JP7675558B2 - Capacity recovery device, secondary battery system, and capacity recovery method - Google Patents

Description

The present invention relates to a capacity recovery device, a secondary battery system, and a capacity recovery method.

The abstract of the following Patent Document 1, which describes the background art of this technical field, states, "[Problem] The object of the present invention is to provide a method for restoring the performance of a deteriorated lithium-ion battery, and a power supply system equipped with a means for restoring battery performance. [Solution] A method for regenerating a battery in which a process of increasing the potential of the negative electrode of a deteriorated lithium-ion battery above the potential of the positive electrode and then decreasing the potential of the negative electrode below the potential of the positive electrode is carried out for at least one cycle. Also, a power supply system equipped with a means for increasing the potential of the negative electrode of a lithium-ion battery above the potential of the positive electrode and then decreasing the potential of the negative electrode below the potential of the positive electrode."

Furthermore, the abstract of Patent Document 2 listed below states, "[Problem] To provide a battery system and a capacity recovery method capable of effectively recovering the capacity of a lithium-ion secondary battery in a short period of time. [Solution] An ECU 960 controls a PCU 920 to execute capacity recovery control to recover the capacity of the battery pack 10. The capacity recovery control includes a discharge mode and a capacity recovery mode. In the discharge mode, the ECU 960 discharges the battery pack 10 to a predetermined over-discharge region. In the capacity recovery mode, the ECU 960 repeatedly executes, in the over-discharge region, a voltage increase of the lithium-ion secondary battery by stopping discharge, and a pulse discharge that discharges the lithium-ion secondary battery while oscillating the discharge current."
The disclosures of these documents are incorporated herein by reference.

JP 2012-169094 A JP 2019-106333 A

However, in the above-mentioned technology, there are cases where the capacity of the secondary battery cannot be properly restored.
The present invention has been made in view of the above circumstances, and has an object to provide a capacity recovery device, a secondary battery system, and a capacity recovery method that can appropriately recover the capacity of a secondary battery.

In order to solve the above problem, the capacity recovery device of the present invention includes a capacity recovery processing unit that performs capacity recovery processing of the secondary battery through one or more cycles of a discharge period in which a discharge current is passed between the positive and negative terminals of a secondary battery, and a pause period in which the positive and negative terminals are opened or the discharge current is made smaller than that during the discharge period, and a capacity recovery stopping unit that stops the capacity recovery processing depending on a change in the battery voltage of the secondary battery during the capacity recovery processing, wherein one cycle is defined as a discharge period in which a discharge current is passed between the positive and negative terminals of a secondary battery, and a pause period in which the discharge current is made smaller than that during the discharge period, and the capacity recovery stopping unit stops the capacity recovery processing when a discharge time, which is the time it takes for the battery voltage to reach a predetermined reference voltage after the discharge period has started, is equal to or shorter than a predetermined first time .

According to the present invention, the capacity of the secondary battery can be appropriately restored.

FIG. 2 is a schematic cross-sectional view of a bipolar battery cell. FIG. 2 is a schematic side view of the electricity storage element. FIG. 1 is a schematic diagram of a presumed mechanism for capacity recovery by overdischarge treatment. FIG. 1 is a block diagram of a secondary battery system according to a preferred embodiment. 4 is a flowchart showing the operation of a control unit. FIG. 4 is a diagram showing the change in battery voltage during a discharging period. 13 is an example showing measurement results of a discharge time, etc.

[Overview of the embodiment]
Lithium-ion batteries are a type of non-aqueous electrolyte secondary battery that have a high energy density and are used as batteries for portable devices and, in recent years, electric vehicles. However, it is known that lithium-ion batteries deteriorate with use, resulting in a decrease in battery capacity.
In lithium-ion batteries, lithium metal oxide is generally used as the active material for the positive electrode, and carbon materials such as graphite are generally used for the negative electrode. The positive and negative electrodes of lithium-ion batteries are formed by adding binders and conductive agents to tiny particles of the active material to form a slurry (mixture), which is then applied to metal foil.

During charging, lithium ions released from the positive electrode active material are absorbed into the negative electrode active material, and during discharging, the lithium ions absorbed into the negative electrode active material are released and absorbed into the positive electrode active material. In this way, a current flows between the electrodes as the lithium ions move between the electrodes.
In these lithium-ion batteries,
(1) Electrical isolation of the positive electrode active material,
(2) Electrical isolation of the negative electrode active material; and (3) Fixation of lithium ions that travel between the electrodes.
This reduces the capacity.

Regarding (3), this is thought to occur when lithium ions are absorbed into the coating component (SEI; Solid Electrolyte Interphase) that is formed at the interface between the negative electrode and the electrolyte, causing a loss of mobility. In addition, if lithium ions are absorbed into the negative electrode active material present on the negative electrode surface that does not face the positive electrode (non-facing surface) during charging, the lithium ions cannot be used in the battery reaction during discharge, which may result in a decrease in capacity.

Of these factors, the capacity loss due to (3) above can be recovered by electrochemically oxidizing the SEI described above and decomposing and removing it, thereby restoring the capacity or battery output. In addition, during the discharge reaction, the negative electrode potential can be reduced to a level that forces the lithium ions captured on the non-opposing surface to be released, and the released lithium ions can be used again as battery capacity to recover the capacity.

The battery capacity is restored by releasing the lithium ions taken into the non-negative electrode facing portion by overdischarging the deteriorated secondary battery. On the other hand, the deterioration of the positive and negative electrodes is accelerated by the extreme drop in the positive electrode potential and the extreme rise in the negative electrode potential due to the overdischarge treatment. If lithium ions remain in the non-negative electrode facing portion, the overdischarge energy is consumed by the movement of these lithium ions, but if no lithium ions remain, the overdischarge energy is consumed to increase the positive and negative electrode potentials and promote the deterioration phenomenon of the positive and negative electrodes. Therefore, it is desirable to perform recovery by overdischarge treatment only when lithium ions remain in the non-negative electrode facing portion. However, the above-mentioned Patent Documents 1 and 2 do not show a method for determining whether or not lithium ions remain in the non-negative electrode facing portion. For this reason, even if the battery capacity is not restored by overdischarge treatment, there is a possibility that the deterioration of the positive and negative electrodes will progress.

In the embodiment described below, the aim is to suppress deterioration due to overdischarge processing in a secondary battery system using a lithium-ion battery in which a cycle of overdischarge (discharge period) and rest (rest period) is performed multiple times between the positive and negative electrodes to release the lithium ions fixed in the negative electrode and restore its capacity. Therefore, in the embodiment described below, the cycle of overdischarge and rest is repeated multiple times, the change over time in voltage during the overvoltage is detected, and based on the result, it is determined whether or not to continue the overdischarge processing.

[Battery cell structure]
First, with reference to Figs. 1 and 2, a structural example of a bipolar battery cell applicable to the embodiment will be described.
FIG. 1 is a schematic cross-sectional view of a bipolar battery cell 100.
1, a battery cell 100 (secondary battery) is a lithium ion battery cell, and includes an electricity storage element 1, a positive electrode terminal 2, a negative electrode terminal 3, and an exterior material 6. A separator 5 is included in the electricity storage element 1. The exterior material 6 is made of a laminate film or a similar material.

FIG. 2 is a schematic side view of the electricity storage element 1. As shown in FIG.
As shown in FIG. 2, in the electric storage element 1, a plurality of positive electrodes 12 and a plurality of negative electrodes 13 are alternately stacked with separators 5 interposed therebetween. The electric storage element 1 shown in FIG. 1 corresponds to the region where the positive electrodes 12 and the negative electrodes 13 appear to overlap. The electric storage element 1 further contains an electrolyte (not shown), and the electrolyte is impregnated into micropores of the positive electrodes 12, the negative electrodes 13, the separator 5, and the like. For example, polypropylene can be used as the separator 5. However, in addition to polypropylene, microporous films and nonwoven fabrics made of polyolefins such as polyethylene can also be used as the separator 5.

The positive electrode 12 includes a positive electrode current collector foil 122 and a positive electrode mixture layer 121 applied thereto. The negative electrode 13 includes a negative electrode current collector foil 132 and a negative electrode mixture layer 131 applied thereto. The positive electrode current collector foil 122 and the negative electrode current collector foil 132 are current collector foils of appropriate metals. The positive electrode mixture layer 121 and the negative electrode mixture layer 131 are mixtures of appropriate electrode active materials, conductive agents, binders, etc. Metal tabs are connected to the positive electrode current collector foil 122 and the negative electrode current collector foil 132 as terminals. These tabs become the positive electrode terminal 2 and the negative electrode terminal 3 shown in FIG. 1. In FIG. 1, the exterior material 6 is sealed with the positive electrode terminal 2 and the negative electrode terminal 3 exposed to the outside of the exterior material 6. This allows the battery cell 100 to be connected to the outside via the positive electrode terminal 2 and the negative electrode terminal 3. Hereinafter, the potentials of the positive electrode 12 and the negative electrode 13 will be referred to as the positive electrode potential Ep and the negative electrode potential En. The difference between the two, i.e., "Ep-En", is the voltage between the positive electrode terminal 2 and the negative electrode terminal 3, and this will be referred to as the battery voltage Epn.

Below, examples of materials and the like for each part of the battery cell 100 are described, but these are only examples, and the battery cell 100 of the lithium ion battery of this embodiment is not limited to materials, shapes, manufacturing methods, etc., and any material, shape, manufacturing method, etc. can be applied.

(Positive electrode 12)
The positive electrode current collector foil 122 (see FIG. 2) may be made of aluminum foil having a thickness of 10 to 100 μm, aluminum perforated foil having a thickness of 10 to 100 μm and a pore size of 0.1 to 10 mm, expanded metal, foam metal plate, or the like. The material of the positive electrode current collector foil 122 may be stainless steel, titanium, or the like, in addition to aluminum. The electrode active material of the positive electrode mixture layer 121 is preferably one that contains a reactive species therein. The reactive species of a lithium-ion battery is lithium ions. In this case, the electrode active material contains a lithium-containing compound that can reversibly insert and remove lithium ions.

Examples of the electrode active material of the positive electrode 12 include lithium cobalt oxide, lithium manganese-substituted cobalt oxide, lithium manganate, lithium nickel oxide, lithium transition metal phosphate such as olivine- _{type lithium} iron phosphate, or _{LiwNixCoyMnzO2} (where _w , _x , y, and z are 0 or positive values). The electrode active material of the positive electrode 12 may contain one or more of the above-mentioned materials.

(Negative electrode 13)
The negative electrode current collector foil 132 may be made of copper foil having a thickness of 10 to 100 μm, perforated copper foil having a thickness of 10 to 100 μm and a pore size of 0.1 to 10 mm, expanded metal, foam metal plate, or the like. In addition to copper, stainless steel, titanium, or the like may also be used as the material for the negative electrode current collector foil 132. The electrode active material in the negative electrode 13 contains a material capable of reversibly inserting and detaching lithium ions.

Examples of the types of electrode active materials applied to the negative electrode mixture layer 131 include natural graphite, composite carbonaceous materials formed by forming a coating on natural graphite by a dry CVD method or a wet spray method, artificial graphite produced by firing resin materials such as epoxy and phenol or pitch-based materials obtained from petroleum or coal, silicon (Si), graphite mixed with silicon, non-graphitizable carbon materials, and lithium titanate Li ₄ Ti ₅ O _12. The electrode active materials applied to the negative electrode mixture layer 131 may contain one or more of the above-mentioned materials.

(electrolyte)
In the case of a lithium ion battery, the electrolyte may be, for example, an aprotic organic solvent such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate (MPC), or ethyl propyl carbonate (EPC).

Alternatively, an electrolyte solution can be used in which a lithium salt such as lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium iodide, lithium chloride, lithium bromide, LiB[ _OCOCF3 ] ₄ , LiB[ _OCOCF2CF3 ] ₄ , _LiPF4 ( _CF3 ) ₂ , LiN( _SO2CF3 ) ₂ , _or LiN( _SO2CF2CF3 ) _{2 is dissolved in a solvent of the above-mentioned mixed organic compound of two or more kinds .}
Alternatively, an electrolyte solution in which a mixed lithium salt of two or more of the above-mentioned types is dissolved can be used.

[Estimated mechanism of battery capacity recovery due to overdischarge]
FIG. 3 is a schematic diagram of a presumed mechanism regarding capacity recovery by overdischarge treatment.
3, state STA shown on the upper side is a state in which the battery cell 100 with reduced capacity is over-discharged. State STB shown on the lower side is a state in which the positive electrode 12 and the negative electrode 13 of the battery cell 100 are open. The portion of the negative electrode 13 of the battery cell 100 that faces the positive electrode 12 is called a facing portion 13A, and the other portion is called a non-facing portion 13B.

In a lithium ion secondary battery such as the battery cell 100, charging and discharging proceeds as lithium ions 22 are exchanged between the positive electrode 12 and the opposing portion 13A of the negative electrode 13. However, during the charging and discharging process, if lithium ions 22 are taken into the non-opposing portion 13B that does not face the positive electrode, they become metallic lithium 24 (hatched in the figure) that does not contribute to charging and discharging, which causes a decrease in capacity.

Usually, the positive electrode potential Ep is set sufficiently higher than the negative electrode potential En _. For example, in _a bipolar battery cell using _{LiwNixCoyMnzO2} for the positive _{electrode and} graphite for the negative electrode, the value of the positive electrode potential Ep relative to the negative electrode potential En, i.e., the battery voltage Epn, is in the range of 2.5 to 4.5 V, and is often in the range of, for example, 2.5 to 4.2 V from the viewpoint of life and safety. In this example, a battery voltage Epn of less than 2.5 V is called an "overdischarge state."

When performing capacity recovery processing, for example as shown in state STA, the DC power supply 30 is connected in reverse polarity to the battery cell 100. That is, when the positive electrode of the DC power supply 30 is connected to the negative electrode 13 and the negative electrode of the DC power supply 30 is connected to the positive electrode 12, the battery cell 100 enters an overdischarge state. However, if the battery voltage Epn = Ep - En is set to less than 2.5 V, the DC power supply 30 does not necessarily have to be in reverse polarity.

In this way, when the battery cell 100 is in an overdischarge state, the negative electrode potential En increases, the discharge reaction in the facing portion 13A progresses, and the lithium ions 22 are almost completely released from the facing portion 13A. This creates a difference in the concentration of lithium ions between the facing portion 13A and the non-facing portion 13B, resulting in a potential difference between the two.

Then, when the battery cell 100 is put into a state STB in which the supply of voltage and current from the outside is stopped, electrons 26 move from the metallic lithium 24 adsorbed to the non-facing portion 13B to the facing portion 13A. This causes lithium ions 22 to leave the non-facing portion 13B, and at the same time, lithium ions 22 are inserted into the facing portion 13A. It is believed that this mechanism causes the capacity of the battery cell 100 to recover.

In the figure, the facing portion 13A and the non-facing portion 13B are arranged to sandwich the current collecting foil (not numbered), but the arrangement of the facing portion and non-facing portion is not limited to this. That is, when the negative electrode 13 is wider than the positive electrode 12, the portion of the surface of the negative electrode 13 facing the positive electrode 12, which is a projection of the positive electrode 12, becomes the facing portion. And, the portion of the surface of the negative electrode 13 facing the positive electrode 12, which is located outside the projected portion, becomes the non-facing portion. In this case, the lithium ions absorbed into this outer non-facing portion also contribute to capacity recovery.

When the battery cell 100 is subsequently charged, the DC power source 30 is connected to the battery cell 100 in the opposite direction to state STA. As a result, if the negative electrode potential En decreases, lithium ions 22 are again taken into the non-facing portion 13B, which may cause a decrease in the capacity of the battery cell 100.

[Secondary battery system]
FIG. 4 is a block diagram of a secondary battery system 500 according to a preferred embodiment.
The secondary battery system 500 includes a battery pack 510 (secondary battery) and a current control device 550 (capacity recovery device). The battery pack 510 includes a battery cell module 512 and a battery heating unit 514. The battery cell module 512 may be one battery cell 100 (see FIG. 1 ) or may be a battery cell 100 connected in series and/or parallel. In this specification, the term "secondary battery" is a concept including a cell, a battery module, or a battery pack of a lithium ion battery. The battery heating unit 514 heats the battery cell module 512 based on a command from the current control device 550. However, the battery heating unit 514 is not essential and may be omitted.

The battery pack 510 has a positive terminal 502 and a negative terminal 503. When the battery cell module 512 has only one battery cell 100, the positive terminal 502 and the negative terminal 503 correspond to the positive terminal 2 and the negative terminal 3 of the battery cell 100, respectively (see FIG. 1). On the other hand, when a plurality of battery cells 100 are connected in series in the battery cell module 512, the positive terminal 502 corresponds to the positive terminal 2 of the battery cell 100 with the highest voltage, and the negative terminal 503 corresponds to the negative terminal 3 of the battery cell 100 with the lowest voltage. In FIG. 4, the positive terminal 502 and the negative terminal 503 of the battery pack 510 are each connected to a current control device 550.

The current control device 550 includes an ammeter 551, a voltmeter 552, a variable resistor 553, a power supply 554, switches 560, 562, and 564, and a control unit 570 (computer). The power supply 554 is primarily for charging, and the variable resistor 553 is for discharging. The current control device 550 also serves as a measuring instrument for monitoring the state of the battery pack 510.

The ammeter 551 measures the current flowing through the positive terminal 502 and the negative terminal 503 of the battery pack 510, and outputs the result to the control unit 570. The voltmeter 552 measures the battery voltage Ev between the positive and negative electrodes, and outputs the result to the control unit 570. The control unit 570 controls the switching of the switches 560, 562, and 564 based on the information of the ammeter 551 and the voltmeter 552. As a result, the control unit 570 sets the positive terminal 502 and the negative terminal 503 of the battery pack 510 to an open state, a state in which the variable resistor 553 is connected, or a state in which the power source 554 is connected. The control unit 570 also controls the discharge current by controlling the resistance value of the variable resistor 553. For example, the battery voltage Ev decreases over time during discharge, but the control unit 570 can keep the discharge current constant by reducing the resistance value of the variable resistor 553 over time.

The control unit 570 is equipped with general computer hardware such as a CPU (Central Processing Unit), RAM (Random Access Memory), and ROM (Read Only Memory), and the ROM stores control programs executed by the CPU and various data. In FIG. 4, the inside of the control unit 570 is shown as blocks representing functions realized by the control programs, etc. That is, the control unit 570 is equipped with a mode setting unit 572, a charge/discharge processing unit 574, a capacity recovery processing unit 576 (capacity recovery processing process), and a capacity recovery stopping unit 578 (capacity recovery stopping process).

(Mode setting unit 572)
The mode setting unit 572 switches the operation mode of the current control device 550 between a charge/discharge mode and a capacity recovery mode in response to a user operation or the state of the battery pack 510 .

(Charge/discharge processing unit 574)
When the operation mode is set to the charge/discharge mode, the charge/discharge processing unit 574 performs charge/discharge operation of the battery pack 510 according to a normal application based on predetermined control parameters. For example, when the battery pack 510 is applied to drive a vehicle, the charge/discharge processing unit 574 performs charge/discharge operation for running the vehicle. The above-mentioned control parameters include a "discharge stop voltage Evs (not shown)". When the battery pack 510 is being discharged, the charge/discharge processing unit 574 stops discharging the battery pack 510 when the battery voltage Ev becomes equal to or lower than the discharge stop voltage Evs. Here, the discharge stop voltage Evs is a voltage at which the terminal voltage per battery cell 100 becomes equal to or lower than 2.5 V, for example.

(Capacity recovery processing unit 576)
The capacity recovery processing unit 576 executes a capacity recovery process when the operation mode is set to the capacity recovery mode. Here, the capacity recovery process is a process in which a "discharge period" and a "rest period" are alternately repeated. The discharge period is a period in which the battery pack 510 is discharged so that the battery voltage Ev becomes equal to or lower than the discharge stop voltage Evs, and may include, for example, a period in which the terminal voltage per battery cell 100 becomes equal to or lower than 2.0 V.

The rest period is a period during which the discharge current flowing between the positive electrode terminal 502 and the negative electrode terminal 503 is made smaller than the discharge current flowing during the above-mentioned discharge period. Or, more preferably, the rest period is a period during which the positive electrode terminal 502 and the negative electrode terminal 503 are opened to make the discharge current "0". The length of the rest period may be determined arbitrarily, but is preferably one minute or longer. In this way, by repeating the discharge period and components, the lithium ions that have been deactivated inside the negative electrode 13 can be reactivated by the mechanism shown in FIG. 3, and the capacity of the battery pack 510 can be restored.

During the discharge period, the overdischarge current can be passed in pulses. The duration of the current is not particularly limited, but can be, for example, 0.1 to 60 seconds. If it is shorter than 0.1 seconds, the effect is weak, and if it is longer than 60 seconds, the active material structure of the positive and negative electrodes is easily destroyed, which may actually shorten the lifespan.

The current value during the discharge period may be set arbitrarily, but if the current value is too large, battery deterioration due to heat generation and overvoltage cannot be ignored, and if the current value is too small, the recovery effect is small. If the current for discharging the capacity of a fully charged battery in 1 hour is 1 CA, and the current for discharging in 2 hours is 0.5 CA, it is preferable to set the current value in the range of 1 to 20 CA. The end conditions for one discharge period may be "when the battery voltage reaches a predetermined lower limit discharge voltage Evd (not shown)", "when discharging continues for a predetermined discharge time td (not shown)", "when a predetermined capacity is discharged", etc. Alternatively, any combination of these conditions may be used as the end conditions for one discharge period.

(Capacity recovery stop unit 578)
The capacity recovery stopping unit 578 stops the capacity recovery process by the capacity recovery processing unit 576 based on the state of the battery pack 510 during the above-mentioned discharge period. For example, the capacity recovery stopping unit 578 measures the discharge time tk (not shown) until the battery voltage Ev reaches a predetermined reference voltage Evk (not shown) after the start of discharge. Then, when the discharge time tk satisfies a predetermined capacity recovery end condition, the capacity recovery stopping unit 578 may stop the operation of the capacity recovery processing unit 576. Note that the reference voltage Evk may be the same as the above-mentioned lower limit discharge voltage Evd, or may be a different voltage.

The above-mentioned capacity recovery ending conditions include, for example, the following conditions #1 to #3. That is, it is preferable to end the capacity recovery process when any one of these conditions #1 to #3 is satisfied.
Condition #1: The discharge time tk in the most recent discharge period is equal to or less than a predetermined time tkm (a first time, not shown).
Here, the predetermined time tkm may be any value, but is preferably about 1 to 10 seconds.
- Condition #2: When the discharge time tk in the first discharge period in the capacity recovery process is an initial discharge time tk1 (not shown), the ratio of the discharge time tk in the most recent discharge period to the initial discharge time tk1 is less than or equal to a predetermined ratio Rtk (first ratio, not shown).
Here, the value of the ratio Rtk may be any value, but is preferably in the range of 0.05 to 0.5.

- Condition #3: When the discharge time tk in the immediately preceding discharge period is defined as the previous discharge time tkp (not shown), the ratio of the discharge time tk in the most recent discharge period to the previous discharge time tkp becomes equal to or greater than a predetermined ratio Rtp (second ratio, not shown).
Here, the value of the ratio Rtp may be any value, but is preferably 0.7 or more.

(Overall operation)
FIG. 5 is a flowchart showing the operation of the control unit 570.
5, when the process proceeds to step S2, the capacity recovery processing unit 576 executes a process for a discharging period. That is, a predetermined discharge current is caused to flow between the positive electrode terminal 502 and the negative electrode terminal 503. Next, when the process proceeds to step S4, the capacity recovery processing unit 576 executes a process for a pause period. For example, the positive electrode terminal 502 and the negative electrode terminal 503 are opened.

Next, when the process proceeds to step S6, the capacity recovery stop unit 578 measures the discharge time tk from the start of discharge until the battery voltage Ev reaches the reference voltage Evk. Next, when the process proceeds to step S8, the capacity recovery stop unit 578 determines whether the discharge time tk satisfies a predetermined capacity recovery end condition (e.g., any one of the above conditions #1 to #3). If the determination here is "No", the process returns to step S2, and the processes of steps S2 to S8 are repeated. On the other hand, if the determination in step S8 is "Yes", the process proceeds to step S10, and the capacity recovery stop unit 578 stops the capacity recovery process.

[Example]
Next, examples that are preferable specific examples of the above-described embodiment will be described.
(Configuration of battery pack 510)
In this embodiment, a lithium ion battery was prototyped as a battery pack 510 with the cell configuration shown in FIG. 1 and FIG. 2. In the prototype lithium ion battery, an organic electrolyte was used that uses a carbonate solvent with a volatilization temperature of 20° C. Specifically, LiPF6 was dissolved in a carbonate-based solvent at a concentration of 1M (=mol/dm ³ ). The solvent composition was ethyl carbonate (EC): dimethyl carbonate (DMC) = 1:2. A laminate cell was produced using LiNi _1/3 Co _1/3 Mn _1/3 O ₂ for the positive electrode and graphite for the negative electrode. A positive electrode material was applied to both sides of an Al current collector foil, and a negative electrode material was applied to both sides of a Cu current collector foil, so that the opposing negative electrode/positive electrode capacity ratio was 1.3. After punching out the positive electrode and negative electrode into a predetermined shape, a porous substrate made of a polyolefin material, a positive electrode, and a negative electrode were laminated. The positive electrode capacity was designed to be about 80 mAh on one side, and the negative electrode capacity was designed to be about 90 mAh on one side. The capacity of the battery pack 510 after initialization was about 280 mAh.

(Deterioration test and capacity recovery process)
As a deterioration test of the above-mentioned battery pack 510, 500 charge/discharge cycles were repeated in a 50°C environment, and it was confirmed that the battery capacity decreased to about 80% of the initial capacity. Next, the battery pack 510 was discharged to adjust the battery voltage Ev to 3.0V, and then a capacity recovery process was performed. In the capacity recovery process, a discharge period and a rest period were defined as one cycle, and this cycle was repeated multiple times. First, in the discharge period, a 10 CA discharge pulse was passed for 5 seconds or until the battery voltage Ev reached the lower limit discharge voltage Evd = 0V. That is, if the battery voltage Ev did not reach the lower limit discharge voltage Evd = 0V, the discharge period was 5 seconds, and if the battery voltage Ev reached the lower limit discharge voltage Evd, the discharge period ended at that point. In addition, the rest period was set to 15 minutes, and the battery pack 510 was in an open state for that time.

6 is a diagram showing the change in battery voltage Ev during the discharge period, where characteristics P1 to P4 show the change characteristics of battery voltage Ev during the discharge periods of the first to fourth cycles, respectively.
Each time the cycle is repeated, the number of lithium ions remaining in the non-facing portion decreases, and as shown in the figure, the rate at which the voltage decreases increases.

Figure 7 shows an example of the measurement results of the discharge time tk, etc. In Figure 7, 1.5 V is used as the reference voltage Evk described above. In this embodiment, the capacity recovery process is terminated when the discharge time tk becomes less than 0.2 times the initial discharge time tk1 (4 seconds in the illustrated example). As a result, the capacity recovery process is terminated after four cycles. Figure 7 also shows the capacity recovery amount in each cycle. According to this embodiment, 3% of the capacity was recovered by repeating four cycles. Also, looking at the capacity recovery amounts for the first to third cycles, it can be seen that the recovery efficiency reaches a plateau each time the cycle is repeated.

[Comparative Example]
Next, in order to clarify the effect of this embodiment, a comparative example will be described.
The configuration of the battery pack 510 in this comparative example is the same as that in the above-mentioned embodiment. The processing contents per cycle of the discharge period and the rest period in the capacity recovery process are also the same as those in the above-mentioned embodiment. However, in the capacity recovery process of this comparative example, the cycle of the discharge period and the rest period was repeated 10 times. The capacity recovery amount after the capacity recovery process was completed was 3% of the initial capacity, which was the same as in the above-mentioned embodiment. However, in this comparative example, the internal resistance of the battery pack 510 after the capacity recovery process was higher than that of the embodiment, and a side effect of battery degradation occurred.

[Effects of the embodiment]
According to the preferred embodiment as described above, the capacity recovery device (550) includes a capacity recovery processing unit (576) that performs a capacity recovery process of the secondary battery (510) by one or more cycles of discharge periods and pause periods, each of which is a discharge period in which a discharge current is passed between the positive electrode terminal (502) and the negative electrode terminal (503) of the secondary battery (510) and a pause period in which the positive electrode terminal (502) and the negative electrode terminal (503) are opened or the discharge current is made smaller than that of the discharge period, and a capacity recovery stopping unit (578) that stops the capacity recovery process according to the change in the battery voltage (Ev) of the secondary battery (510) during the capacity recovery process. As a result, the capacity recovery process can be stopped according to the change in the battery voltage (Ev), so that deterioration of the secondary battery (510) can be suppressed and the capacity of the secondary battery can be appropriately restored.

Moreover, it is more preferable that the discharge period is 0.1 seconds or more and 60 seconds or less, and the capacity recovery processing unit (576) passes a pulsed current through the secondary battery (510) during the discharge period and the rest period. This makes it possible to more appropriately recover the capacity of the secondary battery.

Moreover, it is more preferable that the capacity recovery processing unit (576) performs the capacity recovery process of the secondary battery (510) by repeating one cycle multiple times. This allows the capacity recovery process to be continued until a sufficient capacity recovery effect is obtained.

Moreover, it is more preferable that the capacity recovery stopping unit (578) stops the capacity recovery process when the discharge time (tk), which is the time it takes for the battery voltage (Ev) to reach a predetermined reference voltage (Evk) after the discharge period has started, is equal to or less than a predetermined first time (tkm). This allows the capacity recovery stopping unit (578) to stop the capacity recovery process when the recovery efficiency reaches a plateau based on the relationship between the discharge time (tk) and the first time (tkm).

Moreover, it is more preferable that the capacity recovery stopping unit (578) sets the discharge time (tk) in the first cycle as the initial discharge time (tk1), and stops the capacity recovery process when the ratio of the discharge time (tk) in the second or subsequent cycles to the initial discharge time (tk1) becomes equal to or less than a predetermined first ratio (Rtk). In this way, the capacity recovery stopping unit (578) can stop the capacity recovery process when the recovery efficiency reaches a plateau, based on the relationship between the initial discharge time (tk1) and the discharge time (tk) in the second or subsequent cycles.

Moreover, it is more preferable that the capacity recovery stopping unit (578) sets the discharge time (tk) in the immediately preceding cycle as the immediately preceding discharge time (tkp), and stops the capacity recovery process when the ratio of the discharge time (tk) in the immediately preceding cycle to the immediately preceding discharge time (tkp) becomes equal to or greater than a predetermined second ratio (Rtp). In this way, the capacity recovery stopping unit (578) can stop the capacity recovery process when the recovery efficiency reaches a plateau, based on the relationship between the immediately preceding discharge time (tkp) and the discharge time (tk) in the immediately preceding cycle.

[Modification]
The present invention is not limited to the above-mentioned embodiment, and various modifications are possible. The above-mentioned embodiment is exemplified to explain the present invention in an easy-to-understand manner, and is not necessarily limited to those having all the configurations described. In addition, other configurations may be added to the configuration of the above-mentioned embodiment, and it is also possible to replace a part of the configuration with another configuration. In addition, the control lines and information lines shown in the figure show those that are considered necessary for explanation, and do not necessarily show all the control lines and information lines necessary on the product. In reality, it may be considered that almost all the configurations are connected to each other. Possible modifications of the above-mentioned embodiment are, for example, as follows.

(1) The hardware of the current control device 550 in the above embodiment can be realized by a general computer, so programs for executing the various processes described above may be stored on a storage medium or distributed via a transmission line.

(2) In the above embodiment, each of the above processes is described as software-based processing using a program, but some or all of it may be replaced with hardware-based processing using an ASIC (Application Specific Integrated Circuit) or an FPGA (Field Programmable Gate Array), etc.

(3) The various processes performed in the above embodiment may be executed by a server computer via a network (not shown), and the various data stored in the above embodiment may also be stored in the server computer.

100 Battery cell (secondary battery)
500: Secondary battery system 502: Positive electrode terminal 503: Negative electrode terminal 510: Battery pack (secondary battery)
550 Current control device (capacity recovery device)
576 Capacity recovery processing unit (capacity recovery processing process)
578 Capacity recovery stop section (capacity recovery stop process)
Ev Battery voltage tk Discharge time Evk Reference voltage Rtk Ratio (first ratio)
Rtp ratio (second ratio)
tk1: initial discharge time tkm: predetermined time (first time)
tkp Last discharge time

Claims (13)

a capacity recovery processing unit that performs a capacity recovery process on the secondary battery by one or more cycles of a discharge period in which a discharge current is passed between a positive electrode terminal and a negative electrode terminal of the secondary battery, and a pause period in which the positive electrode terminal and the negative electrode terminal are opened or the discharge current is made smaller than that in the discharge period;
a capacity recovery stopping unit that stops the capacity recovery process in response to a change in a battery voltage of the secondary battery during the capacity recovery process ,
The capacity recovery stopping unit stops the capacity recovery process when a discharge time, which is a time until the battery voltage reaches a predetermined reference voltage after the discharge period starts, is equal to or shorter than a predetermined first time.
A capacity recovery device. The discharge period is from 0.1 seconds to 60 seconds,
The capacity recovery device according to claim 1 , wherein the capacity recovery processing unit causes a pulsed current to flow through the secondary battery during the discharge period and the pause period. 3 . The capacity recovery device according to claim 1 , wherein the capacity recovery processing unit performs the capacity recovery process for the secondary battery by repeating the one cycle a plurality of times. 4 . 4. The capacity recovery device according to claim 3, wherein the capacity recovery stop unit determines a time from when the discharge period starts until the battery voltage reaches a predetermined reference voltage as a discharge time, determines the discharge time in a first cycle as an initial discharge time, and stops the capacity recovery process when a ratio of the discharge time in a second or subsequent cycle to the initial discharge time becomes equal to or less than a predetermined first ratio. 4. The capacity recovery device according to claim 3, wherein the capacity recovery stop unit determines a time until the battery voltage reaches a predetermined reference voltage after the discharge period has started as a discharge time, determines the discharge time in the immediately preceding cycle as a previous discharge time, and stops the capacity recovery process when a ratio of the discharge time in the most recent cycle to the previous discharge time becomes equal to or greater than a predetermined second ratio. A secondary battery;
a capacity recovery processing unit that performs a capacity recovery process on the secondary battery by one or more cycles of a discharge period in which a discharge current is passed between a positive electrode terminal and a negative electrode terminal of the secondary battery, and a pause period in which the positive electrode terminal and the negative electrode terminal are opened or the discharge current is made smaller than that in the discharge period;
a capacity recovery stopping unit that stops the capacity recovery process in response to a change in a battery voltage of the secondary battery during the capacity recovery process ,
The capacity recovery stopping unit stops the capacity recovery process when a discharge time, which is a time until the battery voltage reaches a predetermined reference voltage after the discharge period starts, is equal to or shorter than a predetermined first time.
A secondary battery system comprising: The secondary battery system according to claim 6 , wherein the capacity recovery processing unit performs the capacity recovery process for the secondary battery by repeating the one cycle a plurality of times. 8. The secondary battery system according to claim 7, wherein the capacity recovery stop unit determines a time until the battery voltage reaches a predetermined reference voltage after the discharge period starts as a discharge time, determines the discharge time in a first cycle as an initial discharge time, and stops the capacity recovery process when a ratio of the discharge time in a second or subsequent cycle to the initial discharge time becomes equal to or less than a predetermined first ratio. 8. The secondary battery system according to claim 7, wherein the capacity recovery stop unit determines a time from when the discharge period starts until the battery voltage reaches a predetermined reference voltage as a discharge time, determines the discharge time in the immediately preceding cycle as a previous discharge time, and stops the capacity recovery process when a ratio of the discharge time in the most recent cycle to the previous discharge time becomes equal to or greater than a predetermined second ratio. a capacity recovery process in which one cycle is defined as a discharge period during which a discharge current is passed between a positive electrode terminal and a negative electrode terminal of a secondary battery, and a rest period during which the positive electrode terminal and the negative electrode terminal are opened or the discharge current is made smaller than that during the discharge period, and a capacity recovery process is performed on the secondary battery through one or more cycles of the discharge period and the rest period;
a capacity recovery stopping step of stopping the capacity recovery process in response to a change in the battery voltage of the secondary battery during the capacity recovery process ,
The capacity recovery stopping step is a step of stopping the capacity recovery process when a discharge time, which is a time until the battery voltage reaches a predetermined reference voltage after the discharge period starts, is equal to or shorter than a predetermined first time.
A capacity recovery method comprising: The capacity recovery method according to claim 10 , wherein the capacity recovery process performs the capacity recovery process for the secondary battery by repeating the one cycle a plurality of times. The capacity recovery method according to claim 11, characterized in that the capacity recovery stopping step determines a time from when the discharge period starts until the battery voltage reaches a predetermined reference voltage as a discharge time, determines the discharge time in a first cycle as an initial discharge time, and stops the capacity recovery process when a ratio of the discharge time in a second or subsequent cycle to the initial discharge time becomes equal to or less than a predetermined first ratio. The capacity recovery method according to claim 11, characterized in that the capacity recovery stopping step determines the time until the battery voltage reaches a predetermined reference voltage after the discharge period is started as the discharge time, determines the discharge time in the immediately preceding cycle as the previous discharge time, and stops the capacity recovery process when a ratio of the discharge time in the most recent cycle to the previous discharge time becomes equal to or greater than a predetermined second ratio.

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