KR20250024345A - Flame retardant electrolyte comprising fluorosulfonate solvent - Google Patents

Abstract

The present invention relates to a flame retardant electrolyte for a lithium secondary battery, and more particularly, to a flame retardant electrolyte capable of imparting flame retardancy to a secondary battery by including a fluorinated sulfonate solvent.

Description

Flame retardant electrolyte comprising fluorosulfonate solvent

The present invention relates to a flame retardant electrolyte for a lithium secondary battery, and more particularly, to a flame retardant electrolyte capable of imparting flame retardancy to a secondary battery by including a fluorinated sulfonate solvent.

With the popularization of mobile devices, the commercialization of electric vehicles, and the increase in demand for power storage devices, secondary batteries with high output, high energy density, and high discharge voltage are being developed. In particular, lithium secondary batteries have high energy density, so they are used as power sources for mobile communication devices and portable information terminals, and their market is growing rapidly along with the spread of terminals. Since these lithium secondary batteries operate at high operating voltages, aqueous electrolytes that are highly reactive with lithium cannot be used. Non-aqueous electrolytes, i.e. organic electrolytes, are used in lithium secondary batteries.

A lithium secondary battery includes a negative electrode, a positive electrode, and a separator along with a non-aqueous electrolyte.

In general, lithium metal oxide that absorbs and releases lithium ions is used as the positive electrode of a lithium secondary battery, carbon or silicon materials, lithium metal, lithium alloy, etc. that absorb and release lithium ions are used as the negative electrode, and an electrolyte prepared by dissolving a lithium salt in an organic solvent that is liquid at room temperature is used as the electrolyte. Examples of organic solvents used in the electrolyte include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, propiolactone, valerolactone, tetrahydrofuran, dimethoxyethane, diethoxyethane, and methoxyethoxyethane.

However, since the organic solvent mentioned above is a flammable substance that is easily volatile and highly flammable, there are problems such as overcharge, overdischarge, and short circuit when used in lithium secondary batteries, especially in relatively large-capacity lithium secondary batteries used for power storage and electric vehicle power sources, and there are safety issues such as the possibility of ignition.

Republic of Korea Publication Patent No. 10-2019-0021160 Republic of Korea Publication Patent No. 10-2020-0080224 Republic of Korea Publication Patent No. 10-2018-0108928

Accordingly, methods to increase flame retardancy by using fluorinated carbonates, fluorinated esters, fluorinated ethers, etc. as flame retardant solvents have been examined. However, these solvents have the problem of insufficient solubility in lithium salts.

The object of the present invention is to provide an electrolyte for a lithium secondary battery, which comprises a solvent having excellent safety properties such that it has low volatility and flammability, thus lowering the possibility of ignition, and reducing the risk of fire and explosion, and in particular, a flame-retardant electrolyte comprising a solvent having low viscosity, thus sufficiently increasing lithium ion conductivity, and sufficient solubility for a lithium salt.

In addition, the task of the present invention is to provide a flame retardant electrolyte that has a wide usable temperature range, high miscibility with other solvents, and high lithium ion conductivity, thereby providing high output performance such as rapid charging and discharging and high performance to a lithium secondary battery.

To achieve the above purpose, the present invention

A flame-retardant electrolyte for a lithium secondary battery comprising a fluorinated sulfonate solvent of the following chemical formula 1 is provided.

[Chemical Formula 1]

In the above chemical formula 1, R1 is a C1-C5 alkyl group having a fluorine substituent.

R2 is a C1-C5 alkyl group with or without a fluorine substituent.

The present invention provides a sparingly soluble electrolyte for a lithium secondary battery comprising a fluorinated sulfonate solvent of the following chemical formula 1 and a lithium salt.

[Chemical Formula 1]

In the above chemical formula 1, R1 is a C1-C5 alkyl group having a fluorine substituent.

R2 is a C1-C5 alkyl group with or without a fluorine substituent.

The present invention provides a sparingly soluble electrolyte for a lithium secondary battery comprising a fluorinated sulfonate solvent of the following chemical formula 1, a lithium salt, and an ionic liquid containing a cation having a fluorine substituent.

[Chemical Formula 1]

In the above chemical formula 1, R1 is a C1-C5 alkyl group having a fluorine substituent.

R2 is a C1-C5 alkyl group with or without a fluorine substituent.

The present invention provides a lithium secondary battery including the flame retardant electrolyte.

The flame retardant electrolyte containing the fluorinated sulfonate solvent of the chemical formula 1 of the present invention has low volatility and flammability of the solvent and excellent flame retardancy, so that when manufacturing a lithium secondary battery using the same, the risk of fire and explosion is significantly reduced.

The flame retardant electrolyte of the present invention has excellent solubility in lithium salts and ions, excellent thermal stability, and low viscosity, so that when used in a lithium secondary battery, it has excellent ion conductivity, thereby achieving high output such as rapid charging and discharging and high performance of the secondary battery.

Hereinafter, the present invention will be described in detail. The terms or words used in this specification and claims should not be interpreted as limited to their usual or dictionary meanings, and should be interpreted as meanings and concepts that conform to the technical idea of the present invention based on the principle that the inventor can appropriately define the concept of the term in order to explain his or her own invention in the best way.

The present invention relates to a flame-retardant electrolyte for a lithium secondary battery comprising a fluorinated sulfonate solvent of the following chemical formula 1.

[Chemical Formula 1]

In the above chemical formula 1, R1 is a C1-C5 alkyl group having a fluorine substituent.

R2 is a C1-C5 alkyl group which is optionally substituted with fluorine.

The inventor of the present invention has endeavored to find a solvent that can increase the flame retardancy of a secondary battery while also having excellent solubility in lithium salts and thus can sufficiently serve as a medium for the movement of lithium ions, and has discovered that this can be solved by using a sulfonate solvent having a fluorine substituent, thereby completing the present invention.

The electrolyte of the present invention has the advantages of excellent solubility in lithium ions for the electrochemical reaction of a battery, sufficiently serving as a medium for the movement of lithium ions, and excellent flame retardancy.

In one embodiment of the present invention, R1 of the fluorinated sulfonate solvent of the chemical formula 1 may be selected from CF ₂ H, CF ₃ , CH ₂ CFH ₂ , CH ₂ CF ₂ H, CH ₂ CF ₃ , CFHCH ₃ , CFHCFH ₂ , CFHCF ₂ H, CFHCF ₃ , CF ₂ CH ₃ , CF ₂ CFH ₂ , CF ₂ CF _{2 H} , CF ₂ CF ₃ , CH ₂ CF ₂ CF ₃ , CF ₂ CF ₂ CF ₃ , CH ₂ CF ₂ CF ₂ CF 3 , CF ₂ CF ₂ CF ₂ CF ₃ , CH ₂ CF ₂ CF ₂ CF ₃ and -CF ₂ CF ₂ CF ₂ CF _{2 CF 3} there is.

In one embodiment of the present invention, R2 of the fluorinated sulfonate solvent of the chemical formula 1 can be selected from CH ₃ , CF ₃ , CH ₂ CH ₃ and CH ₂ CF ₃ .

When R1 and/or R2 of chemical formula 1 are as described above, the solubility in lithium salts is excellent, and the lifespan, resistance characteristics, and flame retardancy of the secondary battery are particularly excellent.

In terms of solubility and flame retardancy in lithium salts, R1 and R2 are each preferably CF ₃ .

The present invention provides a sparingly soluble electrolyte for a lithium secondary battery comprising a fluorinated sulfonate solvent of the above chemical formula 1 and a lithium salt.

When a lithium secondary battery electrolyte is manufactured using the above-mentioned poorly soluble electrolyte, the lithium salt is easily dissolved and dissociated in a solvent, and the dissociated salt moves well, thereby exhibiting high lithium ion conductivity while also having high chemical stability and suppressing the deterioration phenomenon of the battery.

The flame retardant electrolyte of the present invention may contain 20 to 100 wt%, preferably 20 to 80 wt%, and more preferably 30 to 60 wt% of the fluorinated sulfonate solvent of Chemical Formula 1, based on the total weight of the electrolyte solvent. When the secondary battery electrolyte solvent contains 100 wt% of the fluorinated sulfonate solvent of Chemical Formula 1, the flame retardancy may be sufficiently excellent. When the fluorinated sulfonate solvent of Chemical Formula 1 is mixed with another solvent, if the miscibility with the other solvent is excellent, the battery may be provided with flame retardancy while sufficiently dissolving lithium salts such as lithium bis(fluorosulfonyl)imide and having low viscosity and excellent lithium ion conductivity.

The present invention provides a sparingly soluble electrolyte for a lithium secondary battery comprising a fluorinated sulfonate solvent of the above chemical formula 1, a lithium salt, and an ionic liquid containing a cation having a fluorine substituent.

The ionic liquid containing the cation having the above fluorine substituent has excellent miscibility with a fluorinated sulfonate solvent and can additionally impart flame retardancy to the electrolyte.

The ionic liquid comprising a cation having the above fluorine substituent preferably comprises at least one of the cations represented by the following chemical formulae 2 to 4.

[Chemical formula 2]

R ₃ R ₄ R ₅ R ₆ N ⁺

In the above chemical formula 2

R3 to R5 are each independently a C1 to C5 alkyl group substituted or unsubstituted with fluorine,

R6 is a C1 to C5 alkyl group in which one or more hydrogens are substituted with fluorine or -R ₇ -OR ₈ ,

R7 is a C1 to C5 alkylene group substituted or unsubstituted with fluorine,

R8 is a C1 to C5 alkyl group in which one or more hydrogens are replaced with fluorine.

[Chemical Formula 3]

In the above chemical formula 3,

n is 1 or 2,

R9 is a C1 to C5 alkyl group which is optionally substituted with fluorine,

R10 is a C1 to C5 alkyl group in which one or more hydrogens are substituted with fluorine or -R ₁₁ -OR ₁₂ ,

R11 is a C1 to C5 alkylene group substituted or unsubstituted with fluorine,

R12 is a C1 to C5 alkyl group in which one or more hydrogens are replaced by fluorine.

[Chemical Formula 4]

In the above chemical formula 4,

R13 is a C1 to C5 alkyl group substituted or unsubstituted with fluorine,

R14 is a C1 to C5 alkyl group, a C2 to C5 alkenyl group, or -R ₁₅ -OR ₁₆ , wherein one or more hydrogens are replaced by fluorine,

R15 is a C1 to C5 alkylene group which is optionally substituted with fluorine,

R16 is a C1 to C5 alkyl group in which one or more hydrogens are replaced with fluorine.

In the above chemical formulas 2 to 4, R6, R8, R10, R12, R14 and R16 are each independently CF ₂ H, CF ₃ , CH ₂ CFH ₂ , CH ₂ CF ₂ H, CH ₂ CF ₃ , CFHCH ₃ , CFHCFH ₂ , CFHCF ₂ H, CFHCF ₃ , CF ₂ CH ₃ , CF ₂ CFH ₂ , CF ₂ CF ₂ H, CF ₂ CF ₃ , CH ₂ CF ₂ CF ₃ , CF ₂ CF ₂ CF ₃ , CH ₂ CF ₂ CF ₂ CF ₃ , CF ₂ CF ₂ CF ₂ CF ₃ , CH ₂ CF ₂ CF _{2 CF 3} and -CF ₂ CF ₂ CF ₂ CF ₂ CF ₃ can be selected.

The anion of the above ionic liquid may be at least one selected from the following anion group.

[Anion group]

, , PF ₆ ^- , BF ₄ ^- , ,

The ionic liquid containing the fluorinated sulfonate solvent of the above chemical formula 1 and the cation having a fluorine substituent can be manufactured and used by a method known in the art or can be obtained and used as a commercial product.

The fluorinated sulfonate solvent of the above chemical formula 1 of the present invention and the ionic liquid containing a cation having a fluorine substituent have excellent mutual compatibility and can be mixed well, and when the mixed solvent is used to prepare a lithium secondary battery electrolyte, the lithium salt is easily dissolved and dissociated in the solvent, and the dissociated salt moves well, thereby exhibiting high lithium ion conductivity while also having high chemical stability and having the effect of suppressing deterioration of the battery.

The above lithium salt serves as an electrolyte salt or additive in a lithium secondary battery electrolyte, and the lithium salt includes at least one salt selected from the group consisting of LiPF ₆ , LiBF ₄ , LiBF ₆ , LiSbF ₆ , LiAsF ₆ , LiN(SO ₂ F) ₂ , LiN(SO ₂ CF ₃ ) ₂ , LiN(SO ₂ C ₂ F ₅ ) 2 , LiN( _{SO 3} C ₂ F ₅ ) ₂ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiC ₆ H ₅ SO ₃ , LiSCN, LiPO ₂ F ₂ , LiClO ₄ , LiAlO ₂ , LiAlCl ₄ , LiCl, LiI, LiB(C ₂ O ₄ )F ₂ and LiB(C ₂ O ₄ ) ₂ . The lithium salt is preferably LiPF ₆ or LiN(SO ₂ F) ₂ .

The lithium bis(fluorosulfonyl)imide (LiN(SO ₂ F) ₂ ) electrolyte salt included in the flame-retardant electrolyte of the present invention operates well even at high temperatures of room temperature to 100°C, compared to electrolytes containing other electrolyte salts that do not operate well at high temperatures, and has the effect of extending the battery cycle life. In addition, the electrolyte containing lithium bis(fluorosulfonyl)imide electrolyte salt has the advantage of maintaining the discharge and input/output performance of the battery even at low temperatures and allowing storage at higher temperatures.

In the flame retardant electrolyte of the present invention, the concentration of the lithium salt is preferably 0.2 to 3.0 M, more preferably 0.5 to 2.5 M. When the concentration of the lithium salt is as described above, the electrolyte has appropriate ion conductivity and viscosity, so that it can exhibit excellent electrolyte performance, and lithium ions can move effectively.

The flame retardant electrolyte of the present invention may contain a fluorinated sulfonate solvent of chemical formula 1 and an ionic liquid including a cation having a fluorine substituent in a weight ratio of 1:9 to 9:1, preferably 3:7 to 7:3. When the weight ratio is as described above, flame retardancy is imparted to the battery while increasing lithium ion conductivity, thereby increasing the charge/discharge efficiency of the battery.

The flame retardant electrolyte of the present invention may contain 20 to 100 wt%, preferably 20 to 80 wt%, and more preferably 30 to 60 wt%, of the fluorinated sulfonate-based solvent of Chemical Formula 1 and the ionic liquid including a cation having a fluorine substituent, based on the total weight of the electrolyte solvent. When the secondary battery electrolyte solvent contains 100 wt% of the fluorinated sulfonate-based solvent of Chemical Formula 1 and the ionic liquid including a cation having a fluorine substituent, the flame retardancy may be sufficiently excellent. Even when the mixed solvent and another solvent are mixed, the electrolyte salt including lithium bis(fluorosulfonyl)imide may be sufficiently dissolved while imparting flame retardancy to the battery and excellent lithium ion conductivity with low viscosity. That is, when the fluorinated sulfonate-based solvent of Chemical Formula 1 and the ionic liquid including a cation having a fluorine substituent are less than 20 wt%, the viscosity may increase and the flame retardancy effect may be reduced.

The secondary battery electrolyte of the present invention may additionally contain a carbonate-based solvent.

The above carbonate solvent may be at least one of a chain-type carbonate solvent and a cyclic carbonate solvent.

The above chain carbonate solvent may be selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylethyl carbonate (MEC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), and combinations thereof, but is not limited thereto.

The above cyclic carbonate solvent may be ethylene carbonate or propylene carbonate.

The above carbonate solvent may be included in an amount of 20 to 80 wt%, preferably 40 to 70 wt%, based on the total weight of the lithium secondary battery electrolyte solvent. When included in the above range, the viscosity of the electrolyte may be lowered and the movement of lithium ions may be promoted, thereby improving ion conductivity.

The present invention provides a lithium secondary battery including the above lithium secondary battery electrolyte.

The lithium secondary battery of the present invention includes a positive electrode and a negative electrode.

The positive electrode includes a positive electrode active material capable of absorbing and desorbing lithium ions, and it is preferable that the positive electrode active material is a composite metal oxide including at least one selected from cobalt, manganese, and nickel and lithium. The solid solution ratio between the metals can be various, and in addition to these metals, an element selected from the group consisting of Mg, Al, Co, K, Na, Ca, Si, Ti, Sn, V, Ge, Ga, B, As, Zr, Mn, Cr, Fe, Sr, V, and rare earth elements can be further included. As a specific example of the positive electrode active material, a compound represented by any one of the following chemical formulas can be used:

Li _a A _1-b B _b D ₂ (wherein 0.90 ≤ a ≤ 1.8, and 0 ≤ b ≤ 0.5); Li _a E _1-b B _b O _2-c D _c (wherein 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05); LiE _2-b B _b O 4 _-c D _c (wherein 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05); Li _a Ni _1-bc Co _b B _c D _α (wherein 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, 0 < α ≤ 2); Li _a Ni _1-bc Co _b B _c O _2-α F _α (in the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, 0 < α <2); Li _a Ni _1-bc Co _b B _c O _2-α F ₂ (in the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, 0 < α <2); Li _a Ni _1-bc Mn _b B _c D _α (in the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, 0 < α ≤ 2); Li _a Ni _1-bc Mn _b B _c O _2-α F _α (in the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, 0 < α <2); Li _a Ni _1-bc Mn _b B _c O _2-α F ₂ (in the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, 0 < α <2); Li _a Ni _b E _c G _d O ₂ (in the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, 0.001 ≤ d ≤ 0.1); Li _a Ni _b Co _c Mn _d GeO ₂ (wherein 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, 0 ≤ d ≤ 0.5, 0.001 ≤ e ≤ 0.1); Li _a NiG _b O ₂ (wherein 0.90 ≤ a ≤ 1.8, 0.001 ≤ b ≤ 0.1); Li _a CoG _b O ₂ (wherein 0.90 ≤ a ≤ 1.8, 0.001 ≤ b ≤ 0.1); Li _a MnG _b O ₂ (wherein 0.90 ≤ a ≤ 1.8, 0.001 ≤ b ≤ 0.1); Li _a Mn ₂ G _b O ₄ (wherein 0.90 ≤ a ≤ 1.8, 0.001 ≤ b ≤ 0.1); QO ₂ ; QS ₂ ; LiQS ₂ ; V ₂ O ₅ ; LiV ₂ O ₅ ; LiIO ₂ ; LiNiVO ₄ ; Li _(3-f) J ₂ (PO ₄ ) ₃ (0 ≤ f ≤ 2); Li _(3-f) Fe ₂ (PO ₄ ) ₃ (0 ≤ f ≤ 2); and LiFePO ₄ .

In the chemical formula, A is Ni, Co, Mn or a combination thereof; B is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, a rare earth element or a combination thereof; D is O, F, S, P or a combination thereof; E is Co, Mn or a combination thereof; F is F, S, P or a combination thereof; G is Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V or a combination thereof; Q is Ti, Mo, Mn or a combination thereof; I is Cr, V, Fe, Sc, Y or a combination thereof; J may be V, Cr, Mn, Co, Ni, Cu or a combination thereof.

The negative electrode includes a negative electrode active material capable of absorbing and desorbing lithium ions, and as such a negative electrode active material, carbon materials such as crystalline carbon, amorphous carbon, carbon composites, and carbon fibers, silicon, lithium metal, and alloys of lithium and other elements can be used. For example, as amorphous carbon, there are hard carbon, coke, mesocarbon microbeads (MCMB) calcined at 1500°C or lower, and mesophase pitch-based carbon fiber (MPCF). As crystalline carbon, there are graphite materials, and specifically, there are natural graphite, graphitized coke, graphitized MCMB, and graphitized MPCF. It is preferable that the carbon material have an interplanar distance of 3.35 to 3.38 Å and an Lc (crystallite size) by X-ray diffraction of at least 20 nm. Other elements that may be used to form alloys with lithium include aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium or indium.

The positive electrode or the negative electrode can be manufactured by dispersing an electrode active material, a binder, a conductive agent, and, if necessary, a thickener in a solvent to manufacture an electrode slurry composition, and applying the slurry composition to an electrode current collector. Aluminum or an aluminum alloy can commonly be used as the positive electrode current collector, and copper or a copper alloy can commonly be used as the negative electrode current collector. The forms of the positive electrode current collector and the negative electrode current collector can include foil or mesh forms.

A binder is a material that plays a role in pasting active materials, mutual adhesion of active materials, adhesion with a current collector, and cushioning against expansion and contraction of active materials. When the battery is repeatedly charged and discharged, the battery's performance, such as battery life and charging time, may be reduced due to volume changes in the negative electrode material. It increases the bonding strength of electrode materials.

Examples of the binder include polyvinylidene fluoride (PVdF), a copolymer of polyhexafluoropropylene-polyvinylidene fluoride (PVdF/HFP)), poly(vinylacetate), polyvinyl alcohol, polyethylene oxide, polyvinyl pyrrolidone, alkylated polyethylene oxide, polyvinyl ether, poly(methyl methacrylate), poly(ethyl acrylate), polytetrafluoroethylene, polyvinyl chloride, polyacrylonitrile, polyvinyl pyridine, styrene-butadiene rubber, and acrylonitrile-butadiene rubber. The binder content is 0.1 to 30 wt%, and preferably 1 to 10 wt%, with respect to the electrode active material. If the content of the above binder is too low, the adhesive strength between the electrode active material and the current collector is insufficient, and if the content of the binder is too high, the adhesive strength improves, but the content of the electrode active material is reduced accordingly, which is disadvantageous for increasing the battery capacity.

The conductive material is used to provide conductivity to the electrode, and any electronically conductive material that does not cause a chemical change in the battery to be formed can be used, and at least one selected from the group consisting of a graphite-based conductive material, a carbon black-based conductive material, and a metal or metal compound-based conductive material can be used. Examples of the graphite-based conductive material include artificial graphite, natural graphite, etc., and examples of the carbon black-based conductive material include acetylene black, ketjen black, denkablack, thermal black, channel black, etc., and CNT (Carbon Nanotube) can be used. Examples of the metal-based or metal compound-based conductive material include tin, tin oxide, tin phosphate (SnPO ₄ ), titanium oxide, potassium titanate, and perovskite materials such as LaSrCoO ₃ , and LaSrMnO ₃ . However, the present invention is not limited to the conductive materials listed above.

The content of the conductive agent is preferably 0.1 to 10 wt% with respect to the electrode active material. When the content of the conductive agent is less than 0.1 wt%, the electrochemical characteristics deteriorate, and when it exceeds 10 wt%, the energy density per weight decreases.

The thickener is not particularly limited as long as it can play a role in controlling the viscosity of the active material slurry, and examples thereof that can be used include carboxymethyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, and hydroxypropyl cellulose.

Non-aqueous solvents or aqueous solvents are used as solvents in which electrode active materials, binders, and conductive agents are dispersed. Non-aqueous solvents include N-methyl-2-pyrrolidinone (NMP), dimethylformamide, dimethylacetamide, N,N-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran.

The lithium secondary battery of the present invention may include a separator that prevents a short circuit between the positive and negative electrodes and provides a passage for lithium ions, and as the separator, a polyolefin-based polymer film such as polypropylene, polyethylene, polyethylene/polypropylene, polyethylene/polypropylene/polyethylene, polypropylene/polyethylene/polypropylene, or a multi-film thereof, a microporous film, a woven fabric, or a non-woven fabric may be used. In addition, a film in which a porous polyolefin film is coated with a resin having excellent stability may be used.

The lithium secondary battery of the present invention may be formed in other shapes, such as a cylindrical shape or a pouch shape, in addition to a square shape.

Hereinafter, preferred embodiments of the present invention will be described in detail. However, in explaining the present invention, descriptions of conditions or configurations already known will be omitted in order to clarify the gist of the present invention.

Example 1

A flame-retardant electrolyte was prepared by dissolving lithium bis(fluorosulfonyl)imide at a concentration of 2 M in a sulfonate solvent of chemical formula 1 (R1= _CF3 , R2= _CH3 ) at 25°C. The sulfonate solvent of chemical formula 1 (R1= _CF3 , R2= _CH3 ) dissolved lithium bis(fluorosulfonyl)imide well.

Example 2

A flame-retardant electrolyte was prepared by dissolving lithium bis(fluorosulfonyl)imide at a concentration of 2 M in a sulfonate solvent of chemical formula 1 (R1=CH ₂ CF ₃ , R2=CH ₃ ) at 25°C. The sulfonate solvent of chemical formula 1 (R1=CH ₂ CF ₃ , R2=CH ₃ ) dissolved lithium bis(fluorosulfonyl)imide well.

Example 3

A flame-retardant electrolyte was prepared by dissolving lithium bis(fluorosulfonyl)imide at a concentration of 2 M in a sulfonate solvent of chemical formula 1 (R1=CF ₂ CF ₃ , R2=CH ₃ ) at 25°C. The sulfonate solvent of chemical formula 1 (R1=CF ₂ CF ₃ , R2=CH ₃ ) dissolved lithium bis(fluorosulfonyl)imide well.

Example 4.

A flame-retardant electrolyte was prepared by dissolving lithium bis(fluorosulfonyl)imide at a concentration of 2 M in a sulfonate solvent of chemical formula 1 (R1=CF ₃ , R2=CF ₃ ) at 25°C. The sulfonate solvent of chemical formula 1 (R1=CF ₃ , R2=CF ₃ ) dissolved lithium bis(fluorosulfonyl)imide well.

Example 5.

A flame-retardant electrolyte was prepared by dissolving lithium bis(fluorosulfonyl)imide at a concentration of 2 M in a sulfonate solvent of chemical formula 1 (R1=CH ₂ CF ₃ , R2=CF ₃ ) at 25°C. The sulfonate solvent of chemical formula 1 (R1=CH ₂ CF ₃ , R2=CF ₃ ) dissolved lithium bis(fluorosulfonyl)imide well.

Example 6

A flame retardant electrolyte was prepared by dissolving lithium bis(fluorosulfonyl)imide at a concentration of ₂ M in a mixed solvent containing a sulfonate solvent of chemical formula 1 (R1= _CF3 , R2= _CH3 ) and an ionic liquid containing a cation of chemical formula 4 (R13= _CH3 , R14= _CH2CF3 ) and an anion of N( _SO2F ) ₂ in a weight ratio of 1:1 at 25°C. The sulfonate solvent of chemical formula 1 and the ionic liquid were sufficiently mixed and had sufficient compatibility as a secondary battery electrolyte solvent.

Example 7

A flame-retardant electrolyte was prepared by dissolving lithium bis(fluorosulfonyl)imide at a concentration of 2 M in a mixed solvent containing a sulfonate solvent of chemical formula 1 (R1=CH ₂ CF ₃ , R2=CH ₃ ) and a ionic liquid of chemical formula 4 (R13=CH ₃ , R14=CH ₂ CF ₃ ) in a weight ratio of 1:1 at 25°C. The sulfonate solvent of chemical formula 1 and the ionic liquid were sufficiently mixed and had sufficient compatibility as a secondary battery electrolyte solvent.

Example 8

A flame retardant electrolyte was prepared by dissolving lithium bis(fluorosulfonyl)imide at a concentration of 2 M in a mixed solvent containing a sulfonate solvent of chemical formula 1 (R1= _CF3 , R2= _CH3 ) and an ionic liquid containing a cation of chemical formula ₃ (n=1, R9= _CH3 , R10= _CH2CF3 ) and an anion of N( _SO2F ) ₂ in a weight ratio of 1:1 at 25°C. The sulfonate solvent of chemical formula 1 and the ionic liquid were sufficiently mixed and had sufficient compatibility as a secondary battery electrolyte solvent.

Example 9

A flame retardant electrolyte was prepared by dissolving lithium bis(fluorosulfonyl)imide at a concentration of 2 M in a mixed solvent containing a sulfonate solvent of chemical formula 1 (R1=CH ₂ CF ₃ , R2=CH ₃ ) and an ionic liquid containing a cation of chemical formula 3 (n=1, R9=CH ₃ , R10=CH ₂ CF ₃ ) and an anion of N(SO ₂ F) ₂ in a weight ratio of 1:1 at 25°C. The sulfonate solvent of chemical formula 1 and the ionic liquid were sufficiently mixed and had sufficient compatibility as a secondary battery electrolyte solvent.

Comparative Example 1

A flame-retardant electrolyte was prepared by dissolving lithium bis(fluorosulfonyl)imide in a 2 M concentration in fluoroethylene carbonate solvent at 25°C.

Comparative example 2.

A flame-retardant electrolyte was prepared by dissolving lithium bis(fluorosulfonyl)imide at a concentration of 2 M in a fluorinated ether solvent of chemical formula a at 25°C.

Chemical formula a

CF ₂ HCF ₂ CH ₂ OCF ₂ CF ₂ H

Experimental Example 1. Solubility of lithium salt in electrolyte

The non-aqueous electrolytes manufactured in the above examples and comparative examples were left at room temperature (25°C) for 24 hours, and the presence or absence of undissolved precipitates was visually observed to evaluate the solubility according to the evaluation criteria below.

<Evaluation criteria>

○: No precipitate occurs

△: No precipitation occurs, but haze is observed.

×: Precipitation of precipitate occurs

Experimental Example 2. Measurement of viscosity of electrolyte

The viscosity of the electrolytes manufactured in the above examples and comparative examples was measured by a method of calculating the viscosity by measuring the change in stress in frequency using a rotational viscometer (digital), and the results are shown in Table 1 below.

Experimental Example 3. Measurement of flame retardancy of electrolyte

The flame retardancy of the electrolytes manufactured in the above examples and comparative examples was measured by the following method, and the results are shown in Table 1 below.

Measurement of flammability: The standard method of UL 94 flammability test was used to distinguish the degree of combustion through a flame combustion experiment of each material. The flammability test was performed by impregnating 0.5 g of electrolyte on a glass fiber measuring 4 cm in width and 1 cm in length, and then touching one end of the glass fiber to an ignition source to measure whether it caught fire and the burning time, thereby measuring the flammability of each flame-retardant electrolyte.

The burning time of 0.5 g of electrolyte was measured and the flame retardancy was determined based on the following criteria.

1) Less than 12 seconds: Non-combustible

2) Less than 12~40 seconds: Flame retardant

3) 40 seconds or more: Combustible

division Lithium salt solubility Viscosity (cP) Flame retardancy Compatibility Example 1 O 47.1 Flame retardant - Example 2 O 49.0 Flame retardant - Example 3 O 48.6 Flame retardant - Example 4 O 48.9 Flame retardant - Example 5 O 49.2 Flame retardant - Example 6 O 68.2 Flame retardant Mix Example 7 O 65.8 Flame retardant Mix Example 8 O 72.1 Flame retardant Mix Example 9 O 66.6 Flame retardant Mix Comparative Example 1 △ - Flame retardant - Comparative Example 2 △ - Flame retardant -

According to Table 1 above, the flame retardant electrolyte of the present invention has excellent solubility in lithium salts. It can be seen that the flame retardant electrolyte of the present invention has a viscosity of 72.1 cP or less, so that it has excellent lithium ion conductivity and is non-flammable or flame retardant. In contrast, the fluorine-substituted carbonate or ether solvents of Comparative Examples 1 and 2 have excellent flame retardancy, but have a disadvantage in that haze is observed due to low solubility in lithium salts. Due to the low solubility in Li salts of Comparative Examples 1 and 2, battery manufacturing was impossible and the output characteristics could not be confirmed.

Experimental Example 2. Confirmation of Lithium Secondary Battery Output Characteristics

A 1.2 Ah pouch battery was assembled using a cathode material containing Li _1.13 Mn _0.463 Ni _0.203 Co _0.203 O ₂ (0.3Li ₂ MnO ₃ · 0.7LiMn _1/3 Ni _1/3 Co _1/3 O ₂ ) as a cathode active material and an anode material containing artificial graphite and natural graphite in a 1:1 weight ratio as a cathode active material using artificial graphite and natural graphite as a cathode active material, and 5 ml of the electrolyte prepared in the above example was injected to complete the secondary battery.

The 1.2 Ah pouch battery obtained through the above-mentioned battery formation process was discharged from a fully charged state to a state of charge (SOC) of 50% at a constant current of 0.2 C at 25 °C, rested for 1 hour, and then discharged for 10 seconds at a constant current of 1.5 C. In the next process, after a rest time of 40 seconds, it was charged at a constant current of 1.125 C. The initial direct current resistance (DC-IR, Rdis) was calculated from the discharge data using the following mathematical expression 1.

[Mathematical formula 1]

Rdis = ΔV/I

In the above equation, ΔV represents OCVdis (voltage just before the start of 10-second discharge) - Vdis (voltage at the end of 10-second discharge), and I is the current value during discharge.

In addition, the resistance of the battery after being fully charged and stored in a high-temperature oven at 60°C for 4 weeks was measured using the same method as above and is shown in Table 2.

Electrolyte DC-IR(mΩ) 25℃ initial 60℃ 4 weeks later Change rate (%) Example 1 41.1 60.7 47.7 Example 2 42.7 62.0 45.2 Example 3 41.0 64.1 56.3 Example 4 40.1 57.1 42.4 Example 5 40.6 63.5 56.4 Example 6 56.5 90.0 59.3 Example 7 66.0 104.6 58.5 Example 8 73.1 115.1 57.5 Example 9 65.7 102.5 56.0

Change rate = (resistance after 4 weeks at 60℃ - initial resistance at 25℃) / initial resistance at 25℃ x 100

As shown in Table 2, it can be seen that the batteries including the electrolytes of Examples 1 to 9 have low internal battery resistance. This shows that the output characteristics of the battery are improved due to the low resistance characteristics.

After manufacturing the battery, the initial resistance value was measured at 25°C, and after the battery was stored in a high-temperature oven at 60°C for 4 weeks, the resistance was measured, and the change rate was measured. As a result, it was found that the batteries containing the electrolytes of Examples 1 to 9 had an increase in resistance value of 59.3% or less. In other words, it can be seen that they are sufficiently stable even at high temperatures.

Claims (11)

A flame retardant electrolyte for a lithium secondary battery comprising a fluorinated sulfonate solvent of the following chemical formula 1.
[Chemical Formula 1]

In the above chemical formula 1, R1 is a C1-C5 alkyl group having a fluorine substituent.
R2 is a C1-C5 alkyl group which is optionally substituted with fluorine. A sparingly soluble electrolyte for a lithium secondary battery comprising a fluorinated sulfonate solvent of the following chemical formula 1 and a lithium salt.
[Chemical Formula 1]

In the above chemical formula 1, R1 is a C1-C5 alkyl group having a fluorine substituent.
R2 is a C1-C5 alkyl group which is optionally substituted with fluorine. A sparingly soluble electrolyte for a lithium secondary battery comprising a fluorinated sulfonate solvent of the following chemical formula 1, a lithium salt, and an ionic liquid containing a cation having a fluorine substituent.
[Chemical Formula 1]

In the above chemical formula 1, R1 is a C1-C5 alkyl group having a fluorine substituent.
R2 is a C1-C5 alkyl group which is optionally substituted with fluorine. In claim 1, claim 2 or claim 3, R1 of the fluorinated sulfonate solvent of the chemical formula 1 is CF ₂ H, CF ₃ , CH ₂ CFH ₂ , CH ₂ CF ₂ H, CH ₂ CF ₃ , CFHCH ₃ , CFHCFH ₂ , CFHCF ₂ H, CFHCF ₃ , CF ₂ CH ₃ , CF ₂ CFH ₂ , CF ₂ CF ₂ H, CF ₂ CF ₃ , CH ₂ CF ₂ CF ₃ , CF ₂ CF ₂ CF ₃ , CH ₂ CF ₂ CF ₂ CF ₃ , CF ₂ CF ₂ CF ₂ CF ₃ , CH ₂ CF ₂ CF ₂ CF ₂ CF ₃ and CF ₂ CF ₂ CF ₂ CF ₂ A flame retardant electrolyte selected from at least one of CF ₃ . A flame retardant electrolyte according to claim 1, claim 2 or claim 3, wherein R2 of the fluorinated sulfonate solvent of the chemical formula 1 is at least one selected from CH ₃ , CF ₃ , CH ₂ CH ₃ and CH ₂ CF ₃ . A flame retardant electrolyte according to claim 3, wherein the ionic liquid containing a cation having a fluorine substituent is at least one of the cations represented by the following chemical formulas 2 to 4.
[Chemical formula 2]
R ₃ R ₄ R ₅ R ₆ N ⁺
In the above chemical formula 2
R3 to R5 are each independently a C1 to C5 alkyl group substituted or unsubstituted with fluorine,
R6 is a C1 to C5 alkyl group in which one or more hydrogens are substituted with fluorine or -R ₇ -OR ₈ ,
R7 is a C1 to C5 alkylene group substituted or unsubstituted with fluorine,
R8 is a C1 to C5 alkyl group in which one or more hydrogens are replaced with fluorine.

[Chemical Formula 3]

In the above chemical formula 3,
n is 1 or 2,
R9 is a C1 to C5 alkyl group which is optionally substituted with fluorine,
R10 is a C1 to C5 alkyl group in which one or more hydrogens are substituted with fluorine or -R ₁₁ -OR ₁₂ ,
R11 is a C1 to C5 alkylene group substituted or unsubstituted with fluorine,
R12 is a C1 to C5 alkyl group in which one or more hydrogens are replaced by fluorine.

[Chemical Formula 4]

In the above chemical formula 4,
R13 is a C1 to C5 alkyl group substituted or unsubstituted with fluorine,
R14 is a C1 to C5 alkyl group, a C2 to C5 alkenyl group, or -R ₁₅ -OR ₁₆ , wherein one or more hydrogens are replaced by fluorine,
R15 is a C1 to C5 alkylene group which is optionally substituted with fluorine,
R16 is a C1 to C5 alkyl group in which one or more hydrogens are replaced with fluorine. A flame retardant electrolyte according to claim 3, wherein the anion of the ionic liquid is at least one selected from the following anion group.
[Anion group]
, , PF ₆ ^- , BF ₄ ^- , , A flame retardant electrolyte according to claim 2 or 3, wherein the lithium salt is at least one selected from the group consisting of LiPF ₆ , LiBF ₄ , LiBF ₆ , LiSbF ₆ , LiAsF ₆ , _{LiN(SO 2 F) 2 , LiN} (SO ₂ CF ₃ ) ₂ , LiN(SO ₂ C ₂ F ₅ ) ₂ , LiN(SO ₃ C ₂ F ₅ ) 2 , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiC ₆ H ₅ SO ₃ , LiSCN, LiPO ₂ F ₂ , LiClO ₄ , LiAlO ₂ , LiAlCl ₄ , LiCl, LiI, LiB(C ₂ O ₄ )F ₂ , and LiB(C ₂ O ₄ ) ₂ . A flame retardant electrolyte according to claim 2 or 3, wherein the concentration of the lithium salt is preferably 0.2 to 3.0 M. A flame retardant electrolyte according to claim 3, comprising a fluorinated sulfonate solvent of chemical formula 1 and an ionic liquid including a cation having a fluorine substituent in a weight ratio of 1:9 to 9:1. A lithium secondary battery comprising the electrolyte of claim 1, claim 2 or claim 3.