JP3157251B2 - Lithium ion 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 an improvement of a lithium secondary battery, which has been attracting attention as a power source for personal equipment, which has been remarkably competed in miniaturization of portable telephones, VTR cameras, book-type personal computers, and the like. The present invention relates to a lithium ion secondary battery capable of improving energy density and having excellent overdischarge characteristics.

[0002]

2. Description of the Related Art Lithium secondary batteries have attracted attention as early as high energy batteries. However, practical use of the lithium secondary battery has been delayed due to the poor reversibility of the lithium anode. Efforts have been made to obtain reversibility of a lithium negative electrode by alloying lithium or storing lithium ions in a carbon material. However, this structure has a disadvantage that the energy density is reduced. For this reason, it has only been put to practical use with some lithium secondary batteries such as coin type batteries.

Recently, a lithium ion secondary battery with a high voltage and a high energy density has been put to practical use by using a lithium-containing oxide such as LiCoO ₂ for the positive electrode and carbon for the negative electrode ( JP-A-62-9086
3 JP, JP-A-63-121260, JP-32nd battery Symposium Proceedings p31~34).

[0004]

However, the secondary batteries described in these publications have a drawback that the energy density is lower than that of batteries using metallic lithium. This is because carbon that does not participate in the reaction is filled in the battery. Further, the lithium secondary battery having this structure has a characteristic that when discharged, 100% of lithium occluded in carbon of the negative electrode cannot be released. This is because the battery voltage drops to 0 V with lithium remaining on the carbon of the negative electrode. For this reason, in order to occlude a large amount of lithium in carbon of the negative electrode, it is necessary to increase the amount of lithium-containing oxides such as LiCoO ₂ that is a positive electrode active material, and this has resulted in further lowering the energy density.

[0005] When assembling the negative electrode, this drawback can be solved by inserting lithium into carbon of the negative electrode. However, if this is realized, the manufacturing process of the negative electrode becomes complicated, and it is difficult to put it to practical use even if it is possible at the prototype level. In addition, in the case where the carbon electrode and the lithium foil are bonded to each other and incorporated into the battery, it is necessary to use an extremely thin lithium foil because the electrodes are thin , but it is difficult to obtain such a thin lithium foil .
There was a problem that the cost was high.

For this reason, it is necessary to incorporate lithium to be stored in the carbon of the negative electrode from the positive electrode active material in the first charging step. Therefore, it is necessary to design a large amount of lithium of the positive electrode active material in consideration of the amount of lithium absorbed by the carbon of the negative electrode and not released even when discharged, and the energy density of the battery is reduced.

[0007] Further, the lithium ion secondary battery having this structure has a characteristic that, when overdischarged, the potential of the negative electrode rises before the potential of the positive electrode falls. Thus, when the battery voltage drops to zero volts, the potential of the negative electrode becomes extremely high. The high-potential negative electrode elutes copper generally used as a negative electrode core. Therefore, this secondary battery has poor overdischarge resistance, and it must be used with particular care so as not to overdischarge, and there is a drawback that it cannot be easily handled.

[0008] The present invention has been developed with the aim of further solving this drawback. An important object of the present invention is to increase the energy density and improve the overdischarge characteristics,
Another object of the present invention is to provide a lithium ion secondary battery capable of reducing the difference between the first charge amount and the subsequent discharge amount.

The lithium ion secondary battery of the present invention has the following configuration to achieve the above object. That is, the lithium ion secondary battery of the present invention is made of LiCoO ₂
Lithium that can release Li ions by charging
A positive electrode containing the compound and a negative electrode containing carbon capable of occluding lithium such as graphite and coke are provided, and the negative electrode capacity is further improved from that designed to be larger than the positive electrode capacity.

[0010] The lithium ion secondary battery is characterized in that when electrolysis is performed, a lithium salt solution is generated in which lithium is occluded on the negative electrode side and gas is generated by decomposition on the positive electrode side.

[0011]

In a lithium secondary battery in which a lithium salt solution is added to an electrolytic solution, when the charging voltage becomes about 3.5 V at the first charging, the lithium salt of the electrolytic solution is electrolyzed. The electrolyzed lithium is stored in the negative electrode.
In this charging process, lithium contained in the positive electrode active material is also ionized and turned into ions, and is simultaneously absorbed by carbon of the negative electrode.

In a conventional lithium ion secondary battery, lithium to be stored in the negative electrode is supplied from a lithium-containing compound as a positive electrode active material. On the other hand, the lithium ion secondary battery of the present invention can supply lithium to the negative electrode not only from the positive electrode active material but also from the lithium salt solution added to the electrolytic solution. The lithium of the first charge / discharge capacity difference,
When occluded in the negative electrode from the lithium salt solution of the electrolytic solution, the amount of lithium occluded in the negative electrode from the positive electrode active material is substantially equal to the amount of lithium transferred from the negative electrode to the positive electrode. Therefore, the difference between the amount of lithium released from the positive electrode active material and the amount of lithium released from the negative electrode active material can be reduced, and the charge / discharge capacity loss can be compensated.

The lithium salt solution added to the electrolytic solution is electrolyzed when charged, and turns into a gas on the positive electrode side. Therefore, when the lithium salt solution is electrolyzed after the battery is sealed, the battery needs to be provided with a return-type pressure release valve so as to release gas outside the battery (FIG. 3). Due to this gas generation, unnecessary anions in the lithium salt solution are released outside the battery. Although it is better to add a small amount of water, even if water is used as the solvent for the lithium salt solution, the charging voltage is as high as about 4.1 V, so that the water is also electrolyzed and discharged out of the battery, and the water is discharged. It does not adversely affect the battery. When the electrolyte is electrolyzed before sealing to occlude lithium in the lithium salt solution in the negative electrode, the battery does not necessarily need to be provided with a pressure release valve.

[0014] and the lithium ion secondary battery of the present invention, the first time of charging and discharging characteristics of the conventional lithium ion secondary battery 1
And FIG. FIG. 1 shows a constant current charge / discharge curve of a conventional lithium ion secondary battery, and FIG. 2 shows a constant current characteristic of the lithium ion secondary battery of the present invention. The charge end voltage is set to V ₁ and the discharge end voltage is set to V ₂ . Also,
The charge capacity of the conventional product is C ₃ , the discharge capacity is C ₁ , the charge capacity of the product of the invention is C ₄ , and the discharge capacity is C ₂ .

As shown in FIG. 1, a conventional lithium ion
The secondary battery has a discharge capacity C₁And charging capacity C_ThreeThe relationship is C₁
<C_ThreeBecomes This difference in capacity is the first charge-discharge capacity
It is a loss. The lithium ion secondary battery of the present invention
Electric capacity C₁<C_Two, Charging capacity C _Three<C_FourAnd discharge capacity
Both of the charging capacities are larger than those of conventional batteries. When
In particular, the lithium salt solution of the present invention has a discharge capacity C_TwoThe traditional
Battery charging capacity C_ThreeCan be approximately equal to I
Therefore, the charge / discharge capacity depends on the lithium salt solution of the electrolyte.
The loss is compensated, and the energy density can be improved.

Further, the lithium salt solution of the present invention comprises:
There is a feature that elution of the core body can be prevented even after complete discharge. This is because lithium remains on the negative electrode in a completely discharged state, so that the potential of the negative electrode is lowered by the lithium, and a high voltage can be prevented from eluted from the copper of the core. For this reason, the lithium ion secondary battery of the present invention can prevent deterioration in characteristics after overdischarge, and can be used extremely conveniently without having to use it under severe conditions. That is,
Even if the battery is overdischarged, the battery shows the original performance when charged, and the battery has excellent overdischarge resistance.

[0017]

Embodiments of the present invention will be described below. However, the following examples illustrate a lithium ion secondary battery for embodying the technical idea of the present invention, and the lithium ion secondary battery of the present invention includes materials, shapes, The structure and arrangement are not limited to the following structures. Various changes can be added to the lithium ion secondary battery of the present invention within the scope of the claims.

[Conventional Lithium Ion] A lithium ion secondary battery includes a positive electrode, a negative electrode, a separator, an electrolytic solution, and a casing. The positive electrode is manufactured by coating an aluminum foil with a lithium cobaltate (LiCoO ₂ ) mixture. The negative electrode is manufactured by coating pitch coke on a copper foil together with a binder. As the separator, a microporous membrane made of polypropylene is used. As the electrolytic solution, one obtained by dissolving 1 mol of lithium hexafluorophosphate in propylene carbonate is used. Using these, an AA size lithium ion secondary battery was assembled. The electrode was spirally wound. This battery had a first charge capacity of 450 mAh and a discharge capacity up to 2.75 V of 340 mAh. The capacity of the positive and negative electrodes was measured as a counter electrode lithium in a beaker cell, and the positive electrode capacity of this battery was 460 mAh and the negative electrode was 540
mAh.

[Lithium ion secondary battery of Example] A solution in which 100 mg of lithium hydroxide was dissolved was added to an electrolyte having exactly the same electrode structure as that of the conventional battery to obtain a lithium ion secondary battery. . 100 mg of lithium hydroxide is 110 mAh, which is the difference between the first negative electrode charge and discharge capacities.
Of 90%. This lithium ion secondary battery is
The charge capacity was 530 mAh and the discharge capacity was 410 mAh.

In this embodiment, the amount of lithium in the lithium hydroxide solution added to the electrolytic solution was set to 90% of the negative electrode charge / discharge capacity difference.
The reason for this is that if lithium is added in excess of 100% of the difference between the charge and discharge capacities of the negative electrode, precipitation of lithium metal occurs on the surface of the negative electrode.

The lithium hydroxide added to the electrolytic solution undergoes the following reaction when charged, causing lithium to be absorbed in the carbon of the negative electrode. LiOH ionization reaction LiOH → L
i ⁺ + OH ^- reaction Li ⁺ + e lithium to the negative electrode are inserted ^- → Li reaction at the positive electrode 4OH ^- → O _{2
^{↑ + 2H 2 O + 4e -}} 2H 2 O → 2H 2 ↑ + O 2 ↑

[0022] Thus, added to the electrolytic solution was hydroxide Li
In the lithium solution, lithium is precipitated on the negative electrode side by electrolysis, and gas is generated by decomposition on the positive electrode side. Therefore, when charging in a sealed state, the lithium ion secondary battery is provided with a return-type pressure release valve 1 as shown in FIG. 3 to discharge gas generated inside to the outside of the battery.

In the pressure relief valve 1 shown in FIG. 3, a sealing plate 4 is sandwiched by a gasket 3 at an upper end opening of an outer can 2. A valve hole 5 is opened at the center of the sealing plate 4. The upper surface of the valve hole 5 is closed by a valve plate 6. The valve plate 6 is elastically pressed by the spring 7 into the valve hole 5. The spring 7 is provided below the anode cap 8 fixed to the upper surface of the sealing plate 4. When the internal pressure of the outer can 2 rises, the pressure relief valve of this structure pushes up the valve plate 6 to release the valve hole 5.
Is opened, and the gas inside is discharged out of the outer can 2.
When the internal pressure decreases, the valve plate 6 is pressed by the valve hole 5, and the valve hole 5 is closed.

[0024]

According to the lithium ion secondary battery of the present invention, a lithium salt solution is added to the electrolytic solution. When the lithium salt solution added to the electrolytic solution is charged, lithium is occluded in the carbon of the negative electrode. Thus, by storing lithium in carbon, the lithium ion secondary battery of the present invention has a feature that the energy density can be increased. This is because, by adding a lithium salt solution to the electrolyte, it is not necessary to incorporate lithium, which cannot be released from the negative electrode even when discharged, from the positive electrode active material into the negative electrode. In other words, lithium that has migrated from the positive electrode active material to the negative electrode in the charging step migrates to the positive electrode with almost no residual in the discharging process. That is, the energy density can be increased by efficiently utilizing the amount of the lithium compound of the positive electrode for charging and discharging. Further, the energy density can be increased without increasing the amount of lithium in the positive electrode active material.

Further, in the lithium ion secondary battery of the present invention, the potential of the negative electrode in an overdischarged state can be lowered.
For this reason, the potential of the negative electrode does not increase, and the core copper is not dissolved. Therefore, there is a feature that the reduction in battery capacity after overdischarge is reduced and the handling is simplified. A conventional lithium ion secondary battery has a battery voltage of 0
When the voltage approaches V, the potential of the negative electrode reaches 2 V, so that the copper in the core begins to elute, and if left for a long period of time, the capacity will not recover even when charged. On the other hand, in the lithium ion secondary battery of the present invention, even if the battery voltage becomes 0 V, the negative electrode potential is 1.5 V or less, and copper does not elute. Therefore, the capacity is restored by charging.

Further, a notable feature of the present invention is that
The feature is that lithium can be stored in the negative electrode very easily and uniformly. This is because a lithium salt solution is added to the electrolytic solution, and lithium can be uniformly absorbed in the carbon of the negative electrode by a charging reaction.

[Brief description of the drawings]

FIG. 1 is a graph showing a voltage change during the first charge / discharge of a conventional lithium secondary battery.

FIG. 2 is a graph showing a voltage change in the first charge / discharge of the lithium ion secondary battery of the present invention.

FIG. 3 is a sectional view showing an upper part of a lithium ion secondary battery having a pressure release valve.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Pressure relief valve 2 ... Outer can 3 ... Gasket 4 ... Sealing plate 5 ... Valve hole 6 ... Valve plate 7 ... Spring 8 ... Anode cap

Continuation of the front page (56) References JP-A-4-328272 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 10/40

Claims (1)

(57) [Claims] 1. A lithium ion secondary battery in which the positive electrode contains a lithium-containing compound capable of releasing lithium ions upon charging, the negative electrode contains carbon capable of absorbing lithium, and the capacity of the negative electrode is larger than the capacity of the positive electrode. A lithium ion secondary battery comprising a lithium salt solution in which lithium is occluded on the negative electrode side by electrolysis and gas is generated by decomposition on the positive electrode side.