CN101070149B - Lithium iron carbonate material prepared by vacuum carbon reduction and method - Google Patents

Abstract

This invention relates to a kind of anode material and method used for reducing preparation of lithium iron phosphate by vacuum carbon. It uses cheap phosphate, iron compound and lithium compound as raw material, compound of cheap heavy metal as adulteration modifying agent, cheap and abundant activated carbon, acetylene black and graphite as reducing agent. Dealing with it using ball milling techniques and makes solid-phase chemical reaction under the vacuum condition, then get the anode material adulterating lithium iron phosphate which contains carbon. Lithium iron phosphate compounded in this invention has a stable performance, physical chemistry of products has a good homogeneity and consistency and a high productivity; lithium iron phosphate produced in this invention has a high tapdensity and electric specific votume, good discharging capability of high-current, stable electrochemistry circulation capability, and low local action, besides, it is easy to control content of tantalum adulterated. The tap density of lithium iron phosphate produced by this method is 1.63-2.04g/cu cm, the highest discharging specific votume is 161.3mAh/g.

Description

Lithium iron phosphate material and method prepared by vacuum carbon reduction

technical field

The invention relates to the technical field of preparation of lithium ion battery materials, in particular to a lithium iron phosphate cathode material prepared by vacuum carbon reduction and a preparation method thereof.

Background technique

Lithium-ion battery is a new generation of green high-energy battery, which has many excellent characteristics such as high working voltage, high energy density, good electrochemical cycle performance, small self-discharge, no memory effect, and wide actual working temperature range. Therefore, since commercial production in 1992, lithium-ion batteries have been widely used in mobile phones, mobile computers, digital cameras, digital cameras, electronic instruments, military portable equipment, etc. Entering the 21st century, especially in the past five years, the lithium-ion battery industry has developed rapidly, and the output of lithium-ion batteries has also increased rapidly, and its application fields have continued to expand. It has become a high-tech product that is of great significance to the national economy and people's lives in this century. However, so far, lithium-ion batteries are still dominated by low-capacity, low-power batteries, and high-capacity, high-power lithium-ion batteries have not yet achieved large-scale production. Medium-sized electric tools, electric bicycles, electric vehicles, etc. have not yet been widely used. One of the important reasons is that the positive electrode active materials of lithium-ion batteries have not yet achieved substantial breakthroughs.

Cathode active materials are an important part of lithium-ion batteries. So far the most detailed and extensive positive electrode materials are LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ and their multiple composite oxides (such as LiNi _0.8 Co _0.2 O ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) and so on. Among them, LiCoO ₂ is the only cathode material that has achieved large-scale industrial production. At present, more than 90% of commercial lithium-ion batteries are also produced with LiCoO ₂ as the cathode material. Although LiCoO ₂ has relatively excellent electrochemical performance, its price is relatively expensive due to the lack of abundant raw materials for its synthesis and the high cost of production technology. Moreover, LiCoO ₂ still has a series of problems such as low capacity (the actual specific capacity is only about 120mAh/g), high toxicity, and poor safety performance.

The raw material and production cost of spinel-type active material LiMn ₂ O ₄ are relatively low, and its safety performance is good, but its electrochemical cycle performance and high-temperature cycle performance are poor, and it dissolves in the electrolyte to a certain extent. It is produced by LiMn ₂ O ₄ Lithium-ion batteries have poor storage performance and short cycle life. The new multi-metal oxide LiNi _1/3 Co _1/3 Mn _1/3 O ₂ material combines the advantages of LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ and other materials: relatively easy to synthesize, relatively low cost, electrochemical cycle Good performance, high reversible capacity, stable structure, and good safety performance, but because it contains more expensive Co and Ni, the cost of raw materials is also relatively high.

Due to the above-mentioned factors, although lithium-ion batteries have many excellent performances, due to factors such as price and safety performance, the research and industrialization of power lithium-ion batteries are struggling. In the production of high-capacity, high-power lithium-ion batteries, the cost, high-temperature performance, and safety of the cathode material are very important. LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ and multi-metal oxide cathode materials cannot meet the production requirements of large-capacity and high-power lithium-ion batteries at all. Therefore, the research and development of new cathode materials suitable for lithium-ion power batteries with large capacity, high power, good high-current discharge performance, and high safety performance has become a hot spot in related research. The olivine-type LiFePO ₄ positive electrode material just has the advantages that meet the requirements of the above-mentioned new positive electrode material for lithium-ion power batteries.

LiFePO ₄ cathode material does not contain more precious or rare metal elements, and the source of raw materials is very rich; and LiFePO ₄ has a stable structure, good electrochemical cycle performance, high theoretical specific capacity (its theoretical capacity is 170mAh/g), moderate discharge voltage (3.2 ~3.4V), extremely stable discharge voltage, high safety performance (PO ₄ ^3- is difficult to decompose and decompose oxygen), high temperature performance and thermal stability are obviously better than all other existing positive electrode materials. In addition, LiFePO ₄ has good storage performance, is non-toxic and non-polluting, and is a truly environmentally friendly power cathode material. Compared with LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ and their derivative cathode materials, LiFePO ₄ cathode materials have outstanding advantages in terms of synthesis cost, electrochemical performance, and safety, and are expected to become medium-large capacity, medium-high power lithium The preferred cathode material for ion batteries. The industrialization and popularization of LiFePO ₄ cathode materials are of great importance to reduce the cost of lithium-ion batteries, improve battery safety, expand the lithium-ion battery industry, especially the high-power lithium-ion power battery industry, and promote the large-scale and high-power lithium-ion batteries. The significance of this will make the wide application of lithium-ion batteries in medium and large capacity UPS, medium and large energy storage batteries, electric tools and electric vehicles a reality.

At present, the preparation methods of LiFePO ₄ mainly include solid-phase reaction method and liquid-phase method (co-precipitation method, hydrothermal method, sol-gel method and emulsion method, etc.), and the common feature of all existing methods is that they need flowing high-purity Inert gas or high-purity nitrogen protection, flowing inert gas protection technology greatly increases the synthesis cost of lithium iron phosphate. Moreover, due to the incompleteness of all existing synthesis technologies, the consistency of the obtained lithium iron phosphate products is poor, that is to say, the physical and chemical uniformity of the products synthesized by different batches is poor, and the electrochemical performance is inconsistent. Make its production cost further increase. In addition, lithium iron phosphate also has its own inherent disadvantages, mainly because of its low bulk density, its theoretical density is 3.6g/cm ³ , and its actual tap density is generally only 1.6-1.8g/cm ³ . Moreover, lithium iron phosphate is a semiconductor material with poor electrical conductivity. The existing immature synthesis technology of lithium iron phosphate and the inherent shortcomings of lithium iron phosphate have seriously hindered the large-scale industrial production process of lithium iron phosphate, and also affected the application of lithium iron phosphate materials in various types of lithium ion batteries, especially lithium iron phosphate. Wide application in the field of ion power battery manufacturing.

Contents of the invention

The purpose of the present invention is to solve the above-mentioned deficiencies in the prior art, and to provide a method for preparing lithium iron phosphate cathode material by vacuum carbon reduction suitable for industrial scale production. The technological synthesis process adopted in the present invention is simple and easy, and the produced lithium iron phosphate product has stable performance, good product uniformity, good consistency, and high yield; the lithium iron phosphate prepared by the present invention has high tap density and high specific capacity. High, stable electrochemical cycle performance, low self-discharge, easy to control the content of doped metal elements.

The present invention is realized through the following technical scheme: the method for preparing lithium iron phosphate cathode material by vacuum carbon reduction comprises the following steps and process conditions:

In the first step, the iron source compound, the lithium source compound and the doping metal compound are mixed in a ratio of P:Fe:Li:M (M represents the doping metal) molar ratio of 1:1:1:(0.001～0.05);

In the second step, the above mixture is placed in a vacuum high-energy ball mill tank, vacuumed to 10 ^-2 ~ 10 ^-4 Pa, and ball milled for 6 to 12 hours;

In the third step, the carbon reducing agent is added to the above-mentioned ball-milled mixture according to the ratio ^{of Fe:C molar ratio of 1:0.8-1.6, and the vacuum is evacuated to 10-2-10-4 Pa} , and ball milling is continued for 6-12 hours. Obtain reaction precursor;

The fourth step is to transfer the above-mentioned reactant precursor to a vacuum reaction furnace, pass high-purity nitrogen gas for 5 to 45 minutes, and then evacuate to 10 ^-2 to 10 ^-4 Pa, heat, control the temperature at 450 to 850°C, and keep the furnace The temperature is kept constant for 8-24 hours, high-purity nitrogen gas is introduced for 5-45 minutes, sealed and cooled to room temperature, and the carbon-coated metal-doped lithium iron phosphate powder is obtained.

The lithium iron phosphate of the present invention is prepared by the above method.

In order to better realize the present invention, the iron source compound includes one or more of iron phosphate, ferrous phosphate, ferric oxide; the lithium source compound includes lithium oxide, lithium hydroxide, lithium phosphate one or more of them; the doped metal compound includes one or more of aluminum dihydrogen phosphate, zinc oxide, magnesium oxide, and cobalt oxide; the carbon reducing agent includes activated carbon, acetylene black, and graphite one or more of.

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

1. The lithium iron phosphate synthesized by the present invention does not need to be protected by high-purity argon at all, and is rarely treated with high-purity nitrogen, which greatly reduces the synthesis cost of lithium iron phosphate;

2. The reaction precursor of lithium iron phosphate synthesized by the present invention is ball milled under vacuum conditions, and the solid-phase chemical reaction is also carried out under vacuum conditions, so that the reactants can be fully contacted so that the solid-phase reaction is carried out more thoroughly, greatly Increased the yield of lithium iron phosphate;

3. The lithium iron phosphate synthesized by the present invention is fully activated due to the reaction precursor being processed by high-energy ball milling, so that the reaction temperature used for the synthesis of lithium iron phosphate is lower and the reaction time is shortened, which also effectively reduces the energy consumption cost of the synthesis technology ,Increase productivity;

4. Using the lithium iron phosphate synthesized by the present invention, because the reaction precursor contains doping metal and carbon reducing agent, and is fully mixed and activated in the high-energy ball milling process, the synthesis, doping and carbon reduction of lithium iron phosphate are many The solid phase reaction is completed in one step, which greatly improves the synthesis efficiency of lithium iron phosphate.

5. Using the lithium iron phosphate synthesized by the present invention, since the reaction precursor containing doped metal ions and carbon reducing agent is fully activated in the high-energy ball milling treatment, the carbon reduction synthesis solid-phase reaction and the doping reaction solid-phase reaction proceed simultaneously, Simplified the synthesis process of lithium iron phosphate;

6. Utilizing the lithium iron phosphate synthesized by the present invention, because the reaction precursor containing doped metal ions and carbon reducing agent is fully mixed and activated in the high-energy ball milling process, the solid-phase chemical reaction of synthesizing lithium iron phosphate is carried out uniformly and completely , so that the phase structure and chemical composition of the lithium iron phosphate material obtained by the technology of the present invention are uniform and do not contain heterogeneous impurity phases;

7. The lithium iron phosphate synthesized by the present invention has relatively perfect crystallization, small particle size, uniform particle size, high tap density, high actual discharge specific capacity, excellent electrochemical cycle performance, abundant and cheap raw materials; obtained by the present invention The particle size range of the lithium iron phosphate is 1-3 μm, the tap density is 1.63-2.04 g/cm ³ , and the highest tap density exceeds the tap density of the synthetic lithium iron phosphate material in the prior art; the phosphoric acid obtained in the present invention The experimental battery composed of iron-lithium positive electrode active material and metal lithium is charged and discharged at a rate of 0.2C, the charging voltage is 3.4-4.2V, and the discharge termination voltage is 2.8V. The highest discharge specific capacity reaches 161.3mAh/g. After 33 charges The highest capacity retention rate after discharge cycles reaches 91.2%.

Description of drawings

Figure 1 is the XRD diffraction pattern of a typical lithium iron phosphate material;

Figure 2 is an SEM image of a typical lithium iron phosphate material;

Fig. 3 is the discharge curve (Fig. 3A) and the charge-discharge cycle discharge specific capacity diagram (Fig. 3B) of a typical experimental lithium battery with lithium iron phosphate as the positive electrode.

Detailed ways

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

Embodiment one

In the first step, ferric phosphate, lithium hydroxide, and aluminum dihydrogen phosphate are mixed in a ratio of 1:1:1:0.05 in a molar ratio of P:Fe:Li:Al;

In the second step, the above mixture is placed in a vacuum high-energy ball mill tank, vacuumed to 10 ^-2 Pa, and ball milled for 12 hours;

The third step is to add activated carbon as a carbon reducing agent to the above-mentioned ball-milled mixture according to the ratio of Fe:C molar ratio of 1:0.8, vacuumize to 10 ^-2 Pa, and continue ball milling for 6 hours to obtain a reaction precursor;

The fourth step is to transfer the above-mentioned reactant precursor to a vacuum reaction furnace, pass high-purity nitrogen gas for 5 minutes, then evacuate to 10 ^-2 Pa, heat, control the temperature at 450°C, keep the furnace temperature constant for 24 hours, and pass high-purity nitrogen gas Nitrogen for 45 minutes, sealed and cooled to room temperature to obtain carbon-coated aluminum-doped lithium iron phosphate powder.

Use a particle size analyzer to measure the particle size of the above-mentioned lithium iron phosphate, the particle size is 1-3 μm, the average particle size is about 2 μm, and the tap density is 1.76g/cm ³ ; it is determined by XRD (X-ray powder diffraction method) Its crystal structure was determined, and the results showed that its crystal structure was olivine-type lithium iron phosphate; its electrochemical performance was _as follows: The experimental battery made of the electrolyte was measured, and its 0.2C rate and discharge termination voltage of 2.8V were measured. The first discharge specific capacity was 142.3mAh/g, the highest discharge specific capacity was 151.9mAh/g, and the capacity after 50 charge-discharge cycles The retention rate is 90.1%, and it has good charge-discharge cycle performance.

Embodiment two

In the first step, ferrous phosphate, lithium phosphate, doped metal compound aluminum dihydrogen phosphate, zinc oxide, magnesium oxide, and cobalt oxide are mixed according to P: Fe: Li: M (M represents doped metal, Al: Zn: Mg: Co Molar ratio is 1: 1: 1: 0.5) and the molar ratio is 1: 1: 1: 0.03 ratio mixing;

In the second step, the above mixture is placed in a vacuum high-energy ball mill tank, vacuumed to 10 ^-3 Pa, and ball milled for 10 hours;

The third step is to add the carbon reducing agent acetylene black and graphite (ratio 1:1) to the above-mentioned treated mixture according to the ratio of Fe:C molar ratio 1:1, vacuumize to 10 ^-3 Pa, and continue ball milling for 8 Hour, obtain reaction precursor;

The fourth step is to transfer the above-mentioned reactant precursor to a vacuum reaction furnace, pass high-purity nitrogen gas for 30 minutes, then evacuate to 10 ^-3 Pa, heat, control the temperature at 650°C, keep the furnace temperature constant for 20 hours, and pass high-purity nitrogen gas Nitrogen for 30 minutes, sealed and cooled to room temperature to obtain carbon-coated multi-metal doped lithium iron phosphate powder.

Measure its crystal structure of above-mentioned lithium iron phosphate with XRD, the result shows that its crystal structure is olivine type lithium iron phosphate (see accompanying drawing 1A); Measure its particle size with particle size analyzer, and observe its outward appearance with SEM, its Its crystal form is close to spherical, its particle size is 1-3μm, and its synthetic average particle size is about 2μm (see attached figure 2); its tap density is 1.89g/cm ³ , and its electrochemical performance adopts lithium iron phosphate as the active material Make a positive electrode, use metal lithium sheet as negative electrode, 1M LiPF ₆ EC/DMC solution as electrolyte, and measure the experimental battery. The first discharge specific capacity of 0.2C rate and discharge termination voltage of 2.8V is 139.6mAh. /g, the highest discharge specific capacity is 161.3mAh/g, and the capacity retention rate is 91.2% after 33 charge-discharge cycles (see accompanying drawings 3A, B).

Embodiment three

The first step mixes ferrous phosphate with lithium phosphate and zinc oxide in a ratio of 1:1:1:0.02 in the molar ratio of P:Fe:Li:Zn;

In the second step, the above mixture is placed in a vacuum high-energy ball mill tank, vacuumed to 10 ^-2 Pa, and ball milled for 11 hours;

In the third step, carbon reducing agent graphite is added to the above-mentioned ball-milled mixture according to the ratio of Fe:C molar ratio of 1:1.4, vacuumed to 10 ^-2 Pa, and ball milled for 9 hours to obtain a reaction precursor;

The fourth step is to transfer the above-mentioned reactant precursor to a vacuum reaction furnace, pass high-purity nitrogen gas for 5 minutes, then evacuate to 10 ^-2 Pa, heat, control the temperature at 550°C, keep the furnace temperature constant for 20 hours, and pass high-purity nitrogen gas Nitrogen for 45 minutes, sealed and cooled to room temperature to obtain carbon-coated aluminum-doped lithium iron phosphate powder.

The particle size of the above-mentioned lithium iron phosphate was measured with a particle size analyzer, and the particle size was 1 to 3 μm, with an average particle size of about 2 μm; the tap density was 1.63 g/cm ³ , determined by XRD (X-ray powder diffraction method). Its crystal structure, the results show that its crystal structure is olivine-type lithium iron phosphate; its electrochemical performance uses lithium iron phosphate as the active material to make the positive electrode, metal lithium sheet as the negative electrode, and 1M LiPF ₆ EC/DMC solution as the electrolyte The manufactured experimental battery was measured, and the first discharge specific capacity was 136.9mAh/g, and the highest discharge specific capacity was 147.3mAh/g.

Embodiment four

In the first step, ferric phosphate is mixed with lithium oxide and zinc oxide in a ratio of P: Fe: Li: Zn molar ratio of 1: 1: 1: 0.04;

In the second step, the above mixture is placed in a vacuum high-energy ball mill tank, evacuated to 10 ^-3 Pa, and ball milled for 7 hours;

In the third step, the carbon reducing agent acetylene black is added to the above-mentioned ball-milled mixture according to the ratio of Fe:C molar ratio of 1:1.2, vacuumed to 10 ^-3 Pa, and ball milled for 10 hours to obtain a reaction precursor;

The fourth step is to transfer the above-mentioned reactant precursor to a vacuum reaction furnace, pass high-purity nitrogen gas for 5 minutes, then evacuate to 10 ^-2 Pa, heat, control the temperature at 750°C, keep the furnace temperature constant for 20 hours, and pass high-purity nitrogen gas Nitrogen for 45 minutes, sealed and cooled to room temperature to obtain carbon-coated aluminum-doped lithium iron phosphate powder.

The particle size of the above-mentioned lithium iron phosphate was measured by a particle size analyzer, and the particle size was 1-3 μm, with an average particle size of about 1.5 μm; the tap density was 1.82 g/cm ³ , determined by XRD (X-ray powder diffraction method). Its crystal structure was determined, and the results showed that its crystal structure was olivine-type lithium iron phosphate; its electrochemical performance was _as follows: The experimental battery manufactured by the electrolyte was measured, and the first discharge specific capacity was 131.3mAh/g, and the highest discharge specific capacity was 152.2mAh/g.

Embodiment five

In the first step, ferric oxide, lithium source compound lithium phosphate and doped metal compound cobalt oxide are mixed in a ratio of P:Fe:Li:Co molar ratio of 1:1:1:0.001;

In the second step, the above mixture is placed in a vacuum high-energy ball mill tank, vacuumed to 10 ^-4 Pa, and ball milled for 11 hours;

The third step is to add carbon reducing agent graphite to the above-mentioned treated mixture according to the ratio of Fe:C molar ratio of 1:1.6, vacuumize to about 10 ^-4 Pa, and continue ball milling for 7 hours to obtain a reaction precursor;

The fourth step is to transfer the above-mentioned reactant precursor to a vacuum reaction furnace, pass high-purity nitrogen gas for 45 minutes, then evacuate to 10 ^-4 Pa, heat, control the temperature at 850°C, keep the furnace temperature constant for 8 hours, and pass high-purity nitrogen gas Nitrogen for 45 minutes, sealed and cooled to room temperature to obtain carbon-coated cobalt-doped lithium iron phosphate powder.

The particle size of the above-mentioned lithium iron phosphate was measured by a particle size analyzer, and the particle size was 1-3 μm, with an average particle size of about 2 μm; its tap density was 2.04 g/cm ³ , and its crystal structure was determined by XRD, and the results showed that its The crystal structure is olivine-type lithium iron phosphate (see accompanying drawing 1B); its electrochemical performance is determined by using lithium iron phosphate as the active material to make the positive electrode, metal lithium sheet as the negative electrode, and 1M LiPF ₆ EC/DMC solution as the electrolyte. The manufactured experimental battery was measured, and the first discharge specific capacity of its 0.1C, 0.2C and 0.5C rate and discharge termination voltage of 2.8V was measured to be 159.6mAh/g, 151.0mAh/g and 136.5mAh/g respectively.

As described above, the present invention can be preferably realized.

Claims (3)

1. a method for preparing lithium iron phosphate cathode material by vacuum carbon reduction, is characterized in that, comprises the steps: The first step is to mix the iron source compound, lithium source compound and doping metal compound according to the molar ratio of P: Fe: Li: M in the ratio of 1:1:1: (0.001-0.05), where M represents the doping metal, The iron source compound is ferric phosphate or ferrous phosphate; The second step: put the mixture obtained in the above first step into a vacuum high-energy ball mill tank, vacuumize to 10 ^-2 ~ 10 ^-4 Pa, and ball mill for 6 ~ 12 hours; The third step is to add the carbon reducing agent into the mixture treated by ball milling in the second step above at the ratio of Fe:C molar ratio of 1:0.8~1.6, vacuumize to 10 ^-2 ~ 10 ^-4 Pa, and continue ball milling for 6 ~ 12 hours, obtain reaction precursor; Described carbon reducing agent is one or more in active carbon, acetylene black, graphite; The fourth step is to transfer the above-mentioned reactant precursor to a vacuum reaction furnace, pass high-purity nitrogen gas for 5 to 45 minutes, and then evacuate to 10 ^-2 to 10 ^-4 Pa, heat, control the temperature at 350 to 850 ° C, and keep the furnace The temperature is kept constant for 8-24 hours, high-purity nitrogen gas is introduced for 5-45 minutes, sealed and cooled to room temperature, and a metal-doped lithium iron phosphate cathode material coated with carbon is obtained; The doping metal compound described in the first step includes one or more of aluminum dihydrogen phosphate, zinc oxide, magnesium oxide, and cobalt oxide. 2. The method according to claim 1, wherein the lithium source compound in the first step comprises one or more of lithium oxide, lithium hydroxide, and lithium phosphate. 3. A lithium iron phosphate cathode material prepared by the method according to claim 1.

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