JP5197904B2 - Non-aqueous electrolyte secondary battery pack charging method - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to a charge control method for a non-aqueous electrolyte secondary battery pack in which a plurality of non-aqueous electrolyte secondary batteries are connected in series.
[0002]
[Prior art]
In recent years, non-aqueous electrolyte secondary batteries have attracted attention as high-density batteries, and active research has been conducted. When used as a power source for information devices such as personal computers and cordless devices such as cordless cleaners, a voltage higher than that obtained with a single non-aqueous electrolyte secondary battery is required. Is used as a battery pack configured by connecting in series a battery set in which the single cells are connected in parallel.
As such a unit cell used for the battery pack, a non-aqueous electrolyte secondary battery known as a battery having a high energy density is used. This non-aqueous electrolyte secondary battery is configured using a lithium-containing cobalt compound, a lithium-containing nickel compound, a lithium-containing manganese compound, or the like as a positive electrode active material for a positive electrode and a carbonaceous material as a negative electrode active material for a negative electrode. In such a non-aqueous electrolyte secondary battery, in the overcharged state, the positive electrode is decomposed, lithium metal is deposited on the negative electrode, and the like, resulting in deterioration of the battery performance, resulting in a decrease in cycle characteristics. To prevent this, the battery voltage during charging is detected and charged at a constant current until it reaches a predetermined voltage, and after reaching the predetermined voltage, charging is performed by controlling the current so that the predetermined voltage is maintained. ing.
[0003]
In a battery pack having an assembled battery in which non-aqueous electrolyte secondary batteries are connected in series, in order to control the voltage of the entire assembled battery, the maximum charging of the assembled battery determined by multiplying the predetermined voltage of the single battery in series The charging control as described above is performed on the voltage.
[0004]
FIG. 4 shows the constant current / constant voltage charging sequence. FIG. 4 shows the progress of application of current voltage when charging is performed by connecting two batteries (A cell and B cell) in series. What is shown as the current in the figure is a diagram showing the transition of the charging current. In the constant current charging region, the charging current is maintained at a constant value and applied to the battery pack for charging. As a result of charging in this region, the charging voltage of each unit cell increases in sequence as indicated by “A unit cell voltage” or “B unit cell voltage”. Then, the time after the point when the charging voltage of the unit cell reaches the maximum charging voltage is a constant voltage charging region, and in this region, a current controlled so that the charging voltage is maintained at a constant value is applied.
[0005]
By the way, if there are variations in capacity and impedance among the cells constituting the assembled battery, the voltage of each cell differs when the battery pack reaches a predetermined voltage, and a battery that is in an overcharged state is generated. Deterioration was caused, and the cycle characteristics of the battery pack were lowered. In particular, when switching from constant-current charging to constant-voltage charging, the voltage of a specific unit cell becomes maximum and is placed in an overcharged state, so that the deterioration is large. That is, in FIG. 4, at the time of transition from the constant current application region to the constant voltage application region (1.76 hours from the start of charging), the charging voltage of the unit cell B exceeds the maximum charging voltage per unit cell. The state is overcharged. Since non-aqueous electrolyte secondary batteries that are actually produced always have variations, this phenomenon has been a factor in reducing the cycle characteristics of the battery pack.
[0006]
In order to avoid such overcharge, a method has been proposed in which all the constituent cells are equipped with a voltage detection device, and even when one of the constituent cells exceeds a predetermined voltage, the voltage is constant at that time and the control proceeds to current control. ing. However, in this method, a device for detecting the voltage of all the constituent cells and a mechanism for controlling charging by the output from the voltage detection device are required, and the configuration of the battery pack is complicated and the price is inevitably increased. Furthermore, since the charging control is performed by the single battery that has reached the predetermined voltage earliest, there is a problem that the charging time becomes long.
[0007]
[Problems to be solved by the invention]
The present invention provides a battery pack having an assembled battery in which non-aqueous electrolyte secondary batteries are connected in series to improve the cycle characteristics by suppressing the unit cells from being overcharged due to variations in the constituent unit cells, and It is an object of the present invention to provide a non-aqueous electrolyte secondary battery pack that can be charged in a short time.
[0008]
That is, the present invention provides a non-aqueous electrolyte secondary battery pack in which a single non-aqueous electrolyte secondary battery or a plurality of batteries connected in parallel is connected in series to a predetermined maximum charging voltage. In the case of performing constant voltage charging at the predetermined maximum charging voltage following current charging, constant current charging is started at an arbitrary initial charging value, and the maximum charging until the charging voltage of the battery pack reaches the maximum charging voltage. In the battery pack in which the voltage arrival time is t, the initial current value I from 0.25C to 1.5C when the current value for constant current charging of the standard capacity of the battery in 1 hour is 1C, and the 0.25C The relationship between the maximum charging voltage arrival time t corresponding to each I of 1.5 C to 1.5 C is represented by I (t), and the charging current at any time t ′ from the start to the end of charging of the battery pack is expressed as I 80% or less of ( t ' ) And charging the non-aqueous electrolyte secondary battery pack, wherein the current from the start to the end of charging of the battery pack is controlled to decrease continuously.
[0009]
Moreover, in this invention, after the said battery pack reaches | attains predetermined | prescribed maximum charging voltage, it is preferable to control charging current so that the said maximum charging voltage may be maintained.
Furthermore, in the battery pack charging method of the present invention, it is preferable that the charging current is linearly reduced in the region from the start of charging until the maximum charging voltage of the battery pack is reached.
In the present invention, constant current / constant voltage charging applies an initial current value to the battery, maintains this current value for a certain period of time, and when the charging voltage of the battery reaches the maximum charging voltage, the charging voltage remains constant. Thus, the controlled current value is applied.
In the present invention, the charging current is set to 80% or less of Imax in constant current / constant voltage charging. When charging is performed at a current exceeding this range, overcharging cannot be effectively suppressed. Because.
[0010]
In the present invention, by performing the current control as described above, the current can be sufficiently narrowed in the vicinity of the time when the predetermined maximum charging voltage of the battery pack is reached, and this is caused by the variation in characteristics of the single cells constituting the battery pack. Variations in the charging voltage of the unit cell can be suppressed, and the cycle characteristics of the battery pack can be improved. In order to actually apply the current control method, it is preferable to narrow down the current so as to be effective for the maximum variation width in consideration of characteristic variations such as the capacity and impedance of the constituent cells. However, in addition to the above-described variation in electrical characteristics, the temperature distribution in the battery pack also produces substantially the same variation in characteristics. For this reason, even when a battery pack is composed of single cells with extremely small variations, it is necessary to make 80% of Imax. In a battery pack made of single cells with actual variations, it must be 80% or less of Imax. Don't be. Among the charging voltage variations, the charging voltage when reaching the maximum charging voltage has the greatest effect on cycle deterioration. Although the detailed mechanism is not clear, it is presumed that the positive and negative electrode balance in the battery is lost before and after reaching the maximum charging voltage, and the negative electrode potential takes a minimum value.
[0011]
Further, the charging current is continuously decreased until the battery pack reaches a predetermined maximum charging voltage, and the charging current Imax when the battery pack is charged at a constant current and a constant voltage, and a constant voltage control to a constant voltage control. When Imax = f (t), the charging current flowing at an arbitrary time t is set to 80% or less of Imax, and charging is controlled so that the current continuously decreases. In this case, since the control can be performed at a predetermined current change rate, an effect can be easily obtained.
Since the actual cell characteristics have a statistical distribution of variations, the maximum charge voltage arrival time also varies and the distribution is continuous. If the current is continuously decreased so as to correspond to this distribution, efficient charging can be performed without suppressing the current more than necessary and extending the charging time, and the above-described effects can be obtained.
In addition, even if the voltage of the entire battery pack does not reach the maximum charging voltage, some of the constituent cells may have a voltage per unit cell exceeding the maximum charging voltage per unit cell. If the current is further reduced, a sufficient effect can be obtained even in the above case.
[0012]
When the charging current is continuously reduced, if the control is performed so as to decrease linearly without considering a strict statistical distribution, a substantially sufficient effect can be obtained, and the control circuit can be simplified. . In addition, after reaching the maximum charging voltage, a large charge / discharge capacity can be obtained without impairing the cycle performance by controlling the current so as to maintain the maximum charging voltage and continuing the charging.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
The charging method of the present invention will be described in detail below.
The charging method of the present invention obtains a charging current allowable limit curve or a function approximate thereto for a battery pack to be charged, and then the current value specified by the charging current allowable limit curve or a function approximated thereto. The battery pack is charged with a charging current of. The procedure will be sequentially described below.
[0014]
The first procedure of the present invention is a procedure for obtaining a charge current allowable limit curve or a function approximating this for a battery pack to be charged.
The battery pack is charged at a constant current and a constant voltage to determine the current upper limit range of the present invention. Constant current and constant voltage charging is a method in which charging is performed at a constant current until the voltage of the battery pack reaches the maximum charging voltage, and thereafter, charging is performed by controlling the current so as to maintain the maximum charging voltage. is there.
In the present invention, an arbitrary initial current value is determined and the constant current / constant voltage charging is performed to obtain a time for shifting from constant current control to constant voltage control, and this is performed for a plurality of initial current values. A value obtained by multiplying the obtained initial current value by 0.8 is obtained as a function of the time. The function is the charge current allowable limit range of the present invention.
This procedure is shown in the drawing as shown in FIG. That is, as shown in FIG. 1, the curve connects the points of transition from the constant current charge region to the constant voltage charge region in the constant current / constant voltage charge curve under a plurality of conditions, thereby obtaining the curve D. A curve E is obtained by plotting a current value of 80% of the charging current at an arbitrary time t on the curve D. This curve E is a charge current allowable limit curve.
FIG. 1 shows an example of constant current and constant voltage charging with initial current values of 0.25C, 0.5C, 1C, and 1.5C. The constant voltage value was 4.15V. A white circle in FIG. 1 shows the obtained initial current value and the transition time from constant current charging to constant voltage charging. A solid line obtained by connecting these is the function Imax (curve D).
The maximum charging voltage is the voltage between the positive and negative terminals of the battery pack, and the maximum charging voltage of the unit cell determined depending on the positive electrode material and the negative electrode material of the nonaqueous electrolyte secondary battery used as the unit cell is the number of series It is doubled.
[0015]
The second procedure of the present invention is a process of charging the battery pack to be charged with a controlled current value. In other words, in the present invention, by charging in the current value region below the charging current allowable limit curve, it is possible to appropriately charge the single cells constituting the battery pack without fear of overcharging.
That is, in the present invention, the intended purpose is achieved by charging in the area below the curve E in FIG.
[0016]
The current at the start of charging is not particularly limited, but it is desirable that the nominal capacity of the battery pack be equal to or higher than the current value (0.5 C current) that can be charged in 2 hours, because the total charging time can be shortened. . Furthermore, it is desirable that the nominal capacity be equal to or less than a current value (3C current) that can be charged in 1/3 hour, because deterioration due to heat generation during charging can be avoided.
[0017]
Further, as a means for controlling the charging current within the charging current allowable limit range, the following method may be mentioned.
In the current reduction control method, a device that constantly monitors the battery pack voltage during charging can be provided, and the current can be reduced according to the battery pack voltage.
Further, the charging current can be continuously decreased along a predetermined current change curve until a predetermined maximum charging voltage is reached. According to this method, since the control can be performed at a predetermined current change rate, the effect can be easily obtained by omitting the control by the voltage monitoring device and its output.
Since the actual cell characteristics have a statistical distribution of variations, the maximum charge voltage arrival time also varies and the distribution is continuous. If the charging current is continuously decreased so as to correspond to this distribution, the charging can be efficiently performed without suppressing the current more than necessary and extending the charging time, and the above-described effects can be obtained. .
When continuously reducing the charging current, it is possible to obtain a substantially sufficient effect if the control is performed so as to decrease linearly without considering a strict statistical distribution, and the current control circuit can be simplified. Can do. Further, after reaching the maximum charge voltage, a larger charge / discharge capacity can be obtained without losing cycle performance by controlling the current so as to maintain the maximum charge voltage and continuing the charge.
In the method of continuously reducing the charging current, the rate of decrease can be reduced immediately after the start of charging, and then increased later, allowing rapid charging in a low battery pack voltage region to shorten the total charging time. . In addition, by performing such control, there is an advantage that a larger charge amount can be obtained even when charging is stopped from the outside by a user or the like during charging.
[0018]
The charging sequence of the present invention will be described with reference to the drawings.
As shown in FIG. 2, one of the charging sequences of the present invention linearly decreases the current value immediately after the start of charging, and after the cell reaches the maximum voltage, the current value is set to constant voltage charging. It can be realized by controlling. According to this charging pattern, a charging method having a feature that an effect can be obtained with the simplest control device is obtained.
[0019]
In addition, as shown in FIG. 3, immediately after the start of charging, the charging current is gradually decreased and controlled so that the rate of decrease gradually increases as charging progresses. After reaching the maximum voltage of the unit cell, the constant voltage charging is performed. It can also be controlled so that According to this charging pattern, charging proceeds at a relatively high speed immediately after the start of charging, so that the charging time can be shortened. In addition, even when charging is interrupted, a relatively high rate of charging results can be obtained.
[0020]
Hereinafter, a non-aqueous electrolyte secondary battery pack suitable for applying the charging method of the present invention will be described in detail.
The non-aqueous electrolyte secondary battery pack includes a non-aqueous electrolyte secondary battery alone or a plurality of non-aqueous electrolyte secondary batteries connected in parallel and a plurality of series batteries connected in series. It consists of an integrated container or binding tool, and may include a device for monitoring and controlling voltage and current. The device for monitoring and controlling the voltage and current may be realized outside the battery pack.
The non-aqueous electrolyte secondary battery has a structure in which an electrode group having a configuration in which a positive electrode, a separator, and a negative electrode face each other is immersed in a non-aqueous organic electrolytic solution.
[0021]
The positive electrode includes various oxides such as lithium cobalt composite oxide, lithium nickel composite oxide, lithium manganese composite oxide, lithium-containing nickel, cobalt oxide, lithium, vanadium oxide, titanium disulfide, molybdenum disulfide. Such a chalcogen compound as a positive electrode active material is formed into a thin plate shape using a binder. The positive electrode preferably contains graphite, carbon black or the like as a conductive material. As the positive electrode active material, lithium cobalt composite oxide, lithium nickel composite oxide, and lithium manganese composite oxide are desirable, and a non-aqueous electrolyte secondary battery that can withstand high capacity and high output can be configured.
[0022]
As said separator, a synthetic resin nonwoven fabric, a polyethylene porous film, a polypropylene porous film etc. can be used, for example.
[0023]
The negative electrode is formed into a thin plate shape by supporting an alkali metal such as lithium or a carbonaceous material that absorbs and releases lithium on a current collector such as a copper foil, a stainless steel foil, or a nickel foil with a binder.
The carbonaceous material is a carbonaceous material obtained by baking and carbonizing low molecular weight organic compounds such as petroleum and coal, low molecular weight organic compounds such as natural gas and lower hydrocarbons, polyacrylonitrile, and phenol resins. It is possible to use artificial, natural graphite or the like.
As said non-aqueous organic electrolyte solution, various well-known electrolyte solutions can be used for non-aqueous electrolyte secondary batteries. Although there is no particular limitation, the solvent includes ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC), γ-butyrolactone (BL), acetonitrile (AN ), Ethyl acetate (EA), toluene, xylene, or methyl acetate (MA). As the electrolyte, lithium perchlorate, lithium hexafluorophosphate, lithium borofluoride, lithium arsenic hexafluoride, trifluoro Lithium salts such as lithium methanesulfonate and lithium bistrifluoromethylsulfonylimide can be used.
[0024]
Furthermore, if necessary, the electrode group is spirally wound or laminated and placed in cans of various shapes or laminated film bags, etc., and by adding components such as electrolyte solution, terminals, insulating plates, etc. Alternatively, a sheet-like battery can be configured.
[0025]
The assembled battery is a non-aqueous electrolyte secondary battery or a plurality of the non-aqueous electrolyte secondary batteries connected in parallel. Although the connection method is not particularly limited, it can be performed by contacting or welding a metal lead such as nickel or aluminum.
[0026]
【Example】
Hereinafter, examples of the present invention will be described in detail with reference to the drawings.
[0027]
Example 1
90% by weight of lithium cobalt oxide (Li ₂ CoO ₂ ) powder, 2% by weight of acetylene black, 3% by weight of graphite, and 5% by weight of polyvinylidene fluoride as a binder were slurried using N-methylpyrrolidone as a solvent and applied onto an aluminum foil. And dried to produce a positive electrode.
A slurry of 87% by weight of mesophase pitch fibrous graphite powder baked at 3000 ° C, 10% by weight of artificial graphite with a uniform particle size of 5μm, 1% by weight of carboxymethylcellulose, and 2% by weight of styrene-butadiene rubber using water as a solvent. Then, it was applied on a copper foil and dried to prepare a negative electrode.
A polyethylene porous film was used as the separator.
The positive electrode, the separator, and the negative electrode were laminated in this order, and then wound in a spiral shape to form an electrode group having an outer diameter of the wound coil of 16.7 mm.
[0028]
The electrode group is placed in a stainless steel cylindrical can (diameter 18 mm, height 65 mm), and 1 M lithium hexafluorophosphate is further dissolved in a mixed solvent of ethylene carbonate and ethyl methyl carbonate (volume ratio 1: 1). The electrolyte prepared in this way was injected, the opening was sealed, and a non-aqueous electrolyte secondary battery was obtained. The non-aqueous electrolyte secondary battery created here is referred to as an A cell.
[0029]
The A cell is 300 mA equivalent to 0.2 C, the maximum charging voltage of the cell is 4.15 V, and is charged with a constant current up to 4.15 V. After reaching 4.15 V, the total time is 10 hours. Charging was performed at a constant voltage. Then, when discharge was performed to 3.0V at 1500 mA, the discharge capacity was 1500 mAh.
A positive electrode was prepared in the same procedure as described above. However, the coating amount per unit area was increased by 5%, and the packing density was improved so that the electrode thickness was the same as described above, to prepare a positive electrode. Other than the above, a cylindrical nonaqueous electrolyte secondary battery was prepared in the same manner as described above. The non-aqueous electrolyte secondary battery created here is a B cell. The B cell is 300 mA equivalent to 0.2 C, the maximum charging voltage of the cell is 4.15 V, and is charged with a constant current up to 4.15 V. After reaching 4.15 V, the total time is 10 hours. Charging was performed at a constant voltage. Then, when discharge was performed to 3.0V at 1500 mA, the discharge capacity was 1490 mAh.
[0030]
An assembled battery was prepared by connecting five A cells and one B cell in series. The battery pack was provided with the current control device. The maximum charging voltage was set to 24.9 V by multiplying 4.15 V per unit cell by 6 as the series number. The current at the start of charging is 1100 mA, the current is decreased linearly at a constant rate of 400 mA / h, and after reaching 24.9 V, the current is controlled while maintaining the voltage at 24.9 V, and charging is performed for a total of 3 hours. It was. FIG. 2 shows the charging current during charging and the voltage of each unit cell with respect to time. Under the above conditions, 24.9 V reached 2.15 hours, and the charging current at that time was 267 mA (corresponding to 40% of the allowable charging current limit). After a 30-minute rest period, discharging was performed at 1500 mA up to 18 V, and the discharge capacity was measured from the discharge time. Charging was performed in the same manner as the previous time after a pause of 30 minutes after discharging. The above charge / discharge operation was performed 400 times, and the change in discharge capacity was measured. The result is shown in FIG.
[0031]
(Example 2)
In the same manner as in Example 1, A cell and B cell were prepared. An assembled battery was prepared by connecting 5 A cells and 1 B cell in series, and a battery pack was provided with the current control device. The maximum charging voltage was 24.9 V as in Example 1. The current at the start of charging was 1000 mA, and the decrease in current was controlled by drawing a curve as shown in FIG. In the initial stage, the rate of decrease was small, and the rate of decrease gradually decreased with a quadratic curve. After reaching 24.9 V, the current was controlled while maintaining the voltage at 24.9 V, and charging was performed for a total of 3 hours. FIG. 2 shows the charging current during charging and the voltage of each unit cell with respect to time. Under the above conditions, reaching 24.9 V was 1.92 hours, and the charging current at that time was 240 mA (corresponding to 36% of the allowable charging current limit). This battery pack was subjected to a cycle test 400 times in the same manner as in Example 1 to measure the change in discharge capacity. The results are also shown in FIG.
[0032]
(Comparative Example 1)
In the same manner as in Example 1, five A cells and B cell were prepared. An assembled battery was prepared by connecting 5 A cells and 1 B cell in series, and a battery pack was provided with the current control device. The maximum charging voltage was 24.9 V as in Example 1. Charging was performed for a total of 3 hours by controlling the current so as to maintain a constant current of 750 mA from the start of charging until reaching 24.9 V, and maintaining 24.9 V after reaching 24.9 V. FIG. 4 shows the charging current during charging and the voltage of each cell with respect to time.
[0033]
2 to 4, the maximum voltage of the B cell in the vicinity of 24.9 V when the battery pack maximum charging voltage is reached is suppressed in FIGS. 2 and 3 in Examples 1 and 2 as compared with Comparative Example 1. You can see that. The maximum voltage of the B cell was 4.182 V in Comparative Example 1, 4.156 V in Example 1, and 4.153 V in Example 2. From these, it can be seen that, when charging a battery pack composed of single cells having variations, the maximum charging voltage of the single cells can be suppressed and overcharging can be prevented by using the method of the present invention.
As is clear from the results of FIG. 5 showing the cycle characteristic measurement results of Examples 1 and 2 and Comparative Example 1, since overcharge was prevented, Examples 1 and 2 had higher cycle characteristics than the comparative example. Is obtained.
[0034]
【Effect of the invention】
As described above in detail, according to the present invention, it is possible to improve the cycle life of the nonaqueous electrolyte secondary battery pack composed of single cells having variations in characteristics.
[Brief description of the drawings]
FIG. 1 is a diagram showing a method of obtaining a charging current allowable limit according to the present invention.
2 is a graph showing changes in current and cell voltage during charging in Example 1. FIG.
3 is a graph showing changes in current and cell voltage during charging in Example 2. FIG.
FIG. 4 is a diagram showing a change in current and a change in cell voltage during comparative charging.
5 is a diagram illustrating cycle characteristics of battery packs of Examples 1 and 2 and Comparative Example 1. FIG.

Claims (3)

Non-aqueous electrolyte secondary battery pack or non-aqueous electrolyte secondary battery pack formed by connecting a plurality of batteries connected in parallel to each other in series, followed by constant current charging up to a predetermined maximum charging voltage, When constant voltage charging is performed at a predetermined maximum charging voltage, constant current charging is started at an arbitrary initial charging value, and the maximum charging voltage arrival time until the charging voltage of the battery pack reaches the maximum charging voltage is t. In the battery pack, an initial current value I of 0.25C to 1.5C when the current value for charging the standard capacity of the battery at a constant current in 1 hour is 1C, and each of the 0.25C to 1.5C A relationship with the maximum charging voltage arrival time t corresponding to I is represented by I (t),
The charging current at an arbitrary time t ′ from the start to the end of charging of the battery pack is 80% or less of I ( t ′ ),
A method for charging a non-aqueous electrolyte secondary battery pack, wherein the current between the start and end of charging of the battery pack is controlled to decrease continuously.   2. The method of charging a non-aqueous electrolyte secondary battery pack according to claim 1, wherein after the battery pack reaches a predetermined maximum charging voltage, a charging current is controlled to maintain the maximum charging voltage. .   2. The non-aqueous battery according to claim 1, wherein in the charging method of the battery pack, the charging current is controlled to linearly decrease in a region from the start of charging to a time when the maximum charging voltage of the battery pack is reached. A method for charging an electrolyte secondary battery pack.

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