JP2730641B2 - Lithium secondary battery - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic electrolyte lithium secondary battery as a power supply for driving portable electronic devices.

2. Description of the Related Art Lithium primary batteries, which have features such as high energy density, excellent storage stability, self-discharge characteristics, and excellent resistance to liquid leakage, have already been made of graphite fluoride / lithium batteries and manganese dioxide /
Lithium batteries and thionyl chloride / lithium batteries have been put into practical use.

On the other hand, with the recent miniaturization and portableness of electronic devices, batteries used as power sources for them have also been required to be smaller and lighter, and conventional secondary batteries cannot provide sufficient electric capacity. There is a great expectation for lithium secondary batteries that can be used repeatedly as many times as long as they take advantage of the features of the above lithium batteries and charge them. At present, only a small number of full-fledged lithium secondary batteries as power supplies for equipment have been commercialized using molybdenum disulfide for the positive electrode. The biggest reason for this is the poor charge-discharge cycle characteristics of lithium as the negative electrode active material. In the negative electrode, lithium dissolves in the electrolyte as ions when the battery is discharged, and lithium ions in the electrolyte conversely precipitate on the negative electrode during charging. The problem is that when lithium ions precipitate during charging, active lithium is formed, which decomposes the electrolyte or forms dendritic products (dendrites), which peel off or passivate as the charge and discharge are repeated. As a result, the dendrite grows excessively in a specific part of the negative electrode, and breaks through the separator and comes into contact with the positive electrode, resulting in a short circuit of the battery and a phenomenon such as a long charge / discharge cycle life. That is. Therefore, when pure lithium is used as the negative electrode, the cycle life of the battery can be said to be about 200 to 300 cycles. Although energy density is inferior as a means to improve this,
Attempts have been made to use aluminum or a low melting point alloy such as lead, bismuth, indium or cadmium and lithium in an alloyed manner. When used as an alloy, decomposition of the electrolyte due to generation of active lithium or passivation due to dendrites is not observed, and the decrease in charge / discharge efficiency is small. In fact, coin-type lithium secondary batteries mainly intended for memory backup use have been put to practical use using these alloys, and the charge / discharge cycle characteristics are said to be several hundred to several thousand cycles. On the other hand, when these alloys are used in a cylindrical lithium secondary battery that is oriented to the battery as the main power supply of the equipment, lithium escapes from the alloy as the battery is charged and discharged because the spiral structure is adopted as the electrode structure In addition, it is impossible to prevent the alloy from collapsing due to repeated shrinkage and expansion of the volume of the alloy when it enters, and sufficient charge / discharge cycle characteristics cannot be expected. As described above, it can be said that the use of either pure lithium or an alloy as the negative electrode of a cylindrical lithium secondary battery as a power source for driving a device is insufficient from the viewpoint of charge / discharge cycle characteristics.

In contrast, B. Scrosati et al.
ADEMIC PRESS 1983 proposed that iron oxide (Fe ₂ O ₃ ) dope lithium into its crystal structure, so that it could be used as a negative electrode of a lithium secondary battery and combined with vanadium pentoxide or titanium disulfide as a positive electrode. ing. Iron oxide can dope lithium up to 6 moles per mole,
That is, 1 Ah of lithium can be doped in 1 g of iron oxide, and 5 Ah can be doped in 1 cc, and unlike the above-mentioned aluminum or low melting point alloy, there is little expansion and contraction of iron oxide due to doping and undoping of lithium during charging and discharging of the battery. It can be an excellent negative electrode with little volume change. However, the potential for doping lithium starts at about 1.5 V with respect to the lithium electrode and is 0.5 V
Therefore, it is desirable that the positive electrode active material has a high potential.

Scrosati and colleagues used vanadium pentoxide for the positive electrode.
From 1V to 2.2V, from 2.2V to 1.
Discharge can be performed up to 1V.

Problems to be Solved by the Invention As described above, iron oxide can be said to be an excellent negative electrode of a lithium secondary battery, but has some problems.

Firstly, since the potential is almost 1.5 V with respect to lithium at the beginning of lithium doping during charging, it is necessary to select a positive electrode with a more noble potential to obtain a battery with a large energy density. Required. Not only the above titanium disulfide but also vanadium pentoxide as soon as discharge starts, the battery voltage immediately drops to 3 V or less, which originally makes it impossible to take advantage of the high energy density of vanadium pentoxide due to the high voltage. Results.

Another is how to dope lithium into iron oxide. Electrochemical doping, which is observed in charging of batteries as lithium doping in iron oxide, is easy, but incorporating electrodes that are electrochemically doped with lithium into the battery in advance is very difficult in terms of process and handling. Difficult. In addition, if lithium can be chemically doped into iron oxide, it can be easily incorporated into a battery as an electrode, but it is difficult to chemically dope lithium into iron oxide, and this method cannot be used. Normally, coin-shaped lithium secondary batteries are made of aluminum,
Alternatively, when a low-melting alloy is used as a negative electrode, lithium is superposed on these, and then an electrolyte is injected and lithium is forcibly electrochemically doped. However, in this case, since lithium and aluminum or the alloy are short-circuited, the surface is rapidly doped with lithium, and the aluminum and alloy expand and collapse. In the case of coin-type batteries, the amount of lithium to be doped is small and the electrodes are tightly pressed.This is not a big problem.However, in the case of cylindrical batteries, the negative electrode may fall off or the battery may be short-circuited. Discharge cycle characteristics are greatly reduced.

As described above, the method of lowering the battery voltage and doping with lithium when using a normal active material is a major problem when using iron oxide for the negative electrode.

Means for Solving the Problems The present invention solves such problems, and comprises a positive electrode made of a lithium-containing oxide and a ferric oxide (Fe ₂ O ₃ ).
It is intended to provide a lithium secondary battery comprising a negative electrode comprising an organic electrolyte.

Action As described above, doping of iron oxide with lithium is chemically difficult and must be performed by an electrochemical method. However, it is not desirable in a process or handling to perform this outside a battery. The present invention, in view of these facts, proposes to use an active material doped with lithium in the positive electrode in advance. By doing so, the lithium contained in the positive electrode is electrochemically doped into the iron oxide of the negative electrode by charging after assembling the battery, and as a result, the iron oxide of the negative electrode is an active material from the beginning. It is the same as containing lithium, and by controlling the charging time at the time of doping, it is possible to prevent the iron oxide from expanding and collapsing due to rapid lithium doping as described above. As the lithium-containing oxide, for example, oxides such as vanadium, molybdenum, cobalt, and manganese are known. Among them, LiCoO ₂ which is a cobalt oxide and LiMn ₂ O ₄ which is a manganese oxide are 4
It is as high as V or more, which is desirable as a positive electrode combined with an iron oxide negative electrode.

 Hereinafter, an embodiment will be described.

EXAMPLES Example 1 FIG. 1 is a sectional view of a battery according to an example of the present invention. In FIG. 1, reference numeral 1 denotes a positive electrode plate, and the positive electrode active material is Li.
An aqueous dispersion of CoO ₂ , carbon powder of conductive material, and polytetrafluoroethylene as binder was mixed at a weight ratio of 100: 10: 10, and kneaded in a paste with water to a thickness of 30.
After coating on both sides of the aluminum foil 2 of μm, it is dried, rolled and cut into a predetermined size. The active material LiCoO ₂ is composed of cobalt oxide (CoO) and lithium carbonate (Li ₂ CO ₃ ) in a molar ratio.
The mixture was mixed at 2: 1 and heated in air at 900 ° C. for 9 hours. In addition, the ratio of the aqueous dispersion of polytetrafluoroethylene in the mixing ratio of the above materials is the ratio of the solid content thereof. At this time, the theoretical charge amount of the positive electrode active material is
700 mAh. Reference numeral 4 denotes an iron oxide negative electrode obtained by kneading an aqueous dispersion of iron oxide powder, conductive graphite and polytetrafluoroethylene as a binder in a weight ratio of 100: 10: 10.
Pressed into a nickel-made expanded metal, dried and cut, and the ratio of polytetrafluoroethylene is the same as the solid content as described above. The weight of iron oxide powder in the negative electrode is 2.5 g
It is. The positive electrode and the negative electrode are spirally wound through three porous film separators made of polypropylene, and inserted into the case 9. After the insertion, the titanium lead 6 is spot-welded to the stainless steel sealing plate 7. Reference numeral 8 denotes a positive electrode cap and a positive electrode terminal which are nickel-plated on iron.
Spot welded. The end of the negative electrode current collector 5 is spot-welded to the case 9 also serving as a negative electrode terminal. Reference numeral 10 denotes a polypropylene insulating plate, and reference numeral 11 denotes a polypropylene insulating gasket. Reference numeral 12 denotes a safety valve attached so that the internal gas is released to the outside when the battery internal pressure rises due to an abnormality in the battery. After the above operation, an electrolyte in which lithium hexafluorophosphate (LiPF ₆ ) is dissolved in propylene carbonate at a rate of 1 mol / l is injected and sealed. The size of the completed battery is AA (14.5φmm × 50
mm). This battery of the present invention is referred to as Battery A.

Next, the positive electrode active material is vanadium pentoxide, the production of the electrodes such as the ratio of the conductive material and the binder is the same as that of the battery A, and the lithium plate is superposed on the side of the negative plate 4 opposite to the positive plate 1 to form a positive electrode. , Wound between the negative electrodes with a separator interposed therebetween, and thereafter the battery manufactured in exactly the same manner as the battery A is referred to as a battery B. At this time, the theoretical charge of the positive electrode active material is 550 mAh, and the theoretical charge of lithium superimposed on the negative electrode plate is 700 mAh.

Next, the positive electrode active material is made of vanadium pentoxide in the same manner as the battery B, the negative electrode plate 4 is lithium pressed to the nickel current collector 5, and the other configuration is the same as the battery A. . The theoretical charge of the positive electrode active material is 550 mAh, the same as that of the battery B, and the theoretical charge of the negative electrode lithium is 2750.
mAh. These batteries A, B and C were charged and discharged at 20 ° C. at a constant current of 70 mA. Charging and discharging were performed on batteries A and C within a voltage range of 4.1 V and 2.2 V, and on battery B within a voltage range of 3.4 V and 2.0 V. FIG. 2 shows charge / discharge curves of the batteries A to C at the 20th cycle. FIG. 3 shows the relationship between the number of charge / discharge cycles of batteries A to C and the energy density of discharge in each cycle. As is clear from FIG. 2, even when the same vanadium pentoxide was used as the positive electrode active material, a comparison was made between the battery B using iron oxide for the negative electrode and the battery C using lithium for the negative electrode.
Obviously, the battery C using lithium as the negative electrode is superior in voltage characteristics.

On the other hand, it can be seen that the battery A of the present invention using LiCoO ₂ having a high voltage as the positive electrode active material shows excellent voltage characteristics similarly to the battery C even when iron oxide is used for the negative electrode. As shown in FIG. 3, batteries A and C show large energy densities, but battery C has a rapid decrease in energy density after 200 cycles. This is due to the passivation or dropout of lithium as the battery is charged and discharged as described above,
This means that lithium was gradually consumed, and lithium which was five times the amount of electricity of the positive electrode at first was lost. Battery B has a low energy density and a large decrease with the cycle because of the low potential of the positive electrode and the fact that lithium is not contained in the positive electrode. It was thought that the doping was so rapid that the surface of the negative electrode partially collapsed. On the other hand, the battery A of the present invention has a high energy density and uses an active material containing lithium for the positive electrode, so that it is not necessary to dope the iron oxide of the negative electrode with lithium during the construction of the battery, as in the case of the battery B. It can be seen that the electrode exhibits good characteristics without collapse of the electrode due to charge and discharge.

Example 2 LiMn ₂ O ₄ was used as the positive electrode active material instead of LiCoO ₂ ,
A battery made by the same method as the battery A of Example 1 in terms of the mixture composition, the battery configuration, and the like is referred to as a battery A '. LiMn ₂ O ₄ was obtained by mixing manganese trioxide (Mn ₂ O ₃ ) and lithium carbonate at a molar ratio of 2: 1 and heating in air at 900 ° C. for 9 hours. The theoretical charge amount of the positive electrode active material is 600 mAh as a reaction of 0.5 Faraday.

Next, the positive electrode active material is LiMn ₂ O ₄ as in the case of the battery A, the negative electrode 4 is lithium pressed to the nickel current collector 5, and the other configuration is the same as the battery A ′. The theoretical charge of the positive electrode active material is 600 mAh as in the case of Battery A ', and the theoretical charge of the negative electrode lithium is 2400 mAh. These batteries A ′ and D were charged and discharged at 20 ° C. at a constant current of 70 mA. Discharge was performed until both batteries reached 2.5 V, but charging was performed up to 4.1 V for battery A 'and 4.3 V for battery D. FIG. 4 shows charge / discharge curves of the second cycle of these batteries. FIG. 5 shows the relationship between the number of charge / discharge cycles and the energy density of discharge in each cycle. As is clear from FIG. 4, the battery D using lithium for the negative electrode shows a high voltage and excellent discharge characteristics. However, when this battery is charged, a phenomenon is seen in which the voltage drops around the time when the voltage of the battery reaches around 4.2 V, and the voltage does not rise any more even if charging is continued. That is, it is inferred from this that a short-circuit phenomenon has occurred in the positive and negative electrodes of the battery due to some factor.
This is supported by the fact that the energy density is greatly attenuated when the charge / discharge cycle is repeated, as shown in FIG. On the other hand, in battery A, LiMn ₂ O was used as the positive electrode active material.
_Because it uses a substance with a high voltage of 4,
As can be seen from the figure, the discharge voltage is high, and as can be seen from FIG. 5, excellent cycle characteristics are exhibited as in the case of Example 1.

Advantages of the Invention As is clear from the above, according to the present invention, a lithium-containing oxide is used as a positive electrode, and a metal oxide doped or dedoped with lithium as a negative electrode, and particularly, iron oxide is used, so that the energy density is large. A lithium secondary battery having excellent charge / discharge cycle characteristics can be obtained.

Conventionally, the biggest problem with lithium secondary batteries was the reversibility that accompanies the charge and discharge of lithium on the negative electrode. By using the same metal oxide as the positive electrode, especially iron oxide, on the negative electrode, the lithium The charge / discharge reversibility of the negative electrode could be improved by making the doping and undoping reactions charge / discharge reactions. However, using only iron oxide instead of lithium lowers the voltage of the battery and inevitably lowers the energy density. However, by combining it with a high-potential lithium-containing oxide as the positive electrode active material, That is, a battery having a large energy density can be obtained. Furthermore, since iron oxide is used for the negative electrode on the negative electrode, lithium as the negative electrode active material must be replenished by some method. However, in the case of laminating lithium to the negative electrode as a commonly used means, in Example 1, As can be seen, the iron oxide electrode collapses.However, since a lithium-containing oxide is used for the positive electrode, a lithium secondary battery with excellent charge / discharge characteristics can be provided without taking any measures. is there.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a cylindrical battery used in an embodiment of the present invention. FIGS. 2 and 4 are charge / discharge characteristics diagrams of the battery of the present invention and a conventional battery. FIGS. It is a charge-discharge cycle life characteristic diagram. 1 ... Positive electrode plate, 2 ... Positive electrode current collector, 3 ... Separator,
4 ... a negative electrode plate, 5 ... a negative electrode current collector, 6 ... a positive electrode case,
7 ... sealing plate, 8 ... positive electrode terminal, 9 ... case (-),
10 ... Insulating plate, 11 ... Insulating gasket, 12 ... Safety valve.

Claims (2)

(57) [Claims] 1. A positive electrode comprising a lithium-containing oxide, a negative electrode comprising a metal oxide to be doped and dedoped with lithium, and an organic electrolyte. Lithium secondary battery using iron (Fe ₂ O ₃ ). 2. The method according to claim 1, wherein the lithium-containing oxide is LiCoO ₂ , LiMn ₂ O
_4. The lithium secondary battery according to claim 1, which is ₄ or a mixture thereof.