Electrolyte adopting double salts and efficient flame-retardant additive and application of electrolyte in lithium iron phosphate battery

Abstract

The invention provides an electrolyte adopting double salts and an efficient flame-retardant additive and application of the electrolyte in a lithium iron phosphate battery, and relates to the technical field of lithium iron phosphate batteries. The electrolyte comprises double salts, a flame retardant additive and a solvent, wherein the double salts are lithium bis (fluorosulfonyl) imide (LiFSI) and lithium hexafluorophosphate (LiPF6). Preferably, the electrolyte contains a high-fluoro diluent, so that the overall solvation ability of solvent molecules can be reduced, and a high-stability solvation structure is formed. The electrolyte provided by the invention can inhibit generation of combustible gas and improve the energy density of the lithium iron phosphate battery, and the additive in the formula is beneficial to formation of a stable SEI film on the surface of an electrode and reduction of loss of active substances, so that the service life of the battery is prolonged.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium iron phosphate batteries, and in particular to an electrolyte using double salts and a high-efficiency flame retardant additive and application thereof in lithium iron phosphate batteries. Background Art

[0002] Lithium iron phosphate batteries, with their excellent long cycle life and high safety characteristics, have become the key power source for the rapid development of electric vehicles and energy storage systems. However, with the rapid expansion of the new energy vehicle market, consumers have increasingly stringent requirements for battery range, safety performance and service life, which has brought unprecedented challenges to the research and development of lithium iron phosphate batteries.

[0003] The performance of electrolyte, as the core component of lithium-ion batteries, directly determines the overall performance of the battery. Although the organic electrolyte used in traditional lithium iron phosphate batteries has high ionic conductivity and good electrochemical stability, its flammable and explosive properties constitute a major obstacle to battery safety. Under extreme conditions of use, such as overcharging, short circuit or high temperature environment, the electrolyte and membrane components inside the battery are prone to thermal runaway, releasing a large amount of flammable gas, which may cause fire or even explosion, seriously threatening the personal and property safety of users.

[0004] Under the current technical level, lithium iron phosphate batteries face great safety risks, limited service life, and are very likely to produce a large amount of flammable gas under thermal runaway conditions. In addition, factors such as electrochemical stability and conductivity also limit the further improvement of battery energy density. These have become key issues that urgently need to be solved in the field of lithium-ion battery technology. Summary of the invention

[0005] In view of the above background and the deficiencies of the prior art, the present invention aims to develop a new electrolyte formula. Through the synergistic effect of the innovative dual salt mechanism and the efficient flame retardant additive, the flame retardant performance of the lithium iron phosphate battery is improved, its service life is extended, and the generation of flammable gas is effectively suppressed, thereby comprehensively improving the safety performance and comprehensive performance of the battery.

[0006] To achieve the above goals, the applicant introduced special flame retardant additives to effectively curb the spread of fire and significantly improve the safety of the battery. At the same time, by adopting a dual salt mechanism to optimize the composition of the electrolyte, the decomposition and consumption of the electrolyte during the cycle is reduced, ensuring efficient mass transfer and stability of the electrolyte, thereby extending the overall service life of the battery. In addition, the applicant has fine-tuned the composition of the electrolyte to reduce the flammable gas generated during thermal runaway, reduce the risk of fire, and ensure safety during use. While improving safety, the new electrolyte formula also takes into account the improvement of conductivity and chemical stability, providing potential possibilities for further increasing the energy density of the battery.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] The invention provides an electrolyte using a double salt and a high-efficiency flame retardant additive, comprising a double salt, a flame retardant additive and a solvent, wherein the double salt is lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate (LiPF6).

[0009] Preferably, in the electrolyte of the present invention, the salt anions are mainly two anions: hexafluorophosphate ion (PF6 - ), bis(fluorosulfonyl)imide (FSI - ), one or more anions selected from the following may be added: bis(trifluorosulfonyl)imide (TFSI - ), perchlorate (ClO4 - ), tetrafluoroborate (BF4 - ), boric acid bis(oxalate) (BOB - ), difluorooxalate borate (DFOB - ).

[0010] Preferably, the flame retardant additive molecules / ions are selected from one or more of the following: fluoroethylene carbonate (FEC), vinylene carbonate (VC), difluoroethylene sulfate (DTD), difluorophosphate (PO2F2 - ), preferably one or more of fluoroethylene carbonate (FEC), vinylene carbonate (VC) and diethylene sulfate (DTD).

[0011] Preferably, the solvent includes cyclic carbonate and chain carbonate. Cyclic carbonate is an important component of the electrolyte and plays a vital role in the electrolyte.

[0012] Preferably, the cyclic carbonate is selected from one or more of the following: ethylene carbonate (EC), propylene carbonate (PC), preferably ethylene carbonate (EC).

[0013] Preferably, the linear carbonate is selected from one or more of the following: dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), preferably ethyl methyl carbonate (EMC).

[0014] Furthermore, the electrolyte contains a highly fluorinated diluent, which can reduce the overall solvation capacity of the solvent molecules, spontaneously allowing more anions to enter the solvation sheath of the cations, thereby forming a highly stable solvation structure. The introduction of the diluted electrolyte with the diluent is conducive to the selective reduction of anions, generating a SEI (Solid Electrolyte Interface) film rich in LiF, Li2SO3, Li2SO4 and Li3N compounds, thereby constructing a thermally inert interface between the negative electrode and the electrolyte, exhibiting excellent thermal decomposition and solubility under thermal abuse conditions, effectively reducing the exothermic side reaction between the highly active lithiated graphite and the electrolyte, thereby reducing the release of heat and combustible gases at the initial stage of thermal failure.

[0015] Preferably, the diluent is an organic liquid that cannot dissolve lithium salts, and is not considered to participate in coordination, but can play a role in modifying the solvation structure composition, increasing ion mobility, and improving the electrochemical window of the electrolyte. The diluent is selected from one or more of the following: 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), 2,2-dimethoxy-4-(trifluoromethyl)-1,3-dioxolane ether (DTDL), 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TFTFE), preferably 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE).

[0016] Preferably, the content of the solvent in the electrolyte is 50-87 wt.%; in the embodiments of the present invention, the total content of EC and EMC is 50.00 wt.% (Example 6) to 86.83 wt.% (Example 3).

[0017] Preferably, the content of the double salt in the electrolyte is 10-14wt.%, and the mass ratio of lithium bis(fluorosulfonyl)imide (LiFSI) to lithium hexafluorophosphate (LiPF6) is (0.75-0.80):1; in an embodiment of the present invention, the total content of LiFSI and LiPF6 is 12.90wt.% (Example 3) to 13.60wt.% (Example 2), and the mass ratio of the two is 3:4 (Example 3) to 4:5 (Example 2).

[0018] Preferably, the content of the flame retardant additive in the electrolyte is 2-3 wt.%; in the embodiments of the present invention, the total content of VC and DTD is 2.00 wt.% (embodiment 3) to 3.00 wt.% (embodiment 2).

[0019] Preferably, the content of the high-fluorinated diluent in the electrolyte is 20-35 wt.%; in the embodiments of the present invention, the total content of TTE is 19.47 wt.% (Example 2) to 33.98 wt.% (Example 6).

[0020] The electrolyte of the present invention is prepared by conventional methods in the art, and the specific preparation process is described in the examples.

[0021] The present invention also provides the use of the electrolyte using the double salt and the high-efficiency flame retardant additive in a lithium iron phosphate battery.

[0022] The core technical features of the present invention include:

[0023] 1) Dual salt mechanism: Combining the high conductivity, low viscosity and thermal stability of LiFSI with the good interface compatibility of LiPF6, it effectively improves the ion conduction efficiency and electrochemical stability of the electrolyte and extends the battery cycle life.

[0024] 2) High-efficiency flame retardant additives: Through the carefully selected combination of additives (such as VC, DTD, etc.), the flame retardant properties of the electrolyte are significantly improved, the risk of thermal runaway is reduced, and the overall safety of the battery is improved.

[0025] The present invention has the following beneficial effects:

[0026] 1) Inhibit the generation of flammable gases: The unique additive formula effectively reduces the gas generation of the battery under extreme conditions such as overcharging and short circuit, further improving the safety of the battery.

[0027] 2) Improve energy density: The optimized electrolyte formula helps to improve the efficiency of ion transmission inside the battery, which indirectly promotes the improvement of battery energy density and meets the demand of new energy vehicles for long driving range.

[0028] 3) Long-life design: The additives in the formula help form a stable SEI film on the electrode surface, reducing the loss of active substances and thus extending the service life of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0030] Figure 1 The main gas component analysis diagrams corresponding to the batteries prepared in the control example, embodiment 2, and embodiment 3 in the thermal runaway experiment;

[0031] Figure 2 It is a comparison chart of the combustible gas and CO2 content corresponding to the batteries prepared in the control example, embodiment 2, and embodiment 3 in the thermal runaway experiment. DETAILED DESCRIPTION

[0032] In order to make the technical problems, technical solutions and advantages to be solved by the present invention more clear, a detailed description will be given below with reference to the accompanying drawings and specific embodiments.

[0033] Control example: LiFSI was not used, LiPF6 12.932wt.%, EC 31.650wt.%, EMC 52.504wt.%, TTE was not used, VC 1.942wt.%, DTD 0.972wt.%. The molar ratio was EC:EMC:TTE=4:5:0.

[0034] Example 2: LiFSI 5.825wt.%, LiPF6 7.767wt.%, EC 9.709wt.%, EMC 54.369wt.%, TTE 19.417wt.%, VC 1.942wt.%, DTD 0.971wt.%, the molar ratio is EC:EMC:TTE=2:11:4.

[0035] Example 3: LiFSI 5.814wt.%, LiPF6 7.089wt.%, EC 30.846wt.%, EMC 54.291wt.%, TTE not used, VC 0.980wt.%, DTD 0.980wt.%, the molar ratio is EC:EMC:TTE=2:3:0.

[0036] Example 4: LiFSI 5.825wt.%, LiPF6 7.767wt.%, EC 29.126wt.%, EMC 34.951wt.%, TTE 19.417wt.%, VC 1.942wt.%, DTD 0.971wt.%, the molar ratio is EC:EMC:TTE=5:5:1.

[0037] Example 5: LiFSI 5.825wt.%, LiPF6 7.767wt.%, EC 19.417wt.%, EMC 34.951wt.%, TTE 29.126wt.%, VC 1.942wt.%, DTD 0.971wt.%, the molar ratio is EC:EMC:TTE=2:4:1.

[0038] Example 6: LiFSI 5.825wt.%, LiPF6 7.767wt.%, EC 14.563wt.%, EMC 34.951wt.%, TTE 33.981wt.%, VC 1.942wt.%, DTD 0.971wt.%, the molar ratio is EC:EMC:TTE=4:8:3.

[0039] The electrolytes of the above embodiments and control examples were prepared in an argon-protected glove box (H2O≤0.1ppm, O2≤0.1ppm). During the preparation, the lithium salt was weighed first, and then the ester solvent and the ether solvent were added in order according to the proportion, and then the additives were added, stirred evenly to form a mixed solution, and matched with a dry battery cell with lithium iron phosphate as the positive electrode material and graphite as the negative electrode material, after the injection and formation process, the battery was cycled at a rate of 0.5C in a charger and discharger, and when the number of cycles was 750, the battery capacity was recorded, and the capacity retention rate (current battery capacity / initial battery capacity·100%) was calculated. The test results are shown in Table 1.

[0040] The electrolyte was subjected to DSC (differential scanning calorimetry) to test the thermal stability of the fully charged battery material. The heating temperature range was 50-550°C and the temperature rise rate was 10°C min -1 , record the heat generation and analyze it to simulate the heating process of the electrolyte inside the lithium iron phosphate battery, and record the self-heating temperature T of the electrolyte during the heating process onset , thermal trigger temperature T peak , where T onset Defined as a temperature rise rate greater than 0.02℃·min -1 , T peak Defined as a temperature rise rate greater than 1°C·s -1 , the test results are shown in Table 2.

[0041] Table 1

[0042]

[0043] Table 2

[0044]

[0045] By analyzing the component differences and test results of the control example, embodiment 4, embodiment 5 and embodiment 6, the following conclusions can be drawn:

[0046] In the control example, the electrolyte contains a higher EC ratio (32.60wt.%). This formulation leads to poor thermal stability of the control example, which is manifested as T onset (self-heating temperature) and T peak(Thermal runaway trigger temperature) is low. Due to the high reactivity of cyclic carbonate EC, the exothermic reaction temperature with the lithium-inserted negative electrode with strong reducibility is low, resulting in poor thermal stability. onset and T peak The test result is low, indicating that the thermal stability of the battery is poor and there may be a greater risk of thermal runaway in actual applications.

[0047] The capacity retention rate of the control example is similar to that of Example 3 and Example 2, but the initial capacity and final capacity of Example 3 and Example 2 are higher than that of the control example, which indicates that the formulations of Example 3 and Example 2 maintain a better cycle state during the charge and discharge process. TTE is introduced into the formulation of Example 2, with a preferred range of 19.00wt.%-20.00wt.%, which may be one of the reasons for its higher capacity retention rate.

[0048] Example 2: Compared with the control example, LiFSI with high thermal stability is introduced, the proportion of LiPF6 is reduced, a double salt synergy is formed, EC with high reactivity is reduced, and TTE is introduced as an additive. This adjustment improves the stability and safety of the battery because LiFSI has high thermal stability and low oxidizability, thereby improving the battery life level. The addition of TTE may help improve the flame retardant properties of the electrolyte.

[0049] Example 3: The proportion of LiFSI is slightly lower than that of Example 2, but still higher than that of the control example, and TTE is not introduced. This formula still improves the stability of the electrolyte to a certain extent, verifying that the dual salts synergistically improve the thermal stability and safety of the battery, but compared with Example 2, more EC is still used, and its performance improvement may not be as significant as that of Example 2.

[0050] Example 4: By reducing the EC ratio and increasing the TTE ratio, the thermal stability of the electrolyte is improved. onset and T peak The test results of the prepared samples were higher than those of the control samples, indicating that the thermal stability was improved, and the heat generation (ΔH) was also lower than that of the control samples.

[0051] Example 5: Compared with Example 4, the EC ratio of Example 5 is further reduced, while the TTE is further increased. onset and T peak The test result of Example 4 is slightly lower than that of Example 4, but still higher than that of the control example, indicating that the thermal stability is improved, accompanied by a significant decrease in heat generation.

[0052] Example 6: Example 6 has the lowest EC ratio and the highest TTE ratio, which leads to T onset and T peakThe test results are slightly higher than those of the control example, indicating that the thermal stability is better, while the heat production is lower than the higher level of the control example, and the heat production is significantly reduced.

[0053] In Examples 4, 5 and 6, the proportion of EC is relatively low, about 30.00 wt.%, about 20.00 wt.% and about 15.00 wt.%, respectively. The proportion of EMC is relatively low, about 35.00 wt.%. This adjustment significantly improves the thermal stability of the electrolyte, T onset This shows that the thermal stability of the electrolyte can be effectively improved by reducing the proportion of EC and increasing the proportion of EMC.

[0054] In summary, the thermal stability of the electrolyte can be effectively improved by adjusting the ratio of EC and EMC and introducing LiFSI to replace part of LiPF6. In addition, the addition of TTE also helps to improve the ionic conductivity and thermal stability of the electrolyte. These findings are of great significance for optimizing the electrolyte formulation, improving the safety performance of the battery, and extending the battery life.

[0055] After the 10Ah battery is prepared, it is tested in a fixed container. During the test, the reaction chamber is replaced by an inert gas (such as nitrogen), and two pressure sensors and eight K-type thermocouples are set up to measure the pressure and temperature changes during the thermal runaway process, record the battery temperature and ambient pressure, and the gas released during the thermal runaway process can be collected using a gas collection bag for further gas composition analysis. The gas composition is accurately quantified by gas chromatography (GC), and the test results are shown in Tables 3 and 4.

[0056] Table 3

[0057]

[0058] Table 4

[0059]

[0060] By analyzing the component characteristics and gas production results of the control example and Example 2, the following conclusions can be drawn:

[0061] In the control example, since LiFSI is not used, the proportion of LiPF6 is higher, which may cause the electrolyte to produce more gas during thermal runaway, especially combustible gases such as H2, methane, ethane and ethylene.

[0062] In Example 2, the addition of LiFSI and the reduction of the proportion of LiPF6, as well as the introduction of TTE, significantly reduced the gas production, especially the generation of combustible gas. This indicates that the optimized electrolyte formula improves thermal stability and reduces decomposition reactions during thermal runaway.

[0063] In Example 3, the addition of LiFSI and the reduction of the proportion of LiPF6 significantly reduced the production of combustible gas. This shows that the electrolyte formula optimized by the dual salt system has improved thermal stability, so that the reaction during thermal runaway is more converted into the output of non-combustible gases such as CO2.

[0064] Figure 1 The following are the main gas component analysis diagrams of the batteries prepared in the control example, example 2, and example 3 in the thermal runaway experiment. Figure 1 It can be seen that the gas production of H2 decreased from 1.3782L in the control example to 1.0115L in Example 2 and 0.4495L in Example 3, reducing the original gas by about 26% and 67%. The gas production of CO decreased from 0.2097L in the control example to 0.0626L in Example 2, reducing the original gas by about 70%. The gas production of hydrocarbon gases such as methane, ethane and ethylene also decreased, indicating that the decomposition of organic solvents in the electrolyte was reduced.

[0065] Figure 2 The following is a comparison chart of the flammable gas and CO2 content corresponding to the batteries prepared in the control example, embodiment 2, and embodiment 3 in the thermal runaway experiment, wherein the flammable gas includes Figure 1 H2, CO, methane, ethane, ethylene and propylene. Figure 2 It can be seen that the amount of combustible gas produced is reduced from 2.4482 L in the control example to 1.4473 L in Example 2 and 1.4702 L in Example 3, which is a reduction of about 40% of the original gas.

[0066] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. An electrolyte using double salt and high-efficiency flame retardant additive, characterized in that: The invention comprises a double salt, a flame retardant additive and a solvent, wherein the double salt is lithium bis(fluorosulfonyl)imide and lithium hexafluorophosphate.

2. The electrolyte using double salt and high-efficiency flame retardant additive according to claim 1, characterized in that: The flame retardant additive molecules / ions are selected from one or more of the following: fluoroethylene carbonate, vinylene carbonate, vinyl sulfate, difluorophosphate, preferably one or more of fluoroethylene carbonate, vinylene carbonate and vinyl sulfate.

3. The electrolyte using double salt and high-efficiency flame retardant additive according to claim 1, characterized in that: The solvent includes cyclic carbonates and chain carbonates.

4. The electrolyte using double salt and high-efficiency flame retardant additive according to claim 3, characterized in that: The cyclic carbonate is selected from one or more of the following: ethylene carbonate, propylene carbonate, preferably ethylene carbonate; The chain carbonate is selected from one or more of the following: dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, preferably ethyl methyl carbonate.

5. The electrolyte using double salt and high-efficiency flame retardant additive according to any one of claims 1 to 4, characterized in that: The electrolyte contains a highly fluorinated diluent selected from one or more of the following: 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 2,2-dimethoxy-4-(trifluoromethyl)-1,3-dioxolane ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, preferably 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

6. The electrolyte using double salt and high-efficiency flame retardant additive according to claim 5, characterized in that: The content of the solvent in the electrolyte is 50-87 wt.%.

7. The electrolyte using double salt and high-efficiency flame retardant additive according to claim 5, characterized in that: The content of the double salt in the electrolyte is 10-14wt.%, and the mass ratio of lithium bis(fluorosulfonyl)imide to lithium hexafluorophosphate is (0.75-0.80):

1.

8. The electrolyte using double salt and high-efficiency flame retardant additive according to claim 5, characterized in that: The content of the flame retardant additive in the electrolyte is 2-3 wt.%.

9. The electrolyte using double salt and high-efficiency flame retardant additive according to claim 5, characterized in that: The content of the high-fluorinated diluent in the electrolyte is 20-35 wt.%.

10. Use of the electrolyte comprising double salt and high-efficiency flame retardant additive according to any one of claims 1 to 9 in a lithium iron phosphate battery.

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