JP2022142030A - Secondary battery system and program - Google Patents

Abstract

To achieve the long lifetime of a secondary battery in a secondary battery system.SOLUTION: A secondary battery system 500 includes: a secondary battery 512 having one or more battery cells 100 that are lithium ion battery cells; a charging/discharging unit 572 that performs charging to the secondary battery 512 or discharging from the secondary battery 512 based on a control parameter that specifies conditions for controlling current or voltage when performing charging to the secondary battery 512 or discharging from the secondary battery 512; a capacity restoration unit 574 that performs, on the secondary battery 512, a capacity restoration process including a process of maintaining the terminal voltage of the battery cell 100 at or below 2.0 V; and a parameter updating unit 580 that updates the control parameter according to the status of the capacity restoration process.SELECTED DRAWING: Figure 3

Description

The present invention relates to a secondary battery system and program.

As a background art of this technical field, the abstract of Patent Document 1 below states, "[Problem] An object of the present invention is to provide a method for recovering the performance of a deteriorated lithium-ion battery and a power supply system provided with means for recovering the battery performance. [Solution] A battery in which a step of increasing the potential of a negative electrode of a deteriorated lithium-ion battery to a level higher than that of a positive electrode and then lowering the potential of the negative electrode to a level lower than that of the positive electrode is performed for at least one cycle. and a power supply system comprising means for increasing the potential of the negative electrode of a lithium-ion battery above the potential of the positive electrode and then lowering the potential of the negative electrode below the potential of the positive electrode.” ing.

In addition, in the abstract of Patent Document 2 below, it is stated that "[Problem] To prevent deterioration of a battery or recover the deterioration, to maximize the charging and discharging performance of the battery, and to maintain the charging and discharging performance of the battery for a long period of time. [Solution] In batteries such as lithium-ion secondary batteries, the cause of various abnormalities and deterioration is the reaction product generated on the surface of the electrode.Specifically, the current that forms the reaction product A signal (reverse pulse current) is applied such that the current flows in the opposite direction to dissolve the reaction product."

In addition, the abstract of Patent Document 3 below states, "[Problem] To provide a technology for controlling charging and discharging of a lithium ion secondary battery in consideration of recoverable deteriorated components. [Solution] Lithium ion secondary battery. The controller (40) controls the charging and discharging of the lithium ion secondary battery (10), the controller performs recovery processing for recovering the capacity of the lithium ion secondary battery (10) due to overdischarge, and after performing the recovery processing, The charging and discharging of the lithium ion secondary battery is controlled according to the deterioration state specified from the capacity of the lithium ion secondary battery."

In addition, in the abstract of Non-Patent Document 1 below, it is stated that ``Analysis of the differentiated discharge curve, which divides the discharge curve of the battery cell into that of the positive electrode and the negative electrode, is useful for investigating the deterioration mechanism (The differential discharge curve analysis that separates the discharge curve of battery cells into those of the positive and negative electrodes is useful for investigating deterioration mechanisms.)…” The descriptions of these documents are incorporated as part of the present specification.

JP 2012-169094 A Japanese Patent Application Laid-Open No. 2014-187002 JP 2011-159545 A

Kohei Honkura, Tatsuo Horiba, "Study of the deterioration mechanism of LiCoO2/graphite cells in charge/discharge cycles using the discharge curve analysis" Journal of Power Sources 264 (2014) 140－146

By the way, in the technology described above, it is preferable if the life of the secondary battery can be further extended.
SUMMARY OF THE INVENTION It is an object of the present invention to provide a secondary battery system and program capable of extending the life of a secondary battery.

In order to solve the above problems, the secondary battery system of the present invention provides a secondary battery having one or more battery cells that are lithium ion battery cells, and charging or discharging the secondary battery. A charge/discharge processing unit that performs charging processing for the secondary battery or discharging processing from the secondary battery based on control parameters that define control conditions for the current or voltage at the time, and for the secondary battery, the a capacity recovery processing unit that performs capacity recovery processing including processing for maintaining the terminal voltage of the battery cell at 2.0 V or less; and a parameter update unit that updates the control parameter according to the status of the capacity recovery processing. It is characterized by

According to the present invention, it is possible to extend the life of the secondary battery.

1 is a diagram conceptually showing an example of a cross section of a battery cell; FIG. FIG. 2 is a side view of the storage element shown in FIG. 1; 1 is a block diagram of a secondary battery system according to an embodiment; FIG. FIG. 4 is a schematic diagram of a presumed mechanism regarding capacity recovery by overdischarge treatment. 4 is a flow chart of a control program executed by a control unit; It is an example of a charge/discharge curve of a battery cell. FIG. 7 is a diagram showing the rate of change in the charge/discharge curve of FIG. 6;

[Premise of the embodiment]
Lithium ion batteries are one type of non-aqueous electrolyte secondary batteries, and because of their high energy density, they are also used as batteries for mobile devices and, in recent years, as batteries for electric vehicles. However, it is known that the lithium ion battery deteriorates with use and the battery capacity decreases. Lithium ion batteries generally use a lithium metal oxide as a positive electrode active material and a carbon material such as graphite as a negative electrode active material. A positive electrode and a negative electrode of a lithium ion battery are formed by adding a binder (binder), a conductive agent, etc. to minute active material particles to form a slurry (mixture), and then applying the slurry (mixture) to a metal foil.

During charging, the lithium ions released from the positive electrode active material are absorbed by the negative electrode active material, and during discharging, the lithium ions absorbed by the negative electrode active material are released and absorbed by the positive electrode active material. Thus, current flows between the electrodes as lithium ions move between the electrodes.
In such lithium-ion batteries,
(1) electrical isolation of the positive electrode active material,
(2) electrical isolation of the negative electrode active material, and (3) immobilization of lithium ions traveling between the electrodes,
capacity is reduced by

Regarding (3), it is considered that lithium ions are taken into a film component (SEI: Solid Electrolyte Interphase) formed at the interface between the negative electrode and the electrolytic solution, resulting in loss of mobility. In addition, if lithium ions are taken into the negative electrode active material present on the negative electrode surface (non-facing surface) that does not face the positive electrode during charging, the lithium ions cannot be used for the battery reaction during discharging, resulting in a decrease in capacity. It is also possible that it will decrease.

Of these factors, the decrease in capacity due to the above (3) can be recovered by electrochemically oxidizing and decomposing and removing the above SEI to restore the capacity or battery output. In addition, the capacity can be recovered by raising the negative electrode potential to the extent that the lithium ions taken into the non-facing surface are forcibly released during the discharge reaction, and using the released lithium ions as battery capacity again.

Due to overdischarge treatment of deteriorated batteries, the film on the surface of the negative electrode is decomposed and removed, and the lithium ions captured in the negative electrode in the non-facing part are released. If the capacity deteriorates again due to discharge, the battery life will not improve. Therefore, it is preferable to perform control to prevent rapid capacity deterioration after recovering the battery capacity by overdischarge treatment. By applying the above-mentioned Patent Document 3, it is considered possible to diagnose the state of deterioration of the battery after recovery of the capacity, and to control the subsequent charge/discharge conditions. It is also conceivable that when the capacity decrease rate exceeds a predetermined threshold, the charge/discharge operation is stopped and the charge/discharge operation conditions are corrected until the capacity decrease rate falls within the threshold.

However, Patent Literature 3 does not clarify a specific correction policy. In addition, the measurement parameter for determining the charging/discharging operation conditions obtained in the technique applying Patent Document 3 is not the direct information about the negative electrode that deteriorates, but the deterioration rate of the entire battery. Therefore, there arises a problem that many steps are required to correct the charging/discharging operation conditions. Therefore, in the embodiments described below, a secondary battery system using a lithium ion battery that recovers the capacity by releasing the lithium ions immobilized in the negative electrode by performing overdischarge between the positive electrode and the negative electrode , the negative electrode information after recovery is detected. Then, based on the detection result, the charging/discharging operation control for suppressing further capacity deterioration is executed to extend the life of the secondary battery system.

[Structure of battery cell of lithium ion battery]
First, with reference to FIGS. 1 and 2, a structural example of a bipolar battery cell applicable to a preferred embodiment will be described.
FIG. 1 is a diagram conceptually showing an example of a cross section of a bipolar battery cell 100 (secondary battery).
In FIG. 1 , a battery cell 100 includes a power 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 storage element 1 . The exterior material 6 is constructed using a laminate film or a similar material.

FIG. 2 is a side view of the storage element 1 shown in FIG.
As shown in FIG. 2 , in the 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 storage element 1 shown in FIG. 1 corresponds to the region where the positive electrode 12 and the negative electrode 13 appear to overlap. The storage element 1 further contains an electrolytic solution (not shown), and the electrolytic solution impregnates micropores of the positive electrode 12, the negative electrode 13, the separator 5, and the like. As the separator 5, for example, polypropylene can be applied. However, as the separator 5, a microporous film made of polyolefin such as polyethylene or a non-woven fabric can be applied in addition to polypropylene.

The positive electrode 12 includes a positive current collector foil 122 and a positive electrode mixture layer 121 applied thereto. Further, the negative electrode 13 includes a negative electrode 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 suitable metal current collector foils. Also, the positive electrode mixture layer 121 and the negative electrode mixture layer 131 are mixtures of suitable electrode active materials, conductive agents, binders, and the like. Metal tabs serving as terminals are connected to the positive current collector foil 122 and the negative current collector foil 132, respectively. These tabs become the positive terminal 2 and the negative terminal 3 shown in FIG. 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 . Thereby, the battery cell 100 can be connected to the outside through the positive terminal 2 and the negative terminal 3 . The potentials of the positive electrode 12 and the negative electrode 13 are hereinafter referred to as a positive electrode potential Ep and a negative electrode potential En. The difference between the two, ie, "Ep-En", is the voltage between the positive terminal 2 and the negative terminal 3, and is called the battery voltage Epn.

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

(Positive electrode 12)
The positive current collector foil 122 (see FIG. 2) is made of an aluminum foil with a thickness of 10 to 100 μm, a perforated aluminum foil with a thickness of 10 to 100 μm and a hole diameter of 0.1 to 10 mm, an expanded metal, a foamed metal plate, or the like. can be applied. In addition to aluminum, stainless steel, titanium, and the like can also be used for the material of the positive electrode current collector foil 122 . The positive electrode mixture layer 121 preferably contains a reactive species therein. The reactive species in lithium ion batteries is lithium ions. In this case, the electrode active material contains a lithium-containing compound capable of reversibly intercalating and deintercalating lithium ions.

Examples of the electrode active material of the positive electrode 12 include lithium cobaltate, manganese-substituted lithium cobaltate, lithium manganate, lithium nickelate, lithium transition metal phosphate such as olivine-type lithium iron phosphate, or Li _w Ni _x . Co _y Mn _z O ₂ (here, w, x, y, and z are 0 or positive values). Further, as the electrode active material of the positive electrode 12, the materials described above may be contained singly or in combination of two or more.

(Negative electrode 13)
As the negative electrode current collector foil 132, a copper foil with a thickness of 10 to 100 μm, a perforated copper foil with a thickness of 10 to 100 μm and a hole diameter of 0.1 to 10 mm, an expanded metal, a foamed metal plate, or the like can be applied. Further, as the material of the negative electrode current collector foil 132, other than copper, stainless steel, titanium, or the like can be applied. The electrode active material in the negative electrode 13 contains a material capable of reversibly intercalating and deintercalating lithium ions.

Examples of the type of electrode active material for the negative electrode 13 include natural graphite, a composite carbonaceous material obtained by forming a film on natural graphite by a dry CVD method or a wet spray method, a resin material such as epoxy or phenol, or a material made from petroleum or coal. Artificial graphite produced by firing the obtained pitch-based material as a raw material, silicon (Si), graphite mixed with silicon, non-graphitizable carbon material, lithium titanate, and Li ₄ Ti ₅ O ₁₂ . Further, as the negative electrode active material of the negative electrode 13, the materials described above may be contained singly or in combination of two or more.

(Electrolyte)
In the case of lithium-ion batteries, the electrolyte is, for example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl Aprotic organic solvents such as propyl carbonate (MPC) and ethyl propyl carbonate (EPC) can be applied.
Alternatively, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium iodide, lithium chloride, lithium bromide, LiB[OCOCF ₃ ] is added to the solvent of the mixed organic compound of two or more kinds described above. ₄ , LiB[ _OCOCF2CF3 ] ₄ , _{LiPF4 ( CF3} ) _{2 ,} LiN _{( SO2CF3} ) ₂ , LiN( _SO2CF2CF3 ) ₂ and other lithium salt _- dissolved electrolytes. .
Alternatively, an electrolytic solution in which two or more kinds of mixed lithium salts described above are dissolved can be used.

The constituent solvent of the aforementioned electrolytic solution is generally highly volatile, and the volatilization temperature of the electrolytic solution is often less than 25°C. Additionally, in one of the preferred embodiments, a lithium ion conductive liquid with an elevated volatilization temperature may be used. Specifically, ionic liquids and solvated ionic liquids can be mentioned. Ionic liquids contain cations and anions. Ionic liquids are classified into imidazolium-based, ammonium-based, pyrrolidinium-based, piperidinium-based, pyridinium-based, morpholinium-based, phosphonium-based, sulfonium-based, and the like, depending on the cation species. Examples of cations constituting imidazolium-based ionic liquids include alkylimidazolium cations such as 1-etyl-3-methylimidazolium and 1-butyl-3-methylimidazolium (BMI).

Examples of cations that make up ammonium-based ionic liquids include N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium (DEME) and tetraamylammonium, as well as N,N,N-trimethyl-N There are alkylammonium cations such as -propylammonium. Examples of cations constituting pyrrolidinium-based ionic liquids include alkylpyrrolidinium cations such as N-methyl-N-propylpyrrolidinium (Py13) and 1-butyl-1-methylpyrrolidinium. Examples of cations constituting piperidinium-based ionic liquids include alkylpiperidinium cations such as N-methyl-N-propylpiperidinium (PP13) and 1-butyl-1-methylpiperidinium.

Examples of cations constituting pyridinium-based ionic liquids include alkylpyridinium cations such as 1-butylpyridinium and 1-butyl-4-methylpyridinium. Examples of cations constituting morpholinium-based ionic liquids include alkylmorpholinium such as 4-ethyl-4-methylmorpholinium. Examples of cations constituting phosphonium-based ionic liquids include alkylphosphonium cations such as tetrabutylphosphonium and tributylmethylphosphonium. Examples of cations constituting sulfonium-based ionic liquids include alkylsulfonium cations such as trimethylsulfonium and tributhylsulfonium. Examples of anions paired with these cations include bis (trifluoromethanesulfonyl) imide (TFSI), bis (fluorosulfonyl) imide, tetrafluoroborate (BF4), hexafluorophosphate (PF6), bis (pentafluoroethanesulfonyl) imide (BETI), trifluoromethanesulfonate (triflate). , acetate, dimethyl phosphate, dicyanamide, trifluoro (trifluoromethyl) borate, etc. These ionic liquids may be used singly or in combination.

Also, the ionic liquid may contain an electrolyte salt. As the electrolyte salt, one that can be uniformly dispersed in the solvent can be used. A lithium salt containing lithium as a cation and the above anion can be used. For example, lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), lithium tetrafluoroborate (LiBF4), lithium hexafluorophosphate Fart (LiPF6), lithium triflate, etc., but not limited to these. These electrolyte salts may be used singly or in combination.

Ether-based solvents constitute solvated electrolyte salts and solvated ionic liquids. Known glyme (a general term for symmetrical glycol diethers represented by R-O(CH2CH2O)n-R' (R, R' are saturated hydrocarbons and n is an integer)) that exhibits similar properties to ionic liquids as an ether solvent. available. In terms of ionic conductivity, tetraglyme (tetraethylene glycol dimethyl ether, G4), triglyme (triethylene glycol dimethyl ether, G3), pentaglyme (pentaethylene glycol dimethyl ether, G5), hexaglyme (hexaethylene glycol dimethyl ether, G6) It can be preferably used.

As an ether solvent, crown ether (a general term for macrocyclic ethers represented by (--CH2--CH2--O)n (where n is an integer)) can be used. Specifically, 12-crown-4, 15-crown-5, 18-crown-6, dibenzo-18-crown-6 and the like can be preferably used, but are not limited thereto. These ether solvents may be used singly or in combination. Tetraglyme and triglyme are preferably used because they can form a complex structure with the solvated electrolyte salt. In the present embodiment, tetraglyme is preferred because it has a coordination structure with excellent reduction stability with respect to lithium ions.

Lithium salts such as LiFSI, LiTFSI, and LiBETI can be used as solvated electrolyte salts, but are not limited to these. Ether-based solvents and mixtures of solvated electrolyte salts may be used singly or in combination as semi-solid electrolyte solvents. The reduction potential of the semi-solid electrolyte solvent is desirably lower than the reduction potential of the negative electrode active material by 0.5 V or more, preferably 0.2 V or more. The weight ratio of the main solvent in the semi-solid electrolyte is not particularly limited, but from the viewpoint of battery stability and high-speed charging and discharging, the weight ratio of the main solvent in the total solvent in the semi-solid electrolyte is 30% to 70%. In particular, it is desirable to be 40% to 60%, more preferably 45% to 55%.

(low viscosity organic solvent)
A low-viscosity organic solvent lowers the viscosity of the semi-solid electrolyte solvent and improves the ionic conductivity. Since the internal resistance of a semi-solid electrolyte solution containing a semi-solid electrolyte solvent is high, the internal resistance of the semi-solid electrolyte solution can be lowered by adding a low-viscosity organic solvent to increase the ionic conductivity of the semi-solid electrolyte solvent. . The low-viscosity organic solvent is desirably a solvent having a viscosity lower than 140 Pa·s at 25° C. of a mixture of, for example, an ether solvent and a solvated electrolyte salt. Low-viscosity organic solvents include propylene carbonate (PC), trimethyl phosphate (TMP), gamma-butyl lactone (GBL), ethylene carbonate (EC), triethyl phosphate (TEP), tris(2,2,2- trifluoroethyl) (TFP), dimethyl methylphosphonate (DMMP), and the like. These low-viscosity organic solvents may be used singly or in combination. The electrolyte salt may be dissolved in a low-viscosity organic solvent.

In the present embodiment, the reason why propylene carbonate is preferable is that the viscosity of the lithium glyme complex salt is lowered to increase the ionic conductivity, and the lithium glyme complex structure, which has excellent reduction stability, is not disturbed, so that the internal resistance of the battery is lowered and the A high-capacity battery can be produced. In addition to ether-based compounds, sulfolane and/or its derivatives can be used as solvents for forming solvated ionic liquids. When a solvated ionic liquid containing sulfolane and/or a derivative thereof is used, sulfolane and/or a derivative thereof and lithium ions take a unique coordination structure, so that the transport rate of lithium ions in the semi-solid electrolyte layer is increased. Become. Therefore, unlike the solvated ionic liquid having an ether solvent and an electrolyte salt, in which the input/output characteristics of the secondary battery deteriorate as the viscosity increases, the solvated ionic liquid can be used even if the viscosity of the solvated ionic liquid is increased. It is possible to suppress deterioration in input/output characteristics of the secondary battery.

Examples of sulfolane derivatives include those in which hydrogen atoms bonded to carbon atoms constituting the sulfolane ring are substituted with fluorine atoms, alkyl groups, or the like. As a specific example, it can be used by appropriately selecting from a group of materials such as fluorosulfolane, difluorosulfolane, and methylsulfolane. For these solvents, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium iodide, lithium chloride, lithium bromide, LiB[ _OCOCF3 ] _{4 ,} LiB[ _OCOCF2CF3 ] ₄ , LiPF ₄ (CF ₃ ) ₂ , LiN(SO ₂ CF ₃ ) ₂ , LiN(SO ₂ CF ₂ CF ₃ ) ₂ and other lithium salts dissolved therein. Alternatively, an electrolytic solution in which two or more of the above mixed lithium salts are dissolved can be used.

[Secondary battery system]
FIG. 3 is a block diagram of a secondary battery system 500 according to a preferred embodiment.
A secondary battery system 500 includes a battery pack 510 and a current control device 550 . The battery pack 510 includes a battery cell module 512 (secondary battery) and a battery heating section 514 . The battery cell module 512 may be a single battery cell 100 (see FIG. 1), or multiple battery cells 100 connected in series and/or in parallel. As used herein, the term "secondary battery" is a concept that includes lithium-ion battery cells, battery modules, and battery packs. Battery heating unit 514 heats battery cell module 512 based on a command from 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 . If the battery cell module 512 comprises 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 (see FIG. 1), respectively. 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 has the lowest voltage. It corresponds to the negative electrode terminal 3 of the battery cell 100 . In FIG. 3, positive terminal 502 and negative terminal 503 of battery pack 510 are each connected to current controller 550 . Note that the power source 554 is mainly for charging, and the resistor 553 is for discharging.

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

Ammeter 551 measures the current flowing through positive terminal 502 and negative terminal 503 of battery pack 510 and outputs the result to control section 570 . The voltmeter 552 measures the voltage Ev between the positive electrode and the negative electrode and outputs the result to the control section 570 . Control unit 570 switches and controls switches 560 , 562 and 564 based on information from ammeter 551 and voltmeter 552 . As a result, control unit 570 sets positive terminal 502 and negative terminal 503 of battery pack 510 to an open state, a state in which resistor 553 is connected, or a state in which power source 554 is connected.

The control unit 570 includes general computer hardware such as a CPU (Central Processing Unit), RAM (Random Access Memory), and ROM (Read Only Memory). Programs, various data, etc. are stored. In FIG. 3, the inside of the control unit 570 shows functions realized by control programs and the like as blocks.

That is, the control unit 570 includes a charge/discharge processing unit 572 (charge/discharge processing means), a capacity recovery processing unit 574 (capacity recovery processing means), and a parameter update unit 580 (parameter update means). Parameter updater 580 also includes a charge/discharge curve acquirer 582 , a correlation acquirer 584 , a discharge amount acquirer 586 , and a parameter determiner 588 .

Charging/discharging processing unit 572 performs charging/discharging operation of battery pack 510 according to a normal application based on predetermined control parameters. For example, if the battery pack 510 is used for driving the vehicle, the charging/discharging processing unit 572 performs charging/discharging operation for driving the vehicle. The control parameters described above include the "discharge stop voltage". When the voltage Ev becomes equal to or lower than the discharge stop voltage while the battery pack 510 is being discharged, the charge/discharge processing unit 572 stops the discharge of the battery pack 510 . Here, the discharge stop voltage is a voltage at which the terminal voltage per battery cell 100 becomes 2.5 V or less, for example.

The capacity recovery processing unit 574 executes capacity recovery processing. Here, the capacity recovery process includes a process of connecting the power supply 554 or the resistor 553 between the positive electrode terminal 502 and the negative electrode terminal 503 to bring the battery pack 510 into an over-discharged state. "Overdischarge state" means that the voltage Ev is made less than the discharge stop voltage. For example, the capacity recovery process includes a process of maintaining the terminal voltage per battery cell 100 at 2.0 V or less. Although the details will be described later, this makes it possible to reactivate lithium ions that have been inactivated inside the negative electrode 13 and restore the capacity thereof.

After the capacity recovery processing of the battery pack 510, the parameter update unit 580 non-destructively analyzes the battery performance, and based on the negative electrode performance data etc. obtained at that time, Determine and update control parameters.
Charge-discharge curve acquisition unit 582 acquires the charge-discharge curve of battery pack 510 in order to determine control parameters. Here, the charge/discharge curve is a charge curve or a discharge curve.

Correlation acquisition unit 584 acquires the correlation between the discharge amount (or charging rate) of battery pack 510 and negative electrode potential En (see FIG. 1) based on this charge/discharge curve. The state of "discharged amount = 0" is the state of "charge rate = 100%", and the state where the discharged amount reaches the maximum value is the state of "charge rate = 0%".

Although the details will be described later, the lithium ion battery has a region where the negative electrode potential En becomes flat with respect to changes in the discharge amount Q (see FIG. 6). These are called flat portions PT1 (first flat portion) and PT2 (second flat portion). call. The discharge amount acquisition unit 586 acquires the discharge amount Q or the charge rate range corresponding to the flat portions PT1 and PT2.

In the charging/discharging control performed by the charging/discharging processing unit 572, the parameter determination unit 588 charges and discharges the battery pack 510 within the above-described range of the flat portion PT1 or more, that is, the charging rate range where the negative electrode potential En is equal to or greater than En1. Determine and update the control parameters as follows. In this way, the parameter updating unit 580 acquires the charge/discharge curve and analyzes its shape as a non-destructive analysis method for battery performance. Henceforth, this analysis method may be called "charge-discharge curve analysis."

[Estimation mechanism and problem of battery capacity recovery due to overdischarge]
FIG. 4 is a schematic diagram of a presumed mechanism regarding capacity recovery by overdischarge treatment.
In FIG. 4, state STA is a state in which capacity recovery processing is being performed on the battery cell 100 whose capacity has decreased, and state STB is a state after the capacity recovery processing is completed. A 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 , charge/discharge proceeds as lithium ions 22 are exchanged between the positive electrode 12 and the opposing portion 13A of the negative electrode 13 . However, when the lithium ions 22 are taken into the non-facing portion 13B that does not face the positive electrode in the charging/discharging process, they become metal lithium 24 (hatched in the figure) that does not contribute to the charging/discharging, which is a factor in the decrease in capacity. becomes.

Here, when the capacity recovery process is performed, the DC power supply 30 is connected to the battery cell 100 with the opposite polarity, as shown in the state STA, for example. That is, when the positive electrode of the DC power source 30 is connected to the negative electrode 13 and the negative electrode of the DC power source 30 is connected to the positive electrode 12, the battery cell 100 is over-discharged. However, if the battery voltage Epn=Ep-En is less than 2.5V, the DC power supply 30 does not necessarily have the reverse polarity.

As described above, when the negative electrode potential En increases, the discharge reaction proceeds in the facing portion 13A, and the lithium ions 22 are released from the facing portion 13A. Metal lithium 24 is also released as lithium ions 22 from the non-facing portion 13B. However, when charging the battery cell 100 after that, the DC power supply 30 is connected to the battery cell 100 in the direction opposite to the state STA. As a result, when the negative electrode potential En drops, the lithium ions 22 are taken into the non-facing portion 13B again, and there is concern that the capacity of the battery cell 100 may drop.

Therefore, it is preferable to increase the negative electrode potential En to some extent after the capacity recovery process is completed. State STB is a state in which the negative electrode potential En is kept high to some extent after the end of the capacity recovery process. When the negative electrode potential En increases, a potential difference occurs inside the negative electrode 13, and the potential of the opposing portion 13A becomes slightly higher than the potential of the non-facing portion 13B. Then, the electrons 26 move from the metallic lithium 24 adsorbed to the non-opposed portion 13B to the opposed portion 13A. As a result, the metallic lithium 24 becomes the lithium ion 22 and leaves the non-facing portion 13B, thereby contributing to charging and discharging again.

[Charging and discharging curve of battery cell 100]
FIG. 6 is an example of a charge/discharge curve of the battery cell 100. FIG.
FIG. 6 shows an example of a lithium ion battery using _{LiwNixCoyMnzO2 (} where _w , _x , _y , and z are 0 or positive values) for the positive electrode 12 and graphite for the negative electrode. . The vertical axis on the left side of FIG. 6 is the potential or voltage (V), the vertical axis on the right side is the potential with respect to the lithium electrode, and the horizontal axis is the discharge amount Q (Ah). Here, the lithium electrode reference is a potential based on the standard electrode potential of lithium, and the standard electrode potential of lithium is the standard electrode potential in a chemical reaction represented by “Li ⁺ +e ⁻ ←→Li”. means. A large number of small circles (unsigned) along the curve of the battery voltage Epn in FIG. 6 are the measured values of the battery voltage Epn, and the curve of the battery voltage Epn is the interpolation result of the measured values. The curves of the positive electrode potential Ep and the negative electrode potential En shown in the figure are also interpolation results of the respective measured values (not shown). As described above, the state of the discharged amount Q=0 corresponds to the fully charged state.

The battery voltage Epn in the charge/discharge curve is desirably close to the open circuit voltage. The charging/discharging curve may be formed by any method, but for example, there is a method of discharging from a fully charged state to a fully discharged state or charging from a fully discharged state to a fully charged state with a minute and constant current. Alternatively, the cycle of discharging at a constant current for a certain period of time and then resting for a certain period of time is repeated from a fully charged state to a fully discharged state, or the cycle of charging at a constant current for a certain period of time and then resting for a certain period of time is repeated from a fully discharged state to a fully charged state. There is a method of measuring the relationship between the charge/discharge amount and the open circuit voltage by repeating up to. Alternatively, there is a method of estimating the open circuit voltage by statistically processing current waveforms and voltage waveforms during operation, or performing regression calculations based on an equivalent circuit, and estimating the relationship between the charge/discharge amount and the open circuit voltage.

Normally, the positive electrode potential Ep is set sufficiently higher than the negative electrode potential En. In a bipolar battery cell using graphite for the negative electrode 13, the value of the positive electrode potential Ep with respect to the negative electrode potential En, that is, the battery voltage Epn (=Ep-En) is in the range of 2.5 to 4.5 V, and the life and safety are improved. from the point of view, it is often in the range of, for example, 2.5 to 4.2V. Here, when the battery voltage Epn is less than the lower limit voltage of 2.5 V, that is, when the battery is in an overdischarged state, the negative electrode potential En is relatively increased. Then, when the negative electrode potential En rises to a potential that decomposes the film on the surface of the negative electrode, battery performance recovery occurs due to film decomposition. In addition, the lithium ions taken into the non-facing portion 13B (see FIG. 4) are released from the negative electrode active material due to the decrease in the negative electrode potential En, thereby recovering the capacity.

In the above-described overdischarged state, when the battery voltage Epn<0 or less, that is, when the negative electrode potential En increases with respect to the positive electrode potential Ep, the decomposition described above tends to proceed. In particular, the reaction proceeds more easily by setting −2<Epn<0.1 [V]. However, when Epn<-2 [V], the active material structures of the positive electrode and the negative electrode are likely to be destroyed, resulting in a shorter life. In addition, the current control device 550 can cause the overdischarge current to flow through the battery pack 510 in pulses. The application time is not particularly limited, but may be 0.1 to 30 seconds. If the time is shorter than 0.1 seconds, the effect is weak, and if it is longer than 30 seconds, the positive electrode and negative electrode active material structures are likely to be destroyed, resulting in a shorter life.

In the characteristics of the negative electrode potential En in FIG. 6, the negative electrode potential En changes stepwise as the discharge amount Q shifts from the low capacity side (the side where the discharge amount Q is large) to the high capacity side (the side where the discharge amount Q is small). do. Here, a flat portion appearing when the amount of discharge Q in the drawing is near 0 to near 0.1 Ah is called PT1. A flat portion appearing in a region where the discharge amount Q is less than 0 is called PT2.

FIG. 7 is a diagram showing the rate of change in the charge/discharge curve of FIG.
The vertical axis of FIG. 7 represents the rate of change of the voltage or potential with respect to the amount Q of discharge. Here, the rates of change corresponding to Epn, Ep, and En are values such that Epn'=|dEpn/dQ|, Ep'=|dEp/dQ|, and En'=|dEn/dQ|, respectively. As shown in FIG. 7, the rate of change Epn' of the battery voltage Epn has a plurality of extreme values. These extreme values are caused by the extreme values of the change rate Ep' of the positive electrode potential Ep or the extreme values of the change rate En' of the negative electrode potential En.

The flat portions PT1 and PT2 of the negative electrode potential En shown in FIG. 6 appear more clearly in FIG. That is, the flat portions PT1 and PT2 include a range in which the rate of change En' is "0", and a range in which the rate of change En' is greater than 0 is included between the flat portions PT1 and PT2. Note that the names of the flat portions PT1 and PT2 are given for convenience in describing the present embodiment, and the names may be different.

The battery voltage Epn and its change rate Epn' shown in FIGS. 6 and 7 correspond to the positive electrode potential Ep, the negative electrode potential En, and the difference between these change rates Ep' and En'. Therefore, the deterioration state of the positive electrode 12 and the negative electrode 13 can be grasped by obtaining the correlation between the discharge amount Q and these values in advance and fitting the measured value to this correlation. As described above, the parameter updating unit 580 (see FIG. 3) determines the charge/discharge conditions after the capacity recovery process, particularly by using the curve showing the relationship between the negative electrode potential En and the discharge amount Q.

In FIG. 6, the negative electrode potential En in the flat portions PT1 and PT2 changes greatly at the boundary point (near 0Ah in FIG. 6), and the negative electrode potential En in the flat portion PT2 is considerably lower than that in the flat portion PT1. . Therefore, when charging and discharging are performed under the condition that the negative electrode potential En becomes the flat portion PT2, many lithium ions 22 are taken into the non-facing portion 13B of the negative electrode 13 (see state STA in FIG. 4), and the recovered capacity immediately decreases. Resulting in.

Therefore, in the present embodiment, the state of charge and the battery voltage Epn are managed so that the negative electrode potential En does not fall below the flat portion PT1, thereby reducing the capacity decrease after the capacity recovery. As described above, the boundaries between the flat portions PT1 and PT2 can be easily determined from the characteristics of the change rates Epn' and En' shown in FIG. That is, it is desirable to charge and discharge at a lower charging rate side (larger discharge amount Q side) than the discharge amount Q at which a large extreme value is obtained in the change rates Epn' and En'.

At this time, the negative electrode potential En at the flat portion PT1 is preferably in the range of 0.1 V to 0.2 V based on the lithium electrode. When the negative electrode potential En is lower than 0.1 V based on the lithium electrode, lithium deactivation at the negative electrode 13 proceeds. On the other hand, when the negative electrode potential En is higher than 0.2 V based on the lithium electrode, a sufficient battery capacity may not be obtained. The parameter updating unit 580 determines the optimum battery charging/discharging conditions after the capacity recovery process by the method described above.

[Operation of Embodiment]
FIG. 5 is a flow chart of a control program executed by the control unit 570. As shown in FIG.
When the process proceeds to step S2 in FIG. 5, the charging/discharging processing unit 572 performs charging/discharging operation of the battery pack 510 according to the normal application. At that time, the charge/discharge processing unit 572 records the voltage Ev (see FIG. 3) or the battery voltage Epn (see FIG. 1) and the discharge amount Q for each predetermined sampling period.

Next, when the process proceeds to step S4, the charge/discharge curve acquisition unit 582 grasps the deterioration state of the battery pack 510, that is, the capacity reduction state of the battery pack 510 by the charge/discharge curve analysis described above. Next, when the process proceeds to step S<b>6 , capacity recovery processing unit 574 determines the capacity recovery amount of battery pack 510 according to the state of deterioration of battery pack 510 . Furthermore, in step S6, capacity recovery processing unit 574 determines an overdischarge condition for battery pack 510 based on the determined capacity recovery amount.

Next, when the process proceeds to step S8, the capacity recovery processing unit 574 performs capacity recovery processing. That is, battery pack 510 is over-discharged based on the determined over-discharge condition. Also, the charge/discharge processing unit 572 records the voltage Ev (see FIG. 3) or the battery voltage Epn (see FIG. 1) and the discharge amount Q for each predetermined sampling period. Next, when the process proceeds to step S10, parameter updating unit 580 acquires the negative electrode charge/discharge curve of battery pack 510 after overdischarge and analyzes it. More specifically, the charge/discharge curve acquisition unit 582 acquires the charge/discharge curve of the battery voltage Epn in FIG. A charge/discharge curve of the negative electrode potential En in FIG. 6 is obtained. Then, the discharged amount acquisition unit 586 acquires the discharged amount Q or the range of the charging rate corresponding to the flat portion PT1.

Next, when the process proceeds to step S12, the parameter determining unit 588 of the parameter updating unit 580 controls the negative electrode potential En to be within an appropriate range in the charge/discharge operation (step S2) according to the normal application. Update parameters. Preferably, parameter updating section 580 updates the control parameters so that negative potential En does not become higher than negative potential En1 in flat portion PT1.

When step S12 ends, the process returns to step S2, and control unit 570 performs charge/discharge operation of battery pack 510 according to a normal application. At that time, control unit 570 charges and discharges battery pack 510 based on the updated control parameters.

[Heat treatment]
In the capacity recovery process of step S8 described above, the battery heating unit 514 may temporarily heat the battery cell module 512 to increase the capacity recovery reaction speed. As the battery heating unit 514, for example, a heater such as a PT heater that converts electric energy by energization into thermal energy can be applied. More specifically, the battery temperature when not heated is set to the ambient temperature T _amb (not shown), and the battery heating unit 514 is used to heat the battery to a heating temperature T _R (not shown) suitable for promoting recovery processing. It is advisable to raise the temperature. Then, it is preferable to advance the recovery process by supplying a pulsed current from the current control device 550 to the battery pack 510 .

By increasing the battery temperature, diffusion of Li ions and film decomposition reaction required for recovery are promoted, and recovery can be performed in a short period of time. In addition, since the diffusion of Li ions inside the battery is promoted, the Li ion distribution in the in-plane direction or thickness direction of the battery is made uniform, so the recovery process is also made uniform within the battery, resulting in a long life. can be expected. However, if the battery _cell temperature is kept high all the time, the deterioration reaction in the battery will continue to progress. is desirable. Although the method for lowering the temperature is not particularly limited, the temperature can be lowered by forced cooling using refrigerant or air in addition to natural cooling.

The optimum environmental temperature T _amb is not particularly limited, but from the viewpoint of suppressing deterioration inside the battery, it is preferably less than 40°C, and more preferably less than 25°C. The environmental temperature T _amb may be less than 10° C. due to the influence of the outside air temperature in which the battery is used, which may cause the output performance of the battery itself to decrease, but does not particularly affect the recovery process of the present invention. The optimum heating temperature T _R is also not particularly limited. From the viewpoint of promoting the Li diffusibility and film decomposition reaction rate, the temperature is preferably 40° C. or higher, and is preferably 60° C. or higher because the temperature is further accelerated. If the temperature is 80° C. or higher, the reaction rate increases, but there is concern about adverse effects on the members themselves that constitute the battery.

In particular, when the heat resistance of the electrolyte is low, increasing the heating temperature T _R makes the electrolyte volatilize or easily decompose electrochemically. Therefore, by using an electrolyte material with high heat resistance, the effect of the secondary battery system of the present embodiment can be easily obtained. If the electrolyte volatilizes at the heating temperature T _R and the battery swells, the recovery process does not proceed properly, so a high volatilization temperature of the electrolyte is desired. Specifically, it desirably exhibits a volatilization temperature higher than the heating temperature T _R , and more desirably has a volatilization temperature exceeding 100°C. As a specific material, the electrolyte using the above-described ionic liquid or solvated ionic liquid can be used.

In addition, as a method of raising the temperature of the lithium ion battery cell to the heating temperature T _R or higher, instead of using the battery heating unit 514 inside the battery pack 510, self-heating accompanying charging and discharging of the battery cell module 512 may be used. . In normal charging and discharging reactions exchanged between the positive and negative electrodes of a lithium-ion battery, _Joule heating represented _by ^I _bat R _cell Occur. For example, the same effect can be obtained by proceeding with the recovery process when the battery temperature exceeds T _R due to self-heating during rapid charging of the lithium ion battery cell or discharge of large current to the external power supply.

[Example]
Preferred embodiments are described below.
In this example, a lithium-ion battery was experimentally produced as a battery pack 510 (see FIG. 3) with the battery cell configuration shown in FIG. In the prototype lithium ion battery, an organic electrolyte with a carbonate solvent having a volatilization temperature of 20° C. was used. Specifically, a solution obtained by dissolving LiPF6 at a concentration of 1M in a carbonate-based solvent was used. The solvent composition was ethyl carbonate (EC):dimethyl carbonate (DMC)=1:2. A laminate cell was fabricated using LiNi1/3Co1/3Mn1/3O2 for the positive electrode and graphite for the negative electrode. A positive electrode material was applied to both sides of the Al current collecting foil, and a negative electrode material was applied to both sides of the Cu current collecting foil, so that the opposing negative electrode/positive electrode capacity ratio was 1.3. After punching out the positive electrode and the negative electrode into a predetermined shape, the porous base material of polyolefin material, the positive electrode, and the negative electrode were laminated. The capacity of the positive electrode was designed to be about 80 mAh on one side, and the capacity of the negative electrode was designed to be about 90 mAh on one side. The capacity of battery pack 510 after initialization was about 280 mAh.

As a deterioration test of the battery pack 510 described above, charge-discharge cycles were repeated 300 times in a 45° C. environment, and charge-discharge curve analysis was performed. 200 mAh) was confirmed to decrease. Therefore, it was confirmed that the capacity was improved by 10 mAh, which corresponds to 3% of the initial capacity, by supplying an overvoltage current so that the difference between the positive electrode potential and the negative electrode potential (Ep-En) was -1 V as a capacity recovery process. After that, when the charge-discharge curve analysis was performed again, it was found that the potential of the first flat portion of the negative electrode was 0.108 V vs Li / Li +, so the negative electrode potential during charging and discharging should not fall below this. Updated pattern. After that, it was found that the battery capacity after repeating 100 cycles maintained 70% of the initial efficiency, and the life could be increased by 25% in the capacity recovery process and the subsequent charge/discharge control parameter update process.

[Comparative example]
Next, a comparative example will be described.
In this comparative example, the configuration of the battery cell is the same as in the above-described example, and the steps up to the step of improving the performance by 3% of the initial capacity in the capacity recovery process are the same as in the above-described first example. However, in the comparative example, charging and discharging were repeated until the potential of the negative electrode fell below the potential of the first flat portion (0.05 V) without updating the charging and discharging control parameter. As a result, during 10 cycles after the capacity recovery treatment, the battery capacity fell below 70% of the initial efficiency, resulting in almost no effect of the capacity recovery treatment.

[Effects of Embodiment]
As described above, according to the preferred embodiment, the secondary battery system 500 includes a secondary battery (512) having one or more battery cells 100, and a charge or secondary battery (512) for the secondary battery (512). ), the charge/discharge processing unit 572 performs charge processing for the secondary battery (512) or discharge processing from the secondary battery (512) based on control parameters that define current or voltage control conditions when discharging from the secondary battery (512). , a capacity recovery processing unit 574 that performs capacity recovery processing for the secondary battery (512), and a parameter update unit 580 that updates control parameters according to the status of the capacity recovery processing. In this way, by updating the control parameters in accordance with the state of the capacity recovery process, it is possible to extend the life of the secondary battery system 500 .

Moreover, it is more preferable that the battery cell 100 is a lithium ion battery and the capacity recovery process includes a process of maintaining the terminal voltage of the battery cell 100 at 2.0 V or less. By maintaining the terminal voltage of the battery cell 100 at 2.0 V or less in this manner, the film on the surface of the negative electrode can be decomposed, and the performance of the battery cell 100 can be recovered.

In addition, the battery cell 100 has a first flat portion (PT1), which is a charging rate region including a range in which the negative electrode potential En is constant at the first potential (En1) with respect to changes in the charging rate, and a charging rate a second flat portion (PT2), which is a charging rate region including a range in which the negative electrode potential En is constant at a second potential (En2) lower than the first potential (En1) with respect to changes in The control parameters include an upper limit voltage designation parameter that designates the upper limit voltage of the secondary battery (512), and the parameter updating unit 580 updates the negative electrode potential En is more than the first potential (En1), it is more preferable to have a function to update the upper limit voltage designation parameter. As a result, the negative electrode potential En can be kept high, and favorable performance can be maintained for a long period of time.

Further, the battery cell 100 is a lithium ion battery, and the first potential (En1) is more preferably 0.1 V or more and 0.2 V or less with respect to the lithium electrode. As a result, the battery cell 100, which is a lithium ion battery, can maintain favorable performance over a long period of time.

Also, the parameter updating unit 580 acquires the charge/discharge curve of the secondary battery (512) based on the current flowing through the secondary battery (512) or the voltage appearing in the secondary battery (512) during the charging process or discharging process. a charge/discharge curve acquisition unit 582; a correlation acquisition unit 584 that acquires the correlation between the charging rate and the negative electrode potential En based on the charge/discharge curve; 2, and based on the obtained charging rate, the negative electrode potential En is the first potential (En1) or higher in the charging rate range where the secondary It is more preferable to have a parameter determination unit 588 that determines control parameters for charging and discharging the battery (512). This makes it possible to determine appropriate control parameters based on the charge/discharge curve.

In addition, the charge/discharge processing unit 572 stops discharging when the terminal voltage of the secondary battery (512) drops below a predetermined discharge stop voltage during discharge of the secondary battery (512). It is more preferable to include a process for maintaining the terminal voltage of the battery cell 100 at a voltage lower than the discharge stop voltage. By maintaining the terminal voltage of the battery cell 100 at a voltage lower than the discharge stop voltage in this manner, the film on the surface of the negative electrode can be appropriately decomposed.

[Modification]
The present invention is not limited to the embodiments described above, and various modifications are possible. The above-described embodiments are illustrated for easy understanding of the present invention, and are not necessarily limited to those having all the described configurations. Further, other configurations may be added to the configurations of the above embodiments, and part of the configurations may be replaced with other configurations. Also, the control lines and information lines shown in the drawings are those considered to be necessary for explanation, and do not necessarily show all the control lines and information lines necessary on the product. In practice, it may be considered that almost all configurations are interconnected. Possible modifications to the above embodiment are, for example, the following.

(1) In the above embodiment, an example in which a lithium-ion battery cell is applied as the battery cell 100 has been described. However, the battery cell 100 is not limited to a lithium ion battery, and various secondary batteries can be applied.

(2) Since the hardware of the control unit 570 in the above embodiment can be realized by a general computer, the flowchart shown in FIG. may be distributed via

(3) The processing shown in FIG. 5 and the other processing described above have been described as software processing using a program in the above embodiment, but some or all of them are implemented in an ASIC (Application Specific Integrated Circuit). It may be replaced with hardware processing using an FPGA (Field Programmable Gate Array) or the like.

100 battery cell 500 secondary battery system 512 battery cell module (secondary battery)
572 charge/discharge processing unit 574 capacity recovery processing unit 580 parameter update unit 582 charge/discharge curve acquisition unit 584 correlation acquisition unit 586 discharge amount acquisition unit 588 parameter determination unit En negative electrode potential En1 negative electrode potential (first potential)
En2 negative electrode potential (second potential)
PT1 flat part (first flat part)
PT2 flat part (second flat part)

Claims (5)

a secondary battery having one or more battery cells that are lithium ion battery cells;
Charging processing for the secondary battery or discharging processing from the secondary battery is performed based on control parameters that define current or voltage control conditions when charging or discharging the secondary battery. A charge and discharge processing unit that performs
a capacity recovery processing unit that performs capacity recovery processing including processing for maintaining the terminal voltage of the battery cell at 2.0 V or less for the secondary battery;
A secondary battery system, comprising: a parameter updating unit that updates the control parameter according to the status of the capacity recovery process. The battery cell has a first flat portion that is the region of the charging rate including a range in which the negative electrode potential is constant at a first potential with respect to changes in the charging rate, and the A second flat portion, which is a region of the charging rate including a range in which the negative electrode potential is constant at a second potential lower than the first potential, appears,
The control parameter includes an upper limit voltage designation parameter that designates an upper limit voltage of the secondary battery,
2. The secondary battery according to claim 1, wherein the parameter updating unit has a function of updating the upper limit voltage specifying parameter so that the negative electrode potential in each of the battery cells becomes equal to or higher than the first potential. battery system. The parameter updating unit
a charge/discharge curve acquisition unit that acquires a charge/discharge curve of the secondary battery based on the current flowing through the secondary battery or the voltage appearing in the secondary battery in the charging process or the discharging process;
a correlation acquisition unit that acquires a correlation between the charging rate and the negative electrode potential based on the charge/discharge curve;
a discharge amount acquiring unit that acquires the charging rate corresponding to the first flat portion and the second flat portion based on the correlation;
a parameter determining unit that determines the control parameter for charging and discharging the secondary battery within a charging rate range in which the negative electrode potential is equal to or higher than the first potential, based on the obtained charging rate. The secondary battery system according to claim 2. The charge/discharge processing unit stops discharging when the terminal voltage of the secondary battery becomes equal to or lower than a predetermined discharge stop voltage during discharging of the secondary battery,
The secondary battery system according to claim 2, wherein the capacity recovery process includes a process of maintaining the terminal voltage of the battery cell at a voltage lower than the discharge stop voltage. the computer,
Based on a control parameter that defines current or voltage control conditions when charging or discharging a secondary battery having one or more battery cells that are lithium ion battery cells, the secondary battery Charging and discharging processing means for performing charging processing for or discharging processing from the secondary battery,
charge and discharge processing means for performing capacity recovery processing including processing for maintaining the terminal voltage of the battery cell at 2.0 V or less for the secondary battery;
parameter update means for updating the control parameter according to the status of the capacity recovery process;
A program to function as

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