ELECTRICITY STORAGE DEVICE AND METHOD FOR SUPPRESSING DETERIORATION OF ELECTRICITY STORAGE ELEMENT

Abstract

A power storage device 2, the power storage device 2 comprising: a battery cell 16 soaked in a non-aqueous electrolyte 18 in a state with a positive electrode P and a negative electrode N separated by a separator 20; and a BMU 46. The BMU 46 executes a detection process that detects a state in which the voltage of the battery cell 16 has not changed substantially, and a discharge process that discharges the battery cell 16 according to the abovementioned state being detected by the detection process.

Description

[Technical field]

[0001] The present invention relates to an electricity storage device and a method for suppressing deterioration of an electricity storage element. [Background technology]

[0002] It is known that storage elements such as lithium ion secondary batteries deteriorate when repeatedly charged and discharged. Deterioration refers to an increase in direct current resistance (DCR) and a decrease in charge capacity (in other words, capacity retention rate). For this reason, efforts have been made to suppress the deterioration of storage elements (for example, see Patent Document 1 and Patent Document 2).

[0003] Specifically, Patent Document 1 describes that lithium condenses on the surface of the graphite negative electrode during charging and grows into whiskers. If some of the lithium that has grown into whiskers peels off, it may adhere to the separator that separates the positive and negative electrodes, causing clogging of the separator and deteriorating the energy storage element. Patent Document 1 describes that the lithium that has grown into whiskers is dissolved by passing a short reverse pulse current during charging of the battery and performing temporary discharge multiple times, i.e., by applying a group of reverse pulses.

[0004] Patent Document 2 describes that when a battery such as a lithium-ion secondary battery is charged or discharged, reactants (also called "scum") are generated and adhere to the electrode surface. The adhered reactants grow larger over time, causing significant deterioration. Patent Document 2 describes that by alternately passing a charging current and a reverse pulse current during charging, or by alternately passing a discharging current and a reverse pulse current during discharging, an electrical stimulus is applied to the electrodes, preventing the reactants generated during charging or discharging from adhering or dissolving the reactants generated. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2014-170741 A [Patent Document 2] JP 2014-187002 A Summary of the Invention [Problem to be solved by the invention]

[0006] The technology described in Patent Document 1 and Patent Document 2 described above is intended to suppress deterioration of an electric storage element during charging or discharging. The voltage of the electric storage element changes substantially during charging or discharging. Therefore, the technology described in Patent Document 1 and Patent Document 2 is intended to suppress deterioration of the electric storage element when the voltage of the electric storage element is substantially changing.

[0007] This specification discloses a technique for suppressing deterioration of an energy storage element when the voltage of the energy storage element is not substantially changing. [Means for solving the problem]

[0008] The energy storage device includes an energy storage element having a positive electrode and a negative electrode separated by a separator and immersed in a non-aqueous electrolyte, and a management unit, wherein the management unit executes a detection process for detecting a state in which a voltage of the energy storage element is not substantially changing, and a discharge process for discharging the energy storage element in response to the state being detected by the detection process. Effect of the Invention

[0009] This makes it possible to suppress deterioration of the storage element when the voltage of the storage element is not substantially changing. [Brief description of the drawings]

[0010] [Figure 1] Schematic diagram of an uninterruptible power supply including a power storage device according to a first embodiment. [Diagram 2] Schematic diagram showing the overall configuration of a power storage device [Diagram 3] A perspective view of a battery cell (for convenience, the case is shown in a transparent state) [Figure 4] A side view of the electrode body as seen from the X direction shown in Figure 3. [Diagram 5] FIG. 1 is a schematic diagram showing an electrical configuration of a power storage device; [Figure 6A] Figure showing the experimental results of capacity retention rate [Figure 6B] Graph showing the results of capacity retention tests [Figure 7A] Figure showing the experimental results of DCR change rate (ambient temperature 25°C) [Figure 7B] Graph showing the experimental results of DCR change rate (ambient temperature 25℃) [Figure 8A] Figure showing the experimental results of DCR change rate (ambient temperature -10℃) [Figure 8B] Graph showing the experimental results of DCR change rate (ambient temperature -10℃) [Figure 9A] Figure showing the experimental results of capacity retention rate [Figure 9B] Graph showing the results of capacity retention tests [Figure 10A] Figure showing the experimental results of DCR change rate (ambient temperature 25°C) [Figure 10B] Graph showing the experimental results of DCR change rate (ambient temperature 25℃) [Figure 11A] Figure showing the experimental results of DCR change rate (ambient temperature -10℃) [Figure 11B] Graph showing the experimental results of DCR change rate (ambient temperature -10℃) [Figure 12] Schematic diagram to explain the mechanism of polymer formation near the electrode [Figure 13] Figure showing the experimental results of DCR change rate (ambient temperature -10℃) DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] (Outline of this embodiment) (1) An energy storage device comprising: an energy storage element having a positive electrode and a negative electrode separated by a separator and immersed in a non-aqueous electrolyte; and a management unit, wherein the management unit executes a detection process for detecting a state in which the voltage of the energy storage element has not substantially changed; and a discharge process for discharging the energy storage element in response to the detection of the state by the detection process.

[0012] The "state in which the voltage of the storage element does not change substantially" typically includes a state in which the storage element is not charged by a charger and the storage element is not discharging to an electric load. The concept of the "state in which the voltage of the storage element does not change substantially" disclosed herein may include a state in which the storage element is charged by a charger at a very small current value that does not substantially change the voltage of the storage element, or the storage element is discharging to an electric load. Therefore, for example, a state in which the storage element is constant-voltage charged by a constant current constant voltage method or a float charging method at a very small current value that does not substantially change the voltage of the storage element (for example, in the case of the constant current constant voltage method, a state in which constant voltage charging is performed at a current value of 1 / 10 or less of the current at a constant current) or a state in which the storage element is discharging to an electric load to supply a dark current are typical examples of the "state in which the voltage of the storage element does not change substantially" as used herein. When the voltage of the storage element is not substantially changing, the amount of change in the voltage of the storage element per unit time is small. Therefore, the "state in which the voltage of the storage element is not substantially changing" may be "a state in which the amount of change in the voltage of the storage element per unit time is equal to or less than a predetermined value." When the voltage of the storage element is not substantially changing, the amount of change in the state of charge (SOC) of the storage element per unit time is small. Therefore, the "state in which the voltage of the storage element is not substantially changing" may be "a state in which the amount of change in the state of charge of the storage element per unit time is equal to or less than a predetermined value."

[0013] "Discharging the storage element in response to the detection of the state in the detection process" includes not only the case where a discharge process is executed when the state is detected, but also the case where a discharge process is executed when another condition is met in addition to the detection of the state.

[0014] In an electricity storage element in which a positive electrode and a negative electrode are separated by a separator and immersed in a non-aqueous electrolyte, polymers derived from the non-aqueous electrolyte are likely to be produced near the electrodes if the element is left at a high temperature or voltage. "Leaving the element unattended" refers to a state in which the voltage of the electricity storage element does not change substantially for a long period of time.

[0015] The mechanism of polymer formation in the vicinity of the electrodes will be described with reference to Fig. 12. The "state immediately after", "state left alone" and "state left further alone" shown in Fig. 12 represent the same region of the same electrode (positive electrode 103 and negative electrode 102). The "state immediately after" indicates the state immediately after the storage element is charged and the voltage is high, in other words, the state of charge (SOC) is high. Monomers 101 that can form polymer 100 are dispersed in the non-aqueous electrolyte. The "state left alone" indicates the state when left alone in a state where the voltage has not substantially changed from the "state immediately after". By leaving the element in a state where the voltage is high, electrophoresis occurs, and a concentration gradient is generated in monomer 101. Therefore, in the "state left alone", the concentration of monomer 101 on the negative electrode 102 side decreases, and the concentration of monomer 101 in the vicinity of positive electrode 103 increases. The "state left further" indicates the state when left alone in a state where the voltage has not substantially changed from the "state left alone". By leaving the element in a state where the concentration of monomer 101 in the vicinity of positive electrode 103 is high, the atmosphere becomes favorable for continuous reaction, and polymer 100 is generated.

[0016] The inventors of the present application have noticed this point and discovered that if an energy storage element is left unattended (especially when the voltage is high), the polymer 100 that is produced during the storage element will block the pores in the separator, causing clogging, and this may lead to deterioration of the energy storage element due to non-uniform current density. According to the above-mentioned storage device, in response to detecting a state in which the voltage of the storage element is not substantially changing, a discharge process is executed to discharge the storage element. In this way, deterioration of the storage element when the voltage of the storage element is not substantially changing can be suppressed. There is no need to clarify the reason why such an effect is obtained, but the following reason is presumed, for example.

[0017] 12, the molecules gathered near the positive electrode 103 are diffused, and the concentration gradient is eliminated. It is presumed that this is because the "left-standing state" changes to the "immediately after-standing state", and the atmosphere in which the polymerization reaction easily proceeds is eliminated. Therefore, according to the above-described electricity storage device, even if the electricity storage elements are left in a high temperature or high voltage state, deterioration of the electricity storage elements can be suppressed.

[0018] Although FIG. 12 illustrates an embodiment in which the concentration of the monomer 101 increases in the vicinity of the positive electrode 103, the effect of the present invention can be obtained even in an embodiment in which the concentration of the monomer 101 increases in the vicinity of the negative electrode . A compound that can act as the monomer 101 in the above-mentioned mechanism is an organic compound with uneven distribution of electric charge. That is, the effect of the present invention can be obtained with any organic compound that contains a highly electronegative element such as nitrogen, oxygen, or a halogen element in the molecule and has an asymmetric molecular structure. Examples of such compounds include cyclic carbonates, linear carbonates, carboxylates, phosphates, sulfonates, ethers, amides, and nitriles.

[0019] (2) The state may be a non-use state in which the power storage device is not in use.

[0020] When the power storage device is not in use, the voltage of the power storage element does not change substantially. Therefore, the unused state can be said to be a state in which the voltage of the power storage element does not change substantially.

[0021] (3) The energy storage element may be a lithium ion battery in which the positive electrode contains a ternary active material.

[0022] There are several types of lithium-ion batteries, including those containing an iron-based active material (such as lithium iron phosphate) in the positive electrode and those containing a ternary active material (nickel, manganese, cobalt) in the positive electrode. In the following explanation, a lithium-ion battery containing an iron-based active material in the positive electrode is simply called an iron-based lithium-ion battery, and a lithium-ion battery containing a ternary active material in the positive electrode is simply called a ternary lithium-ion battery. Ternary lithium-ion batteries can be charged to a higher voltage than iron-based lithium-ion batteries. For this reason, ternary lithium-ion batteries are more likely to produce polymer 100 than iron-based lithium-ion batteries.

[0023] According to the above-mentioned storage device, the discharge process is executed in response to detection of a state in which the voltage of the storage element is not substantially changing, so that the deterioration of the storage element when the voltage of the storage element is not substantially changing can be suppressed. Therefore, it is particularly useful in the case of a ternary lithium ion battery that can be charged to a high voltage (in other words, a lithium ion battery that is likely to generate polymer 100).

[0024] (4) In the discharging process, the management unit may discharge the energy storage element intermittently, or may discharge the energy storage element while alternating between strong and weak currents.

[0025] As a method for discharging the storage element, a method of discharging the storage element only once at a constant current is considered. However, there is a possibility that discharging only once at a constant current does not sufficiently eliminate the atmosphere in which the polymerization reaction is likely to proceed. According to the above-mentioned storage device, the storage element is intermittently discharged in the discharge process, or the current is discharged while alternating between strong and weak currents, so that the atmosphere in which the polymerization reaction is likely to proceed can be more reliably eliminated than when discharging only once at a constant current.

[0026] (5) In the discharging process, the management unit may discharge the power storage element and then cause a charger to charge the power storage element.

[0027] When the storage element is discharged, the voltage drops, so there is a possibility that the storage element is not sufficiently charged when it is used. According to the above-mentioned storage device, the storage element is charged after being discharged, so that the amount of electricity discharged can be charged. Therefore, it is possible to reduce the possibility that the storage element is not sufficiently charged when it is used.

[0028] (6) In the discharging process, the management unit may discharge the energy storage element through a circuit other than a main circuit to which the energy storage element is connected.

[0029] The state in which the voltage of the storage element does not substantially change is a state in which power is not being supplied from the storage element to an external electrical load, for example, because the main circuit to which the storage element is connected is interrupted. Therefore, the storage element cannot be discharged by the external electrical load. According to the above-mentioned storage device, the storage element is discharged by a circuit other than the main circuit, so that the storage element can be discharged even in a state in which power is not being supplied from the storage element to an external electrical load.

[0030] (7) The power storage device may include a circuit breaker connected in series with the storage element, and in the discharge process, the management unit may discharge the storage element by passing a current for opening or closing the circuit breaker from the storage element to the circuit breaker.

[0031] According to the above-described power storage device, the power storage elements are discharged by the circuit breaker, so that the power storage elements can be discharged without adding new hardware for discharging the power storage elements.

[0032] (8) The power supply may include a plurality of the storage elements; and an equalization circuit having a discharge resistor and equalizing the voltages of the storage elements by discharging a storage element having a relatively high voltage among the plurality of storage elements through the discharge resistor, and the management unit may discharge the storage elements through the equalization circuit during the discharge process.

[0033] Generally, a power storage device includes an equalization circuit. In the above-described power storage device, the equalization circuit discharges the power storage elements in the discharging process, so that the configuration of the power storage elements can be simplified compared to a case where a discharging circuit is provided separately from the equalization circuit.

[0034] (9) The storage device may include a plurality of the storage elements, an equalization circuit having a first discharge resistor and discharging a storage element having a relatively high voltage among the plurality of storage elements through the first discharge resistor to equalize the voltages of the storage elements, and a discharge circuit having a second discharge resistor, wherein the management unit may discharge the storage elements through the discharge circuit during the discharge process.

[0035] Generally, a power storage device is equipped with an equalization circuit. When discharging a power storage element, it is possible to discharge the power storage element using an equalization circuit. However, an equalization circuit generally has a small current that can be passed due to constraints such as manufacturing cost and size. For this reason, when an equalization circuit is used, a large current cannot be passed, and the effect of suppressing deterioration of the power storage element may be small. A large current can be passed by increasing the discharge resistance of the equalization circuit, but increasing the discharge resistance has the disadvantage that it becomes difficult to finely adjust the voltage during equalization.

[0036] According to the above-mentioned storage device, a discharge circuit is provided in addition to the equalization circuit, so that it is possible to more easily make fine voltage adjustments during equalization while increasing the effect of suppressing deterioration of the storage elements, compared to the case where the discharge resistance of the equalization circuit is increased. Providing a discharge circuit in addition to the equalization circuit has the advantage that the same effect can be obtained by simply adding a simple circuit to an existing equalization circuit, compared to increasing the size of the equalization circuit.

[0037] (10) The management unit may execute the discharge process when a state in which the voltage of the energy storage element remains substantially unchanged continues for a predetermined period of time or more.

[0038] When the time period during which the voltage of the storage element does not change substantially continues is short, the storage element is less likely to deteriorate. According to the above-described storage device, when the time period during which the voltage of the storage element does not change substantially continues is less than a predetermined time, the storage element is not discharged, so that ineffective discharge can be suppressed.

[0039] (11) The management unit may execute the discharge process when the detection process detects a state in which the voltage of the storage element has not substantially changed and when the voltage or state of charge of the storage element is equal to or greater than a predetermined value.

[0040] When the voltage is low (in other words, when the SOC is low), it is difficult to produce the polymer 100. According to the above-described electricity storage device, when the voltage or SOC of the electricity storage element is below a predetermined value, the electricity storage element is not discharged, so that ineffective discharge can be suppressed.

[0041] (12) The power storage device may be used in an uninterruptible power supply.

[0042] As described above, the technology described in Patent Document 1 and the technology described in Patent Document 2 are intended to suppress deterioration of an electric storage element during charging or discharging. The voltage of the electric storage element changes substantially during charging or discharging. For this reason, it can be said that the technology described in Patent Document 1 and the technology described in Patent Document 2 are based on the attribute that the electric storage element deteriorates when the voltage of the electric storage element changes substantially. In response to this, the inventor of the present application discovered an unknown attribute that the storage element may deteriorate even when the voltage of the storage element does not change substantially. The above storage device utilizes this unknown attribute discovered by the inventor of the present application. Since the uninterruptible power supply is not used when there is no power outage, the voltage of the storage element does not change substantially for a long period of time. For this reason, there is a concern that the storage element may deteriorate when there is no power outage, and the original performance may not be exhibited during a power outage. If the above storage device is used as an uninterruptible power supply, the deterioration of the storage element when there is no power outage can be suppressed, and the original performance may be exhibited during a power outage.

[0043] (13) The power storage device may be mounted on a vehicle.

[0044] A replacement power storage device for a power storage device installed in a vehicle may be stored for a long period of time as inventory at a dealer (such as an automobile dealer or an automobile parts store) after it is manufactured. The voltage of the power storage element does not change substantially during storage. For this reason, there is a concern that the power storage element may deteriorate during storage and not be able to perform as intended when installed in a vehicle. The above-mentioned power storage device utilizes the unknown attribute described above. When the above-mentioned power storage device is used as a power storage device mounted on a vehicle, the deterioration of the power storage element during storage can be suppressed, so that the original performance can be exhibited when mounted on the vehicle.

[0045] (14) The power storage device may be used in a power storage system.

[0046] An energy storage system (ESS) is a system that stores electricity for purposes such as peak shifting, in which electricity generated at night is used during the day, and peak shaving, in which the amount of electricity that exceeds the contracted power is supplied from the energy storage system when a large amount of electricity is temporarily required. Energy storage systems are sometimes left unattended until the power grid is operational. For example, the energy storage elements installed in the early stages of the construction of the energy storage system may be left unattended for several months or more. The voltage of the energy storage elements does not change substantially while the system is left unattended. For this reason, there is a concern that the energy storage elements may deteriorate while left unattended, and may not be able to perform as intended when the power grid is operational. The above-mentioned energy storage device utilizes the unknown attribute described above. When the above-mentioned energy storage device is used in an energy storage system, the deterioration of the energy storage element while left unused can be suppressed, so that the original performance can be expected when the power grid is operated.

[0047] (15) A method for suppressing deterioration of an energy storage element in which a positive electrode and a negative electrode are separated by a separator and immersed in a nonaqueous electrolyte, the method including: a detection step for detecting a state in which a voltage of the energy storage element is not substantially changing; and a discharge step for discharging the energy storage element in response to the state being detected in the detection step.

[0048] According to the deterioration suppression method, in response to detection of a state in which the voltage of the storage element is not substantially changing, a discharge process is executed to discharge the storage element, thereby suppressing deterioration of the storage element when the voltage of the storage element is not substantially changing.

[0049] The invention disclosed in this specification can be realized in various forms, such as an apparatus, a method, a computer program for implementing the functions of these apparatus or methods, and a recording medium on which the computer program is recorded.

[0050] <Embodiment 1> The first embodiment will be described with reference to Figures 1 to 11. In the following description, the reference numerals of the drawings may be omitted for the same components, with some exceptions.

[0051] An uninterruptible power supply (UPS) 1 including a power storage device 2 according to a first embodiment will be described with reference to Fig. 1. The UPS 1 is a device that stores power supplied from a commercial power source 12 and supplies the power to an electric load 11 when the power from the commercial power source 12 is cut off due to a power outage or the like. As shown in Fig. 1, the UPS 1 is connected to a power line 14 that branches off from a power line 13 that connects the commercial power source 12 and the electric load 11.

[0052] The UPS 1 includes an AC / DC converter 3 that converts AC voltage supplied from a commercial power source 12 into DC voltage, and an energy storage device 2. The energy storage device 2 is float charged by the power supplied from the commercial power source 12. Float charging is a charging method that constantly keeps the energy storage device 2 fully charged by continuously applying a constant voltage.

[0053] (1) Configuration of the Energy Storage Device The overall configuration of the power storage device 2 will be described with reference to Fig. 2. The power storage device 2 includes a plurality of (four in Fig. 2) power storage units 15, and a BMU 46 (see Fig. 5) described below. Each power storage unit 15 includes a plurality of (four in Fig. 2) battery cells 16. The power storage device 2 may include a bus bar (not shown) that electrically connects the plurality of battery cells 16, and a bus bar (not shown) that electrically connects the plurality of power storage units 15.

[0054] (2) Battery cell configuration The battery cell 16 is a non-aqueous electrolyte secondary battery, which is an example of a non-aqueous electrolyte storage element, and specifically, a ternary lithium ion battery. 3, the battery cell 16 according to the first embodiment is a rectangular battery, and includes an electrode assembly 17, a non-aqueous electrolyte 18, and a case 19 in which these are accommodated. The electrode assembly 17 is accommodated in the case 19 in a state where it is immersed in the non-aqueous electrolyte 18.

[0055] As shown in Fig. 4, the electrode body 17 is formed by winding a sheet-like positive electrode P and a negative electrode N in a flat shape with a separator 20 sandwiched between them, with the electrodes shifted in the Y direction (the direction perpendicular to the paper surface in Fig. 4). The electrode body 17 shown in Fig. 4 is a vertically wound electrode body with a winding axis extending horizontally. As shown in Fig. 3, the positive electrode P is electrically connected to a positive electrode terminal 22 via a positive electrode lead 21. The negative electrode N is electrically connected to a negative electrode terminal 24 via a negative electrode lead 23.

[0056] The electrode body 17 may be a horizontally wound type in which the winding axis extends vertically, or a stack type in which sheet-like positive electrodes P and negative electrodes N are laminated with a separator 20 interposed therebetween. The battery cells 16 are not limited to square batteries, and may be cylindrical batteries, laminated film batteries, flat batteries, coin batteries, button batteries, etc.

[0057] (2-1) Positive electrode The positive electrode P has a conductive positive electrode substrate and a positive electrode active material layer disposed on the positive electrode substrate directly or via an intermediate layer. The configuration of the intermediate layer is not particularly limited. The material of the positive electrode substrate is a metal such as aluminum, titanium, tantalum, stainless steel, or an alloy thereof. Among these, aluminum or an aluminum alloy is preferred from the viewpoints of potential resistance, high conductivity, and cost. The positive electrode substrate may be a foil, a vapor deposition film, or the like, and foil is preferred from the viewpoint of cost. Therefore, aluminum foil or an aluminum alloy foil is preferred as the positive electrode substrate. Examples of aluminum or an aluminum alloy include A1085, A3003, etc., as specified in JIS-H-4000 (2014).

[0058] The positive electrode active material layer contains a positive electrode active material and, as necessary, optional components such as a conductive agent, a binder, a thickener, and a filler.

[0059] The positive electrode active material can be appropriately selected from known positive electrode active materials. A material capable of absorbing and releasing lithium ions is usually used as the positive electrode active material for a lithium ion secondary battery. Examples of the positive electrode active material include lithium transition metal composite oxides having an α-NaFeO2 type crystal structure, lithium transition metal oxides having a spinel type crystal structure, polyanion compounds, chalcogen compounds, and sulfur. Examples of the lithium transition metal composite oxides having an α-NaFeO2 type crystal structure include Li[Li x Ni 1-x ]O2(0≦x<0.5), Li[Li x Ni γ Co (1-x-γ) ]O2(0≦x<0.5, 0<γ<1), Li[Li x Co (1-x) ]O2(0≦x<0.5), Li[Li x Ni γ Mn (1-x-γ) ]O2(0≦x<0.5, 0<γ<1), Li[Li x Ni γ Mn β Co (1-x-γ-β)]O2(0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1) (ternary system), Li[Li x Ni γ Co β Al (1-x-γ-β) ]O2 (0≦x<0.5, 0<γ, 0<β, 0.5<γ+β<1), etc. Lithium transition metal oxides with spinel crystal structure include Li x Mn2O4,Li x Ni γ Mn (2-γ) O4, etc. Examples of polyanion compounds include LiFePO4 (iron-based), LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, Li2CoPO4F, etc. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, molybdenum dioxide, etc. The atoms or polyanions in these materials may be partially substituted with atoms or anion species of other elements. The surfaces of these materials may be coated with other materials. Among these materials, it is preferable to use a ternary positive electrode active material. In a ternary lithium ion battery that can be charged to a high voltage, the deterioration of the storage element occurs easily when the voltage of the storage element does not change substantially. Therefore, the effect of the present invention that solves the problem can be fully enjoyed. In the positive electrode active material layer, one of these materials may be used alone, or two or more of them may be used in combination.

[0060] (2-2) Negative electrode The negative electrode N has a conductive negative electrode substrate and a negative electrode active material layer disposed on the negative electrode substrate directly or via an intermediate layer. The configuration of the intermediate layer is not particularly limited. The material of the negative electrode substrate is a metal such as copper, nickel, stainless steel, nickel-plated steel, aluminum, or an alloy thereof. Among these, copper or a copper alloy is preferred. The negative electrode substrate may be a foil, a vapor deposition film, or the like, and from the viewpoint of cost, a foil is preferred. Therefore, the negative electrode substrate is preferably a copper foil or a copper alloy foil. Examples of the copper foil include rolled copper foil and electrolytic copper foil.

[0061] The negative electrode active material layer contains a negative electrode active material and, if necessary, any optional components such as a conductive agent, a binder, a thickener, and a filler.

[0062] The negative electrode active material layer may contain typical nonmetallic elements such as B, N, P, F, Cl, Br, and I, typical metallic elements such as Li, Na, Mg, Al, K, Ca, Zn, Ga, and Ge, and transition metallic elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Zr, Ta, Hf, Nb, Sn, Sr, Ba, and W as components other than the negative electrode active material, conductive agent, binder, thickener, and filler.

[0063] The negative electrode active material can be appropriately selected from known negative electrode active materials. A material capable of absorbing and releasing lithium ions is usually used as the negative electrode active material for lithium ion secondary batteries. Examples of the negative electrode active material include metal Li; metals or semimetals such as Si and Sn; metal oxides or semimetal oxides such as Si oxide, Ti oxide, and Sn oxide; Li4Ti5O 12 Examples of the material include titanium-containing oxides such as LiTiO2 and TiNb2O7; polyphosphate compounds; silicon carbide; and carbon materials such as graphite and non-graphitic carbon (easily graphitized carbon or non-graphitizable carbon). Among these materials, graphite and non-graphitic carbon are preferred. In the negative electrode active material layer, one of these materials may be used alone, or two or more of them may be used in combination.

[0064] (2-3) Separator The separator 20 can be appropriately selected from known separators. For example, a separator consisting of only a substrate layer, a separator in which a heat-resistant layer containing heat-resistant particles and a binder is formed on one or both surfaces of the substrate layer, etc. can be used as the separator 20. Examples of the material of the substrate layer of the separator 20 include woven fabric, nonwoven fabric, and porous resin film. Among these materials, a porous resin film is preferable from the viewpoint of strength, and a nonwoven fabric is preferable from the viewpoint of the liquid retention of the nonaqueous electrolyte 18. As the material of the substrate layer of the separator 20, a polyolefin such as polyethylene or polypropylene is preferable from the viewpoint of shutdown function, and a polyimide or aramid is preferable from the viewpoint of oxidation decomposition resistance. A material obtained by combining these resins may be used as the substrate layer of the separator 20.

[0065] The porosity of the separator 20 is preferably 80% by volume or less from the viewpoint of strength, and is preferably 20% by volume or more from the viewpoint of discharge performance. Here, the "porosity" is a volume-based value, and refers to a value measured with a mercury porosimeter.

[0066] (2-4) Non-aqueous electrolyte The nonaqueous electrolyte 18 can be appropriately selected from known nonaqueous electrolytes 18. The nonaqueous electrolyte 18 contains a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent.

[0067] The non-aqueous solvent can be appropriately selected from known non-aqueous solvents. Examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, carboxylates, phosphates, sulfonates, ethers, amides, and nitriles. As the non-aqueous solvent, those in which some of the hydrogen atoms contained in these compounds are substituted with halogens may be used.

[0068] Examples of cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), vinylethylene carbonate (VEC), chloroethylene carbonate, fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), styrene carbonate, 1-phenylvinylene carbonate, 1,2-diphenylvinylene carbonate, etc. Among these, EC is preferred.

[0069] Examples of the chain carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diphenyl carbonate, trifluoroethyl methyl carbonate, bis(trifluoroethyl) carbonate, etc. Among these, EMC is preferable.

[0070] As the non-aqueous solvent, it is preferable to use a cyclic carbonate or a chain carbonate, and it is more preferable to use a combination of a cyclic carbonate and a chain carbonate. By using a cyclic carbonate, it is possible to promote dissociation of the electrolyte salt and improve the ionic conductivity of the non-aqueous electrolyte 18. By using a chain carbonate, it is possible to keep the viscosity of the non-aqueous electrolyte 18 low. When a cyclic carbonate and a chain carbonate are used in combination, it is preferable that the volume ratio of the cyclic carbonate to the chain carbonate (cyclic carbonate:chain carbonate) is, for example, in the range of 5:95 to 50:50.

[0071] The electrolyte salt can be appropriately selected from known electrolyte salts. Examples of the electrolyte salt include lithium salts, sodium salts, potassium salts, magnesium salts, onium salts, etc. Among these, lithium salts are preferred.

[0072] Examples of the lithium salt include inorganic lithium salts such as LiPF6, LiPO2F2, LiBF4, LiClO4, and LiN(SO2F)2, and lithium salts having a halogenated hydrocarbon group such as LiSO3CF3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(SO2CF3)(SO2C4F9), LiC(SO2CF3)3, and LiC(SO2C2F5)3. Among these, inorganic lithium salts are preferred, and LiPF6 is more preferred.

[0073] The content of the electrolyte salt in the nonaqueous electrolyte 18 is preferably from 0.1 M to 2.5 M, more preferably from 0.3 M to 2.0 M, even more preferably from 0.5 M to 1.7 M, and particularly preferably from 0.7 M to 1.5 M. By setting the content of the electrolyte salt within the above range, the ionic conductivity of the nonaqueous electrolyte 18 can be increased.

[0074] The non-aqueous electrolyte solution 18 may contain additives. Examples of additives include halogenated carbonates such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC); oxalates such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalateborate (LiFOB), and lithium bis(oxalate)difluorophosphate (LiFOP); imide salts such as lithium bis(fluorosulfonyl)imide (LiFSI); aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; partial halides of the aromatic compounds such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, and p-cyclohexylfluorobenzene; halogenated anisole compounds such as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, and 3,5-difluoroanisole; vinylene carbonate, methylvinylene carbonate, and ethylvinylene carbonate. , succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, propane sultone, propene sultone, butane sultone, methyl methanesulfonate, busulfan, methyl toluenesulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, 4 ,4'-bis(2,2-dioxo-1,3,2-dioxathiolane, 4-methylsulfonyloxymethyl-2,2-dioxo-1,3,2-dioxathiolane, thioanisole, diphenyl disulfide, dipyridinium disulfide, perfluorooctane, tristrimethylsilyl borate, tristrimethylsilyl phosphate, tetrakistrimethylsilyl titanate, lithium monofluorophosphate, lithium difluorophosphate, and the like. These additives may be used alone or in combination of two or more.

[0075] The content of the additive contained in the nonaqueous electrolyte 18 is preferably 0.01% by mass to 10% by mass, more preferably 0.1% by mass to 7% by mass, even more preferably 0.2% by mass to 5% by mass, and particularly preferably 0.3% by mass to 3% by mass, relative to the total mass of the nonaqueous electrolyte 18. By setting the content of the additive within the above range, it is possible to improve the capacity retention performance or cycle performance after high-temperature storage, and to further improve safety.

[0076] (3) Electrical configuration of the power storage device The electrical configuration of the power storage device 2 will be described with reference to Fig. 5. As described above, the power storage device 2 includes a plurality of power storage units 15 (only one power storage unit 15 is shown in Fig. 5) and a BMU 46 (Battery Management Unit) that manages the plurality of power storage units 15.

[0077] The power storage unit 15 includes a positive external terminal 52, a negative external terminal 53, four battery cells 16 connected in series to a main circuit 60 connecting the positive external terminal 52 and the negative external terminal 53, and a CMU 40 (Cell Management Unit). In the following description, the four battery cells 16 will be referred to as a battery pack 51.

[0078] The CMU 40 includes a current sensor 41 , a voltage sensor 42 , a breaker 43 , an equalization circuit 44 , and a discharge circuit 45 . The current sensor 41 is connected in series with the battery pack 51. The current sensor 41 measures the charge / discharge current of the battery pack 51 and outputs the result to the BMU 46. The voltage sensor 42 is connected in parallel to each battery cell 16. The voltage sensor 42 measures the terminal voltage of each battery cell 16 and the voltage across the battery pack 51, and outputs the results to the BMU 46. The circuit breaker 43 is connected in series with the battery pack 51. The circuit breaker 43 is a relay, a field effect transistor (FET), etc. The circuit breaker 43 is turned on / off (open / closed) by the BMU 46.

[0079] The equalization circuit 44 is a circuit for equalizing the voltages of the battery cells 16. The equalization circuit 44 includes discharge resistors 44A connected in parallel with each battery cell 16, and switches 44B connected in series to each discharge resistor 44A. The switches 44B are relays, FETs, or the like, and are turned on / off by the BMU 46.

[0080] The discharge circuit 45 is a circuit used when discharging the battery cells 16 by the deterioration suppression process described below. The discharge circuit 45 includes a discharge resistor 45A connected in parallel with each battery cell 16, a capacitor 45B connected in parallel with the discharge resistor 45A, and a switch 45C connected in series with the discharge resistor 45A and the capacitor 45B.

[0081] The resistance value of the discharge resistor 45A is greater than the resistance value of the discharge resistor 44A of the equalization circuit 44. The switch 45C is a relay, FET, or the like, and is turned on / off by the BMU 46. When the switch 45C is turned on, a voltage is generated across the capacitor 45B. This charges the capacitor 45B, and the charging current to the capacitor 45B is discharged from the battery cell 16. When charging of the capacitor 45B is completed, discharging from the battery cell 16 also stops. When the switch 45C is turned off, the capacitor 45B is discharged by the discharge resistor 45A connected across the capacitor 45B, and the voltage across both ends becomes zero. Therefore, the next time the switch 45C is turned on, the capacitor 45B can be charged again. The discharge circuit 45 is an example of a circuit other than the main circuit 60 to which the battery cell 16 is connected.

[0082] The BMU 46 includes a microcomputer 49 in which a CPU 49A, a RAM 49B, etc. are integrated into one chip, and a ROM 50. Various software and data are stored in the ROM 50. The BMU 46 manages the power storage unit 15 by executing the software stored in the ROM 50. The CMU 40 and the BMU 46 are examples of a management section.

[0083] (4) Processing performed by the BMU Among the processes executed by the BMU 46, the SOC estimation process, the protection process, the equalization process, and the degradation suppression process will be described.

[0084] The SOC estimation process is a process for estimating the SOC of the power storage unit 15. For example, a current integration method is known as a method for estimating the SOC. The current integration method is a method for estimating the SOC by measuring the current value of the current flowing through the battery pack 51 at a predetermined time interval by the current sensor 41 and adding or subtracting the measured current value from the initial capacity. The method for estimating the SOC is not limited to the current integration method. For example, since there is a relatively accurate correlation between the open circuit voltage (OCV) of the power storage unit 15 and the SOC, the SOC may be estimated from the OCV.

[0085] The protection process is a process for protecting the battery cells 16 from overcharging, over-discharging, overcurrent, etc. Specifically, the protection process includes a process for opening the circuit breaker 43 to protect the battery cells 16 from overcharging or over-discharging when the SOC rises above a predetermined upper limit value or falls below a predetermined lower limit value, and a process for opening the circuit breaker 43 to protect the battery cells 16 from overcurrent when the current sensor 41 detects a current value equal to or higher than a predetermined upper limit value.

[0086] The equalization process is a process in which the equalization circuit 44 discharges the battery cell 16 with a relatively high voltage so that the difference in voltage between the battery cell 16 with the highest voltage and the battery cell 16 with the lowest voltage among the four battery cells 16 that make up one storage unit 15 is equal to or less than a predetermined reference value.

[0087] The deterioration prevention process is a process for suppressing deterioration of the battery cells 16 by discharging the battery cells 16 in response to detection of a state in which the voltage of the battery cells 16 is not substantially changing. In the deterioration prevention process according to the first embodiment, the battery cells 16 are discharged using the discharge circuit 45. For this reason, the equalization circuit 44 is not used to discharge the battery cells 16 in the deterioration prevention process according to the first embodiment.

[0088] The deterioration suppression process includes a detection process and a discharge process. In the detection process, the BMU 46 measures the discharge current of the battery cell 16 at a predetermined time interval using the current sensor 41, and when the value of the discharge current changes from a predetermined reference value (e.g., 0.001 C) or more to less than the predetermined reference value, it determines that a state in which the voltage of the battery cell 16 has not substantially changed has been detected. When there is no power outage, the UPS 1 is in a standby state (not in use) in which it does not supply power to the electrical load 11, so there is substantially no change in the voltage of the battery cells 16. For this reason, when the UPS 1 is not in use, it is detected as a state in which the voltage of the battery cells 16 is not substantially changing.

[0089] When the BMU 46 detects the above-mentioned state in the detection process, the BMU 46 starts the discharge process. In the discharge process, the BMU 46 discharges the battery cell 16 by turning on / off the switch 45C of the discharge resistor 45A according to the conditions shown in Table 1 below. [Table 1]

[0090] Specifically, in the discharge process, a discharge period in which the battery cell 16 is discharged and a charge period in which the battery cell 16 is charged are alternately repeated. The duration of the discharge period (=Y3+Y4) is the same as the duration of the charge period (=Z). The duration of the discharge period and the duration of the charge period do not necessarily have to be the same. The unit of the magnitude of the discharge pulse is CmA (sea milliampere). CmA is a unit that expresses the magnitude of the charge / discharge current of a storage element, and is generally referred to as the C rate. The C rate is defined as 1C, which is the magnitude of the current that flows when a storage element with an SOC of 100% is discharged to 0% in one hour (or the magnitude of the current that flows when a storage element with an SOC of 0% is charged to 100% in one hour). For example, if a storage element is discharged from an SOC of 100% to 0% in 30 minutes, the C rate is 2C. If the charging capacity of the battery cell 16 is different, the current value of the charge / discharge current will be different even if the C rate is the same.

[0091] Under the conditions shown in Table 1, the discharge pulse is discharged for the first Y3 hours (pulse discharge time) during the discharge period, and then discharge is paused for the following Y4 hours (discharge pause time). During the first Y3 hours of discharge, the current is constantly flowing while alternating between strong and weak currents. Specifically, discharge pulse 1 (main pulse) is flowed for Y1 hours at a discharge rate of X1 CmA, and discharge pulse 2 (weak pulse) is flowed for Y2 hours at a discharge rate of X2 CmA, and this is repeated alternately for Y3 hours. The discharge pulse may be discharged only once during the pulse discharge time. Specifically, when the discharge time (Y1 time) of the discharge pulse 1 is the same as the pulse discharge time (Y3 time), the discharge pulse 1 is discharged only once during the pulse discharge time.

[0092] In the discharge for the Y3 hour period, the current may be discharged intermittently. Specifically, the discharge pulse 1 (main pulse) may be discharged at a discharge rate of X1CmA for only Y1 hour, and then the discharge may be stopped for the following Y2 hour period.

[0093] The upper limit of the discharge time of the discharge pulse is preferably 1 second, more preferably 750 milliseconds, and even more preferably 520 milliseconds, so that the effects of the present invention can be reliably enjoyed even when the voltage of the storage element does not change substantially for a long period of time. The lower limit of the discharge time of the discharge pulse is not particularly limited, and may be the shortest time that can be realized by electrical control. The lower limit of the discharge time of the discharge pulse may be, for example, 0.1 milliseconds, 0.3 milliseconds or more, or 0.5 milliseconds. The discharge time of the discharge pulse may be, for example, 0.1 milliseconds or more and less than 1 second, 0.3 milliseconds or more and less than 750 milliseconds, or 0.5 milliseconds or more and less than 520 milliseconds.

[0094] The lower limit of the magnitude of the discharge pulse is preferably 0.1 CmA. This ensures that the effects of the present invention can be achieved. The lower limit of the magnitude of the discharge pulse may be 0.1 CmA, 0.5 CmA, or 1 CmA. The upper limit of the magnitude of the discharge pulse is preferably 10 CmA, more preferably 5 CmA, and even more preferably 3 CmA, which allows the size of the discharge pulse circuit to be reduced. The magnitude of the discharge pulse may be 0.1 CmA or more and 10 CmA or less, 0.5 CmA or more and 5 CmA or less, or 1 CmA or more and 3 CmA or less.

[0095] The charging current can be appropriately set depending on the environment and conditions of use. As a preferred example, the upper limit of the charging current during the charging period is preferably 0.4 CmA, more preferably 0.2 CmA, and even more preferably 0.1 CmA. This can suppress lithium electrodeposition during charging. The lower limit of the charging current during the charging period is not particularly limited, and may be the shortest time that can be realized by electrical control. The lower limit of the charging current during the charging period may be, for example, 0.01 CmA, 0.02 CmA, or 0.03 CmA. The magnitude of the charging current during the charging period may be 0.01 CmA or more and 0.4 CmA or less, 0.02 CmA or more and 0.2 CmA or less, or 0.03 CmA or more and 0.1 CmA or less.

[0096] The BMU 46 ends the discharge process when power supply from the battery cell 16 to the external electrical load 11 starts. Specifically, when power is supplied from the battery cell 16 to the external electrical load 11, the current value of the discharge current of the battery cell 16 increases. Therefore, when the BMU 46 measures a discharge current greater than a preset current value, it determines that the voltage of the battery cell 16 has substantially changed and ends the discharge process. The BMU 46 may also end the discharge process when the amount of change in the voltage of the battery cell 16 per unit time becomes greater than a predetermined value, determining that the voltage of the battery cell 16 has substantially changed.

[0097] In the electricity storage device 2, the battery cells 16 are discharged while the strength of the discharge current is alternately changed during the discharge process, so that an atmosphere in which polymerization reactions tend to progress can be more reliably eliminated compared to a case in which the battery cells 16 are discharged only once.

[0098] According to the energy storage device 2, the battery cells 16 are discharged by a circuit (discharge circuit 45) other than the main circuit 60 to which the battery cells 16 are connected, so that the battery cells 16 can be discharged even when no power is being supplied from the battery cells 16 to the external electrical load 11.

[0099] The power storage device 2 includes the discharge circuit 45 in addition to the equalization circuit 44, which makes it easier to make fine voltage adjustments during equalization and increases the effect of suppressing deterioration of the battery cells 16 compared to increasing the discharge resistance 44A of the equalization circuit 44. Providing the discharge circuit 45 in addition to the equalization circuit 44 has the advantage that, compared to increasing the size of the equalization circuit 44, the effect can be obtained by simply adding a simple circuit to the existing equalization circuit 44.

[0100] According to the power storage device 2, the power storage device 2 is used in the UPS 1. When the power storage device 2 is used in the UPS 1, deterioration of the battery cells 16 during non-power outages can be suppressed, and the power storage device 2 is more likely to exhibit its original performance during a power outage.

[0101] <Embodiment 2> The second embodiment is a modification of the first embodiment. The power storage device according to the second embodiment does not include a discharge circuit, and the battery cells 16 are discharged using a circuit breaker 43.

[0102] Specifically, the circuit breaker 43 according to the second embodiment is a latch relay, and includes a movable core and an excitation coil that drives the movable core. In the discharge process, the BMU 46 according to the second embodiment passes a current to the latch relay, which is already in a closed state, to close the latch relay. This current is supplied from the battery cell 16. As a result, the battery cell 16 is discharged by the excitation coil.

[0103] According to the electricity storage device of the second embodiment, the battery cells 16 are discharged by the circuit breaker 43, so that the discharge process can be performed without adding new hardware for discharging the battery cells 16.

[0104] Here, a latching relay has been described as an example of the circuit breaker 43, but the circuit breaker 43 may be a normally closed relay, a normally open relay, or a FET. However, a latching relay or a normally closed relay is more preferable in order to pass a current for closing the circuit breaker 43 when the circuit breaker 43 is closed. Here, an example has been described in which a current is passed to close the latch relay when the latch relay is closed, but a current may be passed to open the latch relay when the latch relay is closed, a current may be passed to close the latch relay when the latch relay is open, or a current may be passed to open the latch relay when the latch relay is open.

[0105] <Other embodiments> The power storage device of the present invention is not limited to the above-described embodiment, and various modifications may be made without departing from the scope of the present invention. For example, the configuration of one embodiment may be added to the configuration of another embodiment, and a part of the configuration of one embodiment may be replaced with the configuration of another embodiment or a well-known technique. Furthermore, a part of the configuration of one embodiment may be deleted. Also, a well-known technique may be added to the configuration of one embodiment.

[0106] (1) In the above embodiment, the discharge process is described by taking as an example a case where the battery cells 16 are discharged under the conditions shown in Table 1. However, the conditions for discharging the battery cells 16 are not limited to these and can be determined as appropriate.

[0107] (2) In the above embodiment, when the discharge current falls below a predetermined reference value (e.g., 0.001 C), it is determined that the voltage of the battery cells 16 is not substantially changing (in other words, the battery is not in use). Alternatively or in addition to this, a switch may be provided to manually turn on and off the circuit breaker 43 connected in series to the battery pack 51, and when the circuit breaker 43 is turned off, it may be determined that the battery is not in use.

[0108] (3) In the above embodiment, the separator 20 is configured separately from the positive electrode P and the negative electrode N, but the separator 20 is not limited to this. For example, the separator 20 may be an insulating coating layer or the like integrated with the positive and negative electrodes. A battery cell 16 having an insulating coating layer or the like integrated with the positive and negative electrodes is sometimes called separatorless. Since an insulating coating layer or the like corresponds to a separator, a battery cell 16 called separatorless is also included in the category of an energy storage element in which the positive electrode P and the negative electrode N are immersed in the nonaqueous electrolyte 18 with the electrode separated by a separator.

[0109] (4) In the above embodiment, an example was described in which the BMU 46 starts a discharge process when the discharge current value of the battery cell 16 falls below a predetermined reference value, determining that the voltage of the battery cell 16 has not substantially changed, and ends the discharge process when a discharge current greater than a preset current value is measured. In contrast to this, when the amount of change per unit time of the voltage of the battery cell 16 measured by the voltage sensor 42 falls below a predetermined reference value, it may be determined that the voltage of the battery cell 16 has not substantially changed and a discharge process may be started, and when the amount of change per unit time becomes greater than the predetermined reference value, the discharge process may be terminated. Alternatively, when the amount of change in SOC per unit time becomes equal to or less than a predetermined reference value, it is determined that the voltage of the battery cell 16 has not substantially changed and a discharge process is started, and when the amount of change in SOC per unit time becomes greater than the predetermined reference value, the discharge process is terminated.

[0110] (5) In the above embodiment, when the battery cell 16 is discharged in the discharge process, the amount of electricity discharged is charged by float charging. Since float charging is not performed under the control of the BMU 46, the BMU 46 is not involved in this charging. In contrast, the UPS 1 may be provided with a charger that operates under the control of the BMU 46, and after the battery cell 16 is discharged in the deterioration suppression process, the BMU 46 may control the charger to charge the battery cell 16. In this way, it is possible to reduce the possibility that the battery cell 16 is not sufficiently charged when it is used.

[0111] (6) In the above embodiment, a discharge circuit 45 is provided in addition to the equalization circuit 44, and in the discharge process, the discharge circuit 45 is used to discharge the battery cells 16. However, the battery cells 16 may be discharged using both the discharge resistor 45A of the discharge circuit 45 and the discharge resistor 44A of the equalization circuit 44.

[0112] (7) In the above embodiment, a discharge circuit 45 is provided in addition to the equalization circuit 44, and the discharge process is performed by discharging the battery cells 16 using the discharge circuit 45. However, the discharge circuit 45 may not be provided, and the battery cells 16 may be discharged using the discharge resistor 44A of the equalization circuit 44. Since the power storage device 2 generally includes the equalization circuit 44, discharging the battery cells 16 using the equalization circuit 44 can simplify the configuration of the battery cells 16 compared to a case in which a discharge circuit is provided in addition to the equalization circuit 44.

[0113] (8) In the above embodiment, the discharge process is started when it is detected that the voltage has not changed substantially. In contrast, the battery cell 16 may be discharged when the state in which the voltage has not changed substantially continues for a predetermined time or more. When the time during which the voltage of the battery cell 16 has not changed substantially continues is short (in other words, when the battery cell 16 is left alone for a short time), the battery cell 16 is unlikely to deteriorate. If the battery cell 16 is discharged when the state in which the voltage has not changed substantially continues for a predetermined time or more, it is possible to suppress discharge, which has little effect.

[0114] (9) In the above embodiment, the discharge process is started when it is detected that the voltage of the battery cell 16 has not changed substantially, regardless of whether the voltage is high or low. In contrast, the discharge process may be executed when the voltage has not changed substantially and the voltage or SOC of the battery cell 16 is equal to or higher than a predetermined value. When the voltage or SOC is low, the polymer 100 is unlikely to be produced. If the discharge process is not executed when the voltage or SOC is less than a predetermined value, it is possible to suppress discharge, which has little effect.

[0115] (10) In the above embodiment, a ternary lithium ion battery is used as an example of the power storage element. However, the lithium ion battery is not limited to a ternary lithium ion battery, and may be, for example, an iron-based lithium ion battery.

[0116] (11) In the above embodiment, the resistance value of the discharge resistor 45A is larger than the resistance value of the discharge resistor 44A of the equalization circuit 44. However, the resistance value of the discharge resistor 45A can be appropriately selected. For example, the resistance value of the discharge resistor 45A may be the same as or smaller than the resistance value of the discharge resistor 44A of the equalization circuit 44.

[0117] (12) In the above embodiment, the power storage device 2 used in the UPS 1 has been described as an example, but the use of the power storage device 2 is not limited to this. For example, the power storage device 2 may be mounted on a vehicle such as an automobile or motorcycle to supply power to a starter or auxiliary devices. The power storage device 2 may be mounted on a forklift or an automatic guided vehicle (AGV) that runs on an electric motor to supply power to the electric motor. The power storage device 2 may be used in a power storage system that stores power generated by solar power generation. The power storage device 2 may be used in a power storage system for performing peak shifting or peak cutting.

[0118] When the power storage device 2 is used as a power storage device mounted on a vehicle such as an automobile or motorcycle, deterioration of the battery cells 16 during storage at a dealership or the like can be suppressed, increasing the likelihood that the battery will perform at its original potential when mounted on the vehicle. The same applies when the battery is used on a moving object such as a forklift. When the energy storage device 2 is used as an energy storage system, deterioration of the battery cells 16 while left unused until the energy storage system is put into operation can be suppressed, increasing the possibility that the original performance will be exhibited when the power grid is put into operation.

[0119] Even when the power storage device 2 is used for something other than the UPS 1, it may be provided with a mechanism for turning on and off the electrical connection between the power storage device 2 and the electrical load, and when the electrical connection between the power storage device 2 and the electrical load is turned off, it may be determined that the voltage of the battery cells 16 is in a state that is substantially unchanged (in other words, an unused state).

[0120] (13) In the above embodiment, the battery cell 16 is a chargeable and dischargeable non-aqueous electrolyte secondary battery, but the type, shape, size, capacity, etc. of the storage element are arbitrary. The present invention can also be applied to various secondary batteries, electric double layer capacitors, lithium ion capacitors, and other capacitors.

[0121] (14) In the above embodiment, the battery cells 16 are charged during the charging period following the discharging period, but the battery cells 16 do not have to be charged. That is, the discharging period and the charging / discharging pause period during which charging / discharging is not performed may be repeated alternately. The length of the charging / discharging pause period is the same as the length of the charging period. For example, when the power storage device 2 is stored as inventory at a dealer, the battery cells 16 are discharged during the discharging period but are not charged after the discharging period, so that the discharging period and the charging / discharging pause period are repeated alternately.

[0122] [Example] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0123] [Example 1] (Preparation of positive electrode) LiNi as a positive electrode active material 0.5 Co 0.2 Mn 0.3 A positive electrode mixture paste was prepared containing O2, polyvinylidene fluoride (PVDF) as a binder, acetylene black as a conductive agent, and N-methylpyrrolidone (NMP) as a dispersion medium. The ratio of the positive electrode active material, binder, and conductive agent was 93:3:4 by mass. The positive electrode mixture paste was applied to the surface of an aluminum foil as a positive electrode substrate, and the mixture layer was compressed to a predetermined density and then dried to form a positive electrode active material layer, thereby obtaining a positive electrode.

[0124] (Preparation of negative electrode) A negative electrode mixture paste was prepared containing graphite as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, carboxymethyl cellulose (CMC) as a thickener, and water as a dispersion medium. The ratio of the negative electrode active material to the binder and the thickener was 98:1:1 by mass. The negative electrode mixture paste was applied to the surface of copper foil as a negative electrode substrate, and the mixture layer was compressed to a predetermined density and then dried to form a negative electrode active material layer, thereby obtaining a negative electrode.

[0125] (Preparation of separator) The separator used was made up of a base layer and a heat-resistant layer. The base layer was a 20 μm thick microporous polyethylene film, and the heat-resistant layer contained aluminosilicate particles. The porosity of the separator was 50%.

[0126] (Preparation of non-aqueous electrolyte) A non-aqueous solvent was prepared by mixing ethylene carbonate, propylene carbonate, and ethyl methyl carbonate in a volume ratio of 20:10:70, and lithium hexafluorophosphate (LiPF6) was added as an electrolyte salt at a concentration of 1.0 mol / dm 3 The non-aqueous electrolyte was prepared by mixing the above components so as to obtain a non-aqueous electrolyte solution having a content of 1:1.

[0127] (Fabrication of Energy Storage Element) The positive electrode, negative electrode, and separator obtained by the above procedure were stacked and wound. After that, the positive electrode active material layer non-forming region of the positive electrode and the negative electrode active material layer non-forming region of the negative electrode were welded to the positive electrode lead and the negative electrode lead, respectively, and sealed in an aluminum container. After welding the container and the cover plate, the non-aqueous electrolyte obtained above was poured in and sealed. In this way, the storage element of Example 1 was produced.

[0128] (Electrochemical measurements) The obtained electricity storage element was subjected to electrochemical measurement according to the following procedure.

[0129] (Measurement of initial discharge capacity) The produced storage element was charged to 4.35 V at a charging current of 900 mA in a thermostatic bath at 25° C., and then further charged at a constant voltage of 4.35 V for a total of 3 hours. After that, the element was discharged at a constant current of 900 mA to 2.75 V to measure the initial discharge capacity.

[0130] (Measurement of DC resistance (DCR) after the first charge / discharge) The storage element after the initial discharge capacity measurement was charged to 3.73 V at a charging current of 900 mA in a thermostatic chamber at 25°C, and then further charged at a constant voltage of 3.73 V for a total of 3 hours to set the SOC to 50%. Each storage element with an SOC adjusted to 50% was discharged at discharge currents of 180 mA, 450 mA, and 900 mA, and the voltage was measured 10 seconds after the start of discharge. Using these measured voltage values, the DCR after the initial charge and discharge at 25°C was calculated. Next, the energy storage elements were left in an environment of -10°C for 5 hours, and then the energy storage elements with SOC adjusted to 50% were discharged at discharge currents of 90 mA, 180 mA, and 270 mA, and the voltage was measured 10 seconds after the start of discharge. Using these voltage measurements, the DCR after the initial charge and discharge at -10°C was calculated.

[0131] (Left-and-right test) The electricity storage element whose initial discharge capacity and DCR after the initial charge / discharge were measured was left in a thermostatic chamber at 60° C. at 4.35 V for 60 days. At this time, a discharge period in which discharging was performed and a charge period in which charging was performed were repeated under each of conditions 1 to 4 shown in Table 2 below. For example, in the discharge period under condition 2, constant discharging while alternating between strong and weak currents for 4 minutes (pulse discharge time) and no discharging for 6 minutes (discharge pause time) were repeated. When constant discharging while alternating between strong and weak currents, discharge pulse 1 was repeatedly applied for 0.5 milliseconds at a discharge rate of 1 CmA and discharge pulse 2 was repeatedly applied for 88.5 milliseconds at a discharge rate of 0.01 CmA. In the charge period under condition 2, a charge current was applied for 2 minutes at a discharge rate of 0.03 CmA, and charging was paused for the following 8 minutes. Under all conditions, the discharging period and the charging period were switched when the SOC of the storage element fluctuated by 0.11%. [Table 2]

[0132] The discharge capacity, DCR at 25° C., and DCR at -10° C. were measured for the energy storage elements 30 days after the storage test and 60 days after the test. The measurement procedures were the same as those for the measurement of the initial discharge capacity and the DCR after the initial charge / discharge. The capacity retention rate was calculated by dividing the discharge capacity after 30 days and 60 days by the initial discharge capacity. The DCR change rate was calculated by dividing the DCR after 30 days and 60 days by the DCR after the initial charge / discharge.

[0133] [Example 2] The non-aqueous electrolyte was a mixture of FEC, PC, and EMC in a volume ratio of 10:10:40:40, with a content of 1.2 mol / dm 3 An electricity storage element of Example 2 was produced in the same manner as in Example 1, except that LiPF6 was dissolved so as to obtain the following: The obtained electricity storage element was subjected to electrochemical measurement under the same conditions as in Example 1, and the capacity retention rate, the DCR change rate at 25°C, and the DCR change rate at -10°C were calculated.

[0134] The experimental results are shown in FIGS. 6A and 6B through 11A and 11B.

[0135] The experimental results of the capacity retention rate of Example 1 will be described with reference to Figures 6A and 6B. As shown in Figures 6A and 6B, the capacity retention rate after 60 days was 95.9% under condition 1, whereas it was 98.5%, 97.8%, and 99.2% under conditions 2-4. Therefore, when the energy storage element is discharged when the voltage of the energy storage element is not substantially changing (condition 2-4), the decrease in the capacity retention rate after 60 days is suppressed compared to when the energy storage element is not discharged (condition 1).

[0136] 7A and 7B, the experimental results of the DCR change rate (ambient temperature 25°C) in Example 1 will be described. As shown in Fig. 7A and 7B, the DCR change rate after 60 days was 96.2% under condition 1, whereas it was 53.3%, 39.3%, and 25.9% under conditions 2-4. Therefore, when the storage element is discharged when the voltage of the storage element is not substantially changed (condition 2-4), the DCR change rate after 60 days is suppressed compared to when the storage element is not discharged (condition 1).

[0137] 8A and 8B, the experimental results of the DCR change rate (ambient temperature -10°C) in Example 1 will be described. As shown in Fig. 8A and 8B, the DCR change rate after 60 days was 32.7% under condition 1, whereas it was 19.2%, 5.7%, and 7.2% under conditions 2-4. Therefore, when the storage element is discharged when the voltage of the storage element is not substantially changed (condition 2-4), the DCR change rate after 60 days is suppressed compared to when the storage element is not discharged (condition 1).

[0138] The experimental results of the capacity retention rate of Example 2 will be described with reference to Figures 9A and 9B. As shown in Figures 9A and 9B, the capacity retention rate after 60 days was 95.4% under condition 1, whereas it was 97.9%, 97.3%, and 97.7% under conditions 2-4. This shows that discharging the energy storage element when the voltage of the energy storage element is not substantially changing (condition 2-4) suppresses the decrease in the capacity retention rate after 60 days compared to when the energy storage element is not discharged (condition 1).

[0139] 10A and 10B, the experimental results of the DCR change rate (ambient temperature 25°C) in Example 2 will be described. As shown in Fig. 10A and 10B, the DCR change rate after 60 days was 19.2% under condition 1, whereas it was 16.9%, 16.2%, and 18.7% under conditions 2-4. Therefore, when the storage element is discharged when the voltage of the storage element is not substantially changed (condition 2-4), the DCR change rate after 60 days is suppressed compared to when the storage element is not discharged (condition 1).

[0140] 11A and 11B, the experimental results of the DCR change rate (ambient temperature -10°C) in Example 2 will be described. As shown in Fig. 11A and 11B, the DCR change rate after 60 days was -7.3% under condition 1, while it was -9.3% and -8.5% under condition 2-3. Therefore, when the storage element is discharged when the voltage of the storage element has not substantially changed (condition 2-3), the DCR change rate after 60 days is suppressed compared to when the storage element is not discharged (condition 1).

[0141] [Example 3] (Preparation of positive electrode) A positive electrode mixture paste was prepared containing LiFePO4 as a positive electrode active material, polyvinylidene fluoride (PVDF) as a binder, acetylene black as a conductive agent, and N-methylpyrrolidone (NMP) as a dispersion medium. The ratio of the positive electrode active material, binder, and conductive agent was 90:5:5 by mass ratio. The positive electrode mixture paste was applied to the surface of aluminum foil as a positive electrode substrate, and the positive electrode mixture was compressed to a predetermined density and a predetermined thickness, and then dried to form a positive electrode active material layer, thereby obtaining a positive electrode.

[0142] (Preparation of negative electrode) A negative electrode mixture paste was prepared containing graphite as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, carboxymethyl cellulose (CMC) as a thickener, and water as a dispersion medium. The ratio of the negative electrode active material, binder, and thickener was 97:2:1 by mass. The negative electrode mixture paste was applied to the surface of copper foil as a negative electrode substrate, and the positive electrode mixture was compressed to a predetermined density and a predetermined thickness, and then dried to form a negative electrode active material layer, thereby obtaining a negative electrode.

[0143] (Preparation of non-aqueous electrolyte) A solvent containing EC, DMC, and EMC in a volume ratio of 20:35:45 was used to 3 LiPF6 was dissolved so that the

[0144] (Preparation of separator) The separator used was a microporous polyethylene film having a thickness of 16 μm and a porosity of 44%.

[0145] (Fabrication of Energy Storage Element) An electricity storage element of Example 3 was produced in the same manner as in Example 1, except that the positive electrode, negative electrode, separator, and non-aqueous electrolyte obtained by the above procedure were used.

[0146] The obtained electricity storage element was subjected to electrochemical measurement according to the following procedure.

[0147] (Measurement of initial discharge capacity) The produced storage element was charged to 3.50 V at a charging current of 550 mA in a thermostatic bath at 25° C., and then further charged at a constant voltage of 3.50 V for a total of 3 hours, and then discharged at a constant current of 550 mA to 2.00 V to measure the initial discharge capacity.

[0148] (Measurement of DC resistance (DCR) after the first charge / discharge) The storage element after the measurement of the initial discharge capacity was discharged at a constant current of 110 mA to 2.00 V in a thermostatic chamber at 25°C, and then charged for 30 minutes at a current value that would allow the initial discharge capacity to be charged in 1 hour, thereby setting the SOC to 50%. The storage element with the SOC adjusted to 50% was left in an environment of -10°C for 5 hours, and then the voltage was measured 10 seconds after the start of discharge when each storage element with the SOC adjusted to 50% was discharged at discharge currents of 55 mA, 110 mA, and 165 mA. Using these measured voltages, the DCR after the initial charge and discharge at -10°C was calculated.

[0149] (Left-and-right test) The electricity storage element whose initial discharge capacity and DCR after the initial charge / discharge were measured was left in a thermostatic chamber at 60° C. at 3.50 V for 60 days. At this time, a discharging period during which discharging was performed and a charging period during which charging was performed were repeated under the conditions shown in Table 3 below. [Table 3]

[0150] The discharge capacity and DCR at -10°C were measured for the energy storage element 60 days after the storage test and for the energy storage element 90 days after the storage test. The measurement procedure was the same as for the measurement of the initial discharge capacity and the DCR after the initial charge and discharge. The DCR change rate was calculated by dividing the DCR after 60 days and 90 days by the DCR after the initial charge and discharge.

[0151] As in the above-described Example 1, the current was constantly flowing while changing the current strength during the pulse discharge time in Example 3. For example, in Condition 5, during the pulse discharge time, a discharge pulse 1 (main pulse) was flowed for 0.5 milliseconds at a discharge rate of 0.1 CmA, and a discharge pulse 2 (weak pulse) was flowed for 4 milliseconds at a discharge rate of 0.02 CmA, and this was repeated alternately for 4 minutes.

[0152] In Example 3, the charging mode during the charging period differed depending on the conditions. For example, during the charging period under condition 5, a charging current was applied for 2 minutes at a charging rate of 0.05 CmA, and charging was suspended for the following 8 minutes. During the charging period under condition 9, a charging current was applied for 4 minutes at a discharging rate of 0.05 CmA, and charging was suspended for the following 6 minutes.

[0153] In Example 3, the fluctuation SOC [%] at which the discharge period and the charge period are switched also differs depending on the conditions. For example, under condition 5, the discharge period and the charge period are switched when the SOC of the storage element fluctuates by 0.111%. Under condition 9, the discharge period and the charge period are switched when the SOC fluctuates by 0.222%.

[0154] The experimental results are shown in Figure 13. As shown in Figure 13, under condition 1 (no charging / discharging), the DCR change rate after 60 days was 27.8%, whereas under conditions 5-12 it was 24.0%, 19.4%, 19.7%, 22.1%, 20.5%, 16.8%, 21.2%, and 20.4%. In condition 1, the DCR change rate at 90 days was 26.5%, while in conditions 5-12 it was 22.8%, 23.5%, 20.0%, 22.1%, 23.2%, 23.0%, 23.7%, and 27.2%.

[0155] Conditions 5 to 11 suppressed the DCR change rate after 60 days and after 90 days compared to condition 1 (no charging or discharging). Therefore, conditions 5 to 11 exhibited the effect of suppressing the DCR change rate even when the storage element was left unused for a long period of time, such as 90 days. Under condition 12, the rate of change in DCR after 90 days was not significantly different from that of condition 1, but after 60 days the rate of change in DCR was suppressed compared to condition 1. From this, condition 12 was also found to be significantly more effective in suppressing the rate of change in DCR than condition 1 up until 60 days had passed.

[0156] As shown in Table 3 above, in conditions 5 to 11, the discharge time of discharge pulse 1 is less than 1 second, and in condition 12, the discharge time of discharge pulse 1 is 1 second or more (specifically, 65 seconds). Since the DCR change rate after 90 days in condition 12 was not significantly different from that in condition 1, in order to reliably achieve the effect of suppressing the DCR change rate even when the storage element is left unused for a long period of time, it is preferable to set the discharge time of discharge pulse 1 to less than 1 second, and more preferably less than 520 milliseconds.

[0157] As shown in Table 3 above, the magnitude of the discharge pulse is either 0.1 CmA, 0.5 CmA, or 1 CmA under Condition 5 to Condition 12. Since the DCR change rate after 60 days was suppressed under Condition 5 to Condition 12 compared to Condition 1, it is preferable that the lower limit of the discharge pulse magnitude is 0.1 CmA or more. The lower limit of the discharge pulse magnitude may be 0.1 CmA, 0.5 CmA, or 1 CmA.

[0158] As shown in Table 3 above, the magnitude of the charge pulse during the charging period is 0.05 CmA under conditions 5 to 11, and 0.1 CmA under condition 12. The DCR change rate after 90 days under condition 12 was not significantly different from that under condition 1, but the DCR change rate after 60 days was suppressed compared to condition 1, so the magnitude of the charge pulse during the charging period is preferably 0.1 CmA or less, and more preferably 0.05 CmA or less.

[0159] As is clear from the above results, by discharging the electric storage element when the voltage of the electric storage element is not substantially changing, deterioration of the electric storage element can be suppressed. Although the reason for such an effect is not clear, it is thought that this is because discharging eliminates the atmosphere in which the polymerization reaction is likely to proceed.

[0160] Although the present invention has been described in detail above, the above-mentioned embodiments are merely examples, and the invention disclosed herein includes various modifications and alterations of the above-mentioned specific examples. [Explanation of symbols]

[0161] 2. Energy storage device 16 Battery cell (an example of a storage element) 18 Nonaqueous electrolyte 20 Separator 40 CMU (Example of Management Department) 43 Circuit Breaker 44 Equalization circuit 44A Discharge resistor (an example of a first discharge resistor) 45 Discharge circuit (an example of a circuit other than the main circuit) 45A Discharge resistor (an example of a second discharge resistor) 46 BMU (Example of Management Department) 60 Main circuit N negative electrode P positive electrode

Claims

1. A power storage device, an electricity storage element in which a positive electrode and a negative electrode are separated by a separator and immersed in a non-aqueous electrolyte; The management department and Equipped with The management unit A detection process for detecting a state in which the voltage of the storage element is substantially unchanged; a discharging process for discharging the storage element in response to the state being detected in the detection process; A power storage device that performs the above.

2. A power storage device, an electricity storage element in which a positive electrode and a negative electrode are separated by a separator and immersed in a non-aqueous electrolyte; The management department and Equipped with The management unit A detection process for detecting a state in which the voltage of the storage element is substantially unchanged; a discharging process for discharging the storage element in response to the state being detected in the detection process; A power storage device that performs the above. (However, this does not include those equipped with the following secondary battery systems. [Secondary battery system] A secondary battery system including a secondary battery and a charge / discharge control means for controlling charging and discharging of the secondary battery, The charge / discharge control means is when the internal resistance value of the secondary battery becomes 1.5 times or more of its initial value, and when the value of A / B, which is the ratio of the average charging current A=∫(Ic)dt / t which is the average charging current Ic of the secondary battery to the average discharging current B=∫(Id)dt / t which is the average discharging current Id of the secondary battery, is 10 or more, and when the average charging current A is 10C or more, a first pulse charging / discharging is performed on the secondary battery in which charging and discharging are repeated with a discharge current value larger than the charging current value; A secondary battery system that performs second pulse charging and discharging of the secondary battery by making the charging current value larger than the discharging current value and repeating charging and discharging when the internal resistance value of the secondary battery becomes 1.5 times or more of the initial value, the value of A / B is 0.1 or less, and the average discharge current value B is 10C or more.

3. The power storage device according to claim 1 or 2, The state of the power storage device is a non-use state in which the power storage device is not in use.

4. The power storage device according to any one of claims 1 to 3, The management unit, in the discharging process, discharges the energy storage element intermittently, or discharges the energy storage element while alternating between strong and weak currents.

5. The power storage device according to any one of claims 1 to 4, In the discharging process, the management unit discharges the power storage element and then causes a charger to charge the power storage element.

6. The power storage device according to any one of claims 1 to 5, In the discharging process, the management unit discharges the energy storage element through a circuit other than a main circuit to which the energy storage element is connected.

7. The power storage device according to claim 6, A circuit breaker connected in series with the storage element, The management unit discharges the energy storage element by causing a current for opening or closing the circuit breaker to flow from the energy storage element to the circuit breaker in the discharging process.

8. The power storage device according to claim 6, A plurality of the storage elements; an equalization circuit having a discharge resistor and discharging a storage element having a relatively high voltage among the plurality of storage elements through the discharge resistor to equalize the voltages of the storage elements; Equipped with The management unit causes the equalization circuit to discharge the energy storage elements in the discharging process.

9. The power storage device according to claim 6, A plurality of the storage elements; an equalization circuit having a first discharge resistor and discharging a storage element having a relatively high voltage among the plurality of storage elements through the first discharge resistor to equalize the voltages of the storage elements; a discharge circuit having a second discharge resistor; Equipped with The management unit controls the discharge circuit to discharge the energy storage element in the discharge process.

10. The power storage device according to any one of claims 1 to 9, The management unit executes the discharge process when a state in which the voltage of the storage element remains substantially unchanged continues for a predetermined period of time or more.

11. The power storage device according to any one of claims 1 to 10, The management unit executes the discharge process when the detection process detects a state in which the voltage of the storage element has not substantially changed and the voltage or state of charge of the storage element is equal to or greater than a predetermined value.

12. A power storage device according to any one of claims 1 to 11, The management unit measures a discharge current of the storage element, and continues the discharge process until the discharge current exceeds a preset value.

13. The power storage device according to any one of claims 1 to 12, The power storage device is used in an uninterruptible power supply.

14. The power storage device according to any one of claims 1 to 12, The power storage device is mounted on a vehicle.

15. The power storage device according to any one of claims 1 to 12, The power storage device is used in a power storage system.

16. A method for suppressing deterioration of an electric storage element in which a positive electrode and a negative electrode are immersed in a nonaqueous electrolyte solution while being separated by a separator, comprising the steps of: a detection step of detecting a state in which the voltage of the storage element is substantially unchanged; a discharging step of discharging the storage element in response to the state being detected in the detecting step; A method for inhibiting deterioration, comprising:

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