Positive electrode active material for lithium secondary batteries and lithium secondary batteries containing the same

Abstract

To provide a positive electrode active material for a lithium secondary battery with improved life and capacity characteristics, and a lithium secondary battery including the same.SOLUTION: A positive electrode active material for a lithium secondary battery according to an embodiment of the present invention includes lithium-transition metal composite oxide particles containing a plurality of primary particles, and the lithium-transition metal composite oxide particles include lithium-potassium-containing portion formed between the primary particles, and can prevent the deformation of a layered structure of the primary particles, remove the residual lithium, and improve the life characteristics and capacity characteristics.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a positive electrode active material for lithium secondary batteries and a method for producing the same. More specifically, it relates to a lithium metal oxide-based positive electrode active material for lithium secondary batteries and a method for producing the same. [Background technology]

[0002] Rechargeable batteries are batteries that can be repeatedly charged and discharged, and with the development of the information and communication and display industries, they are widely used as power sources for portable electronic communication devices such as camcorders, mobile phones, and laptop computers. More recently, battery packs containing rechargeable batteries have also been developed and applied as power sources for environmentally friendly vehicles such as hybrid cars.

[0003] Examples of rechargeable batteries include lithium-ion batteries, nickel-cadmium batteries, and nickel-metal hydride batteries. Among these, lithium-ion batteries are being actively developed due to their high operating voltage and energy density per unit weight, as well as advantages in charging speed and weight reduction.

[0004] A lithium secondary battery may include an electrode assembly comprising a positive electrode, a negative electrode, and a separator, and an electrolyte that impregnates the electrode assembly. The lithium secondary battery may further include an outer packaging material, for example, a pouch, that houses the electrode assembly and the electrolyte.

[0005] Lithium-transition metal composite oxides can be used as the positive electrode active material for lithium secondary batteries. Examples of such lithium-transition metal composite oxides include nickel-based lithium metal oxides.

[0006] As the range of applications for lithium-ion secondary batteries expands, there is a growing demand for longer lifespan, higher capacity, and greater operational stability. In lithium-transition metal composite oxides used as positive electrode active materials, if chemical structural inhomogeneities occur due to lithium deposition or other factors, it can be difficult to realize lithium-ion secondary batteries with the desired capacity and lifespan. Furthermore, if deformation or damage occurs to the lithium-transition metal composite oxide structure during repeated charge-discharge cycles, lifespan stability and capacity maintenance characteristics may deteriorate.

[0007] For example, Korean Patent Publication No. 10-0821523 discloses a method for removing lithium salt impurities by washing lithium-transition metal composite oxides with water, but there are limitations to sufficient impurity removal, and the water washing process can cause damage to the particle surface. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Korean Patent Publication No. 10-0821523 [Overview of the project] [Problems that the invention aims to solve]

[0009] The object of the present invention is to provide a positive electrode active material for lithium secondary batteries with improved operational stability and electrochemical properties, and a method for producing the same.

[0010] The object of the present invention is to provide a lithium secondary battery with improved operational stability and electrochemical properties. [Means for solving the problem]

[0011] The positive electrode active material for a lithium secondary battery according to an embodiment of the present invention comprises lithium-transition metal composite oxide particles containing a plurality of primary particles, wherein the lithium-transition metal composite oxide particles include lithium-potassium-containing portions formed between the primary particles.

[0012] In some embodiments, the lithium-potassium-containing part may include a lithium-potassium-sulfur-containing part containing lithium, potassium, and sulfur.

[0013] In some embodiments, the primary particles may have a hexagonal close-packed structure.

[0014] In some embodiments, the lithium-transition metal composite oxide particles may not include primary particles having a face centered cubic structure.

[0015] In some embodiments, the sulfur content of the lithium-transition metal composite oxide particles measured by a carbon-sulfur analyzer may be 1,100 to 4,500 ppm with respect to the total weight of the lithium-transition metal composite oxide particles.

[0016] In some embodiments, the potassium concentration of the lithium-potassium-containing part measured by Energy Dispersive Spectroscopy (EDS) may be greater than the potassium concentration in the primary particles measured by the EDS.

[0017] In some embodiments, the average value of the potassium signal of the lithium-potassium-containing part measured by the EDS may be 1.2 to 4 times the average value of the potassium signal in the primary particles measured by the EDS.

[0018] In some embodiments, the content of lithium carbonate (Li2CO3) remaining on the surface of the lithium-transition metal composite oxide particles is 2,500 ppm or less, and the content of lithium hydroxide (LiOH) remaining on the surface of the lithium-transition metal composite oxide particles may be 2,500 ppm or less.

[0019] A method for producing a positive electrode active material for a lithium secondary battery according to an embodiment of the present invention includes the steps of: preparing preliminary lithium-transition metal composite oxide particles; mixing the preliminary lithium-transition metal composite oxide particles with an aqueous potassium compound solution; and heat-treating the mixed preliminary lithium-transition metal composite oxide particles and the aqueous potassium compound solution to form lithium-transition metal composite oxide particles including a plurality of primary particles and lithium-potassium-containing portions formed between the primary particles.

[0020] In some embodiments, the aqueous potassium compound solution is formed by mixing a solvent and potassium compound powder, and the amount of potassium compound powder added may be 0.2 to 1.9% by weight relative to the total weight of the pre-lithium-transition metal composite oxide particles.

[0021] In some embodiments, the amount of solvent added may be 2 to 15% by weight relative to the total weight of the pre-lithium-transition metal composite oxide particles.

[0022] In some embodiments, the potassium compound powder may be potassium bisulfate (KHSO4) powder.

[0023] In some embodiments, the heat treatment can be carried out at 200-400°C in an oxygen atmosphere.

[0024] In some embodiments, the preliminary lithium-transition metal composite oxide particles can be mixed with the aqueous potassium compound without washing.

[0025] A lithium secondary battery according to an embodiment of the present invention includes a positive electrode comprising a positive electrode active material layer containing the positive electrode active material for lithium secondary batteries according to the above embodiment, and a negative electrode facing the positive electrode. [Effects of the Invention]

[0026] The positive electrode active material according to embodiments of the present invention may include lithium-transition metal composite oxide particles comprising a plurality of primary particles, and the lithium-transition metal composite oxide particles may include lithium-potassium-containing portions formed between the primary particles. In this case, residual lithium located on the surface of the lithium-transition metal composite oxide reacts with a potassium-containing compound and is converted into the lithium-potassium-containing portions, thereby improving the initial capacity and battery efficiency characteristics.

[0027] In some embodiments, by forming lithium-potassium-containing regions having a hexagonal close-packed structure between primary particles in a lithium-transition metal composite oxide, the surface of the primary particles is protected by the lithium-potassium-containing regions, thereby improving lifetime characteristics and driving stability.

[0028] In the method for producing a positive electrode active material according to an embodiment of the present invention, a potassium compound aqueous solution can be prepared by mixing a solvent in an amount of 2 to 15% by weight relative to the total weight of the prelithium-transition metal composite oxide particles with a potassium compound powder in an amount of 0.2 to 1.9% by weight relative to the total weight of the prelithium-transition metal composite oxide particles, without including a water washing step. The potassium compound aqueous solution can be mixed with the prelithium-transition metal composite oxide particles.

[0029] In this case, it is possible to prevent the primary particle structure of the lithium-transition metal composite oxide particles from changing from a hexagonal close-packed structure to a face-centered cubic structure during the water washing process. This prevents a decrease in the initial capacity and life characteristics of the secondary battery. In addition, residual lithium located on the surface of the lithium-transition metal composite oxide particles and between the primary particles is removed, preventing a decrease in life characteristics due to gas generation, and reducing battery resistance, thereby improving the initial capacity. [Brief explanation of the drawing]

[0030] [Figure 1] Figure 1 is a process flowchart illustrating a method for producing a positive electrode active material according to an exemplary embodiment. [Figure 2] Figure 2 is a schematic plan view showing a lithium secondary battery according to an exemplary embodiment. [Figure 3] Figure 3 is a schematic cross-sectional view showing a lithium secondary battery according to an exemplary embodiment. [Figure 4] Figure 4 is an HR-TEM image of lithium-transition metal composite oxide particles according to Example 1. [Figure 5] Figure 5 shows an HR-TEM image of lithium-transition metal composite oxide particles according to Comparative Example 1. [Figure 6] Figure 6 shows the FFT images of region A in Figure 4(b) and region B in Figure 5(b). [Figure 7] Figure 7 is a graph showing the potassium signal values ​​in the primary particle region and the inter-primary particle region (e.g., lithium-potassium-containing region) of Examples 1 to 5. [Modes for carrying out the invention]

[0031] Embodiments of the present invention provide a positive electrode active material containing lithium-transition metal composite oxide particles and a lithium secondary battery containing the same.

[0032] The embodiments of the present invention will be described in detail below, but these are merely illustrative examples and the present invention is not limited to any particular embodiment.

[0033] In exemplary embodiments, the positive electrode active material may include lithium-transition metal composite oxide particles comprising a plurality of primary particles, and the lithium-transition metal composite oxide particles may include lithium-potassium (Li-K) containing portions formed between the primary particles.

[0034] In some embodiments, the primary particles may include a crystallographic single-crystal or polycrystalline structure.

[0035] For example, the primary particles may contain nickel (Ni) and further contain at least one of cobalt (Co) or manganese (Mn).

[0036] For example, the primary particle can be represented by the following chemical formula 1.

[0037] [ka]

[0038] In chemical formula 1, a may be 0.9 ≤ a ≤ 1.2, x may be 0.5 ≤ x ≤ 0.99, and y may be -0.1 ≤ z ≤ 0.1. M can represent one or more elements selected from Na, Mg, Ca, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Co, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, Ba, or Zr.

[0039] In some preferred embodiments, the molar ratio or concentration x of Ni in chemical formula 1 may be 0.8 or greater.

[0040] For example, if a nickel-high (High-Ni) composition with x of 0.8 or more is adopted, the firing of lithium-transition metal composite oxide particles can be performed at a relatively low temperature. In this case, the amount of residual lithium generated on the surface of the lithium-transition metal composite oxide particles may increase. Therefore, a water washing process or a non-water washing process (e.g., initial wetting method) can be performed to remove it. Thus, if x is 0.8 or more, the aforementioned process for removing residual lithium becomes substantially significant.

[0041] Ni can be provided as a transition metal related to the output and capacity of lithium secondary batteries. As described above, by employing a high-Ni composition in the lithium-transition metal composite oxide particles, a high-output cathode and a high-output lithium secondary battery can be provided.

[0042] However, increasing the Ni content can relatively decrease the long-term storage stability and lifespan stability of the positive electrode or secondary battery. In this regard, according to exemplary embodiments, by including Co, electrical conductivity can be maintained, while by including Mn, lifespan stability and capacity maintenance characteristics can be improved.

[0043] In some embodiments, the lithium-potassium-containing portion may include a lithium-potassium-sulfur (Li-KS)-containing portion containing lithium, potassium, and sulfur (S). For example, the lithium-potassium-containing portion may include LiKSO4. In this case, the excellent conductivity of LiKSO4 can improve the output characteristics of the secondary battery.

[0044] In some embodiments, the primary particles of the lithium-transition metal composite oxide can have a hexagonal close-packed structure. This allows for the inclusion of many lithium and transition metal elements in a stable, layered structure even in a small space, thereby improving the capacity and lifespan characteristics of the secondary battery.

[0045] In some embodiments, the potassium concentration in the lithium-potassium-containing region, as measured by energy dispersive spectroscopy (EDS), may be greater than the potassium concentration in the primary particles, as measured by EDS. In this case, the lithium-transition metal composite oxide particles can form a concentration gradient across the primary particles and the lithium-potassium-containing region.

[0046] For example, the average potassium signal value of the lithium-potassium-containing portion measured by the EDS may be 1.2 to 4 times the average potassium signal value in the primary particles.

[0047] When the ratio of the average potassium signal satisfies the aforementioned range, a lithium-potassium-containing portion having a hexagonal close-packed structure can be sufficiently formed between the primary particles contained in the lithium-transition metal composite oxide particles. In this case, the surface of the primary particles is protected by the lithium-potassium-containing portion, and the area of ​​the primary particles exposed to the electrolyte can be reduced. This improves the life characteristics of the secondary battery. Furthermore, since residual lithium on the surface of the lithium-transition metal composite oxide particles is sufficiently removed, the electrochemical properties of the secondary battery can be improved.

[0048] In some embodiments, the content of lithium precursors remaining on the surface of lithium-transition metal composite oxide particles can be adjusted.

[0049] For example, the content of lithium carbonate (Li2CO3) remaining on the surface of lithium-transition metal composite oxide particles may be 2,500 ppm or less, and the content of lithium hydroxide (LiOH) remaining on the surface of lithium-transition metal composite oxide particles may be 2,500 ppm or less.

[0050] When the content of lithium carbonate and lithium hydroxide meets the above range, the resistance during lithium ion movement is reduced, improving the initial capacity characteristics and output characteristics of the lithium secondary battery, and improving the lifespan characteristics during repeated charge and discharge cycles.

[0051] In some embodiments, the sulfur content in the lithium-transition metal composite oxide particles may be 1,100 to 4,500 ppm relative to the total weight of the lithium-transition metal composite oxide particles. For example, the lithium-sulfur compound present on the surface of the lithium-transition metal composite oxide particles not only protects the particle surface from the electrolyte but can also favorably facilitate the movement of lithium ions between the electrolyte and the surface. In this case, along with the potassium compound described later, residual lithium can be sufficiently removed while preventing a decrease in capacity characteristics and lifespan characteristics due to excessive potassium addition. This makes it possible to maintain output characteristics while improving the capacity retention rate of the secondary battery.

[0052] For example, the sulfur content can be measured using a carbon-sulfur analyzer (CS analyzer).

[0053] Figure 1 is a process flowchart illustrating a method for producing a positive electrode active material according to an exemplary embodiment.

[0054] The following provides a manufacturing method for the positive electrode active material for lithium secondary batteries according to an exemplary embodiment described above, with reference to Figure 1.

[0055] Referring to Figure 1, preliminary lithium-transition metal composite oxide particles can be prepared (e.g., step S10).

[0056] For example, a transition metal precursor can be reacted with a lithium precursor to produce pre-lithium-transition metal composite oxide particles. The transition metal precursor (e.g., Ni-Co-Mn precursor) can be produced by a coprecipitation reaction.

[0057] For example, transition metal precursors can be produced by the coprecipitation reaction of metal salts. The metal salts may include nickel salts, manganese salts, and cobalt salts.

[0058] Examples of nickel salts include nickel sulfate, nickel hydroxide, nickel nitrate, nickel acetate, and their hydrates. Examples of manganese salts include manganese sulfate, manganese acetate, and their hydrates. Examples of cobalt salts include cobalt sulfate, cobalt nitrate, cobalt carbonate, and their hydrates.

[0059] The metal salt can be mixed with a precipitating agent and / or chelating agent in proportions that satisfy the content or concentration ratio of each metal as described with reference to Chemical Formula 1 to prepare an aqueous solution. The aqueous solution can be co-precipitated in a reactor to produce a transition metal precursor.

[0060] The precipitating agent may include alkaline compounds such as sodium hydroxide (NaOH) and sodium carbonate (Na2CO3). The chelating agent may include, for example, aqueous ammonia (e.g., NH3H2O) and ammonium carbonate (e.g., NH3HCO3).

[0061] The temperature of the coprecipitation reaction can be adjusted, for example, within a range of approximately 40°C to 60°C. The reaction time can be adjusted within a range of approximately 24 to 72 hours.

[0062] The lithium precursor compound may include, for example, lithium carbonate, lithium nitrate, lithium acetate, lithium oxide, or lithium hydroxide. These can be used individually or in combination of two or more.

[0063] In an exemplary embodiment, the preliminary lithium-transition metal composite oxide particles can be mixed with an aqueous potassium compound solution (for example, step S20).

[0064] In some embodiments, the aqueous solution of the potassium compound may include a solvent and a potassium compound powder that is added to the solvent.

[0065] For example, the potassium compound powder can be added in an amount of 0.2 to 1.9% by weight relative to the total weight of the reserve lithium-transition metal composite oxide particles. In this case, the residual lithium and potassium compound can react sufficiently while preventing a decrease in capacity and lifetime characteristics due to excessive addition of the potassium compound. This makes it possible to realize a cathode active material with an appropriate sulfur content and excellent lifetime and capacity characteristics.

[0066] For example, the solvent can be used in an amount of 2 to 15% by weight relative to the total weight of the reserve lithium-transition metal composite oxide particles. In this case, the potassium compound powder can be sufficiently dissolved while preventing changes in the layered structure of the primary particles due to excessive solvent addition. This makes it possible to maintain capacity characteristics and output characteristics while improving lifetime characteristics.

[0067] In some embodiments, potassium compound powder can be added to a solvent in such a concentration of 50% by weight or less relative to the solvent weight to prepare an aqueous potassium compound solution. When potassium compound powder and solvent are added within this range, the potassium compound powder can be sufficiently dissolved in the solvent while the residual lithium reacts sufficiently with the potassium compound, thereby improving processability.

[0068] In some embodiments, the potassium compound powder may be potassium bisulfate (KHSO4) powder. In this case, the aqueous solution of the potassium compound may be an aqueous solution of KHSO4.

[0069] For example, the solvent may be deionized water (DIW).

[0070] In an exemplary embodiment, pre-lithium-transition metal composite oxide particles can be mixed with the aqueous potassium compound solution. In this case, the potassium and / or sulfur contained in the aqueous potassium compound solution can react with residual lithium present on the surface of the pre-lithium-transition metal composite oxide particles to be converted into lithium-potassium-containing portions (e.g., lithium-potassium-sulfur-containing portions). This makes it possible to obtain lithium-transition metal composite oxide particles containing primary particles and lithium-potassium-containing portions.

[0071] For example, the mixing step can remove impurities present on the surface of the pre-lithium-transition metal composite oxide particles. For example, an excess of lithium precursor (lithium salt) may be used to improve the yield of lithium metal oxide particles or to stabilize the synthesis process. In this case, lithium precursors containing lithium hydroxide (LiOH) and lithium carbonate (Li2CO3) may remain on the surface of the pre-lithium-transition metal composite oxide particles.

[0072] Furthermore, for example, lithium-transition metal composite oxide particles containing a higher Ni composition can be fired at a lower temperature during cathode manufacturing. In this case, the residual lithium content on the surface of the lithium-transition metal composite oxide particles may increase.

[0073] When the residual lithium is removed by washing with substantially the same amount of water as the positive electrode active material (water washing treatment), the residual lithium can be removed, but oxidation of the surface of the preliminary lithium-transition metal composite oxide particles and side reactions with water may occur, causing damage or collapse of the layered structure of the primary particles. In addition, the layered structure may be changed by water to a face-centered cubic structure, spinel structure, and / or rock salt structure instead of a hexagonal close-packed structure, and the lithium-nickel oxide may be hydrolyzed, generating nickel impurities such as NiO or Ni(OH)2.

[0074] In contrast, according to exemplary embodiments of the present invention, a mixing step (e.g., an initial wetting method) is performed using an aqueous solution of a potassium compound without a water washing treatment, so that a mixing step is performed and passivation by a potassium-containing compound can be achieved on the surface of lithium-transition metal composite oxide particles. For example, lithium-potassium-containing regions in which lithium and potassium are bonded can be formed between primary particles containing a hexagonal close-packed structure.

[0075] As used in this invention, the term "initial wetting method" may mean, for example, a method in which a washing treatment is not performed by adding a substantially equal or similar amount of water to the total weight of the lithium-transition metal composite oxide particles and stirring, but rather a method in which water or an aqueous potassium compound solution is added in an amount of 15% by weight or less relative to the total weight of the lithium-transition metal composite oxide particles, for example by spraying.

[0076] Furthermore, since no water washing is performed, the lithium-transition metal composite oxide particles do not need to contain primary particles with a face-centered cubic structure. This prevents oxidation of the particle surface by water and damage to the layered structure, while effectively removing residual lithium.

[0077] For example, when potassium compound powder is directly mixed with lithium-transition metal composite oxide particles instead of an aqueous potassium compound solution, the potassium compound powder lacks capillary force and therefore cannot penetrate between primary particles. As a result, most of the potassium compound powder can react with residual lithium present on the surface of secondary particles formed by the aggregation of primary particles. For instance, the lithium-potassium content may be formed as a coating on the surface of the secondary particles. In this case, the surface of the primary particles may not be adequately protected during electrolyte impregnation, and residual lithium may remain on the surface between primary particles, increasing battery resistance. Consequently, the battery's capacity and output characteristics may decrease.

[0078] According to exemplary embodiments of the present invention, as described above, the initial wetting method can be performed using an aqueous potassium compound solution. In this case, the aqueous potassium compound solution penetrates between primary particles by capillary force and reacts with residual lithium between primary particles to form lithium-potassium-containing regions between primary particles.

[0079] In some embodiments, the potassium compound content in the aqueous potassium compound solution may be 0.1 to 2% by weight relative to the total weight of the pre-lithium-transition metal composite oxide particles. In this case, the lithium-potassium content is formed on the surface of the pre-lithium-transition metal composite oxide particles and at the locations where residual lithium was present between the primary particles, while preventing damage or collapse of the layered structure of the primary particles, as would occur in a substantially water-washing process.

[0080] After the mixing step, a heat treatment (calcination) step is performed to obtain a positive electrode active material containing primary particles and lithium-potassium-containing portions (for example, step S30).

[0081] For example, the pre-mixed lithium-transition metal composite oxide particles and lithium-potassium-containing portion can be heat-treated using a firing furnace. This makes it possible to obtain lithium-transition metal composite oxide particles in which the lithium-potassium-containing portion is fixed between the primary particles.

[0082] For example, the heat treatment can be carried out at 200-400°C in an oxygen atmosphere. In this case, the residual lithium on the surface of the preliminary lithium-transition metal composite oxide particles and the potassium compound in the potassium compound aqueous solution can bond sufficiently to form a lithium-potassium-containing portion.

[0083] Figures 2 and 3 are schematic plan and cross-sectional views, respectively, of a lithium secondary battery according to an exemplary embodiment.

[0084] In the following, with reference to Figures 2 and 3, a lithium secondary battery including a positive electrode containing the positive electrode active material for lithium secondary batteries described above is provided.

[0085] Referring to Figures 2 and 3, the lithium secondary battery may include a positive electrode 100 containing the lithium-potassium-containing portion described above, a negative electrode 130, and a separation membrane 140.

[0086] The positive electrode 100 may include a positive electrode active material layer 110 formed by coating the positive electrode active material containing the aforementioned lithium-transition metal oxide particles onto the positive electrode current collector 105.

[0087] For example, a slurry can be produced by mixing and stirring pre-lithium-transition metal oxide particles, which have been mixed with an aqueous potassium compound solution, with a binder, conductive material, and / or dispersant in a solvent. After coating a positive electrode current collector 105 with the slurry, the positive electrode can be produced by compressing and drying it.

[0088] The positive electrode current collector 105 can include, for example, stainless steel, nickel, aluminum, titanium, copper, or alloys thereof, and preferably aluminum or an aluminum alloy.

[0089] The binder may include, for example, organic binders such as vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, or polymethyl methacrylate, or aqueous binders such as styrene-butadiene rubber (SBR), and can be used together with a thickener such as carboxymethyl cellulose (CMC).

[0090] For example, a PVDF-based binder can be used as the binder for forming the positive electrode. In this case, the amount of binder required to form the positive electrode active material layer 110 can be reduced, and the amount of positive electrode active material can be relatively increased. This makes it possible to improve the output and capacity of the secondary battery.

[0091] The conductive material may be included to facilitate the transfer of electrons between active material particles. For example, the conductive material may include carbon-based conductive materials such as graphite, carbon black, graphene, and carbon nanotubes, and / or metallic conductive materials such as tin, tin oxide, titanium oxide, and perovskite materials such as LaSrCoO3 and LaSrMnO3.

[0092] The negative electrode 130 may include a negative electrode current collector 125 and a negative electrode active material layer 120 formed by coating the negative electrode current collector 125 with a negative electrode active material.

[0093] The negative electrode active material can be any material known in the art that can intercept and deintercept lithium ions, without any particular limitations. For example, carbon-based materials such as crystalline carbon, amorphous carbon, carbon composites, and carbon fibers; lithium alloys; silicon or tin can be used. Examples of amorphous carbon include hard carbon, coke, mesocarbon microbeads (MCMB) fired at 1500°C or below, and mesophase pitch-based carbon fiber (MPCF). Examples of crystalline carbon include graphite-based carbon such as natural graphite, graphitized coke, graphitized MCMB, and graphitized MPCF. Elements contained in the lithium alloy include aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, or indium.

[0094] The negative electrode current collector 125 can include, for example, gold, stainless steel, nickel, aluminum, titanium, copper, or alloys thereof, and preferably, copper or a copper alloy.

[0095] In some embodiments, the negative electrode active material can be mixed and stirred in a solvent with a binder, a conductive material and / or a dispersant to produce a slurry. After coating the negative electrode current collector with the slurry, the negative electrode 130 can be produced by compressing and drying it.

[0096] The binder and conductive material can be substantially the same as or similar to the materials described above. In some embodiments, the binder for forming the negative electrode may include an aqueous binder such as styrene-butadiene rubber (SBR) for compatibility with the carbon-based active material, and can be used together with a thickener such as carboxymethyl cellulose (CMC).

[0097] A separation membrane 140 can be interposed between the positive electrode 100 and the negative electrode 130. The separation membrane 140 may include a porous polymer film made from polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, or ethylene / methacrylate copolymer. The separation membrane 140 may also include a nonwoven fabric formed from high-melting-point glass fibers, polyethylene terephthalate fibers, etc.

[0098] According to an exemplary embodiment, an electrode cell is defined by a positive electrode 100, a negative electrode 130, and a separator membrane 140, and a plurality of such electrode cells can be stacked to form an electrode assembly 150, for example, in the form of a jelly roll. For example, the electrode assembly 150 can be formed by winding, laminating, or folding the separator membrane 140.

[0099] A lithium secondary battery can be defined by housing the electrode assembly together with the electrolyte in the outer case 160. According to an exemplary embodiment, a non-aqueous electrolyte can be used as the electrolyte.

[0100] The non-aqueous electrolyte contains a lithium salt as an electrolyte and an organic solvent. The lithium salt is represented by, for example, Li + X - . Examples of the anion (X - ) of the lithium salt include F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - , and (CF3CF2SO2)2N - etc. can be exemplified.

[0101] Examples of the organic solvents that can be used include propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite, and tetrahydrofuran. These can be used individually or in combination of two or more.

[0102] As shown in Figure 3, electrode tabs (positive electrode tab and negative electrode tab) can protrude from the positive electrode current collector 105 and the negative electrode current collector 125 belonging to each electrode cell and extend to one side of the outer casing 160. These electrode tabs can be fused together with the aforementioned side of the outer casing 160 to form electrode leads (positive electrode lead 107 and negative electrode lead 127) that extend or are exposed outside the outer casing 160.

[0103] The aforementioned lithium secondary battery can be manufactured, for example, in the form of a cylindrical, rectangular, pouch, or coin-shaped container using a can.

[0104] According to exemplary embodiments, doping or coating with a potassium-containing compound improves the chemical stability of the positive electrode active material, enabling the realization of a lithium secondary battery with improved lifespan and long-term stability while suppressing a decrease in capacity and average voltage.

[0105] The following are preferred embodiments to aid in understanding the present invention, but these embodiments are merely illustrative and do not limit the scope of the appended claims. It will be apparent to those skilled in the art that various changes and modifications can be made to these embodiments within the scope of the present invention and the technical concept, and that these variations and modifications will naturally fall within the scope of the appended claims.

[0106] Example 1 Preparation of preliminary lithium-transition metal composite oxide particles (S10) Distilled water, from which dissolved oxygen was removed by bubbling with N2 for 24 hours, was used to mix NiSO4, CoSO4, and MnSO4 in a ratio of 0.88:0.09:0.03, respectively. The solution was placed in a reactor at 50°C, and a coprecipitation reaction was carried out for 48 hours using NaOH and NH3H2O ​​as precipitating and chelating agents, respectively, to obtain Ni as a transition metal precursor. 0.88 Co 0.09 Mn 0.03 (OH)2 was obtained. The obtained precursor was dried at 80°C for 12 hours, and then re-dried at 110°C for 12 hours.

[0107] Lithium hydroxide and the transition metal precursor were added to a dry high-speed mixer in a ratio of 1.01:1 and mixed uniformly for 5 minutes. The mixture was placed in a calcination furnace and heated to 710-750°C at a heating rate of 2°C / min, and maintained at 710-750°C for 10 hours. Oxygen was continuously passed through at a flow rate of 10 mL / min during heating and maintenance. After calcination, the mixture was allowed to cool naturally to room temperature, and after pulverization and classification, the positive electrode active material LiNi 0.88 Co 0.09 Mn 0.03 Preliminary lithium-transition metal composite oxide particles in the primary particle form of O2 were obtained.

[0108] Preparation and mixing of potassium compound aqueous solution (S20), and heat treatment (S30) To 5% by weight of de-ionized water (DIW) relative to the total weight of the obtained pre-lithium-transition metal composite oxide particles, 0.8% by weight of potassium bisulfate (KHSO4) powder relative to the total weight of the pre-lithium-transition metal composite oxide particles was added, and the mixture was stirred to thoroughly dissolve the potassium bisulfate powder in the de-ionized water to prepare an aqueous potassium compound solution.

[0109] The prepared aqueous potassium compound solution was added to the reserve lithium-transition metal composite oxide particles and mixed.

[0110] The mixture was placed in a calcination furnace, and while supplying oxygen at a flow rate of 10 mL / min, the temperature was raised to 200-400°C at a heating rate of 2°C / min, and maintained at the raised temperature for 10 hours. After calcination, the material was classified using a 325-mesh filter to obtain the cathode active material.

[0111] Manufacturing of lithium-ion batteries A secondary battery was manufactured using the aforementioned positive electrode active material. Specifically, the positive electrode active material, Denka black as a conductive material, and PVDF as a binder were mixed in a mass ratio of 93:5:2 to produce a positive electrode mixture. This mixture was then coated onto an aluminum current collector, and the positive electrode was manufactured by drying and pressing. The target electrode density of the positive electrode after pressing was adjusted to 3.0 g / cc.

[0112] Lithium metal was used as the negative electrode active material.

[0113] As described above, the positive and negative electrodes were notched into circular shapes with diameters of Φ14 and Φ16, respectively, and then stacked. A separation membrane (polyethylene, 13 μm thick) notched with a diameter of Φ19 was interposed between the positive and negative electrodes to form an electrode cell. The electrode cell was placed inside the outer casing of a coin cell with a diameter of 20 mm and a height of 1.6 mm, and the electrolyte was poured in to assemble it. The electrode was aged for more than 12 hours to allow the electrolyte to permeate the inside of the electrodes.

[0114] As the electrolyte, a mixture of EC / EMC (30 / 70; volume ratio) solvent in which 1M LiPF6 was dissolved was used.

[0115] The secondary batteries manufactured as described above were subjected to chemical charging and discharging (charging conditions: CC-CV 0.1C 4.3V 0.005C CUT-OFF; discharging conditions: CC 0.1C 3V CUT-OFF).

[0116] Example 2 A positive electrode active material and a lithium secondary battery were obtained in the same manner as in Example 1, except that potassium bisulfate (KHSO4) powder was added in an amount of 0.4% by weight relative to the total weight of the preliminary lithium-transition metal composite oxide particles.

[0117] Example 3 A positive electrode active material and a lithium secondary battery were obtained in the same manner as in Example 1, except that potassium bisulfate (KHSO4) powder was added in an amount of 1.6% by weight relative to the total weight of the preliminary lithium-transition metal composite oxide particles.

[0118] Example 4 A positive electrode active material and a lithium secondary battery were obtained in the same manner as in Example 1, except that potassium bisulfate (KHSO4) powder was added in an amount of 0.1% by weight relative to the total weight of the preliminary lithium-transition metal composite oxide particles.

[0119] Example 5 A positive electrode active material and a lithium secondary battery were obtained in the same manner as in Example 1, except that potassium bisulfate (KHSO4) powder was added in an amount of 2.0% by weight relative to the total weight of the preliminary lithium-transition metal composite oxide particles.

[0120] Comparative Example 1 A positive electrode active material and a lithium secondary battery were obtained in the same manner as in Example 1, except that the step of mixing the pre-lithium-transition metal composite oxide particles with an aqueous potassium compound solution was omitted, and the pre-lithium-transition metal composite oxide particles were added to 100% by weight of pure water relative to the total weight of the pre-lithium-transition metal composite oxide particles, stirred for 10 minutes and washed with water, and after filtration, dried under vacuum at 130-170°C for 12 hours.

[0121] Comparative Example 2 A positive electrode active material and a lithium secondary battery were obtained in the same manner as in Example 1, except that pure water was added at a concentration of 5% by weight relative to the total weight of the pre-lithium-transition metal composite oxide particles and mixed instead of an aqueous potassium compound solution.

[0122] In the above-mentioned examples and Comparative Example 2, an initial wetting method was performed by adding a small amount of solution or water, rather than a water washing treatment in which substantially the same amount of water as the positive electrode active material was added. In Comparative Example 1, the water washing treatment was performed.

[0123] Experimental Example 1 (1) Analysis using HR-TEM (High Resolution Transmission Electron Microscope) and FFT (Fast Fourier Transform) The cross-sections of the lithium-transition metal composite oxide particles obtained in the above examples and comparative examples were analyzed by HR-TEM analysis and FFT image analysis to determine the structures of compounds present in the primary particle region and the lithium-potassium-containing region (inter-primary particle region).

[0124] (2) Calculation of the average potassium signal For the lithium-transition metal composite oxide particles obtained in the above examples and comparative examples, the potassium signal values ​​in the primary particle region and the inter-primary particle region (e.g., lithium-potassium-containing region) were continuously measured by line scan using STEM-EDS. Subsequently, the potassium signal values ​​for each region were averaged to calculate the average potassium signal value in the primary particles and the lithium-potassium-containing region.

[0125] (3) Measurement of sulfur content A carbon / sulfur analyzer (C / S analyzer; model name: CS844, manufacturer: LECO) was used to measure the sulfur (S) content. The sample volume was selected based on the range of measurement values ​​of standard samples measured during calibration curve creation.

[0126] Specifically, 0.02 to 0.04 g of the lithium-transition metal composite oxide particles obtained in the above examples and comparative examples were placed in a ceramic crucible. At this time, a combustion aid (LECOCEL II) and an iron chip were added together in a 1:1 ratio.

[0127] Subsequently, O2 was supplied as a combustion gas at a rate of 3 L / min using a high-frequency induction device and burned at approximately 2,600-2,700°C. The sulfur oxide-based inorganic compound gas (e.g., sulfuric acid gas) generated by the combustion was passed through an infrared detection cell, and the change in infrared absorption relative to a blank was measured. This allowed for the quantitative detection of the sulfur content in the lithium-transition metal composite oxide particles.

[0128] The amounts of potassium bisulfate powder and solvent used in the examples and comparative examples, as well as the results of the measurements and evaluations described above, are shown in Table 1 below.

[0129] [Table 1]

[0130] Figure 4 shows HR-TEM images of lithium-transition metal composite oxide particles according to Example 1. Specifically, Figure 4(a) is an HR-TEM image of lithium-transition metal composite oxide particles from Example 1, and Figure 4(b) is an enlarged HR-TEM image of the surface region (1 region) of the primary particles in Figure 4(a).

[0131] Figure 5 shows HR-TEM images of lithium-transition metal composite oxide particles according to Comparative Example 1. Specifically, Figure 5(a) is an HR-TEM image of lithium-transition metal composite oxide particles from Comparative Example 1, and Figure 5(b) is an enlarged HR-TEM image of the internal region (2 regions) of the primary particle in Figure 5(a).

[0132] Figure 6 shows the FFT images of region A in Figure 4(b) and region B in Figure 5(b). Specifically, Figure 6(a) is an enlarged FFT image of region A in Figure 4(b), and Figure 6(b) is an enlarged FFT image of region B in Figure 5(b).

[0133] Referring to Figures 4 to 6, in Comparative Example 1, a water washing process was performed instead of the initial wetting method. As a result, even the layered structure in the internal regions of primary particles (for example, regions 2 in Figure 5(a) and region B in Figure 5(b)), where the probability of damage to the layered structure is relatively low, changed from a hexagonal close-packed structure to a face-centered cubic structure, as shown in Figure 6(b).

[0134] In contrast, in Example 1, where an aqueous potassium compound solution was added and a mixing step (e.g., the initial wetting method) was performed, the layered structure of primary particles with a relatively high probability of damage to the layered structure (e.g., region 1 in Figure 4(a) and region A in Figure 4(b)) maintained a hexagonal close-packed structure as shown in Figure 6(a).

[0135] Figure 7 is a graph showing the potassium signal values ​​in the primary particle region and the inter-primary particle region (e.g., lithium-potassium-containing region) of Examples 1 to 5.

[0136] Referring to Figure 7, in Examples 1 to 3, the ratio of the average potassium signal value in the inter-primary particle region to the average potassium signal value within the primary particle (potassium signal ratio) was in the range of 1.2 to 4.

[0137] However, in Example 4, where the amount of potassium compound powder added was less than 0.2% by weight relative to the reserve lithium-transition metal composite oxide particles, the potassium signal ratio was shown to be low, less than 1.2 times, making it difficult to determine the lithium-potassium content in the lithium-transition metal composite oxide particles.

[0138] Furthermore, in Example 5, where the amount of potassium compound powder added exceeded 1.9% by weight relative to the reserve lithium-transition metal composite oxide particles, the potassium signal ratio exceeded 4 times.

[0139] In contrast, in Example 1, the signal ratio of potassium from the primary interparticle region (e.g., lithium-potassium-containing region) to the 50 nm interval was uniformly shown at 2.63 times.

[0140] Experimental Example 2 (3) Measurement of residual lithium (Li2CO3, LiOH) content 1.5 g of the positive electrode active material from the examples and comparative examples was quantified into a 250 mL flask, 100 g of deionized water was added, and a magnetic bar was inserted and the mixture was stirred at a speed of 60 rpm for 10 minutes. After that, the solution was filtered using a vacuum flask, and 100 g was separated. The separated solution was placed in an automatic titrator and automatically titrated with 0.1 N HCl according to the Warder Method to measure the content of Li2CO3 and LiOH in the solution.

[0141] (4) Measurement of initial charge / discharge capacity and evaluation of initial capacity efficiency The lithium secondary batteries manufactured in the above-described examples and comparative examples were charged in a 25°C chamber (CC-CV 0.1C 4.3V 0.005C CUT-OFF), and their battery capacity (initial charge capacity) was measured. They were then discharged again (CC 0.1C 3.0V CUT-OFF), and their battery capacity (initial discharge capacity) was measured.

[0142] The initial capacity efficiency was evaluated by dividing the measured initial discharge capacity by the measured initial charge capacity and converting the result to a percentage (%).

[0143] (5) Measurement of capacity retention rate (life characteristics) during repeated charge and discharge The lithium secondary batteries of the examples and comparative examples were subjected to 300 cycles of charging (CC / CV 0.5C 4.3V 0.05C CUT-OFF) and discharging (CC 1.0C 3.0V CUT-OFF). The life retention rate was evaluated by dividing the discharge capacity over 300 cycles by the discharge capacity over one cycle and converting the result to a percentage. The evaluation results are shown in Table 2 below.

[0144] [Table 2]

[0145] Referring to Table 2, in the example where an aqueous potassium compound solution was mixed and the initial wetting method was performed, the lithium content remaining on the surface of the lithium-transition metal composite oxide particles was reduced overall compared to the comparative example, resulting in good initial capacity efficiency and excellent lifetime characteristics.

[0146] Among the examples in which the ratio of the average potassium signal in the primary particles and the lithium-potassium-containing portion satisfies a predetermined range (e.g., 1.2 to 4), in the case of Example 1, not only was the initial volume maintained compared to Comparative Example 2, in which only the same weight percent of pure potassium compound was used instead of the aqueous potassium compound solution in the initial wetting method, but improved lifetime characteristics were also ensured by the passivation effect of the lithium-potassium-containing compound formed on the surface of the lithium-transition metal composite oxide particles by reaction with residual lithium.

[0147] However, in Example 4, where potassium compound powder was added in an amount of less than 0.2% by weight, the amount of potassium-containing compound that reacted with residual lithium was insufficient, resulting in a slight increase in residual lithium and a slight decrease in volume retention compared to Examples 1-3.

[0148] Furthermore, in Example 5, where potassium compound powder was added in an amount exceeding 1.9% by weight, the amount of potassium reacting with residual lithium increased, ensuring an excellent residual lithium reduction effect. However, the excessive addition of potassium compound resulted in a slight decrease in discharge capacity, efficiency, and life characteristics compared to Examples 1-3. In addition, the amount of residual lithium carbonate (Li2CO3) increased compared to Example 3 due to unreacted potassium bisulfate and lithium-potassium compounds on the active material surface. Moreover, Example 5 exhibited a trade-off phenomenon where discharge capacity decreased while the capacity retention rate relatively improved.

[0149] In Comparative Example 1, which used a general water washing method, the effect of reducing residual lithium was excellent. However, due to a change in the layered structure of primary particles during the water washing process, the initial capacity, efficiency, lifespan, and electrochemical properties were significantly reduced compared to the Examples and Comparative Example 2. [Explanation of Symbols]

[0150] 100: Positive electrode 105: Positive electrode current collector 107: Positive lead 110: Positive electrode active material layer 120: Negative electrode active material layer 125: Negative electrode current collector 127: Negative electrode lead 130: Negative electrode 140: Separation membrane 150: Electrode assembly 160: Case

Claims

1. It contains lithium-transition metal composite oxide particles that include multiple primary particles, The lithium-transition metal composite oxide particles include a lithium-potassium-containing portion formed between the primary particles. A positive electrode active material for a lithium secondary battery, wherein the average potassium signal value of the lithium-potassium-containing portion, measured by Energy Dispersive Spectroscopy (EDS), is 1.2 to 4 times that of the average potassium signal value in the primary particles, measured by EDS.

2. The lithium-potassium-containing portion comprises a lithium-potassium-sulfur-containing portion containing lithium, potassium, and sulfur, as described in claim 1.

3. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the primary particles have a hexagonal close-packed structure.

4. The lithium-transition metal composite oxide particles do not include primary particles having a face-centered cubic structure, as described in claim 3, for a positive electrode active material for a lithium secondary battery.

5. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the sulfur content of the lithium-transition metal composite oxide particles, as measured by a carbon-sulfur analyzer, is 1,100 to 4,500 ppm relative to the total weight of the lithium-transition metal composite oxide particles.

6. Lithium carbonate (Li) remaining on the surface of the lithium-transition metal composite oxide particles ２ CO ３ The positive electrode active material for a lithium secondary battery according to claim 1, wherein the content of ) is 2,500 ppm or less, and the content of lithium hydroxide (LiOH) remaining on the surface of the lithium-transition metal composite oxide particles is 2,500 ppm or less.

7. The steps include preparing preliminary lithium-transition metal composite oxide particles, The steps include mixing the aforementioned pre-lithium-transition metal composite oxide particles with an aqueous solution of a potassium compound, The step of heat-treating the mixed preliminary lithium-transition metal composite oxide particles and the aqueous potassium compound solution to form lithium-transition metal composite oxide particles that include a plurality of primary particles and lithium-potassium-containing portions formed between the primary particles, The aforementioned aqueous potassium compound solution is formed by mixing a solvent and potassium compound powder. The amount of potassium compound powder added is 0.2 to 1.9% by weight relative to the total weight of the preliminary lithium-transition metal composite oxide particles. A method for producing a positive electrode active material for a lithium secondary battery, wherein the amount of solvent added is 2 to 15% by weight relative to the total weight of the pre-lithium-transition metal composite oxide particles.

8. The potassium compound powder is potassium bisulfate (KHSO4). ４ A method for producing a positive electrode active material for a lithium secondary battery according to claim 7, wherein the material is a powder.

9. The method for producing a positive electrode active material for a lithium secondary battery according to claim 7, wherein the heat treatment is performed at 200 to 400°C in an oxygen atmosphere.

10. The method for producing a positive electrode active material for a lithium secondary battery according to claim 7, wherein the preliminary lithium-transition metal composite oxide particles are mixed with the potassium compound aqueous solution without washing.

11. A positive electrode comprising a positive electrode active material layer containing the positive electrode active material for lithium secondary batteries described in claim 1, A lithium secondary battery comprising a positive electrode and a negative electrode facing the positive electrode.

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