JPH0917451A - Charging method of secondary battery - Google Patents

Abstract

PURPOSE: To carry out charging at high speed while preventing overvoltage phenomenon caused at the time of reaching the final voltage by keeping charging current large in an initial stage of charging with constant current and at low voltage and gradually lowering the current before the voltage reaches the final voltage. CONSTITUTION: At the time of charging a secondary battery composed of a cathode, an anode, and a non-aqueous electrolyte, a target voltage is set lower than the final charging voltage. After the voltage reaches the target voltage, the charging current is gradually lowered while the voltage being made closer to the final charging voltage. For example, after the voltage reaches the target voltage, rated current lower than the charging current is set and by the time when the voltage reaches the final charging voltage, charging is carried out at constant current lower than the rated current. Further, a plurality of rated current values are set and based on the time and charging voltage, the rated current values are gradually lowered step by step.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for charging a secondary battery using a positive electrode, a negative electrode and a non-aqueous electrolyte.

[0002]

2. Description of the Related Art In recent years, with the spread of portable devices such as video cameras, mobile phones, and notebook computers, demand for small, lightweight, high-capacity secondary batteries is increasing. Most of the secondary batteries currently used are nickel-cadmium batteries using an alkaline electrolyte, but it is difficult to increase the energy density because the average battery voltage is as low as 1.2V. For this reason, research on high energy secondary batteries has been conducted using lithium metal, which is the most basic metal for the negative electrode.

However, in a secondary battery using lithium metal for the negative electrode, there is a risk that lithium will grow into a resin (dendrites) due to repeated charging and discharging, causing a short circuit and ignition. In addition, since highly active metal lithium is used, the risk is inherently high, and there are many problems in using it for consumer use. In recent years, a lithium ion secondary battery using various carbonaceous materials has been devised as a device that solves such a safety problem and enables high energy peculiar to a lithium electrode. This method utilizes the fact that lithium ions are occluded (doped) in the carbonaceous material during charging, have the same potential as metallic lithium, and can be used as a negative electrode instead of metallic lithium. Also,
During discharge, doped lithium ions are released from the negative electrode
(Dedoping) and returns to the original carbonaceous material. When such a carbonaceous material doped with lithium ions is used as the negative electrode, there is a small problem of dendrite formation, and since there is no metallic lithium, there is a problem that it is also excellent in safety. , Research is actively done.

As a secondary battery using an electrode using the above-mentioned carbonaceous material doped with lithium ions,
JP-A-57-208079, JP-A-58-93176, JP-A-58-192
266, JP-A-62-90863, JP-A-62-122066 and JP-A-2-66856 are known.

As a method of charging these lithium ion secondary batteries, generally, constant current / constant voltage charging is performed in which charging is performed at a constant current until the target charging voltage is reached, and constant voltage charging is performed after the target charging voltage is reached. The method is general.

[0006]

However, the charging voltage of the lithium-ion secondary battery is higher than that of other secondary batteries, and is generally 4 V or more. Therefore, on the high potential side of the positive electrode,
The higher the potential, the more likely the positive electrode material / electrolyte will decompose, and on the negative electrode side, there is a risk that dendrite will precipitate when the potential becomes 0 V, and the battery life will be greatly adversely affected. become. Since battery charging is controlled by the potential difference between the positive and negative electrodes, the types and filling amounts of the positive electrode material, negative electrode material, and electrolyte are optimized in order to charge and discharge at potentials that do not cause danger in the positive electrode and negative electrode. Must have been, not easy. In addition,
If the balance of the diffusion rate of lithium ions and the internal resistance of the electrolyte cannot be fully optimized, when the difference between the positive electrode potential and the negative electrode potential reaches the final charging voltage, the charging potential at the positive electrode is temporarily below the optimum value. There is a case where a so-called overpotential state in which the charge potential becomes too high or the charge potential is too low at the negative electrode is generated. In particular, when the charging current is increased for high-speed charging, the overpotential state becomes more prominent, which poses a serious problem in terms of battery life and safety.

An object of the present invention is to realize high-speed charging while preventing the overpotential phenomenon in the positive electrode and the negative electrode in order to eliminate the drawbacks of the prior art.

[0008]

The present invention has the following constitution in order to achieve such an object.

[In a charging method for charging a secondary battery using a positive electrode, a negative electrode and a non-aqueous electrolyte, the final charging voltage V
A method of charging a secondary battery, wherein a target charging voltage Va lower than f is set, and after reaching the voltage Va, the charging current I is gradually reduced while bringing the charging voltage V close to the final charging voltage. That is, by increasing the charging current Io in the constant current charging stage at a low voltage at the initial stage of charging and gradually decreasing the current before reaching the target charging voltage, an overpotential phenomenon that occurs when the final charging voltage Vf is reached is caused. It is possible to reduce adverse effects.

Here, the simplest method for reducing the charging current I is to set a constant current Io 'smaller than Io after reaching the target charging voltage Va until the final charging voltage Vf is reached. Two-stage charging, such as constant current charging with ', is preferable. Furthermore, set multiple constant current values,
For example, a method of gradually decreasing the constant current value in a stepwise manner depending on the time or the charging voltage is also possible.

It is also effective to continuously reduce the current at a constant rate after reaching Va, depending on the time and the charging voltage. Here, as a method for reducing the charging current I, a so-called proportional control method in which a value obtained by dividing a difference between the final charging voltage Vf and the charging voltage V by a constant R is a charging current value is simple and effective. That is, in setting the target charging voltage Va, [initial charging current Io] × [constant R] = [final charging voltage Vo]
f] − [Target charging voltage Va] The charging current I after reaching Va is [charging current I] × [constant R] = [final charging voltage Vf] −
By continuously changing the charging voltage to the charging voltage V, it is possible to reduce the adverse effect due to the overpotential phenomenon which is likely to occur in the conventional constant current / constant voltage charging.

Here, the constant R used is not limited, but it is preferable to set a value equal to or higher than the internal resistance existing inside the battery because the overpotential phenomenon can be completely prevented. However, if the constant R is increased, the value of the target charging voltage Va decreases and the constant current I
The charging time at o tends to be shortened, leading to a lower charging speed. In view of these, it can be said that it is desirable to match the constant R with the larger internal resistance of each of the positive and negative single poles. Therefore, by setting the constant R to 50% or more and 100% or less of the internal resistance, the charging speed and overpotential It is preferable in terms of the phenomenon.

The secondary battery used in the present invention is not particularly limited as long as it is suitable for constant current and constant voltage discharge, but in the case of a battery utilizing the cation or anion doping of a carbonaceous material, that is, When used in a non-aqueous electrolyte secondary battery containing an alkali metal salt, a cation-doped carbonaceous material is used for the negative electrode and an anion-doped material is used for the positive electrode.

The positive electrode material is not particularly limited, but carbonaceous materials such as carbon fiber, artificial or natural graphite powder, fluorinated carbon, or an inorganic compound such as metal or metal oxide or organic material. A polymer compound or the like can be used. The positive electrode containing no carbonaceous material, inorganic compounds such as transition metal oxides and transition metal chalcogens containing alkali metal, polyacetylene, polyparaphenylene, polyphenylene vinylene, polyaniline, polypyrrole, conjugated polymer such as polythiophene, disulfide bond Cross-linked polymer having, thionyl chloride, etc.,
The positive electrode used in a normal secondary battery can be mentioned. Among these, in the case of a secondary battery using a non-aqueous electrolytic solution containing a lithium salt, transition metal oxides and transition metals such as cobalt, nickel, manganese, molybdenum, vanadium, chromium, iron, copper, and titanium are used. Chalcogen is preferably used. In particular, LiCoO ₂ and LiNiO ₂ are most preferably used because of their high voltage and large energy density. When an inorganic compound such as a metal or a metal oxide is used as the positive electrode, a charge / discharge reaction occurs by utilizing cation doping and dedoping. When an organic polymer compound is used, a charge / discharge reaction occurs due to doping and undoping of an anion. As described above, various charge / discharge reaction modes are adopted depending on the substance, and these are appropriately selected according to the required positive electrode characteristics of the battery.

The negative electrode material can be used without any particular limitation. Examples of the carbonaceous material include carbon fiber, artificial or natural graphite powder, fluorinated carbon, inorganic compounds such as metal or metal oxide, and organic materials. A polymer compound or the like can be used. The carbon fiber used here is not particularly limited, but generally is obtained by firing an organic substance. Specifically, PAN-based carbon fiber obtained from polyacrylonitrile (PAN),
Pitch-based carbon fibers obtained from pitch such as coal or petroleum, cellulosic carbon fibers obtained from cellulose, vapor-grown carbon fibers obtained from a gas of a low molecular weight organic substance, and the like, in addition to them, polyvinyl alcohol, lignin, Carbon fibers obtained by firing polyvinyl chloride, polyamide, polyimide, phenol resin, furfuryl alcohol, etc. are also suitably used. Among these carbon fibers, carbon fibers satisfying the characteristics are appropriately selected and used according to the characteristics of the electrode and the battery in which the carbon fibers are used.

Among the above carbon fibers, PAN-based carbon fibers and pitch-based carbon fibers are preferable when used in the negative electrode of a secondary battery using a non-aqueous electrolyte containing an alkali metal salt. In particular, PAN-based carbon fibers are preferably used in that the doping of alkali metal ions, particularly lithium ions, is good. By the way, when the carbon fiber is used as an electrode, it may have any form, but a preferred form is such that it is uniaxially arranged, or a fabric-like or felt-like structure is formed. Examples of the structure such as a fabric or a felt include a woven fabric, a knitted fabric, a braid, a lace, a net, a felt, a paper, a nonwoven fabric, a mat, and the like. Felt and the like are preferred. Further, when arranging in a uniaxial direction, a method of aligning carbon fibers on a metal current collector such as a copper foil and applying a solution in which a resin as an adhesive is dissolved to adhere to the current collector is used. To be Furthermore, in the case of a cylindrical battery, the method of arranging the cells in a direction perpendicular to the winding direction is also preferable, because there is no peeling or the like. The diameter of the carbon fibers should be determined so that each form can be easily adopted, but carbon fibers having a diameter of 1 to 1000 μm are preferably used, and 1 to 20 μm is more preferable. It is also preferable to use several types of carbon fibers having different diameters. Furthermore, it is also preferable to use short fibers of carbon fibers, and it is also preferable to cut the carbon fibers to 1 mm or less, 100 μm or less, and further 30 μm or less.

The secondary battery of the present invention is not particularly limited, but a non-aqueous electrolyte containing an alkali metal salt such as lithium perchlorate, lithium borofluoride, and phosphorus hexafluoride / lithium is used. It is preferably used as the secondary battery used.

The electrolytic solution of the secondary battery used in the present invention is not particularly limited, and a conventional electrolytic solution may be used, and examples thereof include an acid or alkaline aqueous solution or a non-aqueous solvent. Among these, as the electrolyte of the secondary battery composed of the above-mentioned non-aqueous electrolyte containing an alkali metal salt, propylene carbonate, ethylene carbonate, dimethyl carbonate, γ-butyrolactone, N-methylpyrrolidone, acetonitrile, N, N- Dimethylformamide,
Dimethylsulfoxide, tetrahydrofuran, 1,3-dioxolane, methyl formate, sulfolane, oxazolidone, thionyl chloride, 1,2-dimethoxyethane, diethylene carbonate, derivatives and mixtures thereof are preferably used. As the electrolyte contained in the electrolytic solution, an alkali metal, particularly a lithium halide, a perchlorate,
Thiocyan salt, borofluoride salt, phosphorous fluoride salt, arsenic fluoride salt, aluminum fluoride salt, trifluoromethyl sulfate salt and the like are preferably used.

[0019]

The present invention will be described more specifically with reference to the following examples. However, the present invention is not limited by this.

Example 1 LiCoO _{2 as} a positive electrode active material, PVDF (manufactured by Kureha Chemical Co., Ltd.) as a binder, artificial graphite SP-20 (manufactured by Nippon Graphite Industry Co., Ltd.) as a conductive material, and aluminum foil (a current collector). 20μ thickness
m) was used to prepare a positive electrode. A PAN fiber Torayca T300 (manufactured by Toray Industries, Inc.) was used as the negative electrode active material, and the same PVDF as the positive electrode was used as the binder. Attach these plates to a porous polypropylene film (Celgard # 2500
, Manufactured by Daicel Chemical Co., Ltd.), and wound into a battery can having an internal volume of 5 cc to obtain a cylindrical electrode body. A dimethyl carbonate containing 1M lithium borofluoride was used as an electrolytic solution, and a charging test was conducted in a three-electrode cell using a metallic lithium foil as a reference electrode. For charging, the constant current at the start is 1000 mA, the final charging voltage Vf is 4.2 V, the target charging voltage Va is 4.0 V, and the relationship between the charging current I and the charging voltage V after reaching the target voltage Va is I [mA] × While changing the charging current so that 0.002 = 4.2-V [V], the charging current becomes 10
It was charged until it reached mA. As a result, the potential curve shown in Fig. 1 was drawn and no overpotential phenomenon was observed. A cycle test was conducted on this battery at a charging time of 2 hours, a discharging current of 600 mA, and an end-of-discharge voltage of 3 V, and the discharge capacity after 100 cycles was 380 mAh.

Comparative Example 1 A battery cell similar to that of Example 1 was prepared and the final charging voltage Vf
Charged with a constant current of 1000mA until reaching the final charge potential, and after reaching the final charge potential Vf, constant current constant voltage charge was performed until the charge current reached 10mA while maintaining the charge voltage at 4.2V.
An overpotential phenomenon was observed as in 2.

The charging time and discharging conditions of this battery are the same as those in Example 1.
When a cycle test was performed under the same conditions as above, the discharge capacity after 100 times was 352 mAh.

[0023]

According to the present invention, it is possible to provide a method for charging a secondary battery in which an overpotential phenomenon is prevented, the battery safety is improved, and the life is improved.

[Brief description of the drawings]

FIG. 1 is a curve showing changes with time in charging potential in Example 1 of the present invention.

FIG. 2 is a curve showing changes with time in charging potential in Comparative Example 1 of the present invention.

Claims (8)

[Claims] 1. A final charging voltage Vf in a charging method for charging a secondary battery using a positive electrode, a negative electrode and a non-aqueous electrolyte.
A method of charging a secondary battery, which comprises setting a target charging voltage Va lower than the target charging voltage Va, and gradually reducing the charging current I while the charging voltage V approaches the final charging voltage after reaching the voltage Va. 2. When decreasing the charging current I, one or a plurality of target charging voltages Va are set until the final charging voltage Vf is reached, and the charging current I is made constant in each charging voltage section, or A method of charging a secondary battery, which is characterized by continuously changing. 3. The target charging voltage Va is [initial charging current Io] × [constant R] = [final charging voltage Vo]
f] − [target charging voltage Va], charging current I after reaching Va is [charging current I] × [constant R] = [final charging voltage Vf] −
The charging method for a secondary battery according to claim 1 or 2, wherein the charging voltage V is continuously changed so that the charging voltage becomes V. 4. The method for charging a secondary battery according to claim 3, wherein the constant R is set to 50% or more and 100% or less of the internal resistance of the secondary battery. 5. The method for charging a secondary battery according to claim 1, wherein the electrolytic solution of the secondary battery uses a lithium salt as an electrolyte. 6. The secondary battery according to claim 1, wherein the positive electrode of the secondary battery contains at least one transition metal compound capable of inserting and extracting lithium ions. How to charge. 7. The charging of a secondary battery according to claim 1, wherein the negative electrode of the secondary battery contains at least one kind of carbon material capable of inserting and extracting lithium ions. Method. 8. The method for charging a secondary battery according to claim 7, wherein at least carbon fiber is used as the carbon material.