CN102856553A - Preparation method of hydrothermal synthesis carbon coated lithium iron phosphate - Google Patents

Abstract

The invention discloses a method for preparing carbon-coated lithium iron phosphate, which is the anode material of a lithium ion battery, by using ^ascorbic acid as a reducing agent and ^a carbon source. Thermal synthesis of LiFePO ₄ ; ascorbic acid was added to LiFePO ₄ as a reducing agent and carbon source to obtain the LiFePO ₄ /C precursor; under the protection of argon atmosphere, the LiFePO ₄ /C precursor was roasted at high temperature to obtain carbon-coated phosphoric acid lithium iron. The method of the invention controls the morphology of crystal grains and improves the lithium ion diffusion coefficient of lithium iron phosphate by coating carbon, thereby obtaining a lithium iron phosphate material with uniform size and excellent electrochemical performance. The preparation method of the invention is simple and economical, and is suitable for large-scale industrial production.

Description

A kind of preparation method of hydrothermal synthesis carbon-coated lithium iron phosphate

technical field

The invention belongs to the technical field of new energy materials, and relates to a preparation method of lithium iron phosphate, in particular to a method for preparing a carbon-coated lithium iron phosphate composite material by a hydrothermal synthesis method.

Background technique

The research on high-capacity and high-current lithium-ion batteries is not only very important to the development of electric vehicles, but also to the effective storage and utilization of clean energy such as wind, electricity, and solar energy. However, the development of lithium-ion batteries is largely restricted by the improvement of the performance of electrode materials, so the research on high-performance cathode materials is an effective way to solve the bottleneck of lithium-ion battery applications.

The earliest positive electrode material proposed is LiCoO ₂ with a layered structure. After a long period of development, the existing positive electrode materials for lithium-ion batteries are mainly represented by layered structures of LiCoO ₂ , LiNiO ₂ , LiMnO ₂ and their solid solutions. LiMn ₂ O ₄ with a spinel structure, and LiFePO ₄ with an olivine structure. At present, LiFePO ₄ is the most concerned new-generation cathode material for lithium-ion batteries in the market. In 1997, goodenough et al. reported for the first time that LiFePO ₄ with olivine structure could reversibly intercalate and deintercalate Li, and this material received great attention. The theoretical capacity of LiFePO ₄ is 170 mAh/g, and the stable discharge platform is 3.4V relative to the lithium metal negative electrode. Compared with the traditional lithium-ion positive electrode materials LiMn ₂ O ₄ and LiCoO ₂ , LiFePO ₄ has a wider source of raw materials and a longer cycle life. Long life, good thermal stability, lower price, environmental friendliness, and no pollution, it is one of the most powerful competitors for the next generation of lithium-ion battery cathode materials.

Due to the special structure of LiFePO ₄ , Li ⁺ has only one-dimensional transport channel, so the diffusion rate of Li ⁺ inside the material is relatively low. Researchers have overcome this shortcoming through modification methods such as doping and coating.

Chiang et al. (Chung S Y, Bloking J T, Chiang Y M. Electronically conductive phospho-olivines as lithium storage electrodes[J], Nature Materials, 2002,1(2):123-128) synthesized cation-deficient LiFePO ₄ , and in Among them, high-valent metals (Nb ⁵⁺ , Mg ²⁺ , Al ³⁺ , Ti ⁴⁺ , W ^{6+ ,} etc.) are doped with solid solution, thereby increasing the conductivity of LiFePO ₄ by 8 orders of magnitude, surpassing the traditional LiCoO ₂ , Electrical conductivity _{of LiMn2O4} . Croce et al. (Croce F, Epifanio A D, Hassoun J, et al. A novel concept for the synthesis of an improved LiFePO ₄ lithium battery cathode [J]. Elecrochem Solid-State Lett, 2002,5(3):47-50 ) doped 1% Cu and Ag in LiFePO4, the specific capacity of the material increased by 25mAh/g. The reason is that the metal particles dispersed in LiFePO ₄ provide LiFePO ₄ with the function of a conductive bridge, enhance the conductivity between particles, and reduce the impedance between particles. At the same time, the doping of metal Cu and Ag can also reduce the resistance of LiFePO ₄ particles. size, thereby increasing the reversible specific capacity of the material. Since Li ⁺ diffuses in one dimension in LiFePO ₄ , effectively controlling the particle size of LiFePO ₄ is the key to improving the diffusion ability of Li ions in LiFePO ₄ . Huang et al. (Huang H, Yings C, Nazra L F. Approachong theoretical capacity of LiFePO ₄ at room temperature at high rates [J]. Electrochemical and solid- State Letters, 2001, 4(10):170-172) adopted in Carbon black treated with HNO ₃ /H ₂ SO ₄ is added to the mixed solution of reactants. Since the partially oxidized carbon black contains carboxyl groups, it acts as a nucleating particle, and LiFePO ₄ with a particle size of less than 200nm is obtained. Discharging at a current of 0.1C can obtain a specific capacity of more than 150mAh/g, and the performance at a high current is also excellent, and a specific capacity of about 130mAh/g can be obtained. Dispersing or coating conductive carbon in LiFePO ₄ can enhance the electronic conductivity between particles and reduce the polarization of the battery on the one hand; on the other hand, it can also provide electron tunneling for LiFePO ₄ to compensate for the Li ⁺ deintercalation process. charge balance in .

Contents of the invention

The purpose of the present invention is to overcome the problems of low electrical conductivity and lithium ion diffusion coefficient of lithium iron phosphate, the cathode material of lithium ion batteries, and provide a preparation method of hydrothermally synthesized carbon-coated lithium iron phosphate (LiFePO ₄ /C composite material).

The present invention adopts following technical scheme:

A preparation method for hydrothermally synthesizing carbon-coated lithium iron phosphate, comprising the following steps:

(1) Synthesis of LiFePO ₄ precursor: In the mixed solution of FeSO ₄ , H ₃ PO ₄ and appropriate amount of ascorbic acid, add LiOH solution, wherein the molar ratio of FeSO ₄ , H ₃ PO ₄ and LiOH is 1:1:3, Heating and carrying out hydrothermal reaction under stirring conditions; after the reaction product is cooled, centrifuge, wash and dry to obtain LiFePO ₄ precursor powder;

(2) Synthesis of LiFePO ₄ /C precursor: After dispersing the LiFePO ₄ precursor powder and ascorbic acid obtained in step (1) with alcohol, mix them evenly by ball milling, and vacuum-dry at 100°C until black powder is obtained;

(3) Synthesis of LiFePO ₄ /C composite material: Under the protection of an inert atmosphere, the LiFePO ₄ /C precursor obtained in step (2) was calcined to obtain carbon-coated lithium iron phosphate.

In step (1), the hydrothermal reaction temperature is 100-140° C., and the reaction time is 1-4 hours.

In step (2), the mass ratio of the LiFePO ₄ precursor powder to ascorbic acid is LiFePO ₄ : ascorbic acid=1:0.005～0.10, preferably 1:0.03～0.07.

In step (3), the calcination temperature is 600-800° C., and the calcination time is 6-15 hours.

More specifically and optimally, the described method comprises the following steps:

(1) Synthesis of LiFePO ₄ precursor: Weigh FeSO ₄ .7H ₂ O, H ₃ PO ₄ and LiOH.H ₂ O at a molar ratio of 1:1:3, first FeSO ₄ .7H ₂ O, H ₃ PO ₄ was dissolved in 50 ml water, then 0.2 g of ascorbic acid was added to prevent Fe ²⁺ from being oxidized to Fe ³⁺ , and finally LiOH.H ₂ O dissolved in water was added, and the solution was quickly transferred to a 500 ml stainless steel reaction kettle, Put it in a collector-type magnetic heating stirrer, set the temperature to 120°C and stir for 2 hours; after the reaction kettle is cooled to room temperature, centrifuge the reaction product, wash with water and disperse with alcohol, and dry in a vacuum oven for 2 hours A light green powder was obtained.

(2) Synthesis of LiFePO ₄ /C precursor: Disperse the LiFePO ₄ precursor powder obtained in step (1) and ascorbic acid in mass ratio LiFePO ₄ : ascorbic acid = 1:0.03～0.07 with alcohol, and mill on a ball mill for 4 hours , placed in a vacuum oven, dried at 100°C for 2 hours to obtain a black powder.

(3) Synthesis of LiFePO ₄ /C composite material: under the protection of argon atmosphere, the above-mentioned LiFePO ₄ /C precursor was transferred to a tube furnace, and treated at 700 ° C for 10 h to obtain lithium iron phosphate uniformly coated with carbon .

For the structural characterization and performance evaluation of the carbon-coated lithium iron phosphate (LiFePO ₄ /C) material prepared by the present invention, the evaluation method is as follows:

The phase structure and crystal structure parameters of the crystal grains, such as lattice constant, unit cell volume, grain size and other microstructural characteristics, were analyzed by wide-angle X-ray diffractometer (XRD). The instrument model is A Rigaku D/Max-3C, and the conditions are: Cu target Kα ray, tube voltage 50kV, tube current 200mA. It can be seen from Figure 1 that the difference in carbon content did not cause the phase change in the samples. There are no stray peaks in the samples, and they all show a single phase structure, which has little effect on the microstructure of lithium iron phosphate.

The morphology, particle size, internal structure of crystalline particles, and carbon coating on the particle surface were analyzed by scanning electron microscopy (SEM) and field emission transmission electron microscopy (TEM). The instrument model is: JSM-5610LV JEOL (SEM), JEM-2100F (TEM), the accelerating voltage is 200kv. It can be seen from Figure 2 that the particle size of LiFePO4 without carbon coating is 200-300nm, and the particle size after carbon coating is about 100-150nm.

The charge-discharge performance of the material (including charge-discharge capacity, cycle life, voltage platform) and the electrochemical kinetic performance of the positive electrode material are studied by charge-discharge test and electrochemical impedance spectroscopy (EIS). The instrument models are: electrochemical workstation (Arbin) and CHI660D. Based on the open circuit voltage, the applied amplitude is 5mV, and the sweep is performed within the frequency range of 10 ⁵ ~0.01H.

The preparation method of hydrothermally synthesized carbon-coated lithium iron phosphate of the present invention comprises the steps of low-temperature hydrothermally synthesizing LiFePO ₄ , using ascorbic acid as a reducing agent and a carbon source, and heating the coated carbon under the protection of an argon atmosphere to prepare carbon-coated lithium iron phosphate. Nanospheres with uniform size and uniform shape are obtained, and the shape of the crystal grains is controlled by coating carbon, and the lithium ion diffusion coefficient of lithium iron phosphate is improved, and its electrochemical performance is excellent. The present invention adopts cheap FeSO ₄ as an iron source, and prepares lithium iron phosphate with excellent electrochemical performance through a hydrothermal synthesis method. The preparation method is simple, the synthesis path is relatively simple, the cost is low, and there is no environmental pollution, which is suitable for large-scale industrial production. .

The present invention will be described in detail below in conjunction with specific embodiments. The protection scope of the present invention is not limited by the specific embodiments, but by the claims.

Description of drawings

Fig. 1 is the XRD patterns of LiFePO ₄ /C composite materials with different carbon contents prepared according to the method of the present invention.

Fig. 2 is the SEM and TEM images of LiFePO ₄ /C composite materials with different carbon contents prepared according to the method of the present invention. Among them, A1 and A2 are the SEM and TEM spectra of LiFePO ₄ -0%C, B1 and B2 are the SEM and TEM spectra of LiFePO ₄ -5%C.

Fig. 3 is a charge-discharge voltage-capacity diagram of LiFePO ₄ /C composite materials with different carbon contents prepared according to the method of the present invention. Among them (a) is LiFePO ₄ -0%C; (b) is LiFePO ₄ -3%C, (c) LiFePO ₄ -5%C is (d) is LiFePO ₄ -7%C.

Fig. 4 is an electrochemical impedance spectrum of LiFePO ₄ /C composite materials with different carbon contents prepared according to the method of the present invention.

Detailed ways

Example 1

A method for hydrothermally synthesizing carbon-coated lithium iron phosphate (LiFePO ₄ /C composite material), comprising the following steps:

(1) Synthesis of LiFePO ₄ precursor: FeSO ₄ .7H ₂ O, H ₃ PO ₄ and LiOH.H ₂ O were weighed at a molar ratio of 1:1:3, and FeSO ₄ .7H ₂ O, H ₃ PO ₄ was dissolved in 50 ml of double distilled water, then 0.2 g of ascorbic acid was added to prevent Fe ²⁺ from being oxidized to Fe ³⁺ , and finally dissolved LiOH.H ₂ O was added, and the solution was quickly transferred to a 500 ml stainless steel reaction kettle , put it in a collector type magnetic heating stirrer, set the temperature to 120° C. and stir for 2 h. After the reactor was cooled to room temperature, the reaction product was centrifuged, washed with double distilled water and dispersed in alcohol, and then dried in a vacuum oven for 2 hours to obtain a light green powder.

(2) Synthesis of LiFePO ₄ /C precursor: put the precursor powder obtained in step (1) and ascorbic acid in a certain mass ratio (LiFePO ₄ : ascorbic acid = 1:0.03, w/w) in an agate jar and disperse with alcohol Finally, after ball milling on a planetary ball mill for 4h (450r), put it in a vacuum drying oven, and dry at 100°C for 2h to obtain a black powder.

(3) Synthesis of LiFePO ₄ /C composite material: Under the protection of argon atmosphere, the above-mentioned LiFePO ₄ /C precursor was transferred to a tube furnace, and treated at 700°C for 10 hours to obtain lithium iron phosphate uniformly coated with carbon.

Example 2

A method for hydrothermally synthesizing carbon-coated lithium iron phosphate (LiFePO ₄ /C composite material), comprising the following steps:

(1) Synthesis of LiFePO ₄ precursor: Weigh FeSO ₄ .7H ₂ O, H ₃ PO ₄ and LiOH.H ₂ O at a molar ratio of 1:1:3, first FeSO ₄ .7H ₂ O, H ₃ PO ₄ was dissolved in 50 ml of double distilled water, then 0.2 g of ascorbic acid was added to prevent Fe ²⁺ from being oxidized to Fe ³⁺ , and finally dissolved LiOH.H ₂ O was added, and the solution was quickly transferred to a 500 ml stainless steel reaction kettle , put it in a collector type magnetic heating stirrer, set the temperature to 120° C. and stir for 2 h. After the reactor was cooled to room temperature, the reaction product was centrifuged, washed with double distilled water and dispersed in alcohol, and then dried in a vacuum oven for 2 hours to obtain a light green powder.

(2) Synthesis of LiFePO ₄ /C precursor: put the precursor powder obtained in step (1) and ascorbic acid in a certain mass ratio (LiFePO ₄ : ascorbic acid = 1:0.05 w/w) in an agate jar and disperse with alcohol , after ball milling on a planetary ball mill for 4h (450r), put it in a vacuum oven and dry at 100°C for 2h to obtain a black powder.

(3) Synthesis of LiFePO ₄ /C composite material: Under the protection of argon atmosphere, the above-mentioned LiFePO ₄ /C precursor was transferred to a tube furnace, and treated at 700°C for 10 hours to obtain lithium iron phosphate uniformly coated with carbon.

Example 3

A method for hydrothermally synthesizing carbon-coated lithium iron phosphate (LiFePO ₄ /C composite material), comprising the following steps:

(1) Synthesis of LiFePO ₄ precursor: FeSO ₄ .7H ₂ O, H ₃ PO ₄ and LiOH.H ₂ O were weighed at a molar ratio of 1:1:3, and FeSO ₄ .7H ₂ O, H ₃ PO ₄ was dissolved in 50 ml of double distilled water, then 0.2 g of ascorbic acid was added to prevent Fe ²⁺ from being oxidized to Fe ³⁺ , and finally dissolved LiOH.H ₂ O was added, and the solution was quickly transferred to a 500 ml stainless steel reaction kettle , put it in a collector type magnetic heating stirrer, set the temperature to 120° C. and stir for 2 h. After the reactor was cooled to room temperature, the reaction product was centrifuged, washed with double distilled water and dispersed in alcohol, and then dried in a vacuum oven for 2 hours to obtain a light green powder.

(2) Synthesis of LiFePO ₄ /C precursor: put the precursor powder obtained in step (1) and ascorbic acid in a certain mass ratio (LiFePO ₄ :ascorbic acid=1:0.07 w/w) in an agate jar and disperse with alcohol , after ball milling on a planetary ball mill for 4h (450r), put it in a vacuum oven and dry at 100°C for 2h to obtain a black powder.

(3) Synthesis of LiFePO ₄ /C composite material: Under the protection of argon atmosphere, the above-mentioned LiFePO ₄ /C precursor was transferred to a tube furnace, and treated at 700°C for 10 hours to obtain lithium iron phosphate uniformly coated with carbon.

Comparative example 1

A method for hydrothermally synthesizing lithium iron phosphate, comprising the following steps:

(1) Synthesis of LiFePO ₄ precursor: FeSO ₄ .7H ₂ O, H ₃ PO ₄ and LiOH.H ₂ O were weighed at a molar ratio of 1:1:3, and FeSO ₄ .7H ₂ O, H ₃ PO ₄ was dissolved in 50 ml of double distilled water, then 0.2 g of ascorbic acid was added to prevent Fe ²⁺ from being oxidized to Fe ³⁺ , and finally dissolved LiOH.H ₂ O was added, and the solution was quickly transferred to a 500 ml stainless steel reaction kettle , put it in a collector type magnetic heating stirrer, set the temperature to 120° C. and stir for 2 h. After the reactor was cooled to room temperature, the reaction product was centrifuged, washed with double distilled water and dispersed in alcohol, and then dried in a vacuum oven for 2 hours to obtain a light green powder.

(2) Place the precursor powder obtained in step (1) into an agate jar and disperse it with alcohol, then ball mill it on a planetary ball mill for 4 hours (450r), then put it in a vacuum drying oven and dry it at 100°C for 2 hours.

(3) Synthesis of LiFePO ₄ : Under the protection of argon atmosphere, the above precursor was transferred to a tube furnace and treated at 700 °C for 10 h to obtain lithium iron phosphate.

Claims (6)

1. the preparation method of a Hydrothermal Synthesis carbon-coated LiFePO 4 for lithium ion batteries is characterized in that, described method may further comprise the steps:

(1) LiFePO ₄Synthesizing of precursor: at FeSO ₄, H ₃PO ₄In the mixed solution of an amount of ascorbic acid, add LiOH solution, wherein FeSO ₄, H ₃PO ₄With the mol ratio of LiOH be 1:1:3, hydro-thermal reaction is also carried out in heating under stirring condition; After the product cooling, centrifugal, cleaning and dry obtains LiFePO ₄The precursor powder;

(2) LiFePO ₄/ C precursor synthetic: the LiFePO that step (1) is obtained ₄After precursor powder and ascorbic acid disperseed with alcohol, ball milling mixed, and 100 ℃ of vacuumizes are to obtaining black powder;

(3) LiFePO ₄/ C composite material synthetic: under inert atmosphere protection, the LiFePO that step (2) is obtained ₄The roasting of/C precursor obtains the LiFePO4 that carbon coats.

2. the preparation method of carbon-coated LiFePO 4 for lithium ion batteries according to claim 1 is characterized in that, in the step (1), described hydrothermal temperature is 100～140 ℃, and the reaction time is 1～4h.

3. the preparation method of carbon-coated LiFePO 4 for lithium ion batteries according to claim 1 is characterized in that, in the step (2), and described LiFePO ₄The mass ratio of precursor powder and ascorbic acid is LiFePO ₄: ascorbic acid=1:0.005～0.10.

4. the preparation method of carbon-coated LiFePO 4 for lithium ion batteries according to claim 3 is characterized in that, in the step (2), and described LiFePO ₄The mass ratio of precursor powder and ascorbic acid is LiFePO ₄: ascorbic acid=1:0.03～0. 07.

5. the preparation method of carbon-coated LiFePO 4 for lithium ion batteries according to claim 1 is characterized in that, in the step (3), described sintering temperature is 600～800 ℃, and roasting time is 6～15h.

6. the preparation method of carbon-coated LiFePO 4 for lithium ion batteries according to claim 1 is characterized in that, described method may further comprise the steps:

(1) LiFePO ₄Synthesizing of precursor: take by weighing FeSO take mol ratio as 1:1:3 ₄.7H ₂O, H ₃PO ₄And LiOH.H ₂O is first with FeSO ₄.7H ₂O, H ₃PO ₄Be dissolved in the water of 50 ml, then add the ascorbic acid of 0.2g to prevent Fe ²⁺Be oxidized to Fe ³⁺, add at last the LiOH.H that has been dissolved in the water ₂O transfers to solution rapidly in the 500 ml stainless steel cauldrons, places it in the heat collecting type magnetic force heating stirrer, and set temperature is 120 ℃ and stirring reaction 2h; After reactor is cooled to room temperature, product is centrifugal, after water cleaning and alcohol disperse, place the dry 2h of vacuum drying chamber to obtain jade-green powder;

(2) LiFePO ₄/ C precursor synthetic: the LiFePO that step (1) is obtained ₄Precursor powder and ascorbic acid be LiFePO in mass ratio ₄: ascorbic acid=1:0.03～0.07 usefulness alcohol behind ball milling 4h on the ball mill, is placed in the vacuum drying chamber after disperseing, and 100 ℃ of dry 2h obtain black powder;

(3) LiFePO ₄Synthesizing of/C composite material: under the argon gas atmosphere protection, with above-mentioned LiFePO ₄/ C precursor is transferred in the tube furnace, at 700 ℃ of lower 10h that process, obtains the LiFePO4 that carbon evenly coats.

Images