CN102623707A - A kind of ferric fluoride cathode material doped with cobalt and coated with carbon and preparation method thereof - Google Patents

Abstract

The invention discloses a cobalt-doped carbon-coated ferric fluoride anode material and a preparation method thereof. The chemical formula of the ferric fluoride anode material of the cobalt coated carbon is Fe(1-x)CoxF3(H2O)0.33/C. The preparation method comprises the steps of adding iron source and cobalt source at the normal temperature into a hydrogen fluoride solution, and stirring the mixture to react in a sealed teflon reactor; continuously stirring the hydrogen fluoride solution to react after the temperature is raised, carrying out sucking and filtering, washing the obtained product with ethanol, drying the obtained product after washing, and carrying out heat treatment in vacuum to obtain cobalt-doped ferric fluoride powder; and pelletizing the cobalt-doped ferric fluoride powder and acetylene black, and carrying out heat treatment in vacuum to obtain the cobalt-doped carbon-coated ferric fluoride anode material. The preparation method disclosed by the invention has the following technical effects that (1) the prepared Fe(1-x)CoxF3(H2O)0.33/C has a complete orthorhombic system structure, the grain diameter of the ferric fluoride anode material of the cobalt coated carbon is small, and the ferric fluoride anode materials of the cobalt coated carbon are uniformly distributed; (2) the materials of Fe(1-x)CoxF3(H2O)0.33/C has favorable electrochemical cycling performance; and (3) the preparation method has the advantages of low required temperature and low cost, and is easy to industrially popularize.

Description

A kind of ferric fluoride cathode material doped with cobalt and coated with carbon and preparation method thereof

technical field

The invention mainly discloses a cobalt-doped and carbon-coated ferric fluoride positive electrode material and a preparation method thereof, and the material can be used as a lithium ion battery positive electrode material.

Background technique

Energy is the driving force for the development of the national economy, and it is also an important indicator to measure the comprehensive national strength of the country, the degree of development of the country's civilization and the living standards of the people. The development of human society is inseparable from the emergence of high-quality energy and the use of advanced energy technologies. In today's world, energy and the environment are common concerns of the whole world and all mankind. Chemical power sources (batteries) have become ideal alternative energy sources because of their convenience, low price and environmental friendliness. With the development of high technology and the improvement of people's living standards, the manufacturing technology of lithium-ion batteries is improving day by day, and the cost of batteries is gradually decreasing. Therefore, lithium-ion batteries will become the mainstream of chemical power sources, and their application prospects are very broad. From the perspective of the development of the entire positive and negative electrode materials, the key factor restricting the improvement of the energy density of lithium-ion batteries is the positive electrode material. Cathode material is an important part of lithium-ion batteries, accounting for about 46% of the cost of lithium-ion batteries, and its performance and price directly affect the performance and price of lithium-ion batteries. The positive electrode material also needs to additionally bear the irreversible capacity loss of the negative electrode material. Therefore, the research and improvement of positive electrode materials has always been the key to the research of lithium ion battery materials.

At present, the most widely used positive electrode materials are mainly layered structure lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), spinel type (LiMn ₂ O ₄ ) and layered structure lithium manganese oxide (LiMnO ₂ ), binary transition metal oxides, ternary transition metal oxides, high voltage Li[M _0.5 Mn _1.5 ]O ₄ and lithium-rich xLi ₂ MnO ₃ ·(1-x)LiMO ₂ , etc.

LiCoO ₂ is a typical representative of layered transition metal oxide cathode materials. Its working voltage is about 3.9V (vs. Li ⁺ /Li), and it has the advantages of stable discharge, suitable for large current discharge, high specific energy, and good cycle performance. It is the most widely used cathode material for lithium-ion batteries. But in Li _x CoO ₂ , x=0.5-1, that is, there are still 0.5 Li ⁺ electrons not participating in the electrode reaction during the charging and discharging process. Although its theoretical specific capacity is 274mAh/g, its actual capacity is only about 140mAh/g . Due to the scarcity and high price of cobalt resources, and the safety of _LiCoO2 is also an important issue in high-current applications, the application range of cobalt-based lithium-ion batteries is greatly limited, especially in electric vehicles and large-scale reserve power sources.

The chemical properties of nickel and cobalt are basically similar, but the price is much lower than that of cobalt. LiNiO ₂ has the same crystal structure as LiCoO ₂ , its theoretical reversible specific capacity is 275mAh·g ^-1 , and its actual capacity is over 170mAh·g ^-1 . LiNiO ₂ has good high temperature stability, low self-discharge rate, good compatibility with various electrolytes, does not pollute the environment, is relatively rich in resources and affordable, but the preparation process of LiNiO ₂ is too complicated, making its application restricted. The main reason is that the stoichiometric _LiNiO2 is easy to decompose under high temperature conditions, and the excess Ni2 ⁺ is in the lithium layer between the _NiO2 planes, which hinders the diffusion of lithium ions and affects the electrochemical activity of the material. At the same time, because Ni ³⁺ is more difficult to obtain than Co ³⁺ , the synthesis of LiNiO ₂ needs to be carried out in an oxygen atmosphere, and the conditions are harsh.

Compared with nickel and cobalt, lithium manganese oxide has the advantages of low price and abundant resources. Currently developed are spinel lithium manganese oxide and layered lithium manganese oxide, which have incomparable physical and chemical properties of other positive electrode materials, and their thermal safety and overcharge resistance are excellent, but LiMn ₂ O _4. During use, the dissolution of Mn can easily lead to defects in the crystal lattice, which makes the crystal structure disordered and blocks the intercalation and deintercalation channels of lithium ions, resulting in a rapid capacity decay of the material. Especially at high temperature (above 45°C), the cycle performance drops sharply and the storage performance is poor, which limits its wide application.

The theoretical capacity of olivine-type LiFePO ₄ is 170mAh/g, and its thermal stability, safety and cycle life are good, but there are also two defects: ①The electronic conductivity is very small, which is not conducive to reversible reactions, especially for high-rate discharges. ②The diffusion of Li ⁺ in the material is slow. In addition, it is not easy to synthesize ideal LiFePO ₄ , and it is necessary to prevent Fe ²⁺ from being oxidized to Fe ³⁺ during synthesis.

Li[Ni _0.5 Mn _0.5 ]O ₂ is a typical representative of binary transition metal oxide cathode materials. It has both the advantages of Mn-based and Ni-based materials. The characteristics can be summarized as follows: (1) The synthesis conditions are relatively mild, and the material cost is relatively low. Low; (2) It has a high discharge specific capacity and can be charged to a potential of 4.5V or higher to obtain a capacity of about 180mAh/g, but the cycle performance is not ideal enough.

Li[M _0.5 Mn _1.5 ]O ₄ high-voltage cathode material has a theoretical capacity of about 150mAh/g and a high-voltage platform of 4.7V, and its energy density is about 20% higher than LiCoO ₂ , which is a very promising high-voltage material. , high specific energy spinel cathode material, but its synthesis method needs to be further studied.

Other positive electrode materials for lithium-ion batteries, such as LiNi _1/3 Mn _1/3 Co _1/3 O ₂ , vanadium series compounds, organic sulfur compounds and other materials are being actively studied at home and abroad. There are many beneficial results, some of which have been applied in industry, but the other part of materials still need to be further perfected and deepened before they can be applied in industry.

As anode materials for lithium secondary batteries, metal fluorides are a promising class of cathode materials for lithium batteries. Due to the high electronegativity of fluorine, the working voltage of metal fluoride cathode materials is much higher than other cathode materials such as metal oxides and metal sulfides.

Most of the metal fluoride MeF ₃ has a PdF ₃ -ReO ₃ type structure, and PdF ₃ -ReO ₃ type MeF ₃ has a structure that allows insertion/extraction of lithium ions. Metal fluoride and lithium metal can not only perform lithium ion intercalation/deintercalation reactions, but also undergo reversible chemical conversion reactions with lithium as shown in formula (1) to store energy.

The reversible chemical conversion reaction is the reversible redox reaction between the cathode material and lithium metal, and its essence is a chemical replacement reaction. This reversible chemical conversion reaction can make full use of various oxidation states of substances in the oxidation-reduction process, exchange all electrons in the material, and its released capacity is much higher than the traditional lithium ion intercalation/deintercalation reaction.

However, metal fluorides have been largely ignored, mainly because of their strong ionic bond characteristics and large energy band gaps, which lead to their poor electronic conductivity. Iron fluoride has attracted much attention due to its cheapness, non-toxicity, and environmental protection. It is one of the most studied metal fluoride cathode materials for lithium secondary batteries. Fe-F compounds have the advantages of high energy density, high voltage, good charge and discharge performance, low cost, high theoretical capacity, etc., so they are very promising cathode materials. In recent years, foreign Amatucci research group, Professor Maier research group, Tarascon research group and Fudan University Professor Fu Zhengwen research group have conducted research on it.

However, FeF ₃ is a wide bandgap insulator, which severely limits its practical commercial application. In order to improve its electrical conductivity, currently FeF ₃ is mainly mixed with conductive materials to improve its electrical conductivity. Amatucci GG et al. prepared carbon-coated nano-FeF ₃ /C composites to improve the conductivity of FeF ₃ , but its cycle performance was still not improved. In order to better shorten the band gap of FeF ₃ and improve its conductivity, It can be achieved by anion-doping and cation-doping _FeF3 . Since the radius of metal Co is similar to that of Fe, Co doping can effectively improve the conductivity of FeF ₃ . With the incorporation of Co, the band gap of _FeF3 will be reduced and the conductivity will be enhanced, thus the electrochemical cycle performance of the material will be significantly improved. FeF ₃ (H ₂ O) _0.33 contains a small amount of crystal water, which plays a role in stabilizing the material structure in the material, and is considered to have better structural stability and electrochemical performance than FeF ₃ .

The research work on this material has made some progress at home and abroad, but its work is far from reaching the application level. The currently reported FeF ₃ (H ₂ O) _0.33 material with the best electrochemical performance is a low-temperature ionic liquid phase method, using 1-butyl-3 methylimidazolium tetrafluoroborate (BmimBF4) ionic liquid as the solvent and soft template, react with Fe(NO ₃ ) ₃ 9H ₂ O, insert FeF ₃ material on the single-walled carbon nanotube (SWNT), and the prepared SWNT/FeF ₃ (H ₂ O) _0.33 composite material, but the cycle performance Not very good, its capacity retention rate is only 70% after 50 cycles, its conductivity and battery life need to be further improved, especially how to further improve electronic conductivity and ion conductivity will affect the application of this type of material The essential.

Contents of the invention

The technical problem to be solved by the present invention is to provide a cobalt-doped carbon-coated ferric fluoride cathode material and a preparation method thereof for the poor electrochemical cycle performance of FeF ₃ (H ₂ O) _0.33 .

The technical solution of the present invention is a cobalt-doped and carbon-encapsulated ferric fluoride cathode material, the chemical expression of which is: Fe _1-x Co _x F ₃ (H ₂ O) _0.33 /C, where 0<x≤0.1 , the mass percentage of C in the material is 5%-20%; the C is acetylene black.

A method for preparing a cobalt-doped carbon-coated ferric fluoride positive electrode material. At normal temperature, iron source and cobalt source are taken, hydrogen fluoride solution is added, and the reaction is stirred in a sealed polytetrafluoroethylene reactor; after the temperature rises, the stirring reaction is continued, and the Filter, wash with ethanol, dry, and heat treat in vacuum to obtain cobalt-doped ferric fluoride powder; ball mill the obtained cobalt-doped ferric fluoride powder with acetylene black, and obtain cobalt-doped carbon-encapsulated fluoride after heat treatment in vacuum. iron cathode material.

The amount of iron source and cobalt source added is determined according to the atomic molar ratio of Fe and Co being 1-X:X, wherein 0<x≤0.1.

The iron source is one of Fe ₂ O ₃ and Fe(OH) ₃ , the cobalt source is one of Co ₂ O ₃ , Co ₃ O ₄ and CoCO ₃ , and the concentration of the hydrogen fluoride solution is 20-40 wt%.

The stirring reaction time at normal temperature is 2-6 hours, the temperature after heating is 70-80° C., and the stirring reaction time after heating is 12-36 hours.

After the ethanol washing, the drying temperature is 60-80°C.

The time for the ball milling is 1-6 hours, the heat treatment time after the ball milling is 2-6 hours, and the temperature is 150-180°C.

The present invention has the following technical effects: (1) the prepared Fe _1-x Co _x F ₃ (H ₂ O) _0.33 /C has a complete orthorhombic crystal structure, small particle size and uniform distribution; (2) Fe The _1-x Co _x F ₃ (H ₂ O) _0.33/ C material has excellent electrochemical cycle performance; (3) This preparation method requires low temperature and low cost, and is easy to industrialize.

Description of drawings

Fig. 1 is a scanning electron micrograph of Fe _0.95 Co _0.05 F ₃ (H ₂ O) _0.33 /C prepared by the present invention.

Fig. 2 is an X-ray diffraction spectrum of Fe _0.95 Co _0.05 F ₃ (H ₂ O) _0.33 /C prepared in the present invention.

Figure 3 shows that Fe _0.95 Co _0.05 F ₃ (H ₂ O) _0.33 /C prepared by the present invention is used as the positive electrode material, and the lithium sheet is used as the negative electrode material, assembled into a button battery, and charged and discharged at 0.1C and 1.0C rates at room temperature The first charge-discharge curve.

Fig. 4 is that Fe _0.95 Co _0.05 F ₃ (H ₂ O) _0.33 /C prepared by the present invention is the positive electrode material, and the lithium sheet is the negative electrode material, assembled into a button battery, at room temperature with 0.1C and 1.0C magnification Cycle life curve.

Detailed ways

The present invention will be further described below in conjunction with specific embodiment

Example 1:

Weigh about 10g of Fe ₂ O ₃ and Co ₂ O ₃ according to the molar ratio of n(Fe):n(Co)=0.95:0.05. After mixing evenly, add 40wt% HF, put it into a sealed polytetrafluoroethylene container, and stir at 25°C for 4h. Then the temperature was raised to 75° C. to fully react until a pink solid was formed. The obtained material was washed with ethanol three times and dried at 60° C. in an air atmosphere for 10 h, and then the material was vacuum-dried at 160° C. for 5 h. Fe _0.95 Co _0.05 F ₃ (H ₂ O) _0.33 product with a purity greater than 96% was obtained, mixed with 15wt% acetylene black, ball milled for 3 hours, kept in vacuum at 150°C for 6 hours, and passed through a 400-mesh sieve to obtain Fe _0.95 Co _0.05 F ₃ (H ₂ O) _0.33 /C composite material.

Example 2:

Weigh about 10g of Fe ₂ O ₃ and Co ₃ O ₄ according to the molar ratio of n(Fe):n(Co)=0.97:0.03, after mixing evenly, add 30wt% % HF, put it into a sealed polytetrafluoroethylene container, and stir at 30°C for 3h. Then the temperature was raised to 80°C to fully react until a gray-black solid was formed. The obtained material was washed with ethanol three times and dried at 80°C for 6 hours in an air atmosphere, and then the material was vacuum-dried at 170°C for 4 hours. Fe _0.97 Co _0.03 F ₃ (H ₂ O) _0.33 product with a purity greater than 96% was obtained, mixed with 20wt% acetylene black, ball milled for 3 hours, kept in vacuum at 160°C for 5 hours, and passed through a 400-mesh sieve to obtain Fe _0.97 Co _0.03 F ₃ (H ₂ O) _0.33 /C composite material.

Example 3:

Weigh about 10 g of industrial Fe(OH) ₃ and CoCO ₃ at a molar ratio of n(Fe):n(Co)=0.92:0.08, add 20 wt% HF at a molar ratio of 1:8, and put them into a sealed polytetrafluoroethylene In an ethylene container, stir at 25°C for 6h to make it evenly mixed. Then the temperature was raised to 75°C to fully react until a pink solid was formed, the resulting material was washed with ethanol three times and dried at 80°C for 6 h in an air atmosphere, and then the material was vacuum-dried at 180°C for 2 h. The product is taken out, crushed, and passed through a 400-mesh sieve to obtain a Fe _0.92 Co _0.08 F ₃ (H ₂ O) _0.33 product with a purity greater than 96%. It was ball milled with 15wt% acetylene black for 3 hours, kept in vacuum at 180°C for 2 hours, and passed through a 400-mesh sieve to obtain a Fe _0.92 Co _0.08 F ₃ (H ₂ O) _0.33 /C composite material.

As shown in Figure 1, it can be seen from the figure that the prepared Fe _0.95 Co _0.05 F ₃ (H ₂ O) _0.33 /C has regular surface morphology, small grains and uniform distribution, and this fine and uniform structure It is beneficial to the intercalation and extraction of Li+, which is conducive to improving the electrochemical performance of the material.

As shown in Figure 2, it can be seen from the figure that the prepared Fe _0.95 Co _0.05 F ₃ (H ₂ O) _0.33 /C has a perfect orthorhombic crystal structure, sharp diffraction peaks, and almost no other impurities.

As shown in Figure 3, the Fe _0.95 Co _0.05 F ₃ (H ₂ O) _0.33 /C prepared by the present invention is used as the positive electrode material, and the lithium sheet is used as the negative electrode material, which is assembled into a button battery. The first charge and discharge curve of charging and discharging at the same rate. It can be seen from the figure that compared with the lithium negative electrode, Fe _0.95 Co _0.05 F ₃ (H ₂ O) _0.33 /C can provide a discharge platform of 3.2-2.8V, and the first discharge specific capacity is as high as 223.7mAh/g, which is close to the theoretical capacity (237mAh/g).

As shown in Figure 4, the Fe _0.95 Co _0.05 F ₃ (H ₂ O) _0.33 /C prepared by the present invention is used as the positive electrode material, and the lithium sheet is used as the negative electrode material, which is assembled into a button battery. The cycle life curve under the magnification. It can be seen from the figure that the battery assembled with Fe _0.95 Co _0.05 F ₃ (H ₂ O) _0.33 /C as the positive electrode material is charged and discharged, and the capacity retention rate is as high as 91% after 100 cycles.

Claims (7)

1. ferric fluoride anode material of mixing cobalt bag carbon, its chemical expression is: Fe _1-xCo _xF ₃(H ₂O) _0.33/ C, 0＜X in the formula≤0.1, the quality percentage composition 5%-20% of C in this material; C is an acetylene black.

2. a preparation method who mixes the ferric fluoride anode material of cobalt bag carbon is characterized in that: under the normal temperature, get source of iron, cobalt source, add hydrogen fluoride solution, stirring reaction in the polytetrafluoroethylene reactor of sealing; Intensification continued stirring reaction, suction filtration is used washing with alcohol, oven dry, heat treatment in a vacuum obtains mixing cobalt ferric flouride powder; Gained is mixed cobalt ferric flouride powder and acetylene black ball milling, obtain mixing the ferric fluoride anode material of cobalt bag carbon in a vacuum after the heat treatment.

3. the preparation method who mixes the ferric fluoride anode material of cobalt bag carbon according to claim 2 is characterized in that: the amount that adds source of iron and cobalt source is that 1～X: X confirms according to the atomic molar ratio of Fe and Co, wherein 0＜X≤0.1.

4. the preparation method who mixes the ferric fluoride anode material of cobalt bag carbon according to claim 2 is characterized in that: said source of iron is Fe ₂O ₃, Fe (OH) ₃In a kind of, the cobalt source is Co ₂O ₃, Co ₃O ₄, CoCO ₃In a kind of, the concentration of hydrogen fluoride solution is 20～40wt%.

5. the preparation method who mixes the ferric fluoride anode material of cobalt bag carbon according to claim 2; It is characterized in that: the time of said stirring reaction at normal temperatures is 2～6 hours; Temperature after the intensification is 70～80 ℃, and the time of the stirring reaction after the intensification is 12～36 hours.

6. the preparation method who mixes the ferric fluoride anode material of cobalt bag carbon according to claim 1 is characterized in that: after the said washing with alcohol, the temperature of oven dry is 60～80 ℃.

7. the preparation method who mixes the ferric fluoride anode material of cobalt bag carbon according to claim 1, it is characterized in that: the time of said ball milling is 1～6 hour, and the heat treatment time behind the ball milling is 2～6 hours, and temperature is 150～180 ℃.

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