Gas detection method and gas detection device

Abstract

To accurately detect gas generation in a power storage devices using an easy and practical method.SOLUTION: A gas detection method includes a preparation step of changing a current value or a voltage value of a power storage device 1, a first measurement step of obtaining a first internal resistance R1 that is the internal resistance when a predetermined first time has elapsed since the start of the change in the current value or the voltage value in the preparation step, a second measuring step of obtaining a second internal resistance R2 which is the internal resistance when a second time that is greater than the first time by a predetermined time has elapsed since the start of the change in the current value or the voltage value in the preparation step, a calculation step of calculating a differential resistance value RG by subtracting the first internal resistance R1 from the second internal resistance R2, and a detection step of detecting gas generation within the power storage device 1 on the basis of the differential resistance value RG.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a gas detection method and a gas detection device for detecting gas generated in a secondary battery. [Background technology]

[0002] The lithium-ion battery (LIB) business has achieved dramatic growth thanks to continuous technological development aimed at making mobile devices smaller, lighter, and more functional (in pursuit of convenience) by leveraging its high energy density, high voltage, and safety characteristics. With global environmental and resource issues now in the spotlight, policies such as promoting eco-cars and shifting to renewable energy are expected to continue to drive the growth of the lithium-ion battery market. Meanwhile, when lithium-ion batteries are applied to electric vehicles (EVs) and stationary energy storage systems (for leveling renewable energy, etc.), the number of lithium-ion batteries required by 2030 is expected to be approximately 10 times that of 2018, raising concerns about a sudden rise in battery costs due to resource shortages and other factors.

[0003] One way to solve these issues has been proposed: the reuse (secondary use) of batteries (secondary batteries / energy storage devices) such as lithium-ion batteries, and EV sharing. However, to realize these, technology to extend the life of the same battery for a long period of time is required. Possible approaches to extend battery life include extending the life of the battery itself (battery design) and making better use of the battery (battery operation). The latter approach includes many techniques, such as cooling the battery, controlling the heating and charging current during low-temperature charging, and controlling the battery's charge / discharge range. However, what is needed is a method to sense the battery's State of Health (SOH), which changes over time with its operation history (aging), and to control the battery according to its current state (SOH) to prevent deterioration.

[0004] Currently, SOH is the SOC (State of Charge) and battery capacity (capacity degradation) calculated from temperature, current, voltage, and time measurements, and in some cases the battery's internal resistance is also used.

[0005] When a side reaction occurs within a battery, gas is generated before battery capacity degradation occurs in most cases. Gas generation causes the pressure inside the battery case to rise. If gas escape is poor or the case cannot expand sufficiently (large batteries are used in a constrained state), gas accumulates between the electrodes, preventing the battery from reacting uniformly (resulting in uneven reaction), leading to secondary degradation accompanied by a rapid decrease in capacity. If the presence or absence of gas generation and its status can be added to the SOH, and the battery can be controlled according to the SOH to prevent degradation, battery life can be extended. Possible gas detection methods include directly measuring the gas pressure inside the battery and sensing the pressure inside the battery pack. Patent Document 1 discloses a method for detecting gas generation by measuring AC impedance, and Patent Document 2 discloses a method for detecting gas generation from voltage changes based on resistance during charging. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 10-92473 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-89311 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the above-mentioned methods of directly measuring the gas pressure inside the battery or the pressure inside the battery pack require the use of a gas sensor, pressure sensor, etc., making it difficult to easily detect gas. Furthermore, the above-mentioned methods of detecting gas generation from AC impedance require a large device configuration, making it difficult to easily detect gas generation. Furthermore, the methods of detecting gas generation from voltage changes based on resistance require complicated processes for gas detection, making it difficult to easily detect gas generation.

[0008] In order to solve the above problems, an object of the present invention is to accurately detect gas generation in an electricity storage device using a simple and practical method. [Means for solving the problem]

[0009] In order to achieve the above object, a characteristic configuration of a gas detection method according to one embodiment of the present invention is that it comprises a preparation step of changing the current value or voltage value of an electricity storage device, a first measurement step of acquiring a first internal resistance, which is the internal resistance when a predetermined first time has elapsed since the current value or voltage value began to change in the preparation step, a second measurement step of acquiring a second internal resistance, which is the internal resistance when a second time, which is at least a predetermined time longer than the first time, has elapsed since the current value or voltage value began to change in the preparation step, a calculation step of calculating a differential resistance value by subtracting the first internal resistance from the second internal resistance, and a detection step of detecting gas generation in the electricity storage device based on the differential resistance value.

[0010] With this configuration, gas generation can be easily confirmed by analyzing the time dependence of the internal resistance, and gas generation in the electricity storage device can be accurately detected using a simple and practical method.

[0011] The change in the current value or voltage value in the preparation step may be performed by stopping charging of the power storage device while it is being charged, or by stopping discharging of the power storage device while it is being discharged.

[0012] With this configuration, the preparation step can be carried out easily and stably, and gas generation in the electricity storage device can be detected accurately using a simple and practical method.

[0013] Furthermore, in the detecting step, it may be determined that the gas has been generated when the differential resistance value is greater than a predetermined threshold value.

[0014] With this configuration, it is possible to easily detect the generation of gas.

[0015] Furthermore, the preparation process, the first measurement process, the second measurement process, the calculation process, and the detection process may be considered as one detection cycle, and the detection cycle may be performed intermittently. If the differential resistance value in each detection process is greater than a predetermined threshold value, it may be determined that the gas has been generated in that detection process.

[0016] With this configuration, it is possible to easily detect the generation of gas.

[0017] In each of the sensing steps, the threshold value may be calculated based on the differential resistance value calculated in the immediately preceding sensing cycle.

[0018] With this configuration, it is possible to easily determine the time dependency of the differential resistance value, and to accurately detect gas generation in the electricity storage device using a simple and practical method.

[0019] The threshold value may also be the differential resistance value in the first detection cycle.

[0020] With this configuration, it is possible to more easily and accurately determine the time dependency of the differential resistance value, and it is possible to accurately detect the generation of gas in the electricity storage device using a simple and practical method.

[0021] Furthermore, the preparation process, the first measurement process, the second measurement process, the calculation process, and the detection process may be considered as one detection cycle, and the detection cycle may be performed intermittently. In each detection process, if the value obtained by subtracting the differential resistance value calculated in the first detection cycle from the differential resistance value calculated in that detection cycle is greater than 0, it may be determined that the gas has been generated in that detection process.

[0022] With this configuration, it is possible to determine the time dependency of the internal resistance more easily and accurately, and it is possible to accurately detect the generation of gas in the electricity storage device using a simple and practical method.

[0023] The second time period may be set to a time period from the start of the preparation step until the internal resistance reaches a predetermined internal resistance that is 80% or more of the steady state.

[0024] With this configuration, the second time period can be appropriately set, and gas generation in the electricity storage device can be detected with high accuracy.

[0025] The first time period may be set to a time period from the start of the preparation step until the internal resistance reaches a predetermined internal resistance that is 10% to 40% of the steady state.

[0026] With this configuration, the first time period can be appropriately set, and gas generation in the electricity storage device can be detected with high accuracy.

[0027] The first time period may be set to be not less than 10 milliseconds and not more than 3 seconds, and the second time period may be set to be not less than 5 seconds and not more than 600 seconds.

[0028] With this configuration, the first time period and the second time period can be set appropriately, and gas generation in the electricity storage device can be detected with high accuracy.

[0029] The preparing step, the first measuring step, the second measuring step, the calculating step, and the detecting step may be performed in a state where the power storage device is maintained within a predetermined temperature range.

[0030] With this configuration, gas detection can be performed using appropriate first and second times and determination conditions according to the temperature range, and gas generation in the electricity storage device can be detected with high accuracy.

[0031] a resistance calculation unit that calculates a first internal resistance of the power storage device from the amount of change in voltage from a change in current or voltage of the power storage device until a predetermined first time has elapsed since the change in current or voltage, and the amount of change in current when the change in current or voltage is made; and a detection unit that calculates a differential resistance by subtracting the first internal resistance from the second internal resistance, and detects gas generation in the power storage device based on the difference in resistance.

[0032] With this configuration, it is possible to easily check the time dependency of the internal resistance caused by gas generation, and it is possible to accurately detect gas generation in the electricity storage device using a simple and practical method. [Brief explanation of the drawings]

[0033] [Figure 1] 10A and 10B are diagrams illustrating examples of changes in voltage over time when the voltage value or current value is changed. [Figure 2] FIG. 10 is a diagram illustrating a configuration for detecting gas generation. [Figure 3] FIG. 1 is a diagram illustrating a flow of a gas detection method. [Figure 4] FIG. 10 is a table summarizing data obtained through experiments. [Figure 5] FIG. 10 is a diagram showing the time course of capacity deterioration based on experimental results and the theoretical time course of capacity deterioration in a state in which no gas is generated. [Figure 6] FIG. 4 is a diagram showing the change over time of the first internal resistance and the change over time of the amount of gas generated. [Figure 7] FIG. 6 is a diagram showing the change over time of the second internal resistance and the change over time of the amount of gas generated. [Figure 8] 10A and 10B are diagrams showing changes over time in the amount of change in resistance value and changes over time in the amount of gas generated. [Figure 9] FIG. 10 is a diagram illustrating an example of improvement in capacity degradation due to changes in battery control. DETAILED DESCRIPTION OF THE INVENTION

[0034] The SOH and degradation level of power storage devices (secondary batteries) such as lithium-ion batteries 1 (see Figure 2) are measured to extend the life of the power storage devices and ensure appropriate use depending on the situation. The degradation of power storage devices is influenced by a combination of various factors.

[0035] One method for determining the deterioration of an electricity storage device is to measure the time dependency of the internal resistance of the electricity storage device. The time dependency of the internal resistance is determined by changing the voltage or current value of the electricity storage device and measuring the internal resistance of the electricity storage device when a predetermined time t1 seconds has elapsed since the voltage or current value was changed. The deterioration of the electricity storage device is determined based on the time dependency of the internal resistance.

[0036] The internal resistance of a power storage device changes when the voltage value or current value of the power storage device changes, and converges to a constant value when the battery reaction reaches a steady state. At this time, the resistance value at time t is expressed as the t-second rate resistance. Also, the resistance varies depending on the device temperature and increases as the temperature decreases. Therefore, the internal resistance of a power storage device can be expressed as a function of time and temperature.

[0037] The time dependence of the internal resistance of a power storage device is determined based on the event (degradation cause) that generates the resistance. The resistance due to the movement of electrons and ions appears relatively quickly, and the resistance due to the reaction (charge transfer), diffusion, and equilibrium on the surface of the electrode active material appears at a later time. This time dependence depends on the type of power storage device, active material, electrode configuration, temperature, etc. For example, in the case of a lithium-ion battery 1, at 25°C, the resistance due to the movement of electrons and ions and the reaction on the surface of the electrode active material are included in the internal resistance of about 1 second.

[0038] When gas is generated inside a power storage device, the internal resistance of the power storage device increases. However, since the causal relationship between gas generation and the internal resistance of the power storage device is not clear, it has been difficult to directly detect gas generation using the internal resistance.

[0039] As a result of intensive research, the inventors found that there is a good correlation between the difference in the t-second rate resistances of two different t-second rate resistances with different elapsed times tx (see Fig. 1) and the gas generation amount. That is, the inventors changed the voltage value or current value of the power storage device, and when the first internal resistance (t1-second rate resistance) of the power storage device at the time when the first time t1 seconds, which is the preset elapsed time tx, has elapsed after changing the voltage value or current value, is R1 (see Fig. 2 described later), and the second internal resistance (t2-second rate resistance) of the power storage device at the time when the second time t2 seconds, which is the preset elapsed time tx, has elapsed after changing the voltage value or current value, is R2 (see Fig. 2) (t1 < t2), they found that it is possible to detect that gas has been generated inside the power storage device based on R2 - R1.

[0040] For example, as shown in Fig. 1, the voltage value or current value starts to change at elapsed time t0. When the voltage value or current value is changed, the voltage of the power storage device increases irregularly and suddenly, and then gradually converges to a predetermined steady voltage Vr (steady state).

[0041] At this time, the voltage value V1 of the power storage device is measured when the elapsed time tx from when the voltage value or current value started to change reaches a first time t1, which is the time beyond the period during which the voltage suddenly rises. Also, the voltage value V2 of the power storage device is measured when the elapsed time tx reaches a second time t2, which is a predetermined time or more longer than the first time t1. Note that the voltage value V1 and the voltage value V2 are potential differences with respect to the voltage value V0 at the time when the voltage value or current value started to change. In other words, the voltage value V1 is the amount of change in the voltage value from when the voltage value or current value started to change until the first time t1 has elapsed, and the voltage value V2 is the amount of change in the voltage value from when the voltage value or current value started to change until the second time t2 has elapsed.

[0042] Furthermore, the first internal resistance R1 at the first time t1 and the second internal resistance R2 at the second time t2 are calculated using Ohm's law from the amount of change ΔI in the current value when the voltage value or current value is changed, and the voltage value V1 and voltage value V2. That is, the first internal resistance R1 is calculated by dividing the voltage value V1 by the amount of change ΔI in the current value (R1=V1 / ΔI), and the second internal resistance R2 is calculated by dividing the voltage value V2 by the amount of change ΔI in the current value (R2=V2 / ΔI).

[0043] Then, based on a differential resistance value RG (see FIG. 2) obtained by subtracting the first internal resistance R1 from the second internal resistance R2, generation of gas inside the electricity storage device is detected using a predetermined criterion.

[0044] In this way, gas detection is performed based on the differential resistance value RG obtained by subtracting the first internal resistance R1, which is the internal resistance immediately after the voltage value (internal resistance) suddenly changes, from the second internal resistance R2, which is the internal resistance when the voltage value (internal resistance) approaches a steady state. The inventors have found that gas generation can be detected accurately using this method. This is thought to be because the effect of gas generation on the internal resistance is small when the voltage value (internal resistance) suddenly changes, but becomes large when the voltage value (internal resistance) approaches a steady state. Furthermore, by calculating the differential resistance value RG, it is possible to suppress (eliminate) the effect on the internal resistance caused by factors other than gas generation and to make the effect of gas generation on the internal resistance apparent, which is thought to enable accurate detection of gas generation.

[0045] A specific example of detecting gas generation inside an electricity storage device based on the differential resistance value RG will be described below.

[0046] [Gas detection device] First, with reference to FIG. 1, an example of the configuration of a gas detection device that detects generation of gas inside an electricity storage device will be described using FIG.

[0047] The lithium ion battery 1 is an example of a chargeable and dischargeable power storage device, and is charged by receiving power from a power source 2. The lithium ion battery 1 supplies power to various devices 3 that are power consumers by discharging the stored power.

[0048] The gas detection device is composed of a voltmeter 7, an ammeter 8, a charge / discharge control unit 9 (corresponding to a "current / voltage control unit"), a current / voltage value acquisition unit 11, a resistance calculation unit 12, a detection unit 15, and a memory unit 16.

[0049] A voltmeter 7 is connected in parallel to the lithium ion battery 1 and measures the voltage value of the lithium ion battery 1. An ammeter 8 is connected in series to the lithium ion battery 1 and measures the current value of the lithium ion battery 1.

[0050] The charge / discharge control unit 9 controls the power supply 2 and the switch 18 to control the charging and discharging of the lithium-ion battery 1, as well as the voltage and current values ​​of the lithium-ion battery 1. When charging the lithium-ion battery 1, the charge / discharge control unit 9 controls the switch 18 to connect the power supply 2 and the lithium-ion battery 1 in series and supply power from the power supply 2 to the lithium-ion battery 1. When the lithium-ion battery 1 supplies power to the device 3 (discharging state), the charge / discharge control unit 9 controls the switch 18 to connect the lithium-ion battery 1 and the device 3 in series. The lithium-ion battery 1 can also be regeneratively charged by the device 3. In this case, the charge / discharge control unit 9 may control the switch 18 and the charging circuit 19 to control the regenerative charging of the lithium-ion battery 1.

[0051] Furthermore, when detecting the generation of gas inside the lithium-ion battery 1, the charge / discharge control unit 9 controls the switch 18 to connect the lithium-ion battery 1 to the gas detection device. When detecting the generation of gas, a detection cycle is performed in which the voltage value or current value of the lithium-ion battery 1 is changed and the change in internal resistance over time is calculated from the voltage value and current value at that time.

[0052] The current and voltage value acquisition unit 11 acquires the voltage value of the lithium ion battery 1 measured by the voltmeter 7 and the current value of the lithium ion battery 1 measured by the ammeter 8, and stores them in the memory unit 16. The current and voltage value acquisition unit 11 acquires a voltage value V1 at a first time t1 and a voltage value V2 at a second time t2, which are different times tx elapsed since the voltage value or the current value was changed.

[0053] The resistance calculation unit 12 calculates the internal resistance of the lithium-ion battery 1 using Ohm's law from the voltage and current values ​​stored in the memory unit 16. The current value used in calculating the internal resistance is the amount of change ΔI in the current value when the voltage or current value is changed. The resistance calculation unit 12 then calculates the first internal resistance R1 and the second internal resistance R2 from the voltage value V1 and the voltage value V2, and stores them in the memory unit 16.

[0054] The detection unit 15 detects the generation of gas inside the lithium ion battery 1 based on the first internal resistance R1 and the second internal resistance R2 stored in the storage unit 16, using a predetermined standard.

[0055] For example, the detection unit 15 determines that gas has been generated when the differential resistance value RG (R2-R1) obtained by subtracting the first internal resistance R1 from the second internal resistance R2 is greater than a predetermined threshold value. The differential resistance value RG (R2-R1) may be calculated by the resistance calculation unit 12.

[0056] Alternatively, the detection cycle may be repeated (continuously) at any timing, the first internal resistance R1 and the second internal resistance R2 may be calculated in each detection cycle, and it may be determined that gas has been generated if the differential resistance value RG in each detection cycle is greater than a predetermined threshold value. The detection unit 15 may also detect gas generation based on a resistance change amount ΔRG (Δ(R2-R1)), which is the difference between the differential resistance value RG in the first detection cycle. In this case, the threshold value may be calculated based on the differential resistance value RG calculated in the immediately preceding detection cycle or may be the differential resistance value RG in the first detection cycle. The detection unit 15 may also determine that gas has been generated if the resistance change amount ΔRG (Δ(R2-R1)), which is the difference between the differential resistance value RG calculated in the detection cycle at the time of detection and the differential resistance value RG calculated in the first detection cycle, is greater than 0. In this way, gas generation can be detected efficiently and accurately by optimal means depending on the gas generation situation and the characteristics of the lithium-ion battery 1 (power storage device). The resistance change amount ΔRG (Δ(R2−R1)) may be calculated by the resistance calculation unit 12.

[0057] [Gas detection method] Next, an example of a gas detection method will be described with reference to Figures 1 to 3. In the following description, a configuration will be described in which a detection cycle is repeated at arbitrary or predetermined intervals during actual use, discharging is stopped during discharging in the detection cycle, and gas generation is detected based on the time dependency of the internal resistance after discharging is stopped. However, charging may be stopped during charging in the detection cycle, and gas generation may be detected based on the time dependency of the internal resistance after charging is stopped, or gas generation may be detected based on the time dependency of the internal resistance after changing the voltage value or current value by any method. Furthermore, as described above, gas generation may be detected in a single detection cycle without repeating the detection cycle.

[0058] First, charging of the lithium-ion battery 1 and supply of power from the lithium-ion battery 1 to the device 3 are repeated. This process is called actual use (step #1 in FIG. 3). During actual use, a detection cycle is repeated at arbitrary or predetermined intervals, and gas generation inside the lithium-ion battery 1 is detected during the detection cycle.

[0059] In each detection cycle, the charge / discharge control unit 9 first stops and restarts the discharge of the lithium-ion battery 1 during discharge. That is, the charge / discharge control unit 9 stops the discharge of the lithium-ion battery 1 during discharge (step #2 in FIG. 3 ), and then restarts the discharge after a predetermined time has elapsed or when a predetermined charge state (charge capacity) is reached (step #3 in FIG. 3 ). The stop and restart of the discharge in the detection cycle may be performed at the same voltage value (charging voltage) or current value (charging current) as the discharge in actual use, or may be performed at a different voltage value (charging voltage) or current value (charging current). This process of stopping and restarting the discharge is referred to as a preparation process. During discharge, the discharge current becomes zero when the discharge is stopped, and discharge is performed at a predetermined discharge current when the discharge is restarted. Therefore, the discharge current during discharge is the amount of change in the current value ΔI. The amount of change in the current value ΔI is stored in the memory unit 16. The discharge current may be acquired by the ammeter 8. In the preparation step of the detection cycle, the voltage value and the current value may be changed instead of stopping and restarting the discharge, in which case the charge / discharge control unit 9 functions as a current / voltage control unit. If the amount of change in the current value ΔI is accurately controlled, the gas detection device does not need to be provided with an ammeter 8.

[0060] In each detection cycle, the current / voltage value acquisition unit 11 acquires the voltage value V0 when the discharge is stopped. Furthermore, the current / voltage value acquisition unit 11 acquires the voltage value at a first time t1, which is one second after the discharge is stopped (step #4 in FIG. 3), and acquires the voltage value at a second time t2, which is 60 seconds after the discharge is stopped (step #5 in FIG. 3). The current / voltage value acquisition unit 11 then stores the potential difference between the voltage value at the first time t1 and the voltage value V0 in the memory unit 16 as the voltage value V1 at the first time t1, and stores the potential difference between the voltage value at the second time t2 and the voltage value V0 in the memory unit 16 as the voltage value V2 at the second time t2.

[0061] Next, resistance calculation unit 12 divides voltage value V1 by the amount of change ΔI in the current value to calculate first internal resistance R1, which is the internal resistance of lithium-ion battery 1 at the first time t1, and divides voltage value V2 by the amount of change ΔI in the current value to calculate second internal resistance R2, which is the internal resistance of lithium-ion battery 1 at the second time t2. The process of acquiring the voltage value at the first time t1 and then calculating first internal resistance R1 is referred to as a first measurement process, and the process of acquiring the voltage value at the second time t2 and then calculating second internal resistance R2 is referred to as a second measurement process. Note that in the first and second measurement processes, voltage values ​​may be measured and the first internal resistance R1 and the second internal resistance R2 may be calculated from the measured voltage values, or the first internal resistance R1 at the first time t1 and the second internal resistance R2 at the second time t2 may be measured directly. In this case, voltmeter 7 does not need to be provided in the gas detection device.

[0062] Then, the resistance calculation unit 12 calculates the differential resistance value RG by subtracting the first internal resistance R1 from the second internal resistance R2. Furthermore, the resistance calculation unit 12 calculates the resistance change amount ΔRG by subtracting the differential resistance value RG calculated in this detection cycle from the differential resistance value RG calculated in the first detection cycle (step #6 in FIG. 3). This process is called the calculation process.

[0063] Next, the detection unit 15 determines a threshold value for detecting gas generation inside the lithium-ion battery 1 (step #7 in FIG. 3). The threshold value is determined based on the resistance change amount ΔRG or the differential resistance value RG calculated before the immediately preceding detection cycle. Note that the threshold value may be set to a predetermined value in advance.

[0064] Then, the detection unit 15 compares the resistance change amount ΔRG with the determined threshold value to determine whether the resistance change amount ΔRG is greater than the threshold value (step #8 in FIG. 3), and if it is greater than the threshold value, it determines that gas has been generated inside the lithium-ion battery 1. This process is referred to as the detection process.

[0065] Thereafter, if the resistance change amount ΔRG is greater than the threshold value (gas generation is detected) (step #8 Yes in FIG. 3), the usage and charging methods of the lithium-ion battery 1 are controlled to suppress gas generation, and actual use is carried out (step #9 in FIG. 3). If the resistance change amount ΔRG is equal to or less than the threshold value (step #8 No in FIG. 3), gas generation is not occurring, so actual use is carried out as is.

[0066] [Experimental results] Next, the results of an experiment in which the differential resistance value RG was used to detect gas generation inside the lithium ion battery 1 will be described with reference to FIG. 1 and with reference to FIGS.

[0067] The experiment was conducted using a rectangular lithium-ion battery 1 with a capacity of 2270mAh [size: 33.8mm x 48.5mm x 10.5mm, rated charging voltage: 4.2V, standard charging current: 1589mAh (0.7CA)]. Assuming actual use, this lithium-ion battery 1 was subjected to a charge-discharge cycle of 0.7CA, charging at 4.2V and discharging at 2.7V in a constant temperature bath at 50°C (CC charging).

[0068] The detection cycle was performed before the charge-discharge cycle and every 1, 2, 3, 5, 7, 10, and 14 weeks (WEEK) during the charge-discharge cycle. In each detection cycle, the battery was charged at 4.2 V and discharged at 2.7 V at 0.2 CA in a 25°C environment to confirm the capacity (this capacity is referred to as the "confirmed capacity"). Then, the battery was discharged in the detection cycle. During this discharge, the discharge was stopped and then restarted. From the time dependence of the resistance near 90% SOC, the voltage value V1 at the first time t1, 1 second after the discharge was stopped, was calculated, and the voltage value V2 at the second time t2, 60 seconds after the discharge was stopped, was calculated. The first internal resistance R1 (R1 1-second rate resistance) and the second internal resistance R2 (R60 60-second rate resistance) were calculated from the charging current, voltage V1, and voltage V2, and the differential resistance RG (R60 - R1) and the resistance change ΔRG (Δ(R60 - R1)) were calculated. ΔR1, the difference between the first internal resistance R1 (R1) in this detection cycle and the first internal resistance R1 calculated in the first detection cycle, and ΔR60, the difference between the second internal resistance R2 (R60) in this detection cycle and the second internal resistance R2 calculated in the first detection cycle, were also calculated. Furthermore, the volume of the lithium-ion battery 1 was measured using the Archimedes method, and the amount of gas generation was calculated by subtracting the volume before the charge-discharge cycle. For confirmation, the AC impedance (near 100% SOC) was also measured, and the bulk resistance was calculated from the X-axis intercept of the Nyquist plot, and the reaction resistance was calculated from the arc.

[0069] Figure 4 is a table summarizing the experimentally measured values. As is clear from Figure 4, gas generation begins when the cycle time is between 3 and 5 weeks. Furthermore, as is clear from Figure 5, the confirmed capacity of lithium-ion battery 1 deteriorates when the cycle time is between 3 and 5 weeks compared to line 5, which shows the theoretical time progression of capacity degradation in a state where no gas is generated. This suggests that gas generation occurs at this timing.

[0070] 4, the first internal resistance R1 (R1), ΔR1, which is the difference between the first internal resistance R1 (R1) calculated in the detection cycle at the time of detection and the first internal resistance R1 (R1) calculated in the first detection cycle, the second internal resistance R2 (R60), and ΔR60, which is the difference between the second internal resistance R2 (R60) calculated in the detection cycle at the time of detection and the second internal resistance R2 (R60) calculated in the first detection cycle, all increase even when no gas is generated. This is also evident from the fact that the time course of ΔR1 deviates from the time course of the amount of gas actually generated in FIG. 6, and that the time course of ΔR60 deviates from the time course of the amount of gas actually generated in FIG. 7. From the above, it is difficult to detect the generation of gas from the first internal resistance R1, ΔR1 which is the difference between the first internal resistance R1 calculated in the detection cycle at the time of detection and the first internal resistance R1 calculated in the first detection cycle, the second internal resistance R2, and ΔR60 which is the difference between the second internal resistance R2 calculated in the detection cycle at the time of detection and the second internal resistance R2 calculated in the first detection cycle.

[0071] On the other hand, as shown in Figure 4, in response to the increase in the amount of gas, the resistance change ΔRG (Δ(R60-R1)) and the differential resistance RG (R60-R1) increase. These are calculated by subtracting the differential resistance RG (R60-R1) calculated in the first detection cycle from the differential resistance RG (R60-R1) calculated in the detection cycle at the time of detection. Also, as shown in Figure 8, the time course of the resistance change ΔRG (Δ(R60-R1)) and the time course of the actual amount of gas generated generally match. From the above, it can be seen that gas generation can be easily detected by calculating the differential resistance RG and the resistance change ΔRG. Note that in Figures 4 and 8, the amount of gas generated at 14 weeks appears to diverge from the resistance change ΔRG (Δ(R60-R1)). This is because the amount of gas generated became too large, restricting the lithium-ion battery 1 and suppressing the volumetric change of the lithium-ion battery 1, making it impossible to accurately measure the amount of gas.

[0072] [Battery control] As described above, when gas generation in the lithium-ion battery 1 is detected, it is appropriate to control the battery. In other words, the battery capacity is maximized until gas generation occurs, and when gas generation is detected, it is appropriate to perform battery control, such as charge / discharge control of the lithium-ion battery 1, so as to suppress gas generation while monitoring the differential resistance value RG and the resistance change amount ΔRG. As a result, secondary deterioration due to gas generation and increase can be suppressed, battery life can be extended, and the total discharge amount (for example, the total driving distance in the case of an electric vehicle) can be increased.

[0073] In the above experimental results, gas generation began after three to five weeks. Based on this, further experiments were conducted on battery control. In this experiment, five batteries were subjected to a four-week charge-discharge cycle in a 50°C thermostatic chamber, with a charge of 4.2 V and a discharge of 2.7 V at 0.7 CA (CC charging). After this, the charge voltage was changed to 4.1 V, 4.05 V, and 4.0 V (CC charging); the charge voltage was changed to 4.1 V and constant-current, constant-voltage charging (CCCV charging) until the current decayed to 1 / 20 CA; and the charge voltage was changed to 4.0 V and constant-current, constant-voltage charging (CCCV charging) until the current decayed to 1 / 20 CA. These cycles were repeated.

[0074] As shown in Figure 9, by controlling the battery and charging / discharging appropriately, the confirmed capacity is larger in both cases than when the control is left unchanged from 0.7 CA (CC charging) with a charge of 4.2 V and a discharge of 2.7 V, and the battery life is longer. Naturally, lowering the charge voltage reduces the charge / discharge amount per cycle. However, in the case of 4.0 V charging, which has the highest capacity retention rate, the total discharge amount after 7 weeks is 1581 Ah, and the capacity retention rate is 82.9%. In contrast, when the charge / discharge method is not changed, the total discharge amount after 7 weeks is 841 Ah, and the capacity retention rate is 79.5%, which means that the battery life is approximately doubled.

[0075] As for battery control, in addition to changing the charging voltage and reducing (attenuating) the charging current to perform constant current and constant voltage charging during charging and discharging, the temperature during charging and discharging may be changed, and the operating conditions such as the operating temperature of the lithium ion battery 1 may be changed.

[0076] [Another embodiment] (1) The first time t1 is not limited to 1 second, but may be any time exceeding the period during which the initial voltage value rises sharply. For example, the first time t1 may be any time until the internal resistance reaches a predetermined internal resistance of 10% to 40% of the steady state. Specifically, in the case of a lithium-ion battery 1, the first time t1 may be any time between 10 milliseconds and 5 seconds, and more preferably between 10 milliseconds and 3 seconds. Furthermore, the second time t2 is not limited to 60 seconds, but may be any time longer than the first time t1 and until the internal resistance reaches a sufficiently steady state. For example, the second time t2 may be any time until the internal resistance reaches a predetermined internal resistance of 80% or more of the steady state. Specifically, in the case of a lithium-ion battery 1, the second time t2 may be any time between 5 seconds and 600 seconds.

[0077] This makes it possible to calculate an appropriate differential resistance value RG and resistance change amount ΔRG, thereby enabling gas generation to be detected with high accuracy.

[0078] (2) In each of the above embodiments, the preparation process may be performed separately from actual use in the detection cycle, as described above. However, charging and discharging during actual use may also be used as the preparation process. In other words, changes in the current or voltage value occur when the lithium-ion battery 1 (secondary battery) is actually being used. For example, during actual use, the current or voltage value changes when the battery is transitioned from a discharging state to a resting state, and similarly, the current or voltage value changes when the battery is transitioned from a resting state to a discharging state. Therefore, the state in which the current or voltage value changes during charging and discharging during actual use may be used as the preparation process, and the remaining detection cycle may be executed when the current or voltage value changes. This allows for efficient and accurate detection of gas generation in the lithium-ion battery 1.

[0079] (3) In each of the above embodiments, each detection cycle is preferably performed while the lithium-ion battery 1 is maintained within a predetermined temperature range. The first time t1, the second time t2, and the relationship between the threshold and the differential resistance value RG or the resistance change amount ΔRG are determined based on this temperature range. Conversely, because the time dependency of the internal resistance depends on temperature, if the environmental temperature of the detection cycle varies, the relationship between the threshold and the differential resistance value RG or the resistance change amount ΔRG may be corrected according to the environmental temperature at which the detection cycle is performed.

[0080] This allows various conditions to be set appropriately, and gas generation in the lithium ion battery 1 to be detected stably and accurately.

[0081] (4) In each of the above embodiments, the gas detection target is not limited to the lithium ion battery 1, but may be a chargeable and dischargeable electricity storage device such as various secondary batteries.

[0082] (5) In each of the above embodiments, the gas detection device is not limited to being configured with functional blocks such as those shown in FIG. 2, but may be configured with any functional blocks. For example, each functional block of the gas detection device may be further subdivided, or conversely, some or all of the functional blocks may be combined. Furthermore, the gas detection method is not limited to being executed by the gas detection device shown in FIG. 2, but may be executed by a gas detection device of any configuration. Furthermore, some or all of the functions of the gas detection device may be configured by software. A software program is stored in any storage device and executed by a processor such as a CPU included in the gas detection device, or by a separately provided processor.

[0083] The configurations disclosed in the above embodiments (including other embodiments, the same applies below) can be applied in combination with configurations disclosed in other embodiments, as long as no contradiction arises. Furthermore, the embodiments disclosed in this specification are examples, and the embodiments of the present invention are not limited to these, and can be modified as appropriate within the scope that does not deviate from the purpose of the present invention. [Industrial Applicability]

[0084] The present invention can be applied to the detection of gas generated inside various electricity storage devices, including lithium ion batteries. [Explanation of symbols]

[0085] 1. Lithium-ion battery (energy storage device) 9. Charge / discharge control section (current / voltage control section) 11 Current and voltage value acquisition section 12 Resistance calculation section 15. Detection unit 16 Memory section R1 First internal resistance R2 2nd internal resistance RG differential resistance value t0 Elapsed time t1 1st hour t2 Second hour tx elapsed time V0 voltage value V1 voltage value V2 voltage value Vr Steady-state voltage ΔI Amount of change in current value ΔRG Resistance change amount

Claims

1. a preparation step of changing the current value or voltage value of the power storage device; a first measurement step of acquiring a first internal resistance, which is the internal resistance when a predetermined first time has elapsed since the current value or voltage value started to change in the preparation step; a second measurement step of acquiring a second internal resistance, which is the internal resistance when a second time, which is a predetermined time or more longer than the first time, has elapsed since the current value or voltage value started to change in the preparation step; a calculation step of calculating a differential resistance value by subtracting the first internal resistance from the second internal resistance; and detecting generation of gas in the electricity storage device based on the differential resistance value.

2. The gas detection method according to claim 1 , wherein the change in the current value or the voltage value in the preparation step is performed by stopping charging of the power storage device while it is being charged, or stopping discharging of the power storage device while it is being discharged.

3. 3. The gas detection method according to claim 1, wherein in the detection step, it is determined that the gas has been generated when the differential resistance value is greater than a predetermined threshold value.

4. the preparation step, the first measurement step, the second measurement step, the calculation step, and the detection step constitute one detection cycle, and the detection cycle is performed intermittently; 3. The gas detection method according to claim 1, wherein in each of the detection steps, if the differential resistance value is greater than a predetermined threshold value, it is determined that the gas has been generated in that detection step.

5. 5. The gas detection method according to claim 4, wherein in each of the detection steps, the threshold value is calculated based on the differential resistance value calculated in the immediately preceding detection cycle.

6. 5. The gas detection method according to claim 4, wherein the threshold value is the differential resistance value in the first detection cycle.

7. the preparation step, the first measurement step, the second measurement step, the calculation step, and the detection step constitute one detection cycle, and the detection cycle is performed intermittently; 3. The gas detection method according to claim 1, wherein in each of the detection steps, if a value obtained by subtracting the differential resistance value calculated in the first detection cycle from the differential resistance value calculated in that detection cycle is greater than 0, it is determined that the gas has been generated in that detection step.

8. 8. The gas detection method according to claim 1, wherein the second time period is set to a time period from when the preparation step starts until the internal resistance reaches a predetermined internal resistance that is 80% or more of a steady state.

9. 9. The gas detection method according to claim 8, wherein the first time period is set to a time period from the start of the preparation step until the internal resistance reaches a predetermined internal resistance that is 10% or more and 40% or less of a steady state.

10. The first time period is set to be equal to or greater than 10 milliseconds and equal to or less than 3 seconds, The gas detection method according to claim 1 , wherein the second time period is set to be not less than 5 seconds and not more than 600 seconds.

11. 11. The gas detection method according to claim 1, wherein the preparation step, the first measurement step, the second measurement step, the calculation step, and the detection step are performed in a state where the electricity storage device is maintained within a predetermined temperature range.

12. A gas detection device that detects generation of gas in a chargeable and dischargeable electricity storage device, a current / voltage control unit that controls at least one of a current value and a voltage value of the power storage device; a voltmeter for measuring a voltage value of the power storage device; an ammeter for measuring a current value flowing through the power storage device; a resistance calculation unit that measures a first internal resistance, which is the internal resistance of the power storage device, from an amount of change in voltage value from a change in a current value or a voltage value of the power storage device until a predetermined first time has elapsed and from an amount of change in current value when the current value or the voltage value is changed, and measures a second internal resistance, which is the internal resistance of the power storage device, from an amount of change in voltage value from a change in current value or a voltage value of the power storage device until a second time, which is a predetermined time longer than the first time, has elapsed and from an amount of change in current value when the current value or the voltage value is changed; a detection unit that calculates a differential resistance value by subtracting the first internal resistance from the second internal resistance, and detects generation of gas in the power storage device based on the differential resistance value.

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