CN104617296A - Method for preparing mesoporous carbon coated LiFePO4 electrode material - Google Patents

Abstract

The invention relates to a preparation method of a mesoporous carbon-coated LiFePO ₄ /C electrode material. Using mesoporous carbon as a hydrothermal reaction matrix, LiFePO ₄ /C composites were prepared. The prepared material particles form a homogeneous mesoporous carbon coating structure: the inner and outer layers are LiFePO ₄ , and the middle is a complete carbon layer. This carbon coating structure can improve the electrical conductivity of the material, and at the same time, it can also prevent the collapse of the LiFePO ₄ structure during the long-term charge and discharge process, and improve the rate cycle performance of the material. The pore size limitation of mesoporous carbon can prevent the excessive growth of LiFePO ₄ particles. The LiFePO ₄ prepared by the method has large specific surface area, high electrical conductivity, high specific capacity, excellent high-rate discharge capability, and stable discharge structure for multiple cycles. In addition, the method has the advantages of wide source of raw materials, low price, less energy consumption, short cycle time and high batch stability.

Description

Preparation method of lithium iron phosphate electrode material coated with mesoporous carbon

technical field

The invention relates to a method for preparing mesoporous carbon-coated LiFePO ₄ /C lithium-ion power battery electrode materials through a hydrothermal process, and belongs to the technical field of lithium-ion battery materials in new energy technologies.

Background technique

With the widespread popularization of mobile communication, electronic appliances and other equipment, the energy crisis, environmental pollution and other issues are becoming more and more serious, the demand for large lithium-ion batteries for renewable energy storage and electric vehicle power technology has become increasingly urgent. The technological developments associated with it are considered to have broad implications for addressing today's energy scarcity and air pollution issues.

Among many cathode materials, the polyanionic compound LiFePO ₄ crystal has a stable olivine structure, which has the advantages of high safety, high theoretical specific capacity (~170 mAh·g ^-1 ), good cycle performance, and moderate working voltage (~3.5 V), good stability at room temperature and high temperature, relatively low cost, and environmental friendliness, has become one of the positive electrode materials for lithium-ion batteries with great development potential in recent years. Its application potential is widely favored by people. However, due to the inherent limitations of the crystal structure of LiFePO ₄ , its electronic conductivity and lithium ion diffusion rate are low, especially under high current discharge conditions, the electrochemical performance of this material decays significantly. At present, the research and development of LiFePO ₄ lithium-ion batteries cannot maximize its advantages. Therefore, how to improve the electronic conductivity and ion diffusion rate of LiFePO ₄ crystals has become a research hotspot in recent years. The commonly used modification methods are coating carbon, reducing the particle size of the material, doping high-valent metal ions and making the material porous to increase the contact area of the material.

There are many methods for preparing lithium iron phosphate, which can be roughly divided into solid-phase method and liquid-phase method. The solid-phase method has a simple process and easy control of conditions. As disclosed in Chinese patent documents CN1762798, CN1753216, CN1767238, and CN1581537, the lithium carbonate (or hydroxide, phosphate), ferrous oxalate (or ferrous acetate, ferrous phosphate) and ammonium dihydrogen phosphate (or diammonium hydrogen phosphate) mixed and roasted at high temperature under the protection of inert gas. However, the particle size distribution range of the product is wide, the crystal size is large, the powder is composed of irregular particles, and the electrical conductivity of lithium iron phosphate is low, and the high rate charge and discharge performance is poor, which makes the practical application of the material difficult. Simultaneously solid state reaction temperature is higher, and reaction time is longer, and production batch is unstable, has increased production cost.

The present invention selects the hydrothermal method, and compared with the solid-phase method, the liquid-phase method is more suitable for preparing LiFePO ₄ particles with fine and uniform particles and narrow particle size distribution. Among various liquid-phase synthesis methods, the preparation process of the hydrothermal method can be better controlled, the hydrothermal reaction is milder, and the reaction consumes less energy. Chinese Patent Document No. CN 102897741 A discloses a method for synthesizing nano-lithium iron phosphate by using a hydrothermal method to synthesize organic compounds with phosphate end groups. The prepared LiFePO ₄ particle size

It is nanoscale and has good uniformity. The method is relatively simple and the cost is low. Document number CN 101007630 A discloses a method of mixing lithium source, iron source and phosphorus source in deionized water or water/alcohol mixture, adding a certain proportion of additives, such as carbon black, tartaric acid, citric acid, etc. A method for obtaining submicron particles of lithium iron phosphate by performing hydrothermal synthesis at low temperature. The synthesized lithium iron phosphate material has a cycle capacity of 145 mAh·g ^-1 at 0.1 C, but at high rates, its capacity loss is large, for example, the cycle capacity is 98.18 mAh·g ^-1 at 4 C, and the cycle performance not good.

Doping with high-valent metal ions can improve the electronic conductivity of the material, but in the process of doping with metal ions, the thermal decomposition of organic groups in the dopant or precursor to generate carbon can also improve the electrical conductivity of the material. Which one of them is the key factor is still inconclusive. Moreover, if the dopant ions replace the position of lithium ions in the crystal structure, the one-dimensional diffusion path of lithium ions may be blocked. Metal ions occupying Fe sites will cause different degrees of distortion in the LiFePO ₄ crystal lattice, resulting in the destruction of some LiFePO ₄ crystals after a certain number of cycles, which will cause a sharp decline in battery performance under high-rate and long-term cycle discharge conditions, affecting charge and discharge performance. For example, the transition element doping disclosed in CN 1785799, the rare earth doping disclosed in CN 1785800, and the phosphorus doping disclosed in CN1785823, although the above methods can partially improve the performance of lithium iron phosphate, it is not easy to realize industrialized mass production.

The conductivity of the material is improved by doping carbon without affecting the crystal structure of the material, and at the same time it can prevent the oxidation of Fe ²⁺ and reduce the formation of impurity phases. Carbon can be used as a nucleating agent during the calcination of LiFePO ₄ to hinder the aggregation and growth of grains, reduce the particle size and increase the specific surface area. The method disclosed in Chinese patent document CN 103367750 is to mix lithium source, iron source and phosphorus source, add carbon source compound and solvent, after ball milling, spray-dry the ball milled slurry, and heat treatment under inert atmosphere. This method obtains a uniform carbon layer coating, and improves the charging and discharging performance of the material, but this method requires ball milling, which consumes a lot of energy; after ball milling, spray drying is required, and the steps are cumbersome, which is not conducive to industrial production. Chinese patent document CN 102306775 B discloses a lithium iron phosphate nanobelt, a cathode material for lithium ion batteries, and a preparation method thereof. The lithium iron phosphate nanobelt has a smooth surface, a thickness of 60-300 nm, a width of 5-25 μm, and a length greater than 100 μm. However, the existing nano-lithium iron phosphate preparation process is complicated, and the thickness is relatively large, resulting in a long transmission distance of lithium ions. In the method disclosed in CN 104201335 A, CATB (cetyltrimethylammonium bromide) and ascorbic acid are dissolved in water, and phosphorus source, lithium source and iron source are added and mixed to carry out hydrothermal reaction. The nanosheet layered LiFePO ₄ was prepared, with a microscopic appearance of 10-100 nm in thickness, 600-800 nm in length and 200-300 nm in width. The unique sheet-like structure can shorten the transmission distance of lithium ions during charge and discharge, thereby improving the electrochemical performance of electrode materials.

At present, there are few researches on making the coated carbon material porous, and making the coated carbon porous is a promising direction to improve the performance of the material. By artificially making the coated carbon porous, firstly, the contact area of the material can be greatly increased, which is more conducive to improving the electronic conductivity of the material; secondly, the electrolyte can also enter the interior of the material through the pores, shortening the time for lithium ions to enter the electrolyte. Diffusion path. Simultaneously improve the conduction of electrons and ions in LiFePO ₄ . It is beneficial to improve the high-rate discharge performance of the material; again, the pore size of the prepared mesoporous carbon is tens of nanometers, and the LiFePO ₄ precursor nucleates and grows in the mesoporous carbon pores until it is filled, and the pore size of the mesoporous carbon is limited. The role can well control the particle size of LiFePO ₄ and prevent the excessive growth of LiFePO ₄ particle size; finally, the special homogeneous junction mesoporous carbon coating structure formed by mesoporous carbon and LiFePO ₄ particles can be charged and discharged at a large rate During the process, the microstructure of the material can be well protected, and the structure of LiFePO ₄ particles can be prevented from collapsing during the process of deintercalating lithium, thereby prolonging the service life of the material.

Contents of the invention

The object of the present invention is to provide a method for preparing LiFePO ₄ /C electrode material coated with mesoporous carbon, the LiFePO ₄ prepared by this method has excellent electrochemical performance, good cycle performance, low raw material price, and high batch stability , reduce production cost and energy consumption, and shorten the production cycle, thereby meeting the requirements of large-scale commercial production.

A kind of preparation method of the lithium iron phosphate electrode material with mesoporous carbon coating of the present invention is characterized in that the method has the following steps:

(1) Add a certain amount of nano-calcium carbonate into the water-soluble sugar solution, stir, heat, and evaporate to dryness to a uniform viscous mixture; heat the mixture to 750 ℃ ~ 850 ℃ in a tube furnace under a protective atmosphere for carbonization, and keep warm for 2 h, to obtain a mixture of black carbon and nano-calcium carbonate; soaking the resulting black mixture in dilute acid solution to wash away the nano-calcium carbonate to obtain the required mesoporous carbon; the mass ratio of calcium carbonate to water-soluble sugar is 0.5 to 1.5;

(2) Add the phosphorus source and the iron source respectively to the pre-dispersed mesoporous carbon suspension, then fully mix and stir with the lithium source solution, and then move it into the reaction kettle for hydrothermal reaction; naturally cool to room temperature, filter, and dry to obtain Powder; the molar ratio of lithium source, iron source and phosphorus source compound consumption is lithium: iron: phosphorus=2.8～3.2:0.8～1.2:0.8～1.2; The mass percentage that mesoporous carbon accounts for total composition is 0.5%～9.5%; The temperature of hydrothermal reaction is 120 ℃ ~ 220 ℃, and the reaction time is 4 ~ 18 h;

(3) Put the powder obtained in step (2) in a tube furnace for heat treatment reaction under a protective atmosphere. The reaction temperature is 550-750 ℃, and the reaction time is 4-12 h. LiFePO4/C electrode material.

The carbon source water-soluble sugar for preparing mesoporous carbon is any one of sucrose, glucose, lactose and maltose; the acid used to remove nano-calcium carbonate is any one of hydrochloric acid, sulfuric acid, phosphoric acid and acetic acid; Described phosphorus source is any in phosphoric acid, ammonium phosphate, ammonium dihydrogen phosphate and diammonium hydrogen phosphate; Described iron source is ferrous nitrate, ferrous sulfate, ferrous chloride, ferrous phosphate and ferrous oxalate any one in; the lithium source is any one in lithium nitrate, lithium chloride, lithium carbonate, lithium phosphate, lithium hydroxide, lithium oxalate and lithium acetate; the protective atmosphere is helium, argon , nitrogen and argon-hydrogen mixed gas in any one.

The beneficial effects of the present invention are:

(1) The nano-calcium carbonate used in the reaction process is usually used as an additive in industrial coatings, rubber, and plastics to improve the performance of the material. It is non-toxic, harmless, easy to obtain, and stable in nature. All dilute acids used to react with calcium carbonate are commonly used acids in industry. The selection and steps of the above raw materials all reduce the production cost. At the same time, compared with other liquid-phase methods, the preparation process of the hydrothermal method has no strict requirements, and the reactants have achieved ion-level combination, which reduces the generation of impurities, and has lower requirements for equipment and processes.

(2) In the preparation process of LiFePO ₄ (step 2), mesoporous carbon is used as the carbon matrix. Under the action of Gibbs free energy, the LiFePO ₄ precursor preferentially nucleates and grows in the mesoporous carbon pores until it is filled with Afterwards, LiFePO ₄ will continue to grow outside the mesoporous carbon shell, forming a homogeneous mesoporous carbon coating structure: the inner and outer layers are LiFePO ₄ , and the middle is a complete carbon layer. This structure can not only improve the electrical conductivity of the material, but also improve the strength of the material during the cycle of charging and discharging LiFePO ₄ particles in the process of deintercalating lithium, prevent the collapse of the LiFePO ₄ structure, and improve the charge and discharge performance of the material at large rates. Extend its cycle life.

(3) During the preparation of LiFePO ₄ (step 2), part of the LiFePO ₄ precursor did not nucleate and grow in the mesopores, and the particle size was on the micron scale. the conductivity of the material.

(4) During the preparation of LiFePO ₄ (step 2), due to the porosity of the coated carbon, the electrolyte can enter the interior of the material through the pores during charge and discharge, shortening the diffusion distance of lithium ions into the electrolyte. Improve the conduction of LiFePO ₄ ions.

(5) During the preparation of LiFePO ₄ (step 2), the pore size limitation of mesoporous carbon can well control the particle size of LiFePO ₄ and prevent the excessive growth of LiFePO ₄ particle size. At the same time, it forms a micro-nano structure with part of the LiFePO ₄ precursor that has not nucleated and grown in the mesopores and the particles in the mesopores, which can effectively increase the tap density of the material.

Description of drawings

Figure 1 is a SEM image of the material obtained in Example 1 of the present invention.

Figure 2 is the XRD spectrum of the material obtained in Example 1 of the present invention.

Figure 3 is a SEM image of the material obtained in Example 1 of the present invention.

Fig. 4 is an XRD pattern of the material obtained in Example 2 of the present invention.

Figure 5 is the HRTEM image of the material obtained in Example 2 of the present invention.

Fig. 6 is the initial charge and discharge curve diagram at 0.1 C of the material obtained in Example 2 of the present invention.

Fig. 7 is a cycle graph of the ratio of the material obtained in Example 2 of the present invention.

Fig. 8 is the initial charge and discharge curve at 0.1 C of the material obtained in Example 3 of the present invention.

Fig. 9 is the initial charge and discharge curve at 0.1 C of the material obtained in Example 4 of the present invention.

Fig. 10 is the initial charge and discharge curve at 0.1 C of the material obtained in Example 5 of the present invention.

Detailed ways

Now illustrate the present invention in conjunction with specific embodiment.

Example 1

Weigh with sucrose:nano-calcium carbonate = 1:1 mass ratio, dissolve sucrose in deionized water, slowly add calcium carbonate, stir and mix on a magnetic stirrer, set the heating temperature to 60°C, to a homogeneous mixture. The mixture was carbonized at high temperature in a tube furnace under the protection of flowing argon atmosphere, the reaction temperature was 800 °C, and the carbonization time was 2 h. Take out the black mixture and soak in dilute hydrochloric acid for 5 h, wash and dry. The scanned picture of the obtained mesoporous carbon is shown in Fig. 1 . Weigh mesoporous carbon with a carbon doping mass percentage of 5.2%, LiOH H ₂ O, FeSO ₄ 7H ₂ O, NH ₄ H ₂ PO ₄ at a molar ratio of Li:Fe:P = 3:1:1 As reactants, the phosphorus source and iron source were added to the ultrasonically dispersed mesoporous carbon suspension, mixed with the lithium source solution and stirred for 30 min, and added to the reactor for reaction. The reaction temperature is 180 °C, and the reaction time is 16 h. Take out, filter, wash and dry. Finally, the resultant was heat-treated in a tube furnace under the protection of a flowing argon atmosphere. The heat treatment temperature was 650 °C and the reaction time was 6 h. As shown in Figure 2, the position of the XRD diffraction peak coincides with the LiFePO ₄ standard card (PDF Card No. 40-1499), indicating that the obtained product is LiFePO ₄ . As shown in Figure 3, the SEM picture shows that the mesoporous carbon is evenly distributed around the lithium iron phosphate, and there are small particles of LiFePO ₄ growing in the mesoporous carbon.

Example 2

The preparation steps in this example are exactly the same as those in Example 1 above. The difference is: the weight ratio of sucrose:nano-calcium carbonate=2:3 is used as the raw material for the preparation of mesoporous carbon. As shown in Figure 4, the position of the XRD diffraction peak coincides with the LiFePO ₄ standard card (PDF Card No. 40-1499), indicating that the obtained product is LiFePO ₄ . Figure 5 is the HRTEM picture of the homojunction mesoporous carbon coating structure of LiFePO ₄ /C material. In the figure, it can be seen that LiFePO ₄ grows in the mesoporous carbon and continues to grow on the outer layer of the mesoporous carbon after being filled with mesoporous carbon. . As shown in Figure 6, the initial discharge specific capacity of the material at a charge-discharge rate of 0.1 C is 160.3 mAh g ^-1 , which is 94.3% of the theoretical value, which is 34% higher than that of pure LiFePO ₄ without carbon doping. %. In Fig. 7, the discharge specific capacities of 0.1 C, 0.5 C, 1 C and 5 C are 160.3, 142.2, 109.1, 93.6 mAh·g ^-1 , respectively, and the high-rate discharge returns to 0.1 C and remains high , Stable discharge specific capacity.

Example 3

The preparation steps in this example are exactly the same as those in Example 2 above. The difference is: the hydrothermal reaction time is 6 h, as shown in Figure 8, the initial discharge specific capacity at 0.1 C is 148.3 mAh·g ^-1 .

Example 4

The preparation steps in this example are exactly the same as those in Example 2 above. The difference is that the heat treatment temperature is 700 ℃ and the heat treatment time is 4 h after the hydrothermal sample is taken out. As shown in Figure 9, the initial discharge specific capacity at 0.1 C is 142.7 mAh·g ^-1 .

Example 5

The preparation steps in this example are exactly the same as those in Example 2 above. The difference is that the carbon source is glucose. The mass ratio of glucose to nano calcium carbonate is still 2:3 as the preparation material of mesoporous carbon. As shown in Figure 10, the initial discharge specific capacity at 0.1 C is 160.6 mAh·g ^-1 .

Claims (2)

1. in a kind of preparation method with mesoporous carbon coated lithium iron phosphate electrode material, it is characterized in that the method has following steps: (1) Add a certain amount of nano-calcium carbonate into the water-soluble sugar solution, stir, heat, and evaporate to dryness to a uniform viscous mixture; heat the mixture to 750 ℃ ~ 850 ℃ in a tube furnace under a protective atmosphere for carbonization, and keep warm for 2 h, to obtain a mixture of black carbon and nano-calcium carbonate; soaking the resulting black mixture in a dilute acid solution to wash off the nano-calcium carbonate to obtain the required mesoporous carbon; the mass ratio of calcium carbonate to water-soluble sugar is 0.5 to 1.5; (2) Add the phosphorus source and the iron source respectively to the pre-dispersed mesoporous carbon suspension, then fully mix and stir with the lithium source solution, and then move it into the reactor for hydrothermal reaction; naturally cool to room temperature, filter, and dry to obtain Powder; the molar ratio of lithium source, iron source and phosphorus source compound consumption is lithium: iron: phosphorus=2.8～3.2:0.8～1.2:0.8～1.2; The mass percentage that mesoporous carbon accounts for total composition is 0.5%～9.5%; The temperature of hydrothermal reaction is 120 ℃ ~ 220 ℃, and the reaction time is 4 ~ 18 h; (3) Put the powder obtained in step (2) in a tube furnace for heat treatment reaction under a protective atmosphere. The reaction temperature is 550~750 ℃, and the reaction time is 4~12 hours, and the mesoporous carbon coating can be obtained. LiFePO4/C electrode material. 2. a kind of preparation method with mesoporous carbon coated lithium iron phosphate electrode material as claimed in claim 1 is characterized in that the carbon source water-soluble sugar of described preparation mesoporous carbon is sucrose, glucose, lactose and maltose any one in; the acid used to remove nano-calcium carbonate is any one of hydrochloric acid, sulfuric acid, phosphoric acid and acetic acid; the phosphorus source is phosphoric acid, ammonium phosphate, ammonium dihydrogen phosphate and diammonium hydrogen phosphate any one of; the iron source is any one of ferrous nitrate, ferrous sulfate, ferrous chloride, ferrous phosphate and ferrous oxalate; the lithium source is lithium nitrate, lithium chloride, lithium carbonate , lithium phosphate, lithium hydroxide, lithium oxalate and lithium acetate; the protective atmosphere is any one of helium, argon, nitrogen and argon-hydrogen mixed gas.