CN103390750A - Method for preparing lithium iron phosphate positive material - Google Patents

Abstract

The invention discloses a preparation method of lithium iron phosphate cathode material. This method aims at the shortcomings of the existing modification of lithium iron phosphate materials. A certain amount of carbon source is added to the synthetic precursor and carbon source gas and silicon source gas are introduced during the sintering process. A layer of SiC coating layer is evenly deposited on the surface to realize the preparation of lithium iron phosphate material with a uniform SiC/C coating layer on the surface. During the synthesis process, the lithium iron phosphate positive electrode with a uniform C/SiC coating layer on the surface was obtained by adjusting the amount of carbon source added in the precursor and the inlet pressure, flow rate and vapor deposition time of the carbon source and silicon source gas during the vapor deposition process. Material. This material has a small primary particle size, a uniform and compact C/SiC coating layer on the surface, a high tap density, good rate charge and discharge performance, and excellent processing performance. It has broad application prospects in the field of power lithium-ion batteries.

Description

A kind of preparation method of lithium iron phosphate cathode material

technical field

The invention relates to the fields of electrochemical material preparation and new energy, in particular to a preparation method of lithium iron phosphate cathode material.

Background technique

With the increasing energy crisis, the whole world is looking for new renewable resources. Renewable resources such as solar energy, wind energy, tidal energy, geothermal energy, etc. have regional and temporal characteristics. To make full use of these resources, smart grids or large-scale energy storage systems are required. Lithium-ion batteries have promising application prospects as energy storage potentials. On the other hand, the depletion of petrochemical energy forces people to develop hybrid electric or pure electric vehicles, and its core component battery has become a research focus. Compared with other batteries, lithium-ion is favored for its high energy density, good safety and long service life.

Lithium iron phosphate material, as the cathode material of lithium-ion batteries, has attracted widespread attention due to its abundant raw materials, low price, environmental protection, moderate working voltage, and good thermal stability.

After years of research and development, its performance has been significantly improved. However, lithium iron phosphate still has the problem of low ion conductivity and electronic conductivity, and its performance is poor when charging and discharging at high rates. At present, the research on the modification of lithium iron phosphate materials is mainly through the following methods: (1) Nano treatment, reducing the grain size of synthesized lithium iron phosphate can improve the ion conductivity of lithium iron phosphate. The smaller the crystal particle radius, the shorter the solid phase diffusion process of lithium ions in the ions, and the easier the intercalation and extraction of lithium ions; (2) element doping, introducing heteroatoms into the material lattice to improve the electrical conductivity of the material, but Its feasibility and working mechanism are still unclear at present, and the effect is not obvious, and the capacity will decrease with the increase of element doping amount; (3) Doping with conductive agent can improve the electronic conductivity of the material, and most of them use carbon Coating technology, due to the complex form of carbon materials, the tap density of most carbon-coated lithium iron phosphate materials will be greatly reduced, and the processing performance will be affected to a certain extent.

There are many studies on the modification of lithium iron phosphate materials, but it has not been found that adding carbon source to the precursor combined with vapor deposition of SiC during the sintering process to synthesize iron phosphate with small particle size and tight SiC/C carbon coating Research on lithium materials.

Contents of the invention

The technical problem to be solved by the present invention is to provide a preparation method of lithium iron phosphate cathode material.

In order to solve the above-mentioned technical problems, the technical solution adopted in the present invention is: a preparation method of lithium iron phosphate cathode material, which is characterized in that it comprises the following steps:

1) Weigh the lithium source, ferrous oxalate, ammonium dihydrogen phosphate, and organic carbon source according to the molar ratio Li:Fe:P:C=1.02:1:1:0.2～0.5, and use alcohol or acetone as the dispersant for ball milling to disperse treatment, and then vacuum drying treatment to obtain the precursor;

2) Place the precursor in a tube furnace at 550°C and pre-burn it under the protection of nitrogen. The heating rate is 5°C/min, the holding time is 5 hours, and it is naturally cooled down to room temperature, and the pre-burned material is coarsely pulverized ;

3) Place the coarsely crushed calcined material in a tube furnace for secondary sintering, raise the temperature to 700-750°C at a rate of 5°C/min under the protection of an inert gas, and keep it warm for 8-15 hours; The inlet pressure of the inert gas is 0.15MPa, and the flow rate is 0.1L/min; when the temperature reaches the set temperature, carbon source gas and silicon source gas are introduced for vapor deposition and coating. During the vapor deposition process, the inlet pressure of carbon source gas is 0.1~ 0.2MPa, the flow rate is 0.04-0.12L/min; the silicon source gas inlet pressure is 0.1-0.2MPa, the flow rate is 0.04-0.12L/min; at the same time, the inert gas inlet pressure is adjusted to 0.1-0.2MPa, and the flow rate is 0.04-0.12L /min; after the vapor deposition time is 30min-240min, close the intake of carbon source gas and silicon source gas, keep the inlet pressure of inert gas at 0.15MPa, and the flow rate at 0.1L/min; complete the rest of the sintering process under this inert atmosphere, and Cool naturally to room temperature to obtain a lithium iron phosphate positive electrode material with a surface C/SiC coating layer.

As preferably, the lithium source described in step 1) is one or a combination of at least two of lithium carbonate, lithium hydroxide, lithium acetate, lithium fluoride; the organic carbon source is glucose, sucrose, phenolic resin, One or a combination of at least two of epoxy resin or polyethylene.

As another preference, the carbon source gas described in step 3) is one or a combination of at least two of methane, ethane, ethylene, acetylene, benzene, toluene; the silicon source gas is monosilane, disilane 1. One or a combination of at least two of silicon tetrafluoride.

Still another preference, the inert gas described in step 3) is one or a mixture of at least two of helium, nitrogen and argon.

The beneficial effects of the present invention are:

Combining the addition of carbon source to the precursor and the introduction of SiC through the chemical vapor deposition process in the subsequent heat treatment process, the growth of crystal particles in the traditional solid-state reaction process is effectively inhibited, and the uniform coating of C on the surface of lithium iron phosphate material is realized. /SiC layer, this material has excellent rate charge and discharge performance and processing performance.

Description of drawings

The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

Fig. 1 is a TEM image of the lithium iron phosphate material obtained in Example 3 of the preparation method of the lithium iron phosphate cathode material of the present invention.

Fig. 2 is an SEM image of the lithium iron phosphate material obtained in Example 3 of the preparation method of the lithium iron phosphate cathode material of the present invention.

3 is a comparison chart of the discharge curves of the lithium iron phosphate material obtained in Example 3 of the preparation method of the lithium iron phosphate cathode material of the present invention and the lithium iron phosphate material obtained in the comparative example at a rate of 0.2C.

In the figure, a is C/SiC vapor deposition coated lithium iron phosphate, b is carbon coated lithium iron phosphate, c is SiC vapor deposition coated lithium iron phosphate, and d is pure phase lithium iron phosphate.

4 is a rate discharge curve of the lithium iron phosphate material obtained in Example 3 of the preparation method of the lithium iron phosphate cathode material of the present invention.

Detailed ways

Example 1

1) Weigh lithium carbonate, ferrous oxalate, ammonium dihydrogen phosphate, and sucrose according to the molar ratio Li:Fe:P:C=1.02:1:1:0.2, and carry out ball milling dispersion treatment with alcohol as a dispersant for 5 hours, and then carry out The precursor was obtained by vacuum drying; the dry precursor obtained above was placed in a tube furnace at 550°C and pre-fired under the protection of nitrogen. The heating rate was 5°C/min, and the holding time was 5 hours. Natural cooling down to room temperature , coarsely pulverize the calcined material;

2) Place the coarsely pulverized calcined material obtained above in a tube furnace for secondary sintering, and raise the temperature to 700° C. at a rate of 5° C./min under the protection of nitrogen, and keep it warm for 10 hours. During the heating process, the nitrogen inlet pressure is 0.15MPa, and the flow rate is 0.1L/min. When the temperature reaches the set temperature, introduce methane gas and monosilane gas for vapor deposition coating. During vapor deposition, the inlet pressure of methane gas is 0.1MPa, the flow rate is 0.12L/min, the inlet pressure of monosilane gas is 0.1MPa, and the flow rate is 0.1MPa. 0.12L/min, while adjusting the nitrogen inlet pressure to 0.1MPa, and the flow rate to 0.04L/min. After 30 minutes of vapor deposition, turn off the intake of methane gas and monosilane gas, keep the nitrogen intake pressure at 0.15MPa, and the flow rate at 0.1L/min, complete the rest of the sintering process in this nitrogen atmosphere, and naturally cool to room temperature, that is, A lithium iron phosphate cathode material with a surface C/SiC coating layer. The material had a tap density of 1.05 g/cm ³ .

Example 2

1) Weigh lithium carbonate, ferrous oxalate, ammonium dihydrogen phosphate, and glucose according to the molar ratio Li:Fe:P:C=1.02:1:1:0.3, and use acetone as a dispersant to carry out ball milling dispersion treatment for 5 hours, and then carry out The precursor was obtained by vacuum drying; the dry precursor obtained above was placed in a tube furnace at 550°C and pre-fired under the protection of argon. The heating rate was 5°C/min, the holding time was 5 hours, and the natural cooling decreased to At room temperature, coarsely pulverize the calcined material;

2) Place the coarsely pulverized calcined material obtained above in a tube furnace for secondary sintering, and raise the temperature to 720° C. at a rate of 5° C./min under the protection of argon, and keep it warm for 10 hours. During the heating process, the argon inlet pressure is 0.15MPa, and the flow rate is 0.1L/min. When the temperature reaches the set temperature, introduce ethane gas and disilane gas for vapor deposition coating. During the vapor deposition process, the inlet pressure of ethane gas is 0.12MPa, the flow rate is 0.1L/min, and the inlet pressure of disilane gas is 0.12MPa , the flow rate is 0.1L/min, and at the same time adjust the argon inlet pressure to 0.1MPa, and the flow rate is 0.08L/min. After 60 minutes of vapor deposition, turn off the ethane gas and disilane gas inlets, keep the argon inlet pressure at 0.15MPa, and the flow rate at 0.1L/min, complete the rest of the sintering process in this argon atmosphere, and naturally cool to room temperature , that is, a lithium iron phosphate positive electrode material with a surface C/SiC coating layer is obtained. The tap density of this material was 1.1 g/cm ³ .

Example 3

1) Weigh lithium carbonate, ferrous oxalate, ammonium dihydrogen phosphate, and phenolic resin according to the molar ratio Li:Fe:P:C=1.02:1:1:0.35, and carry out ball milling dispersion treatment with alcohol as a dispersant for 5 hours, and then Carry out vacuum drying to obtain the precursor; place the dry precursor obtained above in a tube furnace at 550°C and pre-burn under the protection of nitrogen. The heating rate is 5°C/min, the holding time is 5 hours, and the natural cooling is reduced to At room temperature, coarsely pulverize the calcined material;

2) Place the coarsely pulverized calcined material obtained above in a tube furnace for secondary sintering, and raise the temperature to 740° C. at a rate of 5° C./min under the protection of nitrogen, and keep it warm for 10 hours. During the heating process, the inlet pressure of nitrogen gas is 0.15MPa, and the flow rate is 0.1L/min. When the temperature reaches the set temperature, introduce methane gas and monosilane gas for vapor deposition coating. During the vapor deposition process, the inlet pressure of methane gas is 0.12MPa, the flow rate is 0.08L/min, the inlet pressure of monosilane gas is 0.12MPa, and the flow rate 0.08L/min, while adjusting the nitrogen gas inlet pressure to 0.1MPa, and the flow rate to 0.1L/min. After 120 minutes of vapor deposition, close the intake of methane gas and monosilane gas, keep the intake pressure of nitrogen gas at 0.15MPa, and the flow rate at 0.1L/min, complete the rest of the sintering process in this nitrogen atmosphere, and naturally cool to room temperature, that is A lithium iron phosphate cathode material with a surface C/SiC coating layer is obtained. The tap density of the material is 1.3g/cm ³ . The TEM and SEM of the obtained lithium iron phosphate material are shown in Figure 1 and Figure 2 respectively, and the half-cell charge and discharge curve is shown in Figure 3 .

Example 4

1) Weigh lithium carbonate, ferrous oxalate, ammonium dihydrogen phosphate, and epoxy resin according to the molar ratio Li:Fe:P:C=1.02:1:1:0.5, and use acetone as a dispersant to carry out ball milling dispersion treatment for 5 hours, Then carry out vacuum drying treatment to obtain the precursor; place the dry precursor obtained above in a tube furnace at 550°C, and pre-burn it under the protection of argon, with a heating rate of 5°C/min, a holding time of 5 hours, and natural cooling Cool down to room temperature, coarsely pulverize the calcined material;

2) Place the coarsely pulverized calcined material obtained above in a tube furnace for secondary sintering, and raise the temperature to 740° C. at a rate of 5° C./min under the protection of argon, and keep it warm for 10 hours. During the heating process, the argon inlet pressure is 0.15MPa, and the flow rate is 0.1L/min. When the temperature reaches the set temperature, acetylene gas and silicon tetrafluoride gas are introduced for vapor deposition coating. 0.2MPa, the flow rate is 0.04L/min, and at the same time, adjust the argon inlet pressure to 0.1MPa, and the flow rate is 0.1L/min. After 220 minutes of vapor deposition, close the acetylene gas and silicon tetrafluoride gas inlet, keep the argon inlet pressure at 0.15MPa, and the flow rate at 0.1L/min, complete the rest of the sintering process in this argon atmosphere, and cool naturally to At room temperature, a lithium iron phosphate positive electrode material with a surface C/SiC coating layer is obtained. The tap density of this material was 1.1 g/cm ³ .

Comparative Example 1

1) Weigh lithium carbonate, ferrous oxalate, ammonium dihydrogen phosphate, and sucrose according to the molar ratio Li:Fe:P:C=i.02:1:1:0.35, and carry out ball milling dispersion treatment with alcohol as a dispersant for 5 hours, Then carry out vacuum drying treatment to obtain the precursor; place the dry precursor obtained above in a tube furnace at 550 ° C, and pre-burn it under the protection of nitrogen. The heating rate is 5 ° C / min, and the holding time is 5 hours. To room temperature, coarsely pulverize the calcined material;

2) Put the coarsely pulverized calcined material obtained above into a tube furnace for secondary sintering, and raise the temperature to 740° C. at a rate of 5° C./min under the protection of nitrogen gas, and keep it warm for 10 hours. During the process, the inlet pressure of nitrogen gas is 0.15MPa, and the flow rate is 0.1L/min. And naturally cooled to room temperature, that is, a carbon-coated lithium iron phosphate positive electrode material that has not undergone a vapor phase deposition process is obtained. The material has a tap density of 0.85 g/cm ³ .

Comparative Example 2

1) Weigh lithium carbonate, ferrous oxalate, and ammonium dihydrogen phosphate according to the molar ratio Li:Fe:P=1.02:1:1, use alcohol as a dispersant for ball milling and dispersing for 5 hours, and then vacuum dry to obtain the precursor ; Place the dry precursor obtained above in a tube furnace at 550°C, under the protection of nitrogen for pre-burning, the heating rate is 5°C/min, the holding time is 5 hours, naturally cool down to room temperature, and the pre-fired material is Coarse crushing;

2) Place the coarsely pulverized calcined material obtained above in a tube furnace for secondary sintering, and raise the temperature to 740° C. at a rate of 5° C./min under the protection of nitrogen, and keep it warm for 10 hours. During the heating process, the inlet pressure of nitrogen gas is 0.15MPa, and the flow rate is 0.1L/min. When the temperature reaches the set temperature, introduce methane gas and monosilane gas for vapor deposition coating. During the vapor deposition process, the inlet pressure of methane gas is 0.12MPa, the flow rate is 0.08L/min, the inlet pressure of monosilane gas is 0.12MPa, and the flow rate 0.08L/min, while adjusting the nitrogen gas inlet pressure to 0.1MPa, and the flow rate to 0.1L/min. After 120 minutes of vapor deposition, close the intake of methane gas and monosilane gas, keep the intake pressure of nitrogen gas at 0.15MPa, and the flow rate at 0.1L/min, complete the rest of the sintering process in this nitrogen atmosphere, and naturally cool to room temperature, that is A lithium iron phosphate cathode material with a surface SiC coating layer is obtained. The material had a tap density of 1.15 g/cm ³ .

Comparative Example 3

1) Weigh lithium carbonate, ferrous oxalate, and ammonium dihydrogen phosphate according to the molar ratio Li:Fe:P=i.02:1:1, use alcohol as a dispersant for ball milling and dispersing for 5 hours, and then vacuum dry to obtain Precursor: Place the dry precursor obtained above in a tube furnace at 550°C under the protection of nitrogen for pre-calcination, the heating rate is 5°C/min, the holding time is 5 hours, and naturally cool down to room temperature. The material is coarsely crushed;

2) Put the coarsely pulverized calcined material obtained above into a tube furnace for secondary sintering, and raise the temperature to 740° C. at a rate of 5° C./min under the protection of nitrogen gas, and keep it warm for 10 hours. During the process, the inlet pressure of nitrogen gas is 0.15MPa, and the flow rate is 0.1L/min. And naturally cooled to room temperature, the pure-phase lithium iron phosphate positive electrode material obtained through solid-state reaction is obtained. The material had a tap density of 1.0 g/cm ³ .

The lithium iron phosphate material obtained in the above-mentioned examples and comparative examples was assembled into a button battery, the ratio of the active material in the pole piece was LiFeP04:SP:PVDF=80:10:10, the Clgard2300 type diaphragm was adopted, and the counter electrode It is a metal lithium sheet, which is discharged at 0.2C, 1C, 4C, and 5C rates, the charge rate is fixed at 0.2C, and the charge and discharge voltage range is 2.0-4.0V. The charge-discharge curves of the C/SiC-coated, C-coated, SiC-coated and pure-phase lithium iron phosphate materials obtained in Example 3 and Comparative Example 1, Example 2, and Example 3 are shown in FIG. 3 .

The results of Examples and Comparative Examples show that: the synthesis of lithium iron phosphate materials, by adding an organic carbon source to the precursor, and performing CVD vapor deposition process in the subsequent sintering process, on the one hand, effectively inhibits the growth of crystal particles during the sintering process , synthesized lithium iron phosphate with small particle size, on the other hand, realized the tight coating of C/CSi on the surface of lithium iron phosphate material, effectively solved the problem of low tap density of pure carbon-coated lithium iron phosphate material. Compared with carbon-coated lithium iron phosphate, vapor deposition SiC coating and pure phase lithium iron phosphate materials, it has higher discharge specific capacity and excellent rate discharge performance.

In the foregoing examples, the organic carbon source has listed the situation of sucrose, and other organic carbon sources such as glucose, phenolic resin, epoxy resin and polyethylene are similar to the experimental results produced by this organic carbon source; in the embodiment , the lithium source has listed the situation of lithium carbonate, the selection of other lithium sources such as lithium hydroxide, lithium acetate, lithium fluoride and lithium carbonate have produced similar experimental results; the selection of other carbon source gases such as methane, ethane, ethylene, acetylene , benzene, and toluene produced similar experimental results, and choosing other silicon source gases such as disilane and silicon tetrafluoride produced similar experimental results, and selecting other protective gases such as helium, argon, carbon dioxide, and nitrogen produced similar experimental results. Experimental results.

The embodiments of the present invention described above are not intended to limit the protection scope of the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the claims of the present invention.

Claims (4)

1. the preparation method of a lithium iron phosphate positive material is characterized in that comprising the following steps:

1) according to mol ratio Li: Fe: P: C=1.02: 1: 1: 0.2～0.5 takes lithium source, ferrous oxalate, ammonium dihydrogen phosphate, organic carbon source, take alcohol or acetone as dispersant, carries out the ball milling dispersion treatment, then carries out vacuum drying treatment and obtain presoma;

2) described presoma is placed under 550 ℃ of conditions of tube furnace, carries out pre-burning under nitrogen protection, 5 ℃/minute of heating rates, temperature retention time 5 hours, the naturally cooling room temperature of being down to, carry out coarse crushing to Preburning material;

3) will be placed in tube furnace through the Preburning material of coarse crushing and carry out double sintering, the heating rate with 5 ℃/minute under inert gas shielding rises to 700-750 ℃, insulation 8-15 hour; In temperature-rise period, the inert gas inlet pressure is 0.15MPa, and flow is 0.1L/min; After temperature reaches design temperature, introduce carbon-source gas and silicon source gas and carry out the vapour deposition coating, carbon-source gas inlet pressure 0.1～0.2MPa in vapor deposition processes, flow is 0.04～0.12L/min; Silicon source gas inlet pressure 0.1～0.2MPa, flow are 0.04～0.12L/min; Regulate simultaneously inert gas inlet pressure 0.1～0.2MPa, flow is 0.04～0.12L/min; After vapour deposition time 30min-240min, close carbon-source gas and silicon source gas air inlet, keep inert gas inlet pressure 0.15MPa, flow is 0.1L/min; Complete remaining sintering process under this inert atmosphere, and naturally cool to room temperature, namely obtain having surface C/lithium iron phosphate positive material of SiC coating layer.

2. preparation method according to claim 1, is characterized in that: step 1) described in the lithium source be the combination of a kind of in lithium carbonate, lithium hydroxide, lithium acetate, lithium fluoride or at least two kinds; Described organic carbon source is the combination of a kind of in glucose, sucrose, phenolic resins, epoxy resin or polyethylene or at least two kinds.

3. preparation method according to claim 1, is characterized in that: step 3) described in carbon-source gas be the combination of a kind of in methane, ethane, ethene, acetylene, benzene, toluene or at least two kinds; Described silicon source gas be monosilane, disilane, a kind of or combination of at least two kinds in silicon tetrafluoride.

4. preparation method according to claim 1, is characterized in that: step 3) described in inert gas be the mist of a kind of in helium, nitrogen and argon gas or at least two kinds.

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