A mixed lithium salt electrolyte, its preparation method and application

Abstract

The present invention belongs to the technical field of lithium-ion batteries, and discloses a mixed lithium salt electrolyte, a preparation method thereof and an application thereof. The mixed lithium salt electrolyte of the present invention comprises the following raw materials: an electrolyte lithium salt, an inorganic lithium compound additive, an organic additive and a carbonate solvent. By adding an inorganic lithium additive mixed with 4 lithium compounds to the electrolyte of the existing lithium-ion battery, the present invention can play a role in efficient film formation, and the high-entropy system formed by these 4 lithium compounds can promote the dissolution of some lithium salts with low solubility, such as lithium nitrate LiNO3. In addition, the weaker solvation effect caused by the higher disorder of the system leads to the improvement of lithium ion kinetics, and the existence of the anion solvation structure can promote the rapid formation of a stable interface on the electrode surface at low temperature, thereby effectively improving the performance of the battery.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and in particular to a mixed lithium salt electrolyte, a preparation method thereof, and an application thereof. Background Art

[0002] Lithium-ion batteries are the most successful commercial secondary batteries nowadays, and they have advantages such as high actual energy density and long cycle life. However, the booming electric vehicle (EV) industry has put forward more stringent requirements for battery performance, such as fast charging ability and safety within a wide temperature range. Specifically, the negative electrode is regarded as a limiting factor for battery safety and fast charging performance due to its slow electrochemical reaction kinetics. In practical applications, graphite electrodes have a high theoretical specific capacity (372 mAh / g), good electronic and ionic conductivity, and long cycle life, plus their low lithium intercalation potential (<0.2 V vs Li + / Li), enabling the battery to achieve a relatively high energy density, so it has become a commonly used negative electrode material for commercial lithium-ion batteries (LIB). However, the graphite negative electrode is still affected by poor Li intercalation kinetics. Especially when charging at low temperatures, lithium ions cannot be intercalated into graphite and are deposited and lithiated on its surface, thus posing a safety hazard. Therefore, the interfacial stability problem between the graphite negative electrode and the electrolyte is a difficult problem that hinders the improvement of electrolyte performance and even the development of lithium-ion battery technology.

[0003] In order to improve battery performance, in recent years, researchers have improved the electrochemical performance of graphite negative electrodes in various ways, including modification of graphite structure, use of composite materials, optimization of charging procedures, and development of advanced electrolytes. Among these methods, researchers have found that advanced electrolytes are extremely effective and convenient for fast charging. Currently, the design of related electrolytes mainly focuses on two aspects: the solid electrolyte interphase (SEI) structure and the solvated lithium ion structure, and is carried out by adjusting the SEI structure and optimizing the solvation structure. It is generally believed that the inorganic components in SEI contribute to improving thermodynamic stability, mechanical properties, structural compactness, and ionic conductivity, while the organic components in SEI provide its flexibility. It should be noted that the ion migration barriers of most inorganic components in SEI (i.e., Li3N, Li2O, and Li2CO3) are lower than those of alkyl lithium carbonates. Therefore, adjusting the electrolyte composition is very important for optimizing the SEI structure.

[0004] Therefore, those skilled in the art are committed to developing new electrolyte additives and electrolytes around existing electrolyte solvents, lithium salts, or mature electrolytes to further improve the performance of lithium-ion batteries. Summary of the Invention

[0005] The object of the present invention is to provide a mixed lithium salt electrolyte, a preparation method and an application thereof, to improve the interfacial problem between the electrolyte and the graphite electrode, enhance the electrochemical performance of the electrolyte in the graphite-based battery, and enable the electrolyte to be better applied in the graphite-based battery.

[0006] In order to achieve the above object of the invention, the present invention provides the following technical solutions:

[0007] The present invention provides a mixed lithium salt electrolyte, comprising the following raw materials: an electrolyte lithium salt, an inorganic lithium compound additive, an organic additive and a carbonate solvent;

[0008] Among them, the inorganic lithium compound additive is a mixture of Li2O, LiF, Li2CO3 and LiNO3;

[0009] The concentrations of Li2O, LiF, Li2CO3 and LiNO3 in the mixed lithium salt electrolyte are independently 0.05 to 0.2 mol / L.

[0010] Preferably, in the mixed lithium salt electrolyte, the electrolyte lithium salt includes one or more of lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium perchlorate, lithium difluoro(oxalato)borate and lithium tetrafluoroborate.

[0011] Preferably, in the mixed lithium salt electrolyte, the concentration of lithium ions contained in the electrolyte lithium salt in the mixed lithium salt electrolyte is 0.5 to 2 mol / L.

[0012] Preferably, in the mixed lithium salt electrolyte, the organic additive includes fluoroethylene carbonate and / or vinylene carbonate.

[0013] Preferably, in the mixed lithium salt electrolyte, the carbonate solvent includes one or more of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate.

[0014] Preferably, in the mixed lithium salt electrolyte, the volumes of fluoroethylene carbonate and vinylene carbonate are independently 1 to 5% of the total volume of the mixed lithium salt electrolyte.

[0015] Preferably, in the mixed lithium salt electrolyte, the particle sizes of Li2O, LiF, Li2CO3 and LiNO3 are independently 80 to 100 nm.

[0016] The present invention also provides a preparation method of the mixed lithium salt electrolyte, comprising the following steps:

[0017] Mix the electrolyte lithium salt, the inorganic lithium compound additive, the organic additive and the carbonate solvent to obtain the mixed lithium salt electrolyte.

[0018] The present invention also provides an application of the mixed lithium salt electrolyte or the mixed lithium salt electrolyte prepared by the preparation method of the mixed lithium salt electrolyte in a lithium ion battery, and the lithium ion battery includes: a positive electrode, a negative electrode, a mixed lithium salt electrolyte, and a separator located between the positive electrode and the negative electrode.

[0019] Preferably, in the application of the mixed lithium salt electrolyte in the lithium ion battery, the negative electrode includes a negative electrode current collector and a negative electrode film; the negative electrode film includes a negative electrode active material, a conductive agent, and a binder; the negative electrode active material includes one or more of metallic lithium, natural graphite, artificial graphite, mesophase microcarbon beads, hard carbon, soft carbon, silicon, and silicon-carbon composites.

[0020] It can be seen from the above technical solutions that, compared with the prior art, the present invention has the following beneficial effects:

[0021] By adding an inorganic lithium additive mixed with 4 lithium compounds to the electrolyte of the existing lithium ion battery, the present invention can play a role in forming a film efficiently, and the high-entropy system formed by these 4 lithium compounds can promote the dissolution of some lithium salts with low solubility, such as lithium nitrate LiNO3. In addition, the weaker solvation effect caused by the higher disorder of the system leads to the improvement of lithium ion kinetics, and the existence of the anion solvation structure can promote the rapid formation of a stable interface on the electrode surface at low temperature, thereby effectively improving the performance of the battery. Description of the Drawings

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art.

[0023] Figure 1 It is the apparent diagram of the electrolyte obtained in Example 1;

[0024] Figure 2 It is the apparent diagram of the electrolytes obtained in Comparative Examples 2 to 6;

[0025] Figure 3 It is the influence of the addition of the inorganic lithium compound additive on the Li + solvation environment in the electrolyte. In the figure, (a) is the Raman spectrum diagram of the electrolytes obtained in Example 1, Comparative Example 1, and Comparative Example 7, (b) is the FTIR characterization diagram of the electrolytes obtained in Example 1, Comparative Example 1, and Comparative Example 7, and (c) is the 7 Li NMR diagram of the electrolytes obtained in Example 1 and Comparative Example 1;

[0026] Figure 4 It is the ionic conductivity, R SEI activation energy, Rct Activation energy. In the figure, (a) is the ionic conductivity, and (b) is R SEI Activation energy, and (c) is R ct Activation energy;

[0027] Figure 5 Charge-discharge curves of the lithium-ion batteries prepared by adding 5 vt% FEC to Comparative Application Example 1, Comparative Application Example 8, Application Example 1, and Comparative Example 1 at a current density of 0.1C in the first three cycles. In the figure, (a) is Comparative Application Example 1, (b) is Comparative Application Example 8, (c) is the lithium-ion battery prepared by adding 5 vt% FEC to Comparative Example 1, and (d) is Application Example 1;

[0028] Figure 6 Charge-discharge curves of Comparative Application Example 7 in the first three cycles at a current density of 0.1C;

[0029] Figure 7 Charge-discharge curves of Comparative Application Examples 2-5 in the first three cycles at a current density of 0.1C. In the figure, (a) is Comparative Application Example 2, (b) is Comparative Application Example 3, (c) is Comparative Application Example 4, and (d) is Comparative Application Example 5;

[0030] Figure 8 Rate performance test results of different application examples. In the figure, (a) is the rate performance results of Application Example 1, Comparative Application Example 1, and Comparative Application Example 8, (b) is the rate performance results of the lithium-ion battery with the lithium salt concentration of Comparative Application Example 1 and Comparative Example 1 replaced by 2 mol / L, (c) is the rate performance results of the lithium-ion battery prepared by adding 5 vt% FEC to Application Example 1, Comparative Application Example 7, and Comparative Example 1, and (d) is the rate performance results of Application Example 1 and Comparative Application Examples 1-4;

[0031] Figure 9 EIS diagrams of Application Example 1 and Comparative Application Example 1;

[0032] Figure 10 Cycle performance results of Application Example 1, Comparative Application Example 1, and Comparative Application Example 8;

[0033] Figure 11 Cycle performance results of Application Example 1 and Comparative Application Examples 1-4. In the figure, (a) is the capacity change, and (b) is the Coulomb efficiency;

[0034] Figure 12 Effectiveness of SEI film formation of the electrolyte before and after adding inorganic lithium compounds at low temperature. In the figure, (a) is the electrochemical performance of graphite in the electrolytes of Comparative Example 1 and Example 1 at different temperatures at a current density of 0.1C, (b) is the charge-discharge curves of graphite in the electrolyte of Comparative Example 1 at different temperatures, and (c) is the charge-discharge curves of graphite in the electrolyte of Example 1 at different temperatures. Detailed implementation mode

[0035] The present invention provides a mixed lithium salt electrolyte, comprising the following raw materials: electrolyte lithium salt, inorganic lithium compound additive, organic additive and carbonate solvent;

[0036] Among them, the inorganic lithium compound additive is a mixture of Li2O, LiF, Li2CO3 and LiNO3;

[0037] The concentrations of Li2O, LiF, Li2CO3 and LiNO3 in the mixed lithium salt electrolyte are independently 0.05 - 0.2 mol / L.

[0038] In the present invention, the electrolyte lithium salt preferably includes one or more of lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium perchlorate (LiClO4), lithium difluoro(oxalato)borate (LiDFOB) and lithium tetrafluoroborate (LiBF4), more preferably includes one or more of lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl)imide and lithium bis(fluorosulfonyl)imide, and still more preferably is lithium hexafluorophosphate.

[0039] When multiple electrolyte lithium salts are selected, the present invention does not limit the ratio between the respective electrolyte lithium salts, and a scheme well-known to those skilled in the art can be adopted.

[0040] In the present invention, the concentration of lithium ions contained in the electrolyte lithium salt in the mixed lithium salt electrolyte is preferably 0.5 - 2 mol / L, more preferably 0.8 - 1.5 mol / L, and still more preferably 1 mol / L.

[0041] In the present invention, the concentrations of Li2O, LiF, Li2CO3 and LiNO3 in the mixed lithium salt electrolyte are independently preferably 0.05 - 0.2 mol / L, more preferably 0.08 - 0.15 mol / L, and still more preferably 0.1 mol / L.

[0042] In the present invention, the particle sizes of Li2O, LiF, Li2CO3 and LiNO3 are independently preferably 80 - 100 nm.

[0043] In the present invention, the organic additive preferably includes fluoroethylene carbonate (FEC) and / or vinylene carbonate (VC), more preferably fluoroethylene carbonate or vinylene carbonate, and still more preferably fluoroethylene carbonate.

[0044] In the technical solution of the present invention, adding fluoroethylene carbonate and / or vinylene carbonate to the electrolyte is beneficial to the formation of an inorganic film and can also increase the organic components in the SEI film; the formed organic-inorganic composite SEI film has high toughness and better ionic conductivity.

[0045] In the present invention, the volume of the fluoroethylene carbonate and the volume of the vinylene carbonate are independently preferably 1-5% of the total volume of the mixed lithium salt electrolyte, more preferably 3-5%, and even more preferably 5%.

[0046] In the present invention, the carbonate solvent preferably includes one or more of ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC), more preferably includes one or more of ethylene carbonate, dimethyl carbonate, and diethyl carbonate, and even more preferably is a mixture of ethylene carbonate and dimethyl carbonate.

[0047] When multiple carbonate solvents are selected, the present invention does not limit the ratio between the carbonate solvents, and a scheme well-known to those skilled in the art can be adopted.

[0048] The present invention also provides a preparation method of the mixed lithium salt electrolyte, including the following steps:

[0049] Mix the electrolyte lithium salt, inorganic lithium compound additive, organic additive, and carbonate solvent to obtain the mixed lithium salt electrolyte.

[0050] The present invention does not limit the mixing conditions in the preparation method, and a scheme well-known to those skilled in the art can be adopted.

[0051] The present invention also provides an application of the mixed lithium salt electrolyte or the mixed lithium salt electrolyte prepared by the preparation method of the mixed lithium salt electrolyte in a lithium-ion battery, and the lithium-ion battery includes: a positive electrode, a negative electrode, a mixed lithium salt electrolyte, and a separator located between the positive electrode and the negative electrode.

[0052] In the present invention, the negative electrode preferably includes a negative electrode current collector and a negative electrode film.

[0053] In the present invention, the negative electrode film preferably includes a negative electrode active material, a conductive agent, and a binder.

[0054] In the present invention, the negative electrode active material preferably includes one or more of metallic lithium, natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, soft carbon, silicon, and silicon-carbon composites, more preferably includes one or more of metallic lithium, natural graphite, artificial graphite, mesophase carbon microspheres, and silicon-carbon composites, and even more preferably is one or two of natural graphite and artificial graphite.

[0055] The present invention does not limit other components in the lithium-ion battery, and materials well-known to those skilled in the art can be used.

[0056] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the protection scope of the present invention.

[0057] The reagents and materials used in the examples and comparative examples, if not otherwise specified, can be obtained through commercial purchase or prepared by conventional methods, and the instruments used can be obtained through commercial purchase.

[0058] Example 1

[0059] This example provides a mixed lithium salt electrolyte, which contains the following raw materials: lithium hexafluorophosphate, inorganic lithium compound additive, fluoroethylene carbonate, ethylene carbonate and dimethyl carbonate;

[0060] Among them, the concentration of lithium hexafluorophosphate is 1 mol / L;

[0061] The inorganic lithium compound additive is a mixture of Li2O, LiF, Li2CO3 and LiNO3, and the concentrations of Li2O, LiF, Li2CO3 and LiNO3 in the mixed lithium salt electrolyte are all 0.1 mol / L; the particle sizes of Li2O, LiF, Li2CO3 and LiNO3 are 80 - 100 nm;

[0062] The volume of fluoroethylene carbonate is 5% of the volume of the mixed lithium salt electrolyte;

[0063] The volume ratio of ethylene carbonate to dimethyl carbonate is 1:1.

[0064] This example also provides a preparation method of the mixed lithium salt electrolyte, which includes the following steps:

[0065] Mix ethylene carbonate and dimethyl carbonate according to the volume ratio to obtain an organic solvent; then dissolve lithium hexafluorophosphate in the organic solvent, and then add fluoroethylene carbonate. After mixing evenly, an electrolyte is obtained, and then the inorganic lithium compound additive is added one by one under stirring to obtain a mixed lithium salt electrolyte, denoted as 1.4M LP30 - 4S + 5% FEC.

[0066] Comparative Example 1

[0067] This comparative example provides an electrolyte, which is different from Example 1 in that: the inorganic lithium compound additive and fluoroethylene carbonate are removed, and other parameter conditions are the same as those in Example 1, denoted as LP30.

[0068] Comparative Example 2

[0069] This comparative example provides a mixed lithium salt electrolyte and a preparation method. The difference from Example 1 is that the inorganic lithium compound additive is only LiF, and the concentration of LiF in the mixed lithium salt electrolyte is 0.4 mol / L. Other parameter conditions are the same as those in Example 1, denoted as 1.4M LP30-LiF + 5% FEC.

[0070] Comparative Example 3

[0071] This comparative example provides a mixed lithium salt electrolyte and a preparation method. The difference from Example 1 is that the inorganic lithium compound additive is only Li2O, and the concentration of Li2O in the mixed lithium salt electrolyte is 0.4 mol / L. Other parameter conditions are the same as those in Example 1, denoted as 1.4M LP30-Li2O + 5% FEC.

[0072] Comparative Example 4

[0073] This comparative example provides a mixed lithium salt electrolyte and a preparation method. The difference from Example 1 is that the inorganic lithium compound additive is only Li2CO3, and the concentration of Li2CO3 in the mixed lithium salt electrolyte is 0.4 mol / L. Other parameter conditions are the same as those in Example 1, denoted as 1.4M LP30-Li2CO3 + 5% FEC.

[0074] Comparative Example 5

[0075] This comparative example provides a mixed lithium salt electrolyte and a preparation method. The difference from Example 1 is that the inorganic lithium compound additive is only LiNO3, and the concentration of LiNO3 in the mixed lithium salt electrolyte is 0.4 mol / L. Other parameter conditions are the same as those in Example 1, denoted as 1.4M LP30-LiNO3 + 5% FEC.

[0076] Comparative Example 6

[0077] This comparative example provides a mixed lithium salt electrolyte and a preparation method. The difference from Example 1 is that the inorganic lithium compound additive is only LiNO3, and the concentration of LiNO3 in the mixed lithium salt electrolyte is 0.1 mol / L. Other parameter conditions are the same as those in Example 1, denoted as 1.1M LP30-LiNO3 + 5% FEC.

[0078] Comparative Example 7

[0079] This comparative example provides a lithium salt electrolyte and a preparation method. The difference from Example 1 is that the inorganic lithium compound additive is replaced with lithium hexafluorophosphate, and the total concentration of lithium hexafluorophosphate in the mixed lithium salt electrolyte is 1.4 mol / L. The other parameter conditions are the same as those in Example 1, recorded as 1.4 M LP30 + 5% FEC.

[0080] Comparative Example 8

[0081] This comparative example provides an electrolyte, designated as LP57, comprising the following raw materials: lithium hexafluorophosphate, a mixed additive of vinylene carbonate and vinyl sulfate (DTD), and a mixed organic solvent of vinyl carbonate and ethyl methyl carbonate;

[0082] Wherein, the concentration of lithium hexafluorophosphate is 1 mol / L;

[0083] The volume of vinylene carbonate is 2% of the volume of the electrolyte, and the volume of vinyl sulfate is 1% of the volume of the electrolyte;

[0084] The volume ratio of ethylene carbonate to ethyl methyl carbonate is 3:7.

[0085] The compositions of the electrolytes described in Example 1 and Comparative Examples 1 to 8 are shown in Table 1.

[0086] Table 1. Composition of the electrolytes described in Example 1 and Comparative Examples 1 to 8

[0087]

[0088] A commercial 1.0M LiPF6-EC / DMC carbonate electrolyte (LP30) was selected as the base electrolyte, also known as Comparative Example 1. The solubility of LiNO3 in carbonates is negligible (≤1000ppm). 0.1mol / L LiF, 0.1mol / L Li2O, 0.1mol / L Li2CO3, 0.1mol / L LiNO3, and 5vt% FEC were added to the base electrolyte to obtain the mixed lithium salt electrolyte of Example 1. Its appearance is shown in the figure below. Figure 1 As shown. Figure 1 It can be seen that the mixed lithium salt electrolyte of Example 1 is in a suspension state, and no precipitation is found at the bottom, indicating that the sparingly soluble salt LiNO3 is completely dissolved therein. The electrolyte prepared by adding 0.4 mol / L of a single inorganic lithium compound and 5vt% FEC to the basic electrolyte, namely comparative examples 2 to 5, has the following appearance diagrams: Figure 2 As shown. Figure 2 It can be seen that Comparative Example 2 is a transparent solution, Comparative Examples 3 and 4 are suspensions, and a large amount of LiNO3 is not dissolved in Comparative Example 5. Even if the amount of LiNO3 added is reduced to 0.1 mol / L, that is, Comparative Example 6, Figure 2It can be found that there is still a large amount of LiNO3 undissolved at the bottom. It is worth noting that all of the LiNO3 in the electrolyte of Example 1 is dissolved, which is mainly attributed to the addition of a variety of inorganic lithium compounds, increasing the mixing entropy of the system, reducing the Gibbs free energy, and improving the solubility of LiNO3. This indicates that the 1.4M LP30-4S+5% FEC electrolyte prepared by introducing 4 inorganic lithium compounds (i.e., Example 1) has a high-entropy electrolyte effect.

[0089] The Raman, FTIR, 7 Li NMR analysis methods were used to study the influence of the addition of inorganic lithium compound additives on the Li + solvation environment in the electrolyte, and the results are as Figure 3 shown. Figure 3 In (a) is the Raman spectra of the electrolytes obtained from Example 1, Comparative Example 1, and Comparative Example 7. By analyzing the coordination strength between Li + and the solvent, it can be found that the peaks of Li coordinated with EC and DMC both increase, mainly because the increase in the lithium salt concentration enables more Li + to coordinate with the solvent; in addition, in the electrolyte of Comparative Example 7, the peaks of Li + coordinated with EC and DMC converge into one coordination peak, indicating that a large amount of organic solvents in the electrolyte of Comparative Example 7 coordinate with Li + . Figure 3 In (b) is the FTIR characterization diagram of the electrolytes obtained from Example 1, Comparative Example 1, and Comparative Example 7. It can be found that the vibration peaks of C=O in EC / FEC and DMC corresponding to 1799 cm -1 and 1749 cm -1 respectively. These two peaks in the electrolyte of Comparative Example 7 are significantly weakened, indicating that a large amount of organic solvents coordinate with Li + ; while in the electrolyte of Example 1, the changes in these two peaks are small, and at 1280 cm -1 the C-O bond corresponding to DMC and at 1070 cm -1 / 838 cm -1 the C-O bond corresponding to EC / FEC also change accordingly, indicating that fewer organic solvents participate in the coordination with Li + . Figure 3 In (c) is the 7 Li NMR diagram of the electrolytes obtained from Example 1 and Comparative Example 1. It can be found that the positive chemical shift of Li in the nuclear magnetic resonance further proves that the addition of 4 inorganic lithium compounds weakens the interaction between Li + and the solvent and enhances its interaction with the anion. In short, the characterization results of the above several electrolytes show that: the simultaneous introduction of 4 inorganic lithium compounds changes the solvation structure of the initial electrolyte, and after the dissociation of the 4 inorganic lithium compounds, the interaction between Li +- Solvent coordination forms a weakly solvated environment in the electrolyte.

[0090] Figure 4 The ionic conductivity, R SEI activation energy, and R ct activation energy of the electrolytes obtained in Example 1 and Comparative Example 1 were tested. Figure 4 (a) in it is the ionic conductivity. The ionic conductivity of Comparative Example 1 at room temperature is 8.05 mS / cm, and that of Example 1 is 6.25 mS / cm. The desolvation barrier of Li + in the two electrolytes was studied by fitting the electrochemical impedance spectra (EIS) of fresh Li||Li symmetric cells at different temperatures, as shown in Figure 4 (b) in Figure 4 and (c) in

[0091]

[0092] In Equation 1, k is the rate constant, T is the thermodynamic temperature, R ct / SEI is the ionic transfer resistance, A is the pre-exponential factor, E a is the activation energy, and R is the standard gas constant. By fitting the separated semi-circles (R ct , R SEI ) of the impedance of the Li||Li symmetric cell, the activation energy E a was obtained. R SEI represents the resistance of Li + passing through the SEI at medium frequency; R ct represents the charge transfer resistance of Li + at the SEI / electrolyte interface at lower frequencies. According to the fitted R SEI and R ct , the corresponding activation energy E a was obtained from Equation 1. Fitting calculations found that, relative to the higher SEI film impedance (85.0 kJ·mol -1 ) of the electrolyte in Comparative Example 1, the activation energy (74.6 kJ·mol -1 ) in the electrolyte of Example 1 decreased by 10%. It shows that the SEI film formed by the electrolyte of Example 1 can significantly reduce the diffusion impedance of lithium ions and accelerate the diffusion kinetics of lithium ions. In addition, the anions contained in the inorganic lithium compound can participate in the solvation layer of lithium ions, replacing some solvent molecules and reducing the desolvation energy of lithium ions. Therefore, it can be found that although the ionic conductivity of Example 1 is lower than that of Comparative Example 1, the activation energies in the diffusion process and charge transfer process of its electrolyte are both lower, thus having better Li + kinetics.

[0093] Application Example 1

[0094] This application example provides a method for preparing a 2032 lithium-ion battery, including the following steps:

[0095] (1) Prepare the positive electrode sheet: Mix lithium iron phosphate, binder polyvinylidene fluoride, and conductive agent acetylene black in a mass ratio of 8:1:1, then add N-methylpyrrolidone solvent (the mass-volume ratio of binder polyvinylidene fluoride and pyrrolidone is 40 mg:1 mL) and mix evenly to obtain the positive electrode slurry; Coat the positive electrode slurry on aluminum foil using a film coater, with a coating thickness of 100 μm, and heat the heating plate of the film coater to 80 °C to preliminarily dry the positive electrode. Place the preliminarily dried electrode in a vacuum drying oven and dry it at 120 °C for 10 h. Finally, cut the dried electrode into positive electrode sheets with a diameter of 10 mm using a wafer slicer.

[0096] Prepare the negative electrode sheet: Mix natural graphite, binder polyvinylidene fluoride, and conductive agent acetylene black in a mass ratio of 90:5:5, then add N-methylpyrrolidone (NMP) (the mass-volume ratio of binder polyvinylidene fluoride and pyrrolidone is 40 mg:1 mL) and mix evenly to obtain the negative electrode slurry; Coat the negative electrode slurry on copper foil using a film coater, with a coating thickness of 100 μm, and heat the heating plate of the film coater to 80 °C to preliminarily dry the negative electrode. Place the preliminarily dried electrode in a vacuum drying oven and dry it at 120 °C for 10 h. Finally, cut the dried electrode into negative electrode sheets with a diameter of 12 mm using a wafer slicer.

[0097] Separator: Use Kuraray glass fiber membrane (GF / A), model: MA-EN-SE-01.

[0098] (2) Assemble the battery in a glove box in the order of negative electrode case, negative electrode sheet, separator, electrolyte of Example 1, gasket, positive electrode sheet, and positive electrode case. The amount of electrolyte added is 90 μL. After the battery is left standing for 10 h, perform electrochemical tests.

[0099] Comparative Application Examples 1-8

[0100] Comparative Application Examples 1-8 all prepared the same lithium-ion battery as Application Example 1, except that: in step (2), the electrolyte of Example 1 was sequentially replaced with the electrolytes of Comparative Examples 1-8, and other parameter conditions were the same as those of Application Example 1.

[0101] Further test the electrochemical performance of graphite negative electrodes in different electrolytes. Figure 5 For the first three charge-discharge curves of the lithium-ion batteries prepared by adding 5 vt% FEC (denoted as LP30 + 5% FEC) to Comparative Application Example 1, Comparative Application Example 8, Application Example 1, and Comparative Example 1 at a current density of 0.1 C, it can be found that Comparative Application Example 1 (Figure 5 The SEI film formed by reduction and decomposition on the graphite surface in (a) of Figure 5 is unstable and results in poor reversibility in the first few cycles; while in Comparative Application Example 8 ( Figure 5 in (b)) and the lithium-ion battery prepared with LP30 + 5% FEC electrolyte ( Figure 5 in (c)) both exhibit excellent reversibility, which is attributed to the additives promoting the formation of the SEI film in these two electrolytes; in addition, in Application Example 1 ( - in (d)) also exhibits excellent reversibility. Moreover, in Comparative Application Example 1, the discharge plateau appears first at around 0.72V, which is attributed to the reduction and decomposition of EC solvent molecules and passivation of the graphite surface; the discharge plateau of LP30 + 5% FEC is around 1.32V because the reduction potential of FEC is much higher than that of the EC solvent, causing FEC to decompose first during the initial lithiation process of the graphite negative electrode; while in Application Example 1, it appears at around 1.48V, which is attributed to the low LUMO energy level of NO3 the low LUMO energy level of NO3 - which will preferentially form products with high ionic conductivity such as Li3N on the graphite surface.

[0102] Figure 6 Figure 15 shows the charge-discharge curves of the first three cycles of Comparative Application Example 7 at a current density of 0.1C. It can be found that Comparative Application Example 7 also has excellent charge-discharge reversibility.

[0103] Figure 7 Figure 16 shows the charge-discharge curves of the first three cycles of Comparative Application Examples 2-5 at a current density of 0.1C. It can be found that when a single inorganic lithium compound is added to the basic ether-based electrolyte (LP30), similar charge-discharge curves exist.

[0104] Figure 8 Figure 17 shows the rate performance test results of different application examples. Figure 8 In (a) of + Figure 17 are the rate performance results of Application Example 1, Comparative Application Example 1, and Comparative Application Example 8. It can be found that the rate performance of Comparative Application Example 1 is the worst because in the dilute electrolyte LP30, a large number of solvent molecules will coordinate with lithium ions, and most anions are excluded from the solvation sheath. The high desolvation energy results in difficult desolvation of Li desolvation of Li Figure 8In (b), it shows the rate performance results of lithium-ion batteries where the lithium salt concentration in Comparative Application Example 1 and Comparative Example 1 is replaced with 2 mol / L. It can be found that increasing the lithium salt concentration in the LP30 electrolyte can increase the interaction between lithium salt anions and lithium ions, reducing the content of free solvent molecules, and to a certain extent, improving its rate performance. Figure 8 In (c), it shows the rate performance results of lithium-ion batteries prepared by adding 5 vt% FEC (denoted as LP30 + 5% FEC) to Application Example 1, Comparative Application Example 7, and Comparative Example 1. It can be found that graphite shows good rate performance in the electrolyte LP30 + 5% FEC because FEC can weaken the binding between the solvent and Li + through dipole-dipole interaction, promoting the rapid migration of Li + , and facilitating the formation of a stable SEI film rich in LiF; although the rate performance of Comparative Example 7 with the same lithium salt concentration as in Example 1 has been significantly improved compared to LP30 + 5% FEC, it is still inferior to Example 1. Figure 8 In (d), it shows the rate performance results of Application Example 1 and Comparative Application Examples 1 - 4. It can be found that the rate performance of the electrolytes obtained by adding an inorganic lithium compound is inferior to that of Example 1.

[0105] Figure 9 The EIS diagram of Application Example 1 and Comparative Application Example 1 shows that the electrolyte of Example 1 has lower impedance.

[0106] Figure 10 The cycle performance results of Application Example 1, Comparative Application Example 1, and Comparative Application Example 8 show that Application Example 1 exhibits high capacity and cycle stability, indicating that a stable SEI film can be quickly formed in 1.4 M LP30 - 4S + 5% FEC; while Comparative Application Example 1 has a lower initial capacity, showing a trend of first decreasing and then gradually increasing, indicating that it takes a longer cycle process to form a stable SEI film in the LP30 electrolyte. And through Figure 9 the EIS test also shows that the electrolyte of Example 1 has lower interfacial film impedance.

[0107] Figure 11 The cycle performance results of Application Example 1 and Comparative Application Examples 1 - 4 show that for the single-salt electrolytes with the same concentration (i.e., Comparative Application Examples 2 - 4), they still need a period of cycling to become stable in the early stage. Therefore, adding 4 inorganic lithium compounds to the LP30 electrolyte can effectively improve the electrochemical performance of graphite.

[0108] Figure 12The effectiveness of the electrolyte in forming the SEI film at low temperatures before and after adding inorganic lithium compounds was tested. The specific operation was to directly cycle a fresh battery at -10 °C to form the SEI film. It was found that in Comparative Application Example 1 (LP30 electrolyte), the capacity at -10 °C was extremely low, and there was almost no capacity at -20 °C and -30 °C. However, Application Example 1 (1.4M LP30-4S + 5% FEC electrolyte) showed a capacity close to 300 mAh / g at -10 °C, and specific capacities of about 160 mAh / g and 50 mAh / g at -20 °C and -30 °C respectively, indicating that the addition of the four inorganic lithium compounds promoted the rapid formation of a stable SEI film by the electrolyte at low temperatures, and the lower desolvation energy and SEI surface activation energy contributed to its excellent rate performance.

[0109] In summary, in the prior art, the interfacial stability problem between the graphite negative electrode and the electrolyte severely restricts the electrochemical performance of graphite, and this phenomenon is more serious at low temperatures. In the present invention, four inorganic lithium compounds (LiF, LiNO3, Li2O, and Li2CO3) were directly added to a commercial carbonate-based electrolyte (1.0 M LiPF6-EC / DMC, LP30), and then combined with 5% FEC (volume fraction) organic additive to prepare a highly disordered suspension electrolyte 1.4M LP30-4S + 5% FEC. The Li / graphite battery showed far better electrochemical performance in 1.4M LP30-4S + 5% FEC than the commercial LP30 electrolyte and LP57 electrolyte. Through electrolyte characterization, it was found that the lithium ions in the electrolyte obtained in the present invention had a weak solvation structure, showing better desolvation kinetics, enabling graphite to exhibit excellent rate performance and cycling performance; more surprisingly, a stable SEI film could be rapidly formed even at -10 °C. The above test results of the present invention show that the idea of improving the mixing entropy of the electrolyte - directly adding multiple lithium salts to the commercial electrolyte - can effectively improve the compatibility between graphite and the electrolyte, and can be further optimized and extended to other systems after further improvement.

[0110] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A mixed lithium salt electrolyte, characterized in that, It includes the following raw materials: electrolyte lithium salt, inorganic lithium compound additive, organic additive and carbonate solvent; Among them, the inorganic lithium compound additive is a mixture of Li2O, LiF, Li2CO3 and LiNO3; The concentrations of Li2O, LiF, Li2CO3 and LiNO3 in the mixed lithium salt electrolyte are independently 0.05 - 0.2 mol / L; The organic additive includes fluoroethylene carbonate and / or vinylene carbonate; The volumes of fluoroethylene carbonate and vinylene carbonate are independently 1 - 5% of the total volume of the mixed lithium salt electrolyte.

2. The mixed lithium salt electrolyte according to claim 1, wherein The electrolyte lithium salt includes one or more of lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium perchlorate, lithium difluoro(oxalato)borate and lithium tetrafluoroborate.

3. A mixed lithium salt electrolyte according to claim 1 or 2, characterized in that, The concentration of lithium ions contained in the electrolyte lithium salt in the mixed lithium salt electrolyte is 0.5 - 2 mol / L.

4. A mixed lithium salt electrolyte according to claim 1, characterized in that, The carbonate solvent includes one or more of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate.

5. A mixed lithium salt electrolyte according to claim 1, characterized in that, The particle sizes of Li2O, LiF, Li2CO3 and LiNO3 are independently 80 - 100 nm.

6. A method for preparing a mixed lithium salt electrolyte according to any one of claims 1 to 5, characterized in that, It includes the following steps: Mix the electrolyte lithium salt, inorganic lithium compound additive, organic additive and carbonate solvent to obtain a mixed lithium salt electrolyte.

7. Use of a mixed lithium salt electrolyte according to any one of claims 1 to 5 or a mixed lithium salt electrolyte prepared by the preparation method of the mixed lithium salt electrolyte according to claim 6 in a lithium ion battery, characterized in that, The lithium ion battery includes: a positive electrode, a negative electrode, a mixed lithium salt electrolyte and a separator located between the positive electrode and the negative electrode.

8. The application of a mixed lithium salt electrolyte in a lithium ion battery according to claim 7, wherein The negative electrode includes a negative electrode current collector and a negative electrode film; the negative electrode film includes a negative electrode active material, a conductive agent and a binder; the negative electrode active material includes one or more of metallic lithium, natural graphite, artificial graphite, mesophase microbeads, hard carbon, soft carbon, silicon and silicon-carbon composite.

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