Electrodeposition detection method for lithium-ion batteries

Abstract

To provide a method for detecting the electrocrystallization of a lithium ion battery which detects the surficial distribution of lithium electrocrystallization in the anode active material layer of the lithium ion battery and know a sign that lithium electrocrystallization will occur, thereby enabling improvement in energy efficiency of the lithium ion battery.SOLUTION: The method comprises: a cathode SOC map creation step of irradiating a lithium ion battery with an X-ray, conducting X-ray diffraction measurement of a cathode active material layer, and creating a cathode SOC map that indicates the distribution of charge depth along the thickness direction of the cathode active material layer; an SOC difference value detection step of detecting an SOC difference value that represents the difference between the maximum and minimum values of the cathode SOC map; and a determination step of comparing the SOC difference value with a preset threshold to determine the presence of lithium electrocrystallization of an anode active material layer.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for detecting electrodeposition in a lithium ion battery, the method detecting lithium electrodeposition occurring in a negative electrode active material layer of the lithium ion battery. [Background technology]

[0002] 2. Description of the Related Art Vehicles such as EVs (Electric Vehicles) and HEVs (Hybrid Electrical Vehicles) are equipped with power storage devices that supply power to motors, etc. The power storage devices are provided with a plurality of secondary batteries.

[0003] Secondary batteries installed in EVs and HEVs are required to have a certain output, but generally, when a secondary battery is left unused for a medium to long period of time, the surface of the electrodes is covered with an oxide film, which reduces the output characteristics. When the output of a secondary battery decreases in this way, it is generally determined that the output performance has deteriorated, and the secondary battery is controlled to reduce its output. This is because if the output of a deteriorated secondary battery is requested to be at the level it was before the deterioration, the secondary battery will be overloaded, and the deterioration of the life of the secondary battery will be accelerated.

[0004] In order to eliminate deterioration of a secondary battery, it is necessary to perform a predetermined discharge process, a so-called refresh process, to activate the secondary battery, but EVs and HEVs are not designed to perform activation processes with the secondary battery mounted on the vehicle. For this reason, for example, Patent Document 1 discloses that when a lead-acid battery is used as the secondary battery, the SOC (State Of Charge) usage range is increased when the battery deteriorates.

[0005] Meanwhile, lithium-ion batteries (LIBs) are widely used as secondary batteries for EVs and HEVs. Lithium-ion batteries are lightweight and have high energy density, making them ideal for use as high-output power sources on board vehicles.

[0006] Lithium-ion batteries generally use carbon materials such as graphite as the negative electrode active material, and the potential changes when lithium ions enter between the layers of the carbon material during charging. However, depending on the charging conditions of the lithium-ion battery, metallic lithium may be deposited on the negative electrode active material (lithium electrodeposition).

[0007] It is known that the occurrence of lithium electrodeposition leads to a deterioration in the capacity of lithium-ion batteries, so charging is controlled to be performed within a preset upper limit voltage range. Here, lithium electrodeposition refers to the deposition of lithium metal on the surface of the negative electrode as a result of the electrical reduction of lithium ions. Therefore, when a lithium-ion battery deteriorates, the charge amount and regenerative output decrease compared to when it was new, resulting in a problem of a shorter driving distance. For this reason, technologies for detecting the occurrence of lithium electrodeposition have been proposed.

[0008] For example, Patent Document 2 discloses a method for detecting the presence or absence of lithium electrodeposition by detecting the presence or absence of a minimum point, which is a point indicating a minimum value at which the charging current changes from a decrease to an increase. Furthermore, Patent Document 3 discloses a method for detecting the amount of change per unit time of a battery voltage that gradually increases during constant current charging, and detecting the presence or absence of lithium electrodeposition from the vicinity of the minimum value of the amount of change per unit time of the battery voltage. Furthermore, Patent Document 4 discloses a method for detecting the presence or absence of lithium electrodeposition by detecting a change in the thickness dimension of a battery case. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] JP 2001-339864 A [Patent Document 2] JP 2012-003863 A [Patent Document 3] JP 2013-089363 A [Patent Document 4] JP 2019-145342 A Summary of the Invention [Problem to be solved by the invention]

[0010] However, in the methods disclosed in Patent Documents 2 to 4, only the occurrence or non-occurrence of lithium electrodeposition can be known, and the planar distribution of lithium electrodeposition in the negative electrode active material layer cannot be grasped. Therefore, there is a problem that it is not possible to know the sign of lithium electrodeposition occurrence, and it is not possible to effectively control the occurrence of lithium electrodeposition.

[0011] The present invention has been proposed in view of the above-mentioned problems, and has an object to provide a method for detecting electrodeposition in a lithium ion battery, which is capable of detecting the planar distribution of lithium electrodeposition in a negative electrode active material layer of a lithium ion battery and detecting signs of lithium electrodeposition, thereby improving the energy efficiency of the lithium ion battery. [Means for solving the problem]

[0012] In light of the above-mentioned background, the present inventors have discovered a new finding that by using a positive electrode SOC map obtained by mapping the distribution of the depth of charge (SOC) of a positive electrode active material layer, it is possible to detect with high accuracy a sign of the occurrence of lithium electrodeposition in a negative electrode active material layer.

[0013] That is, the electrodeposition detection method for a lithium ion battery of the present invention is a method for detecting lithium electrodeposition occurring in a negative electrode active material layer of a lithium ion battery in which a positive electrode current collector and a positive electrode active material layer located on at least one surface of the positive electrode current collector, a negative electrode current collector and a negative electrode active material layer located on at least one surface of the negative electrode current collector, a negative electrode facing the positive electrode, and an electrolyte layer located between the positive electrode and the negative electrode are laminated, the method comprising the steps of: irradiating the lithium ion battery with X-rays to perform X-ray diffraction measurement of the positive electrode active material layer, and creating a positive electrode SOC map showing a distribution of the depth of charge along the thickness direction of the positive electrode active material layer; detecting an SOC difference value, which is the difference between the maximum value and the minimum value of the positive electrode SOC map; and comparing the SOC difference value with a preset threshold value to determine the presence or absence of lithium electrodeposition in the negative electrode active material layer.

[0014] According to the present invention, a positive electrode SOC map of the positive electrode active material layer, which is easier to measure by X-ray diffraction compared to lithium metal in the negative electrode active material layer due to a large atomic scattering factor of X-rays, is created, and an SOC difference value, which is the difference between the maximum and minimum depth of charge (SOC) values ​​in such a positive electrode SOC map, is calculated, thereby making it possible to easily detect signs of lithium electrodeposition in the negative electrode active material layer or the actual occurrence of lithium electrodeposition, thereby enabling the energy efficiency of the lithium ion battery to be improved.

[0015] In the present invention, in the electrodeposition determining step, the threshold value may be set to 15%, and it may be determined that lithium electrodeposition has occurred in the negative electrode active material layer when the SOC difference value is 15% or more.

[0016] In the present invention, in the positive electrode SOC map creation step, the X-rays used in the X-ray diffraction measurement are preferably high-energy X-rays having an energy of 40 keV or more. Effect of the Invention

[0017] According to the present invention, it is possible to provide a method for detecting electrodeposition in a lithium ion battery, which is capable of detecting the planar distribution of lithium electrodeposition in a negative electrode active material layer of a lithium ion battery and detecting a sign of the occurrence of lithium electrodeposition. [Brief description of the drawings]

[0018] [Figure 1] FIG. 2 is a schematic cross-sectional view showing an example of a layer structure of a lithium ion battery. [Diagram 2] 1 is a flowchart showing a step-by-step method for detecting electrodeposition in a lithium ion battery according to one embodiment of the present invention. [Diagram 3] 1 is an example of a positive electrode SOC map. [Figure 4] 1 is a map of lithium peak intensity of a negative electrode active material layer. [Diagram 5] 1 is an enlarged photograph of a negative electrode active material layer taken under visible light. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Hereinafter, a method for detecting electrodeposition in a lithium ion battery according to one embodiment of the present invention will be described with reference to the drawings. The following embodiment is specifically described to better understand the gist of the invention, and does not limit the present invention unless otherwise specified. In addition, the drawings used in the following description may show essential parts in an enlarged scale for the sake of convenience in order to make the features of the present invention easier to understand, and the dimensional ratios of each component may not necessarily be the same as in reality.

[0020] (Lithium-ion battery) First, the layer structure of a typical lithium ion battery will be described. FIG. 1 is a schematic cross-sectional view showing an example of a layer structure of a lithium ion battery. The lithium-ion battery (LIB) 10 includes a positive electrode 13 having a positive electrode current collector 11 and a positive electrode active material layer 12 located on one surface of the positive electrode current collector 11, a negative electrode 16 having a negative electrode current collector 14 and a negative electrode active material layer 15 located on one surface of the negative electrode current collector 14 and facing the positive electrode 13, and an electrolyte layer 17 located between the positive electrode 13 and the negative electrode 16, which are laminated.

[0021] The positive electrode active material layer 12 has a positive electrode active material and a binder, and optionally has a conductive aid. The positive electrode active material can reversibly carry out ion occlusion and release, ion desorption and insertion (intercalation), or doping and dedoping with ions and ion counter anions (for example, PF6 - ).) using an electrode active material.

[0022] Specific examples of the positive electrode active material include, for example, lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), lithium manganate (LiMnO2), lithium manganese spinel (LiMn2O4), and the general formula: LiNi x Co y 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), lithium vanadium compound (LiV2O5), olivine type LiMPO4 (where M is 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, polyacene, etc.

[0023] The conductive additive includes, for example, carbon powders such as carbon blacks, carbon nanotubes, carbon materials, metal fine powders such as copper, nickel, stainless steel, and iron, mixtures of carbon materials and metal fine powders, and conductive oxides such as ITO. When sufficient conductivity can be ensured only by the positive electrode active material, the positive electrode active material layer 12 may not contain a conductive additive.

[0024] Also, the positive electrode active material layer 12 may contain a binder. Known binders can be used. For example, fluororesins 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) can be mentioned.

[0025] The negative electrode active material layer 15 has a negative electrode active material and a binder, and may have a conductive additive as necessary. As the negative electrode active material, known negative electrode active materials can be used. Examples of the negative electrode active material include metallic lithium, graphite (natural graphite, artificial graphite) capable of occluding and releasing lithium ions, carbon nanotubes, graphitization-resistant carbon, graphitization-easy carbon, carbon materials such as low-temperature calcined carbon, metals that can combine with lithium such as aluminum, silicon, and tin, x (0 < x < 2), amorphous compounds mainly composed of oxides such as tin dioxide, and particles containing lithium titanate (Li4Ti5O 12 ) etc. can be mentioned.

[0026] The conductive additive and the binder of the negative electrode active material layer 15 can be the same as those of the positive electrode active material layer 12. In addition to those mentioned for the positive electrode active material layer 12, binders that can be used for the negative electrode active material layer 15 include, for example, carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyimide (PI), polyamideimide (PAI), polyacrylic acid (PAA), etc.

[0027] When the lithium-ion battery 10 is charged, the potential of the negative electrode 16 including the negative electrode active material layer 15 changes as lithium ions enter between layers in a carbon material, which is an example of a negative electrode active material, but depending on the charging conditions, lithium electrodeposition occurs in the negative electrode active material layer 15, in which metallic lithium is deposited, causing capacity degradation (capacity reduction) of the lithium-ion battery 10. By controlling the charging conditions of the lithium-ion battery 10, the increase in lithium electrodeposition can be suppressed and the capacity degradation of the lithium-ion battery 10 can be slowed down. For this reason, it is important to detect the signs of lithium electrodeposition in the negative electrode 16.

[0028] (Electrodeposition detection method) FIG. 2 is a flow chart showing a step-by-step method for detecting electrodeposition in a lithium ion battery according to one embodiment of the present invention. The electrodeposition detection method for a lithium ion battery according to the present embodiment includes a positive electrode SOC map creation step S1, an SOC difference value detection step S2, and an electrodeposition determination step S3.

[0029] When detecting lithium electrodeposition in a lithium ion battery by the electrodeposition detection method for a lithium ion battery of this embodiment, first, X-rays are irradiated to the lithium ion battery 10 for which the occurrence of electrodeposition is to be confirmed, and X-ray diffraction measurement of the positive electrode active material layer is performed (positive electrode SOC map creation step S1).

[0030] The X-rays used in the X-ray diffraction measurement are preferably high-energy X-rays (synchrotron radiation X-rays) with an energy of, for example, 40 keV or more. Such high-energy synchrotron radiation X-rays are irradiated in the stacking direction of the lithium-ion battery. The positive electrode active material layer 12 has a large atomic scattering factor of X-rays, and therefore is easier to measure by X-ray diffraction than the negative electrode active material layer 15, etc.

[0031] Then, based on the X-ray diffraction data obtained by irradiating the lithium ion battery with X-rays, a positive electrode SOC map showing the distribution of the charge depth along the thickness direction of the positive electrode active material layer 12 is created. The positive electrode SOC map may be, for example, a map in which the charge depth (SOC (%)) is divided into several numerical ranges by isopleths in a cross section in the thickness direction from one surface to the other surface of the positive electrode active material layer 12, and each range is color-coded. 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 or the like. An example of such a positive electrode SOC map is shown in FIG. 3.

[0032] According to an example of the positive electrode SOC map shown in FIG. 3, it can be seen that in positive electrode active material layer 12, a region where the depth of charge (SOC) is about 35% to 45% is formed in a range of −5 mm to −10 mm in the x direction along one surface, and from 0 mm to 20 mm in the z direction, which is the thickness direction, and a region where the depth of charge (SOC) is about 0% to 10% is formed in a range of 0 mm to 10 mm in the x direction, and from 5 mm to 25 mm in the z direction.

[0033] Next, the maximum and minimum state of charge (SOC) values ​​are extracted from the positive electrode SOC map obtained in the positive electrode SOC map creation step S1. Then, a SOC difference value, which is the difference between the maximum and minimum state of charge (SOC) values, is calculated (SOC difference value detection step S2). Such a SOC difference value can be easily calculated from the numerical data of the state of charge (SOC) when creating the positive electrode SOC map.

[0034] Then, the SOC difference value obtained in the SOC difference value detection step S2 is compared with a preset threshold value to determine the presence or absence of lithium electrodeposition in the negative electrode active material layer 15 (electrodeposition determination step S3). In this electrodeposition determination step S3, the above-mentioned threshold value may be set to, for example, 15%, and it may be determined that lithium electrodeposition has occurred in the negative electrode active material layer 15 when the SOC difference value calculated in the SOC difference value detection step S2 is 15% or more.

[0035] As described above, according to the method for detecting electrodeposition in a lithium ion battery of the present embodiment, a positive electrode SOC map of the positive electrode active material layer 12, which has a large atomic scattering factor of X-rays and is therefore easier to measure by X-ray diffraction compared to, for example, lithium metal in the negative electrode active material layer 15, is created, and an SOC difference value, which is the difference between the maximum and minimum depth of charge (SOC) values ​​in such a positive electrode SOC map, is calculated, thereby making it possible to easily detect signs of lithium electrodeposition occurring in the negative electrode active material layer 15 or the actual occurrence of lithium electrodeposition.

[0036] This makes it possible to control charging conditions so as not to cause lithium electrodeposition in the negative electrode of the lithium ion battery, thereby suppressing the decrease in the charge capacity of the lithium ion battery and extending its life.

[0037] In the method for detecting electrodeposition in a lithium-ion battery according to the present embodiment, if the high-energy synchrotron X-ray source used in the X-ray diffraction measurement is miniaturized, then, for example, by incorporating such a small X-ray source in an on-board lithium-ion battery, it will be possible to monitor the occurrence of lithium electrodeposition in the lithium-ion battery in real time and control the charging conditions in accordance with the occurrence status of lithium electrodeposition.

[0038] In addition, the electrodeposition detection method for lithium ion batteries of the present embodiment can be applied to any of various lithium ion batteries in which the electrolyte is liquid and all-solid-state lithium ion batteries in which the electrolyte is solid, and is not limited to being applied to a specific type of lithium ion battery.

[0039] Although the embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope of the invention and its equivalents described in the claims, as well as in the scope and spirit of the invention. EXAMPLES

[0040] The effects of the present invention were verified. For the verification, a lithium ion battery (sample) having a configuration as shown in Fig. 1 was prepared. This lithium ion battery sample had been fully charged and discharged multiple times.

[0041] This lithium-ion battery (sample) was irradiated with high-energy synchrotron radiation X-rays, and the X-ray diffraction measurement of the positive electrode active material layer was performed using an XRD device. Then, based on the obtained X-ray diffraction results of the positive electrode active material layer, a positive electrode SOC map was created. This positive electrode SOC map is shown in Figure 3.

[0042] Next, the lithium ion battery (sample) was disassembled to remove the negative electrode active material layer, and the negative electrode active material layer was subjected to X-ray diffraction measurement using an XRD device. Based on the results, a map of lithium peak intensity was created. This map of lithium peak intensity of the negative electrode active material layer is shown in Figure 4. Also, an enlarged photograph of the negative electrode active material layer under visible light is shown in Figure 5. In Figure 5, the lighter the color, the more lithium electrodeposition has occurred in the area.

[0043] When comparing the positive electrode SOC map of the positive electrode active material layer shown in FIG. 3 with the lithium peak intensity map of the negative electrode active material layer shown in FIG. 4 and the enlarged photograph of the negative electrode active material layer shown in FIG. 5, it can be confirmed that lithium electrodeposition has occurred in the corresponding negative electrode active material layer in the area with high SOC in the positive electrode SOC map of the positive electrode active material layer. This demonstrated that it is possible to detect lithium electrodeposition in the negative electrode active material layer based on the positive electrode SOC map of the positive electrode active material layer, which is easier to measure with X-ray diffraction than the negative electrode active material layer. [Industrial Applicability]

[0044] The method for detecting electrodeposition in a lithium ion battery of the present invention uses an XRD device to create a positive electrode SOC map of a positive electrode active material layer, which is easier to measure by X-ray diffraction than a negative electrode active material layer, and enables detection of lithium electrodeposition in the negative electrode active material layer based on this positive electrode SOC map. Then, by controlling the charging conditions according to the detection result of such lithium electrodeposition, it becomes possible to suppress the decrease in the charge capacity of the lithium ion battery, extend the life of the battery, and improve the energy efficiency of the lithium ion battery. Therefore, the method has industrial applicability. [Explanation of symbols]

[0045] 10...Lithium-ion battery 11...Positive electrode current collector 12...Cathode active material layer 13...Positive electrode 14...Negative electrode current collector 15...Negative electrode active material layer 16...Negative electrode 17...Electrolyte layer

Claims

1. A method for detecting electrodeposition of lithium in a lithium ion battery, the method comprising: a positive electrode having a positive electrode current collector and a positive electrode active material layer located on at least one surface of the positive electrode current collector; a negative electrode having a negative electrode current collector and a negative electrode active material layer located on at least one surface of the negative electrode current collector, the negative electrode facing the positive electrode; and an electrolyte layer located between the positive electrode and the negative electrode, the method comprising the steps of: a positive electrode SOC map creating step of irradiating the lithium ion battery with X-rays to perform X-ray diffraction measurement of the positive electrode active material layer and creating a positive electrode SOC map showing a distribution of depth of charge along a thickness direction of the positive electrode active material layer; an SOC difference value detection step of detecting an SOC difference value which is a difference between a maximum value and a minimum value of the positive electrode SOC map; and an electrodeposition determination step of comparing the SOC difference value with a preset threshold value to determine the presence or absence of lithium electrodeposition in the negative electrode active material layer.

2. 2. The method for detecting electrodeposition in a lithium ion battery according to claim 1, wherein in the electrodeposition determination step, the threshold value is set to 15%, and when the SOC difference value is 15% or more, it is determined that lithium electrodeposition is occurring in the negative electrode active material layer.

3. 3. The method for detecting electrodeposition in a lithium ion battery according to claim 1, wherein in the positive electrode SOC map creation step, the X-rays used in the X-ray diffraction measurement are high-energy X-rays having an energy of 40 keV or more.

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