WO2009088174A2

WO2009088174A2 - Non-aqueous electrolyte solution for lithium secondary battery and lithium secondary battery containing the same

Info

Publication number: WO2009088174A2
Application number: PCT/KR2008/007850
Authority: WO
Inventors: Soo-Jin Kim; Jeong-Ju Cho; Mi-Young Son; Su-Jin Yoon
Original assignee: Lg Chem, Ltd.
Priority date: 2008-01-09
Filing date: 2008-12-31
Publication date: 2009-07-16
Also published as: KR20090076677A; WO2009088174A3

Abstract

A non-aqueous electrolyte solution for a lithium secondary battery includes a lithium salt and an organic solvent. The non-aqueous electrolyte solution includes 0.5 to 5 weight% of vinylethylene carbonate and 0.1 to 5 weight% of 1,3-divinyltetramethyldisiloxane simul¬ taneously, based on the total weight of the non-aqueous electrolyte solution. 1,3-divinyltetramethyldisiloxane may significantly solve side effect of battery swelling without deteriorating effects such as improvement of performance deterioration and life cycle according to the addition of vinylethylene carbonate.

Description

NON-AQUEOUS ELECTROLYTE SOLUTION FOR LITHIUM

SECONDARY BATTERY AND LITHIUM SECONDARY

BATTERY CONTAINING THE SAME

Technical Field

[1] The present invention relates to an electrolyte solution for lithium secondary batteries, which may solve the swelling phenomenon caused by the addition of vinylethylene carbonate, and a lithium secondary battery containing the same. Background Art

[2] Recently, interests on energy storage technologies are more increased. As the energy storage technologies are extended to cellular phones, camcorders and notebook PC, and further to electric vehicles, the demand for high-energy concentration of a battery used as a power source of such an electronic device is increased. A lithium ion secondary battery is one of the most satisfactory batteries, and many studies are now in active progress.

[3] Among secondary batteries currently used, a lithium secondary battery developed in the early 1990' includes an anode made of carbon material capable of intercalating or disintercalating lithium ions, a cathode made of lithium-containing oxide, and a nonaqueous electrolyte solution obtained by dissolving a suitable amount of lithium salt in a mixed organic solvent.

[4] The lithium secondary battery has an average discharge voltage of about 3.6 to 3.7V, which is advantageously higher than those of other batteries such as alkali batteries or nickel-cadmium batteries. To give such a high operation voltage, an electrolyte composition electrochemically stable in a charging/discharging voltage range from 0 to 4.5 V is required. For this purpose, a mixed solvent in which a cyclic carbonate compound such as ethylene carbonate or propylene carbonate and a linear carbonate compound such as dimethyl carbonate, ethylmethyl carbonate or diethyl carbonate are suitably mixed is used as a solvent of electrolyte. A solute of electrolyte commonly uses a lithium salt such as LiPF₆, LiBF₄ and LiClO₄, which acts as a source for supplying lithium ions in a battery and thus enables the lithium battery to operate.

[5] Lithium ions coming out from a cathode active material such as lithium metal oxide during an initial charging process of a lithium secondary battery are moved to an anode active material such as graphite and then intercalated between layers of the anode active material. At this time, due to the strong reactivity of lithium, electrolyte reacts with carbon of the anode active material on the surface of the anode active material such as graphite, thereby generating compounds such as Li₂CO₃, Li₂O and LiOH. These compounds form a kind of SEI (Solid Electrolyte Interface) film on the surface of the anode active material such as graphite.

[6] The SEI film plays a role of ion tunnel, which allows only lithium ions to pass. Due to the effects of ion tunnel, the SEI film prevents an organic solvent molecule moving together with lithium ions in the electrolyte solution and having a great molecular weight from being intercalated into layers of the anode active material and thus breaking down the anode structure. Thus, since the electrolyte solution is not contacted with the anode active material, the electrolyte solution is not decomposed, and also an amount of lithium ions in the electrolyte solution is reversibly kept, thereby ensuring stable charging/discharging.

[7] However, the SEI film is insufficient to play a role of a continuous protective film of an anode, so the life cycle and performance of a battery are deteriorated as the battery repeats charging/discharging. In particular, the SEI film of a lithium secondary battery is thermally unstable. Thus, if a battery is operated or left alone under a high temperature circumstance, the battery may be easily collapsed due to electrochemical energy and thermal energy, which are increased as time goes. In this reason, the battery performance is further deteriorated under a high temperature circumstance.

[8] In order to solve the above problem, there have been proposed non-aqueous electrolyte solutions having various additives. Particularly, as disclosed in Japanese Laid-open Patent Publication No. 1996-45545, vinylethylene carbonate (VEC) is known as being very effective in solving the deterioration of life cycle and performance of a battery when it is added to a non-aqueous electrolyte solution. However, vinylethylene carbonate is easily decomposed in a cathode to generate gas when a battery is kept at a high temperature or repeatedly fully charged and discharged over a normal temperature, thereby giving side effects of causing swelling of the battery. In this reason, the thickness of the battery is increased to cause problems in a set such as cellular phones and notebooks, which is one of most serious claims from customers.

[9] Meanwhile, Korean Laid-open Patent Publication No. 2003-59729, Japanese Laid- open Patent Publication No. 2003-323915, No. 2002-134169 and No. 2003-173816 disclose non-aqueous electrolyte solutions containing 1,3-divinyltetramethyldisiloxane. However, 1,3-divinyltetramethyldisiloxane is just disclosed as an additive to improve life cycle or low-temperature characteristics of a battery, and the above documents do not give any teaching in relation to addition of vinylethylene carbonate or improvement of side effects caused by vinylethylene carbonate. Disclosure of Invention Technical Problem

[10] The present invention is designed to solve the problems of the prior art, and therefore it is an object of the present invention to provide a non- aqueous electrolyte solution for a lithium secondary battery, which may significantly improve side effects of causing swelling of a battery without deteriorating the improvement of deterioration of life cycle and performance of the battery, caused by the addition of vinylethylene carbonate, and a lithium secondary battery containing the same. Technical Solution

[11] In order to accomplish the above objective, the present invention provides a nonaqueous electrolyte solution for a lithium secondary battery, which includes a lithium salt and an organic solvent, wherein the non-aqueous electrolyte solution includes 0.5 to 5 weight% of vinylethylene carbonate and 0.1 to 5 weight% of 1,3-divinyltetramethyldisiloxane simultaneously, based on the total weight of the nonaqueous electrolyte solution.

[12] In the non-aqueous electrolyte solution for a lithium secondary battery according to the present invention, the organic solvent is preferably any one selected from the group consisting of a cyclic carbonate selected from the group consisting of propylene carbonate, ethylene carbonate and vinylene carbonate, or their mixtures, a linear carbonate selected from the group consisting of diethyl carbonate, dimethyl carbonate, methylethyl carbonate and dipropyl carbonate, or their mixtures, dimethyl sulfoxide, acetonitrile, dimethoxyethane, sulforan, gamma-butyrolactone, ethylene sulfite, tetrahydrofuran, ethyl propionate and propyl propionate, or their mixtures. In particular, the organic solvent more preferably includes ethylene carbonate or a mixture of ethylene carbonate and propylene carbonate.

[13] This the non-aqueous electrolyte solution is usefully applied to a common lithium secondary battery, which includes an anode and a cathode. Brief Description of the Drawings

[14] FIG. 1 is a graph showing the changes of capacity and thickness of batteries according to repeated charging/discharging at a normal temperature according to an embodiment and a comparative example;

[15] FIG. 2 is a graph showing the changes of capacity and thickness of batteries according to repeated charging/discharging at 45⁰C according to the embodiment and the comparative example; and

[16] FIG. 3 is a graph showing the change of thickness of batteries according to time when the batteries according to the embodiment and the comparative example are kept under a high temperature circumstance.

Best Mode for Carrying Out the Invention

[17] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to the description, it should be understood that the terms used in the specification and the appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present invention on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation.

[18] A non-aqueous electrolyte solution for a lithium secondary battery according to the present invention includes a lithium salt and an organic solvent. Here, the non-aqueous electrolyte solution includes 0.5 to 5 weight% of vinylethylene carbonate and 0.1 to 5 weight% of 1,3-divinyltetramethyldisiloxane simultaneously, based on the total weight of the non-aqueous electrolyte solution.

[19] As mentioned above, vinylethylene carbonate (VEC) is very effective in solving the deterioration of life cycle and performance of a battery when it is added to a nonaqueous electrolyte solution. However, when a battery is kept at a high temperature or repeatedly fully charged and discharged over a normal temperature, vinylethylene carbonate is easily decomposed in a cathode to generate gas, thereby giving side effects of causing swelling of a battery. To solve the problem caused by the addition of vinylethylene carbonate, the inventors had been screening various materials, and then found that swelling of a battery may be significantly improved without deteriorating the effects obtained by the addition of vinylethylene carbonate, when 1,3-divinyltetramethyldisiloxane and vinylethylene carbonate are added simultaneously. The present invention has been completed under such a background.

[20] In the non-aqueous electrolyte for a lithium secondary battery according to the present invention, if the content of vinylethylene carbonate is less than 0.5 weight%, the improvement of life cycle and performance of a battery, obtained by the addition of vinylethylene carbonate, is insufficient. If the content exceeds 5 weight%, resistance of the battery is greatly increased, so much gas is generated when the battery is kept at a high temperature. Also, if the content of 1,3-divinyltetramethyldisiloxane is less than 0.1 weight%, the improvement of swelling of a battery, obtained by the addition of vinylethylene carbonate, is not expected. If the content exceeds 5 weight%, ion conductivity of the electrolyte solution itself is decreased.

[21] In the non-aqueous electrolyte for a lithium secondary battery according to the present invention, the lithium salt included as an electrolyte may use any one commonly used for electrolyte solutions for lithium secondary batteries. Representatively, the lithium salt may be LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiClO₄, LiN(C₂F₅SOz)₂, LiN(CF₃SO₂),, CF₃SO₃Li, LiC(CF₃SO₂),, LiC₄BO₈ and so on. Besides, other compounds such as lactone, ether, ester, acetonitrile, lactam, and ketone may be added to the non-aqueous electrolyte of the lithium secondary battery without deteriorating the purpose of the present invention. [22] In addition, the organic solvent included in the non-aqueous electrolyte solution of the present invention may use any one commonly used for lithium secondary batteries. Representatively, the organic solvent may use a cyclic carbonate such as propylene carbonate, ethylene carbonate and vinylene carbonate, a linear carbonate such as diethyl carbonate, dimethyl carbonate, methylethyl carbonate and dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, sulforan, gamma-butyrolactone, ethylene sulfite, tetrahydrofuran, ethyl propionate and propyl propionate, in single or in mixture. In particular, ethylene carbonate or a mixture of ethylene carbonate and propylene carbonate may more easily dissociate a lithium salt in an electrolyte due to high dielectric constants, so it contributes to improvement of charging/discharging capacity of a battery. In case propylene carbonate is mixed, a volume ratio of propylene carbonate is preferably 1/4 to 1 with respect to ethylene carbonate. In addition to the above cyclic carbonate, linear carbonate with low viscosity and low dielectric constant such as dimethyl carbonate and diethyl carbonate may be more preferably used in mixture, which allows making an electrolyte solution with high electric conductivity.

[23] The non-aqueous electrolyte solution for a lithium secondary battery according to the present invention is applied to a lithium secondary battery having an anode made of carbon material, metal alloy, lithium-containing oxide, silicon-containing material bondable to lithium or the like, which may intercalate or disintercalate lithium ions, and a cathode made of lithium-containing oxide or the like.

[24] The carbon material capable of intercalating or disintercalating lithium ions may employ any material capable of being used as a carbon material anode of a lithium secondary battery such as low-crystalline carbon and high-crystalline carbon. The low- crystalline carbon is representatively soft carbon or hard carbon, and the high- crystalline carbon is representatively natural graphite, Kish graphite, pyrolytic carbon, mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature sintered carbon such as petroleum or coal tar pitch derived cokes. In addition, alloys containing silicon or oxides such as Li₄Ti₅Oi₂ may be used for an anode. At this time, the anode may have a binding agent, which may use various kinds of binder polymer such as PVDF-co-HFP, polyvinylidenefluoride, poly aery lonitrile, polymethylmethacrylate, and styrene-butadiene rubber (SBR).

[25] Also, a cathode active material made of lithium-containing oxide preferably employs a lithium-containing transition metal oxide, for example any one material selected from the group consisting Of LiCoO₂, LiNiO₂, LiMnO₂, LiMn₂O₄, Li(Ni_aCo_bMn_c)O₂ (0<a<l, 0<b<l, 0<c<l, a+b+c=l), LiNii__yCo_yO₂, LiCo_{l y}Mn_yO₂, LiNii__yMn_y0₂ (0<y<l), Li(Ni₃ Co_bMn_c)O₄ (0<a<2, 0<b<2, 0<c<2, a+b+c=2), LiMn₂__zNi_z0₄, LiMn₂__zCo_zO₄ (0<z<2), LiCoPO₄ and LiFePO₄, or their mixtures. [26] In addition, a separator is commonly interposed between the cathode and the anode, and the separator may use common porous polymer films used as conventional separators, such as porous polymer films made using ethylene homopolymer, propylene homopolymer, ethylene/butene copolymer, ethylene/hexene copolymer and ethylene/methacrylate copolymer, in single or in laminate. In other cases, the separator may use common porous non-woven fabrics such as a non- woven fabric made of glass fiber with a high melt point or polyethylene terephthalate fiber, but not limitedly.

[27] The secondary battery of the present invention has an appearance, not specially limited, but the appearance may be a cylindrical shape using a can, an angled shape, a pouch shape or a coin shape. Mode for the Invention

[28] Hereinafter, the present invention is explained in more detail using embodiments.

However, the following embodiments may be modified in various ways, and the present invention should not be interpreted as being limited thereto. The following embodiments are just given for persons having ordinary skill in the art to understand the present invention in a better way.

[29]

[30] Embodiment 1

[31] LiPF₆ was added to a solvent in which ethylene carbonate (EC), propylene carbonate

(PC) and diethyl carbonate (DEC) were mixed in a weight ratio of 3:2:5 to make a IM LiPF₆ solution, and then 0.2 weight% of vinylethylene carbonate and 0.5 weight% of 1,3-divinyltetramethyldisiloxane were additionally added to the solution, based on the entire weight of the solution, to make a non-aqueous electrolyte solution.

[32]

[33] Embodiment 2

[34] A non-aqueous electrolyte solution was prepared in the same way as the embodiment

1, except that the added contents of vinylethylene carbonate and 1,3-divinyltetramethyldisiloxane were respectively changed into 2 weight% and 1 weight%.

[35]

[36] Comparative Example 1

[37] A non-aqueous electrolyte solution was prepared in the same way as the embodiment

1, except that only 2 weight% of vinylethylene carbonate was added to the prepared solution without 1,3-divinyltetramethyldisiloxane.

[38]

[39] Pouch-type lithium secondary batteries with a thickness of 3.8 mm were made in a common way using the non-aqueous electrolyte solutions prepared in the embodiments and the comparative example, LiCoO₂ as a cathode and artificial graphite as an anode. The made pouch-type batteries to which the electrolyte solutions were injected were impregnated in the electrolyte solutions at a normal temperature for a certain time, and then an activation process was executed. Subsequently, the batteries were degassed and resealed, and then charged/discharged one time at a room temperature. At this time, the batteries were charged to 4.2V under a constant current/constant voltage condition, and then discharged to 3.0V under a constant current condition. For the made batteries, life cycle, performance characteristics and status of components were measured in the following way.

[40]

[41] Life Cycle Characteristics

[42] The batteries prepared in the above ways (two batteries for the comparative example

1 and the embodiment 1, respectively, and one battery for the embodiment 2 were initially charged/discharged, and then charged/discharged at 1.0 C-rate 400 times at a normal temperature in the same voltage region. The changes of capacity and thickness of the batteries according to the repetition number of charging/discharging are shown in FIG. 1. In the graph of FIG. 1, upper lines represent the change of capacity of the batteries, and lower lines represent the change of thickness of the batteries. Both embodiments 1 and 2 exhibited improved sustainable yields of capacity in comparison to the comparative example, and it was found that battery swelling was lowered as the life cycle test was executed.

[43] Also, the batteries were charged/discharged 200 times under 45⁰C circumstance in the above way. The experiment results are shown in FIG. 2. Even in the 45⁰C life cycle evaluation, the embodiments 1 and 2 exhibited excellent characteristics in aspect of both sustainable yield of capacity and the change of shape, in comparison to the comparative example.

[44]

[45] High Temperature Storage Characteristics

[46] The batteries made according to the embodiments and the comparative example (two batteries for each case) were initially charged/discharged, and then charged to 4.2V, respectively. The batteries were put into a temperature control oven and heated from 25⁰C to 9O⁰C during 1 hour, then preserved at 9O⁰C for 4 hours, then cooled to 25⁰C during 1 hour, and then preserved at 25⁰C for a hour. The thickness of the batteries was measured using suitable equipment at 30 minute intervals. The measurement results are shown in FIG. 3. Seeing the graph of FIG. 3, it would be found that both of the embodiments 1 and 2 exhibit a meaningfully decreased swelling in comparison to the comparative example.

[47] In addition, 0.2C discharge capacity and 1C discharge capacity of the batteries were measured before the test, and residual capacity, recovery capacity and recovery rate were measured after the test. Also, a difference between maximum thickness of each battery and a thickness of the battery when the test is initiated is shown in Table 1.

[48] [49] Table 1 [Table 1] [Table ]

[50] [51] Seeing the results in Table 1, it would be understood that the batteries of the embodiments to which a non-aqueous electrolyte solution added by a small amount of 1,3-divinyltetramethyldisiloxane together with vinylethylene carbonate is applied exhibit greatly improvements not only in swelling but also residual capacity and recovery capacity after high-temperature storage, in comparison to batteries to which a non-aqueous electrolyte containing only vinylethylene carbonate. Industrial Applicability

[52] The non-aqueous electrolyte solution for a lithium secondary battery according to the present invention may exhibit sufficient effects in capacity, life cycle and improvement in performance deterioration of a battery by the addition of vinylethylene carbonate, and also greatly solve the problem of battery swelling caused by the addition of vinylethylene carbonate when a battery is stored at a high temperature or repeatedly fully charged/discharged over a normal temperature.

Claims

[1] A non-aqueous electrolyte solution for a lithium secondary battery, which includes a lithium salt and an organic solvent, wherein the non-aqueous electrolyte solution includes 0.5 to 5 weight% of vinylethylene carbonate and 0.1 to 5 weight% of

1,3-divinyltetramethyldisiloxane simultaneously, based on the total weight of the non-aqueous electrolyte solution.

[2] The non-aqueous electrolyte solution for a lithium secondary battery according to claim 1, wherein the organic solvent is any one selected from the group consisting of a cyclic carbonate selected from the group consisting of propylene carbonate and vinylene carbonate, or their mixtures, a linear carbonate selected from the group consisting of diethyl carbonate, dimethyl carbonate, methylethyl carbonate and dipropyl carbonate, or their mixtures, dimethyl sulfoxide, acetonitrile, dimethoxy ethane, sulforan, gamma-butyrolactone, ethylene sulfite, tetrahy- drofuran, ethyl propionate and propyl propionate, or their mixtures.

[3] The non-aqueous electrolyte solution for a lithium secondary battery according to claim 1, wherein the organic solvent is ethylene carbonate or a mixture of ethylene carbonate and propylene carbonate.

[4] The non-aqueous electrolyte solution for a lithium secondary battery according to claim 1, wherein the lithium salt is any one selected from the group consisting of LiPF₆ LiBF₄, LiSbF₆, LiAsF₆, LiClO₄, LiN(C₂F₅SOz)₂, LiN(CF₃SO₂),, CF₃SO₃Li, LiC(CF 3SO₂)₃ and LiC₄BO₈, or their mixtures.

[5] A lithium secondary battery, which includes an anode, a cathode and a nonaqueous electrolyte solution, wherein the non-aqueous electrolyte solution is a non-aqueous electrolyte solution defined in any one of the claims 1 to 4.