Preparation and application of LiFePO4/Li3V2 (PO4)3 composite cathode materials for lithium ion batteries

Abstract

A method of preparing LiFePO4/Li3V2(PO4)3 composite cathode materials and their applications as cathode materials for lithium ion batteries are disclosed. The preparation method includes the following steps: (A) providing a mixture of iron powder, lithium salt, vanadium salt, and a phosphate salt whereafter these compounds are dissolved into a mixed acid solution; (B) drying the solution in order to obtain precursor powders; and (C) heating the precursor powders at a temperature ranging between 400 and 1000° C. to form LiFe1-y′Vy′PO4/Li3V2-y″Fey″(PO4)3 composite powders. Alternatively, prepare the composite cathode by preparing olivine LiFe1-y′Vy′PO4 and monoclinic Li3V2-y′Fey″(PO4)3 powders as in previous procedures followed by mixing adequately. The low cost of iron powder thus facilitates to prepared composite cathode materials exhibiting higher electrical conductivity and superior cycling performance at high C rates than those of olivine LiFe1-y′Vy′PO4 and monoclinic Li3V2-y″Fey″(PO4)3. The invention will help the development of the lithium ion batteries and related industries.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an X-ray diffraction pattern of the embodiment 1 in the present invention;

FIG. 2 is an SEM and mapping micrograph of the embodiment 1 in the present invention;

FIG. 3 is a cyclic charge/discharge diagram of the embodiment 1 in the present invention;

FIG. 4 is a cyclic charge/discharge curve of the embodiment 1 in the present invention; and

FIG. 5 is a comparative diagram of cyclic charge/discharge results of the embodiment 2 in the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Embodiment 1

To Prepare Olivine Phase LiFe_1-y′V_y′PO₄ and Monoclinic Phase Li₃V_2-y″Fe_y″(PO₄)₃ Composite Cathode Material Powders by Direct Solution-based Methods and Spray Drying Methods

0.5 mole iron powders, 0.5 mole NH₄VO₃ powders, 1 mole LiOH powders, and 400 mL 0.5 mole citric acid solution are added in 1 mole (NH₄)₂HPO₄ solution. In the mixed solution, the molar ratio of Li⁺, Fe²⁺, V³⁺, and PO₄³⁻ is 1: 0.5: 0.5: 1. Subsequently, 4.7 g PEG dissolved in optimal water, which becomes 3 wt % PEG solution, is added into the mixed solution. After reacting iron powders, LiOH, NH₄VO₃, citric acid, and (NH₄)₂HPO₄ solution completely, this solution is dried by a spray drying method to obtain precursor powders of LiFe_1-y′V_y′PO₄/Li₃V_2-y″Fe_y″(PO₄)₃ composite cathode material. The LiFe_1-y′V_y′PO₄/Li₃V_2-y″Fe_y″(PO₄)₃ composite cathode material precursor powders are put into nitrogen gas and then heated at 750° C. for 6 hours, whereafter 282 g of the LiFe_1-y′V_y′PO₄/Li₃V_2-y″Fe_y″(PO₄)₃ composite cathode material powders is obtained.

Testing Results

a. X-ray diffraction analysis

As shown in FIG. 1, the X-ray diffraction patterns of the prepared LiFePO₄/Li₃V₂(PO₄)₃ composite cathode material powders with LiFe_1-y′V_y′PO₄/Li₃V_2-y″Fe_y″(PO₄)₃ in this embodiment mainly show two signal peaks of olivine LiFePO₄ and monoclinic Li₃V₂(PO₄)₃, and the more V added means the more distinct peak signals (i.e. peaks with *).

According to the preparation of LiFe_1-y′V_y′PO₄/Li₃V_2-y″Fe_y″(PO₄)₃ composite cathode material powders in the present invention, as long as exact ratio mixture of iron powders, lithium salt, vanadium salt, and ammonium phosphate salt is reacted in a mixed acid solution, LiFe_1-y′V_y′PO₄/Li₃V_2-y″Fe_y″(PO₄)₃ composite cathode material powders would be prepared by any conventional method of drying and heating.

b. SEM and mapping analysis

As the SEM and mapping micrographs shown in FIG. 2, the primary particle diameter of the LiFe_1-y′V_y′PO₄/Li₃V_2-y″Fe_y″(PO₄)₃ composite cathode material powders prepared by the spray drying method in this embodiment is about 1˜2 μm. Any elemental distribution of the composite cathode material powders after adding various dosages of vanadium is observed through mapping. As shown in elemental distribution plots when the added dosage x of vanadium is 0.5, the olivine phase LiFePO₄ and monoclinic phase Li₃V₂(PO₄)₃ exist together in the mixed powders.

c. Electrical conductivity analysis of powders detected by conductivity measurement system

Compared data of electrical conductivity of powders analyzed by conductivity measurement system are shown in table 1. Among single phase LiFePO₄ prepared by a solid-state reaction method, and LiFePO₄ (i.e. LFP), Li₃V₂(PO₄)₃ (i.e. LVP), and LiFePO₄/Li₃V₂(PO₄)₃ composite (i.e. LFVP) prepared in this embodiment, the LFVP has improved electrical conductivity in the similar carbon content and particle diameter.

TABLE 1
comparing electrical conductivity of LiFePO4, Li3V2(PO4)3,
and LiFePO4/Li3V2(PO4)3 cathode materials
	content of		electrical conductivity
sample	carbon (wt %)	D50 (μm)	(Scm−1)
single phase	—	8.44	3.63 × 10−8
LiFePO4
LFP	3.31	3.769	6.75 × 10−3
LVP	3.18	4.037	2.88 × 10−4
LFVP	3.52	3.123	2.5 × 10−2

d. Tests of cyclic voltammetry

The prepared LiFePO₄/Li₃V₂(PO₄)₃ composite cathode material, acetylene black, and polyvinylidene fluoride (PVDF) mixed at ratio of 83: 10: 7 by weight are mixed with N-methylpyrollidone (NMP) to become a slurry spread evenly on aluminum foil. The slurry is prepared to form a suitable cathode test slice through drying. In glove box filled with argon gas, lithium foil used as a counter and reference electrode, 1 M LiPF₆ in EC/DEC (1:1 vol.) used as electrolyte, and Celgard 2400 used as separation membrane are set into a tri-electrode battery in which cyclic voltammetry processes occur. The result of cyclic voltammetry shows that redox reactions of olivine phase LiFePO₄ and monoclinic phase Li₃V₂(PO₄)₃ happen in the LiFePO₄/Li₃V₂(PO₄)₃ composite cathode material (i.e. the redox feature peaks of LiFePO₄ are 3.35 V reductive peaks and 3.5 V oxidative peaks; the redox feature peaks of Li₃V₂(PO₄)₃ are redox peak pairs of reductive peaks of 3.56 V vs. oxidative peaks of 3.6 V, reductive peaks of 3.64 V vs. oxidative peaks of 3.7 V, and reductive peaks of 4.02 V vs. oxidative peaks of 4.11 V.). According to these tests, the cathode material in the present invention is a composite of LiFePO₄/Li₃V₂(PO₄)₃, and different from conventional LiFePO₄ cathode materials. Additionally, the LiFePO₄/Li₃V₂(PO₄)₃ composite cathode material in the present invention tested by cyclic voltammetry has an improved working voltage of LiFePO₄ due to redox reaction of Li₃V₂(PO₄)₃ therein.

e. Tests of cyclic charge/discharge

The LiFePO₄/Li₃V₂(PO₄)₃ composite cathode material prepared in this embodiment, acetylene black, and polyvinylidene fluoride (PVDF) mixed at ratio of 83: 10: 7 by weight are mixed with N-methylpyrollidone (NMP) to become a slurry spread evenly on aluminum foil. The slurry is prepared to form a suitable cathode test slice through drying. In a glove box filled with argon gas, lithium foil used as negative electrode, 1 M LiPF₆ in EC/DEC (1:1 vol.) used as electrolyte, and Celgard 2400 used as separation membrane are set into a coin battery for cyclic charge/discharge tests to be processed therein.

As per the cyclic charge/discharge tests in this embodiment shown in FIG. 3, the results of charge/discharge tests are shown at various charge/discharge rates (between C/10 and 10C) in cutoff voltages ranging from 2.5 V to 4.3 V. FIG. 3 shows that specific capacity of the coin battery made of the LiFePO₄/Li₃V₂(PO₄)₃ composite cathode material prepared in the embodiment 1 is between specific capacity of olivine phase LiFePO₄ and monoclinic phase Li₃V₂(PO₄)₃ in conditions of room temperature and C/10 charge/discharge rate (0.06 mA/cm²), and initial specific capacity thereof is maintained at 127 mAh/g even though 5 charge/discharge cycles have elapsed. When the olivine LiFePO₄/Li₃V₂(PO₄)₃ composite cathode material powders in this embodiment are used as cathode materials, the capacity thereofis not faded, but it still has good charge/discharge characteristics at slower charge/discharge rate of C/10. Further, when testing at faster charge/discharge rates (1C, 5C, 8C, and 10C), the results thereof demonstrate that the battery made of powders synthesized in this embodiment still have good charge/discharge characteristics. The initial specific capacity of the battery so far is 110 mAh/g at fast charge/discharge rate of 1C, and 105 mAh/g at faster charge/discharge rate of 5C, even though 15 charge/discharge cycles have elapsed. In the figures, all of the composite cathode material comparing to olivine LiFePO₄ or monoclinic Li₃V₂(PO₄)₃ cathode material with similar carbon content synthesized according to the same method of the present invention have higher specific capacity. At charge/discharge rates of 8C and 10C, the battery made of the olivine LiFePO₄/Li₃V₂(PO₄)₃ composite cathode material powders in this embodiment still maintain discharge capacity of 100 mAh/g almost without any capacity fading.

f. Tests of batteries charge/discharge curve

Tests of cyclic charge/discharge at various rate processes in the coin battery prepared similarly through the method of mentioned “tests of cyclic charge/discharge”, and the results are plotted into a voltage-specific capacity curve (see FIG. 4). The voltage-specific capacity curve of LiFePO₄/Li₃V₂(PO₄)₃ composite cathode material at different charge/discharge rates is a curve with plural plateaus. In addition to plural plateaus improving working voltage of olivine composite cathode materials, those are also applied to calculate the residual electrical quantity of batteries and to prevent overcharge of lithium batteries.

Embodiment 2

Individually Preparing Olivine Phase LiFePO₄ and Monoclinic Phase Li₃V₂(PO₄)₃ Cathode Material by Way of Indirect Solution-based Methods and Mixing These to Obtain the LiFePO₄/Li₃V₂(PO₄)₃ Composite Cathode Material

5 mole iron powders, 5 mole LiOH, 5 mole (NH₄)₂HPO₄, and 1700 mL 4 mole citric acid solution are mixed to form a solution. In the mixed solution, the molar ratio of Li⁺, Fe²⁺, and PO₄³⁻ is 1: 1: 1. Subsequently, 23.66 g PEG dissolved in optimal water, which becomes 3 wt % PEG solution, is added into the mixed solution. After reacting iron powders, LiOH, citric acid, and (NH₄)₂HPO₄ completely, this solution is dried by a spray drying method to obtain precursor powders of pure phase LiFePO₄. The olivine LiFePO₄ cathode material precursor powders put into nitrogen gas are heated at 750° C. for 6 hours, and then 800 g of the olivine phase LiFePO₄ cathode material powders is obtained.

1 mole NH₄VO₃, 1.5 mole LiOH, 1.5 mole (NH₄)₂HPO₄, and 170 mL 1.5 mole citric acid solution are mixed to form a solution. In the mixed solution, the molar ratio of Li⁺, V³⁺, and PO₄³⁻ is 3: 2: 3. Subsequently, 6.11 g PEG dissolved in optimal water, which becomes a 3 wt % PEG solution, is added into the mixed solution. After reacting NH₄VO₃, LiOH, citric acid, and (NH₄)₂HPO₄ completely, this solution is dried by the spray drying method to obtain precursor powders of pure phase Li₃V₂(PO₄)₃. The Li₃V₂(PO₄)₃ cathode material precursor powders are then put into nitrogen gas and heated at 750° C. for 6 hours, after which 200 g of the monoclinic phase Li₃V₂(PO₄)₃ cathode material powders is obtained.

Further, in the aforementioned preparation, 800 g of the olivine LiFePO₄ and 200 g of the monoclinic Li₃V₂(PO₄)₃ and cathode material powders are mixed together. In the mixed powders, the molar ratio of LiFePO₄ and Li₃V₂(PO₄)₃ is 5: 0.5. After adding the mixed powders into the 3 wt % PEG solution, the mixed solution is dried by the spray drying method to obtain composite cathode material precursor powders with even distribution of olivine LiFePO₄ and monoclinic Li₃V₂(PO₄)₃. The powders obtained by the spray drying method are put into nitrogen gas and heated at 750° C. for 1 hours, after which 1000 g LiFePO₄/Li₃V₂(PO₄)₃ composite cathode material powders is obtained.

Testing Results

Tests of cyclic charge/discharge

The three kinds of powders prepared in the embodiment 2, acetylene black, and polyvinylidene fluoride (PVDF) mixed at ratio of 83: 10: 7 by weight are mixed with N-methylpyrollidone (NMP) to become aslurry spread evenly on aluminum foil. The slurry is prepared to form a suitable cathode test slice through drying. The lithium foil used as negative electrode, 1 M LiPF₆ in EC/DEC (1:1 vol.) used as electrolyte, and Celgard 2400 used as separation membrane are set into a coin battery in which cyclic charge/discharge tests are processed. As per the results shown in FIG. 5, the discharge capacity of the LiFePO₄/Li₃V₂(PO₄)₃ composite cathode material powders is higher than the discharge capacity of pure olivine LiFePO₄ and monoclinic Li₃V₂(PO₄)₃ cathode material powders prepared in the embodiment 2

The LiFePO₄/Li₃V₂(PO₄)₃ composite cathode material powders prepared in the present invention have higher electric conductivity than olivine and monoclinic phase cathode material powders to improve high charge/discharge rate in the lithium batteries, and are suitably applied in the present lithium batteries. Further, the material with composite microstructure prepared in the present invention is a novel material dramatically differing from, and better than, conventional LiFePO₄ material.

Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the scope of the invention as hereinafter claimed.

Claims

1. A method for preparing LiFePO4/Li3V2(PO4)3 composite cathode materials comprising the following steps: (A) providing a mixture of iron powder, lithium salt, vanadium salt, and phosphate salt dissolved into a mixed acid solution to form a mixed solution of LixFe1-yVy(PO4)z, wherein x is between 0.9 and 1.5, y is between 0 and 1, and z is between 0.9 and 1.5;(B) stirring the mixed solution;(C) drying the mixed solution in order to obtain solid powders; and(D) heating the solid powders at temperature ranging between 400° C. and 1000° C.

2. The method as claimed in claim 1, wherein the step (C) is a conventional method for directly heat drying or spray drying the mixed solution.

3. The method as claimed in claim 1, wherein the step (D) is a heating process of the solid powder in nitrogen or argon gas.

4. The method as claimed in claim 1, wherein the mixed acid solution is a mixture of organic acid and inorganic acid.

5. The method as claimed in claim 4, wherein the organic acid is acetic acid, citric acid, oxalic acid, tartaric acid, propionic acid, butyric acid, or a mixture thereof; the inorganic acids are hydrochloric acid, sulfuric acid, nitric acid, perchloric acid, hypochlorous acid, hydrofluoric acid, or a mixture thereof.

6. The method as claimed in claim 1, wherein the step (A) further comprises adding a carbohydrate or a polymer, which is heated at high temperature to supply a trace of carbon increasing electrical conductivity, and the content of the carbohydrate or the polymer is between 1 and 25 percent by weight of the total powders.

7. The method as claimed in claim 1, wherein the duration of the heating process in the step (D) is from 1 to 15 hours.

8. The method as claimed in claim 1, wherein the lithium salt is lithium hydrate, lithium fluoride, lithium nitrate, lithium chloride, lithium bromide, lithium acetate, lithium oxide, lithium phosphate, lithium hydrophosphate, lithium dihydrophosphate, lithium ammonium phosphate, lithium diammonium phosphate, or a mixture thereof.

9. The method as claimed in claim 1, wherein the phosphate salt is diammonium hydrophosphate, ammonium dihydrophosphate, triammonium phosphate, phosphorus pentoxide, phosphoric acid, lithium hydrophosphate, lithium dihydrophosphate, lithium ammonium phosphate, lithium diammonium phosphate, or a mixture thereof.

10. The method as claimed in claim 1, wherein the vanadium salt is VO2, V2O3, V2O5, NH4VO3, or a mixture thereof.

11. The method as claimed in claim 1, wherein the heating process means heating the solid powders at a temperature between 400° C. and 1000° C.

12. A method for preparing LiFePO4/Li3V2(PO4)3 composite cathode materials comprising the following steps: (A) providing olivine phase LiFePO4 and monoclinic phase Li3V2(PO4)3 cathode materials;(B) mixing the olivine phase LiFePO4 and monoclinic phase Li3V2(PO4)3 cathode materials dispersed into an aqueous solution to form a mixed solution or slurry, wherein the molar ratio thereof is between 1:0.06 and 1:2;(C) drying the mixed solution or slurry to obtain solid powders; and(D) heating the solid powders at a temperature between 400 and 1000° C.

13. The method as claimed in claim 12, wherein the formation of the olivine phase LiFePO4 cathode material powders comprises: mixing iron powder, lithium salt, and phosphate salt dissolved into a mixed acid solution to form a mixed precursor solution of LiFePO4; subsequently stirring the mixed precursor solution, and drying the mixed precursor solution to obtain precursor powders of LiFePO4; and forming the precursor powders of LiFePO4 through a heating process.

14. The method as claimed in claim 12, wherein the formation of the monoclinic phase Li3V2(PO4)3 cathode material powders comprises: mixing vanadium salt, lithium salt, and phosphate salt dissolved into a mixed acid solution to form a mixed precursor solution of Li3V2(PO4)3; subsequently stirring the mixed precursor solution, and drying the mixed precursor solution to obtain precursor powders of Li3V2(PO4)3; and forming the precursor powders of Li3V2(PO4)3 through a heating process.

15. The method as claimed in claim 13, wherein the molar ratio of the iron powder, the lithium salt, and the phosphate salt dissolved into the mixed acid solution is 0.9˜1.2: 0.9˜1.2: 0.9˜1.2.

16. The method as claimed in claim 13 or 14, wherein the mixed acid solution is organic acid, or inorganic acid, or a mixture thereof.

17. The method as claimed in claim 16, wherein the organic acid is acetic acid, citric acid, oxalic acid, tartaric acid, propionic acid, butyric acid, or a mixture thereof; the inorganic acids are hydrochloric acid, sulfuric acid, nitric acid, perchloric acid, hypochlorous acid, hydrofluoric acid, or a mixture thereof.

18. The method as claimed in claim 13 or 14, wherein the mixed precursor solution has further added thereto a carbohydrate or a polymer, and the content of the carbohydrate or the polymer is between 1 and 25 percent by weight of the total powders.

19. The method as claimed in claim 13 or 14, wherein the lithium salt is lithium hydrate, lithium fluoride, lithium nitrate, lithium chloride, lithium bromide, lithium acetate, lithium oxide, lithium phosphate, lithium hydrophosphate, lithium dihydrophosphate, lithium ammonium phosphate, lithium diammonium phosphate, or a mixture thereof.

20. The method as claimed in claim 13 or 14, wherein the phosphate salt is diammonium hydrophosphate, ammonium dihydrophosphate, triammonium phosphate, phosphorus pentoxide, phosphoric acid, lithium hydrophosphate, lithium dihydrophosphate, lithium ammonium phosphate, lithium diammonium phosphate, or a mixture thereof.

21. The method as claimed in claim 14, wherein the molar ratio of the lithium salt, the vanadium salt, and the phosphate salt dissolved into the mixed acid solution is 2.9˜3.2: 1.9˜2.2: 2.9˜3.2.

22. The method as claimed in claim 14, wherein the vanadium salt is VO2, V2O3, V2O5, NH4VO3, or a mixture thereof.

23. The method as claimed in claim 13 or 14, wherein the heating process means heating the solid powders at a temperature between 400° C. and 1000° C.

24. A composite cathode material comprising a compound of the following formula (I): LixFe1-yVy(PO4)z formula (I);wherein x is between 0.9 and 1.5, y is between 0 and 1, and z is between 0.9 and 1.5; and the cathode material at least has two evenly distributed crystalline phases.

25. The composite cathode material as claimed in claim 24, wherein the two crystalline phases of the cathode material are respectively olivine phase LiFePO4 and monoclinic phase Li3V2(PO4)3.

26. The composite cathode material as claimed in claim 24, wherein the electrical conductivity of the olivine composite cathode material is more than 10−2 Scm−1.

27. The composite cathode material as claimed in claim 24, wherein the composite cathode material in the voltage-specific capacity curve has plural plateaus.

28. A battery comprising: an anode;a cathode; anda non-aqueous electrolyte which is formed between the anode and the cathode, wherein the cathode comprises a composite cathode material of the following formula (I): LixFe1-yVy(PO4)z,   formula (I);wherein x is between 0.9 and 1.5, y is between 0 and 1, and z is between 0.9 and 1.5; and the cathode material at least has two evenly distributed crystalline phases.

29. The battery as claimed in claim 28, wherein the two crystalline phases of the cathode material are respectively, olivine LiFePO4 and monoclinic Li3V2(PO4)3.

30. The battery as claimed in claim 28, wherein the olivine composite cathode material in the voltage-specific capacity curve has plural plateaus.