Positive electrode active material precursor for secondary battery and method for preparing same, positive electrode active material and method for preparing same, and positive electrode for secondary

Abstract

The present invention relates to a positive electrode active material precursor for a secondary battery and a method for preparing the same, a positive electrode active material and a method for preparing the same, and a positive electrode for a secondary battery and a lithium secondary battery comprising the same. The positive electrode active material precursor for a secondary battery contains primary particles of Co3O4 or CoOOH, in which the primary particles contain a doping element in an amount of 3,000 ppm or more and have an average particle diameter (D50) of 15 [mu] m or more, and the primary particles are non-spherical. The positive electrode active material for a secondary battery includes primary particles of a lithium cobalt-based oxide, in which the primary particles contain a doping element in an amount of 2,500 ppm or more and have an average particle diameter (D50) of 15 [mu] m or more, in which the doping element is Al. The positive electrode active material for a secondary battery has structural stability even at a high voltage.

Description

[0001] This patent application is a divisional application of Chinese Patent Application for Invention No. 201880008449.9, filed on September 19, 2018, entitled "Positive Electrode Active Material Precursor for Secondary Battery, Positive Electrode Active Material, and Lithium Secondary Battery Comprising the Same." TECHNICAL FIELD

[0002] This application claims the benefit of Korean Patent Application Nos. 10-2017-0120645, filed on September 19, 2017, and 10-2018-0111642, filed on September 18, 2018, the disclosures of which are incorporated herein in their entirety by reference.

[0003] The present application relates to a positive electrode active material precursor for a secondary battery, a positive electrode active material, and a lithium secondary battery comprising the same. BACKGROUND

[0004] Recently, as electronic devices using batteries such as mobile phones, notebook computers, and electric vehicles have rapidly spread, the demand for secondary batteries having a relatively high capacity and being small in size and light in weight has rapidly increased. In particular, since lithium secondary batteries are light in weight and have a high energy density, lithium secondary batteries are attracting attention as a driving power source for portable devices. Accordingly, efforts to improve the performance of lithium secondary batteries have been actively made.

[0005] A lithium secondary battery refers to a battery in which a positive electrode comprising a positive electrode active material capable of intercalating / deintercalating lithium ions, a negative electrode comprising a negative electrode active material capable of intercalating / deintercalating lithium ions, and an electrolyte containing lithium ions are contained in an electrode assembly having a microporous separator disposed between the positive electrode and the negative electrode.

[0006] A lithium transition metal oxide is used as a positive electrode active material of a lithium secondary battery, and a lithium metal, a lithium alloy, a crystalline or amorphous carbon, or a carbon composite material is used as a negative electrode active material. An electrode current collector can be coated with an active material of an appropriate thickness and length, or the active material itself can be coated in the form of a film, and then the resulting product is wound or stacked together with an insulating separator to prepare an electrode assembly. Thereafter, the electrode assembly is put into a can or a container similar thereto, and then a secondary battery is prepared by injecting an electrolyte solution.

[0007] As a cathode active material of a lithium secondary battery which has been actively researched, developed and used up to now, there is a lithium cobalt oxide (LiCoO2) having a layered structure. The lithium cobalt oxide (LiCoO2) is advantageous in that it has a high operating voltage and excellent capacity characteristics, but the lithium cobalt oxide is limited in that it has poor thermal properties due to instability of a crystal structure caused by delithiation, and its structure becomes unstable at a high voltage.

[0008] Recently, there has been a gradual increase in demand for a high-capacity lithium secondary battery, and for a lithium cobalt oxide (LiCoO2) different from a ternary cathode active material, since the lithium cobalt oxide (LiCoO2) can increase capacity only by increasing a voltage, there is a need to develop a lithium cobalt oxide (LiCoO2) which can secure structural stability even at a voltage higher than 4.5 V above a typical voltage of 4.45 V.

[0009] In order to prepare a lithium cobalt oxide (LiCoO2) which stably operates at a high voltage of 4.5 V or more, a technique of doping an excess of a doping element has been attempted, but in this case, since the excess of the doping element inhibits growth of a cathode active material, there is a limitation in that it is difficult to prepare a cathode active material having a large particle size. SUMMARY

[0010] TECHNICAL PROBLEM

[0011] An aspect of the present application provides a cathode active material of a lithium cobalt oxide having a large particle size of 15 μm or more in average particle diameter (D 50 ) while solving a problem of particle growth inhibition caused by a doping element by doping an excess of the doping element, even at a high voltage, thereby securing structural stability.

[0012] TECHNICAL SOLUTION

[0013] According to an aspect of the present application, there is provided a cathode active material precursor for a secondary battery, the cathode active material precursor comprising primary particles of Co3O4 or CoOOH, wherein the primary particles contain a doping element in an amount of 3,000 ppm or more and have an average particle diameter (D 50 ) of 15 μm or more.

[0014] According to another aspect of the present application, there is provided a cathode active material for a secondary battery, the cathode active material comprising primary particles of a lithium cobalt-based oxide, wherein the primary particles contain a doping element in an amount of 2,500 ppm or more and have an average particle diameter (D 50 ) of 15 μm or more.

[0015] According to another aspect of the present application, there is provided a method of preparing a positive electrode active material precursor for a secondary battery, the method including: preparing a precursor forming solution including a cobalt-containing starting material and a dopant element source; and performing a co-precipitation reaction on the precursor forming solution to form a Co3O4 or CoOOH precursor containing a dopant element in an amount of 3,000 ppm or more and having a primary particle average particle diameter (D 50 ) of 15 μm or more.

[0016] According to another aspect of the present application, there is provided a method of preparing a positive electrode active material for a secondary battery, the method including mixing a positive electrode active material precursor powder with a lithium source and calcining to form a lithium cobalt-based oxide containing a dopant element in an amount of 2,500 ppm or more and having a primary particle average particle diameter (D 50 ) of 15 μm or more.

[0017] According to another aspect of the present application, there is provided a positive electrode including the positive electrode active material and a lithium secondary battery.

[0018] Advantageous Effects

[0019] According to the present application, the problem of particle growth inhibition caused by a dopant element is solved while having structural stability even at a high voltage by doping with an excess amount of a dopant element, so that it is possible to provide a positive electrode active material of a lithium cobalt oxide having a large particle size of 15 μm or more (D 50 ). BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 is a magnified scanning electron microscope (SEM) image of a positive electrode active material precursor prepared according to Example 1 of the present application; and

[0021] Figure 2 is a magnified scanning electron microscope (SEM) image of a positive electrode active material precursor prepared according to Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0022] Hereinafter, the present application will be described in greater detail to allow for a clearer understanding of the present application. In this case, it should be understood that the words or terms used in the specification and claims should not be interpreted as having common meanings or meanings defined in generally used dictionaries, and should be interpreted as having meanings and concepts corresponding to technical concepts of the present application based on the principle that inventors can define words or terms to best explain the present application.

[0023] The present application produces a large particle size precursor by doping a precursor with an excess amount of a doping element during production of a positive electrode active material precursor and growing the precursor particle size to an average particle diameter (D 50 ) of 15 μm or more. When a positive electrode active material is produced by using the above-described large particle size precursor doped with an excess amount, a large particle size positive electrode active material can be produced without increasing the calcination temperature and the amount of added lithium.

[0024] Specifically, the positive electrode active material precursor for secondary batteries of the present application comprises primary particles of Co3O4 or CoOOH, wherein the primary particles contain a doping element in an amount of 3,000 ppm or more and have an average particle diameter (D 50 ) of 15 μm or more.

[0025] The positive electrode active material precursor of the present application is composed of primary particles of Co3O4 or CoOOH. The positive electrode active material precursor of the present application is not a secondary particle formed by agglomeration of primary particles, but is preferably primary particles that are not physically separated.

[0026] The positive electrode active material precursor of the present application can have a primary particle average particle diameter (D 50 ) of 15 μm or more, and can more preferably have a primary particle average particle diameter (D 50 ) of 17 μm or more. In the case where the primary particle average particle diameter (D 50 ) of the positive electrode active material precursor is less than 15 μm, it can be difficult to produce a positive electrode active material having an average particle diameter (D 50 ) of 15 μm or more because the doping element suppresses particle growth when a positive electrode active material is produced by a calcination process using a precursor having an average particle diameter (D 50 ) of less than 15 μm. If a large particle size positive electrode active material having an average particle diameter (D 50 ) of 15 μm or more is not produced, there is a limitation in increasing the compression density of the positive electrode, and it is difficult to increase the battery capacity.

[0027] Further, the primary particles of the positive electrode active material precursor of the present application can contain a doping element in an amount of 3,000 ppm or more, for example, 4,000 ppm or more. In the case where the primary particles of the positive electrode active material precursor contain a doping element in an amount of less than 3,000 ppm, it is difficult to ensure the structural stability of a lithium cobalt-based oxide positive electrode active material, and particularly, since the structural stability is reduced at a high voltage of 4.5 V or more, battery characteristics such as room temperature and high temperature life characteristics can be deteriorated.

[0028] Instead of forming a highly doped precursor as in the present invention, in the case where an undoped precursor is formed into a large particle and high content doping is performed by additionally adding a doping element at the time of calcination with a lithium source, a high content of the doping element can not be doped at a uniform concentration, and there can be limitations in improving battery characteristics such as battery capacity, rate performance, and life characteristics.

[0029] The doping element can be at least one selected from the group consisting of aluminum (Al), titanium (Ti), manganese (Mn), zirconium (Zr), magnesium (Mg), niobium (Nb), calcium (Ca), fluorine (F), and nickel (Ni), and can be more preferably Al. With respect to the Al doping element, because the Al doping element has a much greater particle growth inhibition effect than other doping elements (e.g., Mg), the Al doping element can be more preferably prepared after preparing a precursor of a large particle size while performing high content doping as in the present invention by using a precursor of a large particle size to prepare a positive electrode active material.

[0030] With respect to the positive electrode active material precursor doped with a doping element during the preparation of the precursor as described above, the doping element can have a predetermined concentration in the primary particles of the precursor.

[0031] Next, a method of preparing the positive electrode active material precursor of the present invention will be described.

[0032] The positive electrode active material precursor of the present invention is prepared by a method including preparing a precursor forming solution containing a cobalt-containing starting material and a doping element source, and performing a coprecipitation reaction on the precursor forming solution to form a Co3O4 or CoOOH precursor containing the doping element in an amount of 3,000 ppm or more and having a primary particle average particle diameter (D 50 ) of 15 μm or more.

[0033] During the preparation of the positive electrode active material precursor in the present invention, the precursor is doped by performing a coprecipitation reaction with the doping element source. Because the precursor can be doped by adding the doping element source in the precursor coprecipitation process, the doping element can be doped at a uniform concentration. Further, because the particle size of the doped precursor can be easily controlled by adjusting the coprecipitation reaction time, the size of the precursor can be easily increased while performing high content doping.

[0034] In the preparation of the precursor, first, a precursor forming solution containing a cobalt-containing starting material and a doping element source is prepared.

[0035] As the cobalt-containing starting material, a cobalt-containing sulfate, halide, acetate, sulfide, hydroxide, oxide, or oxyhydroxide can be used, and these materials are not particularly limited as long as it can be dissolved in water. For example, the cobalt-containing starting material can include Co(S04)2-7H20, CoCl2, Co(OH)2, Co(OCOCH3)2-4H20, or Co(N03)2-6H20, and any one of the above materials or a mixture of two or more thereof can be used.

[0036] The dopant element source can include a dopant element-containing sulfate, nitrate, acetate, halide, hydroxide, or oxyhydroxide, and any one of the above materials or a mixture of two or more thereof can be used. The dopant element can be at least one selected from the group consisting of Al, Ti, Mn, Zr, Mg, Nb, Ca, F, and Ni, and more preferably, Al can be contained as the dopant element.

[0037] The precursor forming solution can be prepared by adding the cobalt-containing starting material and the dopant element source to a solvent, specifically water or a mixture of water and an organic solvent (specifically alcohol or the like) that can be uniformly mixed with water, or these solutions can be mixed after separately preparing a solution containing the cobalt-containing starting material and a solution containing the dopant element source and used.

[0038] Next, a coprecipitation reaction of the precursor forming solution is performed to form a Co304 or CoOOH precursor containing the dopant element in an amount of 3,000 ppm or more and having a primary particle average particle diameter (D 50 ) of 15 μm or more.

[0039] A Co304 or CoOOH precursor containing the dopant element in an amount of 3,000 ppm or more and having a primary particle average particle diameter (D 50 ) of 15 μm or more can be prepared by introducing the precursor forming solution into a reactor and adding a chelating agent and an aqueous alkali solution to perform a coprecipitation reaction.

[0040] The chelating agent can include NH4OH, (NH4)2SO4, NH4NO3, NH4Cl, CH3COONH4, or (NH4)2CO3, and any one of the above materials or a mixture of two or more thereof can be used. In addition, the chelating agent can be used in the form of an aqueous solution, and in this case, as the solvent, water or a mixture of water and an organic solvent (specifically alcohol or the like) that can be uniformly mixed with water can be used.

[0041] The basic compound can include a hydroxide of an alkali metal or an alkaline earth metal such as NaOH, KOH, or Ca(OH)2or a hydrate thereof, and any one of the above materials or a mixture of two or more thereof can be used. The basic compound can also be used in the form of an aqueous solution, and in this case, as the solvent, water or a mixture of water and an organic solvent (specifically, an alcohol or the like) that can be uniformly mixed with water can be used. In this case, the basic aqueous solution can have a concentration of 2 M to 10 M.

[0042] The coprecipitation reaction for preparing the cathode active material precursor can be performed under conditions in which the pH is in the range of 10 to 12. In the case where the pH is outside the above range, there is a problem in that the size of the prepared cathode active material precursor can change or particle breakage can occur. Specifically, the coprecipitation reaction can be performed under conditions in which the pH is in the range of 11 to 12. The adjustment of the above pH can be controlled by adding a basic aqueous solution.

[0043] The coprecipitation reaction for preparing the cathode active material precursor can be performed in a non-reactive atmosphere such as nitrogen at a temperature in the range of 30°C to 80°C. A stirring process can be optionally performed to increase the reaction rate during the reaction, and in this case, the stirring speed can be in the range of 100 rpm to 2,000 rpm.

[0044] Primary particles of the Co3O4or CoOOH precursor doped with an excess amount of a doping element are precipitated as a result of the coprecipitation reaction. The amount of the doping element doped in the precursor can be 3,000 ppm or more, for example, 4,000 ppm or more. A large amount of the doping element can be doped by doping the precursor as described above. Furthermore, the thus-prepared precursor can be uniformly doped with the doping element, and there is no concentration gradient from the center to the surface of the cathode active material precursor particle.

[0045] Furthermore, because the particle size of the doped precursor can be easily controlled by adjusting the coprecipitation reaction time during the preparation of the precursor, the size of the precursor can be easily increased while performing high-content doping. The coprecipitation reaction time can be in the range of 10 hours to 40 hours, for example, 10 hours to 30 hours. By adjusting the coprecipitation reaction time as described above, the Co3O4or CoOOH precursor having a primary particle average particle diameter (D 50 ) of 15 μm or more can be formed.

[0046] The precipitated Co3O4or CoOOH precursor can be separated according to a conventional method, and then a drying process can be optionally performed, and in this case, the drying process can be performed at 110°C to 400°C for 15 hours to 30 hours.

[0047] Further, the present application provides a positive electrode active material prepared by using the above-described over-doped large particle size precursor. Because the positive electrode active material is prepared by using a large particle size precursor as the over-doped primary particle of the present application, a large particle size positive electrode active material containing an over-doped doping element and having a large primary particle average particle diameter can be prepared.

[0048] Specifically, the positive electrode active material for secondary batteries of the present application comprises primary particles of a lithium cobalt-based oxide, wherein the primary particles contain a doping element in an amount of 2,500 ppm or more and have an average particle diameter (D 50 ) of 15 μm or more.

[0049] If a precursor in the form of a secondary particle in which primary particles are agglomerated is used, it is difficult to prepare a positive electrode active material having primary particles with a diameter of 15 μm or more due to the particle growth inhibition effect of a high content of a doping element during the calcination process. In particular, regarding Al doping elements, because Al doping elements have a much greater particle growth inhibition effect than other doping elements (e.g., Mg), it can be more preferable that Al doping elements be prepared after a large particle size precursor is prepared while a high content of doping is performed as in the present application, and then the positive electrode active material is prepared by using the large particle size precursor.

[0050] In the present application, because a precursor containing a doping element in an amount of 3,000 ppm or more and having a primary particle average particle diameter (D 50 ) of 15 μm or more is used, an active material containing a doping element in an amount of 2,500 ppm or more and having a primary particle average particle diameter (D 50 ) of 15 μm or more can be prepared without increasing the calcination temperature and the amount of added lithium.

[0051] The positive electrode active material of the present application is composed of primary particles of a lithium cobalt-based oxide.

[0052] The positive electrode active material of the present application can have a primary particle average particle diameter (D 50 ) of 15 μm or more, and can more preferably have a primary particle average particle diameter (D 50 ) of 17 μm or more. Because the primary particle average particle diameter (D 50 ) of the positive electrode active material satisfies 15 μm or more, the capacity, energy density, and life characteristics of the battery can be improved. In particular, because the compression density of the positive electrode can be significantly improved by mixing a large particle size positive electrode active material having an average particle diameter (D 50 ) of 15 μm or more with a small particle size positive electrode active material in a predetermined ratio, the battery capacity can be increased.

[0053] Further, the primary particles of the positive electrode active material of the present application can contain a dopant element in an amount of 2,500 ppm or more, for example, 3,000 ppm or more. Because a lithium source is also added at the time of production of the positive electrode active material, the content ratio (ppm) of the dopant element of the positive electrode active material can be slightly smaller than the content ratio (ppm) of the dopant element contained in the positive electrode active material precursor. In the case where the primary particles of the positive electrode active material contain a dopant element in an amount of less than 2,500 ppm, it is difficult to ensure the structural stability of the lithium cobalt-based oxide positive electrode active material, and particularly, because the structural stability is reduced at a high voltage of 4.5 V or more, battery characteristics such as room temperature and high temperature life characteristics can be deteriorated.

[0054] The dopant element can be at least one selected from the group consisting of Al, Ti, Mn, Zr, Mg, Nb, Ca, F, and Ni, and Al can be more preferable. With respect to the Al dopant element, because the Al dopant element has a particle growth inhibitory effect much greater than other dopant elements (e.g., Mg), the Al dopant element can be more preferable to produce the positive electrode active material by using a large particle size precursor after producing a large particle size precursor with high content doping as in the present application.

[0055] With respect to the positive electrode active material produced by using a positive electrode active material precursor doped with a dopant element during production of the precursor as described above, the dopant element can have a predetermined concentration in the primary particles of the positive electrode active material. Further, the primary particles of the positive electrode active material can contain 50% or more of the total amount of the dopant element in a central portion located near the center, which corresponds to 50% of the radius from the particle center to the particle surface.

[0056] The molar ratio of lithium to metal elements other than lithium (Co, M, etc.) (molar ratio of lithium / metal elements (Co, M, etc.)) in the lithium cobalt-based oxide can be in the range of 0.98 to 1.1.

[0057] Further, the positive electrode active material according to the embodiment of the present application further includes a surface layer on the surface of the lithium cobalt-based oxide particle, and the surface layer can include an oxide of at least one element selected from the group consisting of Mg, Ti, iron (Fe), copper (Cu), Ca, barium (Ba), tin (Sn), antimony (Sb), sodium (Na), zinc (Zn), silicon (Si), yttrium (Y), Zr, Nb, molybdenum (Mo), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), cadmium (Cd), scandium (Sc), cerium (Ce), praseodymium (Pr), neodymium (Nd), gadolinium (Gd), dysprosium (Dy), ytterbium (Yb), erbium (Er), cobalt (Co), Al, gallium (Ga), and boron (B).

[0058] Next, a method of producing the positive electrode active material of the present application will be described.

[0059] As for the positive electrode active material of the present application, a lithium cobalt-based oxide containing a dopant element in an amount of 2,500 ppm or more and having a primary particle average particle diameter (D 50 ) of 15 μm or more is formed by mixing the positive electrode active material precursor of the present application with a lithium source and calcining.

[0060] As the lithium source, a lithium-containing sulfate, nitrate, acetate, carbonate, oxalate, citrate, halide, hydroxide, or oxyhydroxide can be used, and these materials are not particularly limited as long as it can be dissolved in water. Specifically, the lithium raw material can include Li2CO3, LiNO3, LiNO2, LiOH, LiOH-H2O, LiH, LiF, LiCl, LiBr, LiI, CH3COOLi, Li2O, Li2SO4, CH3COOLi, or Li3C6H5O7, and any one of the above materials or a mixture of two or more thereof can be used.

[0061] Further, the amount of the lithium source used can be determined depending on the amount of lithium and metal elements other than lithium (Co, etc.) in the lithium cobalt-based oxide finally produced, and specifically, the lithium source can be used in an amount such that the molar ratio of lithium to metal elements other than lithium in the lithium cobalt-based oxide finally produced is in the range of 0.98 to 1.1.

[0062] During the mixing of the precursor with the lithium source, a sintering agent can also be added optionally. The sintering agent can specifically include: a compound containing an ammonium ion such as NH4F, NH4NO3, or (NH4)2SO4; a (semi)metal oxide such as B2O3 or Bi2O3; or a metal halide such as NiCl2 or CaCl2, and any one of the above materials or a mixture of two or more thereof can be used. The sintering agent can be used in an amount of 0.01 mol to 0.2 mol based on 1 mol of the precursor. If the amount of the sintering agent is too small, less than 0.01 mol, the sintering property improvement effect of the positive electrode active material precursor can not be significant, and if the amount of the sintering agent is too large, more than 0.2 mol, there is a problem in that the performance as a positive electrode active material can be deteriorated due to the excess sintering agent, and the initial capacity of the battery can decrease during charging and discharging.

[0063] Further, during the mixing of the precursor with the lithium source, a water-removing agent can also be added optionally. Specifically, the water-removing agent can include citric acid, tartaric acid, glycolic acid, or maleic acid, and any one of the above materials or a mixture of two or more thereof can be used. The water-removing agent can be used in an amount of 0.01 mol to 0.2 mol based on 1 mol of the precursor.

[0064] The calcination can be performed in a temperature range of 900°C to 1,100°C, for example, 1,000°C to 1,050°C. If the calcination temperature is lower than 900°C, there is a problem that the discharge capacity per unit weight can decrease due to the residual of unreacted raw materials, the cycle characteristics can deteriorate, and the operating voltage can decrease, and if the calcination temperature is higher than 1,100°C, there is a problem that the discharge capacity per unit weight can decrease due to the generation of by-products, the cycle characteristics can deteriorate, and the operating voltage can decrease.

[0065] The calcination can be performed in an oxidizing atmosphere (such as air or oxygen) or an inactive atmosphere (including nitrogen or hydrogen) for 5 hours to 30 hours.

[0066] A surface layer containing an inorganic oxide can be further formed on the surface of the particles of the lithium cobalt-based oxide prepared as described above.

[0067] The surface layer can contain an oxide of at least one element selected from the group consisting of Mg, Ti, Fe, Cu, Ca, Ba, Sn, Sb, Na, Zn, Si, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Sc, Ce, Pr, Nd, Gd, Dy, Yb, Er, Co, Al, Ga, and B, and can be formed by mixing and heat-treating a coating material containing the element constituting the surface layer.

[0068] The positive electrode active material of the lithium cobalt oxide prepared as described above can be prepared to have large particles having a primary particle average particle diameter (D 50 ) of 15 μm or more while solving the problem of particle growth inhibition caused by the doping element by doping with an excess of the doping element, thereby having structural stability even at a high voltage. Thereby, the positive electrode active material can be used in a high-voltage secondary battery of 4.5 V or more, the positive electrode active material can achieve a high capacity and at the same time, the life characteristics can be significantly improved.

[0069] According to another embodiment of the present application, a positive electrode for a lithium secondary battery and a lithium secondary battery containing the positive electrode active material are provided.

[0070] Specifically, the positive electrode contains a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and containing the positive electrode active material.

[0071] In the positive electrode, the positive electrode current collector is not particularly limited as long as it has conductivity and does not cause an adverse chemical change in the battery, and, for example, stainless steel; aluminum; nickel; titanium; baked carbon; or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, or the like can be used. Furthermore, the positive electrode current collector can generally have a thickness of 3 μm to 500 μm, and a slight concavo-convex can be formed on the surface of the positive electrode current collector to improve the adhesion of the positive electrode active material. For example, the positive electrode current collector can be used in various shapes such as a film, a sheet, a foil, a mesh, a porous body, a foam body, a nonwoven fabric body, or the like.

[0072] Furthermore, the positive electrode active material layer can contain a conductive agent and a binder in addition to the above-described positive electrode active material.

[0073] In this case, the conductive agent serves to provide conductivity to the electrode, and any conductive agent can be used without particular limitation as long as it has suitable electronic conductivity and does not cause an adverse chemical change in the battery. Specific examples of the conductive agent can be graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal crack carbon black, and carbon fiber; a powder or fiber of a metal such as copper, nickel, aluminum, and silver; a conductive whisker such as zinc oxide whisker and potassium titanate whisker; a conductive metal oxide such as titanium oxide; or a conductive polymer such as a polyphenylene derivative, and one of the above materials or a mixture of two or more of the above materials can be used. The conductive agent can generally be contained in an amount of 1% by weight to 30% by weight based on the total weight of the positive electrode active material layer.

[0074] Furthermore, the binder improves the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Specific examples of the binder can be polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers of the above materials, and any of the above materials or a mixture of two or more of the above materials can be used. The binder can be contained in an amount of 1% by weight to 30% by weight based on the total weight of the positive electrode active material layer.

[0075] In addition to the use of the above-described positive electrode active material, the positive electrode can be prepared according to a typical method for preparing a positive electrode. Specifically, a composition for forming a positive electrode active material layer containing the above-described positive electrode active material and optionally a binder and a conductive agent is coated on a positive electrode current collector, and then the coated positive electrode current collector can be dried and rolled to prepare the positive electrode. In this case, the types and amounts of the positive electrode active material, the binder, and the conductive agent are the same as described above.

[0076] The solvent can be a solvent typically used in the art, and can include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methyl pyrrolidone (NMP), acetone, or water, and any one of the above solvents or a mixture of two or more thereof can be used. The amount of the solvent used can be sufficient in that the solvent can dissolve or disperse the positive electrode active material, the conductive agent, and the binder, and can allow a viscosity that can provide excellent thickness uniformity during subsequent coating for preparing the positive electrode, if the coating thickness and the manufacturing yield of the slurry are taken into consideration.

[0077] Further, as another method, the positive electrode can be prepared by casting the above-described composition for forming a positive electrode active material layer on a separate support, and then laminating the film separated from the support on a positive electrode current collector.

[0078] According to another embodiment of the present application, an electrochemical device including the positive electrode is provided. The electrochemical device can specifically be a battery or a capacitor, and can be, for example, a lithium secondary battery.

[0079] The lithium secondary battery specifically includes a positive electrode, a negative electrode disposed to face the positive electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode is the positive electrode as described above. Further, the lithium secondary battery can also optionally include a battery container accommodating an electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member sealing the battery container.

[0080] In the lithium secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector.

[0081] The negative electrode current collector is not particularly limited as long as it has high conductivity and does not cause an adverse chemical change in the battery, and can use, for example, copper; stainless steel; aluminum; nickel; titanium; calcined carbon; copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, and the like; and an aluminum-cadmium alloy. Further, the negative electrode current collector can generally have a thickness of 3 μm to 500 μm, and like the positive electrode current collector, a minute concavo-convex can be formed on the surface of the current collector to improve the adhesion of the negative electrode active material. For example, the negative electrode current collector can be used in various shapes such as a film, a sheet, a foil, a mesh, a porous body, a foam body, a non-woven fabric body, and the like.

[0082] The negative electrode active material layer selectively contains a binder and a conductive agent in addition to the negative electrode active material. The negative electrode active material layer can be prepared, for example, by coating a negative electrode-forming composition containing the negative electrode active material and selectively containing the binder and the conductive agent on the negative electrode current collector and drying the coated negative electrode current collector; or can be prepared by casting the negative electrode-forming composition on a separate carrier and then laminating the film separated from the carrier on the negative electrode current collector.

[0083] A compound capable of reversibly intercalating and deintercalating lithium can be used as the negative electrode active material. Specific examples of the negative electrode active material can be: carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; (semi)metallic materials capable of forming an alloy with lithium such as silicon (Si), aluminum (Al), tin (Sn), lead (Pb), zinc (Zn), bismuth (Bi), indium (In), magnesium (Mg), gallium (Ga), cadmium (Cd), Si alloys, Sn alloys, or Al alloys; metal oxides that can or cannot be doped with lithium such as SiO x (0 < x < 2), SnO2, vanadium oxides, and lithium vanadium oxides; or composite materials containing (semi)metallic materials and carbonaceous materials such as Si-C composite materials or Sn-C composite materials, and any one of the above materials or a mixture of two or more thereof can be used. In addition, a thin film of metallic lithium can be used as the negative electrode active material. Furthermore, low-crystalline carbon and high-crystalline carbon can be used as the carbon material. Typical examples of the low-crystalline carbon can be soft carbon and hard carbon, and typical examples of the high-crystalline carbon can be irregular, planar, flaky, spherical, or fibrous natural graphite or artificial graphite, condensed graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbead, mesophase pitch, and high-temperature calcined carbon such as petroleum or coal tar pitch-derived coke.

[0084] Furthermore, the binder and the conductive agent can be the same as those described previously in the positive electrode.

[0085] In the lithium secondary battery, a separator separates the negative electrode from the positive electrode and provides a path for movement of lithium ions, wherein any separator can be used as the separator without particular limitation, as long as it is generally used in lithium secondary batteries, and in particular, a separator having a high moisture retaining ability with respect to an electrolyte and a low resistance to migration of electrolyte ions can be used. Specifically, a porous polymer film, such as a porous polymer film prepared from a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butylene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminate structure having two or more layers among the above porous polymer films can be used. In addition, a typical porous nonwoven fabric, such as a nonwoven fabric formed of high-melting point glass fibers or polyethylene terephthalate fibers, can be used. In addition, a coated separator including a ceramic component or a polymeric material can be used to secure heat resistance or mechanical strength, and a separator having a single layer or a multi-layer structure can be selectively used.

[0086] In addition, the electrolyte used in the present application can include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a molten-type inorganic electrolyte, which can be used in the preparation of a lithium secondary battery, but the present application is not limited thereto.

[0087] Specifically, the electrolyte can include an organic solvent and a lithium salt.

[0088] Any organic solvent can be used as the organic solvent without particular limitation, as long as it can be used as a medium through which ions participating in an electrochemical reaction of a battery can move. Specifically, as the organic solvent, an ester-based solvent such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; an ether-based solvent such as dibutyl ether or tetrahydrofuran; a ketone-based solvent such as cyclohexanone; an aromatic hydrocarbon-based solvent such as benzene and fluorobenzene; or a carbonate-based solvent such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methylethyl carbonate (MEC), ethylmethyl carbonate (EMC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and propylene carbonate (PC); an alcohol-based solvent such as ethanol and isopropanol; a nitrile such as R-CN (wherein R is a linear, branched, or cyclic C2 to C20 hydrocarbon group and can include a double-bonded aromatic ring or an ether bond); an amide such as dimethylformamide; a dioxolane such as 1,3-dioxolane; or a sulfolane can be used. Among these solvents, a carbonate-based solvent can be preferably used, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and a high dielectric constant, which can improve the charge / discharge performance of a battery, and a low-viscosity linear carbonate compound (e.g., methylethyl carbonate, dimethyl carbonate, or diethyl carbonate) can be more preferably used. In this case, when the cyclic carbonate and the linear carbonate are mixed at a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte solution can be excellent.

[0089] The lithium salt can be used without particular limitation, as long as it is a compound capable of providing lithium ions used in lithium secondary batteries. Specifically, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2may be used as the lithium salt. The lithium salt can be used in a concentration range of 0.1 M to 2.0 M. In the case where the concentration of the lithium salt is included in the above range, since the electrolyte can have appropriate conductivity and viscosity, excellent electrolyte performance can be obtained, and lithium ions can move efficiently.

[0090] To improve the life characteristics of the battery, suppress the decrease in the capacity of the battery, and increase the discharge capacity of the battery, at least one additive, such as a halogenated alkylene carbonate compound (e.g., difluoroethylene carbonate), pyridine, triethyl phosphite, triethanolamine, a cyclic ether, ethylenediamine, n-glyme, hexamethylphosphoric triamide, a nitrobenzene derivative, sulfur, a quinonimine dye, an N-substituted oxazolidinone, an N,N-substituted imidazolidine, an ethylene glycol dialkyl ether, an ammonium salt, a pyrrole, 2-methoxyethanol, or aluminum trichloride, can be added to the electrolyte in addition to the electrolyte components. In this case, the additive can be included in an amount of 0.1% to 5% by weight, based on the total weight of the electrolyte.

[0091] As described above, since the lithium secondary battery including the positive electrode active material according to the present application stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, the lithium secondary battery is suitable for: portable devices, such as mobile phones, notebook computers, and digital cameras; and electric vehicles, such as hybrid electric vehicles (HEVs).

[0092] Therefore, according to another embodiment of the present application, a battery module including the lithium secondary battery as a unit cell and a battery pack including the battery module are provided.

[0093] The battery module or the battery pack can be used as a power source for at least one of medium- and large-sized devices, such as: power tools; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0094] Hereinafter, embodiments of the present application will be described in detail in such a manner that those skilled in the art to which the present application pertains can easily implement the present application. However, the present application can be implemented in many different forms and should not be construed as being limited to the embodiments set forth herein.

[0095] Example 1 - Preparation of a positive electrode active material precursor

[0096] In a 5 L batch reactor set at 60°C, CoSO4 was mixed in water, and further mixed with Al(OH)3 in an amount of 0.5 wt% based on CoSO4 to prepare a precursor forming solution having a concentration of 2 M. A vessel containing the precursor forming solution was connected to the reactor, and in addition, a 25 wt% aqueous NaOH solution and a 15 wt% aqueous NH4OH solution were each prepared and connected to the reactor. 1 L of deionized water was put into the co-precipitation reactor (5 L in capacity), and then the reactor was purged with nitrogen at a rate of 2 L / min to remove dissolved oxygen in the water and form a non-oxidizing atmosphere in the reactor. Thereafter, 10 ml of the 25 wt% aqueous NaOH solution was added, and then stirring was performed at a temperature of 60°C at a speed of 1200 rpm to maintain the pH at 12.0. Subsequently, while the precursor forming solution, the aqueous NaOH solution, and the aqueous NH4OH solution were added at rates of 4 mL / min, 1 mL / min, and 1 mL / min, respectively, a co-precipitation reaction was performed for 12 hours to prepare Co3O4 doped with 5,000 ppm Al of about 15 μm. The resulting Co3O4 particles doped with 5,000 ppm Al were separated, washed, and then dried in an oven at 120°C to prepare a positive electrode active material precursor.

[0097] Example 2 - Preparation of a positive electrode active material precursor

[0098] A positive electrode active material precursor was prepared in the same manner as in Example 1, except that Al(OH)3 was mixed in an amount of 0.3 wt% based on CoSO4 during doping of the precursor and a co-precipitation reaction was performed for 12 hours to prepare a Co3O4 precursor (about 15 μm) doped with 3,000 ppm Al.

[0099] Example 3 - Preparation of a positive electrode active material

[0100] The positive electrode active material precursor (Co3O4 doped with 5,000 ppm Al) prepared as in Example 1 was mixed with Li2CO3 as a lithium source at a Li / Co molar ratio of 1.035, and calcined at 1,000°C for about 17 hours to prepare lithium cobalt oxide doped with 4,500 ppm Al.

[0101] Example 4 - Preparation of a positive electrode active material

[0102] The cathode active material precursor (Co304 doped with 3,000 ppm Al) prepared as in Example 2 and Li2CO3 as a lithium source were mixed at a Li / Co molar ratio of 1.035 and calcined at 1,000°C for about 17 hours to prepare lithium cobalt oxide doped with 2,500 ppm Al.

[0103] Comparative Example 1 - Preparation of a positive electrode active material precursor

[0104] The cathode active material precursor was prepared in the same manner as in Example 1, except that Al(OH)3 was mixed based on CoSO4 in an amount of 0.3 wt% during doping of the precursor and the co-precipitation reaction was performed for 6 hours to prepare a Co304 precursor (about 7 μm) doped with 3,000 ppm Al.

[0105] Comparative Example 2 - Preparation of a positive electrode active material

[0106] The cathode active material precursor (Co304 doped with 3,000 ppm Al) prepared as in Comparative Example 1 and Li2CO3 as a lithium source were mixed at a Li / Co molar ratio of 1.045 and calcined at 1,020°C for about 20 hours to prepare lithium cobalt oxide doped with 2,500 ppm Al.

[0107] Comparative Example 3 - Preparation of a positive electrode active material

[0108] The cathode active material was prepared in the same manner as in Example 1, except that an undoped Co304 precursor (about 17 μm) was used and doping was performed by mixing 3,000 ppm of Al(OH)3 with the lithium source during calcination.

[0109] The cathode active material thus prepared was doped to have a concentration gradient in which Al gradually decreases from the surface of the cathode active material to the inside thereof.

[0110] Comparative Example 4 - Preparation of a positive electrode active material

[0111] The precursor doped with 3,000 ppm Al in the form of secondary particles in which 5 μm primary particles are agglomerated was mixed with Li2CO3 as a lithium source at a Li / Co molar ratio of 1.045 and calcined at 1,020°C for about 20 hours to prepare lithium cobalt oxide doped with 2,500 ppm Al.

[0112] The primary particles of the cathode active material thus prepared grew to 12 μm, but did not grow any further due to the particle growth inhibition effect of Al.

[0113] [Experimental Example 1: Observation of a positive electrode active material precursor]

[0114] Enlarged scanning electron microscope (SEM) images of the positive electrode active material precursor powders prepared in Example 1 and Comparative Example 1 are shown in Figure 1 (Example 1) and Figure 2 (Comparative Example 1), respectively.

[0115] Referring to Figure 1 and Figure 2 , the Co3O4 positive electrode active material precursors prepared in Example 1 and Comparative Example 1 are composed of primary particles, in which the primary particles of Example 1 ( Figure 1 ) are large particles having a diameter of about 15 μm, and the primary particles of Comparative Example 1 ( Figure 2 ) are small particles having a diameter of about 7 μm.

[0116] [Experimental Example 2: Particle size measurement]

[0117] The primary particle average diameters of the positive electrode active material precursors and positive electrode active materials prepared in Examples 1 to 4 and Comparative Examples 1 and 2 were measured using a particle size distribution (PSD) analyzer, and the results thereof are shown in Table 1 below.

[0118]

[0119] Referring to Table 1, it can be confirmed that the primary particle average diameters (D 50 ) of the precursors of Examples 1 and 2 are 15 μm or more, and, for Examples 3 and 4 in which the positive electrode active materials were prepared by using the precursors of Examples 1 and 2, respectively, it is possible to prepare large particles having a primary particle average diameter (D 50 ) of 15 μm or more, which are doped with a large amount of Al of 2,500 ppm or more without increasing the calcination temperature and the amount of added lithium.

[0120] In contrast, it can be confirmed that Comparative Example 1 has a primary particle average diameter (D 50 ) of 7 μm. For Comparative Example 2 in which the positive electrode active material was prepared by using the precursor of Comparative Example 1, since the particle growth was inhibited due to the large amount of doped Al doping elements, the primary particle average diameter (D 50 ) was increased only to 12 μm despite the fact that the calcination temperature and the amount of added lithium were increased, thereby possibly failing to prepare a positive electrode active material having a large particle size.

[0121] For Comparative Example 4 in which the positive electrode active material was prepared by using a precursor in the form of secondary particles in which primary particles are agglomerated, the primary particles were grown to 12 μm, but did not grow any further due to the particle growth inhibition effect of Al.

[0122] [Experimental Example 3: Evaluation of battery performance]

[0123] Each of the positive electrode active materials prepared in Examples 3 and 4 and Comparative Examples 2 to 4, carbon black, and a PVDF binder were mixed in a weight ratio of 96:2:2 in an N-methylpyrrolidone solvent to prepare a composition for forming a positive electrode, one surface of an aluminum current collector was coated with the composition, dried at 130°C, and then roll-pressed to prepare a positive electrode.

[0124] Lithium metal was used as a negative electrode.

[0125] An electrode assembly was prepared by disposing a porous polyethylene separator between the thus-prepared positive electrode and a negative electrode, and a lithium secondary battery was prepared by disposing the electrode assembly in a case and then injecting an electrolyte solution into the case. In this case, the electrolyte solution was prepared by dissolving 1.0 M lithium hexafluorophosphate (LiPF6) in an organic solvent composed of ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate (a mixed volume ratio of EC / DMC / EMC = 3 / 4 / 3).

[0126] Each of the lithium secondary battery cells (half cells) prepared as described above was subjected to a charge and discharge test to measure the capacity and rate performance at 2.0 C / 0.1 C, and the results thereof are shown in Table 2 below.

[0127] In addition, while performing 50 charge and discharge cycles, each of the lithium secondary battery cells (half cells) prepared as described above was charged at 0.5 C to a voltage of 4.55 V in a constant current / constant voltage (CC / CV) mode, cut off charged at 0.05 C, and discharged at a constant current of 1.0 C to a voltage of 3.0 V at 25°C and 45°C to measure the capacity retention rate [%], and the results thereof are shown in Table 2.

[0128] Referring to Table 2, for Examples 3 and 4 in which the positive electrode active material was prepared by using a precursor having a large particle size (D 50 ) of 15 μm or more according to the embodiments of the present application, the rate performance was better, the cycle characteristics were better, and in particular, the high-temperature cycle characteristics were significantly superior to those of Comparative Example 2 in which the positive electrode active material was prepared by using a precursor having a primary particle average particle size (D 50 ) of 7 μm of Comparative Example 1 and Comparative Example 3 in which the positive electrode active material was prepared without doping the precursor and by primary calcination of the precursor. In addition, the battery performance of Examples 3 and 4 was superior to that of Comparative Example 4.

Claims

1. A positive electrode active material precursor for a secondary battery, the positive electrode active material precursor comprising primary particles of Co304 or CoOOH, wherein the primary particles contain a doping element in an amount of 3,000 ppm or more, and The primary particles have an average particle size D of 15 μm or more 50 , wherein the primary particles are non-spherical.

2. The positive electrode active material precursor for a secondary battery according to claim 1, wherein the primary particles have an average particle diameter D of 17 μm or more 50 .

3. The positive electrode active material precursor for a secondary battery according to claim 1, wherein the doping element includes at least one selected from the group consisting of aluminum (Al), titanium (Ti), manganese (Mn), zirconium (Zr), magnesium (Mg), niobium (Nb), calcium (Ca), fluorine (F), and nickel (Ni).

4. The positive electrode active material precursor for a secondary battery according to claim 1, wherein the doping element is Al.

5. The positive electrode active material precursor for a secondary battery according to claim 1, wherein the primary particles contain the doping element in an amount of 4,000 ppm or more.

6. The positive electrode active material precursor for a secondary battery according to claim 1, wherein the doping element has a predetermined concentration in the primary particles.

7. A positive electrode active material for a secondary battery, the positive electrode active material comprising primary particles of a lithium cobalt-based oxide, wherein the primary particles contain a doping element in an amount of 2,500 ppm or more, and The primary particles have an average particle size D of 15 μm or more 50 , wherein the doping element is Al.

8. The positive electrode active material for a secondary battery according to claim 7, wherein the primary particles contain 50% or more of the total amount of the doping element in a central portion located near the center, the central portion corresponding to 50% of the radius from the center of the particle to the surface of the particle.

9. The positive electrode active material for a secondary battery according to claim 7, wherein the primary particles have an average particle diameter D of 17 μm or more 50 .

10. The positive electrode active material for a secondary battery according to claim 7, wherein the primary particles contain the doping element in an amount of 3,000 ppm or more.

11. The positive electrode active material for a secondary battery according to claim 7, wherein the doping element has a predetermined concentration in the primary particles.

12. The positive electrode active material for a secondary battery according to claim 7, further comprising a surface layer on the surface of the particle of the lithium cobalt-based oxide, wherein the surface layer comprises an oxide of at least one element selected from the group consisting of magnesium (Mg), titanium (Ti), iron (Fe), copper (Cu), calcium (Ca), barium (Ba), tin (Sn), antimony (Sb), sodium (Na), zinc (Zn), silicon (Si), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), cadmium (Cd), scandium (Sc), cerium (Ce), praseodymium (Pr), neodymium (Nd), gadolinium (Gd), dysprosium (Dy), ytterbium (Yb), erbium (Er), cobalt (Co), aluminum (Al), gallium (Ga), and boron (B).

13. The positive electrode active material for a secondary battery according to claim 7, wherein the molar ratio of lithium to metal elements other than lithium, i.e., the molar ratio of lithium / metal elements, in the lithium cobalt-based oxide is in the range of 0.98 to 1.

1.

14. A method of producing a positive electrode active material precursor for a secondary battery, the method comprising: a precursor forming solution containing a cobalt-containing starting material and a dopant element source is prepared; and The precursor formation solution is subjected to a coprecipitation reaction to form a Co3O4 or CoOOH precursor containing a dopant element in an amount of 3,000 ppm or more and having a primary particle average particle diameter D of 15 μm or more 50 , wherein the primary particles are non-spherical.

15. The method according to claim 14, wherein the dopant element comprises at least one selected from the group consisting of aluminum (Al), titanium (Ti), manganese (Mn), zirconium (Zr), magnesium (Mg), niobium (Nb), calcium (Ca), fluorine (F), and nickel (Ni).

16. The method according to claim 14, wherein the time of the coprecipitation reaction is in the range of 10 hours to 40 hours.

17. A method of preparing a positive electrode active material for a secondary battery, the method comprising mixing the positive electrode active material precursor according to claim 1 with a lithium source and calcining to form a lithium cobalt-based oxide containing a dopant element in an amount of 2,500 ppm or more and having a primary particle average particle diameter D of 15 μm or more 50 .

18. A positive electrode for a secondary battery, the positive electrode comprising the positive electrode active material according to claim 7.

19. A lithium secondary battery comprising the positive electrode according to claim 18.

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