WO2009117871A1

WO2009117871A1 - A method of preparing a lithium iron phosphate cathode material for lithium secondary batteries

Info

Publication number: WO2009117871A1
Application number: PCT/CN2008/070611
Authority: WO
Inventors: Quan DAI; Julin Shen; Feng Xiao
Original assignee: Byd Company Limited
Priority date: 2008-03-28
Filing date: 2008-03-28
Publication date: 2009-10-01
Also published as: JP5291179B2; EP2238638A1; JP2011515813A; KR20100139085A; EP2238638A4

Abstract

Described is a method of preparing a cathode material for lithium secondary batteries, the method comprising the following steps: sintering a first mixture containing a lithium compound, an iron compound, a phosphorous compound and a carbon additive at a first temperature, to obtain a first sintering product; mixing the first sintering product and a carbon additive, to obtain a second mixture; and sintering the second mixture at a second temperature, to obtain the cathode material. The cathode material so produced exhibits superior electrical properties.

Description

A METHOD OF PREPARING A LITHIUM IRON PHOSPHATE CATHODE MATERIAL FOR LITHIUM SECONDARY BATTERIES

FIELD OF THE INVENTION The present invention relates to a method of preparing a cathode material for batteries, more specifically, to a method of preparing a lithium iron phosphate cathode material for lithium secondary batteries.

BACKGROUND Lithium secondary batteries are widely used and can be found in laptop computers, cameras, camcorders, PDAs, cell phones, iPods and other portable electronic devices. These batteries are also growing in popularity for defense, automotive and aerospace applications because of their high energy density. Lithium phosphate-based cathode materials for secondary battery have long been known in the battery industry. People have used metal intercalation compound to improve the electrical property of lithium phosphate. One popular intercalation compound is lithium iron phosphate (LiFePO₄). Because of its non-toxicity, thermal stability, safety characteristics and good electrochemical performance, there is a growing demand for rechargeable lithium secondary batteries with LiFePO₄ as the cathode material.

However, the electrical conductivity of LiFePO₄ is not ideal, and the density of LiFePO₄, which is only 3.6g/ml, is lower than other cathode material for lithium secondary batteries, therefore, the application of LiFePO₄ in the field of lithium secondary batteries is restricted. Nowadays, the solid phase method of preparing LiFePO₄ is simple, the equipment for it is available, and thus it is the foremost method to be used in industry. CN1401559A discloses a method for preparing a lithium iron phosphate, comprising the steps of:

1 ) mixing lithium compound, iron compound and phosphate compound in a molar ratio of Li : Fe : P being (0.97-1.2) : 1 : 1 ;

2) grinding the mixture obtained in step (1 ) with ethanol for 1 -2 hours;

3) in an inert atmosphere with the velocity of flow of 0.01 -50 L/min, preferably 2-10 L/min, heating the material obtained in step (2) to 100-500°C at a rate of 1 -20°C /minute, and pre-treating it at this temperature for 1 -30 hours in a pyrolysis furnace;

4) Cooling the material to the room temperature, and grinding the mixture of the material obtained in step (3) with ethanol and carbon black again, wherein the amount of the carbon black is 1 -10% by weight; and

5) heat treating the material obtained in step(4) at 500-900°C for 10-48 hours in the pyrolysis furnace, and then cooling the same to the room temperature.

CN1677718A discloses a method for preparing a lithium iron phosphate, comprising the steps of: 1 ) Mixing the precursor: lithium compound, iron compound and phosphate compound are mixed stoichiomethcally;

2) Pretreatment: the mixture obtained in step 1 ) is heated at 200-500°C for 0.5-24 hours in a protection atmosphere, and then the mixture is cooled down naturally and ground to obtain a powdery material; 3) Sintering reaction: the powdery material obtained in step 2) is heated at 400-1200 0C for 4-48 hours in a protection atmosphere to obtain the lithium iron phosphate cathode material for lithium secondary batteries.

Moreover, the method also comprises a step of adding carbon additive to the precursor obtained in step 1 ), or adding carbon additive to the powdery material obtained in step 2), the amount of carbon additive is 0.01 -20% by weight based on the total weight of the cathode material, and the carbon additive is carbohydrate, acetylene black or graphite.

Unfortunately, the electrical conductivity of the lithium iron phosphate obtained by said methods is not ideal all the same. The prior art methods often result in incomplete reduction of thvalent iron (Fe³⁺). This incomplete reduction and poor electrical conductivity cause poor electrical performance of the battery, especially when high electrical discharge is required such as for batteries used in electrical vehicles.

As such, there is a need for a better manufacturing process for lithium iron phosphate cathode material.

SUMMARY OF THE INVENTION The object of the present application is to overcome the shortcomings of batteries made up of lithium iron phosphate prepared by the prior technology, that is, the poor performance of high electrical discharge, and provide a method for preparing lithium iron phosphate that provides batteries with better performance of high electrical discharge.

It is found by the inventor that, carbon additives are always added to increase the electrical conductivity of material so as to improve the electrical conductivity performance of material during the prior solid phase method for preparing lithium iron phosphate using bivalent iron compound as the raw material. As described in the method disclosed by CN1401559A, after sintering the mixture of lithium, ferrite and phosphate for the first time, the sintering product is mixed with carbon black, and then the resulting mixture is put into the pyrolysis furnace for the second sintering reaction. In this method, since the carbon additives are added after the first sintering reaction during which the materials are basically completely decomposed and lithium iron phosphate particles are preliminarily formed, and the particles are very solid, it's difficult for the carbon deposited during the second sintering reaction to infiltrate into the particles, and the carbon are mostly deposited on the surface of the particles. Hence the carbon is easy to disengage from the particles and is not mixed into the particles of lithium iron phosphate. Consequently, the electrical conductivity of the material is still low. In case of low rate discharge, the batteries prepared by this anode material have a relatively good performance of electrochemistry, however, when the material is applied to the power batteries which require high rate discharge, the performance will be poor. In the method disclosed by CN1677718A, the carbon coating material is added in the step of mixing the materials, and then the mixture is sintered; or the carbon coating material is added into the resultant of first sintering reaction before the second sintering reaction and then the second sintering reaction is carried out.

For bivalent iron compound, although carbon can be partly mixed into the resultant of the first sintering reaction by adding carbon coating material in the step of mixing materials before the first sintering reaction, a lot of gas is released during the reaction, which makes the structure of materials incompact and the density low, hence the improvement to the circulation performance and high electrical discharge performance of the materials still can not meet the requirement. For trivalent iron compound, the main purpose of adding the carbon coating material in step of mixing the materials is to provide a function of deoxidizing, but a function of mixing in real sense is not realized.

No matter for bivalent iron compound or trivalent iron compound, when second sintering the mixture of resultant of first sintering reaction and carbon coating material, because the structure of resultant of the first sintering reaction has already been basically formed, the carbon can not be well mixed into the materials, and the increase of electrical conductivity of the prepared material is restricted, resulting in that the circulation performance and high electrical discharge performance of the material still can not meet the requirement.

The present invention discloses a method of preparing a cathode material for lithium secondary batteries comprising the following steps: sintering a first mixture containing a lithium compound, an iron compound, a phosphorous compound and a carbon additive at a first temperature, to obtain a first sintering product; mixing the first sintering product and a carbon additive, to obtain a second mixture; and sintering the second mixture at a second temperature, to obtain the cathode material. In one optimum embodiment of the present invention, the first mixture further contains a metal M-containing compound, the high temperature of sintering can disperse the grains of the metal into the crystal structure of the lithium iron phosphate (LiFePO₄), resulting to the better consistency and electrical conductivity of the lithium iron phosphate (LiFePO₄). The present invention teaches a better method of making lithium iron phosphate and other metal doped compound for cathode materials of secondary batteries. The invention utilizes two carbon processes in a way to significantly improve the electrical property of the cathode material. The first carbon process, i.e., sintering the first mixture containing a carbon additive, provides a reducing environment to ensure the conversion of trivalent iron to divalent iron, or in the case where divalent iron compound is used, to prevent divalent iron from oxidation. The carbon additives in the first carbon process may be evenly distributed among the chemical precursors of lithium iron phosphate before the heating. This characteristic results in the maximal reducing effect of the first carbon process and the carbon formation in the spaces between lithium iron phosphate crystals. During the second carbon process, i.e., sintering the second mixture containing a carbon additive, the carbon particles between lithium iron phosphate crystals tend to coat the surface of the lithium iron phosphate crystals formed during the second carbon process, which is referred to as "carbon coating." The carbon coating, other than providing a layer of conductive material, helps limit the size of the lithium iron phosphate crystals and promotes the desired homogeneity of the crystal size. Further, where a thvalent iron compound is used as a starting material, the second carbon process also ensures the reduction of residual trivalent iron left over from the first carbon process to divalent iron to minimize impurity contents in the cathode material.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 illustrates a scanning electron microscope (SEM) image of a lithium iron phosphate cathode material according to one example of the present invention;

Fig. 2 illustrates an x-ray diffraction (XRD) pattern of the lithium iron phosphate cathode material of Fig. 1.

DETAILED DESCRIPTION It will be appreciated by those of ordinary skill in the art that the invention can be embodied in other specific forms without departing from the spirit or essential character thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. According to the present invention, the first mixture contains 0.5-20% by weight of carbon additive based on the total weight of the first mixture, and the weight ratio of the carbon additive contained in the second mixture to the carbon additive contained in the first mixture is about 1 -10 : 1 ; optimally, the first mixture contains 5-15% by weight of carbon additive based on the total weight of the first mixture, and the weight ratio of the carbon additive contained in the second mixture to the carbon additive contained in the first mixture is about 1 -5 : 1. During the first carbon process, the amount of carbon additive may be important to the performance of the resulting LiFePO₄ cathode material. When the amount of carbon doping is too low, there may be minimal improvement in electrical conductivity. When the amount of carbon doping is too high, the electrical conductivity may improve, but it may lead to excessive carbon particles during the second carbon process thereby lowering the density of the resulting cathode material. This can be especially true if a thvalent iron compound is utilized wherein optimal amounts of carbon additives can be included to maximize the reduction properties of the trivalent iron compound while maintaining the appropriate cathode density. The amount of first carbon additive may ensure that it will not become completely depleted during the first carbon process and that reduction of the trivalent iron compound (or maintaining the reduced state of the divalent iron) will continue to take place during the second carbon process. Accordingly, the first mixture contains 0.5-20% by weight of carbon additive based on the total weight of the first mixture; optimally, the first mixture contains 5-15% by weight of carbon additive based on the total weight of the first mixture. For bivalent iron sources, the first mixture contains 0.5-10%, optimally, 5-9% by weight of carbon additive based on the total weight of the first mixture; the weight ratio of the carbon additive contained in the second mixture to the carbon additive contained in the first mixture is about 1 -5 : 1 , optimally, the ratio is about 1 -4:1. For trivalent iron sources, the first mixture contains 7-20%, optimally, 8-15% by weight of carbon additive based on the total weight of the first mixture; the main effect of the carbon additives of the second mixture is for coating the material, and especially for the trivalent iron sources, the effect of the carbon additives of the second mixture is further for reducing the partial trivalent iron which is not reduced. The weight ratio of the carbon additive contained in the second mixture to the carbon additive contained in the first mixture is about 1 -10 : 1 , optimally, the ratio is about 1 - 5:1. In one embodiment, if a trivalent iron compound is incorporated, the amount of carbon additive utilized is about 5-10 grams, optimally, the amount of carbon additive utilized is about 6 to 8 grams relative to one molar trivalent iron compound in the first mixture.

Prior to the first carbon process, the mixture of lithium, iron, phosphorous compounds and carbon additive can be ground or milled by known techniques. In one embodiment, the mixture can be ground in a ball mill. In other embodiments, the mixture can be ground or milled with a vibrating or pulverizing mill. The mixture can also be ground or milled by other mixing apparatus along with the use of organic solvents. In some instances, the weight ratio of ethanol, alcohol, acetone or other organic solvents relative to the mixture is about 1 -5 to 1. No specific limitations on the rotational speed of the ball mill or the milling time are necessary. In other words, various rotational speed and time may be used to grind or mill the mixture as desired to provide a uniform mixture. Additionally, various temperatures and conditions for removing the organic solvents and drying the mixture may be utilized.

Prior to the second carbon process but after the first carbon process, the residual or un-reacted carbon additive may be ground or milled again to provide a more uniform mixture. The grinding or milling process can include hand milling, hand crushing or the processes described above. In one embodiment, the mixture can be subjected to a ball mill at a rate of between 100 to 300 revolutions per minute for 30 to 100 minutes. In other instances, the milling rate is between 150 to 400 revolutions per minute for 1 to 8 hours. In other embodiments, different milling rate and time may be incorporated. The mixture can also be grounded or milled using organic solvents and other mixing apparatus. The weight ratio of ethanol, alcohol, acetone or other organic solvents relative to the mixture of lithium, iron, phosphorous compounds and carbon additive is about 1 -5 to 1. In other embodiments, instead of grinding or milling the remaining or residual carbon additive, different carbon additives may be incorporated in the mixture. And like above, various temperatures and conditions of removing the organic solvents and drying the mixture may be utilized. In one optimum embodiment, the method further comprises bhquetting the second mixture before sintering the same; and the conditions of the briquetting provide cathode material with a compact density of 2.5-4.0 gram/cm³. The step of briquetting may shorten the diffusing distance of ions in the solid phase material. The size of the dollop obtained by briquetting may not be restricted, as long as the conditions of the briquetting provide cathode material with a compact density of 2.5-4.0 gram/cm³, optimally 2.8-3.5 gram/cm³. The press of briquetting is preferably 1 -30 MPa, and more preferably 5-20 MPa, under which the material may possess the compact density. The variety of the carbon additives is well known for the person skilled in the art, for example, the carbon additives contained in the first and second mixtures may each independently selected from benzene naphthalene phenanthrene copolymer, benzene phenanthrene binary copolymer, benzene anthracene binary copolymer, poly benzene, soluble starch, polyvinyl alcohol, polyethylene glycol, polypropylene, polyacrylamide, sucrose, glucose, urea, phenolic resin, furfural resin, urea- formaldehyde resin, epoxy resin, artificial graphite, natural graphite, acetylene black, and various types of carbon black. It will be appreciated by one skilled in the art that the carbon additives can include other carbon-containing materials. Because small organic molecules can be more evenly dispersed throughout milled particles, the resulting carbon particles from the organic solvents can be better distributed throughout the mixture during a sintering process. Accordingly, in a preferred embodiment, small organic molecules including glucose, sucrose and urea may be used as the carbon additive contained in the first mixture, while polymers including polyvinyl alcohol, polypropylene, polyacrylamide, polyethylene glycol, phenol-formaldehyde resin, urea-formaldehyde resin and epoxy resin may be best suited to be the carbon additive contained in the second mixture. To further facilitate and optimize the role of the carbon additive in the first mixture, the method of sintering comprises heating the first mixture to the first temperature at a rate of 1 -5°C/minute, and then sintering the first mixture at the first temperature, to obtain the first sintering product; cooling the first sintering product to the 10-60⁰C, preferably room temperature, and mixing the first sintering product and carbon additive to obtain the second mixture; and heating the second mixture to the second temperature at a rate of 2-10⁰C /minute, and sintering the second mixture at the second temperature.

The first mixture is sintered at the first temperature of 300-700⁰C for 5-20 hours, preferably 5-15 hours; and the second mixture is sintered at the second temperature of 600-900°C for 5-20 hours, preferably 5-15 hours. In other embodiments, the lithium, iron and phosphorous compounds of the first mixture provide Li : Fe : P molar ratios of about (0.9-1.2) : (0.95-1 ) : 1.

The variety of the lithium compound is well known for the person skilled in the art, for example, the lithium compound may include Li₂CO₃, LiOH, Li₂C₂O₄, CH₃COOLi, LiH₂PO₄, Li₃PO₄ and other lithium-containing compounds known in the art. The variety of the hosphorous compound is well known for the person skilled in the art, for example, the phosphorous compound may include NH₄H₂PO₄, (NH₄)₂HPO₄, Li₃PO₄, (NH₄)₃PO₄ and other phosphorous-containing compounds known in the art. The variety of the iron compound is well known for the person skilled in the art, for example, the iron compound may be a divalent iron compound such as FeC₂O₄, Fe(CH₃COO)₂, FeCI₂, FeSO₄ and Fe₃(PO₄)₂ or any combination thereof. The iron compound can also be a trivalent iron compound such as Fe₂O₃, FePO₄, Fe(NO₃)₃ and Fe₃O₄ Or any combination thereof. It will be appreciated by one skilled in the art that mixed-valence iron-containing compounds may also be utilized. In one instance, if a trivalent iron compound is utilized, the first sintering temperature in the first carbon process can be about 500 to 700 °C, and the first sintering temperature can be maintained at a constant level for 5 to 20 hours. The second sintering temperature in the second carbon process can be about 700 to 900 °C, and the second sintering temperature can be maintained at a constant level also for 5 to 20 hours.

In another instance, when a divalent iron compound is utilized, the first sintering temperature in the first carbon process can be about 300 to 500 °C, and the first sintering temperature can be maintained at a constant level for 5 to 15 hours. The second sintering temperature in the second carbon process can be about 600 to 800 °C, and the second sintering temperature can be maintained at a constant level also for 5 to 20 hours.

The second sintering temperature may be higher than the first temperature by at least 80°C in many instances. In other instances, the second temperature may be higher than the first temperature by at least 100°C. In other instances, the second sintering temperature need not be higher than the first temperature.

Most organic molecules can be pyrolyzed or decomposed at 350 °C, but there's no guarantee that the mixture will achieve complete decomposition. Therefore, the first sintering temperature can be high enough to ensure sufficient decomposition of the carbon additive and to facilitate the reaction of any remaining or additional carbon additives during the second sintering step. In one embodiment, the first sintering temperature can range from about 300 to 500°C for 5-15 hours while the second sintering temperature can range from about 600 to 800°C for 5-20 hours. In other embodiments, the first sintering temperature can range from about 500 to 700 °C for 5-10 hours while the second sintering temperature can range from about 700 to 900°C for 5-20 hours. In other instances, the first sintering temperature can range from 550-650°C for 6-15 hours while the second sintering temperature can range from 725-800 °C for 6-15 hours. The heat treatment at the first temperature is such that a portion of the carbon additive may be consumed by the mixture while residues or unconsumed portions of the carbon additive remain. The mixture can then be subjected to a second temperature heat treatment to exhaust the remainder of the carbon additive. The heat treatments can include sintering or heating without melting, calcinations and pyrolysis. It is understood that other heating processes may be utilized. In other embodiments, the phosphorous-containing compound may not be necessary if either the lithium or iron compound incorporates a phosphate group. In addition, the mixture may contain more than one lithium, iron, phosphorous compound or carbon additive. For example, the lithium iron phosphate (LiFePO₄) cathode material may be made from two lithium-containing compounds, one iron compound and three phosphorous compounds. Additionally, more than one carbon additive may be introduced during either carbon process. In other embodiments, the first mixture may further contain one or more metal- containing compounds, wherein metal ions may be distributed throughout the cathode material during the sintering processes. Additional grinding or milling of the mixture can further facilitate in evenly distributing the metal ions from the metal- containing compound to provide a homogeneous cathode material. Furthermore, the two-step heat treatment can provide further penetration of the metal ions from the metal compound throughout the crystal structure of the LiFePO₄ cathode material thereby enhancing the electrical conductivity of the material.

The metal-containing compound may include oxides, hydroxides and carbonates of Mg, Mn, Ca, Sn, Co, Ni, Cr, Zr and Mo. For example, magnesium oxide, manganese dioxide, calcium carbonate, tin oxide, cobalt oxide, nickel oxide, chromium oxide, zirconium oxide and molybdenum oxide, to name a few. It will be appreciated by one skilled in the art that other metal-containing compounds known in the art may be incorporated. In these embodiments, the lithium, metal, iron and phosphorous compounds are able to provide Li : M : Fe : P molar ratios of 0.9-1.2 : 0.01 -0.05 : 0.95-1 : 1. And like above, the metal-containing compound can be mechanically mixed, hand ground or milled or grounded using similar techniques as described above to provide an even mixture. Also, organic solvents may be utilized to facilitate in providing a uniform mixture. To minimize the amount of oxidation to iron salts, the sintering processes may be carried out in an inert atmosphere with gases or gas mixtures that will not trigger a chemical reaction with the mixture. For example, inert gases including helium, neon, argon, krypton and xenon may be utilized. In other instances, hydrogen, nitrogen, carbon monoxide, ammonia and other gas mixtures may be incorporated. The inert atmosphere can be static with gas flow rate of between 2-50 L/min. In addition, air or mortar pulverization may be utilized to provide powder form of the LiFePO₄ cathode material. In other embodiments, the sintering processes can be conducted in-situ. The cathode material that the present invention produces exhibits superior particle homogeneity and high electrical capacity especially during high electrical discharges. Such cathode materials are better suited for applications including without limitations electrical vehicles and notebook computers (laptop).

The following are various embodiments of the lithium iron phosphate (LiFePO₄) composite cathode material according to the presently disclosed invention. The tap density of LiFePO₄ of the undermentioned examples is measured by type JZ- 1 tap density testing apparatus manufactured by CHENG DU JINGXIN Powder Testing Apparatus Corporation.

EXAMPLE A1 (1 ) 0.1 mole Li₂CO₃, 0.2 mole FeC₂O₄-2H₂O, 0.2 mole NH₄H₂PO₄, 1.4 gram glucose and 250 mL ethanol were mixed to provide Li : Fe : P molar ratio of 1 : 1 : 1 , and the mixture was ground in a ball mill for 12 hours at 300 revolutions per minute, and then dried at 70 °C.

(2) In an argon flow rate of 10 L/min, the mixture of (1 ) was heated to 450 °C at a rate of 2°C/min, maintained at this temperature for 6 hours, and then ambient cooled to room temperature. Sucrose and 100 mL ethanol were added thereto, and then the mixture was ground in a ball mill for 1 hour at 150 revolutions per minute, and dried at 70°C (the weight ratio of glucose in (1 ) to sucrose in (2) was about 1 : 2.5).

(3) In an argon flow rate of 10 L/min, the mixture of (2) was heated to 700 °C at a rate of 10°C/min, maintained at this temperature for 10 hours, and ambient cooled to room temperature, followed by air pulverizing to provide a carbon-doped LiFePO₄ composite powder.

The tap density of carbon-doped LiFePO₄ was 1.03 g/ cm³. A scanning electron microscope (SEM) image of the carbon-doped LiFePO₄ composite powder was performed on a Shimadzu SSX-550 as shown in Fig. 1 , while an x-ray diffraction (XRD) pattern was carried out with a Rigaku D/MAX-2200 as shown in Fig. 2.

EXAMPLE A2

(1 ) 0.1 mole Li₂CO₃, 0.2 mole FeC₂O₄-2H₂O, 0.2 mole NH₄H₂PO₄, 1.4 gram glucose and 250 mL ethanol were mixed to provide Li : Fe : P molar ratio of 1 : 1 : 1 , and the mixture was ground in a ball mill for 12 hours at 300 revolutions per minute, and then dried at 70 °C.

(2) In an argon flow rate of 10 L/min, the mixture of (1 ) was heated to 450 °C at a rate of 2°C/min, maintained at this temperature for 6 hours, and then ambient cooled to room temperature. Sucrose was added thereto (the weight ratio of glucose in (1 ) to sucrose in (2) was about 1 : 2.5), and the mixture was stirred uniformly; and then the mixture was bhquetted with the press of 5MPa until the compact density of the mixture was 2.6g/cm³.

The tap density of carbon-doped LiFePO₄ was 1.1 1 g/ cm³.

EXAMPLE A3

(1 ) 0.21 mole LiOH, 0.2 mole FeC₂O₄-2H₂O, 0.2 mole (NH₄)₂HPO₄, 1.0 gram sucrose and 250 mL ethanol were mixed to provide Li : Fe : P molar ratio of 1 : 1 : 1 , and the mixture was ground in a ball mill for 12 hours at 300 revolutions per minute, and then dried at 70 °C.

(2) In an argon flow rate of 10 L/min, the mixture of (1 ) was heated to 450 °C at a rate of 2°C/min, maintained at this temperature for 6 hours, and then ambient cooled to room temperature. Polyacrylamide and 100 mL ethanol were added thereto, and then the mixture was ground in a ball mill for 30 minutes at 200 revolutions per minute, and dried at 70°C (the weight ratio of sucrose in (1 ) to polyacrylamide in (2) was about 1 : 3); and (3) In an argon flow rate of 10 L/min, the mixture of (2) was heated to 700 °C at a rate of 10°C/min, maintained at this temperature for 12 hours, and ambient cooled to room temperature, followed by air pulverizing to provide a carbon-doped LiFePO₄ composite powder The tap density of carbon-doped LiFePO₄ composite powder was 1.04 g/ cm³.

EXAMPLE A4

(1 ) 0.21 mole LiOH, 0.2 mole FeC₂O₄-2H₂O, 0.2 mole (NH₄)₂HPO₄, 1.0 gram sucrose and 250 mL ethanol were mixed to provide Li : Fe : P molar ratio of 1 : 1 : 1 , the mixture was ground in a ball mill for 12 hours at 300 revolutions per minute, and then dried at 70 °C.

(2) In an argon flow rate of 10 L/min, the mixture of (1 ) was heated at 450 °C at a rate of 2°C/min, maintained at this temperature for 6 hours, and then ambient cooled to room temperature. Polyacrylamide was added thereto (the weight ratio of sucrose in (1 ) to polyacrylamide in (2) was about 1 : 3), and the mixture was stirred uniformly.

(3) In an argon flow rate of 10 L/min, the mixture of (2) was heated to 700 °C at a rate of 10°C/min, maintained at this temperature for 12 hours, and ambient cooled to room temperature, followed by air pulverizing to provide a carbon-doped LiFePO₄ composite powder. The tap density of carbon-doped LiFePO₄ composite powder was 1.05 g/ cm³.

EXAMPLE A5

(1 ) 0.102 mole Li₂CO₃, 0.2 mole Fe(CH₃COO)₂, 0.2 mole (NH₄)₃PO₄, 1.5 gram urea and 250 mL ethanol were mixed to provide Li : Fe : P molar ratio of 1 : 1 : 1 , the mixture was ground in a ball mill for 12 hours at 300 revolutions per minute, and then dried at 70 °C.

(2) In an argon flow rate of 10 L/min, the mixture of (1 ) was heated to 450 °C at a rate of 2°C/min, maintained at this temperature for 6 hours, and then ambient cooled to room temperature. Polyvinyl alcohol and 100 mL ethanol were added thereto, and then the mixture was ground in a ball mill for 1 hour at 180 revolutions per minute, and dried at 70°C (the weight ratio of urea in (1 ) to polyvinyl alcohol in (2) was about 1 : 5). (3) In an argon flow rate of 10 L/min, the mixture of (2) was heated to 700 °C at a rate of 10°C/min, maintained at this temperature for 10 hours, and ambient cooled to room temperature, followed by air pulverizing to provide a carbon-doped LiFePO₄ composite powder. The tap density of carbon-doped LiFePO₄ composite powder was 1.00 g/ cm³.

EXAMPLE A6

(1 ) 0.2 mole LiH₂PO₄, 0.2 mole FeC₂O₄*2H₂O, 1.2 gram glucose and 200 mL ethanol were mixed to provide Li : Fe : P molar ratio of 1 : 1 : 1 , the mixture was ground in a ball mill for 12 hours at 350 revolutions per minute, and then dried at 70 °C.

(2) In an argon flow rate of 10 L/min, the mixture of (1 ) was heated to 450 °C at a rate of 2°C/min, maintained at this temperature for 6 hours, and then ambient cooled to room temperature. Polypropylene and 100 mL ethanol were added thereto, and then the mixture was ground in a ball mill for 1 hour at 200 revolutions per minute, and dried at 70°C (the weight ratio of glucose in (1 ) to polypropylene in (2) was about 1 : 3).

The tap density of carbon-doped LiFePO₄ composite powder was 1.02 g/ cm³.

EXAMPLE A7

(1 ) 0.2 mole CH₃COOLi, 0.2 mole FeC₂O₄-2H₂O, 0.2 mole NH₄H₂PO₄, 1.8 gram glucose and 250 mL acetone were mixed to provide Li : Fe : P molar ratio of 1 : 1 : 1 , the mixture was ground in a ball mill for 10 hours at 350 revolutions per minute, and then dried at 70 °C.

(2) In an argon flow rate of 10 L/min, the mixture of (1 ) was heated to 450 °C at a rate of 2°C/min, maintained at this temperature for 6 hours, and then ambient cooled to room temperature. Urea-formaldehyde resin and 100 mL ethanol were added thereto, and then the mixture was ground in a ball mill for 1 hour at 180 revolutions per minute, and dried at 70°C (the weight ratio of glucose in (1 ) to urea-formaldehyde resin in (2) was about 1 : 2). (3) In an argon flow rate of 10 L/min, the mixture of (2) was heated to 700 °C at a rate of 10°C/min, maintained at this temperature for 15 hours, and ambient cooled to room temperature, followed by air pulverizing to provide a carbon-doped LiFePO₄ composite powder. The tap density of carbon-doped LiFePO₄ composite powder was 1.02 g/ cm³.

EXAMPLE A8

(1 ) 0.1 mole Li₂C₂O₄, 0.2 mole FeC₂O₄-2H₂O, 0.2 mole NH₄H₂PO₄, 1.1 gram glucose and 200 mL acetone were mixed to provide Li : Fe : P molar ratio of 1 : 1 : 1 , the mixture was ground in a ball mill for 12 hours at 300 revolutions per minute, and then dried at 70 °C.

(2) In an argon flow rate of 10 L/min, the mixture of (1 ) was heated to 450 °C at a rate of 2°C/min, maintained at this temperature for 6 hours, and then ambient cooled to room temperature. Epoxy resin and 100 mL acetone were added thereto, and the mixture was ground in a ball mill for 1 hour at 180 revolutions per minute, and then the mixture was briquetted with the press of 15MPa, the compact density of the mixture was 3.2g/cm³; and then dried at 70 °C (the weight ratio of glucose in (1 ) to epoxy resin in (2) was about 1 : 3).

(3) In an argon flow rate of 10 L/min, the mixture of (2) was heated to 700 °C at a rate of 10°C/min, maintained at this temperature for 10 hours, and then ambient cooled to room temperature, followed by air pulverizing to provide a carbon-doped LiFePO₄ composite powder.

The tap density of carbon-doped LiFePO₄ composite powder was 1.14 g/ cm³.

EXAMPLE A9

(1 ) 0.1 mole Li₂CO₃, 0.2 mole FePO₄, 1.5 gram carbon black and 250 mL ethanol were mixed to provide Li : Fe : P molar ratio of 1 : 1 : 1 , the mixture was ground in a ball mill for 8 hours at 300 revolutions per minute, and then dried at 70 °C.

(2) In an argon flow rate of 10 L/min, the mixture of (1 ) was heated to 650 °C at a rate of 5°C/min, maintained at this temperature for 6 hours, and then ambient cooled to room temperature. 4.3 gram glucose and 150 mL ethanol were added thereto, the mixture was ground in a ball mill for 6 hour at 250 revolutions per minute; and then dried at 70 °C. (3) In an argon flow rate of 10 L/min, the mixture of (2) was heated to 750 °C at a rate of 10°C/min, maintained at this temperature for 10 hours, and ambient cooled to room temperature, followed by air pulverizing to provide a carbon-doped LiFePO₄ composite powder. The tap density of carbon-doped LiFePO₄ composite powder was 1.08 g/ cm³.

EXAMPLE A10

(1 ) 0.1 mole Li₂CO₃, 0.1 mole Fe₂O₃, 1.3 gram carbon black and 250 mL ethanol were mixed to provide Li : Fe : P molar ratio of 1 : 1 : 1 , the mixture was ground in a ball mill for 8 hours at 300 revolutions per minute, and then dried at 70 °C.

(2) In an argon flow rate of 10 L/min, the mixture of (1 ) was heated to 650 °C at a rate of 5°C/min, maintained at this temperature for 6 hours, and then ambient cooled to room temperature. 4.5 gram glucose was added thereto, and the mixture was stirred uniformly. (3) In an argon flow rate of 10 L/min, the mixture of (2) was heated to 750 °C at a rate of 10°C/min, maintained at this temperature for 10 hours, and ambient cooled to room temperature, followed by air pulverize to provide a carbon-doped LiFePO₄ composite powder. The tap density of carbon-doped LiFePO₄ composite powder was 1.05 g/ cm³.

EXAMPLE A1 1

(1 ) 0.1 mole Li₂CO₃, 0.198 mole FePO₄, 0.002mole Mg(OH)₂, 1.3 gram carbon black and 200 mL ethanol were mixed to provide Li : Fe :Mg: P molar ratio of 1 : 0.99 :0.01 : 1 , the mixture was ground in a ball mill for 12 hours at 300 revolutions per minute, and then dried at 70 °C.

(2) In an argon flow rate of 10 L/min, the mixture of (1 ) was heated to 670°C at a rate of 8°C/min, maintained at this temperature for 6 hours, and then ambient cooled to room temperature. 4.5 gram glucose and 200 mL ethanol were added thereto, and the mixture was ground in a ball mill for 6 hour at 200 revolutions per minute; and dried at 70 °C.

(3) In an argon flow rate of 10 L/min, the mixture of (2) was heated to 750 °C at a rate of 10°C/min, maintained at this temperature for 10 hours, and ambient cooled to room temperature, followed by air pulverizing to provide a carbon-doped LiFePO₄ composite powder.

The tap density of carbon-doped LiFePO₄ composite powder was 1.1 O g/ cm³.

EXAMPLE A12

(1 ) 0.21 mole Li₂OH, 0.098 mole Fe₂O₃, 0.2 mole (NH₄)₃PO₄,0.004 mole ZrO₂, 2.6 gram urea and 200 mL ethanol were mixed to provide Li : Fe : Zr : P molar ratio of 1.05 : 0.98 : 0.02 : 1 , the mixture was ground in a ball mill for 8 hours at 300 revolutions per minute, and then dried at 70°C. (2) In an argon flow rate of 10 L/min, the mixture of (1 ) was heated to 700 °C at a rate of 5°C/min, maintained at this temperature for 6 hours, and then ambient cooled to room temperature. 3.5 gram polyacrylamide and 200 mL ethanol were added thereto, the mixture was ground in a ball mill for 4 hours at 300 revolutions per minute; and dried at 70 °C. (3) In an argon flow rate of 10 L/min, the mixture of (2) was heated to 850 °C at a rate of 10°C/min, maintained at this temperature for 8 hours, and ambient cooled to room temperature, followed by air pulverizing to provide a carbon-doped LiFePO₄ composite powder. The tap density of carbon-doped LiFePO₄ composite powder was 1.11 g/ cm³.

EXAMPLE A13

(1 ) 0.3 mole CH₃COOLi, 0.099 mole Fe₃O₄, 0.3 mole NH₄H₂PO₄, 0.003 mole Cr(NO₃)₃, 4.0 gram sucrose and 200 mL ethanol were mixed to provide Li : Fe :Cr: P molar ratio of 1 : 0.99 :0.01 : 1 , the mixture was ground in a ball mill for 12 hours at 300 revolutions per minute, and then dried at 70 °C.

(2) In an argon flow rate of 10 L/min, the mixture of (1 ) was heated to 650 °C at a rate of 7°C/min, maintained at this temperature for 8 hours, and then ambient cooled to room temperature. 5.5 gram polyethylene glycol and 200 mL ethanol were added thereto, and the mixture was ground in a ball mill for 4 hours at 250 revolutions per minute; and dried at 70°C.

(3) In an argon flow rate of 10 L/min, the mixture of (2) was heated to 800 °C at a rate of 10°C/min, maintained at this temperature for 7 hours, and ambient cooled to room temperature, followed by air pulverizing to provide a carbon-doped LiFePO₄ composite powder.

The tap density of carbon-doped LiFePO₄ composite powder was 1.1 O g/ cm³.

EXAMPLE A14

(1 ) 0.2 mole LiH₂PO₄, 0.096 mole Fe₂O₃, 0.008 mole CaCO₃, 3.0 gram phenolic resinand 250 mL acetone were mixed to provide Li : Fe :Ca: P molar ratio of 1 : 0.96 :0.04: 1 , the mixture was ground in a ball mill for 8 hours at 300 revolutions per minute, and then dried at 70 °C. (2) In an argon flow rate of 10 L/min, the mixture of (1 ) was heated to 700 °C at a rate of 10°C/min, maintained at this temperature for 6 hours, and then ambient cooled to room temperature. 3.5 gram epoxy resin and 200 mL acetone were added thereto, and the mixture was ground in a ball mill for 6 hours at 300 revolutions per minute; and dried at 70 °C. (3) In an argon flow rate of 10 L/min, the mixture of (2) was heated to 800 °C at a rate of 10°C/min, maintained at this temperature for 12 hours, and ambient cooled to room temperature, followed by air pulverizing to provide a carbon-doped LiFePO₄ composite powder. The tap density of carbon-doped LiFePO₄ composite powder was 1.13 g/ cm³.

EXAMPLE A15

(1 ) 0.1 mole Li₂CO₃, 0.2 mole FeC₂O₄-2H₂O, 0.2 mole NH₄H₂PO₄, 1.4 gram glucose and 250 mL ethanol were mixed to provide Li : Fe : P molar ratio of 1 : 1 : 1 , and the mixture was ground in a ball mill for 12 hours at 300 revolutions per minute, and then dried at 70 °C;

(2) In an argon flow rate of 10 L/min, the mixture of (1 ) was heated to 450 °C at a rate of 2°C/min, maintained at this temperature for 6 hours, and then ambient cooled to room temperature. Sucrose and 100 mL ethanol were added thereto, and the mixture was ground in a ball mill for 1 hour at 150 revolutions per minute, and dried at 70°C (the weight ratio of glucose in (1 ) to sucrose in (2) can be about 1 : 2.5); and then the mixture was bhquetted with the press of 10 MPa, the compact density of the mixture was 2.8g/cm³; and (3) In an argon flow rate of 10 L/min, the mixture of (2) was heated to 700 °C at a rate of 10°C/min, maintained at this temperature for 10 hours, and ambient cooled to room temperature, followed by air pulverizing to provide a carbon-doped LiFePO₄ composite powder. The tap density of carbon-doped LiFePO₄ was 1.21 g/ cm³.

The following are various references of lithium iron phosphate (LiFePO₄) composite cathode material for comparison purposes.

COMPARISON EXAMPLE AC1

(1 ) 0.1 mole Li₂CO₃, 0.2 mole FeC₂O₄*2H₂O, 0.2 mole NH₄H₂PO₄, 4.5 gram glucose and 250 mL ethanol were mixed to provide Li : Fe : P molar ratio of 1 : 1 : 1 , and the mixture was ground in a ball mill for 12 hours at 300 revolutions per minute, and then dried at 70 °C; (2) In an argon flow rate of 10 L/min, the mixture of (1 ) was heated to 450 °C at a rate of 2°C/min, maintained at this temperature for 6 hours, and then ambient cooled to room temperature, 100 mL ethanol was added to thereto, and the mixture was ground in a ball mill for 1 hour at 150 revolutions per minute, and dried at 70°C; and (3) In an argon flow rate of 10 L/min, the mixture of (2) was heated to 700 °C at a rate of 10°C/min, maintained at this temperature for 10 hours, and ambient cooled to room temperature, followed by air pulverizing to provide a carbon-doped LiFePO₄ composite powder. The tap density of carbon-doped LiFePO₄ composite powder was 0.86 g/ cm³.

COMPARISON EXAMPLE AC2

(1 ) 0.1 mole Li₂CO₃, 0.2 mole FeC₂O₄-2H₂O, 0.2 mole NH₄H₂PO₄, and 250 mL ethanol were mixed to provide Li : Fe : P molar ratio of 1 : 1 : 1 , and the mixture was ground in a ball mill for 12 hours at 300 revolutions per minute, and then dried at 70 °C; (2) In an argon flow rate of 10 L/min, the mixture of (1 ) was heated to 450 °C at a rate of 2°C/min, maintained at this temperature for 6 hours, and then ambient cooled to room temperature. 3.6 gram polyacrylamide and 100 mL ethanol were added thereto, and then the mixture was ground in a ball mill for 1 hour at 150 revolutions per minute, and dried at 70 °C; and

(3) In an argon flow rate of 10 L/min, the mixture of (2) was heated to 700 °C at a rate of 10°C/min, maintained at this temperature for 10 hours, and ambient cooled to room temperature, followed by air pulverizing to provide a carbon-doped LiFePO₄ composite powder. The tap density of carbon-doped LiFePO₄ composite powder was 1.02 g/ cm³.

COMPARISON EXAMPLE AC3 (1 ) 0.1 mole Li₂CO₃, 0.2 mole FePO₄, 3.2 gram graphite, and 250 mL ethanol were mixed to provide Li : Fe : P molar ratio of 1 : 1 : 1 , and the mixture was ground in a ball mill for 8 hours at 300 revolutions per minute, and then dried at 70 °C;

(2) In an argon flow rate of 10 L/min, the mixture of (1 ) was heated to 650 °C at a rate of 5°C/min, maintained at this temperature for 6 hours, and then ambient cooled to room temperature, and

(3) In an argon flow rate of 10 L/min, the mixture of (2) was heated to 750 °C at a rate of 10°C/min, maintained at this temperature for 10 hours, and ambient cooled to room temperature, followed by air pulverizing to provide a carbon-doped LiFePO₄ composite powder. The tap density of carbon-doped LiFePO₄ composite powder was 1.07 g/ cm³.

COMPARISON EXAMPLE AC4

(1 ) 0.1 mole Li₂CO₃, 0.2 mole FePO₄, and 250 mL ethanol were mixed to provide Li : Fe : P molar ratio of 1 : 1 : 1 , the mixture was ground in a ball mill for 8 hours at 300 revolutions per minute, and then dried at 70°C;

(2) In an argon flow rate of 10 L/min, the mixture of (1 ) was heated to 650 °C at a rate of 5°C/min, maintained at this temperature for 6 hours, and then ambient cooled to room temperature. 8 gram sucrose and 150 mL ethanol were added thereto, and the mixture was ground in a ball mill for 6 hours at 250 revolutions per minute, and dried at 70 °C; and

The tap density of carbon-doped LiFePO₄ composite powder was 1.09 g/ cm³.

TESTING OF EXAMPLES A1 -A15 and COMPARISON EXAMPLES AC1 -AC4

(1 ) Battery preparation (a) Cathode preparation

100 grams of each of the lithium iron phosphate (LiFePO₄) composite material from examples A1 -A15 and references AC1 -AC4 was separately combined with 5 grams of polyvinylidene fluoride (PVDF) binder, 5 grams of acetylene black, and 50 grams of N-methylpyrrolidone (NMP), and mixed in a vacuum mixer into a uniform slurry. The slurry was applied about 20 microns thick to each side of an aluminum foil, dried at 150°C, rolled and cropped to a size of 540 x 43.5 mm² to provide about 2.8 grams of LiFePO₄ as the active ingredient. (b) Anode preparation

100 grams of natural graphite was combined with 5 grams of conductive acetylene black, 5 grams of polyvinylidene fluoride (PVDF) binder, and 100 grams of N- methylpyrrolidone (NMP), and mixed in a vacuum mixer into a uniform slurry. The slurry was applied about 12 microns thick to each side of a copper foil, dried at 90 °C, rolled and cropped to a size of 500 x 44 mm² to provide about 2.6 grams of natural graphite as the active ingredient, (c) Battery assembly

Each of the cathode and anode was separately wound with polypropylene film into a lithium secondary battery core, followed by dissolving 1 M LiPF₆ in a mixture of non- aqueous electrolyte solvent EC/EMC/DEC to provide a ratio of 1 : 1 : 1 , injecting and sealing the electrolyte having a capacity of 3.8 g/Ah into the battery to provide separate lithium secondary batteries for the testing of examples A1 -A15 and references AC1 -AC4.

(2) Testing cycle (a) Performance test

Each of the lithium secondary batteries A1 -A15 was placed on a test cabinet. The battery was charged using a constant current of 0.1 C with an upper limit of 3.8 volts, then charged for 2.5 hours at constant voltage and set aside for 20 minutes. After charging, using a current of 0.1 C, the battery was discharged from 3.8 volts to 3.0 volts, the battery discharge capacity was recorded, and the capacity of mass ratio and the capacity of volume ratio of the material were calculated according to the following equations, respectively. Capacity of quality ratio= (Initial discharge capacity/the weight of the cathode material) x 100%

Capacity of volume ratio = the tap density x capacity of quality ratio The above steps were repeated 50 times. After the battery has been subjected to the 50 cycles of charge/discharge, the discharge capacity was recorded to calculate the rate of discharge capacity with the following equation:

Rate of discharge capacity = (Discharge capacity at 50^th cycle / Initial discharge capacity) x 100%

(b) Large current discharge performance test

Each battery was charged using a constant current of 0.1 C with an upper limit of 3.8 volts, and then charged for 2.5 hours at constant voltage and set aside for 20 minutes. Each battery was discharged from 3.8 volts to 3.0 volts separately using currents of 1 C, 2C and 5C, the battery discharge capacity was recorded relative to the discharge capacity with 0.1 C to provide discharge capacity ratios, namely: C"ic/Co ic: Current discharge capacity of 1 C from 3.8 volts to 3.0 volts relative to current discharge capacity of 0.1 C from 3.8 volts to 3.0 volts;

C₂c/Co ic: Current discharge capacity of 2C from 3.8 volts to 3.0 volts relative to current discharge capacity of 0.1 C from 3.8 volts to 3.0 volts; and C₅c/Co ic: Current discharge capacity of 5C from 3.8 volts to 3.0 volts relative to current discharge capacity of 0.1 C from 3.8 volts to 3.0 volts. The testing cycle results for examples A1 -A1 1 were shown in Table 1.

Reference is now made to Fig. 1 illustrating a scanning electron microscope (SEM) image at 5000X magnification of a lithium iron phosphate cathode material according to example A1 of the presently disclosed invention. From the figure, it can be observed that the crystals of the LiFePO₄ composite cathode material are relatively uniform with uniform particle size distribution with majority of particles having diameters ranging between 1 to 3 microns. TABLE 1. Test results of LiFePO₄ composite cathode materials and reference samples.

Reference is now made to Fig. 2 illustrating an x-ray diffraction (XRD) pattern of the lithium iron phosphate cathode material according to example A1 of the presently disclosed invention having olivine-type crystal structure and good crystal growth and development.

From the data in Table 1 , it can be observed that the LiFePO₄ composite cathode materials according to examples A1 -A15 of the presently disclosed invention provide higher initial discharge capacity than references AC1 -AC4. In addition, examples A1 - A15 are able to maintain greater than 90% discharge capacity after 50 cycles. More importantly, the large current discharge performance of A1 -A8 maintained, on average, greater than 97%, 94% and 90% at 1 C, 2C, and 5C discharge currents, respectively. Accordingly, the lithium iron phosphate cathode materials for lithium secondary batteries and methods of manufacturing such according to the presently disclosed invention provide superior performance relative to the reference samples and other similar lithium iron phosphate cathode materials currently on the market.

Claims

What is claimed is:

1. A method of preparing a cathode material for lithium secondary batteries comprising the following steps: sintering a first mixture containing a lithium compound, an iron compound, a phosphorous compound and a carbon additive at a first temperature, to obtain a first sintering product; mixing the first sintering product and a carbon additive, to obtain a second mixture; and sintering the second mixture at a second temperature, to obtain the cathode material.

2. The method of claim 1 , wherein the first mixture contains 0.5-20% by weight of carbon additive based on the total weight of the first mixture; and the weight ratio of the carbon additive contained in the second mixture to the carbon additive contained in the first mixture is about 1 -10 : 1.

3. The method of claim 2, wherein the first mixture contains 5-15% by weight of carbon additive based on the total weight of the first mixture; and the weight ratio of the carbon additive contained in the second mixture to the carbon additive contained in the first mixture is about 1 -5 : 1.

4. The method of claim 1 , wherein the method comprises the steps of: heating the first mixture to the first temperature at a rate of 1 -5°C/minute, and then sintering the first mixture at the first temperature, to obtain the first sintering product; cooling the first sintering product to the 10-60°C, and mixing the first sintering product and carbon additive to obtain the second mixture; and heating the second mixture to the second temperature at a rate of 2-10⁰C /minute, and sintering the second mixture at the second temperature.

5. The method of claim 1 or claim 4, wherein the first mixture is sintered at the first temperature of 300-700°C for 5-20 hours; and the second mixture is sintered at the second temperature of 650-900°C for 5-20 hours.

6. The method of claim 1 , wherein the method further comprises grinding the second mixture before sintering the same.

7. The method of claim 1 , wherein the method further comprises bhquetting the second mixture before sintering the same; and the conditions of the bhquetting provide the second mixture with a compact density of 2.5-4.0gram/ cm³.

8. The method of claim 1 , wherein the lithium, iron and phosphorous compounds of the first mixture provide Li : Fe : P molar ratios of about (0.9-1.2) : (0.95-1 ) : 1.

9. The method of claim 1 , 2 or 3, wherein the lithium compound is selected from the group consisting of Li₂CO₃, LiOH, Li₂C₂O₄, CH₃COOLi, LiH₂PO₄ and Li₃PO₄; the iron compound is selected from the group consisting of Fe₂C₂O₄, Fe(CH₃COO)₂, FeCI₂, FeSO₄ and Fe₃(PO₄)₂, Fe₂O₃, FePO₄, Fe(NO₃)₃ and Fe₃O₄; the phosphorous compound is selected from the group consisting of NH₄H₂PO₄, (NH₄)₂HPO₄, Li₃PO₄ and (NH₄)₃PO₄; and the carbon additive is selected from the group consisting of benzene naphthalene phenanthrene copolymer, benzene phenanthrene binary copolymer, benzene anthracene binary copolymer, soluble starch, polyvinyl alcohol, polyethylene glycol, polypropylene, polyacrylamide, sucrose, glucose, urea, phenolic resin, furfural resin, urea-formaldehyde resin, epoxy resin, artificial graphite, natural graphite, superconducting acetylene black, acetylene black, fumed silicon, and various types of carbon black.

10. The method of claim 1 , 2 or 3, wherein the carbon additive contained in the first mixture is selected from the group consisting of sucrose, glucose and urea.

1 1. The method of claim 1 , wherein the sintering processes are carried out in an inert or reducing atmosphere.

12. The method of claim 1 , wherein the first mixture further contains a metal M- containing compound selected from the group consisting of oxides, hydroxides and carbonates of Mg, Mn, Ca, Sn, Co, Ni, Cr, Zr and Mo; and the lithium compound, metal M-containing compound, iron compound and phosphorous compound of the first mixture provide Li : M : Fe : P molar ratios of about (0.9-1.2) : (0.01 -0.05) : (0.95- 1 ) : 1.