JP3596097B2 - Method for detecting micro short circuit of rechargeable lithium battery, charging method and charger - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to a method for detecting a micro short circuit occurring before a short circuit, a method for charging a rechargeable lithium battery, and a charger in order to avoid a danger due to a short circuit in a rechargeable lithium battery .
[0002]
[Prior art]
The biggest problem with rechargeable lithium batteries is how to prevent short-circuiting that occurs when lithium metal precipitates in the form of dendrites at the negative electrode during charging. The heat generated inside the battery due to the short circuit causes the electrolyte to evaporate, causing the battery to rupture, or even reaching a temperature higher than the ignition point, sometimes causing ignition.
[0003]
As a countermeasure, an ion battery of a type in which lithium is occluded in the negative electrode during charging by using carbon or an alloy without using metallic lithium for the negative electrode is known. As a result, deposition of metallic lithium was less likely to occur, and the life of the battery was greatly extended.
[0004]
However, the problem of lithium dendrite has not been basically solved. This is because, at the end of life after repeated charge and discharge, the ability of the negative electrode to occlude lithium is reduced, so that metallic lithium is generated in the form of dendrite. For this reason, as a countermeasure after a short circuit has occurred, measures have been taken to prevent rupture and ignition by the following method.
[0005]
In order to ensure the safety of rechargeable lithium batteries, lithium carbonate is added to the positive electrode of the battery, and even if the battery is short-circuited and generates heat, the valve is opened by the gas generated by decomposition of lithium carbonate to avoid rupture. . In this method, the contents of the battery will eventually erupt. Furthermore, in this method, since lithium carbonate is added to the positive electrode, the amount of the active material, which is a power generating substance, in the battery is reduced by the amount of lithium carbonate, and the capacity of the battery is reduced. That is, the performance of the battery is reduced to ensure safety.
[0006]
There is also a method in which a polyethylene separator is used in place of a polypropylene separator, and is melted by heat generated at the time of a short circuit so as to close a hole of the separator, thereby reducing a short circuit current. In this method, since the mechanical strength of the polyethylene separator is lower than that of the polypropylene separator, a thicker separator must be used to stably manufacture the battery. Also in this method, the separator volume occupied in the battery is increased, the amount of the active material is reduced correspondingly, and the battery capacity is reduced.
[0007]
[Problems to be solved by the invention]
An object of the present invention is to detect an internal short circuit of a battery leading to rupture or ignition in advance and prevent an accident from occurring, unlike the conventional countermeasures after an internal short circuit has occurred.
[0008]
[Means for Solving the Problems]
The present invention is to detect a minute short circuit due to disturbance caused by repeated rise and fall of terminal voltage with time during charging with controlled current in a rechargeable lithium battery. Further, a minute short circuit is detected when the terminal voltage at a preset time becomes smaller than the preset voltage. Further, the present invention is characterized in that a minute short circuit is detected when the terminal voltage during charging becomes lower than the voltage determined by learning from the charging behavior up to now.
[0009]
Further, the present invention is to detect a minute short circuit due to disturbance by repeatedly increasing and decreasing the charging current with time during charging with voltage control. Further, a minute short circuit is detected when the charging current at a preset time becomes larger than the preset current. Further, the present invention is characterized in that a minute short circuit is detected when the charging current becomes larger than the current determined by learning from the charging behavior up to now.
In addition, a charger having the above-mentioned minute short-circuit detection means, and a function of stopping charging and locking so that charging is not performed until battery replacement is performed after the danger due to the short-circuit is detected. It is characterized by having a charger.
[0010]
[Action]
The present invention, in a rechargeable lithium battery, before the occurrence of an internal short circuit of the battery leading to explosion or ignition, paying attention to the fact that a minute short circuit occurs inside the battery while charging the battery, detecting this minute short circuit, It detects that the battery is in a state of being easily short-circuited, and locks the battery so that it cannot be charged until the battery is replaced by the detection.
[0011]
Unlike rechargeable lithium batteries, such as conventional nickel-cadmium storage batteries or nickel-metal hydride storage batteries, oxygen cycles due to overcharging do not work. Therefore, the voltage increases monotonically in constant current charging, and the charging current monotonically decreases with time in constant voltage charging. The present invention focuses on a charging behavior different from the conventional rechargeable battery.
[0012]
When no short circuit or minute short circuit occurs, only ionic conductivity exists between the positive electrode and the negative electrode inside the battery through the electrolyte. When a micro short circuit occurs, electronic conductivity exists between the positive electrode and the negative electrode inside the battery. In the state of the minute short circuit having a small electron conductivity, the battery has self-discharged due to the electron conductivity, but does not cause an accident because the heat generation is small compared to the heat radiation capability of the battery. If the degree of short circuit increases and heat is generated beyond the heat dissipation capability, the battery temperature rises, causing rupture and ignition.
[0013]
Therefore, it is important to stop charging during a minute short-circuit having a small electron conductivity due to an internal short-circuit to ensure safety.
[0014]
The battery equivalent circuit when a micro short circuit occurs is the same as the case where the resistor 2 is connected in parallel between the positive electrode and the negative electrode of the battery 1 as shown in FIG. The state where there is no minute short circuit corresponds to the case where this resistance value is infinite. Therefore, when charging the battery that has caused a short circuit with a constant current, the charging current I is divided into Ir flowing through the resistance of the minute short circuit and the current Ic used for charging the battery. To Ic. Due to the decrease in the charging current, the terminal voltage of the battery being charged decreases. That is, with respect to the constant charging current I, the terminal voltage is lower when a micro short-circuit occurs than when the short-circuit does not occur.
[0015]
In charging at a constant voltage, the charging current increases from Ic to I due to a minute short circuit. That is, at the time of constant voltage charging, the charging current increases when a micro short circuit occurs, as compared with the charging current when no short circuit occurs.
[0016]
When the minute short circuit is further slight, a fluctuation such that the terminal voltage decreases or increases during constant current charging is observed. This is a state in which the short-circuited portion is detached or stuck inside the battery, and a micro short-circuit occurs or does not occur.
[0017]
During constant voltage charging, the charging current increases, decreases and fluctuates. In this way, by detecting a drop or change in the terminal voltage during charging and a change or increase in the charging current, it is possible to detect a micro short circuit that is likely to occur at the end of the life of the battery. The replacement can prevent the internal short circuit of the battery from exploding or firing.
[0018]
In addition to providing the above functions to the charger, in order to surely replace the battery that has detected the micro short circuit, stop charging after detecting the micro short circuit, but do not allow charging until battery replacement is completed. There is a sure way to give the charger a locking function.
[0019]
【Example】
As a representative rechargeable lithium battery, the same coin-type battery using LiCoO ₂ as a positive electrode active material and graphite as a negative electrode active material was prototyped for 6 cells. The amount of LiCoO _{2 in} the positive electrode was 0.5 g, and the amount of graphite in the negative electrode was 0.9 g. Each was press-molded to a diameter of 18 mm to form an electrode. The theoretical capacity of the battery is 75 mAh, and the capacity of the positive electrode is regulated. A 0.1 mm thick polypropylene nonwoven fabric is used as a separator separating the positive electrode and the negative electrode, and 1 mol / l of LiPF ₆ is dissolved in the electrolyte as ethylene carbonate and diethyl carbonate at a volume ratio of 1: 1. -Bond mixed solution was used. The same prototype batteries are referred to as A, B, C, D, E, and F, respectively. FIG. 2 shows a longitudinal sectional view of this battery. In the drawing, a positive electrode 3 and a negative electrode 4 are sealed with a gasket 6 in a sealing plate 7 and a case 8 in a state where they face each other via a separator 5.
[0020]
The cycle behavior and short-circuiting of these batteries were examined in constant current charging as current-controlled charging and constant voltage charging as voltage-controlled charging.
[0021]
(1) Constant current charging: A, B, and C batteries were charged at 2.5 mA until the terminal voltage reached 4.2 V, and then discharged at 2.5 mA until they reached 3 V. In the case where charging takes more than 40 hours to reach the voltage, the charging is stopped in 40 hours. The cycle test was repeated under these charge and discharge conditions. Up to 500 cycles, the batteries A, B, and C all exhibited the same charge / discharge behavior, and no short circuit was observed. Therefore, after 500 cycles, the battery A was disassembled and the inside was examined. No deposition of metallic lithium was observed at all, confirming that no short circuit was caused. FIG. 3 shows the charge curves of the battery of A at the second cycle, the 200th cycle, and the 500th cycle, respectively. The numbers in the figure are the number of cycles. It can be seen that the charging voltage increases as the cycle elapses. This is because, as charging and discharging are repeated, the negative electrode expands due to charging and contracts due to discharging, resulting in poor contact of the negative electrode active material, resulting in a decrease in utilization and an overvoltage.
[0022]
Therefore, if the charge / discharge cycle is repeated in this state, the utilization rate of the negative electrode is further reduced, and the graphite as the active material cannot absorb all lithium by charging, and finally lithium starts to precipitate, forming dendrites, A short circuit occurs after a micro short circuit.
[0023]
For batteries B and C, charging and discharging were further repeated under the same conditions. Battery B generated a disturbance in the charge curve at the charge of the 534th cycle such that the terminal voltage decreased or increased. FIG. 4 shows good charging curves at the 533th cycle and the 534th cycle. The reason why the voltage decreases during charging is that a minute short circuit has occurred, and the reason for the increase is because the minute short circuit portion has come off.
[0024]
That is, when the battery is charged with such a constant current, a minute short circuit can be detected by disturbance due to a decrease or increase in the terminal voltage.
[0025]
As a result of repeating charging and discharging of the battery B, a short circuit occurred completely at the 539th cycle, and an accident that the sealing plate of the battery was opened occurred.
[0026]
In battery C, no disturbance such as a decrease in terminal voltage occurred in the 551th cycle, but the terminal voltage did not reach 4.2 V even 40 hours after the start of charging. FIG. 5 shows charging curves at the 550th cycle and the 551th cycle. This is because, in the 551th cycle, a minute short-circuit occurred immediately after the start of charging, and the minute short-circuit portion did not come off. Before the 550th cycle in which a micro short circuit did not occur, the terminal voltage exceeded 4 V three hours after the start of charging. From this, it can be seen that a short circuit occurs when the terminal voltage after a lapse of a prescribed time (after 3 hours in this example) does not exceed a preset voltage (in this case, for example, 4 V).
[0027]
That is, the terminal voltage measured after the elapse of the preset time is compared with the preset voltage, and when it is lower than the preset voltage, it is possible to detect that a micro short circuit has occurred. The setting of the time and the voltage can be determined in advance, and the setting time and the voltage can be determined more finely by learning from the behavior in the charging in each cycle so far. In this example, as the cycle proceeds, the terminal voltage at the time of charging increases due to overvoltage as shown in FIG. For example, the terminal voltage three hours after the start of charging gradually increases to 4.04 V in the second cycle, 4.07 V in the 500th cycle, and 4.11 V in the 550th cycle. Therefore, it is more effective to set the voltage to be compared gradually higher as the cycle elapses to detect a minute short circuit. The charging and discharging of the battery C was further continued. At the 552th cycle, a short circuit occurred completely, and the sealing plate of the battery was opened.
[0028]
(2) Constant voltage charging: Batteries D, E and F were charged at a constant voltage of 4.2V. When the charging current decreased to 0.25 mA, the charging was terminated. The discharge was performed at 2.5 mA until the terminal voltage became 3 V. It should be noted that charging is stopped in 20 hours if it takes more than 20 hours to reach the current. The cycle test was repeated under these charge and discharge conditions.
[0029]
Up to 100 cycles, batteries D, E, and F all exhibited the same charge / discharge behavior, and no short circuit was observed. Then, after 100 cycles, the battery D was disassembled and the inside was examined. No deposition of metallic lithium was observed at all, confirming that no short circuit was caused. FIG. 6 shows charge curves of the second cycle, the 50th cycle, and the 100th cycle of the battery D, respectively. The numbers in the figure are the number of cycles. It can be seen that as the cycle elapses, the charging current immediately after the start of charging decreases and the charging time increases accordingly. This is because, as charging and discharging were repeated, the contact of the active material became poor due to expansion and contraction of the negative electrode, and the overvoltage was generated along with the decrease in the utilization factor.
[0030]
Therefore, if the charge / discharge cycle is repeated in this state, the utilization rate of the negative electrode is further reduced, and the graphite as the active material cannot absorb all lithium by charging, and finally lithium starts to precipitate, forming dendrites, A short circuit occurs after a micro short circuit.
[0031]
For the batteries E and F, charging and discharging were further repeated under the same conditions. Battery E generated a disturbance such that the charging current increased or decreased in the charging curve at the 124th cycle of charging. FIG. 7 shows the charging curves at the 123rd cycle and the 124th cycle which were good. The reason why the current increases during charging is that a micro-short circuit has occurred, and the reason for the decrease is because the micro-short circuit has been removed.
[0032]
That is, when charging is performed at such a constant voltage, a minute short circuit can be detected by disturbance due to an increase or decrease in the charging current.
As a result of repeating charging and discharging of the battery E, the battery E was completely short-circuited at the 127th cycle and the sealing plate of the battery was opened.
[0033]
In the battery F, no disturbance such as an increase or decrease in the charging current occurred at the 127th cycle, but the charging current did not reach 0.25 mA even after 20 hours from the start of charging. FIG. 8 shows charging curves at the 126th cycle and the 127th cycle. This is because in the 127th cycle, a minute short-circuit occurred immediately after the start of charging, and the minute short-circuit portion did not come off. Before the 126th cycle in which a micro short circuit did not occur, the charging current immediately after the start of charging was 20 mA or less, and tended to decrease with the cycle. From this, it can be seen that a short circuit has occurred when the charging current after the lapse of a certain specified time (immediately in this example) exceeds a preset current (in this case, for example, 20 mA).
[0034]
That is, the current after the elapse of a preset time is compared with the measured charging current, and if it is larger than the preset current, it can be detected that a micro short circuit has occurred. The setting of the time and the current can be determined in advance, and the setting time and the current can be determined more finely by learning from the behavior in the charging in each cycle so far. In this example, as the cycle proceeds, the charging current immediately after the start of charging becomes smaller due to overvoltage as shown in FIG. In this example, the current decreases to 20 mA in the second cycle, 16 mA in the 50th cycle, and 15 mA in the 100th cycle as the cycle progresses. Therefore, it is more effective to detect a minute short circuit if the current to be compared is gradually set smaller as the cycle elapses. Further, the charging and discharging of the battery F was further continued. At the 128th cycle, a complete short circuit occurred, and the battery sealing plate was opened.
[0035]
As described above, before the battery is short-circuited and ruptures or ignites, there is a disorder of a charging curve during charging or a state of a micro short-circuit that behaves differently from the charging behavior up to now. An accident can be prevented beforehand by detecting this state. Further, it is preferable that the above-mentioned minute short-circuit detection function is provided in the charger.
[0036]
Furthermore, a battery that has suffered a micro short circuit cannot be used as it is. Replacement with a new battery is required. Therefore, it is preferable to provide the charger with a lock function so that charging cannot be performed until the battery is replaced.
[0037]
【The invention's effect】
As described above, the voltage and current during charging of a rechargeable lithium battery are measured, and the voltage and current set in advance and the voltage and current determined by learning the charging behavior in a state where no abnormality has occurred are determined. By detecting a micro short circuit and replacing the battery, rupture or ignition of the battery can be prevented.
[Brief description of the drawings]
FIG. 1 is an equivalent circuit diagram of a battery in which a micro short circuit has occurred. FIG. 2 is a longitudinal sectional view of a coin-type battery. FIG. 3 is a charging curve diagram in a constant current charging in a cycle in which a micro short circuit has not occurred in the battery. 4 Charging curves at constant current between the cycle in which the battery did not have a micro short circuit and the cycle in which the micro short circuit occurred. Charging curve diagram with current. [Fig. 6] Charging curve diagram with constant voltage charging in a cycle where a micro short circuit does not occur in the battery. [Charge curve diagram at constant voltage of FIG. 8] [Charge curve diagram at constant voltage of a cycle in which a micro short-circuit does not occur in a battery and a cycle in which a micro short-circuit occurs.
3 Positive electrode 4 Negative electrode 5 Separator 6 Gasket 7 Sealing plate 8 Case

Claims (3)

A method for detecting a micro short circuit in a rechargeable lithium battery, wherein a micro short circuit is detected by disturbance caused by repeated increase and decrease of the charging current with time during voltage-controlled charging. A charging method for a rechargeable lithium battery, comprising: detecting a minute short circuit due to a disturbance caused by repeated increase and decrease of a charging current with time during charging with voltage control, and then stopping charging. Rechargeable battery charger lithium battery, which comprises using a method of charging 請 Motomeko 2.

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