Methods for diagnosing degradation of secondary batteries

Abstract

To provide a secondary battery deterioration diagnosis method that easily and accurately acquires residual capacity information on a post-deteriorated secondary battery with an usage history being unknown.SOLUTION: A secondary battery deterioration diagnosis method using an AC impedance method comprises a pre-deterioration battery measurement step (S1); a characteristic preparation step (S3); a post-deterioration battery measurement step (S4); a deviation-amount estimation step (S7); and a state-of-health determination step (S8). The deviation-amount estimation step (S7) is configured to estimate a deviation degree of charge capacity of positive and negative poles with respect to before and after the deterioration on the basis of a relation characteristic prepared by the characteristic preparation step (S3) and a plurality of pieces of measurement data on impedance acquired by the post-deterioration battery measurement step (S4). The state-of-health determination step (S8) is configured to determine a state of health SOH of the post-deterioration secondary battery, deeming the deviation degree of the charge capacity of the positive and negative poles equivalent to a deterioration amount of the post-deterioration second battery.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for diagnosing the degree of degradation (remaining capacity) of a degraded secondary battery whose usage history is unknown. [Background technology]

[0002] Lithium-ion batteries have been used as power sources for portable electronic devices for over 30 years since their commercialization, and recently their use has rapidly expanded to include power sources for electric vehicles and smart grids. Furthermore, from a sustainability perspective, there is a growing movement to reuse batteries after their initial use has ended. For this reason, it is necessary to understand the health of lithium-ion batteries, but there are not enough evaluation and diagnostic methods.

[0003] Traditionally, to evaluate battery performance (initial performance, remaining performance), charge and discharge curves have been drawn by charging and discharging under stable conditions to obtain the charge and discharge capacities. In addition, internal resistance has been obtained from the change in response voltage when energized to an open-circuit voltage using the electrochemical pulse method (DC method) or the AC impedance method. In particular, the rate of change in charge and discharge capacity before and after degradation has been used as an indicator of the degradation state, known as the battery's state of health or degradation (SOH). Hereinafter, the battery's state of health or degradation will be referred to as "SOH".

[0004] The SOH of a lithium-ion battery can be explained by the deviation in the state of charge or charge capacity (State of Charge: SOC) of the positive and negative electrodes, which is the deviation in the reaction regions of the positive and negative electrodes constituting the battery (see Non-Patent Document 1). However, the causes of the deviation are roughly classified into the formation of SEI (abbreviation for "Solid Electrolyte Interphase") due to the deterioration of the electrolyte with the consumption of lithium ions, the deterioration of the positive electrode with the consumption of lithium ions, and the deterioration of the negative electrode with the consumption of lithium ions. Also, the deviation ratio for each cause varies depending on the usage history of the battery (such as battery temperature, battery voltage, number of charge-discharge cycles, etc.). In particular, lithium-ion batteries for driving electric vehicles, etc., are used in a wide temperature environment (-40°C to +60°C), and during acceleration and deceleration, complex charging and discharging (charge-discharge) due to output / regeneration are performed, so it is considered that the deterioration reactions are various. Hereinafter, the state of charge or charge capacity of the battery is referred to as "SOC".

[0005] Generally, for the SOH diagnosis of a lithium-ion battery with an unknown usage history, it is ideal to evaluate the amount of electricity (charge capacity) from the discharged state to full charge under a stable environment, but considering the cost and operation aspects, it is not realistic. Therefore, as another SOH diagnosis, for batteries deteriorated under various conditions in advance, the SOH and the alternating current impedance spectrum are acquired, and after data processing, they are made into a database. And a method has been proposed to collate the alternating current impedance spectrum obtained for a battery with an unknown usage history with the database to predict the SOH (see Patent Documents 1 and 2).

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0007]

Non-Patent Document 1

[0008] The conventional methods for SOH (State of Health) diagnosis to determine remaining capacity, as disclosed in Patent Documents 1 and 2, have the following problems. While measuring charge and discharge capacity in SOH diagnosis is highly accurate because it directly measures capacity, it requires a significant amount of time. Furthermore, depending on the operating conditions of the product (for example, lithium-ion batteries in vehicles), simple charging and discharging may be difficult. Analysis by matching AC impedance spectra with a database in SOH diagnosis requires time and cost to prepare the database. While the diagnosis is quick and simple, depending on the range of degradation data conditions prepared in advance and the amount of information in the database, there is a concern that it may contain large errors for lithium-ion batteries, which exhibit diverse degradation behaviors.

[0009] Detailed analysis of degradation factors using charge-discharge curve analysis (charge-discharge differential curve analysis) to understand the mechanism of capacity reduction can separate the causes of the state of charge (SOC) difference between the positive and negative electrodes: "SEI formation due to electrolyte degradation accompanied by lithium ion consumption," "positive electrode degradation accompanied by lithium ion consumption," and "negative electrode degradation accompanied by lithium ion consumption." However, this analysis requires charge-discharge measurements at low current values ​​(low C-rate) for each battery with an unknown usage history, which is time-consuming. Furthermore, curve analysis requires intermediate to advanced analytical techniques.

[0010] This invention has been made in view of the above-mentioned problems, and aims to provide a degradation diagnosis method that can easily and accurately obtain information on the remaining capacity of a degraded secondary battery whose usage history is unknown. [Means for solving the problem]

[0011] To achieve the above objective, the present invention has developed a method for quantitatively evaluating the difference in charge capacity (SOC difference) between the positive and negative electrodes before and after degradation using impedance measurement data that is easy to obtain, and for diagnosing the degree of degradation (SOH) of a degraded secondary battery with an unknown usage history. The solution is a method for diagnosing the degradation of a secondary battery using the AC impedance method, comprising a pre-degradation battery measurement step, a characteristic creation step, a post-degradation battery measurement step, a difference amount estimation step, and a degradation degree determination step. The difference amount estimation step estimates the amount of SOC difference between the positive and negative electrodes before and after degradation based on the relationship characteristics created in the characteristic creation step and multiple impedance measurement data obtained in the post-degradation battery measurement step. The degradation degree determination step considers the amount of SOC difference between the positive and negative electrodes to be equivalent to the amount of capacity degradation of the degraded secondary battery, and determines the degree of degradation of the degraded secondary battery. The relationship characteristic of the impedance (Ω) curve with respect to the charge capacity of the secondary battery before degradation, created in the characteristic creation step, is called the pre-normalization characteristic. The normalization step involves determining the charge capacity with the lowest impedance (Ω) from the pre-normalization characteristic, calculating the impedance ratio of the charge capacity at other measurement points so that the lowest impedance (Ω) becomes 100%, and normalizing it to the relationship characteristic of the impedance ratio (%) curve with respect to the charge capacity of the secondary battery before degradation. The deviation amount estimation step estimates the SOC deviation amount of the positive and negative electrodes of the degraded secondary battery using the relationship characteristic of the impedance ratio (%) curve with respect to the normalized charge capacity and the impedance ratio (%) calculated from measurement data of impedances depending on the charge state, which includes at least the full charge range and the discharge range. [Effects of the Invention]

[0012] Thus, the degradation diagnosis method of the present invention utilizes the state of charge (SOC) dependence of impedance in a secondary battery and determines the degree of degradation of a secondary battery after degradation by estimating the amount of SOC difference between the positive and negative electrodes before and after degradation. As a result, information on the remaining capacity of a degraded secondary battery with an unknown usage history can be obtained simply and accurately. Furthermore, by normalizing the curve characteristics, the effects of electrode size and the degradation of the positive or negative electrode are eliminated, allowing the SOC dependence of the secondary battery's impedance to be evaluated on the same scale before and after degradation. In addition, when diagnosing the degradation of a degraded secondary battery with an unknown usage history, it is only necessary to obtain impedance measurement data for at least two charge states that allow for the calculation of the impedance ratio (%) for the degraded secondary battery. As a result, impedance measurements do not require a significant amount of time, enabling quick and easy impedance measurement of degraded secondary batteries and facilitating a simple SOH diagnosis that accurately determines the degree of degradation. [Brief explanation of the drawing]

[0013] [Figure 1] This flowchart shows the procedure for diagnosing the State of Health (SOH) of a degraded lithium-ion battery with an unknown usage history. [Figure 2] This is a flowchart showing the procedure for normalizing the charge state (capacity) vs. impedance curve. [Figure 3] This is a flowchart showing the procedure for assigning the impedance of a lithium-ion battery. [Figure 4] This is an example showing the discharge curve characteristics of a lithium-ion battery (a) and the state of charge (SOC) dependence of the impedance ratio (b). [Figure 5]This is an example showing the amount of SOC deviation in SOH diagnosis for a pre-degradation lithium-ion battery (SOH 100%) and an artificially created post-degradation lithium-ion battery (SOH 90%, SOH 80%) (a), and a comparison table (b) of measured capacity, measured SOH, SOC deviation, and estimated SOH. [Figure 6] This is an example showing the curve characteristics before normalization (a) and after normalization (b) using the positive electrode impedance of an undegraded battery and four different types of cycle-degraded batteries. [Figure 7A] This is an example showing the state of charge (SOC) dependence of the impedance of a lithium-ion battery, as well as the Nyquist plot and Bode plot at 50% SOC. [Figure 7B] This is an example showing the state of emergency (SOC) dependence of the impedance of a positive half-cell, and the Nyquist plot and Bode plot at 50% SOC. [Figure 7C] This is an example showing the state of emergency (SOC) dependence of the impedance of a negative half-cell, and the Nyquist plot and Bode plot at SOC 50%. [Modes for carrying out the invention]

[0014] Hereinafter, an embodiment for implementing the secondary battery degradation diagnosis method according to the present invention will be described based on Example 1 shown in the drawings. [Examples]

[0015] The degradation diagnosis method of Example 1 is applied to a lithium-ion battery with an unknown usage history (an example of a secondary battery that charges and discharges by the movement of lithium ions between the positive and negative electrodes) and is a degradation diagnosis method that determines the state of health (SOH) of the lithium-ion battery after degradation. The degradation diagnosis method of Example 1 will be explained below in the following sections: "Explanation of the degradation diagnosis procedure," "Degradation diagnosis effect," "Normalization effect of the impedance curve," "Positive and negative electrode assignment effect of impedance," and "Effects of the degradation diagnosis method." Here, the lithium-ion battery with an unknown usage history to which the degradation diagnosis method of Example 1 was applied has LiNi as the positive electrode active material. 1 / 3 Co 1 / 3 Mn 1 / 3This is a secondary battery that uses O2, graphite as the negative electrode active material, and lithium-ion battery electrolyte as the electrolyte.

[0016] [Explanation of deterioration diagnosis procedure] Figure 1 is a flowchart for SOH (State of Overheating) diagnosis, which estimates the SOC difference between the positive and negative electrodes of a lithium-ion battery with an unknown usage history and determines the degree of degradation (SOH). S1-S3 in Figure 1 are preparation steps, and S4-S8 are the SOH diagnosis steps for a battery with an unknown usage history.

[0017] In step S1 (pre-degradation battery measurement step), capacity measurement and impedance measurement under different charge states are performed on a battery with 100% SOH (pre-degradation secondary battery). However, the impedance measurement method is not limited to applying an AC voltage or AC current and measuring the current or voltage; it may also be a method in which a voltage waveform or current waveform with multiple frequency signals superimposed is applied, the current waveform or voltage waveform is measured, and the voltage waveform and current waveform are respectively subjected to a Discrete Fourier Transform (DFT) to obtain the ratio for each frequency. Furthermore, impedance measurement is not limited to the steady state; it may also be measured by superimposing an AC current or current waveform for impedance measurement onto the charging and discharging current. Measurements are performed for multiple frequencies.

[0018] In step S2 (Assignment Determination Step), the resistive components observed in the Nyquist plot obtained in S1 are determined to belong to the positive electrode or the negative electrode. In particular, for resistive components whose magnitude changes depending on the charge state, it is necessary to accurately determine which of the positive and negative electrodes that constitute the battery's impedance belongs to. A detailed explanation of the positive and negative electrode assignment of impedance will be provided later (Figure 3).

[0019] In step S3 (characteristic creation step), the impedance is determined for each resistance component assigned based on the judgment result in S2, and a charge state (capacitance) vs. impedance curve is created. Here, it is desirable to normalize the impedance curve using the method described in S6. The impedance can be Z or Z' at a specific frequency, or the resistance value R obtained from the fitting analysis. Furthermore, it is desirable to obtain a regression curve for the charge state (capacitance) vs. impedance curve. The regression curve is not limited to an approximate straight line, but can be selected from polynomial approximation curves, exponential approximation curves, logarithmic approximation curves, etc. The regression curve can also be a simple linear interpolation.

[0020] In step S4 (degraded battery measurement step), impedance measurements are performed on a degraded lithium-ion battery (degraded secondary battery) with an unknown usage history, under different charge states. Ideally, impedance measurements should be performed in a way that allows for the creation of a charge state (capacity) vs. impedance curve, but it is sufficient to perform impedance measurements under at least two charge states that are compatible with the standardization method in S6. When performing impedance measurements under two charge states, impedance measurements should be performed for charge states that include at least a full charge range (e.g., SOC 80% or higher) and a discharge range (e.g., SOC 5% to 10%), and measurement data should be acquired. Note that if the full charge range is set to SOC 80% or higher, measurement data that is almost the same as that of a fully charged state with an SOC of 100% can be obtained. Also, if the discharge range is set to SOC 5% to 10%, measurement data that is not affected by variations due to the increase of Z' originating from the negative electrode when the SOC is near 0% can be obtained.

[0021] In step S5 (temperature compensation step), temperature compensation is performed on the impedance acquired in S3 and S4 as needed. The temperature compensation is performed based on the ambient temperature at the time of impedance measurement, according to the temperature dependence of the lithium-ion battery's impedance. Specifically, in S3 and S4, charge state (capacity) vs. impedance curves are acquired at multiple temperatures, an Arrhenius plot is created from the relationship between temperature and impedance, and temperature compensation is performed.

[0022] In step S6 (normalization step), the impedance curve obtained in S5 (temperature-corrected) or in S3 and S4 is normalized to evaluate the impedance of the lithium-ion battery on the same scale before and after degradation, if necessary. Here, "normalization of the impedance curve" means converting the relationship between the charge state (capacity) vs. impedance (Ω) curve of a battery with 100% SOH to the relationship between the charge state (capacity) vs. impedance ratio (%) curve of a battery with 100% SOH (see Figures 6(a) and (b)). A detailed explanation of impedance curve normalization will be given later (Figure 2).

[0023] In S7 (Sequence of Odds Estimation Step), the SOC (State of Charge) deviation between the positive and negative electrodes of a lithium-ion battery with an unknown usage history is estimated before and after degradation. Specifically, the impedance ratio (e.g., ΔZ') is calculated using impedance measurement data from at least two charge states obtained in S4. SOC10% / ΔZ' SOC80% The impedance ratio is then calculated. The calculated impedance ratio is then compared with the relationship between the charge state (capacity) and impedance ratio (%) curve of a battery with S6 normalized to 100% SOH to estimate the amount of SOC deviation between the positive and negative electrodes (see Figure 5(a)).

[0024] In S8 (Degradation Determination Step), the State of Health (SOH) of a degraded lithium-ion battery with an unknown usage history is determined. Specifically, the SOC difference between the positive and negative electrodes estimated in S7 is considered equivalent to the capacity degradation amount, and the SOH of a degraded lithium-ion battery with an unknown usage history is determined as follows: SOH = 1 - (Amount of SOC shift between positive and negative electrodes / Capacity at 100% SOH) ... (1) The following formula is used for calculation. The degradation rate (%) of a degraded lithium-ion battery is obtained by multiplying the degradation level (SOH) by 100%. The remaining capacity information is obtained by multiplying the degradation level (SOH) by the full charge capacity of a new battery.

[0025] Figure 2 is a normalization flowchart of the charge state (capacity) vs. impedance curve, supplementing S6 in Figure 1. This normalization compensates for the increase in impedance due to the degradation of the positive and negative electrodes.

[0026] In S61, the charge state (capacity) vs. impedance (Ω) curve of the pre-degradation lithium-ion battery with 100% SOH, created in S3 of Figure 1, is obtained. In S62, the SOC with the lowest impedance (Ω) is determined in the curve characteristics of the low frequency range attributed to the positive electrode. Then, in S63, the impedance ratio (%) of the charge capacity SOC at other measurement points is calculated so that the lowest impedance (Ω) becomes 100%. In S64, based on the calculation of the impedance ratio (%) at multiple measurement points, the calculation points of the multiple impedance ratio (%) are connected to create a charge state (capacity) vs. impedance ratio (%) curve.

[0027] Figure 3 is a flowchart for determining whether a component belongs to the positive or negative electrode that constitutes the impedance of a lithium-ion battery, and it supplements S2 in Figure 1.

[0028] In S21, the charge state (capacity) vs. impedance curve of a pre-degradation lithium-ion battery with 100% SOH is obtained separately for different frequency ranges (e.g., low frequency range and high frequency range).

[0029] In S22, a pre-degradation lithium-ion battery with 100% SOH is disassembled. In S23, positive electrode half-cells and negative electrode half-cells are reassembled from the cells of the disassembled battery so that the positive and negative electrodes can be evaluated. Here, there are no particular restrictions on the configuration or shape of the positive electrode half-cells and negative electrode half-cells as long as they are cells that can be evaluated as single electrodes. Preferably, the configuration is a symmetrical cell, a counter electrode lithium metal half-cell, a cell with a reference electrode, etc., and preferably, the shape is a coin-type cell, a laminate-type cell, an evaluation cell, etc.

[0030] In S24, the charge state (capacity) vs. impedance curves of the positive and negative half-cells are acquired separately for different frequency ranges (e.g., low frequency range and high frequency range).

[0031] In S25, the charge state (capacity) vs. impedance curve of the lithium-ion battery obtained in S21 is compared with the charge state (capacity) vs. impedance curves of the positive electrode half-cell and negative electrode half-cell obtained in S24. By determining the approximation of the impedance curve characteristics and finding curves that draw similar lines, the impedance of the lithium-ion battery is attributed to either the positive or negative electrode. In other words, it is determined which of the curve characteristics in different frequency ranges belongs to the positive or negative electrode.

[0032] [Deterioration diagnostic effect] The degradation diagnostic process for degraded lithium-ion batteries with an unknown usage history will be explained with reference to the flowchart in Figure 1. First, if a pre-degradation lithium-ion battery (100% SOH battery) exists, but comparison data using charge state (capacity) vs. impedance curves does not exist, the process proceeds from S1 to S2 to S3 in Figure 1. In S1, capacity measurement and impedance measurement with varying charge states are performed on the pre-degradation lithium-ion battery. In the next S2, it is determined whether the resistance component, which can be seen in the Nyquist plot obtained in S1, belongs to the positive electrode or the negative electrode. In the next S3, the impedance is calculated for each resistance component assigned based on the determination result in S2, and comparison data using charge state (capacity) vs. impedance curves is created. Here, when performing a degradation diagnosis on a degraded lithium-ion battery with an unknown usage history, if comparison data using charge state (capacity) vs. impedance curves for the pre-degradation lithium-ion battery is already prepared, the preparation steps of proceeding from S1 to S2 to S3 in Figure 1 can be omitted.

[0033] Then, once comparison data is created using the charge state (capacity) vs. impedance curve of the pre-degradation lithium-ion battery, or if comparison data is available, the process proceeds from S4 to S5 to S6 to S7 to S8 in Figure 1, and the SOH diagnosis is performed.

[0034] In S4, impedance measurements are performed on a degraded lithium-ion battery with an unknown usage history, under different charge states. In S5, temperature correction is applied to the impedances obtained in S3 and S4 as needed. In S6, the temperature-corrected impedance from S5, or the impedances obtained in S3 and S4, are normalized as needed to evaluate the lithium-ion battery's impedance on the same scale before and after degradation. In S7, the SOC deviation between the positive and negative electrodes of the degraded lithium-ion battery with an unknown usage history is estimated. In S8, the SOH (State of Health) of the degraded lithium-ion battery with an unknown usage history is determined.

[0035] Thus, for degraded lithium-ion batteries with an unknown usage history, impedance measurement data taken under multiple charge conditions is used to estimate the "deviation between the reaction regions of the positive and negative electrodes (state of charge deviation between positive and negative electrodes)," which is the main cause of capacity reduction in lithium-ion batteries. Then, the amount of the deviation between the positive and negative electrodes is considered equivalent to the amount of capacity degradation of the degraded lithium-ion battery, and the degree of degradation (SOH) of the degraded lithium-ion battery is determined.

[0036] Next, we will explain, with reference to Figures 4 and 5, the degradation diagnosis verification mechanism that supports the fact that when performing degradation diagnosis on a degraded lithium-ion battery with an unknown usage history, the amount of SOC deviation between the positive and negative electrodes can be considered equivalent to the amount of capacity degradation, and that degradation diagnosis can be performed accordingly.

[0037] Fabrication of a lithium-ion battery with 100% SOH: LiNi punched out to a diameter of 14mm 1 / 3 Co 1 / 3 Mn 1 / 3Using the O2 positive electrode as the working electrode, a graphite negative electrode with a diameter of 15.9 mm as the counter electrode, a polypropylene porous membrane as the separator, and 1 mol / L LiPF6 in EC:DEC (50:50 v / v%) + VC (1 wt%) + PS (1 wt%) as the electrolyte, a 2032-type coin cell was fabricated.

[0038] Using a charge-discharge test device, after performing conditioning treatment by initial charge-discharge, capacity measurement at a 50-hour rate with a current value of 1 / 50 of the rated capacity (C / 50) and AC impedance measurement at a state of charge of 10% intervals with respect to the 50-hour rate capacity were carried out. For the AC impedance measurement, for example, the amplitude was set to 10 mV, the frequency range was 7 MHz to 5 mHz, and the number of measurement points was 10 points / decade.

[0039] The discharge curve of a lithium-ion battery with SOH 100% was created by 50-hour rate capacity measurement (the solid line characteristic in Fig. 4(a)). From the AC impedance spectrum (Nyquist plot) at a state of charge of 10% intervals, the difference in Z' between 12 Hz and 0.01 Hz (ΔZ' = Z' 0.01Hz - Z' 12Hz ) was obtained, and a charge state (capacity) vs. impedance (ΔZ') curve was created (the solid line characteristic connecting the ● in Fig. 4(b)).

[0040] Fabrication of a lithium-ion battery with unknown usage history: In order to fabricate a battery in which an "offset between the reaction regions of the positive and negative electrodes (SOC offset between the positive and negative electrodes)" was artificially generated, first, a positive electrode half-cell and a negative electrode half-cell were fabricated.

[0041] The positive electrode half-cell was a 2032-type coin cell using a LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 positive electrode with a diameter of 14 mm as the working electrode, a lithium metal with a diameter of 16.0 mm as the counter electrode, a polypropylene porous membrane as the separator, and 1 mol / L LiPF6 in EC:DEC (50:50 v / v%) + VC (1 wt%) + PS (1 wt%) as the electrolyte.

[0042] The negative electrode half-cell was a 2032 type coin cell, using a graphite negative electrode punched to a diameter of 15.9 mm as the working electrode, a lithium metal electrode with a diameter of 16.0 mm as the counter electrode, a polypropylene porous membrane as the separator, and 1 mol / L LiPF6in EC:DEC (50:50v / v%) + VC (1wt%) + PS (1wt%) as the electrolyte.

[0043] For the positive electrode half-cell, after initial charging and discharging conditioning, the coin cell was disassembled and the electrodes removed while the cell was charged to 10% or 20%. For the negative electrode half-cell, after initial charging and discharging conditioning, the coin cell was disassembled and the electrodes removed while the cell was discharged.

[0044] By combining a positive electrode charged to 10% and a negative electrode in a discharged state, we fabricated a lithium-ion battery with approximately 90% SOH. By combining a positive electrode charged to 20% and a negative electrode in a discharged state, we fabricated a lithium-ion battery with approximately 80% SOH.

[0045] Using a charge / discharge test device, initial charging and discharging conditioning was performed. Following this, 50-hour rate capacity measurements were taken at a current value of 1 / 50 of the rated capacity (C / 50), and AC impedance measurements were performed at 10% increments relative to the 50-hour rate capacity. For example, the AC impedance measurements were performed with an amplitude of 10mV, a frequency range of 7MHz to 5mHz, and 10 measurement points per decade.

[0046] Discharge curves for lithium-ion batteries with SOH 90% and SOH 80% were created by measuring the 50-hour rate capacity (dotted and dashed lines in Figure 4(a)). The difference between Z' at 12Hz and 0.01Hz (ΔZ'=Z') was calculated from the AC impedance spectrum (Nyquist plot) at 10% charge intervals. 0.01Hz -Z' 12Hz After calculating the capacitance and normalizing it by ΔZ' / ΔZ'min, a capacitance vs. ΔZ' / ΔZ'min curve was created (the dotted line characteristic connecting the squares and the dashed line characteristic connecting the triangles in Figure 4(b)).

[0047] The charge state (capacity) vs. impedance curves of lithium-ion batteries with 100% SOH, approximately 90% SOH, and approximately 80% SOH were fitted to overlap. As a result, it was confirmed that the fitting was performed with a shift in the charge state (capacity) corresponding to the difference in SOH, as shown in Figure 4(b) for the SOH 100%, SOH 90%, and SOH 80% characteristics representing SOC dependence.

[0048] For degraded lithium-ion batteries with an unknown usage history (lithium-ion batteries with approximately 90% SOH and lithium-ion batteries with approximately 80% SOH), calculate the ΔZ' for SOC 10% and SOC 80%, and determine the ratio (ΔZ'). SOC10% / ΔZ' SOC80% We then calculated (ΔZ') for each of them. Then, as shown in Figure 5(a), we obtained (ΔZ') SOC10% / ΔZ' SOC80% The value of ) is compared with the capacity vs. ΔZ' / ΔZ'min curve of a lithium-ion battery with 100% SOH, and the SOC deviation amount of 0.79 mAh for a lithium-ion battery with approximately 90% SOH and the SOC deviation amount of 1.22 mAh for a lithium-ion battery with approximately 80% SOH are determined. Then, the estimated SOH is calculated using equation (1) above, and the results are shown in the comparison table in Figure 5(b).

[0049] As is clear from the comparison table in Figure 5(b), for a lithium-ion battery with approximately 90% SOH, the measured capacity was 2.88 mAh, the measured SOH was 87%, the SOC deviation was 0.79 mAh, and the estimated SOH was 76%. For a lithium-ion battery with approximately 80% SOH, the measured capacity was 2.33 mAh, the measured SOH was 70%, the SOC deviation was 1.22 mAh, and the estimated SOH was 63%. In other words, even for degraded lithium-ion batteries with an unknown usage history, it was confirmed that the estimated SOH value could be estimated with an accuracy of about 10% deviation from the measured SOH. Thus, through verification experiments conducted by artificially fabricating lithium-ion batteries with approximately 90% SOH and lithium-ion batteries with approximately 80% SOH in advance, the verification result was obtained that the amount of SOC deviation between the positive and negative electrodes can be considered equivalent to the amount of capacity degradation.

[0050] [Normalization effect of impedance curves] The normalization of the impedance curve is performed by proceeding from S61 to S62 to S63 to S64 in the flowchart of Figure 2. Specifically, in S61, the charge state (capacity) vs. impedance (Ω) curve of the pre-degradation lithium-ion battery, created in S3 of Figure 1, is obtained. In S62, the state of charge (SOC) with the lowest impedance (Ω) is determined in the curve characteristics of the low frequency range attributed to the positive electrode. In S63, the impedance ratio (%) of the SOC at other measurement points is calculated so that the lowest impedance (Ω) becomes 100%. In S64, based on the calculation of the impedance ratio (%), the charge state (capacity) vs. impedance ratio (%) curve is created by connecting the calculation points of multiple impedance ratios (%).

[0051] Next, we will explain, with reference to Figure 6, the verification effect of normalizing the impedance curve, which eliminates the effects of electrode size and degradation of the positive or negative electrode.

[0052] LiRing 1 / 3 Co 1 / 3 Mn 1 / 3 With O2 as the positive electrode and Li4Ti5O as the working electrode, 12 Laminate cells were fabricated using a polypropylene porous membrane as the counter electrode and 1 mol / L LiPF6in EC:DEC (50:50v / v%) + VC (1wt%) + PS (1wt%) as the electrolyte. The laminate cells were subjected to charge-discharge cycle tests at 25°C or 45°C with a current value of 1 / 10 of the rated capacity (C / 10). Laminate cells that reached 100, 200, and 300 cycles were disassembled, and the cycle-degraded positive electrode was removed. A positive electrode half-cell was fabricated using the removed positive electrode.

[0053] For each fabricated positive electrode half-cell, initial charging and discharging conditioning was performed using a charge / discharge test apparatus. Following this, 50-hour rate capacity measurements were taken at a current value of 1 / 50 of the rated capacity (C / 50), and AC impedance measurements were performed at 10% increments relative to the 50-hour rate capacity. For example, the AC impedance measurements were performed with an amplitude of 10mV, a frequency range of 7MHz to 5mHz, and 10 measurement points per decade.

[0054] The difference between Z' at 12Hz and 0.01Hz (ΔZ'=Z') is calculated from the AC impedance spectrum (Nyquist plot) at 10% charge intervals. 0.01Hz -Z' 12Hz We calculated the capacitance vs. ΔZ'(Ω) curve and created a capacitance vs. ΔZ'(Ω) curve (Figure 6(a) before normalization). As is clear from Figure 6(a), the impedance curve before normalization confirms that the impedance (Ω) of the cycle-degraded positive electrode is larger than the impedance (Ω) of the undegraded positive electrode.

[0055] On the other hand, a capacitance vs. ΔZ'(Ω) curve was normalized so that the lowest impedance (Ω) in the graph would be 100%, and a capacitance vs. ΔZ'(%) curve was created (Figure 6(b) after normalization). As is clear from Figure 6(b), the normalized impedance curve was confirmed to be in good agreement with the pre- and post-degradation versions in terms of both scale and shape.

[0056] The impedance of a lithium-ion battery changes not only due to its SOC (State of Charge) dependence but also due to factors such as the size of the electrodes being evaluated and the degradation of the positive or negative electrode. Therefore, in order to evaluate only the SOC dependence, it is necessary to eliminate the effects of electrode size and the degradation of the positive or negative electrode.

[0057] In contrast, by normalizing the relationship between the capacity vs. ΔZ'(Ω) curve so that the lowest impedance (Ω) in the graph becomes 100%, and using this as the relationship between the capacity vs. ΔZ'(%) curve (Figure 6(b) after normalization), the scale and shape agree well before and after degradation. In other words, it was confirmed that the effects of electrode size and the degradation of the positive or negative electrode are eliminated. Therefore, the verification result shows that normalizing the impedance curve eliminates the effects of electrode size and the degradation of the positive or negative electrode before and after the degradation of the impedance of lithium-ion batteries with an unknown usage history, and allows evaluation on the same scale based on SOC dependence.

[0058] [Positive and negative electrode assignment of impedance] The assignment of impedance to positive and negative electrodes is performed by proceeding through S21→S22→S23→S24→S25 in the flowchart of Figure 3. Specifically, in S21, the charge state (capacity) vs. impedance curve of a lithium-ion battery with 100% SOH is acquired separately for different frequency ranges (e.g., low frequency range and high frequency range). In S22, the lithium-ion battery with 100% SOH (before degradation) is disassembled. In S23, positive electrode half-cells and negative electrode half-cells that allow evaluation of the positive and negative electrodes are reassembled from the cells of the disassembled battery. In S24, the charge state (capacity) vs. impedance curves of the positive electrode half-cell and negative electrode half-cell are acquired separately for different frequency ranges (e.g., low frequency range and high frequency range). In S25, the charge state (capacity) vs. impedance curves obtained in S21 and S24 are compared, and it is determined which of the curve characteristics in different frequency ranges belongs to the positive or negative electrode.

[0059] Next, regarding the positive and negative electrode assignment effect of impedance, we will explain the verification effect that the low-frequency range characteristics of a lithium-ion battery with 100% SOH are attributed to the positive electrode of the lithium-ion battery, referring to Figures 7A, 7B, and 7C.

[0060] For both the positive and negative half-cells, initial charging and discharging conditioning was performed using a charge / discharge test apparatus. Following this, 50-hour rate capacity measurements were taken at a current value of 1 / 50 of the rated capacity (C / 50), and AC impedance measurements were performed at 10% increments relative to the 50-hour rate capacity. For example, the AC impedance measurements were performed with an amplitude of 10mV, a frequency range of 7MHz to 5mHz, and 10 measurement points per decade.

[0061] For the positive electrode half-cell, the difference between 220,000 Hz and 126 Hz (ΔZ') was obtained from the AC impedance spectrum (Nyquist plot) at 10% charge intervals. 正極高周波 =Z' 126Hz -Z' 220,000Hz ) and the difference in Z' between 12Hz and 0.01Hz (ΔZ' 正極低周波 =Z' 0.01Hz -Z' 12Hz We calculated the respective capacities and created the vs. ΔZ' curves.

[0062] For the negative electrode half-cell, the difference between 6,400Hz and 85Hz (ΔZ') was obtained from the AC impedance spectrum (Nyquist plot) at 10% charge intervals. 負極高周波 =Z' 85Hz -Z' 6,400Hz ) and the difference between 85Hz and 0.1Hz (ΔZ' 負極低周波 =Z' 0.1Hz -Z' 85Hz We calculated the capacity vs. ΔZ' curve and created the capacity vs. ΔZ' curve.

[0063] Figures 7A, 7B, and 7C show the capacity vs. ΔZ' curves, Nyquist plots, and Bode plots for a lithium-ion battery with 100% SOH, a positive electrode half-cell, and a negative electrode half-cell. The shapes of the capacity vs. ΔZ' curves for the positive electrode half-cell and the negative electrode half-cell were compared to those of the lithium-ion battery with 100% SOH. The shape comparison showed that the low-frequency capacity vs. ΔZ' curve of the lithium-ion battery with 100% SOH (solid line characteristic connecting the dots in Figure 7A) was in good agreement with the positive electrode low-frequency curve of the positive electrode half-cell (solid line characteristic connecting the dots in Figure 7B). Therefore, it was verified that the low-frequency range of the lithium-ion battery with 100% SOH originates from the positive electrode and is attributable to the positive electrode of the lithium-ion battery.

[0064] [Effects of deterioration diagnosis methods] As explained above, the lithium-ion battery (secondary battery) degradation diagnosis method of Example 1 has the following effects.

[0065] (1) A method for diagnosing the degradation of a secondary battery using the AC impedance method, comprising: a pre-degradation battery measurement step (S1) in which multiple impedances are measured by changing the charge state of a pre-degradation secondary battery; a characteristic creation step (S3) in which a relationship characteristic of the impedance curve with respect to the charge capacity of the pre-degradation secondary battery is created based on the measurement data of the multiple impedances; a post-degradation battery measurement step (S4) in which multiple impedances are measured by changing the charge state of a post-degradation secondary battery with an unknown usage history; a deviation amount estimation step (S7) in which the deviation amount of the charge capacity difference between the positive and negative electrodes before and after degradation (SOC deviation amount) is estimated based on the relationship characteristic created in the characteristic creation step (S3) and the measurement data of the multiple impedances obtained in the post-degradation battery measurement step (S4); and a degradation degree determination step (S8) in which the deviation amount of the charge capacity difference between the positive and negative electrodes is considered equivalent to the capacity degradation amount of the post-degradation secondary battery and the degradation degree of the post-degradation secondary battery is determined.Therefore, information on the remaining capacity of a post-degradation secondary battery with an unknown usage history can be obtained simply and accurately. In other words, by utilizing the SOC dependence of the impedance of a degraded secondary battery with an unknown usage history, the amount of SOC deviation between the positive and negative electrodes of the degraded secondary battery is estimated from the impedance measured in multiple charge states, and the degree of degradation (SOH) is determined.

[0066] (2) In the characteristic creation step (S3), the relationship characteristic of the impedance (Ω) curve with respect to the charge capacity of the secondary battery before degradation is called the pre-normalization characteristic. The normalization step (S6) involves determining the minimum charge capacity (SOC) with the lowest impedance (Ω) from the pre-normalization characteristic, calculating the impedance ratio (%) of the charge capacity at other measurement points so that the minimum impedance (Ω) becomes 100%, and normalizing it to the relationship characteristic of the impedance ratio (%) curve with respect to the charge capacity of the secondary battery before degradation. The deviation amount estimation step (S7) uses the relationship characteristic of the normalized impedance ratio (%) curve with respect to the charge capacity and the impedance ratio (%) calculated from the impedance measurement data for charge states including at least the full charge range and the discharge range to estimate the deviation amount of the charge capacity deviation (SOC deviation amount) of the positive and negative electrodes of the degraded secondary battery. Therefore, by normalizing the curve characteristics, the effects of electrode size and degradation of the positive or negative electrode are eliminated, and the SOC dependence of the impedance of the secondary battery can be evaluated on the same scale before and after degradation. Furthermore, when diagnosing the degradation of a degraded secondary battery with an unknown usage history, it is only necessary to obtain impedance measurement data for at least two charge states that allow for the calculation of the impedance ratio (%) for the degraded secondary battery. As a result, impedance measurements do not require a significant amount of time, and impedance measurements for degraded secondary batteries can be completed quickly and easily, enabling a simple SOH diagnosis that accurately determines the degree of degradation (SOH).

[0067] (3) A positive electrode half-cell and a negative electrode half-cell are fabricated to evaluate the positive and negative electrodes of the secondary battery before degradation, and impedance curves for the charge capacity of the secondary battery before degradation, impedance curves for the charge capacity of the positive electrode half-cell and impedance curves for the charge capacity of the negative electrode half-cell are obtained for different frequency ranges. The impedance curves for the charge capacity of the secondary battery before degradation, the positive electrode half-cell and the negative electrode half-cell are compared, and an assignment determination step (S2) is performed to determine which of the curve characteristics of the secondary battery before degradation at different frequency ranges belongs to the positive electrode or the negative electrode. The characteristic creation step (S3) and the normalization step (S6) create a relationship characteristic of the impedance curve for the charge capacity of the secondary battery before degradation based on the assignment determination result of the measurement data for the positive electrode or the negative electrode. Therefore, when determining the degree of degradation (SOH) of the secondary battery after degradation, it is possible to determine it by evaluation separated into positive electrode assignment and negative electrode assignment. In addition, the relationship characteristic of the impedance curve for the charge capacity of the secondary battery before degradation can be created using measurement data obtained by impedance measurement at a specific frequency range. In other words, creating the relationship characteristics of the impedance curve with respect to the charging capacity of a secondary battery before degradation, and normalizing those relationship characteristics, does not require a significant amount of time.

[0068] (4) The measurement data obtained by impedance measurement is subject to a temperature correction step (S5) based on the ambient temperature. Therefore, when obtaining information on the remaining capacity of a degraded secondary battery with an unknown usage history, it is possible to prevent a decrease in the accuracy of the degradation diagnosis due to differences in ambient temperature.

[0069] Next, we will explain the usefulness and incidental effects obtained by providing the degradation diagnosis method of Example 1. The degradation diagnosis method of Example 1 can be used for valuing used electric vehicles, deciding whether to reuse lithium-ion batteries, and so on. By incorporating the lithium-ion battery degradation diagnosis system into an in-vehicle device, charger, or inspection machine, the accuracy of evaluating the remaining capacity of lithium-ion batteries can be improved. Furthermore, if this lithium-ion battery degradation diagnosis system is standardized, it will lead to safer secondary battery use and revitalize the market. In addition, it is possible to provide design guidelines for secondary batteries that make the lithium-ion battery degradation diagnosis system easier to use, such as selecting active materials with high impedance SOC dependence for the positive and negative electrodes, and manufacturing electrodes to increase the impedance difference between the positive and negative electrodes.

[0070] The method for diagnosing the degradation of a secondary battery according to the present invention has been described above based on Example 1. However, the specific type of secondary battery and procedure are not limited to this Example 1, and changes or additions to the steps in the degradation diagnosis method are permitted as long as they do not deviate from the gist of the invention as described in each claim of the patent.

[0071] In Example 1, a lithium-ion battery, whose impedance changes depending on the charge state of the positive or negative electrode, was shown as an example of a secondary battery to which the degradation diagnosis method is applied. However, the type of secondary battery is not limited to lithium-ion batteries; other secondary batteries such as solid-state batteries, sodium-ion batteries, and magnesium-ion batteries may also be used.

[0072] In Example 1, the type of positive electrode active material for the secondary battery is LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 An example using O2 was shown. However, the type of positive electrode active material is not limited to this. LiRing 0.5 Co 0.2 Mn 0.3 O2, LiLiLi 0.6 Co 0.2 Mn 0.2 O2, LiLiLi 0.8Co 0.1 Mn 0.1 LiNi, represented by O2 1-x-y Co x Mn y O2, LiLiLi 0.8 Co 0.15 Al 0.05 LiNi, represented by O2 1-x-y Co x Al y O2, LiMn2O4, LiNi 0.5 Mn 1.5 LiNi 2-x Mn x It may also be O2, or a mixture thereof, etc.

[0073] Example 1 shows an example in which graphite is used as the negative electrode active material for a secondary battery. However, the type of negative electrode active material is not limited to this, and graphite-based, silicon-based alloys, Li4Ti5O 12 , or a mixture thereof, etc.

[0074] Example 1 shows an example where lithium-ion battery electrolyte is used as the type of electrolyte for the secondary battery. However, the type of electrolyte is not limited to this, and solid electrolytes for all-solid-state batteries, or mixtures thereof, etc., may also be used.

[0075] Example 1 shows an example of AC impedance measurement with an amplitude of 10mV, a frequency range of 7MHz to 5mHz, and 10 measurement points per decade. However, AC impedance measurement is not limited to this. For example, the frequency range of the AC impedance spectrum may be 10MHz to 0.1mHz. Also, the number of measurement points may be at least two points within the frequency range of the AC impedance spectrum. [Explanation of symbols]

[0076] S1 Pre-degradation battery measurement step S2 Attribution Determination Step S3 Characterization Step S4 Battery degradation measurement step S5 Temperature compensation step S6 Standardization Step S7 Step for estimating the amount of deviation S8 Degradation Determination Step

Claims

1. A method for diagnosing the degradation of a secondary battery using the AC impedance method, A pre-degradation battery measurement step that measures multiple impedances by changing the charge state of a pre-degradation secondary battery, A characteristic creation step to create a characteristic relationship between the impedance curve and the charging capacity of the secondary battery before degradation, based on the measurement data of the plurality of impedances, A degraded battery measurement step that measures multiple impedances by changing the charge state of a degraded secondary battery with an unknown usage history, A deviation amount estimation step is performed to estimate the amount of SOC deviation between the positive and negative electrodes before and after degradation, based on the relational characteristics created in the characteristic creation step and the measurement data of multiple impedances obtained in the post-degraded battery measurement step. The system includes a degradation degree determination step in which the amount of SOC deviation between the positive and negative electrodes is considered equivalent to the amount of capacity degradation of the degraded secondary battery, and the degree of degradation of the degraded secondary battery is determined. When the relationship between the impedance (Ω) curve and the charging capacity of the pre-degradation secondary battery, created in the aforementioned characteristic creation step, is referred to as the pre-normalization characteristic, The process includes a normalization step in which the charging capacity with the lowest impedance (Ω) is determined from the pre-normalization characteristics, the impedance ratio of the charging capacity at other measurement points is calculated so that the lowest impedance (Ω) becomes 100%, and the process is normalized to the relationship characteristics of the impedance ratio (%) curve with respect to the charging capacity of the pre-degradation secondary battery. The deviation amount estimation step estimates the SOC deviation amount of the positive and negative electrodes of the degraded secondary battery using the relationship characteristics of the impedance ratio (%) curve with respect to the normalized charging capacity and the impedance ratio (%) calculated from measurement data of the impedance depending on the charging state, which includes at least the full charging range and the discharge range. A method for diagnosing the degradation of a secondary battery, characterized by the features described above.

2. In the method for diagnosing the degradation of a secondary battery as described in Claim 1, A positive electrode half-cell and a negative electrode half-cell are fabricated to evaluate the positive electrode and negative electrode of the aforementioned secondary battery before degradation. The impedance curve for the charge capacity of the pre-degradation secondary battery, the impedance curve for the charge capacity of the positive electrode half-cell, and the impedance curve for the charge capacity of the negative electrode half-cell are obtained for each different frequency range. The system includes an assignment determination step which compares the impedance curves of the pre-degraded secondary battery, the positive electrode half-cell, and the negative electrode half-cell with respect to their charging capacity, and determines, based on the approximation of the curve characteristics, which of the curve characteristics in different frequency ranges of the pre-degraded secondary battery belongs to the positive electrode or the negative electrode. The characteristic creation step and the normalization step create a characteristic relationship between the impedance curve and the charging capacity of the pre-degradation secondary battery, based on the determination result of whether the measurement data is assigned to the positive or negative electrode. A method for diagnosing the degradation of a secondary battery, characterized by the features described above.

3. In the method for diagnosing the degradation of a secondary battery as described in claim 1 or claim 2, The measurement data obtained by the impedance measurement has a temperature correction step in which temperature correction is performed according to the ambient temperature. A method for diagnosing the degradation of a secondary battery, characterized by the features described above.

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