CN103247801A - Preparation method of high-conductivity lithium iron phosphate cathode material - Google Patents

Abstract

The invention provides a preparation method of lithium iron phosphate cathode material, adding a doping compound to phosphoric acid and ferric salt solution, respectively adding a pyrolytic carbon source and a carbonization accelerator in two ways, adding an alkaline solution, Control the pH of the system in the range of 1 to 4, react for a certain period of time, wash and dry to obtain the iron phosphate precursor; mix the lithium source, precursor, pyrolytic carbon source and carbonization accelerator, and stir (or ball mill)— - vacuum drying - heat treatment - grinding to prepare doped and carbon-coated lithium iron phosphate. In the present invention, a doping compound, a pyrolytic carbon source and a carbonization accelerator are added in the process of preparing the iron phosphate precursor, which improves the electrical conductivity inside the lithium iron phosphate lattice and the electrical conductivity between particles, and significantly improves its The electrochemical performance at the rate; and the present invention no longer uses high-priced ferrous iron, which greatly reduces the production cost.

Description

A kind of preparation method of lithium iron phosphate cathode material with high conductivity

technical field

The invention relates to a preparation method of a positive electrode material of a lithium ion battery, in particular to a preparation method of a lithium iron phosphate positive electrode material.

Background technique

LiCoO ₂ , LiMn ₂ O ₄ and LiFePO ₄ are commonly used cathode active materials for lithium-ion batteries. Among them, lithium iron phosphate materials have stable structure, good thermal stability, low raw material cost, low toxicity, environmental friendliness, good safety performance and It has many advantages such as good high temperature performance, and is a kind of positive electrode active material with great potential. In particular, its oxidation exothermic temperature exceeds 400°C, and it has excellent thermal stability.

However, the electronic conductivity of LiFePO ₄ is low, only 10 ^-9 ~ 10 ^-10 S/cm, so the rate performance is poor, which limits its application. Generally, methods such as carbon coating, coating conductive materials, doping ions, and particle size reduction are used to improve its conductivity and electrochemical performance. Add phenolic resin, carbon gel, glucose, citric acid, acetylene black, carbon black, graphite and graphene to the raw materials for the synthesis of LiFePO ₄ , which can form conductive carbon during pyrolysis, thereby improving the conductivity of LiFePO ₄ rate; or coating the surface of LiFePO ₄ with electrochemically inert Ag and Fe ₂ P and other conductive materials can also increase its conductivity and improve its electrochemical performance.

Coating LiFePO ₄ with carbon can not only avoid the formation of Fe ³⁺ , but also significantly improve the conductivity of the particle surface; but it cannot improve the conductivity inside the LiFePO ₄ lattice, so ion doping of LiFePO ₄ is also Very important. Many metal ions can be used to dope LiFePO ₄ , which can stabilize its crystal structure, improve the migration of electrons and Li ⁺ , thereby increasing the internal conductivity of its crystal and improving the rate performance.

The method of doping metal ions and coating carbon can improve its conductivity and improve the rate performance. However, at present, the +2-valent iron source is generally used, and its raw material cost is relatively high, and the subsequent requirements for the heating furnace and protective gas in the production process are very high.

It has also been reported that the method of preparing LiFePO ₄ /C by reducing the low-priced trivalent precursor FePO ₄ with carbon source can significantly reduce the production cost. However, most of them use pure-phase FePO ₄ , and add doping elements and carbon sources when mixing with Li sources to prepare doped and carbon-coated LiFePO ₄ /C; these doping elements and carbon sources are surrounded by FePO ₄ In the process of forming LiFePO ₄ /C in the subsequent heat treatment, it is difficult for the inside of the core to enter the particle in large quantities only by the diffusion of ions and carbon during the sintering process, and only a very small amount of doping metal elements and The carbon source has entered the surface layer, so the conductivity is not high; although the electrochemical performance at low rates is also good, the performance at higher rates is not fundamentally improved.

Contents of the invention

The present invention aims to provide a method that can effectively reduce production cost and prepare lithium iron phosphate with high electrical conductivity and good rate performance. Implementation scheme of the present invention is as follows:

A method for preparing a lithium iron phosphate positive electrode material, which consists of two steps of precursor preparation and lithium iron phosphate preparation, wherein the precursor preparation step chooses one of the following two methods, the first method: phosphoric acid solution and ferric iron Mix the salt solution, then add the doping compound, pyrolysis carbon source compound and carbonization accelerator compound to the solution; then mix the above mixed liquid system with the alkaline solution for a certain period of time, and then the obtained powder is washed - dried , to obtain the iron phosphate precursor, control the pH value of the reaction system in the range of 1 to 4, and the temperature in the range of 50 to 80°C; the second method: after mixing the phosphoric acid solution and the ferric salt solution, add doping Use compound; then mix the above mixed liquid system with alkaline solution, then add pyrolysis carbon source compound and carbonization accelerator compound to it, after reacting for a certain period of time, the obtained powder is washed-dried to obtain the iron phosphate precursor , controlling the pH value of the reaction system in the range of 1-4, and the temperature in the range of 50-80°C. That is, pyrolysis carbon source compounds and carbonization accelerator compounds are added in two ways, one is added to the mixed solution of phosphoric acid, ferric salt and doping compound; the other is added to the mixed solution containing alkaline solution in the system. In the present invention, the carbonization accelerator compound refers to a substance that can accelerate the formation and regular arrangement of carbon crystallites during the pyrolysis process of carbon source compounds. The alkaline solution generally adopts ammonia water or alkali metal hydroxide solution.

The preparation steps of lithium iron phosphate are: mix the lithium source and the precursor obtained in the first step according to the molar ratio of lithium atom: iron atom (1-1.05): 1, and then add pyrolytic carbon source compound and The carbonization accelerator compound is obtained as a mixed raw material; the similar process method in the preparation of the existing lithium iron phosphate is used to obtain the lithium iron phosphate positive electrode material, that is, one of the following two methods is adopted, the first (dry mixing): ball milling-vacuum Drying—heat treatment—grinding; the second type (wet mixing): add an appropriate amount of organic solvent to the mixed raw materials obtained above, stir or ball mill, then vacuum dry—heat treatment—grinding.

Add an appropriate amount of organic solvent to the mixed raw materials, ball mill or stir, then vacuum dry-heat treatment-grinding.

The doping compound is one of the following substances or a mixture thereof, Mg oxide, hydroxide compound, chloride or organic compound; Cr oxide, hydroxide compound, chloride or organic compound; Ti oxide, hydroxide compound, chloride or organic compound; oxide, hydroxide, chloride or organic compound of Y; oxide, hydroxide, chloride or organic compound of Al; oxide, hydroxide, chloride or organic compounds. Further, the ratio of the total moles of doping atoms to the moles of Fe atoms in the doping compound is (0.005˜0.05):1.

The pyrolysis carbon source compound is selected from one of the following substances or a mixture thereof, such as polyvinyl alcohol, polyvinyl chloride, glucose, sucrose, phenolic resin, epoxy resin or ascorbic acid. The carbonization accelerator compound is selected from one of the following substances or a mixture thereof, H ₃ BO ₃ , B ₂ O ₃ , B ₄ C, MnO ₂ , Al ₂ O ₃ , NiO, Si, SiC or SiO ₂ .

Compared with the prior art, the present invention has the following advantages:

1. The present invention adds doping compounds and pyrolytic carbon sources in the process of preparing the iron phosphate precursor, so as to ensure that the interior is evenly doped with metal ions and disperses pyrolytic carbon, which greatly improves the performance of LiFePO ₄ /C The internal lattice conductivity significantly improves its electrochemical performance at high rates.

2. The addition of a carbonization accelerator accelerates the pyrolysis of the carbon source during the synthesis process, and the conductivity of the prepared LiFePO ₄ reaches a high 10 ^-1 S/cm, and its discharge capacity at 10C is 100mAh/g, which has a good rate performance.

3. The present invention no longer uses high-priced ferrous iron, which greatly reduces the cost of raw materials; the preparation of LiFePO ₄ /C by combining the prepared precursor with pyrolysis carbon reduction method reduces the need for heating equipment and protective atmosphere during the synthesis process. requirements, reducing production costs.

Detailed ways

Example 1

(1) Prepare H ₃ PO ₄ solution with mass concentration of 75%, Fe(NO ₃ ) ₃ solution with 1.5mol/L, and NaOH solution with 6mol/L respectively.

(2) Mix the above H ₃ PO ₄ solution and Fe(NO ₃ ) ₃ solution at a molar ratio of 1:1, and then add the doping compound MgO according to the ratio of Mg:Fe atomic molar ratio of 0.02:0.98. The mass ratio of ₄ is 8% and 0.12% respectively, and the pyrolysis carbon source polyvinyl alcohol and the carbonization accelerator H ₃ BO ₃ are added. After stirring for 30 minutes, add it into the reaction kettle together with NaOH solution, stir at a speed of 800 r/min, control the pH to 1.5, and react at 80°C for 5 hours. Afterwards, the precipitate was filtered, washed with distilled water and ethanol until the pH was 7, and dried to obtain the precursor Fe _0.98 Mg _0.02 PO ₄ /C (that is, carbon-coated Fe _0.98 Mg _0.02 PO ₄ ).

(3) Mix the lithium source material LiOH and the Fe _0.98 Mg _0.02 PO ₄ /C prepared in the previous step into the ball mill tank at a molar ratio of 1.03:1, and then use the mass ratio to LiFe _0.98 Mg _0.02 PO ₄ /C as 2% and 0.04%, add polyvinyl alcohol and H ₃ BO ₃ , then add an appropriate amount of absolute ethanol, ball mill for 3 hours, and dry in a vacuum oven at 80°C for 12 hours. Then put the raw material into a tubular resistance furnace with Ar gas, heat it to 350°C at a rate of 5°C/min and hold it for 5h; to room temperature. The prepared sample was fully ground with an agate mortar in a glove box filled with Ar gas and then passed through a 400-mesh standard sieve to obtain a powdery positive electrode active material LiFe _0.98 Mg _0.02 PO ₄ /C.

Using N-methyl-pyrrolidone as a solvent, the active material, conductive agent acetylene black and binder polyvinylidene fluoride were magnetically stirred at a mass ratio of 80:15:5, and then the slurry was coated on aluminum foil to prepare The 30 μm thick positive electrode sheet was dried in a vacuum oven at 100°C for 12 hours, and then the 2016 battery was assembled in a glove box filled with high-purity argon. The electrolyte is 1mol/LLiPF ₆ /EC+DMC+EMC (1:1:1, volume ratio), and the diaphragm is Celgard2400. The battery is subjected to a constant current charge and discharge test within the potential range of 2.5-4.1V. Investigate its discharge capacity at 0.1C and 10C, where 1C=170mA/g. The test temperature is 25±0.5°C.

The charge and discharge test results show that the conductivity of the active powder material is 1.0×10 ^-1 S/cm, the discharge capacity at 0.1C is 148mAh/g, and the discharge capacity at 10C is 100mAh/g.

Comparative example 1

(1) Prepare H ₃ PO ₄ solution with mass concentration of 75%, Fe(NO ₃ ) ₃ solution with 1.5mol/L, and NaOH solution with 6mol/L respectively.

(2) Mix the above H ₃ PO ₄ solution and Fe(NO ₃ ) ₃ solution at a molar ratio of 1:1, stir for 30 minutes, then add it into the reaction kettle together with NaOH solution, stir at a speed of 800r/min, and control the pH The value is 1.5, react at 80°C for 5h. Afterwards, the precipitate was filtered, washed with distilled water and ethanol until the pH was 7, and dried to obtain the precursor FePO ₄ .

(3) Mix the lithium source material LiOH and the FePO ₄ prepared in the previous step into the ball mill tank at a molar ratio of 1.03:1, add the doping compound MgO at a ratio of Mg:Fe atomic molar ratio of 0.02:0.98, add and implement The same amount of polyvinyl alcohol used in Example 1 was prepared according to the method of Example 1 to obtain a powdery positive electrode active material LiFe _0.98 Mg _0.02 PO ₄ /C.

The charge and discharge test results show that the conductivity of the active powder material is 1.1×10 ^-4 S/cm, the discharge capacity at 0.1C is 145mAh/g, and the discharge capacity at 10C is 52mAh/g.

Compared with the material of Example 1, the electrical conductivity of the two differs by nearly 3 orders of magnitude. The comparison results of the 10C rate discharge capacity show that the material of Comparative Example 1 is only half of that of Example 1.

Example 2

(1) Prepare 80% H ₃ PO ₄ solution, 2.0mol/L Fe(NO ₃ ) ₃ solution, and 8mol/L NaOH solution;

(2) Mix the above H ₃ PO ₄ solution and Fe(NO ₃ ) ₃ solution at a molar ratio of 1:1, and add the doping compound Cr ₂ O ₃ into the In the mixed solution, after stirring for 30 minutes, add it into the reaction kettle together with the NaOH solution, and add pyrolysis carbon source epoxy resin and carbonization accelerator to the mixed solution at a ratio of 7% and 0.10% by mass with FePO ₄ B ₂ O ₃ . Stir at a rate of 1000r/min, control the pH value to 1.6, and react at 80°C for 5 hours; then filter the precipitate and wash it with distilled water and ethanol until the pH is 7, and dry to obtain the precursor Fe _0.97 Cr _0.02 PO ₄ /C.

(3) Mix the lithium source material Li ₂ CO ₃ with the precursor Fe _0.97 Cr _0.02 PO ₄ /C obtained in the previous step at a molar ratio of 0.52:1, and the mass ratio to LiFe _0.97 Cr _0.02 PO ₄ /C is respectively 3% and 0.06%, adding epoxy resin and B ₂ O ₃ to obtain mixed raw materials; then add the obtained mixed raw materials to an appropriate amount of organic solvent acetone, after stirring for 5 hours, basically follow the same process as in Example 1 Conditions, vacuum drying - heat treatment - grinding to prepare the powder material LiFe _0.97 Cr _0.02 PO ₄ /C.

Test results show that the electrical conductivity of the above powder material is 1.2×10 ^-1 S/cm, the discharge capacity at 0.1C is 152mAh/g, and the discharge capacity at 10C is 102mAh/g.

Example 3

(1) Prepare 70% H ₃ PO ₄ solution, 1.8mol/L FeCl ₃ solution, and 6mol/L NaOH solution respectively;

(2) Mix the above H ₃ PO ₄ solution and FeCl ₃ solution at a molar ratio of 1:1, and then add the doping compound TiO ₂ to the mixed solution at a ratio of Ti:Fe atomic molar ratio of 0.02:0.96, and NaOH Add _{the solution together into the reaction kettle; then add pyrolysis carbon source phenolic resin and carbonization accelerator B 4 C} , after stirring for 30 min, stirring at a rate of 1000 r/min, controlling the pH value to 1.5, and reacting at 80° C. for 5 h. The precipitate was filtered, washed with distilled water and ethanol until the pH was 7, and dried to obtain the precursor Fe _0.96 Ti _0.02 PO ₄ /C.

(3) Mix the lithium source material LiF and the precursor Fe _0.96 Ti _0.02 PO ₄ /C prepared in the previous step in a ball mill tank at a molar ratio of 1.05:1, and mix it with Fe _0.96 Ti _0.02 PO ₄ /C in a mass ratio 4% and 0.08% respectively, add phenolic resin and B ₄ C into the ball mill tank, add an appropriate amount of absolute ethanol, according to the process conditions of Example 1, through ball milling-vacuum drying-heat treatment-grinding, LiFe _0.96 Ti _0.02 PO ₄ /C was obtained.

Test results show that the electrical conductivity of the above powder material is 1.8×10 ^-1 S/cm, the discharge capacity at 0.1C is 150mAh/g, and the discharge capacity at 10C is 105mAh/g.

Example 4

(1) Prepare 80% H ₃ PO ₄ solution, 0.8mol/L FeCl ₃ solution, and 8mol/L NaOH solution;

(2) Mix the above H ₃ PO ₄ solution and FeCl ₃ solution at a molar ratio of 1:1, and then add the doping compound Y ₂ O ₃ to the mixed solution at a ratio of Y:Fe atom molar ratio of 0.02:0.97, After stirring for 30 minutes, add it into the reaction kettle together with NaOH solution, and add pyrolysis carbon source glucose and carbonization accelerator MnO ₂ according to the mass ratio of FePO ₄ to 8% and 0.12% respectively, at 800r/min Stir at a high speed, control the pH value to 1.5, and react at 80°C for 8h. The precipitate was filtered, washed with distilled water and ethanol until the pH was 7, and dried to obtain the precursor Fe _0.97 Y _0.02 PO ₄ /C.

(3) Mix the lithium source material LiF and the precursor Fe _0.97 Y _0.02 PO ₄ /C prepared in the previous step in a ball mill tank at a molar ratio of 1.04:1, and mix it with Fe _0.97 Y _0.02 PO ₄ /C in a mass ratio The ratios are 2% and 0.04%, respectively, glucose and MnO ₂ are added to the ball mill tank, and an appropriate amount of absolute ethanol is added. According to the process conditions of Example 1, through ball milling-vacuum drying-heat treatment-grinding, LiFe _0.97 Y _0.02 PO ₄ /C.

Test results show that the electrical conductivity of the above powder material is 1.5×10 ^-1 S/cm, the discharge capacity at 0.1C is 153mAh/g, and the discharge capacity at 10C is 106mAh/g.

Example 5

(1) Prepare 70% H ₃ PO ₄ solution, 1.8mol/L Fe ₂ (SO ₄ ) ₃ solution, and 6mol/L NaOH solution;

(2) Mix the above H ₃ PO ₄ solution and Fe ₂ (SO ₄ ) ₃ solution at a molar ratio of 2:1, and then add a doping compound to the mixed solution at a ratio of Al:Fe atomic molar ratio of 0.02:0.97 Al ₂ O ₃ , and then add pyrolytic carbon source ascorbic acid and carbonization accelerator Al ₂ O ₃ to the above mixed solution at a mass ratio of 7% and 0.10% to FePO ₄ , stir for 30 minutes, and mix with NaOH Add the solution together into the reactor; stir at a speed of 600r/min, control the pH value to 1.5, and react at 80°C for 6h. After the precipitate was filtered, washed with distilled water and ethanol until the pH was 7, and dried to obtain the precursor Fe _0.97 Al _0.02 PO ₄ /C.

(3) Mix the lithium source material LiOH and the precursor Fe _0.97 Al _0.02 PO ₄ /C prepared in the previous step in a ball mill tank at a molar ratio of 1.05:1, and mix it with Fe _0.97 Al _0.02 PO ₄ /C in a mass ratio 3% and 0.06% respectively, add ascorbic acid and Al ₂ O ₃ into the ball mill tank, add an appropriate amount of absolute ethanol, according to the process conditions of Example 1, go through ball milling-vacuum drying-heat treatment-grinding, LiFe _0.97 Al _0.02 PO ₄ /C is obtained.

Test results show that the electrical conductivity of the above powder material is 1.6×10 ^-1 S/cm, the discharge capacity at 0.1C is 151mAh/g, and the discharge capacity at 10C is 105mAh/g.

Example 6

(1) Prepare 75% H ₃ PO ₄ solution, 1.5mol/L Fe ₂ (SO ₄ ) ₃ solution, and 8mol/L NaOH solution;

(2) Mix the above H ₃ PO ₄ solution and Fe ₂ (SO ₄ ) ₃ solution at a molar ratio of 2:1, and then add the doping compound La ₂ O ₃ according to the La:Fe atomic molar ratio of 0.02:0.97 Into the mixed solution, according to the mass ratio of FePO ₄ is 6% and 0.09% respectively, add pyrolysis carbon source sucrose and carbonization accelerator SiO ₂ to the mixed solution; after stirring for 30min, add it to the reaction kettle together with the NaOH solution , stirred at a speed of 600r/min, controlled the pH value to 1.6, and reacted at 80°C for 6h; then filtered the precipitate and washed it with distilled water and ethanol until the pH was 7, and then dried to obtain the precursor Fe _0.97 La _0.02 PO ₄ / c.

(3) Mix the lithium source material Li ₂ CO ₃ and the precursor Fe _0.97 La _0.02 PO ₄ /C obtained in the previous step at a molar ratio of 0.52:1, and the mass ratio to Fe _0.97 La _0.02 PO ₄ /C is respectively 4% and 0.08%, add pyrolysis carbon source sucrose and carbonization accelerator SiO ₂ to obtain mixed raw materials; then add the obtained mixed raw materials to an appropriate amount of organic solvent acetone, stir for 8 hours, and basically follow the same method as in Example 1 Under the same process conditions, the powder material LiFe _0.97 Cr _0.02 PO ₄ /C was prepared by vacuum drying-heat treatment-grinding.

Test results show that the electrical conductivity of the above powder material is 1.4×10 ^-1 S/cm, the discharge capacity at 0.1C is 153mAh/g, and the discharge capacity at 10C is 102mAh/g.

Claims (6)

1. the preparation method of a LiFePO 4 material prepares two steps by presoma preparation and LiFePO4 and forms, and it is characterized in that: the presoma preparation process is selected one of following dual mode,

First kind of mode: phosphoric acid solution is mixed with ferric salt solution, in mixed solution, add doping compound, RESEARCH OF PYROCARBON source compound and carbonization promoter compound again; Afterwards with behind above-mentioned mixture system and the alkaline solution hybrid reaction certain hour, resulting powder obtains the ferric phosphate presoma through washing---drying, and the pH value of control reaction system is in 1～4 scope, and temperature is 50～80 ℃ scope;

The second way: phosphoric acid solution is mixed with ferric salt solution, use compound to wherein adding to mix again; Afterwards above-mentioned mixture system is mixed with alkaline solution, again to wherein adding RESEARCH OF PYROCARBON source compound and carbonization promoter compound, behind the reaction certain hour, resulting powder is through washing---drying, obtain the ferric phosphate presoma, the pH value of control reaction system is in 1～4 scope, and temperature is 50～80 ℃ scope;

The LiFePO4 preparation process is: with lithium source and two kinds of materials of the resultant presoma of the first step by lithium atom: the mol ratio of iron atom (1～1.05): 1 prepares burden, and to wherein adding RESEARCH OF PYROCARBON source compound and carbonization promoter compound, gets mixed material again;---vacuumize---heat treatment---is ground can to adopt one of following dual mode to obtain the iron phosphate powder material that nano-sized carbon coats, first kind: ball milling afterwards; Second kind: add appropriate amount of organic in above-mentioned gained mixed material, behind stirring or the ball milling, vacuumize---heat treatment---is ground.

2. the preparation method of LiFePO 4 material as claimed in claim 1, it is characterized in that: described doping compound is one of following material or its mixture, the oxide of Mg, oxyhydroxide, chloride or organic compound; The oxide of Cr, oxyhydroxide, chloride or organic compound; The oxide of Ti, oxyhydroxide, chloride or organic compound; The oxide of Y, oxyhydroxide, chloride or organic compound; The oxide of Al, oxyhydroxide, chloride or organic compound; The oxide of La, oxyhydroxide, chloride or organic compound.

3. the preparation method of LiFePO 4 material as claimed in claim 2 is characterized in that: described doping is (0.005～0.05) with the ratio of the total mole number of foreign atom in the compound and Fe atomic molar number: 1.

4. as the preparation method of the described LiFePO 4 material of one of claim 1～3, it is characterized in that: described RESEARCH OF PYROCARBON source compound is selected one of following material or its mixture, polyvinyl alcohol, polyvinyl chloride, glucose, sucrose, phenolic resins, epoxy resin or ascorbic acid.

5. as the preparation method of the described LiFePO 4 material of one of claim 1～3, it is characterized in that: described carbonization promoter compound is selected one of following material or its mixture, H ₃BO ₃, B ₂O ₃, B ₄C, MnO ₂, Al ₂O ₃, NiO, Si, SiC or SiO ₂

6. the preparation method of LiFePO 4 material as claimed in claim 4 is characterized in that: one of following material of described carbonization promoter compound selection or its mixture, H ₃BO ₃, B ₂O ₃, B ₄C, MnO ₂, Al ₂O ₃, NiO, Si, SiC or SiO ₂