KR20110103130A - Lithium secondary battery comprising a fast-charging anode without life cycle shortening - Google Patents

Abstract

The present invention relates to a quick-charging negative electrode and a lithium secondary battery including the same, without deterioration of life, and more particularly, in a lithium ion secondary battery having an aqueous negative electrode, the aqueous negative electrode is a negative electrode active material, a conductive agent and a binder The lithium ion secondary battery has a charge rate of 1.7C and a charge amount of 80% or more in 30 minutes, and a capacity of the battery after 80 charge / discharge cycles is 80% or more of its initial capacity. The lithium ion secondary battery which can avoid the battery life degradation by this is provided. Meanwhile, in one aspect of the present invention, the porosity of the aqueous negative electrode may be greater than 30% to 40%, and the total lithium ion concentration may be 1.1 to 2.0 M in the electrolyte of the lithium ion secondary battery.
According to the present invention, it is possible to provide a fast charging type lithium ion secondary battery capable of fast charging at a high charging and discharging speed, including a charging speed of 1.7 C. The present invention provides a battery for charging at 1.7 C, like a lithium ion battery of the prior art. It is characterized by the fact that the cycle life is not lowered, and at the same time, it is eco-friendly because it adopts an aqueous cathode.

Description

Lithium secondary battery comprising a fast-charging anode without life cycle shortening}

The present invention relates to a structure of a lithium polymer battery. More specifically, the present invention relates to a lithium ion battery including a quick charge negative electrode.

As technology develops and demand increases, the performance of portable electronic products continues to increase. Accordingly, the demand for rechargeable secondary batteries as an energy source for portable electronic products is increasing rapidly, and the level required for the performance of secondary batteries is increasing.

Currently, steady and active researches are being conducted in the direction of miniaturization, thinning, weight reduction and high performance of a battery for driving an electronic device. Among such secondary batteries, lithium batteries are used as main driving power sources for these portable devices because of their light weight and high energy density.

In the performance of a lithium battery, the active material constituting the electrode is very important. Lithium-containing transition metal oxides such as LiCoO ₂ , LiNiO ₂ and chalcogen compounds such as MoS2 are widely used as positive electrode active materials of lithium ion batteries. Since these compounds have a layered crystal structure, lithium ions are reversibly inserted or Because it can detach. Metal lithium may also be used as the active material for the negative electrode. However, when the metal lithium is inserted and removed, the needle-shaped precipitates are formed on the surface to reduce the charge and discharge efficiency, and the needle-shaped precipitates come into contact with the anode to cause an internal short circuit. have. Therefore, recently, a material capable of reversibly inhaling and releasing lithium ions has been used as a negative electrode active material. As such a cathode material, a carbon material such as graphite is widely used.

The lithium ion secondary battery has a structure in which a lithium electrolyte is impregnated into an electrode assembly including an anode, a cathode, and a separator. Electrodes of secondary batteries are generally prepared by coating a slurry of an electrode active material on a metal foil. In preparing such an electrode slurry, an electrode mixture composed of an active material and a binder or a binder for adhering it to a thin film for an electrode is prepared by mixing an organic solvent such as N-methylpyrrolidone (NMP). However, NMP is a substance that is harmful to the human body, and thus, the manufacturing process is complicated, and a mechanical device must be used over various processes, and environmental pollution is also a problem.

In order to solve this problem, an alternative method of preparing an active material slurry of an aqueous electrode using a styrene-butadiene rubber (SBR) -based binder which is water as a solvent and is soluble in water is known in the art. SBR is used in the form of dispersion in aqueous solution. Aqueous cathodes using such aqueous binders can simplify the manufacturing process and produce less organic solvent waste than non-aqueous cathodes. However, when only a small amount of the styrene-butadiene rubber (SBR) -based binder is used, the binding force and battery characteristics of the active material are inferior, and thus there is a limit in increasing the bondability of the lithium plate electrode.

On the other hand, as an important part of efforts to improve the performance of the lithium secondary battery, attempts to manufacture a lithium battery with a fast charging speed has been steadily repeated. In summary, the fast-charged battery that can be charged within a short time is a performance that can charge more than 80% of the battery capacity in 30 minutes. When describing the battery behavior during charging and discharging of the battery, the concept of charge rate (C-rate) is used. The charge rate (C-rate) is a concept relative to the capacity of the battery. The charging rate of 1 C is a current for charging or discharging the amount of charge corresponding to the capacity of the battery at one hour. For example, in a 1.2 amp-hour battery, the C / 2 charge rate is 0.6 amps, 1 C is 1.2 amps, and the 2 C charge rate is 2.4 amps.

In the case of commercial lithium ion secondary batteries, a charging rate of 0.5 C during charging and discharging is a standard. On the other hand, although the charging speed of 1.7 C may be used for rapid charging, in this case, the life of the battery is shortened, resulting in a decrease in capacity. For example, a commercially available lithium-ion battery is considered adequate when the battery capacity maintains 80% of its initial capacity after 300 cycles at the standard charge / discharge rate. It was less than 80%.

Due to such a problem, the demand for a product capable of rapid charging as a lithium ion secondary battery having an aqueous negative electrode is very large, but it is not satisfactorily satisfying this.

SUMMARY OF THE INVENTION An object of the present invention is to provide a battery having no lifespan deterioration as a lithium ion secondary battery capable of rapid charging at a high charging speed including 1.7 C.

In order to solve this technical problem, the present invention provides a quick-charge lithium ion secondary battery having an aqueous negative electrode. The aqueous negative electrode of the lithium battery according to the present invention includes a negative electrode active material, a conductive agent and a binder. At this time, the composition ratio may be in the range of the negative electrode active material: conductive agent: binder: 95.0 to 99.9: 0.1 to 1.0: 2.0 to 4.0% by weight, where the binder may be a mixture of an aqueous binder and a cellulose-based dispersant. . The lithium ion secondary battery of the present invention has a charge rate of 1.7C and a charge amount of 80% or more within 30 minutes, and a capacity of the battery after 80 charge / discharge cycles is 80% or more of its initial capacity. .

In one embodiment of the present invention, the porosity of this aqueous cathode is greater than 30% and 40%. In another embodiment of the present invention, the solvent in the electrolyte of the lithium ion secondary battery is ethylene carbonate (EC), propylene carbonate (PC), and Dimethyl carbonate (DMC), any one or a mixture of two or more thereof, characterized in that the total lithium ion concentration is 1.1 ~ 2.0M.

The present invention is a fast-charge type lithium ion secondary battery capable of fast charging at high charge and discharge rates, including a charge rate of 1.7 C, and does not cause a decrease in cycle life of the battery when charging 1.7 C as in the lithium ion battery of the prior art. In addition, the lithium ion battery of the present invention employs an aqueous negative electrode, which is environmentally friendly and the manufacturing process is simple.

Figure 1 after the discharge of the secondary battery of the present invention at 0.5C and repeated 300 cycles at 1.7 C charging rate (a) and the discharge of the prior art secondary battery at 0.5C and repeated 300 times at 1.7C charging rate It is a graph showing the change of the filling rate with respect to the initial capacity of (b).

Hereinafter, the present invention will be described in detail so that those skilled in the art to which the present invention pertains (hereinafter, referred to as those skilled in the art) can easily reproduce the same.

The present invention relates to a lithium ion secondary battery capable of rapid charging at a high charging speed, including 1.7 C, and having no adverse effect on lifespan. The lithium ion secondary battery of the present invention employs an aqueous negative electrode.

The aqueous negative electrode of the present invention includes a negative electrode active material, a binder and a conductive agent.

In the aqueous negative electrode of the present invention, as the negative electrode active material, any material used in this field may be used as a material capable of reversibly sucking and releasing lithium ions. For example, amorphous graphite such as natural graphite, artificial graphite, graphite material of coke or carbon fiber, non-graphitized carbon, and crystalline carbon can be used. Moreover, the composite material which added a small amount of the compound containing a metal element to such carbon-type material can also be used. For example, a compound containing at least one metal element selected from the group consisting of Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, and Ti and the carbon material may be used as a negative electrode active material. It may be.

Such a negative electrode active material plays a very important role in the performance of the battery, it is preferable to include as much content in the total composition as possible in the performance of the battery. In the present invention, 95.9 to 97.9% by weight of the total negative electrode composition is used as the content of the negative electrode active material, and the productivity of the process of manufacturing the water-based negative electrode is ensured while ensuring maximum lithium ion insertion and desorption capacity. When the amount of the negative electrode active material is less than 95.9% by weight, battery performance is deteriorated due to the lack of active material. When the amount of the negative electrode active material is higher than 97.9% by weight, the content of the conductive agent needs to be reduced, resulting in poor electrical conductivity. The fall occurs, and the dispersibility and bonding strength of the active material to the electrode deteriorate, which is not preferable.

In the present invention, the conductive agent serves to support a large amount of current for high speed charge and discharge by increasing the conductivity of the aqueous cathode. The conductive agent is not particularly limited as long as it has conductivity without causing chemical change in the battery. Examples of the conductive agent that can be used include graphite such as natural graphite and artificial graphite; Carbon blacks such as conductive carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fibers and metal fibers; Carbon nanotubes; Metal powders such as carbon fluoride, aluminum and nickel powders; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

In one embodiment of the present invention, conductive carbon black, which is relatively inexpensive and has good conductivity, is used as the conductive agent. Preferably the particle diameter of such conductive carbon black is on the nanometer scale, for example less than 1 μm. When the nanometer-scale conductive carbon black is used as a conductive agent, the effect of the aqueous cathode is improved since the conductivity of the water-based negative electrode is improved than when the micrometer-scale particles are used.

In the present invention, the conductive agent accounts for 0.1 to 1.0% by weight of the total negative electrode composition. Within this range, the conductivity of the battery is increased to support rapid charging while preventing the lifespan from deteriorating. On the contrary, when the content of the conductive agent is less than 0.1% by weight, it is difficult to achieve a sufficient level of conductivity to prevent the deterioration of life, and when the content is more than 1.0% by weight, the binder content must be increased to prevent the weakening of the binding force on the metal thin film, which is the current collector. As a result, there is less conductivity.

In the present invention, the binder serves to bond and evenly disperse the components of the negative electrode, and includes an aqueous binder and a cellulose dispersant suitable for the aqueous negative electrode.

In the present invention, the aqueous binder may include any one of styrene-butadiene rubber, nitrile-butadiene rubber, methyl (meth) acrylate-butadiene rubber, chloroprene rubber, carboxy-modified styrene-butadiene rubber, and modified polyorganosiloxane-based polymer. The above mixture can be used, Preferably styrene-butadiene rubber can be used. In the present invention, the term (meth) acrylic acid refers to a methacrylic acid compound and an acrylic acid compound.

Examples of the cellulose dispersant include carboxymethyl cellulose, carboxyethyl cellulose, aminoethyl cellulose, oxyethyl cellulose or mixtures thereof. Such cellulosic dispersants may be substituted with sodium or ammonium salts.

In the present invention, the binder accounts for 2.0 to 4.0% of the weight of the negative electrode composition. When the content of the binder is within this range, peeling and internal short circuit due to a decrease in the binding capacity can be prevented without impairing the conductivity between the current collector and the carbon material, thereby improving charge / discharge cycle characteristics of the lithium battery. In addition, by improving the dispersibility and increasing the solid content of the carbon material, a lithium battery having a high energy density and excellent safety can be obtained. On the contrary, when the content of the binder is less than 2.0% by weight, the negative electrode material may peel off from the core material or internal short circuit may occur, resulting in a high defect rate. It is disadvantageous because the high rate discharge capacity is reduced.

In order to view the above-described effects using the binder of the present invention, the formulation of the aqueous binder and the cellulose dispersant in a weight ratio of binder: dispersant = 1: 1 to 1: 2 is appropriate.

When the blending ratio of the binder of the cellulose-based dispersant is less than 1: 1, there is almost no viscosity, so that casting of the negative electrode material slurry is difficult, and when it exceeds 1: 2, the viscosity increases, resulting in an inappropriate state for coating the slurry. Since the content of the carbon material should be reduced, it becomes a factor to lower the electrode characteristics of the negative electrode.

Among the components of the aqueous negative electrode material in the present invention, a component referred to as a "binder" may be used alone or in combination of two or more other components such as a dispersion medium, a viscosity modifier, a filler, a coupling agent, an adhesion promoter, etc., in addition to the aqueous binder and a cellulose dispersant. It may further include as.

In one preferred embodiment of the present invention, the porosity of the aqueous negative electrode having the structure described above is 30% or more as a means for supporting the 1.7 C charge / discharge rate more smoothly, thereby securing a high porosity. When the porosity of the water-based negative electrode using the carbon-based active material is 30% or more, the conductivity of lithium ions is improved, which is advantageous in preventing battery life decrease due to rapid charge and discharge. If the porosity is greater than 40%, the contact resistance of the negative electrode active material and the metal thin film as the current collector is increased, and thus the electrical conductivity of the electrode is reduced, so that the porosity is preferably 40% or less as the maximum value.

The current collector to which the negative electrode active material is coated may use a configuration generally used in the art. For example, the current collector is made of a thickness of 10 to 20 ㎛ and is not particularly limited as long as it is conductive without causing chemical changes in the battery. As the current collector, for example, stainless steel, aluminum, nickel, titanium, calcined carbon, a surface treated with carbon, nickel, titanium, silver, or the like on the surface of copper or stainless steel, an aluminum-cadmium alloy, or the like may be used. Preferably, copper excellent in conductivity can be used.

The electrolyte solution of the present invention may be any solvent and lithium salt suitable for the aqueous negative electrode, and is not particularly limited. Examples of suitable solvents for the electrolyte of the lithium battery of the present invention include any one of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (dimethyl carbonate, DMC), or a mixture of two or more thereof. The amount of such a solvent is appropriate if it is a normal level used in lithium batteries.

The lithium salt contained in the electrolyte may be used in the present invention as long as it is a lithium compound used in an aqueous negative electrode type lithium ion battery, and is not particularly limited. For example, lithium hexafluorophosphate (LiPF ₆ ), lithium perchlorate (LiClO ₄ ), lithium tetrafluoroborate (LiBF ₄ ), lithium trifluoromethane sulfonate (LiCF ₃ SO ₃ ) and lithium bistrifluoromethane sulfide Ponylamide (LiN (CF 3 SO ₂ ) ₂ ) and the like can be used.

In one preferred embodiment of the present invention, the lithium salt is used at a higher concentration than the conventional values of the prior art to support high charge and discharge rates and to prevent degradation of life. In the lithium ion battery of the present invention using an aqueous negative electrode, the concentration of lithium salt is more precisely, +1 is the total concentration of lithium species, which is all lithium chemistry containing +1 lithium atom or lithium ion. The concentrations of all +1 lithium species, including species such as free lithium ions (Li ⁺ ) as well as ionic lithium salts such as LiPF ₆ , refer to the concentration of lithium species. In this preferred embodiment the total concentration of the +1 valent lithium species is preferably from 1.1 M to 2.0 M in concentration.

When the total concentration of the +1 lithium species is within this range, the lithium ion conductivity of the electrolyte is improved, thereby preventing a decrease in battery cycle life. That is, the lithium species concentration of 1.1M or more can increase the lithium ion conductivity of the electrolyte solution and prevent the battery cycle life decrease. However, if the total concentration of the lithium species exceeds 2.0M, the electrolyte viscosity becomes excessive and electrolyte injection becomes difficult, which is not preferable in terms of processability. As a solvent that smoothly supports such a high concentration of lithium species, ethylene carbonate and propylene propylene Preference is given to using any one of carbonate, PC), and dimethyl carbonate (DMC) or a mixture of two or more thereof.

The electrolyte defined as described above may be used without particular limitation in a method of manufacturing a lithium battery, which is commonly used. For example, the electrolyte solution according to the present invention may be prepared by accommodating an electrode assembly including a cathode / anode / separation membrane in a battery case. Can be added to produce a lithium battery.

In the present invention, the positive electrode, the positive electrode material, and other components may adopt a configuration commonly used in this field.

Lithium battery of the present invention can be used as it is, except for the above-described general manufacturing process of a lithium ion battery as follows briefly.

First, a positive electrode plate is manufactured according to a conventional method used in manufacturing a lithium battery. Such a positive electrode plate includes an active material and a binder dissolved in a solvent, and is obtained by casting and drying a positive electrode coating material on the aluminum thin film, which may further include a plasticizer or a conductive agent. In this case, the above-described lithium composite oxide, sulfur-containing cathode active material, and the like can be used as the cathode active material. Any separator used in the lithium battery can be used without limitation as long as it is used in the lithium battery. Particularly, it is preferable that the separator be low in resistance to ion migration of the electrolyte and excellent in electrolyte solution moisture-immunity. In more detail, as a material selected from glass fiber, polyester, Teflon, polyethylene, polypropylene, polytetrafluoroethylene (PTFE) or a combination thereof, it may have a nonwoven or woven fabric. Preferably, polyethylene and / or polypropylene porous membranes which are less reactive with organic solvents and suitable for safety may be used.

The lithium ion battery according to the present invention can be applied to not only a stacked electrode assembly but also a jelly roll type electrode assembly, and shapes can also be applied to both rectangular and cylindrical shapes.

The lithium ion secondary battery of the present invention can be used for rapid charging at high charge and discharge rates, including 1.7 C, and supports high ion and current conductivity. It can maintain more than 80%.

Hereinafter, the performance of the electrode assembly of the present invention will be compared with the prior art by giving an experimental example for an example as an embodiment of the present invention and a comparative example compared with the above example.

Example

After mixing 97% by weight of natural graphite (Osaka), 1.0% by weight of carboxymethyl cellulose (Dicel), 1.0% by weight of styrene-butadiene rubber (Zeon) and 0.5% by weight of conductive carbon black (chevron) The ceramic balls were added and kneaded for about 10 hours. This mixture was cast on a copper foil having a thickness of 10 µm with a doctor blade spaced at 300 µm to obtain a cathode. This was placed in an oven at about 130 ° C., dried for about 10 hours, and then roll-pressed and cut into predetermined dimensions to prepare a negative electrode plate having a thickness of 120 μm.

Comparative example  One

Example 1 except that 98% by weight of natural graphite (Osaka), 1.0% by weight of carboxy methyl cellulose (CMC) (Dicel), 1.0% by weight of styrene butadiene rubber (SBR) (Zeon) A negative electrode plate was produced using the same method.

<Production of Lithium Battery>

First, the anode was mixed with 96 wt% of LiCoO2, PVDF (Kureha Co., Ltd.) as a binder, and kneaded well for 10 hours with conductive carbon black (Chevron Co., Ltd.) as a conductive material. Then, casting was performed using a doctor blade of 250 µm on an aluminum foil having a thickness of 15 µm to obtain a positive electrode plate. This was placed in an oven at about 110 ° C., dried for about 12 hours, and then roll-pressed and cut into predetermined dimensions to prepare a positive electrode plate having a thickness of 120 μm.

As the separator, a polyethylene / polypropylene porous membrane (Celgard, USA) having a thickness of 20 μm was used.

The porous membrane was disposed between the positive electrode plate and the negative electrode plates obtained in Example 1 and Comparative Example 1 and wound to make a battery assembly. The jelly-rolled battery assembly was placed in an aluminum cylindrical battery case, and then a non-aqueous electrolyte was injected and sealed to complete a 2200 mAh lithium secondary battery.

As the aqueous electrolyte solution, 5 g of a mixed organic solvent of ethylene carbonate (EC) in which 1.5 M of LiPF 6 was dissolved was used.

<Quick Charge Test>

The cylindrical lithium secondary battery of the present invention having a standard capacity of 1800 mAh was charged at 3.0 to 4.2 V at a 1.7 C charging rate.

The fast charge test results are summarized in Table 1 below.

Charge ratio to initial battery capacity 3.0-4.2 V, 1.7 C, 30 minutes charge 80% 3.0-4.2 V, 1.7 C, 40 minutes charge 90% 3.0-4.2 V, 1.7 C, 60 minutes charge 95%

<Life characteristics test>

1 is discharged at 0.5 C for a cylindrical lithium secondary battery of the present invention having a standard capacity of 1800 mAh after 300 cycles at a charge rate of 1.7 C and 1.7 C after discharging the prior art secondary battery of the comparative example at 0.5 C. The change in filling rate with respect to the initial capacity after 300 repetitions at the filling rate is shown. Example secondary battery according to the present invention maintained a level of about 35% higher than the comparative battery by maintaining a level of more than 80% of the initial capacity even after 300 cycles.

From these results, it can be seen that the life characteristics of the lithium ion battery according to the present invention are superior to those of the prior art, and can be rapidly charged at a high charge / discharge rate.

(a): The change of the charging rate with respect to the initial capacity after discharging the secondary battery of this invention to 0.5C, and repeating 300 cycles at 1.7C charge rate.
(b): change in charge rate with respect to initial capacity after a prior art secondary battery was discharged at 0.5C and then repeated 300 times at a 1.7C charging rate.

Claims (10)

In a lithium ion secondary battery equipped with an aqueous negative electrode,
The aqueous negative electrode includes a negative electrode active material, a conductive agent and a binder,
The lithium ion secondary battery has a charge rate of 1.7C and a charge amount of 80% or more in 30 minutes, and a capacity of the battery after 80 charge / discharge cycles is 80% or more of its initial capacity.
The method of claim 1, wherein the ratio of the negative electrode active material, the conductive agent and the binder is a weight percentage of the negative electrode active material: conductive agent: binder = 95.0 to 99.9: 0.1 to 1.0: 2.0 to 4.0%, characterized in that the lithium ion secondary battery .
The lithium ion secondary battery of claim 1, wherein the binder is a mixture of an aqueous binder and a cellulose dispersant.
The method of claim 1,
The porosity (porosity) of the aqueous negative electrode is a lithium ion secondary battery, characterized in that 30% to 40%.
The method of claim 1, wherein the electrolyte of the lithium ion secondary battery is any one of ethylene carbonate, propylene carbonate (PC), dimethyl carbonate (dimethyl carbonate, DMC) or a mixture of two or more thereof. A lithium ion secondary battery using as a solvent, wherein the total concentration of the +1 valent lithium species is 1.1 to 2.0 M.
The lithium ion secondary battery of claim 1, wherein the conductive agent is selected from conductive carbon black and carbon nanotubes.
The lithium ion secondary battery according to claim 6, wherein the conductive carbon black has a particle diameter of less than 1 micrometer.
The lithium ion secondary battery of claim 1, wherein the aqueous binder is styrene-butadiene rubber.
The lithium ion secondary battery of claim 1, wherein the cellulose dispersant is carboxymethyl cellulose.
The lithium ion secondary battery according to claim 1, wherein the mixing of the aqueous binder and the cellulose dispersant in a weight ratio of binder: dispersant = 1: 1 to 1: 2.

Images