JP5699970B2 - Lithium ion secondary battery system and deposition determination method - Google Patents

Description

  The present invention relates to a lithium ion secondary battery system and a lithium metal deposition determination method.

  A lithium ion secondary battery is a secondary battery that can be charged and discharged by moving lithium ions in a non-aqueous electrolyte between a positive electrode and a negative electrode that occlude and release lithium ions. In a lithium ion secondary battery, lithium metal may be deposited on the negative electrode over time by repeating charge and discharge. Thus, when lithium metal deposits on the negative electrode, the lithium ion in the non-aqueous electrolyte (lithium ions contributing to charge / discharge of the battery) decreases, so the battery capacity of the lithium ion secondary battery decreases.

  Patent Document 1 discloses a technique related to a battery system that can suppress the negative electrode potential from being lower than the Li potential and prevent lithium metal from being deposited on the negative electrode during charging of the lithium ion secondary battery. Yes. The battery system disclosed in Patent Document 1 includes a positive electrode, a negative electrode, a non-aqueous electrolyte, and a lithium ion secondary battery having a reference electrode in contact with the non-aqueous electrolyte, and a potential difference ΔV1 between the negative electrode and the reference electrode. A potential difference measuring device for measuring the voltage, a determination means for determining whether or not the potential difference ΔV1 is a negative value when charging the lithium ion secondary battery, and if the potential difference ΔV1 is determined to be a negative value, the charging Control means for performing control to reduce the charging current value sometimes.

JP 2010-218900 A

  In the technique disclosed in Patent Document 1, when the potential of the negative electrode with respect to the reference electrode is a negative value, it is determined that lithium metal may be deposited on the negative electrode. The lithium metal is prevented from precipitating.

  However, even if the ultimate potential of the negative electrode is a negative value, lithium metal may not be deposited on the negative electrode depending on the amount of current supplied to the lithium ion secondary battery. For this reason, the method for determining whether lithium metal is deposited on the negative electrode based on the potential reached by the negative electrode has a problem that the accuracy of the determination is low.

  That is, when the current value at the time of charging is relatively low (for example, smaller than 5C), it is only necessary to consider the potential change due to the insertion reaction of lithium into the negative electrode. Can be determined. However, when the current value during charging is relatively high (for example, 5C or more), in addition to the potential change due to the insertion reaction of lithium into the negative electrode, resistance components (resistance of the non-aqueous electrolyte, film resistance, etc.) Since it appears in the potential change, it was difficult to determine the deposition of lithium metal using the potential reached by the negative electrode.

  Moreover, when the negative electrode has a planar shape (see FIG. 3), when charging / discharging at a high current value, reaction unevenness occurs on the electrode plane, and the charge amount and the discharge amount may be different depending on the part. That is, even in a portion where the amount of charge is large (that is, the potential reached by the negative electrode is low), lithium metal may not be deposited if the amount of discharge is large. For this reason, there is a problem that the accuracy of determination is low in the method of determining whether lithium metal is deposited on the negative electrode based on the potential reached by the negative electrode.

  In view of the above problems, an object of the present invention is to provide a lithium ion secondary battery system and a lithium metal deposition determination method capable of accurately determining the deposition of lithium metal on the negative electrode.

  A lithium ion secondary battery system according to an aspect of the present invention includes a positive electrode, a negative electrode, a nonaqueous electrolyte, and a reference electrode serving as a reference when measuring the potential of the negative electrode. And a control unit for controlling the lithium ion secondary battery, wherein the control unit has a current value of 10C or more for a predetermined time. The lithium-ion secondary battery is discharged at the same current value as the current value during the charging for the predetermined time after the charging, and the negative electrode potential before the charging and the post-charging are performed. When the first potential difference, which is the difference between the negative electrode potentials, is greater than the second potential difference, which is the difference between the negative electrode potential after the discharge and the negative electrode potential before the discharge, it is determined that lithium metal has precipitated on the negative electrode To do.

  20C or more and 30C or less may be sufficient as the electric current value at the time of the said charge and the said discharge.

  The predetermined time during the charging and discharging may be not less than 10 seconds and not more than 20 seconds.

  The control unit provides a first pause time during which no charge / discharge is performed after the charge, sets the negative electrode potential after the first pause time as the negative electrode potential after the charge, and performs charge / discharge after the discharge A second non-operating time may be provided, and the negative electrode potential after the second non-operating time may be set as the negative electrode potential after the discharge. The first and second non-operating times may be negative electrode potentials per second. | ΔV / s |, which is the absolute value of the amount of change, may be a time until the condition | ΔV / s | <0.5 mV is satisfied.

  The control unit may determine that lithium metal is deposited on the negative electrode when a difference between the first potential difference and the second potential difference is larger than a predetermined threshold.

  When the battery voltage corresponding to the potential difference between the positive electrode and the negative electrode is 4.1 V or more and the current value when the battery voltage reaches 4.1 V is 5 C or more, the control unit The charging and discharging may be performed on a secondary battery, and it may be determined that lithium metal has been deposited on the negative electrode when the first potential difference is greater than the second potential difference.

  When the battery voltage corresponding to the potential difference between the positive electrode and the negative electrode is 4.1 V or more and the current value when the battery voltage reaches 4.1 V is smaller than 5 C, the control unit When the ultimate potential is lower than 0V, it may be determined that lithium metal is deposited on the negative electrode.

  The control unit may control the lithium ion secondary battery so as to suppress deposition of lithium metal on the negative electrode when it is determined that lithium metal is deposited on the negative electrode.

  The control unit may suppress deposition of lithium metal on the negative electrode by limiting a current value when charging the lithium ion secondary battery.

  The control unit may suppress deposition of lithium metal on the negative electrode by placing the lithium ion secondary battery in an overdischarged state.

  In the lithium metal deposition determination method according to one aspect of the present invention, the lithium ion secondary battery is charged at a current value of 10 C or more for a predetermined time, and during the predetermined time after the charging. The lithium ion secondary battery is discharged at the same current value as the first current difference, and the first potential difference, which is the difference between the negative electrode potential before charging and the negative electrode potential after charging, is the negative electrode potential after discharging. And the second potential difference that is the difference between the negative electrode potentials before the discharge, it is determined that lithium metal has been deposited on the negative electrode.

  20C or more and 30C or less may be sufficient as the electric current value at the time of the said charge and the said discharge.

  The predetermined time during the charging and discharging may be not less than 10 seconds and not more than 20 seconds.

  A first pause time during which no charge / discharge is performed after the charging is provided, a negative electrode potential after the first pause time is set as the negative electrode potential after the charge, and a second pause in which charging / discharging is not performed after the discharge The negative electrode potential after the second pause time has elapsed may be used as the negative electrode potential after the discharge, and the first and second pause times are changes in the negative electrode potential per second. The time until ΔV / s satisfies the condition of ΔV / s <0.5 mV may be used.

  When the difference between the first potential difference and the second potential difference is larger than a predetermined threshold value, it may be determined that lithium metal is deposited on the negative electrode.

  According to the present invention, it is possible to provide a lithium ion secondary battery system and a lithium metal deposition determination method capable of accurately determining the deposition of lithium metal on the negative electrode.

1 is a block diagram showing a lithium ion secondary battery system according to a first embodiment. 1 is a diagram illustrating an example of a lithium ion secondary battery according to a first embodiment; 1 is a perspective view illustrating an example of a negative electrode of a lithium ion secondary battery according to a first embodiment. 3 is a flowchart for explaining an operation of lithium metal deposition determination in the lithium ion secondary battery system according to the first exemplary embodiment; It is a figure for demonstrating the timing of charging / discharging of a lithium ion secondary battery at the time of lithium metal precipitation determination. It is a figure which shows the change of the negative electrode electric potential at the time of charge of FIG. It is a figure which shows the change of the negative electrode electric potential at the time of discharge of FIG. It is a table | surface which shows the relationship between the electric current amount (current value) supplied to a lithium ion secondary battery, and precipitation of lithium metal to a negative electrode. 6 is a flowchart for explaining an operation of lithium metal deposition determination in the lithium ion secondary battery system according to the second embodiment; It is a figure for demonstrating the timing of charging / discharging of a lithium ion secondary battery at the time of lithium metal precipitation determination. It is a figure which shows the change of the negative electrode electric potential at the time of charge of FIG. It is a figure which shows the change of the negative electrode electric potential at the time of discharge of FIG. It is a table | surface which shows the relationship between the detection conditions at the time of lithium metal precipitation determination, and the detection limit of lithium metal. It is a figure which shows the relationship between the detection conditions at the time of lithium metal precipitation determination, and resistance increase rate.

<Embodiment 1>
Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a block diagram illustrating a lithium ion secondary battery system according to the first embodiment. As shown in FIG. 1, the lithium ion secondary battery system according to the present embodiment includes a lithium ion secondary battery 1 and a control unit 2 that controls the lithium ion secondary battery 1. The control unit 2 includes a charge / discharge control unit 3, a potential measurement unit 4, and a lithium metal deposition determination unit 5.

  The lithium ion secondary battery 1 can supply power to a predetermined load (not shown). The lithium ion secondary battery 1 may be a single lithium ion secondary battery or an assembled battery configured by electrically connecting a plurality of lithium ion secondary batteries.

  The lithium ion secondary battery 1 includes a positive electrode carrying a positive electrode active material, a negative electrode carrying a negative electrode active material, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The lithium ion secondary battery 1 includes, for example, a battery having a wound electrode body obtained by winding a strip-shaped positive electrode and a strip-shaped negative electrode through a strip-shaped separator, or a plurality of positive electrodes and a plurality of negative electrodes. And those having a laminated electrode body laminated alternately.

  For example, a lithium ion secondary battery including a stacked electrode body is configured by stacking a positive electrode 11, a negative electrode 13, and a separator 12 interposed between the positive electrode 11 and the negative electrode 13, as shown in FIG. can do.

The positive electrode 11 can be produced by applying a positive electrode active material to a positive electrode current collector such as aluminum and drying it. The positive electrode active material is a material capable of inserting and extracting lithium. For example, lithium cobaltate (LiCoO ₂ ), lithium manganate (LiMn ₂ O ₄ ), lithium nickelate (LiNiO ₂ ), and a mixture thereof are used. Can do.

  The negative electrode 13 can be produced by applying a negative electrode active material to a negative electrode current collector such as copper and drying it. The negative electrode active material is a material capable of inserting and extracting lithium, and for example, a powdery carbon material made of graphite or the like can be used.

As the non-aqueous solvent of the non-aqueous electrolyte, one kind selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and the like Alternatively, two or more kinds of materials can be used. In addition, as the supporting salt of the non-aqueous electrolyte, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) One or two or more lithium compounds (lithium salts) selected from ₃ and LiI can be used. As the separator, a porous polymer film such as a porous polyethylene film, a porous polyolefin film, and a porous polyvinyl chloride film can be used.

Moreover, the lithium ion secondary battery 1 is provided with the reference electrode used as the reference | standard at the time of measuring a negative electrode electric potential. For example, the reference electrode is provided between the positive electrode and the negative electrode of a lithium ion secondary battery so as to be in contact with the nonaqueous electrolytic solution. The negative electrode potential can be obtained by measuring the potential of the negative electrode with respect to the reference electrode, that is, the potential difference between the negative electrode and the reference electrode. As the reference electrode, for example, a reference electrode containing lithium metal or a reference electrode having an active material such as LiFePO ₄ or LiMnPO ₄ can be used.

  FIG. 3 is a perspective view showing an example of the negative electrode 13 of the lithium ion secondary battery 1. As shown in FIG. 3, negative electrode active materials 22 and 23 are provided on both surfaces of the negative electrode current collector 21 of the negative electrode 13, respectively. The negative electrode current collector 21 is provided with a terminal 24 for supplying electrons to the negative electrode current collector 21 and taking out electrons from the negative electrode current collector.

  And in the lithium ion secondary battery concerning this Embodiment, a reference electrode is arrange | positioned in the location (for example, shown with the code | symbol 25) furthest away from the terminal 24, for example. A portion of the negative electrode 13 that is farthest from the terminal 24 is a portion where the reaction unevenness occurs most when charged with a high current value. Therefore, by arranging the reference electrode at a place where uneven reaction tends to occur, it is possible to determine the deposition of lithium metal under more severe conditions. The reference electrode may be provided in the vicinity of the side 26 farthest from the terminal 24, for example.

  The charge / discharge control unit 3 included in the control unit 2 controls charging and discharging of the lithium ion secondary battery 1. The potential measuring unit 4 measures the battery voltage and negative electrode potential of the lithium ion secondary battery 1. Here, the battery voltage is a potential difference between the positive electrode and the negative electrode, and the negative electrode potential is a potential difference between the negative electrode and the reference electrode. The lithium metal deposition determination unit 5 determines whether lithium metal has been deposited on the negative electrode based on the battery voltage and negative electrode potential measured by the potential measurement unit 4. Further, when the lithium metal deposition determination unit 5 determines the deposition of lithium metal on the negative electrode, the charge / discharge control unit 3 charges the lithium ion secondary battery with a predetermined current value for a predetermined time. Can be instructed to discharge the lithium ion secondary battery at a predetermined current value for a predetermined time.

  Next, the lithium metal deposition determination operation in the lithium ion secondary battery system according to the present embodiment will be described with reference to FIGS. FIG. 4 is a flowchart for explaining the operation of determining lithium metal deposition in the lithium ion secondary battery system according to the present embodiment. FIG. 5 is a diagram for explaining the charge / discharge timing of the lithium ion secondary battery at the time of lithium metal deposition determination.

  When performing the lithium metal deposition determination, the charge / discharge control unit 3 performs charge / discharge of the lithium ion secondary battery 1 at a predetermined current value for a predetermined time (step S1). That is, as shown in FIG. 5, the charge / discharge control unit 3 charges the lithium ion secondary battery 1 with a predetermined current value from timing t1 to timing t2. Thereafter, during the period from the timing t2 to the timing t3, a resting time (first resting period) during which the lithium ion secondary battery 1 is not charged / discharged is provided. Then, the lithium ion secondary battery 1 is discharged at a predetermined current value from timing t3 to timing t4. Thereafter, during the period from the timing t4 to the timing t5, a resting time (second resting period) in which the lithium ion secondary battery 1 is not charged / discharged is provided.

  Here, the predetermined time (between timing t1 and timing t2) that is the charging time of the lithium ion secondary battery 1 is the predetermined time (between timing t3 and timing t4) that is the discharging time of the lithium ion secondary battery 1. ). The predetermined current value when charging the lithium ion secondary battery 1 is the same as the predetermined current value when discharging the lithium ion secondary battery 1. The rest time after charging and the rest time after discharging are, for example, a condition that | ΔV / s |, which is the absolute value of the amount of change in negative electrode potential per second, is | ΔV / s | <0.5 mV. The time until it is satisfied.

  In the lithium ion secondary battery system according to the present embodiment, when performing lithium metal deposition determination, it is preferable to perform charge and discharge in a relatively short time and at a relatively high current value. Therefore, for example, the predetermined current value at the time of charging and discharging of the lithium ion secondary battery 1 is 10C or more, more preferably 20C or more and 30C or less. For example, it is preferable that the charging time and discharging time of the lithium ion secondary battery 1 are 10 seconds or more and 20 seconds or less, respectively. Further, for example, it is preferable to provide a rest time of about 60 seconds as a rest time after charging and a rest time after discharging.

  The lithium metal deposition determining unit 5 is configured such that the negative electrode potential difference (first potential difference) during charging, which is the difference between the negative electrode potential before charging of the lithium ion secondary battery 1 and the negative electrode potential after charging, is the negative electrode potential after discharging and the discharging. It is determined whether or not the negative electrode potential difference during discharge (second potential difference), which is the difference between the previous negative electrode potentials, is greater (step S2). That is, it is determined whether or not a condition of (negative potential before charging−negative potential after charging)> (negative potential after discharging−negative potential before discharging) is satisfied. At this time, each potential of the lithium ion secondary battery is measured by the potential measuring unit 4. Further, for example, as the negative electrode potential after charging, the negative electrode potential after a pause time after charging has been used is used. Further, as the negative electrode potential after discharge, the negative electrode potential after the rest time after discharge has elapsed is used.

  And the lithium metal precipitation determination part 5 is lithium, when satisfy | filling the conditions of (negative electrode potential before charge-negative electrode potential after charge)> (negative electrode potential after discharge-negative electrode potential before discharge) (step S2: Yes). It is determined that lithium metal is deposited on the negative electrode of the ion secondary battery (step S3). On the other hand, the lithium metal deposition determination unit 5 does not satisfy the condition of (negative electrode potential before charging−negative electrode potential after charging)> (negative electrode potential after discharging−negative electrode potential before discharging) (step S2: No). Then, it is determined that lithium metal is not deposited on the negative electrode of the lithium ion secondary battery (step S4). Here, the case of not satisfying the condition of (negative potential before charging−negative potential after charging)> (negative potential after discharging−negative potential before discharging) is, for example, (negative potential before charging−after charging) Negative electrode potential) = (negative electrode potential after discharge−negative electrode potential before discharge) (that is, the negative electrode potential difference during charging is equal to the negative electrode potential difference during discharging).

  FIG. 6 is a diagram illustrating a change in the negative electrode potential during charging in FIG. 5. As shown in FIG. 6, the negative electrode potential before charging is Vc1. When charging is started at a predetermined current value at timing t1, the negative electrode potential decreases from Vc1, and reaches the minimum value Vc_min immediately before timing t2 when charging ends. Then, when charging is completed at timing t2, the negative electrode potential increases, and the negative electrode potential becomes Vc2 at timing t3 after the pause time. At this time, the negative electrode potential difference ΔVc during charging can be expressed as ΔVc = Vc1−Vc2 using the negative electrode potential Vc1 before charging and the negative electrode potential Vc2 after charging. The negative electrode potential difference ΔVc during charging corresponds to the charge amount of the lithium ion secondary battery at the timings t1 to t2.

  FIG. 7 is a diagram showing a change in the negative electrode potential during the discharge of FIG. As shown in FIG. 7, the negative electrode potential before discharge is Vd1. When discharge is started at a predetermined current value at timing t3, the negative electrode potential rises from Vd1, and reaches the maximum value Vd_max immediately before timing t4 when the discharge ends. When the discharge ends at timing t4, the negative electrode potential decreases, and the negative electrode potential becomes Vd2 at timing t5 after the rest time. At this time, the negative electrode potential difference ΔVd during discharge can be expressed as ΔVd = Vd2−Vd1 using the negative electrode potential Vd2 after discharge and the negative electrode potential Vd1 before discharge. The negative electrode potential difference ΔVd during discharge corresponds to the discharge amount of the lithium ion secondary battery at timings t3 to t4.

  When the negative electrode potential difference ΔVc at the time of charging and the negative electrode potential difference ΔVd at the time of discharging are the same, the lithium metal deposition determining unit 5 determines that the charging amount and the discharging amount of the lithium ion secondary battery are the same. It can be determined that lithium metal is not deposited. On the other hand, when the negative electrode potential difference ΔVc during charging and the negative electrode potential difference ΔVd during discharging satisfy the relationship ΔVc> ΔVd, the lithium metal deposition determining unit 5 has a larger charge amount than the discharge amount. Therefore, it can be determined that lithium metal is deposited on the negative electrode. That is, when the amount of charge of the lithium ion secondary battery is greater than the amount of discharge, in other words, when the amount of discharge of the lithium ion secondary battery is less than the amount of charge, lithium released from the negative electrode during discharge (released) The amount of lithium ion) is less than the amount of lithium accumulated in the negative electrode during charging, so that the lithium accumulated in the negative electrode remains even after discharge, and as a result, lithium metal is deposited on the negative electrode. Therefore, by comparing the negative electrode potential difference ΔVc during charging and the negative electrode potential difference ΔVd during discharging as described above, it can be determined whether or not lithium metal is deposited on the negative electrode.

  In the above description, it is determined that lithium metal is deposited on the negative electrode when the relationship ΔVc> ΔVd (that is, ΔVc−ΔVd> 0) is satisfied. For example, a predetermined threshold value Va (where Va> 0) May be determined that lithium metal is deposited on the negative electrode when the relationship of ΔVc−ΔVd> Va is satisfied. Thus, the erroneous determination of lithium metal deposition on the negative electrode can be suppressed by providing the predetermined threshold value. In addition, the determination described above is performed a plurality of times, and determination of lithium metal deposition on the negative electrode based on the majority vote can suppress erroneous determination.

  In the technique disclosed in Patent Document 1 above, it is determined that lithium metal may be deposited on the negative electrode when the potential reached by the negative electrode is a negative value. The metal was prevented from precipitating. However, even if the ultimate potential of the negative electrode is a negative value, lithium metal may not be deposited on the negative electrode depending on the amount of current supplied to the lithium ion secondary battery. Here, the reached potential of the negative electrode is the potential of the negative electrode reached at the time of charging. For example, the minimum value of the negative electrode potential at the time of charging (minimum value Vc_min in FIG. 6) can be set as the reached potential of the negative electrode.

  The table shown in FIG. 8 is a table showing the relationship between the amount of current (current value) supplied to the lithium ion secondary battery and the deposition of lithium metal on the negative electrode. In the table shown in FIG. 8, charging is performed at each current value (4C, 4.5C, 5C) until the negative electrode potential of the lithium ion secondary battery reaches −0.05 V, and then the lithium ion secondary battery is The result of disassembling and examining the deposition of lithium metal on the negative electrode using ICP emission spectroscopy is shown. Here, when the negative electrode potential is −0.05 V, it is determined that there is a possibility that lithium metal may be deposited on the negative electrode in the technique according to Patent Document 1.

  As shown in FIG. 8, when the current value during charging was 4C, the amount of lithium metal deposited on the negative electrode was 2.5 mg, and when the current value during charging was 4.5C, the lithium metal deposited on the negative electrode When the deposition amount was 1.8 mg and the current value during charging was 5 C, the deposition amount of lithium metal on the negative electrode was below the detection limit (<0.005 mg). Therefore, when the current value during charging was less than 5C, lithium metal was deposited on the negative electrode, and when the current value during charging was 5C or more, lithium metal was not deposited on the negative electrode.

  Thus, even when the negative electrode potential reaches −0.05 V, lithium metal may not be deposited on the negative electrode depending on the amount of current supplied to the lithium ion secondary battery. Therefore, the method for determining whether lithium metal is deposited on the negative electrode based on the potential reached by the negative electrode has a problem that the accuracy of the determination is low.

  That is, when the current value at the time of charging is relatively low (for example, smaller than 5C), it is only necessary to consider the potential change due to the insertion reaction of lithium into the negative electrode. Can be determined. However, when the current value during charging is relatively high (for example, 5C or more), in addition to the potential change due to the insertion reaction of lithium into the negative electrode, resistance components (resistance of the non-aqueous electrolyte, film resistance, etc.) Since it appears in the potential change, it was difficult to determine the deposition of lithium metal using the potential reached by the negative electrode.

  Moreover, when the negative electrode has a planar shape (see FIG. 3), when charging / discharging at a high current value, reaction unevenness occurs on the electrode plane, and the charge amount and the discharge amount may be different depending on the part. That is, even in a portion where the amount of charge is large (that is, the potential reached by the negative electrode is low), lithium metal may not be deposited if the amount of discharge is large. For this reason, there is a problem that the accuracy of determination is low in the method of determining whether lithium metal is deposited on the negative electrode based on the potential reached by the negative electrode.

  Therefore, in the lithium ion secondary battery system according to the present embodiment, the lithium ion secondary battery is charged at a predetermined current value for a predetermined time, and at a predetermined current value for a predetermined time after charging. A lithium ion secondary battery is discharged, and a first potential difference that is a difference between a negative electrode potential before charging and a negative electrode potential after charging is a difference between a negative electrode potential after discharging and a negative electrode potential before discharging. When it is larger than the potential difference, it is determined that lithium metal is deposited on the negative electrode.

  By using such a determination method, it is possible to accurately determine the deposition of lithium metal on the negative electrode even when the current value during charging is relatively high. In other words, by charging / discharging the lithium ion secondary battery with a relatively high current value, a minute difference in charge / discharge amount can be detected, and the deposition of lithium metal on the negative electrode can be accurately determined. it can.

  Therefore, the invention according to the present embodiment can provide a lithium ion secondary battery system and a lithium metal deposition determination method that can accurately determine the deposition of lithium metal on the negative electrode.

  Further, in the lithium ion secondary battery system according to the present embodiment, when it is determined that lithium metal has been deposited on the negative electrode by the above method, the control unit 2 controls the lithium so as to suppress the deposition of lithium metal on the negative electrode. The ion secondary battery 1 may be controlled.

  For example, the control unit 2 can suppress the deposition of lithium metal on the negative electrode by limiting the current value when charging the lithium ion secondary battery 1. For example, when the lithium metal deposition determination unit 5 determines that lithium metal is deposited on the negative electrode, the charge / discharge control is performed so that the charge / discharge control unit 3 limits the current value when charging the lithium ion secondary battery 1. Command part 3. By limiting the current value at the time of charging in this way, it is possible to suppress the negative electrode potential from becoming a negative value at the time of charging, and it is possible to suppress the deposition of lithium metal on the negative electrode.

  Moreover, the control part 2 can suppress precipitation of the lithium metal to a negative electrode by making the lithium ion secondary battery 1 into an overdischarge state. For example, overdischarge is performed at a predetermined current value (for example, 0.1 C), and when the battery voltage of the lithium ion secondary battery 1 reaches 1.5 V, the overdischarge of the lithium ion secondary battery 1 is stopped. . The current value at the time of overdischarge and the battery voltage at which overdischarge is stopped are not limited to the above example, and can be arbitrarily set. Lithium ions that no longer contribute to the chemical reaction may be able to contribute to the chemical reaction again by overdischarge. That is, the lithium ion secondary battery 1 includes a component that is primarily deteriorated, and the deteriorated component can be returned to a normal state by overdischarge. In addition, about the lithium ion secondary battery 1 which performed overdischarge, it is preferable to leave about 12 hours.

  Further, for example, by keeping the temperature of the lithium ion secondary battery 1 within a range of 35 ° C. or more and 55 ° C. or less for a predetermined time, it is possible to suppress the deposition of lithium metal on the negative electrode. For example, by providing heating means such as a heater in the lithium ion secondary battery, the temperature of the lithium ion secondary battery 1 can be maintained within a range of 35 ° C. or more and 55 ° C. or less for a predetermined time. In this case, the control unit 2 controls the heating means. Thus, the lithium metal deposited on the negative electrode of the lithium ion secondary battery can be returned to the lithium ion by keeping the temperature of the lithium ion secondary battery 1 within a range of 35 ° C. or more and 55 ° C. or less for a predetermined time. it can.

  The method described above is an example, and any other method may be used as long as it can suppress the deposition of lithium metal on the negative electrode.

  With the lithium ion secondary battery system according to the present embodiment described above, it is possible to provide a lithium ion secondary battery that can suppress the deposition of lithium metal on the negative electrode.

<Embodiment 2>
Next, a second embodiment of the present invention will be described.
In the lithium ion secondary battery system according to the second embodiment, compared with the lithium ion secondary battery system described in the first embodiment, the battery voltage of the lithium ion secondary battery and the ultimate potential of the negative electrode are used as the initial determination. The difference is that the deposition of lithium metal on the negative electrode is determined. Other than this, since it is the same as the lithium ion secondary battery system described in the first embodiment, a redundant description is omitted as appropriate.

  FIG. 9 is a flowchart for explaining an operation of determining lithium metal deposition in the lithium ion secondary battery system according to the present embodiment. In the lithium ion secondary battery system according to the present embodiment, first, it is determined whether or not the battery voltage (potential difference between the positive electrode and the negative electrode) measured by the potential measuring unit 4 shown in FIG. 1 is 4.1 V or higher. (Step S11). When the battery voltage is lower than 4.1 V (step S11: No), the lithium metal deposition determination unit 5 determines that lithium metal is not deposited on the negative electrode, and uses the lithium ion secondary battery as usual (step S12). ).

  On the other hand, when the battery voltage is 4.1 V or higher (step S11: Yes), it is determined whether or not the current value when the battery voltage reaches 4.1 V is 5 C or higher (step S13). If the current value when the battery voltage reaches 4.1V is smaller than 5C (step S13: No), it is determined whether the ultimate potential of the negative electrode is smaller than 0V (step S14).

  When the ultimate potential of the negative electrode is smaller than 0 V (step S14: Yes), it can be determined that lithium metal has been deposited on the negative electrode (step S15). On the other hand, when the ultimate potential of the negative electrode is 0 V or higher (step S14: No), it can be determined that lithium metal is not deposited on the negative electrode (step S16).

  On the other hand, when the current value when the battery voltage reaches 4.1 V is 5C or more (step S13: Yes), the determination of steps S17 to S20 is performed. Here, the operations in steps S17 to S20 are the same as the operations in steps S1 to S4 in FIG. 4 described in the first embodiment, and thus redundant description is omitted.

  In the lithium ion secondary battery system according to the present embodiment, before charging / discharging of the lithium ion secondary battery to determine the deposition of lithium metal on the negative electrode (that is, steps S17 to S20), the battery voltage and The precipitation potential of lithium metal on the negative electrode is determined using the potential reached by the negative electrode.

  That is, in the lithium ion secondary battery system according to the present embodiment, when the current value at the time of charging is relatively low (less than 5C), lithium to the negative electrode is obtained using the battery voltage and the potential reached by the negative electrode. When metal deposition is determined (steps S11 to S16) and the current value during charging is relatively high (5C or more), the lithium ion secondary battery is charged and discharged to determine lithium metal deposition on the negative electrode. (Steps S17 to S20). Therefore, since it is possible to determine the deposition of lithium metal on the negative electrode using a simple method (steps S11 to S16) using the battery voltage and the ultimate potential of the negative electrode, the deposition of lithium metal on the negative electrode can be more easily performed. Can be determined.

  Next, examples of the present invention will be described. In the examples described below, the lithium ion secondary battery described above was used.

<Example 1>
FIG. 10 is a diagram for explaining the charge / discharge timing of the lithium ion secondary battery at the time of lithium metal deposition determination. As shown in FIG. 10, in this example, the lithium ion secondary battery was charged with a current value of 20 C for 10 to 20 seconds. Thereafter, a resting time during which charging / discharging was not performed on the lithium ion secondary battery was provided for 20 to 80 seconds. Then, the lithium ion secondary battery was discharged at a current value of 20 C for 80 seconds to 90 seconds. Thereafter, a pause time during which charging / discharging was not performed on the lithium ion secondary battery was provided for 90 seconds to 150 seconds.

  FIG. 11 is a diagram showing a change in negative electrode potential during charging shown in FIG. As shown in FIG. 11, the negative electrode potential Vc1 before charging was 0.161V. Then, charging was carried out at a current value of 20 C for 10 seconds. After the start of charging, the negative electrode potential decreased from 0.161 V, and reached the minimum value (the potential reached by the negative electrode) −0.278 V immediately before the end of charging, and then the negative electrode potential increased. The negative electrode potential Vc2 after the 60 second rest period was 0.125V. Therefore, the negative electrode potential difference ΔVc during charging was ΔVc = 0.161 V−0.125 V = 0.036 V.

  FIG. 12 is a diagram showing a change in the negative electrode potential during the discharge shown in FIG. As shown in FIG. 12, the negative electrode potential Vd1 before discharge was 0.125V. And it discharged for 10 second with the electric current value of 20C. After the start of discharge, the negative electrode potential increased from 0.125 V, reached a maximum value of 0.595 V immediately before the end of discharge, and then the negative electrode potential decreased. The negative electrode potential Vd2 after the 60 second rest period was 0.152V. Therefore, the negative electrode potential difference ΔVd during discharge was ΔVd = 0.152V−0.125V = 0.027V.

  Therefore, since the negative electrode potential difference ΔVc during charging was 0.036 V and the negative electrode potential difference ΔVd during discharging was 0.027 V, the relationship of ΔVc> ΔVd was satisfied, and therefore lithium metal was deposited on the negative electrode. I was able to judge.

<Example 2>
In Example 2, a plurality of samples in which different amounts of lithium metal are deposited on the negative electrode are prepared, and these samples are determined using lithium metal deposition determination methods with different conditions, and the lithium metal in each detection condition is determined. Comparison of detection limits was performed.

  A plurality of samples in which different amounts of lithium metal were deposited on the negative electrode were prepared as follows. First, the lithium ion secondary battery was charged with a current value of 10 C, and the charging was terminated when the negative electrode potential became −0.5 V. Thereafter, discharging was performed with a current value of 10 C and a discharging time equal to the charging time. With this charging / discharging as one cycle, samples having different numbers of charging / discharging cycles, such as 10 cycles and 20 cycles, were produced. That is, as the number of cycles increases, the amount of lithium metal deposited on the negative electrode increases. In addition, charging / discharging of the lithium ion secondary battery was performed in the environment of 0 degreeC.

  Thereafter, the prepared samples were examined for the presence or absence of lithium metal deposition on the negative electrode. In this example, the current value of charge / discharge at the time of lithium metal deposition determination was set to 10C, 20C, and 30C, the charge / discharge time was set to 5 seconds, 10 seconds, and 20 seconds, and the presence or absence of lithium metal deposition on the negative electrode was examined. . At this time, the lithium metal deposition determination is performed from a sample with a small number of cycles (that is, a sample with a small amount of lithium metal deposited on the negative electrode), and then the sample for which the lithium metal deposition determination is performed is gradually By using a large number of samples (that is, by gradually increasing the amount of lithium metal deposited on the negative electrode of the sample), the detection limit of lithium metal under each detection condition was examined.

  FIG. 13 is a table showing the relationship between the detection conditions at the time of lithium metal deposition determination and the detection limit of lithium metal. For example, as shown in FIG. 13, when the current value is 10 C and the charge / discharge time is 5 s (hereinafter described as 10 C 5 s), the battery is charged for 5 seconds at a current value of 10 C, paused for 60 seconds after charging, and the current of 10 C The value was charged for 5 seconds and rested for 60 seconds after discharge. When the potential difference between the negative electrode potential before charging and the negative electrode potential after charging (after resting) is larger than the potential difference between the negative electrode potential after discharging (after resting) and the negative electrode potential before discharging, lithium metal is deposited on the negative electrode. It was determined that By such a method, lithium metal deposition was judged in order from a sample with a small amount of lithium metal deposited on the negative electrode. For example, when the detection condition was 10C5s, the deposition of lithium metal on the negative electrode could be detected when the amount of lithium metal deposited on the negative electrode was 5.00 mg. In other words, since the potential difference of the negative electrode potential during charging does not become larger than the potential difference of the negative electrode potential during discharge until the amount of lithium metal deposited on the negative electrode reaches 5.00 mg, the deposition of lithium metal can be detected. There wasn't.

  As shown in FIG. 13, when the current values during charging and discharging were the same, the detection limit of lithium metal became smaller as the charging and discharging time became longer (that is, the detection sensitivity improved). Moreover, when the charge / discharge time was the same, the detection limit of lithium metal became smaller (that is, the detection sensitivity improved) as the current value during charge / discharge increased. At this time, the detection sensitivity was improved when the current value during charging / discharging was 20 C or more and 30 C or less and the charging / discharging time was 10 seconds or more and 20 seconds or less. In particular, when the detection conditions were 20C20s, 30C10s, and 30C20s, the detection sensitivity was better than that of 20C10s.

  On the other hand, depending on the conditions for performing the lithium metal deposition determination, the lithium ion secondary battery may be adversely affected. In order to investigate such a case, the resistance of the lithium ion secondary battery after the lithium metal deposition determination is repeated 100 cycles (that is, after this operation is repeated 100 cycles with charge, pause, discharge, and pause as one cycle). I investigated. The detection conditions (current value, charge / discharge time) at this time were 20C10s, 30C10s, 20C20s, and 30C20s. FIG. 14 is a diagram illustrating a relationship between the detection condition of the lithium ion secondary battery and the resistance increase rate. As shown in FIG. 14, when the detection condition is 30C10s, the resistance increase rate is 1.10, when the detection condition is 20C20s, the resistance increase rate is 1.15, and when the detection condition is 30C20s, the resistance increase rate is 1. It was 20. In contrast, when the detection condition was 20C10s, the resistance increase rate was 1.01, and the resistance increase rate was lower than when the detection conditions were 30C10s, 20C20s, and 30C20s.

  Therefore, it was found that 20C10s having a low resistance increase rate and good detection sensitivity of the lithium ion secondary battery is the best condition for performing the lithium metal deposition determination.

  As mentioned above, although this invention was demonstrated according to the said embodiment and Example, this invention is not limited only to the structure of the said embodiment or Example, Claim of a claim of this-application claim It goes without saying that various changes, modifications, and combinations that can be made by those skilled in the art within the scope of the invention are included.

DESCRIPTION OF SYMBOLS 1 Lithium ion secondary battery 2 Control part 3 Charging / discharging control part 4 Electric potential measurement part 5 Lithium metal precipitation determination part 11 Positive electrode 12 Separator 13 Negative electrode 21 Negative electrode collector 22, 22 Negative electrode active material 24 Terminal

Claims (15)

A lithium ion secondary battery comprising a positive electrode, a negative electrode, a non-aqueous electrolyte, and a reference electrode serving as a reference when measuring the potential of the negative electrode;
A lithium ion secondary battery system having a control unit for controlling the lithium ion secondary battery,
The controller is
The lithium ion secondary battery is charged with a current value of 10C or more for a predetermined time,
During the predetermined time after the charging, discharging the lithium ion secondary battery at the same current value as the current value at the time of charging,
When the first potential difference that is the difference between the negative electrode potential before charging and the negative electrode potential after charging is larger than the second potential difference that is the difference between the negative electrode potential after discharging and the negative electrode potential before discharging. It is determined that lithium metal is deposited on the negative electrode.
Lithium ion secondary battery system.   2. The lithium ion secondary battery system according to claim 1, wherein a current value during charging and discharging is 20 C or more and 30 C or less.   The lithium ion secondary battery system according to claim 1 or 2, wherein the predetermined time during the charging and discharging is 10 seconds or more and 20 seconds or less. The controller is
Providing a first pause time during which no charge / discharge is performed after the charge, and setting the negative electrode potential after the first pause time as the negative electrode potential after the charge;
Providing a second pause time during which no charge / discharge is performed after the discharge, and setting the negative electrode potential after the second pause time as a negative electrode potential after the discharge;
The first and second pause times are times until | ΔV / s |, which is the absolute value of the change amount of the negative electrode potential per second, satisfies the condition | ΔV / s | <0.5 mV. ,
The lithium ion secondary battery system as described in any one of Claims 1 thru | or 3.   5. The control unit according to claim 1, wherein the control unit determines that lithium metal is deposited on the negative electrode when a difference between the first potential difference and the second potential difference is larger than a predetermined threshold. The lithium ion secondary battery system described in 1.   When the battery voltage corresponding to the potential difference between the positive electrode and the negative electrode is 4.1 V or more and the current value when the battery voltage reaches 4.1 V is 5 C or more, the control unit The charging and discharging are performed on a secondary battery, and it is determined that lithium metal is deposited on the negative electrode when the first potential difference is larger than the second potential difference. The lithium ion secondary battery system according to one item.   When the battery voltage corresponding to the potential difference between the positive electrode and the negative electrode is 4.1 V or more and the current value when the battery voltage reaches 4.1 V is smaller than 5 C, the control unit The lithium ion secondary battery system according to any one of claims 1 to 5, wherein it is determined that lithium metal is deposited on the negative electrode when an ultimate potential is lower than 0V.   The said control part controls the said lithium ion secondary battery so that precipitation of the lithium metal to the said negative electrode may be suppressed, when it determines with the lithium metal having precipitated on the said negative electrode. The lithium ion secondary battery system according to item.   The said control part is a lithium ion secondary battery system of Claim 8 which suppresses precipitation of the lithium metal to the said negative electrode by restrict | limiting the electric current value at the time of charging the said lithium ion secondary battery.   The said control part is a lithium ion secondary battery system of Claim 8 which suppresses precipitation of the lithium metal to the said negative electrode by making the said lithium ion secondary battery into an overdischarge state. The lithium ion secondary battery is charged at a current value of 10C or more for a predetermined time,
During the predetermined time after the charging, discharging the lithium ion secondary battery at the same current value as the current value at the time of charging,
When the first potential difference that is the difference between the negative electrode potential before charging and the negative electrode potential after charging is larger than the second potential difference that is the difference between the negative electrode potential after discharging and the negative electrode potential before discharging. It is determined that lithium metal is deposited on the negative electrode.
Lithium metal deposition determination method.   The lithium metal deposition determination method according to claim 11, wherein a current value during the charging and during the discharging is 20 C or more and 30 C or less.   The lithium metal deposition determination method according to claim 11 or 12, wherein the predetermined time during the charging and discharging is 10 seconds or longer and 20 seconds or shorter. Providing a first pause time during which no charge / discharge is performed after the charge, and setting the negative electrode potential after the first pause time as the negative electrode potential after the charge;
Providing a second pause time during which no charge / discharge is performed after the discharge, and setting the negative electrode potential after the second pause time as a negative electrode potential after the discharge;
The first and second pause times are times until ΔV / s, which is the amount of change in negative electrode potential per second, satisfies the condition of ΔV / s <0.5 mV.
The method for determining lithium metal deposition according to any one of claims 11 to 13.   15. The lithium metal according to claim 11, wherein when the difference between the first potential difference and the second potential difference is larger than a predetermined threshold value, it is determined that lithium metal is deposited on the negative electrode. Precipitation determination method.

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