JP2011034699A - Lithium ion secondary battery and secondary battery system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery that facilitates detecting an over-discharging state. <P>SOLUTION: The lithium ion secondary battery 20 includes a battery can 4. The battery can 4 has an electrode group G housed with a cathode plate 1 and an anode plate 2 wound around through a separator 3. Nonaqueous electrolyte solution is injected into the battery can 4. The cathode plate 1 has a cathode mixture containing lithium cobaltate having a stratified crystal structure coated on a collector. The anode plate has an anode mixture containing an amorphous carbon material of an anode active material coated on a collector. The anode mixture has lithium titanate capable of storing and releasing lithium ion at a voltage of 1 V to 2 V blended at a ratio of ≥1 wt.%. Discharge speed at the voltage of 1 V to 2 V is alleviated at discharge. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a lithium ion secondary battery and a secondary battery system, and in particular, a lithium ion secondary battery including a positive electrode including a lithium transition metal composite oxide, a negative electrode including a negative electrode active material, and an electrolyte including a lithium salt. The present invention relates to a secondary battery and a secondary battery system including the lithium ion secondary battery.

  Development of power supply devices such as lithium ion secondary batteries and capacitors has been actively promoted in order to be applied to hybrid vehicles and electric vehicles, and some of them have been put into practical use. In recent years, from the viewpoint of environmental problems such as carbon dioxide reduction, expectations for practical use of hybrid vehicles and the like have increased, and battery performance and battery control technology have made remarkable progress. Moreover, a lithium ion secondary battery can be used for uses, such as a backup power supply (uninterruptible power supply apparatus, UPS), a mobile telephone, etc., and is anticipated as those battery systems.

  In general, in a lithium ion secondary battery, a positive electrode material including a positive electrode active material that can absorb and release lithium ions is applied to a positive electrode current collector, and a negative electrode active material that can release and absorb lithium ions. The negative electrode in which the negative electrode mixture is applied to the negative electrode current collector is infiltrated into the non-aqueous electrolyte and accommodated in the battery container. A lithium transition metal composite oxide is widely used for the positive electrode active material, and a carbon material is widely used for the negative electrode active material.

  However, due to frequent accidents and recalls of equipment using lithium ion secondary batteries, the reliability and safety of lithium ion secondary batteries have attracted attention. For example, when copper is used as the material of the negative electrode current collector, if the negative electrode potential is about 3.0 V or more (battery voltage is about 0.6 V or less, positive electrode potential is 3.6 V or less), The elution of positive electrode may cause abnormalities in the positive electrode and dissolution of the battery container, which may cause leakage of the non-aqueous electrolyte and further damage the battery function. For this reason, in lithium ion secondary batteries, measures against over-discharge are usually taken by voltage control using secondary devices such as protective elements and protective circuits.

  It is also important to improve the charge / discharge characteristics of the lithium ion secondary battery. For example, for the purpose of improving charging characteristics in a low temperature environment, a technique is disclosed in which scaly graphite is used as a negative electrode active material, and an intermediate layer is disposed between the negative electrode mixture and the negative electrode current collector (see Patent Document 1). ). In this technique, since scaly graphite overlaps in layers, an intermediate layer containing lithium titanate is arranged in order to ensure the movement of lithium ions between the negative electrode active material and the negative electrode current collector.

JP 2007-200902 A

  However, although the technology of Patent Document 1 improves the charging characteristics in a low temperature environment, it cannot be said that the above-described safety measures during overdischarge are sufficient. In the case of a lithium ion secondary battery using a general carbon-based negative electrode and a lithium transition metal composite oxide-based positive electrode, for example, the resistance increases at the end of discharge when the battery voltage is 2.5 V or less. For this reason, the discharge rate increases as the voltage decreases, and it becomes difficult to detect and control overdischarge due to the voltage. Furthermore, since the discharge capacity also decreases at the end of such a discharge, it becomes difficult to detect and control overdischarge due to capacity change. If a delay occurs in the detection of the overdischarge state, the battery voltage is likely to be 0.6 V or less, that is, the elution voltage of copper or less as described above.

  An object of the present invention is to provide a lithium ion secondary battery capable of easily detecting an overdischarge state and a secondary battery system including the lithium ion secondary battery.

  In order to solve the above-described problems, a first aspect of the present invention includes a positive electrode in which a positive electrode mixture containing a lithium transition metal composite oxide is applied to a positive electrode current collector, and a negative electrode capable of inserting and extracting lithium ions A negative electrode in which an active material and a negative electrode mixture containing a titanium-containing lithium compound capable of occluding and releasing lithium ions when the battery voltage is 1 V or more and 2 V or less is applied to a negative electrode current collector of a metal foil mainly composed of copper And a non-aqueous electrolyte containing a lithium salt infiltrating the positive electrode and the negative electrode, wherein the titanium-containing lithium compound is contained in the negative electrode mixture in a proportion of 1 wt% or more. It is an ion secondary battery.

  In the first aspect, when the battery voltage is 1 V or more and 2 V or less, a titanium-containing lithium compound capable of occluding and releasing lithium ions is contained in the negative electrode mixture at a ratio of 1 wt% or more. Lithium ions are also released from the titanium-containing lithium compound in the following range, and the discharge rate is relaxed. Therefore, it is possible to easily detect the overdischarge state without reaching the elution voltage of copper used for the negative electrode current collector. it can.

  In this case, lithium titanate can be used for the titanium-containing lithium compound. Further, the lithium transition metal composite oxide may have a layered crystal structure, and an amorphous carbon material may be used as the negative electrode active material.

  In addition, the second aspect of the present invention is a battery unit configured by one or more of the lithium ion secondary batteries of the first aspect, and detects the voltage of each lithium ion secondary battery to determine the battery state. And a battery control unit for controlling the secondary battery system.

  According to the present invention, the lithium ion is also released from the titanium-containing lithium compound in the range of the battery voltage of 1 V or more and 2 V or less at the time of discharge, and the discharge rate is relaxed, leading to the elution voltage of copper used for the negative electrode current collector. Therefore, the effect that the overdischarge state can be easily detected can be obtained.

It is a block circuit diagram showing the outline of the secondary battery system of the embodiment to which the present invention is applied. It is sectional drawing which shows typically the cylindrical lithium ion secondary battery which comprises the secondary battery system of embodiment. It is a graph which shows the change of the voltage with respect to the depth of discharge at the time of discharge of the cylindrical lithium ion secondary battery of an Example and a comparative example.

  DESCRIPTION OF EMBODIMENTS Hereinafter, an embodiment of a secondary battery system including a cylindrical lithium ion secondary battery to which the present invention is applied will be described with reference to the drawings.

(Constitution)
As shown in FIG. 1, the secondary battery system 40 includes a battery unit 25 configured by a cylindrical lithium ion secondary battery 20 and a battery control unit 27 for controlling the battery state of each lithium ion secondary battery 20. And.

  In this example, the battery unit 25 is configured by connecting six lithium ion secondary batteries 20 in series. The battery control unit 27 is a central processing unit CPU, a basic control program, a ROM that stores various setting values and the like, a RAM that functions as a work area for the CPU and temporarily stores various data, and an internal connection between them. It consists of a microcomputer A having a bus. The microcomputer A is operated by a power source from a power supply unit (not shown).

  The voltage etc. of each lithium ion secondary battery 20 constituting the battery unit 25 is detected by the battery control unit 27. That is, the negative electrode terminal of the lowest-order lithium ion secondary battery 20 is connected to the ground. The positive terminal of the uppermost lithium ion secondary battery 20 is connected to one end of the switch SW2. The positive terminal of each lithium ion secondary battery 20 and the negative terminal of the lowest-order lithium ion secondary battery 20 constituting the battery unit 25 are input side terminals of a voltage measurement circuit 29 that measures the voltage of each lithium ion secondary battery 20. It is connected to the. The voltage measurement circuit 29 can be configured by a differential amplifier circuit or the like that converts the voltage of each lithium ion secondary battery 20 into a voltage based on the negative electrode terminal. The output side terminal of the voltage measurement circuit 29 is connected to the A / D input port of the microcomputer A for A / D converting the voltage of the lithium ion secondary battery 20. The voltage measurement circuit 29 is connected to the battery designation port of the microcomputer A in order to receive designation of the lithium ion secondary battery 20 to be voltage measured from the microcomputer A. Therefore, the microcomputer A can capture the voltage data of each lithium ion secondary battery 20.

  The positive terminal of each lithium ion secondary battery 20 is connected to one end of a bypass resistor R for capacity adjustment (the same resistance value for each lithium ion secondary battery), and the other end of the bypass resistor R is a lithium ion secondary. It is connected to one end of a switch SW1 that adjusts the capacity of the battery 20. The other end of the switch SW1 is connected to the negative terminal of each lithium ion secondary battery 20. The switch SW1 is connected to an output port of the microcomputer A that outputs a control signal (high level signal, low level signal). Therefore, when the switch SW1 is turned on by the control signal from the microcomputer A, the current flowing through the lithium ion secondary battery 20 is consumed by the bypass resistor R, and the capacity of each lithium ion secondary battery 20 can be adjusted. is there.

  Further, the microcomputer A has an output port that outputs a control signal to the switch SW2. The other end of the switch SW2 is connected to one end of the external load 32, and the other end of the external load 32 is connected to the ground. For this reason, the switch SW <b> 2 is turned on by the control signal from the microcomputer A, so that power from the secondary battery system 40 is supplied to the external load 32.

  As the switches SW1 and SW2, for example, FETs that function as switching elements can be used. That is, the output port of the microcomputer A is connected to the gate of the FET. Accordingly, when a weak high level signal is input from the output port of the microcomputer A to the gate of the FET, a current flows between the drain and the source, and the switches SW1 and SW2 are turned on.

  As shown in FIG. 2, the lithium ion secondary battery 20 constituting the battery unit 25 includes a bottomed cylindrical battery can 4 made of steel plated with nickel. The battery can 4 accommodates an electrode group G in which the positive electrode plate 1 and the negative electrode plate 2 are wound with the separator 3 interposed therebetween.

  On the upper side of the electrode group G, a positive electrode current collecting lead portion 7 made of aluminum for collecting a potential from the positive electrode plate 1 is disposed on a substantially extension line of the winding center. The positive electrode current collecting lead portion 7 is ultrasonically bonded to the end portion of the positive electrode current collecting lead piece 5 led out from the positive electrode plate 1. A disk-shaped battery lid 9 serving as a positive electrode external terminal is disposed above the positive electrode current collecting lead portion 7.

  The battery lid 9 is composed of a steel disk-shaped terminal plate whose central portion protrudes upward, and an aluminum annular plate having a gas discharge opening formed in the central portion. An annular positive terminal portion 11 is disposed between the protruding portion of the terminal plate and the flat plate. The upper surface and the lower surface of the positive electrode terminal portion 11 are in contact with the lower surface of the terminal plate and the upper surface of the flat plate, respectively. The inner diameter of the positive electrode terminal portion 11 is larger than the inner diameter of the opening formed in the flat plate. On the upper side of the opening of the flat plate, a rupture valve 10 that cleaves when the battery internal pressure rises is disposed so as to close the opening. The peripheral edge portion of the rupture valve 10 is sandwiched between the lower surface of the inner edge portion of the positive electrode terminal portion 11 and the flat plate. The peripheral part of the terminal plate and the peripheral part of the flat plate are fixed. The upper surface of the positive electrode current collector lead portion 7 is joined to the lower surface of the flat plate, that is, the bottom surface of the battery lid 9 (surface on the electrode group G side) by resistance welding.

  On the other hand, a negative electrode current collecting lead portion 8 made of nickel for collecting a potential from the negative electrode plate 2 is disposed below the electrode group G. The negative electrode current collecting lead portion 8 is ultrasonically bonded to the end portion of the negative electrode current collecting lead piece 6 led out from the negative electrode plate 2. The negative electrode current collecting lead portion 8 is joined by resistance welding to the inner bottom portion of the battery can 4 that also serves as a negative electrode external terminal.

In addition, a non-aqueous electrolyte is injected into the battery can 4. In this example, the non-aqueous electrolyte is 1 mol / liter of lithium hexafluorophosphate (LiPF ₆ ) in a mixed organic solvent having a volume ratio of 1: 3 of ethylene carbonate (EC) and ethyl methyl carbonate (EMC). What was dissolved so that it might become the density | concentration of a liter is used. A battery lid 9 is caulked and fixed to the upper part of the battery can 4 via a gasket 12. For this reason, the inside of the lithium ion secondary battery 20 is sealed.

  The electrode group G accommodated in the battery can 4 is such that the positive electrode plate 1 and the negative electrode plate 2 are not in contact with each other via a microporous separator 3 made of polyethylene or the like. Has been wounded. The positive electrode current collecting lead piece 5 and the negative electrode current collecting lead piece 6 are respectively disposed on opposite end surfaces of the electrode group G. The entire outer peripheral surface of the electrode group G is provided with an insulating coating to prevent electrical contact with the battery can 4.

The positive electrode plate 1 has an aluminum foil as a positive electrode current collector. The thickness of the aluminum foil is set to 20 μm in this example. On both surfaces of the aluminum foil, a positive electrode mixture containing a positive electrode active material is applied substantially evenly. In this example, lithium cobaltate (LiCoO ₂ ) having a layered crystal structure is used as the positive electrode active material. In addition to the positive electrode active material, the positive electrode mixture is mixed with graphite as a conductive material and polyvinylidene fluoride as a binder (binder) (hereinafter abbreviated as PVDF). In this example, the mixing ratio of the positive electrode active material, graphite, and PVDF is adjusted to a weight ratio of 85: 10: 5. The positive electrode plate 1 is obtained by applying a positive electrode mixture kneaded by a kneader to an aluminum foil, and after rolling, is rolled and formed by a press machine. A positive electrode current collecting lead piece 5 is led out to a side edge on one side in the longitudinal direction of the aluminum foil.

  On the other hand, the negative electrode plate 2 has a copper foil as a negative electrode current collector. The thickness of the copper foil is set to 10 μm in this example. A negative electrode mixture containing a negative electrode active material is applied to both sides of the copper foil substantially evenly. In this example, an amorphous carbon material is used for the negative electrode active material. In addition to the negative electrode active material, the negative electrode mixture contains graphite as a conductive material, PVDF as a binder, and lithium titanate as a titanium-containing lithium compound for overdischarge countermeasures. Lithium titanate has a characteristic capable of occluding and releasing lithium ions when the battery voltage is 1 V or more and 2 V or less. In this example, the blend ratio of the negative electrode active material, graphite, PVDF, and lithium titanate is adjusted to a weight ratio of (90-x): 5: 5: x. The compounding ratio x of lithium titanate is preferably 1 wt% (wt%) or more. If the blending ratio x is less than 1 wt%, it will be difficult to exert an effect on overdischarge. The negative electrode plate 2 is prepared by applying a negative electrode mixture kneaded with a kneader to a copper foil, and after rolling, is roll-molded with a press machine. A negative electrode current collecting lead piece 6 is led out to the side edge on one side in the longitudinal direction of the copper foil.

(Battery assembly)
In the manufacture of the lithium ion secondary battery 20, the produced positive electrode plate 1 and negative electrode plate 2 are vacuum dried at 100 ° C. for 24 hours, and then wound through the separator 3 to produce the electrode group G. At this time, the positive electrode plate 1 and the negative electrode plate 2 are wound so that the positive electrode current collecting lead piece 5 and the negative electrode current collecting lead piece 6 are positioned in opposite directions. Next, all of the positive electrode current collector lead pieces 5 are ultrasonically bonded to the positive electrode current collector lead part 7, and all of the negative electrode current collector lead pieces 6 are ultrasonically bonded to the negative electrode current collector lead part 8. Insulate the surrounding area. The electrode group G to which the positive electrode current collecting lead part 7 and the negative electrode current collecting lead part 8 are connected is inserted into the battery can 4 with the negative electrode current collecting lead part 8 facing the bottom. After the electrode rod is passed through the winding center portion of the electrode group G and the negative electrode current collector lead portion 8 and the inner bottom portion of the battery can 4 are resistance welded, the positive electrode current collector lead portion 7 and the battery lid 9 are joined by resistance welding. . Then, after pouring a non-aqueous electrolyte into the battery can 4, the battery lid 9 is caulked and fixed to the battery can 4 via the gasket 10, thereby completing the lithium ion secondary battery 20 having a battery capacity of 7 Ah. Let

(Operation)
Next, charge / discharge control of the secondary battery system 40 will be described focusing on battery state control of the lithium ion secondary battery 20.

  When charging each lithium ion secondary battery 20 constituting the battery unit 25, a power supply unit (not shown) is connected in parallel with the battery unit 25. Each lithium ion secondary battery 20 is charged with electric power from the power supply unit. Calculation of the state of charge SOC of the lithium ion secondary battery 20 at this time will be described.

  The microcomputer A detects the open circuit voltage data for each lithium ion secondary battery 20 at a rate of once every fixed time (for example, 5 minutes). In other words, the microcomputer A designates the lithium ion secondary battery 20 to be measured from the battery designation port to the voltage measurement circuit 29, whereby the measurement target lithium ion secondary battery is designated from the voltage measurement circuit 29 via the A / D input port. The open circuit voltage of the battery 20 is taken in. The microcomputer A calculates the state of charge SOC of each lithium ion secondary battery 20 from the data of each open circuit voltage according to the battery state map (or relational expression) stored in advance in the ROM and developed in the RAM in the initial setting. In this battery state map, the open circuit voltage and the state of charge SOC are developed in the RAM in a corresponding state. Moreover, the microcomputer A calculates the average charge state of all the lithium ion secondary batteries as the charge state SOC of the battery unit 25. It is also possible to detect the temperature data of the battery unit 25 with a thermistor or the like and add temperature correction when calculating the state of charge SOC.

  In addition, for each lithium ion secondary battery 20, the microcomputer A has a (charge state SOC of the lithium ion secondary battery 20−average charge state of all lithium ion secondary batteries 20) exceeding 5 points (capacity adjustment target) It is determined whether or not the lithium ion secondary battery 20 is present. If the determination is affirmative, the capacity adjustment time t depending on the resistance value of the bypass resistor R is calculated only for the lithium ion secondary battery. . The switch SW1 is turned on for the capacity adjustment time t and the bypass resistance R is connected to the lithium ion secondary battery 20 to be capacity adjusted, thereby averaging the capacity difference between the batteries. When the average charge state of the battery unit 25 is 100%, that is, when the battery unit 25 reaches the full charge state, the microcomputer A determines that the charge is finished, and notifies the outside of the charge by a not-shown notification means. At this time, the connection between the power supply unit (not shown) and the battery unit 25 is released. The calculation of the capacity adjustment time t is disclosed in, for example, Japanese Patent Application Laid-Open No. 2000-31443.

  Next, control during discharge will be described. When the microcomputer A determines that the average charging state of all the lithium ion secondary batteries 20 is 30% or more, the microcomputer A outputs a control signal from the output port to turn on the switch SW2. Thereby, the electric power from the battery unit 25 is supplied to the external load 32.

  Normally, in a lithium ion secondary battery, when discharged from a fully charged state, the battery voltage gradually decreases as the state of charge SOC decreases, that is, the discharge depth DOD (100-SOC) increases. When the discharge proceeds and reaches the end of discharge (a state where the depth of discharge DOD exceeds approximately 90%), the battery voltage rapidly decreases, and when the battery is further discharged, an overdischarge state occurs. On the other hand, in the lithium ion secondary battery 20 in which lithium titanate is mixed in the negative electrode mixture at a ratio of 1 wt% or more, the lithium titanate occludes and releases lithium ions when the battery voltage is 2 V or less and 1 V or more. To do. That is, during discharge, lithium ions are also released from lithium titanate, so that the discharge capacity from 2V to 1V increases. For this reason, the discharge rate with respect to the voltage fluctuation from 2V to 1V becomes moderate (slow) as compared with the conventional lithium ion secondary battery.

  In the secondary battery system 40, the discharge depth DOD is calculated from the state of charge SOC of each lithium ion secondary battery 20 calculated by the microcomputer A at regular intervals in the same manner as when charging, and the average discharge depth of the battery unit 25 is calculated. calculate. The microcomputer A calculates the average depth of discharge for three times from the time when it is determined that the voltage of the all lithium ion secondary battery 20 has decreased to 2 V or less, and outputs a control signal when the third average depth of discharge is 90% or less. To output the switch SW2. Thereby, the electric power supply path with respect to the external load 32 is interrupted | blocked, and the discharge of the battery part 25 stops. When the discharge of the battery unit 25 is stopped, power is supplied to the external load 32 from another battery unit configured in the same manner as the battery unit 25, and each lithium ion secondary battery 20 of the battery unit 25 is charged. To control.

  Next, examples of the lithium ion secondary battery 20 constituting the secondary battery system 40 of the present embodiment will be described. A comparative example manufactured for comparison is also shown. Further, the present invention is not limited to the examples described below.

Example 1
As shown in Table 1 below, in Example 1, lithium ion secondary battery 20 was manufactured by blending lithium titanate as a countermeasure against overdischarge in a proportion of 50 wt% in the negative electrode mixture.

(Example 2 to Example 3)
As shown in Table 1, Examples 2 to 3 were the same as Example 1 except that the proportion of lithium titanate blended in the negative electrode mixture was changed. That is, the blending ratio of lithium titanate was 35 wt% in Example 2 and 1 wt% in Example 3.

(Comparative Example 1 and Comparative Example 2)
As shown in Table 1, in Comparative Example 1, a lithium ion secondary battery was produced in the same manner as in Example 1 except that lithium titanate was not added to the negative electrode mixture. That is, Comparative Example 1 is a conventional lithium ion secondary battery. In Comparative Example 2, a lithium ion secondary battery was produced in the same manner as in Example 1 except that the proportion of lithium titanate blended in the negative electrode mixture was 0.1 wt%.

(Evaluation)
About the lithium ion secondary battery of each Example and the comparative example, the voltage change with respect to the depth of discharge DOD, the discharge capacity ratio, and the discharge rate were evaluated. That is, after each lithium ion secondary battery is initialized, it is discharged from a fully charged voltage of 4.2 V to a discharge end voltage of 1 V at a current rate of 1 C (1 hour rate of rated electrical capacity), and the voltage with respect to the discharge depth DOD. The change of was measured. The discharge capacity ratio is the ratio of the discharge capacity from voltage 2V to 1V to the discharge capacity from voltage 4.2V to 1V (discharge capacity from 2V to 1V / 4) when discharged at a current rate of 1C (1CA). .2V to 1V discharge capacity) was determined as a percentage. As the discharge rate, a discharge rate (mV / s) with respect to a voltage from 2 V to 1 V was obtained. Table 2 below shows the results of discharge capacity ratio and discharge rate.

  As shown in FIG. 2, in each of the lithium ion secondary batteries 20 of Examples 1 to 3 using a negative electrode plate in which 1 wt% or more of lithium titanate is mixed in the negative electrode mixture, discharge at a voltage from 2 V to 1 V is performed. It turned out that a curve becomes gentle compared with the discharge curve of the lithium ion secondary battery of the comparative example 1 which does not mix | blend lithium titanate. Further, in the lithium ion secondary battery of Comparative Example 2 using the negative electrode plate mixed with 0.1 wt% of lithium titanate, the discharge curve is almost the same as that of Comparative Example 1, and the effect of lithium titanate does not appear. understood.

  Moreover, as shown in Table 2, in each lithium ion secondary battery 20 of Examples 1 to 3 using a negative electrode in which 1 wt% or more of lithium titanate was mixed, the ratio of the discharge capacity from voltage 2V to 1V Was found to increase. Moreover, in each lithium ion secondary battery 20 of Example 1- Example 3, it turned out that the discharge rate with respect to the voltage fluctuation from voltage 2V to 1V becomes slow. In particular, in the lithium ion secondary batteries 20 of Example 1 and Example 2 in which lithium titanate is blended at a ratio of 35 wt% and 50 wt%, respectively, the discharge capacity ratio is significantly increased, and the discharge speed is greatly decreased. confirmed. Therefore, by blending lithium titanate at a ratio of 1 wt% or more, the discharge rate at the time of discharging from a voltage of 2 V to 1 V becomes moderate (slow) compared to a conventional lithium ion secondary battery. It was revealed that the lithium ion secondary battery 20 can easily detect and control the overdischarge state. Moreover, in the secondary battery system 40 provided with the lithium ion secondary battery 20, it turned out that the effect excellent in the countermeasure against overdischarge was exhibited and safety and reliability were securable.

(Action etc.)
Next, the operation and the like of the secondary battery system 40 of the present embodiment will be described focusing on the operation of the lithium ion secondary battery 20.

  Conventionally, in a lithium ion secondary battery, a carbon-based negative electrode and a lithium transition metal composite oxide-based positive electrode are generally used. In such a lithium ion secondary battery, for example, the resistance increases at the end of the discharge when the battery voltage is 2.5 V or less, so the discharge rate increases as the voltage decreases, and overdischarge detection and control by the voltage can be performed. It becomes difficult. Furthermore, since the discharge capacity also decreases at the end of such a discharge, it becomes difficult to detect and control overdischarge due to capacity change. When copper is used as the material of the negative electrode current collector, the negative electrode potential is about 3.0 V or more (battery voltage is about 0.6 V or less, the positive electrode potential is 3. 6V or less) may occur. In this state, since the elution voltage of copper is less than 0.6 V, abnormality of the positive electrode due to elution of copper and dissolution of the battery container may occur, the nonaqueous electrolyte may leak, and the battery function may be impaired. In order to avoid this, countermeasures against overdischarge by voltage control using secondary devices such as a protection element and a protection circuit are taken, but the battery configuration becomes complicated and the size is increased. The present embodiment is a lithium ion secondary battery that can solve these problems without using a secondary device.

  In the present embodiment, the lithium ion secondary battery 20 constituting the secondary battery system 40 includes a negative electrode plate including lithium titanate capable of inserting and extracting lithium ions when the battery voltage is 1 V or more and 2 V or less in the negative electrode mixture. 2 has. For this reason, at the time of discharge, lithium ions are also released from lithium titanate in the battery voltage range of 1 V to 2 V, so the ratio of the capacity of 1 V to 2 V to the capacity of 1 V to 4.2 V increases. As a result, the discharge rate is reduced. Thereby, the overdischarge state can be easily detected from the discharge capacity and the gentle discharge speed without reaching the elution voltage of copper used for the negative electrode current collector.

  In the present embodiment, the negative electrode mixture contains lithium titanate at a ratio of 1 wt% or more. Since the effect is insufficient if the mixing ratio of lithium titanate is less than 1 wt% (see Comparative Example 1 described above), the overdischarge state is detected by adding 1 wt% or more of lithium titanate. Can be made easy and reliable (see also Examples 1 to 3). On the other hand, when the blending ratio of lithium titanate exceeds 50 wt%, the blending ratio of the negative electrode active material is relatively lowered, so that it is difficult to obtain a sufficient capacity during normal charge / discharge.

  Furthermore, the secondary battery system 40 of the present embodiment includes the lithium ion secondary battery 20 having the negative electrode plate 2 containing lithium titanate in the negative electrode mixture. For this reason, by detecting the overdischarge state of the lithium ion secondary battery 20, the discharge stop can be controlled by the battery control unit constituting the secondary battery system 40. Thereby, since the discharge of the battery unit 25 constituted by the lithium ion secondary battery 20 can be stopped without being in an overdischarged state, the safety and reliability of the secondary battery system as a whole can be ensured. Can do. In this case, charging / discharging can be repeated by performing charging control from the power supply unit. Such a secondary battery system 40 can be suitably used for a wide range of applications such as a hybrid vehicle, an electric vehicle, a backup power supply (UPS), and a mobile phone power supply.

  In addition, in the lithium ion secondary battery 20 which comprises the secondary battery system 40 of this embodiment, the example which mix | blends lithium titanate with the ratio of 1 wt% or more in the negative electrode compound material was shown. If the blending ratio of lithium titanate is less than 1 wt%, it is difficult to obtain the effect. However, as the blending ratio increases, the proportion of the negative electrode active material relatively decreases, and the normal charge of the lithium ion secondary battery is reduced. The performance in the discharge range from 4.3 V to 2.5 V is deteriorated. For this reason, it is preferable to reduce the mixing ratio of lithium titanate as much as possible. Further, in place of lithium titanate, any compound may be used as long as it is a titanium-containing lithium compound capable of inserting and extracting lithium ions when the battery voltage is 1 V or more and 2 V or less, and constitutes a titanium-containing lithium compound. It is also possible to use a compound in which a part of lithium or titanium is substituted or doped with another element.

  Moreover, in the lithium ion secondary battery 20, although the example which uses lithium cobaltate for the positive electrode active material was shown, this invention is not limited to this, A lithium transition metal complex oxide can be used. Examples of the lithium transition metal composite oxide include lithium nickelate and the like, and a part of the transition metal such as nickel or cobalt can be substituted and doped with one or more transition metals. In addition, since the lithium transition metal composite oxide having a spinel crystal structure has a higher resistance at the end of discharge, a lithium transition metal composite oxide having a layered crystal structure having a resistance at the end of discharge lower than that of the spinel crystal structure is used. It is preferable to use it.

  Further, in the lithium ion secondary battery 20, an example in which amorphous carbon is used as the negative electrode active material has been shown. However, the present invention is not limited to this, and a carbon material that can occlude and release lithium ions is used. be able to. Examples of such a carbon material include graphite materials such as natural graphite and artificial graphite, in addition to amorphous carbon. Since the graphite carbon material has a high resistance at the end of discharge, it is preferable to use an amorphous carbon material whose resistance at the end of discharge is lower than that of the graphite carbon material. Moreover, it is preferable that the irreversible capacity | capacitance of a negative electrode active material is more than the lithium transition metal complex oxide of a positive electrode active material. This takes into account the effective use of the lithium transition metal composite oxide, which is more expensive than the carbon material of the negative electrode active material.

  Furthermore, in the lithium ion secondary battery 20, although the example which mix | blends binders, electrically conductive materials, etc. other than an active material with the positive electrode compound material and the negative electrode compound material was shown, it was mentioned above according to these kinds and compounding ratios. The effect is not impaired at all.

Furthermore, in the lithium ion secondary battery 20, a nonaqueous electrolytic solution in which LiPF ₆ as a lithium salt is dissolved in a mixed organic solvent having a volume ratio of EC and EMC of 1: 3 to a concentration of 1 mol / liter. However, the present invention is not limited to this. Examples of organic solvents other than EC and EMC include propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, γ-valerolactone, methyl acetate, ethyl acetate, methyl propionate, tetrahydrofuran, and 2-methyltetrahydrofuran. 1,2-dimethoxyethane, 1-ethoxy-2-methoxyethane, 3-methyltetrahydrofuran, 1,2-dioxane, 1,3-dioxane, 1,4-dioxane, 1,3-dioxolane, 2-methyl- At least one selected from 1,3-dioxolane, 4-methyl-1,3-dioxolane and the like can be used. As the lithium salt other than LiPF _6, for _example, can be used _{LiBF 4, LiClO 4, LiN (} C 2 F 5 SO 2) at least one selected from ₂ or the like. Furthermore, an electrolyte generally used in a lithium ion secondary battery using a carbon-based material as a negative electrode active material, for example, a known electrolyte such as a solid electrolyte having a lithium ion conductivity, a gel electrolyte, or a molten salt is used. May be.

  Moreover, although copper foil was illustrated as a negative electrode electrical power collector, this invention is not limited to this. The negative electrode current collector may be a material mainly composed of copper. Phosphorus, lead, iron, tin, zinc, nickel, arsenic, bismuth, silver, sulfur, cadmium are used to improve the tensile strength against heat. And other elements such as mercury, selenium, tellurium, zirconium, indium, gallium, titanium, cobalt, antimony and gold.

  Furthermore, although the example which interposes the microporous separator 3 made from polyethylene between the positive electrode plate 1 and the negative electrode plate 2 was shown, this invention is not limited to this. For example, a material such as polypropylene may be used, and when the above-described solid electrolyte is used, the effect of the present invention is not impaired at all even if a lithium ion secondary battery is configured without using a separator. Further, instead of the electrode group G in which the positive electrode plate 1 and the negative electrode plate 2 are wound, it is also possible to use an electrode group configured by stacking positive and negative electrode plates. Furthermore, the battery shape is not limited to a cylindrical shape, and it is needless to say that the battery shape can be applied to a battery having an elliptical or rectangular shape as a cross-sectional shape.

  Furthermore, in the secondary battery system 40, the example in which the battery unit 25 is configured by six lithium ion secondary batteries 20 has been shown, but the present invention is not limited to this. For example, the present invention can be applied to one lithium ion secondary battery 20 and an overdischarge state can be detected easily and reliably. Of course, it may be composed of two or more lithium ion secondary batteries 20.

  Since the present invention provides a lithium ion secondary battery capable of easily detecting an overdischarge state and a secondary battery system including the lithium ion secondary battery, the lithium ion secondary battery and the secondary battery are provided. Since it contributes to the manufacture and sale of systems, it has industrial applicability.

G Electrode group 1 Positive electrode plate (positive electrode)
2 Negative electrode plate (negative electrode)
4 Battery can 9 Battery cover 20 Cylindrical lithium ion secondary battery 25 Battery part 27 Battery control part 40 Secondary battery system

Claims (4)

A positive electrode in which a positive electrode mixture containing a lithium transition metal composite oxide is applied to a positive electrode current collector;
A negative electrode of a metal foil in which the negative electrode active material capable of occluding and releasing lithium ions and the negative electrode mixture containing a titanium-containing lithium compound capable of occluding and releasing lithium ions when the battery voltage is 1 V or higher and 2 V or lower is mainly composed of copper A negative electrode coated on a current collector;
An electrolyte containing a lithium salt;
With
The lithium ion secondary battery, wherein the titanium-containing lithium compound is contained in the negative electrode mixture in a proportion of 1 wt% or more.   2. The lithium ion secondary battery according to claim 1, wherein the titanium-containing lithium compound is lithium titanate.   The lithium ion secondary battery according to claim 2, wherein the lithium transition metal composite oxide has a layered crystal structure, and an amorphous carbon material is used for the negative electrode active material. A battery unit composed of one or more of the lithium ion secondary batteries according to claim 1;
A battery control unit for detecting the voltage of each lithium ion secondary battery and controlling the battery state;
A secondary battery system comprising:

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