Wide-temperature type electrolyte and preparation method and application thereof

Abstract

The invention relates to a wide-temperature type electrolyte and a preparation method and application thereof, the electrolyte comprises a lithium salt, a solvent and an additive, the additive is a PVDF-based multi-component copolymer, and the PVDF-based multi-component copolymer is obtained by copolymerization of at least three of vinylidene fluoride, trifluoroethylene, chlorofluoroethylene, trichlorofluoroethylene, trifluorobromoethylene and trifluoroiodo ethylene; the solvent is a carbonic ester solvent; according to the invention, the PVDF-based multipolymer obtained by copolymerization of the specific compound is added into the electrolyte as an additive, so that the transmission and interface reaction of the electrolyte can be efficiently regulated and controlled to improve the wide-temperature-range charge-discharge performance of the battery, and the electrolyte can keep excellent cycle stability and electrochemical performance at the temperature of-50-60 DEG C.

Description

Technical Field

[0001] The present invention relates to the technical field of electrolytes, and more specifically, to a wide-temperature electrolyte and a preparation method and application thereof. Background Art

[0002] Due to the significant changes in regional ambient temperature caused by factors such as different latitudes, altitudes and seasons, emerging lithium battery application terminals such as electric vehicles and energy storage base stations often need to face the dual test of low temperature and high temperature during long-term service. Therefore, the development of electrolytes that can take into account both high and low temperature performance (or wide temperature range electrolytes) is of great significance for improving the applicability of lithium batteries in the full temperature range and further promoting the application of lithium batteries in emerging application scenarios. However, compared with the development of a single low-temperature or high-temperature electrolyte, the development of full-temperature range electrolytes faces new and complex challenges. Battery attenuation at low temperatures is mainly related to ion migration, charge transfer dynamics and deposition morphology of lithium metal in the electrolyte. Therefore, the design of low-temperature electrolytes needs to meet high ionic conductivity, stable interface films and high desolvation capabilities at the same time. The attenuation of batteries at high temperatures is mainly related to interface stability and thermal stability. Therefore, the design of high-temperature electrolytes mainly focuses on improving the stability of the electrode interface and the electrolyte. It can be seen that the design of wide-temperature electrolytes is a very arduous task because it needs to meet different performance requirements at high and low temperatures at the same time.

[0003] At present, many researchers have proposed various modification strategies around the structure and properties of electrolytes, mainly including: ① Introducing electrolyte functional additives. The functional additives form a stable protective film on the surface of the electrode material before the solvent is oxidized and decomposed, which can enhance the interfacial stability of the electrode / electrolyte, thereby improving the high-temperature or low-temperature electrochemical properties of the material. ② Local high-concentration lithium salt electrolyte. The diluent in the local high-concentration electrolyte can improve the physicochemical properties of the electrolyte, thereby reducing the viscosity of the electrolyte, improving the ionic conductivity, and benefiting the low-temperature performance. Since almost all the solvents in the local high-concentration electrolyte participate in solvation, there are fewer free organic solvent molecules, which can reduce the volatilization of the solvent at high temperatures and improve the thermal stability of the electrolyte at high temperatures. ③ Weakly solvated electrolyte system. The use of weakly solvated solvents helps to weaken the binding energy between lithium ions and solvents, thereby improving the electrode reaction kinetics at low temperatures. However, the cost of weak interaction is that the oxidation resistance of the solvent molecules cannot be improved by coordinating with lithium ions, and the interfacial side reactions may increase as a result. Summary of the invention

[0004] Based on the above technical problems existing in the prior art, the present invention provides a wide temperature range electrolyte, which can be applied to batteries and has excellent electrochemical properties under temperature conditions of -50 to 60°C.

[0005] In order to achieve the above object, the technical solution of the present invention is as follows:

[0006] A wide temperature electrolyte comprises a lithium salt, a solvent and an additive, wherein the additive is a PVDF-based multipolymer obtained by copolymerizing at least three of vinylidene fluoride (VDF), trifluoroethylene (TrFE), chlorofluoroethylene (CFE), trichlorofluoroethylene (CTFE), trifluorobromoethylene (BTFE) and trifluoroiodoethylene (ETFE); and the solvent is a carbonate solvent.

[0007] In some embodiments, the PVDF-based multi-polymer is obtained by copolymerizing at least one of chlorofluoroethylene, trichlorofluoroethylene, bromotrifluoroethylene, and iodotrifluoroethylene with vinylidene fluoride and trifluoroethylene; specifically, the copolymers include but are not limited to P(VDF+TrFE+CFE), P(VDF+TrFE+CTFE), P(VDF+TrFE+BTFE), P(VDF+TrFE+ITFE), P(VDF+TrFE+CFE+CTFE), P(VDF+TrFE+CFE+ITFE), P(VDF+TrFE+CFE+BTFE), P(VDF+TrFE+CFE+ITFE), etc.

[0008] In some embodiments, the mass ratio of vinylidene fluoride, trifluoroethylene to at least one of chlorofluoroethylene, trichlorofluoroethylene, trifluorobromoethylene, and trifluoroiodoethylene is 60-70:23-30:5-10; that is, vinylidene fluoride and trifluoroethylene are component A and component B, respectively, and at least one of trichlorofluoroethylene, trifluorobromoethylene, and trifluoroiodoethylene is component C, then the mass ratio of component A, component B, and component C is 60-70:23-30:5-10.

[0009] In some embodiments, the amount of the additive added is 0.5-10% of the mass of the electrolyte; preferably, the amount of the additive added is 0.5-5% of the mass of the electrolyte.

[0010] In some embodiments, the lithium salt is at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, and lithium difluorooxalatoborate.

[0011] In some embodiments, the concentration of the lithium salt in the electrolyte is 0.5-3 mol / L.

[0012] In some embodiments, the solvent is at least one of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and fluoroethylene carbonate.

[0013] The present invention also provides a method for preparing the wide temperature range electrolyte of any of the above embodiments, the method comprising the following steps:

[0014] The lithium salt, solvent and additive are mixed evenly to obtain the wide temperature range electrolyte.

[0015] The present invention also provides the use of the wide temperature electrolyte of any of the above embodiments in a lithium ion battery. Specifically, the lithium ion battery includes but is not limited to a lithium ion battery with NCM811, lithium iron phosphate, or lithium-rich manganese-based oxide as a positive electrode.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] The present invention adds a PVDF-based multipolymer obtained by copolymerization of specific compounds as an additive to the electrolyte, which can effectively regulate the electrolyte transmission and interface reaction to improve the wide temperature range charge and discharge performance of the battery, so that the electrolyte maintains excellent cycle stability and electrochemical performance at a temperature of -50 to 60°C; firstly, the multipolymer obtained by copolymerization of the specific compound can be well soluble in carbonate solvents, which can effectively improve the ionic conductivity, lithium ion migration number and oxidative decomposition resistance of the electrolyte; secondly, different monomers are used to disrupt the Great Wall ordered ferroelectric crystal region in a typical polymer to obtain an electric domain structure with short-range disorder and more chaotic microcrystalline regions, further improving the maximum polarization value of the material, reducing the Curie temperature and coercive field of the material, so that the material can promote the dissociation and transportation of lithium salts through a higher dielectric constant at low temperatures, and under high temperature conditions, the long-range ordered CF dipoles in the multipolymer structure can also complex with most solvent molecules to improve the oxidative decomposition resistance of the electrolyte. In addition, since the nano-sized ferroelectric domains greatly reduce the coercive field, they can also perform polarization reversal in time to respond to the changes in the charging and discharging voltage of the lithium battery, and coordinately regulate the electrolyte-electrode interface reaction through the internal electric field formed by self-polarization. Finally, by utilizing the dielectric constant and strong self-polarization characteristics of the multi-polymer itself, the reaction kinetics of lithium ions at the electrode interface are accelerated and the interface side reactions caused by anions or solvent molecules are reduced, thereby achieving a balance between high-temperature and low-temperature performance of the electrolyte. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The conductivity test results of the electrolytes of Example 1, Example 6 and Comparative Example 1 are shown;

[0019] Figure 2 The impedance test results of the batteries of Example 1, Example 6 and Comparative Example 1 are shown;

[0020] Figure 3 The charge and discharge curves of Example 1 and Comparative Example 1 at a rate of 1C at a temperature of -50°C;

[0021] Figure 4 The charge and discharge curves of Example 1 and Comparative Example 1 at 1C rate at -20°C;

[0022] Figure 5 The charge and discharge curves of Example 1 and Comparative Example 1 at 1C rate at -10°C;

[0023] Figure 6 The charge and discharge curves of Example 1 and Comparative Example 1 at 1C rate and 25°C;

[0024] Figure 7 The charge and discharge curves of Example 1 and Comparative Example 1 at 1C rate and 60°C;

[0025] Figure 8 This is a TEM image of the interface layer on the positive electrode surface of the battery of Example 1 after 200 cycles at 25° C. and 1C rate;

[0026] Fig. 9 This is a TEM image of the interface layer on the positive electrode surface of the battery of Comparative Example 1 after 200 cycles at 25°C and 1C rate. DETAILED DESCRIPTION

[0027] Many specific details are set forth in the following description to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways than those described herein, and those skilled in the art can make similar modifications without violating the connotation of the present invention, so the present invention is not limited to the specific implementation disclosed below.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0029] Example 1

[0030] A wide temperature range electrolyte, comprising a lithium salt, an additive and a solvent; wherein the lithium salt concentration is 1 mol / L, and the mass fraction content of the additive in the electrolyte is 2%; the lithium salt is lithium hexafluorophosphate, the solvent is composed of fluoroethylene carbonate, ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate, the fluoroethylene carbonate in the solvent accounts for 10% of the total volume percentage, the volume ratio of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate is 1:1:1, and the additive is a PVDF-based multipolymer, which is obtained by copolymerizing VDF, TrFE and CFE in a mass ratio of 65.4:26.2:8.8;

[0031] The preparation of the wide temperature range electrolyte comprises the following steps:

[0032] S1. Put lithium salt and additives into a vacuum oven for drying. The temperature of the vacuum oven is set to 60°C and the drying time is 12 hours. Ensure that the moisture content detected by the moisture detector is less than 100ppm.

[0033] S2. The molecular sieve was washed with anhydrous ethanol for 3 to 5 times, then dried in an oven at 100°C for 12 h, and finally calcined at 350-400°C for 2 h to 4 h in an air atmosphere, and then transferred to a glove box for use after cooling to room temperature;

[0034] S3. In the glove box, use the pre-treated Molecular sieve soaking and drying treatment of fluoroethylene carbonate, ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, soaking for 10-12 hours to ensure that the moisture content detected by the moisture detector is less than 50ppm;

[0035] S4. In a glove box, add solvents in order according to the ratio to an empty glass bottle, then add lithium salt. After the lithium salt is completely dissolved, add additives and stir thoroughly at room temperature for 12 hours until a clear and transparent solution is obtained, that is, a wide-temperature electrolyte.

[0036] Example 2

[0037] The content of the additive in the electrolyte of this embodiment is 0.5 wt %, and the rest is the same as that of embodiment 1.

[0038] Example 3

[0039] The content of the additive in the electrolyte of this embodiment is 1 wt %, and the rest is the same as that of embodiment 1.

[0040] Example 4

[0041] The content of the additive in the electrolyte of this embodiment is 3 wt %, and the rest is the same as that of embodiment 1.

[0042] Example 5

[0043] The content of the additive in the electrolyte of this embodiment is 5 wt %, and the rest is the same as that of embodiment 1.

[0044] Example 6

[0045] The solvent of this embodiment is composed of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate, and the volume ratio of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate in the solvent is 1:1:1. Other aspects are the same as those in Embodiment 1.

[0046] Example 7

[0047] The solvent of this embodiment is composed of ethylene carbonate and dimethyl carbonate, and the volume ratio of ethylene carbonate to dimethyl carbonate in the solvent is 3:7. Others are the same as those in Embodiment 1.

[0048] Example 8

[0049] The solvent of this embodiment is composed of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate, and the volume ratio of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate in the solvent is 1:1:1. Other aspects are the same as those in Embodiment 1.

[0050] Example 9

[0051] The electrolyte of this embodiment is added with 1% by mass of lithium difluorooxalatoborate (LiDFOB), the solvent is composed of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate, the volume ratio of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate in the solvent is 1:1:1, and the rest is the same as in Embodiment 1.

[0052] Comparative Example 1

[0053] No additives were added to the electrolyte of this comparative example, and the rest was the same as that of Example 1.

[0054] The conductivity test was performed on the electrolytes of Examples 1, 6 and Comparative Example 1. The test results are as follows: Figure 1 As shown. Figure 1 , the ion conductivities of Examples 1, 6 and Comparative Example 1 are 1.51 mS / cm, 0.89 mS / cm and 0.62 mS / cm, respectively, and the lithium ion migration numbers are 0.73, 0.56 and 0.40, respectively. Obviously, the ion conductivity and lithium ion migration number of the electrolyte obtained by the scheme of the present application are both superior to those of the electrolyte in the prior art.

[0055] The electrolytes obtained in Examples 1-9 and Comparative Example 1 were used to prepare lithium ion button cells, wherein the positive electrode was NCM811 and the negative electrode was metallic lithium, and then relevant performance tests were performed, as follows:

[0056] 1. The batteries prepared with the electrolytes obtained in Examples 1-9 and Comparative Example 1 were subjected to discharge tests at different temperatures. The test results are shown in Tables 1 to 3, where:

[0057] The test results of the highest discharge capacity at 25°C and 0.1C-10C rate are shown in Table 1:

[0058] Table 1 Maximum discharge capacity (mAh / g) of different embodiments and comparative examples when assembled into batteries at different rates at 25°C

[0059] 0.1C 0.5C 1C 2C 3C 5C 10C Example 1 231.2 217.3 205.1 192.2 187.5 180.3 156.2 Example 2 228.6 216.6 203.1 190.5 181.7 175.6 150.6 Example 3 229.4 217.1 202.8 191.3 182.4 179.8 141.0 Example 4 227.3 214.8 202.2 188.5 179.6 176.8 144.1 Example 5 221.5 217.9 198.3 182.7 177.2 170.5 143.3 Example 6 225.2 214.2 200.4 189.8 172.1 167.2 142.4 Example 7 230.1 215.4 202.7 186.3 176.5 169.1 156.6 Example 8 221.5 210.6 195.6 183.1 177.8 168.3 148.3 Example 9 228.2 213.5 199.1 187.1 183.5 170.7 141.2 Comparative Example 1 208.1 193.1 182.5 170.6 156.2 140.8 111.7

[0060] -50-60℃, the highest discharge capacity at 1C rate is shown in Table 2:

[0061] Table 2 Maximum discharge capacity (mAh / g) of different embodiments and comparative examples when assembled into batteries at -50-60°C and 1C rate

[0062] 60℃ 25℃ -10℃ -20℃ -50℃ Example 1 225.7 205.1 153.2 136.7 101.4 Example 2 220.3 203.3 151.2 135.3 100.6 Example 3 71.6 202.4 149.1 135.3 100.1 Example 4 69.4 202.2 148.3 132.1 99.1 Example 5 68.2 198.6 147.4 131.2 98.7 Example 6 71.1 200.9 148.2 133.5 100.4 Example 7 67.2 202.7 144.4 130.4 95.3 Example 8 66.5 195.7 143.2 126.7 92.5 Example 9 69.1 199.2 142.0 130.8 94.7 Comparative Example 1 194.9 182.5 125.1 84.2 57.4

[0063] The capacity retention rate after 200 cycles at -50-60℃ and 1C rate is shown in Table 3:

[0064] Table 3 Retention rate (%) of different embodiments and comparative examples when assembled into batteries at -50-60°C and 1C rate for 200 cycles

[0065] 60℃ 25℃ -10℃ -20℃ -50℃ Example 1 72.1％ 87.5％ 91.2％ 86.2％ 82.1％ Example 2 70.3％ 84.3％ 89.3％ 85.1％ 80.3％ Example 3 71.5％ 86.4％ 90.7％ 85.4％ 81.4％ Example 4 69.4％ 83.6％ 88.6％ 82.8％ 79.2％ Example 5 68.3％ 82.5％ 87.5％ 81.3％ 78.9％ Example 6 71.1％ 85.7％ 90.2％ 83.7％ 80.0％ Example 7 67.2％ 82.1％ 84.4％ 80.1％ 75.1％ Example 8 66.7％ 80.5％ 83.2％ 76.6％ 72.2％ Example 9 69.1％ 81.2％ 82.0％ 80.1％ 74.7％ Comparative Example 1 38.9％ 71.6％ 80.1％ 63.5％ 50.1％

[0066] 2. After 0, 50, 100 and 200 cycles at room temperature (25°C) and 1C rate, the AC impedance test results of the batteries of Examples 1, 6 and Comparative Example 1 are as follows: Figure 2 As shown; the results show that the battery made by the electrolyte of the present invention shows lower interface transfer impedance, thus having a higher Li + Migration rate.

[0067] 3. The charge and discharge curves of the batteries prepared with the electrolytes of Example 1 and Comparative Example 1 at a rate of 1C at -50°C, -20°C, -10°C, 25°C and 60°C are as follows: Figure 3-Figure 7 shown.

[0068] 4. After the batteries prepared with the electrolytes of Example 1 and Comparative Example 1 were cycled 200 times at 25°C and 1C, the batteries were disassembled and the positive electrode surfaces were observed by high-resolution transmission electron microscopy. The results were as follows: Figure 8 and Fig. 9 As shown. Figure 8 and Fig. 9 The electrolyte interface on the positive electrode surface of the battery prepared with the electrolyte of the present invention (Example 1) after 200 cycles is thinner and more uniform, while the electrolyte interface layer on the positive electrode surface of the battery prepared with the electrolyte of Comparative Example 1 after 200 cycles is thicker and irregular.

[0069] In summary, the electrolyte of the present invention can exhibit excellent electrochemical performance and cycle stability under conditions ranging from low temperature (-50°C) to high temperature (60°C).

[0070] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0071] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A wide temperature range electrolyte, characterized in that: The invention comprises lithium salt, solvent and additive, wherein the additive is a PVDF-based multi-polymer obtained by copolymerizing at least three of vinylidene fluoride, trifluoroethylene, chlorofluoroethylene, trichlorofluoroethylene, trifluorobromoethylene and trifluoroiodoethylene; and the solvent is a carbonate solvent.

2. The wide temperature range electrolyte according to claim 1, characterized in that: The PVDF-based multi-component copolymer is obtained by copolymerizing at least one of chlorofluoroethylene, trichlorofluoroethylene, trifluorobromoethylene, and trifluoroiodoethylene with vinylidene fluoride and trifluoroethylene.

3. The wide temperature range electrolyte according to claim 2, characterized in that: The mass ratio of the vinylidene fluoride, trifluoroethylene to at least one of chlorofluoroethylene, trichlorofluoroethylene, trifluorobromoethylene and trifluoroiodoethylene is 60-70:23-30:5-10.

4. The wide temperature range electrolyte according to any one of claims 1 to 3, characterized in that: The additive is added in an amount of 0.5-10% of the mass of the electrolyte.

5. The wide temperature range electrolyte according to claim 4, characterized in that: The additive is added in an amount of 0.5-5% of the mass of the electrolyte.

6. The wide temperature range electrolyte according to claim 1, characterized in that: The lithium salt is at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide and lithium difluorooxalatoborate.

7. The wide temperature range electrolyte according to claim 1, characterized in that: The solvent is at least one of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate and fluoroethylene carbonate.

8. The method for preparing a wide temperature range electrolyte according to any one of claims 1 to 7, characterized in that: The following steps are involved: The lithium salt, solvent and additive are mixed evenly to obtain the wide temperature range electrolyte.

9. Use of the wide temperature electrolyte according to any one of claims 1 to 7 in a lithium ion battery.

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