shape of the void may range from a shape close to a sphere in which the major axis length b and minor axis length a are approximately the same (when the average value of the ratio b / a, which is the ratio of the major axis length b to the minor axis length a of the void, is 1) to a rod shape in which the major axis length b is longer than the minor axis length a (when the average value of b / a, which is the ratio of the major axis length b to the minor axis length a of the void, is 3).

[0046] On the other hand, if the average value of the long axis length b of the voids exceeds 15% of the average particle size D50 of the lithium composite oxide, the size of the voids where the average ratio b / a of the long axis length b of the voids to the short axis length a of the voids is in the range of 1 to 3 is excessively large. This is unfavorable to the particle strength of the lithium composite oxide, and there is a risk of cracks occurring within the lithium composite oxide during charging and discharging of the lithium secondary battery. Furthermore, if the size of the voids in the lithium composite oxide becomes excessively large, the amount of electrolyte impregnated into the voids increases, which may lead to side reactions.

[0047] Furthermore, it is preferable that the proportion of voids in the total voids observed from the cross-sectional SEM (scanning electron microscope) image of the lithium composite oxide in which the ratio of the long axis length b of the void to the short axis length a of the void (b / a) exceeds 3 is less than 50%.

[0048] When the proportion of voids in the overall void structure observed from a cross-sectional SEM (scanning electron microscope) image of the lithium composite oxide where the ratio of the long axis length b to the short axis length a of the void (b / a) exceeds 3 exceeds 50%, this is unfavorable to the particle strength of the lithium composite oxide and may lead to cracks occurring within the lithium composite oxide during charging and discharging of the lithium secondary battery. Furthermore, if the size of the voids in the lithium composite oxide becomes excessively large, the amount of electrolyte impregnated into the voids increases, which may lead to side reactions.

[0049] Furthermore, the average value of the major axis length b of the voids in the lithium composite oxide is (0.x)d to (0.y)d, the average value of the ratio b / a of the major axis length b of the voids to the minor axis length a of the voids satisfies 1 to 3, and the average area of ​​the voids observed from the cross-sectional SEM (scanning electron microscope) image of the lithium composite oxide is 0.02 to 1.5 μm². 2 It is preferable that this be the case.

[0050] Furthermore, the occupancy rate of the voids in the cross-section of the lithium composite oxide, as observed from the cross-sectional SEM (scanning electron microscope) image of the lithium composite oxide, may be 0.3 to 3.5%.

[0051] When the average value of the long axis length b of the voids in the lithium composite oxide is less than 15% of the average particle size D50 of the lithium composite oxide, preferably less than 13.7%, the average value of the ratio b / a of the long axis length b of the voids to the short axis length a of the voids satisfies 1 to 3, and the void occupancy rate (also called porosity) in the cross-section of the lithium composite oxide is 0.3 to 3.5%, then a better synergistic effect in electrochemical properties can be expected compared to a positive electrode active material in which only the porosity of the lithium composite oxide exists within the aforementioned range.

[0052] Furthermore, the surface of the lithium composite oxide contained in the positive electrode active material is a region where side reactions with the electrolyte can occur during charging, discharging, and / or storage of the lithium secondary battery. The larger the surface area of ​​the positive electrode active material (for example, it can be expressed as an indicator called the BET specific surface area), the higher the possibility of side reactions, and the crystalline structure within the surface of the positive electrode active material can undergo phase transformation (for example, layered structure → rock salt structure). Such phase transformation of the crystalline structure within the surface of the positive electrode active material has been proposed as one of the causes of reduced lifespan characteristics of lithium secondary batteries.

[0053] As proposed in this invention, when the ratio of ammonia and caustic soda used in the coprecipitation reaction to synthesize the precursor of the lithium composite oxide is adjusted in order to control the void area and void shape of the lithium composite oxide, the void area and void shape of the synthesized lithium composite oxide are controlled, and at the same time, the BET specific surface area of ​​the synthesized lithium composite oxide is increased to 0.2 to 2.0 m². 2 It can be adjusted within the range of / g.

[0054] In further embodiments, the positive electrode active material may further include a coating layer that covers at least a portion of the surface of the lithium composite oxide.

[0055] In this case, the coating layer may contain an alloy oxide represented by the following chemical formula 2. That is, the coating layer can be defined as a region in which the alloy oxide represented by the following chemical formula 2 exists.

[0056] [Chemical formula 2] Li a M3 b O c

[0057] (Here, M3 is at least one selected from Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, Ce, V, Ba, Ta, Sn, Hf, Gd, and Nd. 0 ≤ a ≤ 10, 0 <b≦8、2≦c≦13である)

[0058] Furthermore, the coating layer may be in a form in which different types of alloy oxides exist simultaneously within a single layer, or in which different types of alloy oxides represented by the chemical formula 2 exist in separate layers.

[0059] The alloy oxide represented by chemical formula 2 may be physically and / or chemically bonded with the lithium composite oxide. Alternatively, the alloy oxide may exist in a solid solution with the lithium composite oxide.

[0060] The alloy oxide is an oxide in which lithium and an element represented by M3 are composited, or an oxide of M3, and the oxide 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 Other examples are also acceptable, but the aforementioned examples are merely provided for convenience to aid understanding, and the oxides as defined in this application are not limited to the aforementioned examples.

[0061] In other embodiments, the alloy oxide may be an oxide in which lithium and at least two elements represented by M3 are composited, or may further contain an oxide in which lithium and at least two elements represented by M3 are composited. For example, an oxide in which lithium and at least two elements represented by M3 are composited is 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 cOther options are also acceptable, but the system is not necessarily limited to these.

[0062] Here, the alloy oxide 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 alloy oxide can decrease from the outermost surface of the secondary particle toward the center of the secondary particle.

[0063] As described above, the residual Li present on the surface of the lithium composite oxide can be further reduced by the alloy oxide exhibiting a concentration gradient that decreases from the surface of the secondary particles toward the center of the secondary particles. Furthermore, the alloy oxide can prevent the crystallinity of the inner region of the lithium composite oxide from decreasing. In addition, the alloy oxide can prevent the overall structure of the positive electrode active material from collapsing during the electrochemical reaction.

[0064] Furthermore, the coating layer may include a first coating layer containing at least one alloy oxide represented by the chemical formula 2, and a second coating layer containing at least one alloy oxide represented by the chemical formula 2, but containing an oxide different from the oxide contained in the first coating layer.

[0065] Lithium-ion battery According to yet another aspect of the present invention, a positive electrode can be provided comprising 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 the same as described above, a detailed explanation will be omitted for convenience, and only the remaining unmentioned components will be described below.

[0066] The positive electrode current collector is not particularly limited as long as it is conductive without inducing chemical changes in the battery. 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 typically have a thickness of 3 to 500 μm, and fine irregularities can be formed on the surface of the current collector to enhance the adhesion 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.

[0067] The positive electrode active material layer can 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. 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 within the above content range, excellent capacity characteristics can be observed, but the material is not necessarily limited to this range.

[0068] 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 without causing chemical changes in the battery it is configured in. 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 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 wt% relative to the total weight of the positive electrode active material layer.

[0069] The binder plays a role in improving the adhesion between positive electrode active material particles and the 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, and one of these alone or a mixture of two or more may be used. The binder may be included in an amount of 0.1 to 15 wt% relative to the total weight of the positive electrode active material layer.

[0070] The positive electrode can be manufactured by a conventional positive electrode manufacturing method, except that the positive electrode active material described above is used. Specifically, it can be manufactured by applying a positive electrode slurry composition, prepared by dissolving or dispersing the positive electrode active material and, selectively, a binder and a conductive material in a solvent, onto a positive electrode current collector, followed by drying and rolling.

[0071] 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 of these may be used alone or in a mixture of two or more. The amount of solvent used is sufficient to dissolve or disperse the positive electrode active material, conductive material, and binder, and to have a viscosity that allows for excellent thickness uniformity when applied for the manufacture of the positive electrode, taking into consideration the coating thickness of the slurry and the manufacturing yield.

[0072] In other embodiments, the positive electrode may also 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.

[0073] Furthermore, according to yet another aspect of the present invention, an electrochemical element including the aforementioned positive electrode can be provided. Specifically, the electrochemical element may be a battery, a capacitor, or the like, and more specifically, a lithium secondary battery.

[0074] 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 the same as described above, a detailed explanation will be omitted for convenience, and only the configuration not mentioned above will be described in detail below.

[0075] 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.

[0076] The negative electrode may include a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector.

[0077] 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.

[0078] The negative electrode active material layer can be manufactured by applying a negative electrode slurry composition, which includes a conductive material and, if necessary, a binder, together with the negative electrode active material, to the negative electrode current collector.

[0079] 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, and any one or a mixture of two or more of these 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 carbon materials. 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.

[0080] 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.

[0081] The binder is a component that assists in bonding between the conductive material, active material, and current collector, and is usually added at a concentration of 0.1 to 10 wt% based on the total weight of the negative electrode 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.

[0082] The conductive material is a component for further improving the conductivity of the negative electrode active material and can be added in an amount of 10 wt% or less, preferably 5 wt% 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 is conductive without inducing a chemical change in the battery, and may be used, for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, or summer black; conductive fibers such as carbon fibers or metal fibers; metal powders such as carbon fluoride, aluminum, or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive materials such as polyphenylene derivatives.

[0083] In one embodiment, the negative electrode active material layer can 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 then drying it, or by casting the negative electrode slurry composition onto a separate support, peeling it off the support, and then laminating the resulting film onto the negative electrode current collector.

[0084] In other embodiments, the negative electrode active material layer can also be manufactured by applying 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.

[0085] 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. Any membrane commonly used as a separation membrane in lithium secondary batteries can be used without particular limitations, and it is especially preferable that the membrane 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, can also be used. Furthermore, to ensure heat resistance or mechanical strength, coated separation membranes containing ceramic components or polymeric substances may be used, and they may be selectively used in single-layer or multi-layer structures.

[0086] 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.

[0087] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0088] The organic solvent can be any solvent that can act 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 hydrocarbon group with 2 to 20 carbon atoms in a linear, branched, or cyclic structure, 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, mixing the cyclic carbonate and the linear carbonate in a volume ratio of about 1:1 to about 1:9 can bring out good performance of the electrolyte.

[0089] 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)2, 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, so it can exhibit excellent electrolyte performance and lithium ions can move effectively.

[0090] In addition to the electrolyte components, the electrolyte may further contain one or more additives for the purpose of improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity, such as haloalkylene carbonate compounds like 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 wt% relative to the total weight of the electrolyte.

[0091] 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).

[0092] The external shape of the lithium secondary battery according to the present invention is not particularly limited, but 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 can also be suitably used as a unit battery in medium- and large-sized battery modules containing a large number of battery cells.

[0093] 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.

[0094] 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.

[0095] The present invention will be described in more detail below based on examples. However, these examples are merely illustrative and should not be interpreted as limiting the scope of the present invention.

[0096] Manufacturing Example 1. Manufacturing of positive electrode active material (1) Example 1 Spherical Ni 0.91 Co 0.08 Mn 0.01 (OH)2 hydroxide precursor was synthesized.

[0097] Specifically, nickel sulfate, cobalt sulfate, and manganese sulfate are mixed in a molar ratio of 91:8:1 in a 90L reactor to make a 2.0M aqueous solution of composite transition metal sulfuric acid, to which a 1.8M solution is added based on the transition metal concentration in the aqueous solution. (That is, 3.6M in the aforementioned composite fiber metal sulfuric acid aqueous solution)NaOH is added to the aqueous solution of the composite transition metal sulfuric acid, and the concentration of the transition metal is adjusted to 0.8 M based on the transition metal concentration. (That is, 1.6M in the aforementioned composite fiber metal sulfuric acid aqueous solution) NH4OH was added to achieve the following result.

[0098] The pH inside the reactor was maintained at 11.5, the reactor temperature was maintained at 60°C, and an inert gas, N2, was added to the reactor to prevent oxidation of the produced precursor. After the synthesis stirring was completed, washing and dewatering were performed using a filter press (F / P), and Ni 0.91 Co 0.08 Mn 0.01 (OH)2 hydroxide precursor was obtained.

[0099] Next, the synthesized precursor was mixed with LiOH (Li / (Ni+Co+Mn)mol ratio=1.01), and then heat-treated in a calcination furnace under an O2 atmosphere, increasing the temperature to 800°C at a rate of 2°C per minute for 10 hours to obtain a lithium composite oxide.

[0100] Subsequently, distilled water was added to the lithium composite oxide, and it was washed with water for one hour. The washed lithium composite oxide was then filtered and dried to obtain the positive electrode active material.

[0101] (2) Example 2 The positive electrode active material was manufactured in the same manner as in Example 1, except that NaOH was added to the aforementioned composite transition metal sulfuric acid aqueous solution to a concentration of 1.6 M based on the transition metal concentration, and NH4OH was added to the aforementioned composite transition metal sulfuric acid aqueous solution to a concentration of 0.4 M based on the transition metal concentration.

[0102] (3) Example 3 The positive electrode active material was manufactured in the same manner as in Example 1, except that NaOH was added to the aforementioned composite transition metal sulfuric acid aqueous solution to a concentration of 2.2 M based on the transition metal concentration, and NH4OH was added to the aforementioned composite transition metal sulfuric acid aqueous solution to a concentration of 1.2 M based on the transition metal concentration.

[0103] (4) Example 4 Spherical Ni 0.91 Co 0.08 Mn 0.01 (OH)2 hydroxide precursor was synthesized.

[0104] Specifically, in a 90L reactor, nickel sulfate, cobalt sulfate, and manganese sulfate were mixed in a molar ratio of 91:8:1 to create a 1.5M aqueous solution of composite transition metal sulfuric acid. NaOH was then added to the aqueous solution to a concentration of 1.6M based on the transition metal concentration, and NH4OH was added to the aqueous solution to a concentration of 0.4M based on the transition metal concentration.

[0105] The pH inside the reactor was maintained at 11.5, the reactor temperature was maintained at 60°C, and an inert gas, N2, was added to the reactor to prevent oxidation of the produced precursor. After the synthesis stirring was completed, washing and dewatering were performed using a filter press (F / P), and Ni 0.91 Co 0.08 Mn 0.01 (OH)2 hydroxide precursor was obtained.

[0106] Next, the synthesized precursor was mixed with LiOH (Li / (Ni+Co+Mn)mol ratio=1.01) and 0.5mol% Zr. Then, while maintaining an O2 atmosphere in a calcination furnace, the mixture was heated to 700°C at a rate of 2°C per minute for 10 hours to obtain a lithium composite oxide.

[0107] Subsequently, distilled water was added to the lithium composite oxide, and it was washed with water for one hour. The washed lithium composite oxide was then filtered and dried to obtain the positive electrode active material.

[0108] (5) Example 5 The cathode active material was produced in the same manner as in Example 1, except that before washing the lithium composite oxide with water, 1 mol% Al2O3, 0.25 mol% TiO2, and 0.05 mol% ZrO2 were mixed with the lithium composite oxide, and further heat treatment was performed for 10 hours at a rate of 2°C per minute up to 680°C while maintaining an O2 atmosphere.

[0109] (6) Example 6 Spherical Ni synthesized by the co-precipitation method 0.80 Co 0.10 Mn 0.10 The cathode active material was prepared in the same manner as in Example 1, except that a (OH)2 hydroxide precursor was used.

[0110] (7) Comparative Example 1 The positive electrode active material was manufactured in the same manner as in Example 1, except that NaOH was added to the aforementioned composite transition metal sulfuric acid aqueous solution to a concentration of 1.2 M based on the transition metal concentration, and NH4OH was added to the aforementioned composite transition metal sulfuric acid aqueous solution to a concentration of 0.3 M based on the transition metal concentration.

[0111] (8) Comparative Example 2 The positive electrode active material was manufactured in the same manner as in Example 1, except that NaOH was added to the aforementioned composite transition metal sulfuric acid aqueous solution to a concentration of 2.5 M based on the transition metal concentration, and NH4OH was added to the aforementioned composite transition metal sulfuric acid aqueous solution to a concentration of 1.5 M based on the transition metal concentration.

[0112] (9) Comparative Example 3 The positive electrode active material was manufactured in the same manner as in Example 1, except that NaOH was added to the aforementioned composite transition metal sulfuric acid aqueous solution to a concentration of 1.8 M based on the transition metal concentration, and NH4OH was added to the aforementioned composite transition metal sulfuric acid aqueous solution to a concentration of 1.5 M based on the transition metal concentration.

[0113] (10) Comparative Example 4 The positive electrode active material was manufactured in the same manner as in Example 1, except that NaOH was added to the aforementioned composite transition metal sulfuric acid aqueous solution to a concentration of 2.5 M based on the transition metal concentration, and NH4OH was added to the aforementioned composite transition metal sulfuric acid aqueous solution to a concentration of 0.4 M based on the transition metal concentration.

[0114] (11) Comparative Example 5 The positive electrode active material was manufactured in the same manner as in Example 1, except that NaOH was added to the aforementioned composite transition metal sulfuric acid aqueous solution to a concentration of 1.2 M based on the transition metal concentration, and NH4OH was added to the aforementioned composite transition metal sulfuric acid aqueous solution to a concentration of 1.5 M based on the transition metal concentration.

[0115] Manufacturing Example 2: Manufacturing of Lithium-ion Rechargeable Batteries A cathode slurry was prepared by dispersing 92 wt% each of the cathode active materials produced by Production Example 1, 4 wt% of artificial graphite, and 4 wt% of PVDF binder in 30 g of N-methyl-2-pyrrolidone (NMP). The cathode slurry was uniformly coated onto a 15 μm thick aluminum thin film and vacuum-dried at 135°C to produce a cathode for a lithium secondary battery.

[0116] A coin cell was manufactured using a lithium foil as the counter electrode for the positive electrode, a porous polyethylene membrane (Celgard 2300, thickness: 25 μm) as the separation membrane, and an 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.

[0117] Experimental Example 1. SEM Analysis of Cathode Active Material Cross-sectional SEM images were taken of the positive electrode active material manufactured according to Manufacturing Example 1 to confirm the void characteristics in the cross-section of the lithium composite oxide contained in the positive electrode active material.

[0118] Specifically, a cross-sectional SEM image was taken of the lithium composite oxide contained in the positive electrode active material after cross-sectional processing using a FIB (Ga-ion source). From the cross-sectional SEM image, the average particle size D50 of the lithium composite oxide, the average value of the major axis length b of the voids, the average value of the minor axis length a of the voids, the average value of b / a, the average area of ​​the voids, and the void occupancy rate were measured. The measurement results are shown in Table 1 below.

[0119] [Table 1] *The length of the major axis shown in parentheses is relative to D50(d).

[0120] [Table 2] *The length of the major axis in parentheses indicates the relative value to D50.

[0121] Experimental Example 2. Measurement of particle strength of positive electrode active material During the manufacturing of positive electrodes for lithium secondary batteries using positive electrode active materials, a slurry containing the positive electrode active material is applied to the positive electrode current collector, followed by drying and rolling (pressing). During this process, high-pressure rolling can cause particle breakdown of the positive electrode active material applied to the current collector, potentially degrading the performance of the positive electrode active material.

[0122] In this experiment, in order to confirm the change in strength of the positive electrode active material due to the composition of aggregates of multiple secondary particles contained in the positive electrode active material, the positive electrode active materials produced in Examples 1 to 3 and Comparative Examples 1, 2, and 5 were dried in a vacuum oven at 60°C for 12 hours. Then, one particle corresponding to D50 (lithium composite oxide) was selected, and the fracture strength of the particle (the pressure at which the particle fractures) was measured.

[0123] Table 3 below shows the average value of the fracture strength measured 10 times for each positive electrode active material.

[0124] [Table 3]

[0125] Referring to the results in Table 3, the positive electrode active materials from Examples 1 to 3 were measured to have slightly lower fracture strengths compared to the positive electrode active material from Comparative Example 1 (porosity 0.1%), which had almost no voids in the positive electrode active material. However, it can be confirmed that they showed improved fracture strengths compared to the positive electrode active materials from Comparative Examples 2 and 5. In particular, when comparing the positive electrode active material from Example 3 with that from Comparative Example 5, it can be confirmed that the fracture strength of the positive electrode active material from Example 3 was even higher, despite having the same porosity.

[0126] In other words, according to the present invention, it can be confirmed that by adjusting the ratio of ammonia and caustic soda used in the coprecipitation reaction for synthesizing the precursor of the positive electrode active material, the area and shape of voids in the lithium composite oxide contained in the positive electrode active material can be controlled, thereby contributing to improving the stability of the positive electrode active material.

[0127] Experimental Example 3. Evaluation of Lithium-ion Battery Capacity and Lifetime Characteristics Charge and discharge experiments were conducted on the lithium secondary battery (coin cell) manufactured in Manufacturing Example 2 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 the charge and discharge capacities were measured.

[0128] Furthermore, the same lithium secondary battery underwent 50 charge-discharge cycles at 25°C and a drive voltage range of 3.0V to 4.4V under 1C / 1C conditions. The ratio of the discharged capacity at the 50th cycle to the initial capacity (cycle capacity retention) was then measured.

[0129] The measurement results are shown in Table 4 below.

[0130] [Table 4]

[0131] Referring to the results in Table 4, it can be confirmed that the electrochemical properties of the positive electrode active material can be further improved not only by controlling the porosity of the lithium composite oxide contained in the positive electrode active material, but also by controlling the area and shape of the voids in the lithium composite oxide by adjusting the ratio of ammonia and caustic soda used in the coprecipitation reaction to synthesize the precursor.

[0132] 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, such as by adding, changing, deleting, or adding components, without departing 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. It contains a lithium composite oxide represented by the following chemical formula 1, which allows for lithium intercalation / deintercalation. [Chemical formula 1] Li ｗ Ni １－（ｘ＋ｙ＋ｚ） Co ｘ M1 ｙ M2 ｚ O ２＋α (Here, M1 is at least one selected from Mn or Al. M2 is at least one selected from Mn, P, Sr, Ba, B, Ti, Zr, Al, Hf, Ta, Mg, V, Zn, Si, Y, Sn, Ge, Nb, W, and Cu. M1 and M2 are different elements. (0.5 ≤ w ≤ 1.5, 0 ≤ x ≤ 0.50, 0 ≤ y ≤ 0.20, 0 ≤ z ≤ 0.20, 0 ≤ α ≤ 0.02) The average value of the ratio b / a of the long axis length of the void to the short axis length a observed from the cross-sectional SEM (scanning electron microscope) image of the lithium composite oxide is 2.3 to 3. The lithium composite oxide is a positive electrode active material comprising primary particles and secondary particles formed by the aggregation of multiple primary particles.

2. The positive electrode active material according to claim 1, wherein, when the average particle size D50 of the lithium composite oxide is denoted as d, the average value of the major axis length b of the voids observed from a cross-sectional SEM (scanning electron microscope) image of the lithium composite oxide is less than 0.15d.

3. The positive electrode active material according to claim 1, wherein, of the total voids observed from a cross-sectional SEM (scanning electron microscope) image of the lithium composite oxide, the proportion of voids where the ratio of the long axis length b of the void to the short axis length a of the void (b / a) exceeds 3 is less than 50%.

4. The average area of ​​the voids observed from the cross-sectional SEM (scanning electron microscope) images of the lithium composite oxide was 0.02 to 1.1 μm. ２ The positive electrode active material according to claim 1.

5. The positive electrode active material according to claim 1, wherein the average particle size D50 of the lithium composite oxide is 5 to 20 μm.

6. The positive electrode active material according to claim 1, wherein the occupancy rate of the voids in the cross-section of the lithium composite oxide, as observed from a cross-sectional SEM (scanning electron microscope) image of the lithium composite oxide, is 0.3 to 3.5%.

7. The BET specific surface area of ​​the aforementioned lithium composite oxide is 0.2 to 2.0 m². ２ The positive electrode active material according to claim 1, wherein the value is / g.

8. The total content of LiOH and Li with respect to the total weight of the positive electrode active material is 1.0% by weight or less. The positive electrode active material according to claim 1. ２ CO ３ The total content of LiOH and Li with respect to the total weight of the positive electrode active material is 1.0% by weight or less. The positive electrode active material according to claim 1.

9. The positive electrode active material according to claim 1, wherein at least a portion of the surface of the lithium composite oxide further comprises an alloy oxide represented by the following chemical formula 2. [Chemical formula 2] Li ａ M3 ｂ O ｃ (Here, M3 is at least one selected from Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, Ce, V, Ba, Ta, Sn, Hf, Gd, and Nd. (0 ≤ a ≤ 10, 0 < b ≤ 8, 2 ≤ c ≤ 13)

10. A positive electrode comprising the positive electrode active material according to any one of claims 1 to 9.

11. A lithium secondary battery using the positive electrode described in claim 10.

Images