KR19990057615A - Lithium secondary battery - Google Patents

Abstract

The present invention is mixed by adding 1 to 10 parts by weight of the lithium ion selective carrier, 20 to 50 parts by weight of the polymer support, 45 to 75 parts by weight of the membrane solvent, and 0.5 to 5 parts by weight of the additive to the carbon material negative electrode surface capable of storing and releasing lithium ions. After the normal methyl pyrrolidene (NMP) is added to completely melt the lithium ion selective permeable membrane prepared by melting, a cathode comprising a lithium composite oxide, and a lithium ion conductive electrolyte comprising a lithium ion conductive electrolyte As a result, the capacity deterioration rate and capacity retention rate are excellent, and the pressure increase inside the battery due to decomposition of the solvent can be suppressed, thereby increasing the stability of the battery.

Description

Lithium secondary battery

The present invention relates to a lithium ion secondary battery, and more particularly, to a lithium ion comprising a negative electrode using a lithium ion selective permeable membrane capable of improving self-discharge characteristics and cycle characteristics of a battery and improving safety at high temperatures. It relates to a secondary battery.

In general, when a charge-discharge cycle is repeated at a charge potential of 4 V or higher in a lithium ion secondary battery and when the battery is stored for a long time in a charged state, the battery characteristics rapidly decrease. This is because electrolytes, especially solvents, decompose at the surface of the electrode at high potentials to generate gas, thereby increasing the internal pressure of the battery, and deteriorating the performance of the battery by deteriorating the electrode plate.

In the interlayer occlusion-type electrode, the surface electrochemical reaction is variable. Therefore, in order to improve the performance, cycle characteristics, and storage of a high capacity lithium secondary battery capable of producing a potential of 4 V or higher, a technique of controlling the reaction between the nonaqueous electrolyte and the electrode is inevitable. It is true.

The primary method of controlling the electrode reaction up to the present is to suppress the decomposition of the solvent as much as possible by changing the type and composition of the solvent in the electrolyte, and the solvent in the electrolyte mainly has a high dielectric constant sufficient to dissolve the salt sufficiently. It is used by mixing a cyclic compound (cyclic compound) having a low dielectric constant but excellent in non-cyclic compound (non-cyclic compound).

When the battery is charged, lithium ions in the electrolyte diffuse into the negative electrode. The non-cyclic solvent in the solvent solvates lithium ions and easily enters the negative electrode. However, a solvent having a cyclic structure and a solvent having a large molecular size When the diffusion into the interlayer of the negative electrode, the decomposition reaction of the solvent occurs at the interlayer inlet of the negative electrode, which is a major cause of lowering the initial charge and discharge effect of the battery.

In addition, the continuous diffusion of the solvent into the pole plate destroys the structure of carbon during repeated charge and discharge cycles, and decreases the binding force between the pole plate and the current collector, copper (Cu), thereby degrading battery performance, in particular charging the battery. When stored for a long time in a state, the solvent in contact with the electrically active negative electrode surface is continuously decomposed, resulting in deterioration of battery performance.

As another method of controlling the electrode reaction, a method of protecting the electrode with a polymer film or the like is used. This method is a cathodic protection film, in which a surfactant is coated on the surface of a cathode to lower the potential of the cathode than the decomposition voltage of the solvent to suppress decomposition of the solvent. will be. However, in this method, an additional reaction may occur at the electrode because the salt used as the surfactant is a different component from the salt in the electrolyte, and the diffusion of lithium ions becomes more difficult due to the film resistance of the two protective films.

An object of the present invention is to solve the conventional problems as described above, the membrane resistance is low and the permeation that can inhibit the decomposition of the solvent and selectively diffuse only lithium ions into the cathode while blocking the contact between the solvent and the electrode plate It is to provide a lithium secondary battery consisting of a negative electrode using a film.

1 is a structural diagram of a negative electrode coated with a lithium ion selective permeable membrane on the negative electrode surface according to the present invention,

2 is a graph showing the change in discharge capacity according to the charge and discharge cycle of the battery,

3 is a graph showing capacity retention rate according to the charge / discharge cycle of the battery.

* Description of the major part symbols in the drawings

DESCRIPTION OF SYMBOLS 1 Copper foil 2 Negative electrode 3 Lithium ion selective permeable membrane

That is, the present invention is characterized in that the lithium secondary battery comprises a negative electrode coated with a lithium ion selective permeable membrane on a carbon material negative electrode surface capable of storing and releasing lithium ions, a positive electrode made of a lithium composite oxide, and a lithium ion conductive electrolyte solution. It relates to a lithium secondary battery.

The negative electrode of the lithium secondary battery of the present invention is a carbon-based material capable of absorbing and releasing lithium ions in order to suppress structural destruction of carbon due to the penetration of a solvent in the electrolyte into the negative electrode and to reduce the self-discharge capacity of the battery. The cathode surface is coated with a membrane that selectively permeates only lithium ions.

At this time, the electrolyte is cyclic carbonate (cyclic carbonate), diethyl carbonate (DEC), dimethyl selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), γ- butyrolactone (GBL) Lithium salt is dissolved in a mixed solvent of at least one selected from carbonate (DMC) and ethyl methyl carbonate (EMC).

The solvent in the electrolyte is preferably composed of 20 to 40% of a cyclic carbonate and 40 to 80% of an acyclic carbonate by volume, more preferably 30% of ethylene carbonate, 30% of diethyl carbonate and 40% of dimethyl carbonate.

The membrane solution for preparing a lithium ion selective permeable membrane used in the present invention is added 1 to 10 parts by weight of a lithium ion selective carrier, 20 to 50 parts by weight of a polymer support, 45 to 75 parts by weight of a membrane solvent, and 0.5 to 5 parts by weight of an additive. After mixing by adding normal methylpyrrolidin (NMP) is prepared completely dissolved.

In the membrane solution for preparing a lithium ion selective permeable membrane, a lithium ion selective carrier is N, N'-diheptyl-N, N'5,5-tetramethyl-3,7-dioxanonandiamide (N, N '). -diheptyl-N, N'5,5-tetramethyl-3,7-dioxanonane diamide (ETH 149), N, N'-dioctylhexyl-N, N'-diisobutyl-cis-cyclohexane-1,2 Dicarboxylic amide (N, N'-dioctylhexyl-N, N'-diisobutyl-cis-cyclohexane-1,2-dicarboxylicamide: ETH1810), 1,4,7,10-tetraoxa-cyclododecane (1, 4,7,10-tetraoxa-cyclododecane: 12-crown-4), 6,6-dibenzyl-1,4,8-11-tetraoxacyclotetradecane (6,6-dibenzyl-1,4,8- 11-tetraoxacyclotetradecane: 6,6-dibenzyl-1,4-crown-4) At least one selected from.

The polymer support of the membrane solution for producing a lithium ion selective permeable membrane is one selected from polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), and polyvinyl chloride (PVC).

As the membrane solvent of the membrane solution for preparing the lithium ion selective permeable membrane (3), o-nitrophenyl-n-octyl ether, tris (2-ethylhexyl phosphate) (tris (2-ethylhexyl) phosphate), dibutyl sebacate, and dioctyl phthalate.

In the lithium secondary battery anode 2 of the present invention, normal methylpyrrolidene is added to a mixture of polyvinylidene fluoride powder with mesocarbon microbead or artificial graphite powder and a binder heat-treated at a temperature of 2000 ° C. or higher. After the slurry was made into a copper foil (1) and coated on a copper foil and dried for a predetermined time to volatilize normal methylpyrrolidin, the lithium ion selective membrane solution prepared above was spray coated to a thickness of 10 to 20 mm on the surface of the negative electrode. It is prepared by coating a membrane and drying at 100 ° C. for 10 minutes to volatilize normal methylpyrrolidin and then press the electrode plate.

The positive electrode was prepared by dissolving lithium cobalt oxide (LiCoO ₂ ) powder, carbon black powder, and polyvinylidene fluoride powder in normal methylpyrrolidene to prepare a slurry, coating both sides of aluminum foil, and drying and rolling in the same manner as the negative electrode. It is prepared by the Lessing is not particularly limited for the present invention.

The separator of the battery uses a porous membrane made of polyethylene, and the electrolyte may be used by dissolving lithium hexafluorophosphorus (LiPF ₆ ) as an electrolyte salt in a mixed solvent of ethylene carbonate, diethyl carbonate, and dimethyl carbonate.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited by the following Examples.

Example

1 to 10 parts by weight of the lithium ion selective carrier, 20 to 50 parts by weight of the polymer support, 45 to 75 parts by weight of the membrane solvent, and 0.5 to 5 parts by weight of the additive are added, mixed, and then dissolved by adding normal methylpyrrolidin (NMP). . N, N'-diheptyl-N, N'5,5-tetramethyl-3,7-dioxanonaneamide (N, N'-diheptyl-N, N'5,5-tetramethyl as a lithium ion selective carrier -3,7-dioxanonane diamide: ETH 149) 1 part by weight, 35 parts by weight of polyvinylidene fluoride (PVDF) as a polymer support, and mixed with 1 part by weight of potassium tetrakis (para-chloride-phenyl) boron salt as an additive, 63 parts by weight of tris (2-ethylhexyl) phosphate is added to the membrane solvent to dissolve the mixture. Add enough normal methylpyrrolidin (NMP) to this and dissolve the mixture completely using a magnetic pole.

As a negative electrode active material, normal methylpyrrolidin is added to 9 parts by weight of polyvinylidene fluoride powder mixture with mesocarbon microbead or artificial graphite powder and binder heat-treated at a temperature of 2000 ° C. or above to make a slurry into copper foil. After coating to dry to volatilize normal methylpyrrolidin, the lithium ion selective membrane solution prepared above is spray-coated to a thickness of 10 to 20mm on the surface of the negative electrode to apply a membrane, and dried at 100 ℃ for 10 minutes to normal methyl blood It was prepared by volatilizing rolledden and then rolling the electrode plate at a pressure of 0.5 t / cm.

The positive electrode was prepared by dissolving 87 parts by weight of lithium cobalt oxide (LiCoO ₂ ) powder, 8 parts by weight of carbon black powder and 5 parts by weight of polyvinylidene fluoride powder in normal methylpyrrolidene to prepare a slurry, and then coating it on both sides of aluminum foil. Drying in the same manner as described above was prepared by roll pressing the bipolar plate at a pressure of 1t / ㎠ and cut to a desired size.

The separator of the battery uses a porous membrane made of polyethylene, and the electrolyte has a concentration of 1 mole of lithium hexafluorophosphorus (LiPF ₆ ) in a mixed solvent of ethylene carbonate, diethyl carbonate, and dimethyl carbonate, each mixed at a volume ratio of ₃₀ , 30, ₄₀ . It was prepared by dissolving to.

Comparative example

Except that the lithium ion selective film was not used for the negative electrode was prepared in the same manner as in Example.

The 18650 type battery was fabricated using the components of the battery, and the charge and discharge characteristics as shown in FIGS. 2 and 3 were investigated.

The capacity of the battery was dependent on the discharge capacity of the lithium cobalt oxide used as the active material of the positive electrode, and the discharge capacity of each battery was determined to be 1350 mAh assuming that the number of reactive electrons of the lithium cobalt oxide was 0.5.

The battery characteristics were examined for charge and discharge cycle characteristics at 20 ° C. In the test conditions, charge and discharge current was charged to 4.2V with 1350mA, discharge was repeated up to 3.0V with a discharge current of 280mA, and the test was stopped when the capacity was deteriorated to 80% of the initial discharge capacity.

As can be seen in FIG. 2, the two batteries show little difference in initial discharge capacity. However, as the cycle progresses, the battery having a lithium ion selective film coated on the surface of the negative electrode has a much lower capacity degradation rate. This can be interpreted as deterioration of the electrode plate due to current loss due to decomposition of the solvent on the surface of the cathode during charge and discharge and destruction of the carbon interlayer structure due to penetration of the solvent. The number of cycles maintaining 80% of the initial capacity was 269 cycles of the battery of the comparative example, while the battery of the present invention was 369 cycles.

As can be seen in FIG. 3, the capacity retention rate is very high in the case of the embodiment in which lithium ions are coated on the negative electrode to suppress decomposition of the solvent in the electrolyte, as compared with the comparative example in which the lithium ion selective film is not coated on the negative electrode.

When the lithium ion selective film is not coated on the negative electrode, the lithium ion selection of the present invention is compared with the deterioration of the battery capacity due to the deterioration of the electrode plate due to the decomposition of the solvent on the surface of the negative electrode and the generation of gas and impurities in the battery. When the film is coated on the surface of the cathode, it is possible to prevent side reactions such as the decomposition salts of the solvent and the reaction of the moisture occurring on the surface of the electrode plate and to protect the electrode plate as charging and discharging proceeds.

In addition, as an additional effect, it is possible to expect an effect of increasing the stability of the battery since suppression of the increase in pressure inside the battery due to decomposition of the solvent is suppressed.

As described above, the lithium secondary battery using the lithium ion selective film of the present invention in the negative electrode plate has excellent capacity degradation rate and capacity retention rate, and can increase the stability of the battery by suppressing pressure increase in the battery due to decomposition of the solvent. .

Claims (5)

A lithium secondary battery comprising a negative electrode coated with a lithium ion selective permeable membrane on a negative electrode surface of a carbon material capable of storing and releasing lithium ions, a positive electrode made of a lithium composite oxide, and a lithium ion conductive electrolyte. The method of claim 1, wherein the lithium ion selective permeable membrane is mixed by adding 1 to 10 parts by weight of the lithium ion selective carrier, 20 to 50 parts by weight of the polymer support, 45 to 75 parts by weight of the membrane solvent, and 0.5 to 5 parts by weight of the additive. Lithium secondary battery, characterized in that prepared by completely dissolving normal methylpyrrolidin (NMP). The method of claim 2, wherein the lithium ion selective carrier is N, N'-diheptyl-N, N'5,5-tetramethyl-3,7-dioxanonandiamide (N, N'-diheptyl-N, N'5,5-tetramethyl-3,7-dioxanonane diamide (ETH 149), N, N'-dioctylhexyl-N, N'-diisobutyl-cis-cyclohexane-1,2-dicarboxyamide (N, N'-dioctylhexyl-N, N'-diisobutyl-cis-cyclohexane-1,2-dicarboxylicamide: ETH1810), 1,4,7,10-tetraoxa-cyclododecane (1,4,7,10 -tetraoxa-cyclododecane: 12-crown-4), 6,6-dibenzyl-1,4,8-11-tetraoxacyclotetradecane (6,6-dibenzyl-1,4,8-11-tetraoxacyclotetradecane: 6 , 6-dibenzyl-1,4-crown-4) Lithium secondary battery, characterized in that at least one selected from the group consisting of. The lithium secondary battery of claim 2, wherein the polymer support is selected from polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), and polyvinyl chloride (PVC). According to claim 2, wherein the membrane solvent is o-nitrophenyl-n-octyl ether (o-nitrophenyl-n-octyl-ether), tris (2-ethylhexyl phosphate) (tris (2-ethylhexyl) phosphate), Lithium secondary battery, characterized in that selected from dibutyl sebacate, dioctyl phthalate (dioctyl phthalate).