A Wide Temperature Range and High Specific Energy Lithium Carbon Monofluoride Battery Electrolyte and Its Preparation Method

Abstract

The present invention discloses a wide-temperature-range and high-specific-energy lithium carbon monofluoride battery electrolyte and a preparation method thereof. The electrolyte is composed of a lithium salt and a solvent. The lithium salt is lithium bis(fluorosulfonyl)imide, and the solvent is composed of dimethyl sulfoxide and an organic solvent. The organic solvent is composed of an ester solvent and an ether solvent. Among them, the volume ratio of dimethyl sulfoxide to the organic solvent is 1:0.5-2, the volume ratio of the ester solvent to the ether solvent is 1:0.1-0.5, and the concentration of the lithium salt in the electrolyte is 0.5-5 mol / L. The preparation process is as follows: Add dimethyl sulfoxide, the ester solvent and the ether solvent into a container, and then add lithium bis(fluorosulfonyl)imide salt, and mix evenly to obtain the lithium carbon monofluoride battery electrolyte. The lithium carbon monofluoride battery electrolyte prepared by the present invention has excellent high and low temperature resistance while having good discharge performance, can significantly improve the voltage platform of the battery, as well as the discharge specific capacity and discharge specific energy of the battery, and is a wide-temperature-range and high-specific-energy battery electrolyte.

Description

Technical Field

[0001] The present invention relates to an electrolyte and a preparation method thereof, and particularly to a wide-temperature-range and high-specific-energy lithium-carbon fluoride battery electrolyte and a preparation method thereof. Background Art

[0002] Although lithium-carbon fluoride batteries have many excellent characteristics, currently, there are still three major problems with commonly used lithium-carbon fluoride batteries.

[0003] The first problem is with the carbon fluoride material itself. Although the carbon fluoride, which is the positive electrode material of the lithium-carbon fluoride battery itself, is a good high-energy battery material, the fluorination process is complex and requires high equipment requirements, resulting in a high cost for battery preparation. In addition, the carbon fluoride material has poor electrical conductivity, making the kinetic process during battery discharge relatively slow, and thus the battery often discharges at a low rate and performs poorly at high rates. At high rates, the internal polarization effect is obvious during the discharge of the lithium-carbon fluoride battery, resulting in a large amount of heat generation in the battery, and thus causing the battery to bulge, posing a certain safety hazard.

[0004] The second problem is the voltage problem during the operation of the lithium-carbon fluoride battery. In a lithium-carbon fluoride battery, due to the spontaneous chemical reaction between the negative electrode metal lithium and the electrolyte solution to form a passivation film, after the battery is started, the passivation film slows down the reaction rate between lithium and carbon fluoride, resulting in a voltage hysteresis phenomenon, and causing the operating voltage of the battery not to immediately reach the required operating state. In addition, the operating voltage of the battery is only about 2.5V, far lower than the open-circuit voltage of more than 3.0V, resulting in a low discharge voltage platform of the battery, and thus the specific capacity and specific energy of the battery cannot be further improved.

[0005] The third problem is the poor performance of the lithium-carbon fluoride battery at low temperatures. When the battery is used at a low temperature, the electronic conductivity of the positive electrode of the lithium-carbon fluoride battery becomes poor and the electrode reaction kinetics becomes slow, resulting in poor low-temperature discharge performance of the battery and low utilization rate of active substances, restricting the wide application of lithium-carbon fluoride batteries. In terms of the discharge current density, it can usually reach 0.5 - 1.0 mA / cm² at 20°C 2 , and drops to 0.1 mA / cm² at -20°C 2 Below, no load can be applied at -40°C, and the performance decays severely.

[0006] In summary, as an important component of the battery, the physical and chemical properties of the electrolyte play a crucial role in the overall performance of the battery, and traditional commercial lithium-carbon fluoride electrolytes have many problems such as a low voltage platform, poor low-temperature performance, and poor rate performance. Summary of the Invention

[0007] Objective of the Invention: To solve the technical problems existing in the prior art, the present invention aims to provide a lithium carbon monofluoride battery electrolyte with excellent high and low temperature resistance, excellent discharge performance, and a high discharge voltage platform. In addition, the present invention also provides a preparation method for the electrolyte.

[0008] Technical Solution: The wide temperature range and high specific energy lithium carbon monofluoride battery electrolyte of the present invention is composed of a lithium salt and a solvent. The lithium salt is lithium bis(fluorosulfonyl)imide (LiFSI), and the solvent is composed of dimethyl sulfoxide and an organic solvent. The organic solvent is composed of an ester solvent and an ether solvent.

[0009] Furthermore, the concentration of the lithium salt in the electrolyte is 0.5 - 5 mol / L. If the concentration is lower than 0.5 mol / L, the conductivity of the electrolyte is too low. If the concentration is higher than 5 mol / L, the viscosity of the electrolyte is too high, both of which will affect the normal performance of the electrolyte.

[0010] Furthermore, the viscosities of both the ester solvent and the ether solvent are lower than 1 mPa·S, and the melting points are lower than -40 °C, which can meet the working requirements of most low-temperature scenarios.

[0011] Furthermore, the volume ratio of dimethyl sulfoxide to the organic solvent is 1:0.5 - 2, and the volume ratio of the ester solvent to the ether solvent is 1:0.1 - 0.5. The ester in dimethyl sulfoxide and the organic solvent is the main solvent, and their volume ratios are close. The ether solvent is only used as an auxiliary solvent, and its volume fraction is relatively low.

[0012] Furthermore, the ester solvent is one or more of methyl acetate, methyl butyrate, methyl formate, ethyl acetate, ethyl butyrate, ethyl propionate, ethylene carbonate, propylene carbonate, ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, 1,3 - propane sultone, vinylene carbonate, fluoroethylene carbonate, etc. Preferably, the ester solvent is methyl acetate.

[0013] Furthermore, the ether solvent is one or more of 1,1,2,2 - tetrafluoroethyl - 2,2,3,3 - tetrafluoropropyl ether, ethyl 1,1,2,2 - tetrafluoroethyl ether, 1,1,2,2 - tetrafluoroethyl - 2,2,2 - trifluoroethyl ether, ethylene glycol diethyl ether, ethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, etc. Preferably, the ether solvent is 1,1,2,2 - tetrafluoroethyl - 2,2,3,3 - tetrafluoropropyl ether.

[0014] The preparation method for the wide temperature range and high specific energy lithium carbon monofluoride battery electrolyte of the present invention is as follows: Add dimethyl sulfoxide, the ester solvent, and the ether solvent into a container, and then add lithium bis(fluorosulfonyl)imide, and mix evenly to obtain the lithium carbon monofluoride battery electrolyte.

[0015] Further, the environment of the preparation process is as follows: in a glove box filled with argon, the contents of water and oxygen in the glove box are both less than 0.1 ppm. The electrolyte is relatively sensitive to the water-oxygen ratio in the environment. An electrolyte with a high water content will affect the discharge performance of the battery. At the same time, a high water-oxygen content is more likely to cause a series of side reactions during the preparation of the electrolyte with salts and solvents, resulting in the failure of the final electrolyte preparation.

[0016] Principle of the invention: The commonly used lithium salts in lithium carbon monofluoride electrolytes can be divided into two categories: inorganic lithium salts and organic lithium salts according to different anions. The mixture of inorganic salts and solvents exists in a dissolved form. Since the solubility is greatly affected by temperature, this will cause salting out of the electrolyte at low temperatures, thereby affecting the low-temperature performance of the battery. Organic salts and solvents exist in a coordinated form and are less affected by temperature, so their performance is more stable at high and low temperatures. Among organic anion salts, LiFSI has a smaller anion radius and a faster migration rate in the solution. Therefore, compared with other organic salts, the prepared electrolyte has a higher conductivity and better discharge performance of the battery.

[0017] Among common electrolyte solvents, dimethyl sulfoxide has a high dielectric constant. For an electrolyte prepared with a solvent having a high dielectric constant, the charge transfer resistance between the solid / liquid interfaces of ions is smaller. As a result, the degree of electrochemical polarization of the cathode is higher during the discharge process, and the reaction kinetics of the whole process is faster. This enables the resistance of lithium ions in the electrolyte to diffuse through the discharge product to be lower, thereby increasing the discharge voltage platform.

[0018] Dimethyl sulfoxide with a high dielectric constant has too high a viscosity. A single dimethyl sulfoxide solvent will cause the viscosity of the electrolyte to be too high, affecting the lithium ion transfer rate during low-temperature discharge. Therefore, an ester solvent with a low viscosity is introduced to form an electrolyte with a high dielectric constant and a low viscosity. At the same time, an appropriate amount of ether is introduced into the solvent. Since the self-reductive decomposition of the ether forms a film on both the positive and negative electrode surfaces, it helps to reduce the dissolution of the active material from the positive electrode and can protect the lithium negative electrode. However, the lithium ion solvation ability of this type of ether is very low, and its boiling point is low and it is easy to volatilize, which is not conducive to the stable operation of the battery in high-temperature or even normal-temperature environments. Therefore, in practical applications, the ether is often added in small amounts as an auxiliary solvent.

[0019] Beneficial effects: Compared with the prior art, the present invention has the following remarkable advantages: The present invention prepares a lithium carbon monofluoride battery electrolyte through the synergistic action of multiple formulations. By combining a dimethyl sulfoxide solvent with a high dielectric constant, an ester solvent with a low viscosity, and an ether solvent, it can improve the conductivity and discharge performance of the electrolyte, making the discharge voltage platform of the battery higher; at the same time, it reduces the viscosity of the battery, further improving the specific capacity and specific energy performance of the battery; and further significantly improves the high and low temperature resistance performance of the battery. Description of the drawings

[0020] Figure 1 This is the comparison chart of the specific capacity of the electrolytes prepared in Example 1 (25°C) and Comparative Example 1 (25°C) of the present invention;

[0021] Figure 2 This is the comparison chart of the specific energy of the electrolytes prepared in Example 1 (25°C) and Comparative Example 1 (25°C) of the present invention;

[0022] Figure 3 This is the comparison chart of the specific capacity of the electrolytes prepared in Example 1 (-20°C) and Comparative Example 1 (-20°C) of the present invention;

[0023] Figure 4 This is the comparison chart of the specific capacity of the electrolytes prepared in Example 1 (0°C) and Comparative Example 1 (0°C) of the present invention;

[0024] Figure 5 This is the comparison chart of the specific capacity of the electrolytes prepared in Example 1 (55°C) and Comparative Example 1 (55°C) of the present invention;

[0025] Figure 6 This is the specific capacity performance chart of the electrolyte prepared in Example 2 of the present invention;

[0026] Figure 7 This is the specific capacity performance chart of the electrolyte prepared in Example 3 of the present invention. Detailed Embodiments

[0027] Next, the present invention will be further described in conjunction with specific embodiments and the accompanying drawings.

[0028] Example 1: The wide-temperature-range and high-specific-energy lithium-carbon monofluoride battery electrolyte of the present invention is composed of LiFSI, dimethyl sulfoxide, methyl acetate, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether. Among them, the volume ratio of dimethyl sulfoxide, methyl acetate, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is 2:2:1, and the concentration of LiFSI in the electrolyte is 1 mol / L.

[0029] The preparation method of the above electrolyte is as follows: In a glove box filled with argon (water ≤ 0.1 ppm, oxygen ≤ 0.1 ppm), place dimethyl sulfoxide, methyl acetate, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether in a transparent corrosion-resistant glass bottle according to the above ratio, and then add LiFSI. Stir magnetically at 500 rpm for 24 hours to obtain the final lithium-carbon monofluoride electrolyte.

[0030] Example 2: The difference from Example 1 is that the methyl acetate solvent is replaced with methyl butyrate.

[0031] Example 3: The difference from Example 1 is that 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is replaced by 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether.

[0032] Example 4: The difference from Example 1 is that the concentration of LiFSI in the electrolyte is 0.5 mol / L.

[0033] Example 5: The difference from Example 1 is that the concentration of LiFSI in the electrolyte is 5 mol / L.

[0034] Example 6: The difference from Example 1 is that the volume ratio of dimethyl sulfoxide, methyl acetate and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is 2:1:0.1.

[0035] Example 7: The difference from Example 1 is that the volume ratio of dimethyl sulfoxide, methyl acetate and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is 0.5:1:0.5.

[0036] Example 8: The difference from Example 1 is that the volume ratio of dimethyl sulfoxide, methyl acetate and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is 2:1:0.5.

[0037] Example 9: The difference from Example 1 is that the volume ratio of dimethyl sulfoxide, methyl acetate and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is 0.5:1:0.1.

[0038] Comparative Example 1: Commercial electrolyte, composed of lithium tetrafluoroborate (LiBF4), propylene carbonate and ethylene glycol dimethyl ether, where the volume ratio of propylene carbonate and ethylene glycol dimethyl ether is 1:2, and the concentration of LiBF4 in the electrolyte is 1 mol / L.

[0039] Comparative Example 2: The difference from Comparative Example 1 is that the lithium salt is replaced by lithium perchlorate (LiClO4).

[0040] Manufacture of lithium carbon monofluoride battery:

[0041] (1) Dissolve 5 g of polyvinylidene fluoride powder in 100 mL of N-methylpyrrolidone (NMP) and stir to obtain the required binder; (2) Carbon monofluoride (CF x) The powder, Ketjen black (KB) powder and binder are mixed in a mass ratio of 8:1:1, and ground into a slurry; (3) The positive electrode slurry is coated on aluminum foil, and the coated electrode is transferred to an oven at 80 °C for drying for 12 - 24 h to obtain the required positive electrode. Further, the above electrode is cut into circular pieces as the battery positive electrode, metallic lithium is used as the battery negative electrode, the electrolytes used are the electrolytes in Examples 1 - 9 and Comparative Examples 1 - 2 in sequence, and the separator is a commercial separator, a 32 μm - specification PP separator produced by Celgard. The button battery case is assembled into a battery using the 2016 specification.

[0042] Battery constant - rate discharge test:

[0043] The above - mentioned batteries are subjected to discharge tests. The test environments for Example 1 and Comparative Example 1 are in a constant - temperature oven at - 20 °C, 0 °C, 25 °C, and 55 °C; the test environments for Examples 2 - 9 are in a constant - temperature oven at 25 °C, and the rate conditions for the tests are all 0.05C; the cut - off voltage for the tests is 1.5V, and the test results are shown in Table 1, Table 2 and Figure 1-7 .

[0044] The battery discharge graph at 25 °C is as shown in Figure 1-2 . The specific capacity of the battery in Example 1 is 835.7 mAh / g, the specific energy is 2153.7 mWh / g, and the discharge voltage plateau is 2.68V. The specific capacity of the battery in Comparative Example 1 is 806.5 mAh / g, the specific energy is 1949.7 mWh / g, and the discharge voltage plateau is 2.52V.

[0045] The battery discharge graph at - 20 °C is as shown in Figure 3 . The specific capacity of the battery in Example 1 is 657.8 mAh / g, the specific energy is 1547.0 mWh / g, and the discharge voltage plateau is 2.46V. The specific capacity of the battery in Comparative Example 1 is 603.0 mAh / g, the specific energy is 1367.7 mWh / g, and the discharge voltage plateau is 2.31V.

[0046] The battery discharge graph at 55 °C is as shown in Figure 4 . The specific capacity of the battery in Example 1 is 826.6 mAh / g, the specific energy is 2213.7 mWh / g, and the discharge voltage plateau is 2.76V. The specific capacity of the battery in Comparative Example 1 is 755.0 mAh / g, the specific energy is 1912.9 mWh / g, and the discharge voltage plateau is 2.61V.

[0047] The battery discharge graph at 0 °C is as shown in Figure 5 . The specific capacity of the battery in Example 1 is 818.2 mAh / g, the specific energy is 1977.8 mWh / g, and the discharge voltage plateau is 2.49V. The specific capacity of the battery in Comparative Example 1 is 751.5 mAh / g, the specific energy is 1663.4 mWh / g, and the discharge voltage plateau is 2.34V.

[0048] The discharge graph of the battery in Example 2 is as Figure 6 shown. The specific capacity of the battery is 827.9 mAh / g, the specific energy is 2107.8 mWh / g, and the discharge voltage plateau is 2.66 V.

[0049] The discharge graph of the battery in Example 3 is as Figure 7 shown. The specific capacity of the battery is 837.5 mAh / g, the specific energy is 2142.1 mWh / g, and the discharge voltage plateau is 2.65 V.

[0050] Table 1 Performance test table of electrolytes prepared in Examples 1-9 and Comparative Examples 1-2 at 25 °C

[0051] Specific capacity (mAh / g) Specific energy (mAh / g) Discharge voltage plateau (V) Example 1 835.7 2153.7 2.68 Example 2 827.9 2107.8 2.66 Example 3 837.5 2142.1 2.65 Example 4 813.6 2059.7 2.61 Example 5 818.5 2080.4 2.63 Example 6 812.1 2065.6 2.61 Example 7 825.7 2095.7 2.66 Example 8 853.2 2209.4 2.68 Example 9 811.6 2081.7 2.66 Comparative Example 1 806.5 1949.7 2.52 Comparative Example 2 728.7 1776.9 2.47

[0052] Table 2 Performance test table of electrolytes prepared in Example 1 and Comparative Example 1 at different temperatures

[0053]

[0054]

[0055] As can be seen from Table 1, under the same temperature conditions, the specific capacity and specific energy of the batteries prepared from the electrolytes of Examples 1-9 are improved to a certain extent compared with those of the batteries prepared from the commercial electrolytes of Comparative Examples 1-2; as can be obtained from Table 2, at each temperature involved in Example 1 and Comparative Example 1, the specific capacity and specific energy of the batteries prepared from the electrolytes of Examples 1-9 are also improved to a certain extent compared with those of the batteries using commercial electrolytes. Generally speaking, the specific capacity is increased by about 5%, and the specific energy is increased by about 10%. On the one hand, this is due to the electrolyte conductivity brought by the organic anion salt and the more stable binding degree in different temperature ranges. On the other hand, the addition of low-viscosity esters and co-solvent ethers reduces the viscosity of the electrolyte and improves the stability during the battery discharge process. The combined effect improves the specific capacity and specific energy of the battery.

[0056] As can be seen from Table 1 and Table 2, under the same conditions, the discharge voltage plateau of the batteries prepared from the electrolyte of Example 1 is higher than that of the batteries prepared from the commercial electrolyte of Comparative Example 1, with an average increase of 0.15 V. Because of the introduction of dimethyl sulfoxide with a high dielectric constant, the charge transfer resistance between the solid / liquid interfaces of ions is smaller, and thus the electrochemical polarization degree of the cathode is higher during the discharge process, and the reaction kinetics of the whole process is faster. This makes the resistance of lithium ions diffusing through the discharge products in the electrolyte lower, thereby increasing the discharge voltage plateau. In addition, there is also a quite obvious increase in the discharge voltage plateau in Examples 2-9 compared with Comparative Examples 1 and 2.

Claims

1. A wide-temperature-range and high-specific-energy lithium carbon monofluoride battery electrolyte, which is composed of a lithium salt and a solvent, is characterized in that The lithium salt is lithium bis(fluorosulfonyl)imide, the solvent consists of dimethyl sulfoxide and an organic solvent, and the organic solvent consists of an ester solvent and an ether solvent; the concentration of the lithium salt in the electrolyte is 0.5 - 5 mol / L; the volume ratio of dimethyl sulfoxide to the organic solvent is 1:0.5 - 2; the volume ratio of the ester solvent to the ether solvent is 1:0.1 - 0.5; the ester solvent is one or more of methyl acetate, methyl butyrate, methyl formate, ethyl acetate, ethyl propionate, ethyl butyrate; the ether solvent is one or more of 1,1,2,2 - tetrafluoroethyl - 2,2,3,3 - tetrafluoropropyl ether, ethyl 1,1,2,2 - tetrafluoroethyl ether, 1,1,2,2 - tetrafluoroethyl - 2,2,2 - trifluoroethyl ether.

2. A preparation method of the electrolyte for the wide-temperature-range and high-specific-energy lithium-carbon monofluoride battery according to claim 1, characterized in that, The preparation process is as follows: Add dimethyl sulfoxide, the ester solvent and the ether solvent into a container, and then add lithium bis(fluorosulfonyl)imide, and mix evenly to obtain the electrolyte for the lithium carbon fluoride battery.

3. The preparation method according to claim 2, characterized in that, The environment of the preparation process is: in a glove box filled with argon, and the contents of water and oxygen in the glove box are both less than 0.1 ppm.

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