Positive electrode active material and lithium-ion secondary battery

Abstract

This invention provides a positive electrode active material comprising particles of lithium nickel manganese oxide and a fluoride film covering the surface of the lithium nickel manganese oxide particles. The fluoride film is composed of a fluoride selected from the group consisting of lanthanum fluoride, yttrium fluoride, and scandium fluoride. This invention also provides a lithium-ion secondary battery comprising a positive electrode, a negative electrode spaced apart from the positive electrode, a separator disposed between the positive and negative electrodes, and a lithium salt electrolyte distributed between the positive electrode, the negative electrode, and the separator. The positive electrode includes a current collector and a positive electrode active layer located on the surface of the current collector, the positive electrode active layer having the aforementioned positive electrode active material. The lithium salt electrolyte includes a lithium salt and an organic solvent, the lithium salt being lithium hexafluorophosphate. The positive electrode active material of this invention is not easily corroded, and the lithium-ion secondary battery of this invention exhibits excellent battery performance.

Description

Technical Field

[0001] This invention relates to a positive electrode material and a secondary battery, particularly to a positive electrode active material and a lithium-ion secondary battery. Prior Technology

[0002] A lithium-ion secondary battery consists of a positive electrode, a negative electrode, a separator, and a lithium salt electrolyte. The positive electrode active material is one of the key factors determining the battery performance of a lithium-ion secondary battery. Common positive electrode active materials include lithium nickel manganese oxide.

[0003] Lithium hexafluorophosphate is often used as a component of lithium salt electrolytes. However, when the charging voltage of a lithium-ion secondary battery is as high as 4.5V or above, lithium hexafluorophosphate will dissociate to produce hydrofluoric acid. Hydrofluoric acid comes into contact with lithium nickel manganese oxide, causing the lithium nickel manganese oxide to be corroded by hydrofluoric acid. This causes the nickel and manganese elements in the lithium nickel manganese oxide to be dissolved by hydrofluoric acid, resulting in the degradation of the battery performance of the lithium-ion secondary battery.

[0004] Therefore, preventing the corrosion of lithium nickel manganese oxide is a crucial technology for improving the performance of lithium-ion secondary batteries, and coating technology is the most effective and practical among these technologies. Summary of the Invention

[0005] Therefore, the first objective of this invention is to provide a positive electrode active material that is not easily corroded.

[0006] Therefore, the positive electrode active material of the present invention comprises particles of lithium nickel manganese oxide and a fluoride film. The fluoride film covers the surface of the lithium nickel manganese oxide particles, and the fluoride film is composed of fluorides selected from at least one of the group consisting of lanthanum fluoride, yttrium fluoride and scandium fluoride.

[0007] Therefore, the second objective of this invention is to provide a lithium-ion secondary battery with better battery performance.

[0008] Therefore, the lithium-ion secondary battery of the present invention comprises: a positive electrode, a negative electrode, a separator, and a lithium salt electrolyte.

[0009] The positive electrode includes: a current collector and a positive electrode active layer on the surface of the current collector, the positive electrode active layer having the positive electrode active material as described above. The negative electrode is spaced apart from the positive electrode. The separator is disposed between the positive electrode and the negative electrode. The lithium salt electrolyte is distributed between the positive electrode, the negative electrode and the separator, the lithium salt electrolyte including a lithium salt and an organic solvent, the lithium salt being lithium hexafluorophosphate.

[0010] The advantages of this invention are: the fluoride film in the positive electrode active material of this invention is uniformly covered on the surface of the lithium nickel manganese oxide particles; the positive electrode active material of this invention is not easily corroded by hydrofluoric acid; and the lithium-ion secondary battery of this invention has good battery performance because it contains the positive electrode active material. Simple Explanation of the Diagram

[0011] Other features and effects of the present invention will be clearly presented in the embodiments with reference to the drawings, wherein: Figure 1 is an X-ray diffraction pattern illustrating the X-ray diffraction analysis results of the lithium nickel manganese oxide particles in Example 1; Figure 2 is an X-ray diffraction pattern illustrating the X-ray diffraction analysis results of the positive electrode active materials in Examples 1 to 3; Figure 3 shows multiple SEM images illustrating the morphology of the lithium nickel manganese oxide particles in Example 1; Figure 4 shows multiple SEM images illustrating the particle morphology of the positive electrode active material in Examples 1 to 3; Figure 5 shows multiple TEM images illustrating the particles of lithium nickel manganese oxide in Example 1, and the microstructure analysis results of the positive electrode active materials in Examples 1 to 3; Figure 6 is a data graph illustrating the results of rate charge / discharge performance tests of lithium-ion secondary batteries in Application Examples 1 to 3 and Comparative Application Examples; Figure 7 is a data graph illustrating the results of long charge-discharge cycle tests of lithium-ion secondary batteries in comparative application examples; Figure 8 is a data graph illustrating the results of the long charge-discharge cycle test of the lithium-ion secondary battery in Application Example 2; Figure 9 is a data graph illustrating the results of rate charge / discharge performance tests of the lithium-ion secondary battery in Application Example 4 at different temperatures; Figure 10 is a data graph illustrating the results of the long charge-discharge cycle test of the lithium-ion secondary battery in Application Example 4 at 25°C; and Figure 11 is a data graph illustrating the results of a long charge-discharge cycle test at 50°C for the lithium-ion secondary battery of Application Example 4. Implementation

[0012] The following details the positive electrode active material of the present invention.

[0013] The positive electrode active material of the present invention comprises particles of lithium nickel manganese oxide and a fluoride film.

[0014] The stoichiometric composition of the lithium nickel manganese oxide is, for example, but not limited to, Li1.2Ni0.2Mn0.6O2.

[0015] The fluoride film covers the surface of the lithium nickel manganese oxide particles, and the fluoride film is composed of a fluoride selected from at least one of the group consisting of lanthanum fluoride, yttrium fluoride, and scandium fluoride. In some embodiments, the fluoride is lanthanum fluoride.

[0016] In some embodiments, based on 100 parts by weight of the total amount of lithium nickel manganese oxide particles, the fluoride content ranges from 1 part by weight to 5 parts by weight, resulting in a lithium-ion secondary battery containing this positive electrode active material exhibiting higher discharge capacity and better rate charge / discharge performance. In some embodiments, based on 100 parts by weight of the total amount of lithium nickel manganese oxide particles, the fluoride content ranges from 1 part by weight to 3 parts by weight, resulting in a lithium-ion secondary battery containing this positive electrode active material exhibiting higher discharge capacity and better rate charge / discharge performance. In some embodiments, based on 100 parts by weight of the total amount of lithium nickel manganese oxide particles, the fluoride content is 2 parts by weight, resulting in a lithium-ion secondary battery containing this positive electrode active material exhibiting higher discharge capacity and superior rate charge / discharge performance.

[0017] The method for preparing the positive electrode active material includes dispersing lithium nickel manganese oxide particles in an aqueous solution containing a fluorine source to form a dispersion; thoroughly mixing the dispersion with an aqueous solution containing a rare earth metal source, such that the fluorine source and the rare earth metal source can form fluorides in situ on the surface of the lithium nickel manganese oxide particles; the fluorides cover the surface of the lithium nickel manganese oxide particles, resulting in an uncalcined purified product containing the positive electrode active material; and drying and calcining the uncalcined purified product to remove impurities and purify the positive electrode active material. The fluorine source is, for example, but not limited to, ammonium fluoride. The rare earth metal source is selected from at least one of the group consisting of lanthanum, yttrium, and scandium sources. The lanthanum source is, for example, but not limited to, lanthanum acetate, lanthanum fluoride, or lanthanum oxide. The yttrium source is, for example, but not limited to, yttrium fluoride or yttrium oxide. The scandium source is, for example, but not limited to, scandium fluoride or scandium oxide. The molar ratio of the fluorine source and the rare earth metal source is, for example, but not limited to, 1:3. The drying temperature range is, for example, but not limited to, 70°C to 90°C; the calcination temperature range is, for example, but not limited to, 400°C to 500°C; and the calcination time range is, for example, but not limited to, 2 hours to 10 hours.

[0018] The lithium-ion secondary battery of the present invention will be described in detail below.

[0019] The present invention relates to a lithium-ion secondary battery, comprising: a positive electrode, a negative electrode, a separator, and a lithium salt electrolyte.

[0020] The positive electrode includes a current collector and a positive electrode active layer on the surface of the current collector. The current collector is, for example, but not limited to, aluminum foil. The positive electrode active layer has a positive electrode active material, a conductive agent, and an adhesive. The positive electrode active material is the positive electrode active material of the present invention described above, and will not be repeated here. The conductive agent is, for example, but not limited to, conductive carbon black Super P. The adhesive is, for example, but not limited to, polyvinylidene fluoride.

[0021] In some embodiments, the content of the positive electrode active material is 80 wt.% based on a total positive electrode active layer of 100 wt.%. In some embodiments, the content of the conductive agent is 10 wt.% and the content of the adhesive is 10 wt.% based on a total positive electrode active layer of 100 wt.%.

[0022] The negative electrode is spaced apart from the positive electrode. In some embodiments, the negative electrode is a lithium metal foil.

[0023] The separator is disposed between the positive electrode and the negative electrode. In some embodiments, the separator is a porous polyethylene membrane.

[0024] The lithium salt electrolyte is distributed between the positive electrode, the negative electrode, and the separator. Specifically, the lithium salt electrolyte is distributed between the positive electrode, the negative electrode, and the separator through the flow and diffusion of the lithium salt electrolyte.

[0025] The lithium salt electrolyte comprises a lithium salt and an organic solvent. The lithium salt is lithium hexafluorophosphate. In some embodiments, the concentration of the lithium salt in the electrolyte is 1M. In some embodiments, the organic solvent is selected from at least one of the group consisting of ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). In some embodiments, the organic solvent is a combination of one of DMC, DEC, and EMC with EC. In some embodiments, the organic solvent is a combination of EC and DMC, and the volume ratio of EC to DMC is 1:1. In some embodiments, the organic solvent is a combination of EC and DEC, and the volume ratio of EC to DEC is 1:1. In some embodiments, the organic solvent is a combination of EC and EMC, and the volume ratio of EC to EMC is 1:1. In some embodiments, the organic solvent is a combination of EC and EMC, and the volume ratio of EC to EMC is 3:7.

[0026] In some embodiments, the lithium salt electrolyte further includes N-Propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PMP-TFSI), adiponitrile (ADM), and lithium difluoro(oxalato)borate (LiDFOB), resulting in improved rate charge / discharge performance of the lithium-ion secondary battery at 50°C. PMP-TFSI is an ionic liquid with high lithium-ion conductivity and low viscosity. The adiponitrile contributes to the improved ionic conductivity of the lithium salt electrolyte. Lithium difluoro(oxalato)borate is used as an additive to provide additional lithium ions. During charge-discharge cycles, both lithium difluorooxalate borate and lithium hexafluorophosphate in this lithium-ion secondary battery are dissociated. The dissociated lithium difluorooxalate borate and the lithium ions released from the dissociated lithium hexafluorophosphate form a stable positive electrode electrolyte interface layer containing lithium fluoride (LiF) on the surface of the positive electrode, and a stable solid electrolyte interface layer containing lithium fluoride (LiF) on the surface of the negative electrode. This results in better rate charge / discharge performance of the lithium-ion secondary battery containing this lithium salt electrolyte at 50°C. In some embodiments, the content of ADM in the lithium salt electrolyte is 0.5 wt.%. In some embodiments, the content of LiDFOB in the lithium salt electrolyte is 0.5 wt.%.

[0027] In some embodiments, the lithium salt electrolyte further includes lithium difluorophosphate (LiDFP) and fluoroethylene carbonate (FEC), which improves the rate charge / discharge performance and capacitance retention of the lithium-ion secondary battery. The fluoroethylene carbonate is used as an additive. During charge-discharge cycles, both lithium difluorophosphate and lithium hexafluorophosphate are dissociated. The dissociated lithium difluorophosphate, together with the lithium ions released from the dissociated lithium hexafluorophosphate, forms a stable positive electrode electrolyte interface layer containing lithium fluoride (LiF) and lithium phosphide (Li3P) on the surface of the positive electrode, and a stable solid electrolyte interface layer containing lithium fluoride (LiF) and lithium phosphide (Li3P) on the surface of the negative electrode. The positive electrode electrolyte interface layer prevents the lithium salt electrolyte from directly contacting the positive electrode, and the solid electrolyte interface layer prevents the lithium salt electrolyte from directly contacting the negative electrode, thereby improving the rate charge / discharge performance and capacitance retention of the lithium-ion secondary battery. During charge-discharge cycles, both the fluoroethylene carbonate and lithium hexafluorophosphate are dissociated. The dissociated fluoroethylene carbonate and the lithium ions released from the dissociated lithium hexafluorophosphate form a stable positive electrode electrolyte interface layer containing lithium fluoride (LiF) on the surface of the positive electrode, and a stable solid electrolyte interface layer containing lithium fluoride (LiF) on the surface of the negative electrode. In some embodiments, the LiDFP content in the lithium salt electrolyte is 1 wt.%. In some embodiments, the FEC content in the lithium salt electrolyte is 1 wt.%. In some embodiments, the FEC content in the lithium salt electrolyte is 5 wt.%. In some embodiments, the lithium salt electrolyte contains 1 wt.% LiDFP, 1 wt.% FEC, and 1 M lithium salt; the organic solvent is a combination of EC and EMC, with a volume ratio of EC to EMC of 3:7. In some embodiments, the lithium salt electrolyte contains 1 wt.% LiDFP, 5 wt.% FEC, and 1 M lithium salt; the organic solvent is a combination of EC and EMC, with a volume ratio of EC to EMC of 3:7.

[0028] By covering the surface of the lithium nickel manganese oxide particles with a fluoride film, the Ni2+ ions in the lithium nickel manganese oxide particles are less likely to be oxidized to Ni3+ or Ni4+, thereby giving the lithium-ion secondary battery good lithium-ion conductivity and reducing the polarization voltage during charge-discharge cycles. Furthermore, when the lithium salt electrolyte is under high charging voltage (e.g., above 4.5V), and the lithium hexafluorophosphate is oxidized and dissociated to produce hydrofluoric acid, the fluoride film can prevent the lithium nickel manganese oxide particles from directly contacting the hydrofluoric acid. This prevents the hydrofluoric acid from reacting chemically with the lithium nickel manganese oxide particles, thus preventing the hydrofluoric acid from dissolving the nickel and manganese elements in the particles, reducing corrosion of the lithium nickel manganese oxide particles by the hydrofluoric acid, and further contributing to the good lithium-ion conductivity and reduced polarization voltage during charge-discharge cycles of the lithium-ion secondary battery. Furthermore, during the charge-discharge cycle of the lithium-ion secondary battery, the lithium salt electrolyte will dissociate and release lithium ions. In addition, since the fluoride film is composed of fluoride, the lithium ions will form a stable positive electrode electrolyte interface layer containing lithium fluoride (LiF) with the fluoride film. This positive electrode electrolyte interface layer can further prevent the lithium nickel manganese oxide particles from being corroded by hydrofluoric acid.

[0029] The present invention will be further described with reference to the following preparation examples, embodiments and application examples. However, it should be understood that the following preparation examples, embodiments and application examples are for illustrative purposes only and should not be construed as limiting the implementation of the present invention.

[0030] [Preparation Example 1] Lithium Salt Electrolyte

[0031] The lithium salt electrolyte of Preparation Example 1 is composed of ethylene carbonate, dimethyl carbonate and lithium hexafluorophosphate, wherein the volume ratio of ethylene carbonate to dimethyl carbonate is 1:1 and the concentration of lithium hexafluorophosphate is 1M.

[0032] [Preparation Example 2] Lithium Salt Electrolyte

[0033] The lithium salt electrolyte of Preparation Example 2 is composed of ethylene carbonate, diethyl carbonate, lithium hexafluorophosphate, adiponitrile, lithium difluorooxalate borate, and PMP-TFSI. The concentration of lithium hexafluorophosphate is 1.2 M, the content of adiponitrile is 0.5 wt.%, the content of lithium difluorooxalate borate is 0.5 wt.%, the volume ratio of ethylene carbonate to diethyl carbonate is 1:1, and the volume ratio of the total volume of ethylene carbonate and diethyl carbonate to the total volume of PMP-TFSI is 7:3.

[0034] [Example 1] Positive electrode active material

[0035] 393.3 g of nickel sulfate hexahydrate and 760.6 g of manganese sulfate hexahydrate were mixed with 3000 mL of deionized water to obtain a nickel-manganese metal source solution. Using a peristaltic pump, the flow rate of the nickel-manganese metal source solution was controlled at 1.7 mL / min, and the flow rate of a 7.5 M ammonium hydroxide aqueous solution was controlled at 0.6 mL / min. The nickel-manganese metal source solution and the ammonium hydroxide aqueous solution were fed into a continuous Taylor-Couette flow reactor (brand: Laminar, model: LCTR-Tera 3100), along with a 4 M sodium hydroxide aqueous solution. After the pH in the continuous Taylor-Couette flow reactor reached 12, a co-precipitation reaction was carried out at a reaction temperature of 60 °C and a rotation speed of 1200 rpm for 10 to 30 hours to form nickel-manganese hydroxide (Ni0.25Mn0.75(OH)2), yielding a crude product containing this nickel-manganese hydroxide. The crude product was taken out from the continuous Taylor-Couette flow reactor, washed with deionized water and then with alcohol, and then the washed crude product was filtered by vacuum and dried in a vacuum oven at 60°C to obtain the nickel manganese hydroxide powder.

[0036] 2.5 g of the nickel-manganese hydroxide powder was mixed with 1.84 g of lithium hydroxide to obtain a precursor mixed powder. Multiple zirconia balls were placed in a ball mill jar at a weight ratio of 5:1 and milled at 400 rpm for 10 hours to obtain a ball-milled material. The ball-milled material was dried at 80°C to obtain a powder mixture. The powder mixture was pre-calcined at 500°C for 5 hours to obtain a pre-calcined product. The pre-calcined product was dispersed in deionized water and then granulated in a spray dryer at 150°C to form particles of the pre-calcined product. The particles of the pre-calcined product were calcined at 900°C for 10 hours to obtain particles of lithium nickel-manganese oxide. The chemical composition of the lithium nickel-manganese oxide is Li1.2Ni0.2Mn0.6O2.

[0037] An aqueous solution of ammonium fluoride was prepared by dissolving 17 mg of ammonium fluoride in 100 mL of water. An aqueous solution of lanthanum acetate was prepared by dissolving 48 mg of lanthanum acetate in 100 mL of water. 3 g of the lithium nickel manganese oxide particles were dispersed in 100 mL of the ammonium fluoride aqueous solution to obtain a dispersion. 100 mL of the lanthanum acetate aqueous solution was slowly introduced into the dispersion using a peristaltic pump to mix and react the ammonium fluoride and lanthanum acetate to form lanthanum fluoride (LaF3). The lanthanum fluoride then coated the surface of the lithium nickel manganese oxide particles to form a fluoride film, resulting in an uncalcined purified product containing a positive electrode active material. The uncalcined purified product is first dried at 85°C, and then calcined at 480°C for 5 hours to remove impurities (such as ammonium acetate), thus purifying the positive electrode active material. The positive electrode active material contains particles of lithium nickel manganese oxide and a fluoride film covering the particles of lithium nickel manganese oxide. The content of lanthanum fluoride is 1 part by weight, based on a total amount of 100 parts by weight of lithium nickel manganese oxide particles.

[0038] [Examples 2 to 3] Positive electrode active material

[0039] In Examples 2 and 3, positive electrode active materials were prepared using a method similar to that in Example 1, and the amounts of the lithium nickel manganese oxide particles, the amount of ammonium fluoride, the amount of lanthanum acetate, and the content of lanthanum fluoride are shown in Table 1.

[0040] [Comparative Example] Positive Electrode Active Material

[0041] In the comparative example, the lithium nickel manganese oxide particles were prepared using a similar method to that in Example 1. However, the lithium nickel manganese oxide particles were used directly as the positive electrode active material. That is, in the comparative example, ammonium fluoride and lanthanum acetate were not used to react and form a fluoride film covering the surface of the lithium nickel manganese oxide particles. Therefore, the positive electrode active material obtained did not contain lanthanum fluoride.

[0042] Table 1 Example 1 Example 2 Example 3 Comparative example Particles (grams) of lithium nickel manganese oxide 3 3 3 3 Ammonium fluoride (mg) 17 34 51 0 Lanthanum acetate (mg) 48 96 144 0 Lanthanum fluoride content (parts by weight) 1 2 3 0

[0043] [X-ray diffraction analysis]

[0044] The particles of lithium nickel manganese oxide in Example 1 and the positive electrode active materials in Examples 1 to 3 were analyzed by X-ray diffraction (XRD, brand: BRUKER, model: D2 PHASER). The obtained X-ray diffraction patterns are shown in Figure 1 and Figure 2.

[0045] [Scanning Electron Microscopy Analysis]

[0046] The appearance morphology of the lithium nickel manganese oxide particles in Example 1 and the positive electrode active material particles in Examples 1 to 3 were analyzed using a scanning electron microscope (SEM, brand: JEOL, model: JSM-IT200). The obtained SEM images are shown in Figure 3 and Figure 4.

[0047] [Transmission Electron Microscopy Analysis]

[0048] The lithium nickel manganese oxide particles in Example 1 and the positive electrode active materials in Examples 1 to 3 were analyzed using a transmission electron microscope (TEM, JEOL, model JEM-2100PLUS). The obtained TEM images are shown in Figure 5.

[0049] Referring to Figure 5, the lithium nickel manganese oxide particles of Example 1 have a distinct and well-formed lattice structure. The positive electrode active materials of Examples 1 to 3 all have an additional layer of amorphous structure compared to the lithium nickel manganese oxide particles of Example 1, indicating that the fluoride film formed by the lanthanum fluoride does indeed cover the surface of the lithium nickel manganese oxide particles.

[0050] [Application Example 1] Lithium-ion secondary battery

[0051] Two grams of the positive electrode active material from Example 1 were mixed with 0.25 grams of conductive carbon black Super P (average particle size: 40 nm, surface area: 62 m² / g, purchased from Timcal Ltd.) and dried at 120°C for 1 to 2 hours to obtain a dried product. 0.25 grams of polyvinylidene fluoride (purchased from Arkema Inc., model: KYNAR® HSV 900 PWD, molecular weight: 900,000) was dissolved in 3.32 grams of N-methylpyrrolidone (as solvent) to obtain a 3.57-gram polyvinylidene fluoride solution. 2.25 grams of the dried product and the 3.57-gram polyvinylidene fluoride solution were mixed at 2000 rpm for 30 minutes to obtain a slurry. An appropriate amount of N-methylpyrrolidone was added to the slurry to obtain a paste with a solid content of 25 wt.%. An aluminum foil was used as a current collector. The aluminum foil was placed in a blade coating machine, with the blade's movement speed set to 2 mm / s and the gap between the blade and the aluminum foil to 200 micrometers. The paste was then applied to the aluminum foil using the blade. The aluminum foil coated with the paste was then dried at 60°C for 12 hours to remove N-methylpyrrolidone, resulting in a laminate. The laminate was then rolled using a roller press to produce a positive electrode (13 mm in diameter and 35 to 40 micrometers thick). The positive electrode comprises the aluminum foil and a positive electrode active layer formed from the paste on the surface of the aluminum foil.

[0052] Based on the specifications and structure of the CR2032 lithium-ion secondary battery, a cover, a lithium metal foil (15.8 mm in diameter and 200 μm thick) serving as the negative electrode, a polyethylene porous membrane (18 mm in diameter and 16 μm thick) serving as the separator, the positive electrode, a gasket, and a spring are assembled. 50 μL of the lithium salt electrolyte from Preparation Example 1 is injected between the separator and the positive electrode. Through the flow and diffusion of the lithium salt electrolyte, the lithium salt electrolyte is distributed between the positive electrode, the negative electrode, and the separator, thus obtaining a lithium-ion secondary battery.

[0053] [Application Examples 2 to 4, Comparative Application Examples] Lithium-ion Secondary Batteries

[0054] In Application Examples 2 to 4 and Comparative Application Examples, positive electrodes and lithium-ion secondary batteries were prepared using a method similar to that in Application Example 1. The types of positive electrode active material, negative electrode, separator, and lithium salt electrolyte are shown in Table 2.

[0055] Table 2 Lithium-ion secondary batteries Positive electrode active material negative electrode Separating membrane lithium salt electrolyte Application Example 1 Example 1 Lithium metal foil Polyethylene porous membrane Preparation Example 1 Application Example 2 Example 2 Lithium metal foil Polyethylene porous membrane Preparation Example 1 Application Example 3 Example 3 Lithium metal foil Polyethylene porous membrane Preparation Example 1 Application Example 4 Example 2 Lithium metal foil Polyethylene porous membrane Preparation Example 2 Comparison Application Examples Comparative example Lithium metal foil Polyethylene porous membrane Preparation Example 1

[0056] [Lithium-ion secondary battery rate charge / discharge performance test]

[0057] Using a battery testing device (brand: Chia-Yu Technology Co., Ltd., model: BAT-750B), the lithium-ion secondary batteries in Examples 1 to 4 and the comparative application examples were subjected to charge-discharge cycles under the test conditions (test temperature, voltage range, charging rate, and discharging rate) listed in Table 3. Each combination of charging and discharging rates was performed for three charge-discharge cycles, and the discharge capacity at that discharge rate was measured in each charge-discharge cycle. The results are shown in Figures 6 and 9.

[0058] [Long-cycle charge-discharge test of lithium-ion secondary batteries]

[0059] Using this battery testing equipment, the lithium-ion secondary batteries in Case 2, Application Example 4, and Comparative Application Example were subjected to charge-discharge cycles under the test conditions (test temperature, voltage range, charging rate, discharging rate, and number of charge-discharge cycles) shown in Tables 4 and 5. The measured values ​​were then substituted into the formulas below to calculate the average coulombic efficiency and discharge capacitance retention. The results of the long-cycle charge-discharge test of the lithium-ion secondary batteries are shown in Tables 4 and 5, Figures 7, 8, 10, and 11. Average Coulomb efficiency (%) = Discharge capacity retention (%) = (Discharge capacity in the Nth charge-discharge cycle / Discharge capacity in the 1st charge-discharge cycle)

[0060] Table 3 Lithium-ion secondary batteries Application Example 1 Application Example 2 Application Example 3 Application Example 4 Comparison Application Examples Test temperature (°C) 25 25 25 25 50 25 Voltage range (V) 2.0 to 4.8 2.0 to 4.8 2.0 to 4.8 2.0 to 4.8 2.0 to 4.8 2.0 to 4.8 Charging rate (C) 0.2 Discharge rate (C) 0.2, 0.5, 1, 3, 5, 10

[0061] Table 4 Lithium-ion secondary batteries Application Example 2 Comparison Application Examples Test temperature (°C) 25 25 Voltage range (V) 2.0 to 4.8 2.0 to 4.8 Charging rate (C) 1 1 Discharge rate (C) 1 1 Number of charge / discharge cycles (times) 100 100 Discharge capacity (mAh / g) during the first charge-discharge cycle 208.20 174.25 Discharge capacity (mAh / g) during the Nth charge-discharge cycle, N=100 176.60 162.03 Average Coulomb efficiency (%) 99.13 99.29

[0062] Table 5 Lithium-ion secondary batteries Application Example 4 Test temperature (°C) 25 50 Voltage range (V) 2.0 to 4.8 2.0 to 4.8 Charging rate (C) 1 1 Discharge rate (C) 1 1 Number of charge / discharge cycles (times) 200 200 Discharge capacity (mAh / g) during the first charge-discharge cycle 187.73 229.39 Discharge capacity (mAh / g) in the Nth charge-discharge cycle, N=200 165.19 183.61 Average Coulomb efficiency (%) 98.65 99.17 Discharge capacitor retention rate (%) 87.99 80.00

[0063] Referring to Figure 6, the lithium-ion secondary batteries of Application Examples 1 to 3 have good rate charge / discharge performance, with the lithium-ion secondary battery of Application Example 2 having the best rate charge / discharge performance. Furthermore, the rate charge / discharge performance of the lithium-ion secondary batteries of Application Examples 1 to 3 is better than that of the lithium-ion secondary batteries of the comparative application examples.

[0064] Referring to Table 4 and Figure 8, the lithium-ion secondary battery of Application Example 2 exhibits good average coulombic efficiency and high discharge capacity. Figures 7 and 8 show that the discharge capacity of the lithium-ion secondary battery of Application Example 2 is higher than that of the lithium-ion secondary battery in the comparative application example during charge-discharge cycles.

[0065] Referring to Figure 9, the lithium-ion secondary battery of Application Example 4 has good rate charge / discharge performance at both 25°C and 50°C, with even better rate charge / discharge performance at 50°C.

[0066] Referring to Table 5, Figure 10 and Figure 11, the lithium-ion secondary battery of Application Example 4 has good average coulombic efficiency and discharge capacitance retention at both 25°C and 50°C, and good charge-discharge stability.

[0067] In summary, this invention uses a fluoride film to cover the surface of the lithium nickel manganese oxide particles, making the positive electrode active material in the lithium-ion secondary battery less susceptible to corrosion by hydrofluoric acid generated by the lithium salt electrolyte at high charging voltages (e.g., above 4.5V). The lithium-ion secondary battery of this invention, comprising this positive electrode active material, exhibits high discharge capacity, excellent rate charge / discharge performance, and high average coulombic efficiency. Therefore, the objectives of this invention are indeed achieved.

[0068] However, the above description is merely an embodiment of the present invention and should not be construed as limiting the scope of the present invention. Any simple equivalent changes and modifications made in accordance with the scope of the patent application and the contents of the patent specification of the present invention shall still fall within the scope of the patent of the present invention.

Claims

1. A positive electrode active material comprising: particles of lithium nickel manganese oxide; and a fluoride film covering the surface of the lithium nickel manganese oxide particles, wherein the fluoride film is composed of a fluoride selected from at least one of the group consisting of lanthanum fluoride, yttrium fluoride and scandium fluoride, and the fluoride content is in the range of 2 to 3 parts by weight, based on 100 parts by weight of the total amount of lithium nickel manganese oxide particles.

2. The positive electrode active material as described in claim 1, wherein, Based on a total of 100 parts by weight of the lithium nickel manganese oxide particles, the content of the fluoride is 2 parts by weight.

3. A lithium-ion secondary battery, comprising: a positive electrode, comprising: A current collector and a positive electrode active layer on the surface of the current collector, the positive electrode active layer having a positive electrode active material as described in any one of claims 1 to 2; A negative electrode is spaced apart from the positive electrode; a separator is disposed between the positive electrode and the negative electrode; and a lithium salt electrolyte is distributed between the positive electrode, the negative electrode and the separator, the lithium salt electrolyte comprising a lithium salt and an organic solvent, the lithium salt being lithium hexafluorophosphate.

4. The lithium-ion secondary battery as described in claim 3, wherein, Based on a total positive electrode active layer of 100 wt.%, the content of the positive electrode active material is 80 wt.%.

5. The lithium-ion secondary battery as claimed in claim 3, wherein, The organic solvent is selected from at least one of the group consisting of ethylene carbonate, dimethyl carbonate, diethyl carbonate and methyl ethyl carbonate.

6. The lithium-ion secondary battery as claimed in claim 5, wherein, The organic solvent is a combination of one of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate with ethylene carbonate.

7. The lithium-ion secondary battery as claimed in claim 5 or 6, wherein, The lithium salt electrolyte also includes 1-methyl-1-propylpyrrolidine bis(trifluoromethanesulfonyl)imine salt, adiponitrile, and lithium difluorooxalate borate.

8. The lithium-ion secondary battery as claimed in claim 5 or 6, wherein, The lithium salt electrolyte also includes lithium difluorophosphate and fluoroethylene carbonate.

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