KR101611198B1 - Cathode Active Material for Lithium Secondary Battery Having Novel Fine Crystal Structure - Google Patents

Abstract

The present invention relates to a positive electrode active material for a lithium secondary battery, wherein a lithium layer in which lithium is inserted and removed in a charge and discharge process and a transition metal layer including two or more kinds of transition metals are alternately arranged And a part of the lithium is dispersed in the transition metal layer in the form of a honeycomb unit in which lithium is positioned at the center in a state where the transition metals are arranged at the hexagonal corners of the lithium transition metal oxide .

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a positive active material for a lithium secondary battery,

The present invention relates to a novel microcrystal structure cathode active material for a lithium secondary battery, and more particularly, to a lithium ion secondary battery comprising a layered crystal in which a lithium layer into which lithium is inserted and removed and a transition metal layer including a transition metal are alternately arranged in a charge- And a part of the lithium is dispersed in the transition metal layer in the form of a honeycomb unit in which lithium is located at the center in a state where the transition metals are arranged at the hexagonal corners of the lithium transition metal oxide To a cathode active material.

As the technology and demand for mobile devices increase, the demand for secondary batteries as energy sources is rapidly increasing. Lithium secondary batteries, which exhibit high energy density and operating potential, long cycle life and low self-discharge rate among such secondary batteries, It has been commercialized and widely used.

In addition, as the interest in environmental issues grows, it is necessary to store remaining electric power, such as gasoline vehicles, electric vehicles, and hybrid electric vehicles, which can replace fossil fuel-based vehicles such as diesel vehicles There are many studies on power storage devices that can be used according to the present invention.

Conventionally, a nickel metal hydride secondary battery is mainly used as a secondary battery. Recently, researches on the use of a lithium secondary battery having a high energy density and a high discharge voltage have been actively conducted, and some of them have been commercialized.

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 being considered.

Of the above cathode active materials, LiCoO ₂ is most widely used because it has excellent lifetime characteristics and charge / discharge efficiency, but it has a disadvantage that its structural stability is poor and its cost competitiveness is limited due to resource limitations of cobalt used as a raw material .

Lithium manganese oxides such as LiMnO ₂ and LiMn ₂ O ₄ have an advantage of being excellent in thermal stability and being inexpensive, but have a problem of small capacity and poor high temperature characteristics.

The LiNiO ₂ cathode active material exhibits a relatively low cost and a high discharge capacity, but exhibits a rapid phase transition of the crystal structure in accordance with the volume change accompanying the charging / discharging cycle. When exposed to air and moisture, There is a problem of deterioration.

In order to solve these problems, many studies have been made on lithium transition metal oxides in the form of selectively mixing Ni, Mn, Co, and the like, but materials providing ideal properties have not yet been developed.

Accordingly, the inventors of the present application have conducted intensive research and various experiments, and have come up with the development of a cathode active material based on a lithium transition metal oxide having a new microcrystal structure. Such a cathode active material has a characteristic fine It has been confirmed that the secondary battery has excellent physical properties such as excellent cycle characteristics and excellent rate characteristics due to the crystal structure, and the present invention has been accomplished.

Therefore, the cathode active material for a lithium secondary battery according to the present invention is characterized in that a lithium layer in which lithium is inserted and removed in a charge and discharge process and a transition metal layer including two or more transition metals are alternate and a lithium transition metal oxide having a layered crystal structure in which the transition metal is arranged in a hexagonal shape in a state where the transition metal is arranged in a hexagonal corner, And is dispersed.

A generally known lithium transition metal oxide has a structure in which two or more transition metals are contained and these transition metals are dispersed at uniform concentrations throughout the material. (Ni-Mn-Co-Ni-Mn-Co) n in which the transition metals are uniformly dispersed in the transition metal layer in the lithium transition metal oxide including Ni, Mn, Co, And has an array structure.

On the other hand, the lithium transition metal oxide of the cathode active material according to the present invention has the characteristic microstructure as described above. That is, the lithium transition metal oxide of the present invention is characterized in that a part of lithium is contained in the transition metal layer and that lithium is uniformly dispersed in the form of a unique honeycomb unit.

With respect to the first aspect, what is known in the art is so-called cation mixing, in which a portion of lithium is present in the transition metal layer and, conversely, a portion of the transition metal is present in the lithium layer. On the other hand, with respect to the second feature, the unique honeycomb unit form as in the present invention is a completely new microcrystalline structure.

In this connection, FIG. 1 schematically shows a part of the microcrystalline structure found in the lithium transition metal oxide of the present invention. Referring to FIG. 1, the lithium transition metal oxide has a layered crystal structure in which a lithium layer in which lithium is inserted and removed in a charge and discharge process, and a transition metal layer including two or more transition metals are alternately arranged. Among them, lithium is dispersed throughout the layer in the form of a honeycomb unit in the transition metal layer.

The honeycomb units are divided into two types.

Specifically, the morphology in which the honeycomb units containing lithium are uniformly dispersed in the transition metal layer independently of each other in the transition metal layer and the morphology in which the honeycomb units containing lithium in the transition metal layer are not detected by X-ray diffraction (XRD) analysis And the honeycomb structure is combined with the transition metal layer while being uniformly dispersed in the transition metal layer.

With respect to the latter form, exemplary honeycomb structures are shown in Fig. However, the number of honeycomb units forming the honeycomb structure is not particularly limited as long as the conditions are not detected in the XRD analysis.

This condition also means a condition in which Li ₂ M'O ₃ (where M 'is a transition metal) formed when a plurality of honeycomb units are combined to form a honeycomb structure is not detected in the XRD analysis.

As described above, the lithium transition metal oxide according to the present invention includes two or more transition metals, and may be a composition including two or more selected from the group consisting of Ni, Co, and Mn.

In one preferred example, the lithium transition metal oxide according to the present invention may be a compound represented by the following formula (1).

Li [(Li _a Ni _x Mn _y Co _z ) _1-b M _b ] O _{2 -c} A _c (1)

In this formula,

0 <a <0.2, 0 <x <0.8, 0 <y <0.8, 0? Z <0.5, 0? B <0.3, 0? C <0.3;

a + x + y + z = 1;

M is one or more selected from the group consisting of Al, Cu, Fe, Mg, Co, Mn, B and Ga;

A is one selected from the group consisting of a halogen element, a sulfur, a chalcogenide-based element, and nitrogen.

In the lithium transition metal oxide according to the present invention, more than 50% of the transition metals constituting the honeycomb unit may preferably be Mn, and more preferably not less than 80% may be Mn. In this case, five or six transition metals among the six transition metals constituting the honeycomb unit may be Mn.

In some cases, the honeycomb unit may include a structure in which lithium as a central element constituting the honeycomb unit forms a physical bonding form by interaction with other lithium atoms in the lithium layer. In this case, the lithium in the lithium layer, which forms a physical connection with lithium, which is a central element constituting the honeycomb unit, remains in a fixed position without being desorbed even during the charging process, that is, the process in which lithium ions in the lithium layer are entirely eliminated.

In this regard, referring to FIG. 1, in the two transition metal layers and the lithium layer interposed therebetween, some lithium of the honeycomb units in the transition metal layers interact with other lithium in the lithium layer Giving. 3, the overall stability of the crystal structure and the inter-layer variation of the lithium layer are increased to improve the lithium mobility (see Case B). Such improvement in lithium mobility contributes to improvement in the rate characteristic do.

On the other hand, as shown in Case A of FIG. 1, in the structure in which the honeycomb units are combined to such an extent as to be detectable by the XRD analysis, the layer thickness of the lithium layers is small despite the high stability of the crystal structure, The temperature drops.

The lithium transition metal oxide having the microcrystalline structure as described above can be preferably obtained under conditions where the average oxidation number of transition metals other than the total lithium contained in the lithium layer and the transition metal layer is larger than +3. Accordingly, in the process of producing the lithium transition metal oxide, the lithium transition metal oxide can be prepared by appropriately controlling the reaction amount of the lithium precursor, the sintering temperature, the sintering conditions, etc., and controlling the average oxidation number of the transition metals to exceed +3 .

In one preferred example, the transition metal may include Ni and Mn, and the content of Ni may be greater than Mn.

In this case, the average oxidation number of the Ni is larger than +2, more preferably the content of Ni equal to the Mn content is higher than +2, and the oxidation number of Ni in the content exceeding the Mn content is +3 .

The present invention also provides a positive electrode comprising the positive electrode active material.

The present invention also provides a positive electrode comprising the positive electrode active material, and a lithium secondary battery comprising the positive electrode. Hereinafter, the anode electrode will be abbreviated as 'anode'.

The lithium secondary battery generally comprises an anode, a cathode, a separator, and a non-aqueous electrolyte containing a lithium salt.

The positive electrode is prepared, for example, by coating a mixture of a positive electrode active material, a conductive material and a binder on a positive electrode current collector, and then drying the mixture. Optionally, a filler may be further added to the mixture.

The cathode current collector generally has a thickness of 3 to 500 mu m. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical change in the battery, and may be formed of a material such as stainless steel, aluminum, nickel, titanium, sintered carbon, or a surface of aluminum or stainless steel Treated with carbon, nickel, titanium, silver or the like may be used. The current collector may have fine irregularities on the surface thereof to increase the adhesive force of the cathode active material, and various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric are possible.

The conductive material is usually added in an amount of 1 to 40% by weight based on the total weight of the mixture including the cathode active material. Such a conductive material is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, for example, graphite such as natural graphite or artificial graphite; Carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fiber and metal fiber; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The binder is a component that assists in bonding of the active material to the conductive material and bonding to the current collector, and is usually added in an amount of 1 to 40% by weight based on the total weight of the mixture containing the cathode active material. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene , Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, various copolymers and the like.

The filler is optionally used as a component for suppressing the expansion of the anode, and is not particularly limited as long as it is a fibrous material without causing a chemical change in the battery. Examples of the filler include olefin polymers such as polyethylene and polypropylene; Fibrous materials such as glass fibers and carbon fibers are used.

The negative electrode is manufactured by applying and drying an anode active material on an anode current collector, and may further include the above-described components as needed.

The negative electrode collector is generally made to have a thickness of 3 to 500 mu m. Such an anode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and may be formed of a material such as copper, stainless steel, aluminum, nickel, titanium, fired carbon, surface of copper or stainless steel A surface treated with carbon, nickel, titanium, silver or the like, an aluminum-cadmium alloy, or the like can be used. In addition, like the positive electrode collector, fine unevenness can be formed on the surface to enhance the bonding force of the negative electrode active material, and it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams and nonwoven fabrics.

Examples of the negative electrode active material include carbon such as non-graphitized carbon and graphite carbon; Li _y Fe ₂ O ₃ (0? _Y? 1), Li _y WO ₂ (0? _Y? 1), Sn _x Me _1-x Me _y O _z (Me: Mn, Fe, Pb, : Metal complex oxides such as Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, Halogen, 0 < x &lt; Lithium metal; Lithium alloy; Silicon-based alloys; Tin alloy; _{SnO, SnO 2, PbO, PbO} 2, Pb 2 O 3, Pb 3 O 4, Sb 2 O 3, Sb 2 O 4, Sb 2 O 5, GeO, GeO 2, Bi 2 O 3, Bi 2 O 4, Bi ₂ O ₅ and the like; Conductive polymers such as polyacetylene; Li-Co-Ni-based materials and the like can be used.

The separation membrane is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The pore diameter of the separator is generally 0.01 to 10 mu m, and the thickness is generally 5 to 300 mu m. Such separation membranes include, for example, olefinic polymers such as polypropylene, which are chemically resistant and hydrophobic; A sheet or nonwoven fabric made of glass fiber, polyethylene or the like is used. When a solid electrolyte such as a polymer is used as an electrolyte, the solid electrolyte may also serve as a separation membrane.

The lithium salt-containing nonaqueous electrolyte solution is composed of an electrolyte solution and a lithium salt. As the electrolyte solution, a nonaqueous organic solvent, an organic solid electrolyte, and an inorganic solid electrolyte may be used.

Examples of the non-aqueous organic solvent include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma -Butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane , Acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate Nonionic organic solvents such as tetrahydrofuran derivatives, ethers, methyl pyrophosphate, ethyl propionate and the like can be used.

Examples of the organic solid electrolyte include a polymer electrolyte such as a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, an agitation lysine, a polyester sulfide, a polyvinyl alcohol, a polyvinylidene fluoride, Polymers containing ionic dissociation groups, and the like can be used.

Examples of the inorganic solid electrolyte include Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Nitrides, halides and sulfates of Li such as Li ₄ SiO ₄ -LiI-LiOH and Li ₃ PO ₄ -Li ₂ S-SiS ₂ can be used.

The lithium salt is a material that is readily soluble in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO 4, LiBF 4, LiB 10 Cl 10, LiPF 6, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF _{6, LiSbF 6, LiAlCl 4,} CH 3 SO 3 Li, CF 3 SO 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate and imide have.

For the purpose of improving the charge / discharge characteristics and the flame retardancy, the electrolytic solution is preferably mixed with an organic solvent such as pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, Benzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, . In some cases, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride may be further added to impart nonflammability, or a carbon dioxide gas may be further added to improve high-temperature storage characteristics.

The present invention also provides an electric device including the lithium secondary battery as a power source.

The electric device is not particularly limited as long as it uses a lithium secondary battery as a power source. The electric device may be a small device such as a mobile phone, a portable computer, and the like. Preferably, the electric device is an electric vehicle, a hybrid electric vehicle, a plug- Power storage device.

As described above, the positive electrode active material according to the present invention has a larger average oxidation number of total transition metals except lithium than +3 and a relatively large amount of Ni ³⁺ among Ni corresponding to Mn as compared with conventional active materials , A uniform and stable layered structure is formed, excellent overall electrochemical characteristics including battery capacity, and particularly excellent high rate charge / discharge characteristics.

1 is a schematic diagram partially showing a microcrystalline structure in a lithium transition metal oxide according to the present invention;
2 is a schematic diagram of exemplary honeycomb structures in a lithium transition metal oxide according to the present invention;
FIG. 3 is a graph showing interlayer changes due to insertion and desorption of lithium in a lithium transition metal oxide (Case A) in which a honeycomb structure is detected in an XRD analysis and a lithium transition metal oxide (Case B) according to the present invention;
4 is a graph showing an XRD pattern of the lithium transition metal oxide prepared in Example 1 of the present invention;
5 is a graph showing Li NMR results of the lithium transition metal oxide prepared in Example 1 of the present invention.

Hereinafter, the present invention will be described in detail with reference to Examples, but the scope of the present invention is not limited thereto.

&Lt; Example 1 >

The nickel salt and the manganese salt were dissolved in distilled water so that the molar ratio (Ni / Mn) was 1.125, and the molar ratio of the cobalt salt in the total transition metal salt was dissolved to 15 mol%. Transition metal complexes were obtained by increasing the basicity of the aqueous solution containing the transition metal. The resulting composite was subjected to removal of the solvent using a vacuum filter and dried in an oven at 110 ° C. for 18 hours to remove the excess solvent. The Li salt was mixed so that the molar ratio of Ni ²⁺ / Mn ⁴⁺ was 0.81, and then the mixture was charged into an electric furnace and heated to 950 ° C at a heating rate of 300 ° C per hour and then calcined at 950 ° C for 10 hours to obtain a lithium transition metal composite oxide &Lt; / RTI &gt;

&Lt; Comparative Example 1 &

The nickel salt and the manganese salt were dissolved in distilled water so that the molar ratio (Ni / Mn) was 1.125, and the molar ratio of the cobalt salt in the total transition metal salt was dissolved to 15 mol%. Transition metal complexes were obtained by increasing the basicity of the aqueous solution containing the transition metal. The resulting composite was subjected to removal of the solvent using a vacuum filter and dried in an oven at 110 ° C. for 18 hours to remove the excess solvent. The Li salt was mixed so that the molar ratio of Ni ²⁺ / Mn ⁴⁺ was 1. Then, the mixture was charged into an electric furnace and heated to 950 ° C at a temperature raising rate of 300 ° C per hour and then calcined at 950 ° C for 10 hours to obtain a lithium transition metal composite oxide &Lt; / RTI &gt;

<Experimental Example 1>

XRD analysis was performed on the lithium transition metal oxides prepared in Example 1 and Comparative Example 1, respectively, and the results of Example 1 are shown in FIG. As shown in FIG. 4, Example 1 according to the present invention is substantially different from Comparative Example 1 in the XRD pattern.

On the other hand, ⁶ Li synchronized spin echo MAS NMR analysis of the lithium transition metal oxide prepared in Example 1 revealed that lithium was present in the honeycomb unit as shown in FIG. Therefore, it can be seen that the lithium transition metal oxide according to the present invention contains a characteristic honeycomb unit type which is not confirmed by the XRD pattern but is confirmed by Li NMR.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims (16)

A positive electrode active material for a lithium secondary battery,
A lithium transition metal oxide having a layered crystal structure in which a lithium layer in which lithium is intercalated and deintercalated and a transition metal layer containing two or more kinds of transition metals are alternately arranged in charge and discharge processes In addition,
Part of the lithium is dispersed in the transition metal layer in the form of a honeycomb unit in which lithium is positioned at the center with the transition metals arranged at hexagonal corners,
Wherein the lithium transition metal oxide is a compound represented by the following formula (1): < EMI ID =
Li [(Li _a Ni _x Mn _y Co _z ) _1-b M _b ] O _{2 -c} A _c (1)
In this formula,
0 <a <0.2, 0 <x <0.8, 0 <y <0.8, 0? Z <0.5, 0? B <0.3, 0? C <0.3;
a + x + y + z = 1;
M is one or more selected from the group consisting of Al, Cu, Fe, Mg, Co, Mn, B and Ga;
A is one selected from the group consisting of a halogen element, a sulfur, a chalcogenide-based element, and nitrogen. The cathode active material according to claim 1, wherein the honeycomb units including lithium in the transition metal layer are independently dispersed in the transition metal layer. The method according to claim 1, wherein the honeycomb units including lithium in the transition metal layer are dispersed in the transition metal layer while forming a honeycomb structure mutually combined in a number not detectable by X-ray diffraction (XRD) analysis Lt; / RTI &gt; The positive electrode active material according to claim 1, wherein the lithium transition metal oxide is Li ₂ M'O ₃ (wherein M 'is a transition metal) in the XRD analysis. delete delete The cathode active material according to claim 1, wherein at least 50% of the transition metals constituting the honeycomb unit are Mn. The positive electrode active material according to claim 1, wherein lithium as a central element constituting the honeycomb unit forms a physical bonding form by interaction with other lithium ions in the lithium layer. 8. The cathode active material according to claim 7, wherein the lithium of the lithium layer physically coupled to the lithium, which is a central element constituting the honeycomb unit, is not desorbed during the charging process. The positive electrode active material according to claim 1, wherein an average oxidation number of transition metals other than total lithium contained in the lithium layer and the transition metal layer is greater than +3. The positive electrode active material according to claim 1, wherein the transition metal contains Ni and Mn, and the content of Ni is greater than Mn. 12. The cathode active material according to claim 11, wherein an average oxidation number of Ni is larger than +2. A cathode electrode comprising a cathode active material according to any one of claims 1 to 4 and 7 to 12. A lithium secondary battery comprising the positive electrode according to claim 13. An electric device comprising the lithium secondary battery according to claim 14 as a power source. 16. The electrical device of claim 15, wherein the device is an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, or a power storage device.

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