Electrolyte additive for secondary battery, preparation method thereof, electrolyte composition including the additive, and secondary battery

Abstract

According to one embodiment of the present invention, there is provided an electrolyte additive as a compound represented by Chemical Formula 1 to Chemical Formula 4. In addition, according to another embodiment of the present invention, there is provided an electrolyte additive preparation method for preparing the compounds of Chemical Formula 1 to Chemical Formula 4, and the electrolyte additive preparation method may include the following steps: reacting hexafluorophosphate with 2-monofluoromalonic acid; further adding an HF scavenger to the mixed solution produced by the reaction; and concentrating and drying the obtained reaction solution.

Description

Technical Field

[0001] The present invention relates to an electrolyte, and more particularly, to an electrolyte additive for a secondary battery and a secondary battery including the electrolyte additive. Background Art

[0002] A secondary battery is a battery that can be used repeatedly because it can be charged and discharged. The working principle of a lithium secondary battery, which is a representative secondary battery, is that lithium ions contained in the positive electrode active material move to the negative electrode through the electrolyte and then embed into the layered structure of the negative electrode active material (charging), and then the lithium ions embedded in the layered structure of the negative electrode active material return to the positive electrode (discharging).

[0003] Recently, as the demand for power sources for next-generation automobiles and the demand for environmentally friendly energy have increased, interest in secondary batteries has increased. In particular, there is a high interest in technologies for increasing the energy density of secondary batteries and extending the life of secondary batteries. In this regard, as previously known, in order to manufacture medium-capacity secondary batteries or large-capacity secondary batteries, lithium-rich layered oxides can be used as positive electrode active materials to achieve high voltage and high capacity secondary batteries. In addition, silicon-based materials as well as carbon-based materials can be used as negative electrode active materials to improve the capacity of the battery.

[0004] Meanwhile, in a typical lithium secondary battery, a lithium salt dissolved in an organic solvent is used as an electrolyte. The lithium-rich positive active material generates oxygen and a high-voltage environment during the first charge, and severe volume expansion occurs in the silicon-based negative electrode material during repeated charge and discharge, thereby forming cracks on the surface of the negative electrode, and eventually causing a decomposition reaction of the electrolyte on the surface of the electrode. As a result, the electrolyte is gradually depleted, the electrochemical performance of the battery deteriorates rapidly, and a thick film acting as a resistor is formed on the surface of each electrode. Therefore, the electrochemical reaction rate of the battery is reduced, and the acidic materials (e.g., HF, etc.) generated by the decomposition of the electrolyte melt each electrode film or damage the positive active material, so the electrochemical stability of the battery cannot be guaranteed. Summary of the invention

[0005] Technical issues

[0006] In order to solve the above technical problems, the present invention provides an electrolyte additive for a secondary battery, a method for manufacturing the same, and an electrolyte including the additive, the electrolyte additive for a secondary battery having low reduction decomposition ability while protecting a uniform protective film formed on an electrode.

[0007] The technical problems of the present invention are not limited to the above-mentioned technical problems, and those skilled in the art will clearly understand other technical problems not mentioned from the following description.

[0008] Technical Solution

[0009] One embodiment of the present invention provides an electrolyte additive. The electrolyte additive is a compound represented by the following Chemical Formula 1 or Chemical Formula 2:

[0010] [Chemical formula 1]

[0011]

[0012] [Chemical formula 2]

[0013]

[0014] M is an alkali metal, and R is hydrogen, a substituted or unsubstituted C1 to C5 alkyl group, a substituted or unsubstituted C1 to C5 perfluoroalkyl group, a substituted or unsubstituted C6 to C30 aryl group, a substituted or unsubstituted C6 to C30 perfluoroaryl group, or CF3.

[0015] The electrolyte additive of Chemical Formula 1 may be a compound represented by the following Chemical Formula 3:

[0016] [Chemical formula 3]

[0017]

[0018] The electrolyte additive of Chemical Formula 2 may be a compound represented by the following Chemical Formula 4:

[0019] [Chemical formula 4]

[0020]

[0021] Another embodiment of the present invention provides a method for manufacturing an electrolyte additive, the method comprising: a step of reacting hexafluorophosphate with 2-monofluoromalonic acid; a step of adding an HF scavenger to a mixed solution resulting from the reaction; and a step of concentrating and drying the solution obtained from the above to prepare a compound represented by Chemical Formula 1 or Chemical Formula 2.

[0022] The hexafluorophosphate may include lithium hexafluorophosphate (LiPF6). The 2-monofluoromalonic acid may include 2-fluoromalonic acid.

[0023] The method may also include a step of preparing 2-monofluoromalonic acid by reacting a 2-monofluoromalonic acid ester with an acid solution before reacting the compound. In the step of preparing 2-monofluoromalonic acid, the step of drying the reaction completed solution using a dehydrating agent may be further included. The dehydrating agent may include R 1 C(OR 2 )3 orthoester, in R 1 C(OR 2 )3, R1 It can be hydrogen or C1 to C5 alkyl, R 2 can be C1 to C5 alkyl, and R 1 and R 2 can be selected independently.

[0024] The step of reacting hexafluorophosphate and 2-monofluoromalonic acid may be performed in a non-aqueous organic solvent. The step of reacting hexafluorophosphate and 2-monofluoromalonic acid may be performed at a reaction temperature of 5°C to 60°C.

[0025] The HF scavenger may include a halide, a silane compound, or a combination thereof. The halide may include an alkali metal halide, a silicon halide, a phosphorus halide, or a combination thereof. The halide may include lithium chloride, silicon tetrachloride, dichlorodimethylsilane, trimethylchlorosilane, phosphorus trichloride, or a combination thereof. The silane compound may include an acyclic silane compound, a cyclic silane compound, or a combination thereof.

[0026] The steps of concentrating and drying the solution obtained from the above may be performed under reduced pressure.

[0027] Another embodiment of the present invention provides an electrolyte composition. The electrolyte composition may include an electrolyte additive represented by Chemical Formula 1 or Chemical Formula 2, a non-aqueous organic solvent, and an alkali metal salt.

[0028] The electrolyte additive may be included in an amount of 0.1 to 10% by weight based on the total amount of the electrolyte composition.

[0029] The electrolyte additive may be a mixture of the electrolyte additive and a film former of Chemical Formula 1. The film former may include fluoroethylene carbonate (FEC), vinylene carbonate (VC), vinylethylene carbonate (VEC), or a combination thereof.

[0030] The non-aqueous organic solvent may be a cyclic carbonate, a linear carbonate, or a combination thereof.

[0031] The alkali metal salt may be MPF6, MAsF6, MCF3SO3, MN(CF3SO2)2, MBF4, MBF6, MSbF6, MN(C2F5SO2)2, MAlO4, MAlCl4, MSO3CF3, MClO4 or a combination thereof, wherein M may be an alkali metal. The concentration of the alkali metal salt may be 0.1M to 3M.

[0032] Another embodiment of the present invention provides a secondary battery. The secondary battery may include a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, an electrolyte including an electrolyte additive represented by Chemical Formula 1 or Chemical Formula 2, and a separator between the positive electrode and the negative electrode.

[0033] The positive electrode active material may include LiNi 0.6 Co 0.2 Mn 0.2 O2. The positive electrode active material may include a lithium-rich positive electrode active material.

[0034] The negative electrode active material may include graphite or a silicon-graphite composite.

[0035] Beneficial Effects

[0036] As described above, according to an embodiment of the present invention, the stability of the secondary battery can be improved by the electrolyte additive. In addition, by simultaneously applying a lithium-rich positive electrode active material and a negative electrode active material of a graphite / graphite-silicon composite, a high-voltage and high-capacity secondary battery can be achieved, and even in this case, the improvement of the secondary battery can be improved by using an electrolyte additive.

[0037] The technical effects of the present invention are not limited to the above-mentioned technical effects, and those skilled in the art will clearly understand other technical effects not mentioned from the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 is a flowchart illustrating a method of manufacturing an electrolyte additive according to an embodiment of the present invention.

[0039] Figure 2 is a schematic diagram showing a secondary battery according to an embodiment of the present invention.

[0040] Figure 3 is a graph showing voltage-current curves measured by linear sweep voltammetry of electrolytes according to Electrolyte Example 2 and Electrolyte Comparative Examples 2-1 and 2-2.

[0041] Figure 4 is a graph showing dQ / dV distributions of secondary batteries according to Secondary Battery Example 1 and Secondary Battery Comparative Examples 1-1 and 1-2.

[0042] Figure 5a to Figure 5c is a graph showing high temperature life characteristics of secondary batteries according to Secondary Battery Example 1 and Secondary Battery Comparative Examples 1-1 and 1-2. Figure 5a is a graph showing specific capacity according to the number of cycles, and Figure 5b is a graph showing the coulombic efficiency according to the number of cycles. Figure 5c is a graph showing changes in open circuit voltage during high temperature storage.

[0043] Figure 6is a graph showing specific discharge capacity according to the number of cycles at room temperature of the secondary batteries according to Secondary Battery Example 1 and Secondary Battery Comparative Examples 1-1 and 1-2.

[0044] Figure 7a and Figure 7b is a graph showing room temperature life characteristics of secondary batteries according to Secondary Battery Example 2 and Secondary Battery Comparative Examples 2-1 and 2-2. Figure 7a is a graph showing specific capacity according to the number of cycles, and Figure 7b is a graph showing coulombic efficiency according to the number of cycles.

[0045] Figure 8 is a graph showing high-rate discharge characteristics of secondary batteries according to Secondary Battery Example 2 and Secondary Battery Comparative Examples 2-1 and 2-2.

[0046] Fig. 9 is a graph showing impedance regarding an interface between an electrode and an electrolyte of secondary batteries according to Secondary Battery Example 2 and Secondary Battery Comparative Examples 2-1 and 2-2. DETAILED DESCRIPTION

[0047] Since the present invention may have various changes and may have various embodiments, specific embodiments are shown in the drawings and described in detail. However, this is not intended to limit the present invention to specific embodiments, and it should be understood that all modifications, equivalents and substitutes included in the spirit and scope of the present invention are included.

[0048] The terms used in the present invention are only used to describe specific embodiments and are not intended to limit the present invention. Unless the context clearly indicates otherwise, singular expressions include plural expressions. In the present invention, terms such as "including" or "having" are intended to indicate the presence of features, quantities, steps, operations, components, parts or combinations thereof described in the specification, but it should be understood that this does not exclude the presence or addition of one or more other features or quantities, steps, operations, components, parts or combinations thereof.

[0049] Unless otherwise defined, all terms (including technical or scientific terms) used herein have the same meaning as those generally understood by those of ordinary skill in the art. It should also be understood that terms (such as those defined in commonly used dictionaries) should be interpreted as having a meaning consistent with their meaning in the context of the relevant art, and will not be interpreted in an idealized or overly formal sense unless explicitly defined as such herein.

[0050] Hereinafter, preferred embodiments of the present invention will be described in more detail with reference to the accompanying drawings. In the description of the present invention, in order to facilitate comprehensive understanding, the same reference numerals are used for the same components in the drawings, and repeated description of the same components is omitted.

[0051] When it is described as "Cx to Cy" in the present specification, it should be interpreted as having carbon atom numbers corresponding to all integers between the carbon number x and the carbon number y.

[0052] As used herein, unless otherwise defined, the term "alkyl" refers to an aliphatic hydrocarbon group. The alkyl group may be a "saturated alkyl group" that does not contain any double or triple bonds. The alkyl group may be a branched, straight chain type or a cyclic type.

[0053] In the present specification, unless otherwise defined, the term "aryl group" may refer to a monocyclic aromatic compound or a polycyclic aromatic compound consisting of condensed aromatic rings.

[0054] As used herein, "substituted" may be a substitution in which one or more or all hydrogens are replaced by a halogen group (specifically, a fluorine group).

[0055] The electrolyte additive according to an embodiment of the present invention may be an electrolyte additive represented by Chemical Formula 1, an electrolyte additive represented by Chemical Formula 2, or a mixture thereof.

[0056] [Chemical formula 1]

[0057]

[0058] [Chemical formula 2]

[0059]

[0060] In Chemical Formula 1 and Chemical Formula 2, M may be an alkali metal or ammonium. The alkali metal may be, for example, Li (lithium), Na (sodium), or K (potassium). R may be hydrogen, a substituted or unsubstituted C1 to C5 alkyl group, a substituted or unsubstituted C1 to C5 perfluoroalkyl group, a substituted or unsubstituted C6 to C30 aryl group, a substituted or unsubstituted C6 to C30 perfluoroaryl group, or CF3.

[0061] The electrolyte additive of Chemical Formula 1 may be represented by the following Chemical Formula 3, and the electrolyte additive of Chemical Formula 3 may be lithium tetrafluoro(2-fluoromalonate)phosphate (LiFMTFP).

[0062] [Chemical formula 3]

[0063]

[0064] The electrolyte additive of Chemical Formula 2 may be represented by the following Chemical Formula 4, and the electrolyte additive of Chemical Formula 4 may be lithium difluorobis(2-fluoromalonate)phosphate.

[0065] [Chemical formula 4]

[0066]

[0067] Figure 1 is a flowchart illustrating a method of manufacturing an electrolyte additive according to an embodiment of the present invention.

[0068] The method for preparing an electrolyte additive may include a step of preparing 2-monofluoromalonic acid (S10).

[0069] [Solution 1]

[0070]

[0071] In Scheme 1, R' may be a substituted or unsubstituted C1 to C10 alkyl group or a substituted or unsubstituted C6 to C30 aryl group. R is hydrogen, a substituted or unsubstituted C1 to C5 alkyl group, a substituted or unsubstituted C1 to C5 perfluoroalkyl group, a substituted or unsubstituted C6 to C30 aryl group, a substituted or unsubstituted C6 to C30 perfluoroaryl group, or CF3.

[0072] As shown in Scheme 1, the step of preparing 2-monofluoromalonic acid may include reacting 2-monofluoromalonic acid ester with an acid (HA). Specifically, the acid may be provided as an acidic solution, which is a solution comprising an acid. The acidic solution may be an aqueous solution comprising an acid.

[0073] 2-monofluoromalonic acid can be, for example, 2-fluoromalonic acid. 2-monofluoromalonic acid can be dried, and heating or vacuum drying can be used to dry. The water concentration in 2-monofluoromalonic acid can be 150wt ppm or lower, and preferably, 120wt ppm or lower. When the water concentration meets the above range, the hydrolysis of the phosphate obtained by the following reaction and the electrolyte additive of the present invention obtained according to this reaction can be suppressed.

[0074] The step of preparing 2-monofluoromalonic acid can be carried out at a reaction temperature of 25°C to 60°C (preferably, 30°C to 50°C), and the reaction can be carried out at room temperature. Any acidic solution can be used as long as it reacts with 2-monofluoromalonic acid ester to produce 2-monofluoromalonic acid. Here, in the case where 2-monofluoromalonic acid ester reacts with an alkaline solution, the reaction must be carried out in two steps, but in the case of using an acidic solution, the reaction is carried out in one step, which simplifies the reaction steps, so the synthesis time can be shortened. In addition, compared with using an alkaline solution, when using an acidic solution, the type of reaction byproducts can be reduced, and there can be an advantage of excellent reaction yield.

[0075] The preparation of 2-monofluoromalonic acid may further include drying the reaction solution using a dehydrating agent. Since the reaction in the step of preparing 2-monofluoromalonic acid is carried out in the presence of an acidic solution, specifically, in the presence of water, the water content of the product is high. Therefore, by drying the reaction solution, 2-monofluoromalonic acid can be obtained. Here, in order to remove the moisture contained in the product, any method except the method using a dehydrating agent can be used without restriction, as long as it can be dried without affecting the structure or content of 2-monofluoromalonic acid.

[0076] The dehydrating agent may be used without limitation as long as it does not affect the reaction while removing water from the product 2-monofluoromalonic acid. For example, R 1 C(OR 2 )3 orthoester as a dehydrating agent, and in this case, as shown in the following Scheme 2, water can be removed. Here, R 1 It can be hydrogen or C1 to C5 alkyl, R 2 can be a C1 to C5 alkyl group, and R 1 and R 2 Each may be independently selected. As an example, the orthoester may be trimethyl orthoformate and triethyl orthoformate, but is not limited thereto.

[0077] [Solution 2]

[0078] H2O+R 1 C(OR 2 )3→2R 2 OH+R 1 CO2R 2

[0079] Besides, an organic dehydrating agent such as silicate may be used, or an inorganic dehydrating agent may be used. As the silicate, tetraethyl silicate or the like may be used.

[0080] As shown in Scheme 3 or Scheme 4 below, the method for preparing an electrolyte additive according to the present invention may include reacting hexafluorophosphate with 2-monofluoromalonic acid ( S20 ).

[0081] [Solution 3]

[0082]

[0083] [Solution 4]

[0084]

[0085] In Scheme 3 and Scheme 4, M is an alkali metal or ammonium.

[0086] The hexafluorophosphate may be an alkali metal hexafluorophosphate or ammonium hexafluorophosphate, but is not limited thereto. The alkali metal hexafluorophosphate may be, for example, lithium hexafluorophosphate (LiPF6), sodium hexafluorophosphate (NaPF6) or potassium hexafluorophosphate (KPF6). The ammonium hexafluorophosphate may be tetraalkylammonium hexafluorophosphate. Preferably, the hexafluorophosphate may be lithium hexafluorophosphate (LiPF6).

[0087] The molar ratio of hexafluorophosphate to 2-monofluoromalonic acid may satisfy the equivalent ratio of the above Scheme 3 or Scheme 4. Specifically, in the case of Scheme 3, the molar ratio of hexafluorophosphate to 2-monofluoromalonic acid may be 1:0.9 to 1:1.1, preferably, 1:0.95 to 1:1.05. In addition, in the case of Scheme 4, the molar ratio of hexafluorophosphate to 2-monofluoromalonic acid may be 1:1.8 to 1:2.2, preferably, 1:1.9 to 1:2.1.

[0088] The above reaction can be carried out in a reaction solvent. The reaction solvent can be an organic solvent, specifically, a non-aqueous organic solvent. The non-aqueous organic solvent can be carbonates, esters, ethers, ketones or alcohols or a combination thereof.

[0089] Specifically, the carbonate organic solvent can be a cyclic carbonate or a linear carbonate. The cyclic carbonate can be ethylene carbonate (EC), fluoroethylene carbonate, propylene carbonate (PC) or butylene carbonate (BC), and the linear carbonate can be dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC) or ethyl propyl carbonate (EPC).

[0090] The ester organic solvent may be methyl formate, methyl acetate, ethyl acetate, isopropyl acetate, isoamyl acetate, methyl propionate, ethyl propionate, methyl butyrate, ethyl butyrate, or the like, but is not limited thereto.

[0091] The ether organic solvent may be dibutyl ether, tetraglyme (tetraethylene glycol dimethyl ether, TEGDME), diglyme (diethylene glycol dimethyl ether, DEGDME), dimethoxyethane, 2-methyltetrahydrofuran or tetrahydrofuran, but is not limited thereto.

[0092] The ketone organic solvent may be cyclohexanone or the like, and the alcohol solvent may be ethanol, isopropanol or the like, but is not limited thereto.

[0093] Specifically, carbonate organic solvents or ester organic solvents can be used as reaction solvents, and preferably, carbonate organic solvents can be ethyl methyl carbonate (EMC), dimethyl carbonate (DMC) or diethyl carbonate (DEC). These non-aqueous organic solvents can be dehydrated non-aqueous organic solvents, and these non-aqueous organic solvents can be used alone or in combination of two or more thereof.

[0094] The water concentration in the nonaqueous organic solvent may be 30 wt ppm or less, preferably, 15 wt ppm or less. When the water concentration satisfies the above range, hydrolysis of the phosphate and the electrolyte additive of the present invention may be suppressed.

[0095] The reaction temperature may be in the range of 5°C to 60°C, preferably, in the range of 10°C to 45°C. When this temperature range is met, the generation of by-products and the decomposition of the solvent may be suppressed, and there is an advantage that the reaction time does not need to be extended due to excellent reactivity. In addition, the yield of the electrolyte additive according to the present invention may be excellent.

[0096] The method for preparing an electrolyte additive according to the present invention may include a step ( S30 ) of further adding an HF scavenger to the mixed solution resulting from the reaction.

[0097] The HF scavenger is used to remove HF gas generated by the reaction of hexafluorophosphate and 2-monofluoromalonic acid, and if the HF scavenger is added to the mixture of hexafluorophosphate and 2-monofluoromalonic acid, the forward reaction of the synthesis reaction can be favorably performed.

[0098] As the HF scavenger, any one can be used as long as it is used to remove HF gas and does not participate in the electrolyte addition reaction according to the present invention. As an example, the HF scavenger may include a halide other than fluoride, a silane compound, or a combination thereof. In addition, the HF scavenger may be lithium carbonate, sodium carbonate, calcium hydroxide, sodium hydroxide, activated carbon, etc., but is not limited thereto.

[0099] The halide may be converted into an acid such as hydrogen chloride (HCl), hydrogen bromide (HBr), or hydrogen iodide (HI) having a high vapor pressure by reacting with HF, and then purified and removed by distillation, etc. The halide may be chloride, bromide, or iodide, and an inorganic halide, an organic halide, or a combination thereof may be used as the halide.

[0100] Inorganic halides can be alkali metal halides, alkaline earth metal halides, post-transition metal halides, metalloid halides, etc. Specifically, alkali metal halides can be lithium chloride, lithium bromide, lithium iodide, sodium chloride, sodium bromide, sodium iodide, potassium chloride, potassium bromide, potassium iodide, cesium chloride, cesium bromide, cesium iodide, etc. Alkaline earth metal halides can be magnesium chloride, magnesium bromide, magnesium iodide, calcium chloride, calcium bromide, calcium iodide, barium chloride, barium bromide, barium iodide, etc. Post-transition metal halides can be aluminum trichloride, aluminum tribromide, aluminum triiodide, etc. Metalloid halides can be silicon halides, and silicon halides can be silicon tetrachloride, silicon tetrabromide, silicon tetraiodide, dichlorodimethylsilane, trimethylchlorosilane, etc. In addition, inorganic halides can also be phosphorus halides. Phosphorus halides can be phosphorus trichloride, phosphorus tribromide, phosphorus triiodide, etc.

[0101] The organic halide may be an ammonium halide, etc. Specifically, the ammonium halide may be tetramethylammonium chloride, tetramethylammonium bromide, tetramethylammonium iodide, etc. In addition, the organic halide may be acetyl chloride, acetyl bromide, acetyl iodide, propionyl chloride, propionyl bromide, propionyl iodide, oxalyl chloride, oxalyl bromide, oxalyl iodide, etc.

[0102] More specifically, the halide may be an alkali metal halide, a silicon halide, a phosphorus halide, or a combination thereof. Preferably, the halogen contained in the halide may be chlorine (Cl), and thus, lithium chloride, silicon tetrachloride, dichlorodimethylsilane, trimethylchlorosilane, phosphorus trichloride, or a combination thereof may be used as the halide. Since they have a fast reaction rate, productivity can be improved, and cations not related to lithium ion batteries do not remain in the electrolyte.

[0103] HF formed according to the present reaction can be removed as much as the number of halogens included in the halide. Therefore, when the number of halogens included in one molecule of the halide is n, the lower limit of the amount of halide used relative to 1 mole of HF may be 1 / n mole, preferably 1.2 / n mole, and the upper limit is 5 / n mole, preferably 3 / n mole. When the above range is satisfied, no acid or unreacted halide that adversely affects battery performance will remain.

[0104] The silane compound refers to a compound including a silyl group, and may be an acyclic silane compound, a cyclic silane compound, or a combination thereof. As an example, the acyclic silane compound may be tris(trimethylsilyl)phosphite (TMSP) or hexamethyldisiloxane (HMDSO), and the cyclic silane compound may be octamethylcyclotetrasiloxane or decamethylcyclopentasiloxane, but is not limited thereto.

[0105] Meanwhile, since the product prepared according to the electrolyte additive is hydrolyzed by moisture, the reaction may be performed under an inert gas atmosphere, which is an atmosphere that does not contain moisture. For example, the reaction may be performed under an inert gas atmosphere such as nitrogen or argon.

[0106] Concentration and drying of the reaction solution (S40) obtained from the above can be performed. Concentration and drying are performed by using a method of vapor pressure difference, and in detail, by-products can be removed by introducing an inert gas into the solution under reduced pressure and discharging it with the inert gas. A vacuum pump, an aspirator, etc. can be used for decompression, and decompression can be performed by maintaining the pressure in the system below atmospheric pressure after the reactor is in a sealed state. Inert gas is introduced into the solution and can be performed by bubbling the solution using nitrogen, helium, neon, argon, krypton or xenon. The decompression of the reaction system and the introduction of an inert gas in the solution can be performed simultaneously. In addition, when decompressing and / or introducing an inert gas, the solution can be heated to 20°C to 50°C. Therefore, the electrolyte additive according to the present invention in solid form can be obtained.

[0107] According to another embodiment of the present invention, an electrolyte composition including an electrolyte additive, an alkali metal salt, and a non-aqueous organic solvent may be provided.

[0108] The electrolyte additive according to the present invention may be one of the electrolyte additives represented by Chemical Formula 1 to Chemical Formula 4 above.

[0109] Based on the total amount of the electrolyte composition, the electrolyte additive according to the present invention may be included in an amount of 0.1% to 10% by weight. Preferably, based on the total amount of the electrolyte composition, the electrolyte additive according to the present invention may be included in an amount of 0.5% to 5% by weight. When the content satisfies the above range, the stability improvement effect of the electrode can be fully demonstrated. In detail, the increase in the internal resistance of the secondary battery caused by the occurrence of side reactions can be suppressed.

[0110] The electrolyte additives of Chemical Formulas 1 to 4 can be used alone as electrolyte additives in an electrolyte composition, or can be used in combination with a film former that forms a stable and uniform protective film on an electrode of a secondary battery. The film former can be, but is not limited to, fluoroethylene carbonate (FEC), vinylene carbonate (VC), vinylethylene carbonate (VEC), or a mixture thereof.

[0111] As an example, lithium tetrafluoro (2-fluoromalonate) phosphate (LiFMTFP) can be used as the electrolyte additive of the present invention, and fluoroethylene carbonate can be used as a film former. When fluoroethylene carbonate is added to an electrolyte composition comprising lithium tetrafluoro (2-fluoromalonate) phosphate (LiFMTFP), lithium tetrafluoro (2-fluoromalonate) phosphate (LiFMTFP) and fluoroethylene carbonate can have a weight ratio of 1:10 to 2:5, specifically, 1:10 to 3:10, preferably, 1:5.

[0112] As the alkali metal salt, an alkali metal salt commonly used in the electrolyte of the secondary battery can be used. Examples of alkali metal salts may be, but are not limited to, MPF6, MAsF6, MCF3SO3, MN(CF3SO2)2, MBF4, MBF6, MSbF6, MN(C2F5SO2)2, MA1O4, MA1Cl4, MSO3CF3 or MClO4. In addition, these alkali metal salts may be used alone, or two or more may be used in combination. Here, M is an alkali metal, and may be Li (lithium), Na (sodium) or K (potassium).

[0113] The concentration of the alkali metal salt in the electrolyte composition may be 0.1 M to 3 M, for example, 0.1 M to 2 M. When this concentration range is satisfied, since the electrolyte may have appropriate conductivity and viscosity, insertion and extraction of alkali metal ions during charge and discharge may be facilitated, and decomposition of the solvent due to side reactions may be suppressed.

[0114] Non-aqueous organic solvents are organic solvents commonly used in secondary batteries, which can minimize the decomposition caused by oxidation reactions during charge and discharge, and can exhibit the desired properties of the present invention together with additives. Examples of non-aqueous organic solvents can be carbonates, esters, ethers, ketones or alcohols. These non-aqueous organic solvents can be used alone, or two or more can be used in combination.

[0115] Preferably, among organic solvents, in particular, carbonate organic solvents can be used. As examples of carbonate organic solvents, there are cyclic carbonates and linear carbonates, and cyclic carbonates can include ethylene carbonate (EC), fluoroethylene carbonate, propylene carbonate (PC) or butylene carbonate (BC), etc., and linear carbonates can include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC) or ethyl propyl carbonate (EPC). Preferably, non-aqueous organic solvents can be ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) or a combination thereof. In this case, the volume ratio of the organic solvent can be applied as EC:EMC:DMC=3:4:3.

[0116] The organic solvent may be an ester solvent, more specifically, methyl formate, methyl acetate, ethyl acetate, isopropyl acetate, isoamyl acetate, methyl propionate, ethyl propionate, methyl butyrate or ethyl butyrate.

[0117] The organic solvent may be an ether solvent, and examples of the ether solvent may include dibutyl ether, tetraglyme (tetraethylene glycol dimethyl ether, TEGDME), diglyme (diethylene glycol dimethyl ether, DEGDME), dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, and the like.

[0118] Ketone solvents may be used as the organic solvent, and the ketone solvents may be cyclohexanone or the like, and the alcohol solvents may be ethanol, isopropanol or the like.

[0119] Another embodiment of the present invention may provide a secondary battery including the electrolyte additive.

[0120] Figure 2 is a schematic diagram showing a secondary battery according to an embodiment of the present invention.

[0121] Reference Figure 2 , the secondary battery 100 includes a negative electrode active material layer 120 including a negative electrode active material, a positive electrode active material layer 140 including a positive electrode active material, and a separator 130 disposed between the negative electrode active material layer 120 and the positive electrode active material layer 140. The electrolyte 160 may be disposed or charged between the negative electrode active material layer 120 and the separator 130 and between the positive electrode active material layer 140 and the separator 130. The negative electrode active material layer 120 may be disposed on the negative electrode collector 110, and the positive electrode active material layer 140 may be disposed on the positive electrode collector 150.

[0122] <Positive electrode>

[0123] The positive electrode can be formed by coating a slurry in which a positive electrode active material, a binder, and a solvent are mixed on the positive electrode current collector 150. The positive electrode can be manufactured using a positive electrode material obtained by mixing an active material for a secondary battery, a conductive material, and a binder.

[0124] The positive electrode active material can be a known layered alkaline composite metal compound. As an example, the positive electrode active material can include at least one of a composite phosphorus oxide or composite oxide of cobalt, nickel, manganese, chromium, or a combination thereof and lithium. More specifically, the positive electrode active material can be M x (Ni a Co b Mn c Al d )O2 (where 0.5 ≤ x ≤ 1.5, 0 ≤ a ≤ 1, 0 ≤ b ≤ 1, 0 ≤ c ≤ 1, 0 ≤ d < 1, and a + b + c + d = 1), M(Ni a Co b Mn c )O4 (0 ≤ a ≤ 2, 0 < b ≤ 2, 0 < c ≤ 2, and a + b + c = 2) or MTmPO4 (Tm is Fe, Mn, Co, Ni, or a combination thereof). Here, M is an alkali metal and can be Li (lithium), Na (sodium), or K (potassium). The positive electrode active material can specifically be MCoO2, MCoPO4, MNiO2, MNiPO4, MMnO2, MMn2O4, MMnPO4, MCrO2, MNi 0.6 Co0 .2 Mn 0.2 O2, MNiMnCoO2, M(Co x Ni 1-x )O2 (0.5 ≤ x < 1), etc., but is not limited thereto. As an example, the positive electrode active material can include LiNi 0.6 Co 0.2 Mn 0.2 O2.

[0125] In addition, the positive electrode active material can be a lithium-rich positive electrode active material. The lithium-rich positive electrode active material can be a compound containing an excess of lithium and can exhibit a high capacity and high energy density of the battery. The lithium-rich positive electrode active material can be a lithium-rich positive electrode active material represented by the following formula 3.

[0126] [Formula 3] Li x Ni y Mn z Co w O2

[0127] In formula 3, 1 < x ≤ 2, 0 < y ≤ 1, 0 < z ≤ 1, and 0 < w ≤ 1.

[0128] In addition, a coating may be included on the surface of the positive active material. The coating is a compound of a coating element and may include an oxide, hydroxide, oxyhydroxide, oxycarbonate or basic carbonate of the coating element. The compound constituting the coating may be amorphous or crystalline. The coating element included in the coating may be Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr or a mixture thereof. For the coating formation process, any coating method may be used as long as it is a coating method (e.g., spraying, dipping, etc.) that does not adversely affect the physical properties of the positive active material.

[0129] The conductive material may be a carbon material, such as natural graphite, artificial graphite, coke, carbon black, carbon nanotubes or graphene. The binder may be a thermoplastic resin, for example, a fluororesin (such as polyvinylidene fluoride, polytetrafluoroethylene, tetrafluoroethylene, vinylidene fluoride copolymer, hexafluorinated propylene) and / or a polyolefin resin (such as polyethylene or polypropylene).

[0130] The positive electrode current collector 150 may be a metal having heat resistance, and may be, for example, iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, platinum, etc. In one embodiment, the positive electrode current collector may be aluminum or stainless steel. The upper surface of the positive electrode current collector 150 may also be roughened to improve adhesion with the positive active material layer 140.

[0131] <Negative electrode>

[0132] The negative electrode active material can intercalate and deintercalate lithium ions or can cause a conversion reaction, and can be a metal, a metal alloy, a metal oxide, a metal fluoride, a metal sulfide, a carbon material such as graphite (natural graphite, artificial graphite), coke, carbon black, carbon nanotubes, graphene, or silicon. Preferably, graphite or a silicon-graphite composite can be used as the negative electrode active material.

[0133] The negative electrode material can be obtained by mixing a negative electrode active material, a conductive material and a binder. In this case, the conductive material can be a carbon material such as natural graphite, artificial graphite, coke, carbon black, carbon nanotubes or graphene. The binder can be a thermoplastic resin, for example, a fluororesin (such as polyvinylidene fluoride, polytetrafluoroethylene, tetrafluoroethylene, vinylidene fluoride copolymer, hexafluorinated propylene) and / or a polyolefin resin (such as polyethylene or polypropylene).

[0134] The negative electrode material can be applied to the negative electrode collector to form the negative electrode. The negative electrode collector can be a conductor such as aluminum (Al), nickel (Ni), stainless steel (SUS) or molybdenum (Mo). Applying the negative electrode material to the negative electrode collector can be performed by pressure molding, or can be performed by preparing a slurry using an organic solvent, etc., then applying the slurry to the collector and pressing the slurry to fix it. The organic solvent can be: an amine solvent such as N,N-dimethylaminopropylamine or diethyltriamine; ethers such as ethylene oxide and tetrahydrofuran; ketones such as methyl ethyl ketone; esters such as methyl acetate; aprotic polar solvents such as dimethylacetamide or N-methyl-2-pyrrolidone. Applying the slurry to the negative electrode collector can be performed using, for example, a gravure coating method, a slit die coating method, a doctor blade coating method or a spray coating method.

[0135] <Electrolytes>

[0136] As described above, the present invention may provide an electrolyte including the electrolyte additives of Chemical Formulae 1 to 4, a lithium salt, and a nonaqueous organic solvent.

[0137] [Chemical formula 1]

[0138]

[0139] [Chemical formula 2]

[0140]

[0141] In Chemical Formula 1 and Chemical Formula 2, M may be an alkali metal or ammonium. The alkali metal may be, for example, Li (lithium), Na (sodium), or K (potassium). R may be hydrogen, a substituted or unsubstituted C1 to C5 alkyl group, a C1 to C5 perfluoroalkyl group, a substituted or unsubstituted C6 to C30 aryl group, a C6 to C30 perfluoroaryl group, or CF3.

[0142] [Chemical formula 3]

[0143]

[0144] [Chemical formula 4]

[0145]

[0146] In particular, in the case of the electrolyte additives according to Chemical Formula 3 and Chemical Formula 4, the electrochemical properties of a secondary battery including the same may be more excellent, compared to a compound in which two hydrogens are connected to the α carbon of a malonate group.

[0147] [Solution 5]

[0148]

[0149] In the case of compounds according to Formula 3 and Formula 4 in which fluorine is attached to the alpha carbon of the malonate group, LiF may be formed. As an example of a reaction for forming LiF, referring to Scheme 5 above, the fluorine attached to the alpha carbon of the malonate group may react with a lithium ion (Li + ) and electrons react to form LiF. When LiF is generated as described above, LiF forms a film around the electrode to suppress the elution of the transition metal of the electrode, and as a result, the electrochemical performance of the secondary battery including the electrolyte additive can be improved in terms of durability and the like.

[0150] [Solution 6]

[0151]

[0152] On the other hand, referring to Scheme 6, in the case of a compound in which two hydrogens are connected to the α carbon of the malonate group, a fluorine group is not bonded to the α carbon, thereby forming an acidic atmosphere in the electrolyte. That is, unlike the compounds of Chemical Formulas 3 and 4, LiF used as a protective layer of an electrode cannot be formed, and further, the formed acid promotes a side reaction of the electrolyte.

[0153] The electrolyte additives of Chemical Formula 1 to Chemical Formula 4 according to the present invention can be used by mixing with a film former. The film former can form a stable and uniform protective film on the electrode of the secondary battery. As an example, the film former can be fluoroethylene carbonate (FEC). In addition, the film former can be vinylene carbonate (VC) or vinyl ethylene carbonate (VEC).

[0154] The film former has a lower reduction potential than the electrolyte additives of Chemical Formulas 1 to 4, and when used in combination with the electrolyte additive according to the present invention, the film former may be reduced and decomposed earlier than the electrolyte additives of Chemical Formulas 1 to 4. That is, the electrolyte additives of Chemical Formulas 1 to 4 may have lower reduction decomposition capabilities.

[0155] Thus, when the electrolyte additive according to the present invention and the film former are mixed, a uniform and stable protective film can be generated on the electrode of the secondary battery including the electrolyte additive according to the present invention by the film former, and then the preformed protective layer and the electrode can be protected by the electrolyte additive of Chemical Formula 1 to Chemical Formula 4. Therefore, since the interface between the electrode material and the electrolyte is stabilized by the electrolyte additive, the lithium secondary battery including the electrolyte additive according to the present invention can have improved cycle life characteristics at room temperature and can improve electrochemical performance.

[0156] In addition, the electrolyte additives of Chemical Formulas 1 to 4 according to the present invention can be used alone as an electrolyte additive in an electrolyte composition. By adding the electrolyte additives of Chemical Formulas 1 to 4 alone as electrolyte additives in the electrolyte composition, the stability of the electrode interface can be improved even when no additional electrolyte additives (such as film formers) are further added, and the secondary battery including the electrolyte additives of Chemical Formulas 1 to 4 can exhibit excellent electrochemical properties.

[0157] In particular, due to the excellent cycle capacity retention rate, life characteristics and coulombic efficiency at high temperature and excellent storage performance at high temperature, the secondary battery can be driven with excellent stability even at high temperature. In addition, even when no additional electrolyte additives (such as film formers) are included, the secondary battery including the electrolyte additive according to the present invention can also have excellent room temperature life performance at room temperature.

[0158] Therefore, a secondary battery including the electrolyte additive according to the present invention can have excellent high temperature life characteristics. In addition, even when a positive electrode including a lithium-rich active material and a negative electrode including a silicon-graphite composite active material are used, the secondary battery can be stably driven while exhibiting high capacity and high voltage. Therefore, it is possible to increase the energy density of the secondary battery and improve the capacity retention rate characteristics.

[0159] <Diaphragm>

[0160] The separator 130 may be disposed between the negative electrode and the positive electrode. The separator may be a material in the form of a porous film, a nonwoven fabric, or a woven fabric, and the porous film, the nonwoven fabric, or the woven fabric is made of a material such as a polyolefin resin (such as polyethylene or polypropylene), a fluororesin, or a nitrogen-containing aromatic polymer. From the perspective of increasing the volume energy density of the battery and reducing the internal resistance, the thickness of the separator is preferably thinner as long as the mechanical strength is maintained. The thickness of the separator may generally be about 5 μm to 200 μm, more specifically, 5 μm to 40 μm.

[0161] <Method for manufacturing secondary battery>

[0162] After the positive electrode, the separator and the negative electrode are sequentially stacked to form an electrode group, if necessary, the electrode group is rolled up and stored in a battery can and a secondary battery can be manufactured by impregnating the electrode group with a non-aqueous electrolyte. Alternatively, after the electrode group is formed by laminating the positive electrode, the solid electrolyte and the negative electrode, if necessary, the electrode group can be rolled up and stored in a battery can to manufacture a secondary battery.

[0163] Hereinafter, examples are provided to help understand the present invention. However, the following examples are provided only to help understand the present invention, and the present invention is not limited to the following examples.

[0164] [Example]

[0165] Preparation example of 2-fluoromalonic acid

[0166] At room temperature, 1000g of 35% hydrogen chloride (HCl) aqueous solution is added to a 2000mL three-necked flask and stirred with agitator. Then, 200g of diethyl 2-fluoromalonate is added dropwise to a flask comprising 35% hydrogen chloride aqueous solution for 1 hour using a dropping funnel. Thereafter, the reaction is performed at 10 ° C for 16 hours. At this time, while raising the temperature, the hydrogen chloride dissolved in the water is produced as a gas, which is absorbed by the tank filled with soda lime. After the reaction is completed, trimethyl orthoformate is added and vacuum dried at 50 ° C, to obtain 2-fluoromalonic acid.

[0167] The obtained 2-fluoromalonic acid was dissolved in dimethyl sulfoxide-d6 and NMR analysis was performed, thereby showing that the yield of 2-fluoromalonic acid based on diethyl 2-fluoromalonate was 92.5%.

[0168] Electrolyte Additive Preparation Example 1

[0169] In a glove box with a dew point of -50°C, 100 g of dehydrated ethyl methyl carbonate (EMC) containing 10 mass ppm of water was placed in a 250 mL three-necked round-bottom flask, and 16.23 g (0.107 mol) of lithium hexafluorophosphate (LiPF6) was added and dissolved. In addition, after the 2-fluoromalonic acid obtained according to the preparation example of 2-fluoromalonic acid was dried to a moisture content of 100 mass ppm, 13.04 g (0.107 mol) of dried 2-fluoromalonic acid was added. The three-necked round-bottom flask was taken out of the glove box, immersed in an oil bath at 30°C, and fully stirred using a magnetic stirrer to obtain a mixed solution.

[0170] Next, 13.79 g (0.107 mol) of dichlorodimethylsilane (Me2SiCl2) as a HF scavenger was added to the mixture using a dropping funnel over 1 hour. While adding, difluorodimethylsilane and hydrogen chloride were generated, and the reaction was terminated after sufficient reaction for 3 hours after the gas generation was completed.

[0171] After the reaction, the obtained reaction solution was concentrated and dried at 40°C and 5 torr under reduced pressure to obtain 23.75 g of solid lithium tetrafluoro(2-fluoromalonate)phosphate (LiFMTFP) with a reaction yield of 95%. The obtained electrolyte additive was dissolved in dimethyl sulfoxide-d6 and subjected to 19 F-NMR analysis showed that lithium tetrafluoro(2-fluoromalonate)phosphate (LiFMTFP) was 99% by mass.

[0172] Electrolyte Additive Preparation Example 2

[0173] The electrolyte additive was prepared in the same manner as in Electrolyte Additive Preparation Example 1, except that 9.06 g (0.214 mol) of lithium chloride (LiCl) was used as an HF scavenger. As a result, 22.5 g of solid lithium tetrafluoro(2-fluoromalonate)phosphate (LiFMTFP) was obtained with a reaction yield of 90%. 19 As a result of F-NMR analysis, it was found to be 97% by mass.

[0174] Electrolyte Additive Preparation Example 3

[0175] The electrolyte additive was prepared in the same manner as in Electrolyte Additive Preparation Example 1, except that 9.83 g (0.072 mol) of phosphorus trichloride (PCl3) was used as an HF scavenger. As a result, 21.25 g of solid lithium tetrafluoro(2-fluoromalonate)phosphate (LiFMTFP) was obtained with a reaction yield of 85%. 19 As a result of F-NMR analysis, it was found to be 45% by mass.

[0176] Electrolyte Additive Preparation Example 4

[0177] The electrolyte additive was prepared in the same manner as in Electrolyte Additive Preparation Example 1, except that the reaction was carried out at a reaction temperature of 60° C. As a result, 24.5 g of solid lithium tetrafluoro(2-fluoromalonate)phosphate (LiFMTFP) was obtained with a reaction yield of 98%. 19 As a result of F-NMR analysis, it was found to be 91% by mass.

[0178] Electrolyte Additive Preparation Example 5

[0179] The electrolyte additive was prepared in the same manner as in Electrolyte Additive Preparation Example 1, except that 7.78 g (0.072 mol) of trimethylsilyl chloride (Me3SiCl) was used as an HF scavenger, diethyl carbonate (DEC) was used as an organic solvent, and the reaction was performed at a reaction temperature of 10°C. As a result, 11.25 g of solid lithium tetrafluoro(2-fluoromalonate)phosphate (LiFMTFP) was obtained with a reaction yield of 45%. 19 As a result of F-NMR analysis, it was found to be 65% by mass.

[0180] Electrolyte Additive Preparation Example 6

[0181] The electrolyte additive was prepared in the same manner as in Electrolyte Additive Preparation Example 1, except that 9.83 g (0.072 mol) of phosphorus trichloride (PCl3) was used as an HF scavenger, diethyl carbonate (DEC) was used as an organic solvent, and the reaction was carried out at a reaction temperature of 45°C. As a result, 15 g of solid lithium tetrafluoro(2-fluoromalonate)phosphate (LiFMTFP) was obtained with a reaction yield of 60%. 19 As a result of F-NMR analysis, it was found to be 57% by mass.

[0182] Electrolyte Additive Preparation Example 7

[0183] The electrolyte additive was prepared in the same manner as in Electrolyte Additive Preparation Example 1, except that 9.08 g (0.053 mol) of silicon tetrachloride (SiCl4) was used as an HF scavenger, diethyl carbonate (DEC) was used as an organic solvent, and the reaction was carried out at a reaction temperature of 10°C. As a result, 19.5 g of solid lithium tetrafluoro(2-fluoromalonate)phosphate (LiFMTFP) was obtained with a reaction yield of 78%. 19 As a result of F-NMR analysis, it was found to be 85% by mass.

[0184] Electrolyte Additive Preparation Example 8

[0185] The electrolyte additive was prepared in the same manner as in Electrolyte Additive Preparation Example 1, except that ethyl acetate (AcOEt) was used as the organic solvent. As a result, 24 g of solid lithium tetrafluoro(2-fluoromalonate)phosphate (LiFMTFP) was obtained with a reaction yield of 96%. 19 As a result of F-NMR analysis, it was found to be 90% by mass.

[0186] Table 1 below summarizes the electrolyte additive preparation examples.

[0187] [Table 1]

[0188]

[0189] Electrolyte Additive Preparation Example 9

[0190] In a glove box with a dew point of -50°C, 100 g of dehydrated ethyl methyl carbonate (EMC) containing 10 mass ppm of water was placed in a 250 mL three-necked round-bottom flask, and 16.23 g (0.107 mol) of lithium hexafluorophosphate (LiPF6) was added and dissolved. In addition, after the 2-fluoromalonic acid obtained according to the preparation example of 2-fluoromalonic acid was dried to a moisture content of 100 mass ppm, 26.08 g (0.214 mol) of dried 2-fluoromalonic acid was added. The three-necked round-bottom flask was taken out of the glove box, immersed in an oil bath at 40°C, and fully stirred using a magnetic stirrer to obtain a mixed solution.

[0191] Next, 27.58 g (0.214 mol) of dichlorodimethylsilane (Me2SiCl2) as a HF scavenger was added to the mixture using a dropping funnel within 1 hour. While adding, difluorodimethylsilane and hydrogen chloride were generated, and the reaction was terminated after fully reacting for 3 hours after the gas generation was completed. As a result, difluorobis(2-fluoromalonic acid) lithium phosphate represented by Chemical Formula 4 was obtained.

[0192] Electrolyte Example 1

[0193] In an electrolyte including ethylene carbonate (EC): ethyl methyl carbonate (EMC): dimethyl carbonate (DMC) in a volume ratio of 3:4:3 as an organic solvent and 1.15M LiPF6 as a lithium salt, lithium tetrafluoro(2-fluoromalonate) phosphate (LiFMTFP) according to Electrolyte Additive Preparation Example 1 was added as an electrolyte additive in an amount of 1 wt % based on the total amount of the electrolyte to prepare an electrolyte.

[0194] Electrolyte comparison example 1-1

[0195] A nonaqueous electrolyte was prepared in the same manner as in Electrolyte Example 1, except that 5% by weight of fluoroethylene carbonate (FEC) was added as an electrolyte additive instead of 1% by weight of lithium tetrafluoro(2-fluoromalonate)phosphate (LiFMTFP).

[0196] Electrolyte Comparison Example 1-2

[0197] A nonaqueous electrolyte was prepared in the same manner as in Electrolyte Example 1, except that 1% by weight of lithium tetrafluoro(oxalato)phosphate (LiTFOP) was added as an electrolyte additive instead of 1% by weight of lithium tetrafluoro(2-fluoromalonate)phosphate (LiFMTFP).

[0198] Electrolyte Example 2

[0199] In an electrolyte including ethylene carbonate (EC): ethyl methyl carbonate (EMC): dimethyl carbonate (DMC) in a volume ratio of 3:4:3 as an organic solvent and 1.3 M LiPF6 as a lithium salt, 5 wt % of fluoroethylene carbonate (FEC) and 1 wt % of lithium tetrafluoro(2-fluoromalonate) phosphate (LiFMTFP) according to Electrolyte Additive Preparation Example 1 were added as electrolyte additives based on the total amount of the electrolyte to prepare an electrolyte.

[0200] Electrolyte Comparison Example 2-1

[0201] An electrolyte was prepared in the same manner as in Electrolyte Example 2, except that fluoroethylene carbonate (FEC) was added only in an amount of 5 wt % among the electrolyte additives.

[0202] Electrolyte comparison example 2-2

[0203] A nonaqueous electrolyte was prepared in the same manner as in Electrolyte Example 2, except that 1% by weight of lithium tetrafluoro(oxalate)phosphate (LiPF4(C2O4), LiTFOP) was added instead of 1% by weight of lithium tetrafluoro(2-fluoromalonate)phosphate (LiFMTFP) as an electrolyte additive.

[0204] Table 2 below summarizes the electrolyte examples and comparative examples.

[0205] [Table 2]

[0206]

[0207] Figure 3 is a graph showing voltage-current curves measured by linear sweep voltammetry of the electrolytes according to Electrolyte Example 2 and Electrolyte Comparative Examples 2-1 and 2-2. Here, scanning was performed at a scan rate of 1 mV / s in a voltage range of 3 V to 6 V at 25°C.

[0208] Reference Figure 3 , the electrolytes according to the electrolyte examples and the comparative examples show similar oxidation stability until approximately 4.3 V, but from then on, it can be seen that the peak current according to the oxidation decomposition reaction increases in the order of electrolyte example 2, electrolyte comparative example 2-1, and electrolyte comparative example 2-2. Therefore, it can be seen that the oxidation decomposition stability of electrolyte example 2 is better than that of electrolyte comparative example 2-2 and electrolyte comparative example 2-1, and in the case of electrolyte example 2, the oxidation decomposition stability of the electrolyte is greatly improved.

[0209] Therefore, when lithium tetrafluoro(2-fluoromalonate) phosphate (LiFMTFP) is used as an additive, the oxidation stability is excellent compared to using lithium tetrafluoro(oxalate) phosphate (LiPF4(C2O4), LiTFOP). Therefore, the electrochemical stability of the electrolyte containing lithium tetrafluoro(2-fluoromalonate) phosphate (LiFMTFP) is better than the electrochemical stability of the electrolyte containing lithium tetrafluoro(oxalate) phosphate (LiPF4(C2O4), LiTFOP), so the electrolyte additive according to the present invention can be more suitable for application in secondary batteries operating at high voltages.

[0210] Secondary battery example 1

[0211] Using LiNi 0.6 Co 0.2 Mn 0.2 O2 (NCM622) was used as a positive electrode active material to form a positive electrode active material layer, and graphite was used as a negative electrode active material to form a negative electrode active material layer. In addition, the electrolyte prepared according to Electrolyte Example 1 was placed between the positive electrode active material layer and the negative electrode active material layer to prepare a lithium secondary battery.

[0212] Secondary battery comparison example 1-1

[0213] A lithium secondary battery was prepared in the same manner as in Secondary Battery Example 1, except that the electrolyte prepared according to Electrolyte Comparative Example 1-1 was used as the electrolyte instead of the electrolyte prepared according to Electrolyte Example 1.

[0214] Secondary battery comparison example 1-2

[0215] A lithium secondary battery was prepared in the same manner as in Secondary Battery Example 1, except that the electrolyte prepared according to Electrolyte Comparative Example 1-2 was used as the electrolyte instead of the electrolyte prepared according to Electrolyte Example 1.

[0216] Figure 4 is a graph showing dQ / dV distributions of secondary batteries according to Secondary Battery Example 1 and Secondary Battery Comparative Examples 1-1 and 1-2.

[0217] After the secondary battery is assembled, performance as a battery is completed only after a predetermined charge / discharge process (ie, a formation process). Figure 4 It is based on the cycle during the formation process. In detail, during the primary formation charge / discharge, the secondary battery was charged to 4.3V, and after charging, a constant voltage (CV) condition was applied at 4.3V, wherein the stop condition was 0.02C, and the secondary battery was discharged to 2.7V under constant current conditions. During the life evaluation, the C-rate was 0.1C.

[0218] Reference Figure 4 , when the main peak is examined, the reduction potential of the secondary battery according to Secondary Battery Example 1 appears at 3.0V or higher. On the other hand, it can be seen that the reduction potential of the secondary battery according to Secondary Battery Comparative Example 1-1 is approximately 2.5V, and the reduction potential of the secondary battery according to Secondary Battery Comparative Example 1-2 is 3.0V or lower. From this, it can be seen that the FEC and LiTFOP of Secondary Battery Comparative Examples 1-1 and 1-2 will be reduced and decomposed earlier than the LiFMTFP included in the secondary battery according to Secondary Battery Example 1. That is, it can be confirmed that the LiFMTFP included in the secondary battery according to Secondary Battery Example 1 has the lowest reduction decomposition tendency.

[0219] Figure 5a to Figure 5c is a graph showing high temperature life characteristics of secondary batteries according to Secondary Battery Example 1 and Secondary Battery Comparative Examples 1-1 and 1-2. Figure 5a is a graph showing specific capacity according to the number of cycles, and Figure 5b is a graph showing coulombic efficiency according to the number of cycles.

[0220] exist Figure 5a and Figure 5b In the experiment, the high temperature life characteristics of each secondary battery after one formation charge / discharge were evaluated. In detail, during one formation charge / discharge, each secondary battery was charged to 4.3V, and after charging, a constant voltage (CV) condition was applied at 4.3V, wherein the stop condition was 0.05C, and the secondary battery was discharged to 2.7V under constant current conditions. During the formation charge / discharge, the C-rate was 0.1C.

[0221] In the evaluation of high temperature life, each secondary battery was charged to 4.3 V at 45° C., and after charging, a constant voltage (CV) condition was applied at 4.3 V, wherein the stop condition was 0.05 C, and the secondary battery was discharged under a constant current condition to 2.7 V. During the life evaluation, the C-rate during charge / discharge was 0.1 C.

[0222] Reference Figure 5a , although there is no difference in the initial capacity according to the electrolyte, it can be seen that there is a difference in the degree of capacity reduction according to the increase in the number of cycles and the specific discharge capacity in the final cycle. Compared with the secondary battery according to the secondary battery comparison example, the secondary battery according to the secondary battery example 1 shows a good specific discharge capacity value even after performing 250 cycles. Therefore, it can be confirmed that the secondary battery including the electrolyte additive according to the present invention has excellent capacity retention and life characteristics at high temperatures.

[0223] Reference Figure 5b, during 160 cycles, the secondary battery according to secondary battery example 1 had a constant coulombic efficiency value from the initial cycle, and the coulombic efficiency reached about 99.9%, so it can be seen that more stable driving of the secondary battery was performed.

[0224] in addition, Figure 5c is a graph showing changes in open circuit voltage during high temperature storage.

[0225] exist Figure 5c In the experiment, the high temperature storage performance was confirmed by evaluating the self-discharge characteristics of each secondary battery while storing at 45°C for about 20 days after a formation charge / discharge and charging. In detail, each secondary battery was charged to 4.3V during a formation charge / discharge, and a constant voltage (CV) condition was applied at 4.3V after charging, wherein the stop condition was 0.05C, and the secondary battery was discharged to 2.7V under constant current conditions. During the formation charge / discharge, the C-rate was 0.1C. In order to evaluate the self-discharge, charging was performed again at room temperature. The charging conditions were the same as those for the formation charge, and the open circuit voltage (OCV) of the battery was measured.

[0226] Reference Figure 5c , it can be seen that the open circuit voltage decrease of the secondary battery according to Example 1 is smaller than that of the secondary battery according to Comparative Example 1-2, and thus it can be seen that the high temperature lifetime storage performance is excellent.

[0227] Thus, compared with lithium tetrafluoro(oxalate)phosphate (LiPF4(C2O4), LiTFOP) and fluoroethylene carbonate (FEC), a secondary battery including lithium tetrafluoro(2-fluoromalonate)phosphate (LiFMTFP) has excellent discharge capacity, coulombic efficiency and life storage performance at high temperatures, so it can show improved life characteristics at high temperatures. Therefore, when the electrolyte additive according to the present invention is included, the life can be excellent even in a secondary battery that generates a large amount of heat, so it can be suitable for use in a medium-sized battery or a large-sized battery.

[0228] Figure 6 is a graph showing specific discharge capacities of secondary batteries according to Secondary Battery Example 1 and Secondary Battery Comparative Examples 1-1 and 1-2 according to the number of cycles at room temperature.

[0229] For each secondary battery, the room temperature life characteristics were evaluated after one formation charge and discharge. In detail, during one formation charge / discharge, each secondary battery was charged to 4.3V, and after charging, a constant voltage (CV) condition was applied at 4.3V, wherein the stop condition was 0.02C, and the secondary battery was discharged to 2.7V under constant current conditions. The C-rate during formation charge and discharge was 0.1C.

[0230] After one formation charge and discharge, in order to ensure the life stability before evaluating the room temperature life characteristics, three additional charge and discharges were performed. Specifically, each secondary battery was charged to 4.3V, and after charging, a constant voltage (CV) condition was applied at 4.3V, where the stop condition was 0.05C, and the secondary battery was discharged to 2.7V under constant current conditions. At this time, the C-rate was 0.2C.

[0231] In the room temperature life evaluation, each secondary battery was charged to 4.3 V at 25° C., and after charging, a constant voltage (CV) condition was applied at 4.3 V, wherein the stop condition was 0.05 C, and the secondary battery was discharged to 2.7 V under a constant current condition. During the life evaluation, the C-rate was 1.0 C. At this time, the charge and discharge experiment was performed up to approximately 200 cycles.

[0232] Reference Figure 6 , although there is no difference in initial capacity according to the electrolyte, the secondary battery according to the secondary battery example 1 has an improved specific discharge capacity value at the final cycle compared to the secondary batteries according to the secondary battery comparison examples 1-1 and 1-2. Therefore, compared with lithium tetrafluoro(oxalato)phosphate (LiPF4(C2O4), LiTFOP) and fluoroethylene carbonate (FEC), the secondary battery containing lithium tetrafluoro(2-fluoromalonate)phosphate (LiFMTFP) can have better room temperature life performance.

[0233] As described above, by adding the electrolyte additive according to the present invention alone as an electrolyte additive in the electrolyte composition, the stability of the electrode interface is improved even without further adding another electrolyte additive (such as a film-forming agent), and the secondary battery including the electrolyte additive according to the present invention can exhibit excellent electrochemical properties.

[0234] Secondary battery example 2

[0235] As the active material of lithium-rich cathode, Li 1.17 Ni 0.17 Mn 0.5 Co 0.17O2, binder (PVDF) and conductive material (Super P) are uniformly mixed in N-methyl-2-pyrrolidone (NMP) solvent at a weight ratio of 8:1:1 (positive electrode active material: conductive material: binder). The positive electrode is prepared by the following steps: the mixture containing the lithium-rich positive electrode active material is uniformly applied to the aluminum (Al) current collector; pressed in a roller press; and vacuum dried in a vacuum oven at 110°C for 2 hours. Here, the electrode density is set to have 1.9g / cc.

[0236] A negative electrode active material layer was formed using a silicon-graphite composite as a negative electrode active material. In addition, a polyethylene separator between the prepared positive electrode and the negative electrode was placed in a battery container, and an electrolyte prepared according to Electrolyte Example 2 was arranged between the positive electrode active material layer and the negative electrode active material layer to manufacture a lithium secondary battery in the form of a 2032 full cell according to a conventional manufacturing method.

[0237] Secondary battery comparison example 2-1

[0238] A lithium secondary battery was prepared in the same manner as in Secondary Battery Example 2, except that the electrolyte prepared according to Electrolyte Comparative Example 2-1 was used as the electrolyte instead of the electrolyte prepared according to Electrolyte Example 2.

[0239] Secondary battery comparison example 2-2

[0240] A lithium secondary battery was prepared in the same manner as in Secondary Battery Example 2, except that the electrolyte prepared according to Electrolyte Comparative Example 2-2 was used as the electrolyte instead of the electrolyte prepared according to Electrolyte Example 2.

[0241] Figure 7a and Figure 7b is a graph showing room temperature life characteristics of secondary batteries according to Secondary Battery Example 2 and Secondary Battery Comparative Examples 2-1 and 2-2. Figure 7a is a graph showing specific capacity according to the number of cycles, and Figure 7b is a graph showing coulombic efficiency according to the number of cycles.

[0242] For each secondary battery, the room temperature life characteristics were evaluated after a formation charge / discharge. In detail, during a formation charge / discharge, each secondary battery was charged to 4.55V, and after charging, a constant voltage (CV) condition was applied at 4.55V, wherein the stop condition was 0.02C, and the secondary battery was discharged to 2.0V under constant current conditions. During the formation charge / discharge, the C-rate was 0.1C.

[0243] After one formation charge and discharge, in order to ensure the life stability before evaluating the room temperature life characteristics, three additional charge and discharges were performed. Specifically, each secondary battery was charged to 4.55V, and after charging, a constant voltage (CV) condition was applied at 4.55V, wherein the stop condition was 0.05C, and the secondary battery was discharged to 2.0V under constant current conditions. At this time, the C-rate was 0.2C.

[0244] In the room temperature life evaluation, each secondary battery was charged to 4.55 V at 25° C., and after charging, a constant voltage (CV) condition was applied at 4.55 V, wherein the stop condition was 0.05 C, and the secondary battery was discharged under a constant current condition to 2.0 V. During the life evaluation, the C-rate was 0.5 C, and the charge and discharge experiments were performed up to approximately 100 cycles.

[0245] Reference Figure 7a , although there is no difference in the initial capacity according to the electrolyte, it can be seen that there are differences in the degree of capacity reduction according to the increase in the number of cycles and the specific discharge capacity in the final cycle. In other words, compared with other secondary batteries, the secondary battery according to the secondary battery comparison example 2-1 exhibits a large degree of capacity reduction and has a small specific discharge capacity at the 100th cycle. Compared with the secondary battery according to the secondary battery comparison example 2-2, the secondary battery according to the secondary battery example 2 shows a similar specific discharge capacity value at the 100th cycle, but has a higher specific discharge capacity value at approximately 20 cycles. Therefore, it can be seen that the secondary battery including the electrolyte additive according to the present invention has excellent cycle capacity retention and initial life characteristics.

[0246] In addition, refer to Figure 7b , it can be seen that the secondary battery has no problem in performing 100 cycles, but the average coulombic efficiency value varies depending on the electrolyte contained in the secondary battery. It can be seen that the coulombic efficiency of the secondary battery according to the secondary battery example is more stable than the coulombic efficiency of the secondary battery according to the secondary battery comparison example.

[0247] Therefore, as described above, a secondary battery containing lithium tetrafluoro(2-fluoromalonate)phosphate (LiFMTFP) has excellent discharge capacity and coulombic efficiency at room temperature compared to lithium tetrafluoro(oxalate)phosphate (LiPF4(C2O4), LiTFOP), so it can exhibit excellent life characteristics at room temperature.

[0248] Figure 8 is a graph showing high-rate discharge characteristics of secondary batteries according to Secondary Battery Example 2 and Secondary Battery Comparative Examples 2-1 and 2-2.

[0249] For each secondary battery, after one formation charge / discharge at 25° C., the charge rate was fixed at 0.5 C, and the discharge rate was changed to 0.5 C, 1 C, 2 C, 3 C, 4 C, 5 C, and 0.5 C, thereby evaluating the high rate discharge characteristics. The evaluation was performed every 3 cycles at each discharge rate.

[0250] Reference Figure 8 , it can be confirmed that the secondary battery has operability despite the current density in the range of 0.5C-rate to 5C-rate, and it can be seen that despite the high current density, the decrease in specific discharge capacity of the other secondary batteries is less than the decrease in specific discharge capacity of the secondary battery according to the secondary battery comparison example 2-1. In addition, compared with the secondary battery according to the secondary battery comparison example 2-2, the secondary battery according to the secondary battery example 2 has no significant difference in 0.5C-rate to 3C-rate, but it can be seen that there is a slight difference between the specific discharge capacity at the current density of 4C-rate and 5C-rate.

[0251] Therefore, the high rate charge and discharge characteristics of the electrolyte containing lithium tetrafluoro(2-fluoromalonate)phosphate (LiFMTFP) are improved compared with lithium tetrafluoro(oxalate)phosphate (LiPF4(C2O4), LiTFOP). Therefore, it can be seen that the secondary battery according to the embodiment of the present invention can have excellent high rate discharge characteristics and excellent life stability against C-rate changes at room temperature.

[0252] Fig. 9 is a graph showing impedance regarding an interface between an electrode and an electrolyte of secondary batteries according to Secondary Battery Example 2 and Secondary Battery Comparative Examples 2-1 and 2-2.

[0253] The interface resistance was measured after the formation process of each secondary battery. Specifically, during the formation charge / discharge, each secondary battery was charged to 4.55V, and after charging, a constant voltage (CV) condition was applied at 4.55V, wherein the stop condition was 0.02C, and the secondary battery was discharged to 2.0V under constant current conditions. Here, the C-rate was 0.1C.

[0254] Reference Fig. 9 It can be predicted that the internal resistance of the secondary battery increases as the interface resistance between the electrode and the electrolyte in the secondary battery according to Secondary Battery Example 2 increases slightly compared to Secondary Battery Comparative Examples 2-1 and 2-2. This is considered to be because a film is formed in the initial stage of the electrode.

[0255] As described above, when the electrolyte additive according to the present invention is mixed with a film-forming agent, a uniform and stable protective film can be produced on the electrode of a secondary battery including the electrolyte additive according to the present invention by the film-forming agent, after which the formed protective film and the electrode are protected by the electrolyte additive according to the present invention, so that the interface between the electrode and the electrolyte can be stabilized.

[0256] Hereinabove, the present invention has been described in detail with reference to the preferred embodiments, but the present invention is not limited to the above embodiments, and those skilled in the art can make various modifications and changes within the spirit and scope of the present invention.

Claims

1. A method for producing an electrolyte additive, the method comprising: A step of reacting an alkali metal hexafluorophosphate with 2-monofluoromalonic acid to produce a compound represented by the following Chemical Formula 1 or Chemical Formula 2 in a mixed solution; The step of adding an HF scavenger to the mixed solution produced by the reaction; and The step of concentrating and drying the solution obtained from the above to prepare the compound represented by the following Chemical Formula 1 or Chemical Formula 2 in a solid form: [Chemical formula 1] [Chemical formula 2] wherein M is sodium, lithium or potassium, and R is hydrogen, unsubstituted C1 to C5 alkyl, unsubstituted C1 to C5 perfluoroalkyl or CF3, Wherein, the HF scavenger is dichlorodimethylsilane.

2. The method according to claim 1, wherein: Hexafluorophosphates include lithium hexafluorophosphate (LiPF6).

3. The method according to claim 1, further comprising: A step of preparing 2-monofluoromalonic acid by reacting 2-monofluoromalonic acid ester with an acid solution before reacting hexafluorophosphate with 2-monofluoromalonic acid.

4. The method according to claim 3, further comprising: A step of drying the solution after the reaction is completed using a dehydrating agent in the step of preparing 2-monofluoromalonic acid.

5. The method according to claim 4, wherein: Dehydrating agents include R 1 C(OR 2 )3 orthoester, in R 1 C(OR 2 )3, R 1 is hydrogen or C1 to C5 alkyl, R 2 is a C1 to C5 alkyl group, and R 1 and R 2 are independently selected.

6. The method according to claim 1, wherein: The step of reacting hexafluorophosphate with 2-monofluoromalonic acid is performed in a non-aqueous organic solvent.

7. The method according to claim 1, wherein: The step of reacting hexafluorophosphate with 2-monofluoromalonic acid is performed at a reaction temperature of 5°C to 60°C.

8. The method according to claim 1, wherein: The steps of concentrating and drying the solution obtained from the above are performed under reduced pressure.

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