ELECTROLYTE SOLUTION FOR LITHIUM SECONDARY BATTERY AND LITHIUM SECONDARY BATTERY INCLUDING SAME

Abstract

An additive for an electrolyte solution improves the electrochemical properties of a lithium secondary battery.

Claims

1. An electrolyte solution for a lithium secondary battery, the electrolyte solution comprising: an electrolyte salt; and an organic solvent; wherein the electrolyte solution further comprises, as additives, vinylene carbonate (VC) represented by Formula 1: ##STR00006## and 4-(allyloxy)phenyl fluoro sulfate represented by Formula 2: ##STR00007## .

2. The electrolyte solution of claim 1, wherein the 4-(allyloxy)phenyl fluoro sulfate comprises 0.1 to 1.0% by weight with respect to the total weight of the electrolyte solution.

3. The electrolyte solution of claim 1, wherein the VC comprises 0.1 to 10% by weight with respect to the total weight of the electrolyte solution.

4. The electrolyte solution of claim 1, wherein the 4-(allyloxy)phenyl fluoro sulfate and the VC are added in a ratio of 1:2.

5. The electrolyte solution of claim 1, wherein the electrolyte salt is any one compound or a mixture of two or more compounds selected from the group consisting of: LiPF.sub.6, LiBF.sub.4, LiClO.sub.4, LiCl, LiBr, LiI, LiB.sub.10Cl.sub.10, LiCF.sub.3SO.sub.3, LiCF.sub.3.0CO.sub.2, Li(CF.sub.3SO.sub.2).sub.3.0C, LiAsF.sub.6, LiSbF.sub.6, LiAlCl.sub.4, LiCH.sub.3SO.sub.3, LiCF.sub.3SO.sub.3, LiN(SO.sub.2.0C.sub.2F.sub.5).sub.2, Li(CF.sub.3SO.sub.2).sub.2N, LiC.sub.4F.sub.9SO.sub.3, LiB(C.sub.6H.sub.5).sub.4, and Li(SO.sub.2F).sub.2N(LiFSI).

6. The electrolyte solution of claim 1, wherein the electrolyte salt is comprised in a concentration range of 0.5 M to 1.0 M.

7. The electrolyte solution of claim 1, wherein the organic solvent is any one or a mixture of two or more solvents selected from the group consisting of: a carbonate-based solvent, an ester-based solvent, an ether-based solvent, and a ketone-based solvent.

8. A lithium secondary battery comprising a cathode, an anode, a separator interposed between the cathode and the anode, and the electrolyte solution of claim 1.

9. The lithium secondary battery of claim 8, wherein the cathode comprises a nickel-cobalt-manganese-based cathode active material.

10. The lithium secondary battery of claim 9, wherein in the cathode active material, nickel, cobalt, and manganese are contained in a ratio of 6:2:2 to 8:1:1.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0022] FIG. 1 is a graph showing the energy levels of HOMO and LUMO of 4-(allyloxy)phenyl fluoro sulfate;

[0023] FIG. 2 is a graph showing the initial cell efficiency for Comparative Examples and Examples;

[0024] FIG. 3 is a graph showing the high-temperature life for Comparative Examples and Examples; and

[0025] FIG. 4 is a graph showing high rate characteristics for Comparative Examples and Examples.

DETAILED DESCRIPTION

[0026] Hereinafter, specific contents for solving the above-described objective and problems will be described in detail with reference to the accompanying drawings. On the other hand, when the detailed description of a known technology in the same field is not helpful in understanding the core content of the disclosure in understanding the present disclosure, the description will be omitted, and the technical spirit of the present disclosure is not limited thereto and may be variously implemented by being changed by those skilled in the art.

[0027] One feature of the present disclosure is that the electrochemical properties of a lithium secondary battery may be increased by using vinylene carbonate (VC) represented by Formula 1 as an additive and 4-(allyloxy)phenyl fluoro sulfate represented by Formula 2 as an additive, simultaneously.

[0028] Formulae 1 and 2 are as follows.

##STR00003##

##STR00004##

[0029] Hereinafter, VC will be represented as additive 1, and 4-(allyloxy)phenyl fluoro sulfate will be represented as additive 2.

[0030] By using additive 1 and additive 2 simultaneously, cathode electrolyte interphase (CEI) is formed on the cathode, and SEI is formed on the anode, thereby improving the lifespan and output characteristics of the battery.

[0031] Specifically, since additive 2 has a low LUMO energy level and a high HOMO energy level compared to other materials included in the electrolyte, it is expected to react first on the cathode and anode surfaces to form CEI and SEI.

[0032] FIG. 1 shows the energy levels corresponding to LUMO and HOMO of additive 2, and MH-161 corresponds to additive 2 of the present disclosure. Referring to FIG. 1, the HOMO energy level of additive 2 is -5.96 eV, and the energy level of LUMO is -1.73 eV, and it may be expected that the reaction will occur as it is easily decomposed at the cathode and the anode than EC and FEC used as solvents, VC used as additives, and LiPO.sub.2F.sub.2.

[0033] In particular, the formation of a film can be expected by inducing radical polymerization through the vinyl group at the end of the additive 2. The polymer formed as described above is a polymer component and may have physical flexibility, thereby suppressing breakage of a film structure due to volume expansion and contraction caused by an anode problem, and a phenomenon in which the film is continuously thickened due to breakage of the film structure and the resulting exposure of the cathode active material.

[0034] In addition, the molecules are dissociated in the electrolyte to have OSO.sup.2- functional groups and are expected to form SEI of Li.sub.2SO.sub.3 and ROSO.sub.2LI components. The sulfone-based components such as Li.sub.2SO.sub.3 and ROSO.sub.2Li are expected to play a role in improving electrochemical properties by forming a film having low resistance and excellent thermal stability.

[0035] In addition, fluorine (F) bonded to the molecular end is expected to form LiF on the surface of the anode, and additive 2 is expected to be more effective in forming LiF because fluorine has a higher deintercalation tendency than other additives, fluoroethylene carbonate (FEC),(The binding energy of the C—F bond inside the FEC is -1.61 eV, and the binding energy of the S—F bond inside the additive 2 is -4.35 eV).

[0036] On the other hand, additive 2 of the present disclosure may be prepared through the following mechanism:

##STR00005##

[0037] ① 4.50 ml, 29.96 mmol of 4-(allyloxy)phenol and 4.88 g, 14.98 mmol of cesium carbonate are dissolved in 45 ml of tetrahydrofuran, and 8.91 g, 44.95 mmol of 1.1′-sulfonyl diimidazole is added.

[0038] ② After the mixture was stirred at room temperature for 12 hours, 4-(allyloxy)phenyl 1H-imidazole-1-sulfonate(MH-159) in the form of a transparent oil was obtained through column chromatography (ethyl acetate/hexanes: 3/7)(93% yield, 7.78 g).

[0039] As a result of H-NMR, it was confirmed that MH-159 could be obtained by obtaining the following result values:

[0040] .sup.1H NMR (300 MHz, CDCl.sub.3) δ 7.71 (t, J = 1.0 Hz, 1H), 7.28 (t, J = 1.5 Hz, 1H), 7.16 (dd, J = 1.6, 0.8 Hz, 1H), 6.86-6.79 (m, 4H), 6.01 (ddt, J = 17.3, 10.5, 5.3 Hz, 1H), 5.39 (dq, J = 17.3, 1.6 Hz, 1H), 5.30 (dq, J = 10.5, 1.4 Hz, 1H), 4.50 (dt, J = 5.3, 1.5 Hz, 2H). .sup.13C{.sup.1H} NMR (101 MHz, CDCl.sub.3) δ 158.4, 142.5, 137.7, 132.6, 131.4, 122.4, 118.5, 118.3, 116.0, 69.3.

[0041] ③ 7.78 g, 27.76 mmol of MH-159 was dissolved in 100 ml of acetonitrile, 6.34 g, 49.97 mmol of silver(I) fluoride was added, and the mixture was stirred at 80° C. for 12 hours, and then 4-(allyloxy)phenyl fluoro sulfate(MH-161) in the form of yellow oil can be obtained through column chromatography (ethyl acetate/hexanes: 3/7) (90% yield, 5.80 g).

[0042] As a result of H-NMR, it was confirmed that MH-161 could be obtained by obtaining the following result values.

[0043] .sup.1H NMR (400 MHz, CDCl.sub.3) δ 7.29 - 7.21 (m, 3H),▫7.01 - 6.91 (m, 2H), 6.04 (ddt, J = 17.3, 10.5, 5.3 Hz, 1H), 5.42 (dq, J = 17.3, 1.6 Hz, 1H), 5.32 (dq, J = 10.5, 1.4 Hz, 1H), 4.55 (dt, J = 5.3, 1.5 Hz, 2H). .sup.19F{.sup.1H} NMR (377 MHz, CDCl.sub.3) δ 36.4.

[0044] Hereinafter, the results of experiments on electrochemical properties by manufacturing a lithium secondary battery using the additive will be described.

[0045] The cathode includes an NCM-based cathode active material made of Ni, Co, and Mn, and in particular, NCM811 was used in this embodiment. As the cathode active material may be used of LiCoO.sub.2, LiMnO.sub.2, LiNiO.sub.2, LiNi.sub.1-xCo.sub.xO.sub.2, LiNi.sub.0.5Mn.sub.0.5O.sub.2, LiMn.sub.2-xM.sub.xO.sub.4 (M is Al, Li or a transition metal), LiFePO, and the like, and all other cathode active materials that can be used for lithium secondary batteries may be used.

[0046] The cathode may further include a conductive material and a binder.

[0047] The conductive material is used to impart conductivity to an electrode. Any electronically conductive material without causing chemical changes in a configured battery can be used as a conductive material. For example, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, metal powder such as copper, nickel, aluminum, silver, metal fiber, and the like can be used as a conductive material, and one type or a mixture of one or more types of conductive materials including polyphenylene derivatives may be used.

[0048] The binder serves to adhere the particles of the active material well to each other or to the current collector, which is to mechanically stabilize the electrode. That is, the active material is stably fixed in the process of repeated intercalation and deintercalation of lithium ions to prevent the loosening of the bond between the active material and the conductive material. The binder may include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer including ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoro ethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, and the like, but is not limited thereto.

[0049] The anode includes any one or more of carbon (C)-based or silicon (Si)-based anode active material, and the carbon-based anode material may include one or more of materials selected from the group consisting of artificial graphite, natural graphite, graphitized carbon fiber, graphitized meso-carbon microbeads, fullerene, and amorphous carbon, and the silicon-based anode active material may include any one of SiO.sub.x and silicon-carbon composite materials. In particular, a graphite anode active material was used in this embodiment.

[0050] Like the cathode, the anode may further include a binder and a conductive material.

[0051] The electrolyte solution is composed of an organic solvent and additives.

[0052] The organic solvent may include any one or two or more solvents selected from the group consisting of a carbonate-based solvent, an ester-based solvent, an ether-based solvent, and a ketone-based solvent.

[0053] At this time, the carbonate-based solvent may include 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), vinylene carbonate (VC), and the like. In addition, γ-butyrolactone (GBL), n-methyl acetate, n-ethyl acetate, n-propyl acetate, and the like may be used as the ester-based solvent, and dibutyl ether may be used as the ether-based solvent but is not limited thereto.

[0054] The solvent may further include an aromatic hydrocarbon-based organic solvent. Specific examples of the aromatic hydrocarbon-based organic solvent may include alone or in combination of benzene, fluorobenzene, bromobenzene, chlorobenzene, cyclohexylbenzene, isopropyl benzene, n-butylbenzene, octyl benzene, toluene, xylene, mesitylene, and the like.

[0055] The separator prevents a short circuit between the cathode and anode and provides a passage for lithium ions to move. Such separators may include known materials such as polyolefin-based polymer membrane such as polypropylene, polyethylene, polyethylene/polypropylene, polyethylene/polypropylene/polyethylene, and polypropylene/polyethylene/polypropylene, or multilayers thereof, microporous films, woven fabrics, and non-woven fabrics. In addition, a film coated with a resin having excellent stability on the porous polyolefin film may be used.

[0056] Preparation and experiment of batteries corresponding to Comparative Examples and Examples

Preparation of Cathode

[0057] For the preparation of the cathode, PVdF was dissolved in NMP to prepare a binder solution.

[0058] A slurry was prepared by mixing the cathode active material, and Ketjen Black used as a conductive material in a binder solution. The slurry was coated on both sides of an aluminum foil and dried.

[0059] After that, a rolling process and a drying process were performed, and the aluminum electrode was ultrasonically welded to prepare a cathode. In the rolling process, the thickness was adjusted to be 120 .Math.m to 150 .Math.m.

[0060] In this case, Li[Ni.sub.1-x-yCo.sub.xMn.sub.y]O.sub.2 (1-x-y≥0.6), a material in which Ni, Co, and Mn were mixed in the ratio of an 8:1:1, was used as the cathode active material.

Preparation of Anode

[0061] A slurry was prepared by mixing the anode active material, and Ketjen Black used as a conductive material in a binder solution. The slurry was coated on both sides of an aluminum foil and dried.

[0062] After that, a rolling process and a drying process were performed, and the aluminum electrode was ultrasonically welded to prepare a cathode. In the rolling process, the thickness was adjusted to be 120 .Math.m to 150 .Math.m.

[0063] At this time, graphite was used as the anode active material.

Preparation of Electrolyte Solution

[0064] A mixture of ethylene carbonate (EC), ethylmethyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 25:45:30 was used as an organic solvent, and 0.5 M LiPF.sub.6 and 0.5 M LiFSI were dissolved in the solvent as lithium salts, and the electrolyte was injected. In addition, according to each Example, different ratios of additive 2 were added to the organic solvent.

Preparation of Coin Cell

[0065] After interposing a separator between the cathode and the anode, and then wound to prepare a jelly roll. A coin cell was prepared using the prepared jelly roll and electrolyte.

Comparative Example 1

[0066] Only additive 1 (1.0% by weight) was used as an additive in the electrolyte, and it is a lithium secondary battery that does not include additive 2.

Comparative Example 2

[0067] It is a lithium secondary battery using an electrolyte solution further including additive 1 (1.0% by weight) and LiPO.sub.2F.sub.2 (0.5% by weight) as an additive.

Example 1

[0068] It is a lithium secondary battery using an electrolyte solution including additive 1 (1.0% by weight) and additive 2 (0.1% by weight).

Example 2

[0069] It is a lithium secondary battery using an electrolyte solution further including additive 1 (1.0% by weight) and additive 2 (0.5% by weight).

Example 3

[0070] It is a lithium secondary battery using an electrolyte solution including additive 1 (1.0% by weight) and additive 2 (0.1% by weight).

[0071] For Comparative Examples 1 to 2 and Examples 1 to 3, the results of measuring the cell initial charging and discharging efficiency, the lifespan characteristics after 100 cycles of charging and discharging at a high temperature (45° C.), and the rate-specific characteristics at a high rate (2 C-rate) are shown in Tables 1, 2 and 3.

TABLE-US-00001 Additives (% by weight) Cell initial efficiency (%) Additive 1 LiPO.sub.2F.sub.2 Additive 2 Comparative Example 1 1.0. - - 96.4% Comparative Example 2 1.0. 0.5. - 96.5% Example 1 1.0. - 0.1. 92.5% Example 2 1.0. - 0.5. 97.1% Example 3 1.0. - 1.0. 92.5%

[0072] Table 1 shows the cell initial efficiencies for Comparative Examples and Examples, and the cell initial efficiency refers to a value obtained by dividing a discharge capacity by a charge capacity after charging once after the manufacturing of a lithium secondary battery is completed, discharging is performed, and then discharging is performed. The cut-off voltage was set to 2.5 V to 4.2 V, and the C-rate was tested at 1C at 45° C.

[0073] As a result of the experiment, in the case of Example 2 in which 0.1% by weight of additive 2 was added, it was confirmed that the initial cell efficiency was the best. In the case of Examples 1 and 3, experimental results were found to be inferior to those of the comparative example.

[0074] A graph for this is shown in FIG. 2.

TABLE-US-00002 Additives (% by weight) High-temperature lifespan @100cycle Additive 1 LiPO.sub.2F.sub.2 Additive 2 Comparative Example 1 1.0. - - 90.8% Comparative Example 2 1.0. 0.5. - 90.9% Example 1 1.0. - 0.1. 92.0% Example 2 1.0. - 0.5. 92.4% Example 3 1.0. - 1.0. 90.9%

[0075] Table 2 shows a high-temperature life for Comparative Examples and Examples and shows how much charging/discharging capacity may be maintained compared to an initial charging/discharging capacity after 100 cycles of charging and discharging are repeated. The cut-off voltage was set to 2.5 V to 4.2 V, and the C-rate was tested at 1 C at 45° C.

[0076] As a result of the experiment, in the case of Example 2 in which 0.1% by weight of additive 2 was added, it was confirmed that the initial cell efficiency was the best. Example 1 showed a high-temperature lifespan similar to that of Comparative Example 2, and Example 3 showed a high-temperature lifespan lower than that of Comparative Examples.

[0077] A graph for this is shown in FIG. 3.

TABLE-US-00003 Additives (% by weight) Rate-specific characteristics @2C-rate Additive 1 LiPO.sub.2F.sub.2 Additive 2 Comparative Example 1 1.0. - - 85.7% Comparative Example 2 1.0. 0.5. - 86.1% Example 1 1.0. - 0.1. 86.3% Example 2 1.0. - 0.5. 87.1% Example 3 1.0. - 1.0. 87.0%

[0078] Table 3 shows the rate-specific characteristics for Comparative Examples and Examples and shows how much charge/discharge capacity can be maintained compared to the existing 1C-rate by increasing the rate by 2 times compared to other experiments. Likewise, the cut-off voltage was set to 2.5 V to 4.2 V, and the experiment was performed at 45° C.

[0079] As a result of the experiment, in the case of Example 2 in which 0.1% by weight of additive 2 was added, it was confirmed that the initial cell efficiency was the best.

[0080] A graph for this is shown in FIG. 4.

[0081] Through the above experiment, when a lithium secondary battery is manufactured using an electrolyte solution using both additive 1 and additive 2, a lithium secondary battery with improved electrochemical properties can be obtained. This is presumed to be due to the fact that additive 2 strongly forms CEI and SEI at the cathode and anode.

[0082] From the experimental results, it was confirmed that additive 1 and additive 2 showed the highest electrochemical properties when added in a ratio of 2:1.

[0083] Although shown and described with respect to specific embodiments of the present disclosure, it is within the art that the present disclosure can be variously improved and changed without departing from the spirit of the present disclosure provided by the following claims. It will be obvious to those of ordinary skilled in the art.