CN100420075C - A kind of preparation method of lithium iron phosphate lithium ion battery cathode material - Google Patents

Abstract

The invention discloses a preparation method of lithium iron phosphate, a cathode material of a lithium ion battery. Ferric salt, lithium salt, phosphate and a reducing agent are mixed in a solvent for reaction, and amorphous iron phosphate is obtained after reacting for 0.5-30 hours Lithium: raise the temperature of amorphous lithium iron phosphate to 450-800°C at a heating rate of 5-30°C/min in an atmosphere of nitrogen or argon or a mixed gas, roast at a constant temperature for 20-600min, and then heat it at 1-20°C/min Cool down to room temperature at a cooling rate to obtain crystalline lithium iron phosphate powder, a positive electrode material for lithium ion batteries. The method for synthesizing the lithium iron phosphate material of the present invention adopts a ferric iron source, and the reducing agent reduces the ferric iron to ferrous iron while generating lithium iron phosphate. The lithium iron phosphate obtained by the process has high specific capacity and excellent cycle performance. It has a stable charge and discharge voltage platform and the first charge and discharge efficiency reaches 100%.

Description

A kind of preparation method of lithium iron phosphate lithium ion battery cathode material

technical field

The invention belongs to a method for preparing a cathode material of a lithium ion battery, in particular to a method for preparing lithium iron phosphate, an anode material of a lithium ion battery, by using a wet process.

Background technique

The cathode material of lithium-ion batteries is the bottleneck restricting the development of lithium-ion batteries, which determines the performance, price and development of lithium-ion batteries. Therefore, the research and development of high-performance cathode materials has become the key to the development of lithium-ion batteries. In 1997, the Goodenough research group [AKPadhi, KSNanjundaswarmy,.B.goodenough, J.Electrochem.Soc., 144(1997))] synthesized lithium iron phosphate for the first time, and found that the use of this material as a lithium-ion battery cathode material has a high theoretical The specific capacity (170mAh/g) is greater than the actual discharge specific capacity of 140mAh/g of commercialized LiCoO ₂ . Considering its non-toxicity, environmental friendliness, rich source of raw materials, low price and excellent thermal stability, it has received great attention from researchers and is considered to be a new generation of lithium-ion batteries with the most potential for development and application. battery cathode material.

At present, the methods for synthesizing lithium iron phosphate mainly include high-temperature solid-phase method, sol-gel method, hydrothermal method, and liquid-phase redox method.

A widely used high-temperature solid-phase method (such as [J]P.P.Prosini et al Electrochem Acta, 46(2001) 3517-3523) is to combine iron salts such as ferrous oxalate or ferric nitrate with ammonium dihydrogen phosphate and lithium salts such as lithium carbonate Or lithium hydroxide is evenly mixed according to the metering ratio, and then the lithium iron phosphate is synthesized by two-step roasting under the protection of an inert atmosphere such as nitrogen or argon. This method tends to make the particle size distribution of the synthesized material wider, and it is difficult to obtain pure lithium iron phosphate. Chinese patent CN1581537A directly adopts metal iron powder and iron phosphate as the iron source, and lithium phosphate as the lithium source. After 36 hours of high-energy ball milling, the lithium iron phosphate is obtained by roasting at a constant temperature of 600°C for 60 minutes. Although the process is simple and easy, the high-energy ball milling takes a long time and consumes a lot of energy.

In [J] Electrochemical and Solid-State Letters, 2002, 5(3)A47-A50, F.croce et al. used the sol-gel method to use ferric acetate or ferric nitrate as a precursor, and then added ascorbic acid after mixing stoichiometric LiOH , and then added to H ₃ PO ₄ , adjusted the pH value with ammonia water, and heated to 60°C to obtain a gel. Then heated at 350°C for 12 hours in a nitrogen atmosphere to decompose the gel, and finally sintered at 800°C for 24 hours to obtain lithium iron phosphate. In literature [J] Electrochemical and Solid-State Letters, 2004, 7 (12), A515-A518, Jingsi Yang et al. dissolved lithium acetate, ferrous acetate, and phosphoric acid in aqueous ethylene glycol with a stoichiometric ratio, and after vigorous stirring The gel was obtained, and then heated at 700°C for 12 hours to obtain lithium iron phosphate powder. Although the materials synthesized by this method have good uniformity, the conditions for gel preparation are harsh and difficult to control.

In [J] Electrochem Solid-State Letter, 2002, 5(10) A231-A223, S.Fanger et al. used Fe ₃ (PO ₄ ) ₂ and Li ₃ PO ₄ as precursors by hydrothermal method, in an autoclave React at 220° C. and 240 bar for 1 hour to prepare lithium iron phosphate powder. This synthesis method needs to use an autoclave, and the reaction must be carried out under high temperature and high pressure, so the cost is relatively high.

Pier Paolo et al. in [J]Journal of the Electrochemical Society, 2002, 149(7)A886-890, using Fe(NH ₄ ) ₂ (SO ₄ ) ₂ , NH ₄ H ₂ PO ₄ and H ₂ O ₂ as raw materials First synthesize FePO ₄ , then reduce FePO ₄ with LiI, and heat for 1 hour at 550° C. in a reducing atmosphere to prepare lithium iron phosphate. This method can produce materials with excellent electrochemical properties, but the LiI used is expensive and not suitable for industrial production.

Invention content:

The invention proposes a wet preparation method of lithium iron phosphate, a cathode material of a lithium ion battery, which simplifies the preparation process, improves product purity, and improves material conductivity. The present invention does not need to pre-prepare or use ferrous salts that are easily oxidized in the air, but uses ferric salts, lithium salts, and phosphates as precursors, and uses clean reducing agents to reduce ferric salts in the precursor solution at low temperatures. It is reduced to ferrous iron, so as to prepare lithium iron phosphate lithium ion battery positive electrode material with high purity and excellent electrochemical performance.

The preparation method of lithium iron phosphate, a positive electrode material of lithium ion battery, mainly adopts a low-temperature liquid phase oxidation-reduction synthesis process, and the specific preparation method is as follows:

Mix ferric salt, lithium salt, phosphate and reducing agent in a solvent for reaction, wherein the molar ratio of iron in ferric salt, lithium in lithium salt, phosphorus in phosphate and reducing agent is 1:0.95- 1.05:1:0.25-2, the concentration of iron in the iron salt in the solvent is 0.1-2mol/L, react for 0.5-30 hours, evaporate the solvent at 60-100°C and wash the obtained amorphous lithium iron phosphate precipitate, Then precipitate the amorphous lithium iron phosphate in nitrogen or argon or a mixed gas atmosphere, heat up to 450-800°C at a heating rate of 5-30°C/min, roast at a constant temperature for 20-600min, and then heat at 1-20°C/min Cool down to room temperature at a cooling rate to obtain a crystal-form lithium iron phosphate lithium ion battery positive electrode material.

The ferric salt used in the present invention is ferric nitrate, ferric chloride or ferric phosphate.

The lithium salt used in the present invention is lithium carbonate, lithium hydroxide, lithium acetate or lithium nitrate.

The phosphate used in the present invention is diammonium hydrogen phosphate, ammonium dihydrogen phosphate or iron phosphate.

The reducing agent used in the present invention is hydrazine hydrate, stannous chloride, sodium borohydride or ascorbic acid.

The solvent used in the present invention is deionized water, absolute ethanol, ethylene glycol or glycerin.

The iron phosphate used in the present invention can use commercially available goods, also can be self-made. The method for preparing iron phosphate is: dissolving equimolar Fe(NH ₄ ) ₂ (SO ₄ ) ₂ .6H ₂ O and NH ₄ H ₂ PO ₄ in deionized water respectively to form a 0.025-0.1mol/L solution, After mixing the two solutions, add 1-4mL of 30% H ₂ O ₂ aqueous solution to each liter of the solution under vigorous stirring, and continue stirring for 2 hours. The resulting precipitate is filtered, washed and dried to obtain FePO ₄ .

The present invention adopts low-temperature liquid phase redox to prepare the method for lithium iron phosphate lithium ion battery anode material has the following significant advantages: 1) the reducing agent that the present invention adopts can reduce ferric iron into ferrous iron in low-temperature liquid phase, simultaneously generates Amorphous lithium iron phosphate precipitation; 2) The obtained lithium iron phosphate powder, the positive electrode material of lithium ion battery, is nano-sized particles, and the chemical composition and phase composition are uniform; 3) The method for preparing lithium iron phosphate by the low-temperature liquid phase reduction synthesis process of the present invention , raw material composition and product formula are easy to control, and electrochemical tests show that the lithium iron phosphate material synthesized by the present invention has high specific capacity, excellent cycle performance, stable charge and discharge voltage platform, and the first charge and discharge efficiency reaches 100%.

Description of drawings

Fig. 1 is the X-ray diffraction pattern of the lithium iron phosphate prepared by embodiment 1.

Fig. 2 is the scanning electron micrograph of the lithium iron phosphate prepared by embodiment 1.

Fig. 3 is the charge-discharge curve of the lithium iron phosphate prepared according to Example 1.

Detailed ways

The electrochemical performance test conditions adopted in the following examples are: the voltage range is 2.5V to 4.2V, the electrolyte is 1mol/L LiPF ₆ /EC:DMC (1:1), the counter electrode is metal lithium sheet, and the charge and discharge current is 17mA ·g ^-1 , the test temperature is 20±2°C.

Example 1:

Weigh 0.01mol Fe(NH ₄ ) ₂ (SO ₄ ) ₂ ·6H ₂ O and 0.01mol NH ₄ H ₂ PO ₄ and dissolve them in 500ml of deionized water respectively. After mixing the two solutions, add 4mL of 30% H ₂ O ₂ aqueous solution, stirring was continued for 2 hours. The resulting precipitate was filtered, washed and dried to obtain FePO ₄ . Mix 0.02mol LiAc·2H ₂ O, 0.01mol FePO ₄ , and 0.01mol ascorbic acid in 50mL of ethanol, stir the reaction for 10 hours, evaporate the solvent at 60°C and wash the obtained amorphous lithium iron phosphate precipitate, then the amorphous lithium iron phosphate Precipitation is placed in a high-temperature furnace, in a mixed atmosphere (95% Ar+5% H ₂ ), heated at a heating rate of 5°C/min, roasted at a constant temperature of 600°C for 2 hours, and then cooled to room temperature at a cooling rate of 20°C/min , to prepare lithium iron phosphate powder of lithium ion battery positive electrode material in crystal form. Fig. 1 is an X-ray diffraction diagram, and the obtained lithium iron phosphate powder has a pure olivine-type orthorhombic single-phase structure. Figure 2 is a scanning electron micrograph of lithium iron phosphate powder, the particle size of the product is basically less than 3 microns, and there is agglomeration of particles.

Weigh 0.4 g of the obtained lithium iron phosphate powder, add 0.05 g of acetylene black and 0.05 g of polyvinylidene fluoride binder dissolved in N-N'dimethylpyrrolidone, mix well and apply it on aluminum foil to make a positive electrode sheet . In an argon atmosphere drying glove box, metal lithium sheet is used as counter electrode, UB3025 (PP/PE/PP) is used as separator, ethylene carbonate (EC) + dimethyl carbonate (DMC) + 1ML LiPF ₆ is used as electrolyte, Assemble into a battery.

At normal temperature (20 degrees), the constant current charge and discharge test is carried out on the battery in the voltage range of 2.5V to 4.2V. Figure 3 is the charge-discharge curve at a rate of 0.1C (17mA·g ^-1 ). It can be seen from the figure that the discharge voltage of the obtained lithium iron phosphate material is about 3.4V, and the reversible specific capacity is as high as 163mAh/g, which is 96% of the theoretical specific capacity. The battery cycle performance is also very good.

Example 2:

Weigh 0.01mol LiAc·2H ₂ O, 0.01mol FePO ₄ (prepared according to Example 1), and dissolve 0.005mol ascorbic acid in 25mL ethanol, stir and react for 20 hours, evaporate the solvent at 60°C and wash the obtained lithium iron phosphate precipitate, and finally dissolve the The lithium iron phosphate precipitate is placed in a high-temperature furnace, heated at a heating rate of 5°C/min in an argon atmosphere, roasted at a constant temperature of 700°C for 10 hours, and then cooled to room temperature at a cooling rate of 10°C/min to obtain a lithium-ion battery positive electrode Material Lithium iron phosphate powder. The discharge specific capacity of this material is about 130mAh/g.

Example 3:

Weigh 0.01molLiAc·2H ₂ O, 0.01molFePO ₄ (prepared according to Example 1), and 0.01mol ascorbic acid were dissolved in 25mL ethanol, after stirring for 20 hours, the precipitate was filtered, washed, dried, and then mixed in a mixed atmosphere (95% Ar +5% H ₂ ) at a heating rate of 5°C/min, roasted at a constant temperature of 650°C for 15 hours, and then cooled to room temperature at a cooling rate of 10°C/min to obtain lithium iron phosphate powder, a positive electrode material for lithium-ion batteries. The discharge specific capacity of this material is about 140mAh/g.

Claims (6)

1. the preparation method of a lithium ion battery anode material lithium iron phosphate is characterized in that the preparation method is as follows:

With trivalent iron salt, lithium salts, phosphate and ascorbic acid are mixed in the solvent and react, iron in the trivalent iron salt wherein, lithium in the lithium salts, the phosphorus in the phosphate and the mol ratio of ascorbic acid are 1: 0.95-1.05: 1: 0.25-2, the concentration of iron in the molysite in solvent is 0.1-2mol/L, reacted 0.5-30 hour, 60-100 ℃ of following solvent flashing also washs resultant unbodied LiFePO4 precipitation, unbodied LiFePO4 is deposited in nitrogen or argon gas or the gaseous mixture atmosphere then, be warming up to 450-800 ℃ with the 5-30 ℃/min rate of heat addition, constant temperature calcining 20-600min, be cooled to room temperature with 1-20 ℃/min rate of temperature fall then, make the lithium ion battery anode material lithium iron phosphate of crystal formation.

2. the preparation method of a kind of lithium ion battery anode material lithium iron phosphate according to claim 1 is characterized in that trivalent iron salt is ferric nitrate, iron chloride or ferric phosphate.

3. the preparation method of a kind of lithium ion battery anode material lithium iron phosphate according to claim 1 is characterized in that lithium salts is lithium carbonate, lithium oxalate or lithium nitrate.

4. the preparation method of a kind of lithium ion battery anode material lithium iron phosphate according to claim 3 is characterized in that lithium salts substitutes with lithium hydroxide.

5. the preparation method of a kind of lithium ion battery anode material lithium iron phosphate according to claim 1 is characterized in that phosphate is diammonium hydrogen phosphate, ammonium dihydrogen phosphate or ferric phosphate.

6. the preparation method of a kind of lithium ion battery anode material lithium iron phosphate according to claim 1 is characterized in that solvent is deionized water, absolute ethyl alcohol, ethylene glycol or glycerine.

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