Method for preparing lithium iron phosphate as positive electrode active material for lithium ion secondary battery

Technical Field The present invention relates to a method for preparing a battery positive electrode active material, more particularly to a method for preparing lithium iron phosphate (LiFePO ₄ ) as positive electrode active material for a lithium ion secondary battery.

Technical Background The olivine-type LiFePO ₄ , with excellent electrochemical properties, is suitably used as positive electrode material for lithium battery. LiFePO ₄ is applicable to the field of various mobile power sources, and particularly has extremely promising market potential in the field of battery for electric vehicle due to its advantages of excellent cycle performance, good high-temperature charge-discharge performance, abundant raw material resources, no environmental pollution, good thermal stability, and good safety of the prepared battery. But as pure LiFePO ₄ has low ion conductivity and electron conductivity, and lithium ions have low diffusion coefficient between phases of LiFePO ₄ and FePO ₄ during charging and discharging, it is difficult to commercialize LiFePO ₄ , which dramatically restricts application of lithium iron phosphate in commercial lithium ion battery. Presently, the problem of poor conductivity of LiFePO ₄ has been substantially solved by coating LiFePO ₄ with carbon or doping carbon into LiFePO ₄ to obtain LiFePO ₄ /C composite by carbothermal reaction. CN 1785799A discloses a solid-phase synthesis method for preparing lithium iron phosphate which adopts ferrous salt as iron source, such as ferrous oxalate, ferrous acetate, and ferrous chloride, and adopts ammonium dihydrogen phosphate, diammonium hydrogen phosphate, or ammonium phosphate as phosphorus source. The method comprises weighing and mixing the lithium salt, aforementioned ferrous salt and phosphate salt, and transition element dopant at Li: Fe: P: TR atomic ratio of (1-x): 1 : 1 : x, adding milling medium, ball milling for 6-12 hr, baking at 40-70 ^° C, heating the dried powder at 400-550 ^° C in inert atmosphere or reducing atmosphere, holding for 5-10 hr for precalcining, second ball milling for 6-12 hr, baking at 40-70 ^° C, and second calcining at 550-850 ^° C in inert atmosphere or reducing atmosphere to give transition element-doped lithium iron phosphate powder. CN 1948133 A discloses a preparation method of lithium iron phosphate, comprising mixing lithium dihydrogen phosphate containing doping element, ferrous oxalate or ferrous acetate, and conductive agent or precursor thereof at certain ratio, placing the mixture in microwave oven under inert atmosphere protection, calcining at 400 ^° C-900 ^° C for 10-60 min, and cooling to room temperature to give lithium iron phosphate.

In the prior art for preparing lithium iron phosphate from ferrous salt, inert gas needs

to be continuously introduced during reaction in order to prevent the ferrous salt from being oxidized, which not only consumes large amount of inert gas, but also readily makes the prepared lithium iron phosphate incorporate Fe ₂ P impurity phase, thus leading to high internal resistance and low specific capacity of the battery prepared therefrom.

Summary of the Invention

The object of the present invention is to overcome the disadvantages of the lithium iron phosphate prepared by the available methods, such as high internal resistance, and low specific capacity of the battery prepared therefrom, and to provide a method for preparing lithium iron phosphate capable of imparting the prepared battery with low internal resistance and high specific capacity.

The present invention provide a method for preparing lithium iron phosphate as positive electrode active material for lithium ion secondary battery, which comprises sintering a mixture containing a lithium source, an bivalent iron source, a phosphorus source, and a carbon source in an inert atmosphere, and cooling the sintered product; wherein during the sintering process, the inert atmosphere is static, and has a normal pressure. In the method provided in the present invention for preparing lithium iron phosphate, during sintering process, the inert atmosphere is a static atmosphere, and the pressure of the inert atmosphere is normal pressure, i.e. the inert gas is not introduced during the sintering process, and only the inert gas introduced before the sintering and the non-oxidative gas generated from decomposition of the raw material during the sintering are used as protective gas for protecting Fe ²⁺ from being oxidized into Fe ³⁺ . The lithium iron phosphate prepared by the method provided in the present invention is free Of Fe ₂ P impurity phase, with which the prepared battery has high capacity, low internal resistance, and good cycle performance. For example, the battery prepared from lithium iron phosphate obtained by the method in example 1 has initial discharge specific capacity up to 150mAh/g, and low internal resistance of 25-30mω; while the battery prepared from lithium iron phosphate obtained by the method in comparative example 1 only has initial discharge specific capacity of 112mAh/g and its internal resistance comes up to 200-300 mω.

Brief Description of Accompanying Drawings Fig. 1 is XRD diffraction pattern of the lithium iron phosphate prepared by the example 1 of the present invention;

Fig. 2 is XRD diffraction pattern of the lithium iron phosphate prepared by the example 2 of the present invention;

Fig. 3 is XRD diffraction pattern of the lithium iron phosphate prepared by the comparative example 1 of conventional methods;

Fig. 4 is XRD diffraction pattern of the lithium iron phosphate prepared by the comparative example 2 of conventional methods;

Fig. 5 is XRD diffraction pattern of the lithium iron phosphate prepared by the example 5 of the present invention.

Preferred Embodiments For Carrying Out the Present Invention The method provided in the present invention comprises sintering a mixture containing a lithium source, an bivalent iron source, a phosphorus source, and a carbon source in an inert atmosphere, and cooling the sintered product; wherein during the sintering process, the inert atmosphere is static, and has a normal pressure. The normal pressure in the present invention refers to one standard atmosphere, i.e. 1.01 X 10 ⁵ Pa. Although geological position, height above sea-level, and temperature may vary at each place, and the real local atmosphere pressure is not identical to the standard atmosphere, the normal pressure in the present invention refers to one standard atmosphere for the purpose of simplicity. The static inert atmosphere in the present invention refers to non-flowed atmosphere, which is compared with continuous introduction of inert gas during the sintering process in the prior art, i.e., introduction of inert gas is stopped during sintering process, and only the inert gas introduced before the sintering and the non-oxidative gas generated from decomposition of the raw material in the sintering process are used as protective gas for protecting Fe ²⁺ from being oxidized into Fe ³⁺ . According to the present invention, the sintering process can be carried out in various reaction apparatus, as long as it is ensured that the inert atmosphere is static inert atmosphere and the pressure of the inert atmosphere is normal pressure during sintering process. For example, according to one preferred embodiment in the present invention, the sintering is carried out in a reactor provided with a gas inlet, a gas outlet and a pipe, wherein the inert gas is introduced into the reactor through the gas inlet before sintering to replace the air in the reactor, the gas inlet is kept closed during sintering, and one end of the pipe is hermetically connected with the gas outlet and the other end is disposed in a hydraulic liquid for ensuring that the gas generated during the sintering process is discharged after passing through the hydraulic liquid so as to satisfy that the inert atmosphere is static inert atmosphere during sintering process while the inert atmosphere pressure is normal pressure.

According to the present invention, the purpose of communicating the gas outlet and the hydraulic liquid via the pipe during sintering process is to prevent air from entering into the reactor and oxidizing lithium iron phosphate and to maintain the normal pressure in the reactor; therefore, in preferred embodiment, the manner for communicating the gas outlet and the hydraulic liquid via the pipe is to place the other end of the pipe in the hydraulic liquid, preferably 5-8 cm below the liquid level of the hydraulic liquid. There is no special restriction on quantity and position of the gas outlets and gas inlets arranged on the reactor, as long as it can be ensured that the inert gas can be introduced into the reactor to replace the air in the reactor, and gas resulted from reaction is discharged via gas outlet to keep the normal pressure of the inert

atmosphere. Preferably, for conveniently replacing air and discharging the gas generated during the sintering process, the gas outlets and gas inlets are disposed at the same side of the reactor, and more preferably at the same vertical plane of the same side, wherein the gas inlet is arranged at lower part and the gas outlet is arranged at upper part. When the inert gas is introduced into the reactor, there is no special restriction on its flow rate, for example 5-20 L/min.

There is no special limitation on volume and material of the reactor. Those skilled in the art can determine material and volume of the reactor according to production requirement. As the mixture containing the lithium source, bivalent iron source, phosphorus source, and carbon source will generate hydrogen, ammonia gas, carbon monoxide, and carbon dioxide during sintering in the inert atmosphere and the sintering temperature is relatively high, the hydraulic liquid is preferably liquid which has a boiling point no lower than 140 ^° C and is unreactive with gases generated during the sintering process in order to prevent backward suction of the hydraulic liquid, such as one of hydraulic oil, quenching oil, and high temperature lubrication oil.

According to the present invention, the inert atmosphere is one or more gases unreactive with reactants or reaction product, such as one or more of nitrogen, carbon monoxide, carbon dioxide, ammonia gas, and Group 0 gases. The lithium source, bivalent iron source, and phosphorus source have Li: Fe: P molar ratio of 0.9-1.2: 1 : 1, preferably the Li: Fe: P stoichiometric ratio in the product lithium iron phosphate, i.e. Li: Fe: P molar ratio of 1 : 1 : 1.

The lithium compound can be selected from well known lithium compounds for preparing lithium iron phosphate, such as one or more Of Li ₂ CO ₃ , LiOH, Li ₂ C ₂ O ₄ and CH ₃ COOLi.

The phosphorus source can be selected from well known phosphorus compounds for preparing lithium iron phosphate, such as one or more of NH ₄ H ₂ PO ₄ , (NH ₄ ) ₂ HPO ₄ , (NH ₄ ) ₃ PO ₄ , LiH ₂ PO ₄ , Li ₂ HPO ₄ and Li ₃ PO ₄ . The carbon source can be additive with conductive effect well known in the field, such as one or more of terpolymer of benzene, naphthalene and phenanthrene, binary copolymer of benzene and phenanthrene, binary copolymer of benzene and anthracene, polyvinyl alcohol, sucrose, glucose, citric acid, starch, dextrin, phenolic resin, furfural resin, artificial graphite, natural graphite, acetylene black, carbon black, and mesocarbon microbeads, and preferably one or more of sucrose, glucose, starch, dextrin, artificial graphite, natural graphite, acetylene black, carbon black, and mesocarbon microbeads. During the sintering process, the carbon source can partially be decomposed and released in form of carbon monoxide and carbon dioxide, and partially be doped into the generated lithium iron phosphate to improve conductivity of the lithium iron phosphate. The amount of carbon source can be 0.5-20wt%, preferably 3-12 wt% relative to the total weight of the lithium source, bivalent iron source, and phosphorus source. The bivalent iron source can be selected from well known ferrous compounds for

preparing lithium iron phosphate, such as one or more of FeC ₂ O ₄ , Fe(CH ₃ COO) ₂ and FeCO ₃ . The iron source is preferably mixture of FeC ₂ O ₄ and FeCO ₃ , and the molar ratio OfFeC ₂ O ₄ to FeCO ₃ can be 1 : 0.5-4, preferably 1 : 1.5-4.

Compared with ferrous oxalate as sole iron source, the mixture of FeC ₂ O ₄ and FeCO ₃ at molar ratio of 1 :0.5-4 as iron source has the advantage as below: the amount of the generated CO is small during sintering of the mixture, and thus the generated CO only prevents Fe ²⁺ from being oxidized into Fe ³⁺ , and will not reduce Fe ²⁺ into Fe or PO ₄ into P, such that generation of Fe ₂ P is prevented, and the obtained lithium iron phosphate has high purity and further improved specific capacity; meanwhile, since no H ₂ O or Fe is generated, no hydrogen will be generated and the operation safety will be improved.

The mixture OfFeC ₂ O ₄ and FeCO ₃ is prepared by one of the following methods: Method 1 : mixing anhydrous ferrous oxalate and anhydrous ferrous carbonate at molar ratio of 1 : 0.5-4; Method 2: heating ferrous oxalate under vacuum at a temperature of 100-350 ^° C, preferably 120-300 ^° C for 0.2-6 hr, and preferably 0.5-5 hr.

Molar ratio of FeC ₂ O ₄ to FeCO ₃ in the product resulted from heating ferrous oxalate can be calculated according to method as below. Provided that FeC ₂ O ₄ -2H ₂ O is added at the mass of X g, and the mass of the mixture of FeC ₂ O ₄ and FeCO ₃ as the product resulted from heating FeC ₂ O ₄ -2H ₂ O is Y g, the molar ratio Of FeC ₂ O ₄ : FeCO ₃ is (179.902Y-115.86X): (143.87X-179.902Y), wherein 179.902 is the molecular weight of FeC ₂ O ₄ -2H ₂ O, 115.86 is the molecular weight of FeCO ₃ , and 143.87 is the molecular weight OfFeC ₂ O ₄ . The heating of ferrous oxalate is carried out under vacuum condition, such that CO generated from decomposition can be removed in time to avoid Fe ²⁺ from being reduced into Fe by CO. The pressure under vacuum condition may be 100-1,000 Pa, preferably 200-700 Pa. Herein the pressure is absolute pressure. Conventional vacuum apparatus, such as vacuum pump or vacuum oven, can be used to realize the aforementioned vacuum condition. The mixture containing the lithium source, bivalent iron source, phosphorus source, and carbon source can be obtained by mechanical mixing or grinding, preferably ball milling. The ball milling method comprises mixing the lithium source, bivalent iron source, phosphorus source, and carbon source with the dispersant, and then ball milling. The dispersant can be conventional organic solvents, such as one or more of methanol, ethanol, and acetone. The weight ratio of the usage amount of the dispersant to the total weight of iron source, lithium source, phosphorus source, and carbon source is 1-5: 1. The ball milling condition is such to ensure that the aforementioned substances can be uniformly mixed, for example, the ball milling time can be 3-12 hr. Preferably, the mixture is further dried after the ball milling process. The drying method and condition are well known for those skilled in the art. The drying condition is such to ensure the dispersant can be evaporated off, for example, the drying temperature can be 30-80 ^° C .

The sintering method can be various sintering methods well known to those skilled in the art, for example one-stage sintering or two-stage sintering.

The one-time sintering is preferably one-stage constant-temperature sintering, which can be carried out at 500-750 ^° C, preferably 700-750 ^° C, for 2-20 hr, preferably 10-20 hr. For further controlling morphology of lithium iron phosphate to ensure integral growth of lithium iron phosphate crystal form, preferably, the one-stage constant-temperature sintering method in the present invention comprises heating at rate of 5-20 ^° C/min, preferably 10-15 ^° C/min, to the temperature for the constant-temperature sintering, and sintering at that temperature. The two-stage sintering method comprises subjecting the mixture to a first sintering at a first sintering temperature of 300-450 ^° C for 4-15 hr, and a second sintering at a second sintering temperature of 600-800 ^° C for 10-25 hr.

The cooling method can be various methods well known in the field, such as natural cooling. For further preventing oxidation of the generated lithium iron phosphate, the sintering product is preferably cooled to room temperature under inert atmosphere.

The inert atmosphere can be static atmosphere, and is preferably flow atmosphere with gas flow rate of 2-20 L/min.

The present invention will be further described by the examples as below.

Example 1

This example describes the method for preparing lithium iron phosphate provided in the present invention.

(1) 369g Of Li ₂ CO ₃ , 1799g of FeC ₂ O ₄ ^H ₂ O, 115Og off NH ₄ H ₂ PO ₄ , 30Og of glucose and 300Og of ethanol were mixed (the molar ratio of Li:Fe:P is 1 :1 :1), ball milled at 300 rpm for 10 hr, and then taken out, and dried at 80 ^° C .

(2) 300g of the mixture obtained in step (1) was fed into a stainless reactor (100L) provided with a gas inlet and a gas outlet, wherein the gas inlet and the gas outlet were arranged in the same vertical plane, the gas inlet was arranged at lower part, and the gas outlet was arranged at upper part. The gas inlet and gas outlet were opened, while argon gas was introduced at flow rate of 5 L/min to replace the air in the reactor. Thereafter, the gas inlet was closed, and the gas outlet was connected with a guide tube which was placed into 25 ^° C hydraulic oil (Caltex, special hydraulic oil 46#) at depth of 5cm below the oil level. Then the mixture was heated at rate of 10 ^° C/min to 750 ^° C , and sintered at that temperature for 20 hr. Finally, the gas inlet was opened, and argon gas was introduced at the rate of 5 L/min for cooling the product to room temperature to obtain lithium iron phosphate.

X-ray powder diffractometer (Rigaku Corporation, D/MAX-2200/PC) was adopted to give XRD pattern of the lithium iron phosphate as shown in Fig.l.

Example 2

This example describes the method for preparing lithium iron phosphate provided in

the present invention.

(1) 239.5g of LiOH, 1158.6g OfFeCO ₃ , 1319.7g OfNH ₄ H ₂ PO ₄ , 32Og of glucose,, and 270Og of ethanol were mixed (the molar ratio of Li:Fe:P is 1 :1 :1), ball milled at 300 rpm for 10 hr, and then taken out and dried at 80 ^° C . (2) 30Og of the mixture obtained in step (1) was fed into a stainless reactor (100L) provided with a gas inlet and a gas outlet, wherein the gas inlet and the gas outlet were arranged in the same vertical plane, the gas inlet was arranged at lower part, and the gas outlet was arranged at upper part. The gas inlet and gas outlet were opened, while argon gas was introduced at flow rate of 10 L/min to replace the air in the reactor. Thereafter, the gas inlet was closed, and the gas outlet was connected with a guide tube which was placed into 25 ^° C hydraulic oil (Caltex, special hydraulic oil 46#) at depth of 5cm below the oil level. Then the mixture was heated at rate of 5 ^° C/min to 700 ^° C , and sintered at that temperature for 20 hr. Finally, the gas inlet was opened, and argon gas was introduced at the rate of 15 L/min for cooling the product to room temperature to obtain lithium iron phosphate.

X-ray powder diffractometer (Rigaku Corporation, D/MAX-2200/PC) was adopted to give XRD pattern of the lithium iron phosphate as shown in Fig.2.

Example 3 This example describes the method for preparing lithium iron phosphate provided in the present invention.

(1) 369g Of Li ₂ CO ₃ , 1799g of FeC ₂ O ₄ -2H ₂ O, 115Og Of NH ₄ H ₂ PO ₄ , 31Og of dextrin and 300Og of ethanol were mixed (the molar ratio of Li:Fe:P is 1 :1 :1), ball milled at 300 rpm for 5 hr, and then taken out and dried at 80 ^° C . (2) 30Og of the mixture obtained in step (1) was fed into a stainless reactor

(100L) provided with a gas inlet and a gas outlet, wherein the gas inlet and the gas outlet were arranged in the same vertical plane, the gas inlet was arranged at lower part, and the gas outlet was arranged at upper part. The gas inlet and gas outlet were opened, while argon gas was introduced at flow rate of 15 L/min to replace the air in the reactor. Thereafter, the gas inlet was closed, and the gas outlet was connected with a guide tube which was placed into 25 ^° C quenching oil (SKFZ-2 quenching oil, Suzhou Special Chemical Co. Ltd.) at depth of 8cm below the oil level. Then the mixture was heated at rate of 15 ^° C/min to 750 ^° C, and sintered at that temperature for 20 hr. Finally, the gas inlet was opened, and argon gas was introduced at the rate of 10 L/min for cooling the product to room temperature to obtain lithium iron phosphate.

Example 4

This example describes the method for preparing lithium iron phosphate provided in the present invention.

(1) 369g Of Li ₂ CO ₃ , 1799g of FeC ₂ O ₄ -2H ₂ O, 1319.7g of (NH ₄ ) ₂ HPO ₄ , 31Og of

carbon black and 300Og of ethanol were mixed (the molar ratio of Li:Fe:P is 1 :1 :1), ball milled at 300 rpm for 10 hr, and then taken out and dried at 80 ^° C .

(2) 300g of the mixture obtained in step (1) was fed into a stainless reactor (100L) provided with a gas inlet and a gas outlet, wherein the gas inlet and the gas outlet were arranged in the same vertical plane, the gas inlet was arranged at lower part, and the gas outlet was arranged at upper part. The gas inlet and gas outlet were opened, while argon gas was introduced at flow rate of 5 L/min to replace the air in the reactor. Thereafter, the gas inlet was closed, and the gas outlet was connected with a guide tube which was placed into 25 ^° C high temperature lubricating oil (NSlOOl high temperature lubrication grease, YAMAICHI) at depth of 5cm below the oil level. Then the mixture was heated at rate of 10 ^° C/min to 700 ^° C, and sintered at that temperature for 20 hr. Finally, the gas inlet was opened, and argon gas was introduced at the rate of 5 L/min for cooling the product to room temperature to obtain lithium iron phosphate.

Comparison Example 1

This comparison example describes the method for preparing lithium iron phosphate provided in prior arts.

Lithium iron phosphate was prepared according to the same method in the exampl 1 , except that in the step (2), 300g of the mixture obtained in step (1) was fed into a stainless reactor (100L) provided with a gas inlet and a gas outlet, wherein the gas inlet and the gas outlet were arranged in the same vertical plane, the gas inlet was arranged at lower part, and the gas outlet was arranged at upper part; the gas inlet and the gas outlet were opened, and argon gas was introduced at flow rate of 5 L/min to replace the air in the reactor; argon gas was continuously introduced, and regulated to the flow rate of 2 L/min, while the mixture was heated at rate of 10 ^° C/min to 750 ^° C, and sintered at that temperature for 20 hr; and finally argon gas was continuously introduced for cooling to room temperature to obtain lithium iron phosphate. X-ray powder diffractometer (Rigaku Corporation, D/MAX-2200/PC) was adopted to give XRD pattern of the lithium iron phosphate as shown in Fig.3.

Comparison Example 2

This comparison example describes the method for preparing lithium iron phosphate provided in prior arts. Lithium iron phosphate was prepared according to the same method in the example 1, except that in the step (2) 300g of the mixture obtained in step (1) was fed into a stainless reactor (100L) provided with a gas inlet and a gas outlet, wherein the gas inlet and the gas outlet were arranged in the same vertical plane, the gas inlet was arranged at lower part, and the gas outlet was arranged at upper part; the gas inlet and the gas outlet were opened, and CO gas was introduced at flow rate of 5 L/min to replace the air in the reactor; after the gas inlet and gas outlet were closed, the mixture was heated at rate of 10 ^° C/min to 750 ^° C, and sintered at that temperature for 20 hr;

and finally the product was cooled to room temperature to obtain lithium iron phosphate as anode active material for lithium ion secondary battery. X-ray powder diffractometer (Rigaku Corporation, D/MAX-2200/PC) was adopted to give XRD pattern of the lithium iron phosphate as shown in Fig.4.

Example 5

This example describes the method for preparing lithium iron phosphate provided in the present invention.

Lithium iron phosphate was prepared according to the same method in the example 1 , except that in step (1), 1799g Of FeC ₂ O ₄ -IH ₂ O was replaced by a mixture of 2.5 mol FeC ₂ O ₄ and 7.5 mol FeCO ₃ which was prepared as follows.

1799 g of FeC ₂ O ₄ -2H ₂ O was heated in 280 ^° C vacuum oven (vacuum degree 500 Pa) for 3 hr to obtain the mixture Of FeC ₂ O ₄ and FeCO ₃ which was then cooled at 5 ^° C/min to room temperature. The molar ratio of FeC ₂ O ₄ to FeCO ₃ in the mixture was calculated as 1 :3.

X-ray powder diffractometer (Rigaku Corporation, D/MAX-2200/PC) was adopted to give XRD pattern of the lithium iron phosphate as shown in Fig.5.

Example 6 This example describes the method for preparing lithium iron phosphate provided in the present invention.

Lithium iron phosphate was prepared according to the same method in the example 1 , except that, in step (1), 1799g of FeC ₂ O ₄ -2H ₂ O was replaced by a mixture of 5 mol

FeC ₂ O ₄ and 5 mol FeCO ₃ which was prepared by directly mixing the FeC ₂ O ₄ and FeCO ₃ ; and in the step (2), two-stage sintering method was used, i.e. the mixture obtained in the step (1) was sintered at 350 ^° C for 5 hr, and then sintered at 700 ^° C for

15 hr.

Examples 7-12 In the following examles, batteries were prepared from lithium iron phosphate as positive electrode active material provided in the present invention, and then were subjected to performance tests.

(1) Battery preparation

Positive electrode preparation: 9Og Of LiFePO ₄ prepared by the examples 1-6 as positive electrode active material respectively, 5g of poly(vinylidene difluoride) (PVDF) as binder, and 5g of acetylene black as conductive agent were added into 5Og of N-methylpyrrolidone, and stirred in a vacuum stirrer to form uniform positive electrode slurry. The positive electrode slurry was uniformly coated on both sides of 20 micron-thick aluminum foil, baked at 150 ^° C, rolled, and cutted into 540mm><43.5mm positive electrode containing LiFePO ₄

5.2g as active component.

Negative electrode preparation:

9Og natural graphite as negative electrode active component, 5g of PVDF as binder, and 5g of carbon black as conductive agent were added into lOOg of N-methylpyrrolidone, and stirred in vacuum stirrer to form uniform negative electrode slurry. The slurry was uniformly coated on both side of 12 micron-thick copper foil, baked at 90 ^° C, rolled, and cutted into 500mmx44mm negative electrode containing 3.8g of natural graphite as active component. Battery assembly:

Aforementioned positive electrode, negative electrode, and polypropylene membrane were respectively wound into an electrode core for a square lithium ion battery, and the electrode core was then placed in a battery aluminum casing. LiPF ₆ was dissolved at concentration of 1 mol/L in mixed solvent of EC/EM C/DEC (1 : 1 :1) to form a non aqueous electrolyte which was then injected at 3.8g/Ah into the battery aluminum casing, and then the casing was sealed to obtain lithium ion secondary batteries Al -A6, respectively. (2) Battery mass specific capacity and cycle performance test

The test comprises respectively charging obtained lithium ion batteries A1-A6 at constant current of 0.2C with charging upper limit set at 4.2V, standing for 20min, discharging at constant current of 0.2C from 4.2V to 2.5V, recording the discharge capacity as initial discharge capacity during the discharge process, and calculating the battery mass specific capacity according to equation as below:

Mass specific capacity = initial discharge capacity of the battery (mAh)/ the weight of the active component in the positive electrode (g)

The discharge capacity was recorded respectively after repeating the aforementioned charging-discharging steps for 30 and 50 times, and the capacity maintenance ratio after and before cycles was calculated as below:

Capacity maintenance ratio = (discharge capacity after Nth cycle/initial discharge capacity) X 100%

The results were shown in Table 1.

(3) Battery internal resistance test The test comprises respectively placing lithium ion batteries A1-A6 on intelligent battery internal resistance tester (Guangzhou Kinte Co. Ltd., BS-VR3), applying IKHz AC signal, and obtaining the internal resistance by measuring the battery AC voltage drop.

Comparison example 3-4

The comparison examples describe batteries prepared from lithium iron phosphate obtained by prior arts, and the battery performance tests.

Comparison batteries AC1-AC2 were prepared, and subjected to the battery initial discharge capacity and cycle performance tests to give mass specific capacity according to the same method in examples 7-12, except that the positive electrode active material for preparing the battery was lithium iron phosphate obtained from the comparison examples 1-2.

The results were shown in Table 1.

Table 1

It can be observed from results in Table 1, the batteries A1-A6 prepared from lithium iron phosphate according to the present invention have significantly higher initial discharge mass specific capacity than that of the comparison batteries AC1-AC2 in the comparison examples, and significantly lower battery internal resistance than that of the comparison batteries; and the batteries A1-A6 have capacity maintenance ratios respectively above 98% and 97% after 30 cycles and 50 cycles, while the comparison batteries have the capacity maintenance ratios respectively only 92.21-93.28% and 88.83-90.85% after 30 cycles and 50 cycles. Therefore, the battery prepared from lithium iron phosphate according to the method in the present invention has high capacity, low internal resistance, and good cycle performance. Figures 1, 2, and 5 show XRD diffraction patterns of lithium iron phosphate prepared in the examples 1, 2, and 5, respectively. It can be observed from the figures that those lithium iron phosphates have standard olivine type structure and good crystal form, and are free of impurity phase. Figures 3 and 4 show XRD diffraction patterns of lithium iron phosphate prepared by the conventional method in the comparison examples 1 and 2, respectively. Compared with PDF standard card (85-1727) Of Fe ₂ P, a highest peak at 2θ of 40°-41°, and a second highest peak at 2θ of 44°-45° appear in the XRD diffraction patterns shown in Figures 3 and 4, and those two peaks are characteristic peaks of Fe ₂ P, indicating that Fe ₂ P impurity phase is incorporated in those lithium iron phosphates.