Power storage system

Abstract

To precisely predict deposition of lithium in a lithium ion secondary battery.SOLUTION: A power storage system can determine a deposition risk of lithium in a lithium ion secondary battery, and includes a battery cell having a first electrode that becomes a positive electrode, a second electrode that becomes a negative electrode, a separator disposed between the first electrode and the second electrode, and a third electrode disposed between the second electrode and the separator, a first detection section for detecting a voltage value between the first and second electrodes, a second detection section for detecting a voltage value between the first and third electrodes, and a determination section for outputting a warning signal when a difference in a second voltage value detected by the second detection section relative to a first voltage value detected by the first detection section becomes equal to or more than a predetermined threshold during charging of the battery cell.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to a power storage system.

Background Art

[0002] In a lithium-ion secondary battery used in a battery electric vehicle (BEV), how to predict and suppress the precipitation of lithium during charging is an issue. For example, in Patent Document 1, when charging a lithium-ion secondary battery, the point at which the rate of increase in the voltage of the battery cell becomes dull is determined as the lithium precipitation occurrence point.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Thus, it is preferable that the precipitation of lithium be estimated from the state of the lithium-ion secondary battery during charging.

[0005] An object of the present disclosure is to provide a power storage system that can accurately predict the precipitation of lithium in a lithium-ion secondary battery.

Means for Solving the Problems

[0006] The power storage system according to the present disclosure is a power storage system capable of determining the risk of lithium precipitation in a lithium-ion secondary battery, and includes a first electrode serving as a positive electrode, a second electrode serving as a negative electrode, a separator disposed between the first electrode and the second electrode, and a battery cell having a third electrode disposed between the second electrode and the separator, a first detection unit that detects a voltage value between the first and second electrodes, a second detection unit that detects a voltage value between the first and third electrodes, and a determination unit that outputs a warning signal when a difference between a second voltage value detected by the second detection unit and a first voltage value detected by the first detection unit during charging of the battery cell is equal to or greater than a predetermined threshold value.

Effect of the Invention

[0007] According to the power storage system of the present disclosure, it is possible to accurately predict the precipitation of lithium in a lithium-ion secondary battery.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

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Figure 7

Embodiments for Carrying Out the Invention

[0009] Hereinafter, embodiments of the power storage system according to the present disclosure will be described with reference to the drawings.

[0010] (Configuration Example of Power Storage System) FIG. 1 is a block diagram showing an example of the configuration of a power storage system 1 according to an embodiment. As shown in FIG. 1, the power storage system 1 of the embodiment includes a battery 50 and a determination unit 10, and is configured to be able to determine the risk of lithium precipitation in the battery 50.

[0011] The battery 50 is, for example, a lithium-ion secondary battery and can supply power to an electric motor or the like. The battery 50 is also charged by being connected to a charging system (not shown) including a power source such as an AC power source.

[0012] The determination unit 10 is configured as a computer including, for example, a CPU (Central Processing Unit), a ROM (Read Only Memory), and a RAM (Random Access Memory) (not shown), and determines the risk of lithium precipitation in the battery 50 while appropriately monitoring the state of the battery 50 during charging.

[0013] However, the above functions of the determination unit 10 may be realized by an FPGA (Field Programmable Gate Array), an ASIC (Application Specific Integrated Circuit), or the like.

[0014] FIG. 2 is a schematic diagram showing an example of the configuration of a battery cell 500 included in the battery 50 according to an embodiment. The battery 50 includes a plurality of battery cells 500, and FIG. 2 shows one of them. FIG. 2 also shows a more detailed connection state between the battery cell 500 and the determination unit 10.

[0015] As shown in FIG. 2, the battery cell 500 includes a positive electrode 511, a negative electrode 512, a measurement electrode 513, an electrolyte 520, a positive electrode active material 531, a negative electrode active material 532, a separator 540, and detectors 552 and 553.

[0016] The electrolyte 520 is partitioned by a separator 540 having a plurality of through holes, and the positive electrode 511 and the negative electrode 512 are immersed on both sides separated by the separator 540. In the electrolyte 520, the positive electrode 511 and the negative electrode 512 are arranged at a substantially equal distance from the separator 540. Further, the measurement electrode 513 is immersed in the electrolyte 520 between the negative electrode 512 and the separator 540. At this time, the measurement electrode 513 is preferably arranged closer to the separator 540 than the intermediate position between the negative electrode 512 and the separator 540.

[0017] Here, the positive electrode 511 is an example of the first electrode, the negative electrode 512 is an example of the second electrode, and the measurement electrode 513 is an example of the third electrode.

[0018] The positive electrode 511 and the negative electrode 512 are electrically connected to the positive electrode and the negative electrode of the power supply PW, respectively. A detector 552 is provided between the positive electrode 511 and the negative electrode 512, and the voltage between the positive electrode 511 and the negative electrode 512, that is, the cell voltage Vcell can be measured. A detector 553 is provided between the negative electrode 512 and the measurement electrode 513, and the voltage Vsub between the negative electrode 512 and the measurement electrode 513 can be measured.

[0019] The detectors 552 and 553 are voltmeters or the like. The detector 552 is an example of the first detection unit, and the detector 553 is an example of the second detection unit. Further, the cell voltage Vcell detected by the detector 552 is an example of the first voltage value, and the voltage Vsub detected by the detector 553 is an example of the second voltage value.

[0020] In the electrolyte 520 between the positive electrode 511 and the separator 540, a plurality of positive electrode active materials 531 are immersed in a layered manner in the vertical direction. In the electrolyte 520 between the negative electrode 512 and the separator 540, a plurality of negative electrode active materials 532 are immersed in a layered manner in the vertical direction.

[0021] As described above, the battery 50 is configured as a lithium-ion secondary battery. In this case, the electrolyte 520 is, for example, a non-aqueous electrolyte having lithium-ion conductivity or the like. In the battery 50 of the embodiment, the electrolyte 520 of the battery cell 500 can have properties such as a liquid state, a semi-solid (quasi-solid) state, or an all-solid state. Further, the positive electrode 511 and the positive electrode active material 531 are, for example, lithium cobaltate (LiCoO 2 ) or the like, and the negative electrode 512 and the negative electrode active material 532 are, for example, graphite (C) or the like. Further, for the measurement electrode 513, a material similar to that of the negative electrode 512, such as graphite, can be used.

[0022] During charging of the battery 50, a charging current Icell flows between the positive electrode 511 and the negative electrode 512 from the power source PW, and lithium ions (Li + ) move through the electrolyte 520 from the positive electrode 511 side to the negative electrode 512 side.

[0023] The determination unit 10 is connected to the above-described detectors 552 and 553 provided in the battery cell 500. During charging of the battery 50, the determination unit 10 acquires the cell voltage Vcell and the voltage Vsub detected by these detectors 552 and 553. Further, the determination unit 10 determines whether or not the difference Vprec between the voltage Vsub and the cell voltage Vcell, that is, "difference Vprec = cell voltage Vcell - voltage Vsub", is equal to or greater than a predetermined threshold value.

[0024] As will be described later, when the difference Vprec between the voltage Vsub and the cell voltage Vcell increases, the risk of lithium precipitation in the battery cell 500 increases. For this reason, when the difference Vprec between the cell voltage Vcell and the voltage Vsub is equal to or greater than a predetermined threshold value, the determination unit 10 outputs a warning signal.

[0025] The warning signal from the determination unit 10 is output to the above-described charging system including a power supply or the like, and is a signal for causing the charging system to perform control to prevent lithium precipitation, such as reducing the charging current Icell flowing through the battery cell 500.

[0026] (Operation mechanism of the power storage system) Next, with reference to FIGS. 3 to 5, the operation mechanism during charging in the power storage system 1 of the embodiment will be described. FIG. 3 is a schematic diagram showing the state of the battery cell 500 being charged according to the embodiment.

[0027] As shown in FIG. 3(a), when charging is proceeding normally, lithium ions moving from the positive electrode 511 to the negative electrode 512 are diffused substantially evenly in the electrolyte 520. For this reason, the voltage between the positive electrode 511 and the negative electrode 512, that is, the cell voltage Vcell detected by the detector 552, and the voltage between the measurement electrode 513 and the positive electrode 511, that is, the voltage Vsub detected by the detector 553 are substantially equal.

[0028] As shown in FIG. 3(b), when the charging current is too high compared to the capacity of the battery 50, the diffusion of lithium ions into the negative electrode 512 cannot keep up, and the lithium ion concentration around the negative electrode 512 increases. When the lithium ion concentration around the negative electrode 512 exceeds a predetermined value, lithium precipitation occurs.

[0029] Here, when the diffusion of lithium ions into the negative electrode 512 stagnates, the potential of the negative electrode 512 decreases. For this reason, the cell voltage Vcell increases and exceeds the voltage Vsub between the measurement electrode 513 and the positive electrode 511. That is, by monitoring the difference Prec between the cell voltage Vcell and the voltage Vsub using the detectors 552 and 553, it is possible to predict that the risk of lithium precipitation is increasing.

[0030] As described above, the measurement electrode 513 is immersed in the electrolyte 520 between the negative electrode 512 and the separator 540. At this time, as described above, the measurement electrode 513 is preferably disposed closer to the separator 540 than the central position between the negative electrode 512 and the separator 540. This is because the closer the measurement electrode 513 is to the position away from the negative electrode 512 and closer to the separator 540, the greater the difference Prec between the cell voltage Vcell and the voltage Vsub when the risk of lithium precipitation increases is detected.

[0031] However, if the measurement electrode 513 is located too far from the negative electrode 512, a difference is likely to occur between the cell voltage Vcell and the voltage Vsub, and the detection sensitivity for lithium precipitation may increase excessively. Thus, it is considered that there is an optimal position for the installation position of the measurement electrode 513, and it is preferable to appropriately adjust the installation position of the measurement electrode 513 in advance by experiments or the like.

[0032] In addition, the risk of lithium precipitation during charging can increase not only when the charging current is too high as described above, but also depending on the temperature conditions during charging and the charging rate of the battery 50, etc.

[0033] FIG. 4 is a graph showing the dependence of the lithium precipitation risk during charging of the battery 50 according to the embodiment on temperature and the state of charge (SOC). The horizontal axis of the graph in FIG. 4 is the SOC, and the vertical axis is the coefficient. Also, the lithium precipitation risk for different temperatures is plotted in the graph of FIG. 4.

[0034] Here, the SOC is a numerical value represented by 0% to 100% and indicates the charging rate of the battery 50. When the SOC is 0%, it indicates that the remaining charge amount is zero, and when the SOC is 100%, it indicates a full charge.

[0035] As shown in FIG. 4, it can be seen that the lithium precipitation risk during charging increases as the SOC increases, and also increases as the temperature during charging decreases.

[0036] Based on the above points, in the power storage system 1 of the embodiment, the above-described determination unit 10 determines the risk of lithium precipitation based on the detection results by the detectors 552 and 553.

[0037] FIG. 5 is a graph for explaining a method for determining the risk of lithium precipitation in the power storage system 1 according to the embodiment. The horizontal axis of the graph in FIG. 5 represents the lithium precipitation risk, and the vertical axis represents the difference Vprec between the voltage Vsub and the cell voltage Vcell. As shown in FIG. 5, it can be seen that as the risk of lithium precipitation increases, the difference Vprec between the voltage Vsub and the cell voltage Vcell becomes larger.

[0038] Therefore, in the power storage system 1 of the embodiment, a threshold value is set for the difference Vprec between the voltage Vsub and the cell voltage Vcell, and the determination unit 10 outputs a warning signal when these differences Vprec are equal to or greater than the threshold value. By performing control such as reducing the charging current, for example, in the charging system that receives this warning signal, it is possible to predict and suppress the precipitation of lithium in the battery cell 500 in advance.

[0039] (Example of determination process of power storage system) Next, with reference to FIG. 6, the determination process of the lithium precipitation risk during charging by the determination unit 10 of the embodiment will be described. FIG. 6 is a flowchart showing an example of the procedure of the determination process of the lithium precipitation risk in the power storage system 1 according to the embodiment.

[0040] As shown in FIG. 6, when charging starts, the determination unit 10 acquires the cell voltage Vcell measured by the detectors 552 and 553 respectively, and the voltage Vsub between the measurement electrode 513 and the positive electrode 511 (step S101). Further, the determination unit 10 calculates the difference Vprec between the voltage Vsub and the cell voltage Vcell (step S102), and determines whether these differences Vprec are equal to or greater than a predetermined threshold value (step S103).

[0041] When it is determined that the difference Vprec between the voltage Vsub and the cell voltage Vcell is equal to or greater than the threshold value (step S103: Yes), the determination unit 10 outputs a warning signal (step S104).

[0042] When it is determined that the difference Vprec between the voltage Vsub and the cell voltage Vcell is less than the threshold value (step S103: No), the determination unit 10 skips step S104 described above.

[0043] Also, during the charging of the battery 50 in the processes of steps S101 to S104, the determination unit 10 monitors whether the state of charge (SOC) has reached the target value (step S105). When the SOC is less than the target value (step S105: No), the determination unit 10 repeats the processes of steps S101 to S104, and when the SOC reaches the target value (step S105: Yes), the determination unit 10 ends the process.

[0044] As described above, the determination process for the lithium precipitation risk in the power storage system 1 of the embodiment ends.

[0045] (Application example of the power storage system) Next, with reference to FIG. 7, an application example of the power storage system 1 of the embodiment will be described. As an example, the power storage system 1 of the embodiment can be applied to a charge / discharge system provided in a vehicle such as an electric vehicle.

[0046] FIG. 7 is a block diagram showing an example of the configuration of a charge / discharge system 2 to which the power storage system 1 according to the embodiment is applied. The charge / discharge system 2 according to the embodiment is mounted on a vehicle using, for example, a lithium-ion secondary battery as a power source, and performs charge and discharge of the lithium-ion secondary battery.

[0047] As shown in FIG. 7, the charge / discharge system 2 includes a charge / discharge electronic control unit (ECU) 100, a safety control system (BMS: Battery Management System) 20, a motor generator (MG) 30, an inverter 40, a battery 50, a charger 60, and a charging connector 70.

[0048] The motor generator 30 is a three-phase AC motor that supplies three-phase AC drive currents of U-phase, V-phase, and W-phase. The motor generator 30 is driven by the power from the battery 50 and mainly outputs power for vehicle running and regenerative braking force. That is, the above-described vehicle equipped with the charge-discharge system 2 runs by the power from the motor generator 30. Also, when the vehicle is running downhill or the like, the battery 50 can be charged by the regenerative braking force output from the motor generator 30.

[0049] The inverter 40 includes a plurality of switching elements (not shown) and drives the motor generator 30 by these switching elements. Also, when outputting the regenerative braking force from the motor generator 30 to charge the battery 50, the inverter 40 adjusts the regenerative torque applied to the motor generator 30.

[0050] The battery 50 is the same as the battery 50 included in the above-described power storage system 1 and includes a measurement electrode 513, detectors 552 and 553, etc. In the charge-discharge system 2, the battery 50 can supply power to the motor generator 30 and can be charged by connecting the charging connector 70 to a power source. Also, the battery 50 can be charged by receiving power supply from the motor generator 30.

[0051] The charger 60 converts an alternating current from a power source connected to the charging connector 70 into a direct current and charges the battery 50 with a predetermined charging current.

[0052] The charging connector 70 is connected to an AC power source or the like and supplies power to the battery 50 via the charger 60.

[0053] The charge-discharge ECU 100 is configured as a computer including, for example, a CPU, a ROM, and a RAM (not shown) and controls the charge and discharge of the battery 50. However, at least a part of the functional units of the charge-discharge ECU 100 may be realized by an FPGA or an ASIC or the like.

[0054] For example, when charging the battery 50, the charge / discharge ECU 100 instructs the charger 60 to charge the battery 50 with a predetermined charging current. Also, when outputting a regenerative braking force from the motor generator 30 to charge the battery 50, the charge / discharge ECU 100 instructs the inverter 40 to adjust the regenerative torque of the motor generator 30 to a predetermined value. Thereby, the battery 50 is charged with a desired charging current.

[0055] The charge / discharge ECU 100 incorporates, for example, the determination unit 10 of the above-described power storage system 1. By the determination unit 10 performing determination while appropriately monitoring the state of the battery 50, the inverters 40 and the charger 60 are controlled. That is, the determination unit 10 acquires the cell voltage Vcell and the voltage Vsub from the detectors 552 and 553 of the battery 50, and when the difference Vprec between these becomes equal to or greater than a predetermined threshold value, a warning signal is issued within the charge / discharge ECU 100.

[0056] When a warning signal is issued from the determination unit 10, the charge / discharge ECU 100 decreases the charging current by increasing the regenerative torque of the motor generator 30 by the inverter 40, or decreases the charging current by controlling the charger 60.

[0057] The safety control system 20 suppresses, for example, the heat generation of the battery 50, calculates the charge amount of the battery 50, and equalizes the cell voltages Vcell of the plurality of battery cells 500 constituting the battery 50. Thereby, the safety control system 20 suppresses the deterioration of the battery 50 due to overcharging, over-discharging, or the like.

[0058] (Summary) In a vehicle such as an electric vehicle equipped with a battery such as a lithium-ion secondary battery, in order to suppress lithium precipitation during charging, for example, a guard value obtained by experiments is set, and control is performed to charge the battery with a charging current less than the guard value.

[0059] However, the guard values obtained through experiments cannot accommodate the characteristic differences of individual batteries, and in batteries with inferior characteristics, there is a risk of lithium precipitation. The precipitated lithium may short-circuit the battery. Also, if the guard value is set according to the battery with inferior characteristics, the good-characteristic batteries will be overly guarded, and the charging time may be prolonged.

[0060] According to the power storage system 1 of the embodiment, it includes a detector 552 that detects the cell voltage Vcell, which is the voltage value between the positive electrode 511 and the negative electrode 512, and a detector 553 that detects the voltage Vsub between the positive electrode 511 and the measurement electrode 513. Thereby, the difference between the cell voltage Vcell and the voltage Vsub, which is correlated with the risk of lithium precipitation, can be actually measured.

[0061] According to the power storage system 1 of the embodiment, when the difference Vprec between the voltage Vsub detected by the detector 553 and the cell voltage Vcell detected by the detector 552 becomes equal to or greater than a predetermined threshold during the charging of the battery cell 500, the determination unit 10 outputs a warning signal. Thereby, the precipitation of lithium in the battery 50 configured as a lithium-ion secondary battery can be accurately predicted.

[0062] Also, when the power storage system 1 of the embodiment is applied to the above-described charge and discharge system 2 or the like, as described above, by appropriately controlling the charging current based on the actually measured values during charging, etc., the precipitation of lithium can be more reliably suppressed even in the battery 50 with inferior characteristics, and it can be suppressed that the good-characteristic batteries are overly guarded, and the charging time can be shortened.

[0063] Also, if the battery 50 is charged using the motor generator 30, the amount of regeneration can be increased to improve the electricity cost. Also, even on a long downhill slope, the regenerative braking force of the motor generator 30 can be maintained for a long time to suppress the decrease in the deceleration rate, and the drivability can be improved.

[0064] According to the energy storage system 1 of the embodiment, the measurement electrode 513 is on the negative electrode 512 side between the positive electrode 511 and the negative electrode 512, and is arranged closer to the intermediate position between the positive electrode 511 and the negative electrode 512. Thereby, the detection sensitivity of the difference Vprec between the voltage Vsub and the cell voltage Vcell can be appropriately adjusted, and the precipitation of lithium can be predicted with higher accuracy.

[0065] As described above, the farther the measurement electrode 513 is from the negative electrode 512, the easier it is for the difference Vprec between the voltage Vsub and the cell voltage Vcell to occur, and the determination sensitivity of lithium precipitation also increases. In the above-described embodiment, one measurement electrode 513 is provided at a position in the battery cell 500 where the determination sensitivity of lithium precipitation is appropriate. However, a plurality of measurement electrodes 513 may be provided at different positions between the separator 540 and the negative electrode 512.

[0066] In this case, the average value of the plurality of voltages Vsub obtained from the plurality of measurement electrodes 513 may be taken to adjust the determination sensitivity of lithium precipitation, or depending on the temperature conditions during charging, the elapsed time since the start of charging, the value of SOC, etc., which of the plurality of measurement electrodes 513 is used to measure the voltage Vsub may be appropriately changed.

Explanation of symbols

[0067] 1 Energy storage system 10 Determination unit 20 Safety control system 30 Motor generator 40 Inverter 50 Battery 60 Charger 70 Charging connector 100 Charge / discharge ECU 500 Battery cell 511 Positive electrode 512 Negative electrode 513 Measurement electrode 540 Separator 552, 553 Detector

Claims

1. A power storage system capable of determining the risk of lithium precipitation in a lithium-ion secondary battery, comprising: a battery cell having a first electrode serving as a positive electrode, a second electrode serving as a negative electrode, a separator disposed between the first electrode and the second electrode, and a third electrode disposed between the second electrode and the separator; a first detection unit that detects a voltage value between the first and second electrodes; a second detection unit that detects a voltage value between the first and third electrodes; a determination unit that outputs a warning signal when a difference between a second voltage value detected by the second detection unit and a first voltage value detected by the first detection unit during charging of the battery cell is equal to or greater than a predetermined threshold; A power storage system.

2. The third electrode is between the second electrode and the separator and is disposed closer to the separator. The power storage system according to claim 1.

3. The third electrode includes a plurality of third electrodes disposed between the second electrode and the separator, The second detection unit includes a plurality of second detection units that respectively detect voltage values between the first electrode and each of the plurality of third electrodes. The power storage system according to claim 1.

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