Lithium battery low-temperature electrolyte and lithium ion battery

Abstract

The invention relates to a lithium battery low-temperature electrolyte and a lithium ion battery, and belongs to the technical field of lithium batteries. Comprising the following steps: reducing the mass concentration (llt of conventional lithium salt by introducing a cosolvent; 13.5%), a synergistic positive / negative electrode film-forming additive, a low-impedance lithium salt additive and a functional additive. According to the invention, common low-melting-point and low-viscosity carboxylic ester is selected as a cosolvent, and the concentration of the lithium salt is reduced, so that the viscosity of the electrolyte at a low temperature is reduced, and the ionic conductivity is improved; a stable low-impedance positive / negative electrode interface film is constructed by cooperating with conventional positive / negative electrode film-forming additives such as vinylene carbonate and fluoroethylene carbonate, and the solvation structure of the electrolyte is regulated and controlled by introducing lithium salt additive anions, so that the desolvation process of lithium ions at low temperature is promoted, the low-temperature performance of the lithium battery is improved, and the service life of the lithium battery is prolonged. And the long cycle life of the electrolyte at normal temperature can also be ensured.

Description

Technical Field

[0001] The present invention relates to a low-temperature electrolyte for lithium batteries and a lithium-ion battery, belonging to the technical field of lithium batteries. Background Art

[0002] In recent years, research reports on the low-temperature application of lithium batteries have also increased exponentially. A large number of studies have shown that the performance of lithium batteries decays significantly under low-temperature conditions, such as severe capacity decay, shortened cycle life, decreased rate performance, and reduced safety performance. The main reason is that low temperature makes the transport and diffusion processes of lithium ions inside the electrode material, in the electrolyte, and at the interface between the two very difficult. Researchers have tried to improve the low-temperature performance of batteries by discovering new electrode materials or improving the structure of electrode materials to enhance the transport of lithium ions inside the electrode material under low-temperature conditions. However, these methods usually require complex and expensive preparation methods to achieve, which is not conducive to large-scale production.

[0003] Compared with electrode materials, optimizing the electrolyte formulation to improve the low-temperature performance of lithium batteries is a cheaper, feasible, and more conducive to large-scale commercial application method. In addition, the composition of the electrolyte determines the diffusion rate of lithium ions in the electrolyte, the solvation structure of lithium ions, and affects the composition and structure of the solid electrolyte interface layer (SEI). Therefore, the battery industry has long believed that the electrolyte has a great impact on the low-temperature performance of lithium batteries. Therefore, the application of lithium batteries at low temperatures is largely restricted by the development of electrolytes. By regulating the electrolyte, broadening the operating temperature range of the electrolyte, increasing the diffusion rate of lithium ions at low temperature, reducing the energy barrier of lithium ion desolvation, constructing a stable and low-impedance solid electrolyte interface, and reducing lithium metal deposition, so as to improve the performance of lithium batteries at low temperature, which has important research significance for promoting the application field of lithium batteries and further expanding the lithium battery market.

[0004] Existing commercial lithium battery electrolytes mainly use carbonate solvents and usually contain a certain amount of ethylene carbonate (EC). As an important component in lithium battery electrolytes, EC is a solid at room temperature and easily causes an increase in the viscosity of the electrolyte at low temperature, resulting in problems such as low ionic conductivity of the electrolyte, seriously affecting the capacity of lithium batteries at low temperature. Currently, most of the research on low-temperature electrolyte formulations is to add low-melting-point and low-viscosity co-solvents to reduce the viscosity of the electrolyte at low temperature and increase the ionic conductivity, such as fluorinated carbonate solvents. However, most of the current fluorinated carbonate solvents have not been commercially produced due to complex production processes and difficult purification, and their high usage costs limit their application in low-temperature electrolytes. In addition, although new electrolyte formulation strategies such as the weak solvation strategy can improve the low-temperature performance of the electrolyte by regulating the solvation structure of the electrolyte and promoting the desolvation process of lithium ions at low temperature, this strategy still requires the use of a large amount of fluorinated ether solvents as diluents, so it is also limited in its commercial application in terms of usage costs. Summary of the Invention

[0005] By selecting common low-melting-point and low-viscosity carboxylic acid esters as co-solvents and reducing the lithium salt concentration, the present invention reduces the viscosity of the electrolyte at low temperatures and improves the ionic conductivity. Low-impedance lithium salt additives such as lithium tetrafluoroborate, lithium difluorooxalate borate, or lithium difluorophosphate are introduced, and conventional positive / negative electrode film-forming additives such as vinylene carbonate and fluoroethylene carbonate are used to construct a stable low-impedance positive / negative electrode interface film. In addition, the introduction of the anions of the lithium salt additives regulates the solvation structure of the electrolyte, promotes the desolvation process of lithium ions at low temperatures, and realizes the improvement of the low-temperature performance of lithium batteries while ensuring the long cycle life of the electrolyte at room temperature.

[0006] The specific technical solution is as follows: A low-temperature electrolyte for lithium batteries, comprising a co-solvent, a lithium salt with a mass concentration < 13.5%, positive / negative electrode film-forming additives, and low-impedance lithium salt additives; by using low-melting-point and low-viscosity carboxylic acid esters as co-solvents and reducing the lithium salt concentration, the viscosity of the electrolyte at low temperatures is reduced and the ionic conductivity is improved. Low-impedance lithium salt additives are introduced, and conventional positive / negative electrode film-forming additives are used to construct a stable low-impedance positive / negative electrode interface film. In addition, the introduction of the anions of the lithium salt additives regulates the solvation structure of the electrolyte, promotes the desolvation process of lithium ions at low temperatures, and realizes the improvement of the low-temperature performance of lithium batteries.

[0007] Furthermore, it further comprises functional additives; adding functional additives ensures the long cycle life of the electrolyte at room temperature and inhibits gas generation at high temperatures.

[0008] Preferably: The co-solvent includes any one or more of the following: ethylene carbonate, ethyl methyl carbonate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, isobutyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate.

[0009] The positive / negative electrode film-forming additives include: vinylene carbonate and fluoroethylene carbonate; with a mass concentration of 0.01 - 10.0%.

[0010] The low-impedance lithium salt additives include any one or more of the following: lithium tetrafluoroborate, lithium difluorooxalate borate, or lithium difluorophosphate; with a mass concentration of 0.01 - 5.0%.

[0011] The functional additives include any one or more of the following: 1,3-propane sultone (PS), ethylene sulfate (DTD), and fluorodimethylsilylbutyronitrile; with a mass concentration of 0.01 - 5.0%.

[0012] The lithium salt is LiPF 6 、LiAsF 6 、LiN(SO 2 CF3 ) 2 、LiN(SO 2 C 2 F 5 ) 2 、LiC(SO 2 C 2 F 3 ) 2 、LiC(SO 2 C 2 F 5 ) 2 and LiN(SO 2 F) 2 One or more of .

[0013] The present invention also provides a lithium-ion battery, which includes a positive electrode sheet and a negative electrode sheet, and a separator and an electrolyte disposed between the positive electrode sheet and the negative electrode sheet, wherein the electrolyte is the low-temperature electrolyte provided by the present invention. The active material of the positive electrode is LiNi x Co y Mn z L (1-x-y-z) O 2 , wherein L is Al, Sr, Mg, Ti, Ca, Zr, Zn, Si or Fe, 0≤x≤1, 0≤y≤1, 0≤z≤1, 0≤x+y+z≤1. The active material of the positive electrode is LiCo x L (1-x) O 2 , wherein L is Al, Sr, Mg, Ti, Ca, Zr, Zn, Si or Fe, 0 <x≤1。所述负极的活性物质为人造石墨、天然石墨或SiO k Silicon-carbon composite material compounded with graphite, k≤2. These materials are applied to the present invention alone or in combination, and graphite (Gr) is preferred in the present invention.

[0014] In a specific embodiment of the present invention, the positive electrode of the lithium-ion battery is composed of different usage ratios of nickel, cobalt and manganese materials, including NMC111, NMC523, NMC622 and NMC811, etc., and the 811 type is used in the specific embodiment of the present invention; the negative electrode is a button battery with graphite (Gr) and a diaphragm of Celgard2325. DETAILED DESCRIPTION

[0015] In this embodiment, common low-melting-point and low-viscosity carboxylic acid esters are selected as co-solvents and a method of reducing the lithium salt concentration (below 13.5%) is adopted to reduce the viscosity of the electrolyte at low temperatures and improve the ionic conductivity. Low-impedance lithium salt additives such as lithium tetrafluoroborate (LiBF4), lithium difluoro(oxalato)borate (LiDFOB), or lithium difluorophosphate (LiDFP) are introduced, and conventional positive / negative electrode film-forming additives such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are used to construct a stable low-impedance positive / negative electrode interface film. In addition, the introduction of the anions of the lithium salt additives is used to regulate the solvation structure of the electrolyte solvent, promote the desolvation process of lithium ions at low temperatures, and achieve the improvement of the low-temperature performance of lithium batteries. At the same time, a small amount of functional additives such as 1,3-propane sultone (PS), ethylene sulfate (DTD), and fluorodimethylsilylbutyronitrile (FDSBN) are added to ensure the long cycle life of the electrolyte at room temperature and inhibit gas generation at high temperatures. The physical parameters of some solvents are shown in Table 1.

[0016] Table 1 Parameters of Some Low-Melting-Point and Low-Viscosity Carboxylic Acid Ester Co-Solvents

[0017] 1. Preparation of Electrolyte The electrolyte formulation design is shown in Table 2, and the percentages in parentheses are all mass fractions. The electrolyte is prepared according to the designed ratio. In a glove box filled with argon (water content ≤ 0.1 ppm, oxygen content ≤ 0.1 ppm), first, according to Table 2, electrolyte solvents 1-3 are mixed according to the designed mass ratio. After mixing evenly, lithium hexafluorophosphate (LiPF 6 ) with the designed mass fraction is added. Then, according to the formulation design, additives 1-4 with the required mass fractions are added respectively, and the mixture is stirred at room temperature for 2 hours to obtain the required electrolyte.

[0018] Additives 1-4 are the positive / negative electrode film-forming additive, low-impedance lithium salt additive, and functional additive in sequence.

[0019] Taking Example 1 as an example, the specific preparation method is as follows: In a glove box filled with argon (water content ≤ 0.1 ppm, oxygen content ≤ 0.1 ppm), ethylene carbonate (EC), ethyl methyl carbonate (EMC), and ethyl acetate (EA) are added to a container according to the mass fraction ratio of 8.8% : 35.2% : 44.0%. The container containing the electrolyte is placed on a stirrer, and the rotation speed is set to 500 r / min. Then, 9.0% of the lithium salt lithium hexafluorophosphate (LiPF 6After stirring until completely dissolved and becoming clear and transparent, add additives of vinylene carbonate (VC) accounting for 1.0% of the total mass of the electrolyte, fluoroethylene carbonate (FEC) of 1.0%, and lithium difluorophosphate (LiDFP) of 1.0% to the container respectively, and stir at room temperature for 2 hours.

[0020] Repeat the above steps to prepare the electrolytes required in Examples 2-25 and Comparative Examples 1-3.

[0021] Table 2 Formulation table of low-temperature electrolytes for each example

[0022] 2. Coin cell electrochemical test Coin cell electrochemical performance: Add the prepared various lithium-ion battery electrolytes to coin cells with a ternary material (NMC811) as the positive electrode, graphite (Gr) as the negative electrode, and Celgard 2325 as the separator. The rated capacity of the battery is about 2.5 mAh, and the low-temperature performance and room-temperature cycle performance of the battery are tested. Low-temperature capacity retention rate test: Place the battery in a constant-temperature oven at 30°C and first charge and discharge at 0.1C at room temperature for 2 cycles, charge and discharge at 1C at room temperature for two cycles, with a voltage range of (2.4V - 4.2V); then fully charge the battery to 4.2V at 1C at room temperature, place it in a -40°C constant-temperature oven and let it stand overnight, then discharge to 2.4V at 0.2C, charge and discharge at 0.2C for 3 cycles, and charge and discharge at 0.5C for 3 cycles; Room-temperature cycle test: Place the battery in a constant-temperature oven at 30°C, charge it at a constant current and constant voltage to 4.2V at a current of 0.1C, with a cut-off current of 0.05C, then discharge it at a current of 0.1C to 3.0V, and cycle for 2 cycles. Starting from the 3rd cycle, charge it at a constant current and constant voltage to 4.2V at a current of 1.0C, discharge it at a constant current of 1.0C to 3.0V, and cycle like this until 202 cycles. Take the discharge specific capacity of the 3rd cycle as the starting discharge specific capacity, and calculate the capacity retention rate as follows.

[0023] Low-temperature 0.2C capacity retention rate (%) = (0.2C 3rd cycle discharge specific capacity at -40°C / Room-temperature 1C 2nd cycle discharge specific capacity) × 100% Low-temperature 0.5C capacity retention rate (%) = (0.5C 3rd cycle discharge specific capacity at -40°C / Room-temperature 1C 2nd cycle discharge specific capacity) × 100% Room-temperature nth cycle capacity retention rate (%) = (nth cycle discharge specific capacity / 3rd cycle discharge specific capacity) × 100% Table 3 Capacity retention rate at low temperature and room-temperature cycle capacity retention rate of each electrolyte formulation

[0024] From the results of Comparative Examples 1-2 and Examples 1-12 in Table 3, it can be seen that using a low-melting-point and low-viscosity carboxylic acid ester as a co-solvent and reducing the lithium salt concentration in the electrolyte can effectively improve the capacity retention rate of the electrolyte at low temperatures and can also improve the cycle life of the lithium battery at room temperature; from Comparative Examples 1-3 and Examples 1-25, it can be seen that low-impedance lithium salt additives such as LiDFP, LiBF 4 and LiDFOB can effectively improve the low-temperature capacity retention rate of the lithium battery. Combining the use of various low-melting-point and low-viscosity carboxylic acid ester solvents can further improve the low-temperature performance, and most of them have no negative impact on the room-temperature cycle performance of the battery, and even improve its room-temperature cycle life; Examples 9 and 11-12 prove that adjusting the proportion of each solvent, such as reducing the use proportion of the carboxylic acid ester solvent, has little effect on the low-temperature performance of the battery and has a certain impact on the room-temperature cycle life; Examples 9 and 13-14 prove that further reducing the lithium salt concentration cannot continue to significantly improve the low-temperature performance, and increasing the lithium salt concentration is beneficial to the improvement of the room-temperature cycle life; Examples 15-20 show that adjusting the dosage of the film-forming additives (VC and FEC) and the low-impedance lithium salt additives slightly reduces the low-temperature performance of the battery, while generally increasing the dosage of the additives can improve its room-temperature cycle life; Examples 21-22 illustrate that other low-impedance additives LiBF 4 and LiDFOB can also effectively improve the low-temperature performance of the battery, but the effect is not as good as LiDFP; while Examples 23-25 illustrate that adding other functional additives such as PS, DTD or FDSBN does not significantly affect the low-temperature performance of the battery.

Claims

1. A low-temperature electrolyte for lithium batteries, characterized in that: Including co-solvents, lithium salts with a mass concentration of <13.5%, positive / negative electrode film-forming additives, and low-impedance lithium salt additives.

2. The low-temperature electrolyte for lithium batteries according to claim 1, characterized in that: Also included are functional additives.

3. The low-temperature electrolyte for lithium batteries according to claim 1, characterized in that: The co-solvent includes any one or more of the following: ethylene carbonate, ethyl methyl carbonate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, isobutyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, and ethyl butyrate.

4. The low-temperature electrolyte for lithium batteries according to claim 1, characterized in that: The positive / negative electrode film-forming additives include: vinylene carbonate and fluoroethylene carbonate; the mass concentration is 0.01-10.0%; The low impedance lithium salt additive includes any one or more of the following: lithium tetrafluoroborate, lithium difluorooxalatoborate or lithium difluorophosphate; the mass concentration is 0.01-5.0%.

5. The low-temperature electrolyte for lithium batteries according to claim 2, characterized in that: The functional additives include any one or more of the following: 1,3-propane sultone PS, vinyl sulfate DTD and fluorodimethylsilyl butyronitrile FDSBN; the mass concentration is 0.01-5.0%.

6. The low-temperature electrolyte for lithium batteries according to claim 1, characterized in that: The lithium salt is one or more of LiPF6, LiAsF6, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiC(SO2C2F3)2, LiC(SO2C2F5)2 and LiN(SO2F)2.

7. A lithium ion battery, characterized in that: It comprises a positive electrode sheet and a negative electrode sheet, and a separator and an electrolyte arranged between the positive electrode sheet and the negative electrode sheet, wherein the electrolyte is a low-temperature electrolyte for a lithium battery according to any one of claims 1 to 6.

8. A lithium ion battery according to claim 7, characterized in that: The active material of the positive electrode is LiNi x Co y Mn z L (1-x-y-z) O2, where L is Al, Sr, Mg, Ti, Ca, Zr, Zn, Si or Fe, 0≤x≤1, 0≤y≤1, 0≤z≤1, 0≤x+y+z≤1; or the active material of the positive electrode is LiCo x L (1-x) O2, wherein L is Al, Sr, Mg, Ti, Ca, Zr, Zn, Si or Fe, 0 <x≤1; The active material of the negative electrode is artificial graphite, natural graphite or SiO k Silicon-carbon composite material compounded with graphite, k≤2.

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