Patent Application
20140191731
The present invention relates to a charging method suitable for a lithium ion secondary battery configured through use of a negative electrode material containing silicon (Si).
A lithium ion secondary battery is one of non-aqueous electrolyte secondary batteries, and has been used widely for its high voltage and high capacity, and a charging method thereof also has been improved variously so that the lithium ion secondary battery can be used more effectively. As a method for charging a lithium ion secondary battery, constant current constant voltage (CCCV) charging generally is used.
The CCCV charging is performed as shown in
In recent years, in order to accomplish the charging in a short period of time, in the CCCV charging, there is a demand for maximizing the current during the CC charging. A charging amount is a value obtained by multiplying the charging current by the period of time, and hence, a procedure for performing the charging with an increased charging current is effective. However, heat generation is involved in charging, and an amount of the generated heat is increased along with an increase in current.
On the other hand, when a secondary battery is charged in a high-temperature environment, there is concern about the degradation of the secondary battery and the decrease in safety thereof. As a solution for avoiding excess temperature rise, for example, it has been known to incorporate a function of suspending the charging when the secondary battery reaches a predetermined temperature during the charging, into a circuit for charting the secondary battery. The temperature of the secondary battery is detected by a temperature detecting device (for example, a thermister) attached to the secondary battery or mounted on a protection circuit included in the secondary battery, and electrically transmitted to an external charger or an equipment with a battery pack mounted thereon.
As shown in
When the charging is suspended due to the excess rise in temperature, there is a possibility that the charging may be completed while the battery has not been charged to a predetermined charge amount, or the total charging time to complete the charging may be extended.
Further, in order to prevent the temperature from reaching the charging suspension temperature Toff, a charging method for controlling as shown in
Patent Document 1 discloses an example of a method for subjecting a lithium iron secondary battery to the CCCV charging, the method involving changing the charging current while monitoring the heat generation of a battery pack, as described above. That is, in a first charging step, a temperature rise gradient of a battery with respect to the charging current is detected, and the temperature of the battery, having been charged to a first set capacity, is predicted based on the detected temperature rise gradient. The battery is charged to the first set capacity with the charging current being controlled so that the battery temperature does not exceed the set temperature, based on the predicted temperature. In a second charging step, after the battery is charged to the first set capacity, the temperature of the battery, having been charged to a second set capacity, is predicted based on the temperature rise gradient. The battery is charged to the second set capacity with the charging current being controlled so that the battery temperature does not exceed the set temperature because of the predicted temperature. Accordingly, the lithium ion secondary battery can be fully charged in a short period of time while the temperature rise of the battery is prevented.
Patent Document 1: JP 2009-148046 A
Patent Document 2: JP 2007-242590 A
According to the charging method disclosed by Patent Document 1, the current is changed in multiple stages while monitoring the heat generation gradient constantly, and hence, it is difficult to sufficiently accomplish the quick charging. Further, when such a method is used, time during which a secondary battery is exposed to a high-temperature state increases although the secondary battery does not reach high temperature to be avoided. Therefore, there is increased concern about the degradation of the secondary battery and the decrease in safety thereof.
On the other hand, a composite material (SiOx) having a structure in which Si ultra-fine particles are dispersed in SiO2 has been known as a high-capacity negative electrode material for increasing the capacity of a secondary battery (for example, Patent Document 2). The inventors of the present invention discovered, as a novel finding, that the heat generation characteristics involved in the charging of a lithium ion secondary battery using a negative electrode material containing Si are not found in any other kinds of lithium ion secondary batteries, in a process of researching a charging method preferable for the above-mentioned lithium ion secondary battery. Then, the inventors of the present invention found that the problems in the above-mentioned conventional charging methods can be solved based on the heat generation characteristics.
Thus, it is an object of the present invention to provide a charging method enabling a lithium ion secondary battery using a negative electrode material containing Si to be charged at high efficiency while the heat generation during the charging is suppressed.
A method for charging a lithium ion secondary battery of the present invention is a method for charging a lithium ion secondary battery by constant current constant voltage (CCCV) charging, including: a step of performing constant current (CC) charging up to a predetermined set voltage; and a step of switching the CC charging to constant voltage (CV) charging after the set voltage is reached, thus performing charging while reducing charging current so as to keep the set voltage.
The lithium ion secondary battery to which the charging method of the present invention is to be applied is composed using a negative electrode material containing Si, thereby having characteristics such that, during a period of the CC charging, a transition point Ta appears in a temperature rise gradient when temperature of the battery rises along with progression of the charging, and with the transition point Ta being a border, the temperature rise gradient in an initial T1 period is steeper than the temperature rise gradient in a T2 period following the T1 period.
A first charging method of a lithium ion secondary battery of the present invention has the feature that changeover time ts is set in a range of tT≦ts≦(tT×1.2), based on charging time tT corresponding to timing at which the transition point Ta appears after start of the CC charging from a condition of the SOC (state of charge) of 0%, obtained by measurement in advance, and during a period of the CC charging, the CC charging is performed at a first current value until the changeover time ts elapses after start of the charging, and after the changeover time ts elapses, the CC charging is performed at a second current value larger than the first current value.
Further, a second method for charging a lithium ion secondary battery of the present invention has the feature that changeover time ts set in a range of tT≦ts≦(tT×1.2), based on charging time tT corresponding to timing at which the transition point Ta appears after start of the CC charging from a condition of the SOC of 0%, obtained by measurement in advance, a charge state of the lithium ion secondary battery is determined before start of the charging, and during a period of the CC charging, when the charge state is before the transition point Ta, the CC charging is performed at a first current value until the changeover time ts elapses from start of the charging, and after the changeover time ts elapses, the CC charging is performed at a second current value larger than the first current value, and when the charge state exceeds the transition point Ta, the CC charging is performed at a second current value larger than the first current value.
According to the charging method of the above-mentioned configuration, during a period of the CC charging, the charging at a first current value is switched to the charging at a second current value larger than the first current value at changeover time that is set so as to correspond to a transition point of a temperature rise gradient involved in the charging. Thus, the charging is performed at smaller current during a period corresponding to a T1 period having a steep temperature rise gradient, and the charging is performed at larger current during a period corresponding to a T2 period having a gentle temperature rise gradient. Consequently, the heat generation in the period of a steep temperature rise gradient is suppressed to minimize temperature rise, while the charging can be performed efficiently during the period of a gentle temperature rise gradient, thereby shortening time required for charging.
Further, the CC charging can be performed up to the SOC exceeding 80% by suppressing heat generation, and hence, the time required for the charging can be shortened remarkably.
The method for charging a lithium ion secondary battery of the present invention can take the following forms based on the above-mentioned configuration.
That is, in the second charging method, it is possible that the SOC of the lithium ion secondary battery is measured before starting the charging, and when the SOC is 10% or less, it is determined that the charge state is before the transition point Ta, and when the SOC exceeds 10%, it is determined that the charge state exceeds the transition point Ta.
Further, in the first or second charging method, the charging time tT can be defined as charging time tT10 that elapses from a start of the charging at a condition of the SOC of 0% to time when the SOC reaches 10%, and changeover time ts1 representing the changeover time ts can be set in a range of tT10≦ts1≦(tT10×1.2).
Alternatively, the charging time tT can be defined as a charging time tTA that elapses from a start of the charging at a condition of the SOC of 0% to time when the transition point of a temperature rise gradient is detected, and changeover time ts2 representing the changeover time ts can be set in a range of tTA≦ts2≦(tTA×1.2).
Further, when 1C is defined as a current value at which the lithium ion secondary battery that is fully charged is discharged within one hour, the first current value can be set in a range of 0.7 to 0.8C.
Further, the second current value can be set to 1.5C or more.
Further, the SOC at completion of the T2 period can be set so as to exceed 80%.
Further, the lithium ion secondary battery can be composed by using a composite material (SiOx) having a structure in which ultra-fine particles of Si are dispersed in SiO2 as the negative electrode material. In this case, the composite material (SiOx) can be formed of a core containing a material in which an atomic ratio x of oxygen with respect to silicon is 0.5≦x≦1.5, and a covering layer of carbon covering a surface of the core.
<Description of Characteristics to be a Basis of the Present Invention>
The charging method of the present invention is directed to a lithium ion secondary battery (hereinafter, referred to as “Si-containing lithium ion secondary battery”) using a negative electrode material containing Si such as a composite material (SiOx) having a structure in which Si ultra-fine particles are dispersed in SiO2, and exhibits peculiar characteristics when charging the secondary battery. Therefore, in the description of this section, prior to the description of embodiments, the peculiar characteristics to be a basis of the present invention are described regarding a Si-containing lithium ion secondary battery.
The Si-containing lithium ion secondary battery can be charged and discharged smoothly to have high capacity due to the use of a high-capacity negative electrode material made of the above-mentioned composite material. As an example of a specific configuration of the Si-containing lithium ion secondary battery to which the present invention is directed, there is a non-aqueous secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, as follows. The positive electrode includes a positive electrode mixture layer containing a lithium-containing transition metal oxide. The negative electrode includes a negative electrode mixture layer containing a negative electrode material formed of a core that contains a material containing silicon and oxygen as constituent elements in which an atomic ratio x of oxygen with respect to silicon is 0.5≦x≦1.5 and a covering layer of carbon covering the surface of the core. See Patent Document 2.
The Si-containing lithium ion secondary battery exhibits the heat generation characteristics as shown in
According to the heat generation characteristics, when the temperature of a battery rises due to the heat generation during the CC charging with charging current being controlled to be constant, a temperature rise gradient is steep in an initial charging period, and after the charging is performed for a short period of time, the temperature rise gradient becomes gentle. Thus, when the steep temperature rise gradient changes to the gentle temperature rise gradient, a transition point Ta of the temperature rise gradient is recognized. With timing at which the transition point Ta appears after the start of the charging being a border, a former period of the CC charging is described as a T1 period (charging time tT1), and a latter period of the CC charging is described as a T2 period (charging time tT2).
The transition point Ta of the temperature rise gradient appears in the vicinity of the SOC of 10% as the characteristics common to Si-containing lithium ion secondary batteries. That is, even when the CC charging is performed at a condition of various SOCs, a transition point Ta appears in the vicinity of the SOC of 10%. Therefore, the time required for the transition point Ta to appear after the start of the charging depends upon the SOC when the charging starts. If the charging is started from a condition of a high SOC, a period during which the temperature rise gradient is steep becomes short, compared with the case of starting the charging from a condition of a low SOC. There also is a case where the temperature rise gradient becomes gentle immediately after the start of the charging.
As described above, two regions: the T1 period and the T2 period are present in a region of the CC charging, and the features of each period are as follows.
As described above, the Si-containing lithium ion secondary battery generates heat greatly in a short period of time in the T1 period, and the heat generation in the T2 period is suppressed compared with that in the T1 period or equivalent thereto. Thus, in order to suppress the total amount of the heat generation in the CC charging period, it is effective to suppress the temperature rise in the T1 period. Considering this, the charging method of the embodiments according to the present invention described later has a feature that the charging is performed at small current in a CC charging region corresponding to the T1 period, and the charging is performed at large current in the same way as in the conventional example in a CC charging region corresponding to the T2 period. Further, the completion period of the T2 period can be extended to a region in which the SOC exceeds 80%.
Hereinafter, the embodiments according to the present invention are described with reference to the drawings.
A method for charging a lithium ion secondary battery according to Embodiment 1 of the present invention is described with reference to
This charging method basically belongs to the CCCV charging method. Specifically, the CC charging is performed up to a predetermined set voltage Vc (not shown). After the set voltage Vc has reached (tcv), the CC charging is switched to CV charging, and the CV charging is performed at the charging current being reduced so as to keep the set voltage. At a time tf when the charging current has reached a set value If, the CV charging is stopped, whereby the charging is completed.
The present embodiment is characterized in a process of the CC charging, and as shown in
The changeover time ts in the above-mentioned charging method is basically set as follows. First, in advance, with respect to a lithium ion secondary battery having the same specification as that of a charging target, the charging is started from a condition of the SOC of 0% and a charging time tT corresponding to timing at which the transition point Ta of the temperature rise gradient appears is measured. As described later, it is not necessary to detect directly the appearance of the transition point Ta when measuring the charging time tT. In short, the charging time tT only needs to be measured based on an event corresponding to the timing at which the transition point Ta appears. If the changeover time ts is set so as to correspond to the measured charging time tT, the changeover time ts set in the vicinity of timing at which the transition point Ta appears. Accordingly, the CC1 charging can be switched to the CC2 charging in the vicinity of the transition point Ta of the temperature rise gradient.
In the present embodiment, one setting example of the changeover time ts corresponding to the charging time tT is described. Considering that the changeover time ts peculiar to the present embodiment, the changeover time ts described as changeover time ts1. First, in a lithium ion secondary battery having the same specification as that of a charging target, charging time tT10 from time when charging is started from a condition of the SOC of 0% to time when the SOC reaches 10% is measured in advance to be used as the charging time tT.
As described above, the transition point Ta of the temperature rise gradient appears in the vicinity of the SOC of 10%. Therefore, if the changeover time ts1 is set so as to correspond to the charging time tT10, the changeover time ts1 is set in the vicinity of timing at which the transition point Ta appears. Thus, the CC1 charging can be switched to the CC2 charging in the vicinity of the transition point Ta of the temperature rise gradient.
As a result, the CC1 charging is performed at the smaller first current value I1 in a region substantially corresponding to the T1 period having a large temperature rise gradient, and the CC2 charging is performed at the larger second current value I2 in a region substantially corresponding to the T2 period having a small temperature rise gradient. Thus, the charging can be performed efficiently while the heat generation is suppressed to minimize the temperature rise, and the time required for the charging can be shortened. In particular, if the CC charging is designed so as to be performed up to the SOC of 80%, the time required for charging can be shortened remarkably.
The reason that the above-mentioned effect can be obtained is as follows. Specifically, the transition point Ta of the temperature rise gradient appears in the vicinity of the SOC of 10%, and hence, a ratio of the T1 period occupying the CC charging period is small, and the temperature rise gradient is sufficiently small in the T2 period. Therefore, even when the charging current is reduced during a period corresponding to the T1 period, there is little influence on the speed of the entire charging. On the other hand, the heat generation is large in the T1 period, and hence, the effect of suppressing the temperature rise by reducing the charging current is large. Further, temperature rises less during a period corresponding to the T2 period having a small temperature rise gradient. Therefore, even when the CC2 charging is performed at large current, the temperature rise is suppressed, and the charging efficiency is enhanced. Thus, throughout the entire period of the CC charging, both suppression of the temperature rise and the high-speed charging can be satisfied.
As is understood from the reason that the above-mentioned effect can be obtained in the present embodiment, even if the changeover time ts1 is set to be shifted from the charging time tT10 to some degree, a sufficient effect or a reasonable effect can be actually obtained when the CC1 charging controlled with the smaller first current value I1 is included in the initial charging period. It should be noted that, according to the result of the study based on an experiment, the changeover time ts1 is desirably set in a range of tT10≦ts1≦(tT10×1.2) based on the charging time tT10. That is, a desirable permissible range for obtaining the above-mentioned effect falls in a range of the time equivalent to the charging time tT10 to the time longer by 20% than the charging time tT10.
Even when the changeover time ts1 is set as described above, the changeover time ts1 is not always matched with the timing at which the transition point Ta of the temperature rise gradient appears after the start of the charging. That is, as described above, the charging amount, or the charging time (tT1) to be required before the transition point Ta appears various depending upon the SOC at the start of the charging. In contrast, as the charging time tT10 for setting the changeover time ts1, a measurement result obtained in the case of starting the charging from a condition of the SOC of 0% is used. Therefore, some shift occurs between the changeover time ts1 and the timing at which the transition point Ta appears.
It should be noted that the charging time (tT1) to be required before the transition point Ta appears may become shorter depending upon the SOC at the start of the charging but does not becomes longer. Thus, by setting the changeover time ts1 in a range of tT10≦ts1≦(tT10×1.2), as described above, the CC1 charging is performed at the smaller first current value I1 without fail in a region corresponding to the T1 period having a large temperature rise gradient, and thus, the temperature rise can be suppressed reliably.
On the other hand, the CC1 charging may be extended to a region corresponding to the T2 period. This is disadvantageous for shortening the time for the CC charging because the charging period with smaller current is long. However, a ratio of the charging time tT10 to be a basis of the changeover time ts1, occupying the CC charging, is small, and hence, influence of shortening of the charging time is small if the period of the CC1 charging is up to +20% as described above. Accordingly, contribution to efficient charging, avoiding temperature rise, can be obtained sufficiently. This effect is obtained reasonably irrespective of the other conditions, if the changeover time ts1 is set in the above range with respect to the charging time tT10.
For example, with respect to a comparison between charging at a 2C rate and charging at a 1C rate in the period of the CC1 charging, temperature rise will be as follows. Herein, it should be noted that although the transition point Ta of the temperature rise gradient highly depends upon the addition amount of Si, a substantial change in the transition point Ta caused by the SOC is not found. Therefore, the charging time up to the SOC of 10% changes substantially in proportion with the SOC.
The Si-containing lithium ion secondary battery can be set, for example, so that the transition point Ta of the temperature rise gradient appears at the SOC of about 10% with a 2C rate of a total amount of charge. In this case, when the Si-containing lithium ion secondary battery is charged at a 2C rate, the charging time before the SOC reaches 10% is 3 minutes, and the temperature rise during that time is about 15° C. On the other hand, when the Si-containing lithium ion secondary battery is charged at a 1C rate, the charging time before the SOC reaches 10% is 6 minutes, and the temperature rise during that time is about 7° C. Thus, even when current of the CC1 charging is reduced, the charging time only needs to be extended by about 3 minutes, and the temperature rise during the CC1 charging can be suppressed to about a half.
Further, when the Si-containing lithium ion secondary battery is charged at a 2C rate, the temperature rise during the period of CC2 charging is about 10° C. Thus, when the CC1 charging (1C) and the CC2 charging (2C) are combined as shown in
Further, if the first current value I1 is set to a value smaller than the second current value I2 in a range applicable to the CC charging according to the well-known CCCV charging method, a practical effect can be obtained reasonably. It is preferred practically to set the first current value I1 in a range of a 0.7C to 0.8C level. This is because the effect of suppressing temperature rise is obtained sufficiently, and influence on an increase in a charging speed is small. It is particularly effective for increasing a charging speed to set the second current value I2 at 1.5C or more.
By replacing the charging time tT10 in the above-mentioned embodiment with the general charging time tT corresponding to the timing at which the transition point Ta of the temperature rise gradient appears, it can be stated that more general changeover time ts set in a range of tT≦ts≦(tT×1.2).
A method for charging a lithium ion secondary battery of Embodiment 2 according to the present invention is substantially the same as that of Embodiment 1. In the present embodiment, the changeover time ts1 in the case of Embodiment 1 is replaced with changeover time ts2. Thus, the contents shown in
The changeover time ts2 in the present embodiment is set as follows. Specifically, in advance, with respect to a lithium ion secondary battery having the same specification as that of a charging target, the charging is started from a condition of the SOC of 0% and a charging time tTA before the transition point Ta of the temperature rise gradient is detected is measured.
If changeover time ts2 is set so as to correspond to the charging time tTA, the changeover time ts2 is set at timing when the transition point Ta appears. Thus, the CC1 charging can be switched to the CC2 charging at the transition point Ta of a temperature rise gradient.
Embodiment 2 is different from Embodiment 1 in that the changeover time ts1 is set so as to indirectly correspond to the transition point Ta of a temperature rise gradient through use of a point of time when the SOC reaches 10%, whereas the changeover time ts2 is set so as to directly correspond to the charging time tTA before the transition point Ta of a temperature rise gradient is detected. Accordingly, the CC1 charging can be switched to the CC2 charging at more precise timing.
As a result, in the same way as in Embodiment 1, the CC1 charging is performed at the smaller first current value I1 in a region corresponding to the T1 period having a large temperature rise gradient, and the CC2 charging is performed at the larger second current value I2 in a region corresponding to the T2 period having a small temperature rise gradient. As a result, charging can be performed efficiently while heat generation is suppressed to minimize temperature rise, and time required for charging can be shortened.
Even if the changeover time tS2 is set to be shifted from the charging time tTA to some degree, when the CC1 charging controlled by the smaller first current value I1 is included in an initial charging period, sufficient effects or corresponding effects can be obtained practically. It is desired that the changeover time ts2 be set in a range of tTA≦ts2≦(tTA×1.2) based on the charging time tTA in the same way as in Embodiment 1. That is, the time equivalent to the charging time tTA to the time that is longer by 20% than the charging time tTA is a desirable permissible range for obtaining the above-mentioned effects.
It should be noted, similarly to the Embodiment 1, that, in actual charging, even when the changeover time ts2 is set as described above, the changeover time ts2 is not always matched with timing at which the transition point Ta of a temperature rise gradient appears after start of the charging. Practically, the SOC at a time of start of charging is not constant, and hence, the charging time (tT1) does not become constant, either. Nevertheless, as the charging time tTA for setting the changeover time ts2, a measurement result in the case of starting the charging from a condition of the SOC of 0% is used. Therefore, the changeover time ts2 may be shifted from the timing at which the transition point Ta appears to some degree.
If the changeover time ts2 is set in a range of tTA≦ts2≦(tTA×1.2) as described above, the CC1 charging is performed at the smaller first current value I1 without failure in a region corresponding to the T1 period having a large temperature rise gradient, and the temperature rise is suppressed reliably. Further, the charging time tTA to be a basis of the changeover time ts2 has a small ratio occupying the CC charging period, and hence, there is small influence of shortening the charging time, as long as the CC1 charging period is up to +20% as described above. Thus, contribution to efficient charging can be obtained sufficiently while avoiding the temperature rise. This effect is obtained reasonably irrespective of the other conditions, if the changeover time ts2 is set in the above-mentioned range with respect to the charging time tTA.
A method for charging a lithium ion secondary battery according to Embodiment 3 of the present invention is substantially similar to that of Embodiment 1. The contents shown in
Determination of a charge state of a lithium ion secondary battery is performed so as to detect whether the charge state is before the above-mentioned transition point Ta of a temperature rise gradient of the battery during the CC charging or the charge state exceeds the transition point Ta. Then, if the charge state is before the transition point Ta, the CC charging is performed at a first current value until the changeover time ts elapses after start of charging, and after the changeover time ts elapses, the CC charging is performed at a second current value. On the other hand, if the charge state exceeds the transition point Ta, the CC charging is performed at a second current value.
Determination of a charge state for detecting whether the charge state exceeds the transition point Ta or not can be performed based on, for example, the SOC of 10%. That is, when the SOC is equal to or less than 10%, it is determined that the charge state is before the transition point Ta, and when the SOC exceeds 10%, it is determined that the charge state exceeds the transition point Ta. The SOC of 10% substantially corresponds to the transition point Ta as described above.
As shown in
On the other hand, when the SOC is equal to or less than 10% (No in Step S11), the process proceeds to Step S1, and the CC1 charging is started at the first current value I1. The subsequent steps are similar to those of Embodiment 1.
According to the charging method of the present embodiment, when charging is started from a state in which the SOC exceeds 10%, the CC1 charging with the first current value I1 is omitted, and hence, the effect of shortening time required for charging can be enhanced.
The step of determining a charge state before start of charging also is applicable to the method using the changeover time ts2 in Embodiment 2.
A method for charging a lithium ion secondary battery of the present invention enables charging to be performed efficiently while suppressing temperature rise, and hence, is useful for charging lithium ion secondary batteries to be used for various applications such as mobile equipment.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-210931 | Sep 2011 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2012/072451 | 9/4/2012 | WO | 00 | 3/7/2013 |
