JPH09283184A - Method for charging non-aqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the cycle lifetime by charging a battery, of which capacity is consumed by the predetermined quantity, after discharging this battery to a voltage lower than the open circuit voltage at the time of consumption before starting charging. SOLUTION: On the surface of a negative electrode 12 after charging a dendrite 11 is formed. (Fig. (a)) When the consumption progresses, the dendrite 11 is eluted in the electrolyte, and voltage is gradually lowered. Thereafter, discharging regulating voltage is detected and the consumption is concluded, but at this stage, a base part of the dendrite 11 is left and a part of the surface of the negative electrode 12 is eluted. (Fig. (b)) When charging is performed in a conventional way, lithium is concentrically deposited in an end of the left dendrite 13 and a recessed part 14 of the negative electrode, and quantity of the lithium, which can be charged and discharged, is reduced. (Fig. (c)) On the other hand, with this method for charging and discharging since discharging is performed to a voltage lower than the open circuit voltage at the time of consumption at least 70%, the dendrite, which is not eluted and left, can be eliminated as much as possible, and the reduction of lithium, which can be charged and discharged, is avoided.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for charging a non-aqueous electrolyte secondary battery. More specifically, the present invention relates to a method for charging a non-aqueous electrolyte secondary battery that can provide good cycle life.

[0002]

2. Description of the Related Art Non-aqueous electrolyte secondary batteries using lithium as a negative electrode active material have been extensively studied because they can realize high voltage and high energy density. However, it has not been put to practical use yet. The reason for this is due to the high reactivity of metallic lithium. That is, metallic lithium reacts with carbon dioxide gas, nitrogen, water or the like in the atmosphere during the production thereof, and a non-uniform coating film of carbonate, nitride, hydroxide or the like is formed on the surface. When metallic lithium on which a non-uniform coating is formed is used as the negative electrode active material of a non-aqueous electrolyte secondary battery, the battery capacity decreases every time charging and discharging are repeated. It is considered that this decrease in battery capacity is due to the non-uniform coating present on the surface of metallic lithium, which interferes with the electrode reaction and locally dissolves and deposits lithium, thereby forming metallic lithium having low reversibility. . Further, the deposited metal lithium has a dendritic form generally called dendrite, and this dendrite not only reduces the battery capacity, but also causes a short circuit between the positive and negative electrodes inside the battery, resulting in a secondary battery. It is known as a factor that significantly reduces the safety of.

In order to prevent a decrease in battery capacity due to repeated charging / discharging, an intermetallic compound formed by alloying lithium with aluminum, strontium (JP-A-61-32954), or the like is used. A method of occluding lithium inside and a method of occluding lithium ions in a carbon material have been reported. However, in these methods, the elements that are not involved in the battery reaction are arranged in the battery, which leads to a decrease in the battery capacity. Further, in the case of an intermetallic compound, it is difficult to process it into an electrode shape, and therefore it is costly, so that it has not been used as a main power source for electric equipment.

As another method for improving charge / discharge efficiency,
A method of performing initial discharge at a predetermined current density for a predetermined time at the end of charging or at the start of discharging has been reported (Japanese Patent Laid-Open No. 7-
65867). According to the discharge according to this publication, the non-uniform coating film existing on the surface of the metallic lithium is removed before the formation of the battery, so that the form of deposition of metallic lithium is improved and, as a result, the charging / discharging efficiency is improved.

[0005]

However, even with the method described in the above publication, it is not possible to suppress the growth of dendrite, which is a problem when metallic lithium is used for the negative electrode, and further improve the charge and discharge efficiency. Was desired.

[0006]

Means for Solving the Problems As a result of intensive studies, the inventors of the present invention have considered that the morphology of the lithium surface determines the life of metallic lithium as a negative electrode active material, and detect a predetermined discharge voltage. It was found that charging and discharging efficiency can be improved by controlling the morphology of the surface of metal lithium, that is, the morphology of precipitation, by further discharging later.

Thus, according to the present invention, in a non-aqueous electrolyte secondary battery having a negative electrode made of metallic lithium or a lithium alloy, a positive electrode and an electrolytic solution, at least 7 of the battery capacity is provided.
A method for charging a non-aqueous electrolyte secondary battery, characterized in that the non-aqueous electrolyte secondary battery that has been consumed by a charge is further discharged before the start of charging to a voltage lower than the open circuit voltage at the time of consumption, and then charged. Will be provided.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION As the non-aqueous electrolyte secondary battery that can be used in the present invention, any known non-aqueous electrolyte secondary battery can be used as long as it contains a negative electrode active material made of lithium. The non-aqueous electrolyte secondary battery will be described more specifically below. First, metallic lithium and lithium alloys can be used for the negative electrode that can be used in the present invention. Examples of the lithium alloy include lithium aluminum and lithium magnesium. These negative electrodes also function as a negative electrode active material. It is preferable to use a lithium-magnesium alloy for the negative electrode. Further, a lithium-magnesium alloy containing 20 to 30% by weight of magnesium is preferable.

On the other hand, the positive electrode comprises, for example, a positive electrode active material, a conductive agent and a binder. Examples of the positive electrode active material include a chalcogenide containing lithium,
More specifically, LiCoO ₂ , LiNiO ₂ , LiM
nO _{2 and the} like can be mentioned. Examples of the conductive agent include acetylene black, graphite and carbon.

Examples of the binder include Teflon resin and ethylene-propylene-diene terpolymer. In addition, the positive electrode and the negative electrode may be formed on a current collector made of a metal such as aluminum or copper, if necessary. The electrolytic solution includes an electrolyte and an organic solvent.

The organic solvent is not particularly limited as long as it can be used in the electrolytic solution for a non-aqueous electrolytic solution secondary battery. For example, propylene carbonate, tetrahydrofuran,
Dimethyl sulfoxide, γ-butyrolaclone, 1,3-
Dioxolane, 4-methyl-1,3-dioxolane,
1,2-dimethoxyethane, 2-methyltetrahydrofuran, sulfolane, diethyl carbonate, dimethylformamide, acetonitrile, dimethyl carbonate,
Examples thereof include ethylene carbonate. These organic solvents may be used alone or in combination.

The electrolyte is not particularly limited as long as it forms lithium ions in the electrolytic solution. For example,
LiPF ₆ , LiClO ₄ , LiBF ₄ , LiAs
F ₆ , LiAlCl ₄ , CF ₃ CO ₂ Li, LiCF ₃
SO ₃ , LiSbF _{6 and the} like can be mentioned. These electrolytes may be used alone or in combination. Furthermore, for holding an electrolytic solution between the positive electrode and the negative electrode, and
A separator may be provided in order to prevent a short circuit between the positive electrode and the negative electrode. The material of the separator is not particularly limited as long as it is an insulator that is not dissolved in the electrolytic solution and can be easily processed. More specifically, porous polypropylene, porous polyethylene and the like can be mentioned.

The shape of the non-aqueous electrolyte secondary battery may be any of cylindrical type, prismatic type, button type, sheet type and the like. FIG. 1 shows a schematic sectional view of a coin type non-aqueous electrolyte secondary battery. This FIG. 1 will be briefly described. First, in the container formed by the battery cans (1 and 2), the positive electrode 3, the negative electrode 4, the electrolytic solution holding material 5 that holds the non-aqueous electrolytic solution, and the separator 6 are installed. A current collector 7 is installed between the battery can 1 and the positive electrode 3. The negative electrode 4 is provided on a current collector 9, and a spring 8 is interposed between the current collector 9 and the battery can 2. Note that FIG. 1 is merely an example and does not limit the present invention.

The present invention charges the above non-aqueous electrolyte secondary battery in the following procedure. First, at least 70% of the battery capacity of the non-aqueous electrolyte secondary battery is consumed. The reason for setting at least 70% here is that if it is less than 70%, the amount of lithium that will die due to the constant voltage discharge according to the present invention will be greater than the amount of lithium that will die due to not performing the charging method of the present invention. is there. Furthermore, it is preferable to discharge 90% or more of the battery capacity. Whether or not 70% has been consumed is determined by, for example, a method of determining the ratio of the amount of electricity consumed to the amount of electricity required to charge the battery, measuring the open circuit voltage of the battery, and measuring the open circuit voltage at the time of 70% consumption. Examples of the method include, but are not limited to.

Next, before charging is started, the battery is further discharged to a voltage lower than the open circuit voltage at the time of consumption. Here, the voltage lower than the open circuit voltage at the time of consumption is preferably in the range of discharge regulation voltage +0.5 V or less, and more preferably in the range of discharge regulation voltage ± 0.1 V. The voltage is the discharge regulation voltage + 0.5V
In the following range, the charge / discharge efficiency can be improved without impairing the battery capacity. Further, when the voltage lower than the open circuit voltage at the time of consumption is in the range of the discharge regulation voltage ± 0.1 V, the maximum battery capacity can be obtained, the deterioration of the positive electrode can be prevented, and the charging / discharging efficiency can be most improved. it can. A specific voltage lower than the open circuit voltage at the time of consumption is 3.5 V or less, preferably 3.1 to 2.9 V when the discharge regulation voltage is 3 V. Further, the discharge until the voltage becomes lower than the open circuit voltage at the time of consumption is 0.1 mA.
It is preferable to carry out at a current density of not more than / cm ² .

Next, the discharged non-aqueous secondary battery is
Charged. The charging method is not particularly limited, and a known method can be used. For example, 0.5-1m
Charging to a charging regulation voltage with a constant current having a current density of A / cm ² can be mentioned. Here, the charge regulation voltage is
It means the voltage when the non-aqueous secondary battery is theoretically fully charged. In addition, it is preferable that the charging is performed up to the charging regulation voltage + 0.05V.

Furthermore, the charging method of the present invention can also be applied to a battery which is judged to be completely consumed (fully discharged) by detecting the discharge regulation voltage in a general electric device. This is for the following reason. That is, the detection of full discharge when the non-aqueous electrolyte secondary battery is used is based on the battery voltage detected when the device is used. Since the device is in operation at the time of voltage measurement, the reaction is proceeding in the battery, and the detected voltage shows a value lower than the actual voltage by the polarization amount caused by the reaction. Therefore, when the full discharge is detected, the consumption of the active material in the electrodes is not completely completed by the voltage based on this polarization, and dendrites are present.
Therefore, in the case of such a battery, generation of dendrite can be suppressed by using the charging method of the present invention.

The discharge regulation voltage in the present invention means
Based on the voltage at which the non-aqueous secondary battery theoretically completely discharges (that is, the voltage at which the voltage sharply decreases in the voltage-aging discharge curve), the device side that uses the battery It is a regulation voltage for determining the end of discharge of the battery.
Hereinafter, the concept of the charging method of the present invention will be further described with reference to FIG. In FIG. 2, 11 and 15 are dendrites and 1
2 is a negative electrode made of metallic lithium, 13 is a residual dendrite, and 14 is a negative electrode that has been eluted.

First, FIG. 2A is a schematic sectional view of the negative electrode 12 after the n-th charge. As shown in this figure, dendrites 11 are formed on the surface of the negative electrode. next,
As the consumption proceeds, the dendrite 11 is eluted into the electrolytic solution, and the voltage gradually decreases. After that, if the discharge regulation voltage is detected, the consumption ends, but at this time, the surface of the negative electrode 12 is in the state shown in FIG. That is, the base of the dendrite 11 remains as the remaining dendrite 13 and the surface of the negative electrode 12 is also partially eluted (in the figure, the negative electrode 1 that has been eluted).
4). The base of the dendrite remains because the actually detected voltage has a value lower by the polarization amount due to the reaction, and the consumption is stopped before the active material is consumed.

In the conventional case, charging is performed next, so that FIG.
As shown in (c), lithium is intensively deposited on the end portion of the residual dendrite 13 and the recessed portion of the negative electrode 14 that has been eluted, and becomes the dendrite 15. As described above, in the conventional method, each time charge / discharge is repeated, a large amount of dendrites are finely deposited, and the chargeable / dischargeable metallic lithium is reduced.

On the other hand, in the charging method of the present invention, as shown in FIG.
After (b), the discharge is performed until the voltage becomes at least lower than the open circuit voltage at the time of 70% consumption, so that the surface of the negative electrode 12 becomes flat as shown in FIG. That is, dendrites that remain without elution can be eliminated as much as possible. Therefore, it is possible to prevent reduction of chargeable / dischargeable metallic lithium. It should be noted that if charging is performed after FIG.
It returns to the state of (a).

[0022]

【Example】

Example 1 Negative electrode made of metallic lithium, LiMn as positive electrode active material
O ₄ , acetylene black as a conductive agent, and polytetrafluoroethylene (PTFE) as a binder are 6: 1: 1.
A non-aqueous secondary battery comprising a positive electrode kneaded at a weight ratio of 1, an electrolytic solution in which 1 mol / liter of lithium hexafluorophosphate was dissolved in an equal amount mixed solvent of ethylene carbonate and diethyl carbonate was used.

First, 0.66 m is added to the non-aqueous secondary battery.
It was charged to a charge regulation voltage of 4.3 V with a constant current of A / cm ² . Then, a constant current discharge (discharge current 1 mA / cm ² ) that further discharges with a constant current up to a discharge depth X ₁ % (X ₁ is an arbitrary value of 20 to 100%), and a constant voltage at a discharge regulation voltage of 3.0 V. The cycle life Y ₁ (times) was measured with one charge / discharge cycle consisting of discharging and constant current charging in which the charging regulation voltage was 4.3 V at a constant current of 0.66 mA / cm ² . The relationship between the depth of discharge X ₁ % and the cycle life Y ₁ (times) is shown in FIG. The cycle life means the number of times charging and discharging are repeated when the battery capacity after charging reaches 80% of the initial capacity.

Comparative Example 1 0.66 mA / cm^TwoRegulated voltage of 4.3V with constant current of
Constant current charging to charge up to and discharge regulation voltage 3.0V
Example 1 except that the constant current discharge is one charge / discharge cycle
Cycle life Y_Two(Times) was measured. What
Oh, the depth of discharge is X _Two% (X_TwoIs 20-100% of any
Value) and the depth of discharge X_Two% And cycle life Y_Two(Times)
The relationship is shown in FIG.

As is apparent from FIG. 3, the depth of discharge is 70.
It has been found that the cycle life is improved by using the charging method described in Example 1 for the battery that has undergone deep discharge of 70% or more (consumption of 70% or more). On the other hand, the battery of Comparative Example 1 was not actually discharged to 3.0 V, so it is considered that lithium that could not be completely eluted remained and the cycle life was reduced.

Example 2 The discharge regulation voltage B (V) is 1.5, 2, 2.5, 2.9,
The cycle life was measured in the same manner as in Example 1 except that the values were 3, 3.1, 3.5 and 3.7. Table 1 shows the results
It was shown to.

[0027]

[Table 1]

Table 1 shows that the discharge regulation voltage is 3.5 to 1.5V.
At the time of, it shows that a good cycle life is obtained. The reason for this is that when the discharge regulation voltage is increased, the charging current, not the discharge current, flows under a voltage lower than the discharge regulation voltage. Therefore, it is considered that dendrites grow instead on the negative electrode and the cycle life is shortened. On the other hand, when the discharge regulation voltage is lowered, it is considered that the over-discharge state is caused and the positive electrode is eluted and deteriorated, so that the cycle life is shortened. Furthermore, a voltage in the range of 3.0 V (discharge regulation voltage) +0.5 V or less gives good cycle life, and a voltage in the range of 3.0 V (discharge regulation voltage) ± 0.1 V gives the best cycle life. I found out to give.

Example 3 and Comparative Example 2 The cycle life was measured in the same manner as in Example 1 and Comparative Example 1 except that a lithium-magnesium alloy containing 25% by weight of magnesium was used as the negative electrode. The results are shown in Table 2. In addition, Example 3 corresponds to Example 1 and Comparative Example 2 corresponds to Comparative Example 1, respectively.

[0030]

[Table 2]

From Table 2, it was found that it is preferable to use a negative electrode made of a lithium-magnesium alloy in the charging method of the present invention.

[0032]

The method for charging a non-aqueous electrolyte secondary battery according to the present invention is a non-aqueous electrolyte secondary battery having a negative electrode made of metallic lithium or a lithium alloy, a positive electrode and an electrolytic solution, and at least 70% of the battery capacity. It is characterized in that the consumed non-aqueous electrolyte secondary battery is further discharged to a voltage lower than the open circuit voltage at the time of consumption before charging and then charged. According to this method, the cycle life of the battery can be improved.

Furthermore, the desired voltage is the discharge regulation capacity +0.
Range of 5 V or less, more preferably discharge regulation capacity ± 0.1
Within the range of V, the cycle life can be further improved.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view of a coin type non-aqueous electrolyte secondary battery.

FIG. 2 is a conceptual explanatory diagram of a charging method of the present invention.

FIG. 3 is a graph showing the relationship between the depth of discharge and cycle life in Example 1 and Comparative Example 1.

[Explanation of symbols]

 1, 2 Battery can 3 Positive electrode 4, 12 Negative electrode 5 Electrolyte holding material 6 Separator 7, 9 Current collector 8 Spring 11, 15 is dendrite 13 Residual dendrite 14 Eluted negative electrode

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shinobu Akashi 4-1-1 Kamitadanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture, Fujitsu Limited (72) Isamu Watanabe 4-chome, Kamitadanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture No. 1 within Fujitsu Limited (72) Inventor Tsutomu Miyashita 4-1-1 Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa Within Fujitsu Limited

Claims (1)

[Claims] 1. A non-aqueous electrolyte secondary battery having a negative electrode made of metallic lithium or a lithium alloy, a positive electrode, and an electrolyte solution, the non-aqueous electrolyte secondary battery having at least 70% of its battery capacity consumed before starting charging. In the method for charging a non-aqueous electrolyte secondary battery, the battery is further discharged to a voltage lower than the open circuit voltage at the time of consumption and then charged.