KR20120088280A - Cathode active material for lithium secondary battery, method for preparing same, and lithium battery comprising same - Google Patents

Abstract

PURPOSE: A positive electrode active material for lithium secondary batteries and a lithium secondary battery including the same are provided to enhance energy density, cycle life time, output characteristic and thermal stability. CONSTITUTION: A positive electrode active material for lithium secondary batteries includes a compound represented by chemical formula 1. The chemical formula 1 is as follows: (Li xNa (1.5-x)) a (Z) z ((V) (O (1.5-f))) (1-z) (XO 4) F. The positive electrode active material has a nasicon structure, and Z is an element selected from Fe, Ni, Co, Mn, Cr, Zr, Nb, Cu, Ti, Zn, Al, Ga, Mg, Ca and a combination thereof. X is an element selected from P, S, Si, Mo, As, W and a combination thereof. a,x, and z respectively indicate real numbers which satisfy 0 < a <= 1, 0 < x <= 1.5, 0 <= z <= 0.5. f is determined to match the electroneutrality of the compound and is a real number which satisfies 0.5 <= f <= 1.5.

Description

Cathode active material for lithium secondary battery and lithium secondary battery comprising same TECHNICAL FIELD

It relates to a cathode active material for a lithium secondary battery and a lithium secondary battery comprising the same.

Recently, as electronic devices such as mobile phones, digital cameras, personal digital assistants (PDAs), notebook computers, and personal computers (PCs) are miniaturized, the need for miniaturization and high capacity of power sources used therein is increasing. In addition, the demand for high-power secondary batteries that can be used in hybrid electronic vehicles (HEVs), power tools, electric motorcycles, and robotics is exploding. To this end, a secondary battery having a high output in a short time, a high energy density, and excellent stability even after repeated charging and discharging with a large current is required.

Among positive electrode active materials used in secondary batteries that can satisfy these requirements, research has recently been actively conducted on NASICON type compounds.

One embodiment of the present invention is to provide a cathode active material for a lithium secondary battery, showing improved cycle life characteristics, output characteristics and energy density.

Another embodiment of the present invention to provide a lithium secondary battery comprising the positive electrode active material.

One embodiment of the present invention includes a compound represented by the following formula (1), and provides a cathode active material for a lithium secondary battery having a nasicon structure.

[Formula 1]

(Li _x Na _1.5-x ) _a (Z) _z ((V) (O _1.5-f )) _1-z (XO ₄ ) F _f

In Chemical Formula 1,

Z is a metal element dopant and is selected from the group consisting of Fe, Ni, Co, Mn, Cr, Zr, Nb, Cu, Ti, Zn, Al, Ga, Mg, Ca, and combinations thereof. Elemental,

X is an element selected from the group consisting of P, S, Si, Mo, As, W, and combinations thereof,

a, x and z are each a real number satisfying 0 <a ≤ 1, 0 <x ≤ 1.5, a real number satisfying 0 <x ≤ 1.5, and a real number satisfying 0 ≤ z ≤ 0.5. At this time, z may be a real number satisfying 0 ≦ z ≦ 0.2.

f is determined to match the electroneutrality of the compound and is a real number that satisfies 0.5 ≦ f ≦ 1.5.

Another embodiment of the invention the positive electrode including the positive electrode active material; A negative electrode including a negative electrode active material; And it provides a lithium secondary battery comprising a non-aqueous electrolyte.

The cathode active material for a rechargeable lithium battery according to one embodiment of the present invention may exhibit excellent energy density, cycle life characteristics, output characteristics, and thermal stability.

1A shows the crystallographic structure of Li _x Na _1.5-x VOPO ₄ F _0.5 .
1B shows LiNa 1 and LiNa 2 sites in the crystallographic structure of Li _x Na _1.5-x VOPO ₄ F _0.5 .
2 is a graph showing an X-ray diffraction (XRD) pattern of specimen S1.
3 is a scanning electron microscope (SEM) photograph of specimen S1.
4 is a graph showing an X-ray diffraction pattern refined by Rietveld method of specimen S2.
5 is a graph showing a neutron diffraction pattern refined by Rietveld method of specimen S2.
6 is a scanning electron micrograph of specimen S2.
FIG. 7 is a graph showing an X-ray diffraction pattern of specimen S2 before dry carbon coating and specimen S3 after dry carbon coating.
8 is a scanning electron micrograph of specimen S3.
Figure 9 is a graph showing in comparison the X- ray diffraction pattern of the chemical Na ⁺ / Li ⁺ ion-exchanged water after the chemical and the specimen S2 Na ⁺ / Li ⁺ ion exchange prior to the (ion-exchange) the specimen S4.
10 is a scanning electron micrograph of specimen S4.
FIG. 11 is a bright-field image obtained using transmission electron microscopy (TEM) of specimen S4. FIG.
12 is a selected area diffraction pattern obtained using a transmission electron microscope of specimen S4.
FIG. 13 is a graph showing comparison of X-ray diffraction patterns of specimen S2 and specimens S5 to S8. FIG.
14 is a graph showing the thermogravimetric analysis (TGA) results of specimen S4.
FIG. 15 is an X-ray diffraction pattern graph with temperature showing thermal phase stability of Li _x Na _1.5-x VOPO ₄ F _0.5 specimen (S4), measured by the method of Experimental Example 2. FIG.
16 is a graph showing the AC impedance results of the Li _x Na _1.5-x VOPO ₄ F _0.5 specimen (S4) measured by the method of Experimental Example 3. FIG.
17 is a graph showing the results of direct current conductivity of Li _x Na _1.5-x VOPO ₄ F _0.5 specimen (S4) measured by the method of Experimental Example 3. FIG.
18 is a graph showing changes in AC impedance and DC conductivity values with temperature of Li _x Na _1.5-x VOPO ₄ F _0.5 specimen S4 measured by the method of Experimental Example 3. FIG.
19 is a graph showing the activation energy of Na _1.5 VOPO ₄ F _0.5 calculated using the NEB method.
20 is a graph showing the activation energy of Na _1.5 VOPO ₄ F _0.5 calculated using the NEB method.
21 is a graph showing the activation energy of an active material (LiNa _0.5 VOPO ₄ F _0.5 ) mixed with Li and Na 2: 1, prepared according to Example 1 calculated using the NEB method.
FIG. 22 is a graph showing a combination of FIGS. 20 and 21.
FIG. 23 is a graph illustrating a cyclic voltammetry result of measuring a coin battery prepared according to Comparative Example 1 by the method of Experimental Example 4. FIG.
24 is a graph illustrating a cyclic voltammetry result measured by a method of Experiment 4 for a coin battery prepared in Example 2. FIG.
25 is a graph showing a result of a constant current charge / discharge curve of a C / 5 current density measured by the method of Experiment 4 for a coin battery prepared according to Example 2 (measurement temperature: 25 ° C., potential region: 2.0 to 4.5 V) ).
FIG. 26 is a graph showing cycle life characteristics during C / 5 charging / discharging when a coin battery prepared according to Example 2 was measured by the method of Experimental Example 4 (measurement temperature: 25 ° C., potential region: 2.0 to 4.5 V) .
FIG. 27 is a graph showing cycle life characteristics during C / 5 charge / discharge, measured by the method of Experiment 4, for the coin battery prepared in Example 2 (measurement temperature: 60 ° C., potential region: 2.5 to 4.5 V).
28 is a graph showing the output characteristics at various current densities (C / 5, C / 2, 1C, 2C, 5C, 10C) measured by the method of Experiment 4 for the coin battery prepared according to Example 2. FIG.
FIG. 29 is a graph showing a comparison of X-ray diffraction patterns of a positive electrode active material before and after performing a charge / discharge of a coin battery prepared according to Example 2 at C / 5 in a 2.0 to 4.5 V potential region for 45 times. FIG.

Hereinafter, embodiments of the present invention will be described in detail. However, this is presented as an example, by which the present invention is not limited and the present invention is defined only by the scope of the claims to be described later.

Unless defined otherwise, all terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. Commonly used predefined terms are further interpreted as having a meaning consistent with the relevant technical literature and the present disclosure, and are not to be construed as ideal or very formal meanings unless defined otherwise.

The cathode active material for a rechargeable lithium battery according to one embodiment of the present invention may be represented by the following Chemical Formula 1.

[Formula 1]

(Li _x Na _1.5-x ) _a (Z) _z ((V) (O _1.5-f )) _1-z (XO ₄ ) F _f

In Chemical Formula 1,

Z is a metal element dopant selected from the group consisting of Fe, Ni, Co, Mn, Cr, Zr, Nb, Cu, Ti, Zn, Al, Ga, Mg, Ca, It is an element.

X is an element selected from the group consisting of P, S, Si, Mo, As, W, and combinations thereof, and may be P, S, Si, As, or a combination thereof.

In addition, each of a, x, and z is a real number satisfying 0 <a ≦ 1, 0 <x ≦ 1.5, 0 ≦ z ≦ 0.5, and z may be a real number satisfying 0 ≦ z ≦ 0.2.

f is determined to match the electroneutrality of the compound, and may be a real number satisfying 0.5 ≦ f ≦ 1.5, and preferably a real number satisfying 0.5 ≦ f <1.5.

Specific examples of the positive electrode active material may include the following formula (2).

[Formula 2]

Li _x Na _1.5-x VOPO ₄ F _f

In Chemical Formula 2, f may be a real number satisfying 0.5 ≦ f ≦ 1.5, and preferably a real number satisfying 0.5 ≦ f <1.5.

As shown in Formula 2, a poly-anion, ie, a fluorophosphate-based compound in which fluorine ions are added to phosphate (PO ₄ ) ^3- ), is conventionally known as (PO ₄ ³ ) has a higher cell voltage compared to the compound. This is a complex effect of the inductive effect of the (PO ₄ ) ³ -polyanion group and the phenomenon that fluorine ions having a large electronegativity (3.98) attract very close electrons. Such a high battery voltage can further improve energy density.

The cathode active material according to one embodiment of the present invention is a compound having a nasicon structure. A tank top cone is Sodium (Na) S uper I onic Con as stands for ductor, pear cone type compounds, and the ionic conductivity is very superior because it can be moved very quickly in the nature of lithium and sodium in the frame structure, such a high ion conductivity Therefore, excellent output characteristics can be exhibited.

In addition, nacicon-type compounds, like olivine-type compounds, as a polyanionic compound that strongly holds a structure, have an advantage of excellent thermal stability and lifespan characteristics.

As represented by Chemical Formula 1, the cathode active material of the present invention may include lithium as an essential component and sodium in combination with lithium. As described above, when lithium is included as an essential component and, in some cases, sodium is further included, specific capacity is superior to that of the positive electrode active material containing no sodium and only sodium as an essential component. In addition, the positive electrode active material containing lithium as an essential component can increase the operating voltage of the positive electrode by about 0.3 V as compared with the positive electrode active material containing sodium as an essential component without lithium, resulting in excellent energy density and output density. A lithium secondary battery can be provided.

In addition, in Chemical Formula 1, it is more preferable that 0 <x <1.5, that is, it is more preferable to include both lithium and sodium. As such, including lithium and sodium together has the advantage that the lithium ion diffusion rate is more excellent.

Therefore, since the cathode active material according to the embodiment of the present invention exhibits excellent energy density, cycle life characteristics, output characteristics, and thermal stability, it may be used instead of a cobalt-based cathode active material, such as LiCoO ₂ , which is mainly used at present. .

In addition, the cathode active material may exhibit an average voltage of about 3.7V to about 4V relative to lithium.

1A and 1B illustrate a crystallographic structure of Li _x Na _1.5-x VOPO ₄ F _0.5 in a cathode active material according to an embodiment of the present invention. As shown in Figure 1a, the positive electrode active material according to an embodiment of the present invention is a three-dimensional structure consisting of VO ₅ F hexahedron (light blue cube in Figure 1a) and PO ₄ tetrahedron (pink cube in Figure 1a), lithium or sodium (Yellow sphere in FIG. 1A) occupies two different crystallographic positions. In FIG. 1A, red spheres represent oxygen ions (O ²⁻ ), green spheres represent fluorine ions (F ⁻ ), light blue spheres represent vanadium ions (V ⁴⁺ ), and pink spheres represent phosphorus ions (P ⁵⁺ ). 1B, it can be seen that the LiNa1 site is surrounded by 6 oxygen atoms (red circle in FIG. 1B) and 1 fluorine atom (green circle in FIG. 1B), and the LiNa2 site is surrounded by 6 oxygen atoms .

The cathode active material of the present invention may have an average particle diameter of 100 μm or less, and may be 2 μm to 100 μm. Thus, even if the cathode active material of the present invention has an average particle diameter of 2㎛ 100㎛, it can exhibit a rate characteristic of the nanometer level. Accordingly, there is an advantage that mass production and commercialization are easy as compared with preparing the cathode active material at the nanometer level.

The positive electrode active material of the present invention may further include a carbon coating layer on the surface. The carbon coating layer may have a thickness of 1 nm to 300 nm, and the content of the carbon coating layer may be 0.001 to 25 wt% based on the total weight of the positive electrode active material. When the thickness and content of the carbon coating layer is in the above range, the electrical conductivity of the positive electrode active material can be further improved, and the capacity decrease per unit volume problem due to the volume increase of the positive electrode active material generated when the carbon coating layer is excessively included It is not appropriate.

Another embodiment of the present invention provides a method of manufacturing the cathode active material. In one embodiment of the present invention, the positive electrode active material may be prepared by the following two processes.

The first method comprises mixing a vanadium compound and an X containing compound (X is an element selected from the group consisting of P, S, Si, Mo, As, W and combinations thereof) to prepare a mixture; First heat treating the mixture at 700 to 750 ° C. to prepare a vanadium-X-containing precursor; Mixing the vanadium-X-containing precursor and the Na compound in a molar ratio of 1: 1.0 to 1: 1.7; Preparing a vanadium-X-Na-containing precursor by performing a second heat treatment on the mixture of the vanadium-X-containing precursor and the Na compound; And mixing the vanadium-X-Na-containing precursor with a lithium compound to prepare a mixture, and oxidizing the mixture.

Hereinafter, the manufacturing method will be described in detail.

The vanadium compound and the X-containing compound (X is an element selected from the group consisting of P, S, Si, Mo, As, W and combinations thereof) are mixed. This mixing process can be performed by the solid-phase process which does not use a solvent.

As the vanadium compound, V ₂ O ₅ , VO ₂ , V ₂ O ₃ , VO, VOSO ₄ , V ₂ (SO ₄ ) _3, or a combination thereof may be used. In addition, the X-containing compound may be an ammonium compound, oxide, nitride, sulfide or a combination thereof containing X. Specific examples of the X-containing compound include NH ₄ H ₂ PO ₄ , (NH ₄ ) ₂ HPO ₄ , NaH ₂ PO ₄ , LiH ₂ PO ₄ , VOSO ₄ , V ₂ (SO ₄ ) ₃ , SiO ₂ , Mo ₃ O ₄ , (NH ₄ ) ₆ Mo ₇ O ₂₄ ? H ₂ O, (NH ₄ ) ₃ Mo ₇ O ₂₄ , NH ₄ H ₂ AsO ₄ , (NH ₄ ) ₁₀ H ₂ (W ₂ O ₇ ) ₆ , (NH ₄ ) ₆ H ₂ W ₁₂ O ₄₀ -xH ₂ O, WO ₃ , or a combination thereof can be used.

Carbon additives may be further added during the mixing process. The carbon additive may be carbon black, acetylene black, ketjen black, Super P, or a combination thereof. When further using the carbon additive, the oxidation number of vanadium can be easily adjusted.

In the primary mixing step, the molar ratio of the vanadium compound and the X-containing compound is appropriately mixed so that the molar ratio of the contained V element and X element is 1: 0.8 to 1: 1.2. When the molar ratio of the vanadium compound and the X-containing compound is mixed such that the molar ratio of the contained V element and the X element is included in the above range, impurities may be minimized.

In addition, the amount of the carbon additive may be 5 to 25% by weight based on the total weight of the vanadium compound and the X-containing compound. When the addition amount of the carbon additive is included in this range, the oxidation number of vanadium can be easily adjusted.

Subsequently, the obtained primary mixture is subjected to primary heat treatment at 700 to 750 ° C. to prepare a vanadium-X-containing precursor. The first heat treatment process may be performed for 1 to 10 hours.

Before the primary heat treatment step, the obtained mixture may be dried and then a step of producing pellets may be further performed.

When the first heat treatment process is performed in the above temperature range, the prepared vanadium-X-containing precursor may have β-VOPO ₄ as a target phase. The vanadium-containing precursor -X target in addition to the merchant β -VOPO ₄ has a γ phase -VOPO _4, polymorph, such as ε-phase -VOPO _4, VOPO _4? Impurities such as H ₂ O or V (PO _{3) 3} Fig. Have. However, despite the presence of such impurities, the final product shows good purity with few impurities. That is, the final product is the desired target β-VOPO ₄ may be included in a high content of 40 to 90% by weight .

Next, the vanadium-X-containing precursor and the Na compound are mixed. In this case, NaF, NaHCO ₃ , Na ₂ CO ₃ , NH ₄ F or a combination thereof may be used.

The mixing ratio of the vanadium-X-containing precursor and the Na compound may be 1: 1.0 to 1: 1.7 molar ratio. When the molar ratio of the vanadium-X-containing precursor and the Na compound is included in the above range, there may be an advantage that the impurities in the final phase are minimized.

The mixture of the obtained vanadium-X-containing precursor and the Na compound is subjected to secondary heat treatment to prepare a vanadium-X-Na-containing precursor.

The secondary heat treatment process may be carried out at 700 to 750 ℃. In addition, the secondary heat treatment process may be performed for 30 minutes to 2 hours, and may be performed in an inert atmosphere such as argon, nitrogen, or helium.

Next, the said vanadium-X-Na containing precursor and a lithium compound are mixed, this mixture is mixed in a solvent, a mixed liquid is produced, and the process of ion-exchanging this mixed liquid is performed.

LiBr, LiCl, LiNO ₃ , LiI, LiF, or a combination thereof may be used as the lithium compound. In addition, 1-hexanol, acetonitrile, ethanol, methanol, water, or a combination thereof may be used as the solvent.

The mixing ratio of the vanadium-X-Na-containing precursor and the lithium compound may be adjusted such that the number of moles of Li ions included in the lithium compound is 1 to 50 times the number of moles of Na ions included in the vanadium-X-Na-containing precursor. When the mixing ratio of the vanadium-X-Na-containing precursor and the lithium compound satisfies the above condition, there may be an advantage that the rate of ion exchange or the amount of ion exchange can be controlled.

In addition, the amount of the solvent used may be used so that the concentration of the mixed solution is 0.1 to 5M, and when the solvent is used so that the concentration of the mixed solution is included in the above range, the rate of exchange of Li ions and Na ions or the amount of ion exchange There may be an advantage that can be controlled.

The ion exchange process may be carried out by reflux at a temperature of 0 to 10 ℃ higher than the boiling point of the solvent. By refluxing at a temperature higher than the boiling point of the solvent, vigorous evaporation of the solvent may aid in uniform mixing of the reactants and provide sufficient energy to accelerate the reaction. In addition, since the solvent always boils at a certain temperature, there is an advantage that the reaction temperature is always kept constant.

The ion exchange process may be carried out for 5 to 96 hours. If the ion exchange process is performed during the above time range, there may be an advantage that the amount of exchange of Li ions and Na ions can be controlled.

In addition, the reflux product may be dried at an inert atmosphere for 5 to 24 hours at 70 to 250 ℃ to improve the purity.

The ion exchange process can be carried out in a reaction vessel having a size of 50 to 500ml. When the ion exchange process is carried out in a reaction vessel having a size in the above range, there may be an advantage that the rate or ion exchange amount of the exchange of Li ions and Na ions can be controlled.

After performing the ion exchange process, it is a matter of course that ordinary washing and drying can be performed.

Subsequently, the product obtained by performing the ion exchange process is subjected to tertiary heat treatment to prepare a cathode active material of Chemical Formula 1. The third heat treatment process may be performed for 1 to 24 hours at 150 to 550 ℃. In addition, the tertiary heat treatment step can be carried out in an inert atmosphere such as argon, nitrogen, helium, vacuum. The third heat treatment step may be performed using a mixture of a product obtained by performing an ion exchange step and a carbon precursor. As the carbon precursor, carbon black, acetylene black, ketjen black, Super P, or a combination thereof may be used. The mixing ratio of the product and the carbon precursor obtained by performing the ion exchange process may be a weight ratio of 24: 1 to 2: 1 .

Another embodiment of the present invention relates to a lithium secondary battery including the cathode active material. The lithium secondary battery includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and a nonaqueous electrolyte.

The positive electrode includes a current collector and a cathode active material layer formed on the current collector.

In the cathode active material layer, as the cathode active material, the compound of Formula 1 according to an embodiment of the present invention may be used as a cathode active material, or a carbon coating layer formed on the compound of Formula 1 may be used as a cathode active material. The carbon coating layer may be formed by mechanically mixing the compound of Formula 1 and the carbon precursor for 12 to 24 hours. As the carbon precursor, carbon black, acetylene black, ketjen black, Super P, or a combination thereof may be used. The mixing ratio of the compound of Formula 1 and the carbon precursor may be 19: 1 to 2: 1 weight ratio.

The amount of the cathode active material in the cathode active material layer may be 70 to 98 wt% based on the total weight of the cathode active material layer.

Al foil may be used as the current collector, but is not limited thereto.

The cathode active material layer also includes a binder and a conductive material. At this time, the content of the binder and the conductive material may be 1 to 15% by weight based on the total weight of the positive electrode active material layer, respectively.

As the binder, polyvinyl alcohol, polyvinyl chloride, polyvinyl fluoride, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene and the like can be used. It is not.

Examples of the conductive material include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black and carbon fiber; Metal powders, such as copper, nickel, aluminum, and silver, etc. can be used.

The negative electrode includes a current collector and a negative electrode active material layer formed on the current collector. Copper foil may be used as the current collector, but is not limited thereto.

As the negative electrode active material of the negative electrode active material layer, a carbon-based material capable of intercalating / deintercalating lithium may be generally used. As the carbon-based material, crystalline carbon such as natural graphite or artificial graphite, or amorphous carbon such as hard carbon or soft carbon may be used. Of course, these can also be mixed and used. The content of the negative electrode active material may be 95 to 99% by weight based on the total weight of the negative electrode active material layer.

The negative electrode active material layer also includes a binder, and optionally may further include a conductive material. The content of the binder in the negative electrode active material layer may be 1 to 5% by weight based on the total weight of the negative electrode active material layer. In addition, when the conductive material is further included, 90 to 98 wt% of the negative electrode active material, 1 to 5 wt% of the binder, and 1 to 5 wt% of the conductive material may be used.

Since the binder and the conductive material are the same as the materials exemplified in the positive electrode, the description of the negative electrode will be omitted.

The positive electrode and the negative electrode are prepared by mixing an active material, a binder, and optionally a conductive material in a solvent to prepare an active material composition, and coating the active material directly on a current collector or casting it on a separate support and peeling the active material film from the support. It can be prepared by the method of lamination.

In the lithium secondary battery, the nonaqueous electrolyte includes a nonaqueous organic solvent and a lithium salt.

Examples of the non-aqueous organic solvent include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate and vinylene carbonate; Chain carbonates such as dimethyl carbonate, methyl ethyl carbonate and diethyl carbonate; Esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate and γ-butyrolactone; Ethers such as 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 1,2-dioxane and 2-methyltetrahydrofuran; Nitriles such as acetonitrile or amides such as dimethylformamide may be used, but the present invention is not limited thereto. Moreover, these can be used individually or in combination of multiple. In particular, a mixed solvent of a cyclic carbonate and a linear carbonate can be preferably used.

Examples of the lithium salt include LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , LiSbF6, LiAlO ₄ , LiAlCl ₄ , One or more selected from the group consisting of LiCl and LiI can be used.

In addition, depending on the type of battery, a separator may be further used between the positive electrode and the negative electrode, and the separator may be used as long as it is commonly used in a lithium secondary battery. As an example of the separator, polyethylene, polypropylene, polyvinylidene fluoride or two or more multilayer films thereof may be used, and polyethylene / polypropylene two-layer separator, polyethylene / polypropylene / polyethylene three-layer separator, polypropylene / polyethylene / poly It goes without saying that a mixed multilayer film such as a propylene three-layer separator can be used.

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are only preferred examples of the present invention and the present invention is not limited to the following examples.

* Synthesis of VOPO ₄ Powder

(Production Example 1)

V ₂ O ₅ (Aldrich), NH ₄ H ₂ PO ₄ (Junsei) and Super P were mixed by dry ball milling. At this time, the molar ratio of V ₂ O ₅ and NH ₄ H ₂ PO ₄ was 1: 2, and the amount of Super P added was 7% by weight based on the total weight of V ₂ O ₅ and NH ₄ H ₂ PO ₄ . The exact amount of compound used is shown in Table 1 below.

Reagent Name manufacturer Formula weight (g / mol) amount Mass (g) V ₂ O ₅ Aldrich 181.88 0.05 moles 9.0494 NH ₄ H ₂ PO ₄ Junsei 115.03 0.1 moles 11.503 Super P Alfa Aesar 12.011 7 weight% 1.44

The resultant mixture was dry ball milled for 24 hours, and then heat treated in air at a temperature of 750 ° C. for 4 hours to prepare a yellow VOPO ₄ powder (Sample S1).

X-ray diffractometer (XRD) using CuKα rays as an impurity The particle size was analyzed using a scanning electron microscope (SEM). The heat treatment temperature and measured present phase and particle size are shown in Table 2 below.

The target phase shown in Table 2 means β-VOPO ₄ having a three-dimensional structure consisting of VO ₆ hexahedron and PO ₄ tetrahedron.

Psalter Heat treatment temperature (℃) Present prize (goal award) Particle Size (μm) S1 750 β-, γ-, ε-VOPO ₄ ,
VOPO _4- H ₂ O, V (PO ₃ ) ₃ 5-20

In addition, X-ray diffraction patterns and scanning electron micrographs of the specimen S1 are shown in FIGS. 2 and 3, respectively.

And Table 2, Figure 2 and as shown in Figure 3, the specimen γ-, ε-polymorph VOPO _4, etc. on the number of VOPO ₄ and _{VOPO 4? H 2 O, V} (PO 3) impurities, such as ₃ It can be seen that this exists.

* Synthesis of Na _1.5 VOPO ₄ F _0.5 Powder

(Production Example 2)

VOPO ₄ (Sample S1) and NaF (Aldrich) prepared in Preparation Example 1 were mixed. At this time, the mixing molar ratio of the VOPO ₄ and NaF was 1: 1.1. At this time, the exact amount of the compound used is shown in Table 3 below.

Reagent Name manufacturer Formula weight (g / mol) amount Mass (g) VOPO ₄ Psalm S1 161.9 0.0243 moles 3.969 NaF Aldrich 41.988 0.0267 moles 1.132

The mixture was prepared into pellets by heating at a temperature of 750 ℃ in an argon atmosphere for 1.5 hours, it was synthesized Na _{1 .5} VOPO ₄ F _{0 .5} powder (sample S2) of green.

The powder of Preparation Example 2 was investigated using an X-ray diffractometer (XRD) using CuKα rays, and the particle size was analyzed using a scanning electron microscope (SEM). The heat treatment temperature and the measured phase and particle size are shown in Table 4 below.

The target phase shown in Table 4 means Na _1.5 VOPO ₄ F _0.5 having a three-dimensional structure consisting of VO ₅ F hexahedron and PO ₄ tetrahedron.

Psalter VOPO ₄ : NaF molar ratio Heat treatment
Temperature (℃) Present prize (goal award) Particle size
(Μm) S2 1: 1.1 750 Na _1.5 VOPO ₄ F _0.5 , Na _1.5 VOPO ₄ F _1.5 1-2

The X-ray diffraction pattern and the neutron diffraction pattern refined by the Rietveld method of the specimen S2 are shown in FIGS. 4 and 5, respectively. The VOPO ₄ F Na _{1 0.5 0 0.5} The structure was crystalline (crystal system) is a tetragonal system (tetragonal) used to Rietveld refinement (Rietveld refinement), space group (space group) is using the I ₄ / mmm It was. Referring to Table 4 and Figures 4 to 5, except for some impurity phase Na _1.5 VOPO ₄ F _1.5 it can be seen that the pure target phase Na _1.5 VOPO ₄ F _0.5 is present. The lattice constants calculated by the Rietveld method were a = b = 6.38 Å and c = 10.63 Å. This value is almost similar to the lattice constant of the previously reported material, indicating that the target phase is well formed.

From this result, when the inside VOPO _4, β-VOPO ₄ used as starting material is present in more than a certain percentage (at least 40% by weight or more), VOPO ₄ within the polymorph (α-, γ-, δ-, ε-VOPO ₄ ), It does not affect the purity of the final target phase (Na _1.5 VOPO ₄ F _0.5 ) synthesized by the addition of sodium and fluorine elements.

In addition, a scanning electron micrograph of the specimen S2 is shown in FIG. 6. Referring to Figure 6, it can be seen that the particle size of the prepared Na _1.5 VOPO ₄ F _0.5 is 1-2 ㎛.

* Dry carbon coating of Na _1.5 VOPO ₄ F _0.5 powder

(Production Example 3)

Prior to preparing the electrode with Na _1.5 VOPO- ₄ F _0.5 powder prepared in Preparation Example 2, a dry carbon coating was performed to improve electrical conductivity. At this time, the mixing weight ratio of the Na _1.5 VOPO- ₄ F _0.5 powder and Super P was 80:20. At this time, the exact amount of the material used is shown in Table 5 below.

Reagent Name manufacturer Mass (g) Na _1.5 VOPO- ₄ F _0.5 Production Example 2 1.6 Super P Alfa Aesar 0.4

The mixture was ball milled for 24 hours without solvent, followed by an additional sieving with the balls for 24 hours, followed by an additional 6 hours to separate the balls and the powders to form a carbon coated Na _1.5 VOPO- ₄ F. _0.5 powder (Sample S3) was obtained.

The powder of Preparation Example 3 was investigated using an X-ray diffractometer (XRD) using CuKα rays, and the particle shape was analyzed using a scanning electron microscope (SEM). The X-ray diffraction patterns of the specimen S2 before the dry carbon coating and the specimen S3 after the dry carbon coating are compared, and are shown in FIG. 7. Referring to Figure 7, it can be seen that there was no significant structural deformation in the dry carbon coating process.

A scanning electron micrograph of the specimen S3 is shown in FIG. 8. Referring to FIG. 8, it can be seen that the dry carbon coating was successful because the specimen S3 of Preparation Example 3 and carbon (Super P) are evenly mixed.

* Synthesis of Li _x Na _1.5-x VOPO ₄ F _0.5 Powder

(Example 1)

Na _1.5 VOPO- ₄ F _0.5 powder prepared in Preparation Example 2 was mixed with LiBr, and the mixture was placed in a 50 ml round flask filled with 1-hexanol in the amount shown in Table 6 below. In this case, the amount of LiBr was used so that the lithium ion was 10 times the number of moles of sodium ions in the Na _1.5 VOPO- ₄ F _0.5 formula, and the amount of the solvent was used such that the molar concentration of the final solution was 5M LiBr solution. The exact amount of compound used is shown in Table 6 below.

Reagent Name manufacturer Formula weight (g / mol) amount Mass (g) Na _1.5 VOPO ₄ F _0.5 Production Example 2 206 0.00971 moles 2 LiBr Aldrich 86.85 0.14563 moles 12.647 1-hexanol Aldrich 102.17 29.13 mL N / A

Under reflux, the circular flask was heated to 160 ° C. and maintained at this temperature for 15 hours to effect chemical Na ⁺ / Li ⁺ ion-exchange, resulting in Li _x Na _1.5-x VOPO ₄ F _0.5 (x is about 1) powder was synthesized.

After the ion exchange process was completed, the obtained product was obtained by centrifugation, which was washed three times with methanol and secondary distilled water, respectively.

The powder obtained by washing was dried under argon atmosphere at 200 ° C. for 10 hours to remove the solvent and the like to obtain pure Li _x Na _1.5-x VOPO ₄ F _0.5 (x is about 1) powder.

The chemical composition of the product prepared according to Example 1 (Sample S4) was investigated using an inductively coupled plasma atomic emission spectrometer (ICP-AES), and the results are shown in Table 7 below. Referring to Table 7, it can be seen that about one Na ⁺ ion is exchanged for Li ⁺ ion. In addition, the crystal structure of the specimen S4 was analyzed using an X-ray diffractometer (XRD), and the results were compared with the X-ray diffraction pattern of the specimen S2 before chemical Na ⁺ / Li ⁺ ion exchange. Shown in Referring to FIG. 9, it can be seen that even after about one Na ⁺ ion is exchanged with Li ⁺ ion, the overall crystal structure is not significantly modified. In addition, the lattice constant was calculated using the Rietveld refinement method to the X-ray diffraction pattern obtained using the X-ray diffractometer, the resulting lattice constant is a = b = 6.36 Å, c = 10.54 Å. It can be seen that all three lattice constants a, b, and c were reduced compared to specimen S2 of Preparation Example 2. This is because the ion radius of Li ⁺ is smaller than the ion radius of Na ⁺ .

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to analyze the shape, size and structure of the particles. The scanning electron micrograph of the specimen S4 is shown in FIG. 10, and the bright-field image and the selected area diffraction pattern obtained using the transmission electron microscope are shown in FIGS. 11 and 12, respectively. It was. 10 and 11, it can be seen that the particle size of the prepared LiNa _0.5 VOPO ₄ F _0.5 is 1-2㎛. Referring to FIG. 12, it can be seen that each particle of FIG. 12 is a single crystal because it shows a diffraction pattern of a typical single crystal. In addition, since the tetragonal system can be used to easily index the diffraction points, it can be seen that the particles have a tetragonal structure of excellent crystallinity.

Psalter Reaction temperature
(℃) Reactant
Mass (g) Reaction vessel
(ml) Reaction time
(h) Composition of the product Example 1 S4 160 36.62 250 15 Li _0.86 Na _0.54 VOPO ₄ F _0.5

* Na _x VOPO ₄ F _{0 .5} Synthesis of powder

(Production Examples 4 to 7)

Na _1.5 VOPO- ₄ F _0.5 powder prepared in Preparation Example 2 was mixed with NO ₂ BF ₄ (Alfa Aesar) oxidizing agent, and the mixture was placed in a sealable container with acetonitrile (Aldrich) solvent. Since the NO ₂ BF ₄ oxidant was sensitive to moisture, all procedures were carried out in a glove box filled with argon. As used amount of the oxidizing agent, 0.25 to 1 mole was used _per 1 mole of Na _1.5 VOPO ₄ F _0.5 as shown in Table 8 below. In addition, the amount of acetonitrile used was determined based on the amount of Na _1.5 VOPO ₄ F _0.5 . For example, 300 mL of acetonitrile per 1 g of Na _1.5 VOPO ₄ F _0.5 powder was used.

The mixture was stirred at a rate of 300 rpm at a temperature of 60 ° C., causing a chemical oxidation reaction. Subsequently, after 15 hours, the mixture was centrifuged and washed six times, each with clean acetonitrile, ethanol, and second distilled water. Thereafter, the obtained powder was dried at 70 ° C. for several hours in a vacuum oven to obtain Na _x VOPO ₄ F _0.5 (0.5 ≦ x ≦ 1.5) compound (Preparation Examples 4 to 7 of Table 8) as a desired product.

The chemical composition of the desired product prepared according to Preparation Examples 4 to 7 was investigated using an inductively coupled plasma atomic emission spectrometer (ICP-AES), and the results are shown in Table 8 below. In addition, the crystal structure of the desired product was analyzed using an X-ray diffractometer (XRD), and the results are shown in FIG. 8. The lattice constant is calculated by using the Rietveld refinement method on the X-ray diffraction pattern obtained using the X-ray diffractometer, and is shown in Table 8 below.

Psalter NO ₂ BF ₄ with Na _1.5 VOPO ₄ F _0.5
Molar ratio Composition of the product a (Å) c (Å) V (Å) Production Example 2 S2 0: 1 Na _1.50 VOPO ₄ F _0.5 6.380 10.630 432.69 Production Example 4 S5 0.25: 1 Na _1.23 VOPO ₄ F _0.5 6.307 10.709 425.99 Production Example 5 S6 0.50: 1 Na _0.93 VOPO ₄ F _0.5 6.298 10.724 425.37 Production Example 6 S7 0.75: 1 Na _0.77 VOPO ₄ F _0.5 6.293 10.740 425.32 Preparation Example 7 S8 1.00: 1 Na _0.62 VOPO ₄ F _0.5 6.292 10.745 425.31

Referring to Table 8, as the Na _{1 .5} VOPO ₄ F _{0 .5} reduce the Na content in the lattice constant a decreases and the lattice constant c can be increased to see that the cell volume V is reduced. Referring to FIG. 13, it can be predicted that a one-phase reaction will be performed since only a continuous position change of an existing peak and no new pick appear.

In addition, the cell volume of Li _0.86 Na _0.54 VOPO ₄ F _0.5 (Sample S4) of Example 1 was 426.34 n ^3, and the cell volume of Na _0.62 VOPO ₄ F _0.5 (Sample S8) of Preparation Example 7 was 425.31 n ³ , It can be seen that the volume change during charge and discharge of the _x Na _1.5-x VOPO ₄ F _0.5 structure is about 2-3%. This volume change is very small compared to other anode materials. If the volume change during charge and discharge is small, the stress strain of the structure will be so small, and thus, there is a big advantage in that better life characteristics can be expected.

Experimental Example 1: Li _x Na _1.5-x VOPO ₄ F _0.5 Of or Na _y VOPO ₄ F _0.5 Thermal stability evaluation

The thermal stability of the Li _x Na _1.5-x VOPO ₄ F _0.5 specimen (S4) prepared in Example 1 was analyzed by thermogravimetric analysis (TGA, product of SETARAM, France). The experiment was carried out in an argon atmosphere, and measured at an elevated temperature rate of 10 ° C. min ⁻¹ from room temperature (25 ° C.) to 600 ° C.

The thermogravimetric analysis of the Li _x Na _1.5-x VOPO ₄ F _0.5 specimen (S4) is shown in FIG. 14. Referring to FIG. 14, it can be seen that Li _x Na _1.5-x VOPO ₄ F _0.5 specimen has little thermal weight loss up to 600 ° C. under argon atmosphere, and thus is thermally stable up to 600 ° C. FIG.

Experimental Example 2: Li _x Na _1.5-x VOPO ₄ F _0.5 Evaluation of Thermal Phase Stability

In order to evaluate the thermal phase stability of the Li _x Na _1.5-x VOPO ₄ F _0.5 specimen (S4) prepared in Example 1, the temperature is raised from room temperature (25 ℃) to 700 ℃ _X -ray diffraction patterns of _x Na _1.5-x VOPO ₄ F _0.5 specimens were obtained at 200, 400, 550, 600, 650 and 700 ° C. The change of the X-ray diffraction pattern with temperature is shown in FIG. 15. Referring to FIG. 15, the Li _x Na _1.5-x VOPO ₄ F _0.5 specimen is structurally stable up to 550 ° C., and at higher temperatures, phase separation may gradually occur into the lithium and sodium phases.

Through this, it can be seen that Li _x Na _1.5-x VOPO ₄ F _0.5 is a low temperature phase or a meta-stable phase. According to these results, ie Li _x Na _1.5-x VOPO ₄ F _0.5 is not structurally stable at high temperatures above 550 ° C., the Li _x Na _1.5-x VOPO ₄ F _0.5 material is prepared in a manner other than Example 1, namely The general high temperature synthesis method (heat treatment temperature of about 600 ° C. or more) such as solid-state reaction or sol-gel method supports the fact that it is difficult to synthesize.

Experimental Example 3: Li _x Na _1.5-x VOPO ₄ F _0.5 Measurement of electrical conductivity (AC and DC)

A pellet having a diameter of 15 mm and a thickness of 1-2 mm was prepared by applying a uniaxial pressure of 400 kgcm ^-2 to the Li _x Na _1.5-x VOPO ₄ F _0.5 powder (Sample S4) prepared in Example 1.

The pellet was densified using a cold isostatic press (C.I.P.) at a pressure of 2,300 bar for 180 seconds. The pellet obtained by densification showed compactness of 66% even though the sintering process was not performed.

Using the pellet, the electrical conductivity was measured by the following method.

On both sides of the pellet, ionically blocking electrodes were deposited to a thickness of 100 nm through platinum (Pt) sputtering.

The ion blocking electrode was recorded using a frequency of 1.3 MHz to 0.01 Hz, and the AC impedance was measured using an AC impedance spectroscopy using 1 V voltage perturbation. Combination of analyzer and SI 1287 electrochemical interface (Solatron mobrey, Berks, UK) and constant voltage / constant current meter (WonA Tech, WBCS 3000, Korea) using 40 ℃, 60 ℃, 80 ℃, 100 ℃, 120 ℃, 140 ℃ And at 160 ° C. The measured AC impedance results are shown in FIG. 16.

In addition, direct current conductivity was measured by polarizing the pellet having the ion blocking electrode formed at 0 V to 5 V. The initial current value immediately after the polarization is mixed with the contribution of all charge carriers of ions and electrons, and the initial current value decreases with time. After the current reaches steady-state, the current remains only the contribution from the electrons. The current value in the steady state is defined as i _ss . The i _ss -V curve representing the steady-state current value (i _ss ) according to the polarization voltage (V) is linear according to Ohm's law, where DC electrical conductivity from the slope of the i _ss -V curve (electronic conductivity) was obtained.

The DC electrical conductivity was measured using a combination of a 1255 HF frequency response analyzer and an SI 1287 electrochemical interface (Solatron mobrey, Berks, UK) and a constant voltage / constant current meter (WonA Tech, WBCS 3000, Korea). It measured at 100 degreeC, 120 degreeC, 140 degreeC, and 160 degreeC. The results are shown in FIG.

On the other hand, the activation energy for AC impedance and DC conductivity was calculated by calculating the slope of the log (σT) -1 / T curve showing the change of AC conductivity and DC conductivity with temperature. In this case, the log (σT) -1 / T curve is a linear function and is a regression curve obtained by using a linear regression method on given data. Since the slope of the curve becomes − (E _a / k _B ) according to the equation σ T = σ ₀ exp (−E _a / (k _B T)), the activation energy E _a value was obtained here. AC impedance and direct current conductivity at room temperature (25 ° C.) were obtained by extrapolating the regression curve to 25 ° C. The results are shown in FIG.

In addition, measured AC conductivity and activation energy at room temperature are shown in Table 9 below.

Referring to Table 9 and FIGS. 16 to 18, the electrical conductivity of the Li _x Na _1.5-x VOPO ₄ F _0.5 powder is compared with other cathode active materials for conventional lithium secondary batteries. ₂ , LiMn ₂ O ₄ ) It is a low level compared to the electrical conductivity of the polyanionic cathode materials (LiFePO ₄ , LiFeSO ₄ F), but the level is considerably higher. In addition, as shown in Table 9, since the activation energy is low, it can be seen that the lithium diffusion is easy to move quickly, that is, the activation energy characteristics are much superior to other conventional cathode active material for lithium secondary batteries.

Positive electrode active material AC Conductivity at 25 ℃ (S cm ^-1 ) Activation energy (eV) Example 1 Li _x Na _1.5-x VOPO ₄ F _0.5 2.8 X 10 ^-7 0.156 Na _1.5 VOPO ₄ F _0.5 ¹⁾ 1.8 X 10 ^-7 0.43 LiFePO ₄ ²⁾ To 10 ^-9 0.6 LiFeSO ₄ F ³⁾ 7 X 10 ^-11 0.99 LiMn ₂ O ₄ ⁴⁾ To 10 ^-5 0.4 LiCoO ₂ ⁵⁾ To 10 ^-3 0.16

In Table 9, 1) to 5) describe the results published in the following paper.

Na _1.5 VOPO ₄ F _0.5 : F. Sauvage et al., Solid State Sciences , 8 (2006)

2) LiFePO ₄ : C. Delacourt et al., J. Electrochem. Soc ., 152 (2005)

3) LiFeSO ₄ F: N. Recham et al., Nat. Mater ., 9 (2010)

4) LiMn ₂ O ₄ : J. Molenda et al., Solid State Ionics , 171 (2004)

5) LiCoO ₂ : H. Tukamoto and AR West, J. Electrochem. Soc ., 144 (1997) or M. Menetrier et al., J. Mater. Chem ., 9 (1999)

In addition, Na _1.5 VOPO ₄ F _0.5 (F. Sauvage et al., Solid State Sciences , 8 (2006)), Li _1.5 VOPO ₄ F _0.5 and activation energy at an absolute temperature of 0K of the positive electrode active material prepared in Example 1 By using the first principle calculation (first principle calculations), measured by the NEB (Nudged Elastic Band Theory) method and the results are shown in FIGS. 19 to 21, respectively. In addition, for comparison, the results of FIGS. 20 and 21 are combined and shown in FIG. 22. As shown in FIGS. 19 to 22, the activation energy (E _a ) required for Li ion diffusion of Na material (x = 0, Na _1.5 VOPO ₄ F _0.5 ) is 310 meV, and the Li material (x = 1.5, Li _1.5 It can be seen that E _a of VOPO ₄ F _0.5 ) is 240 meV, and E _a of 200 meV (x = 1, LiNa _0.5 VOPO ₄ F _0.5 ) in which Li and Na are mixed in a certain ratio (here 2: 1). have.

Taken together, these materials (x = 1, LiNa _0.5 VOPO ₄ F _0.5 ) with Li and Na in a certain ratio (here 2: 1) have the lowest activation energy required for Li ion diffusion. It can be expected to show the fastest Li ion diffusion within. In this aspect, there is a positive aspect in which the Li diffusion rate is improved as Na is replaced with Li in pure Na material, and a certain amount of Na remains in the structure rather than all Na is replaced by Li. It can also be seen that it is more advantageous from the side.

* Manufacture of lithium secondary battery

(Comparative Example 1)

Dry carbon coated Na _1.5 VOPO ₄ F _0.5 powder prepared in Preparation Example 3 was used as a positive electrode active material.

After mixing the positive electrode active material, the carbon black (Super-P) conductive material and polyvinylidene fluoride (PVdF) in a weight ratio of 75: 15:10, the mixture was mixed with N-methyl-2-pyrrolidone (NMP ) To prepare a positive electrode active material slurry.

The slurry was uniformly applied to 20 μm thick aluminum foil, and dried in air at 120 ° C. for 2 hours to evaporate N-methyl-2--pyrrolidone to prepare a positive electrode.

LiPF ₆ was added to a solvent in which the prepared anode and lithium metal were used as counter electrodes, and a porous separator (Celgard 2400), ethylene carbonate (EC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 1: 1. A coin battery was prepared using a liquid dissolved at a concentration of 1 M as an electrolyte. The coin cell manufacturing process was done in a glove box filled with argon.

(Example 2)

Li _x Na _1.5-x VOPO ₄ F _0.5 powder prepared in Example 1 and carbon black were prepared in a weight ratio of 85:15, followed by dry-ball milling of Li _x Na _1.5-x VOPO ₄ F _0.5 coated with carbon. A positive electrode active material was prepared.

The positive electrode active material, the carbon black (Super-P) conductive material, and the polyvinylidene fluoride (PVdF) binder were mixed at a weight ratio of 83: 8: 9, and then the mixture was mixed with N-methyl-2-pyrrolidone ( NMP) to prepare a positive electrode active material slurry.

The slurry was uniformly applied to 20 μm thick aluminum foil, and dried in air at 120 ° C. for 2 hours to evaporate N-methyl-2-pyrrolidone to prepare a positive electrode.

LiPF ₆ was added to a solvent in which the prepared anode and lithium metal were used as counter electrodes, and a porous separator (Celgard 2400), ethylene carbonate (EC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 1: 1. A coin battery was prepared using a liquid dissolved at a concentration of 1 M as an electrolyte. The coin cell manufacturing process was done in a glove box filled with argon.

Experimental Example 4: Na _1.5 VOPO ₄ F _0.5  Or Li _x Na _1.5-x VOPO ₄ F _0.5 Evaluation of Electrochemical Properties of

Each coin cell prepared in Comparative Examples 1 and 2 was used at an temperature of 25 ° C. or 60 ° C., using an electrochemical analyzer (WonA Tech, WBCS 3000, Korea or MACCOR, series 4000, USA). Constant current at various current densities (10C, 5C, 2C, 1C, C / 2, C / 5, C / 10, C / 20, C / 50) in the 4.5 V or 2.5 to 4.5 V or 1.0 to 4.95 V potential range A galvanostatic charge / discharge experiment was performed. Here, 1C is a current density that can charge or discharge a positive electrode active material or negative electrode active material by the theoretical capacity of the active material for 1 hour, and in the case of Li _x Na _1.5-x VOPO ₄ F _0.5 material, when x = 1, 1C = 141 mA g ^-1 . For Na _1.5 VOPO ₄ F _0.5 material, 1C = 130.1 mA g ⁻¹ .

Cyclic voltammetry of the coin cells prepared according to Comparative Examples 1 and 2 was measured at a scanning speed of 0.05 or 0.1 mV sec ⁻¹ over a potential region of 2.0 to 4.5 V or 2.8 to 4.9 V It was. The results are shown in FIG. 23 (Comparative Example 1) and FIG. 24 (Example 2), respectively. As shown in FIG. 23, the coin cell of Comparative Example 1 had two sharp picks at average voltages of 3.84 V and 4.21 V, respectively, during a cathodic scan or an anodical scan. It can be seen that the battery voltage is about 4V. On the contrary, as shown in FIG. 24, the coin cell of Example 2 has a similar average cell voltage of about 4 V, but it can be seen that it has one wide pick instead of two sharp picks. Through this, it can be seen that the electrochemical reaction of the two batteries are not the same.

The coin battery prepared according to Example 2 was charged and discharged twice at 25 ° C. (25 ° C.) in a potential region of 2.0 to 4.5 V, and the second charge and discharge capacity was measured. The results are shown in FIG. As shown in FIG. 25, it can be seen that one smooth inclined region appears in place of two distinct voltage plateaus, which are close to theoretical capacity at C / 5.

The coin battery prepared according to Example 2 was subjected to charge / discharge about 40 times (25 ° C) at C / 5 in a 2.0 to 4.5 V potential region, and the charge and discharge capacity thereof was measured. As shown in FIG. 26, it can be seen that the coin battery manufactured according to Example 2 has an excellent capacity retention rate.

The coin cell prepared according to Example 2 was subjected to C / 5 charge / discharge about 40 times (60 ° C.) in a 2.5 to 4.5 V potential region, and the result of measuring the charge / discharge capacity thereof is shown in FIG. 27. As shown in FIG. 27, it can be seen that the coin battery manufactured according to Example 2 was also excellent in capacity retention at high temperature.

The charging / discharging capacity obtained by charging and discharging the coin battery prepared according to Example 2 at C / 5, C / 2, 1C, 2C, 5C, and C / 5 while changing the charge / discharge rate is shown in FIG. It was. As shown in FIG. 28, even when the coin battery manufactured according to Example 2 was charged and discharged at a high rate (5C), when the battery was charged and discharged at a low rate (C / 5), the capacity was almost recovered. .

The coin battery of Example 2 was charged and discharged at C / 5 at a potential of 2.0 to 4.5 V at 45 times, and thereafter, the coin battery was decomposed to sufficiently clean the positive electrode with propylene carbonate, dried and recovered, and then X-rayed. The diffraction test was carried out to obtain an X-ray diffraction pattern. For comparison, an X-ray diffraction pattern was also obtained by performing an X-ray diffraction test on the anode (the anode before charge and discharge) prepared in Example 2. The two X-ray diffraction patterns are compared and shown in FIG. 29. As shown in FIG. 29, almost no change in the X-ray diffraction pattern was observed even after 45 charge / discharge cycles were performed at C / 5. This means that after many charge and discharge processes, the structure does not collapse or deform and remains intact. Thus, these results demonstrate the high stability of the Li _x Na _1.5-x VOPO ₄ F _0.5 structure and the excellent reversibility of lithium insertion / desorption.

It will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the following claims.

Claims (6)

To include a compound represented by the formula (1),
A cathode active material for a lithium secondary battery having a nasicon structure.
[Formula 1]
(Li _x Na _1.5-x ) _a (Z) _z ((V) (O _1.5-f )) _1-z (XO ₄ ) F _f
(In the above formula 1,
Z is a metal element dopant and is selected from the group consisting of Fe, Ni, Co, Mn, Cr, Zr, Nb, Cu, Ti, Zn, Al, Ga, Mg, Ca, and combinations thereof. Elemental,
X is an element selected from the group consisting of P, S, Si, Mo, As, W, and combinations thereof,
a, x and z are each a real number satisfying 0 <a ≤ 1, 0 <x ≤ 1.5, 0 ≤ z ≤ 0.5,
f is determined to match the electroneutrality of the compound and is a real number that satisfies 0.5 ≦ f ≦ 1.5.) The method of claim 1,
Z is a real number satisfying 0 ≦ z ≦ 0.2. The method of claim 1,
The compound is a cathode active material for lithium secondary batteries having an average particle diameter of 100 μm or less. The method of claim 1,
X is P, S, Si, As, or a combination thereof. The method of claim 1,
The cathode active material is a cathode active material for a lithium secondary battery that is represented by the following formula (2).
(2)
Li _x Na _1.5-x VOPO ₄ F _f
(In the above formula 2,
f is a real number that satisfies 0.5 ≤ f ≤ 1.5.) A positive electrode comprising the positive electrode active material of any one of claims 1 to 5;
A negative electrode comprising a negative electrode active material; And
Nonaqueous electrolyte
&Lt; / RTI &gt;

Images