Flame-retardant electrolyte, preparation method and application in graphite negative electrode

Abstract

The invention discloses a flame-retardant electrolyte, a preparation method and application in a graphite cathode, the electrolyte comprises a lithium salt, an organic solvent and a film-forming additive, the organic solvent is a mixed solvent comprising fluorinated carboxylic ester and hydrofluoroether, the concentration of the lithium salt in the organic solvent is 2-6 mol / L, and the film-forming additive is lithium bis (oxalate) difluorophosphate. The electrolyte has the beneficial effects that the high-concentration lithium salt in the electrolyte not only ensures the ionic conductivity of the carboxylic ester-based electrolyte, but also forms a solvation structure rich in anions, so that the compatibility of the electrolyte and a graphite negative electrode is improved. The perfluoro solvent and the additive ensure that the SEI film is rich in inorganic components such as LiF, LixPOyFz and the like, and particularly the SEI film rich in LiF effectively improves the compatibility of the EDFA carboxylate-based electrolyte and a graphite negative electrode and the transmission property of interface Li < + >.

Description

Technical Field

[0001] The invention relates to the technical field of lithium ion batteries, and in particular to a flame retardant electrolyte, a preparation method and application in a graphite negative electrode. Background Art

[0002] Lithium-ion batteries are widely used in all aspects of human life due to their advantages such as high specific energy, long cycle life, no memory and low self-discharge rate. Lithium-ion batteries are composed of four main raw materials, namely positive and negative electrodes, separators and electrolytes. Among them, the negative electrode material is one of the key factors affecting the capacity, cycle and rate (fast charging) performance of lithium-ion batteries. Traditionally, graphite has been widely used as the negative electrode material of lithium-ion batteries due to its stable electrochemical properties and good cycle stability. However, with the rapid development of fields such as energy storage systems, higher requirements have been placed on the energy density, cycle life and safety of lithium-ion batteries. In particular, the flammability of the electrolyte and the compatibility of the electrolyte with the graphite negative electrode have become key factors restricting the further application of lithium-ion batteries. Summary of the invention

[0003] The purpose of the present invention is to overcome the deficiencies of the prior art, provide a flame-retardant electrolyte, a preparation method and an application in a graphite negative electrode, wherein the high concentration of LiFSI in the electrolyte not only ensures the ionic conductivity of the carboxylate-based electrolyte, but also forms an anion-rich solvation structure, thereby improving the compatibility of the electrolyte with the graphite negative electrode. The perfluoro solvent TTE and the additive LiDFBOP ensure that the SEI film is rich in inorganic components such as LiF and LixPOyFz, especially the LiF-rich SEI film, which effectively improves the compatibility of the EDFA carboxylate-based electrolyte with the graphite negative electrode and the transportability of interfacial Li+.

[0004] The objective of the present invention is achieved through the following technical measures: a flame retardant electrolyte, comprising a lithium salt, an organic solvent and a film-forming additive, wherein the organic solvent is a mixed solvent comprising a fluorinated carboxylic acid ester and a hydrofluoroether, the concentration of the lithium salt in the organic solvent is 2 mol / L to 6 mol / L, and the film-forming additive is lithium difluorophosphate bis(oxalate).

[0005] In some embodiments, the volume percentage of the fluorinated carboxylic acid ester in the organic solvent is 50% to 80%.

[0006] In some embodiments, the fluorinated carboxylic acid ester is ethyl difluoroacetate.

[0007] In some embodiments, the hydrofluoroether is 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether.

[0008] In some embodiments, the concentration of the film-forming additive in the organic solvent is 0.1 mol / L to 0.6 mol / L.

[0009] In some embodiments, the lithium salt is bis(fluorosulfonyl)imide lithium salt.

[0010] A method for preparing a flame retardant electrolyte comprises the following steps:

[0011] The fluorinated carboxylic acid ester and the hydrofluoroether are mixed evenly to prepare a mixed solvent, the lithium salt and the film-forming additive are added into the mixed solvent, and the mixture is stirred evenly to obtain a flame-retardant electrolyte.

[0012] The present invention also provides an application of a flame retardant electrolyte in a graphite negative electrode.

[0013] In some embodiments, a graphite negative electrode is combined with a positive electrode plate and a separator to form a lithium / graphite battery, and an electrolyte is injected.

[0014] In some embodiments, the positive electrode plate is made of lithium nickel cobalt manganese oxide positive electrode material or lithium iron phosphate positive electrode material.

[0015] Compared with the prior art, the present invention has the following beneficial effects: the present invention adopts an organic solvent containing fluorinated carboxylic acid ester, introduces fluorine element into the solvent to achieve the non-flammability of the solvent, and can also improve the compatibility of the electrolyte with the graphite negative electrode through the strong electron-withdrawing effect of fluorine. A high-concentration lithium salt electrolyte is used to increase the content of contact ion pairs (CIP) formed by the reaction of lithium ions and anions in the electrolyte and the aggregates (AGG) formed by lithium ions, anions, and solvent molecules, reduce the presence of free solvent molecules, and construct an anion-rich solvation structure, further enhancing the non-flammability of the electrolyte.

[0016] By optimizing the lithium salt concentration and solvent ratio, an anion-rich solvation structure is constructed, which effectively inhibits the direct contact between EDFA and the graphite negative electrode, thereby improving the compatibility of the electrolyte and the graphite negative electrode, and ensuring the formation of a stable and uniform SEI film on the surface of the graphite negative electrode, thereby extending the cycle life of the battery and improving the overall performance of the battery.

[0017] The high concentration of lithium salt used not only ensures the ionic conductivity of the electrolyte, but the generated LiF-rich SEI film also promotes the rapid transmission of lithium ions at the interface, which helps to improve the charge and discharge performance of lithium-ion batteries, enabling them to exhibit better performance in high power density and fast charge and discharge scenarios.

[0018] Through the synergistic effect of TTE and LiDFBOP, the SEI film is rich in inorganic components such as LiF and LixPOyFz, especially the formation of LiF-rich SEI film, which improves the compatibility of carboxylate-based electrolyte with graphite negative electrode and the transportability of interfacial Li+, thereby further improving the energy density, cycle stability and safety of the battery.

[0019] The introduction of LiF promotes the uniform distribution of lithium nitride and lithium sulfide produced by the decomposition of LiFSI in the SEI film, significantly reducing the heterogeneity of the interface. This uniform distribution pattern not only optimizes the transmission path of lithium ions and reduces the transmission resistance, but also promotes the uniform deposition and dissolution of lithium ions, laying a solid foundation for the long-cycle stability of lithium-ion batteries. In addition, the chemical bonds or strong interaction forces formed between LiF and other components greatly enhance the cohesion and mechanical strength of the SEI film. This enhancement effect further improves the stability of the SEI film and provides a more reliable guarantee for effectively curbing the risks of thermal runaway and deflagration of the electrolyte.

[0020] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is the Raman spectrum of the electrolyte in Example 1 of the present invention.

[0022] Figure 2 This is the XPS F1s spectrum of the SEI film on the surface of the graphite negative electrode after the battery is formed in Example 1 of the present invention.

[0023] Figure 3 This is the impedance diagram of the Gr / NCM622 soft-pack battery after formation in Example 1 of the present invention.

[0024] Figure 4 The time-temperature and time-voltage curves of the puncture test of the Gr / NCM622 soft-pack battery in Example 1 of the present invention.

[0025] Figure 5 This is a rate performance diagram of the Gr / NCM622 soft-pack battery in Example 1 of the present invention.

[0026] Figure 6 This is a graph showing the cycle data of the NCM622 / Gr soft pack battery in Example 1 of the present invention.

[0027] Figure 7 This is a graph of the cycle data of the LFP / Gr soft-pack battery in Example 13 of the present invention. DETAILED DESCRIPTION

[0028] All raw materials of the present invention have no particular limitation on their sources, and can be purchased from the market or prepared according to conventional methods known to those skilled in the art.

[0029] There is no particular limitation on the purity of all raw materials in the present invention, and analytically pure materials are preferably used in the present invention.

[0030] A flame retardant electrolyte comprises a lithium salt, wherein the lithium salt is preferably lithium bis(fluorosulfonyl)imide (LiFSI), an organic solvent and a film-forming additive, wherein the organic solvent is a mixed solvent comprising a fluorinated carboxylic acid ester and a hydrofluoroether, wherein the fluorinated carboxylic acid ester is preferably ethyl difluoroacetate (EDFA), and the hydrofluoroether is preferably 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE), wherein the concentration of the lithium salt in the organic solvent is 2 mol / L to 6 mol / L, and the film-forming additive is lithium difluorophosphate bis(oxalate) (LiDFBOP).

[0031] The volume percentage of the fluorinated carboxylic acid ester in the organic solvent is 50% to 80%.

[0032] The concentration of the film-forming additive in the organic solvent is 0.1 mol / L to 0.6 mol / L.

[0033] A method for preparing a flame retardant electrolyte comprises the following steps:

[0034] The fluorinated carboxylic acid ester and the hydrofluoroether are mixed evenly to prepare a mixed solvent, and the lithium salt and the film-forming additive are added into the mixed solvent at room temperature and stirred evenly to obtain a flame-retardant electrolyte.

[0035] The present invention also provides an application of a flame-retardant electrolyte in a graphite negative electrode. The graphite negative electrode, the positive electrode plate, and the separator form a lithium / graphite battery, and the electrolyte is injected. The present invention has no special restrictions on the preparation method of the battery, and the battery preparation method known to those skilled in the art can be used. The preferred preparation method is:

[0036] Prepare the flame-retardant electrolyte, combine the graphite negative electrode with the positive electrode plate and the separator to form a lithium / graphite soft-pack battery, and inject the electrolyte into the battery. It is worth noting that you can also directly buy a soft-pack battery and inject the electrolyte configured in the present invention.

[0037] Place the injected battery at 45°C for 24 hours to allow the electrolyte to fully penetrate the battery cell.

[0038] At a constant temperature of 25° C., the soaked battery is formed. The specific formation steps are preferably: using a constant current value of 0.05C, and gradually charging the battery to a voltage level of 4.2V.

[0039] Release the gas in the battery after formation and perform secondary packaging.

[0040] The secondary packaged batteries were placed at 45°C for 48 hours for high temperature aging.

[0041] The positive electrode plate is made of lithium nickel cobalt manganese oxide positive electrode material or lithium iron phosphate positive electrode material.

[0042] Example 1

[0043] Under nitrogen atmosphere, EDFA and TTE were mixed in a volume ratio of 2:1 to prepare 100 mL of mixed solvent. 74.83 g of LiFSI and 12.6 g of LiDFBOP were weighed respectively. LiFSI and LiDFBOP were added to the mixed solvent at 25 °C to prepare 4.0 mol·L -1 LiFSI+0.5mol·L -1 LiDFBOP+EDFA / TTE (vol% 2:1) electrolyte.

[0044] The electrolyte is injected into the finished soft-pack battery with a capacity of 5Ah, and the injection coefficient is 3.0g / Ah. The battery uses graphite (Gr) as the negative electrode material and NCM622 as the positive electrode material to assemble the Gr / NCM622 soft-pack battery.

[0045] After the injection, the soft-pack battery is placed at 45°C for 24 hours to ensure that the electrolyte fully infiltrates the battery cell. Under a constant temperature of 25°C, the soft-pack battery after infiltration is subjected to a formation process. The specific formation steps are as follows: using a constant current value of 0.05C, the battery is gradually charged to a voltage level of 4.2V. The gas in the soft-pack battery after formation is released, and the side is repackaged. The soft-pack battery after secondary packaging is placed at 45°C for 48 hours for high-temperature aging.

[0046] The above battery was subjected to a cycle test: the soft-pack battery was placed at 25°C for a long cycle test with a cycle current of 0.5C.

[0047] Figure 1 is the Raman spectrum of the electrolyte in Example 1 of the present invention. According to the classification of FSI-SNS bending signals by Raman spectroscopy, it can be seen that the solvation in the electrolyte mainly contains contact ion pairs (CIP, 730.6 cm -1 , a Li + With an FSI - Combined) and aggregated ions (AGG, 746.3 cm -1 , two or more Li + With an FSI - This indicates that the main solvation structure of the electrolyte is anion clusters, which enhances the non-flammability of the electrolyte.

[0048] Figure 2This is the XPS F1s spectrum of the SEI film on the surface of the graphite negative electrode after the battery is formed in Example 1 of the present invention. It can be seen that the SEI film contains abundant inorganic components such as LiF and LixPOyFz. The formation of such SEI film can effectively isolate the direct contact between the carboxylic acid ester solvent and the graphite negative electrode, thereby greatly increasing the compatibility of the carboxylic acid ester solvent and the graphite negative electrode, and at the same time greatly increasing the cycle life of the lithium ion battery using graphite as the negative electrode in this type of electrolyte. In addition, the high content of inorganic components in the SEI film also accelerates the transmission of interfacial lithium ions and enhances the rate performance of the battery.

[0049] Figure 3 This is the impedance diagram of the Gr / NCM622 soft pack battery after formation in Example 1 of the present invention. Figure 5 The rate performance of the Gr / NCM622 soft pack battery in Example 1 of the present invention is: Figure 6 This is the cycle data of the NCM622 / Gr soft pack battery in Example 1 of the present invention. It can be seen that the impedance of the Gr / NCM622 soft pack battery is small after formation, indicating that in the use of carboxylic acid ester non-flammable electrolyte, the solvation structure of polyanionic clusters and LiDFBOP as film-forming additives can effectively reduce the interface impedance, thereby ensuring that the lithium ion battery has good rate performance, which helps to improve the charge and discharge performance of the lithium ion battery, so that it can show excellent performance in high power density and fast charge and discharge scenarios.

[0050] Figure 4 The time-temperature and time-voltage curves of the puncture test of the Gr / NCM622 soft pack battery in Example 1 of the present invention are shown. After the NCM622 / Gr soft pack battery is formed and in a fully charged state, during the puncture test, it is found that its maximum thermal runaway temperature is 450°C.

[0051] Example 2

[0052] A lithium electrolyte and a battery were prepared according to the method in Example 1. The only difference was that the volume ratio of EDFA / TTE was 1:1.

[0053] Example 3

[0054] A lithium electrolyte and a battery were prepared according to the method in Example 1. The only difference was that the volume ratio of EDFA / TTE was 3:1.

[0055] Example 4

[0056] A lithium electrolyte and a battery were prepared according to the method in Example 1. The only difference was that the volume ratio of EDFA / TTE was 4:1.

[0057] Example 5

[0058] A lithium electrolyte and a battery were prepared according to the method in Example 1. The only difference was that the concentration of LiFSI in the organic solvent was 3 mol / L.

[0059] Example 6

[0060] A lithium electrolyte and a battery were prepared according to the method in Example 1. The only difference was that the concentration of LiFSI in the organic solvent was 5 mol / L.

[0061] Example 7

[0062] A lithium electrolyte and a battery were prepared according to the method in Example 1. The only difference was that the concentration of LiFSI in the organic solvent was 6 mol / L.

[0063] Example 8

[0064] A lithium electrolyte and a battery were prepared according to the method in Example 1. The only difference was that the concentration of LiDFBOP in the organic solvent was 0.1 mol / L.

[0065] Example 9

[0066] A lithium electrolyte and a battery were prepared according to the method in Example 1. The only difference was that the concentration of LiDFBOP in the organic solvent was 0.2 mol / L.

[0067] Example 10

[0068] A lithium electrolyte and a battery were prepared according to the method in Example 1. The only difference was that the concentration of LiDFBOP in the organic solvent was 0.3 mol / L.

[0069] Embodiment 11

[0070] A lithium electrolyte and a battery were prepared according to the method in Example 1. The only difference was that the concentration of LiDFBOP in the organic solvent was 0.4 mol / L.

[0071] Example 12

[0072] A lithium electrolyte and a battery were prepared according to the method in Example 1. The only difference was that the concentration of LiDFBOP in the organic solvent was 0.6 mol / L.

[0073] Embodiment 13

[0074] A lithium electrolyte and a battery were prepared according to the method in Example 1. The only difference was that the positive electrode material was LFP.

[0075] The thermal runaway temperatures of the NCM622 / Gr soft pack battery puncture tests in Examples 1 to 12 are shown in Table 1.

[0076] Table 1. Thermal runaway temperature of NCM622 / Gr soft pack battery puncture test

[0077]

[0078]

[0079]

[0080] As can be seen from Table 1, the maximum temperature of thermal runaway in the needle puncture test of the present invention is about 450°C, which shows that the electrolyte of the present invention has good non-flammability and has excellent performance in ensuring the safety performance of lithium-ion batteries.

[0081] Figure 7 This is the cycle data of the LFP / Gr soft pack battery in Example 13 of the present invention. It can be seen that the LFP / Gr battery using the electrolyte of the present invention also has good cycle stability. The electrolyte of the present invention is not only applicable to the NCM622 / Gr system, but also to the LFP / Gr system.

[0082] In the present invention, the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" etc. mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the described specific features, structures, materials or characteristics may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0083] Although the above embodiments have been shown and described, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations on the present invention. Changes, modifications, substitutions and variations of the above embodiments by those of ordinary skill in the art are all within the scope of protection of the present invention.

Claims

1. A flame retardant electrolyte, characterized in that: The invention comprises lithium salt, organic solvent and film-forming additive, wherein the organic solvent is a mixed solvent comprising fluorinated carboxylic acid ester and hydrofluoroether, the concentration of the lithium salt in the organic solvent is 2 mol / L to 6 mol / L, and the film-forming additive is lithium bis(oxalate)difluorophosphate.

2. The flame-retardant electrolyte according to claim 1, characterized in that: The volume percentage of the fluorinated carboxylic acid ester in the organic solvent is 50% to 80%.

3. The flame retardant electrolyte according to claim 1, characterized in that: The fluorinated carboxylic acid ester is ethyl difluoroacetate.

4. The flame retardant electrolyte according to claim 1, characterized in that: The hydrofluoroether is 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether.

5. The flame retardant electrolyte according to claim 1, characterized in that: The concentration of the film-forming additive in the organic solvent is 0.1 mol / L to 0.6 mol / L.

6. The flame retardant electrolyte according to claim 1, characterized in that: The lithium salt is bis(fluorosulfonyl)imide lithium salt.

7. A method for preparing a flame retardant electrolyte, characterized in that: The electrolyte is the electrolyte according to any one of claims 1 to 6, comprising the following steps: The fluorinated carboxylic acid ester and the hydrofluoroether are mixed evenly to prepare a mixed solvent, the lithium salt and the film-forming additive are added into the mixed solvent, and the mixture is stirred evenly to obtain a flame-retardant electrolyte.

8. Application of a flame retardant electrolyte in a graphite negative electrode, characterized in that: The electrolyte is the electrolyte according to any one of claims 1 to 6.

9. The use of the flame-retardant electrolyte in a graphite negative electrode according to claim 8, characterized in that: The graphite negative electrode, positive electrode plate and separator are combined into a lithium / graphite battery, and the electrolyte is injected.

10. The use of the flame-retardant electrolyte in a graphite negative electrode according to claim 9, characterized in that: The positive electrode plate is made of lithium nickel cobalt manganese oxide positive electrode material or lithium iron phosphate positive electrode material.

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