Method for evaluating the internal state of a battery

Abstract

To sufficiently and accurately estimate and evaluate an internal state, including a state of an electrolyte, of a battery.SOLUTION: An evaluation method of an internal state of a battery according to the present disclosure is a method for estimating and evaluating the internal state of the battery including an electrolyte. The evaluation method includes: preparing an initial battery and a degraded battery; simultaneously irradiating the initial battery and the degraded battery with first X-ray having first peak energy, and acquiring first intensity of the first X-ray transmitted through each of the initial battery and the degraded battery; simultaneously irradiating the initial battery and the degraded battery with second X-ray having second peak energy, and acquiring second intensity of the second X-ray transmitted through each of the initial battery and the degraded battery; and calculating changes in the salt concentration and thickness of the electrolyte in the degraded battery from the initial battery based on the first intensity and the second intensity.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for evaluating the internal state of a battery containing an electrolyte solution.

Background Art

[0002] As a method for estimating and evaluating the deterioration of a secondary battery containing an electrolyte solution, for example, in Patent Document 1, a lithium-ion secondary battery in which only initial charging is performed (hereinafter referred to as "initial battery") and a battery of the same type in which performance deterioration due to charge and discharge is suspected (hereinafter referred to as "deteriorated battery") are used, and a method for evaluating the internal state of the battery based on the change rate of the X-ray absorption intensity ratio due to the change in the crystal structure (structural distortion) of the positive electrode active material has been proposed.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, when charging and discharging a secondary battery, it is known that a salt concentration distribution of the electrolyte solution occurs between electrodes and within the electrode surface, and accordingly, the internal resistance of the secondary battery increases. This salt concentration distribution becomes prominent during high-current charging or high-current discharging (high-rate charge and discharge), and it is also known that the internal resistance increases significantly accordingly. Such phenomena are called so-called "high-rate deterioration" or "transient deterioration". That is, in a secondary battery, not only the change in the properties of the positive electrode active material but also the performance deterioration due to the change in the properties of the electrolyte solution is recognized. Therefore, it is difficult to sufficiently estimate and evaluate the internal state of the battery by the above conventional method.

[0005] More specifically, in the case of lithium-ion secondary batteries, for example, high-rate charging and discharging can cause an uneven distribution of salt concentration in the thickness direction between the positive and negative electrodes. Furthermore, due to the expansion of the electrolyte caused by heat generation, and / or the expansion of the negative electrode due to changes in charge rate or state of charge (SOC), the electrolyte on the negative electrode side, where the salt concentration is diluted, is pushed out to the outer edge of the electrode, resulting in an uneven distribution of salt concentration in the surface direction of the electrode. In addition, the distance between the positive and negative electrodes may increase due to the expansion of the electrolyte and electrodes. When these phenomena occur, performance degradation progresses due to an increase in the internal resistance of the battery. Therefore, in order to use the battery safely and to eliminate degradation, it is effective to appropriately control the output and input of the battery (charge / discharge control) and to properly design the battery's constraints.

[0006] Therefore, in order to understand the transient internal state related to the degradation of secondary batteries, it is desirable to non-destructively measure the salt concentration distribution of the electrolyte. Considering that as the salt concentration of the electrolyte increases, X-rays are absorbed and become less likely to penetrate, it is conceivable to apply the conventional X-ray irradiation method described above to the electrolyte as well, and to determine the salt concentration distribution of the electrolyte from the X-ray absorption intensity or X-ray transmission intensity of the electrolyte.

[0007] However, since the X-ray absorption intensity and X-ray transmission intensity change depending not only on the salt concentration of the electrolyte but also on its thickness (volume), even if the above conventional method is applied directly to the electrolyte, it is not possible to distinguish between fluctuations in the salt concentration and fluctuations in the thickness of the electrolyte. Therefore, it is conceivable to use multiple types of X-rays with different energies and measure the X-ray absorption intensity or X-ray transmission intensity for each to calculate a solution (unknown) for the two variables: the salt concentration and thickness of the electrolyte.

[0008] However, since X-ray sources typically generate continuous-energy X-rays by colliding an electron beam with a target and causing bremsstrahlung radiation, variations in X-ray output (dose) are likely to occur. Furthermore, the degree of variation in X-ray output differs depending on the energy of the X-rays, and tends to change randomly over time. While it is theoretically possible to suppress such variations in X-ray output by predicting them and controlling the X-ray emission time, such control is extremely difficult in practice from the standpoint of sufficiently improving prediction accuracy and measurement sensitivity. Moreover, if such control is not possible, the variation in X-ray output may become excessive, potentially reducing the accuracy of measuring the salt concentration of the electrolyte to an undesirable degree.

[0009] Therefore, the present invention has been made in view of these circumstances, and aims to provide a method that can sufficiently and accurately estimate and evaluate the internal state of a battery, including the state of the electrolyte. [Means for solving the problem]

[0010] To solve the above problems, an example of a method for evaluating the internal state of a battery according to the present disclosure is a method for evaluating the internal state of a battery containing an electrolyte, and includes: preparing an initial battery and a degraded battery; simultaneously irradiating the initial battery and the degraded battery with first X-rays having a first peak energy and obtaining a first intensity of the first X-rays that have penetrated each of the initial battery and the degraded battery; simultaneously irradiating the initial battery and the degraded battery with second X-rays having a second peak energy and obtaining a second intensity of the second X-rays that have penetrated each of the initial battery and the degraded battery; and calculating the change from the initial battery in the salt concentration and thickness of the electrolyte in the degraded battery based on the first and second intensities.

[0011] In a method for evaluating the internal state of a battery with such a configuration, multiple types of X-rays with different energies are simultaneously irradiated onto the initial battery and the degraded battery, and the intensity of the X-rays that pass through the initial battery and the degraded battery is obtained. Then, using these intensity measurements, the concentration and volume distribution of the electrolyte in the degraded battery is calculated. [Effects of the Invention]

[0012] Based on the above, this disclosure eliminates the need to consider variations in X-ray output irradiated to initial and degraded batteries, thereby suppressing the deterioration of measurement accuracy that has been a concern in the past. As a result, the internal state of the battery, including the state of the electrolyte in degraded batteries, can be estimated and evaluated sufficiently and accurately, making it possible to appropriately implement measures and constraint designs to improve battery performance. [Brief explanation of the drawing]

[0013] [Figure 1] This schematic block diagram shows an example of a hardware configuration for a system for performing a battery state evaluation method according to one embodiment of this disclosure. [Figure 2] This flowchart shows an example of the procedure for evaluating the internal state of a battery using a system according to one embodiment of this disclosure. [Figure 3] This is a schematic conceptual diagram illustrating the process of irradiating an initial cell Ca and a degraded cell Cb with X-rays Rx from an X-ray source 10, and measuring the two-dimensional intensity distribution of X-rays Ra and Rb that have passed through the initial cell Ca and the degraded cell Cb using an X-ray detector 20. [Figure 4] This is a schematic diagram showing an example of X-ray transmission images 50 of an initial cell Ca and a degraded cell Cb, acquired under the arrangement conditions shown in Figure 3. [Figure 5] This is a conceptual diagram schematically showing the relationship between the electrolyte salt concentration distribution that may occur in a degraded cell Cb and the two-dimensional intensity distribution in the X-ray transmission image 50b. [Modes for carrying out the invention]

[0014] Hereinafter, embodiments of the present invention will be described in detail while referring to the drawings. The same elements are denoted by the same reference numerals, and redundant descriptions will be omitted. Also, the ratios of shapes and dimensions are merely examples and are not limited to the illustrations.

[0015] FIG. 1 is a schematic block diagram showing an example of the hardware configuration of a system 100 for executing a method for evaluating the internal state of a battery according to an embodiment of the present disclosure. The system 100 includes, for example, an X-ray source 10 for irradiating an initial cell Ca (initial battery) and a degraded cell Cb (degraded battery) of a lithium-ion secondary battery with an X-ray Rx, and an X-ray detector 20 for measuring the intensities of the X-rays Ra and Rb that have passed through the initial cell Ca and the degraded cell Cb, respectively, to which a control arithmetic unit 30 is connected.

[0016] The X-ray source 10 is not limited as long as it can output an X-ray Rx having a plurality of different types of energies (either a monochromatic spectrum or a continuous spectrum), and in particular, it is preferable that it can output an X-ray Rx having a plurality (at least two) of different energy peaks. In this case, the X-ray Rx emitted at one time may be a dual X-ray having a continuous spectrum including a plurality of energy peaks, or a plurality of X-rays Rx having different single energy peaks may be configured to be emitted separately. More specifically, for example, a generator of dual X-rays by the K filter method using a K absorption edge described in "Takashi Suzuki, Noboru Sakurai: Generation method of dual X-rays and application to bone density measurement, Research Report of Tokyo Metropolitan Institute of Industrial Science and Technology No. 8 (2005), pages 44-46" can be preferably used as the X-ray source 10.

[0017] Furthermore, the X-ray detector 20 is not particularly limited as long as it can simultaneously detect the two-dimensional intensity distribution of X-rays Ra and Rb that have passed through an object such as an initial cell Ca or a degraded cell Cb, and may be configured to visualize or image the two-dimensional intensity distribution. More specifically, in the case of the dual X-rays Rx described above, a multi-channel pulse height analyzer, which is configured to measure the two-dimensional intensity distribution of X-rays Ra and Rb using a NaI(Tl) detector, a signal amplifier, and a pulse height analyzer as described in the same document, can be suitably used as the X-ray detector 20. In this case, for example, a Ge semiconductor detector can be used instead of the NaI(Tl) detector. Also, when an X-ray Rx having a single energy peak is used, the X-ray detector 20 may be configured to measure the two-dimensional intensity distribution of X-rays Ra and Rb using an X-ray film or an FPD (flat panel detector).

[0018] Furthermore, the control calculation unit 30 is configured to include, for example, a control calculation unit 31, a communication interface (I / F) unit 32, a storage unit 33, an input unit 34, and an output unit 35, and these components are connected to each other so as to be able to communicate with one another via a bus line 36.

[0019] More specifically, the control arithmetic unit 31 includes a CPU, a RAM, a ROM, etc., and performs control of each component and various arithmetic operations according to information processing. The communication I / F unit 32 is a communication module for communicating with other components, and the communication method is not particularly limited and is arbitrary. The X-ray source 10 and the X-ray detector 20 are provided so as to be able to communicate with the control arithmetic unit 31 etc. via this communication I / F unit 32. Further, the storage unit 33 is an auxiliary storage device such as an HDD or an SSD, and stores various arithmetic operations, controls, and calculation programs, various setting parameters, various data (bases), etc. As such, by the various programs stored in the storage unit 33 being executed by the control arithmetic unit 31, the processing in the method for evaluating the internal state of a battery, which is an example of the present disclosure, is realized. On the other hand, the input unit 34 is an interface device (for example, a mouse, a keyboard, a touch panel, a voice microphone, etc.) for receiving various input operations from the operation user of the system 100, and the output unit 35 is an interface device (a display, a speaker, a printer, etc.) for notifying the operation user of various information by display, voice, printing, etc.

[0020] Next, FIG. 2 is a flowchart showing an example of a procedure when evaluating the internal state of a battery (deteriorated cell Cb) using the system 100 according to an embodiment of the present disclosure. FIG. 3 is a conceptual diagram schematically showing a state in which the initial cell Ca and the deteriorated cell Cb are irradiated with X-rays Rx from the X-ray source 10, and the two-dimensional intensity distributions of the X-rays Ra and Rb transmitted through the initial cell Ca and the deteriorated cell Cb are measured by the X-ray detector 20. Note that each of the processes described below is merely an example, and each process may be changed as much as possible within the scope of the technical idea of the present disclosure. Also, the processes described below can be appropriately omitted, replaced, and added in steps.

[0021] First, in step S10, the X-ray source 10 and the X-ray detector 20 are prepared, and the initial cell Ca and the degraded cell Cb are placed side by side at a predetermined X-ray irradiation position between them, arranged in an appropriate geometric relationship as shown in Figure 3. Here, the initial cell Ca and the degraded cell Cb have the same cell structure and electrode design, and only the state of the internal electrolyte differs, and in this evaluation method, the initial cell Ca serves as the reference cell for the degraded cell Cb. Also, since the X-rays Rx emitted from the X-ray source 10 generally have an output distribution that attenuates concentrically from the beam center, it is desirable to place the initial cell Ca and the degraded cell Cb so as to be symmetrical with respect to the output distribution of the X-rays Rx. Thus, step S10 corresponds to the process of "preparing the initial cell and the degraded cell".

[0022] Next, in step S20, the initial cell Ca and the degraded cell Cb are simultaneously irradiated with X-rays Rx (first X-ray) having a predetermined peak energy (first peak energy), and the two-dimensional intensity distribution (first intensity) of the X-rays Ra and Rb that pass through the initial cell Ca and the degraded cell Cb, respectively, is measured. Subsequently, the initial cell Ca and the degraded cell Cb are simultaneously irradiated with X-rays Rx (second X-ray) having a different predetermined peak energy (second peak energy), and the two-dimensional intensity distribution (second intensity) of the X-rays Ra and Rb that pass through the initial cell Ca and the degraded cell Cb, respectively, is measured. At this time, it is desirable to irradiate and measure the X-rays Rx at the same position without changing the installation position of the initial cell Ca and the degraded cell Cb when irradiating with X-rays Rx having the first peak energy and X-rays Rx having the second peak energy. As described above, step S20 corresponds to the process of "acquiring the first intensity of the first X-ray" and the process of "acquiring the second intensity of the second X-ray" (i.e., step S20 combines both processes).

[0023] Here, Figure 4 is a schematic diagram showing an example of an X-ray transmission image 50 of an initial cell Ca and a degraded cell Cb acquired under the arrangement conditions shown in Figure 3. In such an X-ray transmission image 50, the X-ray transmission images 50a and 50b of the initial cell Ca and the degraded cell Cb, which have a positive electrode Ct and a negative electrode An, respectively, are projected, and the two-dimensional intensity distribution of transmitted X-rays Ra and Rb can be visualized.

[0024] Figure 5 is a conceptual diagram schematically showing the relationship between the electrolyte salt concentration distribution that may occur in a degraded cell Cb and the two-dimensional intensity distribution in the X-ray transmission image 50b. As mentioned above, in a degraded cell Cb, there is a possibility of uneven distribution of salt concentration in the thickness direction and the planar direction between the positive electrode Ct and the negative electrode An. As shown by curve L1 in Figure 5, for example, the salt concentration is relatively high in the central part L1C in the planar direction, while it is relatively low in the outer edge L1P. Accordingly, in the X-ray transmission image 50b, as shown by curve L2 in Figure 5, for example, the amount of X-ray absorption is relatively large in the central part L2C in the planar direction (the amount of X-ray transmission is relatively small, making the image brightness darker, or the X-ray count is lower), while the amount of X-ray absorption is relatively small in the outer edge L2P (the amount of X-ray transmission is relatively large, making the image brightness brighter, or the X-ray count is higher).

[0025] Next, in step S30, the changes in the salt concentration and thickness of the electrolyte in the degraded cell Cb from the initial cell Ca are calculated, for example, by the following procedure. That is, first, the intensity (brightness) of X-rays Ra and Rb that have passed through the initial cell Ca and the degraded cell Cb are expressed by the relationship shown in equation (1) below. IB i =IA i ·exp{(-μV·ρV·Δt)+(-μL·ρB·Δt·Δc)} …(1) i: Subscript used to distinguish X-rays Rx with a given peak energy. i=1: X-ray Rx (first X-ray) with the first peak energy. i=2: X-ray Rx (second X-ray) with a second peak energy. IA i : Intensity (brightness) of X-rays Ra transmitted through the initial cell Ca. IBi : Intensity (brightness) of X-rays Rb that have passed through a degraded cell Cb. μV: X-ray absorption coefficient of the electrolyte solvent (dependent on the energy of X-rays Rx) μL: X-ray absorption coefficient of the solute in the electrolyte (dependent on the energy of X-ray Rx) ρV: Density of the solvent in the electrolyte solution ρB: Density of the solute in the electrolyte Δc: Change in the salt concentration of the solute in the electrolyte of the degraded cell Cb from the initial cell Ca. Δt: Change in the solvent thickness of the electrolyte in the degraded cell Cb from the initial cell Ca.

[0026] Measured values ​​IA when X-rays Rx (first X-ray) with a first peak energy and X-rays Rx (second X-ray) with a second peak energy are irradiated onto the initial cell Ca and the degraded cell Cb. i IB i Substituting this into equation (1), we obtain the following equations (2) and (3). Note that the measured value IA i IB i This uses measurements taken at the same in-plane position in the initial Ca cell and the degraded Cb cell. IB1=IA1·exp{(-μV·ρV·Δt)+(-μL·ρB·Δt·Δc)}…(2) IB2=IA2·exp{(-μV·ρV·Δt)+(-μL·ρB·Δt·Δc)}…(3)

[0027] Thus, two equations are obtained for the two unknowns Δc and Δt. By solving these simultaneous equations, the change in the salt concentration of the solute in the electrolyte of the degraded cell Cb from the initial cell Ca (Δc) and the change in the solvent thickness of the electrolyte of the degraded cell Cb from the initial cell Ca (Δt) can be determined. In this way, step S30 corresponds to the process of "calculating the changes in the salt concentration and thickness of the electrolyte in the degraded battery from the initial battery."

[0028] Then, in step S40, the internal state of the degraded cell Cb can be evaluated from the change in salt concentration Δc and the change in thickness Δt of the obtained electrolyte, which can contribute to optimizing the control processing of the output and input of the secondary battery cell.

[0029] According to the battery internal state evaluation method and system 100 for performing it, which are part of one embodiment of the present disclosure including the steps described above, X-rays Rx having different peak energies are simultaneously irradiated onto the initial cell Ca and the degraded cell Cb, and the intensity of X-rays Ra and Rb transmitted through each cell is measured simultaneously. This suppresses the deterioration of measurement accuracy caused by variations in conventional X-ray output. In other words, conventionally, the intensity of transmitted X-rays Ra and Rb measured in advance for the initial cell Ca is used. However, this inevitably leads to measurement errors in the intensity of transmitted X-rays Ra and Rb for the degraded cell Cb because the output of X-rays Rx when measured separately for the degraded cell Cb is different. In contrast, according to one embodiment of the present disclosure, the occurrence of such measurement errors can be suppressed. As a result, the distribution of electrolyte concentration and amount in the degraded cell Cb can be accurately grasped, making it possible to appropriately implement measures to improve battery performance. Furthermore, since the in-plane distribution of electrolyte concentration and volume in degraded cells (Cb) can be determined, the locations and extent to which fluctuations in electrolyte concentration and volume should be controlled can be identified, enabling effective implementation of constraint control design to improve battery performance.

[0030] The above-described embodiments, which serve as an example of this disclosure, have been explained in detail. However, the above-described explanation is merely an example of this disclosure in all respects, and it goes without saying that various improvements and modifications can be made without departing from the scope of this disclosure. Furthermore, the above embodiments may be partially substituted or combined as appropriate. [Explanation of Symbols]

[0031] 10...X-ray source, 20...X-ray detector, 30...Control calculation unit, 31...Control calculation unit, 32...Communication interface (I / F) unit, 33...Storage unit, 34...Input unit, 35...Output unit, 36...Bus line, 50, 50a, 50b...X-ray transmission image, 100...System, An...Negative electrode, Ca...Initial cell (initial battery), Cb...Degraded cell (degraded battery), Ct...Positive electrode, L1, L2...Curve, L1C, L2C...Center, L1P, L2P...Outer edge, Ra, Rb...Transmitted X-ray, Rx...X-ray (1st X-ray, 2nd X-ray), S10~S40...Step, Δc...Change in salt concentration of solute in electrolyte in degraded cell Cb from initial cell Ca, Δt...Change in solvent thickness of electrolyte in degraded cell Cb from initial cell Ca.

Claims

[Claim 1] A method for evaluating the internal state of a battery containing an electrolyte, Prepare both new and degraded batteries, The initial battery and the degraded battery are simultaneously irradiated with a first X-ray having a first peak energy, and the first intensity of the first X-ray that has passed through each of the initial battery and the degraded battery is obtained. The initial battery and the degraded battery are simultaneously irradiated with a second X-ray having a second peak energy, and the second intensity of the second X-ray that has passed through each of the initial battery and the degraded battery is obtained. Based on the first and second intensities, the change in the salt concentration and thickness of the electrolyte in the degraded battery from the initial battery is calculated, A method for evaluating the internal state of a battery, including its internal state.

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