JP7764586B2 - Lithium secondary battery - Google Patents

Description

This application claims the benefit of priority from Korean Patent Application No. 10-2021-0139182 filed with the Korean Intellectual Property Office on October 19, 2021, and Korean Patent Application No. 10-2022-0134631 filed with the Korean Intellectual Property Office on October 19, 2022, the entire contents of which are incorporated herein by reference.

The present invention relates to an electrolyte containing a specific combination of additives and a lithium secondary battery containing a high-nickel positive electrode material.

Lithium secondary batteries are generally manufactured by forming an electrode assembly by interposing a separator between a positive electrode containing a positive electrode active material made of a lithium-containing transition metal oxide and a negative electrode containing a negative electrode active material capable of storing lithium ions. The electrode assembly is then inserted into a battery case, which is then filled with a non-aqueous electrolyte solution that acts as a medium for transferring lithium ions, and sealed.

Lithium secondary batteries can be miniaturized and have high energy density and operating voltage, making them applicable in a variety of fields, including mobile devices, electronic products, and electric vehicles. As the range of applications for lithium secondary batteries becomes more diverse, the required physical properties are also becoming more stringent. In particular, there is a demand for the development of lithium secondary batteries that can operate stably even under high-temperature conditions.

On the other hand, when a lithium secondary battery is operated under high temperature conditions, PF ₆ ^- anions from lithium salts such as LiPF ₆ contained in the electrolyte may be thermally decomposed to generate Lewis acids such as PF ₅ , which react with moisture to generate HF. Such decomposition products such as PF ₅ and HF may not only destroy the coating formed on the surface of the electrode, but may also cause decomposition reactions of organic solvents.

In addition, the decomposition products of the electrolyte react with the decomposition products of the positive electrode active material, eluting transition metal ions. These eluted transition metal ions may then be electrodeposited on the negative electrode, destroying the coating formed on the surface of the negative electrode, further reducing battery performance.

In particular, when the nickel content in the positive electrode active material is increased to realize a high-capacity battery, the initial capacity characteristics are improved, but repeated charge and discharge cycles result in the generation of a large amount of highly reactive Ni ⁴⁺ ions, which causes the structure of the positive electrode active material to collapse, resulting in a problem of accelerating the rate of battery degradation.

Therefore, there is a need to develop secondary batteries that contain high-nickel positive electrode materials and can maintain excellent performance even under high-temperature conditions.

The present invention aims to solve the problems described above by providing a lithium secondary battery containing a high-nickel positive electrode active material with improved high-temperature performance by incorporating a nonaqueous electrolyte solution containing a combination of two specific additives.

According to one embodiment, the present invention comprises:
a non-aqueous electrolyte solution containing a lithium salt, an organic solvent, a first additive represented by the following chemical formula 1, and a second additive represented by the following chemical formula 2;
a positive electrode including a positive electrode active material including a lithium transition metal composite oxide in which the content of nickel in the total content of transition metals is 70 mol % or more;
a negative electrode including a negative electrode active material;
and a separator interposed between the positive electrode and the negative electrode.

In the above Chemical Formula 1,
R1 and R2 each independently represent a direct bond; or an alkylene group having 1 to 5 carbon atoms;
In the above Chemical Formula 2,
A is —NR3R4, a nitrogen-containing heterocyclic group having 3 to 5 carbon atoms, or a nitrogen-containing heteroaryl group having 3 to 5 carbon atoms;
R3 and R4 each independently represent an alkyl group having 1 to 5 carbon atoms,
R5 is an alkylene group having 1 to 3 carbon atoms.

The lithium secondary battery according to the present invention contains a high-nickel (High-Ni) positive electrode active material and an electrolyte containing a specific combination of additives, thereby exhibiting excellent high-temperature stability.

The present invention is described in more detail below.

Generally, anions contained in lithium salts such as _LiPF6, which are widely used in electrolytes for lithium secondary batteries, form decomposition products such as hydrogen fluoride (HF) and _PF5 due to thermal decomposition or moisture. These decomposition products have acid properties and deteriorate the coating or electrode surface in the battery.

Transition metals in the positive electrode are easily eluted into the electrolyte due to decomposition products of the electrolyte and structural changes in the positive electrode caused by repeated charging and discharging. The eluted transition metals are then redeposited on the positive electrode, increasing the resistance of the positive electrode. Furthermore, if the eluted transition metals migrate to the negative electrode via the electrolyte, they are electrodeposited on the negative electrode, causing destruction of the solid electrolyte interphase (SEI) film and further electrolyte decomposition reactions, resulting in problems such as increased lithium ion consumption and resistance.

Furthermore, during initial activation of the battery, a protective coating is formed on the positive and negative electrodes due to electrolyte reactions. However, if the coating becomes unstable for the reasons mentioned above, further decomposition of the electrolyte occurs during charging/discharging or exposure to high temperatures, accelerating battery degradation and generating gas.

In particular, batteries containing positive electrode active materials with a high nickel content have improved initial capacity characteristics, but repeated charging and discharging can lead to increased lithium by-products and gas generation due to side reactions, which can worsen the decomposition reaction of the electrolyte.

To solve this problem, the inventors added a first additive represented by the following Chemical Formula 1 and a second additive that undergoes a reduction reaction at a higher potential than the first additive to a non-aqueous electrolyte solution, thereby allowing the second additive to form a film first during initial charging, maximizing the gas reduction effect of the first additive. Furthermore, the second additive binds with _PF5 to suppress the generation of HF, thereby preventing destruction of the cathode electrolyte interphase (CEI) film formed on the surface of the positive electrode by the first additive, thereby maximizing the effect of the first additive and suppressing further electrolyte decomposition reactions.

The following describes each component of the present invention in more detail.

Nonaqueous Electrolyte The lithium secondary battery according to the present invention includes a nonaqueous electrolyte containing a lithium salt, an organic solvent, a first additive represented by the following Chemical Formula 1, and a second additive represented by the following Chemical Formula 2.

The components of the non-aqueous electrolyte are explained in detail below.

(1) First Additive and Second Additive The nonaqueous electrolyte solution of the present invention contains a first additive represented by the following chemical formula 1.

In the above Chemical Formula 1,
R1 and R2 each independently represent a direct bond; or an alkylene group having 1 to 5 carbon atoms.

The first additive represented by Chemical Formula 1 has a structure containing succinic anhydride, which allows it to form a film on the negative electrode during initial charging and later decomposes on the surface of the positive electrode at high potential, forming a film thereon. This makes it effective in reducing the amount of gas generated by the battery.

In one embodiment of the present invention, R1 and R2 in Chemical Formula 1 may each independently represent a direct bond; or a linear or branched alkylene group having 1 to 5 carbon atoms.

In one embodiment of the present invention, R1 and R2 in Chemical Formula 1 may each independently represent a direct bond or a linear alkylene group having 1 to 5 carbon atoms.

In one embodiment of the present invention, R1 and R2 in Chemical Formula 1 may each independently represent a direct bond or a methylene group.

In one embodiment of the present invention, R1 in Chemical Formula 1 may be a direct bond, and R2 may be an alkylene group having 1 to 5 carbon atoms.

In one embodiment of the present invention, R1 in Chemical Formula 1 may be a direct bond and R2 may be a methylene group.

In one embodiment of the present invention, the first additive may be represented by the following chemical formula 1A:

In one embodiment of the present invention, the content of the first additive may be 0.01 wt % to 5 wt %, preferably 0.5 wt % to 3 wt %, and more preferably 1.5 wt % to 2.5 wt %, based on the total weight of the non-aqueous electrolyte. A content of the first additive of 5 wt % or less is preferred because it reduces gas generation during battery operation and minimizes an increase in battery resistance.

The nonaqueous electrolyte of the present invention contains a second additive represented by the following chemical formula 2:

In the above Chemical Formula 2,
A is —NR3R4, a nitrogen-containing heterocyclic group having 3 to 5 carbon atoms, or a nitrogen-containing heteroaryl group having 3 to 5 carbon atoms;
R3 and R4 each independently represent an alkyl group having 1 to 5 carbon atoms,
R5 is an alkylene group having 1 to 3 carbon atoms.

The second additive represented by Chemical Formula 2 contains a propargyl functional group, which undergoes reductive decomposition to form a highly passivating SEI film on the surface of the negative electrode, improving the high-temperature durability of the negative electrode itself and preventing electrodeposition of transition metals on the negative electrode surface. Furthermore, the propargyl group adsorbs to the surface of metallic impurities contained in the positive electrode, making the impurities less likely to dissolve. This prevents internal short circuits that can occur when dissolved metal ions are deposited on the negative electrode.

Furthermore, since the second additive has a higher potential at which a reduction reaction occurs at the negative electrode than the first additive, the second additive is consumed first to form a film on the negative electrode during initial charging. As a result, a large amount of the first additive remains unreduced during initial charging. Later, the first additive is oxidized and decomposed on the high-potential positive electrode surface, forming a film on the surface of the positive electrode and suppressing side reactions between the positive electrode and electrolyte, thereby maximizing the gas reduction effect.

Specifically, the reduction potential of the first additive may be 1.1 V to 1.3 V, and the reduction potential of the second additive may be 1.5 V to 1.7 V. In the present invention, the reduction potential refers to the reduction decomposition potential, and refers to the value measured by the method described in Experimental Example 1 below.

In one embodiment of the present invention, the second additive may be represented by the following chemical formula 2-1 or 2-2.

In the chemical formulas 2-1 and 2-2,
R3 to R5 are as defined in Chemical Formula 2 above.

In one embodiment of the present invention, A in Chemical Formula 2 may be a nitrogen-containing heteroaryl group having 3 to 5 carbon atoms, and preferably, the second additive may be represented by Chemical Formula 2-2.

In one embodiment of the present invention, R5 in Chemical Formula 2 may be a linear or branched alkylene group having 1 to 3 carbon atoms, preferably a linear alkylene group having 1 to 3 carbon atoms, and more preferably a methylene group.

In one embodiment of the present invention, the second additive may be represented by the following chemical formula 2A:

In one embodiment of the present invention, the content of the second additive may be 0.01 wt % to 5 wt %, preferably 0.5 wt % to 3 wt %, and more preferably 1.5 wt % to 2.5 wt %, based on the total weight of the non-aqueous electrolyte. A content of the second additive of 5 wt % or less is preferred because it allows a coating to be formed on the negative electrode and minimizes an increase in battery resistance.

Furthermore, based on the total weight of the non-aqueous electrolyte, the contents of the first additive and the second additive may each be 1 wt % or more, preferably 1.5 wt % or more, and most preferably 2.0 wt % or more. In this case, the amount of gas generated can be reduced and the voltage drop phenomenon can be alleviated. However, in order to prevent an increase in battery resistance, it is preferable that the contents of the first additive and the second additive each be 5 wt % or less.

In one embodiment of the present invention, the weight ratio of the first additive to the second additive in the non-aqueous electrolyte may be 1:2 to 2:1, preferably 1:1 to 2:1.

(2) Third Additive The nonaqueous electrolyte solution of the present invention may further contain a third additive as needed to prevent the electrolyte solution from being decomposed in a high-voltage environment, which may cause electrode collapse, or to further improve low-temperature high-rate discharge characteristics, high-temperature stability, overcharge prevention, and the effects of suppressing battery expansion at high temperatures.

The third additive may be one or more compounds selected from cyclic carbonate compounds, halogen-substituted carbonate compounds, sultone compounds, sulfate compounds, phosphate or phosphite compounds, nitrile compounds, amine compounds, silane compounds, benzene compounds, and lithium salt compounds.

The cyclic carbonate compound may be one or more compounds selected from vinylene carbonate (VC) and vinyl ethylene carbonate (VEC), and specifically may be vinylene carbonate.

The halogen-substituted carbonate compound may be fluoroethylene carbonate (FEC).

The sultone-based compound is a material capable of forming a stable SEI film on the surface of the negative electrode through a reduction reaction, and may be one or more compounds selected from 1,3-propane sultone (PS), 1,4-butane sultone, ethene sultone, 1,3-propene sultone (PRS), 1,4-butene sultone, and 1-methyl-1,3-propene sultone, and specifically 1,3-propane sultone (PS).

The sulfate-based compound is a material that can be electrically decomposed on the surface of the negative electrode to form a stable SEI film that does not crack even when stored at high temperatures, and may be one or more selected from ethylene sulfate (Esa), trimethylene sulfate (TMS), and methyl trimethylene sulfate (MTMS).

The phosphate or phosphite compound may be one or more selected from lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite, tris(2,2,2-trifluoroethyl)phosphate, and tris(trifluoroethyl)phosphite.

The nitrile compound may be one or more selected from succinonitrile (SN), adiponitrile (ADN), acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, 4-fluorophenylacetonitrile, ethylene glycol bis(2-cyanoethyl)ether (ASA3), 1,3,6-hexanetricarbonitrile (HTCN), 1,4-dicyano-2-butene (DCB), and 1,2,3-tris(2-cyanoethyl)propane (TCEP).

The amine compound may be one or more selected from triethanolamine and ethylenediamine, and the silane compound may be tetravinylsilane.

The benzene-based compound may be one or more selected from monofluorobenzene, difluorobenzene, trifluorobenzene, and tetrafluorobenzene.

The lithium salt-based compound is a compound different from the lithium salt contained in the non-aqueous electrolyte, and may be one or more compounds selected from lithium difluorophosphate (LiDFP; LiPO ₂ F ₂ ), lithium bis(oxalato)borate (LiBOB; LiB(C ₂ O ₄ ) ₂ ), lithium tetrafluoroborate (LiBF ₄ ), lithium tetraphenylborate, and lithium difluoro(bis(oxalato)phosphate) (LiDFOP).

Preferably, the non-aqueous electrolyte according to one embodiment of the present invention may further include one or more third additives selected from vinylene carbonate (VC), vinylethylene carbonate (VEC), fluoroethylene carbonate (FEC), 1,3-propane sultone (PS), 1,3-propene sultone (PRS), ethylene sulfate (ESa), succinonitrile (SN), adiponitrile (ADN), ethylene glycol bis(2-cyanoethyl)ether (ASA3), 1,3,6-hexanetricarbonitrile (HTCN), 1,4-dicyano-2-butene (DCB), 1,2,3-tris(2-cyanoethyl)propane (TCEP), lithium difluorooxalatoborate (LiODFB), lithium tetrafluoroborate (LiBF ₄ ), lithium difluoro(bisoxalato)phosphate (LiDFOP), and lithium difluorophosphate (LiDFP).

More preferably, the nonaqueous electrolyte according to one embodiment of the present invention may further contain one or more third additives selected from vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propane sultone (PS), ethylene sulfate (ESa), succinonitrile (SN), lithium tetrafluoroborate (LiBF ₄ ), lithium difluorooxalatoborate (LiDFOB), and lithium difluorophosphate (LiDFP). In this case, in addition to the film-forming effect of the first and second additives, the formation of dense films on the negative and positive electrodes produces a synergistic effect of improving ion transport characteristics and cycle performance and suppressing side reactions at high temperatures.

Meanwhile, the content of the third additive may be 0.1 wt % to 10 wt % based on the total weight of the non-aqueous electrolyte solution, and preferably 0.2 wt % to 2 wt %. When the content of the third additive is within the above range, it is possible to suppress side reactions by forming a dense coating on the positive and negative electrodes, and to increase the long-term life and durability of the battery.

(3) Organic Solvent The nonaqueous electrolyte solution of the present invention contains an organic solvent.

The organic solvent can be any of the various organic solvents commonly used in lithium electrolytes, without limitation. For example, the organic solvent may be a cyclic carbonate solvent, a linear carbonate solvent, a linear ester solvent, a cyclic ester solvent, a nitrile solvent, or a mixture thereof. Preferably, the organic solvent may include a mixture of a cyclic carbonate solvent and a linear carbonate solvent.

The cyclic carbonate solvent is a highly viscous organic solvent that has a high dielectric constant and therefore easily dissociates the lithium salt in the electrolyte. It may be one or more solvents selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, and vinylene carbonate, and preferably contains ethylene carbonate (EC) or propylene carbonate (PC).

The linear carbonate solvent is an organic solvent having low viscosity and low dielectric constant, and may be one or more selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate, and preferably includes ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), or diethyl carbonate (DEC).

In order to produce an electrolyte solution with high ionic conductivity, it is preferable to use a mixture of a cyclic carbonate solvent and a linear carbonate solvent as the organic solvent.

The linear ester solvent may be one or more selected from methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate, and preferably methyl propionate, ethyl propionate, or propyl propionate.

The cyclic ester solvent may be one or more selected from gamma-butyrolactone, gamma-valerolactone, gamma-caprolactone, sigma-valerolactone, and epsilon-caprolactone.

The nitrile solvent may be one or more selected from succinonitrile, acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile, and preferably succinonitrile.

Unless otherwise specified, the remainder of the total weight of the nonaqueous electrolyte solution, excluding the organic solvent, other components, such as the first additive represented by Chemical Formula 1, the second additive represented by Chemical Formula 2, the third additive, and the lithium salt, is the organic solvent.

(4) Lithium Salt The nonaqueous electrolyte solution of the present invention contains a lithium salt.

The lithium salt may be any of those commonly used in electrolytes for lithium secondary batteries, without any limitation. Specifically, the lithium salt may contain Li ⁺ as a cation and F ⁻ , Cl ⁻ , Br ⁻ , I ⁻ , NO ₃ ⁻ , N(CN) ₂ ⁻ , BF ₄ ⁻ , ClO ₄ ⁻ , B ₁₀ Cl ₁₀ ⁻ , AlCl ₄ ⁻ , AlO ₄ ⁻ , PF ₆ ⁻ , CF ₃ SO ₃ ⁻ , CH ₃ CO ₂ ⁻ , CF ₃ CO ₂ ⁻ , AsF ₆ ⁻ , SbF ₆ ⁻ , CH ₃ SO ₃ ⁻ , (CF ₃ CF ₂ SO ₂ ) ₂ N ⁻ , (CF ₃ SO ₂ ) ₂ N ⁻ , (FSO ₂ ) ₂ N ⁻ , BF ₂ C ₂ O ₄ ⁻ , BC ₄ O ₈ ⁻ , BF ₂ C ₂ O ₄ CHF ⁻ , PF ₄ C ₂ O ₄ ⁻ , PF ₂ C ₄ O ₈ ⁻ , PO ₂ F ₂ ⁻ , (CF ₃ ) ₂ PF ₄ ^- , (CF ₃ ) ₃ PF ₃ ^- , (CF ₃ ) ₄ PF ₂ ^- , (CF ₃ ) ₅ PF ^- , (CF ₃ ) ₆ P ^- , C ₄ F ₉ SO ₃ ^- , CF ₃ CF ₂ SO ₃ ^- , CF ₃ CF ₂ (CF ₃ ) ₂ CO ^- , (CF ₃ SO ₂ ) ₂ CH ⁻ , CF ₃ (CF ₂ ) ₇ SO ₃ ⁻ , and SCN ⁻ may be included.

Specifically, the lithium salt may be LiPF ₆ , LiClO ₄ , LiBF ₄ , LiN(FSO ₂ ) ₂ (LiFSI), LiTFSI, lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), LiSO ₃ CF ₃ , LiPO ₂ F ₂ , lithium bis(oxalate)borate (LiBOB), lithium difluoro(oxalate)borate (LiFOB), lithium difluoro(bisoxalato)phosphate (Li The fluoroisotope may be one or more selected from lithium tetrafluoro(oxalato)phosphate (LiDFOP), lithium difluoro(bisoxalato)phosphate (LiDFOP), lithium tetrafluoro(oxalate)phosphate (LiTFOP), and lithium fluoromalonato(difluoro)borate (LiFMDFB), and preferably may be _LiPF6 .

In one embodiment of the present invention, the concentration of the lithium salt in the non-aqueous organic solution containing the lithium salt and an organic solvent may be 0.5 M to 4.0 M, specifically 0.5 M to 3.0 M, and more specifically 0.8 M to 2.0 M. When the lithium salt concentration is within the above range, the effects of improving low-temperature output and cycle characteristics are sufficiently ensured, while excessive increases in viscosity and surface tension are prevented, and appropriate electrolyte impregnation is achieved.

Positive Electrode The lithium secondary battery according to the present invention includes a positive electrode containing a positive electrode active material including a lithium composite transition metal oxide in which the content of nickel in the total content of transition metals is 70 mol % or more.

High-nickel (High-Ni) positive electrode active materials, in which the nickel content of the total transition metal content is 70 mol% or more, are structurally unstable and can collapse and crack at high temperatures and high potentials. The increased surface area of such collapsed active materials increases the number of sites where side reactions with the electrolyte can occur, resulting in the generation of large amounts of gas when exposed to high temperatures and high potentials. If this gas generation continues, the internal pressure of the battery increases, causing premature venting and resulting in the battery ceasing operation before its end of life. Therefore, it is important to prevent these side reactions and gas generation, which can be improved by strengthening the formation of a coating using a nonaqueous electrolyte containing the first and second additives of the present invention.

In one embodiment of the present invention, the lithium composite transition metal oxide may be represented by the following chemical formula 3:

[Chemical formula 3]
Li _1+x ( _Nia Co _b Mn _c M _d ) O ₂

In the above Chemical Formula 3,
M is one or more selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo;
0≦x≦0.2, 0.70≦a<1, 0<b≦0.25, 0<c≦0.25, 0≦d≦0.1, and a+b+c+d=1.

Preferably, a, b, c, and d may be in the ranges 0.80≦a≦0.95, 0.025≦b≦0.15, 0.025≦c≦0.15, and 0≦d≦0.05, respectively.

The positive electrode according to the present invention can be manufactured by coating a positive electrode current collector with a positive electrode slurry containing a positive electrode active material, a binder, a conductive material, a solvent, etc., followed by drying and rolling.

The positive electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, stainless steel; aluminum; nickel; titanium; baked carbon; or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like may be used.

The positive electrode active material may be included in an amount of 80% to 99% by weight, specifically 90% to 99% by weight, based on the total weight of the solids in the positive electrode slurry. In this case, if the content of the positive electrode active material is less than 80% by weight, the energy density may be low, resulting in a decrease in capacity.

The binder is a component that aids in binding the active material and conductive material, etc., and in binding them to the current collector, and is typically added in an amount of 1 to 30% by weight based on the total weight of the solids in the positive electrode slurry. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, or various copolymers thereof.

The conductive material is a substance that provides conductivity without causing any chemical changes to the battery, and may be added in an amount of 0.5% to 20% by weight based on the total weight of solids in the positive electrode slurry.

Examples of the conductive material may be selected from the following conductive materials: carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powders such as natural graphite, artificial graphite, carbon nanotubes, and graphite; conductive fibers such as carbon fibers and metal fibers; conductive powders such as carbon fluoride powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

The solvent for the positive electrode slurry may also include an organic solvent such as NMP (N-methyl-2-pyrrolidone), and may be used in an amount that provides a suitable viscosity when the positive electrode active material, binder, conductive material, etc. are included. For example, the positive electrode slurry containing the positive electrode active material, binder, and conductive material may be included so that the solids concentration is 40% to 90% by weight, preferably 50% to 80% by weight.

A lithium secondary battery according to the present invention includes a negative electrode including a negative electrode active material, and the negative electrode can be manufactured by coating a negative electrode current collector with a negative electrode slurry including a negative electrode active material, a binder, a conductive material, a solvent, etc., followed by drying and rolling.

The negative electrode current collector generally has a thickness of 3 μm to 500 μm. There are no particular restrictions on the negative electrode current collector, so long as it does not cause chemical changes in the battery and has high conductivity. Examples of materials that may be used include copper; stainless steel; aluminum; nickel; titanium; sintered carbon; copper or stainless steel surfaces treated with carbon, nickel, titanium, silver, or the like; and aluminum-cadmium alloys. As with the positive electrode current collector, the surface may be made finely uneven to strengthen the bonding strength of the negative electrode active material, and the negative electrode current collector may be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

The negative electrode active material may also include one or more selected from the group consisting of carbonaceous materials capable of reversibly intercalating/deintercalating lithium ions; metals or alloys of these metals with lithium; metal composite oxides; materials capable of doping and dedoping lithium; lithium metal; and transition metal oxides.

The carbonaceous material capable of reversibly intercalating/deintercalating lithium ions can be any carbonaceous negative electrode active material commonly used in lithium ion secondary batteries, without any particular limitations. Representative examples include crystalline carbon, amorphous carbon, or a combination of these. Examples of crystalline carbon include graphite, such as amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite. Examples of amorphous carbon include soft carbon (low-temperature calcined carbon), hard carbon, mesophase pitch carbide, and calcined coke.

The metals or alloys of these metals with lithium can be selected from the group consisting of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn, or alloys of these metals with lithium.

The metal composite oxide may be one or _more selected _from the group consisting of PbO, _PbO2 , _Pb2O3 , _Pb3O4 , _Sb2O3 , _Sb2O4 , _Sb2O5 , GeO, GeO2, _Bi2O3 , _{Bi2O4 , Bi2O5 , LixFe2O3} (0≦ _x ≦ ₁ ), _LixWO2 (0≦ _x ≦1), and _{SnxMe1 - xMe'yOz (} Me: _Mn , _Fe , Pb, Ge; Me _' : Al, B _, P, Si, _elements of Groups 1, ₂ and 3 of the periodic table, halogens; 0<x≦1;1≦y≦3; 1≦z≦8).

Examples of the substance capable of doping and dedoping lithium include Si, SiO _x (0<x<2), Si-Y alloy (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Si), Sn, SnO ₂ , Sn-Y (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Sn), and at least one of these may be mixed with SiO ₂ for use. The element Y may be selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db (dubnium), Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof.

Examples of the transition metal oxide include lithium-containing titanium oxide (LTO), vanadium oxide, and lithium vanadium oxide.

In one embodiment of the present invention, the negative electrode active material may include graphite.

The negative electrode active material may be contained in an amount of 80% to 99% by weight based on the total weight of solids in the negative electrode slurry.

The binder is a component that aids in bonding between the conductive material, active material, and current collector, and is typically added in an amount of 1 to 30% by weight based on the total weight of solids in the negative electrode slurry. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, and various copolymers thereof.

The conductive material is a component for further improving the conductivity of the negative electrode active material and may be added in an amount of 0.5% to 20% by weight based on the total weight of the solids in the negative electrode slurry. There are no particular restrictions on the conductive material, so long as it does not cause chemical changes in the battery and is conductive. Examples of such conductive materials include carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal black; graphite powder such as natural graphite with a highly developed crystalline structure, artificial graphite, carbon nanotubes, or graphite; conductive fibers such as carbon fiber or metal fiber; conductive powders such as carbon fluoride powder, aluminum powder, or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive materials such as polyphenylene derivatives.

The solvent for the negative electrode slurry may include water or an organic solvent such as NMP or alcohol, and may be used in an amount that results in an appropriate viscosity when the negative electrode active material, binder, conductive material, etc. are contained. For example, the solvent may be contained so that the solids concentration in the slurry containing the negative electrode active material, binder, and conductive material is 30% to 80% by weight, preferably 40% to 70% by weight.

Separator The lithium secondary battery according to the present invention includes a separator between the positive electrode and the negative electrode.

The separator separates the negative electrode and positive electrode and provides a path for lithium ions to move. Any separator commonly used in lithium secondary batteries can be used without any particular restrictions. In particular, separators that offer low resistance to the movement of ions in the electrolyte, excellent electrolyte impregnation capacity, and safety are preferred.

Specifically, the separator may be a porous polymer film, such as a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, or an ethylene/methacrylate copolymer; or a laminate structure of two or more layers of these. Ordinary porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may also be used. Furthermore, to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymeric substance may also be used, and may be used in a single-layer or multi-layer structure.

The lithium secondary battery according to the present invention as described above can be useful in portable devices such as mobile phones, laptop computers, and digital cameras, as well as in electric vehicles such as hybrid electric vehicles (HEVs).

Accordingly, according to another embodiment of the present invention, a battery module including the lithium secondary battery as a unit cell and a battery pack including the same are provided.

The battery module or battery pack can be used as a power source for one or more medium- to large-sized devices, including power tools; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); and power storage systems.

The external shape of the lithium secondary battery of the present invention is not particularly limited, but may be a cylindrical shape using a can, a rectangular shape, a pouch shape, a coin shape, or the like.

The lithium secondary battery of the present invention can be used not only as a battery cell used as a power source for small devices, but also as a unit battery in medium- to large-sized battery modules containing multiple battery cells.

The present invention will be explained in more detail below using specific examples.

<Example>
Example 1.
(Production of non-aqueous electrolyte)
Ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a volume ratio of 30:70, and then _LiPF6 was dissolved therein to a concentration of 1.2 M to prepare a non-aqueous organic solution. 0.5 wt % of the compound represented by Chemical Formula 1A, 0.5 wt % of the compound represented by Chemical Formula 2A, 1 wt % of vinylene carbonate (VC), 1 wt % of 1,3-propane sultone (PS), and the remainder of the non-aqueous organic solution were mixed together to prepare a 100 wt % non-aqueous electrolyte.

(Manufacturing of lithium secondary batteries)
A positive _{electrode slurry (solid content: 60 wt%) was prepared by adding LiNi0.8Co0.1Mn0.1O2 as a positive electrode active material ,} a conductive material (carbon black), and a binder (polyvinylidene fluoride) to N-methyl-2-pyrrolidone (NMP) in a weight ratio of 98:1:1. The positive electrode slurry was applied to a 13.5 μm-thick aluminum (Al) thin film as a positive electrode current collector, dried, and then roll-pressed to prepare a positive electrode.

Anode active material (graphite), binder (SBR-CMC), and conductive material (carbon black) were mixed in a weight ratio of 97.6:2.2:0.2 with water as a solvent to prepare anode slurry (solid content: 60 wt%). The anode slurry was applied to a 6 μm-thick copper (Cu) thin film as anode current collector, dried, and then roll-pressed to prepare the anode.

The positive electrode, a polyolefin-based porous separator coated with inorganic particles (Al ₂ O ₃ ), and a negative electrode were sequentially stacked to prepare an electrode assembly.

The electrode assembly assembled above was placed in a pouch-shaped battery case, and the non-aqueous electrolyte prepared above was poured into it to produce a lithium secondary battery.

Example 2.
A lithium secondary battery was fabricated in the same manner as in Example 1, except that the content of the compound represented by Formula 1A was changed to 1.0 wt % when preparing the non-aqueous electrolyte.

Example 3.
A lithium secondary battery was fabricated in the same manner as in Example 1, except that the content of the compound represented by Formula 2A was changed to 1.0 wt % when preparing the non-aqueous electrolyte.

Example 4.
A lithium secondary battery was fabricated in the same manner as in Example 1, except that the content of the compound represented by Chemical Formula 1A was changed to 1.0 wt % and the content of the compound represented by Chemical Formula 2A was changed to 1.0 wt % during the preparation of the non-aqueous electrolyte solution.

Example 5.
A lithium secondary battery was fabricated in the same manner as in Example 1, except that the content of the compound represented by Chemical Formula 1A was changed to 2.0 wt % and the content of the compound represented by Chemical Formula 2A was changed to 2.0 wt % during the preparation of the non-aqueous electrolyte solution.

Comparative Example 1.
A lithium secondary battery was manufactured in the same manner as in Example 1, except that the compound represented by Chemical Formula 1A and the compound represented by Chemical Formula 2A were not added during the preparation of the non-aqueous electrolyte solution.

Comparative Example 2.
A lithium secondary battery was fabricated in the same manner as in Example 1, except that the compound represented by Formula 2A was not added during the preparation of the non-aqueous electrolyte solution.

Comparative Example 3.
A lithium secondary battery was fabricated in the same manner as in Example 1, except that the compound represented by Formula 1A was not added during the preparation of the non-aqueous electrolyte.

<Experimental Example>
Experimental Example 1. Measurement of Reduction Potential Ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a volume ratio of 20:80, and _LiPF6 was dissolved therein to prepare a non-aqueous organic solution to a concentration of 1.0 M. 1 g of the compound represented by Formula 1A was added to 99 g of the non-aqueous organic solution and mixed to prepare a non-aqueous electrolyte.

The nonaqueous electrolyte solution thus prepared was poured into a 2032 coin cell having a polyethylene (PE) separator sandwiched between a graphite electrode and a lithium foil electrode. Then, lithium ions were inserted into the graphite electrode until the potential of the graphite electrode reached 0.05 V, and the potential at which the compound represented by Chemical Formula 1A reacted was measured. The measured potential values are listed in Table 1 below as the reduction potential range, and are potential values relative to the reference electrode, Li.

The reduction potential range of the compound represented by chemical formula 2A was also measured using a similar procedure and is shown in Table 1 below.

Experimental Example 2. Evaluation of Gas Generation Amount Each of the lithium secondary batteries manufactured in the above examples and comparative examples was activated at 0.2 C/CV at 25°C and then degassed. After aging at room temperature, the batteries were charged to 4.20 V at 0.33 C/CV and then stored at 70°C for 4 weeks. After high-temperature storage, the amounts of gases generated in the lithium secondary batteries, such as CO and _CO2 , were measured. The relative gas generation amount of each battery was calculated, assuming the gas generation amount measured in Comparative Example 1 as 100%, and is shown in Table 2 below.

Experimental Example 3: Evaluation of Voltage Drop Each of the lithium secondary batteries prepared in the Examples and Comparative Examples was activated at 0.1 C cc and then degassed. The OCV (Open Circuit Voltage, mV) was measured at 25°C. The batteries were then stored at 60°C for 8 weeks, after which the OCV was measured again. The difference in OCV (dOCV) before and after high-temperature storage is shown in Table 2 below.

The results in Table 2 confirm that the batteries of Examples 1 to 5, which used electrolytes containing both the first additive represented by Chemical Formula 1 and the second additive represented by Chemical Formula 2, showed less gas generation and OCV reduction after high-temperature storage than the batteries of Comparative Examples 1 to 3, despite containing a high-nickel positive electrode active material.

Among these, Examples 4 and 5, in which the contents of both the first additive and the second additive were 1% by weight or more, were found to have low measured gas generation and OCV reduction. In particular, Example 5 was found to have the lowest measured gas generation and OCV reduction.

In contrast, the battery of Comparative Example 1, which uses an electrolyte containing neither the first nor the second additive, the battery of Comparative Example 2, which uses an electrolyte containing only the first additive, and the battery of Comparative Example 3, which uses an electrolyte containing only the second additive, were found to have higher gas generation and OCV reduction rates after high-temperature storage than the battery of Example 1.

Claims (7)

a non-aqueous electrolyte solution containing a lithium salt, an organic solvent, a first additive represented by the following chemical formula 1, and a second additive represented by the following chemical formula 2-1 or 2-2 ;
a positive electrode including a positive electrode active material including a lithium transition metal composite oxide in which the content of nickel in the total content of transition metals is 70 mol % or more;
a negative electrode including a negative electrode active material;
a separator interposed between the positive electrode and the negative electrode,
A lithium secondary battery, wherein the contents of the first additive and the second additive are from 0.01 wt % to 5 wt % based on the total weight of the non-aqueous electrolyte solution :
In the above Chemical Formula 1,
R1 and R2 each independently represent a direct bond; or an alkylene group having 1 to 5 carbon atoms;
In the chemical formulas 2-1 and 2-2 ,
R3 and R4 are each independently an alkyl group having 1 to 5 carbon atoms;
R5 is an alkylene group having 1 to 3 carbon atoms . The lithium secondary battery of claim 1, wherein R1 in Chemical Formula 1 is a direct bond and R2 is an alkylene group having 1 to 5 carbon atoms. The lithium secondary battery of claim 1, wherein the contents of the first additive and the second additive are each 1% by weight or more and 5% by weight or less, based on the total weight of the non-aqueous electrolyte. 2. The lithium secondary battery of claim 1, wherein the non-aqueous electrolyte further comprises one or more third additives selected from vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propane sultone (PS), ethylene sulfate (ESa), succinonitrile (SN), lithium tetrafluoroborate (LiBF ₄ ), lithium difluorooxalatoborate (LiDFOB), and lithium difluorophosphate (LiDFP). The lithium secondary battery of claim 1, wherein the organic solvent includes a mixture of a cyclic carbonate solvent and a linear carbonate solvent. The lithium secondary battery according to claim 1 , wherein the lithium composite transition metal oxide is represented by the following Chemical Formula 3:
[Chemical formula 3]
Li _1+x ( _Nia Co _b Mn _c M _d ) O ₂
(In the above chemical formula 3,
M is one or more selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo;
0≦x≦0.2, 0.70≦a<1, 0<b≦0.25, 0<c≦0.25, 0≦d≦0.1, a+b+c+d=1.) 7. The lithium secondary battery according to claim 6 , wherein a, b, c, and d in Chemical Formula 3 are in the ranges of 0.80≦a≦0.95, 0.025≦b≦0.15, 0.025≦c≦0.15, and 0≦d≦0.05, respectively.