KR100736909B1 - Nonaqueous electrolyte for lithium battery and lithium secondary battery comprising the electrolyte - Google Patents

Abstract

Provided is a non-aqueous electrolyte for a lithium secondary battery, which uses a cost-efficient stabilizer for a solid electrolyte interface layer to prevent decomposition of the electrolyte, and ensures excellent charge/discharge characteristics and safety of the battery. The non-aqueous electrolyte comprises: 100 wt% of at least one organic solvent selected from the group consisting of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, ethyl methyl carbonate, dimethoxyethane, diethyoxyethane, 2-methyltetrahydrofuran and dimethyl sulfoxide, in which 0.8-2M of at least one lithium salt selected from the group consisting of lithium perchlorate, lithium hexafluorophosphate, lithium triflate, lithium bistrifluoromethylsulfonyl amide, lithium tetrafluoroborate salts is dissolved; and 0.1-10 wt% of a vinyl silane compound represented by a formula 1 as a stabilizer for a solid electrolyte interface layer and a borate compound represented by a formula 2 as an anion fixing agent. In the formula 1, each of R_1, R_2 and R_3 is H, and R_4 is CH_3. In the formula 2, each of R_1, R_2 and R_3 is (OCH_2CH_2)_nOCH_3, and n is a number of 4-10.

Description

Non-aqueous electrolyte for lithium batteries and lithium secondary battery comprising the same {Nonaqueous electrolyte for lithium battery and lithium secondary battery comprising the electrolyte}

1 relates to charge and discharge characteristics under various conditions of a unit cell to which the nonaqueous electrolyte solution of Comparative Example 1 of the present invention is applied.

2 is related to charge and discharge characteristics under various conditions of a unit cell to which the nonaqueous electrolyte solution of Example 2 of the present invention is applied.

FIG. 3 relates to capacity retention characteristics during continuous charge and discharge under various conditions of the unit cell to which the nonaqueous electrolyte of Example 2 of the present invention is applied.

The present invention can be used to stabilize the passivation film and improve the charge and discharge characteristics as an additive of the non-aqueous electrolyte solution for lithium secondary batteries, and more particularly, a functional non-aqueous electrolyte solution containing a new additive made of a vinyl silane-based material and a borate-based material and applying the same. It relates to a lithium secondary battery.

In recent years, with the rapid advancement of information and communication technology, small secondary batteries used as essential power sources for mobile information technology-related electronic devices such as mobile phones, notebook PCs, and PDAs are rapidly growing. Demand is growing exponentially.

The weight of battery in various portable information and communication equipment accounts for 10-20% for notebook PC and 50% for mobile phone. Secondary battery not only affects the small size and light weight of the device body but also continuously for a long time. Has become an important competitive factor for portable information and communication devices, and the development of small secondary batteries that can meet the demands of such devices will be a key factor in determining the competitiveness of not only the battery industry but also electronic information and communication products.

As the transmission speed of mobile communication systems increases, the existing secondary batteries (nickel-cadmium batteries, nickel-metal hydride batteries) are used to increase energy consumption due to the high functionalization of portable information communication devices and small electronic devices (wireless Internet and wireless data communication services). Since it cannot be satisfied, high energy density, high performance, and high stability of small secondary batteries are required.

To this end, advanced countries such as Japan and the United States have been actively promoting national-led R & D for a long time, and the lithium secondary battery is the world's most popular small secondary battery. Lithium secondary battery system is one of the chemical energy converters that can bring the change of free energy generated by electrochemical oxidation and reduction into electrical energy. It consists of a positive electrode, a negative electrode and a liquid electrolyte (organic solvent + salt). Thin film separators are used to prevent contact.

In order to secure high energy density, high performance, and high safety of lithium secondary batteries, development of high functional electrolyte materials as well as electrode materials is essential. A high functional electrolyte system that can supplement the existing electrolyte function is a non-aqueous electrolyte containing functional additives such as passivation film stabilizer, overcharge inhibitor, flame retardant.

In the case of a lithium ion secondary battery, during initial charging, lithium ions derived from lithium metal oxide, which is a positive electrode, move to a graphite electrode, which is a negative electrode, and are inserted into graphite. At this time, decomposition products such as lithium ions and non-aqueous electrolytes or anions of salts react to form a thin film on the graphite surface. Such a film is referred to as a solid electrolyte interface layer (SEI layer).

The passivation film allows lithium ions to pass but prevents electrons from moving. In addition, the lithium ion is inserted into the graphite anode together with the organic solvent reduction by-product of the large molecular weight electrolyte to prevent the graphite structure from decaying. Therefore, after the passivation film is formed, lithium ions are prevented from further side reactions with other organic solvents or anions of salts, thereby helping to maintain the discharge capacity even during long charge and discharge. That is, the components and morphology of the passivation film formed on the surface of the negative electrode active material particles during initial charging of the battery determine the stability and charge / discharge characteristics of the battery.

If the passive film, which can expect such a positive effect, is not formed stably unlike the original intention, it may lead to further decomposition of the organic solvent. This reduces the number of lithium ions reversibly moving, decreases the charge and discharge capacity and decreases the efficiency. This tendency is more severe when driven at high temperatures. Therefore, in order to implement a high performance secondary battery, a functional material that can decompose first at a lower potential than a mixed organic solvent component to form a stable passivation film is required. If no functional substance is added, lithium ions and electrons are consumed by the reductive decomposition reaction of the organic solvent used in the initial charging of the battery, increasing irreversible capacity, and the formed resistance layer decreases the battery continuously during repeated charging and discharging. Will cause.

The passive film stabilizing material studied to minimize the deterioration of battery performance due to the reduction and decomposition of organic solvents is disclosed in Japanese Patent Application Laid-open No. Hei 7-176323, which discloses a method of adding carbon dioxide (CO ₂ ) to an electrolyte solution. Patent Publication No. 7-320779 discloses a method of suppressing decomposition of an electrolyte by adding a sulfide compound to the electrolyte. In addition, Korean Patent No. 10-0412527 attempted to make a stable passivation film by making an electrolyte solution containing a vinyl ester compound.

In this way, by adding a small amount of organic or inorganic materials, it was tried to form a more stable film on the surface of the negative electrode by reducing decomposition at a lower potential than the organic solvent during the initial charging. However, depending on the nature of the compound added, rather than increasing the irreversible capacity, and may interact with the carbon as the negative electrode to form a decomposed or unstable coating, this tendency is more severe at high temperatures or high rates.

Meanwhile, the related arts related to the present invention include Korean Patent Registration 10-0412527 (a non-aqueous electrolyte and a lithium secondary battery including the same) and Abach et al. (D.Aurbach, K.Gamolsky, B.Marcovsky, Y.Gofer, M. Schmidt, U. Heider, On the use of vinylene caebonate (VC) as an additive to electrolyte solutions for Li-ion batteries, Electrochimica Acta 47, 2002, 1423-1439).

The present invention provides a non-aqueous electrolyte by adding a vinyl silane-based and a borate-based to a mixed organic solvent containing lithium salts to prevent a decrease in battery performance, which is a problem of the prior art. It is not only to decompose earlier to form a stable film to prevent decomposition of the organic solvent, but also to maintain battery performance in continuous charge and discharge under various conditions such as high temperature and high rate.

Hereinafter, the present invention will be described in more detail. The nonaqueous electrolyte solution for lithium batteries of the present invention is composed of an organic solvent, a lithium salt, and an additive.

The organic solvent is one selected from ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, gamma butyrolactone, ethyl methyl carbonate, dimethoxyethane, diethoxy ethane, 2-methyltetrahydrofuran, and dimethyl sulfoxide. Or two or more mixed solvents are used. The mixing ratio of the organic solvent is not particularly limited as long as the object of the present invention is not impaired, and it depends on the mixing ratio at the time of manufacturing a nonaqueous electrolyte solution for ordinary lithium batteries.

In the lithium salt, one or two or more mixed salts selected from lithium perchlorate, lithium hexafluorophosphate, lithium triflate, lithium bistrifluoromethylsulfonylamide, and lithium tetrafluoroborate salts are used. The lithium salt is added at a concentration of 0.8 to 2.0 M. When the concentration of the salt is less than 0.8M, the conductivity of the electrolyte is lowered and the performance of the electrolyte is lowered. When the salt concentration is higher than 2.0M, the mobility of lithium ions due to the increase in viscosity at low temperatures may decrease, resulting in a problem of low temperature performance.

In the nonaqueous electrolyte solution, 0.1 to 10% by weight, more preferably 1 to 5% by weight, of the vinylsilane-based and borate-based additives represented by Formulas 1 and 2 are added to 100% by weight of the organic solvent including lithium salt Manufacture. If the amount of the additive is less than 0.1% by weight, there is a problem that a stable passivation film cannot be formed, and when the amount of the additive exceeds 10% by weight, a problem of deterioration of battery performance may occur.

(화학식 1)

(Formula 1)

In Formula 1, R ₁ , R ₂ , and R ₃ represent one or two or more selected from alkyl groups such as H, CH ₃ , and CH ₂ CH ₃ . R ₄ represents selected from H and CH ₃ .

(화학식 2)

(Formula 2)

In Formula 2, R ₁ , R ₂ , and R ₃ are (OCH ₂ CH ₂ ) _n OCH ₃ and represent one selected from n = 1 to 10.

The lithium secondary battery can be manufactured by a conventional method using the nonaqueous electrolyte solution for lithium batteries of the present invention.

Hereinafter, the contents of the present invention will be described in detail through Examples, Comparative Examples, and Test Examples. However, these are only for illustrating the present invention in detail, and are not intended to limit the present invention.

<Example 1>

1M LiPF ₆ is added to a nonaqueous organic solvent in which ethylene carbonate / dimethyl carbonate (EC / DMC) is mixed in a 1: 1 volume ratio, and triacetoxyvinylsilane represented by Chemical Formula 3 as a vinylsilane-based passivation film stabilizer. ) Was added 3% by weight relative to the electrolyte solution to prepare an electrolyte solution.

(화학식 3)

(Formula 3)

<Example 2>

An electrolyte solution was prepared in the same manner as in Example 1, and 3% by weight of triacetoxy vinylsilane and 2% by weight of diethylene glycol borate shown in Chemical Formula 4 were added to the basic electrolyte solution.

(화학식 4)

(Formula 4)

Comparative Example 1

An electrolyte solution was prepared in the same manner as in Example 1, except that triacetoxy vinyl silane and diethylene glycol borate were not added.

Comparative Example 2

An electrolyte solution was prepared in the same manner as in Example 1, except that 2 wt% of the vinylene carbonate represented by Formula 5 was added without adding triacetoxy vinyl silane and diethylene glycol borate.

(화학식 5)

(Formula 5)

<Test Example 1>

In order to measure initial discharge capacities of the functional nonaqueous electrolytes prepared in Comparative Examples 1 and 2 and Examples 1 and 2, unit cells were manufactured. The battery negative electrode was composed of graphite as the active material and polyvinylidene fluoride as the binder, and the positive electrode was composed of LiCoO ₂ as the active material, polyvinylidene fluoride as the binder and acetylene black as the conductive agent. The prepared unit cell was charged under CC conditions with a current of C / 10 and a charging voltage of 4.2V, and then discharged to 3.0V with a current of C / 10, and the initial discharge capacity was measured. The results are shown in Table 1 below.

Table 1. Initial discharge capacity of nonaqueous electrolyte of Comparative Example and Example

division Comparative Example 1 Comparative Example 2 Example 1 Example 2 Initial discharge capacity (mAh) 8.4 8.7 8.5 8.7

<Test Example 2>

The interfacial resistance of the passive film stabilizer in which the functional nonaqueous electrolytes prepared in Comparative Examples 1 and 2 and Examples 1 and 2 were formed during initial charging was investigated. The change is shown in Table 2 below.

Table 2.

division Comparative Example 1 Comparative Example 2 Example 1 Example 2 Interface resistance (Ω) 7 4.5 15 5

<Test Example 3>

In order to determine the charge and discharge characteristics of the functional nonaqueous electrolyte prepared by Comparative Example 1 and Example 2 under various conditions, the unit cell was configured by the same procedure as in Test Example 1, and the C / 2 current and 4.2V were charged at room temperature. After charging under a CC-CV condition with a voltage, charging and discharging were performed under a C / 2 current to 3.0V, a C / 2 current at the high temperature and the same conditions, and a 1C current at the same temperature and the same conditions. In FIG. 1, the discharge capacities of the first charge / discharge were compared to confirm the degree of maintaining the performance when the various charge and discharge conditions were applied.

<Test Example 4>

In order to determine the charge and discharge characteristics under various conditions of the functional nonaqueous electrolyte prepared by Comparative Example 1 and Example 2, the unit cell was configured in the same procedure as in Test Example 1, and the C / 2 current and 4.2V were charged at room temperature. After charging under a CC-CV condition with a voltage, the battery was discharged to a current of 3.0 C with a C / 2 current, charged with a current of C / 2 at high temperature and the same condition, and a current of 1C at room temperature and the same condition. In FIG. 2, the capacity maintenance characteristics were confirmed during the continuous charge and discharge under these various charge and discharge conditions.

As can be seen in Table 1, Example 2 showing a similar performance to Comparative Example 2, which is known to show the best performance in the existing lithium secondary battery industry was developed. In addition, as can be seen in Table 2, when the diethylene glycol borate is introduced as in Example 2, the large interfacial resistance that is a problem when only triacetoxy vinyl silane is introduced as in Example 1, the vinyl of Comparative Example 2 It fell to a level similar to that of carbonate. In Example 1, although the charge and discharge characteristics were improved under high temperature conditions, the charge and discharge characteristics under high rate conditions were deteriorated due to the problem of high interface resistance. The solution was solved as in Example 2 to show improved charge and discharge characteristics even at high rates as shown in FIG. 1, and nearly 100% retention characteristics were shown even during continuous charge and discharge as shown in FIG. 3.

In the case of the non-aqueous electrolyte added with the vinylsilane-based passivation film stabilizer and the borate-based anion-immobilizing material presented in the present invention, it is decomposed before the non-aqueous electrolyte at the time of initial charging to make a stable passivation film to prevent the decomposition of the non-aqueous electrolyte and suppress side reactions to discharge. To increase the dose. In addition, even when used in an electrolyte solution containing a lithium salt, such as lithium hexafluorophosphate, which is weak in heat, it exhibits excellent charge and discharge characteristics during high temperature charge and discharge, and also improves charge and discharge characteristics at high rates. In addition, it is advantageous in terms of price competitiveness over other materials.

Claims (5)

Ethylene carbonate in which at least one or two or more lithium salts selected from lithium perchlorate, lithium hexafluorophosphate, lithium triflate, lithium bistrifluoromethylsulfonylamide, and lithium tetrafluoroborate salt are dissolved at 0.8 to 2 M, One or two or more organic solvents selected from propylene carbonate, dimethyl carbonate, diethyl carbonate, gamma butyrolactone, ethyl methyl carbonate, dimethoxyethane, diethoxyethane, 2-methyltetrahydrofuran, and dimethyl sulfoxide % Non-aqueous electrolyte solution, characterized in that 0.1 to 10% by weight of the vinyl silane-based material represented by the following formula (1) and the borate-based material represented by the following formula (2) as an anionic immobilization material is added to the passivation film stabilizer

(Formula 1) In Formula 1, R ₁ , R ₂ , R ₃ are H, and R ₄ is CH ₃ .

(Formula 2) In Formula 2, R ₁ , R ₂ , and R ₃ are (OCH ₂ CH ₂ ) _n OCH ₃ and are selected from n = 4 to 10. delete delete delete Lithium secondary battery comprising the nonaqueous electrolyte of claim 1.

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