CN101591012B - Preparation method of lithium iron phosphate as cathode material of lithium ion battery - Google Patents

Abstract

The invention relates to a preparation method of lithium iron phosphate, a cathode material for lithium ion batteries. Mix metal iron powder, lithium compound, and phosphorus compound according to the atomic ratio Li:Fe:P=(0.95~1.1):1:1, then add carbon or carbon precursor, and mix uniformly in the medium for 1~20 hours, then dried and granulated, then treated in an inert atmosphere at 300°C to 500°C for 1 to 20 hours, and then synthesized at 600°C to 850°C for 5 to 36 hours to obtain a lithium iron phosphate cathode material. The present invention uses relatively cheap metal iron powder as a raw material, adds carbon or carbon precursors, and performs mechanical granulation, which effectively improves the tap density of lithium iron phosphate and improves its electrical conductivity. High capacity, excellent cycle performance, good rate performance and other characteristics. The method for preparing lithium iron phosphate of the present invention has simple preparation process, strong operability and easy realization of large-scale production.

Description

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

technical field

The invention relates to a preparation method of lithium iron phosphate (LiFePO ₄ ), which is used as a positive electrode material for lithium ion batteries, and belongs to the technical field of preparation of positive electrode materials for lithium ion batteries.

Background technique

Since Japan's Sony Corporation developed lithium-ion batteries in 1990, the research and development of positive electrode materials has attracted people's attention. At present, the positive electrode materials used in commercialized lithium-ion batteries are mainly lithium-intercalated transition metal oxides, including lithium cobaltate, lithium nickelate, lithium manganate, etc. Among them, the most widely used lithium cobalt oxide material has been developed for its application in large-capacity batteries due to its shortcomings such as scarcity of resources, high price, and poor safety performance. Lithium manganese oxide materials with abundant resources and low raw material prices have low capacity and poor high-temperature cycle performance, which limits the wide application of this material. Lithium nickelate has not been widely used because of its difficulty in preparation, poor thermal stability and safety performance. Compared with the current practical lithium-ion battery cathode materials (lithium cobaltate, lithium manganese oxide, nickel-cobalt lithium manganese oxide ternary system, etc.), lithium iron phosphate is a compound with an olivine structure, which is rich in sources, low in price, and environmentally friendly. , high specific capacity, excellent cycle performance and thermal stability, and high safety are considered to be the most potential second-generation lithium-ion battery cathode materials, and have broad development prospects in energy storage lithium-ion batteries, especially The preferred system for positive electrode materials for lithium-ion power batteries that is rapidly developing.

The preparation methods of lithium iron phosphate mainly include high-temperature solid-phase synthesis and low-temperature liquid-phase synthesis. Typical methods in high-temperature solid-phase synthesis include high-temperature synthesis methods using expensive ferrous organic compounds as precursors. In this method, ferrous iron will be oxidized even under the protection of an inert atmosphere, and the purity of the synthetic product Not high, the tap density is low; the carbothermal reduction method using ferric iron as the precursor has a long synthesis time and high energy consumption. The advantage of the low-temperature liquid-phase synthesis method is that the mixing of raw materials can reach the molecular level, but the degree of crystallization of the product is not high, and there is a phenomenon of mixed displacement of Fe and Li, which still needs to be processed at high temperature, which complicates the preparation process and is not suitable for large-scale production.

Contents of the invention

The object of the present invention is to provide a preparation method of lithium iron phosphate, a positive electrode material for lithium ion batteries. The preparation process of the method is simple, the preparation cost is low, and it is easy to implement in industry. The prepared product has high purity and excellent electrochemical performance.

In order to achieve the above object, the present invention adopts the following technical solutions.

A preparation method for lithium iron phosphate lithium ion battery cathode material, comprising the following steps:

Mix metal iron powder, phosphorus compound, and lithium compound according to the atomic ratio Li:Fe:P=(0.95-1.1):1:1, then add carbon or carbon precursor, and mix uniformly in the medium for 1-20 hours, then dry. The dried mixture is added with a granulating agent and mechanically mixed and granulated. The granulated mixture is placed in a high-temperature shaft furnace, under the protection of an inert gas of 1-30 liters/min, at a heating rate of 1-15°C/min. Treat at 300°C to 500°C for 1 to 20 hours, then continue to heat up and synthesize at 600°C to 850°C for 5 to 36 hours, then cool down to room temperature to prepare the lithium iron phosphate cathode material with olivine structure for lithium ion batteries.

The phosphorus compound is one or more mixtures of lithium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, lithium phosphate and iron phosphate.

The lithium compound is one or more mixtures of lithium hydroxide, lithium carbonate, lithium dihydrogen phosphate, lithium phosphate, and lithium acetate.

The carbon is graphite and acetylene black, and the carbon precursor is one or more of polyethylene, polyethylene glycol, polyvinyl alcohol and sugar, and the added amount accounts for 1-30 wt% of the total weight. The total weight mentioned here is the total weight of raw materials, that is, the total weight of iron powder, lithium compound, phosphorus compound, and carbon or carbon precursor.

The carbon precursor is an organic or polymer compound that can be decomposed into carbon substances by pyrolysis.

Said medium is one or more mixtures of water, ethanol, acetone, n-butanol, n-propanol, isopropanol, acetonitrile and ethanolamine.

The granulating agent is polyvinyl alcohol, polyacrylamide, castor oil, water or a mixture of several kinds, and the addition amount is 1-10wt% relative to the total weight of raw materials. The total weight of raw materials mentioned here refers to the total weight of iron powder, lithium compounds, phosphorus compounds, and carbon or carbon precursors.

Said drying temperature is 50°C-200°C. The drying time is subject to the volatilization of the liquid medium.

The invention adopts iron powder as a raw material, adds carbon or a carbon precursor to the raw material, and mechanically granulates, thereby reducing the synthesis temperature and accelerating the process of heat transfer and mass transfer. During the reaction, the existence of carbon provides a good reducing atmosphere for the reaction, and the lithium iron phosphate product with high purity and excellent electrochemical performance is obtained; the tap density of _LiFePO4 material prepared by using metal iron powder with high density is relatively low. High (from 1.1g/cm ³ to 1.3g/cm ³ or more), high specific capacity; moreover, with metal iron powder as the nucleation center, fine control of the wet process can obtain a controllable particle size and a narrow particle size distribution Metal iron powder, using the inheritance of high temperature reaction, is convenient to control the particle size and particle size distribution of the final product from the raw material; the raw material used in the present invention has a wide range of sources, easy to obtain, no pollution, and low cost; the process is simple and operable Strong, easy to realize large-scale production, the lithium iron phosphate prepared by the present invention shows excellent electrochemical performance as the positive electrode of lithium ion battery, the prepared lithium ion battery positive electrode material is widely used in fields such as electronic equipment, electric vehicles, has broad application prospects.

Description of drawings

FIG. 1 is a scanning electron microscope image of the lithium iron phosphate material prepared in Example 1.

Fig. 2 is the crystal diffraction pattern of the lithium iron phosphate material prepared according to Example 1.

Fig. 3 is the first charge and discharge curve diagram of the lithium iron phosphate material test battery prepared according to embodiment 1, the voltage range is 2.2V～4.25V, and the electrolyte is 1M (mol/L) LiPF ₆ /EC+DMC (1: 1 ), the charge and discharge rate is 0.1C.

Fig. 4 is the cycle performance figure of the lithium-ion battery prepared according to Example 1, the voltage range is 2.2V～4.25V, the electrolyte is 1M (mol/L) LiPF ₆ /EC+DMC (1: 1), charge and discharge The magnification is 0.1C.

Figure 5 is the charge and discharge curves at different charge and discharge rates after the lithium ion battery material prepared in Example 1 is assembled into a 10Ah high power lithium ion power battery, the discharge rates are 1C, 6C, 10C, 20C, and the voltage range is 2.2 V～4.25V, the electrolyte is 1M (mol/L) LiPF ₆ /EC+DMC (1:1).

Detailed ways

The total weight of raw materials mentioned in the following examples refers to the total weight of iron powder, lithium compounds, phosphorus compounds, and carbon or carbon precursors. For example, "mix 0.5 mole of iron powder, 0.5 mole of ammonium dihydrogen phosphate, 0.55 mole of lithium hydroxide, and 5% glucose of the total weight of raw materials", the weight of glucose is iron powder, ammonium dihydrogen phosphate, 5% of the total weight of lithium hydroxide and glucose raw materials.

Example 1

Mix 0.5 mole of iron powder, 0.5 mole of lithium dihydrogen phosphate, and 8% sucrose accounting for the total weight of the raw materials and put them into a rod mill jar, use ethanol as the medium, and fully mix them on the rod mill for 5 hours. Drying, the dried mixture is mechanically stirred and granulated with a polyvinyl alcohol solution of 5wt% relative to the total weight of the raw material, and the granulated mixture is put into a shaft furnace, under an argon atmosphere of 5 liters/min, at 6°C The temperature was raised to 300°C for 10 hours at a rate of 1/min, then synthesized at 700°C for 20 hours, and then lowered to room temperature to obtain lithium iron phosphate. The tap density of the synthetic material was measured to be 1.43 g/cm ³ . Figure 1 is a scanning electron micrograph of the synthesized product LiFePO ₄ , as can be seen from the figure, the particle size of the synthesized product is basically less than 2 microns. Figure 2 is the XRD pattern of the synthesized product LiFePO ₄ , the XRD analysis results show that the prepared product LiFePO ₄ powder has a single olivine crystal structure, no impurity peaks are observed, and the product has high purity.

The positive electrode material synthesized in Example 1 is used to make an electrode according to the following method:

Weigh the prepared lithium iron phosphate at a mass ratio of 83:10:7: binder PVDF (polyvinylidene fluoride): acetylene black, mix them into a slurry, apply them on both sides of the aluminum foil, and dry in the air , to make electrodes. The counter electrode is a lithium metal sheet to form a test cell. The electrolyte is 1M (mol/L) LiPF ₆ /EC+DMC, etc., EC is ethylene carbonate, and DMC is dimethyl carbonate. The charge and discharge current density is 0.1C, and the charge and discharge upper and lower limit voltages are 2.2 to 4.25V. Electrochemical capacity and cycle tests were performed with a computer-controlled galvanostatic tester. Fig. 3 is the first charge and discharge curve of the corresponding battery in the voltage range of 2.2 to 4.25V at a rate of 0.1C. It can be seen from the figure that the synthesized product has a good charge and discharge voltage platform at about 3.4V, and the reversible specific capacity is 157mAh/ g. Figure 4 shows the cycle performance of the battery at a rate of 0.1C. It can be seen from the figure that the synthesized material has excellent cycle performance. After 25 cycles, the capacity decay is very small. Figure 5 shows the discharge curves of the synthesized materials assembled into a 10Ah high-power lithium-ion power battery under different charge and discharge rates. It can be seen from the figure that the capacity decay is not obvious at 1C, 6C, and 10C, indicating that the synthetic material has a large current. The discharge performance is relatively good.

Example 2

Mix 0.5 mole of iron powder, 0.5 mole of ammonium dihydrogen phosphate, 0.55 mole of lithium hydroxide, and glucose accounting for 5% of the total weight of raw materials and put them into a rod mill jar, use water as the medium, and fully mix them on a rod mill Dry at 120°C for 12 hours. The dried mixture is mechanically stirred and granulated with a polyacrylamide solution of 10% by weight relative to the total weight of the raw materials. After granulation, it is placed in a shaft furnace under an argon atmosphere of 10 liters/min. , heated up to 350°C at a rate of 10°C/min for 20 hours, then synthesized at 700°C for 24 hours, and lowered to room temperature to obtain lithium iron phosphate. The tap density of lithium iron phosphate was measured to be 1.38g/cm ³ , and electrode sheets were also prepared according to the method of Example 1. After being assembled into batteries, they were charged and discharged at a rate of 0.1C, and the measured reversible capacity was 153mAh/g.

Example 3

Mix 0.5 mole of iron powder, 0.26 mole of lithium carbonate, 0.5 mole of diammonium hydrogen phosphate, and polyethylene accounting for 20% of the total weight of raw materials and put them into a rod mill tank, using ethanol as the medium, and fully mix them on a rod mill 5 hours, dry at 80 ℃, the mixture after drying is mechanically agitated and granulated with the castor oil solution relative to the total weight of raw material 3wt%, after granulation, this mixture is put into the shaft furnace, under 10 liters/min of argon Under an air atmosphere, the temperature was raised to 400°C at a rate of 10°C/min for 15 hours, then synthesized at 750°C for 24 hours, and then lowered to room temperature to obtain lithium iron phosphate. The tap density of lithium iron phosphate was measured to be 1.41g/cm ³ . The electrode sheet was prepared according to the method of Example 1, assembled into a test battery and then charged and discharged at a rate of 0.1C. The reversible capacity was 155mAh/g.

Example 4

Mix 0.5 mole of iron powder, 0.5 mole of lithium dihydrogen phosphate, and acetylene black accounting for 10% of the total weight of the raw materials and put them into a rod mill tank, use ethanol as the medium, and fully mix them on the rod mill for 8 hours. Under drying, the mixture after drying is mechanically agitated and granulated with respect to the polyvinyl alcohol solution of 8wt% of the total weight of raw materials. The rate of °C/min was raised to 350 °C for 10 hours, then synthesized at 800 °C for 20 hours, and then lowered to room temperature to obtain lithium iron phosphate. The tap density of lithium iron phosphate was measured to be 1.36g/cm ³ . The electrode sheet was prepared according to the method of Example 1, assembled into a test battery and then charged and discharged at a rate of 0.1C. The reversible capacity was 151mAh/g.

Claims (6)

1. preparation method who is used for lithium ion battery anode material lithium iron phosphate is characterized in that:

1) with the compound of the compound of metal iron powder, lithium, phosphorus according to atomic ratio Li: Fe: P=(0.95～1.1): prepare burden at 1: 1; The presoma that adds carbon or carbon again; The presoma of carbon is one or more in polyethylene, polyethylene glycol, polyvinyl alcohol and the sugar, and addition accounts for 1～30wt% of total weight, and said here total weight is the total weight of raw material; Be the compound of iron powder, lithium, the compound of phosphorus and the total weight of carbon or carbon matrix precursor; The compound of said phosphorus is one or more mixtures in lithium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, the lithium phosphate, in medium, evenly mixes 1～20 hour, and is dry then;

2) dried mixture adds granulating agent; Said granulating agent is one or more of polyvinyl alcohol, polyacrylamide, castor oil, water; Addition accounts for 1～10wt% of raw material total weight; The total weight of the compound that said here raw material total weight is iron powder, lithium, the compound of phosphorus and carbon or carbon matrix precursor, the mechanical agitation granulation;

3) mixture after the above-mentioned granulation is put in the shaft furnace; Flow velocity be the 1-30 liter/minute inert gas shielding under heat-treat; Heating rate is 1～15 ℃/minute, is warming up under 300 ℃～500 ℃ conditions heat treatment 1～20 hour, continues then to be warming up under 600 ℃～850 ℃ conditions synthetic 5～36 hours; Reduce to room temperature then, obtain lithium iron phosphate positive material.

2. the preparation method who is used for lithium ion battery anode material lithium iron phosphate according to claim 1 is characterized in that: the compound of described lithium is one or more mixtures in lithium hydroxide, lithium carbonate, lithium phosphate, the lithium dihydrogen phosphate.

3. the preparation method who is used for lithium ion battery anode material lithium iron phosphate according to claim 1 is characterized in that: described carbon is graphite, acetylene black.

4. the preparation method who is used for lithium ion battery anode material lithium iron phosphate according to claim 1 is characterized in that: described medium is one or more mixtures in water, ethanol, acetone, n-butanol, normal propyl alcohol, isopropyl alcohol, the monoethanolamine.

5. the preparation method who is used for lithium ion battery anode material lithium iron phosphate according to claim 1 is characterized in that: described baking temperature is 50 ℃～200 ℃.

6. the preparation method who is used for lithium ion battery anode material lithium iron phosphate according to claim 1 is characterized in that: described protective gas is a kind of in high pure nitrogen, the high-purity argon gas, or its mist.

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