1. Field of the Invention
This invention relates to a lithium salt, and more particularly, to a lithium salt used for a lithium battery, and an electrolyte solution and a lithium battery containing said lithium salt.
2. Description of Related Art
In recent years, due to the uses of lithium batteries in the new energy vehicles and energy storage systems, the market share of the lithium batteries increases from 2.5% to 20%. As forecasted by Zpryme Research & Consulting, LLC (US), the market size of the global intelligent network is projected to reach 171.4 billion US dollars by 2014. In the markets for terminal utilization, the latest Pike Research report also indicates that there will be 32,000 vehicles driven by alternative energy for the global bus markets in 2015. It will gradually drive the business expansion to increase the global demands of lithium batteries dramatically. This represents that the market for the electric drive vehicles and power batteries is approaching. Therefore, it is increasingly important to consider the design for the new thermostable lithium-ion batteries.
The so-called secondary lithium batteries are referred to the utilization of lithium ion in anode and cathode materials as circulated rechargeable and dischargeable batteries. Typically, graphite materials (such as maso carbon micro board, MCMB) are still employed as the anode materials in the commercially available secondary lithium batteries. In the initial charge-discharge cycle, the surface of graphite reacts with the electrolyte to form the passive protective layer (such as solid electrolyte interface, SEI) in the anode. Such passive protective layer is provided to prevent the collapse of the surface of the anode material and the degradation of the electrolyte, so as to stabilize the charge-discharge cycle of the batteries. This passive protective layer has a critical impact on the battery life. However, as for the batteries in high temperature environment for a long period of time, the lithium salt (typically lithium hexafluorophosphate, LiPF6) of the electrolyte solution within the batteries can be easily degraded to strong Lewis acids, PF5− and HF, and they thereby destroy the structure of the electrode materials and the properties of the passive protective layer. Therefore, the battery's performance will be decayed along with the increasing temperature.
Typically, the commercially available electrolyte solution containing lithium hexafluorophosphate has high capacities and low costs, but its chemical structure is easily degradable at high temperature causing the battery expansion and performance deterioration, that affect the practical uses of the lithium batteries in electric drive vehicles. Most of the proposed solutions are as follows: using other electrode materials without the formation of the passive protective layer, adding different kinds of additives to the electrolyte solution to improve the properties of the passive protective layer, or modifying the surface of the particle prior to the preparation of the cathode/anode for preventing attacks. However, all these methods make the preparation steps of the battery more elaborate and complicated.
Accordingly, in order to overcome the above-mentioned problems of the lithium battery and the electrolyte solution, there are demands for the improvement in heat resistance and conductivity of the lithium salt.
The present invention provides a lithium salt comprising a lithium ion and an anion represented by formula (I),
wherein R1 to R5 are independently selected from the group consisting of hydrogen atom, cyano group, fluorine atom, and C1-C5 alkyl group, in which the C1-C5 alkyl group is substituted with at least one fluorine atom.
In one embodiment, R2 to R5 of the lithium ion are cyano groups and R1 is —C2H4CF3.
In another embodiment, R2 to R5 of the lithium ion are cyano groups and R1 is —CF3.
The present invention further provides an electrolyte solution, which comprises an organic solvent and the lithium salt of the present invention.
The present invention also provides a lithium battery comprising the lithium salt of the present invention.
The electrolyte solution comprising the thermostable lithium salt of the present invention can provide good ionic conductivities and very positive results for the cycle life of the batteries at high temperatures. Thus, it can be effectively applied in the operation environment of the engines for electric drive vehicles.
The following specific examples are used for illustrating the technical content and embodiments of the present invention in details. A person skilled in the art can easily conceive the advantages and effects of the present invention based on the disclosure. This invention can also be implemented or applied through other different embodiments. While some of the embodiments of the present invention have been described in detail, it is, however, possible for those of ordinary skilled in the art to make various modifications and changes to the particular embodiments shown without substantially departing from the teaching and advantages of the present invention.
It should be understood that all structures, ratios, sizes and the like included in the drawings are merely illustrative to demonstrate the disclosure of the specification to aid the understanding and reading for those of ordinary skilled in the art without providing substantial technical meanings, and not intended to limit the scope of the present invention. Any modifications of the structures, such as the changes of ratio relationships and size adjustments, should be covered under the scopes of the technical content of the present invention, as long as they do not affect the produced effects and achievable goals of the present invention. Further, the terms of “upper”, “lower”, “top”, “bottom”, “one” and more are merely for illustrative purpose and should not be construed to limit the scope of the present invention. The changes and adjustments of the relative relationships should be covered under the implementation scope of the present invention without changing technical content substantially.
The present invention provides a lithium salt comprising a lithium ion and an anion represented by formula (I),
wherein R1 to R5 are independently selected from the group consisting of hydrogen atom, cyano group, fluorine atom and C1-C5 alkyl group, in which the C1-C5 alkyl group is substituted with at least one fluorine atom. In one embodiment, R1 is fluoro or C1-C3 perfluoroalkyl group; R2 and R3 are independently selected from fluorine atom and cyano group. The C1-C5 alkyl group substituted with at least one fluorine atom can be partially substituted as —CH2F, —C2H4CF3, —C3H6C2F5, —C1H2C2F5, —C2F4CH3, or the like, or also can be C1-C5 perfluoroalkyl group.
The term “perfluoroalkyl group” used herein refers to the alkyl group, in which all hydrogen atoms in the carbon chains are substituted with fluorine atoms, such as —CF3, —C2F5, —C3F7 or C5F11 and the like.
In one embodiment, each of R2 to R5 of the lithium salt is cyano group and R1 is —C2H4CF3.
In another embodiment, each of R2 to R5 of the lithium salt is cyano group and R1 is —CF3.
In one embodiment, an electrolyte solution comprising organic solvents and the lithium salt of the present invention is provided.
The electrolyte solutions may be, but not limited to, γ-butyrolactone (GBL), ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), propyl acetate (PA), dimethyl carbonate (DMC) or ethylmethyl carbonate (EMC).
In addition, besides using the lithium salt of the present invention, the electrolyte solution of the present invention can also be mixed with other lithium salts, for example, LiPF6, LiBF4, LiAsF6, LiSbF6, LiClO4, LiAlCl4, LiGaCl4, LiNO3, LiC(SO2CF3)3, LiN(SO2CF3)2, LiSCN, LiO3SCF2CF3, LiC6F5SO3, LiO2CCF3, LiSO3F, LiB(C6H5)4, or LiCF3SO3.
The electrolyte solution of the present invention can be used in a lithium battery and the structure of the lithium battery is illustrated in
In a preferred embodiment, the anode 10 comprises a first conductive element 110 and an anode metal foil 120 which is formed on the first conductive element 110, so that the first conductive element 110 is interposed between the separation membrane 30 and the anode metal foil 120. The separation membrane 30 is disposed on the first conductive element 110 and has a through opening 300 for partially exposing the first conductive element 110.
The cathode 20 comprises a second conductive element 210 and a cathode metal foil 220 formed on the second conductive element 210, so that the second conductive element 210 is interposed between the separation membrane 30 and the cathode metal foil 220, and an accommodating space 40 is formed by the separation membrane 30, the first conductive element 110 and the second conductive element 210 for accommodating the electrolyte solution.
In one example, the lithium battery further comprises an encapsulating structure 50 (like encapsulated plastic) for encapsulating the anode 10, the cathode 20 and the separation membrane 30.
In the lithium battery of the present invention, the material of the first conductive element 110 may be lithium or carbide, wherein the carbide is at least one selected from the group consisting of carbon powder, graphite, carbon fiber, carbon nanotube and graphene. In an embodiment, the carbide is carbon powder having the average particle diameter of 100 nm to 30 μm.
In the lithium battery of the present invention, the second conductive element can be a transition metal oxide mixed with lithium, wherein the transition metal oxide mixed with lithium can be at least one selected from the group consisting of LiMnO2, LiMn2O4, LiCoO2, Li2Cr2O7, Li2CrO4, LiNiO2, LiFeO2, LiNixCo1-xO2, LiFePO4, LiMn0.5Ni0.5O2, LiMn1/3Co1/3Ni1/3O2 and LiMc0.5Mn1.5O4, wherein 0<x<1 and Mc is the divalent 3d transition metal.
The anode metal foil 120 and the cathode metal foil 220 can be common metal foils, such as copper foil, aluminum foil, nickel foil, silver foil, gold foil, platinum foil and stainless steel sheet.
The lithium battery of the present invention can further comprise a binder (not shown in the drawings) for adhering the anode metal foil and the first conductive element and for adhering the cathode metal foil and the second conductive element, wherein the binder may be polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), polyamide or melamine resin.
The separation membrane 30 can be the insulating material which is selected from polyethylene (PE), polypropylene (PP) and a combination thereof. The insulating material can be a multilayer structure, such as composite multilayer structure of PE-PP-PE.
In order to improve the cycle life of the lithium battery at high temperature, the present invention utilizes the chemical synthesis method to synthesize the specific functional groups (such as —CH3, —F, —CN, —CF3 and the like) in the main structure of the benzimidazole molecules. The lithium salt which is formed by this anion group is used in the electrolyte solution of the commercial lithium battery. The ionic conductivities of the electrolyte mixture are determined at different temperatures.
The following examples are described in details to illustrate the above and other goals, features and advantages of the present invention.
1 mole of 3,4,5,6-4-cyano-nitroaniline as the precursor was dissolved in 50 g of ethanol and added with an excess (approximately 1.1 mole) of hydrazine (N2H4). The reaction of the mixture was performed at 80° C. for 3 to 4 hours. After completing the reaction, the residual and unreacted solid impurities in the solution were filtered and removed, and then the solvent was partially (about 90% of the quantity) removed by using a rotating evaporator. The remaining 10% of solution was recrystallized at 4 to 5° C.
Light brown crystals appeared in the glass vial after overnight incubation. The crystals were taken out and determined the melting point as 104 to 106° C. using a differential scanning calorimeter (DSC). The compound structures of the crystals were also determined as 3,4,5,6-4-cyano-diamine benzene by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR).
0.05 mole of the above 3,4,5,6-4-cyano-diamine benzene and 0.01 mole of 4,4,4-trifluorobutyric acid were dissolved in 50 g of ethanol to form a mixture, and the mixture was proceeded to the azeotropic reaction for 3 hours (100° C.). After completion of the reaction, the mixed solution was cooled to room temperature. The reaction scheme is shown as follows.
Subsequently, the pH of the mixed solution was adjusted to the range of 7 to 8 by sodium hydroxide solution (20% wt). Activated carbon was added to the pH-adjusted solution, and then the solution was heated to 100° C. to react for 45 minutes. The solution was filtered and cooled to room temperature, and then left overnight at 4 to 5° C. Yellow crystals were formed on the wall of the vial. The structures of the crystals at the second stage were determined and confirmed as 4-cyano-2-trifluorobutyl benzimidazole using FTIR and NMR, while the melting point test showed the interval of 150 to 152° C. According to the infrared spectrum shown in
Subsequently, the reaction shown in the following reaction scheme was carried out using Li2CO3.
1 mole of 4-cyano-2-trifluorobutyl benzimidazole and 0.5 mole of Li2CO3 were dissolved in 100 g of ethanol to form a mixed solution and stirred evenly at room temperature for 4 hours. After filtering the solid impurities and concentrating, light yellow lithium crystals were obtained.
According to the infrared spectrum shown in
2 parts by volume of EC, 3 parts by volume of PC and 5 parts by volume of DEC were mixed as the organic solvents of the electrolyte solution. 1M of electrolyte solution B was prepared by using the lithium salt of this example.
75.3 mmol of benzimidazole and 50 mL of toluene were added to a reactor, mixed and stirred at circulation of high purity nitrogen and ice bath for about 1 hour, and then 74.2 mmol of n-butyl lithium were added dropwise to the reactor using quantitative buret. The mixture was stirred at ice bath for 3 to 4 hours until white smoke and exothermic phenomenon disappeared completely, rinsed three times with toluene, filtered and dried, and a light yellow lithium salt having the following formula (II) was obtained. The completion of the replacement of lithium ions was confirmed by NMR spectrum. As shown in
The above synthesized lithium salt was dissolved in the electrolyte solution which was prepared by 10 parts by weight of BF3((C2H5)2O) and 90 parts by weight of mixed solvent (the solvent contained 2 parts by volume of EC, 3 parts by volume of PC and 5 parts by volume of DEC) to form 1M of electrolyte solution C.
The commercially available lithium hexafluorophosphate (UNION CHEMICAL IND. CO., LTD.) was purchased as a lithium salt and used to prepare the electrolyte solution A in the same manner as the preparation of solvent in Example 1 with same composition and ratio.
At room temperature, 50° C., 70° C. and 90° C., the impedance variations of the electrolyte solutions in Example 1 and Comparative Example 1 were determined at a constant voltage (5 mV) by an AC impedance spectroscopy (Biologic), and the conductivity (σ) value and the activation energy (Ea) were also calculated.
The conductivity (σ) is calculated by the following equation:
wherein L is the distance between two electrodes, A is the area of the electrodes and R is the impedance value obtained by the AC impedance spectroscopy.
The prepared electrolyte solution of Example 1 changed from transparent to transparent light yellow, and this represented that the lithium salt was completely dissolved in the electrolyte solution without precipitation. Similarly, the lithium salt was completely dissolved in the electrolyte solution of Example 2.
In the conductivity test,
The above-mentioned embodiments are described to illustrate the principles and effects of the present invention, and they do not impose any limitations to the present invention. It is, however, possible for those of ordinary skills in the art to make modifications to the above-mentioned embodiments without substantially departing from the teaching and advantages of the present invention. Such modifications and changes are encompassed in the spirit and scope of the present invention as set forth in the appended claim.
Number | Date | Country | Kind |
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101140673 | Nov 2012 | TW | national |