Method for charging a secondary battery by supplying pulsed current as charging current

Abstract: In a method for charging a secondary battery having a positive electrode, a negative electrode, and an electrolyte, a pulsed current is supplied to the secondary battery to make the pulsed current flow between the positive electrode and the negative electrode through the electolyte to thereby charge the secondary battery. The pulsed current suitably comprises positive pulse current which has a positive pulse amplitude corresponding to a first current density of 1 µA/cm² to 100 mA/cm² in the positive electrode. However, it may have negative pulse current each following after each positive pulse and having a negative amplitude corresponding to a second current density which preferably is not greater than a quarter of the first current density.

Description: Background of the Invention: The present invention relates to a method for charging a secondary battery. Recently, electrical or electronic machines have been reduced in size and weight and have been made cordless, with development of electronics. The secondary battery, for example, a lithium (Li) secondary battery, a nickel-cadmium (Ni-Cd) battery, and a nickel-zinc (Ni-Zn) battery, or the like, is known and used as a power source of such electrical or electronic machines. Under the circumstances, it is strongly desired that the secondary battery has a long cycle life so that the secondary battery can be repeatedly charged and discharged many times. The secondary battery generally comprises, as well known, a positive electrode, a negative electrode, and an electrolyte. The secondary battery is conventionally provided with a separator which separates the positive electrode and the negative electrode. The secondary battery is conventionally charged by supplying an electric direct current (DC current). Accordingly, a conventional method for charging the secondary battery comprises the steps of producing a DC current, and supplying the DC current to the secondary battery to make the DC current flow from the positive electrode to the negative electrode through the electrolyte to thereby charge the secondary battery. Thus, it is cycled that the secondary battery is charged after being discharged, so that the secondary battery can be used for a long time. It is known in the art that dendrite crystal grows on a surface of the negative electrode, when the secondary battery is charged. The growth of the dendrite crystal is accelerated, as the charging-discharging cycle is repeated many times. As a result, the grown dendrite crystal often breaks through the separator and comes into contact with the positive electrode, so that the positive electrode and the negative electrode are short-circuited. Eventually, the secondary battery becomes unusable. Thus, the growth of the dendrite crystal makes the cycle life of the secondary battery short. In case of the Li secondary battery, the short circuit between the positive electrode and the negative electrode often causes fire. Accordingly, the growth of the dendrite crystal unfortunately brings the Li secondary battery into danger of catching fire. When the Ni-Cd and the Ni-Zn batteries are rapidly charged by use of large DC current, those batteries rise in temperature by the Joule's heat due to an internal resistance of those batteries. The temperature rise inevitably deteriorates a charge acceptability on the positive electrode. As a result, the Ni-Cd and the Ni-Zn batteries are reduced in capacity. The Ni-Cd battery suffers from another particular problem, what is called, a "memory effect". Namely, the Ni-Cd battery memorizes a residual discharging capacity when charging starts. After completion of charging, the Ni-Cd battery stops discharging at the memorized residual discharging capacity. Consequently, a dischargeable capacity of the Ni-Cd battery is considerably deteriorated, when charged before discharge is completed up to 100% of the discharging capacity. It is unknown why the Ni-Cd battery suffers from the "memory effect". In order to protect the Ni-Cd battery from the "memory effect", charging of the Ni-Cd battery should strictly be restricted so that the Ni-Cd battery is charged only after the discharge has completely come up to 100%. Alternatively, a charging apparatus for use in charging the Ni-Cd battery is provided with a circuit which enables the Ni-Cd battery to be charged only after it has been discharged up to 100%. EP-A-0034003 describes a battery charger which supplies pulses of charging current to a battery and discharges the battery between each charging pulse. US-A-4,878,007 describes a method for charging Ni-Cd batteries with a repetitive and alternating sequence of direct current charging and discharging periods. A similar arrangement is described in US-A-4,947,124. US-A-4,730,153 relates to a process for charging a reversible electrochemical generator consisting of a plurality of cells, which are electrically in series, comprising a zinc negative electrode and a positve electrode between which a basic electrolyte, containing solubilized zinc, circulates, consisting in making the electrolyte in the cells circulate successively from one cell to the next and in applying to it a charging current forming a succession of approximately rectangular pulses. The direction of circulation of the electrolyte during charging is periodically reversed. Summary of the Invention: It is therefore an object of this invention to provide a method of charging a secondary battery which enables the secondary battery to have a long cycle life. It is another object of this invention to provide a method of the type described, which can prevent a Li secondary battery from catching fire. It is still another object of this invention to provide a method of the type described, which can rapidly charge the secondary battery by use of large current. Other objects of this invention will become clear as the description proceeds. According to this invention, there is provided a method for charging a secondary battery comprising a positive electrode, a lithium negative electrode and an electrolyte, said method comprising the steps of: (a) producing a pulsed current having a predetermined repetition frequency which is between 0.1 Hz and 10KHz, inclusive, said pulsed current comprising a positive pulsed current and a negative pulsed current following thereto, which are repeated, said positive pulsed current having a positive amplitude which corresponds to a first current density of 1µA/cm 2 to 100 mA/cm 2 , inclusive, in said positive electrode, said negative electrode pulsed current having a negative amplitude which corresponds to a second current density which is not greater than a quarter of the first current density; and (b) supplying the pulsed current to the secondary battery to make the pulsed current flow between the positive electrode and the lithium negative electrode through the electrolyte to thereby charge the secondary battery. Brief Description of the Drawing Fig. 1 is a schematic sectional view of a known Li secondary battery; Fig. 2 is a block diagram of a charging apparatus which may be used in the method of the present invention; Fig. 3 is a graph for illustrating cycle life characteristics of Li secondary battery charged by different pulsed currents according to this invention in comparison with a conventional charging method by use of DC current; Fig. 4(a) and 4(b) are photographs each showing the microstructure of a surface of a negative electrode of Li secondary battery, Fig. 4(a) being after charged by a pulsed current, Fig. 4(b) being after charged by a DC current; Fig. 5 is a graph illustrating cycle life characteristics of Li secondary battery charged by use of different pulsed current according to a second embodiment of this invention in comparison with a conventional charging method by use of DC current; Fig. 6 shows a waveform of a pulsed current which is used in a method according to a third embodiment of this invention; and Fig. 7 and 8 are graphs illustrating cycle life characteristics of Li secondary battery charged according to the third embodiment of this invention, numbers of curves corresponding to test numbers in Table 1. Description of the Preferred Embodiment A secondary battery to which the present invention is applied is a known one. Description is, however, at first made about the secondary battery for the better understanding of the present invention. Referring to Fig. 1, a typical example of a lithium (Li) secondary battery 30 comprises a battery case 31 which is generally used as a negative electrode terminal, and an insulator ring 32 which is used as a cap of the battery case 31. A positive electrode terminal 33 is adapted to the insulator ring 32. A positive electrode 34, a negative electrode 35, and a separator 36 between the positive and the negative electrodes 34 and 35 are contained in the battery case 31. The separator 36 is impregnated with an electrolyte which is not shown in the figure. The positive and the negative electrodes 34 and 35 are connected to the positive electrode and the negative electrode terminals 33 and 31, respectively. In the shown example, the battery case is used as the negative electrode terminal. However, there is known another type where the battery case is used as the positive electrode terminal with a negative electrode terminal being provided separate from the battery case. Further, the positive and the negative electrodes are shown wound together with the separator in spiral form in the figure, but another type is also known where they are concentrically disposed in the battery case. In the conventional charging method, a positive DC voltage is applied across the positive and the negative electrode terminals 33 and 31 of the battery 30 to make a DC current flow from the positive electrode 34 to the negative electrode 35 through the electrolyte so as to charge the battery. According to the present invention, a pulsed current is supplied to the battery so as to charge the battery. Referring to Fig. 2, a charging apparatus suitable for use in the method of the present invention comprises a DC power supplier 41, a pulse generator 42, a function generator 43, and an output port 44. The DC power supplier 41 has a DC power source 45 and a DC/DC converter 46. The DC power source 45 includes an AC power source 47 such as a commercial AC power source and a rectifier 48 for rectifying the AC power to produce a rectified power. The DC/DC converter 46 regulates the rectified power to produce a regulated DC power with a constant current. The pulse generator 42 is connected to the DC power supplier 41 and generates a pulsed power from the DC power. The pulsed power is repeated with a controllable frequency, a controllable duty ratio, and a controllable waveform. The function generator 43 is connected to the pulse generator 42 and controls the pulsed power to set the controllable frequency, controllable duty ratio, and controllable waveform into a predetermined frequency, a predetermined duty ratio, and a predetermined waveform. The output port 44 comprises a positive terminal (+) and a negative terminal (-) and is connected to the pulse generator 42. The output port 44 is used for charging a secondary battery therethrough. Besides, the charging apparatus shown in the figure is further provided with a voltage detector 47 for detecting completion of charge where the secondary battery is completely charged up to 100%. In use of the charging apparatus, the secondary battery 30 is connected to the output port 44 with the positive and the negative electrode terminals 33 and 31 being connected to the positive and the negative terminals of the output port 44. A power switch (not shown) is turned on at the DC power supplier 41. Then, the pulsed current is produced at the pulse generator 42 and is supplied to the secondary battery 30. Thus, the pulsed current flows from the positive electrode terminal 33 to the negative electrode terminal 31 of the secondary battery 30. In detail, the pulsed current flows from the positive electrode 34 to the negative electrode 35 through the electrolyte in the secondary battery 30 to thereby charge the secondary battery. Now, examples according to the present invention will be described below. Example 1 In order to estimate effects of the charging method of the present invention as to the cycle life characteristics of a lithium (Li) secondary battery, a charge-discharge cycle test was carried out for several samples of Li secondary battery. In the Li secondary battery, the positive electrode is made of a manganese dioxide, while the negative electrode is made of lithium metal. The electrolyte is such a solution that a LiClO 4 is melted in propylene carbonate (PC) with a concentration of 1 N (normal). In the charge-discharge cycle test, the charging operation was performed to charge each of samples to a charged voltage of 3.5 V by use of the charging apparatus shown in Fig. 2. The pulsed current used for charging was differently adjusted for different samples to have different pulse repetition frequencies of 100 Hz, 10 kHz, and 0.1 Hz and a constant pulse duty ratio of 50%. Each pulse of the pulsed current was also adjusted to have a constant positive pulse amplitude sufficient to make a current of 0.1 mA flow per 1 cm 2 of the positive electrode of the battery. That is, the positive pulse amplitude is corresponding to a current density of 0.1 mA/cm 2 in the positive electrode of the battery. Thus, the maximum current density flowing through the positive electrode is 0.1 mA/cm 2 . For comparison, one of the samples was charged to 3.5 V by use of a DC current having a positive level corresponding to a current density of 0.1 mA/cm 2 in the positive electrode of the battery. The discharge was performed by continuously supplying a current from the charged sample to a load at a rate of a current density of 0.1 mA/cm 2 in the positive electrode until the battery voltage became 2.0 V. A time period was measured for the voltage of each sample battery dropped from 3.5 V to 2.0 V. A supplying current was also measured when the supplying voltage became 2.0 V. A discharging capacity of each sample battery after each charging operation was calculated from the measured time period and the supplying current. Providing that an initial discharging capacity after an initial charging operation is 100%, variation of the discharging capacity after each charging operation is shown in Fig. 3 as a relation between numbers of charge-discharge cycle and a deterioration rate of the battery discharging capacity. It is noted from Fig. 3 that the samples charged by use of a pulsed current according to the present invention are considerably low in deterioration of battery discharging capacity in comparison with the sample charged by use of DC current according to the conventional charging method. In order to seek for the grounds that cycle life characteristics of the samples charged by the pulsed current are superior to those charged by the DC current, as suggested in Fig. 3, microstructure of a surface of a negative electrode of each one of the samples is observed by use of a scanning electron microscope (SEM). Fig. 4(a) shows the SEM photo of the sample charged by the pulsed current with the pulse repetition frequency of 10 kHz and the pulse amplitude corresponding to the current density of 0.1 mA/cm 2 . Fig. 4(b) shows that of the sample charged by the DC current corresponding to the current density of 0.1 mA/cm 2 . It is noted from Figs. 4(a) and 4(b) that lithium has been deposited in a form of granules on the surface of the negative electrode of the sample charged by the pulsed current, while dendrite crystal has grown on the surface of the negative electrode of the sample charged by the DC current. In view of results of the above charge-discharge cycle test and the SEM photos, it is readily understood that a growth of the dendrite crystal on a surface of a negative electrode causes a deterioration of cycle life characteristics and a short-circuit between a positive and a negative electrodes of a Li secondary battery. Thus, the method according to the embodiment of the present invention can prevent the Li secondary battery from being deteriorated in the cycle life characteristics and short-circuited due to such a growth of the dendrite crystal. Example 2 From different point of view, another charge-discharge cycle test was carried out for several samples of Li secondary battery which are experimentally produced by use of the similar materials to those of the samples in Example 1. In the charge-discharge cycle test, the charging operation was performed to charge each of samples to a charged voltage of 3.5 V by use of the same charging apparatus as that used in Example 1. Each pulsed current used for charging was adjusted to have the constant pulse repetition frequency of 100 Hz and a pulse duty ratio of 50%. The pulse of the pulsed current was differently adjusted for different samples to have different positive pulse amplitudes which are corresponding to different current densities of 1 µA/cm 2 , 0.1 mA/cm 2 , 1 mA/cm 2 , and 100 mA/cm 2 in the positive electrode of the battery. For comparison, two of the samples were charged to 3.5 V by use of each DC current having different positive levels corresponding to current densities of 0.1 mA/cm 2 and 1 mA/cm 2 , respectively, in the positive electrode of the battery. It is also noted from Fig. 5 that the samples charged by use of a pulsed current according to the present invention are considerably low in deterioration of battery discharging capacity in comparison with the sample charged by use of DC current according to the conventional charging method. Example 3 A further charge-discharge cycle test was carried out for several samples of Li secondary battery which were experimentally produced by use of the similar materials to those of the samples in Examples 1 and 2. The charge-discharge cycle test was carried out under the conditions described in Table 1. Referring to Fig. 6, a pulsed current supplied to each of test samples shown in Table 1 comprises a positive pulse 61 and a negative pulse 62 following thereto which are repeated. The positive pulse 61 has a positive amplitude, while the negative pulse 62 has a negative amplitude. CHARGING CONDITION (PULSED CURRENT) DISCHARGING CONDITION (DC CURRENT) PULSE REPETITION FREQUENCY/Hz CURRENT VALUE/ µAcm -2 CURRENT VALUE/ µAcm -2 (+) CHARGING DIRECTION (-) DISCHARGING DIRECTION DISCHARGE TEST SAMPLE 1 100 1 -0.25 -100 TEST SAMPLE 2 100 100 -1 -100 TEST SAMPLE 3 100 100 -10 -100 TEST SAMPLE 4 100 1 x 10 5 -1 -100 TEST SAMPLE 5 100 1 x 10 5 -10 -100 TEST SAMPLE 6 0.1 100 -10 -100 TEST SAMPLE 7 10000 100 -10 -100 COMPARED SAMPLE 100 (DC CURRENT) -100 DUTY RATIO: 50% FINAL VOLTAGE: CHARGE 3.5V, DISCHARGE 2.0V The positive amplitude was, as described in Table 1, corresponding to first current densities of 1 µA/cm 2 to 1 x 10 5 µA/cm 2 (100 mA/cm 2 ) in positive electrodes in the test samples 1 to 7. The negative amplitude was corresponding to second current densities of 0.25 µA/cm 2 to 10 µA/cm 2 which are less than the first current density, as described in Table 1. As described in Table 1, the pulsed current used for charging each of the test samples 1 to 5 was adjusted to have a constant pulse repetition frequency of 100 Hz and a constant pulse duty ratio of 50%. The pulsed current used for charging test samples 6 and 7 was adjusted to have pulse repetition frequencies of 0.1 Hz and 10 kHz, respectively, and the constant pulse duty ratio of 50%. The charging and the discharging operations were performed in the similar manner to that in Examples 1 and 2, as will be understood in Table 1. For comparison, one of the samples was charged to 3.5 V by use of DC current having a positive level corresponding to a current density of 0.1 mA/cm 2 in the positive electrode of the battery. Results of the charge-discharge cycle tests are shown in Figs. 7 and 8. Fig. 7 shows a result of test samples 1 to 5 which were charged by use of pulsed currents having the same pulse repetition frequency but different positive and negative pulse amplitudes. Fig. 8 shows a result of test samples 3, 6, and 7 which were charged by use of pulsed currents having different pulse repetition frequencies but the same positive and negative pulse amplitudes. It is noted from Figs. 7 and 8 that the samples charged by use of the pulsed current according to the present Example are very low in deterioration of battery discharging capacity in comparison with the sample charged by use of DC current according to the conventional charging method. The aforesaid second current density is not greater than a quarter of the aforesaid first current density so as not to elongate a charging time period. While this invention has thus far been described with respect to only several embodiments thereof, it will be readily possible for those skilled in the art to put this invention into practice in various other manners. For example, the pulsed current is not limited, to a pulse current as illustrated in Fig. 6. Namely, a word "pulsed current" in the instant specification may include such a current as having a sinusoidal waveform, such a current as having a sawtooth waveform, and the like. Moreover, the charging apparatus illustrated in Fig. 2 does not have to be used to supply the pulsed current to the secondary battery. Alternatively, a half-wave rectified current may be supplied to the secondary battery by use of an AC power source and a rectifier.

Claims: A method for charging a secondary battery (30) comprising a positive electrode (34), a lithium negative electrode (35) and an electrolyte, said method comprising the steps of: (a) producing a pulsed current having a predetermined repetition frequency which is between 0.1 Hz and 10KHz, inclusive, said pulsed current comprising a positive pulsed current and a negative pulsed current following thereto, which are repeated, said positive pulsed current having a positive amplitude which corresponds to a first current density of 1µA/cm 2 to 100 mA/cm 2 , inclusive, in said positive electrode, said negative pulsed current having a negative amplitude which corresponds to a second current density which is not greater than a quarter of the first current density; and (b) supplying the pulsed current to the secondary battery to make the pulsed current flow between the positive electrode (34) and the lithium negative electrode (35) through the electrolyte to thereby charge the secondary battery.

Inventors: HASEBE AKIO, MOMOTANI HIROSHI, OTSUKI ETSUO, TAKEUCHI TAMIKO, HASEBE, AKIO,, MOMOTANI, HIROSHI,, OTSUKI, ETSUO,, TAKEUCHI, TAMIKO,, HASEBE, AKIO, C/O TOKIN CORPORATION, MOMOTANI, HIROSHI, C/O TOKIN CORPORATION, OTSUKI, ETSUO, C/O TOKIN CORPORATION, TAKEUCHI, TAMIKO, C/O TOKIN CORPORATION
Applicants: TOKIN CORP, TOKIN CORPORATION
Application year: 1993
Main classification: H02J7
IPC codes: H02J7