Positive electrode active material for lithium secondary battery, and lithium secondary battery comprising same

Abstract

The present invention relates to a positive electrode active material satisfying a specific parameter criterion for surface functional groups contained in the positive electrode active material. The present invention also relates to a lithium secondary battery comprising the positive electrode active material. According to the present invention, by introducing a parameter capable of evaluating the active material, the active material can be selected without evaluating a separate cell, thereby contributing to improving the development speed of cells. In addition, the lifespan of the secondary battery can be improved by improving the capacity retention rate of the positive electrode active material.

Description

Cathode active material for lithium secondary batteries and lithium secondary batteries containing the same

[0001] This application claims the benefit of priority to Korean Patent Application No. 10-2024-0105129, filed August 7, 2024, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a cathode active material for a lithium secondary battery. Specifically, the present invention relates to a cathode active material that satisfies specific parameter criteria for surface functional groups contained in the cathode active material. The present invention also relates to a lithium secondary battery comprising the cathode active material.

[0003] For lithium iron phosphate (LFP, LiFePO4) active materials, forming a carbon coating is essential to ensuring conductivity. To ensure conductivity and optimize cell performance through the carbon coating process, uniform formation of the carbon coating layer, control of carbon content, and control of carbon functional groups are crucial factors.

[0004] While LFP active materials offer advantages over other active materials, such as being economical and abundant in raw materials, they suffer from relatively low electrical conductivity at room temperature. To address this shortcoming, ongoing efforts are being made to improve electrical conductivity by coating the surface of LFP active materials with conductive materials. Increasing the electrical conductivity of LFP active materials can lead to improved capacity retention (C-rate) characteristics and capacity.

[0005] Carbon coating is typically applied to improve the electrical conductivity of LFP active materials. During the carbon coating process, coating uniformity and surface properties control are crucial. In particular, the presence of oxidizing and undesirable functional groups on the surface of the carbon coating layer can significantly degrade cell performance, requiring the establishment of standards for controlling surface functional groups in the carbon coating layer.

[0006] An object of the present invention is to provide a positive electrode active material that satisfies specific parameter criteria for surface functional groups contained in a coating layer of the positive electrode active material, and a method for evaluating the same. Another object of the present invention is to provide a lithium secondary battery comprising the positive electrode active material.

[0007] According to one aspect of the present invention, a lithium iron phosphate (LFP, LiFePO4) active material for a lithium secondary battery is provided, which satisfies mathematical expressions 1 and 2.

[0008] [Mathematical Formula 1]

[0009] Carbon functional group ratio ≤ 0.01,

[0010] [Equation 2]

[0011] Carbon functional group ratio = (carbon functional group IR peak area) / (LFP IR peak area)

[0012] In mathematical expression 2, the IR peak area is measured in IR diffuse reflection (DRIFT) mode, and the LFP IR peak is the PO band.

[0013] According to one embodiment, the absorption wavenumber of the carbon functional group is 2500 to 1700 cm -1 And, the absorption wavenumber of the above LFP(PO) is 1300~900cm -1 It could be.

[0014] According to one embodiment, the carbon functional group may include C=C, C=O, C=N, C≡C or C≡N.

[0015] According to another embodiment of the present invention, a method for evaluating a lithium iron phosphate (LFP, LiFePO4) active material for a lithium secondary battery is provided, comprising a step of measuring the active material in an IR diffuse reflectance (DRIFT) mode and calculating a carbon functional group ratio according to mathematical formula 2, and a step of confirming whether the carbon functional group ratio according to mathematical formula 2 satisfies mathematical formula 1:

[0016] [Mathematical Formula 1]

[0017] Carbon functional group ratio ≤ 0.01,

[0018] [Equation 2]

[0019] Carbon functional group ratio = (Carbon functional group IR peak area) / (LFP IR peak area).

[0020] According to another embodiment of the present invention, a lithium secondary battery is provided, which includes a lithium iron phosphate (LFP, LiFePO4) active material for a lithium secondary battery as described above.

[0021] Specific details of other implementation examples according to the present invention are included in the detailed description below.

[0022] According to the present invention, the introduction of parameters for evaluating active materials allows for the selection of active materials without the need for separate cell evaluation, thereby contributing to accelerated cell development. Furthermore, the capacity retention rate of positive electrode active materials can be improved, thereby extending the lifespan of secondary batteries.

[0023] Figure 1 is an example graph of detection of a carbon functional group according to an embodiment.

[0024] The terms and words used in this specification and claims should not be interpreted as limited to their conventional or dictionary meanings, but rather should be interpreted with meanings and concepts that conform to the technical spirit of the present invention, based on the principle that the inventor can appropriately define the concept of the term to best explain his or her own invention. In addition, it should be understood that the configurations exemplified in the embodiments described in this specification are merely the most preferred embodiments of the present invention and do not represent all of the technical spirit of the present invention, and therefore, various equivalents and modified examples may exist that can replace them at the time of this application.

[0025] Hereinafter, the active material for a lithium secondary battery according to the present invention will be described in detail.

[0026] One embodiment of the present invention relates to a lithium iron phosphate (LFP, LiFePO4) active material for a lithium secondary battery, satisfying mathematical expressions 1 and 2.

[0027] [Mathematical Formula 1]

[0028] Carbon functional group ratio ≤ 0.01,

[0029] [Equation 2]

[0030] Carbon functional group ratio = (carbon functional group IR peak area) / (LFP IR peak area)

[0031] In the past, attempts have been made to measure the carbon content using a CS analyzer to detect carbon functional groups contained in the positive electrode active material coating layer, and to evaluate the uniform formation of the carbon coating layer through SEM / TEM. The CS analyzer is a device that detects the content of carbon and sulfur generated by combusting a sample in an oxygen stream. In this field, a positive electrode active material sample is introduced in the form of dried particles together with a combustion agent into the ceramic furnace of a CS analyzer commonly used in the field, and oxygen gas is supplied from a high-frequency induction device, so that the contents of C and S components contained in the sample can be measured. However, it is difficult to obtain sufficient information to analyze the coating layer formed very thinly on the surface of the active material using this method.

[0032] Accordingly, in the present invention, IR analysis was applied to analyze the carbon functional group contained in the coating layer of the positive electrode active material.

[0033] IR analysis methods can be broadly divided into attenuated total reflection (ATR), diffuse reflectance spectroscopy (DRIFT), and transmission modes.

[0034] IR ATR mode analyzes infrared radiation reflected from a sample surface. It involves transmitting infrared radiation through special crystals, such as diamond, ZnSe, or Ge, and measuring the total reflected spectrum from the contact interface between the infrared radiation and the sample. IR ATR mode requires simple sample preparation and allows for analysis with small sample volumes.

[0035] IR DRIFT mode analyzes infrared radiation diffusely reflected from a sample surface. It can be applied to samples with rough surfaces or powdery forms, and chemical composition can be identified through the infrared spectrum reflected from the sample.

[0036] IR transmission mode measures the amount of infrared radiation that passes through a sample without being absorbed. This requires manufacturing the sample thin enough to allow infrared radiation to pass through. IR transmission mode can reveal structural information about molecules through absorption spectra specific to the sample's characteristics.

[0037] Among these, IR ART mode is applicable to various sample types and is widely used in the past due to its advantages of sample preparation and analysis convenience. However, there is a problem that peak detection is often difficult depending on the absorption wavenumber when the positive and negative active materials used in batteries are materials with high IR absorption. Accordingly, in the present invention, the carbon functional group was detected by analyzing the LFP active material in IR DRIFT mode and comparing the peak of the carbon functional group and the PO band.

[0038] In the IR analysis results, the peak position may vary depending on the functional group and the absorption wavelength range may vary. Specifically, 4000-2500 cm -1 Absorption region caused by stretching motion of NH, CH or OH single bond, 3300~3600 cm -1 Absorption region in NH or OH bond, 3000 cm -1The area around is the absorption region of CH bonds, 2500-2000 cm -1 The absorption region for triple bond stretching motion is C≡C or C≡N bond absorption peak. Also, 2000–1500 cm -1 is the absorption region of C=O, C=N or C=C double bonds, and generally the carbonyl group (C=O) absorption range is 1680~1750 cm -1 , and the C=C stretching movement is 1640~1680 cm -1 It can appear in a narrow range of 1500 cm -1 The following is the fingerprint region, where many absorptions can occur due to vibrations of various single bonds, such as CC, CO, CN, and CX.

[0039] In the present invention, the absorption wavenumber of the carbon functional group is, for example, 2500 to 1700 cm -1 It can be, and the absorption wavenumber of the PO band is 1300~900cm -1 It can be. Carbon functional groups can be detected by comparing the peaks of the carbon functional group and the PO band in any specific area like this. Since LFP active materials have structural stability due to the strong covalent phosphorus-oxygen (PO) bond of the olivine structure, the area representing LFP was based on the area of ​​the PO band. The determination of the baseline and absorption wavenumber range may vary depending on the characteristics of the sample.

[0040] According to another embodiment of the present invention, a method for evaluating a lithium iron phosphate (LFP, LiFePO4) active material for a lithium secondary battery is provided. Specifically, the method for evaluating an active material includes a step of measuring the active material in an IR diffuse reflectance (DRIFT) mode and calculating a carbon functional group ratio according to mathematical formula 2, and a step of confirming whether the carbon functional group ratio according to mathematical formula 2 satisfies mathematical formula 1.

[0041] [Mathematical Formula 1]

[0042] Carbon functional group ratio ≤ 0.01,

[0043] [Equation 2]

[0044] Carbon functional group ratio = (Carbon functional group IR peak area) / (LFP IR peak area).

[0045] In mathematical formula 2, the LFP IR peak is related to the PO band. The carbon functional group can be detected by comparing the peak of the carbon functional group and the PO band from the IR analysis results.

[0046] The carbon coating layer of the active material may contain various functional groups, such as carbon double bonds or triple bonds, depending on the degree of oxidation. Specifically, the carbon functional groups may include C=C, C=O, C=N, C≡C, or C≡N. Since these functional groups are closely related to cell performance, high-precision detection of carbon functional groups is important in setting parameters for evaluating the active material.

[0047] According to another aspect of the present invention, a lithium secondary battery including a lithium iron phosphate (LFP, LiFePO4) active material as described above is provided.

[0048]

[0049] Hereinafter, the present invention will be described in detail by way of examples to aid understanding. However, the examples according to the present invention may be modified in various ways, and the scope of the present invention should not be construed as being limited to the following examples. These examples are provided to more fully explain the present invention to those of average skill in the art.

[0050]

[0051] Example 1

[0052] A mixed solution of lithium iron phosphate (LFP) cathode active material precursor LiFePO4 and conductive carbon precursor sucrose at a weight ratio of 1:0.75 was powdered using a spray pyrolysis process. The obtained powder was calcined at 715°C for 5 hours to produce a carbon-coated LiFePO4 cathode active material.

[0053]

[0054] Comparative Example 1

[0055] A mixed solution of the lithium iron phosphate (LFP) cathode active material precursor LiFePO4 and the conductive carbon precursor sucrose, identical to Example 1, was powdered using a spray pyrolysis process. The obtained powder was calcined at 600°C for 5 hours to produce a carbon-coated LiFePO4 cathode active material.

[0056]

[0057] Comparative Example 2

[0058] A mixed solution of the same lithium iron phosphate (LFP) cathode active material precursor LiFePO4 and the conductive carbon precursor sucrose as in Example 1 was powdered using a spray pyrolysis process. The obtained powder was calcined at 500°C for 5 hours to produce a carbon-coated LiFePO4 cathode active material.

[0059]

[0060] Experimental Example 1: Evaluation of the Lifetime Characteristics of Active Materials

[0061] Each of the positive electrode active materials, carbon black conductive agent, and polyvinylidene fluoride (PVdF) binder according to the examples and comparative examples was mixed in an N-methyl pyrrolidone (NMP) solvent at a weight ratio of 85:10:5 to prepare a positive electrode slurry. The prepared slurry was applied to an aluminum current collector, dried at 80°C, and then rolled to prepare a positive electrode.

[0062] A porous polyethylene film was used as the separator, and lithium metal was used as the cathode. The electrolyte was prepared by dissolving 1.0 M lithium hexafluorophosphate (LiPF6) in an organic solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) in a volume ratio of 1:1:1.

[0063] An example graph of the detection of carbon functional groups according to the IR DRIFT analysis results is shown in Figure 1. The area of ​​the detection target was calculated using software (OMNIC) that can calculate the baseline and area for the detected spectrum. At this time, the range of the baseline and absorption wavenumber can be determined according to the characteristics of the sample. As shown in Figure 1, the carbon functional group is 2500-1700 cm -1 and LFP (PO band) 1300~900cm -1 The carbon functional group ratio was calculated from the peak corresponding to the absorption wavenumber region according to mathematical formula 2.

[0064] [Equation 2]

[0065] Carbon functional group ratio = (carbon functional group IR peak area) / (LFP IR peak area)

[0066] The resulting carbon functional group ratios are listed in Table 1.

[0067] Carbon functional group ratio Example 10.01 Comparative example 20.02 Comparative example 30.04

[0068] In addition, for each manufactured lithium secondary battery half cell, the life characteristics were evaluated at a cut-off voltage of 2.5 to 4.0 V and charge / discharge rates of 1C, 2C, 3C, and 4C. The results of the capacity retention rate (C-rate) evaluation are shown in Table 2.

[0069] Capacity retention rate (%) C-rate 1 C 2 C 3 C 4 C Example 1 8 1.3 7 5.0 7 0.0 6 7.5 Comparative example 1 62.5 1 8.1 0.6 fail Comparative example 2 fail fail fail fail

[0070] As can be seen from Tables 1 and 2, the capacity retention rate decreases when the carbon functional group ratio exceeds 0.01.

[0071] As described above, the method according to the present invention can provide an active material satisfying specific parameters. Furthermore, when the parameters of the present invention are satisfied, a secondary battery with improved capacity retention can be provided.

[0072]

[0073] While specific aspects of the present invention have been described in detail above, it should be apparent to those skilled in the art that these specific descriptions are merely preferred implementation examples and are not intended to limit the scope of the present invention. Those skilled in the art will be able to make various applications and modifications within the scope of the present invention based on the above description. Therefore, the actual scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. Lithium iron phosphate (LFP, LiFePO4) active material for lithium secondary batteries, satisfying mathematical formulas 1 and 2: [Mathematical Formula 1] Carbon functional group ratio ≤ 0.01, [Equation 2] Carbon functional group ratio = (carbon functional group IR peak area) / (LFP IR peak area) In mathematical expression 2, the IR peak area is measured in IR diffuse reflection (DRIFT) mode, and the LFP IR peak is the PO band.

2. In paragraph 1, The absorption wavenumber of the above carbon functional group is 2500~1700cm -1 And the absorption wavenumber of the above LFP is 1300~900cm -1 Lithium iron phosphate (LFP, LiFePO4) active material for lithium secondary batteries.

3. In paragraph 1, A lithium iron phosphate (LFP, LiFePO4) active material for a lithium secondary battery, wherein the carbon functional group includes C=C, C=O, C=N, C≡C or C≡N.

4. A method for evaluating lithium iron phosphate (LFP, LiFePO4) active material for lithium secondary batteries. It includes a step of measuring the above active material in IR diffuse reflectance (DRIFT) mode and calculating the carbon functional group ratio according to mathematical formula 2. An active material evaluation method comprising a step of checking whether the carbon functional group ratio according to mathematical formula 2 satisfies mathematical formula 1: [Mathematical Formula 1] Carbon functional group ratio ≤ 0.01, [Equation 2] Carbon functional group ratio = (Carbon functional group IR peak area) / (LFP IR peak area).

5. In paragraph 4, The absorption wavenumber of the above carbon functional group is 2500~1700cm -1 And the absorption wavenumber of the above LFP is 1300~900cm -1 A method for evaluating lithium iron phosphate (LFP, LiFePO4) active material for secondary batteries.

6. In paragraph 4, A method for evaluating an active material, wherein the carbon functional group includes C=C, C=O, C=N, C≡C or C≡N.

7. A lithium secondary battery comprising a lithium iron phosphate (LFP, LiFePO4) active material according to Article 1.

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