CN101572305B - A preparation method of LiFePO4/C cathode material with high rate performance - Google Patents

Abstract

The invention discloses a preparation method of LiFePO ₄ /C positive electrode material with high rate performance, comprising: weighing raw material FePO ₄ and lithium source compound according to the molar ratio of Li to Fe being 1-1.05:1, adding carbon source compound and ferrocene catalyst, ball milling with absolute ethanol as ball milling medium for 8 to 12 hours to obtain a slurry, the obtained slurry is dried and then heat treated under the protection of an inert gas. During this process, with the thermal cracking of the carbon source compound, the catalyst and The interaction between them promotes the formation of a carbon coating film with a better degree of graphitization and crystallization. The lithium iron phosphate positive electrode material prepared by the method of the invention has higher electronic conductivity, higher specific capacity, especially greatly improved high-rate performance, and has good application value in the field of power batteries.

Description

A preparation method of LiFePO₄/C cathode material with high rate capability

technical field

The invention belongs to the technical field of energy materials, and in particular relates to a preparation method of LiFePO ₄ /C cathode material with high rate performance.

Background technique

With the rapid development and popularization of the automobile industry, the emission of automobile exhaust has brought unprecedented urban air pollution and greenhouse effect, which has attracted the attention of governments all over the world. Coupled with the depletion of global oil resources, it is imminent to find new clean energy to replace oil. An electric vehicle powered by a secondary battery realizes zero emission of vehicle exhaust. Lithium-ion secondary batteries, in particular, are favored for their superior performance. Lithium-ion battery is a kind of popular green battery, which has the advantages of high energy density, long life, no memory effect, etc. It is widely used in mobile phone batteries, digital cameras, notebook computers and other portable electronic products with high comprehensive performance requirements field of.

However, traditional lithium-ion batteries such as LiCoO ₂ and LiNiO ₂ are not suitable for large-scale application of power batteries due to factors such as the price and safety of raw materials, and LiMn ₂ O ₄ is not suitable for its cycle performance, especially high temperature performance. It limits its development in power batteries. In contrast, orthorhombic olivine structure LiFePO ₄ has become a research hotspot at home and abroad because of its excellent comprehensive properties. Compared with traditional lithium-ion battery cathode materials, lithium iron phosphate has the advantages of stable charging and discharging platform voltage, excellent cycle performance, non-toxic, non-polluting, good safety performance, wide source of raw materials, and low price, and has become the most potential power battery. positive electrode material.

However, due to the relatively low intrinsic electronic conductivity and ionic conductivity of lithium iron phosphate, it must be modified before it can be applied. At present, the commonly used methods are surface coating of conductive materials, co-firing with carbon source organics to obtain conductive networks, selective doping of cations or anions, and preparation of nano-sized uniform particles. The preparation methods are various: traditional solid-phase method, solid-phase method such as carbothermal reduction method, liquid-phase method such as co-precipitation method, hydrothermal method and sol-gel method.

In the preparation process of the solid-phase method, the raw materials used, such as ferrous oxalate or ferrous acetate, are expensive, and the preparation process of the traditional solid-phase method is relatively complicated, and the cost is high due to the extensive use of high-energy ball milling. The preparation process of the liquid phase method is also relatively complicated, and requires strict control conditions.

The Chinese patent application number 200610049953.2 discloses a method for preparing carbon-coated lithium ferrous phosphate, which is a positive electrode material for lithium ion batteries, by using ferric iron source and metal niobium ion doping at the same time. , trivalent iron source compound, niobium source compound, and carbon source compound are mixed evenly and finely ground, sintered in a reducing atmosphere at 500°C-800°C for 4-30 hours, and finely ground to obtain a lithium-ion battery positive electrode material niobium-doped and carbon-coated Lithium iron phosphate-coated composite material (LiFePO ₄ /C), its high-current discharge capacity is significantly improved.

Chinese patent application number 200410099216.4 discloses a method for preparing a carbon-coated lithium iron phosphate (LiFePO ₄ /M/C) composite positive electrode material containing a metal conductive agent. Its Li-Fe-PO ₄ -M precursor is made of lithium salt, iron compound, phosphate, silver salt (or copper salt) and organic acid, and is synthesized by sol-gel method; then mixed with a certain amount of polymer The precursor of the polymer is pyrolyzed in an inert atmosphere to obtain a lithium iron phosphate (LiFePO ₄ /M/C) composite positive electrode material containing both carbon and a single metal conductive agent. The invention realizes uniform mixing of Li, Fe, PO ₄ ^3- and doping element M at the atomic level, and the obtained product LiFePO ₄ /M/C powder has uniform chemical composition and phase composition, fine and uniform particles. The carbon and hydrogen from high-temperature pyrolysis of high-molecular polymers are used as reducing agents to reduce Ag ⁺ or Cu ²⁺ to Ag or Cu simple substance, and at the same time obtain carbon-coated LiFePO ₄ /M (M=Ag or Cu) powder, so it is not The electronic conductivity of the material can be improved by post-treatment of carbon or metal simple substance coating or doping.

The Chinese patent application number 200410017382.5 discloses a preparation method of a lithium ion battery cathode composite material containing ferrous phosphate lithium salt-carbon, which uses a one-step solid-phase method to combine a certain proportion of lithium salt, Fe3 ⁺ compound and phosphoric acid The salt is mixed evenly, and then the mixture is pyrolyzed in an inert atmosphere, and a certain amount of high molecular polymer is added before pyrolysis to obtain a ferrous phosphate-based lithium salt-carbon cathode composite material. The method does not use expensive Fe ²⁺ raw materials, and the production process is simple, safe, and low in cost. The obtained positive electrode composite material has high purity, improved electrical conductivity, greatly improved electrochemical performance, high specific capacity, and excellent cycle performance. Stable discharge voltage platform around 3.4V.

In contrast, the preparation of LiFePO ₄ /C by using cheap ferric sources such as ferric oxide or ferric phosphate by solid-phase reduction of organic matter has the advantages of simple process, low raw material price, and good material performance. However, the structure of the surface-coated carbon has a great influence on the electronic conductivity. Materials with a higher sp ² /sp ³ ratio have a higher degree of graphitization and crystallization of the surface-coated carbon, and have higher electronic conductivity. The electrochemical performance of the positive electrode material, especially the high rate performance, can be greatly improved. In traditional preparation methods, heat treatment at 750°C or above is required to obtain electrode materials with a higher sp ² /sp ³ ratio. Such a high temperature can easily increase the grain size, which makes the diffusion of Li ⁺ in LiFePO ₄ more difficult, and virtually increases the cost of preparation. Therefore, finding a good preparation process of carbon-coated _LiFePO4 cathode material at low temperature has become a research hotspot. On the other hand, the increase of carbon content in the cathode material will significantly reduce the tap density of the material, thereby reducing its volumetric energy density. Therefore, it is important to find the balance between the coating effect of carbon, the carbon content and the electrochemical performance of the material.

Contents of the invention

The present invention provides a preparation method of LiFePO ₄ /C positive electrode material with high rate performance, which uses the interaction between organic matter and catalyst during the roasting process of positive electrode material precursor, so that the degree of graphitization and crystallization of coated carbon is higher , can effectively improve the electrical conductivity of the positive electrode powder material, and at the same time can reduce the carbon coating amount and increase the tap density of the powder material, and the method has simple process, low heat treatment temperature and low production cost.

A method for preparing a LiFePO ₄ /C positive electrode material with high rate performance, comprising the following steps:

(1) Weigh the raw material _FePO4 and lithium source compound according to the molar ratio of Li to Fe as 1～1.05:1, add carbon source compound and ferrocene catalyst, and use absolute ethanol as ball milling medium for ball milling for 8～12h to obtain slurry material;

(2) drying the slurry obtained in step (1) at 50-70° C. for 8-24 hours, and then grinding to obtain a precursor powder;

(3) heat-treating the precursor powder obtained in step (2) at 500-700° C. for 5-20 hours under the protection of an inert gas to obtain a LiFePO ₄ /C cathode material with high rate capability.

The lithium source compound is preferably LiOH·H ₂ O or Li ₂ CO ₃ , and the amount of the lithium source compound is to ensure that the molar ratio of Li and Fe in the product LiFePO ₄ /C positive electrode material is 1:1. There will be a small amount of loss of Li, and generally the amount of lithium source compound can be appropriately increased.

The carbon source compound can be selected from carbon-containing inorganic substances or carbon-containing organic substances. Considering easy availability of raw materials and cost reduction, any one of glucose, starch and polypropylene is preferred. The amount of carbon source compound can be added as needed.

The volume of the absolute ethanol is preferably 1-2 times the total volume of the raw material FePO ₄ , lithium source compound, carbon source compound and ferrocene catalyst. Absolute ethanol in this dosage range is conducive to the full mixing of raw materials. When the dosage is too small, the ball-milled slurry will be too viscous, which is not conducive to the full mixing of raw materials. When the dosage is too large, there are more solvents in the ball-milled slurry, which is not conducive to drying.

The inert gas is preferably nitrogen or argon.

The consumption of described ferrocene catalyst can be added by the conventional dosage of this area, considers that ferrocene is easy to volatilize loss at high temperature, and can bring the deviation of stoichiometric ratio when consumption is too much, so ferrocene and _FePO4 are preferred The molar ratio is 0.005-0.02.

The invention adopts cheap trivalent iron source FePO ₄ and fully mixes with lithium source, carbon source, catalyst and the like to prepare the precursor. During the calcination process, with the thermal cracking of the carbon source, the catalyst interacts with the cracked product, which promotes the formation of a carbon film with a high degree of graphitization and crystallization to cover the surface of the LiFePO ₄ particle. The LiFePO ₄ /C composite material formed in this way has high electronic conductivity, the high rate performance is greatly improved, the polarization is reduced in the case of high current charge and discharge, and the discharge platform is extended, making the LiFePO ₄ /C composite The material-wrapped carbon structure is improved and the production cost is low.

Description of drawings

Fig. 1 is the XRD diffractogram of the LiFePO ₄ /C cathode material prepared according to Example 2;

Fig. 2 is the SEM photo of the LiFePO ₄ /C cathode material prepared according to Example 2;

Fig. 3 is the Raman spectrum of the LiFePO ₄ /C cathode material prepared according to Example 2;

FIG. 4 is a cycle performance graph of the LiFePO ₄ /C cathode material prepared according to Example 2. FIG.

Detailed ways

Example 1

Weigh analytically pure LiOH·H ₂ O, FePO ₄ ·2H ₂ O, catalyst ferrocene, the molar ratio of which is Li:Fe:ferrocene=1.02:1:0.01, and weigh _14g poly Propylene (Fe:C=1:1) was milled on a planetary ball mill for 10 h with anhydrous ethanol as the ball milling medium at a speed of 250 r/min to obtain a slurry.

Dry the above slurry in an oven at 60°C for 12h, then carefully grind it in a mortar for 1h to obtain a precursor, roast the precursor in a nitrogen-protected vacuum well-type furnace, keep it at 500°C for 2h, then raise the temperature to 700°C for 10h, Cool down to room temperature with the furnace to obtain LiFePO ₄ /C cathode material. It is measured that the weight percentage of carbon in the positive electrode material is 1.28%.

A CR2025 button cell was assembled to test the electrochemical performance of the LiFePO ₄ /C cathode material prepared above. The aluminum sheet is used as the positive electrode collector, and the mass ratio of the positive active material (i.e. LiFePO ₄ /C), acetylene black, and polyvinylidene fluoride (PVDF) is 75:15:10, the negative electrode is metal lithium sheet, and the separator is Celgard- 2325, the electrolyte is ethylene carbonate (EC) and dimethyl carbonate (DMC) solution with equal volume ratio of 1M LiPF ₆ . Assembly of the cells was performed in an argon-filled vacuum glove box. The voltage range of the constant current charge and discharge test is 2.0~4.3V. The charging and discharging system is calculated according to the theoretical capacity of 170mA h g ^-1 , that is, 1C is 170mA g ^-1 .

The measured specific capacity at 0.1C is 168mA h g ^-1 , and the specific capacity at 10C is 135mA h g ^-1 .

Example 2

Weigh analytically pure LiOH·H ₂ O, FePO ₄ ·4H ₂ O, catalyst ferrocene, its molar ratio is Li:Fe:ferrocene=1.02:1: _0.01 , and weigh 42g poly Propylene (Fe:C=1:3) was ball milled on a planetary ball mill for 10 h with anhydrous ethanol as the ball milling medium at a rotational speed of 250 r/min to obtain a slurry.

Dry the above slurry in an oven at 50°C for 24h, then carefully grind it in a mortar for 1h to obtain a precursor, roast the precursor in a nitrogen-protected vacuum pit furnace, keep it at 500°C for 2h, then raise the temperature to 700°C for 10h, Cool down to room temperature with the furnace to obtain LiFePO4/C cathode material. It is measured that the weight percentage of carbon in the positive electrode material is 2.62%.

The LiFePO ₄ /C cathode material prepared above was tested according to the electrochemical performance test method in Example 1, and its 0.1C specific capacity was 171 mA h g ^-1 , and its 10C specific capacity was 144 mA h g ^-1 .

Example 3

Weigh analytically pure LiOH·H ₂ O, FePO ₄ ·4H ₂ O, catalyst ferrocene, its molar ratio is Li:Fe:ferrocene=1.02:1: _0.01 , and weigh 33g C ₆ H ₁₂ O ₆ ·H ₂ O (Fe:C=1:1), using absolute ethanol as the ball milling medium, ball milled on a planetary ball mill for 10 h at a rotational speed of 250 r/min to obtain a slurry.

Dry the above slurry in an oven at 70°C for 8 hours, then carefully grind it in a mortar for 1 hour to obtain a precursor, roast the precursor in a nitrogen-protected vacuum well-type furnace, keep it at 500°C for 2 hours, then raise the temperature to 700°C for 10 hours, Cool down to room temperature with the furnace to obtain LiFePO4/C cathode material. It is measured that the weight percentage of carbon in the positive electrode material is 3.42%.

The LiFePO ₄ /C cathode material prepared above was tested according to the electrochemical performance testing method in Example 1, and its 0.1C specific capacity was 162 mA h g ^-1 , and its 10C specific capacity was 119 mA h g ^-1 .

Example 4

Weigh analytically pure LiOH·H ₂ O, FePO ₄ ·4H ₂ O, catalyst ferrocene, its molar ratio is Li:Fe:ferrocene=1.02:1:0.01, and weigh 27g of starch per mole of FePO ₄ (Fe:C=1:3), with absolute ethanol as the ball milling medium, ball milled on a planetary ball mill for 10 h at a rotational speed of 250 r/min to obtain a slurry.

Dry the above slurry in an oven at 60°C for 12h, then carefully grind it in a mortar for 1h to obtain a precursor, roast the precursor in a nitrogen-protected vacuum well-type furnace, keep it at 500°C for 2h, then raise the temperature to 700°C for 10h, Cool down to room temperature with the furnace to obtain LiFePO ₄ /C cathode material. It is measured that the weight percentage of carbon in the positive electrode material is 3.30%.

The LiFePO ₄ /C cathode material prepared above was tested according to the electrochemical performance test method in Example 1, and its 0.1C specific capacity was 161 mA h g ^-1 , and its 10C specific capacity was 129 mA h g ^-1 .

Example 5

Weigh analytically pure Li ₂ CO ₃ , FePO ₄ 4H ₂ O, catalyst ferrocene, its molar ratio is Li:Fe:ferrocene=1.02:1:0.01, and weigh _42g polypropylene per mole of FePO (Fe:C=1:3), with absolute ethanol as the ball milling medium, ball milled on a planetary ball mill for 10 h at a rotational speed of 250 r/min to obtain a slurry.

Dry the above slurry in an oven at 60°C for 12h, then carefully grind it in a mortar for 1h to obtain a precursor, roast the precursor in a nitrogen-protected vacuum pit furnace, keep it at 500°C for 2h, then raise the temperature to 700°C for 10h, Cool down to room temperature with the furnace to obtain LiFePO4/C cathode material. It is measured that the weight percentage of carbon in the positive electrode material is 2.60%.

The LiFePO ₄ /C cathode material prepared above was tested according to the electrochemical performance test method in Example 1, and its 0.1C specific capacity was 163 mA h g ^-1 , and its 10C specific capacity was 120 mA h g ^-1 .

Example 6

Weigh analytically pure LiOH·H ₂ O, FePO ₄ ·4H ₂ O, catalyst ferrocene, its molar ratio is Li:Fe:ferrocene=1.02:1:0.02, and weigh _42g poly Propylene (Fe:C=1:3) was ball milled on a planetary ball mill for 10 h with anhydrous ethanol as the ball milling medium at a rotational speed of 250 r/min to obtain a slurry.

Dry the above slurry in an oven at 60°C for 12h, then carefully grind it in a mortar for 1h to obtain a precursor, roast the precursor in a nitrogen-protected vacuum well-type furnace, keep it at 500°C for 2h, then raise the temperature to 700°C for 10h, Cool down to room temperature with the furnace to obtain LiFePO ₄ /C cathode material. The weight percent content of carbon in the positive electrode material was measured to be 2.59%.

The LiFePO ₄ /C cathode material prepared above was tested according to the electrochemical performance testing method in Example 1, and its 0.1C specific capacity was 162mAh g ^-1 , and its 10C specific capacity was 128mAh g ^-1 .

Example 7

Weigh analytically pure LiOH·H ₂ O, FePO ₄ ·4H ₂ O, catalyst ferrocene, its molar ratio is Li:Fe:ferrocene=1.04: ₁ :0.005, and weigh 28g poly Propylene (Fe:C=1:2) was milled on a planetary ball mill for 10 h with anhydrous ethanol as the ball milling medium at a speed of 250 r/min to obtain a slurry.

Dry the above slurry in an oven at 60°C for 12h, then carefully grind it in a mortar for 1h to obtain a precursor, roast the precursor in a nitrogen-protected vacuum well-type furnace, keep it at 500°C for 2h, then raise the temperature to 700°C for 10h, Cool down to room temperature with the furnace to obtain LiFePO ₄ /C cathode material. It is measured that the weight percentage of carbon in the positive electrode material is 2.03%.

The LiFePO ₄ /C cathode material prepared above was tested according to the electrochemical performance test method in Example 1, and its 0.1C specific capacity was 165 mA h g ^-1 , and its 10C specific capacity was 138 mA h g ^-1 .

Claims (5)

1. LiFePO who possesses high rate capability ₄The preparation method of/C positive electrode comprises the steps:

(1) be to take by weighing raw material FePO at 1.02～1.05: 1 by the mol ratio of Li and Fe ₄And Li source compound, add carbon-source cpd and ferrocene catalyst, be ball-milling medium ball milling 8～12h with the absolute ethyl alcohol, obtain slurry;

Described carbon-source cpd is any one in glucose, starch, the polypropylene;

(2) with the slurry of step (1) gained in 50～70 ℃ of drying 8～24h, obtain precursor powder through grinding then;

(3) with presoma powder heat treatment under inert gas shielding of step (2) gained, promptly obtain LiFePO ₄/ C positive electrode;

Described heat treatment adopts 500 ℃ of insulations to be warming up to 10 hours mode of 700 ℃ of insulations after 2 hours.

2. preparation method as claimed in claim 1 is characterized in that: described Li source compound is LiOHH ₂O or Li ₂CO ₃

3. preparation method as claimed in claim 1 is characterized in that: the volume of described absolute ethyl alcohol is raw material FePO ₄, Li source compound, carbon-source cpd and ferrocene catalyst cumulative volume 1～2 times.

4. preparation method as claimed in claim 1 is characterized in that: described inert gas is nitrogen or argon gas.

5. preparation method as claimed in claim 1 is characterized in that: described ferrocene catalyst and FePO ₄Mol ratio be 0.005～0.02.

Images