Secondary Battery System

Abstract

To provide a secondary battery system capable of easily separating a high-rate deterioration of a secondary battery and a deterioration mode different from the high-rate deterioration.SOLUTION: A secondary battery system 100 includes a secondary battery 101, an impedance measurement unit 125, and a diagnosis unit 116. The secondary battery 101 has electrode bodies 107 and 108 impregnated with an electrolytic solution 109 containing metal ions. The impedance measurement unit 125 is configured to measure an impedance of the secondary battery 101. The diagnosis unit 116 is configured to detect high-rate deterioration caused by uneven concentration of the metal ions in the electrolytic solution 109 impregnated into the electrode bodies 107 and 108 on the basis of a change in the impedance of the secondary battery 101 measured by the impedance measurement unit 125 during direct current charging or discharging of the secondary battery 101.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a secondary battery system. [Background technology]

[0002] A specific degradation mode of secondary batteries, which differs from normal degradation due to aging, is known. This specific degradation mode is called high-rate degradation, for example. To improve the safety of secondary batteries, it is desirable to quickly detect the occurrence of high-rate degradation and to suppress the progression of high-rate degradation.

[0003] High-rate degradation is a temporary increase in internal resistance that occurs when a secondary battery is charged or discharged at high input and output values, and is thought to be caused by an imbalance in the electrolyte inside the secondary battery.

[0004] Therefore, a system is proposed in, for example, Patent Document 1, which obtains the DC resistance of a secondary battery from the impedance of the secondary battery measured by a measuring device and estimates the state of high-rate degradation using the difference between the DC resistance and the initial DC resistance of the secondary battery. In Patent Document 1, a change in the DC resistance component of the multiple impedance components of the secondary battery is used as an index showing the progress of high-rate degradation.

[0005] Specifically, the DC resistance component, which is represented as the starting point of an arc in a complex impedance plot of the impedance measurement results, is acquired. High-rate degradation is then determined based on the difference from the initial DC resistance component, i.e., the increase in the DC resistance component. If high-rate degradation is determined, control is performed to suppress the charge / discharge current of the secondary battery. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2018-190502 Summary of the Invention [Problem to be solved by the invention]

[0007] The above-mentioned conventional technology is based on the premise that there is no change in the DC resistance component during normal deterioration due to aging. However, the inventors' investigations have revealed that even during normal deterioration, the DC resistance component increases due to, for example, the formation of a solid electrolyte interphase (SEI) film on the surface of the electrode active material due to storage deterioration. In this case, it becomes difficult to accurately distinguish between high-rate deterioration and a deterioration mode other than high-rate deterioration.

[0008] In view of the above, an object of the present invention is to provide a secondary battery system that can easily distinguish between high-rate degradation of a secondary battery and a degradation mode other than high-rate degradation. [Means for solving the problem]

[0009] In order to achieve the above object, claim 1 、3 In the invention described above, the secondary battery system includes a secondary battery (101), an impedance measuring unit (125), and a diagnostic unit (116).

[0010] The secondary battery has electrode bodies (107, 108) impregnated with an electrolyte solution (109) containing metal ions. The impedance measuring unit measures the impedance of the secondary battery.

[0011] The diagnostic unit detects high-rate deterioration caused by uneven concentration of metal ions in the electrolyte that has permeated the electrode body. In a secondary battery, based on the difference between the impedance during DC charging / discharging and the impedance when DC charging / discharging is not performed, Detect. In the invention described in claim 1, the diagnostic unit detects high-rate deterioration of the secondary battery before the secondary battery is used at high power. In addition, in the invention described in claim 3, the impedance measurement unit uses a frequency for measuring the impedance during DC charging and discharging of the secondary battery that is different from the frequency of the AC signal component contained in the DC current that charges and discharges the secondary battery and the frequency of the harmonic components contained in the AC signal component.

[0012] The inventors discovered that during DC charging and discharging of a secondary battery, the salt concentration in the electrolyte solution permeated into the electrode assembly increases unevenly, and that this uneven salt concentration appears as a change in impedance. Therefore, the diagnostic unit can detect high-rate degradation based on the change in impedance during DC charging and discharging of the secondary battery. This makes it easy to distinguish between high-rate degradation of the secondary battery and a degradation mode other than high-rate degradation.

[0013] The symbols in parentheses for each means described in this section and in the claims indicate the correspondence with the specific means described in the embodiments described later. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a diagram showing a configuration of a secondary battery system according to a first embodiment. [Figure 2] FIG. 2 is a diagram showing the configuration of the secondary battery shown in FIG. [Figure 3] FIG. 10 is a diagram showing terms included in a theoretical formula for the measurement effect due to an external magnetic field. [Figure 4] FIG. 1 is a diagram showing a simulated cycle. [Figure 5] FIG. 10 is a diagram showing impedances during and after discharge when a simulated cycle is performed. [Figure 6] FIG. 6 is a diagram showing the change in DC resistance at 1116 Hz shown in FIG. 5 according to the number of simulated cycles. [Figure 7] FIG. 10 is a diagram showing changes in impedance during a storage deterioration test of a secondary battery. [Figure 8] FIG. 10 is a diagram showing the transition of impedance during high-rate degradation of a secondary battery when not energized. [Figure 9] 10 is a diagram showing the degradation determination output values ​​when high-rate degradation is determined without passing a DC current through the secondary battery, and when high-rate degradation is determined by passing a DC current through the secondary battery. FIG. [Figure 10] 10A and 10B are diagrams showing the feature amounts of a DC current component and a reaction resistance component. [Figure 11] FIG. 10 is a diagram showing the relationship between the magnitude of a direct current and impedance fluctuations. [Figure 12] 4 is a flowchart showing the contents of a diagnosis of high-rate deterioration of a secondary battery performed by a diagnostic unit of a control device. [Figure 13] FIG. 10 is a diagram showing a change in the resistance increase rate when input and output of a secondary battery are limited after repeated simulation cycles. [Figure 14] 10 is a flowchart showing the contents of a diagnosis of high-rate deterioration of a secondary battery according to a second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following embodiments, parts that are the same or equivalent to each other are denoted by the same reference numerals in the drawings. (First embodiment) The secondary battery system according to this embodiment extracts high-rate degradation as degradation of a secondary battery mounted on a vehicle. As shown in FIG.

[0016] As shown in Fig. 2, the secondary battery 101 constitutes a battery module in which a plurality of cells 103 are connected in series. Each cell 103 is, for example, a lithium-ion secondary battery. The secondary battery 101 constitutes a power supply unit for an electrically powered vehicle such as an electric vehicle or a hybrid vehicle. The battery module may also include a configuration in which the cells 103 are connected in parallel.

[0017] The secondary battery 101 is connected via an in-vehicle MG 200 (Motor Generator: MG), a switch circuit unit 300, and a PCU 400 (Power Control Unit: PCU). The secondary battery 101 supplies power to drive the MG 200, and is also capable of recovering generated power during regeneration.

[0018] The secondary battery 101 is provided with a current sensor 104, a voltage sensor 105, and a temperature sensor 106. Detection signals from the sensors 104 to 106 are output to the control device 102 as needed.

[0019] Each cell 103 includes electrode bodies 107 and 108, an electrolyte 109, and a separator 110. The electrode body 107 is a positive electrode. The electrode body 107 includes, as a positive electrode active material, a lithium composite oxide containing lithium and a transition metal such as Ni, Co, Fe, or Mn. The electrode body 108 is a negative electrode. The electrode body 108 includes, as a negative electrode active material, a carbon-based material such as graphite.

[0020] The electrolyte solution 109 is a solution containing an electrolyte. For example, a non-aqueous electrolyte solution in which a lithium salt is dissolved in a solvent such as ethylene carbonate is used as the electrolyte solution 109. The electrode bodies 107, 108 are impregnated with the electrolyte solution 109. That is, the electrolyte solution 109 contains lithium ions as metal ions. The separator 110 is disposed between the electrode bodies 107, 108. The separator 110 is a porous film that electrically insulates the electrode bodies 107 and 108 from each other.

[0021] Each cell 103 of the secondary battery 101 is housed in a case 111. The secondary battery 101 is electrically connected to an external device via a positive terminal and a negative terminal that are extended to the outside of the case 111. The positive terminal is connected to one end of a positive electrode current collector 112 that is provided integrally with the electrode body 107 of each cell 103. The negative electrode terminal is connected to one end of a negative electrode current collector 113 that is provided integrally with the electrode body 108 of each cell 103. The positive electrode current collector 112 and the negative electrode current collector 113 are made of, for example, metal foil.

[0022] When the secondary battery 101 is being charged, lithium contained in the electrode assembly 107 dissolves in the electrolyte 109, and lithium ions move through the electrolyte 109 and are held in the negative electrode active material of the electrode assembly 108. On the other hand, when the secondary battery 101 is being discharged, lithium ions are released from the negative electrode active material of the electrode assembly 108, move through the electrolyte 109, and are held in the positive electrode active material of the electrode assembly 107. The deterioration modes of the secondary battery 101 include normal deterioration, in which the internal resistance increases due to repeated charging and discharging over time, and idiosyncratic deterioration.

[0023] Normal deterioration is irreversible deterioration caused by changes in the electrode structure, decomposition of the electrolyte 109, etc. In other words, normal deterioration progresses over time. In contrast, anomalous deterioration is reversible deterioration that indicates a temporary increase in internal resistance. In other words, anomalous deterioration can be recovered from a deteriorated state.

[0024] High-rate degradation, an example of specific degradation, occurs when charging at a high input value or discharging at a high output value is biased toward either one. High-rate degradation is degradation in which the internal resistance increases due to the occurrence of a lithium ion concentration distribution in the electrolyte 109.

[0025] Specifically, high-rate degradation occurs due to uneven distribution of the lithium ion concentration in the electrolyte 109 that permeates the electrode bodies 107 and 108. As high-rate degradation progresses, there is a possibility that, for example, lithium deposition may occur in the electrode body 108.

[0026] Therefore, it is desirable to quickly detect high-rate degradation by distinguishing it from normal degradation and to perform control to suppress the progression of high-rate degradation. When the charge / discharge operation is stopped, the imbalance of the electrolyte 109 is alleviated over time, and the state of high-rate degradation is also resolved.

[0027] The control device 102 appropriately controls charging and discharging of the secondary battery 101 so as to safely use the secondary battery 101 according to the deterioration state of the secondary battery 101. The control device 102 includes a memory unit 114, a battery state monitoring unit 115, a diagnosis unit 116, and a charge and discharge control unit 117.

[0028] The storage unit 114 stores programs for control by the control device 102, characteristic data necessary for control, etc. The storage unit 114 also stores data input from the battery state monitoring unit 115 and the diagnosis unit 116 as needed.

[0029] The battery state monitoring unit 115 monitors the state of the secondary battery 101. The battery state monitoring unit 115 includes a current value / voltage value acquiring unit 118, a temperature acquiring unit 119, and a state estimating unit 120.

[0030] The current value / voltage value acquiring unit 118 acquires the detection signals of the current sensor 104 and the voltage sensor 105. The temperature acquiring unit 119 acquires the detection signal of the temperature sensor 106. The state estimating unit 120 estimates the state of charge or the state of degradation of the secondary battery 101 based on the acquired values ​​of the acquiring units 118 to 120.

[0031] Specifically, the state estimation unit 120 calculates state quantities such as a state of charge (SOC) indicating the remaining battery capacity of the secondary battery 101 and a state of health (SOH) indicating the degree of deterioration of the secondary battery 101. The SOH is expressed as, for example, the ratio of the full charge capacity in the deteriorated state to the full charge capacity in the initial state.

[0032] The configuration of the battery module of the secondary battery 101 and the arrangement of the sensors 104 to 106 are conceptual and are set arbitrarily according to the application, etc. The number of cells 103 of the battery module is not particularly limited, and the secondary battery 101 may be configured by connecting multiple battery modules in parallel or in series, and each of the sensors 104 to 106 may be provided for each cell 103.

[0033] Any method can be used to estimate the state quantities in the state estimation unit 120. For example, the SOC is estimated using a relationship with the open circuit voltage (OCV) of the secondary battery 101 or a relationship with an integrated value of charge / discharge current. The SOH is estimated using a relationship with an integrated current amount or temperature that indicates the battery state or usage environment. Specifically, characteristic data that indicates these relationships is acquired in advance and stored in the storage unit 114 as map values ​​or relational expressions, and the SOC and SOH can be estimated based on the values ​​acquired by the sensors 104 to 106.

[0034] A switch circuit unit 300 is provided between the secondary battery 101 and the PCU 400. The switch circuit unit 300 includes a charge switch that is turned on when the MG 200 is driven, i.e., discharging, or when the MG 200 is powered, i.e., charging, and a discharge switch that is turned on when charging. The PCU 400 is configured as a power conversion device that includes an inverter that converts DC power from the secondary battery 101 to AC power, a step-up / step-down converter, etc.

[0035] The charge / discharge control unit 117 controls the opening and closing of the charge / discharge switch of the switch circuit unit 300 and the operation of the PCU 400. In addition, the charge / discharge control unit 117 outputs control signals to the switch circuit unit 300 and the PCU 400 according to the SOC and SOH estimated by the battery state monitoring unit 115 so that the charge power or discharge power of the secondary battery 101 falls within an allowable range.

[0036] The diagnosis unit 116 detects high-rate degradation based on a change in the impedance of the secondary battery 101 measured during DC charging and discharging of the secondary battery 101. Here, "during DC charging and discharging of the secondary battery 101" refers to a state in which a DC current is flowing through the secondary battery 101. In other words, "during DC charging and discharging of the secondary battery 101" refers to a state in which a DC current is flowing through the secondary battery 101 to charge it, or a state in which a DC current is flowing through the secondary battery 101 to discharge it. The diagnosis of high-rate degradation according to this embodiment only requires that a salt concentration gradient is generated in the secondary battery 101 due to current flow. Therefore, it can be used in both charging and discharging.

[0037] Specifically, the diagnosis unit 116 detects high-rate degradation in the secondary battery 101 based on the difference between the impedance during DC charging / discharging and the impedance when DC charging / discharging is not being performed.

[0038] Furthermore, the impedance of the secondary battery 101 includes a DC resistance component and a reaction resistance component. The diagnosis unit 116 utilizes the fact that both the DC resistance component and the reaction resistance component obtained by measuring the impedance of the secondary battery 101 are affected by the liquid phase concentration during current flow, and diagnoses the progression of high-rate degradation, which is a specific type of degradation, by distinguishing it from other degradation modes. That is, the diagnosis unit 116 detects high-rate degradation by using both the DC resistance component and the reaction resistance component. This allows for improved detectability of high-rate degradation compared to when only the DC resistance component is used to detect high-rate degradation.

[0039] The diagnosis unit 116 includes a resistance calculation unit 121 , a change amount acquisition unit 122 , and a deterioration determination unit 123 .

[0040] The resistance calculation unit 121 is a device that acquires the impedance of the secondary battery 101 by electrochemical impedance spectroscopy (EIS). Based on the calculation results of impedance at a plurality of measurement frequencies, the resistance calculation unit 121 calculates a DC resistance R01 and a reaction resistance Rct1 during DC charging / discharging of the secondary battery 101, and a DC resistance R02 and a reaction resistance Rct2 when the secondary battery 101 is not DC charging / discharging.

[0041] The resistance calculation unit 121 includes a superimposed current application unit 124 , an impedance measurement unit 125 , a DC resistance calculation unit 126 , and a reaction resistance calculation unit 127 .

[0042] The superimposed current application unit 124 applies a superimposed current in which a plurality of frequency components are superimposed to the secondary battery 101. By using the superimposed current, it is possible to collectively acquire the battery voltage when currents of a plurality of frequencies are applied to the secondary battery 101.

[0043] For example, a multiple sine wave can be used as the superimposed current. A square wave, a sawtooth wave, or a triangular wave can also be used as the superimposed current. Here, the current value of harmonics relative to the fundamental frequency as the superimposed frequency increases significantly as the order increases, whereas the multiple sine wave does not. Therefore, by using a multiple sine wave as the superimposed current, high measurement accuracy can be maintained. The superimposed frequency of the multiple sine wave is not particularly limited and can be set arbitrarily within the frequency range corresponding to the DC resistances R01 and R02 and the reactive resistances Rct1 and Rct2.

[0044] The impedance measuring unit 125 measures the impedance of the secondary battery 101. To this end, the impedance measuring unit 125 acquires the current value of the superimposed current applied to the secondary battery 101 by the superimposed current application unit 124. The impedance measuring unit 125 also acquires the response voltage when the superimposed current is applied to the secondary battery 101. Therefore, the impedance is a value calculated by measuring the response voltage corresponding to the AC current applied to the secondary battery 101, and then dividing the response voltage as a complex number having information on the absolute value and phase by the AC current. In other words, the impedance includes a real component Zreal and an imaginary component Zimage.

[0045] Specifically, the impedance measurement unit 125 calculates the impedance of the secondary battery 101 for each of a plurality of frequency components using a discrete Fourier transform. The current value and voltage value when the superimposed current is applied can be the detected values ​​of the current sensor 104 and the voltage sensor 105. The discrete Fourier transform can be a fast discrete Fourier transform (FFT).

[0046] Here, DC charging and discharging current generates a magnetic field around it. This magnetic field is also generated in the voltage detection line and current detection line for impedance measurement. If these magnetic fields contain AC components and their frequency is the same as the measurement frequency for impedance measurement, they become noise through the magnetic field. This means that they introduce errors into the measured impedance.

[0047] Therefore, the impedance measuring unit 125 uses the following frequency as the frequency for measuring the impedance during DC charging and discharging of the secondary battery 101. That is, the impedance measuring unit 125 uses a frequency that is different from the frequency of the AC signal component contained in the DC current that charges and discharges the secondary battery 101 and the frequency of the harmonic components contained in the AC signal component. The AC signal component includes pulsation contained in the DC current that charges and discharges, resonance frequency components during feedback, and the like. This makes it possible to reduce errors in the impedance measurement value.

[0048] The theoretical formula for the measurement effect due to the external magnetic field includes the term shown in FIG. 3. This term includes the modulation current I m The modulation current I m is the measured current on the substrate of the secondary battery 101.

[0049] Also, the term includes the modulation current I m The same frequency as the modulation current I m or an external current I that has an AC signal component contained in the DC current that is charging and discharging. ex The external current I ex is the current flowing through the secondary battery 101 other than the substrate. Note that θ in this term is the modulation current I m and the external current I ex is the phase difference between

[0050] Therefore, in measuring the impedance during DC charging and discharging of the secondary battery 101, the impedance measuring unit 125 measures the modulation current I m and the external current I ex The modulation current I is set so that the current ratio between m The influence of noise due to disturbances is reduced by adjusting the modulation current I m and the external current I ex Since it is proportional to the ratio of the measured current to the measured current, the error will be smaller.

[0051] The impedance measuring unit 125 outputs the calculated impedance for each of the plurality of frequency components to the DC resistance calculating unit 126 and the reaction resistance calculating unit 127. The impedance measuring unit 125 may store the impedance data in the storage unit 114.

[0052] The DC resistance calculation unit 126 calculates DC resistances R01 and R02 from a complex impedance plot based on the impedance for each frequency component. Specifically, the values ​​at the intersections of the real axis and the arc portions of the complex impedance plot are obtained as the DC resistances R01 and R02. Similarly, the reaction resistance calculation unit 127 obtains the sizes of the arc portions starting from the intersections of the real axis and the arc portions of the complex impedance plot as the reaction resistances Rct1 and Rct2.

[0053] The DC resistance calculation unit 126 and the reaction resistance calculation unit 127 correct the impedance measured by the impedance measurement unit 125 to an impedance corresponding to a predetermined temperature and a predetermined SOC. The DC resistance calculation unit 126 and the reaction resistance calculation unit 127 implement an algorithm that converts the temperature dependency of the impedance into a polynomial and normalizes it to an impedance at 25°C. The same applies to the SOC.

[0054] The predetermined temperature is, for example, 25°C. The predetermined SOC is, for example, 50%. By standardizing the impedance to a predetermined temperature or a predetermined SOC in this way, it becomes easier to compare the impedance at each temperature and to determine the control threshold.

[0055] The resistance calculation unit 121 is configured, for example, using a power conversion device that configures the in-vehicle PCU 400. This eliminates the need to separately provide the superimposed current application unit 124 and the impedance measurement unit 125, which include a superimposed current generation unit. Furthermore, a large superimposed current can be generated. This allows for a device configuration suitable for on-board diagnosis of the in-vehicle secondary battery 101. Alternatively, the superimposed current generation unit can be configured to be disposed in an in-vehicle charging device (not shown) or an external charging device.

[0056] The change amount acquiring unit 122 calculates the absolute value of the difference between the impedance during DC charging / discharging and the impedance when DC charging / discharging is not being performed. That is, the change amount acquiring unit 122 calculates |R01-R02| and |Rct1-Rct2| as the amounts of change in impedance.

[0057] The deterioration determination unit 123 compares the amount of change in impedance calculated by the change amount acquisition unit 122 with a reference value indicating high-rate deterioration, and determines whether high-rate deterioration has occurred due to charging and discharging of the secondary battery 101. The reference value is set for each of the DC resistance and the reactive resistance.

[0058] The charge / discharge control unit 117 controls the charging and discharging of the secondary battery 101. When the deterioration determination unit 123 of the diagnosis unit 116 determines that high-rate deterioration has occurred, the charge / discharge control unit 117 performs control to limit the charging and discharging current of the secondary battery 101. The above is the overall configuration of the secondary battery system 100 according to this embodiment.

[0059] Next, the reason why high-rate deterioration is detected based on the change in impedance measured during DC charging and discharging of the secondary battery 101 as described above will be explained.

[0060] First, the inventors investigated changes in impedance when the secondary battery 101 was degraded under predetermined degradation conditions. The secondary battery 101 was a prismatic cell with a capacity of 25 Ah, a positive electrode made of NMC, and a negative electrode made of C. The impedance was measured using a lock-in amplifier as the detection method, an amplitude of 500 mA, and a measurement frequency of 1 kHz to 2 Hz.

[0061] Furthermore, as shown in FIG. 4, a simulated cycle was performed in which the secondary battery 101 was repeatedly charged and discharged with DC to cause high-rate degradation. In the simulated cycle, the secondary battery 101 was CC charged at 25 A, further CC charged at 10 A, and then CC discharged at 100 A. For example, the simulated cycle was repeated 100 times. The impedance was measured during the simulated cycle.

[0062] In addition, the direct current internal resistance (DCIR) is measured every time the simulated cycle reaches 100 cycles. In this case, the SOC of the secondary battery 101 is adjusted to 50% and the DCIR of the secondary battery 101 is obtained. The direct current internal resistance is measured at currents of 0.5C, 1C, and 2C, for example. The DCIR measurement is performed to compare with the impedance measured during the simulated cycle. The above simulated cycle is then repeated 500 times.

[0063] The impedance measurement results after repeating the above simulated cycle 500 times are shown in Figure 5. As shown in Figure 5, looking at the DC resistance on the horizontal axis, the resistance value decreased from during discharge to after discharge. Similarly, looking at the reaction resistance on the vertical axis, the resistance value decreased from during discharge to after discharge.

[0064] FIG. 6 shows the DC resistance at 1116 Hz as a function of the number of cycles, among the DC resistances shown in FIG. 5 . As shown in FIG. 6 , the DC resistance at the end of discharge when a 100 A DC current was passed through the secondary battery 101 was higher than the DC resistance when no current was flowing a certain time after the 100 A discharge had ended. As the number of simulated cycles increased, the difference in DC resistance between when current was flowing and when no current was flowing increased. This is because while the influence of high-rate degradation is strongly reflected when current is flowing through the secondary battery 101, the DC resistance when no current is flowing includes the influence of not only high-rate degradation but also other degradation modes, such as normal degradation. Therefore, this resistance difference can be used to determine high-rate degradation.

[0065] The inventors also investigated the transition of impedance during normal degradation and during high-rate degradation, and the results are shown in FIGS.

[0066] FIG. 7 shows the results of a degradation test in which the secondary battery 101 was stored at 60°C for several months, as an example of normal degradation. No current flows through the secondary battery 101. In this case, the impedance changes along the horizontal axis over time. Specifically, the DC resistance increases over time.

[0067] 8 shows the impedance during high-rate degradation when no current is applied to the secondary battery 101. In the impedance during high-rate degradation, the DC resistance also increases as the number of simulated cycles increases.

[0068] As described above, when no current is flowing through the secondary battery 101, the DC resistance increases both during the storage degradation test and during high-rate degradation, making it difficult to distinguish between high-rate degradation and other degradation modes. However, the increase in DC resistance shown in Figures 5 and 6 does not appear in the storage degradation test of Figure 7. The same is true for the increase in reaction resistance. From this, the inventors discovered that by utilizing the impedance when current is flowing through the secondary battery 101, it is possible to distinguish between high-rate degradation and other degradation modes.

[0069] 9, in a conventional method for determining high-rate degradation based on impedance when no direct current flows through the secondary battery 101, the cell after the high-rate degradation cycle described above could be determined to have undergone high-rate degradation. However, the cell after the storage degradation test, enclosed by a bold frame, was determined to have undergone high-rate degradation, but in fact it was not high-rate degradation.

[0070] On the other hand, in the method of the present invention, which determines high-rate degradation based on the impedance when a direct current is passed through the secondary battery 101, it was possible to determine high-rate degradation for both the cell after the storage degradation test and the cell after the high-rate degradation cycle. Therefore, by detecting high-rate degradation based on the resistance increment during DC charging and discharging of the secondary battery 101, it becomes easy to distinguish high-rate degradation from other degradation modes.

[0071] Furthermore, in the high-rate degradation cell, increases in both DC resistance and reaction resistance were confirmed at the end of discharge of the secondary battery 101. For example, the results when a cycle of 4C discharge and 1C charge was repeated are shown in FIG.

[0072] 10 refers to the conventional method of determining high-rate degradation based on the impedance when no direct current flows through the secondary battery 101. Furthermore, "after high-rate degradation" refers to the proposed method of determining high-rate degradation based on the impedance when a direct current flows through the secondary battery 101.

[0073] As shown in Figure 10, in the initial stage, Re, which is the real part of the impedance at 1 kHz and is a feature of the DC current component, fluctuates up and down as the cycle is repeated. Similarly, -Im, which is the imaginary part of the impedance at 69 Hz and is a feature of the reaction resistance component, also fluctuates up and down as the cycle is repeated. However, no distinctive behavior was observed.

[0074] In contrast, characteristic peaks appeared for each of the feature quantities of the DC current component and the reaction resistance component after high-rate degradation. As such, increases in both the DC resistance and the reaction resistance were confirmed at the end of discharge of the secondary battery 101. Therefore, by utilizing both the DC resistance component and the reaction resistance component, it is possible to improve the detectability of high-rate degradation.

[0075] To utilize the resistance increase, the inventors conducted a simulation to investigate the relationship between the magnitude of the DC discharge current and the resistance increase. The results are shown in Figure 11. The secondary battery 101 had a capacity of 25 Ah and was at a temperature of 25°C.

[0076] As shown in FIG. 11, the resistance value increased as the SOC decreased from 90% to 20% by passing a discharge current through the secondary battery 101. In particular, the increase in resistance was greatest when the discharge current was 150 A. Since the discharge current was set to 6 C, the resistance increase behavior appeared in 10 minutes. In other words, if the discharge current is DC 5 A = 0.2 C or higher, it is expected that the increase in resistance during current flow through the secondary battery 101 will be observable.

[0077] Therefore, it is preferable that the average value of the DC current flowing through the secondary battery 101 over a period of 10 minutes or more is equal to or greater than 0.2 C of the battery capacity. As a result, the salt concentration gradient increases as the DC current value increases and the time the current flows becomes longer, thereby improving the detectability of high-rate degradation.

[0078] Next, a specific flow for diagnosing high-rate deterioration of the secondary battery 101 will be described with reference to Fig. 12. The flow shown in Fig. 12 is executed by the diagnosing unit 116 of the control device 102.

[0079] First, in step S10, the current voltage, current Idc, and temperature of the secondary battery 101 are acquired. Also, the continuous current-flow time of the secondary battery 101 is acquired. The data on the voltage, current Idc, temperature, and continuous current-flow time are acquired by the battery state monitoring unit 115.

[0080] The continuous current-carrying time is, for example, the time during which current is continuously flowing from the secondary battery 101 to the MG 200. For example, the continuous current-carrying time within a certain period of time is acquired.

[0081] In step S11, the resistance calculation unit 121 acquires the DC resistance R01 and the reaction resistance Rct1 when the current Idc is applied to the secondary battery 101. The current Idc is a current that flows through the secondary battery 101 during DC charging or DC discharging.

[0082] In step S12, the DC application is stopped. That is, the DC current flowing to the secondary battery 101 is stopped. In other words, the DC discharge of the secondary battery 101 is stopped. As a result, the secondary battery 101 is in a state where it is not being charged or discharged with DC.

[0083] In step S13, the OCV and temperature of the secondary battery 101 are acquired. The OCV is used to calculate the SOC of the secondary battery 101.

[0084] The temperature is the temperature when no direct current flows through the secondary battery 101. There is a temperature difference between the temperature when a direct current flows through the secondary battery 101, i.e., the temperature acquired in step S10, and the temperature when no direct current flows through the secondary battery 101, i.e., the temperature acquired in this step. Therefore, this temperature difference is taken into consideration when converting the impedance into a predetermined temperature.

[0085] In step S14, the resistance calculation unit 121 acquires the DC resistance R02 and the reaction resistance Rct2 after the current Idc is applied to the secondary battery 101. That is, the DC resistance R02 and the reaction resistance Rct2 when the secondary battery 101 is not being DC charged or discharged are acquired.

[0086] In step S15, the DC resistance R01 and reaction resistance Rct1 during DC charging / discharging of the secondary battery 101, and the DC resistance R02 and reaction resistance Rct2 when the secondary battery 101 is not DC charging / discharging are normalized by temperature correction. Each resistance is converted into an impedance at a temperature of, for example, 25°C.

[0087] In step S16, the absolute values ​​ΔR0 and ΔRct of the change in impedance, i.e., the difference between the resistance of the secondary battery 101 during DC charging / discharging and the resistance of the secondary battery 101 when no current is applied, are obtained. The absolute value ΔR0 of the change in DC resistance is obtained by ΔR0 = |R01 - R02|. The absolute value ΔRct of the change in reaction resistance is obtained by ΔRct = |Rct1 - Rct2|.

[0088] In step S17, it is determined whether ΔR0>reference value A and ΔRct>reference value B are satisfied. Reference value A is a predetermined value set in advance based on the relationship between the absolute value of the change in DC resistance and high-rate degradation. Reference value B is a predetermined value set in advance based on the relationship between the absolute value of the change in reaction resistance and high-rate degradation. If ΔR0>reference value A and ΔRct>reference value B are satisfied in step S17, proceed to step S18.

[0089] In step S18, control for suppressing the DC charge / discharge current of the secondary battery 101, that is, control for suppressing high-rate deterioration, is performed. For example, the charge / discharge control unit 117 controls the current Idc of the secondary battery 101 such that |Idc|max < f(ΔR0, ΔRct). f is a function represented by ΔR0 and ΔRct. When performing the control, each data acquired in step S10 may be used. Then, the process returns to step S10 and the flow is repeated.

[0090] In step S17, when ΔR0 > reference value A and ΔRct > reference value B are not satisfied, the process returns to step S10 and the flow is repeated. The above is the diagnostic flow for high-rate deterioration.

[0091] After repeating the above-described simulation cycle, the inventors examined the resistance increase rate when the input / output of the secondary battery 10 was restricted. Note that the SOC was adjusted to 50%, and the application of current to the secondary battery 10 was set to 120 seconds. The results are shown in FIG. 13.

[0092] As shown in FIG. 13, by repeating the simulation cycle up to 521 cycles, the resistance increase rate increased as the high-rate deterioration progressed. After 521 cycles of the simulation cycle, when the input / output was restricted and the secondary battery 101 was left unattended, the resistance decreased after 12 days. The resistance further decreased after 20 days.

[0093] This is presumably because after detecting the salt concentration unevenness deterioration, the increase in resistance could be reversibly relaxed by restricting the input / output of the secondary battery 101. Therefore, by restricting the input / output of the secondary battery 101, high-rate deterioration can be alleviated.

[0094] As described above, in this embodiment, the property that the salt concentration unevenness tends to become apparent in the liquid phase during DC charging and discharging of the secondary battery 101 is utilized, and high-rate degradation, which is an abnormality in the salt concentration unevenness of the secondary battery 101, is detected from the amount of change in the impedance during current flow and the impedance during no current flow. This makes it easy to distinguish between high-rate degradation of the secondary battery 101 and a degradation mode other than high-rate degradation.

[0095] Furthermore, high-rate degradation is diagnosed in the following order: first, DC resistance R01 and reaction resistance Rct1 are acquired during DC charging / discharging of the secondary battery 101, and then DC resistance R02 and reaction resistance Rct2 are acquired when DC charging / discharging is not occurring. This allows high-rate degradation to be diagnosed when the SOC of the secondary battery 101 is not fluctuating, eliminating the need to convert the SOC to a predetermined SOC and enabling simple diagnosis.

[0096] (Second embodiment) In this embodiment, the differences from Embodiment 1 will be mainly described. In this embodiment, high-rate degradation of the secondary battery 101 is diagnosed according to the flow shown in FIG.

[0097] First, in step S20, similarly to step S10, the current voltage, current Idc, temperature, and continuous current application time of the secondary battery 101 are acquired.

[0098] Next, in step S21, the OCV and temperature of the secondary battery 101 are acquired, similarly to step S13.

[0099] In step S22, the resistance calculation unit 121 acquires the DC resistance R01 and the reaction resistance Rct1 before the current Idc is applied to the secondary battery 101.

[0100] Thereafter, in step S23, DC application is started. That is, a direct current flows through the secondary battery 101. As a result, the secondary battery 101 enters a state of being charged or discharged with a direct current.

[0101] In step S24, similarly to step S11, the DC resistance R01 and the reaction resistance Rct1 when the current Idc is applied to the secondary battery 101 are acquired.

[0102] In step S25, the DC resistance R01 and reaction resistance Rct1 during DC charging / discharging of the secondary battery 101, and the DC resistance R02 and reaction resistance Rct2 when the secondary battery 101 is not DC charging / discharging are normalized by temperature correction and SOC correction. Each resistance is converted to an impedance at a temperature of 25°C and an SOC of 50%, for example.

[0103] In steps S26, S27, and S28, the same processing as in steps S16, S17, and S18 is performed. The above is the diagnostic flow for high-rate degradation according to this embodiment.

[0104] As described above, high-rate degradation may be diagnosed in the following order: first, the DC resistance R02 and the reaction resistance Rct2 are obtained when the secondary battery 101 is not being DC charged or discharged, and then the DC resistance R01 and the reaction resistance Rct1 are obtained during DC charging or discharging. In this case, high-rate degradation can be diagnosed during DC charging or discharging of the secondary battery 101.

[0105] (Other embodiments) The configuration of the secondary battery system 100 shown in each of the above embodiments is an example, and the present invention is not limited to the above-described configuration, and other configurations that can realize the present invention are also possible. For example, the secondary battery 101 is not limited to being mounted on an electric vehicle, and may also be installed in a predetermined location.

[0106] Furthermore, the secondary battery 101 is not limited to a lithium ion battery. Since high-rate degradation is an event caused by uneven distribution of salt concentration in the liquid phase, high-rate degradation can be diagnosed by the above method in any liquid-phase battery other than a lithium ion battery. [Explanation of symbols]

[0107] 101 Secondary battery 107, 108 Electrode body 109 Electrolyte 116 Diagnostic Department 117 Charge / discharge control unit 125 Impedance measurement unit

Claims

1. a secondary battery (101) having electrode bodies (107, 108) impregnated with an electrolyte (109) containing metal ions; an impedance measurement unit (125) that measures the impedance of the secondary battery; a diagnostic unit (116) that detects high-rate deterioration caused by uneven concentration of the metal ions in the electrolyte solution that has permeated the electrode body based on a difference between impedance during DC charging and discharging and impedance during no DC charging and discharging in the secondary battery; Including, The diagnostic unit detects the high-rate deterioration of the secondary battery before the secondary battery is used at high power.

2. 2. The secondary battery system according to claim 1, wherein the impedance measurement unit uses, as a frequency for measuring the impedance during DC charging and discharging of the secondary battery, a frequency that is different from the frequency of an AC signal component contained in the DC current that charges and discharges the secondary battery and the frequency of a harmonic component contained in the AC signal component.

3. a secondary battery (101) having electrode bodies (107, 108) impregnated with an electrolyte (109) containing metal ions; an impedance measurement unit (125) that measures the impedance of the secondary battery; a diagnostic unit (116) that detects high-rate deterioration caused by uneven concentration of the metal ions in the electrolyte solution that has permeated the electrode body based on a difference between impedance during DC charging and discharging and impedance during no DC charging and discharging in the secondary battery; Including, a secondary battery system in which the impedance measurement unit uses a frequency for measuring impedance during DC charging and discharging of the secondary battery that is different from the frequency of an AC signal component contained in the DC current that charges and discharges the secondary battery and the frequency of a harmonic component contained in the AC signal component.

4. The impedance of the secondary battery includes a DC resistance component and a reaction resistance component, 4. The secondary battery system according to claim 1, wherein the diagnosing unit detects the high-rate degradation by using both the DC resistance component and the reaction resistance component.

5. 5. The secondary battery system according to claim 1, wherein the average value of the direct current flowing through the secondary battery over a period of 10 minutes or more is 0.2 C or more of the battery capacity.

6. 6. The secondary battery system according to claim 1, wherein the diagnostic unit corrects the impedance measured by the impedance measuring unit to an impedance corresponding to a predetermined temperature and a predetermined SOC.

7. 7. The secondary battery system according to claim 1, further comprising a charge / discharge control unit (117) that performs control to suppress the charge / discharge current of the secondary battery when a difference between the resistance of the secondary battery during DC charging / discharging and the resistance of the secondary battery when no current is applied is greater than a predetermined value.

8. 8. The secondary battery system according to claim 1, wherein the impedance measurement unit adjusts the modulation current used in the impedance measurement during DC charging and discharging of the secondary battery so that a current ratio between the modulation current used in the impedance measurement and an external current having the same frequency as the modulation current or a frequency of a harmonic component contained in the modulation current, or having an AC signal component contained in the DC current used for charging and discharging, is equal to or less than a threshold value.

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