JP4767566B2 - Lithium secondary battery charger and electronic device including the charger - Google Patents

Description

  The present invention relates to a lithium secondary battery charger.

  In recent years, processing capacity, display capacity, power performance, driving in portable electronic equipment such as information equipment, communication equipment, receiving equipment, video / music recording / playback equipment, transport equipment such as electric vehicles, etc. Improvements such as time are required. For this reason, the electrochemical element used for the above apparatuses is required to have a higher capacity and higher output, and to improve the cycle life. In response to such demands, development of electrode materials to achieve higher energy density of batteries, and charging system to drive the equipment efficiently by maximizing the performance of batteries with higher energy density. Development is underway.

  As an electrode material for the lithium secondary battery, a carbon material typified by graphite is already put to practical use as a negative electrode active material because it is excellent in reversibility and reliability. However, since the capacity of graphite used approaches the theoretical capacity, higher capacity electrode materials have been studied. There have been proposed elements such as silicon (Si) and tin (Sn) that can be expected to have a significantly higher capacity than conventional ones, alloys containing them, and the like (see, for example, Patent Document 1 and Patent Document 2).

FIG. 5 shows a schematic diagram of an electronic device including a conventional charging system.
An electronic device 50 shown in FIG. 5 includes a power supply unit 51, an electrochemical element 52, a load unit 53, a voltage comparator 54 that measures the voltage of the electrochemical element 52 with reference to a threshold voltage _Vth , a charge control unit 55, a charge The circuit 56, current measuring means 57 for measuring the current flowing through the electrochemical element 52, and temperature detecting means 58 for measuring the temperature of the electrochemical element 52 are included. Here, the charging system includes a voltage comparator 54, a charging control unit 55, a charging circuit 56, a current measuring unit 57, and a temperature detecting unit 58. The threshold voltage V _th is applied by the reference power source 59.

  With such a charging system, the voltage of an electrochemical element such as a lithium secondary battery or a nickel metal hydride battery is detected, and the electrochemical element is considered in consideration of differences in conditions such as current and temperature at the time of voltage detection. Detect the remaining amount.

  In the conventional charging system as described above, when the electrochemical element is fully charged, the charging of the electrochemical element is stopped, and when the voltage of the electrochemical element falls below the minimum operating voltage of the device, The discharge of the electrochemical element is stopped. The electrochemical element is then charged until it is fully charged. As a result, the electrochemical element always ensures the maximum remaining amount before use of the device, so that stable power supply can be performed. Therefore, the stable supply of electric power makes it possible to ensure the maximum drive time of the device.

However, in the charge / discharge method in which the electrochemical element is charged to the remaining amount of 100% and discharged to the minimum operating voltage of the device, the battery capacity at the time of full charge is greatly reduced each time the charge / discharge cycle is repeated. The problem is that the cycle life is reduced. In order to solve this problem, two charge end voltages, that is, a first charge end voltage at which the charge capacity of the electrochemical device is increased as much as possible and a second end charge voltage lower than the first charge end voltage. And charging according to the situation up to one of the charge termination voltages has been proposed (see, for example, Patent Document 3 and Patent Document 4).
Japanese Patent Application Laid-Open No. 07-29602 JP 2002-352797 A Japanese Patent Laid-Open No. 2002-218668 (page 9, FIG. 1) JP 2002-359008 A

  However, in the charging methods described in Patent Document 3 and Patent Document 4, how to determine the voltage and the mechanism of deterioration of the electrochemical element are not clearly described. For this reason, even if it uses the said charge method for charge of an electrochemical element, the charge end voltage must be determined by trial and error as a result. When charge / discharge control is performed based on the end-of-charge voltage thus determined, the degradation of the electrochemical device is not completely analyzed, so the set end-of-charge voltage is lower than the preferred end-of-charge voltage. There is a possibility. In this case, since the electrochemical device is not fully charged, there arises a problem that the usage time of the electronic device or the transportation device is shortened.

  On the other hand, when the set end-of-charge voltage is higher than the preferred end-of-charge voltage, not only the battery life but also the reliability, safety and maintainability of the charge / discharge system are lowered.

  Further, in the charging method described in Patent Document 3, the second charge end voltage is used in trickle charging in which the capacity reduced by the self-discharge of the battery incorporated in the notebook computer is charged by the AC power from the AC adapter. It has been. Here, the deterioration of the battery in a high temperature environment is prevented by setting the second end-of-charge voltage to a voltage lower than the first end-of-charge voltage. In this case, it is considered that the second charge end voltage lower than the first charge end voltage is determined on the assumption that the decomposition of the electrolytic solution is prevented even in a high temperature environment. The second end-of-charge voltage is not determined on the assumption that the cycle life is reduced.

In addition, in a material that can be expected to have a high capacity, such as Si, when the amount of lithium stored in the material at the time of charging increases, the cycle life of the battery may decrease. The reason is described below.
When the negative electrode contains lithium-containing silicon represented by the composition formula Li _x Si, the molar ratio x of Li to Si representing the charge / discharge depth of the negative electrode and the charge / discharge cycle life are related.
It is known that the lithium-containing silicon undergoes a phase change at a predetermined molar ratio x as the molar ratio x of Li to Si increases, and this phase change results in five phases: x = 0 (Si: _{cubic); x = 1.71 (Li 12} Si 7: _{orthorhombic); x = 2.33 (Li 14} Si 6: _{rhombohedral); x = 3.25 (Li 13} Si 4: Orthorhombic); x = 4.4 (Li ₂₂ Si ₅ : cubic). In the above lithium-containing silicon, the larger the x value, the higher the theoretical capacity.

  On the other hand, as described above, when a phase change occurs with a change in the x value, the volume of the lithium-containing silicon changes due to the phase change. For example, the volume of the lithium-containing silicon when x = 4.4 increases by about 4.1 times compared to the volume when x = 0. When complete charging and discharging of a battery containing lithium-containing silicon as a negative electrode active material are repeated, the volume change of the lithium-containing silicon increases. When such charge and discharge are continued, the lithium-containing silicon cannot cope with the volume change, the lithium-containing silicon itself is not damaged, and the cycle life of the battery is reduced.

  Further, when the x value increases and the expansion of the negative electrode increases, the current collection performance of the lithium-containing silicon decreases, so that the actual battery capacity of the battery having the negative electrode made of lithium-containing silicon decreases. Problems also arise.

  Therefore, the present invention can charge a lithium secondary battery using lithium-containing silicon as a negative electrode active material to a high capacity without accelerating the decrease in cycle life, and can be charged to a higher capacity as required. It is an object of the present invention to provide a charger that can be used and an electronic device including such a charger.

In the present invention, the positive electrode, lithium-containing silicon represented by the composition formula Li _x Si, and MeSi ₂ (Me is at least one selected from the group consisting of Ti, Ni, Co, and Fe) are set to 1: 0. Regarding a charger for a secondary battery comprising a negative electrode comprising a molar ratio of 03 to 1: 1.5 and an electrolyte,
(1) a voltage detector for detecting the voltage of the charged secondary battery;
(2) Based on the output of the voltage detector, an x value in Li _x Si included in the secondary battery is obtained, and when the obtained x value reaches a preset threshold value, A charging control unit for stopping charging is provided. The charge control unit has at least one settable threshold value including a first threshold value, and the first threshold value is 2.33 or less.

  In the charger, the charge control unit has at least two settable thresholds including a first threshold and a second threshold, and the first threshold and the second threshold can be arbitrarily switched, and the first threshold is It is 2.33 or less, and the second threshold is preferably larger than 2.33 and not larger than 4.4.

In the negative electrode, M eSi ₂ preferably contains TiSi _2.

Moreover, this invention relates to the electronic device which consists of the said secondary battery, a load part, and the said charger.

According to the present invention, a secondary battery containing Li _x Si in the negative electrode can be charged to a high capacity without accelerating the decrease in cycle life. Further, if necessary, the secondary battery can be further increased in capacity. A charger that can be charged and an electronic device including such a charger can be provided.

Hereinafter, the present invention will be described with reference to the drawings.
Embodiment 1
FIG. 1 shows a schematic diagram of an electronic device including a positive electrode, a negative electrode made of lithium-containing silicon represented by the composition formula Li _x Si, and a charger for a secondary battery made of an electrolyte.
The electronic device 10 shown in FIG. 1 includes a load unit 12, a secondary battery 13, and a charger. The charger includes a voltage detection unit 14, a charging control unit 15, and a charging circuit 16.
For example, the portable electronic device 10 is driven by supplying power from the secondary battery 13 to the load unit 12.
In the present embodiment, the secondary battery 13 is charged with electric power supplied from the power supply unit 11 through the charging circuit 16.
Examples of the power supply unit 11 include power supply means such as a fuel cell, a solar cell, or a generator provided in an engine. Further, power may be supplied from a commercial power source via an AC adapter.

Next, the charger will be described.
As described above, the secondary battery 13 is charged by the power supplied from the power supply unit 11 through the charging circuit 16. In the present invention, charging is performed such that Li of lithium silicon-containing (Li _x Si) contained in the secondary battery 13 has a desired molar ratio x ′. The charging of the secondary battery 13 is controlled by the charging circuit 16 based on the output from the charging control unit 15. In the charge control unit 15, at least one desired molar ratio x ′ is preset as the threshold value x _th .

The Li molar ratio x of Li _x Si contained in the charged secondary battery 13 is calculated by the charge control unit 15 based on the voltage of the charged secondary battery 13, for example. In the present embodiment, the voltage of the charged secondary battery 13 is measured by the voltage detection unit 14 connected to the secondary battery 13. The molar ratio x of Li increases as the secondary battery is charged, and the voltage of the secondary battery increases as the molar ratio x increases. At this time, since there is a predetermined relationship between the increase in the Li molar ratio x and the increase in the secondary battery voltage, the secondary battery voltage is calculated from the Li molar ratio x or the secondary battery voltage. It becomes possible to predict the molar ratio x of Li.

Further, when calculating the molar ratio x of Li, as shown in FIG. 1, a current measuring means 17 for measuring the charging current of the secondary battery 13 connected in series with the secondary battery 13, and the secondary battery The x value can also be calculated in consideration of the output from the temperature measuring means 18 for measuring 13 temperatures. This is because the battery voltage when the value of the molar ratio of Li is x may change depending on the temperature of the secondary battery 13 and / or the charging current. The range of V _th corresponding to the threshold value x _th takes into account changes in the charging current to the secondary battery 13, the temperature of the secondary battery 13, and / or the impedance of the secondary battery 13, and the x value thereof. Is preferably in the range of ± 10% to 20% of the voltage determined in accordance with the above, and the upper limit is set to + 0% and the lower limit is set to −10%, most preferably.

Voltage detecting unit 14 connected to the secondary battery 13, the voltage V _th of the secondary battery 13 in the threshold value x _th is the threshold voltage, and used as a voltage reference, the voltage of the secondary battery 13 being charged Is detected. Here, the threshold voltage refers to the voltage V _{th of the} secondary battery when the molar ratio x of Li of lithium-containing silicon contained in the negative electrode of the secondary battery is a predetermined threshold x _th .

In the charging control unit 15, the calculated molar ratios x, is compared with a threshold value x _th set in advance is, if the calculated molar ratio x exceeds the threshold value x _th set, the charging control unit Based on the output from 15, the charging circuit 16 stops charging the secondary battery 13. When the calculated x value does not exceed the set threshold value, the charging of the secondary battery 13 is continued by the charging circuit 16 based on the output from the charging control unit 15. The charging of the secondary battery 13 by the charging circuit 16 can be performed by constant current charging or a combination of constant current charging and constant voltage charging. In the present invention, the voltage of the charged secondary battery 13 is obtained by the voltage detection unit, and the molar ratio x of lithium is obtained by the charge control unit based on the output from the voltage detection unit. Therefore, basically, the secondary battery 13 can be charged by constant current charging.
Also, the secondary battery 13 can be charged by combining constant current charging and constant voltage charging. In this case, first, the lithium molar ratio x of Li _x Si contained in the secondary battery 13 is charged to a value close to a desired molar ratio x ′ by constant current charging, and then the constant voltage charging is performed. It is charged so that the molar ratio x of lithium is the desired molar ratio x ′.

  Here, FIG. 2 shows an example (A) of a discharge curve of a lithium secondary battery comprising a negative electrode, a positive electrode and an electrolyte containing a carbon material as a negative electrode active material, and a negative electrode, a positive electrode and an electrolyte containing lithium-containing silicon as a negative electrode active material. An example (B) of the discharge curve of the lithium secondary battery which consists of is shown.

Generally, in a lithium secondary battery including a carbon material as a negative electrode active material, a charge end voltage is 4.2V and a discharge end voltage is 3.0V.
On the other hand, V _th2 in FIG. 2 (which is also a charge end voltage) is set to 4.2 V, and a lithium secondary battery including lithium-containing silicon as a negative electrode active material and a lithium secondary battery including a carbon material as a negative electrode active material. And compare. In this case, in the lithium secondary battery containing lithium-containing silicon used in the present invention as the negative electrode active material, the voltage is somewhat reduced, but the battery capacity is increased. For example, a lithium secondary battery including a conventional carbon material as a negative electrode active material cannot be used, or even if it can be used, lithium-containing silicon is included as a negative electrode active material up to 2.5 V, which has almost no merit in capacity. Assume that a secondary battery is used. Under such conditions, the battery capacity of the battery can be increased.

Moreover, even if the charge end voltage of the lithium secondary battery containing lithium-containing silicon used in the present invention as a negative electrode active material is set to V _th1 lower than V _th2 , the lithium secondary battery containing a carbon material as the negative electrode active material It can be seen that a higher battery capacity can be obtained.

  In the present invention, the charge control unit 15 has at least one settable threshold including the first threshold or at least two settable thresholds including the first threshold and the second threshold as the desired molar ratio x ′. . Here, the first threshold is 2.33 or less, and the second threshold is greater than 2.33 and 4.4 or less.

As the first threshold value, a settable value of 2.33 or less is selected. For example, when charging is performed until the molar ratio x of Li becomes 2.33, the capacity can be increased and the cycle life can be maintained high as compared with a conventional battery using a carbon material for the negative electrode. . This is because if the x value is 2.33 or less, the volume change of Li _x Si accompanying charging is remarkably suppressed.

As the second threshold, a settable value greater than 2.33 and less than or equal to 4.4 is selected. In this case, it is possible to charge to a higher capacity than when charging to the first threshold. As a result, it can be used dramatically for a long time. Here, the amount of lithium occluded in Li _x Si is maximized when x = 4.4.

Furthermore, when the charge control unit 15 has at least two settable threshold values including the first threshold value and the second threshold value, at least the first threshold value and the second threshold value can be arbitrarily switched. Therefore, by simply switching the threshold value set in the charge control unit, it is possible to easily charge a secondary battery containing Li _x Si as a negative electrode with a high cycle life and high capacity or higher capacity. Is possible.

In a secondary battery including a negative electrode containing lithium-containing silicon, a positive electrode, and an electrolyte, the negative electrode is a metal for the purpose of suppressing volume change of the lithium-containing silicon during charge and discharge in addition to lithium-containing silicon as a negative electrode active material. It is preferable to include intermetallic compounds, oxides such as SiO, SiO ₂ , and Li ₂ O, and conductive ceramics such as TiN, TiB ₂ , and TiC. Examples of the intermetallic compound include those represented by the general formula: MeSi ₂ (Me is at least one selected from the group consisting of Ti, Ni, Co, and Fe).

Among the above, MeSi ₂ (Me is at least one selected from the group consisting of Ti, Ni, Co, and Fe) is preferable from the viewpoint of electron conductivity. That is, MeSi ₂ can improve electron conduction to Si. Furthermore, since MeSi ₂ does not occlude or release lithium during charging and discharging, the volume change of the negative electrode containing lithium-containing silicon can be efficiently suppressed, and damage to the lithium-containing silicon as the active material can be prevented. . Thus, by including MeSi ₂ or the like in the negative electrode, it is possible to maintain cycle characteristics even in a region where the x value is large as compared with the case where they are not included. For this reason, it is possible to achieve both high capacity and cycle characteristics over a wide range of x.

Also, if the negative electrode containing lithium-containing silicon and MeSi _2, which is preferably complexed to the lithium-containing silicon and MeSi _2. At this time, in the composite, both the lithium-containing silicon phase and the MeSi ₂ phase are amorphous, or both phases are composed of microcrystals having a crystallite size of several nanometers to several tens of nanometers, More preferably, they are mixed together. As a result, the effect of achieving both high capacity and cycle characteristics can be further improved.

One type of MeSi ₂ may be included in the negative electrode, and two or more types of MeSi ₂ may be included. In addition, MeSi ₂ preferably contains TiSi ₂ . This is because TiSi ₂ has high electron conductivity among those represented by MeSi ₂ . Similarly, when two or more types of MeSi ₂ are included in the negative electrode, it is preferable that one of them is TiSi ₂ .

In addition, when the negative electrode active material is composed of lithium-containing silicon and another material as described above, the x value representing the amount of lithium in the lithium-containing silicon can be changed depending on the chemical composition of the active material. it can. For example, when the negative electrode active material includes a phase of lithium-containing silicon (Li _x Si) and a phase of MeSi ₂ , the content ratio of Li _x Si and MeSi ₂ is expressed as a molar fraction of 1-y: y (0 ≦ When y <1), the negative electrode active material is represented by the composition formula: Li _{x (1-y)} Me _y Si _{(1 + y)} . The range of the x value is defined as, for example, x ≦ 2.33 in Li _x Si, but x (1-y) ≦ 2.33 (1-y) is also expressed in the negative electrode active material. be able to.

The mixing ratio (molar ratio) between Li _x Si and MeSi ₂ is preferably 1: 0.03 to 1: 1.5. When the molar ratio of MeSi ₂ is smaller than 0.03, the electron conductivity may be impaired, and the cycle characteristics may be deteriorated. When the molar ratio of MeSi ₂ is larger than 1.5, the ratio of Li _x Si is decreased, and it may be impossible to increase the capacity.

Next, a case where the charge control unit has the first threshold value and the second threshold value will be described with reference to FIG.
In the charge control unit 15 shown in FIG. 1, a first threshold value x _th1 and a second threshold value x _th2 are set. The voltage detection unit 14 includes two voltage comparison units 141 a and 141 b and a switching unit 142. Two threshold voltages, a voltage V _th1 corresponding to the first threshold x _th1 set in the charge controller 15 and a voltage V _th2 corresponding to the second threshold x _th2 , are set in the voltage detector 14. The voltage detection unit 14 detects the voltage of the secondary battery using the threshold voltage as a voltage reference.

The voltage comparison unit 141a has a threshold voltage V _th1 corresponding to the first threshold as a voltage reference, and the voltage comparison unit 141b has a threshold voltage V _th2 corresponding to the second threshold as a voltage reference. In voltage comparators 141a and 141b, threshold voltages V _th1 and V _th2 are applied by reference power supplies 143a and 143b, respectively.

For example, when the second threshold value x _th2 is selected as the desired molar ratio x ′ in the charge control unit 15, the voltage comparison unit 141 b and the charge control unit 15 are switched by the switching unit 142 as shown in FIG. Connected selectively. As a result, the threshold voltage V _th2 that serves as a voltage reference when detecting the voltage of the secondary battery 13 can be selected.

Information on the voltage detected as described above is sent to the charge control unit 15 as an output from the voltage detection unit 14.
In the charging control unit 15, the x value of Li _x Si included in the negative electrode of the charged secondary battery 13 is calculated based on the output from the voltage detection unit 14. This calculated x value is compared with a set threshold value x _th .

The charging circuit 16 controls charging of the secondary battery 13 based on the output from the charging control unit 15. When the calculated x value is set and smaller than the threshold value x _th , the charging control unit 15 sends an instruction to the charging circuit 16 so as to continue charging the secondary battery 13. When it is determined that the calculated x value has reached the set threshold value x _th , the charging control unit 15 sends an instruction to the charging circuit 16 to stop the charging of the secondary battery 13, and the secondary battery 13 Stop charging.

Thus, in the present invention, the charging control unit 15 selects one threshold value, and the voltage detection unit 14 selects the voltage of the secondary battery corresponding to this threshold value as the voltage reference. This makes it possible to easily charge the battery so that the x value of Li _x Si contained in the negative electrode of the secondary battery becomes a predetermined value.

  In the above description, the voltage of the charged secondary battery 13 is detected using analog hardware such as an electronic circuit, and the molar ratio x is obtained. In addition to this method, an AD converter may be used as a voltage comparator, and the charge control unit 15 may process the digital data from the AD converter with software to obtain the molar ratio x.

  In addition, the user may perform threshold value selection and switching unit operation in the charge control unit. In addition, when the user selects a threshold value in the charge control unit, the operation of the switching unit may be automatically performed. Furthermore, the electronic device including the charger according to the present invention may learn the usage status of the user, select a threshold value in the charging control unit, and operate the switching unit accordingly.

  Further, the charging control unit 15 may have the function of the charging circuit 16. In this case, the charging circuit 16 may not be provided.

Embodiment 2
FIG. 3 shows a schematic diagram of an electronic device including a charger in which the charge control unit has three threshold values and the voltage detection unit has three voltage comparison units.
In the present embodiment, the electronic device 30 includes a load unit 32, a secondary battery 33, and a charger. The charger includes a voltage detection unit 34, a charge control unit 35, and a charging circuit 36.

  Similarly to the first embodiment, the electronic device 30 is driven by supplying power from the load unit secondary battery 33 to the load unit 32. The secondary battery 33 is charged with electric power supplied from the power supply unit 31 through the charging circuit 36.

In the present embodiment, in the electronic device 30, the charge control unit 35 has three threshold values of a first threshold value x _th1 , a second threshold value x _th2, and a third threshold value x _th3 , and the voltage detection unit 34 has three threshold values. It is the same as that of the said Embodiment 1 except having the voltage comparison parts 341a, 341b, and 341c.

The three voltage comparators 341a, 341b, and 341c respectively supply the secondary battery voltages V _th1 , V _th2, and V _th3 corresponding to the first threshold value x _th1 , the second threshold value x _th2 , and the third threshold value x _th3. Has as a voltage reference. In these three voltage comparators 341a, 341b and 341c, the three voltages are applied by reference power sources 343a, 343b and 343c, respectively. FIG. 3 shows a state in which the charging control unit 35 is connected to the voltage comparator 341b.

  In the present embodiment, the first threshold is 2.33 or less, and the second threshold is greater than 2.33 and 4.4 or less. The third threshold value is 4.4 or less, and a value different from both the first threshold value and the second threshold value is set.

FIG. 4 shows an example (B) of a discharge curve of a secondary battery comprising a negative electrode, a positive electrode and an electrolyte containing lithium-containing silicon as a negative electrode active material. As in the case of FIG. 2 described in the first embodiment, the secondary battery (B) including a negative electrode, a positive electrode, and an electrolyte containing lithium-containing silicon as a negative electrode active material has a threshold voltage V _th1 , V _th2, or V _th3. It can be seen that the battery has a higher capacity than the battery (A) containing a carbon material as a negative electrode active material under the condition that it can be charged up to 2.5 V and can be used up to 2.5 V.

  For example, the third threshold value is set to an x value that can be set to 2.33 or less, and the charge control unit 35 uses these threshold values by switching according to the situation, so that not only high capacity but also high cycle life is maintained. Thus, a charger suitable for charging the secondary battery 33 can be obtained.

  The third threshold value is a settable value between 2.33 and 4.4, and the charge control unit 35 uses these threshold values by switching them according to the situation. It is possible to provide a charger suitable for charging the secondary battery 33 with a higher capacity.

  As in the first embodiment, the user may perform selection of the threshold value and operation of the switching unit 342 in the charging control unit 35. Further, when the user selects a threshold value in the charging control unit 35, the operation of the switching unit 342 may be automatically performed. Furthermore, the electronic device including the charger according to the present invention may learn the usage status of the user, select a threshold value used in the charging control unit 35, and operate the switching unit together with the threshold value.

  Similarly to the first embodiment, the charging control unit 35 can calculate the x value in consideration of the outputs from the current measuring unit 37 and the temperature measuring unit 38.

Embodiment 3
Here, a case where the charge control unit has at least one settable threshold value including the first threshold value, and the first threshold value is 2.33 or less will be described.
In this case, at least one voltage including a voltage corresponding to the first threshold is set as the threshold voltage in the voltage detection unit.
The parts other than the voltage detection unit are the same as those in the first embodiment.

  As described above, by setting a first threshold value of 2.33 or less and charging until the molar ratio x of Li in the lithium-containing silicon contained in the secondary battery reaches the first threshold value, high capacity and The secondary battery can be charged so as to have a high cycle life.

  In addition, for example, when the charge control unit has other threshold values smaller than 2.33 other than the first threshold value, the secondary battery can be charged so as to maintain a higher cycle life. .

  At this time, in the voltage detection unit, in addition to the voltage of the secondary battery corresponding to the first threshold, the voltage of the secondary battery corresponding to another threshold smaller than 2.33 is set as the threshold voltage as the voltage reference. . In this case, as many voltage comparators and reference power supplies as the threshold voltage set as the voltage reference are required.

  The selection of the threshold value in the charge control unit and the selection of the threshold voltage in the voltage detection unit can be performed in the same manner as in the first embodiment.

  In this way, the charge control unit has at least one settable threshold value including the first threshold value, so that it is possible to charge the secondary battery with a high capacity and particularly to improve the cycle life. Become.

  Also in this case, similarly to the first embodiment, the charge control unit can calculate the x value in consideration of the outputs from the current measuring unit and the temperature measuring unit.

(Production of secondary battery)
(Preparation of negative electrode)
[Negative electrode A]
Si was used for the negative electrode active material. A Si layer having a thickness of 5 μm was formed on a 14 μm copper foil by sputtering to obtain a negative electrode. This was designated as negative electrode A.
[Negative electrode B]
A negative electrode active material was synthesized using a mechanical alloying method in an argon atmosphere using a Ti—Si alloy obtained by a melting method as a starting material. Here, as the starting Ti—Si alloy, an alloy having 9 wt% Ti and 91 wt% Si was used. The synthesized negative electrode active material was confirmed to contain two phases of Si and TiSi ₂ by electron beam diffraction using a transmission electron microscope. The ratio (molar ratio) between Si and TiSi ₂ was 1: 0.07.

  Next, 80 parts by weight of the negative electrode active material obtained as described above, 15 parts by weight of acetylene black as a conductive agent, and 5 parts by weight of SBR resin as a binder are added to water and kneaded, A negative electrode mixture paste was prepared. This negative electrode mixture paste was applied onto a copper foil (thickness: 14 μm) functioning as a current collector using a doctor blade method, and then sufficiently dried to obtain a negative electrode. This was designated as negative electrode B.

(Preparation of positive electrode)
90 parts by weight of LiCoO ₂ as a positive electrode active material, 3 parts by weight of acetylene black as a conductive agent, and 5 parts by weight of polyvinylidene fluoride powder as a binder were mixed. N-methylpyrrolidone was added to the resulting mixture to prepare a positive electrode mixture paste. This positive electrode mixture paste was applied onto an aluminum foil (thickness: 15 μm) as a current collector by a doctor blade method and sufficiently dried to obtain a positive electrode.

(Preparation of electrolyte)
Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 1: 1. An electrolyte solution was prepared by dissolving LiPF ₆ at a concentration of 1 M in the obtained mixed solvent.

(Battery assembly)
The negative electrode A and the positive electrode obtained as described above were arranged through a microporous film made of polyethylene, and this was wound to obtain an electrode body. Next, the obtained electrode body was inserted into a rectangular outer case mainly composed of aluminum, and an electrolytic solution was injected. After that, the outer case was sealed to obtain a battery. This battery was designated as battery A.
Further, a battery B was obtained in the same manner as described above except that the negative electrode B was used instead of the negative electrode A.
A lithium secondary battery was produced in the same manner as Battery A, except that artificial graphite was used as the negative electrode active material. The obtained secondary battery was designated as battery C.

(Evaluation of cycle life)
In this example, a charger as shown in FIG. 1 was used. The charge control unit has a first threshold value and a second threshold value. Here, the first threshold value was 2.33, and the second threshold value was 4.4.

Corresponding to this, two voltage comparators were prepared in the voltage comparator. The threshold voltage V _th1 that is one voltage reference of the two voltage comparators was set to 4.05 V that is the voltage of the battery A or B corresponding to the first threshold (x = 2.33). Further, the threshold voltage V _th2 that is the voltage reference of the other voltage comparator is set to 4.3 V that is a voltage corresponding to the second threshold (x = 4.4).

  Conventionally, the discharge end voltage of lithium secondary batteries has been about 3 V to 3.2 V, but in order to realize the high capacity that is characteristic of the battery A and the battery B, the discharge end voltage of these batteries is 2 .5V.

Regarding the battery A and the battery B, charge and discharge cycles for charging when the x value of Li _x Si included in the negative electrode becomes the first threshold and charging when the x value becomes the second threshold Was repeated. In addition, the voltage detection unit switches between the threshold voltage V _th1 and the threshold voltage V _th2 using the switching unit in accordance with the first threshold value or the second threshold value selected in the charge control unit.

Battery C was charged and discharged using a conventional apparatus with a charge end voltage of 4.2 V and a discharge end voltage of 3.0 V.
Capacity retention rate, the following formula:
Capacity retention ratio (%) = (battery capacity at the 100th cycle) / (battery capacity at the first cycle) × 100. Further, the ratio of the battery capacity of the first cycle of the battery A or the battery B to the battery capacity of the first cycle of the battery C as a percentage was defined as the battery capacity ratio.
The obtained capacity retention ratio and battery capacity ratio are shown in Table 1.

  In both the battery A and the battery B, when x = 2.3, the battery capacity was higher than the battery capacity of the battery C that was charged and discharged under the conventional conditions. Moreover, despite these high battery capacities, these capacity retention rates were comparable to those of the battery C charged under the conventional conditions.

When the battery was charged so that x = 4.4, the capacity retention rates of battery A and battery B were lowered. On the other hand, the battery capacity could be charged to increase 1.5 to 1.6 times. Furthermore, it can be seen that in the battery B in which Li _x Si and TiSi ₂ are complexed, a decrease in capacity retention rate can be suppressed as compared with the battery A that does not contain TiSi ₂ . This effect became more prominent as the x value increased.

  In this example, the battery B produced in Example 1 was used, and a charger as shown in FIG. 1 was used. The charge control unit has a first threshold value, a second threshold value, and a third threshold value. Here, the first threshold value was set to 2.33, the second threshold value was set to 3.25, and the third threshold value was set to 4.4.

Corresponding to this, three voltage comparators were prepared in the voltage comparator. The threshold voltage V _th1 was set to 4.05 V that is the voltage of the battery B corresponding to the first threshold (x = 2.33). The threshold voltage V _th2 was set to 4.2 V, which is the voltage of the battery B corresponding to the second threshold (x = 3.25). Further, the threshold voltage V _th3 was set to 4.3 V, which is the voltage of the battery B corresponding to the third threshold (x = 4.4). The discharge end voltage was set to 2.5V.

When charging so that the x value of Li _x Si included in the battery B becomes the first threshold, charging when the x value becomes the second threshold, and charging when the x value of the Li _x Si becomes the third threshold The charge / discharge cycle was repeated. In addition, the voltage detection unit switches _between the threshold voltage V _th1 , the threshold voltage V _th2, and the threshold voltage V _th3 according to the first threshold value, the second threshold value, or the third threshold value selected in the charge control unit. By means.
In the same manner as in Example 1, the capacity retention ratio and the battery capacity ratio were obtained. The obtained results are shown in Table 2.

As shown in Table 2, by using the charger according to the present invention, a battery containing Li _x Si in the negative electrode is obtained from the battery C, which is a conventional lithium secondary battery, even when the molar ratio x is 2.33. It can be seen that the capacity can be increased.

Furthermore, in the case of x = 2.33, the capacity maintenance rate of the battery B containing Li _x Si in the negative electrode was almost the same as the capacity maintenance rate of the battery C. In this way, the battery containing Li _x Si in the negative electrode has a higher capacity than the conventional lithium secondary battery, and the capacity retention rate of the conventional lithium secondary battery can be maintained at the same level as the capacity retention rate. did it.

  Further, when the value of x was increased to 3.25, 4.4, the capacity retention rate was slightly reduced, but the battery capacity could be increased greatly.

  As described above, the charge control unit has at least one settable threshold value including the first threshold value, or at least one settable threshold value including the first threshold value and the second threshold value. You can select and switch between these thresholds. For this reason, the battery can be charged to maintain a high capacity and an excellent cycle life or to a higher capacity by switching the threshold depending on the situation.

  Furthermore, use of the charger of the present invention can prevent unnecessary acceleration of charge / discharge cycle life in a power supply system including a lithium secondary battery, which has been a problem in the past. For this reason, maintainability, such as stability of the whole power supply system, reliability, and battery replacement frequency, can be improved.

  In Examples 1 and 2, Si or Ti—Si alloy was used as the negative electrode active material, but other Si compounds capable of inserting and extracting Li can be used as the negative electrode active material. The charger of the present invention can also be used when charging a battery using a metal such as Sn or Ge, and an alloy made thereof as a negative electrode active material.

As the positive electrode active material of the battery using Si or Ti—Si alloy as the negative electrode active material, conventionally known materials can be used other than LiCoO ₂ as the positive electrode active material of the lithium secondary battery. Examples of such a positive electrode active material include LiNiO ₂ , LiMn ₂ O ₄ , those obtained by adding other elements to these compounds, and those obtained by replacing some of the elements of these compounds with other elements. Can be used.

  Moreover, although the said Embodiment 1-3 demonstrated the electronic device provided with the charger of this invention, the above effects are acquired even when only a charger is used.

  The charger according to the present invention can be appropriately selected as to charge the secondary battery to a high capacity or to have a long life, and thus can be used for a mobile phone using a lithium secondary battery. The charger of the present invention can also be applied to transport equipment such as a rechargeable automobile that does not have a power generation unit.

It is the schematic which shows an electronic device provided with the charger which concerns on one Embodiment of this invention. A discharge curve (A) of a battery using a carbon material as a negative electrode active material and a discharge curve (B) of a battery using silicon as a negative electrode active material used in the present invention are shown. It is the schematic which shows an electronic device provided with the charger which concerns on another embodiment of this invention. A discharge curve (A) of a battery using a carbon material as a negative electrode active material and a discharge curve (B) of a battery using silicon as a negative electrode active material used in the present invention are shown. It is the schematic which shows the electronic device containing the conventional charger.

Explanation of symbols

10, 30, 50 Electronic device 11, 31, 51 Power supply unit 12, 32, 53 Load unit 13, 33 Secondary battery 14, 34 Voltage detection unit 141a, 141b, 341a, 341b, 341c, 54 Voltage comparison unit 142, 342 Switching means 143a, 143b, 343a, 343b, 343c, 59 Reference power supply 15, 35, 55 Charging controller 16, 36, 56 Charging circuit 17, 37, 57 Current measuring means 18, 38, 58 Temperature measuring means 52 Electrochemical element

Claims (4)

A positive electrode, lithium-containing silicon represented by the composition formula Li _x Si and MeSi ₂ (Me is at least one selected from the group consisting of Ti, Ni, Co and Fe) 1: 0.03 to 1: A battery charger for a secondary battery comprising a negative electrode and an electrolyte containing at a molar ratio of 1.5 ,
(1) a voltage detector for detecting the voltage of the charged secondary battery;
(2) An x value in Li _x Si included in the secondary battery is obtained based on an output of the voltage detection unit, and when the obtained x value reaches a preset threshold value, the secondary battery A charging control unit for stopping the charging of
The charging controller has at least one settable threshold value including a first threshold value, and the first threshold value is 2.33 or less.   The charge control unit has at least two settable threshold values including a first threshold value and a second threshold value. The first threshold value and the second threshold value can be arbitrarily switched, and the first threshold value is 2.33 or less. The charger according to claim 1, wherein the second threshold value is greater than 2.33 and not greater than 4.4. The MeSi ₂ is a charger of claim 1, further comprising a TiSi _2. The electronic device which consists of the said secondary battery, a load part, and the charger in any one of Claims 1-3 .

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