ELECTROLYTE FOR LITHIUM SECONDARY BATTERY AND LITHIUM SECONDARY BATTERY COMPRISING SAME

Abstract

The electrolyte for a lithium secondary battery comprises: a lithium salt; a solvent; and a functional additive, wherein the functional additive comprises naphthalen-1-yl sulfurofluoridate, represented by the following formula 1:

##STR00001##

Claims

1. An electrolyte for a lithium secondary battery, the electrolyte comprising: a lithium salt; a solvent; and a functional additive, wherein the functional additive comprises naphthalen-1-yl sulfurofluoridate, represented by the following formula 1: ##STR00006##

2. The electrolyte of claim 1, wherein the naphthalen-1-yl sulfurofluoridate is added in an amount of 0.5-1.0 wt %, based on a total weight of the electrolyte.

3. The electrolyte of claim 1, wherein the functional additive further comprises lithium difluoro(oxalato)borate, represented by the following formula 2: ##STR00007##

4. The electrolyte of claim 3, wherein the lithium difluoro(oxalato)borate is added in an amount of 0.5-1.0 wt %, based on a total weight of the electrolyte.

5. The electrolyte of claim 1, wherein the functional additive further comprises vinylene carbonate (VC).

6. The electrolyte of claim 5, wherein the vinylene carbonate (VC) is added in an amount of 0.5-3.0 wt %, based on a total weight of the electrolyte.

7. The electrolyte of claim 1, wherein the lithium salt is at least one compound selected from a 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.3CO.sub.2, LiAsF.sub.F, LiSbF.sub.6, LiAlCl.sub.4, CH.sub.3SO.sub.3Li, CF.sub.3SO.sub.3Li, LiN(SO.sub.2C.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, Li(SO.sub.2F).sub.2N (LiFSI), and (CF.sub.3SO.sub.2).sub.2NLi.

8. The electrolyte of claim 1, wherein the solvent is at least one substance selected from a group consisting of: a carbonate-based solvent, an ester-based solvent, an ether-based solvent, and a ketone-based solvent.

9. A lithium secondary battery, comprising an electrolyte comprised of: a lithium salt; a solvent; and a functional additive, wherein the functional additive comprises naphthalen-1-yl sulfurofluoridate, represented by the following formula 1: ##STR00008##

10. The lithium secondary battery of claim 9, further comprising: a positive electrode containing a positive electrode active material composed of Ni, Co, and Mn; a negative electrode containing at least one negative electrode active material selected from a carbon (C)-based material and silicon (Si)-based material; and a separator interposed between the positive electrode and the negative electrode.

11. The lithium secondary battery of claim 10, wherein the positive electrode contains Ni at a content of 80 wt % or more.

Description

DRAWINGS

[0023] In order that the disclosure may be well understood, there will now be described various forms thereof, given by way of example, reference being made to the accompanying drawings, in which:

[0024] FIG. 1 is a view showing an event that occurs in an electrolyte to which the first negative electrode film additive and the positive electrode film additive according to one form of the present disclosure are added together; and

[0025] FIGS. 2 and 3 are graphs showing charge/discharge test results in Examples and Comparative Examples, respectively.

[0026] The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

DETAILED DESCRIPTION

[0027] The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.

[0028] An electrolyte for a lithium secondary battery according to one form of the present disclosure comprises a lithium salt, a solvent, and a functional additive.

[0029] The lithium salt may be at least one compound 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.3CO.sub.2, LiAsF.sub.6, LiSbF.sub.F, LiAlCl.sub.4, CH.sub.3SO.sub.3Li, CF.sub.3SO.sub.3Li, LiN(SO.sub.2C.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, Li(SO.sub.2F).sub.2N (LiFSI), and (CF.sub.3SO.sub.2).sub.2NLi.

[0030] The lithium salt may exist at a total concentration of 0.1-1.2 moles in the electrolyte.

[0031] The solvent may be at least one substance selected from the group consisting of a carbonate-based solvent, an ester-based solvent, an ether-based solvent, and a ketone-based solvent.

[0032] In this regard, examples of the carbonate-based solvent include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), ethylmethyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC). The ester-based solvent may be exemplified by γ-butyrolactone (GBL), n-methyl acetate, n-ethyl acetate, n-propyl acetate, etc. As the ether-based solvent, dibutyl ether may be used, but without limitations thereto.

[0033] In addition, the solvent may further comprise an aromatic hydrocarbonate solvent. Examples of the aromatic carbohydrate solvent include benzene, fluorobenzene, bromobenzene, chlorobenzene, cyclohexyl benzene, isopropyl benzene, n-butylbenzene, octyl benzene, toluene, xylene, and mesitylene, which may be used alone or in combination.

[0034] The functional additive used in the electrolyte according to one form of the present disclosure may employ naphthalen-1-yl sulfurofluoridate (hereinafter referred to as “S3”), represented by the following formula 1, as a first negative electrode film additive

##STR00004##

[0035] In this regard, naphthalen-1-yl sulfurofluoridate (S3), which serves as the first negative electrode film additive, stabilize the negative electrode by forming a film protective of the surface of the negative electrode material, and may be preferably added in an amount of 0.5-1.0 wt %, based on the total weight of the electrolyte.

[0036] When added in an amount less than 0.5 wt %, the first negative electrode film additive contributes only little effect because it cannot sufficiently form a surface protecting film on the surface of the negative electrode active material. More than 1.0 wt % of the first negative electrode film additive causes the excessive formation of a surface protecting layer, increasing a cell resistance, which results in a decreased lifetime.

[0037] Meanwhile, the functional additive may further comprise a positive electrode film additive functioning to form a film on a positive electrode active material. As the positive electrode film additive, lithium difluoro(oxalato)borate (hereinafter referred to as “LiFOB”), represented by the following formula 2, may be used:

##STR00005##

[0038] The positive electrode film additive, which is lithium difluoro(oxalato)borate (LiFOB), plays a role in stabilizing the positive electrode by forming a protecting film on the surface of the positive electrode active material. In addition, the positive electrode film additive inhibits the oxidative degradation of S3, which is the first negative electrode film additive, contributing to the formation of a film superb in thermal stability.

[0039] The positive electrode film additive may be preferably added in an amount of 0.5-1.0 wt %, based on the total weight of the electrolyte.

[0040] When added in an amount less than 0.5 wt %, the positive electrode film additive contributes only little effect because it cannot sufficiently form a surface protecting film on the surface of the positive electrode active material. More than 1.0 wt % of the positive electrode film additive causes the excessive formation of a surface protecting layer, increasing a cell resistance, which results in a decreased lifetime.

[0041] FIG. 1 is a view showing an event that occurs in an electrolyte to which the first negative electrode film additive and the positive electrode film additive according to one form of the present disclosure are added together.

[0042] The first negative electrode film additive S3 may be subjected to oxidative degradation on the positive electrode to produce sulfuric acid, with the resultant continuous degradation of the cell. However, as shown in FIG. 1, when added together with the first negative electrode film additive, the positive electrode film additive is decomposed on the surface of the positive electrode, prior to the first negative electrode film additive S3, to form stable CEI, thereby inhibiting the degradation of the cell.

[0043] Meanwhile, the functional additive may further comprise a second negative electrode film additive functioning to form a film on a negative electrode in addition to the first negative electrode film additive. For example, vinylene carbonate (hereinafter referred to “VC”) may be used as the second negative electrode film additive.

[0044] The second negative electrode film additive may be preferably added in an amount of 0.5-3.0 wt %, based on the weight of the electrolyte. More preferably, the second negative electrode film additive may be added in an amount of 1.5-2.5 wt %.

[0045] Less than 0.5 wt % of the second negative electrode film additive reduces long-term lifetime characteristics of the cell. When exceeding 3.0 wt %, the amount of the second negative electrode film additive excessively forms a surface protecting layer causes increased cell resistance, resulting in a reduction in battery output.

[0046] According to one form thereof, the present disclosure provides a lithium secondary battery comprising the electrolyte described above, a positive electrode, a negative electrode, and a separator.

[0047] The positive electrode includes an NCM-based positive electrode active material composed of Ni, Co, and Mn. Particularly, the positive electrode active material in the positive electrode according to one form is composed only of an NCM-based positive electrode active material containing 80 wt % or more of Ni.

[0048] The negative electrode includes at least one substance selected from carbon (C)- and silicon (Si)-based negative electrode active materials.

[0049] The carbon (C)-based negative electrode active material may be at least one material selected from the group consisting of artificial graphite, natural graphite, graphitized carbon fibers, graphitized mesocarbon microbeads, fullerene, and amorphous carbon.

[0050] The silicon (Si)-based negative electrode active material may include a silicon oxide, a silicon particle, and a silicon alloy particle.

[0051] For the positive electrode and the negative electrode, the corresponding active material is mixed with a conductive material, a binder, and a solvent to prepare an electrode slurry. This electrode slurry is applied directly on a current collector and dried to manufacture the positive electrode or the negative electrode. In this regard, the current collector may be formed of aluminum (Al), but with no limitations thereto. Such electrode manufacturing methods are well known in the art and thus a detailed description is not given thereof.

[0052] The binder acts to well aggregate active material particles each other or strongly attach them to a current collector. Examples of the binder include, but are not limited to, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-bearing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene butadiene rubber, acrylated styrene butadiene rubber, an epoxy resin, and nylon.

[0053] The conductive material is used to confer conductivity on the electrodes. So long as it is of electron conductivity without causing a chemical change in the battery, any conductive material is available. Examples of the conductive material include natural graphite, artificial graphite, carbon black, acetylene black, Ketjenblack, carbon fibers, and metal particles or fibers such as copper, nickel, aluminum, silver, etc. In addition, at least one conductive material such as a polyphenylene derivative may be further used.

[0054] The separator inhibits the formation of a short circuit between the positive electrode and the negative electrode while providing migration channels of lithium ions. This separator may well-known one, for example, a polyolefinic membrane such as polypropylene, polyethylene, polyethylene/polypropylene, polyethylene/polypropylene/polyethylene, polypropylene/polyethylene/polypropylene, etc., or a multiple membrane, microporous film, woven fabric, or non-woven fabric thereof. Alternatively, a porous polyolefin film coated with highly stable resin may be used.

[0055] Hereinafter, the present disclosure is explained with reference to Examples and Comparative Examples.

[0056] <EXPERIMENT 1> Test for cell resistance and high-temperature lifetime characteristic at high temperature (45° C.) according to type and amount of functional additive

[0057] To examine cell resistance and high-temperature lifetime characteristics according to amounts and types of functional additives used in an electrolyte, a measurement was made of initial cell resistance and discharge retention after 100 cycles at a high temperature (45° C.) while types of functional additives were changed as shown in Table 1, below. The results are summarized in Table 1 and depicted in FIGS. 1 and 2.

[0058] In this regard, the cycles were performed under the conditions of cut-off: 2.7-4.35V, C-rate 1C, and temperature: 45° C. For preparing electrolytes, 0.5M LiPF.sub.6+0.5 LiFSI was used as a lithium salt and a mixture of ethylene carbonate (EC):ethylmethyl carbonate (EMC):diethyl carbonate (DEC) at a volume ratio of 25:45:30.

[0059] The positive electrode was made of NCM811 while the negative electrode was a graphite electrode.

TABLE-US-00001 TABLE 1 High temp. Initial cell lifetime Additive resistance (%) No. VC LiPO.sub.2F.sub.2 S3 LiFOB (%) @100 cyc 1 Comparative 1.0 — — — 100 77.4 Example 2 Comparative — — 0.2 — 97 76.2 Example 3 Example — — 0.5 — 98 89.9 4 Example — — 1.0 — 103 81.2 6 Comparative 1.0 0.5 — — 102 89.9 Example 7 Example 1.0 — 0.5 — 102 90.5 8 Example 1.0 — 0.5 0.5 101 91.4

[0060] First, as shown in Table 1 and FIG. 2, an improvement in high-temperature lifetime was observed in Nos. 3 and 4 where the negative electrode film additive according to the present disclosure was added within the amount range suggested herein, compared to No. 1 where the typical functional additive VC was used.

[0061] Meanwhile, No. 2 to which the first negative electrode film additive according to the present disclosure was added in an amount less than the lower limit of the amount range suggested herein was inferior in terms of high-temperature lifetime compared to No. 1.

[0062] As can be understood from the data of Table 1 and FIG. 3, the high-temperature lifetime was further improved in No. 7, where the typical functional additive VC and the first negative electrode film additive according to the present disclosure were added, compared to No. 6, where the typical functional additive VC and the conventional negative electrode film additive LiPO.sub.2F.sub.2 were added in the same amounts.

[0063] Particularly, the highest improvement of high-temperature lifetime was detected in No. 8 where the typical functional additive VC was used together with a combination of the first negative electrode film additive and the positive electrode film additive.

[0064] As described hitherto, according to various forms of the present disclosure, the electrolyte can form CEI on the surface of a negative electrode active material to inhibit the degradation of the cell, thereby improving the lifetime of the lithium secondary battery.

[0065] Furthermore, the electrolyte guarantees lifetime stability at high temperatures and high voltages, contributing to an improvement of commercial value in the battery.

[0066] It will be appreciated by those having ordinary knowledge in the art to which the present disclosure pertains that the present disclosure may be practiced in other specific forms without changing the technical spirit and essential features of the present disclosure. Therefore, it should be understood that the above-described forms are illustrative but not restrictive in all aspects. The scope of the present disclosure is defined by the scope of the attached claims, rather than the detailed description. It should be appreciated that all variations and modifications derived from the scope of the claims and the equivalent concepts thereof are included in the scope of the present disclosure.