Electrolytic solution for lithium secondary batteries and lithium secondary batteries containing the same

Abstract

Electrolytic solution for lithium secondary batteries, the electrolytic solution comprising: lithium salt; a solvent; and a functional additive comprising a high voltage additive, wherein the high voltage additive comprises 1-fluoroethyl methyl carbonate (FEMC) represented by [Formula 1], wherein the high-voltage additive is 1 to 3 wt.%, based on the weight of the electrolytic solution, wherein the functional additive further comprises a negative electrode foil additive and wherein the negative electrode foil additive comprises vinylene carbonate (VC), wherein the negative electrode foil additive is 0.5 to 3.0 wt.% based on the weight of the electrolytic solution, and wherein the high-voltage additive and the negative electrode foil additive total 1.5 to 6 wt.%, based on the weight of the electrolytic solution.

Description

TECHNICAL FIELD

[0001] The present invention relates to an electrolytic solution for lithium secondary batteries and lithium secondary batteries comprising the same. BACKGROUND

[0002] A lithium secondary battery is an energy storage device comprising: a positive electrode configured to supply lithium during charging (e.g., recharging), a negative electrode configured to accept lithium during charging (e.g., recharging), an electrolyte serving as a lithium-ion transfer medium, and a separator configured to separate the positive and negative electrodes. When lithium ions are deposited and released at the positive and negative electrodes, the lithium secondary battery generates and stores electrical energy through a change in chemical potential.

[0003] The lithium secondary battery is mainly used in portable electronic devices. However, in recent years, the lithium secondary battery has also been used as an energy storage medium of an electric vehicle (EV) and a hybrid electric vehicle (HEV) due to the commercialization of the electric vehicle and the hybrid electric vehicle.

[0004] Research has been conducted to increase the energy density of the lithium secondary battery to extend the driving distance of the electric vehicle (e.g., to increase the cruising range of the electric vehicle). The energy density of the lithium secondary battery can be increased by increasing the capacity of the positive electrode.

[0005] The capacity of the positive electrode can be increased by using a Ni enrichment method (e.g., a Ni enrichment method), which is a method in which the content of Ni in a Ni-Co-Mn oxide constituting a positive electrode active material is increased, or by increasing the charging voltage of the positive electrode.

[0006] However, the nickel-rich Ni-Co-Mn oxide has an unstable crystal structure and high interfacial reactivity, which accelerates degradation during cycling and makes it difficult to ensure the long-term performance of the lithium secondary battery.

[0007] The contents disclosed in this section are intended only to enhance the understanding of the general background of the invention and should not be construed as an acknowledgment or any form of suggestion that these contents constitute the related prior art already known to those skilled in the art.

[0008] US 2018 / 0 233 778 A1, DE 10 2019 200 320 A1, WO 2019 / 203 622 A1 and US 2018 / 0 287 118 A1 also describe electrolytic solutions for lithium secondary batteries and lithium secondary batteries comprising them. SHORT DESCRIPTION

[0009] The present invention has for its object to provide an electrolytic solution for lithium secondary batteries capable of improving the cycle life and output characteristics (e.g., output power characteristics) of lithium secondary batteries, and a lithium secondary battery containing the same.

[0010] To achieve the object, the invention provides an electrolytic solution for a lithium secondary battery according to claim 1, a lithium secondary battery according to claim 4, and a lithium secondary battery according to claim 7. Preferred embodiments are specified in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The above and other objects, features (e.g., characteristics) and other advantages of the present invention will be better understood from the following detailed description, taken in conjunction with the accompanying drawings, in which: Fig. 1 to 3 are diagrams showing the results of charging (e.g., charging) and discharging of examples and comparative examples; Fig.4 is an illustration (e.g., a photographic illustration) showing the surfaces of the positive electrodes after charging (e.g., charging) and discharging of examples and comparative examples; and Fig. 5 is a simple diagram illustrating a lithium secondary battery according to an embodiment of the present invention. DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0012] Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0013] However, the present invention is not limited to the embodiments disclosed below and may be implemented (e.g., implemented) in various different ways, and the embodiments described herein serve (only) to supplement the disclosure of the present invention and to fully convey the scope of the invention to those skilled in the art.

[0014] An electrolytic solution for lithium secondary batteries according to an embodiment of the present invention is a material that provides an electrolyte containing a lithium salt and applied to a lithium secondary battery, a solvent, and a functional additive.

[0015] The lithium salt may be any one or a mixture of two or more selected from the group consisting of LiPF6, LiBF4, LiClO4, LiCl, LiBr, LiI, LiB 10 Cl 10, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, LiN(SO2C2F5)2, Li(CF3SO2)2N, LiC4F9SO3, LiB(C6H5)4, Li(SO2F)2N (LiFSI), and (CF3SO2)2NLi.

[0016] The lithium salt may be present in the electrolytic solution such that the total amount of lithium salt has a concentration of 0.1 to 1.2 mol.

[0017] As the solvent, any one or a mixture of two or more selected from the group consisting of a carbonate-based solvent, an ester-based solvent, an ether-based solvent, and a ketone-based solvent can be used.

[0018] Dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), fluoroethylene carbonate (FEC), or vinylene carbonate (VC) can be used as the carbonate-based solvent. γ-Butyrolactone (GBL), n-methyl acetate, n-ethyl acetate, or n-propyl acetate can be used as the ester-based solvent. Dibutyl ether can be used as the ether-based solvent. However, the present invention is not limited thereto.

[0019] In addition, the solvent may further contain an aromatic hydrocarbon-based organic solvent. Specific examples of the aromatic hydrocarbon-based organic solvent may include benzene, fluorobenzene, bromobenzene, chlorobenzene, cyclohexylbenzene, isopropylbenzene, n-butylbenzene, octylbenzene, toluene, xylene, and mesitylene, which can be used either alone or as a mixture of two or more.

[0020] The functional additive used to add to the electrolytic solution comprises the high voltage additive 1-fluoroethyl methyl carbonate (FEMC), represented by [Formula 1].

[0021] The FEMC serves to improve the oxidation stability of the electrolytic solution and to stabilize the interface between the electrolytic solution and a positive electrode, and is added in an amount of 1 to 3 wt% based on the weight of the electrolytic solution.

[0022] When the content of the high-voltage additive is less than 1 wt%, it is difficult to sufficiently form a surface protective layer, resulting in insufficient performance. When the content of the high-voltage additive is greater than 3 wt%, the surface protective layer is excessively formed, increasing cell resistance and thus reducing battery output (e.g., battery output power).

[0023] A negative electrode foil additive, which serves to form a foil on a negative electrode, is added as a functional additive, wherein the negative electrode foil additive comprises vinylene carbonate (VC).

[0024] The negative electrode foil additive is added to be 0.5 to 3.0 wt%, preferably 1.5 to 2.5 wt%, based on the weight of the electrolytic solution.

[0025] When the negative electrode foil additive content is less than 0.5 wt%, the long-term life characteristics of the cell deteriorate. When the negative electrode foil additive content is greater than 3.0 wt%, the surface protective layer is excessively formed, increasing the cell resistance and consequently reducing the battery output (e.g., battery output power).

[0026] As in Fig.5, a lithium secondary battery 100 according to an embodiment of the present invention includes a battery case 102, a positive electrode 104 having a portion within the battery case 102, a negative electrode 106 having a portion within the battery case 102, a separator 108 disposed between the positive electrode 104 and the negative electrode 106, and the electrolytic solution described herein within the battery case 102.

[0027] The positive electrode 104 contains an NCM-based positive electrode active material consisting of Ni, Co, and Mn. Specifically, in this embodiment, the positive electrode active material contained in the positive electrode 104 may exclusively contain an NCM-based positive electrode active material containing 60 wt% or more of Ni.

[0028] The negative electrode 106 contains one or more negative electrode active materials selected from among negative electrode active materials based on carbon (C) and based on silicon (Si).

[0029] As negative electrode active materials based on carbon (C), at least one selected from the group consisting of artificial graphite, natural graphite, graphitized carbon fibers, graphitized mesocarbon microspheres, fullerene, and amorphous carbon can be used.

[0030] The negative electrode active materials based on (Si) contain silicon oxide, silicon particles and silicon alloy particles.

[0031] Meanwhile, each of the positive electrode 104 and the negative electrode 106 is manufactured by mixing a conductive agent, a binder, and a solvent with the associated active material to prepare an electrode slurry, and directly applying the electrode slurry to a current collector and drying it. Aluminum (Al) can be used as the current collector. However, embodiments of the present invention are not limited thereto. A method for manufacturing electrodes is well known in the technical field to which the present invention relates, and therefore, a detailed explanation will be omitted in this specification.

[0032] The binder serves to appropriately bond the active material particles to each other or to appropriately bond the active material particles to the current collector, and examples of the binder may include, but are not limited to, polyvinyl alcohol, carboxymethylcellulose, hydroxypropylcellulose, diacetylcellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer including an ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, an epoxy resin, and nylon.

[0033] In addition, the conductivity agent is used to impart conductivity to an electrode. Any conductivity agent can be used as long as the conductivity agent is formed from an electrically conductive material and does not cause chemical changes in a battery. For example, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or metal powder or metal fibers such as copper, nickel, aluminum, or silver can be used as the conductivity agent. In addition, conductive materials such as polyphenylene derivatives can be used, either alone or as a mixture of two or more.

[0034] The separator 108 prevents a short circuit between the positive electrode 104 and the negative electrode 106 and provides a movement path for lithium ions. A polyolefin-based polymer film such as polypropylene, polyethylene, polyethylene / polypropylene, polyethylene / polypropylene / polyethylene, or polypropylene / polyethylene / polypropylene, a multilayer film, a microporous film, a woven fabric, or a nonwoven fabric can be used as the separator. Furthermore, a film obtained by coating a porous polyolefin film with a resin having high stability can be used.

[0035] In the following, the present invention will be described by means of examples and comparative examples. <Versuch 1> Room temperature (25 °C) property testing using voltage based on the type of functional additive (half-cell)

[0036] To investigate the voltage-dependent properties based on the type of functional additive added to the electrolytic solution, the initial capacities and capacity retentions were measured at room temperature (25 °C) while changing the type of functional additive and the voltage, as shown in Table 1 below. The results are shown in Table 1 and Fig. 1 and Fig. 2 shown.

[0037] To prepare the electrolytic solution, 0.5M LiPF6 and 0.5M LiFSI were used as lithium salt and a mixture of ethylene carbonate (EC) : ethyl methyl carbonate (EMC) : diethyl carbonate (DEC) in the ratio 25:45:30 as solvent.

[0038] NCM622 was used as the positive electrode, and Li metal was used as the negative electrode. [Table 1] Classification Additive Tension Initial capacity @1C 1st cycle (mAh / g) Capacity Retention @1C 50th Cycle (%) VC FEMC Nr. 1 Comparison example 2 - 4,2 156 99,2 Nr. 2 Comparison example 2 - 4,3 175 98,9 Nr. 3 Comparison example 2 - 4,4 191 98 Nr. 4 Comparison example 2 - 4,5 207 97,7 Nr. 5 Comparison example 2 - 4,6 200 96,5 Nr. 6 Example - 2 4,2 162 101 Nr. 7 Example - 2 4,3 194 99,5 Nr. 8 Example - 2 4,4 202 97,9 Nr. 9 Example - 2 4,5 212 97,1 Nr. 10 Example - 2 4,6 215 93,2

[0039] From Table 1 and the Fig. 1 and Fig.2, it can be seen that under the same stress condition, in the case where FEMC was used as a functional additive according to the examples, the initial capacitances increased more than in the case where only VC, which is a conventional general functional additive, was used.

[0040] Furthermore, it can be seen that in the case where the same functional additive was used, the cell capacities increased with increasing voltage, initially showing high capacities and (furthermore) maintaining high capacity retention at voltages between 4.2 V and 4.5 V. <Versuch 2> Test on charging and discharging properties at high temperature (45 °C) based on the type and content of the functional additive (half-cell)

[0041] To investigate the charging and discharging characteristics based on the type and content of the functional additive added to the electrolytic solution, initial capacities and capacity retentions were measured at high temperature (45 °C) while changing the type and content of the functional additive, as shown in Table 2 below. The results are shown in Table 2 and Fig. 3 shown. [Table 2] Classification Additive Initial capacity @1C 1st cycle (mAh / g) Capacity Retention @1C 50th Cycle (%) VC FEMC Nr. 11 Comparison example 2 - 205 84,5 Nr. 12 Example 2 1 208 88,7 Nr. 13 Example 2 2 210 88,9 Nr. 14 Example 2 3 212 86,2

[0042] From Table 2 and Fig. 3, it can be seen that in the case according to the examples in which VC, which is a conventional general functional additive, was used and FEMC was used, while the content of the functional additive was changed, the initial capacities increased with the increase of the content of FEMC.

[0043] Furthermore, it can be seen that in the case according to the examples in which FEMC was used as a functional additive, higher capacity retentions were maintained than in the case in which only VC, which is a conventional general functional additive, was used. <Versuch 3> Experiment to investigate the surface of the positive electrode before and after charging and discharging, according to the type of functional additive.

[0044] In the case where the electrolytic solutions according to No. 11 and No. 13 were used, the surfaces were observed before and after the charging and discharging tests at high temperature (45 °C) and the results are shown in Fig. 4 shown.

[0045] Out of Fig. 4, it is clear that in the case of No. 11, in which only VC, which is a conventional general functional additive, was used as the functional additive, cracks were formed in the positive electrode.

[0046] However, it is found that in the case of No. 13, in which FEMC, which is a functional additive according to the present invention, was used as the functional additive, no cracks were formed in the positive electrode.

[0047] As apparent from the above description, according to embodiments of the present invention, an electrolytic solution containing a high-voltage additive is used, thereby improving the long-term life characteristics of a lithium secondary battery.

[0048] In addition, when using the electrolytic solution containing the high-voltage additive, the cell resistance of the lithium secondary battery is reduced, thereby improving the output characteristics (e.g., output power characteristics) of the lithium secondary battery.

[0049] Although the preferred embodiments of the present invention have been described above with reference to the accompanying drawings, it will be understood by those skilled in the art that the present invention can be implemented in various other embodiments without changing the technical concept or characteristics (e.g., features) thereof.

Claims

[1] Electrolytic solution for lithium secondary batteries, the electrolytic solution comprising: lithium salt; a solvent; and a functional additive comprising a high voltage additive, wherein the high voltage additive comprises 1-fluoroethyl methyl carbonate (FEMC) represented by [Formula 1], wherein the high-voltage additive is 1 to 3 wt.%, based on the weight of the electrolytic solution, wherein the functional additive further comprises a negative electrode foil additive and wherein the negative electrode foil additive comprises vinylene carbonate (VC), wherein the negative electrode foil additive is 0.5 to 3.0 wt.% based on the weight of the electrolytic solution, and wherein the high-voltage additive and the negative electrode foil additive total 1.5 to 6 wt.%, based on the weight of the electrolytic solution. [2] The electrolytic solution according to claim 1, wherein the lithium salt is any one or a mixture of two or more selected from the group consisting of LiPF6, LiBF4, LiClO4, LiCl, LiBr, LiI, LiB 10 Cl 10 , LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, LiN(SO2C2F5)2, Li(CF3SO2)2N, LiC4F9SO3, LiB(C6H5)4, Li(SO2F)2N (LiFSI), and (CF3SO2)2NLi. [3] The electrolytic solution according to claim 1 or 2, wherein the solvent is any one or a mixture of two or more selected from the group consisting of a carbonate-based solvent, an ester-based solvent, an ether-based solvent and a ketone-based solvent. [4] A lithium secondary battery (100) comprising an electrolytic solution, wherein the electrolytic solution comprises lithium salt, a solvent and a functional additive, wherein the functional additive comprises a high-voltage additive, and wherein the high-voltage additive comprises 1-fluoroethyl methyl carbonate (FEMC) represented by [Formula 1]. wherein the high-voltage additive is 1 to 3 wt.%, based on the weight of the electrolytic solution, wherein the functional additive further comprises a negative electrode foil additive and wherein the negative electrode foil additive comprises vinylene carbonate (VC), wherein the negative electrode foil additive is 0.5 to 3.0 wt.% based on the weight of the electrolytic solution, and wherein the high-voltage additive and the negative electrode foil additive total 1.5 to 6 wt.%, based on the weight of the electrolytic solution. [5] Lithium secondary battery (100) according to claim 4, further comprising: a positive electrode (104) comprising a positive electrode active material consisting of Ni, Co and Mn; a negative electrode (106) comprising one or more negative electrode active materials selected from negative electrode active materials based on carbon (C) or based on silicon (Si); and a separator (108) arranged between the positive electrode and the negative electrode. [6] The lithium secondary battery (100) according to claim 5, wherein the content of Ni in the positive electrode is 60 wt% or more. [7] Lithium secondary battery (100), comprising: a battery housing (102); a positive electrode (104) having a portion within the battery case (102); a negative electrode (106) having a portion within the battery case (102); a separator (108) disposed between the positive electrode and the negative electrode; and an electrolytic solution within the battery case, the electrolytic solution comprising lithium salt, a solvent and a functional additive, the functional additive comprising a high voltage additive represented by [Formula 1]. wherein the high-voltage additive is 1 to 3 wt.%, based on the weight of the electrolytic solution, wherein the functional additive further comprises a negative electrode foil additive and wherein the negative electrode foil additive comprises vinylene carbonate (VC), wherein the negative electrode foil additive is 0.5 to 3.0 wt.% based on the weight of the electrolytic solution, and wherein the high-voltage additive and the negative electrode foil additive total 1.5 to 6 wt.%, based on the weight of the electrolytic solution. [8] The lithium secondary battery (100) according to claim 7, wherein the positive electrode (104) contains an NCM-based positive electrode active material comprising Ni, Co and Mn. [9] The lithium secondary battery (100) according to claim 8, wherein the NCM-based positive electrode active material contains 60 wt% or more of Ni. [10] The lithium secondary battery (100) according to any one of claims 7 to 9, wherein the negative electrode (106) comprises one or more negative electrode active materials, wherein the negative electrode active materials are carbon (C)-based negative electrode active materials or silicon (Si)-based negative electrode active materials. [11] The lithium secondary battery (100) according to claim 10, wherein the carbon (C)-based negative electrode active materials contain at least one of artificial graphite, natural graphite, graphitized carbon fibers, graphitized meso-carbon microspheres, fullerene, or amorphous carbon, and wherein the silicon (Si)-based negative electrode active materials contain a silicon oxide, silicon particles, or silicon alloy particles. [12] The lithium secondary battery (100) according to any one of claims 7 to 11, wherein the separator (108) comprises a polyolefin-based polymer film, a multilayer film, a microporous film, a woven fabric, or a non-woven fabric. [13] The lithium secondary battery (100) according to any one of claims 7 to 12, wherein the high-voltage additive comprises 1-fluoroethyl methyl carbonate (FEMC). [14] The lithium secondary battery (100) according to any one of claims 7 to 13, wherein the lithium salt is any one or a mixture of two or more selected from the group consisting of LiPF6, LiBF4, LiClO4, LiCl, LiBr, LiI, LiB 10 Cl 10 , LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, LiN(SO2C2F5)2, Li(CF3SO2)2N, LiC4F9SO3, LiB(C6H5)4, Li(SO2F)2N (LiFSI), and (CF3SO2)2NLi. [15] The lithium secondary battery (100) according to any one of claims 7 to 14, wherein the solvent is any one or a mixture of two or more selected from the group consisting of a carbonate-based solvent, an ester-based solvent, an ether-based solvent, and a ketone-based solvent.

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