KR20170009557A - Cylindrical secondary battery with enhanced stability by regulating strength of particle of positive active material - Google Patents

Abstract

Provided are a positive electrode active material for a secondary battery, and a preparation method thereof. The positive electrode active material for a secondary battery increases the amount of deformation in a battery under an impact by reducing the particle strength of a positive electrode active material, and causes electric short throughout a large area, thereby reducing riskiness of ignition through distribution of electric current and securing stability of the battery.

Description

TECHNICAL FIELD [0001] The present invention relates to a cylindrical secondary battery having improved safety by changing the strength of a cathode active material particle. BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a cylindrical secondary battery,

The present invention relates to a secondary battery capable of reducing the particle strength of a cathode active material to increase the amount of deformation of the battery during impact to cause a short circuit in a wide area, .

Recently, demand for batteries as an energy source has been increasing rapidly as technology development and demand for mobile devices have increased, and researches on batteries capable of meeting various demands as HEV, PHEV, Are carried out in various ways. Particularly, research on a lithium secondary battery having a high energy density and excellent lifetime and cycle characteristics as a power source of such a device is being actively conducted.

Lithium-containing cobalt oxide (LiCoO ₂ ) is mainly used as a positive electrode active material of the lithium secondary battery. In addition, a lithium-containing manganese oxide such as LiMnO _{2 having a} layered crystal structure and LiMn ₂ O _{4 having a} spinel crystal structure, (LiNiO ₂ ) is also considered.

Of the above cathode active materials, LiCoO ₂ has excellent properties such as excellent cycle characteristics and is widely used. However, LiCoO ₂ is low in safety, is expensive due to the resource limit of cobalt as a raw material, and is a power source in fields such as electric vehicles and hybrid electric vehicles There is a limit to mass use.

Lithium manganese oxides such as LiMnO ₂ and LiMn ₂ O ₄ have attracted much attention as a cathode active material capable of replacing LiCoO ₂ because they have the advantage of using manganese rich in resources and environment friendly as a raw material. However, these lithium manganese oxides have disadvantages such as small capacity and poor cycle characteristics.

Lithium nickel oxide such as LiNiO ₂ is lower in cost than the cobalt oxide, but a rapid phase transition of the crystal structure occurs due to the volume change accompanying the charging / discharging cycle, thereby causing cracks in the particles and voids in the grain boundaries There is a problem.

In order to solve the above problems, a lithium transition metal oxide in which a part of nickel is substituted with another transition metal such as manganese or cobalt has been proposed. Such metal-substituted nickel-based lithium transition metal oxides have an advantage in that they have excellent cycle characteristics and capacity characteristics.

However, it has disadvantages that the rate capability and the life characteristics at high temperature are poor. In order to overcome such disadvantages, many researches have been conducted on a method of coating a metal having a good conductivity on the surface of a cathode active material or a method of doping the inside of the cathode active material.

On the other hand, the secondary battery includes a cylindrical battery and a prismatic battery in which an electrode assembly is embedded in a cylindrical or rectangular metal can according to the shape of a battery case, and a pouch-type battery in which an electrode assembly is embedded in a pouch-shaped case of an aluminum laminate sheet .

The electrode assembly embedded in the battery case is a charge / dischargeable power generating device formed of a laminate structure of a positive electrode / separator / negative electrode. The electrode assembly includes a jelly-roll type battery having a separator interposed between a positive electrode and a negative electrode coated with an active material, , And a plurality of positive electrodes and negative electrodes of a predetermined size are sequentially stacked in a state in which the separator is interposed therebetween. Among them, the jelly-roll type electrode assembly has an advantage of being easy to manufacture and having a high energy density per weight.

Particularly when a cylindrical lithium secondary battery is used in, for example, a notebook PC, it is mostly subjected to a safety test such as an impact test. As a result of performing the impact test, the center pin is deformed, A phenomenon occurs and becomes a serious problem.

Therefore, there is a high need for a technique capable of effectively preventing internal short-circuiting due to deformation of the center pin and thus igniting the battery, thereby improving the safety of the battery.

Accordingly, the present inventors have invented a cathode active material for a secondary battery and a method of manufacturing the same, which can reduce the risk of ignition by increasing the deformation amount of the battery upon impact by reducing the particle strength of the cathode active material.

It is an object of the present invention to provide a secondary battery positive electrode capable of reducing the particle strength of the positive electrode active material and increasing the amount of deformation of the battery upon impact to cause short circuiting over a wide area, And to provide a method for producing the same.

SUMMARY OF THE INVENTION The present invention is to solve the above problems and provides a positive electrode active material comprising a lithium-transition metal composite oxide represented by the following general formula (1), wherein the positive electrode active material particles have a compressive fracture strength of 50 to 200 MPa. Thereby providing a cathode active material.

[Chemical Formula 1]

Li _x Ni _(1-abc) Co _a Mn _b M _c O ₂

In Formula 1,

M is Al, B, Mg, Ti, or Zr,

0.95? X? 1.2, 0 <a? 0.4, 0 <b? 0.4, 0? C? 0.01, and 0? A + b + c? 0.4.

A) mixing a nickel-containing compound, a cobalt-containing compound, a manganese-containing compound, a complexing agent and a precipitating agent to form a mixture solution; b) coprecipitating the mixture solution to form lithium Forming a transition metal complex oxide precursor; c) introducing a metal element M into the precursor; And d) mixing and firing the precursor with a lithium-containing compound to form a lithium-transition metal composite oxide represented by the general formula (1). The present invention also provides a method for producing a cathode active material for a secondary battery.

The present invention also provides a positive electrode for a secondary battery in which a positive electrode current collector for a secondary battery according to the present invention is applied to a positive electrode current collector, followed by drying and rolling, and a secondary battery including the same.

By reducing the particle strength of the cathode active material for a secondary battery of the present invention, it is possible to increase the deformation amount of the battery at the time of impact to cause a short-circuit of a large area, thereby reducing the risk of ignition through current dispersion and securing the stability of the battery.

1 is a SEM photograph of a cathode active material for a secondary battery according to Example 1 of the present invention.
2 is a SEM photograph of a cathode active material for a secondary battery according to a comparative example of the present invention.
3 is a graph showing the particle size distribution before and after pellet molding of the cathode active material for a secondary battery according to Example 1 of the present invention.
4 is a graph showing particle size distribution before and after pellet molding of a cathode active material for a secondary battery according to a comparative example of the present invention.
5 is a graph showing the particle size distribution before and after the heat treatment (600 ° C) of the positive electrode coated with the positive electrode active material for a secondary battery according to Example 1 of the present invention.
6 is a graph showing the particle size distribution before and after the heat treatment (600 ° C) of the positive electrode coated with the positive electrode active material for a secondary battery according to the comparative example of the present invention.
7 is a graph showing the particle strength of the cathode active material for a secondary battery according to Examples 1 and 2 and Comparative Example of the present invention.
8 is a photograph showing a modified state of the cylindrical lithium secondary battery after the impact test of the secondary battery according to the first embodiment of the present invention.
9 is a photograph showing a modified state of a cylindrical lithium secondary battery after a shock test of a secondary battery according to a comparative example of the present invention.
10 is a photograph showing an exploded view of a cylindrical lithium secondary battery after the impact test of the secondary battery according to the first embodiment of the present invention.
11 is a photograph showing an exploded view of a cylindrical lithium secondary battery after a shock test of a secondary battery according to a comparative example of the present invention.

Hereinafter, the present invention will be described in detail in order to facilitate understanding of the present invention. Herein, terms and words used in the present specification and claims should not be construed to be limited to ordinary or dictionary meanings, and the inventor may appropriately define the concept of the term to describe its own invention in the best way. It should be construed as meaning and concept consistent with the technical idea of the present invention.

The present invention provides a positive electrode active material comprising a lithium-transition metal composite oxide represented by the following formula (1), wherein the positive electrode active material particles have a compressive fracture strength of 50 to 200 MPa.

[Chemical Formula 1]

Li _x Ni _(1-abc) Co _a Mn _b M _c O ₂

In Formula 1,

M is Al, B, Mg, Ti, or Zr,

0.95? X? 1.2, 0 <a? 0.4, 0 <b? 0.4, 0? C? 0.01, and 0? A + b + c? 0.4.

The cathode active material for a secondary battery according to the present invention can prevent side reaction with an electrolyte by coating or doping the surface of the cathode active material with a metal element M having low reactivity with the electrolyte. The metal element M to be coated or doped may be Al, B, Mg, Ti, or Zr, and particularly preferably B or Zr.

The metal elements may be coated on the surface of the lithium transition metal composite oxide according to a heat treatment condition or doped into the lithium transition metal composite oxide.

The coating or doped metal element M may reduce HF formation by reducing the amount of lithium impurities present on the lithium-transition metal composite oxide, or may form a coating layer on the surface of the lithium-transition metal composite oxide, It is possible to prevent the reaction with lithium impurities existing on the surface of the substrate.

The lithium impurity that may be present on the surface of the lithium transition metal oxide may include at least one of LiOH and Li ₂ CO ₃ .

On the other hand, the cathode active material for a secondary battery is subjected to a rolling process as one of manufacturing processes. The rolling process refers to pressing the active material layer a plurality of times at a predetermined pressure in order to increase the density and increase the crystallinity. During the rolling process, some of the positive electrode active material particles may fail to break due to the compressive stress received during rolling.

The compressive stress can be measured by, for example, using a micro compression tester (electronic device researcher) to measure the time at which cracks are generated in the cathode active material under a pressure of 0.5 to 10 mN to obtain a pressure unit (MPa) .

Here, the cathode active material for a secondary battery according to the present invention has a compressive fracture strength of 50 to 200 MPa, and more preferably 50 to 150 MPa. When it is less than 50 MPa, the cathode active material is easily broken and there is a problem of performance deterioration due to breakage of the active material when the electrode is rolled. On the other hand, in the case of exceeding 150 MPa, the particle strength is too strong, and the deformation amount of the battery is small when the battery is impacted, so that it may be difficult to achieve the object of reducing the ignition risk to be achieved by the present invention.

The cathode active material for a secondary battery according to the present invention has such low compressive fracture strength and low particle strength. Therefore, when the battery is impacted, the amount of deformation of the battery is increased to cause a short-circuiting of a large area, so that the risk of ignition is reduced through current dispersion, thereby ensuring the stability of the battery.

The porosity of the cathode active material particles for a secondary battery according to the present invention may be 10 to 50%. If it is less than 10%, there is an advantage that the reaction area with the electrolyte is not large and the side reaction is suppressed. However, since the particle strength is too strong and the deformation amount of the battery is small when the battery is impacted, It can be difficult to achieve. When the content exceeds 50%, the particle strength is excessively low, and the cathode active material is broken too easily, and there is a problem of performance deterioration due to breakage of the active material when the electrode is rolled.

The average particle diameter (D ₅₀ ) of the cathode active material particles for a secondary battery according to the present invention may be 10 to 20 μm. If it is less than 10 탆, there is a problem that the processability is lowered and the strength is decreased due to the differentiation. On the other hand, when the thickness exceeds 20 μm, there is a problem that the capacity of the cathode active material decreases.

In the present invention, the average particle diameter (D ₅₀ ) can be measured using, for example, a laser diffraction method or a scanning electron microscope (SEM) photograph. The laser diffraction method generally enables measurement of a particle diameter of several millimeters from a submicron region, resulting in high reproducibility and high degradability. The average particle diameter (D ₅₀ ) of the cathode active material particles can be defined as a particle diameter on the basis of 50% of the particle diameter distribution.

The present invention also provides a method for preparing a mixture comprising: a) mixing a nickel-containing compound, a cobalt-containing compound, a manganese-containing compound, a complexing agent and a precipitating agent to form a mixture solution; b) coprecipitating the mixture solution to form a lithium-transition metal composite oxide precursor; c) introducing a metal element M into the precursor; And d) mixing and firing the precursor with a lithium-containing compound to form a lithium-transition metal composite oxide represented by the following formula (1).

[Chemical Formula 1]

Li _x Ni _(1-abc) Co _a Mn _b M _c O ₂

In Formula 1,

M is Al, B, Mg, Ti, or Zr,

0.95? X? 1.2, 0 <a? 0.4, 0 <b? 0.4, 0? C? 0.01, and 0? A + b + c? 0.4.

As described above, the cathode active material for a secondary battery is prepared by mixing a metal-containing compound, a complexing agent, and a precipitant as described above with a solvent and performing a co-precipitation method.

The coprecipitation method is a method of simultaneously precipitating various different ions in an aqueous solution or a non-aqueous solution. The nickel cobalt manganese mixed metal is reacted by supplying a pH adjusting agent as a nickel cobalt manganese metal mixed solution, a complexing agent, Complex hydroxide Ni _a Co _b Mn _c (OH) ₂ .

In the method a) for producing a cathode active material for a secondary battery according to the present invention, the nickel containing compound is selected from the group consisting of nickel oxide, nickel hydroxide, nickel carbonate, nickel oxide, nickel sulfate, nickel halide and nickel carbonate Or more.

 The cobalt-containing compound may be at least one selected from the group consisting of cobalt oxide, cobalt hydroxide, cobalt halide, and cobalt carboxylate.

The manganese-containing compound may be at least one member selected from the group consisting of manganese oxide, manganese carbonate, manganese oxide, manganese sulfate, manganese halide, and manganese manganese salt.

The concentration of the metal ion in the metal mixed solution containing the nickel-containing compound, the cobalt-containing compound and the manganese-containing compound is preferably 1 to 3 M. If the concentration of the metal ion is less than 1 M, the amount of the produced substance is low and the productivity is poor. If the concentration of the metal ion is more than 3 M, the metal salt may be precipitated in the storage tank or the input pipe. , Since the concentration of the metal solution is high and the reaction can proceed rapidly, there is a disadvantage that it is difficult to control the coprecipitated particles.

The complexing agent (chelating agent) are generally aqueous ammonia (NH ₄ OH), ammonium sulfate _{((NH 4) 2 SO 4} ), ammonium nitrate (NH ₄ NO ₃₎ and the first ammonium phosphate ((NH _{4) 2} HPO ₄ ) And the like can be used, and ammonia water is preferably used. The ammonia generated in the complexing agent functions to control the shape of the metal complex hydroxide to be formed.

The content of the complexing agent can be appropriately selected as needed within a range in which the charge / discharge cycle characteristics of the secondary battery are improved and the result of facilitating the formation of the chelate is obtained. For example, NH ₄ OH may be used in an amount of 10 to 50 parts by weight based on 100 parts by weight of the metal-containing compound.

As the pH adjuster, an alkali aqueous solution such as lithium hydroxide (LiOH), sodium hydroxide (NaOH) and potassium hydroxide (KOH) may be used as the precipitating agent. The pH adjusting agent acts as a precipitating agent and functions to maintain a pH suitable for coprecipitation in the mixed solution. In the present invention, it is preferable to use an alkaline aqueous solution which provides a hydroxyl group to maintain the total pH of 10 to 12.

The content of the precipitant may be appropriately selected as needed within a range that can improve the charge-discharge cycle characteristics of the secondary battery and facilitate formation of coprecipitation. For example, 70 to 90 parts by weight of a 4 to 8 M NaOH solution based on 100 parts by weight of the metal-containing compound may be used.

Next, in step b), the mixture solution may be coprecipitated to form a lithium-transition metal composite oxide precursor.

Specifically, the lithium-transition metal composite oxide precursor may be formed by coprecipitation of the mixture solution at a pH of 10 to 12. In the preparation of the lithium transition metal complex oxide precursor by coprecipitation, the pH region forming the hydroxide of each metal is a very important factor for controlling the uniformity and strength of the coprecipitated transition metal complex oxide particle.

The cathode active material for a secondary battery according to the present invention is characterized in that the particle strength is reduced as compared with the conventional cathode active material. Therefore, the method for preparing a cathode active material for a secondary battery of the present invention is characterized in that it coprecipitates under a low pH condition. In addition, in order to reduce the particle strength, the addition amount of the complexing agent may be decreased and the firing temperature may be kept low.

Next, in step c), a metal element M may be introduced into the precursor. The introduction of the metal element M into the precursor may be performed by mixing a solution containing a metal element M, that is, a metal salt aqueous solution containing a metal element M or a metal acid into a mixture solution.

The introduction of the metal element M is carried out by a wet production method. a solution containing the lithium transition metal complex oxide precursor obtained in step b) and the metal element M and a pH adjuster as a precipitant are mixed to prepare a slurry, and the resulting slurry is washed and dried. The above-mentioned wet production method is divided into two methods, that is, a metal salt is used as a metal source, a basic solution is used as a precipitant, a metal acid is used as a metal source, and an acidic solution is used as a precipitant .

When the solution containing the metal element M is an aqueous solution of a metal salt in order to adjust the pH to cope with the metal element M, the precipitating agent is a basic solution, and when the solution containing the metal element M is a metal acid, Solution.

Specifically, the metal element M may be at least one selected from Al, B, Mg, Ti, and Zr.

The slurry prepared by the wet preparation method is filtered and washed with distilled water of high purity and then dried in a vacuum oven at 100 to 130 ° C for 10 to 15 hours to prepare a nickel cobalt manganese metal complex hydroxide Ni _{(1-abc )} Co _a Mn _b M _c (OH) ₂ can be obtained.

Next, as a step d), the method for producing a cathode active material for a secondary battery according to the present invention may include a step of mixing and firing the precursor with a lithium-containing compound to form a lithium-transition metal composite oxide represented by the general formula (1).

The precursor may be mixed with the lithium-containing compound in a weight ratio of 1: 1 to 1.1: 1.

The firing step may be carried out while flowing the dried air at 800 to 1000 ° C. The heat treatment time in the firing step can be appropriately selected depending on the firing temperature, and can be heat-treated for 10 to 20 hours, for example.

The positive electrode active material for a secondary battery thus formed is applied to a positive electrode current collector, followed by drying and rolling, thereby obtaining a positive electrode for a secondary battery.

Meanwhile, the positive electrode for a secondary battery according to the present invention may have a loading amount of 20 to 30 mg / cm 2 of a positive electrode active material for a secondary battery. If the concentration is less than 20 mg / cm 2, the capacity may decrease and the life characteristics of the battery may deteriorate. If the concentration exceeds 30 mg / cm 2, electrolyte wetting of the electrode may not be sufficiently performed, It is not preferable.

The present invention also provides a secondary battery including a positive electrode for a secondary battery according to the present invention.

The secondary battery of the present invention can be produced by a conventional method known in the art. For example, a separation membrane may be placed between the anode and the cathode, and an electrolyte in which a lithium salt is dissolved may be added.

The electrode of the secondary battery may also be manufactured by a conventional method known in the art. For example, a slurry is prepared by mixing and stirring a solvent, a binder, a conductive material, and a dispersant, if necessary, in a cathode active material or a negative electrode active material, applying the coating to a current collector of a metal material, Can be manufactured.

As the cathode active material, a lithium-containing transition metal oxide can be preferably used and, as described above, will be omitted.

The anode active material is typically a carbonaceous material, lithium metal, silicon, or tin, from which lithium ions can be occluded and released. Preferably, carbon materials can be used, and carbon materials such as low-crystalline carbon and highly-crystalline carbon can be used. Examples of the low crystalline carbon include soft carbon and hard carbon. Examples of highly crystalline carbon include natural graphite, Kish graphite, pyrolytic carbon, liquid crystal pitch carbon fiber high temperature sintered carbon such as mesophase pitch based carbon fiber, mesocarbon microbeads, mesophase pitches and petroleum or coal tar pitch derived cokes are representative.

The current collector of the metal material is a metal having high conductivity and easily adhered to the slurry of the electrode active material, and any material can be used as long as it is not reactive in the voltage range of the battery. Non-limiting examples of the positive electrode current collector include aluminum, nickel, or a foil produced by a combination of these. Non-limiting examples of the negative electrode current collector include copper, gold, nickel, or a copper alloy or a combination thereof Foil and so on.

The conductive material is not particularly limited as long as it can be generally used in the art, and examples thereof include synthetic graphite, natural graphite, carbon black, acetylene black, ketjen black, denka black, thermal black, channel black, A metal selected from the group consisting of aluminum, tin, bismuth, silicon, antimony, nickel, copper, titanium, vanadium, chromium, manganese, iron, cobalt, zinc, molybdenum, tungsten, silver, gold, lanthanum, ruthenium, , Polythiophene, polyacetylene, polypyrrole, or a combination thereof. In general, a carbon black-based conductive material may be used.

The binder can be generally used in the art, and can be applied to any kind of known binders without limitation, and generally, polyvinylidene fluoride (PVdF), polyhexafluoropropylene-polyvinylidene Poly (vinyl acetate), polyvinyl alcohol, polyethylene oxide, polyvinyl pyrrolidone, alkylated polyethylene oxide, polyvinyl ether, poly (methyl methacrylate), poly (ethyl acrylate) Butadiene rubber, ethylene propylene diene monomer (EPDM), or a mixture thereof, and the like can be used as the polyolefin-based resin composition of the present invention. Examples of the polyolefin-based resin composition of the present invention include polytetrafluoroethylene, polytetrafluoroethylene, polyvinyl chloride, polyacrylonitrile, polyvinylpyridine, styrene-butadiene rubber, acrylonitrile- Can be used.

The lithium salt that may be included as the electrolyte may be any of those conventionally used in an electrolyte for a lithium secondary battery. Examples of the anion of the lithium salt include F ^- , Cl ^- , Br ^- , I ^- , NO ₃ ^{- _{N (CN) 2 -, BF}} 4 -, ClO 4 -, PF 6 -, (CF 3) 2 PF 4 -, (CF 3) 3 PF 3 -, (CF 3) 4 PF 2 -, (CF 3) ₅ PF ^- , (CF ₃ ) ₆ P ^- , F ₃ SO ₃ ^- , CF ₃ CF ₂ SO ₃ ^- , (CF ₃ SO ₂ ) ₂ N ^- , (FSO ₂ ) ₂ N ^- , CF ₃ CF _{2 ^{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 ^- .

Examples of the electrolyte include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel polymer electrolyte, a solid inorganic electrolyte, and a molten inorganic electrolyte that can be used in the production of a lithium secondary battery, but are not limited thereto.

The secondary battery according to the present invention may be a cylindrical, square, or pouch type secondary battery, but may be particularly preferably cylindrical.

The present invention also provides a battery module including the secondary battery as a unit cell and a battery pack including the same.

The battery pack includes a power tool; An electric vehicle including an electric vehicle (EV), a hybrid electric vehicle (HEV), and a plug-in hybrid electric vehicle (PHEV); Or a system for power storage. &Lt; RTI ID = 0.0 &gt; [0027] &lt; / RTI &gt;

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

1) Preparation of cathode active material

After dissolving NiSO ₄ · 6H ₂ O, CoSO ₄ · 7H ₂ O and MnSO ₄ · H ₂ O at a molar ratio of 8: 1: 1 in water at room temperature, aqueous ammonia (NH ₄ OH), 4M sodium hydroxide (NaOH) To form a mixture solution to maintain a pH of 11.1. And co-precipitation and the mixture solution, and after a mixture of the metal-containing compounds of boron oxide LiNi _{0. 8} Co _{0 . 1} Mn _{0 . 1} B _{0 . 05} &lt; _{RTI ID =} 0.0 &gt; O2 &lt; / RTI &gt;

2) Manufacture of cylindrical lithium secondary battery

LiNi ₀ as the positive electrode active material in the solvent is N- methyl-2-pyrrolidone _{(NMP). 8} Co _{0 . 1} Mn _{0 . 1} B _{0 . 05} O ₂ , carbon black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder at a weight ratio of 92: 4: 4, respectively, to prepare a positive electrode mixture slurry. The positive electrode mixture slurry was applied to an aluminum (Al) thin film as a positive electrode collector having a thickness of about 20 占 퐉 and dried to prepare a positive electrode, followed by roll pressing to process the positive electrode (loading amount: 25 mg / cm ² ).

Further, carbon powder as a negative active material, styrene-butadiene rubber (SBR) as a binder, and carbon black as a conductive material were added to NMP as a solvent at a weight ratio of 92: 4: 4 respectively to prepare a negative electrode mixture slurry. The negative electrode mixture slurry was applied to a copper (Cu) thin film as an anode current collector having a thickness of 10 mu m and dried to produce a negative electrode, followed by roll pressing to process the negative electrode.

Into the above through a separation membrane between the manufactured positive electrode and the negative electrode sheet-like laminate in the center pin 350 g / cm ² So as to apply a stress to the jelly-roll. The prepared jelly-roll was placed in a cylindrical case, lead wires were connected, and an electrolyte having 1 M of LiPF ₆ dissolved in a carbonate electrolyte was injected to complete the production of a cylindrical lithium secondary battery.

Example  2

1) Preparation of cathode active material

A cathode active material for a secondary battery was prepared in the same manner as in Example 1, except that the mixture solution was formed to maintain the pH at 11.1.

2) Manufacture of cylindrical lithium secondary battery

A lithium secondary battery was prepared in the same manner as in Example 1 using the positive electrode containing the positive electrode active material for the secondary battery.

Comparative Example

A cylindrical lithium secondary battery was prepared in the same manner as in Example 1, except that the pH was maintained at 13.

Experimental Example  1: Scanning electron microscope ( SEM ) Picture

A scanning electron microscope (SEM) photograph of the cathode active material for a secondary battery according to Example 1 and Comparative Example of the present invention was observed.

As shown in FIGS. 1 and 2, the positive electrode active material (FIG. 2) for a secondary battery according to the comparative example has no voids in the active material, whereas the positive electrode active material for the secondary battery according to Example 1 Can be observed.

Experimental Example  2: Pellets  Measurement of particle size distribution before and after molding

The cathode active material for a secondary battery according to Example 1 and Comparative Example of the present invention was compressed at a constant pressure using a tablet machine and pelletized to measure the particle size distribution.

As shown in FIG. 3 and FIG. 4, it was observed that there was no difference in particle size distribution before and after pellet molding of the cathode active material (FIG. 4) for a secondary battery according to the comparative example. However, the cathode active material for a secondary battery according to Example 1 (FIG. 3) shows that the particle size distribution after the pellet formation is shifted to the left, and the difference in particle size distribution before and after the pellet formation is observed.

This means that the cathode active material particles of the present invention have a small particle strength and can not withstand the pressure during the pellet molding process and are broken into smaller particle sizes.

Experimental Example  3: Heat treatment of anode ( 600 ℃ ) Measurement of particle size distribution before and after

The particle size distribution of the cathode active material was measured by heat treating the cathode for a secondary battery coated with the cathode active material for a secondary battery according to Example 1 and Comparative Example of the present invention and dried and rolled at 600 ° C.

As shown in FIG. 5 and FIG. 6, it was observed that there was no difference in particle size distribution between the cathode active material for a secondary battery (FIG. 6) according to the comparative example before and after the heat treatment. However, it can be seen that the particle size distribution after heat treatment is shifted to the left and the distribution curve is broadened in the cathode active material for secondary battery according to Example 1 (FIG. 5), and a difference in particle size distribution is observed before and after the heat treatment.

This means that the cathode active material particles of the present invention have small particle strength and can not withstand high temperatures during the heat treatment of the electrode and are broken into smaller particle sizes.

Experimental Example  4: Measurement of strength of cathode active material particles

In order to measure the particle strength of the cathode active material for the secondary battery according to Examples 1 and 2 and Comparative Example of the present invention, a micro compression tester was used. The results are shown in FIG.

The pressure was measured using a sample of the cathode active material according to Examples 1 and 2 and Comparative Example under a pressure of 0.5 to 10 mN to measure the point at which the cracks occurred and converted into a pressure unit (MPa) .

As shown in FIG. 7, it was found that the cathode active material according to the comparative example had much higher particle strength than the cathode active material according to Examples 1 and 2.

As a result, the positive electrode active material particles of the present invention have a small particle strength, increase the amount of deformation of the battery upon impact, short-circuit a large area, and reduce the risk of ignition through current dispersion, have.

Experimental Example  5: Shock test of battery

In order to test the ignition of the secondary battery according to Example 1 and Comparative Example of the present invention, the impact test of the battery was performed.

In this experiment, a rod-shaped object having a diameter of 15.8 mm and a weight of 9.1 kg was placed in a center portion of a battery packed with 4.2 V in each of 10 secondary batteries according to Example 1 and Comparative Example, And checking the occurrence of ignition or explosion of the battery.

The results of the ignition or explosion of the battery after the impact test are shown in Table 1 (number of ignited or exploded cells / number of cells used in the experiment).

Number of ignited or exploded cells / Number of cells used in the experiment Example 1 0/10 Comparative Example 4/10

As shown in Table 1, in Example 1, none of ignited or exploded cells among the 10 cells used in the experiment was observed, whereas in the comparative example, ignition or explosion was observed in four cells.

In order to measure the deformation rate of the cylindrical battery after the impact test, the diameters of the upper and lower portions A and B of the battery were measured, and the results are shown in Table 2, FIG. 8, and FIG. The strain (%) was calculated by (A-B) / A.

A B Strain (%) Example 1 17 11.27 33.7 Comparative Example 17.25 12.94 25.0 17.34 12.85 25.9 17.34 12.92 25.5

As shown in Table 2, it was confirmed that the deformation rate of the secondary battery after the impact test was significantly larger than that of the Comparative Example. 10 and 11, the batteries of Example 1 and Comparative Example were disassembled and examined. As a result, it was found that the separator of the secondary battery and the positive electrode collector of Example 1 (Fig. 10) It was observed that the damage occurred in a wider range than the secondary battery.

As a result, in the case of a secondary battery including a cathode active material having a reduced particle strength according to the present invention, a deformation amount of the battery during impact increases to cause a short-circuit in a large area, thereby reducing the risk of ignition through current dispersion, .

It will be understood by those skilled in the art that the foregoing description of the present invention is for illustrative purposes only and that those of ordinary skill in the art can readily understand that various changes and modifications may be made without departing from the spirit or essential characteristics of the present invention. will be. It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive.

Claims (13)

1. A positive electrode active material comprising a lithium-transition metal composite oxide represented by the following formula (1)
Wherein the positive electrode active material particles have compressive fracture strength of 50 to 200 MPa.
[Chemical Formula 1]
Li _x Ni _(1-abc) Co _a Mn _b M _c O ₂
In Formula 1,
M is Al, B, Mg, Ti, or Zr,
0.95? X? 1.2, 0 <a? 0.4, 0 <b? 0.4, 0? C? 0.01, and 0? A + b + c? 0.4.
The method according to claim 1,
Wherein the positive electrode active material particles have a compressive fracture strength of 50 to 150 MPa.
The method according to claim 1,
Wherein the positive electrode active material particles have a porosity of 10 to 50%.
The method according to claim 1,
Wherein the average particle diameter (D ₅₀ ) of the cathode active material particles is 10 to 20 μm.
a) mixing a nickel-containing compound, a cobalt-containing compound, a manganese-containing compound, a complexing agent and a precipitant to form a mixture solution;
b) coprecipitating the mixture solution to form a lithium-transition metal composite oxide precursor;
c) introducing a metal element M into the precursor; And
(d) mixing and firing the precursor with a lithium-containing compound to form a lithium-transition metal composite oxide represented by the following formula (1).
[Chemical Formula 1]
Li _x Ni _(1-abc) Co _a Mn _b M _c O ₂
In Formula 1,
M is Al, B, Mg, Ti, or Zr,
0.95? X? 1.2, 0 <a? 0.4, 0 <b? 0.4, 0? C? 0.01, and 0? A + b + c? 0.4.
6. The method of claim 5,
The nickel-containing compound is at least one selected from the group consisting of nickel oxide, nickel hydroxide, nickel carbonate, nickel oxide, nickel sulfate, nickel halide and nickel carbonate,
Wherein the cobalt-containing compound is at least one selected from the group consisting of cobalt oxide, cobalt hydroxide, cobalt halide, and cobalt carbonate,
Wherein the manganese-containing compound is at least one selected from the group consisting of manganese oxide, manganese carbonate, manganese oxide, manganese sulfate, manganese halide, and manganese manganese salt.
6. The method of claim 5,
Wherein the coprecipitation is performed at a pH of 10 to 12.
A positive electrode for a secondary battery according to any one of claims 1 to 4, wherein the positive electrode active material for a secondary battery is applied to a positive electrode current collector, followed by drying and rolling.
9. The method of claim 8,
Wherein the loading amount of the cathode active material for the secondary battery is 20 to 30 mg / cm &lt; 2 &gt;.
9. A secondary battery comprising a positive electrode for a secondary battery according to claim 8 or 9.
A battery module comprising the secondary battery according to claim 10 as a unit cell.
A battery pack comprising the battery module according to claim 11.
13. The method of claim 12,
Wherein the battery pack is used as at least one medium to large-sized device power source selected from the group consisting of a power tool, an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, and a power storage system.

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