KR100751206B1 - Additives for lithium secondarty battery with charge-cutoff voltages over 4.35 - Google Patents

Abstract

The present invention is an anode (C); Cathode A; Separator; And in the lithium secondary battery comprising an electrolyte, the weight ratio (A / C) per unit area of the negative electrode (A) to the positive electrode (C) is in the range of 0.45 to 0.70, the electrolyte is composed of 2-toluene fluoride and 3-fluoro toluene Provided is a lithium secondary battery comprising at least one toluene fluoride compound selected from the group.

The high voltage lithium secondary battery of 4.35V or more according to the present invention can improve safety and high temperature storage characteristics of a high voltage battery without deteriorating cycle characteristics by using a toluene fluoride compound as an additive.

High voltage, end-of-charge voltage, toluene fluoride, high temperature storage, safety, lithium secondary battery

Description

ADDITIVES FOR LITHIUM SECONDARTY BATTERY WITH CHARGE-CUTOFF VOLTAGES OVER 4.35}

1 is a 4.35V lithium secondary battery of Comparative Example 1 without using an electrolyte additive, 4.35V lithium secondary battery of Comparative Example 2 to which cyclohexylbenzene (CHB) is added and 4-fluoro toluene (para-FT) is added It is a graph which showed the high temperature (45 degreeC) cycling characteristics of the lithium secondary battery of the comparative example 3, respectively.

FIG. 2 is a graph showing the high temperature (45 ° C) cycle characteristics of the 4.35V lithium secondary battery of Example 1 using 3-toluene fluoride compound (3-FT) as an electrolyte additive.

3 is a result diagram after performing a hot box experiment on the lithium secondary battery of Comparative Example 2 to which CHB was added.

4 is a diagram illustrating a result of performing a hot box experiment on a lithium secondary battery of Comparative Example 3, to which 4- toluene fluoride (4-FT) was added.

5 is a high-temperature exposure test result of the 4.35V lithium secondary battery of Example 1 using 3-fluorotoluene (3-FT) as an electrolyte additive.

FIG. 6 shows the lithium secondary battery of Comparative Example 2 to which CHB was added, the lithium secondary battery of Comparative Example 3 to which 4-toluene fluoride (Para-) was added, and the lithium secondary battery of Example 1 to which 3-toluene was added, respectively This is the result after the experiment with high temperature storage (30 cycles: 80 ° C 3 hours + 25 ° C 7 hours)

7 is a high temperature short-term storage of the lithium secondary battery of Example 1 using 3-toluene fluoride as an electrolyte additive, the lithium secondary battery of Comparative Example 1 not using the electrolyte additive and the lithium secondary battery of Comparative Example 2 to which CHB was added ( 90 degreeC 4 hours) is a result of an experiment.

The present invention relates to a high voltage lithium secondary battery of 4.35V or more with improved safety and high temperature storage without deteriorating cycle characteristics.

In recent years, with the trend of miniaturization and weight reduction of electronic devices, miniaturization and weight reduction of batteries acting as power sources are also required. BACKGROUND ART Lithium-based secondary batteries have been put to practical use as small, light weight, high capacity rechargeable batteries, and are used in portable electronic and communication devices such as small video cameras, mobile phones, and notebook computers.

The lithium secondary battery is composed of a positive electrode, a negative electrode, and an electrolyte, and transfers energy while reciprocating both electrodes such that lithium ions from the positive electrode active material are inserted into a negative electrode active material, such as carbon particles, and are detached again when discharged. Since it is possible to charge and discharge.

In order to manufacture a conventional 4.2V class lithium secondary battery with a high capacity, high output and high voltage battery of 4.35V or more, a process of increasing the theoretical usable capacity of the cathode active material in the battery is required. In order to increase the available capacity of the positive electrode active material, the positive electrode active material may be doped using a transition metal or a non-transition metal such as aluminum or magnesium, or the charging end voltage of the battery may be increased. As the end-of-charge voltage of a lithium secondary battery increases to 4.35V or more, the usable capacity in the battery increases by 15% or more, but as the reactivity between the positive electrode and the electrolyte increases, degradation of the surface of the positive electrode and oxidation of the electrolyte occur. Therefore, there was a problem that the high temperature cycle characteristics, safety and high temperature storage characteristics of the battery are deteriorated.

In the conventional 4.2V class lithium secondary battery, an overcharge inhibitor such as cyclohexyl benzene (CHB) or biphenyl (BP) was used to improve battery safety and high temperature long-term storage characteristics. However, when the electrolyte additives such as CHB or BP are added to a high voltage battery of 4.35V or more, the temperature and temperature cycle characteristics are drastically degraded, and the additives are decomposed too much during high temperature storage, so that a very thick insulator is formed on the anode. Formation prevents the movement of lithium ions and causes a problem that no recovery capacity is obtained.

In view of the above problems, the present invention is intended to improve the safety and capacity reduction at high temperature storage without lowering the cycle of a high voltage battery by adding a compound having a reaction potential of 4.7 V or more to the electrolyte.

Accordingly, an object of the present invention is to provide a high voltage and high capacity lithium secondary battery of 4.35V or more having excellent high temperature cycle characteristics, safety and high temperature storage characteristics.

The present invention is an anode (C); Cathode A; Separator; And in the lithium secondary battery comprising an electrolyte, the weight ratio (A / C) per unit area of the negative electrode (A) to the positive electrode (C) is in the range of 0.45 to 0.70, the electrolyte is composed of 2-toluene fluoride and 3-fluoro toluene Provided is a lithium secondary battery comprising at least one toluene fluoride compound selected from the group.

Hereinafter, the present invention will be described in detail.

The present invention is an electrolyte additive for a high-voltage lithium secondary battery of 4.35 V or higher class, which has a high reaction potential among fluorotoluen (FT) compounds having a reaction potential of 4.7 V or higher, and also has little change in reaction potential with the progress of the cycle. Toluene (2-FT) or 3- toluene fluoride (3-FT) is used.

Due to the above characteristics, it is possible to improve the safety and high temperature storage characteristics in the 4.35V or higher class high voltage lithium secondary battery without deteriorating cycle characteristics.

That is, in case of using a conventional electrolyte in a high voltage lithium secondary battery of 4.35V or higher, an increase in reactivity between the positive electrode and the electrolyte causes degradation of the surface of the positive electrode and oxidation of the electrolyte, leading to deterioration of battery safety and performance. It became. In addition, in order to improve the safety and high temperature storage characteristics of a high voltage battery, cyclohexyl benzene (CHB) or biphenyl, which is an electrolyte additive of a conventional 4.2 V or higher battery, may be obtained by decomposition of the additive. This resulted in a significant cycle deterioration.

In contrast, the present invention uses 2-toluene fluoride (2-FT) and / or 3- toluene fluoride (3-FT) as an additive, thereby reducing side reaction-producing contact surfaces between the positive electrode and the electrolyte, thereby improving battery safety. You can. In addition, due to the characteristics of the high reaction potential of the 2-toluene fluoride (2-FT) or 3-toluene fluoride (3-FT) and little change in the reaction potential according to the cycle progress, conventional additive decomposition and rapid change of the reaction potential It is possible to prevent the performance degradation of the battery due to.

2-fluoro toluene and / or 3-fluoro toluene are physically stable among the fluorine-substituted toluene compounds, have a high boiling point, are difficult to thermally decompose, and have a reaction potential of about 0.1 V than conventional CHB and biphenyl. By having a high reaction potential of 4.7V or higher, unlike the CHB and biphenyl additives when the electrolyte is added, it is possible to improve the high temperature storage characteristics and safety of the battery. In addition, since there is no change in the reaction potential according to the cycle as compared with the conventional toluene fluoride compound, it is possible to prevent deterioration in cycle characteristics of a high voltage battery. Indeed, 4-fluorine toluene (4-FT), whose reaction potential is similar to that of CHB, is characterized by a cycle characteristic of the positive electrode active material and the fluorine substituted at the para- position as the cycle progresses in a battery of 4.35 V or higher. Deterioration occurred and it was confirmed that the safety and high temperature storage characteristics of the battery could not be improved.

The electrolyte used for the high voltage battery of 4.35 V or higher according to the present invention is composed of a conventional electrolyte, that is, an electrolyte comprising a solvent solvent and a lithium salt, including the 2-toluene fluoride and / or 3-fluoride toluene compound.

The lithium salt may be selected from one or more of LiClO ₄ , LiCF ₃ SO ₃ , LiPF ₆ , LiBF ₄ , LiAsF ₆ and LiN (CF ₃ SO ₂ ) ₂ , and the electrolyte solvent is ethylene co (carbonate) carbonate. , Propylene carbonate, butylene carbonate, vinylene carbonate, sulfolane, γ-butylo lactone, dimethyl carbonate, diethyl carbonate, 1,2-dimethoxy ethane, tetrahydrofuran and mixtures thereof The above can be used.

The amount of 2-FT and / or 3-FT compound added is preferably 0.1 to 10% by weight per 100% by weight of the total electrolyte. If it is less than 0.1% by weight, the safety and performance improvement effect due to the addition of the additive is insignificant. If it exceeds 10% by weight, there is a risk that excessive heat may be generated by the viscosity decrease of the electrolyte and the exothermic reaction of the additive.

The negative electrode active material of the present invention may use a carbon material, a lithium metal or an alloy thereof that may occlude and release lithium ions, and other TiO ₂ , SnO ₂ , which may occlude and release lithium and have a potential for lithium less than 2V. Metal oxides such as Li ₄ Ti ₅ O ₁₂ may also be used.

The positive electrode active material of the present invention is _a lithium transition metal complex oxide such as LiM _x O _y (M = Co, Ni, Mn, Co _a Ni _b Mn _c ) (for example, lithium manganese composite oxides such as LiMn ₂ O ₄ , LiNiO _2, and the lithium nickel oxide, LiCoO _2, and the lithium cobalt oxide and combinations of manganese, nickel, a vanadium oxide, etc.), or chalcogenide containing a one or a lithium substituted with other transition metal for a portion of cobalt oxide (for example, g., manganese dioxide, titanium disulfide, molybdenum disulfide, etc.), etc. are used from, and preferably a lithium cobalt-based composite oxide, more preferably LiCoO ₂ can be used.

In order to provide a high voltage lithium secondary battery having a range of 4.35 V or more, preferably 4.35 to 4.7 V, the end-of-charge voltage of the battery may be 4.35 V or more, preferably 4.35 to 4.7 V, using the positive electrode active material such as LiCoO ₂ . The positive electrode active material may be increased to a range or doped with a metal selected from Al, Mg, Zr, Fe, Zn, Ga, Sn, Si, Ge, or a mixture thereof.

In order to improve the safety of the battery in the above-described voltage range, the weight ratio (A / C) per unit area of the negative electrode (A) to the positive electrode (C) can be adjusted, wherein the weight ratio per unit area of the negative electrode (A) to the positive electrode (C) ( A / C) is preferably in the range of 0.45 to 0.70. If it is less than 0.45, it is the same as the design of the existing battery, and the capacity balance during the overcharging of 4.35V or more is broken, causing problems such as lithium dendrite growth on the surface of the negative electrode, resulting in short-circuit of the battery, and rapid capacity reduction. Will occur. When it exceeds 0.70, unnecessarily the lithium site of a negative electrode generate | occur | produces and it is unpreferable since the energy density per volume / mass of a battery falls.

Since the positive electrode active material such as LiCoO ₂ used in the present invention has a problem of deteriorating thermal characteristics when charged to 4.35 V or more, the specific surface area of the positive electrode active material may be adjusted to prevent this. The larger the particle size of the positive electrode active material, that is, the smaller the specific surface area, the lower the reactivity with the electrolyte, thereby improving thermal safety. Therefore, the present invention provides a positive electrode active material having a larger particle size than that of a conventionally used positive electrode active material. It is preferable to use. In addition, the loading amount per unit area of the positive electrode active material and the negative electrode active material may be adjusted in order to prevent a decrease in the rate of the overall battery reaction caused by the positive electrode active material having a larger particle size than the conventional particles.

In the above, the particle size of the positive electrode active material is preferably 5 to 20㎛. If the particle size is less than 5㎛ may increase the reactivity of the positive electrode and the electrolyte may cause side effects such as lack of safety of the battery, if the particle size exceeds 20㎛ may cause a problem that the reactivity of the battery is slow.

In the above, it is preferable that the loading amount per unit area of the positive electrode is 0.01 to 0.03 g / cm ² . When the loading amount of the positive electrode is less than 0.01 g / cm ^2, problems such as deterioration of the capacity and efficiency of the battery may occur. When the amount of the positive electrode exceeds 0.03 g / cm ² , the thickness of the positive electrode increases, thereby decreasing the reactivity of the battery.

The electrode according to the present invention can be prepared according to a conventional method known in the art, for example, it is usually obtained by applying, rolling and drying the positive electrode mixture and the negative electrode mixture on a current collector made of metal foil, respectively. Can be.

In this case, the positive electrode mixture and the negative electrode mixture may be obtained by mixing the positive electrode active material and the negative electrode active material, respectively, with a binder, a dispersion medium, and the like, and the positive electrode mixture and the negative electrode mixture may preferably contain a small amount of a conductive agent.

As the conductive agent, any electron conductive material which does not cause chemical change in the battery constructed can be used. For example, carbon black, such as acetylene black, Ketjen black, Farnes black, and thermal black; Natural graphite, artificial graphite, conductive yarn fibers, and the like can be used. Carbon black, graphite powder and carbon fiber are particularly preferable.

As the binder, any one of a thermoplastic resin and a thermosetting resin may be used, or a combination thereof may be used. Among these, polyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE) is preferable, and PVdF is particularly preferable.

As the dispersion medium, an organic dispersion medium such as an aqueous dispersion medium or N-methyl-2-pyrrolidone can be used.

In the above-described positive electrode of the lithium secondary battery, the electrode plate thickness ratio A / C of the negative electrode A to the positive electrode C is appropriately 0.7 to 1.4, and particularly preferably 0.8 to 1.2. If it is less than 0.7 may cause a loss of energy density per volume of the battery, and if it exceeds 1.4 may cause a problem that the reaction rate of the entire battery is slow.

The lithium secondary battery of the present invention may be prepared by inserting a porous separator between the positive electrode and the negative electrode according to a conventional method known in the art, and adding an electrolyte solution having 2-FT and / or 3-FT added thereto.

The separator may be a polypropylene-based, polyethylene-based, or polyolefin-based porous separator, but is not limited thereto.

The external shape of the lithium secondary battery manufactured by the above method is not limited, but a cylindrical, coin type, square or pouch type of cans is possible.

The invention is explained in more detail based on the following examples and experimental examples. However, Examples and Experimental Examples are for illustrating the present invention and are not limited to these.

Example 1. Manufacture of 4.35V class lithium secondary battery.

1-1. Manufacture of anode

A slurry was prepared by mixing 95% by weight of LiCoO ₂ having a particle size of 10 μm, 2.5% by weight of a superconductor (Super P) and 2.5% by weight of a binder (PVDF). Uniformly coated on both sides and rolled to prepare a positive electrode having an active material weight of 19.44 mg / cm ² .

1-2. Preparation of Cathode

A negative electrode slurry was prepared by adding and mixing 95.3 wt% of graphite material, 4.0 wt% of binder (PVDF) and 0.7 wt% of conductive black (Acetylene Black), and uniformly applying the negative electrode slurry on both sides of a copper plate having a thickness of 10 μm. By rolling, a negative electrode having an active material weight of 9.56 mg / cm ² was prepared. At this time, the weight ratio (A / C) per unit area of the anode (A) to the cathode (C) was 0.49.

1-3. Preparation of Electrolyte

An electrolyte was prepared by dissolving 1 mol of LiPF ₆ in a solvent mixture having a volume ratio of ethylene carbonate and dimethyl carbonate 1: 2, and 3 wt% of 3-toluene fluoride (3-FT) was added per 100 wt% of the electrolyte.

1-4. Manufacture of batteries

The square battery was manufactured using the positive electrode and negative electrode which were manufactured by the above-mentioned method.

Comparative Example 1

A lithium secondary battery was manufactured in the same manner as in Example 1, except that no additive was used in the electrolyte.

Comparative Example 2

A lithium secondary battery was prepared in the same manner as in Example 1, except that 3 wt% of cyclohexylbenzene (CHB) was used in place of 3-toluene fluoride.

Comparative Example 3

A lithium secondary battery was prepared in the same manner as in Example 1, except that 3 wt% of 4-fluoro toluene (4-FT, para-F) was used instead of 3-fluoro toluene.

Comparative Example 4

A lithium secondary battery was manufactured in the same manner as in Example 1, except that 0.44 was used as the weight ratio (A / C) of the negative electrode (A) to the positive electrode (C) and 3 wt% of CHB was used in the electrolyte. Prepared.

Comparative Example 5

A lithium secondary battery was manufactured in the same manner as in Example 1, except that the weight ratio (A / C) per unit area of the negative electrode (A) to the positive electrode (C) was 0.44.

Experimental Example 1. Cycle evaluation of a high voltage lithium secondary battery

According to the present invention, a high temperature cycle evaluation experiment was performed on a 4.35V or higher lithium secondary battery including a toluene fluoride compound as follows.

The lithium secondary battery of Example 1 prepared by adding 3-toluene fluoride (3-FT) to the electrolyte solution was used, and the comparison prepared by adding CHB to the electrolyte of Comparative Example 1, which did not use the electrolyte additive as a control, to the electrolyte solution The battery of Example 2 and the battery of Comparative Example 3 prepared by adding 4-toluene fluoride (4-FT) to the electrolyte solution were used.

Each cell was run in a charge and discharge voltage range of 3.0 to 4.35V and cycled under 1C (= 880 mA) charge and discharge current conditions. In the 4.35V constant voltage section, it was maintained at 4.35V until the current dropped to 50mA. The experiment was carried out at a temperature of 45 ℃.

As a result, the lithium secondary battery of Comparative Example 2 including the electrolytic solution using the CHB additive showed that the battery cycle characteristics are significantly lowered at high temperature than the lithium secondary battery of Comparative Example 1 without the additive (see FIG. 1). This means that the charge transfer reaction of the positive electrode active material is inhibited and the positive electrode resistance is increased due to the film formed by the electropolymerization of CHB having a reaction potential of 4.7 V or less, thereby decreasing the cycle characteristics of the battery. In addition, the battery of Comparative Example 3 using 4-fluorotoluene having a reaction potential similar to that of CHB also rapidly lost the cycle characteristics due to the reaction between the positive electrode active material and the fluorine in the para-position according to the 4.35V cycle ( See FIG. 1).

In comparison, the lithium secondary battery of Example 1 using a 3-fluorotoluene (3-FT) additive having a reaction potential of 4.7 V or more did not show a special effect on the high temperature cycle characteristics compared to FIG. 1 (see FIG. 2). .

As a result, it was confirmed that the high-voltage lithium secondary battery prepared using 3-toluene fluoride as an additive according to the present invention can prevent high temperature cycle characteristics from being lowered, unlike conventional CHB.

Experimental Example 2 Safety Evaluation

In order to evaluate the safety of the 4.35V or higher lithium secondary battery containing a toluene fluoride compound according to the present invention, a hot box experiment was performed as follows.

The lithium secondary battery of Example 1 prepared by adding 3-toluene fluoride to the electrolyte was used, and the comparative comparison of the lithium secondary battery of Comparative Example 2 and 4-toluene fluoride (4-FT) added to the electrolyte using CHB as a control The lithium secondary battery of Example 3 was used.

Each battery was charged to 4.4V at 1C (= 880 mA) for 2 hours and 30 minutes, and then maintained in constant voltage. For a while. Then, it was observed whether each battery was ignited.

As a result of the experiment, the batteries of Comparative Examples 2 and 3 to which CHB and 4-FT were added, respectively, exploded after time elapsed (see FIGS. 3 and 4). In comparison, the lithium secondary battery of Example 1 using the 3-fluorotoluene additive in the electrolyte showed a safe state at a high temperature of 150 ° C. (see FIG. 5).                     

Experimental Example 3. High Temperature Storage Evaluation

According to the present invention, a high temperature storage experiment was performed on the 4.35V or higher lithium secondary battery containing the toluene fluoride compound as follows.

3-1. High Temperature Long Term Preservation Experiment

The lithium secondary battery of Example 1 prepared by adding 3-toluene fluoride to the electrolyte was used, and the batteries of Comparative Examples 2 and 3 prepared by adding CHB and 4-fluoro toluene (4-FT) as controls, respectively. Was used.

Each battery was charged to 4.35V at 1C charge current, and 1C discharged to 3V to confirm the initial discharge capacity. Subsequently, the battery was recharged to 4.35V and the thickness was measured while repeating 30 cycles of 3 hours storage at 80 ° C / 7 hours storage at 25 ° C, and then discharged at 1 C to measure the remaining capacity of the battery. After 3 cycles of charge and discharge after measuring the remaining capacity, the recovery capacity of the battery was measured.

As a result of repeating the process four times and measuring the reproducibility, the battery of Comparative Example 2 containing CHB showed a significant bulging phenomenon even before the charge-discharge cycle was performed five times (see FIG. 6). In addition, in the battery of Comparative Example 3 using 4-fluorotoluene having a reaction potential similar to that of CHB, when the charge / discharge cycle proceeded about 10 times, the swelling phenomenon of the battery occurred remarkably (see FIG. 6). In comparison, the battery of Example 1 to which the 3-fluoride toluene compound was added showed an excellent cell swelling reduction phenomenon (see FIG. 6).

3-2. High temperature short term preservation experiment

The lithium secondary battery of Example 1, prepared by adding 3-toluene fluoride to the electrolyte, was used as a control, and the lithium secondary battery of Comparative Example 1, which did not use the electrolyte additive, and the battery of Comparative Example 2, prepared by adding CHB. Was used.

Each battery was charged to 4.35V at 1C charge current, and 1C discharged to 3V to confirm the initial discharge capacity. Then, after charging to 4.35V again, the thickness was measured while preserving at 90 ° C for 4 hours. After storage, the battery was discharged at 1 C and the remaining capacity of the battery was measured. After the remaining capacity was measured, charging and discharging were carried out three times to measure the recovery capacity of the battery.

After 4 hours of storage at 90 ° C., the thickness of the 4.35V or higher cell of Comparative Examples 1 and 2 was significantly increased. Particularly, the battery of Comparative Example 2 using CHB was larger than Comparative Example 1 without using the additive. Cell thickness increase was shown (see FIG. 7). This indicates that the thickness of the battery is increased by forming a thick insulator due to decomposition of the electrolyte due to the increased reactivity of the positive electrode and the electrolyte in a high voltage battery, and an electrolyte additive such as CHB used in a conventional 4.2V class battery is 4.35V. It means that it is not suitable for the ideal high voltage battery.

On the other hand, the high-voltage lithium secondary battery of 4.35V or higher of Example 1 using the 3-toluene fluoride compound as an electrolyte additive did not swell even after storage at 90 ° C., and it was found that the performance deterioration was hardly observed (FIG. 7).

As a result, the present invention was able to confirm that a toluene fluoride compound having a reaction potential of 4.7 V or more is suitable as an electrolyte additive for a high voltage battery of 4.35 V or more.

 The high voltage lithium secondary battery of 4.35V or more according to the present invention can improve safety and high temperature storage characteristics of a high voltage battery without deteriorating cycle characteristics by using a toluene fluoride compound as an additive.

Claims (3)

Anode (C); Cathode A; In a lithium secondary battery comprising a separator and an electrolyte solution, The active material weight ratio (A / C) per unit area of the negative electrode (A) to the positive electrode (C) is in the range of 0.45 to 0.70, The electrolyte solution includes at least one toluene fluoride compound selected from the group consisting of 2-toluene fluoride and 3-toluene fluoride, A lithium secondary battery, characterized in that the end-of-charge voltage of the battery is 4.35V or more. The lithium secondary battery of claim 1, wherein the amount of the toluene fluoride compound is in the range of 0.1 to 10 wt% per 100 wt% of the electrolyte. The method of claim 1, wherein the lithium secondary battery has a charge end voltage range of 4.35 to 4.7V or the positive electrode (C) is a positive electrode active material capable of occluding and releasing lithium, Al, Mg, Zr, Fe, Zn, Lithium secondary battery, characterized in that the doped with at least one metal selected from the group consisting of Ga, Sn, Si and Ge.

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