KR20080087344A - Lithium rechargeable battery - Google Patents

Abstract

A lithium secondary battery is provided to prevent degradation of lifespan caused by swelling/shrinking of an electrode plate, thereby realizing improved lifespan characteristics and charge/discharge cycle characteristics. A lithium secondary battery comprises a casing in which an anode, a cathode, a separator interposed between the anode and the cathode and an electrolyte are received. In the battery, the electrolyte comprises a non-aqueous organic solvent, a lithium salt and succinic anhydride. The anode comprises a silicon thin film or silicon-graphite composite material as an anode active material, and the cathode comprises a lithium intercalation compound as a cathode active material. The succinic anhydride additive is used in an amount of 0.5-2 wt% based on the total weight of the electrolyte.

Description

Lithium secondary battery {LITHIUM RECHARGEABLE BATTERY}

1 is a cross-sectional view of a rectangular lithium secondary battery

Figure 2 is a graph showing the charge and discharge cycle characteristics of the lithium secondary battery of Examples 1 to 3 and Comparative Example 1.

The present invention relates to a lithium secondary battery, and more particularly, to a lithium secondary battery capable of improving the life characteristics of a lithium secondary battery by suppressing electrode expansion.

In recent years, with the spread of portable electronic devices such as portable personal digital assistants (PDAs), mobile telephones, notebook computers, digital cameras, and the like, devices are becoming smaller and lighter. Accordingly, interest in batteries suitable for use as a power source for such portable electronic devices is increasing. In particular, among secondary batteries capable of charging and discharging, the self discharge rate is lower than that of conventional lead batteries and nickel cadmium batteries, and the energy density is high. Lithium secondary batteries are widely used in advanced electronic devices.

The miniaturized and slimmed lithium secondary battery has a positive electrode material made of a lithium metal mixed oxide capable of detaching and inserting lithium ions, a negative electrode capable of forming a compound with lithium ions such as carbon material or metal lithium, and a lithium salt in a non-aqueous organic solvent. It consists of the electrolyte solution melt | dissolved in a suitable quantity. The average discharge voltage of about 3.6 to 3.7 V of the lithium battery is one of the biggest advantages of obtaining high power compared to other alkaline batteries or Ni-MH or Ni-Cd batteries.

In order to exhibit such a high driving voltage, an electrochemically stable electrolyte composition is required at a charge region of 0 to 4.2 V. Therefore, ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (diethyl) are required. Carbonate-based organic solvents such as carbonate and DEC) and fluorobenzene (FB) were used as electrolyte solvents to increase the wickability of the separator.

As the solute of the electrolyte, lithium salts such as LiPF ₆ , LiBF ₄ , LiClO ₄ , and LiN (C ₂ F ₅ SO ₃ ) ₂ are commonly used. They act as a source of lithium ions in the battery, thereby preventing the basic operation of the lithium battery. Made it possible. However, the electrolyte prepared as described above has a disadvantage in high rate charging and discharging because the ion conductivity is significantly lower than that of the aqueous electrolyte used in Ni-MH or Ni-Cd batteries.

In the past, lithium metal having a high energy density was proposed as a negative electrode active material of a lithium secondary battery, but the dendrite formed on the negative electrode during charging may cause internal short circuit through the separator, thereby causing a problem in terms of safety. In addition, the formed dendrite has a very large specific surface area and thus has high reactivity, and reacts with the electrolyte to form a polymer film having low electron conductivity on the surface of the cathode. For this reason, the resistance of the battery increases rapidly, or particles isolated from the network of electron conduction exist, which acts as a factor that inhibits charging and discharging.

Due to this problem of a battery using lithium metal, a graphite material capable of absorbing and releasing lithium ions instead of lithium metal has been proposed and put into practical use as a negative electrode active material. In the case of using graphite as the active material of the negative electrode, since the metallic lithium does not easily precipitate, the above problems due to the dendrites do not occur significantly. However, although the theoretical discharge capacity when lithium metal is used as the negative electrode reaches 3860 mAh / g, the discharge capacity when graphite is used as the negative electrode active material is only 372 mAh / g, which is higher than that of the graphite material. Development of new active materials has been required.

As a method for increasing the discharge capacity of a battery, recently, metals such as Sn, Al, Zn and non-metallic materials such as Si, Ge, B, P, or oxides thereof forming a compound with lithium have been proposed as a negative electrode active material. . The active materials based on the above metals and nonmetallic substances or oxides thereof have a larger capacity than graphite in theory and have a large initial capacity, but have low electrochemical reversibility and thus a rate of decrease in capacity due to charge and discharge cycles. There is a quick disadvantage.

This shortcoming appears as a result of shortening the battery life. As a method for solving the problem, a method of using an active material (hereinafter, referred to as a metal-carbon composite active material) in which the lithium and the compound forming the compound and the carbonaceous material are combined is proposed. It became. The metal-carbon composite active material may be prepared by embedding the metal and nonmetal materials, or oxide particles thereof, which form a compound with lithium in a carbonaceous material, or coating with carbon.

 The negative electrode of a lithium ion battery is comprised based on the negative electrode active material which participates in a battery reaction, an electrical power collector, and the polymeric binder which fixes these to a collector plate. The active material used for the negative electrode of a lithium ion battery is a particle form, and is fixed to an electrical power collector by a binder. The active material particles fixed by the binder are connected to the current collector plate in electrical connection with each other by a point contact. Therefore, when the degree of point contact is low, that is, when the point contact area is small, the flow path of lithium ions is limited and the internal resistance of the battery has a large value. Will not contribute. Therefore, it is important to maintain a large contact area between the active material particles.

On the other hand, the battery reaction occurs in the negative electrode and the positive electrode inside the battery during charging and discharging of the battery. When the battery reaction occurs, lithium ions are inserted or released into the active material particle structure of the negative electrode, and the active material particles expand and contract due to insertion and release of lithium ions.

Although there is a difference depending on the type of material used as the active material, the active material is undifferentiated by repeated volume change due to charging and discharging. Therefore, electrical connection by point contact between active material particles becomes unstable as charging and discharging progresses. As a result, the capacity of the battery gradually decreases as the lithium ion battery is repeatedly charged and discharged. The life degradation due to the volume change of the active material can be alleviated to some extent by adding a material such as carbon black to the conductive material, but the use of an excessive conductive material has the disadvantage of reducing the discharge capacity of the battery.

Particularly, when a silicon thin film or a silicon-graphite composite material is used as a negative electrode active material, a high capacity battery can be manufactured. However, since the life degradation due to expansion of the electrode plate is severe, a silicon thin film or a silicon-graphite composite material is used for a lithium ion battery. There was a difficulty in applying.

The present invention is to solve the problem of deterioration of the life characteristics of the lithium secondary battery by shrinking and expanding the electrode plate by the charge and discharge as described above, to provide a lithium secondary battery having a high life characteristics, excellent charge and discharge cycle characteristics It aims to do it.

In order to achieve the above object of the present invention, the present invention comprises an electrolyte comprising a lithium salt, an organic solvent and a succinic anhydride additive; It provides a lithium secondary battery comprising a silicon thin film or a silicon-graphite composite active material as the negative electrode active material.

Hereinafter, the present invention will be described in more detail.

An example of the structure of the non-aqueous lithium secondary battery 1 is shown in FIG. 1. The battery uses a material capable of detaching and inserting lithium ions as the positive electrode 2 and the negative electrode 4, inserting a separator 6 between the positive electrode 2 and the negative electrode 4, and winding the separator to form an electrode assembly 8. ) And then put into the case 10 is manufactured. The top of the cell is sealed with a cap plate 12 and a gasket 14. The cap plate 12 may be provided with a safety vent and an electrolyte injection hole 16 to prevent overpressure of the battery. The positive electrode tab 18 and the negative electrode tab 20 are respectively installed in the positive electrode 2 and the negative electrode 4, and separate insulating tapes 22 and 24 prevent the internal short circuit of the battery and insertability of the electrode assembly 8. It can be installed for ease. The electrolyte 26 is injected through the electrolyte injection hole 16 before sealing the battery. The injected electrolyte 26 is impregnated into the separator 6. The electrolyte injection opening is sealed by a sealing material.

In the present invention, a conventional material capable of reversible detachment and insertion of lithium ions or a material capable of reversibly reacting with lithium to form a lithium-containing compound is used as a cathode active material, and a silicon thin film or a silicon-graphite composite material is used. Provided is a lithium secondary battery that can improve the swelling characteristics, lifespan characteristics, and low temperature characteristics of a lithium secondary battery by adding a specific compound to an organic solvent, preferably a high boiling point organic solvent, used as an electrolyte while being used as an anode active material. .

The silicon or silicon graphite composite active material, which is newly attracting attention as a negative electrode active material of a secondary battery, has a large discharge capacity, but has a severe deterioration in life due to the expansion of the electrode plate. Therefore, a silicon thin film or a silicon-graphite composite material is applied to a lithium ion battery. There was a difficulty. However, when the electrolyte of the present invention is used, even if a silicon thin film or a silicon-graphite composite material is used as the negative electrode active material, excellent life characteristics can be obtained.

The electrolyte of the present invention is a high boiling point organic solvent; Lithium salts; And succinic anhydride additives.

By adding a succinic anhydride additive to a high boiling organic solvent containing lithium salt as in the present invention, it is possible to suppress swelling due to gas generation inside the battery and to prevent micronization by inhibiting expansion of the electrode plate. It is possible to solve the problem of deteriorating the life characteristics.

The succinic anhydride additive is added in an amount of 0.5 to 3% by weight, preferably 0.5 to 2% by weight, based on the total amount of the electrolyte.

Examples of the lithium salt include LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , LiAlO ₄ , LiAlCl ₄ , LiN ( C _x F _2x + 1SO ₂ ) (C _y F _2y + 1SO ₂ ) (where x and y are natural numbers), LiCl, and LiI, and one or more selected from the group consisting of LiI can be used in combination.

The concentration of the lithium salt is preferably used in the range of 0.6 to 2.0M, more preferably in the range of 0.7 to 1.6M. When the concentration of the lithium salt is less than 0.6M, the conductivity of the electrolyte is lowered, and the performance of the electrolyte is lowered. When the concentration of the lithium salt is higher than 2.0M, the viscosity of the electrolyte is increased to reduce the mobility of lithium ions.

Lithium salt acts as a source of lithium ions in the battery to enable the operation of the basic lithium battery, the non-aqueous organic solvent serves as a medium to move the ions involved in the electrochemical reaction of the battery.

The high boiling point organic solvent means a solvent having a boiling point of 100 ° C or higher, preferably 150 ° C or higher, more preferably 200 ° C or higher. Preferred examples thereof include gamma-butyrolactone, ethylene carbonate, dipropyl carbonate, acid anhydride, N-methyl pyrrolidone, N-methylacetamide, N-methyl formamide, acetonitrile, dimethyl formamide, sulfolane, Dimethyl sulfoxide, dimethyl sulfite and the like.

The electrolyte of the present invention can be applied to all lithium secondary batteries such as lithium ion batteries and lithium polymer batteries.

In the present invention, as a cathode material of a lithium secondary battery, a material capable of reversible detachment and insertion of lithium ions, a material capable of reversibly forming a lithium-containing compound with lithium ions, and a sulfur-based compound may be used. Examples of materials capable of reversible detachment and insertion of lithium ions include LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , or LiNi _(1-xy) Co _x M _y O ₂ (0 <x <1, 0 < y <1, 0 <x + y <1, M is a metal such as Al, Sr, Mg, La), or a metal oxide or chalcogenide compound such as LiFeO ₂ , V ₂ O ₅ , TiS ₂ , MoS ₂ . Examples of a material capable of reversibly forming a lithium-containing compound with lithium ions include tin dioxide (SnO ₂ ) and titanium nitrate. The sulfur compound is a positive electrode active material of a lithium-sulfur battery, elemental sulfur, Li ₂ S _n (n ≧ 1), Li ₂ S _n (n ≧ 1), organic sulfur compound, and carbon- dissolved in catholyte. Sulfur polymers ((C ₂ S _x ) _n : x = 2.5 to 50, n ≧ 2) and the like.

As a negative electrode material of the lithium secondary battery of the present invention, a silicon thin film or a silicon-graphite composite material may be used. As described above, a silicon thin film or a silicon-graphite composite material may be used as the negative electrode material of the lithium secondary battery of the present invention. Even when used, the battery characteristics are not deteriorated. Therefore, a lithium secondary battery having a high discharge capacity may be manufactured using a silicon thin film or a silicon-graphite composite material as a negative electrode material of the battery.

The positive electrode including the active material is prepared by applying a slurry to a current collector of a thin plate with a suitable thickness and length, or by coating the active material itself in a film form.

Several methods are already known for the preparation of the negative electrode including the silicon thin film and the negative electrode including the silicon-graphite composite active material. For example, silicon powder is mixed with graphite powder using a liquid such as alcohol, and then alcohol is blown off, or a mixture of silicon and graphite that is used to mediate silicon and graphite in alcohol, such as alcohol, is used so that the medium remains after the alcohol is removed. Graphite composite active materials can be prepared.

The prepared positive electrode and negative electrode are wound or laminated together with a separator which is an insulator to make an electrode group, and then placed in a can or a similar container, and then a lithium secondary battery is manufactured by injecting the non-aqueous electrolyte of the present invention. As the separator, a polyethylene separator, a polypropylene separator, a polyethylene / polypropylene two-layer separator, a polyethylene / polypropylene / polyethylene three-layer separator, or a polypropylene / polyethylene / polypropylene three-layer separator may be used.

The following presents a preferred embodiment to aid the understanding of the present invention. However, the following examples are merely provided to more easily understand the present invention, and the present invention is not limited to the following examples.

Examples and Comparative Examples

(Example 1)

1.15 M in a non-aqueous organic solvent mixed with gamma-butyrolactone / ethylene carbonate / ethylmethyl carbonate / dimethyl carbonate / fluorobenzene (GBL / EC / EMC / DMC / FB) in 10/3/5/1/1 LiPF ₆ was added and 0.5 wt% of succinic anhydride was added to the electrolyte to prepare an electrolyte.

A slurry was prepared by adding LiCoO ₂ (average particle diameter: 10 μm), a conductive material (super P), and a binder (PVDF) as a positive electrode active material to N-methylpyrrolidone (NMP) at a weight ratio of 94: 3: 3. The slurry was coated on aluminum foil, dried and rolled with a roll press to prepare a positive electrode plate having a width of 4.9 cm and a thickness of 147 μm.

A silicon-graphite composite material in which silicon powder adheres to the graphite powder surface is prepared as a negative electrode active material, and SBR is prepared as a slurry using a solvent as a silicon graphite composite material and a binder. This slurry was applied to a copper current collector, dried and rolled with a roll press to prepare a negative electrode plate having a width of 5.1 cm and a thickness of 178 μm.

A separator made of a polyethylene (PE) porous film (width: 5.35 cm, thickness: 18 μm) was inserted between the positive electrode plate and the negative electrode plate, wound, compressed, and placed in a pouch case, followed by injection of 2.3 g of the electrolyte to 660 mAh. A pouch-type lithium secondary battery was produced.

(Example 2)

A lithium secondary battery was manufactured in the same manner as in Example 1, except that 1% by weight of succinic anhydride was added to prepare an electrolyte.

(Example 3)

A lithium secondary battery was manufactured in the same manner as in Example 1, except that 2 wt% of succinic anhydride was added to prepare an electrolyte.

(Example 4)

A lithium secondary battery was manufactured in the same manner as in Example 1, except that 3% by weight of succinic anhydride was added to prepare the electrolyte.

(Comparative Example 1)

A lithium secondary battery was manufactured in the same manner as in Example 1, except that the electrolyte was prepared without adding succinic anhydride in the preparation of the electrolyte.

After charging the lithium secondary batteries of Examples 1 to 3 and Comparative Example 1 at 25 ° C. under a constant current-constant voltage (CC-CV) condition with a current of 0.8 C, a 100 mA cut-off, and a charging voltage of 4.2 V, The charge and discharge characteristics were evaluated by discharging from 1.0C to 3.0V under constant current conditions. The capacity change according to charging and discharging is shown in FIG. 1.

On the other hand, the table below is a result graph showing the capacity (%) at 100 charges in Examples 1 to 4 and Comparative Example 1. At this time, the capacity at 100 charges indicates the discharge capacity after 100 charge and discharge cycles under the charging and discharging conditions for the constant current constant voltage (CCCV) test as described above when the initial discharge capacity is 100.

As shown in Figure 1 and Table 1 it can be seen that the charge and discharge cycle characteristics of Examples 1 to 4 according to the present invention is superior to Comparative Example 1, in particular 1 wt%, 2 wt% of succinic anhydride (succinic anhydride) Examples 2 and 3 added showed very good life characteristics results.

Table 1

Succinic anhydride added amount (% by weight) Capacity at 100 charges (%) Example 1 0.5 42 Example 2 One 49 Example 3 2 57 Example 4 3 29 Comparative Example 1 0 21

According to the present invention, an electrolyte prepared by using an anhydrous succinic acid additive in a high boiling point solvent is used in a secondary battery using a silicon thin film or a silicon-graphite composite electrode. This excellent lithium secondary battery can be obtained.

Claims (8)

In a lithium secondary battery in which a negative electrode, a positive electrode, a separator and an electrolyte interposed between the negative electrode and the positive electrode are accommodated in a case, The electrolyte includes a non-aqueous organic solvent, a lithium salt, and succinic anhydride of the following formula, wherein the negative electrode includes a silicon thin film or a silicon-graphite composite material as a negative electrode active material, and the positive electrode is lithium as a positive electrode active material. A lithium secondary battery comprising an intercalation compound. The method of claim 1, wherein the lithium salt is LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , LiAlO ₄ , LiAlCl ₄ , LiN (C _x F _2x + 1SO ₂ ) (C _y F _2y + 1SO ₂ ) (where x and y are natural numbers), Lithium, characterized in that at least one selected from the group consisting of Secondary battery. The lithium secondary battery according to claim 1 or 2, wherein the lithium salt is dissolved in a non-aqueous organic solvent at a concentration of 0.6 to 2 M. The lithium secondary battery of claim 1, wherein the non-aqueous organic solvent is at least one selected from the group consisting of carbonate, ester, ether, and ketone. The method of claim 4, wherein the carbonate is one or more selected from the group of cyclic carbonate organic solvents consisting of ethylene carbonate (EC) or vinylene carbonate (VC), propylene carbonate (PC) and butylene carbonate (BC). And linear carbonate organic solvents composed of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylpropyl carbonate (EPC), ethylmethyl carbonate (EMC), methylpropyl carbonate (MPC), dipropyl carbonate (DPC) Lithium secondary battery, characterized in that one or more mixtures selected from the group. The method of claim 4, wherein the ester is butyrolactone, decanolide, valerolactone, mevalonolactone, caprolactone, n-methylacetate, n-ethyl acetate, and n Lithium secondary battery, characterized in that at least one compound selected from the group consisting of -propyl acetate. The lithium secondary battery according to claim 1, wherein the mixed amount of succinic anhydride is 0.5 to 2% by weight of the total electrolyte. The method of claim 1, wherein the lithium intercalation compound capable of reversible detachment and insertion of lithium ions is LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNi _(1-xy) Co _x M _y O ₂ (0 <x <1, 0 <y <1, 0 <x + y <1, M is a metal such as Al, Sr, Mg, La, etc.), LiFeO ₂ , V ₂ O ₅ , TiS ₂ , MoS ₂ Lithium secondary battery, characterized in that one selected from the group is used.

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