KR20180076647A - Positive electrode active material for secondary battery, method for producing the same, positive electrode and secondary battery comprising the same - Google Patents

Abstract

The present invention relates to a positive electrode active material for a secondary battery, a production method thereof, and a positive electrode and a secondary battery comprising the same, wherein the positive electrode active material comprises: a lithium composite metal oxide represented by chemical formula 1, Li_(1+x) [Ni_aCo_bMn_cM_d]_(1-x)O_(2-y)X_y; and a cobalt-rich layer formed on the surface of the lithium composite metal oxide and having a cobalt content higher than that of the lithium composite metal oxide. In the chemical formula 1, 0.7<a<1, 0<b<0.3, 0<c<0.1, 0<d<0.1, a+b+c+d=1, 0<=x<=0.05, 0<=y<=0.2, M is one or more metal elements selected from the group consisting of Al, Mg, Ge, V, Mo, Nb, Si, Ti, Zr, and W, and X is P or F.

Description

TECHNICAL FIELD The present invention relates to a positive electrode active material for a secondary battery, a method for producing the same, a positive electrode and a secondary battery for a secondary battery including the positive electrode active material,

The present invention relates to a positive electrode active material for a secondary battery, a method for producing the same, and a positive electrode and a secondary battery for the same. More particularly, the present invention relates to a positive electrode active material having a low residual amount of lithium byproduct on the active material surface, Ni-rich cathode active material, a method for producing the same, and a cathode and a secondary battery for the secondary battery comprising the same.

As technology development and demand for mobile devices increase, the demand for secondary batteries as energy sources is rapidly increasing. Among such secondary batteries, lithium secondary batteries having a high energy density and voltage, a long cycle life, and a low self-discharge rate are commercially available and widely used.

Lithium transition metal composite oxides are used as the cathode active material of lithium secondary batteries, and among them lithium cobalt composite metal oxides of LiCoO ₂ having high action voltage and excellent capacity characteristics are mainly used. However, since LiCoO ₂ has a poor thermal characteristic due to destabilization of crystal structure due to depolythium, and is expensive, it can not be used as a power source in fields such as electric vehicles and the like.

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 as materials for replacing LiCoO ₂ . Among them, research and development of a lithium nickel composite metal oxide having a high reversible capacity of about 200 mAh / g and facilitating the realization of a large capacity battery has been actively studied. However, LiNiO ₂ has poor thermal stability as compared with LiCoO _2, and if an internal short circuit occurs due to external pressure or the like in a charged state, there is a problem that the cathode active material itself is decomposed to cause rupture and ignition of the battery.

Accordingly, a method for improving low thermal stability while maintaining an excellent reversible capacity of LiNiO ₂ is known as a nickel-cobalt manganese-based lithium composite metal oxide in which a part of Ni is substituted with Mn and Co (hereinafter simply referred to as 'NCM-based lithium oxide' ) Was developed. However, the conventional NCM-based lithium oxide developed to date has a limited capacity because of insufficient capacitance characteristics.

In order to overcome such a problem, studies have recently been made to increase the content of Ni in NCM-based lithium oxide. However, in the case of a high-concentration nickel cathode active material having a high nickel content, there is a problem that the structural stability and chemical stability of the active material are deteriorated and the thermal stability is rapidly deteriorated. Also, as the content of nickel in the active material increases, the residual amount of lithium by-products existing in the form of LiOH and Li ₂ CO ₃ on the surface of the cathode active material is increased to cause gas generation and swelling, There is also a problem of degradation.

Therefore, development of a high-concentration nickel-rich cathode active material which is in conformity with high capacity and which has a small residual amount of lithium by-products and is excellent in high temperature stability is required.

Korean Patent Laid-Open No. 10-2016-0063982 (published on June 06, 2016)

Disclosure of the Invention The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide a high-concentration nickel cathode active material capable of simultaneously realizing excellent capacity characteristics and high temperature stability by forming a cobalt- A manufacturing method thereof, and a positive electrode and a secondary battery for the secondary battery including the same.

In one aspect, the present invention relates to a lithium composite metal oxide represented by Formula 1 below; And a cobalt-rich layer formed on the surface of the lithium composite metal oxide, the cobalt-rich layer having a higher content of cobalt than the lithium composite metal oxide.

[Chemical Formula 1]

Li _{1 + x} [Ni _a Co _b Mn _c M _d ] _1-x O _2-y X _y

In the above formula (1), 0.7 <a <1, 0 <b <0.3, 0 <c <0.1, 0 <d <0.1, a + b + c + d = 1, 0≤x≤0.05, 0≤y≤0.2 And M is one kind of metal element selected from the group consisting of Al, Mg, Ge, V, Mo, Nb, Si, Ti, Zr and W, and X is P or F.

The ratio of the number of atoms of cobalt to the total number of atoms of nickel, cobalt, manganese and M in the cobalt-rich layer and the number of atoms of cobalt relative to the sum of the numbers of atoms of nickel, cobalt, manganese and M in the lithium composite metal oxide May be 0.05 to 0.2, preferably 0.05 to 0.15.

Specifically, the ratio of the number of atoms of cobalt to the sum of the numbers of atoms of nickel, cobalt, manganese and M in the cobalt-rich layer may be 0.05 to 0.45, preferably 0.05 to 0.35.

The cobalt-rich layer may have a thickness of 10 to 100 nm, preferably 10 to 70 nm.

On the other hand, when the heat flow is measured by differential scanning calorimetry (DSC), the cathode active material exhibits a peak in a temperature range of 230 ° C to 250 ° C, preferably 235 ° C to 245 ° C, The maximum value of the heat flow may be 1000 mW or less, preferably 600 mW or less, and more preferably 400 mW or less.

The content of lithium by-products in the cathode active material may be 1 wt% or less, preferably 0.5 wt% or less, more preferably 0.3 wt% or less.

According to another aspect of the present invention, there is provided a lithium secondary battery comprising: preparing a lithium composite metal oxide represented by Formula 1; Washing the lithium composite metal oxide with water; And mixing the washed lithium composite metal oxide with the cobalt-containing starting material, and then heat-treating the lithium composite metal oxide in an oxidizing atmosphere at a temperature of 700 ° C or higher.

The preparation of the lithium composite metal oxide represented by Formula 1 may include a method of mixing the nickel-cobalt precursor, the manganese-containing raw material, the M-containing raw material, and the lithium-containing raw material and firing the mixture at 600 ° C. to 900 ° C. Lt; / RTI &gt; The washing may be performed at a temperature of 20 DEG C or lower.

In the heat treatment step, the cobalt-containing raw material may be mixed in an amount of 0.001 to 0.01 parts by weight, preferably 0.002 to 0.008 parts by weight based on 100 parts by weight of the lithium composite metal oxide.

Also, the heat treatment may be performed at a temperature of 700 ° C to 800 ° C within 10 hours.

According to another aspect of the present invention, there is provided a secondary battery including a positive electrode for a secondary battery including the positive electrode active material for a secondary battery according to the present invention, and a secondary battery comprising the positive electrode for the secondary battery.

The positive electrode active material for a secondary battery according to the present invention has a cobalt-rich layer containing a high content of cobalt having excellent output characteristics and stability on its surface, and is structurally stable unlike the conventional high-concentration nickel positive electrode active material. It is possible to realize a secondary battery having excellent capacity characteristics and a small resistance increase rate.

When the cobalt-containing raw material is mixed with water and subjected to a high-temperature heat treatment in an oxidizing atmosphere, the residual amount of the lithium by-product is reduced, and the structural stability is improved while recrystallization occurs, A cathode active material can be produced.

FIG. 1 is a graph showing the heat flow of the cathode active material according to Example 1 and Comparative Example 3 according to temperature. FIG.
FIG. 2 is a graph showing the pH titration curves of the cathode active materials of Example 1, Comparative Examples 1 and 4 to 6. FIG.
3 is a graph showing a capacity retention rate of a battery cell manufactured using the cathode active material of Example 1 and Comparative Examples 2 and 3 according to the charge reversal cycle.
FIG. 4 is a graph showing the rate of resistance increase according to charge-discharge cycles of a battery cell manufactured using the cathode active material of Example 1 and Comparative Examples 2 and 3. FIG.

Below. The present invention will be described in more detail.

The present inventors have conducted extensive research to produce a high-concentration nickel-based cathode active material having a low residual amount of lithium by-products and excellent high-temperature stability. As a result, they have found that a lithium composite metal oxide containing a specific metal element M in combination with nickel, cobalt, The nickel-based positive electrode active material having a low residual amount of lithium by-products and an excellent high-temperature stability can be produced by mixing the cobalt-containing raw material with water, washing with water and then heat-treating the mixture at a temperature of 700 ° C or higher in an oxidizing atmosphere. Respectively.

First, a method of manufacturing a cathode active material for a secondary battery according to the present invention will be described.

Method for manufacturing cathode active material for secondary battery

The method for producing a cathode active material according to the present invention comprises the steps of: (1) preparing a lithium composite metal oxide represented by Chemical Formula 1; (2) washing the lithium composite metal oxide; and (3) Containing raw material and then heat-treating the cobalt-containing raw material at a temperature of 700 ° C or higher in an oxidizing atmosphere. Hereinafter, each step of the present invention will be described in more detail.

(1) preparing a lithium composite metal oxide

First, a lithium composite metal oxide is prepared.

The lithium composite metal oxide used in the present invention may be at least one selected from the group consisting of nickel, cobalt, manganese, and four elements of M, which is a metal element selected from the group consisting of Al, Mg, Ge, V, Mo, Nb, Si, Ti, Zr, Component lithium-metal composite oxide, and the molar ratio of nickel in the four components is more than 0.7, and is a high-nickel-based four-component lithium composite metal oxide.

Specifically, the lithium composite metal oxide may be a lithium composite metal oxide represented by the following general formula (1).

[Chemical Formula 1]

Li _{1 + x} [Ni _a Co _b Mn _c M _d ] _1-x O _2-y X _y

Wherein M is one kind of metal element selected from the group consisting of Al, Mg, Ge, V, Mo, Nb, Si, Ti, Zr and W and X is P or F.

A, b, c, and d are atomic numbers of respective elements with respect to the sum of the atomic fractions of Ni, Co, Mn, and M elements of the metal elements other than lithium, 0 <b <0.3, 0 <c <0.1, 0 <d <0.1, a + b + c + d = 1, where 0.7 <a <1, 0 <b <0.3.

Specifically, a represents an atomic fraction of nickel in metal elements other than lithium, and a may be more than 0.7 and less than 1, preferably 0.8 to 0.95, and more preferably 0.8 to 0.9.

B represents the atomic fraction of cobalt among metal elements other than lithium, and b may be more than 0 and less than 0.3, preferably 0.001 to 0.25, more preferably 0.01 to 0.2.

C represents the atomic fraction of manganese among the metal elements other than lithium, and c may be more than 0 and less than 0.1, preferably 0.001 to 0.05.

D represents the atomic fraction of the M element among the metal elements other than lithium, and d may be more than 0 and less than 0.1, preferably 0.001 to 0.05.

X represents the degree to which a part of metal elements other than lithium is replaced by lithium, y represents the degree of substitution of oxygen by X element, and 0? X? 0.1, 0? Y? 0.5. Preferably, 0? X? 0.05 and 0? Y? 0.2.

The lithium composite metal oxide having the same composition as that of Formula 1 has a higher structural stability than that of the conventional NMC-based lithium oxide while exhibiting high capacity characteristics. Specifically, the lithium composite metal oxide of the above formula (1) has a high content of nickel having a high capacity characteristic and can realize excellent capacity characteristics. In addition, Mn and M elements are doped in the active material, It is structurally stable. In addition, when oxygen is substituted for P or F, the separation of oxygen during charging and discharging of the lithium secondary battery is prevented, the interface reaction with the electrolyte is suppressed, and the surface stability is improved.

The lithium composite metal oxide of the above formula (1) is not limited to this. For example, the nickel-cobalt precursor, the manganese-containing raw material, the M-containing raw material and the lithium- Lt; 0 &gt; C. &Lt; / RTI &gt;

In this case, the nickel-cobalt precursor may be, for example, a nickel-cobalt hydroxide, specifically, a compound represented by Ni _{a '} Co _b' OOH wherein a 'is 0.7 to 0.95, May be 0.8 to 0.95, and b 'may be 0.05 to 0.3, preferably 0.05 to 0.2. The nickel-cobalt precursor may be purchased from commercially available nickel-cobalt hydroxide, or may be prepared according to a method for producing a nickel-cobalt precursor well known in the art.

For example, the nickel-cobalt precursor may be prepared by adding an ammonium cation-containing complexing agent and a basic compound to a metal solution containing a nickel-containing starting material and a cobalt-containing starting material, followed by a coprecipitation reaction.

The nickel-containing source material is, for example, or the like the nickel-containing acid salt, a nitrate, a sulfate, a halide, a sulfide, a hydroxide, an oxide or oxy-hydroxide, and, specifically, Ni (OH) _2, NiO, NiOOH, NiCO ₃ and 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, fatty acid nickel salts, But is not limited thereto.

The cobalt-containing raw material may be a cobalt-containing acetic acid salt, a nitrate salt, a sulfate salt, a halide, a sulfide, a hydroxide, an oxide or an oxyhydroxide and specifically Co (OH) ₂ , CoOOH, Co (OCOCH ₃ ) ₂ .4H ₂ O , Co (NO ₃ ) ₂ .6H ₂ O, Co (SO ₄ ) ₂ .7H ₂ O, or a combination thereof, but is not limited thereto.

The metal solution is prepared by adding a nickel-containing raw material and a cobalt-containing raw material to a mixed solvent of a solvent, specifically water, or an organic solvent (for example, alcohol or the like) which can be uniformly mixed with water, or Or may be one prepared by mixing an aqueous solution of the nickel-containing raw material and an aqueous solution of the cobalt-containing raw material.

The ammonium cation-containing complexing 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 , But is not limited thereto. On the other hand, the ammonium cation-containing complex-forming agent may be used in the form of an aqueous solution. As the solvent, water or a mixture of water and an organic solvent which can be uniformly mixed with water (specifically, alcohol or the like) and water may be used.

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

On the other hand, if necessary, it may be used by dissolving the above-mentioned X element, that is, an anionic compound containing P and / or F, in the above basic compound. In this case, the X element derived from the anionic compound is partly substituted in the oxygen position of the precursor, so that it is possible to obtain an effect of suppressing the oxygen desorption during the charge and discharge of the secondary battery and the reaction with the electrolyte.

The basic compound is added to adjust the pH of the reaction solution. The basic compound may be added in an amount such that the pH of the metal solution becomes 11-13.

On the other hand, the coprecipitation reaction can be carried out under an inert atmosphere such as nitrogen or argon at a temperature of 40 ° C to 70 ° C.

Particles of nickel-cobalt hydroxide are formed by the above-described process and precipitated in the reaction solution. The precipitated nickel-cobalt hydroxide particles can be separated and dried by a conventional method to obtain a nickel-cobalt precursor.

The nickel-cobalt precursor, the manganese-containing raw material, the M-containing raw material and the lithium-containing raw material prepared by the above method are mixed and calcined at 600 to 900 ° C, preferably 600 to 800 ° C to obtain a lithium composite metal oxide Can be obtained.

The manganese-containing raw material may be, for example, a manganese-containing acetate salt, a nitrate salt, a sulfate salt, a halide, a sulfide, a hydroxide, an oxide, an oxyhydroxide or a combination thereof. More specifically, Mn ₂ O ₃ , MnO ₂ , Mn ₃ O _4, and the like; _{MnCO 3, Mn (NO 3)} 2, MnSO 4, manganese acetate, dicarboxylic acid manganese, manganese citrate, manganese, such as fatty acid manganese; Manganese oxyhydroxide, manganese chloride, or a combination thereof, but is not limited thereto.

The M-containing raw material may be an acetate salt, a nitrate salt, a sulfate salt, a halide, a sulfide, a hydroxide, an oxide, an oxyhydroxide, or a combination thereof containing M element. Specific examples thereof include Al ₂ O ₃ , AlSO ₄ , AlCl ₃ , Al-isopropoxide, AlNO ₃ , or a combination thereof, but is not limited thereto.

The lithium-containing source material may be at least one selected from the group consisting of a lithium-containing carbonate (for example, lithium carbonate), a hydrate (for example, lithium hydroxide I hydrate (LiOH.H ₂ O) (E.g., lithium nitrate (LiNO ₃ ) and the like) and chlorides (e.g., lithium chloride (LiCl) and the like), and a single or a mixture of two or more of them may be used.

On the other hand, the mixing of the nickel-cobalt precursor, the manganese-containing raw material, the M-containing raw material and the lithium-containing raw material may be performed by solid-phase mixing such as jet milling. On the other hand, the mixing ratio of the nickel-cobalt precursor, the manganese-containing raw material, the M-containing raw material and the lithium-containing raw material can be determined within a range that satisfies the atomic fraction of each metal component in the finally produced lithium composite metal oxide.

Further, although not essential, in order to dope a part of the oxygen of the lithium composite metal oxide with the X element, the X-containing raw material may be further mixed at the time of firing. The X-containing raw material may be, for example, Na ₃ PO ₄ , K ₃ PO ₄ , Mg ₃ (PO ₄ ) ₂ , AlF ₃ , NH ₄ F, LiF or the like, but is not limited thereto.

When a part of the oxygen is replaced by the element X as described above, the effect of suppressing the oxygen at the time of charge and discharge of the secondary battery and the reaction with the electrolyte can be obtained.

(2) Washed  step

When the lithium composite metal oxide is prepared as described above, it is washed with water to remove lithium by-products remaining in the lithium composite metal oxide.

Lithium composite metal oxides containing a high concentration of nickel are structurally unstable compared to lithium complex metal oxides having a low nickel content, so that lithium by-products such as unreacted lithium hydroxide and lithium carbonate are more generated in the manufacturing process. Specifically, in the case of a lithium composite metal oxide having a nickel fraction of less than 80 at%, the amount of lithium by-product after synthesis is about 0.5 to 0.6 wt%, whereas in the case of a lithium composite metal oxide having a nickel fraction of 80 at% The amount of by-products is as high as 1% by weight. On the other hand, when a large amount of lithium by-products is present in the cathode active material, the lithium by-product and the electrolyte react with each other to generate a gas and swell, thereby significantly deteriorating the high temperature stability. Therefore, a water washing step for removing lithium by-products from the lithium composite metal oxide containing high-concentration nickel is essentially required.

The water washing step may be performed, for example, by charging a lithium composite metal oxide into ultrapure water and stirring the solution. At this time, the washing temperature may be 20 ° C or less, preferably 10 ° C to 20 ° C, and the washing time may be 10 minutes to 1 hour. When the washing temperature and the washing time satisfy the above range, the lithium by-product can be effectively removed.

(3) Step of heat treatment

When the washing is completed, the washed lithium composite metal oxide and the cobalt-containing raw material are mixed and heat-treated. At this time, the heat treatment is performed in an oxidizing atmosphere at a temperature of 700 ° C or higher. The heat treatment step is to improve structural stability and thermal stability by further removing lithium by-product and recrystallizing the metal elements in the cathode active material through a high-temperature heat treatment.

In the cobalt-containing source material, and the like cobalt-containing acid salt, a nitrate, a sulfate, a halide, a sulfide, a hydroxide, an oxide or oxy-hydroxide, specifically, the _{Co (OH) 2, CoOOH,} Co (OCOCH 3) 2 and 4H ₂ O, Co (NO ₃ ) ₂揃 6H ₂ O, Co (SO ₄ ) ₂揃 7H ₂ O, or combinations thereof, but are not limited thereto.

The cobalt-containing raw material may be mixed in an amount of 0.001 to 0.01 part by weight, preferably 0.002 to 0.008 part by weight based on 100 parts by weight of the lithium composite metal oxide. When the content of the cobalt-containing raw material satisfies the above range, the output characteristics can be effectively improved without inhibiting the capacity characteristics of the lithium composite metal oxide. Specifically, when the amount is less than 0.001 parts by weight, the output improvement effect is insignificant. When the amount is more than 0.01 parts by weight, nickel in the lithium composite metal oxide may be replaced by cobalt and the capacity characteristics may be deteriorated.

When the cobalt-containing raw material is further added and heat-treated, cobalt is coated on the surface of the lithium composite metal oxide during the heat treatment, and the cobalt-rich layer having a relatively higher cobalt content than the lithium composite metal oxide Cobalt-Rich Layer) is formed. When the cobalt-rich layer is formed on the surface of the lithium composite metal oxide as described above, the effect of improving the output characteristics can be obtained.

Meanwhile, the heat treatment is performed in an oxidizing atmosphere, for example, an oxygen atmosphere. Specifically, the heat treatment may be performed while supplying oxygen at a flow rate of 0.5 to 10 L / min, preferably 1 to 5 L / min. When the heat treatment is performed in an oxidizing atmosphere as in the present invention, lithium byproducts are effectively removed. According to the studies of the present inventors, the effect of removing lithium byproducts is significantly lowered when the heat treatment is performed under the atmosphere. In particular, when the heat treatment is performed at 700 ° C or higher in the atmosphere, the amount of lithium byproduct is increased rather than before heat treatment.

The heat treatment is preferably performed at a temperature of 700 ° C or higher, for example, 700 ° C to 800 ° C within 10 hours, for example, 1 hour to 10 hours. When the heat treatment temperature and time satisfy the above range, the effect of improving the thermal stability is excellent. According to the studies of the present inventors, when the heat treatment temperature is less than 700 캜, the thermal stability improving effect is hardly exhibited.

Cathode active material for secondary battery

Next, the cathode active material for a secondary battery according to the present invention will be described.

The cathode active material for a secondary battery according to the present invention comprises a lithium composite metal oxide represented by the following Chemical Formula 1 and a cobalt-rich layer formed on the surface of the lithium composite metal oxide.

[Chemical Formula 1]

Li _{1 + x} [Ni _a Co _b Mn _c M _d ] _1-x O _2-y X _y

M is at least one metal element selected from the group consisting of Al, Mg, Ge, V, Mo, Nb, Si, Ti, Zr and W, X is P or F, 0 <b <0.3, 0 <c <0.1, 0 <d <0.1, a + b + c + d = 1, 0? X? 0.05 and 0? Y? 0.2. Specific specifications of the lithium composite metal oxide represented by the above formula (1) are the same as those described in the above-mentioned production method, and thus a detailed description thereof will be omitted.

The cobalt-rich layer is a layer formed by coating a surface of a lithium composite metal oxide with a cobalt component derived from a cobalt-containing raw material in a process of mixing a lithium composite metal oxide with a cobalt-containing raw material and heat- Is a layer containing a relatively large amount of cobalt as compared with the metal oxide.

Specifically, the ratio of the number of atoms of cobalt to the total number of atoms (i.e., the sum of the numbers of atoms of nickel, cobalt, manganese and M) of metal elements other than lithium in the cobalt-rich layer (hereinafter, ) And the lithium composite metal oxide may be about 0.05 to 0.2, preferably about 0.05 to 0.15, in terms of the cobalt atomic fraction. More specifically, the atomic fraction of nickel, cobalt, manganese and manganese in the cobalt-rich layer (that is, the ratio of the number of atoms of cobalt to the sum of the numbers of atoms of nickel, cobalt, manganese and M) 0.45, preferably 0.05 to 0.35. When the cobalt atomic fraction in the cobalt-rich layer satisfies the above range, the output characteristics can be effectively improved without hindering the capacity characteristics of the lithium composite metal oxide.

Meanwhile, the cobalt-rich layer may have a thickness of 10 to 100 nm, preferably 30 to 70 nm. When the thickness of the cobalt-rich layer satisfies the above-mentioned range, the initial discharge capacity may decrease when the thickness of the cobalt-rich layer is more than 100 nm, and the output and cycle characteristics are degraded when the thickness of the cobalt- .

The cathode active material according to the present invention is manufactured through a high temperature heat treatment process in an oxidizing atmosphere after the water rinse so that the residual amount of lithium byproduct is remarkably smaller than that of the conventional high-concentration nickel-containing cathode active material, and excellent high temperature stability can be realized.

Specifically, when the heat flow is measured by differential scanning calorimetry (DSC), the cathode active material according to the present invention is heated to a temperature in the range of 230 ° C. to 250 ° C., preferably 235 ° C. to 245 ° C. A peak appears and the maximum value of the heat flow is 1000 mW or less, preferably 600 mW or less, more preferably 400 mW or less. If the heat treatment is not performed after washing with water or if the heat treatment temperature and atmosphere do not satisfy the conditions of the present invention even at the heat treatment, a peak appears at a lower temperature, that is, below 230 캜, and a high heat flux value exceeding 1000 mW appears. When a positive electrode active material having a peak value in a low temperature range is used and a positive electrode active material having a high maximum value of the heat flow rate is used, an explosion may occur due to a sudden increase in the heat flow rate when the temperature inside the battery rises due to overcharging. In contrast, the cathode active material of the present invention has a relatively high temperature range at which the peak appears and a small maximum value of the heat flow rate, so that the risk of explosion is low even when the internal temperature of the battery rises due to overcharging or the like.

Also, the content of the lithium by-product in the cathode active material according to the present invention is 1% by weight or less, preferably 0.5% by weight or less, more preferably 0.3% by weight or less. Therefore, when the secondary battery is manufactured using the positive electrode active material according to the present invention, gas generation and swelling phenomenon can be effectively suppressed during charge and inversion.

Anode and secondary battery

The cathode active material for a secondary battery according to the present invention can be usefully used for manufacturing a cathode for a secondary battery.

Specifically, the cathode for a secondary battery according to the present invention includes a cathode current collector and a cathode active material layer formed on the cathode current collector, wherein the cathode active material layer includes the cathode active material according to the present invention.

The positive electrode may be produced according to a conventional positive electrode manufacturing method, except that the positive electrode active material according to the present invention is used. For example, the positive electrode may be prepared by dissolving or dispersing the components constituting the positive electrode active material layer, that is, the positive electrode active material, the conductive material and / or the binder, in a solvent to prepare a positive electrode composite material, Or by a method in which the film is coated on at least one side and then dried and rolled or by casting the positive electrode material on a separate support and then peeling off the support from the support to laminate the positive electrode current collector.

The positive electrode current collector is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery. For example, the positive electrode current collector may be made of a metal such as stainless steel, aluminum, nickel, titanium, sintered carbon, aluminum or stainless steel, , Titanium, silver, or the like may be used. In addition, the cathode current collector may have a thickness of 3 to 500 탆, and fine unevenness may be formed on the surface of the current collector to increase the adhesive force of the cathode 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 nonwoven fabric.

The current collector includes the cathode active material according to the present invention on at least one surface thereof and optionally includes at least one of a conductive material and a binder.

The cathode active material includes the cathode active material according to the present invention, that is, the lithium composite metal oxide represented by Formula 1 and a cobalt-rich layer formed on the surface of the lithium composite metal oxide. Since the specific contents of the cathode active material according to the present invention are the same as those described above, detailed description thereof will be omitted.

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

The conductive material is used for imparting conductivity to the electrode. The conductive material is not particularly limited as long as it has electron conductivity without causing chemical change. Specific examples thereof include graphite such as natural graphite and artificial graphite; Carbonaceous materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black and carbon fiber; Metal powder or metal fibers such as copper, nickel, aluminum and silver; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; And polyphenylene derivatives. These may be used alone or in admixture of two or more. The conductive material may be included in an amount of 1% by weight to 30% by weight based on 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 adhesion between the positive electrode active material and the current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose ), Starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, and various copolymers thereof. One kind or a mixture of two or more kinds of them may be used. The binder may be included in an amount of 1% by weight to 30% by weight based on the total weight of the cathode active material layer.

On the other hand, the solvent used in the preparation of the positive electrode material may be a solvent commonly used in the art, for example, dimethyl sulfoxide (DMSO), isopropyl alcohol, (NMP), acetone or water may be used alone or in combination. The amount of the solvent to be used can be appropriately adjusted in consideration of the application thickness of the slurry, the production yield, the viscosity, and the like.

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

The secondary battery according to the present invention includes a positive electrode, a negative electrode disposed opposite to 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.

The secondary battery may further include a battery container for storing the positive electrode, the negative electrode and the electrode assembly of the separator, and a sealing member for sealing the battery container.

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

The negative electrode may be manufactured according to a conventional negative electrode manufacturing method generally known in the art. For example, the negative electrode may be produced by dissolving or dispersing the components constituting the negative electrode active material layer, that is, the negative electrode active material, the conductive material and / or the binder in a solvent to form a negative electrode mixture, Or a method in which the negative electrode material is coated on at least one surface and then dried or rolled or casting the negative electrode material onto a separate support and then peeling the support from the support to laminate the negative electrode current collector.

The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, the negative electrode current collector may be formed on the surface of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, Carbon, nickel, titanium, silver or the like, aluminum-cadmium alloy, or the like may be used. In addition, the negative electrode collector may have a thickness of 3 to 500 탆, and similarly to the positive electrode collector, fine unevenness may be formed on the surface of the collector to enhance the binding force 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 nonwoven fabric.

As the negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. Specific examples thereof include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber and amorphous carbon; Metal compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys or Al alloys; Metal oxides such as SiO _v (0 <v <2), SnO ₂ , vanadium oxide, and lithium vanadium oxide, which can dope and dedoped lithium; Or a composite containing the metallic compound and the carbonaceous material such as Si-C composite or Sn-C composite, and any one or a mixture of two or more thereof may be used. Also, a metal lithium thin film may be used as the negative electrode active material. The carbon material may be both low-crystalline carbon and high-crystallinity carbon. Examples of the low-crystalline carbon include soft carbon and hard carbon. Examples of the highly crystalline carbon include natural graphite, artificial graphite, artificial graphite or artificial graphite, Kish graphite graphite, pyrolytic carbon, mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches and petroleum or coal tar coke derived cokes).

In addition, 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 moving path of lithium ion. The separator can be used without any particular limitation as long as it is used as a separator in a secondary battery. Particularly, It is preferable that it is a resistance and excellent in an ability to impregnate the electrolyte. Specifically, porous polymer films such as porous polymer films made of polyolefin-based polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers and ethylene / methacrylate copolymers, May be used. Further, a nonwoven fabric made of a conventional porous nonwoven fabric, for example, glass fiber of high melting point, polyethylene terephthalate fiber, or the like may be used. In order to secure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer material may be used, and may be optionally used as a single layer or a multilayer structure.

The electrolyte may be an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, a melt-type inorganic electrolyte, and the like, which are usable in the production of a secondary battery.

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

The organic solvent may be used without limitation as long as it can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate,? -Butyrolactone and? -Caprolactone; Ether solvents such as dibutyl ether or tetrahydrofuran; Ketone solvents such as cyclohexanone; Aromatic hydrocarbon solvents such as benzene and fluorobenzene; Dimethyl carbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), propylene carbonate PC) and the like; Alcohol solvents such as ethyl alcohol and isopropyl alcohol; Ra-CN (Ra is a linear, branched or cyclic hydrocarbon group having 2 to 20 carbon atoms, which may contain a double bond aromatic ring or ether bond); Amides such as dimethylformamide; Dioxolanes such as 1,3-dioxolane; Or sulfolane may be used. Among these, a carbonate-based solvent is preferable, and a cyclic carbonate (for example, ethylene carbonate or propylene carbonate) having a high ionic conductivity and a high dielectric constant, for example, such as ethylene carbonate or propylene carbonate, For example, ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate) is more preferable. In this case, when the cyclic carbonate and the chain carbonate are mixed in a volume ratio of about 1: 1 to 9, the performance of the electrolytic solution may be excellent.

The lithium salt can be used without particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the lithium _{salt, 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 may be used. The concentration of the lithium salt is preferably within the range of 0.1M to 2.0M. When the concentration of the lithium salt is within the above range, the electrolyte has an appropriate conductivity and viscosity, so that it can exhibit excellent electrolyte performance and the lithium ion can effectively move.

The electrolyte may contain, for example, a haloalkylene carbonate-based compound such as difluoroethylene carbonate or the like for the purpose of improving the lifetime characteristics of the battery, suppressing the reduction of the battery capacity, and improving the discharge capacity of the battery. N, N &lt; RTI ID = 0.0 &gt; (N, &lt; / RTI &gt; N, N'-tetramethyluronium hexafluorophosphate), pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, And at least one additive such as substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol or aluminum trichloride may be further included. The additive may be included in an amount of 0.1 wt% to 5 wt% based on the total weight of the electrolyte.

As described above, the secondary battery including the cathode active material according to the present invention has excellent capacity characteristics and high temperature stability, and can be used as a portable device such as a mobile phone, a notebook computer, a digital camera, and a hybrid electric vehicle (HEV) Electric vehicles, and the like.

In addition, the secondary battery according to the present invention can be used as a unit cell of a battery module, and the battery module can be applied to a battery pack. The battery module or the battery pack may include a power tool; An electric vehicle including an electric vehicle (EV), a hybrid electric vehicle, and a plug-in hybrid electric vehicle (PHEV); Or a power storage system, as shown in FIG.

Example

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily carry out the present invention. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

Example  One

Ni _{0 . 9} Co _{0 . 1 OOH, Mn 3 O 4,} Al 2 O 3 , and then a solution of the solid phase the LiOH, then fired at 750 ℃ lithium-metal composite oxide _{Li (Ni 0. 86 Co 0} . 1 Mn 0. 02 Al 0. 02) O 2 .

300 g of the lithium composite metal oxide was added to 300 mL of ultrapure water, stirred for 30 minutes, washed with water, and filtered for 20 minutes. The filtered lithium composite metal oxide was dried in a vacuum oven at 130 캜 and then subjected to seiving. Then, Co (OH) ₂ was added to the lithium composite metal oxide at a concentration of 5,000 ppm and heat treatment was performed at 700 ° C. for 5 hours while oxygen was supplied at a flow rate of 1 L / min to prepare a cathode active material.

Comparative Example  One

The cathode active material was prepared in the same manner as in Example 1, except that the heat treatment was not performed.

Comparative Example  2

The cathode active material was prepared in the same manner as in Example 1, except that Co (OH) ₂ was not added during the heat treatment.

Comparative Example  3

A cathode active material was prepared in the same manner as in Example 1, except that Co (OH) ₂ was not added during the heat treatment and the heat treatment was performed at 300 ° C without oxygen supply.

Comparative Example  4

The cathode active material was prepared in the same manner as in Example 1, except that Co (OH) ₂ was not added during the heat treatment at the heat treatment and the heat treatment was performed at 500 ° C without oxygen supply.

Comparative Example  5

A cathode active material was prepared in the same manner as in Example 1, except that Co (OH) ₂ was not added during the heat treatment at the heat treatment and the heat treatment was performed at 600 ° C without oxygen supply.

Comparative Example  6

A cathode active material was prepared in the same manner as in Example 1, except that Co (OH) ₂ was not added during the heat treatment and the heat treatment was performed at 700 ° C without oxygen supply.

Experimental Example  One - Heat flux  evaluation

Heat flux according to temperature of the cathode active materials of Example 1 and Comparative Example 3 was measured using a SETARAM Instrumentation (Sensys evo DSC). Specifically, 16 mg of the cathode active material of Example 1 and Comparative Example 1 was charged into a pressure-resistant pen for DSC measurement, and then 20 μL of an electrolyte (EVPS) was injected. The temperature range for DSC analysis was 25 ° C to 400 ° C and the rate of temperature increase was 10 ° C / min. DSC measurement was performed on each cathode active material 3 times or more and the average value was measured.

The measurement results are shown in Fig. 1, the cathode active material of Example 1 exhibited a peak at 240 ° C and the maximum value of the heat flux was less than 200 mW, whereas the cathode active material of Comparative Example 3 had a peak at 225 ° C and a maximum value of the heat flux exceeded 1000 mW can confirm. This indicates that the positive electrode active material of Example 1 has excellent high temperature stability as compared with the positive electrode active material of Comparative Example 3.

Experimental Example  2 - Evaluation of Lithium Byproduct Residue

5 g of the cathode active material prepared in Example 1 and Comparative Examples 1, 2, and 4 to 6 was dispersed in 100 ml of water, and the pH value was changed while titrating with 0.1 M HCl to obtain a pH titration curve. . The pH titration curve is shown in FIG.

The LiOH residual amount and the LiCO ₃ residual amount in each cathode active material were calculated using the pH titration curve, and the sum of the LiOH residual amount and the LiCO ₃ residual amount was evaluated as the total lithium byproduct residual amount.

LiOH residual amount
(wt%) LiCO ₃ residual amount
(wt%) Total residual lithium by-product
(wt%) Example 1 0.158 0.063 0.221 Comparative Example 1 0.165 0.119 0.284 Comparative Example 4 0.111 0.334 0.445 Comparative Example 5 0.146 0.372 0.518 Comparative Example 6 0.228 0.605 0.833

When the residual amount of lithium by-product in the cathode active material of Example 1 satisfying the heat treatment conditions of the present invention was not subjected to the heat treatment process (Comparative Example 1) or the heat treatment was performed without oxygen supply (Comparative Example 4 to 6). In addition, as in Comparative Examples 4 to 6, when the heat treatment is performed in an atmospheric environment without supplying oxygen, it can be seen that the residual amount of lithium by-product is increased as compared with Comparative Example 1 in which heat treatment is not performed.

Experimental Example  3 - Battery performance evaluation

The cathode active material, the carbon black conductive material and the PVdF binder prepared in Example 1, Comparative Example 2 and Comparative Example 3 were mixed in a N-methylpyrrolidone solvent in a weight ratio of 95: 2.5: 2.5, (Viscosity: 5000 mPa 占 퐏) was coated on one surface of the aluminum current collector, dried at 130 占 폚 and rolled to prepare a positive electrode.

Also, as a negative electrode active material, natural graphite, a carbon black conductive material and a PVdF binder were mixed in a N-methylpyrrolidone solvent in a weight ratio of 85: 10: 5 to prepare a composition for forming a negative electrode active material layer, To prepare a negative electrode.

A lithium secondary battery was prepared by preparing an electrode assembly between a positive electrode and a negative electrode manufactured as described above through a separator of porous polyethylene, positioning the electrode assembly inside a case, and then injecting an electrolyte into the case. The electrolyte solution was prepared by dissolving 1.0 M lithium hexafluorophosphate (LiPF ₆ ) in an organic solvent composed of ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate (mixed volume ratio of EC / DMC / EMC = 3/4/3) Respectively.

The capacity of the lithium secondary battery thus produced was subjected to 50 cycles of charging and discharging at 45 ° C. under the conditions of a charging end voltage of 4.25 V and a discharge end voltage of 2.5 V and 0.5 C / 0.5 C, and a capacity retention [%] (DCR Incress [%]) was measured. The measurement results are shown in FIG. 3 and FIG. 3 is a graph showing the capacity retention rate, and FIG. 4 is a graph showing the rate of resistance increase.

3 and 4, in the case of the secondary battery to which the cathode active material of Example 1 was applied, compared with the secondary battery to which the cathode active materials of Comparative Examples 2 and 3 were applied, the capacity reduction rate and the resistance increase rate were significantly Low.

Claims (14)

A lithium complex metal oxide represented by the following formula (1); And
And a cobalt-rich layer formed on the surface of the lithium composite metal oxide, the cobalt-rich layer having a higher content of cobalt than the lithium composite metal oxide.
[Chemical Formula 1]
Li _{1 + x} [Ni _a Co _b Mn _c M _d ] _1-x O _2-y X _y
In the above formula (1), 0.7 <a <1, 0 <b <0.3, 0 <c <0.1, 0 <d <0.1, a + b + c + d = 1, 0≤x≤0.05, 0≤y≤0.2 And M is one kind of metal element selected from the group consisting of Al, Mg, Ge, V, Mo, Nb, Si, Ti, Zr and W, and X is P or F.
The method according to claim 1,
The ratio of the number of atoms of cobalt to the sum of the number of atoms of nickel, cobalt, manganese and M in the cobalt-rich layer and the ratio of the number of atoms of nickel, cobalt, manganese and M in the lithium composite metal oxide to the total number of atoms of cobalt Wherein the ratio of the number of the positive electrode active material to the negative active material is 0.05 to 0.2.
The method according to claim 1,
Wherein the ratio of the number of atoms of cobalt to the total number of atoms of nickel, cobalt, manganese and M in the cobalt-rich layer is 0.05 to 0.45.
The method according to claim 1,
Wherein the cobalt-rich layer has a thickness of 10 nm to 100 nm.
The method according to claim 1,
Wherein the positive electrode active material exhibits a peak at a temperature range of 230 to 250 ° C when a heat flow is measured by differential scanning calorimetry (DSC).
The method according to claim 1,
Wherein the cathode active material has a maximum heat flow of 1000 mW or less as measured by differential scanning calorimetry (DSC).
The method according to claim 1,
Wherein the content of lithium by-products in the positive electrode active material is 1 wt% or less.
Preparing a lithium composite metal oxide represented by Formula 1 below;
[Chemical Formula 1]
Li _{1 + x} [Ni _a Co _b Mn _c M _d ] _1-x O _2-y X _y
0 <d <0.1, a + b + c + d = 1, 0? X? 0.05, 0? Y? 0.2 and M is one kind of metal element selected from the group consisting of Al, Mg, Ge, V, Mo, Nb, Si, Ti, Zr and W, and X is P or F)
Washing the lithium composite metal oxide with water; And
Mixing the washed lithium composite metal oxide with the cobalt-containing starting material, and then heat-treating the lithium composite metal oxide at a temperature of 700 ° C or higher in an oxidizing atmosphere.
9. The method of claim 8,
The preparation of the lithium composite metal oxide represented by Formula 1 may be performed by mixing a nickel-cobalt precursor, a manganese-containing raw material, an M-containing raw material, and a lithium-containing raw material and firing the mixture at 600 ° C. to 900 ° C. Wherein the positive electrode active material is a negative electrode active material.
9. The method of claim 8,
Wherein the washing step is performed at a temperature of 20 DEG C or less.
9. The method of claim 8,
Wherein the cobalt-containing raw material is mixed in an amount of 0.001 to 0.01 part by weight based on 100 parts by weight of the lithium composite metal oxide in the heat treatment step.
9. The method of claim 8,
Wherein the heat treatment is performed at a temperature of 700 ° C to 800 ° C within 10 hours.
A positive electrode for a secondary battery comprising the positive electrode active material for a secondary battery according to any one of claims 1 to 7.
14. A secondary battery comprising a positive electrode for a secondary battery according to claim 13.

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