KR102738147B1 - 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 composite transition metal oxide having a layered structure in which the nickel content among the total transition metals is 80 atm% or more, and a rock-salt phase having a thickness of 20 nm or more is formed on at least a portion of the surface of the lithium composite transition metal oxide, 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 cathode active material having excellent continuous charging characteristics under high voltage and/or high temperature conditions, and a cathode and a lithium secondary battery including the same.

As technology development and demand for mobile devices increase, the demand for secondary batteries as an energy source is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which have high energy density and voltage, long cycle life, and low self-discharge rate, are commercialized and widely used.

Lithium transition metal composite oxides are used as positive electrode active materials for lithium secondary batteries, and among these, LiCoO ₂ has high operating voltage and excellent capacity characteristics. Lithium cobalt composite metal oxides are mainly used. However, LiCoO ₂ has very poor thermal properties due to instability of the crystal structure due to delithiation, and is expensive, so it is limited in mass use as a power source in fields such as electric vehicles.

As materials to replace the above LiCoO ₂ , lithium manganese composite metal oxides (such as LiMnO ₂ or LiMn ₂ O ₄ ), lithium iron phosphate compounds (such as LiFePO ₄ ), or lithium nickel composite metal oxides (such as LiNiO ₂ ) have been developed. Among these, lithium nickel composite metal oxides, which have a high reversible capacity of about 200 mAh/g and thus make it easy to implement large-capacity batteries, are being researched and developed more actively. However, LiNiO ₂ has inferior thermal stability compared to LiCoO ₂ , and there is a problem that when an internal short circuit occurs due to external pressure in a charged state, the positive electrode active material itself decomposes, causing rupture and ignition of the battery. Accordingly, as a method to improve the low thermal stability while maintaining the excellent reversible capacity of LiNiO ₂ , lithium nickel cobalt manganese oxide has been developed in which some of the Ni is replaced with Mn and Co.

Recently, as the demand for high-capacity cathode active materials has increased, high-nickel (high-Ni) cathode active materials with a nickel content of 80 atm% or higher in lithium nickel cobalt manganese oxide have been developed. However, high-nickel cathode active materials have a problem in that they have poor stability in high-voltage and/or high-temperature regions instead of realizing high energy density. For example, when a lithium secondary battery using a high-nickel cathode active material is continuously charged under high-voltage and/or high-temperature conditions, current leakage occurs, significantly reducing battery stability.

Therefore, there is a need for the development of high-nickel cathode active materials with excellent stability under high-voltage and/or high-temperature conditions.

The present invention is intended to solve the above problems, and provides a cathode active material capable of minimizing current leakage even during continuous charging at high voltage and/or high temperature, by forming a rock-salt phase having a thickness of 20 nm or more on the surface of a layered lithium composite transition metal oxide particle having a nickel content of 80 atm% or more.

In addition, the present invention aims to provide a positive electrode using the positive electrode active material, and a lithium secondary battery having excellent continuous charging performance even at high voltage and/or high temperature using the positive electrode.

In one aspect, the present invention provides a cathode active material comprising a lithium composite transition metal oxide having a layered structure in which a nickel content of 80 atm% or more among the total transition metals is present, and a rock-salt phase having a thickness of 20 nm or more is formed on at least a portion of the surface of the lithium composite transition metal oxide.

In another aspect, the present invention provides a positive electrode and a lithium secondary battery including the positive electrode active material of the present invention as described above.

The cathode active material of the present invention includes a thick rock-salt phase of 20 nm or more on the surface of lithium composite transition metal oxide particles, and due to the rock-salt phase, oxygen desorption is minimized under high voltage and/or high temperature conditions, thereby exhibiting superior stability compared to conventional high-nickel cathode active materials.

In particular, a secondary battery using the positive electrode active material of the present invention has excellent continuous charging characteristics because current leakage is suppressed during high-voltage continuous charging.

Figure 1 is a transmission electron microscope (TEM) image of a positive electrode active material manufactured according to Example 1.
Figure 2 is a transmission electron microscope (TEM) image of the positive electrode active material manufactured by Comparative Example 1.
Figure 3 is a graph showing the results of a high-voltage continuous charging test of a secondary battery using a positive electrode active material manufactured according to Example 1.
Figure 4 is a graph showing the results of a high-voltage continuous charging test of a secondary battery using a positive electrode active material manufactured by Example 2.
Figure 5 is a graph showing the results of a high-voltage continuous charging test of a secondary battery using a positive electrode active material manufactured by Comparative Example 1.
Figure 6 is a graph showing the results of a high-voltage continuous charging test of a secondary battery using a positive electrode active material manufactured by Comparative Example 2.

The terms or words used in this specification and claims should not be interpreted as limited to their usual or dictionary meanings, but should be interpreted as having meanings and concepts that conform to the technical idea of the present invention, based on the principle that the inventor can appropriately define the concept of the term in order to explain his or her own invention in the best manner.

In this specification, the average particle diameter (D ₅₀ ) can be defined as the particle diameter at 50% of the particle diameter distribution, and can be measured using a laser diffraction method. Specifically, the average particle diameter (D ₅₀ ) can be calculated by dispersing target particles in a dispersion medium, introducing them into a commercially available laser diffraction particle size measuring device (e.g. Microtrac MT 3000), and irradiating them with ultrasonic waves of about 28 kHz at an output of 60 W, and then calculating the average particle diameter (D ₅₀ ) at 50% of the particle volume cumulative distribution according to the particle diameter in the measuring device.

In this specification, 'primary particle' means a primary structure of a single particle, and 'secondary particle' means an aggregate formed by a plurality of primary particles cohesively cohesively through physical or chemical bonding between the primary particles.

In this specification, % means weight % unless otherwise stated.

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

The present inventors have conducted repeated studies to improve the high-voltage and/or high-temperature stability of a high-nickel cathode active material having a nickel content of 80 atm% or more among the total transition metals, and as a result, have discovered that the high-voltage and/or high-temperature stability of a high-nickel cathode active material can be dramatically improved by forming a rock salt phase of a certain thickness or more on the surface of a lithium composite transition metal oxide having a layered structure, thereby completing the present invention.

Specifically, the cathode active material according to the present invention is characterized in that it includes lithium composite transition metal oxide particles having a layered structure in which the nickel content among the total transition metals is 80 atm% or more, and a rock-salt phase having a thickness of 20 nm or more is formed on at least a portion of the surface of the lithium composite transition metal oxide particles.

In the past, it was believed that the formation of a salt phase on the surface of a lithium composite transition metal oxide having a layered structure would act as a factor that lowered the performance of the positive electrode active material, and thus the formation of a salt phase was avoided. However, as a result of the research of the present inventors, it was revealed that in a high-nickel positive electrode active material having a nickel content of 80 atm% or higher, the high-voltage continuous charge characteristics are dramatically improved when a salt phase of a certain thickness or more is formed.

<Bipolar active material>

Hereinafter, the positive electrode active material according to the present invention will be described in more detail.

The cathode active material of the present invention comprises lithium composite transition metal oxide particles having a layered structure in which the nickel content among the total transition metals is 80 atm% or more, and a rock-salt phase formed on at least a portion of the surface of the lithium composite transition metal oxide particles and having a thickness of 20 nm or more, preferably 30 nm to 500 nm.

In the present invention, a lithium composite transition metal oxide having a nickel content of 80 atm% or higher is used. When a lithium composite transition metal oxide having a high nickel content like this is used, high capacity characteristics can be implemented. However, a lithium composite transition metal oxide having a high nickel content of 80 atm% or higher has low structural stability, so that when the cycle is repeated or when exposed to high voltage and/or high temperature, gas generation and rapid battery performance deterioration occur due to a side reaction with the electrolyte. In addition, in the case of a lithium secondary battery using the above-mentioned high-nickel lithium composite transition metal oxide as a cathode active material, there is a problem that current leakage occurs during high-voltage continuous charging, which significantly deteriorates battery stability.

However, as in the present invention, when a rock-salt phase having a thickness of 20 nm or more is formed on at least a portion of the surface of the lithium composite transition metal oxide particles, the surface stability of the positive electrode active material is improved, thereby minimizing performance degradation and current leakage under high voltage and/or high temperature conditions.

At this time, the rock salt phase may have a thickness of 20 nm or more, preferably 30 nm to 500 nm. When the rock salt phase has a thickness of less than 20 nm, the effect of improving particle surface stability and suppressing current leakage during high-voltage continuous charging are minimal.

Meanwhile, the thickness of the above rock salt can be measured using a transmission electron microscope (TEM).

Meanwhile, in the present invention, the rock salt phase may have a portion having a thickness of 20 nm or more in at least a portion of the surface of the lithium composite transition metal oxide particle, and the thickness need not be 20 nm or more over the entire surface of the lithium composite transition metal oxide particle.

Preferably, in the cathode active material of the present invention, the area where the rock-salt phase having a thickness of 20 nm or more is formed may be 10% to 100%, preferably 10% to 75%, of the total surface area of the lithium composite transition metal oxide particles. When the area where the rock-salt phase having a thickness of 20 nm or more is formed satisfies the above range, the effect of suppressing structural degradation of the active material in a high temperature and/or high voltage region can be obtained.

Meanwhile, the lithium composite transition metal oxide may be, for example, represented by the following [chemical formula 1].

[Chemical Formula 1]

Li _x [Ni _y Co _z M ¹ _w M ² _v ]O ₂

In the above chemical formula 1, M ¹ is at least one selected from Mn and Al,

The above M ² is at least one selected from Ba, Ca, Zr, Ti, Mg, Ta, Nb, and Mo, and 0.9≤x≤1.5, 0.8≤y<1, 0<z≤0.15, 0<w<0.2, 0≤v≤0.2.

At this time, the above x means the atomic fraction of lithium in the lithium composite transition metal oxide, and may be 0.9≤x≤1.5, preferably 0.9≤x≤1.2, and more preferably 0.9≤x≤1.1.

The above y refers to the atomic fraction of nickel among the total transition metals of the lithium composite transition metal oxide, and may be 0.8≤y<1, preferably 0.8≤y≤0.98, and more preferably 0.81≤y≤0.98.

The above z represents the atomic fraction of cobalt among the total transition metals of the lithium composite transition metal oxide, and may be 0<z≤0.15, preferably 0.01≤z≤0.15, and more preferably 0.01≤z≤0.1.

The above w refers to the atomic fraction of the M ¹ element among the total transition metals of the lithium composite transition metal oxide, and may be 0<w<0.2, preferably 0.01≤w≤0.15.

The above v represents the atomic fraction of the M ² element among the total transition metals of the lithium composite transition metal oxide, and may be 0≤v≤0.2, preferably 0≤v≤0.15.

Meanwhile, the lithium composite transition metal oxide may further include a coating layer comprising at least one coating element selected from the group consisting of one or more elements selected from the group consisting of Al, Ti, W, B, F, P, Mg, Ni, Co, Fe, Cr, V, Cu, Ca, Zn, Zr, Nb, Mo, Sr, Sb, Bi, Si, and S, if necessary. When the coating layer is included as described above, contact between the lithium composite transition metal oxide and the electrolyte is blocked, thereby suppressing the occurrence of side reactions, and thus improving the life characteristics when applied to a battery, and further increasing the packing density of the cathode material.

As described above, when the coating element is additionally included, the content of the coating element in the coating layer may be 100 ppm to 10,000 ppm, preferably 200 ppm to 5,000 ppm, based on the total weight of the lithium composite transition metal oxide. For example, when the coating element is included in the above range based on the total weight of the lithium composite transition metal oxide, the occurrence of side reactions with the electrolyte is more effectively suppressed, and the life characteristics can be further improved when applied to a battery.

The coating layer may be formed on the entire surface of the lithium composite transition metal oxide, or may be formed partially. Specifically, when the coating layer is formed partially on the surface of the lithium composite transition metal oxide, it may be formed in an area of 5% or more and less than 100%, preferably 20% or more and less than 100% of the entire surface area of the lithium composite transition metal oxide.

The above lithium composite transition metal oxide may be a lithium composite transition metal oxide particle in the form of a secondary particle in which a plurality of primary particles are aggregated, and at this time, the average particle diameter (D ₅₀ ) of the secondary particles may be 10 to 20 μm, preferably 10 to 17 μm, and more preferably 10 to 15 μm.

When lithium composite transition metal oxides in the form of secondary particles are used, sintering is possible at relatively low temperatures, and active material particles can be formed into spherical shapes. In addition, since secondary particles have a wider reaction area than primary particles, they have excellent interfacial resistance characteristics, and since the diffusion reaction distance is short, they are also advantageous in terms of diffusion resistance. However, if the average particle diameter of the secondary particles becomes too large, the above effect cannot be obtained, so it is preferable to use secondary particles having an average particle diameter of 10 to 20 μm as described above.

<Method for manufacturing positive electrode active material>

Next, a method for manufacturing a positive electrode active material according to the present invention will be described.

A method for manufacturing a cathode active material according to the present invention comprises the steps of 1) mixing a transition metal precursor having a nickel content of 80 atm% or more among the total transition metals and a lithium raw material, and 2) calcining the mixture at a temperature of 850° C. to 1000° C. to form lithium composite transition metal oxide particles having a rock-salt phase having a thickness of 20 nm or more formed on at least a portion of the particle surface.

First, a transition metal precursor having a nickel content of 80 atm% or more among the total transition metals and a lithium raw material are mixed.

The above transition metal precursor may be a transition metal hydroxide or oxyhydroxide having a nickel content of 80 atm% or more.

The above transition metal precursor can be purchased and used as a commercially available cathode active material precursor, or can be manufactured according to a method for manufacturing a cathode active material precursor known in the art.

For example, the transition metal precursor may be prepared by adding an ammonium cation-containing complex forming agent and a basic compound to a transition metal solution containing a nickel-containing raw material, a cobalt-containing raw material, an M ¹ -containing raw material, and/or an M ^2- containing raw material, and performing a co-precipitation reaction.

The above nickel-containing raw material may be, for example, a nickel-containing acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide or oxyhydroxide, and specifically, may be, but is not limited to, Ni(OH) ₂ , NiO, NiOOH, NiCO ₃ ㆍ2Ni(OH) ₂ ㆍ4H ₂ O, NiC ₂ O ₂ ㆍ2H ₂ O, Ni(NO ₃ ) ₂ ㆍ6H ₂ O, NiSO ₄ , NiSO ₄ ㆍ6H ₂ O, a fatty acid nickel salt, a nickel halide or a combination thereof.

The above cobalt-containing raw material may be a cobalt-containing acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide or oxyhydroxide, and specifically, may be Co(OH) ₂ , CoOOH, Co(OCOCH ₃ ) ₂ ㆍ4H ₂ O, Co(NO ₃ ) ₂ ㆍ6H ₂ O, CoSO ₄ , Co(SO ₄ ) ₂ ㆍ7H ₂ O or a combination thereof, but is not limited thereto.

The above M ¹ containing raw material may be an M ¹ containing acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide, oxyhydroxide or a combination thereof, and specifically may be a manganese containing acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide, oxyhydroxide and/or an aluminum containing acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide, oxyhydroxide. For example, the above M ¹ containing raw material may be a manganese oxide such as Mn ₂ O ₃ , MnO ₂ , Mn ₃ O ₄ ; a manganese salt such as MnCO ₃ , Mn(NO ₃ ) ₂ , MnSO ₄ , manganese acetate, manganese dicarboxylic acid salt, manganese citrate, and fatty acid manganese salt; It can be, but is not limited to, manganese oxyhydroxide, manganese chloride, Al ₂ O ₃ , Al(OH) ₃ , AlSO ₄ , AlCl ₃ , Al-isopropoxide, AlNO ₃ , AlF or a combination thereof.

The above M ² containing raw material may be an M ² containing acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide, oxyhydroxide or a combination thereof.

The above transition metal solution may be prepared by adding a nickel-containing raw material, a cobalt-containing raw material and an M ^1- containing raw material to a solvent, specifically, water, or a mixed solvent of an organic solvent (e.g., alcohol, etc.) that can be uniformly mixed with water, or may be prepared by mixing an aqueous solution of a nickel-containing raw material, an aqueous solution of a cobalt-containing raw material and an aqueous solution of a M ¹ -containing raw material.

The above ammonium cation-containing complex forming agent may be, for example, NH ₄ OH, (NH ₄ ) ₂ SO ₄ , NH ₄ NO ₃ , NH ₄ Cl, CH ₃ COONH ₄ , NH ₄ CO ₃ or a combination thereof, but is not limited thereto. Meanwhile, the ammonium cation-containing complex forming agent may be used in the form of an aqueous solution, and at this time, water or a mixture of water and an organic solvent (specifically, alcohol, etc.) that can be uniformly mixed with water may be used as a solvent.

The above basic compound may be a hydroxide of an alkali metal or alkaline earth metal, such as NaOH, KOH or Ca(OH) ₂ , a hydrate thereof or a combination thereof. The above basic compound may also be used in the form of an aqueous solution, and in this case, water or a mixture of water and an organic solvent (specifically, alcohol, etc.) that can be evenly mixed with water may be used as a solvent.

The above basic compound is added to adjust the pH of the reaction solution, and can be added in an amount such that the pH of the metal solution becomes 10.5 to 13, preferably 11 to 13.

Meanwhile, the coprecipitation reaction can be performed at a temperature of 40 to 70° C. under an inert atmosphere such as nitrogen or argon. In addition, a stirring process can be optionally performed to increase the reaction speed during the reaction, and at this time, the stirring speed can be 100 to 2000 rpm.

Transition metal precursor particles are generated by the above process and precipitated in the reaction solution. The precipitated transition metal precursor particles can be separated and dried according to a conventional method to obtain a transition metal precursor.

Next, the transition metal precursor obtained by the above method is mixed with a lithium raw material and then calcined. The lithium raw material may include a lithium-containing carbonate (e.g., lithium carbonate, etc.), a hydrate (e.g., lithium hydroxide hydrate (LiOH H ₂ O)), a hydroxide (e.g., lithium hydroxide, etc.), a nitrate (e.g., lithium nitrate (LiNO ₃ )), a chloride (e.g., lithium chloride (LiCl)), etc., and one of these may be used alone or a mixture of two or more thereof may be used.

Meanwhile, the mixing of the transition metal precursor and the lithium source material can be achieved by solid-state mixing such as jet milling.

Meanwhile, it is preferable that the transition metal precursor and the lithium raw material be mixed so that the molar ratio of lithium to transition metal is 1:1 to 1.25:1, preferably 1:1 to 1.15:1. If the mixing amount of the lithium raw material exceeds the above range, residual lithium byproducts such as LiOH and Li ₂ CO ₃ are excessively generated, the composition ratio of the active material changes, and the crystallinity decreases so that the capacity is not properly expressed.

Meanwhile, when mixing the transition metal precursor and the lithium raw material, the M ¹ raw material and/or the M ² raw material may be additionally mixed as needed, and at this time, the mixing ratio of the M ¹ and M ² containing raw materials may be appropriately adjusted in consideration of the atomic ratio of each element in the lithium composite transition metal oxide ultimately produced.

For example, when a lithium composite transition metal oxide contains both Mn and Al as the M ¹ element, a transition metal precursor containing Ni, Co, Mn, and Al may be used as the transition metal precursor, but a precursor containing Ni, Co, and Mn may be used as the transition metal precursor, and an aluminum raw material may be additionally mixed in when mixing with the lithium raw material. At this time, the aluminum raw material may be Al ₂ O ₃ , Al(OH) ₃ , AlSO ₄ , AlCl ₃ , AlNO ₃ , AlF or a combination thereof, but is not limited thereto.

Meanwhile, the calcination can be performed at a temperature of 850° C. to 1000° C., preferably 850° C. to 900° C., for 12 to 24 hours. When the heat treatment temperature and time satisfy the above ranges, a lithium composite transition metal oxide in which a rock salt phase having a thickness of 20 nm or more is formed on at least a portion of the particle surface can be obtained.

<Bipolar and secondary batteries>

The cathode material according to the present invention as described above can be usefully used in the manufacture of a cathode of a lithium secondary battery.

Specifically, the positive electrode according to the present invention includes the positive electrode active material according to the present invention described above. More specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, and at this time, the positive electrode active material layer may include the positive electrode active material according to the present invention. Since the specific content of the positive electrode active material according to the present invention is the same as described above, a detailed description is omitted.

The above positive electrode can be manufactured according to a conventional positive electrode manufacturing method, except that the positive electrode active material according to the present invention is used as the positive electrode active material. For example, the positive electrode can be manufactured by a method in which components constituting the positive electrode active material layer, i.e., the positive electrode active material, a conductive material, and/or a binder, are dissolved or dispersed in a solvent to manufacture a positive electrode composite, and the positive electrode composite is applied to at least one surface of a positive electrode current collector, and then dried and rolled, or by casting the positive electrode composite on a separate support, and then peeling the film from the support and laminating the obtained film on a positive electrode current collector.

At this time, the positive electrode current collector is not particularly limited as long as it is conductive and does not cause a chemical change in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. may be used. In addition, the positive electrode 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 increase the adhesive strength of the positive electrode active material. For example, it 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.

A cathode active material layer comprising a cathode active material according to the present invention is positioned on at least one surface of the cathode current collector, and optionally further comprising at least one of a conductive material and a binder, as needed.

The above-mentioned positive electrode active material may be included in an amount of 80 to 99 wt%, more specifically 85 to 98 wt%, based on the total weight of the positive electrode active material layer. When included in the above-mentioned amount range, excellent capacity characteristics may be exhibited.

The conductive material is used to provide conductivity to the electrode, and in the battery to be formed, as long as it does not cause a chemical change and has electronic conductivity, it can be used without special restrictions. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and the like, and one of these may be used alone or a mixture of two or more may be used. The conductive material may be included in an amount of 1 wt% to 30 wt% with respect to the total weight of the cathode active material layer.

In addition, the binder serves to improve the adhesion between the positive electrode active material particles and the adhesive strength between the positive electrode active material and the current collector. Specific examples thereof include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of these may be used alone or a mixture of two or more thereof. The binder may be included in an amount of 1 wt% to 30 wt% with respect to the total weight of the positive electrode active material layer.

Meanwhile, the solvent used in the manufacture of the positive electrode composite may be a solvent generally used in the relevant technical field, and for example, dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water may be used alone or in a mixture thereof. The amount of the solvent used may be appropriately adjusted in consideration of the coating thickness of the slurry, manufacturing yield, viscosity, etc.

Next, a secondary battery according to the present invention will be described.

A secondary battery according to the present invention comprises a positive electrode, a negative electrode positioned opposite the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode is the positive electrode according to the present invention described above.

Meanwhile, the secondary battery may optionally further include a battery container that accommodates the electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

In the above secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode active material layer positioned on at least one surface of the negative electrode current collector.

The above negative electrode can be manufactured according to a conventional negative electrode manufacturing method generally known in the art. For example, the negative electrode can be manufactured by dissolving or dispersing components constituting the negative electrode active material layer, i.e., the negative electrode active material, a conductive material, and/or a binder, etc., in a solvent to manufacture a negative electrode composite, applying the negative electrode composite to at least one surface of a negative electrode current collector, and then drying and rolling it, or by casting the negative electrode composite on a separate support, and then peeling the film from the support and laminating it on a negative electrode current collector.

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

As the negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium can be used. Specific examples thereof include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, and Al alloy; metallic oxides capable of doping and dedoping lithium, such as SiO _v (0<v<2), SnO ₂ , vanadium oxide, and lithium vanadium oxide; or composites containing the metallic compounds and carbonaceous materials, such as Si-C composites or Sn-C composites, and any one or a mixture of two or more of these can be used. In addition, a metallic lithium thin film can be used as the negative electrode active material. In addition, the carbon material can be both low-crystalline carbon and high-crystalline carbon. Representative examples of low-crystallization carbon include soft carbon and hard carbon, and representative examples of high-crystallization carbon include amorphous, plate-like, flaky, spherical or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesophase pitches, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch derived cokes.

Additionally, the binder and the conductive material may be the same as those described above for the anode.

Meanwhile, in the secondary battery, the separator separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. Any separator commonly used in secondary batteries can be used without special restrictions, and in particular, one having 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 optionally be used in a single-layer or multi-layer structure.

Meanwhile, the electrolyte may include, but is not limited to, an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, etc. that can be used in the manufacture of a secondary battery.

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 Ra-CN (where Ra 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 lithium salt above can be used without limitation as an electrolyte for a lithium secondary battery, and for example, the cation of the lithium salt includes Li ⁺ and the anion includes F ^- , Cl ^- , Br ^- , I ^- , NO ₃ ^- , N(CN) ₂ ^- , BF ₄ ^- , ClO ₄ ^- , AlO ₄ ^- , AlCl ₄ ^- , PF ₆ ^- , SbF ₆ ^- , AsF ₆ ^- , BF ₂ C ₂ O ₄ ^- , BC ₄ O ₈ ^- , PF ₄ C ₂ O ₄ ^- , PF ₂ C ₄ O ₈ ^- , (CF ₃ ) ₂ PF ₄ ^- , (CF ₃ ) ₃ PF ₃ ^- , (CF ₃ ) ₄ PF ₂ ^- , (CF ₃ ) ₅ PF ^- , (CF ₃ ) ₆ P ^- , At least one selected from the group consisting of CF ₃ SO ₃ ^- , C ₄ F ₉ SO ₃ ^- , 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 ^- . Specifically, the lithium salt may include a single substance or a mixture of two or more substances selected from the group consisting of LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCH ₃ CO ₂ , LiCF ₃ CO ₂ , LiAsF ₆ , LiSbF ₆ , LiAlCl ₄ , LiAlO ₄ , and LiCH ₃ SO ₃ .

The above lithium salt may be appropriately changed within a generally usable range, but may be specifically included in the electrolyte at 0.8 M to 3 M, specifically 0.1 M to 2.5 M.

In addition to the electrolyte components, various additives may be used in the electrolyte for the purpose of improving the life characteristics of the battery, suppressing battery capacity decrease, and improving the discharge capacity of the battery. Examples of such additives include haloalkylene carbonate compounds such as difluoroethylene carbonate; or 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-methoxyethanol, or aluminum trichloride. The additives may be used alone or in combination. At this time, the additives may be included in an amount of 0.1 wt% to 5 wt% with respect to the total weight of the electrolyte.

The lithium secondary battery according to the present invention as described above can be usefully used in portable devices such as mobile phones, laptop computers, digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs).

Accordingly, according to another embodiment of the present invention, a battery module including the lithium secondary battery as a unit cell and a battery pack including the same are provided.

The above battery module or battery pack can be used as a power source for one or more medium- to large-sized devices, including power tools; electric vehicles including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

There is no particular limitation on the external shape of the lithium secondary battery of the present invention, but it may be in the shape of a cylinder, a square, a pouch, or a coin using a can.

The lithium secondary battery according to the present invention can be used not only as a battery cell used as a power source for a small device, but can also be preferably used as a unit battery in a medium- to large-sized battery module including a plurality of battery cells.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. However, the present invention may be implemented in various different forms and is not limited to the embodiments described herein.

Example 1

[Ni _{0 . 85} Co _{0 . 1} Mn _{0 . 05} ]O OH and lithium raw material LiOH were mixed so that the molar ratio of Li: transition metal (Ni+Co+Mn) was 1.1: 1, and the mixed powder was placed in an alumina crucible and calcined at 870°C for 20 hours in an oxygen (O ₂ ) atmosphere to form a lithium composite transition metal oxide Li[Ni _0.85 Co _0.1 Mn _0.05 ]O ₂ in the form of secondary particles having an average particle size D ₅₀ of 10 μm.

As a result of measuring the manufactured lithium composite transition metal oxide particles using a transmission electron microscope, it was confirmed that a rock salt phase existed on the surface of the lithium composite transition metal oxide particles, and the maximum thickness of the rock salt phase was approximately 30 nm. In addition, the formation area of the rock salt phase with a thickness of 20 nm or more was approximately 50% of the total surface area of the lithium composite transition metal oxide particles.

Example 2

[Ni _{0 . 9} Co _{0 . 05} Mn _{0 . 05} ]O OH and lithium raw material LiOH were mixed so that the molar ratio of Li: transition metal (Ni+Co+Mn) was 1.1:1, and the mixed powder was placed in an alumina crucible and calcined at 880℃ for 20 hours in an oxygen (O ₂ ) atmosphere to form a lithium composite transition metal oxide Li[Ni _0.9 Co _0.05 Mn _0.05 ]O ₂ in the form of secondary particles with an average particle size D ₅₀ of 14 μm.

As a result of measuring the manufactured lithium composite transition metal oxide particles using a transmission electron microscope, it was confirmed that a rock salt phase existed on the surface of the lithium composite transition metal oxide particles, and the maximum thickness of the rock salt phase was approximately 50 nm. In addition, the formation area of the rock salt phase with a thickness of 20 nm or more was approximately 30% of the total surface area of the lithium composite transition metal oxide particles.

Comparative example 1

[Ni _{0 . 85} Co _{0 . 1} Mn _{0 . 05} ]O OH and lithium raw material LiOH were mixed so that the molar ratio of Li: transition metal (Ni+Co+Mn) was 1.1:1, and the mixed powder was placed in an alumina crucible and calcined at 800°C for 20 hours in an oxygen (O ₂ ) atmosphere to form a lithium composite transition metal oxide Li[Ni _0.85 Co _0.1 Mn _0.05 ]O ₂ in the form of secondary particles having an average particle size D ₅₀ of 10 μm.

As a result of measuring the manufactured lithium composite transition metal oxide particles using a transmission electron microscope, it was confirmed that no rock salt phase existed on the surface of the lithium composite transition metal oxide particles.

Comparative example 2

[Ni _{0 . 9} Co _{0 . 05} Mn _{0 . 05} ]O OH and lithium raw material LiOH were mixed so that the molar ratio of Li: transition metal (Ni+Co+Mn) was 1.1:1, and the mixed powder was placed in an alumina crucible and calcined at 800°C for 20 hours in an oxygen (O ₂ ) atmosphere to form a lithium composite transition metal oxide Li[Ni _0.9 Co _0.05 Mn _0.05 ]O ₂ in the form of secondary particles with an average particle size D ₅₀ of 14 μm.

As a result of measuring the manufactured lithium composite transition metal oxide particles using a transmission electron microscope, it was confirmed that a rock salt phase existed on the surface of the lithium composite transition metal oxide particles, and the maximum thickness of the rock salt phase was approximately 10 nm.

Experimental example 1

The positive electrode active materials of Example 1 and Comparative Example 1 were observed using a transmission electron microscope (TEM) to confirm whether a rock salt phase was formed.

FIG. 1 shows a TEM image of the positive electrode active material of Example 1, and FIG. 2 shows a TEM image of the positive electrode active material of Comparative Example 1.

Through Fig. 1, it can be confirmed that the positive electrode active material of Example 1 has a rock salt phase with a thickness of about 30 nm formed on the surface of the lithium composite transition metal oxide, and that the interior of the positive electrode active material has a layered structure. On the other hand, as shown in Fig. 2, it can be confirmed that the positive electrode active material of Comparative Example 1 has a layered structure on both the surface and the interior of the lithium composite transition metal oxide, and that no rock salt phase is formed.

<Coin Half-Cell Manufacturing>

The positive electrode active material, carbon black conductive material, and PVdF binder manufactured by the above examples and comparative examples were mixed in a weight ratio of 96.5:1.5:2.0 in an N-methylpyrrolidone solvent to manufacture a positive electrode composite, which was then applied to one surface of an aluminum current collector, dried at 130°C, and rolled to manufacture a positive electrode.

Lithium metal was used as the cathode.

An electrode assembly was manufactured by interposing a porous polyethylene separator between the positive and negative electrodes manufactured as described above, and after positioning the electrode assembly inside a case, an electrolyte was injected into the case to manufacture a coin halfcell.

At this time, the electrolyte was prepared by dissolving 1.0 M lithium hexafluorophosphate (LiPF ₆ ) in an organic solvent consisting of ethylene carbonate/ethyl methyl carbonate/diethyl carbonate/(mixed volume ratio of EC/EMC/DEC = 3/4/3).

Experimental example 2 - Continuous charging evaluation

A continuous charging test was performed on coin half-cells manufactured by the above method using the positive electrode active materials of Examples 1 and 2 and Comparative Examples 1 and 2. Specifically, three coin half-cells each were charged at 0.2 C/0.2 C at 50° C., then charged at 0.2 C in CC-CV mode until the voltage reached 4.7 V, and the termination time was set to 120 hours. Then, the current and voltage changes for 120 hours were measured.

The measurement results are shown in Figs. 3 to 6. Fig. 3 shows the results of a continuous charging test of a coin half-cell using the positive electrode active material of Example 1, and Fig. 4 shows the results of a continuous charging test of a coin half-cell using the positive electrode active material of Example 2. In addition, Figs. 5 and 6 show the results of a continuous charging test of a coin half-cell using the positive electrode active material of Comparative Example 1 and the positive electrode active material of Comparative Example 2, respectively.

As shown in FIGS. 3 to 6, in the case of the coin half-cells to which the positive active materials of Examples 1 and 2 were applied, the current and voltage were stably maintained even during continuous charging at high temperatures and high voltages, whereas in the case of the coin half-cells to which the positive active materials of Comparative Examples 1 and 2 were applied, it was confirmed that current leakage occurred.

Claims (11)

Contains layered lithium composite transition metal oxide particles having a nickel content of 80 atm% or more among the total transition metals,
A rock-salt phase having a thickness of 20 nm or more is formed on at least a portion of the surface of the lithium composite transition metal oxide particle,
The above lithium composite transition metal oxide particles are cathode active materials represented by the following [chemical formula 1]:
[Chemical Formula 1]
Li _x [Ni _y Co _z M ¹ _w M ² _v ]O ₂
In the above chemical formula 1,
The above M ¹ is at least one selected from Mn and Al,
The above M ² is at least one selected from Ba, Ca, Zr, Ti, Mg, Ta, Nb, and Mo,
0.9≤x≤1.5, 0.8≤y<1, 0<z≤0.15, 0<w<0.2, 0≤v≤0.2.
delete In the first paragraph,
In the above chemical formula 1, a positive electrode active material where 0.85≤y<1.
In the first paragraph,
A cathode active material having a rock-salt phase having a thickness of 30 nm to 500 nm.
In the first paragraph,
A cathode active material in which an area of a rock-salt phase having a thickness of 20 nm or more is formed is 10% to 100% of the total surface area of the lithium composite transition metal oxide particles.
In the first paragraph,
The above lithium composite transition metal oxide particles are cathode active materials in the form of secondary particles in which a plurality of primary particles are aggregated.
In Article 6,
A cathode active material having an average particle diameter D ₅₀ of the above secondary particles of 10 to 20 μm.
A step of mixing a transition metal precursor and a lithium raw material having a nickel content of 80 atm% or more among the total transition metals; and
A method for producing a cathode active material according to claim 1, comprising the step of calcining the mixture at a temperature of 850° C. to 1000° C. to form lithium composite transition metal oxide particles in which a rock-salt phase having a thickness of 20 nm or more is formed on at least a portion of the particle surface.
In Article 8,
A method for producing a cathode active material, wherein the above transition metal precursor and lithium raw material are mixed so that the molar ratio of lithium:transition metal is 1:1 to 1.25:1.
A positive electrode comprising the positive active material of claim 1.
A lithium secondary battery comprising the positive electrode of claim 10.