TW201526342A - Electrolyte composition for lithium-ion battery - Google Patents

Abstract

The electrolyte composition for lithium-ion battery is provided by this invention. There are advantage as long time of useful life, high discharge capacity and more safety. The electrolyte composition for lithium-ion battery can effectively increase the conductivity, electrochemical stability, cycle life, etc. Therefore, this invention can reduce the cost of process comprises equipment, human resource, and simplify the operation or the step of the process.

Description

Electrolyte composition of lithium ion battery

The present invention relates to an electrolyte composition of a battery, and more particularly to an electrolyte composition of a lithium ion battery.

Since the successful commercialization of lithium-ion secondary batteries in 1990 and the introduction of portable electronic products, it has rapidly developed and improved the appearance of many technological products, and lithium-ion secondary batteries have high energy density. Flexible design of product specifications and long service life, it has become the main energy storage component of portable electronic products.

The main components of the lithium ion battery include a positive electrode (LiCoO ₂ , LiMn ₂ O ₄ , LiFePO _{4 ,} etc.), an electrolytic solution, a separator, and a negative electrode (carbonaceous material, titanium-based material, etc.). The working principle of the lithium ion battery is to use lithium ions to perform charging and discharging reactions between the positive and negative electrodes, and the behavior of embedding and embedding. The charge-discharge chemical reaction can be summarized into the following chemical reaction formulas, wherein M is Co, Ni, or Mn; and the reaction direction is toward the right when charging, and the reaction direction is toward the left when discharging: Positive reaction: LiMO ₂ Li(1-x)MO ₂ +xLi ⁺ +xe ^-

Negative reaction: C ₆ +xLi ⁺ +xe ^- LixC ₆

Total reaction: LiMO ₂ + C ₆ Li(1-x)MO ₂ +LixC ₆

In the lithium ion secondary battery, the electrolyte conducts lithium ions between the yin and yang electrodes in a molecular and ion manner, but there is no chemical reaction with any of the electrodes. In the process of charging and discharging the whole battery, determining the energy of the battery is determined by the difference in chemical potential between the cathode and the anode, and the amount of active powder in the cathode and anode. Therefore, the electrolyte must be in a stable state in the battery and is not affected by the working potential. If any chemical reaction occurs between the electrolyte and the electrode, it will cause electron transfer and consume energy. The working potential window of the liquid electrolyte is expressed as follows: Eg=E _LOME -E _HOMO

Where E _LOMO is the lowest air rail domain and E _HOMO is the highest air rail domain.

The commercialized lithium ion secondary battery electrolyte system has been mainly composed of a carbonate solvent and a conductive salt solute as a main component, and the formulation contains two or more solvents or solute components. In fact, due to the poor thermal stability of carbonate electrolytes and the disadvantages of low vapor pressure and low ignition point, commercially available carbonate electrolytes have very poor thermal stability at about 100 °C. When an unexpected local overheating or short circuit occurs in a lithium ion secondary battery, the battery temperature may rise rapidly, causing explosion and combustion of the battery.

Ionic liquids have good thermal stability, low vapor pressure, high operating potential window and non-flammable properties. Many studies have used ionic liquids in lithium ion secondary batteries as electrolytes for improved safety and battery performance. There are quite a lot of results on it. However, due to the large molecular group of the ionic liquid, which has the disadvantages of low conductivity and high viscosity, there are still many shortcomings to be applied to the lithium ion secondary battery, and the TFSI-anionic ionic liquid system is For example, its large anion-cationic group and high viscosity characteristics have a great influence on the conductivity of the battery, so its conductivity is significantly worse than that of today's commercial carbonate-based electrolytes. Further, when it is used for a carbon material negative electrode, the growth of a solid electrolyte interface film (SEI) between the carbon material and the electrolyte solution is considerably unstable. Since the pure ionic liquid is used as the electrolyte of the lithium ion secondary battery, there are still many disadvantages. Therefore, many studies have mixed the carbonate electrolyte with the ionic electrolyte to improve the problem of high viscosity and poor conductivity. It is expected to meet the needs of commercialization.

Document J. Power Sources 2010, 195, 845. and J. Power Sources 2011, 196, 4801. Revealed by the use of 1-ethyl-3-methylimidazolium-bis(fluorsulfonyl)imide (EMIm-TFSI) and N-butyl-N- The ionic liquid mixed carbonate electrolyte of methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI) has a remarkable flame retardant effect, which greatly improves the safety defects caused by the most ill electrolyte of lithium ion batteries. In the improvement of conductivity and viscosity, the electrolyte formula mixed with the ionic liquid and the carbonate electrolyte can also effectively improve the high viscosity of the ionic liquid itself, wherein the content of the carbonate electrolyte is overall. The conductivity of the electrolyte has a very important influence. In the TFSI ^- series ionic liquid mixed carbonate electrolyte solution electrolyte, if ethyelen carbonate (EC) or vinylene carbonate (Vinylene, VC) is added, the SEI film can be effectively promoted. The growth of the graphite negative electrode improves the disadvantage that the ionic liquid cannot be stably grown on the graphite negative electrode, and effectively increases the reversibility of the capacitance. In the influence of aluminum current collectors, Electrochimica Acta 2011, 56, 4092. and Electrochimica Acta 2002, 47, 2787. reveal that TFSI ^- anions contained in ionic liquids have anodized pitting characteristics on aluminum foil, so There is still a bad influence on the lithium ion battery, but the pitting behavior, if mixed with the carbonate electrolyte, will have a considerable degree of inhibition, and due to the wide operating potential window of the ionic liquid, for the overall mixed electrolysis The working potential window of the liquid has a considerable contribution, effectively improving the flammability characteristics of the carbonate electrolyte itself and improving the safety of the lithium ion battery.

In view of the above-mentioned background of the invention, in order to meet the requirements of the industry, an object of the present invention is to provide an electrolyte formula for mixing an ionic liquid, which is used for a lithium ion battery, improves the service life and safety of the lithium ion battery, and can be mass-produced. Lithium-ion batteries have an important improvement in the application of portable electronic energy storage components and electric vehicles.

One of the objects of the present invention is to provide a triethylmethylammonium bis(trifluoromethylsulfonyl)amide (N1222-TFSI) as a main ionic liquid formulation. And by mixing N1222-TFSI with a linear carbonate electrolyte, in addition to making the electrolyte have flame resistance characteristics, and improving the problem of low conductivity caused by excessive molecular group and high viscosity of the ionic liquid itself, and by adding After N1222-TFSI, it enhances the electrochemical stability of the electrolyte and makes the operable potential range wider.

According to the above object of the present invention, the present invention provides a methyltriethyl bistrifluoromethanesulfonimide ionic liquid, which is selected from the above-mentioned methyl triethyl bistrifluoromethane sulfonimide ionic liquid Lithium iron cathode material, and assembled into a coin-type lithium iron phosphate battery, cycle life test and high rate discharge test under normal temperature environment. The electrolyte formulation was formulated using TGA for thermal stability analysis and the flame resistance of the formulated electrolyte was identified by direct combustion. It is obtained from the results of the examples that the addition of the above-mentioned methyltriethyl bistrifluoromethanesulfonimide ionic liquid to the electrolyte can ensure that the lithium iron phosphate battery has good cycle life, high discharge characteristics, and improved electricity. The capacity and lifting capacitance are reversible, and the disadvantage of poor thermal stability of the carbonate electrolyte is improved. And the electrochemical method is used to identify the working potential window and the corrosion behavior of the aluminum foil, wherein the formulated electrolyte has the effects of improving the working potential window and suppressing the pitting behavior. In summary, in the selection of the formula ratio of the formula electrolyte, after a plurality of tests, the formula range is determined, and the total composition is 100% of the three-component formula: the above-mentioned methyl triethyl The bis-trifluoromethanesulfonimide ionic liquid is 24 to 50 wt% and the DMC is 24 to 50 wt%, and the EC is 16 to 33 wt% is the optimum value, and one of the formulas is selected, and the additive vinylene carbonate is added. After (1% by weight of Vinylene, VC), in addition to having stable battery performance, the formulation has a stable cycle life by charging and discharging at a high temperature of 60 °C.

According to the above object of the present invention, the present invention provides an electrolyte composition for a lithium ion battery, the electrolyte composition for a lithium ion battery comprising: an imine ion liquid, the imine ion liquid to the electrolyte The weight percentage is 24 to 50% by weight, wherein the composition formula of the content of the imine ion liquid is (I) Wherein R ¹ , R ² , R ³ and R ^{4 above} may be selected from one or a combination of the following: a hydroalkyl group, a heterocyclic ring, a linear alkyl group, a branched alkyl group, as shown in FIG. Dimethyl Carbonate, the weight percentage of the dimethyl carbonate to the electrolyte is 24 to 50 wt%; Ethylene Carbonate, the weight percentage of the ethylene carbonate to the electrolyte is 16 to 33 wt %; Lithium hexafluorophosphate (LiPF ₆ ), the weight percentage of the lithium hexafluorophosphate to the electrolyte is 6 to 15% by weight; and lithium bis(trifluoromethylsulfonimide) (LiTFSI), the ditrifluoro The weight percentage of lithium sulfonimide is 4 to 12% by weight.

According to the above method of the present invention, the imine ionic liquid may be a triethylmethylammonium bis(trifluoromethylsulfonyl)amide, the methyltriethyl group. The composition of the content of the bistrifluoromethanesulfonimide ionic liquid is as shown in (II), as shown in the first B:

According to the above method of the present invention, wherein the weight percentage of the imine ion-type liquid to the electrolyte further comprises 27 to 37% by weight, wherein the weight percentage of the dimethyl carbonate to the electrolyte further comprises 27 to 37% by weight, wherein the above The weight percentage of the ethylene carbonate to the electrolyte further comprises 11 to 21% by weight, wherein the above-mentioned R ¹ , R ² , R ³ and R ^{4 each} have a carbon number of C1 to C4, wherein the above-mentioned lithium hexafluorophosphate The weight percentage of the electrolyte further comprises 6-11% by weight, wherein the above-mentioned lithium bistrifluoromethylsulfonimide further comprises 4-7 wt% to the electrolyte, wherein the vinylene carbonate is the electrolyte The weight percentage further includes 0.1 to 5 wt%.

10‧‧‧Lithium ion battery electrolyte composition

11‧‧‧imine ion liquid

12‧‧‧Dimethyl carbonate

13‧‧‧ Vinyl carbonate

14‧‧‧Lithium hexafluorophosphate

15‧‧‧Lithium trifluoromethylsulfonimide

100‧‧‧Lithium ion battery electrolyte composition

110‧‧‧Methyltriethyldifluoromethanesulfonimide ionic liquid

120‧‧‧Dimethyl carbonate

130‧‧‧Vethyl carbonate

140‧‧‧Lithium hexafluorophosphate

150‧‧‧Lithium trifluoromethylsulfonimide

160‧‧‧ vinylene carbonate

170‧‧‧1-Methyl-3-propylpyrrole bistrifluoromethanesulfonimide ionic liquid

200‧‧‧Electrochemical Analyzer

210‧‧‧Working electrode

220‧‧‧relative electrode

230‧‧‧ reference electrode

240‧‧‧ electrolyte

380‧‧‧ electrolyte

300‧‧‧ coin type lithium ion battery

310‧‧‧Upper cover

320‧‧‧Stainless steel disc

330‧‧‧Spring washer

340‧‧‧ positive pole piece

350‧‧‧Separator

360‧‧‧Lithium foil

370‧‧‧Lower cover

The first diagram is for explaining the structural formula disclosed by the electrolyte of the present invention; the first diagram is the structural formula disclosed by the electrolyte of the present invention; the second diagram is the measurement configuration of the electrochemical measurement equipment (Auto lab PGSTAT30). The third figure is the composition and structure of the button type battery; the fourth figure is the change of the conductive behavior at different temperatures in the application example 1 and the comparative example; the fifth figure is an application example using the platinum as the working electrode (1) And the electrochemical window stability of 0 to 5.5 V of Comparative Examples (1) and (2); the sixth figure is an application example (1) and Comparative Examples (1) and (2) using platinum as a working electrode The electrochemical window stability at an oxidation potential of 3 to 5.5 V; the seventh figure is an application example (1) and comparative examples (1) and (2) using a platinum as a working electrode at 3.5 to 5.5 V aluminum foil anode The first cycle of cyclic voltammetry test results of the test; The eighth figure is a first-cycle cyclic voltammetry test result illustrating the application of the aluminum foil as the working electrode (1) and the comparative examples (1) and (2) in the 3.5-5.0 V aluminum foil anodization test; It is a surface observation of SEM 30000 times that the aluminum foil is not subjected to cyclic voltammetry according to an embodiment of the present invention; the tenth figure is a SEM 30000 times of the aluminum foil after the five-turn scanning of the comparative example (1) The eleventh surface observation shows the surface observation of the SEM 30000 times of the aluminum foil after five cycles of cyclic voltammetry. The twelfth figure shows the application example (1) after five cycles of cyclic voltammetry. The surface of the aluminum foil SEM is 30000 times. The thirteenth figure is the cycle life test result of the comparative example (1) (2) and the application example (1) at room temperature 25 ° C; the fourteenth figure is for comparison Example (1) and application example (1) high temperature cycle life test results at 60 ° C; the fifteenth figure is a cycle illustrating application examples (2) to (17) and comparative examples (1) at room temperature 25 ° C Life test result; The sixteenth figure is a test result showing the different discharge rates of 0.1, 0.5, 1.0, 2.0, 3.0, and 5.0 C-rate at room temperature 25 ° C in the application examples (2) to (17) and the comparative example (1); The seventeenth figure is used to illustrate the discharge performance test results of the application examples (2) to (17) and the comparative example (1) at a temperature interval of 25 ° C and -10 ° C; the eighteenth figure is for explaining the comparative example (1), Application Example (18) The discharge performance test result at 60 ° C; the nineteenth figure is the flame resistance test result of Comparative Example (1), Application Example (2) (3) (4); To illustrate the thermogravimetric analysis results of the application examples (2), (3), (4) and the comparative example (1); and the twenty-first diagram for explaining the application examples (2), (3), (4) and the comparative example (1) The difference is the result of the thermal analysis.

The present invention is directed to the electrolyte composition of a lithium ion battery and its method of manufacture. In order to fully understand the present invention, detailed structures, elements, and method steps thereof are set forth in the following description. Obviously, the practice of the present invention is not limited to the specific details familiar to those skilled in the art of lithium ion battery technology. On the other hand, well-known structures and elements thereof are not described in detail to avoid unnecessary limitation of the invention. In addition, in order to provide a clearer description and to enable those skilled in the art to understand the present invention, the various parts of the drawings are not drawn according to their relative sizes, and the ratio of certain dimensions to other related scales will be highlighted. The exaggerated and irrelevant details are not completely drawn, in order to simplify the illustration. The preferred embodiments of the present invention are described in detail below, but the present invention may be widely practiced in other embodiments and the scope of the present invention is not limited by the scope of the appended claims.

According to an embodiment of the present invention, an electrolyte composition 10 of a lithium ion battery is provided. The electrolyte composition 10 of the lithium ion battery comprises: an imine ion liquid 11 is 24 to 50 wt%, and dimethyl carbonate is dimethyl carbonate. , DMC) 12 series is 24~50wt%, Ethylene Carbonate (EC) 13 series is 16~33wt%, hexafluorophosphorus lithium (LiPF ₆ ) 14 series is 6~12wt%, and bistrifluoromethyl Lithium sulfonate (LiTFSI) 15 is 4 to 12% by weight. Here, the structural formula disclosed by the above imine ion liquid 11 is as shown in FIG.

According to an embodiment of the present invention: Application Example 1 provides an electrolyte composition 100 of a lithium ion battery, the electrolyte composition 100 of the lithium ion battery comprising: methyl triethyl bistrifluoromethane sulfonimide ionic liquid ( The triethylmethylammonium bis (trifluoromethylsulfonyl)amide, N1222-TFSI) 110 system is 48 wt%, the dimethyl carbonate (DMC) 120 system is 21.1 wt%, and the ethylene carbonate (EC) 130 system is 21.1 wt%. Lithium hexafluorophosphate (LiPF ₆ ) 140 was 6.2 wt%, and was _3.6 wt% with lithium bistrifluoromethylsulfonimide (LiTFSI) 150. Wherein, the above methyltriethyl bistrifluoromethanesulfonimide ionic liquid 110 reveals a structural formula as shown in FIG. In order to maintain the concentration of Li ⁺ in the fully formulated electrolyte at 0.53 mol kg ^-1 , the rotor was stirred for 8 hours.

An embodiment of the present invention, Application Example 2-17: The formulation range is as shown in Table 1, N1222-TFSI 110 is 24.49~40.82wt%, EC 130 is 16.33~32.66wt% and DMC 120 is 24.49~40.82wt % and maintain an equal amount of lithium salt in the fully formulated electrolyte, with LiPF ₆ 140 being 11.63 wt% and LiTFSI 150 being 6.72%.

In one embodiment of the present invention, Application Example 18: Application Example 11 was carried out by adding 1 wt% of vinylene carbonate (VC) 160 and stirring with a rotor for 8 hours.

Comparative Example 1: EC 130 and DMC 120% by weight each were 45 wt%, and 10 wt% of LiPF ₆ 140 was added as a base electrolyte formulation.

Comparative Example 2: DMC 120 was taken as 21.1 wt%, ethylene carbonate EC 130 was 21.1 wt%, LiPF ₆ 140 was 6.2 wt%, and LiTFSI 150 was 3.6 wt%, and 48 wt% of 1-methyl-3-propene was added. The pyryrrole bistrifluoromethanesulfonyl imide (Pyr13-TFSI) ionic liquid 170 was mixed as a solvent, and the concentration of Li ⁺ in the fully formulated electrolyte was maintained at 0.53 mol kg ^-1 , and the rotor was stirred for 8 hours.

Comparative Example 3: A methyltriethyl bistrifluoromethanesulfonimide ionic liquid (N1222-TFSI) 110 was white salt solid at room temperature and had a melting point of 402.15 K.

Test 1, conductivity test: Take the comparative example (1) (2) and the application example (1) to fill the formulation electrolyte in a two-pole (without reference electrode) canister for measurement, wherein the working electrode and the opposite electrode All are made of stainless steel, and the tank filled with the formula electrolyte is placed in the stable environment control box for the test temperature for one hour. The test temperature parameters are 10 °C, 0 °C, -10 °C, -20 °C, respectively. The configuration is shown in the second figure. The electrochemical measuring device (Auto lab PGSTAT30) is connected to the test tank and tested by Electrochemical impedance spectroscopy. After inputting an AC voltage to the sample to be tested, the AC current fed back by the sample is passed through the device. After calculation, a complex Nyquiste plot can be obtained, and the conductivity value (σ, Conductivity) can be obtained by extrapolation of the spectrum.

Test 2, Cyclic voltammetry (CV) test: Cyclic voltammetry (CV) test was carried out in Comparative Example (1) (2) and Application Example (1), and the configuration was as shown in the second figure. . The principle is that the electrochemical analyzer 200 applies a circulating voltage to the working electrode 210 at a fixed rate, from the initial potential to the end potential, and then returns to the initial potential at the same rate, by which the current is responsive. The value can plot the current versus voltage, and the redox reaction of the electrode at different potentials, as well as the electrochemical reaction and reaction rate at the electrode surface can be observed. In this experiment, the working potential window is tested by using a three-electrode device with a CV. The working electrode 210 is platinum, the counter electrode 220 and the reference electrode 230 are all lithium metal. Each of the formulation electrolytes 240 was injected into the test can and tested at a scan rate of 20 mV s ^-1 . The scanning direction is first scanned from 3.0V to 5.5V, then from 5.5V to 0V for negative scanning, and then from 0V to 3.0V for one cycle.

Test 3, aluminum foil corrosion test: using the comparative example (1) (2) and the application example (1), the experiment was conducted with an electrochemical measuring device (Auto Lab PGSTAT30) three-electrode device with CV for aluminum foil corrosion test. The test configuration is shown in the second figure. The purpose is to observe the electrochemical characteristics of each formulation electrolyte 240 on the aluminum foil, wherein the working electrode 210 is an aluminum foil, and the opposite electrode 220 and the reference electrode 230 are both lithium metal. After injecting each of the formulated electrolytes into the test can, the test was performed at a scan rate of 1.0 mV S-1. The scanning direction is first scanned by the open circuit voltage to 5.0V, and then the negative scanning is performed from 5.0V to the open circuit voltage. After 5 cycles of testing, the tested aluminum foil is scanned by Scanning Electron Microscope (SEM). Surface observation.

Test 4, charge and discharge cycle test: lithium iron phosphate powder (manufactured by Changyuan Co., Ltd.), and a conductive material (carbon black; manufactured by Timcal, model Super-P), adhesive (PVDF; by Solef Manufacture, Model 6020, molecular weight approximately 304,000), and oxalic acid (manufactured by Fluka, 97.0% purity) doped into a solid composition at a weight ratio of 84.9:5:10:0.1, followed by 57 wt% of the composition The solvent (NMP; manufactured by Taiwan Boeing Co., Ltd., purity 99.94%) is uniformly mixed with the solid composition to form a slurry mixture; the mixture is applied to a foil-like current collector (aluminum foil; The positive electrode tab 340 was formed by drying and then pressing and cutting it, manufactured by Nippon foil Co., Ltd., having a thickness of 15 μm.

Thereafter, each of the positive electrode tabs 340 is prepared with a battery component and an electrolyte 380 as described later, and a glove box having a water oxygen content of 10 ppm or less and provided with a battery capping machine (manufactured by Unilab Mbraum, model number is 150B-G) is assembled internally, and the battery capper is used to cover (to ensure its sealing property), and a coin-type lithium ion battery 300 to be tested is respectively obtained.

The above electrolyte solution is shown in the above Comparative Examples (1) and (2) and Application Examples (1) to (17).

The above information about the battery components is as follows: the upper cover 310 and the lower cover 370: manufactured by Taiwan Haoju Industrial Co., Ltd., model number 2032. Spring washer 330, stainless steel disc 320: manufactured by Taiwan Haoju Industrial Co., Ltd. Membrane 350: manufactured by Celgard Corporation, model number Celgard 2300. Negative electrode pole piece: Lithium foil 360, manufactured by FMC Corporation, is a wafer having a purity of 99.9% and a diameter of 1.65 cm, as shown in the third figure.

Test 5: Take the comparative example (1) and the application examples (2) to (4) for the flame resistance test setting. As shown in the nineteenth figure, the container is filled with the electrolyte of each formula, and the lamp core is placed and Fully wetted, experimented by direct combustion observation. After burning the fire source close to the lamp core for three seconds, remove the fire source and observe the burning phenomenon, and record it with a camera.

Test 6: Take Comparative Example (1) (3) and Application Examples (2) to (4) with Thermogravimetry Analyzer (TGA, TATGA-Q50). This test is mainly to observe the electrolyte of each formulation. The degree of weight loss at high temperatures is used to more accurately identify the thermal stability of the formulated electrolyte. In the TGA test method, the ceramic sample dish is filled with 5-10 mg of each formula electrolyte, and the ceramic sample dish is placed in a narrow platinum plate, and placed in an instrument for testing. The test temperature ranged from room temperature to 550 ° C and the scan rate was 10 ° C min ^-1 . Derivative thermogravimetry (DTG) was drawn to investigate the rate of change of mass with time.

The efficacy test of the invention into a lithium battery is as follows: Conductivity test: comparing the ionic liquids of different cations, the conductivity of the same solvent formulation conditions, the results are shown in the fourth figure. Application Examples (1) and Comparative Examples (1) corresponded to the electrical conductivity of Comparative Example (2). The linear regression of the experimental data is shown in Table 2. Considering that the salt and the solvent are homogeneous materials, the conductive behavior of the ionic liquid and the non-aqueous electrolyte at different temperatures is in accordance with the Vogel-Tamman-Fuicher (VTF) equation, as follows Formula: σ =AT ^-1/2 exp ^{[-Ea/R(T-To)]} or σ = σ ₀ exp ^[-B/(T-To)] , where A is the pre-exponential term and T is the absolute temperature. Ea is considered as the activation energy, T _o is the thermal critical temperature of ion conduction, and T _{o is} approximately the glass transition temperature T _{g of the} polymer. It can be known from the VTF equation that the slope of the conductivity versus temperature represents the activation energy Ea of the formulation electrolyte. The activation energy E _{a of the} formulation electrolyte is discussed by the slope of the conductivity versus temperature according to the VTF equation. By comparison, in the electrolyte formulation of the mixed ionic liquid, the melting point of the ionic liquid itself does not affect the conductivity of the overall formulation electrolyte, and the main reason that the experimental results confirm the conductivity is derived from The ionic structure of the ionic liquid in the electrolyte is related to the size. Even though the ionic liquid N1222-TFSI used in the application example (1) is solid at normal temperature, the ionic liquid Pyr13-TFSI used in the comparative example (2) is liquid at normal temperature, but the electrolyte of the two formulations is not conductive. Divided into Xuanyuan, and approached the comparative example (1). Table 2. Slope and regression results of conductivity change at different temperatures of application example (1) and comparison example (1) (2)

Electrochemical test: using the mixed formula of Comparative Example (1) (2) and Application Example (1), the test results of the working potential window are as shown in the fifth figure, and the reduction potential of the working potential window is 0V-3.0V. As shown in the six figures, the reaction current of lithium metal and platinum in the application example (1) is more obvious than that in the comparative example, indicating that the application example (1) is easier for the lithium dissolution behavior of lithium metal than the comparative example, so it is more advantageous for lithium ion. Conductive behavior in the electrolyte. In the oxidation reaction potential of 3.0V-5.5V, as shown in the seventh figure, the application example (1) has better electrochemical stability than the comparative example at a higher potential, indicating that the application example is compared with the comparison. Example (2) has a wider working potential window, indicating that its electrochemical performance is also relatively stable.

Aluminum foil anodization test: The aluminum foil anodization test was carried out using the formulation electrolyte of Comparative Example (1) (2) and Application Example (1), and the test results of the first cycle cyclic voltammetry were as shown in the eighth figure. The scanning result reaction current obtained in the comparative example (1) is contributed by the literature Electrochimica Acta 2005, 47, 2677. It is contributed by PF ₆ ^- which is derived from the passivation layer of surface-grown AlF ₃ and uniformly covers the aluminum. On the substrate, the protective aluminum foil is no longer corroded by subsequent oxidation and reduction potential cycles. Its chemical formula is as follows: Al + 3F → AlF ₃ . In Comparative Example (2) and Application Example (1), since LiPF ₆ is contained, the reaction current of the same AlF ₃ passivation layer as in Comparative Example (1) is different, but the magnitude of the oxidation current is different. The reaction current of the application example (1) was the smallest.

The aluminum foil which was not subjected to the cyclic voltammetry test was subjected to SEM surface-identification with the aluminum foil of Comparative Example (1) (2) and Application Example (1) after five cycles of cyclic voltammetry. The SEM results are shown in the ninth figure, showing defects in corrosion of the aluminum foil which have not been subjected to cyclic voltammetry and particles of alumina. The surface of the aluminum foil using the comparative example (1) was observed as shown in the tenth figure, and the results were in accordance with the literature, and the surface was covered with a uniform and dense AlF ₃ passivation layer, so that the defects on the surface were less obvious. However, in Comparative Example (2), as shown in Fig. 11, the surface has an apparent corrosion behavior as compared with the application example (1) shown in Fig. 12, and an unknown substance grows on the surface. It is speculated that the surface electrolyte is different from the application example (1) because the electrochemical characteristics of the Pyr13 ⁺ cation are relatively unstable, causing the formulation electrolyte to react at the oxidation potential and causing pitting behavior at the same time. Although the surface of the application example (1) still has some pitting behavior, the cationic structure of the application example (1) has an advantage of lowering the pitting behavior of the aluminum foil than the comparative example (2).

Charge and discharge cycle test 1: by charge and discharge tester (made by Acutech), model BAT-700S), to 0.120mA. a current of cm ^-2 (about 0.1 C-rate), and the batteries of the application example (1) and the comparative example (1) (2) are subjected to constant current charging until the voltage of the battery circuit reaches 4.2 V. The first charge capacity of the battery (mAh g ^-1 ); followed by 0.120mA. The current of cm ^-2 is fixed current discharge to each battery until the circuit voltage reaches 2.5V, and the first discharge capacity value obtained is as shown in Table 3, wherein the formulation electrolyte of the application example (1), It has the best reversible capacity compared to each of the comparative examples. The cycle life result at normal temperature is as shown in Fig. 13, and the application example (1) is superior to the comparative example (2) in the stability of the cycle life, wherein the comparative example (2) is at the 10th. After the charge and discharge, the circle began to have some micro-recession phenomenon, and there was a rapid decline on the 49th circle. It is speculated that the electrochemical stability of Pyr13-TFSI in the LiFePO ₄ battery system is inferior to that of N1222-TFSI, and the cycle life of the application example (1) is quite stable compared with the comparative example (1). Thus, the application example (1) is obtained as a formulation having the best cycle life at room temperature.

Subsequent to 1.20 mA at room temperature and 60 ° C, respectively. The current of cm-2 is recorded according to the charging and discharging steps described above, and the discharge capacity values measured at the respective discharges are recorded synchronously. The test results are shown in Fig. 14, compared with the comparative examples (1) and (2). ), the application example (1) has the best performance in high temperature cycle life. This behavior should a content ratio with a lithium salt (LiPF _6, of LiTFSI) of the relevant, because the thermal stability of LiPF ₆ as compared to the TFSI ^- poor.

And because Application Example (1) and the PF ^6- TFSI ^- the ratio of 1: 9, it is speculated that due to the high concentration of TFSI within the system ^- the electrolyte is added so that the formulation has a better performance in the high temperature cycle life, but Comparative Example (1) has a rapid decay phenomenon compared to Application Example (1). From this result, it was found that the cationic structure of the ionic liquid has a considerable influence on the thermal stability of the LiFePO ₄ battery, and the application example (1) is the optimum formulation for the high temperature cycle life test.

Charge/Discharge Test 2: From the above research results, we obtained an electrolyte mixed with N1222-TFSI, which has good electrical performance, electrical conductivity and can effectively inhibit the corrosion behavior of aluminum foil. More importantly, this formulation electrolyte improves the thermal stability of the organic electrolyte compared to the conventional carbonate electrolyte.

In order to obtain the optimum range of formula electrolyte, use N1222-TFSI as the main ionic liquid, DMC as the formulation solvent and match EC, define a formula range, and use the weight percentage as the application example (2)~( 17) and comparative example (1), at room temperature at 1.20 mA. The current of cm ^-2 (1C-rate) was charged and discharged as described in [Charge/Discharge Test 1], and the discharge capacity value measured at each discharge was recorded. The results of the application examples (2) to (17) and the comparative example (1) normal temperature cycle life test are shown in Fig. 15. It can be clearly found that most of the application examples have a significantly higher discharge capacity than the comparative example (1). A lot. The reversible capacitance percentage, capacitance decay rate and capacitance of the planned application examples (2) to (17) are summarized in Table 4. The percentage of reversible capacitance is the percentage of charge and discharge of the first circle, such as: percentage of reversible capacitance (%) ) = discharge capacity / charge capacity × 100%. The capacity decay rate is linearly regression (y=-ax-b) by taking the capacitance of 20-80 cycles of the cycle life, and then taking the obtained coefficient x of the x term for regression analysis.

Within the range of selected formulas (application examples (2) to (17)), the performance of reversible capacity, capacity decay rate and capacitance is not far from or even slightly better than the comparative example (1), indicating N1222-TFSI and The composition ratio of EC and DMC has an effect on the reversible capacity of the system. It is known that the content of DMC must be sufficient to effectively increase the reversible capacity. It is speculated that the viscosity of the electrolyte is increased due to the high content of N1222-TFSI and EC, so more energy must be lost to form. And there must be a suitable ratio between EC and N1222-TFSI to effectively interfere with the ionization that may form inside. Comparing the capacity decline rate, it is found that the content of N1222-TFSI has the most significant effect on the capacity decay behavior. However, if the proportion of solvent is increased, the capacity decay behavior will also be aggravated, and at the same time, the ratio of N1222-TFSI to DMC can be slowed down. As for the performance of the capacitance, it can be known that the content of N1222-TFSI may have a large influence on the capacitance of the mixed formulation. The formulation range of the selected application examples (2) to (17) is compared with the comparative example (1). The discharge capacity of the LiFePO ₄ battery at normal temperature and the slowing down of the decay behavior are indeed improved.

High-rate discharge tests were first performed at 0.12 mA. After cm ⁺² (0.1C-rate) is charged and discharged for five cycles, 0.60 mA is performed in sequence. Cm ^-2 (0.5C-rate), 1.20mA. Cm ^-2 (1.0C-rate), 2.40mA. Cm ^-2 (2.0C-rate), 3.60mA. Cm ^-2 (3.0C-rate), 6.00mA. The discharge performance test was performed for five turns of cm ^-2 (5.0C-rate). The charge cutoff voltage of the experiment was 4.2V, and the discharge cutoff voltage was 2.5V. In order to obtain the effect of the formulation content on the system at different discharge rates, the calculation of the percentage of the capacity of the high rate discharge (2.0C, 3.0C, 5.0C) compared to the low speed (0.5C) is defined. Such as: high rate capacity percentage (%) = high speed discharge capacity (2.0C, 3.0C, 5.0C) / low speed discharge capacity (0.5C) × 100%, the results are shown in Table 5.

The results show that in the interval of the application examples (2) to (17), the content of EC is not additive to the discharge at a higher rate. As the discharge rate increases, it can be found that the content of the solvent DMC has a significant effect on the system. Therefore, it is speculated that the effect of high-rate discharge characteristics is that if the content of EC and N1222-TFSI is increased and the solvent DMC is insufficient, the viscosity of the electrolyte will increase. This makes ion transport difficult. The high-rate discharge test results are shown in Fig. 16. As the discharge rate increases, the application rate in the application examples (2) to (17) is similar to the high-rate discharge performance of the comparative example (1). It indicates that the planned application range varies with the formulation ratio, and the high-rate discharge performance is also improved to a considerable extent. The formulation of the application example is used to improve the conductivity caused by the addition of ionic liquid ions ( The viscosity is too high, the molecular group is too large, etc.), the effect of high-rate discharge is enhanced, and the high-rate discharge performance similar to that of Comparative Example (1) can be maintained.

The low temperature discharge performance is 0.12 mA at room temperature. The cm ^-2 (0.1C-rate) current was charged and discharged for three cycles, and then subjected to a low-temperature discharge test. First with 0.12mA. Cm ^-2 (0.1C-rate) charging, 1.20mA. Cm ^-2 (1.0C-rate) discharge current discharge observation of its discharge characteristics at room temperature, and then 0.12mA. After the current of cm ^-2 (0.1C-rate) is fully charged, put the battery into the refrigerator with constant temperature control temperature for 30 minutes, to 1.20mA. The discharge current rate of cm ^-2 (1.0C-rate) is discharged in a low temperature environment to a set cutoff voltage of 2.5V. Then remove the battery and leave it at room temperature for 30 minutes, then at 1.20 mA. The cm ^-2 (1.0C-rate) discharge is used to observe the capacitance remaining in the battery. In order to observe whether the battery affects the capacitance at low temperature, after each low temperature discharge test, it is then 0.12 mA at normal temperature. Cm ^-2 (0.1C-rate) charging, 1.20mA. Cm ^-2 (1.0C-rate) rate of discharge and discharge, that is, the battery performance test of a low temperature parameter is completed, the low temperature parameter is 0 ° C, -10 ° C, the charge cutoff voltage of the experiment is 4.2 V, discharge cutoff voltage It is 2.5V. The discharge test results, as shown in Fig. 17, the application examples (2) to (17) of the plan greatly improved the behavior of the ionic liquid at a low temperature to affect the discharge performance, and compared with the comparative example (1) at 0. The discharge characteristics of °C and -10 °C have a performance that is significantly closer to that of Comparative Example (1). The percentage of discharge capacity of each formulation electrolyte at different temperatures is defined as: low temperature discharge capacity percentage = discharge capacity _{(low temperature 0 ° C, -10 ° C)} / charge capacity _{(normal temperature 25 ° C)} × 100%, the result is Table 6 shows.

The results show that in the low temperature environment, the content of DMC has a significant effect on the discharge performance at -10 ° C, indicating that the formulation of the example requires sufficient solvent and must have a certain ratio with the EC to make N1222-TFSI is effectively dissociated in the electrolyte, reducing its crystallization behavior at low temperature, indicating that the electrolyte formulation of the embodiment is applied to a lithium ion battery, which can effectively improve the shortcomings of poor ionic liquid low temperature characteristics, and more A wider range of applications for a wider temperature range, high discharge rate performance requirements, and the like.

Charge/Discharge Test 3: It has been known from the foregoing test that the range of the examples taken has good performance, and Comparative Example (1) and Application Example (18) are taken to verify that Application Example (11) is added to the additive VC 1 wt%. The cycle life test was carried out at a high temperature of 60 ° C, and the results are shown in Fig. 18. The stability of the application example (18) at a high temperature of 60 ° C cycle life is significantly better than that of the comparative example (1), and the application example (18) is one of the formulation examples. After adding the additive, the chemical stability can be maintained and the additive can be maintained. Optimization.

Flame resistance test: To further compare the safety of the example formulation with the comparative example, take the comparative example (1) and the application example (2) (3) (4) for the flame resistance test, as shown in the nineteenth (a) chart. The comparative example (1) is shown, and there is a significant spontaneous combustion phenomenon at the same time as the ignition is burned, and when the fire source is removed, the comparative example (1) itself continues to burn. The combustion results of the application examples (2) and (3) (4) are as shown in the nineteenth (b) (c) (d) diagram. After the fire source is removed, no spontaneous combustion continues to occur, indicating that the implementation has been carried out. In the formulation range, since the carbonate-based electrolyte containing N1222-TFSI has the function of suppressing the combustion of the carbonate solvent after ignition, the flammability of the electrolyte can be greatly improved, and the safety of the lithium ion battery can be improved.

Thermogravimetric analysis: Comparing the thermal stability of the more precise examples with the comparative examples, the thermogravimetric analysis was carried out by comparing the comparative example (1) with the application examples (2) (3) (4), and the TGA results are as shown in the twentieth figure. . Obviously, Comparative Example (1), due to its high volatility, began to volatilize continuously at normal temperature, and began to lose weight quickly at 50 ° C, and burned out after 180 ° C. However, in the formulation electrolytes of the application examples (2), (3) and (4), due to the addition of N1222-TFSI, the electrolyte has a range of weight loss hysteresis, from about 50 ° C to 200 ° C, and from 200 ° C to 400 ° C. The platform range is the contribution of N1222-TFSI. The TGA data is differentiated from time to obtain a differential thermogravimetric curve (DTG). As shown in the twenty-first figure, it can be more clearly seen that the application example (2) (3) (4) is compared with the comparative example (1). The thermal stability is improved a lot, and the results corroborate the aforementioned flame resistance test results, demonstrating the application examples (2)(3)(4) of the mixed N1222-TFSI, which have improved thermal stability of the carbonate electrolyte at high temperatures. efficacy. It also indicates that the selected formulation range has the effect of improving the thermal stability of the electrolyte system, and observing the ratio of the thermal weight loss, it can be seen that the embodiment can reduce the generation of gas, and the electrolyte formula proposed by the invention is matched with the lithium ion battery. Can be more widely applied to systems with high security requirements.

Obviously, many modifications and differences may be made to the invention in light of the above description. It is therefore to be understood that within the scope of the appended claims, the invention may be The above are only the preferred embodiments of the present invention, and are not intended to limit the scope of the present invention; all other equivalent changes or modifications which are not departing from the spirit of the present invention should be included in the following. Within the scope of the patent application.

10‧‧‧Lithium ion battery electrolyte composition

Claims (9)

An electrolyte composition for a lithium ion battery, the electrolyte composition for a lithium ion battery comprising: an imine ion liquid, the weight percentage of the imine ion liquid to the electrolyte is 24 to 50 wt%, wherein The composition formula of the content of the imine ionic liquid is (I) Wherein R ¹ , R ² , R ³ and R ^{4 above} may be selected from one or a combination of the following: hydrogen alkyl, heterocyclic, linear alkyl, branched alkyl; Dimethyl Carbonate The weight percentage of the dimethyl carbonate to the electrolyte is 24 to 50% by weight; the ethylene carbonate (Ethylene Carbonate), the weight percentage of the ethylene carbonate to the electrolyte is 16 to 33% by weight; and the lithium hexafluorophosphate ( LiPF ₆ ), the weight percentage of the lithium hexafluorophosphate to the electrolyte is 6 to 15 wt%; and the lithium bis trifluoromethylsulfonimide (LiTFSI), the weight of the lithium bistrifluoromethylsulfonimide The percentage is 4~12wt%. The electrolyte composition for a lithium ion battery according to claim 1, wherein the imine ion liquid is more preferably methyltriethyl bistrifluoromethane sulfonimide ionic liquid (triethylmethylammonium). Bis(trifluoromethylsulfonyl)amide), the composition of the content of the methyltriethyl bistrifluoromethanesulfonimide ionic liquid is (II): The electrolyte composition for a lithium ion battery according to claim 1, wherein the weight ratio of the imine ion liquid to the electrolyte further comprises 27 to 37 wt%. The electrolyte composition for a lithium ion battery according to claim 1, wherein the dimethyl carbonate to the electrolyte further comprises 27 to 37% by weight. The electrolyte composition for a lithium ion battery according to claim 1, wherein the ethylene carbonate to the electrolyte further comprises 11 to 21% by weight. The electrolyte composition for a lithium ion battery according to claim 1, wherein the above-mentioned R ¹ , R ² , R ³ and R ^{4 each} have a carbon number of C1 to C4. The electrolyte composition for a lithium ion battery according to claim 1, wherein the hexafluorophosphorus lithium further comprises 6 to 11 wt% by weight of the electrolyte. The electrolyte composition for a lithium ion battery according to claim 1, wherein the above-mentioned lithium bistrifluoromethylsulfonimide further comprises 4 to 7 wt% by weight of the electrolyte. The electrolyte composition for a lithium ion battery according to claim 1, wherein the vinylene carbonate (Vinylene) further comprises 0.1 to 5 wt% of the electrolyte.