JPS63314778A - Nonaqueous electrolyte lithium secondary battery - Google Patents

Abstract

PURPOSE:To provide possibility of electric discharge to a battery voltage of 0V by incorporating such a construction that the dischargeable electric capacity of Li in pressure contact with an alloy neg. pole goes out while the dischargeable capacity of No.2 active substance of positive pole consisting of specific No.1, No.2 active substances is maintained. CONSTITUTION:No.1 active substance selected from MnO2, chromium oxide, and vanadium oxide is mixed with No.2 active substance such as MoO2, MoO3, MoS2, MoS3, and thereto carbon black and binder are added followed by pressure molding to accomplish a positive pole 5. An alloy neg. pole 2 is spot welded to the inner surface of a mouth sealing plate 1 consisting of Ni-plated stainless steel, and thereto Li 3 is attached by pressure. The neg. pole 2 consists of alloy prepared through melting of Pb, In, Cd in a proportion 75:6:20 by weight. Electrolyte (Li perchlorate) is impregnated in a propylene separator 4. Current collector 7 of Ti is spot welded to a case 6, and the positive pole 5 is attached by pressure. At the time of electric discharge, No.1 active substance of the positive pole 5 makes discharge for a specified period of time, and the Li capacity of the neg. pole 2 goes out while No.2 active substance makes discharge, and the battery voltage becomes 0V.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an improvement of a non-aqueous electrolyte lithium secondary battery as a power source for portable equipment or a power source for memory backup.

Conventional technology Non-aqueous electrolyte lithium secondary batteries have high voltage and energy density compared to conventional aqueous batteries, and have excellent self-discharge resistance and leakage resistance, making them highly anticipated batteries in the future. Although some lithium secondary batteries using molybdenum disulfide as a positive electrode are currently in practical use, full-scale practical use has not yet been achieved.

The biggest reason for this is that, compared to, for example, nickel-cadmium batteries, which are conventional aqueous batteries, they have a major problem in that they have inferior charge-discharge cycle characteristics.

This takes the form that lithium, which is the negative electrode active material of lithium secondary batteries, dissolves in the electrolyte during discharging and deposits on the electrode again during charging, but lithium is not deposited uniformly during charging, and the resin This is because so-called dendrites are generated, and the next time the discharge occurs, the discharge does not occur smoothly, or the dendrites penetrate the separator and come into contact with the positive electrode, causing a short circuit phenomenon.

In contrast, Japanese Patent Application Laid-open No. 59-163756,
In No. 163758, alloys on two-element plates such as cadvaram, lead, tin, bismuth, antimony, mercury, and indium, so-called low-melting point fusible alloys, easily absorb and release lithium in nonaqueous electrolytes. We have discovered this and are proposing a non-aqueous electrolyte lithium secondary battery that uses these alloys as the negative electrode.

In other words, in batteries using these alloys as negative electrodes,
During discharging, the lithium occluded within the alloy is released into the electrolyte, and conversely, during charging, lithium ions from the electrolyte precipitate on the alloy, quickly react with the alloy, diffuse into the alloy, and become occluded. Therefore, dendrites do not form on the surface of the alloy negative electrode, and therefore it exhibits good charge-discharge cycle characteristics.

Among the above elements, lead, tin, bismuth, and indium in particular have a large lithium storage capacity, and easily release lithium during charging and discharging of the battery. On the other hand, if a battery is constructed using an alloy that combines these elements as a negative electrode and is repeatedly charged and discharged, the disadvantage is that the electrode will collapse and its life will end due to the large amount of lithium that can be absorbed. have Normally, to compensate for this drawback, an appropriate amount of cadmium, zinc, etc. is added as an element that acts as a kind of binder, although the amount of lithium absorbed is small, to form an alloy.

Also, these alloys have a voltage of about 0.4 to o, eV with respect to lithium.
Therefore, in order to obtain a high-voltage, high-energy-density lithium secondary battery, it is necessary to select a positive electrode active material that has a large theoretical capacity and a high potential.

Manganese dioxide,
Chromium oxide, vanadium oxide, etc. are known.

That is, by combining the above alloy negative electrode and positive electrode active material, a nonaqueous electrolyte lithium secondary battery with high voltage, high energy density, and excellent charge/discharge cycle characteristics can be obtained.

Problems to be Solved by the Invention On the other hand, over-discharge must be taken into consideration when actually using the battery. This also applies to secondary batteries, but if a battery is installed in a device and left in use, the battery will continue to discharge forever. Particularly when used for memory backup, which has been increasingly used in recent years, the battery is put into operation from the moment it is installed in the device, and the device user connects it to an AC power source to start charging the battery. In extreme cases, the battery may be completely discharged and the battery voltage may reach OV. Conventional batteries, such as nickel-cadmium batteries, are resistant to over-discharge, and even if they are discharged until the battery voltage reaches Ov, they can be re-strung and used again as long as they are charged at that point.

In the case of a lithium secondary battery, this overdischarge characteristic is extremely weak, and if it is discharged until the voltage reaches OV, it will not return to its original state even if it is charged. This is one of the major reasons why lithium secondary batteries are currently not widespread.

Means for Solving the Problem In order to solve this problem, the present invention provides a positive electrode consisting of a first active material that discharges at a high potential and a second active material that discharges at a low potential, and a negative electrode that is composed of a first active material that discharges at a high potential and a second active material that discharges at a low potential. is made of an alloy in which lithium is compressed to an alloy that occludes and releases lithium, or an alloy in which lithium is occluded in advance, and the first active material is discharged at a potential higher than the dissolution potential of the alloy in a non-aqueous electrolyte. , the second active material discharges at a low potential, and the theoretical amount of electricity of the chargeable and dischargeable lithium of the negative electrode is higher than that of the first active material.
To provide a non-aqueous electrolyte lithium secondary battery designed to have a dischargeable quantity of electricity greater than that of an active material and smaller than the sum of the dischargeable quantity of electricity of a first active material and a second active material. It is.

Function As mentioned above, when a lithium secondary battery is over-discharged, it does not return to its original state for the following reason: When the battery is over-discharged, the discharge voltage decreases, and eventually the battery voltage decreases to Ov. However, this usually indicates that the capacity of either the positive electrode or the negative electrode has been exhausted. When the capacity of the positive electrode is exhausted, that is, when the positive electrode capacity is regulated, inorganic compounds such as manganese dioxide, chromium oxide, vanadium oxide, etc. are usually used as the positive electrode active material of lithium secondary batteries. When discharged to below a certain potential, the crystal structure changes. Therefore, even if the battery is charged, it will not return to its original state.

On the other hand, when the capacity of the negative electrode is exhausted, that is, when considering the case of negative electrode capacity regulation, the lithium occluded in the alloy negative electrode dissolves as ions in the electrolyte as it discharges, but all of the lithium is consumed. The potential of the negative electrode increases, and a dissolution reaction of the alloy negative electrode begins to occur, eventually dissolving it all into the electrolyte. Therefore, even if the battery is charged next time, since there is no alloy to absorb lithium, the battery will not be charged.

The above is the reason why lithium secondary batteries cannot be overdischarged.

As a way to overcome this drawback, first of all, in the case of batteries with a limited positive electrode capacity, even if the capacity of the positive electrode active material is exhausted, some method is taken to raise the potential of the positive electrode to a potential higher than the potential at which the crystal structure of the active material changes. Just keep it.

Conversely, in the case of negative electrode capacity regulation, even if the capacity of the lithium compressed or occluded in the alloy negative electrode is exhausted, some means may be taken to keep the potential of the negative electrode below the potential of the alloy. The above is theoretically true, but when considering the case of positive electrode capacity regulation as a practical matter, all of the above-mentioned positive electrode active materials usually have a lithium ratio of 1.0 to 2. The crystal structure changes within the Ov range. Therefore, in order to prevent the potential of the positive electrode from falling below that level, it is necessary to discharge the negative electrode at a potential higher than the potential at which the crystal structure of each positive electrode active material changes. The voltage as a battery is too low and cannot be compared to a high voltage, high energy density battery. Therefore, in the case of lithium secondary batteries, the negative electrode capacity must be regulated, and when the negative electrode capacity is exhausted, it is necessary to consider using some means to keep the negative electrode potential below the dissolution potential of the alloy in the non-aqueous electrolyte. Must be.

Lead, bismuth, and
Table 1 shows the dissolution potentials of tin, indium, cadmium, mium, and zinc relative to lithium in nonaqueous electrolytes.

Table 1 Manganese dioxide, chromium oxide, and vanadium oxide, all of which are considered as positive electrode active materials, have a discharge potential of 3 V or more in a nonaqueous electrolyte. All the metal elements listed in Table 1 will be dissolved, except for some such as zinc.

Therefore, as it is, it cannot be used as a secondary battery capable of overdischarging.

The present invention uses, as a positive electrode active material, a second active material that discharges at a potential lower than the dissolution potential of various metal elements shown in Table 1, together with the above-mentioned positive electrode active material (first active material). This is what we propose.

In other words, although the battery as a whole is regulated by negative electrode capacity,
At the positive electrode, the first active material is first discharged at a potential of 3 V or more, and when its capacity is exhausted, the second active material is discharged. If the battery is designed so that the capacity of the lithium intercalated or crimped into the negative electrode alloy is exhausted while this second active material is discharging, the potential of the negative electrode will eventually decrease to that of the positive electrode. When the discharge potential of the second active material is reached, the voltage of the battery becomes OV, and no more current flows.
The discharge of the battery will end. At that point, as mentioned above, since the second active material is being discharged at a potential lower than the dissolution potential of the various alloying elements, these alloying elements are kept at that potential without being dissolved. It turns out.

In other words, an ideal non-aqueous electrolyte lithium secondary battery with excellent overdischarge resistance can be obtained.

Note that the second active material is molybdenum oxide (MoO2
), molybdenum trioxide, molybdenum disulfide, molybdenum trisulfide, titanium disulfide, titanium oxide, tungsten oxide, niobium disulfide, niohium trioxide (Nb205),
Possible materials include niobium selenide (NbSe3).

All of these are active materials that discharge at a potential of 2.4 V or less with respect to lithium. Above (Example 1) Manganese dioxide, which is the first positive electrode active material, and titanium disulfide, which is the second positive electrode active material. Mix this mixture at a weight ratio of 2:1, and add this mixture to an aqueous dispersion of carbon black as a conductive material and a copolymer of tetrafluoroethylene and hexafluoropropylene as a binder (however, the aqueous dispersion The mixture was mixed in a ratio of 100:5:10 (in terms of solid content), and after drying, it was press-formed into a disk shape with a diameter of 15 mm and a thickness of 0.5 mm to form a positive electrode. Using this positive electrode, a flat battery shown in FIG. 1 was assembled. The negative electrode is made of lead, indium,
Cadmium was melted at a weight ratio of 75:5:20, and lithium was pressed onto an alloy.

In Figure 1, reference numeral 1 denotes a sealing plate made of nickel-plated stainless steel, with an alloy negative electrode 2 spot-welded on its inner surface, and lithium 3 crimped onto it. 4 is a polypropylene separator impregnated with an electrolyte in which lithium perchlorate is dissolved in a solvent of 1,3-dioxolane at a ratio of 1 mol/l. 5 is the disk-shaped positive electrode, and a titanium current collector 7 is spot-welded to a stainless steel case 6.
It is crimped. 8 is a polypropylene gasket,
The dimensions of the completed battery are 20 mm in diameter and 1.6 mm in thickness. The theoretical capacitance of manganese dioxide, which is the first active material in the positive electrode, is 2 a mAh, the theoretical capacitance of titanium disulfide, which is the second active material, is 8 mAh, and the theoretical capacitance of lithium bonded to the alloy negative electrode is (4mm h. Discharge this battery to 2v with a current of 1mm at 20°C, and then
After three cycles of charging to 3.4V with a current of
Similarly, FIG. 2 shows the behavior of the positive and negative electrodes and the battery voltage when the battery was discharged at a current of 20'C11m.

As is clear from FIG. 2, manganese dioxide, which is the first active material of the positive electrode, first discharges, and after about 19 hours, titanium disulfide, which is the second active material, starts discharging. While titanium disulfide is discharging lithium from about 2.2V, the capacity of the lithium in the negative electrode is exhausted, and the potential of the negative electrode rapidly rises to 1.9V. Therefore, at this point, the battery voltage becomes OV. If the discharge continues, the potentials of the positive and negative electrodes will reverse, but in actual use, the battery is generally discharged with a constant resistance, so the discharge ends when the battery voltage reaches OV. Therefore, even if the battery is left as is, the positive electrode still has the capacity of the second active material and no change will occur, and the negative electrode will also be lower than the dissolution potential of the lead, indium, and cadmium components of the alloy. Since the voltage is kept at a much lower level of 1.9V, no change occurs.

In this case, the actual amount of electricity discharged is 24 mAh even though the amount of lithium charged in the negative electrode is 40 mAh.The reason why the amount of lithium charged in the negative electrode is 24 mAh is because the amount of lithium discharged is 16 mAh, which is taken up by the manganese dioxide in the alloy negative electrode and the positive electrode. It is shown that. Next, after leaving this battery in this state for one month, it was heated for 1 m at 20°C.
When the battery was charged with a human current until the battery voltage reached 3.4V and then discharged again with a 1m person, it exhibited exactly the same behavior as shown in Figure 2. From this, it can be seen that in this battery system, even if the battery is over-discharged until the voltage reaches OV, no change occurs in the system itself.

(Example 2) Chromium oxide (Cr20s) was used as the first active material, and the second
Molybdenum disulfide is mixed as an active material at a weight ratio of 6:2, and this mixture is mixed with carbon black as a conductive material and a copolymer of ethylene tetra7tide and propylene hexafluoride as a binder. The weight ratio of each dispersion (however, aqueous dispersion is calculated as solid content) is 100:5:1.
Mix at a ratio of 0 and after drying, the diameter is 15 mm and the thickness is 0.5
Pressure mold it into a disk shape of mm and use it as a positive electrode. Using this positive electrode, the flat battery shown in FIG. 1 was assembled in the same manner as in Example 1. The negative electrode used was an alloy obtained by melting lead, bismuth, and cadmium in a weight ratio of 50:25:25, respectively, and pressing lithium onto the alloy. The structure of the battery is exactly the same as in Example 1, but the electrolyte is a mixture of propylene carbonate and 1,3-dioxolane in a volume ratio of 1:1, and 1 mol/l of lithium perchlorate.
A solution dissolved at the following ratio was used. The theoretical electric capacity of chromium oxide, which is the first active material in the positive electrode, is 24111 h.
The theoretical electric capacity of the second active material, molybdenum disulfide, is 5.5! IIAh, and the theoretical electric capacity of lithium compressed to the alloy negative electrode is 3811Ah.

As in the case of Example 1, this battery was discharged to 2V at 20°C with a current of 1 meter, and then charged to 3.4V with a current of 1 carbonate, which was repeated three times. A hole was made in the battery, and lithium was used as a reference electrode in the same electrolyte used in the battery in a dry air atmosphere, and the behavior of the positive and negative electrodes and the voltage of the battery when discharged at 20°C and a current of 1 m were measured in a third experiment. As shown in the figure.

As can be seen from Figure 3, as in the case of Figure 2, chromium oxide, which is the first active material of the positive electrode, is first discharged, and after about 19 hours, molybdenum disulfide, which is the second active material, is discharged. Start. Molybdenum disulfide also starts discharging at 2.2V, similar to the case of titanium disulfide. During that time, the capacity of the lithium in the negative electrode is exhausted, and the potential of the negative electrode increases, reaching 2.OV, which is the discharge potential of the positive electrode at that point, and the battery voltage becomes O
When the voltage reaches V, the discharge ends. As in Example 1, since this potential is much lower than the dissolution potential of the constituent elements of the alloy negative electrode, no change occurs in the battery itself. Next, insert this battery 1
After leaving the battery for several months, the battery was charged again at 20°C with a current of 1m until the battery voltage reached 3.4V, and then discharged under the same conditions, showing exactly the same behavior as shown in Figure 3. I repeated the same pattern several times after that, but the behavior hardly changed.

Effects of the Invention As is clear from the above, according to the present invention, the first active material and the second active material suitable for the positive electrode are selected, and while the discharge capacity of the second active material is maintained, By designing a battery so that the dischargeable capacity of lithium crimped or occluded in an alloy negative electrode is exhausted, it is possible to provide a non-aqueous electrolyte lithium secondary battery that can be over-discharged to a battery voltage of OV. The effect can be changed. At the same time, in the present invention, the selection of the second active material of the positive electrode is an important point, and in order to enable the battery to be overdischarged many times, this active material itself must have reversibility of charging and discharging. No. Further, it goes without saying that the active materials are not limited to those listed in the examples, but include all those that satisfy the above conditions.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a flat battery used in an example of the present invention.
FIGS. 2 and 3 are overdischarge characteristic diagrams of the battery of the present invention. 1...Sealing plate, 2...Alloy negative electrode, 3.
...Lithium, 4...Separator, 6.
Old: Positive electrode, 6: Case, 7: Current collector, 8: Gasket. Name of agent: Patent attorney Toshio Nakao and 1 other person
-Hokes2 Figure 3

Claims (3)

[Claims] (1) A positive electrode consisting of a chargeable and dischargeable first active material and a second active material, and an alloy in which lithium is compressed or pre-occluded to an alloy that occludes lithium during charging and releases lithium during discharging. In a battery comprising a negative electrode made of The first active material is one that discharges at a low potential, and the amount of dischargeable electricity of the lithium compressed to the alloy or the lithium occluded in the alloy is larger than the amount of electricity that can be discharged of the first active material, and the first active material A non-aqueous electrolyte lithium secondary battery characterized in that the amount of electricity is smaller than the sum of dischargeable electricity of the second active material and the second active material. (2) The first active material of the positive electrode is manganese dioxide (MnO_
2), chromium oxide (CrO_x), vanadium oxide (VO_x), and the second active material is molybdenum oxide (MoO_2), molybdenum trioxide (MoO_3), molybdenum disulfide (MoS_2). )
, molybdenum trisulfide (MoS_3), titanium disulfide (T
iS_2), titanium oxide (TiO_2), tungsten oxide (WO_3), niobium disulfide (NbS_2),
The nonaqueous electrolyte lithium secondary battery according to claim 1, which is a type selected from the group consisting of niobium oxide (Nb2O_5) and niobium selenide (NbSo_3). (3) The alloy that absorbs and desorbs lithium in the negative electrode is tin (S
n), lead (Pb), bismuth (Bi), indium (I
n) at least one selected from the group consisting of
n) and one type selected from the group consisting of cadmium (Cd).