KR20240154468A - Positive electrode active material, positive electrode and lithium secondary battery comprising the same - Google Patents

Abstract

The present invention relates to a cathode active material comprising a lithium manganese oxide represented by the chemical formula 1 and having a sulfur content measured on the surface of 4,000 ppm or more based on the total weight of the cathode active material, a method for producing the same, a cathode comprising the same, and a lithium secondary battery.

Description

POSITIVE ELECTRODE ACTIVE MATERIAL, POSITIVE ELECTRODE AND LITHIUM SECONDARY BATTERY COMPRISING THE SAME

The present invention relates to a manganese-rich cathode active material and a method for producing the same, which improve the lifespan by controlling the content of S-based impurities present on the surface, and to a cathode and lithium secondary battery including the cathode active material.

Lithium secondary batteries have been used as energy storage media in various fields since their commercialization in 1991. As the market for products equipped with lithium secondary batteries expands, research is being actively conducted to increase the energy density of lithium secondary batteries, and one of the most notable methods is the development of a cathode active material with a composition that can utilize a larger amount of lithium than before.

As positive electrode active materials that can utilize more lithium, perlithium-based transition metal oxides having a layered structure and a molar ratio of lithium to transition metal exceeding 1 are being developed. These perlithium-based transition metal oxides can implement high capacities because they implement capacity by utilizing not only the cation redox reaction of the transition metal but also the anion redox reaction using oxygen in the positive electrode structure. A representative perlithium-based transition metal oxide that is currently being actively studied is a perlithium manganese-based oxide (hereinafter referred to as Mn-rich perlithium manganese-based oxide) in which the molar ratio of lithium to transition metal exceeds 1 and the manganese content among the total transition metal exceeds 50 mol%.

However, since Mn-rich lithium manganese oxides have low rate characteristics, that is, high resistance, they are mainly manufactured in the form of secondary particles in which primary particles are aggregated to reduce the lithium movement distance and lower the diffusion resistance. However, if the sintering temperature is raised to improve the structural stability, the resistance cannot be improved as the size of the primary particles increases. Therefore, a technology is needed that can increase the structural completeness by raising the sintering temperature without increasing the size of the primary particles beyond a certain level.

KR 2019-0052103 A

The present invention aims to provide a cathode active material and a method for manufacturing the same, which increases structural completeness while not excessively increasing the size of primary particles by controlling S-based impurities to remain on the surface at a certain level during the synthesis of the cathode active material.

According to one embodiment, the present invention,

Contains a lithium manganese oxide represented by the following chemical formula 1,

A cathode active material is provided, wherein the sulfur content is 4,000 ppm or more based on the total weight of the cathode active material.

[Chemical Formula 1]

Li _1+a [Mn _1-(b+c) Ni _b M _c ]O ₂

In the above chemical formula 1,

M is at least one selected from the group consisting of Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr,

0.1≤a≤0.5, 0≤b<0.5, 0≤c≤0.1, 0≤a+b+c≤0.5.

According to another embodiment of the present invention,

A step of forming precursor particles for a cathode active material by supplying a transition metal-containing solution containing nickel and manganese sulfate or sulfide, an ammonium cation complex forming agent, and a basic compound to a reactor and performing a co-precipitation reaction;

A step of manufacturing a precursor for a cathode active material by washing the precursor particles for the cathode active material with water; and

A method for producing the above-described positive electrode active material is provided, comprising the step of mixing a precursor for the positive electrode active material and a lithium raw material and calcining at 800°C to 950°C.

According to another embodiment, the present invention provides a positive electrode for a lithium secondary battery comprising the positive electrode active material described above.

According to another embodiment, the present invention provides a lithium secondary battery including the above-mentioned positive electrode; a negative electrode including a negative electrode active material; a separator interposed between the positive electrode and the negative electrode; and an electrolyte.

The present invention provides a method for manufacturing a cathode active material including a Mn-rich lithium manganese oxide having low rate characteristics, wherein the structural stability can be increased while suppressing excessive growth of primary particles by controlling the content of sulfur-based impurities without a separate additional process.

The cathode active material manufactured in this way has the advantage of excellent resistance and capacity characteristics because the lithium movement distance is short and the diffusion resistance is low.

Figure 1 is a photograph of the positive electrode active material particles manufactured in Example 1 and Comparative Example 1 observed at a magnification of 50k using a scanning electron microscope (SEM).
Figure 2 is a photograph of the positive electrode active material particles manufactured in Example 1 and Comparative Example 1 observed at a magnification of 20k using a scanning electron microscope (SEM).
Figure 3 is a photograph of the positive electrode active material particles manufactured in Example 1 and Comparative Example 1 observed at a magnification of 5k using a scanning electron microscope (SEM).
Figure 4 is a photograph of the positive electrode active material particles manufactured in Comparative Example 2 observed at magnifications of 5k, 20k, and 50k using a scanning electron microscope (SEM).

Hereinafter, the present invention will be described in more detail.

A cathode active material including a Mn-rich lithium manganese oxide having a manganese content exceeding 50 mol% of all metals excluding lithium not only has a higher energy density than the currently commercialized lithium nickel cobalt manganese (NCM) active material, but also has the advantage of reducing the manufacturing cost because it can reduce the amount of expensive cobalt used.

However, Mn-rich cathode materials have the disadvantages of high resistance and low rate characteristics because the oxidation/reduction reactions of oxygen ions and Mn ions proceed relatively slowly. Therefore, improvements in these are essential for commercialization of lithium secondary batteries using Mn-rich cathode materials.

Accordingly, the inventors of the present invention controlled the content of residual sulfur to 4,000 ppm or more with respect to the total weight of the cathode active material by washing only with water that did not contain a separate flux material during the precursor cleaning process when manufacturing a cathode active material including Mn-rich lithium manganese oxide, thereby increasing the structural completeness of the cathode active material while lowering the resistance characteristics.

Specifically, when the heat treatment temperature is increased in the step of mixing and heat-treating the precursor for the positive electrode active material and the lithium raw material during the manufacture of the positive electrode active material, the crystallinity of the particles can be improved and the degree of sintering completion can be improved. However, in the case of lithium manganese oxide, the problem occurs that the resistance increases as the size of the primary particles increases due to the high heat treatment temperature. In this case, when the sulfur content in the positive electrode active material is 4,000 ppm or more, the process of growing the primary particles is hindered, so that excessive growth of the primary particles can be prevented by heat treatment at a high temperature.

That is, by increasing the sintering temperature, the crystal size can be secured to a certain level while preventing the growth of primary particles, so that the shape of secondary particles in which primary particles are aggregated is maintained while the movement distance of lithium ions is shortened, thereby reducing the diffusion resistance.

In the present invention, the sulfur may exist in the form of, for example, metal sulfate (MeSO ₄ ), and the content of sulfur refers only to the content of the S element among these.

In the present invention, the “sulfur content” means the content of elemental sulfur in a cathode active material obtained through ICP analysis, and specifically means the result of mixing 0.1 g of the cathode active material to be analyzed with 2 mL of distilled water and 1 mL of concentrated nitric acid, then diluting it with 50 mL of ultrapure water, and then analyzing it using an ICP-OES (PERKIN-ELMER, Optima 7300DV) device.

In the present invention, “primary particle” means a particle unit that has no apparent grain boundary when observed at a magnification of 5,000 to 20,000 times using a scanning electron microscope, and “secondary particle” means a particle formed by the agglomeration of multiple primary particles.

In the present invention, the "average particle diameter of primary particles" means an arithmetic mean value calculated by measuring the particle diameters of at least 20 primary particles observed in a scanning electron microscope image. In this case, the particle diameter means the longest axis diameter of the primary particles.

In the present invention, "D ₅₀ " means a particle size corresponding to 50% of the volume accumulation in the volume accumulation particle size distribution of the corresponding particle powder, and can be measured using a laser diffraction method. For example, after dispersing the positive active material powder in a dispersion medium, it can be measured by introducing it into a commercially available laser diffraction particle size measuring device (e.g., S-3500 of Microtrac Corporation) and irradiating it with ultrasonic waves of about 28 kHz with an output of 60 W to obtain a volume accumulation particle size distribution graph, and then finding the particle size at a point where the volume accumulation amount is 50% in the obtained volume accumulation particle size distribution graph.

In the present invention, "crystallite" means a particle unit having substantially the same crystal orientation, which can be confirmed through EBSD (Electron Backscatter Diffraction) analysis. Specifically, it is the minimum particle unit displayed in the same color in an IPF map obtained by EBSD analysis of a cross-section of a cathode active material cut through ion milling.

Meanwhile, in the present invention, the "average crystallite size" can be quantitatively analyzed using X-ray diffraction analysis (XRD) by Cu Kα X-rays. Specifically, the average crystallite size of the crystal grains can be quantitatively analyzed by putting the particles to be measured in a holder, irradiating the particles with X-rays, and analyzing the diffraction grating that appears. Sampling was prepared by putting a powder sample of the target particle into the groove in the middle of a general powder holder, smoothing the surface using a slide glass, and making the sample height the same as the edge of the holder. Then, X-ray diffraction analysis was performed using a Bruker D8 Endeavor (light source: Cu Kα, λ=1.54Å) equipped with a LynxEye XE-T position sensitive detector under the conditions of a step size of 0.02 degrees, a total scan time of about 20 minutes, and an FDS of 0.5°, 2θ=15° to 90°. For the measured data, Rietveld refinement was performed considering the charge (+3 for metal ions at the transition metal site and +2 for Ni ions at the Li site) and cation mixing at each site. When analyzing the grain size, instrumental brodadening was considered using the Fundamental Parameter Approach (FPA) implemented in the Bruker TOPAS program, and the entire peaks of the measurement range were used for fitting. Among the peak types available in TOPAS, only the Lorenzian contribution was used for the peak shape fitting with the First Principle (FP), and strain was not considered.

In the present invention, the “BET specific surface area” is calculated using the BET (Brunauer-Emmett-Teller) multi-point method from a nitrogen adsorption isotherm under a 77K liquid nitrogen atmosphere obtained using BELSORP-MAX (MicrotracBEL corp.).

Bipolar active material

The cathode active material according to the present invention comprises a lithium manganese oxide represented by the following chemical formula 1, and the sulfur content may be 4,000 ppm or more, preferably 4,000 to 8,000 ppm, and more preferably 5,000 to 7,000 ppm based on the total weight of the cathode active material.

[Chemical Formula 1]

Li _1+a [Mn _1-(a+b+c) Ni _b M _c ]O ₂

In the above chemical formula 1,

M is at least one selected from the group consisting of Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr,

0.1≤a≤0.5, 0≤b<0.5, 0≤c≤0.1, 0≤a+b+c≤0.5.

As described above, however, when the sulfur content in the positive electrode active material is 4,000 ppm or more, excessive growth of primary particles can be prevented even during high-temperature heat treatment. However, when the sulfur content becomes excessively high, the problem of capacity reduction due to an increase in the impurity ratio can be aggravated. Therefore, it is necessary to control the sulfur content by taking this into consideration.

Meanwhile, in the chemical formula 1, a is the molar ratio of Li in the lithium manganese oxide, and may be 0.1≤a≤0.5, 0.1≤a≤0.4, or 0.12≤a≤1.17. When a satisfies the above range, high capacity can be achieved.

The above b is the molar ratio of Ni in the lithium manganese oxide, and may be 0≤b<0.5, 0.1≤b≤0.4, or 0.25≤b≤0.35.

The above c is a molar ratio of the doping element M in the lithium manganese oxide, and may be 0≤c≤0.1, 0≤c≤0.05, or 0≤c≤0.01. The above doping element M may preferably be Co, and if the content of the doping element is too high, not only may it have a negative effect on the capacity of the active material, but also there is a concern that the life characteristics may be deteriorated due to the increased oxygen-redox reaction, which may aggravate gas generation and deterioration of the positive electrode active material.

The above 1-(a+b+c) is a molar ratio of Mn in the lithium manganese oxide, and may be 0≤a+b+c≤0.5, 0≤a+b+c<0.5, or 0.35≤a+b+c≤0.45. When a+b+c exceeds 0.5, that is, when 1-(a+b+c) is less than 0.5, the ratio of the rock salt phase becomes too small, so the effect of improving the structural stability is minimal.

Preferably, in the chemical formula 1, 0.1≤a≤0.4, 0.1≤b≤0.4, 0≤c≤0.05, 0≤a+b+c<0.5, and more preferably, 0.12≤a≤0.17, 0.25≤b≤0.35, 0≤c≤0.01, 0.35≤a+b+c≤0.45.

Meanwhile, in the lithium manganese oxide represented by the above chemical formula 1, the molar ratio of Li to the molar number of total metal elements excluding Li (Li/Me) may be 1.1 to 1.5, preferably 1.1 to 1.4, and more preferably 1.12 to 1.17. When the Li/Me ratio satisfies the above range, the rate characteristics and capacity characteristics are excellent. If the Li/Me ratio is too high, the electrical conductivity may decrease and the rock salt phase (Li ₂ MnO ₃ ) may increase, which may accelerate the degradation rate, and if it is too low, the effect of improving the energy density is minimal.

Meanwhile, in the case of lithium manganese oxide containing an excess of lithium, it has a structure in which a layered phase (LiM'O ₂ ) and a rock salt phase (Li ₂ MnO ₃ ) are mixed, and the composition of the lithium manganese oxide can be represented by the following chemical formula 2.

[Chemical formula 2]

X Li ₂ MnO ₃ ·(1-X)Li[Ni _1-yz Mn _y M _z ]O ₂

In the above chemical formula 2,

M is at least one selected from the group consisting of Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr,

0.1≤X≤0.5, 0.4≤y<1, 0≤z≤0.2.

The above X represents the ratio of the Li ₂ MnO ₃ phase in the lithium manganese oxide, and the above y and z represent the molar ratios of Mn and the doping element M in the LiM'O ₂ layer, respectively.

Meanwhile, the cathode active material according to the present invention may further include a coating layer on the surface of the lithium manganese oxide, if necessary. When the cathode active material includes a coating layer, the contact between the lithium manganese oxide and the electrolyte is suppressed by the coating layer, thereby reducing the electrolyte side reaction, and thereby improving the life characteristics.

The coating layer may include a coating element M ¹ , and the coating element M ¹ may be at least one selected from the group consisting of Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr and Zr, preferably Al, Co, Nb, W and combinations thereof, and more preferably Al, Co and combinations thereof. The coating element M ¹ may include two or more types, and may include, for example, Al and Co.

The above coating element may exist in the coating layer in the form of an oxide, i.e., M ¹ Oz (1≤z≤4).

The above coating layer can be formed through methods such as dry coating, wet coating, chemical vapor deposition (CVD), physical vapor deposition (PVD), and atomic layer deposition (ALD). Among these, it is preferable to form the coating layer through atomic layer deposition because it can form a large coating layer area.

The formation area of the above coating layer may be 10% to 100%, preferably 30% to 100%, and more preferably 50% to 100%, based on the total surface area of the lithium manganese oxide particles. When the formation area of the coating layer satisfies the above range, the effect of improving the life characteristics is excellent.

Meanwhile, the positive electrode active material according to the present invention may be in the form of secondary particles in which a plurality of primary particles are aggregated, and the average particle diameter of the primary particles may be 0.01 ㎛ to 5 ㎛, preferably 0.01 ㎛ to 2 ㎛, and more preferably 0.01 ㎛ to 0.1 ㎛. In order to improve structural stability, the average particle diameter of the primary particles increases as sintering is performed above a certain temperature, but when the average particle diameter of the primary particles exceeds 5 ㎛, the lithium movement distance increases, which is not preferable because the diffusion resistance increases.

In addition, the D ₅₀ of the positive electrode active material, that is, the D ₅₀ of the secondary particles, may be 2 μm to 15 μm, preferably 2 μm to 13 μm, and more preferably 5 μm to 12 μm. When the D ₅₀ of the positive electrode active material satisfies the above range, the electrode density can be excellently implemented, and the deterioration of the capacity and rate characteristics can be minimized.

In addition, the average crystallite size of the positive electrode active material may be 20 nm to 150 nm, preferably 20 nm to 100 nm, and more preferably 60 nm to 100 nm. In terms of reinforcing crystallinity, a larger average crystallite size is desirable, but considering the resulting increase in resistance, it is desirable that it does not exceed 150 nm.

In addition, the BET specific surface area of the positive electrode active material may be 0.1 m ² /g to 10 m ² /g, specifically 1 m ² /g to 6 m ² /g, and more specifically 1.2 m ² /g to 3 m ² /g. If the BET specific surface area of the positive electrode active material is too low, the reaction area with the electrolyte is insufficient, making it difficult to implement sufficient capacity, and if the specific surface area is too high, moisture absorption is fast and side reactions with the electrolyte are accelerated, making it difficult to secure life characteristics.

Method for manufacturing positive electrode active material

The method for producing a cathode active material according to the present invention comprises the steps of: supplying a transition metal-containing solution containing sulfate or sulfide of nickel and manganese, an ammonium cation complex forming agent, and a basic compound to a reactor while performing a co-precipitation reaction to form precursor particles for a cathode active material; washing the precursor particles for a cathode active material with water to produce a precursor for a cathode active material; and mixing the precursor for a cathode active material with a lithium raw material and calcining the mixture at 800°C to 950°C.

The step of forming the precursor particles for the above-mentioned positive electrode active material can be produced by, for example, dissolving each transition metal-containing raw material in a solvent to produce a transition metal-containing solution, then mixing the transition metal-containing solution, an ammonium cation complex forming agent, and a basic compound, and then performing a coprecipitation reaction. In addition, an oxidizing agent or oxygen gas may be additionally added during the coprecipitation reaction, if necessary.

Meanwhile, the transition metal-containing raw material may be an acetate, carbonate, nitrate, sulfate, halide, sulfide, etc. of each transition metal. However, the transition metal-containing solution of the present invention contains a sulfate or sulfide of nickel and manganese. Specifically, the transition metal-containing raw material may be NiO, NiCO ₃ .2Ni(OH) ₂ .4H ₂ O, NiC ₂ O ₂ .2H ₂ O, Ni(NO ₃ ) ₂ .6H ₂ O, NiSO ₄ , NiSO ₄ .6H ₂ O, nickel sulfide, Mn ₂ O ₃ , MnO ₂ , Mn ₃ O ₄ , MnCO ₃ , Mn(NO ₃ ) ₂ , MnSO ₄ .H ₂ O, manganese acetate, manganese halides, manganese sulfide, etc., and the transition metal-containing solution includes at least one selected from the group consisting of NiSO ₄ , NiSO ₄ .6H ₂ O, nickel sulfide, MnSO ₄ .H ₂ O, and manganese sulfide.

The above ammonium cation complex forming agent may be at least one selected from the group consisting of NH ₄ OH, (NH ₄ ) ₂ SO ₄ , NH ₄ NO ₃ , NH ₄ Cl, CH ₃ COONH ₄ , and NH ₄ CO ₃ .

The above basic compound may be at least one selected from the group consisting of NaOH, Na ₂ CO ₃ , KOH, and Ca(OH) ₂ . The form of the precursor may vary depending on the type of the basic compound used. For example, when NaOH is used as the basic compound, a precursor in the form of a hydroxide may be obtained, and when Na ₂ CO ₃ is used as the basic compound, a precursor in the form of a carbonate may be obtained. In addition, when the basic compound and the oxidizing agent are used together, a precursor in the form of an oxide may be obtained.

Meanwhile, the precursor for the positive electrode active material may be in the form of a hydroxide, oxide or carbonate.

In one embodiment of the present invention, the step of washing the precursor particles for the positive electrode active material with water can be performed using deionized water, and it is preferable that no other substance is added during the washing. In a typical precursor washing step, a flux material such as NaOH is used, and in this case, it is difficult for the sulfur content to remain at 4,000 ppm or more.

Specifically, the washing is preferably performed by stirring the precursor particles at 1,000 rpm to 3,000 rpm for 10 minutes at a temperature of 20°C to 30°C, adjusting the weight ratio of the precursor to water to 2:1 to 10:1. After the washing, the particles are filtered and dried by exposing them to a temperature of 100°C to 150°C for 5 to 15 hours.

Meanwhile, examples of the lithium raw material include lithium-containing carbonates (e.g., lithium carbonate, etc.), hydrates (e.g., lithium hydroxide hydrate (LiOH H ₂ O)), hydroxides (e.g., lithium hydroxide, etc.), nitrates (e.g., lithium nitrate (LiNO ₃ )), chlorides (e.g., lithium chloride (LiCl)), etc., and one of these may be used alone or a mixture of two or more may be used.

Meanwhile, the precursor for the positive electrode active material and the lithium raw material can be mixed in an amount such that the molar ratio of the total transition metal (Ni+Co+Mn):Li is 1:1.1 to 1:1.5, preferably 1:1.1 to 1:1.4, and more preferably 1:1.12 to 1:1.17.

Meanwhile, the sintering can be performed at a temperature of 800°C to 950°C, preferably 850°C to 950°C, and more preferably 870°C to 920°C. When the sintering temperature is 800°C or higher, the degree of structural perfection can be improved, but it is preferable not to exceed 950°C in order to prevent excessive growth of primary particles. The sintering time can be 5 hours to 20 hours, and preferably 10 hours to 15 hours. In addition, the sintering atmosphere can be an air atmosphere or an oxygen atmosphere, and for example, can be an atmosphere containing 20 to 100 volume% of oxygen.

In general, when manufacturing a cathode active material including Mn-rich lithium manganese oxide, there is a problem that resistance increases as the primary particle size becomes excessively large when the sintering temperature is increased to 800°C or higher, but this can be prevented when 4,000 ppm or more of sulfur remains on the surface as in the present invention.

Bipolar and lithium secondary batteries

The positive electrode according to the present invention includes the positive electrode active material, and the lithium secondary battery according to the present invention includes the positive electrode, the negative electrode including the negative electrode active material; a separator interposed between the positive electrode and the negative electrode; and an electrolyte.

The positive electrode according to the present invention and the lithium secondary battery including the same can be manufactured through a conventional method for manufacturing a positive electrode and a lithium secondary battery in the art, except that the positive electrode active material described above is used.

For example, a lithium secondary battery can be manufactured by sequentially stacking and drying electrode assemblies by interposing a separator between a positive electrode including a positive active material and a negative electrode including a negative active material, inserting the electrode assembly into a case, injecting an electrolyte, and sealing the assembly. The lithium secondary battery can be a cylindrical, square, coin-shaped, or pouch-shaped battery.

The above positive and negative electrodes can be manufactured by applying a composition for forming an active material layer including an electrode active material on a current collector and then drying it.

The composition for forming a positive electrode active material layer may optionally further contain a binder, a conductive material, a filler, etc. in addition to the positive electrode active material, as needed. The composition for forming a negative electrode active material layer may optionally further contain a binder, a conductive material, a filler, etc. in addition to the negative electrode active material, as needed.

The current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., an aluminum-cadmium alloy, etc. can be used. In addition, the current collector can typically have a thickness of 3 ㎛ to 500 ㎛, and fine unevenness can be formed on the surface of the current collector to strengthen the binding force of the active material. For example, the current collector can be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven fabric, etc.

The above positive electrode may further include a conventional positive electrode active material in addition to the above-described positive electrode active material, and for example, may further include at least one positive electrode active material selected from the group consisting of LCO (LiCoO ₂ ), LNO (LiNiO ₂ ), LFP (LiFePO ₄ ), and NCM (Li[Ni _p Co _q Mn _r1 ]O ₂ , 0＜p＜1, 0＜q＜1, 0＜r1＜1, p+q+r1=1), but preferably, only the positive electrode active material may be used alone.

The above positive electrode active material may be included in an amount of 80 wt% to 99 wt% based on the total weight of the positive electrode active material layer.

In one embodiment of the present invention, the negative electrode may include at least one selected from the group consisting of a carbon-based material; a silicon-based material; a metal or an alloy of these metals and lithium; a metal composite oxide; a material capable of doping and dedoping lithium; lithium metal; and a transition metal oxide as a negative electrode active material, and preferably may include a carbon-based material, a silicon-based material, or a mixture thereof.

As the above carbon-based material, any carbon-based negative electrode active material generally used in lithium secondary batteries may be used without particular limitation, and representative examples thereof include crystalline carbon, amorphous carbon, or a combination of these. Examples of the crystalline carbon include graphite such as amorphous, plate-shaped, flake-shaped, spherical, or fibrous natural graphite or artificial graphite, and examples of the amorphous carbon include soft carbon (low-temperature calcined carbon), hard carbon, mesophase pitch carbide, calcined coke, and the like.

The above silicon-based material is at least one selected from Si, SiO _x (0<x<2), and a Si-Y alloy (wherein Y is an element selected from an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a transition metal, a rare earth element, and a combination thereof, and cannot be Si), and is preferably SiO. The silicon-based negative electrode active material has a capacity that is about 10 times higher than that of graphite, and thus can lower the mass loading (mg cm ^-2 ) and improve the rapid charging performance of the battery.

As the above metal or an alloy of these metals with lithium, a metal selected from the group consisting of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al and Sn or an alloy of these metals with lithium can be used.

As the above metal composite oxide, _{at least one selected from the group consisting of PbO, PbO 2 , Pb 2 O 3 , Pb 3 O 4 , Sb 2 O 3 , Sb 2 O 4 , Sb 2 O 5 , GeO , GeO 2 , Bi 2 O 3 , Bi 2 O} 4 , Bi 2 O ₅ , Li _x Fe ₂ O ₃ (0≤x≤1), Li _x WO ₂ (0≤x≤1), and Sn _x Me _1-x Me' _y O _z (Me: Mn, Fe, Pb, Ge; Me': Al, B, P, Si, elements of group 1, 2, and 3 of the periodic table, halogen; 0<x≤1;1≤y≤3; 1≤z≤8) may be used.

Materials capable of doping and dedoping the lithium include Sn, SnO ₂ , Sn-Y (wherein Y is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof, but is not Sn), and at least one of these may be mixed with SiO ₂ for use.

In the above Si-Y and Sn-Y, the element Y can be selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db(dubnium), Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof.

Examples of the above transition metal oxides include lithium-containing titanium composite oxide (LTO), vanadium oxide, and lithium vanadium oxide.

The above negative active material may be included in an amount of 80 wt% to 99 wt% based on the total weight of the solid content in the negative electrode slurry.

The above binder is a component that assists in bonding between the conductive material, the active material, and the current collector, and is typically added in an amount of 0.1 to 10 wt% based on the total weight of the active material layer. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, various copolymers thereof, and the like.

The conductive agent is a component for further improving the conductivity of the active material, and can be added in an amount of 10 wt% or less, specifically 5 wt% or less, based on the total weight of the active material layer. The conductive agent is not particularly limited as long as it has conductivity and does not cause a chemical change in the battery, and for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black; conductive fibers such as carbon fiber or metal fiber; metal powders such as fluorinated carbon, aluminum, or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, etc. can be used.

Meanwhile, 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 commonly used as a separator in a lithium secondary battery can be used without special restrictions, and in particular, one that has low resistance to ion movement of the electrolyte and excellent electrolyte moisture retention capacity is preferable. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, and an ethylene/methacrylate copolymer, or a laminated structure of two or more layers thereof, can be used. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of high-melting-point glass fibers, polyethylene terephthalate fibers, etc. can also be used. In addition, a coated separator containing a ceramic component or a polymer material to secure heat resistance or mechanical strength can be used, and can be selectively used in a single-layer or multi-layer structure.

In addition, examples of the electrolyte include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.

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

As the organic solvent, any solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move may be used without particular limitation. Specifically, the organic solvent may include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; Examples of solvents that can be used include carbonate solvents such as dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (wherein R represents a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms and may include a double-bonded aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; and sulfolanes. Among these, a carbonate solvent is preferable, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate, etc.) having high ionic conductivity and high dielectric constant that can improve the charge/discharge performance of the battery and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate, etc.) is more preferable.

The above lithium salt can be used without any special restrictions as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the anion of the lithium salt may be at least one selected from the group consisting of F ^- , Cl ^- , Br ^- , I ^- , NO ₃ ^- , N(CN) ₂ ^- , BF ₄ ^- , CF ₃ CF ₂ SO ₃ ^- , (CF ₃ SO ₂ ) ₂ N ^- , (FSO ₂ ) ₂ N ^- , CF ₃ CF ₂ (CF ₃ ) ₂ CO ^- , (CF ₃ SO ₂ ) ₂ CH ^- , (SF ₅ ) ₃ C ^- , (CF ₃ SO ₂ ) ₃ C ^- , CF ₃ (CF ₂ ) ₇ SO ₃ ^- , CF ₃ CO ₂ ^- , CH ₃ CO ₂ ^- , SCN ^- and (CF ₃ CF ₂ SO ₂ ) ₂ N ^- , and the lithium salt may be at least one selected from the group consisting of LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAl0 ₄ , LiAlCl ₄ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN(C ₂ F ₅ SO ₃ ) ₂ , LiN(C ₂ F ₅ SO ₂ ) ₂ , LiN(CF ₃ SO ₂ ) _2. LiCl, LiI, or LiB(C ₂ O ₄ ) ₂ can be used. It is preferable to use the concentration of the lithium salt within the range of 0.1 to 2.0 M. When the concentration of the lithium salt is within the above range, the electrolyte can exhibit excellent electrolyte performance because it has appropriate conductivity and viscosity, and lithium ions can move effectively.

In addition to the electrolyte components, the electrolyte may further contain one or more additives, such as, for example, haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethylphosphite, triethanolamine, cyclic ethers, ethylene diamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxy ethanol, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing battery capacity decrease, and improving the discharge capacity of the battery. In this case, the additives may be contained in an amount of 0.1 to 5 wt% with respect to the total weight of the electrolyte.

Hereinafter, the present invention will be described in more detail through specific examples.

[Example: Production of positive electrode active material]

Example 1.

After adding 4 L of distilled water to a co-precipitation reactor (capacity 20 L), the temperature was maintained at 50°C, and 100 mL of an ammonia aqueous solution having a concentration of 28 wt% was added. Then, a transition metal solution in which NiSO ₄ and MnSO ₄ were mixed so that the molar ratio of nickel:manganese was 35:65, an ammonia aqueous solution, and a sodium hydroxide solution were added to the co-precipitation reactor, and a co-precipitation reaction was performed to form a precursor. The precursor particles were separated, washed by stirring at 2,000 rpm for 10 minutes at a weight ratio of the precursor to ultrapure water of 5:1 at a temperature of 25°C, and then dried in an oven at 130°C to produce a precursor.

The precursor synthesized by the coprecipitation reaction and LiOH were mixed so that the molar ratio of transition metal:Li was 1:1.3, and heat-treated at 900°C for 15 hours in an oxygen atmosphere to manufacture a cathode active material having a composition of Li _1.13 Ni _0.31 Mn _0.56 O ₂ . The manufactured cathode active material had an average primary particle size of 0.05 μm, a D ₅₀ of secondary particles of 10 μm, an average crystallite size of 80 nm, and a BET surface area of 1.5 m ² /g.

Comparative example 1.

A cathode active material was manufactured in the same manner as in Example 1, except that a 1 M NaOH aqueous solution was used instead of ultrapure water when washing the precursor particles in the above Example 1. The cathode active material manufactured in Comparative Example 1 had an average primary particle size of 0.2 μm, a D ₅₀ of secondary particles of 10 μm, an average crystallite size of 80 nm, and a BET specific surface area of 1.0 m ² /g.

Comparative example 2.

A cathode active material was manufactured in the same manner as in Example 1, except that a 0.5 M NaOH aqueous solution was used instead of ultrapure water when washing the precursor particles in Example 1. The cathode active material manufactured in Comparative Example 2 had an average primary particle size of 0.1 μm, a D ₅₀ of secondary particles of 10 μm, an average crystallite size of 80 nm, and a BET specific surface area of 1.2 m ² /g.

Example 2.

A cathode active material was manufactured in the same manner as in Example 1, except that the precursor precipitation reaction time in Example 1 was controlled to a level where the D50 value of the precipitated precursor was 3 μm and the weight ratio of the precursor to ultrapure water was changed from 5:1 to 2:1 when washing the precursor particles. The cathode active material manufactured in Example 2 had an average primary particle size of 0.05 μm, a _D50 of secondary particles of 3 μm, an average crystallite size of 80 nm, and a BET specific surface area of 2.5 m ² /g.

Comparative example 3.

A cathode active material was manufactured in the same manner as in Example 2, except that a 0.5 M NaOH aqueous solution was used instead of ultrapure water when washing the precursor particles in Example 2. The cathode active material manufactured in Comparative Example 3 had an average primary particle size of 0.1 μm, a D ₅₀ of secondary particles of 3 μm, an average crystallite size of 80 nm, and a BET specific surface area of 1.7 m ² /g.

Comparative example 4.

A cathode active material was manufactured in the same manner as in Example 2, except that a 1 M NaOH aqueous solution was used instead of ultrapure water when washing the precursor particles in Example 2. The cathode active material manufactured in Comparative Example 4 had an average primary particle size of 0.2 μm, a D ₅₀ of secondary particles of 3 μm, an average crystallite size of 80 nm, and a BET specific surface area of 1.4 m ² /g.

[Experimental example]

Experimental Example 1. Measurement of S content in positive electrode active material

0.1 g of the positive electrode active material powder manufactured in the above Examples 1-2 and Comparative Examples 1-4 was mixed with 2 mL of distilled water and 1 mL of concentrated nitric acid, then diluted with 50 mL of ultrapure water. The content of elemental sulfur was measured using an ICP-OES (PERKIN-ELMER, Optima 7300DV) device. The results are shown in Table 1 below.

Example 1 Example 2 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 6,000ppm 6500ppm 1,500ppm 3,000ppm 3000ppm 2000ppm

Experimental Example 2. SEM Analysis

The photographs of the positive electrode active materials manufactured in Example 1 and Comparative Examples 1-2 observed at magnifications of 50k, 20k, and 5k using a scanning electron microscope (SEM) are shown in FIGS. 1 to 3.

It can be confirmed through the photograph that the cathode active material of Comparative Example 1-2 is composed of larger primary particles than the cathode active material of Example 1.

Experimental Example 3. Coin Cell Performance Evaluation

1) Manufacturing of the anode

The positive electrode active material of Example 1, a conductive agent (carbon black), and a binder (polyvinylidene fluoride) were added to N-methyl-2-pyrrolidone (NMP) in a weight ratio of 94:3:3 to prepare a positive electrode slurry having a solid content of 60 wt%. The positive electrode slurry was applied to a 15 μm thick aluminum (Al) film as a positive electrode collector to a thickness of 45 μm and dried, and then roll pressed to prepare a positive electrode having a loading of 2.8 mAh/cm ² . Positive electrodes using the positive electrode active materials of Example 2 and Comparative Examples 1-4 were also prepared in the same manner, respectively.

2) Manufacturing of coin cells

A negative electrode slurry having a solid content of 60 wt% was prepared by adding a negative electrode active material (graphite), a binder (SBR-CMC), and a conductive agent (carbon black) to a solvent (water) in a weight ratio of 95:3.5:1.5. The negative electrode slurry was applied to a 10 μm thick copper (Cu) film as a negative electrode collector to a thickness of 67 μm, dried, and then roll pressed to prepare a negative electrode having a loading of 3.0 mAh/cm ² .

For each of the positive electrode of Example 1-2 and the positive electrode of Comparative Example 1-4 manufactured in 1) above, a 15 ㎛ thick polyethylene separator was interposed between the positive electrode and the negative electrode to manufacture an electrode assembly, which was then placed inside a coin-shaped (2032 type) secondary battery case, and an electrolyte was injected into the case to manufacture a lithium secondary battery. At this time, an electrolyte in which 1 M LiPF ₆ is dissolved in a mixed organic solvent of ethylene carbonate (EC): ethyl methyl carbonate (EMC) in a volume ratio of 1:2 was injected to manufacture a lithium secondary battery.

3) Life evaluation

Coin cells to which the positive active materials of the above Examples 1-2 and Comparative Examples 1-4 were applied were formed at 45°C, and then charged at a rate of 0.5C until the voltage reached 4.4 V in CCCV mode at 25°C. Thereafter, discharge was performed at a constant current of 0.5C until the voltage reached 2.5 V, and the initial discharge capacity and initial resistance were measured. The discharge capacity retention rate and the resistance increase rate were measured while performing 50 such charge and discharge cycles, and the results are shown in Table 2 below.

Example 1 Example 2 Comparative Example 1 Comparative Example 2 Comparative Example 4 Comparative Example 5 1 cycle charge/discharge at room temperature Initial discharge capacity [mAh/g] 200 205 198 198 200 195 Initial Resistance [Ohm @SOC 100%] 28 25 30 29 28 30 50 cycles of charge/discharge at room temperature Discharge capacity retention rate [%] 95 90 89 91 87 86 Resistance Increase Rate [%] 20 30 55 40 45 60

Through the results in Table 2 above, it can be confirmed that the coin cell using the cathode active material of Example 1-2 having a sulfur element content of 4,000 ppm or more in the cathode active material shows improved values in terms of initial capacity and initial resistance compared to the coin cell using the cathode active material of Comparative Example 1-4 having a sulfur element content of less than 4,000 ppm. This is a result that proves that even when the same firing temperature is applied, when the sulfur content in the cathode active material is controlled, there is an effect of improving crystallinity and suppressing the growth of primary particles, thereby preventing an increase in resistance. In addition, in terms of capacity retention rate and resistance increase rate after 50 cycles, the coin cell using the cathode active material of Example 1-2 shows superior performance than the coin cell using the cathode active material of Comparative Example 1-4. This is because the stress during repeated charge and discharge processes is reduced as the primary particle size is controlled, thereby improving the change in capacity and resistance according to the progression of the cycle.

Claims (11)

Contains a lithium manganese oxide represented by the following chemical formula 1,
Cathode active material having a sulfur content of 4,000 ppm or more based on the total weight of the cathode active material:
[Chemical Formula 1]
Li _1+a [Mn _1-(a+b+c) Ni _b M _c ]O ₂
In the above chemical formula 1,
M is at least one selected from the group consisting of Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr,
0.1≤a≤0.5, 0≤b<0.5, 0≤c≤0.1, 0≤a+b+c≤0.5.
In claim 1,
A cathode active material having an average primary particle size of 0.01 ㎛ to 5 ㎛.
In claim 2,
A cathode active material having an average primary particle diameter of 0.01 ㎛ to 0.1 ㎛.
In claim 1,
D ₅₀ is a cathode active material having a diameter of 2㎛ to 15㎛.
In claim 1,
A cathode active material having an average crystallite size of 20 nm to 150 nm.
In claim 1,
A positive electrode active material, wherein in the chemical formula 1, 0.1≤a≤0.4, 0.1≤b≤0.4, 0≤c≤0.05, and 0≤a+b+c<0.5.
In claim 1,
A cathode active material having a sulfur content of 4,000 ppm to 8,000 ppm based on the total weight of the cathode active material.
A step of forming precursor particles for a cathode active material by supplying a transition metal-containing solution containing nickel and manganese sulfate or sulfide, an ammonium cation complex forming agent, and a basic compound to a reactor and performing a co-precipitation reaction;
A step of manufacturing a precursor for a cathode active material by washing the precursor particles for the cathode active material with water; and
A method for producing a cathode active material according to claim 1, comprising the step of mixing a precursor for the cathode active material and a lithium raw material and calcining at 800°C to 950°C.
In claim 8,
A method for manufacturing a positive electrode active material, wherein no other substance is added other than water during the washing process.
A positive electrode comprising a positive electrode active material according to any one of claims 1 to 7.
Anode according to claim 9;
A cathode comprising a cathode active material;
A separator interposed between the positive and negative electrodes; and
A lithium secondary battery containing an electrolyte.