JP7116484B2 - Electrochemical system and method of operating electrochemical system - Google Patents

Description

Embodiments of the present invention relate to electrochemical systems for measuring electrochemical cells that include multiple electrodes and electrolytes, and methods of operating electrochemical systems that include multiple electrodes and electrolytes.

Impedance measurement of an electrochemical cell including a plurality of electrodes and an electrolyte is widely used to elucidate the mechanism of electrochemical reaction. As an impedance measurement method, an AC impedance method is known in which the frequency of a sine wave signal applied to an electrochemical cell to be measured is scanned.

In the AC impedance method, a frequency response analyzer (FRA) and a potentiostat are used. The FRA outputs a frequency response signal for applying a sinusoidal signal of a given frequency to the electrochemical cell. The potentiostat controls the voltage (current) applied to the electrochemical cell based on the frequency signal from the FRA.

By scanning the frequency of the sine wave signal, the impedance at a plurality of frequencies, that is, the frequency characteristics of the impedance are obtained. The frequency characteristics of impedance are expressed in a complex plane diagram with the Z' (real impedance) axis as the resistance component and the Z'' (imaginary impedance) axis as the reactance component (usually capacitive). call plot).

The Nyquist plot shown in FIG. This is the case of the model considering the diffusion Zw. That is, an electrochemical reaction in an electrochemical cell system using a reference electrode is composed of ion migration in the electrolyte, charge transfer reaction at the electrode interface, and accompanying ion diffusion. In an electrochemical cell that does not use a reference electrode, the impedance of two electrodes (positive electrode and negative electrode) is included, so the semicircular trajectory is a trajectory in which at least two semicircles overlap. By analyzing the trajectory using an equivalent circuit model, it is possible to grasp the characteristics of each constituent element such as a plurality of electrodes and an electrolyte that constitute the electrochemical cell.

For example, when the diameter of the semicircle indicating the reaction resistance Rct(R2) increases, it indicates that the cell has deteriorated. That is, when the electrochemical cell is a secondary battery, the active material itself deteriorates due to changes in the crystal structure, and the ionic electrolyte components and organic solvent in the electrolyte decompose. It is deposited in the form of ions and inorganic substances, and the intercalation and deintercalation of ions is inhibited, resulting in an increase in resistance.

In JP-A-2014-126532, a second signal obtained by superimposing a rectangular wave on a first signal (action signal) of constant voltage (immersion potential: 0.221 V) is applied to the battery, and the response signal is subjected to Fourier transform. Thus, a measurement device is disclosed for obtaining impedance at multiple harmonic frequencies.

However, when the first signal changes with time, the response signal also changes, so the conventional method cannot perform dynamic measurement. For varying signals, the impedance was statically measured by stopping the increase and decrease of the signal.

Also, when measuring electrochemical reactions, it is important to perform general electrochemical measurements and impedance measurements of the measurement system.

However, in conventional measurement methods, it was necessary to set all measurement conditions including measurement points before starting measurement. Therefore, when there are multiple measurement items, it is necessary to determine other measurement conditions based on the result of one measurement item. For example, after obtaining the current-potential characteristics (cyclic voltammetry: CV) of the measurement system, a measurement point for measuring impedance is set based on the CV curve, and remeasurement is performed.

That is, when measuring two types of measurement items (CV characteristics and impedance), measurements were performed twice. However, in the second measurement performed after the first measurement, the state of the electrochemical cell to be measured may have changed. Therefore, so-called operand measurement, in which impedance measurement is performed simultaneously with electrochemical measurement of an electrochemical cell, has been desired. Operando means working in Latin.

Itagaki et al. have proposed a 3D impedance method that can acquire the impedance in an arbitrary measurement range that has not been set before the measurement after the measurement. However, in the 3D impedance method, the impedance in an arbitrary measurement range obtained after measurement is calculated data obtained by complementary processing based on the impedance measurement results of several times to several tens of times, and is not actual measurement data. rice field.

JP 2014-126532 A M. Itagaki, et al., Journal of Power Sources, 148 (2005) 78-84

Embodiments of the present invention aim to provide an electrochemical system that obtains impedance from a varying response signal and a method of operating an electrochemical system that obtains impedance from a varying response signal.

An electrochemical system according to an embodiment comprises an electrochemical cell including a working electrode, a counter electrode, and an electrolyte; a power source applied between the working electrode and the counter electrode of a chemical cell; a storage section for continuously storing response signals of the second signal; and the changing response stored in the storage section. A selection unit that arbitrarily selects a signal analysis range, and a calculation unit that performs a Fourier transform on the response signal in the analysis range and calculates each impedance at the plurality of frequencies.

In the method of operating an electrochemical system according to an embodiment, a second signal obtained by superimposing an auxiliary signal including a plurality of frequency components on a first signal that changes with time is an electrochemical cell including a working electrode, a counter electrode, and an electrolyte . a step of applying a signal applied between the working electrode and the counter electrode; a step of storing a response signal to the second signal, which is performed simultaneously with the step of applying the signal, in a storage unit; a selection step of arbitrarily selecting an analysis range of the changing response signal stored in the storage unit, performed after the application step and the storage step; and a calculating step of converting to calculate respective impedances at the plurality of frequencies.

Embodiments of the present invention can provide an electrochemical system that obtains impedance at any point in time from a changing response signal, and a method of operating an electrochemical system that obtains impedance at any point in time from a changing response signal. .

It is a figure which shows an example of a Nyquist plot. 1 is a configuration diagram of an electrochemical system according to a first embodiment; FIG. FIG. 3 is a diagram showing a first signal of the electrochemical system of the first embodiment; FIG. FIG. 4 is a diagram showing auxiliary signals of the electrochemical system of the first embodiment; FIG. 4 is a diagram showing a second signal of the electrochemical system of the first embodiment; FIG. 4 is an enlarged view showing a second signal of the electrochemical system of the first embodiment; FIG. FIG. 4 is a diagram showing a response signal of the electrochemical system of the first embodiment; FIG. 4 is an enlarged view showing response signals of the electrochemical system of the first embodiment; FIG. FIG. 4 is a diagram showing Fourier transform results of response signals of the electrochemical system of the first embodiment; FIG. 4 is a diagram showing Fourier transform results of response signals of the electrochemical system of the first embodiment; FIG. 4 is a Nyquist plot of the response signal of the electrochemical system of the first embodiment; FIG. 4 is a Nyquist plot of the response signal of the electrochemical system of the first embodiment; 4 is a flow chart of a method of operating the electrochemical system of the first embodiment; FIG. 4 is a Nyquist plot of the response signal of the electrochemical system of the first embodiment; FIG. 2 is a configuration diagram of an electrochemical system of a second embodiment; FIG. FIG. 11 shows a second signal of the electrochemical system of the second embodiment; FIG. 10 is a Nyquist plot of the response signal of the electrochemical system of the second embodiment; FIG. 10 is a diagram showing the impedance of the response signal of the electrochemical system of the second embodiment; FIG. 10 is a diagram showing a second signal of the electrochemical system of the modification of the second embodiment; FIG. 10 is a diagram showing a Nyquist plot of the response signal of the electrochemical system of the modified example of the second embodiment; FIG. 10 is a diagram showing the impedance of the response signal of the electrochemical system of the modified example of the second embodiment;

<First Embodiment>
As shown in FIG. 1, the electrochemical system 1 of this embodiment includes an electrochemical cell 10, a power supply 20, a storage unit 30, a selection unit 40, a calculation unit 50, and an analysis unit 60. .

As will be described later, in the electrochemical system 1, measurement data is stored in the storage unit 30. FIG. After the measurement is finished, analysis is performed using the measurement data stored in the storage unit 30 . This method of operation is referred to as the "time shift method".

The electrochemical system 1 is a system for analyzing the redox reaction of iron hexacyano. The electrochemical cell 10 includes a working electrode (WE) 11 made of glassy carbon, a counter electrode (CE) 12 made of platinum wire, a reference electrode (RE) 13 made of silver/silver chloride and 3M-NaCl, and an electrolyte 14. and including. The electrolyte 14 is an aqueous solution consisting of 5 mM K ₄ [Fe(CN) ₆ ], 5 mM K ₃ [Fe(CN) ₆ ] and 0.5 M KNO ₃ .

The power supply 20 includes a first signal generating section (first signal generating circuit) 21, an auxiliary signal generating section (auxiliary signal generating circuit) 22, and a superimposing section (superimposing circuit) 23 that superimposes the first signal and the auxiliary signal. including.

The first signal generator 21 generates a first signal, which is a measurement signal (action signal) for measuring the oxidation-reduction reaction of the electrochemical cell 10 . The first signal is based on the reference electrode (RE) 13, and the voltage changes linearly with time. The first signal may be a signal based on current instead of voltage.

As shown in FIG. 3, the first signal is a linear sweep signal in which the voltage V1 linearly increases and decreases with time. In the electrochemical system 1, the first signal has a period of 300 seconds, an amplitude ΔV1 of 0.6 V, and a sweep speed of 4 mV/s (frequency, 0.0033 Hz).

The auxiliary signal generator 22 generates an auxiliary signal containing multiple frequency components. As shown in FIG. 4, in the electrochemical system 1, the auxiliary signal is a continuous rectangular wave signal with one period of (1/f1), that is, the frequency of f1. Frequency f1 is, for example, 3.968 Hz. A square wave signal of frequency f1 potentially contains multiple harmonics of frequency f1, ie, frequencies f2, f3, f4, etc., which are integer multiples of frequency f1.

The amplitude ΔVs of the auxiliary signal is set to 1/50 or less, preferably 1/100 or less of the amplitude ΔV1 of the voltage V1 of the first signal so as not to affect the measurement result. For example, when the amplitude ΔV1 is 1.2V, ΔVs is set to 10mV.

The superimposing unit 23 applies a second signal obtained by superimposing the auxiliary signal on the first signal to the working electrode (WE) 11 and the counter electrode (CE) 12 of the electrochemical cell 10 .

5 and 6 show a second signal (input signal) in which the auxiliary signal is superimposed on the first signal, which is output by the superimposing unit 23 and input to the electrochemical cell 10. FIG.

The power source 20 may be a second signal generator that generates a second signal in which the first signal and the auxiliary signal are superimposed. For example, the power supply 20 may have a function generator that generates a signal with a predetermined waveform and a bipolar power supply that outputs a second signal by amplifying the signal with a predetermined waveform.

In addition, a second signal in which the auxiliary signal is superimposed on the measurement signal based on the counter electrode (CE) of the electrochemical cell 10 that does not have the reference electrode (RE) 13 is the working electrode (WE) 11 and the counter electrode ( CE) 12 may be applied.

The storage unit 30 continuously stores the response signal of the electrochemical cell 10 to which the second signal is applied. The storage unit 30 stores the digitized response signal. That is, the electrochemical system 1 has an AD converter that digitizes the response signal.

The storage unit 30 is a storage circuit that stores voltage and current at a predetermined sampling rate and a predetermined bit rate from the start of application of the second signal until the end of application. A sampling rate of 1 kHz or more and a bit rate of 8 bits or more are preferable in order to obtain highly accurate data. Particularly preferably, the sampling rate is 10 kHz or higher and the bit rate is 16 bits or higher.

The storage unit 30 is preferably a semiconductor memory that records digital signals at high speed. After the measurement is completed, the data stored in the semiconductor memory may be transferred to a magnetic recording device capable of storing a large amount of data, and the subsequent steps may be performed.

7 and 8 show response signals (voltage/current) of the electrochemical cell 10 to which the second signal was applied, which are stored in the storage section 30 after the measurement. (a) to (h) in FIG. 7 are analysis ranges to be analyzed selected by the selection unit 40, which is a selection circuit.

The analysis range can be arbitrarily selected based on the measurement results (V/I curve) shown in FIG. The analysis range is preferably 1 cycle or more and 50 cycles or less of the auxiliary signal. If it is more than the said analysis range, the data with little influence of a noise component will be obtained. Dynamic data are obtained if the range is less than or equal to the aforementioned analysis.

(a) Voltage application start (b) Voltage and current increase (current 0A)
(c) Current is maximum (d) Current is decreasing (e) Voltage is maximum (0.6V)
(f) Voltage and current decrease (current 0A)
(g) Current is minimum (h) Voltage decreases, current increases

(a) to (h) are the range of the second auxiliary signal (f1=39.68 Hz), 5 cycles.

9 and 10 show the Fourier transform results of the selected analysis range (a) calculated by the calculation unit 50, which is the calculation circuit. The voltage and current contain harmonics of the frequency f1 of the auxiliary signal.

The calculator 50 calculates the impedance (Z′, Z″) from the voltage and current in the harmonics.

Figures 11 and 12 show the Nyx plots for selected analysis ranges (a)-(h). 11 and 12 show responses measured by applying a second signal superimposed with an auxiliary signal (f1=39.68 Hz, ΔVs=10 mV) separately in order to obtain Nyx plots over a wide frequency range. The signal impedance (Z', Z'') is also plotted.

Impedances of 3.968 Hz to 35.712 Hz were obtained from the response signal of the second signal superimposed with the first auxiliary signal (f1=3.968 Hz). Impedances of 39.68 Hz to 4.008 kHz were obtained from the response signal of the second signal superimposed with the second auxiliary signal (f1=39.68 Hz).

Table 1 shows the characteristics of the electrochemical cell 10 by the analysis unit 60, which is an analysis circuit, using an equivalent circuit model based on the Nyquist plot.

(Table 1)

As the potential difference ΔE increases, the reaction resistance Rct (R2) increases, and a dynamic change in the ionic state of the interface is obtained. The potential difference ΔE is the difference between the measured potential E and the half-wave potential E _1/2 , which is the potential when the current is 50% of the maximum or minimum.

The electrochemical system 1 can obtain the impedance at any time from the changing response signal. Furthermore, unlike the conventional system, the electrochemical system 1 simply sets the analysis range (measurement conditions) after performing cyclic voltammetry measurement and acquiring the CV curve, without performing the second measurement. can obtain the impedances Z′ and Z″ of the measured potential E (analysis range). In other words, the same dynamic measurement result is obtained as the operando measurement, in which the impedance measurement is performed simultaneously with the cyclic voltammetry measurement. The electrochemical system 1 can obtain impedance in dynamic processes with high reliability.

<Method of operating the electrochemical system>
As shown in FIG. 3, the operating method of the electrochemical system 1 includes a signal application step S10, a storage step S20, a selection step S30, a calculation step S40, and an analysis step S50.

<Step S10> Signal application step In the signal application step, a second signal obtained by superimposing an auxiliary signal including a plurality of frequency components on the first signal that changes over time is applied to an electrochemical cell including a plurality of electrodes and an electrolyte. applied.

<Step S20> Storage Step The storage step is performed at the same time as the signal application step S10, in which the response signal of the second signal is continuously AD-converted and continuously stored as a digital signal.

<Step S25>
The operation from step S10 is repeated until the measurement is completed (No). When the measurement ends (Yes), the process proceeds to step S30.

<Step S30> Selection step The selection step is performed after the signal application step S10 and the storage step S20, and an analysis range to be analyzed is selected (set) from the stored response signals that are changing. The analysis range may be selected by the user based on the response signal (see FIG. 7) displayed on the monitor, or the analysis range may be automatically selected based on preset conditions.

<Step S40> Calculation step In the calculation step, the impedance of each of the plurality of harmonics is calculated by Fourier transforming the response signal in the selected analysis range.

<Step S50> Analysis Step In the analysis step, the characteristics of the electrochemical cell 10 are analyzed using an equivalent circuit model based on Nyquist plots of impedances at a plurality of frequencies.

Note that the storage unit 30, the selection unit 40, the calculation unit 50, and the analysis unit 60 may be physically configured circuits, or may be a CPU that operates according to a program. Also, one circuit may constitute a plurality of functional units. For example, an AD conversion section (AD conversion circuit) that converts an analog response signal into a digital response signal and a CPU that constitutes the calculation section 50 may be integrated in one semiconductor chip.

FIG. 14 shows a second auxiliary signal (f1=39.68 Hz, ΔVs=10 mV) superimposed on the first auxiliary signal (f1=3.968 Hz, ΔVs=10 mV). FIG. 10 shows a Nyx plot based on the response signals of the two signals; FIG.

FIG. 14 is in good agreement with FIG. That is, a plurality of square waves having different frequencies may be superimposed on the auxiliary signal. Also, an auxiliary signal in which sine waves of different frequencies are superimposed may be used.

The time-varying first signal includes measurement signals for measuring general electrochemical reactions such as cyclic voltammetry (CV) and linear sweep voltammetry (LCV). After superimposing a very small auxiliary signal for impedance measurement and performing electrochemical measurement by CV or LCV, etc., it is possible to measure impedance in an arbitrary range (potential / current), so according to the electrochemical system 1 , the impedance in the measurement range (measurement point) required for analysis can be obtained.

The auxiliary signal for impedance measurement is applied with a current or current of predetermined amplitude, depending on the method of measurement.

Furthermore, other operand measurements may be combined in synchronism with electrochemical measurements in addition to impedance measurements. Other operando measurements include neutron beam diffraction analysis, X-ray diffraction analysis, X-ray absorption fine structure analysis, X-ray photoelectron spectroscopy, infrared spectroscopy, ultraviolet spectroscopy, Raman spectroscopy, and optical microscopy and electron microscopy. , X-ray analysis, spectroscopic analysis and microscopic observation.

<Second embodiment>
As shown in FIG. 15, the electrochemical system 1A of this embodiment has a similar basic configuration to the electrochemical system 1 of the first embodiment and has the same effects. Therefore, constituent elements having the same function are denoted by the same reference numerals, and descriptions thereof are omitted.

In the electrochemical system 1A, the electrochemical cell 10A is a secondary battery (storage battery) 10A. The battery 10A is, for example, a lithium ion battery including a positive electrode containing lithium cobalt oxide or the like, a negative electrode containing a carbon material or the like, and an electrolyte in which LiPF ₆ is dissolved in cyclic and chain carbonates. Note that the electrochemical cell is not limited to the secondary battery 10A as long as it is a power storage unit that can store and discharge electricity, and may be, for example, an electric double layer capacitor.

When the electrochemical system 1A is a power storage system for an electric vehicle, the load 25 is a motor. An AD conversion section (AD conversion circuit) and a CPU forming the storage section 30 are integrated in one semiconductor chip. Furthermore, the storage unit 30, the selection unit 40, the calculation unit 50, and the analysis unit 60 are configured by one computer including a monitor and a keyboard. The CPU of the computer is preferably a multi-core processor in order to operate multiple functional units at high speed.

FIG. 16 shows the response signal when the first signal acquired by the electrochemical system 1A is the charging signal. The first signal is a constant current signal whose voltage is limited to 3V or more and 4.2V or less. The auxiliary signals are a current-controlled first rectangular wave signal with a frequency f1=5 Hz and an amplitude ΔIs=0.5 A, and a second current-controlled rectangular wave signal with a frequency f1=50 Hz and an amplitude ΔIs=0.5 A. and is a superimposed signal. The amplitude ΔIs of the auxiliary signal obtained by superimposing the first rectangular wave signal and the second rectangular wave signal is 1A.

The sampling rate is 10 kHz and the bit rate is 24 bits. The response signal gradually increases in voltage as charging progresses.

FIG. 17 shows a Cole-Cole plot that changed with charging time, and FIG. 18 shows R1 and R2 obtained by analyzing the Cole-Cole plot.

With charging, R1 (Rs) and R2 (Rct) decreased, indicating that the battery characteristics changed with charging.

Note that, in the electrochemical system 1A, even when the first signal is the discharge signal, the battery 10A can be analyzed in the same manner as in the case of the charge signal.

FIG. 19 shows the response signal when the first signal acquired by the electrochemical system 1A is the discharge signal of the secondary battery 10A. The discharge signal is a constant current signal. The auxiliary signal is a current-controlled square wave superimposed signal of frequency f1=50 Hz and 50 Hz. The discharge signal gradually decreases in voltage as charging progresses.

FIG. 20 shows a Cole-Cole plot that changed with discharge time, and FIG. 21 shows R1 and R2 obtained by analyzing the Cole-Cole plot.

Due to discharge, R1 and R2 changed, and a change in battery characteristics accompanying discharge was obtained.

Comparing FIG. 17 and FIG. 20, the changes are not the same in both, so it can be seen that dynamic changes are acquired instead of static changes.

To simplify the explanation, the electrochemical system 1A has one battery 10A as an electrochemical cell. However, the electrochemical cell is a battery unit in which a plurality of batteries 10A are connected in series, a battery unit in which a plurality of batteries 10A are connected in parallel, or a battery unit in which a plurality of batteries 10A are connected in series and in parallel It is also possible to detect changes in the characteristics of the battery unit as a whole.

Moreover, the electrochemical systems 1 and 1A were equipped with an analysis section 60 that analyzes the characteristics of the electrochemical cells 10 and 10A using an equivalent circuit model based on the Nyquist plot.

However, the electrochemical system of the embodiment does not have to have the analysis unit 60 that analyzes using the equivalent circuit model. For example, if at least one impedance of a specific frequency can be obtained, it is possible to grasp a rough characteristic change of the electrochemical cell by comparing it with previously obtained impedances.

The present invention is not limited to the above-described embodiments and the like, and various modifications and alterations, such as combinations of constituent elements of the embodiments, are possible within the scope of the present invention.

DESCRIPTION OF SYMBOLS 1, 1A... Electrochemical system 10, 10A... Electrochemical cell 11, 12, 13... Electrode 14... Electrolyte 20... Power source 21... First signal generator 22... Auxiliary signal generator 23... Superimposed part 25... Load 30... Memory Part 40... Selection part 50... Calculation part 60... Analysis part

Claims (12)

an electrochemical cell comprising a working electrode, a counter electrode and an electrolyte;
a power supply that applies a second signal obtained by superimposing an auxiliary signal containing a plurality of frequency components on a first signal that changes over time between the working electrode and the counter electrode of the electrochemical cell;
a storage unit that continuously stores response signals from between the working electrode and the counter electrode of the second signal;
a selection unit that arbitrarily selects an analysis range of the changing response signal stored in the storage unit;
an electrochemical system, comprising: a calculator for Fourier-transforming the response signal in the analysis range to calculate respective impedances at the plurality of frequencies. wherein the auxiliary signal is a square wave of a first frequency;
2. The electrochemical system according to claim 1, wherein the calculator calculates impedances of the plurality of harmonics of the first frequency. A plurality of rectangular waves having different frequencies are superimposed on the auxiliary signal,
2. The electrochemical system according to claim 1 , wherein said calculator calculates respective impedances at respective multiple harmonics of multiple different frequencies. 1 to 1, further comprising an analysis unit that analyzes the characteristics of the electrochemical cell using an equivalent circuit model based on the Nyquist plot consisting of the respective impedances at the plurality of frequencies. Item 4. The electrochemical system according to any one of Item 3. 5. The electrochemical system according to any one of claims 1 to 4, wherein said first signal is a measurement signal for measuring an oxidation-reduction reaction of said electrochemical cell. the electrochemical cell is a secondary battery,
5. An electrochemical system according to any one of claims 1 to 4, wherein said first signal is a charging signal. 7. The electrochemical system of claim 6, wherein the secondary battery discharge signal is analyzed in the same manner as the charge signal. An AD conversion unit that digitizes the response signal,
8. The electrochemical system according to any one of claims 1 to 7, wherein the digitized response signal is stored in the storage unit. A second signal obtained by superimposing an auxiliary signal containing a plurality of frequency components on a first signal that changes over time is applied between the working electrode and the counter electrode of an electrochemical cell including a working electrode, a counter electrode, and an electrolyte. a signal applying step of
a storage step in which the response signal from between the working electrode and the counter electrode of the second signal is stored continuously in a storage unit, which is performed simultaneously with the signal applying step;
a selection step of arbitrarily selecting an analysis range of the changing response signal stored in the storage unit, performed after the signal application step and the storage step;
and a calculating step of Fourier transforming the response signal in the analysis range to calculate impedances at the plurality of frequencies. wherein the auxiliary signal is a square wave of a first frequency;
10. The method of operating an electrochemical system of claim 9, wherein the step of calculating comprises calculating respective impedances at a plurality of harmonics of the first frequency. A plurality of rectangular waves having different frequencies are superimposed on the auxiliary signal,
10. The method of operating an electrochemical system of claim 9 , wherein in the calculating step, respective impedances at a plurality of respective harmonics of a plurality of different frequencies are calculated. From claim 9, further comprising an analysis step of analyzing the characteristics of the electrochemical cell using an equivalent circuit model based on the Nyquist plot consisting of the respective impedances at the plurality of frequencies. 12. A method of operating an electrochemical system according to any one of claims 11.

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