KR101063934B1 - Manufacturing Method of Active Material - Google Patents

Abstract

The present invention relates to a method for producing a Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z active material used as a positive electrode active material of a lithium secondary battery, more specifically inexpensive Fe (III) By selecting PO ₄ as the starting material for the active material synthesis, the material cost can be lowered. Formic acid is used as a reducing agent to reduce the process cost by simplifying the synthesis process. Li _x A _1-x Fe _y B _{1- It} relates to a method for preparing _y (PO ₄ ) _z C _1-z active material.

Lithium Secondary Battery Cathode Active Material Formic Acid Reducing Agent LiFePO4

Description

Manufacturing Method of Active Material {Manufacturing Method of Active Material}

The present invention relates to a method of producing active material such as lithium secondary batteries, especially Li _x A ₁ using formic acid as a reducing agent _{- x Fe y B 1 -y (} PO 4) z C 1-z electrode active material and a carbon-coated Li _x A _{1- x} Fe _y B _1-y (PO ₄ ) _z C _1-z relates to a method for producing an active material.

In general, a lithium secondary battery is a broad concept including a lithium ion secondary battery as well as a secondary battery using a lithium metal, and has a high voltage and a high energy density. It is also divided into a battery, a gel polymer battery that uses a mixture of liquid and polymer, and a solid polymer battery using pure polymer.

The core three components of the lithium secondary battery are a positive electrode, a negative electrode, and an electrolyte, and are mainly composed of a positive electrode, a negative electrode, an electrolyte, a separator, and an exterior material.

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, and these materials have electrochemical potentials due to the intercalation / deintercalation of lithium ions into the crystal structure. Is high.

In addition, 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.

In addition, the electrolyte is mainly composed of LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ , LiPF ₆ , LiBF ₄ , LiClO ₄ , A salt containing lithium ions such as LiN (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. In addition, a packaging material composed of a metal can or aluminum and several layers of polymer layers is mainly used as an exterior material that protects the contents of the battery and provides an electrical passage to the outside of the battery.

The application field of the lithium secondary battery has broadened its scope in small electronic products, making portable or mobile electronic products more convenient for daily life, and improving the efficiency of industrial activities.

However, thermal instability and high prices of batteries are still major drawbacks of lithium secondary batteries, which prevent the market from expanding in earnest not only for cell makers but also for consumers. Therefore, there is a demand for the development of a material having a low cost and excellent safety, and a lot of demands for this are in progress.

From this point of view, the positive electrode material is the material that can produce the greatest effect by inducing improvement in price, safety, cost, and the like among the battery materials. 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 doped with a small amount of alkali metal to improve performance. In addition, research is being conducted to improve safety by coating such a cobalt oxide.

However, while cobalt-containing materials have good conductivity and performance, there are high cost and safety problems, and materials that can replace them have been studied. Among the potential candidates, LiFePO ₄ has a theoretical specific capacity of 170mAh / g and, depending on the conditions, may provide near-theoretical capacity. LiCoO ₂ , on the other hand, achieves only about 140 mAh / g, which is about half the theoretical value. LiFePO ₄ is attracting attention because of its superior price and safety.

General LiFePO Synthesis of ₄ particles are _{Fe (SO 4), Fe (} acetate), Fe (oxalate), Fe (NO 3) 2 , such as the Fe ^{2 +} Li salt, in a high temperature electric furnace mixed in an equivalent ratio with PO ₄ salt Firing. The reason for using the Fe ^{2 +} as described above as well as the burden of having to add a process or the material to be reduced to the Fe ^{2 +} The Fe ^{3 +,} Fe ^{3 +} ions are not all reduced part remains, battery This is because the performance as a material is impaired. However, since the Fe ^{2 +} materials as described above are higher in price than the Fe ^{3 +} materials, there is a problem of reducing the attractiveness of LiFePO ₄ as an inexpensive material.

The present invention is to solve the above problems, to provide an improved Li _x A _1-x Fe _y B _1-y (PO ₄ ) _z C _1-z active material, especially LiFePO ₄ particles using formic acid as a reducing agent Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z active material and carbon coated Li _x A _{1 - x} Fe It is an object of the present invention to provide a method for producing a _y B _{1- y} (PO ₄ ) _z C _1-z active material.

In order to achieve the above object, the present invention is to prepare a mixture containing FePO ₄ , lithium salt and reducing agent, to crush the mixture in a ball mill (ball mill) to prepare a slurry and the prepared slurry Method for producing a Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z active material comprising the step of heat treatment under inert gas conditions (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 at least one of anions). The reducing agent is formic acid (formic acid) characterized in that the manufacturing method of Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _{1- z} active material using formic acid as the technical gist.

It is also preferable to further include MgO in the mixture.

In addition, the Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z active material is preferably LiFePO ₄ .

In addition, the heat treatment step is preferably heat treatment for 3 to 6 hours in the range of 550 ℃ to 600 ℃.

In addition, the inert gas is preferably nitrogen or argon.

In addition, the step of mixing Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z active material prepared by the above method with cooking oil; And it is preferable to prepare a Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z active material using a carbon coated formic acid comprising the step of carbonizing by heat treatment in a heat treatment furnace (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 transition metal, and C is selected from at least one anion do.).

In addition, the cooking oil is preferably at least one selected from milk fat, olive oil, soybean oil, butter.

In addition, the carbonization by heat treatment is preferably performed for 1 to 3 hours within the range of 500 ℃ to 600 ℃.

The method for preparing Li _x A _{1 - x} Fe _y B _{1 -y} (PO ₄ ) _z C _1-z active material using formic acid according to the present invention simplifies the synthesis of the active material and uses formic acid as an inexpensive starting material, in particular a reducing agent. By providing a method of reducing the process cost of the active material and the lithium secondary battery, there is an industrially useful effect to secure price competitiveness.

Hereinafter, a method of preparing Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z active material using formic acid according to the present invention will be described in more detail.

First, FePO ₄ , a lithium salt and a reducing agent are mixed to prepare a mixture. When selecting a starting material for the synthesis of Li _x A _1-x Fe _y B _1-y (PO ₄ ) _z C _1-z active material Fe (III) PO ₄ inexpensive instead of selecting the existing Fe ^{2 +} ions Select.

The lithium salt is not particularly limited, but is preferably lithium hydroxide (LiOH). Lithium hydroxide is water soluble and thus not only easy to ionize, but also has high density (1.51 g / cm ² ), which is advantageous for mass production. In addition, although the solubility in water is slightly lower than that of lithium hydroxide described above, lithium carbonate (Li 2 CO 3), which is soluble by adding water to an acid such as polyphosphoric acid and using an acid-base reaction, also belongs to the scope of the present invention. The lithium hydroxide is used as an aqueous solution dissolved in distilled water, wherein the concentration range of the lithium hydroxide aqueous solution is suitably 1 to 3M.

The mixing ratio of the lithium salt and Fe (III) PO ₄ is preferably 1 to 1.5: 1 in an equivalent ratio. In addition, various materials may be used as the reducing agent, and an organic material or an organic acid including C, H, and O is preferably used as a constituent element, and among them, formic acid

(formic acid) is preferred. Such formic acid is less expensive than other organic acids as a reducing agent and is simple in the manufacturing method.

The formic acid participates in the reaction at the time of synthesis of the active material and leaves carbon. Thus, the carbon remaining after participating in the reaction also increases the conductivity of the active material, and thus has an additional function of doping or coating carbon on the produced LiFePO ₄ . The reducing agent is also preferably included in an amount of 1 to 2 equivalents relative to the equivalent of the mixture of the lithium salt and Fe (III) PO ₄ . If the amount of the reducing agent is in the range of 1 equivalent part or less, the effect of adding a reducing agent is insufficient.

Next, the prepared mixture is crushed with a ball mill to prepare a slurry. Alcohol is added when the mixture is crushed with a ball mill, although the alcohol to be added is not particularly limited, but ethanol is preferred. In addition, the volume of the added alcohol is preferably 2 to 2.5 times the volume of the lithium salt, Fe (III) PO ₄ and the reducing agent.

In addition, the mixture is not particularly limited, but preferably crushed in a ball mill for 1.5 days, and in the case of crushing a slurry through a ball milling method as described above, a nano-sized uniform particle size Can be obtained.

The size and shape (determined) of the particles having a great influence on the performance of the active material is LiFePO ₄ And its analogs are very poor in conductivity of the particles, the particles of the micron unit is significantly reduced in performance, in particular due to the rapid deterioration in high current discharge (high rate discharge) is a major obstacle to commercialization. These shortcomings can be improved in many ways by particle size or aging. Since the movement distance of the ions during charging and discharging is shortened through the nanoparticles of particles, the space of most active materials is used for charging and discharging, so that the specific capacity is increased and the high-rate characteristic can be greatly improved. Here, nano-size means 1 ~ 999nm, and includes the meaning commonly used in the art.

Next, Li _x A _{1- x} Fe _y B _1-y (PO ₄ ) _z C _1-z active material is prepared by heat-treating the prepared slurry under inert gas conditions. Here, 0 <x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 1, and A is an ion capable of substituting a part of Li ions, and includes sodium (Na), potassium (K), and rubidium (Rb). At least one of an alkali metal such as magnesium and an alkaline earth metal such as magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba) is selected. B is an ion capable of substituting some or all of Fe ions, and is selected from at least one of transition metals such as manganese (Mn), chromium (Cr), vanadium (V), nickel (Ni), copper (Cu), and the like. , C is an ion capable of substituting part or all of the PO ₄ ions, at least one selected from anions such as SO ₄ , NO ₃ , O ₂ and the like. The values of x, y, and z also include a range that can be seen as an error range and an equivalent range.

As the doping material for doping the A, B or C, it is preferable to use alkaline earth metal. MgO, SrCO ₃ , TiO _2, etc. can be added as a raw material using alkaline earth metals, and MgO is particularly preferable. For example, when MgO is added to prepare Li _x A _1-x Fe _y B _1-y (PO ₄ ) _z C _1-z active material, part of Li ions are partially substituted with Mg ions, which are alkaline earth metals, and Mg is Trace doped Li _x A _{1 - x} Fe _y B _{1 -y} (PO ₄ ) _z C _{1 -z} active materials can be prepared.

As described above, when a doping element such as Mg is used, a passage through which lithium ions move well during charging and discharging of the battery is secured, so that the initial capacity of the battery is good and the capacity does not decrease rapidly even after several cycles. In addition, the doping element functions to smoothly change the crystal structure in the process of changing FePO ₄ to LiFePO ₄ .

The heat treatment is not limited, but is preferably performed for 3 to 6 hours in an inert gas atmosphere at a temperature of 550 ℃ ~ 600 ℃. Nitrogen or argon may be used as the inert gas. This is to prevent oxidation of the metal (Fe). If the temperature of the heat treatment is lower than 500 ℃ crystalline material is difficult to form.

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

Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z active material according to the present invention is a method for producing a Li _x A _{1 - x} Fe _y B _{1 -y} (PO ₄ ) _{z The} C _1-z active material is mixed with cooking oil and stirred, and heat treatment in a heat treatment furnace comprising the step of carbonizing. Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z active material and the like as described above are the same as the above _- described bar dedicated and will not be described.

First, Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z active material prepared by the above method is mixed with cooking oil and stirred. The mixed cooking oil is preferably at least one selected from olive oil, soybean oil, butter, milk (fat milk), the weight of the Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z active material It is preferable to mix in 2 to 4 parts by weight. Below the above range, the coating effect is insignificant, and beyond the above range, the adverse effect of the carbon coating may occur. The cooking oil may be used after dissolving in a solvent such as alcohol, in particular isopropyl alcohol or ethanol. At this time, the amount of the solvent to be used is sufficient to dissolve the cooking oil.

Next, the carbon coating is completed by carbonization of the Li _x A _1-x Fe _y B _1-y (PO ₄ ) _z C _1-z active material mixed with the cooking oil by heat treatment in a heat treatment furnace and stirred. The heat treatment is not limited, but may be performed for 1 to 3 hours in an inert gas atmosphere at a temperature of 500 ℃ ~ 600 ℃. Nitrogen or argon may be used as the inert gas. This is to prevent oxidation of the metal (Fe) and carbon. The carbon coated Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _{1 -z} active material is improved in conductivity compared to before carbon coating.

Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z active material or carbon coated Li _x A _1-x Fe _y B _1-y (PO ₄ ) _z C _{1 The -z} active material may be used as an electrode active material, preferably a cathode active material of a lithium secondary battery, and the Li _x A _{1 - x} Fe _y B _{1 -y} (PO ₄ ) _z C _1-z active material according to a conventional method After mixing with a binder to prepare an electrode slurry, the electrode can be prepared by coating the prepared electrode slurry on a current collector. As a manufacturing method of such an electrode, the conventional method known in the art can be used, and is not particularly limited.

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

<Examples>

(a) Preparation of Carbon Coated LiFePO ₄ Particles

First, LiOH and FePO ₄ were mixed in an equivalent ratio of 1.1: 1, 1 equivalent part of formic acid was mixed with FePO ₄ , and the ethanol was then mixed with two volumes of the entire mixture. Then, it was crushed into a ball mill for 1.5 days to obtain a slurry having a uniform particle size of nano size. The prepared slurry was heat-treated at 570 ° C. for 5 hours in an electric furnace to synthesize LiFePO ₄ particles. At this time, argon gas was used to prevent oxidation of Fe.

The synthesized LiFePO ₄ particles and 3 parts by weight of soybean oil to 100 parts by weight of the particles were mixed, and heat-treated at 550 ° C. for 1 hour in an electric furnace to synthesize carbon-coated LiFePO ₄ particles. At this time, argon gas was used as the electricity.

(b) Preparation of Lithium Secondary Battery

Coin cells were prepared to observe the performance of the prepared carbon-coated LiFePO ₄ particles 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 for 10 minutes in an appropriate amount of N-methylpyrrolidone (NMP) and a conditioning mixer, and then the active material and the conductive material are mixed therein, and then a little N-methylpyrrolidone (NMP) is added. After further addition to adjust the viscosity, the mixture was further stirred for 30 minutes. The A1 current collector was laid on a glass plate, and the slurry prepared on the A1 current collector was cast by a doctor blade method, and then dried overnight at 100 ° C. The electrode thus dried was pressed at a compression ratio of 20-25% with a roll press machine at 120 ° C. to complete the electrode.

A coin cell was fabricated to evaluate the characteristics of the manufactured 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 LiPF ₆ , EC / DEC (1: 1).

1 is a graph showing the results of measuring capacity and high rate discharge characteristics by fabricating an electrode using a carbon-coated LiFePO ₄ active material synthesized according to an embodiment of the present invention. Although less than 170mAh / g of theoretical capacity, it provided 130mAh / g at 0.2C discharge rate, which can be improved by additional coating and doping. It can be seen that under the condition of no coating and doping, the performance of the carbon-coated LiFePO ₄ particles as an active material is excellent.

Therefore, in the case of using LiFePO ₄ particles as the cathode material of the lithium secondary battery using formic acid as a reducing agent according to the present invention, it can contribute to the reduction in unit cost and reliability of the lithium secondary battery.

1-a graph showing the results of measuring capacity and high rate discharge characteristics by fabricating an electrode using a carbon-coated LiFePO ₄ active material synthesized according to an embodiment of the present invention.

Claims (8)

Preparing a mixture comprising FePO ₄ , a lithium salt, and a reducing agent, adding alcohol to the mixture at 2 to 2.5 times the volume of the reducing agent, and crushing the mixture in a ball mill for 1.5 days to prepare a slurry; and Method for producing a Li _x A _1-x Fe _y B _1-y (PO ₄ ) _z C _1-z active material comprising the step of heat-treating the prepared slurry under inert gas conditions (in the above, 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 at least one selected from anions). Lithium salt is LiOH, the reducing agent uses formic acid (formic acid), Li _x A _1-x Fe _y B _1-y (PO ₄ ) characterized in that the mixture further comprises an alkaline earth metal as a doping material _z C _1-z manufacturing method. delete The Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z active material is LiFePO ₄ , characterized in that Li _x A _{1 - x} Fe _y B _{1 -y} ( PO ₄ ) _z C _1-z active material production method. The method of claim 1, wherein the heat treatment is Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1- characterized in that the heat treatment for 3 to 6 hours in the range of 550 ℃ to 600 ℃. _z Manufacturing method of active material. The method according to claim 1, wherein the inert gas is nitrogen or argon. Li _x A _1-x Fe _y B _1-y (PO ₄ ) _z C _1-z active material manufacturing method. The method of claim 1 and the method of any one of claims 3 to 5 Li _x A _1-x Fe _y B _1-y (PO ₄ ) _z C _1-z active material mixed with cooking oil and stirring step ; And carbonizing by heat treatment in a heat treatment furnace, the method for producing a carbon-coated Li _x A _1-x Fe _y B _1-y (PO ₄ ) _z C _1-z active material (wherein, 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 at least one anion). The carbon-coated Li _x A _{1 - x} Fe _y B _{1- y} (PO ₄ ) _z C _1-z characterized in that the cooking oil is at least one selected from milk fat, olive oil, soybean oil, butter. Method for producing an active material. The carbon-coated Li _x A _{1 - x} Fe _y B _{1 -y} (PO ₄₎ according to claim 6, wherein the carbonization by heat treatment is performed for 1 to 3 hours in a range of 500 ° C. to 600 ° C. _8. ) _z C _1-z active material production method.

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