Locally concentrated ionic liquid electrolyte, preparation method thereof and lithium ion battery

Abstract

The invention relates to a locally concentrated ionic liquid electrolyte, a preparation method thereof and a lithium ion battery. The locally concentrated ionic liquid electrolyte comprises a lithium salt, an ionic liquid and a low dielectric constant diluent, the low dielectric constant diluent comprises 2, 2, 2-trifluoroethyl trifluoromethane sulfonate, and the low dielectric constant diluent comprises 2, 2, 2-trifluoroethyl trifluoromethane sulfonate. The locally concentrated ionic liquid electrolyte provided by the invention has high conductivity and lithium ion transference number, the overpotential of a symmetrical battery assembled by the locally concentrated ionic liquid electrolyte is smaller, the electrolyte is modified by selecting a low-dielectric-constant diluent, and the low-dielectric-constant diluent can induce the appearance of nano anion clusters, so that the stability of the electrolyte is improved. And the redox kinetics of anions at the interface is enhanced, the assembled lithium ion soft package battery can pass a needling experiment under a full charge condition without any fire and explosion, and the safety performance is greatly improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and particularly to a locally concentrated ionic liquid electrolyte, a preparation method thereof, and a lithium ion battery. Background Art

[0002] Lithium metal anodes are regarded as ideal anode materials for high energy density batteries due to their excellent theoretical capacities (3860 mAh·g -1 and 2061 mAh·cm -3 ) and low electrochemical potential (-3.04 V relative to the standard hydrogen electrode). However, the commercialization process of lithium metal batteries still faces significant challenges, and its core bottleneck stems from the thermodynamic instability at the electrode / electrolyte interface and the safety issues of traditional electrolyte systems.

[0003] Traditional carbonate-based electrolytes react violently with lithium metal, forming a solid electrolyte interface (SEI) mainly composed of organic components with weak mechanical properties, which is difficult to effectively inhibit the growth of lithium dendrites; while ether-based electrolytes have better lithium metal compatibility, but their insufficient oxidation stability limits the charging cut-off voltage to below 4.0 V and cannot be adapted to high-voltage battery systems. More notably, ester / ether solvents generally have safety hazards such as high flash points, high volatility, and flammability, severely restricting the practical application of batteries.

[0004] Ionic liquids are molten salts at room temperature, and their synergistic advantages of non-flammability, low volatility, and wide electrochemical stability windows provide new ideas for developing electrolyte systems with both safety and high performance. Due to their large ionic structures and strong cation-anion Coulomb interactions, ionic liquids generally have problems such as high viscosity and low lithium ion transference numbers. Although electrolytes using a single ionic liquid as a solvent have extremely high safety, they have poor wettability to diaphragms and electrode sheets and high costs. Blending the flame retardant triethyl phosphate (TEP) with ionic liquids can optimize the physical and chemical properties, but the strong solvation ability of TEP will cause a decrease in the mechanical strength of the interfacial film, ultimately damaging the long-term cycle stability of the battery.

[0005] In recent years, the locally concentrated ionic liquid electrolyte system developed introduces hydrofluoroether (HFE) diluents (such as 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, bis(2,2,2-trifluoroethyl) ether, and 1,2-bis(1,1,2,2-tetrafluoroethoxy) ethane, etc.), which maintain the ionic solvation structure while reducing the system viscosity through low-viscosity diluents. However, the low flash points (<50 °C) and high vapor pressures (4.8 - 5.3 kPa) of such HFE diluents cause the electrolyte to volatilize easily and form flammable gases under thermal abuse conditions, leading to serious safety hazards.

[0006] Therefore, due to the above-mentioned defects existing in the prior art, how to develop a locally concentrated ionic liquid electrolyte with a low diluent content for lithium metal batteries to achieve a comprehensive technical improvement in the electrolyte system of lithium metal batteries in terms of safety, interfacial stability, and solvation structure regulation has become an urgent problem to be solved at present. Summary of the Invention

[0007] To solve the above technical problems, the object of the present invention is to provide a locally concentrated ionic liquid electrolyte with a low diluent content for lithium metal batteries to solve the comprehensive technical problems of the electrolyte system of lithium metal batteries in terms of safety, interfacial stability, and solvation structure regulation.

[0008] To achieve this purpose, the present invention adopts the following technical solutions:

[0009] In the first aspect, the present invention provides a locally concentrated ionic liquid electrolyte, and the locally concentrated ionic liquid electrolyte includes a lithium salt, an ionic liquid, and a low dielectric constant diluent;

[0010] The low dielectric constant diluent includes 2,2,2-trifluoroethyl trifluoromethanesulfonate.

[0011] The locally concentrated ionic liquid electrolyte provided by the present invention modifies the electrolyte by selecting a low dielectric constant diluent, and not only has high conductivity and lithium ion transference number, but also the overpotential of the assembled symmetric battery is smaller. The low dielectric constant diluent will induce the appearance of nano-anion clusters, enhancing the redox kinetics of anions at the interface. The diluent improves the ionic conductivity and lithium ion transference number of the system by reducing the viscosity of the ionic liquid electrolyte system.

[0012] In addition, compared with the commonly used fluorinated ether diluents, the low dielectric constant diluent adopted by the present invention has not been reported in the prior art. 2,2,2-trifluoroethyl trifluoromethanesulfonate perfectly adapts to the high-voltage battery system and is a new type, efficient, and environmentally friendly diluent.

[0013] The "local concentration" in the locally concentrated ionic liquid electrolyte of the present invention is due to the addition of the diluent, which promotes the formation of ion clusters (such as the coordination domain of Li + and FSI - in the electrolyte at the nano-scale) and the diluent enrichment domain. Through the precise design of the diluent, the locally concentrated ionic liquid electrolyte realizes the coexistence of ion clusters and diluent phases at the nano-scale. The ionic liquid itself has a high ion concentration and a wide electrochemical window, while the diluent reduces the overall viscosity and improves the ion mobility. This "local concentration" structure not only retains the interfacial stability and lithium metal compatibility of the high-concentration electrolyte, but also improves the fluidity and kinetic advantages.

[0014] The following are the preferred technical solutions of the present invention, but do not limit the technical solutions provided by the present invention. Through the following preferred technical solutions, the technical objectives and beneficial effects of the present invention can be better achieved and realized.

[0015] Preferably, the molar ratio of the lithium salt, ionic liquid and low dielectric constant diluent is 1:(2 - 4):(0.5 - 1.5), for example, it can be 1:2:0.5, 1:2:1, 1:2:1.5, 1:2.5:0.5, 1:2.5:1, 1:2.5:1.5, 1:3:0.5, 1:3:1, 1:3:1.5, 1:3.5:0.5, 1:3.5:1, 1:3.5:1.5, 1:4:0.5, 1:4:1 or 1:4:1.5, but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0016] Preferably, the molar ratio of the lithium salt, ionic liquid and low dielectric constant diluent is 1:(2 - 2.5):(0.5 - 1), for example, it can be 1:2:0.5, 1:2:1, 1:2.5:0.5 or 1:2.5:1, but not limited to the listed values, and other unlisted values within the numerical range are equally applicable, and preferably 1:2:0.5.

[0017] The present invention further controls the molar ratio of the lithium salt, ionic liquid and low dielectric constant diluent to be 1:(2 - 2.5):(0.5 - 1). The added amount in the present invention is lower than that of the conventionally used fluoroethers. Excellent technical effects can be achieved at a lower content. If the content of the diluent is too high, phase precipitation will occur, which is not conducive to the electrochemical stability of the battery; if the content of the diluent is too low, the conductivity and lithium ion transference number of the ionic liquid electrolyte cannot be improved, and the formation of nano-anion clusters cannot be induced. If the content of the ionic liquid is too low, the viscosity increases, and if the content is too high, the Coulombic efficiency of the battery is low.

[0018] Preferably, the lithium salt includes any one or at least two combinations of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, LiPF6, lithium perchlorate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate or lithium difluorophosphate. Typical but non-limiting combinations include the combination of lithium bis(trifluoromethanesulfonyl)imide and lithium bis(fluorosulfonyl)imide, the combination of LiPF6 and lithium perchlorate, the combination of lithium bis(oxalato)borate and lithium difluoro(oxalato)borate, the combination of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide and LiPF6, the combination of lithium bis(oxalato)borate, lithium difluoro(oxalato)borate and lithium difluorophosphate, and preferably lithium bis(trifluoromethanesulfonyl)imide.

[0019] Preferably, the ionic liquid includes an ionic liquid with bis(fluorosulfonyl)imide or bis(trifluoromethanesulfonyl)imide as the anion and pyrrolidine or pyridinium as the cation.

[0020] Preferably, the ionic liquid includes N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (Pyr 13 FSI), N-methyl-N-butylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Pyr 14 TFSI), or N-methyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Pyr 13 TFSI), and any one or a combination of at least two of them. Typical but non-limiting combinations include Pyr 13 FSI and Pyr 14 TFSI combination, Pyr 14 TFSI and Pyr 14 TFSI combination, Pyr 13 FSI and Pyr 14 TFSI combination, Pyr 13 FSI, Pyr 14 TFSI and Pyr 13 TFSI combination, preferably Pyr 13 FSI.

[0021] In a second aspect, the present invention provides a method for preparing a locally concentrated ionic liquid electrolyte as described in the first aspect. The preparation method includes the following steps:

[0022] Mix a lithium salt, an ionic liquid, and a low dielectric constant diluent according to a molar ratio to obtain a locally concentrated ionic liquid electrolyte.

[0023] The preparation process of the present invention is simple, and a locally concentrated ionic liquid electrolyte with excellent performance can be obtained through simple mixing.

[0024] In a third aspect, the present invention provides a lithium-ion battery, which includes a positive electrode, a negative electrode, a separator, and the locally concentrated ionic liquid electrolyte described in the first aspect.

[0025] The lithium-ion battery provided by the present invention has excellent long-cycle stability and safety. The surface of the negative electrode material of the lithium-ion battery has a thin and dense SEI film rich in inorganic substances. LiF formed at the interface can inhibit the growth of lithium dendrites, promote the uniform deposition of lithium ions, effectively improve the battery interface stability, not only improve the safety performance of the battery, but also its high ion transference number reduces the overpotential of the battery, and the Coulomb efficiency is higher. At the same time, it improves the long-cycle stability performance of the battery, and the service life of the battery is longer.

[0026] Preferably, the lithium-ion battery is a lithium metal negative electrode battery.

[0027] Preferably, the surface of the negative electrode of the metal negative electrode battery contains a LiF solid electrolyte interface film.

[0028] The 2Ah Li / NCM622 soft-pack battery provided by the present invention successfully passes the nail penetration test under full charge conditions without any fire or explosion, and its safety performance is greatly improved.

[0029] The numerical ranges described in the present invention include not only the above-listed point values, but also any point values between the above numerical ranges not listed. Due to space limitations and for the sake of simplicity, the specific point values included in the described ranges are not exhaustively listed in the present invention.

[0030] Compared with the prior art, the present invention has at least the following beneficial effects:

[0031] (1) The locally concentrated ionic liquid electrolyte provided by the present invention modifies the electrolyte by adding a low-dose low-dielectric constant diluent. It not only has high conductivity and lithium ion transference number, but also has a smaller overpotential for the assembled symmetric battery. The low-dielectric constant diluent will induce the appearance of nano-anion clusters, enhancing the redox kinetics of anions at the interface. 2,2,2-Trifluoroethyl trifluoromethanesulfonate perfectly adapts to the high-voltage battery system and is a new type, efficient and environmentally friendly diluent.

[0032] (2) The lithium ion battery provided by the present invention has excellent long-cycle stability and safety. The surface of the negative electrode material of the lithium ion battery has a thin and dense SEI film rich in inorganic substances. LiF formed at the interface can inhibit the growth of lithium dendrites and promote the uniform deposition of lithium ions, effectively improving the battery interface stability. It not only improves the safety performance of the battery, but also its higher ion transference number reduces the overpotential of the battery, and the Coulomb efficiency is higher. At the same time, it improves the long-cycle stability performance of the battery, and the service life of the battery is longer. Description of the Drawings

[0033] Figure 1 is the ternary phase diagram of the lithium salt, ionic liquid and low-dielectric constant diluent of the present invention;

[0034] Figure 2 is the Coulomb efficiency spectrum of the Li / Cu battery assembled with the locally concentrated ionic liquid electrolyte of Example 1, Example 4 and Example 6 of the present invention;

[0035] Figure 3 is the charge-discharge curve spectrum of the Li / Cu battery assembled with the locally concentrated ionic liquid electrolyte of Example 6 of the present invention;

[0036] Figure 4 is the Coulomb efficiency spectrum of the Li / Cu battery assembled with the locally concentrated ionic liquid electrolyte of Example 4 of the present invention;

[0037] Figure 5 is the charge-discharge curve spectrum of the Li / Cu battery assembled with the locally concentrated ionic liquid electrolyte of Example 1 of the present invention;

[0038] Figure 6 is the partial concentrated ionic liquid electrolyte of Example 1 of the present invention 7 Li NMR spectrogram;

[0039] Figure 7 is the graph of the conductivity and transference number of the electrolytes of Example 1 and Comparative Example 1 of the present invention;

[0040] Figure 8 is the Coulombic efficiency graph of the Li / Cu batteries assembled with the electrolytes of Example 1, Comparative Example 1, and Comparative Example 2 of the present invention;

[0041] Figure 9 is the polarization voltage curve of the Li / Li symmetric battery assembled with the electrolyte of Example 1 and Comparative Example 1 of the present invention;

[0042] Figure 10 is the polarization voltage curve of the Li / Li symmetric battery assembled with the electrolyte of Comparative Example 2 of the present invention;

[0043] Figure 11 is the surface SEM image of the copper foil after cycling in the Li / Cu battery assembled with the electrolyte of Example 1 of the present invention;

[0044] Figure 12 is the surface SEM image of the copper foil after cycling in the Li / Cu battery assembled with the electrolyte of Comparative Example 1 of the present invention;

[0045] Figure 13 is the surface SEM image of the copper foil after cycling in the Li / Cu battery assembled with the electrolyte of Comparative Example 2 of the present invention;

[0046] Figure 14 is the time-of-flight secondary mass spectrometry (TOF-SIMS) image of the lithium negative electrode side after cycling the copper foil in the Li / Cu battery assembled with the electrolyte of Example 1;

[0047] Figure 15 is the time-of-flight secondary mass spectrometry (TOF-SIMS) image of the lithium negative electrode side after cycling the copper foil in the Li / Cu battery assembled with the electrolyte of Comparative Example 1;

[0048] Figure 16 is the surface SEM image of the positive electrode material interface after cycling in the Li / NCM622 battery assembled with the electrolyte of Example 1 of the present invention;

[0049] Figure 17 is the surface SEM image of the positive electrode material interface after cycling in the Li / NCM622 battery assembled with the electrolyte of Comparative Example 1 of the present invention;

[0050] Figure 18The long cycle spectra of Li / NCM622 button cells assembled with the electrolytes of Example 1 and Comparative Example 1 of the present invention at cut-off voltages of 4.3V and 4.5V respectively;

[0051] Figure 19 The coulombic efficiency spectra of Li / NCM622 button cells assembled with the electrolytes of Example 1 and Comparative Example 1 of the present invention at cut-off voltages of 4.3V and 4.5V respectively;

[0052] Figure 20 This is a long cycle spectrum of a 1.4Ah Li / NCM622 soft pack battery assembled with the electrolyte of Example 1 of the present invention at a cut-off voltage of 4.5V;

[0053] Figure 21 The voltage and temperature curves of the 2Ah Li / NCM622 soft pack battery assembled with the electrolyte of Example 1 of the present invention before and after the puncture test;

[0054] Figure 22 It is the voltage and temperature curve of the 2Ah Li / NCM622 soft pack battery assembled with the electrolyte of comparative example 1 of the present invention before and after the puncture test. DETAILED DESCRIPTION

[0055] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and through specific implementation methods. However, the following examples are only simplified examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention shall be subject to the claims.

[0056] In the following examples and comparative examples, unless otherwise specified, all reagents and consumables were purchased from conventional reagent manufacturers in the field; unless otherwise specified, the experimental methods and technical means used were conventional methods and means in the field.

[0057] Example 1

[0058] This embodiment provides a locally concentrated ionic liquid electrolyte, wherein the locally concentrated ionic liquid electrolyte comprises LiFSI, Pyr 13 FSI and 2,2,2-trifluoroethyl trifluoromethanesulfonate;

[0059] The method for preparing the locally concentrated ionic liquid electrolyte provided in this embodiment comprises the following steps:

[0060] (1) Activate 3 angstrom molecular sieves at 300°C for 12 h, add them into 2,2,2-trifluoroethyl trifluoromethanesulfonate, remove water and dry for 3 days; 13 FSI was placed in a vacuum oven at 110°C to remove water for 24 hours; LiFSI was placed in a vacuum oven at 80°C to dry for 12 hours.

[0061] (2) Dissolve LiFSI in Pyr in a molar ratio of 1:2 13 in FSI to obtain an ionic liquid electrolyte;

[0062] (3) Mix the ionic liquid electrolyte and 2,2,2-trifluoroethyl trifluoromethanesulfonate in a molar ratio of LiFSI, Pyr 13 FSI and 2,2,2-trifluoroethyl trifluoromethanesulfonate at 1:2:0.5 to obtain a locally concentrated ionic liquid electrolyte, which is named TAILE.

[0063] Figure 1 is the ternary phase diagram of lithium salt, ionic liquid, and low dielectric constant diluent. TFS represents the low dielectric constant diluent, Figure 1 indicating that the region above the dashed line is an immiscible system, that is, the system will exhibit supersaturation, the system will be turbid, and the deposition of lithium salt is not desired either during the experiment or in industrial production. Below the dashed line, such as molar ratios of 1:2:0.5, 1:2:0.7, 1:2:1, 1:3:2, 1:4:4 are all homogeneous systems.

[0064] Figure 2 and Figure 5 are the Coulomb efficiency and charge-discharge curves of the Li / Cu battery assembled with the locally concentrated ionic liquid electrolyte of Example 1 respectively. It can be seen from the figure that TAILE can stably cycle no less than 600 times at a current density of 1 mA·cm -2 and a deposition capacity of 0.5 mAh·cm -2 , with a Coulomb efficiency of no less than 98% and a normal charge-discharge curve; while Figure 3 and Figure 4 show that the batteries assembled with the electrolytes of Example 6 and Example 4 fail rapidly, with a lifespan of no more than 300 cycles.

[0065] Figure 6 is the 7 Li NMR spectrogram of the locally concentrated ionic liquid electrolyte of Example 1. It can be seen from the figure that 7 the Li NMR spectrum shows that from the 1LiFSI:4Pyr13FSI sample to Comparative Example 1 and then to Example 1, the NMR peak position gradually shifts to the high field. This upfield shift indicates that the FSI - anion's shielding effect on Li + is enhanced, indicating that the interaction between Li + -FSI - is stronger, forming a solvation structure enhanced by nano-anion clusters.

[0066] Figure 7It is the graph of the conductivity and transference number of the electrolytes in Example 1 and Comparative Example 1. It can be seen that the ionic conductivity and lithium ion transference number of Example 1 are 3.228 mS·cm -1 and 0.45, which are higher than 2.588 mS·cm -1 and 0.38 of Comparative Example 1.

[0067] Figure 8 It is the Coulombic efficiency graph of the Li / Cu batteries assembled with the electrolytes of Example 1, Comparative Example 1 and Comparative Example 2. It can be seen from the graph that Example 1 has a high Coulombic efficiency of 98.7%, while Comparative Example 1 and Comparative Example 2 show poor performance in Li / Cu batteries.

[0068] Figure 9 It is the polarization voltage curve of the Li / Li symmetric battery assembled with the electrolytes of Example 1 and Comparative Example 1. It can be seen from the graph that the cycling overpotential of the Li / Li symmetric battery in Example 1 is only 32 mV, while Comparative Example 1 shows a high polarization voltage of 64 mV in the Li / Li symmetric battery, which is twice that of TAILE in Example 1. Figure 10 The cycling polarization voltage of the Li / Li symmetric battery of Comparative Example 2 shown increases continuously, and the interface is extremely unstable.

[0069] Figure 11 It is the surface SEM image of the copper foil after cycling of the Li / Cu battery assembled with the local concentrated ionic liquid electrolyte of Example 1. It can be seen from the graph that the deposition morphology of the liquid electrolyte in Example 1 is dense and uniform.

[0070] Figure 12 It is the surface SEM image of the copper foil after cycling of the Li / Cu battery assembled with the electrolyte of Comparative Example 1. It can be seen from the graph that the surface of the copper foil of the ionic liquid electrolyte in Comparative Example 1 is in a rod-like morphology, and the dendrite growth is inhibited to a certain extent, but the deposited lithium is relatively loose.

[0071] Figure 13 It is the surface SEM image of the copper foil after cycling of the Li / Cu battery assembled with the electrolyte of Comparative Example 2. It can be seen from the graph that the conventional carbonate electrolyte is in a classic dendritic shape, and as the battery cycles, the dendrites penetrate the separator, posing a risk of short circuit.

[0072] Figure 14 It is the time-of-flight secondary mass spectrometry (TOF-SIMS) of the lithium negative electrode side of the copper foil after cycling of the Li / Cu battery assembled with the electrolyte of Example 1. It can be seen from the graph that the system in Example 1 retains a relatively thick fresh metal Li layer after cycling, and only a thin surface layer reacts with the electrolyte.

[0073] Figure 15It is the time-of-flight secondary mass spectrometry (TOF-SIMS) of the lithium anode side after the copper foil of the Li / Cu battery assembled with the electrolyte of Comparative Example 1 is cycled. It can be seen from the figure that, in contrast, the system of Comparative Example 1 shows a significant decrease and uneven distribution in the fresh lithium content; in addition, the LiF2 - signal is usually related to inorganic LiF species;

[0074] The SEI formed in Example 1 has a higher LiF content on the surface. As the sputtering time increases, the LiF signal decreases rapidly; in contrast, the ILE system shows a lower LiF content on the surface, and a large amount of LiF is retained in the inner layer; these results together indicate that the nano-anion clusters enhance the solvation structure and promote the formation of a thinner, denser, and more inorganic-rich SEI layer; this optimized SEI structure effectively solves the problem of high interfacial impedance caused by the formation of excessive LiF due to anion decomposition.

[0075] Figure 16 It is the SEM image of the cathode material interface after the Li / NCM622 battery assembled with the electrolyte of Example 1 is cycled. It can be seen from the figure that the cycled cathode shows significantly fewer cracks, and the particles maintain a complete morphology and a clear cross-section, indicating a complete structure.

[0076] Figure 17 It is the SEM image of the cathode material interface after the Li / NCM622 battery assembled with the electrolyte of Comparative Example 1 is cycled. It can be seen from the figure that the cycled cathode shows extensive cracks. This is because during the charge and discharge process, the polycrystalline cathode material undergoes repeated lattice volume expansion and contraction, generating internal mechanical stress within the particles; when the electrolyte penetrates into the cracks and triggers side reactions with the cathode material, it will lead to irreversible crack formation and expansion.

[0077] Figure 18 It is the long-term cycling profiles of the Li / NCM622 coin cells assembled with the electrolytes of Example 1 and Comparative Example 1 at cut-off voltages of 4.3V and 4.5V respectively. It can be seen from the figure that the Li / NCM622 coin cells using TAILE have a cycle life of no less than 300 cycles and 200 cycles at cut-off voltages of 4.3V and 4.5V respectively.

[0078] Figure 19 It is the Coulombic efficiency profiles of the Li / NCM622 coin cells assembled with the electrolytes of Example 1 and Comparative Example 1 at cut-off voltages of 4.3V and 4.5V respectively. It can be seen from the figure that the Coulombic efficiency is not less than 99.3%; while the Li / NCM622 coin cells using ILE are lower than TAILE in terms of cycle life and Coulombic efficiency; among them, the loading is 10.6mg·cm -2 , and the specific capacity is not less than 155mAh·g -1 .

[0079] Figure 20 It is the long - cycle graph of the 1.4Ah Li / NCM622 soft - package battery assembled with the electrolyte of Example 1 at a cut - off voltage of 4.5V. It can be seen from the figure that at 4.5V, the life of the 1.4Ah Li / NCM622 soft - package battery is not less than 80 cycles, and the loading amount is 15mg·cm -2 , and the Coulomb efficiency is not less than 99.3%.

[0080] Figure 21 It is the voltage and temperature curves of the 2Ah Li / NCM622 soft - package battery assembled with the electrolyte of Example 1 before and after the nail - penetration test. It can be seen from the figure that there is no explosion or fire during the nail - penetration test, and the battery has excellent safety under abusive conditions.

[0081] Figure 22 It is the voltage and temperature curves of the 2Ah Li / NCM622 soft - package battery assembled with the electrolyte of Comparative Example 1 before and after the nail - penetration test. It can be seen from the figure that there is a violent explosion during the nail - penetration test, and the temperature reaches 3500℃ instantly, indicating that the battery has serious potential safety hazards under abusive conditions.

[0082] Example 2

[0083] This example provides a locally - concentrated ionic - liquid electrolyte. The locally - concentrated ionic - liquid electrolyte includes LiFSI, Pyr 13 FSI and 2,2,2 - trifluoroethyl trifluoromethanesulfonate with a molar ratio of 1:3:0.5;

[0084] The preparation method of the locally - concentrated ionic - liquid electrolyte provided in this example includes the following steps:

[0085] (1) Activate 3 - angstrom molecular sieve at 270℃ for 15h, then add it to 2,2,2 - trifluoroethyl trifluoromethanesulfonate, and carry out water removal and drying for 3 days; Place Pyr 13 FSI in a vacuum oven at 110℃ for 24h to remove water; Place LiFSI in a vacuum oven at 80℃ for 12 hours to dry.

[0086] (2) Dissolve LiFSI in Pyr 13 FSI according to the molar ratio of 1:3 to obtain an ionic - liquid electrolyte;

[0087] (3) Mix the ionic - liquid electrolyte and 2,2,2 - trifluoroethyl trifluoromethanesulfonate according to the molar ratio of LiFSI, Pyr 13 FSI and 2,2,2 - trifluoroethyl trifluoromethanesulfonate of 1:3:0.5 to obtain a locally - concentrated ionic - liquid electrolyte.

[0088] Example 3

[0089] This embodiment provides a locally concentrated ionic liquid electrolyte, and the locally concentrated ionic liquid electrolyte includes LiFSI, Pyr, and 2,2,2-trifluoroethyl trifluoromethanesulfonate with a molar ratio of 1:2:1. 13 ;

[0090] The preparation method of the locally concentrated ionic liquid electrolyte provided in this embodiment includes the following steps:

[0091] (1) Activate the 3-angstrom molecular sieve at 350 °C for 8 h, and then add it to 2,2,2-trifluoroethyl trifluoromethanesulfonate to remove water and dry for 3 days; place Pyr 13 FSI in a vacuum oven at 100 °C to remove water for 24 h; place LiFSI in a vacuum oven at 90 °C and dry for 12 hours.

[0092] (2) Dissolve LiFSI in Pyr 13 FSI according to the molar ratio of 1:2 to obtain an ionic liquid electrolyte;

[0093] (3) Mix the ionic liquid electrolyte and 2,2,2-trifluoroethyl trifluoromethanesulfonate according to the molar ratio of LiFSI, Pyr 13 FSI and 2,2,2-trifluoroethyl trifluoromethanesulfonate of 1:2:1 to obtain a locally concentrated ionic liquid electrolyte.

[0094] Example 4

[0095] This embodiment provides a locally concentrated ionic liquid electrolyte, which is only different from Example 1 in that the molar ratio of the lithium salt, the ionic liquid, and the low dielectric constant diluent is 1:5:0.5.

[0096] Example 5

[0097] This embodiment provides a locally concentrated ionic liquid electrolyte, which is only different from Example 1 in that the molar ratio of the lithium salt, the ionic liquid, and the low dielectric constant diluent is 1:1:0.5.

[0098] Example 6

[0099] This embodiment provides a locally concentrated ionic liquid electrolyte, which is only different from Example 1 in that the molar ratio of the lithium salt, the ionic liquid, and the low dielectric constant diluent is 1:2:4.

[0100] Example 7

[0101] This embodiment provides a locally concentrated ionic liquid electrolyte, which is only different from Example 1 in that the molar ratio of the lithium salt, the ionic liquid, and the low dielectric constant diluent is 1:2:0.1.

[0102] Comparative Example 1

[0103] This comparative example provides an ionic liquid electrolyte, which is only different from that of Example 1 in that no diluent with a low dielectric constant is added, that is, step (3) is not carried out during the preparation process, and it is named ILE.

[0104] Comparative Example 2

[0105] This comparative example provides a conventional liquid electrolyte, and its composition and ratio are: 1 mol / L LiPF6 / EC:DEC = 1:1 vol%.

[0106] Comparative Example 3

[0107] This comparative example provides a locally concentrated ionic liquid electrolyte, which is only different from that of Example 1 in that 2,2,2-trifluoroethyl trifluoromethanesulfonate is replaced by 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether.

[0108] The ionic liquid electrolytes prepared in the examples and comparative examples and the batteries assembled using them were tested:

[0109] (1) Long cycle stability test: Assemble a Li / Cu battery, and the test conditions are a current density of 1 mA·cm -2 and a deposition capacity of 0.5 mAh·cm -2 .

[0110] (2) Lithium nuclear magnetic resonance was used to characterize the solvation structure: Detect the strength of the interaction between lithium ions and anions.

[0111] (3) Interface stability test: Assemble a Li / Li symmetric battery, and as the number of charge and discharge cycles increases, test the magnitude of its polarization voltage.

[0112] (4) Time-of-flight secondary mass spectrometry (TOF-SIMS) characterization: Perform time-of-flight secondary mass spectrometry detection on the lithium metal interface of the battery after long cycling, test the composition and content of its surface, and at the same time, as the sputtering time extends, detect the composition and content of its inner surface layer.

[0113] (5) Safety performance test of lithium metal soft-pack battery: Perform a needle-punch test on the assembled lithium metal soft-pack battery, and at the same time test the voltage and temperature of the battery before and after the needle-punch test.

[0114] The test results are shown in Table 1 below.

[0115] Table 1

[0116]

[0117]

[0118] It can be seen from the test results that:

[0119] (1) It can be seen from Examples 1 to 3 that by adding a low dose of low-dielectric-constant diluent 2,2,2-trifluoroethyl trifluoromethanesulfonate to modify the electrolyte, the obtained local-concentrated ionic liquid electrolyte not only has high conductivity and lithium-ion transference number, but also the assembled symmetric cell has a smaller overpotential. The lithium-ion battery has excellent long-cycle stability and safety. The surface of the negative electrode material of the lithium-ion battery has a thin and dense SEI film rich in inorganic substances. LiF formed at the interface can inhibit the growth of lithium dendrites, promote the uniform deposition of lithium ions, and effectively improve the interface stability of the battery. The Li / NCM622 button battery has a cycle life of not less than 300 cycles and 200 cycles respectively at the cut-off voltages of 4.3V and 4.5V, and the Coulomb efficiency is not less than 99.3%. The 2Ah Li / NCM622 soft-pack battery successfully passes the nail penetration test under full charge conditions without any fire or explosion, and the safety performance is greatly improved.

[0120] (2) By comparing Example 1 with Examples 4 to 7, it can be seen that by further controlling the molar ratio of lithium salt, ionic liquid and low-dielectric-constant diluent to 1:(2 - 2.5):(0.5 - 1), the addition amount of the low-dielectric-constant diluent in the present invention is lower than that of the commonly used fluoroethers. Excellent technical effects can be achieved at a lower content. If the content of the diluent is too high, phase separation will occur, which is not conducive to the electrochemical stability of the battery. If the content of the diluent is too low, the conductivity and lithium-ion transference number of the ionic liquid electrolyte cannot be improved, and the formation of nano-anion clusters cannot be induced. When the content of the ionic liquid is too low, the viscosity increases, and when the content is too high, the Coulomb efficiency of the battery is low.

[0121] (3) By comparing Example 1 with Comparative Example 1, it can be seen that when the low-dielectric-constant diluent 2,2,2-trifluoroethyl trifluoromethanesulfonate is not added, the lithium-ion transference number and conductivity of the obtained ionic liquid electrolyte will both decrease significantly. During the charge and discharge process, the deposition of metallic lithium is relatively loose, the growth of lithium dendrites is uncontrolled, the Coulomb efficiency and long-cycle stability performance of the battery decrease, and the safety performance of the battery is also worse, far inferior to the excellent effect of the example of the present invention.

[0122] (4) By comparing Example 1 with Comparative Example 2, it can be seen that when the battery uses a traditional electrolyte, since a dense and uniform SEI film cannot be formed, the polarization voltage of the symmetric cell during the battery cycle increases continuously, the interface is extremely unstable, and the Coulomb efficiency and long-cycle performance of the battery are also worse than those of Example 1.

[0123] (5) It can be seen from Example 1 and Comparative Example 3 that by adding the low-dielectric-constant diluent 2,2,2-trifluoroethyl trifluoromethanesulfonate, the present invention can achieve enhanced lithium-ion anion interactions. However, when using the conventional 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether, the technical effect of forming nano-anion clusters cannot be achieved, and its ionic conductivity and lithium-ion transference number are both low. The essential reason is that the dielectric constant of 2,2,2-trifluoroethyl trifluoromethanesulfonate is 8.5, while the dielectric constant of the conventional fluorinated ether is 14.2, which is higher than the proposed 2,2,2-trifluoroethyl trifluoromethanesulfonate diluent. The interaction between ions is inversely proportional to the dielectric constant of the electrolyte system. Therefore, the interaction between lithium ions and anions in the electrolyte system using 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether diluent is weaker than that in the electrolyte system using 2,2,2-trifluoroethyl trifluoromethanesulfonate diluent, and the anion nano-cluster structure cannot be formed. The movement of lithium ions in the anion nano-cluster structure formed in Example 1 is a structural movement, and the lithium-ion transport efficiency is higher, so the conductivity and transference number are higher.

[0124] In summary, the local-concentrated ionic liquid electrolyte provided by the present invention modifies the electrolyte by adding a low dose of low-dielectric-constant diluent, not only has high conductivity and lithium-ion transference number, but also the assembled symmetric battery has a smaller overpotential. The low-dielectric-constant diluent will induce the appearance of nano-anion clusters, enhance the redox kinetics of anions at the interface. 2,2,2-Trifluoroethyl trifluoromethanesulfonate perfectly fits the high-voltage battery system. The assembled lithium-ion battery has excellent long-cycle stability and safety. The surface of the negative electrode material of the lithium-ion battery has a thin and dense SEI film rich in inorganic substances. The LiF formed at the interface can inhibit the growth of lithium dendrites, promote the uniform deposition of lithium ions, effectively improve the battery interface stability, not only improve the safety performance of the battery, its higher lithium-ion transference number reduces the overpotential of the battery, and the Coulomb efficiency is higher. At the same time, it improves the long-cycle stability performance of the battery, and the service life of the battery is longer.

[0125] The applicant declares that the above description is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by any person skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A local concentrated ionic liquid electrolyte, characterized in that, The local concentrated ionic liquid electrolyte includes a lithium salt, an ionic liquid, and a low dielectric constant diluent; The low dielectric constant diluent includes 2,2,2-trifluoroethyl trifluoromethanesulfonate.

2. The partial concentration ionic liquid electrolyte according to claim 1, wherein The molar ratio of the lithium salt, the ionic liquid, and the low dielectric constant diluent is 1:(2 - 4):(0.5 - 1.5).

3. The partial concentration ionic liquid electrolyte according to claim 1 or 2, characterized in that The molar ratio of the lithium salt, the ionic liquid, and the low dielectric constant diluent is 1:(2 - 2.5):(0.5 - 1).

4. The partial concentration ionic liquid electrolyte according to any one of claims 1-3, characterized in that, The lithium salt includes any one or a combination of at least two of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, LiPF6, lithium perchlorate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, or lithium difluorophosphate.

5. The partial concentration ionic liquid electrolyte according to any one of claims 1-4, characterized in that The ionic liquid includes an ionic liquid with bis(fluorosulfonyl)imide or bis(trifluoromethylsulfonyl)imide as the anion and pyrrolidine or pyridinium as the cation.

6. The partial concentration ionic liquid electrolyte according to any one of claims 1-5, characterized in that, The ionic liquid includes any one or a combination of at least two of N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide, N-methyl-N-butylpyrrolidinium bis(trifluoromethylsulfonyl)imide, or N-methyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide.

7. A method for preparing a locally concentrated ionic liquid electrolyte according to any one of claims 1-6, characterized in that, The preparation method includes the following steps: Mix the lithium salt, the ionic liquid, and the low dielectric constant diluent according to the molar ratio to obtain the local concentrated ionic liquid electrolyte.

8. A lithium-ion battery, characterized in that, The lithium ion battery includes a positive electrode, a negative electrode, a separator, and the local concentrated ionic liquid electrolyte according to any one of claims 1 - 6.

9. The lithium ion battery according to claim 8, characterized in that, The lithium ion battery is a lithium metal negative electrode battery.

10. The lithium ion battery according to claim 9, characterized in that, The surface of the negative electrode of the metal negative electrode battery contains a LiF solid electrolyte interface film.

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