JP3043175B2 - Lithium secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery in which adverse effects due to overcharge and overdischarge are prevented.

[0002]

2. Description of the Related Art A carbonaceous material used as a negative electrode active material of a lithium secondary battery has attracted attention because it exhibits excellent durability with little deterioration during a charge / discharge cycle.

[0003] The negative electrode potential of a battery using this carbonaceous material shifts to a noble direction by discharging lithium ions during normal discharge, and shifts to a noble direction during charging, by absorbing lithium ions. Note that, in this case, the negative electrode potential does not reach the lithium metal potential, but even in this type of battery, the following problem occurs due to overcharging or overdischarging.

[0004]

[Problems to be Solved by the Invention] Problems due to overcharging: When charging a battery, a sudden failure of the charger causes a temporary large current to flow into the battery, or the battery is still energized even if it reaches the charge end voltage. May follow.

When the negative electrode potential of the battery in such an overcharged state becomes lower than the metallic lithium potential, metallic lithium is deposited on the lithium storage material as the negative electrode. At this time, if all the lithium intercalated to the positive electrode is released, the electrolytic solution is decomposed at the positive electrode, and this reaction involves gas generation, so that the electrolytic solution deteriorates, the internal pressure of the battery increases, and the battery ruptures. , Can lead to ignition and is very dangerous.

In the negative electrode, as described above, the electrolyte in the electrolytic solution is reduced, and metallic lithium is deposited. However, since the deposited form is dendrite, the deposited lithium penetrates through the separator and enters inside. A short circuit may occur, which will render the battery inoperable, and may even explode or ignite.

Problems due to overdischarge: When a plurality of batteries of this type are connected in series and used, and batteries having different dischargeable capacities are mixed, batteries having a small dischargeable capacity have a large discharge capacity. The battery is forcibly discharged.

In a battery which has been forcibly discharged, if this state continues, the positive and negative electrodes will be inverted, and metallic lithium will precipitate on the positive electrode. As described above, electrodeposited lithium grows in a dendrite shape, which penetrates through the separator to cause an internal short circuit, which may lead to rupture or ignition. At this time, when all of the lithium occluded in the negative electrode is released, the electrolytic solution is decomposed at the negative electrode, and this reaction involves gas generation. , Can lead to ignition and is very dangerous.

The present invention has been made to solve the above-mentioned problems caused by overcharge and overdischarge, and has as its object to prevent the deposition of metallic lithium on the positive and negative electrode surfaces due to overcharge and overdischarge and to prevent the electrolytic solution It is an object of the present invention to provide a lithium secondary battery in which a decomposition reaction of a lithium battery is prevented, thereby preventing the battery from firing or bursting.

[0010]

To achieve the above object, the present invention relates to a lithium secondary battery comprising a positive electrode capable of doping and undoping lithium, a negative electrode made of a carbonaceous material, and a non-aqueous electrolyte. And 0.01 to 0.1 mol of a cobalt complex represented by the general formula: [Co (Olefin) (PR ₃ ) ₃ ] (where Olefin is ethylene or propylene, and R is a methyl group or an ethyl group) in the electrolyte. It is added in a compounding ratio.

The cobalt complex is used when graphite is intercalated with an alkali metal such as potassium, sodium or lithium in a solvent, and various properties have been found.

Taking lithium as an example, powdered lithium metal and graphite are represented by the general formula: [Co (Olefin
) (PR ₃ ) ₃ ] (where Olefin is ethylene or propylene, and R is a methyl group or an ethyl group) when stirred in an organic solvent, the cobalt complex acts as shown in FIG. 1 to form graphite as a lithium carrier. It is known to intercalate lithium. FIG.
First, [Co (Olefin) (PR ₃ ) ₃ ] contacts and reacts with lithium metal powder to form Li [Co (Olefin)
(PR ₃ ) ₃ ], and Li [Co (Olefin) (PR
₃ ) ₃ ] shows the process of returning to [Co (Olefin) (PR ₃ ) ₃ ] again when lithium is intercalated into graphite.

At this time, Li [Co (Olefin) in FIG.
(PR ₃ ) ₃ ] intercalates lithium into graphite by Li [Co (Olefin) (P
R ₃ ) ₃ ] is more precious than the lithium metal potential, but has a lower potential than the lithium-graphite-intercalation compound.

The amount of the cobalt complex of the present invention is as follows:
If the amount is too small, the effect as a chemical shuttle described later is not sufficiently obtained, and if the amount is too large, the internal resistance increases and the continuous discharge time until reaching the discharge end voltage is shortened.
The addition amount is limited to the above range.

[0015]

According to the above arrangement, the cobalt complex added to the electrolyte reacts during overcharge and overdischarge as shown in FIGS.

(1) At the time of overcharging (see FIG. 2): If any amount of metallic lithium is deposited on the surface of the negative electrode due to overcharging,
It reacts easily with the deposited lithium to form Li [Co (Olefin) (PR ₃ ) ₃ ] as shown by the arrow (a).

This Li [Co (Olefin) (PR ₃ ) ₃ ]
Indicates that lithium is no longer occluded in the overcharged negative electrode, and the positive electrode potential is clearly nobler than the negative electrode potential, as described above. Therefore, an electrochemical reaction easily occurs with the positive electrode (a kind of short-circuit reaction), and lithium intercalates into the positive electrode as shown by the arrow (b).

On the other hand, since the positive electrode is in an overcharged state, the intercalated lithium is quickly deintercalated, and metal lithium is deposited again at the negative electrode as shown by an arrow (c).

The cobalt complex in the electrolytic solution is obtained by the above (a) →
By repeating the cycle of (b) → (c), it acts as a kind of chemical shuttle at the time of overcharge, suppresses the growth of electrodeposited lithium on the negative electrode, and suppresses the electrolytic solution decomposition reaction by supplying lithium to the positive electrode. Suppress.

(2) Overdischarge (see FIG. 3): If any metallic lithium precipitates on the positive electrode due to overdischarge, it easily reacts with the deposited lithium, and as shown by arrow (d), Li [ Co (Olefin) (PR ₃ ) ₃ ] is formed.

Li [Co (Olefin) (PR ₃ ) ₃ ] is
The negative electrode potential is clearly more noble than the positive electrode potential due to the over-discharged positive electrode being in a state where lithium can no longer be inserted into the positive electrode, and the negative electrode is now cathodicly polarized. Therefore, an electrochemical reaction easily occurs with the negative electrode (a kind of short-circuit reaction), and lithium intercalates into the negative electrode as shown by an arrow (e).

Actually, Li [Co (Olefin) (PR ₃ ) ₃ ]
As described above, it is known that even when the carbonaceous material is not in a polarized state, it reacts with the carbonaceous material in an organic solvent to form a binary intercalation compound of a lithium-carbonaceous material. However, since the negative electrode is in an overdischarged state and is cathodicly polarized as described above, the intercalated lithium is quickly deintercalated, and again as shown by the arrow (f). Thus, metallic lithium is deposited on the positive electrode.

Therefore, the cobalt complex in the electrolytic solution functions as a kind of chemical shuttle at the time of overdischarge by repeating the above-mentioned circulation of (d) → (e) → (f), and the deposited lithium on the positive electrode In addition to suppressing the growth, the decomposition reaction of the electrolytic solution is suppressed by supplying lithium to the negative electrode.

[0024]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 4 shows the structure of an AA lithium secondary battery according to the present invention. Basically, this lithium secondary battery is spirally wound with a separator 3 made of a porous film made of polypropylene between the positive electrode 1 and the negative electrode 2 to form a winding element. A positive electrode lead plate 4 connected to the positive electrode 1 side and a negative electrode lead plate 5 connected to the negative electrode 2 side protrude below the PP insulating plate 6.
a, the negative electrode lead plate 5 is connected to the center of the inner bottom surface of the bottomed cylindrical case 6 by spot welding, and the positive electrode is connected to the bottom of the positive terminal plate 7 with a safety valve. The lead plate 4 is spot-welded, and then a non-aqueous electrolyte is injected into the case 6, the positive electrode terminal plate 7 is fitted into the opening of the case 6 via the sealing gasket 8, and completed by caulking.

The positive electrode 1 is made of LiCo as a positive electrode active material.
O ₂ , a conductive material such as carbon powder and an aqueous dispersion of PTFE were mixed at a weight ratio of 100: 10: 10, and kneaded in a paste with water to form a current collector having a thickness of 30 μm. Is coated on both sides of an aluminum foil, dried, rolled, and cut into a predetermined size. A portion of the mixture is scraped off at right angles to the longitudinal direction of the strip, and the positive electrode lead plate 4 Was spot welded. The ratio of the aqueous dispersion of PTFE in the above mixture ratio is the ratio of the solid content.

The active material LiCoO ₂ is obtained by mixing cobalt oxide (CoO) and lithium carbonate (LiCO ₃ ) at a molar ratio of 2: 1 and heating in air at 900 ° C. for 9 hours. Was. The theoretical charge amount of the positive electrode 1 is 500 mAh.

The negative electrode 2 was formed by kneading with water a carbonaceous powder and an aqueous dispersion of PTFE at a weight ratio of 100: 3, and pressing the mixture into a nickel expanded metal, followed by drying and cutting to form a strip. The negative electrode lead plate 5 is spot-welded to a part thereof perpendicular to the longitudinal direction and spot-welded thereto. In addition, the ratio of the aqueous dispersion of PTFE is the ratio of the solid content as described above,
The weight of the carbon powder in the negative electrode was 3.5 g.

When the counter electrode is made of metallic lithium and the doping amount of lithium in the first cycle is a constant current of a current density of 1 mA / cm ^2, a voltage between both electrodes of up to 0.01 V is obtained. 500 mAh when the dedoping amount is 800 mAh and the dedoping amount is a constant current of 1 mA / cm ^2.
It is.

Further, the non-aqueous electrolyte contains lithium perchlorate (LiCl O ₄ ) in propylene carbonate and 1,2
- obtained by dissolving at a rate of 1 mol / l in a mixed solvent of dimethoxyethane, to the electrolytic solution, as the cobalt complex _{[Co (C 2 H 2)} (PCH 3) 3] respectively 0
mol, 0.001 mol, 0.005 mol, 0.01 mol
, 0.05 mol, 0.1 mol or more at various concentrations.

Next, the performance of the battery completed by injecting an electrolytic solution to which the cobalt complex was added at various concentrations was compared and investigated.

(1) Overcharge test: The upper limit voltage was 4.2 V, the lower limit voltage was 3.0 V, and charging and discharging were performed for 20 cycles at a constant current of 0.5 A, and the discharge capacity at the 20th cycle was measured.
The characteristics shown in FIG. 5 were obtained. In this graph, when the amount exceeds the upper limit of 0.1 mol of the range of the present invention, the discharge capacity decreases rapidly, which suggests that the internal resistance of the battery increases due to the addition of an excessive amount of the cobalt complex. ing. In general, the discharge capacity of the battery to which the cobalt complex was added was lower than that of the battery to which no cobalt complex was added, but the discharge capacity was 200 mAh.
This is within the allowable range of capacity reduction.

Next, after the completion of 20 cycles, these batteries were continuously charged with a constant current of 0.5 A again to artificially create an overcharged state, and the time required to reach 10 V was measured. As a result, as shown in FIG. 6, those added below the lower limit of the range of the present invention have a longer time to reach 10 V depending on the amount added, but have a cobalt complex of 0.01 mol or more exceeding the lower limit of the present invention. The time to reach 10V is 1
It was confirmed that the time was more than 00 hours, which was a long time before the data could not be posted on this graph. In addition, in the case of the addition amount which is less than the range of the overcharged present invention, it has been confirmed that dendrites pierce the separator and cause an internal short circuit, so that the battery cannot be used.

(2) Overdischarge test: After 20 cycles of charging / discharging, 5 batteries recharged to 4.2 V and 1 uncharged battery are connected in series, and the voltage of 6 batteries is reduced. The battery was forcibly discharged at a constant current of 0.5 A until the voltage became 0 V.

Here, as the electrolytic solution of the battery which is not charged, those without the addition of the cobalt complex and those with the addition of the cobalt complex at various concentrations were tested to confirm the presence or absence of a short circuit. The results are shown in the table below.

[0035]

[Table 1] As is evident from the results shown in this table, a secondary battery using an electrolyte solution containing at least 0.005 mol of a cobalt complex has no internal short circuit at the time of overcharge and has high safety. However, the addition range of the cobalt complex is 0.01 to 0.1 mol with respect to the non-aqueous electrolyte in consideration of the above-mentioned discharge capacity and characteristics at the time of overcharging.
The range up to is preferable.

In this embodiment, [Co (C ₂ H ₂ ) (PCH ₃ ) ₃ ] is used as the cobalt complex.
General formula: [Co (Olefin) (PR ₃ ) ₃ ] (However, Olefin
Is an ethylene or propylene, and R is a methyl group or an ethyl group), any of which can be adopted because the effect as a chemical shuttle is confirmed.

The present invention can be applied not only to the above-mentioned spiral type battery but also to a flat type battery such as a coin type.

[0038]

As described in detail in the above embodiments, in the lithium secondary battery according to the present invention, the cobalt complex in the electrolytic solution reacts reversibly with lithium metal to form a kind of overcharge. It acts as a chemical shuttle, suppressing the growth of electrodeposited lithium on the negative electrode, and suppressing the decomposition reaction of the electrolyte by supplying lithium to the positive electrode.This cobalt complex also acts as a kind of chemical shuttle during overdischarge. In addition, while suppressing the growth of electrodeposited lithium on the positive electrode, the decomposition reaction of the electrolytic solution is suppressed by supplying lithium to the negative electrode. Therefore, in the present invention, the risk of internal short circuit of the battery at the time of overcharging and overdischarging and the danger of fire and explosion due to this can be avoided beforehand, and the safety of this type of secondary battery can be improved.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing the function of a cobalt complex.

FIG. 2 is a schematic diagram showing a function of a cobalt complex during overcharge.

FIG. 3 is a schematic diagram showing the function of a cobalt complex during overdischarge.

FIG. 4 is a cross-sectional view of a spiral-type lithium secondary battery according to an embodiment of the present invention.

FIG. 5 is a graph showing the discharge capacity at the 20th charge / discharge cycle for each addition amount of the cobalt complex.

FIG. 6 is a graph showing the time required to reach 10 V overcharge at each addition amount of a cobalt complex.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Separator 4 Positive electrode lead plate 5 Negative electrode lead plate 6 Case 7 Positive electrode terminal plate 8 Sealing gasket

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hidetetsu Nakura 5-36-11 Shimbashi, Minato-ku, Tokyo Inside Fuji Electric Chemical Co., Ltd. (72) Takashi Suzuki 5-36-11 Shimbashi, Minato-ku, Tokyo Fuji Electric Chemical Co., Ltd. (56) References JP-A-4-242074 (JP, A) JP-A-1-206571 (JP, A) JP-A-5-47416 (JP, A) Proc. Pymp. Recharge enable Lithium Batt eries 1989, page. 77-86 (1990) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 10/40 H01M 6/16

Claims (1)

(57) [Claims] 1. A lithium secondary battery comprising a positive electrode capable of doping and dedoping lithium, a negative electrode made of a carbonaceous material, and a non-aqueous electrolyte, wherein the electrolyte has a general formula: [Co (Olefin) ( P
R ₃ ) ₃ ] (where Olefin is ethylene or propylene,
(R is a methyl group or an ethyl group) a cobalt complex represented by the formula (1) in an amount of 0.01 to 0.1 mol.