CN101847711A - Porous carbon coated ferrous silicate lithium anode material and preparation method thereof - Google Patents

Abstract

The invention relates to a porous carbon-coated lithium ferrous silicate positive electrode material and a preparation method. Polyethylene glycol is used as a pore-forming agent to form a gel through hydrolysis and condensation of ethyl orthosilicate. During the calcination process of the precursor, due to polyethylene glycol Diol pyrolysis forms a continuous skeleton and through macropores/mesoporous channels. The average pore diameter of macropores is 0.5-3.9 μm, the average pore diameter of mesopores is 18-40 nm, and the average porosity is 57.2-71.9%. The three-dimensional network structure is formed through the hydrolysis and condensation of ethyl orthosilicate, which reduces the migration distance and activation energy necessary for the migration of ions during lattice reorganization, which is beneficial to reduce the reaction temperature and shorten the reaction time. A product that conforms to the stoichiometric ratio. The porous structure increases the specific surface area of the material, which is not only conducive to the penetration of the electrolyte in the particles, but also improves the probability of lithium ion deintercalation inside the crystal, improves the utilization rate of lithium ions, and avoids the growth of the crystal grains during the calcination process. The aggregation during charging and discharging improves the electrochemical performance of the positive electrode material.

Description

Porous carbon-coated lithium ferrous silicate positive electrode material and preparation method

technical field

The invention relates to a porous carbon-coated lithium ferrous silicate positive electrode material and a preparation method, belonging to the technical field of energy material preparation.

Background technique

With the rapid growth of human demand for energy, energy has become an important problem encountered in the economic development of countries all over the world, which has led to urgent problems such as energy crisis and environmental pollution. In energy development, it is of great significance to make full use of natural forces such as wind energy, tidal energy, and solar energy. Due to the discontinuity of these energy sources, to use these natural energies on a large scale requires a matching energy storage device. Lithium-ion battery is a new type of secondary battery. Due to its advantages such as high voltage, high capacity, small size, light weight, long cycle life, good safety performance, and no memory effect, it has become the object of development all over the world. Li ₂ FeSiO ₄ has the advantages of high specific capacity (theoretical capacity is 166mAh/g), good safety, non-toxicity, low price, abundant resources, etc. Material.

Orthosilicate Li ₂ FeSiO ₄ has a structure similar to that of low-temperature Li ₃ PO ₄ , belongs to the orthorhombic crystal system, and the space group is Pmn2 ₁ , in which the oxygen atoms are arranged in a regular tetrahedral close-packed manner, and Fe and Si are each in the oxygen atom The center position of the tetrahedron. The currently reported Li ₂ FeSiO ₄ preparation methods mainly include: (1) High-temperature solid-phase method: A. Nyten et al. used lithium silicate, ferrous oxalate and ethyl orthosilicate as raw materials, and after fully grinding and mixing with C gel, Li ₂ FeSiO ₄ was synthesized by heating to 750°C in CO/CO ₂ airflow (50:50) and keeping it warm for 24h. Electrochemical test results show that: at 60°C, C/16 rate, 2.0-3.7V voltage range, the discharge capacity is stable at about 130mAh/g, showing good charge-discharge reversibility, and the capacity retention rate after 10 cycles was 84%. Chinese patent CN101582495A uses a microwave synthesis method to prepare lithium ion battery cathode material Li ₂ FeSiO ₄ , and obtains a Li ₂ FeSiO ₄ /C composite cathode material, and the discharge specific capacities at 10mA/g and 30mA/g are 119.6mAh/g and 103.7 mAh/g. Japanese patent JP2001266882 (A) uses FeO and Li ₂ SiO ₃ as raw materials, ball mills for 4 hours, and then calcines at 800° C. for 4 hours to obtain the target product Li ₂ FeSiO ₄ . However, because the raw materials are only mechanically mixed in this method, long-term calcination at high temperature is required, resulting in coarser particle size of the product, wide particle size distribution range, more impurity phases, and insufficient electrochemical activity. (2) Hydrothermal synthesis method: R.Dominko et al use lithium hydroxide, colloidal silicon dioxide and ferrous chloride tetrahydrate as raw materials, and uniformly disperse lithium hydroxide and colloidal silicon dioxide in ferrous chloride solution, then transferred to a closed stainless steel autoclave, and reacted at a constant temperature of 150°C under _N2 atmosphere for 14d, and then the obtained green powder was repeatedly washed with distilled water under an argon atmosphere, and then dried at 50°C for 1d to obtain Li ₂ FeSiO ₄ powder. C/30 rate at room temperature, charge and discharge in the voltage range of 2.0-4.2V, the reversible capacity is about 91mAh/g. The electrochemical performance of the product prepared by this method is poor, the reaction time is long, and it is not conducive to ion doping modification. (3) Sol-gel method: R.Dominko et al. proposed to use a mixture of ferric citrate and ferric nitrate, lithium acetate and colloidal SiO with a molar ratio of 1: ₁ as raw materials to combine lithium acetate and iron (III) ions After the mixture was dissolved in water respectively, the three were mixed and stirred for 1 h, and preserved to form a sol. The obtained sol was dried at 60 °C for more than 24 h. After grinding, react the obtained powder under an inert atmosphere at 700° C. for 1 h and then cool to room temperature to obtain Li ₂ FeSiO ₄ . The reversible capacity retention rate is 75% after 10 cycles at 60°C and C/2 rate. The relevant patent applied by Yang Yong et al. proposes: add lithium acetate, ferrous oxalate and ethyl orthosilicate to 150ml of ethanol, react in an oil bath at 80°C for 20h, transfer to an evaporating dish and dry at 100°C. The obtained mixture was transferred to an agate ball mill jar, sucrose was added, acetone was used as a dispersant, and ball milled for 5 hours. After the acetone is volatilized, it is transferred to a porcelain boat and placed in a tubular resistance furnace under the protection of N ₂ , heat-treated at 650°C for 10 hours, and cooled naturally to room temperature to obtain a Li ₂ FeSiO ₄ composite material. In addition, the Chinese patent CN101499527A firstly prepared the ferrosilicon blend by precipitation method, washed and dried, then mixed with lithium source compound and organic carbon source for ball milling, and dried to obtain the precursor material of Li ₂ FeSiO ₄ , and then protected The Li ₂ FeSiO ₄ cathode material is obtained by calcining at low temperature under the atmosphere. In this method, the soluble ions in the ferrosilicon blend are inevitably washed away during the washing process, resulting in a mismatch of the stoichiometric ratio, thereby affecting the electrochemical performance of the product.

The main reason restricting the development of lithium ferrous silicate as positive electrode material is the low electronic conductivity and small lithium ion diffusion coefficient of lithium ferrous silicate material itself, which seriously affects the charge and discharge current density and specific capacity of the material, limiting the Practical application of lithium ferrous silicate. The current methods for improving the conductivity of lithium ferrous silicate include: doping, surface carbon coating, and grain size refinement, etc., all of which can improve the electrochemical performance of lithium ferrous silicate to a certain extent. In addition, if lithium ferrous silicate is made into a porous material, the porous structure increases the specific surface area of the material, which not only facilitates the penetration of the electrolyte in the particles, but also increases the solid-liquid contact interface area and improves the lithium ion density inside the crystal. The probability of deintercalation also avoids the growth of crystal grains during the calcination process and the aggregation during the charge and discharge process, thereby significantly improving the electrochemical performance of the positive electrode material.

Contents of the invention

The present invention uses polyethylene glycol as a pore-forming agent to form a gel through the hydrolysis and condensation of tetraethyl orthosilicate, and forms continuous skeletons and through macropores/mesopores due to the pyrolysis of polyethylene glycol during the calcination process of the precursor In the pores, the average pore diameter of macropores is between 0.5-3.9 μm, the average pore diameter of mesopores is between 18-40 nm, and the average porosity is between 57.2-71.9%.

The object of the present invention is to provide a method for preparing a porous Li ₂ FeSiO ₄ /C positive electrode material. The porous carbon-coated lithium ferrous silicate powder prepared according to the method has a continuous skeleton and through macropores/intermediates. Hole channel. This structure improves the porosity of the material and increases the specific surface area of the material, which not only facilitates the penetration of the electrolyte in the particles, but also avoids the growth of the grains during the calcination process and the aggregation during the charging and discharging process, thereby improving Electrochemical properties of cathode materials.

The present invention is achieved through the following technical solutions:

The porous carbon-coated lithium ferrous silicate positive electrode material of the present invention forms a continuous skeleton and through macropores/mesoporous channels, the average pore diameter of the macropores is between 0.5 and 3.9 μm, and the average pore diameter of the mesopores is between 18 and 40 nm. The average porosity is between 57.2% and 71.9%. The calcination temperature in the synthesis process is relatively low, 600° C., and the stoichiometric ratio of the product is accurate.

The preparation method of porous carbon-coated lithium ferrous silicate cathode material of the present invention, the steps are as follows:

1) Lithium acetate dihydrate, ferrous acetate, and ethyl orthosilicate are used as raw materials, and the corresponding substances are weighed according to the mass ratio of the substances, so that the mass ratio of Li:Fe:Si is 2:1:1; with ethanol as the solvent, The concentration of ethyl orthosilicate is 0.5-0.8mol/L; the additive polyethylene glycol pore-forming agent, acetic acid or ammonia water catalyst, and ascorbic acid are antioxidants, in which polyethylene glycol accounts for 10% of the mass of theoretically synthesized lithium ferrous silicate. % to 30%, the antioxidant accounts for 3% of the mass of theoretically synthesized lithium ferrous silicate, and the catalyst is made into an aqueous solution of 0.1 to 0.3 mol/L; lithium salt, tetraethyl orthosilicate and polyethylene glycol are dissolved in ethanol , stir and mix evenly, then add ferrous acetate and ascorbic acid to the mixed solution, stir to dissolve, add a catalyst to adjust the pH of the solution system between 2 and 7;

2) Transfer the mixed liquid in step 1) to the reaction kettle, put the reaction kettle into a constant temperature box and react at 120~180°C for 12~20h, and dry the obtained gel mixture in a drying oven at 60~100°C get xerogel;

3) Grinding the xerogel in step 2) and transferring it to a temperature-programmed tube furnace, raising the temperature to 600-750°C at a rate of 10°C·min ^-1 under an argon atmosphere and keeping the temperature constant for 7-10 hours, Cool down to room temperature with the furnace to obtain porous Li ₂ FeSiO ₄ /C powder.

The advantage of the present invention is that a three-dimensional network structure is formed through the hydrolysis and condensation of tetraethyl orthosilicate, various ions in the reactants are fixed in the network structure, and the mixing at the molecular level is achieved, thereby reducing the migration of ions during lattice recombination The distance and the activation energy necessary for migration are beneficial to reduce the reaction temperature and shorten the reaction time, and produce products with high phase purity and stoichiometric ratio. In addition, using polyethylene glycol as a pore-forming agent, the formed grains have a continuous skeleton and through macropores/mesoporous channels, and the pores can be controlled by adjusting the concentration of reactants, pH and the amount of polyethylene glycol added. Distribution and size, this porous structure increases the specific surface area of the material, which not only facilitates the penetration of the electrolyte in the particles, but also increases the area of the solid-liquid contact interface, improves the probability of deintercalation of lithium ions inside the crystal, and improves the lithium ion density. The utilization rate is conducive to improving the specific capacity of the material, and also avoids the growth of crystal grains during the calcination process and the aggregation during the charging and discharging process, thereby improving the electrochemical performance of the positive electrode material.

Description of drawings

The XRD pattern of Li ₂ FeSiO ₄ /C synthesized in Fig. 1 embodiment one;

The SEM figure of Li ₂ FeSiO ₄ /C synthesized in Fig. 2 embodiment one;

Figure 3 is the charge-discharge curve of Li ₂ FeSiO ₄ /C synthesized in Example 1;

Fig. 4 is the cycle performance curve of Li ₂ FeSiO ₄ /C synthesized in Example 1.

Detailed ways

Example 1:

Lithium acetate, ferrous acetate, and ethyl orthosilicate are used as raw materials, and the corresponding substances are weighed according to the ratio of substances, so that the mass ratio of Li:Fe:Si is 2:1:1, and 1.640g of lithium acetate is accurately weighed, 1.8ml of tetraethyl orthosilicate and 0.399g of polyethylene glycol were dissolved in 13.3ml of ethanol so that the concentration of tetraethyl orthosilicate was 0.6mol/L. The obtained solution system was fully stirred for 10 min and mixed evenly. Accurately weigh 1.391g ferrous acetate, accurately weigh 0.039g ascorbic acid, add it to the above solution, stir to dissolve, and adjust the pH value to 7 with 0.3mol/L ammonia water as a catalyst. The mixed solution was transferred to a reaction kettle, and the reaction kettle was placed in a constant temperature oven to react at 120°C for 20 hours, and the obtained gel mixture was dried in a vacuum oven at 60°C to obtain a xerogel.

Under the protection of _N2 atmosphere, the above lithium ferrous silicate precursor was calcined at 600 °C for 10 h to obtain carbon-coated porous lithium ferrous silicate powder. The average pore diameter of the macropores is about 3.9 μm, and the distribution is wide, the average pore diameter of the mesopores is 30 nm, and the average porosity is 71.9%. The XRD pattern of Li ₂ FeSiO ₄ /C is shown in Figure 1. The characteristic peaks of the XRD in the figure correspond to the diffraction peaks of the XRD pattern of the standard Li ₂ FeSiO ₄ powder, indicating that the synthesized powder is Li ₂ FeSiO ₄ . The SEM of Li ₂ FeSiO ₄ is shown in Figure 2. The product has a continuous skeleton and through porous channels, and the average pore size of the macropores is about 3.9 μm. The charge-discharge curves at C/10 rate at 60°C are shown in Figure 3. The initial charge-discharge capacities of the synthesized Li ₂ FeSiO ₄ /C cathode materials are 142.2mAh/g and 138.7mAh/g, respectively. The cycle performance curve is shown in Fig. 4. After 20 cycles of Li ₂ FeSiO ₄ /C, the capacity remains at 128.3mAh/g, and the attenuation is small.

Example 2:

Lithium acetate, ferrous acetate, and ethyl orthosilicate are used as raw materials, and the corresponding substances are weighed according to the ratio of substances, so that the mass ratio of Li:Fe:Si is 2:1:1, and 1.640g of lithium acetate is accurately weighed, 1.8ml of tetraethyl orthosilicate and 0.129g of polyethylene glycol were dissolved in 10.0ml of ethanol so that the concentration of tetraethyl orthosilicate was 0.8mol/L. The obtained solution system was fully stirred for 10 min and mixed evenly. Accurately weigh 1.391g of ferrous acetate and 0.039g of ascorbic acid, add them to the above solution, stir and dissolve, and adjust the pH value to 6 with 0.2mol/L ammonia water as a catalyst. The mixed solution was transferred to a reaction kettle, and the reaction kettle was placed in a constant temperature oven at 140° C. for 17 hours. The obtained gel mixture was dried in a vacuum oven at 70° C. to obtain a xerogel.

Under the protection of _N2 atmosphere, the above lithium ferrous silicate precursor was calcined at 650°C for 9h to obtain carbon-coated porous lithium ferrous silicate powder. The average pore diameter of the macropores is about 0.5 μm, and the distribution is narrow, the average pore diameter of the mesopores is 40 nm, and the average porosity is 57.2%.

Example 3:

Lithium acetate, ferrous acetate, and ethyl orthosilicate are used as raw materials, and the corresponding substances are weighed according to the ratio of substances, so that the mass ratio of Li:Fe:Si is 2:1:1, and 1.640g of lithium acetate is accurately weighed, 1.8ml of tetraethyl orthosilicate and 0.399g of polyethylene glycol were dissolved in 16.0ml of ethanol so that the concentration of tetraethyl orthosilicate was 0.53mol/L. The obtained solution system was fully stirred for 10 min and mixed evenly. Accurately weigh 1.391g of ferrous acetate, accurately weigh 0.039g of ascorbic acid, add it to the above solution, stir and dissolve, and adjust the pH value to 4 with 0.3mol/L ammonia water as a catalyst. The mixed solution was transferred to a reaction kettle, and the reaction kettle was placed in a constant temperature oven at 160° C. for 15 hours. The obtained gel mixture was dried in a vacuum oven at 80° C. to obtain a xerogel.

Under the protection of _N2 atmosphere, the above lithium ferrous silicate precursor was calcined at 700°C for 8h to obtain carbon-coated porous lithium ferrous silicate powder. The average pore diameter of macropores is about 3.4 μm, the average pore diameter of mesopores is 33 nm, and the average porosity is 68.5%.

Example 4:

Lithium acetate, ferrous acetate, and ethyl orthosilicate are used as raw materials, and the corresponding substances are weighed according to the ratio of substances, so that the mass ratio of Li:Fe:Si is 2:1:1, and 1.640g of lithium acetate is accurately weighed, 1.8ml of tetraethyl orthosilicate and 0.258g of polyethylene glycol were dissolved in 11.4ml of ethanol so that the concentration of tetraethyl orthosilicate was 0.53mol/L. The obtained solution system was fully stirred for 10 min and mixed evenly. Accurately weigh 1.391g of ferrous acetate, accurately weigh 0.039g of ascorbic acid, add it to the above solution, stir and dissolve, and adjust the pH value to 2 with 0.1mol/L acetic acid as a catalyst. The mixed solution was transferred to a reaction kettle, and the reaction kettle was placed in a constant temperature box to react at 180°C for 12 hours, and the obtained gel mixture was dried in a vacuum oven at 100°C to obtain a xerogel.

Under the protection of _N2 atmosphere, the above lithium ferrous silicate precursor was calcined at 750°C for 7h to obtain carbon-coated porous lithium ferrous silicate powder. The average pore diameter of macropores is about 2.0 μm, the average pore diameter of mesopores is 18 nm, and the average porosity is 63.4%.

Claims (2)

1. porous carbon coated ferrous silicate lithium anode material, it is characterized in that forming the macroporous/mesoporous duct of continuous skeleton and perforation, three-dimensional net structure, the macropore average pore size is between 0.5～3.9 μ m, mesoporous average pore size is between 18～40nm, and mean porosities is between 57.2～71.9%.

2. the preparation method of porous carbon coated ferrous silicate lithium anode material is characterized in that step is as follows:

1) be raw material with acetate dihydrate lithium, ferrous acetate, tetraethoxysilane, take by weighing respective substance by the amount ratio, make Li: Fe: Si amount ratio is 2: 1: 1; With ethanol is solvent, and the concentration of tetraethoxysilane is 0.5～0.8mol/L; The additives polyethylene glycol pore creating material, acetic acid or ammonia-catalyzed agent, ascorbic acid are antioxidant, wherein reasonable 10%～30% of the ferrous lithium quality of synthetic silicic acid of discussing of polyethylene glycol, 3% of the ferrous lithium quality of antioxidant reasonable opinion synthetic silicic acid, catalyst is made into the aqueous solution of 0.1～0.3mol/L; Lithium salts, tetraethoxysilane and polyethylene glycol are dissolved in the ethanol, mix, add ferrous acetate and ascorbic acid to mixed liquor again, stirring and dissolving adds catalyst regulator solution system pH between 2～7;

2) mixed liquor with step 1) is transferred to reactor, reactor is put into insulating box react 12～20h down at 120～180 ℃, and the gel mixture that obtains obtains xerogel after 60～100 ℃ of oven dry in drying box;

3) with step 2) the xerogel porphyrize after be transferred in the tube furnace of temperature programmed control, under argon gas atmosphere with 10 ℃ of min ^-1Speed be warming up to 600～750 ℃ and constant temperature 7～10h, cool to room temperature with the furnace, obtain porous Li ₂FeSiO ₄/ C powder.

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