JP2023175531A - Secondary battery deterioration diagnosis method - Google Patents

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

The present invention relates to a secondary battery deterioration diagnosis method for diagnosing the degree of deterioration (remaining capacity) of a degraded secondary battery whose usage history is unknown.

Lithium-ion batteries have been used as power sources for portable electronic devices for over 30 years since they were commercialized, and their use as power sources for electric vehicles and smart grids has recently expanded rapidly. Furthermore, from a sustainability perspective, there is a growing movement to reuse storage batteries after their temporary use has ended. For this reason, it is necessary to understand the health of lithium-ion batteries, but evaluation and diagnosis methods are not sufficient.

Conventionally, in order to evaluate battery performance (initial performance, remaining performance), charging and discharging in a stable environment draws a charge-discharge curve to obtain the charge capacity and discharge capacity, and the electrochemical pulse method is also used. (DC method) or AC impedance method has been used to obtain internal resistance from the change in response voltage during energization with respect to open circuit voltage. In particular, the rate of change in charge/discharge capacity before and after deterioration is used as a parameter called the state of health (SOH) of the battery, and is used as an index representing the state of deterioration. Hereinafter, the state of health or deterioration of the battery will be referred to as "SOH".

The SOH of a lithium ion battery can be explained by the difference in the state of charge or state of charge (SOC) of the positive and negative electrodes, which is the difference in the reaction area between the positive and negative electrodes that make up the battery (see Non-Patent Document 1). reference). However, the causes of misalignment are the formation of SEI (abbreviation for "Solid Electrolyte Interphase") due to 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. It is broadly classified into deterioration. Furthermore, the deviation ratio for each cause changes depending on the usage history of the battery (battery temperature, battery voltage, number of times of charging and discharging, etc.). In particular, lithium-ion batteries for driving electric vehicles and the like are used in a wide range of temperature environments (-40°C to +60°C), and complex charging and discharging (charging and discharging) is performed by output/regeneration during acceleration and deceleration. Therefore, its deterioration reactions are thought to vary. Hereinafter, the charging state or charging capacity of a battery will be referred to as "SOC".

In general, for SOH diagnosis of lithium-ion batteries whose usage history is unknown, it is ideal to evaluate the amount of electricity (charge capacity) from the discharge state to full charge in a stable environment, but the cost and operational aspects are It's not realistic when you think about it. Therefore, as another SOH diagnosis, SOH and AC impedance spectra are obtained for batteries that have been degraded under various conditions in advance, and the data are processed and compiled into a database. A method has been proposed in which the AC impedance spectrum obtained for a battery whose usage history is unknown is compared with a database to predict the SOH (see Patent Documents 1 and 2).

JP2016-90346A JP2013-26114A

K. Ando et al., Journal Power Sources, 390 (2018) 278-285.

The conventional methods of SOH diagnosis for determining remaining capacity disclosed in Patent Documents 1 and 2 have the following problems. Measurement of charge/discharge capacity in SOH diagnosis is highly accurate because it directly measures capacity, but requires a considerable amount of time. Furthermore, depending on the operating conditions of the product (for example, a lithium ion battery installed in a vehicle), it is difficult to perform simple charging and discharging. For SOH diagnosis, analysis based on checking AC impedance spectrum and database requires time and cost to prepare the database. While diagnosis is quick and easy, there is a concern that it may contain large errors for lithium-ion batteries, which have varying deterioration behavior, depending on the condition range of the deterioration data prepared in advance and the amount of information in the database. .

For detailed analysis of deterioration factors using charge-discharge curve analysis (charge-discharge differential curve analysis) to understand the mechanism of capacity decrease, see SEI formation caused by lithium ions," "positive electrode deterioration accompanied by lithium ion consumption," and "negative electrode deterioration accompanied by lithium ion consumption." It is necessary to measure charge and discharge at C rate), which takes a considerable amount of time. Also, curve analysis requires medium-to-advanced analysis technology.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a deterioration diagnosis method that easily and accurately acquires remaining capacity information of a degraded secondary battery whose usage history is unknown.

In order to achieve the above object, the present invention quantitatively evaluates the charge capacity difference (SOC difference) of the positive and negative electrodes before and after deterioration using impedance measurement data that is easy to obtain, and We have developed a method to diagnose the degree of deterioration (SOH). The problem solving means is a method for diagnosing deterioration of a secondary battery using an AC impedance method, which includes a step of measuring the battery before deterioration, a step of creating characteristics, a step of measuring the battery after deterioration, a step of estimating the amount of deviation, and a step of determining the degree of deterioration. , has. The deviation amount estimation step estimates the charge capacity deviation amount of the positive and negative electrodes before and after deterioration based on the relational characteristics created in the characteristic creation step and the measurement data of a plurality of impedances obtained in the post-deterioration battery measurement step. . In the deterioration degree determination step, the amount of charge capacity deviation between the positive and negative electrodes is considered to be equivalent to the capacity deterioration amount of the degraded secondary battery, and the deterioration degree of the degraded secondary battery is determined.

As described above, the deterioration diagnosis method of the present invention utilizes the SOC dependence of impedance in a secondary battery and estimates the amount of charge capacity difference between the positive and negative electrodes before and after deterioration, thereby determining the degree of deterioration of the secondary battery after deterioration. is being determined. As a result, remaining capacity information of a degraded secondary battery whose usage history is unknown can be easily and accurately acquired.

It is a flowchart which shows the procedure of the SOH diagnosis method of the lithium ion battery after deterioration whose usage history is unknown. It is a flowchart which shows the procedure of the normalization method of a state of charge (capacity) vs. impedance curve. It is a flowchart which shows the procedure of the impedance attribution method of a lithium ion battery. This is an example showing the discharge curve characteristics (a) and SOC dependence of impedance ratio (b) of a lithium ion battery. SOC deviation amount (a) in SOH diagnosis of lithium ion battery before deterioration (SOH 100%) and artificially created lithium ion battery after deterioration (SOH 90%, SOH 80%), measured capacity, measured SOH, SOC deviation amount, estimated SOH This is an example showing the comparison table (b). This is an example showing the curve characteristics before normalization (a) and the curve characteristics after normalization (b) using the impedance of the positive electrode of an undegraded battery and four different types of cycle-aged batteries. This is an example showing the SOC dependence of the impedance of a lithium ion battery, and a Nyquist plot and a Bode diagram when the SOC is 50%. This is an example showing the SOC dependence of the impedance of a positive electrode half cell, and a Nyquist plot and a Bode diagram when the SOC is 50%. This is an example showing the SOC dependence of the impedance of a negative electrode half cell, and a Nyquist plot and a Bode diagram when the SOC is 50%.

EMBODIMENT OF THE INVENTION Hereinafter, the form for carrying out the deterioration diagnosis method of the secondary battery by this invention is demonstrated based on Example 1 shown in drawing.

The deterioration diagnosis method of Example 1 is applied to a lithium ion battery whose usage history is unknown (an example of a secondary battery where lithium ions are charged and discharged by moving between the positive electrode and the negative electrode). This is a deterioration diagnosis method for determining the degree of deterioration SOH of. The deterioration diagnosis method of Example 1 will be described below as follows: "Explanation of deterioration diagnosis procedure", "Deterioration diagnosis effect", "Impedance curve standardization effect", "Positive and negative pole assignment effect of impedance", and "Effects of the deterioration diagnosis method" I will explain it separately. Here, the lithium ion battery with unknown usage history applied to the deterioration diagnosis method of Example 1 uses LiNi _1/3 Co _1/3 Mn _1/3 O ₂ as the positive electrode active material and graphite as the negative electrode active material. This is a secondary battery that uses an electrolyte for lithium ion batteries as the electrolyte.

[Explanation of deterioration diagnosis procedure]
FIG. 1 is an SOH diagnosis flowchart for estimating the SOC deviation of the positive and negative electrodes of a lithium ion battery whose usage history is unknown and determining the degree of deterioration SOH. S1 to S3 in FIG. 1 are preparation steps, and S4 to S8 are SOH diagnosis steps for a battery whose usage history is unknown.

In S1 (pre-deterioration battery measurement step), capacity measurement and impedance measurement with different charging states are performed on a 100% SOH battery (pre-deterioration secondary battery). However, the impedance measurement method is not limited to applying an alternating voltage or current to measure the current or voltage, but also applying a voltage waveform or current waveform on which multiple frequency signals are superimposed to measure the current waveform or voltage. A method may also be used in which the waveform is measured, the voltage waveform and the current waveform are each subjected to discrete Fourier transform (DFT), and the ratio for each frequency is determined. Moreover, impedance measurement is not limited to a steady state, and may be measured by superimposing an alternating current or a current waveform for impedance measurement on the charging/discharging current. Measurements are made at multiple frequencies.

In S2 (belonging determination step), it is determined whether the resistance component that can be confirmed in the Nyquist plot obtained in S1 is a resistance component that belongs to the positive electrode or a resistance component that belongs to the negative electrode. In particular, with regard to the attribution of a resistance component whose magnitude changes depending on the state of charge, it is accurately determined to which of the positive and negative electrodes that make up the impedance of the battery it belongs. Note that a detailed explanation of the positive and negative pole assignment of impedance will be given later (FIG. 3).

In S3 (characteristic creation step), impedance is determined for each resistance component to which it belongs based on the determination result in S2, and a state of charge (capacity) vs. impedance curve is created. Here, it is desirable that the impedance curve be normalized by the method of S6, which will be described later. The impedance may be Z or Z' at a specific frequency, or a resistance value R obtained by fitting analysis. Furthermore, it is desirable to obtain a regression curve for the state of charge (capacity) vs. impedance curve. The regression curve is not limited to an approximate straight line, and can be selected from a polynomial approximate curve, an exponential approximate curve, a logarithmic approximate curve, and the like. The regression curve may be a simple linear interpolation.

In S4 (degraded battery measuring step), impedance measurement is performed with the state of charge changed for a degraded lithium ion battery (deteriorated secondary battery) whose usage history is unknown. Here, it is desirable to perform impedance measurement that can create a charging state (capacity) vs. impedance curve, but it is sufficient to perform impedance measurement in at least two charging states that are compatible with the standardization method in S6. Here, when performing impedance measurements in two charging states, impedance measurements are performed in charging states that include at least a full charge range (for example, SOC 80% or more) and a discharge range (for example, SOC about 5% to 10%), and the measured data is get. Note that when the SOC is set to 80% or higher as the full charge range, measurement data that is almost the same as a fully charged state with an SOC of 100% can be obtained. Furthermore, by setting the SOC to about 5% to 10% as the discharge region, it is possible to obtain measurement data that is not affected by variations due to an increase in Z' caused by the negative electrode when the SOC is near 0%.

In S5 (temperature correction step), temperature correction of the impedance obtained in S3 and S4 is performed as necessary. Temperature correction is performed based on the temperature dependence of the impedance of the lithium ion battery, using the environmental temperature at the time of impedance measurement. Specifically, in S3 and S4, state of charge (capacity) vs. impedance curves are obtained at a plurality of temperatures, an Arrhenius plot is created from the relationship between temperature and impedance, and temperature correction is performed.

In S6 (normalization step), the impedance curves temperature-corrected in S5 or the impedance curves obtained in S3 and S4 are normalized to evaluate the impedance of the lithium ion battery on the same scale before and after deterioration, if necessary. do. Here, "normalization of the impedance curve" refers to the relationship between the state of charge (capacity) vs. impedance (Ω) curve of a 100% SOH battery, and the state of charge (capacity) vs. impedance ratio of a 100% SOH battery ( %) is converted into a curved relational characteristic (see Figures 6(a) and (b)). Note that a detailed explanation of the normalization of the impedance curve will be given later (FIG. 2).

In S7 (deviation amount estimation step), the SOC deviation amount of the positive and negative electrodes before and after deterioration of the lithium ion battery whose usage history is unknown is estimated. Specifically, an impedance ratio (for example, ΔZ' _SOC10% /ΔZ' _SOC80% ) is calculated based on the impedance measurement data obtained in at least two charging states acquired in S4. Then, by comparing the calculated impedance ratio with the relationship characteristic of the state of charge (capacity) vs. impedance ratio (%) curve of a 100% SOH battery standardized in S6, the amount of SOC deviation between the positive and negative electrodes is estimated (Fig. 5(a)).

In S8 (deterioration degree determination step), the deterioration degree SOH of the degraded lithium ion battery whose usage history is unknown is determined. Specifically, the amount of SOC deviation of the positive and negative electrodes estimated in S7 is considered to be equivalent to the amount of capacity deterioration, and the degree of deterioration SOH of the degraded lithium ion battery whose usage history is unknown is calculated as follows:
SOH=1-(SOC deviation amount of positive and negative electrodes/capacity at 100% SOH)...(1)
Calculate using the formula. Note that the deterioration ratio (%) of the lithium ion battery after deterioration is determined by a formula in which the degree of deterioration SOH is multiplied by 100%. The remaining capacity information is obtained by multiplying the degree of deterioration SOH by the full charge capacity of a new battery.

FIG. 2 is a normalization flowchart of the state of charge (capacity) vs. impedance curve, and supplements S6 in FIG. 1. This normalization corrects the impedance that increases due to deterioration of the positive and negative electrodes.

In S61, the state of charge (capacity) vs. impedance (Ω) curve of the SOH 100% lithium ion battery before deterioration created in S3 of FIG. 1 is acquired. In S62, the SOC with the lowest impedance (Ω) in the curve characteristics in the low frequency range belonging to the positive electrode is determined. Then, in S63, impedance ratios (%) of the charging capacity SOC at other measurement points are calculated so that the lowest impedance (Ω) becomes 100%. In S64, based on calculation of impedance ratio (%) at a plurality of measurement points, a state of charge (capacity) vs. impedance ratio (%) curve is created by connecting a plurality of calculation points of impedance ratio (%).

FIG. 3 is a flowchart for determining which of the positive electrode and the negative electrode that constitutes the impedance of the lithium ion battery belongs to, and supplements S2 in FIG. 1.

In S21, the charging state (capacity) vs. impedance curve of the SOH 100% lithium ion battery before deterioration is obtained by dividing it into different frequency ranges (for example, a low frequency range and a high frequency range).

In S22, the SOH 100% lithium ion battery before deterioration is disassembled. In S23, the positive electrode half cell and the negative electrode half cell whose positive electrode and negative electrode can be evaluated are reassembled from the cells of the disassembled battery. Here, the configuration and shape of the positive electrode half cell and negative electrode half cell are not particularly limited as long as they can be evaluated as a single electrode.The configuration is preferably a symmetrical cell, a counter electrode lithium metal half cell, a cell with a reference electrode, etc. Coin-type cells, laminate-type cells, evaluation cells, etc. are desirable.

In S24, the state of charge (capacity) vs. impedance curves of each of the positive half cell and the negative half cell are obtained in different frequency ranges (for example, a low frequency range and a high frequency range).

In S25, the state of charge (capacity) vs. impedance curve of the lithium ion battery acquired in S21 and the state of charge (capacity) vs. impedance curve of each of the positive electrode half cell and negative electrode half cell acquired in S24 are calculated. compare. The impedance of the lithium ion battery is attributed to the positive electrode or the negative electrode by determining the similarity of impedance curve characteristics to find curves that draw similar lines. That is, it is determined which of the curve characteristics in different frequency ranges belongs to the positive pole or the negative pole.

[Deterioration diagnosis effect]
A deterioration diagnosis operation for performing a deterioration diagnosis on a degraded lithium ion battery whose usage history is unknown will be explained with reference to the flowchart of FIG. First, if there is a lithium ion battery (SOH 100% battery) before deterioration, but there is no comparison data based on the state of charge (capacity) vs. impedance curve, the process proceeds from S1 to S2 to S3 in FIG. In S1, capacity measurement and impedance measurement with different charging states are performed on the lithium ion battery before deterioration. In the next S2, it is determined whether the resistance component that can be confirmed in the Nyquist plot obtained in S1 belongs to the positive electrode or the negative electrode. In the next step S3, the impedance is determined for each resistance component based on the determination result in S2, and comparison data based on the state of charge (capacity) vs. impedance curve is created. Here, when performing a deterioration diagnosis on a degraded lithium-ion battery whose usage history is unknown, if comparative data based on the state of charge (capacity) vs. impedance curve of the pre-degraded lithium-ion battery is prepared in advance, the The preparation procedure of proceeding from S1 to S2 to S3 in step 1 can be omitted.

When the comparison data based on the state of charge (capacity) vs. impedance curve of the lithium ion battery before deterioration is created or when the comparison data is prepared, in FIG. 1, S4 → S5 → S6 → S7 → S8 Then, SOH diagnosis is performed.

In S4, impedance measurement is performed on the degraded lithium ion battery whose usage history is unknown, with the state of charge changed. In S5, temperature correction of the impedance obtained in S3 and S4 is performed as necessary. In S6, the impedance temperature-corrected in S5 as necessary or the impedance obtained in S3 and S4 is standardized in order to evaluate the impedance of the lithium ion battery on the same scale before and after deterioration. In S7, the SOC deviation amount of the positive and negative electrodes of the degraded lithium ion battery whose usage history is unknown is estimated. In S8, the degree of deterioration SOH of the degraded lithium ion battery whose usage history is unknown is determined.

In this way, for degraded lithium-ion batteries whose usage history is unknown, we use impedance measurement data measured in multiple charging states to investigate the "reaction between the positive and negative electrodes," which is the main cause of capacity reduction in lithium-ion batteries. "region deviation (SOC deviation between positive and negative poles)" is estimated. Then, the amount of SOC deviation between the positive and negative electrodes is considered to be equivalent to the amount of capacity deterioration of the lithium ion battery after deterioration, and the degree of deterioration SOH of the lithium ion battery after deterioration is determined.

Next, when performing a deterioration diagnosis on a degraded lithium-ion battery whose usage history is unknown, we will explain the deterioration diagnosis verification function that supports deterioration diagnosis by assuming that the amount of SOC deviation between the positive and negative electrodes is equivalent to the amount of capacity deterioration. , will be explained with reference to FIGS. 4 and 5.

Fabrication of 100% SOH lithium ion battery: A LiNi _1/3 Co _1/3 Mn _1/3 O ₂ positive electrode punched to a diameter of 14 mm is used as a working electrode, a graphite negative electrode with a diameter of 15.9 mm is used as a counter electrode, and a polypropylene porous membrane is used as a separator. Next, a 2032-type coin cell was fabricated using 1 mol/L LiPF ₆ in EC:DEC (50:50v/v%)+VC(1wt%)+PS(1wt%) as the electrolyte.

After performing conditioning treatment by initial charging and discharging using a charge/discharge test device, 50-hour rate capacity measurement was performed at a current value (C/50) of 1/50 of the rated capacity, and 10% of the 50-hour rate capacity was measured. AC impedance measurements were performed under each charging state. Note that the AC impedance measurement is performed at, for example, amplitude: 10 mV, frequency range: 7 MHz to 5 mHz, and number of measurement points: 10 points/decade.

A discharge curve of a 100% SOH lithium ion battery was created by 50-hour rate capacity measurement (solid line characteristics in FIG. 4(a)). Find the difference between Z' between 12Hz and 0.01Hz (ΔZ' = Z' _0.01Hz - Z' _12Hz ) from the AC impedance spectrum (Nyquist plot) at every 10% charging state, and calculate the charging state (capacity) vs. impedance. (ΔZ') curve was created (solid line characteristic connecting ● in FIG. 4(b)).

Fabrication of a lithium ion battery with unknown usage history: In order to fabricate a battery with an artificial "misalignment of the reaction regions between the positive and negative electrodes (SOC deviation between positive and negative electrodes)", a positive half cell and a negative half cell were first fabricated.

The positive electrode half cell consists of a LiNi _1/3 Co _1/3 Mn _1/3 O ₂ positive electrode punched to a diameter of 14 mm as a working electrode, a lithium metal with a diameter of 16.0 mm as a counter electrode, and a polypropylene porous membrane as a separator. A 2032 type coin cell was made using L LiPF ₆ in EC:DEC (50:50v/v%)+VC(1wt%)+PS(1wt%) as the electrolyte.

The negative electrode half cell consists of a graphite negative electrode punched to a diameter of 15.9 mm as a working electrode, a lithium metal with a diameter of 16.0 mm as a counter electrode, a polypropylene porous membrane as a separator, and 1 mol/L LiPF ₆ in EC:DEC (50:50v /v%)+VC(1wt%)+PS(1wt%) was used as the electrolyte to make a 2032 type coin cell.

After the positive electrode half cell was subjected to conditioning treatment by initial charging and discharging, the coin cell was disassembled and the electrode was taken out in a state where it was charged only 10% or 20%. After the negative electrode half cell was subjected to conditioning treatment by initial charging and discharging, the coin cell was disassembled in the discharged state and the electrode was taken out.

By combining a 10% charged positive electrode and a discharged negative electrode, you can create a lithium ion battery with an SOH of approximately 90%, and by combining a 20% charged positive electrode and a discharged negative electrode, you can create a lithium ion battery with an SOH of approximately 80%. were prepared respectively.

After performing conditioning treatment by initial charging and discharging using a charge/discharge test device, 50-hour rate capacity measurement was performed at a current value (C/50) of 1/50 of the rated capacity, and 10% of the 50-hour rate capacity was measured. AC impedance measurements were performed under each charging state. Note that the AC impedance measurement is performed at, for example, amplitude: 10 mV, frequency range: 7 MHz to 5 mHz, and number of measurement points: 10 points/decade.

Discharge curves of lithium ion batteries with 90% SOH and 80% SOH were created by 50-hour rate capacity measurement (dotted line characteristics and broken line characteristics in FIG. 4(a)). Find the difference between Z' between 12Hz and 0.01Hz (ΔZ' = Z' _0.01Hz - Z' _12Hz ) from the AC impedance spectrum (Nyquist plot) at every 10% charging state, and standardize it by ΔZ'/ΔZ'min. After that, a capacitance vs. ΔZ'/ΔZ'min curve was created (the dotted line characteristic connecting ■ and the broken line characteristic connecting ▲ in FIG. 4(b)).

Fitting was performed so that the state of charge (capacity) vs. impedance curves of a lithium ion battery with 100% SOH, a lithium ion battery with about 90% SOH, and a lithium ion battery with about 80% SOH overlapped with each other. As a result, as shown in the SOH 100% characteristic, SOH 90% characteristic, and SOH 80% characteristic representing SOC dependence in Figure 4(b), it was confirmed that the fitting was performed with the state of charge (capacity) deviated by the difference in SOH. did.

For degraded lithium ion batteries with unknown usage history (lithium ion batteries with SOH of about 90% and lithium ion batteries with SOH of about 80%), calculate ΔZ' at SOC10% and SOC80%, and calculate the ratio (ΔZ' _SOC10% /ΔZ ' _SOC80% ) was determined for each. Then, as shown in FIG. 5(a), the obtained value of (ΔZ' _SOC10% /ΔZ' _SOC80% ) is compared with the capacity vs. ΔZ'/ΔZ'min curve of a 100% SOH lithium ion battery, An SOC deviation amount of 0.79 mAh in a lithium ion battery with an SOH of approximately 90% and an SOC deviation amount of 1.22 mAh in a lithium ion battery with an SOH of approximately 80% are determined. Then, the estimated SOH is calculated using the above equation (1), and the results are shown in the comparison table in FIG. 5(b).

As is clear from the comparison table in Figure 5(b), in the case of a lithium-ion battery with an SOH of approximately 90%, results were obtained with an actual capacity of 2.88 mAh, an actual SOH of 87%, an SOC deviation of 0.79 mAh, and an estimated SOH of 76%. Ta. In the case of a lithium ion battery with an SOH of approximately 80%, results were obtained with an actual capacity of 2.33 mAh, an actual SOH of 70%, an SOC deviation of 1.22 mAh, and an estimated SOH of 63%. That is, it was confirmed that the estimated SOH value could be estimated with an accuracy of approximately 10% deviation from the actually measured SOH even though the battery was a degraded lithium ion battery whose usage history was unknown. In this way, through verification experiments in which lithium ion batteries with an SOH of approximately 90% and lithium ion batteries with an SOH of approximately 80% were artificially prepared in advance, the amount of SOC deviation between the positive and negative electrodes was equivalent to the amount of capacity deterioration. We obtained verification results that it can be considered as

[Impedance curve normalization effect]
The normalization of the impedance curve is executed by proceeding from S61 to S62 to S63 to S64 in the flowchart of FIG. That is, in S61, the state of charge (capacity) vs. impedance (Ω) curve of the lithium ion battery before deterioration created in S3 of FIG. 1 is acquired. In S62, the SOC with the lowest impedance (Ω) in the curve characteristics in the low frequency range belonging to the positive electrode is determined. In S63, impedance ratios (%) of the SOCs at other measurement points are calculated so that the lowest impedance (Ω) becomes 100%. In S64, based on the impedance ratio (%) calculation, a charging state (capacity) vs. impedance ratio (%) curve is created by connecting a plurality of impedance ratio (%) calculation points.

Next, the effect of verifying that the influence of electrode size and deterioration of the positive electrode or negative electrode is eliminated by normalizing the impedance curve will be described with reference to FIG. 6.

_{1 mol} / _L LiPF ₆ in _EC : _{DEC ( 50} :50v/v%)+VC(1wt%)+PS(1wt%) was used as the electrolyte to fabricate a laminate cell. The laminate cell was subjected to a charge/discharge cycle test at a current value of 1/10 of the rated capacity (C/10) in an environment of 25°C or 45°C. The laminate cells that had reached 100, 200, and 300 cycles were disassembled, and the cycle-deteriorated positive electrodes were taken out. A positive electrode half cell was produced using the taken out positive electrode.

After carrying out conditioning treatment by initial charging and discharging for each of the produced positive electrode half cells using a charging and discharging test device, a 50-hour rate capacity measurement was performed at a current value (C/50) of 1/50 of the rated capacity, and a AC impedance measurement was performed at every 10% charging state with respect to the time rate capacity. Note that the AC impedance measurement is performed at, for example, amplitude: 10 mV, frequency range: 7 MHz to 5 mHz, and number of measurement points: 10 points/decade.

Calculate the difference in Z' between 12Hz and 0.01Hz (ΔZ' = Z' _0.01Hz - Z' _12Hz ) from the AC impedance spectrum (Nyquist plot) at every 10% charging state, and calculate the capacity vs. ΔZ' (Ω). ) A curve was created (Figure 6(a) before normalization). As is clear from the impedance curve before normalization in FIG. 6(a), it was confirmed that the impedance (Ω) of the cycle-degraded positive electrode is larger than the impedance (Ω) of the undegraded positive electrode.

On the other hand, a capacitance vs. ΔZ' (%) curve was created by normalizing the capacitance vs. ΔZ' (Ω) curve so that the lowest impedance (Ω) in the graph was 100% (Figure 6(b) standard (after). As is clear from FIG. 6(b), it was confirmed that the impedance curves after normalization matched well in both scale and shape before and after deterioration.

In addition to SOC dependence, the impedance of a lithium ion battery changes depending on the size of the electrode to be evaluated and the deterioration of the positive or negative electrode. Therefore, in order to evaluate only the SOC dependence, it is necessary to It is necessary to eliminate the effects of deterioration.

On the other hand, the relationship characteristics of the capacitance vs. ΔZ' (Ω) curve are normalized so that the lowest impedance (Ω) in the graph is 100%, and the capacitance vs. ΔZ' (%) curve (Figure 6(b) ) After normalization), the scale and shape match well before and after deterioration. In other words, it was confirmed that the influence of electrode size and deterioration of the positive electrode or negative electrode was eliminated. Therefore, by normalizing the impedance curve, the influence of the electrode size and deterioration of the positive or negative electrode before and after deterioration of the impedance of a lithium-ion battery whose usage history is unknown is eliminated, and it is possible to evaluate it on the same scale due to SOC dependence. Obtained verification results.

[Positive and negative polarity attribution effect of impedance]
Attribution of impedance to positive and negative polarities is executed by proceeding from S21 to S22 to S23 to S24 to S25 in the flowchart of FIG. That is, in S21, the state of charge (capacity) vs. impedance curve of a 100% SOH lithium ion battery is obtained divided into different frequency ranges (for example, a low frequency range and a high frequency range). In S22, the lithium ion battery with 100% SOH (before deterioration) is disassembled. In S23, the positive electrode half cell and the negative electrode half cell whose positive electrode and negative electrode can be evaluated from the cells of the disassembled battery are reassembled. In S24, the state of charge (capacity) vs. impedance curves of each of the positive half cell and the negative half cell are obtained separately for different frequency ranges (for example, a low frequency range and a high frequency range). In S25, the state of charge (capacity) vs. impedance curves acquired in S21 and S24 are compared, and it is determined which of the curve characteristics in different frequency ranges belongs to the positive electrode or the negative electrode.

Next, with reference to FIGS. 7A, 7B, and 7C, we will verify that the characteristics in the low frequency range of a 100% SOH lithium-ion battery are attributable to the positive electrode of the lithium-ion battery in terms of impedance attribution to the positive and negative electrodes. explain.

For the positive electrode half cell and the negative electrode half cell, after conditioning treatment by initial charging and discharging using a charging/discharging test device, a 50-hour rate capacity measurement was performed at a current value (C/50) of 1/50 of the rated capacity, and a 50-hour AC impedance was measured at every 10% charge state relative to the rate capacity. Note that the AC impedance measurement is performed at, for example, amplitude: 10 mV, frequency range: 7 MHz to 5 mHz, and number of measurement points: 10 points/decade.

For the positive electrode half cell, the difference between 220,000 Hz and 126 Hz (ΔZ' _{positive electrode high frequency} = Z' 126 Hz - Z' 220,000 _Hz ) and the difference between ₁₂ Hz and 0.01 Hz from the AC impedance spectrum (Nyquist plot) at every 10% charging state. The difference in Z'(ΔZ' _{positive low frequency} = Z' _{0.01 Hz} - Z' _{12 Hz} ) was determined, and a capacitance vs. ΔZ' curve was created for each.

For the negative electrode half cell, the difference between 6,400 Hz and 85 Hz (ΔZ' _{negative electrode high frequency} = Z' ₈₅ Hz - Z' _{6,400 Hz) and the difference between 85 Hz and 0.1 Hz are determined from the AC impedance spectrum (Nyquist plot} ) at every 10% charging state. The difference (ΔZ' _{negative low frequency} = Z' _{0.1 Hz} - Z' _{85 Hz} ) was determined, and a capacitance vs. ΔZ' curve was created.

7A, FIG. 7B, and FIG. 7C show capacity vs. ΔZ′ curves, Nyquist plots, and Bode diagrams of 100% SOH lithium ion batteries, positive electrode half cells, and negative electrode half cells, respectively. The shapes of the capacity vs. ΔZ' curves of the positive electrode half cell and the negative electrode half cell were compared with the capacity vs. ΔZ' curve of a 100% SOH lithium ion battery. When comparing the shapes, it was found that the low frequency capacity vs. ΔZ' curve of the 100% SOH lithium ion battery (the solid line characteristic connecting the ● in Figure 7A) is the positive electrode low frequency curve of the positive electrode half cell (the solid line characteristic connecting the ● in Figure 7B). ) was in good agreement. Therefore, it was verified that the low frequency range of the 100% SOH lithium ion battery originates from the positive electrode and belongs to the positive electrode of the lithium ion battery.

[Effects of deterioration diagnosis method]
As explained above, the method for diagnosing deterioration of a lithium ion battery (secondary battery) of Example 1 has the following effects.

(1) A method for diagnosing deterioration of a secondary battery using an AC impedance method, which includes a pre-deterioration battery measurement step (S1) of measuring a plurality of impedances by changing the charging state of the pre-deterioration secondary battery, and a plurality of impedances. A characteristic creation step (S3) of creating a relational characteristic of the impedance curve to the charging capacity of the secondary battery before deterioration based on the measurement data of Based on the degraded battery measurement step (S4) for measuring the impedance of the battery, the relational characteristics created in the characteristic creation step (S3), and the plurality of impedance measurement data obtained in the degraded battery measurement step (S4). A deviation amount estimation step (S7) of estimating the charge capacity deviation amount of the positive and negative electrodes (SOC deviation amount) before and after deterioration, and a deviation amount estimation step (S7) in which the charge capacity deviation amount of the positive and negative electrodes is equivalent to the capacity deterioration amount of the secondary battery after deterioration. and a deterioration degree determination step (S8) of determining the deterioration degree SOH of the degraded secondary battery. Therefore, remaining capacity information of a degraded secondary battery whose usage history is unknown can be easily and accurately acquired. That is, using the SOC dependence of the impedance of a degraded secondary battery whose usage history is unknown, the SOC deviation amount of the positive and negative electrodes of the degraded secondary battery is estimated from the impedance measured in multiple charging states, and the degree of degradation SOH is estimated. A determination is made.

(2) When the relationship characteristic of the impedance (Ω) curve against the charging capacity of the secondary battery before deterioration created in the characteristic creation step (S3) is called the pre-standardization characteristic, the impedance (Ω) is the lowest from the pre-standardization characteristic. Determine the charging capacity (SOC) of the secondary battery before deterioration, calculate the impedance ratio (%) of the charging capacity at other measurement points so that the lowest impedance (Ω) becomes 100%, and calculate the charging capacity of the secondary battery before deterioration. It has a normalization step (S6) of normalizing the relational characteristic of the impedance ratio (%) curve to the impedance ratio (%) curve. The deviation amount estimation step (S7) calculates the impedance ratio (%) calculated from the relationship characteristic of the impedance ratio (%) curve to the standardized charging capacity and the impedance measurement data according to the charging state including at least the full charging region and the discharging region. %) and, the charge capacity deviation amount (SOC deviation amount) of the positive and negative electrodes of the degraded secondary battery is estimated. Therefore, by normalizing the curve characteristics, the influence of the electrode size and deterioration of the positive electrode or the negative electrode is eliminated, and the SOC dependence of the impedance of the secondary battery can be evaluated on the same scale before and after deterioration. In addition, when diagnosing the deterioration of a degraded secondary battery whose usage history is unknown, all that is required is to obtain impedance measurement data in at least two charging states that can be used to calculate the impedance ratio (%) for the degraded secondary battery. That's fine. As a result, it is not necessary to take a long time to measure the impedance, and the impedance measurement for the degraded secondary battery can be easily completed in a short period of time, thereby making it possible to perform a simple SOH diagnosis that accurately determines the degree of SOH deterioration.

(3) Create a positive half cell and a negative half cell that can evaluate the positive and negative electrodes of the secondary battery before deterioration, and draw the impedance curve for the charging capacity of the secondary battery before deterioration, the impedance curve for the charging capacity of the positive half cell, and the impedance curve for the negative half cell. Obtain impedance curves for charging capacity for different frequency ranges. Compare the impedance curves for charging capacity of the secondary battery before deterioration, the positive electrode half cell, and the negative electrode half cell, and determine which of the curve characteristics in different frequency ranges of the secondary battery before deterioration belongs to the positive electrode or the negative electrode. It has a belonging determination step (S2). The characteristic creation step (S3) and the standardization step (S6) create a relational characteristic of the impedance curve to the charging capacity of the secondary battery before deterioration, based on the result of determining whether the measurement data belongs to the positive electrode or the negative electrode. For this reason, when determining the degree of deterioration SOH of a secondary battery after deterioration, it can be determined by evaluating whether the positive electrode belongs or the negative electrode belongs. In addition, the relationship characteristic of the impedance curve with respect to the charge capacity of the secondary battery before deterioration can be created using measurement data obtained by impedance measurement in a specific frequency range. That is, it does not take a long time to create the relationship characteristics of the impedance curve to the charge capacity of the secondary battery before deterioration and to standardize the relationship characteristics.

(4) A temperature correction step (S5) is included in which the measurement data obtained by impedance measurement is subjected to temperature correction based on the environmental temperature. Therefore, when acquiring remaining capacity information of a degraded secondary battery whose usage history is unknown, it is possible to prevent deterioration diagnosis accuracy from decreasing due to differences in environmental temperature.

Next, the usefulness and accompanying effects obtained by providing the deterioration diagnosis method of Example 1 will be explained. The deterioration diagnosis method of Example 1 can be utilized for determining the value of used electric vehicles, determining whether to reuse lithium ion batteries, etc. By installing a system using a lithium-ion battery deterioration diagnosis method as an in-vehicle device or incorporating it into a charger or inspection device, it is possible to improve the accuracy of evaluating the remaining capacity of a lithium-ion battery. Additionally, if it becomes standardized as a deterioration diagnosis system for lithium-ion batteries, it will lead to safe secondary use of batteries and revitalize the market. Furthermore, electrodes can be created to make it easier to use in a lithium-ion battery deterioration diagnosis system, such as by selecting active materials with high impedance dependence on SOC for the positive and negative electrodes, and by creating electrodes that have a large impedance difference between the positive and negative electrodes. It is possible to provide design guidelines for secondary batteries such as:

The method for diagnosing deterioration of a secondary battery according to the present invention has been described above based on the first embodiment. However, the specific types and procedures of secondary batteries are not limited to this Example 1, and as long as they do not deviate from the gist of the invention according to each claim, for example, in the deterioration diagnosis method. Step changes and step additions are permitted.

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

In Example 1, an example was shown in which LiNi _1/3 Co _1/3 Mn _1/3 O ₂ was used as the type of positive electrode active material of the secondary battery. However, the type of positive electrode active material is not limited to this,
LiNi _1-xy Co _x Mn _y O ₂ , LiNi _0.8 Co _0.15 Al _0.05 O ₂ represented by LiNi _0.5 Co _{0.2 Mn 0.3} O ₂ , LiNi _0.6 Co _0.2 Mn _0.2 O 2 , _{LiNi 0.8} Co 0.1 _Mn 0.1 O ₂ It may be LiNi _1-xy Co _x Al _y O ₂ , LiMn ₂ O ₄ , LiNi _2-x Mn _x O ₂ typified by LiNi _0.5 Mn _1.5 O ₄ , or a mixture thereof. .

In Example 1, an example was shown in which graphite was used as the type of negative electrode active material of the secondary battery. However, the type of negative electrode active material is not limited thereto, and may be graphite-based, silicon-based alloy, Li ₄ Ti ₅ O ₁₂ , or a mixture thereof.

In Example 1, an example was shown in which a lithium ion battery electrolyte was used as the type of electrolyte for the secondary battery. However, the type of electrolyte is not limited to this, and may be a solid electrolyte for all-solid-state batteries, a mixture thereof, or the like.

In Example 1, an example was shown in which the AC impedance was measured with amplitude: 10 mV, frequency range: 7 MHz to 5 mHz, and number of measurement points: 10 points/decade. However, AC impedance measurement is not limited to this. For example, the frequency range of the AC impedance spectrum may be 10 MHz to 0.1 mHz. Further, the number of measurement points may be at least two points in the frequency range of the AC impedance spectrum.

S1 Pre-deterioration battery measurement step S2 Attribution determination step S3 Characteristic creation step S4 Deterioration battery measurement step S5 Temperature correction step S6 Standardization step S7 Deviation amount estimation step S8 Degradation degree determination step

Claims (4)

A method for diagnosing deterioration of a secondary battery using an AC impedance method,
a pre-deterioration battery measurement step of measuring a plurality of impedances by changing the charging state of the pre-deterioration secondary battery;
a characteristic creation step of creating a relationship characteristic of an impedance curve to a charging capacity of the secondary battery before deterioration based on the plurality of impedance measurement data;
a degraded battery measurement step of measuring a plurality of impedances by changing the charging state of a degraded secondary battery whose usage history is unknown;
A deviation amount estimation step of estimating the amount of charge capacity deviation of the positive and negative electrodes before and after deterioration based on the relational characteristics created in the characteristic creation step and the plurality of impedance measurement data acquired in the post-deterioration battery measurement step. and,
A deterioration degree determining step of determining the degree of deterioration of the deteriorated secondary battery by regarding the charge capacity deviation amount of the positive and negative electrodes to be equivalent to the capacity deterioration amount of the deteriorated secondary battery. A method for diagnosing deterioration of secondary batteries. In the method for diagnosing deterioration of a secondary battery according to claim 1,
When the relationship characteristic of the impedance (Ω) curve against the charging capacity of the secondary battery before deterioration created in the characteristic creation step is referred to as the characteristic before normalization,
The charging capacity with the lowest impedance (Ω) is determined from the characteristics before normalization, and the impedance ratio of the charging capacity at other measurement points is calculated so that the lowest impedance (Ω) becomes 100%, and the impedance ratio of the charging capacity at other measurement points is determined. a normalization step of normalizing the relational characteristics of the impedance ratio (%) curve to the charge capacity of the previous secondary battery;
In the step of estimating the amount of deviation, the impedance ratio (%) is calculated based on the relationship characteristic of the impedance ratio (%) curve with respect to the standardized charging capacity and the impedance measurement data according to the charging state including at least the full charge region and the discharge region. ) to estimate the charge capacity difference between the positive and negative electrodes of the degraded secondary battery. In the method for diagnosing deterioration of a secondary battery according to claim 2,
Producing a positive electrode half cell and a negative electrode half cell that can evaluate the positive electrode and negative electrode of the secondary battery before deterioration,
Obtaining an impedance curve for the charging capacity of the secondary battery before deterioration, an impedance curve for the charging capacity of the positive electrode half cell, and an impedance curve for the charging capacity of the negative electrode half cell for each different frequency range,
The impedance curves for the charging capacity of the secondary battery before deterioration, the positive electrode half cell, and the negative electrode half cell are compared, and based on the similarity judgment of the curve characteristics, among the curve characteristics in different frequency ranges of the secondary battery before deterioration, an attribution determination step of determining which characteristic belongs to the positive electrode or the negative electrode;
The characteristic creation step and the standardization step create a relational characteristic of an impedance curve to a charge capacity of the secondary battery before deterioration based on a determination result of whether the measurement data belongs to a positive electrode or a negative electrode. How to diagnose battery deterioration. In the method for diagnosing deterioration of a secondary battery according to any one of claims 1 to 3,
A method for diagnosing deterioration of a secondary battery, comprising a temperature correction step of performing temperature correction based on an environmental temperature on the measurement data obtained by measuring the impedance.