KR100762799B1 - Carbon-coated composite material, manufacturing method thereof, positive active material, and lithium secondary battery comprising the same - Google Patents

Abstract

A carbon-coated composite material is provided to ensure improved electrical conductivity and ion conductivity and to obtain performances in a high discharge current(high rate characteristic) and capacity approximate to the theological capacity. A carbon-coated Li_(x)A_(1-x)Fe_(y)B_(1-y)(PO4)_(z)C_(1-z) composite is prepared by the steps: of dissolving a vegetable or animal oil carbon precursor in a solvent to prepare a coating solution; (b) adding Li_(x)A_(1-x)Fe_(y)B_(1-y)(PO4)_(z)C_(1-z) particles to the coating solution to stir the admixture; and (c) heating the stirred solution in a heat treatment furnace to be carbonized, wherein 0<x<=1, 0<=y<=1, 0<=z<=1, A is at least one selected from alkali metals and alkaline earth metals, B is at least one selected from transition metals, and C is one selected from anions.

Description

Carbon-coated composite, its manufacturing method, positive electrode active material and lithium secondary battery having the same {Carbon-Coated Composite Material, Manufacturing Method, Positive Active Material, And Lithium Secondary Battery Comprising The Same}

1 is a SEM photograph of a carbon coated composite according to the present invention,

Figure 2 is a measure of the capacity of a battery using an uncoated composite as an active material,

Figure 3 is a measure of the rate characteristics of a battery using a carbon coated composite (205nm) as an active material using 3% by weight of olive oil according to an embodiment of the present invention,

4a to 4c are measured the rate characteristics of a battery using a carbon coated composite (121nm) as an active material using 3% by weight of olive oil according to an embodiment of the present invention,

Figure 5 is a measure of the rate characteristics of a battery using a carbon-coated composite as an active material using 3% by weight of butter according to an embodiment of the present invention,

Figure 6 is a measure of the rate characteristics of a battery using a carbon-coated composite as an active material using 3% by weight of soybean oil according to an embodiment of the present invention,

7 is a configuration diagram of a lithium secondary battery according to an embodiment of the present invention.

** Description of the main symbols in the drawings *

1: lithium secondary battery 2: negative electrode

3: anode 4: separator

5: battery container 6: sealing member

The present invention relates to a carbon coated composite, a method of manufacturing the same, a cathode active material, and a lithium secondary battery having the same.

Lithium secondary battery is a broad concept that includes not only secondary battery using lithium metal but also lithium ion secondary battery, and has high voltage and high energy density. It is also divided into a gel polymer battery using a mixture of polymers and a solid polymer battery using pure polymers.

The core components of a lithium secondary battery are the positive electrode, the negative electrode, and the electrolyte.

The lithium secondary battery is mainly composed of a positive electrode, a negative electrode, an electrolyte, a separator, a packaging material, and the like. The positive electrode is formed by binding a mixture of a positive electrode active material, a conductive agent, and a binder to a current collector. Lithium transition metal compounds such as LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , and LiMnO ₂ are mainly used as positive electrode active materials. These materials have an electrochemical potential that is increased by intercalation / deintercalation of lithium ions into the crystal structure. high.

Lithium metal, carbon or graphite is mainly used as the negative electrode active material and has a low electrochemical reaction potential as opposed to the positive electrode active material.

The electrolyte is mainly composed of LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ , LiPF ₆ , LiBF ₄ , LiClO ₄ , LiN ( A salt containing lithium ions such as SO ₂ C ₂ F ₅ ) ₂ is dissolved and used.

In addition, the separator, which electrically insulates the positive electrode and the negative electrode and provides a passage of ions, mainly uses a polyoletin-based polymer such as porous polyethylene. As an exterior material that protects the contents of the battery and provides an electrical passage to the outside of the battery, a metal can or a packaging material composed of aluminum and several layers of polymer layers is mainly used.

Lithium secondary battery applications have broadened the scope of small electronics, making portable or mobile electronics more convenient for everyday life and improving the efficiency of industrial activities.

However, the thermal instability and high price of batteries are still a major disadvantage of lithium secondary batteries that prevent the market expansion in earnest not only for cell makers but also for consumers. Therefore, the development of a material having a low cost and excellent safety is required, and much research is being conducted on this.

Of the battery materials, cathode materials are the ones that can bring about the greatest effect by inducing improvements in price, safety, and cost. Conventional cathode materials are metal oxides containing cobalt (Co), the most common of which is LiCoO ₂ . Accordingly, a part of cobalt is replaced with transition metals such as Ni and Mn, or the doping of a small amount of alkali metal is intended to improve performance. In addition, research is being conducted to improve safety by coating such a cobalt oxide.

However, while materials containing cobalt have good conductivity and performance, materials that can replace the cathode material containing cobalt due to high cost and safety issues are being studied. Among the promising candidates, LiFePO ₄ has a theoretical specific capacity of 170 mAh / g and, depending on the conditions, provides near-theoretical capacity. LiCoO _2, on the other hand, achieves only about 140 mAh / g, which is about half the theory. LiFePO ₄ is attracting attention as it has superior advantages in terms of price and safety.

However, the biggest disadvantage of LiFePO ₄ is its low conductivity. In addition to the poor electrical conductivity of the material itself, the larger the particle size, the lower the ion conductivity.

The present invention is to solve the above problems, by improving the electrical conductivity and ionic conductivity at the same time through the modification of the low-cost and high-efficiency carbon coating and nanoparticles of the active material, the performance and theoretical capacity at high discharge current (high rate characteristics) The purpose is to obtain a capacity close to.

When such a material is developed, it is possible to make an inexpensive battery, which makes it possible to make the battery larger in size and to ensure high safety. Therefore, it has been difficult to realize the battery of a hybrid vehicle and renewable energy. It will be possible to produce large scale batteries in energy systems.

The present invention for achieving the above object,

Li _x A _{1 - x} Fe _y B _{1 -y} (PO ₄ ) _z C _{1 -z} Carbon coated Li _x A _{1 - x} Fe _y B _{1 -y} using vegetable or animal oil carbon precursors (PO ₄ ) _z C _1-z complex.

In addition, as the carbon precursor, it provides a carbon coated Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z composite, characterized in that at least one selected from olive oil, soybean oil, butter, milk fat. do.

In addition, the carbon-coated Li _x A _{1 - x} Fe _y B _{1- y} characterized in that the size of the Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z particles are nano-sized (PO ₄ ) _z C _1-z complex.

In addition, the carbon precursor is used in the carbon-coated Li, characterized in that in the range of 0.1 to 10 parts by weight based on 100 weight of the Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z particles _x A _1-x Fe _y B _1-y (PO ₄ ) _z C _1-z complex.

The present invention also provides a cathode active material comprising a carbon coated Li _x A _{1 - x} Fe _y B _{1 -y} (PO ₄ ) _z C _{1 -z} composite.

The present invention also comprises the steps of dissolving a vegetable or animal oil carbon precursor in a solvent to prepare a coating solution; Adding Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z particles into the coating solution and stirring; And heat treating and carbonizing the same in a heat treatment furnace, thereby providing a method of preparing a carbon-coated Li _x A _{1 - x} Fe _y B _{1 -y} (PO ₄ ) _z C _{1 -z} composite.

In addition, the carbon-coated Li _x A _{1 - x} Fe _y B _{1- y} characterized in that the size of the Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z particles are nano-sized It provides a method for preparing (PO ₄ ) _z C _1-z composite.

In addition, the solvent provides a method for producing a carbon-coated Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z composite, characterized in that the isopropyl alcohol or ethanol.

In addition, the carbonization by heat treatment, carbon-coated Li _x A _{1- x} Fe _y B _1-y (PO ₄ ) _z C, characterized in that carried out for 0.5 to 3 hours in an inert gas atmosphere at 400 ~ 1000 ℃ _It provides a method for producing a _1-z composite.

The present invention also provides a lithium secondary battery having a negative electrode, a positive electrode including a positive electrode active material and an ion conductor, wherein the positive electrode active material is the carbon-coated Li _x A _1-x Fe _y B _1-y (PO ₄ ) _It provides a lithium secondary battery comprising a _z C _1-z composite.

In addition, the positive electrode provides a lithium secondary battery, characterized in that further comprises a carbon black (Super P Black) as a conductive material.

In addition, the positive electrode provides a lithium secondary battery, characterized in that further comprises a binder and a current collector.

In addition, the ion conductor provides a lithium secondary battery, characterized in that the electrolyte or polymer electrolyte.

Hereinafter, the present invention will be described in more detail.

First, the carbon-coated Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z composite according to the present invention and a cathode active material including the same will be described.

LiFePO ₄ as a cathode active material and its analog Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z composites preferably use a carbon coating to improve the conductivity of the material. The reason for this is that although carbon is inexpensive, it is also used as a conductive material used in electrode production, and thus has affinity as a material already used. Compared with the conventional electrode manufacturing method, the carbon coating ensures high electrical contact between the active materials, thereby significantly lowering the battery resistance than when manufacturing the electrode only by stirring the electrode material (active material, conductive material, binder). . In addition, an appropriate coating can take a large part of the role of the conductive material, so that it is possible to use less conductive material in the electrode, thereby increasing the capacity.

In the development of the carbon coating composite according to the present invention, and the active material including the same, the following three were considered.

Considerations include the surface orientation of the precursor in consideration of the surface properties of the active material, the particle size of the active material, the economics and the like.

end. Surface orientation of the precursor in consideration of the surface properties of the active material.

Under the premise that the surface characteristics of the active material vary depending on the synthesis conditions, the use of precursors capable of active interaction with these various surfaces will lead to the improvement of carbon coating quality and battery characteristics.

For example, if the surface properties of the active material are largely divided into polarity, nonpolarity, hydrophilicity, and hydrophobicity, irrespective of these surface properties, if a precursor having similar properties to the surface is nearby, the surface is well surrounded and carbonized. A dense carbon coating with no defects is possible. As the carbon precursor capable of incorporating the above properties in a single material, a carbon precursor having both hydrophobicity and hydrophilicity is preferable. Examples thereof include fatty acids which are raw materials for soaps and detergents, alcohols derived from them, and surfactants. These fatty acids and surfactants contain all of the above polar, nonpolar, hydrophilic, and hydrophobic functional groups, and depending on the nature of the surface, functional groups similar to the surface properties will be approached to closely surround the active material surface. At this time, for even contact between the precursor and the active material, the precursor is dissolved in a suitable solvent (water or various alcohols) and stirred. This friendly interaction will provide a uniform and low defect of carbon coating after carbonization of the precursor. Although the thickness of the carbon layer may be limited depending on the type of precursor, the characteristics may suggest various properties depending on the length of the precursor, the functional group, and the concentration of the precursor. The material that can be used as such a precursor may be an organic material including C, H, O having 10 or more carbon atoms, and is a coating material in which fatty acids and alcohols have a cheap and active surface orientation.

B. Particle size

The size and shape of the particles (crystals) have a great effect on the performance of the active material, and also play an important role in the production cost. Many LiCoO ₂ -based active materials have excellent conductivity, so that nanoparticles are not used, and particle sizes of about 10 microns are used. However, since LiFePO ₄ and its analogs are very inferior in conductivity, the particles of the micron unit have a significant deterioration in performance, and in particular, due to the rapid deterioration in high current discharge (high rate discharge), a major obstacle to commercialization. Acts as. This drawback will be possible with many improvements as nanoparticle size. Since the movement distance of the ions during charging and discharging is shortened through the nanoparticles of the particles, since most of the space of the active material is used for charging and discharging, specific capacity may be increased and high rate characteristics may be greatly improved.

All. Economic aspects

If the price of carbon precursors or LiFePO ₄ and its analogs for coating is high, the economic meaning is lost, so economics are just as important as good performance. Since the price of the active material is much better than LiCoO ₂ , it has a good strength. As the carbon precursor, it is preferably selected from the above-mentioned fatty acids, alcohol derivatives of the fatty acids, and surfactants. Once these precursors are so inexpensive, they can be used either, but they are easily selected from harmless materials that are readily available and friendly to people. They are animal or vegetable oils, such as olive oil, soybean oil, butter, milk (milk fat). All of them contain fatty acids.

This comprehensive consideration has led to the present invention.

Carbon coated composite according to the present invention and an active material including the same are as follows.

The carbon coated composite according to the present invention is Li _x A _{1 - x} Fe _y B _{1 -y} (PO ₄ ) _z C _1-z particles carbon coated using a vegetable or animal oil carbon precursor using Li _x A _{1 - x} Fe _y B _{1 -y} (PO ₄ ) _{z It} is characterized in that the complex _1-z .

In the above, 0 <x≤1, 0≤y≤1, 0≤z≤1, A is an ion capable of substituting a part of Li ions, at least one selected from alkali metals and alkaline earth metals, and B is Fe As an ion capable of substituting some or all of the ions, at least one of transition metals is selected, and C is an ion capable of substituting some or all of PO ₄ ions and at least one of negative ions. The values of x, y, and z also include a range that can be seen as an error range and an equivalent range.

Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z particles can be obtained by applying a known method (Japanese Patent Laid-Open No. 9-134724, Japanese Patent Laid-Open No. 9- 134725, Japanese Patent Laid-Open No. 11-261394, Japanese Patent Laid-Open 2001-110414, Japanese Patent Laid-Open 2001-250555, Japanese Patent Laid-Open 2000-294238, Domestic Patent Laid-Open No. 2003-0066396, Domestic Patent Laid-Open See Publication No. 2004-0007591, Domestic Patent Publication No. 2006-0025842, etc.).

The size of the Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z particles is not limited, but the nano-size has a shorter moving distance of the ions during charging and discharging. Since it is used for charging and discharging, it is preferable because specific volume increases and a high rate characteristic can also be improved significantly. Here, the nano size means 1 to 999 nm and includes the meanings commonly used in the art.

As described above, the carbon precursor may be used without limitation as long as it is a vegetable or animal oil containing a lot of fatty acids and the like in which hydrophobicity and hydrophilicity coexist. Since these components coexist with hydrophobicity and hydrophilicity in one molecule, they closely surround the surface of the Li _x A _1-x Fe _y B _1-y (PO ₄ ) _z C _1-z particles, and the carbon coating after carbonization of the precursor It provides a uniform surface with less defects.

The carbon precursor is not limited, but at least one selected from olive oil, soybean oil, butter, and milk fat. They are easy to get around and cheaply available.

The amount of the carbon precursor is not limited, but is preferably in the range of 0.1 to 10 parts by weight based on 100 parts by weight of the Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z particles. Below the above range, the coating effect is insignificant, and beyond the above range, the adverse effect of the carbon coating may occur.

The positive electrode active material according to the present invention includes the aforementioned carbon coated Li _x A _{1 - x} Fe _y B _{1 -y} (PO ₄ ) _z C _{1 -z} composite. It may be used alone as an active material, it may be used by mixing other active materials and included in the present invention.

Hereinafter, a method for preparing a carbon coated Li _x A _{1 - x} Fe _y B _{1 -y} (PO ₄ ) _z C _1-z composite according to the present invention.

In the method for preparing a carbon coated Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z composite according to the present invention, a coating solution by dissolving a vegetable or animal oil carbon precursor in a solvent To prepare a step, including Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z particles into the coating solution and stirring, and heat treatment in a heat treatment furnace to carbonize It is characterized by. Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z composite, carbon precursors and the like as described above are dedicated to the above bar and will not be described.

First, a coating solution is prepared. Vegetable or animal oil carbon precursors are dissolved in a solvent to form a coating solution. The solvent is not limited as long as it is a solvent capable of dissolving the carbon precursor, and preferably alcohol, in particular isopropyl alcohol or ethanol. The amount of solvent used is sufficient to dissolve the carbon precursor.

Next, Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z particles are added to the coating solution and stirred. The size of the Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z particles is not limited but is preferably nano-sized. Li _x A _{1 - x} Fe _y B _{1 -y} (PO ₄ ) _z C _1-z particles can be obtained in nano size, micro-sized particles by ball milling (Ball milling) method It is also possible to obtain nanoscale uniform particle sizes.

Next, heat treatment is carbonized to complete the carbon coating. The heat treatment is not limited, but is preferably performed in an electric furnace for 0.5 to 3 hours in an inert gas atmosphere at a temperature in the range of 400 ~ 1000 ℃. Nitrogen or argon may be used as the inert gas. This is to prevent oxidation of metal and carbon.

Hereinafter, a lithium secondary battery according to an embodiment of the present invention will be described in detail.

Lithium secondary battery according to the present invention, a lithium secondary battery having a negative electrode, a positive electrode including a positive electrode active material and an ion conductor, the positive electrode active material is the carbon-coated Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _{1 -z It} is characterized by comprising a complex. Except for the carbon coated Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z composites, the other configurations can be selected and applied without limitation to those known in the art. If necessary, the positive electrode may further include carbon black (Super P Black) as a conductive material, and the positive electrode may further include a binder and a current collector. In addition, the ion conductor may be an electrolyte solution or a polymer electrolyte.

The lithium secondary battery according to an embodiment of the present invention may further include a cathode active material known in the art, in addition to the cathode active material described above. In addition, the negative electrode may also include an active material.

7 shows a lithium secondary battery 1 which is an embodiment of the present invention. The lithium secondary battery 1 includes a negative electrode 2, an electrode 3, a separator 4 disposed between the negative electrode 2 and the positive electrode 3, the negative electrode 2, the positive electrode 3, and the separator 4. ), The ion conductor impregnated in the ()), the battery container (5), and the sealing member (6) for sealing the battery container (5) as a main portion. The shape of the lithium secondary battery illustrated in FIG. 2 may be in a cylindrical shape, but in addition, various shapes such as a cylindrical shape, a square shape, a coin shape, a sheet shape, and the like.

The positive electrode 3 is provided with a positive electrode mixture composed of a positive electrode active material, a conductive material and a binder.

As the separator, an olefin porous film such as polyethylene or polypropylene can be used.

The ion conductor may be propylene carbonate (hereinafter referred to as PC), ethylene carbonate (hereinafter referred to as EC), butylene carbonate, benzonitrile, acetonitrile, tetrahydrofuran, 2-methyl tetrahydrofuran, γ-butyrolactone, dioxane, and the like. Solan, 4-methyldioxolane, N, N-dimethylformamide, dimethylacetoamide, dimethyl sulfoxide, dioxane, 1,2-dimethoxyethane, sulfolane, dichloroethane, chlorobenzene, nitrobenzene, dimethyl carbonate ( Hereinafter, DMC), ethyl methyl carbonate (hereinafter EMC), diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, ethyl butyl carbonate, dipropyl carbonate, diisopropyl carbonate, dibutyl carbonate, diethylene glycol, dimethyl ether To aprotic solvents such as or mixed solvents in which two or more of these solvents are mixed, such as LiCF 3 SO 3, Li (CF 3 SO 2) 2, LiPF 6, LiBF 4, LiClO 4, LiN (SO 2 C 2 F 5) 2, and the like. The thing which melt | dissolved the mixture of 1 type (s) or 2 or more types of electrolyte which consists of lithium salts can be used.

In addition, a polymer solid electrolyte may be used instead of the electrolyte solution. In this case, it is preferable to use a polymer having high ion conductivity with respect to lithium ions, and polyethylene oxide, polypropylene oxide, polyethyleneimine and the like can be used. A gel obtained by adding the solvent and the solute to this polymer can also be used.

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

<Example>

(a) Preparation of Carbon Coated LiFePO ₄ Composite

LiFePO ₄ having a diameter of about several tens of micrometers was crushed through ball milling to obtain LiFePO ₄ having a nano-sized uniform particle size (particle size of 250 nm (D ₁₀ = 0.13 μm, D _{50). = 0.25μm, D 90 = 0.47μm)} , 121 nm (D 10 = a _{0.097μm, D 50 = 0.121μm, D} 90 = 0.167μm) two kinds were prepared, respectively).

LiFePO ₄ 100 3 wt% by weight, 5% by weight of olive oil and 3% by weight of butter, by dissolving 3% by weight of soybean oil, each with a carbon precursor to a sufficient amount of isopropyl alcohol to make a coating solution. And LiFePO ₄ nanoparticles are put into the coating solution and stirred. The amount of alcohol may be further added to the coating solution so that the active material is sufficiently submerged. After stirring for about 30 minutes, it was put in a crucible and heat-treated in an electric furnace at 550 ° C. for 1 hour to prepare a carbon-coated LiFePO ₄ composite. At this time, an argon gas is used to prevent oxidation of Fe.

(b) Preparation of Lithium Secondary Battery

Coin cells were prepared to observe the performance of the prepared carbon-coated LiFePO ₄ composite prepared as an active material. The active material is made of a plate, the order of which is as follows. As the conductive agent and the binder, carbon black (Super-P Black) and polyvinylidene fluoride (PVDF) were used. The weight ratio of active material: conductive material: binder was 85: 8: 7. First, the binder is stirred in an appropriate amount of N-methylpyrrolidone (NMP) and a conditioning mixer for 10 minutes, mixed with an active material and a conductive agent, and a little more N-methylpyrrolidone (NMP) is added to the viscosity. After adjusting, stir for another 30 minutes. After spreading the Al current collector on the glass plate and casting the slurry prepared in the Al current collector by the doctor blade method (doctor blade) method, the overnight drying at 100 ℃ (overnight). The dried electrode is pressed at a compression rate of 20 to 25% with a roll press machine at 120 ° C. to complete the electrode.

A coin cell was fabricated to evaluate the characteristics of the fabricated electrode. Coin cell was used as the standard 2032, the electrode was used as a positive electrode, lithium (Li) metal as the negative electrode (electrolyte) was used 1.0M LiPF6, EC / DEC (1: 1).

For the experiment, FE-SEM used the S-4800 model of HITACHI Co., Ltd., the charger and the discharger was conducted using the TOSCAT-3100U model of TOYO, and the results will be described later.

Comparative Example

(a) Preparation of electrode comprising LiFePO ₄ composite of uncoated nanoparticles

In the preparation of the electrode comprising the carbon-coated LiFePO ₄ composite described above, it was performed in the same manner except that no carbon coating. However, two types of electrodes were prepared, respectively, in which the weight ratio of active material: conductive material: binder was 85: 8: 7 and 81: 12: 7.

(b) Preparation of an electrode comprising a LiFePO ₄ composite coated with another carbon precursor

In preparing the electrode including the carbon-coated LiFePO ₄ composite described above, it was performed in the same manner except that citric acid, ethylene glycol, and various hydrocarbon gases were used as the carbon coating precursor.

1 is a SEM photograph of the carbon-coated composite using 3% by weight, 5% by weight of olive oil. (a) is a photograph of the uncoated composite, and (b) and (c) are photographs of the carbon-coated composite using 3% by weight and 5% by weight of olive oil, respectively. Although the coating amount of carbon is small, it is not clearly distinguished by the picture, but it shows a marked difference in performance. In the case of coating using 3% by weight, the coating amount contained by the TGA method was measured, and it was detected that about 1.60% by weight was coated.

Figure 2 is a graph measuring the capacity of the uncoated nanoparticles of the comparative example. The charge and discharge conditions are the charge and discharge boundaries of 4V and 2V and the same charge discharge rate of 0.2C. (a) The graph is an electrode of active material: conductive material: binder = 85: 8: 7, and (b) the graph is an electrode of active material: conductive material: binder = 81: 12: 7.

(a) shows a capacity of about 42 mAh / g. To achieve similar performance with significantly smaller particle sizes, a much larger amount of conductive material must fill the gap between the particles. (b) shows a slight improvement in capacity by increasing the conductive material. This shows the need for enormous conductive materials for nanoparticles. The increase in specific amount due to the increase of the conductive material means a relatively small amount of the active material, so the uncoated active material is not practical. The demand for many conductive materials due to the surface area of these nanomaterials can be solved by carbon coating.

3 is a rate characteristic curve of a material using 3% by weight of olive oil as a carbon precursor. When the boundary voltage of charge / discharge is charged at 0.2C under 4.5V, 2.0V, CC (constant current) / CV (constant voltage) condition, when charging boundary voltage is reached , Right to left 0.2C, 0.5C, 1C, 2C, 3C conditions.

As shown, 125.3 mAh / g at 2C and 146.8 mAh / g at 0.2C, the ratio of the capacity of 2C / 0.2C is 85.4%. As such, by coating the nanoparticles, it shows superior performance compared to simply increasing the conductive material.

4A-4C show clear performance improvements when the particle size is smaller. a is the boundary voltage of charge and discharge at 4.0V and 2.0V, b is the boundary voltage of charge and discharge is 4.2V and 2.0V, c is the boundary voltage of charge and discharge is 4.5V and 2.0V, CC (constant current) / After charging at 0.2C under CV (constant voltage) condition, when the charging boundary voltage is reached, charging is completed when the limit current of 1 / 20C is reached, and discharge is 0.2C, 0.5C, 1C, 2C, 3C conditions from right to left. to be.

The improved data was obtained when the electrode plate was fabricated and measured with 121 nm particles (D ₁₀ = 0.097 μm, D ₅₀ = 0.121 μm, D ₉₀ = 0.167 μm) whose diameter was reduced by half. In addition, it can be seen that as the charging voltage is increased, the high rate characteristic and capacity are improved. Its numerical comparison is shown in Table 1.

TABLE 1

Charge voltage (V) Discharge Capacity at 0.2C (mAh / g) Discharge Capacity at 2C (mAh / g) Capacity retention rate of 2C / 0.2C (%) 4.0 162.0 149.0 92.5 4.2 161.8 152.8 94.4 4.5 163.8 153.3 93.9

At this time, the thickness of the electrode including the Al current collector was 55μm. The thickness of the collector was 15 μm. At 2C, this capacity gradually increased with increasing charging voltage. The capacity at 0.2C was about the same or increased slightly with increasing voltage. At this time, the packing density of LiFePO ₄ was 0.96 g / cm ³ .

5 and 6 are graphs showing the rate characteristics of 121 nm LiFePO ₄ composite using 3 wt% butter and 3 wt% soybean oil as carbon precursors, respectively. When the boundary voltage of charge / discharge is charged at 0.2C under 4.5V, 2.0V and CC (constant current) / CV (constant voltage) conditions, when charging boundary voltage is reached, charging is completed when the limit current of 1 / 20C is reached. , Right to left 0.2C, 0.5C, 1C, 2C, 3C conditions.

As shown, for the butter precursor of FIG. 5, it was 153.8 mAh / g at 0.2C, 152.0 mAh / g at 2C and the 2C / 0.2C capacity retention provided 98.9%. In the case of the soybean oil precursor of FIG. 6, a capacity of 159.3 mAh / g at 0.2C, 156.4 mAh / g at 2C, and a 2C / 0.2C retention rate provided 98.2%.

7 is a view showing the cycle characteristics of 121nm LiFePO4 coated with olive oil 3%. As shown, it can be seen that the cycle characteristics are excellent.

The results are superior to the active material obtained from the comparative example in terms of high rate characteristics and cycle characteristics. In addition, even in the case of carbon coating at a high temperature by mixing various hydrocarbon gas and inert gas, even using more carbon coating (4-15%) than the method shown in the example, it did not show excellent performance. . In addition, the coating material reported in the present invention shows an excellent result while the price and coating method are extremely advantageous.

Li _x A _{1 - x} Fe _y B _{1 -y} (PO ₄ ) _z C _1-z Uncoated Li _x A _{1 - x} Fe _y B _{1 -y} ( PO ₄ ) significantly reduced the high resistance of _z C _1-z . The active material thus formed can be applied to high power and large sized batteries, which have been difficult to realize in lithium secondary batteries due to the safety and improved high rate characteristics of Li _x A _{1 - x} Fe _y B _{1 -y} (PO ₄ ) _z C _1-z . .

Claims (13)

Li _x A _{1 - x} Fe _y B _{1 -y} (PO ₄ ) _z C _{1 -z} Carbon coated Li _x A _{1 - x} Fe _y B _{1 -y} using vegetable or animal oil carbon precursors (PO ₄ ) _z C _1-z complex. (In the above, 0 <x≤1, 0≤y≤1, 0≤z≤1, A is selected from at least one of alkali metals and alkaline earth metals, B is selected from at least one of transition metals, and C is selected from anions Is chosen) The carbon coated Li _x A _1-x Fe _y B _1-y (PO ₄ ) _z C ₁ according to claim 1, wherein the oil carbon precursor is selected from at least one of olive oil, soybean oil, butter, and milk fat. _-z complex. The carbon-coated Li _x A _{1 - x} Fe _y according to claim 1, wherein the Li _x A _{1 - x} Fe _y B _{1 -y} (PO ₄ ) _z C _{1 -z} particles have a nano size. B _{1 -y} (PO ₄ ) _z C _1-z complex. The method of claim 1, wherein the carbon precursor is used in an amount of 0.1 to 10 parts by weight based on 100 parts by weight of Li _x A _{1 - x} Fe _y B _{1 -y} (PO ₄ ) _z C _1-z particles. Carbon coated Li _x A _1-x Fe _y B _1-y (PO ₄ ) _z C _1-z composite. The cathode active material comprising the carbon coated Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z composite of any one of claims _{1 to 4} . Preparing a coating solution by dissolving a vegetable or animal oil carbon precursor in a solvent; Adding Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z particles into the coating solution and stirring; And Carbonization by heat treatment in a heat treatment furnace; comprising a carbon coated Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z composite. (In the above, 0 <x≤1, 0≤y≤1, 0≤z≤1, A is selected from at least one of alkali metals and alkaline earth metals, B is selected from at least one of transition metals, and C is selected from anions Is chosen) The carbon-coated Li _x A _{1 - x} Fe _y according to claim 6, wherein the size of the Li _x A _{1 - x} Fe _y B _{1 -y} (PO ₄ ) _z C _1-z particles is nanosize. B _{1 -y} (PO ₄ ) _{z A} method for preparing a C _1-z composite. The method of claim 6, wherein the solvent is isopropyl alcohol or ethanol, characterized in that the carbon coated Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z composite. The carbonized Li _x A _{1 -x} Fe _y B _1-y (PO) method according to claim 6, wherein the carbonization by heat treatment is performed at 400 to 1000 ° C. for 0.5 to 3 hours in an inert gas atmosphere. ₄ ) Preparation of _z C _1-z composite. In a lithium secondary battery provided with a negative electrode, a positive electrode containing a positive electrode active material and an ion conductor, The cathode active material is a lithium secondary characterized in that it comprises a carbon coated Li _x A _{1- x} Fe _y B _1-y (PO ₄ ) _z C _1-z composite of any one of claims _{1 to 4} . battery. The lithium secondary battery of claim 10, wherein the cathode further comprises carbon black (Super P Black) as a conductive material. The lithium secondary battery of claim 10, wherein the cathode further comprises a binder and a current collector. The lithium secondary battery of claim 10, wherein the ion conductor is an electrolyte or a polymer electrolyte.

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