TWI533494B - Preparation of Lithium Iron Phosphate / Carbon Cathode Composites - Google Patents

Description

Method for preparing lithium iron phosphate/carbon cathode composite material

The invention relates to a method for preparing a cathode composite material, in particular to a method for preparing a lithium iron phosphate/carbon (LiFePO ₄ /C) cathode composite material.

In view of the earth's oil and natural gas reserves, it is estimated that in the next 30 to 50 years, and the problems caused by the increasingly serious greenhouse effect, Europe and the United States will not be able to develop "new energy" or "alternative energy." And because of the advancement of today's technology, handheld electronic products are gradually increasing and different styles, such as: notebook computers, mobile phones, PDAs, video cameras, digital cameras, Bluetooth headsets, etc. Small things and high functions, so the battery has high energy, long loop life, low cost, environment, etc., so most of the technology products on the market are replaced by secondary batteries, especially lithium-ion batteries ( Li-ion battery is the mainstay, and with the rapid development of 3C and electric vehicle industry, the market demand for high-capacity lithium-ion battery cathode materials has grown significantly.

The research team led by Professor Goodenough of the University of Texas in the United States discovered the LiFePO ₄ cathode material in 1997. It belongs to the Olivine structure and its theoretical theoretical capacitance is about 170 mAh g ^-1 when lithium ions are released. ^{When +} ) leaves the lithium ion in the olivine structure, LiFePO ₄ will become FePO ₄ , and the stable structure can be maintained during charge/discharge (because the bond of PO bond in PO ₄ ^3- is strong and not It will release good oxygen heat stability, unlike materials such as LiCoO ₂ and LiNiO _{2 which} will collapse due to the layer structure. However, the LiFePO ₄ material has a problem of poor electronic conductivity (about 10 ^-9 ~10 ^-10 S cm ^-1 ) and Fe ^{2+ is} easily oxidized to Fe ³⁺ , and the lithium ion diffusion coefficient is low, which causes There is a bottleneck in the application of the electro-capacitor capacity, but the process of preparing the LiFePO ₄ material needs to be protected in an inert atmosphere to prevent the oxidation of Fe ²⁺ to Fe ³⁺ . In addition, the carbon source may be added to the calcination or doping other Metal ions or other metal substances improve the conductivity of the material, which can solve the problem of low capacity.

However, the basic structure of a lithium ion secondary battery can be mainly divided into four parts: (1), anode material (Anode material), (2), electrolyte (Electrolyte), (3), separator (Separator), and 4), cathode material (Cathode material), the cathode material lattice structure should be stable and there are lattice vacancies, so that lithium ions can be freely embedded and embedded. If LiFePO _{4 is taken} as an example, the cathode electrode is charged and discharged. The semi-reaction is as follows:

Charging: LiFePO ₄ →Li _1-x FePO ₄ +xLi ⁺ +xe ^- (1)

Discharge: Li _1-x FePO ₄ +xLi ⁺ +xe ^- →LiFePO ₄ (2)

Because LiFePO ₄ cathode materials are high-safety, low-cost, and non-toxic in lithium-ion secondary batteries, they are in line with the concept of green earth. Therefore, many lithium battery research teams are working toward this material. However, currently used LiFePO ₄ There are many synthetic methods, such as a Sol-gel method, a Co-precipitation method, a solid-state method, a spray pyrolysis method, and a hydrothermal method. Hydrothermal method, etc.

The advantages of the sol-gel method by Croce et al. are that the particle size can be miniaturized, the gel treatment temperature is low, and the reaction is easy to control. Usually, metal salts are used as a starting material, and hydrolysis, condensation, and The steps of aging and polymerization form a gel, which is then sintered to remove impurities and by-products into nano powder. Generally, Fe(NO ₃ ) _{3 is} used as a starting material, and the pH is controlled by ammonia water, and the prepared gel is heat-treated at 60-80 ° C, and finally, under an inert atmosphere (N ₂ or Ar) environment. After sintering for 12 hours, the pure phase olivine structure of LiFePO ₄ can be obtained at a rate of 0.2 C discharge at a small current density, and its discharge capacity can reach 120 mAh g ^-1 .

The coprecipitation method is to dissolve a suitable raw material, add another ionic solution to precipitate a precipitate together, and after drying, the powder is dried and sintered to obtain a product. Park et al. used the co-precipitation method to achieve uniform mixing of the precursors, and added LiOH to the mixed solution of (NH ₄ ) ₂ Fe(SO ₄ ) ₂ ‧6H ₂ O and H ₃ PO ₄ to obtain a coprecipitate, which was filtered and washed. Under the environmental protection of inert atmosphere, LiFePO ₄ with olivine structure is obtained after sintering, and the electric capacity can reach 151 mAh g ^-1 at a discharge rate of 0.1C.

LiFePO _{4 was} prepared by other researchers such as Kang and the simple solid state synthesis method. These scholars have found that excessive calcination temperature and excessive calcination time will cause LiFePO ₄ particles to become larger, and at 600 ° C and calcination 3 The hourly product was tested for discharge at a rate of 0.1 C, and its discharge capacity was 160 mAh g ^-1 . It can be seen that the calcination temperature controls the particle size and grain shape of LiFePO ₄ , and it is found by electrochemical analysis that the larger the crystal, the smaller the capacitance. It is indicated that the preparation temperature affects the crystal particle size of the material, which in turn affects the level of LiFePO ₄ gram capacity.

The spray pulverization method is a method in which a solution of a metal salt is sprayed into a high-temperature environment in a mist form, causing instantaneous evaporation of a solvent or thermal decomposition of a metal salt, so that a supersaturation phenomenon is easily formed and a desired powder material is precipitated. Yang et al. used spray lysis and prepared LiFePO ₄ at different ambient temperatures. They found that the product at 450 ° C was discharged at a rate of 0.1 C, and the product had a gram capacity of 150 mAh g ^-1 .

The hydrothermal synthesis method uses a high-temperature and high-pressure condition in a closed high-pressure vessel to make the metal salt inside the vessel highly active, and further to carry out a crystallization reaction in an aqueous solution. The purity of LiFePO ₄ synthesized by Yang et al. using hydrothermal method is almost 100%, and the product has uniform phase and small particle size, which significantly reduces the particle size of LiFePO ₄ material, which can increase the contact area. Therefore, the lithium ion diffusion distance can be shortened. And researchers such as Dokko, LiOH, FeSO ₄ and (NH ₄ ) ₂ HPO _{4 were} mixed into an aqueous solution, and the molar ratio of Li:Fe:P was 2:1:1, and the hydrothermal synthesis method was used at 170 °C. The product of LiFePO ₄ was prepared under the operation for 12 hours, and the particle size was about 0.5 μm, and the discharge test was performed at a rate of 0.1 C, and the optimum discharge capacity was 150 mAh g ^-1 .

However, the electronic conductivity of the LiFePO ₄ material is too low, mainly because the FeO ₆ octahedron in LiFePO ₄ is separated by the PO ₄ tetrahedron between its layers, compared to other structural layers such as LiMO _{2 in a} layered structure ( M=Co, Ni), spinel-structured LiMn ₂ O ₄ , the continuous structure of the two eutectic MO ₆ octahedrons is different, so the electronic conductivity of LiFePO ₄ is relatively low, and on the other hand In the structure of LiFePO ₄ , the oxygen atoms of the LiO ₆ octahedron are arranged close to the hexagonal closest packing, so that lithium ions can be provided with a limited channel, which hinders the incorporation of lithium ions into and out of it. In order to improve the conductivity of LiFePO ₄ , there are two methods commonly used: (1) adding metal powder; (2) adding carbon source, carbon mixed calcination to form carbon coating, lifting material Electronic conductivity.

One object of the present invention is to provide a method for preparing a lithium iron phosphate/carbon (LiFePO ₄ /C) cathode composite by adding a polystyrene (PS) carbon source to prepare a LiFePO ₄ /C cathode composite, and adding polyphenylene. After the ethylene carbon source, the problem of poor electronic conductivity of the composite material can be effectively improved, and the electrochemical performance can be improved.

Another object of the present invention is to provide a method for preparing a lithium iron phosphate/carbon (LiFePO ₄ /C) cathode composite material, wherein the prepared LiFePO ₄ /C cathode composite material can be used as a cathode of a coin type (Coin cell) battery. electrode.

For the above purpose, a method for preparing a lithium iron phosphate/carbon (LiFePO ₄ /C) cathode composite material of the present invention comprises the following steps: a method for preparing a lithium iron phosphate/carbon cathode composite material, comprising the following steps: (a) dissolving a lithium (Li) source, an iron (Fe) source, a phosphorus (P) acid source, and a carbon (C) source in water and an organic solvent to form a mixed aqueous solution; (b) placing in an oil bath or an oven And uniformly heating the mixed aqueous solution; (c) drying the mixed aqueous solution to obtain a carbon-containing LiFePO ₄ /C material solid powder; and (d) sintering and heating the solid powder.

The detailed description of the drawings and the preferred embodiments are set forth in the accompanying drawings.

Please refer to FIG. 1 , which is a schematic flow chart of a method for preparing a lithium iron phosphate/carbon (LiFePO ₄ /C) cathode composite according to a preferred embodiment of the present invention.

As shown in FIG. 1 , a method for preparing a lithium iron phosphate/carbon (LiFePO ₄ /C) cathode composite according to a preferred embodiment of the present invention comprises the following steps: a method for preparing a lithium iron phosphate/carbon cathode composite material, The method comprises the following steps: (a) dissolving a lithium (Li) source, an iron (Fe) source, a phosphorus (P) acid source and a carbon (C) source in water and an organic solvent to form a mixed aqueous solution; (b) placing The mixed aqueous solution is uniformly heated in an oil bath or an oven; (c) drying the mixed aqueous solution to obtain a carbon-containing solid powder of LiFePO ₄ /C material; and (d) sintering and heating the solid powder.

In the step (a), a lithium (Li) source, an iron (Fe) source, a phosphorus (P) acid source and a carbon (C) source are dissolved in water and an organic solvent to form a mixed aqueous solution; wherein the Li: Fe: molar ratio of P, such as but not limited to 1:1:1, 2:1:1 or 3:1:1, and the lithium source is, for example but not limited to, lithium hydroxide, lithium nitrate, lithium acetate, Lithium chloride, lithium hydrogen phosphate, and lithium phosphate are exemplified by lithium hydroxide (LiOH) in the present embodiment, but are not limited thereto; and the iron source is, for example but not limited to, iron sulfate or oxalic acid. Iron, iron phosphate, iron acetate, iron nitrate, ferric chloride, in the present embodiment, iron sulfate (FeSO ₄ ) is taken as an example, but not limited thereto, and the FeSO _{4 is} easy to be in the air. Oxygen is used for the reaction. It is preferred to use this solution for a day; for example, but not limited to, ammonium phosphate, ammonium hydrogen phosphate, ammonium dihydrogen phosphate, lithium phosphate, lithium hydrogen phosphate, lithium ammonium phosphate, phosphoric acid, sodium phosphate, In the present embodiment, ammonium phosphate (NH ₄ H ₂ PO ₄ ) is taken as an example, but it is not limited thereto.

The organic solvent is, for example, but not limited to, acetone, methanol, ethanol, isopropanol, ethylene glycol or butanol. In the present embodiment, acetone is taken as an example, but not limited thereto. The amount is between 5% and 60% (v/v), and the optimum amount is between 20 and 40%.

The carbon source is, for example but not limited to, polystyrene (PS), which is formed by high temperature calcination to increase the electronic conductivity. The molecular weight (MW) of the polystyrene is, for example but not limited to, between 500,000 and 10,000. The carbon source content after sintering is LiFePO ₄ /C, and the weight percentage of the powder is, for example but not limited to, between 0.10% and 20 wt.%, and the optimum carbon source content is between 3 and 10 wt.%. If the polystyrene polymer is to be uniformly dispersed in the organic solvent acetone, the amount of acetone needs to exceed 10 to 15 wt.%.

In the step (b), the oil is placed in an oil bath or an oven and the mixed aqueous solution is uniformly heated; wherein the mixed aqueous solution is hydrothermally synthesized without stirring, and the temperature thereof is, for example but not limited to, 150 to 200 ° C. Between the best, between 150 ~ 180 ° C.

In the step (c), the mixed aqueous solution is dried to obtain a carbon-containing solid powder of LiFePO ₄ /C material; wherein, in an oven, heating is continued at 170 ° C for 19 hours to heat dry the mixed aqueous solution to facilitate LiFePO ₄ nucleates to obtain a carbon-containing solid powder of LiFePO ₄ /C material.

In the step (d), the solid powder is sintered by heating; wherein the solid powder is dried, placed in a quartz boat (not shown), and the quartz boat is placed in a high temperature forging furnace (not shown) In the medium calcination, the high-temperature calciner is calcined by a mixed gas of a reducing gas, wherein the type and volume ratio (v/v ratio) of the reducing gas are respectively H ₂ : Ar=5%: 95%, H ₂ : Ar = 3%: 97%, H ₂ : Ar = 2%: 98% and H ₂ : Ar = 1%: 99%, wherein the argon (Ar) inert gas may also be substituted with nitrogen (N ₂ ), and the gas The flow rate is, for example but not limited to, 250 mL min ^-1 , and the calcination reaction temperature rise condition is 10 ° C min ^-1 . When it rises to 100 ° C, it stays for one hour, and then continues to rise to the set temperature of 600-900 ° C. Preferably, the temperature is 850 ° C, and the temperature is between about 8 and 24 hours, preferably 16 to 20 hours. After the end, the temperature is lowered to room temperature, that is, the preparation work is completed.

In the present invention, in addition to the addition of a polystyrene carbon source, the problem of poor electronic conductivity of the composite material can be effectively improved, and the electrochemical performance can be improved, and the organic solvent acetone is also added. In addition to being used as a solvent for polystyrene, acetone is also a structure-orient agent. After adding an organic solvent to the aqueous solution of the precursor mixture, the solubility of the salt is lowered to provide more positions for the growth of the LiFePO ₄ nucleus. At the same time, it is also possible to suppress the enlargement of crystal grains and the formation of a more favorable pure phase.

Figure 2 is a schematic view showing the flow of a cathode electrode made of the lithium iron phosphate/carbon (LiFePO ₄ /C) cathode composite material of the present invention. As shown in FIG. 2, the cathode composite prepared by the method for preparing a lithium iron phosphate/carbon (LiFePO ₄ /C) cathode composite according to the present invention can be applied to a cathode electrode of a button type battery, and the manufacturing method thereof is as follows: The cathode electrode is prepared by the following composition: LiFePO ₄ /C composite prepared by the above hydrothermal method, Polyvinylidene difluoride (PVDF) / methylpyrrolidone (NMP) (14) Wt.%) Adhesive, methylpyrrolidone (NMP) solvent and Super P carbon black conductive agent produced by Timcal Corporation. Initially, 9 g of LiFePO ₄ /C composite powder and 13.89 g of PVDF/NMP were weighed according to the ratio of LiFePO ₄ /C:PVDF:Super P carbon black conductive agent=90 wt.%: 5 wt.%: 5 wt.%. (about 14 wt.%), 0.5 g of Super P, slowly add Super P to 13.89 g of NMP and stir with a blender for 1 hour (step 1); after stirring evenly, add PVDF/NMP adhesive to the slurry. Stirring was continued for 30 minutes (step 2); then the LiFePO ₄ /C composite powder was slowly added to the slurry and stirring was continued for 3.7 hours (step 3); after being completely stirred uniformly, the prepared slurry was coated with a doctor blade. On the aluminum foil (Al foil), the prepared cathode electrode is placed in an oven and dried at 110 ° C for 1.5 hours to remove residual organic solvent (step 4); the dried cathode electrode is rolled The machine was milled twice to level the treatment, each time reducing the thickness by about 10% (step 5); finally, the circular cathode electrode was cut using a 13 mm cutter (step 6). The solid-liquid ratio of the cathode sheet is controlled to be 1:3, and the average active material of the cathode sheet is about 8 to 12 mg.

FIG. 3 is a schematic flow chart showing the assembly of the lithium iron phosphate/carbon (LiFePO ₄ /C) cathode electrode of the present invention in a button type battery. As shown in FIG. 3, the cathode electrode prepared according to the method shown in FIG. 3 can be applied to a button type battery, and the manufacturing method is as follows: first, each component of the button type battery (2032 coin cell) is firstly 95 wt. The % C ₂ H ₅ OH was cleaned and placed in a vacuum oven. After the C ₂ H ₅ OH was completely evaporated, it was placed in an argon (Ar) glove box. In the Ar glove box, a cut lithium metal (Li foil) round ingot 6 (diameter 13 mm) was used as an anode, placed in the center of the lower cover 7 of the button type battery, and then immersed in the electrolyte. A separator 5 is placed above the lithium metal ingot 6, and some electrolyte is dropped on the separator 5 by a dropper, and then the cut and weighed LiFePO ₄ /C cathode electrode sheet 4 is placed. On the upper side of the separator 5 (the electrode surface faces the separator 5), the gasket 3 and the spring 2 are placed on the cathode electrode 4 in sequence, and finally, the upper cover 1 is covered, and the button is sealed using a battery packaging machine. The battery can be used to complete the packaging of the button battery.

Figure 4a is a SEM analysis of the surface analysis structure of a lithium iron phosphate cathode composite without a carbon source under an electron microscope; Figure 4b is an SEM analysis of the surface analysis structure of a lithium iron phosphate cathode composite with a carbon source under an electron microscope. Figure. As shown in Figures 4(a) and 4(b), the SEM surface analysis structure of the lithium iron phosphate/carbon cathode material prepared by hydrothermal synthesis method, the olivine structure shape can be seen in the figure, A little carbon layer, and the grain size of LiFePO ₄ becomes smaller as the carbon content of the carbon source increases. We can see that no carbon is added to LiFePO ₄ (as shown in Figure 4a), and the particle size is significantly larger. This also indicates that the carbon source can effectively inhibit the growth of crystal grains during the calcination process, and the LiFePO ₄ /C particles are smaller in size.

Therefore, the method for preparing a lithium iron phosphate/carbon (LiFePO ₄ /C) cathode composite of the present invention comprises adding a polystyrene (PS) carbon source to prepare a LiFePO ₄ /C cathode composite, and adding a polystyrene carbon source After that, the problem of poor electronic conductivity of the composite material can be effectively improved, and the electrochemical performance can be improved; and the organic solvent acetone is added. In addition to being used as a solvent for polystyrene, acetone is also a structural adjustment aid. After adding an organic solvent to the aqueous solution of the precursor mixture, the solubility of the salt is lowered, and more LiFePO ₄ nucleation sites are provided, and the crystal grains can be suppressed. The advantages of larger, more favorable generation of pure phase. Therefore, the preparation method of the lithium iron phosphate/carbon (LiFePO ₄ /C) cathode composite of the present invention is indeed more advanced than the prior art.

Figure 5a is an XRD analysis of a lithium iron phosphate cathode composite with no carbon source treated at different calcination temperatures; Figure 5b is a lithium iron phosphate cathode composite with carbon source added under different heating methods. Analysis chart of XRD. The lithium iron phosphate/carbon cathode material prepared by the hydrothermal synthesis method is ground into a powder by a stainless steel mortar, filled into a stainless steel stage and flattened, and then placed in an X-ray diffraction analyzer (hardware equipment). :X'Pert Pro system, Philip, USA). The experimental conditions were as follows: X'Pert voltage was 45 KV, current was 40 mA, scanning range was 2θ = 10° to 50°, and scanning rates were 0.05°/step and 4 seconds/step. The obtained diffraction spectrum is compared and analyzed according to the material database in the storage computer, and the material lattice parameter identification is compared. As shown in Figure 5a, the X-ray analysis of the LiFePO ₄ sample (no carbon source, 0 wt.% PS) treated at different calcination temperatures (no sinter, 750, 800, 850 ° C), and Figure 5b, XRD analysis of LiFePO ₄ /C samples (with carbon source, 5 wt.% PS) treated in different heating methods (for example, comparing oil bath and oven water heat and heat treatment at 850 ° C). The LFP material was added with 5 wt.% of PS polymer, and the XRD diffraction pattern and crystal constant value of LiFePO ₄ /C treated at different calcination temperatures were compared. The detailed results are shown in Table 1. The materials a, b, c, and V parameters prepared by the hydrothermal method are close to the theoretical values of the literature. After comparison with the XRD database in the computer, a small amount of impurity phase can be found in the XRD pattern (this is Fe ₂ P = ☆, ☆ is the impurity (phase) generated when the LiFePO4 cathode material is prepared at a high temperature, However, due to the higher electron conductivity of Fe ₂ P, there is some improvement in the conductivity of LiFePO ₄ , but too much is not good. However, if it is a carbon source additive, if it contains a large amount of carbon. Or hydrogen, or the calcination temperature is too high, it is easy to produce Fe ₂ P impurities. Although Fe ₂ P is a highly conductive property, it is possible to repel each other with LiFePO ₄ so that the gram capacity does not rise and fall. In addition, according to the literature, the formation of Fe ₂ P impurities is conceivable as a carbon source with an excessively high content or an excessively high calcination temperature, so that an appropriate carbon source amount and an appropriate calcination temperature are added for quality.

Figure 6a is a full-range micro-Raman spectrum of the carbon-free source LiFePO ₄ treated at different calcination temperatures according to the hydrothermal method of the present invention; Figure 6b is prepared according to the hydrothermal method of the present invention. The full range of Raman spectra of LiFePO ₄ /C. The prepared lithium iron phosphate/carbon cathode material sample is placed on the microscope test piece at about 5 mg of the sample to be tested, and flattened with a spatula, and the microscope test piece is placed on a microscopic Raman spectrometer (Confocal micro-Renishaw) On the microscope test piece, and using Raman spectroscopy to analyze the surface of lithium iron phosphate/carbon cathode material with different addition ratios and different calcination temperatures, Raman spectroscopy analysis as shown in Fig. 6a and Fig. 6b can be obtained. Fig. 6a is a full-range micro-Raman spectrum of the carbon-free source LiFePO ₄ treated by hydrothermal method at different calcination temperatures (no sinter, 750, 800, 850 ° C), phosphate (PO The main positions of ₄ ^3- ) are 940 cm ^-1 , 990 cm ^-1 , 1060 cm ^-1 ; and Figure 6b is LiFePO ₄ /C prepared by hydrothermal method (adding 5wt.% PS carbon source at 850 °C) Full-scale Raman spectroscopy of heat treatment), while the Raman peak of carbon is mainly D-band (I _D ) at 1320 cm ^-1 and G-band (I _G ) at around 1580 cm ^-1 . It has also been found from the Raman diagram that when the temperature is uniformly heated, the degree of graphitization of the polymer material (for example, polystyrene) as a carbon source is better; and (I _D /I) _The ratio of _G )(=R ₁ ) (R ₁ =1.07~1.09 in water bath and oven water heat) is lower, which indicates that there are more graphite structures in the carbon source, which is beneficial to the electronic conductivity of LFP, generally Super P conductive material. The R ₁ value is between 1.2 and 1.4. It can be seen that the carbon coating formed by the PS carbon source has excellent quality, that is, the crystallinity of carbon is good. In addition, the higher the ratio of (I _D +I _G )/PO ₄ ^3- (=R ₂ ) (R ₂ =4.57 when the oven is hot), which indicates that the amount of carbon source coated on LiFePO ₄ is increased and the uniformity is good. . The experimental analysis results are shown in Table 2; however, from the figure we can find that some Fe ₂ O ₃ and FeO _{x are} generated, and the Fe ₂ O ₃ position is between 145 and 275 cm ^-1 , which represents A small amount of Fe ²⁺ is oxidized to Fe ³⁺ , which will reduce its gram capacity; FeO _x is located at 440 cm ^-1 and 625 cm ^-1 , and a little FeO _{x is} formed, which is beneficial to the increase of gram capacity. In the present embodiment, comparative analysis was carried out by using different heating medium methods such as an oil bath and an oven during preparation.

Figure 7 is a sample of lithium iron phosphate/carbon cathode material prepared by hydrothermal method, sealed by a battery packaging machine, and then subjected to cyclic voltammetry in a potential range (2.5-4.2 V) to obtain an oxidation/reduction peak. CV analysis chart. As shown in FIG. 7 , after the lithium iron phosphate/carbon cathode material sample of the present invention is sealed by a battery packaging machine, the oxidation/reduction peak is mainly tested by applying a cyclic potential, from the initial potential. A fixed rate is applied to the endpoint potential and then changed back to the onset potential at the same rate. This is a cycle that analyzes the reversibility (CV plot) of the reversible oxidation/reduction electrochemical reaction of LiFePO ₄ when scanning from a low potential to a high potential. At this time, the analyte will generate an oxidation peak of the oxidation current. Conversely, when scanning from a high potential to a low potential, the analyte will produce a reduction peak of the reduction current. This CV diagram can help determine at which potential. The redox reaction, the CV parameter value calculated by the LiFePO ₄ /C electrode prepared by the CV diagram analysis, is shown in Table 3. As shown in Table 3, R' ₁ represents I _p,a (A/g)/I _p,c (A/g), and R' ₂ represents I' _p,a (A/cm ² )/I' _{p, c} (A/cm ² ), if the values of R' ₁ and R' ₂ are closer to 1, the reversibility of the oxidation/reduction fraction of LFP is better, and as a result, R' ₁ and R of LFP prepared by hydrothermal method are found. ' ₂ values are between 0.87 and 0.90.

Figure 8 is a graph of the lithium iron phosphate/carbon cathode material sample prepared when the oil bath is a heating medium, and the gram capacity of the sealed button type battery through the battery packaging machine is plotted against different discharge rates; Figure 9 is prepared by using the oven as a heating medium. A lithium iron phosphate/carbon cathode material sample, a graph of the gram capacity of a button-type battery through a battery packer versus different discharge rates. As shown in the figure, when charging/discharging at 0.2C, the LiFePO ₄ /C synthesized by hydrothermal method can reach 152 mAh g ^-1 , while charging at 0.5C, 1C, 3C, 5C and 7C. When discharged, the gram capacity of 140 mAh g ^-1 , 131 mAh g ^-1 , 121 mAh g ^-1 , 113 mAh g ^-1 , and 92 mAh g ^-1 was generated. As a result of summarizing the experimental results, as shown in Table 4 (Sample #1) and V (Sample #2), it was found that the LiFePO ₄ /C powder prepared by the present invention has a high gram capacity, and a polystyrene polymer carbon can be obtained. It is derived from the hydrothermal method to effectively increase the LiFePO ₄ /C gram capacity and improve the high power discharge capacity.

Fig. 10 is a graph showing experimental results of a life cycle of a lithium iron phosphate/carbon cathode material sample prepared by a hydrothermal method and sealed by a battery packer to a button type battery at different rates. As shown in Fig. 10, the lithium iron phosphate/carbon cathode material sample prepared by the hydrothermal method of the present invention is subjected to a cycle test of a sealed life of a button-type battery through a battery packaging machine, and the experimental result is found to After charging/discharging the charge/discharge rate of 0.1C, 0.2C and 1C for 20 cycles, it was found that the battery still has very good and stable gram capacity, but there are some changes (ie, between 130~160 mAh g ^-1 ) ).

The disclosure of the present invention is a preferred embodiment. Any change or modification of the present invention originating from the technical idea of the present invention and being easily inferred by those skilled in the art will not deviate from the scope of patent rights of the present invention.

In summary, this case, regardless of its purpose, means and efficacy, is showing its technical characteristics that are different from the conventional ones, and its first invention is practical and practical, and it is also in compliance with the patent requirements of the invention. I will be granted a patent at an early date.

1. . . Upper cover

2. . . spring

3. . . Gasket

4. . . LiFePO ₄ /C electrode sheet

5. . . Isolation film

6. . . Lithium metal ingot

7. . . lower lid

Step (a): dissolving a lithium (Li) source, an iron (Fe) source, a phosphorus (P) acid source and a carbon (C) source in water and an organic solvent to form a mixed aqueous solution;

Step (b): placing in an oil bath or an oven and uniformly heating the mixed aqueous solution;

Step (c): drying the mixed aqueous solution to obtain a carbon-containing LiFePO ₄ /C material solid powder;

Step (d): Sintering heats the solid powder.

1 is a schematic flow chart of a method for preparing a lithium iron phosphate/carbon (LiFePO ₄ /C) cathode composite of the embodiment.

2 is a schematic flow chart of a cathode electrode prepared by a lithium iron phosphate/carbon (LiFePO ₄ /C) cathode composite.

3 is a flow chart showing the assembly of a lithium iron phosphate/carbon (LiFePO ₄ /C) cathode electrode in a button type battery.

Figure 4a is a SEM analysis of the surface analysis structure of a lithium iron phosphate cathode composite without a carbon source under an electron microscope.

Figure 4b is a SEM analysis of the surface analysis structure of a lithium iron phosphate cathode composite with a carbon source under an electron microscope.

Figure 5a is an XRD analysis of a lithium iron phosphate cathode composite with no added carbon source treated at different calcination temperatures.

Fig. 5b is an analysis diagram of XRD of a lithium iron phosphate cathode composite material in which carbon source is added under different heating methods.

Figure 6a is a full range microscopic Raman spectrum of the carbon-free source LiFePO ₄ treated at different calcination temperatures for the samples prepared according to the hydrothermal method of the present invention.

Figure 6b is a full range microscopic Raman spectrum of LiFePO ₄ /C prepared according to the hydrothermal method of the present invention.

7 is a sample of a lithium iron phosphate/carbon cathode material prepared by a hydrothermal method, sealed by a battery packer, and then subjected to cyclic voltammetry in a potential range (2.5-4.2 V) to obtain an oxidation/reduction peak. CV analysis chart.

Figure 8 is a graph showing the gram capacity of a lithium carbonate/carbon cathode material sample prepared by an oil bath as a heating medium, and the sealing capacity of the button type battery through a battery packer to different discharge rates.

Fig. 9 is a graph showing the gram capacity of a sealed button type battery through a battery packer for different discharge rates, using a lithium iron phosphate/carbon cathode material sample prepared by using an oven as a heating medium.

Fig. 10 is a graph showing experimental results of a life cycle of a lithium iron phosphate/carbon cathode material sample prepared by a hydrothermal method and sealed by a battery packer to a button type battery at different rates.

Claims (1)

A method for preparing a lithium iron phosphate/carbon cathode composite material, comprising the steps of: (a) dissolving a lithium (Li) source, an iron (Fe) source, a phosphorus (P) acid source and a carbon (C) source in water; Forming a mixed aqueous solution with an organic solvent, wherein the lithium source is lithium hydroxide, lithium nitrate, lithium acetate, lithium chloride, lithium hydrogen phosphate, lithium phosphate; the iron source is iron sulfate, ferrous oxalate, iron phosphate, acetic acid Iron, ferric nitrate, ferric chloride; the phosphoric acid source is ammonium phosphate, ammonium hydrogen phosphate, ammonium dihydrogen phosphate, lithium phosphate, lithium hydrogen phosphate, lithium ammonium phosphate, phosphoric acid, sodium phosphate, the organic solvent is acetone, the carbon source Is polystyrene (PS), wherein the molar ratio of Li:Fe:P is 1:1:1, 2:1:1 or 3:1:1, and the amount of acetone added is 5%~60% Between (v/v), the polystyrene (PS) generates conductive carbon by high-temperature calcination to increase the electronic conductivity. The molecular weight (MW) of the polystyrene is between 500,000 and 10,000, after sintering. carbon content of LiFePO ₄ / C, the percentage weight of the powder between 0.10% ~ 20wt%;. ( b) placed in an oil bath or an oven heated and uniformly mixed aqueous solution, the aqueous-based mixture without Mixed hydrothermally synthesized at a temperature between 150 ~ 200 ℃; (c) mixing the aqueous solution is directly heated and dried, to give a LiFePO ₄ / C material of the solid polymer containing a carbon source powder; and (d) sintering The solid powder is heated, and the solid powder is subjected to a sintering heat treatment in a reducing gas, and the type and volume ratio (v/v ratio) of the reducing gas are respectively H ₂ : Ar = 5%: 95%, H ₂ : Ar = 3%: 97%, H ₂ : Ar = 2%: 98% and H ₂ : Ar = 1%: 99%, wherein the argon (Ar) inert gas may also be substituted with nitrogen (N ₂ ), sintered and heated. The time is between 8 and 24 hours and the temperature is between 600 and 900 °C.