JP6437407B2 - Battery pack and charge control method - Google Patents

Description

  The present invention relates to a battery pack and a charge control method for a non-aqueous electrolyte secondary battery.

  Rechargeable batteries can be used repeatedly for charging and discharging, helping to reduce waste, as well as portable devices that cannot use alternating current (AC) power, and backup power when AC power is cut or stopped. Widely used. In recent years, expansion of the usage range of secondary batteries, such as in-vehicle applications, backup of solar cells, and power leveling applications, has been studied. Along with this, demands for improving the performance, such as capacity, temperature characteristics, safety, etc. of secondary batteries are increasing.

Among the secondary batteries, the non-aqueous electrolyte secondary battery is a secondary battery that performs charge and discharge by movement between positive and negative electrodes of lithium ions. Such a non-aqueous electrolyte secondary battery is characterized in that a voltage higher than that of a nickel cadmium secondary battery or a nickel hydride secondary battery using an aqueous solution can be obtained because an organic solvent is used for the electrolyte. Currently, non-aqueous electrolyte secondary batteries in practical use use lithium-containing cobalt composite oxide or lithium-containing nickel composite oxide as the positive electrode active material, and use carbon-based material or titanium-containing oxide as the negative electrode active material. A lithium salt such as LiPF ₆ or LiBF ₄ is used as an electrolyte dissolved in an organic solvent such as cyclic carbonate or chain carbonate. The positive electrode active material has an average operating potential of about 3.4 to 3.8 V with respect to the lithium metal potential, and a maximum ultimate potential during charging of 4.1 to 4.3 V. On the other hand, in the case of the carbon-based material that is the negative electrode active material, the average operating potential is about 0.05 to 0.5 V with respect to the lithium metal potential. Moreover, it is 1.55V in the most typical lithium titanate (Li ₄ Ti ₅ O ₁₂ ) among the titanium-containing composite oxides. By combining these positive and negative electrode materials, the battery voltage becomes 2.2 to 3.8 V, and the maximum charging voltage becomes 2.7 to 4.3 V.

  Secondary batteries using a titanium-containing oxide as the negative electrode can increase the charge / discharge cycle life, and such batteries have been put into practical use. However, in conventional portable device applications, a lifetime of 2 to 3 years is required, whereas secondary batteries are required to have a lifetime of 10 years or more for in-vehicle applications and power generation-related stationary applications. Correspondingly, it is necessary to further improve the life of these secondary batteries.

  The most typical cycle performance in the life performance is represented by a decrease in battery capacity when charging and discharging are repeated. The number of repeated charging and discharging in the 2 to 3 years until the secondary battery for portable equipment reaches the end of its life is about several hundred times, and in 10 years or more when the secondary battery for in-vehicle / stationary use is continued. The number of repeated charging and discharging reaches several thousand to 10,000 times or more.

  The main cause of cycle deterioration in a battery using the titanium-containing oxide as a negative electrode is not clear. In the case of a battery using a titanium-containing oxide as a negative electrode, the operating potential is different from a battery using a conventional carbon-based material as a negative electrode, and there is a sudden change in the negative electrode potential at the end of charging. No noticeable film formation on the surface of the negative electrode as seen in the negative electrode containing benzene has been reported. From these things, it is estimated that the titanium-containing oxide as the negative electrode active material may have a different deterioration mechanism.

In addition to the most frequently reported spinel type lithium titanate (for example, Li ₄ Ti ₅ O ₁₂ ; hereinafter referred to as LTO), titanium-containing oxides include monoclinic titanium dioxide (TiO ₂ ) and niobium-titanium composites. An oxide (for example, Nb ₂ TiO ₇ ; hereinafter referred to as NTO) exists. Although these titanium-containing oxides share characteristics different from those of the carbon-based active material as described above, it is considered that there are also differences due to differences in crystals and elemental configurations. Up to now, long life has been reported in batteries using LTO among such titanium-containing oxides, and it is becoming possible to repeatedly charge and discharge about several thousand times. Nevertheless, further improvement is required. Further, with TiO ₂ and NTO, a larger capacity can be obtained, but the cycle life is inferior compared with LTO. Therefore, in a battery using TiO ₂ or NTO, improvement of life performance is a more important issue.

JP2013-243823A

  It is an object of the present invention to provide a battery pack that includes a nonaqueous electrolyte secondary battery including a negative electrode including a titanium-containing oxide and can realize high cycle performance.

  According to the embodiment, a battery pack including a nonaqueous electrolyte secondary battery, a measurement unit, a calculation unit, and a charge / discharge controller is provided. The nonaqueous electrolyte secondary battery included in the battery pack includes a positive electrode, a negative electrode including a titanium-containing composite oxide, a nonaqueous electrolyte, and a separator. The measurement unit measures the voltage of the nonaqueous electrolyte secondary battery at regular intervals when the nonaqueous electrolyte secondary battery is charged or discharged. The calculation unit calculates a voltage change per time during charging or discharging based on the output value output from the measuring unit, and based on this voltage change, a charging curve during charging or a discharging curve during discharging The potential position of one or more voltage flat portions in is calculated. The charge / discharge controller lowers the maximum charge voltage below the specified value by the same value as the potential drop amount whose potential position has dropped from the reference potential.

  Another embodiment provides a charge control method in a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode including a titanium-containing composite oxide, a non-aqueous electrolyte, and a separator. In this charge control method, the maximum charge voltage is lowered from a specified value. The maximum charging voltage is reduced by the same value as the voltage reduction amount from one of the potential positions of one or more voltage flat portions in the charging curve for discharging the nonaqueous electrolyte secondary battery or the discharging curve for discharging. Let

The block diagram of the control circuit of the battery pack of an example which concerns on embodiment. The graph which shows the charge curve of the nonaqueous electrolyte secondary battery which concerns on embodiment. The graph which shows the differential curve of the electric current calculated from the charge curve of the nonaqueous electrolyte secondary battery which concerns on embodiment, and a voltage. 1 is a cross-sectional view schematically showing a nonaqueous electrolyte secondary battery according to an embodiment. The expanded sectional view of the A section of FIG.

  Hereinafter, embodiments will be described with reference to the drawings.

(First embodiment)
[Battery pack]
The battery pack of the embodiment includes a non-aqueous electrolyte secondary battery, a measurement unit, a calculation unit, and a charge / discharge controller. The nonaqueous electrolyte secondary battery includes a positive electrode, a negative electrode including a titanium-containing composite oxide, a nonaqueous electrolyte, and a separator. The measurement unit measures the voltage of the non-aqueous electrolyte secondary battery at regular time intervals when charging and discharging the non-aqueous electrolyte secondary battery, and transmits the measurement result as an output value to the calculation unit. The calculation unit calculates a voltage change per time during charging / discharging based on the output value transmitted from the measurement unit, and derives the potential position of the voltage flat part in the charging / discharging curve based on the voltage change. The charge / discharge controller calculates the amount by which the potential position has decreased with respect to the potential set as a reference, and resets the maximum charge voltage to a value that is decreased from the specified value by the same amount as the potential decrease amount.

  That is, in the said battery pack, charge control is implemented with respect to the nonaqueous electrolyte secondary battery contained there by operation | movement of a measurement part, a calculating part, and a charge / discharge controller. Here, it can be said that the measurement unit, the calculation unit, and the charge / discharge controller constitute a charge control mechanism for performing charge control. In the charge control method implemented in the battery pack, the maximum charge voltage that can be achieved by the nonaqueous electrolyte secondary battery during charging is corrected. Specifically, the maximum charging voltage is lower than the specified value by the amount by which the voltage of the voltage flat portion in the charge / discharge curve obtained when charging or discharging the nonaqueous electrolyte secondary battery is lower than the reference voltage. To be corrected. By carrying out like this, it can charge using the charging voltage suitable for the occasional state of a nonaqueous electrolyte secondary battery.

  By using such a charge control method, the balance fluctuation between the negative electrode containing a titanium-containing oxide (hereinafter referred to as titanium-containing negative electrode) contained in the nonaqueous electrolyte secondary battery and the positive electrode can be minimized. . Thereby, the cycle performance of the non-aqueous electrolyte secondary battery can be extended.

  At the interface between the electrode and the non-aqueous electrolyte, side reactions between the electrode surface and the non-aqueous electrolyte may occur in addition to the insertion and release of lithium ions by the electrode material. In a negative electrode containing a carbon-based active material (hereinafter referred to as a carbon-based negative electrode), a remarkable film is formed on the surface, and such side reactions are suppressed. However, in the titanium-containing negative electrode, there is a tendency that a remarkable surface coating is not formed as in the case where the carbon-based active material as described above is used. For this reason, in a battery including a titanium-containing negative electrode, the capacity balance between the positive and negative electrodes tends to fluctuate due to side reactions. In such a non-aqueous electrolyte secondary battery, when charge / discharge control is performed using the voltage between the positive and negative electrodes as an index, the individual potential applied to the positive and negative electrodes may deviate from the original design range. is assumed.

Such behavior is presumed to be remarkably exhibited in a battery containing LTO as a negative electrode active material, which is considered to have very little surface coating or is not generated. Further, TiO ₂ and NTO have a higher reactivity on the surface of the active material than LTO, so that it is presumed that the TiO ₂ and NTO are particularly prominent in the initial charge / discharge.

  In the carbon-based negative electrode, the reaction at the negative electrode shifts from lithium occlusion to the carbon-based material to generation of lithium metal at the end of charging, and the potential of the negative electrode gradually converges to the lithium metal potential. On the other hand, in the titanium-containing negative electrode, there is a rapid decrease in potential at the end of charging. Therefore, when the balance between the positive and negative electrodes is lost, the negative electrode potential may decrease more than expected, and conversely, the positive electrode potential may increase, which may cause collapse of the positive and negative electrode crystal structure and deterioration due to another side reaction. There is.

  As a result of investigating behaviors peculiar to batteries using such a titanium-containing negative electrode, when the positive / negative electrode balance is lost as described above, a charge curve (charge curve) or discharge curve showing the transition of the battery voltage during charge / discharge of the battery It turned out that the voltage flat part in (discharge curve) moves. It has also been found that controlling the maximum voltage during charging in accordance with the amount of movement of the voltage flat portion effectively corresponds to controlling the individual potentials of the positive and negative electrodes. In view of this, by performing charge control of the battery including the titanium-containing negative electrode, it is possible to suppress the above-described collapse of the positive and negative electrode crystal structure and deterioration due to another side reaction, and to improve the cycle life.

  FIG. 1 shows a block diagram of a control circuit of an example of a battery pack. In addition, the aspect of the nonaqueous electrolyte secondary battery of embodiment is not restricted to FIG. For example, the connection order of each part shown in FIG. 1 can be changed as needed.

  As shown in FIG. 1, a plurality of nonaqueous electrolyte secondary batteries 1 are electrically connected in series. Details of the nonaqueous electrolyte secondary battery 1 will be described later. In FIG. 1, three non-aqueous electrolyte secondary batteries 1 are connected in series, but the number of non-aqueous electrolyte secondary batteries 1 is not limited as long as the number is one or more. Further, the plurality of nonaqueous electrolyte secondary batteries may be connected in parallel. Alternatively, the plurality of nonaqueous electrolyte secondary batteries may be connected in combination of a series arrangement and a parallel arrangement.

  As shown in FIG. 1, the measurement unit 2 is electrically connected to the negative electrode side and the positive electrode side of the nonaqueous electrolyte secondary battery 1 by wiring 8. As shown in the figure, when there are a plurality of nonaqueous electrolyte secondary batteries 1, the measuring units 2 are connected to the negative electrode side and the positive electrode side of each nonaqueous electrolyte secondary battery. In FIG. 1, one measuring unit 2 is connected so as to correspond to a plurality of nonaqueous electrolyte secondary batteries 1, but a plurality of measuring units 2 respectively corresponding to the plurality of nonaqueous electrolyte secondary batteries are used. May be. As long as the measurement part 2 is connected so that the battery voltage and electric current of each nonaqueous electrolyte secondary battery 1 in charge / discharge can be measured, the form is not limited. For example, the measurement unit 2 may include a measurement circuit including a circuit for measuring voltage and current and a circuit for converting an analog value (measured value) into a digital value.

  The calculation unit 3 receives the output value output from the measurement unit 2 as a signal S1. Although details will be described later, the output value output by the measurement unit 2 as the signal S1 includes the measurement results of the battery voltage and current measured by the measurement unit 2 for the nonaqueous electrolyte secondary battery. In the calculation part 3, the calculation process including the derivation | leading-out of the electric potential position of the voltage flat part about a nonaqueous electrolyte secondary battery based on this output value is performed. Although details of this calculation process will be described later, the calculation unit 3 may be a calculation circuit, for example. In addition, when there are a plurality of measurement units 2, one calculation unit 3 may receive signals S <b> 1 from the plurality of measurement units 2, or the plurality of calculation units 3 correspond to the plurality of measurement units 2, respectively. The signal S1 may be received.

  The charge / discharge controller 4 receives the calculation result obtained by the calculation process in the calculation unit 3 as the signal S2. Although details will be described later, the charge / discharge controller 4 performs charge control of the nonaqueous electrolyte secondary battery based on the calculation result received as the signal S2.

  The battery pack may include the non-aqueous electrolyte secondary battery 1, the measurement unit 2, the calculation unit 3, and the charge / discharge controller 4. Further, the battery pack can include an AC / DC conversion circuit 5. When charging a non-aqueous electrolyte secondary ground, charging is often performed using direct current (DC). Since what can be used as an external power source is often alternating current (AC), this alternating current is converted into direct current during charging. The AC / DC conversion circuit 5 that can be provided in the battery pack converts alternating current from an external power source into direct current. When the battery pack does not have the AC / DC conversion circuit 5, it is preferable to connect the battery pack via an external AC / DC conversion circuit when connecting the battery pack to an AC power source for charging.

  When charging the battery pack, it is desirable to electrically connect the AC / DC conversion circuit 5 and the AC power source 6 via the charge / discharge controller 4 as shown in FIG. By connecting to an external power source via the charge / discharge controller 4, charge control can be easily performed.

  When the battery pack itself is used as a power source, the battery pack and the external load 7 are electrically connected. The external load 7 varies depending on the use of the battery pack. The nonaqueous electrolyte secondary battery 1 that is electrically connected to the external load 7 can be discharged.

  In the block diagram of FIG. 1, the connection to the AC power source 6 and the connection to the external load 7 are indicated by white circles. However, the AC power source 6 or the external load 7 is connected to the wiring 9 at this connection point (white circle). 10 can be physically connected or disconnected. Alternatively, when using the battery pack, the connection points (white circles) may be electrically connected or disconnected, and the wirings 9 and 10 are not necessarily physically separated.

  In the battery pack, a charge / discharge control method performed on the non-aqueous electrolyte secondary battery 1 by the measurement unit 2, the calculation unit 3, and the charge / discharge controller 4 will be described.

  In the charge control method, first, a voltage flat portion in the charge / discharge curve (charge curve or discharge curve) of the nonaqueous electrolyte secondary battery 1 is detected. Specifically, charging / discharging of the nonaqueous electrolyte secondary battery 1 is performed by the charging / discharging controller 4. At the time of charging / discharging, the measurement unit 2 measures and records the voltage of the nonaqueous electrolyte secondary battery 1 at regular intervals, and outputs an output value including the measurement result to the calculation unit 3. The calculation unit 3 calculates a charge / discharge curve for charging / discharging based on the output value, detects a voltage flat portion in the charge / discharge curve, and calculates a potential position of the voltage flat portion.

  Subsequently, the maximum charging voltage for charging the nonaqueous electrolyte secondary battery 1 is corrected based on the obtained potential position of the voltage flat portion. Specifically, the calculation unit 3 outputs a calculation result including the calculated potential position of the voltage flat portion to the charge / discharge controller 4. The charge / discharge controller 4 reduces the maximum charge voltage by the same value as the potential decrease amount from the reference potential of the voltage flat portion at this potential position.

  Details of the charge / discharge control method performed on the nonaqueous electrolyte secondary battery 1 are as follows.

  Charging / discharging of the nonaqueous electrolyte secondary battery 1 is controlled by the charge / discharge controller 4. Here, with respect to charging, charging / discharging is performed with the specified value of the maximum charging voltage set for the battery as the upper limit of the charging voltage. For this charging / discharging, for example, a cycle life test in which charging / discharging is repeated between 100% charging and 100% discharging, that is, between a fully charged state and a fully discharged state may be used. Here, the fully charged state can be, for example, a state where the battery is charged to the rated capacity set to the battery or a state where the SOC is 100%. Further, the complete discharge state may be, for example, a SOC 0% state. The charge / discharge may not be 100% charge / discharge.

  Further, it is desirable to charge the nonaqueous electrolyte secondary battery 1 by constant voltage charging subsequent to constant current charging. More specifically, the battery is charged with a constant current until reaching a specified voltage, and after reaching the voltage, constant voltage charging is performed. Alternatively, the constant voltage process described above can be omitted and only constant current charging can be performed. The constant voltage charging is performed until the constant voltage charging time or the total charging time reaches a predetermined value, or until the current value during constant voltage charging converges to the predetermined value. Moreover, the voltage at the time of constant voltage charging makes the maximum charging voltage set about a battery an upper limit.

  The specified value of the maximum charging voltage set in the nonaqueous electrolyte secondary battery 1 can be recorded in the calculation unit 3 or the charge / discharge controller 4. Further, the specified value of the maximum charging voltage may be the maximum charging voltage set for the battery in the initial state immediately after manufacture or immediately after shipment or acquisition. Alternatively, the specified value of the maximum charging voltage may be a corrected maximum charging voltage after performing the charging control, as will be described later.

  In this charging / discharging, the measurement part 2 measures and records the voltage of the nonaqueous electrolyte secondary battery 1 for every fixed time. Here, the time interval for measuring the voltage is desirably 1/100 or less of the time required for the charge or the discharge. In order to further improve accuracy, it is preferable to shorten the time interval to about 1/1000. Note that a preferable time interval corresponds to about 1 to 5 seconds under normal charging conditions. Moreover, the measurement part 2 may further measure the electric current value in the case of charging / discharging for every fixed time. The voltage and current value of the nonaqueous electrolyte secondary battery 1 are desirably measured at the same time.

  After the measurement in charging / discharging is completed, the measurement unit 2 outputs an output value including the measurement result to the calculation unit 3 as a signal S1. This output value may include information such as the measured voltage and the measurement time interval. The output value may include a current value in charge / discharge.

  The calculation unit 3 calculates a voltage change per time when the nonaqueous electrolyte secondary battery 21 is charged or discharged based on the output value output from the measurement unit 2 as the signal S1. For example, the calculation unit 3 calculates a charge curve based on the battery voltage measured by the measurement unit 2 at regular intervals when the nonaqueous electrolyte secondary battery 1 is charged. Alternatively, the calculation unit 3 calculates a discharge curve based on the battery voltage measured by the measurement unit 2 at regular intervals when the nonaqueous electrolyte secondary battery 1 is discharged. And the calculating part 3 calculates the electric potential position of the voltage flat part in charging / discharging from the calculated voltage change per time.

  The voltage flat portion in charging / discharging may be a portion where the potential does not change abruptly in the charging / discharging curve and the potential changes with time. In addition, in the voltage flat part, the inflection point where the charge / discharge curve changes from a shape that is convex down (concave up) to a shape that is convex downward (or changes from convex downward to convex upward) The potential at the (inflection point) may be the potential position of the voltage flat portion.

As an example, FIG. 2 shows a charge curve of a non-aqueous electrolyte secondary battery using lithium-containing iron phosphate (LiFePO ₄ ) for the positive electrode and monoclinic titanium dioxide (TiO ₂ ) for the negative electrode. In FIG. 2, when the first charge / discharge after assembling the battery is the first cycle, the charge curve in the third cycle is shown by a solid line, and the charge curve in the 100th cycle is shown by a broken line. Moreover, in FIG. 2, the voltage flat part in each charge curve is shown by the arrow.

  When the output value from the measurement unit 2 includes a current value for each charge / discharge time, the calculation unit 3 can calculate a differential (dQ / dV) curve between the current amount Q and the voltage V. Although mentioned later for details, the voltage flat part in the charging / discharging curve of the nonaqueous electrolyte secondary battery 1 can also be derived | led-out from the dQ / dV curve corresponding to this charging / discharging.

  In the calculation unit 3, the dQ / dV curve is calculated as follows, for example. First, the dQ / dV value is calculated based on the output value output as the signal S2 from the measurement unit 2. The battery voltage value and current value at the first time A are defined as voltage E (A) and current I (A), respectively, and these values at the second time B are defined as voltage E (B) and current value I (respectively). When B), the dQ / dV value at the voltage (E (B) + E (A)) / 2 can be obtained as follows. The dQ / dV value at the voltage (E (B) + E (A)) / 2 can be calculated as the current value difference at the time interval (B−A) by the following equation. If the measured voltage is the same at the first time and the second time (E (A) = E (B)), the denominator of the following equation is zero. It is necessary to cope with this by widening the interval between the first time and the second time until changes. In order to obtain sufficient accuracy, the time interval, that is, the interval between the first time A and the second time B is preferably 1/100 or less of the time required for the charging or discharging. . In order to further improve accuracy, it is preferable to shorten the time interval to about 1/1000. Under normal charging conditions, a preferable time interval corresponds to about 1 to 5 seconds.

  When the dQ / dV value is calculated by the above calculation, the voltage difference (E (B) -E (A)) corresponding to the denominator becomes small in the voltage flat portion. For this reason, in the dQ / dV curve obtained by plotting dQ / dV against voltage, the dQ / dV value of the portion corresponding to the voltage flat portion becomes large and peaks at the voltage corresponding to the potential position of the voltage flat portion. Form.

  FIG. 3 shows a graph (dQ / dV curve graph) obtained by plotting the dQ / dV obtained by the above-described method against the voltage for the nonaqueous electrolyte secondary battery having the charging curve shown in FIG. Similar to FIG. 2, in FIG. 3, the dQ / dV curve in the third cycle charging is indicated by a solid line, and the dQ / dV curve in the 100th cycle charging is indicated by a broken line. FIG. 3 shows that there are a plurality of voltage flat portions in the charging curve of the nonaqueous electrolyte secondary battery. Of the plurality of peaks, the peak having the maximum peak intensity is indicated by an arrow.

  As described above, the potential position of the voltage flat portion calculated by the calculation unit 3 based on the output value from the measurement unit 2 is set as the reference potential of the voltage flat portion.

  In addition, the calculation unit 3 can record the reference potential of the voltage flat portion. The reference potential of the voltage flat portion may be the potential position of the voltage flat portion obtained for the nonaqueous electrolyte secondary battery 1 immediately after manufacture or in an initial state immediately after shipment or acquisition. In this case, using the non-aqueous electrolyte secondary battery in the initial state, the reference potential of the voltage flat portion is derived in the same manner as the above-described reference potential of the voltage flat portion is calculated. Can be requested. Specifically, the nonaqueous electrolyte secondary battery in the initial state is subjected to charge / discharge, a voltage flat portion in a charge / discharge curve during the charge / discharge is detected, and the potential position is used as a reference value of the voltage flat portion. be able to. Here, the nonaqueous electrolyte secondary battery in the initial state may be the nonaqueous electrolyte secondary battery 1 included in the battery pack in the initial state, or the same type as the nonaqueous electrolyte secondary battery 1. The battery may be in an initial state. Alternatively, the reference potential of the voltage flat portion recorded in the calculation unit 3 may be the reference potential of the voltage flat portion calculated when the non-aqueous electrolyte secondary battery 1 is subjected to charge control before.

  The calculation unit 3 compares the reference potential with the reference potential of the voltage flat part, and calculates the potential decrease amount of the voltage flat part. Here, the value of the potential drop amount corresponds to the difference between the reference potential of the voltage flat portion and the reference potential. That is, the calculation unit 3 calculates, as the amount of potential decrease, the amount by which the potential position of the voltage flat portion detected when charging / discharging the nonaqueous electrolyte secondary ground 1 has decreased from the reference potential.

  When the calculation unit 3 derives the potential position of the voltage flat portion from dQ / dV, the peak top position in the dQ / dV curve corresponds to the potential position of the voltage flat portion. Here, a peak having a peak top position corresponding to the reference potential of the voltage flat portion is defined as a reference peak. Further, a peak having a peak top position corresponding to the reference potential of the voltage flat portion is set as a reference peak. The amount (potential) by which the position of the peak top of the reference peak is shifted compared to the position of the peak top of the reference peak corresponds to the amount of potential decrease in the voltage flat portion from the reference potential.

  The calculation unit 3 outputs a calculation result including the voltage drop amount calculated as described above to the charge / discharge controller 4 as a signal S2. At this time, the calculation unit 3 can reset the reference potential of the voltage flat portion recorded in the calculation unit 3 to the calculated reference potential value of the voltage flat portion. In addition, when the specified value of the maximum charging voltage set for the nonaqueous electrolyte secondary battery 1 is recorded in the calculation unit 3, the value of the maximum charging voltage can be included in the signal S2. The specified value of the maximum charging voltage recorded in the calculation unit 3 can be reset to a corrected specified value that is decreased by the same value as the reference potential of the voltage flat portion at the same time as the signal S2 is output.

  The voltage flat portion appears at a specific potential at which the amount of occlusion and release of lithium ions at the positive electrode or the negative electrode increases during charge / discharge of the battery. Therefore, the position and number of voltage flat portions vary depending on the type of titanium-containing negative electrode or positive electrode.

  Moreover, it is also possible to mix several types of active materials with the positive electrode and negative electrode in the nonaqueous electrolyte secondary battery 1, and in that case, more voltage flat parts may appear. Thus, in the case where there are many voltage flat portions, it is preferable to control by paying attention to the voltage flat portion where the amount of change from the reference potential (potential drop amount) is maximized. By doing so, it is possible to perform the control reflecting the balance between the positive and negative electrodes most faithfully.

  When the voltage flat part derived from the positive electrode and the voltage flat part derived from the negative electrode overlap, the voltage change of the voltage flat part seen for the whole battery tends to be small or unclear. However, by mixing a plurality of active materials and using them for the positive electrode and the negative electrode, a plurality of voltage flat portions can appear as described above. And the charge control most suitable for the state of the nonaqueous electrolyte secondary battery 1 can be implemented by using the maximum potential decrease amount. Therefore, it is desirable to use a mixture of a plurality of active materials.

  In particular, the balance fluctuation due to the side reaction on the negative electrode surface, which is often seen in the titanium-containing negative electrode, is largely reflected by the voltage flat portion derived from the positive electrode. Therefore, it is desirable to mix a plurality of active materials with the positive electrode. Among these, the lithium-containing cobalt composite oxide has a relatively high voltage flat portion, and is therefore more preferable because it can be easily distinguished from other voltage flat portions.

  When there are a plurality of voltage flat portions, the calculation unit 3 calculates a plurality of potential decrease amounts based on the reference potentials of the plurality of voltage flat portions and the reference potentials of the voltage flat portions corresponding to the reference potentials. It is desirable that the calculation unit 3 is set so as to output the maximum value of the calculated plurality of potential decrease amounts to the charge / discharge controller 4. Alternatively, in the battery pack in the initial state, the expected range of the reference potential may be set in advance for the voltage flat portion that should be noted by the calculation unit 3. In this case, for example, a charge / discharge cycle test is performed using a nonaqueous electrolyte secondary battery of the same type as the nonaqueous electrolyte secondary battery 1 included in the battery pack, and the correlation between the number of cycles and the potential position of the voltage flat portion is calculated. Ask. The expected range of the reference potential can be set based on the result of the charge / discharge cycle test.

  When the calculation unit 3 derives a dQ / dV curve, a plurality of reference peaks can be detected in the dQ / dV curve. In this case, the calculation unit 3 calculates the shift amount of the peak top position from the reference peak corresponding to each of the plurality of reference peaks. The calculation unit 3 determines the highest value among the obtained plurality of peak shift amounts. It is desirable that the calculation unit 3 is set so as to calculate the potential decrease amount of the voltage flat portion based on the highest shift amount. Alternatively, in the battery pack in the initial state, the expected range of the peak top position may be set in advance for the reference peak that should be noted by the calculation unit 3. In this case, for example, a charge / discharge cycle test is performed using a nonaqueous electrolyte secondary battery of the same type as the nonaqueous electrolyte secondary battery 1 included in the battery pack, and the cycle number and the peak top position of the dQ / dV peak are determined. Find the correlation. The expected range of the peak top position of the reference peak can be set based on the result of the charge / discharge cycle test. Alternatively, when a plurality of reference peaks are detected in the dQ / dV curve, control may be performed using any peak. In particular, since the reference peak having the maximum peak intensity has high detection accuracy, it is desirable to perform charge control based on the reference peak.

  In the battery pack of the embodiment, in the charge control method performed on the nonaqueous electrolyte secondary battery 1, the potential decrease amount of the voltage flat portion included in the calculation result output as the signal S <b> 2 from the calculation unit 3 is calculated. As an index, the charge / discharge controller 4 lowers the maximum charge voltage from a specified value. Specifically, in the battery pack, the charge / discharge controller 4 reduces the maximum charge voltage from the set specified value by the same value as the potential decrease amount calculated by the calculation unit 3. In this way, the charge / discharge controller 4 corrects the charge setting so that the maximum charge voltage is appropriate for the current state of the nonaqueous electrolyte secondary battery 1. That is, in the battery pack of the embodiment, the charging voltage is corrected so as to decrease from the specified charging voltage set for the battery by the amount by which the voltage flat portion has decreased. In this manner, the charging of the nonaqueous electrolyte secondary battery 1 is controlled so that the charging is performed in a state where the charging voltage is corrected to a state suitable for the current state of the battery.

  After correcting the maximum charge voltage, the charge / discharge controller 4 can set the corrected charge voltage to a new specified value of the maximum charge voltage and record it. Next, when the charge control is performed, the charge / discharge controller 4 can use a new reference potential of the voltage flat portion and a new specified value of the maximum charge voltage.

  As described above, in the charge control method performed on the nonaqueous electrolyte secondary battery 1, in order to reflect the amount of change in the voltage flat portion from the reference potential in the value for correcting the maximum charge voltage, the amount of change is simply used. May be subtracted from the specified value to calculate a new specified value. This method is desirable because it is simple.

  In the above description, the reference potential of the voltage flat portion calculated when the charge control was previously performed on the nonaqueous electrolyte secondary battery 1 can be newly set as the standard value. In the charge control method implemented in the secondary battery 1, it is not necessary to reset the reference value of the voltage flat portion. For example, the reference value of the voltage flat portion used every time the charge control is performed may be the potential position of the voltage flat portion obtained for the nonaqueous electrolyte secondary battery in the initial state. In this case, the reference value of the voltage flat portion is not reset even after the charge control is performed. Further, the specified value of the maximum charging voltage used every time the charging control is performed is also set to the initial state non-aqueous electrolyte secondary battery, and the corrected charging voltage is not newly set as the specified value.

  In the battery pack, the above-described charge / discharge control including the detection of the potential decrease amount of the voltage flat portion and the correction of the charging voltage value based on the charge / discharge of the battery uses the battery after a certain number of cycles or for a certain period. Set to be implemented later. That is, the reference potential of the voltage flat portion or the peak top position of the reference peak for the nonaqueous electrolyte secondary battery 1 is obtained at regular intervals. For example, if the potential drop amount of the voltage flat portion is detected at an extremely short interval, the shift amount of the dQ / dV peak becomes less than the error in the calculated dQ / dV curve, and the charge voltage cannot be corrected accurately. . On the other hand, if the interval is too long, the charge / discharge cycle may be repeated without correcting the charging voltage. As a result, the electrode of the nonaqueous electrolyte secondary battery 1 will be deteriorated, which is not desirable. Specifically, it is preferable that correction is performed every time the potential position of the voltage flat portion decreases by about 5 mV to 10 mV.

  The interval at which the charging control is performed can be determined based on various criteria. For example, detection of the potential drop amount of the voltage flat portion and correction of the charging voltage value are performed every predetermined number of cycles such as every 50 cycles or 100 cycles. The general number of cycles is 10 to 100 cycles. Alternatively, it is possible to set so that the charge control is performed when the charge amount or the discharge amount, that is, the capacity of the nonaqueous electrolyte secondary battery 1 is reduced by a certain amount or more. In this case, it is appropriate that the battery capacity is implemented at a timing when the battery capacity is reduced by, for example, about 5% or 10%, compared with the case where the charge control is performed previously. It is also possible to set so that charging control is performed at regular time intervals. In this case, it is desirable to calculate a period during which charging and discharging of about 50 to 100 cycles are performed in the usage situation assumed for the battery pack, and to set this period as a time interval. For mobile devices, the standard is about one month to half a year, and for in-vehicle devices, the standard is about half a year to one year.

  Further, the voltage flat portion for calculating the voltage drop amount for charge control may be a charge curve during charging of the nonaqueous electrolyte secondary battery 1 or a discharge curve during discharge. The battery can be discharged with a simple constant current, whereas the charging is performed with a constant current and constant voltage. Therefore, it is more convenient and desirable to use a voltage flat portion in discharging. However, if the target battery requires a large discharge current and the charging current is smaller than the discharging current, it is possible to measure the voltage flat portion more accurately during charging than during discharging. is there. Therefore, when such a battery is included in the battery pack, it is desirable to use a voltage flat portion during charging.

  Furthermore, unlike the charge / discharge in the cycle life test, the amount of change in the charge voltage can be calculated even in the case where the battery does not go back and forth between the fully charged state and the fully discharged state in that application. In such a battery, a voltage flat portion to be noted is determined in advance in the initial charge / discharge. When the battery is charged / discharged in a voltage range including the voltage flat portion determined to be focused, the amount of change from the reference potential is calculated using the potential position of the voltage flat portion as a reference potential. By implementing the charge control using this amount of change, the cycle life of the battery can be improved.

[Nonaqueous electrolyte secondary battery]
The nonaqueous electrolyte secondary battery 1 included in the battery pack of the embodiment will be described. In the following, the non-aqueous electrolyte secondary battery having a laminate outer package enclosing the stack electrode group will be described with reference to FIGS. 4 and 5, but the type and outer package of the electrode group of the non-aqueous electrolyte secondary battery are as follows. The present invention is not limited to the following example, and may be a conventionally known wound electrode group or a metal can exterior.

  The nonaqueous electrolyte secondary battery includes a positive electrode 11 connected to a positive electrode terminal 15, a negative electrode 12 connected to a negative electrode terminal 16, and an electrode group in which separators 13 are stacked in a zigzag manner, and a nonaqueous solution impregnated in the electrode group. It is composed of an electrolyte, a pre-nonaqueous electrolyte, and an exterior container formed by sealing the exterior material 14 that houses the electrode group (FIG. 4). The positive electrode 11 has a positive electrode active material layer 11a on both sides or one side of the positive electrode current collector 11b. Similarly, the negative electrode 12 has a negative electrode active material layer 12a on both sides or one side of the negative electrode current collector 12b (FIG. 5). .

(Positive electrode)
The positive electrode 11 contains a positive electrode active material, and can further contain a material having electronic conductivity such as carbon and a binder, and a base material such as a metal having electronic conductivity is used as a current collector. Used in contact with the current collector.

As a positive electrode active material, a chalcogen compound such as lithium-containing cobalt composite oxide, lithium-containing nickel composite oxide, lithium-containing nickel cobalt composite oxide, lithium manganese composite oxide, or lithium-containing nickel cobalt manganese composite oxide is used alone or in combination. Can be used. Among them, lithium-containing cobalt composite oxide, lithium-containing nickel-cobalt composite oxide, lithium-containing manganese composite oxide, etc. having a charge / discharge potential of (vs. Li / Li ⁺ ) 3.8 V or higher with respect to the lithium metal potential were used. In this case, it is desirable because a high battery capacity can be realized.

  As the conductive material, a substance having electronic conductivity such as carbon or metal can be used. The conductive agent desirably has a shape such as powder or fibrous powder.

  Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), an ethylene-propylene-diene copolymer, and styrene-butadiene rubber.

  As the current collector, a metal foil made of a metal such as aluminum, stainless steel, or titanium, a thin plate or mesh, a wire mesh, or the like can be used.

  The positive electrode active material and the conductive material can be formed into a sheet by adding the binder and kneading and rolling. Alternatively, after dissolving and suspending in a solvent such as toluene and N-methylpyrrolidone (NMP) to form a slurry, it can be applied onto the current collector and dried to form a sheet.

  The positive electrode terminal 15 is made of a metal ribbon, a metal plate, or a metal rod that is electrically connected to the positive electrode 11, and is connected to the positive electrode 11 to electrically bridge the outside of the battery and the positive electrode 11. As shown in FIG. 4, the positive electrode current collector 11b may be welded to a metal ribbon, and the metal ribbon may be drawn out of the exterior material to serve as a positive electrode terminal. Aluminum, titanium, or the like can be used for the positive electrode terminal 15.

(Negative electrode)
The negative electrode 12 includes a titanium-containing composite oxide as a negative electrode active material, and the negative electrode active material is formed into a pellet shape, a thin plate shape, or a sheet shape using a conductive material, a binder, or the like.

Examples of the titanium-containing composite oxide include spinel type lithium titanate (Li ₄ Ti ₅ O ₁₂ ; LTO), monoclinic titanium dioxide (TiO ₂ ), and niobium-titanium composite oxide (NTO). . Further, as the niobium-titanium composite compound (NTO), monoclinic Nb ₂ TiO ₇ , orthorhombic Li ₂ Na _2−x Ti _6−x Nb _x O _14, or the like can be used. .

  When LTO is used as the negative electrode active material, since the charge / discharge curve of LTO is flat, a voltage flat portion derived from LTO is remarkably observed in the charge / discharge curve of the battery. Thus, it is desirable to use LTO as the negative electrode active material from the viewpoint that it is easy to distinguish the origin of the positive and negative electrodes in the voltage flat portion.

On the other hand, in a battery using TiO ₂ or NTO, the amount of side reaction on the surface of the active material is larger than in the case of using LTO. Therefore, in a battery using TiO ₂ or NTO, it can be expected that the effect obtained by performing the charge control becomes more remarkable. In particular, as NTO, Li _a TiM _b Nb _{2 ± β} O _{7 ± σ} (0 <a <5, 0 <b <0.3, 0 <β <0.3, 0 <σ <0 .3, when M is a compound represented by at least one element of Fe, V, Mo, Ta), the amount of side reactions tends to increase, and the effect can be expected to be particularly remarkable.

  As the conductive material, a substance having electronic conductivity such as carbon or metal can be used. The conductive agent desirably has a shape such as powder or fibrous powder.

  As the binder, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), styrene-butadiene rubber, carboxymethyl cellulose (CMC), or the like can be used.

  As the current collector, a metal foil made of a metal such as copper, stainless steel, nickel or the like, a thin plate or mesh, a metal mesh, or the like can be used.

  The negative electrode active material and the conductive material can be pelletized or sheeted by adding the binder and kneading and rolling. Alternatively, after dissolving and suspending in a solvent such as water or N-methylpyrrolidone (NMP) to form a slurry, it can be coated on the current collector and dried to form a sheet.

  The negative electrode terminal 16 is formed of a metal ribbon, a metal plate, or a metal rod that is electrically connected to the negative electrode 12, and is connected to the negative electrode 12 to electrically bridge the battery exterior and the negative electrode 12. As shown in FIG. 4, the negative electrode current collector 12 b may be welded to a metal ribbon and drawn out of the exterior material to form the negative electrode terminal 16. As the negative electrode terminal 16, aluminum, copper, stainless steel, or the like can be used. Aluminum that is lightweight and has excellent weldability is desirable.

(Separator)
As the separator 13, a polyolefin porous film, a cellulose nonwoven fabric, a polyethylene terephthalate nonwoven fabric, or a polyolefin nonwoven fabric can be used. Polyolefin porous membranes and polyolefin nonwoven fabrics are desirable because they can prevent the introduction of water and alcohol impurities. Nonwoven fabrics are desirable because they are excellent in impregnation properties of sulfonic nonaqueous electrolytes with high viscosity. However, although the cellulose nonwoven fabric has the advantage of being inexpensive, it has high hygroscopicity and can easily bring water into the battery as an impurity. Therefore, it is desirable to remove water by vacuum drying when used. It is also possible to use a separator in which different kinds of films are laminated, or a separator in which a layer of a nonconductive material is formed on the separator 3 to have a short-circuit prevention function or an impregnation improving function.

(Nonaqueous electrolyte)
The nonaqueous electrolytic solution is obtained by dissolving a lithium salt in a solvent. As a solvent, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC), γ-butyrolactone (gamma) -butyrolactone (GBL), acetonitrile (acetonitrile; AN), ethyl acetate (EA), toluene, xylene, methyl acetate (MA), or the like can be used.

Examples of the lithium salt include lithium perchlorate (LiClO ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), and trifluoromethanesulfonic acid. Lithium salts such as lithium, bistrifluoromethylsulfonylimide lithium (LiN (CF ₃ SO ₂ ) ₂ ; TFSI), and bispentafluoroethylsulfonylimide lithium (LiN (C ₂ F ₅ SO ₂ ) ₂ ) can be used. Since better cycle performance can be obtained, LiPF ₆ and LiBF ₄ are desirable, and a mixed salt thereof may be used.

(Exterior material)
A metal or resin can or a laminate sheath can be used as the exterior member 14. As the metal can, a rectangular container such as aluminum, iron, and stainless steel can be used. Also, a square container such as plastic or ceramic can be used. As a laminate exterior, a laminate material in which a resin layer is combined with a metal such as aluminum, copper, or stainless steel is used in a bag shape by heat fusion. In particular, a laminate exterior is desirable because it can be measured as a change in battery appearance when internal gas is generated.

  According to the embodiment, a battery pack including a non-aqueous electrolyte secondary battery including a titanium-containing negative electrode, a measurement unit, a calculation unit, and a charge / discharge controller is provided. In this battery pack, the maximum charging voltage of the non-aqueous electrolyte secondary battery is set to the same value as the potential decrease amount of the voltage flat portion in the charging curve when charging the non-aqueous electrolyte secondary battery or the discharging curve during discharging. A charge control method for lowering the value from the specified value is performed. By using this charge control method, the cycle life of the battery pack provided with the non-aqueous electrolyte secondary battery can be improved.

(Second Embodiment)
In the charge control method according to the embodiment, the maximum charge voltage is varied during charging of a nonaqueous electrolyte secondary battery including a positive electrode, a negative electrode including a titanium-containing composite oxide, a nonaqueous electrolyte, and a separator. Here, the voltage of the voltage flat portion in the charge / discharge curve obtained when charging or discharging the non-aqueous electrolyte secondary battery is the same as the amount of decrease compared to the reference voltage of the voltage flat portion. Is lower than the specified value. Specifically, in the charge control method, when charging or discharging the target nonaqueous electrolyte secondary battery, the voltage of the nonaqueous electrolyte secondary battery is measured at regular intervals, and the measured voltage value is set. Based on the voltage change, the voltage change per time during the charge or discharge is calculated, and one of one or more voltage flat portions in the charge curve during the charge or the discharge curve during the discharge is calculated based on the voltage change. Calculating a potential position, and reducing the maximum charging voltage from a predetermined value by the same value as a potential decrease amount at which the potential position has decreased from a reference potential.

  Examples of the charge control method include a charge control method implemented by a charge control mechanism including a measurement unit, a calculation unit, and a charge / discharge controller in the battery pack of the first embodiment. This charge control method can be implemented not only for the non-aqueous electrolyte secondary battery included in the battery pack but also for the non-aqueous electrolyte secondary battery as a single battery cell.

  A non-aqueous electrolyte secondary battery that is subject to charge control includes a positive electrode, a negative electrode including a titanium-containing composite oxide, a non-aqueous electrolyte, and a separator. Details of the nonaqueous electrolyte secondary battery include the details of the nonaqueous electrolyte secondary battery 1 included in the battery pack according to the first embodiment.

  Details of the charge control method of the embodiment can include details of charge control performed on the nonaqueous electrolyte secondary battery 1 in the battery pack of the first embodiment.

  However, as in the case of the battery pack of the first embodiment, when the measurement unit, the calculation unit, and the charge / discharge controller are provided, the charge control can be automatically performed. In the case of using a battery that does not include (a measurement unit, a calculation unit, and a charge / discharge controller), for example, the charge control method of the embodiment can be manually performed.

  When manually detecting the potential position of the voltage flat portion in the charge / discharge curve of the battery, it is desirable to derive from the dQ / dV curve. As shown in FIG. 3, since the position of the peak top in the dQ / dV curve is clear, the potential position of the voltage flat portion can be easily determined. Further, when there are a plurality of voltage flat portions, the peak having the maximum peak intensity can be clearly identified.

  According to the above embodiment, there is provided a charge control method for reducing the maximum charge voltage below a specified value by the same value as the potential drop amount of the voltage flat portion in the charge / discharge curve during charge / discharge of the nonaqueous electrolyte secondary battery. Is done. By using this charge control method, the cycle life of a nonaqueous electrolyte secondary battery comprising a positive electrode, a negative electrode containing a composite oxide containing titanium, a nonaqueous electrolyte, and a separator can be improved. it can.

[Example]
Hereinafter, examples according to the embodiment will be described in detail with reference to the drawings. The non-aqueous electrolyte secondary battery in the following examples employs the battery structure shown in FIG.

Example 1
Using 20 wt% lithium-containing cobalt composite oxide (LiCoO ₂ ) powder and 70 wt% lithium-containing nickel cobalt manganese composite oxide (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) powder as the positive electrode active material, 2% by weight of acetylene black and 3% by weight of graphite were used as a conductive agent, and 5% by weight of polyvinylidene fluoride (PVdF) was used as a binder. Using N-methylpyrrolidone (NMP) as a solvent, these positive electrode active material, conductive agent and binder were slurried. The obtained slurry was applied to both sides of an aluminum foil having a thickness of 15 μm, dried and rolled to prepare a plurality of positive electrode sheets having a width of 67 mm and a length of 92 mm. In each positive electrode sheet, the 5 mm end in the length direction of the aluminum foil was an uncoated portion where no slurry was applied. An aluminum ribbon having a width of 5 mm and a thickness of 0.1 mm was welded to the uncoated portion at three locations to form a positive electrode tab. Thus, a belt-like positive electrode sheet was produced.

90% by weight of spinel-type lithium titanate (Li ₄ Ti ₅ O ₁₂ ) powder as the negative electrode active material, 5% by weight of artificial graphite and 5% by weight of polyvinylidene fluoride (PVdF) as the conductive material, N-methylpyrrolidone (NMP) solution In addition to mixing, a slurry was obtained. The obtained slurry was applied to both sides of an aluminum foil having a thickness of 25 μm, dried and rolled. The obtained negative electrode sheet was cut out to have a width of 68 mm and a length of 93 mm. About each of the cut-out negative electrode sheet, the length direction terminal 5mm part of the aluminum foil was made into the non-application part to which the slurry was not apply | coated. Three aluminum ribbons having a width of 5 mm and a thickness of 0.1 mm were welded to the uncoated portion to form a negative electrode tab. In this way, a strip-shaped negative electrode sheet was produced.

  As the separator, a polyethylene porous membrane having a width of 93 mm (a kind of polyolefin porous membrane) was used.

  The strip-like positive electrode sheet, the separator, the strip-like negative electrode sheet, and the separator were each laminated 10 times in this order to form an electrode coil. The positive electrode tab 5 was overlapped and welded to an aluminum sheet having a thickness of 0.1 mm, a width of 30 mm, and a length of 50 mm to form a positive electrode terminal 5. Similarly, the negative electrode tab 6 was overlapped and welded to an aluminum sheet having a thickness of 0.1 mm, a width of 30 mm, and a length of 50 mm to obtain a negative electrode terminal 6.

The electrode group was housed in an aluminum laminate exterior. 1M LiPF ₆ was dissolved in propylene carbonate (PC) to obtain a non-aqueous electrolyte. 9 g of a non-aqueous electrolyte was added to the aluminum laminate case containing the electrode group, and the aluminum laminate case was heat-sealed and closed to produce a non-aqueous electrolyte secondary battery. A measurement unit, a calculation unit, and a charge / discharge controller similar to those described in the first embodiment were mounted on the non-aqueous electrolyte secondary battery to produce a battery pack. The battery pack thus obtained was used as the battery pack of Example 1.

(Example 2)
As the positive electrode active material, 90% by weight of olivine type lithium iron composite phosphate compound (LiFePO ₄ ) powder, 5% by weight of acetylene black as a conductive agent, and 5% by weight of polyvinylidene fluoride (PVdF) as a binder were used. Otherwise, a belt-like positive electrode sheet was produced in the same manner as in Example 1.

As the negative electrode active material, 90% by weight of monoclinic titanium dioxide (TiO ₂ ) powder, 5% by weight of artificial graphite as a conductive material, and 5% by weight of polyvinylidene fluoride (PVdF) as a binder were used. Other than that was carried out similarly to Example 1, and produced the strip | belt-shaped negative electrode sheet.

As the non-aqueous electrolyte, a solution obtained by dissolving 1M LiPF ₆ in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 1: 2 was used.

  A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the above-described materials were used as the belt-like positive electrode sheet, the belt-like negative electrode sheet, and the nonaqueous electrolyte. Moreover, the battery pack of Example 2 was produced using this nonaqueous electrolyte secondary battery.

Example 3
Niobium titanium composite oxide (Nb ₂ TiO ₇ ) powder is used as the negative electrode active material, and 1M LiPF ₆ is dissolved in a mixture of propylene carbonate (PC) and diethyl carbonate (DEC) in a volume ratio of 1: 2 as the nonaqueous electrolyte. A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the obtained one was used. Moreover, the battery pack of Example 3 was produced using this nonaqueous electrolyte secondary battery.

(Comparative Example 1)
A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1. In this nonaqueous electrolyte secondary battery, a battery pack having the same configuration as that of Example 1 was manufactured except that a charge control mechanism including a measurement unit, a calculation unit, and a charge / discharge controller was not mounted. This was designated as the battery pack of Comparative Example 1.

(Comparative Example 2)
A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 2. In this nonaqueous electrolyte secondary battery, a battery pack having the same configuration as that of Example 2 was manufactured except that a charge control mechanism including a measurement unit, a calculation unit, and a charge / discharge controller was not mounted. This was designated as the battery pack of Comparative Example 2.

(Comparative Example 3)
A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 3. In this nonaqueous electrolyte secondary battery, a battery pack having the same configuration as that of Example 3 was prepared except that a charge control mechanism including a measurement unit, a calculation unit, and a charge / discharge controller was not mounted. This was designated as the battery pack of Comparative Example 3.

(Charge / discharge cycle test)
A charge / discharge cycle test was performed on the battery packs of Examples 1 to 3 and Comparative Examples 1 to 3. The total number of cycles in the charge / discharge cycle test was 200 cycles. Details of the cycle test for each battery pack are as follows.

  For the battery pack of Example 1, first, the initial maximum charging voltage (specified value) was 2.8 V, the discharge starting voltage was 1.5 V, and charging / discharging at 45 ° C. with a 1 C equivalent current (one hour rate current). A cycle was performed. One charge and one discharge are defined as one cycle. The potential position of the voltage flat part in the charge / discharge of the third cycle was used as the reference potential of the voltage flat part used for charge control. The potential position of the voltage flat part in the charging curve at that time was 2.386V and 2.212V. When the amount of movement of the potential position of these two voltage flat portions for each charge / discharge cycle was examined, the amount of movement at the 100th cycle was 5 mV for both. Therefore, for the battery pack of Example 1, it was determined that the potential decrease amount of the voltage flat portion from the 3rd cycle to the 100th cycle was 5 mV. In the next 100 cycles, the maximum charge voltage was set to 2.795 V obtained by subtracting 5 mV from the initial specified value of 2.8 V, and a charge / discharge cycle was performed.

  For the battery pack of Example 2, the conditions for the initial charge / discharge cycle (first 100 cycles) were set in the same manner as the battery pack of Example 1 except that the temperature condition was 60 ° C. The potential positions of the voltage flat portion in the charge curve at the third cycle were 1.874V and 1.934V. Among these, charge control was performed focusing on a peak corresponding to a potential of 1.874 V where the dQ / dV peak is larger and the peak top movement amount associated with the cycle is large, and a charge / discharge cycle test was performed. Specifically, since the peak shift amount corresponding to the potential of 1.874 V in the 100th cycle is 8 mV, the maximum charge voltage is set to the initial specified value 2 in the charge and discharge from the 101st cycle to the 200th cycle. Charging / discharging was performed by setting to 2.792 V obtained by subtracting 8 mV from 8 V.

  For the battery pack of Example 3, the conditions of the initial charge / discharge cycle (first 100 cycles) were set in the same manner as the battery pack of Example 1. The potential positions of the voltage flat portion in the charge curve at the third cycle were 2.095V and 2.265V. Since the potential decrease amount at the 100th cycle was 1.3 mV in all cases, the potential decrease amount of the voltage flat portion from the 3rd cycle to the 100th cycle was determined to be 1.3 mV. In charging and discharging from the 101st cycle to the 200th cycle, charging and discharging were performed with the maximum charging voltage set to 2.7987 mV obtained by subtracting 1.3 mV from the initial 2.8 V.

  Since the measurement part, the calculating part, and the charge / discharge controller were not mounted on the battery pack of Comparative Example 1, 200 cycles of charge / discharge were performed without performing charge control. Specifically, similarly to the initial charge / discharge of the battery pack of Example 1, the maximum charge voltage in each cycle is 2.8 V, the discharge start voltage is 1.5 V, and the charge at 45 ° C. is performed at a current equivalent to 1 C. A discharge cycle was performed.

  Since the measurement part, the calculating part, and the charge / discharge controller were not mounted on the battery pack of Comparative Example 2, 200 cycles of charge / discharge were performed without performing charge control. Specifically, similarly to the initial charge / discharge of the battery pack of Example 2, the maximum charge voltage in each cycle was 2.8 V, the discharge start voltage was 1.5 V, and the charge at 60 ° C. was performed at a current equivalent to 1 C. A discharge cycle was performed.

  Since the measurement part, the calculating part, and the charge / discharge controller were not mounted on the battery pack of Comparative Example 3, 200 cycles of charge / discharge were performed without performing charge control. Specifically, similarly to the initial charge / discharge of the battery pack of Example 3, the maximum charge voltage in each cycle was 2.8 V, the discharge start voltage was 1.5 V, and the charge at 45 ° C. was performed at a current equivalent to 1 C. A discharge cycle was performed.

  For the obtained battery packs of Examples 1 to 3 and Comparative Examples 1 to 3, the discharge capacity after carrying out 200 cycles of charge / discharge was measured and divided by the discharge capacity at the third cycle in which it was the initial state. The value was defined as a 200 cycle discharge capacity retention rate. The 200 cycle discharge capacity retention rates obtained for the battery packs of Examples 1 to 3 and Comparative Examples 1 to 3 are shown in the following table.

As shown in Table 1, the discharge capacity maintenance rates in the battery packs of Examples 1 to 3 in which the charge control was performed by the charge control mechanism (measurement unit, calculation unit, and charge / discharge controller) included in the battery pack were It was higher than the discharge capacity maintenance rate of the battery packs of Comparative Examples 1 to 3 in which the charge control mechanism was not provided and the charge control was not performed. Moreover, the difference of the discharge capacity maintenance rate of the battery pack in which charge control was implemented and the battery pack in which charge control was not implemented is with respect to the difference between Example 1 and Comparative Example 1 using LTO as the negative electrode active material. When compared, the difference between Example 2 and Comparative Example 2 using TiO ₂ and the difference between Example 3 and Comparative Example 3 using NTO were shown to be larger.

  As a result, it was found that the charge / discharge cycle life of the nonaqueous electrolyte secondary battery can be improved by controlling the maximum charge voltage based on the amount of movement of the voltage flat portion.

  According to the embodiments and examples described above, a battery pack including a nonaqueous electrolyte secondary battery, a measurement unit, a calculation unit, and a charge / discharge controller is provided. The nonaqueous electrolyte secondary battery included in the battery pack includes a positive electrode, a negative electrode including a titanium-containing composite oxide, a nonaqueous electrolyte, and a separator. The measurement unit measures the voltage of the nonaqueous electrolyte secondary battery at regular intervals when the nonaqueous electrolyte secondary battery is charged or discharged. The calculation unit calculates a voltage change per time during charging or discharging based on the output value output from the measuring unit, and based on this voltage change, a charging curve during charging or a discharging curve during discharging The potential position of one or more voltage flat portions in is calculated. The charge / discharge controller lowers the maximum charge voltage below the specified value by the same value as the potential drop amount whose potential position has dropped from the reference potential. This battery pack exhibits a high charge / discharge cycle life.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

DESCRIPTION OF SYMBOLS 1 ... Nonaqueous electrolyte secondary battery, 2 ... Measurement part, 3 ... Calculation part, 4 ... Charge / discharge controller, 5 ... AC / DC conversion circuit, 6 ... AC power supply, DESCRIPTION OF SYMBOLS 7 ... External load, 11 ... Positive electrode, 11a ... Positive electrode active material layer, 11b ... Positive electrode collector, 12 ... Negative electrode, 12a ... Negative electrode active material layer, 12b ... Negative electrode current collector, 13 ... Separator, 14 ... Exterior material (laminate outer package), 15 ... Positive electrode terminal, 16 ... Negative electrode terminal

Claims (8)

A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode including a titanium-containing composite oxide, a non-aqueous electrolyte, and a separator;
When charging or discharging the nonaqueous electrolyte secondary battery, a measurement unit that measures the voltage of the nonaqueous electrolyte secondary battery at regular intervals;
Based on the output value output from the measurement unit, the voltage change per time during the charge or discharge is calculated, and the charge curve during the charge or the discharge curve during the discharge based on the voltage change An arithmetic unit that calculates a potential position of one or more voltage flat portions;
A battery pack comprising: a charge / discharge controller that reduces the maximum charge voltage below a specified value by the same value as a potential decrease amount at which the potential position has decreased from a reference potential.   The charge / discharge controller reduces the maximum charge voltage below the specified value by the same value as the potential decrease amount of the voltage flat portion having the maximum potential decrease amount among the one or more voltage flat portions. The battery pack described.   The measurement unit further measures a current value of the nonaqueous electrolyte secondary battery for each voltage measured at regular intervals, and the calculation unit calculates the current value based on an output value output from the measurement unit. A differential curve with respect to the voltage is further calculated, and the charge / discharge controller lowers the maximum charge voltage from the specified value by the same value as a shift amount of any one of one or more peaks in the differential curve. Item 6. The battery pack according to Item 1.   4. The battery pack according to claim 3, wherein the charge / discharge controller reduces the maximum charge voltage from the specified value by the same value as a shift amount of a peak position of a peak having a maximum peak intensity among the one or more peaks. .   A charge control method for a non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode containing a titanium-containing composite oxide, a non-aqueous electrolyte, and a separator, and a charging curve for charging the non-aqueous electrolyte secondary battery Alternatively, a charge control method in which the maximum charge voltage is lowered from a specified value by the same value as the potential drop amount at which any one of the one or more voltage flat portions in the discharge curve during discharge is lowered from the reference potential.   6. The charge control method according to claim 5, wherein, among the one or more voltage flat portions, the maximum charge voltage is decreased from the specified value by the same value as the potential decrease amount of the voltage flat portion having the maximum potential decrease amount. .   A differential curve of current value and voltage per time during the charging or discharging is calculated, and the maximum charging voltage is set to the same value as the shift amount of any one of the one or more peaks in the differential curve. The charge control method according to claim 5, wherein the charge control method lowers the specified value.   The charge control method according to claim 7, wherein the maximum charge voltage is reduced from the specified value by the same value as a shift amount of a peak position of a peak having a maximum peak intensity among the one or more peaks.