Method for detecting electrocrystallization of lithium-ion batteries

Abstract

The present application provides a lithium ion battery crystallization detection method capable of detecting the face distribution of lithium electric crystallization in the negative electrode active material layer of a lithium ion battery, thereby knowing the precursor of lithium electric crystallization and improving the energy efficiency of the lithium ion battery. The lithium ion battery crystallization detection method has: a positive electrode SOC map making process, X-rays are irradiated to the lithium ion battery, X-ray diffraction measurement of the positive electrode active material layer is performed, and a positive electrode SOC map is made, which represents the distribution of the depth of charge of the positive electrode active material layer in the thickness direction; a SOC difference value detection process, which detects the SOC difference value, which is the difference between the maximum value and the minimum value of the positive electrode SOC map; and a determination process, which compares the SOC difference value with a pre-set threshold value to determine whether the negative electrode active material layer has lithium electric crystallization.

Description

Technical Field

[0001] This invention relates to a method for detecting the formation of lithium-ion crystals in the negative electrode active material layer of a lithium-ion battery. Background Technology

[0002] Vehicles such as EVs (Electric Vehicles) and HEVs (Hybrid Electric Vehicles) are equipped with energy storage devices that supply power to motors and other components. These energy storage devices contain multiple secondary batteries.

[0003] Secondary batteries used in EVs and HEVs are required to have a specified output, but in general, when secondary batteries are stored for a medium to long period of time, the surface of the electrodes is covered by an oxide film, which reduces the output characteristics.

[0004] Generally, when the output from the secondary battery decreases as it is in this case, it is considered a degradation in output performance, and control measures are implemented to reduce the output from the secondary battery. This is because if the output is still required to maintain its pre-degradation level despite the degradation of the secondary battery, the secondary battery becomes overloaded, accelerating its lifespan degradation.

[0005] To eliminate the degradation of secondary batteries, a prescribed discharge treatment, also known as a recovery treatment, is required to reactivate the secondary battery. However, EVs and HEVs do not envision performing this activation treatment while the secondary battery is mounted in the vehicle. Therefore, for example, Japanese Patent Application Publication No. 2001-339864 discloses a method for increasing the State of Charge (SOC) range during degradation when a lead-acid battery is used as a secondary battery.

[0006] On the other hand, lithium-ion batteries (LIBs) are widely used as secondary batteries for EVs and HEVs. Due to their lightweight nature and high energy density, lithium-ion batteries are preferred as high-output power sources for vehicle applications.

[0007] In lithium-ion batteries, carbon materials such as graphite are generally used as the negative electrode active material. During charging, lithium ions enter the interlayer of the carbon material, causing a potential change. However, depending on the charging state of the lithium-ion battery, metallic lithium (lithium-ion crystals) sometimes precipitates in the negative electrode active material.

[0008] When lithium-ion battery crystallization occurs, it is known to cause capacity degradation, and therefore charging is controlled to maintain a pre-set upper voltage limit. Here, lithium-ion battery crystallization refers to the electroreduction of lithium ions, causing them to deposit as lithium metal on the negative electrode surface. As lithium-ion batteries gradually deteriorate due to this, the charging capacity and regeneration output decrease compared to when they were new, resulting in problems such as a shorter driving range. Therefore, techniques for detecting the occurrence of lithium-ion battery crystallization have been proposed in the past.

[0009] For example, Japanese Patent Application Publication No. 2012-003863 discloses a method for detecting the presence or absence of lithium-ion crystals by detecting the presence or absence of a minimum point, where the minimum point represents the point where the charging current changes from decreasing to increasing.

[0010] In addition, Japanese Patent Application Publication No. 2013-089363 discloses a method for detecting the change in battery voltage per unit time as it gradually increases through constant current charging, and for detecting the presence or absence of lithium-ion crystals based on the proximity of the minimum value of the change in battery voltage per unit time.

[0011] Furthermore, Japanese Patent Application Publication No. 2019-145342 discloses a method for detecting the presence or absence of lithium-ion battery crystals by detecting changes in the thickness of the battery casing. Summary of the Invention

[0012] However, the methods disclosed in Japanese Patent Application Publication Nos. 2012-003863, 2013-089363, and 2019-145342 can only determine whether lithium-ion battery crystallization has occurred, but cannot determine the surface distribution of lithium-ion battery crystals in the negative electrode active material layer. Therefore, there is a problem that the precursors to lithium-ion battery crystallization cannot be known, and effective control for suppressing lithium-ion battery crystallization cannot be implemented.

[0013] The purpose of this invention is to provide a method for detecting lithium-ion battery crystallization, which can detect the surface distribution of lithium-ion battery crystals in the negative electrode active material layer of a lithium-ion battery to identify the signs of lithium-ion battery crystallization and thus improve the energy efficiency of the lithium-ion battery.

[0014] Based on the above background, the inventors of this application have discovered new knowledge that allows for the high-precision detection of signs of lithium-ion crystallization in the negative electrode active material layer by using a positive electrode SOC map obtained by mapping the distribution of the positive electrode active material layer's depth of charge (SOC).

[0015] In one aspect of the present invention, a method for detecting the electrocrystallization of a lithium-ion battery comprises a positive electrode, a negative electrode, and an electrolyte layer stacked together. The positive electrode has a positive current collector and a positive active material layer located on at least one side of the positive current collector. The negative electrode has a negative current collector and a negative active material layer located on at least one side of the negative current collector, and is opposite to the positive electrode. The electrolyte layer is located between the positive and negative electrodes. The method for detecting the electrocrystallization of the lithium-ion battery detects lithium-ion crystals generated on the negative active material layer. The electrocrystallization detection method includes the following steps: a positive electrode SOC map fabrication step, in which X-ray diffraction measurement of the positive electrode active material layer is performed by irradiating the lithium-ion battery with X-rays, and a positive electrode SOC map representing the distribution of the charging depth of the positive electrode active material layer in the thickness direction is fabricated; a SOC difference value detection step, in which the difference between the highest and lowest values ​​of the positive electrode SOC map, i.e., the SOC difference value, is detected; and an electrocrystallization determination step, in which the SOC difference value is compared with a preset threshold to determine whether lithium-ion battery crystals are present in the negative electrode active material layer.

[0016] According to the present invention, by creating a positive electrode SOC map of the positive electrode active material layer, which is easier to measure based on X-ray diffraction compared to lithium metal and other negative electrode active material layers due to the large atomic scattering factor of X-rays, and calculating the difference between the highest and lowest values ​​of the depth of charge (SOC) in such a positive electrode SOC map, i.e., the SOC difference value, it is possible to easily detect the signs of lithium battery crystallization in the negative electrode active material layer, or the actual occurrence of lithium battery crystallization. Therefore, it is possible to improve the energy efficiency of lithium-ion batteries.

[0017] In the above scheme, the threshold is set to 15% in the electrocrystallization determination process. If the SOC difference is greater than 15%, it is determined that lithium battery crystallization has occurred in the negative electrode active material layer.

[0018] In the above scheme, it is also possible that the X-rays used for the X-ray diffraction measurement in the positive electrode SOC pattern fabrication process are high-energy X-rays with an energy of 40 keV or higher.

[0019] According to the present invention, a method for detecting lithium-ion battery crystallization can be provided, which can detect the surface distribution of lithium-ion battery crystals in the negative electrode active material layer of the lithium-ion battery to know the signs of lithium-ion battery crystallization. Attached Figure Description

[0020] Figure 1 This is a schematic cross-sectional view illustrating an example of the layered structure of a lithium-ion battery.

[0021] Figure 2This is a flowchart illustrating step-by-step an embodiment of the present invention for detecting electrocrystallization in a lithium-ion battery.

[0022] Figure 3 This is an example of a positive electrode SOC diagram.

[0023] Figure 4 This is a graph showing the peak lithium intensity of the negative electrode active material layer.

[0024] Figure 5 It is a magnified photograph of the negative electrode active material layer under visible light. Detailed Implementation

[0025] Hereinafter, with reference to the accompanying drawings, a method for detecting electrocrystallization in a lithium-ion battery according to an embodiment of the present invention will be described. It should be noted that the embodiments described below are specific in order to better understand the gist of the invention, and unless otherwise specified, the invention is not limited. Furthermore, the drawings used in the following description sometimes show enlarged portions of the main parts for the purpose of facilitating understanding of the features of the invention; the dimensions and ratios of the constituent elements may not be the same as in reality.

[0026] (Lithium-ion battery)

[0027] First, let's explain the typical layer structure of a lithium-ion battery.

[0028] Figure 1 This is a schematic cross-sectional view illustrating an example of the layered structure of a lithium-ion battery.

[0029] The lithium-ion battery (LIB) 10 is formed by stacking a positive electrode 13, a negative electrode 16 and an electrolyte layer 17. The positive electrode 13 has a positive current collector 11 and a positive active material layer 12 located on one side of the positive current collector 11. The negative electrode 16 has a negative current collector 14 and a negative active material layer 15 located on one side of the negative current collector 14 and is opposite to the positive electrode 13. The electrolyte layer 17 is located between the positive electrode 13 and the negative electrode 16.

[0030] The positive electrode active material layer 12 comprises a positive electrode active material and a binder, and may contain conductive additives as needed. The positive electrode active material can utilize electrode active materials capable of ion adsorption and release, ion desorption and insertion (intercalation), or ion-to-ion balance anions (e.g., PF6). - The doping and dedoping processes between the dopant and dopant can proceed reversibly.

[0031] Specific examples of positive electrode active materials include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2), lithium manganese spinel (LiMn2O4), and those derived from the general formula: LiNi x Coy Mn z M a O2 (x+y+z+a=1, 0≤x<1, 0≤y<1, 0≤z<1, 0≤a<1, M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, Cr) represents a composite metal oxide, lithium vanadium compound (LiV2O5), olivine-type LiMPO4 (however, M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr or VO), lithium titanate (Li4Ti5O) 12 LiNi x Co y Al z O2 (0.9 < x + y + z < 1.1) and other composite metal oxides, polyacetylene, polyaniline, polypyrrole, polythiophene, polybenzoxene, etc.

[0032] Examples of conductive additives include carbon powders such as carbon black, carbon nanotubes, carbon materials, metal powders such as copper, nickel, stainless steel, and iron, mixtures of carbon materials and metal powders, and conductive oxides such as ITO. Where sufficient conductivity can be ensured using only the positive electrode active material, the positive electrode active material layer 12 may not contain conductive additives.

[0033] Additionally, the positive electrode active material layer 12 may also include a binder. Known binders can be used. Examples include fluoropolymers such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), and polyvinyl fluoride (PVF).

[0034] The negative electrode active material layer 15 contains a negative electrode active material and a binder, and may contain conductive additives as needed. Known negative electrode active materials can be used. Examples of negative electrode active materials include carbon materials such as graphite (natural graphite, artificial graphite) capable of adsorbing and releasing metallic lithium and lithium ions, carbon nanotubes, difficult-to-graphitize carbon, easily-graphitize carbon, and low-temperature sintered carbon, as well as metals capable of combining with lithium such as aluminum, silicon, and tin, and SiO₂. x (0<x<2), amorphous compounds mainly composed of oxides such as tin dioxide, and lithium titanate (Li4Ti5O) 12 Particles such as )

[0035] The conductive additives and binders used in the negative electrode active material layer 15 can be the same as those used in the positive electrode active material layer 12. In addition to the binders mentioned in the positive electrode active material layer 12, the binders used in the negative electrode active material layer 15 can also include carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyimide (PI), polyamide-imide (PAI), polyacrylic acid (PAA), etc.

[0036] When the negative electrode 16, containing such a negative electrode active material layer 15, is charged with the lithium-ion battery 10, the potential changes as lithium ions enter the interlayer of the carbon material, which is an example of a negative electrode active material. However, depending on the charging conditions, lithium metal crystals are formed in such a negative electrode active material layer 15, resulting in capacity degradation (capacity reduction) of the lithium-ion battery 10. By controlling the charging conditions of the lithium-ion battery 10, the increase of such lithium crystal formation can be suppressed, and the capacity degradation of the lithium-ion battery 10 can be delayed. Therefore, it is important to detect the signs of lithium crystal formation in the negative electrode 16.

[0037] (Electrocrystallization detection method)

[0038] Figure 2 This is a flowchart illustrating step-by-step an embodiment of the present invention for detecting electrocrystallization in a lithium-ion battery.

[0039] The method for detecting electrocrystallization of lithium-ion batteries in this embodiment includes a positive electrode SOC diagram fabrication step S1, an SOC differential value detection step S2, and an electrocrystallization determination step S3.

[0040] According to the lithium-ion battery electrocrystallization detection method of this embodiment, when detecting lithium-ion battery electrocrystallization, firstly, the lithium-ion battery 10 to which electrocrystallization is confirmed is irradiated with X-rays to perform X-ray diffraction measurement of the positive electrode active material layer (positive electrode SOC pattern fabrication step S1).

[0041] For X-rays used in X-ray diffraction measurements, high-energy X-rays (synchrotron radiation X-rays) with energies of 40 keV or higher are preferably used. Such high-energy synchrotron radiation X-rays are irradiated towards the stacking direction of the lithium-ion battery. In the positive electrode active material layer 12, the atomic scattering factor of the X-rays is large, thus making it easier to perform X-ray diffraction-based measurements compared to the negative electrode active material layer 15, etc.

[0042] Furthermore, a positive electrode SOC map is created based on X-ray diffraction data obtained by irradiating the lithium-ion battery with X-rays. This positive electrode SOC map represents the distribution of the depth of charge (SOC) of the positive electrode active material layer 12 in the thickness direction. For example, the positive electrode SOC map can be represented by dividing the depth of charge (SOC%) into contour lines on a cross-section of the positive electrode active material layer 12 in the thickness direction from one side to the other, and then color-coding each range. Such a positive electrode SOC map can be created, for example, by inputting the X-ray diffraction data of the positive electrode active material layer 12 into a computer and using image processing software. Figure 3 The diagram shows an example of such a positive electrode SOC diagram.

[0043] according to Figure 3 As shown in the example of the positive electrode SOC diagram, the positive electrode active material layer 12 forms a region with a depth of charge (SOC) of 35% to 45% in the range of -5 mm to -10 mm in the x direction along one side and from 0 mm to 20 mm in the z direction as the thickness direction, and forms a region with a depth of charge (SOC) of 0% to 10% in the range of 0 mm to 10 mm in the x direction and from 5 mm to 25 mm in the z direction.

[0044] Next, the highest and lowest depth of charge (SOC) values ​​are extracted from the positive electrode SOC map obtained in the positive electrode SOC map fabrication process S1. Furthermore, a SOC difference value is calculated, which is the difference between the highest and lowest SOC values ​​(SOC difference detection process S2). This SOC difference value can be easily calculated based on the SOC numerical data obtained during the positive electrode SOC map fabrication.

[0045] Furthermore, the SOC difference value obtained in the SOC difference detection step S2 is compared with a preset threshold to determine whether lithium-ion crystals have formed in the negative electrode active material layer 15 (electrocrystallization determination step S3). In this electrocrystallization determination step S3, the threshold is set to, for example, 15%. If the SOC difference value calculated in the SOC difference detection step S2 is 15% or higher, it is determined that lithium-ion crystals have formed in the negative electrode active material layer 15.

[0046] As described above, according to the lithium-ion battery electrocrystallization detection method of this embodiment, a positive electrode SOC diagram of the positive electrode active material layer 12 is prepared, which is easier to measure based on X-ray diffraction than lithium metal in the negative electrode active material layer 15 due to the large atomic scattering factor of X-rays. The SOC difference value, which is the difference between the highest and lowest values ​​of the depth of charge (SOC) in such a positive electrode SOC diagram, can be calculated, making it easy to detect the signs of lithium battery crystallization occurring in the negative electrode active material layer 15, or the actual occurrence of lithium battery crystallization.

[0047] Therefore, it is possible to control the charging conditions to prevent the progress of lithium crystallization in the negative electrode of the lithium-ion battery, thereby suppressing the reduction of the charging capacity of the lithium-ion battery and seeking to extend its lifespan.

[0048] It should be noted that in the lithium-ion battery electrocrystallization detection method of this embodiment, as long as the high-energy synchrotron radiation X-ray source used for X-ray diffraction measurement is miniaturized, for example, by assembling such a small X-ray source into a lithium-ion battery for vehicle use, the occurrence of lithium-ion battery electrocrystallization in lithium-ion battery can be monitored in real time, and the charging conditions can be controlled according to the occurrence of lithium-ion battery electrocrystallization.

[0049] Furthermore, the method for detecting electrocrystallization of lithium-ion batteries according to this embodiment is applicable to all types of lithium-ion batteries with liquid electrolytes, as well as to all-solid-state lithium-ion batteries with solid electrolytes, and is not limited to specific types of lithium-ion batteries.

[0050] The embodiments of the present invention have been described above, but such embodiments are given as examples and are not intended to limit the scope of the invention. Such embodiments can be implemented in a variety of other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included within the scope and spirit of the invention, and likewise within the scope of the invention described in the technical solution and its equivalents.

[0051] Example

[0052] The effects of the embodiments of the present invention were verified. During the verification, [the following were prepared]... Figure 1 A lithium-ion battery (sample) with the structure shown. The lithium-ion battery used as the sample was repeatedly charged and discharged.

[0053] The lithium-ion battery (sample) was irradiated with high-energy synchrotron X-rays, and X-ray diffraction of the positive electrode active material layer was performed using an XRD apparatus. Then, based on the obtained X-ray diffraction results of the positive electrode active material layer, a positive electrode SOC pattern was prepared. Figure 3 The diagram shows the SOC of the positive electrode.

[0054] Next, the lithium-ion battery (sample) was disassembled and the negative electrode active material layer was removed. X-ray diffraction of the negative electrode active material layer was performed using an XRD device, and a graph of the lithium peak intensity was made based on the results. Figure 4 The graph shows the peak lithium intensity of the negative electrode active material layer. Additionally, Figure 5 The image shows a magnified photograph of the negative electrode active material layer under visible light. It should be noted that... Figure 5 In the image, the lighter-colored areas are where more lithium battery crystals have formed.

[0055] When such Figure 3 The positive electrode SOC diagram of the positive electrode active material layer shown is consistent with... Figure 4 The graph shows the peak lithium intensity of the negative electrode active material layer. Figure 5 When comparing the magnified images of the negative electrode active material layer shown, it can be confirmed that the high SOC portion in the positive electrode active material layer indicates that lithium-ion crystallization has occurred in the corresponding negative electrode active material layer.

[0056] Therefore, it can be confirmed that the positive electrode SOC pattern of the positive electrode active material layer, which is easier to measure by X-ray diffraction compared to the negative electrode active material layer, can be used to detect lithium battery crystallization in the negative electrode active material layer.

[0057] Industrial availability

[0058] The lithium-ion battery electrocrystallization detection method of the present invention uses an XRD apparatus to generate a positive electrode SOC pattern of the positive electrode active material layer, which is easier to measure with X-ray diffraction than the negative electrode active material layer. Based on this positive electrode SOC pattern, lithium-ion battery crystallization in the negative electrode active material layer can be detected. Furthermore, by controlling the charging conditions based on the detection results of lithium-ion battery crystallization, the reduction in the charging capacity of the lithium-ion battery can be suppressed, thereby extending its lifespan and improving its energy efficiency. Therefore, the lithium-ion battery electrocrystallization detection method of the present invention is industrially applicable.

Claims

1. A method for detecting lithium-ion battery crystallization, the lithium-ion battery comprising a positive electrode, a negative electrode, and an electrolyte layer stacked together, wherein the positive electrode has a positive current collector and a positive active material layer located on at least one side of the positive current collector, the negative electrode has a negative current collector and a negative active material layer located on at least one side of the negative current collector and opposite to the positive electrode, and the electrolyte layer is located between the positive electrode and the negative electrode, the method for detecting lithium-ion battery crystallization detecting lithium-ion crystals generated on the negative active material layer, wherein... The method for detecting electrocrystallization in lithium-ion batteries includes the following steps: The positive electrode SOC map fabrication process involves irradiating the lithium-ion battery with X-rays to perform X-ray diffraction measurements of the positive electrode active material layer, and based on the X-ray diffraction data obtained from the X-ray diffraction measurements of the positive electrode active material layer, a positive electrode SOC map representing the distribution of charging depth of the positive electrode active material layer in the thickness direction is fabricated, where the charging depth represents SOC, i.e., the state of charge. The SOC difference detection process detects the difference between the highest SOC value and the lowest SOC value in the positive electrode SOC diagram produced by the positive electrode SOC diagram production process, which is the SOC difference value. as well as The electrocrystallization determination process compares the SOC difference value detected by the SOC difference value detection process with a preset threshold to determine whether there is lithium-ion crystallization in the negative electrode active material layer.

2. The method for detecting electrocrystallization in lithium-ion batteries according to claim 1, wherein, In the electrocrystallization determination process, the threshold is set to 15%. If the SOC difference is greater than 15%, it is determined that lithium-ion crystals have formed in the negative electrode active material layer.

3. The method for detecting electrocrystallization in lithium-ion batteries according to claim 1 or 2, wherein, In the process of creating the positive electrode SOC pattern, the X-rays used for the X-ray diffraction measurement are high-energy X-rays with an energy of 40 keV or higher.

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