JP2000123872A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the amount of an electrolyte of a lithium secondary battery without decreasing the performance of the battery. SOLUTION: In a lithium secondary battery, a belt-shaped positive electrode and a belt-shaped negative electrode are wound through a separator, then an electrolyte is impregnated into them, and the amount of the electrolyte is made less than the void of each of the positive electrode, the negative electrode, and the separator. Battery performance is not decreased even if the amount of the electrolyte is limited to about 80% of each void.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a lithium secondary battery.

[0002]

2. Description of the Related Art Due to its high energy density, lithium secondary batteries are not only used as power sources for portable equipment, but also as E-powers.
The use of V and HEV as a power source is promising. However, at present, since the cost of lithium secondary batteries is high, it is an important issue to reduce the manufacturing cost especially for use as a power source for EVs and HEVs equipped with a large number of batteries.

Conventionally, in a lithium secondary battery formed by winding a strip-shaped positive electrode and a negative electrode with a separator interposed therebetween, an electrolyte is sufficient to fill gaps between the positive electrode, the negative electrode and the separator. An amount or more is impregnated. In order to reduce the cost of the lithium secondary battery, it is effective to reduce the amount of use of the separator and the electrolyte, in which the value of each component material is particularly high in the battery price. However, if the amount of the separator used is reduced by, for example, thinning the separator, it is not practical because a short circuit is likely to occur. On the other hand, if the use of the electrolytic solution is not adversely affected on the battery performance, it is effective not only for cost reduction but also for weight reduction and safety improvement.

[0004] However, in general, a battery in a case where the electrolyte solution is insufficient runs down due to a decrease in ionic conductivity.
It is said that the internal resistance of the battery increases and the discharge capacity decreases, and no technique has been found to suppress the amount of electrolyte impregnated in the battery.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and it is an object of the present invention to reduce the performance of a battery by reducing the amount of electrolyte used to fill the voids of a positive electrode, a negative electrode and a separator. It is an object to reduce it without any problem.

[0006]

The lithium secondary battery of the present invention is formed by winding a strip-shaped positive electrode and a negative electrode with a separator interposed therebetween, and applying an electrolytic solution to the positive electrode, the negative electrode and the separator. In the lithium secondary battery obtained by impregnation, the amount of the electrolytic solution impregnated in the positive electrode, the negative electrode, and the separator is smaller than the total amount of the voids in the positive electrode, the negative electrode, and the separator. It is characterized by having.

[0007] The amount of the electrolyte is preferably less than 100% and more than 80% of the void amount of each of the positive electrode, the negative electrode and the separator.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The lithium secondary battery of the present invention is formed by winding a strip-shaped positive electrode and a negative electrode through a separator, and impregnating the positive electrode, the negative electrode and the separator with an electrolytic solution. A lithium secondary battery comprising the electrolyte solution impregnated in an amount smaller than the total amount of the voids in the positive electrode, the negative electrode, and the separator.

The strip-shaped positive electrode is formed by applying a mixed material of a powdered positive electrode active material, a powdered conductive material and a binder to a current collector foil. The obtained electrode has a void portion that can be impregnated with the electrolytic solution. The void amount of the positive electrode can be calculated as follows. It is calculated as (volume of positive electrode)-(volume of active material + volume of binder + volume of conductive material). For example, volume of active material = (weight of positive electrode)
X (weight ratio of active material in positive electrode / true density of active material), and similarly in the case of binder and conductive material.

[0010] The strip-shaped negative electrode is formed by mixing a carbon powder and a binder and applying the mixture to a current collector foil, and has a void portion that can be impregnated with an electrolytic solution. The void amount of this negative electrode is
(Volume of negative electrode) − (volume of carbon + volume of binder), and volume of carbon = (weight of negative electrode) × (weight of carbon / true density of carbon). It can be calculated similarly.

The separator is sandwiched between the positive electrode and the negative electrode by a porous sheet-like member and wound together with the quantity electrode. The void amount of this separator is
It was calculated by (porosity of sheet material) × (volume of separator). The total amount of the voids of the positive electrode, the negative electrode, and the separator is calculated as (the void of the positive electrode + the void of the negative electrode + the void of the separator). The weight of the electrolyte filling the above-mentioned all voids can be calculated as (total void amount) × (true density of the electrolyte).

The weight of the electrolyte solution calculated above is set to 100%
In the present invention, a battery having charge / discharge characteristics equal to or higher than that of the conventional battery impregnated with the reference amount can be formed by using the electrolyte solution in an amount smaller than the amount filling the voids of 100%. That is, the amount of the electrolyte is 1
In the charge / discharge cycle in the case of the impregnation by the amount of 00%, the discharge capacity and the charge capacity are almost equal from the beginning, and the decrease in the discharge capacity with the increase in the number of cycles is small.

In the charge / discharge cycle at 60 ° C. when the electrolyte is impregnated with the amount of the electrolyte reduced to 85% of the void volume, the discharge capacity and the charge capacity are almost equal from the beginning, and the discharge capacity and the discharge capacity with the increase in the number of cycles 1 was substantially on the same curve as shown in FIG. 1, and was equivalent to the case where the amount of the electrolyte was a reference amount of 100%. Although the discharge capacity was confirmed at 20 ° C., the discharge capacity did not decrease even when the amount of the electrolytic solution was reduced to 85%. Therefore, the cycle characteristics deteriorate as shown in the examples when the electrolyte is excessively injected into the voids, but the battery characteristics deteriorate even when the amount of the electrolyte is as small as about 85% of the voids. Indicates that there is no.

[0014]

The present invention will be specifically described below with reference to examples. (Example) In this example, LiMn ₂ O ₄ (manufactured by Honjo Chemical Co., Ltd., having an average particle size of 25 μm, true density of 4.28 g)
/ Cm ³ ) and conductive material (TB # 550 made by Tokai Carbon)
0, true density 1.825 g / cm ³ ), polyvinylidene fluoride (KF polymer manufactured by Kureha Chemical Industry, true density 1.
765 g / cm ³ ) at 86.54: 6.73 respectively.
A mixture of 6.73% by weight was coated and dried on both sides of a 20 μm-thick aluminum foil, and roll-pressed to use as a strip-shaped positive electrode. The shape of the strip-shaped positive electrode is 54 m
mx 450 mm, the thickness of one side of the aluminum foil of the positive electrode is 45 µm, and the weight of the positive electrode is 4.857 g.

Therefore, the volume of the positive electrode portion = 5.4 ×
A 45 × 45 × 2/10000 = 2.187cm 3. Volume of LiMn ₂ O ₄ in the above electrode portion = 4.857 ×
0.8654 / 4.28 = 0.982 cm ³ Volume of the conductive material in the above electrode portion = 4.857 × 0.067
3 / 1.825 = 0.185 cm ³ Volume of the binder in the above electrode portion = 4.875 × 0.067
3 / 1.765 = 0.185 cm ³ , and the gap in the positive electrode portion = 2.187− (0.982 + 0.179 +
0.185) = a 0.841cm ^3.

The negative electrode is made of artificial graphite (MCMB25-28 manufactured by Osaka Gas Chemicals, average particle size 25 μm, true density 2.22 g / cm ³ ) as a negative electrode active material, and polyvinylidene fluoride (Kureha Chemical Industry) as a binder. KF polymer, true density 2.22 g / cm ³ ) were mixed at a ratio of 95: 5% by weight, coated and dried on both sides of a 10 μm-thick copper foil, and roll-pressed to obtain a negative electrode.

The negative electrode is 56 × 500 mm and has a negative electrode.
The thickness of one side of the copper foil is 45 μm, and the weight of the negative electrode is
3.054 g. Therefore, the volume of the negative electrode part = 5.6 × 50 × 2/10000 =
2.52cm^Three Volume of artificial graphite in negative electrode part = 3.054 × 0.95
/2.22=1.307cm^Three Volume of binder in negative electrode portion = 3.05 × 0.05 / 1.
765 = 0.087cm^Three Void of negative electrode part = 2.52- (1.307 + 0.08)
7) = 1.026 cm ^ThreeBecomes

The battery is manufactured by winding the above-mentioned band-shaped positive and negative electrodes on a roll via a separator (manufactured by Tonen Talpis, PE, 25 μm thick, 58 mm wide, 1.5 m long, 40% void ratio). Turn, insert into 18650 battery can, inject electrolyte (1M LiPF ₆ EC + DEC (solvent: 1/1 volume ratio), true density: 1.23 g / cm ³ , manufactured by Toyama Pharmaceutical Co., Ltd.), then close the top cap and seal did.

Therefore, the space in the separator portion = 5.8
× 150 × 0.0025 × 0.4 = 0.87 cm ³ , and the total gap of the positive electrode part, the negative electrode part, and the separator part =
0.841 + 1.126 + 0.87 = 2.837 cm ³
Becomes Weight of electrolyte filling all voids = 2.837 × 1.
23 = 3.490 g.

In this embodiment, the amount of the electrolyte to be injected is set to 85% of the weight of the electrolyte filling all the voids (the weight of the injected electrolyte is 2.9).
7 g), 100% (injection electrolyte weight 3.49 g), 15
0% (injection electrolyte weight 5.24 g) and 200%
(Injection electrolyte weight) and various batteries were prototyped, and cycle characteristics and rate characteristics were examined. When the electrolyte is injected at 100% or more, the pressure is reduced (−630 mmHg) and the pressure is increased (2 kg / c) while the electrolyte is supplied in excess.
m ² ) was alternately repeated 5 times for 30 seconds each time, so that the roll-shaped electrode was sufficiently impregnated with the electrolytic solution, the excess electrolytic solution was discharged with the opening of the battery can facing downward, and then the insufficient electrolytic solution was removed. Further injections were made. When excess electrolyte was discharged after the electrolyte was sufficiently impregnated, 100% calculated above was used.
%, The above calculation result is appropriate.

In the case of 85%, after 2.97 g of electrolyte was injected, the pressure was reduced (-630 mmHg) and the pressure was increased (2
kg / cm ² ) were alternately repeated 5 times for 30 seconds, and the top cap was caulked and sealed. Each battery was left at room temperature for one week after injecting the electrolyte,
The cycle characteristics were evaluated under the following atmosphere. The charge and discharge conditions at the time of the cycle characteristic evaluation were constant current charging (4.2 V, 1 mA / c).
m ² ) / constant current discharge (3.0 V, 1 mA / cm ² ). After injecting the electrolytic solution and leaving it at room temperature for one week, the rate characteristics were evaluated in a 20 ° C. atmosphere. The charge / discharge conditions at the time of rate characteristic evaluation are constant current and constant voltage charge (4.2
V, 1 mA / cm ² , 3 hr) / constant current discharge (3.0
V, 0.5 to 8 mA / cm ² ).

FIG. 1 shows the cycle characteristics at 60 ° C. of the batteries having different amounts of injected electrolyte on the horizontal axis.
The vertical axis represents the discharge capacity. 1 electrolyte
00% and 85% are almost on the same curve, indicating that the cycle characteristics are the same. When the amount of the electrolyte was 200%, discharging and discharging became impossible in 20 cycles. The reason for this is that the charge capacity is clearly larger than the discharge capacity from the beginning of the evaluation, and it is considered that there is a high possibility that a micro short circuit has occurred due to the large amount of the electrolytic solution. This phenomenon was obtained with good reproducibility.

In the case where the amount of the electrolyte is 150%, the charge capacity is slightly larger than the discharge capacity from the initial stage of the evaluation. However, the charge / discharge does not become impossible, and the decrease in the discharge capacity with the increase in the number of cycles does not occur. The liquid volume was suppressed more than the case of 200%. When the amount of the electrolyte was 100%, the discharge capacity and the charge capacity were almost equal from the initial stage of the evaluation, and the decrease in the discharge capacity with the increase in the number of cycles was further suppressed as compared with the case where the amount of the electrolyte was 150%.

When the amount of the electrolyte is further reduced to 85%, the discharge capacity and the charge capacity are almost equal from the initial stage of the evaluation, and the decrease in the discharge capacity with the increase in the discharge capacity and the number of cycles is smaller than that in the case of the 100% electrolyte. It was equivalent. In addition, 20
The discharge capacity was also checked at 0 ° C.
Even when the discharge capacity was reduced to 5%, the discharge capacity did not decrease. Therefore, the cycle characteristics are deteriorated when the electrolyte is excessively injected into the voids, and are improved by injecting only 100% into the voids, but deteriorate even with a small amount of about 85% of the voids. There is no.

FIG. 2 is a graph showing the rate characteristics at 20 ° C. of the batteries differing only in the amount of the injected electrolyte, in which the horizontal axis represents the discharge current density and the vertical axis represents the discharge capacity. 8 electrolytes
The discharge capacity of the battery reduced to 5% is such that when the discharge current density is low at 4 mA / cm ² or less, the amount of the electrolyte is 100%.
~ 150%, but the discharge current density is 6 m
At a high discharge current density of A / cm ² or more, the amount of the electrolytic solution was larger than that when the amount of the electrolyte was 100 to 150% as shown by the mark ○. Therefore, the rate characteristic is that the electrolyte is 85% of the gap.
It was found that when the amount was reduced to the extent, the value was slightly improved.

It is generally said that, when the electrolyte is insufficient, the ionic conductivity is reduced, so that the internal resistance of the battery is increased and the discharge capacity is reduced.
If the gap is about 85%, the amount of the electrolyte can be reduced without impairing the battery performance.

[0027]

The lithium secondary battery of the present invention is impregnated with the electrolyte in an amount smaller than the total amount of the voids in the positive electrode, the negative electrode and the separator. Since the electrolyte used for lithium secondary batteries is non-aqueous, expensive and flammable, the amount of electrolyte can be reduced as compared to the conventional case where the total amount of voids is filled, and the price of lithium secondary batteries is reduced and the This is effective for improving the safety of the vehicle.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of the amount of electrolyte on the 60 ° C. cycle characteristics in Examples.

FIG. 2 is a graph showing the effect of the amount of electrolyte on rate characteristics in Examples.

Claims (1)

[Claims] 1. A lithium secondary battery formed by winding a strip-shaped positive electrode and a negative electrode with a separator interposed therebetween, wherein the positive electrode, the negative electrode, and the separator are impregnated with an electrolytic solution. A lithium secondary battery characterized in that the amount of electrolyte impregnated in the positive electrode, the negative electrode, and the separator is smaller than the total amount of voids in the positive electrode, the negative electrode, and the separator.