JP2002298933A - Lithium ion secondary battery pack - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery improved in determining accuracy of the discharging end while maintaining its high capacity and restrained from capacity degradation in repeated cycle. SOLUTION: This lithium ion secondary battery pack is provided with a main unit including a first cell or a battery pack of the first cell, and a voltage detecting unit including a second cell or a battery pack of the second cell and used for determining the last discharging stage of the discharging voltage. The main unit and the voltage detecting unit are connected in series, and a relational expression (1) is established between the first cell and the second cell. X1 <X2 (1).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a lithium ion secondary battery pack.

[0002]

2. Description of the Related Art Lithium-ion secondary batteries are attracting attention as batteries capable of obtaining a high energy density, and are being actively studied. When a lithium ion secondary battery is used as a power source for information devices such as a personal computer, a cordless device such as a cordless cleaner, and an electric vehicle, a voltage higher than a voltage obtained from a unit cell is required. A battery pack including an assembled battery in which the above batteries are connected in series is used. When the assembled batteries are divided according to the connection method of the batteries constituting the assembled batteries, in addition to the assembled batteries in which the cells are connected in series, the batteries in which the cells are connected in parallel, the assembled battery in which the cells are connected in series Examples include a unit in which units are connected in parallel, and a unit in which assembled battery units in which unit cells are connected in parallel are connected in series.

When a lithium ion secondary battery is overdischarged, a copper current collector used for a negative electrode is ionized,
When recharging, the phenomenon of precipitation as metal occurs, which leads to a decrease in battery capacity or a decrease in cycle life, and sometimes causes a shortage in safety due to, for example, a short circuit caused by the precipitated copper. Therefore, it is necessary to forcibly stop the discharge before the overdischarge occurs. In the assembled battery, it is unavoidable that there is a difference between the cells constituting the battery.Therefore, when the termination of the discharge is determined from the voltage of the entire battery pack, a cell having a low capacity or a cell having advanced deterioration has already been detected. Since the battery may be overdischarged, some cells may be rapidly deteriorated. Since the cycle life of the deteriorated unit cell is shortened, the life as a battery pack is inevitably shortened in accordance with the unit cell, and also the safety is reduced. For this reason, it is necessary to detect not only the voltage of the entire battery pack but also the voltage of each single cell or each assembled battery, and to control the discharge so that all the cells constituting the battery pack are not overdischarged. is there. However, when trying to individually detect the voltage of a single cell or a battery pack, a complicated external control circuit is required, which not only increases the size of the battery pack but also increases the manufacturing cost of the battery pack. In recent years, research on application of lithium ion secondary batteries to fields requiring large current discharge, such as cordless devices, power tools, and electric vehicles, has been advanced. In a large current discharge, even if the discharge end is slightly exceeded for a short time, a large overdischarge will occur, and thus the importance of accurate discharge end detection is increasing more and more.

Further, in a battery pack for a cordless device or an electric vehicle, a more accurate display of the remaining capacity has been demanded in order to improve the convenience in addition to the large current characteristics.

[0005] By the way, a carbonaceous material is used for a negative electrode of a lithium ion secondary battery. As a carbonaceous material,
For example, graphite, non-graphitizable carbon, low-temperature calcined carbon (for example, coke) having a calcining temperature of 2000 ° C. or less, and a calcined polymer compound can be used. Lithium ion secondary batteries provided with a negative electrode containing graphite as a carbonaceous material are widely used because of their high charge / discharge capacity and excellent voltage flatness, and have a large technical accumulation.

However, in a lithium ion secondary battery provided with a negative electrode containing graphite as a carbonaceous material, the voltage curve at the time of discharging is flat until immediately before the end of discharging. It has been difficult to accurately detect the discharge end even if the voltage is individually detected. In addition, to display the remaining capacity of the battery pack,
In addition to the need for a complex dedicated circuit for integrating the amount of discharged electricity, even if such a dedicated circuit is provided, the remaining capacity will decrease if the discharge capacity decreases as the charge / discharge cycle progresses. Detection accuracy decreases.

[0007] On the other hand, according to non-graphitizable carbon, low-temperature calcined carbon, and a calcined product of a polymer compound which are carbonaceous materials other than graphite, it is easy to display the remaining capacity because the discharge voltage curve has a slope from the beginning of discharge. In addition, since it can be obtained at a low firing temperature, the firing cost is low and the cost can be reduced. However, carbonaceous materials other than graphite have a significantly lower discharge capacity or initial charge / discharge efficiency than graphite, so that when a lithium ion secondary battery is constructed, a reduction in battery capacity is inevitable.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a lithium ion secondary battery pack in which the accuracy of the termination of discharge is improved while maintaining a high capacity, and the capacity deterioration upon repeated use is suppressed. To provide.

[0009]

A first lithium ion secondary battery pack according to the present invention comprises a main unit including a first unit cell or an assembled battery of the first unit cell, and a second unit cell or a main unit including the first unit cell. A lithium ion secondary battery pack including an assembled battery of a second unit cell, and a voltage detection unit whose discharge voltage is used for determination of the end of discharge, wherein the main unit and the voltage detection unit are , And are connected in series, and the following relational expression (1) is established between the first unit cell and the second unit cell.

X ₁ <X ₂ (1) However, in the relational expression (1), X ₁ is the first
The unit cells in magnitude of slope at 50% depth of discharge in the voltage time course curves obtained when discharged at a single, the X ₂ is obtained when discharged solely the second unit cell This is the magnitude of the slope at a discharge depth of 50% in the voltage aging curve.

[0011] A second lithium ion secondary battery pack according to the present invention comprises a main unit including a first cell or an assembled battery of the first cell, and a main unit including the second cell or the second cell. A lithium ion secondary battery pack including an assembled battery and having a voltage detection unit used for determination of a discharge voltage at the end of discharge, wherein the main unit and the voltage detection unit are connected in series. The first cell includes a negative electrode containing a negative electrode active material having a graphite material content of 70% by weight or more, and the second cell includes:
A negative electrode containing a negative electrode active material having an amorphous carbon content of 50% by weight or more is provided.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a first lithium ion secondary battery pack according to the present invention will be described.

The first lithium ion secondary battery pack includes a first single battery or a first single battery pack.
One or more main units, and one or more voltage detection units including a second cell or an assembled battery of the second cells, wherein a discharge voltage is used to determine the end of discharge of the battery pack. . The one or more main units and the one or more voltage detection units are connected in series.

The following relational expression (1) is established between the first unit cell and the second unit cell.

X ₁ <X ₂ (1) In the relational expression (1), X ₁ is the first type.
The unit cells in magnitude of slope at 50% depth of discharge in the voltage time course curves obtained when discharged at a single, the X ₂ is obtained when discharged solely the second unit cell This is the magnitude of the slope at a discharge depth of 50% in the voltage aging curve. X ₂ is preferably 1.5 times or more of X ₁ .

In the first lithium ion secondary battery pack, it is preferable that the discharge voltage of the entire pack is used for determining the end of discharge. Since both the discharge voltage of the entire pack and the discharge voltage of the voltage detection unit are used for determining the end of discharge, the reliability of the battery pack can be improved. In addition, the voltage detection device for detecting the discharge voltage may be mounted in the first lithium ion secondary battery pack, or the voltage detection device is incorporated in the device, and the device is provided with the battery pack. By incorporating the battery pack, a voltage detection device may be connected to the battery pack.

In the first lithium ion secondary battery pack, it is preferable that the discharge voltage of the voltage detecting unit is used for detecting the remaining capacity. The remaining capacity detection mechanism may be mounted in the first lithium ion secondary battery pack, or the remaining capacity detection mechanism may be incorporated in the device, and the remaining battery capacity may be stored in the battery pack by incorporating the battery pack in the device. A capacitance detection mechanism may be connected. Here, the remaining capacity detection mechanism is to check in advance the voltage change up to the discharge termination of the voltage detection unit, determine the correspondence between the voltage and the depth of discharge, and then calculate the remaining capacity from the actual detected voltage based on that. It is a mechanism for calculating. The remaining capacity detection mechanism may include a remaining capacity display mechanism such as a liquid crystal display for displaying the remaining capacity.

The first lithium ion secondary battery pack comprises:
A temperature monitoring device can be provided.

Further, the first lithium ion secondary battery pack can have a final safety mechanism.

The assembled battery included in the main unit and the voltage detecting unit includes a battery in which a plurality of cells are connected in series, a battery in which a plurality of cells are connected in parallel, and a battery in which a plurality of cells are connected in series. And a plurality of unit cells connected in parallel, and a plurality of unit cells connected in parallel are connected in series. Among them, an assembled battery in which a plurality of unit cells are connected in parallel is preferable. When a unit is constructed from such a battery pack,
Since the voltage of the unit cell and the voltage of the assembled battery unit can be made equal, it is easy to perform safety control using the voltage of the unit. As a result, when the final safety mechanism is mounted on the battery pack, the final safety mechanism can be reliably operated, and the safety of the battery pack can be further enhanced. Here, the final safety mechanism refers to a mechanism that sets a voltage that cannot be reached in normal use and stops the function of the battery pack itself when the voltage is reached.
The voltage is, for example, 4.4 V as a measure during charging,
For example, 2 V can be used as a measure during discharging.

The first lithium ion secondary battery pack will be described in detail.

1) Main Unit This main unit includes a first unit cell or an assembled battery of the first unit cell.

It is preferable that the total capacity of all the main units is larger than the total capacity of the voltage detecting unit.

The first cell has a positive electrode, a negative electrode, and a non-aqueous electrolyte.

A) Positive Electrode This positive electrode has a positive electrode current collector and a positive electrode layer supported on one or both surfaces of the positive electrode current collector and containing an active material and a binder.

Examples of the positive electrode active material include lithium-containing nickel oxide, lithium-containing cobalt oxide, lithium-containing iron oxide, lithium-containing nickel-cobalt oxide, lithium-manganese composite oxide, lithium-containing vanadium oxide, and disulfide. Chalcogen compounds such as titanium and molybdenum disulfide can be exemplified. In particular, lithium-containing cobalt oxide, lithium-containing nickel oxide,
The use of a lithium manganese composite oxide is preferable because a lithium ion secondary battery with high capacity and high output can be realized.

Examples of the binder include polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVd).
F), ethylene-propylene-diene copolymer (EPDM) and the like can be used.

The positive electrode layer can contain a conductive agent. As the conductive agent, graphite, carbon black,
Acetylene black or the like can be used.

As the current collector, a perforated or non-porous foil of aluminum, stainless steel, nickel or the like can be used.

The positive electrode is coated with a coating solution obtained by mixing the positive electrode active material, the conductive material and the binder in an appropriate solvent, and then drying the resultant. It is produced by molding.

B) Negative Electrode This negative electrode has a negative electrode current collector and a negative electrode layer supported on one or both surfaces of the negative electrode current collector and containing a negative electrode active material and a binder.

Examples of the negative electrode active material include materials that occlude and release lithium ions. A carbonaceous material can be used as a material that stores and releases lithium ions. The carbonaceous material is, for example, carbonized by firing natural and artificial graphite, coke and pitch such as petroleum and coal, low molecular weight organic compounds such as natural gas and lower hydrocarbons, and synthetic polymers such as polyacrylonitrile and phenolic resin. It is obtained by doing. In particular, the content of the graphite material is preferably 70 to 100% by weight. When the content of the graphitic material is less than 70% by weight, there is a possibility that the charge / discharge efficiency or the discharge capacity in the initial stage may be reduced. In particular, the content of the graphite material is desirably set to 80 to 100% by weight in order to improve the discharge capacity and the electrode density. Such a negative electrode active material can further increase the discharge capacity of the main unit.

Examples of the binder include polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVd).
F), styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC) and the like can be used.

As the current collector, a perforated or non-perforated foil of copper, stainless steel, nickel or the like can be used.

The negative electrode is prepared, for example, by applying a coating liquid obtained by mixing the negative electrode active material and the binder in an appropriate solvent on the current collector, drying and applying pressure. It is made.

C) Nonaqueous Electrolyte The form of the nonaqueous electrolyte can be, for example, a liquid, a gel, or a solid.

The liquid non-aqueous electrolyte contains a solvent containing an organic solvent and an electrolyte dissolved in the solvent.

The gel non-aqueous electrolyte contains the liquid non-aqueous electrolyte and a polymer material that is composited with the liquid non-aqueous electrolyte.

The solid non-aqueous electrolyte contains an electrolyte.

Examples of the polymer material include polyvinylidene fluoride (PVdF) and polyacrylonitrile (PA
N), polyacrylate, polyethylene oxide (P
EO).

Examples of the organic solvent include ethylene carbonate (EC) and propylene carbonate (P
C), dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethyl carbonate (DE
C), γ-butyrolactone (BL), acetonitrile (AN), ethyl acetate (EA), toluene, xylene,
Methyl acetate (MA) or the like can be used.

As the electrolyte, for example, lithium salts such as lithium perchlorate, lithium hexafluorophosphate, lithium borofluoride, lithium arsenide hexafluoride, lithium trifluoromethanesulfonate, and lithium bistrifluoromethylsulfonylimide are used. be able to.

A separator can be provided between the positive electrode and the negative electrode. As the separator, for example, a synthetic resin nonwoven fabric, a polyethylene porous film, a polypropylene porous film, or the like can be used.

2) Voltage Detecting Unit The voltage detecting unit includes the second unit cell or an assembled battery of the second unit cell. If the pack capacity is small, such as a battery pack used for portable electronic devices,
By using only one voltage detection unit, a battery pack having the largest capacity can be obtained. On the other hand, when an electric vehicle or the like has a large pack capacity or a large number of constituent batteries, the environment such as the temperature at which the constituent units are placed may differ depending on the position in the battery pack. This is also valid. In that case, the voltage detection unit that detected the termination first is
Determine the end of the entire battery pack.

The second cell has a positive electrode, a negative electrode, and a non-aqueous electrolyte. Further, a separator can be arranged between the positive electrode and the negative electrode. As the positive electrode, the non-aqueous electrolyte, and the separator, the same ones as those described in the first cell can be used.

As the negative electrode, a negative electrode current collector and a negative electrode layer supported on one or both surfaces of the negative electrode current collector and containing a negative electrode active material and a binder can be used.

Examples of the negative electrode active material include materials that occlude and release lithium ions. As the material for inserting and extracting lithium ions, the same materials as those described in the first cell can be used. Above all, the content of amorphous carbon is 50 to 100% by weight.
It is preferable that Since the characteristic that the negative electrode potential almost always rises during discharging can be obtained, the battery voltage can be steadily reduced in almost all areas during discharging as a lithium ion secondary battery, and the discharge termination can be determined. Accuracy can be improved, and the remaining capacity can be accurately displayed at almost all stages during discharging. In particular, the depth of discharge before reaching the end of discharge is 30% to 70%.
%, It is possible to improve the accuracy of detecting the discharge state in the vicinity of%.

Here, the amorphous carbon means a carbonaceous material having a plane distance d ₀₀₂ of (002) plane by powder X-ray diffraction of 0.342 nm or more. Amorphous carbon includes, for example, coke and pitch such as petroleum and coal, low molecular weight organic compounds such as natural gas and lower hydrocarbons, and synthetic polymers such as polyacrylonitrile and phenol resin.
It is produced by firing at 000 ° C. and carbonizing. In particular, non-graphitizable carbon obtained by baking a thermosetting resin or the like is desirable. Non-graphitizable carbon has a high capacity among amorphous carbons and can realize charge / discharge characteristics closer to the main unit.

When the content of amorphous carbon in the negative electrode active material is 50
When the content is not less than 100% by weight and less than 100% by weight, graphite is preferably added as another carbonaceous material. Such a negative electrode active material can further improve the initial charge / discharge efficiency of the secondary battery.

As the binder and the current collector, the same ones as described in the first cell can be used.

The negative electrode is prepared by, for example, applying a coating liquid obtained by mixing the negative electrode active material and the binder in a suitable solvent, onto the current collector, drying, and then press-molding. It is made.

It is preferable that the nominal capacity of the second cell is equal to or smaller than the nominal capacity of the first cell.
With such a configuration, the first cell can be a discharge termination in a shallower discharge state,
The effect of preventing overdischarge of the unit cells of the battery pack is enhanced. In particular, in order to reduce deterioration due to cycle use without impairing the capacity of the battery pack, the nominal capacity of the second cell is set to 90 to 9 times the nominal capacity of the first cell.
It is preferable to be within the range of 8%.

3) Relational expression (1) The voltage gradient X _{1 of the first} cell in the relational expression (1)
And the voltage gradient X2 of the _second cell is calculated by the method described below.

One cell is arbitrarily selected from among the first cells constituting the main unit. The first unit cell is discharged alone to obtain a voltage change curve with time. Seeking the gradient of tangent at discharge depth 50% in the voltage time course curve, to the magnitude of the inclination (absolute value) and X _1. On the other hand, one cell is arbitrarily selected from the second cells constituting the voltage detection unit. The second cell is discharged independently to obtain a voltage aging curve. The slope of the tangent line at a discharge depth of 50% in the voltage aging curve is determined, and the magnitude (absolute value) of this slope is defined as X ₂ .

Here, the 50% depth of discharge means that a single cell is charged at a constant current of 0.2 C to 4.2 V, and after reaching 4.2 V, the total charging time (total of the constant current charging time and the constant voltage charging time) is reached. ) Is 10 hours, and is a discharge time required for discharging at 0.2 C until the discharge capacity reaches 50% of the nominal capacity after charging at 4.2V constant voltage. 1C is a current value required to discharge or charge the nominal capacity (Ah) in one hour. Therefore, 0.2C
Is a current value required to discharge or charge the nominal capacity (Ah) in 5 hours.

If the voltage gradient X ₁ of the first unit cell is larger than the voltage gradient X _{2 of the second} unit cell, the discharge voltage of the main unit continuously drops from the middle stage of discharge, so that the main unit And the capacity of the battery pack is reduced. At the same time, since the flatness of the voltage curve of the voltage detection unit after the middle stage of discharge is high, it becomes difficult to accurately determine the discharge end of the battery pack. If the voltage gradient X1 of the _first cell is equal to the voltage gradient X2 of the _second cell, the discharge termination cannot be determined accurately or the capacity of the battery pack decreases.

By making the voltage gradient X ₂ of the _second unit cell larger than the voltage gradient X _{1 of the first} unit cell, the discharge voltage is continuously reduced in the voltage detecting unit. The discharge voltages can be made to correspond substantially one-to-one, and the end of discharge of the voltage detecting unit can be accurately determined from the discharge voltage. The voltage detection unit is
Since it is connected in series with the main unit, the current drawn from the battery pack always passes through the voltage detection unit and is discharged by the same amount of current in synchronization with the main unit. It is guaranteed to be the same as the discharge state of the whole pack. Therefore,
By detecting the discharge voltage of the voltage detection unit, the discharge of each unit can be terminated at an appropriate time without detecting the voltage of each unit constituting the battery pack. As a result, it is possible to avoid that some of the cells constituting the battery pack become overdischarged, so that the discharge capacity is reduced when the battery pack is repeatedly used and the specific cell or unit is not discharged. Unsafety of the battery pack due to rapid deterioration due to overdischarge can be suppressed.

Further, in a case where a discharge at a large current is assumed or a variation in capacity of the lithium ion secondary battery constituting the battery pack becomes a problem, the voltage at which the discharge is terminated should be set higher. Thus, the risk of overdischarge of the lithium ion secondary battery constituting the battery pack can be greatly reduced. If the flatness of the discharge voltage curve of the voltage detection unit is high, if the discharge end is advanced, the discharge end is detected at the voltage of the voltage flat portion, which is extremely difficult or practically impossible.

In the first lithium ion secondary battery pack according to the present invention, the voltage gradient X ₂ of the _second cell is set to be 1.5 times or more the voltage gradient X ₁ of the _first cell. preferable. With such a configuration, the accuracy of the determination of the discharge end can be increased, and the cycle life of the pack can be further improved. Further, the discharge capacity of the battery pack can be further improved.

Further, since the discharge voltage of the voltage detecting unit is continuously reduced, the discharge voltage is utilized for the display of the remaining capacity of the battery pack, whereby the display of the remaining capacity with high accuracy can be realized. .

Next, a second lithium ion secondary battery pack according to the present invention will be described.

[0062] The second lithium ion secondary battery pack according to the present invention comprises one or more main units including a first cell or an assembled battery of the first cell, and a second cell or a second cell. And one or more voltage detection units whose discharge voltage is used to determine the end of discharge. The one or more main units and the one or more voltage detection units are connected in series.

The first unit cell includes a negative electrode containing a negative electrode active material having a graphite material content of 70% by weight or more. On the other hand, the second cell includes a negative electrode containing a negative electrode active material having an amorphous carbon content of 50% by weight or more. As the negative electrode active material included in the first unit cell and the second unit cell, those similar to those described in the first lithium ion secondary battery pack described above can be used.

According to the second lithium ion secondary battery pack, the first negative electrode active material having a graphite material content of 70% by weight or more is used for the negative electrode of the first unit cell in the main unit. Therefore, the initial charge / discharge efficiency and the discharge capacity can be increased. The negative electrode of the second cell in the voltage detecting unit has an amorphous carbon content of 50% by weight.
Since the above-described second negative electrode active material is used, the discharge voltage of the voltage detection unit can be continuously reduced over almost the entire region. As a result, the discharge depth and the discharge voltage can be made to correspond one-to-one, so that the discharge termination can be accurately determined from the discharge voltage of the voltage detection unit. Since the voltage detection unit is connected in series with the main unit, the discharge of each unit is terminated at an appropriate time by terminating the discharge of the battery pack based on the discharge termination of the voltage detection unit. be able to. As a result, it is possible to avoid that some of the cells constituting the battery pack become overdischarged, so that a decrease in the discharge capacity when the battery pack is repeatedly used in cycles can be suppressed. Further, it is possible to save the trouble of detecting and monitoring the voltage of each unit constituting the battery pack.

Further, in applications where discharge at a large current is assumed, or when variations in the capacity of the lithium ion secondary battery constituting the battery pack pose a problem, it is necessary to set a higher voltage as the discharge termination. Thus, the risk of overdischarge of the lithium ion secondary battery constituting the battery pack can be greatly reduced. If the first negative electrode is used as the negative electrode of both the main unit and the voltage detecting unit, if the discharge termination is to be advanced, the discharge termination is detected at the voltage of the voltage flat portion, which is extremely difficult or practical. Impossible.

Further, since the main unit and the voltage detecting unit are connected in series, the current taken out of the battery pack always passes through the voltage detecting unit, so that the same amount of current is synchronized with the main unit. It is guaranteed that the discharged state of the voltage detecting unit is the same as the discharged state of the entire battery pack.

Further, since the discharge voltage of the voltage detecting unit is continuously reduced, the discharge voltage is used for displaying the remaining capacity of the battery pack, so that the display of the remaining capacity with high accuracy can be realized. .

The first cell and the second cell included in the second lithium ion secondary battery pack can satisfy the above-mentioned relational expression (1).

[0069]

Embodiments of the present invention will be described below in detail with reference to the drawings.

(Example 1) <Manufacture of lithium ion secondary battery (first unit cell) for main unit> Lithium cobalt oxide (Li ₂ Co)
O ₂ ) 90% by weight powder, 2% by weight acetylene black,
A slurry containing 3% by weight of graphite and 5% by weight of polyvinylidene fluoride as a binder was prepared, the slurry was applied to an aluminum foil, dried, and then pressed to obtain a positive electrode.

The mesophase pitch-based carbon fiber was 3,000
By firing at ℃, a mesophase pitch-based fibrous graphite powder was obtained as a graphite material. After preparing a slurry containing 87% by weight of graphite material, 10% by weight of artificial graphite having an average particle size of 5 μm, 1% by weight of carboxymethylcellulose and 2% by weight of styrene-butadiene rubber, the slurry is applied to copper foil and dried. After that, pressure molding was performed to obtain a negative electrode.

Further, a polyethylene porous film was prepared as a separator.

On the other hand, 1M lithium hexafluorophosphate was dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate (volume ratio 1: 1) to prepare a liquid non-aqueous electrolyte.

After laminating the positive electrode, the separator, and the negative electrode in this order, they were spirally wound to form an electrode group having an outer diameter of a wound coil of 16.7 mm.

After the electrode group was placed in a stainless steel cylindrical can (diameter: 18 mm, height: 65 mm), a liquid nonaqueous electrolyte was injected, the opening of the can was sealed, and the structure shown in FIG. 2 was obtained. The lithium ion secondary battery for the main unit was assembled.

As shown in FIG. 2, a bottomed cylindrical container 1 made of, for example, stainless steel has an insulator 2 disposed at the bottom. The electrode group 3 is housed in the container 1.
The electrode group 3 has a structure in which a band formed by laminating a positive electrode 4, a separator 5 and a negative electrode 6 in this order is spirally wound.

The container 1 contains a non-aqueous electrolyte. A PTC element 7 having an opening in the center,
A safety valve 8 disposed on the TC element 7 and a hat-shaped positive terminal 9 disposed on the safety valve 8 are caulked and fixed to an upper opening of the container 1 via an insulating gasket 10. The positive electrode terminal 9 has a built-in safety mechanism serving as a gas vent hole (not shown). One end of a positive electrode lead 11 is connected to the positive electrode 4 and the other end is connected to the PTC element 7.
Connected to each other. The negative electrode 6 is connected to the container 1 as a negative terminal via a negative lead (not shown).

The main unit lithium-ion secondary battery was charged at a constant current of 4.2 V at 280 mA corresponding to 0.2 C, and after reaching 4.2 V, the voltage was maintained at 4.2 V until the total time reached 10 hours. The battery was charged with voltage. Then 280
When the battery was discharged to 2.7 V at mA, the discharge capacity was 1
It was 400 mAh.

<Manufacture of Lithium Ion Secondary Battery (Second Single Cell) as Voltage Detection Unit> As an amorphous carbon, an isotropic pitch was fired at 1500 ° C. to obtain a non-graphitizable carbon powder. After preparing a slurry containing 90% by weight of non-graphitizable carbon powder and 10% by weight of polyvinylidene fluoride, the slurry was applied to a copper foil, dried, and then pressed to obtain a negative electrode.

A lithium ion secondary battery for voltage detection having the same configuration as that of the lithium ion secondary battery for the main unit, except that such a negative electrode is used and the outer diameter of the wound coil is set to 17.3 mm. Assembled.

The lithium ion secondary battery for the voltage detection unit was charged at a constant current of 4.2 V at 280 mA corresponding to 0.2 C. After reaching 4.2 V, the battery was charged with 4.2 V until 10 hours in total. The battery was charged at a constant voltage. afterwards,
When discharge was performed at 280 mA to 2.7 V, the discharge capacity was 1400 mAh.

As shown in FIG. 1, one lithium ion secondary battery for the main unit was prepared as the main unit 12, and one lithium ion secondary battery for the voltage detection unit was prepared as the voltage detection unit 13. . Five main units 12 and one voltage detection unit 13 were connected in series. A first voltage detecting device 14 is connected so that the voltage between both ends of the voltage detecting unit 13 can be detected, and a second voltage detecting device 14 is connected to both ends of six units connected in series so that the voltage of the entire battery pack can be detected. Voltage detector 1
5 to form a lithium ion secondary battery pack.

The battery pack was charged at 280 mA until the voltage of the entire battery pack reached 25.2 V, and after reaching 25.2 V, the battery pack was charged at a constant voltage of 25.2 V so that the total charging time was 10 hours. After the opening time of 30 minutes, the voltage of the voltage detecting unit 13 detected by the first voltage detecting device 14 becomes 2.8 V, or the voltage of the entire battery pack detected by the second voltage detecting device 15 becomes Constant current discharge was performed at 280 mA until the voltage reached 16.2 V. This was repeated 100 times.

(Example 2) A lithium ion secondary battery for a main unit was assembled in the same manner as described in Example 1 except that the outer diameter of the wound coil of the electrode group was 17.2 mm. Was.

The lithium ion secondary battery for the main unit was charged at a constant current of up to 4.2 V at 0.2 C.
After reaching 2 V, charging was performed at a constant voltage of 4.2 V until the total time reached 10 hours. Thereafter, when discharging was performed at 2.7 C to 2.7 V, the discharging capacity was 1500 mAh. Therefore, the lithium ion secondary battery for voltage detection (second
Of the lithium ion secondary battery for the main unit (first unit cell) corresponds to 93% of the nominal capacity of the main unit lithium ion secondary battery (first unit cell).

A lithium ion secondary battery pack was constructed in the same manner as described in Example 1 except that such a lithium ion secondary battery for a main unit was used.

The obtained battery pack of Example 2 was
The charge / discharge cycle was repeated 100 times under the same conditions as described in Example 1 described above.

Example 3 A lithium ion secondary battery 16 for the main unit and a lithium ion secondary battery 17 for voltage detection were manufactured in the same manner as described in Example 1.

As shown in FIG. 3, an assembled battery in which two main unit lithium ion secondary batteries 16 were connected in parallel was prepared as a main unit 18. Further, an assembled battery in which two lithium ion secondary batteries 17 for voltage detection were connected in parallel was prepared as a unit 19 for voltage detection. Five main units 18 and one voltage detection unit 19 were connected in series. The voltage detecting device 14 was connected so that the voltage at both ends of the voltage detecting unit 19 could be detected. Furthermore, the voltage detection device 1 is connected to both ends of a serial connection of five main units 18 and one voltage detection unit 19.
5 to form a lithium ion secondary battery pack.

The battery pack was charged at 560 mA until the voltage of the entire battery pack reached 25.2 V, and after reaching 25.2 V, the battery pack was charged at a constant voltage of 25.2 V so that the total charging time was 10 hours. Until the voltage of the voltage detecting unit 19 detected by the first voltage detecting device 14 becomes 2.7 V after the opening time of 30 minutes, or the second voltage detecting device 1
The battery was discharged at a constant current of 560 mA until the voltage of the entire battery pack detected at 5 was 16.2 V. This was repeated 100 times.

(Comparative Example 1) A lithium ion battery having the same configuration as that described in Example 1 above, except that the lithium ion secondary battery for the main unit described in Example 1 was also used for the voltage detecting unit. A secondary battery pack was obtained.

The obtained battery pack of Example 1 was
The battery was charged at 280 mA until the voltage of the entire battery pack reached 25.2 V, and after reaching 25.2 V, charging was performed at a constant voltage so that the total charging time was 10 hours. After an opening time of 30 minutes, the voltage of the entire battery pack detected by the second voltage detecting device 15 becomes 16.2 V or the voltage of the main unit detected by the first voltage detecting device 14 becomes 2.7 V. Until 2
Constant current discharge was performed at 80 mA. This was repeated 100 times.

(Comparative Example 2) A lithium ion battery having the same configuration as that described in Example 3 described above, except that the lithium ion secondary battery for the main unit described in Example 3 was also used for the voltage detection unit. A secondary battery pack was obtained.

The obtained battery pack of Example 3 was
The battery was charged at 560 mA until the voltage of the entire battery pack reached 25.2 V, and after reaching 25.2 V, charging was performed at a constant voltage of 25.2 V so that the total charging time was 10 hours. After the opening time of 30 minutes, the voltage of the main unit detected by the first voltage detecting device 14 becomes 2.7 V or the voltage of the entire battery pack detected by the second voltage detecting device 15 becomes 16.
Constant current discharge was performed at 560 mA to 2 V. This one
Repeated 00 times.

FIG. 4 shows charge / discharge cycle characteristics of the battery packs of Examples 1 to 3 and Comparative Examples 1 and 2. In Examples 1 to 3, the discharge termination was determined by the voltage of the voltage detection unit in any of the battery packs, and the discharge termination was not determined by the voltage of the entire battery pack. On the other hand, in Comparative Examples 1 and 2, the voltage of the entire battery pack determined the discharge termination.

As is apparent from FIG. 4, the deterioration of the battery packs of Examples 1 to 3 when the cycles were repeated was suppressed as compared with Comparative Examples 1 and 2. In particular,
In the battery pack of Example 2, since the voltage detection unit was formed from a lithium ion secondary battery having a smaller capacity than the lithium ion secondary battery forming the main unit, cycle deterioration was further suppressed.

In the battery pack of Example 1, the lithium ion secondary battery (first unit cell) for the main unit of the main unit 12 alone was charged at a constant current of 0.2 C to 4.2 V and reached 4.2 V. after constant voltage charge at 4.2V so that the total charging time (the sum of the constant current charging time and a constant voltage charging time) of 10 hours, a voltage change with time curves V ₁ of the time of discharging under 0.2C FIG later It is shown in FIG. Further, the voltage detection unit 13 alone charges the voltage detection lithium ion secondary battery (second cell) at a constant current of 0.2 C to 4.2 V, and after reaching 4.2 V, a total charging time (constant current). after charged at 4.2V constant voltage such that the sum of the charging time and the constant voltage charging time) of 10 hours, are also shown the voltage changes over time curve V ₂ at the time of discharge at 0.2C in FIG. FIG. 5 also shows a discharge voltage curve V of the entire battery pack in the second cycle of the charge / discharge cycle test described above.

FIG. 5 shows that the lithium ion
The time-dependent change curve of the on-secondary battery (first unit cell)
At a depth of discharge of 50% (discharge time 1.92 hours)
Line L ₁When the slope of this slope was calculated, the magnitude of this slope (absolute
Value) X₁Was 43 mA / hr. On the other hand, for voltage detection
Temporal change in voltage of lithium ion secondary battery (second cell)
50% (discharge time 1.92 hours)
Tangent L at_TwoWhen we calculated the slope of
Size (absolute value) X_TwoWas 120 mA / hr. Yo
The lithium ion secondary battery for voltage detection (the second single
Pond) tilt X_TwoIs the lithium ion secondary battery for the main unit.
Pond (first cell) tilt X₁2.8 times.

As is clear from FIG. 5, the discharge voltage curve V ₂ of the lithium ion secondary battery for the voltage detection unit used in the voltage detection unit 13 is larger than the discharge voltage curve V of the entire battery pack. It can be seen that the voltage curve has a slope over the discharge region, and that the end of the discharge can be easily detected. Furthermore, if the discharge termination is made at 3V or more by the detection voltage of the voltage detection unit 13, the discharge can be completed with a sufficient margin, so that the lithium ion secondary batteries constituting the battery pack have variations in capacity. Also, the discharge termination can be performed before all the lithium ion secondary batteries reach overdischarge. On the other hand, in the discharge voltage curve V of the whole battery pack, since the voltage at the discharge time of 4.5 hours near the discharge end is in the voltage flat portion near 22.5 V, it is not possible to determine the discharge end from the discharge voltage. You can see that. In the discharge voltage curves V ₁ of the main unit lithium ion secondary battery used in the main unit 12, the voltage at close to the discharge end discharge time 4.5 hours in the flat portion, the voltage of the entire battery pack As in the case, it can be seen that the discharge end cannot be determined from the discharge voltage.

(Embodiment 4) With respect to the lithium ion secondary battery pack of Embodiment 1, the voltage of the whole battery pack was 25.
The battery was charged at 280 mA to 2 V, and after reaching 25.2 V, the battery was charged at a constant voltage of 25.2 V so that the total charging time was 10 hours. After the opening time of 30 minutes, the voltage of the voltage detection unit 13 detected by the first voltage detection device 14 becomes 2.
560 until the voltage reaches 8 V or until the voltage of the entire battery pack detected by the second voltage detection device 15 becomes 16.2 V.
Constant current discharge was performed at 0 mA. This was repeated 50 times.

Comparative Example 3 With respect to the lithium ion secondary battery pack of Comparative Example 1, the voltage of the whole battery pack was 25.
The battery was charged at 280 mA to 2 V, and after reaching 25.2 V, the battery was charged at a constant voltage so that the total charging time was 10 hours.
After an opening time of 30 minutes, the voltage of the entire battery pack detected by the second voltage detection device 15 becomes 16.2 V or the voltage of the main unit detected by the first voltage detection device 14 becomes 2.8 V. Until then, constant current discharge was performed at 5600 mA. This was repeated 50 times.

FIG. 6 shows the cycle evaluation results of Example 4 and Comparative Example 3 when large current discharge was performed. As is clear from FIG. 6, the battery pack of Example 4 has more suppressed cycle deterioration than Comparative Example 3. It can be seen that a larger effect can be obtained when using a large current discharge as compared with the cycle test result at 0.2 C shown in FIG.

(Embodiment 5) The correspondence between the voltage and the remaining capacity of the lithium ion secondary battery for voltage detection prepared in advance was input to the voltage detector 14 of the battery pack of the first embodiment. Example 5 with connection of a remaining capacity detection mechanism
Of the battery pack.

The lithium ion secondary battery pack of Example 5 was charged and discharged four times under the same conditions as in Example 1,
Charged. At the time of the subsequent discharge, the remaining capacity calculation value by the remaining capacity detection mechanism is recorded at 1 hour, 2 hours, and 3 hours after the start of the discharge. Just calculated what was. The results are shown in Table 1 below.

(Comparative Example 4) The correspondence between the voltage and the remaining capacity of the lithium ion secondary battery for voltage detection prepared in advance was input to the voltage detector 14 of the battery pack of Comparative Example 1. Comparative Example 4 with connection of remaining capacity detection mechanism
Of the battery pack.

The remaining capacity measurement test was performed on the lithium ion secondary battery pack of Comparative Example 4 in the same manner as described in Example 5 above, and the results are shown in Table 1 below.

[0107]

[Table 1]

As is evident from Table 1, the battery pack of Example 5 has a more accurate display of the remaining capacity in the entire discharging process. On the other hand, at the time when 2 hours have passed in Comparative Example 4, since the change in voltage is small, 500 to
It was 900 mAh, which was a wide value, and could not be calculated.

In the above-described embodiment, an example in which a cylindrical cell is used as the unit cell has been described. However, the outer structure of the unit cell is not particularly limited. For example, a rectangular cylindrical container having a bottom is used. A rectangular lithium ion secondary battery, a thin lithium ion secondary battery using a container formed of a film material such as a laminated film, or the like can be used as a unit cell.

[0110]

As described in detail above, according to the lithium ion secondary battery pack of the present invention, it is possible to accurately detect the discharge termination of the battery pack while maintaining high capacity, and to repeat the battery pack. There is a remarkable effect that the deterioration of the battery pack when the battery is used in cycles can be reduced.

[Brief description of the drawings]

FIG. 1 is an electric circuit diagram showing a schematic configuration of a lithium ion secondary battery pack of Example 1.

FIG. 2 is a partially cutaway perspective view showing a cylindrical lithium ion secondary battery which is an example of a single cell used in the lithium ion secondary battery pack of Embodiment 1.

FIG. 3 is an electric circuit diagram illustrating a schematic configuration of a lithium ion secondary battery pack according to a third embodiment.

FIG. 4 is a characteristic diagram showing a change in a discharge capacity retention ratio when changing the number of charge / discharge cycles in the lithium ion secondary battery packs of Examples 1 to 3 and Comparative Examples 1 and 2.

FIG. 5 shows the entire pack and the main unit cell (first cell) in the lithium ion secondary battery pack of Example 1.
FIG. 6 is a characteristic diagram showing a discharge voltage curve of a cell for voltage detection (second cell).

FIG. 6 is a characteristic diagram showing a change in a discharge capacity retention ratio when changing the number of charge / discharge cycles in the lithium ion secondary battery packs of Example 4 and Comparative Example 3.

[Explanation of symbols]

 12: Main unit, 13: Voltage detection unit, 14: First voltage detection device, 15: Second voltage detection device

Continued on front page F term (reference) 5H029 AJ03 AJ12 AK03 AL06 AL07 AL08 AM03 AM05 AM07 BJ06 BJ27 CJ16 HJ18 HJ19 5H030 AA06 AS08 AS11 BB22 FF41 FF44 FF51 5H040 AA36 AA40 AT01 AY04 DD26 NN05 5A08 BA08 A08 A08 HA19

Claims (3)

[Claims] 1. A main unit including a first unit cell or an assembled unit of a first unit cell, and a main unit including a second unit cell or an assembled unit of a second unit cell. A lithium ion secondary battery pack including a voltage detection unit to be used, wherein the main unit and the voltage detection unit are connected in series, and the first unit cell and the second cell unit are connected in series. A lithium ion secondary battery pack characterized in that the following relational expression (1) is established between the battery pack and a single cell. X ₁ <X ₂ (1) However, in the relational expression (1), the X ₁ is the first
X ₂ is obtained when the second cell is discharged alone, with the magnitude of the slope at a discharge depth of 50% in the voltage aging curve obtained when the single cell is discharged alone. This is the magnitude of the slope at a discharge depth of 50% in the voltage aging curve. 2. A main unit including a first unit cell or an assembled unit of the first unit cell, and a main unit including an assembled unit of the second unit cell or the second unit cell. A lithium ion secondary battery pack including a voltage detection unit to be used, wherein the main unit and the voltage detection unit are connected in series, and the first cell is a graphite material. Content of 70% by weight
The second cell includes the negative electrode containing the above negative electrode active material, and the content of the amorphous carbon is 50% by weight.
A lithium ion secondary battery pack comprising a negative electrode containing the above negative electrode active material. 3. The lithium ion secondary battery pack according to claim 1, wherein a discharge voltage of the voltage detection unit is used for detecting a remaining capacity.