JP2000277164A - Battery performance recovery method of lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively recover battery performance against continuous charge- discharge in a high-temperature environment, capacity degradation during storage and power degradation by keeping the battery in an overdischarge state of which overcharge voltage is in a specific range when the battery performance is degraded. SOLUTION: By bringing a battery of which battery performance is degraded into an overdischarge state, a film of a Li containing compound formed on the surface of a negative electrode is decomposed and Li in the film is dissolved in an electrolyte and returns to a positive electrode. Thereby, the positive and negative electrodes return to a state like that of a new product, so that the battery performance is recovered. The battery voltage in overdischarge is set to 1.2-2.5. When the voltage is too low, the film of the Li containing compound formed on the surface of the negative electrode is entirely decomposed, this situation is not desirable for the stabilization of charge and discharge, and metal ions are dissolved from a negative electrode collector so that the adhesion between the collector and a negative electrode material is degraded. When the voltage is too high, the recovery performance of the battery tends to be degraded as well.

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, and more particularly, to a method for restoring battery performance when battery performance is reduced during use or during storage.

[0002]

2. Description of the Related Art In recent years, attention has been paid to the fact that lithium secondary batteries have a high voltage and a high energy density and can be reduced in size and weight. As a positive electrode material of the lithium secondary battery, lithium cobalt oxide (LiCoO
₂ ) is used, and then this material is considered to be problematic in terms of cost, resources, etc., and a lithium manganese composite oxide (LiMn ₂ O ₄ ) -based material having a spinel type crystal structure has been used. Further, the above-mentioned LiCoO ₂
Development of a lithium-nickel composite oxide (LiNiO ₂ ) -based material having the same crystal structure (ordered layered rock salt type structure) as that described above is also underway.

As the negative electrode material, carbon materials such as artificial graphite and carbon black are generally used as materials capable of absorbing lithium ions in an electrolytic solution and releasing the lithium once absorbed into the electrolytic solution. I have.

Further, non-aqueous ethylene is used as an electrolyte.
Carbonate (EC) and propylene carbonate (P
C) and diethyl carbonate (DEC)
Lithium hexafluorophosphate (LiPF)₆) And 4
Lithium borate (LiBF ₄) Etc. are used
You.

In this lithium secondary battery, lithium on the positive electrode elutes into the electrolytic solution during charging, lithium ions in the electrolytic solution are occluded by the negative electrode, and lithium occluded by the negative electrode during discharging discharges the lithium in the electrolytic solution. The lithium ions in the electrolytic solution are returned to the positive electrode, and the battery performance is exhibited by repeating this behavior. Its operating voltage range is about 3 to 4.2 V, and the average discharge voltage is about 3.7 V, which is higher than that of a conventional battery.

[0006]

However, in this type of lithium secondary battery, battery capacity and power are reduced by repeated use of charge and discharge. The main reason is that the Li-containing compound is gradually deposited on the surface of the negative electrode due to repeated charge and discharge, and the amount of movable Li ions is reduced. And this causes a decrease in battery capacity,
In addition, the formation of a passivation film on the negative electrode surface increases the DC resistance of the battery and significantly reduces the battery power. In addition, there is a problem that the capacity and power decrease during repeated charge / discharge and high-temperature storage under a high-temperature use environment (for example, 60 ° C.) are greater than those of a nickel-metal hydride storage battery.

[0007] On the other hand, conventionally, it has been attempted to suppress a decrease in capacity by adding an additive to an electrolytic solution or surface treatment of a negative electrode. However, no drastic solution was achieved, and at most about several hundred cycles, 80% of the initial capacity could be maintained by repeated charging and discharging at a high temperature (60 ° C.).
This level is sufficient for applications in portable electronic devices that have a mild use environment.However, considering future applications to energy sources for electric vehicles, continuous driving in high-temperature environments and long-term parking There is a serious problem later that in the worst case, you get stuck.

The problem to be solved by the present invention is that when using a lithium secondary battery, a high temperature environment (for example, 60 ° C.)
It is an object of the present invention to provide a method of restoring the battery performance when the capacity or power of the battery decreases during continuous charge / discharge or storage in the above.

[0009]

In order to solve this problem, a method for restoring the performance of a lithium secondary battery according to the present invention is described by using Li
In a lithium secondary battery using a transition metal oxide containing as a positive electrode, when battery performance is reduced, the battery is maintained in an overdischarge state in an overdischarge voltage range of 1.2 V to 2.5 V. The gist is that.

In this case, the “composite oxide containing Li” used for the positive electrode includes LiCoO ₂ , LiNiO
₂ , a composite oxide of lithium and a transition metal such as LiMn ₂ O ₄ is preferred.

The negative electrode is not limited as long as it is a carbon material capable of occluding and releasing lithium ions in the electrolyte during charge and discharge, and artificial graphite and carbon black are preferably used.

When the battery performance is reduced, the over-discharge state is set to recover the battery performance, whereby the film of the Li-containing compound formed on the surface of the negative electrode is decomposed, and the Li in the film is dissolved in the electrolyte. It will melt and return to the positive electrode. Therefore, the positive electrode and the negative electrode are as if they were new, and the battery performance is restored.

In this case, if the applied voltage (overdischarge voltage) in the overdischarged state is too low, the L
The coating of the i-containing compound is completely decomposed. It is desirable that the film of the Li-containing compound on the surface of the negative electrode is slightly formed to stabilize charge and discharge. In particular, since a film of the Li-containing compound is formed on the surface of the negative electrode during the first charge, stable charge and discharge can be performed thereafter.
It is necessary to avoid that the coating of the i-containing compound is completely decomposed.

If the battery voltage at the time of overdischarge is low, metal ions (copper ions) elute from the negative electrode current collector (usually a copper foil is used). Adhesion with the film extremely decreases. As a result, the electrode resistance increases, causing a reduction in battery capacity and power. For this reason, the range of the overdischarge voltage needs to be "1.2 V or more", so that the battery performance reduced during use or storage of the battery can be recovered.

Incidentally, even if the battery voltage at the time of overdischarge is too high, the recovery performance of the battery tends to decrease.
5 V or less "(not exceeding 2.5 V).

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a schematic configuration of a cylindrical lithium secondary battery. The lithium secondary battery 10 has a battery can 12 having an electrode assembly 14 formed by spirally winding a positive electrode sheet and a negative electrode sheet via a separator. The positive current collecting lead 16 drawn from the positive electrode sheet is connected to a cap 17 attached to the battery can 12, and the negative current collecting lead 18 drawn from the negative sheet is connected to the battery can 12. I have. The inner bottom surface of the battery can 12 and the electrode assembly 1
An insulator (insulating plate 20) is mounted on the upper part of each of the four.

Next, a method of manufacturing this battery will be described.
You. The positive electrode active material includes a lithium matrix having a spinel crystal structure.
A gangue composite oxide was used. And for the production of the positive electrode,
Lithium carbonate (Li₂CO₃) And manganese dioxide (Mn)
O₂) In a molar ratio of Li: Mn = 1.1: 1.9.
Mix well using ethanol as solvent. mixture
Is performed by a ball mill, and after drying the mixed particles, 85
Hold at 0 ° C. for 8 hours in an air stream, then reach room temperature for 1 hour.
The furnace was cooled at a rate of ° C / minute, and lithium manganate (Li _1.1M
n_1.9O₄) A powder was obtained. Using this as an active material, conductive material
5% by weight as graphite, polyfluoride as binder
After mixing 5% by weight of vinylidene, n-methylpyrrolid
Was added as a solvent to prepare a positive electrode mixture paste. This
Paste on both sides of the current collector made of aluminum foil.
The cloth was then dried and rolled to form a positive electrode sheet.

Next, artificial graphite is used as the negative electrode active material.
After 5% by weight of polyvinylidene fluoride as a binder was mixed with the artificial graphite powder, n-methylpyrrolidone was added as a solvent to prepare a negative electrode mixture paste. This paste was applied to both sides of a current collector made of copper foil, dried and rolled to obtain a negative electrode sheet.

Both the positive electrode sheet and the negative electrode sheet are provided with an uncoated portion other than the coated portion, and a current collecting lead is welded to the uncoated portion, and the positive electrode sheet and the negative electrode sheet are made porous. It was wound together with a separator made of a polypropylene film to form a spiral electrode assembly 14.
After inserting the electrode assembly 14 into the battery can 12, a non-aqueous electrolyte is injected into the battery can.
One solution is prepared by mixing ethylene carbonate (EC) and diethylene carbonate (DEC) in a volume ratio of 3: 7.
mol / l lithium hexafluorophosphate (Li
It was prepared by dissolving the PF _6).

Next, a charge / discharge test was performed on this lithium secondary battery, which will be described. First, initial charge / discharge conditions when the first initial charge / discharge is performed will be described. This first charge is 0.2 mA / c
The measurement was performed at a constant current of m ² until the voltage reached 4.2 V (hereinafter, this is referred to as “charging end voltage” as 4.2 V). Then, the charging at the constant voltage of 4.2 V was continued, and the total charging time was 6 hours. Next, after the completion of the charging, the discharging was started. This first discharge was performed at a constant current of a discharge current density of 0.2 mA / cm ² until the voltage reached 3 V (hereinafter, this is referred to as “discharge end voltage” is 3 V). Such a charging method is referred to as “constant current / constant voltage charging−
This is referred to as “constant current discharge”. The ambient temperature was 20 ° C.

After the first charge / discharge, the charge end voltage is further reduced to 4.2 V and the discharge end voltage is set to 3 V.
Constant current constant voltage charge-constant current discharge was performed. The current density was 1 mA / cm ^{2 for} both charging and discharging, the total charging time was 2 hours, and the ambient temperature was 20 ° C. This charge / discharge was defined as one cycle, and a total of four cycles of charge / discharge were performed. The discharge capacity of the fourth cycle (fifth cycle in total) is referred to as “reference capacity”.
And The above five cycles of charging and discharging are hereinafter referred to as “initial charging and discharging”.

After the five cycles of "initial charge / discharge", a cycle durability test was performed. The conditions will be described. After measuring the reference capacity by the “initial charge / discharge”, the lithium secondary battery is charged to a final charge voltage of 4.
The battery was charged and discharged at a discharge termination voltage of 3 V at 2 V. In this case, immediately after the charging end voltage is reached, the state is immediately shifted to the discharging state.
Such a charging / discharging method is referred to as “constant current charging-constant current discharging”. The current density was 1 mA / cm ² for both charging and discharging, and the ambient temperature was 60 ° C. With this charge / discharge as one cycle, a durability test was performed for a total of 100 cycles (105 cycles in total). This battery was referred to as “Comparative Sample C1”. This one
The repetition of the charge and discharge of 00 cycles is referred to as “100 cycle durability test”.

After the cycle endurance test, in order to recover the performance of this lithium secondary battery, a discharge end voltage of 1.5
V, constant-current constant-voltage discharge-constant-current charging was performed at a charge end voltage of 3 V. The current density is 0.1 mA / cm for both charging and discharging.
^2. The holding time at a discharge end voltage of 1.5 V was 1 hour, and the ambient temperature was 20 ° C. This battery was referred to as “Sample 1 of this example”. The batteries after the 100-cycle endurance test different from Sample 1 of this example were charged at a constant current of 0.1 mA / cm ² , and the discharge end voltages were set to 0.5, 1, ² , and ^2.5 V, respectively. At a final voltage of 3 V, constant-current constant-voltage discharge-constant-current charging was performed. The retention time at the discharge termination voltage was 1 hour each.
These batteries were referred to as “Comparative Sample C2” and “Comparative Sample C”, respectively.
3 "," Example 2 ", and" Example 3 ". Hereinafter, this one-cycle discharging and charging will be referred to as “performance recovery processing”.

Next, after performing the “performance recovery processing”,
Each battery was charged at a final charge voltage of 4.2V and a final discharge voltage of 3V.
Constant current constant voltage charge-constant current discharge was performed. The current density was 1 mA / cm ^{2 for} both charging and discharging, the total charging time was 2 hours, and the ambient temperature was 20 ° C. The discharge capacity at this time was defined as “capacity after durability”. Hereinafter, this one-cycle charge / discharge will be referred to as “capacity check”.

As described above, the initial charge / discharge (1 cycle), the 100-cycle durability test, the performance recovery process (1 cycle), and the capacity check (1 cycle) are sequentially performed on each battery, and then the performance recovery of each battery is performed. I checked the degree. This degree of performance recovery is determined by the ratio of the capacity after 100 cycles durability and the reference capacity. The results are shown in Table 1 below.
Note that the performance recovery processing was not performed on “Comparative Example C1”.

[0026]

[Table 1]

As is clear from the data in Table 1, Samples 1, 2, and 3 of the present invention, which had been subjected to the performance recovery processing in which the discharge end voltage was higher than 1 V, were Comparative Examples C1 to which the performance recovery processing was not performed. This indicates that the decrease in capacity after cycle durability is greatly suppressed. On the other hand, when the discharge end voltage is 1
The post-durability capacities of Comparative Examples C2 and C3 subjected to the performance recovery processing of V or less were lower than Comparative Example C1. This was because the surface coating of the graphite-like negative electrode was almost completely decomposed, and the copper foil used for the negative electrode core material was dissolved in the electrolytic solution to lower the adhesiveness of the negative electrode material.

Next, the "discharge end voltage" was selected over more steps to test the tendency of the battery performance to recover. The results are shown in FIG. In FIG. 2, the recovery tendency of the battery performance is shown by the relationship between "discharge end voltage" and "capacity after 100 cycles durability / reference capacity". From FIG. 2, in order for the capacity after 100 cycles of durability to maintain 90% or more of the reference capacity, the range of the discharge end voltage is 1.2 V or more.
It can be understood that the voltage is desirably 2.5 V or less.

Further, for each sample, a 100-cycle durability test at 60 ° C., a performance recovery process at 20 ° C., a 100-cycle durability test at 60 ° C., and a performance recovery process at 20 ° C. Cycle endurance test and 20 ℃
, And the durability test was repeated for a total of 1000 cycles. FIG. 3 shows a relationship between the discharge capacity and the number of endurance cycles in constant current charging-constant current discharging in comparison between the sample 1 of the present embodiment and the comparative sample C1.
As shown in FIG. 3, when compared with the comparative sample C1, the capacity of the sample 1 of the present embodiment returned to almost the initial value every 100 cycles, and the tendency of the discharge capacity to decrease after the endurance of 1000 cycles was almost recognized. Not confirmed.

Although no data was shown, not only the battery capacity but also the power were recovered by the current performance recovery method. Furthermore, by performing the performance recovery method of the present invention, a capacity of 90% or more of the reference capacity was obtained for a battery having a reduced capacity after storage in a high-temperature environment (about 60 ° C.). In addition, LiCoO ₂ or its partially substituted system, LiCoO ₂
It was also confirmed that the performance of a battery using NiO ₂ or a partial substitution system thereof was almost similarly restored by intentional overdischarge, and as a result, deterioration of capacity due to long-term high-temperature cycle durability could be suppressed.

The present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, in the above embodiment, the charging end voltage is 3 V and the current density is 0.1 m in the performance recovery process.
A / cm ² (both during charging and discharging), but this condition can be changed as appropriate. In the above embodiment, LiMn ₂ O ₄ was described as a positive electrode material.
Needless to say, the present invention can be applied to iCoO ₂ , LiNiO _{2, and the} like. Further, spherical graphite was used as the negative electrode material. Compounds, lithium alloys and the like can also be used.

[0032]

According to the method for restoring the performance of a lithium ion secondary battery according to the present invention, the battery performance is restored by maintaining the battery in an overdischarged state under predetermined conditions, thereby extending the life of the battery. , Not only the economic effect is great,
Since the battery capacity and the power are maintained at a high level even by repeated use, the usage as a power supply battery for various information communication devices such as a personal computer and a mobile phone is further increased. Above all, the battery capacity and power are maintained even in a use environment at a relatively high temperature (about 60 ° C.) such as a power supply for an automobile, so that the application to severe use under the use temperature condition is expanded. There is also a benefit that can be achieved.

[Brief description of the drawings]

FIG. 1 is a diagram showing a schematic configuration of a lithium secondary battery as one embodiment to which the present invention is applied.

FIG. 2 is a diagram showing a relationship between “discharge end voltage” and “after 100 cycles of durability / reference capacity” in a battery performance recovery process.

FIG. 3 shows a comparison between “Sample 1 of the present example” subjected to the performance recovery processing every 100 cycles and “Comparative Sample C1” not subjected to the performance recovery processing. FIG. 4 is a diagram illustrating a relationship between a capacity and the number of cycles.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tatsuo Noritake 41-Cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central Research Laboratory Co., Ltd. No. 41, Yokomichi, 1st place, Toyota Central Research Laboratory Co., Ltd. F term (reference) 5H029 AJ02 AK03 AL07 AL08 AM03 AM05 AM07 BJ02 BJ14 HJ18 5H030 AA01 AA10 AS08 BB01 FF43

Claims (1)

[Claims] 1. In a lithium secondary battery using a composite oxide containing Li for a positive electrode, an overdischarge voltage of 1.2 V or more in order to recover battery performance that has deteriorated during use or during storage. A method for recovering battery performance of a lithium secondary battery, comprising maintaining the battery in an overdischarge state of 5 V or less.