JP5485080B2 - Discharge control system - Google Patents

Description

  The present invention relates to a discharge control system.

  Lithium ion secondary batteries have a high energy density, so for example, batteries used in vehicles such as railways and automobiles, or used for storing electric power generated by solar power generation or wind power generation and supplying it to an electric power system. It is attracting attention as. For example, when a lithium ion secondary battery (hereinafter referred to as “battery”) is used in an automobile, such an automobile includes a zero emission electric vehicle without an engine, an engine and a secondary battery. There are hybrid electric vehicles equipped with both, and plug-in hybrid electric vehicles that are charged directly from the system power supply. In addition, it is expected to be used as a stationary power storage system that supplies power in an emergency when the power system is cut off.

  For such various uses, excellent durability is required for the battery. For example, even when the environmental temperature becomes high or the charge / discharge cycle is repeated, the reduction rate of the chargeable battery capacity (that is, discharge capacity) is low, and the discharge capacity maintenance ratio is required to be high over a long period of time. Further, due to radiant heat from the road surface or heat conduction from the inside of the vehicle, for example, storage characteristics and cycle life in a high temperature environment of 60 ° C. or higher are important required performances.

  However, the discharge capacity of a lithium ion secondary battery can usually be reduced by leaving it in a high temperature environment or performing a charge / discharge cycle. The degree of decrease in the discharge capacity tends to increase particularly when the discharge capacity is left at a high voltage or a charge / discharge cycle is repeated in a wide voltage range.

  In addition, for example, when the performance of the battery deteriorates due to deactivation of lithium ions in the lithium ion secondary battery, the resistance inside the battery may increase. In such a case, an overvoltage is generated by an increase in resistance inside the battery, so that the chargeable / dischargeable voltage range is shifted, and it may not be possible to discharge to a voltage that can be discharged when the battery is discharged.

  In view of such circumstances, for example, Patent Document 1 discloses a discharge capacity so that an increase / decrease in SOC (State of charge) of a charged state of a battery is within a predetermined range including the optimum SOC of the battery. It is described that is set.

  For example, Patent Document 2 describes that a battery voltage immediately before battery deterioration occurs is set as a discharge end voltage, and the battery is discharged to the discharge end voltage.

  Furthermore, for example, Patent Document 3 describes that a target SOC is set according to battery deterioration and charge / discharge is controlled to the target SOC.

  For example, Patent Document 4 describes that the voltage drive range is expanded so that the discharge capacity becomes constant even when the battery deteriorates.

JP 2006-54958 A JP 2008-113545 A JP 2000-30753 A JP-A-8-214469

  However, the technique described in Patent Document 1 does not control charging / discharging of the battery according to deterioration of the battery.

  In the techniques described in Patent Documents 2 and 3, the battery is only discharged more strictly up to a preset end-of-discharge voltage, and the reduction in discharge capacity due to battery deterioration is not improved.

  And in the technique of patent document 4, since the charging / discharging voltage of a battery becomes an excessive wide range, the internal resistance of a battery may increase excessively.

  The present invention has been made to solve the above problems, and an object of the present invention is to provide a discharge control system that can improve the cycle characteristics of a lithium ion secondary battery.

  As a result of intensive studies to solve the above problems, the present inventors can improve the cycle characteristics of a lithium ion secondary battery by appropriately changing the discharge end voltage of the battery when a predetermined condition is satisfied. It has been found that a discharge control system can be provided, and the present invention has been completed.

  ADVANTAGE OF THE INVENTION According to this invention, the discharge control system which enables the improvement of the cycle characteristic of a lithium ion secondary battery can be provided.

It is a circuit diagram showing the structure of the discharge control system which concerns on embodiment of this invention. It is a graph showing the relationship between battery capacity and potential in a battery to which a discharge control system according to an embodiment of the present invention is applied. It is a flowchart showing the discharge control method in the discharge control system which concerns on embodiment (1st embodiment) of 1st this invention. It is a flowchart showing the discharge control method in the discharge control system which concerns on embodiment (2nd embodiment) of 2nd this invention. It is a figure which represents typically an example of the internal structure of the battery which can apply the discharge control system which concerns on embodiment of this invention.

  Hereinafter, a mode for carrying out the present invention (hereinafter referred to as “the present embodiment” as appropriate) will be described in detail, but the present embodiment is not limited to the following contents and does not depart from the gist thereof. Any change can be made within the range.

[1. Configuration of Discharge Control System According to this Embodiment]
The discharge control system according to the present embodiment controls discharge of a lithium ion secondary battery having a positive electrode and a negative electrode. FIG. 1 is a circuit diagram illustrating a configuration of a discharge control system according to the present embodiment. As shown in FIG. 1, the discharge control system 201 according to this embodiment includes at least a discharge end voltage correction unit 202 and a charge / discharge control circuit 203.

  In addition to the discharge control system 201 according to the present embodiment, FIG. 1 shows a lithium ion secondary battery (battery) 204 to which the discharge control system 201 according to the present embodiment is applied, and the present embodiment. An arithmetic processing unit 205 that performs various calculations in the discharge control system 201, an external load 206, a charging power source 207, a current measuring unit 208, a voltage measuring unit 209, and switches 210a and 210b are also shown. In the present embodiment, the calculation processing unit 205 is provided with a discharge end voltage correction unit 202.

The end-of-discharge voltage correcting unit 202 changes (that is, corrects) the end-of-discharge voltage V0 of the battery 204 to the corrected end-of-discharge voltage V1.
Incidentally, a secondary battery such as the battery 204 cannot be discharged to 0 V, but can only be discharged to a certain voltage, and if the battery is further discharged, the performance of the secondary battery is deteriorated. This certain voltage is the discharge end voltage V0, and the value varies depending on the type of the secondary battery. The battery capacity is represented by the amount of electricity (Ah) that can be taken out from the fully charged state to the end-of-discharge voltage V0.
Here, the significance of changing the discharge end voltage V0 to the corrected discharge end voltage V1 will be described.

  FIG. 2 is a graph showing the relationship between battery capacity and potential in a battery to which the discharge control system according to this embodiment is applied. In the graph shown in FIG. 2, the horizontal axis represents the battery capacity (Ah), and the vertical axis represents the potential (V). Note that the vertical axis represents the potential when the current is zero. Reference numeral 101 denotes a positive electrode potential curve of the battery, reference numeral 102 denotes a negative electrode potential curve of the battery, reference numeral 103 denotes a battery voltage curve, reference numeral 104 denotes a negative electrode potential curve after battery deterioration, and reference numeral 105 denotes a battery voltage curve after battery deterioration. . In this embodiment, the difference between the “potential” of the positive electrode and the “potential” of the negative electrode is defined as the “voltage” of the battery. That is, the battery voltage curve 103 is defined by the difference between the positive electrode potential curve 101 and the negative electrode potential curve 102. α, β, and γ will be described later.

  Actually, since the irreversible capacity is generated during the first charge / discharge cycle of the battery, the positions of the positive electrode potential curve 101 and the negative electrode potential curve 102 may be shifted. However, for convenience of explanation, the following description will be made on the assumption that the irreversible capacity is constant between the positive electrode and the negative electrode, and the positions of the positive electrode potential curve 101 and the negative electrode potential curve 102 are not shifted. Further, in FIG. 2, only the negative electrode potential curve after deterioration (graph represented by reference numeral 104) is described, and it appears that the positive electrode potential curve after deterioration does not appear. Although there is a curve, for the convenience of explanation, the positive electrode potential curve after deterioration is matched with the positive electrode potential curve before deterioration (that is, the positive electrode potential curve 101).

  According to the study by the present inventors, the cause of the decrease in the discharge capacity as a result of repeating the charge / discharge cycle was found to be mainly due to the following two points. That is, (1) electrode materials (for example, positive electrode active material, negative electrode active material, etc.) contributing to charge and discharge of the positive electrode and negative electrode are reduced, and (2) lithium ions are deactivated, Two points are that the potential range is shifted. It was also found that the cause of the decrease in discharge capacity during the low charge state was derived from the positive electrode, and the cause of the decrease in discharge capacity during the high charge state was derived from the negative electrode.

  As a result of examining the increase / decrease of the DC resistance with respect to the charging depth of the positive electrode and the negative electrode, it was found that the resistance of the electrode was particularly large in a region where the charging depth was small in the positive electrode, that is, in a region where the potential of the positive electrode was low (low potential region). For this reason, when the charge / discharge cycle is repeated using the low potential region of the positive electrode, the cycle characteristics tend to deteriorate. Although it is not clear about the reason why the resistance increases in the region where the charging depth in the positive electrode is small, according to the study by the present inventors, a large amount of lithium ions are present on the positive electrode surface in the low charging state. One of the causes is considered to be a decrease in the diffusibility of. In the present embodiment, the “charge depth” represents a value indicating the charge capacity as a percentage of the rated capacity of the battery (that is, the battery capacity at the time of full charge).

  In particular, regarding the point (2) above, it has been found that inactivation of lithium ions mainly occurs in the negative electrode during a high charge state. That is, the lithium ion occluded in the negative electrode reacts with, for example, a decomposition product of the nonaqueous electrolytic solution to be inactivated, so that the potential range of the positive electrode and the negative electrode becomes narrow. Since such a reaction is mainly a reduction reaction, the reaction tends to be larger in a high charge state.

  In the battery to which the graph shown in FIG. 2 is applied, the low potential region of the positive electrode having high resistance (specifically, the region having a positive electrode potential of α or less) is not used when the battery is discharged, and is represented by the battery voltage curve 103. As described above, it is assumed that the battery is charged / discharged by setting the discharge end voltage to V0 and the minimum battery capacity to A0 before charging. However, for example, when the battery deteriorates due to deactivation of lithium ions or the like, the negative electrode potential curve after deterioration becomes a curve represented by reference numeral 104, and the battery voltage becomes a curve represented by reference numeral 105. Therefore, if the end-of-discharge voltage remains V0 and is not changed, even if the battery deteriorates and the battery voltage curve is represented by reference numeral 105, the battery voltage is V0, that is, the battery capacity is A1. At this point, the battery is completely discharged.

  However, as described above, the minimum capacity of the battery increases to A1 due to the deterioration of the battery even though the battery is set so that the positive electrode utilization area is set to α or more before charging. By doing so (that is, by reducing the utilization area of the positive electrode), the discharge capacity is reduced.

  In view of this point, the present invention has been invented. That is, when the battery is deteriorated and the battery voltage becomes a curve represented by reference numeral 105, the discharge end voltage V0 is changed to the corrected discharge end voltage V1, thereby discharging the original positive electrode utilization area until α. Can be performed. Therefore, according to the present invention, it is possible to suppress a decrease in discharge capacity even when the battery is deteriorated.

  In FIG. 2, the above description is made with the positive electrode potential curve 101 fixed for convenience of explanation. However, in actuality, the decrease in the electrode material that contributes to charging / discharging of the positive electrode and the negative electrode of the battery (that is, (1 ), Side reactions in the positive electrode and the negative electrode may occur. Therefore, since the positive electrode potential curve 101 also changes, when a method in which the discharge capacity is simply made constant is applied, a region where the positive electrode utilization region is α or less, that is, a region where the positive electrode resistance is high is also used during charging and discharging. The cycle characteristics of the battery will deteriorate.

  As described above, the end-of-discharge voltage correcting unit 202 changes the preset end-of-discharge voltage V0 to the corrected end-of-discharge voltage V1. The timing at which the discharge end voltage correction unit 202 changes the discharge end voltage V0 is that the potential difference between the positive electrode and the negative electrode (that is, the battery voltage measured by the voltage measuring unit shown in FIG. 1) of the battery 204 is the discharge end voltage of the battery 204. This is performed when the discharge end voltage correction unit 202 determines (that is, detects) that the discharge end voltage change condition is satisfied at the time when V0 is reached. Such a condition for changing the discharge end voltage is not particularly limited. For example, a method of estimating the change in the positive electrode potential from the positive and negative electrode potential, the number of battery cycles, the elapsed storage time, the battery life, the deterioration degree of the battery, etc. Etc. Further, it is particularly preferable that the potential of the positive electrode included in the battery 204 includes a predetermined value α or more. By correcting the discharge end voltage based on the potential of the positive electrode, it is possible to easily suppress a decrease in discharge capacity. In FIG. 2, the positive electrode potential is set to α when the battery voltage is the final discharge voltage V0.

  α is a parameter that can be arbitrarily determined by the user, and is not particularly limited. For example, α may be determined according to discharge capacity, battery life, battery output, battery deterioration, positive and negative electrode materials and combinations, and the like. However, as the value of α, it is preferable to set α as the potential when the DC resistance value is increased by a specific value with respect to the DC resistance of the positive electrode at the charging depth of 50% of the battery 204. The specific value of the “specific value” is usually 5% or more, preferably 10% or more, more preferably 20% or more, and the upper limit is usually 100% or less, preferably 80% or less, more preferably Is 50% or less, more preferably 40% or less, and particularly preferably 30%. The initial discharge positive electrode potential setting method is also a parameter that can be arbitrarily determined by the user, and is not particularly limited. For example, discharge capacity, battery output, battery life, battery deterioration, positive and negative electrode materials and combinations, etc. It may be determined according to. In FIG. 2, the initial discharge positive electrode potential and α coincide with each other.

  Moreover, it is also preferable that the discharge end voltage changing condition includes that the potential of the negative electrode of the battery 204 is not more than a predetermined value γ. For example, by changing the discharge end voltage based on both the positive electrode potential and the negative electrode potential, a decrease in discharge capacity can be more strictly suppressed. Similarly to α, the value of γ is a parameter that can be arbitrarily determined by the user, and is not particularly limited. For example, the value of γ is determined according to the discharge capacity, battery life, battery deterioration, positive and negative electrode materials and combinations, and the like. That's fine. For example, when natural graphite is used as the negative electrode material, γ is usually 0.3 V or higher, preferably 0.4 V or higher, and the upper limit is usually 1.0 V or lower, preferably 0.7 V or lower. In FIG. 2, γ is set so that the negative electrode potential is γ or less when the battery voltage is the final discharge voltage V0.

Furthermore, as the above-mentioned discharge end voltage changing condition, regardless of the value of the discharge end voltage V0, the potential difference between the positive electrode and the negative electrode of the battery 204 (that is, the battery voltage measured by the voltage measuring unit 209 shown in FIG. 1). It is also preferable to include being equal to or greater than a predetermined value V ′. Such V ′ is a parameter that can be arbitrarily determined by the user, and is not particularly limited. For example, it is determined according to the discharge capacity, battery output, battery life, battery deterioration, positive electrode and negative electrode materials and combinations, and the like. For example, when LiNi _1/3 Mn _1/3 Co _1/3 O ₂ is used for the positive electrode material and natural graphite is used for the negative electrode material, it is usually 2.5 V or higher, preferably 3.5 V or higher, The upper limit is usually 3.9 V or less, preferably 3.7 V or less.

  The corrected discharge end voltage V1 is a value that satisfies the value obtained by subtracting β from the discharge end voltage V0, that is, V1 = V0−β (see FIG. 2). The value of the management value β is also a parameter that can be arbitrarily determined by the user, and is not particularly limited. Like the management value α, for example, the discharge capacity, the battery life, the battery deterioration degree, the positive electrode and negative electrode materials and combinations, etc. It may be determined according to. Such a management value β is usually set within a range of 10 mV or more and usually 500 mV or less.

  In the present embodiment, the end-of-discharge voltage correction unit 202 measures the potentials of the positive electrode and the negative electrode of the battery 204 via the charge / discharge control circuit 203. Although there is no restriction | limiting in the measuring method of an electric potential, For example, it can measure using the battery voltage value of the battery 204, the electric current value which is flowing, and temperature etc. as needed. Specifically, for example, it is calculated using the positive and negative charge / discharge curves stored in the discharge end voltage correction unit 202 in advance and the voltage change of the battery voltage curve, or lithium metal or the like is incorporated in the battery 204 in advance. In addition, it can be calculated by using the difference in potential between the lithium metal or the like and each electrode. However, from the viewpoint that a general battery can be used as the battery 204 without changing the structure of the battery 204, the former method is preferably used.

  In the former method, the voltage can be calculated from the voltage change rate (ΔV / ΔQ), which is the ratio between the potential change ΔV and the battery capacity change ΔQ, from the positive and negative unipolar curves measured and stored in advance. Here, the integrated value of the battery capacity change ΔQ is calculated as the product of the current value flowing through the battery 204 and time (that is, the cycle time). As an apparatus for measuring the battery capacity change ΔQ, for example, any known electronic element can be used.

  The more detailed method of the former is described in, for example, Japanese Unexamined Patent Application Publication No. 2009-80093.

  The charge / discharge control circuit 203 charges and discharges the battery 204. The charge / discharge control circuit 203 discharges the battery 204 until the battery voltage of the battery 204 reaches the final discharge voltage V <b> 0 after starting the discharge of the charged battery 204. When the discharge end voltage V0 is changed to the corrected discharge end voltage V1 by the discharge end correction unit 202, the discharge is further performed from the discharge end voltage V0 to the corrected discharge end voltage V1.

  The battery 204 is applied with the discharge control system 201 according to this embodiment, and has a positive electrode and a negative electrode. The specific configuration of the battery 204 is not particularly limited. For example, the following [3. The battery described in [Configuration of Battery] can be used.

  Note that the battery 204 may be a single battery made of one battery or an assembled battery in which two or more batteries are arbitrarily combined.

  In addition, the arithmetic processing unit 205 measures and integrates, for example, battery voltage and current, discharge time, rest time (standby time), non-use time, and the like of the battery 204, and performs calculation, processing, and the like according to the integration time. Is. Then, the charge / discharge control circuit 203 can be controlled based on the results of these calculations, processes, and the like. Specifically, for example, the arithmetic processing unit 205 measures the battery voltage from the charge / discharge control circuit 203 instead of the end-of-discharge voltage correction unit 202, and the charge / discharge control parameters of the battery 204 (for example, discharge or charge time, discharge Alternatively, charging / discharging of the battery 204 may be controlled by determining a charging voltage, discharging or charging current, and transmitting the charging / discharging control parameter to the charging / discharging control circuit 203.

  Note that the battery voltage of the battery 204 can be measured by the voltage measurement unit 209 shown in FIG. Further, the current at the time of charging the battery 204 and the current at the time of charging from the battery 204 can be measured by the current measuring unit 208 shown in FIG.

  As a specific method of charge / discharge control, for example, when the battery 204 is discharged, the arithmetic processing unit 205 transmits control parameters to the charge / discharge control circuit 203 so that the discharge voltage and discharge current become desired, and the switch 210a. Is closed, and a signal for opening the switch 210b is transmitted to each switch, whereby the battery 204 and the external load 206 are electrically connected, and the battery 204 is discharged. On the other hand, when the battery 204 is charged, the arithmetic processing unit 205 transmits control parameters to the charge / discharge control circuit 203 so that the charging voltage and charging current are as desired, and the switch 210a is opened and the switch 210b is further closed. By transmitting a signal to each switch, the battery 204 and the charging power source 207 are electrically connected, and the battery 204 is charged.

  For example, in order to measure the temperature of the battery 204, temperature measuring means (not shown) such as a thermocouple or a thermistor may be provided. By providing such means, if the arithmetic processing unit 205 acquires the temperature measured by these means, for example, the discharge of the battery 204 can be controlled according to the temperature, and more accurate discharge control can be performed. Is possible.

  Furthermore, for example, a recording unit (not shown) for recording various measured information may be provided. There is no particular limitation on the specific configuration of such a recording unit, for example, a magnetic recording medium such as a floppy (registered trademark) disk (FD) or a hard disk drive (HDD); a random access memory (RAM), a flash memory (USB memory) Semiconductor media such as: compact discs (CD-R, CD-RW, etc.), digital versatile discs (DVD-R, DVD + R, DVD + RW, DVD-RW, DVD-RAM, etc.), HD-DVD, Blu-ray disc, etc. An optical recording medium can be used.

[2. Discharge Control Method in Discharge Control System According to Present Embodiment]
Next, a discharge control method in the discharge control system according to the present embodiment will be described with reference to the flowcharts shown in FIGS.

  FIG. 3 is a flowchart showing a discharge control method in the discharge control system according to the first embodiment. In the description of the flowchart shown in FIG. 3, in the following description, the discharge control system 201 according to the present embodiment is described as including the arithmetic processing unit 205, but as described above, Since the charge / discharge control circuit 203 or the discharge end voltage correction unit 202 has the function, the arithmetic processing unit 205 may not be provided. Further, V0, α, and γ described below are the same as those described in [1. The same thing as what is described in the structure of the discharge control system concerning this embodiment] is represented.

  First, the arithmetic processing unit 205 receives a discharge trigger for starting the discharge of the battery 204. This discharge trigger is not particularly limited. For example, when operating a mass control in a railway, when stepping on an accelerator in a car, when a timer is activated in a distributed power storage system, when a user presses a button switch Etc., which are emitted at an arbitrary timing.

  When the arithmetic processing unit 205 receives the generated discharge trigger, the arithmetic processing unit 205 transmits a discharge command signal for discharging the battery 204 to the charge / discharge control circuit 203 (step S101). Then, after the charge / discharge control circuit 203 receives the discharge command signal, the charge / discharge control circuit 203 starts discharging the battery 204. The discharge trigger may be transmitted directly to the charge / discharge control circuit 203 instead of the arithmetic processing unit 205, and the discharge may be started without going through the arithmetic processing unit 205.

  After the battery 204 starts discharging, the discharge end voltage correction unit 202 measures the potentials of the positive electrode and the negative electrode of the battery 204 at a predetermined timing. The measured potentials of the positive electrode and the negative electrode can be recorded in the storage unit when the storage unit is provided, for example. Any known method can be used to measure the potential. For example, [1. The method described in “Configuration of Discharge Control System According to Present Embodiment]” may be performed. The timing of measuring the potential of the positive electrode and the negative electrode is not particularly limited. However, if the measurement time interval is too long, the dead time after the battery voltage of the battery 204 reaches the discharge end voltage V0 and the discharge is stopped. There is a possibility of increasing. Further, when the measurement time interval is too short, there is a possibility that the load on the end-of-discharge voltage correction unit 202 becomes excessive in order to perform measurement excessively.

  As described in FIG. 2, when the discharge end voltage correction unit 202 detects that the battery voltage of the battery 204 measured by the voltage measurement unit 209 becomes the discharge end voltage V0 (step S102), the discharge end voltage correction unit. 202 again measures the positive electrode potential Vp of the battery 204 (step S103).

  When the measured positive electrode potential Vp is less than the preset control value α (ie, Vp <α) (No direction in step S104), the discharge end voltage correction unit 202 performs discharge end determination (step S106), and the calculation is performed. A discharge end signal is transmitted to the charge / discharge control circuit 203 via the processing unit 205, and the discharge of the battery 204 is completed.

On the other hand, when the measured positive electrode potential Vp is greater than or equal to the preset management value α (ie, Vp ≧ α) (Yes direction in step S104), the battery voltage of the battery 204 is the battery voltage V2 (V2 = V0−nω). Until it becomes, the charge / discharge control circuit 203 further discharges the battery 204 (step S105). At this time, V2 is a battery voltage for measuring the next positive electrode potential, and ω is a step size of the battery voltage for measuring and calculating the next positive electrode potential. Such a management value ω is usually set within a range of 0.1 mV or more and usually 100 mV or less. Further, n is an integer of 1 or more, and is a value that increases by 1 as the number of times of calculation of the positive electrode potential Vp increases. That is, in S105, the first discharge is performed to V2 where V2 = V0−ω, the second discharge is performed to V2 where V2 = V0−2ω, and the third discharge is performed to V2 where V2 = V0−3ω. It has become. The same applies to the fourth and subsequent times.
Then, when the battery voltage of the battery 204 reaches the battery voltage V2, the positive electrode potential Vp is calculated again (step S103). If the positive electrode potential Vp is less than α (No direction of step S104), the same as the above case. Then, the discharge ends (step S106). If the positive electrode potential Vp is greater than or equal to α (Yes direction in step S104), the same steps are repeated until the positive electrode potential Vp becomes less than α, and the finally measured positive electrode potential Vp is set in advance. Discharge is carried out to a value α or more (in terms of battery voltage, corrected discharge end voltage V1), and the discharge ends (step S106).

  Further, in the flowchart shown in FIG. 3, it is determined whether or not to change the discharge end voltage V0 using the positive electrode potential, but instead of using the positive electrode potential, the battery life and the deterioration degree of the battery are used as judgment criteria. Whether or not to change the corrected discharge final voltage V1 may be determined by using the negative electrode potential in addition to the positive electrode potential.

  FIG. 4 is a flowchart showing a discharge control method in the discharge control system according to the second embodiment. Since S201 to S204 in FIG. 4 represent the same as S101 to S104 in FIG.

  In the embodiment shown in FIG. 4 as well, as in the first embodiment shown in FIG. 3, the discharge end voltage correction unit 202 determines whether or not the positive electrode potential Vp is greater than or equal to a predetermined management value α in step S204. Determination is made (step S204). When the positive electrode potential Vp is less than the preset management value α (that is, Vp <α) (No direction in step S204), the discharge end voltage correction unit 202 performs a discharge end determination (step S207), and the arithmetic processing A discharge end signal is transmitted to the charge / discharge control circuit 203 via the unit 205, and the discharge of the battery 204 is completed.

  On the other hand, when the measured positive electrode potential Vp is equal to or greater than the preset management value α (ie, Vp ≧ α) (Yes direction in step S204), the discharge end voltage correction unit 202 calculates the negative electrode potential Vn. Then, the discharge end voltage correction unit 202 determines whether or not the negative electrode potential Vn is equal to or lower than a predetermined management value γ (ie, Vn ≦ γ) (step S205). When the negative electrode potential Vn is larger than γ (No direction in step S205), the discharge end determination is made in the same manner as in step S106 (step S207).

  On the other hand, when the negative electrode potential Vn is equal to or lower than the control value γ (Yes in step S205), the charge / discharge control circuit 203 operates until the battery voltage of the battery 204 reaches the battery voltage V2 (V2 = V0−nω). Is further discharged (step S206). These V0, V2, n, and ω are the same as those described above.

  When the battery voltage of the battery 204 reaches the battery voltage V2, the positive electrode potential Vp is calculated again (step S203). If the positive electrode potential Vp is less than α (No direction in step S204), the same as the above case. Then, the discharge is completed (step S207). When the positive electrode potential Vp is greater than or equal to α (Yes direction in step S204), the positive electrode potential Vp is less than α, or the negative electrode potential Vn is greater than γ even if the positive electrode potential Vp is greater than or equal to α. The same steps are repeated until the finally measured positive electrode potential Vp is greater than or equal to a preset management value α or the negative electrode potential Vn is greater than γ (in terms of battery voltage, up to the corrected discharge end voltage V1). Discharging is performed and the discharging ends (step S207).

  In the flowcharts shown in FIG. 3 and FIG. 4, the discharge is automatically terminated based on the values of V0, V2, α, γ, and ω. The charge / discharge control circuit 203 may be directly instructed to end the discharge. The method for instructing the end of discharge by the user is not particularly limited. For example, the discharge can be ended by pressing an arbitrary button or the like. Further, when the user directly instructs the end of the discharge, step S106 or step S207 is reached regardless of what the current step is.

  As described above with reference to FIGS. 3 and 4, the discharge control system 201 according to the present embodiment has the above-described configuration, and each part operates as described above. It is possible to provide a discharge control system capable of improving cycle characteristics such as suppressing an increase in internal resistance and suppressing a decrease in discharge capacity.

[3. Configuration of Battery to which Discharge Control System According to this Embodiment is Applicable]
The discharge control system according to the present embodiment can be applied to any known lithium ion secondary battery. Hereinafter, a battery to which the discharge control system according to this embodiment can be applied (hereinafter referred to as “battery according to this embodiment” as appropriate) will be described with reference to FIG. However, the configuration of the battery shown in FIG. 5 is merely an example of the internal structure of the lithium ion secondary battery to which the discharge control system according to this embodiment can be applied, and the battery to which the discharge control system according to this embodiment can be applied. Is not limited to that shown in FIG.

  FIG. 5 is a diagram schematically showing the internal structure of the battery according to the present embodiment. A battery 1 according to this embodiment shown in FIG. 5 includes a positive electrode 10, a separator 11, a negative electrode 12, a battery container (that is, a battery can) 13, a positive electrode current collecting tab 14, a negative electrode current collecting tab 15, an inner lid 16, and an internal pressure release valve. 17, a gasket 18, a positive temperature coefficient (PTC) resistance element 19, a battery lid 20, and an axis 21. The battery lid 20 is an integrated part composed of the inner lid 16, the internal pressure release valve 17, the gasket 18, and the PTC resistance element 19. A positive electrode 10, a separator 11, and a negative electrode 12 are wound around the shaft center 21.

The positive electrode 10 includes a positive electrode active material, a conductive agent, a binder, and a current collector. Illustrative examples of the positive electrode active material include LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O ₄ . In addition, LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ , Li ₄ Mn ₅ O ₁₂ , LiMn _2−x M _x O ₂ (however, selected from the group consisting of M = Co, Ni, Fe, Cr, Zn, Ti) Li ₂ Mn ₃ MO ₈ (however, at least one selected from the group consisting of M = Fe, Co, Ni, Cu, Zn), Li _{1− xA x} Mn ₂ O ₄ (where A = Mg, B, Al, Fe, Co, Ni, Cr, Zn, Ca, at least one selected from the group consisting of x, 0.01 to 0.1), _{LiNi 1-x M x O 2} ( however, M = Co, Fe, at least one selected from the group consisting of _{Ga, x = 0.01~0.2), LiFeO} 2, Fe 2 (SO 4) 3, _{LiCo 1-x M x O 2} ( where M = Ni, Fe, at least one selected from the group consisting of _{Mn, x = 0.01~0.2), LiNi} 1-x M x O 2 ( however, M = Mn, Fe, Co , Al, Ga , Ca, Mg, at least one selected from the group consisting of x, 0.01 to 0.2), Fe (MoO ₄ ) ₃ , FeF ₃ , LiFePO ₄ , LiMnPO _{4 and the} like.
Among the above, the positive electrode 10 preferably contains a lithium composite oxide represented by the following formula (1), and particularly contains LiNi _1/3 Mn _1/3 Co _1/3 O _2. preferable.
_{LiNi a Mn b Co c M d} O 2 ··· (1)
(In the above formula (1), M represents at least one selected from the group consisting of Fe, V, Ti, Cu, Al, Sn, Zn, Mg, B and W, and a, b, c and d are Are values satisfying 0.2 ≦ a ≦ 0.8, 0.1 ≦ b ≦ 0.4, 0 ≦ c ≦ 0.4, 0 ≦ d ≦ 0.1, and a + b + c + d = 1. Note that, in each of the above examples, for example, there are characters that are duplicated in each of the examples such as “M” and “x”, but these characters are independent in each example. It shall be. The same applies to the following description unless otherwise specified.

  The particle size of the positive electrode active material is usually defined so as to be equal to or less than the thickness of the mixture layer formed from the positive electrode active material, the conductive agent, and the binder. When there are coarse particles having a size equal to or greater than the thickness of the mixture layer in the positive electrode active material powder, the coarse particles can be removed in advance by sieving classification or wind classification to produce particles having a thickness of the mixture layer thickness or less. preferable.

  In addition, since the positive electrode active material is generally oxide-based and has high electrical resistance, a conductive agent made of carbon powder for supplementing electrical conductivity is used. Since both the positive electrode active material and the conductive agent are usually powders, a binder can be mixed with the powders, and the powders can be bonded together and simultaneously bonded to the current collector.

  For the current collector of the positive electrode 10, an aluminum foil having a thickness of 10 to 100 μm, an aluminum perforated foil having a thickness of 10 to 100 μm and a pore diameter of 0.1 to 10 mm, an expanded metal, or a metal foam plate is used. . In addition to aluminum, materials such as stainless steel and titanium are also applicable. In the present invention, any current collector can be used without being limited by the material, shape, manufacturing method and the like.

  A positive electrode slurry in which a positive electrode active material, a conductive agent, a binder, and an organic solvent are mixed is attached to a current collector by a doctor blade method, a dipping method, or a spray method, and then the organic solvent is dried and applied by a roll press. The positive electrode 10 can be produced by pressure forming. In addition, a plurality of mixture layers can be laminated on the current collector by performing a plurality of times from application to drying.

The negative electrode 12 includes a negative electrode active material, a binder, and a current collector. When high rate charge / discharge is required, a conductive agent may be added. Examples of the negative electrode active material that can be used in the present invention include graphite, non-graphite carbon, metals such as aluminum, silicon, and tin, and alloys thereof, lithium-containing transition metal nitride Li _(3-x) M _x N, silicon The lower oxide Li _x SiO _y (0 ≦ x, 0 <y <2) and the tin lower oxide Li _x SnO _y are selected from materials that form an alloy with lithium or materials that form an intermetallic compound. be able to.

  The material of the negative electrode active material is not particularly limited and can be used other than the above materials. However, when a part of the material such as a material having a large expansion and contraction is selected, if the range used by the negative electrode is too large, Resistance rise may be large. In this case, it is preferable to confirm whether or not the negative electrode potential is below a certain level as a condition for changing the battery voltage.

However, the negative electrode 12 preferably contains graphite, and the graphite preferably has a graphite interlayer distance (d ₀₀₂ ) of 0.335 nm or more and 0.338 nm or less. By including such graphite in the negative electrode 12, many reduction reactions occur at the negative electrode, so that the cycle characteristics of the lithium ion secondary battery can be further improved. The graphite used for the negative electrode 12 is natural graphite, artificial graphite, mesophase carbon, expanded graphite, carbon fiber, vapor grown carbon fiber, pitch-based carbonaceous material, needle coke capable of chemically occluding and releasing lithium ions. , Petroleum coke, and polyacrylonitrile-based carbon fiber. Note that the graphite interlayer distance _{(d 002)} can be measured using the XRD (X-ray powder diffractometry) (X-Ray Diffraction Method) or the like.

  The non-graphite carbon used for the negative electrode 12 is a carbon material excluding the above graphite, and can occlude or release lithium ions. This includes a carbon material that has a graphite layer spacing of 0.34 nm or more and changes to graphite by high-temperature heat treatment at 2000 ° C. or more, a 5-membered or 6-membered cyclic hydrocarbon, or a cyclic oxygen-containing material. Amorphous carbon materials synthesized by pyrolysis of organic compounds are included.

As described above, a material that forms an alloy with lithium or a material that forms an intermetallic compound may be added to the negative electrode 12 having a voltage change rate different from that of the positive electrode 10 as a third negative electrode active material. Examples of the third negative electrode active material include metals such as aluminum, silicon, and tin and alloys thereof, lithium-containing transition metal nitrides Li _(3-x) M _x N, and silicon lower oxide Li _x SiO _y ( 0 ≦ x, 0 <y <2), and lower oxide of tin Li _x SnO _y . There is no restriction | limiting in particular in the material of a 3rd negative electrode active material, It can utilize also other than said material.

  Since the negative electrode active material generally used is a powder, a binder is mixed with the negative electrode active material so that the powders are bonded together and simultaneously bonded to the current collector. In the negative electrode 12 included in the battery according to the present embodiment, it is desirable that the particle size of the negative electrode active material be equal to or less than the thickness of the mixture layer formed from the negative electrode active material and the binder. When there are coarse particles having a size equal to or greater than the thickness of the mixture layer in the negative electrode active material powder, the coarse particles may be removed in advance by sieving classification or wind classification, and particles having a thickness of the mixture layer thickness or less may be used. preferable.

  For the current collector of the negative electrode 12, a copper foil having a thickness of 10 to 100 μm, a copper perforated foil having a thickness of 10 to 100 μm and a pore diameter of 0.1 to 10 mm, an expanded metal, a foam metal plate, or the like is used. In addition to copper, materials such as stainless steel, titanium, or nickel are also applicable. In the present invention, any current collector can be used without being limited by the material, shape, manufacturing method and the like.

  A negative electrode slurry in which a negative electrode active material, a binder, and an organic solvent are mixed is attached to a current collector by a doctor blade method, a dipping method, a spray method, or the like, and then the organic solvent is dried and pressure-molded by a roll press. Thereby, the negative electrode 12 can be produced. Moreover, it is also possible to form a multilayer mixture layer on a current collector by carrying out a plurality of times from application to drying.

  The separator 11 is inserted between the positive electrode 10 and the negative electrode 12 produced by the above method to prevent a short circuit between the positive electrode 10 and the negative electrode 12. The separator 11 can be a polyolefin polymer sheet made of polyethylene, polypropylene, or the like, or a two-layer structure in which a polyolefin polymer and a fluorine polymer sheet typified by tetrafluoropolyethylene are welded. It is. A mixture of ceramics and a binder may be formed in a thin layer on the surface of the separator 11 so that the separator 11 does not shrink when the battery temperature increases. Since these separators 11 need to allow lithium ions to permeate during charging and discharging of the battery, they can generally be used for lithium ion secondary batteries if the pore diameter is 0.01 to 10 μm and the porosity is 20 to 90%. It is.

  Such a separator 11 is inserted between the positive electrode 10 and the negative electrode 12, and an electrode group wound around the axis 21 is produced. As the axis 21, any known one can be used as long as it can support the positive electrode 10, the separator 11, and the negative electrode 12. In addition to the cylindrical shape shown in FIG. 5, the electrode group has various shapes such as a stack of strip electrodes, or a positive electrode 10 and a negative electrode 12 wound in an arbitrary shape such as a flat shape. Can do. The shape of the battery case 13 may be selected from shapes such as a cylindrical shape, a flat oval shape, a flat oval shape, and a square shape according to the shape of the electrode group.

  The material of the battery case 13 is selected from materials that are corrosion-resistant to the nonaqueous electrolyte, such as aluminum, stainless steel, and nickel-plated steel. Further, when the battery container 13 is electrically connected to the positive electrode 10 or the negative electrode 12, the material is not deteriorated due to corrosion of the battery container 13 or alloying with lithium ions in the portion in contact with the nonaqueous electrolyte. Thus, the material of the battery container 13 is selected.

  The electrode group is housed in the battery container 13, the negative electrode current collecting tab 15 is connected to the inner wall of the battery container 13, and the positive electrode current collecting tab 14 is connected to the bottom surface of the battery lid 20. The electrolyte is injected into the battery container interior 13 before the battery is sealed. As a method for injecting the electrolyte, there are a method of adding directly to the electrode group in a state where the battery cover 20 is released, or a method of adding from an injection port installed in the battery cover 20.

  Thereafter, the battery lid 20 is brought into close contact with the battery container 13 to seal the entire battery. If there is an electrolyte inlet, seal it as well. As a method for sealing the battery, there are known techniques such as welding and caulking.

As a typical example of the electrolytic solution that can be used in the present invention, lithium hexafluorophosphate (LiPF ₆ ) or lithium borofluoride as an electrolyte in a solvent obtained by mixing dimethyl carbonate, diethyl carbonate, or ethyl methyl carbonate with ethylene carbonate There is a solution in which (LiBF ₄ ) is dissolved. The present invention is not limited to the type of solvent and electrolyte, and the mixing ratio of solvents, and other electrolytes can be used.

  Examples of non-aqueous solvents that can be used in the electrolyte include propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, 1,2-dimethoxyethane, 2 -Methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, methyl propionate, ethyl propionate, phosphate triester, trimethoxymethane, dioxolane, diethyl ether, sulfolane, 3-methyl-2- There are nonaqueous solvents such as oxazolidinone, tetrahydrofuran, 1,2-diethoxyethane, chloroethylene carbonate, or chloropropylene carbonate. Other solvents may be used as long as they do not decompose on the positive electrode 10 or the negative electrode 12 incorporated in the battery of the present invention.

In addition, examples of the _{electrolyte, LiPF 6, LiBF 4, LiClO} 4, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF 6, LiSbF 6, or imide salts such as lithium represented by lithium trifluoromethane sulfonimide, multi There are different types of lithium salts. A nonaqueous electrolytic solution obtained by dissolving these salts in the above-mentioned solvent can be used as a battery electrolytic solution. An electrolyte other than this may be used as long as it does not decompose on the positive electrode 10 and the negative electrode 12 included in the battery according to the present embodiment.

  When a solid polymer electrolyte (polymer electrolyte) is used, an ion conductive polymer such as polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, polymethyl methacrylate, polyhexafluoropropylene, and polyethylene oxide can be used for the electrolyte. When these solid polymer electrolytes are used, there is an advantage that the separator 11 can be omitted.

Furthermore, an ionic liquid can be used. For example, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF ₄ ), lithium salt LiN (SO ₂ CF ₃ ) ₂ (LiTFSI), a mixed complex of triglyme and tetraglyme, a cyclic quaternary ammonium cation (N— methyl-N-propylpyrrolidinium is exemplified) and an imide anion (example is bis (fluorsulfonyl) imide), and a combination that does not decompose at the positive electrode 10 and the negative electrode 12 is selected and this embodiment is adopted. It can be used for such a battery.

  The lithium ion secondary battery having the above configuration is used as the battery 204, and the discharge control system according to this embodiment can be applied to such a battery.

  Hereinafter, the present embodiment will be described in more detail with reference to examples. However, the present embodiment is not limited to the following examples, and may be arbitrarily modified without departing from the scope of the present invention. Can do.

[Example 1]
First, a lithium ion secondary battery (battery) having the structure shown in FIG. 5 was produced. At this time, LiNi _1/3 Mn _1/3 Co _1/3 O ₂ was used as the positive electrode active material, and natural graphite (graphite interlayer distance (d ₀₀₂ ) = 0.336 nm by X-ray structural analysis) was used as the negative electrode active material. . Further, an aluminum foil was used as the positive electrode, and a copper foil was used as the negative electrode.

  Using the produced battery as the battery 204, the discharge control system 201 shown in FIG. 1 was applied, and the battery characteristics were evaluated by the following method.

  The manufactured battery was charged to 4.0 V at a current equivalent to 0.3 C around room temperature (25 ° C.), and then constant voltage charging was performed until the current became 0.03 C at 4.0 V. After a 30-minute pause, constant current discharge was performed to 3.60 V with a constant current corresponding to 0.3 C. This was initialized by performing four cycles, and the discharge capacity at the fourth cycle was measured, and the measured discharge capacity was defined as the initial discharge capacity. The initial discharge capacity was 0.920 Ah.

  Next, it discharged at 50 degreeC for 10 second in order of electric current 4CA, 8CA, 12CA, and 16CA. The relationship between the discharge current at that time and the voltage at 10 seconds was plotted, and the initial DC resistance was obtained from the slope of the obtained straight line. The measured initial DC resistance was 54.2 mΩ.

  Next, 1000 charge / discharge cycles were performed at 50 ° C. In each cycle, the battery was charged to 4.0 V with a current corresponding to 0.3 C, and then constant voltage charging was performed until the current became 0.03 C at 4.0 V. Thereafter, after resting for 30 minutes, constant current discharge was performed at a constant current corresponding to 0.3C. Discharging was performed according to the flowchart shown in FIG. 3, and the final discharge voltage V0 = 3.60V, α = 3.75V, and ω = 0.01V. Note that the value of α was the potential of the positive electrode when the value of DC resistance of the positive electrode increased by 30% at a charge depth of 50% of the battery.

  When the discharge capacity of the battery after 1000 cycles was measured, it was 0.842 Ah. Further, when the DC resistance after 1000 cycles was determined in the same manner as in the measurement of the initial DC resistance, it was 66.7Ω.

Using the results obtained above, the discharge capacity retention rate and the DC resistance increase rate were calculated according to the following formula. The results are shown in Table 1 below.
Discharge capacity retention rate (%) = (discharge capacity after 1000 cycles) / (initial discharge capacity)
DC resistance increase rate (%) = (DC resistance after 1000 cycles) / (initial DC resistance)

[Example 2]
The discharge capacity retention rate and the DC resistance increase rate were calculated in the same manner as in Example 1 except that discharging was performed according to the flowchart shown in FIG. 4 in 1000 charge / discharge cycles. The results are shown in Table 1. During discharge, γ = 0.25 V, the initial discharge capacity was 0.92 Ah, the initial DC resistance was 54.2 mΩ, the discharge capacity after 1000 cycles was 0.831 Ah, and the DC resistance after 1000 cycles was 64.0 mΩ. It was.

[Comparative Example 1]
In the 1000 charge / discharge cycles, the discharge capacity retention rate and the DC resistance increase rate were calculated in the same manner as in Example 1 except that the discharge end voltage was not changed and the discharge end voltage was kept constant at 3.6V. . The results are shown in Table 1. The initial discharge capacity was 0.920 Ah, the initial DC resistance was 54.2 mΩ, the discharge capacity after 1000 cycles was 0.713 Ah, and the DC resistance after 1000 cycles was 64.0 mΩ.

  As shown in Table 1, in Examples 1 and 2 in which the discharge end voltage was changed, the DC resistance increase rate was maintained at the same level as that in Comparative Example 1 in which the discharge end voltage was not changed. However, the discharge capacity maintenance rate was greatly improved. In particular, when the discharge end voltage was changed based on both the positive electrode potential and the negative electrode potential (Example 2), the DC resistance increase rate showed the same value as that of Comparative Example 1, but Comparative Example 1 Compared to the above, the discharge capacity retention rate was particularly high.

  As described above, according to the present invention, even after 1000 cycles of charge and discharge, it is possible to suppress an increase in resistance inside the lithium ion secondary battery and a decrease in discharge capacity, and to improve cycle characteristics. A discharge control system can be provided.

DESCRIPTION OF SYMBOLS 1 Lithium ion secondary battery 10 Positive electrode 11 Separator 12 Negative electrode 13 Battery container 14 Positive electrode current collection tab 15 Negative electrode current collection tab 16 Inner cover 17 Internal pressure release valve 18 Gasket 19 Positive temperature coefficient resistance element 20 Battery cover 21 Shaft center 101 Positive electrode potential curve DESCRIPTION OF SYMBOLS 102 Negative electrode potential curve 103 Battery voltage curve 104 Negative electrode potential curve 105 after battery deterioration Battery voltage curve 201 after battery deterioration Discharge control system 202 End-of-discharge voltage correction unit 203 Charge / discharge control circuit 204 Lithium ion secondary battery (battery)
205 arithmetic processing unit 206 external load 207 charging power source 208 current measuring unit 209 voltage measuring unit 210a switch 210b switch

Claims (7)

A discharge control system for controlling discharge of a lithium ion secondary battery having a positive electrode and a negative electrode,
A discharge end voltage correcting unit for changing the discharge end voltage of the lithium ion secondary battery to a corrected discharge end voltage;
A charge / discharge control circuit for discharging the lithium ion secondary battery;
With
After the charge / discharge control circuit starts discharging the lithium ion secondary battery, when the potential difference between the positive electrode and the negative electrode becomes the discharge end voltage of the lithium ion secondary battery, the discharge end voltage correcting unit When it is detected that the end voltage change condition is satisfied, the discharge end voltage correction unit changes the discharge end voltage to the corrected discharge end voltage, and the charge / discharge control circuit detects that the potential difference is the corrected discharge end. There are more rows of discharge of the lithium ion secondary battery until a voltage,
The discharge end voltage changing condition includes that the potential of the positive electrode is a predetermined value or more,
The potential of the positive electrode when the predetermined value increases by a preset value within a range of 5% or more and 100% or less of the DC resistance value of the positive electrode at a charge depth of 50% of the lithium ion secondary battery. A discharge control system, characterized in that 2. The discharge control system according to claim 1, wherein the discharge end voltage change condition includes a preset first reference potential of the negative electrode or less. 3.   The material constituting the negative electrode is natural graphite,
  The first reference potential of the negative electrode is 1.0V
The discharge control system according to claim 2, wherein: The discharge end voltage change condition, characterized in that it is intended to include that a preset positive electrode and the negative second reference potential difference more poles, discharge according to any one of claims 1 to 3 Control system.   The material constituting the positive electrode is LiNi _1/3 Mn _1/3 Co _1/3 O ₂ And the material constituting the negative electrode is natural graphite,
  The second reference potential difference between the positive electrode and the negative electrode is 2.5V.
The discharge control system according to claim 4, wherein: The discharge control system according to any one of claims 1 to 5, wherein the positive electrode contains a lithium composite oxide represented by the following formula (1).
_{LiNi a Mn b Co c M d} O 2 ··· (1)
(In the above formula (1), M represents at least one selected from the group consisting of Fe, V, Ti, Cu, Al, Sn, Zn, Mg, B and W, and a, b, c and d are Are values satisfying 0.2 ≦ a ≦ 0.8, 0.1 ≦ b ≦ 0.4, 0 ≦ c ≦ 0.4, 0 ≦ d ≦ 0.1, and a + b + c + d = 1. There is a relationship that satisfies.) The negative electrode includes graphite;
The discharge control system according to any one of claims 1 to 6, wherein a graphite interlayer distance of the graphite is 0.335 nm or more and 0.338 nm or less.

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