KR20160135513A - Nonaqueous electrolyte solution for lithium secondary battery and Lithium secondary battery comprising the same - Google Patents

Abstract

The present invention relates to a non-aqueous electrolyte solution for a lithium secondary battery and a lithium secondary battery comprising the same. The non-aqueous electrolyte solution for a lithium secondary battery comprises: lithium bis(fluorosulfonyl)imide (LiFSI) as an electrolyte; and a phosphate-based compound as a non-aqueous electrolyte solution additive. A lithium secondary battery manufactured by using the non-aqueous electrolyte solution maintains the capacity retention rate after high temperature storage, suppresses battery expansion, and has improved low temperature output properties.

Description

TECHNICAL FIELD [0001] The present invention relates to a non-aqueous electrolyte for a lithium secondary battery and a lithium secondary battery including the same,

The present invention relates to a non-aqueous electrolyte for a lithium secondary battery and a lithium secondary battery including the same.

As technology development and demand for mobile devices are increasing, the demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries having high energy density and voltage have been commercialized and widely used.

In recent years, technology has been developed not only for power supply for small appliances, but also for power supply for power storage facilities and medium and large-sized industrial power supplies for vehicles.

Lithium metal oxide is used as the positive electrode active material of the lithium secondary battery, and lithium metal, lithium alloy, crystalline or amorphous carbon or carbon composite material is used as the negative electrode active material. The active material is coated on the current collector with an appropriate thickness and length, or the active material itself is coated in a film form and wrapped or laminated with a separator as an insulator to form an electrode group. The electrode group is then placed in a can or similar container, Thereby manufacturing a secondary battery.

This lithium secondary battery is charged and discharged while repeating the process of lithium ion intercalation and deintercalation from the anode to the cathode. At this time, since lithium is highly reactive, it reacts with the carbon-based cathode to form a coating on the surface of the cathode. This film is called Solid Electrolyte Interface (SEI) film. The SEI film formed at the beginning of charging prevents the reaction between lithium ion and carbonaceous cathode or other materials during charging and discharging. It also acts as an ion tunnel, allowing only lithium ions to pass through. This ion tunnel serves to prevent the organic solvent of the electrolyte having a large molecular weight, which is solvated by lithium ion, to co-intercalate with the carbon-based anode to collapse the structure of the carbon-based cathode.

However, the SEI film may fail to form a continuous protective film as the battery is charged and discharged, and thus the life of the battery may be deteriorated. In addition, the SEI coating is thermally unstable and may be destroyed by increased electrochemical energy and thermal energy when the cell is operated at high temperature or left at high temperature.

If the SEI coating is not stable, the performance of the battery due to the collapse of the SEI coating is degraded, and the CO ₂ gas is continuously generated due to the decomposition of the electrolyte, thereby increasing the internal pressure and thickness of the battery.

Alternatively, even when the electrolyte additive is not contained or the electrolyte additive having poor characteristics is contained, a non-uniform SEI coating is formed on the surface of the negative electrode, which makes it difficult to expect improvement in low temperature output characteristics. Further, even if the amount of the electrolyte additive is included, if the amount of the additive can not be controlled to an appropriate amount, the surface of the anode may be decomposed or the oxidation reaction may occur during the high temperature reaction due to the electrolyte additive, thereby ultimately increasing the irreversible capacity of the secondary battery, There was a problem.

Therefore, in order to improve the performance of the lithium secondary battery, it is important to form a solid SEI film on the electrode of the lithium secondary battery.

The present invention provides a nonaqueous electrolyte for a lithium secondary battery and a lithium secondary battery including the same, which can improve not only the low-temperature output characteristics but also the capacity retention after high temperature storage.

In one aspect of the present invention, lithium bis (fluorosulfonyl) imide: LiFSI, a first lithium salt; And a non-aqueous electrolyte for a lithium secondary battery comprising a phosphate compound.

The lithium bisfluorosulfonylimide may have a concentration in the range of 0.01 to 1 M (mole / liter) in the nonaqueous electrolyte solution.

The phosphate compound may be contained in an amount of 2 to 10% by weight based on the total weight of the non-aqueous electrolyte.

The phosphate-based compound may be represented by the following Formula 1:

In this formula,

a is a carbon atom (C) or a silicon atom (Si)

b is a hydrogen atom (H) or a fluorine atom (F)

n is 1 to 5;

Wherein the phosphate compound is selected from the group consisting of tris (2,2,2-trifluoroethyl) phosphate, tris (1,1,2,2-tetrafluoroethyl) phosphate, tris (hexafluoroisopropyl) (2,2,3,3-tetrafluoropropyl) dimethyl phosphate, bis (2,2,3,3-tetrafluoropropyl) methyl phosphate, tris (2,2,3,3-tetrafluoropropyl) (4-fluorophenyl) phosphate, and pentafluorophenylphosphate.

The non-aqueous electrolyte solution, in addition to the first lithium _{salt, LiPF 6, LiAsF 6, LiCF} 3 SO 3, LiN (CF 3 SO 2) 2, LiBF 6, LiSbF 6, LiN (C 2 F 5 SO 2) 2, LiAlO ₄ , LiAlCl ₄ , LiSO ₃ CF _3, and LiClO ₄ as a second lithium salt.

The second lithium salt may preferably LiPF _6.

The first lithium salt and the second lithium salt may be mixed in a molar ratio of 0.01: 1 to 1: 1.

According to another aspect of the present invention, there is provided a lithium secondary battery comprising a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a separator interposed between the positive electrode and the negative electrode, -Cobalt oxide, and the negative electrode active material is graphite.

The lithium secondary battery comprising the non-aqueous electrolyte of the present invention is formed with a solid SEI film on the cathode at the time of initial charging, and the decomposition of the anode surface and the oxidation reaction of the electrolyte during the high-temperature cycle are prevented, The battery expansion is suppressed, and the low temperature output characteristic is improved.

Hereinafter, the present invention will be described in detail in order to facilitate understanding of the present invention. The terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary terms and the inventor may appropriately define the concept of the term in order to best describe its invention It should be construed as meaning and concept consistent with the technical idea of the present invention.

The non-aqueous electrolyte according to an embodiment of the present invention includes lithium bisfluorosulfonylimide (LiFSI) as an electrolyte and a phosphate-based compound as a non-aqueous electrolyte additive.

The SEI film formed on the carbonaceous anode of the secondary battery may be durably deteriorated when stored at a high temperature in a fully charged state, for example, at 100% at 4.2 V and then left at 85 DEG C for 4 days. That is, when the high-temperature storage in the fully charged state the cathode while slowly SEI film is collapsed and exposed with the lapse of time, the surface of the thus exposed anode reacts with an electrolyte around, causing a side reaction continued to CO, CO _2, CH ₄ , C ₃ H _6, and the like are generated, thereby increasing the internal pressure and thickness of the battery.

The present invention is a phosphate-based compound which does not participate in the SEI film forming reaction but which enables smooth migration of lithium ions by reducing the resistance which inhibits the charge transfer reaction of lithium ions during charging and discharging, Phosphate-based compounds may be used:

 [Chemical Formula 1]

In this formula,

a is a carbon atom (C) or a silicon atom (Si)

b is a hydrogen atom (H) or a fluorine atom (F)

n is 1 to 5;

Non-limiting examples of the phosphate based compound include fluorinated alkyl phosphates such as tris (2,2,2-trifluoroethyl) phosphate, tris (1,1,2,2-tetrafluoroethyl) phosphate, tris Hexafluoroisopropyl) phosphate, (2,2,3,3-tetrafluoropropyl) dimethyl phosphate, bis (2,2,3,3-tetrafluoropropyl) methyl phosphate and tris , 3-tetrafluoropropyl) phosphate; But are not limited to, fluorinated aromatic phosphates such as tris (4-fluorophenyl) phosphate and pentafluorophenylphosphate. Tris (2,2,2-trifluoroethyl) phosphate (TFEP) is preferred in terms of low temperature output and high temperature characteristics.

The phosphate compound may be contained in an amount of 2 to 10% by weight based on the total weight of the non-aqueous electrolyte. If the content is less than 2% by weight, sufficient output improvement effect can not be exhibited. If the content is more than 10% by weight, mobility of the electrolyte decreases and resistance increases, and excessive side reactions in the electrolyte at high temperature It causes swelling phenomenon, so that it can not be used in an excessive amount.

When the phosphate compound is used in a secondary battery at a room temperature or a high temperature, it improves the output by improving the interface between the electrode and the electrolyte to reduce the battery resistance, but the present inventors have found that the phosphate compound is used as an electrolyte additive The secondary battery exhibits a reduced output at low temperatures. As a result of studies for solving these problems, the present inventors have found that the use of LiFSI as a lithium salt improves the output effect at room temperature and high temperature by the phosphate compound, and also improves the low temperature output.

The LiFSI acts not only as an electrolyte in a non-aqueous electrolyte, but also as a solid and thin SEI film on the cathode to facilitate the movement of lithium ions, thereby improving the low-temperature output characteristics and, at the same time, The decomposition of the surface can be suppressed and the oxidation reaction of the electrolytic solution can be prevented.

According to one embodiment of the present invention, the LiFSI has a concentration in the range of 0.01 to 1 M in the non-aqueous electrolyte. If the LiFSI concentration is lower than 0.01 M, the effect of improving the low-temperature output and the high-temperature cycle characteristics of the lithium secondary battery is insignificant. If the LiFSI concentration exceeds 1.0 M, side reactions within the electrolyte occur excessively during charging / discharging of the battery, ) Phenomenon may occur.

The non-aqueous electrolyte of the present invention may further include a lithium salt (second lithium salt) in addition to LiFSI (first lithium salt). The second lithium salt in the conventional lithium salts used in the art, for example, _{LiPF 6, LiAsF 6, LiCF 3} SO 3, LiN (CF 3 SO 2) 2, LiBF 6, LiSbF 6, LiN (C 2 F ₅ SO ₂ ) ₂ , LiAlO ₄ , LiAlCl ₄ , LiSO ₃ CF _3, and LiClO ₄ , or a mixed lithium salt of two or more kinds. Since LiPF ₆ can form a film on a metal, particularly aluminum, to effectively prevent metal corrosion by LiFSI, LiPF ₆ is preferable as the second lithium salt.

The first lithium salt and the second lithium salt may be used in a molar ratio of 0.01: 1 to 1: 1. If the first lithium salt is used excessively beyond the above range, side reactions within the electrolyte may occur excessively during charging / discharging of the battery, resulting in swelling, and in the electrolytic solution, If the first lithium salt is used below the above-described range, the process of forming the SEI film in the battery and the process of forming the SEI film by the lithium ion solvated by the carbonate- An improvement in the low-temperature output of the secondary battery and the improvement in the cycle characteristics and the capacity characteristics after storage at a high temperature by peeling of the negative electrode surface layer (for example, carbon surface layer) and decomposition of the electrolytic solution .

The organic solvent that can be included in the nonaqueous electrolyte solution is not limited as long as it can minimize decomposition due to oxidation reaction during charging and discharging of the battery and can exhibit desired properties together with additives. Carbonates, linear carbonates, esters, ethers or ketones. These may be used alone or in combination of two or more. (EC), propylene carbonate (PC), and butylene carbonate (BC) may be used as the cyclic carbonate, dimethyl carbonate (DMC) may be used as the linear carbonate, Diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC) and ethyl propyl carbonate (EPC).

In another embodiment of the present invention, the non-aqueous electrolyte may further comprise one or more kinds of mixtures selected from the group consisting of a carbonate compound, a sulfate compound and a sulfone compound as an additive capable of forming a stable SEI film on the electrode surface .

The carbonate compound may include at least one selected from the group consisting of vinylene carbonate and vinyl ethylene carbonate. When the carbonate compound is included as an additive in the non-aqueous electrolyte, it may be contained in an amount of 1.5 to 3% by weight based on the total amount of the electrolytic solution.

The sulfate compound may be ethylene sulfate. When the sulfate compound is included as an additive in the non-aqueous electrolyte, the sulfate compound may be contained in an amount of 0.5 to 1.5% by weight based on the total weight of the electrolyte solution.

The sulfone compound may be 1,3-propane sultone. When the sulfone compound is included as an additive in the non-aqueous electrolyte, it may be contained in an amount of 0.5 to 1% by weight based on the total amount of the electrolyte solution. have.

Meanwhile, the lithium secondary battery according to an embodiment of the present invention may include an anode, a cathode, a separator interposed between the anode and the cathode, and the non-aqueous electrolyte. The positive electrode and the negative electrode may include a positive electrode active material and a negative electrode active material, respectively.

Here, the cathode active material may include a lithium metal oxide or a mixture thereof. Non-limiting examples of the lithium metal oxide may be selected from the group consisting of lithium manganese oxide, lithium nickel-manganese oxide, lithium manganese-cobalt oxide and lithium nickel-manganese-cobalt oxide, _{LiCoO 2, LiNiO 2, LiMnO 2} , LiMn 2 O 4, Li (Ni a Co b Mn c) O 2 ( where, 0 <a <1, 0 <b <1, 0 <c <1, a + b _{+ c = 1), LiNi 1} -Y Co Y O 2, LiCo 1 - Y Mn Y O 2, LiNi 1 - Y Mn Y O 2 ( here, 0≤Y <1), Li ( Ni a Co b Mn _{c) O 4 (0 <a} <2, 0 <b <2, 0 <c <2, a + b + c = 2), LiMn 2 - z Ni z O 4, LiMn 2 -z Co z O 4 ( Where 0 &lt; z &lt; 2).

On the other hand, as the negative electrode active material, carbon-based negative electrode active materials such as crystalline carbon, amorphous carbon or carbon composite may be used alone or in combination of two or more.

The separator may be a porous polymer film such as a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer Or may be a laminate of two or more kinds. In addition, nonwoven fabrics made of conventional porous nonwoven fabrics, for example, glass fibers having a high melting point, polyethylene terephthalate fibers, or the like can be used, but the present invention is not limited thereto.

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

Example  One

Preparation of electrolytic solution

An electrolytic solution containing 0.5 M LiFSI and 0.5 M LiPF ₆ electrolyte in an organic solvent having a composition of ethylene carbonate (EC): ethylmethyl carbonate (EMC): dimethyl carbonate (DMC) = 3: 3: 4 As a standard, 2% by weight of TFEP, 3% by weight of vinylene carbonate (VC) and 1.5% by weight of 1,3-propane sultone were added, and 1% by weight of vinylethylene sulfite was further added to prepare a nonaqueous electrolytic solution.

Manufacture of lithium secondary battery

As a positive electrode active material Li (Ni Co _{0 .6 0 0 .2 .2} Mn) O ₂ 92% by weight, the conductive agent was used and carbon black (carbon black) 4% by weight, polyvinylidene fluoride (PVdF) as a binder polymer Were mixed to prepare a positive electrode slurry. The positive electrode slurry was coated on an aluminum current collector, followed by drying and rolling to prepare a positive electrode.

Also, a negative electrode slurry was prepared by mixing graphite anode active material, PVdF binder polymer, and thickener, and dispersing in water. The negative electrode slurry was coated on a copper collector, followed by drying and rolling to prepare a negative electrode.

A battery was fabricated by a conventional method using the positive electrode, the negative electrode, and the polyethylene porous film thus prepared, and the nonaqueous electrolyte thus prepared was injected to complete the production of the lithium polymer secondary battery.

Comparative Example  One

A nonaqueous electrolytic solution was prepared in the same manner as in Example 1 except that only 1.0M LiPF _{6 was} used as an electrolyte and TFEP additive was not used as an organic solvent component, and then a lithium secondary battery was prepared.

Comparative Example  2

A nonaqueous electrolytic solution was prepared in the same manner as in Example 1 except that only 1.0 M LiPF _{6 was} used as an electrolyte, and then a lithium secondary battery was produced.

Comparative Example  3

A nonaqueous electrolytic solution was prepared in the same manner as in Example 1 except that the TFEP additive was not used, and then a lithium secondary battery was produced.

Evaluation example  One: After high temperature storage  Capacity retention rate and thickness increase rate

The cell capacity (cell capacity ₁ ) and thickness (thickness ₁ ) of each of the cells prepared in Example 1 and Comparative Examples 1 to 3 were measured. The battery was stored at 60 ° C. for 20 weeks at a high temperature and then charged at 1 C up to 4.2 V / 38 mA at a constant current / constant voltage condition at room temperature (25 ° C.), discharged at 3 C to 2.5 V under constant current conditions, Cell capacity ₂ ) and thickness (thickness ₂ ) were measured again. The battery capacity ₂ or the thickness ₂ was divided by the cell capacity ₁ or the thickness ₂ , and then multiplied by 100 to calculate the capacity retention ratio and the thickness increase rate after high temperature storage, respectively, and the results are shown in Table 1 below.

Evaluation example  2: Low temperature output

The voltage difference output generated by discharging the battery manufactured in Example 1 and Comparative Examples 1 to 3 at -30 ° C and SOC 50 at 5 ° C for 10 seconds was calculated and the results are shown in Table 1 below.

Day 20 After high temperature storage Low temperature output (W) Capacity retention rate (%) Thickness increase rate (%) Example 1 94.8 6.531 7.45584 Comparative Example 1 82.5 18.469 5.32619 Comparative Example 2 86.4 10.762 6.03709 Comparative Example 3 89.9 9.846 6.49013

Claims (9)

Lithium bisfluorosulfonylimide (LiFSI, primary lithium salt); And a non-aqueous electrolyte for a lithium secondary battery.
The method according to claim 1,
Wherein the lithium bisfluorosulfonylimide has a concentration in the range of 0.01 to 1 M in the non-aqueous electrolyte.
The method according to claim 1,
Wherein the phosphate compound is contained in an amount of 2 to 10% by weight based on the total weight of the non-aqueous electrolyte.
The method according to claim 1,
The non-aqueous electrolyte solution for a lithium secondary battery according to claim 1, wherein the phosphate compound is represented by the following formula (1)
[Chemical Formula 1]

In this formula,
a is a carbon atom (C) or a silicon atom (Si)
b is a hydrogen atom (H) or a fluorine atom (F)
n is 1 to 5;
The method according to claim 1,
Wherein the phosphate compound is selected from the group consisting of tris (2,2,2-trifluoroethyl) phosphate, tris (1,1,2,2-tetrafluoroethyl) phosphate, tris (hexafluoroisopropyl) (2,2,3,3-tetrafluoropropyl) dimethyl phosphate, bis (2,2,3,3-tetrafluoropropyl) methyl phosphate, tris (2,2,3,3-tetrafluoropropyl) (4-fluorophenyl) phosphate, and pentafluorophenylphosphate. 4. A non-aqueous electrolyte solution for a lithium secondary battery, comprising:
The method of claim 1,
Wherein the non-aqueous electrolyte, in addition to the first lithium _{salt, LiPF 6, LiAsF 6, LiCF} 3 SO 3, LiN (CF 3 SO 2) 2, LiBF 6, LiSbF 6, LiN (C 2 F 5 SO 2) 2, LiAlO ₄ , LiAlCl ₄ , LiSO ₃ CF _3, and LiClO ₄ as a second lithium salt. The non-aqueous electrolyte according to claim 1,
The method according to claim 6,
Wherein the second lithium salt is LiPF ₆ .
The method according to claim 6,
Wherein the first lithium salt and the second lithium salt are mixed in a molar ratio of 0.01: 1 to 1: 1.
An anode including a cathode active material,
A negative electrode including a negative electrode active material,
A separator interposed between the anode and the cathode,
A nonaqueous electrolytic solution according to any one of claims 1 to 8,
Wherein the positive electrode active material is a lithium nickel-manganese-cobalt oxide and the negative electrode active material is graphite.