JPH05258771A - Lithium secondary battery - Google Patents

Abstract

PURPOSE:To prevent a battery from firing and rupture by adding cobalt complex to an electrolyte in a certain mix proportion, and thereby precluding electro- eduction of metal lithium onto the surfaces of a positive and a negative elec trode due to over-charging and over-discharging and also decomposing reactions of the electrolyte. CONSTITUTION:To an electrolyte, cobalt complex expressed by a general formula Co(Olefin)(PR3)3 is added in a mix proportion of 0.01-0.1mol, where Olefin is ethylene or propylene and R is methyl group or ethyl group. The cobalt complex in the electrolyte reacts reversibly with lithium metal and works as a sort of chemical shuttle at the time of over-charging to suppress growth of electro-educed lithium on the negative electrode 2 and to suppress decomposing reactions of the electrolyte by supplying lithium to the positive electrode 1. Therefore, the battery can be precluded from internal shortcircuiting and firing or rupture resulting therefrom at the time of over-charging and over- discharging, which enhances the safety of the lithium secondary battery.

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 which prevents harmful effects due to overcharge and overdischarge.

[0002]

2. Description of the Related Art Carbonaceous materials used as negative electrode active materials for lithium secondary batteries have been attracting attention because they show excellent durability with little deterioration during charge and discharge cycles.

The negative electrode potential of a battery using this carbonaceous material shifts to a noble direction by discharging lithium ions when normally discharged, and shifts to a noble direction by storing lithium ions when charging. In this case, the negative electrode potential does not reach the lithium metal potential, but even in this type of battery, the following problems occur due to overcharge or overdischarge.

[0004]

[Problems to be Solved by the Invention] Problems due to overcharge: When charging a battery, a large current is temporarily passed to the battery due to a sudden failure of the charger, or the battery is still energized even when the battery reaches the end-of-charge voltage. May continue.

When the negative electrode potential of the battery in such an overcharged state becomes baser than the metallic lithium potential, metallic lithium is deposited on the lithium occlusion body which is the negative electrode. At this time, if all of the lithium that was intercalated in the positive electrode is released, the electrolytic solution decomposes in the positive electrode, and this reaction involves the generation of gas, so the electrolytic solution deteriorates, the internal pressure of the battery rises, and the battery bursts. , Can lead to ignition and is very dangerous.

Further, at the negative electrode, the electrolyte in the electrolytic solution is reduced as described above, and metallic lithium is deposited, but since this deposition form is dendrite-like, the electrodeposited lithium penetrates the separator and the internal A short circuit will occur, which will make the battery inoperable after that, and it may also explode or ignite.

Problem due to over-discharge: When a plurality of batteries of this type are connected in series and the batteries having different dischargeable capacities are mixed, a battery having a small dischargeable capacity has a large discharge capacity. It is forcibly discharged by the battery.

In a battery that is forcibly discharged, if this state continues, the positive and negative electrodes will be inverted, and metallic lithium will be deposited on the positive electrode. As described above, the electrodeposited lithium grows in the form of dendrite, which penetrates the separator to cause an internal short circuit, which may lead to rupture and ignition. At this time, if all the lithium stored in the negative electrode is released, the electrolytic solution decomposes in the negative electrode, and this reaction involves gas evolution, which deteriorates the electrolytic solution and increases the internal pressure of the battery, causing rupture. , Can lead to ignition and is very dangerous.

The present invention is intended to solve the above problems caused by overcharge and overdischarge, and its object is to prevent the electrodeposition of metallic lithium on the surface of the positive and negative electrodes due to overcharge and overdischarge and to prevent the electrolytic solution. Another object of the present invention is to provide a lithium secondary battery which prevents the decomposition reaction of the battery and thereby prevents the battery from catching fire and bursting.

[0010]

To achieve the above object, the present invention provides a lithium secondary battery comprising a positive electrode capable of lithium doping and dedoping, a negative electrode made of a carbonaceous material, and a non-aqueous electrolyte. In the electrolytic solution, 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) is added. It is added at a compounding ratio.

This 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 the case of lithium as an example, powdery lithium metal and graphite are prepared 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), the cobalt complex acts as shown in FIG. 1 on graphite as a lithium carrier when stirred. It is known to intercalate lithium. Figure 1
First, [Co (Olefin) (PR ₃ ) ₃ ] contacts and reacts with lithium metal powder, and Li [Co (Olefin)
(PR ₃ ) ₃ ] to generate Li [Co (Olefin) (PR
₃ ) ₃ ] shows the process of returning lithium to [Co (Olefin) (PR ₃ ) ₃ ] when lithium is intercalated into graphite.

At this time, Li [Co (Olefin) in FIG.
(PR ₃ ) ₃ ] intercalates lithium into graphite because Li [Co (Olefin) (P
R _{3) 3]} While it is nobler than a lithium metal potential, the lithium - Graphite - are explained to be due to having a lower potential than the intercalation compound.

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

[0015]

With the above structure, the cobalt complex added to the electrolytic solution reacts during overcharge and overdischarge as shown in FIGS. 2 and 3 below.

(1) At the time of overcharging (see FIG. 2): If metallic lithium deposits on the surface of the negative electrode even a little due to overcharging,
The electrodeposited readily reacts with lithium to form Li as shown by arrow (a) [Co (Olefin) (PR 3) 3].

This Li [Co (Olefin) (PR ₃ ) ₃ ]
Indicates that the overcharged negative electrode is in a state in which lithium can no longer be occluded, and the positive electrode potential is obviously nobler than the negative electrode potential, and at this time the positive electrode is cathodic polarized. Therefore, an electrochemical reaction easily occurs with the positive electrode (a kind of short-circuit reaction), and lithium intercalates with 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 promptly deintercalated, and metallic lithium is electrodeposited again at the negative electrode as shown by arrow (c).

The cobalt complex in the electrolytic solution is (a) above →
By repeating the cycle of (b) → (c), it acts as a kind of chemical shuttle during overcharge, suppresses the growth of electrodeposited lithium on the negative electrode, and also supplies the lithium to the positive electrode to decompose the electrolytic solution. Suppress.

(2) During over-discharging (see FIG. 3): When even a small amount of metallic lithium is deposited on the positive electrode due to over-discharging, it reacts easily with the electrodeposited lithium, and as shown by the arrow (d), Li [ Co (Olefin) (PR ₃ ) ₃ ].

Li [Co (Olefin) (PR ₃ ) ₃ ] is
It is clear that the positive electrode in the over-discharged state can no longer store lithium, and the negative electrode potential is obviously nobler than the positive electrode potential due to the reversal due to over-discharging. Therefore, an electrochemical reaction easily occurs with the negative electrode (a kind of short-circuit reaction), and lithium intercalates with the negative electrode as shown by 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 this in an organic solvent to form a binary intercalation compound of the lithium-carbonaceous material. However, since the negative electrode is in the over-discharged state and is polarized cathodically as described above, the intercalated lithium is promptly deintercalated, and again shown by the arrow (f). As described above, metallic lithium is electrodeposited on the positive electrode.

Therefore, the cobalt complex in the electrolytic solution acts as a kind of chemical shuttle at the time of overdischarge by repeating the above cycle of (d) → (e) → (f), so that the deposited lithium on the positive electrode The growth is suppressed and 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 in detail below with reference to the drawings. FIG. 4 shows the structure of an AA lithium secondary battery according to the present invention. This lithium secondary battery is basically wound in the same manner as in the past with a separator 3 made of a polypropylene porous film sandwiched between a positive electrode 1 and a negative electrode 2 to be spirally wound 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 projected to the lower part of the PP insulating plate 6
It is housed in a bottomed cylindrical case 6 via a, the negative electrode lead plate 5 is connected by spot welding to the center of the inner bottom surface of the bottomed cylindrical case 6, and the positive electrode is attached to the bottom of the positive electrode terminal plate 7 with a safety valve. The lead plate 4 is spot-welded, then a non-aqueous electrolyte is poured into the case 6, and the positive electrode terminal plate 7 is fitted into the opening of the case 6 through the sealing gasket 8 and completed by caulking.

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

The active material LiCoO ₂ is prepared 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. I was there. The theoretical charging electricity amount of the positive electrode 1 is 500 mAh.

The negative electrode 2 is formed into a strip shape by pressing carbonaceous powder and an aqueous dispersion of PTFE in a weight ratio of 100: 3 and kneading with water into a nickel expanded metal, drying and cutting. Further, a part of the negative electrode lead plate 5 is scraped off orthogonally to the longitudinal direction and spot-welded with the negative electrode lead plate 5. Incidentally, the ratio of the aqueous dispersion of PTFE is the ratio of the solid content as described above,
The weight of carbon powder in the negative electrode is 3.5 g.

When the counter electrode of this negative electrode is metallic lithium and the lithium doping amount in the first cycle is a constant current with a current density of 1 mA / cm ² , the voltage between the electrodes is up to 0.01 V. 500 mAh when the de-doping amount is 800 mAh and the current density is 1 mA / cm ^{2 at} a constant current.
Is.

The non-aqueous electrolytic solution contains lithium perchlorate (LiCl O ₄ ) and 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, 0.001mol, 0.005mol, 0.01mol
, 0.05 mol, 0.1 mol or higher.

Next, the performance of the completed batteries was comparatively investigated by injecting an electrolytic solution containing various concentrations of this cobalt complex.

(1) Overcharge test: Charge and discharge were performed for 20 cycles at a constant current of 0.5 A with an upper limit voltage of 4.2 V and a lower limit voltage of 3.0 V, and the discharge capacity at the 20th cycle was measured.
The characteristics shown in FIG. 5 were obtained. In this graph, when the upper limit of 0.1 mol, which is the range of the present invention, is exceeded, the discharge capacity decreases rapidly, which suggests that the addition of an excessive amount of cobalt complex increases the internal resistance of the battery. ing. It should be noted that, generally, the discharge capacity of the one with the cobalt complex added is lower than that without the cobalt complex, but the discharge capacity is 200 mAh.
To the extent that it is within the allowable range of capacity reduction.

Next, after 20 cycles, these batteries were continuously charged again with a constant current of 0.5 A to artificially make an overcharged state, and the time to reach 10 V was measured. As a result, as shown in FIG. 6, when the amount added was below the lower limit of the present invention, the time required for reaching 10 V was prolonged depending on the addition amount, but 0.01 mol or more of the cobalt complex exceeding the lower limit of the present invention was added. It takes 1 time to reach 10V with the added battery.
It was confirmed that the time was more than 00 hours, and that it was a long time before it could not be published in this graph. It should be noted that, in the case of the overcharged addition amount below the range of the present invention, the dendrite broke through the separator to cause an internal short circuit, and it was confirmed that the battery cannot be used.

(2) Over-discharge test: After 20 cycles of charging / discharging, 5 batteries charged to 4.2 V and 1 battery not charged again were connected in series, and the voltage of 6 batteries was changed. It was forcibly discharged at a constant current of 0.5 A until it reached 0V.

Here, as the electrolytic solution of the battery which is not charged, the one to which the cobalt complex was not added and the one to which various concentrations of the cobalt complex were added 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 clear from the results shown in this table, the secondary battery using the electrolytic solution containing 0.005 mol or more of the cobalt complex has high safety without internal short circuit during overcharge. 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 discharge capacity and the characteristics at the time of overcharge.
Is preferable.

Although [Co (C ₂ H ₂ ) (PCH ₃ ) ₃ ] is used as the cobalt complex in this example,
General formula: [Co (Olefin) (PR 3) 3] ( where Olefin
Any of the cobalt complexes represented by ethylene or propylene and R is a methyl group or an ethyl group) can be used because their effects as a chemical shuttle have been confirmed.

Further, it is needless to say that the present invention can be applied not only to the spiral type battery described above, but also to a flat type battery such as a coin type.

[0038]

As described above in detail with reference to the embodiments, in the lithium secondary battery according to the present invention, the cobalt complex in the electrolytic solution reversibly reacts with the lithium metal to cause a kind of overcharge. It functions as a chemical shuttle, suppresses the growth of electrodeposited lithium on the negative electrode, and suppresses the decomposition reaction of the electrolyte solution by supplying lithium to the positive electrode.This cobalt complex also functions as a kind of chemical shuttle during overdischarge. The growth of electrodeposited lithium on the positive electrode is suppressed, and the decomposition reaction of the electrolytic solution is suppressed by supplying lithium to the negative electrode. Therefore, according to the present invention, it is possible to avoid the risk of internal short circuit of the battery during overcharging and overdischarging, and ignition and rupture due to this, and improve the safety of this type of secondary battery.

[Brief description of drawings]

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

FIG. 2 is a schematic diagram showing the 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 lithium secondary battery according to an embodiment of the present invention.

FIG. 5 is a graph showing discharge capacities at 20th charge / discharge cycles in each amount of cobalt complex added.

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

[Explanation 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

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hidenori Nagura, 5-3-11 Shimbashi, Minato-ku, Tokyo Fuji Electric Chemical Co., Ltd. (72) Takashi Suzuki 5-36-11, Shimbashi, Minato-ku, Tokyo Fuji Electrochemical Co., Ltd.

Claims (1)

[Claims] 1. A lithium secondary battery comprising a positive electrode capable of lithium doping and dedoping, a negative electrode made of a carbonaceous material, and a non-aqueous electrolytic solution, wherein a general formula: [Co (Olefin) ( P
R ₃ ) ₃ ] (where Olefin is ethylene or propylene,
The lithium secondary battery is characterized in that a cobalt complex represented by R is a methyl group or an ethyl group) is added at a compounding ratio of 0.01 to 0.1 mol.