Positive electrode active material and lithium secondary battery including the same

Abstract

The present invention relates to a cathode active material and a lithium secondary battery using a cathode including the cathode active material. More specifically, the present invention relates to a cathode active material having improved electrochemical properties and stability by controlling the area and shape of voids in a lithium composite oxide included in the cathode active material by adjusting the ratio of ammonia and caustic soda used in a co-precipitation reaction for synthesizing a precursor of the cathode active material, and a lithium secondary battery using a cathode including the cathode active material.

Description

[Technical field]

[0001] The present invention relates to a positive electrode active material and a lithium secondary battery using a positive electrode including the positive electrode active material. More specifically, the present invention relates to a positive electrode active material having improved electrochemical properties and stability by controlling the area and shape of voids in a lithium composite oxide included in the positive electrode active material by adjusting the ratio of ammonia and caustic soda used in a co-precipitation reaction for synthesizing a precursor of the positive electrode active material, and a lithium secondary battery using a positive electrode including the positive electrode active material. [Background technology]

[0002] Batteries store electricity by using materials capable of electrochemical reactions at the positive and negative electrodes. A representative example of such batteries is the lithium secondary battery, which stores electrical energy by the difference in chemical potential when lithium ions are intercalated / deintercalated at the positive and negative electrodes.

[0003] The lithium secondary battery is manufactured by using a material capable of reversible intercalation / deintercalation of lithium ions as a positive electrode and a negative electrode active material, 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 lithium secondary batteries, and examples of such composite oxides that have been researched include LiCoO2, LiMn2O4, LiNiO2, and LiMnO2.

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

[0006] Lithium manganese oxides such as LiMnO2 and LiMn2O4 have the advantages of being thermally safe and inexpensive, but have problems with small capacity and poor high-temperature characteristics. In addition, LiNiO2-based positive electrode active materials show high discharge capacity battery characteristics, but are difficult to synthesize due to the problem of cation mixing between Li and transition metals, which causes major problems in rate characteristics.

[0007] In addition, a large amount of Li impurities are generated depending on the depth of this cation mixing, and most of these Li impurities consist of compounds of LiOH and Li2CO3, which causes gelation during the manufacture of the positive electrode paste and gas generation during charge and discharge after the electrode is manufactured. Residual Li2CO3 not only increases the swelling phenomenon of the cell, reducing cycles, but also causes the battery to swell.

[0008] In addition, micropores may be present in the lithium composite oxide constituting the positive electrode active material. The presence of pores in the lithium composite oxide allows the electrolyte to pass through, which allows the lithium composite oxide to exhibit its electrochemical properties. However, if there are too many pores in the lithium composite oxide (usually, the porosity measured from a cross-sectional SEM (scanning electron microscope) image is referred to as an index), the possibility of a side reaction between the lithium composite oxide and the electrolyte increases, and stability may decrease.

[0009] For this reason, attempts have been made to achieve both electrochemical properties and stability of the lithium composite oxide by controlling the porosity in the lithium composite oxide. However, since the shapes of the pores observed in the cross-sectional SEM images of the lithium composite oxide are various, there is a limit to simply controlling the porosity, which can only improve the stability of the lithium composite oxide. Summary of the Invention [Problem to be solved by the invention]

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

[0011] In particular, the applicant has confirmed that the area and shape of the voids in the lithium composite oxide can be controlled by adjusting the ratio of ammonia and caustic soda used in the coprecipitation reaction for synthesizing the precursor of the lithium composite oxide constituting the positive electrode active material. In this way, by more actively controlling the area and shape of the voids, rather than simply 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 cathode active material having improved electrochemical properties and stability by controlling the area and shape of voids in a lithium composite oxide contained in the cathode active material by adjusting the ratio of ammonia and caustic soda used in a co-precipitation reaction for synthesizing a precursor of the cathode active material.

[0013] It is also an object of the present invention to provide a positive electrode comprising the positive electrode active material defined herein.

[0014] It is also an object of the present invention to provide a lithium secondary battery using the positive electrode defined herein. [Means for solving the problem]

[0015] According to one aspect of the present invention, there is provided a positive electrode active material comprising 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] (where: 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 different elements, 0.5≦w≦1.5, 0≦x≦0.50, 0≦y≦0.20, 0≦z≦0.20, 0≦α≦0.02)

[0018] In this case, the average ratio b / a of the major axis length b of the voids to the minor axis length a of the voids observed in a cross-sectional SEM (scanning electron microscope) image of the lithium composite oxide may be 1-3.

[0019] Furthermore, when the average particle diameter D50 of the lithium composite oxide is designated 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 may be less than 0.15d. In other words, the average value of the major axis length b of the voids is preferably less than 15% of the average particle diameter D50 of the lithium composite oxide. Here, the average particle diameter D50 of the lithium composite oxide refers to the average particle diameter D50 of the lithium composite oxide as secondary particles.

[0020] Furthermore, the lithium composite oxide may further include an alloy oxide represented by the following Chemical Formula 2 on at least a portion of the surface thereof.

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

[0022] (where: 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] According to another aspect of the present invention, there is provided a positive electrode comprising the above-described positive electrode active material.

[0024] According to yet another aspect of the present invention, there is provided a lithium secondary battery using the above-mentioned positive electrode. Effect of the Invention

[0025] According to the present invention, it is possible to obtain the lithium composite oxide having a controlled pore area and pore shape by adjusting the ratio of ammonia and caustic soda used in the co-precipitation reaction for synthesizing the precursor of the lithium composite oxide constituting the positive electrode active material. By using the positive electrode active material including the lithium composite oxide having a controlled pore area and pore shape, it is possible to improve various electrochemical characteristics such as capacity characteristics, life characteristics, charge and discharge efficiency characteristics, which are important indicators for evaluating the performance of a lithium secondary battery.

[0026] In addition, as described above, when the void area and void shape are controlled while the void ratio in the lithium composite oxide is controlled, a better synergistic effect of the electrochemical properties described above can be expected.

[0027] In addition, the conventional methods for controlling the porosity in the lithium composite oxide could improve stability by suppressing side reactions between the lithium composite oxide and the electrolyte, but they were limited in that they were insufficient in terms of improving the particle strength of the lithium composite oxide and suppressing the occurrence of cracks during charging and discharging.

[0028] However, as introduced in the present invention, when the pore area and pore shape are controlled by adjusting the ratio of ammonia and caustic soda used in the coprecipitation reaction, the particle strength and crack generation suppression power of the lithium composite oxide can be improved. Similarly, when the pore area and pore shape in the lithium composite oxide are simultaneously controlled together with the porosity, a synergistic effect on the stability of the lithium composite oxide can be expected. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Hereinafter, a cathode active material according to the present invention, a cathode including the cathode active material, and a lithium secondary battery using the cathode will be described in more detail.

[0030] positive electrode active material According to one aspect of the present invention, there is provided a positive electrode active material including a lithium composite oxide capable of intercalating / deintercalating lithium.

[0031] The lithium composite oxide may be oxide particles having a single crystal or polycrystalline form, but is preferably a particle having a polycrystalline form. The lithium composite oxide having a polycrystalline form means an aggregate including a primary particle and a secondary particle in which a plurality of the primary particles are aggregated.

[0032] The primary particle means a single crystal grain or crystallite, and the secondary particle means an aggregate formed by agglomeration of a plurality of primary particles. A gap and / or a grain boundary may exist between the primary particles constituting the secondary particle.

[0033] For example, the primary particles may be separated from adjacent primary particles within the secondary particle to form internal voids, and the primary particles may contact the internal voids without contacting adjacent primary particles to form grain boundaries, thereby forming a surface present within the secondary particle.

[0034] The surface of the primary particle that is present on the outermost surface of the secondary particle and is exposed to the outside air forms the surface of the secondary particle.

[0035] Here, the average particle diameter D50 of the primary particles is within a range of 0.1 μm to 5 μm, preferably 0.1 μm to 3 μm, so that the optimal density of the positive electrode manufactured using the positive electrode active material according to various embodiments of the present invention can be realized. Also, the average particle diameter D50 of the secondary particles may vary depending on the number of aggregated primary particles, and may be 5 μm to 20 μm.

[0036] Additionally, the primary particles and / or the 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] (where: 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 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 crystal structure containing at least Ni and Co. In addition, the lithium composite oxide is preferably a high-Ni type lithium composite oxide in which x+y+z in 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 having a controlled pore area and pore shape. By using the positive electrode active material including the lithium composite oxide, various electrochemical characteristics, such as capacity characteristics, life characteristics, charge / discharge efficiency characteristics, etc., which are important indicators for evaluating the performance of a lithium secondary battery, can be improved.

[0042] Pore-related indices such as the pore area, pore shape, and porosity of the lithium composite oxide can be measured from a cross-sectional SEM (scanning electron microscope) image of the lithium composite oxide.

[0043] Specifically, when the average particle diameter D50 of the lithium composite oxide is designated 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 may be less than 0.15d, preferably less than 0.137d. In other words, the average value of the major axis length b of the voids is preferably less than 15% of the average particle diameter D50 of the lithium composite oxide. Here, the average particle diameter D50 of the lithium composite oxide refers to the average particle diameter D50 of the lithium composite oxide as secondary particles.

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

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

[0046] On the other hand, when the average value of the major axis length b of the voids exceeds 15% of the average particle diameter D50 of the lithium composite oxide, 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 is in the range of 1 to 3, which is excessively large, and this is disadvantageous to the particle strength of the lithium composite oxide, and cracks may occur in the lithium composite oxide during charging and discharging of the lithium secondary battery. In addition, the excessively large size of the voids in the lithium composite oxide increases the amount of electrolyte impregnated in the voids, which may cause side reactions.

[0047] In addition, it is preferable that the ratio of voids having a ratio b / a of the long axis length b of the void to the short axis length a of the void, which is greater than 3, among all voids observed in a cross-sectional SEM (scanning electron microscope) image of the lithium composite oxide, is less than 50%.

[0048] When the ratio of voids having a ratio b / a of the major axis length b of the void to the minor axis length a of the void exceeds 3, which is greater than 50%, the particle strength of the lithium composite oxide is degraded and cracks may occur in the lithium composite oxide during charging and discharging of the lithium secondary battery. In addition, when the size of the voids in the lithium composite oxide becomes too large, the amount of electrolyte impregnated in the voids increases, which may cause side reactions.

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

[0050] The void occupancy rate in a cross section of the lithium composite oxide observed from a 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% and preferably less than 13.7% of the average particle size D50 of the lithium composite oxide, 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 occupancy rate of the voids in the cross section of the lithium composite oxide (also referred to as porosity) is 0.3 to 3.5%, 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 is within the above-mentioned range.

[0052] In addition, the surface of the lithium composite oxide contained in the positive electrode active material is an area where side reactions with the electrolyte may occur during charging / discharging and / or storage of the lithium secondary battery, and the larger the surface area of ​​the positive electrode active material (which can be expressed as an index called BET specific surface area, for example), the higher the possibility of side reactions, and the phase transformation of the crystal structure within the surface of the positive electrode active material may occur (for example, from layered structure to rock salt structure). Such phase transformation of the crystal structure within the surface of the positive electrode active material has been suggested as one of the causes of the decrease in the life characteristics of lithium secondary batteries.

[0053] As proposed in the present invention, in order to control the pore area and pore shape of the lithium composite oxide, the ratio of ammonia and caustic soda used in the coprecipitation reaction for synthesizing the precursor of the lithium composite oxide is adjusted, and the pore area and pore 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 m. 2 / g.

[0054] In yet another embodiment, the positive electrode active material may further include a coating layer covering at least a portion of a surface of the lithium composite oxide.

[0055] In this case, the coating layer may include an alloy oxide represented by the following Chemical Formula 2. That is, the coating layer may 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] (where: 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] In addition, the coating layer may have a form in which different alloy oxides are simultaneously present in one layer, or different alloy oxides represented by the above Chemical Formula 2 are present in separate layers.

[0059] The alloy oxide represented by Chemical Formula 2 may be in a state of being physically and / or chemically bonded to the lithium composite oxide, and the alloy oxide may exist in a state of forming a solid solution with the lithium composite oxide.

[0060] The alloy oxide is an oxide of lithium and an element represented by M3 combined together, or an oxide of M3. 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 However, the above examples are merely given for the sake of convenience to facilitate understanding, and the oxide defined in the present application is not limited to the above examples.

[0061] In another embodiment, the alloy oxide may be an oxide of lithium and at least two elements represented by M3, or may further include an oxide of lithium and at least two elements represented by M3. The oxide of lithium and at least two elements represented by M3 may be, for example, Li a (W / Ti) b O c , Li a (W / Zr) b O c , Li a (W / Ti / Zr) b O c , Li a (W / Ti / B) b O cIt may be, but is not necessarily limited to, this.

[0062] Here, the alloy oxide may exhibit a concentration gradient that decreases from the surface portion of the secondary particle toward the center portion of the secondary particle, whereby the concentration of the alloy oxide may decrease from the outermost surface of the secondary particle toward the center portion of the secondary particle.

[0063] As described above, the alloy oxide exhibits a concentration gradient that decreases from the surface of the secondary particle toward the center of the secondary particle, so that the residual Li present on the surface of the lithium composite oxide can be further reduced. In addition, the alloy oxide can prevent the crystallinity of the surface 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 an electrochemical reaction.

[0064] Furthermore, the coating layer may include a first coating layer including at least one alloy oxide represented by Chemical Formula 2, and a second coating layer including at least one alloy oxide represented by Chemical Formula 2 and an oxide different from the oxide included in the first coating layer.

[0065] Lithium secondary battery According to yet another aspect of the present invention, a positive electrode may be provided, the positive electrode including 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 a positive electrode active material according to various embodiments of the present invention. Therefore, since the positive electrode active material is the same as that described above, a detailed description thereof will be omitted for convenience, and only the remaining components not described above will be described below.

[0066] The positive electrode current collector is not particularly limited as long as it is conductive without inducing a chemical change in the battery, and may be, for example, stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. The positive electrode current collector may usually have a thickness of 3 to 500 μm, and fine irregularities may be formed on the surface of the current collector to increase the adhesive strength of the positive electrode active material. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0067] The positive electrode active material layer may be prepared by applying a positive electrode slurry composition containing the positive electrode active material, a conductive material, and optionally a binder, to the positive electrode current collector. In this case, the positive electrode active material may be contained in a content of 80 to 99 wt%, more specifically, 85 to 98.5 wt%, based on the total weight of the positive electrode active material layer. When contained in the above content range, excellent capacity characteristics can be exhibited, but the content is not necessarily limited thereto.

[0068] The conductive material is used to impart conductivity to the electrode, and can be used without any particular limitation as long as it has electronic conductivity without causing chemical changes in the battery that is constructed. Specific examples include graphite such as natural graphite and artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fibers; 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; and conductive polymers such as polyphenylene derivatives, and one or more of these may be used alone or in combination. The conductive material may be contained in an amount of 0.1 to 15 wt% based on the total weight of the positive electrode active material layer.

[0069] The binder serves to improve adhesion between the positive electrode active material particles and adhesion between the positive electrode active material and the current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one or more of these may be used alone or in combination. The binder may be included in an amount of 0.1 to 15 wt% based on the total weight of the positive electrode active material layer.

[0070] The positive electrode may be manufactured by a conventional method for manufacturing a positive electrode, except for using the positive electrode active material. Specifically, the positive electrode may be manufactured by dissolving or dispersing the positive electrode active material and, optionally, a binder and a conductive material in a solvent to prepare a positive electrode slurry composition, which is then coated on a positive electrode current collector, followed by drying and rolling.

[0071] The solvent may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and one or more of these may be used alone or in combination. The amount of the solvent used is sufficient to dissolve or disperse the positive electrode active material, conductive material, and binder, and to have a viscosity that allows excellent thickness uniformity when applied to manufacture a positive electrode, taking into consideration the coating thickness of the slurry and the production yield.

[0072] In another embodiment, the positive electrode may be prepared by casting the positive electrode slurry composition on a separate support, peeling the positive electrode slurry composition from the support, and laminating the resulting film on a positive electrode current collector.

[0073] According to yet another aspect of the present invention, there is provided an electrochemical device including the above-mentioned positive electrode. The electrochemical device may be, for example, a battery or a capacitor, and more specifically, a lithium secondary battery.

[0074] Specifically, the lithium secondary battery may include a positive electrode, a negative electrode facing 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 that described above, a detailed description will be omitted for convenience, and only the components not described above will be described in detail below.

[0075] The lithium secondary battery may optionally further include a battery container that houses an electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member that seals 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, and may be, for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or aluminum-cadmium alloy. The negative electrode current collector may generally have a thickness of 3 μm to 500 μm, and like the positive electrode current collector, the surface of the current collector may be formed with fine irregularities to strengthen the binding force of the negative electrode active material. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, or nonwoven fabric.

[0078] The negative electrode active material layer may be prepared by coating a negative electrode slurry composition containing the negative electrode active material, a conductive material, and optionally a binder, on the negative electrode current collector.

[0079] The negative electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium. Specific examples of the negative electrode active material 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, and Al alloys; SiO β (0<β<2), metal oxides capable of doping and dedoping lithium, such as SnO2, vanadium oxide, and lithium vanadium oxide; or composites containing the metallic compounds and carbonaceous materials, such as Si-C composites or Sn-C composites, and any one or a mixture of two or more of these may be used. Also, a thin film of metallic lithium may be used as the negative electrode active material. Also, the carbon material may be low-crystalline carbon or high-crystalline carbon. Representative examples of low crystalline carbon include soft carbon and hard carbon, and representative 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, carbon microbeads, mesophase pitches, and high-temperature fired carbon such as petroleum or coal tar pitch derived cokes.

[0080] The negative electrode active material may be included 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 aids in bonding between the conductive material, the active material, and the current collector, and may be added in an amount 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, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, 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 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 active material layer. Such a conductive material is not particularly limited as long as it has conductivity without inducing a chemical change in the battery, and may be, for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, summer black; conductive fiber such as carbon fiber or metal fiber; metal powder such as carbon fluoride, aluminum, nickel powder; conductive whisker such as zinc oxide, potassium titanate; conductive metal oxide such as titanium oxide; conductive material such as polyphenylene derivatives, etc.

[0083] In one embodiment, the negative electrode active material layer may be prepared by coating a negative electrode slurry composition prepared by dissolving or dispersing a negative electrode active material, and optionally a binder and a conductive material, in a solvent on a negative electrode current collector and drying the coating. Alternatively, the negative electrode slurry composition may be cast on a separate support, and then peeled off from the support to obtain a film, which may be laminated on the negative electrode current collector.

[0084] In another embodiment, the negative electrode active material layer may be prepared by coating a negative electrode slurry composition prepared by dissolving or dispersing a negative electrode active material, and optionally a binder and a conductive material, in a solvent on a negative electrode current collector, and drying the coating. Alternatively, the negative electrode slurry composition may be cast on a separate support, and then peeled off from the support to obtain a film, which may be laminated on the negative electrode current collector.

[0085] In the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. Any separator that is generally used in lithium secondary batteries may be used without any particular limitation. In particular, it is preferable that the separator has low resistance to ion movement of the electrolyte and has excellent electrolyte humidification ability. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof may be used. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of a high-melting point glass fiber, a polyethylene terephthalate fiber, or the like, may be used. In addition, a coated separator containing a ceramic component or a polymeric material may be used to ensure heat resistance or mechanical strength, and may be selectively used in a single-layer or multi-layer structure.

[0086] In addition, examples of the electrolyte used in the present invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the production of lithium secondary batteries, but are not limited to these.

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

[0088] The organic solvent may be used without any particular limitation as long as it can act as a medium through which ions involved in the electrochemical reaction of the battery can move. 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; dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), or propylene carbonate (propylene carbonate). Carbonate-based solvents such as ethylene carbonate (PC); alcohol-based solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (R is a hydrocarbon group having a linear, branched or cyclic structure with 2 to 20 carbon atoms, which may contain a double bond aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes may be used. Among these, carbonate-based solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ion conductivity and high dielectric constant that can improve the charge / discharge performance of the battery and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate) is more preferred. In this case, the cyclic carbonate and the chain carbonate are mixed in a volume ratio of about 1:1 to about 1:9, so that the performance of the electrolyte can be well exhibited.

[0089] The lithium salt may be used without any particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. 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. The concentration of the lithium salt is preferably used within the range of 0.1 to 2.0M. When the concentration of the lithium salt is within the above range, the electrolyte has appropriate conductivity and viscosity, so that 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 such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing the decrease in battery capacity, improving the discharge capacity of the battery, etc. In this case, the additives may be contained in an amount of 0.1 to 5 wt% based on the total weight of the electrolyte.

[0091] As described above, the lithium secondary battery including the positive electrode active material according to the present invention stably exhibits excellent discharge capacity, output characteristics and life characteristics, and is therefore useful in portable devices such as mobile phones, notebook computers and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs).

[0092] The external shape of the lithium secondary battery according to the present invention is not particularly limited, and may be a cylindrical shape using a can, a square shape, a pouch shape, a coin shape, etc. Furthermore, the lithium secondary battery can be used not only as a battery cell used as a power source for a small device, but also suitably as a unit battery for a medium- to large-sized battery module including a large number of battery cells.

[0093] According to yet another aspect of the present invention, there can be provided a battery module including the lithium secondary battery as a unit cell and / or a battery pack including the same.

[0094] The battery module or the battery pack can be used as a power source for one or more medium- to large-sized 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.

[0095] The present invention will be described in more detail below with reference to examples. However, these examples are merely for the purpose of illustrating the present invention, and it is not to be understood that the scope of the present invention is limited by these examples.

[0096] Production Example 1. Production of positive electrode active material (1) Example 1 Spherical Ni was prepared by the co-precipitation method. 0.91 Co 0.08 Mn 0.01 The (OH)2 hydroxide precursor was synthesized.

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

[0098] The pH in the reactor was maintained at 11.5, the temperature of the reactor was maintained at 60°C, and inert gas N2 was introduced into the reactor to prevent the precursor from being oxidized. After the synthesis and mixing was completed, the reactor was washed and dehydrated using a filter press (F / P) equipment, and Ni 0.91 Co 0.08 Mn 0.01 The (OH)2 hydroxide precursor was obtained.

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

[0100] Thereafter, distilled water was added to the lithium composite oxide, and the lithium composite oxide was washed with water for 1 hour. The washed lithium composite oxide was filtered and dried to obtain a positive electrode active material.

[0101] (2) Example 2 A positive electrode active material was prepared in the same manner as in Example 1, except that NaOH was added to the composite transition metal sulfate aqueous solution so that the transition metal concentration was 1.6 M, and NH4OH was added to the composite transition metal sulfate aqueous solution so that the transition metal concentration was 0.4 M.

[0102] (3) Example 3 A positive electrode active material was prepared in the same manner as in Example 1, except that NaOH was added to the composite transition metal sulfate aqueous solution so that the transition metal concentration was 2.2 M, and NH4OH was added to the composite transition metal sulfate aqueous solution so that the transition metal concentration was 1.2 M.

[0103] (4) Example 4 Spherical Ni was prepared by the co-precipitation method. 0.91 Co 0.08 Mn 0.01 The (OH)2 hydroxide precursor was synthesized.

[0104] Specifically, in a 90 L reactor, a 1.5 M composite transition metal sulfate aqueous solution was prepared by mixing nickel sulfate, cobalt sulfate, and manganese sulfate in a molar ratio of 91:8:1. NaOH was added to the composite transition metal sulfate aqueous solution so that the transition metal concentration in the composite transition metal sulfate aqueous solution was 1.6 M, and NH4OH was added to the composite transition metal sulfate aqueous solution so that the transition metal concentration in the composite transition metal sulfate aqueous solution was 0.4 M.

[0105] The pH in the reactor was maintained at 11.5, the reactor temperature was maintained at 60°C, and inert gas N2 was introduced into the reactor to prevent the precursor from being oxidized. After the synthesis and mixing was completed, the precursor was washed and dehydrated using a filter press (F / P) equipment, and Ni 0.91 Co 0.08 Mn 0.01 The (OH)2 hydroxide precursor was obtained.

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

[0107] Thereafter, distilled water was added to the lithium composite oxide, and the lithium composite oxide was washed with water for 1 hour. The washed lithium composite oxide was filtered and dried to obtain a positive electrode active material.

[0108] (5) Example 5 A positive electrode active material was prepared in the same manner as in Example 1, except that before washing the lithium composite oxide with water, 1 mol % of Al2O3, 0.25 mol % of TiO2, and 0.05 mol % of ZrO2 were mixed with the lithium composite oxide, and the mixture was further heat-treated for 10 hours by increasing the temperature to 680° C. at a rate of 2° C. per minute 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 A positive electrode active material was prepared in the same manner as in Example 1, except that the (OH)2 hydroxide precursor was used.

[0110] (7) Comparative Example 1 A positive electrode active material was prepared in the same manner as in Example 1, except that NaOH was added to the composite transition metal sulfate aqueous solution so that the transition metal concentration was 1.2 M, and NH4OH was added to the composite transition metal sulfate aqueous solution so that the transition metal concentration was 0.3 M.

[0111] (8) Comparative Example 2 A positive electrode active material was prepared in the same manner as in Example 1, except that NaOH was added to the composite transition metal sulfate aqueous solution so that the transition metal concentration was 2.5 M, and NH4OH was added to the composite transition metal sulfate aqueous solution so that the transition metal concentration was 1.5 M.

[0112] (9) Comparative Example 3 A positive electrode active material was prepared in the same manner as in Example 1, except that NaOH was added to the composite transition metal sulfate aqueous solution so that the transition metal concentration was 1.8 M, and NH4OH was added to the composite transition metal sulfate aqueous solution so that the transition metal concentration was 1.5 M.

[0113] (10) Comparative Example 4 A positive electrode active material was prepared in the same manner as in Example 1, except that NaOH was added to the composite transition metal sulfate aqueous solution so that the transition metal concentration was 2.5 M, and NH4OH was added to the composite transition metal sulfate aqueous solution so that the transition metal concentration was 0.4 M.

[0114] (11) Comparative Example 5 A positive electrode active material was prepared in the same manner as in Example 1, except that NaOH was added to the composite transition metal sulfate aqueous solution so that the transition metal concentration was 1.2 M, and NH4OH was added to the composite transition metal sulfate aqueous solution so that the transition metal concentration was 1.5 M.

[0115] Manufacturing Example 2. Manufacturing of lithium secondary batteries A positive electrode slurry was prepared by dispersing 92 wt% of the positive electrode active material prepared in Preparation Example 1, 4 wt% of artificial graphite, and 4 wt% of PVDF binder in 30 g of N-methyl-2-pyrrolidone (NMP). The positive electrode slurry was uniformly applied to a thin aluminum film having a thickness of 15 μm and dried in a vacuum at 135° C. to prepare a positive electrode for a lithium secondary battery.

[0116] A coin battery was fabricated using a lithium foil as a counter electrode for the positive electrode, a porous polyethylene film (Celgard 2300, thickness: 25 μm) as a separator, and an electrolyte of LiPF6 at a concentration of 1.15 M in a solvent of ethylene carbonate and ethyl methyl carbonate mixed in a volume ratio of 3:7.

[0117] Experimental Example 1. SEM analysis of positive electrode active material A cross-sectional SEM image of the positive active material prepared in Preparation Example 1 was taken to confirm the void characteristics in the cross section of the lithium composite oxide contained in the positive active material.

[0118] Specifically, the lithium composite oxide contained in the positive electrode active material was processed by using an FIB (Ga-ion source) to obtain a cross-sectional SEM image, and the average particle diameter D50 of the lithium composite oxide, the average void major axis length b, the average void minor axis length a, the average b / a, the average void area, and the void occupancy rate were measured from the cross-sectional SEM image. The measurement results are shown in Table 1 below.

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

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

[0121] Experimental Example 2: Measurement of particle strength of positive electrode active material When manufacturing a positive electrode for a lithium secondary battery using a positive electrode active material, a process is carried out in which a slurry containing the positive electrode active material is applied onto a positive electrode current collector, followed by drying and rolling (pressing). During this process, when the positive electrode active material is rolled under high pressure, the particles of the positive electrode active material applied onto the positive electrode current collector may break down, resulting in a decrease in the performance of the positive electrode active material.

[0122] In this experimental example, in order to confirm the change in strength of the positive electrode active material depending on the composition of the aggregate of multiple secondary particles contained in the positive electrode active material, the positive electrode active materials prepared according to Examples 1 to 3 and Comparative Examples 1, 2, and 5 were dried in a vacuum oven at 60° C. for 12 hours, and then one particle (lithium composite oxide) corresponding to D50 was selected and the breaking strength of the particle (the pressure when the particle breaks) was measured.

[0123] The breaking strength of each positive electrode active material was measured 10 times, and the average value of the measured values ​​is shown in Table 3 below.

[0124] [Table 3]

[0125] Referring to the results in Table 3, the positive electrode active materials according to Examples 1 to 3 were measured to have slightly lower breaking strengths than the positive electrode active material according to Comparative Example 1 (porosity: 0.1%) in which almost no voids exist in the positive electrode active material, but were found to have improved breaking strengths compared to the positive electrode active materials according to Comparative Examples 2 and 5. In particular, when comparing the positive electrode active material according to Example 3 with the positive electrode active material according to Comparative Example 5, it was found that the positive electrode active material according to Example 3 had a higher breaking strength despite having the same porosity.

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

[0127] Experimental Example 3. Evaluation of capacity and life characteristics of lithium secondary batteries A charge-discharge experiment was carried out on the lithium secondary battery (coin cell) manufactured in Manufacturing Example 2 using an electrochemical analyzer (Toyo, Toscat-3100) at 25° C., voltage range of 3.0 V to 4.3 V, and discharge rate of 0.1 C to measure the charge and discharge capacities.

[0128] In addition, the same lithium secondary battery was charged and discharged 50 times at 25°C and within a driving voltage range of 3.0 V to 4.4 V at 1 C / 1 C, and then the ratio of the discharge capacity at the 50th cycle to the initial capacity (cycle capacity retention) was measured.

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

[0130] [Table 4]

[0131] Referring to the results of Table 4, it can be seen that the electrochemical properties of the positive electrode active material can be further improved 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 co-precipitation reaction for synthesizing the precursor, rather than simply controlling the porosity in the lithium composite oxide included in the positive electrode active material.

[0132] Although the embodiments of the present invention have been described above, a person having ordinary knowledge in the art may modify and change the present invention in various ways by adding, changing, deleting or adding elements without departing from the spirit of the present invention as described in the claims, and this can also be said to be included in the scope of the present invention.

Claims

1. The lithium composite oxide is represented by the following formula 1 and is capable of lithium intercalation / deintercalation, [Chemical formula 1] Li ｗ Ni １－（ｘ＋ｙ＋ｚ） Co ｘ M1 ｙ M2 ｚ O ２＋α (where: 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 different elements, 0.5≦w≦1.5, 0≦x≦0.50, 0≦y≦0.20, 0≦z≦0.20, 0≦α≦0.02) an average value of a ratio b / a of a major axis length b of the voids to a minor axis length a of the voids observed from a cross-sectional SEM (scanning electron microscope) image of the lithium composite oxide is 1 to 3; The positive electrode active material, wherein the void occupancy rate in a cross section of the lithium composite oxide observed from a cross-sectional SEM (scanning electron microscope) image of the lithium composite oxide is 1.2 to 2.9%.

2. 2. The positive electrode active material according to claim 1, wherein, when the average particle diameter D50 of the lithium composite oxide is referred to 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. 2. The positive electrode active material according to claim 1, wherein a ratio of voids having a major axis length b of the void to a minor axis length a of the void, b / a, exceeds 3, among all voids observed from a cross-sectional SEM (scanning electron microscope) image of the lithium composite oxide, is less than 50%.

4. The average area of ​​voids observed from a cross-sectional SEM (scanning electron microscope) image of the lithium composite oxide is 0.02 to 1.0 μm ２ The positive electrode active material according to claim 1 ,

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

6. The BET specific surface area of ​​the lithium composite oxide is 0.2 to 2.0 m ２ The positive electrode active material according to claim 1 , wherein the molecular weight of the positive electrode active material is 1 / g.

7. LiOH and Li based on the total weight of the positive electrode active material ２ CO ３ The positive electrode active material according to claim 1 , wherein the total content is 1.0% by weight or less.

8. The positive electrode active material according to claim 1 , further comprising an alloy oxide represented by the following formula 2 on at least a portion of a surface of the lithium composite oxide: [Chemical formula 2] Li ａ M3 ｂ O ｃ (where: 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)

9. A positive electrode comprising the positive electrode active material according to claim 1 .

10. A lithium secondary battery using the positive electrode according to claim 9.

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