Power storage device and charge / discharge control method

Abstract

The power storage device includes: a plurality of secondary batteries including a negative electrode including crystalline carbon; and a charge / discharge control portion that controls charge / discharge of the plurality of secondary batteries in such a manner that a state of charge does not become in a range of 50% or more and 70% or less.

Description

Technical Field

[0001] This invention relates to energy storage devices and charging / discharging control methods. Background Technology

[0002] The state of charge (SOC) of all batteries included in an energy storage device is usually the same. In contrast, patent documents 1 and 2 describe techniques that allow for uneven SOC among the batteries.

[0003] Patent Document 1 describes a power supply device that maintains a low SOC of a first battery and a high SOC of a second battery, thereby primarily receiving regenerative current from the motor from the first battery with high regenerative power density, and outputting drive current to the motor from the second battery with high output density.

[0004] Patent document 2 describes a monitoring device for an energy storage device, which sets a standby SOC based on the lifespan characteristics of the energy storage device, representing the SOC during standby time when the energy storage device is not charging or discharging.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 9-308013

[0008] Patent Document 2: Japanese Patent Application Publication No. 2016-12983. Summary of the Invention

[0009] The problem that the invention aims to solve

[0010] However, both the power supply device and the energy storage device monitoring device described in Patent Documents 1 and 2 use a high SOC standby battery for discharging and a low SOC standby battery for charging. Furthermore, the high SOC standby battery needs to be discharged to a predetermined SOC and then recharged to return to its original standby SOC, while the low SOC standby battery needs to be charged to a predetermined SOC different from that of the high SOC standby battery and then discharged to return to its original standby SOC. However, the battery used to suppress system frequency fluctuations would be subjected to continuous, repeated, minute charge-discharge cycles, making it difficult to implement the recovery operation described in Patent Documents 1 and 2.

[0011] The purpose of this invention is to provide an energy storage device and a charge / discharge control method that can suppress the degradation of a secondary battery due to the minute charge / discharge even when the minute charge / discharge is repeated continuously.

[0012] Solution for solving the problem

[0013] To solve the above-mentioned problems, the first invention is an energy storage device comprising: a plurality of secondary batteries, each secondary battery having a negative electrode containing crystalline carbon; and a charge / discharge control unit that controls the charging and discharging of the plurality of secondary batteries in a manner that prevents the state of charge from falling within a range of 50% to 70%.

[0014] The second invention is a charge-discharge control method comprising the following steps: calculating the state of charge of a plurality of secondary batteries having a negative electrode containing crystalline carbon; and controlling the charge-discharge of the plurality of secondary batteries in a manner that prevents the state of charge of the secondary batteries from becoming more than 50% and less than 70%.

[0015] The effects of the invention

[0016] According to the present invention, even when there is continuous and repeated minute charging and discharging, it is possible to suppress the deterioration of the secondary battery due to the minute charging and discharging. Attached Figure Description

[0017] Figure 1 This is a block diagram illustrating an example of the configuration of an energy storage device according to an embodiment of the present invention.

[0018] Figure 2 This is a flowchart illustrating an example of the charging and discharging operation of an energy storage device according to an embodiment of the present invention.

[0019] Figure 3 This is a flowchart used to illustrate a test involving repeated partial charge and discharge.

[0020] Figure 4 This is a graph showing the SOC range of a partial cyclic test.

[0021] Figure 5 This is a coordinate graph showing the cycle number dependence of capacity maintenance.

[0022] Figure 6 This is a coordinate graph showing the SOC dependence of the positive electrode capacity retention rate.

[0023] Figure 7 This is a coordinate graph showing the SOC dependence of the negative electrode capacity retention rate.

[0024] Figure 8 This is a coordinate graph showing the SOC dependence of the reactive resistance determined by the GITT method. Detailed Implementation

[0025] [Composition of an energy storage device]

[0026] Figure 1This is a block diagram illustrating an example of the configuration of an energy storage device 10 according to an embodiment of the present invention. The energy storage device 10 includes battery modules 11A and 11B, a charge / discharge control unit 20, and an external input / output unit 12. The energy storage device 10 is a peak-assistance energy storage device for improving system stability. In one embodiment, the case where the energy storage device 10 includes two battery modules 11A and 11B will be described, but the number of battery modules is not limited to this; it may include one, three, or more.

[0027] (Battery module)

[0028] Battery modules 11A and 11B are used in either a low SOC range or a high SOC range. Here, "low SOC range" refers to a range with an SOC of 0% or higher but less than 50%, and "high SOC range" refers to a range with an SOC of 70% or higher but less than 100%.

[0029] Battery modules 11A and 11B each have multiple secondary batteries. These secondary batteries can be connected in series, parallel, or a series-parallel connection. Each secondary battery has a negative electrode containing crystalline carbon as the negative electrode active material. Examples of secondary batteries include non-aqueous electrolyte secondary batteries such as lithium-ion batteries, or all-solid-state batteries.

[0030] Crystalline carbon is, for example, graphite. Graphite includes, for example, at least one of natural graphite and artificial graphite. The secondary battery may include materials other than crystalline carbon as the negative electrode active material. Materials other than crystalline carbon include, for example, at least one of metallic and semi-metallic elements capable of forming alloys with lithium. Specifically, for example, at least one selected from the group consisting of Mg, B, Al, Ti, Ga, In, Si, Ge, Sn, Pb, Bi, Cd, Ag, Zn, Hf, Zr, Y, Pd, and Pt.

[0031] Crystalline carbon exhibits a phase transition between stage 1 and stage 2 within a state of charge (SOC) range of 50% to 70%. These inter-stage phase transitions are accompanied by larger changes in lattice constants than other inter-stage phase transitions, resulting in significant changes in the particle size of the negative electrode active material and consequently, deformation within the negative electrode. By reducing the number of phase transitions between stage 1 and stage 2, even under continuous and repeated minor charge-discharge cycles in the energy storage device 10, degradation caused by changes in the particle size of the negative electrode active material and deformation within the negative electrode due to these minor charge-discharge cycles can be suppressed. In other words, even under continuous and repeated minor charge-discharge cycles in the energy storage device 10, degradation of the secondary battery due to these minor charge-discharge cycles can be suppressed.

[0032] (External Input / Output Section)

[0033] The external input / output unit 12 is connected to a load that is continuously and repeatedly subjected to minute charging and discharging.

[0034] (Charge / Discharge Control Unit)

[0035] The charge / discharge control unit 20 measures the state of charge (SOC) of each of the battery modules 11A and 11B. Based on the SOC measurement results, the charge / discharge control unit 20 controls the charging and discharging of the battery modules 11A and 11B in a manner that prevents the SOC from falling to a range of 50% or more and 70% or less. Specifically, based on the SOC measurement results, the charge / discharge control unit 20 controls the charging and discharging of the battery modules 11A and 11B in a manner that makes one of the battery modules 11A and 11B fall into a low SOC range and the other fall into a high SOC range. That is, the charge / discharge of the battery modules 11A and 11B is controlled in a manner that makes the stage structure of the crystalline carbon contained in each secondary battery constituting the negative electrode active material in one of the battery modules 11A and 11B reach a stage 2 or higher, while making the stage structure of the crystalline carbon contained in each secondary battery constituting the negative electrode active material in the other battery reach a stage 1.

[0036] The charge / discharge control unit 20 periodically changes the SOC range of battery modules 11A and 11B. By periodically changing the SOC range of battery modules 11A and 11B, the cycle life of battery modules 11A and 11B can be extended.

[0037] The charge and discharge control unit 20 controls the charging and discharging of battery module 11A and battery module 11B in such a way that both battery module 11A and battery module 11B are charged when the energy storage device 10 is charging and discharged when the energy storage device 10 is discharging.

[0038] The charge / discharge control unit 20 includes bidirectional voltage conversion units 21A and 21B, SOC detection units 22A and 22B, SOC adjustment units 23A and 23B, and a timer 24.

[0039] (Bidirectional voltage conversion section)

[0040] The bidirectional voltage conversion unit 21A performs voltage conversion between the voltage of the battery module 11A connected to it and the voltage of the external input / output unit 12. The bidirectional voltage conversion unit 21B performs voltage conversion between the voltage of the battery module 11B connected to it and the voltage of the external input / output unit 12.

[0041] (SOC Testing Department)

[0042] The SOC detection unit 22A monitors the voltage of the battery module 11A and the current flowing between the battery module 11A and the bidirectional voltage conversion unit 21A, and calculates the SOC of the battery module 11A. The SOC detection unit 22A smooths the calculated SOC by performing a moving average with a time constant larger than that for the minute charge and discharge cycles of the energy storage device 10.

[0043] The SOC detection unit 22B monitors the voltage of the battery module 11B and the current flowing between the battery module 11B and the bidirectional voltage conversion unit 21B, and calculates the SOC of the battery module 11B. The SOC detection unit 22B smooths the calculated SOC by performing a moving average with a time constant larger than that for the minute charge and discharge cycles of the energy storage device 10.

[0044] (SOC Adjustment Department)

[0045] SOC adjustment units 23A and 23B have a low SOC charge / discharge mode (first charge / discharge mode) and a high SOC charge / discharge mode (second charge / discharge mode), and SOC adjustment units 23A and 23B are set to different charge / discharge modes. SOC adjustment units 23A and 23B periodically change the charge / discharge mode according to the notification from timer 24.

[0046] The SOC adjustment unit 23A controls the bidirectional voltage conversion unit 21A based on the smoothed SOC calculated by the SOC detection unit 22A. Specifically, the SOC adjustment unit 23A determines whether the smoothed SOC calculated by the SOC detection unit 22A falls within the SOC range of the currently set charge / discharge mode, and controls the bidirectional voltage conversion unit 21A based on the determination result. As a result, the battery module 11A performs charge / discharge corresponding to the set charge / discharge mode.

[0047] The SOC adjustment unit 23B controls the bidirectional voltage conversion unit 21B based on the smoothed SOC calculated by the SOC detection unit 22B. Specifically, the SOC adjustment unit 23B determines whether the smoothed SOC calculated by the SOC detection unit 22B falls within the SOC range of the currently set charge / discharge mode, and controls the bidirectional voltage conversion unit 21B based on the determination result. As a result, the battery module 11B performs charge / discharge corresponding to the set charge / discharge mode.

[0048] The SOC adjustment unit 23A controls the bidirectional voltage conversion unit 21A in a manner that the battery module 11A charges when the energy storage device 10 is charging and discharges when the energy storage device 10 is discharging. Similarly, the SOC adjustment unit 23B controls the bidirectional voltage conversion unit 21B in a manner that the battery module 11B charges when the energy storage device 10 is charging and discharges when the energy storage device 10 is discharging.

[0049] (Timer)

[0050] The timer 24 of the time measurement unit measures the time for periodically replacing the SOC range of battery module 11A and battery module 11B. If the measurement time reaches the replacement time, it notifies the SOC adjustment units 23A and 23B of the replacement of the SOC range (charge and discharge mode) of battery module 11A and battery module 11B.

[0051] [Operation of the energy storage device]

[0052] Figure 2 This is a flowchart illustrating an example of the charging and discharging operation of the energy storage device 10 having the above-described configuration. It should be noted that the bidirectional voltage conversion unit 21A, SOC detection unit 22A, and SOC adjustment unit 23A operate identically to the bidirectional voltage conversion unit 21B, SOC detection unit 22B, and SOC adjustment unit 23B. Therefore, the operation of the bidirectional voltage conversion unit 21A, SOC detection unit 22A, and SOC adjustment unit 23A will be described below only.

[0053] First, in step S1, the SOC detection unit 22A monitors the voltage of the battery module 11A and the current flowing between the battery module 11A and the bidirectional voltage conversion unit 21A to calculate the SOC of the battery module 11A.

[0054] Next, in step S2, the SOC detection unit 22A smooths the SOC calculated in step S1 by performing a moving average with a time constant larger than that for the slight charge and discharge of the energy storage device 10.

[0055] Next, in step S3, the SOC adjustment unit 23A determines whether the smoothed SOC calculated in step S2 exceeds the SOC range corresponding to the currently set SOC mode.

[0056] If, in step S3, it is determined that the smoothed SOC exceeds the aforementioned SOC range, in step S4, the SOC adjustment unit 23A adjusts the discharge direction smoothed current I... A > Smoothing current I in the charging direction B The bidirectional voltage converter 21A is controlled in a manner that allows for control.

[0057] On the other hand, if it is determined in step S3 that the smoothed SOC does not exceed the above-mentioned SOC range, in step S5, the SOC adjustment unit 23A determines whether the smoothed SOC obtained in step S2 is insufficient for the SOC range corresponding to the currently set SOC mode.

[0058] If, in step S5, it is determined that the smoothed SOC is insufficient within the aforementioned SOC range, in step S6, the SOC adjustment unit 23A adjusts the discharge direction smoothing current I... A Smoothing current I in the charging directionB The bidirectional voltage converter 21A is controlled in a manner that allows for control.

[0059] On the other hand, if it is determined in step S5 that the smoothed SOC is not below the aforementioned SOC range, in step S7, the SOC adjustment unit 23A adjusts the discharge direction smoothed current I to ensure that the discharge direction is within the specified range. A = Smoothing current I in the charging direction B The bidirectional voltage converter 21A is controlled in a manner that allows for control.

[0060] [Effects]

[0061] As described above, in the energy storage device 10 according to one embodiment, the charge / discharge control unit 20 controls the charge and discharge of battery modules 11A and 11B in a manner that prevents the State of Charge (SOC) from becoming more than 50% and less than 70%. Therefore, even when the energy storage device 10 continuously repeats minute charge and discharge cycles, the number of phase transitions in the crystalline structure of the crystalline carbon contained in the negative electrode between stage 1 and stage 2 can be reduced. Thus, the degradation of the secondary battery characteristics caused by changes in the particle size of the negative electrode active material and the generation of deformation within the negative electrode can be suppressed. Therefore, even when the system energy storage device continuously repeats its characteristic minute charge and discharge cycles, the degradation of the secondary battery characteristics due to these minute charge and discharge cycles can be suppressed.

[0062] [Variation Example]

[0063] (Variation Example 1)

[0064] In the above embodiment, the charge / discharge control unit 20 has been described in a manner that causes one of the battery modules 11A and 11B to be in a low SOC range and the other to be in a high SOC range. However, it is also possible to control the charge / discharge of the battery modules 11A and 11B in a manner that changes the charging state of both battery modules 11A and 11B to a low SOC range or a high SOC range. That is, it is also possible to control the charge / discharge of the battery modules 11A and 11B in a manner that changes the stage structure of the crystalline carbon contained in each secondary battery constituting the negative electrode active material to a stage 2 or higher state or a stage 1 state.

[0065] Specifically, SOC adjustment units 23A and 23B can also be set to the same charging and discharging mode. That is, both SOC adjustment units 23A and 23B can be set to a low SOC charging and discharging mode or a high SOC charging and discharging mode.

[0066] The charge / discharge control unit 20 can periodically change the SOC range of both battery modules 11A and 11B. Specifically, the SOC adjustment units 23A and 23B can periodically change the charge / discharge mode based on notifications from the timer 24.

[0067] (Variation Example 2)

[0068] In one embodiment described above, the charge / discharge control unit 20 is equipped with a timer 24, and the charge / discharge mode of the SOC adjustment units 23A and 23B is changed according to the notification from the timer 24 to the SOC adjustment units 23A and 23B. However, it is also possible that the charge / discharge control unit 20 does not have a timer 24 and does not change the charge / discharge mode of the SOC adjustment units 23A and 23B.

[0069] Example

[0070] The present invention will be specifically described below through embodiments, but the present invention is not limited to these embodiments.

[0071] [Examples 1-7, Comparative Examples 1-3]

[0072] First, as test samples, 10 commercially available cylindrical lithium-ion secondary batteries (positive electrode active material: NCM (nickel-cobalt-manganese) based positive electrode material, negative electrode active material: graphite) were prepared. Next, the 10 prepared secondary batteries were connected to CH1 to CH10 of a secondary battery charge-discharge testing device manufactured by SECI (Soft Energy Controls Co., Ltd.).

[0073] Next, based on Figure 3 The flowchart shown illustrates the repeated partial charge-discharge cycles performed on the prepared secondary battery. It should be noted that the SOC range conditions are 10 conditions, and the temperature condition is 35℃. Figure 4 The SOC range of each partial cycle of CH1 to 10 (Examples 1 to 7, Comparative Examples 1 to 3) is shown.

[0074] First, in step S11, each secondary battery was charged and discharged at full speed using a secondary battery charge / discharge testing device. Next, in step S12, the State of Charge (SOC) of each secondary battery was adjusted using the same device. Next, in step S13, 100 partial cycles (ΔSOC = 5%) were performed. Steps S11 to S13 were repeated until the number of partial cycles reached 11,000.

[0075] (Evaluation of capacity maintenance rate)

[0076] The cycle number dependence and SOC dependence of the capacity retention rate of the charge / discharge control methods of Examples 1-7 and Comparative Examples 1-3 were determined. It should be noted that the capacity was calculated based on the discharge curve (0.05C) of full-speed charge / discharge. Furthermore, the capacity retention rate was calculated based on the initial full-speed measurement value. Additionally, the amplification of the positive and negative electrodes was calculated by analyzing the OCV of the discharge curves of full-speed charge / discharge.

[0077] (Evaluation of reactive resistance)

[0078] The reactive resistance of the secondary batteries used in the charge-discharge control methods of Examples 1-7 and Comparative Examples 1-3 above was determined by the GITT (Galvanostatic Intermittent Titration Technique).

[0079] (Evaluation Results)

[0080] Figure 5 This is a graph showing the cycle number dependence of capacity retention for secondary batteries with CH1 (SOC range: 15%–20%). For degradation caused by continuous, repeated, minute charge-discharge cycles, the negative electrode degrades faster than the positive electrode; it can be said that the negative electrode dominates. This trend is also observed in secondary batteries with CH2–10. Therefore, it is believed that battery degradation is strongly correlated with negative electrode degradation. That is, it is thought that by implementing charge-discharge control that minimizes negative electrode degradation, battery life can be extended.

[0081] Figure 6 This is a graph showing the SOC dependence of cathode capacity retention. The cathode capacity retention shows a decrease as the SOC increases, i.e., the further to the right of the graph, the lower the retention. This trend is believed to be related to the phase transition of the ternary (NMC) cathode material used as the cathode active material.

[0082] Figure 7 This is a graph showing the SOC dependence of the negative electrode capacity retention rate. The negative electrode capacity retention rate shows a mortar-like change in SOC decreasing in the range above 50% and below 70% (i.e., near the center of SOC). This trend is considered to be related to the phase transition between phase 1 and phase 2 of graphite.

[0083] Figure 8 This is a graph showing the SOC dependence of the reaction resistance measured by the GITT method. A relatively large reaction resistance exists between 50% and 70% SOC, associated with the phase transition between stage 1 and stage 2. Therefore, it is believed that within the SOC range of 50% to 70%, there is degradation associated with changes in the particle size of the negative electrode active material, as well as with localized heating and localized overvoltage.

[0084] (in conclusion)

[0085] To improve system stability, the secondary battery used undergoes continuous, repeated, minute charge-discharge cycles. Furthermore, it was previously believed that secondary batteries used to improve system stability would ideally be kept in standby mode around 50% of their State of Charge (SOC) to maximize both power reception and supply. However, based on the above evaluation results, extending the lifespan of the negative electrode is considered more effective in extending the lifespan of the secondary battery. It is desirable to control the charge-discharge cycle in a manner that avoids continuous, minute charge-discharge cycles within an SOC range of 50% to 70%.

[0086] The embodiments and variations of the present invention have been described in detail above. However, the present invention is not limited to the above embodiments and variations, and various modifications can be made based on the technical concept of the present invention.

[0087] For example, the structures, methods, processes, materials, and values ​​listed in the above-described embodiments and their variations are merely examples, and different structures, methods, processes, materials, and values ​​may be used as needed.

[0088] The above-described embodiments and their variations, including their structure, method, process, materials, and values, can be combined with each other as long as they do not depart from the spirit of the present invention.

[0089] In the numerical ranges described in stages in the above-described embodiments and their variations, the upper or lower limit of the numerical range in a certain stage can be replaced by the upper or lower limit of the numerical range in other stages.

[0090] Unless otherwise specified, the materials exemplified in the above embodiments and their variations can be used alone or in combination of two or more.

[0091] Symbol Explanation

[0092] 10. Energy storage device; 11A, 11B. Battery module; 12. External input / output unit; 20. Charge / discharge control unit; 21A, 21B. Bidirectional voltage conversion unit; 22A, 22B. SOC detection unit; 23A, 23B. SOC adjustment unit; 24. Timer.

Claims

1. An energy storage device, characterized in that, have: A plurality of secondary batteries, each secondary battery having a negative electrode comprising crystalline carbon, wherein the negative electrode configuration of each secondary battery is identical; and The charge / discharge control unit controls the charging and discharging of the plurality of secondary batteries in a manner that prevents the state of charge from falling to a range of 50% or more and 70% or less. The charge / discharge control unit controls the charging and discharging of the plurality of secondary batteries in such a way that the state of charge of a portion of the secondary batteries becomes greater than 0% but less than 50%, and the state of charge of the remaining secondary batteries becomes greater than 70% but less than 100%. The charge / discharge control unit periodically changes the charging state range of a portion of the secondary batteries and the charging state range of the remaining secondary batteries.

2. The energy storage device according to claim 1, characterized in that, The charge / discharge control unit controls the charging and discharging of the plurality of secondary batteries in such a way that the stage structure of the crystalline carbon contained in a portion of the secondary batteries is changed to stage 2 or higher, and the stage structure of the crystalline carbon contained in the remaining secondary batteries is changed to stage 1.

3. The energy storage device according to claim 1, characterized in that, The charge / discharge control unit controls the charging and discharging of multiple secondary batteries in such a manner that a portion of the secondary batteries whose charging state is in the range of more than 0% and less than 50% and the remaining secondary batteries whose charging state is in the range of more than 70% and less than 100% are charged when the energy storage device is charging and discharged when the energy storage device is discharging.

4. A charging and discharging control method, characterized in that, The following steps are required: The state of charge of multiple secondary batteries is calculated, each secondary battery having a negative electrode comprising crystalline carbon, and the negative electrode configuration of each secondary battery being identical; and The charging and discharging of the multiple secondary batteries are controlled in a manner that prevents the state of charge of the secondary battery from falling within a range of 50% or more and 70% or less. The charge / discharge control method controls the charging and discharging of the multiple secondary batteries in such a way that the state of charge of a portion of the secondary batteries is in the range of more than 0% and less than 50%, and the state of charge of the remaining secondary batteries is in the range of more than 70% and less than 100%. The charge / discharge control method periodically changes the range of the charging state of a portion of the secondary batteries and the range of the charging state of the remaining secondary batteries.

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