KR20220053353A - Secondary battery having an excellent fire resistance and improved safety - Google Patents

Abstract

The present invention relates to a lithium secondary battery with improved flame retardancy and safety. Specifically, the present invention provides a lithium secondary battery including a positive electrode, a negative electrode, a separator and an electrolyte, wherein the separator includes inorganic particles and a binder, and the electrolyte includes a fluorinated ether-based compound represented by chemical formula 1, a nitrile compound and ionizable lithium salt.

Description

Lithium secondary battery with improved flame retardancy and safety {SECONDARY BATTERY HAVING AN EXCELLENT FIRE RESISTANCE AND IMPROVED SAFETY}

The present invention relates to a lithium secondary battery having improved flame retardancy and safety.

With the development of personal IT devices and computer networks due to the development of the information society, and with it, the overall society's dependence on electric energy increases, the development of technology for efficiently storing and utilizing electric energy is required.

In particular, in the case of secondary battery-based technology, since it can be miniaturized and can be applied to electric vehicles and power storage devices, it is emerging as the most suitable technology for various uses. Lithium ion secondary batteries are in the spotlight as a battery system with the highest theoretical energy density among secondary batteries, and are currently being applied to various devices.

Such a lithium ion secondary battery is composed of a positive electrode, a negative electrode, an electrolyte, and a separator.

Among them, the separator separates and electrically insulates the positive electrode and the negative electrode, and in order to provide lithium ion permeability, physical properties such as electrical insulation properties and chemical resistance are required. Polyolefin-based polymer materials such as inexpensive polyethylene are used as main components.

However, since the polyolefin-based polymer material has very weak heat resistance at a temperature of 130° C. or higher, it has a disadvantage of being contracted when exposed to a high temperature exceeding 130° C. As a result, there is a problem in that the insulation characteristics of the secondary battery are deteriorated, such as an internal short circuit between the positive electrode and the negative electrode, and the safety of the lithium secondary battery is deteriorated.

Furthermore, since the lithium ion secondary battery uses an electrolyte containing a non-aqueous organic solvent as a main component, a flammable gas is generated in a battery abuse situation such as high temperature exposure and overcharging, and the non-aqueous organic solvent There is a disadvantage in that the possibility of ignition is high due to the low flash point of the solvent.

Accordingly, various attempts have been made to develop a lithium ion secondary battery with improved safety as well as flame retardancy.

Korean Patent Publication No. 2020-0023409

An object of the present invention is to solve the above problems, and to provide a lithium secondary battery with improved flame retardancy and safety.

According to one embodiment, the present invention

a positive electrode, a negative electrode, a separator and an electrolyte;

The separator consists of inorganic particles and a binder,

The electrolyte is a compound represented by the following formula (1); Provided is a lithium secondary battery comprising a nitrile compound and an ionizable lithium salt.

[Formula 1]

R1-O-CH ₂ -R2

In Formula 1,

R1 and R2 are an alkyl group having 1 to 8 carbon atoms substituted with at least one fluorine.

Since the lithium secondary battery of the present invention includes a ceramic separator made of inorganic particles and a binder instead of the polyolefin-based separator, it is possible to prevent the contraction of the separator when exposed to high temperatures, thereby suppressing an internal short circuit between the positive electrode and the negative electrode. improvement effect can be realized. In addition, the lithium secondary battery of the present invention includes an electrolyte including a fluorinated ether-based compound and a nitrile compound, thereby improving the moisture content of the ceramic separator, so that not only the flame retardancy of the secondary battery but also overall performance can be further improved.

The following drawings attached to the present specification illustrate preferred embodiments of the present invention, and serve to further understand the technical spirit of the present invention together with the above-described content of the present invention, so the present invention is limited to the matters described in those drawings It should not be construed as being limited.
1 is a photograph showing a flame retardancy evaluation result for an electrolyte according to Experimental Example 1.
2 is a photograph showing a flame retardancy evaluation result for a separator according to Experimental Example 2. Referring to FIG.
3 is a graph showing a compression resistance evaluation result of a separator according to Experimental Example 4;
4 is a graph showing the wettability evaluation results according to Experimental Example 5.
5 is a graph showing the thickness change rate of the separator after impregnation with the electrolyte according to Experimental Example 6.

Hereinafter, the present invention will be described in more detail with reference to the drawings. At this time, since the configuration shown in the drawings described in this specification is only the most preferred embodiment of the present invention and does not represent all the technical spirit of the present invention, various equivalents and It should be understood that there may be variations.

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

The terms or words used in the present specification and claims should not be construed as being limited to their ordinary or dictionary meanings, and the inventor may properly define the concept of the term in order to best describe his invention. Based on the principle that there is, it should be interpreted as meaning and concept consistent with the technical idea of the present invention.

<Lithium secondary battery>

According to one embodiment, the present invention

a positive electrode, a negative electrode, a separator and an electrolyte;

The separator consists of inorganic particles and a binder,

The electrolyte is a compound represented by the following formula (1); Provided is a lithium secondary battery comprising a nitrile compound and an ionizable lithium salt.

[Formula 1]

R1-O-CH ₂ -R2

In Formula 1,

R1 and R2 are an alkyl group having 1 to 8 carbon atoms substituted with at least one fluorine.

(1) Anode

The positive electrode may be prepared by coating a positive electrode active material slurry including a positive electrode active material, a binder for an electrode, a conductive material for an electrode, a solvent, and the like on a positive electrode current collector.

The positive electrode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or carbon on the surface of aluminum or stainless steel. , nickel, titanium, silver, etc. may be used. In this case, the positive electrode current collector may form fine irregularities on the surface to strengthen the bonding force of the positive electrode active material, and may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a nonwoven body.

The positive active material is a compound capable of reversible intercalation and deintercalation of lithium, and specifically may include a lithium composite metal oxide including lithium and one or more metals such as cobalt, manganese, nickel, or aluminum. there is. More specifically, the lithium composite metal oxide is a lithium-manganese oxide (eg, LiMnO ₂ , LiMn ₂ O ₄ , etc.), a lithium-cobalt-based oxide (eg, LiCoO ₂ , etc.), lithium-nickel-based oxide (eg, LiNiO ₂ , etc.), lithium-nickel-manganese oxide (eg, LiNi _1-Y1 Mn _Y1 O ₂ (here, 0<Y1<1), LiMn _2-z1 Ni _z1 O ₄ ( Here, 0<Z1<2, etc.), lithium-nickel-cobalt-based oxides (eg, LiNi _1-Y2 Co _Y2 O ₂ (here, 0<Y2<1), etc.), lithium-manganese-cobalt based oxides (eg, LiCo _1-Y3 Mn _Y3 O ₂ (here, 0<Y3<1), LiMn _2-z2 Co _z2 O ₄ (here, 0<Z2<2), etc.), lithium-nickel -manganese-cobalt oxide (eg, Li(Ni _p1 Co _q1 Mn _r1 )O ₂ (here, 0 < p1 < 1, 0 < q1 < 1, 0 < r1 < 1, p1+q1+r1= 1) or Li(Ni _p2 Co _q2 Mn _r2 )O ₄ (where 0<p2<2, 0<q2<2, 0<r2<2, p2+q2+r2=2), etc.), or lithium- Nickel-cobalt-transition metal (M) oxide (eg, Li(Ni _p3 Co _q3 Mn _r3 M _S1 )O ₂ , where M is Al, Fe, V, Cr, Ti, Ta, Mg and Mo is selected from the group consisting of, and p3, q3, r3 and s1 are atomic fractions of each independent element, 0<p3<1, 0<q3<1, 0<r3<1, 0<s1<1, p3+q3 +r3+s1=1) and the like), and any one or two or more compounds may be included.

Among them, the lithium composite metal oxide is LiCoO ₂ , LiMnO ₂ , LiNiO ₂ , lithium nickel manganese cobalt oxide (for example, Li(Ni _0.6 Mn _0.2 Co _0.2 )O ₂ , Li(Ni _0.5 Mn _0.3 Co _0.2 )O ₂ , or Li(Ni _0.8 Mn _0.1 Co _0.1 )O ₂ , etc.), or lithium nickel cobalt aluminum oxide (eg, LiNi _0.8 Co _0.15 Al _0.05 O ₂ etc.), etc. In consideration of the significant improvement effect according to the control of the type and content ratio of the element forming the lithium composite metal oxide, the lithium composite metal oxide is Li(Ni _0.6 Mn _0.2 Co _0.2 )O ₂ , Li(Ni _0.5 Mn _0.3 Co _0.2 )O ₂ , Li(Ni _0.7 Mn _0.15 Co _0.15 )O ₂ , or Li(Ni _0.8 Mn _0.1 Co _0.1 )O ₂ , and the like, and any one or a mixture of two or more thereof may be used there is.

The binder for the electrode is a component that assists in bonding the positive electrode active material and the electrode conductive material and the like to the current collector. Specifically, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene (PE) , polypropylene, ethylene-propylene-dienter polymer, sulfonated ethylene-propylene-dienter polymer, styrene-butadiene rubber, styrene-butadiene rubber-carboxymethylcellulose (SBR-CMC), fluororubber, various copolymers, etc. can be heard

The conductive material for the electrode is a component for further improving the conductivity of the positive electrode active material. The electrode conductive material is not particularly limited as long as it has conductivity without causing a chemical change in the battery. For example, graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskeys such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives may be used. Specific examples of commercially available conductive materials include acetylene black-based products such as Chevron Chemical Company, Denka Singapore Private Limited, Gulf Oil Company, etc.), Ketjenblack, EC series (products of the Armak Company), the Vulcan XC-72 (products of the Cabot Company) and the Super P (products of the Timcal Company).

The solvent may include an organic solvent such as N-methyl-2-pyrrolidone (NMP), and may be used in an amount having a desirable viscosity when including the positive electrode active material, and optionally a positive electrode binder and positive electrode conductive material. there is.

(2) cathode

The negative electrode may be manufactured by forming a negative electrode mixture layer on the negative electrode current collector. The negative electrode mixture layer may be formed by coating a negative electrode slurry including a negative electrode active material, a binder, a conductive material and a solvent on a negative electrode current collector, drying and rolling.

The negative electrode current collector generally has a thickness of 3 to 500 μm. Such a negative current collector is not particularly limited as long as it has high conductivity without causing a chemical change in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel. A surface treated with carbon, nickel, titanium, silver, etc., an aluminum-cadmium alloy, etc. may be used on the surface. In addition, like the positive electrode current collector, the bonding strength of the negative electrode active material may be strengthened by forming fine irregularities on the surface, and may be used in various forms such as a film, sheet, foil, net, porous body, foam, non-woven body, and the like.

In addition, the negative active material is lithium metal, a carbon material capable of reversibly intercalating/deintercalating lithium ions, a metal or an alloy of these metals and lithium, a metal oxide, a material capable of doping and dedoping lithium , and may include at least one selected from the group consisting of transition metal oxides.

As a carbon material capable of reversibly intercalating/deintercalating lithium ions, any carbon-based negative active material generally used in lithium ion secondary batteries may be used without particular limitation, and representative examples thereof include crystalline carbon, Amorphous carbon or these may be used together. Examples of the crystalline carbon include graphite such as amorphous, plate-like, flake, spherical or fibrous natural graphite or artificial graphite, and examples of the amorphous carbon include soft carbon (low-temperature calcined carbon). or hard carbon, mesophase pitch carbide, and calcined coke.

Examples of the above metals or alloys of these metals with lithium include Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al. And a metal selected from the group consisting of Sn or an alloy of these metals and lithium may be used.

Examples of the metal oxide include PbO, PbO ₂ , Pb ₂ O ₃ , Pb ₃ O ₄ , Sb ₂ O ₃ , Sb ₂ O ₄ , Sb ₂ O ₅ , GeO, GeO ₂ , Bi ₂ O ₃ , Bi ₂ O ₄ , Bi ₂ O ₅ , Li _x Fe ₂ O ₃ (0≤x≤1), Li _x WO ₂ (0≤x≤1), and Sn _x Me _1-x Me' _y O _z (Me: Mn, Fe, Pb, Ge; Me': Al, B, P, Si, group 1, 2, 3 elements of the periodic table, halogen; 0<x≤1;1≤y≤3; 1≤z≤8) One selected from may be used.

Materials capable of doping and dedoping lithium include Si, SiO _x (0<x≤2), Si-Y alloy (wherein Y is an alkali metal, alkaline earth metal, group 13 element, group 14 element, transition metal, An element selected from the group consisting of rare earth elements and combinations thereof, but not Si), Sn, SnO ₂ , Sn-Y (wherein Y is an alkali metal, an alkaline earth metal, a group 13 element, a group 14 element, a transition metal, a rare earth) It is an element selected from the group consisting of elements and combinations thereof, and is not Sn) and the like, and at least one of these and SiO ₂ may be mixed and used. The element Y includes Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, 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, It may be selected from the group consisting of Te, Po, and combinations thereof.

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

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

The binder is a component that assists in bonding between the conductive material, the active material, and the current collector, and is typically added in an amount of 1 to 30% by weight based on the total weight of the solid content in the negative electrode slurry. Examples of such a binder include a fluororesin-based binder including polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE); styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber, and styrene-rubber-based binders including isoprene rubber; Cellulose binders including carboxyl methyl cellulose (CMC), starch, hydroxypropyl cellulose, and regenerated cellulose; a polyalcohol-based binder comprising polyvinyl alcohol; Polyolefin-based binders including polyethylene and polypropylene; polyimide-based binders; polyester-based binders; and silane-based binders.

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 1 to 20 wt % based on the total weight of the solid content in the negative electrode slurry. The conductive material is not particularly limited as long as it has conductivity without causing a chemical change in the battery, and for example, carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black. carbon powder; Graphite powder, such as natural graphite, artificial graphite, or graphite with a highly developed crystal structure; 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; Conductive materials such as polyphenylene derivatives may be used.

The solvent may include water or an organic solvent such as NMP, alcohol, and the like, and may be used in an amount to have a desirable viscosity when the negative electrode active material and, optionally, a binder and a conductive material are included. For example, it may be included so that the solid content concentration in the slurry including the negative active material, and optionally the binder and the conductive material is 50 wt% to 75 wt%, preferably 50 wt% to 65 wt%.

(3) separator

The lithium secondary battery of the present invention provides a separator comprising inorganic particles and a binder.

That is, since the separator of the present invention does not include a polyolefin-based substrate, but consists only of inorganic particles and a binder for bonding the same, the resistance to ion migration and thermal shrinkage at high temperature is low. safety can be ensured. In addition, since the compression resistance is strong, it is possible to suppress a change in thickness during battery manufacturing.

(3-1) inorganic particles

The inorganic particles constituting the separator preferably include boehmite (AlOOH) particles.

The boehmite may be represented by a chemical composition of AlO(OH) or Al ₂ O ₃ ·H 2 O, and is generally chemically stable alumina monohydrate prepared by heat treatment or hydrothermal treatment of alumina trihydrate in air. Boehmite has a high dehydration temperature of 450°C to 530°C, and various shapes such as plate-shaped boehmite, needle-shaped boehmite, and hexagonal plate-shaped boehmite can be controlled by adjusting manufacturing conditions. Moreover, an aspect-ratio and a particle diameter can be suitably controlled by controlling manufacturing conditions according to various uses.

The boehmite particles applied to the separator of the present invention preferably have an average particle diameter (D50) of 15 nm to 95 nm, specifically 30 nm to 35 nm.

When the average particle diameter of the boehmite particles satisfies the above numerical range, it is possible to secure adhesion to the binder, and at the same time to implement a separator having a low resistance value and improved electrolyte moisture content with respect to the electrolyte.

On the other hand, the average particle size (D50) is a particle size based on 50% of the particle size distribution. More specifically, the average particle size D50 is 50% of the total volume when the volume is accumulated from small particles by measuring the particle size with a particle size analyzer It means the particle size of the particles. The average particle diameter may be measured using a laser diffraction method. In general, the laser diffraction method can measure a particle diameter of several nm from a submicron region, and high reproducibility and high resolution results can be obtained.

Further, the specific surface area of the boehmite particles may be 80 m 2 /g to 180 m 2 /g, specifically 95 m 2 /g to 100 m 2 /g.

When the specific surface area of the boehmite particles satisfies the above numerical range, it is possible to secure adhesion to the binder and, at the same time, to implement a separator having a remarkably low resistance value.

Meanwhile, the specific surface area may be measured using a BET specific surface area analysis device (Brunauer Emmett Teller, BET). Specifically, it can be calculated from the amount of nitrogen gas adsorbed under liquid nitrogen temperature (77 K) using BELSORP-mino II manufactured by BEL Japan.

Since the boehmite particles of the present invention have an average particle diameter and a specific surface area within the above-described ranges, adhesion to the binder can be secured, and a separator with reduced resistance to ion migration can be implemented. In this case, when the average particle diameter and specific surface area of the boehmite particles do not fall within the numerical ranges, as well as when the average particle diameter of the boehmite does not satisfy the numerical ranges or the specific surface area does not satisfy the numerical ranges, the binder It is not possible to improve adhesion with and to achieve a low resistance value at the same time. For example, when the average particle diameter of the boehmite particles is in the range of 15 nm to 95 nm, but the specific surface area is less than 80 m 2 /g, the resistance value is equivalent to or similar to that of the separator according to an aspect of the present invention, whereas boehmite It is difficult to disperse and there is a problem in that the coating property is deteriorated. If the average particle diameter of the boehmite particles is in the range of 15 nm to 95 nm, but the specific surface area exceeds 180 m 2 /g, the resistance value is high and the physical properties of the slurry are lowered, so that it is difficult to uniformly disperse the boehmite particles. there is. On the other hand, although the specific surface area of the boehmite particles is included in 80 m 2 /g to 180 m 2 /g, when the average particle diameter is small, less than 15 nm, it is difficult to disperse the boehmite particles, so that the resistance value of the separator is increased, Even if the same amount of the binder polymer is added, it is difficult to bond between the boehmite particles, so the adhesive force seems to be low.

Therefore, the boehmite particles used in the separator of the present invention have an average particle diameter of 15 nm to 95 nm, specifically 30 nm to 35 nm, and a specific surface area of 80 m 2 in order to improve adhesion with the binder while having low resistance. It is preferred that /g to 180 m 2 /g, specifically 95 m 2 /g to 100 m 2 /g.

(3-2) binder

In addition, the separator of the present invention may include a binder to connect and fix the inorganic particles.

As the binder, a polymer commonly used in forming the separator may be used. In particular, in order to improve mechanical properties such as flexibility and elasticity of the separator, the glass transition temperature (T _g ) is -200° C. to 200° C. A polymer having a temperature of ℃ can be used.

These polymers serve as a binder to connect and stably fix the inorganic particles, so that a separator made of inorganic particles can be formed in the form of a film, and the effect of improving ionic conductivity and preventing deterioration of mechanical properties can be obtained. there is.

Specifically, the binder does not necessarily have an ion-conducting ability, but when a polymer having an ion-conducting ability is used, the performance of the electrochemical device may be further improved. Accordingly, the binder polymer preferably includes a fluororesin-based binder having a high possible dielectric constant.

The fluororesin binder may include polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP), polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE) as a representative example thereof. Among them, it is preferable to include polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP) in order to further secure the binding force between inorganic substances.

Meanwhile, the inorganic particles constituting the separator: the binder is preferably mixed in a weight ratio of 1:0.1 to 1:0.35, specifically, 1:0.2 to 1:0.25 by weight.

When the weight ratio of the inorganic particles to the binder satisfies the above range, high dispersibility and adhesion may be realized. If the content of the binder exceeds 0.35, the performance of the secondary battery may be deteriorated due to an increase in resistance due to an excessive amount of binder, and if it is less than 0.1, adhesion may be deteriorated.

Meanwhile, the separator of the present invention may have a thickness of about 10 μm to 30 μm, specifically, 15 μm to 30 μm in consideration of ion conductivity. When the thickness of the separator is 10 μm or more, mechanical strength can be secured, and when the thickness is 30 μm or less, protons (H ⁺ ), which are ion transporters, can easily pass through, thereby preventing an increase in volume per unit performance of the secondary battery specification and thus high performance of secondary batteries can be manufactured.

Meanwhile, the separator of the present invention preferably does not include a polyolefin substrate in order to secure flame retardancy and heat resistance.

That is, polyolefin-based substrates commonly used in secondary batteries have very weak heat resistance at a temperature of 130° C. or higher, so when exposed to a high temperature exceeding 130° C., they shrink and the insulation properties are lowered, thereby preventing an internal short circuit between the positive electrode and the negative electrode. cause Accordingly, there is a disadvantage in that the heat resistance and safety of the secondary battery are deteriorated. Moreover, in the case of a polyolefin-based substrate, there is a disadvantage in that wettability to the electrolyte of the present invention including a nitrile compound and an ionizable lithium salt to be described later is very low.

As described above, in the lithium secondary battery of the present invention, by applying a ceramic separator made of inorganic particles and a binder without a polyolefin-based substrate, the contraction of the separator is prevented when exposed to high temperatures, thereby suppressing an internal short circuit between the positive electrode and the negative electrode. , and further, wettability to an electrolyte, which will be described later, may be improved by the capillary effect. As a result, flame retardancy and safety of the lithium secondary battery may be improved.

(4) electrolyte

The electrolyte of the present invention is a compound represented by the following formula (1); nitrile compounds and ionizable lithium salts.

[Formula 1]

R1-O-CH ₂ -R2

In Formula 1,

R1 and R2 are an alkyl group having 1 to 8 carbon atoms substituted with at least one fluorine.

(4-1) a compound represented by the formula (1)

The electrolyte of the present invention may further include a fluorinated ether-based compound represented by Formula 1 as an additive to improve viscosity and ionic conductivity.

Specifically, in Formula 1, R1 may be an alkyl group having 2 to 4 carbon atoms substituted with at least one fluorine, and R2 may be an alkyl group having 2 to 6 carbon atoms substituted with at least one fluorine.

In an exemplary embodiment of the present invention, the ratio of carbon atoms and fluorine atoms included in each of R1 and R2 may be 1:2.

Specifically, the compound represented by Formula 1 is 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether or 1H, 1H, 5H-octafluoropentyl 1,1 ,2,2-tetrafluoroethyl ether (1H, 1H, 5H-octafluoropentyl 1,1,2,2-tetrafluroethyl ether), more specifically 1H, 1H, 5H-octafluoropentyl 1,1, 2,2-tetrafluoroethyl ether.

The fluorinated ether-based compound represented by Chemical Formula 1 included in the electrolyte of the present invention improves electrolyte flame retardancy and at the same time improves electrolyte impregnation properties for a separator not containing a polyolefin substrate, thereby shortening the electrolyte injection time during battery manufacturing. there is. That is, when the compound represented by Formula 1 and the nitrile compound and ionizable lithium salt described later are included as electrolyte components together, the viscosity of the electrolyte is reduced, the ion conductivity is increased, and the electrolyte impregnation property is improved, thereby improving the high-temperature stability of the secondary battery It is possible to implement the effect of further improving the battery performance characteristics, such as.

On the other hand, the compound represented by Formula 1 is 0.1 wt% to 30 wt%, specifically 0.5 wt% to 20 wt%, more specifically 0.5 wt% to 10 wt%, most preferably 0.5 wt%, based on the total weight of the electrolyte It may be included in weight % to 7 weight %.

When the content of the compound represented by Formula 1 is less than 0.1% by weight, the effect of improving the viscosity and ionic conductivity and the effect of improving the flame retardancy may be insignificant. In addition, when the content of the compound represented by Formula 1 exceeds 30% by weight, the concentration of the lithium salt is lowered and the ionic conductivity of the electrolyte decreases, so that when applied to a battery, the charging speed and output characteristics may decrease, and safety may be reduced. may deteriorate.

(4-2) Nitrile compound

The nitrile compound is a material having a solid form at room temperature, and refers to a compound including a -C≡N functional group.

Representative examples of the nitrile compound include succinonitrile (SN), adiponitrile (Adn), acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptannitrile, cyclopentane carbonitrile, cyclohexane Carbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile It may be at least one selected from the group consisting of. Specifically, as a solvent that can significantly improve the safety of the electrolyte, succinonitrile (SN), which exists in a solid form at room temperature to lower the volatility of the solvent, is most preferable.

Meanwhile, the nitrile compound may be included in an amount of 50 wt% to 99.9 wt%, specifically 50 wt% to 95 wt%, more specifically 50 wt% to 80 wt%, based on the total weight of the electrolyte.

When the nitrile compound is included in the above content range, the volatility of the electrolyte is lowered to improve the safety of the battery, and there is an advantage that a stable common mixture can be formed by mixing with a lithium salt.

On the other hand, when the content of the nitrile compound is less than 50% by weight, the flame retardancy and safety of the battery may be reduced.

(4-3) ionizable lithium salt

The ionizable lithium salt is used as an electrolyte salt in a lithium secondary battery, and is not particularly limited as long as it is used as a medium for transferring ions, and representative examples thereof include LiPF ₆ , LiBF ₄ , LiN (SO ₂ CF ₂ CF ₃ ) ) ₂ (lithium bis(perfluoroethanesulfonyl) imide; LiBETI), LiN(SO ₂ F) ₂ (lithium bis(fluorosulfonyl) imide; hereinafter abbreviated as “LiFSI”), and LiN(SO ₂ CF ₃ ) ₂ (lithium bis (trifluoromethanesulfonyl) imide, LiTFSI) may include at least one selected from the group consisting of.

In particular, the ionizable lithium salt may include an imide-based lithium salt such as LiFSI. This imide-based lithium salt can be added to the electrolyte to form a strong and thin SEI film on the negative electrode, thereby improving low-temperature output characteristics, suppressing decomposition of the positive electrode surface that may occur during high-temperature cycle operation, and reducing the electrolyte It is known as a substance capable of preventing oxidation reactions.

The lithium salt may be included in a concentration of 1.0M to 3.0M, preferably 1.0M to 1.5M in the electrolyte. If the concentration of the lithium salt is less than 1.0M, the effect of improving the low-temperature output and cycle characteristics of the lithium secondary battery is insignificant, and if it exceeds 3.0M, the electrolyte impregnation property will be lowered as the viscosity and surface tension of the non-aqueous electrolyte excessively increase. can

Meanwhile, the electrolyte of the present invention may maintain a viscosity of 5 cP to 15 cP by including the compound represented by Formula 1, a lithium salt, and a nitrile compound. In general, when only the high concentration lithium salt and the nitrile compound are included, the viscosity of the electrolyte may increase by 15 cP or more. can be maintained as

On the other hand, in the electrolyte of the present invention, the ionizable lithium salt and the nitrile compound are preferably mixed in a weight ratio of 1:2 to 1:8, specifically 1:2 to 1:5.

When the weight ratio of the nitrile compound and the ionizable lithium salt is within the above range, lithium ions are dissociated to a certain level or more in the electrolyte, so that charging and discharging of the battery can be smoothly performed, and the wetting in the battery can be maintained constant. It can maintain a level of viscosity that can be achieved. If the weight ratio of the nitrile compound is less than 2, the lithium salt may not be completely dissolved, and if it exceeds 8, the ionic conductivity of the electrolyte may be reduced.

Therefore, when the weight ratio of the nitrile compound and the ionizable lithium salt satisfies the numerical range, the electrolyte of the present invention can not only realize high thermal and chemical stability, but also ensure particularly excellent flame retardancy.

On the other hand, the nitrile compound contained in the electrolyte of the present invention, for example, succinonitrile and ionizable lithium salt is mixed with each other to be transformed into a liquid form, and at the same time, the trans isomer (trans-isomer) for the central C-C bond below the melting point of SN gauche isomerism) exists, so it has ionic conduction behavior. Therefore, it can serve as an electrolyte. In this case, the trans isomer contributes to aggravation of lattice defects and lowering of activation energy for ion conduction, thereby enhancing the effect of an ion transport mechanism. Accordingly, it is possible to have a wider electrochemical window than a liquid electrolyte including a conventional organic solvent and an ionic liquid due to such physical stability. Practically, the upper limit of the potential window of the conventional liquid electrolyte is about 4.0V to 4.5V, whereas the upper limit of the potential window of the electrolyte of the present invention is in the range of 4.5V to 5.5V, and has a potential window wider than that of the liquid electrolyte. Accordingly, it is possible to extend the operating voltage range of the electrochemical device.

The electrolyte of the present invention is less likely to volatilize and generate gas compared to conventional solvents, there is no problem of evaporation and depletion of the electrolyte, and has flame retardancy to improve the safety of the electrochemical device. In addition, since the electrolyte itself of the present invention is in a very stable form, it is possible to implement suppression of side reactions in the device. In fact, since the electrolyte of the present invention has a wide temperature range as a liquid, high solvating ability, non-coordinating binding properties, etc., it is known to have physicochemical properties as an environmentally friendly solvent that can replace the existing toxic organic solvent. there is.

(4-4) additional additives

In addition, the electrolyte of the present invention suppresses corrosion of Al foil due to the use of imide-based lithium salt, etc., and at the same time prevents the anode from collapsing when exposed to high temperatures or in a high-output environment, high-temperature stability, overcharge prevention, high-temperature swelling In order to further improve the improvement effect, etc., additional additives may be further included as necessary.

Such additional additives include vinylene carbonate, vinylethylene carbonate, ethylene sulfate (Esa), lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, lithium oxalyldifluoroborate (LiODFB) , lithium bisoxalatoborate (LiBOB, LiB(C ₂ O ₄ ) ₂ ), 1,3-propane sultone (PS), 1,4-butane sultone, ethensultone, 1,3-propene sultone (PRS), 1,4-butene sultone, and 1-methyl-1,3-propene sultone, LiPO ₂ F ₂ , LiBF ₄ , or aluminum fluoride (AlF ₃ ).

The additional additive may be included in an amount of 10 wt% or less, specifically 0.1 wt% to 7 wt%, based on the total weight of the electrolyte.

When the content of the additional additive is less than 0.1% by weight, the effect of improving low-temperature output and high-temperature storage characteristics and high-temperature lifespan characteristics of the battery is insignificant, and when the content of the additional additive exceeds 10% by weight, charging and discharging of the battery There is a possibility that side reactions in the electrolyte may occur excessively. In particular, when the additives for forming the SEI film are added in excess, they may not be sufficiently decomposed at high temperatures, and thus may remain unreacted or precipitated in the electrolyte at room temperature. Accordingly, a side reaction in which the lifespan or resistance characteristic of the secondary battery is deteriorated may occur.

(4-5) non-aqueous organic solvent

In addition, the electrolyte of the present invention may further contain a non-aqueous organic solvent such as a carbonate-based organic solvent, a non-fluorinated ether-based solvent, and an ester-based organic solvent, if necessary, in order to improve ionic conductivity.

The carbonate-based organic solvent is a general organic solvent used in the electrolyte, and may be a non-fluorine-based cyclic carbonate-based solvent, a non-fluorine-based linear carbonate-based solvent, a fluorine-based cyclic carbonate solvent, a fluorine-based linear carbonate solvent, or a mixture thereof.

The non-fluorine-based cyclic carbonate-based solvent is a high-viscosity organic solvent, which has a high dielectric constant and can well dissociate lithium salts in the electrolyte, and includes ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2 It may be at least one selected from ,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, and vinylene carbonate, and specifically, it may be ethylene carbonate and propylene carbonate.

In addition, the non-fluorine-based linear carbonate-based solvent is an organic solvent having a low viscosity and a low dielectric constant, and includes dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethylmethyl carbonate (EMC), methylpropyl carbonate, and ethyl It may be at least one selected from among propyl carbonate, and specifically, it may be dimethyl carbonate, diethyl carbonate, and ethylmethyl carbonate.

The fluorine-based cyclic carbonate solvent is fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoroethylene carbonate, tetrafluoroethylene carbonate, 3,3,3-trifluoropropylene carbonate, and 1-fluoro It may be at least one selected from among ropropylene carbonate, and specifically, it may be fluoroethylene carbonate.

The fluorine-based linear carbonate solvent may be at least one selected from fluoromethyl methyl carbonate (FMMC) and fluoroethyl methyl carbonate (FEMC), and specifically, fluoroethyl methyl carbonate (FEMC).

Examples of the non-fluorinated ether-based organic solvent include dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, or ethyl propyl ether.

The ester-based organic solvent may include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, butyrolactone, or caprolactone.

On the other hand, the non-aqueous organic solvent is 30% or less, specifically 20% by weight or less, preferably 1% to 15% by weight, based on the total weight of the electrolyte, in order to reduce the possibility of ignition and increase safety under environments such as high temperature exposure and overcharging. It is preferably included in weight %. When the non-aqueous organic solvent is included in the above range, it is possible to implement a secondary battery having improved electrode and separator impregnation properties with high energy density.

As described above, in the present invention, instead of including a polyolefin-based substrate, a ceramic separator made of inorganic particles and a binder, an electrolyte including a compound represented by Formula 1, a nitrile compound, and an ionizable lithium salt are provided together By doing so, it is possible not only to improve the electrolyte wettability for the ceramic separator, but also to prevent the shrinkage of the separator when exposed to high temperatures, thereby suppressing an internal short circuit between the positive electrode and the negative electrode, thereby implementing a lithium secondary battery with improved flame retardancy and safety. .

The external shape of the lithium secondary battery of the present invention is not particularly limited, but may be variously applied such as a cylindrical shape, a prismatic shape, a pouch type, or a coin type, depending on the purpose for which it is performed. The lithium secondary battery according to an embodiment of the present invention may be a pouch-type secondary battery.

Hereinafter, examples will be given to describe the present invention in detail. However, the embodiments according to the present invention may be modified in various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described in detail below. The embodiments of the present invention are provided to more completely explain the present invention to those of ordinary skill in the art.

Example

I. Electrolyte Preparation

Example 1.

2 g (1.3M) of LiFSI and 7 g of succinonitrile (manufactured by Sigma Aldrich) were put into a round-bottom flask and stirred slowly for 5 minutes under a nitrogen atmosphere at room temperature (25° C.) to form a liquid phase.

Then, 0.5 g of 1H, 1H, 5H-octafluoropentyl 1,1,2,2-tetrafluoroethyl ether (manufactured by TCI) and 0.5 g of vinylene carbonate were further added, and 12 under a nitrogen atmosphere at 25° C. The electrolyte of the present invention was prepared by stirring for a period of time.

Comparative Example 1 .

Ethylene carbonate (EC): ethyl methyl carbonate (EMC) was mixed in a 3:7 volume ratio to prepare a non-aqueous organic solvent. LiFSI was dissolved in 99 g of the non-aqueous organic solvent to a concentration of 1.3M.

Then, 0.5 g of 1H, 1H, 5H-octafluoropentyl 1,1,2,2-tetrafluoroethyl ether cynonitrile (manufactured by TCI) and 0.5 g of vinylene carbonate were additionally added to the non-aqueous electrolyte for lithium secondary batteries was prepared.

II. Separator Manufacturing

Example 2.

Boehmite particles (average particle diameter 35 nm, specific surface area 95 m / g) and PVDF-co-HFP as a binder were mixed in a weight ratio of 8:2 (1:0.25), and stirred for 240 minutes under a nitrogen atmosphere at 25°C to prepare a precursor slurry. prepared. Then, the precursor slurry was coated on a release film by a die coating method, dried, and separated to prepare a separator having a thickness of 15 μm.

Comparative Example 2.

A slurry of inorganic particles (Al ₂ O ₃ ) and a binder (PVDF-HFP) in a weight ratio of 8:2 (1:0.25) was applied to both sides of a polyethylene/polypropylene substrate by die coating, and a 15 μm thick polyolefin-based slurry was applied. A porous separator was prepared .

III. Lithium secondary battery manufacturing

Example 3.

A cathode active material (LiCoO ₂ ; LCO), carbon black as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were mixed in a weight ratio of 97:1.7:1.3, and then N-methyl-2-pyrrolidone as a solvent (hereinafter, NMP) to prepare a cathode active material slurry having a solid content of 60 wt%. The positive electrode active material slurry was applied to an aluminum (Al) thin film as a positive electrode current collector having a thickness of about 12 μm, dried to prepare a positive electrode, and then a positive electrode was manufactured by performing a roll press.

The negative electrode active material (graphite), styrene-butadiene rubber-carboxymethyl cellulose (SBR-CMC) as a binder, and carbon black as a conductive material are mixed in distilled water as a solvent in a 96:3.5:0.5 weight ratio to slurry the negative electrode active material (solid content: 50% by weight) was prepared. The negative electrode active material slurry was applied to a negative electrode current collector (Cu thin film) having a thickness of 6 μm, dried and roll pressed to prepare a negative electrode.

An electrode assembly was prepared by sequentially stacking the positive electrode, the separator prepared in Example 2, and the negative electrode, and then the electrode assembly was accommodated in a battery case, and the electrolyte of Example 1 was injected to prepare a lithium secondary battery.

Comparative Example 3.

A lithium secondary battery was prepared in the same manner as in Example 4, except that the separator prepared in Comparative Example 2 was used instead of the separator prepared in Example 2, and the electrolyte of Comparative Example 1 was injected instead of the electrolyte of Example 1 prepared.

Experimental example

Experimental Example 1. Flame retardant evaluation experiment for electrolyte (1)

1 g of the electrolyte of Example 1 and 1 g of the electrolyte of Comparative Example 1 were placed in a metal container, respectively, and the surface of the electrolyte was ignited with a gas lighter to check whether the electrolyte was ignited, and the results are shown in Table 1 and FIG. 1 it was

At this time, the case where the fire was ignited and combusted was indicated by O, and the case where the fire was not ignited and not burned was indicated by X.

burning or not Example 1 X Comparative Example 1 O

Referring to Table 1 and FIG. 1 , it was confirmed that the electrolyte of Example 1 of the present invention did not burn, whereas the electrolyte of Comparative Example 1 including the nitrile-based compound caught fire and burned.

Experimental Example 2. Flame retardant evaluation experiment for separator (2)

It was confirmed that the separator prepared in Example 2 and the porous separator prepared in Comparative Example 2 were ignited by igniting a fire with a gas lighter, and the results are shown in Table 2 and FIG. 2 below.

At this time, the case where the fire was ignited and combusted was indicated by O, and the case where the fire was not ignited and not burned was indicated by X.

burning or not Example 2 X Comparative Example 2 O

Referring to Table 2 and FIG. 2, it was confirmed that the separator of the present invention did not burn, whereas the polyolefin-based porous separator of Comparative Example 2 caught fire and burned.

Experimental Example 3. Heat resistance evaluation experiment for separator

After exposing the separator prepared in Example 2 and the porous separator prepared in Comparative Example 2 to 180° C. for 30 minutes, respectively, the shrinkage rate was checked using an optical measuring device (Keyence, im-600), and the results are shown in the table below 3 is shown.

Shrinkage (%) Example 2 0 Comparative Example 2 8.0

Referring to Table 3, it can be seen that the separator of Example 2 of the present invention hardly shrinks, whereas the polyolefin-based porous separator of Comparative Example 2 exhibits shrinkage of about 8.0%.

Experimental Example 4. Compression resistance evaluation experiment for separator

Compression resistance was evaluated by applying a pressure of 7.49 MPa at 70° C. to the separator prepared in Example 2 and the porous separator prepared in Comparative Example 2, respectively, and the results are shown in FIG. 3 .

Referring to FIG. 3 , it can be seen that the separator of Example 2 of the present invention has higher compression resistance than the polyolefin-based porous separator of Comparative Example 2.

Experimental Example 5. Wettability evaluation experiment

10 μl of the electrolyte prepared in Example 1 and 10 μl of the electrolyte prepared in Comparative Example 1 were added dropwise to the separator prepared in Example 2, respectively, under a temperature condition of 25° C., and then the moisture content was evaluated for 3 minutes and 3 hours, The results are shown in FIG. 4 below.

In addition, 10 μl of the electrolyte prepared in Example 1 and 10 μl of the electrolyte prepared in Comparative Example 1 were added dropwise to the separator prepared in Comparative Example 2, respectively, under a temperature condition of 25° C., and then the moisture content was evaluated for 3 minutes and 3 hours. and the results are shown in FIG. 4 below.

Referring to FIG. 4 , in the case of the electrolyte of Comparative Example 1, it can be seen that both the polyolefin-based porous separator of Comparative Example 2 and the ceramic-based separator of Example 2 have excellent moisture content. On the other hand, in the case of the electrolyte of Example 1, it can be seen that the moisture content of the polyolefin-based porous separator of Comparative Example 2 is not good, but the moisture content of the ceramic separator of Example 4 is excellent.

Experimental Example 6. Evaluation of thickness change rate of separator after impregnation

The ceramic separator of Example 2 was immersed in the electrolyte of Comparative Example 1 and the electrolyte of Example 1 for 1 hour under a temperature condition of 25° C., and then the dimensional change rate of the separator was measured. The results are shown in FIG. 5 .

Referring to FIG. 5 , it can be seen that the thickness change rate of the separator impregnated in the electrolyte of Example 1 is 1.0%, while the thickness change rate of the separator impregnated in the electrolyte of Comparative Example 1 is as high as 1.9%. Referring to these results, when the electrolyte of Comparative Example 1 is used, the swelling of the battery is expected to increase.

What has been described above is only one embodiment for implementing a secondary battery including a negative active material, a method for manufacturing the same, and a negative active material according to the present invention, and the present invention is not limited to the above-described embodiment, and the following claims are made. As claimed in the scope, it is included in the technical spirit of the present invention to the extent that various modifications can be made by anyone skilled in the art without departing from the gist of the present invention.

Claims (15)

a positive electrode, a negative electrode, a separator and an electrolyte;
The separator consists of inorganic particles and a binder,
The electrolyte is a lithium secondary battery comprising a compound represented by the following Chemical Formula 1, a nitrile compound, and an ionizable lithium salt.
[Formula 1]
R1-O-CH ₂ -R2
In Formula 1,
R1 and R2 are an alkyl group having 1 to 8 carbon atoms substituted with at least one fluorine.
The method according to claim 1,
The inorganic particle is a lithium secondary battery of boehmite (boehmite, AlOOH).
The method according to claim 1,
The binder is a lithium secondary battery comprising a fluororesin binder.
4. The method according to claim 3,
The fluororesin binder is polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride, or polytetrafluoroethylene lithium secondary battery.
The method according to claim 1,
The inorganic particles: the binder is a lithium secondary battery that is mixed in a weight ratio of 1:0.1 to 1:0.35.
The method according to claim 1,
In Formula 1, R1 is an alkyl group having 2 to 4 carbon atoms substituted with at least one fluorine, and R2 is an alkyl group having 2 to 6 carbon atoms substituted with at least one fluorine.
The method according to claim 1,
The compound represented by Formula 1 is 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether or 1H, 1H, 5H-octafluoropentyl 1,1,2, A lithium secondary battery that is 2-tetrafluoroethyl ether.
The method according to claim 1,
The compound represented by Chemical Formula 1 is 1H, 1H, 5H-octafluoropentyl 1,1,2,2-tetrafluoroethyl ether.
The method according to claim 1,
The nitrile compound is succinonitrile, adiponitrile, acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptannitrile, cyclopentane carbonitrile, cyclohexane carbonitrile, 2-fluorobenzonitrile , at least one selected from the group consisting of 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile lithium secondary battery.
The method according to claim 1,
The nitrile compound is a lithium secondary battery of succinonitrile.
The method according to claim 1,
The ionizable lithium salt is LiPF ₆ , LiBF ₄ , LiN(SO ₂ CF ₂ CF ₃ ) ₂ , LiN(SO ₂ F) ₂ , and LiN(SO ₂ CF ₃ ) ₂ At least one selected from the group consisting of lithium secondary battery.
The method according to claim 1,
The ionizable lithium salt is a lithium secondary battery of LiFSI.
The method according to claim 1,
The ionizable lithium salt and the nitrile compound is a lithium secondary battery that is included in a weight ratio of 1:2 to 1:8.
The method according to claim 1,
The electrolyte includes vinylene carbonate, vinylethylene carbonate, ethylene sulfate, lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, lithium oxalyldifluoroborate, lithium bisoxalatoborate, 1,3- propane sultone, 1,4-butane sultone, ethensultone, 1,3-propene sultone, 1,4-butene sultone, and 1-methyl-1,3-propene sultone, LiPO ₂ F ₂ , LiBF ₄ , and at least one additional additive selected from the group consisting of aluminum fluoride.
15. The method of claim 14,
The additional additive is a lithium secondary battery of vinylene carbonate.

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